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Iron-incorporated ordered carbonaceous frameworks (OCFs) have been synthesized by the pyrolysis of iron porphyrin with ethynyl groups as polymerizable/carbonizable moieties at a temperature higher than 600 °C. The pyrolysis behavior is analyzed by thermogravimetry-differential thermal analysis-mass spectrometry, and the obtained carbon materials are characterized by X-ray diffraction, X-ray absorption fine structure, and high-angle annular dark-field scanning transmission electron microscopy. The introduction of ethynyl groups at the meta positions of the peripheral phenyl groups is essential for efficient cross-linking upon thermal polymerization. The thermally stable polymer thus obtained can be transformed into OCFs with high carbonization yield of 93 % at the subsequent carbonization. OCFs possess periodic structural regularity and porphyrin Fe-N4 coordination structure, and exhibit electrocatalysis for the conversion of CO2 into CO.

One of the challenging targets in materials synthesis is a direct conversion of molecular based crystalline substances into crystalline ordered carbonaceous frameworks (OCFs). Though we have discov...

Force-responsive ordered carbonaceous frameworks (OCFs) are synthesized for the first time. Carbonization of Ni porphyrin monomers having eight polymerizable ethynyl groups yields OCFs with atomically dispersed divalent Ni species and developed micropores. The highest specific surface area (673 m2 g−1) among the OCFs has been achieved. The OCFs thus synthesized comprise non-stacked graphene sheets, affording a unique mechanical flexibility that enables force-driven reversible phase transition.

Despite recent advances in the carbonization of organic crystalline solids like metal-organic frameworks or supramolecular frameworks, it has been challenging to convert crystalline organic solids into ordered carbonaceous frameworks. Herein, we report a route to attaining such ordered frameworks via the carbonization of an organic crystal of a Ni-containing cyclic porphyrin dimer (Ni2-CPDPy). This dimer comprises two Ni–porphyrins linked by two butadiyne (diacetylene) moieties through phenyl groups. The Ni2-CPDPy crystal is thermally converted into a crystalline covalent-organic framework at 581 K and is further converted into ordered carbonaceous frameworks equipped with electrical conductivity by subsequent carbonization at 873–1073 K. In addition, the porphyrin’s Ni–N4 unit is also well retained and embedded in the final framework. The resulting ordered carbonaceous frameworks exhibit an intermediate structure, between organic-based frameworks and carbon materials, with advantageous electrocatalysis. This principle enables the chemical molecular-level structural design of three-dimensional carbonaceous frameworks., Carbon-based materials are promising alternatives to noble metal catalysts, but their structures are typically disordered and difficult to control. Here, the authors obtain ordered carbonaceous frameworks with advantageous electrocatalytic properties via the carbonization of nickel-containing porphyrin dimer networks.

To gain insight into the singlet oxygen (1O2) generation properties of supramolecular complexes of cyclic free-base porphyrin dimer with fullerene C60, we evaluated the 1O2 quantum yield (ΦΔ) and rate constant (Kobs) of 1O2 generation for a cyclic free-base porphyrin dimer (CPD) linked by butadiyne bearing four 4-pyridyl groups and its inclusion complex (C60⊂CPD) with C60. We demonstrate that CPD and C60⊂CPD possess the ability to generate 1O2 under visible light irradiation. Moreover, it was found that the ΦΔ value of C60⊂CPD is lower than that of CPD. Based on the kinetic and thermodynamic consideration concerning the electron transfer processes between the porphyrin dimer and C60, this work revealed that the lower ΦΔ value of the C60 inclusion complex would be attributed to the formation of the charge-separated state C60·−-CPD·+, leading to a low intersystem crossing (ISC) efficiency for the formation of the triplet excited state 3(CPD)*.

The monomer, dimer, and trimer of 5,15-diphenyl-10,20-di(pyridin-4-yl)porphyrin are used to investigate the multianchoring effect on TiO2 for visible light-driven photocatalytic hydrogen production in a water medium. Further, the porphyrin trimer is prepared and analyzed by nuclear magnetic resonance (NMR) spectroscopy, absorption spectroscopy, electrochemical voltammetry, fast atom bombardment (FAB) mass spectroscopy, and density functional theory (DFT) computation. The results of this study indicate that the peak intensities of the absorption spectra increase as the number of porphyrin units increases, while changes could be barely observed in the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps. The porphyrin dimer in a 1 wt % Pt-loaded TiO2 powder photocatalyst system exhibited optimal hydrogen production performance in a stable state over a period of 80 h and at a superior rate of 1023 μmol·g–1·h–1. Further, the stability of the photocatalytic system was systematically investigated using films containing dyes on 1 wt % Pt-loaded TiO2/FTO. For a film containing the dimer, almost no change was observed in the hydrogen-bond coordination mode of the dimer and the photocurrent during the photocatalytic reaction. However, the photocurrents of the monomer and trimer were altered during visible light irradiation without altering the coordination mode, indicating that the arrangements and orientations of the porphyrins on TiO2 surfaces were altered. These results indicate that the presence of multiple anchoring groups enhance the stability of the photocatalytic system and the rate of hydrogen production.

