Photophysical Processes in ‘Supramolecular Balls’ Formed by Lanthanide Chloride with 2,2′‐Bipyridine

Abstract

With gratitude to Prof. Jean-Claude G. Bünzli for induced interest to supramolecular lanthanide chemistry The europium complex [EuCl 2 (bpy) 2 (H 2 O) 2 ]Cl · 1.25 C 2 H 6 O · 0.37 H 2 O, where bpy is 2,2'-bipyridine, was synthesized and investigated with the aim to relate its molecular geometry and crystal packing to the efficiency of energy-transfer processes. The presence of H-bonds between noncoordinated Cl À ions and coordinated H 2 O molecules leads to the formation of discrete trimers assembled by a number of CÀH ··· Cl and stacking interactions into supramolecular balls which contain Cl À ions and solvate molecules (H 2 O and EtOH). The additional stabilization of the complex is due to intramolecular N ··· C interactions between two bpy ligands that causes some shortening of the EuÀN bonds. Deciphering the luminescence properties of the Eu complex was performed under consideration of both the composition of the inner coordination sphere and the peculiarities of the crystal packing. The influence of the latter and the bpy orientation on the energy of the ligand ! Eu charge-transfer state (LMCT) was established, and an additional excited state induced by the p-stacking interaction (SICT) was identified.Introduction. -Weak noncovalent forces (H-bonding, coordination bonds, electrostatic and charge-transfer attractions, aromatic p-stacking interaction, etc.) are the subject of intensive study as a new approach of developing materials science [1]. In coordination chemistry, these weak interactions are important in determining the conformations, the selectivities of the reactions, and the crystal structures of metal complexes [2]. The p -p and CH -p interactions are noncovalent forces which contribute to self-assembly and/or recognition processes when extended structures are formed from building blocks with aromatic moieties [3]. Recently it has been reported that the photophysical properties of transition-metal complexes are affected by intermolecular p -p and CH -p interactions [4]. The introduction of rare-earth metals into self-assembling molecular architectures can lead to the design of advanced luminescent materials. The unique ability of the rare-earth metals to emit well-defined narrow bands in different spectral ranges from VIS to near-IR with relatively long lifetimes and high quantum yields [5] makes them perfect candidates for many fields of materials science. In spite of the fact that numerous lanthanide complexes have been intensively studied to find a method to manage the efficiency of photophysical processes, many questions of the influence of the supramolecular structure on the properties of the lanthanide systems are still open, and no systematic studies of the effect of noncovalent interactions on photophysical properties have been performed.The 2,2'-bipyridine (bpy) is a well known ligand of highly luminescent complexes with transition and rare-earth metals. In the former case, diimine complexes of

Ab Initio Study of Phosphorescent Emitters Based on Rare-Earth Complexes with Organic Ligands for Organic Electroluminescent Devices

“…We used experimental data of refs and – for tta, phen, and bipy in Gd 3+ complexes. References and give the position of the triplet level of tta obtained from the phosphorescence of Gd(tta) 3 , and ref gives the position of the triplet level of phen obtained from the phosphorescence of Gd(phen) 2 Cl 3 (H 2 O) 2 .…”

“… a References and . b Reference data for Gd(R 2 NCS 2 ) 3 (bipy) and Gd(R 2 NCS 2 ) 3 (phen). c Reference . d Reference . …”

“…References and give the position of the triplet level of tta obtained from the phosphorescence of Gd(tta) 3 , and ref gives the position of the triplet level of phen obtained from the phosphorescence of Gd(phen) 2 Cl 3 (H 2 O) 2 . Reference gives the position of the triplet level of bipy in Gd(bipy) 2 Cl 3 (H 2 O) 2 . Reference gives the triplet energies of phen and bipy in heteroleptic complexes of Gd 3+ with various dithiocarbamates and phen or bipy, respectively.…”

DFT evaluation of the effects of OH, NH2 and Br substituents on the properties of 2,2′-bipyridine derivatives

“…The presence of lone pairs of electrons on both nitrogen atoms makes 2,2 -bipyridine basic and as such, functions as a bidentate ligand by donating these lone pairs electrons to a transition metal [3]. Bipyridines have a long history of being used in forming a variety of transition metal complexes for both catalysis and material applications, [4][5][6][7][8][9] but their chemistry is less studied compared to phosphine [10]. 2,2 -Bipyridine was found to give excellent results when used as a ligand in homogenous catalytic transfer hydrogenation of acetophenone [3].…”

Microwave-assisted synthesis of a neodymium trichloride complex with phenanthroline containing infinite chains, NdCl3(H2O)(phen)

“…They accommodate two phenanthroline ligands in the coordination sphere of the rare-earth, which prevent any further infinite condensation with other cations. This chelating mode was encountered in many other complexes with other multidentate pyridine-based ligands (2,2′-bipyridine [23][24][25] or 2,2′:6′,2″-terpyridine [26,27] for instance). Here, the unexpected chain-like complex is based on the chelation of only one phenanthroline with one neodymium cation.…”