Hydrogen bond donor ability alpha(H)(2) is a good index for finding an appropriate color developer for the molecular design of functional thermal paper

摘要

Thermal paper, which is comprised of sulfonylurea molecule (RSO2NH)-S-1-CO-NH-R-2 (color developer) and the leuco-dye, can develop strong black color, and the developed color is durable. This is because sulfonylurea functional group has sufficient hydrogen bond donor ability to stabilize a pi-delocalized carbocation generated from the leuco-dye due to heat-induced lactone ring opening. Thus, the color-developing complex (CDC) that appears can be stabilized by hydrogen-bond formation between two N-H bonds of the sulfonylurea and the carboxylate anion from the leuco-dye. The hydrogen bond donor ability of the color developer can be evaluated in terms of the alpha(H)(2) H parameter proposed by Abraham. H-1 NMR chemical shift values of the N-H protons in the sulfonylurea and HPLC retention times of the sulfonylurea on a silica column show a good correlation with the alpha(H)(2) parameter values. The essential factors necessary for the sulfonylurea to give rise to high color developing by forming a stable hydrogen-bond complex are also discussed on the basis of the structure and the electronic properties of the sulfonylureas.