fluorene base material
TRANSCRIPT
HIGHLIGHT
Fluorene-Based Materials and Their Supramolecular Properties
ROBERT ABBEL, ALBERTUS P. H. J. SCHENNING, E. W. MEIJERLaboratory of Macromolecular and Organic Chemistry, Eindhoven University ofTechnology, Den Dolech 2, 5612 AZ Eindhoven, The Netherlands
Received 18 March 2009; Accepted 4 May 2009DOI: 10.1002/pola.23499Published online in Wiley InterScience (www.interscience.wiley.com).
Robert Abbel
Robert Abbel studied chemistry at the Universities of Mainz, Germany
and Toronto, Canada. In 2004, he received the degree of Diplom-Chem-
iker after having finished a research project on rod-coil block copolymers
under the guidance of Dr. Andreas Kilbinger and Prof. Holger Frey.
Afterwards, he joined the group of Prof. E. W. Meijer at the Eindhoven
University of Technology, the Netherlands, as a PhD student, working on
fluorene-based polymers and oligomers and their supramolecular chemis-
try. He received his PhD degree in 2008 and is currently working at TNO
Industries.
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 4215–4233 (2009)VVC 2009 Wiley Periodicals, Inc.
Correspondence to: A. P. H. J. Schenning (E-mail: [email protected]) or E. W. Meijer (E-mail: [email protected])
ABSTRACT: Fluorene-based p-conjugated polymers and oligo-
mers combine several advanta-
geous properties that make them
well-suited candidates for appli-
cations in organic optoelectronic
devices and chemical sensors.
This review highlights strategies
to synthesize these materials and
to tune their absorption and
emission colors. Furthermore,
methods to control their supra-
molecular organization will be
discussed. In many cases, a deli-
cate interplay between the chem-
ical structure and the processing
conditions are found, resulting in
a high sensitivity of both struc-
tural features and optical proper-
ties. VC 2009 Wiley Periodicals, Inc. J
Polym Sci Part A: Polym Chem 47:
4215–4233, 2009
Keywords: p-conjugated oligo-
mers; p-conjugated polymers; fluo-
rene copolymers; morphology;
oligofluorenes; phase behavior;
polyaromatics, polyfluorenes; self-
assembly; self-organization; supra-
molecular structures; thin films
4215
Albertus P. H. J. Schenning
Albertus P. H. J. Schenning is associate professor at the Eindhoven Uni-
versity of Technology, the Netherlands. He received his PhD degree at
the Radboud University of Nijmegen in 1996 on supramolecular architec-
tures based on porphyrin and receptor molecules with Dr. M. C. Feiters
and Prof. R. J. M. Nolte. Between June and December 1996, he was a
postdoctoral fellow in the group of Prof. E. W. Meijer at the Eindhoven
University of Technology working on dendrimers. In 1997, he joined the
group of Prof. F. Diederich at the ETH in Zurich, where he investigated
p-conjugated triacetylenes. His current research interests are self-
assembled p-conjugated systems.
Prof. E. W. ‘‘Bert’’ Meijer
Prof. E. W. ‘‘Bert’’ Meijer is Distinguished University Professor in the
Molecular Sciences and Professor of Organic Chemistry at the Eindhoven
University of Technology, the Netherlands. After a PhD in 1982 from the
University of Groningen (Organic Chemistry with Prof. Hans Wynberg)
and a 10-year career in industry (Philips and DSM), he became head of
the Laboratory of Macromolecular and Organic Chemistry at the Eind-
hoven University of Technology. His research is focused on supramolec-
ular chemistry, functional organic materials, chemical biology, and
stereochemistry.
INTRODUCTION
p-Conjugated polymers and oligomers based on fluorene
building blocks have gained importance as the active
materials in various types of organic optoelectronic de-
vices, most notably organic light-emitting diodes1,2 and
organic photovoltaic cells.3 Recently, their use as sensing
and imaging agents has emerged as a growing second
field of application.4 The optical and electronic proper-
ties of fluorene-based materials highly depend on both
the chemical structures and the supramolecular organiza-
tion.5,6 Here, an overview is presented highlighting sci-
entific literature on fluorene-based optoelectronic materi-
als such as oligomers, homo-, and copolymers. The most
common synthetic routes will be described together with
both their optical properties and phase behavior. Further-
more, the fields of application are introduced and some
recent key developments concerning research in these
areas are depicted. Finally, strategies are highlighted that
have been successfully employed to gain understanding
of and control over the supramolecular organization of
fluorene-based materials.
