Reversibly Thermochromic, Fluorescent Ultrathin Films with a Supramolecular Architecture

Molecular design and architectonics towards film-based fluorescent sensing

The past few decades have witnessed encouraging progress in the development of high-performance film-based fluorescent sensors (FFSs) for detecting explosives, illicit drugs, chemical warfare agents (CWAs), and hazardous volatile organic chemicals (VOCs), among others. Several FFSs have transitioned from laboratory research to real-world applications, demonstrating their practical relevance. At the heart of FFS technology lies the sensing films, which play a crucial role in determining the analytes and the resulting signals. The selection of sensing fluorophores and the fabrication strategies employed in film construction are key factors that influence the fluorescence properties, active-layer structures, and overall sensing behaviors of these films. This review examines the progress and innovations in the research field of FFSs over the past two decades, focusing on advancements in fluorophore design and active-layer structural engineering. It underscores popular sensing fluorophore scaffolds and the dynamics of excited state processes. Additionally, it delves into six distinct categories of film fabrication technologies and strategies, providing insights into their advantages and limitations. This review further addresses important considerations such as photostability and substrate effects. Concluding with an overview of the field's challenges and prospects, it sheds light on the potential for further development in this burgeoning area.

Exploration of Coumarin Derivative: Experimental and Computational Modeling for Dipole Moment Estimation and Thermal Sensing Application

The Optical properties of the FBTC (1-((4-((5-chlorobenzo[d]oxazol-2-ylthio)methyl)-1H-1,2,3-triazol-1-yl)methyl)-3H-benzo[f]chromen-3-one) molecule were studied experimentally and theoretically. The spectra of absorption and fluorescence were recorded in various solvents to explore their Solvatochromic behavior and dipole moment at room temperature. To determine the ground and excited state of dipole moment experimentally and theoretically, we employed different Solvatochromic techniques, including microscopic solvent polarity functions developed by Lippert, Bakhshiev, Kawaski-Chamma-Viallet, and Reichardt's, as well as density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The stability of the excited state dipole moment in FBTC is higher. Using prime functional, FBTC was optimized in its ground state, and its HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital), energies were estimated. These values were then compared with those obtained through cyclic voltammetry. Based on the HOMO and LUMO values given, we calculated the global reactivity parameter and energy gap, which was found to be low at 3.77 eV. This study also includes an estimation of electron absorption energies and oscillator strength. Natural population analysis (NPA), Milliken atomic charge, and molecular electrostatic potential (MESP) map are correlated. In addition, FBTC exhibited exceptional physiological temperature sensing behaviour from 20 °C -65 °C with high relative sensitivity and firm stability. Hence these results confirm that FBTC is a potential candidate for photonic devices and it's also applicable in optical temperature sensing.

Stimuli-fluorochromic smart organic materials

Smart materials based on stimuli-fluorochromic π-conjugated solids (SFCSs) have aroused significant interest due to their versatile and exciting properties, leading to advanced applications. In this review, we highlight the recent developments in SFCS-based smart materials, expanding beyond organometallic compounds and light-responsive organic luminescent materials, with a discussion on the design strategies, exciting properties and stimuli-fluorochromic mechanisms along with their potential applications in the exciting fields of encryption, sensors, data storage, display, green printing, etc. The review comprehensively covers single-component and multi-component SFCSs as well as their stimuli-fluorochromic behaviors under external stimuli. We also provide insights into current achievements, limitations, and major challenges as well as future opportunities, aiming to inspire further investigation in this field in the near future. We expect this review to inspire more innovative research on SFCSs and their advanced applications so as to promote further development of smart materials and devices.

Leveraging Crystalline and Amorphous States of a Metal-Organic Complex for Transformation of the Photosalient Effect and Positive-Negative Photochromism

Uncovering differences between crystalline and amorphous states in molecular solids would both promote the understanding of their structure-property relationships, as well as inform development of multi-functional materials based on the same compound. Herein, for the first time, we report an approach to leverage crystalline and amorphous states of a zero-dimensional metal-organic complex, which exhibited negative and positive photochromism, due to the competitive chemical routes between photocycloaddition and photogenerated radicals. Furthermore, different polymorphs lead to the on/off toggling of photo-burst movement (photosalient effect), indicating the controllable light-mechanical conversion. Three demos were further constructed to support their application in information encryption and anti-counterfeiting. This work provides the proof-of-concept of a state- and polymorph-dependent photochemical route, paving an effective way for the design of new dynamically responsive systems.

