Bridging Azahelicene Enables Circularly Polarized Luminescence in a Dinuclear Ytterbium(III) Single-Molecule Magnet

Modern coordination chemistry employs increasingly sophisticated building blocks to design compounds and molecule-based materials with unusual combinations of properties relevant to emerging technologies. A notable example is the incorporation of helicenes as chiral ligands in luminescent and magnetic lanthanide complexes, which can give rise to circularly polarized luminescence (CPL) or magneto-chiral dichroism (MChD). Lanthanides are particularly wellsuited for CPL owing to their intraconfigurational 4 f -4 f transitions with long excited-state lifetimes and sharp, well-defined emission bands. Herein we present a new low-coordinate Yb III dinuclear complex [{Yb(BHT) 3 } 2 (azahelicene)] (BHT = 2,6-di-tert-butyl-4-methylphenolate; azahelicene = benzo[4,5]imidazo[1,2-a]benzo[4,5]imidazo[2,1-k][1,10]phenanthroline), where the two Yb III luminophores are bridged by the optically pure azahelicene ligand as the source of chirality. Additionally, the reported compound is a single-molecule magnet (SMM) thanks to the synthetic strategy that enables control of the coordination sphere around the Yb III ion. The combined experimental and theoretical magneto-optical studies conducted for both enantiomers of [{Yb(BHT) 3 } 2 (azahelicene)] reveal a direct link between the crystal structure, the related crystalfield splitting, and the observed magnetic and chiroptical properties. First of all, the introduction of the azahelicene into the coordination sphere of the achiral luminescent Yb(BHT) 3 parent compound was achieved with the retention of its SMM behavior. Most importantly, however, its incorporation as an integral component of the molecule enabled the observation of circularly polarized near-infrared (NIR) emission from Yb III . Three CPL bands were detected in the NIR, including the most intense signal at 978 nm with the luminescence dissymmetry factor glum = |0.035|, as well as a rare CPL emission above 1050 nm.