Photophysical and bioactivity behavior of fac-rhenium(I) derivatives containing ditopic sulfurpyridine ligands

Luminescent fac-rhenium(I) derivatives have proven their great potential as cell imaging agents. However, there is still a lack of information regarding the structure-bioactivity and biodistribution relationship specially in those cases where the axial ligand is a S-donor ligand. Therefore, new [Re(bipy)(CO)3L]0/+ complexes, where L is pyridine-4-thiolate (L1), pyridine-2-thiolate (L2) and 6-methylpyridine-2(1H)thione (L3) were synthesised. The ditopic thiol/thione-pyridine derivatives (Spy derivatives) behaved as sulfur donor in all cases, affording two neutral (1 and 2) and a cationic (3) complex respectively. X-ray diffraction revealed the different coordination mode presented by L2 and L3, a thiolate and a thione donor respectively. Photophysical studies showed that they have moderate emission intensities that are tentatively assigned to 3MLCT transitions (1, 2) and a mixture of 1IL and 3MLCT transition (3) with lifetimes in the ns range. Possibly the presence of both, donor and acceptor ligands, SPy and bipyridine respectively, is facilitating a non-radiative LLCT transition to take place, which diminishes the probability of dissipating the energy by a radiative 3MLCT transition. In addition, cytotoxicity assays performed in human cancerous A549 lung and HeLa cervix cells disclosed their poor cytotoxic activity (IC50 > 100 μM), which would enable their applications in cell imaging. However, confocal cell microscopy studies performed in A549 cells were not decisive; probably the low emission intensity prevented a clear visualization of the probe.Authors thank the Ministerio de Economía y Competitividad (MINECO-FEDER CTQ2013-48635-C2-1-P, and CTQ2015-70371-REDT) and Gobierno de Aragón-Fondo Social Europeo (E77) for financial support.Peer reviewe