Theoretical and experimental spectroscopic investigation of new Eu(III)-FOD complex containing 2-pyrrolidone ligand.

The preparation and photoluminescent properties of the new [Eu(FOD)3(2-Pyr)2] complex (FOD = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadionate; 2-Pyr = 2-pyrrolidone) are reported. The obtained complex was characterized by elemental analysis, complexometric titration using EDTA, infrared spectroscopy, and single-crystal X-ray diffraction studies. The coordination polyhedron of the complex is described as a distorted square antiprismatic with both 2-Pyr monodentate ligands coordinated to Eu(III) via the oxygen atoms, in neutral form, while the three FOD molecules are coordinated in the anionic form. Structural modeling at the PBE1PBE/SVP/MWB52 level of theory provided a geometry in excellent agreement with the one obtained experimentally. Spectroscopy properties such as intensity parameters (Ω2 and Ω4), radiative emission rate (Arad), and chemical partition of Arad for [Eu(FOD)3(2-Pyr)2] and [Eu(FOD)3(H2O)2] were calculated by using the QDC model with help of the semiempirical wavefunctions. The modeling of the ligand-to-metal energy transfer for both complexes was performed, allowing to obtain the theoretical emission quantum yield and to characterize the most relevant molecular orbitals involved.

Lanthanide Contraction in Lanthanide Organic Frameworks: A Theoretical and Experimental Study.

In this work, the lanthanide (Ln) contraction phenomenon has been analyzed for three-dimensional structures in the solid state. We chose to study an isostructural series of lanthanide organic frameworks (LOFs) of formula [Ln2(C4H4O4)3(H2O)2]n·H2O and 14 crystallographic structures (except promethium complex). The analysis of Ln contraction was made by analyzing the sum of all Ln-O bond lengths and the sum of all O-O distances, for the oxygen atoms of the coordination polyhedra, calculated with different semiempirical quantum mechanical models. The ∑Ln-O and ∑O-O for this LOF can be fit to a second-order polynomial. Based on the crystallographic structures, it is concluded that the phenomenon of Ln contraction is observed. Our results also suggest that the semiempirical Sparkle/PM3 and Sparkle/RM1 models reproduce the Ln contraction phenomenon well, and similar fits were obtained for ∑Ln-O and ∑O-O bond lengths.

Parameters for the RM1 Quantum Chemical Calculation of Complexes of the Trications of Thulium, Ytterbium and Lutetium.

The RM1 quantum chemical model for the calculation of complexes of Tm(III), Yb(III) and Lu(III) is advanced. Subsequently, we tested the models by fully optimizing the geometries of 126 complexes. We then compared the optimized structures with known crystallographic ones from the Cambridge Structural Database. Results indicate that, for thulium complexes, the accuracy in terms of the distances between the lanthanide ion and its directly coordinated atoms is about 2%. Corresponding results for ytterbium and lutetium are both 3%, levels of accuracy useful for the design of lanthanide complexes, targeting their countless applications.

Chemical Partition of the Radiative Decay Rate of Luminescence of Europium Complexes.

The spontaneous emission coefficient, Arad, a global molecular property, is one of the most important quantities related to the luminescence of complexes of lanthanide ions. In this work, by suitable algebraic transformations of the matrices involved, we introduce a partition that allows us to compute, for the first time, the individual effects of each ligand on Arad, a property of the molecule as a whole. Such a chemical partition thus opens possibilities for the comprehension of the role of each of the ligands and their interactions on the luminescence of europium coordination compounds. As an example, we applied the chemical partition to the case of repeating non-ionic ligand ternary complexes of europium(III) with DBM, TTA, and BTFA, showing that it allowed us to correctly order, in an a priori manner, the non-obvious pair combinations of non-ionic ligands that led to mixed-ligand compounds with larger values of Arad.

Europium Luminescence: Electronic Densities and Superdelocalizabilities for a Unique Adjustment of Theoretical Intensity Parameters.

