Tunable Fluorescence, Morphology, and Antibacterial Behaviors of Conjugated Oligomers via Host-Guest Supramolecular Self-Assembly

Host-guest supramolecular self-assembly has become one facile but efficient way to regulate the optical properties of conjugated oligomers and construct promising photofunctional materials. Herein, we design two linear conjugated oligomers terminated with two or four pyridinium moieties, which show different 1:1 'head-to-tail' binding patterns with cucurbit[8]uril (CB[8]) to form host-guest supramolecules. After being encapsulated in the hydrophobic cavity of the CB[8] host, the fluorescence emission of the conjugated oligomers undergoes significant changes, resulting in tunable fluorescence color with enhanced quantum yields. Triggered by the aggregation of supramolecules, the regular or rigid binding modes lead to the formation of cuboids and spheroids in nanoscale, respectively. Due to the macrocyclic-confinement effect, the light-driven reactive oxygen species (ROS) production of the host-guest complex is increased significantly, thereby improving the photodynamic antibacterial performance toward Staphylococcus aureus (S. aureus).

Excitation-Selective and Double-Emissive Lead-Free Binary Hybrid Metal Halides for White Light-Emitting Diode and X-Ray Scintillation

Zero-dimensional (0D) organic metal halides comprising heterogeneous metal cations in single phase can achieve multiple luminous emissions enabling them toward multifunctional light-emitting applications. Herein, A novel single crystal of (C8H20N)4SbMnCl9 containing two luminescent centers of [SbCl5]2- pentahedrons and [MnCl4]2- tetrahedrons is reported. The large distance between Sb-Sb, Mn-Mn, and Sb-Mn as well as theory calculation indicate negligible interaction between individual centers, thus endowing (C8H20N)4SbMnCl9 with excitation-dependable and efficient luminescence. Under near-UV excitation, only orange emission originates from self-trapped excitons recombination in [SbCl5]2- pentahedron occurs with photoluminescence quantum yield (PLQY) of 91.5%. Under blue-light excitation, only green emission originating from 4T1-6A1 transition of Mn2+ in [MnCl4]2- tetrahedrons occurs with PLQY of 66.8%. Interestingly, upon X-ray illumination, both emissions can be fully achieved due to the high-energy photon absorption. Consequently, (C8H20N)4SbMnCl9 is employed as phosphors to fabricate white light-emitting diodes optically pumped by n-UV chip and blue-chip thanks to its excitation-dependable property. Moreover, it also shows promising performance as X-ray scintillator with low detection limit of 60.79 nGyair S-1, steady-state light yield ≈54% of commerical scintillaotr LuAG:Ce, high resolution of 13.5 lp mm-1 for X-ray imaging. This work presents a new structural design to fabricate 0D hybrids with multicolor emissions.

Interfacing Lanthanide Metal-Organic Frameworks with ZnO Nanowires for Alternating Current Electroluminescence

Alternating current electroluminescence (ACEL) devices are attractive candidates in cost-effective lighting, sensing, and flexible displays due to their uniform luminescence, stable performance, and outstanding deformability. However, ACEL devices have suffered from limited options for the light-emitting layer, which presents a significant constraint in the progress of utilizing ACEL. Herein, a new class of ACEL phosphors based on lanthanide metal-organic frameworks (Ln-MOFs) is devised. A synthesis of lanthanide-benzenetricarboxylate (Ln-BTC) thin film on a brass grid substrate seeded with ZnO nanowires (NWs) as anchors is developed. The as-synthesized Ln-BTC thin film is employed as the emissive layer and shows visible electroluminescence driven by alternating current (2.9 V µm-1 , 1 kHz) for the first time. Mechanistic investigations reveal that the Ln-based ACEL stems from impact excitation by accelerated electrons from ZnO NWs. Fine-tuning of the ACEL color is also demonstrated by controlling the Ln-MOF compositions and introducing an extra ZnS emitting layer. The advances in these optical materials expand the application of ACEL devices in anti-counterfeiting.

