PHOTOREDUCTION OF CRYSTAL VIOLET IN ISOPROPYL ALCOHOL. MECHANISMS INVOLVING A LEUCO ETHER DERIVATIVE AND DYE ION PAIRS

Article abstract of DOI:10.1021/j100412a104

The photoreduction of crystal violet (CV⁺) has been studied as a function of excitation wavelenght and the nature of halide counterions in 90 percent isopropyl alcohol (IPA).The dye photobleaches upon excitation in the UV (283 nm) (but not in the visible) and on the selection of chloride and bromide (but not iodide) counterions.Leuco dye (CVH), acetone, and hydrogen ion are the products of photolysis.The presence of a leuco ether (CVOR) in equilibrium with CV⁺ in alcohol solution was confirmed by equilibration studies.The leuco ether and CV⁺ (with excess halide) werre subjected to laser flash photolysis (283-nm excitatiom).The transient radical, CV^., was observed, consistent with homolytic cleavage of the leuco ether or with photoinduced electron transfer within a CV⁺X^- ion pair involving an upper excited singlet state of CV⁺.The ion pair of CV⁺I^- gave rise to a new absorption band on addition band of excess iodide; an ion-pair formation constant could be calculated (K_ip= 30 M^-1).Photoinduced (reversible) electron transfer for CV⁺I^- was observed by flash photolysis (355-nm excitation), and the decay rates for recombination of CV^. and I2^.- radicals were obtained.Mechanisms of dye photoreduction-alcohol oxidation are propsed to involve hydrogen abstraction from solvent by halogen atoms (photogenerated from ion pairs) or indirectly via homolytic ether cleavage.The pathways of homolytic vs. heterolytic cleavage of CVOR in IPA were inspected by using the laser flash technique.Relevance of the mechanisms of leuco ether cleavage and ion-pair eklectron transfer to the general problem of dye photodegradation is emphasized.