Cyclic free-base porphyrin dimers (H4-C4-CPDPy(TEO) and H4-Ptz-CPDPy(TEO)) linked by butadiyne or phenothiazine bearing four 4-pyridyl groups and their inclusion complexes (C60⊂H4-C4-CPDPy(TEO) and C60⊂H4-Ptz-CPDPy(TEO)) with fullerene C60 have been applied to dye-sensitized solar cells (DSSCs) as a new class of porphyrin dye sensitizers with pyridyl anchoring groups for attachment on a TiO2 electrode. The FTIR spectra of the porphyrin dimers adsorbed on TiO2 nanoparticles demonstrated that these porphyrin dimers are adsorbed on the TiO2 surface through the formation of hydrogen bonding of pyridyl groups and/or pyridinium ions at Bronsted acid sites on the TiO2 surface. The adsorption amount of the porphyrin dimers adsorbed on the TiO2 electrode is 2.0 × 1017 molecules per cm2, that is, the adsorption amount of the porphyrin unit is 4.0 × 1017 cm−2, which is higher than that of dye sensitizers with pyridyl groups reported so far. The photovoltaic performance of DSSCs based on phenothiazine-bridged cyclic porphyrin dimer H4-Ptz-CPDPy(TEO) is higher than that of DSSCs based on butadiyne-linked cyclic porphyrin dimer H4-C4-CPDPy(TEO). Moreover, the photovoltaic performances of DSSCs based on cyclic free-base porphyrin dimers are higher than those of DSSCs based on their C60 inclusion complexes C60⊂H4-C4-CPDPy(TEO) and C60⊂H4-Ptz-CPDPy(TEO). On the basis of the electrochemical measurements (voltammetry and electrochemical impedance spectroscopy) and the transient absorption spectroscopy, the differences in the photovoltaic performances among these cyclic free-base porphyrin dimers are discussed from kinetic and thermodynamic considerations concerning the electron transfer processes in DSSCs.

The induced-fit accommodation of a variety of gaseous molecules including non-polar molecules has been demonstrated in porphyrin-based supramolecular architectures for the first time. Moreover, the gas uptake behaviour can be modulated by changing the central cation of porphyrin.

C60, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), lithium-cation-encapsulated C60 (Li+@C60), and [6,6]-diphenyl-C62-bis(butyric acid methyl ester) (bis-PCBM) were included into a phenothiazine-bridged cyclic free-base porphyrin dimer (H4-Ptz-CPDPy(TEO)) in a polar solvent (benzonitrile) with large association constants of 1.3 × 106, 6.4 × 105, 3.2 × 106, and 2.5 × 105 M–1, respectively. Based on the electrochemical data, the lowest energy levels of the charge-separated (CS) states for the inclusion complexes of H4-Ptz-CPDPy(TEO) with C60, PCBM, Li+@C60, and bis-PCBM (designated as C60⊂H4-Ptz-CPDPy(TEO), PCBM⊂H4-Ptz-CPDPy(TEO), Li+@C60⊂H4-Ptz-CPDPy(TEO), and bis-PCBM⊂H4-Ptz-CPDPy(TEO)) composed of the phenothiazine donor and fullerene acceptors were determined to be 1.30, 1.40, 0.66, and 1.51 eV, respectively. Both C60⊂H4-Ptz-CPDPy(TEO) and PCBM⊂H4-Ptz-CPDPy(TEO) underwent electron transfer upon photoexcitation of the porphyrin and fullerene chromophores, and the resultant photoinduced CS states comp...

As new donor–acceptor hybrids for energy and/or electron transfer, zinc complex and free-base of a porphyrin-quinoidal porphyrin dyad linked by a diphenylethynyl bridge were prepared via Sonogashira and subsequent Takahashi coupling reactions. The quinoidal porphyrin units have two dicyanomethylene groups at the opposite meso-positions. The UV-vis absorption spectra of the dyads were almost linear summations of the absorption spectra of each component in comparison with those of the monomeric reference compounds, indicating very weak electronic interactions between the chromophores at the ground state. Both zinc complex and free-base of the reference porphyrin exhibited fluorescence in the range of ca. 600–700 nm upon photoexcitation, while the quinoidal porphyrin monomers showed no fluorescence. The considerable quenching (more than 99%) of the porphyrin fluorescence in the dyads suggested significant electronic communications between the subunits of the excited state. The electrochemical analysis confirmed that the first oxidations of the dyads occur at the porphyrin units and the first reductions take place at the quinoidal porphyrin units. Femtosecond laser flash photolysis of the zinc dyad with photoexcitation at 393 nm showed both ultrafast (

We have newly constructed supramolecular photovoltaic cells using inclusion complexes of lithium-ion-encapsulated [60]fullerene ( Li +@ C 60) and cyclic porphyrin dimers (M-CPDPy, M = H 4 and Ni 2). First, supramolecular inclusion complexes of Li +@ C 60 and M-CPDPy were prepared in MeCN/PhCN (3/1, v/v) by rapid injection method. The molecular aggregates with spherical nanoparticles demonstrated a broad absorption property in the visible region. The macroscopic structures were also estimated to be ca. 200 nm in diameter by transmission electron microscope (TEM) and dynamic light scattering (DLS) measurements. The photoelectrochemical solar cells composed of these assemblies on nanostructured SnO 2 electrode were fabricated by electrophoretic deposition method. The photoelectrochemical behavior of the nanostructured SnO 2 film of supramolecular nanoassemblies of Li +@ C 60 and M-CPDPy is significantly higher than those of the single component films ( Li +@ C 60 or M-CPDPy) and supramolecular inclusion complexes of pristine C 60 and M-CPDPy.

