Effect of substituents on charge carrier dynamics in thiophene oligomers

Anion and cation radicals of substituted thiophene oligomers were generated and isolated from respective solutions at low temperature by gamma irradiation at a total dose of 4 kGy. The absorption spectra of these radicals were investigated using low temperature matrix isolation technique. Absorption bands in two typical regions: visible and infrared (IR) were observed experimentally for both anion (HOMO → SOMO and SOMO → LUMO) and cation (SOMO → LUMO and HOMO → SOMO) radicals of all molecules. The values of oscillator strength (f) decreases with the increase in the number of phosphate bridges in thiophene molecule for both the anion and cation radicals, suggesting the breaking conjugation by the dihedral angle of thiophene rings at centre of the molecule. The optical transitions in substituted thiophene oligomers were also investigated on the basis of a theoretical approach using ZINDO CI at B3LYP/6-31G(d) DFT geometry. The most stable backbone geometries of the anion and cation radicals are the planner ones in spite of twisted helical structures calculated for the steady-state molecules as the plausible structures. Transformation from twisted (helical) to planner structure plays a crucial role in the determination of energy levels of the molecules bearing negative and positive charge carriers. Incomplete transformation due to the low temperature matrix effects causes considerable decrease in the value of f.