Multicolor Emission Research Articles

Luminescent Color Controller-Based Bilayer PC-UCNPs-PC Dynamically Modulate Photon Propagation

Lanthanide-doped upconversion nanoparticles (UCNPs) exhibit abundant emission wavelengths under near infrared (NIR) excitation. This property enables UCNPs to have potential for multicolor output. However, the realization of multicolor-tunable emission output in UCNPs at a fixed composition has remained challenging. Here, a luminescent color controller that can dynamically control the upconversion luminescence (UCL) colors from UCNPs is reported. By constructing bilayer photonic crystals (PCs) in which the UCNPs are distributed between the two-layer PCs, the propagation of green and red emission can be modulated by the top and bottom PCs, respectively. Moreover, the photonic bandgap (PBG) could be varied by changing the reflected angles, according to Bragg diffraction. Thus, the variation in the viewing angles also results in the yellow to blue and red to cyan structural color changes on the top and bottom bilayer PC-UCNPs-PC films. Correspondingly, the emission color outputs are changed from red to yellow-green and green to yellow-green, respectively. These findings provide a general method to achieve multicolor-tunable UCL by a bilayer PC-UCNPs-PC film. Moreover, the UCNPs with multicolor emissions in this work enrich the upconversion system and should have potential applications in display and anticounterfeiting.

Multicolor emitting luminescent MgO nanocubes for implication in ratiometric optical thermometry

Optical thermometry employing dual emission from two distinct wavelengths for sensing performances has advantages over conventional thermometers due to the self-reference and noninvasive mode of operation. Here, we have demonstrated the synthesis of the multiple colors emitting luminescent MgO nanocubes and explored their temperature-dependent tunable emissions for application in intensity ratio-based optical thermometry. MgO nanocubes are prepared by heating Mg metal ribbons at different temperatures in air ambient. The as-prepared nanocubes exhibit two distinguished photoluminescence emission bands under UV (355 nm) and visible (532 nm) excitations. An excitation and synthesis condition-dependent multicolor emissions are observed in the visible to NIR regions due to the presence of various intermediate defect levels within the band gap of the materials. A temperature-dependent tunable dual emission is realized for different nanocubes, where the two peaks respond to the applied temperature differently, forming an isosbestic point. The difference in the temperature dependency of the two peaks is essential for the fluorescence intensity ratio-based optical thermometry application. The reason for the different responses of the two peaks is explained depending on the presence of the defect levels. From the temperature-dependent intensity ratio variation of MgO nanocubes, the highest sensitivity value of 6.79% K − 1 is achieved, which is very high compared to the various reported ratiometric sensors. The MgO nanocubes demonstrate better stability and reproducibility for application in the 83–513 K wide temperature range.

Multicolor emission in potassium-zinc phosphate glasses activated with Dy3+, Tb3+ and Dy3+/ Tb3+ for photonic device applications

We report the spectroscopy analysis of potassium zinc phosphate glasses activated with Dy3+, Tb3+ and Dy3+/Tb3+. Radiative parameter values (stimulated emission cross section, effective bandwidth, gain bandwidth and optical gain) were obtained for the dysprosium 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 emissions upon 349 nm excitation. Emission of warm white light can be attained in the KZDy glass upon excitation at 349 nm. The co-doped glass can emit yellowish-green light from Tb3+ and Dy3+ luminescence sensitized by Dy3+ (upon 394 nm excitation) and Tb3+ (upon 377 nm excitation), respectively. The Dy3+ to Tb3+ energy transfer efficiency is higher (38.5%) than that (22.7%) of the Tb3+ to Dy3+ energy transfer.

