Electron transfer from methoxybenzenes to the excited state of an iridium(III) complex studied by the pulsed laser technique

Abstract

A new reversible photochemical electron-transfer system involving the photosensitizer (Ir(bpy){sub 2}(C{sub 3},N'bpy)){sup 2+}, Ir(III){sup 2+}, and its acid form Ir(III){sup 3+} and the neutral methoxybenzenes as electron donors is described. The quenching kinetics and back reactions between the photochemical products have been studied both by steady-state emission measurements of the Ir(III)* excited states and by the pulsed laser technique. The effect of a charged polymer, poly(styrenesulfonate) (PSS), is also reported. The iridium complex is reduced to Ir(II){sup 2+} with high quantum yields. Tetramethoxybenzene (TMB) is more efficiently oxidized as compared with dimethoxybenzene (DMB). The acid-base equilibrium of Ir(III)* (pK = 3.5) has only a small, if any, effect on the yields. PSS affects both product yields and reaction kinetics. The back-electron-transfer reaction in the presence of PSS proceeds in two steps. The major step, accounting for most of the initial absorption, is attributed to a reaction between the electron-transfer products in the same polymer aggregate. The slower process is attributed to DMB{sup +} ion radicals which escaped the initial stage of the back reaction and diffused to other polymer aggregates. The positive DMB{sup +} ions are converted to Br{sub 2}{sup {minus}} in the presence of Br{sup {minus}}. The effect ofmore » PSS in the presence as well as in the absence of Br{sup {minus}} ions is discussed. The results indicate an effect of hydrophobic interactions between PSS and DMB.« less