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Constructing organic materials with rigid‐planar molecules provides a unique platform for obtaining efficient photoluminescence (PL) by suppressing vibrational relaxation and extending the π‐conjugation. However, the presence of H‐aggregates/excimers resulting from interactions between π electrons and lone pairs during crystalization leads to aggregation‐caused quenching. Here, an alternative pressure‐treatment approach is presented for regulating molecular assembly to brighten rigid‐planar phthalic anhydride (PA) crystals with initial non‐emission. The pressure‐treated PA crystals exhibit significant blue‐light emission under ambient conditions, accompanied by a PL quantum yield of up to 15.7% and Commission International de I'Eclairage color coordinates of (0.20, 0.19). Detailed experiments and calculations reveal that pressure‐treated hydrogen bonds promote the planar rotation among PA molecules, which facilitates intermolecular π‐conjugation. This extended π‐conjugation alters the exciton relaxation channel, thereby triggering blue‐light emission based on charge transfer (CT) states. Remarkably, the obtained PA crystals also exhibit linear piezochromism (4.56 nm GPa−1) as a result of the structural sensitivity of the CT process. This work demonstrates the feasibility of regulating molecular assembly to achieve solid‐state emission in rigid‐planar molecules with initial non‐emission and opens up a new platform toward stimuli‐responsive crystalline materials. [ABSTRACT FROM AUTHOR]

Piezochromic materials refer to a class of matters that alter their photoluminescence (PL) colors in response to the external stimuli, which exhibit promising smart applications in anti‐counterfeiting, optoelectronic memory and pressure‐sensing. However, so far, most reported piezochromic materials have been confined to organic materials or hybrid materials containing organic moieties with limited piezochromic range of less than 100 nm in visible region. Here, we achieved an intriguing piezochromism in all‐inorganic zero‐dimensional (0D) Cs3Cu2Cl5 nanocrystals (NCs) with a considerable piezochromic range of 232 nm because of their unique inorganic rigid structure. The PL energy shifted from the lowest‐energy red fluorescence (1.85 eV) to the highest‐energy blue fluorescence (2.83 eV), covering almost the entire visible wavelength range. Pressure‐modulated self‐trapped exciton emission between different energy levels of self‐trapped states within Cs3Cu2Cl5 NCs was the main reason for this piezochromism property. Note that the quenched emission, which is over five times more intense than that in the initial state, is retained under ambient conditions upon decompression. This work provides a promising pressure indicating material, particularly used in pressure stability monitoring for equipment working at extreme environments. [ABSTRACT FROM AUTHOR]

Organic piezochromic materials that manifest pressure‐stimuli‐responses are important in various fields such as data storage and anticounterfeiting. The manipulation of piezofluorochromic behaviors for these materials is promising but remains a great challenge. Herein, a non‐luminous components regulated strategy is developed and organic molecular cages (OMCs), a burgeoning class of crystalline organic materials with structural dynamics, are first explored for the design of piezofluorochromic materials with tunable luminescence. A series of OMCs based on aggregation‐induced emission (AIE) chromophores, termed Cage 1–3, are synthesized and their piezofluorochromic behaviors are investigated by diamond anvil cell technique. Due to the sufficient voids between its flexible chromophores offered by the OMC structure, Cage 1 exhibits thermofluorochromic and piezofluorochromic properties. Moreover, the piezofluorochromic performance of this OMC could be further promoted by replacing its non‐luminous components with improved flexibilities, and a remarkable luminescence peak shift by 150 nm together with a response sensitivity of 13.8 nm GPa−1 was achieved upon hydrostatic compression. The cage structure plays a vital role in facilitating efficient and reversible piezofluorochromic behaviors. This study has shed light on the rational design and exploitation of OMCs as an exceptional platform to accomplish customizable piezofluorochromic behaviors and enlarge their potential applications in pressure‐based luminescence. [ABSTRACT FROM AUTHOR]

