Nonorthogonal Configuration Interaction for Singlet Fission: Beyond the Dimer

Non-orthogonal configuration interaction with fragment calculations are presented for a number of compounds that show singlet fission properties: (i) four perylene-diimide derivatives, (ii) crystalline pentacene and its (B,N)-substituted variant, and (iii) a regular and a distorted stack of three indolonaphthyridine molecules. The electronic couplings between the singlet excitonic states (S₁) and the singlet-coupled double triplet (T₁T₁), the so-called singlet fission coupling, were computed from ensembles with two and three molecules, and except for some small deviations when charge transfer states were included, results are virtually the same. Ensembles of three molecules were used to study the mechanisms of triplet separation, double triplet diffusion, and singlet and triplet exciton diffusion. The calculations show that apart from the standard mechanism for the generation of two uncoupled triplet states (S₁ → T₁T₁ → T₁...T₁), there are two other possible pathways: the direct generation from the singlet excitonic state (S₁ → T₁...T₁) and the process in which the excitonic state evolves in a superposition of T₁T₁ and T₁...T₁ states. The electronic coupling for triplet diffusion is in general much smaller than for singlet diffusion.