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 Eu^III and ClO₄^-. In fc1∩Eu, UV light irradiation triggers non-covalent bond cleavage to release a ferrocenium and Eu^II 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 Eu^III 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.