Singlet Fission in Quinoidal Oligothiophenes

The electronic properties of quinoidal oligithiophenes make them interesting for applications in semiconductor technology. Because of their very large singlet–triplet splitting, they are promising candidates for singlet fission (SF), a process in which an initially excited singlet state is converted into two triplet excitons. Thus, the efficiency of solar cells could be increased to overcome the Shockley-Queisser limit. Here, we investigate the ability of a quinoidal bithiophene to undergo SF in solution. We calculated the ground state and low-lying excited states using a combined density functional theory and multireference configuration interaction approach including dispersion corrections. Potential energy curves along normal mode displacements were computed to detect avoided crossings between the initially excited bright singlet state and a dark doubly excited state which can be interpreted as a triplet pair overall coupled to a singlet 1 (TT). The studied quinoidal bithiophene meets the energetic requirement for SF. A path enabling intramolecular SF could not be found. In contrast, we were able to identify two vibrational modes relevant for an intermolecular SF process in the slip-stacked dimer: A promoting coordinate that couples a bright singlet state with the 1 (TT) state and a separating coordinate that localizes the triplet states on the respective monomers. These results elucidate the mechanism underlying the formation of a triplet pair and the separation of the triplet excitons after initial photoexcitation of the bright singlet state.