Photochemistry of coordination compounds

Absorption band Electronic transition

The excited states are treated in the "localised MO approximation": the transition is considered to involve two predominant orbitals, the

electron being promoted from OM1 to OM2, ignoring more or less the other orbitals

What are the most important excited states, as far as electron transfer is concerned?

1. Ligand-Field bands/states, often referred to as LF, d-d or MC (for metal-centered )

These states are dissociative, because an antibonding orbital is populated by the electronic transition. Two classical examples:

a) Ti(H2O)63+ : the electronic configuration of Ti(III) is d1. This implies that, to

generate the excited state, the electron be taken from a t2gorbital so as to populate an eg

* level, which is strongly antibonding.

b) Rh(NH3)63+ : the electronic configuration of Rh(III) is d6. To generate the

excited state, the electron has also to be taken from a t2gorbital to populate an eg

* level (strongly antibonding).

t2g

eg*

As a consequence, Rh(NH3)63+ is rapidly aquated in water, under

UV light irradiation, whereas it is thermally very stable. The excited state is 1014 times more reactive than the ground state!

Rh(NH3)63+ + H2O Rh(NH3)5(H2O)3+ + NH3

h

In conclusion, LF states are not adapted to photosynthesis and photoredox reactions in general. On the other hand, if we want to

exchange ligands under the action of light, they could be particularly useful ( light-driven machines and motors)

2. Charge-transfer states: MLCT and LMCT

LMCT: electron-rich ligand and high oxidation state for the metal

[CoIII(NH3)5Br]2+ IrIVCl62-

MLCT: electron-poor ligand and relatively low oxidation state for the metal. These states are the most important ones in relation to electron transfer and mimics of the natural photosynthesis by transition metal complexes:

ReI(bipy)(CO)3Cl, Mo(bipy)(CO)4, Ru(bipy)32+

For these three complexes, the transition is a metal-to-ligand charge transfer: dof the metal-*of the bipy ligand. The ligand must have relatively low-lying orbitals available

A paradoxical statement: an excited state is generally a better oxidant and a better reductant than the ground state

0

energy

level of the free electron

Ground State Excited State

t2g

eg*

*h

a d6 metal centre with a -accepting ligand

0

energy

level of the free electron

Ground State Excited State

t2g

eg*

*h

EA, the electronic affinity, is directly related to the oxidizing character of the molecule, whereas the ionisation potential, IP, is roughly

inversely proportional to its reducing power.

EA EA

IP

IP

The prototype: Ru(bipy)32+

2+

N

N

N

NN

NRu

This complex is chiral. It can be regarded as a triple-stranded helix

or

Ru(bipy)32+

Ground State

*Ru(bipy) 32+

MLCT Excited State

h

visible

Ru(bipy)32+ has unique properties, related to its light absorbing

properties, nature and lifetime of excited state, emission properties and ability to transfer an electron or accept it in its excited state. In

addition, it is a very robust complex, both thermally and photochemically.

Absorption:

max = 452 nm in H2O ; 13 500 (deep red-orange complex in solution)

The excited state is a triplet Metal-to-Ligand Charge Transfer state

(3MLCT). In its excited state, the complex is best described as Ru(III) and bipy-. (for one of the three bipy ligands):

Emission:max ~ 605 nm in H2O (red emission) em ~ 0.05 (emission quantum yield)° ~ 600 ns in water to ~ 1s in organic solvents. This means that the excited state has a very long lifetime, which will allow, in particular, bimolecular reactions with various reagents.

