Reversible Photochromic System Based on Rhodamine B Salicylaldehyde Hydrazone Metal Complex

A Nonlinear Photochromic Reaction Based on Sensitizer-Free Triplet-Triplet Annihilation in a Perylene-Substituted Rhodamine Spirolactam

Nonlinear photochromic reactions that work with weak incoherent light are important for molecular operations with high spatial resolution and multiple photofunctions based on single molecules. However, nonlinear photochromic compounds generally require complex molecular design, restricting accessibility in various fields. Herein, we report nonlinear photochromic properties in a perylene-substituted rhodamine spirolactam derivative (Rh-Pe), which is synthesized from rhodamine B in facile procedures. Direct excitation of Rh-Pe produces the triplet excited state via the charge-transfer (CT) state. The triplet excited state causes triplet-triplet annihilation to bring the generation of the intensely colored ring-open form with nonlinear behavior. Furthermore, green- and red-light-induced photochromism was achieved in Rh-Pe using triplet sensitizers, although Rh-Pe can be directly excited only by ultraviolet and blue light. Our findings are expected to contribute to the development of photofunctional materials showing nonlinear behavior and low-energy light responsivity.

A Multimode Optical Sensor for Selective and Sensitive Detection of Harmful Heavy Metal Cr(VI) in Fresh Water and Sea Water

Water pollution originating from heavy metals has shown great impacts on the ecological environment and human health due to their extremely low biodegradability. Hexavalent chromium Cr(VI), as one harmful heavy metal with strong oxidation, high biological permeability, and high carcinogenicity, is becoming an increasingly serious threat to human health. Therefore, conveniently but accurately, monitoring the Cr(VI) level in water to maintain its normal level and ensuring the stability of the ecosystem and human health become very valuable. However, most of these heavy metal sensors reported are turn-off type single-emission sensors. In this work, a ratiometric fluorescence/colorimetry/smartphone triple-mode turn-on optical sensor for Cr(VI) was developed based on a multifunctional metal-organic framework platform. The detection limits for these three mutual verification modes were only 1.28, 4.89, and 68.4 nM, respectively. Additionally, the color changes of the detection system under sunlight can also be observed directly by the naked eye. The accuracy and practicability of this multimode sensor were further proved by the detection of Cr(VI) in actual water and seawater samples, and the recovery rate ranged from 97.308 to 104.041%.

Suppressive Photochromism and Promotive Mechanochromism of Rhodamine Mechanophore by the Strategy of Poly(methyl acrylate)/Polyurethane Interpenetrating Polymer Network

As molecular design and the structure-property relationships of photochemical molecules established in the literature serve as a convenient reference for mechanophore exploration, many typical mechanophores suffer undesired responses to UV light or even sunlight in bulk polymers. We developed a strategy of a poly(methyl acrylate)/polyurethane (PMA/PU) interpenetrating polymer network (IPN) to suppress the photochromic property of the mechanophore and promote its mechanochromic property. A widely used rhodamine mechanophore (Rh-2OH) was first incorporated into polyurethane (P1). Then P1 was swollen in methyl acrylate and photopolymerized to prepare a PMA2.8/PU IPN (P2). Different from photo/force-responsive P1, P2 selectively responded to force because the low free volume in IPN greatly hinders photoisomerization of the rhodamine spirolactam, suggesting that a simple IPN strategy successfully resolves the giant problem of nonselective response to photo/force for photochromic mechanophores. Moreover, PMA/PU IPN enhanced the mechanical property, resulting in a higher mechanochemical activation ratio than PU, and the prestretching effect of PMA/PU IPN promoted the force sensitivity of rhodamine mechanophores significantly. We believe that the strategy can be applied to other mechanophores, promoting their application in more complicated environments.

The photodynamic approach to the molecular-level origin of metal-guided photochromism and ultrafast absorption spectroscopy

Metal-guided photochromic material (photochromic complex) is one of the latest versions of photo-responsive materials due to their smart behaviour and promising real-world applications. The present work explores the molecular-level origin of metal-guided photochromism using a photodynamic approach and ultrafast absorption spectroscopy, to address all existing lacunas. Here, rhodamine B (RhB) dye containing the Schiff base zinc complex is considered a representative photochromic complex for both theoretical treatment and experimental observations. Detailed theoretical studies, including geometry optimization, frontier molecular orbital (FMO) analysis, transition state (TS) identification, and natural bond orbital (NBO) analysis, along with spectral studies, are employed to investigate the photodynamic equilibrium (enol-form keto-form). This equilibrium is regulated by the interplay of intrinsic factors (push-pull effect) and extrinsic factors (such as UV-light, the phenolic-OH group, metal ions, and solvents). The potential energy surface (PES) of the photo-conversion (enol →enol*→keto*→ meta-stable keto) is evaluated. While, the PES of the reversion (meta-stable keto →enol) is constructed based on the studies of thermo-reversion and photo-reversion. Finally, the theoretical findings related to the photodynamic equilibrium are validated by direct experimental evidence obtained through femtosecond transient absorption (fs-TA) spectroscopy.

