Mechanoresponsive Luminescence Research Articles

Milling-Induced "Turn-off" Luminescence in Copper Nanoclusters.

Atomically precise copper nanoclusters (NCs) attract research interest due to their intense photoluminescence, which enables their applications in photonics, optoelectronics, and sensing. Exploring these properties requires carefully designed clusters with atomic precision and a detailed understanding of their atom-specific luminescence properties. Here, we report two copper NCs, [Cu4(MNA)2(DPPE)2] and [Cu6(MNA-H)6], shortly Cu4 and Cu6, protected by 2-mercaptonicotinic acid (MNA-H2) and 1,2-bis(diphenylphosphino)ethane (DPPE), showing "turn-off" mechanoresponsive luminescence. Single-crystal X-ray diffraction reveals that in the Cu4 cluster, two Cu2 units are appended with two thiols, forming a flattened boat-shaped Cu4S2 kernel, while in the Cu6 cluster, two Cu3 units form an adamantane-like Cu6S6 kernel. High-resolution electrospray ionization mass spectrometry studies reveal the molecular nature of these clusters. Lifetime decay profiles of the two clusters show the average lifetimes of 0.84 and 1.64 μs, respectively. These thermally stable Cu NCs become nonluminescent upon mechanical milling but regain their emission upon exposure to solvent vapors. Spectroscopic data of the clusters match well with their computed electronic structures. This work expands the collection of thermally stable and mechanoresponsive luminescent coinage metal NCs, enriching the diversity and applications of such materials.

Reversible mechanoresponsive luminescence based on an unsymmetrical D-π-A type phenanthroimidazole derivative

Abstract A kind of D-π-A mechanoresponsive luminescent molecule 2- (4- (1H-phenanthro[9,10-d] imidazol) benzylidene) malononitrile (ANPPI) was synthesized and characterized by UV absorption, photoluminescence, and other techniques. The obvious mechanoresponsive properties of ANPPI were reversible upon grinding and fuming, and were demonstrated by X-ray diffraction, scanning electron microscopy, and differential scanning calorimetry.

Dinuclear tricarbonylrhenium(I) complexes: impact of regioisomerism on the photoluminescence properties.

Dinuclear Re(I) complexes have proportionally been much less studied than mononuclear analogues. In particular, very little information is available about their solid-state emission properties. In this work, two structural isomers of dinuclear complexes (Bi-Re-metaPhe and Bi-Re-paraPhe), which differ by the relative position of the coordination spheres on a central phenyl ring, were synthesized and compared with each other and with the parent mononuclear compound (Mono-Re-Phe), from a theoretical and experimental point of view. In solution, the electronic, electrochemical and spectroscopic properties of the dinuclear complexes were almost identical, and rather close to those of the monomer. In the solid state, the photoluminescence (PL) efficiency of dimers was not higher than that of the monomer, but a clear mechanoresponsive luminescence (MRL) effect appeared only for the former ones. The positional isomerism influenced the amplitude of this effect, as well as the aggregation-induced emission (AIE) properties in a water-acetonitrile mixture. This study reveals the importance of positional isomerism to modulate the emission properties in the solid state. It also shows the advantage of dinuclear structures to access new MRL-active materials.

Mechanoresponsive luminescence triggered by phase transition of a supercooled copper(I) complex.

Under mechanical stimulation, a copper(I) complex in its supercooled liquid state transforms into a crystalline phase, showing a dramatic emission color change from red to green that is accompanied by a 20-fold increase in the photoluminescence quantum yield up to 87%. This reversible phase transition relies on the intriguing ability of this copper complex to form a supercooled metastable state.

Mechanoresponsive luminescence switching by polymorphs and doped crystals of donor–acceptor-type benzothiadiazoles

Mechanoresponsive luminescence switching by polymorphs and doped crystals of donor–acceptor-type benzothiadiazoles

Palladium-Catalyzed C-H Arylation/Arene Dearomatization Domino Reaction: Expeditious Access to Spiro[4,5]fluorenes.

