Ricardo L. Longo

Spectroscopic properties and design of highly luminescent lanthanide coordination complexes

Abstract In this paper recent advances in the development of efficient light conversion molecular devices (LCMD) based on lanthanide complexes are reviewed, with emphasis on the work of our group. We have adopted a strategy based upon both theoretical and experimental (synthesis and methodological) investigations. The theoretical aspects are described in terms of the well known theory of 4f–4f transitions and a recently developed model of intramolecular energy transfer processes in lanthanide coordination compounds. The necessary structural data (coordination geometries and electronic structures of the organic parts of the compounds) are obtained from a sparkle model also recently developed. The results lead us to achieve a better understanding of the factors determining the quantum yields and other relevant properties of these complexes, establishing the basis of a framework for the modeling of new complexes which are promising LCMDs. In addition, the fluorinated compounds, which are sufficiently volatile and thermodynamically stable, are candidates for a number of applications. We illustrate their use as LCMDs devices for sensing UV radiation (dosimeter) and as antireflection coatings (ARC) on silicon solar cells with beneficial effects on device performance.

Emission quantum yield of europium (III) mixed complexes with thenoyltrifluoroacetonate and some aromatic ligands

Abstract We report the experimental determination and theoretical calculation of the 4f–4f emission quantum yield (q) and 5D0 excited state lifetime in the Eu (TTA)3.nL compounds (where TTA=thenoyltrifluoroacetonate, L denotes 1,10-phenantroline orp-tolyl sulfoxide and n=1 or 2). The experimental q values for these compounds vary significantly by changing the L ligand. The relation between the emission quantum yield and photophysical characteristics of the compounds is discussed. The calculations are carried out by using theoretical models for ligand–rare earth ion energy transfer processes and numerical solutions of the rate equations. Pathways for the intramolecular energy transfer process between the ligands and the rare earth ion are proposed. The theoretical results agree well with the experimental data for the compounds analyzed.

Tristriazolotriazines: a core for luminescent discotic liquid crystals

The synthesis and structural, thermal, optical and theoretical characterization of new tris[1,2,4]triazolo[1,3,5]triazines were performed to support their application as liquid crystals and advanced materials.

THEORETICAL MODEL FOR THE PREDICTION OF ELECTRONIC SPECTRA OF LANTHANIDE COMPLEXES

A technique is introduced for the theoretical prediction of electronic spectra of lanthanide complexes by replacing the metal ion by a point charge with the ligands held in their positions as determined by the SMLC/AM1, and by computing the theoretical spectra via the intermediate neglect of differential overlap/spectroscopic-configuration interaction (INDO/S-CI). As a test case, we report the absorption spectrum of tris(picolinate-N-oxide)(2,2′: 6′,2″-terpyridine) of EuIII complex which has been synthesized in our laboratory. The predicted absorption spectra (complex and free ligands) compare well with the UV region experimental data. Moreover, the computed triplet energy levels display transitions near 470 and 560 nm which are due to the N-oxide and which may be relevant for the luminescence.

On the dependence of the luminescence intensity of rare-earth compounds with pressure: a theoretical study of Eu(TTF)32H2O in polymeric solution and crystalline phases

Abstract Theoretical models for ligand–rare-earth-ion energy-transfer processes and numerical solutions of the rate equations are used to analyse the dependence of the 4f–4f emission intensity and the 5 D 0 excited state lifetime with an applied external pressure, in Eu 3+ complexes. Pathways for the intramolecular energy-transfer process between the ligands and the rare-earth ion are proposed. The theoretical results agree well with the experimental data for the Eu(TTF) 3 2H 2 O compound in a polymeric solution and crystalline form.

A theoretical study of the energy-transfer process in [Eu⊂bpy.bpy.bpy]3+ cryptates: a ligand-to-metal charge-transfer state?

Abstract A complete theoretical model to calculate the luminescent properties of lanthanide coordination compounds is used to analyze the emission quantum yield of the [Eu⊂bpy.bpy.bpy] 3+ and [Eu⊂bpy.bpy.bpy] 3+ ·2H 2 O compounds. This theoretical model includes the calculation of the molecular structure, of the ligand electronic structure, of the ligand–lanthanide energy-transfer, of the temporal dependence of the ligand and lanthanide populations, which lead to the emission quantum yield, relative emission intensity and lifetime of the emitting state. In the present case, this theoretical approach was used to ascertain the presence of a ligand-to-metal charge-transfer (LMCT) state in these compounds. In addition, this approach has provided indications of the location of the LMCT state as well as of the magnitudes of the energy-transfer rates involving this state.

