Mechanofluorochromic Properties Research Articles

Mechanochromic Organic Micro-Laser.

This work presents the first demonstration of a mechanochromic organic micro-laser, which exhibits remarkable wide range pressure sensing characteristics. The gain material, pinacolato boronate ester functionalized anthanthrene (AnBPin), is designed by incorporating mechanofluorochromic (MFC) properties into organic laser dye. The AnBPin exhibits a reversible transition between green and orange fluorescence upon grinding annealing and recrystallization cycle, and its micro-crystal exhibits typical organic micro-laser behaviors. Applying localized mechanical pressure as low as 0.1 MPa inhibits micro-laser behavior at the given spot. In contrast, under 1 to 2 GPa hydrostatic pressure, the organic laser maintains narrow emission while showing a pressure-dependent shift in emission wavelength. By combining theory and experimentation, we attribute the unusual pressure-correlated emission spectroscopy to the unique interleaved locked crystal structure. Understanding the mechanochromic laser behavior in AnBPin micro-crystals under an unprecedented pressure range significantly expands the family of organic micro-lasers and provides a new route for wide-range photonic pressure detection.

Aromatic Difluoroboron β-Ketoiminate Complexes: Effect of Substituents on Mechanofluorochromism and Polymorphic Behavior.

Aromatic difluoroboron β-ketoiminate complexes (ketimBF2) are structural nitrogen-containing analogues of aromatic difluoroboron β-diketonates (diketBF2). Aggregation-induced emission (AIE) and polymorphic behavior allow ketimBF2 to exhibit mechanofluorochromic (MFC) properties. A detailed comparative study of the luminescence of a wide range of ketimBF2 with H- and CH3-substituents of nitrogen atom and diketBF2 with various substituents of the chelate ring (methyl, phenyl, toluoyl, anisoyl, biphenyl, naphthyl, anthracyl) in the solid state was carried out. As a result, regularities of the influence of substituents on the luminescent and MFC properties for 21 dyes have been established. Replacing one oxygen atom in diketBF2 with nitrogen atom in the chelate ring (H-substituted ketimBF2) changes the nature of the molecular stacking in crystals, which is manifested in different MFC properties. The introduction of a methyl group into the chelate ring (CH3-substituted ketimBF2) induces steric hindrance, which prevents the efficient formation of supramolecular structures and, consequently, leads to the shortest-wavelength monomeric emission in the solid state in comparison with oxygen and H-substituted nitrogen analogues. Methoxy derivative of H-substituted ketimBF2 exhibits non-reversible fluorochromic switching after annealing and can be used as temperature indicator to control unauthorized heating of temperature-sensitive substances during transportation and storage.

Cyano-Substituted Oligo(p-phenylene vinylene) Derivatives with Aggregation-Induced Enhanced Emissions and Mechanofluorochromic Luminescence.

Developing red fluorescence emitters with simple structures via convenient synthetic routes is highly desirable yet challenging. Herein, two novel donor-acceptor-type red emitters, DCFOPV-TPA and SCFOPV-TPA, featuring the intramolecular charge transfer effect were designed by integrating triphenylamine and trifluoromethyl into a CN-substituted oligo(p-phenylene vinylene) backbone. Both chromophores exhibited aggregation-induced enhanced emission and solvatochromic behavior. Moreover, DCFOPV-TPA also displayed reversible mechanofluorochromic properties under external force.

Isomer engineering of benzofuran derived AIE luminogens: Synthesis, mechanochromism, pH responsive fluorescent switching, cell-imaging and Fe3+ sensing

The influence of multiple factors often leads to the unexpected performances of the designed AIEgens. It is difficult to precisely conclude the influence of the substituted sites and role of isomers on the photophysical properties of the AIEgens which therefore brings uncertainty in the molecular design strategies. Isomer engineering of AIEgens is a less explored domain. There are only a few reports that substantiate the effects and positions of the substituents on the AIE properties. The present investigation explores two isomeric benzofuran-derived luminogens GBY-1 and GBY-4, both featuring a TPE moiety and showcasing AIE and AIEE properties respectively inherited from TPE. The isomeric effect resulting from TPE incorporation at different positions across benzofuran significantly influenced their photophysical properties, mechanofluorochromic properties, and AIE effect. Their distinctive luminogenic features were evident in mechanochromic property as well as in cancer cell imaging. GBY-1 was found to be uniformly dispersed in the cytoplasm, not co-localized with the nucleus, while GBY-4 accumulated near the cell membrane. Furthermore, post-functionalization of one of the luminogens, GBY-1, derived another AIEgen, GBY-5 with a dramatic improvement in the PLQE along with its ability to selectively sense Fe3+ in aqueous environments, within a detection limit of 0.00035 mM.