Mechanochemical Duff Reaction in Solid Phase for Easy Access to Mono- and Di-formyl Electron-Rich Arenes

A sustainable alternative to the century-old Duff reaction was developed by adopting a solid-phase mechanochemical route. A series of mono-formyl electron-rich arenes were prepared in high yields in silica as the solid reaction media using a combination of hexamethylenetetramine (HMTA) as the formyl source and a small amount of H2SO4 in a mixer mill. The use of toxic, costly, and low-boiling trifluoroacetic acid was avoided in the new mold of the mechanochemical Duff reaction. The mono-formyl phenols were obtained with exclusive ortho-selectivity, whereas unprecedented para-formylation was observed for other electron-rich aromatics. By controlling the stoichiometry of HMTA, the method offers easy access to di-formylated phenols as well. The scalability of the reaction was validated with selected substrates at the gram-scale level. In a case study, a mechanochemical tandem reaction was explored in the synthesis of a rhodol derivative. The solvent-free, metal-free mild method of formylation, with the absence of tedious work-up steps and shorter reaction times using an inexpensive mineral acid, is a sustainable alternative to the available methods for aromatic formylation.

Fabricating a photochromic benzonitrile Schiff base into a low-cost reusable paper-based wearable sensor for naked-eye dosimetry of UV radiations

We report in this study a photochromic benzonitrile Schiff base, (E)-4-((2-hydroxy-4-methoxybenzylidene)amino)benzonitrile (HMBAB). The molecular design, synthesis, aggregation-induced emission (AIE) as well as the quantum chemical calculations were outlined. In particular, HMBAB would undergo a reversible tautomerism in response to UV exposure, exhibiting remarkable changes in both absorption and emission: the compound shows yellow color and green-yellow luminescence; after UV exposure, the changes into orange-red while the luminescence is dramatically quenched, accompanied by a large bathochromic-shift. In addition, the photochromic state can be fully recovered via thermal treatment. Such reversible dual-channel photochromism was investigated using UV-vis reflectance spectroscopy and colorimeter, wherein a gradient change with time and a high fatigue resistance in cycle use was recorded. The photochromism is quantified by well-established RGB and Lab color space, in which the color change can be accurately analyzed by the chromatic aberration (ΔE*_Lab). Sensitivity test gives a two-stage linear relation between ΔE*_Lab and UV intensity, by which a limit of detection (LOD) as low as 67 μW/cm² is obtained. HMBAB was further fabricated into a paper-based wearable sensor, capable of being integrated into a chest card or a bracelet. It exhibits various degrees of color change in different sunlight environments, which can be readily observed by naked eyes, providing an early warning for high-dose UV radiations.

A Cu2+ triggered reversible photochromic system: the structure photochromic response relationship study and potential applications

Fifteen rhodamine B hydrazide hydrazone (RhBHH) derivatives (compounds a-o) with various substituent groups at different position and their photochromic property triggered by Cu²⁺ were studied to illustrate the structure photochromic response relationship (SPRR). Three of them (compounds f-h) with a para-position hydroxyl group and two meta-position halogen substituents display Cu²⁺-triggered photochromic which is significantly different from the previous reports. It was found that halogen atoms, which were generally considered to have no remarkable regulation effect, exhibited great influences on the photochromic behaviour of RhBHH derivatives. Detail photochromic properties of the developed photochromic system were revealed by using compound g as the model substrate, and only Cu²⁺ displayed high selective trigger effect. Good reversible photochromic phenomenon was observed after stimulated with visible light irradiation and dark (or heat) bleaching consecutively. Furthermore, this photochromic system could be used in the preparation of photochromic glass, special security ink, molecular logic gate and two-dimensional code for security information storage.

