Optical and vibrational properties of conjugated polymeric heterostructures

We present an optical and vibrational analysis of two copolymers consisting of alternating thienylene and phenylene blocks linked by azomethines moieties which might represent models of a one-dimensional superlattice. Both the pristine polymers and their GaCl3 complexes are studied. Their electronic structure is investigated with absorption-emission spectra, preresonant and resonant Raman scattering. Though the backbone conjugation is not interrupted by the phenylene moieties, resonant Raman modes of the thienylene and phenylene blocks are selectively enhanced by exciting in the visible and in the UV regions, respectively. Radiative recombination occurs within the thienylenic blocks which act as efficient trapping centers for the photoexcitations. While the thienylene ring vibrations feel the presence of the potential barriers and are confined within the thienylenic blocks of the copolymers, phenylene vibrations are characteristic of a phenylenic chain segment longer than the biphenylene block. By GaCl3 complexation, copolymers with lower energy gap are obtained. A weakening of the electronic density on the thienylene blocks is proposed to account for their vibrational and electronic properties