Fluorescence probes for prokaryotic and eukaryotic cells using Re(CO) 3 + complexes with an electron withdrawing ancillary ligand

Research in fluorescence microscopy presents new challenges, especially with respect to the development of new metal-based fluorophores. In this work, new fac -[Re(CO) 3 (bpy)L]PF 6 ( C3 ) and fac -[Re(CO) 3 (dmb)L]PF 6 ( C4 ) complexes, where L is an ancillary ligand, E -2-((3-amino-pyridin-4-ylimino)-methyl)-4,6-di- tert -butylphenol, both exhibiting an intramolecular hydrogen bond, have been synthesized for use as preliminary probes for fluorescence microscopy. The complexes were characterized using chemical techniques such as UV-vis, 1 H-NMR, TOCSY, FT-IR, cyclic voltammetry, mass spectrometry (EI-MS 752.22 M + for C3 and 780.26 M + for C4 ) and DFT calculations including spin–orbit effects. The electron withdrawing nature of the ancillary ligand L in C3 and C4 explains their electrochemical behavior, which shows the oxidation of Re I at 1.84 V for C3 and at 1.88 V for C4 . The UV-vis absorption and emission properties have been studied at room temperature in acetonitrile solution. The complexes show luminescent emission with a large Stokes shift ( λ ex = 366 nm, λ em = 610 nm for C3 and λ ex = 361 nm, λ em = 560 nm for C4 ). The TDDFT calculations suggest that an experimental mixed absorption band at 360 nm could be assigned to MLCT (d(Re) → π*(dmb)) and LLCT (π(L) → π*(dmb)) transitions. We have also assessed the cytotoxicity of C3 and C4 in an epithelial cell line (T84). We found that 12.5 μg ml −1 of C3 or C4 is the minimum concentration needed to kill 80% of the cell population, as determined by neutral red uptake. Finally, the potential of C3 and C4 as biological dyes for use in fluorescent microscopy was assessed in bacteria ( Salmonella enterica ) and yeasts ( Candida albicans and Cryptococcus spp.), and in an ovarian cancer cell line (SKOV-3). We found that in all cases, both C3 and C4 are suitable compounds to be used as fluorescent dyes for biological purposes. In addition, we present evidence suggesting that these rhenium( i ) tricarbonyl complexes may be also useful as differential fluorescent dyes in yeasts ( Candida albicans and Cryptococcus spp.), without the need for antibodies.