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pi-Building blocks for organic electronics: Revaluation of “inductive” and “resonance” effects of pi-electron deficient units Kazuo Takimiya, Itaru Osaka, and Masahiro Nakano Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm4021063 • Publication Date (Web): 16 Aug 2013 Downloaded from http://pubs.acs.org on August 17, 2013

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π-Building blocks for organic electronics: Revaluation of “inductive” and “resonance” effects of π-electron deficient units Kazuo Takimiya†,§*, Itaru Osaka†,§*, Masahiro Nakano† †

Emergent Molecular Function Research Group, RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan §

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 7398527, Japan KEYWORDS: π-Building block, electronic structure, highest occupied molecular orbital, lowest unoccupied molecular orbital, semiconducting polymer ABSTRACT: Organic electronics has rapidly advanced in the last two decades, owing to the development of semiconducting materials and the innovation of device technologies. One of the critical issues in the materials development, for achieving high performances in the organic devices, is to precisely control their frontier orbitals, i.e., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. These energy levels are largely dependent on the chemical structures, and thus to evaluate the electronic properties of representative π-building blocks and to understand their structure-property relationship are of particular importance. In this review, we focus on various π-building blocks for semiconducting polymers and oligomers, especially electron poor heterocycles (acceptor units), and revaluate their electronic structures focusing on the model compounds and the corresponding polymers. A clear difference in the electronic structure is found depending on the chemical structure, which can be explained in terms of “inductive effect” and “resonance effect”. We hope that this review would give new insight into the electronic structure of the semiconducting materials and be an important guideline for the materials design.

Introduction The field of organic electronics has been significantly developed in the last two decades, aiming at practical applications to ultra-thin, large-area, and/or flexible devices of future generation, consisting of organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaics (OPVs). Since these devices can be fabricated by low-cost and low-energy consumption processes, organic electronics has been expected as a powerful and fascinating technology that offers great differentiation from the conventional electronics technology based on inorganic materials.1 Despite its high expectation, performances of the organic devices had been lower than those of conventional inorganic-based electronic devices. However, continuous research efforts have contributed to the great advances in the performances of, in particular, OFETs and OPVs during the last several years.2 For example, impressively high carrier mobility of up to 30 cm–1 V–1 s–1 has been achieved in solution-processed thin film OFETs using C8-BTBT as the active material,3 and high power conversion efficiencies (PCE) approaching or even exceeding 10% have been recorded in the state-of-the-art solar cells with both vapor-deposition and solution-processed systems.4 In order to achieve such high performances, one of the most critical contributions is the materials development, along with innovations and optimizations in the device structure and processing. In

fact, vast numbers of new active materials, often called as organic semiconductors, have been developed and evaluated in various electronic devices.5 In the development of organic semiconductors, regardless of the molecular size, e.g., from small molecules to oligomers and polymers, there are many factors that should be taken into account for achieving high performances in the devices, which are charge carrier transport, carrier injection, light absorption, and charge separation, and so on. Chemical stability and durability of the materials under the ambient conditions are also important factors in terms of the practical application. In order to realize the desirable properties in these factors, one of the most important issues is to control the frontier orbitals of organic semiconductors, namely highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). For instance, the energy levels of HOMO (EHOMO) and LUMO (ELUMO) should be as close as the work function of the electrode for the efficient carrier (hole or electron) injection, and as low as possible for the ambient stability and durability of p- and n-channel OFETs, respectively (Figure 1a).6 In OPVs, on the other hand, the open circuit voltage (Voc) is generally proportional to Ediff, which is defined by the difference between the EHOMO of the donor material and ELUMO of the acceptor materials (Figure 1b).7 From the viewpoint of synthetic chemistry, on the other hand, recent evolution in the cross coupling chemistry,

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Figure 2. Th-A-Th type triads with the central π-electron deficient building units discussed.