SYNTHETIC PROCEDURES
The fluorene molecule (C13H10) is an isocyclic aromatic
hydrocarbon composed of two benzene rings that are
connected via a direct carbon–carbon bond and an adja-
cent methylene bridge (Scheme 1). The methylene bridge
forces the two phenyl rings to be planar,7 which
increases their orbital overlap and the degree of conjuga-
tion of the aromatic system. In bare fluorene, the protons
at the sp3 carbon in the methylene bridge (9-position) ex-
hibit a significant CH acidity (pKA ¼ 22.98) as the result-
ing aromatic fluorenyl anion is efficiently resonance sta-
bilized.9,10 Oxidation to 9-fluorenone is another fre-
quently observed reaction,11–13 which is also favored by
resonance stabilization because the p-conjugated system
is extended. Such chemical reactions can be suppressed
by double alkylation11,14 or arylation11,15 of fluorene,
which has the additional advantage of introducing side
groups that enhance the solubility in organic solvents.2
Whereas alkylation is easily achieved by reaction of the
fluorenyl anion with bromoalkanes, arylation requires
more elaborate synthetic procedures and is therefore less
frequently used.11
Fluorene monomers can be directly connected to each
other via aromatic coupling at the 2 and 7 positions,
yielding a series of oligofluorenes (OFs) with increasing
conjugation lengths.16 Because of the torsional freedom
of the biphenyl bond, the planes of the fluorene units are
usually tilted with respect to each other.5 This can be
overcome by full planarization using the so-called
Scheme 1. Chemical structure and atomic numbering of
fluorene.
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ladder-type materials,11,17 but these are not the subject of
this highlight article. The first systematic study involving
a considerable number of well-defined fluorene oligo-
mers (up to ten repeat units) used the Ni(0)-mediated
Yamamoto coupling18 of 2,7-dibromo-9,9-dihexylfluo-
rene in the presence of 2-bromofluorene as an end capper
(Scheme 2).19 Monodisperse molecules were obtained
from the oligomer mixture by high-performance liquid
chromatography. Later, several research groups20–26 have
prepared OFs by stepwise approaches that also allowed
variation of the alkyl substitution pattern along the
oligomer backbone in a defined manner (Scheme 2).23,25
The central carbon–carbon coupling step of all these
routes is the Pd-catalyzed Suzuki reaction of aromatic
halogen compounds with boronic acids or esters.27 To
allow the synthetic strategy toward longer oligomers to
proceed selectively, protective groups such as trimethyl-
silyl units are required.22,23,26 Alternatively, the higher
reactivity of aromatic iodine compounds20 or diazonium
salts21 in Suzuki couplings compared to bromine deriva-
tives has been exploited.
Connection of fluorene monomers with two reactive
sites in a one-step synthesis produces polydisperse
oligomer mixtures or, when no endcappers are present,
polyfluorenes (PFs).2,5,7,11,28,29 Although insoluble poly-
(dimethylfluorene) has been obtained by electropolymeri-
zation in 1987,30 the first example of a synthetic
approach toward soluble PFs was the oxidative coupling
of dihexylfluorene with FeCl3 published in 1989.31 The
material obtained, however, had a low-molecular weight
(Mn up to 500032) and additionally contained structural
defects, as the oxidative coupling does not proceed
strictly regioselectively.5,11 An enormous synthetic
improvement was the introduction of metal-catalyzed
aryl–aryl coupling reactions that require monomers func-
tionalized in the 2 and 7 positions, since they guarantee
perfect regioselectivity.5,11 As in the case of OF synthe-
sis, the most prominent types of reactions used to prepare
PFs are the Ni(0)-mediated Yamamoto and the Pd-
catalyzed Suzuki condensations7,11,33–35 (Scheme 2).
With the appropriate reaction conditions applied, high-
molecular-weight PF (Mn [ 100,000 g mol�1) can be
obtained with both strategies.28,29,36
PFs have been prepared with a wide variety of alkyl
chains attached to the 9-position, both linear (methyl up
to n-hexadecyl),30,37,38 branched (most notably 2-methyl-
butyl, 2-ethylhexyl and 3,7-dimethyloctyl),39–42 and
spirocyclic.43 Poly(dimethylfluorene) and poly(diethyl-
fluorene) are poorly soluble, but the solubility of PFs
increases with the length of the alkyl chains and the
degree of branching. When methylbutyl, hexyl, or longer
substituents are attached to the 9-position, the resulting
PFs are highly soluble in common apolar organic sol-
vents.17,44 Arylated OFs and PFs most commonly bear
(substituted) phenyl groups at the 9-position,15,45 but
other groups such as spirolinked fluorenes46,47 have also
been used.
Metal-catalyzed aryl–aryl cross couplings are also
very useful for the synthesis of fluorene copolymers.
These polymers are of particular interest due to the possi-
bility to tune the electronic band gap and thus the emis-
sion colors of the materials by an appropriate choice of
the comonomers (vide infra). Although statistical copoly-
mers can be prepared via the Yamamoto route,48 the
Suzuki polycondensation is the standard procedure used,
because it allows the synthesis of strictly alternating fluo-
rene copolymers (APFs49; Scheme 2), which have espe-
cially advantageous materials properties.3,28,44 As with
PF homopolymers, the most commonly used solubilizing
side chain in APFs is n-octyl, followed by n-hexyl,50–52
but also n-decyl,53 ethylhexyl,54 or trimethyldodecyl55
are occasionally used.
Scheme 2. Illustration of the two most commonly used synthetic strategies toward fluorene
polymers: PF homopolymer synthesis by (a) Yamamoto and (b) Suzuki polycondensation.
(c) APF synthesis by Suzuki polycondensation. R ¼ alkyl, R0 ¼ H or alkyl.
HIGHLIGHT 4217
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OPTICAL PROPERTIES
OFs, starting from the dimer, absorb in the UV region and
emit blue light when excited at their absorption maximum.
