Excited-state intramolecular proton transfer (ESIPT) in 2-(2′-hydroxyphenyl)-oxazole and -thiazole

Theoretical Modelling for the Ground State Rotamerisation and Excited State Intramolecular Proton Transfer of 2-(2’-hydroxyphenyl)oxazole, 2-(2’-hydroxyphenyl)imidazole, 2-(2’-hydroxyphenyl)thiazole and Their Benzo Analogues

“…The differential behaviour of HBT compared to that of HBO and HBI may be incurred due to the low electronegativity and the bulkier size of the sulfur atom [44] The fact that the rotational behaviours of HPO, HPI and HPT are grossly similar to those of the corresponding benzo analogues is obvious from figures 1-6. isolated nor solvated conditions. The recent experimental study of Le Gourriérec et al [27] rules out the existence of II and corroborates our proposition. The non-existence of the rotamer of HPT has been rationalized from a greater single bond character of the bond joining the phenol and the azole rings and thus allowing for more twisting vibrations whereby the rotamer is unable to get any well-defined stability.…”

“…From the proximity of the spectral data and our calculated transitions, we ascribe that the S 1 and T 1 states effective for the ESIPT process are both of ππ* nature. This is consistent with the literature reports [16,25,27,33,50]. *Since the experimental values for HPT system are not available, the data in the table refer to the spectral positions for 2-(2′-hydroxyphenyl)-4-methylthiazole and they have been taken from reference [27].…”

“…There is also a difference in the temporal behaviour of the fluorescence decay of the rotamer and the formation of the tautomer [18,39]. These evidences indicate that the electronic excitation of the species leads to the ESIPT product from only one of the two rotameric forms in selected solvents [23,25,27,28,[33][34][35]37,[39][40][41][42][43][44][45][46][47][48][49][50]. Mordzinski and Grellmann suggested that a thermally activated radiationless transition dominates the decay of the photoproduced keto form at room temperature, whereas, at lower temperature, fluorescence and intersystem crossing are the main deactivation processes [40].…”

“…Spectroscopic and theoretical studies have also been performed with compounds of the second series, viz., 2-(2′-hydroxyphenyl)oxazole (HPO), 2-(2′-hydroxyphenyl)imidazole (HPI), 2-(2′-hydroxyphenyl)thiazole (HPT) [27,38]. Though reports are limited, the experimental observations with this group of compounds grossly run parallel to those for the compounds of the other group.…”

“…The family of molecules in the azole category has been dealt with by several research groups in explaining the various aspects of excited state intramolecular proton transfer [21][22][23][24][25][26][27][28][29][30]. The early studies on 2-(2′-hydroxyphenyl)benzoxazole (HBO) and 2-(2′-hydroxyphenyl)benzothiazole (HBT) were done by Cohen and Flavian [31,32], who compared the behaviour of these compounds with Nnitrosalicylideneanilines in different solvents.…”