Modulation of electronic properties in neutral and oxidized oligothiophenes substituted with conjugated polyaromatic...

The structural and electronic properties of neutral and oxidized terthiophenes substituted with polyaromatic systems have been investigated using a combination of both Raman and electronic absorption spectroscopy in conjunction with density functional theory calculations. Naphthylethenyl terthiophene exhibits structural and electronic properties, in both the neutral and oxidized species, that are dominated by the terthiophene backbone, in a manner similar to that previously reported for phenylethenylterthiophene. Anthracenylethenyl terthiophene, on the other hand, displays properties that are dominated by the anthracene group. Unlike both phenylethenyl and naphthylethenyl terthiophene, which have electronic absorption spectra dominated by transitions between molecular orbitals that are delocalized throughout the molecules, the absorption spectrum of anthracenylethenyl terthiophene consists of a simple addition of the absorption bands of the separate terthiophene and anthracenylethene chromophores. This is the result of a spatial partitioning of its molecular orbitals that effectively electronically decouples the anthracene and terthiophene moieties. Upon oxidation, the naphthylethenylterthiophene sigma-dimerizes to form sexithiophene charged species and spectral signatures of the sexithiophene backbone are evident in both the electronic absorption and resonance Raman spectra. In contrast, these signatures are absent in the corresponding spectra of the oxidized anthracenylethenylterthiophene, suggesting that the anthracene group is the primary site of the structural changes induced by oxidation.