A palladium-catalyzed C-H arylation/arene dearomatization of α-aryl-β-naphthol with o-dihalobenzenes was realized in a redox-neutral manner. This bimolecular domino reaction was initiated by an in situ-formed Pd(II) species generated from the dihalobenzene, followed by phenolic-group-directed C-H activation, biaryl cross-coupling, and naphthol dearomatization, thus rendering the rapid assembly of a class of spiro[4,5]fluorenes in high yields with good chemoselectivity. Remarkably, malononitrile-derived spirofluorene 6 was found to exhibit mechanoresponsive luminescence, which can be applied to optical memory devices.

Tuning the photoluminescence properties of SLE- and MRL-active tricarbonylrhenium(I) complexes through minor structural changes of the organic ligand

Solid-state luminescence properties depend on numerous parameters, including the molecular geometry and the intermolecular interactions that take place in the solid. To clarify the role played by these parameters on the photoluminescence (PL) properties of tricarbonylrhenium(I) complexes, four molecules incorporating a 3-(2-pyridyl)-1,2,4-triazole (pyta) ligand with appended phenylbenzoxazole (PBO) unit were compared. One or two methyl groups were inserted at different places of the organic ligand of the parent compound RePBO, resulting in three new complexes RePBO-Me1, RePBO-Me2 and RePBO-Me3. As shown by NMR, the presence of the methyl group(s) induced some changes in molecular flexibility, but the electronic effects were relatively weak. As a result, the electrochemical and optical properties were little impacted, and the four complexes behaved almost similarly in organic solution, in agreement with theoretical calculations. In contrast, marked differences appeared between the complexes when considering the aggregation-induced emission (AIE) effect, mainly due to the formation of tiny microcrystals in aqueous medium. In the same way, the three methylated complexes in the form of microcrystalline powders showed clear crystallization-induced emission enhancement (CIEE) with respect to solutions, but with distinct characteristics. They emitted less intensely and at longer wavelengths than the unsubstituted complex. Most likely, the methyl groups strongly affect the geometry and the packing mode of the molecules in the crystals, which influence the PL properties in the solid state. After grinding the powders, the emission spectra of the three methylated complexes were shifted to the red, although this shift was weaker than that previously observed for RePBO. This effect was almost reversible after THF fuming. It was attributed to transitions between the crystalline and amorphous phases. Remarkably, in the amorphous phase where molecules regain their mobility, the emission differences between the four complexes almost disappeared. It was then concluded that the amplitude of the mechanoresponsive luminescence (MRL) effect strongly depends on the geometry of the molecules in the pristine powder. This study is one more step toward the rational design of photoluminescent tricarbonylrhenium(I) complexes. More generally, it is also a good example of how very small structural modifications can drastically govern the PL and MRL properties.

Y-Shaped D-A Type Thioxanthone Derivatives: Mechano-Induced Thermally Activated Delayed Fluorescence and High-Contrast Mechano-Responsive Luminescence.

High-contrast mechano-responsive luminescence (MRL) materials with mechano-induced emission enhancement properties are fascinating candidates but few, for applications in rewritable media and recording devices. Here, an interesting design strategy of "Y-shape" donor-acceptor (D-A) type molecules for high-contrast MRL materials was presented, based on substituted diphenylamine donor and planar acceptor. Interestingly, their D-A torsion angles are small in crystals but increased after ground, resulted in planar and twist intramolecular charge transfer (PICT and TICT) states, respectively. Therefore, high-contrast MRL switching between weak blue (450 nm) fluorescence and bright yellow (552 nm) thermally activated delayed fluorescence (TADF) can be achieved for compound TXDO (4,4'-dimethoxydiphenylamine donor), which photoluminescence quantum yield increased from 2.8% to 54.7% after ground. Most importantly, the two independent D-A conjugation dihedral angles are actually independent in the "Y-shape" molecules. Especially for compound TXDT (4,4'-di-tert-butyldiphenylamine donor), its crystal exhibited both PICT and TICT processes inside, resulted from the different dihedral angles of 11.8° and 35.5°, respectively. The TXDT crystal thus showed dual-peak emission, including both TICT fluorescence and PICT room-temperature phosphorescence. Therefore, this strategy of "Y-shape" D-A type molecules provide a new approach to design advanced luminescent materials with mechano-induced TADF feature, for high-contrast MRL and single-component white luminescence.

