Aggregation-induced Emission Enhancement Properties Research Articles

Aggregation-induced emission enhancement (AIEE) active bispyrene-based fluorescent probe: “turn-off” fluorescence for the detection of nitroaromatics

The detection of nitroaromatic explosives in real samples is essential for environmental monitoring because of their strongly powerful nature and wide applications in industries. Aggregation-induced emission enhancement (AIEE) active fluorescent probe has been widely employed to detect nitroaromatic explosives. Hereby, a simple V-shaped bispyrene-based fluorescent probe (called py-o) with AIEE properties was designed and synthesized, which was fully charactered by 1D NMR, ESI, FTIR, and 2D NOESY spectra. The py-o displayed bright blue-green fluorescence excimer emission at 480 nm in DMF/H2O (v/v 1:1). It is observed that the fluorescence excimer emission of py-o at 480 nm was quenched by PA in solution with a quenching constant of 5.45 × 104 M–1, and the limit of detection was approximately 0.139 μM. The details of the sensing mechanism were explained using 1H NMR titrations, Job’s plot and Bensi-Hildebrand methods, which revealed a 1:1 binding ratio via the π-π interactions between PA and py-o. Meanwhile, it exhibited outstanding anti-interference ability in the detection of PA when interfering analytes were added under the same conditions. Furthermore, low-cost thin-layer chromatography (TLC) plates coated with py-o were developed as fluorescent tools for naked-eye detection of PA in the solid state. Therefore, this work provides a new method for constructing an AIEE fluorescent probe for the detection of nitroaromatic explosives to utilize in environmental monitoring.

A triphenylamine derivative with multistimuli responsive behavior and high-contrast vapochromism of its Zn(II) complex

In this work, we designed and synthesized a novel multifunctional molecule, (E)-2-(4-(diphenylamino)styryl)-5-(4-pyridyl)-8-quinolinol (HL), which possessed D-π-A structure and exhibited optical responses to acid-base vapor and Zn(II) ions. Zn(II) complex of HL (W1) was obtained and characterized by SC-XRD. Spectral experiments demonstrated that complex W1 displayed aggregation-induced emission enhancement (AIEE) property, which was exact opposite to the aggregation-caused quenching effect of HL. The quantum yield of W1 (15%) was much higher than that of HL (2.1%) in the solid state. Both HL and complex W1 involved sp2 hybrid N atoms, whose protonation and deprotonation gave rise to reversible acidochromism with high contrast. The mechanism was clarified by XPS, 1H NMR, infrared spectroscopy, elemental analysis, PXRD and thermogravimetry results. Benefiting from excited state intramolecular proton transfer in HL together with AIEE of W1, HL could recognize Zn(II) sensitively in aqueous media. The detection limit of HL for Zn(II) reached as low as 5.02 × 10−8 M. An anti-counterfeiting application was realized using HL-loaded test strip, which exhibited fluorescence response, but no color change upon exposure to Zn(II) solution. In addition, HL and W1 solids were combined to develop a stimuli induced encryption/decryption strategy successfully based on their reversible acidochromic behaviors.

Tetraphenylethylene fused pyrrolo[3,2-b]pyrrole derivatives with reversible mechanofluorochromic and aggregation-induced emission enhancement characteristics

Modification on typical of molecular unit with aggregation-induced emission (AIE)/aggregation-induced emission enhancement (AIEE) can play a important role in obtaining solid-state fluorescence materials and mechanofluorochromic (MFC) materials. We utilize the multicomponent reaction of TPE-containing aldehyde, arylamine and butane-2,3-dione in the presence of iron(III) perchlorate to design and synthesize two novel MFC materials by combination of the tetraphenylethylene (TPE) and pyrrolo [3,2-b]pyrrole core units. Investigation of CHW-1 and CHW-2 aggregation-induced emission behaviors indicated that these materials all exhibited AIEE properties. Subsequent mechanochromic luminescence tests exhibited reversible mechanochromism natures that involve fluorescent color change from green to yellow (Grinding-induced spectral shift (△λ): 35 nm and 38 nm, respectively). Then the datas (including SEM, PXRD and DSC) revealed that this mechanofluorochromism was ascribed to the transformation of crystal state to the amorphous form. Moreover, CHW-1 and CHW-2 could be applied for optical recording devices and security printing.

