Ko Furukawa

Charge Dynamics in A Donor–Acceptor Covalent Organic Framework with Periodically Ordered Bicontinuous Heterojunctions

The donor–acceptor heterojunction is a key structure in current technologies, including transistors, light-emitting diodes, and photovoltaics, because it controls the charge dynamics in the devices. Covalent organic frameworks (COFs) are crystalline molecular skeletons that allow atomically precise integration of building blocks into periodic array structures. In this regard, we have demonstrated arene, porphyrin, and phthalocyanine COFs that provide periodically ordered columnar arrays of p-components and show outstanding semiconducting and photoconductive properties. We recently synthesized a donor–acceptor COF that gives rise to a periodically ordered bicontinuous heterojunction structure and self-sorted donor and acceptor columnar arrays separated at nanometer-scale intervals. This nanoscopic segregation morphology forms a broad interface for charge separation, provides ambipolar pathways for charge collection, and would be ideal for the current semiconducting devices that involve photoenergy transformations; however, the charge dynamics, which is a key mechanism that controls the energy transformation, remains unclear. Here, we report the charge dynamics of a donor–acceptor COF, which were determined using time-resolved spectroscopy to elucidate the photochemical processes of the free charges from their generation to delocalization and retention. In the COF, the heterojunctions allow an ultrafast electron transfer from the donor to the acceptor columns. Consequently, the light absorption is directly coupled with charge dissociation to generate free charges in the donor and acceptor p-columns within 2 ps. On the other hand, the stacked p-columns delocalize the charges, suppress charge recombination, and retain the charges for a prolonged period of time. We show that both solvated and solid-state COFs enable rapid charge separation and exceptional long-term charge retention, thereby providing a key mechanistic basis to envisage the high potential of donor–acceptor COFs for photoelectric applications. The donor–acceptor COF (Scheme 1a, DZnPc-ANDI-COF) is a tetragonal, mesoporous 2D framework that is composed of zinc phthalocyanine as an electron donor and naphthalene diimide as an acceptor. In the COF, the two p-units are alternately linked within an electron-transfer distance and at a dihedral angle of approximately 428. The COF provides selfsorted, bicontinuous columnar arrays and constitutes periodically structured heterojunctions in which each donor column is interfaced with four acceptor columns that are equally active in capturing photo-generated electrons (Scheme 1b). The DZnPc-ANDI-COF absorbs light over a broad visible and near-infrared region up to 1100 nm (Figure S1 in the Supporting Information). Elemental analysis, infrared spectroscopy, nuclear magnetic resonance spectroscopy, and electron microscopy confirmed the formation of the COF (Figure S2–S4 and Table S1). The same COF has been reported as a thin film. The DZnPc-ANDI-COF exhibited a type IV nitrogen sorption curve that is characteristic of mesoporous frameworks (Figure 1a). The Brunauer–Emmett–Teller surface area and pore volume were calculated as 1410 mg 1 and 1.25 cmg , respectively. The pore-size distribution profile with a range up

Photoelectric covalent organic frameworks: converting open lattices into ordered donor-acceptor heterojunctions.

Ordered one-dimensional open channels represent the typical porous structure of two-dimensional covalent organic frameworks (COFs). Here we report a general synthetic strategy for converting these open lattice structures into ordered donor-acceptor heterojunctions. A three-component topological design scheme was explored to prepare electron-donating intermediate COFs, which upon click reaction were transformed to photoelectric COFs with segregated donor-acceptor alignments, whereas electron-accepting buckyballs were spatially confined within the nanochannels via covalent anchoring on the channel walls. The donor-acceptor heterojunctions trigger photoinduced electron transfer and allow charge separation with radical species delocalized in the π-arrays, whereas the charge separation efficiency was dependent on the buckyball content. This new donor-acceptor strategy explores both skeletons and pores of COFs for charge separation and photoenergy conversion.

