Enhanced Sensitivity and Piezochromic Contrast through Single-Direction Extension of Molecular Structure

Tunable Chromic Properties of Viologen-Metal Polymers Modulated by Coordination Modes for Inkless Erasable Printing

Inkless and erasable printing (IEP) based on chromic materials holds great promise to alleviate environmental and sustainable problems. Metal-organic polymers (MOPs) are bright platforms for constructing IEP materials. However, it is still challenging to design target MOPs with excellent specific functions rationally due to the intricate component-structure-property relationships. Herein, an effective strategy was proposed for the rational design IEP-MOP materials. The stimuli-responsive viologen moiety was introduced into the construction of MOPs to give it potential chromic behaviors and two different coordination models (i. e. bilateral coordination model, M1 ; unilateral coordinated model, M2 ) based on the same viologen ligand were designed. Aided by theoretical calculations, model M1 was recommended secondarily as a more suitable system for IEP materials. Along this line, two representative viologen-ZnII MOPs 1 and 2 with models M1 and M2 were synthesized successfully. Experiments exhibit that 1 does have quicker stimuli response, stronger color contrast and longer radical lifetime compared to 2. Significantly, the obtained 1-IEP media brightly inherits the excellent chromic characteristics of 1 and the flexibility of the paper at the same time, which achieves most daily printing requirements, as well as enough resolution and durability to be used in identification by smart device.

Anisotropic flexibility and rigidification in a TPE-based Zr-MOFs with scu topology

Tetraphenylethylene (TPE)-based ligands are appealing for constructing metal-organic frameworks (MOFs) with new functions and responsiveness. Here, we report a non-interpenetrated TPE-based scu Zr-MOF with anisotropic flexibility, that is, Zr-TCPE (H4TCPE = 1,1,2,2-tetra(4-carboxylphenyl)ethylene), remaining two anisotropic pockets. The framework flexibility is further anisotropically rigidified by installing linkers individually at specific pockets. By individually installing dicarboxylic acid L1 or L2 at pocket A or B, the framework flexibility along the b-axis or c-axis is rigidified, and the intermolecular or intramolecular motions of organic ligands are restricted, respectively. Synergistically, with dual linker installation, the flexibility is completely rigidified with the restriction of ligand motion, resulting in MOFs with enhanced stability and improved separation ability. Furthermore, in situ observation of the flipping of the phenyl ring and its rigidification process is made by 2H solid-state NMR. The anisotropic rigidification of flexibility in scu Zr-MOFs guides the directional control of ligand motion for designing stimuli-responsive emitting or efficient separation materials.

Near-Infrared Piezochromism from an o-Carborane Dyad: Boron Clusters Facilitating a Wide-Range Redshift and High Sensitivity

Piezochromic materials, which exhibit a fluorescence response with large emission spectral shifts and high sensitivity, may be useful in important applications, but there have been few reports of such organic luminophores. Herein, we report a new high-sensitivity piezochromic material based on the incorporation of an o-carborane unit, which exhibits aggregation-induced emission properties. In a high-pressure experiment, compared to carborane-free MTY, which exhibits an emission spectral shift of 75 nm and a sensitivity of 19.1 nm ⋅ GPa^-1 , the o-carborane dyad MTCb shows a larger emission wavelength difference of 131 nm and a higher sensitivity of 32.8 nm ⋅ GPa^-1 , demonstrating a performance that ranks among the best of organic piezochromic materials reported thus far. MTCb molecules adopt a J-aggregated pattern and have relatively loose molecular packing in the crystalline state. Interestingly, nonconjugated spherical carborane can disrupt the π-π interactions between adjacent molecules during compression, which results in excellent piezochromic performance.

Stimuli-Responsive Sponge for Imaging and Measuring Weak Compression Stresses

Imaging and measuring compression stresses secure a safe and healthy life. Compression stresses in kPa range are not easily detected by conventional mechanoresponsive materials because microscopic molecular motion of the chromophores is not induced by such weak stresses. Moreover, imaging of the stress distribution is not achieved so far. The present study shows a sponge device combining two stimuli-responsive materials, a capsule releasing interior liquid and color-changing polymer in responses to compression stress and chemical stimulus, respectively. The stimuli-responsive capsule is dispersed on a melamine sponge comprised of the fibers with coating the layered polydiacetylene (PDA). The application of weak compression stresses induces collapse of the capsules, outflow of the interior liquid, and subsequent irreversible color change of PDA. The cascading response in the sponge device colorimetrically enables imaging of the distribution and measuring the strength of the compression stresses in kPa range. Furthermore, the device demonstrates imaging and measuring unknown weak compression stresses applied by the irregular-shaped objects. A couple of clinical issues in surgical operation of intestine are studied using the stress-imaging sponge device. The device and its design strategy can be applied to stress imaging in a variety of fields.

