Emission Color Variation Research Articles

Formation of Na2Mo2xW2(1 − x)O7 solid solutions and derived phase diagram

Na2Mo2xW2(1 − x)O7 (x = 0..1 with step 0.1) compositions were obtained by solid-state synthesis and spontaneous crystallization. The sample series were studied by XRD, Raman, DSC and SEM methods. The obtained data indicates formation of solid solutions, with unit cell gradually increasing with increasing tungsten ratio. A refined Na2Mo2O7 - Na2W2O7 phase diagram with unlimited miscibility was constructed. The solidus line was built based on DSC data and the liquidus line was constructed based on spontaneous crystallization experiments data. A luminescence study showed possibility of emission color variation in Na2Mo2xW2(1 − x)O7 solid solutions in dependence from excitation wavelength.

Achieving thermochromic upconversion of Tm3+ for high-sensitive nanoprobe and information encryption

Upconversion-based optical thermometry has many merits, such as quick temperature response, higher thermal sensitivity and robust reliability. However, the previous works were mainly focused on the thermally coupled emissions, limiting their thermal sensitivity due to the intrinsic constraint by small energy gaps (e.g., <2000 cm−1) according to Boltzmann distribution theory. Here, we report a design of NaYF4:Yb3+/Tm3+@NaYF4 core-shell nanostructure to achieve thermochromic upconversion and high thermal sensitivity by taking advantage of nonthermally coupled upconversion emissions of Tm3+. We show that the blue upconversion emission of Tm3+ at 475 nm (1G4→3H6) exhibits a regular decrease with elevating temperature, while its deep-red emission at 696 nm (3F2,3→3H6) presents an increase. Such a feature leads to a rapid rise of luminescence intensity ratio (I696 nm/I475 nm) and the optimal relative sensitivity (Sr) reaches 2.79 % K−1 (503 K). More interestingly, the sample gives rise to a gradual emission color change from blue to purplish-red with lifting temperature. Our results offer a new class of upconversion-based nonoprobe with high thermal sensitivity and observable emission color variation, showing a great potential for nanoprobe and information security.

High-Temperature, Reversible, and Robust Thermochromic Fluorescence Based on Rb2 MnBr4 (H2 O)2 for Anti-Counterfeiting.

Thermochromic fluorescent materials (TFMs) characterized by noticeable emission color variation with temperature have attracted pervasive attention for their frontier application in stimulus-response and optical encryption technologies. However, existing TFMs typically suffer from weak PL reversibility as well as limited mild operating temperature and severe temperature PL quenching. PL switching under extreme conditions such as high temperature will undoubtedly improve encryption security, while it is still challenging for present TFMs. In this work, high-temperature thermochromic fluorescence up to 473 K and robust structural and optical reversibility of 80 cycles are observed in Rb2 MnBr4 (H2 O)2 and related crystals, which is seldom reported for PL changes at such a high temperature. Temperature-driven nonluminous, red and green light emission states can be achieved at specific temperatures and the modulation mechanism is verified by in situ optical and structural measurements and single particle transition. By virtue of this unique feature, a multicolor anti-counterfeiting label based on a broad temperature gradient and multidimensional information encryption applications are demonstrated. This work opens a window for the design of inorganic materials with multi-PL change and the development of advanced encryption strategies with extreme stimuli source.

Substantial Improvement of Operating Stability by Strengthening Metal‐Halogen Bonds in Halide Perovskites

