Helically annelated and cross-conjugated β-oligothiophenes:: A Fourier transform Raman spectroscopic and quantum chemical density functional theory study

This article describes Fourier transform Raman/infrared spectroscopic studies to determine the conjugational properties of novel beta-oligothiophenes, in which thiophene rings are helically annelated, forming a cross-conjugated pi-system. These helicenes may be viewed as fragments of the unprecedented carbon-sulfur (C2S)(n) helix, having a sulfur-rich molecular periphery. The B3LYP/6-31G** vibrational analysis of the experimental spectroscopic data for beta-oligothiophenes with three, seven, and eleven thiophene rings indicates selective enhancement of a limited number of Raman scatterings. In particular, the enhancement of the Raman-active skeletal nu(CC) stretching modes in the 1400-1300-cm(-1) region is related to the occurrence of a vibronic coupling between the highest-occupied molecular orbital and lowest-unoccupied molecular orbital frontier molecular orbitals. Decreased dispersion of enhanced Raman scatterings and greatly increased near degeneracy of the highest occupied MOs with the increasing number of annelated thiophene rings suggest significant electron localization in beta-oligothiophenes, similar to that in cross-conjugated pi-systems.