large breathing effects in three-dimensional porous hybrid matter

This article was published as part of the

2009 Metal–organic frameworks issueReviewing the latest developments across the interdisciplinary area of

metal–organic frameworks from an academic and industrial perspective Guest Editors Jeffrey Long and Omar Yaghi

Please take a look at the issue 5 table of contents to access the other reviews.

Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online / Journal Homepage / Table of Contents for this issue

http://www.rsc.org/Publishing/Journals/CS/article.asp?JournalCode=CS&SubYear=2009&Issue=5&type=Issue

http://www.rsc.org/publishing/journals/CS/Index.asp

http://dx.doi.org/10.1039/b804302g

http://pubs.rsc.org/en/journals/journal/CS

http://pubs.rsc.org/en/journals/journal/CS?issueid=CS038005

Large breathing effects in three-dimensional porous hybrid matter:

facts, analyses, rules and consequencesw

Gerard Ferey*ab and Christian Serreb

Received 2nd October 2008

First published as an Advance Article on the web 26th February 2009

DOI: 10.1039/b804302g

This critical review focusses on a strange behaviour of crystallized solid matter: its reversible

swelling with large magnitude. This will be of interest for experts in porous solids but also for

solid state chemists and physicists. Some examples, classified according to the dimensionality of

the inorganic subnetwork, present the general requirements and the structural rules which govern

the existence of this phenomenon. Its consequences concern specific applications related to

sensors, energy savings, sustainable development and health (100 references).

1. Introduction

Breathing is associated with life, at the macroscopic level and

at the molecular scale as well. Biological matter is known for a

long time to imply molecular movements with large magnitude

during biochemical reactions, and can be mimicked by clever

organic chemists.1,2 On the contrary, inorganic matter is often

associated with rigid bodies and a quasi-immobility except

during phase transitions in the solid state.

However, some recent papers run counter to these pre-

conceived ideas and describe solids which, under an external

stimulus (temperature, pressure, chemical inclusion. . .),

exhibit a very important flexibility, while retaining the same

or similar topologies. The atomic displacements reach some-

times several A. This paper will analyze some of these solids

and try to explain the different origins of such flexibility. We

shall call this phenomenon ‘breathing’3–6which, according to

other authors, is also labelled as ‘dynamic frameworks’,7

‘springlike’,8 ‘sponge like’9,10 or ‘accordion’11 effects.

Breathing is associated with reversible movements between

two states corresponding to expansion and contraction,

respectively. It is obvious for humans who inhale oxygen

and breathe out carbon dioxide. However, as seen below,

two situations are possible: many times, expansion is

associated with an input and contraction with an output but

the inverse phenomenon can occur: expansion 3 output and

contraction3 input. Whatever the sense of the breathing, the

external stimulus generates the movement.

An important point of the breathing is also its magnitude.

Generally, in solid state matter, the movements are very small

(o0.8 A) and often occur during displacive (without bond

breaking) phase transitions. A good example is provided by

the distortions of the perovskites12 or the FeF3 structure

(Fig. 1) with the ReO3 type structure.13 The rhombohedral

room temperature variety (contracted form) corresponds to

the tilting of octahedra represented in Fig. 1(a). It transforms

reversibly by heating at 400 1C into the ideal cubic ReO3 type,

a Institut universitaire de France, France.E-mail: [email protected]. E-mail: [email protected]

b Institut Lavoisier, (CNRS 8180), University of Versailles, 45,Avenue des Etats-Unis, F-78035 Versailles Cedex, France

w Part of the metal–organic frameworks themed issue.

Gerard Ferey

Gerard Ferey received his PhDfrom Paris VI University in1977. He was Professor ofInorganic Chemistry firstin Le Mans University(1981–1996) and then inVersailles University wherehe created the InstitutLavoisier (1996–. . .). He isnow Professor at Institutuniversitaire de France andmember of the French Acad-emy des Sciences. He has re-ceived many internationalAwards. After working on themagnetism of transition metal

inorganic fluorides, his current interests concern the structuralchemistry of inorganic and hybrid porous solids, their mechan-isms of formation and their applications in gas storage, energy,drug delivery and nanosciences.

Christian Serre

Christian Serre, 38 years old,is an engineer from the EcoleSuperieure de Physique et deChimie Industrielles de Paris.He obtained a PhD inInorganic Chemistry in 1999.After a post-doctoral fellow-ship in the USA, he moved toa permanent CNRS researchposition in 2001 in the group ofProf. Ferey in Versailles. Heis currently working on thesynthesis, structure determina-tion and applications of poroushybrid solids. He received theCNRS bronze medal in 2006

and a European research council young researchers’ grantin 2008.

1380 | Chem. Soc. Rev., 2009, 38, 1380–1399 This journal is �c The Royal Society of Chemistry 2009

CRITICAL REVIEW www.rsc.org/csr | Chemical Society Reviews

Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


showing no tilting, after a cooperative movement of all the

fluoride ions of 0.56 A (Fig. 1(b)).

This phenomenon, despite being general in inorganic solid

state chemistry and in zeolites,14 implies however too small

atomic displacements for being further considered in this

paper, devoted exclusively to large breathing effects in three-

dimensional organic–inorganic hybrid solids containing only

strong bonds. For this reason, the purely organic or bio-

organic compounds, which exhibit similar displacements15,16

and are governed by hydrogen bond interactions between

molecular species, will not be considered here. In a similar

way, two-dimensional hybrids are also excluded. Only the

layers are hybrid. Guests are incorporated between the sheets,

leading to a rich guest exchange chemistry, but the guest

extraction can render (reversibly or not9) the solid amorphous,

due a disordered arrangement of the layers to a (turbostratic

disorder).

Large breathing effects represent a new aspect of non-

organic matter. From the description of some flexible solids

and of some counter examples, this paper will analyze the

structural reasons of breathing and try to generate some

building rules for obtaining such swelling phases. In a final

part, the consequences of this breathing effect will be examined

in several domains such as energy, energy savings, sensors,

molecular recognition, and gas storage for sustainable devel-

opment and health.

2. General requirements for breathing in

crystalline hybrid solids

The need of a chemical or physical stimulus for generating

reversible movements was already noticed. From the

structural point of view, the flexibility inherent to breathing

implies also a contrast in the strength, the directivity and the

nature of the different bonds existing in the solid. Such effects

are particularly evident in hybrid solids. In their skeleton,

these compounds generally associate ionic or ionocovalent

bonds in the inorganic part, and covalent and sometimes

p–p ones in the organic moieties. Hydrogen or van der Waals

interactions18,19 occur between the framework and the

occluded species.

The spatial occupancy of the different parts of the hybrid

solid plays a role. Indeed, the inorganic part is often associated

with a rigid behaviour, whereas the organic moieties corres-

pond to flexibility. The amount of each component in the

final solid will influence the degree of breathing of the solid.

Therefore, one must consider not only the whole dimension-

ality of the hybrid solid, but also the dimensionality of each

subnetwork. In a first step, if one excepts the case of molecular

solids, two groups of three situations occur with non-

interpenetrated solids.

� for three-dimensional frameworks, the inorganic subnet-

work can be either two-, one ot zero-dimensional and linked

by organic moieties in one, two or three directions.

� the same situation occurs for the inorganic contribution in

two-dimensional solids, but this time, the organic linkage

occurs only within the layers, the space between the latter

being occupied by the occluded species (solvents,

templates. . .). Hydrogen and p–p bonds are involved, and

have been discussed in papers of Kitagawa.17

Fig. 1 Perspective view of (a) rhombohedral and (b) cubic FeF3

which correspond to the contracted and expanded forms of this solid

(see text).

Fig. 2 The six classes of Kitagawa. In all the classes, G is the symbol for Guest. In the 1D class, the voids between the chains are occupied by small

molecules and can exhibit ion exchange. In the first case of 2D class, the manner of stacking of the layers (superimposed or shifted) is strongly

dependent on the nature of the guest and the weak interactions they have with the layers. In the second case, the interdigitated layers are

superimposed and form 1D channels. Closed without guests, they open with some of them, resulting in an elongation of the stacking parameter. In

the 3D cases, three situations occur. When pillared layers are concerned, the reversible phenomenon of interlayer elongation and shortening is

realized by non-rigid pillars. The expanding and shrinking frameworks act as sponges. Keeping the same topology, the drastic volume change is

induced by strong host–guest interactions. Depending on the structure, the volume increase is associated with either the evacuation or the inclusion

of the guests. Finally, in the case of interpenetrated grids, they are densely packed in the absence of guests and the introduction of molecules

generates a sliding of one network.

This journal is �c The Royal Society of Chemistry 2009 Chem. Soc. Rev., 2009, 38, 1380–1399 | 1381

Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


A special situation concerns the interpenetrated or inter-

woven networks17,18,20,21 which must take also into account

the interactions between the two (or more) framework contri-

butions. All these cases led Kitagawa17 to propose six classes for

the breathing of dynamic coordination polymers (Fig. 2)

A third requirement obviously concerns the need of a free

space in the structure (which is often realized for hybrid

porous solids) in order to accommodate the modifications of

the steric hindrance of the flexible part during the movements.

Finally, and related to the contrast between the nature of

the bonds (ionic, covalent. . .), non-rigid areas (hereafter

denoted as ‘weak points’) must exist in the solid to allow the

breathing modes. Even if it is not obvious, their existence is

necessary but not sufficient for generating breathing, as seen

later. The topology of some elements of the structure also

plays a crucial role. All these requirements will be illustrated in

the following examples.

3. Structural analysis of some illustrative examples

of three-dimensional networks

3.1 Two-dimensional inorganic subnetworks. The case of

pillared c-zirconium phosphates

The interesting two forms, a and g, of hydrated zirconium

phosphates (often labelled a- and b-ZrP) have been the subject

of many studies and applications since their discovery thirty

years ago (see for instance the references contained in refs. 11

and 22–25) They are lamellar solids with guest water molecules

within the layers, and correspond to type a-1 in the Kitagawa’s

classification.17 The layers are built from the dense corner-

sharing connection of zirconium octahedra and phosphorus

tetrahedra. For our purpose, g-ZrP or Zr2(HPO4)4�4H2O

(Fig. 3(a) and (b)) is of particular interest. Indeed, Alberti11

and Clearfield25 proved that the phosphate groups of the

layers could be substituted, totally or partially, by mono- or

diphosphonate groups (linking two layers) with variable

length of aliphatic chains. Alberti11 alone looked at

the thermal behaviour of these phases which were, to

our knowledge, the first to exhibit such a drastic breathing

(B8 A in the best case).

The study of the system Zr/phosphorous acid/

alkanediphosphonic acids/acetone/water at 350 K lead

to a series of phases of general formula ZrPO4[O2P(OH)2]1�x-

[O2P(OH)–(CH2)n–(HO)PO2]x/2�mH2O with 4 r n r 16

corresponding to the number of carbons in the alkyl chain;

x was allowed to vary from 0 to 1, x = 0 corresponding to

g-ZrP, x = 1 to the fully pillared Zr diphosphonate, and

intermediate values between 0 and 1 to an increasing degree of

pillaring between the layers of g-ZrP. Two examples of

complete pillaring are shown in Fig. 3(c) (for n = 4) and

Fig. 3 (a) Polyhedral projection of the structure of g-ZrP along [010] and (b) along [001]; (c) and (d) [010] projections of the completely pillared Zr

1,4- and 1,10-alkane diphosphonates, (e) a schematic view of the fully hydrated phosphate/diphosphonate and (f) of the corresponding dehydrated

sample showing the contraction of the carbon chain. Zr octahedra are in pale blue, phosphate groups in yellow, water molecules in dark blue,

OH groups in red and white and the carbons in black.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


Fig. 3(d) (for n = 10) and the distance between the layers

increases linearly following the law d001 = 0.93 + 0.127n sin a(a being the angle �52.71 in this family—between the alkyl

chain and the plane of the layer). In the case of full pillaring,

three features are noteworthy: (i) the configuration of the

chains corresponds to the most stable arrangement (maximum

elongation), (ii) the distance between the chains is fixed by the

distance separating the phosphorus atoms within the same side

of the layer (ca. 5.4 A) and therefore (iii) the steric hindrance

of the chains authorizes neither degrees of freedom for the

conformation of the chains nor the insertion of additional

water molecules, since m, which represents the number of H2O

molecules per Zr remains 1. As a consequence, the fully

pillared series do not exhibit any flexibility.

