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

Abstract

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 sulfonatepyridinium 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 (λ max = 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 (f w ) value of 10, i.e., 90:10 (THF/H 2 O 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: NH 3 , Et 3 N, 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.

Stimuli-responsive fluorochromic organic salt

“…12,43,51 Recently, we realized solid-state emission tuning in organic complexes based on ionic interactions. [39][40][41] Although common in charge transfer co-crystals, solid-state emission, and its tuning have rarely been reported for hydrogen-bonded co-crystals or salts. 37,52 Only a few instances of the realization of emission tuning in hydrogen-bonded binary crystal systems involve solvates.…”

“…39 Notably, we have reported the realization of conformer-dependent emission tuning as well as ''on-off'' switching in hydrogen-bonded ionic cocrystals. [40][41][42] Although less reported, their being predominantly kinetic solid forms and the presence of an acidic hydrogen in the hydrogenbonded emissive crystal structures makes them suitable candidates for the investigation of stimuli-responsive behavior.…”

Photoluminescent organic crystals and co-crystals

“…168 Using 1,10-phenanthroline and 2,2′-bipyridyl as coformers, emission “on–off” switching was achieved with functionalised organo-sulfonate, which was attributed to the rigid structure of the former due to the better π-interactions, and flexible and twisted structure of the latter, resulting in the breakdown of the π-interactions. 169 The combined impact of functionalization and co-crystallization was employed to explore emission tuning in sulfonate-pyridinium-based organic salts. 170…”

Engineering the solid-state luminescence of organic crystals and cocrystals

“…Recently we have reported emission tuning of the organic precursor by its cocrystallization, as the pyridyls of the same shape but the different positions of the nitrogen lead to different packing arrangements. 49,50 However, advancement in the area remains limited and requires more vigorous endeavours.…”