We advance the concept that the charge factors of the simple overlap model and the polarizabilities of Judd-Ofelt theory for the luminescence of europium complexes can be effectively and uniquely modeled by perturbation theory on the semiempirical electronic wave function of the complex. With only three adjustable constants, we introduce expressions that relate: (i) the charge factors to electronic densities, and (ii) the polarizabilities to superdelocalizabilities that we derived specifically for this purpose. The three constants are then adjusted iteratively until the calculated intensity parameters, corresponding to the (5)D0→(7)F2 and (5)D0→(7)F4 transitions, converge to the experimentally determined ones. This adjustment yields a single unique set of only three constants per complex and semiempirical model used. From these constants, we then define a binary outcome acceptance attribute for the adjustment, and show that when the adjustment is acceptable, the predicted geometry is, in average, closer to the experimental one. An important consequence is that the terms of the intensity parameters related to dynamic coupling and electric dipole mechanisms will be unique. Hence, the important energy transfer rates will also be unique, leading to a single predicted intensity parameter for the (5)D0→(7)F6 transition.

RM1 Semiempirical Quantum Chemistry: Parameters for Trivalent Lanthanum, Cerium and Praseodymium.

The RM1 model for the lanthanides is parameterized for complexes of the trications of lanthanum, cerium, and praseodymium. The semiempirical quantum chemical model core stands for the [Xe]4fn electronic configuration, with n =0,1,2 for La(III), Ce(III), and Pr(III), respectively. In addition, the valence shell is described by three electrons in a set of 5d, 6s, and 6p orbitals. Results indicate that the present model is more accurate than the previous sparkle models, although these are still very good methods provided the ligands only possess oxygen or nitrogen atoms directly coordinated to the lanthanide ion. For all other different types of coordination, the present RM1 model for the lanthanides is much superior and must definitely be used. Overall, the accuracy of the model is of the order of 0.07Å for La(III) and Pr(III), and 0.08Å for Ce(III) for lanthanide-ligand atom distances which lie mostly around the 2.3Å to 2.6Å interval, implying an error around 3% only.

Unusual photoluminescence properties of the 3D mixed-lanthanide-organic frameworks induced by dimeric structures: a theoretical and experimental approach.

The present work describes a complementary experimental and theoretical investigation of the spectroscopic properties of the four isostructural 3D Ln-MOFs (wherein PDC = pyrazole-3,5-dicarboxylate, [La2(PDC)3(H2O)4]·2H2O (1), [(La0.9Eu0.1)2(PDC)3(H2O)4]·2H2O (2), [(La0.9Tb0.1)2(PDC)3(H2O)4]·2H2O (3) and [(La0.9Eu0.5Tb0.5)2(PDC)3(H2O)4]·2H2O (4)). The experimental data and theoretical calculations show that the singular photophysical properties presented by these Ln-MOFs are induced by strong interaction between the Ln(3+) ions.

LUMPAC lanthanide luminescence software: Efficient and user friendly.

In this study, we will be presenting LUMPAC (LUMinescence PACkage), which was developed with the objective of making possible the theoretical study of lanthanide-based luminescent systems. This is the first software that allows the study of luminescent properties of lanthanide-based systems. Besides being a computationally efficient software, LUMPAC is user friendly and can be used by researchers who have no previous experience in theoretical chemistry. With this new tool, we hope to enable research groups to use theoretical tools on projects involving systems that contain lanthanide ions.

Semiempirical quantum chemistry model for the lanthanides: RM1 (Recife Model 1) parameters for dysprosium, holmium and erbium.

Complexes of dysprosium, holmium, and erbium find many applications as single-molecule magnets, as contrast agents for magnetic resonance imaging, as anti-cancer agents, in optical telecommunications, etc. Therefore, the development of tools that can be proven helpful to complex design is presently an active area of research. In this article, we advance a major improvement to the semiempirical description of lanthanide complexes: the Recife Model 1, RM1, model for the lanthanides, parameterized for the trications of Dy, Ho, and Er. By representing such lanthanide in the RM1 calculation as a three-electron atom with a set of 5 d, 6 s, and 6 p semiempirical orbitals, the accuracy of the previous sparkle models, mainly concentrated on lanthanide-oxygen and lanthanide-nitrogen distances, is extended to other types of bonds in the trication complexes' coordination polyhedra, such as lanthanide-carbon, lanthanide-chlorine, etc. This is even more important as, for example, lanthanide-carbon atom distances in the coordination polyhedra of the complexes comprise about 30% of all distances for all complexes of Dy, Ho, and Er considered. Our results indicate that the average unsigned mean error for the lanthanide-carbon distances dropped from an average of 0.30 Å, for the sparkle models, to 0.04 Å for the RM1 model for the lanthanides; for a total of 509 such distances for the set of all Dy, Ho, and Er complexes considered. A similar behavior took place for the other distances as well, such as lanthanide-chlorine, lanthanide-bromine, lanthanide, phosphorus and lanthanide-sulfur. Thus, the RM1 model for the lanthanides, being advanced in this article, broadens the range of application of semiempirical models to lanthanide complexes by including comprehensively many other types of bonds not adequately described by the previous models.

