Supramolecular Polymers with Photoswitchable Multistate Fluorescence for Anti-Counterfeiting and Encryption

Dual AIE and Visible-Light-Driven Photoswitchable Polymer for Super-resolution Imaging

Photochromic polymers and aggregation-induced emission (AIE) materials show great potential for many applications. To explore the synergy of both characteristics in polymer material areas, we reported the first synthesis of tetraphenylethylene (TPE)-diarylethene (DAE) polymer and its application as a super-resolution probe for imaging self-assembled cylindrical micelles of PSt38k-b-PEO11k. The polymer exhibits high fluorescence ON/OFF ratios, visible-light-driven photocycloreversion, and AIE properties. Compared with other DAE materials studied in super-resolution imaging, our polymer shows advantages of visible-light-driven photocycloreversion, higher resolution, higher fluorescence quantum yield, or higher thermal stability.

Copper-Based Metal-Organic Framework as Photoelectric Material

Photo-stimulated Polymers have garnered significant attention for their potential applications ranging from optical memory to sensing. Herein, by changing coordination metal and the position of nitrogen atom in pyridine-based photo-stimulated ligand, we successfully synthesised a novel photo-stimulated copper-based MOF (Cu-MOF) using 9,10-bis(di(pyridine-3-yl)methylene)-9,10-dihydroanthracene as the photo-stimulated ligand. Structural analysis revealed a 3D porous architecture, offering a distinct advantage over previously reported 1D coordination polymer using similar photo-stimulated ligand. The Cu-MOF exhibited exceptional photoelectric properties, demonstrating potential as an anode material for lithium-ion batteries and a luminescent MnO₄- sensor. Its electrochemical performance revealed high reversibility, with discharge capacities retaining stability under varying current densities. Additionally, the Cu-MOF displayed notable photo-stimulated behavior under UV light irradiation, enhancing its optical properties. Although the structure changes from 1D coordination polymer to 3D MOF, the photocyclized form (c-Cu MOF) still exhibited superior fluorescence quenching efficiency and sensitivity for MnO₄- detection with Ksv (quenching constant) 5.24×105 M-1. This work highlights the utility of Cu-MOF as a multifunctional material for energy storage and environmental sensing applications.

Visible-Light-Driven Fluorescence Turn-on Photoswitches With Near Quantitative Photocyclization Yield

Photoswitchable fluorescent materials have gained significant attention for their potential in advanced information encryption and anti-counterfeiting applications. However, the common use of UV light to trigger the isomerization processes leads to photobleaching and poor fatigue resistance. Visible-light-driven fluorescent photoswitches are highly desirable, but achieving high cyclization yield remains challenging. Herein, it is reported that all visible-light-driven turn-on fluorescence in dimethoxyphenyl functionalized diarylethene isomers. The open-ring form of o-DMPB and p-DMPB exhibits near quantitative conversion yields (up to 94%) under 405 nm visible light, attributed to the strong electron-donating character. In contrast, the meta isomer m-DMPB shows limited response to visible light, with a ring-closing yield of 22%. Furthermore, all photoswitches display good thermal stability, photostability, and fatigue resistance. Notably, o-DMPB demonstrates promising applications in anti-counterfeiting, information encryption, and photorewritable patterns. This work provides a valuable strategy for the development of high-performance fluorescent photoswitches.

Novel core-shell materials SiO2@Tb-MOF for the incorporation of spiropyran molecules and its application in dynamic advanced information encryption

