The windmill, the dragon, and the frog: geometry control over the spectral, magnetic, and electrochemical properties of cobalt phthalocyanine regioisomers.

For the first time, we have prepared non-aggregating phthalocyanine cobalt complexes as a set of resolved positional isomers. These compounds comprise a unique test bed for the structure-properties studies, as their optical and electrochemical properties are influenced by the planarity of the phthalocyanine macrocycle, which can be controlled by the positional isomerism of the bulky aromatic substituents at the α-phthalo sites. We support our conclusions with molecular modelling studies, which show a perfect match between the calculated and experimentally determined spectral/electrochemical values. We challenge a common perception that the NMR spectra of cobalt phthalocyanines cannot be measured due to the paramagnetic nature of Co(II). We suggest instead that the key factors affecting the NMR spectral resolution are molecular aggregation and π-π stacking. These interactions are suppressed by the bulky peripheral substituents on the cobalt phthalocyanines prepared, making these isomeric compounds an excellent tool for paramagnetic NMR studies.

Supermolecular Nanoentities of Vitamin B12 Derivative as a Link in the Evolution of the Parent Molecules During Self-Assembly at the Air-Water Interface.

Nanoarchitectures with promising properties have now been formed from many important biomolecules. However, the preparation of nanoparticles of vitamin B12 and its derivatives remains an ongoing research challenge. This paper describes the formation of supermolecular nanoentities (SMEs) of vitamin B12 derivatives, unique nanoparticles with strong noncovalent intermolecular interactions, emerging properties, and activity. These were created by a nanoarchitectonic approach using directed assembly of layers at the air-water interface as a link in the chain of evolution of the parent molecules under specially created conditions. Such layers can be represented as a nanocosm, where, at a critical density, the assemblies act as nanoreactors in which the transformation of the original material occurs. The discovered SMEs not only replicate the functioning of vitamin B12 assemblies with proteins in living organisms and act as vitamin B12-depended enzymes but also demonstrate important advantages over vitamin B12. They are more efficient in oxygen reduction/evolution reactions and in transformation into other forms. These SMEs, in performing advanced tasks, are an alternative to widely used materials based on noble metals for catalysis, medicine, and environment protection. Our findings open new perspectives both for the fabrication of novel SMEs of biomolecules and for a better understanding of the evolution of biomolecules in nature.

Iron-Catalyzed Biomimetic Dimerization of Tryptophan-Containing Peptides.

Biomimetic oxidative dimerization of tryptophan derivatives in aqueous media with oxygen as a bulk oxidant catalyzed by an iron octacarboxy phthalocyanine complex was established. The discovery of the extremely active iron catalyst enables aerobic enzyme-mimetic oxidation to be performed in a flask. This method was applicable to the oxidative dimerization of a wide range of tryptophan derivatives, including various dipeptides and oligopeptides, with remarkable functional-group tolerance without the protection of the amino acid residues. Furthermore, oxidative dimerization of tryptophan derivatives bearing dioxopiperazine units enabled the convergent total synthesis of five natural pyrroloindole compounds and unnatural congeners. The established chemical method provides facile access to a broad range of dimerized peptides with a unique scaffold to link two turn structures, which will serve as a powerful tool to create new small- and medium-sized-molecules as drug candidates.

Dianion and Dication of Tetracyclopentatetraphenylene as Decoupled Annulene-within-an-Annulene Models.

The dianion and dication of tetramesityl-substituted tetracyclopentatetraphenylene, a circulene consisting of alternating five- and six-membered rings, have been generated by reduction with alkali metals and oxidation with antimony(V) halides, respectively. They are theoretically predicted to adopt double annulenoid structures called annulene-within-an-annulene models in which the outer and inner conjugation circuits are significantly decoupled. The theoretical structures were experimentally proven by X-ray crystallographic analyses and the electronic configurations were supported by MCD spectra. Based on the 13 C NMR chemical shifts, negative and positive charges are shown to be located mainly at the outer periphery, indicating that the dianion and dication have delocalized 22-π and 18-π electron outer perimeters, respectively, and 8-π electron structure at the inner ring. Notably, the dianion has an open-shell character, whereas the dication has a closed-shell ground state.

