Advances in Pure Organic Mechanoluminescence Materials

A Novel High-Strength Dental Resin Composite Based on BaSi2O2N2 for Caries Restoration

Dental resin composite (DRC) is the most widely used restorative material for caries filling treatments. However, DRC has limitations, including incomplete curing and suboptimal mechanical properties, which restrict its clinical application. In this study, we combined the mechanoluminescent material BaSi2O2N2:Eu2+ (BSON) with DRC to create a novel DRC (BD) capable of emitting blue light under occlusal force. This study explored a resin composite with enhanced curing efficiency and improved mechanical properties. Characterization of the mechanical properties demonstrated that, as the BSON doping ratio increased (2, 4, 8, 16, and 32 wt %), the compressive strength, flexural strength, and surface hardness of BD initially increased and then decreased. The composite doped with 4 wt % BSON (BD4) exhibited the best mechanical properties. Compared to DRC, BD4 showed an 11% increase in compressive strength (211.9 ± 13.9 MPa), a 36% increase in flexural strength (71.9 ± 8.4 MPa), and a 7% increase in surface hardness (111.0 ± 6.4 HV). Based on these findings, BD4 was selected for further experiments. The study of luminescent properties revealed that the mechanoluminescent wavelength of BD4 (470-720 nm) partially overlapped with the wavelength range of the light-curing unit (420-490 nm). Additionally, after cyclic loading, BD4's compressive strength and degree of conversion (DC) improved. After applying a cyclic load of 300 N for 240 s, BD4's compressive strength increased by 70% (142.2 ± 1.2 MPa), and the DC increased by 8% (74.4%). Moreover, biocompatibility evaluations showed that the cell survival rate of L929 fibroblast cells exceeded 90%. Thus, we developed an effective strategy to enhance DRC by incorporating BSON, resulting in a resin composite with superior mechanical properties, enhanced curing efficiency, and favorable biocompatibility, offering a promising new solution for caries restorations.

Naphthalimide and Carbazole Based Mechanochromic Molecular Dyads and Triads for Selective Lysosome Imaging

In this study, we report the design and development of a stable fluorescent probe that is selectively localized in the cytosol of Hela cells. We designed two probes, 1 and 2, with D-π-A (carbazole (Cbz)-vinyl-naphthalimide (NPI)) and A-π-D-π-A (NPI-vinyl-Cbz-vinyl-NPI) architecture, respectively. Probes 1 and 2 exhibit broad photoluminescence (PL) spectra ranging from green (550 nm) to far-red (800 nm) in solutions and aggregated states. In the solid-state, the PL of these probes shows a bathochromic shift, which can be attributed to intermolecular interactions. In a water-rich medium, Probe 1, with a single NPI moiety, shows aggregation-caused quenching (ACQ) but retains a moderate quantum yield of 13.7 % (Φsoln=61.4 %). On the other hand, probe 2, with two NPI units, showed aggregation-induced enhanced emission AIEE, where the PLQY is increased nearly 4 times (Φsoln=3.5 %, Φagg=12.8 %). In-vitro cell studies revealed that these probes are non-toxic and effectively stain cells in green and red channels. Notably, Probe 1 demonstrated excellent cellular uptake and selectivity for lysosome, with a Pearson overlap coefficient of 0.91.

Hydrogen Bonding-Induced Multicolor and Thermochromic Emissions of Triphenylamines

The fluorescence tuning of stimuli-responsive materials is crucial but challenging. The deep understanding of thermal induced fluorescence change, however, has rarely been conducted. Herein, the thermochromic emission of a triphenylamine (TPA) derivative (1), with one acetyl and two 1-hydroxy-1-methylethyl units on each o-phenyl group around the nitrogen atom, has been investigated. Upon heating, the bright blue-emitting solid 1 turns to a strong green-emitting liquid. Moreover, 1 is green emissive in ethanol but blue emissive with high absolute quantum yields in dimethyl sulfoxide (DMSO). Another TPA derivative (2) is green emitting. X-ray crystallography studies reveal that the strong solid-state fluorescence arises from a rigid pyramidal structure around central nitrogen of 1, due to the OH•••OH, OH•••O = C hydrogen bonding interactions. Comparatively, 2 has a planar configuration of three C─N bonds around central nitrogen atom. This work will provide a new route for constructing multiemission materials using unimolecular platforms.

