Meso-nitrogen Atoms Research Articles

An n-type semiconducting diazaporphyrin-based hydrogen-bonded organic framework.

Significant effort has been devoted to the development of materials that combine high electrical conductivity and permanent porosity. This paper discloses a diazaporphyrin-based hydrogen-bonded organic framework (HOF) with porosity and n-type semiconductivity. A 5,15-diazaporphyrin Ni(ii) complex with carboxyphenyl groups at the meso positions afforded a HOF due to hydrogen-bonding interactions between the carboxy groups and meso-nitrogen atoms. The thermal and chemical stabilities of the HOF were examined using powder X-ray diffraction analysis, and the charge-carrier mobility was determined to be 2.0 × 10-7 m2 V-1 s-1 using the flash-photolysis time-resolved microwave conductivity (FP-TRMC) method. An analogous diazaporphyrin, which does not form a HOF, exhibited mobility that was 20 times lower. The results presented herein highlight the crucial role of hydrogen-bonding networks in achieving conductive pathways that can tolerate thermal perturbation.

Synthesis, Structure, and Redox and Optical Properties of 5,10,15,20-Tetraaryl-5-azaporphyrinium Salts.

The fundamental properties of azaporphyrins can be modulated over a wide range by changing the number of meso-nitrogen atoms. We report herein the first examples of 5,10,15,20-tetraaryl-5-azaporphyrinium (MTAMAP) salts, which were prepared via metal-templated cyclization of the corresponding zinc(II) and copper(II) complexes of 10-aryl-1-chloro-19-benzoyl-5,15-dimesityl-10-azabiladiene-ac. The inclusion of one meso-nitrogen atom in the 5,10,15,20-tetraarylporphyrin skeleton considerably changes the redox and optical properties as well as the degree of aromaticity of the porphyrin ring. The present findings suggest that MTAMAP salts would be promising scaffolds for the development of new azaporphyrin-based ionic fluorophores and photosensitizers.

Easily Switchable 18π-, 19π-, and 20π-Conjugation of Diazaporphyrin Double-Pincer Bispalladium Complexes.

NiII 3,7,13,17-tetrapyridyl-5,15-diazaporphyrin serves as a double tridentate ligand to PdII ions to provide a pincer-type bispalladium complex. Electrochemical analysis revealed that the bispalladium complex shows excellent ability to accept electrons and reversible redox properties due to the coordination of the two cationic PdII centers to the meso-nitrogen atoms. We isolated and characterized one- and two-electron reduction species of the bispalladium complex. The 20π antiaromatic nature of the two-electron reduction species was confirmed by 1 H NMR spectroscopy, UV/Vis-near-IR (NIR) absorption spectra, and density functional theory (DFT) calculations. X-ray diffraction revealed highly twisted structures for the bispalladium complexes regardless of the oxidation state.

Recent advances in the synthesis of diazaporphyrins and their chalcogen derivatives.

Porphyrins are highly influential functional materials in the field of science owing to their fascinating colors and diverse electrochemical and optical properties. Peripheral modification of porphyrin rings with nitrogen is an attractive approach for imparting unprecedented functions to both natural and artificial pophyrins. Diazaporphyrins (DAPs) are a class of compounds that have two meso-nitrogen atoms. DAPs exhibit characteristic redox and optical properties that differ significantly from those of porphyrins. The preparation and chemical functionalization of DAP-based sensitizers, catalysts, and materials have gained significant attention in various fields such as medical science, organic synthesis, and materials chemistry. This review focuses on recent advances in the synthesis of diazaporphyrins and their chalcogen derivatives, including historically important examples.