Ru(bipy)32+ *Ru(bipy) 3

2+ ~ Ru(III) (bipy-.)(bipy)22+

1MLCT/ 3MLCT

h

visible

oxidant reductant

Ru(III) (bipy-.)(bipy)22+

*Ru(bipy) 32+ is thus an interesting electron transfer

reagent: it can act as an electron donor (reductant) or as an electron acceptor (oxidant)

Ru(bipy) 32+ Ru(bipy) 3

3+" Ru(bipy) 3+ "

h

-e-

-e-e-

e-

formally Ru(I) but, in fact, Ru(II)

the redox potentials of the various couples involving the excited state can be deduced from the electrochemical

properties of the ground state and the energy level of the excited state (E0-0 )

*Ru(bipy) 32+ Ru(bipy) 3

2+ + h

h605 nm E0-0 ~ 2.1 eV

ground state

3MLCT

3d-d state

1MLCT excited state

h

ABSORPTION

h

EMISSION

E0-0 ~ 2.1 eV

Electrochemical properties of Ru(bipy)32+ and *Ru(bipy)3

2+

For *Ru(bipy) 32+ in its 3MLCT state:

E (redox potential), in Volts

0E0-0 ~ 2.1 eV

RuIII/RuII

*RuII/RuI

RuIII/*RuII

RuII/RuI

+1.26

+0.84

-0.86

-1.28

E0-0

E0-0

*Ru(bipy) 32+ is thus able to react with an electron donor (D) or

an electron acceptor (A) : quenching of luminescence : quenching of the excited state by an electron transfer reaction:

RuII *RuIIh

*RuII + A RuIII + A-.

kQ

kQ is the quenching rate constant; in this particular case, we have an oxidative quenching (the complex is oxidized). kQ is often referred to as keT

(electron transfer) or kCS, CS meaning "charge separation" . The value of this rate constant can be large (typically 108-109 mol L-1s-1). It is

nevertheless limited by diffusion (~ 1010 in usual solvents).

What should be done to circumvent this difficulty if we want a really fast electron transfer from the excited state?

*RuII + A RuIII + A-.

kQ

Once the CS state has been formed (forward electron transfer), recombination of the charges (backward eT) can

also be very fast, and sometimes even faster than the forward reaction:

RuIII + A-. RuII + A

kCR or kb

These rate constants can be roughly predicted from Marcus theory

(1MLCT)*Ru(bipy) 3

2+

*Ru(bipy) 32+

3MLCT

RuII(bipy) 32+RuIII(bipy) 3

+ RuI(bipy) 3+

-1.28 V+1.26 V

+0.84-0.86

InterSystem Crossing (yield=1)

max = 452 nmmax = 14 000

habs

h'em

E0-0 ~ 2.1 eV

= non radiative = radiative = redox properties

E0-0 ~ 2.1 eV = 600 nsem = 0.04

Cu(dpp)2

+ and related tetrahedral complexes (d10) MLCT

dpp = 2,9-diphenyl-1,10-phenanthroline

Other complexes of interest, whose excited states are relatively long-lived and which can participate in electron transfer processes in the ground state and in the excited

state (3MLCT excited state):

Re(bipy)(CO)3Cl or Re(bipy)(CO)3L+ , L = py, CH3CN, etc..

Absorption centred around 400 nm; E0-0 ~ 2.3 eV

N N

Photochemical splitting of water to H2 and O2 : approaches based on RuII(bipy) 3

2+

Hypothetical sequence of reactions:

RuII *RuIIh

*RuII + A RuIII + A-.

eT

A-. + H2O A + 1/2 H2 + OH

-

RuIII + OH- RuII + 1/4 O2 + 1/2 H2O

kQ

overall reaction:

1/2 H2O 1/2 H2 + 1/4 O2

h

Does it really work as describedon the previous slide?

h

RuII *RuII

RuIII

A

A-.

D+.

D H2O

H2O

O2

H2

A = MV2+, RhIII, CoIII, etc…D = sacrificial agent, which is unable to oxidize water once oxidized to D

+., but which reacts rapidly with RuIII in its

reduced state D ( amine: EDTA, TOA, …)

Energy transfer: two important mechanisms:

D D*

D* + A D + A*

D : energy donorA : energy acceptor

1) Förster (dipole-dipole interaction or coulombic mechanism)

2) Dexter (double exchange of electrons : collisional mechanism)

