Water Switched Reversible Thermochromism in Organic Salt of Sulfonated Anil

We report concomitant stoichiomorphs [(4,4′-BPY-2H)2+(SA-SY-H)−2·4H2O] 1 and [(4,4′-BPY-H)+(SA-SY-H)−·4H2O] 2 formed in the same reaction vessel, with formation of 2 following 1. The expeditious formation of 1 with half equivalent of bipyridyl former, attributed to 2-fold bifurcated methoxy assisted hydroxyl-pyridinium synthon, leaves behind the solution with an excess of bipyridyl, resulting in the formation of 2. Organic salts 1 and 2 crystallize as triclinic (P̅1) and monoclinic (P21/c) tetrahydrates, with D3 acyclic non-branched and branched lattice water clusters. Hirshfeld and fingerprint studies reveal reciprocal O–H···H–O and weak dispersive H···H contacts are the major intermolecular interactions responsible for supramolecular aggregation. Interestingly, 1 exhibits reversible thermochromism switched by lattice water, while 2 is not thermochromic. Photochromism in both sulfonated anil based organic salts 1 and 2 is ruled out due to the significantly lower Vfree value than required for the free rotation about the imine bond to generate cis–trans isomers. Keto–enol prototropism-triggered thermochromism is also ruled out because of the intramolecular sulfonate proton shift to the imine nitrogen as well as the position of absorption curves in the Kubelka–Munk plots. The lattice water switched mechanism for reversible thermochromism in 1 is proposed and substantiated by FT-IR, TGA-DTA, DR-UV, and P-XRD studies and further validated by the Tauc plots. To the best of our knowledge, 1 represents the first example of water-triggered all-organic reversible thermochromic material and has been utilized as a thermal sensor for the reflux reactions in low boiling solvents.