Recent Progress of Mechanofluorochromism and Mechanoluminescence for Phenothiazine Derivatives and Analogues

Mechanofluorochromism (MFC) and mechanoluminescence (ML) materials have garnered significant attention from researchers due to their potential applications in anti-counterfeiting, optical recording, photodynamic therapy, bioimaging, stress sensing, display technology, and ink-free printing paper. Among the various building blocks utilized in these materials, phenothiazine (PTZ) has emerged as a widely employed fundamental component owing to its distinctive electronic and optical properties as well as its facile modification capabilities. Summarizing the recent progress of PTZ derivatives and analogues in this field holds practical significance. In this review article, we classify over one hundred compounds into a few classes based on the positions of substituents and provide detailed descriptions of their contributions to MFC and ML research respectively. This comprehensive review aims to offer theoretical insights and practical examples for researchers engaged in designing and developing new phenothiazine functional materials while serving as a bridge for further exploration of MFC or ML studies.

Room-Temperature Reversible Control of Fluorescently Distinct Polymorphs Using Pressure and E-Field: Writing and Erasing Information without Thermal Treatment

This paper presents the reversible transformation between two polymorphs of a hexacatenar liquid crystal (1) with distinct fluorescence colors at room temperature (RT). This method utilizes mechanical pressure (mechanochromism) and an electric field (E-field-chromism). The molecule (1), designed with a pyrene core and 1,2,3-triazole linkers, exhibits a blue-emissive crystalline (CRY) polymorph (1-B) and a green-emissive liquid crystalline (LC) polymorph (1-G) at RT, depending on the cooling rate from the liquid phase. The metastable 1-G is stabilized by hydrogen bonding (H-bonding) between 1,2,3-triazole linkers, forming a helical columnar structure. Mechanical pressure converts thermodynamically stable 1-B to 1-G, while the application of an alternating current (AC) E-field to 1-G transforms it back to 1-B. Notably, this study reports the first instance of an E-field-induced polymorphic transformation. Using mechanical pressure and E-field application at RT, patterns were successfully recorded and erased on substrates, demonstrating potential applications in data storage, anticounterfeiting, and sensor technologies.

Achieving Ultrasound-Excited Emission with Organic Mechanoluminescent Materials

Unlike traditional photoluminescence (PL), mechanoluminescence (ML) achieved under mechanical excitation demonstrates unique characteristics such as high penetrability, spatial resolution, and signal-to-background ratio (SBR) for bioimaging applications. However, bioimaging with organic mechanoluminescent materials remains challenging because of the shallow penetration depth of ML with short emission wavelengths and the absence of a suitable mechanical force to generate ML in vivo. To resolve these issues, the present paper reports the achievement of ultrasound (US)-excited fluorescence and phosphorescence from purely organic luminogens for the first time with emission wavelengths extending to the red/NIR region, with the penetrability of the US-excited emission being considerably higher than that of PL. Consequently, US-excited subcutaneous phosphorescence imaging can be achieved using a mechanoluminescent-luminogen-based capsule device with a quantified intensity of 9.15 ± 1.32 × 104 p s-1 cm-2 sr-1 and an SBR of 24. Moreover, the US-excited emission can be adequately tuned using the packing modes of the conjugated skeletons, dipole orientation of mechanoluminescent luminogens, and strength and direction of intermolecular interactions. Overall, this study innovatively expands the kind of excitation sources and the emission wavelengths of organic mechanoluminescent materials, paving the way for practical biological applications based on US-excited emission.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Recent innovations in the technology and applications of low-dimensional CuO nanostructures for sensing, energy and catalysis

Low-dimensional copper oxide nanostructures are very promising building blocks for various functional materials targeting high-demanded applications, including energy harvesting and transformation systems, sensing and catalysis. Featuring a very high surface-to-volume ratio and high chemical reactivity, these materials have attracted wide interest from researchers. Currently, extensive research on the fabrication and applications of copper oxide nanostructures ensures the fast progression of this technology. In this article we briefly outline some of the most recent, mostly within the past two years, innovations in well-established fabrication technologies, including oxygen plasma-based methods, self-assembly and electric-field assisted growth, electrospinning and thermal oxidation approaches. Recent progress in several key types of leading-edge applications of CuO nanostructures, mostly for energy, sensing and catalysis, is also reviewed. Besides, we briefly outline and stress novel insights into the effect of various process parameters on the growth of low-dimensional copper oxide nanostructures, such as the heating rate, oxygen flow, and roughness of the substrates. These insights play a key role in establishing links between the structure, properties and performance of the nanomaterials, as well as finding the cost-and-benefit balance for techniques that are capable of fabricating low-dimensional CuO with the desired properties and facilitating their integration into more intricate material architectures and devices without the loss of original properties and function.

