1 INTRODUCTION

One of the most desirable findings for materials scientists are the correlations between properties of materials and their components’ molecular structure according to the reliably studied relationships. Quantum-chemical simulation offers promising and reliable instruments to succeed in these tasks, especially useful in the case of lanthanide(III) (Ln(III)) based materials and their mesogenic representatives. Prospective liquid-crystalline (LC) behavior and unique physico-chemical properties determine the application of mesogenic Ln(III) complexes in optoelectronics, photonics and biomedicine, as biomarkers and contrast agents in tomography, components of solar cells, optical amplifiers and fibers, diodes, information storage and many other photonic devices.[1-4] Distinctive f ­ftransitions in the inner 4f shell of Ln(III) ion define their high luminescence intensity, narrow emission bands, significant lifetimes of excited states and Stokes shifts.[5,6] Their emission efficiency is firstly determined by the nature of the central ion and the intramolecular energy transfer between excited states of ligands and Ln(III) (ʺantenna effectʺ). Thermostable and polymorphic mesogenic Ln(III) complexes can be also easily oriented by weak external magnetic or electric fields.[7-9] Their magnetic properties and significant anisotropy of the magnetic susceptibility depend on the type of the coordination polyhedron and the nature of the Ln(III) ion.[10] Europium(III) (Eu(III)) mesogenic complexes exhibit increased anisotropy of the magnetic susceptibility, large spins and strong electron correlation, effective optical properties, high emission intensity and density of excited states.[7-11]
The presence of long alkyl substituents in mesogenic Ln(III) complexes make it difficult to experimentally obtained their molecular structure and to grow a single crystal for X-ray diffraction analysis.[7-9,11] Consequently, quantum-chemical simulation is one of the main opportunities to study this compounds and to predict structures with improved properties. Though most of the quantum-chemical studies of Ln(III)-containing compounds refer to the simulation of non-LC molecules with a much simpler ligand environment than in mesogenic Ln(III) complexes.[12-26]
Semi-empirical Sparkle model was specially constricted for the simulations of the equilibrium geometry of Ln(III) complexes, their IR spectra or optical properties.[12,13] It allows one to perform fast calculations with potentially overestimated results due to the strong dependence of the parameterization method. The study of quasi-degenerate excited states of Ln(III) complexes, complicated conical intersections can be performed with reliable multireferenceab initio methods. These approaches process singlet and triplet states with equal accuracy for further simulation of spin-orbit coupling, rate constants and quantum yields.[14-16] They were effectually used for the study of the emission efficiency of binuclear Ln(III) complexes without LC properties,[16] energy transfer processes,[17] luminescence mechanisms of Ln(III)-doped phosphors,[18] zero-field splitting calculations in the ground state,[18,19] and excited states of Ln(III) complexes with much simpler ligand environment than in mesogenic compounds.[14,23-26]Nevertheless, such qualitatively correct and accurate approaches are much more computationally expensive than semi-empirical and density functional theory (DFT) methods especially for the study of polyatomic mesogenic Ln(III) complexes. Therefore, it becomes clear why DFT with its time-dependent variant TDDFT are often used in theoretical studies of Ln(III) compounds for their aqua-complexes,[20,21] IR spectra calculations,[22] magnetic and optical behaviour in Ln-doping systems,[23] study of mechanisms of intramolecular energy transfer and excited state simulations,[8,11,17,24-26] etc. In our recent work ab initio molecular dynamics together with DFT were used to study inter- and intramolecular interactions in mesophases, the supramolecular organization and LC behavior of La(III) complexes.[27]
In this work, we studied mesogenic Eu(III) complexes that show specific LC behavior, low viscosity smectic and nematic mesophases in a wide temperature range in combination with unique photophysical and magnetic properties.[7-9,11] We optimized the geometry of complexes in the ground and triplet excited states. The calculated values of geometric anisotropy allowed us to evaluate their LC behavior depending on the ligand environment. The calculated energies of the lowest triplet excited states and parameters of VDP in the optimized geometries of the excited states were used for characterization of their intramolecular energy transfer and correlation between polyhedra structure, magnetic behavior and emission efficiency.