Upconversion Emission Research Articles

Ultrafast upconversion superfluorescence with a sub-2.5 ns lifetime at room temperature.

Photon upconversion through lanthanide-doped nanoparticles is of great significance for various applications. However, the current development of upconversion nanoparticles is hindered by the low quantum efficiency and long radiative lifetimes of lanthanide ions, restricting their applications in time-dependent nanophotonics. Herein, we report ultrafast upconversion superfluorescence with a lifetime of sub-2.5 ns in lanthanide-doped nanoparticles at room temperature. Upon excitation with an 800-nm fs-pulsed laser, we achieve a large number (N = 912) of correlated dipoles in Nd3+-concentrated nanoparticles, resulting in collective coherent emission with two orders of magnitude amplification in intensity and more than three orders of magnitude improvement in the radiative decay rate. Furthermore, we demonstrate that the control of excitation power and emitting sample length enables the lifetime manipulation of upconversion emission in a wide range from μs to sub-ns, accompanied by the typical superfluorescence signature of Burnham-Chiao ringing. These findings may benefit applications in many advanced technologies such as quantum counting and high-speed super-resolution bioimaging.

Laser-Controlled Information Releasing and Hiding Based on Perovskite Phosphors.

Laser-active interference with high confidentiality and convenience opens up a cutting-edge path for releasing and hiding key targets; however, its development still faces enormous challenges owing to the difficulty of concealing objects. Herein, a novel conceptual design for laser-controlled information release and hiding (LIRH) is proposed and successfully realized. Cs2NaInCl6:Er3+, Yb3+ (CNIC:Er, Yb) perovskite microcrystal is adopted as a carrier for LIRH implementation, exhibiting excellent up-conversion (UC) emission under NIR (980 and 1530 nm) irradiation due to its ultralow phonon energy. The fluorescence intensity crossover and outstanding photon output capacity are revealed in comparison with Er3+/Yb3+ codoped and Er3+ single-doped CNIC phosphors under different laser sources, and the obvious difference in quantum yields (QY) under 980 and 1530 nm excitation provides theoretical possibility for LIRH. More importantly, the obtained LIRH features high stability at temperatures up to 413 K, showing good adaptability in various potential scenarios. Moreover, CNIC:Er, Yb is further combined with polyacrylonitrile (PAN) polymer to form fluorescent fibers with exceptional crystal stability and composite flexibility, thus making the LIRH code a reality based on perovskite composite phosphors. The laser-active invisibility offers an innovative idea for LIRH, further extending the application of LIRH in the field of information encryption, which has promising prospects in information safety, advanced anticounterfeiting, and smart responsive materials.

Untypical tuneable emission activated by NIR radiation observed in gadolinium borate nanomaterials doped with Yb3+, Ho3+ and Ce3+ ions.

New luminescent nanoparticles with adjustable emission properties were synthesized via a hydrothermal method to investigate the effect of Ce3+ concentration on the tuneable emission properties of Yb3+/Ho3+ co-doped gadolinium borates. The synthesized samples' comprehensive structural and morphological analysis was conducted utilizing X-ray diffraction and transmission electron microscopy. To obtain highly crystalline nanoparticles GdBO3:Yb3+/Ho3+/Ce3+ characterized by intense and bright emission, they were annealed at 525°C and 925°C. Under the excitation of 972nm, the tunable yellow to green up-conversion (UC) emission was observed, while the Ce3+ concentration increased. This change correlated with a difference in the correlated color temperature from 4345K to 5442K, respectively. The untypical changes observed in the red-to-green (R/G) ratio, displaying a declining trend contrary to conventional expectations, allowed for the proposed mechanism for understanding their UC luminescence properties. The results obtained indicate that the Ce3+ doped GdBO3:Yb3+/Ho3+ nanoparticles demonstrate high application potential for anti-counterfeiting.

