Switching the Solid-State Emission of Organic Crystals through Coformer Choice and Vapochromism

Luminescence in aggregated systems is an intriguing phenomenon that can be exploited for the development of smart commercial materials. The establishment of a structure-property relationship is crucial to designing and improvising solid-state emitters. We report an organo-sulfonate hydrate (1) that exists in zwitterionic form and forms an isolated head-to-tail dimer without long-range p-stacking to form a nonemissive solid. Utilizing the understanding of the sulfonate-pyridinium supramolecular synthon, the emission of 1 is turned on and off by cocrystallization with 1,10-phenanthroline and 2,2'-bipyridine (2,2'-Bpy) in 2 and 3, respectively. Structural and Hirshfeld studies validate that the packing modulations triggered by the pyridyl precursors are responsible for the emission switching. Charge-transfer dimers formed in 2 stacks through pi-interactions to form emissive mixed-stack aggregates (lambda(max) = 610 nm and Phi 1.1%), while the charge-transfer complex formed in 3 exhibits poor pi-overlap due to the twisted conformation of 2,2'-Bpy and poor extended p-interactions to form a nonemissive mixed stack 3. The aggregation-induced emission (AIE) is observed in both 1 and 2, which exhibit green emission with maximum intensity at 500 nm (Phi 58.2%) and 465 nm (Phi 77.6%) for a water fraction (f(w)) value of 10, i.e., 90:10 (THF/H2O v/v). AIE behavior is validated by dynamic light scattering and scanning electron microscopy studies. 1 exhibits vapochromic behavior and undergoes emission turn-on exposure to fumes of organic bases: NH3, Et3N, and Py. Plausibly due to proton abstraction by the bases, the vapochromic change is reverted by HCl fumes, and the process cycles. The salt forms of 2 and 3 respond to basic fumes only after prior exposure to the fumes of HCl and undergo a red shift (0.98 nm) in 2 and an emission-turn-on (612 nm) in 3. Furthermore, 3 exhibits irreversible thermochromic behavior at 75 degrees C, which is attributed to the loss of lattice water. The results are supported by the thermal, diffuse reflectance, powder X-ray diffraction, and Hirshfeld studies.