Breathing occurs (Fig. 3(e) and (f)) for low values of x. They

correspond to situations where two different alkanediphos-

phonic chains are separated, in a direction parallel to the

layer, by [O2P(OH)2] groups. For a given n, either when the

solids are wet or exposed to an atmosphere with 100% of

relative humidity (r.h.) (which represents the stimulus at

300 K)), the distance is, whatever x, the same as for the

completely pillared material. In this case, additional water

molecules take the place of the missing chains, which explains

why m is important for low x (for n= 10, m= 3.0 for x= 0.1

instead of m = 1 for x = 1). Their role implies, at this stage,

only space filling up to an amount which preserves the

maximum stability of the carbon chain.

However, a thorough study of the term n = 10 proves that,

varying the r.h. conditions and/or temperature, efficient

breathing occurs mainly for low values of x (Fig. 4(a) and

(b)). Compared to the fully hydrated sample for r.h. = 100%,

the decrease of r.h. to 75% leads, for instance for x= 0.1, to a

small (2.6 ) 2.4) but significant decrease of m and, correla-

tively, an important decrease of the d001 distance (19.2 A )14.1 A). For the same x, the completely dehydrated sample

(which can be considered as either the 0% r.h. or the result of

the stimulus of temperature), corresponds to the following

changes: 2.6 ) 1.0 for m and 19.2 A ) 11.2 A (decrease

of 8 A!) for d001. Rehydration of the dehydrated solid

regenerates the initial fully hydrated sample with all its

specifications. Fig. 4(a) and (b) show the evolution of the

different characteristics of breathing as a function of the

pillaring percentage x in this series.

This example provides a first analysis of the reasons for such

a large breathing, whereas the structural topology remains the

same. Not only the stimulus is necessary but also its intensity

(in this case, water vapor pressure). Keeping the dense and

rigid arrangement of the layers, breathing (here uniaxial)

indeed occurs only for low values of the pillaring percentage x.

It is noteworthy that a threshold around 66%maximum seems

to be necessary for such an effect. Below, this corresponds

to the creation of enough space for the mobility of species and,

therefore, a flexibility up to a certain amount, depending on

the intensity of the stimulus.

Two features are responsible of these movements: (i) as soon

as the carbon chains have sufficient space between them, they

can develop their extraordinary plasticity due to the covalent

bonding and the free rotation around the axis of the bonds.

The torsion of these chains by increasing the cis-positions can

lead to variations in length of more than 50% even if,

thermodynamically, they are unstable as such. In the case of

the Alberti’s phosphonates, and even if no structural determi-

nation was given in their paper, the evolution of the d001distance between the fully hydrated and the dehydrated phase

can be explained by an helicoidal arrangement of the chain. It

is indeed easy to calculate26 that, for n = 10, such a helix coils

around a cylinder with a radius of 1.08 A, the pitch of the helix

being 0.668 A. The length between the two terminal carbons of

the chains is then 6.012 A (instead of 11.2 A for the elongated

chain) and explains the 19.2 A ) 11.2 A decrease for d001.

The ‘weak point’ is then the carbon chain which acts as a

spring, which is compressed in the anhydrous form and

elongated for the fully hydrated one. This spring effect (ii) is

allowed because the water molecules are only loosely bound to

the skeleton and can move easily in the structure and give

enough space to the chain for its contraction.

3.2 One-dimensional inorganic subnetworks

3.2.1 The case of trivalent metal terephthalates (MIL-47

and -53)3–6. This situation was recently discovered in the

laboratory3,4 during the search of functionalized hybrid

nanoporous solids with large pores.27,28 It concerns metal

1,4-benzenedicarboxylates (BDC) [M(X)[BDC]�xG (M is

VIV, with X = O for MIL-47, and trivalent Al, Cr, Fe, Ga

and X = OH, F for MIL–53). The templates (G) are

terephthalic acid H2BDC, water or solvents such as DMF.

Fig. 4 (a) For n = 10, evolution of the number m of occluded water

molecules per Zr atom and of the decrease of the d001 parameter

between the layers vs. the pillaring percentage x for different values of

r.h. (b) Evolution of the d001 parameter vs. x and different situations of

conditioning of the samples (n = 10).


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


At variance to the others, MIL-47 does not breathe. Indeed,

there are no OH groups in the skeleton in MIL-47, and it will

be shown below that these hydroxyl groups play a major role

in MIL-53 for the creation of symmetrical hydrogen bonds

with the oxygens of the occluded water.

At variance to MIL-47, MIL-53 breathes. According to the

different treatments, three variants of MIL-53 (hereafter

labelled -as, -ht and -lt) appear. They have in common a

three-dimensional network composed of corner-shared trans

chains of metallic octahedra (the OH groups being the shared

species) linked in the two other directions by terephthalate

ions. This determines large unidimensional lozenge-based

tunnels (Fig. 5(a)). In terms of nets, the topology of MIL-53

corresponds to a 44 lattice.

In the as-synthesized form MIL-53-as (V = 1440 A3) the

tunnels are occupied by structurally disordered molecules of

neutral terephthalic acid (Fig. 5(b)). They leave the structure

by heating at 573 K, and create the structure MIL-53-ht, with

empty tunnels (V = 1486 A3). By cooling in air, the structure

reabsorbs water. This leads to the third form MIL-53-lt

(V = 1012 A3). This transition is fully reversible when water

is involved, but the structure shows a very high selectivity

toward other adsorbable species. Indeed, whereas MIL-53-ht

is strictly inactive for the readsorption of acetone and ethanol

even after 4 days, it immediately adsorbs DMF in a reversible

way. Moreover, in MIL-53-lt, water is easily exchanged by

DMF, showing the great affinity of DMF for this type of

structure. The resulting product has cell parameters inter-

mediate between MIL-53-as and MIL-53-lt, proving the

adaptability of the breathing network to the shape of the

chemical stimulus. Note that the introduction of large

amounts of some gases leads to the largest cell (see section 4.2).

The structures of the three forms, solved from X-ray powder

diffraction, allow to quantify the large breathing effects during

the transformations, characterized, at constant topology, by

movements of large amplitude each time, created when either

adsorbed species or temperature act as stimuli.

For MIL-53(Cr) [X: O,F], the disappearance of the template

H2BDC during the transition MIL-53-as ) MIL-53-ht

corresponds to an increase by 3.16% of the volume of the

cell (Fig. 5(c)) with significant variations of two of the cell

parameters (17.34 A ) 16.73 A and 12.18 A ) 13.04 A). The

third, which corresponds to the direction of the metallic

octahedral chains, remains invariant (B6.8 A). The breathing

out paradoxically relates to an expansion of the cell instead of

the expected contraction. During the transition MIL-53-ht )MIL-53-lt, the adsorption of water (the inhaling step) corres-

ponds (Fig. 5(d)) to a drastic decrease of the cell volume

(�32%) associated this time with very large variations of the

two parameters involved above (16.7 A ) 19.7 A (Dd: 3 A)

and 13.0 A ) 7.85A (Dd: 5.15 A)).

The reasons for such changes are not the same as for the

diphosphonates. Indeed, now, if the rigidity of the inorganic

moieties remains almost the same, the sp2 hybridization in the

phenyl rings renders the carbon skeleton rigid. The area of

flexibility of the structure, the ‘weak point’, is restricted to the

connection between the inorganic chain and the carboxylic

function of the terephthalate (Fig. 5(e) and (f)). The two

oxygens of the latter are covalently bound to carbon but are

linked to chromium via a ionocovalent bond. This means

a possible rotation around the O–O axis (which acts as a

‘kneecap’) of the two planes O–Cr–Cr–O and O–C–O

(Fig. 5(f)). The corresponding dihedral angle a between these

two planes (177.51 for -as, 1801 for -ht and 1391 for -lt)

characterizes mechanically the extent of breathing better than

the angles between two chains, depicted in Fig. 5(b)–(d). It is

of note that the most expanded structure corresponds to a

dihedral angle of 1801.

This potential flexibility is activated by the presence of

inserted species in the tunnels and the creation of weak bonds

between the guest and the skeleton. Here, the oxygens of the

chromium octahedra are involved. The onset of these addi-

tional bonds decreases the distance between the chains along

the short diagonal of the lozenge. They are separated by 13 A

in the expanded empty MIL-53-ht (Fig. 5(c)). The distance is

lowered by 0.9 A in MIL-53-as when the template H2BDC is

present and the shrinkage reaches 5.2 A in MIL-53-lt when

fixed water molecules are involved and create a ‘lock-in’

between the chains, assimilating inhalation with contraction.

The strong rotation around the O� � �O axis of the carboxylate

functions simultaneously induces some small changes for the

tilting angle M–(OH)–M in the chains, as proved by NMR

and IR measurements. Therefore, the rigidity of the inorganic

moiety is less strict for a 1D subnetwork than it was for a 2D

Fig. 5 (a) Perspective view of MIL-53-ht; (b)–(d) projection along the

direction of the tunnels of (b) MIL-53-as (with some atoms of the

disordered terephthalic acid in blue), (c) MIL-53-ht, and (d) MIL-53-lt

with variable parameters; (e), (f) perspective views of the connection

between the chromium chains and the terephthalate ions. Chromium

octahedra are in green, water molecules in dark blue, OH groups in

pale blue, oxygens in red and carbons in black.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


one. This allows more degrees of freedom for this type of

breathing which is, at variance to the first example, essentially

two-dimensional.

Moreover, in situ solid-state 13C, 1H and 27Al NMR

experiments of the Al-homologue5 during rehydration have

shown that the water molecules show three types of hydrogen

bonds (Fig. 6). The first corresponds to a guest–guest inter-

action of the water molecules along the axis of the tunnels. The

two others relate to host–guest interactions between (i) the

oxygen of the guest with the OH group of the host alterna-

tively with one chain and the other and (ii), between one

hydrogen of the water molecule and one oxygen of the

carboxylate linked to Al. The latter is clearly evidenced

by 13C NMR experiments. The COO contribution of the

dehydrated sample progressively disappears during hydration

and is replaced by the COO–H2O signal, which exists alone for

the fully hydrated sample. 27Al NMR also confirms the

structural results and the small distortions and tiltings of the

Al(OH)2O4 octahedra occuring during hydration.

The above interactions seemed to be sufficient for explaining

the breathing phenomenon but, very recently, the study of the

Fe(III) analog, with the same topology, provided surprising

results.29 Indeed, whereas its hydrated form is identical to the

other ones, the full dehydration of MIL-53(Fe) increases

the shrinkage of the structure in two subtle steps, instead of

the expansion of the others. This means that, beside the above

interactions, host–host interactions must also be considered.

The comparison of the various distances observed in all the

hydrated forms does not allow to find a geometrical criterion

able to explain the above difference.

Anyhow, two possible reasons could be assumed for the

moment: (i) the influence of the nature of the metal and

(ii) strong p–p interactions. In most cases, the cation is a 3d

transition metal and magnetic dipolar interactions between the

chains may be taken into account. These interactions are

proportional to the number of unpaired d electrons. While

they can be considered as very weak with chromium(III) (d3)

and vanadium (d2), they become significant with iron (d5) and

may participate in the stability of the structure. In the case of

MIL-53(Fe), this could explain that they are strong enough for

keeping the structure closed in the absence of water within the

tunnels. However, even if it is possible, the argument is

unlikely due to the long distance between the closest Fe(III)

along the short diagonal of the tunnel (7.64 A (hydrated) and

6.87 A (anhydrous)).

The distances between the phenyl rings of the terephthalates

can be used to estimate the strength of p–p interactions.