Chromatic tuning of upconversion emission upon dual infrared wavelength co-excitation in Er3+ doped Ba3Lu4O9 phosphor

Emission color change of the upconversion luminescence materials is often required for applications in optical anti-counterfeiting, laser lighting, and 3D displays. In this work, we developed a new route for chromatic tuning of upconversion emission upon two wavelength co-excitation by changing excitation power density. The proposed route was used for Er3+ single-doped Ba3Lu4O9 phosphor which was derived from a traditional solid-state reaction. The crystal structure of the obtained Ba3Lu4O9:Er3+ phosphor was characterized by X-ray diffraction and Rietveld refinement modeling. Under individual excitation of 980 or 1550 nm, it was found that the luminescence intensity ratio of red to green changes slightly with respectively increasing excitation power density (working current of the laser) of 980 or 1550 nm laser. However, upon both 980 and 1550 nm co-excitation at the same time and by adjusting the excitation power density of one laser amongst the two lasers, the luminescence intensity ratio of red to green changes greatly with increasing excitation power density. The change of luminescence intensity ratio of red to green implies the upconversion emission color tuning. The mechanism for the color tuning was discovered. It was found that with changing excitation power density the ratio of red to green changed greatly when the phosphor was irradiated by 980 and 1550 together, but the ratio of red to green changed slightly when the phosphor was irradiated individually by 980 or 1550. It is proved that it is feasible to turn color by the method of two infrared wavelengths excitation.

Luminescent and magnetic Y2O3: Er3+-Yb3+@γ-Fe2O3 bifunctional broadband emitting nanocomposites

The development of novel nanocomposites containing upconverting luminescent materials and magnetic nanoparticles have shown their potential applications in the hyperthermia and bio-imaging. In this regards, the search for new class of nanocomposites, which is responsive to magnetic and optical guiding in medical fields are in demand. To resolve the problem, bifunctional upconverting Y2O3: Er3+-Yb3+@γ-Fe2O3 nanocomposites have been synthesized through ball milling approach. Nanocomposites are able to upconvert NIR photons into overall yellowish-white colour broadband visible emission in 400–900 nm wavelength range at room temperature under 980 nm CW laser diode excitation. Thermal conductivity study shows an improved phonon scattering process in the Y2O3: Er3+-Yb3+@γ-Fe2O3 (∼ 0.0681 W/mK) nanocomposites as compared to Y2O3: Er3+-Yb3+ (∼ 0.0750 W/mK) phosphors due to grain boundary expansion. Broadband UC emission is originated via Yb-Yb dimer formation and heat associated continuum emission is reported due to photon avalanche (∼ 6 NIR photons) phenomenon. Besides, it also exhibits the attraction ability towards the magnet. Therefore, the developed nanocomposites can be used in converting the NIR radiation into broadband upconversion emission and luminescence imaging applications.

New Horizons for Temperature Sensing and Bioimaging Using Er3+‐Doped Core@Shell Nanoparticles and Effects of H2O and D2O Solvents on Their Spectroscopic Properties

AbstractLanthanide‐doped nanoparticles (NPs) exhibit temperature‐dependent luminescence, enabling the design of luminescent nanothermometers for industrial and medical applications. This research demonstrates the temperature‐sensing properties of NaYF4:7.5%Er3+@NaYF4 and NaErF4@NaYF4 NPs, which have a hexagonal shape and average size of 17 nm. Their core@shell structure is confirmed using high‐resolution transmission electron microscopy, and they exhibit intense upconversion (UC) emission under 1532 nm excitation in H2O and D2O colloids. The recorded spectra show Er3+ emission bands with varying intensity ratios depending on the Er3+ concentration, chosen solvent, and temperature. The spectroscopic properties of the studied NPs allow for their excitation and observation of emission within biological windows, which makes them useful for bio‐related applications. The emission of prepared NPs is analyzed as a function of temperature from 298 up to 358/363 K in H2O and D2O. The ratios for thermally‐coupled levels and non‐TCLs and their relative sensitivities are studied. For the high dopant concentration sample in water, the O─H vibrations and blue shift in the absorption spectrum lead to a record relative sensitivity of 2.50% K−1 (at 363 K) for the 2H11/2/4I11/2 ratio. The use of synthesized NPs for bioimaging under 1550 nm excitation is also demonstrated to observe their accumulation in the guts of Daphnia magna.

