Tailored Near-Infrared Photoemission in Fluoride Perovskites through Activator Aggregation and Super-Exchange between Divalent Manganese Ions

Photoluminescent, dielectric, and magnetic responsivity to the humidity variation in SHG-active pyroelectric manganese(ii)-based molecular material

Multifunctional response to external stimuli which engages various properties, including optical, dielectric, magnetic, or mechanical, can be the source of new generations of highly sensitive sensors and advanced switches. Such responsivity is expected for molecular materials based on metal complexes whose properties are often sensitive to even subtle changes in a particular stimulus. We present a novel hybrid organic-inorganic salt based on earth-abundant divalent manganese ions forming two types of complexes, octahedral [MnII(Me-dppmO2)3]2+ cations with methyl-functionalized bis(diphenylphosphino)methane dioxide ligands and tetrahedral [MnIICl4]2- anions. These ions crystallize with water molecules leading to the molecular material [MnII(Me-dppmO2)3][MnIICl4]·H2O (1). We show that, due to the simple methyl substituent on the diphosphine-type ligand, 1 reveals a polar crystal structure of the Cc space group as confirmed by the single-crystal X-ray diffraction, second-harmonic generation (SHG) effect, piezoelectric response, and pyroelectricity. Besides these non-centrosymmetricity-related non-linear optical and electrical features, this material combines three other physical properties, i.e., visible room-temperature (RT) photoluminescence (PL) originating from d-d electronic transitions of octahedral Mn(ii) complexes, dielectric relaxation in ca. 170-300 K range related to Bjerrum-type orientation defects of water molecules, and slow magnetic relaxation below 3 K related to spin-phonon interactions involving paramagnetic Mn(ii) centers. We demonstrate that these three physical effects detected in 1 are sensitive to humidity variation that governs the RT-PL intensity, leads to the ON/OFF switching of dielectric relaxation around RT, and non-trivially modulates the magnetic relaxation at cryogenic temperatures. Thus, we report a unique molecular material revealing broadened multifunctionality and triple physical responsivity to the humidity change exploring luminescent, dielectric, and magnetic properties.

Strong Coupling between Mn2+ Dopants and CdSe Nanoplatelets Enables Charge-Transfer Transition and Dual Emission

Doping transitional metals into colloidal nanocrystals can significantly modify their excited-state dynamics and enrich their optical and magneto-optical functionalities. Here we synthesize Mn-doped CdSe nanoplatelets and investigate their excited-state dynamics and light-emission mechanisms. Extensive characterizations suggest that Mn2+ ions are situated near the surface-region of the nanoplatelets. The atomic thinness of nanoplatelets allows for a strong host-dopant coupling, manifested as broadband charge-transfer absorption and emission (near 575 nm) between the host valence band and the dopant d-orbitals. Photoexcitation of the host leads to rapid (a few ps) electron transfer from the conduction band to the d-orbitals, and the resultant charge-transfer state decays within a few ns not only through charge-transfer emission but also generating an excited-state species (likely Mn-Mn dimer) with a characteristic near-infrared emission. These novel photophysics and photochemistry uncovered for quasi-two-dimensional Mn-doped nanocrystals form the basis for optical, magneto-optical, and energy conversion applications using such materials.

High Defect Tolerance Breaking the Design Limitation of Full-Spectrum Multimodal Luminescence Materials

With the development of optical anti-counterfeiting and the increasing demand for high-level information encryption, multimodal luminescence (MML) materials attract much attention. However, the discovery of these multifunctional materials is very accidental, and the versatile host suitable for developing such materials remains unclear. Here, a grossite-type fast ionic conductor CaGa4O7, characterized by layered and tunnel structure with excellent defect tolerance, is found to meet the needs of various luminescent processes. Almost all luminescent modes, including down/up-conversion luminescence (DCL/UCL), long persistent luminescence (LPL), mechanoluminescence (ML), and X-ray excited optical luminescence (XEOL), are realized in this single host. Full-spectrum (from violet to near-infrared) photoluminescence and ML as well as multicolor XEOL are achieved by simply changing the doped luminescent center. A series of anti-counterfeiting devices, including the quasi-dynamic display of famous paintings, digital information encryption, and multi-color handwritten signatures, are designed to show the encryption of information in temporal and spatial dimensions. This study clarifies the importance of defect tolerance of the host for the development of MML materials, and provides a unique insight into the cross-field applications of special functional materials, which is a new strategy to accelerate the development of novel MML materials.

