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. [ABSTRACT FROM AUTHOR]