Dual-performance devices based on electrochromism and electrofluorochromism have recently gained much attention in display technology. However, getting electrochromism and electrofluorochromism from the same material is very challenging. Herein, we have synthesized a low-band-gap donor− acceptor conjugated thiazolothiazole-containing organic polymer, P-TzTz, that exhibited a quasi-reversible single-electron reduction to demonstrate a pale yellow-to-black (NIR region) electrochromism and a red-to-quenched electrofluorochromism at a low potential window of −1.7 to +0.6 V with suitable electrochromic/electrofluorochromic parameters and stability. Possible redoxinduced modulation of π−π* and intramolecular charge transfer in the P-TzTz polymer, which was validated through density functional theory calculations, interplayed the key role of tunable absorption from vis to NIR and on−off fluorescence switching. Finally, mimicking a flip-flop logic gate model with optical memory functions was also demonstrated by taking the potentials as inputs and electrochromic and electrofluorochromic dual performance signals as outputs.
A comparative new strategy to enhance thermally activated delayed fluorescence (TADF) of through-space charge transfer (CT) molecules in organic light-emitting diodes (OLEDs) is investigated. Generally, TADF molecules adopt a twisted donor and acceptor structure to get a sufficiently small ΔE ST and a higher value of the spin−orbit coupling matrix element (SOCME). However, molecules containing donor−phenyl bridge−acceptor (D−p−A) units and featuring π-stacked architectures have intramolecular CT contribution through space and exhibit high TADF efficiency. We have explored the insights into the TADF mechanism in D−p−A molecules using the density functional theory (DFT) and time-dependent DFT methods. The calculated optical absorption and ΔE ST values are found to be in good agreement with available experimental data. Interestingly, we found the origin of the SOCME to be the twisted orientation of the donor and bridge moieties. Also, we predicted similar molecules with enhanced OLED efficiency with different substitutions.
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.
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