Suppressing Energy Migration via Antiparallel Spin Alignment in One-Dimensional Mn2+ Halide Magnets with High Luminescence Efficiency

Photoexcited energy migration is prone to causing luminescence quenching in Mn2+ luminescent materials, presenting a formidable challenge for optoelectronic applications. Although various strategies and mechanisms have been proposed to mitigate this issue, the role of spin alignment between adjacent Mn2+ ions has remained largely unexamined. In this study, we have elucidated the influence of spin alignment on energy migration within the one-dimensional Mn2+-metal halide compound (CH3)4NMnCl3 (TMMC) through variable-temperature photoluminescence (PL) and magnetic-optical spectroscopy. This investigation was conducted with reference to (CH6N3)2MnCl4 (GUA) with isolated [Mn3Cl12]6- trimers and Cd2+-doped TMMC. The spin order in TMMC below approximately 55 K is demonstrated by the disorder-order transition observed in the temperature-dependent magnetic susceptibility. This finding is further corroborated by the negligible shift in the temperature- and field-dependent emission peaks, a consequence of magnetic saturation. Our results indicate that the antiparallel spin alignment along the Mn2+ chain in TMMC effectively suppresses energy migration and multiphonon relaxation, thereby reducing nonradiative transitions and enhancing the photoluminescence quantum yield (PLQY). This research casts new light on the potential for developing high-performance Mn2+-doped phosphors for optoelectronic and spin-photonic applications, offering insights into the manipulation of spin and energy dynamics in these materials.

Does Mn2+-Mn2+ Spin-Exchange Interaction Involve Mn2+ Luminescence of Mn2+-Doped/Concentrated Materials?

Mn2+-doped luminescent quantum dots play a vital role in the fields of optoelectronic materials and devices. The presence of five unpaired d electrons in Mn2+ ions facilitates spin-exchange interactions, profoundly influencing the spin state of the exciton and thereby impacting the optical behaviors. However, the involvement and specific effects of spin-exchange interactions on optical properties of Mn2+ in insulating bulk phosphors remain a subject of controversy, attributed to the scarcity of solid evidence and the interference of various factors. In this Perspective, we delve into the fundamentals and recent advancements concerning the Mn2+-Mn2+ spin-exchange interaction in Mn2+ luminescent materials. The discussion encompasses various aspects, such as types of magnetic coupling, the coupling mechanism in optical ground state and excited state, as well as effective measures for verification. This Perspective underscores the existing knowledge gaps in Mn2+-doped bulk phosphors, highlighting significant opportunities for further exploration and advancement in both fundamental and applied research within this domain.

Lanthanide-Doped KMgF3 Upconversion Nanoparticles for Photon Avalanche Luminescence with Giant Nonlinearities

Lanthanide (Ln3+)-doped photon avalanche (PA) upconversion nanoparticles (UCNPs) have great prospects in many advanced technologies; however, realizing efficient PA luminescence in Ln3+-doped UCNPs remains challenging due to the deleterious surface and lattice quenching effect. Herein, we report a unique strategy based on the pyrolysis of KHF2 for the controlled synthesis of aliovalent Ln3+-doped KMgF3 UCNPs, which can effectively protect Ln3+ from luminescence quenching by surface and internal OH- defects and thereby boost upconversion luminescence. This enables us to realize efficient PA luminescence from Tm3+ at 802 nm in KMgF3: Tm3+ UCNPs upon 1064 nm excitation, with a giant nonlinearity of ∼27, a PA response time of 281 ms, and an excitation threshold of 16.6 kW cm-2. This work may open up a new avenue for exploring highly nonlinear PA luminescence through aliovalent Ln3+ doping and crystal lattice engineering toward diverse emerging applications.

