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Keywords:

  • Artificial photosynthesis;
  • Energy transfer;
  • Electron transfer;
  • Metallo­cenes;
  • Porphyrins

Abstract

New amide-linked porphyrin–ferrocene conjugates [M(PAr)–Fc] were prepared from aminoferrocene and a carboxy-substituted meso-tetraaryl-porphyrin [M = 2H, Zn; Ar = mesityl (Mes), C6F5: 3a, 3e, Zn-3a, Zn-3e]. A further porphyrin building block was attached to the second cyclopentadienyl ring of the ferrocene moiety to give the metallopeptides M(PMes)–Fc–M(PAr) (M = 2H, Zn; Ar = C6H5, 4-C6H4F: 6b, 6c, Zn-6b, Zn-6c). The effects of the Ar substituents, the porphyrin central atom M and the presence of the second porphyrin at the ferrocene hinge on the excited-state dynamics was studied by optical absorption spectroscopy, electrochemistry, steady-state emission, time-resolved fluorescence measurements and transient absorption pump–probe spectroscopy in addition to density functional theory calculations. In the ground state, only weak interactions were revealed between the ferrocene and porphyrin units by optical absorption spectroscopy and electrochemical measurements. However, the porphyrin emission is strongly quenched with respect to that of the reference porphyrins without the ferrocene moieties. Fluorescence is partially recovered at lower temperatures, which suggests an activated excited-state decay process. All excited-state lifetimes are reduced with respect to those of the reference porphyrins. The quantum yields and lifetimes correlate with the porphyrin and ferrocene redox potentials. All observations point to photoinduced electron transfer from ferrocene to the porphyrin in the normal Marcus region as the dominant excited-state reactivity. The resulting charge-separated states of selected conjugates were studied by ns–μs transient absorption pump–probe spectroscopy. Hints for the feasibility of singlet–singlet energy transfer between cofacial porphyrins were found in tweezers 6 and Zn-6.