Ultrafast electron-transfer in a fully conjugated coumarin-ferrocene donor-acceptor dyads

A series of a fully conjugated ferrocene-containing coumarins and B,O-chelated coumarins was prepared and characterized by a variety of spectroscopic (UV–vis, NMR, and mass spectrometry), electrochemical (CV and DPV) and spectroelectrochemical methods. The photophysical properties of the target compounds were investigated by steady-state fluorescence and ultrafast transient absorption spectroscopy methods. Selective photoexcitation of the coumarin core was followed by ultrafast electron transfer from the ferrocene fragment, resulting in formation of a charge-separated Fc+-coumarin-. state. This charge-separated state undergoes ultrafast ground state recovery in 20–25 ps, shortening the excited state lifetime of the organic coumarin precursors (1.2–1.5 ns) by almost two orders of magnitude. Formation of the charge-separated states was supported by the experimental (steady-state fluorescence oxidative titrations) and theoretical DFT and TDDFT calculations.