A narrowing of the electronic band gaps56,57 and thus red
shifts in both absorption and fluorescence are observed
with increasing conjugation length19–23 (Fig. 1). Con-
stancy of the spectral properties is reached at about 12
repeat units for the absorption, but already at six repeat
units for the emission,19 indicating distinct differences in
the geometries of the ground and the excited states.7,11
Similar to OFs, PFs absorb UV (p-p* transition at
about 380 nm2) and emit blue light, with two photolumi-
nescence maxima around 420–425 and 445 nm.7,11 The
reported fluorescence quantum yields exceed 50% both
in solution and in the solid state.58,59 Because the 9-
position is not conjugated to the fluorene p-system and is
far away from the aryl coupled 2- and 7-positions, the
influence of the alkyl side chain architecture on the opti-
cal properties of PFs is negligible in good solvents under
dilute conditions.2,16 Strong influences have, however,
been found on the aggregation behavior in poor
solvents37,60–62 and in the solid state,62–64 which are
reflected in distinct differences in the optical proper-
ties.65,66 For example, certain thermal treatments65,67,68
or solvent annealing69,70 have been found to induce
poly(dioctylfluorene) and poly(dihexylfluorene) to adopt
a conformation in which the fluorene repeat units are pla-
narized with respect to each other (the so-called b-phase;Fig. 2), giving rise to significant red shifts in the optical
Figure 1. Chain-length dependence of absorption and fluorescence maxima and energies in
OFs in chloroform solution (data taken from refs. 22 and 24).
Figure 2. Absorption (dotted line) and fluorescence (solid line) spectra of amorphous (top) and
b-phase (bottom) of a low-molecular-weight poly(dialkyl)fluorene film with a schematic repre-
sentation of the fluorene backbone in a twisted and planarized conformation. (Reproduced with
permission from ref. 69. Copyright 2008 Wiley – VCH Verlag GmbH & Co. KGaA).
4218 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 47 (2009)
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spectra.71–73 No such phase has been observed when the
side chains were branched.5,60,74
In contrast to fluorene homopolymers, which are re-
stricted to a rather high band gap,1,11 copolymerization
with appropriate aromatic moieties allows easy adjust-
ment of the frontier orbital energies. If the comonomers
have a sufficiently high electron acceptor character, i.e.,
a low-lying LUMO, orbital mixing between the donor
(fluorene) and the acceptor parts occurs, by which the
effective band gap is decreased.3 Alternatively, this phe-
nomenon can also be explained in terms of the valence
bond theory, where copolymerization with acceptor
materials increases the contribution of quinoid resonance
structures, thereby decreasing the degree of bond length
alternation, which also reduces the band gap energy by
suppression of the Peierls effect.3 This effect is most pro-
nounced when the acceptor and donor moieties regularly
alternate in the polymer backbone, and therefore the fol-
lowing discussion will be restricted to APFs. These have
been prepared with an impressive range of aromatic
comonomers,3,29,44,50–55,75–81 and their emission colors
span the entire visible range, extending even into the
near IR region81,82 (Scheme 3).
Band gap tuning by incorporation of electron-accept-
ing comonomers has also been applied in oligofluorene
derivatives, although not all of them can strictly be called
alternating.83–86 Alternatively, bis(difluorenyl)amino sub-
stituted aromatics with low band gap energies (e.g., py-
rene and perylene) have been reported that allowed varia-
tion of the emission color by energy transfer to the aro-
matic acceptor moieties.87
MORPHOLOGY
At elevated temperatures, PFs generally develop nematic
mesophases,2,5 which can become chiral (cholesteric) in
the case of enantiomerically pure branched side chains.88
Another important structural parameter that governs the
solid state phase behavior is the average molecular
weight.89,90 For example, for poly(di(2-ethylhexyl)fluo-
rene), a transition from a nematic to a hexagonal order-
ing has been found at a threshold molecular weight of
�10 kg/mol90 (Fig. 3). As usually observed for liquid-
crystalline polymers, the transition temperatures increase
with chain length.5
In the case of PF homopolymers with enantiomeri-
cally pure chiral side chains, circular dichroic (CD) spec-
troscopy can be used as a sensitive tool to study phase
behavior as they exhibit extraordinarily high CD effects
in thermally annealed thin films40,91–93 [Fig. 4(a,b)]. In
case of (S)-3,7-dimethyloctyl chains, the degree of circu-
lar polarization in absorption (gabs), defined as gabs ¼2�(AL � AR)/(AL þ AR), increases with film thickness,
corresponding to a nonlinear rise of CD [Fig. 4(c)].93
This demonstrates that the optical activity of chiral PFs
is not only an intrinsic property of the material but is
also related to a mesoscopic phenomenon. Circular
Scheme 3. Position of the fluorescence colors of several APFs in the visible spectrum (top;
emission from solution). Values are taken from refs. 50–55 and 75–81. Chemical structures of
three frequently used APFs (bottom).
HIGHLIGHT 4219
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differential scattering88 or selective reflection due to a
cholesteric mesophase23,25 or a helical arrangement of
the polymer chains in the film41,91 have been proposed as
the cause.