Synthesis and Characterization of Peralkylated Pyrrole-Fused Azacoronene.

A hexapyrrolohexaazacoronene (HPHAC) with 12 less-bulky peripheral ethyl groups than its aryl-containing HPHAC counterpart was synthesized to investigate the innate character of HPHAC. X-ray diffraction analysis revealed that HPHAC had a planar structure and close packing because of CH-π interactions between the alkyl groups and the HPHAC core. Compared to the previously reported HPHAC decorated with 12 peripheral aryl groups, this electron-rich π-system exhibited reversible multistep oxidations at low potentials and easily formed mono- and dicationic salts and charge-transfer (CT) complexes with 7,7,8,8-tetracyano-p-quinodimethane. These oxidized species exhibited clear changes in the bond-length alternation of the pyrrole units in the crystal state, indicating charge and spin delocalization. The distinct upfield shift of the central carbon signal of the dication in the 13C NMR spectrum affirms the global aromaticity from the viewpoint of a magnetic criterion. In the UV-vis/NIR spectra, broad absorption in the NIR region was observed only for HPHAC2+ and not the structurally similar cyclo[6]pyrrole. Magnetic circular dichroism measurements and time-dependent density functional theory calculations revealed that the broad absorption was assigned to the CT transition from the central benzene ring to the outer pyrrole rings.

Radially π-Extended Pyrrole-Fused Azacoronene: A Series of Crystal Structures of HPHAC with Various Oxidation States.

Radially π-extended hexapyrrolohexaazacoronene (HPHAC 2) was synthesized in two steps with 2,5-di-tert-butyl-acenaphtho[1,2-c]pyrrole and hexafluorobenzene. Like its parent HPHAC 1a, 2 exhibited reversible oxidation behavior. Crystal structures of not only the neutral and dicationic compounds but also the radical cation were all revealed. While analyzing its peculiar structure, the formation of a 1:1 complex with C60 was observed in solution by NMR and absorption analyses. The spectroscopic analyses of 2 and 1a were performed based on magnetic circular dichroism and theoretical calculations.

Periphery-Fused Chiral A2B-Type Subporphyrin.

Despite significant interest, the chiroptical properties of subporphyrins have rarely been investigated because chiral subporphyrins are elusive. Here, inherently chiral subporphyrins are elaborated by forming a fused pyran ring at the periphery of an A2B-type meso-aryl-substituted subporphyrin. Their circular dichroism (CD) properties are largely affected by the peripheral substituents and the dihedral angles between the meso-aryl substituents and the subporphyrin core: the ß-perbromo subporphyrin with an orthogonal arrangement of the meso-phenyl substituents to the subporphyrin core exhibits weak CD signals corresponding to the Q bands, whereas the unsubstituted species with smaller dihedral angles shows relatively intense CD signals. A detailed structure-property relationship of these chiral subporphyrins was elucidated by time-dependent (TD) DFT calculations. This study reveals that the CD properties of chiral subporphyrins can be controlled by peripheral substitution and meso-aryl substituents.

Preparation, Spectroscopic Characterization and Theoretical Study of a Three-Dimensional Conjugated 70 π-Electron Thiophene 6-mer Radical Cation π-Dimer.

A radical cation, generated from an extended π-conjugated thiophene 6-mer composed of four ethynylene-thienylene and two vinylene-thienylene units, was observed to form a stable three-dimensional π-dimer containing 70 π-electrons. The π-dimer prepared in solution was investigated by using magnetic circular dichroism (MCD), ESR spectroscopy, and UV-vis-NIR absorption spectroscopy. Probing the individual NIR absorption bands showed that the MCD signals can be assigned to the pseudo Faraday A term, indicating that the absorption bands are comprised of nearly degenerate electronic transitions. X-ray crystallographic analysis revealed that the π-dimer has a three-dimensional face-to-face and continuous π-conjugated donutlike structure. Analysis of the UV-vis-NIR and ESR spectra of the π-dimer in the solid state confirmed that it possesses the dimer structure. The prediction made by using TD-DFT calculations that the dimer would have a 70 π-electron diatropic nature was confirmed by using solid state 1H NMR spectroscopy.

