Optical and redox properties of meso-diphenyltetrabenzoporphyrins

Abstract

The meso-diphenyltetrabenzoporphyrin zinc complex ( 1b, ZnP 2TBP) having two phenyl substituents at the neighboring meso-positions was isolated by the careful purification of the reaction product of 3-benzylidenephthalimidine (3-BPI) with zinc acetate. The treatment of 1b with trifluoroacetic acid led to the successful demetalation-of 1b to its metal free compound ( 1e, H 2P 2TBP). Hence, the other divalent metallic complexes such as magnesium ( 1a), cadmium ( 1c) and palladium ( 1d) were readily prepared by metal insertion into 1e with the appropriate metal sources. The well-defined structural elucidation of these meso-diphenyltetrabenzoporphyrins (P 2TBP) has enabled us to determine their electrochemical and optical properties. Among the metallo P 2TBPs discussed here, the Mg complex possesses both the lowest oxidation and reduction potentials whilst the Pd complex has the highest oxidation and reduction potentials. This result was explained on the basis of the induction effect on the porphyrin ring current by the central metal ion. The absorption maxima of these metallo P 2TBPs were also affected by the central metal ions. Since these TBPs reasonably fluoresce at room temperature and phosphoresce at 77 K as in the case of the corresponding metallotetraphenylporphyrins (TPP), their excited state energies were measurable. Their fluorescence quantum yields (φ f) and lifetimes (τ f) obeyed the heavy atom effect by the central metal ions. Finally, we also discussed their redox properties in the excited stares.