Visible and Near-Infrared Emission from Lanthanoid ß-Triketonate Assemblies Incorporating Cesium Cations.

The reaction of the ß-triketonate ligands tris(4-methylbenzoyl)methanide and tribenzoylmethanide with the trivalent lanthanoids Eu3+, Er3+, and Yb3+ in the presence of Cs+ afforded polymeric structures where the repeating units are represented by bimetallic tetranuclear assemblies of formulation {[Ln(Cs)(ß-triketonate)4]2}n. The only exception is the structure formed by the reaction of tris(4-methylbenzoyl)methanide, Yb3+, and Cs+, which yielded a polymeric assembly where the repeating units are mononuclear Yb3+ complexes bridged by Cs+ cations. Photophysical measurements on the obtained materials confirmed efficient sensitization from the ligand excited states to the 4f* excited states of the three lanthanoids. According to transient absorption data, Er3+ and Yb3+ are sensitized via energy transfer from the triplet state of the ß-triketonate ligands. On the other hand, energy transfer to Eu3+ seems to occur via an alternative pathway, possibly directly via the singlet state or through ligand to metal charge transfer states. The emission measurements confirm efficient sensitization for all three lanthanoids and bright near-infrared emission for Er3+ and Yb3+, a characteristic that seems to be linked to the specific chemical structure of the ß-triketonate ligands.

Lanthanoid/Alkali Metal ß-Triketonate Assemblies: A Robust Platform for Efficient NIR Emitters.

The reaction of hydrated lanthanoid chlorides with tribenzoylmethane and an alkali metal hydroxide consistently resulted in the crystallization of neutral tetranuclear assemblies with the general formula [Ln(Ae⋅HOEt)(L)4 ]2 (Ln=Eu(3+) , Er(3+) , Yb(3+) ; Ae=Na(+) , K(+) , Rb(+) ). Analysis of the crystal structures of these species revealed a coordination geometry that varied from a slightly distorted square antiprism to a slightly distorted triangular dodecahedron, with the specific geometrical shape being dependent on the degree of lattice solvation and identity of the alkali metal. The near-infrared (NIR)-emitting assemblies of Yb(3+) and Er(3+) showed remarkably efficient emission, characterized by significantly longer excited-state lifetimes (τobs ≈37-47 µs for Yb(3+) and τobs ≈4-6 µs for Er(3+) ) when compared with the broader family of lanthanoid ß-diketonate species, even in the case of perfluorination of the ligands. The Eu(3+) assemblies show bright red emission and a luminescence performance (τobs ≈0.5 ms, ${{\Phi}{{{\rm L}\hfill \atop {\rm Ln}\hfill}}}$≈35-37 %, ηsens ≈68-70 %) more akin to the ß-diketonate species. The results highlight that the ß-triketonate ligand offers a tunable and facile system for the preparation of efficient NIR emitters without the need for more complicated perfluorination or deuteration synthetic strategies.

Photophysical and photochemical trends in tricarbonyl rhenium(I) N-heterocyclic carbene complexes.

A family of tricarbonyl Re(I) complexes of the formulation fac-[Re(CO)3(NHC)L] has been synthesized and characterized, both spectroscopically and structurally. The NHC ligand represents a bidentate N-heterocyclic carbene species where the central imidazole ring is substituted at the N3 atom by a butyl, a phenyl, or a mesityl group and substituted at the N1 atom by a pyridyl, a pyrimidyl, or a quinoxyl group. On the other hand, the ancillary L ligand alternates between chloro and bromo. For the majority of the complexes, the photophysical properties suggest emission from the lowest triplet metal-to-ligand charge transfer states, which are found partially mixed with triplet ligand-to-ligand charge transfer character. The nature and relative energy of the emitting states appear to be mainly influenced by the identity of the substituent on the N3 atom of the imidazole ring; thus, the pyridyl complexes have blue-shifted emission in comparison to the more electron deficient pyrimidyl complexes. The quinoxyl complexes show an unexpected blue-shifted emission, possibly occurring from ligand-centered excited states. No significant variations were found upon changing the substituent on the imidazole N3 atom and/or the ancillary ligand. The photochemical properties of the complexes have also been investigated, with only the complexes bound to the pyridyl-substituted NHC ligands showing photoinduced CO dissociation upon excitation at 370 nm, as demonstrated by the change in the IR and NMR spectra as well as a red shift in the emission profile after photolysis. Temperature-dependent photochemical experiments show that CO dissociation occurs at temperatures as low as 233 K, suggesting that the Re-C bond cleaves from excited states of metal-to-ligand charge transfer nature rather than thermally activated ligand field excited states. A photochemical mechanism that takes into account the reactivity of the complexes bound to the pyridyl-NHC ligand as well as the stability of those bound to the pyrimidyl- and quinoxyl-NHC ligands is proposed.