Figure 1. EHOMO and ELUMO are important metrics in organic electronics: (a) determination of carrier type and ambient stability in OFET devices. Experimentally, EHOMO and ELUMO can be conveniently estimated from oxidation and reduction potentials determined by electrochemical measurements such as cyclic voltammetry (CV) and dif8 ferential pulse voltammetry (DPV). (b) Voc of OPVs can be proportional to Ediff.

such as Suzuki-Miyaura,9 Kosugi-Migita-Stille,10 and Kumada-Tamao11 reactions, with which the carbon-carbon σbonds between aryl groups can be effectively and reliably constructed, have significantly contributed to the materials development in the organic electronics field, especially in semiconducting polymers. In fact, a number of πbuilding blocks have been easily combined and utilized into versatile polymers. Together with the development of new building blocks, the advance in the cross coupling chemistry has made great success in developing the semiconducting polymers, leading to high field-effect mobility (> 1 cm2 V–1 s–1) in OFETs12 and high PCEs (> 8%) in OPVs.13 As one can freely design and synthesize polymers with such powerful synthetic tools, it becomes then critical to design the target polymers with appropriate EHOMOs and ELUMOs. In general they could be estimated by taking into account the EHOMOs and ELUMOs of individual building blocks in the backbone structures. Although this would afford decent estimation of the electronic structures of resulting polymers in most cases, such simple estimation does not match with the actual experimental results in some cases, like the isomeric naphthodithiophene (NDT) systems. In this review, we first show the “unconventional” behavior of the electronic structure in the NDT isomers that are electron rich building blocks (donor or D-units). We then

shift our focus on several representative electron deficient building blocks that are often employed in semiconducting polymers, so-called acceptor units or A-units, namely, 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (diketopyrrolopyrrole; DPP),14 benzo[c][1,2,5]thiadiazole (BTz),15 thiazolo[5,4-d]thiazole (TzTz),16 naphtho[1,2-c:5,6c']bis[1,2,5]thiadiazole (NTz),17 thieno[3,4-c]pyrrole-4,6dione (TPD),18 and 1,4,5,8-naphthalenedicarboximide (NDI)19 (Figure 2). The reasons why we focus on the Aunits are as follows. (i) The donor–acceptor (D–A) motif is the state-of-the-art methodology to tune the EHOMOs, ELUMOs, and HOMO-LUMO gaps (Egs) of the polymers. (ii) Since the energy levels of the D–A polymers are determined by the mixing of the EHOMO of the D-unit and ELUMO of the A-unit and the EHOMOs of the polymers are required to be relatively low, one might need to tune the Egs by tuning the ELUMO while preserving the EHOMO low. We therefore reevaluate the electronic structures of these A-units using their dithiophene derivatives, i.e., thiophene-(A-unit)-thiophene (Th-A-Th) triads, which is regarded as the model compound of D–A polymers. By comparing the electronic structures of the A units, the triads, and the corresponding oligothiophene compolymers, we discuss their nature as π-building blocks in terms of “inductive effect”, ability to supply or withdraw electrons through the σ-bonds, and “resonance effects”, ability to delocalize the π-system with neighboring building blocks.20 Electronic structure of naphthodithiophenes and their conjugated polymers In 2010 we have developed a straightforward synthesis of naththodithiophene isomers,21 and this chemistry allowed us to synthesize the NDT-based small molecule organic semiconductors and the NDT-based polymers (PNDTnBTs) (Figure 3).22 The electronic structure of the parent NDT isomers is largely governed by the shape of molecules, i.e., the conjugation manner of the four aromatic rings. Electrochem-

Figure 3. Isomeric naphthodithiophenes (NDTns) and their copolymers with bithiophene units (PNDTnBT).

