Understanding the photophysical properties of rhenium(I) compounds coordinated to 4,7-diamine-1,10-phenanthroline: synthetic, luminescence and biological studies

In the present study, the photophysical properties and preliminary time-dependent density functional theory (TD-DFT) data of new rhenium(I) polypyridyl compounds, fac-[Re(L)(Am2phen)(CO)3]0/+ where Am2phen = 4,7-diamine-1,10-phenanthroline and L = Cl or ethyl isonicotinate (et-isonic) provided new insights on the excited-state deactivation through an unusual inversion between two metal-to-ligand charge-transfer excited states. Additionally, their cellular uptake using breast cancer (MCF-7) and melanoma (SkMel-147 and SkMel-29) cell lines and bioactivity were investigated as well as their cell-killing mechanism and the protein expression. Both compounds exhibited a strong and broad absorption band around 300-400 nm that by preliminary TD-DFT corresponds to a mixture of ILAm2phen and MLCTRe→Am2phen transitions and a strong contribution of the charge transfer transition MLCTRe→et-isonic for fac-[Re(et-isonic)(Am2phen)(CO)3]+. In contrast to typical Re(I) polypyridyl complexes, the change of Cl for et-isonic ligand promoted a bathochromic shift of the emission maxima, relatively low emission quantum yield and fast lifetime. Photophysical investigation for fac-[ReCl(et-isonic)2(CO)3] compound provided meaningful information on the excited state manifold of fac-[Re(L)(Am2phen)(CO)3]0/+ complexes. As in the absorption profile, a remarkable inversion of the lowest-lying excited state is taking place from the usual observed MLCTRe→Am2phen to the unusual MLCTRe→et-isonic. The lipophilicity of the positive-complex was higher than the non-charge compound and the same trend for the activity against cells was observed, in the absence of light. In addition, the flow cytometry and the Western Blotting analyses showed an overexpression on pro-caspase-9, suggesting a Caspase proteolytic cascade through the intrinsic-pathway apoptosis mechanism. The photophysics of these compounds reported herein provide new fundamental insights into the understanding of substituent groups on the polypyridyl ligands that are relevant to practical development.