Structure, photophysics and magnetism of a europium mixed complex, Eu(HFAA)₃bipy.H₂O, in the solid state and solution

Abstract

Lanthanide β-diketones were used recently in organic-layered electroluminescence diodes and can also find other optical applications. An important characteristic of these materials can be correlated with donor–acceptor properties of the substituents in ligands. Single crystals of C₃₅H₂₁EuF₁₈N₄O₇ were obtained. The X-ray diffraction analysis shows the triclinic system and space group $\text{P}\text{1}$. The europium ion is coordinated by six oxygen atoms of hexafluoroacetylacetone and two nitrogen atoms of the 2,2′-dipyridine molecule. The coordination number of Eu(III) is completed to nine by one water molecule. Besides, one non-coordinated 2,2′-dipyridine is located in the crystal lattice which is disordered over two positions. Absorption, emission and emission excitation spectra at 293, 77 and 4 K, as well as luminescence decay time measurements are used to characterize the title compound in the solid state and in solution. The emission properties are strongly dependent on the energy of the excitation beam and on temperature. Magnetic susceptibility measurements were carried out down to 1.7 K. Correlation of the spectra and magnetic properties with details of the structure of C₃₅H₂₁EuF₁₈N₄O₇ was studied. These results are compared with previously reported data for related systems.

Introduction

In recent years interest increased in the development of efficient light conversion molecular devices (LCMD) based on lanthanide complexes [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. The design of efficient lanthanide complexes has become an important research goal and covers many different classes of ligands.

Efficient light conversion molecules may find several applications mainly in optical devices, luminescence sensors for chemical species, luminescence probes in biomedical assays, fluorescent lighting, and electroluminescent devices. In the latter area heteroleptic europium complexes of the Euβ₃L type can be used [2], [4], [11], [12], [13], [14], [15], [16], [17]. Photophysical characteristics of two types of chelates based on a wide group of derivatives of both β-diketones and Lewis base (L) (1,10-phenathroline) ligands were reported by us [13], [14], [15]. In this field, important results were distinguished by many scientific groups [2], [10], [11], [12], [13], [14], [15], [19], [20], [21], [22], [23].

Based on experimental and theoretical results it was shown that the quantum yield of lanthanide complexes is controlled by the rates of several processes: ligand to metal energy transfer, multiphonon relaxation, back-transfer and cross-over to charge-transfer states [18], [19], [20], [23]. It was also shown that a number of important characteristics of the chelates (which can be applied in electroluminescence diodes) can be correlated with the donor–acceptor properties of the substituents of ligands. Furthermore, it affects the intensity of electroluminescence and remains in close relation with mobility of charge carriers and height of the potential barrier for charge carrier injection. Selection of ligands allows us to develop some promising LCMDs with high room temperature emission efficiency leading to new applications.

Undoubtedly, analysis of the total spectral data permits evaluation of the applicability of different β-diketones in electroluminescence diodes (OLEDs). The aim of the present work are X-ray, photophysical, and magnetic studies of a new mixed europium β-diketone of formulae; Euβ₃L.H₂OxL, where β is hexafluoroacetylacetone (HFAA) and L is the Lewis base 2,2′-bipyridine (bipy).