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ical and optical evaluation of the parent isomers revealed that the linear-shaped NDT1 has high-lying EHOMO (–5.3 eV) and small Eg (3.1 eV) compared to the angular-shaped NDT3 with low-lying EHOMO (–5.8 eV) and large Eg (3.9 eV). These significant differences can be clearly explained by considering their isoelectronic hydrocarbons, naphthacene and chrysene, respectively. On the other hand, when these NDT units are integrated into the polymer main chain as a building block combined with a bithiophene co-monomer unit, PNDTn-BTs (Figure 3), it turned out that the order of the EHOMOs is indeed turned around compared to the parent NDT1 and NDT3 monomers: the experimentally-determined EHOMO of the NDT3-based polymer (–5.0 eV) was higher than that of the NDT1-based polymer (–5.1 eV) (Figure 4). Although this result was surprising to us at first glance, experimental evaluations and theoretical MO calculations on NDTnBTs, namely thiophene-NDTn-thiophene triads (model compounds of polymers), affords reasonable explanation (Figure 4).23 The extent of upwards shift of EHOMOs of NDTnBTs is markedly different between the two systems; a slight shift (0.1 eV) for NDT1BT, whereas a much pronounced shift (0.4 eV) for NDT3BT are observed in both the experimental24 and theoretical results. This difference can be understood by taking into account how the molecular orbitals of two units interact to each other: owing to relatively small energetic difference of HOMO between NDT3 (–5.66 eV, calculated) and thiophene (–6.6 eV, calculated), effective orbital mixing occurs to delocalize over the whole π-conjugation systems. In case of NDT1BT with much larger energetic difference between the two units, the HOMO coefficients mostly localize inside the NDT1 core, likely owing to the stabilization effect from peripheral 18π aromaticity of the NDT1 core. As a result, effective delocalization of HOMO onto the attached thiophene rings is characteristic for NDT3BT. Such localization/delocalization effects to the EHOMOs seem to be further pronounced in the actual polymers. As a result, the significantly large upward shift of EHOMO for PNDT3BT by

Figure 4. Electronic structures of the parent, dithiophene derivatives, and bithiophene copolymers of NDT1 and NDT3. The values in parentheses are calculated ones with DFT methods at B3LYP/6-31G(d) level.

0.8 eV compared to the parent NDT3 building block is rationalized. These results strongly imply that one must pay attention on both “static” and “dynamic” electronic nature of π-building unit, where the static nature is expressed as the EHOMO and ELUMO of given building unit, whereas the dynamic nature is the tendency to delocalize π-conjugation with neighboring π-units. These two natures can be likened to the inductive and resonance effects in organic chemistry, respectively.20 The important lessons taken from the study on the NDT system are (i) the electronic nature of given πbuilding block should be described in terms of the EHOMO and ELUMO themselves (“inductive effect”) and the effectiveness of π-delocalization with neighboring groups (“resonance effect”). (ii) The triad type structure with two additional thiophene rings, i.e., NDTnBT, can afford useful information on the electronic natures by evaluating the EHOMO and ELUMO with electrochemical measurements, which is supplemented by the theoretical MO calculations that give the geometries of HOMO and LUMO to estimate effectiveness of π-delocalization. With these viewpoints, we expect that revaluation of various πbuilding blocks, especially the A-units, which are regarded as vital building blocks for developing high performance semiconducting polymers with a so-called “DonorAcceptor” motif, would provide an important perspective to understand their electronic natures and hence to control the energy level of frontier orbitals, in particularly ELUMO. In the following sections, we discuss the electronic structure of six A-units by evaluating their Th-A-Th triads together with the parent A-units and corresponding copoymers. EHOMOs and ELUMOs of A-units and Th-A-Th triads Since the several A-units without substituents can not be accessible owing to the lack of suitable method for the synthesis, the electronic structures of the six A-units were estimated by the theoretical MO calculations with the DFT methods at B3LYP/6-31G(d) level (Figure 5, Table 1). It is interesting to note that the calculated EHOMOs and ELUMOs of the units relatively are widespread from –5.52 to –7.03 eV for HOMO, and from –1.50 to –3.40 eV for LUMO, indicating that the “inductive effect” of the Aunits vary significantly. The large differences, especially in the ELUMOs, should be reflected their electronic nature as discussed below. On the other hand, the electronic structures of the ThA-Th triads were evaluated by both the DFT-MO calculations and cyclic voltammograms (CVs). Figure 6 shows the CVs of Th-A-Th triads, namely, DPP-2T,25 BTz-2T,17b TzTz-2T,26 NTz-2T,17b TPD-2T,18 and NDI-2T27 measured under identical conditions.28 Note that all these compounds show both oxidation and reduction waves in a typical electrochemical window, and thus their EHOMO and ELUMO can be directly estimated experimentally, which is an additional merit of using the triad structure in evaluating the electronic nature of the given π-units. The experimental and theoretical EHOMOs of the triads are similar to