The solid-state phase behavior of OFs has been dem-
onstrated to depend strongly on chain length, but also on
the architecture of the alkyl substituents at the 9-posi-
tions.16 Short linear side chains, such as propyl, generally
give rise to crystalline materials, whereas longer residues
such as pentyl chains lead to an amorphous glassy
state.26 By contrast, partial or full replacement of linear
with branched side chains such as methylbutyl,23,25,26,94
ethylhexyl,21,26,95 or dimethyloctyl23,25,26 generates liq-
uid crystalline phases whose structures are preserved
upon cooling by vitrification into a nematic21,24,95,96 or
cholesteric23 glassy state. Furthermore, it is of impor-
tance whether racemic26 or enantiomerically pure23
branched alkyl chains are used. Within one series of OFs
with identical side chain architecture, the transition tem-
peratures increase with the number of fluorene repeat
units.21,23,95 Comparable observations have also been
made for some fluorene ‘‘co-oligomers.’’84,85
Similar to PFs, several APFs are also liquid crystal-
line,51,79,96–99 but not much detailed information about
the nature of the mesophases is available. Only few sys-
tematic studies have been published on the influence of
the side chain architecture on the aggregation behavior.
In one example, the spectral properties of thin films of
poly(fluorene-alt-dithienylbenzothiadiazole) (PFDTBT;
Scheme 3) have been found to vary moderately with the
length of the alkyl substituents,80,100 which might point
to different degrees of aggregation. The influence of sub-
stituents attached to the comonomers has also been
investigated and found to have a significant effect on the
transition temperatures to the liquid crystalline state51
and the degree of aggregation in thin films.101 The molec-
ular weight has been demonstrated to exert a decisive
impact on the melting temperature, the alignment speed,
the degree of alignment and the chain packing in
the liquid crystalline poly(fluorene-alt-benzothiadiazole)
Figure 4. (a) Chemical structure of poly(di-(S)-3,7-dimethyloctyl)fluorene). (b) CD spectra of a
film of this chiral PF after annealing at various temperatures. (Reproduced with permission from
ref. 40. Copyright 2000 Elsevier). (c) Film thickness dependence of the degree of circular polar-
ization in absorption of an annealed chiral PF film. (Reproduced with permission from ref. 93.
Copyright 2003 Wiley – VCH Verlag GmbH & Co. KGaA).
Figure 3. Chemical structure and phase diagram of poly(di(2-ethylhexyl)fluorene) with sche-
matic representation of the supramolecular chain packing. (Reproduced with permission from
ref. 89. Copyright 2005 American Physical Society).
4220 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 47 (2009)
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(PFBT; Scheme 3).97,102 Besides chiral PF homopolymer,
two chiral APFs have also been shown to develop intense
CD effects after annealing of thin spin-coated films99
[Fig. 5(a,b)]. The CD intensities were strongly depended
on the values of various parameters, such as the applied
temperatures and annealing times and the film thick-
nesses. Interestingly, also a dependence on the molecular
weight was obtained, with an optimum at intermediate
chain lengths [Fig. 5(c)].
OPTOELECTRONIC APPLICATIONS
PFs combine a number of advantageous properties mak-
ing them attractive materials for use in polymer
electronic devices.1 High-molecular-weight samples are
easily accessible via metal-mediated polymerization
reactions. Their good solubility in organic solvents
allows simple processing techniques such as spin coating
and ink jet printing, whereas small molecules have to be
deposited by technologically more demanding techniques
such as high vacuum deposition.103 Furthermore, thin
films are flexible and resistant to decomposition until
temperatures above 400 �C.2 OFs lack some of these
advantages because of their low-molecular weights, but
on the other hand can be prepared in a monodisperse
fashion and purified by high-performance purification
techniques.16,104 Because of the rather large band gap of
both classes of materials (3.0–3.2 eV for PF1,28,105), their
main field of application are blue polymer light-emitting
diodes (PLEDs).1,2,16,28,59 The first example of a blue
PLED based on PF was published in 1991, contained
poly(dihexylfluorene) and had a rather poor perform-
ance.106 Since then, however, impressive improvements
have been achieved2 and PF-based optoelectronic de-
vices are now believed to have the potential of intermedi-
ate-term commercialization.103 The color of the electro-
luminescent light depends on the energy difference
between the excited and the ground state28 and thus blue
emission occurs when PF is used as the active layer. The
nematic liquid crystallinity of many PFs has been used to
align them on rubbed substrates, resulting in polarized
blue photo-107,108 and electroluminescence2,36 with di-
chroic ratios up to 21 for poly(di(2-ethylhexyl)fluorene)
(Fig. 6). The formation of monodomain nematic glasses
by a set of OFs containing five to ten repeat units has
also been used with comparable results.26,94 Application
of chiral PFs in PLEDs led to circularly polarized elec-
troluminescence with degrees of circular polarization
(gCPEL)109 up to 0.25,40,41 thereby exceeding those
reported for other chiral p-conjugated polymers.110
The alternating fluorene copolymers (APFs;49 vide su-pra) share with PF homopolymers many of their advanta-
geous properties, such as good solubility and thus con-
venient processability, mechanical flexibility, and ther-
mal stability. Because of their more complex chemical
structures, some are synthetically less easily accessible,
especially if the comonomers require elaborate multistep
preparation. A crucial advantage of APFs compared to
Figure 5. (a) Chemical structure of a chiral PFDTBT. (b) CD spectra of annealed films of this
polymer with various molecular weights. (c) Molecular weight dependence of the circular polar-
ization in absorption of annealed films of this chiral PFDTBT. (Reproduced with permission
from ref. 99. Copyright 2008 American Chemical Society).