Influence of ortho group in rhodamine B hydrazide based Schiff base for selective recognition of Cu2+ and Fe3+ ions: A mechanistic approach by DFT and colorimetric studies

Herein we have implemented a computational approach in designing sensor molecules for the selective recognition of Cu²⁺ and Fe³⁺ ions. Seven rhodamine B hydrazide-based Schiff base derivatives were designed and analysed their chemosensing properties against Cu²⁺ and Fe³⁺ ions in ethanol solution theoretically. The theoretical calculations revealed that the selective recognition of Cu²⁺ and Fe³⁺ ions takes place via spirolactam ring-opening and there is a pivotal role of ortho substituents and N-heteroatoms. The two best chemosensors were synthesised and used for the detection of Cu²⁺ and Fe³⁺ ions by colorimetric methods.

Scalable Production of Self-Toughening Plant Oil-Based Polyurethane Elastomers with Multistimuli-Responsive Functionalities

Plant oils are becoming of high industrial importance due to the persisting challenges befalling with the utilization of fossil fuels. Thus, developing methodologies to produce multifunctional materials by taking advantage of the unique structure of plant oil is highly desired. In this study, castor oil served as a cross-linker and soft segments, by incorporating scalable rhodamine 6G derivatives, to systematically synthesize a series of smart polymers that possess self-toughening and multistimuli-responsive capabilities. The polyurethane elastomers showed 10 times and 60 times increases in tensile strength and toughness, respectively, in comparison with the unmodified polyurethane due to the existence of large amounts of hydrogen bonding, dynamic C-N spiro bonds, rigid benzene ring, and high cross-link densities. The novel polyurethane elastomers exhibited excellent reversible multichromic behaviors in response to light, pH, and mechanics. Notably, the resulting polyurethane elastomers exhibited ultrasensitive sustained photochromism with tunable white emission and rapid reversibility. This study provides a simple and effective strategy to utilize plant oil for multifunctional material preparation and paves the way to open access for application of plant oil-based products in a variety of industry applications, such as sensors, self-fitting tissue scaffolds, and switchable devices.

A turn-on fluorescent probe with high selectivity for Hg2+ and its applications in living cells

A novel fluorescent probe L with a rhodamine B lactam structure modified with 3-methyl-2-thiophenecarboxaldehyde has been prepared based on the thiophilicity of Hg²⁺. The probe L exhibits a unique response with an “off–on-type” mode towards Hg²⁺ among other biologically relevant metal cations. The limit of detection (LOD) for probe L is 1.5 ppb. In addition, in the presence of Hg²⁺, the probe L shows a colorimetric response from colorless to pink. The recognition behavior of probe L towards Hg²⁺ has been investigated by ¹H NMR titration experiments, Job's plot, and MS and IR analyses. As a result, the ligation between the probe and Hg²⁺ leads to the scission of the spirolactam bond of free L and the restoration of its conjugated structure, which can give rise to the fluorescence enhancement of the probe L. Besides, it also can be used as a sensitivity probe in living cells for Hg²⁺ sensing, which can meet various needs in genetic and environmental samples.

A novel colorimetric fluorescent probe with low detection limits has been designed and synthesized, which shows an unique response for Hg²⁺ in living cells.

Mixed-Linker Strategy for the Construction of Metal-Organic Framework Combined with Dyes toward Alcohol Detection

Herein, a N-rich metal-organic framework (MOF) with four kinds of cages, Zn₄(ade)₂(TCA)₂(H₂O) (NENU-1000, Hade = adenine, H₃TCA = 4,4',4″-tricarboxytriphenylamine, NENU = Northeast Normal University), was prepared by the mixed-ligand strategy. Cationic dyes can be selectively absorbed by NENU-1000 at proper concentrations, but not neutral and anionic dyes, which perhaps can be assigned to the N-rich neutral framework of NENU-1000. When NENU-1000 was introduced to a relatively lower concentration of cationic dye solutions (e.g., rhodamine B or basic red 2), the colors of these systems faded quickly. Furthermore, the faded solutions can be used for the detection of methanol and other small alcohol molecules with either the naked eye or common UV-vis spectra. The effect of the length of carbon chain, the position of the -OH group, and the number of the hydroxyl group of the alcohols was explored for the color development rate. In addition, the performance of NENU-1000 in iodine sorption and release was also studied.

