Pressure-Induced Emission Enhancement of Carbazole: The Restriction of Intramolecular Vibration

Pressure-induced luminescence evolution of 3,3'-diamino-4,4'-azofurazan: Role of restricting chemical bond vibration and conformational modification

The luminescence and electronic structure of 3,3'-Diamino-4,4'-azofurazan (DAAzF) were studied under high pressure conditions through experimental and calculation approaches. The transition of π* → π was primarily responsible for DAAzF's broad light emission. Upon applying pressure to DAAzF, high-pressure-stiffened hydrogen-bond interactions enable the restriction of the stretching vibration of NH2 group. The reduced energy loss through nonradiative rotational relaxation and molecular motions lead to a ∼20 times luminescent enhancement of DAAzF from 1 atm to 8.9 GPa. With the further strengthening of interlayer hydrogen bond interactions at higher pressure, the deviation of hydrogen atoms in amino groups from the molecular plane lessens the radiation transition efficiency. In addition, the bending of the C-C-N=N bond further leads to molecular conformation changes at approximately 20.7 GPa, which induces an abrupt redshift and moderate quenching of the luminescence. Furthermore, the band gap of DAAzF is significantly influenced by pressure. As the color undergoes a transition from yellow to red, and becomes darker as the pressure increases, the absorption edge shifted towards red. At 3.4, 9, and 21 GPa, three conformational variations were identified in conjunction with electronic structural alterations.

Pressure effects on both fluorescent emission and charge transport properties of organic semiconductors: a computational study

External pressure can regulate the photophysical property and charge transport performance of organic semiconductors, however, the underlying mechanism at the microscopic level is still elusive. Using thermal vibrational correlation function coupled quantum mechanics/molecular mechanics and full quantum charge transfer rate theory, we systematically explore the influence of pressure on fluorescence emission and charge transport behaviours of representative cyclooctatetrathiophene (COTh). It is found that, upon pressurization, the intramolecular configurations of COTh became more twisted, leading to the blue-shifted emission. The fluorescence quantum efficiency (FQE) of COTh crystals decreases monotonically in a wide pressure range of 0-4.38 GPa, because the increase of intermolecular electronic energy transfer rate constant (keet) is larger than the decrease of internal conversion rate constant (kic), and the variation of keet is dominant. The decrease in kic is attributed to the decreasing reorganization energy, reflecting the suppression of the low-frequency flipping vibrations of four thiophene rings and the high-frequency stretching vibrations of central cyclooctatetraene, while the keet increase is due to the simultaneous increase in exciton coupling and spectra overlap. Moreover, we predicted that the hole mobility of COTh increases monotonically by nearly an order of magnitude from 0.39 to 3.00 cm2 V-1 s-1 upon compression, because of the increase in transfer integral and the decrease of charged reorganization energy. Furthermore, its hole mobility exhibits obvious anisotropy. Our work systematically builds the external pressure, molecular packing, luminescence and transport properties relationships of organic semiconductors and provides theoretical guidance for the rational design of pressure responsive organic semiconductors with excellent photoelectric performance.

Multiple responses of 1,6-diphenyl-1,3,5-hexatriene to mechanical stimulation: emission enhancement, piezochromism and negative linear compressibility

The properties of mechanoresponsive materials are mainly affected by intermolecular interaction, in which anisotropic grinding and hydrostatic high-pressure compression are the powerful tools used for modulation. Upon applying high pressure to 1,6-diphenyl-1,3,5-hexatriene (DPH), the reduced molecular symmetry results in the originally forbidden S₀ → S₁ transition to become allowed that then leads to a 13-times emission enhancement, and π-π interactions result in piezochromism (red-shifted up to 100 nm). With increasing pressure, high-pressure-stiffened H⋯C/C⋯H and H⋯H interactions enable the DPH molecules to generate a NLC mechanical response (9-15 GPa) with K_b = -5.8764 TPa^-1 along the b-axis. As a contrast, upon destroying the intermolecular interactions by grinding, the DPH luminescence blue-shifts from cyan to blue. Based on this research, we investigate a new pressure-induced emission enhancement (PIEE) mechanism and enabled NLC phenomena by controlling weak intermolecular interactions. In-depth research of the evolution of intermolecular interactions has important reference value for developing new fluorescence materials and structural materials.

