Multiple Anti‐Counterfeiting Guarantees from a Simple Tetraphenylethylene Derivative – High‐Contrasted and Multi‐State Mechanochromism and Photochromism

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

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

Tetraphenylethene-modified polysiloxanes: Synthesis, AIE properties and multi-stimuli responsive fluorescence

Herein, polysiloxane-based aggregation-induced emission (AIE) polymers and rubbers were prepared which display interesting multi-stimuli responsive fluorescence. TPE-modified polydimethylsiloxanes (PDMS-TPE) as polysiloxane-based AIE polymers were synthesized through Heck reaction of bromo-substituted tetraphenylethene (TPE-Br) and vinyl polysiloxanes. As expected, TPE moiety endows the modified polysiloxane with typical AIE behavior. However, limited by the long polymer chains, the aggregation process of PDMS-TPE shows obvious differences compared with the small molecule TPE-Br. The fluorescence of PDMS-TPE in THF/H2O starts to increase when the H2O fraction (fw) is 70% while TPE-Br is nearly non-luminous until the fw is up to 99%. The fluorescence intensity ratio (I/I0) of PDMS-TPE in the aggregated state and dispersed state is over 1300, greater than that of TPE-Br (I/I0 = 380). More importantly, the exceptional thermal motion of Si-O-Si chains and AIE characteristic of TPE moiety work together, enabling PDMS-TPE to show specific temperature-dependent fluorescence with a wider response range of room temperature to 190°C, which is distinguished from TPE-Br. And such fluorescence responsiveness possess good fatigue-resistance. Furthermore, fluorescent silicone rubbers, r-PDMS-TPE were prepared by using PDMS-TPE as additive of the base gum. They display interesting solvent-controllable fluorescence and higher tensile strength (4.42 MPa) than the control sample without TPE component (1.96 MPa). Notably, a unique stretching-enhanced emission (SEE) phenomenon is observed from these TPE-modified silicone rubbers. When being stretched, the rubbers' fluorescent emission intensity could increase by 143%.

Robust luminogens as cutting-edge tools for efficient light emission in recent decades

Blue luminogens play a vital role in white lighting and potential metal-free fluorescent materials and their high-lying excited states contribute to harvesting triplet excitons in devices. However, in TADF-OLEDs (ΔEST < 0.1 eV), although T1 excitons transfer to S1via RISC with 100% IQE, the longer lifetime of blue TADF suffers from efficiency roll-off (RO). In this case, hybridized local and charge transfer (HLCT) materials have attracted significant interest in lighting owing to their 100% hot exciton harvesting and enhanced efficiency. Both academics and industrialists widely use the HLCT strategy to improve the efficiency of fluorescent organic light-emitting diodes (FOLEDs) by harvesting dark triplet excitons through the RISC process. Aggregation-induced emissive materials (AIEgens) possess tight packing in the aggregation state, and twisted AIEgens with HLCT behaviour have a shortened conjugation length, inducing blue emission and making them suitable candidates for OLED applications. TTA-OLEDs are used in commercial BOLEDs because of their moderate efficiency and reasonable operation lifetime. In this review, we discuss the devices based on TTA fluorophores, TADF fluorophores, HLCT fluorophores, AIEgens and HLCT-sensitized fluorophores (HLCT-SF), which break through the statistical limitations.

A Reversible MnO2 Deposition-Enabled Multicolor Electrochromic Device with Efficient Tunability of Ultraviolet-Visible Light

Electrochromic technology offers exciting opportunities for smart applications such as energy-saving and interactive systems. However, achieving dual-band regulation together with the multicolor function is still an unmet challenge for electrochromic devices. Herein, an ingenious electrochromic strategy based on reversible manganese oxide (MnO2) electrodeposition, different from traditional ion intercalation/deintercalation-type electrochromic materials is proposed. Such a deposition/dissolution-based MnO2 brings an intriguing electrochromic feature of dual-band regulation for the ultraviolet (UV) and visible lights with high optical modulation (93.2% and 93.6% at 400 and 550 nm, respectively) and remarkable optical memory. Moreover, a demonstrative smart window assembled by MnO2 and Cu electrodes delivers the electrochromic properties of effective dual-band regulation accompanied by multicolor changes (transparent, yellow, and brown). The robust redox deposition/dissolution process endows the MnO2-based electrochromic device with excellent rate capability and an areal capacity of 570 mAh m-2 at 0.1 mA cm-2. It is believed that the metal oxide-based reversible electrodeposition strategy would be an attractive and promising electrochromic technology and provide a train of thought for the development of multifunctional electrochromic devices and applications.

Fluorescence Switching and Photoisomerization of a Spiropyran Molecular Photoswitch through Confined Spaces Regulation in Crystals

Spiropyran (SP) and its derivatives are highly attractive owing to their distinctive merits in high contrast and fast response read-out systems. However, the realization of photoswitching properties of SP is hindered in the aggregate state, particularly in crystals owing to the dense packing of molecules leading to insufficient study of the relationship between the molecular structure/stacking mode and photoswitching behavior. Herein, we report three SP derivatives: different flexible chains (carboxyl group for SP-0 and ester group for SP-1) are attached to the indoline moiety, while the ester group is attached to the chromene moiety for SP-2. SP-1 exhibits fluorescent photoswitching properties in crystals due to the weak intermolecular interactions resulting in enough free space for the photoisomerization. The presence of hydrogen bonds in SP-0 enhances the molecular interactions to restrict the photoisomerization, and the ester group of SP-2 impacts the thermodynamic properties, thereby limiting the realization of photoswitching of SP-2.

