Solvatochromism of Polymethine Dyes as a Basis for the Development of New Molecular Probes

The solvatochromism of polymethine (cyanine) dyes (PDs) was studied to determine the influence of solvent properties on their spectral characteristics. For this purpose, the absorption and fluorescence spectra of PDs of various classes were measured in solvents of different nature (from highly polar to nonpolar and from proton-donating to aprotic). Based on the experimental results obtained and literature data available, a correlation was established between the wavenumbers of the absorption and fluorescence maxima of PDs (ν_abs and ν_fl, respectively) and the polarity (dielectric constant ε) and polarizability (refractive index n) of the solvent. It has been shown that the ν_abs and ν_fl values poorly correlate with Bayliss function f(n²) = (n² – 1)/(2n² + 1). However, good linear correlations are achieved by combining the Bayliss function with the solvent dielectric constant functions f(ε) or ϕ(ε) in the form f(n²) + αf(ε) or f(n²) + αf(ε) (where f(ε) = (ε – 1)/(2ε + 1) and ϕ(ε) = (ε – 1)/(ε + 2)). Such correlations were found for symmetrical PDs – carbo-, dicarbo-, and, partially, tricarbocyanines and monomethines. No significant effects of the proton-donating ability or nucleophilicity of the solvent on such correlations were found. Therefore, solvent polarizability and polarity are the dominant factors determining the spectral shifts of symmetrical PDs. The results obtained provide a basis for the use of symmetrical PCs as probes and sensors of the polarity/polarizability of the molecular environment in various systems, with potential applications in biochemistry, biophysics and other research fields.

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