A ferrocene∩europium assembly showing phototriggered anticancer activity and fluorescent modality imaging

Two macrocyclic ferrocenophanes containing a coumarin fluorophore, Se2N[7]ferrocenophane (fc1), and Se4N2[7,7]ferrocenophane (fc2), construct an assembly of fc1-H+⋯ClO4⋯[Eu(C10H21COO)2(H2O)2(ClO4)] (fc1∩Eu) and fc2-2H+⋯{ClO4⋯[Eu(C10H21COO)2(H2O)2(ClO4)]}2 (fc2∩Eu) via a N–H⋯O hydrogen bond and a coordinate bond between EuIII and ClO4−. In fc1∩Eu, UV light irradiation triggers non-covalent bond cleavage to release a ferrocenium and EuII complex, accompanying chromism and luminescence signals. Investigations through the steady-state UV-vis absorption, fluorescence, time-resolved fluorescence, femtosecond transient absorption spectra and electrochemical characterization elucidate a stepwise mechanism: firstly, an effective electron transfer occurs from a ferrocene unit to the singlet state of a coumarin unit; the following proton-coupled electron transfer (PCET) reduces EuIII and results in a non-covalent interaction cleavage. Further in vitro exploration of fc1∩Eu in HepG2 cells demonstrated phototriggered integrated cell cytotoxicity and fluorescent modality imaging.