Manipulation of aurophilicity in constructed clusters of gold(I) complexes with boosted luminescence and smart responsiveness

High-performance luminescent gold(I) complexes have attracted considerable attention due to their potential applications in various fields, but their construction is a significantly challenging task. Herein, we designed and synthesized a series of novel dinuclear gold(I) complexes 1-4 based on 1,2-bis(diphenylphosphino)benzene and 1,4-bis(diphenylphosphino)benzene frameworks, where para-substitutions of benzene ring were employed for comparison and bulky t-butyl groups were introduced into carbazole ligands to assist flexibly regulating the aurophilicity. Among them, the structure of complex 1 was confirmed by single-crystal X-ray diffraction, and all the complexes exhibited typical aggregation-induced emission characteristics. Due to the construction of intramolecular aurophilicity and the formation of molecular clusters, noticeable enhancement of the luminescent efficiency was achieved for the core complex 1. Together with the introduction of flexible t-butyl groups, good responsiveness towards external mechanical force and solvent vapors were also realized. Moreover, the specific bioimaging ability of complex 1 towards cancer cells was demonstrated. Thus, this work presents the crucial capability of aurophilic manipulation in tuning the luminescence and smart behaviors of gold complexes, and it will open a new route to developing high-performance luminescent materials.

A solvent-controlled photoresponsive ionic hydrogen-bonded organic framework for encryption applications

Two novel ionic hydrogen-bonded organic frameworks (iHOF-17 and iHOF-18) were obtained by integrating organosulfonic acids with amidine salts. Among them, iHOF-18 exhibits fast, reversible, and high-contrast UV-induced photochromic properties, and this property is solvent-controlled. This work provides valuable insights for designing advanced anti-counterfeiting techniques and encryption applications.

A Heat-Resistant Polymer Based on the Reversible Change in Polymer Skeleton Structure for Self-Anticounterfeiting

Heat-resistant polymer materials have been widely used in many fields, but their anticounterfeit is still a significant challenge. This work has successfully constructed a heat-resistant polymer material that can achieve self-anticounterfeit. In response to changes in the external environment, the color of polymer changes from yellow-green to red reversibly, which is due to the fact that polymer material's backbone undergoes isomerization. Therefore, this high-performance polymer material can not only be used in a high-temperature environment for a long time but also achieve its anticounterfeit and be used in advanced security applications.

Natural Acceptor of Coumarin-Isomerized Red-Emissive BioAIEgen for Monitoring Cu2+ Concentration in Live Cells via FLIM

Artificial aggregation-induced emission luminogens (AIEgens) have flourished in bio-applications with the development of synthetic chemistry, which however are plagued by issues like singularity in structures and non-renewability. The unique structures and renewability of biomass moieties can compensate for these drawbacks, but their properties are hard to design and regulate due to their confined structures. Therefore, it appears to be a reasonable approach to derive AIEgens from abundant biomass (BioAIEgens), integrating the bilateral advantages of both synthetic and natural AIEgens. In this work, the blue-violet emissive coumarin with its lactone structure serving as a rare natural acceptor, is utilized to construct donor-π-acceptor typed BioAIE isomers incorporating the propeller-like and electron-donating triphenylamine (TPA) unit. The results show that Cm-p-TPA undergoes charge transfer with its keto form, emitting red light at 600 nm, which can be applied to monitor Cu2+ concentration during mitophagy using fluorescence lifetime imaging microscopy because of the excellent biocompatibility, photostability, and specific recognition to Cu2+ . This work not only demonstrates the feasibility of utilizing positional isomerization to modulate excited-state evolutions and resultant optical properties, but also provides evidence for the rationality of constructing biologically-active BioAIEgens via a biomass-derivatization concept.

Force-triggered hypso- and bathochromic bidirectional fluorescence switching beyond 120 nm and its anticounterfeiting applications

Achieving high-contrast tricolor emissive regulation of a single-component molecule using a single type of external stimulus is highly desirable but challenging. In the present study, we report a symmetric acceptor-donor-acceptor (A-D-A)-type aggregation-induced emission-active luminogen, which displays a sequential high-contrast fluorescence switching just by anisotropic mechanical grinding. Specifically, upon light grinding, an orange-yellow-to-blue hypsochromic mechanofluorochromic response with a distinct color contrast (change in the maximum emission wavelength, Δλem,max = 122 nm) is noticed, and the slightly ground solid exhibits a blue-to-red high-contrast (Δλem,max = 185 nm) bathochromic mechanofluorochromic conversion upon vigorous grinding. Thus, using a single luminogen developed here, we can realize wide-range (Δλem,max > 100 nm) hypso- and bathochromic fluorescence mechanochromisms simultaneously. The tricolored mechanofluorochromic phenomenon is attributed to two different morphological transitions involving crystalline-to-crystalline and crystalline-to-amorphous states. Furthermore, three information anticounterfeiting systems are developed using the luminogen presented here.

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.

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.

Programmable Dynamic Information Storage Composite Film with Highly Sensitive Thermochromism and Gradually Adjustable Fluorescence

The development of an integrated material system capable of effectively organizing and combining multisource information, such as dynamic pigmentary, structural, and fluorescent colors, is significant and challenging. Achieving such programmable dynamic information storage can considerably enhance the diversity and security of information deliveries. Here, a polymer-stabilized cholesteric liquid crystal system with highly temperature-sensitive structural color and light-sensitive pigmentary and fluorescence colors is presented. The prepared cholesteric liquid crystals (clcs) can reversibly change their structural color from red to blue within variational 3 °C near room temperature, and exhibit a gradually adjustable fluorescence which can transform from blue to pink and finally to bright red. All this dynamic information is programmable and tailored, hundreds of thousands of (>540 000) pattern combinations can easily be achieved by optical writing with a "bagua" pattern photomask. Therefore, if the corresponding code combinations to the pattern are assigned particular meanings, encrypted transmission of information with very high security can be achieved by utilizing applicable information encoding tables and decryption rules.