Pressure-Tuned Multicolor Emission of 2D Lead Halide Perovskites with Ultrahigh Color Purity

The chemical diversity and structural flexibility of lead halide perovskites (LHPs) offer tremendous opportunities to tune their optical properties through internal molecular engineering and external stimuli. Herein, we report the wide-range and ultrapure photoluminescence emissions in a family of homologous 2D LHPs, [MeOPEA]₂ PbBr_4-4x I_4x (MeOPEA=4-methoxyphenethylammonium; x=0, 0.2, 0.425, 0.575, 1) enabled through internal chemical pressure and external hydrostatic pressure. The chemical pressure, induced by the C-H⋅⋅⋅π interactions and halogen doping/substitution strengthens the structural rigidity to give sustained narrow emissions, and regulates the emission energy, respectively. Further manipulation of physical pressure leads to wide-range emission tuning from 412 to 647 nm in a continuous and reversible manner. This work could open up new pathways for developing 2D LHP emitters with ultra-wide color gamut and high color purity which are highly useful for pressure sensing.

Tailoring the high-brightness "warm" white light emission of two-dimensional perovskite crystals via a pressure-inhibited nonradiative transition

Efficient warm white light emission is an ideal characteristic of single-component materials for light-emitting applications. Although two-dimensional hybrid perovskites are promising candidates for light-emitting diodes, as they possess broadband self-trapped emission and outstanding stability, they rarely achieve a high photoluminescence quantum yield of warm white light emissions. Here, an unusual pressure-induced warm white emission enhancement phenomenon from 2.1 GPa to 9.9 GPa was observed in two-dimensional perovskite (2meptH2)PbCl4, accompanied by a large increase in the relative quantum yield of photoluminescence. The octahedral distortions, accompanied with the evolution of organic cations, triggered the structural collapse, which caused the sudden emission enhancement at 2.1 GPa. Afterwards, the further intra-octahedral collapse promotes the formation of self-trapped excitons and the substantial suppression of nonradiative transitions are responsible for the continuous pressure-induced photoluminescence enhancement. This study not only clearly illustrates the relationship between crystal structure and photoluminescence, but also provides an experimental basis for the synthesis of high-quality warm white light-emitting 2D metal halide perovskite materials.

Blue Emission of Tetrafluorobenzocarbazole Under the Interactions of Nitrogen–oxygen Saturated hydrogen Bonds with Aggregated Proton Acid

Two novel tetrafluorobenzocarbazole and containing the amino branch introduced at the end of the molecule are synthesized by a simple method. The tetrafluorobenzocarbazole as the electron donor with electron-rich fluoride ions connected by π-benzyl ring conjugation structure, which affects the overall electron cloud density. Moreover, the amino branch introduced at the end of the molecule, which makes it easy to form intermolecular hydrogen bonds and affected photophysical properties. Meanwhile, the photophysical property of both compounds are discussed under different acidic conditions. The UV-absorption show that around ~286 nm is mainly attributed to the strong structural absorption band peak of the π-π ∗ transition of the carbazole moiety, and the irregular absorption band around ~314 nm and ~326 nm are mainly attributed to the n-π ∗ transition of the carbazole group conjugate with the adjacent molecule. The emission spectrum of both compounds showed that the intensity of fluorescence decreased in different degrees after the addition of the acidic solution. Furthermore, the electrochemical properties were evidenced by cyclic voltammetry (CV) and density functional theory (DFT) calculations, and the orbital conformation (HOMOs-LUMOs) was simulated by Gaussian 09 software and its crystal structure was observed by X-ray diffraction (XRD). The results exhibited that both compounds are electrochemically stable blue small-molecule fluorescent substances, and expected that both compounds can be novel and stable acid-sensitive organic blue-light materials.

Mechanical Force-Induced Blue-Shifted and Enhanced Emission for AIEgens

Mechanochromic (MC) luminescence of organic molecules has been emerging as a promising smart material for optical recording and memory devices. At the same time, pressure-induced blue-shifted and enhanced luminescence are rarely reported now. Herein, a series of cyanostilbene-based AIEgens with different substituents were synthesized to evaluate the influence of morphology transformation and push-pull electronic effect on the MC luminescence. Among these luminophores, compound 1 with one cyano group and diethylamino group was more susceptible to mechanical stimuli and obtained blue-shifted and enhanced fluorescence in response to anisotropic grinding. Powder X-ray diffraction patterns indicated that the MC behaviors were ascribed to the solid-state morphology transition from crystal-to-crystal. Analysis of crystal structures revealed that loose molecular packing is a key factor for high high-contrast MC luminescence. The smart molecular design, together with the excellent performance, verified that luminophores with twisted structures are ideal candidates for MC luminogens.

Fluorescence-based monitoring of the pressure-induced aggregation microenvironment evolution for an AIEgen under multiple excitation channels

The development of organic solid-state luminescent materials, especially those sensitive to aggregation microenvironment, is critical for their applications in devices such as pressure-sensitive elements, sensors, and photoelectric devices. However, it still faces certain challenges and a deep understanding of the corresponding internal mechanisms is required. Here, we put forward an unconventional strategy to explore the pressure-induced evolution of the aggregation microenvironment, involving changes in molecular conformation, stacking mode, and intermolecular interaction, by monitoring the emission under multiple excitation channels based on a luminogen with aggregation-induced emission characteristics of di(p-methoxylphenyl)dibenzofulvene. Under three excitation wavelengths, the distinct emission behaviors have been interestingly observed to reveal the pressure-induced structural evolution, well consistent with the results from ultraviolet-visible absorption, high-pressure angle-dispersive X-ray diffraction, and infrared studies, which have rarely been reported before. This finding provides important insights into the design of organic solid luminescent materials and greatly promotes the development of stimulus-responsive luminescent materials.