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each other, whereas the ELUMOs are quite different between the experimental and theoretical ones. However, the qualitative trends of ELUMOs are mostly similar to those experimentally determined ones. Comparison of the theoretical EHOMOs and ELUMOs of the parent A-units and triads shows several interesting aspects: first, upward shift of EHOMO is very large for all the triads. This is quite natural to consider that the electron rich thiophenes act as the D unit, and thus, the HOMOs of the triads tend to be largely influenced by the two thiophene subunits. In fact, the HOMOs of the triads tend to delocalize over the whole π-conjugation system including the thiophene units. However, the extent of upward shift (∆EHOMO1 in Table 1) is somewhat dependent on the Aunits incorporated. Compared to significantly large ∆EHOMO1 for the TPD system (1.7 eV) and moderately large shift ( > 1.0 eV) for the BTz, TzTz, NTz, and NDI systems, the ∆EHOMO1 for the DPP is rather small (0.55 eV). This

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can be qualitatively understood by different efficiency in the HOMO mixing of the A-unit with the thiophene units (EHOMO: –6.33 eV calculated by the same protocol). For the DPP unit with relatively high-lying EHOMO (–5.52 eV), perturbation from two thiophene units are rather limited compared to other system with low-lying EHOMOs (–6.45 ~ –7.30 eV). On the contrary, shift of ELUMO (∆ELUMO1) are relatively small compared to ∆EHOMO1. However, the TzTz and TPD units with high-lying ELUMO show large downward shift (∆ELUMO1: 0.68 and 0.41 eV, respectively), whereas ∆ELUMO1 for the naphthalene-based NTz- and NDI-units with extended π-system show negligible ∆ELUMO1 by addition of two thiophenes.

Figure 5. Electronic structures of the A-unit, triad, and co-polymers of DPP (a), BTz (b), TzTz (c), NTz (d), TPD (e), and NDI (f). The colored values in parentheses are calculated with DFT methods at B3LYP/6-31G(d) level, which corresponds to the left scale in each color. EHOMOs and ELUMOs of the triads estimated from the CVs (Figure 6), and EHOMOs of polymers are estimated from reported values of oxidation potentials, whereas ELUMOs of polymers were obtained from their EHOMOs and optical energy gaps (see Table 1). The right scale in black corresponds to the experimentally determined EHOMOs and ELUMOs (black). Definitions of ∆EHOMO1/∆ELUMO1 and ∆EHOMO2/∆ELUMO2 are shown only in Figure 5(b).

