Optical sensitization and upconversion in discrete polynuclear chromium–lanthanide complexes

Taming 2,2'-biimidazole ligands in trivalent chromium complexes

Complete or partial replacement of well-known five-membered chelating 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) ligands with analogous didentate 2,2'-biimidazole (H2biim) provides novel perspectives for exploiting the latter pH-tuneable bridging unit for connecting inert trivalent chromium with cationic partners. The most simple homoleptic complex [Cr(H2biim)3]3+ and its stepwise deprotonated analogues are only poorly soluble in most solvents and their characterization is limited to some solid-state structures, in which the pseudo-octahedral [CrN6] units are found to be intermolecularly connected via peripheral N-H⋯X hydrogen bonds. Moreover, the associated high-energy stretching N-H vibrations drastically quench the targeted near infrared (NIR) CrIII-based phosphorescence, which makes these homoleptic building blocks incompatible with the design of molecular-based luminescent assemblies. Restricting the number of bound 2,2'-biimidazole ligands to a single unit in the challenging heteroleptic [Cr(phen)2(Hxbiim)](1+x)+ (x = 2-0) complexes overcomes the latter limitations and allows (i) the synthesis and characterization of these [CrN6] chromophores in the solid state and in solution, (ii) the stepwise and controlled deprotonation of the bound 2,2'-biimidazole ligand and (iii) the implementation of Cr-centered phosphorescence with energies, lifetimes and quantum yields adapted for using the latter chromophores as sensitizers in promising 'complex-as-ligand' strategies.

Symmetry and Rigidity for Boosting Erbium-Based Molecular Light-Upconversion in Solution

Previously limited to highly symmetrical homoleptic triple-helical complexes [Er(Lk)3 ]3+ , where Lk are polyaromatic tridentate ligands, single-center molecular-based upconversion using linear optics and exploiting the excited-state absorption mechanism (ESA) greatly benefits from the design of stable and low-symmetrical [LkEr(hfa)3 ] heteroleptic adducts (hfa- =hexafluoroacetylacetonate anion). Depending on (i) the extended π-electron delocalization, (ii) the flexibility and (iii) the heavy atom effect brought by the bound ligand Lk, the near-infrared (801 nm) to visible green (542 nm) upconversion quantum yield measured for [LkEr(hfa)3 ] in solution at room temperature can be boosted by up to three orders of magnitude.

Excited-Multimer Mediated Supramolecular Upconversion on Multicomponent Lanthanide-Organic Assemblies

Upconversion (UC) is a fascinating anti-Stokes-like optical process with promising applications in diverse fields. However, known UC mechanisms are mainly based on direct energy transfer between metal ions, which constrains the designability and tunability of the structures and properties. Here, we synthesize two types of Ln8L12-type (Ln for lanthanide ion; L for organic ligand L1 or L2R/S) lanthanide-organic complexes with assembly induced excited-multimer states. The Yb8(L2R/S)12 assembly exhibits upconverted multimer green fluorescence under 980 nm excitation through a cooperative sensitization process. Furthermore, upconverted red emission from Eu3+ on the heterometallic (Yb/Eu)8L12 assemblies is also realized via excited-multimer mediated energy relay. Our findings demonstrate a new strategy for designing UC materials, which is crucial for exploiting photofunctions of multicomponent lanthanide-organic complexes.

Donor Radii in Rare-Earth Complexes

We present a set of donor radii for the rare-earth cations obtained from the analysis of structural data available in the Cambridge Structural Database (CSD). Theoretical calculations using density functional theory (DFT) and wave function approaches (NEVPT2) demonstrate that the Ln-donor distances can be broken down into contributions of the cation and the donor atom, with the minimum in electron density (ρ) that defines the position of (3,-1) critical points corresponding well with Shannon's crystal radii (CR). Subsequent linear fits of the experimental bond distances for all rare earth cations (except Pm3+) afforded donor radii (rD) that allow for the prediction of Ln-donor distances regardless of the nature of the rare-earth cation and its oxidation state. This set of donor radii can be used to rationalize structural data and identify particularly weak or strong interactions, which has important implications in the understanding of the stability and reactivity of complexes of these metal ions. A few cases of incorrect atom assignments in X-ray structures were also identified using the derived rD values.

Recent Progress in Photonic Upconversion Materials for Organic Lanthanide Complexes

Organic lanthanide complexes have garnered significant attention in various fields due to their intriguing energy transfer mechanism, enabling the upconversion (UC) of two or more low-energy photons into high-energy photons. In comparison to lanthanide-doped inorganic nanoparticles, organic UC complexes hold great promise for biological delivery applications due to their advantageous properties of controllable size and composition. This review aims to provide a summary of the fundamental concept and recent developments of organic lanthanide-based UC materials based on different mechanisms. Furthermore, we also detail recent applications in the fields of bioimaging and solar cells. The developments and forthcoming challenges in organic lanthanide-based UC offer readers valuable insights and opportunities to engage in further research endeavors.

