Quantum Master Equation Approach to Singlet Fission Dynamics of Realistic/Artificial Pentacene Dimer Models: Relative Relaxation Factor Analysis

Abstract

The singlet fission (SF) dynamics of realistic/artificial pentacene dimer models are investigated using the quantum master equation method in order to obtain new insight into the SF dynamics and its rational design guidelines. We comprehensively clarify the effects of the energy offsets of diabatic Frenkel exciton (FE) and charge transfer (CT) exciton states to the double-triplet (TT) exciton state, excitonic couplings, and state-dependent vibronic couplings on the exciton population dynamics using relative relaxation factors (RRFs) between the adiabatic exciton states. As shown in previous studies, efficient sequential/superexchange CT-mediated SF is observed in the energy level matching region (E(TT) – E(FE) < 0). On the other hand, it is predicted that almost the perfect energy level matching (E(TT) – E(FE) ∼ 0) causes the significant reduction of TT yields though exhibits remarkably fast SF rates, when the corresponding adiabatic double-triplet (TT′) and Frenkel exciton (FE′) states are near-degenerate to each other with common diabatic configurations. The excitonic coupling is also found to have a possibility of causing significant change of SF dynamics when it has a large amplitude comparable to those of the other electronic coupling elements. Furthermore, the large vibronic coupling of CT state shows striking enhancement of SF rates with keeping high TT yields in the CT-mediated superexchange region, while the large vibronic couplings of FE and TT states do not show such striking enhancement. These features are understood by analyzing their RRFs, which are proportional to the product of the square of common diabatic exciton configuration coefficients in the concerned two adiabatic exciton states, multiplied by the spectral density (vibronic coupling).