Excitation-Controlled Host-Guest Multicolor Luminescence in Lanthanide-Doped Calcium Zirconate for Information Encryption

Efficient control over lanthanide luminescence by regulating excitations offers a real-time and reversible luminescence-managing strategy, which is of great importance and highly desirable for various applications, including multicolor display and information encryption. Herein, we studied the crystal structure, luminescence properties, and mechanisms of undoped and Tb3+/Eu3+-doped CaZrO3 in detail. The intrinsic purple-blue luminescence from host CaZrO3 and the introduced green/red luminescence from guest dopants Tb3+/Eu3+ were found to have different excitation mechanisms and, therefore, different excitation wavelength ranges. This enables the regulation of luminescent color through controlling the excitation wavelengths of Tb3+/Eu3+-doped CaZrO3. Furthermore, preliminary applications for information encryption with these materials were demonstrated using portable UV lamps of 254 and 302 nm. This study not only promotes the development of multicolor luminescence regulation in fixed-composition materials, but also advances the practical applications of lanthanide luminescent materials in visually readable, high-level anti-counterfeiting and information encryption.

Formation and Band Gap Tuning Mechanism of Multicolor Emissive Carbon Dots from m-Hydroxybenzaldehyde

Reported in 2004, carbon dots (CDs) have been widely used in various fields due to their excellent optical properties. However, the mechanism of their fluorescence modulation is still a controversial issue, which also seriously affects the further development of carbon dots. In this paper, m-hydroxybenzaldehyde is used as a raw material to obtain multicolor luminescent CDs by pyrolysis under different reaction conditions, thereby revealing the forbidden band tuning and formation mechanism of CDs. Different acid-base conditions lead to different reaction paths of the precursors, forming molecular fluorophores with different conjugated structures, which aggregate to eventually form CDs and further enhance the photoluminescence of the system by inhibiting the movement of the fluorescent centers.

Multifunctional and Multicolor Perovskite-CdSe Quantum Dots Diodes for Pulse Oximetry

A conventional pulse oximeter system is composed of two light sources with different peak emission wavelengths and a photodetector. Integrating these three independent components into one single device will absolutely simplify the system design and create a miniaturized size of the product. Here, we demonstrate a bilayer perovskite-CdSe quantum dot (hereafter perovskite-QD) diode capable of voltage-tunable green/red emission and photodetection. The proposed diode also presents an intriguing feature of simultaneous light emission and detection, which is explored as regards the diode being used as a photoconductor when the positive bias is larger than the built-in voltage. The multifunctional and multicolor diode is further employed in a reflective pulse oximeter system, as either the multicolor light sources or the sensing unit in the system provide accepted and trustful results for heart rate and arterial blood oxygenation. Our work provides a possible avenue for the simplification of the pulse oximetry with a compact and miniaturized design in the future.

Monolithic Multicolor Emissions of InGaN-Based Hybrid Light-Emitting Diodes Using CsPbBr3 Green Quantum Dots

To address the increasing demand for multicolor light-emitting diodes (LEDs), a monolithic multicolor LED with a simple process and high reliability is desirable. In this study, organic-inorganic hybrid LEDs with violet and green wavelengths were fabricated by depositing CsPbBr₃ perovskite green quantum dots (QDs) as the light-converting material on InGaN-based violet LEDs. As the injection current was increased, the total electroluminescence (EL) intensities of the hybrid LEDs increased, whereas the light-converted green emission efficiency of the CsPbBr₃ QDs decreased. The maximum green-to-violet EL spectral intensity ratio of the hybrid LEDs with CsPbBr₃ QDs was achieved with the injection current of <10 mA. Moreover, the EL spectral ratio of the green-to-violet emission decreased at an injection current of 100 mA. The light-conversion intensity of the CsPbBr₃ QDs decreased linearly as the junction temperature of the hybrid LEDs was increased with increasing injection current, similar to the temperature-dependent photoluminescence degradation of CsPbBr₃ QDs. In addition, the junction temperature of the hybrid LED was minimized by pulse injection to suppress the thermal degradation of QDs and increase the light conversion efficiency to green emission. Therefore, the overall emission spectrum color coordinates of the hybrid LEDs exhibited a red shift from violet to blue in the low-current region and a blue shift toward violet as the green emission of the QDs was decreased above 10 mA.