Figure 6. Polarized electroluminescence from aligned
poly(di(2-ethylhexyl)fluorene). (Reproduced with permission
from ref. 2. Copyright 2001 Wiley – VCH Verlag GmbH &
Co. KGaA).
HIGHLIGHT 4221
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PFs is the almost unrestricted tuneability of their band
gap energies by the appropriate choice of the comonomer
(Scheme 3), making them attractive for a manifold of
applications.
The use of APFs has allowed to extend the range of
electroluminescence colors far beyond blue, eventually
encompassing the entire visible spectrum. With purely
isocyclic or nitrogen-containing comonomers, the emis-
sion is usually restricted to blue and cyan,50,51,76 but a
higher stability of the spectra toward fluorenone emission
(vide infra) has been reported using dialkylbenzenes or
carbazole as comonomers.111,112 APFs with triaryl
amines as copolymers, such as poly(fluorene-alt-bis-(alkylphenyl)-bisphenyl-phenylenediamine) (PFB; Sch-
eme 3), combine deep blue emission44 with good hole
mobilities.75 Increasing the electron acceptor character
of the comonomers resulted in green,50,79,113,114 yel-
low,50,115,116 orange,116,117 or red118,119 PLEDs. White
electroluminescence has been achieved by partial energy
transfer in blends of blue emitting matrices and lower
band gap APFs.120,121 Similar results have been obtained
with fluorene co-oligomers, which additionally could be
aligned due to their liquid crystallinity, allowing the gen-
eration of polarized white light86 (Fig. 7).
Although some examples are known of APFs that are
applied in polymer field effect transistors,81,82 their sec-
ond main field of application is organic photovoltaics,3
either in combination with other polymers or with [6,6]-
phenyl-C61-butyric acid methyl ester ([60]PCBM).
Because of the broad shape of the solar emission that
reaches far into the IR region of the electromagnetic
spectrum, efficient solar cells have to collect radiation
over a wide range of wavelengths. PFBT blended with
PFB has been used in organic solar cells,122–124 but due
to the rather large band gaps of PFBT (2.3 eV50) and
PFB (2.8 eV125), these devices only collected light of
wavelengths shorter than 550 nm. More promising candi-
dates for efficient organic solar cells are so-called low-
band-gap polymers that are able to absorb far into the
red and even near infrared part of the visible spectrum.
To achieve this goal, fluorene-based polymers were
designed that at the same time contain strong electron
acceptor and donor units. An impressive number of
APFs has been prepared, using mainly thiophene as the
donor moiety and various acceptors, most notably benzo-
thiadiazole, thienopyrazine, and thiadiazoloquinoxaline.
In these polymers, which are generally applied as inti-
mately mixed blends with [60] PCBM in so-called bulk
heterojunction devices,126–128 band gap energies as low
as 1.3 eV80 made light collection possible down to 800
nm.129,130 An especially interesting material is PFDTBT,
Figure 7. Chemical structures of blue (top) and yellow (bottom) emissive fluorene based
oligomers and polarized white electroluminescence form their aligned nematic blends. The arrow
indicates spectral changes upon increasing the relative amount of yellow emissive oligomer.
(Reproduced with permission from ref. 86. Copyright 2004 Wiley – VCH Verlag GmbH & Co.
KGaA).
Figure 8. External (EQE) and calculated internal (IQE)
quantum efficiencies of a bulk heterojunction solar cell con-
taining PFDTBT and PCBM. Inset: current-voltage measure-
ment in the dark (squares) and under simulated solar illumi-
nation (solid line). (Reproduced with permission from ref.
131. Copyright 2007 American Institute of Physics).
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which is not an extremely low band gap polymer (1.8
eV80), but still absorbs until 700 nm and gives excellent
power conversion efficiencies of above 4% (Fig. 8).131
CHEMICAL DEGRADATIONPROCESSES IN PF
A major obstacle toward successful introduction of PF-
based light-emitting devices into the markets is their lim-
ited stability during processing and device operation.132
Consequently, an improved understanding of the degra-
dation of active components in existing devices is critical
for the long-term success of the emerging field of plastic
electronics. Poor device lifetimes are especially problem-
atic for blue PLEDs103 that require high operation
voltages and are therefore especially susceptible to the
formation of chemical defects. Additionally, transfer of
excitation energy is especially likely to occur in the
emissive materials in blue PLEDs due to their high band
gap energies. For example, this is a well-known problem
in PF-based PLEDs, whose desired blue electrolumines-
cence frequently changes into an unwanted green emis-
sion band at 500–550 nm132 [Fig. 9(a)]. Originally, this
Figure 9. (a) Increasing green electroluminescence from a PF containing PLED after thermal
treatment (130 �C) in air. (Reproduced with permission from ref. 136. Copyright 2007 Wiley –
VCH Verlag GmbH & Co. KGaA). (b) Simplified depiction of two competing suggestions for
the mechanism of fluorenone formation from monosubstituted fluorene repeat units. (c) Proposed
mechanism for the formation of fluorene defects in a fully alkylated model oligofluorene.
(Reproduced with permission from ref. 104. Copyright 2009 Wiley – VCH Verlag GmbH & Co.