Förster : the oscillating dipole of D* creates an electrostatic field, which induces the transition A A* when D*

deactivates

D*

D

A*

A

energy transfer

Förster's mechanism does not imply that the two reagents collide in order to exchange energy : the

process can occur at large distance (up to 60-70 Å)

kF = Constant x em

n4 r6JF

kF : rate constant of the energy transfer reactionem : emission quantum yield of the donor D* excited state lifetime of the donor D*n: refraction index of the solventr : centre-to-centre distance between D* and AJF : overlap integral between the emission spectrum

of D and the absorption spectrum of A

Dexter's mechanism requires that the two reagents D* and A exchange a particle (electron) in the course of the energy transfer process: they thus have to come to close contact.This mechanism is operative at short distances only

D* DA A*

Dexter's mechanism: exchange of electrons implies orbitals overlapping; the distance between D and A will

thus have a great importance

= attenuation factorr = edge-to-edge distance between the donor and the acceptor

kD = constant x exp(-r)

Very fast decay of the energy transfer rate with the distance; this reflects the nature of

the process, similar to electron transfer

Generally, transition metal complexes are prompted to undergo energy transfer processes

according to Dexter mechanism (exp-r) whereas porphyrins, in their singlet excited state, react

according to Förster's mechanism (1/r6)

RuII OsII

Zn H2

PZn PH2

Ru(bipy)32+ derivative Os(bipy)3

2+ derivative

Marcus theory is too complex to be treated in less than a few hours…Sorry!

D + A D+ + A- kel

kel(r) = n(r) el(r) n(r)

n(r) : nuclear vibration frequency

el(r) : electronic transmission factor: electronic coupling which contains, in particular, the distance between D and A and the nature of the chemical functions located in between D and A

n(r) : nuclear factor: thermodynamics of the process and reorganization parameter

The electronic factorel is proportional to HAB

2, HAB being the coupling matrix

element or the coupling Hamiltonian.

HAB = Ho exp(-r)

= attenuation factor, in Å-1, if r is expressed in Å. The

nature of the bridge will determine the coupling and it is quantified by If ~ 0.1 or 0.2 Å

-1, the electronic coupling is very strong

and the electron can travel over very large distances (aromatic or conjugated bridges between D and A).If ~ 0.8 or 1 Å

-1, D and A are only very slightly coupled

and the electron transfer rate will fall off rapidly (a few Å);The rate of electron transfer in DNA has recently been

the subject of intense debates…

The nuclear factor :

n = exp(- G≠ / RT)

G≠ = "classical" activation barrier

Rreagents(D + A) P

products( D+ + A-)

G≠

Marcus quadratic equation:

G≠ = /4 ( 1 + G° / )2

G° = free energy difference between the reagents and the products. For a spontaneous reaction, G° is negative.

= reorganization parameter, which measures the energy cost that the system would have to pay to undergo the distorsion leading to P from R, without electron transfer.

For a self-exchange process, G° = 0.The energy curves are at the same level

PR

In general, G° ≠ 0 :

G≠

G°

P

R

contains 2 distinct terms: in , the internal reorganization parameter( internal coordination sphere), and out , which takes

into account the solvent molecules, the counterions, etc…

Let us increase the driving force (IG°I = - G°) of the electron transfer reaction:

PR

RP

the activation barrier decreases, which is in agreement with our intuition: very generally, if we

increase the driving force of a reaction, we expect it to become

faster

RP

We continue to increase the driving force (- G°) so as to reach the "activation-less" situation; in this case, - G° =

R

P

G≠ = 0

The "inverted" region:

when the reaction is more and more

favourable from a thermodynamic

viewpoint, the rate of electron transfer

decreases! This is, of course, countreintuitive.

To any "classical" chemist, the rate of any reaction is expected to

increase with the driving force.

R

PG≠ = 0

R

P

G≠ ≠ 0

R P

G≠ ≠ 0

RP

G≠ increases!

