Water Switched Reversible Thermochromism in Organic Salt of Sulfonated Anil

We report concomitant stoichiomorphs [(4,4'-BPY-2H)(2+)(SA-SY-H)(-)(2)center dot 4H(2)O] 1 and [(4,4'-BPY-H)(+)(SA-SY-H)(-)center dot 4H(2)O] 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 ((P) over bar1) and monoclinic (P21/c) tetrahydrates, with D3 acyclic non-branched and branched lattice water clusters. Hirshfeld and fingerprint studies reveal reciprocal O-H center dot center dot center dot H-O and weak dispersive H center dot center dot center dot 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 V-free 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.