Химия и химические технологии/2. Теоретическая химия

Romanova K.A., Galyametdinov Yu.G.

Kazan National Research Technological University, Russia

Theoretical simulation of lanthanide(III) complexes promising for luminescent functional materials

Lanthanide (Ln) coordination compounds have a great potential as luminescent materials due to their remarkable magnetic and optical properties especially for optoelectronic devices, flat and flexible displays, organic light emitting diodes, luminescent biological probes and solar cells. Liquid-crystalline Ln(III) complexes can form optically transparent films with polarized luminescence, whose intensity can be controlled by magnetic and electric fields, temperature, and laser irradiation.

Photophysical properties of Ln(III) complexes are mainly defined by their ligand environment [1]. Organic ligands in these molecules provide the transfer of the excitation energy onto the emissive Ln(III) ion. Theoretical calculations can help one to find the ligands that guarantee the most efficient energy transfer to the Ln(III) ion and enable the design of highly efficient luminescence materials. In this study, the influence of the ligand substituents on luminescent properties, the molecular anisotropy and on the subsequent supramolecular organization of some Ln(III) complexes was invesigated. Quantum-chemical methods were applied for the simulation of equilibrium geometries, absorption, IR, NMR spectra and excited states of some Ln(III) complexes with different ligand environment.

Theoretical calculations of equilibrium geometries and IR spectra of Ln(III) complexes were performed in the gas phase using the density functional theory and the exchange-correlation functional PBE. NMR simulations were carried out by GIAO method. The energies of the lowest singlet and triplet excited states were found by TDDFT method (functionals PBE, B3LYP) in program Firefly 8. For Ln(III) ions the scalar relativistic 4f-in-core pseudopotentials with the associated valence basis sets were used (ECP52MWB for Eu(III), ECP53MWB for Gd(III), ECP54MWB for Tb(III) and ECP57MWB for Er(III)). For other atoms 6-31G(d,p) basis set was applied. Absorption spectra were calculated using SMLC model in ORCA 3.0.3 program.

It was established that the coordination polyhedrons of the studied Ln(III) complexes were a square antiprism and a dodecahedron. However, it should be mentioned that calculations do not take into account the influence of neighboring molecules. Therefore the geometric distortions that occur while molecules are packaging in crystal are eliminated. Optimization of the Ln(III) complexes was accompanied by a slight distortion of the coordination polyhedron due to the steric hindrance caused by terminal substituents in the ligands. The isomer with a crosswise arrangement of β-diketones in the complex when alkyl substitutes do not sterically hinder each other was chosen for calculations because of its lowest energy. The optimized structure of one of the studied Ln(III) complexes is shown in Fig. 1.

Fig. 1. Optimized structure and geometric parameters of one of the studied Ln(III) complexes, where l and d - length and width of the molecule, respectively

The calculated UV, IR, NMR spectra and the excited states were confirmed experimentally. Good agreement between simulated and experimental data showed that the proposed methodology allows to predict the photophysical properties of Ln(III) complexes and can be used to describe intramolecular energy transfer processes. Correlations between the positions of the excited levels and the values of absolute quantum yield were established, the main intramolecular energy transfer channels were determined. Influence of the nature of substituents on luminescence properties of the complexes and the effectiveness of their use in optoelectronics was revealed. It was revealed that β-diketones play a major role during the photoexcitation of the Ln(III) complexes since their geometry considerably changes in comparison with other ligands. Quantum-chemical simulations discovered the structural features of the Ln(III) compounds that predetermine their effective luminescence and further application as luminescence functional materials.

The calculations were performed using the facilities of the Joint Supercomputer Center of Russian Academy of Sciences and the Supercomputing Center of Lomonosov, Moscow State University [2]. This work was supported by the grant of the President of the Russian Federation for the state support of the young Russian scientists - candidates of sciences (No МК-7320.2016.3).

References:

1. Romanova K.A., Freidzon A.Ya., Bagaturyants A.A., Galyametdinov Yu.G. Ab initio study of energy transfer pathways in dinuclear lanthanide complex of europium(III) and terbium(III) ions // Journal of Physical Chemistry A. 2014. V. 118. № 47. P. 11244-11252.

2. Voevodin Vl.V., Zhumatiy S.A., Sobolev S.I., Antonov A.S., Bryzgalov P.A., Nikitenko D.A., Stefanov K.S., Voevodin Vad.V. Practice of “Lomonosov” Supercomputer // Open Systems J. 2012. V. 7. P. 36-39.