Zn-Salphen Acrylic Films Powered by Aggregation-Induced Enhanced Emission for Sensing Applications.

Zn(II) complexes possess attractive characteristics for supramolecular chemistry, catalysis, and optoelectronic applications, while Zn-Salphen counterparts are also suitable as chemical sensors, although limited by solution-based to date. In this study, we report the synthesis of new polymers from methyl methacrylate, n-butyl acrylate, and a non-symmetrical Zn-Salphen complex. We show that this low-fluorescent complex exhibits aggregation-induced emission enhancement (AIEE) properties and that, the incorporation of AIEE complexes into a polymeric matrix make it possible to achieve fluorescent films with enhanced fluorescence suitable for sensing applications. As a proof of concept, these films could detect acetic acid, showing a decrease of up to 73% in the original fluorescence. Host/guest studies showed a subtle disruption of the emission in aggregates upon treatment with anion guests. These results indicate that an interaction between the guest and Zn-Salphen complex may occur, stabilizing or destabilizing the complex and causing a concomitant increase or decrease in emission.

2,3-Diphenylquinoxalin-6-yl based dyes: Synthesis and studies of photophysical, aggregation-induced emission enhancement and electrochemical properties

Herein, 2,3-diphenylquinoxalin-6-yl based derivatives were synthesized, characterized by various spectroscopic method and studied for photophysical, aggregation-induced emission enhancement (AIEE) and electrochemical properties. Existence of intramolecular charge transfer transition (ICT) at 372−379 nm in absorption spectra of molecules confer weak push-pull interaction within it. Broad range of emission from blue to orange with λem = 412−592 nm in dyes arose from different energy state emission viz., locally excited (LE), Twisted intramolecular charge transfer (TICT) and excimer emission. Switching in emission state of molecules from excimer to TICT emission were observed by changing the polarity of solvent due to stabilization of TICT in polar media. Dyes have good solid state emission and AIEE activity desirable for its solid/aggregate state application in OLED's. Good Stokes shift (2183−9703 cm−1), thermal stability and comparable HOMO (−5.65 to −6.02 eV) energy levels with reported hole transporting materials confer desirable role of molecules in various optoelectronic applications. Further, DFT calculation of dyes, designate comparable experimental and theoretical photophysical and electrochemical properties with respect to structure of dyes.

Unraveling the substituent group effects on the emission properties of fluorenone derivatives

Elucidating the structure–property relationship is vital for developing organic luminescent materials with excellent properties and performance optimization. Herein, we designed and synthesized three types of fluorenone-based molecules named FO-Ph, FO-Na and FO-An, with similar structures but different sizes of substituent groups (benzene, naphthalene and anthracene). This slight modification resulted in significantly different optical properties of luminogens in dilute solutions and powders. FO-Ph as an aggregation-caused quenching (ACQ) emitter displayed strong emissions in solution; moreover, after the addition of a poor solution, the fluorescence intensity of FO-Ph gradually decreased. On the contrary, FO-Na displayed aggregation-induced emission enhancement properties, and FO-An was non-emissive in solutions but displayed enhanced emission owing to its molecular aggregation, indicating that FO-An comprises an AIE-active molecule. The characterization of the photophysical properties, theoretical calculations and an X-ray single crystal analysis showed that the emission properties of the three compounds could be affected by the size of substituent groups, twisted geometry and molecular packing modes. The three fluorenone luminogens exhibited different force-induced emission enhancement properties. Moreover, FO-Ph can be used as a water detector and showing high sensitivity. This study elucidates the AIE and ACQ phenomena of fluorenone complexes and provides effective ways for developing organic luminescent materials.

