Photochromic transformations in solutions of 8,8′-diquinolyl disulfide and di(mercaptoquinolinato)nickel(II)

Abstract

The nature of intermediate species and their reactions were studied by laser pulse photolysis for a photochromic system consisting of 8,8′-diquinolyl disulfide (RSSR) and a planar NiII complex di(mercaptoquinolinato)nickel(II) (Ni(SR)2) in toluene and benzene solutions. Under exposure to laser radiation, disulfide RSSR dissociates to two RS· radicals, whose spectrum has an intense absorption band with a maximum at λ = 400 nm (e = 8400 L mol−1 cm−1). The radicals disappear by recombination (2krec = 4.6 · 109 L mol−1 s−1). In the presence of the Ni(SR)2 complex, coordination of the radical (kcoord = 4.4 · 109 L mol−1 s−1) competes with recombination to form a radical complex RS· Ni(SR)2 having an intense absorption band with a maximum at 460 nm (e = 16 600 L mol−1 cm−1). This species decays in the second-order reaction (2k = 4.6 · 104 L mol−1 s−1). Since the photochromic system returns to the initial state, the reaction of two radical complexes is assumed to produce radical recombination and reduction of the disulfide and Ni(SR)2 complex. Analysis of the kinetic data showed that some RS· radicals decay in the microsecond time interval due to the reaction with the RS· Ni(SR)2 radical complex (k = 3.1 · 109 L mol−1 s−1).