Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications
Transcript of Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications
Feature Article
Molecular Engineering of the Band Gapof p-Conjugated Systems: FacingTechnological Applications
J. Roncali
For almost two decades, the search of an intrinsically-conductive organic metal hasrepresented the major driving force for research on control of the band gap of extendedp-conjugated systems. However, the emergence of the application of p-conjugated oligomersand polymers in field-effect transistors, light-emitting diodes, electrochromic devices andsolar cells has introduced major changes in the chemistry of gap engineering. Besidescontrolled band gap, activematerials for electron-ic and photonic applications must present appro-priate absorption and/or emission properties,highest occupied and lowest unoccupied molec-ular orbital (HOMO and LUMO) energy levels andcharge-transport properties. The aim of this shortreview is to present an overview of the recenttrends in this area in order to identify possibledirections for future research.
Introduction
The control of the highest occupied molecular orbital
(HOMO)-lowest unoccupied molecular orbital (LUMO)
energy gap of p-conjugated systems and hence of the
band gap of the corresponding materials has been in
the centre of the synthetic chemistry of functional
p-conjugated systems for more than twenty years.
Following the discovery of the metallic conductivity of
doped polyacetylene,[1] conjugated polymers derived from
J. RoncaliGroupe Systemes Conjugues Lineaires, Laboratoire d’IngenierieMoleculaire d’Angers, UMR CNRS 6200, Universite d’Angers,Bd Lavoisier 49045 Angers FranceFax: (þ33) 241735405; E-mail: [email protected]
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heteroaromatic units such as polypyrrole[2] or polythio-
phene (PT)[3] emerged in the early eighties as an answer to
the problems posed by the lack of stability of polyacety-
lene in atmospheric conditions. Owing to a unique
combination of environmental stability, conductivity,
moderate band gap and structural versatility, thiophene-
based p-conjugated systems have progressively sup-
planted other classes of systems in both fundamental
and technologically-oriented research.[4]
Whereas band gap control has been a target already at
an early stage of research on conjugated polymers, during
the past two decades the field has undergone major
changes in both its finalities and synthetic approaches.
During the 1980–1990 period conducting polymers were
essentially considered as possible alternatives for metals
or metal oxides in bulk applications such as electrode
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J. Roncali
Jean Roncali received his PhD in 1984 from theUniversity of Paris 13 under the supervision ofFrancis Garnier. After successive positions astechnician, engineer and associate researcherin the Laboratory of Molecular Materials nearParis, he moved to the University of Angers, in1991, where he became Director of Research andcreated the Linear Conjugated Systems Group.His research interests encompass the develop-ment of functional p-conjugated systems withtailored electronic properties, in view of appli-cations in energy conversion, electronic andphotonic devices and nanoscience.
Scheme 1. Unlike in polyenes, in aromatic systems, the two limit-ing mesomeric forms obtained by the flip of the double bonds arenot energetically equivalent.
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materials for energy storage or anti-static coatings.
Consequently, while considerable research effort was
invested in the optimization of the conductivity of doped
conjugated polymers, the search for a zero-band-gap
polymer capable of forming an intrinsically conductive
material, namely a true organic metal, represented the
‘‘Holy Grail’’ for the chemistry of conjugated systems.[5]
The turn of the nineties has been marked by a
progressive decline of research on the bulk applications
of conducting polymers, with the parallel emergence of
research focused on the applications of conjugated
systems in electronic and photonic devices such as
field-effect transistors (FETs), light-emitting diodes (LEDs)
and solar cells.[6–8] These applications, based on the
electronic properties of the neutral semiconducting form
of conjugated systems, have had a profound impact on the
chemistry of these systems by providing new finalities.
This new situation has generated a drastic change in point
of view regarding the molecular engineering of the
electronic properties ofmaterials derived fromp-conjugated
systems and hence on the structural control of their band
gap. The aim of this short review is to survey recent
advances of the field of conjugated systemswith a reduced
band gap in the light of the prerequisites imposed by their
applications in modern organic electronic and photonic
devices.
Structural Factors and Band Gap
The origin of the finite energy gap in conjugated systems
lies in the alternation of single and double bonds. In the
hypothetical case of complete electron delocalization, all of
the C–C bonds should have an equal length and the
extension of the conjugated chain would produce a
progressive closure of the energy gap making the corre-
sponding material a ‘‘one-dimensional graphite’’. How-
ever, theoretical and experimental work has shown that
one-dimensional conjugated systems are unstable; the
combined effects of electron-phonon coupling and electron-
electron correlation leads to the localization of p-electrons
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with the aperture of a gap of a least�1.50 eV in the case of
a simple polyenic system. Bond length alternation (BLA)
thus represents the major contribution EBLA to the
magnitude of the energy gap (Scheme 1). Consequently
synthetic approaches leading to structural modifications
resulting in a reduced BLA can be expected to produce a
decrease in the HOMO-LUMO gap DE.[5]
Unlike polyenes, aromatic systems like poly(p-pheny-
lene) or polythiophene (PT), have a non-degenerate ground
state. Thus, contrary to polyenic systems, the two limiting
mesomeric forms obtained by the flip of the double bonds
are not energetically equivalent (Scheme 1). While the
aromatic form is energetically more stable, the quinoid
form has a higher energy and a lower energy gap.[9]
Simple considerations show that the energy needed to
switch from the aromatic to the quinoid form directly
depends on the aromatic stabilisation resonance energy of
the aromatic unit. This resonance effect tends to confine
the p-electrons within the aromatic ring and hence to
prevent their delocalization along the whole conjugated
chain. This effect thus contributes to the magnitude of DE
by a quantity ERes. Another specificity of polyaromatic
systems concerns the rotational disorder around inter-
annular single bonds. A mean dihedral angle u between
consecutive units thus contributes to limit the delocaliza-
tion of p-electrons along the conjugated backbone and
hence to increase DE by a quantity Eu. The introduction of
electron-withdrawing or electron-releasing substituents is
the most direct way to modulate the HOMO and LUMO
levels, and hence their difference, of a conjugated system.