Seeking Brightness in Molecular Erbium-Based Light Upconversion

Whereas dye-sensitized lanthanide-doped nanoparticles represent an unquestionable advance for pushing linear near-infrared (NIR) to visible-light upconversion within the frame of applications, analogous improvements are difficult to mimic for related but intramolecular processes induced at the molecular level in coordination complexes. Major difficulties arise from the cationic nature of the target cyanine-containing sensitizers (S), which drastically limits their thermodynamic affinities for catching the lanthanide activators (A) required for performing linear light upconversion. In this context, the rare previous design of stable dye-containing molecular SA light-upconverters required large S···A distances at the cost of the operation of only poorly efficient intramolecular S → A energy transfers and global sensitization. With the synthesis of the compact ligand [L2]⁺, we exploit here the benefit of using a single sulfur connector between the dye and the binding unit for counterbalancing the drastic electrostatic penalty which is expected to prevent metal complexation. Quantitative amounts of nine-coordinate [L2Er(hfac)₃]⁺ molecular adducts could be finally prepared in solution at millimolar concentrations, while the S···A distance has been reduced by 40% to reach circa 0.7 nm. Detailed photophysical studies demonstrate the operation of a three times improved energy transfer upconversion (ETU) mechanism for molecular [L2Er(hfac)₃]⁺ in acetonitrile at room temperature, thanks to the boosted heavy atom effect operating in the close cyanine/Er pair. NIR excitation at 801 nm can thus be upconverted into visible light (525-545 nm) with an unprecedented brightness of B^up(801 nm) = 2.0(1) × 10^-3 M^-1·cm^-1 for a molecular lanthanide complex.

4f → 3d sensitization: a luminescent EuII-MnII heteronuclear complex with a near-unity quantum yield

A new heteronuclear Eu^II-Mn^II complex [Eu(N2O6)]MnBr4 (N2O6 = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) is designed and synthesized, which shows an intense green emission from Mn^II with a near-unity photoluminescence quantum yield. Measurement of excited-state dynamics demonstrated the sensitization process from Eu^II to Mn^II, which represents the first example of f → d molecular sensitization. Due to the large optical absorption cross-section of the Eu^II center, [Eu(N2O6)]MnBr4 shows an emission intensity 7 to 2500 times stronger than that of the Sr^II-Mn^II control complex [Sr(N2O6)]MnBr4 upon the excitation of near ultraviolet to blue light.

Lanthanide Hexacyanidoruthenate Frameworks for Multicolor to White-Light Emission Realized by the Combination of d-d, d-f, and f-f Electronic Transitions

We report an effective strategy toward tunable room-temperature multicolor to white-light emission realized by mixing three different lanthanide ions (Sm³⁺, Tb³⁺, and Ce³⁺) in three-dimensional (3D) coordination frameworks based on hexacyanidoruthenate(II) metalloligands. Mono-lanthanide compounds, K{Ln^III(H₂O)_n[Ru^II(CN)₆]}·mH₂O (1, Ln = La, n = 3, m = 1.2; 2, Ln = Ce, n = 3, m = 1.3; 3, Ln = Sm, n = 2, m = 2.4; 4, Ln = Tb, n = 2, m = 2.4) are 3D cyanido-bridged networks based on the Ln-NC-Ru linkages, with cavities occupied by K⁺ ions and water molecules. They crystallize differently for larger (1, 2) and smaller (3, 4) lanthanides, in the hexagonal P6₃/m or the orthorhombic Cmcm space groups, respectively. All exhibit luminescence under the UV excitation, including weak blue emission in 1 due to the d-d ³T_1g → ¹A_1g electronic transition of Ru^II, as well as much stronger blue emission in 2 related to the d-f ²D_3/2 → ²F_5/2,7/2 transitions of Ce^III, red emission in 3 due to the f-f ⁴G_5/2 → ⁶H_{5/2,7/2,9/2,11/2} transitions of Sm^III, and green emission in 4 related to the f-f ⁵D₄ → ⁷F_6,5,4,3 transitions of Tb^III. The lanthanide emissions, especially those of Sm^III, take advantage of the Ru^II-to-Ln^III energy transfer. The Ce^III and Tb^III emissions are also supported by the excitation of the d-f electronic states. Exploring emission features of the Ln^III-Ru^II networks, two series of heterobi-lanthanide systems, K{Sm_xCe_1-x(H₂O)_n[Ru(CN)₆]}·mH₂O (x = 0.47, 0.88, 0.88, 0.99, 0.998; 5-9) and K{Tb_xCe_1-x(H₂O)_n[Ru(CN)₆]}·mH₂O (x = 0.56, 0.65, 0.93, 0.99, 0.997; 10-14) were prepared. They exhibit the composition- and excitation-dependent tuning of emission from blue to red and blue to green, respectively. Finally, the heterotri-lanthanide system of the K{Sm_0.4Tb_0.599Ce_0.001(H₂O)₂[Ru(CN)₆]}·2.5H₂O (15) composition shows the rich emission spectrum consisting of the peaks related to Ce^III, Tb^III, and Sm^III centers, which gives the emission color tuning from blue to orange and white-light emission of the CIE 1931 xy parameters of 0.325, 0.333.