Light-Responsive Supramolecular Liquid-Crystalline Metallacycle for Orthogonal Multimode Photopatterning

The development of multimode photopatterning systems based on supramolecular coordination complexes (SCCs) is considerably attractive in supramolecular chemistry and materials science, because SCCs can serve as promising platforms for the incorporation of multiple functional building blocks. Herein, we report a light-responsive liquid-crystalline metallacycle that is constructed by coordination-driven self-assembly. By exploiting its fascinating liquid crystal features, bright emission properties, and facile photocyclization capability, a unique system with spatially-controlled fluorescence-resonance energy transfer (FRET) is built through the introduction of a photochromic spiropyran derivative, which led to the realization of the first example of a liquid-crystalline metallacycle for orthogonal photopatterning in three-modes, namely holography, fluorescence, and photochromism.

Thermally activated delayed fluorescence emitters showing wide-range near-infrared piezochromism and their use in deep-red OLEDs

Organic small molecules exhibiting both thermally activated delayed fluorescence (TADF) and wide-ranging piezochromism (Δλ > 150 nm) in the near-infrared region have rarely been reported in the literature. We present three emitters MeTPA-BQ, tBuTPA-BQ and TPPA-BQ based on a hybrid acceptor, benzo[g]quinoxaline-5,10-dione, that emit via TADF, having photoluminescence quantum yields, ΦPL, of 39-42% at photoluminescence (PL) maxima, λPL, of 625-670 nm in 2 wt% doped films in 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP). Despite their similar chemical structures, the PL properties in the crystalline states of MeTPA-BQ (λem = 735 nm, ΦPL = 2%) and tBuTPA-BQ (λem = 657 nm, ΦPL = 11%) are significantly different. Further, compounds tBuTPA-BQ and TPPA-BQ showed a significant PL shift of ∼98 and ∼165 nm upon grinding of the crystalline samples, respectively. Deep-red organic light-emitting diodes with MeTPA-BQ and tBuTPA-BQ were also fabricated, which showed maximum external quantum efficiencies, EQEmax, of 10.1% (λEL = 650 nm) and 8.5% (λEL = 670 nm), respectively.

Structure-activity strategies for mechanically responsive fluorescent materials: a molecular perspective

Mechanical response luminescence (MRL) describes the photophysical properties triggered by mechanical stimulation. Usually, MRL can be regulated by intermolecular interactions, molecular conformation or molecular packing, to achieve the desirable optical properties. Herein, at the molecular level, this review covers the factors that influence mechanically responsive fluorescent materials, involving the single- or multifactorial modulation of aliphatic chains, donor-receptor switch, substituent adjustment, and position isomerism. According to these factors, the structure-activity strategies can be summarized as: (i) the self-recovery of optical properties, from the final to initial state, can be regulated by introducing long alkyl chains to a fluorophore. (ii) The sensitivity of MRL materials can be controlled by modifying the donor-acceptor structure via the changed ICT (intramolecular charge transfer) and intramolecular interaction. (iii) The electronic and steric effects of substituents can affect ICT and intermolecular interactions, thereby resulting in high quantum yield and high-contrast MRL materials via changing the molecular stacking of crystalline states. (iv) Intermolecular interaction is modulated by the position isomerism of the substituents, which results in switched molecular packing for the extended response toward a wide range of stimuli. It is anticipated that the molecular mechanisms of these structure-activity relationships will serve as a significant reference for developing novel, high contrast, recyclable mechanical response luminous materials.

Polymorphism and Light-Driven Forward Movement of TPE Derivative Micro-Crystal due to ArH-pi Interactions Difference

Aggregation-induced emission (AIE) and aggregation-caused quenching (ACQ) are two classes of opposite luminescence phenomena. It is almost impossible to show both AIE and ACQ effect simultaneously by the same molecule. However, here we report that a simple TPE derivative TAP-TPE grows into both AIE crystals and ACQ ones. It is found that equatorial, contact distance-longer and weak ArH-π interactions exist in AIE crystals while vertical, contact distance-shorter and strong ArH-π interactions appear in ACQ crystals. Theoretical calculation of electron density on the interaction atoms unveils that ACQ crystals have much larger change in electron density than AIE ones, suggesting that the intermolecular electron transfer aroused by the strong ArH-π hydrogen bonds leads to ACQ phenomenon. This result provides a new insight into the emission mechanism in aggregation state. Interestingly, due to the ArH-pi interactions difference, only one of five kinds of crystals shows rapid photochromism, and can act as multimode anti-counterfeiting materials. Very exceptionally, for the first time we find that the photochromic micrometric rod-like crystal even makes forward rolling movement as it repeatedly bends and straightens by responding to on and off of the ultraviolet light irradiation, displaying potential for photo-actuator and light-driven micro-vehicle.

Functionalized Fluorescent Organic Nanoparticles Based AIE Enabling Effectively Targeting Cancer Cell Imaging

We report a fluorescent dye TM by incorporating the tetraphenylethylene (TPE) and cholesterol components into perylene bisimides (PBI) derivative. Fluorescence emission spectrum shows that the dye has stable red emission and aggregation-induced emission (AIE) characteristics. The incorporation of cholesterol components triggers TM to show induced chirality through supramolecular self-assembly. The cRGD-functionalized nanoparticles were prepared by encapsulating fluorescent dyes with amphiphilic polymer matrix. The functionalized fluorescent organic nanoparticles exhibit excellent biocompatibility, large Stokes' shift and good photostability, which make them effective fluorescent probes for targeting cancer cells with high fluorescence contrast.

