Luminescence Modulations of Rhenium Tricarbonyl Complexes Induced by Structural Variations

Octahedral
d6 low-spin Re­(I) tricarbonyl complexes are
of considerable interest as noninvasive imaging probes and have been
deeply studied owing to their biological stability, low toxicity,
large Stokes shifts, and long luminescence lifetimes. We reported
recently the bimodal IR and luminescence imaging of a Re­(I) tricarbonyl
complex with a Pyta ligand (4-(2-pyridyl)-1,2,3-triazole) in cells
and labeled such metal–carbonyl complexes SCoMPIs for single-core
multimodal probes for imaging. Re­(I) tricarbonyl complexes have unique
photophysical properties allowing for their unequivocal detection
in cells but also present some weaknesses such as a very low luminescence
quantum yield in aqueous medium. Further optimizations would thus
be desirable. We therefore developed new Re­(I) tricarbonyl complexes
prepared from different ancillary ligands. Complexes with benzothiadiazole–triazole
ligands show interesting luminescent quantum yields in acetonitrile
and may constitute valuable luminescent metal complexes in organic
media. A series of complexes with bidentate 1-(2-quinolinyl)-1,2,3-triazole
(Taquin) and 1-(2-pyridyl)-1,2,3-triazole (Tapy) ligands bearing various
4-substituted alkyl side chains has been designed and synthesized
with efficient procedures. Their photophysical properties have been
characterized in acetonitrile and in a H2O/DMSO (98/2)
mixture and compared with those of the parent Quinta- and Pyta-based
complexes. Tapy complexes bearing long alkyl chains show impressive
enhancement of their luminescent properties relative to the parent
Pyta complex. Theoretical calculations have been performed to further
characterize this new class of rhenium tricarbonyl complexes. Preliminary
cellular imaging studies in MDA-MB231 breast cancer cells reveal a
strong increase in the luminescence signal in cells incubated with
the Tapy complex substituted with a C12 alkyl chain. This study points
out the interesting potential of the Tapy ligand in coordination chemistry,
which has been so far underexploited.