FIGURE 5 Shapes of the VDP of the optimized Eu(III) complexes: (а) Eu(CPDK3-5)3Bpy17-17, (b) Eu(CPDK3-5)3Phen, (c) Eu(CPDK5­Th)3Bpy17-17, (d) Eu(CPDK5-Th)3Phen, (e) Eu(CPDK3­Ph)3Phen, (f) Eu(DK12-14)3Bpy17-17
The asymmetry of the nearest environment of Eu(III) can be qualitatively evaluated by the G3 parameter which is the normalized second moment of inertia of the VDP.[71] This parameter determines the degree of nonsphericity or distortion of the coordination shell and considers the shape of the molecule, its chemical composition, and intermolecular interactions between the molecule and its environment. High degree of sphericity of the central atom’s environment is characterized by small values of G3 , while high values mean significant asymmetry in the arrangement of ligands.
According to the calculated data (Table 2), the optimized geometries of Eu(III) complexes in the ground state have similar values ofG3 in the range 0.0812-0.0828, while in the triplet excited state this parameter varies from 0.0811 to 0.0838. Typical G3 values for Ln(III) with a simpler ligand environment are in the range 0.081-0.085.[70,72] The lowestG3 parameters for optimized ground state structures correspond to Eu(DK12-14)3Bpy17-17(0.0812) and Eu(CPDK3-5)3Phen (0.0813). The replacement of CPDK3-5 in this complex 2 by CPDK3-Ph and CPDK5-Th in complexes4 and 5 leads to a slight increase ofG3 to 0.0820 and 0.0821 and, apparently, to a greater asymmetry in the arrangement of ligands. This effect can be explained by conjugation between С6H5- substituents in CPDK3-Ph and С4H3S- in CPDK5-Th.
When triplet excitation is localized on β-diketones,G3 consistently increases (Table 2) due to a decrease in the uniformity of the molecule’s structure, significant distortions of coordination polyhedra, and thermal vibrations of molecules upon photoexcitation. A more significant increase inG3 is observed for complexes 2 ,4 and 5 with Phen. Complexes 3 and 6with CPDK5-Th and DK12-14 have the highest G3 values - 0.0833 and 0.0838, while the lowest G3 value (0.0811) corresponds to complexes2 and 6 with the localization of the excitations on Lewis bases.
For optimized structures with the excited state localization on Lewis base, G3 parameter slightly decreases in comparison with the ground state. This indicates insignificant changes in the geometry of Eu(III) complexes and small distortions of coordination polyhedra in photophysical processes involving Bpy17-17 and Phen. Thus, photoexcitation of β-diketones leads to a greater change in the coordination sphere of the Eu(III) complexes. Previously it was shown that the localization of the triplet excitation on β-diketones causes more significant changes in structural parameters in comparison with Lewis bases. Consequently, while the structure of the Lewis base regulates the LC properties of mesogenic Eu(III) complexes, the choice of a certain β­diketone significantly affects their absorbance and emission efficiency.
The volume of VDP (VVDP ) is related to the valence state of the central atom, the nature and electronegativity of ligand atoms in polyhedron.[69,70,73] All optimized ground state structures of Eu(III) complexes have similar VDP volumes (Table 2) due to identical atoms (six O from β-diketones and two N from the Lewis base) that form bonds with the Eu(III) ion in similar ligands.
Upon photoexcitation, an increase in VVDP is observed by 4% for triplet state localization on Lewis bases and by 4-7% for β-diketone localized excited states. Such changes indicate significant distortions in the geometry of the polyhedra.
The radius R of a certain sphere with a volume equal to the volume of the VDP describes the state of the central atom in a certain environment. R is constant for an ion in the same oxidation state, surrounded by atoms of the same type.[74]Therefore, its values are very similar for all the studied Eu(III) complexes in the ground state (Table 2). Since R also correlates with the energy of intermolecular interactions between molecule and its environment, it increases upon photoexcitation. Some of the highest values are observed for triplet excitation on β-diketones in complexes2 and 4 , which do not have LC properties. Significant distortions in the coordination polyhedron of Eu(DK12­14)3Bpy17­17with three branchy β-diketones result in notable VDP’ volume of 14.18 Å3 and radius of 1.896 Å.
Therefore, luminescence, LC and magnetic properties of the Ln(III) complexes are determined not only by the ligand environment. Such factors as Ln(III) ions’ nature, interactions between ions and ligands, the crystal field potential and the type of polyhedra can make a big difference on their behaviour. The relationship between the type of the coordination polyhedron of various Ln(III) complexes and their room-temperature magnetic anisotropy (the difference between magnetic susceptibilities parallel and perpendicular to the magnetic field director) was reported.[10] Authors noticed the influence of the Ln(III) ion nature and the degree of the distortion of high-symmetry polyhedra on the sign and magnitude of the magnetic anisotropy. Since the magnetic susceptibility of high-symmetry polyhedra is isotropic their magnetic anisotropy is zero in the absence of distortions. Therefore low-symmetry polyhedra of the studied mesogenic Eu(III) complexes may result in significant magnetic anisotropy at room temperature and easy alignment even in a weak in external magnetic field.