Photochromism, anomalous multi-banded fluorescence and laser properties of some amino- and tosyl-amino derivatives of oxadiazole

A new class of N-H excited-state intramolecular proton transfer (ESIPT) molecules bearing localized zwitterionic tautomers

A series of new amino (NH)-type intramolecular hydrogen-bonding (H-bonding) compounds have been strategically designed and synthesized. These molecules comprise a 2-(imidazo[1,2-a]pyridin-2-yl)aniline moiety, in which one of the amino hydrogens was replaced with substituents of different electronic properties. This, together with the versatile capability for modifying the parent moiety, makes feasible comprehensive spectroscopy and dynamics studies of excited-state intramolecular proton transfer (ESIPT) as a function of N-H acidity. Different from other (NH)-type ESIPT systems where the ESIPT rate and exergonicity increase with an increase in the N-H acidity and hence the H-bonding strength, the results reveal an irregular relationship among ESIPT dynamics, thermodynamics and H-bond strength. This discrepancy may be rationalized by the localized zwitterionic nature of 2-(imidazo[1,2-a]pyridin-2-yl)aniline in the proton-transfer tautomer form, which is different from the π-delocalized tautomer form in other (NH)-type ESIPT systems.

Effect of PEG-200 and Tween-20 on photoisomerization of 1-alkyl-2-(arylazo)imidazoles in toluene

The photoisomerization of 1-alkyl-2-(arylazo)imidazole, trans-to-cis, has been studied in the matrix of PEG-200 and Tween-20 in toluene medium by UV light irradiation. The trans and cis-isomers have different absorption spectra. The cis-to-trans isomerization proceeds slowly in visible light irradiation while it is appreciably fast in thermal process. The rate of trans-to-cis isomerization is decreased by 30-60% in presence of PEG-200 and Tween-20. The quantum yield of the photoisomerization is also decreased by 35-55% and follows the rate sequence: free state>PEG-200-phase>Tween-20 phase. The activation energy (E(a)) of cis→trans, thermal backward isomerization, is reduced in PEG-200 and Tween-20 phase following free state>PEG-200-phase>Tween-20-phase. The branched polyhydroxo structure of Tween-20 may help to wrap the polar photochrome more efficiently than major ether functionalized PEG-200 and stabilizes trans-isomer.