Hydrogen Bond Donor Ability α2H Is a Good Index for Finding an Appropriate Color Developer for the Molecular Design of Functional Thermal Paper

Thermal paper, which is comprised of sulfonylurea molecule R1SO2NH–CO–NH–R2 (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 π-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 α2H parameter proposed by Abraham. 1H 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 α2H 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.