Dynamic fluorescent switches with multiple light outputs offer promising opportunities for advanced security encryption. However, the achievement of dynamic emission, particularly when based on the timing of external stimuli, continues to present a significant challenge. Herein, a unique dynamic fluorescent switch was developed by integrating spiropyran molecules (SP) into a core-shell structure (SiO2@Tb-MOF). The core-shell structure, derived from lanthanide complexes and silica microspheres, was synthesized under solvothermal conditions. This structure not only preserves the green fluorescence emission of Tb-MOF, but also results in a substantial specific surface area and mesoporous pore size from SiO2, which is advantageous for incorporating SP molecules to create a dynamic fluorescent switch, SP ⊂ SiO2@Tb-MOF. Upon exposure to ultraviolet light, SP gradually transitions into the merocyanine form (MC), displaying a pronounced absorption band at approximately 550 nm. Concurrently, a fluorescence resonance energy transfer (FRET) process is initiated between Tb3+ and the merocyanine isomers. With prolonged exposure to UV light, the fluorescence color shifts progressively from green to red, facilitated by the ongoing FRET process. Moreover, SP ⊂ SiO2@Tb-MOF is doped with polydimethylsiloxane to fabricate a film. Utilizing time-dependent fluorescence, dynamic encryption patterns and advanced information encryption were investigated. This work provides a design basis for how to better construct core-shell structures and combine them with SP molecules to prepare dynamic fluorescent materials, and paves a way for constructing advanced encryption materials with higher safety requirements.

Antiswelling Photochromic Hydrogels for Underwater Optically Camouflageable Flexible Electronic Devices

Optical camouflage offers an effective strategy for enhancing the survival chances of underwater flexible electronic devices akin to underwater organisms. Photochromism is one of the most effective methods to achieve optical camouflage. In this study, antiswelling hydrogels with photochromic properties were prepared using a two-step solvent replacement strategy and explored as underwater optically camouflaged flexible electronic devices. The hydrophobic network formed upon polymerization of hydroxyethyl methacrylate (HEMA) ensured that the hydrogels possessed outstanding antiswelling properties. Internetwork hydrogen bonding interactions allowed the hydrogels to exhibit tissue-adaptable mechanical properties and excellent self-bonding capabilities. The introduction of polyoxometalates further enhanced the hydrogels' mechanical and self-bonding properties while imparting photochromic capability. The hydrogels could be rapidly and reversibly colored under 365 nm UV irradiation. The bleaching rate of the colored hydrogels increased with temperature, bleaching within 12 h at 60 °C but maintaining the color for more than 5 days at room temperature. The self-bonding and photochromic properties enabled the hydrogels to be easily assembled into optically camouflaged underwater flexible electronic devices for underwater motion sensing and wireless information transmission. An optically camouflaged strain sensor was first assembled for underwater limb motion sensing. Additionally, an underwater optically camouflaged wireless information exchange device was assembled to enable wireless communication with a smartphone. This work provided an effective strategy for the optical camouflage of underwater flexible electronic devices, presenting opportunities for next-generation underwater hydrogel-based flexible devices.

Multi-Functional Lanthanide Metallopolymer: Self-Healing and Photo-Stimuli-Responsive Dual-Emitting Luminescence for Diverse Applications

Photoluminescent metallopolymers displaying photo-stimuli-responsive properties are emerging as promising materials with versatile applications in photo-rewritable patterns, wearable UV sensors, and optical encryption anti-counterfeiting. However, integrating these materials into practical applications that require fast response times, lightweight qualities, fatigue resistance, and multiple encryption capabilities poses challenges. In this study, luminescent photochromic lanthanide (Ln) metallopolymers with rapid self-healing properties are developed by cross-linking terpyridine (Tpy)- and spiropyran (SP)- functionalized polyurethane chains through Ln-Tpy coordination bonds and H-bonds among polymer chains. The resulting products exhibit a range of intriguing features: i) photo-stimuli responsiveness using spiropyran monomers without additional dopants; ii) dual-emitting performance under UV-light due to Ln-Tpy and open-ring spiropyran moieties; iii) satisfactory mechanical properties and self-healing abilities from polymer chains; iv) multiple control switches for luminescence colors through photostimulation or feed ratio adjustments. Leveraging these attributes, the developed material introduces novel opportunities for light-writing applications, advanced information encryption, UV-sensing wearable devices, and insights into designing multifunctional intelligent materials for the future.

