Computational Modelling of Singlet Fission: From the static picture to fission dynamics

Singlet fission, the conversion of an optically singlet excited state into a pair of two triplet excitons, which are coupled into an overall spin singlet, is one promising strategy to increase the efficiency of single junction solar cells beyond the theoretical limit. The renewed focus of this process driven by fundamental interests and its applications has become prominent. Experimental measurements and theoretical studies on the detailed mechanism through which the singlet fission occurs in pentacene, tetracene, and other polyacenes derivatives have been done intensively. Nevertheless, singlet fission is still a rare and not fully understood phenomenon in other chromophores. The aim of the research presented here is to understand the singlet fission mechanism and dynamics of recently proposed potential singlet fission chromophores, namely the bis(inner salt) of 2,5-dihydroxy-1,4-dimethyl-pyrazinium (DHDMPY) and 2,5-bis(fluorene-9-ylidene)-2,5-dihydrothiophene (ThBF), with the aid of theoretical chemistry and computational modelling. First, static quantum chemical calculations based on the nonorthogonal configuration interaction approach were performed to calculate effective electronic couplings of pairs of DHDMPY molecules, followed by a preliminary exploration of its crystal structure and dynamics. The resulting couplings showed that DHDMPY is indeed a potential singlet fission chromophore. Second, nonadiabatic dynamics simulations were performed based on the trajectory surface hopping approach with ‘on-the-fly’ calculations of electronic wave functions and energies by the semiempirical floating occupation molecular orbital—configuration interaction method. The aim was to investigate the fission dynamics of a pair of ThBF molecules embedded in their crystal environment and to predict the excited state lifetimes and the singlet fission quantum yield. It turned out that the process is fast, being almost complete in few picoseconds, and the quantum yield is very high.