Solution and Solid-State Emissive Organophosphonates with High-Contrast Reversible Mechanofluorochromism: Beyond the Classical Frameworks
Banchhanidhi Prusti, Shivani Tripathi, Pandiyan Sivasakthi, Pralok K. Samanta, Manab Chakravarty
Abstract
Organic fluorogens with the solution and solid-state emissive features appeal dramatically due to their unique photophysical properties and wide applications in bioimaging, sensing, and optoelectronic devices. However, a well-defined design strategy has yet to be progressed to produce such fluorogens. With recently detected needs, we herein develop an easily affordable doubly twisted and thermally stable organophosphonate as a first emitter in both solution and solid states with high-contrast mechanofluorochromism in the ubiquitous phosphonate family. However, no typical conjugated donor–acceptor core is linked to this system. Still, it maintains highly conjugated structures with smart rigidity, planarity, restricted intramolecular motion, and minimum π···π interactions in the crystal/powder state, which offer such features to these phosphonates. The steady-state/lifetime fluorescence studies disclose an intense emission in the solution [quantum yield (Φ f ) 68–84%] and solid state (Φ f = 14–18%). The geometrical changes between singlet ground and excited states are theoretically calculated to support the outcomes. A detailed crystal structure and molecular packing analysis support the doubly molecular twists with noncovalent interactions, resulting in solid-state emission. Hirshfeld surface analysis and energy framework calculations elucidate the reversible high-contrast blue-to-green (68 nm red-shift) emission switching that arises upon applying mechanical grinding. The molecular packing shows the participation of the phosphonate part in multiple noncovalent interactions and imposes a twisted molecular structure. Such a prominent and efficient color contrast is hitherto unfamiliar for small organophosphonates. The essential role of intermolecular P=O···H–O bonding and other weak supramolecular interactions are recognized in a structural variation upon grinding and verified with an analogous molecule having only −OMe in place of −OH. This study would enrich the molecular assets of the phosphonate kingdom and generate an avenue to produce a thermally stable organophosphonate as a potential smart material.