A Stable Non-Kekulé Singlet Biradicaloid from meso-Free 5,10,20,25-Tetrakis(Pentafluorophenyl)-Substituted [26]Hexaphyrin(1.1.1.1.1.1)

A meso,meso-diketohexaphyrin was isolated and characterized as a chemically stable non-Kekulé singlet biradicaloid. Two unpaired electrons are seemingly delocalized on two tripyrrolic units separated by C=O bonds. These results underscore the potential of expanded porphyrins to achieve unique electronic states.

Gd2@C79N: Isolation, Characterization, and Monoadduct Formation of a Very Stable Heterofullerene with a Magnetic Spin State of S = 15/2

The dimetallic endohedral heterofullerene (EHF), Gd(2)@C(79)N, was prepared and isolated in a relatively high yield when compared with the earlier reported heterofullerene, Y(2)@C(79)N. Computational (DFT), chemical reactivity, Raman, and electrochemical studies all suggest that the purified Gd(2)@C(79)N, with the heterofullerene cage, (C(79)N)(5-) has comparable stability with other better known isoelectronic metallofullerene (C(80))(6-) cage species (e.g., Gd(3)N@C(80)). These results describe an exceptionally stable paramagnetic molecule with low chemical reactivity with the unpaired electron spin density localized on the internal diatomic gadolinium cluster and not on the heterofullerene cage. EPR studies confirm that the spin state of Gd(2)@C(79)N is characterized by a half-integer spin quantum number of S = 15/2. The spin (S = ½) on the N atom of the fullerene cage and two octet spins (S = 7/2) of two encapsulated gadoliniums are coupled with each other in a ferromagnetic manner with a small zero-field splitting parameter D. Because the central line of Gd(2)@C(79)N is due to the Kramers doublet with a half-integer spin quantum number of S = 15/2, this relatively sharp line is prominent and the anisotropic nature of the line is weak. Interestingly, in contrast with most Gd(3+) ion environments, the central EPR line (g = 1.978) is observable even at room temperature in a toluene solution. Finally, we report the first EHF derivative, a diethyl bromomalonate monoadduct of Gd(2)@C(79)N, which was prepared and isolated via a modified Bingel-Hirsch reaction.

Creation of Superheterojunction Polymers via Direct Polycondensation: Segregated and Bicontinuous Donor–Acceptor π-Columnar Arrays in Covalent Organic Frameworks for Long-Lived Charge Separation

By developing metallophthalocyanines and diimides as electron-donating and -accepting building blocks, herein, we report the construction of new electron donor-acceptor covalent organic frameworks (COFs) with periodically ordered electron donor and acceptor π-columnar arrays via direct polycondensation reactions. X-ray diffraction measurements in conjunction with structural simulations resolved that the resulting frameworks consist of metallophthalocyanine and diimide columns, which are ordered in a segregated yet bicontinuous manner to form built-in periodic π-arrays. In the frameworks, each metallophthalocyanine donor and diimide acceptor units are exactly linked and interfaced, leading to the generation of superheterojunctions-a new type of heterojunction machinery, for photoinduced electron transfer and charge separation. We show that this polycondensation method is widely applicable to various metallophthalocyanines and diimides as demonstrated by the combination of copper, nickel, and zinc phthalocyanine donors with pyrommellitic diimide, naphthalene diimide, and perylene diimide acceptors. By using time-resolved transient absorption spectroscopy and electron spin resonance, we demonstrated that the COFs enable long-lived charge separation, whereas the metal species, the class of acceptors, and the local geometry between donor and acceptor units play roles in determining the photochemical dynamics. The results provide insights into photoelectric COFs and demonstrate their enormous potential for charge separation and photoenergy conversions.

