Intramolecular Singlet Fission in an Antiaromatic Polycyclic Hydrocarbon

Singlet fission (SF), in which one singlet exciton (S₁) splits into two triplets (T₁) on adjacent molecules through a correlated triplet‐pair ¹(TT) state, requires precise but difficult tuning of exciton energetics and intermolecular electronic couplings in the solid state. Antiaromatic 4nπ dibenzopentalenes (DPs) are demonstrated as a new class of single‐chromophore‐based intramolecular SF materials that exhibit an optically allowed S₂ state with E(S₂)>2×E(T₁) and an optically forbidden S₁ state. Ultrafast population transfer from a high‐lying S₂ state to a ¹(TT) state was observed in monomeric solution of styryl‐substituted DP (SDP) on a sub‐picosecond timescale. There is evidence of exciton diffusion (ED) of the ¹(TT) state to yield two individual long‐lived triplets in SDP thin film. The overall triplet yield via intramolecular SF and subsequent triplet‐pair diffusion can be as high as 142±10 % in thin film.