Singlet fission in pancake-bonded systems.

We theoretically investigate the potential of pancake-bonded systems for efficient singlet fission by examining phenalenyl radical dimer and tetramer models. In the dimer model, we propose an efficient way to satisfy the excitation energy level matching required for singlet fission by tuning the diradical character by changing the intermolecular distance. In the tetramer model, electronic couplings required for both singlet fission and charge mobility are found to be maximized simultaneously in the face-to-face configuration, which is usually known to be difficult to achieve due to a trade-off relationship between them in conventional π-conjugated molecules like pentacene. In addition, on the basis of the quasi-degenerate second-order perturbation analysis, we obtain a simplified and more efficient crystal-structure design guideline that pancake-bonded systems cause highly-active singlet fission.