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 (H4La and H4Lb) can be constructed via the condensation reactions between rigid o-phenylenediamine and extended dialdehydes (H2hpdd/H2pdd) in the presence of either LnIII or ZnII template with remarkable distinction on the ion radii and charge. X-ray single-crystal diffraction analyses reveal the formation of mononuclear LnIII complexes (1–4 and 7) and dinuclear ZnII complexes (5 and 6). It is noted that LnIII macrocyclic complexes have eight-coordinate sandwich-like mononuclear structures fully surrounded by flexible and large-sized macrocyclic ligands. Photophysical studies have demonstrated that both H4La and H4Lb can serve as effective sensitizers for the YbIII ion (2 and 7) exhibiting near-infrared emission at 974 nm with high quantum yields in solution (C2H5OH and CH3OH, ∼1%). Moreover, the quantum yields of two YbIII complexes 2 and 7 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 YbIII complexes 2 and 7 play important roles in simultaneously guaranteeing the effective match of the energy levels of YbIII centers as well as shielding from the solvent molecules and counterions.