Stereocontrolled Self-Assembly of Ln(III)-Pt(II) Heterometallic Cages with Temperature-Dependent Luminescence

Structurally well-defined discrete d/f heterometallic complexes show diverse application potential in electrooptic and magnetic materials. However, precise control of the component and topology of such heterometallic compounds with fine-tuned photophysical properties is still challenging. Herein, we report the stereocontrolled syntheses of a series of Ln^III-Pt^II heterometallic cages through coordination-driven self-assembly of enantiopure alkynylplatinum-based metalloligands (L₁^R/S, L₂^R/S) with lanthanide ions (Ln = Eu^III, Yb^III, Nd^III, Lu^III). Taking advantage of the metal-to-ligand charge transfer (MLCT) excited state on the designed alkynylplatinum ligands, the excitation window for the sensitized near-infrared (NIR) luminescence on the Yb^III- and Nd^III-containing cages can be extended to the visible region (up to 500 nm). Linear temperature-dependent red and NIR emissions observed on the Ln₄(L₂^R/S)₆ (Ln^III = Eu^III and Yb^III, respectively) complexes suggest their potential applications as luminescent temperature sensors, with sensitivities of -0.54% (Ln^III = Eu^III, 77-250 K) and -0.17% (Ln^III = Yb^III, 77-300 K) per K achieved. This work not only offers a good strategy to prepare new d/f heterometallic supramolecular cages but also paves the way for the design of stimuli-responsive luminescent materials.

Investigation of Intermetallic Energy Transfers in Lanthanide Coordination Polymers Molecular Alloys: Case Study of Trimesate-Based Compounds

Reactions in water at ambient temperature and pressure between a lanthanide ion and benzene-1,3,5-tricarboxylate (or trimesate) lead to two series of iso-structural coordination polymers. Their general chemical formula is [Ln(tma)(H₂O)₆]_∞ for the lightest lanthanide ions (Ln = La-Dy except Pm), while it is [Ln(tma)(H₂O)₅·3.5H₂O]_∞ for the heaviest ones (Ho-Lu plus Y). For the heaviest lanthanide ions, reactions at 50 °C lead to a third structural family with the general chemical formula [Ln(tma)(H₂O)₃·1.5H₂O]_∞ with Ln = Ho-Lu plus Y. Homo-lanthanide coordination polymers that belong to the latter two families do not exhibit luminescence in the visible region. Therefore, we used a phase induction strategy to obtain molecular alloys that belong to these structural families and show sizeable emission. The random distribution of the lanthanide ions over the metallic sites has been investigated using ⁸⁹Y and ¹³⁹La solid-state NMR spectroscopy experiments. Luminescent properties of homo- and hetero-nuclear coordination polymers based on Eu³⁺ and Tb³⁺ have been studied in detail and compared. As a result, this study strongly suggests that exchange-based intermetallic energy transfer mechanisms play an important role in these systems. It also suggests the presence of an intermetallic exchange pathway through π-stacking interactions.

Complex-as-Ligand Strategy as a Tool for the Design of a Binuclear Nonsymmetrical Chromium(III) Assembly: Near-Infrared Double Emission and Intramolecular Energy Transfer

The chromium(III) polypyridyl complexes are appealing for their long-lived near-infrared (NIR) emission reaching the millisecond range and for the strong circularly polarized luminescence of their isolated enantiomers. However, harnessing those properties in functional polynuclear Cr^III devices remains mainly inaccessible because of the lack of synthetic methods for their design and functionalization. Even the preparation and investigation of most basic nonsymmetrical Cr^III dyads exhibiting directional intramolecular intermetallic energy transfer remain unexplored. Taking advantage of the inertness of heteroleptic chromium(III) polypyridyl building blocks, we herein adapt the "complex-as-ligand" strategy, largely used with precious 4d and 5d metals, for the preparation of a binuclear nonsymmetrical Cr^III complex (3d metal). The resulting [(phen)₂Cr(L)Cr(tpy)]⁶⁺ dyad shows dual long-lived NIR emission and a directional intermetallic energy transfer that is controlled by the specific arrangements of the different coordination spheres. This strategy opens a route for building predetermined polynuclear assemblies with this earth-abundant metal.

Efficient Triplet-Triplet Annihilation Upconversion Sensitized by a Chromium(III) Complex via an Underexplored Energy Transfer Mechanism

Sensitized triplet-triplet annihilation upconversion (sTTA-UC) mainly relies on precious metal complexes thanks to their high intersystem crossing (ISC) efficiencies, excited state energies, and lifetimes, while complexes of abundant first-row transition metals are only rarely utilized and with often moderate UC quantum yields. [Cr(bpmp)₂ ]³⁺ (bpmp=2,6-bis(2-pyridylmethyl)pyridine) containing earth-abundant chromium possesses an absorption band suitable for green light excitation, a doublet excited state energy matching the triplet energy of 9,10-diphenyl anthracene (DPA), a close to millisecond excited state lifetime, and high photostability. Combined ISC and doublet-triplet energy transfer from excited [Cr(bpmp)₂ ]³⁺ to DPA gives ³ DPA with close-to-unity quantum yield. TTA of ³ DPA furnishes green-to-blue UC with a quantum yield of 12.0 % (close to the theoretical maximum). Sterically less-hindered anthracenes undergo a [4+4] cycloaddition with [Cr(bpmp)₂ ]³⁺ and green light.

