title: The mechanism of the photochromic transformation of spirorhodamines.
abstract: We investigate the equilibrium, kinetics, and mechanism of the photochromic transformation of a series of amido spirorhodamine compounds-differing in the nature of the substituents of the amido group and in the rhodamine chromophore-in ethanol at room temperature in the presence of trifluoroacetic acid. A proton participates in the equilibrium between the spiro form and the open rhodamine form. The relaxation times in the dark or under continuous irradiation show a linear dependence on the proton concentration. The slopes of these plots show a linear free energy relation with the equilibrium constant of the transformation. A mechanism involving reversible reaction steps between four states: the two thermodynamically stable isomers, a protonated spiro form, and a deprotonated open form, can account for the kinetic observations in the dark and under irradiation.
authors.affiliation: INQUIMAE and Departamento de Química Inorgánica, Analítica y Química Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón 2, Ciudad Universitaria, 1428 Buenos Aires, Argentina.
authors.forename: Hugo
authors.initials: H
authors.lastname: Montenegro
authors.forename: Matías
authors.initials: M
authors.lastname: Di Paolo
authors.forename: Daiana
authors.initials: D
authors.lastname: Capdevila
authors.forename: Pedro F
authors.initials: PF
authors.lastname: Aramendía
authors.forename: Mariano L
authors.initials: ML
authors.lastname: Bossi