Intramolecular Singlet Fission in an Antiaromatic Polycyclic Hydrocarbon

Singlet fission (SF), in which one singlet exciton (S-1) splits into two triplets (T-1) 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 4n pi dibenzopentalenes (DPs) are demonstrated as a new class of single-chromophore-based intramolecular SF materials that exhibit an optically allowed S-2 state with E(S-2)>2XE(T-1) and an optically forbidden S-1 state. Ultrafast population transfer from a high-lying S-2 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.