Processes in photochromic system containing xanthate (S2COEt)2 disulfide and xanthate Ni(S2COEt)2 complex

Abstract

It has been shown that the solutions of xanthate disulfide (S2COEt)2 and xanthate Ni(II) complex Ni(S2COEt))2 in acetonitrile are the photochromic systems. In this solution after laser pulse (266 nm) the absorption band of xanthate S2COEt radical appears in visible region (640 nm) due to the photodissociation of disulfide. Within a few microseconds, the S2COEt radical disappears in two competing reactions. The first is recombination with reverse formation of disulfide, and the second is coordination of radical to Ni(S2COEt))2 complex with the formation of Ni(S2COEt))2(S2COEt) radical complex which has the intense absorption bands in blue part of spectrum (370 and 450 nm). The S2COEt radical is coordinated, apparently, by one sulfur atom above the planar Ni-S4 unit, while the other sulfur atom remains free. The analysis of kinetics and transient absorption spectra after the laser pulse shows that the Ni(S2COEt))2(S2COEt) radical complex disappears in two competing reactions. The first is determined by escape of S2COEt radical from coordination sphere, and the second is formation of dimer [Ni(S2COEt)2(S2COEt)]2 in reaction of two radical complexes.The [Ni(S2COEt))2(S2COEt)]2 dimer slowly loses one of S2COEt radical, turning into a new [(S2COEt)Ni(S2COEt)2Ni(S2COEt)2] dimer. This new dimer slowly dissociates into a Ni(S2COEt))2(S2COEt) radical complex and an initial Ni(S2COEt))2 complex. The Ni(S2COEt))2(S2COEt) radical complex, in turn, enters a new chain of reactions, which extends the kinetics of photochromic transformations to a couple of hundred seconds. This model of photochromic cycle is based on the preservation of form of transient absorption spectrum from 20 μs up to hundreds of seconds. Thus, the photochromic properties of the ((S2COEt)2 and Ni(S2COEt)2 system are determined by the reversible coordination of S2COEt radical and the back recombination to (S2COEt)2 disulfide.