Long-lived photoinduced charge separation in new Ru(bipyridine)₃²⁺-phenothiazine dyads

Abstract

Synthesis and photophysical properties of three Ru(bpy)₃²⁺-Ptz (bpy = 2,2'-bipyridine and Ptz = phenothiazine) dyads, where the number of Ptz groups increased from one to three, are reported. The MLCT absorption bands of these compounds were slightly red shifted compared to Ru(bpy)₃²⁺. The emission, however, was highly quenched and this is attributed to electron transfer from the Ptz moiety to the excited Ru(bpy)₃²⁺ to generate the charge separated state Ru(bpy)₃⁺-Ptz^{· +}. Observed electron transfer rates (k_et > 10⁸ s^-1) were much faster than those previously reported (k_et < 10⁷ s^-1) for linked Ru(bpy)₃²⁺-Ptz systems. Compared to the previous systems, back electron transfer rates in these systems were about 100 times slower. This has enabled us to observe the charge separated state in nanosecond flash photolysis experiments. Transient absorptions assignable to Ru(bpy)₃⁺ and Ptz^{· +}, having lifetimes in the range of 10-30 ns were observed. In order to explain the fast charge separation and slow charge recombination rates, formation of a folded conformer where the Ptz group attached to one bpy residue comes closer to and associates with another bpy moiety was invoked. A scheme which explains the fast electron transfer and slow recombination in this pre-associated state is proposed.