Ab Initio Study of the Structure, Spectral, Ionochromic, and Fluorochromic Properties of Benzoazacrown-Containing Dyes as Potential Optical Molecular Sensorsстатья

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Дата последнего поиска статьи во внешних источниках: 18 июля 2013 г.

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[1] Ab initio study of the structure, spectral, ionochromic, and fluorochromic properties of benzoazacrown-containing dyes as potential optical molecular sensors / A. Y. Freidzon, A. A. Bagatur'yants, E. N. Ushakov et al. // International Journal of Quantum Chemistry. — 2011. — Vol. 111, no. 11. — P. 2649–2662. Experimental absorption and emission spectra of benzoaza-18-crown-6-containing styryl dye 2a and its complexes with Li(+), Na(+), and Ca(2+) are recorded. It is found that complexation causes large hypsochromic shifts of the long-wave absorption band of the dye. In addition, the spectra of Li(+) and Na(+) complexes exhibit an additional band or a shoulder with lambda(abs)(max) virtually coinciding with the absorption band of the free dye. Complexation also enhances the fluorescence of the dye. Simulating the structure and electronic spectra of the dye and its complexes with Li(+), Mg(2+), Na(+), and Ca(2+) by DFT and TD DFT allowed us to explain these effects. It is found that cation coordination breaks the conjugation of the crown ether nitrogen with the chromophore, thus reducing the effective chromophore size, and the positive charge of the cation simultaneously decreases the donor nature of the amino group. This explains large hypsochromic shift of the long-wave absorption band. It is found that Li(+) and Na(+) complexes can form two types of conformers, with and without cation-nitrogen bond. The conformers with the cation-nitrogen bond exhibit a very strong blue shift of the absorption maximum, while the absorption spectra of the complexes without cation-nitrogen bond are almost unshifted relative to the spectra of the free dye. The ground-state recoordination of the cation in the macrocycle cavity involves the change between these two forms. Recoordination is promoted by the interaction of the cation with solvent molecules. This explains the presence of the additional long-wave band or shoulder in the spectra of the complexes. Cation-induced fluorescence enhancement is explained by the changes in the excited-state structure of the free dye and its complexes. The free dye changes its structure from the "quinoid” form in the ground state to the "benzenoid” form in the excited state, thus facilitating the rotation of the alkoxyaminophenyl group and causing near-degeneracy of the potential energy surfaces of the ground and excited states. This results in rapid nonradiative deactivation and fluorescence quenching. Complexation with cations reverses the dye structure to the "benzenoid” form in the ground state and to the “quinoid” form in the excited state thus preventing alkoxyaminophenyl group from rotation, which results in fluorescence enhancement. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 2649-2662, 2011. [ DOI ]

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