AbstractSolution‐processed lead halide perovskites (LHPs) hold great promise for low‐cost high‐performance solar cells and light‐emitting devices, but they also suffer from a serious operating instability problem due to the ionic migration and lattice decomposition driven by strong electric fields. Here, considerably suppressed ionic migration and enhanced lattice stability in LHPs with partial substitution of Pb with 3d transition metal (TM: Mn and Ni) are reported. It is experimentally shown that the energy barrier for ionic migration in CsPbBr3 can be increased fourfold by Mn and Ni substitution, even with a small doping level (&lt;4%). However, post‐TM Zn and non‐TM Bi incorporations are less efficient in suppressing ionic migration. The theoretical results reveal that Ni and Mn ions with partially filled 3d orbitals can passivate the active lone‐pair electron of surrounding Pb‐Br octahedrons via a coordination effect and reduce the Pb 6s‐Br 4p antibonding states, resulting in long‐range lattice stabilization and suppressed ionic migration. The Ni incorporation strategy in mixed‐halogen CsPbBr1.5I1.5 is further demonstrated, for which the field‐driven halogen segregation is significantly mitigated and the associated emission color variation is reduced sixfold. This study paves the way for improving the operating stability in LHP‐based optoelectronic and electronic devices.

Promising Tb3+-doped gallium tungsten-phosphate glass scintillator: Spectroscopy, energy transfer and UV/X-ray sensing

Undoped and Tb3+-doped (0.5–10 mol%) gallium tungsten-phosphate glasses, in the new compositional system NaPO3-Ga2O3-Na2WO4, were prepared and characterized by UV–visible absorption, photoluminescence excitation and emission, excited state decay measurements and X-ray excited radioluminescence measurements. A detailed study of ion-ion energy transfer, based on the analysis of decay curves by the Inokuti-Hirayama model, indicated that the energy transfer process affecting the emitting level 5D3 of Tb3+ is related to dipole–dipole interaction. Additionally, a theoretical model was successfully employed to establish a correlation between the emission color variation due to changes in the green to blue emission ratio (IG/IB). The emission spectra of the undoped and Tb3+-doped glasses were measured under UV and X-ray excitation. From the viewpoint of UV-sensing, samples doped with up to 7.0 mol% Tb3+ do not present visible emission quenching, whereas X-ray sensing is not prone to quenching at all up to 10 mol% Tb3+ doping. These characteristics, associated to a relatively high density, highlight the potential of these glasses as X-ray scintillators.

A carbon dioxide responsive fluorescent system based on micellar transformation

Carbon dioxide (CO2) responsive fluorescent systems mainly focus on modification of fluorophores with specific CO2-responsive groups, the processes of which are usually tedious and time-consuming. Here, we report a facile strategy for fabricating CO2-responsive fluorescent system by employing a surfactant, N,N-Dimethyl-1- dodecanamine (C12A), to form an oil-in-water (O/W) emulsion and a fluorophore with vibration-induced emission, which could be solubilized in the oil core. At the presence of CO2, the tertiary amine groups of C12A were protonated due to the reaction between C12A and carbonic acid, resulting in a transition of the semitransparent emulsion mixture to a transparent micellar solution due to the increased amphipathicity of C12A. Correspondingly, the micro-environment of the fluorophores in the oil core had been changed, leading to an obvious emission color variation from bright pink to blue. Thus, these responsive behaviors were visible to the naked eyes and could be simultaneously monitored by fluorescence and transmittance measurements. Repeatability and cyclability of this system were easily achieved by bubbling nitrogen (N2) into this system or heating, demonstrating great potential application of this system for CO2 detection. As a concept-of-proposal, the CO2-responsive system was proved to be effective in monitoring respiration during the process of seed germination.

Efficient and reversible acidofluorochromic features on a solid platform for reusable security writing: A structure-property relationship study on anthracenyl π-conjugates