Dependence of the Lifetime upon the Excitation Energy and Intramolecular Energy Transfer Rates: The 5D0 EuIII Emission Case

In many Eu(III)-based materials, the presence of an intermediate energy level, such as ligand-to-metal charge transfer (LMCT) states or defects, that mediates the energy transfer mechanisms can strongly affect the lifetime of the (5)D(0) state, mainly at near-resonance (large transfer rates). We present results for the dependence of the (5)D(0) lifetime on the excitation wavelength for a wide class of Eu(III)-based compounds: ionic salts, polyoxometalates (POMs), core/shell inorganic nanoparticles (NPs) and nanotubes, coordination polymers, β-diketonate complexes, organic-inorganic hybrids, macro-mesocellular foams, functionalized mesoporous silica, and layered double hydroxides (LDHs). This yet unexplained behavior is successfully modelled by a coupled set of rate equations with seven states, in which the wavelength dependence is simulated by varying the intramolecular energy transfer rates. In addition, the simulations of the rate equations for four- and three-level systems show a strong dependence of the emission lifetime upon the excitation wavelength if near-resonant non-radiative energy transfer processes are present, indicating that the proposed scheme can be generalized to other trivalent lanthanide ions, as observed for Tb(III)/Ce(III). Finally, the proper use of lifetime definition in the presence of energy transfer is emphasized.

Theoretical modelling of the low quantum yield observed in an Eu(III) triple helical complex with a tridentate aromatic ligand

The dramatic decrease in the quantum yield of the Eu-centred luminescence recently observed in going from [Eu(NO3)3L(MeOH)] (2.8%) to the triple helical [Eu(L)3](ClO4)3 complex (8.2 × 10−5%) prompted us to perform a theoretical analysis in order to find out which parameter might be responsible for this quenching. In particular, we explore the influence of a resonance between a ligand-to-metal charge-transfer state (LMCT) and the 1ππ* and/or 3ππ* states on the emission quantum yield. A good agreement between the theoretical and experimental values is reached when high rates for non-radiative deactivation on the ligand are considered and, for the 1: 3 complex, when a LMCT state close in energy to the ligand 1ππ* state is taken into account. This type of modelling opens the way for a better predictability of the photophysical properties of luminescent europium-containing edifices.

Reaction-field–supermolecule approach to calculation of solvent effects

The self-consistent reaction-field formalism has been implemented into the AM1 and MNDO molecular structure codes and has been used to calculate the solvation energies of water and ions in aqueous solution. The interaction of nearby solvent molecules with the solute has been included explicitly by combining the reaction-field formalism with the supermolecule model. The self-consistent reaction-field method was also used to study dielectric effects on tautomeric equilibria. The agreement with the experimental results is good and indicates the usefulness of the combined reaction-field–supermolecule approach for studying solvent effects on chemical processes.

Modeling Lanthanide Complexes: Towards the Theoretical Design of Light Conversion Molecular Devices

Theoretical techniques have been developed and/or improved to predict the molecular structure of lanthanide complexes which were used to calculate their electronic properties, in particular, their electronic spectra and energy levels necessary to calculate the rates of energy transfer from the ligands to the metal ion. The molecular structure has been obtained by the SMLC/AM1 (Sparkle Model for the Calculation of Lanthanide Complexes – Austin Model 1) model where the lanthanide ion is simulated by a sparkle implemented into the AM1 Hamiltonian used to perform a HF-SCF (Hartree-Fock Self-Consistent Field) calculation. The previous implementation of the SMLC/AM1 model (sparkle/1) involving only two parameters has been generalized to be consistent with the AM1 Hamiltonian and the new model (sparkle/2) significantly improved the prediction of molecular structures of Eu(III) complexes. For the electronic spectra and energy level calculations of the lanthanide complexes the model replaces the metal ion by a point charge with the ligands held in their positions as determined by the SMLC/AM1 model, and uses a INDO/S-CI (intermediate neglect of differential overlap/spectroscopic-configuration interaction) model. A preliminary study of the solvent effects on the absorption spectra of the free ligand is also presented. For the ligand-lanthanide ion energy transfer Fermis golden rule is used with the multipolar and exchange mechanisms being implemented and tested for several complexes. These theoretical techniques have been applied to several complexes yielding very good results when compared to experimental data as well as predictions for the molecular and electronic structures and the relative contributions of the mechanisms for the energy transfer rates.