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Table 1. EHOMO and ELUMO of the A-units, triads, and co-polymers. A unit

triads

polymer

A unit -> triads

triads-> polymer

ref.

a EHOMO

a ELUMO

a,b EHOMO

a,c ELUMO /

a EHOMO

d ELUMO

∆EHOMO1

∆ELUMO1

∆EHOMO2

∆ELUMO2

/ eV

/eV

/ eV

eV

/ eV

/ eV

/ eV

/ eV

/ eV

/ eV

-5.18

-3.25

5.2

–4.0

~0

~0.75

25

(-5.52)

(-2.17)

(-4.97)

(-2.51)

(0.55)

(0.34)

-5.46

-3.23

~0.4

~0.2

17b

(-6.61)

(-2.31)

(-5.35)

(-2.61)

(1.26)

(0.30)

-5.53

-2.66

~0.4

~0.65

26

(-5.46)

(-2.18)

-5.54

-3.44

~0.4

~0.15

17b

(-2.92)

~0.2

~0.8

18

~0.7

~0.1

29

DPP BTz TzTz (-6.69)

(-1.50)

NTz (-6.45)

(-2.85)

(-5.44) -5.68

-2.91

(-7.30)

(-1.78)

(-5.59)

(-2.19)

-6.01

-3.82

(-5.99)

(-3.35)

TPD NDI (-7.03)

(-3.40)

–5.07

e

–3.42

–5.1

–3.3

–5.16

–3.6

(1.23)

–5.47 –5.36

e

f

(0.68)

(1.01)

(0.07)

(1.71)

(0.41)

(1.02)

(–0.05)

–3.71 d

–3.91

a

f

b

Values in the parentheses are theoretically calculated EHOMO and ELUMO by the DFT method (B3LYP/6-31(d) level). EHOMO = – + c d 4.80 – Eox(onset). Eox(onset) was electrochemically determined vs Fc/Fc . ELUMO = –4.80 – Ered(onset). EHOMO = Eg(opt) + ELUMO. e ELUMO = Eg(opt) + EHOMO. Eg(opt) was determined from absorption onset. ∆EHOMO1 and ∆ELUMO1 are differences between calculatf ed EHOMOs and ELUMOs of the A-units and triads, respectively. ∆EHOMO2 and ∆ELUMO2 are differences between EHOMOs and ELUMOs of the triads and polymers experimentally determined.

These different perturbations from two thiophene units

Figure 6. Cyclic voltammograms (CVs) of Th-A-Th triads. Among triads, BTz-2T, TzTz-2T, and NTz-2T have an alkyl group on each thiophene attached, which is designated with “(R)” after the compound abbreviations. CVs were recorded in dichloromethane solution containing tetrabutylammonium hexafluorophosphate as supporting electrolyte at a scan rate of 100 mV/s. Counter and working electrodes were made of Pt, and the reference electrode was Ag/AgCl. All + the potentials were calibrated with the standard Fc/Fc redox couple (E1/2 = +0.47 V measured under identical conditions).

in the triad system indicate that estimation of EHOMO and ELUMO of π-system, even for rather simple Th-A-Th triads, from the individual building blocks is not that simple, and the extent of ∆EHOMO1 and ∆ELUMO1 are largely dependent on the electronic structure of π-building blocks involved. From the viewpoint of structural chemistry, the six A-units can be classified into four categories. The first class, DPP, can be described as a non-aromatic 1,3butadiene structure being bridged with amide moieties, where the 1,3-butadiene substructure would help the effective conjugation between two thiophene subunits resulting in small Eg with quite high-lying EHOMO and lowlying ELUMO. In addition to the resonance effect, the strong electron withdrawing nature of two amide units further stabilizes the ELUMO, resulting in impressively low lying ELUMO of the triad. The second class incudes BTz and NTz, which can be expressed as 1,2,5-thiadiazole-fused obenzoquinoidal structure(s). Owing to the electron deficiency of thiadiazole ring(s) together with the nonaromatic quinoidal structure(s), these units also have lowlying ELUMO (strong “inductive effect”). On the other hand, three rest A-units are based on aromatic systems. In the TzTz unit (the third class), there are two electron deficient pyridine-type nitrogen atoms in the aromatic ring, but its electron withdrawing nature is not that strong compared to amide or imide functionality, resulting in high-lying ELUMO for the TzTz unit. The last two A-units, TPD and NDI, can be described as aromatic carboxyl imides (forth class), and their ELUMOs seem to be scaled by the number of the imide moiety: with two imide moieties with strong electron withdrawing property, NDI2T has