Origin of Nonmonotonical Variation of Luminescence Efficiency under Pressure in Organic Molecule

The nonmonotonical variation of luminescence efficiency under extra pressure occurs frequently in organic molecules; however, the mechanism behind this is still elusive. Using a theoretical protocol combining thermal vibration function rate formalism coupled quantum mechanics/molecular mechanics models, we explored the relationship between extra pressure, molecular packing, and fluorescent quantum efficiency (FQE) of the representative 1,2,3,4-tetraphenyl-1,3-cyclopentadiene (TPC). It is found that the first increase and then decrease of FQE in TPC crystalline aggregates upon pressurization is cooperatively attributed to the continuous reduction of the radiative decay rate constant and nonmonotonical change of the nonradiative decay rate constant (k_ic). The initial decrease of k_ic originates from the effective suppression of electron-vibration coupling and the Duschinsky rotation effect by extra pressure, whereas the following increase of k_ic comes from the surge of nonadiabatic electronic coupling and the reduction of adiabatic excitation energy upon further compression. This study can provide a theoretical basis for the rational design and performance control of the piezochromic luminescent materials.

Pressure-Induced Restricting Intermolecular Vibration of a Herringbone Dimer for Significantly Enhanced Multicolor Emission in Rotor-Free Truxene Crystals

Organic solid-state luminescent materials exhibit numerous exciting photoelectric properties that are central to emergent organic light-emitting diodes, smart sensors, and data encryption. However, the luminescence of pure organic rotor-free materials has been afflicted with strong intermolecular π-π stacking interactions. Herein, an unprecedented pressure-induced emission enhancement (PIEE) is realized in a system of rigid planar pure polycyclic aromatics, i.e., truxene crystals. The emission intensity is enhanced 7-fold below 3.0 GPa with a photoluminescence quantum yield increased to 10.17% compared with the initial value of 1.78%, and the emission colors change from green (520 nm) to red (640 nm) within 11.8 GPa. Spectral characterizations and first-principles calculations reveal that the PIEE and piezochromism can mainly be attributed to the restricted intermolecular vibration and the decreased energy gap. Our findings enrich the PIEE mechanism and provide a new guideline for designing pressure-responsive luminescent materials in advancing their photoelectric applications.

Pressure-Induced Local Excitation Promotion: New Route toward High-Efficiency Aggregate Emission Based on Multimer Excited State Modulation

Achieving high-efficiency solid state emission is essential for practical applications of organic luminescent materials. However, intermolecular interactions generally induce formation of multimeric aggregate excited states with deficient emissive ability, making it extremely challenging to enhance emission in aggregated states. Here we demonstrate a novel strategy of continuously regulating multimeric excitation constituents with a high-pressure technique successfully enhancing the emission in a representative organic charge-transfer material, Laurdan (6-lauroyl-N,N-dimethyl-2-naphthylamine). The Laurdan crystal exhibits distinct emission enhancement up to 4.1 GPa accompanied by a shift in the emission color from blue to cyan. Under compression, the π-π interplanar distance in Laurdan multimers is reduced, and intermolecular wave function diffusion is demonstrated to be improved simultaneously, which results in local excitation promotion and thus enhanced emission. Our findings not only provide new insights into multimeric excited state emission modulation but also pave the way for the further design of high-performance aggregated luminophores.

Understanding the solid state luminescence and piezochromic properties in polymorphs of an anthracene derivative

Luminescent molecular crystals have gained significant research interest for optoelectronic applications. However, fully understanding their structural and electronic relationships in the condensed phase and under external stimuli remains a significant challenge. Here, piezochromism in the molecular crystal 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA) is studied using a combination of density functional theory (DFT) and time-dependent TD-DFT. We investigate the effects that molecular packing and geometry have on the electronic and phonon structure and the excited state properties in this archetypal system. We find that the luminescence properties are red-shifted with the transition from a herringbone to a sheet packing arrangement. An almost continuous red-shift in the band gap is found with the application of an external pressure through the enhancement of π-π and CH-π interactions, and is a mechanism in fine tuning an emissive response. The analysis of the phonon structure of the molecular crystal suggests restriction of motion in the herringbone packing arrangement, with motion restricted at higher pressure. This is supported by the Huang-Rhys factors which show a decrease in the reorganisation energy with the application of pressure. Ultimately, a balance between the decrease in reorganisation energies and the increase in exciton coupling will determine whether nonradiative decay is enhanced or decreased with the increase in pressure in these systems.

