Tunable Photochromism of Spirooxazine in the Solid State: A New Design Strategy Based on the Hypochromic Effect

Flexible Organic Chiral Crystals with Thermal and Excitation Modulation of the Emission for Information Transmission, Writing, and Storage

Organic single crystals quickly emerge as dense yet light and nearly defect-free media for emissive elements. Integration of functionalities and control over the emissive properties is currently being explored for a wide range of these materials to benchmark their performance against organic emissive materials diluted in powders or films. Here, we report mechanically flexible emissive chiral organic crystals capable of an unprecedented combination of fast, reversible, and low-fatigue responses. UV-excited single crystals of both enantiomers of the material, 4-chloro-2-(((1-phenylidene)imino)methyl)phenol, exhibit a drastic yet reversible change in the emission color from green to orange-yellow within a second and can be cycled at least 2000 times. The photoresponse was found to depend strongly on the excitation intensity and temperature. Combining chirality, mechanical compliance, rapid emission switching, multiple responses, and writability by UV light, this material provides a unique and versatile platform for developing organic crystal-based materials for on-demand signal transfer, information storage, and cryptography.

Photo-Controllable Ultralong Room-Temperature Phosphorescence: State of the Art

In this concept, we showcase the upsurge in the studies of dynamic ultralong room-temperature phosphorescence (RTP) materials containing inorganic and/or organic components as versatile photo-responsive platforms. The goal is to provide a comprehensive analysis of photo-controllable RTP, and meanwhile delve into the underlying RTP properties of various classes of photochromic materials including metal-organic complexes, organic-inorganic co-crystals, purely organic small molecules and organic polymers. In particular, the design principles governing the integration of the photochromic and RTP moieties within a single material system, and the tuning of dynamic RTP in response to light are emphasized. As such, this concept sheds light on the challenges and opportunities of using these tunable RTP materials for potential applications in optoelectronics, particularly highlighting their use of reversible information encryption, erasable light printing and rewritable smart paper.

Light and Heat-Driven Flexible Solid Supramolecular Polymer Displaying Phosphorescence and Reversible Photochromism

Herein, a type of light- and heat-driven flexible supramolecular polymer with reversibly long-lived phosphorescence and photochromism is constructed from acrylamide copolymers with 4-phenylpyridinium derivatives containing a cyano group (P-CN, P-oM, P-mM), sulfobutylether-β-cyclodextrin (SBCD), and polyvinyl alcohol (PVA). Compared to their parent solid polymers, these flexible supramolecules based on the non-covalent cross-linking of copolymers, SBCD, and PVA efficiently boost the phosphorescence lifetimes (723.0 ms for P-CN, 623.0 ms for P-oM, 945.8 ms for P-mM) through electrostatic interaction and hydrogen bonds. The phosphorescence intensity/lifetime, showing excellent responsiveness to light and heat, sharply decreased after irradiation with a 275 nm flashlight or sunlight and gradually recovered through heating. This is accompanied by the occurrence and fading of visible photochromism, manifesting as dark green for P-CN and pink for P-oM and P-mM. These reversible photochromism and phosphorescence behaviors are mainly attributed to the generation and disappearance of organic radicals in the 4-phenylpyridinium derivatives with a cyano group, which can guide tunable luminescence and photochromism.

Preparation of polylactic acid reinforced with cellulose nanofibers toward photochromic self-healing adhesive for anti-counterfeiting applications

There are a number of drawbacks with photochromic adhesives, including their poor durability, high price tag, and lackluster performance. On the other hand, self-healable adhesives have shown to be durable and robust than conventional alternatives. Hydrogel adhesives that change color in response to ultraviolet light were created for usage in self-healable authenticating stamps. In this context, a combination of cellulose nanofibers (CNFs), polylactic acid (PLA) and nanoparticles of lanthanide aluminate (NLA) were prepared to generate an organic-inorganic hybrid hydrogel adhesive with self-healing properties. NLA agglomerates were avoided due to the use of CNFs as a nanofiller and dispersion agent. Colorless stamps require that NLA to be dispersed consistently in the CNFs/PLA hydrogel without clumping. This film becomes green when irradiated with ultraviolet, as indicated by luminescence spectra and CIE Lab coordinates. When illuminated at 365 nm, the paper sheets emitted light with a wavelength of 519 nm. The morphologies of prints were analyzed by different analytical methods. Diameter measurements from a transmission electron microscope (TEM) of the synthesized NLA ranged from 5 to 9 nm, whereas CNFs displayed diameters of 40-60 nm. The current NLA@CNFs/PLA hydrogel presents a reliable anti-counterfeiting solution for various authenticating products.

Photoswitchable Photochromic Chelating Spironaphthoxazines: Synthesis, Photophysical Properties, Quantum-Chemical Calculations, and Complexation Ability

The stable and efficient photochromic and photoswitchable molecular systems designed from spirooxazines are of increasing scientific and practical interest because of their present and future applications in advanced technologies. Among these compounds, chelating spironaphthoxazines have received widespread attention due to their efficient optical response after complexation with some metal ions being of biomedical interest and environmental importance, as well as their good cycle performance and high reliability, especially by metal ion sensing. In this mini-review, we summarize our results in the design of novel photoswitchable chelating spironaphthoxazines with specific substituents in their naphthoxazine or indoline ring systems in view of recent progress in the development of such molecular systems and their applications as metal ion sensors. The design, synthesis methods, and photoresponse of such spirooxazine derivatives relevant to their applications, as well as quantum-chemical calculations for these compounds, are presented. Examples of various design concepts are discussed, such as sulfobutyl, hydroxyl, benzothiazolyl, or ester and carboxylic acid as substituents in the chelating spironaphthoxazine molecules. Further developments and improvements of this interesting and promising kind of molecular photoswitches are outlined.

