Christian Lenoble

Photophysics, photochemistry and kinetics of photochromic 2H-pyrans and chromenes

Abstract Nine naturally occurring and synthetic chromenes and 2 H -pyrans (oxo and sulfo) derivatives have been examined using microsecond and nanosecond laser flash techniques. Triplet transients have only been observed for the coumarin-substituted chromenes (xanthyletin and seselin). Photochromism of the coumarin-substituted chromenes arose dominantly via an excited triplet state in about 800 ns. For the other chromenes and 2 H -pyrans, the colored forms arose only from an excited singlet state and subnanosecond rise times were observed. Fluorescence lifetimes have been determined for some chromenes at room temperature as well as 77 K. For xanthyletin, a fluorescence lifetime of less than 30 ps at 295 K in hexane was observed.

Photophysics, photochemistry and kinetics of indolinospiropyran derivatives and an indolinothiospiropyran

Abstract Three indolinospiropyran derivatives and an indolinothiospiropyran were studied in solution using nanosecond and microsecond laser flash techniques. A fluorescence lifetime was also obtained for the indolinothiospiropyran. All these compounds showed photochromism under laser flash conditions. No triplet transients were observed and oxygen (versus nitrogen) had no effect on the spectra, the kinetics of formation (shorter than 1 ns) or the disappearance (longer than 400 μs) of the colored photoproduct. The transients were in the transoid opened form (merocyanine) existing in several trans conformations coming exclusively from the singlet excited state on the picosecond time scale. No cisoid form was observed down to 1 ns. No evidence of aggregation of the species in hexane was observed in contrast to 6′-nitroindolinospiropyran.

NEW FEATURES IN THE PHOTOPHYSICS and PHOTOCHEMISTRY OF 2‐(2'‐HYDROXY‐PHENYL)BENZOTHIAZOLES INTRODUCED BY AMINE SUBSTITUTION

Abstract— –The photophysics and photochemistry of the 4′‐diethylamino derivative of both 2‐phenyl‐benzothiazole and 2‐(2′‐hydroxyphenyl)benzothiazole have been studied by nanosecond and microsecond laser flash photolysis and picosecond emission spectroscopy. For the non‐hydroxy substituted molecule, the singlet excited state was shown to relax primarily via fluorescence emission, and a very weak triplet transient was observed after laser flash excitation. The 2‐(2′‐hydroxy‐4′‐diethylamino‐phenyl)benzothiazole (AHBT) was shown to undergo excited state intramolecular proton transfer (ESIPT) in the picosecond timescale (k > 3 times 1010 s‐1) to form a colored zwitter‐ion/keto form in solution at room temperature while the ground state back proton transfer was slower by a factor of?105. However, in marked contrast with other derivatives of 2‐(2′‐hydroxyphenyl)benzothiazole and related molecules, the ESIPT was not the only deactivation process of the lowest singlet excited state of the enol form. Under steady‐state excitation at room temperature (and low temperature), the fluorescence emission of the enol form was observed. The T‐T absorption of the enol form was also observed and furthermore, the ESIPT was shown to have an activation energy which was estimated to be ? 4 kJ. None of the foregoing, fluorescence and T‐T absorption of the enol nor activation energy for proton transfer have been observed for the parent or derivatives of 2‐(2′ hydroxyphenyl)benzothiazoles. The striking new features for the ESIPT photochemistry and photo‐physics for the 4′‐diethylamino derivative of 2‐(2′‐hydroxyphenyl)benzothiazole are discussed and MO calculations are used to aid in the interpretation of some of the experimental results.