Versatile Hydrochromic Fluorescent Materials Based on a 1,8-Naphthalimide Integrated Fluorophore-Receptor System
Atmospheric moisture uptake by materials of varying hygroscopicity can substantially alter their physicochemical properties and thus has long been a point of interest in the development and manufacture of various commodities. In particular, hydrochromic materials have achieved much commercial success as sensors and for their applications in the paint and coatings industry. It is essential that these materials are scalable, easily processed from their raw materials, exhibit point-of-use versatility, and are sufficiently sensitive to changes in water content. Herein we report the design, synthesis, and material processing of two integrated fluorophore-receptor systems that undergo reversible fluorescence hydrochromism in the solid state in response to changes in atmospheric relative humidity. The systems are comprised of a 1,8-naphthalimide fluorophore coupled to an amphiphilic pyridinium acceptor and its bromide counterion, which serves as a water-sensitive receptor capable of engaging the emissive unit by way of excited state charge transfer. Dynamic vapor sorption isotherms revealed that the pristine raw material experiences a blue-to-green 27 nm bathochromic shift in its emission upon 3.8 weight % water uptake. Processing of the materials into hydrochromic ultra-light agar-based hygroscopic aerogels significantly enhanced their hydrochromic sensitivity, effectively decreasing their critical relative humidity from ≈75 % to less than 20 % with bathochromic shifts up to 93 nm. Fluorescent hydrochromic silica gel/carboxymethyl cellulose films and inkjet printer compatible inks were developed and demonstrated increased point-of-use versatility. This work makes the case for the inclusion of the integrated fluorophore-receptor system as a design principle for the construction of compounds that express aggregation induced emission, and/or aggregation induced emission enhancement in the solid state, and their continued post-synthetic development, and materials processing.