Semiconductors with Chiral Crystal Structure in Group IVB Transition Metal Pernitrides

Group IVB transition metal (TM) nitrides rarely exhibit semiconductor phase, except for TM3N4 (TM = Ti, Zr, and Hf) compounds. In this study, using the ab-initio calculations based on density...

Pressure-induced polymerization and bandgap-adjustment of TPEPA

Organic solar cells have become an important development direction in solar cell materials because of their low cost, light weight, and good flexibility. However, the size of their bandgap is difficult to continuously regulate, resulting in a low power conversion efficiency. In this work, an organic molecule TPEPA was synthesized, and its luminescence performance and polymerization under high pressure were studied by performing in situ Raman, IR, fluorescence, and UV-vis spectroscopy. The Raman and IR spectroscopic results show that single bonds (C–H, C–Ph) and long chains (C–C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C–C) are more unstable and prone to amorphization under high pressure. At 10 GPa, the TPEPA molecule undergoes a transition of amorphization accompanied by a few polymerizations in the CC bond structure. After holding pressure at 20 GPa for one day and releasing to ambient pressure, the other peaks almost disappeared, while the new peak of C(sp³)–H from the polymerization of the benzene ring was observed, indicating that the irreversible amorphization and polymerization did occur. UV-vis spectra results show that the bandgap is reduced from 2.9 eV to 1.3 eV, which is just in the maximum conversion efficiency bandgap range (1.3–1.4 eV) of p–n junction solar cell materials. This pressure is within the working pressure range of a large volume press, which is favorable in applications of large-scale synthesis. Our strategy may provide a method for the large-scale synthesis of novel organic solar cell materials.

Organic solar cells have become an important development direction in solar cell materials because of their low cost, light weight, and good flexibility.

In Situ Exploration of Stimulus-Induced Emission Changes in Mechanochromic Dyes

The development of in situ analysis techniques for visualizing and linking macro- and nanoscopic features of external stimulus-responsive materials is crucial for their rational design and applications. Herein, we investigate the mechanical stress-induced emission changes in electron donor-acceptor type organic dye molecules in solid states through in situ single-particle fluorescence spectroscopy combined with macroscopic and nanoscopic stimulation systems. The change in emission color from green to yellow was attributed to repeated rubbing or scratching of the crystal surface, and not to simple cutting. This change was due to partial amorphization, which changed the intra- and intermolecular charge-transfer interactions of stacked molecules near the surface. We believe that this study will facilitate the efficient design of mechano-responsive materials with finely controlled and responsive properties.

Mechanofluorochromism with Aggregation-Induced Emission (AIE) Characteristics: A Perspective Applying Isotropic and Anisotropic Force

Organic mechanofluorochromic (MFC) materials (that change their emission under anisotropic and isotropic pressure) have attracted a great attention in recent years due to their promising applications in sensing pressure, storage devices, security inks, three-dimensional (3D) printing, etc. Stimuli-responsive organic materials with aggregation-induced emission (AIE) characteristics would be an interesting class of materials to enrich the chemistry of MFC compounds. A diamond anvil cell (DAC) is a small tool that is employed to generate high and uniform pressure on materials over a small area. This article discusses the relationship between the chemical structure of AIE compounds and the change in emission properties under anisotropic (mechanical grinding) and isotropic (hydrostatic) pressure. The luminescent properties of such materials depend on the molecular rearrangement in the lattice, conformational changes, excited state transitions and weak intermolecular interactions. Hence, studying the change in luminescent property of these compounds under varying pressure will provide a deeper understanding of the excited-state properties of various emissive compounds with stress. The development of such materials and studies into the effect of pressure on their luminescence properties are summarized.

Computational and infrared spectroscopic investigations of N-substituted carbazoles

The carbazole moiety is a commonly identified structural motif in the high-molecular-weight components of petroleum, known as asphaltenes. Detailed characterization of carbazoles is important for understanding the structure of asphaltenes and addressing challenges in the areas of heavy oil recovery, transportation, upgrading, and oil spills, arising from asphaltene properties and composition. In this work we study carbazole and the four N-substituted carbazoles 9-methylcarbazole, 9-ethylcarbazole, 9-vinylcarbazole and 9-phenylcarbazole. Experimental far- and mid-infrared spectra of these five carbazoles are measured using transmission and photoacoustic techniques. The molecular structures of the monomers and the respective dimers, optimized at the ωB97X-D/6-311++G(d,p) level of the density functional theory (DFT), are subjected to harmonic vibrational frequency calculations. The effect of changing substituents on the N-H bond, π-π stacking distances, and angles between monomers within the dimers, in addition to intermolecular interactions, is investigated. Noncovalent interaction analysis is employed to highlight the areas of attractive and repulsive interactions in the dimers. Thermochemistry calculations show that the formation of dimers of all carbazoles is spontaneous at 298 K. Comparison of the calculated vibrational spectra of these compounds with experimental spectra indicates that the existence of both monomers and dimers must be invoked to account for the observed bands in the infrared spectra. Excellent correlations between the experimentally-determined and calculated harmonic vibrational energies are obtained, with an experimental-to-calculated scaling factor of 0.95-0.96. These findings highlight the coupled computational-experimental approach for the interpretation of vibrational spectra and are essential for improving the spectroscopic characterization of N-substituted carbazoles.

