Picosecond Absorption Spectroscopy of Polymethine Cis-Trans Isomerization

Abstract

Results are presented of time resolved fluorescence and absorption investigations of DDEOCI (3,3'-diethyl-6,6'-diphenyl-9-ethyl-oxacarbocyanine iodide) in solvents which differ in polarity and viscosity. The measured picosecond absorption and fluorescence kinetics probed at various wavelengths of the polymethine dye studied can be fitted to a double exponential decay with a minor fast component of lifetime τ₁ and a major slower component of lifetime τ₂. Furthermore, probing in the S1←S0 absorption region and in the fluorescence region results in a residual with a lifetime of some ns and belonging to the detected red-shifted photoisomer with a low quantum yield (φ < 10%). The lifetime and the preexponential factors of the two components are in methanolic solution uneffected by changes of the anion.The detected two picosecond lifetime components are supposed to belong to two isomeric species (all-trans and mono-cis isomer) with strongly overlapping absorption spectra.