Excited-state energy-transfer dynamics in self-assembled triads composed of two porphyrins and an intervening bis(dipyrrinato)metal complex

The synthesis and characterization of various triads composed of a linear array of two zinc porphyrins joined via an intervening bis(dipyrrinato)metal(II) complex are reported. The preparation exploits the facile complexation of dipyrrins with divalent metal ions to give bis(dipyrrinato) metal (II) complexes [abbreviated (dP)(2)M]. Copper(II) and palladium(II) chelates of dipyrrins (available by oxidation of dipyrromethanes) were prepared in 50-80% yield. A one-flask synthesis of bis(dipyrrinato)zinc(II) complexes was developed by oxidation of a dipyrromethane with DDQ or p-chloranil in the presence of Zn(OAc)(2).2H(2)O in THF (similar to80% yield). Three routes were developed for preparing porphyrin-dipyrrins: (1) Suzuki coupling of a boronate-substituted zinc porphyrin (ZnP) and bis[5-(4-iodophenyl)dipyrrinato]Pd(II) to give the (ZnP-dP)(2)Pd triad (50% yield), followed by selective demetalation of the (dP)(2)Pd unit by treatment with 1,4-dithiothreitol under neutral conditions (71% yield); (2) oxidation of a porphyrin-dipyrromethane with p-chloranil in the presence of Zn(OAc)(2).2H(2)O followed by chromatography on silica gel (71% yield); and (3) condensation of a dipyrrin-dipyrromethane and a dipyrromethane-dicarbinol under InCl3 catalysis followed by oxidation with DDQ (10-16% yield). Four triads of form (ZnP-dP)(2)Zn were prepared in 83-97% yield by treatment of a porphyrin-dipyrrin with Zn(OAc)(2).2H(2)O at room temperature. Free base dipyrrins typically absorb at 430-440 nm, while the bis(dipyrrinato) metal complexes absorb at 460-490 nm. The fluorescence spectra/yields and excited-state lifetimes of the (ZnP-dP)(2)Zn triad in toluene show (1) efficient energy transfer from the bis(dipyrrinato)zinc(II) chromophore to the zinc porphyrins (98.5% yield), and (2) little or no quenching of the resulting excited zinc porphyrin relative to the isolated chromophore. Taken together, these results indicate that bis(dipyrrinato)zinc(II) complexes can serve as self-assembling linkers that further function as secondary light-collection elements in porphyrin-based light-harvesting arrays.