Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

15
Molecular Engineering of the Band Gap of p-Conjugated Systems: Facing Technological Applications J. Roncali 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 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 Feature Article For almost two decades, the search of an intrinsically-conductive organic metal has represented the major driving force for research on control of the band gap of extended p-conjugated systems. However, the emergence of the application of p-conjugated oligomers and polymers in field-effect transistors, light-emitting diodes, electrochromic devices and solar cells has introduced major changes in the chemistry of gap engineering. Besides controlled band gap, active materials 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 and charge-transport properties. The aim of this short review is to present an overview of the recent trends in this area in order to identify possible directions for future research. J. Roncali Groupe Syste `mes Conjugue ´s Line ´aires, Laboratoire d’Inge ´nierie Mole ´culaire d’Angers, UMR CNRS 6200, Universite ´ d’Angers, Bd Lavoisier 49045 Angers France Fax: (þ33) 241735405; E-mail: [email protected] Macromol. Rapid Commun. 2007, 28, 1761–1775 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/marc.200700345 1761

Transcript of Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

Page 1: 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]

Macromol. Rapid Commun. 2007, 28, 1761–1775

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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

DOI: 10.1002/marc.200700345 1761

Page 2: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

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.

1762

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

Macromol. Rapid Commun. 2007, 28, 1761–1775

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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)

DOI: 10.1002/marc.200700345

Page 3: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

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

Macromol. Rapid Commun. 2007, 28, 1761–1775

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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

www.mrc-journal.de 1763

Page 4: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

J. Roncali

1764

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.

Macromol. Rapid Commun. 2007, 28, 1761–1775

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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

DOI: 10.1002/marc.200700345

Page 5: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

Molecular Engineering of the Band Gap of p-Conjugated Systems: . . .

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)

Macromol. Rapid Commun. 2007, 28, 1761–1775

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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

www.mrc-journal.de 1765

Page 6: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

J. Roncali

1766

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.

Macromol. Rapid Commun. 2007, 28, 1761–1775

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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.

DOI: 10.1002/marc.200700345

Page 7: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

Molecular Engineering of the Band Gap of p-Conjugated Systems: . . .

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

Macromol. Rapid Commun. 2007, 28, 1761–1775

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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).

www.mrc-journal.de 1767

Page 8: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

J. Roncali

1768

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

Macromol. Rapid Commun. 2007, 28, 1761–1775

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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

DOI: 10.1002/marc.200700345

Page 9: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

Molecular Engineering of the Band Gap of p-Conjugated Systems: . . .

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

Page 10: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

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

Page 11: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

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

Page 12: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

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

Page 13: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

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

Page 14: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

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

[1] H. Shirakawa, E. J. Lewis, A. G. McDiarmid, C. K. Chiang, A. J.Heeger, J. Chem. Soc., Chem. Commun. 1977, 578.

[2] A. F. Diaz, K. K. Kanazawa, G. P. Gardini, J. Chem. Soc., Chem.Commun. 1979, 635.

[3] G. Tourillon, F. Garnier, J. Electroanal. Chem. 1982, 135, 173.

Macromol. Rapid Commun. 2007, 28, 1761–1775

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

[4] [4a] J. Roncali, Chem. Rev. 1992, 92, 711; [4b] ‘‘Handbook ofOligo- and Polythiophene’’, D. Fichou, Ed., Wiley-VCH, Wein-heim 1999.

[5] J. Roncali,, Chem. Rev. 1997, 97, 173.[6] H. Koezuka, A. Tsumura, T. Ando, Synth. Met. 1987, 18,

699.[7] J. H. Burroughes, D. D. C. Bradley, R. N. Marks, R. H. Friend,

P. L. Burn, A. B. Holmes, Nature 1990, 347, 539.[8] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science

1995, 270, 1789.[9] J.-L. Bredas, J. Chem. Phys. 1985, 82, 3808.[10] A. C. Grimsdale, A. B. Holmes, ‘‘Chapter 4’’ ‘‘Handbook of

Conducting Polymers’’, 3rd edition, T. A. Skotheim, J. R.Reynolds, Eds., CRC Press, Boca Raton, FL 2007.

[11] K.-Y. Jen, M. R. Maxfield, L. W. Shacklette, R. L. Elsenbaumer,J. Chem. Soc., Chem. Commun. 1987, 309.