Photonic Bandgap and Excitation Wavelength Modulated Multicolor Upconverted Circularly Polarized Luminescence in Liquid Crystal Microcapsule

AbstractUpconverted circularly polarized luminescence (UC‐CPL) which is coined by integrating two concepts of photon upconversion and circularly polarized luminescence, has aroused enormous attention from scientists owing to the great performance and potential applications in various fields. To date, although various types of UC‐CPL systems are reported, constructing a UC‐CPL system with multicolor emission remains a big challenge. In this work, by incorporating lanthanide‐doped upconversion nanoparticles (UCNPs) into chiral nematic liquid crystal (N*LC), multicolor UC‐CPL emission can be realized upon two kinds of modulations: the regulation of photonic bandgap (PBG) and adjustment of excitation wavelength of the doped UCNPs. More importantly, this kind of multicolor UC‐CPL system can be used as internal materials for constructing polymer‐shell microcapsules. The PBG effect can still be well preserved in the microcapsules, providing a kind of easy processable luminescent material with multicolor UC‐CPL properties, which can be used as an advanced anti‐counterfeiting material.

Solvatochromism of high-efficient and long-wavelength emissive carbon dots for wide-range water-sensing in organic solvents

A novel solvent-dependent fluorescent CDs (R-CDs) were prepared by solvothermal method from neutral red and urea precursors. The multicolor emissions ranging from blue to red from R-CDs could be achieved by tuning the solvents polarity. The highest quantum yield of 61.11% is achieved in R-CDs acetone solution with bright yellow emission at 565 nm. The solvent-dependent fluorescence is ascribed to the variation of CDs’ band-gap and surface potential, which are induced by polarity. R-CDs were used as a quantitative fluorescent sensor for detecting water in different solvents. An excellent linear relationship between water content (0–90%) and emission intensity is found in ethanol, methanol, acetonitrile, isopropanol, DMSO, and acetone. The limit of detection (LOD) is 1.55%, the recoveries range from 95% to 110%, and the relative standard deviations (RSD) are 0.15%–5.43%. All results indicate that our R-CDs for water sensing exhibit good linear relationship and accuracy.

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.

Organic nanoparticles with tunable AIE derived from amino acids appended naphthalenediimide based amphiphiles

Fluorescent organic nanoparticles (FONPs) are unique cooperative manifestations of supramolecular self-aggregates and luminescent nanoparticles. In this work, we developed naphthalenediimide (NDI) based amphiphiles (NDI-1-9) having amino acids substitution from L-alanine to L-phenyl alanine to 3-(2-naphthyl)-L-alanine in absence and presence of alkyl spacer (C-6, C-11). Formation of self-aggregated spherical organic nanoparticles of ∼50 nm took place at and above 50 vol% water or methyl cyclohexane (MCH) in different binary solvent systems (DMSO/DMF-water, CHCl3-MCH) via J-aggregation. In case of NDI-1-3 (amino acids are directly connected to the NDI) self-assembled FONPs (MCH in CHCl3) exhibited aggregation-induced emission (AIE) through excimer formation. Red shift in the emission maxima were observed from 484 nm to 495 nm, and 590 nm, respectively with increasing π electron cloud at substituted donor from NDI-1 to NDI-3. Alongside, in case of aromatic amino acids substitution, FONPs of 6-amino caproic acid spacer containing NDI derivatives (NDI-5 and NDI-6) showed AIE at 505 nm and 545 nm (water in DMF/DMSO). Corresponding 11-amino undecanoic acid containing analogues (NDI-8 and NDI-9) showed AIE at 480 nm and 585 nm in CHCl3-MCH and DMF/DMSO-water. The multi colour emission from blue-green to yellow-orange with enrichment of π-electron cloud density in the side chain substitution of NDI was observed possibly due to the facilitated electron transfer at a lesser energy to the electron deficient NDI core that resulted in red shifted emission maxima. This was further clarified by regulating the availability of π-electrons of phenyl ring of NDI-2 by including electron withdrawing –NO2 and donating -OMe groups at the molecular backbone (NDI-2a and NDI-2b) that showed blue shifted (475 nm) and red shifted (570 nm) emission maxima with respect to NDI-2 (495 nm). However, increase in alkyl spacer length between side chain substitution and NDI core yielded relatively lower quantum yield presumably due to the impeded electron transfer.

Luminescence Temperature Sensing and First Principles Calculation of Photoelectric Properties in C12A7 Co-Doped Eu3+ Ions.

