Theoretical Engineering of Singlet Fission Kinetics in Perylene Bisimide Dimer with Chromophore Rotation

We investigated the potential of chromophore's rotations to tune singlet fission (SF) kinetics in perylene bisimide (PBI) dimers in addition to relative horizontal displacements. The total number of 250 PBI dimers (five displacements along the long and short axis of PBI, respectively, and ten rotation angle changes from parallel to perpendicular alignment) was examined. Ground-state energies showed that dimer formation is favored in all orientations with some differences in interaction strength. Time-dependent density functional theory predicted S₁ and T₁ excitons' energy, and the thermodynamic feasibility of SF process was judged by the energy difference between a S₁ exciton and twice of T₁ excitons. In addition, we also estimated the relative rate of multiexciton generation step by the three-state kinetic model with the results of restricted active space employing double spin-flip. Nine promising orientations including two parallel PBI dimers and seven twisted ones were discussed. Wave function composition analysis showed that SF occurs mainly through the superexchange mechanism in various twisted PBI dimers, but the direct two-electron and coherent pathway could be operative at the particular positions. Quantum chemical simulations suggested the rotation as an effective tool to tune SF efficiency in PBI dimers, which is helpful to substantiate more efficient SF material.