MnBr₂/18-crown-6 coordination complexes showing high room temperature luminescence and quantum yield

Supramolecular Metal Halide Complexes for High-Temperature Nonlinear Optical Switches

Nonlinear optical (NLO) switching materials, which exhibit reversible intensity modulation in response to thermal stimuli, have found extensive applications across diverse fields including sensing, photoelectronics, and photonic applications. While significant progress has been made in solid-state NLO switching materials, these materials typically showcase their highest NLO performance near room temperature. However, this performance drastically deteriorates upon heating, primarily due to the phase transition undergone by the materials from noncentrosymmetric to centrosymmetric phase. Here, we introduce a new class of NLO switching materials, solid-state supramolecular compounds 18-Crown-6 ether@Cu2Cl4·4H2O (1·4H2O), exhibiting reversible and stable NLO switching when subjected to near-infrared (NIR) photoexcitation and/or thermal stimuli. The reversible crystal structure in response to external stimuli is attributed to the presence of a weakly coordinated bridging water molecule facilitated by hydrogen bonding/chelation interactions between the metal halide and crown-ether supramolecules. We observed an exceptionally high second-harmonic generation (SHG) signal under continuous photoexcitation, even at temperatures exceeding 110 °C. In addition, the bridging water molecules within the complex can be released and recaptured in a fully reversible manner, all without requiring excessive energy input. This feature allows for precise control of SHG signal activation and deactivation through structural transformations, resulting in a high-contrast off/on ratio, reaching values in the million-fold range.

Manganese(II) Bromide Compound with Diprotonated 1-Hydroxy-2-(pyridin-2-yl)-4,5,6,7-tetrahydrobenzimidazole: Dual Emission and the Effect of Proton Transfers

An organic–inorganic cation–anion manganese(II) tetrabromide compound with diprotonated 1-hydroxy-2-(pyridin-2-yl)-4,5,6,7-tetrahydrobenzimidazole, [H3L][MnBr4][H2O], has been synthesized and investigated. The compound has a few possible pathways for proton transfers, which play an important role in the observed luminescence, optical, and magnetic properties. The proton transfers result in the appearance of two-band luminescence. One band is caused by the Mn(II) d-d transitions. The other band is caused by the transition from the triplet state of organic cation and the d-d transition of manganese(II) coupled through {[H3L]}-{[MnBr4]}-{[H2O]} vibrations. The optical absorption spectra of [H3L][MnBr4][H2O] indicate the presence of two direct and one indirect band transitions. The reason for the two-band luminescence and complex optical absorption in [H3L][MnBr4][H2O] were additionally considered using the DFT calculations.

Structural Origin of Enhanced Circularly Polarized Luminescence in Hybrid Manganese Bromides

Chiral hybrid metal halides with a high dissymmetry factor (g_lum ) and a superior photoluminescence quantum yield (PLQY) are promising candidates for circularly polarized luminescence (CPL) light sources. Here, we report eight new chiral hybrid manganese halides, crystallizing in the non-centrosymmetric space group P2₁ 2₁ 2₁ and showing intense CPL emissions. Oppositely-signed circular dichroism (CD) and CPL signals are detected according to the R- and S-configurations of the chiral alkanolammonium cations. Time-resolved PL spectra show long averaged decay lifetimes up to 1 ms for (R-3-quinuclidinol)MnBr₃ (R-1). The g_lum of polycrystalline samples for coordinated structures (23×10^-3 ) is more than doubled compared with the non-coordinated ones (8.5×10^-3 ), due to the structural variations. R-1 exhibit both a high g_lum and a high PLQY (50.2 %). The effective chirality transfer mechanism through coordination bonds, with strongly emissive Mn^II centers, enables a new class of high-performance CPL materials.

Supramolecular Assembly of Halide Perovskite Building Blocks

The structural diversity and tunable optoelectronic properties of halide perovskites originate from the rich chemistry of the metal halide ionic octahedron [MX₆]^n- (M = Pb²⁺, Sb³⁺, Te⁴⁺, Sn⁴⁺, Pt⁴⁺, etc.; X = Cl^-, Br^-, and I^-). The properties of the extended perovskite solids are dictated by the assembly, connectivity, and interaction of these octahedra within the lattice environment. Hence, the ability to manipulate and control the assembly of the octahedral building blocks is paramount for constructing new perovskite materials. Here, we propose a systematic supramolecular strategy for the assembly of [MX₆]^n- octahedra into a solid extended network. Interaction of alkali metal-bound crown ethers with the [M(IV)X₆]^2- octahedron resulted in a structurally and optoelectronically tunable "dumbbell" structural unit in solution. Single crystals with diverse packing geometries and symmetries will form as the solid assembly of this new supramolecular building block. This supramolecular assembly route introduces a new general strategy for designing halide perovskite structures with potentially new optoelectronic properties.