Control of Fluorescence of Organic Dyes in the Solid-State by Supramolecular Interactions

Fluorescent organic dyes play an essential role in the creation of new "smart" materials. Fragments and functional groups capable of free rotation around single bonds can significantly change the fluorescent organic dye's electronic structure under analyte effects, phase state transitions, or changes in temperature, pressure, and media polarity. Dependencies between steric and electronic structures become highly important in transition from a solution to a solid-state. Such transitions are accompanied by a significant increase in the dye molecular structure's rigidity due to supramolecular associates' formation such as H-bonding, π···π and dipole-dipole interactions. Among those supramolecular effects, H-bonding interactions, first of all, lead to significant molecular packing changes between loose or rigid structures, thus affecting the fluorescent dye's electronic states' energy and configuration, its fluorescent signal's position and intensity. All the functional groups and heteroatoms that are met in the organic dyes seem to be involved in the control of fluorescence via H-bonding: C-H···N, C-H···π, S = O···H-C, P = O···H, C-H···O, NH···N, C - H···C, C - H···Se, N-H···O, C - H···F, C-F···H. Effects of molecular packing of fluorescent organic dyes are successfully used in developing mechano-, piezo-, thermo- fluorochromes materials for their applications in the optical recording of information, sensors, security items, memory elements, organic light-emitting diodes (OLEDs) technologies.

Bionic Assembly of Layered Double Hydroxides Nanosheets and Positively Charged Micelles by Counterions Balance and Their Selective Detection of Mannose

The selective detection of mannose is significant for tumor early diagnosis. However, current methods for detecting mannose are expensive and time-consuming, limiting their application. In this paper, we have obtained a 25-layer positively charged micellar/LDHs nanocomposite film system by electrostatic layer-by-layer assembly with reference to the unique properties of homogeneous charge ion attraction and charge overcompensation in biomolecules: hexadecyl trimethylammonium bromide (CTAB) was used to coat neutral molecules of fluorescein (FLU) to form (FLU@CTAB) cationic micelles, which were electrostatically assembled with laminate positively charged layered double hydroxides (LDHs) nanosheets to form (FLU@CTAB/LDHs)_n ultrathin films (UTFs) by the layer-by-layer electrostatic assembly, where the mediating role of the Br^- counteranion had a profound effect on the success of the assembly. Moreover, compared to pure FLU solution, the fluorescence intensity and the lifetime of (FLU@CTAB/LDHs)₂₀ UTFs were enhanced by 1.6 and 2 times, respectively. (FLU@CTAB/LDHs)₂₀ UTFs exhibited selective detection for d-mannose with a detection limit of 0.05 mg·mL^-1. Therefore, the (FLU@CTAB/LDHs)_n UTFs can be a novel biosensor. Compared to conventional powder sensors, (FLU@CTAB/LDHs)_n thin-film fluorescent sensors are more promising for device implementation. Moreover, the design strategy of positively charged micellar/LDHs nanocomposite systems breaks the current limitation that LDHs can only be assembled with anions or neutral molecules and extends the scope of counterion-mediated host-guest to the nanosheet-micellar system.

Solid-State Fluorescence of A Quasi-Isostructural Polymorphic Biphenyl Based Michael Addition Product

Polymorphic materials have gained significant attention owing to their fascinating physicochemical properties. Herein, a biphenyl based Michael addition product (Compound A) with an active methylene group (dimedone) was synthesized. Compound...

Panchromatic Fluorescence Emission from Thienosquaraines Dyes: White Light Electrofluorochromic Devices

Electrofluorochromic devices (EFCDs) that allow the modulation of the light emitted by electroactive fluorophores are very attractive in the research field of optoelectronics. Here, the electrofluorochromic behaviour of a series of squaraine dyes was studied for the first time. In solutions, all compounds are photoluminescent with maxima located in the range 665-690 nm, characterized by quantum yields ranging from 30% to 4.1%. Squaraines were incorporated in a polymer gel used as an active layer in all-in-one gel switchable EFCDs. An aggregation induced quenching occurs in the gel phase, causing a significant decrease in the emission quantum yield in the device. However, the squaraines containing the thieno groups (thienosquaraines, TSQs) show a panchromatic emission and their electrofluorochromism allows the tuning of the fluorescence intensity from 500 nm to the near infrared. Indeed, the application of a potential difference to the device induces a reversible quenching of their emission that is significantly higher and occurs at shorter switching times for TSQs-based devices compared to the reference squaraine dye (DIBSQ). Interestingly, the TSQs fluorescence spectral profile becomes more structured under voltage, and this could be explained by the shift of the aggregates/monomer equilibrium toward the monomeric species, due to electrochemical oxidation, which causes the disassembling of aggregates. This effect may be used to modulate the colour of the fluorescence light emitted by a device and paves the way for conceiving new electrofluorochromic materials based on this mechanism.

