Novel lanthanide(iii)/gallium(iii) metallacrowns with appended visible-absorbing organic sensitizers for molecular near-infrared imaging of living cells

Optical imaging of biological samples in the near-infrared (NIR) domain is of major interest for non-invasive experiments due to reduced light scattering, absorption and autofluorescence. Taking advantage of the maleimide moieties in the [Ln₂Ga₈(shi)₈(mip)₄]²⁻ metallacrowns (MCs; shi³⁻ = salicylhydroximate; mip²⁻ = 5-maleimido-isophthalate; Ln = Nd^III, Gd^III, Er^III, Yb^III and Y^III), we appended visible-absorbing thiol-coumarin (C) sensitizers through a thiol-Michael addition click reaction, affording the functionalized [Ln₂Ga₈(shi)₈(C-mip)₄]²⁻ MCs. By using this approach, the sensitization of NIR-emitting Nd^III, Er^III and Yb^III was achieved upon excitation in the visible range (λ_exc = 420 nm) through the appended coumarins. NIR epifluorescence microscopy of living HeLa cells incubated with the Nd^III derivative confirmed unambiguous detection of the signal arising from the ⁴F_3/2 → ⁴I_11/2 electronic transition in the range of 1050–1080 nm in the cells. These results demonstrate that the [Ln₂Ga₈(shi)₈(mip)₄]²⁻ MC can be used as a versatile molecule for tuning the chemical and photophysical properties of the [Ln₂Ga₈] MC family.