Piezochromic Behavior Research Articles

Piezofluorochromism in Covalent Organic Frameworks: Pressure-Induced Emission Enhancement and Blue-Shifted Emission.

Achieving enhanced or blue-shifted emission from piezochromic materials remains a major challenge. Covalent organic frameworks (COFs) are promising candidates for the development of piezochromic materials owing to their dynamic structures and adjustable optical properties, where the emission behaviors are not solely determined by the functional groups, but are also greatly influenced by the specific geometric arrangement. Nevertheless, this area remains relatively understudied. In this study, a successful synthesis of a series of bicarbazole-based COFs with varying topologies, dimensions, and linkages was conducted, followed by an investigation of their structural and emission properties under hydrostatic pressure generated by a diamond anvil cell. Consequently, these COFs exhibited distinct piezochromic behaviors, particularly a remarkable pressure-induced emission enhancement (PIEE) phenomenon with a 16-fold increase in fluorescence intensity from three-dimensional COFs, surpassing the performance of CPMs and most organic small molecules with PIEE behavior. On the contrary, three two-dimensional COFs with flexible structures exhibited rare blue-shifted emission, whereas the variants with rigid and conjugated structures showed common red-shifted and reduced emission. Mechanism research further revealed that these different piezochromic behaviors were primarily determined by interlayer distance and interaction. This study represents the first systematic exploration of the structures and emission properties of COFs through pressure-treated engineering and provides a new perspective on the design of piezochromic materials.

Piezofluorochromism in Covalent Organic Frameworks: Pressure‐Induced Emission Enhancement and Blue‐Shifted Emission

AbstractAchieving enhanced or blue‐shifted emission from piezochromic materials remains a major challenge. Covalent organic frameworks (COFs) are promising candidates for the development of piezochromic materials owing to their dynamic structures and adjustable optical properties, where the emission behaviors are not solely determined by the functional groups, but are also greatly influenced by the specific geometric arrangement. Nevertheless, this area remains relatively understudied. In this study, a successful synthesis of a series of bicarbazole‐based COFs with varying topologies, dimensions, and linkages was conducted, followed by an investigation of their structural and emission properties under hydrostatic pressure generated by a diamond anvil cell. Consequently, these COFs exhibited distinct piezochromic behaviors, particularly a remarkable pressure‐induced emission enhancement (PIEE) phenomenon with a 16‐fold increase in fluorescence intensity from three‐dimensional COFs, surpassing the performance of CPMs and most organic small molecules with PIEE behavior. On the contrary, three two‐dimensional COFs with flexible structures exhibited rare blue‐shifted emission, whereas the variants with rigid and conjugated structures showed common red‐shifted and reduced emission. Mechanism research further revealed that these different piezochromic behaviors were primarily determined by interlayer distance and interaction. This study represents the first systematic exploration of the structures and emission properties of COFs through pressure‐treated engineering and provides a new perspective on the design of piezochromic materials.

Piezochromic Behavior of 2,4,6‐Triphenylpyrylium Tetrachloroferrate

In advanced photonics, there is a growing interest in piezochromic luminescent materials that exhibit multicolor switching, driven by their potential applications in optical recording, memory, and sensors. Here, the piezochromic behavior of 2,4,6‐triphenylpyrylium tetrachloroferrate (Py‐FeCl4) under high pressures from 0 to 9 GPa is reported. The observed multicolor changing properties of Py‐FeCl4 (yellow–orange–red–maroon–black) are found to be fully reversible upon decompression to ambient conditions. The mechanism of Py‐FeCl4 piezochromism is investigated via Raman, infrared, and UV–vis spectroscopy combined with powder X‐ray Diffraction. The absence of structural phase transitions as well as the abrupt shifts of bandgap values together with characteristic Raman and IR peaks within 0‐9 GPa suggests that the Py‐FeCl4 multicoloring switching behavior is driven by an electron transfer between the inorganic FeCl4− anion and the organic pyrylium cation. The obtained results demonstrate that Py‐FeCl4 dye is a good candidate for developing high‐pressure sensing technologies designed to function in extreme environments. Moreover, due to the inherent role of molecular‐structure relationships in the pyrylium salt's photophysical properties, findings suggest the potential discovery of piezochromic behavior in other pyrylium compounds.

The reversible piezochromic luminescence behavior of carbon dots under a cycle of loading/unloading pressure.

