Why does a color-developing phenomenon occur on thermal paper comprising of a fluoran dye and a color developer molecule?

The mechanism of the color development on thermal paper, comprising a fluoran dye (S-205) and a color developer molecule, such as bisphenol A, was elucidated based on spectroscopic analyses (IR, and NMR) on an isolated black-colored compound prepared by a reaction of S-205 and bisphenol A. We propose that this black-colored compound is, indeed, a color-developing complex (CDC) with a definite molar ratio (S-205: bisphenol A = 1: 4) which has been formed between the open-form S-205 (generated by the cleavage of the lactone ring of S-205: zwitterion) and bisphenol A through hydrogen-bonding. The enthalpy gain associated with the formation of CDC plays an important role for black-color formation on thermal paper.