Large pore donor–acceptor covalent organic frameworks

We report the synthesis and structural characterization of large pore covalent organic frameworks (COFs) integrated with donor and acceptor building blocks. The donor and acceptor, based on triphenylene and diimide, respectively, are topologically linked to form COFs with stacked donor and acceptor columns and 5.3 nm width channels, which show high crystallinity and large surface area. By varying donor–acceptor structure in conjunction with time-resolved spectroscopy, these large-pore COFs constitute benchmark frameworks to elucidate not only the importance of donor–acceptor pairing but also the role of lattice structure in charge transfer and separation, thereby casting a general principle for the structural design of optoelectronic and photovoltaic COFs.

Ferromagnetic spin coupling between endohedral metallofullerene La@C82 and a cyclodimeric copper porphyrin upon inclusion.

The cyclic host cyclo-[P(Cu)](2) carrying two covalently connected Cu(II) porphyrin units can accommodate La@C(82), a paramagnetic endohedral metallofullerene, in its cavity to form the inclusion complex cyclo-[P(Cu)](2)⊃La@C(82), which can be transformed into the caged complex cage-[P(Cu)](2)⊃La@C(82) by ring-closing olefin metathesis of its side-chain olefinic termini. On the basis of electron spin resonance (ESR) and electron spin transient nutation (ESTN) studies, cyclo-[P(Cu)](2)⊃La@C(82) is the first ferromagnetically coupled inclusion complex featuring La@C(82), whereas cage-[P(Cu)](2)⊃La@C(82) is ferrimagnetic.

Switchable Intermolecular Communication in a Four-Fold Rotaxane†

In biological systems, precise intermolecular electronic communication occurs in, and is regulated by, proteins and their complexes. For example, light-harvesting complexes have metal ions or metal complexes, such as manganese clusters, and organic cofactors, such as chlorophylls, carotenoids, or ubiquinones, in the “right” spatial arrangement so that electrons and energy can be efficiently transferred. 2] Electronic communication between molecules is sensitive to their relative orientation. Large molecular scaffolds of proteins provide precise frameworks for molecular arrays as well as the ability of dynamic switching intermolecular communication, accomplished by conformational changes induced by external stimuli. These natural molecular frameworks are programmed as sequences of amino acids, but we could achieve flexible intermolecular communication in a simpler molecular architecture by using mechanically interlocked supramolecular motives, such as catenanes and rotaxanes, in which two or more molecular components are inseparable but their interactions are flexibly convertible. 4] Syntheses of rotaxanes have been well-established, and a large variety of fascinating examples of rotaxane-based molecular systems has been reported. As a specific example of switchable rotaxanes, Stoddart, Credi and co-workers reported elevatorlike motion of a molecule with crown ethers along the strings on another molecule in a three-fold rotaxane, in which the relative position between the molecules was switchable by redox or acid-base reactions. Sauvage and co-workers made a porphyrin dimer in a cyclic [4]rotaxane and showed its ability to act as switchable receptor. Porphyrin, phthalocyanine, and their metal complexes have been applied to a broad range of functionalized molecular systems. Intermolecular electronic communication in their programmed arrays particularly has attracted a lot of interests in a wide range of fields, from photomaterials to pigments, molecular devices, and catalysts. Herein, we report a mechanically linked cofacially stacked dimer of a metalloporphyrin and metallophthalocyanine units by formation of a four-fold rotaxane and its switchable spin–spin communication induced by external stimuli. A rotaxane consisting of a secondary ammonium ion and a crown ether has been recognized as a versatile building block for supramolecular systems. R2NH2 + ions are complexed by dibenzo[24]crown-8 in a threaded manner as a result of electrostatic stabilization and hydrogen bonds between the negatively charged interior of the crown ether and the cationic ammonium moiety. We designed a porphyrin unit with four alkylammonium chains [1·4H] and phthalocyanine unit with peripheral crown ethers 2 which are expected to form a facially stacked dimer through formation of a four-fold rotaxane (Figure 1). The four-fold rotaxane [3·8 H]·4 Cl was obtained in 41% yield through formation of a pseudo-rotaxane between the porphyrin [1·5H]·5 BARF (BARF = tetrakis[(3,5-bistrifluoromethyl)phenyl]borate) and the phthalocyanine 2 in chloroform and acetone at a ratio of 4:1, followed by locking through Staudinger-phosphite reaction, which converts the azide groups to larger phosphoramidate units. The structure of the four-fold [2]rotaxane [3·8 H]·4Cl was given unambiguously by ESI-TOF mass spectrometry and NMR spectroscopy (Figure 2). The ESI-MS data clearly showed m/z values of 1365.0 (z = 3), 1377.0 (z = 3), and 1024.0 (z = 4), which confirmed the interlocked structure between [1·4H] and 2 (calculated m/z values for [3 + 7H], [3 + 8H + Cl], and [3 + 8H] are 1365.0, 1377.0, and 1024.0, respectively; Figure 2b). The highly symmetric structure of [3·8H]·4 Cl was shown in the H NMR spectrum (Figure 2a). All signals in the spectrum were assignable as one-quarter part of the four-fold rotaxane. Especially diagnostic are the high-fieldshifted signals of the pyrrolic NH protons of both porphyrin and phthalocyanine units (Ha and Ha in Figure 2a, respectively), indicating a p–p stacking interaction and a close spatial arrangement between the crown ethers and the [*] Dr. Y. Yamada, Prof. Dr. K. Tanaka Department of Chemistry, Graduate School of Science Nagoya University Furo-cho, Chikusa-ku, Nagoya 464-8602 (Japan) E-mail: kentaro@chem.nagoya-u.ac.jp

