Construction of Identical [2 + 2] Schiff-Base Macrocyclic Ligands by Ln^III and Zn^II Template Ions Including Efficient Yb^III Near-Infrared Sensitizers

Identical 34-membered [2 + 2] pendent-armed Schiff-base macrocyclic ligands (H₄L_a and H₄L_b) can be constructed via the condensation reactions between rigid <i>o</i>-phenylenediamine and extended dialdehydes (H₂hpdd/H₂pdd) in the presence of either Ln^III or Zn^II template with remarkable distinction on the ion radii and charge. X-ray single-crystal diffraction analyses reveal the formation of mononuclear Ln^III complexes (<b>1</b>–<b>4</b> and <b>7</b>) and dinuclear Zn^II complexes (<b>5</b> and <b>6</b>). It is noted that Ln^III macrocyclic complexes have eight-coordinate sandwich-like mononuclear structures fully surrounded by flexible and large-sized macrocyclic ligands. Photophysical studies have demonstrated that both H₄L_a and H₄L_b can serve as effective sensitizers for the Yb^III ion (<b>2</b> and <b>7</b>) exhibiting near-infrared emission at 974 nm with high quantum yields in solution (C₂H₅OH and CH₃OH, ∼1%). Moreover, the quantum yields of two Yb^III complexes <b>2</b> and <b>7</b> could be increased ∼15% in CH₃OH under weak alkaline condition (pH = 8–9), while no significant changes are observed in C₂H₅OH by contrast. We think the unique sandwich-like macrocyclic structures of Yb^III complexes <b>2</b> and <b>7</b> play important roles in simultaneously guaranteeing the effective match of the energy levels of Yb^III centers as well as shielding from the solvent molecules and counterions