Near-Infrared Fluorescent Probes:  Synthesis and Spectroscopic Investigations of A Few Amphiphilic Squaraine Dyes

With the objective of developing near-infrared fluorescence probes for biological applications, a few squaraine dyes 3a − d, containing amphiphilic substituents, were synthesized and their photophysical properties have been investigated in the presence and absence of the organized media. These dyes exhibited absorption in the range 630−650 nm, with significant absorption coefficients (ε = 1−3 × 105 M-1 cm-1) in the aqueous medium. The fluorescence spectra of these dyes showed emission maximum from 660 to 675 nm, depending on the nature of substituents. The fluorescence quantum yields were in the range from 0.15 to 0.21 in ethanol, but 10 times lower values were observed (Φf = 0.01−0.02) in the aqueous medium. In the presence of micelles such as cetyltrimethylammonium bromide, sodium dodecyl sulfate, and Triton X-100, these dyes showed negligible changes in their absorption properties, whereas a significant enhancement (5−10-folds) in their fluorescence yields was observed. Picosecond time-resolved studies indicated that these dyes show single-exponential decay in ethanol and ethanol−water mixtures; however, they exhibit biexponential decay with longer lifetimes in the presence of the micellar media. The results indicate that these novel amphiphilic squaraine dyes 3a − d, which exhibit favorable photophysical properties, good solubility in the aqueous medium, and interact efficiently with micelles, can have potential biological applications as near-infrared fluorescence sensors.