Conformational profile, energy barriers and optical properties of quinquethiophene-S, S-dioxides

Theoretical calculations, dynamic NMR experiments and absorption and photoluminescence data in solution are reported for a series of quinquethiophene S,S-dioxides substituted with alkyl groups of variable size and steric hindrance. Ab initio B3LYP/6-31G* and force field MM3 theoretical calculations show that the energy barriers for rotation around the inter-ring C-C bonds amount to a few kcal/mol even in the presence of very bulky substituents such as the cyclohexyl group. Dynamic NMR data were in agreement with the results of theoretical calculations. It was found that changing the steric hindrance of the substituents leaves the emission and photoluminesce properties unaltered. However, the photoluminesce intensities and wavelengths of all compounds were found to be very sensitive to solvent variations.