Reversible solid-state acidochromic (SSAC) behavior offers facile and handy fluorescence (FL)-based pH detection and secured ink or rewritable platform for anti-counterfeiting applications. The acidochromism based detection primarily depends on the pyridine-based linkers in the solution state but remains ill-defined in the solid-state. This work describes the degree of emission responses, primarily in the solid-state if the N-atom of pyridyl ring alters the position in toluene/thiophene-connected structurally twisted anthracenyl π-conjugates. The role of N-atom at the 4th position is more sensitive against acid fumes, especially in the thiophene-linked analog that displays 108 nm redshift, compared to the toluene analog (85 nm). Changing N-atom's position from 4th to 3rd in the ring, only tolyl-linked analog is significantly responsive with 48 nm redshift. Single-crystal structures of protonated molecules have been judiciously determined and found more planar compared to the originals. The crystal structure and molecular packing analyses dictate that the rotors are significantly twisted with multiple noncovalent interactions, crucially responsible for the admirable solid-state emission. The gained planarity of the molecules upon protonation can elucidate the redshifted emission with quenching. Further, stronger H-bonding and excited state stabilization for a few pyridinium salts may be accountable for reducing the emission intensity and redshifted emission, respectively. High-contrast emission color variation on protonation-deprotonation has been employed as a handy polymer-blended emitting platform for convenient and reusable security writings.

Pumping-controlled multicolor modulation of upconversion emission for dual-mode dynamic anti-counterfeiting

Abstract Lanthanide up-conversion features stepwise multi-photon processes, where the difference in photon number that is required for specific up-conversion process usually leads to significant variance in pumping-related processes/properties. In this work, a pumping-controlled dual-mode anti-counterfeiting strategy is conceived by taking advantage of the combination of up-conversion processes with different photon numbers. The combination of Er3+ and Tm3+, which are spatially separated within a designed core/triple-shell nano-architecture, is taken as an example to illustrate such idea. Upon infrared excitation, the emission color of a designed pattern can be switched from red to purple by increasing the excitation power density from 5 to 11 W/cm2, while a bright luminescent trajectory including red, white and blue-green color with different length is observed when rotating the pattern above 600 rpm. In addition, the relative up-conversion emission intensities of the Er3+ and Tm3+ ions can be manipulated through tailoring interfacial or inner defects in the core/triple-shell nano-crystals, which enable an ultrahigh sensitivity for the pumping-controlled emission color variation to be observed under excitation power well below 11 W/cm2.

Low-Temperature Solid-State Synthesis and Upconversion Luminescence Properties in (Na/Li)Bi(MoO4)2:Yb3+,Er3+ and Color Tuning in (Na/Li)Bi(MoO4)2:Yb3+,Ho3+,Ce3+ Phosphors.

In this Article, we reported the synthesis and the upconversion luminescence (UCL) properties of a series of novel (Na/Li)Bi(MoO4)2:Yb3+,Er3+ [(N/L)BMO:Yb3+,Er3+] and (Na/Li)Bi(MoO4)2:Yb3+,Ho3+,Ce3+ [(N/L)BMO:Yb3+,Ho3+,Ce3+] phosphors. X-ray diffraction patterns and Rietveld refinements for several representative samples indicated the pure phase of as-prepared samples. The Yb3+,Er3+ codoped (N/L)BMO presented bright green luminescence under 975 nm laser excitation with UCL spectra showing two main green bands around 529 nm (Er3+, 2H11/2 → 4I15/2) and 551 nm (Er3+, 4S3/2 → 4I15/2), in addition to a very weak one at 655 nm (Er3+, 4F9/2 → 4I15/2). The (N/L)BMO:Yb3+,Ho3+ mainly showed a green band around 544 nm (5S2,5F4 → 5I8) and a red band around 654 nm (5F5 → 5I8) upon 975 nm laser excitation. With increasing Yb3+ concentrations in (N/L)BMO:Yb3+,0.01Ho3+, the red/green ratios decreased monotonously corresponding to the emission color variation from light red to light yellow. Both UCL mechanisms of Yb3+,Er3+ and Yb3+,Ho3+ were determined to be two-phonons absorption processes in (N/L)BMO:Yb3+,Er3+/Ho3+. The Ce3+ ions were introduced into Yb3+,Ho3+ codoped (N/L)BMO to show the color tuning from light yellow to light red originating from the cross relaxation processes of (CR1) Ho3+ (5F4,5S2) + Ce3+ (2F5/2) → Ho3+ (5F5) + Ce3+ (2F7/2) and (CR2) Ho3+(5I6) + Ce3+ (2F5/2) → Ho3+ (5I7) + Ce3+ (2F7/2), which is based on the energy matching of Ce3+2F7/2-2F5/2 level pairs with Ho3+5I6-5I7 and 5F4,5S2-5F5 level pairs and confirmed by the decay times. These results suggest good UCL properties of (N/L)BMO:Yb3+, Er3+ and (N/L)BMO:Yb3+, Ho3+, Ce3+ materials, and color modulation is easily controlled by varying Yb3+ concentration and a cross relaxation process between Ce3+ and Ho3+, which provides efficient methods to regulate the emission color of UCL phosphors.