This contribution to DE is represented by a term ESub. These
four structural factors can serve as starting points for the
definition of synthetic approaches for the molecular
engineering of the HOMO-LUMO gap DE of an isolated
conjugated system. However, when assembling individual
molecules or polymer chains into a material, the band gap
Eg involves a fifth contribution (EInt) related to inter-
molecular interactions, which in some cases can have a
considerable impact on the magnitude of Eg (Figure 1).
The band gap of a material derived from a linear
p-conjugated system can be expressed by the sum of these
five contributions:
Eg ¼ EBLA þ ERes þ ESub þ Eu þ EInt (1)
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Molecular Engineering of the Band Gap of p-Conjugated Systems: . . .
Figure 1. Structural factors determining the band gap of materials derived from linearp-conjugated systems.
Although due to the intricacy of these various individual
contributions, this expression is not analytical, it has the
advantage of suggesting possible directions for the
definition of synthetic strategies for band gap control.
Synthetic Principles for Band-GapEngineering ‘‘the Tool Box’’
The considerable development of research on organic
field-effect transistors (OFET), light-emitting devices and
solar cells during the past decade represents a major
driving force for the synthesis of p-conjugated systems
with tailored electronic properties. Since these applica-
tions require active materials with a specific combination
of properties, the search for the lowest possible energy gap
is no longer the exclusive goal of band-gap engineering,
which must more and more take into account more
complex prerequisites. The engineering of the energy gap
of p-conjugated systems can be consid-
ered along different synthetic approaches
which can be illustrated by the use of
different synthetic tools related to one or
several of the elemental contributions
discussed above.
Figure 2. The chemical structures of oligomers 1–3, showing the effect of addingdouble bonds between the thiophene units in the molecule.
Resonance Energy
Based on their combination of stability
and structural flexibility, polyaromatic
systems such as PPP or PT are the most
appropriate basic structures for the design
and synthesis of low-band-gap p-conju-
gated systems. The insertion of double
bonds between the aromatic rings of PPP
or PT, as shown in Figure 2, represents a
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simple and straightforward way of
reducing DE. This structural modifica-
tion which leads to poly(p-phenylene
vinylene) (PPV)[10] and poly(thienylene
vinylene) (PTV),[11] respectively, has
several consequences. In the case of
PPV, ethylenic linkages eliminate the
torsion angle due to steric interactions
between adjacent phenyl rings, thus
allowing the conjugated system to
adopt a planar structure. For PT, the
double bonds suppress the rotational
freedom around the thiophene-thio-
phene single bonds and lead also to a
planar geometry. Another important
effect is that the insertion of double
bonds leads to a decrease of the overall
aromaticity of the system and thus to a further reduction
of the gap. These synergistic effects result in a decrease of
Eg from 3.20 to 2.60 eV between PPP and PPV and from 2.00
to 1.70 eV for PT and PTV.[11]
These effects can be evidenced by comparing the
electronic properties of conjugated oligomers (1–3) con-
taining a constant number of p-electrons in different
combinations of thiophene and ethylene units.
While the insertion of ethylenic linkages between the
thiophene rings produces the expected decrease of DE, as
shown by a 50 nm red shift of lmax between 1 and 2,
insertion of two consecutive double bonds (3) has the
opposite effect as shown by the blue shift of lmax from 490
to 487 nm between 2 and 3. This result shows that the
decrease of aromaticity is in fact counterbalanced by an
increase of the vibrational freedom of the system.[12]
Together with previous extensive studies on oligo(thieny-
lene vinylene)s,[13–16] these results confirm that, in the
absence of an electronic-substituent effect, the thienylene
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vinylene system leads to the lowest value of DE. On
the other hand, this conclusion poses the question of the
definition of synthetic strategies allowing this limit to
be surpassed.
Rigidification of the Conjugated System
A first possible solution to this problem involves the
rigidification of the conjugated system by covalent
fastening of elemental units. Application of this approach
to bithiophene (4),[17] dithienylethylene (6),[18–20] terthio-
phene (8),[21] or dithienylhexatriene (10),[22] shown in
Figure 3, leads to fully-planar conjugated structures with
significantly lower DE values than the parent open-chain
compounds. As expected, rigidification also produces a
considerable increase in the photoluminescence effi-
ciency.[18–22] As shown by crystallographic and theoretical
results, the reduction of DE results largely from a decrease
in the BLA.[19,20] However, for purely thiophenic systems,
the suppression of rotational disorder clearly plays amajor
role. As expected, covalent rigidification of the precursor
produces a significant reduction of the band gap of the
corresponding polymer; however, the magnitude of this
reduction is limited by the remaining possible inter-ring
rotations in the conjugated backbone. More recent work
Figure 3. Rigidification of bithiophene (4), dithienylethylene (6),terthiophene (8) and dithenylhexatriene (10) leading to fully-planar structures.
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has shown that covalent rigidification is also effective
for reducing the gap of extended oligo(thienylene
vinylene)s.[23] To summarize, covalent rigidification of
p-conjugated systems represents an efficient strategy
for band gap control. Furthermore, the large enhancement
of electron delocalization and photoluminescence effi-
ciency inherent to this approach opens interesting
perspectives for applications in photonic systems such
as nonlinear optics,[24] light-emitting devices or solar
cells. However, a major limitation is that this approach
implies, in general, complex multi-step syntheses,
which may be the source of problems in technological
applications requiring cost-effective, large-scale pro-
duction.