Chiral Dual Core Chromophores Based on Binaphthyl Acrylonitrile Motif with Tunable Dual Emission Bands and Anti-counterfeiting

Small organic molecules which can emit fluorescence with tunable dual emission bands are significant for fundamental research and broad applications. In this work, two binaphthyl based arylacrylonitrile derivatives with pyrene and triphenylamine unit (BiNp-Py and BiNp-TPA) were designed and synthesized, respectively, featuring chiral backbone and dual AIE-active cyanostyrene-linked chromophores. Excellent fluorescence emissions in a range of solution and solid states were observed with high quantum yields, indicative of the solvatochromism and mechanochromism. More interestingly, dual emission bands were found and tunable by the water fraction in THF, and speculatively attributed to the balancing of intramolecular charge transfer (ICT) and locally excited (LE) emission in solution and aggregate states. Furthermore, the potential application in anti-counterfeiting ink was also explored, indicating the very low concentration (5 ppm) for sufficient distinguishable vision and small colour migration (28 nm) for printing on the filter. The present work provides a new strategy to design organic luminescent structure having widely fluorescent emissions in dual states and a valuable reference for the study of chiral optical materials.

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.

Supramolecular Liquid Crystal Carbon Dots for Solvent-Free Direct Ink Writing

Recent years have witnessed the major advances of nanolights with extensive exploration of nano-luminescent materials like carbon dots (CDs). However, solvent-free processing of these materials remains a formidable challenge, impeding endeavors to develop advanced manufacturing techniques. Herein, in response to this challenge, liquid crystallization is demonstrated as a versatile and robust approach by deliberately anchoring flexible alkyl chains on the CDs surface. Alkyl chain grafting on the CDs surface is observed to substantially depress the common aggregation-caused quenching effect, and results in a shift of self-assembly structure from the crystalline phase to smectic liquid crystalline phase. The liquid-crystalline phase-transition temperature is ready to adjust by varying the alkyl chain length, endowing low-temperature (<50 °C) melt-processing capabilities. Consequently, the first case of direct ink writing (DIW) with liquid crystal (LC) carbon dots is demonstrated, giving rise to highly emissive objects with blue, green and red fluorescence, respectively. Another unexpected finding is that DIW with the LC inks dramatically outperforms DIW with isotropic inks, further highlighting the significance of the LC processing. The approach reported herein not only exhibits a fundamental advance by imparting LC functions to CDs, but also promises technological utility in DIW-based advanced manufacturing.

General Synthesis of Carbon Dot-Based Composites with Triple-Mode Luminescence Properties and High Stability

Carbon dot (CD)-based luminescent materials have attracted great attention in optical anti-counterfeiting due to their excellent photophysical properties in response to ultraviolet-to-visible excitation. Hence, there is an urgent need for the general synthesis of CD-based materials with multimode luminescence properties and high stability; however, their synthesis remains a formidable challenge. Herein, CDs were incorporated into a Yb,Tm-doped YF3 matrix to prepare CDs@YF3:Yb,Tm composites. The YF3 plays a dual role, not only serving as a host for fixing rare earth luminescent centers but also functioning as a rigid matrix to stabilize the triplet state of the CDs. Under the excitation of 365 nm ultraviolet light and 980 nm near-infrared light, CDs@YF3:Yb,Tm exhibited blue fluorescence and green room-temperature phosphorescence of CDs and upconversion luminescence of Tm3+, respectively. Due to the strong protection of the rigid matrix, the stability of CDs@YF3:Yb,Tm is greatly improved. This work provides a general synthesis strategy for achieving multimode luminescence and high stability of CD-based luminescent materials and offers opportunities for their applications in advanced anti-counterfeiting and information encryption.

Two New Red/Near-Infrared Ir(Ⅲ) Complexes with Reversible and Force-Induced Enhanced Mechanoluminescence

Two novel ionic red/near-infrared Ir(III) complexes (Ir1 and Ir2) were reasonably designed and prepared using 2-(1-isoquinolinyl)-9,10-anthraquinone as the main ligand and 4,4'-dimethyl-2,2'-bipyridyl and 4,4'-dimethoxy-2,2'-bipyridyl as the auxiliary ligands, respectively. Both complexes showed bright phosphorescence in solution (peak at 618 nm with a shoulder at 670 nm). Interestingly, the phosphorescence peak of two Ir(III) complexes showed a blue-shift of about 36 nm after being ground. Simultaneously, both complexes exhibited mechanical force-induced enhanced emission, and the intensity of the luminescence for Ir1 and Ir2 increased by around two times compared to the one before being ground, respectively. Powder X-ray diffraction (PXRD) and time-dependent density functional theory (TD-DFT) calculation were utilized to understand well the mechanism of this phenomenon and suggested that the destruction of the well-ordered crystalline nature and the decline in triplet-triplet annihilation maybe responsible for the pressure-induced blue-shift and the enhancement of the phosphorescence.

Synthesis, structure and mechanofluorochromic properties of phenothiazine-S-oxide and phenothiazine-S,S-dioxide derivatives

In this work, two novel luminescent molecules containing distorted phenothiazine-S-oxide and phenothiazine-S,S dioxide skeletons were synthesized by oxidation reactions using different oxidants (m-chloroperoxybenzoic acid, acetic acid /hydrogen peroxide). The target compounds were all confirmed by ¹H NMR, ¹³C NMR and EI-MS. Combined with the results of UV-vis absorption spectra and fluorescence emission spectra, we found that the different oxidation states of S-atom, from sulfide (+2) to sulfoxide (+4) and sulfone (+6), led to the blue, yellow-green and yellowish fluorescence of these compounds in the solid states. Subsequent studies showed that the molecule containing the phenothiazine-S-oxide skeleton exhibited obvious solvatochromism, and the increase of solvent polarity induced a red-shift in the emission wavelength. Moreover, this molecule also exhibited a rare self-recovery mechanochromatic behavior. In addition, these properties were further confirmed by theoretical calculations and X-ray single-crystal diffraction analysis.