Usually, they are considered as strong when the smallest

distances between the rings lie in the range 2.9–3.5 A. Here,

they are in the range 3.7 (Fe)–4,1 A (Al, Cr) for the whole

family of MIL-53 hydrated solids. The significantly smaller

distance in hydrated MIL-53(Fe) (Dd E0.4 A) could explain

why the anhydrous MIL-53(Fe) remains closed. In such a case,

this would imply that it corresponds to the strongest host–host

interaction in this topology. The evolution of the short ring–

ring distances in MIL-53(Fe) [3.73 A (hydrated form) and

3.41 A when anhydrous], indicates an enhancement of the p–pinteraction when the steric influence of the occluded water

disappears. It is probably sufficient for keeping the structure

closed. On the contrary, the larger distances observed for

the Al and Cr solids (4–4.1 A), allow the opening of the

structure upon dehydration. The p–p interactions obviously

play a role and a threshold seems to exist in the series of

values, below which opening is forbidden. This is confirmed

by MIL-69,30 an aluminium naphthalene-dicarboxylate

AlIII(OH)[O2C–C10H6–CO2], with the same topology, with

naphthalene rings replacing phenyl ones. It also remains

closed after dehydration (shorter ring–ring distances 3.78 A).

3.2.2 The structural counter example of vanadium

terephthalate MIL-68. and the influence of the topology. Besides

MIL-53, another three-dimensional vanadium(III) phase

(MIL-686) appears in the same system, when using a non-

aqueous solvent. It has the same chemical formula as MIL-53:

MIII(OH)[BTC]�xG but its density is ca. 75% of that of

MIL-53. It represents a lacunar variant (25% vacancies)

of MIL-53 (Fig. 7). In terms of nets, MIL-68 is 6.3.6.3. instead

of 44 for MIL-53. According to O’Keeffe and Hyde,31 no direct

transition can occur between these two nets. However, both

structures are formed with the same octahedral chains and

linked in the same way by the same ligands. Only the topology

is different: MIL-68 contains hexagonal and triangular tunnels

parallel to the metallic chains, with angles being strictly 120 and

601, respectively, instead of lozenge sections in MIL-53.

The thermodiffractometry of MIL-68 shows that the cell

remains unchanged, whatever the temperature, up to 350 1C,

which is the temperature of collapse of the structure. This

time, the absence of breathing has only one reason: the

existence of triangular cycles, already known to be strictly

rigid. They forbid any flexibility at the weak points of the

previous type of structure and MIL-68 does not breathe. This

example illustrates the role of the whole topology on the

possibility of breathing.

As a first conclusion of this part, breathing effects result

from the combination of several factors: (i) the existence of

only even cycles in the structure and of ‘weak points’, which

allow the structure to distort under the action of a guest

stimulus, and (ii) the occurrence of strong guest–guest and

host–guest interactions, stronger than host–host ones. The

Fig. 6 (a) Pespective view of the disposition of the water molecules

within the tunnel of MIL-53-lt with the hydrogen bonds.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


latter, if the strongest, prevent the opening of the structure.

This point will be later developed in section 3.4.

3.3 Zero-dimensional inorganic subnetworks. Examples and

counter examples

For such a type of dimensionality, it is now common to speak

about porous coordination polymers (PCP), a continuously

expanding domain. In such solids, the inorganic part has

finite dimensions and corresponds either to single metallic

polyhedra or to oligomeric clusters. The dynamic effects have

already been documented by the groups of Kitagawa

and Rosseinsky from the experimental point of view, with

numerous examples,7,32–36 and the reader is recommended to

return to the reviews and the references they contain.

Without going into detail, one can distinguish two types of

flexibility resulting from (i) displacive phase transitions in

which the skeleton remains the same, while distorted or

(ii) reconstructive transitions with reversible cleavage/

formation of a metal–carboxylate bond and a scissoring

motion induced by the size of the guest.36

The first case is illustrated by the ‘jungle gym’ solid

[Zn2(1,4-BDC)2(DABCO)]�G.37–41 It is based on Zn2 dimers

which, by linkage with four 1,4-BDC molecules, provide 2D

square grids. The pillaring of the latter by the nitrogens of

DABCO ensures the 3D structure (Fig. 8).

For example, during the adsorption of propane 2-ol

(IPA),41 a monoclinic distortion of the primitive tetragonal

unit cell first occurs after the introduction of up to three IPA

molecules per cell. The volume decreases by 21% with a

significant (0.3 and 0.9 A) unexpected decrease of the length

of the edges, since it corresponds to the length of both rigid

linkers. The distortion corresponds to a shift of the square

grids from one to the other by 161. By increasing the amount

of IPA up to 4.5 IPA regenerates the dimensions of the

original cell. This structural behaviour is obviously similar to

that observed with MIL-53(Cr) (section 4.2). The sequence

dimer–DABCO–dimer corresponds to a rigid moiety, as also

for the chains in MIL-53. The O� � �O axis of the BDC plays the

same role of ‘kneecap’ in both cases. Unfortunately, the high

disorder of isopropanol molecules prevents an accurate struc-

tural determination and therefore, any discussion about the

nature of interactions, at least for three IPA in the cell. The

same behaviour also occurs in a solid recently discovered by

Long et al.42 with a cobalt (1,4-benzenedipyrazolate).

Displacive transitions are also observed in interpenetrating

and interdigitated 3D frameworks. This time, it is only the

slipping motion of the rigid interpenetrated layers which

occurs under the action of the stimulus. This behaviour is

one of the aspects of the ‘gate effect’.43 The original phase can

Fig. 7 Perspective view of MIL-68, with hexagonal and triangular

tunnels.

Fig. 8 Structure of the ‘jungle gym’ [Zn2(1,4-BDC)2(DABCO)]�xIPA, and a scheme of its intermediary distortion.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


be non-porous although, under a stimulus, it opens.

For instance, the (ZnI2)3(tpt)2 framework44 undergoes

considerable motion of the two interpenetrating (10,3b)

networks during nitrobenzene guest removal and reintroduc-

tion. In the same way, Kitagawa described45 a copper(II)

coordination polymer [Cu(dhbc)2(4-40-bipy]�H2O which is

non-porous up to 50 bar but opens above, when the pressure

of nitrogen increases.

The reconstructive phase transitions are by far more

rare. Up to now, only [Cu2(pzdc)2(dpyg)]�8H2O46 and

[Ni2(4,40bipy)3(NO3)4]

35,47,48 exhibit this behaviour. The first

solid is a 3D pillared structure. It undergoes a transformation

during the adsorption process, attributed to cleavage/

formation of the Cu–carboxylate bond. During adsorption

of methanol and water, it shrinks with a 27,9% contraction

and then expands, but not with methane.

These few examples (and many others) exhibit significant

changes for the cell volume which reached, until recently,

ca. 40%. They also identify the effect of the stimulus on the

structural changes in the skeleton but, for several reasons (high

disorder of the guests, partial structure determination. . .) in

most of the cases, attention was not paid enough to the

localization of the guests in order to determine what part of

the skeleton is affected by host–guest interactions, and to

evaluate the nature and the intensity of the latter, either

structurally or using computer simulations.49,50 Moreover,

in situ X-ray powder diffraction studies in real time are not

enough developed in order to understand the dynamics of the

evolution during adsorption. This will be discussed later

(section 4.2) in detail from recent examples.

This was recently performed in our group on a new series of

isoreticular solids (families MIL-8851,52 and -8953,54) which

represents the largest expansions ever evidenced: the ratio

between the volumes of the open and closed forms can

reach more than 300%. These solids are iron(III) or

chromium(III) carboxylates with a general formula

[MIII3O(X)3[

�OOC–(CH)n–COO�]3]�xG (X = CH3OH, H2O, F).

The organic moieties, always rigid, can be either an alkyl chain

with double bonds (fumarate [�OOC–(CH)2–COO�] (MIL-

88A) and muconate [�OOC–(CH)4–COO�]) (MIL-89) or ben-

zyl moieties (1,4-benzene dicarboxylate (1,4-BDC) [MIL-88B],

naphthalene dicarboxylate (NDC) [MIL-88C], or diphenyl

dicarboxylate (DPDC) [MIL-88D]. They all have an hexago-

nal symmetry (P�62c).

The inorganic cluster is a trimer of MIII3O octahedra

sharing a m3-oxygen (Fig. 9). Each of the six carboxylate

functions links two octahedra of the trimer, with three above

the trimer and three below. The second COO function of the

dicarboxylate is grafted on another trimer of the structure,

leading to a three-dimensional network.

In the as-synthesized products, the corresponding arrange-

ment gives rise to two types of cavities: large tunnels and large

cages having a triangular bipyramid (TBP) shape, within

which solvent molecules are located (Fig. 9). The vertices of

the bipyramid are occupied by the trimers. The distance

between two trimers of the TBP corresponds to the ahex cell

parameter, whereas the distance between the apical trimers of

the TBP refers to the chex parameter. The linking dicarboxylates

are on the oblique edges of the bipyramid; there is no link in

the equatorial plane of the TBP. This absence of a rigid

linker in this case is an essential point for breathing because

the distance between the three equatorial trimers can vary

without constraints under the influence of a stimulus (in this

case, the hydrogen and van der Waals bonds created by the

occluded species). As in MIL-53, the ‘kneecap’ role of the

O� � �O axis of the COO group is preserved. For a given length

of the linker, its flexibility allows the bipyramid to breathe

because it has two degrees of freedom: (i) the absence of

linkage between equatorial trimers (which avoids any

constraint in the equatorial plane of the bipyramid), and

(ii) the possibility of rotation of the COO group around its

O� � �O axis (Fig. 10).

When the solvent molecules leave the structure, the original

space group is preserved (P�62c), but one observes a drastic

increase of the length between the two non-equatorial trimers

(chex cell parameter), correlated with a decrease of the distance

between the equatorial ones (ahex cell parameter). On the other

hand, when the solvent is incorporated in the structure, the

bipyramid flattens and its volume increases (Fig. 9). In the

MIL-88D structure using diphenyl dicarboxylate as a linker,

the passage from the empty to the filled form corresponds to

an increase of ahex from 10.1 (as) to 20.04 A (Dd = 10 A)

whereas chex decreases from 27.8 to 22.9 A (Dd = 4.9 A).

Correlatively, the cell volume increases from 2362 to 7968 A3

Fig. 9 Views of the cages (top) and of the (001) projections (down) of

the different forms: dry, as (as synthesized) and open of MIL-88D. The

inorganic trimers are clearly shown on the projection.

Fig. 10 Local movements of the inorganic trimer and of the linker

during the swelling.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


which means a final volume more than three times the initial

one and reversible atomic displacements of ca. 10A. . .

The most surprising feature concerns the fact that this

breathing occurs without any apparent bond breaking. It is

difficult to imagine that such variations could occur without a

phase transition. However, if the latter occurred, both phases

would coexist, at least during a few minutes, in the X-ray

powder pattern. This is definitively not the case. During the

dehydration–rehydration process of MIL-88D a continuous

evolution of the pattern, with drastic changes in the position of

the Bragg peaks is observed.

The evolution of the volume can be calculated analytically.

Indeed, a flexible bipyramid is a polyhedron in which the

length a of the oblique edges is fixed, whereas the length xa

((ahex) of the edges of the equatorial plane can vary. In these

conditions, it is easy to show that chex can be expressed as h =

[aO( 3 � x2)]/O3 and the volume as V = (1/6)x2a3O(3 � x2) ,

maximum for x = O2. The value x = O3, for which V = 0,

corresponds to the complete flattening of the bipyramid

(Fig. 11). The value of the experimental cell parameters for

MIL-88A-D shows that their open form always corresponds

to a complete filling.