Lanthanide Single‐Molecule Magnets are fascinating objects that break magnetic performance records with observable magnetic bistability at the boiling temperature of liquid nitrogen, paving the way for potential applications in high‐density data storage. The switching of lanthanide SMM has been successfully achieved using several external stimuli such as redox reaction, pH titration, light irradiation or solvation/desolvation thanks to the high sensitivity of the magnetic anisotropy to any structural change in the lanthanide surrounding. Nevertheless, the use of applied high pressure as an external stimulus is largely underused, especially considering that it can be combined with high pressure X‐ray diffraction to establish a complementary structure‐property relationship. This Concept article summarizes the few relevant examples of investigations of lanthanide SMMs under applied high pressure, provides conclusions on the effect of such stimulus on molecular structures and magnetic anisotropy, and finally draws perspective on the future development of magnetic measurements under applied pressure. [ABSTRACT FROM AUTHOR]

Inkless and erasable printing (IEP) based on chromic materials holds great promise to alleviate environmental and sustainable problems. Metal–organic polymers (MOPs) are bright platforms for constructing IEP materials. However, it is still challenging to design target MOPs with excellent specific functions rationally due to the intricate component‐structure‐property relationships. Herein, an effective strategy was proposed for the rational design IEP‐MOP materials. The stimuli‐responsive viologen moiety was introduced into the construction of MOPs to give it potential chromic behaviors and two different coordination models (i. e. bilateral coordination model, M1; unilateral coordinated model, M2) based on the same viologen ligand were designed. Aided by theoretical calculations, model M1 was recommended secondarily as a more suitable system for IEP materials. Along this line, two representative viologen‐ZnII MOPs 1 and 2 with models M1 and M2 were synthesized successfully. Experiments exhibit that 1 does have quicker stimuli response, stronger color contrast and longer radical lifetime compared to 2. Significantly, the obtained 1‐IEP media brightly inherits the excellent chromic characteristics of 1 and the flexibility of the paper at the same time, which achieves most daily printing requirements, as well as enough resolution and durability to be used in identification by smart device. [ABSTRACT FROM AUTHOR]

Manipulating the radical concentration to modulate the properties in solid multifunctional materials is an attractive topic in various frontier fields. Viologens have the unique redox capability to generate radical states through reversible electron transfer (ET) under external stimuli. Herein, taking the viologens as the model, two kinds of crystalline compounds with different molecule‐conjugated systems were designed and synthesized. By subjecting the specific model viologens to pressure, the cross‐conjugated 2‐X all exhibit much higher radical concentrations, along with more sensitive piezochromic behaviors, compared to the linear‐conjugated 1‐X. Unexpectedly, we find that the electrical resistance (R) of 1‐NO3 decreased by three orders of magnitude with the increasing pressure, while that in high‐radical‐concentration 2‐NO3 remained almost unchanged. To date, such unusual invariant conductivity has not been documented in molecular‐based materials under high pressure, breaking the conventional wisdom that the generations of radicals are beneficial to improve conductivity. We highlight that adjusting the molecular conjugation modes can be used as an effective way to regulate the radical concentrations and thus modulate properties rationally. [ABSTRACT FROM AUTHOR]

Piezochromic materials, which exhibit a fluorescence response with large emission spectral shifts and high sensitivity, may be useful in important applications, but there have been few reports of such organic luminophores. Herein, we report a new high‐sensitivity piezochromic material based on the incorporation of an o‐carborane unit, which exhibits aggregation‐induced emission properties. In a high‐pressure experiment, compared to carborane‐free MTY, which exhibits an emission spectral shift of 75 nm and a sensitivity of 19.1 nm ⋅ GPa−1, the o‐carborane dyad MTCb shows a larger emission wavelength difference of 131 nm and a higher sensitivity of 32.8 nm ⋅ GPa−1, demonstrating a performance that ranks among the best of organic piezochromic materials reported thus far. MTCb molecules adopt a J‐aggregated pattern and have relatively loose molecular packing in the crystalline state. Interestingly, nonconjugated spherical carborane can disrupt the π‐π interactions between adjacent molecules during compression, which results in excellent piezochromic performance. [ABSTRACT FROM AUTHOR]

