Precise Control of the Electronic Coupling Magnitude between the Electron Donor and Acceptor in Perylenebisimide Derivatives via Conformation Restriction and Its Effect on Photophysical Properties

Abstract

Perylenebisimide (PBI)–phenothiazine (PTZ) and PBI–diphenylamine (DPA) dyads were prepared, with the PTZ or DPA as the electron donor and the 6-subsituted PBI unit as the electron acceptor. The dyads were designed in such a way that electronic coupling (magnitude is the matrix elements, VDA and VDA*) between the electron donor and acceptor is controlled by conformation restriction. The effects of the electronic coupling on UV–Vis absorption and emission were studied. More significant charge-transfer (CT) absorption and CT fluorescence emission bands were observed for dyads with smaller dihedral angles between the electron donor and acceptor, thus stronger electronic coupling (VDA = 3290 cm–1 and VDA* = 4317 cm–1) was observed than those with larger dihedral angle, showing weaker coupling effect (VDA = 1210 cm–1 and VDA* = 2770 cm–1). Triplet state production was observed only for PBI–DPA but not for PBI–PTZ dyads. With an intermolecular triplet photosensitizing method, the triplet state of PBI–PTZ dyads was observed. The femtosecond transient absorption spectra confirmed the ultrafast charge separation (CS, 0.3 ps ∼ 0.6 ps) and slow charge recombination (CR, 130 ps ∼ 240 ps) process. These results indicate that the CR does not produce any triplet state in the PBI–PTZ dyads, for instance by the expected spin–orbital charge transfer intersystem crossing (SOCT-ISC). We propose that the lack of triplet state production in PBI–PTZ dyads is due to the large separation of the 1CT and the 3CT/3PBI states, and the orthogonal geometry and the CR are not exclusively sufficient criteria for SOCT-ISC. Our results on controlling the absorption, emission, and ISC by tuning the VDA magnitudes between the donor and acceptor will be useful for molecular design of compact electron donor/acceptor dyads.