Upconversion in a d-f [RuYb3] Supramolecular Assembly

We have prepared a hetero-tetrametallic assembly consisting of three ytterbium ions coordinated to a central [Ru(bpm)₃]²⁺ (bpm = 2,2'-bipyrimidine) motif. Irradiation into the absorption band of the peripheral ytterbium ions at 980 nm engenders emission of the ³MLCT state of the central [Ru(bpm)₃]²⁺ core at 636 nm, which represents the first example of f → d molecular upconversion (UC). Time-resolved measurements reveal a slow rise of the UC emission, which was modeled with a mathematical treatment of the observed kinetics according to a cooperative photosensitization mechanism using a virtual Yb centered doubly excited state followed by energy transfer to the Ru centered ¹MLCT state.

Synthesis, Crystal Structure, and Luminescence Properties of the Iso-Reticular Series of Lanthanide Coordination Polymers Synthesized from Hexa-Lanthanide Molecular Precursors

Microwave-assisted reactions in DMSO, between a hexa-lanthanide octahedral complex ([Ln₆(μ₆-O)(μ₃-OH)₈(NO₃)₆(H₂O)₁₂·2NO₃·2H₂O] with Ln = Nd-Yb plus Y) and either 3-halogenobenzoic acid (hereafter symbolized by 3-xbH with x = f or c for fluoro or chloro, respectively) or 4-halogenobenzoic acid (hereafter symbolized by 4-xbH with x = f, c, or b for fluoro, chloro, or bromo, respectively), lead to 1D lanthanide coordination polymers. These coordination polymers are almost iso-reticular. The crystal structure is described on the basis of the coordination polymer with chemical formula [Tb(4-fb)₃(DMSO)(H₂O)₂·DMSO]_∞ obtained from 4-fluorobenzoic acid (4-fbH) and the Tb³⁺-based octahedral complex: It crystallizes in the triclinic system, space group P1̅ (n°2), with the following cell parameters: a = 9.8561(9) Å, b = 10.5636(9) Å, c = 15.1288(15) Å, α = 100.840(3)°, β = 95.552(3)°, γ = 110.482(3)°, V = 1426.4(3) ų, and Z = 2. It can be described on the basis of 1D molecular chains. Luminescence properties of the Tb and Eu derivatives have been measured and compared vs the halogeno-function and its position (meta or para). Some molecular alloys have also been prepared to estimate the strength of the intermetallic energy transfers. To confirm that the hexa-nuclear complexes (and not the halogenated ligand) have a structuring effect for the formation of the straight chain-like molecular motif, another coordination polymer with chemical formula [Tb(4-npa)₃DMSO·DMSO·H₂O]_∞ where 4-npaH symbolizes 4-nitro-phenyl-acetic acid has been prepared. It crystallizes in the triclinic system, space group P1̅ (n°2) with the following cell parameters: a = 7.8784(8) Å, b = 14.8719(16) Å, c = 15.2753(17) Å, α = 73.612(4)°, β = 86.406(4)°, γ = 83.104(4)°, V = 1703.8(3) ų, and Z = 2. Its crystal structure can be described on the basis of a molecular motif that is similar to the one observed in the five previous crystal structures which confirms the structuring effect of the hexa-nuclear complexes.

Heteroleptic mer-[Cr(N∩N∩N)(CN)3] complexes: synthetic challenge, structural characterization and photophysical properties

The substitution of three water molecules around trivalent chromium in CrBr₃·6H₂O with the tridentate 2,2′:6′,2′′-terpyridine (tpy), N,N′-dimethyl-N,N′-di(pyridine-2-yl)pyridine-2,6-diamine (ddpd) or 2,6-di(quinolin-8-yl)pyridine (dqp) ligands gives the heteroleptic mer-[Cr(L)Br₃] complexes. Stepwise treatments with Ag(CF₃SO₃) and KCN under microwave irradiations provide mer-[Cr(L)(CN)₃] in moderate yields. According to their X-ray crystal structures, the associated six-coordinate meridional [CrN₃C₃] chromophores increasingly deviate from a pseudo-octahedral arrangement according to L = ddpd ≈ dpq ≪ tpy; a trend in line with the replacement of six-membered with five-membered chelate rings around Cr^III. Room-temperature ligand-centered UV-excitation at 18 170 cm⁻¹ (λ_exc = 350 nm), followed by energy transfer and intersystem crossing eventually yield microsecond metal-centered Cr(²E → ⁴A₂) phosphorescence in the red to near infrared domain 13 150–12 650 cm⁻¹ (760 ≤ λ_em ≤ 790 nm). Decreasing the temperature to liquid nitrogen (77 K) extends the emission lifetimes to reach the millisecond regime with a record of 4.02 ms for mer-[Cr(dqp)(CN)₃] in frozen acetonitrile.

The heteroleptic mer-[Cr(L)(CN)₃] (L = tpy, ddpd, dqp) complexes with their C_2v-symmetrical [CrC₃N₃] luminescent chromophores represent the missing links between pseudo-octahedral [CrN₆] and [CrC₆] units found in their well-known homoleptic parents.

