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.