Daisuke Sakamaki

Nitrogen-embedded buckybowl and its assembly with C60.

Curved π-conjugated molecules have attracted considerable interest because of the unique properties originating from their curved π surface. However, the synthesis of such distorted molecules requires harsh conditions, which hamper easy access to heteroatom-containing curved π systems. Here we report the synthesis of a π-extended azacorannulene with nitrogen in its centre. The oxidation of 9-aminophenanthrene provides tetrabenzocarbazole, which is converted to the azabuckybowl through palladium-catalysed intramolecular coupling. The electron-donating nature and curved π surface of the azabuckybowl enable its tight association with C60 in solution and solid states. High charge mobility is observed for the azabuckybowl/C60 assembly. This compound may be of interest in the fields of curved π systems as fullerene hosts, anisotropic π donors and precursors to nitrogen-containing nanocarbon materials.

Effects of Carbon–Metal–Carbon Linkages on the Optical, Photophysical, and Electrochemical Properties of Phosphametallacycle-Linked Coplanar Porphyrin Dimers

5-(Diphenylphosphanyl)-10,15,20-triarylporphyrins (meso-phosphanylporphyrins) underwent complexations with palladium(II) and platinum(II) salts to afford phosphapalladacycle- and phosphaplatinacycle-fused coplanar porphyrin dimers, respectively, via regioselective peripheral β-C-H activation of the meso-phosphanylporphyrin ligands. The optical and electrochemical properties of these metal-linked porphyrin dimers as well as their porphyrin monomer/dimer references were investigated by means of steady-state UV-vis absorption/fluorescence spectroscopy, cyclic and differential pulse voltammetry, time-resolved spectroscopy (fluorescence and transient absorption lifetimes and spectra), and magnetic circular dichroism spectroscopy. All the observed data clearly show that the palladium(II) and platinum(II) linkers play crucial roles in the electronic communication between two porphyrin chromophores at the one-electron oxidized state and in the singlet-triplet intersystem-crossing process at the excited state. It has also been revealed that the C-Pt-C linkage makes more significant impacts on these fundamental properties than the C-Pd-C linkage. Furthermore, density functional theory calculations on the metal-linked porphyrin dimers have suggested that the antibonding dπ-pπ orbital interaction between the peripherally attached metal and adjacent pyrrolic β-carbon atoms destabilizes the highest occupied molecular orbitals of the porphyrin π-systems and accounts for the observed unique absorption properties. On the basis of these experimental and theoretical results, it can be concluded that the linear carbon-metal-carbon linkages weakly but definitely perturb the optical, photophysical, and electrochemical properties of the phosphametallacycle-linked coplanar porphyrin dimers.

A Facile and Versatile Approach to Double N‐Heterohelicenes: Tandem Oxidative CN Couplings of N‐Heteroacenes via Cruciform Dimers

Novel double N-hetero[5]helicenes that are composed of two nitrogen-substituted heteropentacenes are synthesized by tandem oxidative CN couplings via the cruciform heteropentacene dimers. The developed method is very facile and enables the synthesis of a double helicene in only two steps from commercially available naphthalene derivatives. These double N-hetero[5]helicenes have larger torsion angles in the fjord regions than typical [5]helicenes, and optical/electrochemical measurements revealed a significant increase in the electronic communication between the two heteropentacene moieties of the double helicenes compared with their cruciform dimers. The optical resolution of one of the double helicenes was successfully carried out, and their stability towards racemization was remarkably higher than those of typical [5]helicenes. The synthetic strategy proposed in this paper should be versatile and widely applicable to the preparation of double helicenes from other N-containing π-conjugated planar molecules.

A Triphenylamine Double-Decker: From a Delocalized Radical Cation to a Diradical Dication with an Excited Triplet State†