Size dependent ratiometric detection of Pb (II) ions in aqueous solution by light emitting biogenic CdS NPs

We have synthesized water soluble CdS quantum dots (QDs) in different sizes (~ 3, 10 and 25nm) via a green rout using pomegranate peel extract as a reducing and stabilizing agent. The particles with the size of ~ 3, 10 and 25nm respectively exhibit blue, green and yellowish red emission under UV light. We have utilized the NPs for the discrimination of metal ions in aqueous solution. Interestingly, the fluorescence from the NPs undergoes a ratiometric variation only in presence of Pb2+ ions, indicating that the system can act as a potential Pb2+ ion sensor. There was a remarkable size dependent stock shift (~ 270nm) upon the addition of Pb2+ ions, which enables the detection by naked eye. DLS study indicates that Pb2+ ions induce aggregation in the system owing to the complex formation between the capping agent and the metal ion. The aggregation, in turn, generates inter plasmon coupling and results in emission color variation. The naked eye detection limit of Pb2+ ions in solution was found to be 10−9M. This is the first study of Pb2+ ion sensing using CdS quantum systems and the detection procedure is easy-to-make, less time consuming and highly re-producible. The present method enables direct application of the system for detecting Pb2+ ions due to the presence of ratiometric fluorescence, leading to a large Stokes shift and associated color change in the visible region.

Filter paper with chitosan membrane as platform for fast and simple blood glucose assay using phone camera detection

We have synthesized water soluble CdS quantum dots (QDs) in different sizes (~ 3, 10 and 25 nm) via a green rout using pomegranate peel extract as a reducing and stabilizing agent. The particles with the size of ~ 3, 10 and 25 nm respectively exhibit blue, green and yellowish red emission under UV light. We have utilized the NPs for the discrimination of metal ions in aqueous solution. Interestingly, the fluorescence from the NPs undergoes a ratiometric variation only in presence of Pb2+ ions, indicating that the system can act as a potential Pb2+ ion sensor. There was a remarkable size dependent stock shift (~ 270 nm) upon the addition of Pb2+ ions, which enables the detection by naked eye. DLS study indicates that Pb2+ ions induce aggregation in the system owing to the complex formation between the capping agent and the metal ion. The aggregation, in turn, generates inter plasmon coupling and results in emission color variation. The naked eye detection limit of Pb2+ ions in solution was found to be 10−9 M. This is the first study of Pb2+ ion sensing using CdS quantum systems and the detection procedure is easy-to-make, less time consuming and highly re-producible. The present method enables direct application of the system for detecting Pb2+ ions due to the presence of ratiometric fluorescence, leading to a large Stokes shift and associated color change in the visible region.

Molecular Origin of Color Variation in Firefly (Beetle) Bioluminescence: A Chemical Basis for Biological Imaging.