[12] P. Frere, J.-M. Raimundo, P. Blanchard, J. Delaunay, P.Richomme, J.-L. Sauvajol, J. Orduna, J. Garin, J. Roncali, J. Org.Chem. 2003, 68, 7254.

[13] J. Roncali, Acc. Chem. Res. 2000, 33, 147.[14] E. Elandaloussi, P. Frere, P. Richomme, J. Orduna, J. Garin, J.

Roncali, J. Am. Chem. Soc. 1997, 119, 10774.[15] I. Jestin, P. Frere, N. Mercier, E. Levillain, D. Stievenard, J.

Roncali, J. Am. Chem. Soc. 1998, 120, 8150.[16] J. J. Apperloo, C. Martineau, P. A. van Hal, J. Roncali, R. A. J.

Janssen, J. Phys. Chem. A, 2002, 106, 21.[17] H. Brisset, C. Thobie-Gautier, A. Gorgues, M. Jubault, J.

Roncali, J. Chem. Soc., Chem. Commun. 1994, 1305.[18] J. Roncali, C. Thobie-Gautier, E. Elandaloussi, P. Frere,

J. Chem. Soc., Chem. Commun. 1994, 2249.[19] H. Brisset, P. Blanchard, B. Illien, A. Riou, J. Roncali, Chem.

Commun. 1997, 569.[20] P. Blanchard, H. Brisset, A. Riou, J. Roncali, J. Org. Chem.

1997, 62, 2401.[21] J. Roncali, C. Thobie-Gautier, Adv. Mater. 1994, 6, 846.[22] P. Blanchard, H. Brisset, A. Riou, R. Hierle, J. Roncali, J. Org.

Chem. 1998, 63, 8310.[23] P. Blanchard, P. Verlhac, L. Michaux, P. Frere, J. Roncali,

Chem. Eur. J. 2006, 12, 1244.[24] J. M. Raimundo, P. Blanchard, I. Ledoux-Rak, R. Hierle, L.

Michaux, J. Roncali, Chem. Commun. 2000, 1597.[25] R. J. Waltman, J. Bargon, A. F. Diaz, J. Phys. Chem. 1983, 87,

1459.[26] T. L. Lambert, J. P. Ferraris, J. Chem. Soc., Chem. Commun.

1991, 752.[27] T. L. Lambert, J. P. Ferraris, J. Chem. Soc., Chem. Commun.

1991, 1268.[28] J. M. Toussaint, J. L. Bredas, Synth. Met. 1993, 61, 103.[29] H. A. Ho, H. Brisset, P. Frere, J. Roncali, J. Chem. Soc., Chem.

Commun. 1995, 2309.[30] H. A. Ho, H. Brisset, E. Elandaloussi, P. Frere, J. Roncali, Adv.

Mater. 1996, 8, 990.[31] J. Roncali, R. Garreau, A. Yassar, P. Marque, F. Garnier, M.

Lemaire, J. Phys. Chem. 1987, 91, 6706.[32] R. D. McCullough, Adv. Mater. 1998, 10, 93.[33] H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K.

Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J.Janssen, E.W. Meijer, P. Hervig, D.M. de Leeuw,Nature 1999,401, 685.

[34] L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J. R.Reynolds, Adv. Mater. 2000, 12, 481.

[35] S. Kirchmeyer, K. Reuter, J. Mater. Chem. 2005, 15, 2077.[36] J. Roncali, P. Blanchard, P. Frere, J. Mater. Chem. 2005, 15,

1589.

DOI: 10.1002/marc.200700345

Page 15: Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing Technological Applications

Molecular Engineering of the Band Gap of p-Conjugated Systems: . . .

[37] D. D. Cunningham, A. Gaa, C. V. Pham, E. T. Lewis, A.Burkhardt, L. L. Davdson, A. N. Kansah, O. Y. Ataman, H.Zimmer, H. B. Mark, J. Electrochem. Soc. 1988, 135, 2750.

