Experimental methods, characterization data and 1 H NMR and 13 C NMR spectra of monomers for all polymers are described below.Materials. Fluorene, 3-bromothiophene, 3-thiophenecarboxaldehyde and malononitrile were obtained from Aldrich Chemical Co. 9,9-Dihexylfluorene. 1 To a solution of 5.0 gm (30.1 mmol) of fluorene in THF (60 ml) at -78 o C was added dropwise, 25 ml (62.5 mmol) of n-butyllithium (2.5 M in hexane).The mixture was stirred at -78 o C for 1 h, and 11.5 gm (70 mmol) of 1-bromohexane in THF (15 ml) was added dropwise to the mixture. The solution was allowed to warm to room temperature and was stirred for overnight. The mixture was poured into water and extracted with ether (3 X 30 ml). The organic layer was washed with brine solution and dried over anhydrous MgSO 4 . The solvent was removed by reduced

There is a growing interest in developing low-band gap conjugated polymers via synthesis of copolymers containing alternating units of different pi-electron-donating/accepting capabilities. In this study, electronic and optical properties of conjugated copolymers containing fluorene and thiophene/cyclopentadithiophene derivatives are determined using density function theory and semiempirical ZINDO calculations. A remarkable linear correlation is found between the amount of charge transfer between the donor-acceptor pair, the band gap, the bandwidth, and the oscillator strength of S(0)-->S(1) electronic transition (ground state to first excited state) of the copolymers. Strong pi-electron withdrawing substituents, such as dicyanoethenyl and carbonyl groups, on the thiophene moiety effectively reduce the band gap of the copolymers. However, the reduction of band gap is frequently accompanied by a linear reduction in bandwidths and in the oscillator strength of S(0)-->S(1) transition. For very strong pi-electron withdrawing thiophene derivatives, the occurrence of maximum oscillator strength may even shift from S(0)-->S(1) to S(0)-->S(n>1) (ground state to a higher excited state), giving a blue shift in maxima absorption peak and a red shoulder in the UV-vis spectra as reported in recent experimental measurements. Therefore, the achievement of low band gap for conjugated polymers with alternating arrangement of pi-electron-donating/accepting moieties may be achieved at a cost of lowering electron mobility and optical efficiency and sometimes a blue-shift in the major optical (UV-vis) absorption.

A series of low band gap, highly soluble alternating conjugated copolymers, comprised of 11,11,12,12-tetrahexylindenofluorene and thiophene derivatives (P1-P4), were synthesized via Pd-catalyzed Suzuki coupling reaction with very good yields. Described here are the synthesis, thermal, optical, and electrochemical properties of these new copolymers as potential new active materials for electronic and optoelectronic device applications. P1 and P2 have electron donating non-psubstituents with ethylenedioxy and propylenedioxy bridging the 3,3 positions of the cyclopentadithiophene groups; whereas P3 and P4 have electron withdrawing psubstituents (carbonyl and pyrazine groups on P3 and P4, respectively). For the main absorptions in UV-vis spectrum, P1 and P2 displayed more red absorptions in comparison with P3 and P4. Nevertheless, much suppressed quantum yields are exhibited by P3 and P4. The behaviors of P3 can be attributed to the significant charge transfer interactions between the p-substituents and the conjugated polymer backbone that leads to a less allowed optical transition between the ground and the lowest excited state. For P4, the weak fluoresence might associate with energy transfer from indenofluorene to the low band gap thiophene-pyrazinethiophene-thiophene (T-PT-T) unit. In comparison with the corresponding polymers containing fluorene instead of indenofluorene, the use of indenofluorene exhibited mixed effects on the optical properties and improved solubility. V

Varying composition of π-donor/acceptor moieties has been considered as an effective strategy for fine-tuning of the electronic properties of D-A conjugated copolymers. In this study, the change of optoelectronic properties with the change of donor/acceptor ratios is investigated on the basis of first-principles density functional calculations. Copolymers containing moieties of similar π-electron donating and/or accepting capabilities, e.g., thiophene (T)-methoxythiophene (OT), exhibit a linear dependence of electronic properties (especially, HOMO/LUMO, band gap, and bandwidth) on the D/A content. In contrast, for strong D/A contrast systems, e.g., thiophene (T)-thienopyrazine (TP), the electronic properties vary nonlinearly with D/A compositions. However, when the block size of one parent monomer in a strong D/A contrast system is fixed, the variation of electronic properties shows a remarkable linear correlation against D/A compositions. We found that the deviation of electronic properties from a linear composition dependence is dominated by the strength of orbital interactions between D and A. Weak orbital interactions between D and A moieties tend to lead to a nonlinear composition dependence. Our results provide useful insights for band gap tuning through the adjustment of D/A compositions in D-A conjugated copolymers.

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