Electron-Withdrawing Groups
Introduction of electron-releasing or electron-withdrawing
groups represents the most immediate way to tune the
HOMO and LUMO energy levels of a conjugated system. As
shown in previous work, introduction of acceptor groups
such as nitro, carboxy or cyano at the 3-position of the
thiophene unit induces a large increase in the oxidation
potential.[25] In 1991 Ferraris and Lambert reported
that poly(cyclopentabithiophene)s, bearing electron-
withdrawing keto (12) or dicyano (13) groups (see
Figure 4) at the bridging carbon, showed Eg values of
1.20 eV and 0.80 eV respectively.[26,27] Based on theoretical
work, the low band gap has been attributed to an increase
of the quinoid character of the ground state.[28] However,
such systems can also be viewed as alternant donor-
acceptor systems where the thiophene ring acts as the
donor (see below).
Work carried out in our group has shown that the
introduction of a cyano group at the vinylene linkage of
dithienylethylene could lead to a considerable reduction of
the band gap of the corresponding polymers.[29] Thus Egvalues as low as 0.60 eV have been observed for poly(14)
(Figure 4). However due to solubility problems, the
polymer contains only a limited fraction of low band
gapmaterial. Furtherwork has shown that this approach is
also effective for longer oligomers (15–17) than can be
eventually electropolymerized into polymers with Egvalues of �1.50–1.60 eV.[30] A major advantage of this
approach lies in the straightforward synthesis of the target
compounds by the simple and efficient Knoevenagel
condensation. Another important point is that the
electron-withdrawing cyano groups induce a decrease of
the HOMO level which results in a stabilization of the
neutral state of the system. This approach, however,
presents several limitations. Firstly, the increase in the
oxidation potential of the precursor can render its
polymerization by chemical or electrochemical oxidation
more difficult. On the other hand, cyano groups generally
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Figure 4. Introduction of electron-withdrawing groups such asketo, dicyano and cyano groups can reduce the band-gap energy.
Figure 5. Introduction of electron-donating groups such as simplealkyl groups, and alkoxy and alkylsulfanyl groups can alsodecrease the band gap.
lead to a strong decrease of solubility, which must be
anticipated when designing the target system.
Scheme 2. Intramolecular interactions between sulfur and oxygenatoms lead to a self rigidification of the structure in which theEDOT unit is incorporated.
Electron-Releasing Groups
Introduction of electron-donor groups to a conjugated
system produces an increase of the HOMO level, generally
accompanied by a reduction of DE. Thus, the inductive
effect of simple alkyl groups (18) (Figure 5) decreases the
oxidation potential of the thiophene ring by approxi-
mately 0.20 V.[31] On the other hand, linear alkyl chains of
sufficient length (typically 6–9 carbons) indirectly con-
tribute to reduce the energy gap by enhancing the
long-range order in the polymer, through lipophilic
interactions between the alkyl chains,[31] this effect being
particularly important in regioregular polymers.[32,33]
Mono- or disubstitution by strong electron donors such
as alkoxy or alkylsulfanyl groups (19, 20) (Figure 5)
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produces a large increase of the HOMO level. However,
excessive stabilization of the corresponding cation radical,
sometimes associated with steric problems can render
polymerization problematic.[4a]
In this context, 3,4-ethylenedioxythiophene (EDOT) (21)
represents a unique building block since, unlike molecules
such as compound 20 (Figure 5), EDOT can be straightfor-
wardly polymerized by chemical or electrochemical
methods into highly-conductive polymers with a lower
band gap than PT.[34,35] Furthermore, as extensively
illustrated by work done in our group, the EDOT unit
generates noncovalent. Intramolecular, sulfur-oxygen
interactions which lead to a self-rigidification of the
conjugated structure in which it is incorporated
(Scheme 2).[36]
However, as a counterpart to the stabilization of the
oxidized form of conjugated systems by the strong donor
effect of EDOT, the neutral state becomes unstable in
ambient conditions, which represents a major problem for
the design of EDOT-containing activematerials for OFET or
solar cells.
Cyclopenta[c]dithiophene (22)[37] (Figure 5) is subject to
a strong renewal of interest as a building block for the
synthesis of low-band-gap polymers for applications in
electronic and photonic devices.[38] Due to the combined
effects of planarization, rigidification and inductive effects
of the covalent bridge, cyclopenta[c]dithiophene can
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lead to polymers with smaller band gaps than poly(3-
alkylthiophenes) while disubstitution of the bridging
carbon enables soluble polymers to be synthesized.[38]
Figure 6. Chemical structures of poly(benzo[c]thiophene) (23),poly(dihexylthieno[3,4b]pyrazine (24), and poly(thieno[3,4-b]-thiophene) (25).
Increasing the Quinoid Character
The conversion of a polyaromatic chain into a conjugated
system with an enhanced quinoid character is one of the
most efficient approaches for gap reduction. In the case of
PT, themost direct way to increase the quinoid character of
the neutral state involves the fusion of the thiophene ring
with an aromatic system with a higher resonance energy
(ERes). Since the aromatic sextet tends to localize in the
system of highest ERes, it follows that the thiophene ring
tends to dearomatize to adopt a quinoid structure. Thus,
comparison of the ERes values for thiophene and benzene
shows that the fusion of the two systems should
contribute to confer a quinoid character on the thiophene
ring (Scheme 3).
This concept was first illustrated by poly(benzo[c]thio-
phene) (23) reported by Wudl and coworkers in 1984.[39]
This polymer presents a band gap of 1.10 eV instead of
2.00 eV for PT. Poly(dihexylthieno[3,4b]pyrazine) (24) is
another example of this approach.[40] Poly(24) was initially
prepared by oxidative polymerization using iron trichlo-
ride and was reported to have a band gap of 0.95 eV.[40]
While not directly resorting to the fusion of cycles with
different ERes, poly(thieno[3,4-b]thiophene) (25) is also a
typical low-band-gap polymer (Eg¼ 0.80–0.90 eV) based on
fused ring systems (Figure 6).[41,42] A major drawback of
some of these fused systems lies in their limited stability,
which has led several groups to combine them with more
stable building blocks to obtain multicyclic precursors (see
below).
