Designing an Efficient\nSinglet Fission Material with B−N Substitution in Pyrene: A\nModel Exact Study

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