Temperature-Dependent Reversible Optical Properties of Mn-Based Organic-Inorganic Hybrid (C8H20N)2MnCl4 Metal Halides

Strategic engineering of cationic systems for spatial & temporal anti-counterfeiting applications in zero-dimensional Mn(II) halides

While spatial and time-resolved anti-counterfeiting technologies have gained increasing attention owing to their excellent tunable photoluminescence, achieving high-security-level anti-counterfeiting remains a challenge. Herein, we developed a spatial-time-dual-resolved anti-counterfeiting system using zero-dimensional (0D) organic-inorganic Mn(II) metal halides: (EMMZ)2MnBr4 (named M-1, EMMZ=1-Ethyl-3-Methylimidazolium Bromide) and (EDMMZ)2MnBr4 (named M-2, EDMMZ=1-Ethyl-2,3-Dimethylimidazolium Bromide). M-1 shows a bright green emission with a quantum yield of 78 %. It undergoes a phase transformation from the crystalline to molten state with phosphorescence quenching at 350 K. Reversible phase and luminescent conversion was observed after cooling down for 15 s. Notably, M-2 exhibits green light emission similar to M-1 but undergoes phase conversion and phosphorescence quenching at 390 K, with reversible conversion observed after cooling down for 5 s. The photoluminescence switching mode of on(green)-off-on(green) can be achieved by temperature control, demonstrating excellent performance with short response times and ultra-high cyclic reversibility. By leveraging the different quenching temperatures and reversible PL conversion times of M-1 and M-2, we propose a spatial-time-dual-resolved photoluminescence (PL) switching system that combines M-1 and M-2. This system enables multi-fold tuning of the PL switch for encryption and decryption through cationic engineering strategies by modulating temperature and cooling time. This work presents a novel and feasible design strategy for advanced-level anti-counterfeiting technology based on a spatial-time-dual-resolved system.

Promoting structural distortion to enhance the crystal field strength of Mn(II) in tetrahedral bromide for near-unity yellow emission

Two zero-dimensional hybrid manganese bromide polymorphs ((DMAPH)2MnBr4) exhibit single-crystal to single-crystal phase transformation, accompanied by an increase in MnBr4 tetrahedral bond angle variance (σ2). This structural change leads to an emission redshift due to enhanced crystal field strength, achieving yellow emission with near-unity quantum yield, which highlights its potential for solid-state lighting applications.

Two Organic-Inorganic Hybrid Manganese Bromides with Highly Efficient Emission toward White LEDs

Organic-inorganic metal halides have attracted great attention due to their tunable structural and spectroscopic properties. Here, two organic-inorganic hybrid manganese bromides, (TEMA)2MnBr4 (TEMA = triethylmethylammonium) and (TEBA)2MnBr4 (TEBA = benzyltriethylammonium), are synthesized using the evaporation crystallization method. Following a heat-induced phase transition at 363 K, the structure and optical properties of (TEMA)2MnBr4 change but return to their initial state upon cooling to room temperature, as confirmed by X-ray diffraction, photoluminescence (PL), and Raman spectra. Meanwhile, (TEBA)2MnBr4, with a larger Mn-Mn distance, exhibits a higher photoluminescence quantum yield of 98.1% and greater thermal quenching temperature. However, due to the poorer thermal stability of the organic cation, the crystal melts at 400 K, leading to fluorescence quenching. White LEDs based on (TEMA)2MnBr4 and (TEBA)2MnBr4 are successfully fabricated with color rendering indices of 97.4 and 97.2, respectively. The investigation provides deep insights into the structural and optical properties of (TEMA)2MnBr4 and (TEBA)2MnBr4, advancing research for LED display design by tuning organic cations.