KGaA).
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has been explained as a result of excimer forma-
tion,133,134 but more recent research has identified energy
transfer from fluorene to 9-fluorenone defects as the
main reason.5,135,136 Despite the consensus concerning
the chemical nature of the defect, the mechanism of its
formation is still uncertain.132 Incomplete dialkylation of
fluorene units in the pristine PFs has been suggested as
the main source of oxidation to fluorenone, either by
deprotonation and subsequent reaction of fluorenyl
anions with oxygen5,135 or via radical reactions [Fig.
9(b)].47,136,137 Several strategies have been employed to
circumvent these color instabilities, e.g., the preparation
of defect-free PFs, either by careful monomer purifica-
tion,138 or by applying a special synthetic route to the
monomers, that ensure complete double alkylation.139
Although these materials display higher stabilities, com-
plete stability in the solid state has turned out to be diffi-
cult to achieve. This is further supported by the recent
observation that even defect-free OFs obtained by high-
performance purification techniques are prone to photo-
oxidative degradation via the formation of carboxylic
intermediates in the side chains [Fig. 9(c)].104 Spirosub-
stituted PFs,136 replacement of the saturated side chains
by aromatic substituents47,140 or using the 9-silicon ana-
logues of fluorene141 seem to be more promising
approaches to avoid oxidative degradation.
SUPRAMOLECULAR APPROACHES TOWARDSOLID-STATE ORGANIZATION
‘‘Classical’’ covalent chemistry offers excellent structural
control up to a length scale of several nanometres, but
for the performance optimalization of optoelectronic
devices, the supramolecular order in the nanometer up to
the micrometer regime is also of paramount impor-
tance6,62,142 (Fig. 10). Morphological control at these
dimensions is difficult to achieve and is largely domi-
nated by noncovalent interactions. As has been shown
earlier, OFs, PFs, and APFs by themselves exhibit al-
ready a richly varied phase behavior, especially as their
liquid crystallinity is concerned. Advanced manipulation
of solid state and solution structures is possible when
special sample preparation procedures are applied or
when a fluorene-based conjugated segment is combined
with additional chemical moieties that further influence
its supramolecular organization.
Because a high contact area of the components in an
electroactive blend promotes desirable electronic pro-
cesses, such as exciton dissociation, several strategies
have been developed to create nanoparticles from fluo-
rene-based materials. An early approach was the produc-
tion of core-shell particles through layer-by-layer deposi-
tion of anionic PSS and a cationic precursor polymer on
colloidal substrates.143 As the precursor contained non-
conjugated fluorene units, oxidative coupling stabilized
these structures by the formation of oligomeric cross-
links. Afterwards, removal of the core templates by
chemical decomposition left back blue fluorescent hol-
low capsules of about 2 lm diameter. Alternatively,
microemulsions consisting of water, a surfactant, and a
PF solution in chloroform have been prepared by ultraso-
nication, and after evaporation of the organic solvent, an
artificial latex remained with an average particle size of
about 100 nm.144,145 In a more recent approach, it has
been shown that even smaller nanoparticles of PFs or
APFs (diameter 5–50 nm) can be produced without the
addition of surfactants, when a water-miscible solvent is
used146,147 [Fig. 11(a)]. Their size could be controlled by
the concentration of the injected stock solution, and in
PF particles, the internal chain organization could be
changed from a glassy state to the b-phase by improving
Figure 10. Schematic representation of the interplay between chemical structure, solid state
morphology and macroscopic properties of p-conjugated materials and the resulting importance
of control over the supramolecular ordering processes.
4224 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 47 (2009)
Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola
the solvent quality.148 In the case of mixed nanoparticles,
their emission properties were tuned by energy transfer
processes from PF to APFs149 [Fig. 11(b,c)].
Several strategies have been developed to control the
ordering of fluorene-based materials by chemical modifi-
cations instead of only relying on their inherent supra-
molecular properties, such as liquid crystallinity. The
most frequently applied approach is the synthesis of
block copolymers with PF or OF as one of the blocks,
because microphase separation of chemically incompati-
ble blocks is known to trigger the formation of several
different solid state morphologies.6,150,151 Because of the
limited motional freedom of the PF backbone, it is
rather rigid (persistence length 8–10 nm152–154) and
serves as a rod block. It has been combined with a wide
variety of flexible polymers such as poly(meth)acry-
lates,155–158 polystyrene,157 poly(ethylene glycol)
(PEG),159,160 and polyglutamate161 to prepare rod-coil
di- and triblock copolymers. Distinct differences in the
solid state order have been found when PF-PEG diblock
Figure 11. (a) AFM height image of poly(dioctylfluorene) nanoparticles. The scale bar corre-
sponds to 100 nm. (Reproduced with permission from ref. 146. Copyright 2006 American Chem-
ical Society). (b) Aqueous dispersions of nanoparticles from PF and a mixture of PF and PFBT
under UV illumination (375 nm). (c) Absorption (dashed line) and fluorescence excitation and
emission spectra (solid lines) of pure PF (top) and mixed (bottom) nanoparticles in water. Exci-
tation wavelength 375 nm. (Reproduced with permission from ref. 149. Copyright 2006 Ameri-
can Chemical Society).
