In situ observation of thermochromic behavior of binary mixtures of phenolic long-chain molecules and fluoran dye for rewritable paper application

This paper reports experimental results of in situ observations of thermochromic processes of two binary mixture systems of 1-(4-hydroxyphenyl)-3-octadecylurea (PU18) + fluoran dye (FD), and 4-hydroxy-N-octadecylbenzamide (PC18) + FD, which were obtained by using FT-IR spectroscopy, reflective optical density, and thermal and X-ray diffraction (XRD) techniques. As the FD, 2 '-anilino-6 '-di-n- butylammospiro [isobenzofuran-1(3H),9 '-[9H] xanthene]-3-one was employed. We called the long-chain molecules of PU18 and PC18 developers, since the FD molecules became either colored or discolored depending on whether the FD and developer mixture formed a supramolecular complex or the FD and developer were separately crystallized. The supramolecular complex, in which FD was colored by opening its lactone ring, was formed when the molten mixture of FD-developer was quenched from elevated temperature to chilled temperature. The separate crystallization of the FD and developer occurred by heating the supramolecular complex. The present study focused to clarify the detailed structure of the supramolecular complex and its decomposition mechanism that, determines the discoloring rate of FD. Small-angle XRD patterns showed that the supramolecular complexes formed lamellar structures with long spacing values of 42.0 angstrom (PU18/FD) and 40.7 angstrom (PC18/FD). However, wide-angle XRD patterns showed broad peaks, indicating disordered lateral packing of the long-chain molecules in the supramolecular complex. The colored supramolecular complexes were thermodynamically metastable, and decomposition occurred by heating to elevated temperatures, making FD discolored. We found that, in the two mixtures of PU18/FD and PC18/FD, the decomposition of the supramolecular complexes included two stages of structural transformations of the developers: (a) the formation of a liquid-crystal-like structure occurred soon after the developer-FD complex was decomposed, and (b) crystallization occurred at elevated temperatures. In accordance with these transformation processes, the degree of discoloring of FD was developed, and a perfect discoloring state was achieved after the crystallization of the developer was completed. The present findings provided new ideas to design the thermochromic mixture system for a high-speed rewritable recording paper system using the FD-developer mixtures.