PCD1 A1 A2D2

h e- e-

e-e-

Charge Separationlong-range and multistep electron transfer

electron

hole

Artificial Photosynthesis

Rhodopseudomonas viridisPhotosynthetic Reaction Centre

Deisenhoffer et al.(D)

(A1)

(A2)(A3)

Mg(II)Fe(II)

O

O

SPBC

FeII

BP

O

O

h

CytoplasmicMembrane

+

-

3ps

200ps1s

1s

1.3eVinvested0.5eVstored

60%energywasted=20sd=120Å

Photosynthetic Reaction Centre

the "molecular triad" approach

PC A1 A2

D APC

h

h

classical work: 1) Porphyrins: Mataga, Gust-Moore-Moore, Wasielewsky2) Ruthenium complexes: Meyer

strict geometrical control is essential

RuIID1 A1 A2D2

h e- e-

e-e-

Charge Separation

Ruthenium(II) as the central photoactive species

electron

hole

Fast forward electron transfer reactionsSlow recombination reactions

In a bis-terpyridine complex, it is easy to identify an axis on which donor (D) and acceptor (A) groups can to be attached

The situation is very different for a tris-bipy complex : a ready-to-functionalize axis is difficult to identify

A

Ru(bipy)32+

D

Synthesis : Collin et al., 1989Photochemistry : Barigelletti, De Cola, Flamigni, Balzani, 1991-1994

Review article : Chambron, Coudret, Collin, Guillerez, Barigelletti, Balzani, De Cola, Flamigni, CHEM. REV., 1994

3 2

1

N

multicomponent systems combining transition metals and

porphyrins

Fabrice OdobelPhotochemistry: A. Harriman

IrM1 M2

an iridium(III)-bis-terpy complex as central species

3+

Isabelle Dixon & Jean-Paul Collin

Photochemistry: Lucia Flamigni

Zn

stepwise electron transfer at 293 K

NN

NN

N

N

N

N N

N N

N

N

N

4+30 ps 75 ps

75 ps

3.5 ns

h

viaPH2tripletstate

H

HAuIr

PZn-Ir-PAu triadefficient charge separation in toluene at 293 K

NN

NN

N

N

N

N N

N N

N

N

N

4+

450 ns

h

long-livedchargeseparatedstate

<20ps <20ps

Zn AuIrZn

Towards multicomponent systems containing an iridium(III) complex as photoactive centre

IrD A IrD+.

A-.

h

1

2 3

Etienne BaranoffJean-Paul Collin

D Ir A

Ir

the "short" triadD-Ir(III)-A

D Ir A

Ir

the "long" triadD-Ir(III)-A

D Ir A

Ir

h

1

2

3

Lucia Flamigni : photochemical study of the "short" triad

D-Ir(III)-A

D Ir A

Ir

+

4(~ 110 s)

Photochemistry: Lucia Flamigni (Bologna)

Mataga, Sakata and coworkersJ. Am. Chem. Soc., 105, 7771-7772 (1983)

CS ~ 120 psCS ~ 300 ps

measurements performed in benzene at r.t.

Wasielewski et al.J. Am. Chem. Soc., 107, 5562 (1985)

D-P-Q → D-P+-Q- → D+-P-Q-

D+-P-Q- → D-P-Q CS ~ 120 ps

Nature, Vol. 307, No.5952, pp. 630-632, 16 February 1984

Quinone-------Porphyrin-------Carotenoid

A DPC

h

(simulation)

Danielson, E.; Elliott, C. M.; Merkert, J. W.; Meyer, T. J. J. Am. Chem. Soc. 1987, 109, 2519.

Water splitting : one of the main difficulties is to reduce or oxidise water to dihydrogen and dioxygen

respectively.

These two reaction are multi-electron processes which makes them extremely difficult to carry out

using homogeneous catalysts

Generation of H2 or O2 from water : highly difficult reactions

heterogeneous catalysis : inspiration from electrochemistry (cathodic and anodic materials used in water electrolysis cells)

H2 formation : colloidal noble metals (mostly, palladium or platinum)

✤✤✤✤✤

O2 generation : colloidal, electrode or deposited (on TiO2, in zeolites, in clays, etc...) :

RuO2, IrO2, cobalt oxides, NiCoO4, cobalt phosphate, SrFeO3, MnO2, etc...