Coordination-Directed Self-Assembly of Functional Polynuclear Lanthanide Supramolecular Architectures

Lanthanide supramolecular chemistry is a fast growing and intriguing research field due to the unique photophysical, magnetic, and coordination properties of lanthanide ions (Ln^III). Compared with the intensively investigated mononuclear Ln-complexes, polymetallic lanthanide supramolecular assemblies offer more structural superiority and functional advantages. In recent decades, significant progress has been made in polynuclear lanthanide supramolecules, varying from structural evolution to luminescent and magnetic functional materials. This review summarizes the design principles in ligand-induced coordination-driven self-assembly of polynuclear Ln-structures and intends to offer guidance for the construction of more elegant Ln-based architectures and optimization of their functional performances. Design principles concerning the water solubility and chirality of the lanthanide-organic assemblies that are vital in extending their applications are emphasized. The strategies for improving the luminescent properties and the applications in up-conversion, host-guest chemistry, luminescent sensing, and catalysis have been summarized. Magnetic materials based on supramolecular assembled lanthanide architectures are given in an individual section and are classified based on their structural features. Challenges remaining and perspective directions in this field are also briefly discussed.

Metal-Based Linear Light Upconversion Implemented in Molecular Complexes: Challenges and Perspectives

The piling up of low-energy photons to produce light beams of higher energies while exploiting the nonlinear optical response of matter was conceived theoretically around 1930 and demonstrated 30 years later with the help of the first coherent ruby lasers. The vanishingly small efficacy of the associated light-upconversion process was rapidly overcome by the implementation of powerful successive absorptions of two photons using linear optics in materials that possess real intermediate excited states working as relays. In these systems, the key point requires a favorable competition between the rate constant of the excited-state absorption (ESA) and the relaxation rate of the intermediate excited state, the lifetime of which should be thus maximized. Chemists and physicists therefore selected long-lived intermediate excited states found (i) in trivalent lanthanide cations doped into ionic solids or into nanoparticles (^2S+1L_J spectroscopic levels) or (ii) in polyaromatic molecules (triplet states) as the logical activators for designing light upconverters using linear optics. Their global efficiency has been stepwise optimized during the past five decades by using indirect intermolecular sensitization mechanisms (energy transfer upconversion = ETU) combined with large absorption cross sections.The induction of light-upconversion operating in a single discrete entity at the molecular level is limited to metal-based units and remained a challenge for a long time because coordination complexes possess high-frequency oscillators incompatible with the existence of (i) scales of accessible excited relays with long lifetimes and (ii) final high-energy emissive levels with noticeable intrinsic quantum yields. In contrast to intermolecular energy transfer processes operating in metal-based doped solids, which require statistical models, the combination of sensitizers and activators within the same molecule limits energy transfers to easily tunable intramolecular processes with first-order kinetic rate constants. Their successful programming in a trinuclear CrErCr complex in 2011 led to the first detectable near-infrared to green light upconversion induced in a molecular unit under reasonable excitation intensity. The subsequent progress in the modeling and understanding of the key factors controlling metal-based light upconversion operating in molecular complexes led to a burst of various designs exploiting different mechanisms, excited-state absorption (ESA), energy transfer upconversion (ETU), cooperative luminescence (CL), and cooperative upconversion (CU), which are discussed in this Account.

Mesoporous Silica Nanoparticles-Embedded Lanthanide Organic Polyhedra for Enhanced Stability, Luminescence and Cell Imaging

We report here a simple but efficient “ship-in-a-bottle” synthetic strategy for increasing the stability and luminescence performance of LOPs by embedding them into mesoporous silica nanoparticles (MSNs). Three types of...

Hexanuclear Molecular Precursors as Tools to Design Luminescent Coordination Polymers with Lanthanide Segregation

Solvothermal reactions between hexanuclear complexes with the general chemical formula [Ln₆(μ₆-O)(μ₃-OH)₈(NO₃)₆(H₂O)₁₂]·2NO₃·2H₂O and 2-bromobenzoic acid (2-bbH) lead to a series of isostructural one-dimensional coordination polymers with the general chemical formula [Ln₂(2-bb)₆]_∞ with Ln = Sm, Eu, Tb, Dy, and Y. These coordination polymers crystallize in the orthorhombic space group Fdd2 (No. 43) with the following cell parameters: a = 29.810(3) Å, b = 51.185(6) Å, c = 11.7913(14) Å, V = 17992(4) ų, and Z = 16. The europium- and terbium-based derivatives show sizable luminescence intensities under UV excitation. Isostructural heterolanthanide coordination polymers have also been prepared. Their luminescent properties suggest that during the synthetic process the starting hexanuclear complexes are destroyed but strongly influence the distribution of the different lanthanide ions over the metallic sites of the crystal structure. Indeed, it is possible to prepare heterolanthanide coordination polymers in which lanthanide-ion segregation is controlled.