Viologen derived from 4,4'‐bipyridine has attracted much attention because of its color changing properties with electron transfer, unique redox stability and structural diversity. These characteristics have led to its successful use in various applications, in particular in color‐changing materials. In the past few years, researchers have been working on the syntheses of viologen‐based color‐changing functional materials, and such materials have been widely used in many fields. In photochromic materials, it is used as anti‐counterfeiting material; in thermochromic, it is used as memory storage material, and in electrochromic, it is used as a battery material. This Review discusses the progress of viologen in organic and inorganic discoloration materials in recent years. The syntheses of viologen and its derivatives are summarized, and its application in the field of discoloration materials is introduced. [ABSTRACT FROM AUTHOR]

Driven by the rapidly increasing demand for intelligent materials, the sensitivity of organic materials to pressure has been intensively investigated in recent years. Many examples describe material responses to both mechanical grinding and hydrostatic pressure, but only few materials have been identified with clear colour difference as well as strong penetrability, especially under high pressure, limiting development of pressure sensors. In this work, an asymmetric luminophore, MTBA was developed by end‐capping a (Z)‐3‐(benzo[c][1,2,5]‐thiadiazol‐4‐yl)‐2‐phenylacrylonitrile (TPAN) core with propeller‐like triphenylamine (TPA) and rigid 12b‐methyl‐5,12b‐dihydroindeno[1,2,3‐kl]acridine (MeIAc) units, revealing aggregation‐induced emission (AIE) behaviour. In high‐pressure and grinding experiments, MTBA exhibits a dramatically large colour difference of up to 164 nm and 112 nm in deep‐red and near‐infrared regions, respectively, for piezochromic performance that is among the best reports for pure organic materials. The experimental and theoretical analyses indicated that the excellent piezochromic performance of MTBA is due to its highly twisty and rigid conformation, which weakens intermolecular π‐π interaction and obstructs emission quenching during compression. [ABSTRACT FROM AUTHOR]

Overcrowded ethylenes composed of 10‐methyleneanthrone and two bulky aromatic rings contain a twisted carbon–carbon double (C=C) bond as well as a folded anthrone unit. As such, they are unique frustrated aromatic enes (FAEs). Various colored crystals of these FAEs, obtained in different solvents, correspond to multiple metastable conformations of the FAEs with various twist and fold angles of the C=C bond, as well as various dihedral angles of attached aryl units with respect to the C=C bond. The relationships between color and these parameters associated with conformational features around the C=C bond were elucidated in experimental and computational studies. Owing to the fact that they are separated by small energy barriers, the variously colored conformations in the FAE crystal change in response to various external stimuli, such as mechanical grinding, hydrostatic pressure and thermal heating. [ABSTRACT FROM AUTHOR]

2D hybrid organic–inorganic perovskites (HOIPs) are highly responsive to external stimuli and therefore have application potential as sensing materials. Though their optical properties upon singular thermal or pressure stimulation have been recently investigated, their dual‐stimuli‐responsive behaviors have not yet been explored. Here, the dual‐stimuli‐responsive luminescence of a pair of new enantiomeric 2D Dion–Jacobson HOIPs, R+[(4‐aminophenyl)ethylamine]PbI4 and S‐[(4‐aminophenyl)ethylamine]PbI4, is reported. The photoluminescence results show that their 485 nm emissions can be red‐shifted by ≈6 nm upon heating, and further increased to 529 nm under pressure. Such dual‐stimuli‐responsive emissions expand their Commission Internationale de L'Eclairage coordinates successively from (0.140, 0.272) to (0.283, 0.473). Detailed structural analysis and first principles calculations reveal that the temperature‐ and pressure‐responsive behaviors arise from the predominant electron–phonon interactions over thermal expansion effect and pressure‐induced in‐plane PbI bond contraction, respectively. The findings open up a new pathway to successively tune the optical emission of 2D HOIPs via a dual‐stimuli‐responsive approach. [ABSTRACT FROM AUTHOR]

