Emission Enhancement through Dual Donor Sensitization: Modulation of Structural and Spectroscopic Properties in a Series of Europium Tetracyanoplatinates

Real-Time In Situ Volatile Organic Compound Sensing by a Dual-Emissive Polynuclear Ln-MOF with Pronounced LnIII Luminescence Response

Lanthanide metal-organic frameworks (Ln-MOFs) are promising for luminescence detection of volatile organic compound (VOC) vapors, but usually suffer from the silent or quenched Ln³⁺ emission. Herein, we report a new dual-emissive Eu-MOF composed of the coordinatively unsaturated Eu₉ clusters that afford abundant open metal sites to form a confined "binding pocket" to facilitate the preconcentration and recognition of VOCs. Single-crystal structural analyses reveal that specific analytes can replace the OH oscillators in the first coordination sphere of Eu³⁺ and form a unique hydrogen-bonding second-sphere adduct tying adjacent Eu₉ clusters together to minimize their nonradiative vibrational decay. With the promoted Eu³⁺ luminescence, the MOF realizes real-time in situ visual sensing of THF vapor (<1 s) and shows a quantitative ratiometric response to the vapor pressure with a limit of detection down to 17.33 Pa. Also, it represents a top-performing ratiometric luminescent thermometer.

Stable Fluorescence of Eu3+ Complex Nanostructures Beneath a Protein Skin for Potential Biometric Recognition

We designed and realized highly fluorescent nanostructures composed of Eu3+ complexes under a protein coating. The nanostructured material, confirmed by photo-induced force microscopy (PiFM), includes a bottom fluorescent layer and an upper protein layer. The bottom fluorescent layer includes Eu3+ that is coordinated by 1,10-phenanthroline (Phen) and oleic acid (O). The complete complexes (OEu3+Phen) formed higher-order structures with diameter 40-150 nm. Distinctive nanoscale striations reminiscent of fingerprints were observed with a high-resolution transmission electron microscope (HRTEM). Stable fluorescence was increased by the addition of Eu3+ coordinated by Phen and 2-thenoyltrifluoroacetone (TTA), and confirmed by fluorescence spectroscopy. A satisfactory result was the observation of red Eu3+ complex emission through a protein coating layer with a fluorescence microscope. Lanthanide nanostructures of these types might ultimately prove useful for biometric applications in the context of human and non-human tissues. The significant innovations of this work include: (1) the structural set-up of the fluorescence image embedded under protein "skin"; and (2) dual confirmations of nanotopography and unique nanofingerprints under PiFM and under TEM, respectively.

Europium(III) Photoluminescence Governed by d8-d10 Heterometallophilic Interactions in Trimetallic Cyanido-Bridged Coordination Frameworks

We report an efficient pathway toward sensitization of red room temperature Eu^III emission by the charge-transfer (CT) states related to d⁸-d¹⁰ heterometallophilic interactions achieved by the simultaneous application of tetracyanidometallates of Pt^II/Pd^II and dicyanidometallates of Au^I/Ag^I in the construction of a trimetallic d-d-f assembly. The combination of Eu³⁺, [M^II(CN)₄]^2- (M = Pt, Pd), and [M^I(CN)₂]^- (M = Au, Ag) ions along with 4,4'-bipyridine N,N'-dioxide (4,4'-bpdo) results in four novel isostructural 2D {[Eu^III(4,4'-bpdo)(H₂O)₂][M^II(CN)₄]}·[M^I(CN)₂]·H₂O (M^II/M^I = Pt/Au, 1; Pt/Ag, 2; Pd/Au, 3; Pd/Ag, 4) coordination networks. They are built of hybrid coordination layers, based on cyanido-bridged {Eu^III[M^II(CN)₄]}_n square grids coexisting with metal-organic {Eu^III(4,4'-bpdo)}_n chains, with the further attachment of [M^I(CN)₂]^- ions through metallophilic {M^II-M^I} interactions. This results in dinuclear {M^IIM^I} units generating an orange emissive metal-to-metal-to-ligand charge-transfer (MMLCT) state, whose energy is tuned by the applied d⁸-d¹⁰ metal centers. Thanks to these CT states, 1-4 exhibit room temperature red Eu^III photoluminescence enhanced by energy transfer from {M^IIM^I} units, with the additional role of 4,4'-bpdo also transferring the energy to lanthanides. These donor CT states lying in the visible range successfully broaden the available efficient excitation range up to 500 nm. The overall emission quantum yield ranges from 8(1)% for 4 to 15(2)% for 1, with the intermediate values for 2 and 3 relatively high among the reported Eu^III-based compounds with tetracyanido- and dicyanidometallates. We found that the sensitization efficiency is equally high for all compounds because of the similar energies of the CT states, while the main differences are related to the observed emission lifetimes ranging from ca. 80 μs for 4 to 120-130 μs for 2 and 3 to ca. 180 μs for 1. This phenomenon was correlated with the energies of the vibrational states, e.g., cyanide stretching vibrations, responsible for nonradiative deactivation of Eu^III excited states, which are the highest for the Pd/Ag pair of 4 and the lowest for the Pt/Au pair in 1. Thus, the heaviest pair of Pt^II/Au^I cyanide metal complexes is proven to be the best candidate for the sensitization of room temperature Eu^III luminescence.

