Thermal and Emission Properties of a Series of Lanthanides Complexes with N-Biphenyl-Alkylated-4-Pyridone Ligands: Crystal Structure of a Terbium Complex with N-Benzyl-4-Pyridone

Florentina L Chiriac,Augustin Madalan,Monica Iliş,Doina Manaila-Maximean,Mihail Secu,Viorel Cîrcu

doi:10.3390/molecules26072017

Abstract

This work focuses on the investigation of the liquid crystalline behavior and luminescence properties of the lanthanide complexes of Eu(III), Sm(III) and Tb(III) with N-biphenyl-alkylated-4-pyridone ligands. The organic ligands having a biphenyl group attached via a long flexible spacer with either 9 or 10 carbon atoms were synthesized by the reaction between 4-hydroxypyridine and the corresponding bromide compounds. The chemical structures of the organic and lanthanide complexes were assigned based on elemental analysis, single-crystal X-ray diffraction, 1H, 13C NMR and IR spectroscopies, and thermogravimetric analysis (TGA). The X-ray diffraction analysis of a parent compound shows that the lanthanide ions are surrounded by three monodentate pyridone ligands and three bidentate nitrate ions, giving a 9-coordinate environment. The mesogenic behavior and the type of liquid crystalline phases exhibited by the new complexes were analyzed by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM), and powder X-ray diffraction (XRD) studies. Only the lanthanide complexes with longer spacer (10) display a monotropic SmA phase, typically on a short thermal range (less than 10 °C). The complexes with shorter flexible chains (9) show no liquid crystalline properties with melting temperatures lower than their analogs with longer spacers. The emission spectra recorded in solid state at room temperatures show typical emission bands for each lanthanide ion employed (Eu(III), Tb(III) and Sm(III)).

Highlights

Liquid crystals based on lanthanide complexes are well-investigated anisotropic materials that develop interesting photophysical properties [1,2,3,4,5,6,7,8]
We have described a new class of lanthanidomesogens based on lanthanide nitrate complexes with 4-pyridone monodentate ligands, which are coordinated to the lanthanide ions via the carbonyl oxygen atom
We report the preparation and characterization of a series of 4-pyridone ligands carrying one biphenyl group linked to the pyridone ring via a flexible spacer with 9 and 10 methylene groups and the corresponding lanthanide complexes (europium(III), samarium(III), terbium(III))

Summary

Introduction

Liquid crystals based on lanthanide complexes (lanthanidomesogens) are well-investigated anisotropic materials that develop interesting photophysical properties [1,2,3,4,5,6,7,8]. The unique photophysical properties of the lanthanide ions provided a strong fundament for the design and study of their complexes with liquid crystal properties and these compounds are mainly built around several categories of organic ligands, among the most used were the Schiff bases, β-diketonates, alkanoates, different macrocyclic ligands (phthalocyanine, porphyrin, etc.), or bis-(benzimidazolyl)pyridines [6,7] The design of such complexes must take into account the use of suitable organic ligands that act on two different complementary perspectives: first, providing thermodynamic stability to the products and second, the capacity to exploit the antenna effect, where the organic chromophore acts as a light-harvesting antenna able to transfer the absorbed energy to the emitting levels of the lanthanide ions. The polar 4-pyridone platform delivers several versatile functionalities: intermolecular interactions through hydrogen bonding [19,20], strong coordination properties to various metal ions [21,22,23], and last, but not least, useful intermediates to liquid crystalline methylene1,4-dihydropyridines [14] or to ionic liquid crystals based on pyridinium salts [24,25,26,27]

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Results

Conclusion