Synthesis and Aromaticity of Benzene-Fused Doubly N-Confused Porphyrins.

A bicyclo[2.2.2]octadiene(BCOD)-fused bis(dipyrromethane) derivative was reacted with methylal in the presence of trifluoroacetic acid followed by oxidation with chloranil to give a doubly N-confused phlorin derivative, which did not undergo the retro-Diels-Alder reaction extruding an ethylene molecule on heating. In contrast, a 7-tert-butoxybicyclo[2.2.1]heptadiene(7-BuOBCHD)-fused bis(dipyrromethane) yielded benzene-ring-fused doubly N-confused porphyrins by following similar reaction sequences.

TTF-Annulated Silicon Phthalocyanine Oligomers and Their External-Stimuli-Responsive Orientational Ordering.

Orientational control of functional molecules is essential to create complex functionalities as seen in nature; however, such artificial systems have remained challenge. Herein, we have succeeded in controlling rotational isomerism of µ-oxo silicon phthalocyanine (SiPc) oligomers to achieve an external-stimuli-responsive orientational ordering using intermolecular interactions of tetrathiafulvalene (TTF). In this system, three modes of orientations, free rotation, eclipsed conformation, and staggered conformation, were interconverted in response to the oxidation states of TTF, which varied interactions from association due to formation of mixed-valence TTF dimer to dissociation due to electrostatic repulsion between TTF dications. Furthermore, a stable performance of oligomers as a cathode material in a Li-ion battery proved that the one-dimensionally stacked, rotatable structure of SiPc oligomers is useful to control the orientation of functional molecules toward molecular electronics.

Boron(III) Carbazosubphthalocyanines: Core-Expanded Antiaromatic Boron(III) Subphthalocyanine Analogues.

Condensation of 1,8-diamino-3,6-dichlorocarbazole with a series of disubstituted 1,3-diiminoisoindolines, followed by treatment with BF3 ⋅OEt2 led to the formation of the corresponding core-expanded boron(III) subphthalocyanine analogues. These air-stable π-conjugated boron(III) carbazosubphthalocyanines possess two boron-containing seven-membered-ring units and a 16 π-electron skeleton, and represent the first examples of antiaromatic boron(III) subphthalocyanine analogues as supported by spectroscopic and theoretical studies. The molecular structure of one of these compounds was unambiguously determined by single-crystal X-ray diffraction analysis. In contrast to typical boron(III) subphthalocyanines, which adopt a cone-shaped structure, the π skeleton of this compound is almost planar.

Increased Light-Harvesting in Dye-Sensitized Solar Cells through Förster Resonance Energy Transfer within Supramolecular Dyad Systems.

Novel pyridine-substituted subphthalocyanines were prepared for an additional harvesting of a green spectral region of the solar light spectrum for zinc phthalocyanine-based dye-sensitized solar cells. These compounds can bind with the central metal of zinc phthalocyanines to form the corresponding supramolecular complexes as monitored by the absorption and fluorescence spectral changes. The stability constants of these complexes were altered by the number and position of pyridine units in the pyridine-substituted subphthalocyanines. On the basis of fluorescence titration study, the complexes efficiently transfer energy from the subphthalocyanine to zinc phthalocyanine. The solar cells using TiO2 electrodes stained with the supramolecular complexes, composed of zinc phthalocyanine sensitizer and pyridine-substituted subphthalocyanines, showed panchromatic responses, and the photocurrent generation in the range of 500-600 nm is attributed to the efficient Förster resonance energy transfer from subphthalocyanine to zinc phthalocyanine on the TiO2 surface.

Magnesium Porphine Supermolecules and Two-Dimensional Nanoaggregates Formed Using the Langmuir-Schaefer Technique.

