Fluorescence lifetimes of adsorbed polymethines

摘要

Polymethine dyes are used as colourants and as spectral sensitizers. Thiacarbocyanine and thiadicarbocyanine dyes were studied in solution, on electronically inert (CaF_2) and active surfaces (AgCl, ZnO), using steady state and time resolved fluorescence spectroscopy. Fluorescence lifetimes are strongly dependent on the environment. Deactivation processes such as nonradiative decay and fluorescence are discussed. The widely accepted model for the S_1 deactivation of polymethines is torsion of the polymethine chain. In the adsorbed state, the quantum yield of fluorescence increases and shows a strong concentration dependence. We studied 3,3'-diethylthiacarbocyanine iodide (DTCC) and 3,3'-diethyl-thiadicarbocyanine chloride (DTDC) in different solvents and., on CaF_2. The adsorption isotherms of DTCC and DTDC on CaF_2 are of non-Langmuir type. The fluorescence lifetimes of adsorbed polymethines are not constant and start to decrease even at surface concentrations which are much less than one monolayer. The quenching was observed at concentrations lower than those at which dirtier formation could be spectroscopically detected. Forster energy transfer between the adsorbed molecules was therefore taken into account. On active surfaces, electron transfer and Forster energy transfer were investigated as competing processes. The results were related to the mechanistic aspects of the spectral sensitization of silver halides.