Highly efficient circularly polarized electroluminescence based on chiral manganese(ii) complexes

Currently reported circularly polarized luminescent devices are primarily based on rare earth and noble metal complexes or lead perovskite materials. Reports on electroluminescent devices employing eco-friendly luminescent materials are notably scarce. In this study, we strategically designed and synthesized manganese complexes featuring Binapo as the chiral ligand. The complex structure reveals a tetrahedral coordination configuration, with the R/S configurations exhibiting a mirror relationship. Leveraging the strong ligand field and chiral structural characteristics of Binapo, the enantiomers display red emission and exhibit significant circularly polarized luminescence with a circularly polarized luminescence asymmetric factor (g lum) of 5.1 × 10-3. The circularly polarized electroluminescent performance was investigated by using a solution processing method and host-guest doping strategy. Our efforts resulted in device performance with an external quantum efficiency (EQE) exceeding 4%, and its electroluminescent asymmetric factor (g EL) reached an impressive -8.5 × 10-3. This surpasses the performance of most devices relying on platinum (Pt) and iridium (Ir) metal complexes and perovskite related materials. Our work establishes a pathway for the development of cost-effective and environmentally friendly chiral electroluminescent materials and devices.

Optical activity levels of metal centers controlling multi-mode emissions in low-dimensional hybrid metal halides for anti-counterfeiting and information encryption

In-depth insight into the electronic competition principles between inorganic units and organic ligands proves to be extremely challenging for controlling multi-mode emissions in low-dimensional hybrid metal halides (LHMHs). Herein, an efficient blue emission from organic ligand was engineered in (DppyH)2MCl4 (Dppy = diphenyl-2-pyridylphosphine, M = Zn2+, Cd2+) due to the reverse type I band alignment constructed by optically inert units with nd10 shell electrons. By contrast, the optically active [MnCl4]2- with semi-fully filled 3d5 shell electrons prompts the band alignment of type II, resulting in the narrowband green emission of Mn2+, along with an energy transfer from DppyH+ to [MnCl4]2-. Beyond that, the band alignment of (DppyH)SbCl4 is further reversed to type I due to the strong stereochemical activity of 5s2 lone-pair electrons, resulting in the triplet-state (3P1 → 1S0) self-trapped exciton (STE) emission of [SbCl4]-. The conclusion is that the electronic configurations of metal centers govern the optical activity levels of inorganic units, which in turn controls the multi-mode emissions by maneuvering the band alignments. This research provides an enlightening perspective on the multi-mode emissions with tunable photoluminescence and resulting electronic transitions of LHMHs, whose derived emitters can be employed in anti-counterfeiting and information encryption.

Unlocking the Potential of Organic-Inorganic Hybrid Manganese Halides for Advanced Optoelectronic Applications

Organic-inorganic hybrid manganese(II) halides (OIMnHs) have garnered tremendous interest across a wide array of research fields owing to their outstanding optical properties, abundant structural diversity, low-cost solution processibility, and low toxicity, which make them extremely suitable for use as a new class of luminescent materials for various optoelectronic applications. Over the past years, a plethora of OIMnHs with different structural dimensionalities and multifunctionalities such as efficient photoluminescence (PL), radioluminescence, circularly polarized luminescence, and mechanoluminescence have been newly created by judicious screening of the organic cations and inorganic Mn(II) polyhedra. Specifically, through precise molecular and structural engineering, a series of OIMnHs with near-unity PL quantum yields, high anti-thermal quenching properties, and excellent stability in harsh conditions have been devised and explored for applications in light-emitting diodes (LEDs), X-ray scintillators, multimodal anti-counterfeiting, and fluorescent sensing. In this review, the latest advancements in the development of OIMnHs as efficient light-emitting materials are summarized, which covers from their fundamental physicochemical properties to advanced optoelectronic applications, with an emphasis on the structural and functionality design especially for LEDs and X-ray detection and imaging. Current challenges and future efforts to unlock the potentials of these promising materials are also envisioned.