Polymorphous meso-Dicyanovinyl-Substituted Tetraphenylethylene Derivative Showing Distinguishing Two-Color Emission and Ultrahigh-Contrast Mechano-Responsive Luminescence Behavior

Polymorphous <i>meso</i>-Dicyanovinyl-Substituted Tetraphenylethylene Derivative Showing Distinguishing Two-Color Emission and Ultrahigh-Contrast Mechano-Responsive Luminescence Behavior

Adjusting emission behaviors of molecules based 2,1,3-benzothiadiazole by intermolecular interaction and mechano-responsive luminescence

Adjusting emission behaviors of molecules based 2,1,3-benzothiadiazole by intermolecular interaction and mechano-responsive luminescence

Tuning the mechanoresponsive luminescence of rotaxane mechanophores by varying the stopper size

The efficiency of force-induced dethreading of rotaxane mechanophores is controlled by the stopper size.

Carborane-thiol protected copper nanoclusters: stimuli-responsive materials with tunable phosphorescence.

Atomically precise nanomaterials with tunable solid-state luminescence attract global interest. In this work, we present a new class of thermally stable isostructural tetranuclear copper nanoclusters (NCs), shortly Cu4@oCBT, Cu4@mCBT and Cu4@ICBT, protected by nearly isomeric carborane thiols: ortho-carborane-9-thiol, meta-carborane-9-thiol and ortho-carborane 12-iodo 9-thiol, respectively. They have a square planar Cu4 core and a butterfly-shaped Cu4S4 staple, which is appended with four respective carboranes. For Cu4@ICBT, strain generated by the bulky iodine substituents on the carboranes makes the Cu4S4 staple flatter in comparison to other clusters. High-resolution electrospray ionization mass spectrometry (HR ESI-MS) and collision energy-dependent fragmentation, along with other spectroscopic and microscopic studies, confirm their molecular structure. Although none of these clusters show any visible luminescence in solution, bright μs-long phosphorescence is observed in their crystalline forms. The Cu4@oCBT and Cu4@mCBT NCs are green emitting with quantum yields (Φ) of 81 and 59%, respectively, whereas Cu4@ICBT is orange emitting with a Φ of 18%. Density functional theory (DFT) calculations reveal the nature of their respective electronic transitions. The green luminescence of Cu4@oCBT and Cu4@mCBT clusters gets shifted to yellow after mechanical grinding, but it is regenerated after exposure to solvent vapour, whereas the orange emission of Cu4@ICBT is not affected by mechanical grinding. Structurally flattened Cu4@ICBT didn't show mechanoresponsive luminescence in contrast to other clusters, having bent Cu4S4 structures. Cu4@oCBT and Cu4@mCBT are thermally stable up to 400 °C. Cu4@oCBT retained green emission even upon heating to 200 °C under ambient conditions, while Cu4@mCBT changed from green to yellow in the same window. This is the first report on structurally flexible carborane thiol appended Cu4 NCs having stimuli-responsive tunable solid-state phosphorescence.

Ultralong room temperature phosphorescence and reversible mechanochromic luminescence in ionic crystals with structural isomerism

Photofunctional ionic crystals have become more and more appealing in the applications of light-emitting devices, optical imaging, and anti-counterfeiting. However, it is still limited to achieve ultralong room temperature phosphorescence (RTP) and stimuli-responsive property in ionic crystals due to the lack of valid design principles. Herein, we developed a new class of organic ionic crystals (sodium o-, m-, p-carboxyphenylborate, o-, m-, p-Nacpb) by a structural isomerization strategy, which could exhibit ultralong RTP and mechanochromic emission. Among the three isomers, m-Nacpb crystal exhibits the longest RTP lifetime up to 732.8 ms, which is ca. 120 times longer than that (6.1 ms) of o-Nacpb crystal. Interestingly, o-Nacpb could display mechano-responsive luminescence with tunable fluorescence-phosphorescence emissions and longer phosphorescence lifetime (increased by about 20-fold). Both experimental and theoretical studies indicate that the distinct RTP and mechanochromic luminescence properties of the organic ionic crystals are attributed to the alternation of molecular conformations and spatial packing modes. Moreover, potential applications including data encryption and decoration are demonstrated, benefiting from the varied phosphorescence lifetimes and good processability. Therefore, this work not only provides a rational isomerization route to prepare various organic ionic crystals with ultralong RTP and stimuli-responsive emission, but also supplies a facile way to develop the advanced information encryption applications.

