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

The role of charge transfer states in multi-exciton mechanismshas recently become a point of discussion due to the difficulty associatedwith modeling their contributions accurately. Intermolecular packinghas been shown experimentally to heavily influence multi-exciton mechanisms,and therefore understanding how this affects the coupling is key tocontrolling these processes. Using a gas phase perylene dimer in aweakly coupled configuration as a case study, we employ two separatemethods to model the coupling between the bright and correlated triplet (1) TT states as a function of relative displacement.For singlet fission, displaced geometries are found to yield largecharge transfer contributions within a wavefunction overlap paradigm,unlike for aligned geometries. Triplet-triplet annihilationcharge transfer couplings are conversely very weak due to a largeenergy gap. We found that slipping of the dimer cofacial geometryis beneficial to both charge transfer-mediated processes within awavefunction overlap scheme. However, within a fragment excitationdifference (FED) scheme, a 1 & ANGS; slip is more beneficial than a2 & ANGS; one. The resulting rates for singlet fission are in the femtosecondrange, up to 22 ps(-1), while for triplet fusion theyare in the nanosecond range, up to 707 & mu;s(-1). By studying the dynamics of the triplet pair following singletfission, we show that the decorrelation time scale depends on thenature of the relative molecular motion, ranging from picosecondsfor fluctuations in the monomer orientations to microseconds for coplanarfluctuations. The direct comparison of the wavefunction overlap andFED methods yields an expected differential due to the method of calculation(linear-response vs multireference) but still strong agreement, suggestingthat the more exact wavefunction overlap method can be substitutedfor the FED method in larger systems with minimal loss in accuracyvs computational complexity. These results provide a good steppingstone for further investigations into singlet fission related problems,correlating well with experiments despite the weakly coupled natureof the dimer.