New highly fluorescent ketocyanine polarity probes

The syntheses and spectral properties of three new and highly fluorescent solvent polarity probes are described. They are found to be extremely sensitive to solvent polarity in that spectral red shifts in both absorption and fluorescence spectra occur upon increasing solvent polarity. Excitation and emission data of the dyes in a set of different polar solvents are given. The emission data are compared with the standard ET N values of solvent polarity and a linear correlation is obtained over a wide range. The origin of the unusual solvatochromic properties is discussed in terms of the resonance structures of this new group of molecular probes. Their outstanding features include high spectral sensitivity to polarity, high molar absorptivities, high fluorescence quantum yields, longwave excitation and emission, insignificant quenching by oxygen, and a sufficient stability in aqueous solution. Therefore, the new probes are considered to be advantageous over other polarity probes used so far in probing biochemical and biological systems.