Energetics and optimal molecular packing for singlet fission in BN-doped perylenes: electronic adiabatic state basis screening.

Singlet fission has the potential to increase the efficiency of photovoltaic devices, but the design of suitable chromophores is notoriously difficult. Both the electronic properties of the monomer and the packing motif in the crystal have a big impact on the singlet fission efficiency. Using perylene as an example, it is shown that doping with boron and nitrogen not only helps to align the energy levels but also shifts the stacking position that is optimal for singlet fission. Among all perylene derivatives doped with one or two BN groups, we identify the most suitable isomer for singlet fission with the help of TD-DFT and CASPT2 calculations. The optimal relative disposition of the two monomer units in a cofacially stacked homodimer is explored using two semiempirical models for the singlet fission rate: The first one is the well-known diabatic frontier orbital model, while the second treats singlet fission as a non-adiabatic transition and approximates the rate as the length squared of the non-adiabatic coupling vector between eigenfunctions of the diabatic Hamiltonian.