Phosphorus(V) Porphyrin-Manganese(II) Terpyridine Conjugates: Synthesis, Spectroscopy, and Photo-Oxidation Studies on a SnO₂ Surface

A major challenge in designing artificial photosynthetic systems is to find a suitable mimic of the highly oxidizing photoactive species P₆₈₀ in photosystem II. High-potential phosphorus(V) porphyrins have many attractive properties for such a mimic but have not been widely studied. Here, we report the synthesis and photophysical characterization of a novel phosphorus(V) octaethylporphyrin-oxyphenyl-terpyridine conjugate (PPor-OPh-tpy, 1) and its corresponding manganese(II) complex (PPor-OPh-Mn(tpy)Cl₂, 2). The X-ray structure of 2 shows that the Mn(II) and P(V) centers are 11.783 Å apart and that the phenoxy linker is not fully conjugated with the terpyridine ligand. The porphyrin fluorescence in 1 and 2 is strongly quenched and has a shorter lifetime compared to a reference compound without the terpyridine ligand. This suggests that electron transfer from tpy or Mn(tpy) to the excited singlet state of the PPor may be occurring. However, femtosecond transient absorbance data show that the rate of relaxation to the ground state in 1 and 2 is comparable to the fluorescence lifetimes. Thus, if charge separation is occurring, its lifetime is short. Because both 1 and 2 are positively charged, they can be electrostatically deposited onto the surface of negatively charged SnO₂ nanoparticles. Freeze-trapping EPR studies of 2 electrostatically bound to SnO₂ suggest that excitation of the porphyrin results in electron injection from ¹PPor∗ into the conduction band of SnO₂ and that the resulting PPor^•+ species acquires enough potential to photo-oxidize the axially bound Mn(II) (tpy) moiety to Mn(III) (tpy).