Long-Lived Charge Separated States in Anthraquinone-Phenothiazine Dyads: Synthesis and Study of the Photophysical Property by Using Transient Optical and Magnetic Resonance Spectroscopies.

In order to obtain long-lived charge separated (CS) states in electron donor-acceptor dyads, herein we prepared a series of anthraquinone (AQ)-phenothiazine (PTZ) dyads, with adamantane as the linker. UV-vis absorption spectra show negligible electronic interaction between the AQ and PTZ units at ground state, yet charge transfer (CT) emission bands were observed. Nanosecond transient absorption shows that the 3 AQ state is populated upon photoexcitation for AQ-PTZ in cyclohexane (CHX), but in acetonitrile (ACN) a 3 CS state is formed. Similar results were observed for AQ-PTZ-M. The 3 CS state lifetimes were determined as 0.52 µs and 0.49 µs, respectively. Upon oxidation of the PTZ unit, the 3 AQ state was observed in both polar and non-polar solvents. For AQ-PTZ, femtosecond transient absorption spectra show fast formation of the 3 AQ state in all solvents, with no charge separation in CHX, while formation of the 3 CS state takes 106 ps in ACN. For AQ-PTZ-M, a 3 CS state is formed in CHX within 241 ps. Time-resolved electron paramagnetic resonance (TREPR) spectra show that a radical ion pair with electron exchange energy of |2 J|≥5.68 mT was observed for AQ-PTZ and AQ-PTZ-M, whereas in the dyads with the PTZ unit oxidized, only the 3 AQ state was observed.