Multi-Stimuli-Responsive Organo-Sulfonated Anil and Its Organic Complex

Abstract

Sensing by molecules is an intriguing phenomenon that can lead to next-generation intelligent materials. Stimuli-responsive organic molecules or molecular complexes, although less reported, can open up new avenues and opportunities in the area of artificial intelligence and memory. Extending the scope of organo-sulfonated Anils as stimuli-responsive materials, we report hydrated sulfonated Anil 1 and its nonstoichiometric bipyridyl complex 1.BPY. The formation of new materials is well supported and further substantiated through the solid-state structural elucidation of 1. Thermal and crystallographic studies validate the presence of two lattice water molecules each in 1 and 1.BPY. Structural studies also indicate that 1 undergoes intramolecular proton transfer between sulfonate and imine groups and exists in the zwitterionic form. Both 1 and 1.BPY respond to external stimuli: temperature, solvent polarity, and ammonia vapor. The thermochromism in both forms is reversible and interestingly triggered by the breathing of lattice water, a rare phenomenon in organic materials, supported by Fourier-transform infrared (FT-IR), thermal, and diffraction studies. Compared with the transition temperature of 130 °C for 1, 1.BPY undergoes a color change at 65 °C, and their heated forms, 1-Heat and 1.BPY-Heat, in turn, can be used as humidity sensors. Both solid forms exhibit solvatochromism and show emission turn-on in nonprotic polar solvents, dimethyl sulfoxide (DMSO) and dimethylformamide (DMF). The incipience of emission of 1.BPY in highly polar solvents DMSO (φ = 12%) and DMF (φ = 46%), and its absence in the solvents of relatively lower polarity such as H2O and MeOH as well as in solid forms, may be attributed to aggregation-induced quenching, which is supported by the solubility and DLS studies of 1 and 1.BPY in the solvents. Very interestingly and to the best of our knowledge, molecular solids 1 and 1.BPY represent the first examples of organic materials that exhibit emission turn-on on exposure to fumes of a base. When exposed to the fumes of ammonia, 1 and 1.BPY undergo a color change from maroon and orange to yellow, respectively, and the 1.NH3 and 1.BPY-NH3 forms show a striking green emission, which may be attributed to proton transfer within the molecular systems most likely involving the excited-state intermolecular proton-transfer (ESIPT) pathway. The overall analyses of the results indicate that the molecular complex 1.BPY is a better material vis-à-vis its response time (thermochromism), intensity (solvatochromic), and fatigue (vapochromism) and substantiate the scope of the crystal-engineering principles in designing next-generation smart materials.