Exploring rhenium (I) complexes as potential fluorophores for walled-cells (yeasts and bacteria): Photophysics, biocompatibility, and confocal microscopy

In the present work, we synthesized and characterized two complexes: fac-Re(CO)₃(4,5-diazafluoren-9-one)Br (ReL1) and fac-Re(CO)₃(5,6-epoxy-5,6-dihydro-1,10-phenanthroline)Br (ReL2), where ReL2 has not been reported at present. In this study, we show a complete structural characterization of both ReL1 and ReL2 by elemental analysis, FTIR, ¹H and ¹³C NMR, DEPT, and HHCOSY. Moreover, we carried out UV–Vis and luminescence experiments in organic solvents showing different polarities. In addition, we performed relativistic theoretical calculations to better understand electronic transitions and optical properties. We also assessed the cytotoxicity of ReL1, ReL2, and their respective N,N ligands (i.e., 4,5-diazafluoren-9-one and 5,6-epoxy-5,6-dihydro-1,10-phenanthroline) against walled-cells, including Gram-negative bacteria (Salmonella enterica serovar Typhimurium), non-sporulated Gram-positive bacteria (Staphylococcus aureus), sporulated Gram-positive bacteria (Bacillus cereus), and yeasts (Candida albicans and Cryptococcus spp.). We observed that these complexes exhibited very low or no cytotoxicity. We also found that only ReL2, and not ReL1, exhibited good properties as a luminescent probe for this kind of cells. Accordingly, we found that ReL2 showed good potential to be directly used as a luminescent probe for walled cells, including yeasts (e.g., Candida albicans) and bacteria (e.g., Salmonella enterica), which can be observed by confocal microscopy.