Porphyrins are functional elements of important biomolecules, whose assemblies play a central role in fundamental processes such as electron transfer, oxygen transport, enzymatic catalysis, and light harvesting. Here we report an approach to formation of porphyrin supermolecules, a particular type of nanoparticles with unusually strong noncovalent intermolecular interactions. Key differences between the supermolecules and noncovalent nanostructures described earlier are as follows. (1) Supermolecules consist of molecules of the same type without side groups promoting the self-assembly and without any spacers; no surfactant or catalyst to assist the process is needed. (2) They exhibit unusual photophysical properties and remain stable even in organic solvents. Their formation occurs under specially selected conditions at the air-water interface at room temperature. Following this route, we have formed supermolecules of magnesium porphine, a functional element of chlorophyll. The properties of these supermolecules are markedly different from those of the constituent molecules. For example, in contrast to the pink color of the monomer solution, solutions of supermolecules are transparent for visible light and absorb in the ultraviolet and near-infrared regions. We also present atomic force microscopy visualization of the porphyrin two-dimensional nanoaggregates forming at the air-water interface that were predicted in our previous works. This approach offers a guideline for the discovery of new supermolecules, including complex biological ones, and the formation of supermolecular materials with novel properties.

Optically Active Porphyrin and Phthalocyanine Systems.

This review highlights and summarizes various optically active porphyrin and phthalocyanine molecules prepared using a wide range of structural modification methods to improve the design of novel structures and their applications. The induced chirality of some illustrative achiral bis-porphyrins with a chiral guest molecule is introduced because these systems are ideal for the identification and separation of chiral biologically active substrates. In addition, the relationship between CD signal and the absolute configuration of the molecule is analyzed through an analysis of the results of molecular modeling calculations. Possible future research directions are also discussed.

Regioregular Phthalocyanines Substituted with Bulky Donors at Non-Peripheral Positions.

Three regioregular phthalocyanines (1-3) were synthesized selectively by the cyclic tetramerization of phthalonitriles bearing a bulky diarylamine substituent at the next position of nitrile. The steric repulsion at the tetramerization of bulky phthalonitriles allowed for the selective formation of regioregular phthalocyanines as confirmed by NMR and single crystal X-ray structural analyses. The absorption spectrum of 1 substituted with di(4-tert-butylphenyl)amine groups at the non-peripheral positions showed a non-split Q-band at 764 nm, which was redshifted by 83 nm compared with that of metal free phthalocyanine (H2 Pc). The TD-DFT calculation and electrochemical studies prove that the substitution of diarylamine groups at the α-positions effectively destabilizes the HOMO energy level, which causes a large redshift of the Q-band. Moreover, 1 can generate a one-electron oxidation species through chemical oxidation. The Q-band position of 2 bearing 4,4'-dimetoxyphenylamine units was further shifted by 10 nm compared with that of 1. In addition, 3 having carbazole units showed a small redshift of the Q-band relative to H2 Pc. The hole-mobility of 2 in thin film was determined to be 1.1×10-5  cm V-1 s-1 by using a space charge limited current method. A perovskite solar cell employing 2 as a hole-transporting layer gave an efficiency of 5.1 % under standard global 100 Wcm-2 AM 1.5 G illumination.

Structural, Photophysical, and Magnetic Circular Dichroism Studies of Three Rigidified meso-Pentafluorophenyl-Substituted Hexaphyrin Analogues.

Detailed electronic, structural, photophysical, and redox studies of a series of meso-pentafluorophenyl-substituted hexaphyrins, namely amethyrin (1), rosarin (2), and rubyrin (3), are described. In prior work, it was found that the electronic states of the antiaromatic hexapyrrolic macrocycle, [24]rosarin 2, could be modified by exposure to several Brønsted acids (e.g., HCl, HBr and HI) to produce either one- and two-electron reduced species, or both. In an effort to gain further insights into the reactivity of hexaphyrins possessing different π-conjugation pathways, the ß-dodecamethyl-substituted [24]amethyrin 1 was prepared and its electronic structure was analyzed along with that of the o-phenylene-bridged [26]rubyrin 3 and rosarin 2 The [4n] and [4n+2] π-conjugated formulations of 2 and 3, respectively, were inferred from steady-state, fs-transient absorption and two photon absorption measurements. Similar photophysical analyses lead to the conclusion that 1 is best considered as nonaromatic or weakly antiaromatic. Magnetic circular dichroism (MCD) spectroscopic analyses of hexaphyrins 1 and 3, as well as comparisons to 2, and theoretical perimeter MO diagram analyses provided support for the electronic assignments. In contrast to what was found for 2, simple protonation of 1 and 3 by halohydric acids did not induce an evident, redox-based change in the electronic structure of the macrocycle.