Zero-dimensional hybrid binuclear manganese chloride with thermally stable yellow emission

Photoluminescence (PL) thermal quenching of hybrid metal halides blocks their applications. Herein, a novel type of 0D hybrid metal halide, [Pb(C₁₂H₂₄O₆)Cl]₂[Mn₂Cl₆], with broad yellow emission and near-unity PL quantum yield is reported. Importantly, it preserves outstanding thermal stability up to 450 K.

Electrochemical and Fluorescent Properties of Crown Ether Functionalized Graphene Quantum Dots for Potassium and Sodium Ions Detection

The concentration of sodium and potassium ions in biological fluids, such as blood, urine and sweat, is indicative of several basic body function conditions. Therefore, the development of simple methods able to detect these alkaline ions is of outmost importance. In this study, we explored the electrochemical and optical properties of graphene quantum dots (GQDs) combined with the selective chelating ability of the crown ethers 15-crown-5 and 18-crown-6, with the final aim to propose novel composites for the effective detection of these ions. The results obtained comparing the performances of the single GQDs and crown ethers with those of the GQDs-15-crown-5 and GQDs-18-crown-6 composites, have demonstrated the superior properties of these latter. Electrochemical investigation showed that the GQDs based composites can be exploited for the potentiometric detection of Na+ and K+ ions, but selectivity still remains a concern. The nanocomposites showed the characteristic fluorescence emissions of GQDs and crown ethers. The GQDs-18-crown-6 composite exhibited ratiometric fluorescence emission behavior with the variation of K+ concentration, demonstrating its promising properties for the development of a selective fluorescent method for potassium determination.

Functionalized triphenylphosphine oxide-based manganese(II) complexes for luminescent printing

Three novel neutral manganese(ii) complexes (TPhPONMe2)2MnBr2, (TPhPOOMe)2MnBr2, and (TPhPOCF3)2MnBr2 have been designed and synthesized based on a functionalized Ph3PO ligand. These structures are clarified by single crystal X-ray diffraction analysis, which reveals that they crystallize in centrosymmetric space groups and feature an isolated mononuclear structure with Mn2+ in a tetrahedral environment. The photoluminescence spectra and emission lifetime decay curves of three manganese(ii) complexes show distinct green emission (λem = 498-512 nm) and phosphorescence lifetime (τ = 362.0-663.0 μs). The results of DFT calculations indicate that the energy levels of (TPhPONMe2)2MnBr2 and (TPhPOOMe)2MnBr2 are higher than that of (TPhPOCF3)2MnBr2 due to the electron-donating effect of the NMe2 or OMe group, which explains the blue-shift of the emission wavelength and the increase of emission lifetime. Furthermore, the prepared neutral manganese(ii) complexes can be used for high-resolution luminescent printing.

A Halomanganates(II) with P,P'-Diprotonated Bis(2-Diphenylphosphinophenyl)ether: Wavelength-Excitation Dependence of the Quantum Yield and Role of the Non-Covalent Interactions

A [H2DPEphos][MnX4] [X = Br, Cl] tetrahalomanganates(II) with P,P'-diprotonated bis[2-(diphenylphosphino)phenyl]ether cation has been designed and investigated in photophysics and EPR terms. The complexes exhibit a green luminescence resulted from the Mn(II) d-d transitions (4T1→6A1) with the wavelength-excitation dependence of the quantum yield. The solid [H2DPEphos][MnBr4] complex exhibits a bright green phosphorescence (λmax = 515 nm) with the high luminescence quantum yield depending on the excitation energy whereas the solid [H2DPEphos][MnCl4] complex exhibits a very weak phosphorescence (λmax = 523 nm). The unexpected shorter luminescence lifetime for the [H2DPEphos][MnCl4] than for the [H2DPEphos][MnBr4] at 300 K can be a result of the higher non-radiative relaxation contribution. On the one hand, the non-covalent PH…X(Mn) interactions quench the manganese(II) luminescence. On the other hand, the PH…X(Mn) interactions are a pathway of the excitation transfer from [H2DPEphos]2+ to [MnX4]2-.

