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.