Tert-Butyl substituted hexaazasubphthalocyanine: Synthesis, isolation of C1 and C3 regioisomers and their spectral-luminescence study

By cyclotrimerization of tert-butyl substituted pyrazine-2,3-dicarbonitrile in the presence of boron trichloride, two positional isomers ([Formula: see text] and [Formula: see text] symmetry) of the corresponding boron(III) tripyrazinosubporphyrazines were first obtained and efficiently separated by column chromatography. The new compounds were characterized by MALDI mass-spectrometry, UV-visible and 1H NMR spectroscopy. A Q-band in the electronic absorption spectra for both fractions was observed at [Formula: see text]530 nm, which is typical for tripyrazinosubporphyrazines. The fluorescence quantum yield for the regioisomer with lower symmetry ([Formula: see text] is 1.5 times higher than for the more symmerical [Formula: see text] species ([Formula: see text] = 0.37 and 0.28, respectively). Comparative spectrophotometric study of the basic properies reveals that hexaaza substitution in benzene rings of tert-butyl substituted subphthalocyanine decreases the basic properties of meso-nitrogen atoms.

Meso-Diazacorrphycenes: Neighboring Effect of Two Nitrogen Atoms.

The neighboring effect of two adjacent heteroatoms influences the structures and properties of heterocyclic compounds. Herein, we demonstrated that the introduction of two adjacent sp2 nitrogen atoms into a porphyrinic skeleton significantly enhanced its Brønsted basicity due to the repulsive interaction between two lone pairs on the nitrogen atoms. Palladium- and copper-templated ring closure of dichlorobis(dipyrrin) with hydrazine hydrate provided meso-diazacorrphycene palladium and copper complexes in good yields. The structural, magnetic, and electrochemical properties of the diazacorrphycene complexes were investigated experimentally and theoretically to elucidate the effect of the meso-nitrogen atoms.

Syntheses, Properties, and Catalytic Activities of Metal(II) Complexes and Free Bases of Redox-Switchable 20π, 19π, and 18π 5,10,15,20-Tetraaryl-5,15-diazaporphyrinoids.

In spite of significant advances in redox-active porphyrin-based materials and catalysts, little attention has been paid to 20π and 19π porphyrins because of their instability in air. Here we report the meso-modification of 5,10,15,20-tetraarylporphyrin with two nitrogen atoms, which led to redox-switchable 20π, 19π, and 18π 5,10,15,20-tetraaryl-5,15-diazaporphyrinoids (TADAPs). Three kinds of metal(II) complexes and free bases of TADAP were prepared by the metal-templated annulation of the corresponding metal-bis(dipyrrin) complexes. The inductive and resonance effects of the meso-nitrogen atoms on the aromatic, optical, electrochemical, and magnetic properties of the entire TADAP π-systems were assessed by using various spectroscopic measurements and density functional theory calculations. The aromaticity and π-π* electronic transition energies of the TADAPs varied considerably, and were shown to be dependent on the oxidation states of the π-systems. In contrast to the isoelectronic 5,10,15,20-tetraarylporphyrin derivatives, the 20π and 19π TADAPs were chemically stable under air. In particular, the 19π TADAP radical cations were extremely stable towards dioxygen, moisture, and silica gel. This reflected the low-lying singly occupied molecular orbitals of their π-systems and the efficient delocalization of their unshared electron spin. The capability of MgTADAP to catalyze aerobic biaryl formation from aryl Grignard reagents was demonstrated, which presumably involved a 19π/20π redox cycle.

Evaluation of the Intramolecular Charge-Transfer Properties in Solvatochromic and Electrochromic Zinc Octa(carbazolyl)phthalocyanines.

2,3,9,10,16,17,23·24-Octakis-(9H-carbazol-9-yl) phthalocyaninato zinc(II) (3) and 2,3,9,10,16,17,23·24-octakis-(3,6-di-tert-butyl-9H-carbazole) phthalocyaninato zinc(II) (4) complexes were prepared and characterized by NMR and UV-vis spectroscopies, magnetic circular dichroism (MCD), matrix-assisted laser desorption ionization mass spectrometry, and X-ray crystallography. UV-vis and MCD data are indicative of the interligand charge-transfer nature of the broad band observed in 450-500 nm range for 3 and 4. The redox properties of 3 and 4 were probed by electrochemical and spectro-electrochemical methods, which are suggestive of phthalocyanine-centered first oxidation and reduction processes. Photophysics of 3 and 4 were investigated by steady-state fluorescence and time-resolved transient absorption spectroscopy demonstrating the influence of the carbazole substituents on deactivation from the first excited state in 3 and 4. Protonation of the meso-nitrogen atoms in 3 results in much faster deactivation kinetics from the first excited state. Spectroscopic data were correlated with density functional theory (DFT) and time-dependent DFT calculations on 3 and 4.