Printable Off-On Thermoswitchable Fluorescent Materials for Programmable Thermally Controlled Full-Color Displays and Multiple Encryption

Thermoswitchable fluorescent materials (TFMs) have received special attention due to their unique fluorescent colorimetric responses to temperature. Conventional TFMs generally display unicolor with switching from one color to another, showing unprintable and unsatisfied performances. These limitations greatly hinder their development and expansion toward advanced applications. Herein, the superior integration of full-color, off-on switching mode, printability, and high performance to TFMs is achieved successfully. The success is due to a thermally induced synchronous "dual/multichannel" stimulus-response mode regulated by a self-crystalline phase-change material; that is, synergistic changes of the molecular existence states and subsequent colors/spectra of the fluorescent modifier and fluorophores, accompanied by corresponding high-efficiency on-off switching of Förster resonance energy transfer. These TFMs are simple to prepare and show good performance, such as high fluorescence emission contrast (>100), great reversibility (>200 cycles), and easy-to-adjust response temperature. Particularly, these R/G/B TFMs can be prepared as tricolor fluorescent inks, and thus full-color emissions on flexible substrate can be easily obtained by printing. Finally, their great potential in switchable dynamic interior decoration, programmatic temperature-control information display, and senior information encryption are illustrated. This successful exploration offers a new perspective for designing and optimizing various other switchable materials with higher comprehensive performances.

Photoluminescent and Chromic Nanomaterials for Anticounterfeiting Technologies: Recent Advances and Future Challenges

Counterfeiting and inverse engineering of security and confidential documents, such as banknotes, passports, national cards, certificates, and valuable products, has significantly been increased, which is a major challenge for governments, companies, and customers. From recent global reports published in 2017, the counterfeiting market was evaluated to be $107.26 billion in 2016 and forecasted to reach $206.57 billion by 2021 at a compound annual growth rate of 14.0%. Development of anticounterfeiting and authentication technologies with multilevel securities is a powerful solution to overcome this challenge. Stimuli-chromic (photochromic, hydrochromic, and thermochromic) and photoluminescent (fluorescent and phosphorescent) compounds are the most significant and applicable materials for development of complex anticounterfeiting inks with a high-security level and fast authentication. Highly efficient anticounterfeiting and authentication technologies have been developed to reach high security and efficiency. Applicable materials for anticounterfeiting applications are generally based on photochromic and photoluminescent compounds, for which hydrochromic and thermochromic materials have extensively been used in recent decades. A wide range of materials, such as organic and inorganic metal complexes, polymer nanoparticles, quantum dots, polymer dots, carbon dots, upconverting nanoparticles, and supramolecular structures, could display all of these phenomena depending on their physical and chemical characteristics. The polymeric anticounterfeiting inks have recently received significant attention because of their high stability for printing on confidential documents. In addition, the printing technologies including hand-writing, stamping, inkjet printing, screen printing, and anticounterfeiting labels are discussed for introduction of the most efficient methods for application of different anticounterfeiting inks. This review would help scientists to design and develop the most applicable encryption, authentication, and anticounterfeiting technologies with high security, fast detection, and potential applications in security marking and information encryption on various substrates.

Exfoliated Single Layers of Layered Cobalt Hydroxide for Ultrafine Co3 O4 Nanoparticles on Graphene Nanosheets as an Efficient Electrocatalyst for Oxygen Reduction

A highly effective way to produce an oxygen reduction electrocatalyst was developed through the self-assembly of exfoliated single layers of cobalt hydroxide (Co(OH)₂ ) and graphene oxide (GO). These 2D materials have complete contact with one another because of their physical flexibility and the electrostatic attraction between negatively charged GO and positively charged Co(OH)₂ layers. The strong coupling induces transformation of the Co(OH)₂ single layer into a discrete nanocrystal of spinel Co₃ O₄ with an average size of 8 nm on reduced GO (RGO) during calcination, which could not be obtained with bulk-layered cobalt hydroxide because of its rapid layer collapse. The ultrafine Co₃ O₄ /RGO hybrid exhibited not only comparable performance in the oxygen reduction reaction but also higher durability compared with the commercial 20 wt % Pt/C catalyst.

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.

Interfacial Coordination Nanosheet Based on Nonconjugated Three-Arm Terpyridine: A Highly Color-Efficient Electrochromic Material to Converge Fast Switching with Long Optical Memory

An electrochromic (EC) hyperbranched coordination nanosheet (CONASH) comprising a three-arm terpyridine (3tpy)-based ligand and Fe(II) ion has been synthesized by interfacial complexation at the liquid-liquid interface. The film can be easily deposited on the desired substrate such as indium tin oxide (ITO) glass. Characterization of CONASH deposited on ITO by microscopic methods reveals the homogeneous nanosheet film with an ∼350 nm thickness after 48 h of reaction. The fabricated solid-state EC device (ECD) undergoes a reversible redox reaction (Fe²⁺ → Fe³⁺) in the potential range of +3 to -2 V in ECDs accompanied with a distinct color change from intense pink to colorless for several switching cycles with a coloration time of 1.15 s and a bleaching time of 2.49 s along with a high coloration efficiency of 470.16 cm² C^-1. Besides, the nonconjugated 3tpy ligand restricts the easy electron redox conduction inside the EC film to enhance the EC memory in open-circuit condition as it shows 50% retention of its colorless state until 25 min. The long EC memory compared to other metallo-supramolecular polymers having a conjugated ligand suggests the potentiality of the 3tpy-Fe CONASH film to be used as a power-efficient EC material for modern display device applications.

