Monomeric and Dimeric Erbium(III) Complexes: Crystal Structure and Photoluminescence Studies

Abstract

The crystal structure and photoluminescence (PL) studies of the monomeric and dimeric Er(III) complexes with two different chelating ligands (anthracene-9-carboxylic acid, 9-ACA; pentaethylene glycol, EO5; and picric acid, HPic) are reviewed. The Er(III) metal ion was coordinated to the attached ligands in eight- and nine-coordination number. The dimeric [Er2(9-AC)6(DMF)2(H2O)2] complex shows the presence of deprotonated 9-AC anions with the negatively charged oxygen atoms bridged two Er(III) ions leads to a great coordinative flexibility via three possibilities of coordination modes, i.e. monodentate, chelation bidentate, chelating–bridging tridentate, where 9-AC = anthracene-9-carboxylate anion. The monomeric [Er(Pic)2(EO5)](Pic) complex displays the important flexible structure of the acyclic EO5 ligand and the role of Pic anions act as bidentate and monodentate chelations. The PL spectra of both Er(III) complexes show a broad band with the center peak position being dependent on the attached aromatic ligands. The heavier lanthanide complexes show the difference in structures, coordination geometry environment, and luminescence properties compared to the lighter lanthanide complexes. The energy transfer process in the complexes could be optimized with maximize the overlap between the emission spectrum of donor atom and absorption spectrum of acceptor atom. The monomeric [Er(Pic)2(EO5)](Pic) and dimeric [Er2(9-AC)6(DMF)2(H2O)2] complexes, where EO5 = pentaethylene glycol, Pic = picrate anion, and 9-AC = anthracene-9-carboxylate anion, have been structurally investigated. The 9-AC anions with the negatively charged oxygen atoms have bridged two Er(III) ions leads to a great coordinative flexibility via three possibilities of coordination modes, i.e. monodentate, chelation bidentate, chelating–bridging tridentate. The pseudo-cyclic conformation of the acyclic EO5 ligand after the formation of complex was observed. The heavier lanthanides and Pic anion have a great important role in formation of complex. The photoluminescence spectra of both Er(III) complexes display a broad band with the center peak position being dependent on the attached aromatic ligands.