Designing an Efficient Singlet Fission Material with B-N Substitution in Pyrene: A Model Exact Study.

The electronic structure of boron (B)-nitrogen (N)-substituted pyrene molecules is the center of attraction in designing an efficient intermolecular singlet fission (x-SF) material. Thermodynamic energy criteria required for x-SF are obtained by captodative substitution with B and N in pristine pyrene to increase the lowest singlet-triplet energy gap. We computed low-lying excited states of BN-embedded pyrene molecules by exactly solving the Pariser-Parr-Pople (PPP) model Hamiltonian and compared these results with the TDDFT and EOM-CCSD values. Exact diagonalization of the PPP model Hamiltonian suggests that pristine pyrene, which is endothermic for x-SF, becomes isoergic with certain (BN)2 substitution. The low-lying excited state energies calculated using the model Hamiltonian match very well with experimental values over EOM-CCSD and TDDFT. Moreover, the low value of the spin-orbit coupling constant calculated for BN-substituted pyrene strengthens its applicability as an SF material.