Synthesis and Optical Properties of Axially Chiral Bibenzo[b]carbazole Derivatives

Pure organic materials with the circularly polarized luminescence (CPL) property have attracted significant research interest over the past few decades. In this study, a series of axially chiral bibenzo[b]carbazole derivatives were synthesized by adopting palladium- and iridium-catalyzed direct C-H functionalization reactions as the key steps. These compounds exhibited CPL characteristics with considerably large dissymmetry factors up to 2.81 × 10^-2 in the solid state, indicating the formation of well-ordered aggregates.

A synergy between the push-pull electronic effect and twisted conformation for high-contrast mechanochromic AIEgens

Mechanochromic (MC) luminogens in response to external stimulus have shown promising applications as pressure sensors and memory devices. Meanwhile, research on their underlying mechanism is still in the initial stage. Here, three pyridinium-functionalized tetraphenylethylenes bearing n-pentyloxy, hydrogen and nitro groups, namely TPE-OP, TPE-H and TPE-NO, are designed to systematically investigate the influence of the push-pull electronic effect and molecular conformation on MC luminescence. Upon anisotropic grinding and isotropic hydrostatic compression, TPE-OP with strong intramolecular charge transfer (ICT) affords the best MC behavior among them. Analysis of three polymorphs of TPE-H clearly indicates that planarization of the molecular conformation plays an important role in their bathochromic shifts under mechanical stimuli. Theoretical calculations also verify that high twisting stress of AIEgens can be released under high pressure. This study presents a mechanistic insight into MC behaviour and an effective strategy to achieve high-contrast MC luminescence.

Pressure-induced phosphorescence enhancement and piezochromism of a carbazole-based cyclic trinuclear Cu(i) complex

Interest in piezochromic luminescence has increased in recent decades, even though it is mostly limited to pure organic compounds and fluorescence. In this work, a Cu3Pz3 (Cu3, Pz: pyrazolate) cyclic trinuclear complex (CTC) with two different crystalline polymorphs, namely 1a and 1b, was synthesized. The CTC consists of two functional moieties: carbazole (Cz) chromophore and Cu3 units. In crystals of 1a, discrete Cz-Cu3-Cu3-Cz stacking was found, showing abnormal pressure-induced phosphorescence enhancement (PIPE), which was 12 times stronger at 2.23 GPa compared to under ambient conditions. This novel observation is ascribed to cooperation between heavy-atom effects (i.e., from Cu atoms) and metal-ligand charge-transfer promotion. The infinite π-π stacking of Cz motifs was observed in 1b and it exhibited good piezochromism as the pressure increased. This work demonstrates a new concept in the design of piezochromic materials to achieve PIPE via combining organic chromophores and metal-organic phosphorescence emitters.

Mechanofluorochromic Anthryl Phosphonate/Benzoic Acid Cocrystals with a Large Blue Shift: The Role of P=O⋅⋅⋅H Interactions

Organophosphonates have a rich and diverse chemistry, but their mechanofluorochromic features have rarely been documented. Herein, we report on cocrystals of anthranylphosphonate with (E)-4-(2-(anthracen-9-yl)vinyl)benzoic acid that exhibit reversible mechanofluorochromic properties with large blue shifts. Anisotropic grinding (or pressure of just 25 kPa) maintains the co-crystals emission features but leads to a pronounced 59 nm (2087 cm^-1 ) blue-shift. Such an impressive blue-shift for a cocrystal has not been reported to date. The underlying causes for the blue-shift were validated by X-ray diffraction (powder and single crystal) studies and Hirshfeld surface analysis. Twisted molecular conformations and P=O⋅⋅⋅ H/C=O⋅⋅⋅H/C⋅⋅⋅H noncovalent interactions afford molecular arrangements via slip-stacks with two different pitch angles and relatively larger interlayer distances. The new compound was established as a potential medium for optical recording and security display.