Figure 12. (a) Solid state organization of PF-PEG diblock copolymers with different volume
fractions of the PEG segment (fPEG; left 0.1, right 0.3) and corresponding chemical structures
(Reproduced with permission from ref. 159. Copyright 2004 Wiley – VCH Verlag GmbH & Co.
KGaA). (b) Solid state organization of a PA-PF-PA block copolymer spin coated from THF and
toluene and corresponding chemical structure. (Reproduced with permission from ref. 168. Copy-
right 2007 Wiley – VCH Verlag GmbH & Co. KGaA).
HIGHLIGHT 4225
Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola
and PEG-PF-PEG triblock copolymers with varying vol-
ume fractions of coil to rod were studied by atomic
force microscopy (AFM; Fig. 12a).159 A low percentage
of PEG gave rise to well-defined organization into nano-
ribbons due to p-p stacking of the PF blocks and inter-
actions of the PEG with the substrate. At higher coil-to-
rod ratios, p-p stacking was prevented and untextured
aggregates were found. The combination of PFs with
thermoresponsive N-isopropylacrylamide blocks in coil-
rod-coil triblock copolymers has allowed the preparation
of thermochromic assemblies in aqueous solution.162 A
well-defined oligomer composed of two 3,4-ethylene-
dioxythiophene units linked by dihexylfluorene has been
equipped with PEG side chains of different lengths,
resulting in the formation of various types of self-
assembled micelles in water.163 PFs have also been com-
bined with other conjugated polymers, such as polyani-
line164–166 (PA) and polythiophene,167 to create rod-rod
block copolymers. PA-PF-PA triblock copolymers have
been found to microphase separate into different kinds
of structures depending on the solvent from which they
were deposited (Fig. 12b).168 In solution, PF-PT diblock
copolymers could be self-assembled by adjusting the
solvent composition, because the two blocks exhibited
strong differences in polarity. The aggregation processes
could be easily followed by fluorescence spectroscopy
due to energy transfer from the PF to the PT block in
the aggregated state.169
Supramolecular approaches different from PF or OF
block copolymers include the use of noncovalent interac-
tions such as hydrogen bonding,22,170,171 metal-ligand
coordination,172,173 or hydrophobic interactions.83,174 For
example, in OFs replacement of alkyl by polar 9-
substituents such as oligo(ethylene glycol) led to collapse
of the apolar parts in water and the formation of nanopar-
ticles.174 A similar result has also been achieved with
bolaamphiphilic fluorene oligomers, in which addition-
ally the emission colors of the individual particles could
be tuned by adjustment of their chemical structures and
composition.83 Supramolecular polymers with high vir-
tual degrees of polymerization and high solution viscos-
ities have been obtained by disubstitution of OFs with
endfunctionalities that are able to dimerize via strong
and directional quadruple hydrogen bonds.171 These
compounds combined the advantages of well-defined
small molecules (e.g., high-performance purification)
with those of covalent polymers (e.g., processing from
solution) and could be applied in electroluminescent
Figure 13. (a) Chemical structure of a hydrogen bonding oligofluorene. (b) Schematic repre-
sentation of the principle of white emission by partial energy transfer within a hydrogen bonded
supramolecular polymer containing energy donor (blue) and energy acceptor (green, red) moi-
eties. (c) White emission from a thin film of a fluorene-based hydrogen bonded polymer. (Repro-
duced with permission from ref. 171. Copyright 2009 American Chemical Society).
4226 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 47 (2009)
Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola
devices. Mixing in other hydrogen bonded conjugated
oligomers allowed tuning of the emission color covering
the entire visible range, including white without any sign
of phase separation (Fig. 13).
The solid-state organization of APFs have been stud-
ied especially intensely in blends with other electroactive
materials due to the importance of these mixtures in or-
ganic solar cells, where the correct thin film morphology
is indispensable for good charge separation, transport,
and extraction.6,175–177 In the scope of device optimaliza-
tion, the influence of different processing conditions has
been investigated. In bulk heterojunction solar cells con-
taining blends of PFB and PFBT, a more intimate mixing
of the components in the active layer has been reached
by an increased evaporation rate during the deposition
process, accomplished either by heating, solvent choice,
or variation of the deposition technique122–124 (Fig. 14a).
In all cases, phase separation on a smaller scale resulted
in an increased device performance. Similar results have
also been obtained in blends of PFDTBT with
[60]PCBM, when the degree of mixing was adjusted by
the solvent composition126 (Fig. 14b).
SENSING AND IMAGING APPLICATIONS
Except for their manifold use in organic and polymer
electronics, fluorene-based p-conjugated materials have
recently found widespread applications as sensors.4 Also
here, the sensing process is dominated by noncovalent
interactions between the analyte and sensor molecules. A
high sensitivity toward subtle changes in the environ-
ments is made possible due to signal enhancement
because of the conjugation via the polymer or oligomer
backbones. Furthermore, fluorescence spectroscopy often
allows an easy and sensitive detection. When metal bind-
ing ligands such as bipyridine derivatives are used as
comonomers, APFs are obtained that display different
sensitivities toward transition metal ions, depending on
the complexation strength178,179 (Fig. 15a). Similar
observations have been made for PFs containing ligands
attached to the side chains, such as imidazole, showing a
high selectivity for Cu(II)180 or phosphonates, which
selectively bind to Fe(III).181 A similar approach has
been published with a (statistical) copolymer of fluorene
and dibenzoborole, which showed fluorescence
Figure 14. (a) AFM height images of a 1:1 blend of PFB and PFBT spin coated from xylene
(left) or chloroform (center) solution and corresponding EQE spectra (left). (Reproduced with
permission from ref. 124. Copyright 2001 American Chemical Society). (b) AFM height images
of a 3:1 blend of [60]PCBM and PFDTBT spin coated from chloroform containing 1.2% xylene
(left) or 1.2% chlorobenzene (center) and corresponding EQE spectra (left). The relevant spectra
are marked with an arrow. (Reproduced with permission from ref. 126. Copyright 2006 Wiley –
VCH Verlag GmbH & Co. KGaA).