Homogeneous catalytic systems

Water oxidation to O2 is difficult to carry out, either in a stoichiometric reaction or in a

catalytic system; a relatively large number of molecular catalysts have been proposed but,

unfortunately, many of them do not seem to be easily reproduced.

To a lesser extent, this is also true for water reduction to H2, although several published

systems are undisputibly operative and sometimes relatively efficient

"Old" molecular catalysts for water reduction to H2

Co(bipy)32+ : Brown, Brunschwig, Creutz,

Matsubara, Sutin*, J. Am. Chem. Soc. 1979, 101, 1298–1300.

[Co(dmgH)2(OH2)2] : Hawecker, Lehn* and Ziessel, NouV. J. Chim. 1983, 7, 271.

Recently reported systems of the same family :

Artero and co-workers, Inorg. Chem., 2007 :

Eisenberg at al., JACS 2008 and Inorg. Chem. 2009 :

Rh and Ir complexes

Very old work with Rh(bipy)33+ : Lehn, Sauvage, New. J. Chem.,

1977 and Lehn, Sauvage, Kirch, Helv. Chim. Acta, 1979

RhI(bipy)2+ is an important intermediate

Recent work on cyclometalated iridium complexes (photoactive species) associated to Rh(bipy)3

3+ : Bernhard and co-workers, Inorg. Chem. 2008

Bernhard et al., Inorg. Chem., 2008Reductive quenching of the Iridium(III) complex by the TEA

donor

Similar studies on reductive quenching of Ru(bipy)32+ by

ascorbic acid have been reported long ago (Sutin at al.). More recently, reductive quenching of Re(I) complexes by TEOA also led to water photo-reduction to H2 (Alberto et al., 2009)

Bernhard et al.

Water oxidation to O2

An historical complex and an historical paper :

Catalytic Oxidation of Water by an Oxo-Bridged Ruthenium Dimer :S.W. Gersten, G.J. Samuels and T.J. Meyer, J.Am. Chem. Soc.,

104, 4029 (1982)

X-ray structure of Meyer's blue dimer

Complex in,in-[Ru2(mbpp)(trpy)2(H2O)2]3+

Llobet et al., Angew. Chem. Int. Ed. 2009, 48, 2842 – 285 and references

3,5-bis(2-pyridyl)pyrazole

Ce4+ as oxidant : high turnover number (~ 200)

Use of a bis-pyridyl pyridazine core with two peripheral naphtyridine as bridging ligand : Thummel et al.

Åkermark, Sun and coworkers, Inorg. Chem. 2009

X-ray structure

Cyclometalated Iridium(III) Aquo Complexes: Efficient andTunable Catalysts for the Homogeneous Oxidation of Water

Neal D. McDaniel, Frederick J. Coughlin, Leonard L. Tinker, and Stefan Bernhard*

JACS, 2008

Bernhard, JACS 2008 and Crabtree & Eisenstein, JACS 2009

Homogeneous Light-Driven Water Oxidation Catalyzed by a Tetraruthenium Complex with All Inorganic Ligands

Tianquan Lian, Craig L. Hill and coworkers

tetraruthenium polyoxometalate complex: [{Ru4O4(OH)2(H2O)4}-(γ-SiW10O36)2]10-

JACS, 2008

Cs10[Ru4(μ-O)4(μ-OH)2(H2O)4(γ-SiW10O36)2] is obtained by reacting the divacant POM, [γ-SiW10O36]8- (γ-SiW10) with μ-oxo-

bispentachlororuthenate(IV), Ru2OCl104-, in aqueous solution

The unprecedented entrapment of the Ru4O6 fragment occurs readily by the complementary assembly of two γ-SiW10 units under

mild temperature conditions.

Highlight by Süss-Fink, AC 2009

recent work on cobalt complexes

(old work : Shafirovich, Khannanov and Strelets, New. J. Chem, 4, 81, 1980)

Kanan and Nocera : "In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+"

Science, 2008

Jiao, Frei : " Nanostructured Cobalt Oxide Clusters in Mesoporous Silica as Efficient Oxygen-Evolving Catalysts"

(Co3O4 nanoclusters)ACIE, 2009