Dielectric Properties Estimation of Solvation Shells and Nano-cavities by Solvatochromic Shift in Organic Dye Spectrum

Computer modeling of the dielectric properties of the cucurbit [7]uril (CB[7]) cavity based on the Onsager-Liptay model was carried out by analyzing the solvatochromic shift of the absorption spectrum of 1- (3ammoniumpropyl) -4 - [(E) -2- (3,4-dimethoxyphenyl) ethynyl]pyridinium upon the formation of the inclusion complex with CB[7] in a water solution. The CB[7] cavity was considered as a polar environment into which the dye chromophore is placed. Positions of the maxima of the dye absorption spectra were measured experimentally in the following solvents: methanol, ethanol, i-propanol, n-butanol and water with known dielectric and optical properties. These values were used for parametrization of the Onsager-Liptay equation. Quantum-chemical calculations were used to determine molecular structures, dipole moments of the ground and excited states, and polarizability of the dye. Theoretical value of the effective dielectric permittivity of the cavity is in good agreement with the literature data derived by another method. Also, we investigated the preferential solvation effect of pyridinium-N-phenolate betaine (organic dye) taken as a molecular probe in two binary solvents (water-BuOH and water-BuNH2) in concentration range 0-100%. The spectral solvatochromic shift in the dye's solvation shell and the environment's dielectric permittivity were linked using the aforementioned Onsager-Liptay model. It was demonstrated that the second component of the binary solvent in water solution greatly depletes the dye's solvation shell. Both the water-BuOH and water-BuNH2 solvents exhibit this property. The water-BuNH2 solvent scenario exhibits a higher absolute spectral shift than the second case. However, in the case of water-BuOH, the relative depletion is greater. Thus, we propose a new method for investigation the dielectric constant both in bulk liquids and in very small liquid volumes. The method's unique potential lies in the creation of nanoprobe devices that may be used to test the dielectric constant of small objects and nano-regions and to adjust the instantaneous solvent concentration in evaporating sessile droplets of binary solvent in various zones of the droplet.