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.