[38] P. Coppo, M. L. Turner, J. Mater. Chem. 2005, 15, 1123.[39] F. Wudl, M. Kobayashi, A. J. Heeger, J. Org. Chem. 1984, 49,

3382.[40] M. Pomerantz, B. Chalonner-Gill, M. O. Harding, J. J. Tseng,

W. J. Pomerantz, J. Chem. Soc., Chem. Commun. 1992, 1672.[41] S. Y. Hong, D. S. Marynick, Macromolecules 1992, 25, 4652.[42] G. A. Sotzing, K. Lee, Macromolecules 2002, 35, 7281.[43] S. A. Jenekhe, Nature 1986, 322, 345.[44] H. Braunling, R. Becker, G. Blochl, Synth. Met. 1993, 55–57,

833.[45] W. C. Chen, S. A. Jenekhe, Macromolecules 1995, 28, 465.[46] E. E. Havinga, W. ten Hoeve, H. Wynberg, Polym. Bull. 1992,

29, 119.[47] E. E. Havinga, W. ten Hoeve, H. Wynberg, Synth. Met. 1993,

55–57, 299.[48] U. Salzner, J. Phys. Chem. B 2002, 106, 9214.[49] C. Kitamura, S. Tanaka, Y. Yamashita, J. Chem. Soc., Chem.

Commun. 1994, 1585.[50] S. Tanaka, Y. Yamashita, Synth. Met. 1993, 55–57, 1251.[51] M. Karikomi, C. Kitamura, S. Tanaka, Y. Yamashita, J. Am.

Chem. Soc. 1995, 117, 6791.[52] S. Akoudad, J. Roncali, Chem. Commun. 1998, 2081.[53] J. Casado, R. P. Ortiz, M. C. Ruiz Delgado, V. Herandez, J. T.

Lopez-Navarette, P. J. M. Raimundo, P. Blanchard, M. Allain,J. Roncali, J. Phys. Chem. B 2005, 109, 16616.

[54] I. F. Perepichka, E. Levillain, J. Roncali, J. Mater. Chem. 2004,14, 1679.

[55] K. Kaneto, K. Yoshino, Y. Inuishi, Jpn. J. Appl. Phys. 1983, 22,L157.

[56] F. Garnier, G. Tourillon,M. Gazard, J. C. Dubois, J. Electroanal.Chem. 1983, 148, 299.

[57] A. L. Dyer, J. R. Reynolds, ‘‘Handbook of Conducting Poly-mers’’, 3rd edition, T. A. Skotheim, J. R. Reynolds, Eds., CRCPress, Boca Raton, FL 2007, Chapter 20.

[58] G. A. Sotzing, V. Seshadri, F. J. Waller, ‘‘Handbook ofConducting Polymers’’, 3rd edition, T. A. Skotheim, J. R.Reynolds, Eds., CRC Press, Boca Raton, FL 2007, Chapter 11.

[59] H. Meng, D. Tucker, S. Chaffins, Y. Chen, R. Helgeson, B.Dunn, F. Wudl, Adv. Mater. 2003, 15, 146.

[60] G. Sonmez, H. Meng, F. Wudl,Macromolecules 2003, 15, 4923.[61] G. Sonmez, F. Wudl, J. Mater. Chem. 2005, 15, 20.[62] U. Mitchke, P. Bauerle, J. Mater. Chem. 2000, 10, 1471.[63] I. F. Perepichka, D. F. Perepichka, H. Meng, F. Wudl, Adv.

Mater. 2005, 17, 2281.[64] M. Berggren, O. Inganas, G. Gustafsson, J. Rasmusson, M. R.

Andersson, T. Hjertberg, O. Wennerstrom, Nature 1994, 372,444.

[65] G. Barbarella, L. Favaretto, M. Zambianchi, O. Pudova, C.Arbizzani, A. Bongini, M. Mastragostino, Adv. Mater. 1998,10, 551.

[66] L. Antolini, E. Tadesco, G. Barbarella, L. Favaretto, G. Sotgiu,M. Zambianchi, D. Casarini, G. Gigli, R. Cingolani, J. Am.Chem. Soc. 2000, 122, 9006.

[67] J.-M. Raimundo, P. Blanchard, H. Brisset, S. Akoudad, J.Roncali, Chem. Commun. 2000, 939.

[68] S. Glenis, G. Tourillon, F. Garnier, Thin Solid Films 1984, 122,9.

[69] N. S. Sariciftici, L. Smilowitz, A. J. Heeger, Science 1992, 258,1474.

Macromol. Rapid Commun. 2007, 28, 1761–1775

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

[70] C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct.Mater. 2001, 11, 15.

[71] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y.Yang, Nat. Mater. 2005, 4, 864.

[72] W. Ma, C. Yang, X. Gong, K. Lee, A. J. Heeger, Adv. Funct.Mater. 2005, 15, 1617.

[73] M. Reyes-Reyes, K. Kim, J. Dewald, M. Lopex-Sandoval, A.Avadhanula, S. Curran, D. S. Carrol, Org. Lett. 2005, 7, 5749.

