Metalloporphyrin triads: Synthesis and photochemical characterization

A porphyrin triad has been synthesized by reaction of 5-(3-hydroxyphenyl)-10,15,20-triphenylporphyrin with the tris(triflate) ether of 2-(hydroxymethyl)-2-methylpropane-1,3-diol in dimethylformamide in the presence of cesium carbonate. This has been fully characterized, and the zinc(II), palladium(II), platinum(II) and manganese(III) metallated triads were prepared from it by modified Alder reactions. Semiempirical (PM6) calculations indicate that the lowest energy geometries of the triads are not strongly influenced by the metal and suggest free access of reagents to the porphyrin units. This is supported by acid–base titrations of the free-base porphyrins with trifluoroacetic acid. Detailed photophysical studies are reported on both free base and metallated triads and suggest little interaction between porphyrin moieties, although molar absorption coefficients are higher than in the corresponding monomers, probably due to decreased symmetry. With the Pd(II) and Pt(II) derivatives, room temperature phosphorescence is observed in solution. This is quenched by oxygen, and it is suggested that these may be good candidates for ratiometric sensing. The mechanism of quenching of triad triplets by oxygen depends on the metal ion, and both type-I, which involves hydrogen-atom abstraction or electron-transfer between the excited sensitizer and a substrate yielding free radicals and type II, in which singlet oxygen is generated via an energy transfer process during a collision of the excited sensitizer with triplet oxygen processes, may be involved. The good solubility and photophysical characteristics of the platinum(II) triad may make this useful as a phosphor in organic light emitting diodes.