Another interesting point must be mentioned. The pore

opening is strongly dependent of the nature of the solvent,

indicating some selectivity in adsorption. The soaking with

various polar and non-polar liquids shows three degrees of

pore opening: (i) small polar molecules (water and methanol)

and polar solvents induce a small swelling magnitude (6–8% in

volume); (ii) more hydrophobic but still polar liquids (butanol,

DMF, dimethyl carbonate) generate larger pore openings

(15–60%) whereas (iii) pyridine and diethylformamide

completely open the structure. The reason of such differences

in the magnitude of opening can be found in the characteristics

of the tunnels of the structure and in the nature of the guest

and its associated interactions. Indeed, along the tunnels,

hydrophilic inorganic parts of constant thickness (2.9 A) and

aperture of 3.7 A alternate with hydrophobic organic parts

(free aperture 3.2 A), the thickness of these depending on the

length of the linker. Therefore, the small polar liquids will

interact with the inorganic trimer, non-polar liquids interact

only with the linker via van der Waals, p–p or C–H–pinteractions but the presence of water on the trimer does not

favour their adsorption.52 Finally, these experiments shed

some light on the kinetics of adsorption, an aspect rarely

taken into account in the literature. It seems that the kinetics

of breathing is distinctive for each MIL-88. For example,

MIL-88A and MIL-88B breathe in a few seconds in the

presence of ethanol, but it takes several days to MIL-88B to

open its pores completely when water and nitrobenzene are

used. In the same way, MIL-88C opens rapidly with pyridine

(o1 min) but slowly with DEF (hours).

The above examples could indicate that breathing is a

general phenomenon, but, on the whole, it is rather scarce

and many MOFs do not swell. This is the case for the well

known zinc (1,4-benzenedicarboxylate) MOF-555 and the

series of IRMOF-n56 which correspond to the replacement

of 1,4-BDC by longer organic ligands. They do not breathe,

despite the existence of large windows (8 A for MOF-5) which

allow an easy access to different molecules and the possibility

of breathing. However, a structure determination at 350 1C

shows21 that, even without guests, the structure remains

unaltered and therefore insensible to the temperature stimulus.

Why MOF-5 does not breathe? Why the jungle-gym do? The

origin of such an inertness must be found not only from the

topology, as before, but from the structure of the zinc cluster

itself. Indeed, if the symmetry of the cell was taken alone into

account, the presence of the guest species could at least lead to

a rhombohedral cell distortion (elongation or compression

along one diagonal of the cube), which corresponds in both

cases to a decrease of its volume (the volume of a rhombo-

hedron V = a3(1 � 3cos2a + 2cos3a)1/2 is maximum for

a = 901) in agreement with the disappearance of the guests.

The terephthalate ions could accommodate such a distortion

owing to the ‘kneecap’ role of the O–C–O moieties, which

would allow to privilege one of the four threefold axes of the

original cubic cell if the cluster had a different symmetry.

However, considering the tetrahedron of tetrahedra forming

the cluster (Fig. 12) a cooperative movement of the ligands is

impossible during a rhombohedral elongation (for example).

If the ligands of the upper part of Fig. 12 satisfy this

movement because the O� � �O axes of the carboxylates are

perpendicular to the elongation axis, those of the lower part

are almost parallel to this axis, which renders impossible a

rotation of the ligands in the same way owing to the rigid sp2

configuration of the COO group. MOF-5 illustrates the case

when the configuration of the cluster prohibits breathing

because the positions of some of the ligands do not allow a

cooperative and symmetrical rotation of the linkers. This is

probably due to the octahedral disposition of the carbons of

the carboxylates around Zn4, which creates odd cycles between

these carbons, a condition which does not favour breathing, as

seen for MIL-68. Therefore, a new condition for swellingFig. 11 The geometry of the bipyramid (left) and the analytical

evolution of the volume vs. x.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


arises from this example: not all inorganic bricks (SBU) can

allow dynamic effects, which are possible only if the carboxylates

have mirror positions around the organic brick.

In conclusion, one can ask a question: why does the jungle

gym [Zn2(1,4-BDC)2(DABCO)], described above, swell

despite the fact that it is cubic like MOF-5, has the same

topology as MOF-5, and has six ligands in octahedral disposi-

tion around the cluster? The ‘brick’ is not the same, however:

instead of a tetrahedron of tetrahedra, it is a dimer, but also

there are two types of ligands: four 1,4-BDC and two DABCO

in trans position. The latter are intrinsically monodentate, with

strong and rigid metal–nitrogen bonds which do not allow

rotations around N. Therefore, the distortion can only occur

in the square grids formed by four carboxylates which can

present cooperative and symmetrical rotation.

A question arises from all these observations: can empirical

rules be extracted for predicting possible breathings of the

skeleton of hybrid porous solids?

3.4 Some empirical structural rules for possible breathing

(a) The first rule concerns the inorganic brick (SBU). Inde-

pendently of the nature of the ligand, it seems clear that the

only bricks which permit swelling are those which possess a

mirror plane with the carboxylates in symmetrical position

towards it. This is the case of the dimer of the jungle gym

[Zn2(1,4-BDC)2(DABCO)] and of the trimer of MIL-88

(Fig. 13(a) and (b)). Otherwise, the situation of MOF-5 occurs

(Fig. 13(c)). This will be an a priori indication for colleagues

discovering new clusters before trying any experiment related

to dynamic effects. This also incites chemists to search for new

inorganic bricks, the currently existing ones being rather

rare.57

(b) It seems that the ratio C/M (C: number of carbons of the

carboxylate surrounding the cluster; M: number of metallic

atoms within the cluster) is a good indicator of the possibility

of the brick to potentially allow swelling. From the few results

in the literature, it seems that C/M must be Z 2 for such an

effect. This is the case for [Zn2(1,4-BDC)2(DABCO)] and of

MIL-88 (4/2 and 6/3, respectively), but not for MOF-5 and

IRMOF-n (6/4) but this rule needs to be validated in many

other examples.

(c) Breathing effects can only occur with ditopic carboxylates.

Between two clusters, the kneecaps provided by the

O� � �O axes of the different carboxylates ought to be parallel

for possible phase rotations. This condition obviously prohi-

bits the use of tri- or tetratopic ligands for expecting swelling.

This is typically the case of MOF-77,58 MIL-10359 which

contains tritopic 1,3,5-benzene trisbenzoate and MIL-10260

formed with the naphthalene 1,4,5,8-tetracarboxylate.

(d) The existence of odd cycles in the structure, both at the

level of the cluster and/or at the level of the topology of the

skeleton, is unfavourable for dynamic effects, owing to their

rigidity (MIL-68, MOF-5 for instance). A particularly illus-

trative example is provided by MIL-10161 which, despite

favorable conditions (trimeric brick, ditopic ligand) does not

breathe only because most of the windows of the cages are

pentagonal. But, at variance to the other rules, it cannot be

predicted because the topology is known a posteriori.

(e) Breathing occurs only if all the conditions above are

satisfied simultaneously.

(f) As it will be shown below, as soon as the above

geometrical conditions are fulfilled, adsorption–desorption

phenomena can exhibit hysteretic effects (gate effect, multi-

ple-step adsorption. . .) depending on the nature of the guest

and on the relative energies of host–guest and guest–guest

interactions.

Fig. 12 Explanation of the absence of breathing in MOF-5. The stars

indicate the impossibility of rotation of the linker in the direction of

the elongation axis.

Fig. 13


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


4. Consequences of breathing for academic

knowledge and possible applications

4.1 Sensors and adsorbate–adsorbant interactions

The extreme sensitivity of breathing solids toward the action

of a stimulus suggests using them as sensors for evaluating

host–guest interactions for a better understanding of their role

in the dynamics of the framework.

This was realized very recently in our group using the

MIL-53 type.62 Starting from the hydrated form of

MIL-53(Fe) immersed in water, and placed in the beam of

the synchrotron radiation source, the solvent molecules were

introduced dropwise (an impregnation of MIL-53 in another

solvent leads to an immediate and full exchange with water,

preventing a kinetic study) and the X-ray powder patterns

were collected after each droplet addition in order to follow

structurally the evolution of the cell volume during the

exchange.

Looking first at the volumes of the fully exchanged samples,

whatever the evolution of the space groups (C2/c - Pnam -

Imcm, when the volume increases), it appears that:

(i) The structural changes occur even for the very smallest

added amounts of guests.

(ii) the topology of the structure of MIL-53 remains

invariant and can be described in all the cases from a reduced

unit cell with a lozenge-based prism. The ratio d/D of the two

diagonals d (smallest distances between chains) and D

therefore characterizes the extent of breathing. Small ratios

relate to the shrunk form of MIL-53 and large ones to the

expanded variety; d is the signature of the strength of the

host–guest interactions and the strong shrinkage of hydrated

MIL-53 shows the strength of the hydrogen bonds which link

the inserted water molecules to both the OH groups and the

oxygens of the carboxylates of the inorganic octahedral

chains.5 All the experimental data related to the different guest

molecules fit nicely with the theoretical evolution of the cell

volume vs. d/D (Fig. 14). This proves that all the molecules

involved in this study have each a selective response in terms of

swelling, and the evolution of the cell volumes provides a

quantitative classification of the strength of the host–guest

interactions: the smaller the expansion, the stronger the

interaction.

(iii) The size of the molecules does not play any steric role

during the swelling. Only host–guest (through the OH groups

of the chains and/or the phenyl rings) and guest–guest inter-

actions are involved in the process.

(iv) Except for the transient phase with methanol and light

alcohols, only the Bragg reflections of the starting and final

phases are observed without any evolution of their position, at

least on the time-scale of the experiment, implying a dramatic

stepwise expansion of the material, even in the presence of

minute amounts of guests which however open up the struc-

ture (Fig. 15). This is what we called the ‘forceps’ effect during

the adsorption, indicating that there is no solid solution during

the exchange which corresponds to a biphasic system.

(v) The time-resolved study of the exchange with pyridine,

lutidine and m-xylene also shows that the kinetics of adsorp-

tion is strongly dependent on the nature of the guest. This is

fast for pyridine (o10 mn) while full exchange occurs in one

hour for lutidine and in 90 mn for m-xylene.

Fig. 14 Comparison of the theoretical evolution of the volume vs.

d/D (black squares) with the fully exchanged experimental ones

(red, green and blue filled circles) associated with the changes of

symmetry and space groups during swelling. These discontinuities

illustrate the influence of host–guest interactions on the phenomenon.

The orange stars correspond to the increase of the swelling of the

transient water–methanol phase at t = 0, 25 and 40 mn.62

Fig. 15 Evolution of the powder patterns of MIL-53(Fe)�H2O during

the dropwise addition of lutidine. The Miller indices on the left refer to

the original MIL-53(Fe)�H2O, and those on the right to MIL-53(Fe)�lutidine. It is clearly seen that the positions of the peaks do not change

during the exchange and that, in this biphasic system, the lutidine

phase appears quasi-immediately after the introduction of the first

drops, even if the contribution is not very visible on the graph during

the first minutes (see text).


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


(vi) For alcohols (methanol and ethanol), the same ‘forceps’

effect occurs when introducing minute amounts of guests,

before the appearance of intermediary phases during the

exchange. The parameters of these transient phases follow

the normal evolution of volumes. This implies that the

methanol–host interactions are rather strong although weaker

than the water–host ones. Moreover, and, at variance to the

observation with pyridine and lutidine, once the transient

phase formed, further addition of methanol provides a slight

increase of the cell volume (and therefore a decrease of

host–alcohol interactions). Within a certain range of composi-

tions (corresponding to a solid solution behaviour) which

stabilise the intermediate phase, the water–alcohol mixture

within the tunnels plays a specific structural role. In terms of

guest–guest interactions, strong hydrogen bonding may be

evoked for explaining this behaviour.