Multiple Stimuli-Responsive Color-Changing Polymer Materials for Reversible Writing and Anti-Counterfeiting

Polymer materials with multiple stimuli-responsive properties have demonstrated many potential and practical applications. By covalently introducing spiropyran (SP1) and spirothiopyran (STP) into the polyurethane backbone, photochromic, mechanochromic, and thermally discolored polymer materials have been prepared. In this work, we report for the first time that white light (violet, blue, and green light) above a certain intensity can activate STP to green color. Based on the above discovery, the polyurethane with SP1 and STP can exhibit reversible three-color changes (brown, green, and purple) in response to four stimuli: ultraviolet irradiation, white light irradiation, mechanical stress, and heat. The color-changing polymer materials have high color contrast and excellent reversibility, and can be used for reversible writing, anticounterfeiting and information encryption, etc.

Preparation of thermochromic ink from anthocyanidin-encapsulated alginate nanoparticles for anticounterfeiting applications

In order to make commercial products less vulnerable to counterfeiting, thermochromic inks have proven to be a viable authentication strategy. Herein, we developed a thermochromic ink for authentication by combining an anthocyanidin (ACYD) extract with alginate (ALG). To increase the anthocyanidin/alginate ink stability, a mordant (ferrous sulfate) was employed to tie up the anthocyanidin biomolecules with alginate. ACYD was extracted from red-cabbage and then immobilized into alginate to serve as an environmentally friendly spectroscopic probe. Thermochromic composite inks (ACYD@ALG) were made by adjusting the content of anthocyanidin. A homogenous blue film (608 nm) was printed on a paper surface and investigated by the CIE Lab coordinate system. The blue color transformed into reddish (477 nm) when heated from 35°C to 65°C. Nanoparticles (NPs) of anthocyanidin/mordant (ACYD/M) were examined for their size and morphology to indicate diameters of 80-90 nm, whereas the ACYD/M-encapsulated alginate nanoparticles showed diameters of 120-150 nm. Multiple analytical techniques were utilized to examine the printed papers. The mechanical and rheological performance of both stamped sheets and ink fluid were explored. The cytotoxicity and antimicrobial efficacy of ink (ACYD@ALG) were investigated.

Recent Development of Photochromic Polymer Systems: Mechanism, Materials, and Applications

Photochromic polymer is defined as a series of materials based on photochromic units in polymer chains, which produces reversible color changes under irradiation with a particular wavelength. Currently, as the research progresses, it shows increasing potential applications in various fields, such as anti-counterfeiting, information storage, super-resolution imaging, and logic gates. However, there is a paucity of published reviews on the topic of photochromic polymers. Herein, this review discusses and summarizes the research progress and prospects of such materials, mainly summarizing the basic mechanisms, classification, and applications of azobenzene, spiropyran, and diarylethene photochromic polymers. Moreover, 3-dimensional (3D) printable photochromic polymers are worthy to be summarized specifically because of its innovative approach for practical application; meanwhile, the developing 3D printing technology has shown increasing potential opportunities for better applications. Finally, the current challenges and future directions of photochromic polymer materials are summarized.

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.

CsBr-Triggered Reversible Phase Transition of Perovskite Nanocrystals for Advanced Information Encryption and Decryption

Luminescent perovskite nanocrystals (NCs), possessing the advantages of low cost, easy detection, and excellent luminescence, are becoming more and more significant in the fields of information encryption and decryption. Most hydrochromic perovskite NCs for information encryption have moderate reversibility and are easily passively decrypted by water in the moist air, limiting their practical applications. Herein, a lyochromic material is synthesized based on reversible phase transition between luminescent CsPbBr3-HBr (pretreating CsPbBr3 with HBr) and nonluminescent Cs4PbBr6, exhibiting excellent reversibility in 50 cycles triggered by CsBr solution. HBr treatment boosts the ion migration of NCs via diminishing surface ligands and passivating Br vacancy, assisting CsBr concentration acting as a crucial factor in dynamic ion exchange equilibrium between the trigger solution and CsPbBr3-HBr. By utilizing CsPbBr3-HBr as a safety ink, the CsBr-triggered photoluminescence switch has been demonstrated to be reproducible, stable, and reliable for information encryption and decryption.

