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Transition metal complexes are well-known for their efficient light emission and are promising for applications ranging from bioimaging to light-emitting diodes. In solution, interactions between the metal centers of two complexes become possible and drastically change the photophysical properties. For real-world devices, solid-state materials consisting of these molecules are preferable. Recently, the ligand-controlled aggregation of platinum(II) and palladium(II) complexes into molecular single crystals and the controlled formation of metal-metal contacts have been demonstrated. Here, we show how the metal-metal distance can be tuned in a controlled way by exerting pressure on the molecular crystal. Using optical spectroscopy inside a diamond anvil cell, we find strong and reversible piezochromism up to 18 GPa. Using time-dependent density functional theory, we attribute the wavelength shift to a reduction in the metal-metal distance and enhanced π orbital overlap in the dimers.

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) Cs 3 Cu 2 Cl 5 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 Cs 3 Cu 2 Cl 5 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., (© 2024 Wiley-VCH GmbH.)

By using complementary experimental methods including in situ high-pressure single-crystal X-ray diffraction and UV-Vis spectroscopy, the intricate connection between solvatochromism and solvatomorphism has been elucidated in a recent publication [Sobczak & Katrusiak (2024). IUCrJ, 11, 528-537]. The connection was demonstrated for an important pigment - Reichardt's dye - with potential applications in nonlinear optoelectronics and molecular pressure sensor development., (open access.)

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., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

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., (© 2024 Wiley-VCH GmbH.)

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, M 1 ; unilateral coordinated model, M 2 ) based on the same viologen ligand were designed. Aided by theoretical calculations, model M 1 was recommended secondarily as a more suitable system for IEP materials. Along this line, two representative viologen-Zn II MOPs 1 and 2 with models M 1 and M 2 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., (© 2023 Wiley-VCH GmbH.)

A library of β-enamino diketones was prepared via base-mediated, three-component reaction of 4-hydroxycoumarins with various aromatic/aliphatic amines and β-nitrostyrenes under microwave irradiation conditions to investigate their photochemical properties. Among the prepared compounds, a thiophene derived β-enamino diketone was found to be light-sensitive and to exhibit unique photochromic behavior, that is, positive photochromism in solution and negative photochromism in crystalline phase. In addition, this prepared photochromic compound was further covalently linked to a structure-related, piezochromic β-enamino diketone moiety to explore its potential multi-stimuli responsive properties., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Manjappa, Fan and Yang.)

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-NO 3 decreased by three orders of magnitude with the increasing pressure, while that in high-radical-concentration 2-NO 3 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., (© 2023 Wiley-VCH GmbH.)

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., (© 2023 Wiley-VCH GmbH.)

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., (© 2022 Wiley-VCH GmbH.)

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., (© 2022 Wiley-VCH GmbH.)

Transition-metal containing halides with Ruddlesden-Popper (RP) perovskite structures have received extensive attention owing to their emerging and anisotropic photoelectric functionalities. Among them, A 2 CuX 4 (A=alkali metal or organic cations, X=Cl, Br, I) series are particular, because of the Jahn-Teller distortion of Cu 2+ sensitive to external stimuli such as temperature and pressure. In this article, we report the structure evolution and physical property responses of RP perovskites Rb 2 CuCl 4-x Br x (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 Cu 2+ cations under compression and crossing the phase transition, respectively. Rb 2 CuCl 4-x Br x 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 Rb 2 CuCl 4-x Br x have been greatly reduced by 5-orders of magnitude. Moreover, all of the pressure-induced phenomena in Rb 2 CuCl 4-x Br x perovskites are reversible upon decompression and no obvious difference is observed for the pressure responses between [CuCl 4 Br 2 ] and [CuCl 4 (Cl,Br) 2 ] coordination environments. The impact of pressure on the structural and physical properties in two-dimensional Rb 2 CuCl 4-x Br x provides in-depth understanding on the structure design of functional halide perovskites at ambient conditions., (© 2021 Wiley-VCH GmbH.)

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.

Metal-organic frameworks (MOFs) provide intriguing platforms for the design of responsive materials. It is challenging to mobilize as many components as possible of a MOF to collaboratively accomplish multiple responsive properties. Here, reversible photochromism, piezochromism, hydrochromism, ionochromism, and luminescence modulation of an ionic Eu(III) MOF is reported furnished by cationic electron-deficient viologen units and exchangeable guest anions. Mechanistically, the extraordinarily versatile responsive properties are owed to electron transfer (ET), charge transfer (CT), and energy transfer, involving viologen as electron acceptor, anion as electron donor, luminescing Eu(III) as energy donor, and anion-viologen CT complex or ET-generated radical as energy acceptor (luminescence quencher). Moreover, guest anions and waters provide flexible handles to control the ET-based responsive properties. Water release/reuptake or exchange with organic solvents can switch on/off the response to light, while reversible anion exchange can disenable or awaken the responses to pressure, light, and water release/reuptake. The impacts of water and anions on ET are justified by the high polarity and hydrogen-bonding capability of water, the different electron donor strength of anions, and the strong I - -viologen CT interactions. The rich responsive behaviors have great implications for applications such as pressure sensors, iodide detection, and chemical logic gates., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We report the synthesis of trisalkoxy substituted 9, 10-bis styrylanthracene derivatives (C8-ant and C12-ant) by Heck coupling with very good yield and their photophysical properties. Both C8-ant and C12-ant exhibit aggregation induced emission (AIE), mechnoflurochromism and thermochromism. Trisubstituted 9, 10-distyrylanthracene molecules having all the luminescent properties in a single molecule are first of its kind.

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., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Mechanofluorochromic or piezochromic fluorescence chemistry involves the switching and tuning of the luminescent properties of solid-state materials induced by exogenous forces, such as grinding, shearing, compression, tension, and so forth. Up until now, most reported mechanochromic systems, including liquid crystals, organic molecules, organometallic compounds, polymers, and dye-doped polymers, have displayed reversible two-color changes, which arise from either supramolecular or chemical structure transformations. However, fluorescent materials that undergo mechanically induced multicolor changes remain rare; this Minireview is focused on such materials. Topics are categorized according to the different applied forces that are required to induce the multicolor change, including mechanical control of either the supramolecular structures or the chemical structures, and mechanical control of both the supramolecular structures and chemical structures., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Atmospherically stable porous frameworks and materials are interesting for heterogeneous solid-gas applications. One motivation is the direct and selective uptake of pollutant/hazardous gases, where the material produces a measurable response in the presence of the analyte. In this report, we present a combined experimental and theoretical rationalization for the piezochromic response of a robust and porous molecular crystal built from an extensively fluorinated trispyrazole. The electronic response of the material is directly determined by analyte uptake, which provokes a subtle lattice contraction and an observable bathochromic shift in the optical absorption onset. Selectivity for fluorinated absorbates is demonstrated, and toluene is also found to crystallize within the pore. Furthermore, we demonstrate the application of electronic structure calculations to predict a physicochemical response, providing the foundations for the design of electronically tunable porous solids with the chemical properties required for development of novel gas-uptake media.