Fine-Tuning Dual Absorption Enables Fluorans Multicolor Hydrochromic Switching

Abstract

With the growing emphasis on green and sustainable development, the development of low-energy, environmentally friendly intelligent responsive materials has become an important direction. Among these, hydrochromic molecular switches have attracted considerable attention in recent years due to their mild stimulus (water), excellent reversibility, and ease of integration with other systems. These attributes make them particularly suitable for applications such as rewritable paper, smart labels, and environmental sensors. However, most existing molecular switches exhibit only a single visible absorption band, which inherently restricts their chromatic versatility and necessitates multi-dye blending to achieve multicolor outputs. To overcome this limitation, we present a rational molecular design strategy based on asymmetric amino modification of fluoran derivatives. By introducing distinct electron- donating and hydrophilic amino groups at the asymmetric 2- and 6-positions of the xanthene ring, we successfully constructed a series of high-performance hydrochromic molecular switches (M1–M3). Upon exposure to water, these molecules undergo a complete ring-opening isomerization, giving rise to intramolecular dual absorption in the visible region with peak separations exceeding 100 nm. Control experiments and theoretical calculations reveal that the two absorption bands originate from different localized π–π* transitions within the xanthene core, and critically depend on the 2-position amino substituent to activate the higher-energy transition channel. This unique electronic architecture enables precise color-switching behavior, yielding three distinct macroscopic colors: magenta, dark green, and purple-black. These hydrochromic molecular switches, when immobilized on solid substrates, demonstrate excellent reversibility over more than 50 write–erase cycles and maintain high optical contrast. Furthermore, they are fully compatible with water-jet printing, allowing for high-resolution, multicolor patterning on cellulose-based media for rewritable paper applications. This work not only provides a simple and efficient route to multicolor hydrochromism but also establishes asymmetric amino modification as a general principle for engineering intramolecular dual absorption, offering a new perspective for the molecular design and precise color regulation of next-generation stimuli-responsive dyes.

Keywords

Hydrochromic material / Molecular switches / Fluoran / Sustainable chemistry / Intra-molecular dual absorption / Water chemistry / Supramolecular chemistry / Water-jet printing

RIGHTS & PERMISSIONS

2025 SIOC, CAS, Shanghai, & WILEY-VCH GmbH