Oligothienoacenes versus oligothiophenes: impact of ring fusion on the optical properties

Abstract

The impact of backbone rigidity on the optical properties of thiophene-based compounds is studied by analyzing in detail the geometrical, electronic, optical and vibronic features of a family of oligothienoacenes (nnTAs) in comparison to non-fused α-oligothiophenes (nnTs) by means of quantum-chemical calculations. Ring fusion in nnTAs provokes a greater conjugation in the ground state. However, the change in the bond length alternation upon electronic excitation is very similar in both systems, which is also reflected in a similar evolution of the first optical transition energy with increasing oligomer size. Larger transition energies in nnTAsvs.nnTs arise from an electronic effect rather than from a structural one. nnTAs present a normal mode predicted at ca. 500 cm⁻¹ which displays significantly higher Franck–Condon activity compared to nnTs and which leads to pronounced differences in the optical spectra. Due to the rigid structure of nnTAs, persistent mirror symmetry of absorption and emission is observed, very different to nnTs.