[a] 1 -
Eu(CPDK35)3Bpy17-17; 2
- Eu(CPDK35)3Phen; 3 -
Eu(CPDK5-Th)3Bpy17-17; 4
- Eu(CPDK5Th)3Phen; 5 -
Eu(CPDK3-Ph)3Phen; 6 -
Eu(DK12-14)3Bpy17-17; 7
-
(2,2’-Bipyridyl)-tris(4,4,4-trifluoro-1-phenyl-1,3-butanedionato)-europium(III);[49]8 -
(2,2’-Bipyridyl)-tris(2-thienoyltrifluoroacetonato)-europium(III);[50]9 –
Tris(Di-benzoylmethanido)-(o-phenanthroline-N,N’)-europium(III).[51][b] OEuO is the average angle between Eu(III) and O centers in a
single β-diketone. [c] NEuO is the average angle between N centers,
Eu(III) ions, and the nearest O center. [d] Separate excited state
localization on βdiketones (top line) and Lewis bases (bottom line).
It was necessary to simulate equilibrium geometries of complexes in the
ground state by high-level quantum-chemical methods to ensure a
sufficiently high reliability of the calculated structural data. The
obtained structures were further applied for the optimization of
excited-state geometries, the study of their correlations with
parameters of polyhedra and intramolecular energy transfer pathways. The
calculated IR spectra, NMR chemical shifts and subsequent energies of
excited states are in good agreement with experimental data, which can
confirm an accuracy of the selected model polyhedra and a successful
optimization of complex geometries. Some examples of the calculated
vibrational modes and chemical shifts compared with experimental data
can be found in the Supporting Information (Tables S2 and S3). Chemical
shifts were simulated with a relative error of 3% compared to
experimental measurements.[8,9,30,37]
One of the main factors that allows one to divine LC properties prior to
the synthesis of compounds is the values of anisotropy of geometry. This
parameter can be calculated as the ratio between the long (l ) and
the short (d ) inertial axes of the molecule (l /d )
(Figure 2). It changes from 4 to 8 for organic liquid crystals. Ligands
with long alkyl substituents increase this ratio for LC Ln(III)
complexes in comparison with other Ln(III) complexes to maximum value of
3.5.[8,9,11,27] According to our earlier studies
and calculations,[8,9,11,27] the minimum value of
the anisotropy parameter at which mesogenic Ln(III) complexes have LC
properties equals 2.5. In this study, the anisotropy of geometry equals
2.81 and 2.52 for
Eu(CPDK35)3Bpy17-17 and
Eu(CPDK5Th)3Bpy17-17(Table 1), respectively. For
Eu(CPDK35)3Phen,
Eu(CPDK5Th)3Phen, and
Eu(CPDK3Ph)3Phen this parameter reaches
the values of 1.95, 2.39 and 2.09. According to experimental studies of
these compounds,[9,30,56] only complexes with
Bpy17-17 exhibit LC properties. Upon the heating,
Eu(CPDK3-5)3Bpy17-17sequentially shows transitions to smectic and nematic mesophases, while
Eu(CPDK5Th)3Bpy17-17has only a smectic mesophase. In optimized structures (Table 1), the
greatest contribution to the anisotropy parameter is made by
Bpy17-17 and terminal alkyl substituents in βdiketones.
Complexes 1 and 3 have the same Lewis base and similarl values, which significantly decrease for complexes 2and 4 when Bpy17-17 is replaced with Phen.
Substitution of CPDK3-5 in complex 1 by
CPDK5-Th in 3 has a greater effect on thed value due to bulky substituents
C4H3S- in CPDK5-Th. The
largest d value for
Eu(CPDK3-5)3Phen in comparison with
other complexes with Phen leads to the smallest l /d ratio
1.95. Therefore, complexes 1 and 3 have an appropriatel /d ratio that is above threshold of 2.5. This leads to
the appearance of mesomorphism, while the complexes with Phen have anl /d value below 2.5 and no LC properties. Complex6 shows smectic mesomorphism and the smallest anisotropy
parameter (2.19) among studied LC complexes with
Bpy17-17 due to long alkyl substituents in βdiketone
DK12-14. The greater substituent’ length in
DK12-14 compared to other βdiketones increases both the
length and the width of this molecule.
One of the pioneer studies of mesogenic rare-earth complexes with Schiff
base ligands[57] showed that the chain length in
ligands has a little influence on the first coordination sphere of
Ln(III) ion but determines the packing of complexes in the crystal
structure. However, these long alkyl chains affect the geometry of the
ligands during photoexcitation and the efficiency of the subsequent
photophysical processes.