All-inorganic Mn2+-doped metal halide perovskite crystals for the late-time detection of X-ray afterglow imaging

All-inorganic metal halide perovskite (MHP) materials have been widely studied because of their unique optoelectronic properties, whereas there has been little research reported on their X-ray afterglow imaging properties. Herein, we report the design and synthesis of Mn2+-doped hexagonal CsCdCl3 MHP crystals with excellent X-ray scintillation and X-ray induced afterglow. The orange emission from Mn2+ shows a red shift due to the strong interaction of the Mn2+-Mn2+ dimers formed at higher doping concentrations. The high-energy X-rays with higher electron filling capacity to feed the shallow (0.71 eV) and deep (0.90-1.08 eV) traps enable a long orange afterglow for more than 300 min. The afterglow emission can be rejuvenated effectively by 870 nm stimulus or heating even after 72 h of decay. Finally, we demonstrate the proof-of-concept applications of the fabricated flexible scintillator films for real-time online X-ray imaging with a spatial resolution of 12.2 lp mm-1, as well as time-lapse X-ray imaging recorded by a cell phone, which shows promise for being able to do offline late-time detection of X-ray afterglow imaging in the future.

Facile Synthesis of Highly Emissive All-Inorganic Manganese Bromide Compounds with Perovskite-Related Structures for White LEDs

Lead-free all-inorganic halide materials with different Mn²⁺-based crystal structures (Cs₃MnBr₅ and CsMnBr₃) were obtained using a convenient synthetic method. Cs₃MnBr₅ had a bright green emission (522 nm), with a unique single-exponential lifetime (τ_avg = 236 µs) and a high photoluminescence quantum yield (82 ± 5%). A red emission was observed in the case of the CsMnBr₃ structure with a two-exponential fluorescence decay curve, and the lifetime was 1.418 µs (93%) and 18.328 µs (7%), respectively. By a judicious tuning of the synthetic conditions, a mixed phase of Cs₃MnBr₅/CsMnBr₃ was also produced that emitted white light, covering almost the entire visible spectrum. White-light-emitting diodes (WLEDs) with color coordinates (0.4269, 0.4955), a color temperature of (3773 K), and a color rendering index (68) were then fabricated using the as-prepared powder of mixed phases of Cs₃MnBr₅/CsMnBr₃ with a commercial UV LED chip (365 nm).

Recent Development in Sensitizers for Lanthanide-Doped Upconversion Luminescence

The attractive features of lanthanide-doped upconversion luminescence (UCL), such as high photostability, nonphotobleaching or photoblinking, and large anti-Stokes shift, have shown great potentials in life science, information technology, and energy materials. Therefore, UCL modulation is highly demanded toward expected emission wavelength, lifetime, and relative intensity in order to satisfy stringent requirements raised from a wide variety of areas. Unfortunately, the majority of efforts have been devoted to either simple codoping of multiple activators or variation of hosts, while very little attention has been paid to the critical role that sensitizers have been playing. In fact, different sensitizers possess different excitation wavelengths and different energy transfer pathways (to different activators), which will lead to different UCL features. Thus, rational design of sensitizers shall provide extra opportunities for UCL tuning, particularly from the excitation side. In this review, we specifically focus on advances in sensitizers, including the current status, working mechanisms, design principles, as well as future challenges and endeavor directions.

All-Inorganic Manganese-Based CsMnCl3 Nanocrystals for X-Ray Imaging

Lead-based halide perovskite nanomaterials with excellent optical properties have aroused great attention in the fields of solar cells, light-emitting diodes, lasing, X-ray imaging, etc. However, the toxicity of lead prompts researchers to develop alternatives to cut down the usage of lead. Herein, all-inorganic manganese-based perovskite derivatives, CsMnCl3 nanocrystals (NCs), with uniform size and morphology have been synthesized successfully via a modified hot-injection method. These NCs have a direct bandgap of 4.08 eV and a broadband emission centered at 660 nm. Through introducing modicum lead (1%) into the CsMnCl3 NCs, the photoluminescence intensity greatly improves, and the quantum yield (PLQY) increases from 0.7% to 21%. Furthermore, the CsMnCl3 :1%Pb NCs feature high-efficiency of X-ray absorption and radioluminescence, which make these NCs promising candidates for X-ray imaging.

