Our previous work on solvatochromism has indicated that probe's pKa and hydrophobic character (indicated by log P) are relevant to solute-solvent interactions. The indicators of widely different properties make evaluation of the relative contribution of these structural properties not viable. In order to remedy this problem, a series of 2,6-dibromo-4-[(e)-2-(1-alkylpyridinium-4-yl)ethenyl]phenolate. For this series, similar pKa´s and variable log P were expected, and indeed determined (pKa = 5.15, and log P = -0.16, 1.12 and 2.71, for R = methyl, n-butyl and n-octyl, respectively). Their solvatochromism was studied at 25 ºC in 34 protic and aprotic solvents. E_T(probe) in kcal/mol correlated linearly with the E_T(30) scale, with slopes of ca. 0.65. In pure solvents, for all indicators, the multi-parameter correlation showed the order of contribution of solvent parameters to E_T(probe): acidity (a) > dipolarity/polarizability (p*) > log P. Thermosolvatochromism was studied in binary mixtures of water with methanol, MeOH, 1-propanol, 1-PrOH, acetonitrile, MeCN, and dimethylsulfoxide, DMSO, respectively. The results were treated according to a model that considers the exchange in the probe solvation micro-sphere of the two pure solvents, and hydrogen bonded species, e.g., MeOH-W. In alcohols, all are preferentially solvated by the organic solvent, and more by the ROH-W species. In MeCN, the probes are more solvated by water and by MeCN-W than by pure MeCN. In DMSO, the probes are more solvated by the pure solvent, whereas the efficiency of DMSO-W in displacing both W and DMSO is lower than the mixed species of the above-mentioned solvents. As expected, E_T(probe) decreases as a function of increasing T, due to the concomitant decrease in solvent structure. The multi-parameter correlation in the above-mentioned alcoholic mixtures was performed and the correlation coefficients showed that the response of E_T(probe) to solvent hydrophobicity increases as a function of increasing the chain length of the probe. We thank FAPESP for financial support and a pre-doctoral research fellowship to C. T. Martins; CNPq for a research productivity fellowship to O. A. El Seoud, and a PIBIC fellowship to M. S. Lima; Paulo A. R. Pires and C. Guizzo for their help.

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1. Solvation in Pure and Mixed Solvents: An Overview