RM1 Model for the Prediction of Geometries of Complexes of the Trications of Eu, Gd, and Tb.

All versions of our previous Sparkle Model were very accurate in predicting lanthanide-lanthanide distances in complexes where the two lanthanide ions directly face each other, and mainly lanthanide-oxygen, and lanthanide-nitrogen distances, which are by far the most common ones in lanthanide complexes. In this article, we are advancing for the first time the RM1 model for lanthanides. Designed to be a much more general NDDO model, the RM1 model for lanthanides is capable of predicting geometries of lanthanide complexes for the cases when the central lanthanide trication is directly coordinated to any other atoms, not only oxygen or nitrogen. The RM1 model for lanthanides is defined by three important attributes: (a) the orbitals, the lanthanide ion has now three electrons and a NDDO basis set made of 5d, 6s, and 6p functions; (b) the parametrization, via cluster analysis and an adequate sampling; and (c), the statistical validation of the parameters to make sure the errors behave as random around a mean. All three aspects are described in detail in the article. Results indicate that the RM1 model does extend the accuracy of the previous Sparkle Models to types of coordinating bonds other than Ln-O and Ln-N; the most common ones for Eu, Gd, and Tb, being Ln-C, Ln-S, Ln-Cl, and Ln-Br. Overall, these other coordinating bonds are now predicted within 0.06 Å of their correct values. Therefore, the RM1 model here presented is capable of predicting geometries of lanthanide complexes, materials, metal-organic frameworks, etc., with useful accuracy.

Synthesis, crystal structure and luminescence properties of the Ln(III)-picrate complexes with 1-ethyl-3-methylimidazolium as countercations.

New anionic complexes of lanthanide picrates containing 1-ethyl-3-methylimidazolium (EMIm) as countercation have been prepared. The Ln(III) complexes were characterized by complexometric titration, elemental analyses, infrared spectroscopy and molar conductivity. The molecular structures of the (EMIm)2[Ln(Pic)4(H2O)2]Pic complexes, Ln(III)=Sm, Eu, Gd and Tb, and Pic=picrate, were determined by X-ray crystallography. In these structures the picrate anion appears in three forms: as uncoordinated counteranion, as monodentated and bidentate ligand. The coordination polyhedron around the Ln(III) atom can be described as tricapped trigonal prismatic molecular geometry. The theoretical molecular structures of the complexes were also calculated using the Sparkle/PM3 model for Ln(III) complexes, allowing analysis of intramolecular energy transfer processes of the Eu(III) compound. The spectroscopic properties of the 4f(6) intraconfigurational transitions of the Eu(III) complex were then studied experimentally and theoretically. The low value of emission quantum efficiency of (5)D0 emitting level (η) of Eu(III) ion (ca. 10%) is due to the vibrational modes of the water molecule that act as luminescence quenching. In addition, the luminescence decay curves, the experimental intensity parameters (Ωλ), lifetimes (τ), radiative (Arad) and non-radiative (Anrad) decay rates, theoretical quantum yield (q) were also determined and discussed.

Theoretical methodologies for calculation of Judd-Ofelt intensity parameters of polyeuropium systems.

