Singlet Fission in Perylenediimide Dimers

Abstract

Singlet fission is a process in which one singlet exciton is converted to two triplets. By using transient absorption and time-resolved emission spectroscopy, recent experimental study (J. Am. Chem. Soc. 2017, 140, 814) investigated how different crystal packing of perylenediimide (PDI) molecules modulates their singlet fission rates and yields. It was observed that the rates vary between 0.33 and 4.3 ns–1. By employing a simple three-state kinetic model and restricted active-space configuration interaction method with double spin-flip, we study the electronic factors (excitation energies and coupling between relevant states) responsible for the variation of singlet fission rates in these PDI derivatives. Our approach reproduces the trends in singlet fission rates and provides explanations for the experimental findings. Our analysis reveals that the electronic energies and the coupling play significant roles in controlling the singlet fission rates. The wave function analysis of the adiabatic electronic states shows that in many model PDI structures, the multiexciton character is spread over several states, in contrast to previously studied systems. This different nature of the multiexciton state poses interesting mechanistic questions. By mapping the relation between the stacking geometries of PDIs and the rates of the singlet multiexciton formation and the binding energies, we suggest favorable PDI structures that should not lead to exciton trapping.