Pressure-induced Emission Enhancement Research Articles

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.

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.

Temperature- and pressure-responsive photoluminescence in a 1D hybrid lead halide

Low-dimensional hybrid lead halides with responsive emissions have attracted considerable attention due to their potential applications in sensing. Herein, a new one-dimensional hybrid lead bromide CyPbBr3 (Cy = cytosine cation) was synthesized to explore its emission evolution in response to temperature and pressure. The compound possesses an edge-sharing 1D double-chain structure and emits warm white light across nearly the entire visible spectrum upon ultraviolet excitation. This emission arises from the self-trapped excitons and its broadband feature is attributed to the strong electron-phonon coupling as revealed by the variable-temperature photoluminescence experiments. Moreover, a 4.5-fold pressure-induced emission enhancement was observed at 2.7 GPa which is caused by the pressure suppressed non-radiative energy loss. Furthermore, in-situ powder X-ray diffraction and Raman experiments reveal the maxima of the emission enhancement is associated with a phase transition at the same pressure. Our work demonstrates that low-dimensional metal halides are a promising class of stimuli-responsive materials which could have potential applications in temperature and pressure sensing.

Pressure-induced emission enhancement: A review

The luminescent properties of some materials undergo significant changes under compression. High pressure generated by a diamond anvil cell (DAC) is often used as an external stimulus to explore the relationship between the structures and luminescent properties of materials, provide traceable color and structural changes, and quantify the environment in which the materials are located. Under high pressure, the luminous intensity or color of materials changes, which has important potential applications in fields such as safety detection, information storage, optoelectronic devices, and mechanical sensing. Recently, many phenomena of pressure-induced luminescence enhancement have been discovered in DAC, commonly referred to as pressure-induced emission enhancement. In this review, recent pressure-induced emission enhancement phenomena have been collected, and the role of pressure in promoting the luminescent enhancement of materials in DAC was revealed and discussed, which helps to design some materials with specific emission characteristics and provides a perspective for in-depth research on the photophysical behavior of materials.

Pressure induced emission enhancement (PIEE) in solid-state 2,3,4,5-tetraphenylthiophene: a QM/MM study.

Organic fluorophores exhibit pressure-dependent behaviors that are often irregular and contingent upon the specific system. Elucidating how pressure influences these behaviors is essential for the accurate design of piezochromic materials. Here, we conducted an exhaustive theoretical analysis of the excited-state decay processes of crystalline 2,3,4,5-tetraphenylthiophene (TPT) at high pressure through a combined quantum mechanics and molecular mechanics (QM/MM) method. The study revealed that the fluorescence quantum yield experiences a pronounced initial increase owing to the decrease of nonradiative decay IC rate (k ic). The suppression of low-frequency modes results in the decrease of λ e, and then reduces the electron-vibration couplings, and finally slows down the non-radiative process. Our research provides detailed mechanism analyses on PIEE properties of solid state TPT, aiding the rational design of advanced PIEE materials.

Blue light emission enhancement and robust pressure resistance of gallium oxide nanocrystals.

Exploration of pressure-resistant materials largely facilitates their operation under extreme conditions where a stable structure and properties are highly desirable. However, under extreme conditions, such as a high pressure over 30.0 GPa, fluorescence quenching generally occurs in most materials. Herein, pressure-induced emission enhancement (PIEE) by a factor of 4.2 is found in Ga2O3 nanocrystals (NCs), a fourth-generation ultrawide bandgap semiconductor. This is mainly attributed to pressure optimizing the intrinsic lattice defects of the Ga2O3 nanocrystals, which was further confirmed by first-principles calculations. Note that the bright blue emission could be stabilized even up to a high pressure of 30.6 GPa, which is of great significance in the essential components of white light. Notably, after releasing the pressure to ambient conditions, the emission of the Ga2O3 nanocrystals can completely recover, even after undergoing multiple repeated pressurizations. In addition to stable optical properties, synchrotron radiation shows that the Ga2O3 nanocrystals remain in the cubic structure described by space group Fd3m upon compression, demonstrating the structural stability of the Ga2O3 nanocrystals under high pressure. This study pays the way for the application of oxide nanomaterials in pressure anti-counterfeiting and pressure information memory devices.

Conformation-determined emission enhancement of phenothiazine derivatives under high pressure

Under high pressure, PTZ-DP-F with ax-conformation exhibited rarely pressure-induced emission enhancement, while eq-conformation showed quenched emission, revealing that molecular conformation significantly influences their photophysical properties.