Poly(heteroarylene methine)s (26), illustrated in Figure 7,
represent a peculiar class of low-band-gap polymers in
Scheme 3. When a thiophene ring is fused with benzene, thethiophene ring dearomatizes and adopts a quinoid character.
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which a variable proportion of aromatic rings are con-
verted to quinoid form by means of elimination reac-
tions.[43–45] Whereas band gap values as low as 0.78 eV
were initially claimed,[43] these results have been con-
troversial. The major drawback of these polymers is that
the final low-band-gap system is produced by elimination
or oxidation reactions leading to materials difficult to
characterize unequivocally. These two aspects strongly
limit the practical possible use of poly(heteroarylene
methine)s for technological applications.
Alternating Donor-Acceptor Groups
The donor-acceptor (D-A) concept for band gap reduction
was proposed byHavinga et al. in 1992.[46] The basic idea is
that conjugated systems involving a regular alternation of
donor and acceptor groups should lead to a broadening of
the valence and conduction bands and thus band gap
reduction. Very low optical band gaps (�0.5 eV) have been
reported for combinations of squaric or croconic acid and
various donor groups.[46,47] Salzner analyzed this concept
from a theoretical viewpoint and concluded that a lack of
band broadening could limit the practical interest of
low-gap systems based on alternant D-A units.[48]
Synergistic Combinations of Structural Effects
This rapid survey of the main synthetic principles for
band-gap control shows that all of them present both
advantages and limitations from the viewpoints of
efficiency and synthetic accessibility. Examination of the
systemswith the smallest band gaps reported so far shows
Figure 7. Chemical structure of poly(heteroarylene methine), inwhich a variable proportion of aromatic rings are converted toquinoid form by elimination reactions.
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that, in fact, their design implicitly or explicitly resorts to
various combinations of some of the synthetic tools
discussed above.
Yamashita and coworkers synthesized a series of
tricyclic systems involving a median proquinoid acceptor
group such as thienopyrazine (27)[49] thienothiadiazole
(28),[50] and thiadiazolothienopyrazine (29)[51] (Figure 8)
and showed that electropolymerization of these systems
leads to polymers with Eg values as low as 0.50 eV for
poly(29) for example.[51]
While these tricyclic systems stabilize the relatively
unstable median groups, they represent a synergistic
combination of alternant D/A groups, quinoidization of
the PT backbone by the proquinoid median groups and
rigidification by intramolecular sulfur-nitrogen interac-
tions. A similar synergistic combination of effects can
explain the very low band gap (0.36 eV) of the polymer
obtained by electropolymerization of precursor (30)
(Figure 8), in which EDOT is associated with thienopy-
razine.[52] The further decrease of Eg compared with
poly(27–29) can be attributed to the stronger donor effect
of EDOT to the additive effects of the sulfur-nitrogen and
sulfur-oxygen noncovalent interactions, and to the regular
1:1 alternation of the D and A groups. The importance of a
regular D-A alternation is clearly shown by the increase of
Eg from 0.36 to 1.10 eV for poly(30) and poly(31) when
the D/A ratio increases from 1:1 to 2:1.[53]
Figure 8. Chemical structures of example tricylcic repeat units withpolymer precursors are also shown (30/31), illustrating the importancompared to that of 0.36 eV for poly(30). The solubility is improved
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Intermolecular Effects
Although these results clearly demonstrate that judicious
combinations of structural effects can indeed lead to very
small Eg values, a major drawback of this class of materials
lies in their almost complete absence of solubility. Thus,
despite the presence of two hexyl chains, poly(30) is quite
insoluble in common solvents. This insolubility is due to
the very compact structure of the polymer in which
intermolecular D-A interactions in the solid state probably
play a major role. A soluble polymer was obtained by
replacing EDOT by an analogue, substitutedwith a dodecyl
chain (32) (Figure 8).[54] In fact, the enhanced solubility of
poly(32) is mainly due to the suppression of tight
p-stacking by the alkyl substituent at an sp3 carbon of
the ethylenedioxy bridge. However, this enhanced solu-
bility is accompaniedwith two deleterious effects. The first
concerns a dramatic loss of stability of the reduced
polymer. Indeed, poly(30) presents an exceptional stability
under repetitive reductive cycling, even in the presence of
oxygen, owing to its highly compact structure which
prevents oxygen permeation in the polymer bulk. How-
ever, this stability is no longer observed for poly(32),
probably because of a much-less- compact polymer
structure. Furthermore, poly(32) shows a band gap of
0.80 eV - approximately twice as large as that of
poly(30).[54] While this result provides an experimental
a median proquinod acceptor group (27–29); structures of similarce of regular D-A alternation, as the band gap of poly(31) is 1.10 eVthrough substitution with a dodecyl chain (32).
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evaluation of the contribution of
inter-chain interactions to the
band gap, it also underlines one
of the major problems posed by
the design of soluble, low-
band-gap polymers.
Figure 9. Electropolymerization of EDOT-based tricyclic precursors (33) containing variousmedian units enables modulation of the band gap of the polymers.
Tailoring the Band Gapof p-Conjugated Systemsfor Specific Applications
Electrochromic Devices
PT derivatives have been widely investigated as electro-
chromic materials.[55–58] However, after a few seminal
papers,[55,56] the field has remained more or less sleeping
for several years due to the lack of stability of PTs in their
oxidized and most transmissive form. The development of
poly(3,4-ethylenedioxythiophene) (PEDOT) at the begin-
ning of the 1990’s has generated a renewal of interest; in
fact, the low oxidation potential of PEDOT (of approxi-
mately 0.0 V versus SCE (saturated calomel electrode)
leads to a stable conducting and optically-transparent
state.[35] This property, together with the excellent cyc-
lability of PEDOT derivatives,[34] has beenwidely exploited
by Reynolds and coworkers who developed a large variety
of EDOT-based polymers for electrochromic applica-
tions.[57] PEDOT is dark blue in its neutral form and
transmissive light blue in its oxidized form. On this basis,
efforts devoted to the electrochromic applications of
PEDOT derivatives have been focused on the improve-
ment of switching time, cyclability, and optical contrast.
