Carbazole-perylenebisimide electron donor/acceptor dyads showing efficient spin orbit charge transfer intersystem crossing (SOCT-ISC) and photo-driven intermolecular electron transfer

Abstract

Perylenebisimide-carbazole (PBI–Cz) dyads were prepared to study charge-recombination (CR) induced intersystem crossing (ISC) in electron donor/acceptor dyads. The distance and the mutual orientation of the perylenebisimide (PBI) and carbazole (Cz) moieties were varied to study their effect on photophysical properties. Steady state and time-resolved optical spectroscopies show that electronic coupling between the electron donor and acceptor is negligible at the ground state. The fluorescence of the PBI moiety is strongly quenched in the dyads, and a larger separation between the donor and the acceptor results in less fluorescence quenching. The Gibbs free energy changes of the electron transfer and the energy level of the charge transfer state were studied using the electrochemical and optical spectra data. The singlet oxygen quantum yields (Φ_Δ) are up to 72% for dyads with the shortest separation between the donor and the acceptor. Nanosecond transient absorption spectra confirmed the formation of the PBI-localized long lived triplet state (the lifetime is up to 190 μs). Notably non-orthogonal dyads show efficient spin orbit charge transfer (SOCT-ISC), which is different from the previously proposed orthogonal molecular structure for SOCT-ISC. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy shows that all three dyads give the same electron spin polarization of eae/aea, and thus the radical pair ISC (RP ISC) mechanism is excluded, which is different from the previously reported PBI-phenothiazine analogues. Efficient and reversible transformation of dyads to their radical anion was observed in the presence of sacrificial electron donor triethanolamine in an inert atmosphere with photo-irradiation.