Carbon dots (CDs) have gained intensive interest owing to their small size, unique structure, excellent photoluminescence (PL) properties and broad applications. In particular, pressure-triggered irreversible piezochromic behavior of fluorescent CDs was previously reported and attributed to the sp2-sp3 transition in the carbon core or aggregation-induced emission under high pressure. Here, we report the reversible piezochromic behavior of microwave-heating synthesized CDs (named M-CDs) using ethylenediamine and aspartic acid as precursors. Under a loading/unloading cycle, the PL intensity of M-CDs decreased continuously with the pressure increasing from 101 kPa up to 20 GPa, and the maximum emission of M-CDs at 101 kPa (λmax = 550 nm) was slightly blue-shifted to 541 nm at 20 GPa, but when the pressure was released from 20 GPa to normal environmental conditions, both the emission wavelength and the PL intensity of M-CDs returned to their initial states at 101 kPa. The control sample was also synthesized using the same precursors but through a hydrothermal method and thus named H-CDs. Both H-CDs and M-CDs have similar particle sizes, morphology and excitation-dependent PL behavior under 101 kPa; however, H-CDs showed a typical piezochromic behavior with the emission blue-shifted from 518 to 491 nm when the pressure was increased from 101 kPa to 0.97 GPa, and then red-shifted from 491 to 530 nm when the pressure was increased up to 10.53 GPa. This irreversible behavior of H-CDs was accompanied by a 2-fold enhancement of their PL intensity after releasing the pressure. The remarkable different behaviors of M-CDs and H-CDs under a loading/unloading cycle are caused by different interior structures of M-CDs and H-CDs due to different synthetic processes, which is worthy of further research.

Narrowband emission from fully-bridged triphenylamine derivatives: insights into effects of structure modification and pressure.

Triphenylamine derivatives with narrowband emission have attracted growing attention in purely organic thermally-activated fluorescence (TADF) materials owing to their enhanced color purity and flexible molecular design strategy. Combined time-dependent density functional theory (TD-DFT) and ONIOM (QM/MM) calculations indicate that the excellent planarity of the experimentally developed DQAO could result in gradually decreased intermolecular interactions in the aggregated state at ambient pressure and upon compression, which is unfavorable for suppressing structural relaxation and achieving narrowband emission in its non-doped practical application. Therefore, three structure-modified derivatives, DQAO-Cb, DQAO-Ph, and DQAO-PhCb, were theoretically designed by introducing the spherical o-carborane and dangling phenyl units positioned para to the N atom of the DQAO to provide additional geometrical distortion and steric hindrance. The explorations on the reported DQAO, OQAO, and SQAO found that small structural relaxations, suppressed low-frequency vibrations, and noticeable short-range charge-transfer (SR-CT) natures of DQAO and OQAO are responsible for their much narrower emission spectral full-width at half-maxima (FWHMs) compared to that of SQAO. Introducing the o-carborane unit directly at the para position of the N atom could result in additional scissoring and stretching vibrations of the corresponding DQAO-Cb while the presence of the phenyl unit in DQAO-Ph is beneficial for suppressing the high-frequency vibrations of the pristine DQAO. More importantly, the bridged phenyl unit incorporated in DQAO-PhCb is of particular importance to inhibit the undesired low-frequency scissoring and high-frequency stretching vibrations of the o-carborane unit, which is crucial to reduce the reorganization energy of DQAO-PhCb and achieve narrowband emission. Also, the phenyl unit in DQAO-Ph and DQAO-PhCb helps to shorten charge transfer distances and improve ISC and RISC processes. Since the o-carborane unit is an adopted building block to achieve piezochromic behaviors, the theoretically structure-modified DQAO-PhCb is expected to exhibit narrowband emission, TADF, and piezochromic features all together. Our findings will hopefully provide ideas for designing triphenylamine-based TADF emitters with narrowband emission and piezochromic behaviors.

Piezochromism and Conductivity Modulations under High Pressure by Manipulating the Viologen Radical Concentration.

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.

Piezochromism in Dynamic Three-Dimensional Covalent Organic Frameworks.

Piezochromic materials with pressure-dependent photoluminescence tuning properties are important in many fields, such as mechanical sensors, security papers, and storage devices. Covalent organic frameworks (COFs), as an emerging class of crystalline porous materials (CPMs) with structural dynamics and tunable photophysical properties, are suitable for designing piezochromic materials, but there are few related studies. Herein, we report two dynamic three-dimensional COFs based on aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ) chromophores, termed JUC-635 and JUC-636 (JUC = Jilin University China), and for the first time, study their piezochromic behavior by diamond anvil cell technique. Due to the various luminescent groups, JUC-635 has completely different solvatochromism and molecular aggregation behavior in the solvents. More importantly, JUC-635 with AIE effect exhibits a sustained fluorescence upon pressure increase (~3 GPa), and reversible sensitivity with high-contrast emission differences (Δλem = 187 nm) up to 12 GPa, superior to other CPMs reported so far. Therefore, this study will open a new gate to expand the potential applications of COFs as exceptional piezochromic materials in pressure sensing, barcoding, and signal switching.

