Self-Assembly Synthesis, Structural Features, and Photophysical Properties of Dilanthanide Complexes Derived from a Novel Amide Type Ligand: Energy Transfer from Tb(III) to Eu(III) in a Heterodinuclear Derivative

A novel amide type ligand benzyl-<i>N</i>,<i>N</i>-bis­[(2′-furfurylaminoformyl)­phenoxyl)­ethyl]-amine (<b>L</b>) has been designed and applied for the self-assembly generation of homodinuclear lanthanide coordination compounds [Ln₂(μ₂-L)₂(NO₃)₆(EtOH)₂] [Ln = Eu (<b>1</b>), Tb (<b>2</b>), and Gd (<b>3</b>)] and a heterodinuclear derivative [EuTb­(μ₂-L)₂(NO₃)₆(EtOH)₂] (<b>4</b>). All the complexes have been characterized by the X-ray single-crystal diffraction analyses. They are isostructural, crystallize in a monoclinic space group <i>P</i>2₁/<i>c</i>, and form [2 + 2] rectangular macrocycle structures. Compound <b>4</b> is the first example of a [2 + 2] rectangular macrocycle heterodinuclear EuTb complex assembled from an amide type ligand. In <b>4</b>, the discrete 0D dimeric [EuTb­(μ₂-L)₂(NO₃)₆(EtOH)₂] units are extended, via the multiple N–H···O hydrogen bonds, into a 2D supramolecular network that has been topologically classified as a uninodal 4-connected underlying net with the <b>sql</b> [Shubnikov tetragonal plane net] topology. The triplet state (³ππ*) of <b>L</b> studied by the Gd­(III) complex <b>3</b> demonstrated that the ligand beautifully populates Tb­(III) emission (Φ = 52%), whereas the corresponding Eu­(III) derivative <b>1</b> shows weak luminescence efficiency (Φ = 0.7%) because the triplet state of <b>L</b> has a poor match with ⁵D₁ energy level of Eu­(III). Furthermore, the photoluminescent properties of heterodinuclear complex <b>4</b> have been compared with those of the analogous homodinuclear compounds. The quantum yield and lifetime measurements prove that energy transfer from Tb­(III) to Eu­(III) is being achieved, namely, that the Tb­(III) center is also acting to sensitize the Eu­(III) and enhancing Eu­(III) emission in <b>4