Spin chemical approach towards long-lived charge-separated states generated by photoinduced intramolecular electron transfer in a donor-bridge-acceptor system

Photo-induced intramolecular electron transfer (ET) and subsequent back ET have been studied for phenothiazine-bridge-pyromellitdiimide (D-B-A) and phenothiazine-bridge-pyromellitdiimide-nitroxide radical (D-B-A-R .) in various solvents by nano-second transient absorption spectroscopy. Upon excitation of the phenothiazine (D) moiety in these compounds, ET from D-1(*) to A took place at a rate of ca. 5 x 10(9) s(-1) to give charge-separated (CS) states, (1)[D.+-B-A(.)] or (2)[D.+-B-A(.-)-R .], in high quantum yields. The decay dynamics of the CS states was determined by the interplay between the spin conversion and the back ET to the ground state. The efficiency of the interconversion between states with different spin multiplicities was strongly affected by the presence of R . and external magnetic fields. A faster spin conversion of [D.+-B-A(.-)-R .] resulted in a slower initial decay of the CS state generated from the excited singlet state of D compared with [D.+-B-A(.-)]. Such a spin effect manifested most markedly in dioxane. On the other hand, the back ET rate (k(BET)) varied by more than two orders of magnitude with the solvent polarity. Tn dimethyl sulfoxide, the back ET was predicted to be almost barrierless, k(BET) greater than or equal to 10(9) s(-1), while in benzene, this process fell in the inverted region, k(BET) = 6 5 x 10(6) s(-1).