Piezochromism in Dynamic Three‐Dimensional Covalent Organic Frameworks

AbstractPiezochromic materials with pressure‐dependent photoluminescence tuning properties are important in many fields, such as mechanical sensors, security papers, and storage devices. Covalent organic frameworks (COFs), as an emerging class of crystalline porous materials (CPMs) with structural dynamics and tunable photophysical properties, are suitable for designing piezochromic materials, but there are few related studies. Herein, we report two dynamic three‐dimensional COFs based on aggregation‐induced emission (AIE) or aggregation‐caused quenching (ACQ) chromophores, termed JUC‐635 and JUC‐636 (JUC=Jilin University China), and for the first time, study their piezochromic behavior by diamond anvil cell technique. Due to the various luminescent groups, JUC‐635 has completely different solvatochromism and molecular aggregation behavior in the solvents. More importantly, JUC‐635 with AIE effect exhibits a sustained fluorescence upon pressure increase (≈3 GPa), and reversible sensitivity with high‐contrast emission differences (Δλem=187 nm) up to 12 GPa, superior to other CPMs reported so far. Therefore, this study will open a new gate to expand the potential applications of COFs as exceptional piezochromic materials in pressure sensing, barcoding, and signal switching.

Precise peripheral design enables propeller-like squaraine dye with highly sensitive and wide-range piezochromism.

Piezochromic fluorescent (PCF) materials that feature high sensitivity and wide-range switching are attractive in intelligent optoelectronic applications but their fabrication remains a significant challenge. Here we present a propeller-like squaraine dye SQ-NMe2 decorated with four peripheral dimethylamines acting as electron donors and spatial obstacles. This precise peripheral design is expected to loosen the molecular packing pattern and facilitate more substantial intramolecular charge transfer (ICT) switching caused by conformational planarization under mechanical stimuli. As such, the pristine SQ-NMe2 microcrystal exhibits significant fluorescence changes from yellow (λem = 554 nm) to orange (λem = 590 nm) upon slight mechanical grinding and further to deep red (λem = 648 nm) upon heavy mechanical grinding. Single-crystal X-ray diffraction structural analysis of two SQ-NMe2 polymorphs provides direct evidence to illustrate the design concept of such a piezochromic molecule. The piezochromic behavior of SQ-NMe2 microcrystals is sensitive, high-contrast, and easily reversible, enabling cryptographic applications.

Thermally activated delayed fluorescence with dual-emission and pressure-induced bidirectional shifting: cooperative effects of intramolecular and intermolecular energy transfer.

Different from the conventional piezochromic materials with a mono-redshift of single emission, our well-designed molecule demonstrates a sensitive turn-on and color-tunable piezochromic luminescence in response to the hydrostatic pressure. The molecule PXZ-W-SOF possesses dual-emission and pressure-induced bidirectional shifting characteristics. On the basis of in-depth experimental studies, on one hand, it is confirmed that the origin of the dual-emission behavior is the intramolecular charge transfer, namely thermally activated delayed fluorescence (TADF), and the intermolecular excimer; on the other hand, the emission of the excimer exhibits three-step variations with increasing pressure, which is mainly attributed to the molecular structure and its crystal packing state. The remarkable color change of PXZ-W-SOF from sky-blue to green to deep-blue during the whole process of boosting and releasing pressure is a result of intramolecular and intermolecular energy-transfer interactions. The PXZ-W-SOF molecular model is an extremely rare example of highly sensitive fluorescence tuning driven by TADF and excimer conversion under mechanical stimulation, thus providing a novel mechanism for the field of piezochromism. The unique molecular design also offers a new idea for rare deep-blue and ultraviolet TADF materials.

