Solvent influences on the photophysics of naphthalene:: Fluorescence and triplet state properties in aqueous solutions and in cyclodextrin complexes

The photophysical properties of naphthalene were studied in aqueous solution (H(2)O and D(2)O), in some organic solvents (ethanol, hexane, and acetonitrile), and in complexes with the cyclodextrins (CDs) alpha-CD and beta-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, tau(F), 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 H(2)O or D(2)O. Inclusion of naphthalene in the CD hosts has the effect, at high CD concentration (>0.01 M), of increasing tau(F) with respect to the aqueous medium, the value in alpha-CD being near to that in the organic media and the value in beta-CD intermediate. The spectral and kinetic data are consistent with the predominant formation of 1:2 host:guest complexes with alpha-CD, and of 1:1 complexes with beta-CD, although 2:2 complexes with beta-CD are also formed and are identified by their excimer like fluorescence. Several experimental results, including the values of tau(F), 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 alpha-CD.