Solvent Influences on the Photophysics of Naphthalene: Fluorescence and Triplet State Properties in Aqueous Solutions and in Cyclodextrin Complexes

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Abstract

The photophysical properties of naphthalene were studied in aqueous solution (H2O and D2O), in some organic solvents (ethanol, hexane, and acetonitrile), and in complexes with the cyclodextrins (CDs) α-CD and β-CD, by means of absorption, steady-state and time-resolved fluorescence, circular dichroism, and triplet−triplet absorption spectroscopies. The structures of the CD inclusion complexes were computed using a dynamic Monte Carlo method. The main difference of the photophysics in the pure aqueous with respect to the organic media consists in a reduction of the fluorescence lifetime, τF, by a factor of about 2.5. Consideration of the triplet properties in aqueous and organic media led to the conclusion that this effect is most probably due to a corresponding increase of the intersystem crossing rate, induced by H2O or D2O. Inclusion of naphthalene in the CD hosts has the effect, at high CD concentration (>0.01 M), of increasing τF with respect to the aqueous medium, the value in α-CD being near to that in the organic media and the value in β-CD intermediate. The spectral and kinetic data are consistent with the predominant formation of 1:2 host:guest complexes with α-CD, and of 1:1 complexes with β-CD, although 2:2 complexes with β-CD are also formed and are identified by their excimer like fluorescence. Several experimental results, including the values of τF, and the temperature dependences of fluorescence and triplet−triplet absorption spectra on one hand and of triplet quenching and triplet−triplet annihilation kinetics on the other, point to a considerable structural flexibility of the 1:2 complex with α-CD.

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