Developing high-resolution, high-accuracy fluorescent thermometers is challenging. In this study, the optical properties and thermal sensing of Yb-, Tm-, and Eu-co-doped C12A7 (C12A7:Yb/Eu/Tm), with flower-like structure upconversion microparticles, were studied. Eu3+ doping induced an approximately 6-fold change in the upconversion luminescence (UCL) output in comparison with C12A7:Yb/Tm microparticles. The maximum relative temperature sensitivity (S) of C12A7:Yb/Eu/Tm reached 3.0% K-1, representing an approximately 5-fold difference compared with the value of C12A7:Yb/Tm. In particular, the multicolor upconversion emission of C12A7:Yb/Eu/Tm can easily change from blue to white UCL with increasing temperature. Moreover, the band structure, total density, and optical coefficient of C12A7:Yb/Eu/Tm were investigated via density functional theory. The total density of O atoms increased in comparison with the total density of pure C12A7, indicating that substitution of Ca2+ by Yb/Eu/Tm produced positive vacancies on the cage structure. The optical coefficient of C12A7 was improved by the Yb/Eu/Tm dopant. The thermally regulated multicolor characteristics and thermally coupled energy levels of Tm3+ provide "dual adjustment temperature sensing", which is a promising strategy for realizing accurate and effective temperature sensors.

Solvents/photo/pillar[5]arene triple responsive morphology and luminescence transformation from an amphiphilic dicyanostilbene-functionalized thiophene

α-Cyanostilbene (CS) based organic luminescent materials with efficient electrical conductivity, aggregation-induced enhanced emission, and controllable multi-colour emission properties, have been aroused wide attention by scientists over the past few years. Self-assembly of CS-motif in aqueous media refers to an environment-friendly method for preparing luminescent materials. However, it is still challenging to control the intrinsic hydrophobic properties of the organic components in aqueous media. In this study, an amphiphilic dicyanostilbene-functionalized thiophene (ACSTP) derivative was synthesized. Z-ACSTP was identified to dissolve in different organic solvents, accompanied with strong and tunable fluorescence emission. However, when Z-ACSTP was dispersed in water, it was self-assembled into nanofibers, and the fluorescence was red shifted, accompanied with sharp decrease of intensity compared with that in DMSO. Furthermore, Z-form of ACSTP to its E-form under 365 nm irradiation led to the morphology transformation from nanofibers to nanosheets. Notably, upon addition of water-soluble pillar[5]arene (WP5), the nanofibers were transformed into fluorescent hollow particles due to the host–guest interactions between the pyridinium group and WP5 and the obtained fluorescent particles can be further applied in living cell imaging.

Polyrotaxane in-situ copolymerization stretchable supramolecular hydrogels for photo-controlled cascade energy transfer

Amphiphilic polysorbate (Tween 80) encapsulated hydrophobic methyl acrylate (MA), spiropyran (SP), nitrobenzoxadiazole (NBDAE) and cyanine 5 (Cy5) was mixed to form micelles, which in-situ copolymerized with acrylamide (AM) and polyrotaxane (PR) containing polyethylene glycol (PEG) threaded into vinyl modified α-cyclodextrin (α-CD) with amantadine end-capping to form stretchable supramolecular hydrogel. Benefiting from the sliding cross-linking points of PR, the hydrogel exhibited excellent mechanical properties (1300 % breaking strain and 0.26 MPa breaking stress). Meanwhile, photoreactive achievement of reversible SP acted as primary energy acceptor tuned cascade energy transfer from energy donor NBDAE to secondary acceptor Cy5 (energy transfer efficiency = 80 % for primary transfer, 65 % for secondary transfer). Therefore, this stretchable supramolecular hydrogel material can achieve phototunable multi-color emission and be applied in the field of time-resolved information anti-counterfeiting.