Free-bases and a nickel(II) complex of phenothiazine-bridged cyclic porphyrin dimers bearing self-assembling 4-pyridyl groups (M2-Ptz-CPDPy(OCn); M = H2 or Ni, OCn = OC6 or OC3) at opposite meso-positions have been prepared as host molecules for fullerenes. The free-base dimer (H4-Ptz-CPDPy(OC6)) includes fullerenes with remarkably high association constants such as 3.9 ± 0.7 × 10(6) M(-1) for C60 and 7.4 ± 0.8 × 10(7) M(-1) for C70 in toluene. This C60 affinity is the highest value ever among reported receptors composed of free-base porphyrins. The nickel dimer (Ni2-Ptz-CPDPy(OC6)) also shows high affinities for C60 (1.3 ± 0.2 × 10(6) M(-1)) and C70 (over 10(7) M(-1)). In the crystal structure of the inclusion complex of C60 within H4-Ptz-CPDpy(OC3), the C60 molecule is located just above the centers of the porphyrins. The two porphyrin planes are almost parallel to each other and the center-to-center distance (12.454 Å) is close to the optimal separation (∼12.5 Å) for C60 inclusion. The cyclic porphyrin dimer forms a nanotube through its self-assembly induced by C-H···N hydrogen bonds between porphyrin β-CH groups and pyridyl nitrogens as well as π-π interactions of the pyridyl groups. The C60 molecules are linearly arranged in the inner channel of this nanotube.

We have studied the supramolecular structures and photoelectronic properties of inclusion complexes of cyclic porphyrin dimers (M2-CPDPy, M = Ni, 2H) and fullerenes (C60, C70, PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) and Li+@C60). The porphyrin dimers have self-assembling 4-pyridyl groups at the meso positions and include fullerenes in their cavities with high affinities (Kassoc ≈ 105M−1). In crystals, C60 ⊂ Ni2-CPDPy and PCBM ⊂ Ni2-CPDPy give self-assembled porphyrin nanotubes containing the linear arrays of fullerene molecules. These self-assemblies are induced mainly by the non-classical hydrogen bonds between pyridyl nitrogens and porphyin β C–H groups. On the other hand, in the crystals of C60 ⊂ H4-CPDPy and C70 ⊂ H4-CPDPy, the included fullerene molecules show zigzag chains through van der Waals contacts with each other. Due to these well-ordered arrays of C60, both crystals of C60⊂ Ni2-CPDPy and C60 ⊂ H4-CPDPy have high charge mobilities (Σμ > 0.1 cm2 V−1 s−1). C60 ⊂ H4-CPDPy shows photoinduced electron transfer from the porphyrin moieties to C60 and photovoltaic activity with IPCEmax = 17% in photoelectrochemical cells. Moreover, the inclusion complexes of lithium-ion-encapsulated C60(Li+@C60) and M2-CPDPy exhibit photoinduced charge separation with very long lifetimes reaching submillisecond order. [TANSO 2013 (No. 260) 284–91.]

The synthesis, characterization, photophysical properties, and theoretical analysis of a series of tetraaza porphyrin analogues (H-Pn: n=1-4) containing a dipyrrin subunit and an embedded 1,10-phenanthroline subunit are described. The meso-phenyl-substituted derivative (H-P1) interacts with a Mg(2+) salt (e.g., MgCl(2), MgBr(2), MgI(2), Mg(ClO(4))(2), and Mg(OAc)(2)) in MeCN solution, thereby giving rise to a cation-dependent red-shift in both the absorbance- and emission maxima. In this system, as well as in the other H-Pn porphyrin analogues used in this study, the four nitrogen atoms of the ligand interact with the bound magnesium cation to form Mg(2+)-dipyrrin-phenanthroline complexes of the general structure MgX-Pn (X=counteranion). Both single-crystal X-ray diffraction analysis of the corresponding zinc-chloride derivative (ZnCl-P1) and fluorescence spectroscopy of the Mg-adducts that are formed from various metal salts provide support for the conclusion that, in complexes such as MgCl-P1, a distorted square-pyramidal geometry persists about the metal cation wherein a chloride anion acts as an axial counteranion. Several analogues (HPn) that contain electron-donating and/or electron-withdrawing dipyrrin moieties were prepared in an effort to understand the structure-property relationships and the photophysical attributes of these Mg-dipyrrin complexes. Analysis of various MgX-Pn (X=anion) systems revealed significant substitution effects on their chemical, electrochemical, and photophysical properties, as well as on the Mg(2+)-cation affinities. The fluorescence properties of MgCl-Pn reflected the effect of donor-excited photoinduced electron transfer (d-PET) processes from the dipyrrin subunit (as a donor site) to the 1,10-phenanthroline acceptor subunit. The proposed d-PET process was analyzed by electron paramagnetic resonance (EPR) spectroscopy and by femtosecond transient absorption (TA) spectroscopy, as well as by theoretical DFT calculations. Taken together, these studies provide support for the suggestion that a radical species is produced as the result of an intramolecular charge-transfer process, following photoexcitation. These photophysical effects, combined with a mixed dipyrrin-phenanthroline structure that is capable of effective Mg(2+)-cation complexation, lead us to suggest that porphyrin-inspired systems, such as HPn, have a role to play as magnesium-cation sensors.