Zn2+-Induced aggregation of a water-soluble iridium complex enhances aggregation-induced emission for intracellular Zn2+ imaging

Iridium(III) complexes show wide applications in biosensing and bioimaging, while Ir(III)-based Zn2+ probes are still rare and suffer from poor selectivity and low water solubility. In this work, we verified the phosphonate group as an efficient Zn2+ recognition moiety and developed a phosphonate-containing Ir(III) complex (Ir-BPA) as a water-soluble phosphorescent Zn2+ probe. Ir-BPA exhibited aggregation-induced emission enhancement (AIEE). Zn2+-binding also induced Ir-BPA aggregation, leading to enhanced phosphorescence and emission color changing from red to yellow. The binding mechanism was studied via spectra analysis, transmission electron microscope (TEM) images, and theoretical simulation. Ir-BPA was further employed in imaging intracellular Zn2+ level fluctuations via supplementary supply of Zn2+ source or endogenous release under oxidative stress. This work positively utilized AIEE properties of Ir(III) complexes in Zn2+ detection and broadened the strategies for developing water-soluble, AIE-active Ir(III) complexes for biological applications.

Synthesis, tuning of photophysical and electrochemical properties of Yellow-Red emissive, AIEE active benzamide dyes

Herein, Benzamide-based dyes were prepared by the multistep synthetic method and then characterized by various spectroscopic methods. The tuning in photophysical, aggregation-induced emission enhancement (AIEE) and electrochemical properties with alteration in molecular structure of dyes were studied in detail. Molecules possess weak push–pull interaction due to the existence of inbuilt charge transfer (CT) transition at 366–387 nm in absorption spectra. Dyes emit in yellow to red region with emission maxima at 560–599 nm in solution and 548–600 nm in the solid state. The notable positive solvatochromic effect and good Stoke’s shift (9004–9717 cm−1) observed in dyes. Additionally, the existence of good solid-state emission and AIEE activity is desirable for its solid/aggregate state application in Organic light-emitting diodes. Besides, HOMO (−5.60 to − 5.74 eV) and LUMO (−3.14 to − 3.17 eV) energy levels comparable with reported ambipolar materials. Density functional theory (DFT) and time-dependent density-functional theory (TDDFT) calculations agree with the experimental photophysical and electrochemical data. The opto-electrochemical properties possessed by dyes undersign its suitable use in various optoelectronic applications.

Dibenzthiophene and carbazole modified dicyanoethylene derivative exhibiting aggregation-induced emission enhancement and mechanochromic luminescence

A novel D-A type dicyanoethylene derivative DBT-Cz-DCN was designed and synthesized using dibenzthiophene, dicyanoethylene and carbazole as raw materials. Photophysical studies indicated that the luminogen DBT-Cz-DCN possessed unique intramolecular charge transfer (ICT) characteristic from electron donating carbazole unit to electron accepting dicyanoethylene moiety and aggregation-induced emission enhancement (AIEE) property. Theoretical calculations indicated that DBT-Cz-DCN exhibited a highly twisted conformation due to the steric hindrance effect of the dicyanoethylene unit. It was worth noting that DBT-Cz-DCN also exhibited distinct mechanochromic luminescence property. Specifically, when the pristine powder of DBT-Cz-DCN underwent grinding, the emitting color shifted from yellow to orange-yellow. The emitting color and wavelength could be restored to the pristine state after fuming with dichloromethane vapor. Powder X-ray diffraction confirmed that the transformation between the amorphous state and the crystalline state under external force stimuli was the reason for the MCL (mechanochromic luminescent) behavior of DBT-Cz-DCN. The current work not only developed a novel molecular structure with AIEE characteristic and MCL behavior, but also laid the foundation for the development of MCL materials with high contrast, high luminescence efficiency and good reversibility.

AIEE-Active Flavones as a Promising Tool for the Real-Time Tracking of Uptake and Distribution in Live Zebrafish.