Large Spectral Shift of Mn2+ Emission Due to the Shrinkage of the Crystalline Host Lattice of the Hexagonal CsCdCl3 Crystals and Phase Transition

All-inorganic halide perovskite crystals are considered excellent optical host lattices for various dopants to obtain wavelength-tunable emissions with ultra-broad bands even over a wide spectral range. Here, a series of Mn²⁺-doped bulk ligand-free CsCdCl₃ (CCC) perovskite crystals with a hexagonal shape and size of about 1 millimeter (mm) have been prepared by a facile hydrothermal method. These CCC:Mn²⁺ (CCC:Mn) crystals emit the representative orange-red photoluminescence (PL) of Mn²⁺ (⁴T₁(G)-⁶A₁(S)) in the centers of hexagonal octahedrons coordinated with six Cl^- ions. A fine-tuning of the Mn²⁺ concentration from 1 to 50 mol % Cd²⁺ induces a substantial red shift of emission spectra from 570 to 630 nm due to the shrinkage of the crystalline host lattice, and the maximum intensity of emission is achieved at 20 mol % Mn²⁺ doping. A further increase in the Mn²⁺ concentration causes a decrease of the PL intensity due to the phase transition from CCC to CsMnCl₃·2H₂O (CMCH). The strong excitation bands at 360, 370, 420, and 440 nm can make the excitation of the emissive CCC:Mn crystals possible with ultraviolet (UV) and blue chips for application in white light-emitting diodes (WLEDs). The similarity of the Mn²⁺-concentration-dependent emission spectra excited by various wavelengths indicates that there is only one type of site for Mn²⁺ occupation in CCC.

Mn2+-activated dual-wavelength emitting materials toward wearable optical fibre temperature sensor

Photothermal sensing is crucial for the creation of smart wearable devices. However, the discovery of luminescent materials with suitable dual-wavelength emissions is a great challenge for the construction of stable wearable optical fibre temperature sensors. Benefiting from the Mn²⁺-Mn²⁺ superexchange interactions, a dual-wavelength (530/650 nm)-emitting material Li₂ZnSiO₄:Mn²⁺ is presented via simple increasing the Mn²⁺ concentration, wherein the two emission bands have different temperature-dependent emission behaviours, but exhibit quite similar excitation spectra. Density functional theory calculations, coupled with extended X-ray absorption fine structure and electron-diffraction analyses reveal the origins of the two emission bands in this material. A wearable optical temperature sensor is fabricated by incorporating Li₂ZnSiO₄:Mn²⁺ in stretchable elastomer-based optical fibres, which can provide thermal-sensitive emissions at dual- wavelengths for stable ratiometric temperature sensing with good precision and repeatability. More importantly, a wearable mask integrated with this stretchable fibre sensor is demonstrated for the detection of physiological thermal changes, showing great potential for use as a wearable health monitor. This study also provides a framework for creating transition-metal-activated luminescence materials.

Dual-wavelength emission materials can provide fluorescence intensity ratio technology with self-calibration features; their fabrication however, remains a challenge. Here, authors design a dual-wavelength emitting material Li₂ZnSiO₄:Mn²⁺ and present a wearable optical fibre temperature sensor, functioning in both contact and noncontact modes.

Compensation effect of electron traps for enhanced fluorescence intensity ratio thermometry performance

How the electron traps in the host matrix impact the fluorescence intensity ratio (FIR) thermometry performance in inorganic phosphors is still unclear. In this work, the relationships between temperature-dependent photoluminescence,...

Broad luminescence tuning in Mn2+-doped Rb2Zn3(P2O7)2via doping level control based on multiple synergies

A series of novel Mn2+-doped Rb2Zn3(P2O7)2 phosphors for tunable emission from green to orange-red due to Mn2+ preferential occupation of different crystallographic sites with an increasing Mn2+ doping level.

Microwave-assisted synthesis of NaMnF3 particles with tuneable morphologies

Here, the synthesis of sub-micron MMnF₃ (M = Na or K) particles by a rapid microwave-assisted approach is reported. Adjustment of the Na⁺-to-Mn²⁺ ratio in the reaction mixture yielded tuneable morphologies, i.e., rods, ribbons, and plates. Relaxometric results indicated that poly(acrylic acid)-capped MMnF₃ particles exhibited characteristic magnetic properties, which endows them with potential T₁-weighted contrast agent capabilities.