Synthesis and photoluminescence of manganese(II) naphtylphosphonic diamide complexes

Manganese(ii) halide complexes with N,N,N',N'-tetramethyl-P-naphtalen-2-ylphosphonic diamide were synthesized from anhydrous MnX2 salts (X = Cl, Br, I) and characterized. Single-crystal X-ray diffraction revealed in all the cases slightly distorted tetrahedral geometry of the coordination sphere. The photoluminescence spectra showed the superimposition of a green emission, related to the 4T1(4G) → 6A1(6S) transition of Mn(ii), with a band in the red range. Different possible attributions to the lowest-energy emission were taken into account. The emission spectra of the isolated products are dependent upon the nature of the halide and, in the case of X = Br, also upon the excitation wavelength.

The moisture-responsive structural transformation of manganochlorine for water-soluble luminescent switching ink

CsMnCl3(H2O)2 (CMCH) has been widely investigated for magnetic and optical applications, including anti-Stokes photoluminescence, microwave absorption, and magnon-assisted optical transitions. Herein, CMCH crystals, which are colorless and transparent (unlike the pink crystals reported previously), were obtained through a unique approach. Consequently, a high-resolution optical absorption spectrum and distinct thermal behavior were observed. The reversible (de)hydration of CMCH being accompanied by photoluminescence switching (mainly in terms of the color temperature) was rationalized using crystal structure analysis. As a result, water-soluble CMCH could be applied as a moisture-responsive luminescent ink. Moreover, density functional theory (DFT) calculations were performed to understand the optical absorption of CMCH.

18-Crown-6 Coordinated Metal Halides with Bright Luminescence and Nonlinear Optical Effects

The crown-ether coordination compounds ZnX₂(18-crown-6), EuX₂(18-crown-6) (X: Cl, Br, I), MnI₂(18-crown-6), Mn₃Cl₆(18-crown-6)₂, Mn₃I₆(18-crown-6)₂, and Mn₂I₄(18-crown-6) are obtained by ionic-liquid-based synthesis. Whereas MX₂(18-crown-6) (M: Zn, Eu) show conventional structural motives, Mn₃Cl₆(18-crown-6)₂, Mn₃I₆(18-crown-6)₂, and Mn₂I₄(18-crown-6) exhibit unusual single MnX₄ tetrahedra coordinated to the crown-ether complex. Surprisingly, some compounds show outstanding photoluminescence. Thus, rare Zn²⁺-based luminescence is observed and unexpectedly efficient for ZnI₂(18-crown-6) with a quantum yield of 54%. Unprecedented quantum yields are also observed for Mn₃I₆(18-crown-6)₂, EuBr₂(18-crown-6), and EuI₂(18-crown-6) with values of 98, 72, and 82%, respectively, which can be rationalized based on the specific structural features. Most remarkable, however, is Mn₂I₄(18-crown-6). Its specific structural features with finite sensitizer-activator couples result in an extremely strong emission with an outstanding quantum yield of 100%. Consistent with its structural features, moreover, anisotropic angle-dependent emission under polarized light and nonlinear optical (NLO) effects occur, including second-harmonic generation (SHG). The title compounds and their optical properties are characterized by single-crystal structure analysis, X-ray powder diffraction, chemical analysis, density functional theory (DFT) calculations, and advanced spectroscopic methods.

Tetrahedral photoluminescent manganese(ii) halide complexes with 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide as a ligand

Photoluminescent Mn(ii) tetrahedral complexes characterized by intense emission in the green region were isolated from the reaction of MnX₂ (X = Cl, Br, I) and the ligand 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide.

Luminescence of the Mn2+ ion in non-Oh and Td coordination environments: the missing case of square pyramid

The luminescence of Mn²⁺ ion in a square-pyramidal (C_4v) ligand field was discovered. The molecular complexes with such coordination geometry exhibit red emission with enhanced Stokes shift and millisecond lifetimes.

Achievement of ligand-field induced thermochromic luminescence via two-step single-crystal to single-crystal transformations

We report here a new approach to achieve thermochromic luminescence through thermally induced ligand-field transformations. A one-dimensional chain manganese(ii) complex exhibits thermochromic luminescence from blue-green (502 nm) to red (617 nm) emission in a wide range of temperatures (480-80 K), ascribed to the conversion of a tetrahedronal to trigonal bipyramidal ligand-field via two-step single-crystal to single-crystal transformations.