Multi-Stimuli-Responsive Amphiphilic Pyridinium Salt and Its Application in the Visualization of Level 3 Details in Latent Fingerprints

Organic luminescent materials that can simultaneously achieve multimode mechanochromism and its water-vapor-induced recovery are desirable for practical applications but rarely reported. Herein, an amphiphilic compound, 4-(9H-carbazol-9-yl)-1-(2-hydroxyethyl)pyridin-1-ium bromide (CPAB), is designed by integrating a lipophilic aromatic unit and hydrophilic end in the molecular architecture. Self-recovered mechanochromism from brown to cyan is observed upon mechanical grinding in air. Comprehensive research by X-ray diffraction, infrared spectroscopy, and single-crystal analysis reveals that the photoluminescence switch originates from the variation in intermolecular hydrogen bonds and molecular packing mode. The amphiphilic nature of CPAB allows water molecules to enter the crystalline lattice, forming two polymorphs of the crystalline phase, namely CPAB-D and CPAB-W. The hydrosoluble CPAB exhibits excellent capability in probing the level 3 details of fingerprints because its lipophilic part can target the fatty acid residues of fingerprints, leading to strong aggregation-induced fluorescence. The research may inspire the design of latent fingerprint developers and application in forensics/anti-counterfeiting.

Multiphysical Field Modulated VO2 Device for Information Encryption

In the information explosion society, information security is highly demanded in the practical application, which raised a surge of interest in designing secure and reliable information transmission channels based on the inherent properties of emerging devices. Here, an innovative strategy to achieve the data encryption and reading during the data confidential transmission based on VO₂ device is proposed. Owing to the specific insulator-to-metal transition property of VO₂ , the phase transitions between the insulator and metallic states are modulated by the combination of electric field, temperature, and light radiation. These external stimulus-induced phase diagram is directly associated with the defined VO₂ device, which are applicable for control the "0" or "1" electrical logic state for the information encryption. A prototype device is fabricated on an epitaxial VO₂ film, which displayed a unique data encryption function with excellent stability. The current study not only demonstrated a multiphysical field-modulated VO₂ device for information encryption, but also supplied some clues for functional devices applications in other correlated oxide materials.

Molecular Cage with Dual Outputs of Photochromism and Luminescence Both in Solution and the Solid State

The rational design of stimuli-responsive materials requires a deep understanding of the structure-activity relationship. Herein, we proposed an intramolecular conformation-locking strategy─incorporating flexible tetraphenylethylene (TPE) luminogens into the rigid scaffold of a molecular cage─to produce a molecular photoswitch with dual outputs of luminescence and photochromism in solution and in the solid states at once. The molecular cage scaffold, which restricts the intramolecular rotations of the TPE moiety, not only helps to preserve the luminescence of TPE in a dilute solution but facilitates the reversible photochromism on account of the intramolecular cyclization/cycloreversion reactions. Furthermore, we demonstrate assorted applications of this multiresponsive molecular cage, e.g., photo-switchable patterning, anticounterfeiting, and selective vapochromism sensing.

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

As an important organic photofunctional material, spirooxazine (SO) usually does not exhibit photochromism in the solid state since the intermolecular π-π stacking impedes photoisomerization. Developing photochromic SO in the solid state is crucial for practical applications but is still full of challenges. Here, a series of spirooxazine derivatives (SO1-SO4) with bulky aromatic substituents at the 4- and 7-positions of the skeleton, which provide them with a large volume with which to undergo solid-state photochromism under mild conditions, is designed and synthesized. All the compounds SO1-SO4 exhibit tunable solid photochromism without ground colors, excellent fatigue resistance, and high thermal stability. Notably, it takes only 15 s for SO4 to reach the saturation of absorption intensity, thought to represent the fastest solid-state photoresponse of spirooxazines. X-ray crystal structures of the intermediate compound SO0 and the products SO1-SO2 as well as computational studies suggest that the bulky aromatic groups can lead to a hypochromic effect, allowing for the photochromism of SO in the solid state. The ideal photochromic properties of these spirooxazines open a new avenue for their applications in UV printing, quick response code, and related fields.

Solvent stimuli-responsive off-on fluorescence induced by synergistic effect of doping and phase transformation for Te4+ doped indium halide perovskite: Giving printable and colorless ink for information encryption and decryption

