Thermo-solvatochromism of Merocyanine indicators: Effects of the Hydrophobic Character of the Alkyl Group
|Clarissa T. Martins 1, Omar A. El Seoud 1, Michelle S. Lima|
1. Instituto de Química, USP (IQUSP), Av. Lineu Prestes 748, Sao Paulo 05508-900, Brazil
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. ET(probe) in kcal/mol correlated linearly with the ET(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 ET(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, ET(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 ET(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.
Presentation: poster at 18th Conference on Physical Organic Chemistry, Posters, by Clarissa T. Martins
See On-line Journal of 18th Conference on Physical Organic Chemistry
Submitted: 2006-05-26 17:47 Revised: 2009-06-07 00:44