Luminescent fac-[ReX(CO)3(phenyl-pyta)] (X = Cl, Br, I) complexes: influence of the halide ligand on the electronic properties in solution and in the solid state.

Tricarbonylrhenium(I) complexes that incorporate a chloride ligand are promising photoluminescent materials, but those incorporating a bromide or iodide ligand have received very little attention regarding their solid-state properties. In this work, three rhenium(I) complexes differing only by the nature of their halide ligand (X = Cl, Br, and I) were compared. They are based on a fac-[ReX(CO)3(N^N)] framework where the N^N bidentate ligand is a 3-(2-pyridyl)-1,2,4-triazole unit functionalized by an appended phenyl group. DFT calculations showed that the character of the lowest energy transitions progressively changes from Re → N^N ligand (MLCT) to X → N^N ligand (XLCT) when increasing the size of the halogen atom. Regarding the electrochemical behavior, the chloride and bromide complexes 1-Cl and 1-Br were similar, while the iodide complex 1-I exhibited a strikingly different electrochemical signature in oxidation. From a spectroscopic viewpoint, all three complexes emitted weak red-orange phosphorescence in dichloromethane solution. However, in the solid state, marked differences appeared. Not only was 1-Cl a good emitter of yellow light, but it had strong solid-state luminescence enhancement (SLE) properties. In comparison, 1-Br and 1-I were less emissiveand they showed better mechanoresponsive luminescence (MRL) properties, probably related to a loose molecular arrangement in the crystal packing and to the opening of vibrational non-radiative deactivation pathways. This study highlights for the first time how the nature of the halide ligand in this type of complex allows fine tuning of the solid-state optical properties, for potential applications either in bio-imaging or in the field of MRL-active materials.

Large effects of tiny structural changes on the AIE-TADF type xanthone derivatives in mechano-responsive luminescence and electroluminescence

In this work, two compounds with aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) properties (XT-T and XT-OT) which have the xanthone moiety as an electron withdrawing group were synthesized and studied. Both molecules exhibited the mechano-responsive luminescence (MRL) effect, in which a crystal of XT-OT could exhibit a bathochromic shift of 101 nm to achieve a wide-range emission transition from blue to yellow after grinding, compared to 34 nm for crystal XT-T. Such a high color-difference in the maximum emission peak was rarely reported in MRL materials. Analysis of crystal structures showed that small differences in structure could give rise to variable conformations and packing patterns in different aggregated states. The application of two compounds as light-emitting layers in organic light-emitting diodes (OLEDs) revealed the impact on device performance by their structural difference. The results presented here could provide a complementary strategy to design new multi-function materials.

Damage detection of fiber reinforced composite based on mechanoresponsive aggregation induced emission luminogen

Mechanoresponsive luminescence (MRL) have great potential applications in the fields of sensing, displaying, optical information storage and damage detection because of their light emission or color changing response under mechanical stimulation. In this work, 1,1,2,2-tetrakis (4-nitrophenyl) ethane (TPE-4N) has been synthesized as a MRL with aggregation induced emission (AIE) properties. Taking advantage of the high contrast of mechanically sensitive fluorescence of TPE-4N under mechanical stimulation, it has been coated on the various laminated fiber reinforced composites (FRCs) to successfully realize the on-site visualization of stress distribution of composites. V-shear experiments were carried out on three types of laminated FRCs coated with TPE-4N coatings. The experimental results show that TPE-4N can transform the invisible stress distribution information into visible fluorescence information. Moreover, the matrix cracking, fibre fracture and delamination of the composites can be qualitatively analyzed by using the fluorescence visualization images. By considering the repairability of FRC, early warning values were introduced by calculating the fluorescence areas. The fluorescence visualization method proposed in this work demonstrates the great potential in on-site monitoring the structural health condition of engineering composite structures.

