Dual-Color Photoluminescent Functionalized Nanoparticles for Static-Dynamic Anticounterfeiting and Encryption: First Collaboration of Spiropyran and Coumarin

β-cyclodextrin/spiropyran-functionalized optical-driven hydrogel film for bisphenol A detection in food packaging

Bisphenol A (BPA), an endocrine disruptor, is widely used in food packaging materials, including drink containers. Sensitive detection of BPA is crucial to food safety. Herein, we have developed a novel optical-driven hydrogel film sensor for sensitive BPA detection based on the displacement of spiropyran (SP) from β-cyclodextrin (β-CD) cavity by BPA followed by the photochromism of the released SP. The released SP converts to the ring-opened merocyanine form which shows an enhanced red fluorescence in the dark. The sensor demonstrates a linear detection range from 0.1 to 20 μg mL-1 with a limit of detection at 0.027 μg mL-1 and a limit of quantification at 0.089 μg mL-1. Notably, the proposed β-CD/SP hydrogel can be reused due to the reversible isomerization of SP and the reversible host-guest interaction. This sensor also shows good performance for BPA determination in real samples, indicating its great potential for food safety monitoring.

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.

Cholesterol-substituted spiropyran: Photochromism, thermochromism, mechanochromism and its application in time-resolved information encryption

Organic multi-stimulus-responsive materials are widely used in anti-counterfeiting and information encryption due to their unique response characteristics and designability. However, progress in obtaining multi-stimulus-responsive smart materials has been very slow. Herein, a spiropyran derivative is constructed, which shows photochromic, thermochromic and mechanical photochromic properties, and has reversible absorption/luminescence adjustment ability. By introducing non-covalent interactions such as van der Waals force and hydrogen bond, this new molecule is more sensitive to external stimuli and exhibits better photochromic, mechanochromic and thermochromic properties with rapid speed and high contrast. Furthermore, these three stimulus responses can be completely restored to the initial state under white light irradiation. The reversible multiple response characteristics of this molecule make it possible to provide dynamic anti-counterfeiting and advanced information encryption capabilities. To demonstrate its application in advanced information encryption, powders treated with different stimuli are combined with fluorescent dyes to encrypt complex digital information. This work puts forward a new time-resolved encryption strategy, which provides important guidance for the development of time-resolved information security materials.

Zero-dimensional halide hybrid bulk glass exhibiting reversible photochromic ultralong phosphorescence

Dynamically responsive materials, capable of reversible changes in color appearance and/or photoemission upon external stimuli, have attracted substantial attention across various fields. This study presents an effective approach wherein switchable modulation of photochromism and ultralong phosphorescence can be achieved simultaneously in a zero-dimensional organic-inorganic halide hybrid glass doped with 4,4´-bipyridine. The facile fabrication of large-scale glasses is accomplished through a combined grinding-melting-quenching process. The persistent luminescence can be regulated through the photochromic switch induced by photo-generated radicals. Furthermore, the incorporation of the aggregation-induced chirality effect generates intriguing circularly polarized luminescence, with an optical dissymmetry factor (glum) reaching the order of 10-2. Exploiting the dynamic ultralong phosphorescence, this work further achieves promising applications, such as three-dimensional optical storage, rewritable photo-patterning, and multi-mode anti-counterfeiting with ease. Therefore, this study introduces a smart hybrid glass platform as a new photo-responsive switchable system, offering versatility for a wide array of photonic applications.

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.

Active control of polariton-enabled long-range energy transfer

Optical control is achieved on the excited state energy transfer between spatially separated donor and acceptor molecules, both coupled to the same optical mode of a cavity. The energy transfer occurs through the formed hybrid polaritons and can be switched on and off by means of ultraviolet and visible light. The control mechanism relies on a photochromic component used as donor, whose absorption and emission properties can be varied reversibly through light irradiation, whereas in-cavity hybridization with acceptors through polariton states enables a 6-fold enhancement of acceptor/donor contribution to the emission intensity with respect to a reference multilayer. These results pave the way for synthesizing effective gating systems for the transport of energy by light, relevant for light-harvesting and light-emitting devices, and for photovoltaic cells.

