Self-Assembled Tetrahedral [CrL] Cage Displaying Near-Infrared Spin-Flip Photoluminescence.

The thermodynamically controlled self-assembly of bis-bidentate quaterpyridine ligand, L = 2,2':5',5″:2″,2‴-quaterpyridine, with Cr and subsequent oxidation to Cr yields the first photoluminescent tetrahedral [CrL] molecular cage. Single-crystal X-ray diffraction reveals the presence of two homochiral cages (ΛΛΛΛ and ΔΔΔΔ) in the unit cell that crystallize as a racemic mixture. Additionally, a PF anion is observed inside the cavity, in line with isostructural cages built with Ni or Fe.

Magnetic circularly polarized luminescence from spin-flip transitions in a molecular ruby.

Magnetic circularly polarized luminescence (MCPL), the possibility of generating circularly polarized luminescence in the presence of a magnetic field in achiral or racemic compounds, is a technique of rising interest. Here we show that the far-red spin-flip (SF) transitions of a molecular Cr(iii) complex give intense MCD (magnetic circular dichroism) and in particular MCPL ( up to 6.3 × 10 T) even at magnetic fields as low as 0.

Molecular Fe(II)-Ln(III) dyads for luminescence reading of spin-state equilibria at the molecular level.

Due to the primogenic effect, the valence shells of divalent iron Fe(II) ([Ar]3d) and trivalent lanthanides Ln(III) ([Xe]4f) are compact enough to induce spin-state equilibrium for the 3d-block metal and atom-like luminescence for the 4f-block partner in Fe(II)-Ln(III) dyads. In the specific case of homoleptic pseudo-octahedral [Fe(II)N] units, programming spin crossover (SCO) around room temperature at normal pressure requires the design of unsymmetrical didentate five-membered ring chelating NN' ligands, in which a five-membered (benz)imidazole heterocycle (N) is connected to a six-membered pyrimidine heterocycle (N'). Benefiting from the influence, the facial isomer -[Fe(II)(NN')] is suitable for inducing SCO properties at room temperature in solution.

Maximizing Nanoscale Downshifting Energy Transfer in a Metallosupramolecular Cr(III)-Er(III) Assembly.

Pseudo-octahedral CrN chromophores hold a unique appeal for low-energy sensitization of NIR lanthanide luminescence due to their exceptionally long-lived spin-flip excited states. This allure persists despite the obstacles and complexities involved in integrating both elements into a metallosupramolecular assembly. In this work, we have designed a structurally optimized heteroleptic Cr building block capable of binding rare earths.

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 (Hbiim) 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(Hbiim)] 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 [CrN] units are found to be intermolecularly connected peripheral N-H⋯X hydrogen bonds. Moreover, the associated high-energy stretching N-H vibrations drastically quench the targeted near infrared (NIR) Cr-based phosphorescence, which makes these homoleptic building blocks incompatible with the design of molecular-based luminescent assemblies.

Unravelling Kinetics of Intramolecular Nd → Fe Energy Transfer in Spin Crossover Single Molecules: Dotting the 's and Crossing the 's.

Compared with the ripple of visible Eu-based emission intensity induced by appended [FeN] spin crossover (SCO) units, as detected in the triple-stranded [EuFe()] helicate, the lanthanide-based luminescent detection of Fe spin-state equilibria could be improved significantly if the luminophore emission is shifted toward the near-infrared (NIR) domain. Replacing Eu with Nd in [NdFe()] (i) maintains the favorable SCO properties in acetonitrile [critical temperature = 322(2) K], (ii) saturates nonradiative vibrational relaxation processes in the 233-333 K range, and (iii) boosts the crucial intramolecular Nd → Fe energy transfer rate processes, which are sensitive to the spin state of the Fe metallic center. Consequently, the steady-state NIR Nd(F → I) emission of the luminophore is amplified and linearly correlated with the low-spin-[FeN] and high-spin-[FeN] mole fractions controlled by the SCO equilibrium.

Programming heterometallic 4f-4f' helicates under thermodynamic control: the circle is complete.

Three non-symmetrical segmental ligand strands L4 can be wrapped around a linear sequence of one Zn and two trivalent lanthanide cations Ln to give quantitatively directional [ZnLn(L4)] triple-stranded helicates in the solid state and in solution. NMR speciations in CDCN show negligible decomplexation at a millimolar concentration and the latter helicate can be thus safely considered as a preorganized -symmetrical -[(L4Zn)(Ln)(Ln)] platform in which the thermodynamic properties of (i) lanthanide permutation between the central N and the terminal NO binding sites and (ii) exchange processes between homo- and heterolanthanide helicates are easy to access (Ln = La, Eu, Lu). Deviations from statistical distributions could be programmed by exploiting specific site recognition and intermetallic pair interactions.

Tuning Selectivity and Stability in Heteroleptic Lanthanide Adducts by Ligand Design.