Discrete Heteropolynuclear Yb/Er Assemblies: Switching on Molecular Upconversion Under Mild Conditions

The salts {[Ln₂ Ln*(Hhmq)₃ (OAc)₃ (hfac)₂ ]⁺ [Ln*(hfac)₃ (OAc)(MeOH)]^- } (Hhmq=2-methanolquinolin-8-oxide, hfac=hexafluoroacetylacetonate; Ln, Ln*=Er, Gd, Yb) feature a discrete heteronuclear cation consisting of two types of lanthanide atoms. The quinolinoxy O-atom serves as a μ₂ -bridge to two Ln atoms and as a μ₃ -bridge to all three atoms, with metal⋅⋅⋅metal distances being around 3.7 Å. For 1 ([Yb₂ Er]⁺ ), near-infrared downshifted luminescence is switched to competitive upconversion luminescence upon irradiation by a 980 nm laser under an extremely low excitation power (0.288 W cm^-2 ) through introduction of fluoride ions. The stability of 1 after addition of fluoride was confirmed by powder X-ray diffraction and multistage mass spectrometry, associated with the ¹ H NMR of 6 ([La₂ Eu]⁺ ). More importantly, the at least 20-fold enhancement of the quantum yield in non-deuterated solvents at room temperature under low power densities (2 W cm^-2 ) is the highest among the few molecular examples reported.

Room-Temperature Upconversion in a Nanosized {Ln15} Molecular Cluster-Aggregate

The successive absorption of low-energy photons to the accumulation of the intermediate excited states leading to higher energy emission is still a challenge in molecular architectures. Contrary to low-phonon solids and nanoparticles, the rational construction of molecular systems containing an excess of donor atoms in relation to acceptor ones is far from trivial. Moreover, the vibrations caused by high-energy oscillators commonly present on coordination compounds result in serious drawbacks on molecular upconversion. To overcome these limitations, we demonstrate that upconversion can be achieved even at room temperatures through the use of molecular cluster-aggregates (MCAs). To achieve the upconverted emission, we synthesized a MCA containing 15 lanthanide ions, {Er₂Yb₁₃}, ensuring an excess of donor atoms. With the excitation on the ytterbium ion, the characteristic green and red emissions from erbium were obtained at room temperature. To prove the mechanism behind the upconversion process, four other compositions were synthesized and studied, namely, {Y₁₃Er₂}, {Y₁₀Er₅}, {Er₁₀Yb₅}, and {Y₁₀Er₁Yb₄}. Upconversion quantum yield values on the order of 10^-3% were obtained, values 100000 times higher than for previously reported lanthanide-based molecular upconverting systems. The presented methodology is an interesting approach to address a fine composition control and harness the upconversion properties of nanoscale molecular materials.

Transient FTIR spectroscopy after one- and two-colour excitation on a highly luminescent chromium(III) complex

The development of photoactive transition metal complexes with Earth-abundant metals is a rapidly growing research field, where a deeper understanding of the underlying photophysical processes is of great importance. A multitude of potential applications in the fields of photosensitizing, optical sensing, photoluminescence and photoredox catalysis motivates demanding spectroscopic studies. We applied a series of high-level spectroscopic methods on the previously reported highly luminescent chromium(iii) complex [Cr(ddpd)2](BF4)3 (ddpd = N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine) possessing two near-IR emissive doublet states with microsecond lifetimes. Luminescence measurements were performed at temperatures down to about 10 K, showing a remarkable rise of the integrated emission intensity by more than a factor of three. The emissive doublet states were structurally characterized by transient FTIR spectroscopy at 290 K and 20 K, supplemented by ground state FTIR and Raman spectroscopy in combination with density functional theory. According to emission and step-scan FT-IR spectroscopy, the stronger luminescence at lower temperature arises from decreased non-radiative decay via energy transfer to CH vibrational overtones and increased radiative decay based on lowered symmetry. Pump/pump/probe (FTIR) and pump/dump/probe (FTIR) schemes were developed to modulate the excited doublet state populations at 290 and 20 K as a function of specific near-IR pump vs. dump wavelengths. The effect of the second near-IR pulse can be explained by combinations of excited state absorption, ground state absorption and stimulated emission. The successful establishment of these two-colour step-scan FTIR experiments is an important step towards profound studies on further transition metal complexes with energetically close-lying excited states in the near future.

Near-IR to Near-IR Upconversion Luminescence in Molecular Chromium Ytterbium Salts

Upconversion photoluminescence in hetero-oligonuclear metal complex architectures featuring organic ligands is an interesting but still rarely observed phenomenon, despite its great potential from a basic research and application perspective. In this context, a new photonic material consisting of molecular chromium(III) and ytterbium(III) complex ions was developed that exhibits excitation-power density-dependent cooperative sensitization of the chromium-centered 2 E/2 T1 phosphorescence at approximately 775 nm after excitation of the ytterbium band 2 F7/2 →2 F5/2 at approximately 980 nm in the solid state at ambient temperature. The upconversion process is insensitive to atmospheric oxygen and can be observed in the presence of water molecules in the crystal lattice.