Nitrogen-Bridged Metallodiazaporphyrin Dimers: Synergistic Effects of Nitrogen Bridges and meso-Nitrogen Atoms on Structure and Properties.

NH-bridged and pyrazine-fused metallodiazaporphyrin dimers have been prepared from nickel(II) and copper(II) complexes of 3-amino-5,15-diazaporphyrin by Pd-catalyzed C-N cross-coupling and oxidative dimerization reactions, respectively. The synergistic effects of the nitrogen bridges and meso-nitrogen atoms play major roles in enhancing the light-harvesting properties and delocalization of an electron spin over the entire π-skeletons of the metallodiazaporphyrin dimers.

Tetrabenzoporphyrin and -mono-, -cis-di- and Tetrabenzotriazaporphyrin Derivatives: Electrochemical and Spectroscopic Implications of meso CH Group Replacement with Nitrogen.

Nonperipherally hexyl-substituted metal-free tetrabenzoporphyrin (2H-TBP, 1a) tetrabenzomonoazaporphyrin (2H-TBMAP, 2a), tetrabenzo-cis-diazaporphyrin (2H-TBDAP, 3a), tetrabenzotriazaporphyrin (2H-TBTAP, 4a), and phthalocyanine (2H-Pc, 5a), as well as their copper complexes (1b-5b), were synthesized. As the number of meso nitrogen atoms increases from zero to four, λmax of the Q-band absorption peak becomes red-shifted by almost 100 nm, and extinction coefficients increased at least threefold. Simultaneously the blue-shifted Soret (UV) band substantially decreased in intensity. These changes were related to the relative electron-density of each macrocycle expressed as the group electronegativity sum of all meso N and CH atom groups, ∑χR. X-ray photoelectron spectroscopy differentiated between the three different types of macrocyclic nitrogen atoms (the Ninner, (NH)inner, and Nmeso) in the metal-free complexes. Binding energies of the Nmeso and Ninner,Cu atoms in copper chelates could not be resolved. Copper insertion lowered especially the cathodic redox potentials, while all four observed redox processes occurred at larger potentials as the number of meso nitrogens increased. Computational chemical methods using density functional theory confirmed 1b to exhibit a Cu(II) reduction prior to ring-based reductions, while for 2b, Cu(II) reduction is the first reductive step only if the nonperipheral substituents are hydrogen. When they are methyl groups, it is the second reduction process; when they are ethyl, propyl, or hexyl, it becomes the third reductive process. Spectro-electrochemical measurements showed redox processes were associated with a substantial change in intensity of at least two main absorbances (the Q and Soret bands) in the UV spectra of these compounds.

Synthesis and Photophysical Properties of Two Diazaporphyrin-Porphyrin Hetero Dimers in Polar and Nonpolar Solutions.

Two diazaporphyrin (DAP)-porphyrin hetero dimers, in β-meso and β-β configurations, were prepared to study their photoinduced intramolecular electron transfer properties. The two meso nitrogen atoms in the porphyrin ring of DAP change its redox potential, making DAP more easily reduced, compared to its porphyrin counterpart. A charge-transfer from porphyrin to DAP in both hetero dimers was verified by versatile optical spectroscopic methods. The steady-state fluorescence spectra indicated an efficient intramolecular exciplex formation for both dimers. For the β-meso dimer, ultrafast time-resolved spectroscopic methods revealed the subpicosecond formation of two types of primary short-living (1-18 ps) intramolecular exciplexes, which relaxed in toluene to form a long-living final exciplex (1.4 ns) followed by a longer-living charge transfer complex (>5 ns). However, in benzonitrile, the lifetime of the final exciplex was longer (660 ps) as was that of the charge transfer complex (180 ps). The β-β analogue formed similar short-living exciplexes in both solvents, but the final exciplex and the charge transfer state had significantly shorter lifetimes. The electrochemical redox potential measurements and density functional theory calculations supported the proposed mechanism.

