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 3AQ state is populated upon photoexcitation for AQ-PTZ in cyclohexane (CHX), but in acetonitrile (ACN) a 3CS state is formed. Similar results were observed for AQ-PTZ-M. The 3CS state lifetimes were determined as 0.52 ?s and 0.49 ?s, respectively. Upon oxidation of the PTZ unit, the 3AQ state was observed in both polar and non-polar solvents. For AQ-PTZ, femtosecond transient absorption spectra show fast formation of the 3AQ state in all solvents, with no charge separation in CHX, while formation of the 3CS state takes 106 ps in ACN. For AQ-PTZ-M, a 3CS 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 3AQ state was observed.