Conformational Flexibility-Controlled Piezochromic Behavior of Organic Fluorophores

The pressure-responsive behaviors of organic fluorophores are often irregular and highly system-dependent. Unraveling the pressure effects on them is the key to the precise design of piezochromic materials. Here, we demonstrate that the different conformational flexibilities of organic fluorophores can lead to their distinct piezochromic behaviors. By combining quantum mechanics/molecular mechanics models and thermal vibration correlation function formalism, we show that two organic fluorophores, dibenzo[b,d]thiophene 5,5-dioxide (DBTS) and carbazole (Cz), with similar chemical structures, display different conformations and conformational flexibilities, which determine the responsiveness of their piezochromic behaviors. It is found that pressurization continuously improves the planarity between the sulfonyl group and the aromatic skeleton in cross-shaped DBTS, leading to red-shifted emission. In addition, the fluorescence quantum efficiency (FQE) of DBTS increases monotonically in a wide range of 0–7 GPa and then shows a dramatic reduction upon further compression to 12 GPa. The non-monotonical FQE of DBTS is attributed to the continuously decreased radiative decay rate constant (kr) and non-monotonically changed non-radiative decay rate constant (kic). The initial decrease in kic is due to the restriction of aromatic skeleton vibrations in the high-frequency region, and the subsequent increase in kic is ascribed to the acceleration of the bending and rotational vibrations of the sulfonyl group in the low-frequency region. In contrast, pressurization hardly influences the conformation of planar Cz, resulting in a slight change in emission and FQE, which is well consistent with experimental results. This study opens an efficient shortcut to the rational design of advanced piezochromic luminescent materials.

Piezochromic luminescence behavior based on the molecular conformation change of nitro-triphenylamine

Mechanoresponsive luminescent (MRL) materials have become a high-profile research system in the fields of force sensing, anti-counterfeiting, and optical information storage because of their obvious variety on luminescence intensity or luminescence color under mechanical force. In this work, nitro-triphenylamine (TPA-NO2) crystal was selected to conduct in situ high pressure experiments based on diamond anvil cell combined with theory calculation, and the piezochromism of TPA-NO2 crystal was discussed in detail. During the compression process, the TPA-NO2 crystal showed the piezochromic behavior and a redshift of the emission peak. The mechanical response behavior of TPA-NO2 provides insight into the study of multicolor properties from a structural perspective, offering a clean means of using pressure to modulate different colors and contributing to the exploration of high-potency MRL materials.

A New Concept of Radiation Detection Based on a Fluorochromic and Piezochromic Nanocluster.

Developing materials that possess colorimetric responses to external stimuli is a promising strategy for addressing the current challenges in radiation dosimetry. Currently, colorimetric ionizing-radiation-responsive materials remain underexplored, and those with multistimuli response are rare. Herein, the integration of thorium cation and photoresponsive terpyridine carboxylate ligand gives rise to a thorium nanocluster, Th-101, which displays the second case of fluorochromic response and unprecedented piezochromic behavior among all actinide materials. The emission color of Th-101 exhibits a gradual transition from blue to cyan to green upon irradiation with accumulated dose, which renders colorimetric dosimetry of ionizing radiation based on a red-green-blue (RGB) concept. Further fabricating Th-101 into a custom-built optoelectronic device allows for on-site quantification of radiation dose with merits of ease of operation, rapid readout, and cost-effectiveness.

AIPE-active Ir(Ⅲ) complexes with reversible piezochromic behavior and its application for data security protection

Three cyclometalated iridium complexes bearing different electron donating or electron withdraw group substitutes (Ir-OCH3), (Ir-H), (Ir-CF3) were synthesized and characterized. All complexes exhibit obvious aggregation-induced phosphorescence emission (AIPE) behavior in CH3CN-water mixtures, and relatively high absolute quantum efficiency in solid-states (ΦF =0.12-0.35). In addition, complexes Ir-OCH3 and Ir-CF3 show distinct revisable mechanochromic response. Upon grinding, the emission color of Ir-OCH3 and Ir-CF3 in solid state can be tuned from yellow to orange and green to yellow (both of Ir-OCH3 and Ir-CF3 show about 30 nm red-shift), respectively. Powder X-ray diffraction spectroscopy (PXRD) were also used to confirm the phase transition of different complexes in their different colored solid state. Moreover, complex Ir-CF3 was conducted as smart luminescent materials in a model of data security protection.

Highly Bright Fluorescence from Dispersed Dimers: Deep‐Red Polymorphs and Wide‐Range Piezochromism

AbstractNear‐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).

Photoactivated Fluorescence Enhancement in F,N-Doped Carbon Dots with Piezochromic Behavior.

