The effect of anionic, cationic and neutral surfactants on the photophysics and isomerization of 3,3′-diethylthiacarbocyanine

The photophysical properties and isomerization of 3,3′-diethylthiacarbocyanine (DTC) were studied in isooctane/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/water reverse micelles and in aqueous solutions containing sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB) and chloride (CTAC) and Triton X-100 (TX100) at concentrations below and above the critical micelle concentrations (c.m.c.). The absorption and fluorescence spectra of DTC in AOT reverse micelles are red-shifted and the nonradiative deactivation rate constant decreases strongly relative to that in water, leading to higher fluorescence quantum yields (Φf) and lifetimes. The DTC fluorescence decay in AOT reverse micelles and TX100 normal micelles is biexponential. SDS induces aggregation of DTC in water at concentrations well below the c.m.c. (10−5–10−3 mol l−1). Addition of SDS and TX100 at concentrations just above the c.m.c. increases Φf steeply, while CTAB (or CTAC) exerts such an effect only at concentrations much higher than the c.m.c. A rise in the isomerization activation energy and the pre-exponent is observed in the reverse micelles with small w0 (w0 = [water]/[AOT]); the isomerization rate constant decreases in SDS, TX100 and CTAC micelles.