Kinetics and Mechanism of the Photochromism of N-Phenyl-Rhodaminelactame

Abstract

The photochromism of N-phenyl-rhodaminelactame (3' ,6' -bis-(diethylamino)-2-phenyl-spiro[1H-isoindole-1,9'[9H] Xanthaen]-3-[2H]-one) was investigated in solutions of acetonitrile, dioxane and mixtures of dioxane with H₂0, methanol and ethanol by laser flash time resolved absorption measurements. The colouring reaction kinetics and the relative quantum yield of the red product were measured using nanosecond excimer laser excitation at 308 nm. In most cases, first order kinetics was.observed. The first order rate constants and the quantum yield were found to be dependent on the solvent and the number of laser pulses that had previously irradiated the solution. The thermal fading reaction occurred on a millisecond timescale and showed large deviations from first order kinetics in all solvents. In addition, picosecond laser pulses at 285 nm were used to measure the fluorescence lifetime of the S₁ state of the rhodamine derivative with picosecond resolution. Single exponential decays were observed in all solvents and the lifetime ranged between 3 and 6 nanoseconds. We conclude that the photoreaction does not proceed directly from the S₁ state, and propose a mechanism for the photochromic process, in which an intermediate decomposes to rotameric isomers of the coloured product, which are not equilibrated on the millisecond time scale of the reverse reaction.