Multicolour Laser Emission Based on the Polystyrene Dust

Multicolour emission, independently of the considered mechanism, constitutes an appealing application for modern technologies and an interesting phenomenon for the various fields of science. Moreover, effective and easy colour tuning seems even more engaging. Through this contribution, we present a totally new approach to the organic-based system, which can provide effective and multicolour emission, including laser action. Polystyrene dust (PSd) particles play the role of the optically passive matrix for highly effective laser dyes (Coumarin 504, Coumarin 540, and DCM), which were successfully introduced into the polymer units by liquid crystals carrier. In such a way, various types of organic hybrid systems were created, starting from a single dye embedded in PSd volume, passing by two dye-component architectures, reaching the final and most complex network containing three chromophores, providing three basic tones (red, green, and blue). Moreover, by playing with an optical pumping magnitude, we were able to easily modulate colour tones/shades in a broad range, which is highly appreciated when considering human eye perception.

Realizing multicolor tunable photoluminescence in La2MgTiO6: Tb3+, Eu3+ phosphors via energy transfer and application for white light-emitting diodes

The achievement of multiple emission color phosphors has a significant importance in screen display, lighting devices, and multicolor imaging, but it still faces formidable challenges. In this work, double perovskite La2MgTiO6: Tb3+, Eu3+ phosphors with tunable emissions are designed and successfully synthesized. The obtained phosphors can emit intense green (Tb3+), red (Eu3+), and yellow emissions (Tb3+/Eu3+) via varying the co-doping concentration of Tb3+ and Eu3+. The CIE coordinates exhibit an excellent linear behavior in La2MgTiO6: Tb3+, Eu3+, revealing the possibility of linear adjustment of multiple emission colors. Detailed material characterization demonstrates that the multicolor tunable emission originates from energy transfer between Tb3+ and Eu3+. The decay dynamics analysis suggests that the multipole interaction between Tb3+ and Eu3+ is quadrupole-quadrupole interaction. A white light-emitting diode device with color rendering index of 84.7, correlated color temperature of 4684 K, and CIE coordinate of (0.3472, 0.3083) is fabricated by combining La2MgTiO6: Tb3+, Eu3+ yellow phosphor and BaMgAl10O17: Eu2+ blue phosphor. Our results reveal that La2MgTiO6: Tb3+, Eu3+ phosphors are promising luminescent materials in white light-emitting diodes.

Preparation and luminescence investigations of garnet-based Y2Ca2Ga3VO12:RE (RE = Eu3+, Sm3+, and Dy3+) phosphors

We report, for the first time, the preparation and luminescence investigations of the YCGVORE (RE = Eu3+, Sm3+, and Dy3+) phosphors. The efficient multicolor emissions at 592 nm (orange), 612 nm (red), and 577 nm (yellow) were obtained from YCGVO:RE (RE = Eu3+, Sm3+, and Dy3+) phosphors with near-UV excitation. The room temperature decay of 5D0 (Eu3+), 4G5/2 (Sm3+), and 4F9/2 (Dy3+) levels in YCGVO:RE phosphors exhibits the average lifetime of 2.59–2.89, 0.82–1.44, and 0.25–0.63 ms, respectively. Upon excitation of VO43- group, the emission intensity of the RE ions increases with increasing the dopant concentration, simultaneously, the intensity of 520 nm emission related to the VO43- group decreases, indicating an efficient VO43− → RE energy transfer (ET). The YCGVO: RE (RE = Eu3+, Sm3+, and Dy3+) phosphors are promising candidate for white LED application.

Bismuth-doped gadolinium oxide films for UV-Vis multicolor conversion: Combined XPS, DFT and photoluminescence study

Results of XPS characterization, theoretical modeling and photoluminescence study of bismuth-implanted gadolinium oxide (Gd2O3:Bi) films are presented. Scenarios for the introduction of bismuth ions into amorphous-crystalline Gd2O3 host and mechanisms for the formation of optically active centers are considered. It has been demonstrated that the optical properties of Gd2O3:Bi films are guided by the interplay of a quite complex system of single Bi2+ ions, (Bi3+–Bi3+) pairs and single Bi3+-ions, oxygen vacancies and "defective" gadolinium ions. Photoinduced excitation-relaxation processes involving various Bi-related centers and defect states of the host are observed. Due to the multicolor emission (red, green and blue) of single Bi2+ ions, (Bi3+- Bi3+) pairs and single Bi3+ ions excited in the ultraviolet spectral range, the UV–VIS energy conversion in a wide spectral range is realized. The advantage of Gd2O3:Bi as a material for energy conversion lies in the fact that due to the polyvalence of bismuth and the variety of different optical centers, a wide range of both absorbed UV light and emitted VIS light is involved in the conversion. This indicates the prospects for practical use of Gd2O3:Bi films as multifunctional material for energy conversion and photo-detection systems.