Supramolecular Construction of Cyanide-Bridged ReI Diimine Multichromophores

The reactions of labile [Re(diimine)(CO)₃(H₂O)]⁺ precursors (diimine = 2,2'-bipyridine, bpy; 1,10-phenanthroline, phen) with dicyanoargentate anion produce the dirhenium cyanide-bridged compounds [{Re(diimine)(CO)₃}₂CN)]⁺ (1 and 2). Substitution of the axial carbonyl ligands in 2 for triphenylphosphine gives the derivative [{Re(phen)(CO)₂(PPh₃)}₂CN]⁺ (3), while the employment of a neutral metalloligand [Au(PPh₃)(CN)] affords heterobimetallic complex [{Re(phen)(CO)₃}NCAu(PPh₃)]⁺ (4). Furthermore, the utilization of [Au(CN)₂]^-, [Pt(CN)₄]^2-, and [Fe(CN)₆]^4-/3- cyanometallates leads to the higher nuclearity aggregates [{Re(diimine)(CO)₃NC} _xM] ^m+ (M = Au, x = 2, 5 and 6; Pt, x = 4, 7 and 8; Fe, x = 6, 9 and 10). All novel compounds were characterized crystallographically. Assemblies 1-8 are phosphorescent both in solution and in the solid state; according to the DFT analysis, the optical properties are mainly associated with charge transfer from Re tricarbonyl motif to the diimine fragment. The energy of this process can be substantially modified by the properties of the ancillary ligands that allows to attain near-IR emission for 3 (λ_em = 737 nm in CH₂Cl₂). The Re-Fe^II/III complexes 9 and 10 are not luminescent but exhibit low energy absorptions, reaching 846 nm (10) due to Re^I → Fe^III transition.

Multi-colour uranyl emission efficiently tuned by hexacyanidometallates within hybrid coordination frameworks

Hexacyanidometallates of transition metal ions govern the topology and the photoluminescence of bimetallic d–f hybrid coordination networks incorporating uranyl cations.

Mixed-Valent Cyanoplatinates Featuring Neptunyl-Neptunyl Cation-Cation Interactions

The tetracyanoplatinate ligand was employed in synthesizing the first neptunyl cyanoplatinate complexes. Results indicate in situ oxidation of Pt(II) by Np(V/VI) to form mixed-valent Pt-Pt stacked columnar chains linked by cation-cation interaction induced chains of Np(V) polyhedra into a two-dimensional sheet structure. The Pt-Pt stacking distances of 3.04-3.05 Å are the longest reported columnar platinophilic interactions among mixed-valent tetracyanoplatinate structures. These complexes further illustrate the marked chemical differences and structural diversity of solid-state Np(V) coordination complexes with regard to Np(VI) and U(VI).

