Color tuning and white light emission by codoping in isostructural homochiral lanthanide metal–organic frameworks

Tunable Dual-emission of Mixed-lanthanide MOFs with no Antenna Effect

Heteronuclear zinc(II):mixed-lanthanide(III) metal organic frameworks (MOFs) bearing the formula [L n x A L n y B ${{{\rm L}{\rm n}}_{{\rm x}}^{{\rm A}}{{\rm L}{\rm n}}_{{\rm y}}^{{\rm B}}}$ Zn3(oda)6(H2O)6]⋅nH2O (Ln = lanthanide(III) ion,L n B ${{{\rm L}{\rm n}}^{{\rm B}}}$ is smaller thanL n A ${{{\rm L}{\rm n}}^{{\rm A}}}$ , x+y=2, n=13 or 14) were obtained by direct reaction of the corresponding metal ion salts and oxydiacetate (-OOC-CH2-O-CH2-COO-) in aqueous solution, followed by slow solvent diffusion. This strategy led to the isolation of thirteen pure crystalline samples of isostructural hexagonal compounds (space group P6/mcc). Characterization by single-crystal X-ray diffraction was complemented by the analytical determination of the finalL n B / L n A ${{{\rm L}{\rm n}}^{{\rm B}}/{{\rm L}{\rm n}}^{{\rm A}}}$ molar ratio by microwave induced plasma atomic emission spectrometry. Results show that the experimentally determinedL n B / L n A ${{{\rm L}{\rm n}}^{{\rm B}}/{{\rm L}{\rm n}}^{{\rm A}}}$ molar ratio is systematically higher than that set out in the synthesis, especially for combinations of Ln that have larger size differences. Dual emission luminescence is observed for the compounds containing a combination of the more efficient visible-emitting Ln (Sm, Eu, Tb, Dy). The selected aliphatic ligand does not promote antenna effect. Thus, tunable color luminescence can be achieved by selectively targeting each Ln preferential excitation wavelength. Dual emission is also observed for CeEu mixed-MOF since the simultaneous excitation of both Ln is possible. Interestingly, GdEu mixed-MOF also shows dual emission when only Gd is directly excited.

Multifunctional Chiral d10-Metal Coordination Polymers: Tunable Photoluminescence and Efficient Second-Harmonic Generation with Circular Dichroic Response

A series of homochiral coordination polymers (HCPs), [M2(SIAP)2(bpy)2] [M(S)] and [M2(RIAP)2(bpy)2] [M(R)] (M = Zn or Cd, SIAP or RIAP = (S,S)- or (R,R)- 2,2'-(isophthaloylbis(azanediyl))di-propionic acid, bpy = 4,4'-bipyridine), is successfully synthesized through solvothermal reactions, self-assembling d10 metal cations, chiral dicarboxylic ligands, and π-conjugated bipyridyl ligands. The HCPs crystallize in the extremely rare triclinic chiral space group, P1, and present 3D framework structures attributed to the strong intermolecular interactions, such as hydrogen bonds and π-π stacking. Due to the unique crystal structures, the title compounds reveal efficient photoluminescence emission across a broad visible range, with significant brightness and color tuning by varying the excitation wavelength. Moreover, they exhibit efficient phase-matched second-harmonic generation (SHG) with very high laser-induced damage thresholds, essential for high-power nonlinear optical (NLO) applications. Intriguingly, the title compounds exhibit a measurable contrast in the SHG response under right- and left-handed circularly polarized excitation, thereby providing a unique case of SHG circular dichroism from the chiral centers of SIAP2- or RIAP2- ligand packed in the noncentrosymmetric environment. These exceptional attributes position these HCPs as promising candidates for multifunctional materials, with potential applications ranging from NLO devices to tailored luminescent systems with polarization control.