Synthesis of Visible Light Excitable Carbon Dot Phosphor-Al2 O3 Hybrids for Anti-Counterfeiting and Information Encryption.

The preparation of room temperature phosphorescent carbon dots still faces great challenges, especially in the case of carbon dots endowed of visible-light excitable room temperature phosphorescence (RTP). To date, a limited number of substrates have been exploited to synthesize room temperature phosphorescent carbon dots, and most of them can emit RTP only in solid state. Here, the synthesis of a composite obtained from the calcination of green carbon dots (g-CDs) blended with aluminum hydroxide (Al(OH)3 ) is reported. The resultant hybrid material g-CDs@Al2 O3 exhibits blue fluorescence and green RTP emissions in an on/off switch process at 365nm. Notably, this composite manifests strong resistance to extreme acid and basic conditions up to 30 days of treatment. The dense structure of Al2 O3 formed by calcination contributes to the phosphorescent emission of g-CDs. Surprisingly, g-CDs@Al2 O3 can also emit yellow RTP under irradiation with white light. The multicolor emissions can be employed for anti-counterfeiting and information encryption. This work provides a straightforward approach to produce room temperature phosphorescent carbon dots for a wide range of applications.

Supramolecular glasses with color-tunable circularly polarized afterglow through evaporation-induced self-assembly of chiral metal–organic complexes

The fabrication of chiral molecules into macroscopic systems has many valuable applications, especially in the fields of optical displays, data encryption, information storage, and so on. Here, we design and prepare a serious of supramolecular glasses (SGs) based on Zn-L-Histidine complexes, via an evaporation-induced self-assembly (EISA) strategy. Metal-ligand interactions between the zinc(II) ion and chiral L-Histidine endow the SGs with interesting circularly polarized afterglow (CPA). Multicolored CPA emissions from blue to red with dissymmetry factor as high as 9.5 × 10−3 and excited-state lifetime up to 356.7 ms are achieved under ambient conditions. Therefore, this work not only communicates the bulk SGs with wide-tunable afterglow and large circular polarization, but also provides an EISA method for the macroscopic self-assembly of chiral metal–organic hybrids toward photonic applications.

2D Inorganic Framework with Self-Transforming Luminescence Centers for Multicolor Emission

Multicolor luminescence is an attractive and emerging field for next-generation optoelectronic devices, including ultrathin displays, optical sensors, and wearable electronic devices. Currently, it is easy to achieve monochromatic emission, while the highly anticipated realization of multicolor luminescence in two-dimensional (2D) material is still blank. This is because the atomic thickness and structural sensitivity of 2D material make it extremely challenging to introduce multiple luminescent centers into it without structural collapse. Here, for the first time, we have designed a 2D inorganic framework with a wide bandgap and easily substitutable sites to embed self-transforming luminescence centers nondestructively and achieved multicolor emissions spanning from green to orange. These findings can provide a new paradigm for designing new 2D multicolor luminescent materials and extend a large margin of diverse applications.

Controllable Aggregation-Induced Emission and Förster Resonance Energy Transfer Behaviors of Bistable [c2] Daisy Chain Rotaxanes for White-Light Emission and Temperature-Sensing Applications.

