Discrete Cyclic Porphyrin Arrays as Artificial Light-Harvesting Antenna

Abstract

The importance of photosynthesis has driven researchers to seek ways to mimic its fundamental features in simplified systems. The absorption of a photon by light-harvesting (antenna) complexes made up of a large number of protein-embedded pigments initiates photosynthesis. Subsequently the many pigments within the antenna system shuttle that photon via an efficient excitation energy transfer (EET) until it encounters a reaction center. Since the 1995 discovery of the circularly arranged chromophoric assemblies in the crystal structure of light-harvesting antenna complex LH2 of purple bacteria Rps. Acidophila, many designs of light-harvesting antenna systems have focused on cyclic porphyrin wheels that allow for efficient EET. In this Account, we review recent research in our laboratories in the synthesis of covalently and noncovalently linked discrete cyclic porphyrin arrays as models of the photosynthetic light-harvesting antenna complexes. On the basis of the silver(I)-promoted oxidative coupling strategy, we have prepared a series of extremely long yet discrete meso-meso-linked porphyrin arrays and covalently linked large porphyrin rings. We examined the photophysical properties of these molecules using steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transient absorption measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly related to the EET processes within the porphyrin rings. Within these structures, the exciton-exciton annihilation time and the polarization anisotropy rise time are well-described in terms of the Forster-type incoherent energy hopping model. In noncoordinating solvents such as CHCl(3), meso-pyridine-appended zinc(II) porphyrins and their meso-meso-linked dimers spontaneously assemble to form tetrameric porphyrin squares and porphyrin boxes, respectively. In the latter case, we have demonstrated the rigorous homochiral self-sorting process and efficient EET along these cyclic porphyrin arrays. The meso-cinchomeronimide appended zinc(II) porphyrin forms a cyclic trimer. We have also shown that the corresponding meso-meso-linked diporphyrins undergo high-fidelity self-sorting assembling to form discrete cyclic trimer, tetramer, and pentamer with large association constants through perfect discrimination of enantiomeric and conformational differences of the meso-cinchomeronimide substituents. Collectively, these studies of covalently and noncovalently linked discrete cyclic porphyrin arrays aid in the understanding of the structural requirements for such very fast EET in natural light-harvesting complexes.

Synthesis and photophysical studies of self-assembled multicomponent supramolecular coordination prisms bearing porphyrin faces

“…Many of these designs incorporate porphyrin-based molecules (17)(18)(19)(20)(21)(22), a pigment that is related to the ubiquitous chlorophyll found in natural light-harvesting systems (23). Porphyrins possess characteristic structural and photophysical properties that make them well suited for adaptation into artificial designs, not only for solar energy conversion but also for applications in photodynamic therapy (24)(25)(26)(27)(28)(29), enzyme mimics (30)(31)(32), catalysis (33)(34)(35)(36)(37), and molecular electronic devices (38,39).…”

Meso-Linked Multiporphyrins as Model for Light Harvesting Systems: Review

“…A series of extremely long yet discrete meso-meso linked porphyrin arrays and covalently linked large porphyrin rings has been synthesized using silver(I)-promoted oxidative coupling strategy by Osuka et al [98]. The photophysical properties of these molecules were studied using steadystate absorption, fluorescence, fluorescence lifetime, fluorescence Meso-pyridine-linked zinc(II) porphyrins and their mesolinked dimers have been reported to get assembled spontaneously in the presence of non-coordinating solvents such as CHCl 3 to form tetrameric porphyrin squares and porphyrin boxes, respectively [98]. In the porphyrin boxes, efficient EET throughout the cyclic porphyrin arrays was well recognized along with rigorous homochiral selfsorting process.…”

Porous–Hybrid Polymers as Platforms for Heterogeneous Photochemical Catalysis

“…17 In order to generate artificial photosynthesis systems, several attempts were made to mimic the energy transfer processes of photo-generated excitons in naturally occurring photosystems. [23][24][25][26] 2 Scheme 1. Synthesis scheme for PRP-1 (left) and a geometrically optimized polymer fragment model showing extension points highlighted in yellow (right).Nitrate ions are omitted for clarity; C (gray), H (white), N (blue), Ru (deep red).…”