Fluorescence lifetimes of adsorbed polymethines

Abstract

Polymethine dyes are used as colourants and as spectral sensitizers. Thiacarbocyanine and thiadicarbocyanine dyes were studied in solution, on electronically inert (CaF₂) 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 theS 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′-diethylthiadicarbocyanine chloride (DTDC) in different solvents and on CaF₂. The adsorption isotherms of DTCC and DTDC on CaF₂ 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 dimer formation could be spectroscopically detected. Förster energy transfer between the adsorbed molecules was therefore taken into account. On active surfaces, electron transfer and Förster energy transfer were investigated as competing processes. The results were related to the mechanistic aspects of the spectral sensitization of silver halides.