New host molecules “Dicavitand-Porphyrins” [H2C2P(syn,syn) (3), H2C2P(syn,anti) (4)] with small cavities on one or both sides of the porphyrin plane were synthesized from 5,10-bis(2,6-dihydroxyphen...

Cyclic nickel and free-base porphyrin dimers (Ni2-CPDPy and H4-CPDPy) include [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) both in solution and in the crystals. In the crystal packing of the i...

Facing zinc bisporphyrins connected with diethanoanthracene and diethanonaphthacene (syn-1 and syn-2) are prepared by double [3+1] porphyrin synthesis of tripyrranedicarbaldehyde with syn-5,11-dimethoxy-4,6,10,12-tetrahydro-4,12;6,10-diethanoanthracene[2,3-c;7,8-c′]dipyrrole and syn-4,7,11,14- tetrahydro-4,14;7,11-diethanonaphthacene[2,3-c;8,9-c′]dipyrrole. These bisporphyrins contain large clefts with different sizes capable for complexation with fullerenes such as C60 and C70 . These designed syn-oriented bisporphyrins serve as effective and selective molecular tweezers for C70 . Binding affinity of the zinc bisporphyrins, namely, syn-1 and syn-2, towards C60 and C70 in solution is determined by employing UV-vis spectrophotometric technique. Values of binding constants (K) in toluene for the non-covalent complexes of syn-1 with C60 and C70 are estimated to be 3.1(4) × 104 and 5.0(2) × 105 M-1, respectively, and those of syn-2 with C60 and C70 are enumerated to be 2.1(4) × 104 and 1.70(13) × 105 M-1, respectively. Binding of C60 and C70 in the clefts of syn-1 and syn-2 is clearly demonstrated by the crystal structures of C60 /syn-1, C70 /syn-1, C60 /syn-2, and C70 /syn-2 complexes. In the crystal structures with C70 , the directions of the long axis of C70 are found to be quite different: in the case of C70 /syn-1 complex the axis lies perpendicularly to the line connecting two zinc atoms; however, for the C70 /syn-2 complex, the axis occupies in-plane to the same line. Both the clefts of syn-1 and syn-2 are induced to fit the included fullerenes by domed out-of-plane distortion of porphyrin rings. Moreover, the bicyclo[2.2.2]octadiene moieties of syn-1 are widened by complexation with the fullerenes, while the same moieties of syn-2 are narrowed by the complexation. Therefore, syn-1 catches the fullerenes at the shallow part of the cleft. On the other hand, syn-2 catches the fullerenes at the bottom part of the cleft. NMR experiments of the complexes in THF-d8 also support these orientations even in solution.

A cyclic free-base porphyrin dimer H 4 -CPD Py (CPD=cyclic porphyrin dimer) linked by butadiyne moieties bearing 4-pyridyl groups self-assembles to form a novel porphyrin nanotube in the crystalline state. The cyclic molecules link together through non-classical C―H···N hydrogen bonds and π―π interactions of the pyridyl groups along the crystallographic a axis. H 4 -CPD Py includes a C 60 molecule in its cavity in solution. In the crystal structure of the inclusion complex (C 60 ⊂H 4 -CPD Py ), the dimer "bites" a C 60 molecule by tilting the porphyrin rings with respect to each other, and there are strong π―π interactions between the porphyrin rings and C 60 . The included C 60 molecules form a zigzag chain along the crystallographic b axis through van der Waals contacts with each other. Femtosecond laser flash photolysis of C 60 ⊂H 4 -CPD Py in the solid state with photoexcitation at 420 nm shows the formation of a completely charge-separated state {H 4 -CPD Py •+ + C 60 •― }, which decays with a lifetime of 470 ps to the ground state. The charge-carrier mobility of the single crystal of C 60 ⊂H 4 -CPD Py was determined by flash photolysis time-resolved microwave conductivity (FP-TRMC) measurements. C 60 ⊂H 4 -CPD Py has an anisotropic charge mobility (Σμ=0.16 and 0.13 cm 2 V ―1 s ―1 ) along the zigzag chain of C 60 (which runs at 45° and parallel to the crystallographic b axis). To construct a photoelectrochemical cell, C 60 ⊂H 4 -CPD Py was deposited onto nanostructured SnO 2 films on a transparent electrode. The solar cell exhibited photovoltaic activity with an incident photon to current conversion efficiency of 17 %.

