Table of Contents
Laser Chemistry
Volume 10, Issue 2, Pages 63-72

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

1Physikalisch-Chemisches lnstitut, Im Neuenheimer Feld 253, Heidelberg D-6900, Germany
2Organisch-Chemisches Institut, Im Neuenheimer Feld 270, Heidelberg D-6900, Germany

Received 12 April 1988; Accepted 7 July 1988

Copyright © 1989 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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.