Stabilization of Charge-Transfer States in Pentacene Crystals and Its Role in Singlet Fission

We theoretically investigate the role of charge-transfer (CT) states being stabilized by surrounding environments on singlet fission (SF) dynamics in a pentacene crystal. Using a polarizable force field to calculate induction energy by the surrounding molecules, the stabilization of the CT dimer energy is assessed for its convergence in massively large crystalline cells. The behavior of the convergence and the converged energies are found to depend on the local dimer configuration, that is, parallel slip-stack or tilted face-to-edge packings. Using the resultant crystalline-effective CT energies, the SF dynamics simulation based on the quantum master equation is performed for both dimers and multimers in the crystal. It is found for the multimer that the populations of the Frenkel excitons and CT excitons exhibit recurrence motions between neighboring sites and that double-triplet excitons are then gradually generated in an ultrafast timescale (<100 fs). From the analysis on relative relaxation factors (RRFs) between the adiabatic exciton states, the mechanism of the rapid SF rate in the crystal is revealed, where the intrusions of the diabatic CT configurations into all the adiabatic states effectively accelerate the SF process. Furthermore, the RRF is used to predict the double-triplet exciton yield at the extreme of the extended large size cell. The present findings will provide a clue to construct design guidelines for efficient SF based on the control of CT energy by environmental engineering.

Automated summary

In this theoretical study, the role of charge-transfer states stabilized by surrounding environments on singlet fission dynamics in a pentacene crystal was investigated. It was found that in multimers, Frenkel excitons and CT excitons exhibit recurrence motions between neighboring sites, leading to the gradual generation of double-triplet excitons in an ultrafast timescale. The rapid singlet fission rate in the crystal was attributed to the intrusion of diabatic CT configurations into all adiabatic states, effectively accelerating the SF process.