Since traditional fluorescent materials are too easily observed by the eyes just under the UV light, off-on fluorescent materials are explored as the new generation of fluorescent labels. In the "off" state, such off-on fluorescent labels cannot be observed by naked eyes under either natural light or UV light. Only after a specific decryption treatment to make the fluorescent materials turning into the "on" state, the fluorescent labels can be observed under the UV light. Up to now, it is still a challenge to prepare fluorescent inks with aforementioned ideal properties by using halide perovskite materials. Herein, we reported the first example of Te⁴⁺ doped indium halide perovskite inks with both off-on fluorescence under solvent stimuli and invisible ink color by the naked eyes. The synergistic effect of doping/undoping of Te⁴⁺ together with the reversible phase transformation between Cs₂InCl₅(H₂O) and Cs₃InCl₆ under solvent stimuli is key for the off-on fluorescence of crystals. Under acid solvent, the substitutional doping of Te⁴⁺ during the process of phase transformation from Cs₃InCl₆ to Cs₂InCl₅(H₂O):Te⁴⁺ gives rise to "turning-on" orange emission from Te-induced self-trap emission (STE). Under the stimuli of methanol, the dissolution of Te⁴⁺ from the crystals destroys the structure of Te⁴⁺ in ligand-field and results in "turning-off" Te-induced emission during the process of phase transformation from Cs₂InCl₅(H₂O):Te⁴⁺ to Cs₃InCl₆. On the basis of the Te⁴⁺ doped indium halide perovskite, printable and colorless ink can be prepared for the confidential information encryption and decryption. Since the mixture of Cs₃InCl₆ crystals and TeCl₄ have no absorption in visible light scope, the printed encrypted information by such off-state fluorescent ink is colorless and invisible by the naked eyes under either ambient light or UV light. After decryption by acid solvent stimuli, the resulted Cs₂InCl₅(H₂O):Te⁴⁺ doping crystals have a large Stokes shift with absorption below 450 nm from the excitation of Te⁴⁺ in ligand-field and emission around 570 nm from Te-induced STE. It makes the decryption information still colorless and invisible by the naked eyes under the ambient light but visible and readable under the UV light. In comparison to traditional undoped CsPbBr₃/CsPb₂Br₅ perovskites with small Stokes shift and eye-visible ink color, the current colorless Te⁴⁺doped indium halide perovskites are no doubt providing better security level for both encrypted and decrypted information.

Full-Color Emissive D-D-A Carbazole Luminophores: Red-to-NIR Mechano-fluorochromism, Aggregation-Induced Near-Infrared Emission, and Application in Photodynamic Therapy

The preparation of multifunctionalized luminophores with full-color emission based on an identical core skeleton is a significative but challenging research topic. In this work, eight donor-donor-acceptor (D-D-A)-type luminogens based on a central carbazole core bearing a C₆ hydrocarbon chain were designed by using different kinds of donor and acceptor units on the left and right, and synthesized in good yields. These D-D-A carbazole derivatives display deep-blue, sky-blue, cyan, green, yellow-green, yellow, orange and red fluorescence in the solid state, achieving full-color emission covering the whole visible light range under UV light illumination. Notably, the dicyano-functionalized triphenylamine-containing carbazole derivative exhibits rare aggregation-induced near-infrared emission and red-to-near-infrared mechano-fluorochromism with high contrast beyond 100 nm. Furthermore, the red-emissive luminogen can serve as a potential candidate for cell imaging and photodynamic therapy (PDT). This work not only provides reference for the construction of full-color emissive systems but also opens a new avenue to the preparation of multifunctionalized luminophores capable of simultaneous application in near-Infrared mechanical-force sensors and PDT fields.

Bright Organic Mechanoluminescence and Remarkable Mechanofluorochromism from Circularly Polarized TADF Enantiomers with Aggregation-Induced Emission Properties

The development of circularly polarized thermally activated delayed fluorescence (CP-TADF) luminogens with stimuli-response characteristics remains challenging. Herein, a pair of organic enantiomers, S-CzTA and R-CzTA, with aggregation-induced emission properties, have been successfully developed by introducing chiral 1,2,3,4-tetrahydronaphthalene and carbazole to phthalimide. They present CP-TADF properties in toluene solutions, giving dissymmetric factors of 0.84×10^-3 and -1.03×10^-3 , respectively. In the crystalline state, both S-CzTA and R-CzTA can emit intense blue TADF and produce very bright sky-blue mechanoluminescence (ML) and remarkable mechanofluorochromism (MFC) under the stimuli of mechanical force. Single-crystal analysis and theoretical calculation results suggest that their ML activities are probably associated with their chiral and polar molecular structures and unique non-centrosymmetric molecular packing modes. Furthermore, the MFC properties of the enantiomers likely originate from the destruction of crystal structure, leading to the planarization of molecular conformation. This work may provide helpful guidance for developing new CP-TADF materials with force-stimuli-responsive properties.

High-contrast reversible multiple color-tunable solid luminescent ionic polymers for dynamic multilevel anti-counterfeiting

Dynamic color-tunable luminescent materials, which possess huge potential applications in advanced multilevel luminescence anti-counterfeiting, are of considerable interest. However, it remains challenging to develop simple high-contrast reversible multiple (triple or more than triple) color-tunable high-efficiency solid luminescent materials with low cost, facile synthesis, and good processability. Herein, by simply grafting charged multi-color AIEgen-based chromophores into polymers, a series of high-efficiency multiple color-tunable luminescent single ionic polymers are constructed through tuning feed ratios, counter anions and reaction solvents. Remarkably, some ionic polymers can not only achieve rare high-contrast reversible multiple color-tunable emission in solid states in response to different solvent stimuli, but also could realize excitation-dependent color-tunable emission. To the best of our knowledge, such charming multiple (triple or more than triple) color-tunable solid polymers responding to multiple external stimuli are still rare. Based on comparative studies of emission spectra, excitation spectra and fluorescence lifetimes before and after swelling, it could be inferred that solvent stimuli could induce microstructure changes of these ionic polymers and then change the aggregated-states of their corresponding AIE-active emission centers. Moreover, the different solvent stimuli could induce to produce different degrees of microstructure changes, resulting in their unique multiple color-tunable emission. More significantly, these smart color-tunable ionic polymers show great promise for applications in dynamic multilevel (three-level or even more than three-level) anti-counterfeiting.