Transition‐metal containing halides with Ruddlesden‐Popper (RP) perovskite structures have received extensive attention owing to their emerging and anisotropic photoelectric functionalities. Among them, A2CuX4 (A=alkali metal or organic cations, X=Cl, Br, I) series are particular, because of the Jahn‐Teller distortion of Cu2+ sensitive to external stimuli such as temperature and pressure. In this article, we report the structure evolution and physical property responses of RP perovskites Rb2CuCl4‐xBrx (x=1, 2) to external pressure. Dramatic structural phase transitions from orthorhombic to monoclinic occur around 3.0 GPa in both materials regardless of their distinct compositions. Structure analyses reveal the suppression and final vanishing of the Jahn‐Teller distortion of Cu2+ cations under compression and crossing the phase transition, respectively. Rb2CuCl4‐xBrx perovskites exhibit abrupt bandgap narrowing (from reddish‐brown to black) along with the structural phase transition, and an overall bandgap narrowing of 75% up to ∼27 GPa but still keeping semiconductive. During the compression processes, the resistances of Rb2CuCl4‐xBrx have been greatly reduced by 5‐orders of magnitude. Moreover, all of the pressure‐induced phenomena in Rb2CuCl4‐xBrx perovskites are reversible upon decompression and no obvious difference is observed for the pressure responses between [CuCl4Br2] and [CuCl4(Cl,Br)2] coordination environments. The impact of pressure on the structural and physical properties in two‐dimensional Rb2CuCl4‐xBrx provides in‐depth understanding on the structure design of functional halide perovskites at ambient conditions. [ABSTRACT FROM AUTHOR]

Transition‐metal containing halides with Ruddlesden‐Popper (RP) perovskite structures have received extensive attention owing to their emerging and anisotropic photoelectric functionalities. Among them, A2CuX4 (A=alkali metal or organic cations, X=Cl, Br, I) series are particular, because of the Jahn‐Teller distortion of Cu2+ sensitive to external stimuli such as temperature and pressure. In this article, we report the structure evolution and physical property responses of RP perovskites Rb2CuCl4‐xBrx (x=1, 2) to external pressure. Dramatic structural phase transitions from orthorhombic to monoclinic occur around 3.0 GPa in both materials regardless of their distinct compositions. Structure analyses reveal the suppression and final vanishing of the Jahn‐Teller distortion of Cu2+ cations under compression and crossing the phase transition, respectively. Rb2CuCl4‐xBrx perovskites exhibit abrupt bandgap narrowing (from reddish‐brown to black) along with the structural phase transition, and an overall bandgap narrowing of 75% up to ∼27 GPa but still keeping semiconductive. During the compression processes, the resistances of Rb2CuCl4‐xBrx have been greatly reduced by 5‐orders of magnitude. Moreover, all of the pressure‐induced phenomena in Rb2CuCl4‐xBrx perovskites are reversible upon decompression and no obvious difference is observed for the pressure responses between [CuCl4Br2] and [CuCl4(Cl,Br)2] coordination environments. The impact of pressure on the structural and physical properties in two‐dimensional Rb2CuCl4‐xBrx provides in‐depth understanding on the structure design of functional halide perovskites at ambient conditions. [ABSTRACT FROM AUTHOR]

Near‐infrared piezochromic materials (PCMs), which show a fluorescence response with a clear color difference and strong penetrability, have important potential applications, but few such organic luminophores have been reported. High‐brightness donor (D)–acceptor (A) luminophores present great potential for preparing near‐IR PCMs with clear color differences. However, the intermolecular π–π interactions are sharply enhanced at high pressures, which is not beneficial for the improvement of the piezochromic properties. Herein, two crystalline polymorphs (TPEB‐α and TPEB‐γ) of a D–π–A‐type dye is presented, which emits deep‐red fluorescence with a high emission efficiency (53.2% and 28.1%, respectively). In crystalline state, dispersed dimers are observed and arranged in a herringbone pattern. Interestingly, each dimer is surrounded by four pairs of twisted tetraphenyl ethylene units, which could disrupt the π–π interactions between adjacent dimers during compression. As a result, this luminophor with dispersed dimeric stacking demonstrates the deep‐red to near‐infrared piezochromic behavior with a remarkable redshift (212 nm) and high sensitivity (19.8 nm GPa−1). [ABSTRACT FROM AUTHOR]

