Strong Luminescent Europium Complexes Induced by the Unprecedented Anti-chelate Effect of Acyl Groups on a N 6 -Hexadentate Ligand Shuhei Ogata, 1,2 Hiroaki Komiya, 1 Naoto Goto, 1 Ryota Tanabe, 1 Kunihisa Sugimoto, 3 Shogo Kawaguchi, 3 Kenta Goto, 4 Miho Hatanaka, 5,6 Ayumi Ishii, 1,2,6 and Miki Hasegawa* 1,2 1 College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan 2 Mirai Molecular Material Design Institute, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan 3 Research & Utilization Division, Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan 4 Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan 5 Institute for Research Initiatives, Division for Research Strategy, Graduate School of Materials Science, Data Science Center, Nara Institute for Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan 6 JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan E-mail: hasemiki@chem.aoyama.ac.jp An acyl group derived-bipyridine ligand and Eu form 3:2 complexes. The structure and luminescence behaviour were evaluated, and we found an unexpected bi-nuclear complex system, which induces efficiently the luminescence of europium ion. This observation is discussed from experimental and theoretical viewpoints. Keywords: Anti-chelate effect | Lanthanide complexes | Solid state luminescence It is observed that multi-dentate moieties in organic com- pounds more strongly coordinate to a metalion than bidentate ligands. This can be explained by the increase in entropy after complexation. 1,2 This phenomenon is well known as a chelate effect, which allows luminescent lanthanide complexes in solu- tions or biological species to keep their structure, and facilitates organic-inorganic hybridization among other things. 3-13 Recently, two helical europium (Eu III ) complexes EuL, and EuLH were developed. These complexes have a ligand consist- ing of two bipyridine moieties bridged by an ethylenediamine coordinated to Eu III . 14,15 EuLH is the reduced form of azome- thine moieties in EuL, which has a single helicate molecular structure and keeps its molecular structure and luminescence even in acetonitrile through chelation (Figure 1a). 15 In our latest work a conformation change of EuLH with acyl groups (EuLC x ) in solution was expected in both experimental and theoretical approaches (Figure 1a). 15 Based on the theoretically optimized geometry of EuLC x , two oxygen atoms of acyl groups coordinate to Eu III instead of six nitrogen atoms. The luminescence spectra including lifetimes and absolute quantum yields of Eu III complexes with various acyl groups showed evidence of a conformation change in solution. 15 Here, we experimentally demonstrated an unusual coordination mode of EuLC 8 in the solid state from synchrotron X-ray single crystal structural analysis. Solid-state luminescence and struc- tural behaviour of Eu III and gadolinium (Gd III ) complexes with various length of acyl groups (EuLC x ) are also reported. The crystal structure and crystallographic data of EuLC 8 are shown inFigure 1b, Figure S1 and Table S1. This complex consists of three ligands coordinated to two Eu III . Oxygen atoms of one of the ligands are coordinated to two different Eu III acting (a) (b) (c) Figure 1. (a) Conformation change of EuLH 15 to EuLCx after alkylation through acylgroups and (b) ORTEP drawing of EuLC 8 as a 2:3 complex of Eu and ligand. Hydrogen atoms and acetonitrile molecule are omitted for clarity. Thermal ellipsoids are drawn at 50% probability. Data is available from Crystallographic Data Centre under deposition number CCDC 1862383. (c) Electrostatic potential of LC 8 and natural charges of selected oxygen and nitrogen atoms calculated with the ωB97XD/cc-pVDZ level of theory. Received: February 22, 2019 | Accepted: March 19, 2019 | Web Released: May 9, 2019 CL-190140 Chem. Lett. 2019, 48, 593–596 | doi:10.1246/cl.190140 © 2019 The Chemical Society of Japan | 593