Substituent effects on the mechanochromic behaviour of pyrene-based AIEgens

It is extremely significant and desirable to integrate the fascinating properties of mechanochromism and aggregation-induced emission (AIE) in one simple molecule. Herein, pyrene-based aggregation-induced emission luminogens (AIEgens) with mechanofluorochromic (MFC) properties solve the traditionally existing aggregation-caused quenching (ACQ) challenge. Three cyanoethylene-functionalized pyrene-based derivatives were designed and synthesized. All compounds exhibit remarkable aggregation-enhanced emission (AEE) activity by a 3–20 times enhancement in mixed H2O/THF solutions. More importantly, high fluorescence efficiency was recorded in the solid state, especially for the luminogen 2c, with more than 200 times observed increased efficiency from 0.3% in solution to 69.8% in the solid state. Meanwhile, excellent mechanochromism properties and reversibility against the grinding treatment was systematically investigated. The excellent fatigue resistance properties are a necessity for potential applications in the field of anti-counterfeiting fluorescent inks and for the encryption of security information.

Novel Dual-Emission Emitters Featuring Phenothiazine-S-Oxide and Phenothiazine-S,S-Dioxide Motifs.

In this study, we have successfully designed and synthesized two novel dual-emission emitters featuring phenothiazine-5-oxide and phenothiazine-5,5-dioxide motifs, characterized by highly lopsided and asymmetric conformational states. Through rigorous spectral examinations and DFT calculations, the compounds exhibit distinctive ICT phenomena, coupled with efficient emission in solid states and AIEE characteristics under high water fractions in DMF/H2O mixtures. These non-planar luminogens exhibit vibrant green and blue solid-state luminescence, with fluorescence quantum yields of 24.1 % and 15.21 %, respectively. Additionally, they both emit green fluorescence in THF solution, with notable emission quantum yields (QYs) 36.4 % and 30.4 %. Comprehensive theoretical investigations unveil well-defined electron cloud density separation between the energies of HOMO/LUMO levels within the two luminogens. Notably, the targeted molecule harboring the phenothiazine-S,S-dioxide motif also demonstrates remarkable reversible mechanofluorochromic properties. Moreover, we testify their potential in applications such as solid-state rewritable information storage and live-cell imaging in solution states. Through theoretical calculations and comparative studies, we have explored the intrinsic relationship between molecular structure and performance, effectively screening and identifying new fluorescent molecules exhibiting outstanding luminescent attributes. These discoveries establish a robust theoretical and technical foundation for the synthesis and application of efficient DSE-based MFC materials, opening new avenues in the realm of advanced luminescent materials.

Tetraphenylethene-based mononuclear aggregation-induced emission (AIE)-active mechanofluorochromism gold(I) complexes with different auxiliary ligands

In this study, a series of tetraphenylethene-containing gold(I) complexes with different auxiliary ligands have been synthesized. These complexes were characterized using a variety of techniques including nuclear magnetic resonance spectroscopy, mass spectrometry, and single crystal X-ray diffraction. Their aggregation-induced emission (AIE) behaviors were investigated through ultraviolet/visible and photoluminescence spectrum analyses, and dynamic light scattering measurements. Meanwhile, their mechanofluorochromic properties were also studied via solid-state photoluminescence spectroscopy. Intriguingly, all these mononuclear gold(I) molecules functionalized by tetraphenylethene group demonstrated AIE phenomena. Furthermore, five gold(I) complexes possessing diverse auxiliary ligands exhibited distinct fluorescence changes in response to mechanical grinding. For luminogens 2–5, their solids showed reversible mechanofluorochromic behaviors triggered by the mutual transformation of crystalline and amorphous states, while for luminogen 1, blue-green-cyan three-color solid fluorescence conversion was realized by sequential mechanical grinding and solvent fumigation. Based on this stimuli-responsive tricolored fluorescence feature of 1, an information encryption system was successfully constructed.