Terdentate ligands - and their heteroleptic [Ln(hfac)] complexes (Ln = La, Eu, Gd, Er, or Y; H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) exhibit multifactorial correlations between the ligand's structural frameworks, including their level of preorganization and steric congestion and their affinities and selectivities for catching the trivalent lanthanide containers [Ln(hfac)]. The polyaromatic ligand scaffolds could be stepwise modulated via lanthanide-template synthetic strategies, using intermolecular rhodium-catalyzed insertion reactions. The increasing level of preorganization along the → → series leads to a duality in which larger thermodynamic formation constants with lanthanides in CDCl are accompanied by an unexpected decrease in the Ln-N affinities in the solid state, which could be assigned to a limited match between the lanthanide size and the enlarged preorganized cavities.

Additional Insights into the Design of Cr(III) Phosphorescent Emitters Using 6-Membered Chelate Ring Bis(imidazolyl) Didentate Ligands.

The interest in Cr(III) complexes has been renewed over the past decades for building practical guidelines in the design of efficient earth-abundant phosphorescent near-infrared emitters. In that context, we report the first family of homoleptic tri(didentate) Cr(III) complexes [Cr] based on polyaromatic ligands inducing 6-membered chelate rings, namely, the bis(1-methylimidazol-2-yl)ketone ( = bik), bis(1-methylimidazol-2-yl)methane ( = bim), and bis(1-methylimidazol-2-yl)ethane ( = bie) ligands. The programmed close-to-perfect octahedral microsymmetry of {CrN} chromophores found in [Cr(bik)](OTf) (), [Cr(bim)](OTf) (), and [Cr(bie)](BF) () ensures a ligand-field strength large enough to induce intense and long-lived Cr-based phosphorescence.

Detecting Fe(II) Spin-Crossover by Modulation of Appended Eu(III) Luminescence in a Single Molecule.

Multifunctionality in spin-crossover (SCO) devices is limited to macroscopic or nanoscopic materials because of the need for long-range effects for inducing favorable cooperativity, efficient energy migration processes, and detectable magnetization transfer. The difficult reproducibility, control, and rational design of doped materials offer some place to SCO processes, modulating the optical properties of neighboring luminescent probes in single molecules. We report here on the combination of a [FeN] chromophore, the SCO temperature and absorption spectra of which have been tuned to induce unprecedented room-temperature modulation of Eu(III)-based line-like luminescence in the molecular triple-helical [EuFe()] complex in solution.

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

Previously limited to highly symmetrical homoleptic triple-helical complexes [Er(Lk) ] , 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) ] 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) ] in solution at room temperature can be boosted by up to three orders of magnitude.

Rational Loading of Linear Multi-Site Receptors with Functional Lanthanide Containers: The Missing Link between Oligomers and Polymers.

Although metal-containing organic polymers are becoming essential for modern applications in lighting, catalysis, and electronic devices, very little is known about their controlled metallic loading, which mainly limits their design to empirical mixing followed by characterization and often hampers rational developments. Focusing on the appealing optical and magnetic properties of 4f-block cations, the host-guest reactions leading to linear lanthanidopolymers already display some unexpected dependence of the binding-site affinities on the length of the organic polymer backbone: a drift usually, and erroneously, assigned to intersite cooperativity. Taking advantage of the parameters obtained for the stepwise thermodynamic loading of a series of rigid linear multi-tridentate organic receptors with increasing length, N = 1 (monomer L1), N = 2 (dimer L2), and N = 3 (trimer L3), with [Ln(hfa)3] containers in solution (Ln = trivalent lanthanide cations, hfa = 1,1,1,5,5,5-hexafluoro-pentane-2,4-dione anion), it is demonstrated here that the site-binding model, based on the Potts-Ising approach, successfully predicts the binding properties of the novel soluble polymer P2 made up of nine successive binding units .

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.

Preorganized Polyaromatic Soft Terdentate Hosts for the Capture of [Ln(β-diketonate) ] Guests in Solution.

The concept of preorganization is famous in coordination chemistry for having transformed flexible bidentate 2,2'-bipyridine scaffolds into rigid 1,10-phenanthroline platforms. The resulting boosted affinities for d-block cations has successfully paved the way for the design of a wealth of functional complexes, devices and materials for analysis and optics. Its extension toward terdentate homologues adapted for the selective complexation of f-block cations with larger coordination numbers remains more overlooked.

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 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 dyads exhibiting directional intramolecular intermetallic energy transfer remain unexplored.

Positive Cooperativity Induced by Interstrand Interactions in Silver(I) Complexes with α,α'-Diimine Ligands.

Invited for the cover of this issue are Davood Zare, Claude Piguet, Edwin C. Constable and co-workers at the University of Basel and the University of Geneva. The image depicts a [Ag L] intermediate about to catch a second α,α'-diimine ligand to form the stable [Ag L ] .

Positive Cooperativity Induced by Interstrand Interactions in Silver(I) Complexes with α,α'-Diimine Ligands.

