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.