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).