Throwing light on dark states of α-oligothiophenes of chain lengths 2 to 6: radical anionphotoelectron spectroscopy and excited-state theory

Abstract

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 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 α-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.