Eb. Tada et al., Solvatochromism in pure and binary solvent mixtures: effects of the molecular structure of the zwitterionic probe, J PHYS ORG, 13(11), 2000, pp. 679-687
The solvatochromic behavior of 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio)
phenolate (WB) was studied by UV-visible spectrophotometry in 32 pure solve
nts, in binary mixtures of l-butanol-cyclohexane (BuOH-Cyhx), and of water
with methanol, ethanol, I-propanol, 2-butoxyethanol (2-BE), acetonitrile, 1
,4-dioxane and THF. The solvent polarity, E-T(33) in kcal mol(-1), was calc
ulated from the position of the longest-wavelength intramolecular charge-tr
ansfer absorption band of WE and the results were compared with those for 2
,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate [RB, E-T(30)] add of 1
-methyl-8-oxyquinolinium betaine [QB, E-T(QB)]. For pure solvents, ET(33) i
s a linear function of ET(30), with a slope of practically unity. Steric cr
owding from the two ortho phenyl rings of RE hinders the formation of H-bon
ds with solvents, which results in similar susceptibilities of WE and RE to
solvent acidity. For binary solvent mixtures, all plots of ET versus the m
ole fraction of l-butanol or water are nonlinear owing to preferential solv
ation of the probe by one component of the mixed solvent and, when applicab
le, to solvent micro-heterogeneity. Preferential solvation due to non-speci
fic and specific probe-solvent interactions was calculated for BuOH-Cyhx an
d water-acetonitrile. Both solvation mechanisms contribute to the non-ideal
behavior in the former binary mixture, whereas probe-solvent specific inte
ractions dominate the solvatochromic behavior in the latter. The compositio
n of the probe solvation shell was calculated. In aqueous alcohols, prefere
ntial solvation is by the alcohol. In water-aprotic solvent mixtures, prefe
rential solvation of RE and WE is by the solvent which is present in lower
concentration, whereas QB seems to form its own, water-rich solvation shell
over a wide range of water concentration. Copyright (C) 2000 John Wiley &
Sons, Ltd.