[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.