Novel colorimetric and fluorescent off–on enantiomers with high selectivity for Fe3+ imaging in living cells

• Two novel colorimetric and fluorescent off–on enantiomers (R/S-RBOL) have been developed and characterized.
• The fluorescence intensity of R-RBOL is linearly proportional with increasing the concentration of Fe3+ ranging from 0 to 20 equiv.
• R/S-RBOL could be successfully applied to monitor Fe3+ in living cells without cytotoxicity under physiological conditions.

Two novel colorimetric and fluorescent off–on enantiomers (R/S-RBOL) were developed to detect Fe3+ in aqueous and biological samples through probing the integrated changes in fluorescence, circular dichroism (CD) and circularly polarized luminescence (CPL). The fluorescence intensity of R/S-RBOL is linearly proportional to the Fe3+ concentration in the range of 0–20 equiv., bringing out a detection limit of 1.83 × 10−7 M. The Job's plot indicates a 1:1 binding stoichiometry of RBOL:Fe3+, which is further confirmed by ESI-MS analysis. Moreover, the RBOLs exhibit a dual-readout response in colour and fluorescence, rendering them suitable to sense Fe3+ in living cells with low cytotoxicity. In order to shed some light on the detection mechanism, the coordination between R/S-RBOL and Fe3+ is unravelled by DFT calculations. From the free RBOL to the RBOL–Fe3+ complex, the spiro-lactam ring is opened to interact with Fe3+ via the lactam N and O, imine N and hydroxyl O atoms in the form of a ferric six-coordinate structure.

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