Excited-State Energy-Transfer Dynamics in Self-Assembled Triads Composed of Two Porphyrins and an Intervening Bis(dipyrrinato)metal Complex

Abstract

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 ( approximately 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)(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 InCl(3) 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.

Supramolecular Chemistry of Acyclic Oligopyrroles

“…With regard to coordinating ligands, dipyrrins (dipyrromethenes), as partial structures of bile pigments consisting of two pyrroles with an sp 2 meso position, are essential π-conjugated bidentate monoanionic ligands for metal ions in natural and artificial systems. [11][12][13] Multitopic dipyrrin derivatives are promising scaffolds for self-assemblies and would provide neutral coordination oligomers, which, in combination with various spacer units, could be used to fabricate supramolecular structures and fine-tuned nanoscale morphologies by means of bridging metal cations. For example, Cohen et al showed infinite coordination networks based on dipyrrin derivatives in the solid state (Figure 2a).…”

“…[12] Lindsey et al reported dipyrrin metal complexes as bridging units for energy transfer systems consisting of porphyrins (Figure 2b). [13] Apart from these studies, the author has also attempted to form coordination oligomers to yield nanoscale spherical architectures and discrete coordination macrocycles by using dimeric dipyrrin derivatives. [14][15][16] In natural systems, the double helix of DNA is constructed by complementary hydrogen-bonding between the base pairs, and the high-dimensional structures of proteins are maintained by means of noncovalent interactions between subunits such as amides.…”

The influence of bridging ligand electronic structure on the photophysical properties of noble metal diimine and triimine light harvesting systems

“…Many photosynthetic light harvesting systems are arranged as noncovalent molecular assemblies, with the degree of systematic order in the ensembles varying widely (Logunov and Rodgers 1992;Fujihira 1993;Cox 1997;Chen et al 2002;Yu et al 2003). While there are a large number of self-assembling synthetic light harvesting systems, there are also many artificial systems that involve covalently linked arrays of chromophores that can be isolated and fully characterized.…”

A Bisfullerene–Bis(dipyrrinato)zinc Complex: Electronic Coupling and Charge Separation in an Easy‐to‐Assemble Synthetic System

“…[6,9] Moreover, Bocian, Holten, Lindsey et al have exploited the (dpy)-Zn-(dpy) linkage to create bisporphyrinbased energy-transfer systems. [10] However, to the best of our knowledge bridging bis(dipyrrinato)zinc motifs have not been used as linking moieties to produce charge-separating devices. Complex 1 was designed to test this potential utility.…”