Photoinduced charge separation of phenothiazine-platinum-naphthalene diimide triads linked by twisted phenylene bridges

Two triads (i.e., 3PTZ-Pt-MNDI and 10PTZ-Pt-MNDI) consisting of 3-phenothiazine (3PTZ) or 10-phenothiazine (10PTZ), bipyridine-diacetylide platinum complex (Pt), and naphthalene diimide (MNDI) chromophores linked by highly twisted biphenylene spacers have been prepared. The formation and decay of the charge-separated (CS) states in toluene were studied by use of picosecond and nanosecond laser photolysis via selective excitation of the Pt moiety. The time required for formation of the CS state, PTZ⁺-Pt-MNDI ^-, from PTZ-³Pt*-MNDI was determined to be τ _CS = 280 ps for 3PTZ⁺-Pt-MNDI^- and τ _CS = 230 ps for 10PTZ⁺-Pt-MNDI^-. The lifetimes of the CS states were determined to be τ _CR1 = 75 ns (95 %) and τ _CR2 = 285 ns (5 %) for 3PTZ⁺-Pt-MNDI^- and τ _CR = 830 ns for 10PTZ⁺-Pt-MNDI^-. Formation and decay of the CS states are discussed in terms the Marcus theory and the spin-correlated radical pair mechanism.