Two cruciform‐shaped piezochromic materials, PBM‐DPA and PBM‐BCbz, have been developed. PBM‐DPA exhibits a higher photoluminescence quantum yields (PLQY) and redder emission than PBM‐BCbz owing to the formation of a hybridized local and charge‐transfer (HLCT) excited state. Furthermore, high‐pressure measurements indicate that PBM‐DPA displays a higher sensitivity (9.4 nm/GPa) than PBM‐BCbz (6.4 nm/GPa) under the same pressure range. Information from the crystal structures suggests that this is because of the easier planarization of diphenylamine (DPA) in PBM‐DPA. In contrast, PBM‐BCbz possesses a twisty conformation, owing to the large dihedral between the central benzene ring and carbazole (Cbz), which hinders the coplanarization of the whole PBM‐BCbz molecule. This phenomenon reveals that molecular structure can efficiently influence piezochromic contrast and sensitivity. Both the planarization and the HLCT excited‐state characteristics make PBM‐DPA a high‐performance piezochromic material. This research will help to design high‐performance pressure‐response materials. [ABSTRACT FROM AUTHOR]

The crystal structure of pyrene‐1‐carbaldehyde (PA), a model polyaromatic hydrocarbon, highly luminescent in the solid state and crystallizing in the triclinic system, has been re‐determined at several pressures ranging from atmospheric up to 3 GPa using a diamond anvil cell. A 'multi‐crystal' approach was used in crystal structure determination, significantly improving completeness of X‐ray diffraction data attainable for such a low‐symmetry system. The crystal structure consists of infinite π‐stacks of PA molecules with discernible dimers, which resemble aggregates formed by pyrene derivatives in solution as well as in the solid state. A series of measurements showed that the average inter‐planar distance between individual molecules within π‐stacks decreases with pressure in the investigated range. This results in piezochromic properties of PA: a significant sample color change as well as a red‐shift of fluorescence with pressure, as studied with UV–vis spectroscopy. Periodic DFT calculations allowed us to relate the variations in the crystal structure with pressure to the changes in the electronic structure of this material. [ABSTRACT FROM AUTHOR]

Two 9,10‐distyrylanthracene‐based luminophores exhibiting aggregation‐induced emission and stimuli‐responsive properties were synthesized. Seven‐ or five‐color luminescence switching based on a single organic molecule was achieved for the first time. These phase transitions can be induced by physical stimuli such as grinding by mortar and pestle, heating, and exposure to the vapors of organic solvents. Moreover, a strategy for the design of new mechanoresponsive materials with π‐conjugated luminophores is proposed. Seven‐color luminophore: Piezochromism based on aggregation‐induced emission with distinct seven‐color luminescence switching built on a single organic molecule was achieved for the first time. The fluorescence switching process (see figure) can be smoothly triggered by disrupting the ordered molecular packing (mechanical grinding) and repacking (heating or solvent annealing). [ABSTRACT FROM AUTHOR]