A cyclic porphyrin dimer (Ni2−CPDPy) linked by butadiyne moieties bearing 4-pyridyl groups includes a C60 molecule inside its cavity in solution to give a 1:1 inclusion complex (C60⊂Ni2−CPDPy). The charge-transfer (CT) band is observed at 645 nm in the UV−vis absorption spectrum of the solution of C60⊂Ni2−CPDPy. In the cyclic voltammogram of C60⊂Ni2−CPDPy, a small anodic shift of the porphyrin oxidation potential and a small cathodic shift of the fullerene reduction potential compared with their original redox potentials are indicative of CT interaction from the porphyrin to C60. In the crystal structure of C60⊂Ni2−CPDPy, a porphyrin nanotube is formed by the self-assembly of Ni2−CPDPy. Ni2−CPDPy molecules link together through nonclassical C−H···N hydrogen bonds and π−π interactions of the pyridyl groups along the crystallographic b axis. The included C60 molecules are linearly arranged in the nanotube to afford a supramolecular peapod. The charge-carrier mobility of the single crystal of C60⊂Ni2−CPDPy w...

We have prepared nickel complexes and freebases of cyclic porphyrin dimers linked by butadiyne or phenothiazine groups. The porphyrin dimers have self-assembling pyridyl groups at the opposite meso positions and include fullerenes such as C60, PCBM, C70, and Li+@C60 to give π-complexes. In particular, the phenothiazine-bridged cyclic porphyrin dimers exhibit notable high affinities toward C60 (Kassoc > 106 M−1) and C70 (Kassoc > 107 M−1). In the crystal structures, the π-complexes of C60 and PCBM within the butadiyne-bridged nickel dimer and the phenothiazine-bridged freebase dimer afford self-assembled porphyrin nanotubes containing the linear arrays of the fullerenes. These self-assemblies are formed by the C-H · · · N hydrogen bonds and π-π interactions of the meso pyridyl groups. On the other hand, the π-complexes of C60 and C70 within the butadiyne-bridged freebase dimer gave the zigzag chains of the fullerenes through van der Waals contacts with each fullerene. These well-ordered C60 arrays yield high electron mobilities (Σμ > 10−1 cm2V−1 s−1). The π-complexes of C60 and Li+@C60 within the butadiyne-bridged dimers perform photoinduced electron transfer from the porphyrins to the fullerenes to give the CS states. The π-complexes of Li+@C60 with the butadiyne-bridged dimers afford the CS states of remarkably long lifetimes reaching submillisecond order.

New capsule-shaped hosts “cavitand-linked porphyrin” metal complexes (Mcp: M = H2, Ni, Zn, and Pd) have been synthesized to mimic the substrate binding functions of metalloenzymes such as cytochrom...

A new and stable quadruple hydrogen-bonded capsule, tetracarboxylcavitand.meso-tetra(2-pyridyl)porphyrin (1 x 2) has been synthesized and spectroscopically characterized. 1 x 2 can encapsulate small molecules of various size from methane to cyclopentane. The guest and host exchange rates were evaluated by EXSY.

Linear assemblies of a 1 : 1 porphyrin-fullerene C60 complex were formed in vertical cylindrical polyether nanodomains of amphiphilic block copolymer films by a simple spin coating-annealing method. The nanocylinder structures were retained even with high contents of the complex in the polymer films.

Inspired by the observation of polar interactions between CO and O 2 ligands and the peptide residues at the active site of hemoglobin and myoglobin, we synthesized two kinds of superstructured porphyrins: TCP-IM, which contains a linked imidazole ligand, and TCP-PY, which contains a linked pyridine ligand, and examined the thermodynamic, kinetic, and spectroscopic (UV/Vis, IR, NMR, and resonance Raman) properties of their CO and O 2 complexes. On both sides of each porphyrin plane, bulky binaphthyl bridges form hydrophobic cavities that are suitable for the binding of small molecules. In the proximal site, an imidazole or pyridine residue is covalently fixed and coordinates axially to the central iron atom. In the distal site, two naphtholic hydroxyl groups overhang toward the center above the heme. The CO affinities of TCPs are significantly lower than those of other heme models. In contrast, TCPs have moderate O 2 binding ability. Compared with reported model hemes, the binding selectivity of O 2 over CO in TCP-IM and TCP-PY complexes is greatly improved. The high O 2 selectivity of the TCPs is mainly attributable to a low CO affinity. The comparison of k o n (CO) values of TCPs with those of unhindered hemes indicates the absence of steric hindrance to the intrinsically linear CO coordination to Fe I I in TCP-IM and TCP-PY. The abnormally large k o f f (CO) values are responsible for the low CO affinities. In contrast, k o f f (O 2 ) of TCP-PY is smaller than those of other pyridine-coordinated model hemes. For the CO adducts of TCPs, unusually low v(Fe-CO) and unusually high v(C-O) frequencies are observed. These results can be ascribed to decreased back-bonding from the iron atom to the bound CO. The lone pairs of the oxygen atoms of the hydroxyl groups prevent back-bonding by exertion of a strong negative electrostatic interaction. On the other hand, high v(Fe-O 2 ) frequencies are observed for the O 2 adducts of TCPs. In the resonance Raman (RR) spectrum of oxyTCP-IM, we observed simultaneous enhancement of the Fe-O 2 and O-O stretching modes. Furthermore, direct evidence for hydrogen bonding between the hydroxyl groups and bound dioxygen was obtained by RR and IR spectroscopy. These spectroscopic data strongly suggest that O 2 and CO binding to TCPs is controlled mainly by the two different electrostatic effects exerted by the overhanging OH groups: destabilization of CO binding by decreasing back-bonding and stabilization of O 2 binding by hydrogen bonding.