Effect of Urea and Thiourea on the color emission of (YxBi1-x)2Zr2O7:Er3+,Yb3+ upconversion phosphors

This study reports the upconversion emission properties of Bi2Zr2O7:Er,Yb (BiZr), Y2Zr2O7:Er,Yb (YZr) and (Y0.25Bi0.75)2Zr2O7:Er,Yb (Y0.25Bi0.75Zr) phosphors. All those samples were synthesized with fixed Er and Yb concentrations of 2 and 20 mol%, respectively. The analysis by X-ray diffraction revealed that the YZr sample had cubic phase, the BiZr sample had a mixture of monoclinic/cubic phases and the Y0.25Bi0.75Zr sample had a mixture of all the phases mentioned above. In addition, the analysis by electron microscopy indicated that all the samples are formed by mixtures of particles with irregular and quasi-spherical shapes. The YZr, BiZr and Y0.25Bi0.75Zr samples were synthesized using urea or thiourea as fuel and an annealing temperature of 775 or 850 °C. For any fuel, the samples produced green (525 and 548 nm) and red (655 nm) emissions by upconversion after exciting them with 975 nm. Interestingly, the Y0.25Bi0.75Zr sample always presented the most intense emission for any fuel or temperature of synthesis. Interestingly, the Y0.25Bi0.75Zr sample made with thiourea had a green emission intensity 49 % higher than that made with urea, but the red emission intensity was 25 % lower in the sample made with thiourea. Color emission tuning was observed in the samples under certain conditions: 1) the color emission for the BiZr sample (made with urea) was changed from yellow to orange-red after increasing the annealing temperature from 775 or 850 °C and 2) the YZr sample (annealed at 775 °C) changed its color emission from green to red after using thiourea instead of urea. Surprisingly, the Y0.25Bi0.75Zr sample always maintained its green emission despite the change of fuel or annealing temperature. In general, the results of this research demonstrate that different fuels or synthesis temperatures can be used to enhance the upconversion emission or to tune the emission color. This avoids the use of high concentrations of rare earths to produce any of these effects as previously reported in the literature.

Unlocking nature’s brilliance: using Antarctic extremophile Shewanella baltica to biosynthesize lanthanide-containing nanoparticles with optical up-conversion

Both lanthanide-containing and fluorine-containing nanomaterials present challenging targets for microbial biosynthesis because these elements are toxic to most bacteria. Here, we overcome these challenges by using an Antarctic Shewanella baltica strain that tolerates these elements and report the first biosynthesis of lanthanide-doped fluoride nanoparticles (NPs) from them. NaYF4 NPs doped with Er3+/Yb3+ are prototypical lanthanide-based upconverting nanoparticles (UCNPs) with upconverted luminescence at visible wavelengths under infrared excitation. However, their synthesis employs high precursor concentrations, organic solvents, and elevated temperatures. Microbial biosynthesis offers a greener alternative but has not been explored for these materials. Here, we harness an extremophile S. baltica strain to biosynthesize UCNPs at room temperature, based upon its high tolerance for fluoride and lanthanide ions and the observation that tolerance of lanthanides increased in the presence of fluoride. Our biosynthesis produces electron-dense nanostructures composed of Na, Y, F, Yb, and Er in the bacterial periplasm, adhered to the outer cell membrane, and dispersed extracellularly, which exhibited up-converted emission under 980 nm excitation. This suggests that extracellular or periplasmic mineralization of lanthanides as fluorides protects the bacteria from lanthanide toxicity. Subsequent heating both enhanced upconverted emission from UCNPs and allowed observation of their crystallinity in transmission electron microscopy (TEM). This work establishes the first biosynthesis of NaYF4:Yb: Er UCNPs, advancing both nanotechnology and biotechnology.Graphical

Rise and Decay of Photoluminescence in Upconverting Lanthanide-Doped Nanocrystals.

Nanocrystals (NCs) doped with lanthanides are capable of efficient photon upconversion, i.e., absorbing long-wavelength light and emitting shorter-wavelength light. The internal processes that enable upconversion are a complex network of electronic transitions within and energy transfer between dopant centers. In this work, we study the rise and decay dynamics of upconversion emission from β-NaYF4 NCs codoped with Er3+ and Yb3+. The rise dynamics of the red and green upconverted emissions are nonlinear, reflecting the nonlinear nature of upconversion and revealing the mechanisms that populate the emitting states. The excited-state decay dynamics are nonexponential. We unravel the underlying decay pathways using photonic experiments. These reveal the contributions of different upconversion pathways visually, as each pathway exhibits a distinct response to systematic variation of the local density of optical states. Moreover, the effect of the local density of optical states on core-only NCs is qualitatively different from core-shell NCs. This is due to the different balance between feeding and decay of the electronic levels that produce upconverted emission. The understanding of the upconversion dynamics provided here could lead to better imaging and sensing methods relying on upconversion lifetimes or guide the rational optimization of the dopant concentrations for brighter upconversion.