In recent years, aggregation-induced emission enhancement (AIEE) molecules have shown great potential for applications in the fields of bio-detection, imaging, optoelectronic devices, and chemical sensing. Based on our previous studies, we investigated the fluorescence properties of six flavonoids and confirmed that compounds 1-3 have good aggregation-induced emission enhancement (AIEE) properties through a series of spectroscopic experiments. Compounds with AIEE properties have addressed the limitation imposed by the aggregation-caused quenching (ACQ) of classic organic dyes owing to their strong fluorescence emission and high quantum yield. Based on their excellent fluorescence properties, we evaluated their performance in the cell and we found that they could label mitochondria specifically by comparing their Pearson correlation coefficients (R) with Mito Tracker Red and Lyso-Tracker Red. This suggests their future application in mitochondrial imaging. Furthermore, studies of uptake and distribution characterization in 48 hpf zebrafish larvae revealed their potential for monitoring real-time drug behavior. The uptake of compounds by larvae varies significantly across different time cycles (between uptake and utilization in the tissue). This observation has important implications for the development of visualization techniques for pharmacokinetic processes and can enable real-time feedback. More interestingly, according to the data presented, tested compounds aggregated in the liver and intestine of 168 hpf larvae. This finding suggests that they could potentially be used for monitoring and diagnosing liver and intestinal diseases.

Tunable Fluorescence via Self-Assembled Switching of AIE-Active Micelle-like Nanoaggregates.

Chemical structures bearing a combination of aggregation-induced emission enhancement (AIEE) and intramolecular charge transfer (ICT) properties attracted the attention of many researchers. Recently, there is an increasing demand to pose tunable AIEE and ICT fluorophores that could present their conformation changes-related emission colors by adjusting the medium polarity. In this study, we designed and synthesized a series of 4-alkoxyphenyl-substituted 1,8-naphthalic anhydride derivatives NAxC using the Suzuki coupling reaction to construct donor-acceptor (D-A)-type fluorophores with alkoxyl substituents of varying carbon chain lengths (x = 1, 2, 4, 6, 12 in NAxC). To explain the observation that molecules with longer carbon chains revealed unusual fluorescence enhancement in water, we study the optical properties and evaluate their locally excited (LE) and ICT states by solvent effects combined with Lippert-Mataga plots. Then, we explored the self-assembly abilities of these molecules in water-organic (W/O) mixed solutions and observed the morphology of its nanostructure using a fluorescence microscope and SEM. The results show that NAxC, x = 4, 6, 12 show different degrees of self-assembly behaviors and corresponding aggregation-induced emission enhancement (AIEE) progresses. At the same time, different nanostructures and corresponding spectral changes can be obtained by adjusting the water ratio in the mixed solution. That is, NAxC compounds present different transitions between LE, ICT and AIEE based on the polarity, water ratio and time changes. We designed NAxC as the structure-activity relationship (SAR) of the surfactant to demonstrate that AIEE comes from the formation of micelle-like nanoaggregates, which causes a restriction of the transfer from the LE state to the ICT state, and micelle formation results in a blue-shift in emission and enhances the intensity in the aggregate state. Among them, NA12C is most likely to form micelles and the most obvious fluorescence enhancement, which will switch over time due to the nano-aggregation transition.

Effects of Central Elements on the Properties of Group 13 Dialdiminate Complexes.

Novel luminescent dialdiminate complexes of the Group 13 elements were prepared to evaluate the effects of the central element on their properties. We demonstrate that their absorption wavelength and the response to Lewis bases apparently depend on the central atom. The aluminum complex exhibited the absorption band in the higher-energy region than the gallium and indium congeners. Theoretical calculations suggest that the aluminum complex has a lower-lying highest-occupied molecular orbital than the other complexes. Additionally, the emission intensity of the aluminum complex clearly changed in response to a Lewis base. Quantum chemical calculations suggest that these element-dependent optical properties could originate from the difference in the electric charges on the central elements. Interestingly, the ligand exchange reactions were observed in the indium complexes together with the changes in the optical properties and controlled by the addition of InCl3 and InMe3 . Furthermore, all the complexes showed aggregation-induced emission enhancement (AIEE) and crystallization-induced emission enhancement (CIEE) properties. These results lead to proposing a practical strategy for manipulating the optoelectronic properties coupled with the reactivities of complexes by choosing the central elements in the same group.