A series of cerium(IV) bis-porphyrinate double-deckers [Ce(bbpp)(2)] (BBPP=5,15-bis(4-butoxyphenyl)porphyrin dianion), [Ce(tmpp)(2)] (TMPP=5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin dianion), [Ce(tfpp)(2)] (TFPP=5,10,15,20-tetrakis(4-fluorophenyl)porphyrin dianion), [Ce(tmcpp)(2)] (TMCPP=5,10,15,20-tetrakis(4-methoxycarbonylphenyl)porphyrin dianion), and [Ce(tmpp)(tmcpp)] was prepared. They bind three Ag(+) ions to their concave porphyrin pi subunits (pi-clefts) according to a positive homotropic allosteric mechanism with Hill coefficients (n(H)) of 1.7-2.7. The rotation rates of the porphyrin ligands in [Ce(bbpp)(2)] were evaluated to be 200 s(-1) at 20 degrees C (DeltaG$\rm{^{\ne }_{293}}$=14.1 kcal mol(-1)) and 220 s(-1) at -40 degrees C (DeltaG$\rm{^{\ne }_{233}}$=11.0 kcal mol(-1)) without and with Ag(+) ions, respectively. These results consistently support our unexpected finding that Ag(+) binding can accelerate rotation of the porphyrin ligand. On the basis of UV-visible, (1)H NMR, and resonance Raman spectral measurements, the rate enhancement of the rotational speed of the porphyrin ligands is attributed to conformational changes of the porphyrin in cerium(IV) bis-porphyrinate induced by binding of Ag(+) guest ions in the clefts. This novel concept of positive homotropic allosterism is applicable to the molecular design of various supramolecular and switch-functionalized systems.

The carbon monoxide (CO) adducts of iron "twin coronet" porphyrins (TCPs) are characterized by UV-vis, resonance Raman (RR), IR, and 13C NMR spectroscopies. A superstructured porphyrin, designated as TCP, was used as a common framework for the four different types of iron complexes. TCP bears two binaphthalene bridges on each side and creates two hydrophobic pockets surrounded by the bulky aromatic rings. In the CO-binding cavities, the hydroxyl groups are oriented toward the center above the heme. The iron complexes investigated are as follows: TCP (which is without a covalently linked axial ligand), TCP-PY (which has a linked pyridine ligand), and TCP-TB and TCP-TG (both of which have a linked thiolate ligand). These complexes were synthesized as ferric forms and identified by the various spectroscopic methods. The UV-vis spectra of TCP-CO and TCP-PY-CO exhibit lambda(max) at 432, 546 and 428, 541 nm, respectively. On the other hand, the CO adducts of TCP-TB and TCP-TG show typical hyperporphyrin spectra for a thiolate-ligated iron(II) porphyrin-CO complex. In the RR spectra, the nu(Fe-CO) bands were observed at 506, 489 cm(-1) (TCP), 465 cm(-1) (TCP-PY), 458, 437 cm(-1) (TCP-TG) and 429 cm(-1) (TCP-TB). Compared with the reported nu(Fe-CO) frequencies of hemoproteins and their model systems, these observed values are unusually low. Further, abnormally high nu(C-O) bands are observed at 1990 cm(-1) (TCP-CO) and 2008 cm(-1) (TCP-PY-CO) in IR spectra. The lower nu(Fe-CO) and the higher nu(C-O) frequencies can be ascribed to the strong negative polar effect caused by the vicinal hydroxyl groups in the cavity. This prediction is further supported by the observation of significant 13C shieldings exhibited by TCP-CO (delta = 202.6 ppm) and TCP-PY-CO (delta = 202.3 ppm), in comparison to hemoproteins and other heme models. The CO affinity of TCP-PY (P1/2CO = 0.017 Torr at 25 C) is unusually lower than other heme models. The unique behavior of these CO adducts is discussed in context of the TCP structures.