Photoactivation in CdSe/ZnS quantum dots (QDs) on UV/Vis light exposure improves photoluminescence (PL) and photostability. However, it was not observed in fluorescent carbon quantum dots (CDs). Now, photoactivated fluorescence enhancement in fluorine and nitrogen co-doped carbon dots (F,N-doped CDs) is presented. At 1.0 atm, the fluorescence intensity of F,N-doped CDs increases with UV light irradiation (5 s-30 min), accompanied with a blue-shift of the fluorescence emission from 586 nm to 550 nm. F,N-doped CDs exhibit photoactivated fluorescence enhancement when exposed to UV under high pressure (0.1 GPa). F,N-doped CDs show reversible piezochromic behavior while applying increasing pressure (1.0 atm to 9.98 GPa), showing a pressure-triggered aggregation-induced emission in the range 1.0 atm-0.65 GPa. The photoactivated CDs with piezochromic fluorescence enhancement broadens the versatility of CDs from ambient to high-pressure conditions and enhances their anti-photobleaching.

Photoactivated Fluorescence Enhancement in F,N‐Doped Carbon Dots with Piezochromic Behavior

AbstractPhotoactivation in CdSe/ZnS quantum dots (QDs) on UV/Vis light exposure improves photoluminescence (PL) and photostability. However, it was not observed in fluorescent carbon quantum dots (CDs). Now, photoactivated fluorescence enhancement in fluorine and nitrogen co‐doped carbon dots (F,N‐doped CDs) is presented. At 1.0 atm, the fluorescence intensity of F,N‐doped CDs increases with UV light irradiation (5 s–30 min), accompanied with a blue‐shift of the fluorescence emission from 586 nm to 550 nm. F,N‐doped CDs exhibit photoactivated fluorescence enhancement when exposed to UV under high pressure (0.1 GPa). F,N‐doped CDs show reversible piezochromic behavior while applying increasing pressure (1.0 atm to 9.98 GPa), showing a pressure‐triggered aggregation‐induced emission in the range 1.0 atm–0.65 GPa. The photoactivated CDs with piezochromic fluorescence enhancement broadens the versatility of CDs from ambient to high‐pressure conditions and enhances their anti‐photobleaching.

Hybrid piezochromic coatings for impact detection on composite substrates for aeronautic

Structural Health Monitoring (SHM) is a major issue in the aeronautic field, especially for impact detection on composite materials which can be time consuming. Impact-sensitive coatings are an interesting way to save costs and time associated with these monitoring operations. This paper describes the successful preparation of a reversible piezochromic coating for impacts detection on composite substrates. It results from the incorporation of piezochromic pigments into a hybrid organic-inorganic matrix. It has been optimized by studying and quantifying their piezochromic behavior by UV-visible spectroscopy. It revealed that the piezochromic properties of the sensitive coating and the pigments alone are quite different and ruled by the hardness of the film. The obtained sensitivity and threshold pressure are determined to be 28% GPa−1 and 0.2 GPa respectively.

Structural tuning enables piezochromic and photochemical properties in N-aryl-β-enaminones.

An efficient synthesis of N-aryl-β-enaminones via Et3N-mediated, one-pot three-component reaction of 4-hydroxycoumarin/dimedone, β-nitrostyrene/2-(2-nitrovinyl)thiophene, and arylamine in toluene under refluxed conditions is herein presented. Some prepared compounds were found to exhibit piezochromic properties. The XRD and SEM measurements of the piezochromic compound showed substantial crystal packing and morphology changes before and after grinding. Further, one prepared compound was found to be light-sensitive and can be converted to a furo[3,2-b]pyridin-2(4H)-one derivative upon UV irradiation. A plausible mechanism for this photochemical reaction was proposed.

AIEE Active Donor-Acceptor-Donor-Based Hexaphenylbenzene Probe for Recognition of Aliphatic and Aromatic Amines.

In the present investigation, an intramolecular charge transfer (ICT) and aggregation induced emission enhancement (AIEE) active donor-acceptor-donor (D-A-D) system 5 having fumaronitrile as the acceptor and hexaphenylbenzene (HPB) as the donor moieties joined through rotatable phenyl rings has been designed and synthesized that is highly emissive in the solid state and exhibits stimuli-responsive reversible piezochromic behavior upon grinding and heating. Because of its AIEE characteristics, HPB derivative 5 undergoes aggregation to form fluorescent aggregates in mixed aqueous media that exhibit ratiometric fluorescence response toward aliphatic amines (primary/secondary/tertiary) and turn-off response toward aromatic amines and hence differentiates between them. Further, the solution-coated portable paper strips of derivative 5 showed pronounced and sensitive response toward aromatic and aliphatic amines with a detection limit in the range of picogram and nanogram level, respectively.