Significantly Enhanced Thermoelectric Properties of Organic-Inorganic Hybrids with a Periodically Ordered Structure

The deficient order in amorphous components severely affects the thermoelectric (TE) properties in polymers. Encouragingly, two-dimensional layered double hydroxides (LDHs) have been regarded as an efficient host material to tune the conformation of guest molecules and construct ordered hybrids. Herein, we report a facile construction of periodically ordered organic-inorganic TE hybrids by alternative stacking of inorganic LDH nanosheets and organic poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) molecules. The ordered structure of PEDOT:PSS-LDH gave rise to the extended molecular configuration of PEDOT:PSS, resulting in the improved carrier mobility in the hybrids. Moreover, the energy filtering was induced by such a periodically ordered structure, which blocked the low-energy carriers preferentially and improved the Seebeck coefficient in the hybrids. Therefore, the power factor of the PEDOT:PSS-LDH hybrid was 120-fold higher than that of pristine PEDOT:PSS. These results not only establish an effective method for the construction of periodically ordered TE materials but also address the significance of an ordered structure of molecules in TE materials.

High Temperature Sensing and Detection for Cementitious Materials Using Manganese Violet Pigment

In recent years, advanced materials have attracted considerable interest in the field of temperature detection and sensing. This study examined the thermochromic properties of inorganic manganese violet (MV) with increasing temperature. According to the thermochromic test, the material was found to have reversible and irreversible color change properties. The MV pigment was then applied to cementitious material at ratios of 1%, 3%, and 5%. The mixed cement samples with MV pigment were heated in a furnace, and digital images were captured at each temperature interval to evaluate the changes in the color information on the surface of the specimen. The mixed samples exhibited an irreversible thermochromic change from dark violet to grayish green above 400 °C. At the critical temperature of 440 °C, the RGB values increased by approximately 22%–55%, 28%–68%, and 7%–25%, depending on the content of MV pigment. In Lab space, the L value increased by approximately 23%–60% at 440 °C. The a value completely changed from positive to negative, and the b value changed from negative to positive. All the values differed according to the content of MV pigment at room temperature but approached similar ranges at the critical temperature, irrespective of the amount of MV pigment. To assess the changes in their microstructure and composition, scanning electron microscopy and energy dispersive X-ray spectroscopy were performed on the samples exposed to temperatures ranging from room temperature to 450 °C.

Smart Responsive Luminescent Aptamer-Functionalized Covalent Organic Framework Hydrogel for High-Resolution Visualization and Security Protection of Latent Fingerprints

Covalent organic frameworks (COFs) have been proposed as alternative candidates for "smart" materials due to their ordered π-columnar structures. However, it remains a challenge to develop external-stimuli-responsive luminescent COFs for confidential information protection. Here, we have designed and synthesized a water-dispersible and smart responsive luminescent carboxymethyl cellulose-COF hydrogel encapsulated 5-(dimethylamino)-N,N-bis (pyridin-2-ylmethyl) napthalene-1-sulfonamide, named CMC-COF-LZU1⊃DPYNS, for latent fingerprint imaging and encryption. We show that the fluorescence of CMC-COF-LZU1⊃DPYNS is reversibly switchable upon addition of Cu²⁺/H₂O. This effect endows potential applications of tunable luminescent COFs based hydrogel as an invisible security probe for imaging, recording, storage, and security of latent fingerprint information. It is shown that the latent fingerprint information incubated by the aptamer-functionalized CMC-COF-LZU1⊃DPYNS hydrogel is invisible in the presence of Cu²⁺, but three levels of fingerprint features with high-resolution patterns could be readable upon addition of H₂O under UV light. The design strategy provides a promising platform for the development of smart responsive luminescent COFs and their detection and protection of valuable latent fingerprint information.

Electroluminochromic Materials and Devices Based on Metal Complexes

Electroluminochromism (ELC) refers to an interesting phenomenon exhibited by a material whose luminescent properties can be reversibly modulated under an electrical stimulus. Such a luminescence-switching property has been widely used in various organic optoelectronic devices because it can simultaneously detect electrical and optical signals. Metal complexes are the promising candidates for ELC materials due to their sensitivity to an electrical stimulus. Herein, recent progress on electroluminochromic materials and devices based on various metal complexes has been summarized. Meanwhile, the applications of these complexes in data recording and security protection have also been discussed. Finally, a brief conclusion and outlook are presented, pointing out that the development of electroluminochromic metal complexes with excellent performance is important because they play a vital role in future intelligent optoelectronic devices.

