2- benzimidazole, 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]. 

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>. 

Rotational isomerization has been described for the phototautomer formed by proton transfer in the excited singlet state of 2,2 -bipyridin-3-ol in hydrocarbon solvents. At room temperature both the primary phototautomer and its rotamer fluoresce, allowing the activation energy for internal rotation to be determined. Excited state tautomerization has also been described for camp-tothecin in acidic aqueous solution and for derivatives of hypericin. Light-induced keto-enol tautomerism has been invoked to explain the fluorescence behaviour of certain benzimidazole compounds. Interconversion of confor-mers of constrained tryptophan derivatives takes place in the first-excited singlet state.The excited state behaviour has been reported for conformationally-distorted porphyrin derivatives. The lifetimes of the S] states of these... 

Measurement of the influence of different micellar environments on proton transfer from excited states of 3-hydroxyflavone allows estimates to be made of micelle concentrations from measurement of the tautomer emission yield. Proton transfer reactions of benzimidazole excited singlet states have also been studied in ionic micelles. Magnetic fields are found to affect the behaviour of radicals generated by the photodissociation of benzil in micellar media. The starburst dendrites which are formed by anionic macromolecules in interaction with both anionic and cationic surfactants have been examined by pyrene fluorescence. Benzo[k]fluoranthrene fluorescence has served as a probe of the effects of metal salts on bile salt aggregation. The incorporation and distribution of benzoquinone into liposomes containing amphilic Zn(II) porphyrin has been followed by its effect on the quenching of the excited state °. A comparison of the photochromism of spirobenzpyran derivatives in unilamellar surfactant vesicles and solvent cast surfactant films has also been reported. ... 

Fores M., M. Duran, M. Sola, and L. Adamowicz. 1999. Excited-state intramolecular proton transfer and rotamerism of 2-(2 -hydroxyvinyl) benzimidazole and 2-(2 -hydroxyphenyl) imidazole. 7. Phys. Chem A, 103(22) 4413 420. 

Fluorescent benzimidazole derivatives have been designed and synthesized using cobalt(ll) hydroxide (15 mol%) as highly efficient catalyst (Scheme 83) [117]. Hydroxy-substituted benzimidazole shows a single prominent emission in polar solvents and dual emission in nonpolar solvents due to excited state intramolecular proton transfer. 

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