Enhanced red UC emission of Er3+ ions by constructing multi-heterojunction core-shell nanoparticles

The construction of core-shell structures with multiple heterojunctions facilitates ion energy transfer and minimizes surface quenching effects, which are essential for enhancing and regulating the upconversion emission of materials. In this study, we effectively regulated the spatial distribution of Yb3+ and Er3+ ions in multilayered heterogeneous core-shell structures to significantly enhance the red upconversion emission of Er3+ ions. Specifically, the red emission intensity of the NaErF4@NaYbF4: 2 %Er3+@NaYF4 core-shell nanoparticles was enhanced nearly 24.4-fold compared to that of NaErF4 nanoparticles. This enhancement results from a bidirectional energy transfer process. Moreover, when we swapped the positions of the NaErF4 core and the NaYbF4: 2 %Er3+ shell and introduced NaYF4 inert isolation layer and Yb3+ ions, the red emission intensity of the NaYbF4:2 %Er3+@NaYF4:20 %Yb3+@NaErF4@ NaYF4 multilayer core-shell structure increased significantly by nearly 254.3-fold compared to NaErF4 nanoparticles. The remarkable enhancement of the red emission of Er3+ ions is primarily attributed to a well-organized bidirectional energy transfer process between the heavily doped core and shell Er3+-Yb3+ ion pairs. The energy transfer behavior of these multi-heterojunction core-shell nanoparticles was studied based on their spectral characteristics. The multilayer heterogeneous core-shell nanoparticles exhibited colorful emissions under different excitation conditions, highlighting their potential for biomedical applications and colorful anti-counterfeiting.

Ultrasensitive and Adjustable Nanothermometers Based on Er3+-Sensitized Core@Shell Nanoparticles for Use in the First Biological Window.

In recent years, intensive research has focused on lanthanide-doped nanoparticles (NPs) used as noncontact temperature sensors, particularly in nanomedicine. These NPs must be capable of excitation and emission within biological windows, where biological materials usually show better transparency for radiation. In this article, we propose that NPs sensitized with Er3+ ions can be applied as temperature sensors in biological materials. We synthesized the NPs through a reaction in high-boiling solvents and confirmed their crystal structure and the formation of core@shell NPs by using X-ray diffraction, high-resolution transmission electron microscopy, and element distribution mapping within the NPs. NaErF4@NaYF4, NaYF4:12.5% Er3+, 2.5% Tm3+@NaYF4, NaYF4:7.5% Er3+@NaYF4, and NaYF4:12.5% Er3+, 2.5% Ho3+@NaYF4 exhibited intense upconversion (UC) emission under 1532 nm laser excitation detectable also in the whole human blood. We propose that this UC results from energy transfer between Er3+ ions and from Er3+ to Tm3+ or Ho3+ codopants. To determine the mechanism of UC, we measured the dependence of the emission band intensities on the laser power densities. Importantly, we also analyzed the temperature-dependent emission of the NPs within the 295-360 K range. Based on the collected emission spectra, we calculated the luminescence intensity ratios (LIRs) of the emission bands to assess their potential for optical temperature sensing. The temperature-sensing properties varied with the concentration of Er3+ ions and the presence of additional Tm3+ or Ho3+ codopants. Depending on the NP composition and the emission bands used for luminescence ratio calculations, the maximum relative temperature sensitivity ranged from 4.55%·K-1 to 1.12%·K-1, with temperature resolution between 0.05 and 2.53 K at room temperature. Finally, as proof of using NPs as temperature sensors in biomedicine, we successfully measured the temperature-dependent emission of NaYF4:7.5% Er3+@NaYF4 NPs dispersed in whole blood under 1532 nm excitation. We demonstrated that the ratio of Er3+ ion emission bands changes with temperature, indicating that these NPs have potential applications in temperature sensing within biological environments. We also confirmed the properties of NPs as temperature sensors by measuring the temperature reading uncertainty and the repeatability of the LIR readings during heating-cooling cycles, thereby confirming the excellent properties of the studied systems as temperature sensors.