Synthesis of MnO2-Ag Nanojunctions with Plasmon-Enhanced Photocatalytic and Photothermal Effects for Constructing Rewritable Mono-/Multi-Color Fabrics

Semiconductor-mediated photoreversible color switching systems (PCSSs) have great potential to replace traditional photochromic materials, and the key is to obtain semiconductors with unique photocatalytic and photothermal features. Herein, we have developed MnO₂-Ag nanojunctions with plasmon-enhanced photocatalytic and photothermal effects for PCSSs. MnO₂-Ag nanojunctions are solvothermally synthesized with Mn(CH₃COO)₃, KMnO₄, and AgNO₃ in diethylene glycol as precursors, and they are composed of MnO₂ nanoparticles (∼30 nm) that are decorated by Ag nanodots (∼6 nm). The presence of Ag confers an enhanced visible photoabsorption with a narrow band gap for MnO₂ (E_g = 1.82 eV) and a weak/broad photoabsorption tail (∼875 nm) compared to that of pure MnO₂ (2.45 eV, ∼625 nm). By coupling MnO₂-Ag nanojunctions with various redox dyes, some PCSS inks can be obtained, and especially, the inks containing hydroxyethyl cellulose could be used to prepare rewritable fabrics. When inks and fabrics are irradiated by 475 nm light, rapid discoloration can occur, resulting from the photocatalytic reduction of the dye. Contrarily, the irradiation of 808 nm light promotes the rapid recoloration since Ag nanodots with plasmonic effects in the nanojunctions can absorb light to generate heat, which facilitates the oxidization of leuco dyes in air. Consequently, remote printing of figures was attained on the rewritable fabrics via 475 nm light illumination, and then, the erasure was performed by 808 nm light illumination in an O₂ atmosphere, with high reversibility and cycling stability. Therefore, MnO₂-Ag nanojunctions have tremendous promise for rewritable media, and the introduction of metal-semiconductor junctions as a nanophotocatalyst offers new insights for PCSSs.

Synthesis of an advanced metal-guided photochromic system for molecular keypad lock: detailed experimental findings and theoretical understanding

The dye-containing Schiff base metal complex is a new member of the photochromic family with advantages, such as long-wavelength absorption, high molar absorption coefficient, quick-photoresponse, and excellent fatigue resistance.

Unveiling electron transfer in a supramolecular aggregate for adaptive and autonomous photochromic response

Interactive and responsive materials are dynamic molecular systems that are capable of modulating their behavior and adapting to environment autonomously. Photochromic materials are among the fascinating class of dynamic responsive systems and widely used in molecular switches and optoelectronic devices. However, the phototriggered color changing largely relies on the conformation transformation of the photochromic motif, which significantly limited their usage in organic liquid solutions. Herein, we demonstrate a photochromic organic supramolecular system by using electron-rich N,N-dimethylacetamide (DMA) and electron-deficient naphthalenediimide (NDI) as a donor and acceptor, respectively. In the binary system, the photo-induced electron transfer through lone pair···π interactions pathway leads to dynamic photochromic response not only in the solution but also in the crystalline aggregated state. Furthermore, by incorporating the supramolecules in the polymer matrix, the transparent polymeric film also exhibits rapid photochromic response, which makes it a promisingly interactive and adaptive photochromic material.

A Tri-state Fluorescent Switch with "Gated" Solid-state Photochromism Induced by an External Force

Mr. Chen Qian, Dr. Zhimin Ma, Mr. Jianwei Liu, Mrs. Xue Zhang, Prof. Shitao Wang and Prof. Zhiyong Ma. In this article, we report a newly designed molecule composed of a dihydroazulene (DHA) group and a phenothiazine (PTZ) moiety, which achieves aggregation-induced emission enhancement (AIEE), mechanochromism and "gated" solid-state photochromism upon stimulation by an external force. Grinding loosens intermolecular interactions in the crystal and causes a red-shift of fluorescence from 570 nm to 600 nm. Meanwhile, the ring-opening reaction of DHA unit is activated by grinding and a remarkable photochromism could be observed from the grinded powder. The reddish emission of the grinded powder peaked at 600 nm weakened gradually and finally became dark, and a new absorption band at 470 nm emerged in the absorption spectra. Time-dependent density functional theory (TD-DFT) calculation results reveal that the intramolecular intramolecular charge-transfer (ICT) process is replaced by a locally excited (LE) emission on the DHA group, which leads to the quenching of fluorescence. Its impressive photochromic property inspired us to a simple but effective way to develop an encryption system which can let the correct information be displayed upon external stimulation.

