2- benzoxazole, ESIPT

Semi-empirical AMI-SCI calculations have been performed to rationalize the photophysical behavior of two series of compounds one comprising of 2-(2 -hydroxyphenyl)benzoxazole, 2-(2 -hydroxyphenyl)benzimidazole (HBI), and 2-(2 -hydroxyphenyl)benzothiazole, and the other of 2-(2 -hydroxyphenyl)oxazole (HPO), 2-(2 -hydroxyphenyl)imi-dazole (HPI), and 2-(2 -hydroxyphenyl)thiazole (HPT). These compounds exhibit intramolecular rotation as well as excited state intramolecular proton transfer (ESIPT). The results suggested that for the first series of compounds two rotational isomers are present in the ground state of HBO and HBI while HBT has a single conformer under similar circumstances. For the other series, existence of rotamers depends very much on the polarity of the environment <2003IMS335, 2002JST(604)87>. 

In early investigations, transfer times of about 100 fs or less were observed for HBT in tetrachloroethylene [16], 2-(2 -hydroxyphenyl)benzoxazole (HBO) in cyclohexane [17], and for methyl salicylate (MS) [18] and OHBA [19] in gas phase. For a number of ESIPT molecules very fast transfer times were found, even at cryogenic temperatures [20]. To resolve the evolution of the transfer and to learn about the mechanism the experimental time resolution had to be improved to better than 50 fs. Such experiments were only possible with the advent of Ti sapphire laser systems and novel nonlinear sources for ultrashort tunable light pulses. Within the last ten years several experiments with extremely high time resolution have been performed. They revealed rich spectroscopic dynamics and resulted in a detailed picture of the ESIPT and the underlying mechanisms. Their comparative discussion is the central issue of this article. 

Isonitrile 19 was detected in ultraviolet (UV) and infrared (IR) studies at low temperatures. They speculated that azirine intermediate 21 may precede the formation of 19 and, indeed, LFP studies on 18 performed by Richard and co-workers allowed observation of an intermediate which was assigned to 21 on the basis of the similarity of its UV absorption spectra to that of the cyclohexadienone chro-mophore. The reaction was not quenchable by piperylene and is presumed to occur via the singlet manifold. The authors did not put forward a mechanism for the formation of 21, although it may be formed by initial ESIPT from the phenol to the cyano group to give tautomer 22, which can then rearrange to give 21. Similar products (substituted benzoxazoles) were observed via presumed initial ESIPT in structurally related o-hydroxy substituted aromatic oximes and oxime ethers. ... 

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