Synthesis, structure and red-light emission of a manganese halide directed by a methyldiphenylphosphine oxide complex

Controlling the optical activity of halide perovskite materials through modulation of the coordination configurations of the metal ions is important. Herein, a novel manganese-based halide, specifically diaquatetrakis(methyldiphenylphosphine oxide)manganese(II) tetrachloridomanganate(II), [Mn(C13H13OP)4(H2O)2][MnCl4] or [Mn(MDPPO)4(H2O)2][MnCl4] (MDPPO is methyldiphenylphosphine oxide), was synthesized through the solvothermal reaction of MnCl2 with the neutral molecule MDPPO. In this compound, [Mn(MDPPO)4(H2O)2]2+ acts as the cation, while [MnCl4]2- serves as the anion, enabling the co-existence of tetrahedral and octahedral structures within the same system. Remarkably, the compound exhibits efficient red-light emission at 662 nm, distinct from the green-light emission typically observed in MnX4-based halides. Theoretical calculations show that the red emission comes from the charge transfer from the MDPPO to the Mn2+ of [MnCl4]2-. This work provides a new perspective for the design and synthesis of red-light-emitting manganese-based halides with unique structures.

Recent strategies for triplet-state emission regulation toward non-lead organic-inorganic metal halides

Organic-inorganic metal halides (OIMHs) have strengthened the development of triplet-state emission materials due to their excellent luminescence performance. Due to the inherent toxicity of lead (Pb) significantly limiting its further advancement, numerous studies have been conducted to regulate triplet-state emission of non-Pb OIMHs, and several feasible strategies have been proposed. However, most of the non-Pb OIMHs reported have a relatively short lifetime or a low luminescence efficiency, not in favor of their application. In this review, we provide a summary of recent reports on the regulation of triplet-state emissions in non-Pb OIMHs to provide benefits for the design of innovative luminescent materials. Our focus is primarily on exploring the internal and external factors that influence the triplet-state emission. Starting from the luminescence mechanism, the current strategies for regulating triplet-state emissions are summarized. Moreover, by manipulating these strategies, it becomes feasible to achieve triplet-state emissions that span a range of colors from blue to red, and even extend into the near-infrared spectrum with high luminescence efficiency, while also increasing their lifetimes. This review not only provides fresh insights into the advancement of triplet-state emissions in OIMHs but also integrates experimental and theoretical perspectives to illuminate the trajectory of future research endeavors.

Ultra-Broad-Band-Excitable Cu-Based Halide (C4H10N)4Cu4I8 with High Stability for LED Applications

Currently, organic-inorganic hybrid cuprous-based halides are receiving substantial attention for their eco-friendliness, distinctive structures, and outstanding photophysical properties. Nevertheless, most of the reported cuprous-based halides demand deep ultraviolet excitation with a narrow excitation range that can meet the commercial requirement. Herein, zero-dimensional (0D) cuprous-based halide (C4H10N)4Cu4I8 single crystals (SCs) were synthesized, with an ultrabroad band excitation ranging 260-450 nm and a greenish-yellow emission band peaking at 560 nm. Excitingly, (C4H10N)4Cu4I8 also features a large Stokes shift of 300 nm, a high photoluminescence quantum yield (PLQY) of up to 84.66%, and a long lifetime of 137 μs. Furthermore, density functional theory calculations were performed to explore the relationship between structure and photophysical properties, and the photoluminescence performance of (C4H10N)4Cu4I8 originates from the electron interactions in [Cu2I4]2- clusters. Taking advantage of broad band excitation and excellent photoluminescent performances, a high luminescence characteristic UV-pumped light-emitting diode (LED) device with remarkable color stability was fabricated by employing the as-synthesized (C4H10N)4Cu4I8 SCs, which present the promising applications of low-dimensional cuprous-based halides in solid-state lighting.

