Atsuhiro Osuka

Discrete Cyclic Porphyrin Arrays as Artificial Light-Harvesting Antenna

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.

Expanded Porphyrins: Intriguing Structures, Electronic Properties, and Reactivities

The chemistry of expanded porphyrins, which are higher homologues of porphyrins, has been intensively explored for the last three decades. Expanded porphyrins exhibit structures, electronic properties, coordination chemistry, and reactivities that are entirely different from those of porphyrins. Through these studies, it has become increasingly apparent that expanded porphyrins are attractive in views of aromaticity and multimetal coordination, or as functional dyes, nonlinear optical materials, ion receptors, or stable organic radicals. As such, we have continuously witnessed the emergence of expanded porphyrins that exhibit unprecedented structures and properties, as is highlighted by the facile realization of Möbius aromatic and even antiaromatic systems with twisted molecular structures. In this Review, the recent progress of the chemistry of expanded porphyrins after the seminal Review by Sessler and Seidel in 2003 is presented.

Cyclic porphyrin arrays as artificial photosynthetic antenna: synthesis and excitation energy transfer

Covalently linked cyclic porphyrin arrays have been explored in recent years as artificial photosynthetic antenna. In this review we present the fundamental aspects of covalently linked cyclic porphyrin arrays by highlighting recent progress. The major emphasis of this tutorial review lies on the synthetic method, the structure, and the excitation energy transfer (EET) of such arrays. The final cyclization steps were often performed with the aid of templates. Efficient EET along the wheel is observed in these cyclic arrays, but ultrafast EET processes with rates of <1 ps, which rival those in the natural LH2, are rare and have been identified only in cyclic arrays 30-32 composed of directly meso-meso linked porphyrins.

Unambiguous Identification of Möbius Aromaticity for meso-Aryl-Substituted [28]Hexaphyrins(1.1.1.1.1.1)

meso-Aryl-substituted [28]hexaphyrins(1.1.1.1.1.1) have been examined by (1)H, (13)C, and (19)F NMR spectroscopies, UV-vis absorption spectroscopy, magnetic circular dichroism spectroscopy, and single-crystal X-ray diffraction analysis. All of these data consistently indicate that [28]hexaphyrins(1.1.1.1.1.1) in solution at 25 degrees C exist largely as an equilibrium among several rapidly interconverting twisted Möbius conformations with distinct aromaticities, with a small contribution from a planar rectangular conformation with antiaromatic character at slightly higher energy. In the solid state, [28]hexaphyrins(1.1.1.1.1.1) take either planar or Möbius-twisted conformations, depending upon the meso-aryl substituents and crystallization conditions, indicating a small energy difference between the two conformers. Importantly, when the temperature is decreased to -100 degrees C in THF, these rapid interconversions among Möbius conformations are frozen, allowing the detection of a single [28]hexaphyrin(1.1.1.1.1.1) species having a Möbius conformation. Detailed analyses of the solid-state Möbius structures of compounds 2b, 2c, and 2f showed that singly twisted structures are achieved without serious strain and that cyclic pi-conjugation is well-preserved, as needed for exhibiting strong diatropic ring currents. Actually, the harmonic-oscillator model for aromaticity (HOMA) values of these structures are significantly large (0.85, 0.69, and 0.71, respectively), confirming the first demonstration of stable Möbius aromatic systems consisting of free-base expanded porphyrins without the assistance of metal coordination.

Confusion, inversion, and creation—a new spring from porphyrin chemistry

This article mainly deals with the recent serendipity of novel porphyrin analogs such as N-confused porphyrin. The unique property of this ligand allows the formation of a variety of metal complexes. The important aspect of dynamic flipping (inversion), induced either by confusion or expansion of the macrocyclic core, that leads to the generation of new porphyrinoids, is emphasized. This review concludes with the recent progress on expanded porphyrins bearing confused, inverted, and fused pyrrole rings.

Kinetically Blocked Stable Heptazethrene and Octazethrene: Closed-Shell or Open-Shell in the Ground State?

Polycyclic aromatic hydrocarbons with an open-shell singlet biradical ground state are of fundamental interest and have potential applications in materials science. However, the inherent high reactivity makes their synthesis and characterization very challenging. In this work, a convenient synthetic route was developed to synthesize two kinetically blocked heptazethrene (HZ-TIPS) and octazethrene (OZ-TIPS) compounds with good stability. Their ground-state electronic structures were systematically investigated by a combination of different experimental methods, including steady-state and transient absorption spectroscopy, variable temperature NMR, electron spin resonance (ESR), superconducting quantum interfering device (SQUID), FT Raman, and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken density functional theory (DFT) calculations. All these demonstrated that the heptazethrene derivative HZ-TIPS has a closed-shell ground state while its octazethrene analogue OZ-TIPS with a smaller energy gap exists as an open-shell singlet biradical with a large measured biradical character (y = 0.56). Large two-photon absorption (TPA) cross sections (σ((2))) were determined for HZ-TIPS (σ((2))(max) = 920 GM at 1250 nm) and OZ-TIPS (σ((2))(max) = 1200 GM at 1250 nm). In addition, HZ-TIPS and OZ-TIPS show a closely stacked 1D polymer chain in single crystals.

Möbius aromaticity and antiaromaticity in expanded porphyrins

Aromaticity is a key concept in chemistry, dating back to Faradays discovery of benzene in 1825 and Kekulés famous alternating-double-bond structure of 1865. In 1858, the Möbius strip was discovered by Möbius and Listing. The Hückel rules for predicting aromaticity, stating that [4n + 2] π electrons result in an aromatic system, work for planar molecules. Although molecules with Möbius geometry are not found in nature, chemists have tried to synthesize such molecules since the first theoretical prediction by Heilbronner in 1964 and the prediction of Möbius aromaticity for suitable compounds with [4n] π electrons. However, Möbius-aromatic molecules have proved difficult to synthesize, and sometimes even to identify. Here we summarize recent contributions of several research groups that have succeeded in synthesizing Möbius-type molecules. The results of this survey lead us to suggest that the generation of Möbius topologies in expanded porphyrins is easier than hitherto appreciated.

WINDMILL-LIKE PORPHYRIN ARRAYS AS POTENT LIGHT-HARVESTING ANTENNA COMPLEXES

Up to 14 porphyrin rings are present in the title compounds 1, which are readily available with high regioselectivity from linear nickel-zinc porphyrins. Upon irradiation with light a rapid energy transfer from the peripheral porphyrin rings to the diporphyrin core takes place.

A Porphyrin Nanobarrel That Encapsulates C60

A porphyrin nanobarrel, 1, that can encapsulate C(60) effectively was prepared via a concise coupling route. The structures of both 1 and C(60)@1 were confirmed by single-crystal X-ray diffraction analysis.

Marriage of porphyrin chemistry with metal-catalysed reactions.

The development of porphyrin synthesis by means of transition metal-catalyzed reactions is explored in this feature article. Porphyrins have been receiving much attention in a wide area of chemistry as functional dyes, non-linear optical materials, ligands for a variety of metals, structural motifs in supramolecules, and so forth. However, they have been merely recognized as a reaction substrate in transition metal-catalyzed transformations. Recently, application of such new methodologies to porphyrin synthesis has proven to be very powerful to create new types of porphyrinic compounds, which have their own intriguing structures and properties. New transformations on porphyrins via transition metal catalysis offer us prospects of new designs of architectures, thus facilitating further development of this important class of functional molecules.