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

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 andmixtures of dioxane with H 2 0, methanol and ethanol by laser flash time resolved absorption measurements.The colouring reaction kinetics and the relative quantum yield of the red product were measuredusing nanosecond excimer laser excitation at 308 nm. In most cases, first order kinetics was.observed. Thefirst order rate constants and the quantum yield were found to be dependent on the solvent and thenumber of laser pulses that had previously irradiated the solution. The thermal fading reaction occurredon 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 S 1 state ofthe rhodamine derivative with picosecond resolution. Single exponential decays were observed in allsolvents and the lifetime ranged between 3 and 6 nanoseconds. We conclude that the photoreaction doesnot proceed directly from the S 1 state, and propose a mechanism for the photochromic process, in whichan intermediate decomposes to rotameric isomers of the coloured product, which are not equilibrated onthe millisecond time scale of the reverse reaction.