[74] M. C. Scharber, D. Mulbacher, M. Koppe, P. Denk, C. Wadauf,A. J. Heeger, C. J. Brabec, Adv. Mater. 2006, 18, 789.

[75] C. J. Brabec, A. Cravino, D. Meissner, N. S. Sariciftci, T.Fromherz, M. T. Rispens, L. Sanchez, K. Hummelen, Adv.Funct. Mater. 2001, 11, 374.

[76] A. Gadisa, M. Svensson, M. Andersson, O. Inganas, Appl.Phys. Lett. 2004, 84, 1609.

[77] A. Henckens, M. Knipper, I. Polec, J. Manca, L. Lutsen, D.Vanderzande, Thin Solid Films 2004, 451–542, 572.

[78] A. P. Smith, R. R. Smith, B. E. Taylor, M. F. Durstock, Chem.Mater. 2004, 16, 4687.

[79] J. J. Aperloo, C. Martineau, P. A. van Hal, J. Roncali, R. A. J.Janssen, J. Phys. Chem. A 2002, 106, 21.

[80] B. C. Thompson, Y.-G. Kim, J. R. Reynolds, Macromolecules2005, 38, 5359.

[81] K. Colladet, S. Fourier, T. J. Clej, L. Lutsen, J. Gelan, D. Van-derzande, L. H. Nguyen, H. Neugebauer, S. Sariciftci, A.Aguirre, G. Janssen, E. Goovaerts, Macromolecules 2007, 40,65.

[82] J. Roncali, P. Frere, P. Blanchard, R. de Bettignies, M. Turbiez,S. Roquet, P. Leriche, Y. Nicolas, Thin Solid Films 2006,511–512, 567.

[83] A. Dhanabalan, J. K. J. van Duren, P. A. van Hal, J. L. J. vanDongen, R. A. J. Janssen, Adv. Funct. Mater. 2001, 11, 255.

[84] C. J. Brabec, C. Winder, N. S. Sariciftci, J. C. Hummelen, A.Dhanabalan, P. A. van Hal, R. A. J. Janssen, Adv. Funct. Mater.2002, 12, 709.

[85] L. M. Campos, A. Tontcheva, S. Gunes, G. Sonmez, H.Neugebauer, N. S. Sariciftci, F. Wudl, Chem. Mater. 2005,17, 4031.

[86] F. Zhang, E. Perzon, X. Wang, W. Mammo, M. R. Andersson,O. Inganas, Adv. Funct. Mater. 2005, 15, 745.

[87] Y. Xia, J. Luo, X. Deng, X. Li, D. Li, X. Zhu, W. Yang, Y. Cao,Macromol. Chem. Phys. 2006, 207, 511.

[88] M. Svensson, F. Zhang, S. Veenstra, W. J. H. Verhees, J. C.Hummelen, J. M. Kroon, O. Inanas, M. R. Andersson, Adv.Mater. 2003, 15, 988.

[89] F. Zhang, K. G. Jespersen, C. Bjornstrom, M. Svensson, M. R.Andersson, V. Sundstrom, K.Magnuson, E. Moons, A. Yartsev,O. Inganas, Adv. Funct. Mater. 2006, 16, 667.

[90] X. Wang, E. Perzon, J. L. Delgado, P. De la Cruz, F. Zhang, F.Langa, M. Andersson, O. Inganas, Appl. Phys. Lett. 2004, 85,5081.

[91] R. Demadrille, M. Firon, J. Leroy, P. Rannou, A. Pron, Adv.Funct. Mater. 2005, 15, 1547.

[92] D. Muhlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller,R. Gaudiana, C. Brabec, Adv. Mater. 2006, 18, 2884.

[93] J. Peet, J. Y. Kim, N. E. Coates,W. L. Ma, D. Moses, A. J. Heeger,G. Bazan, Nat. Mater. 2007, DOI: 10.1038/Nmat1928.

[94] S. Roquet, P. Leriche, A. Cravino, O. Aleveque, P. Frere, J.Roncali, J. Am. Chem. Soc. 2006, 128, 3459.

[95] A. Cravino, P. Leriche, O. Aleveque, S. Roquet, J. Roncali,Adv.Mater. 2006, 18, 3033.

[96] J. Roncali, A. Cravino, P. Leriche, Adv. Mater. 2007, (in press).

www.mrc-journal.de 1775