Polymers based on 3,4-propylenedioxythiophene deriva-
tives, which lead to more porous polymers with enhanced
optical contrast and faster redox switching, have been
developed in the search for solutions to these pro-
blems.[33,57] Another important line of research concerns
Figure 10. Chemical structures of low-band-gap polymers from hybrid precursors combiningEDOT and benzo[c]thiophene-N-200-ethylhexyl-4,5-dicarbodiimide (34, 35) and a precursorcontaining a median dithienylthienopyrazine group (36).
the engineering of the band gap of
the polymer in order to develop
electrochromic devices with dif-
ferent colors. Thus, electropoly-
merization of EDOT-based tricyclic
precursors (33) containing various
median units (Figure 9) enables
modulation of the band gap of the
polymers and thus the color of the
oxidized and reduced states.[33,57]
Sotzing and coworkers have
investigated various derivatives
of the low-band-gap poly(thieno-
[3,4-b]thiophene (25) as electro-
chromic materials.[58] Wudl and
coworkers have developed low-
band-gap polymers from hybrid
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precursors combining EDOT and benzo[c]thiophene-
N-200-ethylhexyl-4,5-dicarbodiimide (34, 35) (Figure 10)
and investigated their electrochromic behavior in the
infrared (IR) spectral range.[59,60] The polymer derived from
the bithiophenic precursor (34) absorbs at a longer wave-
length (lmax¼ 807 nm) than the parent polymer derived
from the tricyclic system 35 (lmax¼ 780 nm) and has a
slightly lower band gap (1.00 versus 1.10 eV) as has already
been observed for EDOT-thienopyrazine polymers.[52,53]
The same group has described a polymer derived from a
precursor containing a median dithienylthienopyrazine
group (36). Reflection of green light was the consequence
of the coexistence of two absorption bands. These
effects might be related to the strong charge localization
generally observed for EDOT-containing polymers. This
type of polymer opens interesting perspectives for
the development of red-green-blue electrochromic dis-
plays.[61]
Light-Emitting Devices
The design of conjugated luminophores for organic LEDs
(OLEDs) implies the simultaneous control of several
parameters that more or less directly resort to band-gap
engineering. In addition to the emission spectrum, which
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Figure 11. Chemical structure of polythiophenes containingbranched alkyl chains.
Figure 12. Chemical structures of polymers derived from 3- and3,4-mono- and di-substituted thiophenes (40–42).
determines the color of the emitted light, the photo-
luminescence quantum yield, and the HOMO and LUMO
levels that determine the efficiency of charge injection in
the active material are crucial parameters for the overall
device efficiency. The various aspects of the molecular
engineering of conjugated luminophores for OLEDs have
been extensively discussed in recent reviews[62,63] and
only a few examples that more specifically resort to the
band-gap engineering of conjugated systems will be
discussed here.
Several of the synthetic principles for band-gap control
discussed above have been applied to the design of
p-conjugated luminophores. As shown in early work, the
effective conjugation length and hence the band gap of PTs
can be varied over a large extent by means of the steric
hindrance to planarity generated by fixation of an
electronically-inert substituent at the 3-position of the
thiophene ring. This principle is illustrated by the effects
produced by the modification of the distance between a
simple isopropyl group and the 3-position of thiophene.
Thus, 3-isopropyl thiophene (37) (Figure 11) could not be
polymerized, presumably because of the interruption of
conjugation between consecutive thiophene rings due to
an excessive torsion angle, poly(3-isobutylthiophene) (38)
(Figure 11) absorbs at 432 nm while finally, insertion of
two carbons between the thiophene ring and the isopropyl
group allows the PT chain to recovers its planarity as
shown by the similar lmax of poly(3-isopentylthiophene)
(39) (Figure 11) and poly(3-pentylthiophene) at 510 nm.[31]
This principle has been exploited by Berggren et al. for
the tuning of the photoluminescence spectrum of a series
of PTs derived from 3- and 3,4-mono- and disubstituted
thiophenes (40–42) (Figure 12). These polymers enable a
full luminescence emission color, extending from red to
blue, to be reached.[64]
Figure 13. Chemical structures of oligiothiophenes containing thiophene-S,S-dioxides.
Oligothiophenes containing thiophene-S,S-
dioxides developed by Barbarella and coworkers
represent one of the most salient recent develop-
ments in thiophene-based luminophores.[65,66] In
these systems, oxidation of some thiophene rings
into non aromatic S,S-dioxide units produces
simultaneously a strong enhancement of photo-
luminescence efficiency, a moderate increase of
the oxidation potential and a large positive shift of
the reduction potential resulting in a �0.7 eV
narrowing of DE between 43 and 44 (Figure 13).[65]
The high electron affinity of these compounds
facilitates electron injection while, for symmet-
rical systems such as 45, the anti-aggregative
effects of the bulky S,S-dioxide group, leads to a
high luminescence efficiency in the solid state.[66]
The association of EDOT with proquinoid
acceptors represents also a powerful approach
for the design of conjugated fluorophores with
Macromol. Rapid Commun. 2007, 28, 1761–1775
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
high luminescence efficiency. Thus, compared to terthio-
phene (46) (Figure 14) the association of EDOT with
benzo[c]thiophene (47) (Figure 14) or benzothiadiazole (48)
(Figure 14) leads to conjugated fluorophores with lower
www.mrc-journal.de 1769
J. Roncali
1770
oxidation potentials, higher reduction potentials and
considerably-enhanced photoluminescence efficiency
(Table 1).[67] These results show that the synergistic effects
associated with this synthetic approach offer interesting
perspectives for both the synthesis of low-band-gap
systems and for the design of tailored luminophores for
light-emitting devices.
