Study of the Spin-Orbit Charge Transfer Intersystem Crossing of Perylenemonoimide-Phenothiazine Compact Electron Donor/Acceptor Dyads with Steady-State and Time-Resolved Optical and Magnetic Spectroscopies

Compact electron donor/acceptor dyads were prepared, with perylenemonoimide (PMI) as the electron acceptor and phenothiazine (PTZ) as the electron donor, to study the relationship between the molecular geometry and the spin-orbit charge transfer (CT) intersystem crossing (SOCT-ISC) efficiency. The photophysical properties of the dyads were studied with steady-state and time-resolved optical and magnetic resonance spectroscopies. We found that PTZ is an ideal chromophore for molecular conformation restriction in the compact dyads, which exerts a significant effect on the electronic coupling between the donor and acceptor and thus on the photophysical properties such as UV-vis absorption, fluorescence, and ISC efficiency. Anomalous intensified fluorescence at elevated temperatures was observed, which can be fully rationalized by the bright twisted charge transfer (CT) state. The singlet oxygen quantum yields (Phi(Delta)) of the dyads are up to 57%, which are highly solvent-polarity-dependent. Femtosecond transient absorption and fluorescence spectroscopies indicate that the charge separation proceeds within 1 ps and the charge-recombination-induced ISC takes 2.8 ns. With time-resolved electron paramagnetic resonance spectroscopy, we confirmed the SOCT-ISC mechanism of the dyad, for which the electron spin polarization and the population rates of the sublevels of the triplet state (zero-field states T-X, T-Y and T-Z) are drastically different from those of the spin-orbit-coupling-induced ISC (for monobrominated PMI). Photodriven intermolecular energy-transfer and electron-transfer processes in the presence of a triplet energy acceptor or a sacrificial electron donor were also studied. The radical anion of PMI was reversibly produced with photoreduction for which near-IR absorption band in the range of 750-850 nm was observed.