The heavy-atom effect on the photophysics of aromatic hydrocarbons in the presence of solid clays

The exchange of the original cation present on a Laponite clay (usually Na) for heavy atoms such as Rb+, Cs+, and Tl+ significantly alters the emissi on characteristics of some aromatic hydrocarbons (p-terphenyl, naphthalene, pyrene, and biphenyl). The increase of the atomic mass of the cation induc es a decrease of the fluorescence emission simultaneous with an increase of the emission in the region of lower energies of the spectra, ascribed to t he phosphorescence of those hydrocarbons. Time-resolved experiments for the pyrene-clay system showed a decrease of singlet lifetimes for the heavier atoms. Hydrocarbon aggregates were also detected from both the emission spe ctra and the time-resolved studies. The "excimer-like" emission showed long er lifetimes (10-25 ns) than the monomolecular hydrocarbons (1-3 ns), as al ready found for other similar systems. The amount of aggregates increased f or the heavier cations due to the smaller surface available on the clay par ticles. Experiments increasing the amount of Tl+ in samples containing a co nstant concentration of naphthalene allowed evaluation of the distance betw een the heavy atoms and the probe on the clay surface. The Perrin model tre atment was used and resulted in approximately R-0 = 9.2 Angstrom. (C) 2001 Academic Press.