Multicolor Photochemical Printing Inside Polymer Matrices for Advanced Photonic Anticounterfeiting

Conventional security inks, generally directly printed on the data page surface, are vulnerable to counterfeiters, thereby raising the risk of chemical structural deciphering. In fact, polymer film-based data pages with customized patterns embedded within polymer matrix, rather than printed on the surface, emerge as a promising solution. Therefore, the key lies in developing fluorophores offering light dose-controlled fluorescent color inside polymer matrices. Though conventional fluorophores often suffer from photobleaching and uncontrolled photoreactions, disqualifying them for this purpose. Herein a diphenanthridinylfumaronitrile-based phototransformers (trans-D5) that undergoes photoisomerization and subsequent photocyclization during photopolymerization of the precursor, successively producing cis- and cyclo-D5 with stepwise redshifted solid-state emissions is developed. The resulting cyclo-D5 exhibits up to 172 nm emission redshift in rigidifying polymer matrices, while trans-D5 experiences a slightly blueshifted emission (≈28 nm), cis-D5 undergoes a modest redshift (≈14 nm). The markedly different rigidochromic behaviors of three D5 molecules within polymer matrices enable multicolor photochemical printing with a broad hue ranging from 38 to 10 via an anticlockwise direction in Munsell color space, yielding indecipherable fluorescent patterns in polymer films. This work provides a new method for document protection and implements advanced security features that are unattainable with conventional inks.

Dual-state emission, mechanofluorochromism, and lipid droplet imaging of asymmetric D-π-A-D'-type triads

Dual-state emission (DSE) is an emerging phenomenon wherein organic luminescent molecules display bright emissions in both molecularly isolated and packed states, addressing the challenge associated with the traditional paradigm of dyes with mono-state emission. This study presents the design and synthesis of two unsymmetrical triads, TPCA and TPCT, featuring a D-π-A-D' electronic structure by integrating phenothiazines, triphenylamines, and cyanostilbene. Photophysical assessments reveal that both molecules serve as robust DSEgens, exhibiting strong emissions in both solution and solid phases. TPCA displays ΦTHF 53.2% and Φsolids 43.2%, while TPCT exhibits ΦTHF 49.6% and Φsolids 37.5%. However, due to differences in molecular conformation and packing, they diverge in solid-state emission wavelengths and mechanofluorochromic behavior. In the solid state, TPCA emits strong red fluorescence, contrasting with TPCT, which emits orange fluorescence. Furthermore, TPCA demonstrates significant mechanofluorochromism (MFC), shifting from yellow to yellow-red upon mechanical grinding, while TPCT exhibits negligible MFC owing to conformational distinctions. As robust and low-toxic bioimaging agents, both TPCA and TPCT prove highly effective for lipid-droplet imaging studies. This research contributes valuable insights to the evolving field of DSE materials, elucidating the promising applications and mechanisms governing their versatile emission behaviors.

A viologen-derived luminescent material exhibiting photochromism, photocontrolled luminescence and selective detection of Cr2O72- in aqueous solution

Stable viologen-derived multifunctional smart materials exhibit widespread practical applications in many areas. In this study, a viologen-derived material with 4-fold interpenetrating diamondoid network, {[Cd(1,4-ndc)(cpbpy)]·2H2O}n, was successfully constructed based on asymmetrical N-(3-carboxyphenyl)-4,4'-bipyridinium (cpbpy) and 1,4-naphthalenedicarboxylic acid (1,4-H2ndc). The compound shows reversible photochromic behavior under a xenon lamp, which are proved by UV-vis spectra and EPR characterizations. Moreover, the compound with good photoluminescence properties displays photocontrolled luminescence quenching behaviors. Owing to its good water stability, the compound is then applied in luminescence sensing for the detection of Cr2O72- in aqueous solution. The corresponding luminescence quenching constant for Cr2O72- is KSV = 4.33 × 104 M-1, and the detection limit is 3.66 μM. Systematic investigations on the luminescence quenching mechanism suggest that the inner filter effect resulted in the selective detection of Cr2O72-. This study provides inspiration for the design and synthesis of target luminescent crystalline materials with rigid and asymmetric viologen-derived ligands.

Main-chain Macromolecular Hydrazone Photoswitches

Hydrazones-consisting of a dynamic imine bond and an acidic NH proton-have recently emerged as versatile photoswitches underpinned by their ability to form thermally bistable isomers, (Z) and (E), respectively. Herein, we introduce two photoresponsive homopolymers containing structurally different hydrazones as main-chain repeating units, synthesized via head-to-tail Acyclic Diene METathesis (ADMET) polymerization. Their key difference lies in the hydrazone design, specifically the location of the aliphatic arm connecting the rotor of the hydrazone photoswitch to the aliphatic polymer backbone. Critically, we demonstrate that their main photoresponsive property, i.e., their hydrodynamic volume, changes in opposite directions upon photoisomerization (λ=410 nm) in dilute solution. Further, the polymers-independent of the design of the individual hydrazone monomer-feature a photoswitchable glass transition temperature (Tg ) by close to 10 °C. The herein established design strategy allows to photochemically manipulate macromolecular properties by simple structural changes.

