Smectic Mesomorphism and Photoluminescence of a Series of Lanthanide Mesogens

A series of new coordination compounds of tris(β-diketonates) of lanthanides(III) with 1,10-phenanthroline containing anisometric ligands with long hydrocarbon substituents was synthesized and characterized. The composition and structure of the complexes were confirmed by elemental analysis and mass spectrometry. By polarized optical microscopy, it was established that the complexes of Eu(III), Gd(III), Tb(III), Ho(III), and Er(III) exhibit smectic C mesomorphism with a mesophase existence temperature range of 18–35°C, whereas Pr(III), Nd(III), Sm(III), and Yb(III) complexes do not exhibit liquid-crystalline properties. A model of molecular packing in the smectic C phase was proposed based on small-angle X-ray scattering data. The spectral-luminescent properties of the obtained coordination compounds were studied in the visible and near-infrared ranges. For the Eu(III), Sm(III), and Tb(III) complexes, the excited-state lifetimes were determined to be 394, 38, and 367 μs, respectively. It was shown that the orientation of liquid-crystalline films of the Eu(III) complex leads to an increase in luminescence intensity by 26% and allows the intensity of polarized emission to be controlled (a 1.5-fold change) by varying the polarization plane of the exciting light. The obtained bifunctional materials are considered as promising tunable sources of polarized luminescence for optoelectronics, photonics, and biomedical applications.