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Figure 7. Molecular structures of D-A copolymers consisting of the A unit and oligothiophene.

the lowest ELUMO and EHOMO among the six triads (strong “inductive effect”). Although it is not easy to compare all the A-units in one scheme to understand the differences of their ELUMOs, the present classification may afford a good overview to correlate the molecular structures and ELUMO as well as EHOMO both for the A-units and triads. The next step is to examine the electronic structures of the corresponding copolymers, focusing on the resonance effects of the A-units. EHOMOs and ELUMOs of D-A type polymers with oligothiophene co-monomers Figure 7 depicts the molecular structures of D-A type polymers consisting of the A-units with oligothiophene comonomers.17b,18,25,26,29 These simple copolymers are expected to be suitable structures for evaluating the electronic effect on the EHOMO and ELUMO caused by polymerization in comparison with the Th-A-Th triads. Table 1 summarizes their EHOMO and ELUMO taken from references already reported.30 Comparing the polymers’ EHOMOs and ELUMOs with those of the corresponding triads, almost all the polymers have high-lying EHOMOs and low-lying ELUMOs, indicating that polymerization causes effective π-extension. However, the extent of upward/downward shift of EHOMO/ELUMO (∆EHOMO2 and ∆ELUMO2 in Table 1) is not equal to each other. Although ∆EHOMO2s are, in general, 0.2–0.4 eV, almost negligible ∆EHOMO2 is observed for the DPP case. This can be explained as follows: the HOMO of the DPP triad mostly localized at the central part of the molecule, i.e., the DPP unit, and does not seep out to the thiophene units, compared to other triads (Figure 5a), which brings about less effective π-extension in the polymer backbone. Another exception is a significantly large upward shift of HOMO in the NDI system (∆EHOMO2 ~0.7 eV). In this case, the distribution of HOMO coefficients in the NDI triad is sharply contrasted to that of the DPP triad. In the NDI system, the HOMO coefficients are almost localized on

the peripheral thiophene rings, which allows effective conjugation with neighboring groups attached at the thiophene α-positions. Similar effective delocalization of HOMO of an NDI-based molecule in the lateral molecular direction has been reported for core-expanded NDI derivatives with aromatic rings.31 Interestingly, very large ∆ELUMO2s (0.6 ~ 0.8 eV) are observed for DPP, TzTz, and TPD, whereas those of BTz, NTz, and NDI are rather smaller (0.1 ~ 0.2 eV). This obvious difference can be qualitatively explained by the LUMO geometry of each triad. As for the former systems with large ∆ELUMO2, there exists large LUMO density at the thiophene α-positions, indicating that the LUMO tends to effectively delocalize over the whole polymer backbone (DPP2T, TzTz2T, and TPD2T in Figure 5), which can be referred as larger “resonance effect” system. On the other hand, in the latter systems with the small ∆ELUMO2, the LUMO coefficients of the triads heavily localize on the central A-units and only slightly seep out to the attached thiophene units, which likely cause the localized LUMO on the A-units also in the polymers, which can be expressed as small “resonance effect” system. Thus, the resonance effect of A-units can qualitatively be evaluated by the LUMO coefficients distribution in the triads (Figure 5) and ∆ELUMO2 (Table 1). Conclusion In this review, we have discussed electronic structures of various electron deficient building blocks utilized in the development of semiconducting polymers, by comparing EHOMO/ELUMO of A-units, thiophene-(A-unit)-thiophene triads, and corresponding polymers. As we discussed above, the evaluation of A-units with the triad structure can manifest two different electronic nature qualitatively, inductive and resonance effects, that strongly affect the LUMO energy levels of resulting polymers. This would give us important insights into how the two different effects influence on the electronic structure of the polymers, which may be a strong guidance for us to control

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when designing polymers with desired electronic structure. Although there are many other factors that govern performances of semiconducting polymer-based devices, such as molecular weight, solubility, crystallinity, and orientation of the backbone, the basic electronic structure and the energy levels of frontier orbitals are one of the most intrinsic properties of electronic materials. We thus hope that the present measure for the evaluation of building blocks using the triad structure could contribute to the design of further superior π-building blocks and their oligomers and polymers.

