Pyridinium N-phenolate betaine dyes as empirical indicators of solvent polarity: Some new findings
C. Reichardt
Department of Chemistry, Philipps University, Hans-Meerwein-Strasse,
D-35032 Marburg, Germany
Abstract: Solutions of the zwitterionic betaine dye 4-(2,4,6-triphenylpyridinium-1-yl)-2,6-di-
phenylphenolate are solvatochromic, thermochromic, piezochromic, and
halochromic. That means the position of its longest-wavelength intramolecular
charge-transfer absorption band depends on solvent polarity, solution
temperature, external pressure, and the nature and concentration of
added salts. The extraordinarily large negative solvatochromism of this
standard betaine dye has been used to establish UV/vis spectroscopically
an empirical scale of solvent polarity, called ET(30), respectively
ETN scale, meanwhile known for many organic solvents
and solvent mixtures. In this review, the solvatochromic properties
of some new hydrophilic and lipophilic betaine dyes with better solubility
in water (respectively, nonpolar solvents), as well as some recent applications
of these betaine dyes in various fields of interest [e.g., microheterogeneous
(micellar) and polymer solutions, chemical sensors, as well as surface
polarity of silicas, aluminas, and cellulose derivatives] are described.
*Plenary lecture presented at the 28th International Conference on Solution Chemistry, Debrecen, Hungary, 23-28 August 2003. Other presentations are published in this issue, pp. 1809-1919.
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