Excitation Wavelength-Dependent Charge Stabilization in Highly Interacting Phenothiazine Sulfone-Derived Donor-Acceptor Constructs

Prolonging the lifetime of charge-separated states (CSS) is of paramount importance in artificial photosynthetic donor-acceptor (DA) constructs to build the next generation of light-energy-harvesting devices. This becomes especially important when the DA constructs are closely spaced and highly interacting. In the present study, we demonstrate extending the lifetime of the CSS in highly interacting DA constructs by making use of the triplet excited state of the electron donor and with the help of excitation wavelength selectivity. To demonstrate this, π-conjugated phenothiazine sulfone-based push-pull systems, PTS2-PTS6 have been newly designed and synthesized via the Pd-catalyzed Sonogashira cross-coupling followed by [2 + 2] cycloaddition-retroelectrocyclization reactions. Modulation of the spectral and photophysical properties of phenothiazine sulfones (PTZSO₂) and terminal phenothiazines (PTZ) was possible by incorporating powerful electron acceptors, 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (exTCBD). The quadrupolar PTS2 displayed solvatochromism, aggregation-induced emission, and mechanochromic behaviors. From the energy calculations, excitation wavelength-dependent charge stabilization was envisioned in PTS2-PTS6, and the subsequent pump-probe spectroscopic studies revealed charge stabilization when the systems were excited at the locally excited peak positions, while such effect was minimal when the samples were excited at wavelengths corresponding to the CT transitions. This work reveals the impact of wavelength selectivity to induce charge separation from the triplet excited state in ultimately prolonging the lifetime of CCS in highly interacting push-pull systems.

Stimulus-Responsive Organic Phosphorescence Materials Based on Small Molecular Host-Guest Doped Systems

Small molecular host-guest doped materials exhibit superiority toward high-efficiency room-temperature phosphorescence (RTP) materials due to their structural design diversity and ease of preparation. Dynamic RTP materials display excellent characteristics, such as good reversibility, quick response, and tunable luminescence ability, making them applicable to various cutting-edge technologies. Herein, we summarize the advances in host-guest doped dynamic RTP materials that respond to external and internal stimuli and present some insights into the molecular design strategies and underlying mechanisms. Subsequently, specific viewpoints are described regarding this promising field for the development of dynamic RTP materials. This Perspective is highly beneficial for future intelligent applications of dynamic RTP systems.

A high-contrast polymorphic difluoroboron luminogen with efficient RTP and TADF emissions

A simple N,S-chelated four-coordinated difluoroboron-based emitter is reported with three polymorphs, which emit high contrast green (G), yellow (Y) and red (R) light. Interestingly, the G and R-Crystals show different thermally activated delayed fluorescence (TADF) at 530 nm and 630 nm with a remarkable emission spectral shift of up to 100 nm, while the Y-Crystal exhibits room temperature phosphorescence (RTP) at around 570 nm with a high solid-state quantum yield of 77%. Single crystal analysis and theoretical calculations reveal that different molecular conformations and packing modes lead to distinct triplet exciton conversion channels.

Smart Mechanoluminescent Phosphors: A Review of Strontium-Aluminate-Based Materials, Properties, and Their Advanced Application Technologies

Mechanoluminescence, a smart luminescence phenomenon in which light energy is directly produced by a mechanical force, has recently received significant attention because of its important applications in fields such as visible strain sensing and structural health monitoring. Up to present, hundreds of inorganic and organic mechanoluminescent smart materials have been discovered and studied. Among them, strontium-aluminate-based materials are an important class of inorganic mechanoluminescent materials for fundamental research and practical applications attributed to their extremely low force/pressure threshold of mechanoluminescence, efficient photoluminescence, persistent afterglow, and a relatively low synthesis cost. This paper presents a systematic and comprehensive review of strontium-aluminate-based luminescent materials' mechanoluminescence phenomena, mechanisms, material synthesis techniques, and related applications. Besides of summarizing the early and the latest research on this material system, an outlook is provided on its environmental, energy issue and future applications in smart wearable devices, advanced energy-saving lighting and displays.

Versatile Method of Generating Triboluminescence in Polymer Films Blended with Common Luminophores

Herein we report a versatile method for the preparation of triboluminescent polymer films by physical blending with common luminophores. This method does not require the presence of a crystalline phase or the use of materials known to be triboluminescent. Emission is generated in response to friction of the polymer surface via triboelectrification, either by rubbing directly or through an inert coating layer, even with low applied stress (<0.1 MPa). Our findings offer a convenient and practical method of preparation of triboluminescent, amorphous polymer films with easily tunable emission properties.