A Class of Organic Units Featuring Matrix-Controlled Color-Tunable Ultralong Organic Room Temperature Phosphorescence

A novel class of organic units (N-1 and N-2) and their derivatives (PNNA-1 and PNNA-2) with excellent property of ultralong organic room temperature phosphorescence (UORTP) is reported. In this work, N-1, N-2, and their derivatives function as the guests, while organic powders (PNCz, BBP, DBT) and polymethyl methacrylate (PMMA) serve as the host matrixes. Amazingly, the color of phosphorescence can be tuned in different states or by varying the host matrixes. At 77 K, all molecules show green afterglow in the monomer state but yellow afterglow in the aggregated state because strong intermolecular interactions exist in the self-aggregate and induce a redshift of the afterglow. In particular, PNNA-1 and PNNA-2 demonstrate distinctive photoactivated green UORTP in the PMMA film owing to the generation of their cation radicals. Whereas the PNNA-1@PNCz and PNNA-2@PNCz doping powders give out yellow UORTP, showing matrix-controlled color-tunable UORTP. In PNCz, the cation radicals of PNNA-1 and PNNA-2 can stay stably and form strong intermolecular interactions with PNCz, leading to a redshift of ultralong phosphorescence.

Carbazole-based aggregation-induced phosphorescent emission-active gold(I) complexes with various phosphorescent mechanochromisms

A series of carbazole-containing gold(I) complexes modified with different substituents were successfully designed and synthesized, and their molecular structures were characterized by nuclear magnetic resonance spectroscopy and mass spectrometry. The aggregation-induced behaviors of these gold(I) complexes were studied by ultraviolet/visible and photoluminescence spectroscopy. Meanwhile, their mechanical force-responsive emissive properties were also investigated via solid-state photoluminescence spectroscopy. Interestingly, all these gold(I)-based luminogenic molecules were capable of exhibiting aggregation-induced phosphorescent emission phenomena. Furthermore, their solids of three gold(I) complexes displayed contrasting mechano-responsive phosphorescence features. More specifically, trifluoromethyl or methoxyl-substituted luminophores 1 and 3 demonstrated mechanochromic behaviors involving blue-shifted phosphorescence changes, and their mechanoluminochromic phenomena were reversible. However, the solid-state phosphorescence of phenyl-substituted luminophor 2 was not sensitive to external mechanical force.

Harnessing molecular isomerization in polymer gels for sequential logic encryption and anticounterfeiting

Using smart photochromic and luminescent tissues in camouflage/cloaking of natural creatures has inspired efforts to develop synthetic stimuli-responsive materials for data encryption and anticounterfeiting. Although many optical data-encryption materials have been reported, they generally require only one or a simple combination of few stimuli for decryptions and rarely offer output corruptibility that prevents trial-and-error attacks. Here, we report a series of multiresponsive donor-acceptor Stenhouse adducts (DASAs) with unprecedented switching behavior and controlled reversibility via diamine conformational locking and substrate free-volume engineering and their capability of sequential logic encryption (SLE). Being analogous to the digital circuits, the output of DASA gel-based data-encryption system depends not only on the present input stimulus but also on the sequence of past inputs. Incorrect inputs/sequences generate substantial fake information and lead attackers to the point of no return. This work offers new design concepts for advanced data-encryption materials that operate via SLE, paving the path toward advanced encryptions beyond digital circuit approaches.

Insight into Isomeric Effect on the Photoluminescence and Mechanoluminescence of Cyanostilbene Derivatives

Molecular structures, packings, and intermolecular interactions significantly affect the photophysical properties of organic luminogens. In this work, the photoluminescence (PL) and mechanoluminescence (ML) of two pairs of isomers, 1/2 and 3/4, were systematically explored. The fluorescence of crystals 1c and 4c is much brighter than that of their isomers 2c and 3c, respectively. Only 1c is ML-active among all four molecules. Single-crystal structural analysis revealed that isomerization of a substituent group affected their molecular packing and intermolecular interactions. Stronger intermolecular interaction and intact three-dimensional hydrogen-bonded networks were formed only in crystal 1c, which were essential for preventing slippage of molecular layers and generating ML; the other molecules were either lacking π-π interactions or C-H···π interactions. Theoretical calculation suggested that the energy barrier between the Franck-Condon (FC) structure and minimum energy crossing point (MECP) structure of 2/3 was much lower than that of 1/4. Nonradiative decay channels of molecules 2 and 3 were thus more easily activated, which led to their lower quantum yield.

Mechanofluorochromism of 2-Biarylyl Cinchoninic Acids with High Sensitivity and Large Mechanochromic Shift

In recent years, organic mechanofluorochromism (MFC) materials have attracted wide attention in many fields. However, the exploration of MFC materials with high-contrast, high-sensitivity and high-responsiveness remains a challenge. Herein, a series of MFC materials with 2-biarylyl cinchoninic acid skeleton were successfully established, which are based on interconversion of classical/ frustrated Brönsted pairs. These compounds have the mechanochromic shift of up to 115 nm, as well as the property of stunning sensitivity and multiple responses to external mechanical force stimuli. The luminescence properties can be easily tuned by changing the substituents.

Deep-Red Emissive Squaraine-AIEgen in Elastomer Enabling High Contrast and Fast Thermoresponse for Anti-Counterfeiting and Temperature Sensing

Two challenges remain for organic thermoresponsive materials; one is to develop high-performance red-emissive thermoresponsive materials, while another is to simultaneously achieve high contrast ratio (CR), fast and reversible thermoresponse in a single element. Herein, we not only develop a new deep-red emissive squaraine-based AIEgen (TPE-SQ12) based on a pyrylium end group, which is suitable for fabricating high-performance thermoresponsive materials, but also show an effective approach to improve both CR (∼ten times increase) and response time (less than 3 seconds), that is, molecularly dispersing AIEgen into an elastomer, attributed to the significantly expanded free volume of elastomer upon increasing the temperature that can activate the AIEgen intramolecular movements more pronouncedly. Double encryption and temperature mapping systems have been separately established by using our designed elastomer/TPE-SQ12 film, showing the great potential for anti-counterfeiting and temperature sensing. Finally, white emission is further achieved by co-doping TPE-SQ12 with cyan dye into elastomer, which enables fluorescent thermochromism for improving the temperature mapping ability.