Optical, electrochemical, and magnetic properties of pyrrole- and thiophene-bridged 5,15-diazaporphyrin dimers.

The first examples of pyrrole- and thiophene-bridged 5,15-diazaporphyrin (DAP) dimers are prepared through Stille coupling reactions of nickel(II) and copper(II) complexes of 3-bromo-10,20-dimesityl-5,15-diazaporphyrin (mesityl=2,4,6-trimethylphenyl) with the respective 2,5-bis(tributylstannyl)heteroles. The effects of the heterole spacers and meso nitrogen atoms on the optical, electrochemical, and magnetic properties of the DAP dimers are investigated by UV/Vis absorption spectroscopy, density functional theory calculations, magnetic circular dichroism spectroscopy, cyclic voltammetry, and EPR spectroscopy. The heterole spacers are found to have a significant impact on the electronic transitions over the entire π-system. In particular, the pyrrole-bridged DAP dimers exhibit high light-harvesting potential in the low-energy visible/near-infrared region owing to the intrinsic charge-transfer character of the lowest excitation.

Synthesis and Photophysical and Photoelectrochemical Properties of Diazaporphyrin-Fullerene Linked Dyad

Over the last two decades much effort has been devoted to explore excellent donor-acceptor (D-A) couples for artificial photosynthesis and solar energy conversion.1-6 In this regard fullerenes have been frequently employed as acceptors to exhibit outstanding electron transfer (ET) properties in D-A systems as well as organic photovoltaics. Meanwhile, various donors such as porphyrins, phthalocyanines, and ruthenium bipyridyl complexes have been involved in such systems and devices. However, the examples of superior donors are still limited in terms of light-harvesting and ET properties.5,15-Diazaporphyrins (DAPs) are a class of porphyrins that bear two meso-nitrogen atoms linking two dipyrromethene units. DAPs have received continuing interest in relation to their analogues, porphyrins and phthalocyanines. It has been shown that 5,15-aza-substitution significantly changes the intrinsic properties of the porphyrin p systems that would be comparable to or even better than their analogues. Nevertheless, due to synthetic difficulties of the cyclization and peripheral modification of the core in DAPs, the opportunity of using DAPs as materials has poorly been addressed. As such, DAPs have yet to be used as donors in D-A linked systems to elucidate the intrinsic ET properties.7-9 Here we report, for the first time, the synthesis and photophysical and photoelectrochemical properties of zinc 5,15-diazaporphyrin (ZnDAP)-C60 linked dyad. To evaluate the intrinsic photophysical properties of ZnDAP as a donor accurately, the corresponding zinc porphyrin (ZnP)-C60 linked dyad with the same spacer was also designed (Figure 1). Owing to the electron-withdrawing character of the 5,15-aza-substitution ZnDAP would be more difficult to be oxidized than ZnP. Accordingly, we expected that once a charge-separated state was generated via photoinduced ET, charge recombination (CR) from the charge-separated state to the ground state in ZnDAP-C60 would be slowed down as compared with ZnP-C60, because CR in ZnDAP-C60 is shifted more deeply into the Marcus inverted region. To attain sufficient solubility of the dyads bulky long alkoxy chains were also introduced to the ortho-positions of the meso-phenyl group.In addition to the better matching of ZnDAP with solar energy distribution in the visible region than ZnP, the lifetime of charge-separated state in ZnDAP-C60 was 4 times longer than that in ZnP-C60 due to the high oxidation potential of ZnDAP relative to ZnP. This difference also paralleled the enhanced photocurrent generation of ZnDAP-C60–based device in the visible region. These results unambiguously exemplify the potential utility of DAPs as an excellent donor in artificial photosynthesis and solar energy conversion. In this talk, we will present the synthesis, optical, electrochemical, photophysical, and photoelectro-chemical properties of ZnDAP-C60 in comparison with the corresponding ZnP-C60.[1] H. Imahori and Y. Sakata, Adv. Mater. (Review Article), 9, 537-546 (1997).[2] H. Imahori and Y. Sakata, Eur. J. Org. Chem. (Microreview), 1999, 2445-2457.[3] H. Imahori, Org. Biomol. Chem. (Perspective), 2, 1425-1433 (2004).[4] H. Imahori, Bull. Chem. Soc. Jpn. (Award Accounts), 80, 621-636 (2007).[5] H. Imahori, T. Umeyama, K. Kurotobi, and Y. Takano, Chem. Commun. (Feature Article), 48, 4032-4045 (2012).[6] Photofunctional Hybrid Nanocarbon Materials, T. Umeyama and H. Imahori, J. Phys. Chem. C (Feature Article), 117, 3195-3209 (2013).[7] Y. Matano, T. Shibano, H. Nakano, and H. Imahori, Chem. Eur. J., 18, 6208-6216 (2012).[8] Y. Matano, T. Shibano, H. Nakano, Y. Kimura, and H. Imahori, Inorg. Chem., 51, 12879-12890 (2012).[9] K. Kurotobi, K. Kawamoto, Y. Toude, Y. Fujimori, Y. Kinjo, S. Ito, Y. Matano, and H. Imahori, Chem. Lett., 42, 725-726 (2013).