Pressure-shortened delayed fluorescence lifetime of solid-state thermally activated delayed fluorescent 4CzIPN: the structure evolution.

The incorporation of mechanochromic luminescence into thermally activated delayed fluorescence (TADF) molecules is a promising strategy for developing multifunctional mechanochromic luminescent materials. Nevertheless, due to the difficulties in systematic design, it is still challenging to controllably exploit the versatility of TADF molecules. In this work, we were surprised to find that the delayed fluorescence lifetime of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene crystals was continuously shortened with increasing pressure, which was ascribed to the increasing HOMO/LUMO overlap by planarization of the molecular conformation, as well as the pressure-induced emission enhancement and obvious multicolor emission at high pressure (green to red), owing to the formation of new interactions and the part-planarization of the molecular conformation, respectively. This study not only developed a new function of TADF molecules, but also provided a route to shorten the delayed fluorescence lifetime, which is beneficial for designing TADF-OLEDs with small efficiency roll-off.

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.

Two different emission enhancement of trans-stilbene crystal under high pressure: Different evolution of structure

Mechanoresponsive luminescent (MRL) materials have drawn extensive concern due to their potential applications in mechanical sensors, memory chips, and security inks; especially these possessing high emission efficiency. In this work, we found trans-stilbene crystal exhibited two different pressure-induced emission enhancement (PIEE) behaviors at different pressure areas. The structural characterizations combined with density functional theory calculation indicate that the first emission enhancement was due to the decrease of nonradiation transition by the weaken of energy exchange process between atoms and lattice. And the second emission enhancement was attributed to the strengthen of C–H…C interactions from the non-planarization comformation. The results regarding the mechanoresponsive behavior of trans-stilbene offered a deep insight into PIEE from the structural point of view, which will facilitate the design of and search for high-performance MRL materials.

Molecular insertion regulates the donor-acceptor interactions in cocrystals for the design of piezochromic luminescent materials

Developing a universal strategy to design piezochromic luminescent materials with desirable properties remains challenging. Here, we report that insertion of a non-emissive molecule into a donor (perylene) and acceptor (1,2,4,5-tetracyanobezene) binary cocrystal can realize fine manipulation of intermolecular interactions between perylene and 1,2,4,5-tetracyanobezene (TCNB) for desirable piezochromic luminescent properties. A continuous pressure-induced emission enhancement up to 3 GPa and a blue shift from 655 to 619 nm have been observed in perylene-TCNB cocrystals upon THF insertion, in contrast to the red-shifted and quenched emission observed when compressing perylene-TCNB cocrystals and other cocrystals reported earlier. By combining experiment with theory, it is further revealed that the inserted non-emissive THF forms blue-shifting hydrogen bonds with neighboring TCNB molecules and promote a conformation change of perylene molecules upon compression, causing the blue-shifted and enhanced emission. This strategy remains valid when inserting other molecules as non-emissive component into perylene-TCNB cocrystals for abnormal piezochromic luminescent behaviors.

Pressure-induced emission enhancement by restricting chemical bond vibration

The effect of intermolecular interactions on intramolecular chemical bonds vibration and luminescence was analyzed. Enhanced intermolecular hydrogen bonds under high pressure suppresses the non-radiative process and result in emission enhancement.

Realization of pressure induced emission enhancement for rare earth luminescent materials: Adopting delta-doped structure

Design and enhancing the luminescence of rare earth materials under pressure have been important topics in the past few years. Here we report a new strategy for realizing pressure induced emission enhancement of rare earth element thulium (Tm) by confining the dopant luminescentions in a two-dimensional hexagonal NaYF4 planes, NaYF4@β-NaYF4:Yb0.2,Tm0.005@NaYF4 delta-doped structure. Upon compression, NaYF4@β-NaYF4:Yb0.2,Tm0.005@NaYF4 exhibits a 2 fold increase in emissions, obviously higher than that of corresponding homogeneous doped samples. When Tm3+ was overdoped, this pressure induced emissions enhancement would absent in the whole pressure region due to concentration quenching. Moreover, both NaYF4@β-NaYF4:Yb0.2,Tm0.005@NaYF4 and corresponding homogeneous doped samples undergo structure transition under pressure. Raman study and calculations show that the delta-doping induced phonon peak red shift and reduced defects density increasing rate could play major roles for the pressure-induced emission enhancement of NaYF4@β-NaYF4:Yb0.2,Tm0.005@NaYF4, and the deformation of the Y-F9 polyhedrons at vertices leading to the structure transitions of both delta and homogeneous doped samples. These findings suggest that the pressure induced emission enhancement of rare earth materials can be realized by passivating inner and surface defects using a delta-doped structure.