Indoloquinoxaline-based organic material enabling multi-stimuli response

A new triphenylethylene-modified indolo[2,3-[Formula: see text]]quinoxaline TPE-IQ has been synthesized by a mild Suzuki-Miyaura coupling reaction. The thermal, electrochemical, photophysical, aggregation, and acidofluorochromic and mechanofluorochromic properties were investigated. TPE-IQ shows typical solvatochromism and aggregation-induced emission enhancement features. TPE-IQ also exhibits interesting reversible acid and mechanical force-responsive properties. This work highlights the development of multi-stimuli responsive organic material containing indolo[2,3-[Formula: see text]]quinoxaline moiety.

Cis‐Linked Cyclotetraphenylenes: Synthesis, Structures and Fluorescence Properties

Abstractcis‐Linked cyclotetraphenylene dimer [2]c‐CTPE and trimer [3]c‐CTPE were synthesized via Pt‐mediated cyclization of cis‐diboronated tetraphenylethene. The structure of [2]c‐CTPE has been confirmed by single crystal X‐ray diffraction. The strain energy of [2]c‐CTPE and [3]c‐CTPE was calculated to be 86.28 ⋅ kJ mol−1 and 9.23 kJ ⋅ mol−1, respectively. [2]c‐CTPE and [3]c‐CTPE are typical AIEgens and show mechanofluorochromic properties. The comparisons of cis‐linked cyclotetraphenylenes with gem‐linked cyclotetraphenylenes were also discussed.

Mechanofluorochromic property of coumarin derivatives containing carbonyl group

Four novel coumarin derivatives with different substituents (CMCO-H, CMCO-F, CMCO-CH3 and CMCO-CF3) functionalized by carbonyl unit were designed and synthesized. The relationship between spatial structure and photophysical properties of these coumarin compounds in different states had been systematically studied. In the single molecule state, with the increase of solvent polarities, the fluorescent peaks exhibited obvious solvatochromic effect, accompanied by a decrease in intensity. The significant solvatochromism effect suggested that these four coumarin derivatives both are typical intramolecular charge transfer molecules. The density functional theory calculation results showed that the electron clouds were transferred from coumarin unit and the connected aromatic ring to carbonyl and styrenyl groups. Meanwhile, compounds CMCO-CH3 and CMCO-CF3 exhibited reversible mechanofluorochromic (MFC) property between yellow and orange emission. The single crystal XRD, powder XRD and SEM experiment suggested that the modulation of photophysical properties come from the change of compounds spatial structures. The study provided an efficient way for the design of organic compounds with MFC property.

Bromine substituent position of tetraphenylethylene-based luminogens effect on enhanced AIE and reversible mechanofluorochromic properties by halogen bonding

The halogen atoms have been exploited for designing highly efficient aggregation-induced emission (AIE) materials in recent years, whereas the development of halogenated AIEgen has been greatly limited by the unclear inherent mechanism. In this paper, a series of 1,1,2,2-tetrakis (4-(phenyl ethynyl) phenyl) ethene (TPPE) derivatives with bromine (Br) substituents in different positions were synthesized and their unique photophysical behaviors based on the anti-heavy atom effect were investigated. The brominated TPPE derivatives possessed higher fluorescence quantum yield (ФF) than that of TPPE, resulting from the formation of intermolecular Br-based bonds. Single crystal analysis and theoretical calculations revealed that changing the positions of the Br substituents could induce the formation of different intermolecular interactions. The Br-based bonds efficiently restricted the intermolecular motion, in which the Br⋯Br bond made a decisive factor in determining the highly efficient fluorescence. TPPE derivatives displayed obvious mechanofluorochromic (MFC) properties. PXRD and SEM results demonstrated that the crystal-to-amorphous transition attributed to the reversibility of MFC properties. This work provides a straightforward and practical method to achieve highly efficient AIE-active materials with higher contrast MFC properties via introducing the halogen atoms.