Synthesis of Fluorophore Based Functional Material for Selective Detection of Al3+ Ion in Water and Decoding the AIEE Property of Its Hydrosol.

A designed aggregation-induced emission enhancement (AIEE) active fluorescence probe 2,3-Bis-[(2-hydroxy-napthalen-1-ylmethylene)-amino]-but-2-enedinitrile (L) was synthesized via one step condensation method. The probe shows swift sensitivity and selectivity toward Al3+over other relevant metal ions and also exhibits significant AIEE phenomena in methanol/water mixture. Significant enhancement of fluorescence intensity is triggered via chelation-enhanced fluorescence through complex (Al3+-L) formation. A 2:1 metal to ligand ratio is observed from Job's plot based on UV - Vis absorption titration and detection limit (LOD) is found as low as 31.14nM. Moreover, 1H NMR titrations and fluorescence reversibility by adding Al3+ and EDTA sequentially had been performed to establish the binding site of sensor complex (Al3+-L). Time-resolved photoluminescence, dynamic light scattering, optical microscopy, and on-site visualization studies have been performed to understand the AIEE mechanism of L in different volume percentage of water and methanol mixture. An INHIBIT molecular logic gate has been constructed utilizing the fluorescence behavior of the probe, L in presence of Al3+ and strong chelating ligand EDTA.

Flexible HSA-interactive phenolphthalein based probe proficient at selective turn-on sensing for Zn2+ & Al3+

A phenolphthalein based fluorescent probe, 3, 3-Bis-[4-hydroxy-3-(pyridine-2-yliminomethyl)-phenyl]-3H-isobenzofu ran-1-one (PHPA), has been synthesized. It exhibits high sensitivity and selectivity towards Zn2+ and Al3+over other relevant water soluble metal ions and also display novel aggregation-induced emission enhancement (AIEE) characteristics. Both complexes followed 2:1 metal to ligand stoichiometry calculated from Job's plot. A significant fluorescence enhancement with a turn-on ratio of ∼94-fold was triggered via chelation-enhanced fluorescence through sensor complex (Zn2+-PHPA) and∼245-fold increase for sensor complex Al3+-PHPA. In addition, the binding sites of both sensor complexes (PHPA-Zn2+ & PHPA-Al3+) have been confirmed through 1H NMR, 13C NMR, FT-IR and ESI-MS spectroscopy. The detection limits is found to be 8.5 × 10−8 M for complex PHPA-Zn2+ and 1.88 × 10−7 M for complex PHPA-Al3+, respectively. The reversible detection of Zn2+ and Al3+ complexes in methanol are also ascertained in the presence of Na2EDTA, enabling the construction of a molecular Logic gate. An advanced-level NOT-AND-OR logic gate has been constructed from a different chemical combinational input and emission output. Human serum albumin (HSA) binding studies were performed by various spectroscopic methods to expound possible bioactivity of PHPA. Interestingly; the AIEE properties of PHPA may advance its impact as studied in methanol-water mixtures with high water contents. To gain the knowledge into the AIEE mechanism of the PHPA, the size and growth process of particles in different volume percentage of water and methanol mixture were studied using time-resolved photoluminescence, dynamic light scattering, and optical microscopic image. The probe PHPA has also been used to detect intracellular Zn2+ ion in A549 cells.

AIEE active J-aggregates of naphthalimide based fluorescent probe for detection of Nitrobenzene: Combined experimental and theoretical approaches for Non-covalent interaction analysis