Multiresponsive Tetra-Arylethene-Based Fluorescent Switch with Multicolored Changes: Single-Crystal Photochromism, Mechanochromism, and Acidichromism

In this article, we report a simple tetra-arylethene-based fluorescent switch TPS═C4 modified with a flexible alkyl chain by the Schiff base structure. The incorporation of C═N retains the excellent photochromic property of tetra-arylethene and endows TPS═C4 with new multiresponsiveness of mechanochromism and acidichromism and multicolor changes. TPS═C4 shows remarkable mechanochromism from a deep blue emission at 420 nm to bright blue with a new shoulder band at 450 nm, which arises from the force-induced phase transition from the crystal state to an amorphous form. Both the original crystalline powder and the ground amorphous sample exhibit interesting acidichromism, and their emission colors turn yellow (530 nm), due to the protonation of C═N. More interestingly, TPS═C4 displays fascinating photochromism in multiple states, especially in the single-crystal state. The flexible alkyl chain offers enough free space for molecular motion and facilitates single-crystal photochromism. Due to the multiresponsiveness and multicolor switch, TPS═C4 can be satisfactorily used for the multidimensional anticounterfeiting application. To the best of our knowledge, TPS═C4 is a rare multifunctional molecule with a simple structure but featuring multiresponsiveness and multicolor switch.

Rewritable PEDOT Film Based on Water-Writing and Electroerasing

Rewritable paper has greatly promoted the sustainable development of society. However, the hydrophilicity/lipophilicity of the poly(3,4-ethylenedioxythiophene) (PEDOT) film limits its application as the rewritable paper. Herein, we constructed a repeatable writing/erasing pattern on a PEDOT film (rewritable PEDOT paper) by combining wettability control, water-induced dedoping, and an electrochemical redox reaction. The treatment with a medium-polarity/high-volatility solvent (MP/HVS) adjusted the wettability of the PEDOT film (water contact angle increased from 6.5° to 146.2°), contributing to the formation of a hydrophobic writable substrate. The treatment with a high-polarity solvent (HPS) induced the dedoping of anions in the PEDOT chain, resulting in the film's color changed from blue to purple and serving as a writing process. The intrinsic electrochemical redox (elimination of color change by doping/dedoping of lithium ions in the PEDOT chain) of the PEDOT film enabled the erasing process. This writing/erasing process can be repeated at least 10 times. The patterned PEDOT film maintained excellent stability to standing diverse solvents (low-polarity solvent (LPS) and MP/HVS), high temperatures (350 °C), and irradiation of different light wavelengths (wavelengths of 365, 380, 460, 520, and 645 nm). Additionally, the conductivity of the PEDOT film was quantitatively measured (impedance: LPS, increased 8.84%; MP/HVS, decreased 6.67%; and HPS, increased 27.97%) by fabricating a micropatterned PEDOT electrode. This work will provide a method for the fabrication of PEDOT-based optoelectronic functional materials.

A color-tunable single-component luminescent molecule with multiple emission centers

For achieving smart materials with color-tunable emissions, the development of single-component systems exhibiting high durability and stability is desired but remains challenging, in comparison to multicomponent systems. Here, a single-component luminescent molecule (3-SPhF) with colorful emissions is successfully reported through different expressions of triplet excitons in radiative transitions. The time-resolved spectra confirm the existence of delayed fluorescence (τ = 282.5 μs), monomeric phosphorescence (τ = 497.7 ms) and aggregated-state phosphorescence (τ = 230.0 ms) in the crystal powder of 3-SPhF, which affords time-dependent afterglow and excitation-dependent emissions in a steady state. Furthermore, the relationships between ultra-long luminescence and stacking of the dibenzofuran group in single crystals are explored, providing evidence for the regularity of multiple emission centers in single-component compounds with dibenzofuran substituents.

Turning On Solid-State Luminescence by Phototriggered Subtle Molecular Conformation Variations

The development of solid-state intelligent materials, in particular those showing photoresponsive luminescence (PRL), is highly desirable for their cutting-edge applications in sensors, displays, data-storage, and anti-counterfeiting, but is challenging. Few PRL materials are constructed by tethering the classic photochromic systems with newly-emerged solid-state emitters. Selective solid-state photoreactions are demanded to precisely manipulate the luminescent behavior of these emitters, which require dramatic structural change and enough free space, thus limiting the scope of the PRL family. Here, a new PRL material, TPE-4N, that features sensitive and reversible fluorescence switching is reported. The interesting on-off luminescent property of TPE-4N can be facilely tuned through fast phototriggering and thermal annealing. Experimental and theoretical investigations reveal that subtle molecular conformation variation induces the corresponding PRL behavior. The crystalline and amorphous state endows an efficient and weak ISC process, respectively, to turn on and off the emission. The readily fabricated thin-film of TPE-4N exhibits non-destructive PRL behavior with high contrast (>10² ), good light transmittance (>72.3%), and great durability and reversibility under room light for months. Remarkably, a uniform thin-film with such fascinating PRL properties allows high-tech applications in invisible anti-counterfeiting and dynamic optical data storage with micro-resolution.

