New Insights on the Photochromism of 2-(2‘,4‘-Dinitrobenzyl)pyridine

The photochromic behavior of 2-(2‘,4‘-dinitrobenzyl)pyridine (α-DNBP) has been followed in poly(methyl methacrylate) (PMMA) films and benzene solutions to clarify the behavior of a precursor state, previously identified in studies on crystalline α-DNBP at low temperatures. In PMMA films, photolysis at temperatures ≤50 K led to the concurrent formation of a NH tautomer and a colorless intermediate, which was stable for several hours. On irradiation at low temperatures and warming the sample, the colorless intermediate was seen to react to produce the NH tautomer in a higher yield than that found in the direct photolysis. Further information on this intermediate has come from flash photolysis studies in benzene solution, in which a new transient absorption has been observed at 335 nm and assigned to this species. This decays within a few microseconds at room temperature to form an OH tautomer, which then interconverts to the NH tautomer. The precursor state is not quenched by oxygen or naphthalene. From consideration of the kinetic and spectral data, it is suggested that this new species corresponds to a nonrelaxed tautomeric form of the OH state of α-DNBP

THE EXCITED STATE OF CIS-THIOINDIGO

Dans le cadre de l'étude de la photoisomérisation cis-trans réversible du thioindigo, des mesures spectroscopiques à basse température des isomères cis et trans isolés en matrice ont été réalisées. C'est la première observation, pour l'isomère cis, de spectres ayant une structure vibrationnelle résolue. Différentes évaluations du rendement quantique de fluorescence de ce composé indiquent que la durée de vie de l'état excité est dans le domaine subpicoseconde.Spectroscopic measurements at low temperature of matrix isolated cis and trans-thioindigo are presented and are discussed in relation to the photoinduced reversible isomerization of this molecule. For the cis isomer these data represent the first vibrationally resolved spectra. Estimates of the emission quantum yield for this species show that the excited state decays on a subpicosecond timescale

LASER INDUCED HYDROGEN MOTION IN ORGANIC CRYSTALS AT LOW TEMPERATURE

We study here a laser induced reaction leading to a rearrangement of the acid protons in benzoic acid crystals at very low temperature. This hydrogen motion results from the electronic excitation of pentacene molecules substitutionally included into a benzoic acid host matrix, protonated or deuterated. The growth of a deuterated benzoic acid crystal doped with pentacene leads to a partial deuteration of pentacene molecules, resulting in several origin bands in the absorption spectrum. The dynamics of the system following non selective and selective laser irradiations of the differently deuterated pentacene molecules is analyzed in terms of hydrogen rearrangements

Transformation photochimique de dérives de la benzoyl-amino-2 Δ

L’étude de la transformation photochimique des dérivés: chloro-4’ benzoyl, méthyl-4’ bénzoyl, chloro-2’ benzoyl et méthyl-2’ benzoyl de l’amino-2-A2-thiazoline en dérivés correspondants de la N-éthyl N’-benzoyl thiourée par rupture d’une liaison C-S, est présentée.La nature des photoproduits est déterminée et leurs spectres UV, IR, de masse, RMN 1H et 13C sont indiqués. La mesure du rendement quantique de la réaction photochimique et les données photophysiques préalablement obtenues ont permis de proposer un modèle cinétique et de déterminer les valeurs relatives des constantes de vitesse de formation d’un biradical intermédiaire