Probing the effect of ß-triketonates in visible and NIR emitting lanthanoid complexes.

An isomorphous series of lanthanoid complexes containing tribenzoylmethanide (tbm) and 1,10-phenanthroline (phen) ligands has been synthesised and structurally characterised. These complexes, formulated as [Ln(phen)(tbm)3] (Ln = Eu3+, Er3+ and Yb3+), were compared with analogous dibenzoylmethanide (dbm) [Ln(phen)(dbm)3] complexes to investigate the effect of changing ß-diketonate to ß-triketonate ligands on the photophysical properties of the complex. The photophysical properties for the Eu3+ complexes were similar for both systems, whereas a modest enhancement was observed for Yb3+ and Er3+ moving from the dbm to the tbm complexes. A detailed study of the NIR photophysical properties was achieved by adapting the integrating sphere method for the calculation of overall quantum yields in the solid state.

Lanthanoid ß-triketonates: a new class of highly efficient NIR emitters for bright NIR-OLEDs.

The reaction of hydrated YbCl3 with potassium tribenzoylmethanide yields a new bimetallic tetranuclear Yb(3+)/K(+) assembly. This species not only possesses the longest excited state lifetime and quantum yield reported for the Yb(3+) diketonate family but is also suitable to be incorporated in NIR-OLEDs, whose performance outclasses any other reported lanthanoid-based device with NIR emission.

Recyclable calix[4]arene-lanthanoid luminescent hybrid materials with color-tuning and color-switching properties.

Inorganic-organic hybrid materials combine the properties of both components providing functionality with a wide range of potential applications. Phase segregation of the inorganic and organic components is a common challenge in these systems, which is overcome here by copolymerizing a metal-free calixarene ionophore and methyl methacrylate. A lanthanoid ion is then added using a swelling-deswelling procedure. The resulting luminescent hybrid materials can be made to emit any required color, including white light, by loading with an appropriate mixture of lanthanoids. The gradation of the emitted color can also be finely adjusted by changing the excitation wavelength. The polymer monolith can be recycled to emit a different color by swelling with a solution containing a different lanthanoid ion. This methodology is flexible and has the potential to be extended to many different ionophores and polymer matrices.

The photochemistry of rhenium(I) tricarbonyl N-heterocyclic carbene complexes.

The photophysical and photochemical properties of the new tricarbonyl rhenium(I) complexes bound to N-heterocyclic carbene ligands (NHC), fac-[Re(CO)3(N^C)X] (N^C = 1-phenyl-3-(2-pyridyl)imidazole or 1-quinolinyl-3-(2-pyridyl)imidazole; X = Cl or Br), are reported. The photophysics of these complexes highlight phosphorescent emission from triplet metal-to-ligand ((3)MLCT) excited states, typical of tricarbonyl rhenium(I) complexes, with the pyridyl-bound species displaying a ten-fold shorter excited state lifetime. On the other hand, these pyridyl-bound species display solvent-dependent photochemical CO dissociation following what appear to be two different mechanisms, with a key step being the formation of cationic tricarbonyl solvato-complexes, being themselves photochemically active. The photochemical mechanisms are illustrated with a combination of NMR, IR, UV-Vis, emission and X-ray structural characterization techniques, clearly demonstrating that the presence of the NHC ligand is responsible for the previously unobserved photochemical behavior in other photoactive tricarbonyl rhenium(I) species. The complexes bound to the quinolinyl-NHC ligand (which possess a lower-energy (3)MLCT) are photostable, suggesting that the photoreactive excited state is not any longer thermally accessible. The photochemistry of the pyridyl complexes was investigated in acetonitrile solutions and also in the presence of triethylphosphite, showing a competing and bifurcated photoreactivity promoted by the trans effect of both the NHC and phosphite ligands.

A "plug-and-play" approach to the preparation of transparent luminescent hybrid materials based on poly(methyl methacrylate), a calix[4]arene cross-linking agent, and terbium ions.

A novel methodology to prepare transparent luminescent hybrid materials is reported. Using a calixarene ionophore as a PMMA cross-linker avoids problems, such as phase segregation, and produces a polymer monolith that can be loaded with the metal ion required for luminescence post-synthesis. This approach is versatile and will simplify the production of such materials.