"Two-in-one" organic-inorganic hybrid MnII complexes exhibiting dual-emissive phosphorescence

Unprecedented organic-inorganic hybrid complexes, [Mn(L)3]MnHal4, containing both four- and hexacoordinated Mn2+ ions were synthesized by reacting MnCl2 or MnBr2 with bis(phosphine oxide) ligands (L) such as dppmO2, dppeO2, and 2,3-bis(diphenylphosphinyl)-1,3-butadiene (dppbO2). In the [Mn(L)3]2+ cation of the complexes, the Mn2+ ion features a [MnO6] octahedral coordination environment (Oh), and the [MnHal4]2- anion adopts a tetrahedral geometry (Td). These "two-in-one" complexes exhibit strong long-lived luminescence (τav = 12-15 ms at 300 K) having interesting thermochromic behavior attributed to the thermal equilibrium between two emission bands. So, in an emission spectrum of the typical complex [Mn(dppbO2)3]MnBr4, the intense "red" (ca. 620 nm) and weak "green" (ca. 520 nm) bands, originating from Mn2+ ions in Oh and Td environments, respectively, are observed. Cooling from 300 to 77 K simultaneously leads to (i) redshift of both bands by ca. 20 nm, (ii) increasing their intensities, and (iii) causing a substantial change of their integral intensity ratio from about 4 : 1 to 2 : 1. As a result, the colour of the emission changes from orange (CIE 0.56, 0.45) at 300 K to deep red (CIE 0.62, 0.39) at 77 K. This behavior was rationalized using steady-state and time-resolved fluorescent spectroscopy at various temperatures. The high photoluminescence quantum yields (up to 61% at 300 K) and fascinating dual-emissive phosphorescence coupled with high thermal stability and solubility suggest a high potential of this novel class of emissive Mn2+ complexes as promising emitters for OLED devices and potential stimuli-responsive materials.

Multifunctional luminescent magnetic cryocooler in a Gd5Mn2 pyramidal complex

Magnetic cooling is a highly efficient refrigeration technique with the potential of replacing expensive and rare helium-3 in the field of ultra-low temperature cooling. However, the visualization of a cryogen at an extremely low temperature and in a strong magnetic field is challenging, but it is crucial for the precise positioning and in situ thermal probe measurements in potential practical applications. Here, the activation of a red-emissive Mn(ii) ion using 3d/4f chemistry produces a luminescent molecule cooler, [Gd5Mn2(LOMe)2(OH)4(Ac)6(MeOH)10Cl2]Cl3·2MeOH (1), with the core of an Mn(ii)-anchored heptanuclear [GdIII5MnII2] pyramid. The photoluminescence (PL) of the Mn2+ emission, with a large Stokes shift (λem ∼ 690 nm) from 4T1(4G) → 6A1(6S), shows not only a sensitive temperature sensing property but also reversible mechanoluminescence (ML). More attractively, these findings reveal a considerable magnetocaloric effect (MCE) coupled with a tunable emission window, opening up new opportunities in the multifunctional applications of PL, ML, and the MCE involving red-light sources, thermometers, and stress imaging. In particular, this provides a novel resolution to design visualized PL coolers.

Temperature- and excitation wavelength-dependent emission in a manganese(ii) complex

A mononuclear manganese(ii) complex with a chelating 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine ligand (L), [MnL₂Cl₂]·H₂O, shows intriguing excitation wavelength-dependent emission. Depending on the excitation wavelength, the complex demonstrates three emission bands with the maxima at 380 nm, 440 nm and 495 nm. The 380 nm and 440 nm emissions originate from the π → π* and n → π* ligand-centered transitions. The long-wave 495 nm emission with microsecond lifetimes is related to the d-d transitions and/or metal-to-ligand and halogen-to-ligand charge transfer. The emission behavior of this complex is strongly temperature-dependent: upon cooling from 300 K down to 77 K, the intensity of emission considerably increases. The enhancement of the luminescence upon cooling is accompanied by the appearance of the vibrational structure. This complex is the first example of manganese(ii) complexes demonstrating excitation wavelength-dependent emission.

Green-Light-Emitting Diodes based on Tetrabromide Manganese(II) Complex through Solution Process

Highly phosphorescent (Ph₄ P)₂ [MnBr₄ ] as a low-cost and environmentally benign emitting material achieves peak current efficiency of 25.4 cd A^-1 and external quantum efficiency (EQE) of 7.2% for nondoped organic light-emitting diodes, and peak current efficiency of 32.0 cd A^-1 and EQE of 9.6% for doped devices with 20% (Ph₄ P)₂ [MnBr₄ ]:27% TCTA:53% 6DCZPPY as a doping emitting layer.