Aspects of lanthanide complexes for selectivity, intensity and sharpness in luminescence bands from twenty-four praseodymium, europium and gadolinium complexes with differently distorted-hexadentate ligands

We structurally and spectroscopically investigated a series of praseodymium (Pr) complexes with eight ligands that form helicate molecular structures. The mother ligand skeleton (L) has two bipyridine moieties bridged with ethylenediamine. The bridged skeleton of PrL was changed to diamines 1-methyl-ethylenediamine, trimethylenediamine and 2,2'-dimethyl-trimethylenediamine, and the corresponding ligands were designated as L^me, L^pr and L^dmpr, for each Pr in these complexes upon UV-excitation. The luminescence quantum yields of PrL and PrL^pr in the visible and near infrared (NIR) regions indicate that PrL is excited by both the electronic state of the ligand and the ff absorption band, whereas PrL^pr is excited through the ligand. The addition of a methyl group to PrL and PrL^pr has a different effect on the Pr emission intensity with the intensity of PrL^me decreasing more than that of PrL and PrL^dmpr and increasing more than that of PrL^pr. Thus, the coordination of Pr and the increased rigidity of the ligand upon methylation enhance luminescence. The azomethine moieties on L^me, L^pr and L^dmpr were reduced and formed the corresponding PrLH, PrL^meH, PrL^prH and PrL^dmprH complexes. The luminescence of the non-methylated series is due to transitions related to the ¹D₂ level and thus the methylated series luminesces due to high energy levels such as ³P_J arising from the shortened π electronic systems. We also discuss the strong red emission of a series of Eu complexes with eight ligands from the viewpoint of their molecular structures and luminescence efficiencies and evaluate the Judd-Ofelt parameters from the luminescence spectra of Eu complexes.

Formation of Heteropolynuclear Lanthanide Complexes Using Macrocyclic Phosphonated Cyclam-Based Ligands

Ligands L₁ and L₂, respectively based on a cyclam and a cross-bridged cyclam scaffold functionalized at N1 and N8 by 6-phosphonic-2-methylene pyridyl groups, are described. While complexation of lanthanide (Ln) cations with L₂ was not possible, a family of complexes has been prepared with L₁, of the general formulae [LnL₁H₂]Cl (Ln³⁺ = Lu, Tb, Yb) or [LnL₁H] (Ln³⁺ = Eu). The solution, structural, potentiometric, and photophysical data for these novel ligands and their complexes have been investigated, including a solid-state study by X-ray diffraction (L₁, L₂, and [EuL₁H]), ¹H NMR complexation investigations (Lu³⁺), as well as UV-vis absorption and luminescence spectroscopy in water and D₂O (pH ≈ 7). L₁ forms 1:1 metal-ligand stoichiometric octadentate complexes in solution. Importantly, the pyridyl phosphonate functions are capable of simultaneous chelation to the metal center and of interaction with a second metal center. ¹H NMR (Lu³⁺) and spectrophotometric titrations of the isolated [TbL₁]^- complex by EuCl₃ salts demonstrated the formation of high-order (hetero)polymetallic species in aqueous solution (H₂O, pH = 7). Global analysis of the luminescence titration experiment points to the formation of 4:1, 3:1, and 3:2 [TbL₁]/Eu heteropolynuclear assemblies, exhibiting a strong preference to forming [TbL₁]₃Eu₂ at increased europium concentrations, with energy transfer occurring between the kinetically inert terbium complex and added europium cations.

Key Strategy for the Rational Incorporation of Long-Lived NIR Emissive Cr(III) Chromophores into Polymetallic Architectures

The Cr^IIIN₆ chromophores are particularly appealing for low-energy sensitization via energy transfer processes since they show extremely long excited state lifetimes reaching the millisecond range in the technologically crucial near-infrared domain. However, their properties were barely harnessed in multimetallic structures because of the lack of both monitoring methods and accessible synthetic pathways. We herein report a remedy to monitor and control the formation of Cr^III-containing assemblies in solution via the design of a Cr^IIIN₆ inert "complex-as-ligand" that can be included into polymetallic architectures. As a proof of concept, these CrN₆ building blocks were reacted in solution with Zn^II or Fe^II to give extended trinuclear linear Cr-M-Cr assemblies, the structure of which could be addressed by NMR spectroscopy despite the presence of two slowly relaxing Cr^III paramagnetic centers. In addition to long Cr^III excited state lifetimes and weak sensitivity to oxygen quenching, these polymetallic assemblies display controlled Cr^III to M^II energy transfers, which pave the way for use of the "complex-as-ligand" strategy for introducing photophysically active Cr^III probes into light-converting polymetallic devices.

Heteronuclear d-d and d-f Ru(ii)/M complexes [M = Gd(iii), Yb(iii), Nd(iii), Zn(ii) or Mn(ii)] of ligands combining phenanthroline and aminocarboxylate binding sites: combined relaxivity, cell imaging and photophysical studies