Reversible Red‐/Blue‐Shifted Mechanofluorochromic Properties of Indole‐Based Difluoroboron Compounds with Aggregation‐Induced Emission

AbstractTo further explore the relationship between structure and property of the mechanofluorochromic materials, two indole‐based aggregation‐induced emission compounds (I−Ph−BF2 and I−PhNp−BF2) with BF2‐core were designed and synthesized. They displayed intramolecular charge transfer characteristics and dissimilar reversible mechanofluorochromic behaviors (red‐shift for I−Ph−BF2 and blue‐shift for I−PhNp−BF2). Based the powder X‐ray diffraction patterns and quantum chemical calculation results, it was responsible for the blue‐shift mechanofluorochromic behavior of I−PhNp−BF2 that the reduced molecular polarity of the excited state and the undermined partially ordered structure. In contrast, the phase change between an ordered arrangement state and a disordered amorphous state of I−Ph−BF2 solid powder, as well as the appropriate molecular polarity, brought out red‐shift mechanofluorochromic effect. It provided a foundation for further exploration of the mechanofluorochromic property.

Mechanofluorochromic Properties of 1,4-Diphenylanthracene Derivatives with Hypsochromic Shift.

Several types of 1,4-diphenylanthracene derivatives 1-4 were prepared, and their photophysical properties were observed in the solid and solution states. Interestingly, the CN-group-substituted 1,4-diphenylanthracene derivative 2 was found to exhibit a higher fluorescence quantum yield (ϕf = 0.71) in the solid state than in the solution state, probably due to the formation of an intermolecular Ar-CN⋯H-Ar hydrogen bond and antiparallel type locked packing structure in the solid state. Furthermore, for some derivatives, an increase in the fluorescence quantum yield was observed in the PMMA film (1 wt%) over both the solid state and the solution state. More interestingly, some of the 1,4-diphenylanthracene derivatives exhibited unusual mechanofluorochromic properties with a "hypsochromic shift" in luminous color depending on the substituents of the phenyl group, and with the derivatives having CF3, OMe, CN, and two F substituents (1d-1f, 2-4) showing a significant luminous color change with a "hypsochromic shift" after grinding. However, no change in the luminous color was observed for the derivatives having H, Me, and one F substituent (1a-1c), and especially for some of the CN-substituted derivatives, a reversible luminous color change with a "hypsochromic shift" was observed, probably due to the formation of an antiparallel type packing structure. These "hypsochromic" anthracene derivatives could probably be utilized as new mechanofluorochromic materials.

Regulation of aggregation-induced emission and reversible remarkable mechanofluorochromism by substitution position of the carbazole unit in D-A type carbazole-functionalized dicyanoethylene π-systems

In this study, two twisted D-A type dibenzothiophene- and carbazole-functionalized dicyanoethylene derivatives were formulated and synthesized. They were labeled as 2-substituted carbazole (BT-CE-C2) and 3-substituted carbazole (BT-CE-C3), respectively. Furthermore, an investigation into the correlation between the molecular structure of these derivates and their photophysical properties was conducted. The characteristic D-A structures and the highly distorted spatial conformations of the two fluorophores led to the emergence of a unique intramolecular charge transfer (ICT) effect and remarkable aggregation-induced emission (AIE) behavior. Notably, the position of substitution of the carbazole unit regulated the reversible high-contrast mechanofluorochromic (MFC) properties demonstrated by BT-CE-C2 and BT-CE-C3. The as-prepared solid powders of BT-CE-C2 and BT-CE-C3 produce blue-green and orange-yellow emissions at 508 and 567 nm, respectively. However, their ground powders emitted orange (at 589 nm) and orange-red (at 619 nm) lights. Notably, BT-CE-C2 exhibited enhanced fluorescence upon external stimulation, increasing its solid-state luminescence efficiency from 0.035 to 0.189. In contrast, the BT-CE-C3 exhibited a decrease in fluorescence, causing its solid-state luminescence efficiency to change from 0.426 to 0.348. The fluorescence lifetimes, powder X-ray diffraction (PXRD), and spectral analysis collectively identified the phase transition between the crystalline and amorphous states as the underlying cause of the MFC behavior observed in BT-CE-C2 and BT-CE-C3. Additionally, the red shift in photoluminescence (PL) spectra under external stimulus can be attributed to planarized-induced charge transfer (PICT), exciton coupling, increased π-π interactions, and enhanced orbital overlap between adjacent molecules. This combination of factors led to the observed MFC properties of these compounds.