Probing the Alcoholysis Degree of Polyvinyl Alcohol by Synergistic Coordination-Regulated Fluorescence

Polyvinyl alcohol (PVA) with abundant hydroxyl groups (-OH) has been widely used for membranes, hydrogels, and films, and its function is largely affected by the alcoholysis degree. Therefore, the development of rapid and accurate methods for alcoholysis degree determination in PVAs is important. In this contribution, we have proposed a novel fluorescence-based platform for probing the alcoholysis degree of PVA by using the (E)-N-(4-methoxyphenyl)-1-(quinolin-2-yl)methanimine (QPM)-Zn2+ complex as the reporter. The mechanism study disclosed that the strong coordination between -OH and Zn2+ induced the capture of the QPM-Zn2+ complex and promoted its subsequent immobilization into the noncrystalline area. The immobilization of the QPM-Zn2+ complex restricted its molecular rotation and reduced the nonirradiative transition, thus yielding bright emissions. In addition, the practical applications of this proposed method were further validated by the accurate alcoholysis degree determination of blind PVA samples with the confirmation of the National Standard protocol. It is expected that the developed fluorescence approach in this work might become an admissive strategy for screening the alcoholysis degree of PVA.

Chameleon-like Photoluminescent Janus Nanoparticles as Full-Color Multicomponent Organic Nanoinks: Combination of Förster Resonance Energy Transfer and Photochromism for Encryption and Anticounterfeiting with Multilevel Authentication

Increasing the security by the multilevel authentication mechanism was the most significant challenge in recent years for the development of anticounterfeiting inks based on photoluminescent nanomaterials. For this purpose, the greatest strategy is the use of multicomponent organic materials and a combination of Förster resonance energy transfer (FRET) with the intelligent behavior of photochromic compounds like spiropyran. Here, the hydroxyl-functionalized polymer nanoparticles were synthesized by emulsion copolymerization of methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) in different compositions (0-30 wt % of HEMA). Results illustrated that the size of the nanoparticles changed from 64 to 204 nm, and a morphology evolution from spherical to Janus shape was observed by increasing the concentration of HEMA. Photoluminescent inks with red, green, and blue (RGB) fluorescence emissions were prepared by modification of nanoparticles containing 15 wt % of HEMA with spiropyran, fluorescein, and coumarin, respectively. To develop dual-color and multicolor photoluminescent inks that display static and dynamic emission, RGB latex samples were mixed together in different ratios and printed on cellulosic paper. Results display that the fluorescence emission of developed inks can be photoswitched between different statuses, including white to blue, green to blue, green to red/orange, purple to pink, and white to pink, utilizing the FRET phenomenon, photochromism, and a combination of both phenomena. Samples containing spiropyran displayed dynamic color changes in the emission to red, orange, and pink depending on the composition. Hence, developed dual-color and multicolor photoluminescent inks were used for printing of security tags and also painting of some hand-drawn artworks, which obtained results indicating high printability, maximum fluorescence intensity, high resolution, and fast responsivity upon UV-light irradiations of 254 nm (for static mode) and 365 nm (for dynamic mode). In addition, the multilevel authentication mechanism by a static emission under UV-light irradiation of 254 nm, a dynamic emission under UV-light irradiation of 365 nm, and photochromic color change was observed, resulting in increasing the security of developed inks. Actually, developed multicolor photoluminescent inks are the most efficient candidates for developing a new category of chameleon-like high-security anticounterfeiting inks that have tunable optical properties and complex multilevel authentication mechanisms.