HIGHLIGHT 4227
Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola
quenching in the presence of halide anions182 (Fig. 15b).
PFs and APFs have also been widely applied as sensors
for biologically interesting analytes, such as DNA or pro-
teins, which are only soluble in water.4 Since conven-
tional fluorene-based materials only dissolve in rather
apolar organic solvents, they require special adaptations
of their chemical structures to be used in aqueous
media.183 The most commonly used strategy to achieve
this goal is the attachment of ionic side chains to
the 9-positions of the fluorenes, such as ammonium
salts,184–186 sulfonates,187,188 or carboxylates.189 These
PF polyelectrolytes have been found to electrostatically
attract oppositely charged substrates in solution, which
gave rise to aggregation of the PF backbone, and the
resulting shifts in the fluorescence spectra were followed
in time to monitor enzyme activity.188 Especially intense
research has been done on interactions of DNA and cati-
onic OFs.190–194 Also in this case, the detection mecha-
nism is usually based on the electrostatic attraction
between the oppositely charged analyte and probe mole-
cules. In one strategy, the hybridization of single strand
DNA with complementary peptide nucleic acids (PNAs)
has been used to discriminate the base sequence of target
DNA strands195,196 (Fig. 15c). In an aqueous solution of
a cationic APF and PNA functionalized with a fluores-
cent label, no energy transfer was observed because PNA
is neutral and does not interact strongly with the poly-
electrolyte. Upon addition of single strand DNA, hybrid-
ization with the PNA occurred when the strands were
complementary. The resulting negatively charged DNA-
PNA hybrid formed a complex with the APF, which
therefore came into close spatial proximity to the fluores-
cent label. Since this label was chosen such that good
spectral overlap of its absorption with the fluorescence
spectrum of the APF was ensured, energy transfer was
observed, resulting in emission of the label. By contrast,
Figure 15. (a) Fluorescence titration of an APF with MnCl2 in THF. The arrow indicates spectral
changes upon increasing Mn(II) concentration from 0 to 10 ppm. (Reproduced with permission
from ref. 178. Copyright 2001 American Chemical Society). (b) Fluorescence titration of a statisti-
cal fluorene copolymer with tetrabutylammonium iodide in THF. The arrow indicates spectral
changes upon increasing iodide concentration from 0 to 1.3 mM. (Reproduced with permission
from ref. 182. Copyright 2008 Wiley – VCH Verlag GmbH & Co. KGaA). (c) Schematic illustra-
tion of the detection principle of DNA with fluorescently labeled PNA and a cationic APF. (Repro-
duced with permission from ref. 195. Copyright 2002 National Academy of Sciences, USA).
Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola
4228 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 47 (2009)
no such interactions were detected when DNA and PNA
were not complementary. Similar strategies have also
been applied that do not require PNA, but work solely
with DNA.191–193
Except for sensing, PFs and APFs have also been pro-
posed for biological imaging applications, for example in
the form of nanoparticles (vide supra).146–149 A first suc-
cessful attempt was the decoration of amyloid fibrils
with a low band gap APF to produce red fluorescent
nanowires.197
CONCLUSIONS
In this highlight article, fluorene-based p-conjugatedmaterials such as OFs and fluorene homo- and copoly-
mers were introduced. The combination of a relatively
easy synthetic availability and their advantageous opto-
electronic properties makes them excellently suited for
applications as the active materials for biological sensing
and imaging procedures and in organic electronic devices
such as light-emitting diodes and photovoltaic cells.
Color tuning can be easily achieved by copolymerization
with appropriate comonomers, resulting in emission col-
ors spanning the entire visible range. Except by the
chemical structure, the optoelectronic properties are also
influenced by the supramolecular ordering, which is gov-
erned by noncovalent interactions. Precise control over
the order in thin films on a mesoscopic length scale is
therefore indispensable for a purposeful manipulation
and optimization of the device performance. As can be
concluded from this highlight article, despite impressive
advances, research in this field is limited, especially
when oligomers are concerned. Therefore, the supra-
molecular chemistry of fluorene-based materials offers a
promising field to control the morphology and macro-
scopic properties of this class of p-conjugated systems.
The authors acknowledge the many discussions with and contri-
butions from all our former and current colleagues. Their names
are given in the references cited. The research in our laboratory
has been supported by the Eindhoven University of Technol-
ogy, the Royal Netherlands Academy of Sciences (KNAW), the
Netherlands Organisation for Scientific Research (NWO) and
the European Young Investigators Awards (EURYI).
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