Tuning reductive and oxidative photoinduced electron transfer in amide-linked anthraquinone-porphyrin-ferrocene architectures

Porphyrin amino acids 3a-3h with meso substituents Ar of tunable electron-donating power (Ar = 4-C₆H₄OnBu, 4-C ₆H₄OMe, 4-C₆H₄Me, Mes, C ₆H₅, 4-C₆H₄F, 4-C₆H ₄CF₃, C₆F₅) have been linked at the N terminus to anthraquinone Q as electron acceptor through amide bonds to give Q-P^Ar dyads 4a-4h. These were conjugated to ferrocene Fc at the C terminus as electron donor to give the acceptor-chromophore-donor Q-P ^Ar-Fc triads 6a-6h. To further modify the energies of the electronically excited and charge-separated states, the triads 6a-6h were metallated with zinc(II) to give the corresponding Q-(Zn)P^Ar-Fc triads Zn-6a-Zn-6h. The Q-P^Ar1 dyad (Ar¹ = C ₆H₅) was further extended with a second porphyrin P ^Ar2 (Ar² = 4-C₆H₄Me) as well as appended to a ferrocene to give the tetrad Q-P^Ar1-P^Ar2-Fc 9. Almost all the conjugates show strongly reduced fluorescence quantum yields and excited-state lifetimes, which has been interpreted as photoinduced electron transfer (PET) either from the excited porphyrin to the quinone (oxidative PET) or from the ferrocene to the excited porphyrin (reductive PET). Electrochemical data, absorption spectroscopy, steady-state emission, time-resolved fluorescence, transient absorption pump-probe spectroscopy as well as DFT calculations have been used to elaborate the preferred PET pathway (reductive vs. oxidative PET) in these architectures with systematically varied electron-donating substituents at the central chromophore.