Incorporation of hexacyanidoferrate(iii) ion in photoluminescent trimetallic Eu(3-pyridone)[Co1−xFex(CN)6] chains exhibiting tunable visible light absorption and emission properties

Tuning of absorption and photoluminescence is achieved in trimetallic EuCoFe cyanido-bridged frameworks through the gradual replacement of [Co^III(CN)₆]³⁻ with [Fe^III(CN)₆]³⁻.

Lanthanide Photoluminescence in Heterometallic Polycyanidometallate-Based Coordination Networks

Solid-state functional luminescent materials arouse an enormous scientific interest due to their diverse applications in lighting, display devices, photonics, optical communication, low energy scintillation, optical storage, light conversion, or photovoltaics. Among all types of solid luminophors, the emissive coordination polymers, especially those based on luminescent trivalent lanthanide ions, exhibit a particularly large scope of light-emitting functionalities, fruitfully investigated in the aspects of chemical sensing, display devices, and bioimaging. Here, we present the complete overview of one of the promising families of photoluminescent coordination compounds, that are heterometallic d–f cyanido-bridged networks composed of lanthanide(3+) ions connected through cyanide bridges with polycyanidometallates of d-block metal ions. We are showing that the combination of cationic lanthanide complexes of selected inorganic and organic ligands with anionic homoligand [M(CN)_x]ⁿ⁻ (x = 2, 4, 6 and 8) or heteroligand [M(L)(CN)₄]²⁻ (L = bidentate organic ligand, M = transition metal ions) anions is the efficient route towards the emissive coordination networks revealing important optical properties, including 4f-metal-centred visible and near-infrared emission sensitized through metal-to-metal and/or ligand-to-metal energy transfer processes, and multi-coloured photoluminescence switchable by external stimuli such as excitation wavelength, temperature, or pressure.

Cyclometalated Platinum(II) Cyanometallates: Luminescent Blocks for Coordination Self-Assembly

A family of cyanide-bridged heterometallic aggregates has been constructed of the chromophoric cycloplatinated metalloligands and coordinatively unsaturated d¹⁰ fragments {M(PPh₃)_n}. The tetranuclear complexes of general composition [Pt(C^N)(CN)₂M(PPh₃)₂]₂ [C^N = ppy, M = Cu (1), Ag (2); C^N = tolpy (Htolpy = 2-(4-tolyl)-pyridine), M = Cu (4), Ag (5); C^N = F₂ppy (HF₂ppy = 2-(4, 6-difluorophenyl)-pyridine), M = Cu (7), Ag (8)] demonstrate a squarelike arrangement of the molecular frameworks, which is achieved due to favorable coordination geometries of the bridging ligands and the metal ions. Variation of the amount of the ancillary phosphine (for M = Ag) afforded compounds [Pt(C^N)(CN)₂Ag(PPh₃)]₂ (C^N = ppy, 3; C^N = tolpy, 6); for the latter one an alternative cluster topology, stabilized by the Pt-Ag metallophilic and η¹-C_ipso(C^N)-Ag bonding, was observed. The solid-state structures of all of the title species 1-8 were determined crystallographically. The complexes exhibit moderately strong room-temperature phosphorescence as crystalline powders (Φ_em = 16-34%, λ_em = 470-511 nm). The luminescence studies and time-dependent density functional theory computational analysis indicate that the photophysical behavior is dominated by the ³π-π* electronic transitions localized on the cyclometalated fragment and mixed with M_PtLCT contribution, while the d¹⁰-phosphine motifs have a negligible contribution into the frontier orbitals and therefore show a little influence on the emission performance of the described compounds.

Synthesis, structure, and photoluminescent behaviour of molecular lanthanide–2-thiophenecarboxylate–2,2′:6′,2′′-terpyridine materials

A lanthanide series incorporating 2-thiophenecarboxylate and terpyridine is presented. Four structure types are observed with differences in the coordination number and nuclearity of the complexes attributed to the effects of the lanthanide contraction.

