Thermo-solvatochromism of Merocyanine indicators: Effects of the Hydrophobic Character of the Alkyl Group

Clarissa T. Martins 1Omar A. El Seoud 1Michelle S. Lima 

1. Instituto de Química, USP (IQUSP), Av. Lineu Prestes 748, Sao Paulo 05508-900, Brazil

Abstract

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.

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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
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