Upconversion enhancement and temperature sensing studies in Li+ ions incorporated GdPO4:Tm3+/Yb3+ phosphor

The GdPO4:Tm3+/Yb3+ phosphor codoped with various concentrations of Li + ions were synthesized for upconversion emission and optical thermometry studies. Excitation using 980 nm laser diode results in three upconversion (UC) emission bands with centre wavelengths of 478, 648 and 692 nm. These bands are originated from 1G4→3H6, 1G4→3F4, and 3F3→3H6 transitions of the Tm3+ ion, respectively. Li+ ions modified the local crystal symmetry around dopant ions, resulting enhanced UC emissions. The lifetime of the 1G4 level of Tm3+ ion was studied using a 980 nm laser excitation. The temperature sensing performances of GdPO4:Tm3+/Yb3+ and GdPO4:Tm3+/Yb3+/Li+ based on fluorescence intensity ratio (FIR) technique were evaluated in the temperature range 301–713 K under 980 nm excitation. Non-thermally coupled levels 3F3 (692 nm) and 1G4 (478, 648 nm) were utilized for FIR estimation. A maximum absolute sensitivity of 6.28 × 10−3 K−1 at 653 K and 18.71 × 10−3 K−1 at 713 K were observed for Li + undoped and codoped phosphors respectively. The result indicates that codoping of Li + ions improved the UC emission as well as optical thermometry. Moreover, CIE colour coordinates and anti-counterfeiting application were also exhibited.

Stokes and anti-Stokes emission characteristics of Er3+/Yb3+ co-doped zinc tellurite glasses under 377 and 1550 nm excitations for solar energy conversion application

One of the best rare earth combinations for down- and up-converting the solar photons is Er3+/Yb3+. Therefore, at present, by using the traditional melt quench technique, zinc tellurite glasses doped with Er3+ and Er3+/Yb3+ were characterised. Several methods of down-conversion that result in the emission of 1000 nm from the acceptor (Yb3+) under excitation of donor (Er3+) were studied. On the other hand, the impact of acceptor (Yb3+) concentration and excitation pump power on the excited state absorption and its influence on the up-conversion emission (1000 nm) properties were investigated in detail under 1535 nm excitation. The studies on emission and decay results are discovered to be very significant. Suggesting that these glasses when used as conversion layers, can append the conversion efficiency of Si-based solar cells.

Tuning the emission color of SrLaAlO4:Er,Yb upconversion phosphors by decorating their surface with CsPbBr3-xIx quantum dots

In this work, SrLaAlO4:Er3+ (2 mol%),Yb3+ (4 mol%) upconversion phosphor was synthesized by using a combustion method. Later, CsPbBr3-xIx (x = 0, 1.5 and 0.75) perovskite quantum dots (QDs) were synthesized by using hot injection method. According the analysis by X-ray diffraction, the SrLaAlO4:Er,Yb (SL:Er,Yb) phosphors and QDs had tetragonal and cubic phases, respectively. Moreover, the analysis by microscopy revealed that the SL:Er,Yb phosphors are composed of micro-grains with irregular morphology, while the CsPbBr3-xIx QDs had a morphology of cubes. The surface of the SL:Er,Yb phosphors was decorated with CsPbBr3-xIx QDs and obtained three composite powders: CsPbBr3+SL:Er,Yb, CsPbBr1.5I1.5+SL:Er,Yb and CsPbBr2.25I0.75+SL:Er,Yb. Those composites were firstly excited with UV light (380 nm) and produced emissions in the green and orange-red regions by downconversion. Interestingly, the emission intensity of the composite powders was 45–75 % higher than that for the individual CsPbBr3-xIx QDs or SL:Er,Yb phosphors. Later, the same composite powders were excited with NIR light (980 nm), in consequence, intense green and yellow emissions were produced by upconversion. In particular, the CsPbBr1.5I1.5+ SL:Er,Yb composite produced strong red emission by upconversion because the presence of the QDs on the SL:Er,Yb surface promoted the following cross relaxation process: 4I11/2 (Er3+)+ 4F7/2 (Er3+)→4F9/2 (Er3+)+ 4F9/2 (Er3+). In general, the quantum dots deposited on the SL:Er,Yb surface not only enhanced the red upconversion emission, but also provoked a color tuning when they are excited with NIR or UV light. Those last effects have not been reported in the literature previously. Thus, the results of this investigation demonstrate that combining perovskite quantum dots and the SL:Er,Yb forms a luminescent material able to tune its emission by upconversion or downconversion. This last property can be utilized for the design of new LED based lamps, which are employed for general illumination in houses and buildings.

