Naoki Aratani

Ultrahigh Porosity in Metal-Organic Frameworks

Network Approaches to Highly Porous Materials Metal-organic frameworks (MOFs), in which inorganic centers are bridged by organic linkers, can achieve very high porosity for gas absorption. However, as the materials develop larger void spaces, there is also more room for growing interpenetrating networks—filling the open spaces not with gas molecules but with more MOFs. Furukawa et al. (p. 424, published online 1 July) describe the synthesis of a MOF in which zinc centers are bridged with long, highly conjugated organic linkers, but in which the overall symmetry of the networks created prevents formation of interpenetrating networks. Extremely high surface areas and storage capacities for hydrogen, carbon dioxide, and methane were observed. The large surface areas of these materials would correspond to that of dispersed nanocubes just 3 to 6 nanometers wide. Crystalline solids with extended non-interpenetrating three-dimensional crystal structures were synthesized that support well-defined pores with internal diameters of up to 48 angstroms. The Zn4O(CO2)6 unit was joined with either one or two kinds of organic link, 4,4′,4″-[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]tribenzoate (BTE), 4,4′,44″-[benzene-1,3,5-triyl-tris(benzene-4,1-diyl)]tribenzoate (BBC), 4,4′,44″-benzene-1,3,5-triyl-tribenzoate (BTB)/2,6-naphthalenedicarboxylate (NDC), and BTE/biphenyl-4,4′-dicarboxylate (BPDC), to give four metal-organic frameworks (MOFs), MOF-180, -200, -205, and -210, respectively. Members of this series of MOFs show exceptional porosities and gas (hydrogen, methane, and carbon dioxide) uptake capacities. For example, MOF-210 has Brunauer-Emmett-Teller and Langmuir surface areas of 6240 and 10,400 square meters per gram, respectively, and a total carbon dioxide storage capacity of 2870 milligrams per gram. The volume-specific internal surface area of MOF-210 (2060 square meters per cubic centimeter) is equivalent to the outer surface of nanoparticles (3-nanometer cubes) and near the ultimate adsorption limit for solid materials.

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.

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.

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.

Synthesis, Structure, and Air‐stable N‐type Field‐Effect Transistor Behaviors of Functionalized Octaazanonacene‐8,19‐dione

Increasing the length of N-heteroacenes or their analogues is highly desirable because such materials could have great potential applications in organic electronics. In this report, the large π-conjugated N-heteroquinone 6,10,17,21-tetra-((triisopropylsilyl)ethynyl)-5,7,9,11,16,18,20,22-octaazanonacene-8,19-dione (OANQ) has been synthesized and characterized. The as-prepared OANQ shows high stability under ambient conditions and has a particularly low LUMO level, which leads to it being a promising candidate for air-stable n-type field-effect transistors (FETs). In fact, FET devices based on OANQ single crystals have been fabricated and an electron mobility of up to 0.2 cm(2) V(-1) s(-1) under ambient conditions is reported. More importantly, no obvious degradation was observed even after one month. Theoretical calculations based on the single crystal are consistent with the measured mobility.

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.

Photochemistry of covalently-linked multi-porphyrinic systems

Abstract Synthesis, structural characteristics, and optical and electrochemical properties of various covalently-linked porphyrin arrays are described. First, aromatic-spacer bridged diporphyrins were prepared in which the diporphyrin geometries were conformationally-restricted and thus suitable for detailed studies on the exciton coupling and the intramolecular energy and/or electron transfer reactions. Secondly, the Ag(I)-salt oxidation of 5,15-diaryl Zn(II) porphyrins provided meso–meso-linked Zn(II)-diporphyrins. This reaction is advantageous in light of its high regioselectivity and easy extension to longer porphyrin arrays. The doubling reaction was repeated up to the synthesis of a discrete 128-mer, which is, to the best of our knowledge, the longest man-made molecule. Finally, the oxidation of meso–meso-linked Zn(II) porphyrin arrays with a combination of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and Sc(III)(OTf)3 produced fused porphyrin arrays with full π-conjugation, which displayed extremely small HOMO–LUMO gaps that reach into the infrared region.

π-Conjugation Enlargement Toward the Creation of Multi-Porphyrinic Systems with Large Two-Photon Absorption Properties

Recent progress in the synthesis of covalently linked porphyrin arrays with large two-Photon absorption (TPA) cross-section values has been reviewed with a particular focus on the relation of TPA properties with molecular structures. Covalently linked porphyrin arrays continue to be important and useful for the creation of functional materials owing to their chemical robustness, fine-tuning, and easy manipulation. More importantly, the porphyrin electronic systems are quite susceptible to periphery conjugative perturbations, hence allowing facile fabrications to extensively delocalized systems. This property has been used for exploration of porphyrin-based molecular systems with large TPA values, demonstrating a general trend that enhancement in electronic interactions leads to large TPA cross-section values. As a representative example, the porphyrin tapes exhibit larger TPA values owing to the fully delocalized nature of the pi-electrons. This Focus Review will help understand the structural requirements of porphyrin arrays with large TPA values, which will be useful for future applications in optical communication in the IR region.

Two-Dimensionally Extended Porphyrin Tapes: Synthesis and Shape-Dependent Two-Photon Absorption Properties

We report the synthesis and characterization of L- and T-shaped porphyrin tapes as extensible structural motifs of two-dimensionally extended porphyrin tapes. The two-photon absorption (TPA) cross-section values (sigma((2))) for L- and T-shaped porphyrin tapes as well as those for linear trimeric and tetrameric porphyrin tapes were measured by an open-aperture Z-scan method at 2300 nm, a wavelength at which the one-photon absorption contribution is either zero or almost negligible. Under these conditions, the sigma((2)) values for the linear porphyrin tape trimer and tetramer were determined to be 18 500 and 41 200 GM, respectively. The sigma((2)) value for the L-shaped trimer was determined to be 8700 GM, which is only half that of the linear trimer, whereas the sigma((2)) value for the T-shaped tetramer was measured to be 35 700 GM. These results clearly indicate the dependence of the TPA cross-section on the molecular shape, which underscores the importance of directionality in the pi-conjugation pathway for the enhancement of TPA cross- section.

Synthesis of Extremely π‐Extended Porphyrin Tapes from Hybrid meso‐meso Linked Porphyrin Arrays: An Approach Towards the Conjugation Length

Directly meso-meso, beta-beta, beta-beta triply linked porphyrin arrays are exceptional pi-conjugated molecules exhibiting remarkably red-shifted absorption bands extending deeply in the IR region. In order to determine the effective conjugated length (ECL), we embarked on the synthesis of the porphyrin tapes far beyond the 12-mer, which is the longest we have prepared so far. In this study, to find the compromise between the feasibility of the meso-meso coupling reaction up to longer arrays and the sufficient solubility and chemical stability of the resultant porphyrin tapes, we prepared hybrid meso-meso linked porphyrin arrays BOn up to 24-mer, which have two different aryl groups, a 2,4,6-tris(3,5-di-tert-butylphenoxy) phenyl group (Ar(1)) and a 3,5-dioctyloxy phenyl group (Ar(2)). All these arrays were effectively converted into the corresponding triply linked porphyrin tapes TBOn by oxidation with DDQ-Sc(OTf)(3). Importantly, the low energy Q-band-like absorption bands of TBOn are progressively red-shifted with an increase in the number of porphyrins n until 16 but the red-shift is saturated at n=16, indicating the ECL of the porphyrin tape to be around 14-16. The regularly introduced meso-aryl bulky substituents impose facial encumbrance, hence leading to the effective suppression of pi-pi interactions as well as improvement of the chemical stabilities of TBOn.