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.