Quantum yield and thermal sensitivity of SrGd2O4:Yb, Tm up-conversion nanoparticles

Luminescence thermometry is an effective method for temperature measurements using the remote operating mode. In this work, a series of Tm3+ (1 at%) and Yb3+ (2, 4, and 6 at%) ions doped SrGd2O4 up-conversion (UC) nanoparticles were synthesized by the modified sol-gel method, and fully characterized toward optimization of their composition and optical characteristic. Crystallization of single-phase nanoparticles with homogeneous doping was confirmed by XRPD, TEM/HRTEM, and STEM/EDS analysis in all samples, independently of their stoichiometry. The UC emissions recorded under 976 nm excitation revealed typical Tm3+ transitions, showing that the optimal sensitizer doping concentration is 4 at%. For this sample, a lifetime of 342 μs, and a total quantum yield of 1.12 % were determined at room temperature for the emission observed in the Vis part of up-conversion spectra. The temperature-dependent emission spectra, measured from 263 to 363 K, implied its excellent temperature-sensing capabilities. The luminescent intensity ratio (LIR) was employed for blue I479/485 and IR I795/807 Tm3+ emissions. The absolute and relative temperature sensitivity continuously decreases with the increase of the temperature, showing the values of 4.05x10−3 K-1 and 0.41 % K−1 (for blue) and 3.8x10−3 K-1 and 0.31 % K−1 (for IR) at the temperature of 300 K. The results indicate that SrGd2O4 doped with 4 % Yb3+ and 1 % Tm3+ may be employed for reliable temperature measurement over an investigated temperature range.

BaMoO4:Nd3+-Yb3+ upconverting phosphors for NIR to NIR contactless temperature reading out

BaMoO4:Nd3+–Yb3+ phosphors have been synthesized by using conventional solid state reaction synthesis technique. Frequency up-conversion (UC) emission studies have been performed under 980 nm excitation. The developed UC phosphors show multicolor upconverted emission bands in the 400–900 nm wavelength range and assigned via the radiative transitions of Nd3+ ion. FIR technique based on frequency upconversion has been applied for the 4F7/2→4I9/2 and 4F5/2→4I9/2 transitions of Nd3+ ion in the NIR (750–804 nm) region arising from the two thermally coupled energy states. The energy gap (ΔE) between the upper and lower excited levels (4F7/2 and 4F5/2) was found to be ∼ 800 cm−1. The maximum sensor sensitivity ∼ 23.2 × 10−4 K−1 at 306 K (operating temperature range 306–483 K) under 980 nm NIR laser irradiation for BaMoO4:Nd3+-Yb3+ phosphors is reported. The prepared phosphors have proven their utility as a novel upconverting material for NIR to NIR contactless temperature reading out.

Blue to UV upconversion properties of Pr3+ doped ACaF3 (A = K, Rb, Cs) phosphors

Materials capable of converting visible light into the ultraviolet C (UVC) region are promising candidates for antimicrobial technology and photocatalytic applications. In this work, the effective UVC upconversion phosphors ACaF3:Pr3+ (A = K, Rb, Cs) were synthesized using solid state reactions. Upconverted emissions in the range from 230 to 315 nm were obtained with 444 nm laser excitation. The dependence of upconverted luminescence on pump power, the concentration of Pr3+ ions, and the decay profiles of upconverted luminescence provide insights into the primary mechanisms responsible for upconversion. Among the obtained fluoroperovskites, the rubidium composition showed the most intense luminescence. Additionally, ACaF3:1 %Pr3+ exhibits a manifold increase in upconversion emission compared to the previously studied reference material LiYF4:1 %Pr3+, indicating the potential use of fluoroperovskites for surface decontamination.

Asymmetric BODIPY Dyes Enabling Triplet-Triplet Annihilation Upconversion.

