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 P-680 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-oxy-phenyl-terpyridine conjugate (PPor-OPh-tpy, 1) and its corresponding manganese(II) complex (PPor-OPh-Mn(tpy)-Cl-2, 2). The X-ray structure of 2 shows that the Mn(II) and P(V) centers are 11.783 angstrom 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 (1)PPor* into the conduction band of SnO2 and that the resulting PPor(center dot+) species acquires enough potential to photo-oxidize the axially bound Mn(II) (tpy) moiety to Mn(III) (tpy).