Photophysics and photochemistry of hydrophilic cyanine dyes in normal and reverse micelles

Abstract

The photophysical and photochemical properties of the negatively charged hydrophilic cyanine dyes 1–3 were studied in isooctane/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/water and cyclohexane–hexanol/Triton X-100 (TX100)/water reverse micelles and in aqueous solutions in the presence of surfactants. In spite of the opposite total charges, 2 and its structural analog, cationic 3,3′-diethylthiacarbocyanine (DTC), show similar photophysical and photochemical behaviour in AOT reverse micelles. However, in normal micelles their photophysical properties are quite different: 2 exhibits unusual behaviour [a sharp increase in the fluorescence quantum yield (Φ_f) in the presence of both cationic and anionic micelles, but very small changes with neutral TX100], whereas the behaviour of DTC is normal. The Φ_f values of 1–3 in AOT reverse micelles suffer a steep drop when w₀ increases from 0 to 10 (w₀ = [H₂O]/[AOT]) and remain essentially constant at higher w₀, while in TX100 reverse micelles the Φ_f values undergo small variations with w₀. A decrease in the photoisomerization quantum yield, and a rise in Φ_f, in S₁→T intersystem crossing, in the back isomerization activation energy and in the preexponential factor were found for 1–3 in AOT reverse micelles with small w₀. The unusual behaviour of 1–3 in micelles is attributed to the bipolar structure of the dyes.