Solvent polarity scales. 1. Determination of ET and π* values for phosphonium and ammonium melts

The solvent polarity of a series of organic salts in their molten state has been determined using the solvatochromic dyes: pyridinium N‐phenolate betaine, the basis of the ET scale, and N,N‐diethyl‐4‐nitroniline, one of the primary standards for the π* scale. Each of these dyes was dissolved in the following liquids: lithium acetate–sodium acetate–potassium acetate eutectic; ethylammonium chloride; dimethylammonium cloride; diethylammonium nitrate; ammonium trifluoracetate; tetraethylammonium acetate; tetrabutylphosphonium chloride; octyltributylphosphonium chloride; dodecyltributylphosphonium chloride; tetrabutylammonium bromide; tetrabutylphosphonium bromide; octyltributylphosphonium bromide; dodecyltributylphosphonium bromide; octyltributylphosphonium iodide; dodecyltributylphosphonium iodide; tetrahexylammonium benzoate; methyltrioctylphosphonium dimethylphosphate; and methyltrioctylammonium chloride. The ultraviolet and visible spectra were measured for each of these. In addition, data were collected as a function of temperature and in the presence of neutral and ionic additives. In general, these molten salts represent solvent polarities equivalent to moderately polar aprotic solvents such as acetone and acetonitrile. The salts with cations capable of being hydrogen bond donors gave considerably higher values, equivalent to or higher than water, the solvent with the highest value on the ET scale. These and the eutectic mixture are thought to involve specific dye–solvent interactions which cause these anomalously high values.