The synthesis and photostability of novel squarylium indocyanine dyes

Novel, water-soluble, squarylium indocyanine dyes with various N-substituents on 3H-indolenine were synthesized. The maxima of absorption and emission wavelengths of the dyes in different solvents were in the range 628–670 nm and exhibited negative solvatochromism with increasing solvent polarity in protic solvents. Dyes with N-benzyl rings (such as N-benzyl, N-carboxylbenzyl and N-fluorobenzyl) displayed greater photostability than dyes containing N-alkyl groups (such as N-ethyl and N-carboxylpentanyl) in aqueous solution. The electron-withdrawing group (such as carboxyl and fluoro group) on the N-benzyl group of cyanine dyes improved photostability compared to dyes that contained an electron-donating group (such as methyl). The fading constant k of the N-carboxylbenzyl dye was considerably lower than that of the N-carboxylpentanyl dye.