Photophysics of Cyanine Dyes on Surfaces: Laser-Induced Photoisomer Emission of 3,3'-Dialkylthiacarbocyanines Adsorbed on Microcrystalline Cellulose

Abstract

The photophysics of three thiacarbocyanine dyes, 3,3'-dimethylthiacarbocyanine iodide (DMTCC), 3,3'-diethylthiacarbocyanine iodide (DETCC), and 3,3′-dipropylthiacarbocyanine iodide (DPTCC) was studied when adsorbed on microcrystalline cellulose in the concentration range from 5.0 · 10-4 to 10.0 μmol g-1. Using ground-state diffuse reflectance absorption technique, only H aggregate formation was detected for all the probes. The amount of aggregate formed depends on the hydration degree of the sample, always decreasing with sample dryness. The fluorescence quantum yields for all the adsorbed dyes are one order of magnitude higher than those observed in nonviscous solvents, being 0.98 for DMTCC, 0.96 for DETCC, and 0.63 for DPTCC. Laser-induced fluorescence emissions were recorded (using an intensified-charge-coupled-device detection system) as a function of the laser power, showing that for dry concentrated samples irradiated with high laser intensity, a second fluorescence emission band (bathochromically shifted relative to the monomer emission) was detected. This emission shows a supralinear dependence on laser power. The new emissions here detected arise from fluorescent photoisomers formed via singlet monomers, by a two-photon absorption process.