The Halochromism of the 1-Methyl-8-Oxyquinolinium Dye

Abstract

The halochromic behaviour of the title dye (1) was studied in methanol, ethanol, 2-propanol, 1-butanol, 2-butanol, dimethyl acetamide, dimethyl sulfoxide, acetone and acetonitrile, in the presence of sodium iodide, potassium perchlorate and lithium perchlorate. The observed shifts of the charge-transfer band of 1, caused by the addition of increasing concentrations of the salts, was interpreted in terms of dye-salt and solvent-salt interactions and compared with the behaviour of other solvatochromic dyes which exhibit cationic halochromism. The phenomenon of halochromism is related to the more thoroughly investigated solvatochromic behaviour of dyes exhibiting a medium-sensitive absorption arising from an internal charge transfer. 1–8 The position of the charge- transfer band of a halochromic dye in solution is sensitive to the nature and concentration of an added salt. We may distinguish two kinds of behaviour, a cationic and an anionic halochromism, depending on which species is mainly responsible for the spectral changes induced by the salt added to the dye solutions.3 The halochromic shifts may be explained in terms of dye-salt associations.9.10 Most studies have been concerned with the halochromism of pyridinium phenoxides. For these compounds, which exhibit cationic halochromism, it is the association of the dye with the metal cation which determines the spectral shifts in solution. The 1-methyl-8-oxyquinolinium betaine 1 (QB) was originally described by Ueda and Schelly 11. More recently, solvatochromic shifts of this dye in 28 solvents were recorded and compared to the solvatochromic behaviour of the widely employed ET(30) polarity probe 212. A good correlation of the transition energies of the solvatochromic band of 1 (ET(QB)) was obtained with the corresponding ET(30) values in these media, with a correlation coefficient of 0.9935. In the present communication we studied the spectral properties of organic solutions of dye 1 in the presence of various salts. Because of the similarity of its solvatochromic behaviour with that of the ET(30) dye, it seemed natural to compare the halochromic properties of 1 with those of other pyridiniophenolate dyes previously studied by us and others. 1,6,8