Symmetric-Key Encryption Based on Bioaffinity Interactions

The research presented here shows a bridge between biochemistry and cryptography. Enzyme-based assays were used in a new methodology linked to ciphers and cipher systems. Three separate enzyme assays, alkaline phosphatase (ALP) (E.C. 3.1.3.1), lysozyme (E.C. 3.2.1.17), and horseradish peroxidase (HRP) (E.C. 1.11.1.7), were used to create a cipher key in order to encrypt a message. By choosing certain parameters for one's experiment that are performed in the same way as a person receiving the message, correct encryption and decryption keys would be produced, resulting in a correct encryption and decryption of a message. It is imperative that both parties perform the same experiment under the same conditions in order to correctly interpret the message. Bioaffinity-based assays, in particular enzymatic assays, provide a specific, yet flexible mechanism to use for the encryption of messages. Because of the nature of this process there are a multitude of sets of parameters that may be chosen, each of which would result in a different key being produced, heightening the security and the robustness of the method. This paper shows that by using this concept of forming encryption keys using a bioaffinity-based approach, one is able to properly encrypt and decrypt a message, which could be viable for other biochemically based techniques.

Construction of Coordination Nanosheets Based on Tris(2,2'-bipyridine)-Iron (Fe2+) Complexes as Potential Electrochromic Materials

The coordination nanosheets (CONASHs) are emerging as a new class of functional two-dimensional materials, which are one of the most active research areas of chemistry and physics in this decade. Despite the success of various structural and functional CONASHs, the development of a new molecular structure to discover alluring functional CONASHs remains challenging. Herein, we report successful preparation of two novel CONASHs (NBP1 and NBP2) through coordination between one of the unexplored molecular frameworks of bis(2,2'-bipyridine)-based ligands (BP1 and BP2) and Fe²⁺ ions. Using a liquid-liquid interface as a platform, large-scale thin films of multilayer CONASHs have been prepared without any support, which can be deposited onto any desired substrate. Detailed characterization of the CONASHs using various microscopic and spectroscopic techniques reveals homogeneous and flat morphology of nanometer thickness with the quantitative formation of tris(2,2'-bipyridine)-Fe²⁺ complex motifs in the nanosheet frameworks. The color of the films has been tuned from blue to magenta by the suitable molecular design of the ligands. Owing to the insolubility of the CONASH films in any solvent and the presence of redox-active Fe²⁺, we explore the functionality of these nanostructured thin films deposited on indium tin oxide as electrochromic materials. The CONASHs exhibit color-to-colorless and color-to-color electrochromic transitions with attractive response times, switching stabilities, and coloration efficiencies. Finally, we demonstrate solid-state electrochromic devices of the CONASHs operated at a potential range of +2.5 to -2.5 V, which are electrochemically stable for several switching cycles, suggesting that these CONASHs are potential electrochromic materials for next-generation display applications.

Investigation of high contrast and reversible luminescence thermochromism of the quantum confined Cs4PbBr6 perovskite solid

Thermochromism of organic/inorganic halide perovskites has attracted particular interest due to their potential applications as photoluminescence (PL)-based temperature sensors. However, despite the outstanding PL characteristics, their use as a thermochromic material in practical temperature ranges has been limited because of their poor thermal stability. In this study, we used the quantum confinement effect and exceptional PL quantum efficiency of the Cs4PbBr6 perovskite to demonstrate their high on/off ratio (20) and reversible PL thermochromism in the solid state in practical temperature ranges including room temperature (RT). Systematic photophysical and optical characterization studies, including exciton-phonon scattering, exciton binding energy, exciton decay dynamics, and crystal structure change, were performed to investigate the origin of this unique thermochromic PL property. The results showed that the efficient and highly reversible thermochromic PL emission of the Cs4PbBr6 perovskite is due to its desirable optical properties such as highly luminescent emission, efficient PL quenching at high temperatures, and thermally reversible structural changes.

Fabrication of dual-stimuli responsive films assembled by flavin mononucleotide and layered double hydroxides

A dual-stimuli responsive strategy is developed for the detection of melamine and temperature. Flavin mononucleotide (FMN) was immobilized onto layered double hydroxide (LDH) nanosheets through a layer-by-layer process. The as-prepared composite films show higher emission intensity, prolonged fluorescence lifetimes and fluorescence responses towards melamine and temperature.