Thinking about the Development of High-Pressure Experimental Chemistry

High-pressure chemistry is an interdisciplinary science which uses high-pressure experiments and theories to study the interactions, reactions, and transformations among atoms or molecules. It has been extensively studied thus far and achieved rapid development over the past decades. However, what is next for high-pressure chemistry? In this Perspective, we mainly focus on the development of high-pressure experimental chemistry from our own viewpoint. An overview of the series of topics is as follows: (I) high pressure used as an effective tool to help resolve scientific disputes regarding phenomena observed under ambient conditions; (II) high-pressure reactions of interest to synthetic chemists; (III) utilizing chemical methods to quench the high-pressure phase; (IV) using high pressure to achieve what chemists want to do but could not do; (V) potential applications of in situ properties under high pressure. This Perspective is expected to offer future research opportunities for researchers to develop high-pressure chemistry and to inspire new endeavors in this area to promote the field of compression chemistry science.

Mechanical-Bending-Induced Fluorescence Enhancement in Plastically Flexible Crystals of a GFP Chromophore Analogue

Single crystals of optoelectronic materials that respond to external stimuli, such as mechanical, light, or heat, are immensely attractive for next generation smart materials. Here we report single crystals of a green fluorescent protein (GFP) chromophore analogue with irreversible mechanical bending and associated unusual enhancement of the fluorescence, which is attributed to the strained molecular packing in the perturbed region. Soft crystalline materials with such fluorescence intensity modulations occurring in response to mechanical stimuli under ambient pressure conditions will have potential implications for the design of technologically relevant tunable fluorescent materials.

Pressure-dependent distinct luminescent evolutions of pyrene and TPA-Py single crystals

The effects of the high pressure on two single crystals, pyrene and N,N-diphenyl-4-(pyren-1-yl)aniline (TPA-Py), were studied by in situ fluorescent and Raman spectroscopies. During the compression, the pyrene with one structureless excimer emission band showed a continuous bathochromic-shift. In contrast, with the pressure increasing to 10.36 GPa, TPA-Py previously dominated with the hybridized local and charge transfer (HLCT) excited state gradually exhibited a new band at longer wavelengths, which is assigned to a new excited state species with the intramolecular charge transfer (ICT) state, caused by the pressure-induced changes on its molecular configuration. Accompanied by the spectral changes, a sequential color variation from blue to cyan was observed, giving a change to yellow and then red. The significant broadening of the full-width half-maximum (FWHM) of the TPA-Py is observed due to the enhanced dipole-dipole interaction and the existence of pressure gradient. Both pyrene and TPA-Py showed the delayed recovery of the luminescence in the compression-decompression cycle, which results from the poor reversibility of electronic structure caused by the compression-induced piezochromic effect. Furthermore, the evolutions of the Raman spectra of pyrene and TPA-Py indicated that the pressure-induced reversible transformation is caused by the molecular conformational change. This study is a deeper understanding of the structure-property relation of the HLCT species and will be a helpful reference for the regulation of photoluminescence in these intramolecular electron donor-acceptor crystal materials.

Mechanism of Different Piezoresponsive Luminescence of 2,3,4,5-Tetraphenylthiophene and 2,3,4,5-Tetraphenylfuran: A Strategy for Designing Pressure-Induced Emission Enhancement Materials

Mechanoresponsive luminescent materials have attracted widespread attention for their potential applications, especially for these behaving pressure-induced emission enhancement (PIEE). Designing and seeking systems with high-efficiency PIEE are desirable and crucial for material science. Here, the mechanisms of different piezoresponsive luminescence of 2,3,4,5-tetraphenylthiophene (TPT) and 2,3,4,5-tetraphenylfuran (TPF) crystals are explored. The experimental results combined with density functional theory (DFT) theory calculation indicate that the PIEE phenomenon is possibly exhibited in V-shape arrangement for the reason of the weak π-π interactions. This study not only gains deep insight into the relationship between optical properties and structural evolution but also puts forward a strategy for designing PIEE materials from the point of molecular arrangement.

Pressure-Induced Blue-Shifted and Enhanced Emission: A Cooperative Effect between Aggregation-Induced Emission and Energy-Transfer Suppression

Most organic piezochromic materials exhibit red-shifted and quenched emission as pressure increases. However, an abnormal phenomenon of pressure-induced blue-shifted and enhanced emission is observed in a 9-(3-(1,2,2-triphenylvinyl)phenyl)anthracene crystal, which is based on discrete π-π anthracene (AN) dimers stacking with tetraphenylethylene (TPE) as spacer. A blue-shifted emission appears and strengthens when the pressure is more than 1.23 GPa, and it reaches the maximum when the pressure is 4.28 GPa. This phenomenon is ascribed to the cooperative effect between the aggregation-induced emission of TPE units and energy-transfer suppression from TPE to an AN excimer. This work reports a new concept in the piezochromic field and provides a novel strategy to achieve luminescence from a high-lying excited state.

