1,1″,1‴-Trialkyl[4,2′;4′,4″;6′,4‴] quaterpyridinium trichlorides as electron transfer agents

Abstract

Photo- and electrochemical properties of a novel class of electron transfer substances (1,1″,1‴-trialkyl[4,2′;4′,4″;6′,4‴] quaterpyridinium trichlorides) have been studied for hydrogen producing systems containing sensitizerS, electron transfer agentR, electron donorD, and platinum catalyst. 1,1″,1‴-trimethyl [4,2′;4′,4″;6′,4‴] quaterpyridinium trichloride (1) and 1,1″,1‴-tribenzyl [4,2′;4′,4″;6′,4‴] quaterpyridinium trichloride (2) quench the excited state of Ru(bipy)3Cl2. Irradiation (λ>400 nm) of solutions containing Ru(bipy)3Cl2,1 or2 and EDTA lead to the formation of radicals of1 or2. A solution of radicals of compound1 can change its colour reversibly with change of pH-values (colourless in acidic, blue in alkaline solutions). Also, compounds1 and2 are able to produce H2 without additional sensitizer (irradiation light: λ>300 nm). Hydrogen production rates, quantum yields, and turnover numbers have been determined in two different systems (S, R, D, Pt, λ>400 nm; andR, D, Pt without additional sensitizer, λ>300 nm). Hydrogen production rates with compounds1 and2 are of the same order of magnitude as those with methyl viologen. The radicals of1 and2 are much more stable against oxygen, than those of methyl viologen. Cyclovoltammetry showed that the redox potentials of compounds1 and2 are dependent on pH: increasing pH values result in more negative redox potentials. Coulometric analysis showed that e.g. compound1 could accept 5 electrons at pH=4·8 and 3·4 electrons at pH=12·6 compared to only one electron for methyl viologen (independent of pH). This behaviour is explained with reference to protonated radicals.