Abstract: Three novel smart fluorescent on‐off materials based on hemicyanine, 3‐(2‐((1E,3E)‐4‐(3‐(4‐Fluorophenyl)‐1‐isopropyl‐1H‐indol‐2‐yl)buta‐1,3‐dien‐1‐yl)naphtho[1, 2‐d]thiazol‐1‐ium‐1‐yl)propane‐1‐sulfonate (NTPS‐FPI), (E)‐3‐(2‐(2‐(2‐hy‐droxynaphthalen‐1‐yl)vinyl)naphtho[1, 2‐d]thiazol‐1‐ium‐1‐yl)propane‐1‐sulfonate (NTPS‐HN) and (E)‐2‐(2‐(2‐hydroxynaphthalen‐1‐yl)vinyl)‐1,1,3‐trimethyl‐1H‐benz‐o[e]indol‐3‐ium iodide (BI‐HN), have been developed. They exhibit red or near‐infrared (NIR) emission and tunable on‐off solid‐state luminescent switch by multi‐stimuli, including physical or chemical mode. Single crystal, powder X‐ray diffraction, differential scanning calorimetry and scanning electron microscopy studies provide deep insight on polymorphism‐dependent piezochromism mechanism. Intriguingly, NTPS‐HN and BI‐HN show reversible solid‐state fluorescent on‐off characteristics by exposing in acid‐base vapor, which are ascribed to chemical deprotonation/protonation of terminal hydroxyl group. On the basis of multi‐stimuli response, the dual molecular level basic logic gates are established by using the appropriate external stimulus channel as the inputs and fluorescent colors as the readout data. [ABSTRACT FROM AUTHOR]

Tetraphenylethylene (TPE) and its derivatives are the most typical and most widely studied organic compounds showing aggregation-induced emission (AIE). Due to their propeller-like structures, V-like clefts exist between the aryl rings, which make them promising host compounds. However, such a possibility is seldom explored. Herein, it is reported that TPE derivatives bearing two or four pyridine rings at the para positions of the phenyl rings (TPE-Pys) can selectively include triangular (Δ-like) m-phthalic acid from a mixture of o-, m-, and p-phthalic acids due to their shape complementary to form host-guest co-crystals, which showed redder emission than the TPE-Pys themselves. The emission of co-crystals 1- 5 could be reversibly switched between yellow and red by alternating exposure to HCl and ammonia vapor. The host-guest co-crystals not only exhibited great potential for selectively recognizing and separating m-phthalic acid and as multicolor emission materials, but are also suitable for use as secret ink due to their reversible color change on varying the host-guest interactions. [ABSTRACT FROM AUTHOR]

Two conjugated molecules based on triphenylamine and 1,3- indandione have been synthesized by employing Knoevenagel condensation. Both materials demonstrated aggregation-induced emission behavior, and solvato- and piezochromic properties. These red luminescence materials are able to respond to F- and I- sensitively. The emission wavelength changed by almost 100 nm in the presence of F- and I-, and could be observed by the naked eye under daylight and UV light. [ABSTRACT FROM AUTHOR]

The straightforward coordination of the Lewis acid B(C6F5)3 to classical, non-emitting aldehydes results in solid-state photoluminescence. Variation of the electronic properties of the carbonyl moieties lead to the modulation of the solid-state emission colors, covering the entire visible spectrum with quantum yields up to 0.64. Steady-state spectroscopy in combination with X-ray diffraction analysis and DFT calculations confirm that intermolecular interactions between the Lewis adducts are responsible for the observed luminescence. Alteration of the latter interactions induces, moreover, remarkable solid-state phenomena such as piezochromism. The versatility and simplicity of our approach facilitate the future development of solid-state emitting materials. [ABSTRACT FROM AUTHOR]

Keywords: metastable structures; mechanochromism; overcrowded ethylene; piezochromism; thermochromism EN metastable structures mechanochromism overcrowded ethylene piezochromism thermochromism 1 1 1 05/18/22 20220516 NES 220516 B In the crystalline state b of sterically frustrated aromatic enes (FAEs) both twisted and folded carbon-carbon double bonds coexist. These color differences correspond to the multiple energetically close metastable conformations of the FAEs, which differ in the twist and fold angles of the C=C bond and the dihedral angle of the aryl unit relative to the C=C bond. [Extracted from the article]

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 aggregation‐enhanced 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. An X‐shaped π architecture that exhibits solid‐state twisted intramolecular charge transfer (TICT) emission is developed, and its luminescent chromism, aggregation‐enhanced emission (AEE) behavior, and bioimaging properties are carefully investigated. Under high pressure, 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). [ABSTRACT FROM AUTHOR]