Thermal/Water-Induced Phase Transformation and Photoluminescence of Hybrid Manganese(II)-Based Chloride Single Crystals

Mn(II)-based hybrid halides have attracted great attention from the optoelectronic fields due to their nontoxicity, special luminescent properties, and structural diversity. Here, two novel organic-inorganic hybrid Mn(II)-based halide single crystals (1-mpip)MnCl4·3H2O and (1-mpip)2MnCl6 (1-mpip = 1-methylpiperazinium, C5H14N2+) were grown by a slow evaporation method in ambient atmosphere. Interestingly, (1-mpip)2MnCl6 single crystals exhibit the green emission with a PL peak at 522 nm and photoluminescence quantum yields (PLQYs) of ≈5.4%, whereas (1-mpip)MnCl4·3H2O single crystals exhibit no emission characteristics. More importantly, there exists a thermal-induced phase transformation from (1-mpip)MnCl4·3H2O to emissive (1-mpip)2MnCl6 at 372 K. Moreover, a reversible luminescent conversion between (1-mpip)MnCl4·3H2O and (1-mpip)2MnCl6 was simply achieved when heated to 383 K and placed in a humid environment or sprayed with water. This work not only deepens the understanding of the thermal-induced phase transformation and humidity-sensitive luminescent conversion of hybrid Mn(II)-based halides, but also provides a guidance for thermal and humidity sensing and anticounterfeiting applications of these hybrid materials.

Electron Spin Decoherence Dynamics in Magnetic Manganese Hybrid Organic-Inorganic Crystals: The Effect of Lattice Dimensionality

Organic-inorganic metal hybrids with their tailorable lattice dimensionality and intrinsic spin-splitting properties are interesting material platforms for spintronic applications. While the spin decoherence process is extensively studied in lead- and tin-based hybrids, these systems generally show short spin decoherence lifetimes, and their correlation with the lattice framework is still not well-understood. Herein, we synthesized magnetic manganese hybrid single crystals of (4-fluorobenzylamine)2MnCl4, ((R)-3-fluoropyrrolidinium)MnCl3, and (pyrrolidinium)2MnCl4, which represent a change in lattice dimensionality from 2D and 1D to 0D, and studied their spin decoherence processes using continuous-wave electron spin resonance spectroscopy. All manganese hybrids exhibit nanosecond-scale spin decoherence time τ2 dominated by the symmetry-directed spin exchange interaction strengths of Mn2+-Mn2+ pairs, which is much longer than lead- and tin-based metal hybrids. In contrast to the similar temperature variation laws of τ2 in 2D and 0D structures, which first increase and gradually drop afterward, the 1D structure presents a monotonous rise of τ2 with the temperatures, indicating the strong correlation of spin decoherence with the lattice rigidity of the inorganic framework. This is also rationalized on the basis that the spin decoherence is governed by the competitive contributions from motional narrowing (prolonging the τ2) and electron-phonon coupling interaction (shortening the τ2), both of which are thermally activated, with the difference that the former is more pronounced in rigid crystalline lattices.

Zero-dimensional organic-inorganic hybrid manganese bromide with coexistence of dielectric-thermal double switches and efficient photoluminescence

Zero-dimensional (0D) hybrid metal halides have attracted much attention due to their rich composition, excellent optical stability, large exciton binding energy, etc. Photoelectric switchable multifunctional materials can integrate multiple physical properties (e.g., ferroelectricity, photoluminescence, magnetic, etc.) into one device and are widely used in many fields such as smart switches, sensors, etc. However, multifunctional materials with thermal energy storage, stimulant dielectric response, and light-emitting properties are rarely reported. Here, we synthesized a new organic-inorganic hybrid metal halide single crystal [TEMA]2MnBr4 (1) (TEMA+ = triethylmethylammonium). Compound 1 undergoes a reversible phase transition at a high temperature of 344/316 K, having a large thermal hysteresis of 28 K and exhibits high stability dielectric switching characteristics near the phase transition temperature. The single crystal exhibits green emission at 513 nm under UV excitation, originating from the 4T1g(G) → 6A1g(S) transition of Mn2+ ions. Excitingly, this single crystal's photoluminescence quantum yield (PLQY) is as high as 80.78%. This work provides a strategy for the development of organic-inorganic hybrid optoelectronic multifunctional materials with high-efficient light emission and switchable dielectric properties.