A few structures of the MIL-53(Fe) type with different

guests have been solved recently.63–68 They shed more light

on the different interactions which are responsible of the

swelling of the tunnels. Three types of interactions are

currently evidenced: (i) guest–guest ones (hereafter denoted

G–G), (ii) host–guest ones through p–p and CH–p interactions

between the guests and the terephthalate of the skeleton

(H–G-p), and (iii) for N containing guests, interactions

between them and the OH of the structure (H–GN).

Depending on their relative importance, three situations

occur (Fig. 16), illustrated by the cases of the room- and

high-temperature forms of MIL-53(Fe)�quinone68 and of

MIL-53(Fe), 0.85pyridine:63

(a) In the room-temperature form of MIL-53(Fe)�quinone(V = 1454 A3), whatever the tunnel, all the guest molecules

have the same disposition, parallel to one half of the

terephthalates (BDC) of the skeleton, and creating CH–pbonds with the other half. Between two rows of the same

tunnel, the O� � �O axes of the quinone are orthogonal. This

indicates three types of interactions: (i) strong p–p ones

between the centers of the phenyl rings of the BDC and of

quinone (d: 4.06 A), (ii) medium CH–p ones between the H of

quinone and the BDC and (iii) weaker guest–guest interactions

(4.99 A). However, despite this ranking, all the interactions are

relatively weak since the volume is very high as seen in the

curve of Fig. 15.

(b) In the high-temperature form of MIL-53(Fe)�quinone,the interactions are a little weaker than above (V = 1494 A3).

The main difference with (a) concerns the disposition of the

guests between two adjacent tunnels. Whereas the disposition

of quinones within one tunnel remains roughly the same apart

of a small deviation, it becomes orthogonal from one tunnel to

the other instead of parallel in case (a) but the characteristic

distances are very close. This corresponds to a thermally

induced transition, showing also that this host–guest inter-

action drastically depends on weak variations of the tempera-

ture. The same situation is encountered with molecules such

as m-xylene66 in MIL-53(Fe), and aniline and thiophene in

MIL-47(V).64,65

(c) When N donors exist in the inserted molecule, the

situation is completely different. This is the case for pyridine

(py), DMF63 and lutidine in MIL-53(Fe).68 This time, the

molecules are stacked head to tail with a small shift along the

axis of the tunnel. The predominant interaction exists between

the N atom and the OH groups linking two octahedra of the

skeleton chains (d = 2.68 A for py). Two facts prove that this

interaction is rather strong: (i) the lower volume (V= 1393 A3),

(ii) the comparison with what is observed in solid pyridine

(4.7 A). This enhancement of the strength relates to confine-

ment effects. Otherwise, the G–G interaction between two

consecutive py is rather strong since the distance between

two rings is 3.47 A. The other interactions are weaker.

Some remarks can be made around these situations, parti-

cularly when N donors are involved. Is has been noted that

MIL-53(Fe), as well as MIL-63(Ga)67 accepts a maximum of

0.85 py per Fe, with no means of introducing more. This

implies some disorder within the tunnels. This is suppressed

with Al, with the 1 : 1 stoichiometry being reached. This time,

the organization of pyridine molecules corresponds to the

situation described in (a).67 Two conclusions emerge from this

fact: (i) within a structure type, the nature of the metal plays a

Fig. 16 The three different dispositions of the guests within the

tunnels of MIL-53(Fe): (a) quinone at 300 K; (b) quinone at 333 K;

pyridine at 300 K.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


role on the relative strength of all the interactions and (ii) small

variations in the amount of the guest in the pore induce drastic

modifications of the relative strengths of all the interactions

which occur within the tunnels. However, by exposure to air,

the stoichiometric Al terephthalate loses 0.15 py to give the

classical MIL-53(Al)�0.85py with the same characteristics as

the Fe and Ga solids.

A second remark concerns the comparison of the

behaviours of pyridine and lutidine (or dimethylpyridine) in

MIL-53(Fe). The cell volume with lutidine (V = 1575 A3) is

considerably larger than for pyridine (V = 1393 A3)

suggesting weaker interactions. A preliminary structural

study68 indicates that while the relative disposition of the

guests remains the same in both solids, the characteristic

distances change. If the lutidine–lutidine interactions, even if

slightly longer (3.66 A), remain almost the same, this is not the

case for the N–OH interactions which increase significantly

because the nitrogen atom is masked by the two methyl groups

surrounding it, weakening the interaction for steric reasons.

The increase of the volume decreases correlatively the

confinement effects of the tunnel and the positions of lutidine

are not as constrained as those of pyridine. It explains why the

local environments of lutidine in its solid state or included in

the tunnels are very close. In pyridine, the two states are

different.

The third remark concerns the existence of another type

of interaction encountered once: acido-basic one. This was

demonstrated in the solid [Cu2(pzdc)2(pyz)] (pzdc = pyrazine-

2,3-dicarboxylate, pyz = pyrazine) with permanent 1D

channels with cross-section 4 � 6 A.70 After adsorption of

C2H2, these molecules reside in the channels in such a way that

the acidic hydrogens of acetylene interact with the terminal

oxygens of the skeleton and electron density calculations69

proved that there is an overlap between the electronic clouds

of the two atoms, indicative of a weak acidobasic reaction.

The final remark is more general and concerns a beginning

of justifications of the conditions of synthesis in chemistry.

Indeed, looking at the recipes leading to hybrid solids (and not

only in this domain), it can be seen that the solvent is just

mentioned by the authors, with of course, no reason for its

choice. ‘It is the art of the chemist’ is replied to those who ask

for the question. . . Art can become science too! And probably,

this study of the breathing effect can provide some elements of

an answer. Indeed, it proves indirectly that, even during the

reaction, and depending on the nature of the solvents and of

the species (inorganic bricks (also called Secondary Building

Units (SBU)) and linkers as well) in the solution, the different

interactions involved in the process can change as a function

of their strength and orientate the crystallization of one phase

or another.

To conclude this section, the reversible sponge-like

properties of MIL-53(Fe) applies to a variety of simple organic

molecules (including both protic and non-protic species and

polar and non-polar molecules). Its extreme sensitivity and

selectivity toward organic molecules have provided interesting

information about the host–guest and guest–guest interactions

which govern the adsorption and separation phenomena, as

well as their different strengths. This could find applications in

the field of sensors for their detection in small amounts.

4.2 Adsorption and diffusion

These phenomena are obviously the first application of

breathing solids since the adsorption of gases or liquids is

one of the possible stimuli for the swelling effect. We shall

restrict our discussions to physisorption, whereas breathing

effects involving chemisorption can arise, particularly with

molecular solids.71 This review does not aim at presenting a

large amount of data relative to adsorption; such data are

found in the reviews of the Kitagawa and Rosseinsky groups.

On the contrary, this review extracts from some selected

examples, the specificity of adsorption by breathing matter

in its different macroscopic behaviours, in relation with the

structures when data are available. As yet this is not general

and most authors simply mention the adsorption curves

without following at least in situ the evolution of the

X-ray diffraction pattern to evaluate the cell parameters. Full

structural determinations of the filled samples are currently

extremely rare.

The isotherms of adsorption–desorption will first depend

simultaneously on the nature of the adsorbate and of the

surface of the adsorbant, on the strength of host–guest

interaction, but also on the state of the starting material,

either filled by guests/closed by previous activation, or opened

by the same treatment.

If the starting structure is closed or filled by initial guests, it

is extremely difficult to force entry into the structure. In

doing so, high pressures are necessary for opening.

[Cu(4,40-bipy)(dhbc)2]�2H2O72 represents a textbook example

for illustrating the ‘gate effect’ in this case (Fig. 17(a)). No

Fig. 17 (a) Adsorption isotherms of N2, O2, CH4 and CO2 in

[Cu(4,40-bipy)(dhbc)2]�2H2O72 showing the gate effect as a function

of the nature of the gas. (b) Adsorption isotherm of N2 in [Co(BDP)�2DEF�H2O] with the strange five steps.74


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


adsorption is observed at low pressures except for CO2 and the

pressure threshold for adsorption strongly depends on the

nature of the gas. The pressure of opening is an indirect

signature of the energy of adsorbate/adsorbant interaction

(or the exothermic enthalpy of adsorption, unfortunately

scarcely measured). The larger the enthalpy, the lower the

threshold. This behaviour was also observed during the

adsorption of methanol by [Cu2(pzdc)2(dpyg)]�8H2O.73 This

effect opens large possibilities in separation processes. This will

be developed in section 4.3.

Until very recently, it was thought that this behaviour

corresponded to a one-step transition, but a recent paper by

Long74 with the study of the adsorption of N2 by cobalt

(1,4-benzenepyrazolate (BDP)) [Co(BDP)�2DEF�H2O] shows

that the problem is more complex. This structure is topo-

logically equivalent to the jungle gym [Zn2(1,4-BDC)2(DABCO)],

with dimeric corner-sharing Co tetrahedra replacing the Zn

dimers. The activation of the tetragonal solid distorts the

structure in a way not solved for the moment, but probably

of the same type as MIL-53. During the first stages of the N2

adsorption at low pressure and 77 K, the isotherm exhibits

an unprecedended five steps (Fig. 17(b)), the last one corres-

ponding to the characteristics of the original unit cell.

Increasing slightly the temperature of adsorption (87 K)

makes the steps less and less visible. The reason of this strange

behaviour is currently unknown but in situ measurements of

the evolution of the diffraction patterns vs. pressure is

currently under way. Anyhow, this example is interesting

because it may be supposed that the successive ‘plateaus’

correspond to the fixation of N2 on different sites of the

distorted structure. The resolution of the different structures

during adsorption will be of paramount importance for the

understanding of this phenomenon. Note that the isotherm of

adsorption of H2 exhibits only a single step.

At variance to the above cases, the structure of

MIL-53(Cr,Al) opens under activation. For a better under-

standing of the adsorption of gases, a detailed study, both

experimental and theoretical, was undertaken when H2, CO2

and CH4 and homologous alkanes are concerned, both on

MIL-53(Cr,Al) solids and their counter example MIL-47 (V),

which remains opened and does not breathe. The main struc-

tural difference between the two types results from the presence

or not of m2-OH groups linking the octahedra of the chains.

Experimentally, two situations occur. In the first, the

adsorption curves exhibit a classical type I behaviour; no

unusual behavior is observed in the curve of amount adsorbed

vs. pressure (Fig. 18). This situation is encountered for all the

solids with hydrogen and methane and only with MIL-47

adsorbing CO2. The second, encountered only a few

times,47,75–77 belongs to a new case of adsorption in the field

of hybrid porous solids. It is a two-step behaviour. After a fast

adsorption at very low pressure, the curve reaches a ‘plateau’

in a certain range of pressure before increasing once more.

Even at 70 kbar, saturation is not reached. This phenomenon

occurs only with MIL-53(Cr,Al) during CO2 adsorption

(Fig. 19). As mentioned in ref. 77 this two-step behaviour is

‘still difficult to understand’.

For this reason, we undertook a series of experiments

combining for each gas in situ powder diffraction in real time

at various pressures, with microcalorimetry and molecular

simulations in order to provide complementary elements of

explanations for such an unusual feature. Powder diffraction

was performed at the ESRF Grenoble using a new set-up

placed in the beam. The gas is introduced at increasing

pressures in the tube containing the activated solid and the

powder patterns are collected for each pressure. This gives

the evolution of the cell parameters for each pressure with the

possibility, if the product is sufficiently well crystallized, to solve

the complete structure in order to provide structural informa-

tion about the location of the guests. Quasi-elastic neutron

scattering (QENS) permits access to the dynamics of diffusion.