Highly bright solid-state carbon dots for efficient anticounterfeiting

Carbon dots (C-dots) as promising fluorescent materials have attracted much attention because of their low toxicity and excellent optoelectronic properties. However, the aggregation-caused quenching (ACQ) of the solid-state C-dots has limited their potential applications in anti-counterfeiting and optoelectronic devices. In this work, C-dot powder was prepared by directly dispersing the as-prepared C-dots in a polymer matrix or in situ formation of the C-dot/Ca-complex by vacuum heating in the presence of boric acid. The as-prepared C-dots have high quantum yields (QYs) in the range of 40-67% with temperature-dependent photoluminescent (PL) properties. As a proof of concept, the as-synthesized C-dots were used to produce a flexible anti-counterfeiting code and showed high-level security. This highlights the potential of C-dots in solid-state information, anti-information encryption and anti-counterfeiting.

Electrospun glass nanofibers to strengthen polycarbonate plastic glass toward photoluminescent smart materials

Electrospun glass nanofibers (GNFs) were used to strengthen polycarbonate (PC) to create long-persistent photoluminescent and fluorescent smart materials such as afterglow concrete and smart window. Physical integration of lanthanide-activated aluminate (LA) nanoparticles (NPs) yielded transparent GNFs@PC smart sheets. Spectral investigations utilizing photoluminescence and CIE Lab parameters were performed to confirm that the translucent appearance of GNFs@PC changed to green when exposed to UV light. This fluorescence activity was quickly reversible for the GNFs@PC hybrids with low concentrations of LANPs, which indicate fluorescence emission. Higher phosphor concentrations in GNFs@PC led to longer-lasting afterglow photoluminescence and slower reversibility. The GNFs@PC hybrids showed an emission band detected at 518 nm upon excitation at 368 nm. The morphological characteristics of LANPs and GNFs were analyzed by transmission electron microscopy (TEM), which revealed sizes of 11-26 nm and 250-300 nm, respectively. GNFs were prepared using electrospinning technology and then used as a roughening agent into PC sheets. Morphological characteristics of GNFs and GNFs@PC smart sheets were examined using energy-dispersive X-ray spectroscopy (EDXA), X-ray fluorescence (XRF) and scanning electron microscopy (SEM). The GNFs@PC smart sheets were shown to have enhanced scratch resistance in comparison to LANPs-free PC control sample. Increases in LANPs concentration enhanced both hydrophobicity and UV protection.

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.

New cationic spiropyrans with photoswitchable NIR fluorescence

Visible-light-mediated photochromic compounds with NIR absorption and fluorescence are of great interest for use in different biomedical applications. In this work, new representatives of spiropyrans with conjugated cationic 3H-indolium substituents in different positions of 2H-chromene moiety were synthesized. The electron-donating methoxy groups were introduced in the uncharged indoline and charged indolium cycles to form the effective conjugation chain between the hetarene moiety and the cationic fragment for reaching NIR absorption and fluorescence. The molecular structure and the effects of cationic fragment position on the mutual stability of the spirocyclic and merocyanine forms of compounds were carefully studied in the solutions and solid state by NMR, IR, HRMS, single-crystal XRD, and quantum chemical calculations. It was found that the obtained spiropyrans demonstrate positive or negative photochromism depending on the cationic fragment's position. One of spiropyrans has shown bidirectional photochromic properties induced exclusively by visible light of different wavelengths in both directions. The photoinduced merocyanine forms of compounds possessed far-red shifted absorption maxima and NIR fluorescence, which makes them prospective fluorescent probes for bioimaging.

UV Irradiation-Induced Cyclic-to-Linear Topological Changes of a Light/pH Dual-Sensitive Cyclic Copolymer for Enhanced Drug Delivery

Cyclic polymers with cleavable backbones triggered by either external or internal stimuli can realize simultaneous extracellular stability and intracellular destabilization of cyclic polymer-based nanocarriers but remain seldom reported. To this end, we prepared herein cyclic-ONB-P(OEGMA-st-DMAEMA) (c-ONB-P(OEGMA-st-DMAEMA)) with a light-cleavable junction in the polymer backbone based on oligo (ethylene glycol) monomethyl ether methacrylate (OEGMA) and N,N-dimethylaminoethyl methacrylate (DMAEMA) using a light-cleavable atom transfer radical polymerization (ATRP) initiator containing an o-nitrobenzyl (ONB) ester group. Together with the pH-sensitivity of DMAEMA, c-ONB-P(OEGMA-st-DMAEMA) shows a light-cleavable mainchain and pH-sensitive side chains. Notably, doxorubicin (DOX)-loaded c-ONB-P(OEGMA4-st-DMAEMA38) (C2) micelles mediated an IC50 value of 2.28 μg/mL in Bel-7402 cells, which is 1.7-fold lower than that acquired without UV irradiation. This study thus reported the synthesis of a cyclic copolymer with a UV-cleavable backbone and uncovered the effects of topological modulation on the in vitro controlled release properties of cyclic polymers.