Multicolor emission based on a N, N'-Disubstituted dihydrodibenzo [a, c] phenazine crown ether macrocycle

Dynamic fluorophore 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) affords a new platform to produce diverse emission outputs. In this paper, a novel DPAC-containing crown ether macrocycle D-6 is synthesized and characterized. Host-guest interactions of D-6 with different ammonium guests produced a variety of fluorescence with hypsochromic shifts up to 130 nm, which are found to be affected by choice of solvent or guest and host/guest stoichiometry. Formation of supramolecular complexes were confirmed by UV-vis titration, ¹H NMR and HRMS spectroscopy.

Multicolor Luminescent Carbon Dots: Tunable Photoluminescence, Excellent Stability, and Their Application in Light-Emitting Diodes

Carbon dots (CDs) are attracting much interest due to their excellent photoelectric properties and wide range of potential applications. However, it is still a challenge to regulate their bandgap emissions to achieve full-color CDs with high emissions. Herein, we propose an approach for producing full-color emissive CDs by employing a solvent engineering strategy. By only tuning the volume ratio of water and dimethylformamide (H₂O/DMF), the photoluminescence (PL) emission wavelengths of the CDs can be changed from 451 to 654 nm. Different fluorescence features of multicolor CDs were systematically investigated. XRD, SEM, TEM, Abs/PL/PLE, XPS, and PL decay lifetime characterizations provided conclusive evidence supporting the extent to which the solvent controlled the dehydration and carbonization processes of the precursors, leading to a variation in their emission color from red to blue. The as-prepared CDs exhibited excellent and stable fluorescence performance even after being heated at 80 °C for 48 h and with UV light continuously irradiated for 15 h. Based on their excellent fluorescent properties and photothermal stability, bright multicolor light-emitting diodes with a high CRI of up to 91 were obtained. We anticipate that these full-color emissive CDs are beneficial for applications in lighting, display, and other fields.

Tuning Multicolor Emission of Manganese-Activated Gallogermanate Nanophosphors by Regulating Mn Ions Occupying Sites for Multiple Anti-Counterfeiting Application

The ability to manipulate the luminescent color, intensity and long lifetime of nanophosphors is important for anti-counterfeiting applications. Unfortunately, persistent luminescence materials with multimode luminescent features have rarely been reported, even though they are expected to be highly desirable in sophisticated anti-counterfeiting. Here, the luminescence properties of Zn₃Ga₂GeO₈:Mn phosphors were tuned by using different preparation approaches, including a hydrothermal method and solid-state reaction approach combining with non-equivalent ion doping strategy. As a result, Mn-activated Zn₃Ga₂GeO₈ phosphors synthesized by a hydrothermal method demonstrate an enhanced red photoluminescence at 701 nm and a strong green luminescence with persistent luminescence and photostimulated luminescence at 540 nm. While Mn-activated Zn₃Ga₂GeO₈ phosphors synthesized by solid-state reactions combined with a hetero-valent doping approach only exhibit an enhanced single-band red emission. Keeping the synthetic method unchanged, the substitution of hetero-valent dopant ion Li⁺ into different sites is valid for spectral fine-tuning. A spectral tuning mechanism is also proposed. Mn-activated Zn₃Ga₂GeO₈ phosphors synthesized by a hydrothermal approach with multimodal luminescence is especially suitable for multiple anti-counterfeiting, multicolor display and other potential applications.

Decagram-Scale Synthesis of Multicolor Carbon Nanodots: Self-Tracking Nanoheaters with Inherent and Selective Anticancer Properties