Dipole-Orientation-Dependent Förster Resonance Energy Transfer from Aromatic Head Groups to MnBr42- Blocks in Organic-Inorganic Hybrids

The understanding and visualization of dipole-dipole interaction on molecular scale are scientifically fundamental and extremely of interest. Herein, two new zero-dimensional (0D) Mn hybrids with aromatic head groups and alkyl tails as organic spacers are selected as models. It was found that the dipole interaction between head groups and Mn blocks could have a huge impact on their crystalline structures as well as the luminescent properties. The parallel-oriented dipoles of the head groups and MnBr₄^2- blocks contribute to an efficient Förster Resonance Energy Transfer (FRET) in cetylpyridinium manganese bromide ([C16Py]₂MnBr₄), while the process is absent in 1-methyl-3-hexadecylimidazolium manganese bromide ([C16mim]₂MnBr₄) with perpendicular-oriented dipoles. This work gives insight into the influence of organic spacers on the geometry and the dipole interaction of Mn polyhedron in the hybrids, which could be of great interest in the future optical regulations and structural design.

Li/Na substitution and Yb3+ co-doping enabling tunable near-infrared emission in LiIn2SbO6:Cr3+ phosphors for light-emitting diodes

Near-infrared (NIR) phosphor-converted light-emitting diode (pc-LED) has great potential in non-invasive detection, while the discovery of tunable broadband NIR phosphor still remains a challenge. Here, we report that Cr³⁺-activated LiIn₂SbO₆ exhibits a broad emission band ranging from 780 to 1400 nm with a full width at half maximum (FWHM) of 225 nm upon 492 nm excitation. The emission peaks are tuned from 970 to 1020 nm together with considerable broadening of FWHM (∼285 nm) via Li/Na substitution. Depending on Yb³⁺ co-doping, a stronger NIR fluorescence peak of Yb³⁺ appears with improved thermal resistance, which is ascribed to efficient energy transfer from Cr³⁺ to Yb³⁺. An NIR pc-LED package has been finally designed and demonstrated a remarkable ability to penetrate pork tissues (∼2 cm) so that the insertion depth of a needle can be observed, indicating that the phosphor can be applied in non-destructive monitoring.

A red-light-chargeable near infrared MgGeO3:Mn2+,Yb3+ persistent phosphor for bioimaging and optical information storage applications

An NIR-emitting MgGeO3:Mn2+,Yb3+ persistent phosphor chargeable with red light has been developed. The features of red-light charging and NIR persistent luminescence make this phosphor hold great potential for biomedical imaging and optical data storage.

Mn2+-Mn2+ Magnetic Coupling Effect on Photoluminescence Revealed by Photomagnetism in CsMnCl3

The magnetic coupling interaction of Mn²⁺-Mn²⁺ in Mn²⁺-included phosphors could induce a shorter emission decay time, compared with that of isolated Mn²⁺, which could overcome the photoluminescence (PL) saturation when stimulated by a high photon flux due to the long lifetime of the Mn²⁺ excited state. However, few studies have directly proved the Mn²⁺-Mn²⁺ coupling effect on the PL decay. In this paper, the effect on PL of CsMnCl₃ (CMC) and its hydrates is revealed by photomagnetism results, excluding the interference effects of site symmetry and phonon energy. The antiferromagnetic interaction of the CMC is larger when Mn²⁺ at a photoexcited state than at a dark state, which is contrary to the hydrates with weak Mn²⁺-Mn²⁺ interaction. This research not only helps researchers to understand the fundamental optical process but also is instructive for designing high performance Mn²⁺-doped phosphors in the field of displays and lighting.

High-security-level multi-dimensional optical storage medium: nanostructured glass embedded with LiGa5O8: Mn2+ with photostimulated luminescence

The launch of the big data era puts forward challenges for information preservation technology, both in storage capacity and security. Herein, a brand new optical storage medium, transparent glass ceramic (TGC) embedded with photostimulated LiGa₅O₈: Mn²⁺ nanocrystals, capable of achieving bit-by-bit optical data write-in and read-out in a photon trapping/detrapping mode, is developed. The highly ordered nanostructure enables light-matter interaction with high encoding/decoding resolution and low bit error rate. Importantly, going beyond traditional 2D optical storage, the high transparency of the studied bulk medium makes 3D volumetric optical data storage (ODS) possible, which brings about the merits of expanded storage capacity and improved information security. Demonstration application confirmed the erasable-rewritable 3D storage of binary data and display items in TGC with intensity/wavelength multiplexing. The present work highlights a great leap in photostimulated material for ODS application and hopefully stimulates the development of new multi-dimensional ODS media.