Firefly shows bioluminescence by "luciferin-luciferase" (L-L) reaction using luciferin, luciferase, ATP and O2. The chemical photon generation by an enzymatic reaction is widely utilized for analytical methods including biological imaging in the life science fields. To expand photondetecting analyses with firefly bioluminescence, it is important for users to understand the chemical basis of the L-L reaction. In particular, the emission color variation of the L-L reaction is one of the distinguishing characteristics for multicolor luciferase assay and in vivo imaging. From the viewpoint of fundamental chemistry, this review explains the recent progress in the studies on the molecular mechanism of emission color variation after showing the outline of the reaction mechanism of the whole L-L reaction. On the basis of the mechanism, the progresses in organic synthesis of luciferin analogs modulating their emission colors are also presented to support further developments of red/near infrared in vivo biological imaging utility of firefly bioluminescence.

Phase Transformation in Ca3(PO4)2:Eu2+ via the Controlled Quenching and Increased Eu2+ Content: Identification of New Cyan‐Emitting α‐Ca3(PO4)2:Eu2+ Phosphor

A case of phosphor is reported where the cooling rate parameter significantly influences the luminescence property. By quenching the sample after the high‐temperature solid‐state reaction at 1250°C, we successfully prepared the Eu2+‐doped α form Ca3(PO4)2 (α‐TCP:Eu2+) as a new kind of bright cyan‐emitting phosphor. The unusual emission color variation (from cyan to blue) depends on the cooling rate after sintering and Eu2+ doping level as it was observed in the TCP‐based phosphors. By the Rietveld analysis, it is revealed that the cyan‐ and blue‐emitting phosphors are two different TCP forms crystallizing in the monoclinic (space group P21/a, α‐TCP) and the rhombohedral structure (space group R3c, β‐TCP), respectively. Upon 365 nm UV light excitation, α‐TCP:Eu2+ exhibits an asymmetric broad‐band cyan emission peaking at 480 nm, while β‐TCP:Eu2+ displays a relatively narrow‐band blue emission peaking at 416 nm. The Eu2+‐doping in Ca3(PO4)2 shifts the upper temperature limit of the stable structural range of β form from 1125°C to ≥1250°C. Moreover, the crystal structures of α/β‐TCP:Eu2+ were compared in the aspects of compactness and cation site sets. The emission thermal stability of α/β‐TCP:Eu2+ was comparatively characterized and the difference was related to the specific host structural features.

Multicomponent Molecular Puzzles for Photofunction Design: Emission Color Variation in Lewis Acid-Base Pair Crystals Coupled with Guest-to-Host Charge Transfer Excitation.

Simple yet ubiquitous multimolecular assembly systems with color-tunable emissions are realized by cooperative electron donor-acceptor interactions, such as the boron-nitrogen (B-N) dative bond as a Lewis acid-base pair and charge transfer (CT) interactions. These are ternary-component systems consisting of a naphthalenediimide derivative (NDI), tris(pentafluorophenyl)borane (TPFB), and aromatic molecules (guest) with an NDI:TPFB:guest ratio of 1:2:2. The crystal shows guest-dependent color-tunable emissions such as deep blue to orange when a guest molecule of benzene is replaced with other π-conjugated systems. A good correlation between the emission wavelength and ionization potential of the guest and electronic structure calculations indicated that the emission is due to the CT transition from the guest to the NDI. The present study suggests that a rational solution of multcomponent molecular puzzles would be useful for obtaining novel photofunctional solid-state systems.

Emission Color Variation of Sr&lt;sub&gt;3&lt;/sub&gt;SiO&lt;sub&gt;5&lt;/sub&gt;:Ce&lt;sup&gt;3+&lt;/sup&gt;, Li&lt;sup&gt;+&lt;/sup&gt; Phosphor for Near-UV Light-Emitting Diodes by Co-Doping with Mg&lt;sup&gt;2+&lt;/sup&gt; and Ba&lt;sup&gt;2+&lt;/sup&gt;

This paper reports the spectral variation of Sr3SiO5: Ce3+, Li+ phosphor. It was synthesized by the conventional solid state method. The effects of calcining temperature and codoption of Mg2+ and Ba2+ on phase composition, structure, photoluminescence and thermal stability were firstly investigated in detail. Under the near-UV excitation of 415 nm, Sr3SiO5: Ce3+, Li+ emits a bright greenish-yellow color centered at 532nm, which originates from 5d to 4f (2F7/2 and 2F5/2) transition of Ce3+. The PL spectra show the different emission color variation with substituting Sr2+ cations with smaller Mg2+ or larger Ba2+, which makes it be applied as a color-tunable phosphor for LED based on near-UV chip/phosphor technology. Simultaneously, the thermal stability of (Sr,Ba)3SiO5: Ce3+, Li+ phosphor is improved markedly.