Carbon nanodots (CDs) are a new class of carbon-based nanoparticles endowed with photoluminescence, high specific surface area, and good photothermal conversion, which have spearheaded many breakthroughs in medicine, especially in drug delivery and cancer theranostics. However, the tight control of their structural, optical, and biological properties and the synthesis scale-up have been very difficult so far. Here, we report for the first time an efficient protocol for the one-step synthesis of decagram-scale quantities of N,S-doped CDs with a narrow size distribution, along with a single nanostructure multicolor emission, high near-infrared (NIR) photothermal conversion efficiency, and selective reactive oxygen species (ROS) production in cancer cells. This allows achieving targeted and multimodal cytotoxic effects (i.e., photothermal and oxidative stresses) in cancer cells by applying biocompatible NIR laser sources that can be remotely controlled under the guidance of fluorescence imaging. Hence, our findings open up a range of possibilities for real-world biomedical applications, among which is cancer theranostics. In this work, indocyanine green is used as a bidentate SO_x donor which has the ability to tune surface groups and emission bands of CDs obtained by solvothermal decomposition of citric acid and urea in N,N-dimethylformamide. The co-doping implies various surface states providing transitions in the visible region, thus eliciting a tunable multicolor emission from blue to red and excellent photothermal efficiency in the NIR region useful in bioimaging applications and image-guided anticancer phototherapy. The fluorescence self-tracking capability of SO_x-CDs reveals that they can enter cancer cells more quickly than healthy cell lines and undergo a different intracellular fate after cell internalization. This could explain why sulfur doping entails pro-oxidative activities by triggering more ROS generation in cancer cells when compared to healthy cell lines. We also find that oxidative stress can be locally enhanced under the effects of a NIR laser at moderate power density (2.5 W cm^-2). Overall, these findings suggest that SO_x-CDs are endowed with inherent drug-independent cytotoxic effects toward cancer cells, which would be selectively enhanced by external NIR light irradiation and helpful in precision anticancer approaches. Also, this work opens a debate on the role of CD surface engineering in determining nanotoxicity as a function of cell metabolism, thus allowing a rational design of next-generation nanomaterials with targeted anticancer properties.

Cucurbituril Based Luminescent Materials in Aqueous Media and Solid State

Cucurbit[n]urils, the pumpkin shaped macrocyclic host molecules possessing a hydrophobic cavity and two identical carbonyl portals, have drawn a lot of attention in recent years due to their high-affinity yet dynamic molecular recognition properties in water. The reversible and stimuli-responsive nature of their host-guest complexes imparts "smart" features leading to materials with intriguing optical, mechanical and morphological properties. In this review, we focus on the design of cucurbituril based luminescent materials in aqueous media as well in solid or film state. The design principles of fluorescent complexes, small assemblies as well as supramolecular polymers along with their stimuli-responsive properties and applications in diverse areas such as optoelectronic devices, light harvesting, anti-counterfeiting and information technology, cell imaging, etc are highlighted with selected examples from recent literature. We also discuss examples of room temperature phosphorescent materials derived from purely organic luminogens in the presence of cucurbiturils.

Magnetofluorescent Nanoprobe for Multimodal and Multicolor Bioimaging

Although, superparamagnetic iron oxide nanoparticles (SPIONs) have extensively been used as a contrasting agent for magnetic resonance imaging (MRI), the lack of intrinsic fluorescence restricted their application as a multimodal probe, especially in combination with light microscopy. In Addition, the bigger size of the particle renders them incompetent for bioimaging of small organelles. Herein, we report, not only the synthesis of ultrasmall carbon containing magneto-fluorescent SPIONs with size ∼5 nm, but also demonstrate its capability as a multicolor imaging probe. Using MCF-7 and HeLa cell lines, we show that the SPIONs can provide high contrast mulicolor images of the cytoplasm from blue to red region. Further, single particle level photon count data revealed that the SPIONs could efficaciously be utilized in localization based super resolution microscopy in future.

Efficient Cascade Resonance Energy Transfer in Dynamic Nanoassembly for Intensive and Long-Lasting Multicolor Chemiluminescence

