Excited state redox properties of phthalocyanines: influence of the axial ligand on the rates of relaxation and electron-transfer quenching of the lowest 3ππ* excited state

Abstract

Laser flash excitations at 640 nm have been used to generate the transient spectra of the lowest-lying ³ππ* state of phthalocyaninatoruthenium(II) complexes. The properties of this excited state such as the positions of the maxima, λ_max.= 500 ± 30 nm, and lifetimes, t_½= 70–4 500 ns, exhibit a large dependence on the electron-accepting and electron-withdrawing tendencies of the axial ligands. A similar influence was observed upon the rate of electron-transfer quenching of the ³ππ* state. Values between 10⁶ and 10⁷ dm³ mol^–1 s^–1 for the self-exchange rate constant have been obtained, according to Marcus–Hush theoretical treatments, for [Ru(pc˙)LL′]⁺/(³ππ*)[Ru(pc)LL′][L and L′= neutral axial ligands; pc = phthalocyaninate(2–)] and isoelectronic cobalt(III) and rhodium(III) couples. The redox properties of the ground and excited states are correlated with axial ligand-induced perturbations of the electronic structure.