Breakdown of the Perturbative Approach to Molecular Packing Dependence of Singlet Fission Rates in Pentacene Dimer Models: A Systematic Comparison with the Quantum Master Equation Approach

Abstract

We have compared the molecular packing dependences of singlet fission (SF) rates calculated by perturbative and quantum master equation approaches using slip-stacked pentacene dimer models to clarify the applicability of the perturbative treatment and the importance of vibronic coupling effects on the SF dynamics. Several significant differences between these two approaches are found in the peak positions as well as in the relative peak amplitudes on the two-dimensional map of the SF rate as a function of the longitudinal and lateral displacements of the molecular backbones. Using the relative relaxation factor analysis, these disagreements are more striking for increasing the electronic couplings between the related diabatic exciton states as well as for reducing the energy difference between the lowest adiabatic Frenkel-like and double-triplet-like exciton states to match the primary vibronic coupling frequency. These results systematically illuminate the breakdown of the perturbative approach to the pr...