Singlet fission in perylenediimide dimers
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 investigated how different crystal packing of the perylenediimide (PDI) modulates the singlet fission rates and yields. It was observed that the rates of the PDIs 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 speed of the singlet fission rates. The wavefunction analysis of the adiabatic electronic states shows that in many model PDI structures the multi-exciton character is spread over several states, in contrast to previously studied systems. By mapping the relation between the stacking geometries of PDIs and the rates of the singlet multi-exciton formation and the binding energies, we suggest favorable PDI structures that should not lead to exciton trapping.