The allosteric positive cooperativity accompanying the formation of compact [Cu (α,α'-diimine) ] building blocks contributed to the historically efficient synthesis of metal-containing catenates and knotted assemblies. However, its limited magnitude can easily be overcome by the negative chelate cooperativity that controls the overall formation of related polymetallic multistranded helicates and grids. Despite the more abundant use of analogous dioxygen-resistant [Ag (α,α'-diimine) ] units in modern entangled metallo-supramolecular assemblies, a related thermodynamic justification was absent.

Heteroleptic -[Cr(NNN)(CN)] complexes: synthetic challenge, structural characterization and photophysical properties.

The substitution of three water molecules around trivalent chromium in CrBr·6HO with the tridentate 2,2':6',2''-terpyridine (tpy), ,'-dimethyl-,'-di(pyridine-2-yl)pyridine-2,6-diamine (ddpd) or 2,6-di(quinolin-8-yl)pyridine (dqp) ligands gives the heteroleptic -[Cr(L)Br] complexes. Stepwise treatments with Ag(CFSO) and KCN under microwave irradiations provide -[Cr(L)(CN)] in moderate yields. According to their X-ray crystal structures, the associated six-coordinate meridional [CrNC] 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.

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.

Bottom-Up Approach for the Rational Loading of Linear Oligomers and Polymers with Lanthanides.

The adducts between luminescent lanthanide tris(β-diketonate)s and diimine or triimine ligands have been explored exhaustively for their exceptional photophysical properties. Their formation, stability, and structures in solution, together with the design of extended metallopolymers exploiting these building blocks, remain, however, elusive. The systematic peripheral substitution of tridentate 2,6-bis(benzimidazol-2-yl)pyridine binding units ( = -), taken as building blocks for linear oligomers and polymers, allows a fine-tuning of their affinity toward neutral [Ln(hfa)] (hfa = hexafluoroacetylacetonate) lanthanide containers in the [Ln(hfa)] adducts.

Ligand-Sensitized Near-Infrared to Visible Linear Light Upconversion in a Discrete Molecular Erbium Complex.

While the low-absorption cross section of lanthanide-based upconversion systems, in which the trivalent lanthanides (Ln) are responsible for converting low- to high-energy photons, has restricted their application to intense incident light, the emergence of a cascade sensitization through an organic dye antenna capable of broadly harvesting near-infrared (NIR) light in upconversion nanoparticles opened new horizons in the field. With the aim of pushing molecular upconversion within the range of practical applications, we show herein how the incorporation of an NIR organic dye antenna into the ligand scaffold of a mononuclear erbium coordination complex boosts the upconversion brightness of the molecule to such an extent that a low-power (0.7 W·cm) NIR laser excitation of [Er(hfa)] (hfa = hexafluoroacetylacetonate) at 801 nm results in a measurable visible upconverted signal in a dilute solution (5 × 10 M) at room temperature.

A Near-Infrared-II Emissive Chromium(III) Complex.

The combination of π-donating amido with π-accepting pyridine coordination units in a tridentate chelate ligand causes a strong nephelauxetic effect in a homoleptic Cr complex, which shifts its luminescence to the NIR-II spectral range. Previously explored Cr polypyridine complexes typically emit between 727 and 778 nm (in the red to NIR-I spectral region), and ligand design strategies have so far concentrated on optimizing the ligand field strength. The present work takes a fundamentally different approach and focusses on increasing metal-ligand bond covalence to shift the ruby-like E emission of Cr to 1067 nm at 77 K.

Molecular light-upconversion: we have had a problem! When excited state absorption (ESA) overcomes energy transfer upconversion (ETU) in Cr(III)/Er(III) complexes.

Nine-coordinate [ErN9] or [ErN3O6] chromophores found in triple helical [Er(L)3]3+ complexes (L corresponds to 2,2',6',2''-terpyridine (tpy), 2,6-(bisbenzimidazol-2-yl)pyridine (bzimpy), 2,6-diethylcarboxypyridine (dpa-ester) or 2,6-diethylcarboxamidopyridine (dpa-diamide) derivatives), [Er(dpa)3]3- (dpa is the 2,6-dipicolinate dianion) and [GaErGa(bpb-bzimpy)3]9+ (bpb-bzimpy is 2,6-bis((pyridin-2-benzimidazol-5-yl)methyl-(benzimidazol-2-yl))pyridine) exhibit NIR (excitation at 801 nm) into visible (emission at 542 nm) linear light upconversion processes in acetonitrile at room temperature. The associated quantum yields 5.5(6) × 10-11 ≤ φuptot(ESA) ≤ 1.

Bright Long-Lived Circularly Polarized Luminescence in Chiral Chromium(III) Complexes.

A series of highly emissive inert and chiral Cr complexes displaying dual circularly polarized luminescence (CPL) within the NIR region have been prepared and characterized. The helical [Cr(dqpR) ] (dqp=2,6-di(quinolin-8-yl)pyridine; R=OCH , Br or C≡CH) complexes were synthesized as racemic mixtures and resolved into their respective PP and MM enantiomers by chiral stationary phase HPLC. The corresponding enantiomers show large g ≈0.