Figure 14. Chemical structure of terthiophene (46), the associ-ation of EDOT with benzo[c]thiophene (47), and the associationof EDOT with benzothiadiazole (48).
Solar Cells
The first examples of solar cells based on PT were reported
in 1984 by Garnier and coworkers, who described Schottky
diodes with 0.15% efficiency under white-light irradiation
at low intensity.[68] In 1992 Sariciftci et al. demonstrated
that photo-excitation of a mixture of poly[2-methoxy,5-
(20-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and
C60 fullerene results in an ultra-fast, photo-induced electron
transfer from the p-conjugated system to C60 with a
quantum efficiency for charge separation close to unity
and a lifetime of the charge-separated state in the milli-
second-range.[69] During the past decade, bulk-hetero-
junction (BHJ) solar cells based on interpenetrated networks
of a soluble C60 derivative, namely 1-(3-methoxycarbonyl)-
propyl-1-phenyl[6,6]methanofullerene (PCBM), and poly-
mers of the PPV or PT series have been subject to a
considerable research effort focused on the analysis of the
elemental processes involved in the photon/electron con-
version and on the optimization of the device technology.[70]
These efforts have generated a continuous improvement in
performance and power-conversion efficiencies of 3.5% have
been obtained for BHJ cells based on PPV derivatives.[70] The
replacement of PPV derivatives with poly(3-hexylhiophene)
(P3HT) represents an important step in the optimization of
BHJ solar cells and several groups have recently reported
power conversion efficiencies in the 4.5 to 5% range.[71–73] In
addition to a higher hole mobility, the lower band gap of
P3HT compared to PPV represents a major contribution to
this better efficiency.
It is generally admitted that a conjugated system
serving as donor in a BHJ solar cell should have a band
gap lower than 1.80 eV in order to achieve a better
harvesting of solar photons, whose maximum flux is
around 1.77 eV.[74] Although this Eg value is relatively easy
Table 1. Electrochemical (in V versus SCE) and optical properties of E
Eox
V
Terthiophene (46) 1.10
EDOT/benzo[c]thiophene (47) 0.56
EDOT/benzothiadiazole (48) 0.92
Macromol. Rapid Commun. 2007, 28, 1761–1775
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
to obtain, the problem is complicated by the fact that,
besides having the appropriate band gap, the conjugated
donor must fulfill other prerequisites regarding the
absolute position of the HOMO and LUMO levels, the
absorption coefficient and the hole mobility. Furthermore,
since the open-circuit voltage of the cell depends on the
difference between the HOMO of the donor and the LUMO
of the acceptor material, the donor must possess a
relatively low HOMO level.[74–76] Finally, in addition to
these electronic properties, the donor material must
combine solution processability and appropriate compat-
ibility with PCBM in order to result in a compositematerial
with optimal nanoscale morphology.
Conjugated polymers combining a band gap slightly
smaller than that of poly(3-alkylthiophene)s and relatively
simple synthetic procedures appears particularly interest-
ing for application in BHJ solar cells. As already discussed,
poly(thienylene vinylene) (PTV) has a smaller band gap
than PT (typically 1.70–180 eV vs. 1.90–2.00 eV) due to the
combined effects of planarization and reduced aromaticity
DOT-based conjugated fluorophores.
Ered lmax lem fem
V nm nm %
�2.00 350 430 6.6
�1.80 450 613 92
�1.40 481 630 75
DOI: 10.1002/marc.200700345
Molecular Engineering of the Band Gap of p-Conjugated Systems: . . .
Figure 16. Chemical structure of a low-band-gap polymer (54)designed specifically for BHJ solar cells.
of the ethylenic linkages. Vanderzande and coworkers
have investigated the synthesis of different PTV deriva-
tives as active material for BHJ solar cells; thus, a device
realized with a PTV based on 3,4-dichlorothiophene (49)
(Figure 15) delivered a short-circuit current density (Jsc) of
1.40mA � cm�2, an open circuit voltage (Voc) of 0.408 V, and
a fill factor of 30% corresponding to a power-conversion
efficiency of 0.18%.[77] Smith et al. fabricated BHJ solar
cells using poly(3-dodecyl-2,5-thienylene vinylene) (50)
(Figure 15). The external quantum efficiency (EQE) photo-
action spectrum shows a peak at 580 nm that corresponds
to the absorption maximum of polymer 50. While this
peak is red-shifted by approximately 100 nm compared to
cells based on PPV, the EQE peak reaches only 4.5% instead
of approximately 70% with P3HT. Under white-light
illumination, the cells gave a maximum efficiency of
0.24% with a Voc of 0.54 V and a Jsc of 0.80 mA � cm�2.[78]
Photophysical experiments on oligo(thienylene viny-
lene)s of increasing chain length of have shown that
whereas the shorter oligomers (n¼ 2 and 3) fluoresce, the
lifetimes of the singlet excited state of the longer systems
are extremely short because of fast thermal decay
resulting in negligible quantum yield for fluorescence.[79]
This short lifetime of the singlet excited state could explain
the low efficiency of BHJ solar cells based on PTV.
Reynolds and coworkers synthesized a cyanovinylene-
dioxythiophene polymer (51) (Figure 15) with a band gap
of 1.70 eV. BHJ solar cells were fabricated with PCBM in a
Figure 15. Chemical structures of polymers based on 3,4-dichloro3-dodecyl-2,5-thienylene vinylene (50), cyanovinylene-dioxythiophen(cyano-2-thienylvinylene)phenylene precursors (52, 53).