Synthesis and photophysical and mechanochromic properties of novel 2,3,4,6-tetraaryl-4H-pyran derivatives

The crystallization abilities caused by introducing different aromatic groups were found to play a very important role in determining the formation of the mechanochromic activities of 2,3,4,6-tetraaryl-4H-pyran derivatives.

Pressure-Induced Emission Enhancement and Multicolor Emission for 1,2,3,4-Tetraphenyl-1,3-cyclopentadiene: Controlled Structure Evolution

Mechanoresponsive luminescent (MRL) materials have attracted considerable attention because of their potential applications in mechanical sensors, memory chips, and security inks; MRL materials possessing high efficiency and multicolor emission qualities are especially interesting. In this Letter, we found 1,2,3,4-tetraphenyl-1,3-cyclopentadiene (TPC) crystal exhibited both pressure-induced emission enhancement (PIEE) and multicolor behavior. In addition, infrared spectroscopy analysis indicated that the ring-opening reaction of the phenyl ring occurred when pressure was beyond 24.7 GPa. The reaction was promoted from 24.7 to 35.9 GPa, which resulted in the redder irreversible color change for the sample released from 35.9 GPa than from 24.7 GPa. The results regarding the mechanoresponsive behavior of TPC offered a deep insight into PIEE and multicolor properties from the structural point of view and inspired the idea of capturing different colors by hydrostatic pressure, which will facilitate the design of and search for high-performance MRL materials.

High-Pressure Band-Gap Engineering and Metallization in the Perovskite Derivative Cs3 Sb2 I9

Among photovoltaic materials, the antimony-based, perovskite-like structure Cs₃ Sb₂ I₉ stands out owing to its low toxicity and air stability. Here, changes in the optoelectronic properties and crystal structure of the lead-free perovskite derivative Cs₃ Sb₂ I₉ are reported, caused by pressure-induced lattice compression. At 20.0 GPa, Cs₃ Sb₂ I₉ with a wide band gap (2.34 eV) successfully broke through the Shockley-Queisser limit (1.34 eV), accompanied by clear piezochromism from orange-yellow to opaque black. Additionally, Cs₃ Sb₂ I₉ experienced completely reversible amorphization at 20.0 GPa. These optical changes could be attributed to atomic-orbital overlap enhancement caused by contraction of the Sb-I bond length and diminution of the Sb-I bond angle. In addition, Cs₃ Sb₂ I₉ underwent a transition from semiconductor to conductor upon compression and obtained metallic properties at 44.3 GPa, indicating new electronic properties. The obtained results may further broaden the research prospects of halide perovskite materials in the field of photovoltaics.

Cooling-Induced NIR Emission Enhancement and Targeting Fluorescence Imaging of Biperylene Monoimide and Glycodendrimer Conjugates

Under high concentrations, strong pressure, and low temperature, fluorophores usually exhibit the fluorescence quenching phenomenon. Of significance, the development of aggregation-induced emission (AIE) and pressure-induced emission (PIE) fluorophores has perfectly prevented fluorescence quenching under high concentrations and strong pressure. However, cooling-induced fluorescence quenching in water is still an urgent problem. In this paper, cooling-induced emission (CIE) enhancement based on a biperylene monoimide (BPMI) derivative, BPMI-18Lac, with a conjugated lactose-based glycodendrimer was developed. BPMI-18Lac, as a non-AIE molecule, exhibited the CIE phenomenon with a fluorescent intensity increasing 7-fold when the temperature decreased from 80 to -40 °C. The mechanism was due to the inhibition of the intramolecular electron interactions between the perylene monoimide moieties linked by the C-C single bond. In addition, BPMI-18Lac, as a multivalent glycodendrimer, showed selective fluorescence imaging for HepG 2 cells through the ASGP receptor on the cell surface. Importantly, this work developed a water-soluble CIE molecule for potential application below freezing temperature.

Insights into supramolecular-interaction-regulated piezochromic carbonized polymer dots

The photoluminescence (PL) mechanism plays a significant role in the study of carbonized polymer dots (CPDs). The supramolecular interaction exists in most materials, which offers innate methods to regulate the optical and physical properties. However, insights into the tunable red- and blue-shifted PL peaks of CPDs by the supramolecular interaction still remain elusive. Herein, the supramolecular interaction-triggered fluorescence change of CPDs is reported by the investigation of the piezochromic behaviors. The π-conjugated system and the hydroxy group are both critical to manipulate the PL of CPDs under high pressure. The π-π stacking of the π-conjugated system was enhanced with increasing pressure, which induces the red-shifting of PL peaks, while the hydroxyl-related hydrogen bond formation eventually causes a blue-shift. In addition, their chemical stability, low toxicity, and the tunable PL properties of CPDs by supramolecular interaction under high pressure would deepen the understanding of the fluorescence mechanism of CPDs, inspiring extensive application prospects in sensing and light-emitting diodes.

