2- benzoxazole, excited

The photolysis of 1,2-benzisoxazole in the absence of air in acetonitrile gave salicylonitrile and benzoxazole (67AHC(8)277). When air-saturated acetonitrile was employed, 2,2 -dimeriz-ation to (38) occurred, accompanied by benzoxazole. Photolysis of the 2,2 -dimer (38) and benzoxazole did not alter the ratio, thus indicating that neither one arose from the other. Selective excitation also ruled out dimer formation from benzoxazole under the reaction conditions (Scheme 9). This dimerization is similar to that observed for benzimidazole, except that in that series no 2,2 -dimerization was observed (74TL375). 

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... 

Ikegami M, Arai T (2000) Laser flash photolysis study on hydrogen atom transfer of 2-(2-hydn>xyphenyl)benzoxazole and 2-(2-hydroxyphenyl)benzothiazole in the triplet excited state. Chem Lett 9 996-997... 

A wide variety of different classes of fluorescent molecules has been investigated in the peroxyoxalate chemiluminescent systems. Among those screened were fluorescent dyes such as rhodamines and fluoresceins, heterocyclic compounds such as benzoxazoles and benzothiazoles, and a number of polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and perylenes. The polycyclic aromatic hydrocarbons and some of their amino derivatives appear to be the best acceptors as they combine high fluorescence efficiency with high excitation efficiency in the chemiluminescent reaction [28],... 

Excitation of o-nitrophenyl alkyl ethers (107 and 108) causes the intramolecular hydrogen abstraction from the njr triplet state of the nitro group to give benzoxazoles 109 and 110 respectively63 according to the mechanism in equation 54. 

The effects of a-, (3-, y-, and 2,6-di-o-methyl-(3-CDs on the ground- and excited-state properties of 2-(2 -hydroxyphenyl)benzoxazole (HBO), 2-(2 -hydrox-yphenyl)benzothiazole (HBT), and 2-(2 -hydroxyphenyl)benzimidazole (HBI) (42) in aqueous media have been investigated using fluorescence [184], The molecules form 1 1 complexes with a reasonably high association constant, and they enter the cavity axially from the wider-rim side of (3-CD with the benzazole... 

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>. 

The majority of reversible photoinduced PT reactions takes place between a donor and an acceptor that are chemically different, being either different atoms or identical atoms in inequivalent positions of a molecule or a molecular assembly [12,13], Examples are systems in which the phenomenon was first discovered and which (sometimes in the form of related compounds and derivatives) continue to be the object of present work methyl-salicylate [6,7], n-salicylidene aniline [8], 3-hydroxyflavone [10], and dimers of 7-aza-indole [9], Other molecules include 2-(2 -hydro-xyphenyl)benzothiazole [163,164] and 2-(2 hydroxyphenyl)benzoxazole, which has been studied in detail by Grellmann and co-workers [165,166], and where in addition to the fast transfer in the excited singlet state, the slower transfer in the triplet manifold could be characterized by transient absorption measurements in solution at temperatures down to 55 K. 

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 ... 

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

The vapour absorption spectrum of benzothiazole near 2850 A, with origins at the wavenumber 34 327 cm, has been described and compared with those of benzoxazole and benzimidazole. In each case the origin is the strongest band, and excited-state fundamentals at ca. 700, 950, and 1225 cm" are prominent. There is no evidence for a (jr n) transition. ... 

The distal arrangement of the substituents in 24 prevents cyclization to a benzoxazole-type intermediate. While stable in alcohols, this intermediate can undergo hydrolysis (in water) to give hydroxyformanilide (25). Further analysis of the reaction revealed that 24 is a secondary photoproduct of the first-formed azirine 26, in contrast with the thermally induced azirine conversion to isonitrile that is observed for 18. Triplet quenchers such as oxygen and methyl acrylate quench the reaction efficiently. The authors propose a mechanism for conversion of 23 to 26 that involves a deprotonation-reprotonation sequence preceded by intramolecular electron transfer from the triplet-excited state of 23. 

Superieure de Chimie, Mulhouse, Fr. J. Photopolym. Sci. Technol. (2000), 13 (2), 237-242 (Eng.). The interactions of excited states of a coumarin or a ketocoumarin photosensitizer with a bisimi-dazole deriv., mercaptobenzoxazole photoinitiators, and titanocene were studied by laser absorption spectroscopy and by photocond. The redox potential of the compds. was measured and used in calcn. of the free enthalpy change for a possible electron transfer reaction. The coumarin forms radicals through an electron transfer reaction, while the ketocoumarin undergoes an energy transfer reaction with bisimidazole and a hydrogen abstraction reaction with a benzoxazole deriv. Thus, coumarin or ketocoumarin/free radical initiator combinations are suitable as initiators of radical polymn. reactions, esp. those applicable to laser imaging. [1707-68-2]. 

Two and Three Component Photoinitiating Systems Based on Coumarin Derivatives. AUonas, X. Fouassier, J.P. Kaji, M. Miyasaka, M. Hidaka, T. Departement de Photochimie Genetale, UMR No. 7525, Ecole Nationale Superieure de Chimie, Mulhouse, Fr. Polymer (2001), 42 (18), 7627-7634 (Eng.). Several coumarin or ketocoumarin/additives combinations (bisimidazole deriv., mercapto-benzoxazole, titanocene, oxime ester) can initiate quite efficiently radical polymn. The interactions between the excited states of coumarins or aketocoumarin and various additives have also been studied by laser absorption spectroscopy, time resolved fluorescence and photocond. the redox potentials of these different compds. have been measured and allowed the calcn. of free enthalpy variations for a possible electron transfer reaction. The whole results explain the interaction mechanism and show that the coumarins are able to form radicals through an electron transfer reaction with the different additives whereas the ketocoumarin leads to an energy transfer with bisimidazole and to an hydrogen abstraction with the benzoxazole deriv. 

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