Influence of Conformational Flexibility on Self-Assembly and Luminescence Properties of Lanthanide Coordination Polymers with Flexible exo-Bidentate Biphenol Derivatives

To explore how nonplanar conformational distortions affect supramolecular self-assembly and properties of lanthanide complexes, we have designed and synthesized two new flexible exo-bidentate ligands derived from biphenol featuring two salicylamide pendant arms, 2,2′-bis{[(2′-benzylaminoformyl)phenoxyl]ethoxyl}-1,1′-biphenylene (L^I) and 5,5′-dibromo-2,2′-bis{[(2′-benzylaminoformyl)phenoxyl]ethoxyl}-1,1′-biphenylene (L^II). These two structurally related ligands can have different conformations and are used for constructing diverse lanthanide polymers with interesting luminescence properties. Among two series of lanthanide nitrate complexes which have been characterized by elemental analysis, X-ray powder diffraction, and IR spectroscopy, four new coordination polymers have been determined using X-ray diffraction analysis. The coordination polymer type {Ln₂(NO₃)₆(L^I)₃·3H₂O}_∞ (Ln = Nd, Sm, Eu, Gd, Tb or Dy) displays a two-dimensional honeycomb-like framework in the ab plane, which can be regarded as a (6,3) topological network with neodymium atoms acting as “three-connected” centers. In contrast, the coordination polymer types {[Nd(NO₃)₃(L^II)(CH₃OH)]·CH₃OH}_∞ and [Ln(NO₃)₃(L^II)(C₂H₅OH)]_∞ (Ln = Sm, Eu, Gd, Tb or Dy) possess single-stranded helix chains which can be further connected through intermolecular hydrogen bonds to form two-dimensional supramolecular sheets. The photophysical properties of trivalent Sm, Eu, Tb, and Dy complexes at room temperature were investigated. The present work substantiates the claim that the supramolecular structure as well as the luminescence properties of the coordination polymer can be tuned by controlling the conformational distortion of a nonplanar flexible ligand in the supramolecular self-assembly.