Phase transitions and chemical reactions of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine under high pressure and high temperature

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is one of the most important energetic materials. Investigations on its phase transitions and chemical reactions under extreme conditions are very important to understand the explosion process and design new energetic materials. By using a diamond anvil cell combined with in situ Raman, IR and X-ray diffraction techniques up to a pressure of ∼40 GPa, we found that β-HMX undergoes four reversible phase transitions without any chemical reaction under external pressure at room temperature. Isostructural phase transitions emerge around 5 GPa (ζ-HMX) and 10–13 GPa (ε-HMX), and another two phases emerge at 16 GPa (η-HMX) and 27 GPa (ϕ-HMX). The unit cells of ζ-HMX and ε-HMX were determined as a = 6.215 Å, b = 10.417 Å, c = 8.272 Å, β = 124.88°, P2₁/c at 6.2 GPa and a = 6.130 Å, b = 9.846 Å, c = 8.258 Å, β = 125.06°, P2₁/c at 12.6 GPa, respectively. The crystal structures of β, ζ, and ε-HMX were obtained by Rietveld refinement, based on which the rotations of NO₂ groups were found to be related to the phase transition at 5 GPa. Additionally, HMX decomposes at 8.7 GPa and 300 °C. Carbon dioxide, hydroxyl, imino and hydroxyimino groups were detected in the IR spectrum, which indicates that the reaction contains a hydrogen transfer process. Our investigation uncovers the structural variation of β-HMX under external pressure and identifies the decomposition products under extreme conditions, which provides new insight to understand the detonation process of energetic materials.

Phase transitions, crystal structures and chemical reactions of β-HMX under extreme conditions were described systematically.

Visible responses under high pressure in crystals: phenolphthalein and its analogues with adjustable ring-opening threshold pressures

The ring-opening reaction of phenolphthalein (PP) crystals under hydrostatic pressure provided by using a diamond anvil cell (DAC) is described in this paper for the first time. The color of the crystals changed into red from colorless visibly. The ring-opening threshold pressures could be adjusted by changing the substituent groups. Mechanochromic responses under hydrostatic pressure could also be achieved in polymer blends, which contributes to their practical applications.

Emission enhancement and high sensitivity of a π-conjugated dye towards pressure: the synergistic effect of supramolecular interactions and H-aggregation

Piezoresponsive fluorescent (PRF) materials are highly promising for applications in deformation, flaw detection and haptic sensing. However, traditional PRF materials generally suffer from low sensitivity and fluorescence quenching processes. This study involved the preparation of H-aggregated dyes with weak supramolecular interactions, which showed enhanced emission under a low pressure.

Spontaneous proton transfer in a series of amphoteric molecules under hydrostatic pressure

Hydrostatic pressure has induced intermolecular proton transfer in the crystals of a series of amphoteric molecules, which results in significant color changes.

Visualization and Quantitative Detection of Friction Force by Self-Organized Organic Layered Composites

Visualization and quantitative detection of external stimuli are significant challenges in materials science. Quantitative detection of friction force, a mechanical stress, is not easily achieved using conventional stimuli-responsive materials. Here, the quantitative detection of friction force is reported, such as the strength and accumulated ammount, from the visible color of organic layered composites consisting of polydiacetylene (PDA) and organic amines without an excitation light source. The composites of the layered diacetylene monomer crystal and interlayer organic amine are synthesized through self-organization from the precursor solution. After topochemical polymerization, the layered composites based on PDA show tunable temperature-responsive and mechanoresponsive color-change properties depending on the types of interlayer amines. The layered composites are homogeneously coated on a filter paper. The change in color of the paper is quantitatively used to visualize the strength and accumulated amount of the applied friction force. Furthermore, writing pressure is measured by friction force using the paper device.

Pressure-induced phase transition of 4-aminobenzonitrile: the formation and enhancement of N–H⋯N weak hydrogen bonds

A reversible pressure-induced structural phase transition of 4-aminobenzonitrile was found at about 0.3 GPa by conducting in situ high-pressure synchrotron angle-dispersive X-ray diffraction (ADXRD) experiments. The discontinuous changes of Raman modes at 0.2 GPa confirmed the occurrence of phase transition. In situ high-pressure Raman spectra indicated that the molecular arrangement and intermolecular interactions changed abruptly. The process of this phase transition continued up to about 1.0 GPa. When the pressure reached 1.1 GPa, the initial N–H⋯N interaction transformed into a new weak hydrogen bond, which was enhanced by further compression. The ab initio calculations and Hirshfeld surfaces were used to illustrate the above views. This study gives an example that demonstrates that the pressure can induce the formation of hydrogen bonds, which contributes to the development of supramolecular chemistry.

The initial N–H⋯N interactions in 4-aminobenzonitrile crystals are enhanced and changed into weak hydrogen bonds by high pressure.