A ligand skeleton combining a 1,10-phenanthroline (phen) binding site and one or two heptadentate N3O4 aminocarboxylate binding sites, connected via alkyne spacers to the phen C3 or C3/C8 positions, has been used to prepare a range of heteronuclear Ru·M and Ru·M2 complexes which have been evaluated for their cell imaging, relaxivity, and photophysical properties. In all cases the phen unit is bound to a {Ru(bipy)2}2+ unit to give a phosphorescent {Ru(bipy)2(phen)}2+ luminophore, and the pendant aminocarboxylate sites are occupied by a secondary metal ion M which is either a lanthanide [Gd(iii), Nd(iii), Yb(iii)] or another d-block ion [Zn(ii), Mn(ii)]. When M = Gd(iii) or Mn(ii) these ions provide the complexes with a high relaxivity for water; in the case of Ru·Gd and Ru·Gd2 the combination of high water relaxivity and 3MLCT phosphorescence from the Ru(ii) unit provides the possibility of two different types of imaging modality in a single molecular probe. In the case of Ru·Mn and Ru·Mn2 the Ru(ii)-based phosphorescence is substantially reduced compared to the control complexes Ru·Zn and Ru·Zn2 due to the quenching effect of the Mn(ii) centres. Ultrafast transient absorption spectroscopy studies on Ru·Mn (and Ru·Zn as a non-quenched control) reveal the occurrence of fast (<1 ns) PET in Ru·Mn, from the Mn(ii) ion to the Ru(ii)-based 3MLCT state, i.e. MnII-(phen˙-)-RuIII → MnIII-(phen˙-)-RuII; the resulting MnIII-(phen˙-) state decays with τ ≈ 5 ns and is non-luminescent. This occurs in conformers when an ET pathway is facilitated by a planar, conjugated bridging ligand conformation connecting the two units across the alkyne bridge but does not occur in conformers where the two units are electronically decoupled by a twisted conformation of the bridging ligand. Computational studies (DFT) on Ru·Mn confirmed both the occurrence of the PET quenching pathway and its dependence on molecular conformation. In the complexes Ru·Ln and Ru·Ln2 (Ln = Nd, Yb), sensitised near-infrared luminescence from Nd(iii) or Yb(iii) is observed following photoinduced energy-transfer from the Ru(ii) core, with Ru → Nd energy-transfer being faster than Ru → Yb energy-transfer due to the higher density of energy-accepting states on Nd(iii).

Solid-state luminescence properties, Hirshfeld surface analysis and DFT calculations of mononuclear lanthanide complexes (Ln = EuIII, GdIII, TbIII, DyIII) containing 4'-phenyl-2,2':6',2″-terpyridine

Four mononuclear lanthanide complexes containing 4'-phenyl-2,2':6',2″-terpyridine (ptpy), [Ln(NO₃)₃(ptpy) (H₂O)] (Ln = Eu (1), Gd (2), Tb (3), Dy (4)), were solvothermally synthesized and characterized via elemental analysis, infrared spectroscopy, thermogravimetric analysis, single-crystal X-ray diffraction, and powder X-ray diffraction. Hirshfeld surfaces and the solid-state luminescence properties of the complexes were investigated. The 3-D Hirshfeld surface and 2-D fingerprint plots show that the main interactions are the O H/H O intermolecular interactions in 1-4. Solid-state luminescence investigation reveals that Gd^III complex 2 displays a ligand-centered emission and the Eu^III, Tb^III and Dy^III complexes 1, 3 and 4 show the characteristic lanthanide-centered luminescence upon UV excitations. The Eu^III and Tb^III complexes exhibit red (CIE: 0.6549, 0.3447) and green (CIE: 0.3760, 0.5412) luminescence in the solid state with quantum yields of 16.8% and 0.8% and lifetimes of 0.545 and 0.043 ms, respectively. Density functional theory (DFT) calculations were conducted to unravel the HOMO-LUMO energy gaps of the structures of ptpy and complexes 1 and 3.

Visible light sensitized near-infrared luminescence of ytterbium via ILCT states in quadruple-stranded helicates

Quadruple-stranded helicates show visible light sensitized near-infrared luminescence of ytterbium via ILCT states.

Electronic Energy Transduction from {Ru(py)4} Chromophores to Cr(III) Luminophores

Despite the large body of work on {Ru(bpy)₂} sensitizer fragments, the same attention has not been devoted to their {Ru(py)₄} analogues. In this context, we explored the donor-acceptor trans-[Ru(L)₄{(μ-NC)Cr(CN)₅}₂]^4-, where L = pyridine, 4-methoxypyridine, 4-dimethylaminopyridine. We report on the synthesis and the crystal structure as well as the electrochemical, spectroscopical, and photophysical properties of these trimetallic complexes, including transient absorption measurements. We observed emission from chromium-centered d-d states upon illuminating into either MLCT or MM'CT absorptions of {Ru(L)₄} or {Ru-Cr}, respectively. The underlying energy transfer is as fast as 600 fs with quantum efficiencies ranging from 10% to 100%. These results document that {Ru(py)₄} sensitizer fragments are as efficient as {Ru(bpy)₂} in short-range energy transfer scenarios.

Antiferromagnetic exchange and long-range magnetic ordering in supramolecular networks constructed of hexacyanido-bridged LnIII(3-pyridone)–CrIII (Ln = Gd, Tb) chains

Exchange interactions and magnetic phase transitions are observed in novel cyanido-bridged lanthanide(iii)–chromium(iii) chains as proved by magnetic and calorimetric studies.

Bringing upconversion down to the molecular scale

After bulk solids and nanoparticles, examples of upconversion are now emerging at the discrete molecular scale in solution.