Covalently Linked 5,15‐Diazaporphyrin Dimers: Promising Scaffolds for a Highly Conjugated Azaporphyrin π System

The first examples of β-β directly linked, acetylene-bridged, and butadiyne-bridged 5,15-diazaporphyrin dimers have been prepared by palladium-catalyzed coupling reactions of nickel(II) and copper(II) complexes of 3-bromo-10,20-dimesityl-5,15-diazaporphyrin (mesityl=2,4,6-trimethylphenyl). The effects of the linking modes and meso-nitrogen atoms on the structural, optical, electrochemical, and magnetic properties of the distributed π systems were investigated by using X-ray crystallography, UV/Vis absorption spectroscopy, DFT calculations, cyclic voltammetry, and ESR spectroscopy. Both the electronic and steric effects of the meso-nitrogen atoms play an important role in the highly coplanar geometry of the directly linked dimers. The direct β-β linkage produces enhanced π conjugation and electron-spin coupling between the two diazaporphyrin units.

Peripheral modification and basicity of (phthalocyaninato)-copper(II) according to the electronic spectroscopy and quantum chemical calculation data

Singly, doubly, and triply protonated forms of tetra- and octacarboxy (phthalocyaninato)copper(II) derivatives were identified and characterized by electronic absorption spectra and PM3 quantum chemical calculations. Molecules of carboxy-substituted phthalocyanine copper(II) complexes have a distorted nonplanar structure with specific charge distribution over meso-nitrogen atoms. Singly, doubly, and triply protonated complexes at the meso-nitrogen atoms and carboxy groups exist as mixtures of isomers, which is reflected in splitting of the Q band in the electronic absorption spectra and reduction of its symmetry. The formation of bifurcated hydrogen bonds O ... H+ ... Nmeso and O ... H+ ... O between the neighboring substituents was revealed.

Perfluorinated porphyrazines 1: Synthesis and UV-vis spectral study of perfluorinated octaphenylporphyrazine and its indium(III) complex, [MPA(F5Ph)8](M= 2H,InIII(OH))

Novel perfluorinated porphyrazine has been prepared as indium(III) complex of octa(pentafluorophenyl)porphyrazine by cyclotetramerization of 1,2-dicyano-1,2-bis(pentafluorophenyl)-ethylene (mixture of cis- and trans-isomers) with basic indium(III) acetate in a melt. The InIIIcomplex is stable in trifluoroacetic and sulfuric acids, but is easily demetalated with formation of the free-base in a chloroform solution containing HCl or slighly acidified with CF3COOH in the presence of tetraalkylammonium chloride as a catalyst. Perfluorination of octaphenylporphyrazine macrocycle leads to a hypsochromic shift of the Q-band maxima in the electronic absorption spectra, to drastic decrease of the basicity of meso-nitrogen atoms of porphyrazine macrocycle and to increase of the acidity of the internal NH -groups.

