Abstract
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 [Ln(2)Ga(8)(shi)(8)(mip)(4)](2-) metallacrowns (MCs; shi(3-) = salicylhydroximate; mip(2-) = 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 [Ln(2)Ga(8)(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 (4)F(3/2) → (4)I(11/2) electronic transition in the range of 1050-1080 nm in the cells. These results demonstrate that the [Ln(2)Ga(8)(shi)(8)(mip)(4)](2-) MC can be used as a versatile molecule for tuning the chemical and photophysical properties of the [Ln(2)Ga(8)] MC family.