Flash photolysis and pulse radiolysis of the Co(sep)3+-X- (sep = sepulchrate; X = I, Br) systems in aqueous solution

Abstract

The Co(sep)/sup 3 +/ complex (sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo(6.6.6)eicosane) in aqueous solution forms ion pairs with the I/sup -/ and Br/sup -/ anions, which exhibit a relatively intense charge-transfer absorption. In deoxygenated aqueous solution no net reaction is obtained upon continuous light excitation in the ion-pair charge-transfer bands, but formation of transient species is observed in flash photolysis experiments. For the Co(sep)/sup 3 +/-I/sup -/ system, I/sub 2//sup -/ ions are formed that decay in the 50-..mu..s time scale to give I/sub 3//sup -/. The latter species disappears in a time scale of seconds, leading the system back to the preexcitation conditions. For the Co(sep)/sup 3 +/-Br/sup -/ system, only formation of the Br/sub 2//sup -/ transient is observed, followed by regeneration of the preexcitation conditions in the 20-..mu..s time scale. In order to elucidate the kinetic aspects of the transient formation and disappearance, pulse radiolysis experiments on the Co(sep)/sup 3 +/-I/sup -/ and Co(sep)/sup 3 +/-Br/sup -/ systems have been carried out. The rate constants of the reactions of Co(sep)/sup 2 +/ with I/sub 2//sup -/, I/sub 3//sup -/, and Br/sub 2//sup -/ have been measured, and a complete picture of the redox processes that follow flash photolysismore » and pulse radiolysis excitations has been obtained. 5 figures.« less