Mono- and Bichromophores Formed from Perylene Monoimide Diesters: Competition between Intramolecular Charge Transfer and Intermolecular Singlet Exciton Fission

Perylene monoimide diesters and the corresponding phenyl-linked bichromophores are strongly fluorescent in dilute solution with minimal triplet population after relaxation of the initial Franck-Condon state. The monomer forms nonemissive face-to-face dimers in solution, wherein illumination leads to formation of a spin-correlated, triplet pair with a yield of ca. 13% and with a time constant of 4 +/- 1 ps. The triplet pair, which is localized on the aggregate, cannot separate and decays with a mean lifetime of 80 +/- 10 ps. The relaxed S1 state of the weakly coupled, phenyl-linked bichromophores establishes an equilibrium with an intramolecular charge-transfer state over a hundred picoseconds or so, depending on the solvent and the geometry of the linkage. This equilibrium mixture, being dominated by the relaxed S1 state, decays on the nanosecond time scale in solution at room temperature without implication of a triplet state. Self-association occurs at higher concentration and, for the para-bridged bichromophore, leads to inefficient triplet formation in tetrahydrofuran at room temperature.