Photophysical and aggregation properties of a long-chain squarylium indocyanine dye

The photophysical properties of a long-chain squarilium indocyanine dye, bis[(1-octadecyl-3,3-dimethylindol-2-ylidene)methyl]-squaraine (SQI1), were studied in various solvents and in normal and reverse micelles. The dependence of these properties on different solvent parameters is compared with the literature data on other squaraines. The absorption and fluorescence maxima of SQI1 and similar heterocyclic squarilium cyanines exhibit a good linear correlation with the Lorentz-Lorenz function. The rate of nonradiative deactivation of the SQI1 fluorescent state increases with the solvent polarity, but the viscosity effect is very small. A reversible photoisomerization process leading to the formation of a transient photoisomer with a long-wavelength absorption band was detected in SQI1. The Lifetime of the photoisomer varies within a wide range, depending on the solvent polarity. SQI1 forms nonfluorescent aggregates in aqueous medium, which dissociate into monomeric molecules upon addition of TX100 above the critical micelle concentration. (C) 2001 Elsevier Science B.V. All rights reserved.