4 CONCLUSIONS

Quantum-chemical simulation of six mesogenic Eu(III) complexes with various substituted β­diketones and Lewis bases, Eu(CPDK3-5)3Bpy17-17(1), Eu(CPDK3-5)3Phen (2), Eu(CPDK5­Th)3Bpy17­17(3), Eu(CPDK5­Th)3Phen (4), Eu(CPDK3­Ph)3Phen (5) and Eu(DK12-14)3Bpy17-17 (6) allowed us to determine relationships between their molecular structure, the anisotropy of geometry, LC properties and luminescence efficiency.
Optimization of the geometry of triplet excited states led to the localization of excitation on individual ligands and significant geometric changes in the corresponding ligands. The calculated values agree well with experimental data. It was found that, due to the separate localization of excitations on individual ligands in Eu(III) complex, the calculated excited states are practically independent on the presence of other ligands in the complex. On the basis of the calculated triplet excited states and intramolecular energy transfer rates, the main channels of intramolecular energy transfer in Eu(III) complexes were determined and their luminescence efficiency and quantum yields were estimated.
It is found that, in the case of Bpy17-17-localized excitations, energy transfer occurs from the triplet level of the ligand to the 5D2 level of Eu(III). From the Phen-localized triplet states, excitation energy transfers to the5D1 multiplet of Eu(III), as well as in the case of β-diketones CPDK3­5 and CPDK3-Ph. According to experimental studies,[66-68] the highest emission efficiency usually corresponds to Eu(III) complexes when energy transfer occurs from ligands to the 5D1 level, which is located above the 5D0 emitting level. In our case, complexes 2 and 5 with Phen best fit this experimental rule. Furthermore, the more rigid geometry of Phen ligand compared to Bpy17-17 increases the emission efficiency of complexes with Phen by minimizing the contribution of nonradiative deactivation to the energy transfer process. At the same time, complexes 1 and 3 with Bpy17-17 have the highest anisotropy parameters above the LC properties demonstration threshold[8,9,11,27] due to alkyl substituents in Lewis base and β­diketones.
The parameters of VDP of the studied Eu(III) complexes in the optimized geometries of their ground and triplet excited states were calculated and evaluated. The smallest G3 parameters of about 0.0817 and 0.0813 were obtained for complexes 1 and 2 with CPDK3­5 in the ground state. Complex 3 has the highestG3 value of 0.0828 possibly due to the conjugation between С4H3S- heterocycles in CPDK5­Th. High values of G3determine the asymmetry in the arrangement of ligands and characterize a decrease of directed intermolecular interactions. For Eu(III) complexes 4 and 5 that results in the absence of LC behavior.
The changes in the coordination polyhedra of the complexes during their photoexcitation were identified. The localization of the triplet excitation on β-diketones results in the increase of the G3 parameter in comparison with the ground state. This indicates significant distortions of the geometry of polyhedra and asymmetry in the arrangement of ligands. The optimization of the geometry of excited states also lead to more significant changes in the geometry of β-diketones compared to the excitations on Lewis bases. Such low-symmetry polyhedra may further result in significant magnetic anisotropy and easy alignment of the studied complexes even in a weak external magnetic field. On the other side the localization of the excitation on the Lewis base lead to a slightly decrease of the G3 parameter. This is can be explained by insignificant changes in the geometry of Bpy17-17- or Phen-localized excitations and small distortions in the coordination polyhedron. The more significant increase in G3 was observed for complexes with Phen which do not exhibit LC properties. Significant asymmetry of polyhedra and lower forces of intermolecular interactions apparently lead to the absence of LC properties for complexes with Phen. Therefore, while the structure of the Lewis base regulates the presence of LC properties in mesogenic Eu (III) complexes, the selection of a certain β­diketone will significantly affect the efficiency of light absorption. The calculated IR spectra, triplet excited states, and geometric anisotropy of the Eu(III) complexes are in good agreement with the corresponding experimental data. This confirms the revealed dependencies and the adequacy of the chosen simulation technique.
Therefore, the main factors (Ln(III), coordination polyhedra, ligands and their substitutes) influencing the multifunctional behavior of the studied metallomesogens in the excited state were established. According to the described dependences and results, their LC properties mostly depend on the ligand environment, the magnetic properties are determined by the structural features of the coordination polyhedra, and both factors can affect the luminescence efficiency. Although the described effects were observed for six exemplary mesogenic Eu(III) complexes with β­diketones and Lewis bases, the proposed simulation methodology has a wide scope. It can be effectively used to study other components of multifunctional materials, including their structure and luminescence efficiency, to predict liquid-crystalline properties and magnetic anisotropy of metal containing LC or supramolecularly organized compounds with promising optical and magnetic properties.