Polyoxometalate/s-triazine hybrid heterostructures with ultrafast photochromic properties

As an emerging class of hybrid complexes, donor-acceptor (D-A) hybrid heterostructures, which combine the advantages of both organic and inorganic photoactive components, provide excellent platforms for the fabrication of photochromic materials with enhanced photo-responsive performances. Herein, four novel hybrid heterostructures, namely H3TPT·(PW12O40)·2NMP (1), (H1.5TPT)2·(PW12O40) (2), (H3TPT)2·(SiW12O40)·2Cl·2MeCN (3), and H3TPT·(HPMo12O40)·Cl·3NMP (4) (TPT is tri(4-pyridyl)-s-triazine, NMP is N-methylpyrrolidone), have been synthesized and characterized. Benefitting from the strong interactions (anion-π interactions) and matching electron energy levels between the donors and acceptors, some of them exhibited ultrafast photochromic behaviour even up to 1 second. Furthermore, based on experimental and theoretical calculations, the plausible PIET process and structure-activity relationship have been discussed in detail.

Customizable High-Contrast Optical Responses: Dual Photosensitive Colors for Smart Textiles

Smart textiles demonstrating optical responses to external light stimuli hold great promise as functional materials with a wide range of applications in personalized decoration and information visualization. The incorporation of high-contrast, vivid, and real-time optical signals, such as color change or fluorescence emission, to indicate light on/off states is both crucial and challenging. In this study, we have developed a dual output photosensitive dye system possessing photochromic and photofluorescent properties, which was successfully applied to the dyeing and finishing processes of cotton fabrics. The design and fabrication of this dye system were based on the unique photoinduced proton transfer (PPT) principle exhibited by the water-soluble spiropyran (trans-MCH) molecule. The dual output response relies on the open-/closed-loop mechanism, wherein light regulates the trans-MCH molecule. Upon excitation by UV or visible light, the dye system and dyed fabrics display significant color changes and fluorescence switching in a real-time and highly reversible manner. Moreover, diverse photosensitive color systems can be tailored by direct blending with commercially available water-soluble dyes. By integrating high-contrast dual optical outputs into this scalable, versatile, and reversible dye system, we envisage the development and design of smart textiles capable of producing high-end products.

Nitrogen Atom Induced Contrast Effect on the Mechanofluorochromic Characteristics of Anthracene-Based Acceptor-Donor-Acceptor Fluorescent Molecules

The mechanofluorochromic (MFC) characteristics of anthracene-based acceptor-donor-acceptor (A-D-A) fluorescent molecules are explored through a comprehensive investigation of their photophysical behaviors. Six 9,10-diheteroarylanthracene derivatives with varying acceptor groups (pyridin-4-yl, pyridin-3-yl, pyridin-2-yl, pyrimidin-5-yl, pyrazinyl and quinoxalinyl) are synthesized and systematically characterized. The photophysical properties in both solution and solid-state are examined, revealing subtle yet significant influences of the spatial arrangement and number of nitrogen atoms within the acceptor group on fluorescence emission. Single-crystal structures of these compounds provide insights into their steric configurations and intermolecular packing modes, offering valuable insights into the fundamental mechanisms that underlie the observed MFC properties. This study illuminates the intricate interplay between MFC properties and the refined molecular structure, thus presenting promising avenues for the design and advancement of novel MFC materials.

Efficient manipulation of Förster resonance energy transfer through host-guest interaction enables tunable white-light emission and devices in heterotopic bisnanohoops

In this study, we synthesized and reported the heterotopic bisnanohoops P5-[8,10]CPPs containing cycloparaphenylenes (CPPs) and a pillar[5]arene unit, which act not only as energy donors but also as a host for binding energy acceptors. We demonstrated that a series of elegant FRET systems could be constructed successfully through self-assembly between donors P5-[8,10]CPPs and acceptors with different emissions via host-guest interaction. These FRET systems further allow us to finely adjust the donors P5-[8,10]CPPs and acceptors (BODIPY-Br and Rh-Br) for achieving multiple color-tunable emissions, particularly white-light emission. More importantly, these host-guest complexes were successfully utilized in the fabrication of white-light fluorescent films and further integrated with a 365 nm LED lamp to create white LED devices. The findings highlight a new application of carbon nanorings in white-light emission materials, beyond the common recognition of π-conjugated molecules.

Smart photochromic materials based on polylactic acid

The smart photochromic materials based on polylactic acid (PLA) were prepared by melt-blending and hot-pressing, in which photochromic microcapsules (PM) were used as a functional additive, and poly(vinyl acetate) (PVAc) was introduced into the photochromic PLA blends for the first time to improve their properties. The crystallization and melting behavior, morphology, and photochromic performance of PLA/PVAc/PM blends were characterized by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and spectrophotometer, respectively. The results showed that PVAc significantly improved the photochromic properties of PLA/PM blends. Under 30s UV irradiation, the blends reached a value of ΔE that could be recognized in 3 s by human eyes. This discriminative ΔE value could be maintained for at least 3 min after removal from UV irradiation. Meanwhile, the blend had outstanding photochromic durability and recyclability. Compared to ΔE for 0.5 h of continuous light irradiation, ΔE for 8 h of continuous light irradiation decreased by only about 1, to 14.1. In 25 cycles of 3 s UV irradiation, the values of ΔE for the first and 25th irradiation were 11.4 and 11.6, respectively. The blend showed different photochromic responses to different light intensities. The ΔE values of 8.6, 14.6, 14.6, and 18.4 for irradiation at 600, 800, 1000, and 1200 W/m² of solar intensity, respectively.