Ghislain Guyot

The cis → trans photoisomerization of azobenzene: an experimental re-examination

Abstract The quantum yields O t for the cis → trans photoisomerization of azobenzene have been redetermined in various solvents. As for the reverse process, there is a quantum yield dependence on the excitation wavelength. Reasons for the discrepancy with previous reported values are briefly discussed.

Fluorescence of soil humic acids and their fractions obtained by tandem size exclusion chromatography–polyacrylamide gel electrophoresis

Humic acids (HAs) extracted from soils of different origin (chernozem, ferralsol and ranker) and their fractions (A, B and C+D) obtained by tandem size exclusion chromatography–polyacrylamide gel electrophoresis were investigated by steady-state fluorescence spectroscopy in the emission mode. Independently of HA source, high molecular size fractions A and B are shown to be weakly fluorescent. The main fluorophores, especially those emitting at long wavelength (around 500–510 nm), are contained in the polar and low molecular size fractions C+D. As indicated by the observed pH effect, aromatic structures bearing carboxylate and OH substituents may be involved in these longer wavelength emissions.

Silver(I) perturbation of (E)–(Z) photoisomerization of stilbene and azobenzene

The influence of AgClO4 on the photochemical behaviour of stilbene and azobenzene in acetonitrile and methanol has been compared with that of Nal, a well known perturber of the excited-state properties via the heavy-atom effect. Both the salts quench the fluorescence and the geometrical photoisomerization yields of stilbene, the effect being greater in CH3OH. l– has no effect on the photoisomerization of azobenzene whatever the solvent, while Ag+ increases the isomerization yield in CH3OH but has no effect in CH3CN. l– interacts with the singlet excited state of stilbene and l–via a charge-transfer complex which favours (owing to the heavy-atom effect) ISC to the isomerizable triplet state of the alkene. The lack of effect of l– on the azobenzene photoisomerization probably results from its exceedingly short lifetime. Ag+ forms a ground-state complex with stilbene and azobenzene in methanol solution, as is apparent from the variations of the absorption spectrum of the two aromatics. The light excitation of the complex causes (E)→(Z) isomerization of the bond chromophore.

Preparation and Photophysics of 2-(1-Pyrenyl)acrylic Acid and Its Methyl and 2′,2′,6′,6′-Tetramethyl-4′-Piperidyl Esters

Novel probes represented connection of pyrene as chromophore and sterically hindered amine stabilizers (HAS) in the form of esters of 2-(1-pyrenyl)acrylic acid were synthesized. HAS was in the form of parent amine (PAP) as well as stable nitroxyl radical form (PAP-NO.). Photophysics of these probes were compared with their precursor as 2-(1-pyrenyl)acrylic acid (PAA) and its methyl ester (PAM). The fluorescence spectrum of PAA strongly depends on the acidity of the solution. The spectrum in neutral methanol indicates that it originates from the anionic form –COO−. Changes of acidity or basicity of methanol solution resulted in the changes of shape, position as well as the intensity of fluorescence band. This is due to the presence of protolytic equilibria, either in the ground state or in the singlet excited state, leading to the formation of molecular form –COOH and the cationic form –COOH2+. The ester analogues did not show any changes in various pH conditions. Fluorescence of all probes depends on the polarity of solvents and the presence of oxygen. Intermolecular quenching was studied with external quenchers TEMPO and oxygen and the data were compared with the intramolecular quenching using 1′-oxo-2′,2′,6′,6′-tetramethyl-4′-piperidinyl-2-(1-pyrenyl)acrylate (PAP-NO.).

Triplet state of 4-biphenylcarbonyl and 2-naphthylcarbonyl chromophores in homopolymers, copolymers and model compounds

Abstract The triplet state of poly(1-(4-biphenyl)-2-propen-1-one), poly(1-(2-naphthyl)-2-propen-1-one), copolymers of the above carbonyl monomers with methyl methacrylate and respective model compounds was sought in solution at room temperature by emission spectroscopy and microsecond flash photolysis. Although 4-biphenylcarbonyl and 2-naphthylcarbonyl yield an intense phosphorescence at 77 K, no emission was observed in solution at room temperature. The intense transient spectrum observed in benzene solution for 1-(4-biphenyl)-3-chloropropan-1-one and 1-(2-naphthyl)-3-chloropropan-1-one was ascribed to the triplet state. Self-quenching was found to decrease the intensity of the transient spectrum and the life-time when building up both chromophores in a macromolecule. More efficient self-quenching was observed for 2-naphthylcarbonyl than for the 4-biphenylcarbonyl chromophore which was suggested as a suitable triplet probe. The limiting triplet life-time of 4-biphenylcarbonyl built up in a copolymer with methyl methacrylate at low concentration in a thermodynamically good solvent is about 0·3 ms.