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)₂M]. 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)₂·2H₂O in THF (∼80% 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)₂Pd triad (50% yield), followed by selective demetalation of the (dp)₂Pd 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)₂·2H₂O 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)₂Zn were prepared in 83−97% yield by treatment of a porphyrin-dipyrrin with Zn(OAc)₂·2H₂O 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)₂Zn 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.