2- benzoxazole, excited state proton transfer

As in the case of benzothiazoles and benzimidazoles, the excited-state proton transfer in 2-(2 -hydroxyphenyl)benzoxazole was studied both experimentally and computationally. The results closely resemble the observations for the other species The cw-enol form is preferred in the Sq ground state and the cw-keto form in the 5i excited state. Moreover, the proton transfer appears to be due to vibrational relaxation rather than thermal activation, suggesting that the aromatic ring has an impact on the transfer reaction of these systems [95JPC12456, 99JST255]. 

Wang H, Zhang H, Abou-Zied OK et al (2003) Femtosecond fluorescence upconversion studies of excited-state proton-transfer dynamics in 2-(20-hydroxyphenyl)benzoxazole (HBO) in liquid solution and DNA. Chem Phys Lett 367 599-608... 

Mechanism of excited-state proton transfer in "double" benzoxazoles bis-2,5-(2-benzoxazolyl)hydroquinone, Chem.Phys.Lett., 111 383 (1984). 

Molecular materials, such as 2-(2-hydroxyphenyl)benzoxazole and 2-(2-hydroxyphenyl)-benzothiazole, which contain intramolecular hydrogen bonds are known to undergo excited state (charge transfer) intramolecular proton transfer upon photo-excitation. 

Jenckhe at al., explored the effects of molecular size, extent of conjugation, concentration quenching, and competition with excimer formation on intramolecular proton transfer as well as on the electro-luminescent device potential of polymeric materials. Materials like 2-(2-hydroxyphenyl)-benzoxazole and 2-(2-hydroxyphenyl)benzothiazole contain intramolecular hydrogen bonds and are known to undergo excited state charge transfer reactions and intramolecular proton transfer upon excitation. They reported, however, that the results of their studies with model compounds and several polymers, whose structure ... 

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>. 

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