Irreversible Thermochromism in Organic Salts of Sulfonated Anils

Condensation of p-amino benzenesulfonic acids with salicylaldehyde and o-vanillin and that of o-aminobenzene-sulfonic acid with o-vanillin via mechanochemical grinding followed by heating under reflux yielded 4-[(2-hydroxy-benzylidene)-amino]-benzenesulfonic acid [4-SA-BA-2H] (1a), 4-[(2hydroxy-3-methoxy-benzylidene)-amino]-benzenesulfonic acid [4SA-oV-2H] (2a), and 2-[(2-hydroxy-3-methoxy-benzylidene)amino]-benzenesulfonic acid [2-SA-oV-2H] (3a), respectively. Anticipating the formation of sulfonate center dot center dot center dot pyridinium supramolecular synthon, organic salts [(4,4'-BPY-2H)(2+)(4-SA-BA-H) 2-] (1), [(4,4'-BPY-2H)(2+) (4-SA-oV-H) 2-center dot 4H(2)O] (2), and [(4,4'-BPY2H)2+(2-SA-oV-H) 2-center dot 2H(2)O] (3) have been obtained by in situ as well as postsynthetic reaction of 1a-3a with 4,4'-bipyridine. Crystallographic investigations of the molecular salts reveal intriguing structural features and indicate the ubiquitous formation of hydroxylate center dot center dot center dot pyridinium interactions over otherwise robust sulfonate center dot center dot center dot pyridinium synthons when the hydroxyl group has an adjacent methoxy group. 1 crystallizes as an anhydrous salt with direct chargeassisted sulfonate center dot center dot center dot pyridinium interactions between crystal formers forming two-dimensional layers which p-interlock to form a rare three-dimensional yellow solid. Crystal formers in hydrated salts 2 and 3 associate through ionic hydroxylate center dot center dot center dot pyridinium interactions instead of anticipated sulfonate center dot center dot center dot pyridinium synthons. The organosulfonate ions in 2 and 3 associate with the help of octameric and tetrameric water clusters into two-dimensional and one-dimensional assemblies, respectively, which are further bridged by bis-pyridinium ions into three-dimensional colored ionic solids. The enhanced optical behavior of 1-3 on formation from 1a-3a is investigated through diffuse reflectance studies. Schiff base 1a and its salt 1 exhibit redshift on heating, both 2a and 2, as well as 3a do not respond notably to heat, while as 3 undergoes a remarkable blue shift on heating. Optical studies vis-a`-vis crystallographic investigations rule out the conventional and general mechanism, i.e., enol. cis-keto isomerization and instead strongly indicate packing changes to be the cause of irreversible thermochromism observed in 1 and 3. Furthermore, the resultant molecular salts exhibit significant thermal and chemical stability, and their formation is also substantiated by spectroscopic and analytical methods.