Bistable [c2] daisy chain rotaxanes with respective extended and contracted forms of [c2]A and [c2]B containing a blue-emissive anthracene (AN) donor and orange-emissive indandione-carbazole (IC) acceptor were successfully synthesized via click reaction. Tunable-emission bistable [c2] daisy chain rotaxanes with fluorescence changes from blue to orange, including bright-white-light emissions, could be modulated by the aggregation-induced emission (AIE) characteristics and Förster resonance energy transfer (FRET) processes through altering water fractions and shuttling processes (i.e., acid/base controls). Accordingly, as a result of excellent fine-tuning AIE (at 60% water content of H2O/THF) and FRET (with a compatible energy transfer of EFRET = 33.2%) behaviors after the shuttling process (by adding base), the brightest white-light emission at CIE (0.31, 0.37) with a quantum yield of Φ = 15.64% was obtained in contracted [c2]B with good control of molecular shuttling to possess higher photoluminescence (PL) quantum yields and better energy transfer efficiencies (i.e., the manipulation of reduced PET and enhanced FRET processes) due to their intramolecular aggregations of blue AN donors and orange IC acceptors with a proper water content of 60% H2O. Furthermore, dynamic light-scattering (DLS) and time-resolved photoluminescence (TRPL) measurements, along with theoretical calculations, were utilized to investigate and confirm AIE and FRET phenomena of bistable [c2] daisy chain rotaxanes. Especially, both bistable [c2] daisy chain rotaxanes [c2]A and [c2]B and noninterlocked monomer M could be exploited for the applications of ratiometric fluorescence temperature sensing due to the temperature effects on the AIE and FRET features. Based on these desirable bistable [c2] daisy chain rotaxane structures, this work provides a potential strategy for the future applications of tunable multicolor emission and ratiometric fluorescence temperature-sensing materials.

Tunable multicolor afterglow including white light emission from poly(acrylic acid)-based room temperature phosphorescence materials via phosphorescence FRET

Room temperature phosphorescence (RTP) materials with multicolor afterglow emission have important application potential in multicolor display and anti-counterfeiting. However, the preparation of polymer-based multicolor RTP materials still faces challenges. In this work, we prepare a kind of ultralong RTP materials by facilely doping phthalimide (Pth) into poly (acrylic acid) (PAA) matrices based on the hydrogen-bonding interaction. Attributed to the wide phosphorescence emission band of PAA-Pth, the ternary doping systems are constructed by using PAA-Pth as donors and commercially available fluorescent dyes rhodamine B (RhB) or fluorescein (FL) as acceptors, which display multicolor afterglow from cyan to red and green through efficient phosphorescence Förster resonance energy transfer (FRET). More impressively, the quaternary doping systems PAA-Pth-RhB-FL with different molar ratios of FL exhibit tunable multicolor afterglow, including nearly white afterglow emission. Due to the different FRET efficiency of PAA-based multicolor RTP materials at different excitation wavelengths, they are successfully applied in color information encryption.

Facile synthesis, novel structure, multicolor luminescence, and potential applications of lanthanide ions doped bismuth titanate phosphors

A variety of lanthanide ions doped bismuth titanate (Bi4Ti3O12) luminescent materials with eminent down-conversion (DC) and up-conversion (UC) luminescence performance have been fabricated via a facile sol-gel approach. The XRD, XPS, and EDX elemental mapping results confirm the phase structure of orthorhombic Bi4Ti3O12 (BTO), and the lanthanide activator ions occupy the Bi3+ lattice sites in the BTO crystal. Under UV or NIR excitation, the Eu3+, Yb3+/Ln3+ (Ln = Er, Tm, and Ho) doped Bi4Ti3O12 samples exhibit characteristic red, green, blue, and green emissions. The luminescent mechanisms of the BTO:Eu3+ and BTO:Yb3+/Ln3+ samples are discussed based on the energy level diagrams. The doping concentrations of Eu3+, Yb3+, Er3+, Tm3+, Ho3+ ions and annealing temperature and time are optimized, whose optimal values are determined to be 14, 8, 1, 0.4, 1 mol% and 800 oC, 4 h. The as-obtained LED devices fabricated by Bi4Ti3O12:Eu3+ and Yb3+/Ln3+ phosphors exhibit dazzling multicolor visible light emissions from different Ln3+ ions. The results indicate that the as-obtained Ln3+ doped BTO phosphors may be potentially utilized in LED devices and solid-state lighting. Furthermore, the Eu3+ and Er3+ co-doped BTO samples exhibit different DC and UC luminescence spectral profiles when excited at various UV, visible, or NIR wavelengths, revealing their eminent feasibility and great potential in anti-counterfeiting applications.