REFERENCES 1.

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AUTHOR INFORMATION 5.

Corresponding Author * [email protected], [email protected]

Kazuo Takimiya received his Ph. D. from Hiroshima University in 1994. Thereafter, he joined Professor T. Otsubo’s group at Hiroshima University where he carried out research on organic conductors/superconductors. He promoted to an associate professor (2003) and then a full professor (2007) at Hiroshima University. In 2013, his group moved to RIKEN Center for Emergent Matter Science (CEMS). His research interests include the syntheses, characterization, and application of organic semiconductors.

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7.

8.

Itaru Osaka received Ph. D from the University of Tsukuba in 2002. After a four-year research at Fujifilm, he joined Professor R. D. McCullough’s group at Carnegie Mellon University as a postdoctoral fellow. He then joined Professor Takimiya’s group at Hiroshima University as an assistant professor in 2009, and moved to RIKEN CEMS as a senior research scientist in 2013. His research interests include the design and synthesis of conjugated polymers for organic electronics.

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ACKNOWLEDGMENT This work was financially supported by Grants-in-Aid for Scientific Research (Nos. 23245041, 24685030) from MEXT, Japan and The Strategic Promotion of Innovative Research and Development from the Japan Science and Technology Agency. The authors thank Mr. Toru Abe, Mr. Masafumi Shimawaki, Mr. Masahiko Saito, and Mr. Akira Tamoto for sample preparations. One of the authors (M.N.) is grateful for the Postdoctoral Fellowship of Japan Society for the Promotion of Science (JSPS).

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D.; Fan, J.; Seifter, J.; Lim, B.; Hufschmid, R.; Heeger, A. J.; Wudl, F. J. Am. Chem. Soc. 2011, 133, 20799–20807. Osaka, I.; Zhang, R.; Sauvé, G.; Smilgies, D. M.; Kowalewski, T.; McCullough, R. D. J. Am. Chem. Soc. 2009, 131, 2521– 2529. (a) Ahmed, E.; Ren, G.; Kim, F. S.; Hollenbeck, E. C.; Jenekhe, S. A. Chem. Mater. 2011, 23, 4563–4577. (b) Gu, C.; Hu, W.; Yao, J.; Fu, H. Chem. Mater. 2013, 25, 2178–2183. Among these triads, BTz-2T, TzTz-2T, and NTz-2T have an alkyl group on each thiophene ring attached. Although the inductive electron donating effect from the alkyl groups can cause upward shifts of both EHOMOs and ELUMOs, we assume that the extent of shits are rather small, up to 0.05 eV per one alkyl group, based on experimental results and theoretical calculations, and thus it should not affect the overall trend of the present discussion. (a) Chen, Z.; Zheng, Y.; Yan, H.; Facchetti, A. J. Am. Chem. Soc. 2009, 131, 8–9. (b) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler, M.; Facchetti, A. Nature 2009, 457, 679–686. Experimental estimation of EHOMO and/or ELUMO of polymers from their cyclic voltammogram sometimes causes problematic results owing to difficulty to obtain reversible redox waves, especially for the reduction wave. The present discussion therefore is based on the polymers EHOMOs estimated from their CVs and ELUMOs calculated from their EHOMOs and optical Egs or vice versa. See footnote of Table 1. Suraru, S.-L.; Zschieschang, U.; Klauk, H.; Wurthner, F. Chem. Commun. 2011, 47, 11504–11506.

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