Highly Efficient Sky-Blue π-Stacked Thermally Activated Delayed Fluorescence Emitter with Multi-Stimulus Response Properties

Organic materials with multi-stimulus response (MSR) properties have demonstrated many potential and practical applications. Herein, a π-stacked thermally activated delayed fluorescence (TADF) material with multi-stimulus response (MSR) properties, named SDMAC, was designed and synthesized using distorted 9,9-dimethyl-10-phenyl-9,10-dihydroacridine as a donor. SDMAC possesses a rigid π-stacked configuration with intramolecular through-space interactions and exhibits aggregation-induced emission enhancement (AIEE), solvatochromic, piezochromic, and circularly polarized luminescence (CPL) under different external stimuli. The rigid molecular structure and efficient TADF properties of SDMAC can be used in displays and lighting. Using SDMAC as an emitter, the maximum external quantum efficiency (EQE) of the fabricated organic light-emitting diodes (OLEDs) is as high as 28.4 %, which make them the most efficient CP-TADF OLEDs based on the through-space charge transfer strategy. The CP organic light-emitting diodes (CP-OLEDs) exhibit circularly polarized electroluminescence (CPEL) signals.

Red to near-infrared piezochromism from AIE-active luminophores: isolated dimers facilitating a wide-range redshift

An AIE-activity architecture with HLCT and highly bright fluorescence was developed and it was the dispersed dimer packing found to be attributable to cholesteryl units. During compression, the dispersed dimers presented a remarkable redshift (157 nm) and high sensitivity (22.1 nm GPa−1).

Ultrahigh-Sensitive Compression-Stress Sensor Using Integrated Stimuli-Responsive Materials

Measurement of mechanical stresses, such as compression, shear, and tensile stresses, contributes toward achieving a safer and healthier life. In particular, the detection of weak compression stresses is required for healthcare monitoring and biomedical applications. Compression stresses in the order of 10⁶ -10¹⁰  Pa have been visualized and/or quantified using mechano-responsive materials in previous works. However, in general, it is not easy to detect compression stresses weaker than 10³  Pa using conventional mechano-responsive materials because the dynamic motion of the rigid mechano-responsive molecules is not induced by such a weak stress. In the present work, weak compression stresses in the order of 10⁰ -10³  Pa are visualized and measured via the integration of stimuli-responsive materials, such as layered polydiacetylene (PDA) and dry liquid (DL), through response cascades. DLs consisting of liquid droplets covered by solid particles release the interior liquid and collapse with application of a weak compression stress. The color of the layered PDA is changed by the spilled liquid as a chemical stress. A variety of weak compression stresses, such as expiratory pressure, are visualized and colorimetrically measured using the paper-based device of the integrated stimuli-responsive materials. Diverse mechano-sensing devices can be designed via the integration of stimuli-responsive materials.

Stimuli-Responsive Luminescent Properties of Tetraphenylethene-Based Strontium and Cobalt Metal-Organic Frameworks

Herein we report two new TPE-based 3D MOFs, that is, Sr-ETTB and Co-ETTB (TPE=Tetraphenylethylene, H₈ ETTB=4',4''',4''''',4'''''''-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1'-biphenyl]-3,5-dicarboxylic acid))). Through tailoring outer shell electron configurations of Sr^II and Co^II cations, the fluorescence intensity of the MOFs is tuned from high emission to complete non-emission. Sr-ETTB with strong blue fluorescence shows reversible fluorescence variations in response to pressure and temperature, which is directly related to the reversible deformation of the crystal structure. In addition, non-emissive Co-ETTB counterpart exhibits a turn-on fluorescent enhancement under the stimulation of analyte histidine. In the process, TPE-cored linkers in the MOFs are released through competitive coordination substitution and subsequently reassembled to perform aggregation-induced luminescence behavior originated from the organic linkers.

Deep insight into reasons for the mechanoluminescence phenomenon of triphenylamine-substituted imidazoles and their distinct mechano-fluorochromic behaviours

Three imidazoles with different numbers of fused aromatic rings have been prepared by the respective introduction of triphenylamine and 4-cyanophenyl at the N1 and C2 positions in the imidazole ring. Each imidazole effectively exhibits positive solvatochromism, and that of benzo[d]imidazole is the most significant. Although these imidazole crystals have centrosymmetric space groups, they are all ML-active. It was verified by DFT calculations based on X-ray crystallography that some molecular couples with strong intermolecular interactions possess large net dipole moments that should be dominantly responsible for the ML behaviours of these crystals. Moreover, the considerably high molecular dipole moments of the three imidazoles also make a great contribution to good ML effects. Based on this triphenylamine-substituted imidazole system, the relationships among space groups, molecular dipole moments, polar molecular couples and the ML phenomenon are made clear for the first time. Unlike the remarkable MFC activities on imidazole and benzo[d]midazole crystals, phenanthro[9,10-d]imidazole is MFC-inert, and this may well be attributed to strong intramolecular C-H⋯π interactions, which make the rotation of triphenylamine nearly impossible under force stimuli.