Light emission induced by chemical reactions, known as chemiluminescence (CL), has been widely used for bioassays, biosensors, imaging, and illumination applications. Most known CL systems exhibit flash-type single-color light emissions, which limit their applications. Long-lasting multicolor CL in aqueous solutions is highly desirable, especially for biological applications, but remains a challenge. Herein, we report a simple strategy of achieving highly efficient cascade Förster resonance energy transfer (FRET) in the dynamic nanoassembly of β-cyclodextrin (β-CD), CL reagents, and fluorophores in aqueous solution, which emits intensive multicolor CL with adjustable wavelength within 410-610 nm. β-CD can bind CL reagents and fluorophores to form a dynamic nanoassembly. These nanoassemblies can bring the included luminescent intermediate and fluorophores into close proximity and proper alignment, which should greatly enhance the FRET efficiency between luminescent intermediate and fluorophores. Indeed, the cascade FRET efficiency in this supramolecular nanoassembly reaches up to 92%, which is comparable with the cascade FRET systems based on covalently linked donors and acceptors. By using hydroxypropyl methylcellulose as the thickener to slow the diffusion (to elongate the CL emission), and using Ca(OH)₂ solid (a low solubility strong base) as buffer to maintain the pH in the optimal range for the CL reaction, this nanoassembly system has been further developed to achieve slow-diffusion-controlled catalytic CL reactions, which enables long-lasting multicolor CL in aqueous solution that is visible to naked eyes and lasts for more than 20 h. The multicolor CL systems can be used to prepare transformable two-dimensional multicolor codes for encryption application.

Amorphous Ionic Polymers with Color-Tunable Ultralong Organic Phosphorescence

Amorphous purely organic phosphorescence materials with long-lived and color-tunable emission are rare. Herein, we report a concise chemical ionization strategy to endow conventional poly(4-vinylpyridine) (PVP) derivatives with ultralong organic phosphorescence (UOP) under ambient conditions. After the ionization of 1,4-butanesultone, the resulting PVP-S phosphor showed a UOP lifetime of 578.36 ms, which is 525 times longer than that of PVP polymer itself. Remarkably, multicolor UOP emission ranging from blue to red was observed with variation of the excitation wavelength, which has rarely been reported for organic luminescent materials. This finding not only provides a guideline for developing amorphous polymers with UOP properties, but also extends the scope of room-temperature phosphorescence (RTP) materials for practical applications in photoelectric fields.

Bright and Multicolor Chemiluminescent Carbon Nanodots for Advanced Information Encryption

The various luminescent properties of carbon nanodots (CDs) reveal fascinating applications in several areas. Here, bright and multicolor chemiluminescence (CL) is realized from CDs, whose CL quantum yield can be optimized by adjusting the energy level alignment between the CDs and 1,2-dioxetanedione intermediate generated from the reaction of peroxalate and hydrogen peroxide. A CL quantum yield of 9.32 × 10-3 Einsteins mol-1, maximal luminance of 3.28 cd m-2, and lifetime of 186.4 s are achieved in red CDs, all of which are the best values ever reported for CDs. As a proof-of-concept prototype, a high-quality information encryption strategy is established via CD based CL imaging techniques by virtue of the high brightness and multicolor CL.

Humidity- and Temperature-Tunable Multicolor Luminescence of Cucurbit[8]uril-Based Supramolecular Assembly

Fabrication of tunable luminescent materials by a single luminophore is a challenge owning to the limit of emissive properties of monofluorophores. Herein, a type of temperature and humidity dual-responsive luminescent material based on host-guest supramolecular self-assembly was developed. Included into the cavity of cucurbit[8]uril (CB[8]) to form a 1:2 host-guest binding motif, the highly blue-emissive thiazolothiazole methyl-viologen (TMV) molecules were promoted to stack closely with a sharp luminescence decrease at 460 nm and rise of the dimer emission at 535 nm, especially at high concentrations in aqueous solution, which was demonstrated by fluorescence spectra, UV-vis absorbance spectra, NMR, and ITC data. Accordingly, when printed on paper, the 1/2 CB[8]/TMV complex presented a reversibly humidity-dependent emissive behavior with luminescent color changing from greenish-yellow in wet to blue upon evaporation. Besides, the  sensitivity of the host-guest interaction endowed the CB[8]/TMV complexes with temperature-tunable emission which showed a considerably enhanced blue luminescence at higher temperature. Subsequently, a ratiometric temperature-responsive emitter which luminesced reversibly from pink to white and then to blue light at temperature ranging from 0 to 70 °C was fabricated by mixing the CB[8]/TMV complex with thermal-sensitized emitting GSH-Au nanoclusters. These fine-tuning abilities make the CB[8]/TMV supramolecular complex applicable in visual luminescent devices such as anti-counterfeiting labels and fluorescent thermometers.