Reversible Mechanochromic Delayed Fluorescence in 2D Metal-Organic Micro/Nanosheets: Switching Singlet-Triplet States through Transformation between Exciplex and Excimer

Mechanochromic luminescent materials have attracted much attention and present a variety of applications in information security, data recording, and storage devices. However, most of these smart luminescent systems are based on typical fluorescence and/or phosphorescence mechanisms; the mechanochromic delayed fluorescence (MCDF) materials involving switching singlet and triplet states are rarely studied to date. Herein, new 2D layered metal-organic micro/nanosheets, [Cd(9-AC)2(BIM)2] (named as MCDF-1; 9-AC = anthracene-9-carboxylate and BIM = benzimidazole) and its solvate form containing interlayer CH3CN (named as MCDF-2), which exhibit reversible mechanochromic delayed fluorescence characteristics, are presented. With applying the mechanical force, the luminescent center of MCDF-1 can be converted from 9-AC/BIM exciplex to 9-AC/9-AC excimer, resulting in alternations of delayed fluorescence. Such luminescent change can be further recovered by CH3CN fumigation, accompanied by the structural transformation from MCDF-1 to MCDF-2. Furthermore, the force-responsive process also refers to the energy redistribution between singlet and triplet states as inferred by both temperature-dependent photophysics and theoretical calculations. Therefore, this work not only develops new 2D micro/nanosheets as MCDF materials, but also supplies a singlet-triplet energy switching mechanism on their reversible mechanochromic process.

Layered rare-earth hydroxide (LRH, R = Tb, Y) composites with fluorescein: delamination, tunable luminescence and application in chemosensoring for detecting Fe(iii) ions

We demonstrate a novel example of tunable luminescence and the application of the delaminated FLN/OS-LRH composites (LRHs are layered rare-earth hydroxides, R = Tb, Y; FLN is the fluorescein named 2-(6-hydroxy-3-oxo-(3H)-xanthen-9-yl)benzoic acid; OS is the anionic surfactant 1-octane sulfonic acid sodium) in detecting Fe(iii) ions. The FLNxOS1-x species (x = 0.02, 0.05, 0.10, and 0.20) are intercalated into the LTbyY1-yH layers (y = 1, 0.9, 0.7, 0.5, 0.3, 0.1 and 0) by ion exchange reactions to yield the composites FLNxOS1-x-LTbyY1-yH. In the solid state, the LYH composites display green emission (564 nm) arising from the organic FLN, while in LTbH composites, the luminescence of the Tb3+ in the layers (545 nm) and the FLN in the interlayers is co-quenched. In the delaminated state in formamide (FM), FLNxOS1-x-LTbH composites display green to yellowish-green luminescence (540-574 nm) following the increasing FLN/OS ratio; while the FLN0.02OS0.98-LTbyY1-yH composites show green emission at ∼540 nm. The fluorescence lifetimes of the composites (4.22-4.63 ns) are comparable to the free FLN-Na, and the quantum yields (31.62-78.70%) of the composites especially that (78.70%) of the FLN0.02OS0.98-LYH are much higher than that (28.40%) of free FLN-Na. The recognition ability of the FLN0.02OS0.98-LYH composite for metal cations is researched. The delaminated FLN0.02OS0.98-LYH colloidal suspension exhibits high selectivity for Fe3+ over other ions (Mg2+, Al3+, Ni2+, Co2+, Cu2+, Zn2+, Mn2+, Pb2+, and Cd2+) with fluorescence quenching, which can work as a kind of turn-off fluorescence sensor for the detection of Fe3+. The detection limit of Fe3+ is determined to be 2.58 × 10-8 M and the quenching constant (Ksv) is 1.70 × 103 M-1. This is the first work on LRH materials working as a chemosensor for recognising metal cations. It provides a new approach for the design of LRH materials to be applied in fluorescence chemosensing.

Optical differential temperature measurement with beat frequency phase fluorometry

We present, to the best of our knowledge, a new method for differential temperature measurement based on thermal sensitivity of the fluorescence lifetime of thermographic phosphors. Pairs of thermographic phosphors are excited with intensity-modulated light at frequencies ω and ω+Δω. The phase shift Δθ of the summary fluorescence intensity beat signal envelope is measured. A prototype of a fluorometric differential temperature sensor is developed, and feasibility of the method is experimentally demonstrated with a Sm²⁺:SrFCl crystal and the D₁5->F7₀ transition for high thermal sensitivity. The observed linear dependence between envelope phase shift Δθ and temperature difference ΔT agrees with the theoretical prediction. Sensitivity of S=-0.97°/°C was achieved. This method could also be applied to differential measurements of any parameter affecting fluorescence lifetime.