Dichloridobis(methanol-κO)[cis-(±)-2,4,5-tris-(pyridin-2-yl)-2-imidazoline-κ(3)N(2),N(3),N(4)]ytterbium(III) chloride

In the crystal structure of the title complex, [YbCl₂(C₁₈H₁₅N₅)(CH₃OH)₂]Cl, the pseudo-penta­gonal–bipyramidal coordination geometry of the Yb^III cation is composed of three N atoms from one cis-(±)-2,4,5-tris­(pyridin-2-yl)imidazoline (HL) ligand, two O atoms from two methanol mol­ecules and two Cl⁻ anions. Chains are formed along [010] through N—H⋯Cl, O—H⋯Cl and O—H⋯N hydrogen bonds.

Novel mixed anion [trans-Pt(C[triple bond, length as m-dash]CTol)(2)(CN)(2)](2-) as a building block of new luminescent Pt(II)-Tl(I) and Pt(II)-Pb(II) coordination polymers

A new class of heteroleptic blue emitter anion, [trans-Pt(C[triple bond, length as m-dash]CTol)(2)(CN)(2)](2-), has been prepared and successfully used in the assembling of luminescent supramolecular heterometallic PtTl(2) and PtPb(2) species.

catena-Poly[[[diaqua-(nitrato-κO,O')(2,2':6',2''-terpyridine-κN,N',N'')ytterbium(III)]-μ-cyanido-κN:C-[dicyanido-platinum(II)]-μ-cyanido-κC:N] acetonitrile monosolvate]

The title compound, {[PtYb(CN)₄(NO₃)(C₁₅H₁₁N₃)(H₂O)₂]·CH₃CN}_n, was isolated from solution as a one-dimensional coordination polymer. The Yb³⁺ site has ninefold coordination with a distorted tricapped trigonal–prismatic geometry, while the Pt^II ion is coordinated by four cyanide groups in an almost regular square-planar geometry. cis-Bridging by the tetra­cyanidoplatinate(II) anions links the Yb³⁺ cations, forming chains. Additionally, each Yb³⁺ is coordinated by two water mol­ecules, one bidentate nitrate anion, and one tridentate 2,2′:6′,2′′-terpyridine mol­ecule. O—H⋯N hydrogen-bonding inter­actions are found between adjacent chains and help to consolidate the crystal packing. In addition, π–π stacking inter­actions exist between the terpyridine ligand and the two corresponding terpyridine ligands along the adjacent chain (average inter­planar distance = 3.667 Å). Moderate Pt⋯Pt inter­actions [3.5033 (4) Å] are observed in the structure.

catena-Poly[[[diaqua-(nitrato-κO,O')(2,2':6',2''-terpyridine-κN,N',N'')neodymium(III)]-μ-cyanido-κN:C-[dicyanidoplatinum(II)]-μ-cyanido-κC:N] acetonitrile solvate 2,2':6',2''-terpyridine hemisolvate]

The title compound, {[NdPt(CN)₄(NO₃)(C₁₅H₁₁N₃)(H₂O)₂]·CH₃CN·0.5C₁₅H₁₁N₃}_n, was isolated from solution as a one-dimensional coordination polymer. The Nd³⁺ site in the structure has a ninefold coordination with a distorted tricapped trigonal-prismatic geometry, while the Pt^II ion is coordinated by four cyanide groups in an almost regular square-planar geometry. Cis-bridging by the tetracyanidoplatinate anions links the Nd³⁺ cations, forming the one-dimensional chains. Additionally, each Nd³⁺ contains coordin­ation by two water mol­ecules, one tridentate 2,2′:6′,2′′-terpyridine mol­ecule, and one bidentate nitrate anion. 2,2′:6′,2′′-Terpyridine and acetonitrile solvent mol­ecules are incorporated between the chains, the former form π-stacking inter­actions (average inter­planar distance 3.33 Å) with terpyridine mol­ecules located in the chains. Relatively long Pt⋯Pt inter­actions [3.847 (1) Å] are observed in the structure. O—H⋯N and O—H⋯O hydrogen bonding interactions between the consituents consolidates the crystal packing.