A rhodamine-based dual chemosensor for the naked-eye detection of Hg2+ and enhancement of the fluorescence emission for Fe3

A novel fluorescent chemosensor based on trimesoyl chloride-rhodamine (TR) was successfully synthesized. Rising chromogenic and fluorogenic spectral enhancements could be observed in trimesoyl chloride-rhodamine (TR) probes when Hg²⁺ and Fe³⁺ were added, respectively. TR has shown selectivity for Hg²⁺ and Fe³⁺ with high sensitivity due to metal ion complexation induced photophysical "turn-on" signaling responses. The detection limit towards Hg²⁺ was 2.46 × 10^-8 M as determined by the 3σ method. At the same time, fluorogenic spectral enhancements were observed in TR, which exhibits a superior sensitive and selective recognition towards Fe³⁺ with 4.11 × 10^-8 M of the detection limit. The test strips were used for colorimetric and simple detection towards Hg²⁺, which might finally enable the advancement of the Hg²⁺ sensor in the field of on-site detection.

Employing a fluorescent and colorimetric picolyl-functionalized rhodamine for the detection of glyphosate pesticide

The ongoing poisoning of agricultural products has pushed the security problem to become an important issue. Among them, exceeding the standard rate of pesticide residues is the main factor influencing the quality and security of agricultural products. Monitoring pesticide residues and developing simple, yet ultrasensitive detection systems for pesticide residues are urgently needed. In this study, we successfully developed a novel rhodamine derivative as fluorescent and colorimetric chemosensor R-G for the rapid, selective and ultrasensitive detection of glyphosate pesticide residue in aqueous solution. Through a Cu²⁺-indicator displacement strategy, glyphosate can displace an indicator (R-G) from a Cu²⁺-indicator complex due to its strong affinity to bind with Cu²⁺ to give a turn-on fluorescence and distinct color change. Moreover, a test strip was also fabricated to achieve a facile detection of glyphosate pesticide. To demonstrate the possibility of practical applications, glyphosate was detected on the surface of cabbage and in a spiked soil sample. The detection limit of 4.1 nM and the response time of 2 min indicate that the method is enough sensitive and rapid to detect the glyphosate residue at or below levels that pose a health risk.

Recent Advances in Noninvasive Biosensors for Forensics, Biometrics, and Cybersecurity

Recently, biosensors have been used in an increasing number of different fields and disciplines due to their wide applicability, reproducibility, and selectivity. Three large disciplines in which this has become relevant has been the forensic, biometric, and cybersecurity fields. The call for novel noninvasive biosensors for these three applications has been a focus of research in these fields. Recent advances in these three areas has relied on the use of biosensors based on primarily colorimetric assays based on bioaffinity interactions utilizing enzymatic assays. In forensics, the use of different bodily fluids for metabolite analysis provides an alternative to the use of DNA to avoid the backlog that is currently the main issue with DNA analysis by providing worthwhile information about the originator. In biometrics, the use of sweat-based systems for user authentication has been developed as a proof-of-concept design utilizing the levels of different metabolites found in sweat. Lastly, biosensor assays have been developed as a proof-of-concept for combination with cybersecurity, primarily cryptography, for the encryption and protection of data and messages.

Multiple Factors Regulate the Spirocyclization Equilibrium of Si-Rhodamines

Si-rhodamine has been extensively used in super-resolution fluorescence imaging in recent years. Its equilibrium between ring-closed nonfluorescent spirolactones and ring-opened fluorescent zwitterions endows Si-rhodamine with excellent fluorogenicity, membrane permeability, and photostability. In this paper, the equilibrium of Si-rhodamine between lactones and zwitterions was revealed to be greatly affected by various environmental factors, including molecular aggregation, solvent polarity, pH, metal ions, irradiation, and temperature. These environmental sensitivities make Si-rhodamine useful as a hydrochromic material, a fluorescent sensor array for metal ions or solvents, and a photoactivatable switch. Importantly, these results indicate that using Si-rhodamine as a fluorogenic probe or a blinking fluorophore in single-molecule localization super-resolution microscopy requires caution on possible false signals caused by its environmental sensitivity.

