Molecular structure and spectral properties of indolenine based norsquaraines versus squaraines

Novel indolenine based norsquaraine dyes, wherein the oxygen of the squaric acid bridge was substituted with a barbituric or a dicyanomethylene group, were synthesized and their molecular structure, spectral and luminescent properties were compared to those of analogous squaraine dyes. The molecular structure was investigated using X-ray analysis, NMR spectroscopy and ab initio DFT B3LYP/6-311G (d, p) simulations. The calculated populations of possible conformers and the barriers of internal rotations were found to be in good agreement with the experimental data. Norsquaraines absorb and emitt light within the same long-wavelength spectral range as the corresponding squaraines but due to intramolecular H-bonds and increased conformational rigidity they were less sensitive to solvent polarity and the presence of protein (BSA).