Lighting Up Fluorescence: Precise Recognition of Halogenated Solvents Through Effective Fluorescence Detection Using Chalcone Derivatives as a D-A-D-A-type Fluorescent Chemosensor.

In this paper, we report a D-A-D-A-type fluorescence sensor, FX, composed of triphenylamine and pyrazine units as electron donors, pyridine units, and α-β unsaturated carbon-based structures as electron acceptors. FX exhibits typical ICT characteristics. As a dual-emission material, FX undergoes acid-base-induced color changes and displays mechanofluorochromic properties in the solid state. In solution, FX, as an AIE material, shows significant fluorescence enhancement behavior in most halogenated solvents. Notably, it achieves a high quantum yield of 0.672 in a chloroform solution. We utilized this phenomenon to quantitatively detect chloroform through fluorescence titration analysis, with a detection limit of 0.061%. Additionally, we developed a test paper to verify the practical applicability of the sensor for detecting halogenated solvents. The fluorescence enhancement behavior was confirmed through DFT calculations. The results indicate that FX is not only a multifunctional dual-state emission material but also provides valuable references for the fluorescence detection of halogenated solvents.

Carbazole-modified dicyanoethylene derivatives exhibiting tert-butyl-regulated aggregation-induced emission and reversible high-contrast mechanofluorochromism

Dibenzothiophene, dicyanoethylene, and carbazole were used as building blocks in the synthesis of two novel donor-acceptor-type molecules, DBT-CE-C and DBT-CE-BC. These compounds exhibit typical intramolecular charge transfer (ICT) behavior. Importantly, DBT-CE-C and DBT-CE-BC exhibit strong reversible high-contrast mechanofluorochromism and aggregation-induced emission (AIE) due to the introduction of tert-butyl groups in the 3- and 6-positions of carbazole. In the solid state, DBT-CE-C and DBT-CE-BC exhibit robust emissions, with fluorescence efficiencies of 0.315 and 0.504, respectively. They also exhibit distinct AIE phenomena, with AIE factors of 63.0 and 26.5, respectively. Notably, during the grinding process, the initially green- and yellow-green-emitting solid powders of DBT-CE-C and DBT-CE-BC undergo a transition to yellow and orange-red emission, respectively. Simultaneously, their emission peaks shift from 511 to 532 nm to 574 and 603 nm, respectively. Experiments using powder X-ray diffraction on as-prepared, ground, and fumed solid samples of these compounds provide evidence that their mechanofluorochromic properties stem from a phase shift between crystalline and amorphous states under external stimuli. The reduction in bandgap due to the ICT effect, coupled with increased π–π interactions, exciton coupling, and orbital overlap between neighboring molecules, accounts for the observed red shift in the photoluminescence spectra when subjected to external stimulation.

Mechanofluorochromic properties of 1-phenylanthracene derivatives with extremely simple structures

Several types of 1-phenylanthracene derivatives 1–4 were prepared and their photophysical properties were observed in the solution and solid states. Interestingly, the fluorine-substituted 1-phenylanthracene derivative 1c was found to exhibit a higher fluorescence quantum yield (Φf=0.57) in the solid state than in the solution state, probably due to the formation of intermolecular Ar–F•••H–Ar hydrogen bonds. Additionally, some of the 1-phenylanthracene derivatives exhibited mechanofluorochromic properties depending on the substituents of the phenyl group, with the derivatives having CF3, OMe, and CN substituents (1d-1f, 2, 3) showing a significant change in luminous color from a bluish to greenish color at “room temperature” after grinding. However, no change in the luminous color was observed for the derivatives having H, Me, and F substituents (1a-1c) at “room temperature” but luminous color change was observed with low temperature grinding (−78 °C), especially for the CN substituent, in which a significant change in the luminous color was observed at “room temperature” probably due to the formation of Ar-CN•••H–Ar hydorogen bond interactions. Interestingly, for the 1-phenylanthracene derivative 3 substituted with two CN groups, which exhibited a solvatochromic phenomenon, the luminous color recovered with controllable rate from green to bluish when heated and fumed with solvent vapor in the solid state, and the luminous color change with controllable recovery rate was repeatedly successfully observed. These anthracene derivatives have extremely simple structures and could probably be applied as new mechanofluorochromic materials.

