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 π-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 π-interactions to form emissive mixed-stack aggregates
(λmax = 610 nm and Φ 1.1%), while the charge–transfer
complex formed in 3 exhibits poor π-overlap due
to the twisted conformation of 2,2′-Bpy and poor extended π-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 (Φ
58.2%) and 465 nm (Φ 77.6%) for a water fraction (fw) 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 °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.