Acid-base properties of thianaphthene-annulated porphyrazine and tetra(pyrazino)porphyrazine complexes with aluminum group metals

Acid-base properties of Al(III), Ga(III), and In(III) complexes with tert-butyl-substituted thianaphthene-annulated porphyrazine and tetra(2,3-pyrazino)porphyrazine in proton-donor medium (CH2Cl2-CF3COOH) were studied by spectrophotometric titration. The concentration stability constants of the singly protonated complexes were determined. The effects of the metal nature and aromatic heterocyclic fragment on the basicity of meso-nitrogen atoms were analyzed. Negative inductive effect of the sulfur atom in the thianaphthene fragment reduces the basicity of the meso-nitrogen atoms as compared to analogous porphyrazine and phthalocyanine complexes. Acid-base transformations of the thianaphthene-annulated tetra(2,3-pyrazino)porphyrazine involve both meso-nitrogen atoms and those in the pyrazine rings.

Comparative electronic structures and UV–vis spectra of tribenzosubporphyrin, tribenzomonoazasubporphyrin, tribenzodiazasubporphyrin, and subphthalocyanine: Insight from DFT and TDDFT calculations

Electronic structures, geometries, and vertical excitation energies of chloroboron subphthalocyanine, tribenzodiazasubporphyrin, tribenzomonoazasubporphyrin, and tribenzosubporphyrin were calculated using density functional theory (DFT) and time-dependent (TD) DFT coupled with polarized continuum model (PCM) approach. Molecular geometries calculated at the BP86/6-311G(d) level reveal bowl-shape, trigonal prismatic conformations for all compounds with a variable bowl-depth that depends on the number of meso-nitrogen atoms in corresponding molecule. TDDFT-PCM calculations predict that the Q-band should undergo gradual high-energy shift, while the B-band should undergo low-energy shift upon stepwise substitution of the meso-nitrogen atoms in subphthalocyanine toward tribenzosubporphyrin. The TDDFT-PCM predicted trend was rationalized on the basis of electronic structures of target macrocycles. When comparison between theory and experiment is available, TDDFT-PCM calculations are in qualitative and quantitative agreement with experimental data.

Acid-base interaction of octaaryltetraazaporphyrin complexes in a proton-donor medium

The processes of acid-base interactions of the octaaryltetraazaporphyrin complexes in a protondonor environment based on acetic or trifluoroacetic acids was studied. The constants of the acidity of the protonated forms were determined. The introduction of electron-withdrawing functional groups was shown to reduce the basicity of the meso-nitrogen atoms of the macrocycle. The decrease in the cobalt oxidation state was shown to reduce the complex basicity.

Metal–ligand coordination in subphthalocyanines and phthalocyanines: DFT, AIM and ELF analyses

A comparative study of subphthalocyanines and phthalocyanines was carried out using high-level quantum chemistry calculations. These calculations represent the highest computational level that has been employed in a systematic study of such oligopyrrolic macrocycles. The boron–phthalocyanine coordination mode is quite different from many other metal–phthalocyanine modes. A metal–ligand bond in a subphthalocyanine is shorter and stronger than that in a phthalocyanine. All metal–ligand bonds are polar bonds with both covalent and ionic natures. The thermodynamic data is consistent with the relative stability of subphthalocyanines and phthalocyanines that have been examined. Fukui functions suggest that the attack perpendicular to the carbon atoms adjacent to the meso nitrogen atoms is probably an initial step for the decomposition of a subphthalocyanine.