Hydrogen-bond formation with the terminal oxygen atom is considered to be the basis of dioxygen molecule activation by cytochrome P450. In order to verify the effect of this hydrogen bond, we have undertaken the synthesis of a model complex: a binaphthyl-bridged porphyrin bearing hydroxy groups at suitable positions (“single coronet” or “SC”). The reactivity of the iron complex of the synthesized compound towards basic ligands, dioxygen and carbon monoxide has been studied. When a bulky axial ligand such as 1-methyladamantyl-2-methylimidazole is used, only the pentacoordinated complex is obtained, and, as expected, dioxygen binds as the sixth ligand only in the cavity of the compound. Under unusually low dioxygen partial pressures and in rigorously anhydrous toluene, the pentacoordinated iron complex is completely transformed into a new species which absorbs in the visible region at 420 and 559 nm, and which we have identified as the oxygenated complex. Surprisingly, this reaction seems to occur irreversibly, based on the fact that the initial complex is not recovered after bubbling nitrogen through the solution for several hours. On the other hand, saturation of the solution with carbon monoxide transforms the complex slowly but completely into the FeII−CO complex which is stable in a dioxygen-saturated toluene solution at 0 °C. However, by raising the temperature, it is spontaneously transformed back into the dioxygen complex; this verifies the reversibility of the dioxygen binding process. Compared with its affinity towards carbon monoxide, the SC iron complex has a much stronger affinity towards dioxygen. This remarkable property may be partly explained by hydrogen bonding between the terminal atom of the dioxygen molecule and the hydroxy groups attached to the binaphthyl bridges of the porphyrin, and also by polar neighbouring-group effects. Ligand binding and debinding constants have been determined by laser flash photolysis. Ligand−iron bond strength and hydrogen bonding have been investigated by IR and Raman spectroscopy. The role of the hydroxy groups has also been emphasized by comparing the properties of SC with those of a binaphthyl-bridged porphyrin bearing methoxy groups instead of hydroxy groups.

Tetraaryl-substituted cerium(IV) double-decker porphyrin (2D), tetraaryl-substituted lanthanum(III) double-decker porphyrin (3D), and diaryl-substituted lanthanum(III) triple-decker porphyrins (4T•Me and 4T•MeO) were newly synthesized and their porphyrin ring rotation rates were systematically estimated by means of a VT NMR spectroscopic method. In 2D the coalescence temperature (Tc) for the porphyrin ring rotation was higher than 110 °C, whereas in 3D it appeared at ca. 0 °C. In diaryl-substituted cerium(IV) double-decker porphyrin (5D) it appeared at 13 °C whereas in 4T•Me and 4T•MeO they were lower than −80 °C. These results consistently support the view that the porphyrin ring rotation rates in lanthanum(III)-based porphyrins are much faster than those in cerium(IV)-based porphyrins. The difference is reasonably explained by the difference in the ion size between these two metal ions. Since these sandwich-type porphyrins can act as novel scaffolds for designing positive allosteric recognition systems,...

Effect of central metals and oxidants on the catalytic and asymmetric epoxidation of simple olefins was evaluated with a chiral binaphthalene-linked twin-coronet porphyrin catalyst, whose Mn derivative gave superior ees of resultant epoxide than the corresponding Fe derivative.

Synthesis and characterization of an iron porphyrin axially-coordinated by thiolate were carried out in order to mimic the active center of cytochrome P-450. O2 binds reversibly to the ferrous form at −80 °C. The oxygenated complex showed a ν(Fe-O2) band at 578 cm−1 in the resonance Raman spectrum.

An amphiphilic iron porphyrin having four hexadecanoic acid chains on each face of the porphyrin was prepared and the catalytic epoxidation of linolenic acid with the heme-casted bilayer membranes was achieved in high regioselectivity at the terminal double bond of the substrate. The orientation of the porphyrin ring in the membrane was determined to be parallel to the aqueous-bilayer interface by means of ESR.

Phenanporphodimethene (1) is a porphyrin analogue which has a dipyrromethene unit replaced by a 1,10-phenanthroline moiety. This modification effectively coverts a dianionic porphyrin to a monoanionic porphyrin analogue with a porphyrin-like N4 coordination sphere. The ligand 1 forms copper and nickel hexafluorophosphate complexes, Cu-1 and Ni-1, respectively. X-ray crystallographic analysis of Cu-1 indicates that the Cu(II) complex is tetracoordinated by two pyrrolic nitrogen atoms and two phenanthrolic nitrogen atoms and includes a non-bonding PF(6) counter-anion. The Ni-1 complex has similar geometry with a tetracoordinate square-planar structure in non-coordinating solvents such as CHCl(3). In coordinating solvents such as MeOH, the coordination structure adopts an octahedral geometry. These results indicate that Ni-1 can be converted from a low-spin to high-spin configuration by the coordination of two solvent molecules to the nickel center. This solvatochromic conversion of Ni-1 is accompanied by thermochromic behavior resulting from the transformation between square planar and octahedral configurations in THF solution. The redox peak responsible for the nickel-centered redox reaction of Ni-1 is observed at -0.66 V (vs. Fc/Fc(+)) in CH(2)Cl(2) solution, which indicates generation of low valent Ni(I) species. Thus, Ni-1 may be useful for future investigations as a novel structural model of the active site of cofactor F(430) in methyl-coenzyme M reductase.

Effect of central metals and oxidants on the catalytic and asymmetric epoxidation of simple olefins was evaluated with a chiral binaphthalene-linked twin-coronet porphyrin catalyst, whose Mn derivative gave superior ees of resultant epoxide than the corresponding Fe derivative.

A binucleating porphyrin with covalently appended copper chelates having a cross-linked imidazole-phenol group as the novel active site model of cytochrome c oxidase has been prepared, and the dioxygen adduct of its iron(II)-copper(I) complex was spectroscopically characterized.

