Synthesis and properties of benzo[c]-, pyrrolo[3,4-c]-, and thieno[3,4-c]-pyrrole-4,6-dione copolymers

New π-conjugated alternating donor-acceptor polymers, BPD-DTT and PPD-DTT, based respectively on benzo[c]pyrrole-4,6-dione (BPD) and pyrrolo[3,4-c]pyrrole-4,6-dione (PPD) acceptor structures with the 2,5-dithienylthieno[3,2-b]thiophene (DTT) donor structure were synthesized via Stille cross-coupling polymerization to study their physical and optoelectronic properties. For comparison, an analogous polymer, TPD-DTT, derived from thieno[3,4-c]pyrrole-4,6-dione (TPD) was also prepared by the same approach. These polymers displayed reasonably good solubility in common organic solvents (e.g., THF, toluene, etc.), and were thermally stable to about 350 °C in nitrogen or air. Both BPD-DTT and PPD-DTT showed blue shifts in their UV-vis absorptions and photoluminescence compared with those of TPD-DTT, suggesting that both possess lower degrees of π-delocalization than TPD-DTT, and thus shorter effective π-conjugation lengths and higher air stability as evidenced from their lower HOMO energy levels. Optical band gaps of 1.61 eV, 1.82 eV, and 1.73 eV and HOMO levels of -5.06 eV, -5.32 eV, and -5.20 eV were estimated for TPD-DTT, BPD-DTT and PPD-DTT respectively from spectral and electrochemical measurements. Electrical characterization showed that all three polymers behaved predominantly as p-type semiconductors in organic field-effect transistor and bulk-heterojunction organic solar cell devices.