THE INFLUENCE OF MOLECULAR-CONFORMATION ON THE STABILITY OF ULTRAVIOLET STABILIZERS TOWARD DIRECT AND DYE-SENSITIZED PHOTOIRRADIATION - THE CASE OF 2-(2'-HYDROXY-5'-METHYPHENYL)BENZOTRIAZOLE (TIN-P)

The mechanism for photodegradation of the ultraviolet photostabilizer 2-(2'-hydroxy-5'-methylphenyl)benzotriazole (TIN P) upon direct and dye-sensitized (singlet molecular oxygen [O-2((1) Delta(g))]mediated) irradiation was studied. From the experimental TIN P photodegradation rate data, and low temperature (77 K) fluorescence and phosphorescence quantum yields, one can conclude that the photodegradative process involves phosphorescent states of TIN P. The open conformer of TIN P quenches O-2((1) Delta(g)) by physical scavenging with a rate constant (k(q)) in dimethylsulfoxide of 2.8 x 10(6) M(-1) s(-1). The intramolecular hydrogen-bonded conformer does not appreciably interact with O-2((1) Delta(g)). In the presence of a relatively high concentration of OH- (either 5 x 10(-2) M KOH in ethanol or water at pH 13), the ionic form of TIN P (with an ionized phenol group) physically and chemically quenches O-2((1) Delta(g)) The reaction rate constant (k(r)) is 1 x 10(8) M(-1) s(-1), and the ratio k(q)/k(r) is approximately three in alkaline aqueous media.