Throwing light on dark states of α-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 α-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 T(1) and T(2) 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 α-oligothiophenes investigated in this work, the T(2) state is situated below S(1). In the gas phase, the S(1) state energies lie higher than in non-polar solution (0.2 to 0.4 eV). The geometry optimizations show that the S(0) 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 S(1) to T(2), followed by internal conversion to T(1). Our theoretical calculations predict that in 6T a doubly excited state becomes nearly isoenergetic to S(1). This state is not observed by PD-PES, which is explained by the analysis of the calculated contributing electron configurations.