LAUSR

Hydrostatic pressure is a common natural stimulus that can alter the photoluminescence (PL) intensity and color properties through a phenomenon referred to as piezochromism.[1] The practical requirements of flaw detection, mechanical sensors, and deformation detection[2] have promoted the pursuit of piezochromic materials (PCMs) with clear color differences and strong penetrability. However, at present, most reported PCMs exhibit changes in the absorption and/or photoluminescence bands in the visible region (less than 700 nm)[3] and thus generally have poor penetrability. Deep-red/nearinfrared (near-IR) fluorescence exhibits longer PL wavelengths and lower energy, which endow it with strong penetrability.[4] Nevertheless, the preparation of near-IRresponsive PCMs remains a significant challenge due to evident PL quenching and a small wavelength shift (faint color difference) in the aggregated state. For the molecular construction of near-IR-responsive fluorophores, one must consider the response sensitivity to improve the color difference. In addition, a strikingly twisted structure should be adopted to acquire high PL quantum yields (PLQY) and avoid the strong intermolecular interactions[5] that give rise to severe fluorescence quenching during the compression process. Owing to the intriguing photophysical properties of donor (D)–acceptor (A) fluorophores, these systems have drawn considerable attention for versatile applications, such as bio/chem sensors, organic light-emitting diodes (OLEDs), and organic sensors.[6] The most frequently discussed topic with regard to D–A fluorophores is probably the generation of a twisted intramolecular charge transfer (TICT) excited state in polar solvents, which arises from twisting of the DA bond.[7] TICT emission, which is influenced by conformational changes and stabilization/charge separation, is highly sensitive to changes in the external environment. As a result, various types of TICTbased fluorophores that show PL chromism depending on the solvent polarity, temperature, and viscosity in solution have been prepared.[8] For example, a TICT-type pyrene-containing triarylboron molecule,[8b] reported by Yang and co-workers, Near-infrared piezochromic materials presenting fluorescence responses with clear color differences and good penetrability have important potential applications, but a few such organic compounds are developed. Twisted intramolecular charge transfer (TICT) emission is versatile in solutions, especially for preparing bio/chem-sensing materials due to the excellent sensitivity of the emission to alterations in the external environment. By analogy, the solid-state TICT-emissive chromophores are probably excellent candidates for the environmentally responsive material. Herein, X-shaped π architectures that exhibit solid-state TICT emission are developed, and their luminescent chromism and bioimaging properties are investigated. Initially, the cruciform fluorophore exhibits anomalous aggregationenhanced emission (AEE) and dual emission due to the existence of a TICT state. Interestingly, TICT emission is observed even in the aggregated state because the spacious environment around the bulky triphenylamine allows for rotation. During the compression process, the TICT-based fluorophore demonstrates deep-red to near-infrared piezochromic behavior with a remarkable redshift (162 nm) and high sensitivity (15.1 nm GPa−1). The bioimaging performance of the TICT-emissive dye suggests its potential utility as a fluorescent probe for biological applications.