Dual-Mode Long-Lived Luminescence of Mn2+-Doped Nanoparticles for Multilevel Anticounterfeiting

Luminescent nanoparticles with dual-mode long-lived luminescence are of great importance for their attractive applications in biosensing, bioimaging, and data encoding. Herein, we report the realization of up- and downconversion emission of Mn²⁺ dopants in multilayer nanoparticles of NaGdF₄:Yb/Tm@NaGdF₄:Ce/Mn@NaYF₄ upon excitation at 980 and 254 nm, respectively. The dual-mode emission of the Mn²⁺ dopants at 531 nm have a long-lived lifetime up to ∼30 ms as a result of the spin-forbidden optical transition of Mn²⁺ within the 3d⁵ configuration. After ceasing steady excitation at the two wavelengths, the long-lived feature of Mn²⁺ luminescence allows a longer persistent time than lanthanide emissions, thereby enabling the ease of data decoding by a cell phone camera under a burst mode. The long-lived green upconversion emission also permits the generation of a long green tail emission upon dynamic excitation at 980 nm. These attributes make the as-prepared Mn²⁺-doped multilayer nanoparticles particularly attractive for multilevel anticounterfeiting.

Long-lived Photon Upconversion Phosphorescence in RbCaF3:Mn2+,Yb3+ and the Dynamic Color Separation Effect

The development of luminescence materials with long-lived upconversion (UC) phosphorescence and long luminescence rise edge (LRE) is a great challenge to advance the technology of photonics and materials sciences. The lanthanide ions-doped UC materials normally possess limited UC lifetime and short LRE, restricting direct afterglow viewing in visual images by the naked eye. Here, we show that the RbCaF₃:Mn²⁺,Yb³⁺ UC luminescence material generates a long UC lifetime of ∼62 ms peaking at 565 nm and an ultralong LRE of ∼5.2 ms. Density functional theory calculations provide a theoretical understanding of the Mn²⁺-Yb³⁺ aggregation in the high-symmetry RbCaF₃ host lattice that enables the formation of the long-lived UC emission center, superexchange coupled Yb³⁺-Mn²⁺ pair. Through screen printing ink containing RbCaF₃:Mn²⁺,Yb³⁺, the visualized multiple anti-counterfeiting application and information encryption prototype with high-throughput rate of authentication and decryption are demonstrated by the dynamic color separation effect.

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Photon upconversion phosphorescence material RbCaF₃:Mn²⁺,Yb³⁺ is developed

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The UC emission center in RbCaF₃:Mn²⁺,Yb³⁺ is ascribed to the Yb³⁺-Mn²⁺ pair

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A multiple anti-counterfeiting prototype based on the RbCaF₃:Mn²⁺,Yb³⁺ is demonstrated

Ultrabright single-band red upconversion luminescence in highly transparent fluorosilicate glass ceramics containing KMnF3 perovskite nanocrystals

With a specially designed composition, highly transparent Yb³⁺/Er³⁺-doped fluorosilicate glass ceramic (GC) containing KMnF₃ perovskite nanocrystals (NCs) is obtained for the first time. The rare-earth ions are preferentially accumulated in regions embedded with KMnF₃ NCs; as a result, a remarkably enhanced (by an order of magnitude) single-band red upconversion luminescence (UCL) is achieved. Absolute quantum efficiency of the red UCL, which cannot be measured in previous GCs owing to insufficiency, reaches as high as 0.10%±0.02% in the GC sample reported in this Letter. This value is even higher than that of the well-known multiband emitting β-NaYF₄:Er³⁺/Yb³⁺ NCs and widely recognized GCs containing NaYF₄:Yb³⁺/Er³⁺NCs.