Nanoparticulate Photoluminescent Probes for Bioimaging: Small Molecules and Polymers

Recent interest in research on photoluminescent molecules due to their unique properties has played an important role in advancing the bioimaging field. In particular, small molecules and organic dots as probes have great potential for the achievement of bioimaging because of their desirable properties. In this review, we provide an introduction of probes consisting of fluorescent small molecules and polymers that emit light across the ultraviolet and near-infrared wavelength ranges, along with a brief summary of the most recent techniques for bioimaging. Since photoluminescence probes emitting light in different ranges have different goals and targets, their respective strategies also differ. Diverse and novel strategies using photoluminescence probes against targets have gradually been introduced in the related literature. Among recent papers (published within the last 5 years) on the topic, we here concentrate on the photophysical properties and strategies for the design of molecular probes, with key examples of in vivo photoluminescence research for practical applications. More in-depth studies on these probes will provide key insights into how to control the molecular structure and size/shape of organic probes for expanded bioimaging research and applications.

Photochromic and Electrochromic Hydrogels Based on Ammonium- and Sulfonate-Functionalized Thienoviologen Derivatives

Ammonium cations and sulfonate anions have been introduced as end-caps for alkyl viologens with thiophene-derived bridges. When the as-prepared thienoviologen derivatives are dispersed in polyacrylamide (PAAm) hydrogels, photochromic (PC) and electrochromic (EC) bifunctional hydrogels can be simply realized. The incorporated thiophene or ethylenedioxylthiophene bridge not only expands the photoresponse range but also stabilizes the photoinduced radical intermediate. Therefore, reversible PC and EC behaviors can be achieved for hydrogels containing thienoviologens N,N'-di(3-(trimethylammonio)propyl)-4,4'-(thien-2,5-diyl)bispyridinium tetrabromide (ATV), N,N'-bis(3-sulfonatopropyl)-4,4'-(thien-2,5-diyl)bispyridinium (STV), N,N'-di(3-(trimethylammonio)propyl)-4,4'-(3,4-ethylenedioxylthien-2,5-diyl)bispyridinium tetrabromide (AETV), and N,N'-bis(3-sulfonatopropyl)-4,4'-(3,4-ethylenedioxylthien-2,5-diyl)bispyridinium (SETV). On the contrary, no photochromism can be observed for PAAm hydrogels based on N,N'-di(3-(trimethylammonio)propyl)-4,4'-bipyridinium tetrabromide (AV) and N,N'-bis(3-sulfonatopropyl)-4,4'-bipyridinium (SV) without thiophene bridges. Furthermore, no significant coloration difference can be observed between the hydrogels containing ammonium- and sulfonate-functionalized viologens. However, during repetitive cycles, the transmittance contrast losses of electrochromic devices (ECDs) based on the hydrogels containing ammonium-modified viologens are lower than those for sulfonate-substituted viologens probably due to their larger number of cation-anion pairs and thus higher solubility in aqueous media. Typically, no observable difference can be found for unsealed ECDs after 15 days in ambient conditions. Additionally, a large-area ECD with a diameter of 10 cm has been facilely fabricated by simply sandwiching the EC hydrogels, and the transparency can be finely tuned upon applying different potentials. Overall, our findings may provide a new path to design multifunctional hydrogels with PC and EC responses.

Interfacial AIE for Orthogonal Integration of Holographic and Fluorescent Dual-Thermosensitive Images

Orthogonal integration of thermosensitive images is of vital significance for advanced anticounterfeiting, which however remains formidably challenging due to the trade-off that facile thermoresponse needs easy molecular motion while robust imaging requires molecular restriction. Herein, a viable approach is demonstrated to tackle the challenge by in situ fixing a predesigned aggregation induced emission luminogen (AIEgen) at the polymer/liquid crystal (LC) interface via precisely controlled interfacial engineering, in which the AIEgen is enriched in LC phases during polymerization induced phase separation and subsequently driven to the interface by the interfacial thiol-ene click reaction. Crosstalk-free integration of holographic and fluorescent dual-thermosensitive images with high sensitivity, high contrast ratio, and robust performance is successfully realized in a single unit, attributed to the simultaneously LC-facilitated AIEgen molecular motion and polymer-restricted AIEgen diffusion at the interface. The exciting characteristics of these orthogonally integrated dual images will enable them to prevent illegal replication and thus are expected to be promising for high-security-level anticounterfeiting applications.

Pressure-Induced Restricting Intermolecular Vibration of a Herringbone Dimer for Significantly Enhanced Multicolor Emission in Rotor-Free Truxene Crystals

Organic solid-state luminescent materials exhibit numerous exciting photoelectric properties that are central to emergent organic light-emitting diodes, smart sensors, and data encryption. However, the luminescence of pure organic rotor-free materials has been afflicted with strong intermolecular π-π stacking interactions. Herein, an unprecedented pressure-induced emission enhancement (PIEE) is realized in a system of rigid planar pure polycyclic aromatics, i.e., truxene crystals. The emission intensity is enhanced 7-fold below 3.0 GPa with a photoluminescence quantum yield increased to 10.17% compared with the initial value of 1.78%, and the emission colors change from green (520 nm) to red (640 nm) within 11.8 GPa. Spectral characterizations and first-principles calculations reveal that the PIEE and piezochromism can mainly be attributed to the restricted intermolecular vibration and the decreased energy gap. Our findings enrich the PIEE mechanism and provide a new guideline for designing pressure-responsive luminescent materials in advancing their photoelectric applications.