A new naphthalimide-based fluorescent probe NS with exceptional J-aggregates based aggregation-induced emission enhancement (AIEE) properties was rationally synthesized through a single-step imidation reaction. Probe NS exhibited excellent AIEE properties in aqueous media through the formation of J-aggregates with remarkable red-shift. The AIEE active probe NS was used for selective and sensitive detection of nitrobenzene (NB) based on fluorescence quenching response. Formation of J-aggregates was assessed through fluorescence titration. These J-aggregates contributed significantly to produce favorable interaction between probe NS and NB. The highly selective fluorescence detection of NB was accredited to the adjustable smaller size of NB that can easily penetrate into interstitial spaces of probe molecules. Ability of sensor to detect NB in solid state was also accomplished through solid state fluorescence spectroscopy. Nature of interaction and sensitivity of probe NS for NB has also been investigated through 1H NMR titration and density functional theory (DFT) including non-covalent interaction (NCI), quantum theory of atom in molecule (QTAIM), electron density differences (EDD), frontier molecular orbitals (FMO) and density of states (DOS) analysis. Advantageously, probe exhibited colorimetric and vapor phase detection of NB. Moreover, probe was quite sensitive for the trace detection of NB in real samples.

氰基苯乙烯基聚集诱导发光材料在光和pH调控下的多颜色和荧光转换

Stimuli-responsive aggregation-induced emission (AIE) materials with regulated color and fluorescence have shown promising applications in smart optical systems, while most of them are only regulated by a single stimulus. Herein, a cyanostilbene-based fluorophore (Z-BDPA) with AIE enhancement properties is elaborately designed, which exhibits multiple regulable color and fluorescence owing to its various photochemical reactions and reversible pH response. After exposure to 420 nm light, the green emission of Z-BDPA in tetrahydrofuran (THF) becomes brighter owing to the Z→E isomerization, which can be recovered under 365 nm irradiation; the yellow fluorescence of Z-BDPA in THF/H2O mixtures with 90% water fraction (fw) becomes stronger yellow-green fluorescence due to the photodimerization. At acidic conditions, Z-BDPA powder with yellow fluorescence can become another AIEgen Z-p-BDPA with blue fluorescence via protonation, which would be recovered through deprotonation under basic conditions. Under ultraviolet light irradiation, the blue fluorescence of Z-p-BDPA is blue-shifted and increased due to the photo-isomerization and subsequent photocyclodehydrogenation reactions. Based on the regulable color and fluorescence upon the photochemical and pH responses, Z-BDPA has been successfully applied in photo-patterning and anti-counterfeiting.

An AIEE Active Anthracene-Based Nanoprobe for Zn2+ and Tyrosine Detection Validated by Bioimaging Studies

Novel anthracene-based Schiff base derivative (4-(anthracen-9-ylmethylene) amino)-5-phenyl-4H-1,2,4-triazole-3-thiol; AT2) is synthesized and utilized as an aggregation-induced emission-enhancement (AIEE) active probe to detect Zn2+ and Tyrosine. Ultraviolet-visible absorption/photoluminescence (UV-vis/PL) spectroscopy studies on the AIEE property of AT2 (in ethanol) with increasing water fractions (fw: 0–97.5%) confirm the J-type aggregation. Excellent sensor selectivity of AT2 to Zn2+ and its reversibility with Tyrosine are demonstrated with PL interrogations. 2:1 and 1:1 stoichiometry and binding sites of AT2-Zn2+ and Tyrosine-Zn2+ complexes are elucidated from Job plots, HR-mass, and 1H-NMR results. Nanomolar-level detection limits (LODs) of Zn2+ (179 nM) and Tyrosine (667 nM) and association constants (Kas) of 2.28 × 10−6 M−2 (for AT2-Zn2+) and 1.39 × 10−7 M−1 (for Tyrosine-Zn2+) are determined from standard deviation and linear fittings. Nanofiber formation in AIEE and aggregated/dispersed nanoparticles in the presence of the Zn2+/Tyrosine are supported by scanning-electron microscope (SEM), transmission-electron microscope (TEM), atomic-force microscope (AFM), and dynamic-light scattering (DLS) investigations. Density-functional theory (DFT) studies confirm an “On-Off” twisted intramolecular charge transfer/photo-induced electron transfer (TICT/PET) and “On-Off-On” PET mechanisms for AIEE and sensors, respectively. B16-F10 cellular and zebrafish imaging are conducted to support the applications of AIEE and sensors.