Macromol. Rapid Commun. 2007, 28, 1761–1775
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ratio of 1:4. The action spectrum of the cell shows an EQE
peak of 11% at 600 nm, and a 0.10% efficiency under
white-light illumination in AM 1.5 conditions.[80] More
recently, Vanderzande and coworkers synthesized a series
of polymers from bis-(cyano-2-thienylvinylene)phenylene
precursors (52, 53) (Figure 15). The polymers show Egvalues of 1.72 and 1.59 eV for 52 and 53 respectively.[81]
The characterization of BHJ solar cells with poly(52b),
poly(53a,b) and PCBM (1:2 ratio) showed that polymer 52
leads to the highest EQE value of 12% at 550 nm and to the
highest conversion efficiency under AM 1.5 illumination
(0.14%). The incorporation of EDOT in the structure was
found to produce an increase of Jsc and a decrease of Voc, in
agreement with the results obtained with EDOT-based
oligomers.[82]
Janssen and coworkers described one of the first
examples of a low-band-gap polymer specifically designed
for BHJ solar cells (54) (Figure 16). Stille coupling of a
thiophene (49),e (51), and bis-
distannyl derivative of a dithienylpyrrole
donor block with the dibrominated pro-
quinoid acceptor benzothiadiazole led to
polymers with Eg values of 1.50–1.60 eV,
depending on the synthesis conditions.[83]
BHJ solar cells have been fabricated with a
1:1 polymer/PCBM ratio. The photo-action
spectrum showed an onset of the photo-
current around 750 nm, while under
white-light illumination the device
gave a Voc of 0.65 V, a Jsc of 0.80 mA � cm�2
2 and a power conversion efficiency of
0.34%.[83] An improved conversion effi-
ciency of �1% has been obtained with a
polymer/PCBM ratio of 1:3.[84]
Campos et al. synthesized a polymer
with a band gap of 1.30 eV (55) (Figure 17)
by oxidative polymerization of a dithienyl-
thienopyrazine using iron trichloride.
Under monochromatic irradiation the
action spectrum of BHJ cells with PCBM
(1:1 ratio) showed an onset of the photo-
current at approximately 800 nm with an
EQE peak of 6% at 660 nm. Under white-
light illumination (100 mW � cm�2) the cell
delivered a Jsc of 1.0 mA � cm�2; the poor
www.mrc-journal.de 1771
J. Roncali
Figure 17. Chemical structure of a polymer (55) synthesized byoxidative polymerization of a dithenyl-thienopyrazine using irontrichloride.
Figure 19. Chemical structure of a donor-acceptor random co-polymer (58) replacing the dithienyl-thienopyrazine in polymer56 with benzothiadiadole.
1772
conversion efficiency (h< 0.1%) was, for a large part, the
consequence of the low Voc of 0.22 V.[85]
Inganas and coworkers developed BHJ solar cells with a
low-band-gap donor-acceptor random copolymer result-
ing from the combination of a dialkylfluorene donor
block with the dithienyl-thienopyrazine acceptor (56)
(Figure 18). The optical spectrum shows a main absorption
band at around 400 nm, a less intense CT band with a
maximum at 615 nm and a band gap of�1.70 eV. BHJ solar
cells were realized with a 1:4 ratio polymer/PCBM. Under
monochromatic irradiation the action spectrum shows an
EQE peak of 14% at approximately 600 nm with a
photocurrent onset at �800 nm. Under white light
irradiation at 100 mW � cm�2, the device gave a Jsc of
3 mA � cm�2 and a Voc of 0.78 V, leading to a maximum
conversion efficiency of 0.96%.[86]
Cao and coworkers have synthesized random copoly-
mers starting from a dimethylthienopyrazine (57)
(Figure 18). Copolymers of varied (m/n) composition were
obtained by changing the feed ratio. The copolymers with
the highest thienopyrazine content showed a band gap of
1.77 eV. The BHJ cells prepared with a 1:1 polymer/PCBM
ratio gave a Jsc of 4.1 mA � cm�2, a Voc of 0.70 V and a
conversion efficiency of 0.83% under white-light irradia-
tion at 100 mW � cm�2.[87]
Replacing the diphenyl-thienopyrazine by benzothia-
diadole (58) (Figure 19), leads to an increase of the band gap
to 1.90 eV. However, rather than intrinsic structural
Figure 18. Chemical structures of polymer 56, a low band-gap random c57, a random copolymer starting from dimethylthienopyrazine.
Macromol. Rapid Commun. 2007, 28, 1761–1775
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
effects, this result might reflect differences in the chain
length between the two polymers.
The photocurrent action spectrum of a BHJ cell made
with poly(58a) and PCBM in a 1:4 ratio shows a
photocurrent onset around 650 nm with an EQE peak of
approximately 40% at 550 nm. Under while-light irradia-
tion (100 mW � cm�2) the cell gave a Jsc of 4.66 mA � cm�2, a
Voc of 1.04 V and an efficiency of 2.2%.[88] More recently the
optimization of cells based on poly(58b) led to an improved
efficiency of 2.8%.[89]
The combination of the dialkylfluorene donor blockwith
the benzothiadiazole-pyrazine acceptor group (Figure 20)
induces large changes in the optical spectrum of the
copolymer with a shift of the absorption onset up to ca
1 000 nm corresponding to a band gap of 1.24 eV for
poly(59).
The photocurrent action spectrum of a BHJ cell made
with a 1:4 polymer:PCBM ratio shows a photocurrent onset
at approximately 1 000 nm. The 8.4% EQE peak observed at
the maximum of the CT band confirms that this band is
photoactive. Under while-light irradiation (100 mW � cm2)
the cell gave a Jsc of 1.76 mA � cm�2 and a Voc of 0.54 V, with
an efficiency of 0.30%.[90]
Demadrille et al. synthesized D-A polymers using
fluorenone as the acceptor block and various thiophene-
based conjugated systems as the donor group.[91] A
polymer containing thienylene vinylene units (60)
(Figure 21) was used as the donor in a BHJ solar cell.
opolymer based on dialkylfluorene and dithienyl-thienopyrazine, and
DOI: 10.1002/marc.200700345
Molecular Engineering of the Band Gap of p-Conjugated Systems: . . .
