Lanthanide ions possess fascinating optical properties and their discovery, first industrial uses and present high technological applications are largely governed by their interaction with light. Lighting devices (economical luminescent lamps, light emitting diodes), television and computer displays, optical fibres, optical amplifiers, lasers, as well as responsive luminescent stains for biomedical analysis, medical diagnosis, and cell imaging rely heavily on lanthanide ions. This critical review has been tailored for a broad audience of chemists, biochemists and materials scientists; the basics of lanthanide photophysics are highlighted together with the synthetic strategies used to insert these ions into mono- and polymetallic molecular edifices. Recent advances in NIR-emitting materials, including liquid crystals, and in the control of luminescent properties in polymetallic assemblies are also presented. (210 references.)

Taking advantage of luminescent lanthanide ions

Interpretation of europium(III) spectra

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Ionic liquids (ILs) are organic salts with melting points near room temperature (or by convention below 100 degrees C). Recently, their unique materials and solvent properties and the growing interest in a sustainable, "green" chemistry has led to an amazing increase in interest in such salts. A huge number of potential cation and anion families and their many substitution patterns allows the desired properties for specific applications to be selected. Because it is impossible to experimentally investigate even a small fraction of the potential cation-anion combinations, a molecular-based understanding of their properties is crucial. However, the unusual complexity of their intermolecular interactions renders molecular-based interpretations difficult, and gives rise to many controversies, speculations, and even myths about the properties that ILs allegedly possess. Herein the current knowledge about the molecular foundations of IL behavior is discussed.

Understanding Ionic Liquids at the Molecular Level: Facts, Problems, and Controversies

Room-temperature ionic liquids (RTILs) are liquids consisting entirely of ions, and their important properties, e.g., negligible vapor pressure, are considered to result from the ionic nature. However, we do not know how ionic the RTILs are. The ionic nature of the RTILs is defined in this study as the molar conductivity ratio (Λimp/ΛNMR), calculated from the molar conductivity measured by the electrochemical impedance method (Λimp) and that estimated by use of pulse-field-gradient spin−echo NMR ionic self-diffusion coefficients and the Nernst−Einstein relation (ΛNMR). This ratio is compared with solvatochromic polarity scales:  anionic donor ability (Lewis basicity), ET(30), hydrogen bond donor acidity (α), and dipolarity/polarizability (π*), as well as NMR chemical shifts. The Λimp/ΛNMR well illustrates the degree of cation−anion aggregation in the RTILs at equilibrium, which can be explained by the effects of anionic donor and cationic acceptor abilities for the RTILs having different anionic and catio...

How Ionic Are Room-Temperature Ionic Liquids? An Indicator of the Physicochemical Properties

Density-functional-based Car−Parrinello molecular dynamics (CPMD) simulations have been performed for the ionic liquid 1,3-dimethylimidazolium chloride, [dmim]Cl, at 438 K. The local structure of the liquid is described in terms of various partial radial distribution functions and anisotropic spatial distributions, which reveal a significant extent of hydrogen bonding. The cation−anion distribution simulated with the BP86 functional is in qualitative agreement with the structural model derived from neutron diffraction data for the liquid, whereas the theoretical cation−cation distribution shows less satisfactory accord. Population analyses indicate noticeable charge transfer from anions to cations, and specific CH···Cl hydrogen bonds are characterized in terms of donor−acceptor interactions between lone pairs on Cl and antibonding σ*CH orbitals.

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Ab initio molecular dynamics of liquid 1,3-dimethylimidazolium chloride.

We report a molecular dynamics study of the interface between water and (macroscopically) water-immiscible room-temperature ionic liquids "ILs", composed of PF6(-) anions and butyl- versus octyl-substituted methylimidazolium+ cations (noted BMI+ and OMI+). Because the parameters used to simulate the pure ILs were found to exaggerate the water/IL mixing, they have been modified by scaling down the atomic charges, leading to better agreement with the experiment. The comparison of [OMI][PF6] versus [BMI][PF6] ILs demonstrates the importance of the N-alkyl substituent on the extent of solvent mixing and on the nature of the interface. With the most hydrophobic [OMI][PF6] liquid, the "bulk" IL phase is dryer than with the [BMI][PF6] liquid. At the interface, the OMI+ cations retain direct contacts with the bulk IL, whereas the more hydrophilic PF6(-) anions gradually dilute in the local water micro-environment and are thus isolated from the "bulk" IL. The interfacial OMI+ cations are ordered with their imidazolium moiety pointing toward the aqueous side and their octyl chains toward the IL side of the interface. With the [BMI][PF6] liquid, the system gradually evolves from an IL-rich to a water-rich medium, leading to an ill-defined interfacial domain with high intersolvent mixing. As a result, the BMI+ cations are isotropically oriented "at the interface". Because the imidazolium cations are more hydrophobic than the PF6(-) anions, the charge distribution at the interface is heterogeneous, leading to a positive electrostatic potential at the interface with the two studied ILs. Mixing-demixing simulations on [BMI][PF6]/water mixtures are also reported, comparing Ewald versus reaction field treatments of electrostatics. Phase separation is very slow (at least 30 ns), in marked contrast with mixtures involving classical organic liquids, which separate in less than 0.5 ns at the microscopic level. The results allow us to better understand the specificity of the aqueous interfaces with hydrophobic ionic liquids, compared with classical organic solvents, which has important implications as far as the mechanism of liquid-liquid ion extraction is concerned.

