Charge Transfer-Mediated Multi-exciton Mechanisms in Weakly Coupled Perylene Dimers

The role of charge transfer states in multi-exciton mechanisms has recently become a point of discussion due to the difficulty associated with modeling their contributions accurately. Intermolecular packing has been shown experimentally to heavily influence multi-exciton mechanisms, and therefore understanding how this affects the coupling is key to controlling these processes. Using a gas phase perylene dimer in a weakly coupled configuration as a case study, we employ two separate methods to model the coupling between the bright and correlated triplet 1TT states as a function of relative displacement. For singlet fission, displaced geometries are found to yield large charge transfer contributions within a wavefunction overlap paradigm, unlike for aligned geometries. Triplet–triplet annihilation charge transfer couplings are conversely very weak due to a large energy gap. We found that slipping of the dimer cofacial geometry is beneficial to both charge transfer-mediated processes within a wavefunction overlap scheme. However, within a fragment excitation difference (FED) scheme, a 1 Å slip is more beneficial than a 2 Å one. The resulting rates for singlet fission are in the femtosecond range, up to 22 ps–1, while for triplet fusion they are in the nanosecond range, up to 707 μs–1. By studying the dynamics of the triplet pair following singlet fission, we show that the decorrelation time scale depends on the nature of the relative molecular motion, ranging from picoseconds for fluctuations in the monomer orientations to microseconds for coplanar fluctuations. The direct comparison of the wavefunction overlap and FED methods yields an expected differential due to the method of calculation (linear-response vs multireference) but still strong agreement, suggesting that the more exact wavefunction overlap method can be substituted for the FED method in larger systems with minimal loss in accuracy vs computational complexity. These results provide a good stepping stone for further investigations into singlet fission related problems, correlating well with experiments despite the weakly coupled nature of the dimer.