Black hydrochromic fluoran molecular switches: substituent positional isomerization effects and multicolor water-jet printing

Abstract

The colors that can be provided by hydrochromic molecular switches, which directly regulate isomerization by water, are very limited, especially for the most used color black, which remains difficult to achieve. Herein, an ideal hydrochromic black display was successfully achieved via the substitution of nitro groups in the non-conjugated chromogenic structure of an acidochromic black fluoran dye. The positional isomerization effect of nitro substitution on the synthesis and hydrochromic properties of the obtained hydrochromic molecular switches both in solution and a solid matrix were explored and discussed in detail. Based on the spectral uniqueness of black dyes, a multicolor water-jet printing display with a single molecular switch was also realized using different ‘aqueous inks’. This work not only promotes the development of hydrochromic molecular switches and water-jet rewritable paper but also provides a reference for the design and synthesis of other new stimuli-responsive molecular switches and materials for different purposes.