Zwitterionic Luminescent 2D Metal-Organic Framework Nanosheets (LMONs): Selective Turn-On Fluorescence Sensing of Dihydrogen Phosphate

While a plethora of organic linkers based on carboxylic acids have been utilized in the construction of MOFs, zwitterionic linkers that typify the attributes of naturally occurring amino acids have been exploited only scarcely to the best of our knowledge. Zwitterionic interior characteristics should be expected to impart unique properties to the resultant MOFs with a high potential to interact with guest species through electrostatic interactions. In our investigations with bis(p-carboxyphenyl)imidazolylarenes as a novel class of linkers for the development of functional MOFs, we have found that bisimidazole-tetracarboxylic acid H₄DMBI undergoes metal-assisted self-assembly with Zn(NO₃)₂ to yield a layered MOF (Zn-DMBI). In the latter, the linker serves as a two-connecting linker with imidazoles and carboxylic acids behaving as zwitterions. The layers are offset stacked in the crystal structure and are bound firmly by hydrogen bonds between imidazolium and carboxylate ions. Such a packing precludes fluorescence from being observed due to self-quenching. However, exfoliation into zwitterionic 2D metal-organic nanosheets (MONs) by sonication in methanol for 1 h liberates palpable fluorescence. Furthermore, the suspension of luminescent MONs (LMONs) in methanol permits selective sensing of anions; in particular, dihydrogen phosphate (H₂PO₄^-) that is complementary to the zwitterions in terms of hydrogen bond donor and acceptor sites is observed with fluorescence enhancement by 120%, leading to its detection at a sub-parts-per-million (0.13 ppm) level. Thus, access to zwitterionic 2D MONs and their application for selective anion sensing with "turn-on" fluorescence are demonstrated by a rational de novo bottom-up approach.

Shedding light on predicting and controlling emission chromaticity in multicomponent photoluminescent systems

Predictable colour tuning in multicomponent photoluminescent (PL) systems is achieved using mixtures of simultaneously emitting organic molecules. By mitigating the potential for energy transfer through the control of concentration, the resulting emission chromaticity of five dichromic PL systems is approximated as a linear combination of the emitting components and their corresponding brightness (χ i , ϕ i , and I ex,i ). Despite being limited to dilute solutions (10-6 M), colour tuning within these systems was controlled by (1) varying the composition of the components and (2) exploiting the differences in the components' excitation intensities at common wavelengths. Using this approach, white light emission (WLE) was realized using a pre-determined mixture of red, green, and blue emitting organic molecules. Based on these results, materials and devices with built-in or programmable emission colour can be achieved, including highly sought-after WLE.

A multifunctional benzothiadiazole-based fluorescence sensor for Al3+, Cr3+ and Fe3+

A benzothiadiazole-based Zn^II MOF (JXUST-3) has been synthesized, which is a good multifunctional chemosensor for the detection of Al³⁺, Cr³⁺ and Fe³⁺.

Three resorcin[4]arene-based lanthanide-coordination polymers with multifunctional photoluminescence sensing properties

By utilizing a novel octacarboxylate-functionalized resorcin[4]arene as organic linkers, three lanthanide-coordination polymers, namely, [(CH3)2NH2][Ln2(HL)(H2O)7]·2H2O (Ln = Tb (1), Eu (2) and Gd (3), H8L = 2,8,14,20-tetra-pentyl-4,6,10,12,16,18,22,24-octa-carboxymethoxy-resorcin[4]arene) have been solvothermally synthesized and structurally characterized. Isostructural 1-3 display unique two dimensional sandwich-based layers built with Ln3+ cations and bowl-shaped HL7- anions. Remarkably, 1 and 2 produce intensive green and red emissions respectively and long lifetimes thanks to the antenna effect of HL7- anions. The energy level testing of 3 indicates that the newly designed ligand H8L has a very efficient intersystem crossing process. More importantly, luminescent investigations reveal that 1 and 2 can selectively detect N,N'-dimethylformamide and Fe3+ ions with turn-on-type and turn-off-type responses, respectively.