Microcalorimetry provides experimentally the thermodynamic

elements, in particular the adsorption enthalpy, which indicates

the strength of the guest–host interactions. Finally, molecular

simulations (DFT, Monte Carlo, molecular dynamics) combine

experiment and theory for the fit of the isotherms and give an

image of the movements of the guests within the tunnels and

their jumps from one adsorption site to another.

Our recent series of papers79–87 shed some light on the

explanation of adsorption phenomena.

Concerning hydrogen,78,79 the enthalpies of adsorption are

very weak (3–7 kJ mol�1) and the solids do not breathe. QENS

shows79–87 however that MIL-47(V) and MIL-53(Cr) have

some points of difference in their behaviours in terms of

self-diffusivity. In both cases, diffusivity coefficients D indicate

an orientationally averaged motion, but MIL-47(V) exhibits a

higher diffusivity than MIL-53(Cr) , which is consistent with a

lower activation energy (experimentally 0.6 kJ mol�1 for

MIL-47; 1.6 kJ mol�1 for MIL-53; calculated 0.6 and

1.3 kJ mol�1, respectively). In particular, the self-diffusivity

coefficient of MIL-47(V) (2 � 10�6 m2 s�1 at 300 K) is much

higher than those observed in liquid hydrogen (10�8 m2 s�1)

and in zeolites (10�8–10�10 m2 s�1). In these conditions, it is

illusory to try to localize H2 molecules in the tunnels.

However, in order to more deeply understand the diffusion

mechanism of H2 in the tunnels, molecular dynamics simula-

tions were performed. They show that the diffusion of H2 in

MIL-53(Cr) is mainly governed by the interaction between H2

and the m2-OH groups of the chains, leading to a 1D diffusion

Fig. 18 Adsorption isotherms of H278 in MOF-5, MIL-53(Al) and

HKUST-1 (a copper 1,3,5-benzene tricarboxylate).


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


along the tunnel via a jump sequence involving these OH

groups. In contrast, in MIL-47(V), the simulation shows a

three-dimensional diffusion process with random motions

within the pore of the material.

More attention has been paid to CH4 and CO2. In terms of

breathing, CH4, like H2 does not lead to swelling as proved by

its Type I isotherms.75 In MIL-47(V), five molecules of

methane per tunnel are present at 30 bar. The experimental

adsorption enthalpies increase from �15 to �19.5 kJ mol�1

between 0 and 20 bar, and suggest that methane probes

MIL-47(V) as a homogeneous energetic surface with no pre-

ferential adsorption sites within the pore. QENS experiments

indicate strong similarities with those obtained for H2:

unidirectional diffusion, high self-diffusivity coefficient

[4.2 � 10�7 m2 s�1 at 300 K], but lower than for hydrogen.

The comparison of the probability density in both solids

MIL-47(V) and MIL-53(Cr) is fruitful (Fig. 20). Except for

adsorption enthalpies which remain constant (�17 kJ mol�1),

it once more evidences the influence of the presence of m2-OH

groups on the unidimensional process. It shows that the lower-

energy regions are centered around the m2-OH as well as at the

middle of the tunnel. In MIL-47(V), this latter region is much

more favourable for the methane molecules. In MIL-53(Cr),

one can imagine a sequence of jumps between two consecutive

OH groups. According to QENS data, it is possible to estimate

average jump lengths of 5.6 A (MIL-53) and 8.1 A (MIL-47)

(with a large distribution) with characteristic intervals of

7.3 and 6.0 ps, respectively.

With CO2, the situation is completely different. It breathes

with the m2-OH–containing MIL-53(Al) and (Cr) but not with

MIL-47(V) which does not possess OH groups. Our in situ

structural study of MIL-53(Cr) during adsorption and

desorption of CO281 proves first that, when activated the

structure is open with a cell volume of 1486 A3. Second,

during the first steps of adsorption (Fig. 19), a large amount

of CO2 is adsorbed (two molecules per unit cell) and leads to a

plateau in the adsorption isotherm) between 1 and 4 bar.

In situ diffraction shows that, at that moment, the structure is

contracted by ca. 32% (V = 1072 A3 instead of 1013 when

water is adsorbed). A further adsorption of CO2 at higher

pressures reopens the framework while accepting additional

CO2 molecules into a newly formed tunnel (ca. 9 molecules per

unit cell). From the structure determination at 2 bar, (Fig. 21),

it appears that the CO2 molecules interact with the two

opposite chains corresponding to the small diagonal of the

rhomblike section of the tunnel. The linear molecules are lined

up along the axis of the latter, with an intermolecular distance

of 3.4 A, in the same order of magnitude as distances existing

in solid CO2 (3.24–3.57 A) and indicates strong guest–guest

interactions within the tunnel. Regarding the interactions

between CO2 molecules and the framework, the interatomic

distances suggest two scenarios: (i) interaction between the

carbon atom of CO2 with the oxygen atom of the OH group of

the chain (dC–O = 2.8 A) and (ii) as the CO2 molecule

has a strong quadrupolar moment (�1.4 � 10�35 C m), a

quadrupolar–dipolar interaction between CO2 and OH

(the distance between the two oxygens is 3.06 A).

To improve the knowledge of these interactions, in situ IR

spectroscopic experiments were performed in the same

conditions as for the diffraction studies. They provide

an elegant signature of the dynamics of swelling during

Fig. 19 Adsorption isotherms of methane and carbon dioxide75 in MIL-53(Al) (left) and the corresponding enthalpies of adsorption.

Fig. 20 2D free-energy maps of methane in MIL-53(Cr) (left) and

MIL-47(V) (right) at 250 K obtained through the xz plane of a given

tunnel for a loading of five CH4 molecules per unit cell. White

corresponds to regions of lower free energy, and black to regions of

higher free energy. The dashed lines are guides to the eye.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


adsorption–desorption.82,83 The increasing amount of

adsorbed CO2 is estimated from the evolution of the integrated

intensity of the n2 bands between 645 and 665 cm�1. The shift

from 1022 to 1017 cm�1of the n18a ring mode of the terephthalate

entities corresponds to the passage from the open to

the closed forms of the skeleton. At lower wavenumbers

(550–600 cm�1), the evolution of the spectra characterizes

the distortions of the framework under the action of the

CO2 molecules. The split of the n2 band into two components

at 653 and 662 cm�1 also reveals that the main interaction

involves the formation of electron donor–acceptor complexes

between the C atom of the CO2 molecule and the electron

donor center of the framework. Clearly, CO2 acts as an

electron acceptor while the OH group plays the role of donor.

Incidently, this is the first time that such an interaction

between CO2 and OH is characterized in a solid.

Computer simulation experiments provide more details

about the phenomenon. For MIL-53(Al),84,85 the calculated

enthalpy of adsorption is �37 kJ mol�1 (exptl. �35 kJ mol�1)

and the distances between the oxygens of CO2 molecules and

the hydrogens of the OH groups are calculated as 1.94 and

2.02 A. In conformity with the structural data at 2 bar

(2 molecules/u.c.), a double interaction between the oxygen

atoms of the adsorbate and the two hydrogen atoms on the

hydroxyl groups on opposite pore walls is the preferential

adsorption arrangement and is responsible of the shrinkage of

the tunnel. At increasing pressures, the favourable geometry is

broken, due to the interaction between the additional CO2

molecules. The resulting arrangement corresponds to a less

energetic single interaction between the adsorbate and the OH

group, and therefore, the structure reopens. The same study

with MIL-53(Cr)86 confirms the results obtained with MIL-

53(Al), with an enthalpy of adsorption is �35 kJ mol�1. It

provides also an important new information which could not

be obtained experimentally due to the extremely fast CO2

uptake: the situation when only one CO2 molecule is in the

tunnel instead of two at the plateau (Fig. 22). For one

molecule, calculations prove that the structure remains open

and that the contraction occurs only above two CO2 molecules

per unit cell (in agreement with the experimental results). The

structure begins to re-expand between 5 and 6 guests cell�1.

This result is very important for the explanation of the

phenomenon. It shows that, even for molecules having a high

enthalpy of adsorption, breathing occurs only for a minimum

number of guests in the cell, a minimum which corresponds to

the establishment (for reasons of proximity) of strong

guest–guest interactions within the tunnel These CO2–CO2

interactions act as a backbone for the adsorbed species and

allow the double interaction between the oxygen atoms of the

adsorbate and two hydrogen atoms on the hydroxyl groups on

opposite chains. This symmetry and a threshold intensity

of the interactions is essential for the occurrence of the

contraction. This last remark regarding intensity is crucial

and explains why MIL-53 does not breathe with methane. Its

enthalpy of adsorption is too low and, mainly due to the

spherical and apolar characters of methane, rules out the

development of strong guest–guest interactions which could

lead to a contraction. In a recent paper,87 we prove that with

superior linear alkanes, both the larger enthalpies (which

increase roughly by 10 kJ per additional carbon in the chain)

and their stronger interactions make that the corresponding

solids breathe once more.

In other words, explaining the swelling phenomenon is

indeed difficult because many parameters occur in order to

allow the dynamic movements. However the general picture is

becoming more clear now. Beside this important improve-

ment, other aspects are of course of potential interest in

relation with the breathing phenomenon. They concern

dedicated properties and applications which are detailed

below. They have not received the same level of attention as

above but future studies should be forthcoming.

4.3 Adsorption and separation

Separation is based on the selective times of adsorption of

different molecules on a surface or by the different boiling

Fig. 21 The CO2 molecules in the shrunk tunnel. The OH groups of

the skeleton appear as large grey spheres in the chains.

Fig. 22 Top: Calculation of the evolution with time of the cell volume

of MIL-53(Cr) for different values of the number of CO2 molecules

within one unit cell; Bottom: the evolution of the cell volume vs. the

number of CO2 molecules within the cell. Lines are simply a guide for

the eye.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


temperatures of the constituents of a mixture. It is a strategic

problem for industry which, most of the time, requires the use

of energy to perform these separations, and an evaluation of

the induced costs. Hybrid solids can provide in some cases a

solution at room temperature in order to circumvent the

problem. We shall present two examples in this area, using

the same family of solids, one breathing (MIL-53(Cr)), the

other non-breathing (MIL-47).

It has been seen above that depending on their nature and

that of the skeleton, molecules are oriented in a specific

manner within the pores, which governs their time of residence

on the surface. It is therefore assumed that isomers, despite

similar orientations, will present different energies of surface

adsorption in relation with the positions of the substituents in

the molecule.

With this in mind, the De Vos group66 has recently proved

that the non-breathing MIL-47, which remains open whatever

the guest (see above) is a very good material for the separation

of the isomers of xylene. Their separation, in a mixture with

ethylbenzene is quantitative and effective in less than 20 min at

room temperature.

The second example uses MIL-53(Cr). It was previously

seen that, when it is activated (open), the CO2 adsorption

corresponds to a two-step behaviour whereas that of methane

shows a type I isotherm. When adsorption measurements are

performed on the hydrated form of this solid, the situation

becomes completely different (Fig. 23). The adsorption of

methane becomes close to zero while, for CO2, the curve is

characteristic of a gate effect, and therefore of a one-step

behaviour.88

All the examples above (threshold of the gate effect depend-

ing on the nature of the gas for the same structure type, two-

step behaviour depending on the activation of the solid,

different temperatures of adsorption. . .) make breathing solids

excellent candidates for separation at room temperature,

playing on the different parameters cited above. They are

highly selective towards mixtures of guests and could represent

a cheap alternative for current processes.