Highly twisted luminogen based on asymmetric D-A-D’ type dicyanovinyl derivative decorated dibenzofuran and carbazole with aggregation-induced emission enhancement and obvious mechanochromic behavior

A novel asymmetric D-A-D’ type dicyanovinyl derivative DBF-DCN-Cz, which was constructed with a dibenzofuran unit and a carbazole moiety bridged by a dicyanovinyl spacer, was designed and synthesized. It was found that the luminogen DBF-DCN-Cz showed unique intramolecular charge transfer (ICT) process from electron-donating carbazole unit to electron-accepting dicyanovinyl moiety. Optical studies and theoretical calculations indicated that DBF-DCN-Cz possessed highly twisted conformation and aggregation-induced emission enhancement (AIEE) property because of the presence of the large steric hindrance. Interestingly, the emitting color of solid powder of DBF-DCN-Cz changed from initial bright yellow to orange after grinding, showing obvious mechanochromic luminescence (MCL). XRD and DSC experiments confirmed that the MCL phenomenon originated from the phase transition from crystalline to amorphous states. This work might broaden the knowledge for AIE-active MCL materials and laid the foundation for their extensive applications.

D-A-D type cyclohexyl and cycloheptyl-modified benzo[d]imidazole derivatives with different aggregation-induced emission enhancement (AIEE) and mechanofluorochromic properties

Four tetraphenylethylene or triphenylamine-functionalized donor-acceptor-donor type benzo[d]imidazole derivatives 1–4 with different cycloalkyl substituents were successfully developed and characterized. All of these fluorogenic compounds showed remarkable aggregation-induced emission enhancement characteristics, and these luminogens emitted high brightness orange or green fluorescence in the aggregation state. Furthermore, these cyclohexyl or cycloheptyl-modified benzo[d]imidazole molecules exhibited tunable solid-state fluorescence effect, and their solids demonstrated contrasting mechanically triggered fluorescence responses. More specifically, luminogens 1–3 showed no observable fluorescence changes under mechanical stimulation, while tetraphenylethylene-based cycloheptyl-modified benzo[d]imidazole luminogen 4 displayed reversible mechanochromic fluorescence conversion between green and yellowish-green emission colors. The mechanofluorochromic mechanism of solid 4 was revealed by the powder X-ray diffraction measurements, and the experimental results proved that the phase interconversion between crystalline and amorphous states was responsible for the noticed switchable mechanoresponsive fluorescence. In addition, the mechanical force-sensitive emission of fluorogenic molecule 4 afforded its promising application in ink-free writing.

Aggregation-induced emission enhancement and mechanofluorochromism based on dicyanovinyl derivatives decorated carbazole or triphenylamine units: Effects of electronic structures and spatial conformations

Based on a rational design strategy by combining an electron-accepting dicyanoethene unit and electron-donating carbazole (CZ) or triphenylamine (TPA) segment, we successfully synthesized two AIE-active luminogens DPMN-CZ and DPMN-TPA using the classical Suzuki-Miyaura coupling and Knoevenagel condensation reactions. It was found that both of them showed specific intramolecular charge transfer (ICT) characteristics and aggregation-induced emission enhancement (AIEE) properties. Spectral studies and theoretical calculations proved that compounds DPMN-CZ and DPMN-TPA had typical donor-acceptor (D-A)π-conjugated structures. More interestingly, DPMN-CZ and DPMN-TPA showed mechanofluorochromic (MFC) behaviors. Upon the treatment of grinding, the initial crystals of DPMN-CZ only showed a low contrast mechanochromism (wavelength changed by 27 nm), while the emitting colors of DPMN-TPA samples changed from orange to red, and a large spectral shift of 57 nm was obtained. The reason was that DPMN-TPA possessed a stronger ICT degree and more twisted molecular conformation than that of DPMN-CZ, which endowed DPMN-TPA with loose molecular packing and weak intermolecular interactions, and its initial crystal was easily destroyed by external force, resulting in a large spectral shift. XRD and DSC studies confirmed that the significant MFC nature of DPMN-CZ and DPMN-TPA was attributed to crystalline-to-amorphous transition.