Multicolor Emission from Non-conjugated Polymers Based on a Single Switchable Boron Chromophore

Multicolor emissive and responsive materials are highly attractive owing to their potential applications in various fields, and polymers are preferred for their good processability and high stability. Herein, we report a series of new polymers based on a methacrylate monomer containing a switchable boron chromophore. In spite of their unconjugated nature, interestingly, the homopolymers from this monomer display rare multicolor fluorescence in solution that is highly dependent on the degree of polymerization (DP). With an increasing DP, the local concentration of the chromophore increases, leading to a higher propensity for switching the blue-emitting tricoordinate boron chromophore to the red-emitting tetracoordinate one. The homopolymers also display temperature- and solvent-dependent emission color change. Furthermore, pure white-light emission could be achieved in various solvents by precisely tuning the homopolymer molecular weight, or in films/solid state by copolymerizing the emissive boron monomer with non-emissive monomers in an appropriate ratio.

Simultaneous Dual-mode Emission and Tunable Multicolor in the Time Domain from Lanthanide-doped Core-shell Microcrystals

The dual-mode emission and multicolor outputs in the time domain from core-shell microcrystals are presented. The core-shell microcrystals, with NaYF₄:Yb/Er as the core and NaYF₄:Ce/Tb/Eu as the shell, were successfully fabricated by employing the hydrothermal method, which confines the activator ions into a separate region and minimizes the effect of surface quenching. The material is capable of both upconversion and downshifting emission, and their multicolor outputs in response to 980 nm near-infrared (NIR) excitation laser and 252 nm, and 395 nm ultraviolet (UV) excitation light have been investigated. Furthermore, the tunable color emissions by controlling the Tb³⁺- Eu³⁺ ratio in shells and the energy transfer of Ce³⁺→Tb³⁺→ Eu³⁺ were discussed in details. In addition, color tuning of core-shell-structured microrods from green to red region in the time domain could be obtained by setting suitable delay time. Due to downshifting multicolor outputs (time-resolved and pump-wavelength-induced downshifting) coupled with the upconversion mode, the core-shell microrods can be potentially applied to displays and high-level security.

Multicolor Tunable Luminescence Based on Tb3+/Eu3+ Doping through a Facile Hydrothermal Route

Ln³⁺-doped fluoride is a far efficient material for realizing multicolor emission, which plays an important part in full-color displays, biolabeling, and MRI. However, studies on the multicolor tuning properties of Ln³⁺-doped fluoride are mainly concentrated on a complicated process using three or more dopants, and the principle of energy transfer mechanism is still unclear. Herein, multicolor tunable emission is successfully obtained only by codoping with Tb³⁺ and Eu³⁺ ions in β-NaGdF₄ submicrocrystals via a facile hydrothermal route. Our work reveals that various emission colors can be obtained and tuned from red, orange-red, pink, and blue-green to green under single excitation energy via codoping Tb³⁺ and Eu³⁺ with rationally changed Eu³⁺/Tb³⁺ molar ratio due to the energy transfer between Tb³⁺ and Eu³⁺ ions in the β-NaGdF₄ host matrix. Meanwhile, the energy transfer mechanism in β-NaGdF₄: x Eu³⁺/y Tb³⁺ (x + y = 5 mol %) submicrocrystals is investigated. Our results evidence the potential of the dopants' distribution density as an effective way for analyzing energy transfer and multicolor-controlled mechanism in other rare earth fluoride luminescence materials. Discussions on the multicolor luminescence under a certain dopant concentration based on single host and wavelength excitation are essential toward the goal of the practical applications in the field of light display systems and optoelectronic devices.

Thermoresponsive Polymer and Fluorescent Dye Hybrids for Tunable Multicolor Emission

Fully reversible emission color change is achieved by blending a thermoresponsive polymer with dye hybrids. The emission color can be tuned by changing the mixing ratio of each polymer-dye hybrid.