4.4 Conducting materials

This title seems surprising as hybrid solids are well known for

being insulators. This relates to a general feature of hybrid

solids. Except in a few cases with copper-based solids, the

metals of the skeleton are in a single valence state. The idea

was therefore to introduce mixed-valency for metals using a

post-treatment procedure. This was performed electro-

chemically by introduction of lithium in MIL-53(Fe).89 It

must be remembered that this solid, in its dehydrated form,

is shrunk (V = 900 A3) at variance to its Cr rand Al

analogues. The process was followed by in situ methods

(diffraction, Mossbauer spectroscopy) in real time. These

prove that (i) both Fe2+ and Fe3+ coexist in the final solid

with a maximum ratio Fe2+ : Fe3+ of 3 : 2; (ii) the trans-

formed solid becomes a conductor and exhibits both electronic

and ionic conductivity; (iii) the structure swells during the

charging process with a complete opening of the structure

(V = 1562 A3) and the process is reversible during discharge.

The breathing occurs not only from the introduction of

lithium but also from the simultaneous insertion of the

electrolyte. The extent of the swelling depends on the nature

of the latter. This presence of electrolyte ensures better

contacts in the half-cell during the conductivity measurements

and opens possibilities of using breathing solids as new

electrode materials for fuel cells.

Incidently, and beside its aspect, it must be noted that, after

this work, the introduction of lithium in MOFs has become

topical for hydrogen storage purposes. From ideas of

Goddard90 and Blomquist,91 some attempts were successfully

realized 92,93 for introducing Li chemically in the structures of

MOFs. This indeed improves the performances of H2 storage

of these MOFs.

4.5 Drug storage and delivery

The synthesis of new bioactive compounds of very high

molecular weight with therapeutic activity and/or with a low

aqueous solubility becomes more and more complex and the

processes of their commercialization very slow. This requires

the use of carrier systems to improve the activity of known

molecules. These systems usually allow a better control of the

drug plasmatic effects, increasing the efficiency and decreasing

the toxicity, as well as an increase in the drug stability by

protection of the biodegradation. Until recently, polymeric

and mixed systems have been proposed for a better controlled

release of drugs.94,95 In particular, mesoporous inorganic

solids such as the ordered mesoporous silicas (pure or func-

tionalized by a post-synthesis modification) are now used.96

However, this process leads to a decrease of the drug storage

capacity.97,98

Recently, our group proposed an alternative route

(the hybrid route), using for the first time porous MOFs as

new controlled delivery systems99 because of their high pore

volume, regular porosity and the presence of tunable organic

groups within the framework which allow an easy modulation

of the size of the pores. The first example concerned the

adsorption of the model molecule ibuprofen in the rigidMOFs

MIL-100 and MIL-101 with mesopores. They exhibit a very

high drug storage capacity, up to an unprecedented 1.4 gram

Fig. 23 Comparison of the performances of anhydrous and hydrated

forms of MIL-53(Cr) in terms of CO2 and CH4 adsorptions, indicating

a high selectivity for the latter.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


of drug per gram of porous solid, and a complete drug

controlled release under physiological conditions from

3 to 6 days (Fig. 24).

For studying the effect of this breathing phenomenon on the

drug adsorption and delivery properties of pharmacological

molecules, Ibuprofen (hereafter denoted Ibu) was once more

chosen as a test molecule for looking at two points (i) the

influence of breathing on drug adsorption and (ii) the

characterizations and the induced effects of the host–drug

interactions (including kinetics of delivery in a simulated

physiological environment). Two flexible materials were used:

(i) MIL-53(Cr) solid as a model material despite the well-

known toxicity of chromium compounds and which, when

activated, presents its open form; (ii) the much less-toxic iron

analogue MIL-53(Fe) (rat oral dose: DL50(Fe) = 30 g kg�1,

DL50(terephthalic acid 46.4 g kg�1) which is closed after

activation.

Both solids adsorb ca. 20 wt% of ibuprofen100 (Fig. 24). The

filled samples are non-porous. The final cell volume after

treatment is roughly the same for the two compounds

(V E 1400 A3), corresponding to a slight contraction (6%)

for the opened MIL-53(Cr) and an important expansion

(55%) for the Fe analogue. Using IR spectroscopy, the shift

of the n(CQO) band of the carboxylic group of Ibu (1695 -

1725 cm�1) correlated to that of the vibrational n(O–H) band

of the inorganic chains (3656 - 3639 cm�1) indicates the

formation of a strong hydrogen bond between these two

groups. The different 1H and 13C solid-state NMR character-

izations confirm this fact and also provide information about

the dynamics of ibuprofen within the tunnel. 13C NMR

indicates a slight conformational distribution of the drug in

the pores, which concerns essentially the aromatic part of Ibu.

However, it shows also that the mean conformation is close to

that of the pure form of ibuprofen. All these features were

confirmed by DFT calculations which also suggest (Fig. 25),

beside the main interaction, the possibility of van der Waals

and/or CH–p interactions between the hydroxyls of the skele-

ton and the methyl groups of Ibu. This provides an estimate of

the drug–matrix interaction energy of �57 kJ mol�1, larger

than that observed for CO2 in MIL-53(Cr) (�35 kJ mol�1).

The most striking feature of the phenomenon concerns the

kinetics of delivery. In the first place, the delivery is complete.

The kinetic study was performed using a simulated body fluid

(SBF) with similar inorganic composition as human plasma, at

37 1C under continuous stirring. Surprisingly, a very slow

delivery, complete only after 3 weeks, is observed with clearly

two steps in the process. Despite the slight changes of slope,

the results were treated as a first approximation, as having a

zero-order kinetics. A model was proposed to explain this fact.

These facts suggest a specific behaviour of flexible frame-

works on the adsorption of species (gases, vapours, drugs...),

compared to rigid ones. They are adaptative and take, within

the same topology, a configuration where the interactions

between guest molecules and the framework are optimised

while taking also into account the steric hindrance of the guest

molecules and their number. If the latter increases due to a

additional stimulus (increase of pressure for instance), the

structure will open more. This adaptability might explain the

long time of delivery. Indeed, due to its flexibility, MIL-53 can

be considered as an intrinsically tailor-made container which

fits with the geometrical and energetical characteristics of the

guest and therefore enhances the confinement effects. If the

Fig. 24 Ibuprofen delivery performances of flexible MIL-53(Cr) and -(Fe) (left) and of rigid MIL-100 and MIL-101 (right).

Fig. 25 (a) Localization of ibuprofen in the pores of MIL-53(Fe);

comparison of the conformations of ibuprofen confined in the tunnels

(b) and in its pure solid form (c).


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


latter are optimized, they will not favour rapid evacuation of

the guests and therefore explain the long times for release.

Finally, we could expect very long therapies using flexible

MOFs for drug delivery. Also, the possibility of having drug

carriers with zero-order kinetics represents an important

progress since a unique administration could be provided.

This would lead to a stable blood concentration, a mini-

misation of the toxicity effects as well as a decrease in patient

discomfort. Moreover, the slow release will protect the

drug from degradation processes by increasing its plasmatic

half-life, its bioavailability and therefore its efficiency.

5. Conclusion

At the outset, breathing effects were considered as laboratory

curiosities. Their careful academic study has allowed to

understand not only the structural requirements for such a

phenomenon, but also its different manifestations and their

reasons. They result from the juxtaposition of structural

characteristics of the skeleton (mainly the existence of weak

points in the structure which allow flexibility), of the nature of

the internal surface, on the complex and competitive nature of

the different interactions (guest–guest and host–guest). Beside

these academic features, some potential applications of this

effect have appeared, due to the large amounts of species that

the expansion of the cells allow to store compared to rigid

frameworks. These applications relate to energy and sustain-

able development (adsorption, separation, storage. . .), and to

health (drug delivery). Their development is ongoing.

Acknowledgements

The authors are grateful to their colleagues in Versailles and

also to the groups of M. Daturi (Caen), H. Jobic (Lyon),

P. L. Llewellyn (Marseille) and G. Maurin (Montpellier) for

their outstanding contributions to the knowledge of the

breathing effect.

References

1 J. P. Collin, C. Dietrich-Buchecker, P. Gavina, M. C. Jimenez-Molero and J. P. Sauvage, Acc. Chem. Res., 2001, 34, 477.

2 M. C. Jimenez-Molero, C. Dietrich-Buchecker and J. P Sauvage,Chem.–Eur. J., 2002, 8, 1456.

3 K. Barthelet, J. Marrot, D. Riou and G. Ferey, Angew. Chem.,Int. Ed., 2002, 41, 281.

4 C. Serre, F. Millange, C. Thouvenot, M. Nogues, G. Marsolier,D. Louer and G. Ferey, J. Am. Chem. Soc., 2002, 124, 13519.

5 T. Loiseau, C. Serre, C. Huguenard, G. Fink, F. Taulelle,M. Henry, T. Bataille and G. Ferey, Chem.–Eur. J., 2004, 10,1373.

6 K. Barthelet, J. Marrot, D. Riou and G. Ferey, Chem. Commun.,2004, 520.

7 K. Uemura, S. Kitagawa, M. Kondo, K. Fukui, R. Kitaura,H.-C. Chang and T. Mizutani, Chem.–Eur. J., 2002, 8, 3587.

8 K. Biradha andM. Fujita, Angew. Chem., Int. Ed., 2002, 41, 3392.9 D. Maspoch, D. Ruiz-Molina, K. Wurst, N. Domingo,M. Cavallini, F. Biscarini, J. Tejada, C. Rovira and J. Veciana,Nat. Mater., 2003, 2, 190.

10 G. Ferey, Nat. Mater., 2003, 2, 136.11 G. Alberti, M. Murcia-Mascaros and R. Vivani, J. Am. Chem.

Soc., 1998, 120, 9291.12 R. H. Mitchell, Perovskites Modern and Ancient, Almaz Press

Inc., Thunderbay, Canada, 2002.

13 G. Ferey and J. Pannetier, Eur. J. Solid State Inorg. Chem., 1994,31, 697.

14 N. Khosrovani and A. W. Sleight, J. Solid State Chem., 1996,121, 2.

15 J.-M. Lehn, Science, 2002, 295, 2400.16 L. Girondini, A. G. Stendardo, S. Geremia, M. Campagnolo,

P. Samori, J. P. Rabe, R. Fokkens and E. Dalcanale, Angew.Chem., Int. Ed., 2003, 42, 1384.

17 S. Kitagawa and K. Uemura, Chem. Soc. Rev., 2005, 34, 109.18 S. R. Batten and R. Robson, Angew. Chem., Int. Ed., 1998, 37,

1460.19 G. Ferey, Chem. Soc. Rev., 2008, 37, 191.20 T. Reneke, M. Eddaoudi, D. B. Moler, M. O’Keeffe and

O. M. Yaghi, J. Am. Chem. Soc., 2000, 122, 4844.21 M. Eddaoudi, D. B. Moler, H. Li, B. Chen, T. Reneke,

M. O’Keeffe and O. M. Yaghi, Acc. Chem. Res., 2001, 34, 319.22 M. E. Thomson, Chem. Mater., 1994, 6, 1168.23 G. Alberti, in Solid State Supramolecular Chemistry: two- and

three-dimensional inorganic networks, ed. G. Alberti and T. Bein(vol. 7 of Comprehensive Supramolecular Chemistry, J.-M. Lehnchairman ed.), Pergamon-Elsevier Science, Oxford, 1996, ch. 5.

24 A. Clearfield, Prog. Inorg. Chem., 1998, 47, 373.25 H. Byrd, A. Clearfield, D. Poojary, K. P. Reis and

M. E. Thomson, Chem. Mater., 1996, 8, 2239.26 G. Ferey, unpublished results.27 G. Ferey, Chem. Mater., 2001, 13, 3084.28 G. Ferey, C. Mellot-Draznieks, C. Serre and F. Millange, Acc.

Chem. Res., 2005, 38, 317.29 F. Millange, N. Guillou, R. I. Walton, J.-M. Greneche,

I. Margiolaki and G. Ferey, Chem. Commun., 2008, 4732.30 T. Loiseau, C. Mellot-Draznieks, H. Muguerra, M. Haouas,

F. Taulelle and G. Ferey, Comptes-Rendus Chimie, 2005, 8, 765.31 M. O’Keeffe and B. G. Hyde, Philos. Trans. R. Soc. London, Ser.

