2- benzothiazole, excited

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]. 

Frey W, Laermer F, Elsaesser T (1991) Femtosecond studies of excited-state proton and deuterium transfer in benzothiazole compounds. J Phys Chem 95 10391-10395... 

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... 

Cheng YM, Pu SC, Hsu CJ et al (2006) Femtosecond dynamics on 2-(2 -hydroxy-4 -diethylaminophenyl)benzothiazole solvent polarity in the excited-state proton transfer. ChemPhysChem 7 1372-1381... 

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],... 

Rate coefficients in the expected range have been observed in excited state intramolecular proton transfers involving 2-(2-hydroxyphenyl)benzothiazole (Barbara et al., 1980a) and salicylideneaniline (Barbara et al., 1980b). In the former, the observation of fluorescence was explained by Scheme 2 and the... 

Organic dye nanoparticles of DHIA and DHBIA (the chemical structures are shown in Fig. 1) have been synthesized in THF/water mixed solvent by a reprecipitation method [32]. These dye molecules possess a 2-(2-hydroxyphenyl) benzothiazole (HBT) unit, which is known to be more stable as an enol imine form in the ground state and as a keto amine form in the excited state [32, 33] (Fig. 6). The nanoparticles exhibited the AIEE phenomenon mainly due to a restricted intramolecular motion, that is, impediment to free rotation of two end-substituted HBT units around single bonds. It is interesting to note that the... 

Fig. 6 The 2-(2-hydroxyphenyl)benzothiazole (HBT) unit that represents the keto-enol equilibrium (tautomerism). Normally, the enol (keto) form is rather stable in the ground state (in the excited state), respectively...

Qian Y, Li S, Zhang G, Wang Q, Wang S, Xu H, Li C, Li Y, Yang G (2007) Aggregation-induced emission enhancement of 2-(2 -hydroxyphenyl)benzothiazole-based excited-state intramolecular proton-transfer compounds. J Phys Chem B 111 5861-5868... 

F. Laermer, T. Elsaeser, and W. Kaiser, Femtosecond spectroscopy of excited-state proton-transfer in 2-(2 -hydroxyphenyl)benzothiazole, Chem, Phys. Lett. 148, 119(1988). 

Time-resolved electron paramagnetic resonance served to detected the short-lived triplet state of the keto tautomer (238) of 2-(2-hydroxyphenyl)benzothiazole (239) generated by excited state intramolecular proton transfer (92CC641). 

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... 

The increase in fluorescence intensity of TO upon binding to dsDNA is due to the restriction of rotation around the monomethine bridge upon intercalation of the dye into the double helical structure as the benzothiazole and quinolinium rings adapt to the propeller twist of the base pairs [49]. The monomethine bridge has a low energy barrier to rotation and hence is free to rotate in solution, allowing for the electronically excited dye to relax by non-radiative decay [49]. The quantum yield of free TO in solution has been reported to be 2 x 10-4 at 25 °C [43]. The binding constant for TO is 106 M 1 while that of ethidium bromide is 1.5 x 105 M 1 [59]. 

Several molecules which can undergo intramolecular proton transfer in the excited state have been found to be unusually photostable. Thus, for example, 2-hydroxybenzophenone is used as a photostabilizer in polymers while benzophenone itself is photoactive (Kysel, 1969). In crystalline 2-(2 -hydroxyphenyl)benzothiazole and its derivatives [3] a proton is transferred in the excited state from an... 

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>. 

AMI semi-empirical MO calculations in both gaseous and condensed phases were employed to study excited-state intramolecular proton transfer from 2-(2 -hydroxyphenyl) benzothiazole <2000CPL(327)23, 2000PCP203>. 

The dependence of intramolecular proton transfer on solvent friction has been established for 2-(2 -hydroxy-5-methylphenyl) benzotriazole in alcohol and other solvents. Excited-state proton transfer in 2-(2 -hydroxyphenyl) benzothiazole has also been studied 2 Photophysical properties and laser performance of w, w -bis ( oxazol-2-yl)- -oligophenylenes in dioxane have been measured at room temperature. +p increases with the number of phenyl rings between terminally positioned oxazoyl groups. 

We have used this approach to investigate the excited state relaxation dynamics of 2-(2 -hydroxyphenyl)benzothiazole, HBR (1J. The fluorescence properties of this molecule are well described by a kinetic scheme outlined in Figure 4. 

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. 

Light-induced intramolecular proton transfer has long been suspected to play an important role in the mechanism for photochromism in both crystals and solutions. One class of compounds that has been extensively studied is the A -salicylidene anilines. The main evidence for invoking an excited-state proton-transfer step in the photochromic mechanism comes from the observation that the colored species exists as the keto amine tautomer (in the ground state) while the bleached form exists as an enol imine (in the ground state). Replacement of the labile hydrogen with a methyl group prevents the photochemical reaction that leads to the colored species. Picosecond transient spectroscopy was used to study iV-salycylidene-o-toluidine (NST) and the related compound 2-(o-hydroxyphenyl)benzothiazole (HBT). ... 

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