Stimuli-responsive fluorochromic organic salt

Abstract

Crystal engineering is emerging as an important approach to design functional materials. In this work, we explore the scope of a sulfonate–pyridinium supramolecular synthon to design the organic salt [(TPT-H)₂⁺(NDSA-H)²⁻·4H₂O] (1), based on 1,5-naphthalenedisulfonic acid (NDSA-2H) and 2,4,6-tris(2-pyridyl)-s-triazine (TPT). Structural studies of 1 establish proton transfer between crystal formers and their aggregation through the water-masked sulfonate–pyridinium synthon to form one-dimensional hydrogen bonded tapes, which further aggregate through weak π–π and C–H⋯O interactions to form a three-dimensional layered solid. The greenish solid 1 exhibits mild charge transfer interactions in the solid-state, as indicated by the red-shift of the absorption beyond 400 cm⁻¹ in the diffuse reflectance spectrum. Interestingly, 1 exhibit mild brownish emission (λ_max = 550 nm), which undergoes remarkable blue-shifting on exposure to ammonia fumes (λ_max = 450 nm), heat (λ_max = 494.8 nm), and grinding (λ_max = 542.9 nm). This multi-stimuli responsive behavior of 1 has been investigated in detail, with the help of diffuse reflectance, fluorescence, thermal, spectroscopic, and diffraction studies, and a plausible commentary of the behavior in light of its structure is provided. The intriguing multi-stimuli response behavior of 1 indicates the potential scope of sulfonate–pyridinium based organic salts in applications as sensors and intelligent materials.