Microenvironments induced ring-closing of halide salts of oxazolidines: a rare inverse proton gradient process and its application in water-jet rewritable paper

Abstract

Proton gradient reactions are common, but those with inverse proton gradients are rare. Inspired by the phenomenon that stomach can release acid with microenvironment changes, an inverse proton gradient process along with a ring-closing of the precursors of oxazolidine molecular switches (i.e., ROF⁺X⁻s) was achieved within specific microenvironments both in solution and on solid substrate. Nucleophilicity and concentration of solution were found playing important roles in this process. Furthermore, benefits from this ring-closing process arose from inverse proton gradient, a potential application of ROF⁺X⁻s replacing their akalization products—ring-closed forms (RCFs) of oxazolidines in water-jet rewritable paper (WJRP) was developed. Influence of dissolving solvent and drying temperature on hydrochromic performances of ROF⁺X⁻ based WJRP were inspected and compared to its corresponding RCF based WJRP. Results indicate that the former outperforms the latter in color depth, coloration speed and retention time. The water-jet prints on ROF⁺X⁻ based WJRP exert excellent legibility, resolution, uniformity, and repeatability. This research achievement will not only promote the industrialization of oxazolidines derivatives based WJRP by greatly reducing production costs from a perspective of synthetic industrialization, but may also provide a reference for better understanding of similar microenvironment-induced phenomena of inverse proton gradient in complex biosystems.