Dynamics on the Electronically Excited State Surface of the Bioluminescent Firefly Luciferase–Oxyluciferin System

Dynamics of the firefly luciferase-oxyluciferin complex in its electronic ground and excited states are studied using various theoretical approaches. By mimicking the physiological conditions with realistic models of the chromophore oxyluciferin, the enzyme luciferase, and solvating water molecules and by performing real time simulations with a molecular dynamics technique on the model surfaces, we reveal that the local chromophore-surrounding interaction patterns differ rather severely in the two states. Because of the presence of protein, the solvation dynamics of water around the chromophore is also peculiar and shows widely different time scales on the two terminal oxygen atoms. In addition, simulations of the emission with the quantum-mechanics/molecular-mechanics approach show a close relationship between the emission color variation and the environmental dynamics, mostly through electrostatic effects from the chromophore-surrounding interaction. We also discuss the importance of considering the time scales of the luminescence and the dynamics of the interaction.

Emission color variation of (Ba,Sr)_3BP_3O_12:Eu^2+ phosphors for white light LEDs

A series of alkaline earth borophosphate phosphors, (Ba,Sr)(3)BP(3)O(12) doped with Eu(2+) ions, were synthesized by a solid state reaction. Two emission bands at 465 nm and 520 nm were attributed to the f-d transitions of doped Eu(2+) ions occupying in two different cation sites in host lattices and emission color variation was observed by substituting the M(2+) sites, which was rationalized in terms of two competing factors of the crystal field strength and bond covalence. Green and bluish-white pc-LEDs were fabricated by combination of a 370 nm near-UV chip and composition-optimized Ba(3)BP(3)O(12):Eu(2+) and (Ba,Sr)(3)BP(3)O(12):Eu(2+) phosphors, respectively. The series of phosphors may serve as a promising green and bluish-white luminescent materials used in fabrication of near UV-based white pc-LEDs.

Emission color variation of M 2SiO 4:Eu 2+ (M=Ba, Sr, Ca) phosphors for light-emitting diode

The luminescent properties of alkaline earth orthosilicates M 2SiO 4 (M=Ba, Sr, Ca) doped with Eu 2+ ions are investigated. Two emission bands are assigned to the f–d transitions of Eu 2+ ions doped into two different cation sites in host lattices confirmed by electron paramagnetic resonance signal. Two emission bands show the different emission color variation with substituting M 2+ cations with smaller cations. This behavior is discussed in terms of two competing factors of the crystal field strength and covalence. Also the decay times are in order of 600–1000 ns. These phosphors with maximum excitation of around 370 nm can be applied as a color-tunable phosphor for light-emitting diode based on ultraviolet chip/phosphor technology.

Efficient color tuning (blue, red-orange, white) of light emitting diodes by excitation energy transfer

We present a color tuning technique allowing to control the emission colors of polymer light emitting diodes (PLEDS), which is of considerable interest for flat panel display applications. The emission color variation of the PLED from blue to red-orange is achieved by blending small amounts of a red light emitting guest polymer with the active PLED layer consisting of a blue emitting laddertype poly (paraphenylene) (LPPP). Using this new technique we realized highly efficient stable single layer PLEDs emitting different colors including white light emission. Besides the color tuning, which is established by an efficient excitation energy transfer (EET) and charge transfer from the host, m-LPPP, to the guest polymer, a significant increase of the photoluminescence (PL) and the electroluminescence (EL) quantum efficiency is observed.