Luminescent Re(I) Carbonyl Complexes as Trackable PhotoCORMs for CO delivery to Cellular Targets

A family
of Re­(I) carbonyl complexes of general formula [ReX­(CO)3(phen)]0/1+ (where X = Cl–, CF3SO3–, MeCN, PPh3, and methylimidazole)
derived from 1,10-phenanthroline (phen) exhibits variable emission
characteristics depending on the presence of the sixth ancillary ligand/group
(X). All complexes but with X = MeCN exhibit moderate CO release upon
irradiation with low-power UV light and are indefinitely stable in
anaerobic/aerobic environment in solution as well as in solid state
when kept under dark condition. These CO donors liberate three, one,
or no CO depending on the nature of sixth ligand upon illumination
as studied with the aid of time-dependent IR spectroscopy. Results
of excited-state density functional theory (DFT) and time-dependent
DFT calculations provided insight into the origin of the emission
characteristics of these complexes. The luminescent rheinum­(I) photoCORMs
uniformly displayed efficient cellular internalization by the human
breast adenocarcinoma cells, MDA-MB-231, while the complex with PPh3 as ancillary ligand showed moderate nuclear localization
in addition to the cytosolic distribution. These species hold significant
promise as theranostic photoCORMs (photoinduced CO releasing molecules),
where the entry of the pro-drug can be tracked within the cellular
matrices.