Химия и химические технологии/2. Теоретическая химия

Romanova K.A., Galyametdinov Yu.G.

Kazan National Research Technological University, Russia

Quantum-chemical study of mono- and binuclear lanthanide(III) complexes for optoelectronic devices

Remarkable emission properties of lanthanide (Ln) complexes (significant lifetime values, large Stokes shifts, narrow emission bands) can be explained by the characteristic f-f transitions in the inner 4f shell of Ln(III) ions. The own absorbance of ions is very weak and the emission efficiency of Ln(III) complexes is mainly defined by its strongly absorbing ligands. Ligands provide the transfer of the excitation energy from own triplet excited levels to the resonant levels of the ion. Dinuclear Ln complexes possess advanced emission efficiency in comparison with complexes with one ion and are used to enhance the luminescence of a certain ion.

Computer simulations can help one to find the ligands that provide the most efficient energy transfer and predict luminescence materials with low energy loss and high radiation efficiency. In this work, quantum-chemical methods for the simulation of the structure and photophysical properties of mono- and dinuclear Ln(III) complexes were selected and the features of their luminescence properties were determined.

Theoretical calculations of equilibrium geometries of Ln(III) complexes were performed in the gas phase using the density functional theory and the exchange-correlation functional PBE. 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 (ECPs) with the associated valence basis sets were used. For other atoms 6-31G(d,p) basis set was applied.

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 structures of some of the studied Ln(III) complexes are shown in Fig. 1.

Fig. 1. The optimized structures of some of the studied Ln(III) complexes

The calculations of the lowest singlet and triplet excited states showed that excitation can be localized on each of the ligands. On the basis of the calculated excited states, the main channels of intramolecular energy transfer were determined and correlations between the positions of the excited levels and the values of absolute quantum yield were established. The theoretical results agree well with experimental data therefore the proposed methodology allows to predict the photophysical properties of Ln(III) complexes and can be used to describe intramolecular energy transfer processes. It is found that, in spite of a significant difference in the luminescence spectra of the Ln(III) ions, their excited states slightly depend on the nature of the Ln(III) ion and on the presence of other ligands.

A computational approach for the simulation of dinuclear Ln(III) complexes was developed, according to which the complex can be divided into small functional fragments to which the ab initio methods are applicable (Fig. 1). It follows from the calculated results that the proposed theoretical technique can be used for the directional variation and regulation of Ln(III) complexes photophysical properties for their further usage in organic electronics and as fluorescent sensors.

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.