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

Abstract

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.

Steering the multiexciton generation in slip-stacked perylene dye array via exciton coupling

“…In this regard, we performed restricted active space with spin-flip (RAS-SF) simulation, which has been widely used to interrogate the MEG process. RAS-3SF calculations for Tris-PBI suggest that the S 1 state exhibits 66% of CR character, consistent with the experimental observation upon LP into the CT band (Table S7 ) 66 , 67 . This feature is indeed quite different from the null-type PBI dimers studied in our previous work 45 that features the LE as the lowest transition.…”

Towards multistate multimode landscapes in singlet fission of pentacene: the dual role of charge-transfer states

“…37 The crystalline polarizable environment was deduced to stabilize vibronically tuned CT states, causing a signicant acceleration in SF generation rates with a slight diminution in the TT yield. 38 Conceptually, the presence of strongly correlated triplet pairs in SF chromophores requires a high-level ab initio multi-congurational wavefunction 39,40 for which the number of signicant determinants grows exponentially with correlated electrons. Dealing with such strongly correlated systems becomes intractable, which limits correlation to only a small number of electrons in these molecules, leaving many profound questions unresolved despite already intriguing ndings.…”

“… 37 The crystalline polarizable environment was deduced to stabilize vibronically tuned CT states, causing a significant acceleration in SF generation rates with a slight diminution in the TT yield. 38 …”

Theoretical Study on Thermal Structural Fluctuation Effects of Intermolecular Configurations on Singlet Fission in Pentacene Crystal Models

“…Furthermore, energy levels of the CT states are affected by surrounding situations, such as the dielectric environment in crystal phase , or solution phase which may lead to the stabilization of the CT states. For example, Nagami and co-workers have theoretically predicted that stabilization of the CT states in the crystal environment can play an important role in the ultrafast SF using quantum dynamics simulations . In the solution phase, solvent polarity was shown to influence the SF mechanism. , …”

“…In this study, we performed exciton dynamics simulations of pentacene multimer models in the crystal phase based on the quantum master equation (QME) approach which has been developed in our group. ,, By combining the classical molecular dynamics (MM/MD) simulations with the force field, we extended our QME approach so that we can understand effects of thermal fluctuations of intermolecular configurations on TT generation in the crystal phase through modulations of excitation energies and electronic coupling. We evaluated how these fluctuations can affect the relaxation dynamics and the spatial distributions of excitons based on the relative relaxation factor (RRF) analysis and its decomposition into the exciton overlap and the VC contributions.…”