Blue/red light-triggered reversible color switching based on CeO2-x nanodots for constructing rewritable smart fabrics

Photoreversible color switching systems (PCSSs) have attracted increasing attention in various applications, but in most PCSSs the discoloration process usually relies on harmful UV light as a stimulus and the recoloration requires high temperature. To solve these problems, we have designed and prepared CeO2-x nanodots as novel photocatalytic components in PCSSs that respond to two kinds of visible light. CeO2-x nanodots are prepared by a solvothermal reaction with l-ascorbic acid as the reducing agent. CeO2-x nanodots with a size of ∼2 nm have a high concentration of oxygen vacancies, which confers a broadened photoabsorption with an edge at 500 nm, as well as a weak photoabsorption tail in the visible region (500-800 nm). To realize the color switching, both the CeO2-x/Dye/H2O solution and CeO2-x/dye/hydroxyethyl cellulose (HEC)-coated fabrics have been prepared. Under blue (450 nm) light irradiation, both the solution and fabric show a rapid discoloration in 30 s and 150 s, respectively, due to the efficient photocatalytic reduction of the redox dye by CeO2-x. Conversely, red (630 nm) light irradiation with air confers a rapid recoloration in 35 s for the solution and 200 s for the fabric, resulting from CeO2-x-mediated self-catalyzed oxidation. In particular, the required images and letters can be remotely printed on CeO2-x/Dye/HEC-coated T-shirts with a 450 nm laser pen, and then erased with 630 nm light, with high reversibility and stability. Therefore, the present CeO2-x/Dye/HEC PCSSs have great potential to construct rewritable smart fabrics for various applications.

On-demand regulation of photochromic behavior through various counterions for high-level security printing

Materials exhibiting reversible changes in optical properties upon light irradiation have shown great potential in diverse optoelectronic areas. In particular, the modulation of photochromic behavior on demand for such materials is of fundamental importance, but it remains a formidable challenge. Here, we report a facile and effective strategy to engineer controllable photochromic properties by varying the counterions in a series of zinc complexes consisting of a spirolactam-based photochromic ligand. Colorability and coloration rate can be finely tuned by conveniently changing their counterions. Through utilization of the reversible feature of the metal-ligand coordination bond between Zn²⁺ and the spirolactam-based ligand, dynamic manipulation of photochromic behavior was achieved. Furthermore, we demonstrated the practical applications of the tunable photochromic properties for these complexes by creating photochromic films and developing multilevel security printing. These findings show opportunities for the development of smart materials with dynamically controllable responsive behavior in advanced optoelectronic applications.

Highly Selective and Sensitive Detection of Biogenic Defense Phytohormone Salicylic Acid in Living Cells and Plants Using a Novel and Viable Rhodamine-Functionalized Fluorescent Probe

Detecting plant-derived signal molecules using fluorescent probes is a key topic and a huge challenge for scientists. Salicylic acid (SA), a vital plant-derived defense hormone, can activate global transcriptional reprogramming to systemically express a network of prominent pathogenesis-related proteins against invasive microorganisms. This strategy is called systemic acquired resistance (SAR). Therefore, monitoring the dynamic fluctuations of SA in subcellular microenvironments can advance our understanding of different physiological and pathological functions during the SA-induced SAR mechanism, thus benefiting the discovery and development of novel immune activators that contribute to crop protection. Here, detection of signaling molecule SA in plant callus tissues was first reported and conducted by a simple non-fluorescent rhodamine-tagged architecture bearing a flexible 2-amino-N,N-dimethylacetamide pattern. This study can markedly advance and promote the usage of fluorescent SA probes for distinguishing SA in the plant kingdom.

