Near-Infrared-Emitting Lanthanide(III)/Gallium(III) Metallacrowns with Appended Coumarins: Tuning the Sensitizer–Emitter Distance through the Linker

Near-infrared (NIR)-emitting trivalent lanthanides (Ln^III) are attractive for applications such as biological imaging and telecommunications. However, their sensitization remains challenging, particularly at excitation wavelengths corresponding to low energies. To address this challenge, a new coumarin-bearing isophthalate-based bridging ligand (C-ip^2–), in which the coumarin sensitizer is directly attached to the bridge, was synthesized. In the corresponding [Ln₂Ga₈(shi)₈(C-ip)₄]^2– metallacrowns (MCs), the sensitization of the NIR-emitting Nd^III, Er^III, and Yb^III cations was achieved upon excitation of appended coumarins in the visible range (λ_exc = 435 nm). Compared to the previous generation of [Ln₂Ga₈(shi)₈(C-mip)₄]^2– MCs, the coumarin antennas are about 5 Å closer to the Ln^III emissive centers in this new series of [Ln₂Ga₈(shi)₈(C-ip)₄]^2– MCs, resulting in an increase in the QLnL values by factors of 6.2, 9.4, and 2.7 for Nd^III, Er^III, and Yb^III analogues, respectively. This allows the detection of their NIR emissions through tissue-mimicking phantoms of 1 mm thickness, validating the potential of this design for NIR imaging applications.