Two kinds of novel cytochrome P450 models, which have alkanethiolate axial ligands and hydroxyl groups inside molecular cavities, were designed and synthesized as functional O(2) binding systems. A superstructured porphyrin, designated as "twin-coronet" porphyrin, was used as the common framework of the model complexes. This porphyrin bears four binaphthalene bridges on the both sides and forms two pockets surrounded by the bulky aromatic rings. Thiobenzyloxy and thioglycolate moieties, which contain an alkanethiolate group exhibiting various electron-donating abilities and degrees of bulkiness, were covalently linked to twin-coronet porphyrin to yield thiolate-coordinated hemes, TCP-TB and TCP-TG (twin-coronet porphyrin with thiobenzyloxy and thioglycolate groups), respectively. Both ferric complexes exhibited high stability during usual experimental manipulation under air and were characterized by MS, UV/vis, ESR spectroscopies, and CV. The ESR spectra exhibited low-spin signals (TCP-TB: g = 2.334, 2.210, 1.959; TCP-TG: g = 2.313, 2.209, 1.966). The cyclic voltammogram of TCP-TB in CH(3)CN gave a quasi-reversible wave which corresponds to the Fe(III)/Fe(II) redox couple: E(p)()(/2) = -1.35 V (vs Fc/Fc(+)). On the other hand, TCP-TG showed a fine reversible wave: E(1/2) (Fe(III)/Fe(II)) = -1.12 V. The stable dioxygen adducts were formed in the reaction of the ferric complexes with KO(2) under an oxygen atmosphere and characterized by UV/vis and resonance Raman (RR) spectroscopies. In the RR spectra, the nu(O--O) bands of the dioxygen adducts were observed at 1138 cm(-1) (TCP-TB) and 1137 cm(-1) (TCP-TG). The hypothesis that hydrogen bonding between the bound oxygen and the hydroxyl groups of the binaphthyl moieties could increase their stability was verified by RR spectroscopy. When all hydroxyl groups were deuterated, only the frequencies of the nu(O--O) bands were upshifted by 2 cm(-1) without any perturbation in the porphyrin skeleton. This work shows the first direct evidence for a hydrogen bond to dioxygen in an oxy form of a thiolate-coordinated heme model system. These results are discussed in context of the process of dioxygen binding and activation in cytochrome P450.

Lithium ion encapsulated [60]fullerene (Li+@C60) is included within a free base and nickel complex of a cyclic porphyrin dimer (M-CPDPy, M = H4 and Ni2) to afford supramolecules (Li+@C60⊂M-CPDPy) in a polar solvent (benzonitrile) with the association constants of 2.6 × 105 M−1 and 3.5 × 105 M−1, respectively. From the electrochemical analysis, the energies of the charge-separated (CS) states are estimated to be 1.07 eV for Li+@C60⊂H4-CPDPy and 1.20 eV for Li+@C60⊂Ni2-CPDPy. Both values are lower than the triplet excited energies of the fullerene and porphyrin. Upon the photoexcitation at the Q-band of the porphyrin chromophore of Li+@C60⊂H4-CPDPy, electron transfer from the triplet excited state of the free base porphyrin to Li+@C60 occurs to produce the CS state. Li+@C60⊂Ni2-CPDPy also undergoes photoinduced electron transfer to produce the CS state. The lifetimes of the resulting CS states are 0.50 ms for Li+@C60⊂H4-CPDPy and 0.67 ms for Li+@C60⊂Ni2-CPDPy. These remarkably long CS lifetimes are the best values ever reported for non-covalent porphyrin-fullerene supramolecules in solution and are attributable to the lower CS energies than the triplet energy of each chromophore.

The resonance Raman spectrum of oxy CoSCP with AdIm(2-H) showed an unusually low ν(O-O) frequency at 1123 cm-1. This result is due to the cooperative effect of strong donation from AdIm(2-H) and hydrogen bonding between the inner hydroxyl groups and coordinated dioxygen.

An amphiphilic iron porphyrin having four hexadecanoic acid chains on each face of the porphyrin was prepared and the catalytic epoxidation of linolenic acid with the heme-casted bilayer membranes was achieved in high regioselectivity at the terminal double bond of the substrate. The orientation of the porphyrin ring in the membrane was determined to be parallel to the aqueous-bilayer interface by means of ESR.

The structure of a 1,2-phenylene linked dicobalt tetraarylporphyrin dimer was determined by X-ray crystallography and its dioxygen adduct was characterized by absorption and ESR spectroscopies. Since the porphyrin dimer has a long metal–metal distance and a rigid framework against inward bending, the cobalt centers were not bridged by the coordinated dioxygen.

Alkylcobalt(III) and alkylperoxocobalt(III) species of porphyrin and porphycene have been prepared and characterized by UV-vis and 1H NMR spectroscopic measurements. Thermal decomposition of the alkylperoxocobalt species proceeds via Co–O or O–O bond cleavage, which depends on the coordination of an axial ligand.

Synthesis and characterization of an iron porphyrin axially-coordinated by thiolate were carried out in order to mimic the active center of cytochrome P-450. O2 binds reversibly to the ferrous form at −80 °C. The oxygenated complex showed a ν(Fe-O2) band at 578 cm−1 in the resonance Raman spectrum.