Isomeric Pair of E/Z Tetraphenylethene-Cored Luminogens Showing Distinguishing Mechanoresponsive Luminescence Turn-On and Two-Color Behavior

A pair of tetraphenylethylene (TPE)-based E/Z isomers (E-TPE2PD and Z-TPE2PD) using two dicyanovinyl groups as the electron-accepting moieties have been synthesized and separated successfully. Reversible E/Z isomerization for each isomer is achieved, in which the isomerization speed from E to Z is higher than that from Z to E. This pair of pure E/Z isomers exhibits obvious AIEE properties. Z-TPE2PD shows highly morphology-dependent solid-state photoluminescent properties, whose solid-state emission is red-shifted from 564 to 588 nm with an enhancement of crystalline degree together with a decrease in the emission intensity. In contrast, E-TPE2PD leads to a slightly blue-shifted emission of 5 nm dependent on its crystalline degree. To the best of our knowledge, the discrepancy in emissive color between TPE-based E and Z isomers is much larger than most of the reported ones. The absence and presence of π–π interactions between adjacent PDCV units observed in their crystal packing structures are suggested to be responsible for the distinct solid fluorescence of E and Z isomers. More interestingly, Z-TPE2PD exhibits blue-shifted mechanoresponsive luminescence (MRL) turn-on behavior when subjected to grinding, while E-TPE2PD shows a remarkable two-color photoluminescence (PL) nature. This paper offers a synthetic strategy by introducing strong polar dicyanovinyl groups into the TPE core to increase the molecular polarity difference of E and Z isomers and facilitate their separation, which makes the exploration of distinguishing morphology-dependent solid-state PL properties possible. This is the first report on an isomeric pair of E/Z tetraphenylethene-cored luminogens displaying different MRL turn-on and dichromic behavior.

An AIE-active acridine functionalized spiro[fluorene-9,9′-xanthene] luminophore with mechanoresponsive luminescence for anti-counterfeiting, information encryption and blue OLEDs

A tetra-acridine substituted spiro[fluorene-9,9′-xanthene] (SFX-Ad) compound is demonstrated as a multifunctional luminophore applied to anti-counterfeiting, information encryption, and blue OLEDs.

ESIPT-regulated Mechanoresponsive Luminescence Process by Introducing Intramolecular Hydrogen Bond in Naphthalimide Derivatives

ESIPT-regulated Mechanoresponsive Luminescence Process by Introducing Intramolecular Hydrogen Bond in Naphthalimide Derivatives

A defect-induced emission material with turn-on mechanoresponsive luminescence serving as a data storage

Most of the mechanoresponsive luminescent (MRL) materials function based on phase transition (crystalline-to-amorphous or crystalline-to-crystalline) or solid-state chemical reaction of the organic solid emitters. However, it is difficult to enable a turn-on type mechanoresponsive luminescence using such strategy. In this work, the defect-induced emission (DIE) of (E)-(4-((2-hydroxy-4-methoxybenzylidene)amino)phenyl)(phenyl) methanone (HMPM) was disclosed, which endows the resulting material with reversible turn-on MRL. The crystalline powder of HMPM is non-emissive, but the emission intensity can be greatly enhanced by external mechanical stimuli, showing high contrast. In the semiquantitative experiment, pressure up to 1.78 N would generate crystal defects, and further induces DIE. Crystallographic data demonstrate that the dark-state stems from the closely-connected plane-style aggregation of neighboring molecules, which allows the intermolecular orbital-overlapping to the disadvantage of radiative transition. Mechanical stimuli would destroy the plane-style aggregation to block intermolecular orbital overlap and the possible energy transfer, resulting in emission-enhancement. Taking advantage of the DIE nature, an information storage film is fabricated, capable of data-writing via mechanical force and data-erasing by fumigation treatment, showing rewritability as well as high signal contrast.