Fluorene Base Material

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.

4216 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 47 (2009)

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

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


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).



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


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).



(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


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).



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.


KGaA).

HIGHLIGHT 4223


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.



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


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


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).



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


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).



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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