Near-Ultraviolet to Near-Infrared Fluorescent Nitrogen-Doped Carbon Dots with Two-Photon and Piezochromic Luminescence

Carbon dots (CDs) have gained intensive interests owing to their unique structure and excellent optoelectronic performances. However, to acquire CDs with a broadband emission spectrum still remains an issue. In this work, nitrogen-doped CDs (N-CDs) with near-ultraviolet (NUV), visible, and near-infrared (NIR) emission were synthesized via one-pot solvothermal strategy, and the excitation-independent NUV and NIR emission and excitation-dependent visible emission were observed in the photoluminescence (PL) spectra of N-CDs. Moreover, the as-synthesized N-CDs displayed two-photon fluorescence emission. It is important to note that N-CDs also exhibited piezochromic luminescence with reversibility, in which the red- and blue-shifted PL with increasing applied pressure (0.07-5.18 GPa) and the red- and blue-shifted PL with releasing applied pressure (5.18 GPa to 1 atm) were developed for the first time. Combined with good hydrophilicity, high photobleaching resistance, and low toxicity, the piezochromic luminescence would greatly boost the valuable applications of N-CDs.

Nano-photosensitizer based on layered double hydroxide and isophthalic acid for singlet oxygenation and photodynamic therapy

Singlet oxygen has won a great deal of attention to catalysis and biological studies due to its strong oxidizing properties. However, the photosensitizers which require for the generation of singlet oxygen remain inadequate because of their lack of long-wavelength absorption, weak hydrophilicity, and poor biocompatibility. Here, we develop near-infrared laser activated supramolecular photosensitizers (isophthalic acid/layered double hydroxide nanohybrids) for efficient two-photon photodynamic therapy. The singlet oxygen quantum yield of nanohybrid is up to 0.74. Critically, in vitro tests verify the superior anti-cancer properties of nanohybrid with an IC50 determine to be 0.153 μg mL-1. The nanohybrids take advantage of the superior tissue penetration of 808 nm laser irradiation and exhibit a dramatically strong ability to ablate tumors in vivo, with extremely low toxicity. This work provides the proof of concept that ultralong-lived triplet excitons can function as two-photon-activated photosensitizers for an effective singlet oxygen generation.

Hydrotalcite monolayer toward high performance synergistic dual-modal imaging and cancer therapy

Recently, theranostic has drawn tremendous attention by virtue of the nanotechnology development and new material exploration. Herein, we reported a novel theranostic system by loading Au nanoclusters (AuNCs) and Chlorin e6 (photosensitizer, Ce6) onto the monolayer nanosheet surface of Gd-doped layered double hydroxide (Gd-LDH). The as-prepared Ce6&AuNCs/Gd-LDH exhibits a largely enhanced fluorescence quantum yield (QY) of 18.5% relative to pristine AuNCs (QY = 3.1%) as well as superior T₁ magnetic resonance imaging (MRI) performance (r₁ = 17.57 mM^-1s^-1) compared with commercial MRI contrast agent (Gd(III)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (Gd-DOTA): r₁ ≈ 3.4 mM^-1s^-1), resulting from a synergistic effect between AuNCs and Gd-LDH. In addition, both in vitro and in vivo therapeutic evaluations demonstrate an efficient dual-modality imaging guided anticancer performance, especially the synergetic enhanced magnetic resonance/fluorescence (MR/FL) visualization of tumor site. Therefore, this work demonstrates a successful paradigm for the design and preparation of LDHs monolayer-based theranostic material, which holds great promises in practical applications.

Polymorphism and solvates of 1-acetyl-3-(phenyl)-5-(1-pyrenyl)pyrazoline: the structures, thermal and optical-physical properties

Single crystals of the β polymorph, formic acid and propanoic acid solvates of the title compound (abbreviation APPP) have been obtained.

YVO4:Nd3+ nanophosphors as NIR-to-NIR thermal sensors in wide temperature range

We report on the potential application of NIR-to-NIR Nd3+-doped yttrium vanadate nanoparticles with both emission and excitation operating within biological windows as thermal sensors in 123-873 K temperature range. It was demonstrated that thermal sensing could be based on three temperature dependent luminescence parameters: the luminescence intensity ratio, the spectral line position and the line bandwidth. Advantages and limitations of each sensing parameter as well as thermal sensitivity and thermal uncertainty were calculated and discussed. The influence of Nd3+ doping concentration on the sensitivity of luminescent thermometers was also studied.

Thermochromic Ion-Exchange Micelles Containing H+ Chromoionophores

Thermochromic composites constitute a classical subfamily of stimuli responsive materials. We report here the thermochromic effect in Pluronic F-127 (F127) micelles containing hydrophobic ion-exchanger and H⁺ chromoionophores. The highly versatile and reversible thermochromism is attributed to the temperature-induced hydration-dehydration of the peripheral layer of the micelles, which in turn controls the ion-exchange process between the core and the periphery of the micelles. The color typically changes abruptly within 3-5 °C, and the color transition temperature can be tuned within 5-25 °C upon varying the F127 concentrations. This work lays the foundation of a new variety of thermochromic materials involving ion-exchange.

