Structure Analysis of a Coloring and Decoloring Unsymmetrical Fluoran Dye by Carbon-13 Nuclear Magnetic Resonance Method

Abstract

Unsymmetrical fluoran dyes whose coloring-to-decoloring reversible reaction depends upon the acidity are used in heat-sensitive recording systems. The reversible reaction corresponds to open-to-closed forms of the lactone ring. We observed remarkable chemical shift differences in 4 species of ¹³C NMR signals between open- and closed-forms of the lactone ring of an unsymmetrical fluoran dye, 3-diethylamino-6-methyl-7-chlorofluoran (DEAMCF) in CDCl₃. According to the optimized molecular structure of DEAMCF by a semi-empirical molecular orbital (MO) Austin Model 1 (AM 1) method, the xanthene moiety in colorless form is folded at 166 degree around the C(spiro)-0(xanthene) axis, and the benzolactone moiety is almost perpendicular to the xanthene moiety. The moiety in color form for a molar DEAMCF/ H₂SO₄ ratio of 1/2.2 becomes almost planar and has canonical structure, while the lactone ring opens, according to NMR and MO analyses.