Dual-Antenna Trimetallic Lanthanide Complexes for Enhanced Near-Infrared Luminescence

Lanthanide [Ln(III)] ions are known for their unique near-infrared (NIR) luminescence, typically achieved through indirect excitation via the antenna effect. Organic ligands, such as N,N-bis(3-methoxysalicylidene)-1,4-diamino butane (L), in combination with Zn, have previously demonstrated their effectiveness in enhancing lanthanide NIR emission, as seen in bimetallic [L-Zn-Ln] complexes, which employs a single antenna. In this study, we present a new series of trimetallic Zn-Ln complexes, [(L-Zn)₂-Ln], featuring two compartmental ligand-Zn complexes, acting as antennas, aimed at further improving energy transfer efficiency to the lanthanide centers. Comprehensive characterization using SCXRD, PXRD, FT-IR, and CHN analyses confirmed the structural integrity of these complexes. Notably, SCXRD and XPS revealed significant structural differences between the bimetallic and trimetallic systems. The impact of the additional antenna, replacing nitrate and methanol-known contributors to nonradiative relaxation in the bimetallic [L-Zn-Ln] complexes-was thoroughly examined. Photophysical studies across both visible and NIR regions demonstrated substantial enhancements in luminescence, particularly in the NIR region, attributed to the inclusion of the second antenna, highlighting its role in improving the overall energy transfer process.