Azaporphyrin phosphorus(v) complexes: synthesis, structure, and modification of optical properties.

Azaporphyrinoids, such as phthalocyanines (Pcs), tetraazaporphyrins (TAPs), and tetrabenzotriazacorroles (TBCs), are some of the most well-known and successful artificial dyes and pigments in modern material chemistry. Modifications of the macrocyclic core, periphery, and central element have attracted a great deal of interest in materials sciences due to generation of unique optical and electronic properties. However, the synthesis of most azaporphyrinoids with novel physical properties generally needs long, tedious procedures. On the other hand, the introduction of a phosphorus(v) atom into simple (known) azaporphyrin macrocycles is not necessarily difficult and can generate changes in the structural and optical properties. This paper provides an overview of the recent development of azaporphyrin phosphorus(v) complexes with unique structural and optical properties. Optical properties are discussed based on a combination of experimental absorption spectra, electrochemical properties, and theoretical molecular orbital calculations. These complexes are relatively easy to synthesize, are robust and free from transition metals, and have predictable properties.

Template Synthesis of Decaphyrin without Meso-Bridges: Cyclo[10]pyrrole.

An acenaphthylene-fused cyclo[10]pyrrole 1b was selectively synthesized via an oxidative coupling reaction of the corresponding 2,2'-bipyrrole with the appropriate dianion template, croconate anion. The structure of 1b as the isolated largest cyclo[n]pyrrole was elucidated by X-ray crystallographic analysis. The absorption spectrum exhibited a markedly red-shifted, intensified L band at 1982 nm, which was interpreted by application of Michl's perimeter and Gouterman's 4-orbital models, supported by magnetic circular dichroism (MCD) data and theoretical calculations.

Pyrene-Bridged Boron Subphthalocyanine Dimers: Combination of Planar and Bowl-Shaped π-Conjugated Systems for Creating Uniquely Curved π-Conjugated Systems.

Pyrene-bridged boron subphthalocyanine dimers were synthesized from a mixed-condensation reaction of 2,7-di-tert-butyl-4,5,9,10-tetracyanopyrene and tetrafluorophthalonitrile, and their syn and anti isomers arising from the result of connecting two bowl-shaped boron subphthalocyanine molecules were successfully separated. Expansion of the conjugated system of boron subphthalocyanine through a pyrene bridge caused a redshift of the Q band absorption relative to the parent pyrene-fused monomer, whereas combining the curved π-conjugation of boron subphthalocyanine with the planar π-conjugation of pyrene enabled facile embracement of C60 molecules, owing to the enhanced concave-convex π-π stacking interactions.

Low-Symmetry Ω-Shaped Zinc Phthalocyanine Sensitizers with Panchromatic Light-Harvesting Properties for Dye-Sensitized Solar Cells.

Two low-symmetry phthalocyanines (Pcs) substituted with thiophene units at the non-peripheral (α) and peripheral (ß) positions were synthesized and their optical, electronic-structure, and electrochemical properties were investigated. The substitution of thiophene units at the α positions of the phthalocyanine skeleton resulted in a red shift of the Q band and significantly modified the molecular-orbital electronic distributions just below the HOMO and just above the LUMO, with distortion of the typical Gouterman four-orbital arrangement of MOs. Two amphiphilic Ω-shaped ZnPcs (αPcS1 and αPcS2) bearing a π-conjugated side chain with an adsorption site at an α position of the Pc macrocycle were synthesized as sensitizers for dye-sensitized solar cells (DSSCs). The absorption spectra of αPcS1 and αPcS2 showed red shifted Q bands and a broad band from 350 to 550 nm assignable to the intramolecular charge-transfer transition from the ZnPc core to the side chains. Time-dependent DFT calculations provided a clear interpretation of the effect of the thiophene conjugation on the typical phthalocyanine core π MOs. Compound αPcS1 was used as a light-harvesting dye on a TiO2 electrode for a DSSC, which showed a panchromatic response in the range 400-800 nm with a power conversion efficiency of 5.5 % under one-sun conditions.