The construction of triplet-triplet annihilation upconversion (TTA-UC) systems with upconversion (UC) emission efficiency at low power densities is still under continuing exploration. From an environmental point of view, the utilization of purely organic pairs is more beneficial than the involvement of transition-metal complexes. In this context, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes, which can be found in a wide range of applications, have been previously used as suitable sensitizers in TTA-UC systems. The versatility of these scaffolds makes them magnificent objectives for designing and synthesizing potential entities with different target abilities. Herein, we prepared several asymmetric BODIPY dyes with excellent optical properties to be applied to a bimolecular TTA-UC system. In the presence of 2,5,8,11-tetra-tert-butylperylene (TBPe) as a suitable annihilator, a green-to-blue light conversion was clearly observed by means of detailed spectroscopic investigations. The results revealed a high UC emission efficiency (ηUC) of ∼8%, together with a low threshold intensity (I th) of ∼40-50 mW/cm2. All data indicated that these asymmetric BODIPY dyes were ideal sensitizers for TTA-UC, providing a particular design for further investigations.

Photoluminescence properties and Mn4+ → Tm3+ energy transfer of La0.557Li0.33TiO3: Mn4+, Tm3+ for thermometry and NIR-LED applications

Over the span of years, Mn4+-doped red phosphors have been utilized to meaningful optoelectronic devices due to their outstanding optical properties, and Tm3+ usually is used for activator ion because of its efficient up-conversion emission, but the down-conversion processes are rarely used. Herein, we have successfully synthesized a series of novel La0.557Li0.33Ti1-xO3:xMn4+ (x = 0.1–7 %) phosphors. The photoluminescence properties have been analysis in detailed. Using Mn4+-Tm3+ co-doped strategy, we successfully synthesis La0.557-yLi0.33Ti0.993O3:0.7 %Mn4+, yTm3+ phosphors. Upon excitation with 360 nm light, this phosphor exhibits two distinct emission peaks at 729 nm and 806 nm. The emission intensity of Tm3+ is greatly improved after Mn4+ doping, which is attributed to the energy transfer of Mn4+ to Tm3+. In terms of potential applications, utilizing fluorescence intensity ratio technology, La0.551Li0.33Ti0.993O3:0.7 %Mn4+, 0.6 %Tm3+ exhibits maximum absolute and relative sensitivities of 3.25 % K−1 at 523 K and 3.49 % K−1 at 298 K, which demonstrating its potential for application in optical thermometry. Furthermore, The NIR-LED using phosphor and 365 nm chips reveals potential applications of the La0.551Li0.33Ti0.993O3:Mn4+, Tm3+ phosphor in NIR night vision and biosensing.

Upconversion Luminescence in a Photostable Ion‐Paired Yb−Eu Heteronuclear Complex

AbstractLanthanide‐based upconversion molecular complexes have potential application in diverse fields and attracted considerable research interest in recent years. However, the similar coordination reactivity of lanthanide ions has constrained the designability of target molecule with well‐defined structure, and many attempts obtained statistical mixtures. Herein, an ion‐paired Yb−Eu heteronuclear complex [Eu(TpPy)2][Yb(ND)4] (TpPy=tris[3‐(2‐pyridyl)pyrazolyl]hydroborate, ND=3‐cyano‐2‐methyl‐1,5‐naphthyridin‐4‐olate) was designed and synthesized. Thanks to the radius difference between Eu3+ (1.07 Å) and Yb3+ (0.98 Å) ions, the hexadentate TpPy ligand was selected to coordinate with Eu3+ and the Yb3+ with a smaller radius was chelated by bidentate ND ligand. As a result, the sites of Eu3+ and Yb3+ in the complex can be clarified by high‐resolution mass spectrometry and single‐crystal structure analysis. Upon the excitation of Yb3+ at 980 nm, the upconversion emission of Eu3+ was realized through a cooperative sensitization process. Furthermore, [Eu(TpPy)2][Yb(ND)4] demonstrated excellent photostability during continuous high‐power density 980 nm laser irradiation, with a LT95 (the time to 95 % of the initial emission intensity) of 420 minutes. This work provides the first example of a pure ion‐paired Yb−Eu heteronuclear complex upconversion system and may bring insights into rational design of lanthanide‐based upconversion molecular complexes.