Tunable Color Temperatures and Emission Enhancement in 1D Halide Perovskites under High Pressure

AbstractDesigning low‐dimensional halide perovskites (LDHPs) for broadband emissions with enhanced emission and preferred chromaticity coordinator or color temperature remains a pressing challenge. Herein, a comprehensive study is conducted about the relationship between the crystal structure and broadband emission properties of 1D halide perovskite C4N2H14PbBr4, one of promising white‐emission LDHPs. It is found that the zigzag distortion degree tuned by pressure not only suppresses the general photoluminescence quenching, but also effectively adjusts the chromaticity coordinator and color temperature of C4N2H14PbBr4 nanocrystals (NCs). The initially broad white emissive C4N2H14PbBr4 NCs can be continuously modulated to a very bright bluish‐white emission with high color‐rendering index of 86, accompanied by a significant emission enhancement. The underlying mechanism of thus pressure‐induced emission enhancement (PIEE) can be attributed to the strengthening of electron–phonon interactions of compressed C4N2H14PbBr4 NCs. Furthermore, the zigzag distortion of [PbBr42−]∞ octahedral chains is able to rearrange the depths of multiple self‐trapped states, which is also corroborated by the wavelength‐dependent decay times under pressure, ultimately giving rise to adjustable emission chromaticity. This study offers a deep insight into the relationship between PIEE and distortion degree of LDHPs at extremes and facilitates the design of new materials with the desired emission properties.

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 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.

Pressure-Induced Emission Enhancement and Piezochromism of Triphenylethylene

Pressure-induced emission enhancement (PIEE) is a new phenomenon that has attracted widespread attention in the past few years for improving the solid-state emission efficiency of fluorophores. How...

Pressure-induced emission enhancement in hexaphenylsilole: a computational study

The pressure-induced emission enhancement in hexaphenylsilole is a consequence of restricted low-frequency molecular rotations. Excessive pressurization over 5.06 GPa hampers the high-frequency molecular vibrations without further limitation of molecular rotations.

Pressure-Induced Large Emission Enhancements of Cadmium Selenide Nanocrystals

Pressure quenching of optical emission largely limits the potential application of many materials in optical pressure-sensing devices, since emission intensity is crucially connected to performance. Boosting visible-light emission at high pressure is, therefore, an important goal. Here, we demonstrate that the emission of CdSe nanocrystals (NCs) can be enhanced by more than an order of magnitude by compression. The brightest emission can be achieved at pressures corresponding to the phase transitions in different sized CdSe NCs. Very bright blue emission can be obtained by exploiting the increase in band gap with increasing pressure. First-principles calculations indicate that the interaction between the capping oleic acid (OA) layer and the CdSe core is strengthened with increased Hirshfeld charge at high pressure. The effective surface reconstruction associated with the removal of surface-related trap states is highly responsible for the pressure-induced emission enhancement of these CdSe NCs. These findings pave the way for designing a stress nanogauge with easy optical readout and provide a route for tuning bright-fluorescence imaging in response to an externally applied pressure.

Pressure‐Induced Emission Enhancement, Band‐Gap Narrowing, and Metallization of Halide Perovskite Cs3Bi2I9

AbstractLow‐toxicity, air‐stable bismuth‐based perovskite materials are attractive substitutes for lead halide perovskites in photovoltaic and optoelectronic devices. The structural, optical, and electrical property changes of zero‐dimensional perovskite Cs3Bi2I9 resulting from lattice compression is presented. An emission enhancement under mild pressure is attributed to the increase in exciton binding energy. Unprecedented band gap narrowing originated from Bi−I bond contraction, and the decrease in bridging Bi‐I‐Bi angle enhances metal halide orbital overlap, thereby breaking through the Shockley–Queisser limit under relatively low pressure. Pressure‐induced structural evolutions correlate well with changes in optical properties, and the changes are reversible upon decompression. Considerable resistance reduction implies a semiconductor‐to‐conductor transition at ca. 28 GPa, and the final confirmed metallic character by electrical experiments indicates a wholly new electronic property.