A rigid-flexible steric blocking strategy for highly bright and ultrawide piezochromism in the deep-red/near-infrared region


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Metal-free near-infrared (NIR) piezochromic materials (PCMs) have critical applications as sensors and probes, particularly those with high luminescence efficiencies, ultrawide wavelength shifts, and pressure resistances when used in hyperbaric environments. Herein, a rigid-flexible steric blocking strategy is proposed to achieve the desired performance by simply introducing an alkyl carbazole at the periphery of the benzothiadiazole cyanostilbene-based luminophore. The as-synthesised molecule, CzC6BT, exhibits highly efficient deep-red (DR) luminescence in both solution and solid states, with an extraordinary photoluminescence quantum yield (PLQY) of 48.5% in the molten glassy state. The polymorphs Crystal-a (lambda(em) = 680 nm, PLQY = 40.1% at atmosphere) and Crystal-b (lambda(em) = 670 nm, PLQY = 21% at atmosphere) exhibited ultrawide piezochromism in the DR/NIR region (up to >900 nm), with wavelength shifts of 270 and 263 nm in large tolerable pressure ranges (16 and 13.2 GPa). These properties were significantly superior to those of the control molecule MPBM (164 nm, 10 GPa) without the alkyl carbazole fraction and achieved state-of-the-art NIR PCMs. The rigid-flexible alkyl carbazole wrapped around the dimeric luminophore has important steric blocking effects in hindering the formation of continuous pi-pi stacking while offering a moderate free volume for molecular rearrangements during compression. Thus, ultrawide NIR piezochromism was observed in CzC6BT, with a limited luminescence quenching effect caused by the enhanced pi-pi interactions during compression. This work demonstrates that inserting rigid-flexible alkyl carbazoles effectively regulates luminophore stacking, adjusts their response towards compression, and simplifies the rational design of advanced NIR PCMs.
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