Throwing light on dark states of [small alpha]-oligothiophenes of chain lengths 2 to 6: radical anion photoelectron spectroscopy and excited-state theory

In this work, we apply photodetachment photoelectron spectroscopy (PD-PES) on radical anions to access the lowest excited electronic states of neutral [small alpha]-oligothiophenes nT (n = 2-6, where n denotes the number of thiophene rings) in the gas phase. Besides electron affinities, the spectra provide the energies of the T1 and T2 states which are otherwise difficult to investigate in neutral molecules due to spin selection rules. The assignment of the spectra is assisted by quantum chemical calculations using a combined density functional theory and multi-reference configuration interaction approach. For all [small alpha]-oligothiophenes investigated in this work, the T2 state is situated below S1. In the gas phase, the S1 state energies lie higher than in non-polar solution (0.2 to 0.4 eV). The geometry optimizations show that the S0 state and especially the excited states gain planarity with increasing chain length. A non-planar structure or out-of-plane vibrational activity is needed to allow an efficient intersystem crossing (ISC) dynamics from S1 to T2, followed by internal conversion to T1. Our theoretical calculations predict that in 6T a doubly excited state becomes nearly isoenergetic to S1. This state is not observed by PD-PES, which is explained by the analysis of the calculated contributing electron configurations.