Intra- to Intermolecular Singlet Fission

Singlet fission, the splitting of one singlet into two triplets, can potentially increase the efficiency of optoelectronic devices beyond conventional limits. Among the singlet fission molecules discovered to date, two mechanisms have emerged: intra- or intermolecular singlet fission. Here we show a combined intra- to intermolecular singlet fission mechanism in the model system of diphenyl-dicyano-oligoene (DPDC). Excitation of DPDC to the first optically bright state leads to the ultrafast formation of an intramolecular triplet pair, which decays in 40 ps in the solution phase but can also split competitively in 30 ps into two long-lived triplets (2xT(1)) on adjacent molecules in solid films. These findings suggest a design principle for efficient singlet fission: the independent tuning of singlet-triplet pair coupling and triplet pair splitting from intra- and intermolecular interactions, respectively.