Metal ions-triggered photo-induced fluorescence change in rhodamine B-based photo-responsive complexes

Photo-responsive materials with tunable properties by multiple stimuli have been widely used as molecular machines, molecular logic gates, optical data storages, etc. In this work, we report a rhodamine B-based photo-responsive system, whose properties could be facilely modulated by metal ions (Zn(II), Ni(II) and Hg(II)). These metal ions endow the complexes (L-Zn, L-Ni and L-Hg) with similar photochromic property but distinctly different photo-induced fluorescence change. Upon UV light irradiation, the spirolactam ring in rhodamine B moiety turned from a close form to an open form, along with enlarged conjugated structure with intense absorbance. Interestingly, fluorescence "turn off", "no change" and "turn on" responses were induced by Zn(II), Ni(II) and Hg(II) respectively upon UV light irradiation. Taking advantage of the prominently different characteristics caused by metal ions, different logic gates were designed by simply varying the inputs of metal ions and UV light. This work provided a new strategy for developing multifunctional photo-responsive materials, which were further beneficial for constructing photo-controlled logic gates with tunable performance.

Solid-State Emissive Metallo-Supramolecular Assemblies of Quinoline-Based Acyl Hydrazone

Development of fluorescence-based sensory materials for metal elements is currently in the mainstream of research due to the simplicity and usability of fluorescence as a method of detection. Herein, we report a novel "bis"-quinoline-based acyl hydrazone-named bQH that could be synthesized by a facile, low-cost method through simple condensation of hydrazide with an aldehyde. This acyl hydrazone showed emissive properties through Zn selective binding, especially in its solid-state, as shown by experiments such as UV-Vis, photoluminescence (PL), nuclear magnetic resonance (NMR), and inductively-coupled plasma-optical emission spectroscopies (ICP-OES), and energy-dispersive X-ray spectroscopy (EDS) mapping. The binding modes in which bQH coordinates to Zn2+ was proved to consist of two modes, 1:1 and 1:2 (bQH:Zn2+), where the binding mode was controlled by the Zn2+ ion content. Under the 1:1 binding mode, bQH-Zn2+ complexes formed a polymeric array through the metallo-supramolecular assembly. The resulting bQH-Zn2+ complex maintained its fluorescence in solid-state and exhibited excellent fluorescence intensity as compared to the previously reported quinoline-based acyl hydrazone derivative (mQH).

Wavelength-Tunable Single-Mode Microlasers Based on Photoresponsive Pitch Modulation of Liquid Crystals for Information Encryption

Information encryption and decryption have attracted particular attention; however, the applications are frequently restricted by limited coding capacity due to the indistinguishable broad photoluminescence band of conventional stimuli-responsive fluorescent materials. Here, we present a concept of confidential information encryption with photoresponsive liquid crystal (LC) lasing materials, which were used to fabricate ordered microlaser arrays through a microtemplate-assisted inkjet printing method. LC microlasers exhibit narrow-bandwidth single-mode emissions, and the wavelength of LC microlasers was reversibly modulated based on the optical isomerization of the chiral dopant in LCs. On this basis, we demonstrate phototunable information authentication on LC microlaser arrays using the wavelength of LC microlasers as primary codes. These results provide enlightenment for the implementation of microlaser-based cryptographic primitives for information encryption and anticounterfeiting applications.

Adsorption Promoted Aggregation-Induced Emission Showing Strong Dye Lateral Interactions

Aggregation-induced emission (AIE) is a powerful method to produce fluorescence for a diverse range of applications. While most previous work induced aggregation by change of solvent, ionic strength, pH, or self-assembly, we herein explored adsorption-induced aggregation using 4,4'-(hydrazine-1,2-diylidene bis(methanylylidene)) bis(3-hydroxybenzoic acid) (HDBB) as an AIE luminogen. HDBB is known to aggregate with AIE at low pH but not at neutral pH, and its aggregation facilitates excited state intramolecular proton transfer for enhanced emission. Using a nonquenching nanomaterial, Y₂O₃ nanoparticles, HDBB showed sevenfold fluorescence increase at pH 7.0. Fluorescence lifetime showed that HDBB was in the aggregated state in the presence of Y₂O₃. For comparison, a fluorescent porphyrin compound showed that adsorption caused quenching after mixing with Y₂O₃, whereas other dyes such as fluorescein, calcein, and rhodamine B failed to be adsorbed by Y₂O₃. Adsorption did not follow a Langmuir isotherm, but it showed strong lateral HDBB interactions because adsorption was only achieved with a high concentration of HDBB. Adsorption was inhibited by salt and by phosphate, indicating the importance of electrostatic and metal-binding interactions. Comparisons were made with other nanomaterials, where graphene oxide and CeO₂ quenched HDBB and a cationic liposome also enhanced its emission, although with a less red-shifted peak wavelength. This study provides a simple method to induce aggregation of an AIE dye and its aggregation in turn-enhanced adsorption.