Substituent Effect on the Optoelectronic Properties of Alternating Fluorene−Cyclopentadithiophene Copolymers.

A novel series of soluble alternating conjugated copolymers, comprised of 9,9-dihexylfluorene and cyclopentadithiophenes ( P1− P5), were synthesized via Pd-catalyzed Suzuki coupling reaction in good yields. The UV−vis absorption spectra, fluorescence spectra, fluorescence quantum yields, and cyclic voltammograms of P1− P5are also reported. The P2and P3with electron-donating non-π-substituents (ethylenedioxy and propylenedioxy bridges the 3,3-positions of the thiophene groups) display high fluorescence quantum yields and red-shifted absorption as compared with nonsubstituted P1. However, the P4and P5are weakly fluorescent and exhibit blue-shifted absorption which are due to the presence of electron-withdrawing π-substituents (carbonyl and dicyanoethenyl). The behavior of P4and P5can be attributed to the significant electronic coupling between the π-substituents and the conjugated polymer backbone that leads to a less allowed optical transition between the ground and the lowest excited state, resembling the metaconjugation effect in phenylene-derived conjugated π-systems. This argument is supported by molecular orbital distribution calculations. [ABSTRACT FROM AUTHOR]