A, 1980, 295, 553.32 S. Kitagawa and K. Uemura, Chem. Soc. Rev., 2005, 34, 109.33 K. Uemura, R. Matsuda and S. Kitagawa, J. Solid State Chem.,

2005, 178, 2420.34 D. Bradshaw, T. J. Prior, E. J. Cussen, J. B. Claridge and

M. J. Rosseinsky, J. Am. Chem. Soc., 2004, 126, 6106.35 X. Zhao, B. Xiao, A. J. Fletcher, K. M. Thomas, D. Bradshaw

and M. J. Rosseinsky, Science, 2004, 306, 1012.36 A. J. Fletcher, K. M. Thomas andM. J. Rosseinsky, J. Solid State

Chem., 2005, 178, 2491.37 K. Seki and W. Mori, J. Phys. Chem. B, 2002, 106, 1380.38 D. N. Dybtsev, H. Chun and K. Kim, Angew. Chem., Int. Ed.,

2004, 43, 5033.39 H. Chun, D. N. Dybtsev, H. Kim and K. Kim, Chem.–Eur. J.,

2005, 11, 3521.40 S. Horike, R. Matsuda, D. Tanaka, S. Matsubara, M. Mizuno,

K. Endo and S. Kitagawa, Angew. Chem., Int. Ed., 2006, 45, 7226.41 K. Uemura, Y. Yamasaki, Y. Komagawa, K. Tanaka and

H. Kita, Angew. Chem., Int. Ed., 2007, 46, 6662.42 H. J. Choi, M. Dinca and J. R. Long, J. Am. Chem. Soc., 2008,

130, 7848.43 D. Tanaka, K. Nakagawa, M. Higuchi, S. Horike, Y. Kubota,

T. C. Kobayashi, M. Takata and S. Kitagawa, Angew. Chem., Int.Ed., 2007, 46, 6662.

44 K. Birhada andM. Fujita, Angew. Chem., Int. Ed., 2002, 41, 3392.45 R. Kitaura, K. Seki, G. Akiyama and S. Kitagawa, Angew.

Chem., Int. Ed., 2003, 42, 428.46 R. Kitaura, K. Fujimoto, S.-I. Noro, M. Kondo and S. Kitagawa,

Angew. Chem., Int. Ed., 2002, 41, 133.47 A. J. Fletcher, E. J. Cussen, T. J. Prior, M. J. Rosseinsky,

C. J. Kepert and K. M. Thomas, J. Am. Chem. Soc., 2001, 123,10001.

48 E. J. Cussen, J. B. Claridge, M. J. Rosseinsky and C. J. Kepert,J. Am. Chem. Soc., 2002, 124, 9574.

49 C. Mellot-Draznieks, J. Dutour and G. Ferey, Angew. Chem., Int.Ed., 2004, 43, 6290.

50 C. Mellot-Draznieks, C. Serre, S. Surble and G. Ferey, J. Am.Chem. Soc., 2005, 127, 16273.

51 S. Surble, C. Serre, C. Mellot-Draznieks, F. Millange andG. Ferey, Chem. Commun., 2006, 284.

52 C. Serre, C. Mellot-Draznieks, S. Surble, N. Audebrand,Y. Filinchuk and G. Ferey, Science, 2007, 315, 1828.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


53 C. Serre, F. Millange, S. Surble and G. Ferey, Angew. Chem., Int.Ed., 2004, 43, 6286.

54 C. Serre, S. Surble, C. Mellot-Draznieks, Y. Filinchuk andG. Ferey, Dalton Trans., 2008, 5462.

55 H. Li, M. Eddaoudi, M. O’Keeffe and O. M. Yaghi,Nature, 1999,402, 276.

56 M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter,M. O’Keeffe and O. M. Yaghi, Science, 2002, 295, 469.

57 G. Ferey, Dalton Trans., 2009, DOI: 10.1039/b817360p.58 H. K. Chae, D. Y. Siberio-Perez, J. Kim, Y. Go, M. Eddaoudi,

A. J. Matzger, M. O’Keeffe and O. M. Yaghi, Nature, 2004, 427,523.

59 T. Devic, C. Serre, N. Audebrand, J. Marrot and G. Ferey, J. Am.Chem. Soc., 2005, 127, 12788.

60 S. Surble, C. Serre, F. Millange, T. Duren, M. Latroche andG. Ferey, J. Am. Chem. Soc., 2006, 128, 14889.

61 G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour,S. Surble and I. Margiolaki, Science, 2005, 309, 2040.

62 F. Millange, C. Serre, N. Guillou, G. Ferey and R. I. Walton,Angew. Chem., Int. Ed., 2008, 47, 4100.

63 T. R. Whitfield, X. Wang, L. Liu and A. J. Jacobson, Solid StateSci., 2005, 7, 1096.

64 E. V. Anokhina, M. Vougo-Zanda, X. Wang and A. J. Jacobson,J. Am. Chem. Soc., 2005, 127, 15000.

65 X. Wang, L. Liu and A. J. Jacobson, Angew. Chem., Int. Ed.,2006, 45, 6499.

66 L. Alaerts, C. E. A. Krschhock, M. Maes, M. A. van der Veen,V. Finsy, A. Depla, J. A. Martens, G. V. Baron, P. A. Jacobs,J. E. M. Denayer and D. E. De Vos, Angew. Chem., Int. Ed., 2007,46, 4293.

67 M. Vougo-Zanda, J. Huang, E. V. Anokhina, X. Wang andA. J. Jacobson, Inorg. Chem., 2008, 47, 11535.

68 F. Millange, N. Guillou, J.-M. Tarascon and G. Ferey,unpublished results.

69 R.Matsuda, R. KItaura, S. Kitagawa, Y. Kubota, R. V. Belosludov,T. C. Kobayashi, H. Sakamoto, T. Chiba, M. Takata, Y. Kawazoeand Y. Mita, Nature, 2005, 436, 238.

70 R. Kitaura, S. Kitagawa, Y. Kubota, T. C. Kobayashi, K. Kindo,Y. Mita, A. Matsuo, M. Kobayashi, H. C. Chang, T. C. Ozawa,M. Suzuki, M. Sakata and M. Takata, Science, 2002, 298, 2358.

71 M. Albrecht, M. Lutz, A. I. Spek and G. van Koten, Nature,2000, 406, 970.

72 R. Kitaura, K. Seki, G. Akiyama and S. Kitagawa, Angew.Chem., Int. Ed., 2003, 42, 428.

73 M. Kondo, T. Okbo, A. Asami, S.-I. Noro, T. Yoshitomi,S. Kitagawa, T. Ishii, H. Matsuzaka and K. Seki, Angew. Chem.,Int. Ed., 1999, 38, 140.

74 H. J. Choi, M. Dinca and J. R. Long, J. Am. Chem. Soc., 2008,130, 7848.

75 S. Bourrelly, P. L. Llewellyn, C. Serre, Millange, T. Loiseau andG. Ferey, J. Am. Chem. Soc., 2005, 127, 13519.

76 K. Yamada, H. Tanaka, S. Yagishita, K. Adachi, T. Uemura,S. Kitagawa and S. Kawata, Inorg. Chem., 2006, 45, 4322.

77 K. Uemura, Y. Yamasaki, Y. Komagawa, K. Tanaka andH. Kita, Angew. Chem., Int. Ed., 2007, 46, 6662.

78 G. Ferey, M. Latroche, C. Serre, T. Loiseau and A. Percheron-Guegan, Chem. Commun., 2003, 2976.

79 F. Salles, H. Jobic, G. Maurin, M. M. Koza, P. L. Llewellyn,T. Devic, C. Serre and G. Ferey, Phys. Rev. Lett., 2008, 100,245901.

80 N. Rosenbach Jr, H. Jobic, A. Ghoufi, F. Salles, G. Maurin,S. Bourrelly, P. L. Llewellyn, T. Devic, C. Serre and G. Ferey,Angew. Chem., Int. Ed., 2008, 47, 6611.

81 C. Serre, S. Bourrelly, A. Vimont, N. A. Ramsahye, G. Maurin,P. L. Llewellyn, M. Daturi, Y. Filinchuk, O. Leynaud, P. Barnesand G. Ferey, Adv. Mater., 2007, 19, 2246.

82 A. Vimont, H. Leclerc, P. Bazin, J.-C. Lavalley, M. Daturi,C. Serre, G. Ferey, S. Bourrelly and P. L. Llewellyn, Chem.Commun., 2007, 3291.

83 A. Vimont, A. Travert, M. Daturi, J.-C. Lavalley, S. Surble,C. Serre and G. Ferey, J. Phys. Chem. C, 2007, 111, 383.

84 N. A. Ramsahye, G. Maurin, S. Bourrelly, P. L. Llewellyn,T. Loiseau and G. Ferey, Phys. Chem. Chem. Phys., 2007, 9, 1059.

85 N. A. Ramsahye, G. Maurin, S. Bourrelly, P. L. Llewellyn,T. Loiseau, C. Serre and G. Ferey, Chem. Commun., 2007, 3261.

86 F. Salles, A. Ghoufi, G. Maurin, R. G. Bell, C. Mellot-Draznieks,P. L. Llewellyn, C. Serre and G. Ferey, Angew. Chem., Int. Ed.,2008, 47, 8487.

87 P. L. Llewellyn, G. Maurin, T. Devic, S. Loera-Serna,N. Rosenbach Jr, C. Serre, S. Bourrelly, P. Horcajada,Y. Filinchuk and G. Ferey, J. Am. Chem. Soc., 2008, 130, 12808.

88 P. Llewellyn, S. Bourrelly, C. Serre, Y. Filinchuk and G. Ferey,Angew. Chem., Int. Ed., 2006, 45, 7751.

89 G. Ferey, F. Millange, M. Morcrette, C. Serre, M.-L. Doublet,J.-M. Greneche and J.-M. Tarascon, Angew. Chem., Int. Ed.,2007, 46, 3259.

90 S. S. Han and W. A. Goddard III, J. Am. Chem. Soc., 2007, 129,8422.

91 A. Blonquist, C. M. Araujo, P. Srepusharawoot and R. Ahuja,Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 20173.

92 K. L. Mulfort and J. T. Hupp, J. Am. Chem. Soc., 2007, 129, 9604.93 K. L. Mulfort and J. T. Hupp, Inorg. Chem., 2008, 47, 7936.94 S. Freiberg and X. X. Zhu, Int. J. Pharm., 2004, 282, 1.95 K. S. Soppimath, T. M. Aminabhavi, A. R. Kulkarni and

W. E. Rudzinski, J. Controlled Release, 2001, 70, 1.96 M. Vallet-Regı, Chem.–Eur. J., 2006, 12, 5934, and references

therein.97 B. Munoz, A. Ramila, J. Perez-Pariente, I. Diaz and M. Vallet-

Regi, Chem. Mater., 2003, 15, 500.98 P. Horcajada, A. Ramila, G. Ferey and M. Vallet-Regı, Solid

State Sci., 2006, 8, 1243.99 P. Horcajada, C. Serre, M. Vallet-regi, M. Sebban, F. Taulelle

and G. Ferey, Angew. Chem., Int. Ed., 2006, 45, 5974.100 P. Horcajada, C. Serre, G. Maurin, N. A. Ramsahye, F. Balas,

M. Vallet-Regi, M. Sebban, F. Taulelle and G. Ferey, J. Am.Chem. Soc., 2008, 130, 6774.


Dow

nloa

ded

by N

AT

ION

AL

TE

CH

NIC

AL

UN

IVE

RSI

TY

OF

AT

HE

NS

on 2

4 Fe

brua

ry 2

013

Publ

ishe

d on

26

Febr

uary

200

9 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

8043

02G

View Article Online


large breathing effects in three-dimensional porous hybrid matter

Documents