Aqueous interfaces with hydrophobic room-temperature ionic liquids: a molecular dynamics study.

We report a molecular dynamics study of the solvation of the UO2(2+) and Eu3+ cations and their chloro complexes in the [BMI][PF6][H2O] "humid" room-temperature ionic liquid (IL) composed of 1-butyl-3-methylimidazolium+ and PF6- ions and H2O in a 1:1:1 ratio. When compared to the results obtained in dry [BMI][PF6], the present results reveal the importance of water. The "naked" cations form UO2(H2O)5(2+) and Eu(H2O)9(3+) complexes, embedded in a shell of 7 and 8 PF6- anions, respectively. All studied UO2Cln(2-n) and EuCln(3-n) chloro complexes remain stable during the dynamics and coordinate additional H2O molecules in their first shell. UO2Cl4(2-) and EuCl6(3-) are surrounded by an "unsaturated" water shell, followed by a shell of BMI+ cations. According to an energy component analysis, the UO2Cl4(2-) and EuCl6(3-) species, intrinsically unstable toward dissociation, are more stable than their less halogenated analogues in the IL solution, due to the solvation forces. The different chloro species also interact better with the humid than with the dry IL, which hints at the importance of solvent humidity to improve their solubility. Humidity markedly modifies the local ion environment, with major consequences as far as their spectroscopic properties are concerned. We finally compare the aqueous interface of [BMI][PF6] and [OMI][PF6] ionic liquids, demonstrating the importance of imidazolium substituents (N-butyl versus N-octyl) to the nature of the interface and miscibility with water.

Solvation of uranyl(II) and europium(III) cations and their chloro complexes in a room-temperature ionic liquid. A theoretical study of the effect of solvent "humidity".

We report a molecular dynamics study of the solvation of M3+ lanthanide cations (La3+, Eu3+ and Yb3+) in two room-temperature ionic liquids: [BMI][PF6] based on 1-butyl-3-methyl-imidazolium+,PF6− and [EMI][TCA] based on 1-ethyl-3-methyl-imidazolium+,AlCl4−. They reveal specific solvation as a function of the cation size and on the solvent. In [BMI][PF6] solution, the three studied M3+ cations are surrounded by six PF6− anions, while in [EMI][TCA] solution, they are surrounded by eight AlCl4− anions. The precise binding mode of the AlCl4− or PF6− solvent anions (denticity, number of coordinated halides and dynamics) depends on the M3+ cation size. The first shell PF6− anions rotate markedly during the dynamics while AlCl4− do not. In both liquids, this first ionic shell is surrounded by about 11–13 imidazolium cations. The solvation of the neutralizing NO3− counterions is similar in both liquids, and involves mainly imidazolium+ cations in the first shell (≈5.0 in [BMI][PF6] and 4.3 in [EMI][TCA]). When simulated in the gas phase, the first anionic shells are similar, but more rigid than in the solutions. Finally, free energy calculations are performed to compare the Eu3+/La3+ and Yb3+/Eu3+ solvation. In both ionic liquids, the smaller cations are better solvated, but the differences from one cation to the other are weaker than in water.

Solvation of M3+ lanthanide cations in room-temperature ionic liquids. A molecular dynamics investigation

The first coordination sphere of the uranyl cation in room-temperature ionic liquids (ILs) results from the competition between its initially bound counterions, the IL anions, and other anions (e.g., present as impurities or added to the solution). We present a joined spectroscopic (UV−visible and extended X-ray absorption fine structure)-simulation study of the coordination of uranyl initially introduced either as UO2X2 salts (X- = nitrate NO3-, triflate TfO-, perchlorate ClO4-) or as UO2(SO4) in a series of imidazolium-based ILs (C4mimA, A- = PF6-, Tf2N-, BF4- and C4mim = 1-methyl-3-butyl-imidazolium) as well as in the Me3NBuTf2N IL. The solubility and dissociation of the uranyl salts are found to depend on the nature of X- and A-. The addition of Cl- anions promotes the solubilization of the nitrate and triflate salts in the C4mimPF6 and the C4mimBF4 ILs via the formation of chloro complexes, also formed with other salts. The first coordination sphere of uranyl is further investigated by molecular dyna...

Uranyl Coordination in Ionic Liquids: The Competition between Ionic Liquid Anions, Uranyl Counterions, and Cl- Anions Investigated by Extended X-ray Absorption Fine Structure and UV−Visible Spectroscopies and Molecular Dynamics Simulations

Alain Chaumont

Papers

Ab initio molecular dynamics of liquid 1,3-dimethylimidazolium chloride.

Aqueous interfaces with hydrophobic room-temperature ionic liquids: a molecular dynamics study.

Solvation of uranyl(II) and europium(III) cations and their chloro complexes in a room-temperature ionic liquid. A theoretical study of the effect of solvent "humidity".

Solvation of M3+ lanthanide cations in room-temperature ionic liquids. A molecular dynamics investigation

Uranyl Coordination in Ionic Liquids: The Competition between Ionic Liquid Anions, Uranyl Counterions, and Cl- Anions Investigated by Extended X-ray Absorption Fine Structure and UV−Visible Spectroscopies and Molecular Dynamics Simulations