Highly stretchable and self-healing photoswitchable supramolecular fluorescent polymers for underwater anti-counterfeiting

Thanks to the non-destructiveness and spatial-controllability of light, photoswitchable fluorescent polymers (PFPs) have been successfully applied in advanced anti-counterfeiting and information encryption. However, most of them are not suitable for use in harsh underwater environments, including high salinity seawater. In this study, by integrating photochromic molecules into a hydrophobic polymer matrix with the fluorine elastomer, including dipole-dipole interactions, we describe a class of novel photoswitchable supramolecular fluorescent polymers (PSFPs) that can adaptively change their fluorescence between none, green and red by the irradiation of different light. The PSFPs not only exhibited excellent photoswitchable properties, including fast photo-responsibility, prominent photo-reversibility, and photostability, but also exhibited some desired properties, including exceptional stretchability, hydrophobicity, antifouling, self-healing ability, simple preparation process, and processability. We thus demonstrated their applications in underwater data encryption and anti-counterfeiting labels.

Recyclable soft photonic crystal film with overall improved circularly polarized luminescence

Existing circularly polarized luminescence materials can hardly satisfy the requirements of both large luminescence dissymmetry factor and high luminescent quantum yield, which hinders their practical applications. Here, we present a soft photonic crystal film embedded with chiral nanopores that possesses excellent circularly polarized luminescence performance with a high luminescence dissymmetry factor as well as a large luminescent quantum yield when loaded with various luminescent dyes. Benefitting from the retention of chiral nanopores imprinted from a chiral liquid crystal arrangement, the chiral soft photonic crystal film can not only endow dyes with chiral properties, but also effectively avoid severe aggregation of guest dye molecules. More importantly, the soft photonic crystal film can be recycled many times by loading and eluting guest dye molecules while retaining good stability as well as circularly polarized luminescence performance, enabling various applications, including smart windows, multi-color circularly polarized luminescence and anticounterfeiting.

Photoresponsive Diarylethene-Containing Polymers: Recent Advances and Future Challenges

Photoresponsive polymers have attracted increasing interest owing to their potential applications in anticounterfeiting, information encryption, adhesives, etc. Among them, diarylethene (DAE)-containing polymers are one of the most promising photoresponsive polymers and have unique thermal stability and fatigue resistance compared to azobenzene- and spiropyran-containing polymers. Herein, the design of DAE-containing polymers based on different types of structures, including main chain polymers, side-chain polymers, and crosslinked polymers, is introduced. The mechanism and applications of DAE-containing polymers in anti-counterfeiting, information encryption, light-controllable adhesives, and photoinduced healable materials are reviewed. In addition, the remaining challenges of DAE-containing polymers are also discussed.

Photochromic Tungsten Oxide Quantum Dots-based Fluorescent Photoswitches towards Dual-mode Anti-counterfeiting Application

Development of new anti-counterfeiting technology to increase the difficulty of imitation and decoding is becoming increasingly important, but still remains challenging yet. In this work, we report the design of new fluorescence photoswitches based on photochromic tungsten oxide quantum dots (WO₃ QDs) for dual-mode anti-counterfeiting applications. Complexing photochromic WO₃ QDs with fluorescent gold nanoclusters (AuNCs) enables the construction of a photoswitchable fluorescence system (WO₃-AuNCs) based on fluorescence resonance energy transfer. Detailed spectral and photophysical characterization showed that WO₃ QDs well-retain the photochromic properties within the WO₃-AuNCs composite. Importantly, photoresponsive and highly reversible switching of both color and fluorescence signals was successfully achieved by simply alternating the irradiation with UV and visible light. Potential utility of photoswitchable WO₃-AuNCs composite as novel dual-mode anti-counterfeiting materials has been successfully demonstrated, including photoswitchable ink, rewritable paper and number encryption. Compared with other anti-counterfeiting materials, the present photochromic WO₃ QDs-based fluorescent switches are easily synthesized and handled, and they can provide dual security mode (color and fluorescence). This work provides a generable WO₃ QDs-assisted strategy of fabricating advanced fluorescence photoswitches for versatile optical counterfeiting applications.