meso‐Hydroxysubporphyrins: A Cyclic Trimeric Assembly and a Stable meso‐Oxy Radical

Treatment of meso-chlorosubporphyrin with potassium hydroxide in DMSO followed by aqueous work up and recrystallization gave a cyclic trimer consisting of meso-hydroxysubporphyrin units linked between the central boron atoms and meso-hydroxy groups. Solutions of this trimer are nonfluorescent, but become fluorescent when exposed to acid or base, since hydrolytic cleavage of the axial B-O bonds generates the meso-hydroxysubporphyrin monomer or its oxyanion. Ring cleavage of the trimer was also effected by reaction with phenylmagnesium bromide to produce meso-hydroxy-B-phenyl subporphyrin, which can be quantitatively oxidized with PbO2 to furnish a subporphyrin meso-oxy radical as a remarkably stable species as a result of spin delocalization over almost the entire molecule.

Triarylporphyrin meso-Oxy Radicals: Remarkable Chemical Stabilities and Oxidation to Oxophlorin π-Cations

5-Hydroxy-10,15,20-triarylporphyrin (oxophlorin) and its Ni(II) and Zn(II) complexes were oxidized with PbO2 to give the corresponding porphyrin meso-oxy radicals as remarkably stable species. These radicals were fully characterized with X-ray diffraction analysis, UV/vis/NIR absorption and ESR spectroscopies, magnetic susceptibility measurement, electrochemical studies, and theoretical calculations. Free-base radical and its Ni(II) complex have been shown to exist as a monoradical in solution, while the Zn(II) complex exists in an equilibrium between monomer (doublet monoradical) and dimer (a non-Kekulé singlet biradicaloid) with a dimerization constant of KD = 3.0 × 10(5) M(-1) in noncoordinating CH2Cl2 but becomes a pyridine-coordinated monoradical upon addition of pyridine. Variable temperature magnetic susceptibility measurements of these radicals revealed different magnetic interactions in the solid-states, which has been interpreted in terms of their different packing structures in a microscopic sense. These radicals undergo one-electron oxidation and reduction in a reversible manner within narrow potential windows of 0.57-0.82 V. Finally, one-electron oxidation of Ni(II) and Zn(II) porphyrin meso-oxy radicals with tris(4-bromophenyl)aminium hexachloroantimonate furnished oxophlorin π-cations, which displayed nonaromatic closed-shell character, NIR absorption, and significant double bond character of the C-O bond.