Synthesis of Strongly Fluorescent Imidazole Derivatives: Structure Property Studies, Halochromism and Fluorescent Photoswitching

New series of methoxy and hydroxyl group substituted triphenylamine (TPA)-imidazole fluorescent molecules (5-(diphenylamino)-2-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol (1), 5-(diphenylamino)-2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenol (2), 5-(diphenylamino)-2-(4,5-diphenyl-1H-imidazol-2-yl)phenol (3), 5-(diphenylamino)-2-(1,4,5-triphenyl-1H-imidazol-2-yl)phenol (4), N-(3-methoxy-4-(1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-N-phenylbenzenamine (5), N-(3-methoxy-4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-N-phenylbenzene amine (6), and N-(3-methoxy-4-(4,5-diphenyl-1H-imidazol-2-yl)phenyl)-N-phenylbenzenamine (7)) have been synthesized that exhibited strong solution fluorescence and molecular structure and conformation controlled fluorescence photoswitching, solid state fluorescence and halochromism. Hydroxyl substituted molecules (1-4) showed moderate to strong fluorescence in solution depend on solvent polarity and very weak solid state fluorescence. Methoxy substituted molecules (5-7) displayed strong fluorescence both in solution and solid state. Solid state structural studies revealed strong intramolecular H-bonding in the crystal lattice. Interestingly, highly twisted structure (6) showed rare light induced reversible fluorescence switching in CHCl₃. The observation of isobestic point in time dependent fluorescence photoswitching studies indicated structural isomer conversion. Further, acid sensitive imidazole nitrogen has been made use to demonstrate solid state fluorescence switching via halochromism. Thus the present studies attempted to develop new fluorescent molecules and establish structure-property relationship for designing fluorescence switching materials. Graphical Abstract Molecular structure controlled solid state fluorescence, halochromism and a rare fluorescence photoswitching in chloroform have been observed with triphenylamine-imidazole derivatives.

Structural tuning enables piezochromic and photochemical properties in N-aryl-β-enaminones

An efficient synthesis of N-aryl-β-enaminones via Et₃N-mediated, one-pot three-component reaction of 4-hydroxycoumarin/dimedone, β-nitrostyrene/2-(2-nitrovinyl)thiophene, and arylamine in toluene under refluxed conditions is herein presented. Some prepared compounds were found to exhibit piezochromic properties. The XRD and SEM measurements of the piezochromic compound showed substantial crystal packing and morphology changes before and after grinding. Further, one prepared compound was found to be light-sensitive and can be converted to a furo[3,2-b]pyridin-2(4H)-one derivative upon UV irradiation. A plausible mechanism for this photochemical reaction was proposed.

N-Aryl-β-enaminones are synthesized via Et₃N-mediated, one-pot three-component reaction of 4-hydroxycoumarin/dimedone, β-nitrostyrene/2-(2-nitrovinyl)thiophene and arylamine, and their piezochromic behavior and photosensitivity explored.

Side chain effects on the solid-state emission behaviours and mechano-fluorochromic activities of 10H-phenothiazinylbenzo[d]imidazoles

A family of 4-cyanophenyl-substituted 10H-phenothiazinylbenzo[d]imidazoles with different side chains at the 10-position are prepared and their physical properties are studied. The detailed structure-property research demonstrates that the cold crystallization temperature of ground samples and the emission wavelengths of pristine samples are in good accordance with the packing density, conformation distortion and intermolecular interactions, but emission wavelengths of ground samples are slightly chain-dependent. For benzimidazoles with alkyl chains, longer and more branched chains can produce looser packings, which cause pristine samples to display red-shifted emission and reduced MFC activity. For benzimidazoles with a phenyl chain, the emission wavelengths of both the pristine and the ground samples are remarkably red-shifted. Moreover, the degree of conformation distortion is larger, and the cold crystallization temperature is higher. Interestingly, the homologue with the n-hexyl chain displays an intense ML effect that is mainly attributed to the heavy discharge quantity on largely enhanced discharge areas under force stimuli due to the great fragility of this crystal.

The solid-state emission behaviours and MFC performances of 10H-phenothiazinylbenzo[d]imidazoles with different side chains are investigated.

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

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

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

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

Unusual fluorescent photoswitching of imidazole derivatives: the role of molecular conformation and twist angle controlled organic solid state fluorescence

Triphenylamine-imidazole molecules exhibited unprecedented light induced fluorescence switching via conformational change.