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 ReLl and ReL2 by elemental analysis, FTIR, H-1 and C-13 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 semvar 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.

Automated summary

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), were synthesized and characterized in this study, with ReL2 being a novel compound. Structural characterization of both complexes was conducted using various analytical techniques, alongside UV-Vis and luminescence experiments in organic solvents. Relativistic theoretical calculations were also performed to understand electronic transitions and optical properties. Cytotoxicity assessments revealed low or no toxicity for both complexes, with ReL2 showing potential as a luminescent probe for walled cells, such as yeasts and bacteria, observed via confocal microscopy.