Tunable Emission Properties of Manganese Chloride Small Single Crystals by Pyridine Incorporation

Pure transition-metal compounds seldom produce luminescence because of electron correlation and spin-spin coupling. The Pb-free perovskite materials, C₁₀H₁₂N₂MnCl₄ and C₅H₆NMnCl₃·H₂O, were obtained by using pyridine-implanted manganese chloride lattices. The single-crystal X-ray diffraction indicates their different crystal structures. In C₁₀H₁₂N₂MnCl₄, MnCl₄ cocoordinated with two pyridine molecules forms a lattice composed of independent mononuclear structures with paramagnetic behavior, which shows a clear emission band at 518 nm from the lowest d-d transition of a single Mn(II) ion in the octahedral crystal field. In C₅H₆NMnCl₅·H₂O crystal, MnCl₅·(H₂O) _x octahedron-cocoordinated with less pyridine molecules than 2 lead to formation arris-share linear chains of Mn-ion octahedra, which give emission band at 620 nm due to the ferromagnetic Mn pair, and ferromagnetism. Pyridine incorporations in the transition-metal halide lattice provide a new channel to modulate the electron correlation and obtain materials with both luminescence and ferromagnetic properties.

Enhanced green upconversion luminescence in tetrahedral LiYF4:Yb/Er nanoparticles by manganese(ii)-doping: the key role of the host lattice

We report the enhancement of green upconversion luminescence in tetrahedral LiYF₄:Yb/Er nanoparticles by Mn²⁺ ion doping, which is different from the enhanced single-band red emission dominated by Mn²⁺ ions in cubic NaLnF₄:Yb/Er (Ln = Y, Gd, Lu) nanoparticles. The energy levels of the first excited state ⁴T₁ of Mn²⁺ in tetrahedral LiYF₄:Gd and cubic NaGd(Y)F₄ are compared by detection of emissions from Mn²⁺via the energy transition from Gd³⁺ to Mn²⁺ with excitation at 275 nm. The coordination environments of Mn²⁺ in these two host lattices have been investigated by X-ray absorption fine structure measurements. The results demonstrate that the formation of tetrahedral MnF₄ in tetragonal LiYF₄ arising from the replacement of Ln³⁺ ions with Mn²⁺ ions leads to a higher energy level of the Mn^{2+ 4}T₁ state than that in octahedral MnF₆ in cubic NaYF₄. The high-lying excited state of tetrahedral MnF₄ is close to the green emitting ⁴S_3/2 state of Er³⁺ and thus enhances green upconversion emission in tetragonal LiYF₄:Yb/Er, while the low-lying excited state of octahedral MnF₆ dominates red emission in cubic NaYF₄:Yb/Er. These findings provide direct evidence for the key roles of the host lattices and more possibilities in modulating the upconversion behaviour of lanthanide-based nanoparticles by transition-metal ion doping to achieve the desired goals of specific applications.

Efficient and Stable Luminescence from Mn2+ in Core and Core-Isocrystalline Shell CsPbCl3 Perovskite Nanocrystals

There has been a growing interest in applying CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) for optoelectronic application. However, research on doping of this new class of promising NCs with optically active and/or magnetic transition metal ions is still limited. Here we report a facile room temperature method for Mn²⁺ doping into CsPbCl₃ NCs. By addition of a small amount of concentrated HCl acid to a clear solution containing Mn²⁺, Cs⁺, and Pb²⁺ precursors, Mn²⁺-doped CsPbCl₃ NCs with strong orange luminescence of Mn²⁺ at ∼600 nm are obtained. Mn²⁺-doped CsPbCl₃ NCs show the characteristic cubic phase structure very similar to the undoped counterpart, indicating that the nucleation and growth mechanism are not significantly modified for the doping concentrations realized (0.1 at. % - 2.1 at. %). To enhance the Mn²⁺ emission intensity and to improve the stability of the doped NCs, isocrystalline shell growth was applied. Growth of an undoped CsPbCl₃ shell greatly enhanced the emission intensity of Mn²⁺ and resulted in lengthening the radiative lifetime of the Mn²⁺ emission to 1.4 ms. The core-shell NCs also show superior thermal stability and no thermal degradation up to at least 110 °C, which is important in applications.