Figure 20. Chemical structure of copolymer based on a dialkyl-fluorene donor block and a benzothiadiazole-pyrazine acceptorgroup.
Figure 22. Chemical structure of polymer 61, synthesized fromcyclopentabithiophene and benzothiadiazole.
The cell was realized with a mixture containing a 1.26
molar equivalent of PCBM and showed a photocurrent
onset at around 700 nm, while under while-light irradia-
tion the device delivered a Jsc close to 3 mA � cm�2, a Voc of
0.50 V and a 1.10% efficiency.[91]
Recently, Mulbacher et al. synthesized a copolymer of
dialky-cyclopentabithiophene and benzothiadiazole (61)
(Figure 22). This polymer shows an absorption maximum
at 705 nm in solution, shifting to 775 nm for film, and a
band gap of �1.40 eV.[92] Solar cells were realized by
blending poly(61) with PCBM or its C70 analogue in a 1:3
weight ratio. The action spectra show a photocurrent onset
beyond 900 nm with a maximum EQE of approximately
30% for PCBM and 35% for the C70 derivative. Under
white-light irradiation, (100 mW � cm�2) current densities
of 9 and 11 mA � cm�2 with a Voc of 0.65 V were obtained,
corresponding to power-conversion efficiencies of 2.67 and
3.16% respectively.[92] Quite recently, Peet et al. have
shown that incorporation of a few percent of alkanedithiol
in the solution used to spin-cast films of 61 and C70 led to
an increase of the conversion efficiency of the cell to
5.5%.[93]
To summarize, the design of low-band-gap polymers
for BJH solar cells is a complex problem that must
simultaneously take into account several, sometimes
contradictory parameters. While the field is still in its
infancy, promising results have recently been reported
with, in particular, a significant extension of the photo-
response towards the near infrared. Although efficiencies
Figure 21. Chemical structure of polymer 60.
Macromol. Rapid Commun. 2007, 28, 1761–1775
� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
of 2.7–2.8% have been obtained, these values are still lower
than those obtained with P3HT; however, it is difficult to
compare these first results to those obtainedwith a system
which has been extensively optimized.
Conclusion and Outlook
Whereas zero band gap and intrinsic conductivity have
represented a major goal for the band-gap control of
p-conjugated polymers for many years, the huge devel-
opment of the applications of these systems in electronic
and photonic devices requiring active materials with
tailored optical and charge-transport properties has
generated major changes in the orientations of the field.
Organic solar cells are, at present, a major driving force
for research in band-gap engineering. Although materials
with optimal band-gap values can be designed using
known synthetic principles, the difficulty of the task lies in
the need to associate optimal light-harvesting properties
to a well-defined chemical structure, a rather-low HOMO
level and a high hole mobility.
Despite an apparent diversity, the various strategies for
band-gap control can be, in fact, roughly divided into two
main lines. The first, ‘‘mechanical approach’’ involves
the planarization, rigidification and quinoidization of
the conjugated system with, as a more or less direct
consequence, a reduction of BLA. This approach leads in
general to a large increase in the photoluminescence
efficiency and furthermore, rigidification can be expected
to decrease the reorganization energy and thus to con-
tribute to improve the charge mobility. A major drawback,
however, is that this ‘‘mechanical approach’’ requires, in
general, long and complex synthetic work.
The second main route lies on the creation of
intramolecular charge-transfer by association of donor
and acceptor units. This approach, which can lead to
very low band gaps, has been until now the preferred one
for the synthesis of polymers for BHJ solar cells. Although
cells with an extended photo-response have been realized,
their performances remain generally inferior to those of
cells based on P3HT. While further device optimization
is certainly needed, the D-A approach still poses
www.mrc-journal.de 1773
J. Roncali
1774
several fundamental problems related to the effects of the
ratio, placement, mode of connection and relative
strengths of the D and A units in the p-conjugated
system. In addition to controlling the width of the band
gap, these structural parameters determine the width
and absorption coefficient of the pp� and ICT bands, as
well as the electronic distribution in the conjugated
system and hence the charge-transport properties of the
material.
While the control of these structural factors can be
difficult in random copolymers with, sometimes, rather
limited molecular weights, recent work has shown that
the creation of a unique ICT in donors based on star-shaped
and/or 3D geometries can lead, at the same time, to an
extension of the photo-response and to an increase of the
Voc.[94,95] These results thus suggest that amorphous
materials based on monodisperse molecular architectures
can represent a possible alternative to polydisperse
low-band-gap polymers for the realization of both multi-
layer and BHJ solar cells.[96]
In addition to stringent prerequisites in terms of optical
and charge-transport properties, p-conjugated systems
designed as active materials for electronic and photonic
devices must combine processability and high environ-
mental and photochemical stability. Furthermore, in order
to be compatible with industrial development, these
future materials will have to be produced by means of
cost-effective simple and straightforward synthetic pro-
cedures taking into account increasing pressure in terms of
environmental constraints: atom economy, minimal use
of organic solvents and toxic metal catalysts, and use of
renewable starting materials eventually derived from
biomass, for example.
Although these multiple and complex problems clearly
represent major challenges for the chemistry of functional
p-conjugated systems, it is clear that their solutions
will, for a large part, determine the future applications
of p-conjugated systems in electronic and photonic
devices.
Received: May 7, 2007; Accepted: June 27, 2007; DOI: 10.1002/marc.200700345
Keywords: band gap; conjugated polymers; donor-acceptor;quinoid; rigidification; solar cells
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