Nitrogen Atom Induced Contrast Effect on the Mechanofluorochromic Characteristics of Anthracene-Based Acceptor-Donor-Acceptor Fluorescent Molecules.

The mechanofluorochromic (MFC) characteristics of anthracene-based acceptor-donor-acceptor (A-D-A) fluorescent molecules are explored through a comprehensive investigation of their photophysical behaviors. Six 9,10-diheteroarylanthracene derivatives with varying acceptor groups (pyridin-4-yl, pyridin-3-yl, pyridin-2-yl, pyrimidin-5-yl, pyrazinyl and quinoxalinyl) are synthesized and systematically characterized. The photophysical properties in both solution and solid-state are examined, revealing subtle yet significant influences of the spatial arrangement and number of nitrogen atoms within the acceptor group on fluorescence emission. Single-crystal structures of these compounds provide insights into their steric configurations and intermolecular packing modes, offering valuable insights into the fundamental mechanisms that underlie the observed MFC properties. This study illuminates the intricate interplay between MFC properties and the refined molecular structure, thus presenting promising avenues for the design and advancement of novel MFC materials.

D-A type π-systems containing tetraphenylethene exhibiting aggregation-induced emission and reversible mechanical force-induced luminescent enhancement and chromism

Two novel D-A type tetraphenylethene-based fluorophores, named XA-BZ-TPE and XA-DCE-TPE, were designed and synthesized. Both fluorophores exhibit typical intramolecular charge-transfer features, good solid-state luminescence, and remarkable aggregation-induced emission (AIE) behavior. The AIE factors for XA-BZ-TPE and XA-DCE-TPE were >243 and 12, respectively. Notably, both XA-BZ-TPE and XA-DCE-TPE display high-contrast mechanofluorochromic (MFC) properties. During the grinding of initially prepared solid powders, the emission colors transformed from blue and blue-green to blue-green and yellow, and their maximum emission wavelengths shifted from 449 to 485 nm to 481 and 557 nm, respectively. Importantly, the two fluorophores attained enhancement in luminescence efficiency and chromism when the mechanical force was applied. After grinding, the solid-state luminescence efficiencies were elevated from 0.243 to 0.132 to 0.484 and 0.278, respectively, for XA-BZ-TPE and XA-DCE-TPE. In XA-BZ-TPE and XA-DCE-TPE, according to the analysis using PXRD, fluorescence lifetimes, and spectral data, the MFC behavior can be attributed to the phase transition between the crystalline and amorphous states. The present study contributes to expanding the repertoire of D-A type MFC materials and provides valuable insights for the development of novel MFC materials incorporating tetraphenylethene units, which exhibit excellent responsiveness under solid-state emission stimuli.

Reversible mechanofluorochromic luminescence behaviors of triphenylamine-based compounds and their application in erasable writing

It is challenging and desirable to develop an effective molecular design strategy to tune mechanofluorochromic (MFC) activity in organic molecules. Herein, three triphenylamine (TPA) derivatives were synthesized to investigate the MFC properties. The inclusion of iminodibenzyl, diphenylamine and carbazolyl units into the molecules causes some changes in the molecular conformation and packing pattern, which can lead to the variation of MFC activities. Among three TPA derivatives, T1 exhibits excellent MFC performance upon grinding. In addition, the color change induced by grinding is revisable upon heating. In consideration of the good repeatability and sensitivity of MFC process, T1 is chosen to prepare the rewritable papers and can be achieved in inkless writing technology.