Interaction between the exchanged Mn2+ and Yb3+ ions confined in zeolite-Y and their luminescence behaviours

Luminescent zeolites exchanged with two distinct and interacted emissive ions are vital but less-studied for the potential applications in white light emitting diodes, solar cells, optical codes, biomedicine and so on. Typical transition metal ion Mn2+ and lanthanide ion Yb3+ are adopted as a case study via their characteristic transitions and the interaction between them. The option is considered with that the former with d-d transition has a large gap between the first excited state 4T1 and the ground state 6A1 (normally >17,000 cm-1) while the latter with f-f transition has no metastable excited state above 10,000 cm-1, which requires the vicinity of these two ions for energy transfer. The results of various characterizations, including BET measurement, photoluminescence spectroscopy, solid-state NMR, and X-ray absorption spectroscopy, etc., show that Yb3+ would preferably enter into the zeolite-Y pores and introduction of Mn2+ would cause aggregation of each other. Herein, cation-cation repulsion may play a significant role for the high valence of Mn2+ and Yb3+ when exchanging the original cations with +1 valence. Energy transfer phenomena between Mn2+ and Yb3+ occur only at elevated contents in the confined pores of zeolite. The research would benefit the design of zeolite composite opto-functional materials.

Transition Metal-Involved Photon Upconversion

Upconversion (UC) luminescence of lanthanide ions (Ln³⁺) has been extensively investigated for several decades and is a constant research hotspot owing to its fundamental significance and widespread applications. In contrast to the multiple and fixed UC emissions of Ln³⁺, transition metal (TM) ions, e.g., Mn²⁺, usually possess a single broadband emission due to its 3d⁵ electronic configuration. Wavelength-tuneable single UC emission can be achieved in some TM ion-activated systems ascribed to the susceptibility of d electrons to the chemical environment, which is appealing in molecular sensing and lighting. Moreover, the UC emissions of Ln³⁺ can be modulated by TM ions (specifically d-block element ions with unfilled d orbitals), which benefits from the specific metastable energy levels of Ln³⁺ owing to the well-shielded 4f electrons and tuneable energy levels of the TM ions. The electric versatility of d⁰ ion-containing hosts (d⁰ normally viewed as charged anion groups, such as MoO₆^6- and TiO₄^4-) may also have a strong influence on the electric dipole transition of Ln³⁺, resulting in multifunctional properties of modulated UC emission and electrical behaviour, such as ferroelectricity and oxide-ion conductivity. This review focuses on recent advances in the room temperature (RT) UC of TM ions, the UC of Ln³⁺ tuned by TM or d⁰ ions, and the UC of d⁰ ion-centred groups, as well as their potential applications in bioimaging, solar cells and multifunctional devices.

Lanthanide upconversion luminescence at the nanoscale: fundamentals and optical properties

Upconversion photoluminescence is a nonlinear effect where multiple lower energy excitation photons produce higher energy emission photons. This fundamentally interesting process has many applications in biomedical imaging, light source and display technology, and solar energy harvesting. In this review we discuss the underlying physical principles and their modelling using rate equations. We discuss how the understanding of photophysical processes enabled a strategic influence over the optical properties of upconversion especially in rationally designed materials. We subsequently present an overview of recent experimental strategies to control and optimize the optical properties of upconversion nanoparticles, focussing on their emission spectral properties and brightness.

Shifting the Sun: Solar Spectral Conversion and Extrinsic Sensitization in Natural and Artificial Photosynthesis

Solar energy harvesting is largely limited by the spectral sensitivity of the employed energy conversion system, where usually large parts of the solar spectrum do not contribute to the harvesting scheme, and where, of the contributing fraction, the full potential of each photon is not efficiently used in the generation of electrical or chemical energy. Extrinsic sensitization through photoluminescent spectral conversion has been proposed as a route to at least partially overcome this problem. Here, we discuss this approach in the emerging context of photochemical energy harvesting and storage through natural or artificial photosynthesis. Clearly contrary to application in photovoltaic energy conversion, implementation of solar spectral conversion for extrinsic sensitization of a photosynthetic machinery is very straightforward, and-when compared to intrinsic sensitization-less-strict limitations with regard to quantum coherence are seen. We now argue the ways in which extrinsic sensitization through photoluminescent spectral converters will-and will not-play its role in the area of ultra-efficient photosynthesis, and also illustrate how such extrinsic sensitization requires dedicated selection of specific conversion schemes and design strategies on system scale.