When Judd-Ofelt intensity parameters of polynuclear compounds with asymmetric centers are calculated using the current procedure, the results are inconsistent. The problem arises from the fact that the experimental intensity parameters cannot be determined for each asymmetric polyhedron, and this precludes the individual theoretical adjustment. In this study, we then propose three different methods for calculation of these parameters of polyeuropium systems. The first, named the centroid method, proposes the calculation considering the center of the dimeric system as the half distances between the two europium centers. The second method, called the overlapped polyhedra method, proposes the calculation considering the overlapping of both europium polyhedra, and the last one, the individual polyhedron method, proposes the use of theoretical mean values of charge factors and polarizabilities associated with each europium-ligand atom bond to calculate the intensity parameters. One symmetric polyeuropium system and one asymmetric system were assessed by using the three methods. Among the methods assessed, the one based on the overlapped polyhedra produced more consistent results for the study of both kinds of systems.

Sparkle/PM7 Lanthanide Parameters for the Modeling of Complexes and Materials.

The recently published Parametric Method number 7, PM7, is the first semiempirical method to be successfully tested by modeling crystal structures and heats of formation of solids. PM7 is thus also capable of producing results of useful accuracy for materials science, and constitutes a great improvement over its predecessor, PM6. In this article, we present Sparkle Model parameters to be used with PM7 that allow the prediction of geometries of metal complexes and materials which contain lanthanide trications. Accordingly, we considered the geometries of 224 high-quality crystallographic structures of complexes for the parameterization set and 395 more for the validation of the parameterization for the whole lanthanide series, from La(III) to Lu(III). The average unsigned error for Sparkle/PM7 for the distances between the metal ion and its coordinating atoms is 0.063Å for all lanthanides, ranging from a minimum of 0.052Å for Tb(III) to 0.088Å for Ce(III), comparable to the equivalent errors in the distances predicted by PM7 for other metals. These distance deviations follow a gamma distribution within a 95% level of confidence, signifying that they appear to be random around a mean, confirming that Sparkle/PM7 is a well-tempered method. We conclude by carrying out a Sparkle/PM7 full geometry optimization of two spatial groups of the same thulium-containing metal organic framework, with unit cells accommodating 376 atoms, of which 16 are Tm(III) cations; the optimized geometries were in good agreement with the crystallographic ones. These results emphasize the capability of the use of the Sparkle Model for the prediction of geometries of compounds containing lanthanide trications within the PM7 semiempirical model, as well as the usefulness of such semiempirical calculations for materials modeling. Sparkle/PM7 is available in the software package MOPAC2012, at no cost for academics and can be obtained from http://openmopac.net.

Synthesis and characterization of the europium(III) pentakis(picrate) complexes with imidazolium countercations: structural and photoluminescence study.

Six new lanthanide complexes of stoichiometric formula (C)(2)[Ln(Pic)(5)]--where (C) is a imidazolium cation coming from the ionic liquids 1-butyl-3-methylimidazolium picrate (BMIm-Pic), 1-butyl-3-ethylimidazolium picrate (BEIm-Pic), and 1,3-dibutylimidazolium picrate (BBIm-Pic), and Ln is Eu(III) or Gd(III) ions--have been prepared and characterized. To the best of our knowledge, these are the first cases of Ln(III) pentakis(picrate) complexes. The crystal structures of (BEIm)(2)[Eu(Pic)(5)] and (BBIm)(2)[Eu(Pic)(5)] compounds were determined by single-crystal X-ray diffraction. The [Eu(Pic)(5)](2-) polyhedra have nine oxygen atoms coordinated to the Eu(III) ion, four oxygen atoms from bidentate picrate, and one oxygen atom from monodentate picrate. The structures of the Eu complexes were also calculated using the sparkle model for lanthanide complexes, allowing an analysis of intramolecular energy transfer processes in the coordination compounds. The photoluminescence properties of the Eu(III) complexes were then studied experimentally and theoretically, leading to a rationalization of their emission quantum yields.

Theoretical spectroscopic study of europium tris(bipyridine) cryptates.

A series of europium cryptates are studied, using semiempirical methods to predict electronic and spectroscopic properties. The results are compared with theoretical (DFT) and experimental results published by Guillaumont and co-workers (ChemPhysChem2007, 8, 480). Triplet energies calculated by semiempirical methods have errors similar to those obtained by TD-DFT methodology but hundreds of times faster. Moreover, the semiempirical results not only reproduce well the experimental values but also help explain the low values of quantum efficiency observed for these complexes.