Jean Faure

Singlet-singlet absorption and intersystem crossing from the 1B3u− state of naphthalene

Abstract By flash photolysis using a pulsed laser, we have examined the following intramolecular processes occuring from the lowest excited singlet state of naphthalene in cyclohexane at room temperature: fluorescence lifetime, excited singlet-singlet absorption and intersystem crossing kinetics.

MECHANISM OF PHOTOREDUCTION OF THIAZINE DYES BY EDTA STUDIED BY FLASH-PHOTOLYSIS-I

Abstract— The photoreduction of thiazine dyes by ethylene diamine tetraacetic acid (EDTA) was investigated by Rash photolysis. This reaction was found to occur according to a three‐step mechanism. the first being the formation of the dye triplet state followed, in weakly acid solutions, by protonation. During the second step, the triplet state of the dye disappears through two competing processes: spontaneous deactivation and reaction with EDTA, which leads to the semireduced dye. The third step leads to the leucodye. It is shown that the overall quantum yield of photoreduction is governed by the second step and can be calculated from the ratio of the rate constants of the two elementary processes involved in this step. This ratio was measured over a wide pH range.

Photoisomerization processes in the IR-140 laser dye

Abstract The photoisomerization processes in the well known IR-140 laser dye and in the closest relevant DTTC model have been investigated by a mode-locked laser spectroscopy in order to observe the influence of a partial rigidization of the polymethine chain. The kinetic, spectral and thermodynamic parameters have been measured; the correlations between the transient quantum yields and the laser efficiency under flash-lamped pumping are discussed. Evidence is given for a surprising solvent effect upon the photoisomerization process.

Transient absorptions in a polymethine laser dye

Abstract Transient absorptions in HITC are investigated in the range 650-950 nm by a conventional flash photolysis technique and “picosecond” spectroscopy with a train of 6 to 7 pulses. This study gives evidence for S 1 -S n absorption, photoisomer and triplet generation. The comparison between experimental data and computer solutions allows the determination of the rate constants of these processes and molar extinction coefficients of the absorption spectra. Taking into account these results, we discuss the limitations of the performances of the flashlamp excited HITC dye laser.

Quenching effects in flashlamp-excited polymethine dye lasers

Abstract The main action of oxygen dissolved in air-saturated polymethine-dye laser solutions is the quenching of the excited singlet state by enhancing transitions between it and the triplet state. A substantial increase of laser efficiency was observed by outgasing the solutions of five di- and tri-carbocyanine dyes. In one case (DOTC) a specific triplet state quencher (COT) was found.

Laser investigation of some oxazole and oxadiazole derivatives

Abstract Fast spectroscopy experiments have been performed on oxazole and oxadiazole compounds. Transient absorption from the first excited singlet state, extinction coefficients of the triplet-triplet spectrum and intersystem crossing rate constants are reported. Direct evidence of the singlet state quenching by oxygen has been obtained. These results make it possible to discuss the stimulated emission of these molecules.

Flashlamp-excited dye lasers in the near infrared

Abstract The laser emission of flashlamp-excited polymethine dyes is investigated. The spectral shift towards longer wavelengths due to relatively slow pumping is discussed in terms of transient triplet-triplet absorption. First results concerning mode locking and tuning on the 785–805 nm and 872–880 nm ranges of two tricarbocyanine dyes are presented.

Photochimie des oligomeres dimeres et trimeres de la polyacrylophenone I. Rendements quantiques de coupures dues aux reactions de norrish II

Abstract Quantum yields of scission of oligomers, dimers and trimers of poly- acrylophenone have been determined by irradiating in benzene solution under nitrogen at 365 nm. The scissions occur preferentially towards the non-substituted carbonyl in the α position, in agreement with the substitution rules of Lewis. The carbonyls situated in the middle of the trimers lead to the same quantum yields of scission, which are equivalent to the quantum yields of the carbonyls substituted by CH 3 in the α position. The photochemical epimerization of the diastereoisomers of the dibenzoyl 2,4-pentane compound supports the existence of a biradical intermediate in the Norrish type II reaction. The quantum yields for the formation of the various photoproducts are independent of the configuration of the oligomers.

Direct measurements of carbonyl triplet lifetime in the nanosecond range: Investigation in a polymer solution

A “picosecond” spectroscopy technique allows direct measurements of the carbonyl triplet lifetime τT and the intersystem crossing constant KST of polymers. We thus obtain the primary step of the Norrish type II photodegradation process and our results for the τT values are in good agreement with these deduced by Wagner from Stern-Volmer experiments.

Theoretical conformational analysis of some hydroxyazoic dyes

Abstract The optimum structures of p-hydroxy azobenzene, 4-phenyl azo 1-naphthalene and 4-phenyl azo 1-napthol are determined theoretically using the quantum mechanical MO method PCILO. The conformational minima are obtained by simultaneous optimization of the geometric parameters. The conformers equilibria and their temperature dependence, suggested by experimental dipole moments results, are discussed on the basis of our PCILO calculations.