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 (Φf) in the presence of both cationic and anionic micelles, but very small changes with neutral TX100], whereas the behaviour of DTC is normal. The Φf 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 Φf values undergo small variations with w0. A decrease in the photoisomerization quantum yield, and a rise in Φf, 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. © 2002 The Royal Society of Chemistry and Owner Societies.