Far-Red-Emitting TEG-Substituted Squaraine Dye: Synthesis, Optical Properties, and Selective Detection of Cyanide in Aqueous Solution

A comprehensive study of the photophysical properties of triethylene glycol monomethyl ether (TEG)-substituted squaraine dye SQ1 is presented. SQ1 exhibits strong absorption and intense fluorescence emission in the far-red spectral region and high photostability. Interestingly, bis-tegylated SQ1 was characterized to exhibit properties different from its tetrahydroxy analog (SQ2), to which we ascribe attenuation of intermolecular interactions as a result of steric hindrance through introduction of two TEG chains adjacent to the squaraine core. The photophysical properties of SQ1 were also investigated as a function of temperature and pH. In cetyltrimethylammonium bromide (CTAB) micellar media (1.0 mm), the fluorescence quantum yield of SQ1 increased dramatically to 0.22, compared with 0.02 in water. Based on this discovery, SQ1 was found to display excellent sensitivity and selectivity to the presence of cyanide ions (CN^–), with a limit of detection of 1.7 µm. The hypothesized CN^– sensing mechanism was probed, and visual test strips were developed. This work broadens the family of squaraine dyes and demonstrates SQ1 for use as a fluorescent or visual sensing probe.