Studying the fluorescence conversion in organic charge transfer cocrystals of chalcone derivatives and TCNB

Four binary co-crystals were mainly formed by halogen bonds and charge transfer interactions, which generated remarkable optical properties.

Layered host-guest long-afterglow ultrathin nanosheets: high-efficiency phosphorescence energy transfer at 2D confined interface

Tuning and optimizing the efficiency of light energy transfer play an important role in meeting modern challenges of minimizing energy loss and developing high-performance optoelectronic materials. However, attempts to fabricate systems giving highly efficient energy transfer between luminescent donor and acceptor have achieved limited success to date. Herein, we present a strategy towards phosphorescence energy transfer at a 2D orderly crystalline interface. We first show that new ultrathin nanosheet materials giving long-afterglow luminescence can be obtained by assembling aromatic guests into a layered double hydroxide host. Furthermore, we demonstrate that co-assembly of these long-lived energy donors with an energy acceptor in the same host generates an ordered arrangement of phosphorescent donor-acceptor pairs spatially confined within the 2D nanogallery, which affords energy transfer efficiency as high as 99.7%. Therefore, this work offers an alternative route to develop new types of long-afterglow nanohybrids and efficient light transfer systems with potential energy, illumination and sensor applications.

A switchable electrochromism and electrochemiluminescence bifunctional sensor based on the electro-triggered isomerization of spiropyran/layered double hydroxides

A bifunctional ultrathin film electrode was fabricated based on layered double hydroxides and spiropyran, which exhibited electrochromism and electrochemiluminescence sensing behavior toward temperature and Zn²⁺ ions.

Novel Visible-Light Photodetector Based on Two-Dimensional Confined Electron Donor-Acceptor Co-Assembled Layered Double Hydroxide Ultrathin Films

Photodetectors are a class of critical optoelectronic devices that can transform incident light into a detectable electrical signal. In this work, we develop a novel photodetector based on two-dimensional (2D) confined electron donor-acceptor co-assembled ultrathin films (UTFs). The (PCDTBT@CN-PPV/LDHs) _n UTFs are composed of an organic electron donor, poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), and an acceptor, poly(5-(2-ethylhexyloxy)-2-methoxy-cyanoterephthalylidene) (CN-PPV), within inorganic Mg₂Al-layered double hydroxides (LDHs). The UTFs exhibit broad-range visible-light absorption, from 400 to 650 nm, resulting from complementary absorption of PCDTBT and CN-PPV. The fluorescence emission of the UTFs is completely quenched, implying the occurrence of photoinduced charge transfer (PCT). As a novel photodetector, the co-assembled UTFs have a high photocurrent and on/off switching ratio (300 nA/∼120), in contrast to those of the PCDTBT/CN-PPV drop-casting thin film (5.4 nA/∼1.6); a fast response; a short recovery time (lower than 0.1 s); and excellent wavelength and light-intensity dependence. The PCT mechanism can be attributed to the formation of a 2D bulk heterojunction of the two polymers within the interlayers of the LDH nanosheets. Furthermore, flexible UTFs on polyethylene terephthalate substrates are also fabricated, which exhibit excellent folding strength and electrical stability.

Tunable Electronic Transport Properties of 2D Layered Double Hydroxide Crystalline Microsheets with Varied Chemical Compositions

Transistors based on layered double hydroxides (LDH) single microcrystal are fabricated, whose conductivity of LDH can be tuned by varying metal cations or interlayer anions, but weakly affected by external electric field. The carrier mobility can reach about 1 × 10(-5) cm(2) V(-1) s(-1) , a value comparable to that of organic C60-based transistors. This work paves a way for future electrical applications of LDH.

Luminescent films for chemo- and biosensing

Luminescent films have received great interest for chemo-/bio-sensing applications due to their distinct advantages over solution-based probes, such as good stability and portability, tunable shape and size, non-invasion, real-time detection, extensive suitability in gas/vapor sensing, and recycling. On the other hand, they can achieve selective and sensitive detection of chemical/biological species using special luminophores with a recognition moiety or the assembly of common luminophores and functional materials. Nowadays, the extensively used assembly techniques include drop-casting/spin-coating, Langmuir-Blodgett (LB), self-assembled monolayers (SAMs), layer-by-layer (LBL), and electrospinning. Therefore, this review summarizes the recent advances in luminescent films with these assembly techniques and their applications in chemo-/bio-sensing. We mainly focused on the discussion of the relationship between the sensing properties of the films and their architecture. Furthermore, we discussed some critical challenges existing in this field and possible solutions that have been or are being developed to overcome these challenges.