Triphenylaminium radical cations are well-known for their remarkable stability; furthermore, the oxidation potential is adjustable by the introduction of a series of substituents, in particular, at the para positions. Thus, they are widely used not only as one-electron oxidative reagents but also as catalysts in organic chemistry. Besides such an application as versatile chemical reagents, the ambient stable radical cations generated from triphenylamine (TPA) derivatives could be utilized as spin sources in molecule-based magnetic materials. In fact, a considerable number of TPA-based molecular multi-spin systems have appeared in the literature. However, from the viewpoint of assemblage of the spin sources into the architecture of the reported molecular systems, the spin–spin exchange coupling pathway has been confined to oneand two-dimensionalities, and accordingly, TPA-based multispin systems with three-dimensional (3D) connectivity are rare. In 1985, Neugebauer and co-workers reported the first synthesis of a TPA double-decker 1 (Scheme 1) in which two TPA moieties are directly connected by threefold o,o’-linkage, in association with an interest in intramolecular interaction between two nitrogen lone pairs. In spite of its sensitivity to oxidation and the formation of persistent radical cation, no detailed information was available concerning the electronic structure. Such a cage-like oligo(triphenylamine) can be regarded as a potent building block to afford the opportunity furnishing new TPA-based macromolecular systems with genuine 3D connectivity. Although neither the parent 1 nor its derivatives have been reported since then, an analogue 2 in which two TPA moieties are connected at para positions of all phenyl groups by three meta-phenylenediamine pillars, was successfully prepared in our laboratory. Slow evaporation of a dilute mixed solution (CH3OH and CH2Cl2) of a pale yellow product 2, which was obtained by using Buchwald–Hartwig cross-coupling reaction between 4,4’,4’’-tribromo-TPA as a deck and meta-phenylenediamine as a pillar in a ratio of 2:3, gave yellow block crystals suitable for X-ray structure analysis. The crystal belongs to the space group P21, and contains two molecules of 2 per unit cell in which there exist three sites for CH2Cl2 molecules so as to fill spaces between the double-deckers (Supporting Information, Table S1). The X-ray structure of 2 has almost C3h symmetrical trichetria-like shape when all the methoxy substituents are ignored, and the upper and lower TPA decks adopt almost planar propeller-like conformation (the tilted angles of phenyl rings range from 428 to 538), and moreover, are totally eclipsed (Figure 1). Three meta-phenylenediamine planes are perpendicular to the TPA planes, and therefore, the distance between two TPA decks are estimated to be about 4.9 at the peripheral three nitrogen pairs and 5.8 at the central nitrogen pair, indicating the direct transannular p–p interaction between two TPA decks is virtually ignorable. The sp planes of the nitrogen atoms in the meta-phenylenediamine as a pillar are found to be at angles of about 608 to the metaphenylene plane. Scheme 1. The first reported TPA double-decker 1 and meta-phenylenediamine-bridged TPA double-decker 2.

Fabrication of enzyme-degradable and size-controlled protein nanowires using single particle nano-fabrication technique

Protein nanowires exhibiting specific biological activities hold promise for interacting with living cells and controlling and predicting biological responses such as apoptosis, endocytosis and cell adhesion. Here we report the result of the interaction of a single high-energy charged particle with protein molecules, giving size-controlled protein nanowires with an ultra-high aspect ratio of over 1,000. Degradation of the human serum albumin nanowires was examined using trypsin. The biotinylated human serum albumin nanowires bound avidin, demonstrating the high affinity of the nanowires. Human serum albumin–avidin hybrid nanowires were also fabricated from a solid state mixture and exhibited good mechanical strength in phosphate-buffered saline. The biotinylated human serum albumin nanowires can be transformed into nanowires exhibiting a biological function such as avidin–biotinyl interactions and peroxidase activity. The present technique is a versatile platform for functionalizing the surface of any protein molecule with an extremely large surface area.

A Polymacrocyclic Oligoarylamine with a Pseudobeltane Motif: Towards a Cylindrical Multispin System

Although several types of belt-shaped compounds with novel structures have been reported over the past 30 years, they are currently receiving increasing attention in conjunction with the synthesis of the shortest possible segments of singlewalled carbon nanotubes. Nanoscaled beltlike molecules are considered to be “cycles of cycles”, and thus they have welldefined shapes with rigid cavities and can conceivably be used to construct solid-state materials with nanoporous networks. In addition, polymacrocycles with electron(or hole-) delocalized (or localized) scaffolds are fascinating for potential applications towards electron (or hole) transport and/or as magnetic materials. In this context, oligoarylamine-based macrocyclic spin systems are being pursued to take advantage of the multi-electron redox properties of oligoarylamines and the relative stability of their poly(radical cation)s. It is well known that strong Coulombic interactions between charged centers in oligoarylamine-based macrocycles hinder the generation of higher oxidation states with maximum spin multiplicity (Coulombic penalty). As we have shown recently, however, the insertion of para-phenylenediamine (PD) units into the molecular backbone can alleviate the Coulombic penalty between charged triarylaminium radical centers in oligoarylamines and lower their oxidation potentials. Tetraaza[14]m,p,m,p-cyclophane, the smallest macrocyclic oligoarylamine bearing the alternating meta–para linkage, is transformed into an almost pure spin-triplet diradical dication upon two-electron oxidation. Moreover, it has been demonstrated that the introduction of this macrocycle into a oligoarylamine backbone with one-dimensional connectivity can can convert the one-dimensional multispin system with a fragile spin-coupling pathway into a robust, aligned highspin system. Thus the polymacrocycles provided by the tetraazacyclophanes may be an indication for the further development of cylindrical multispin systems, which could culminate in nanotube-like surfaces with multi-electron redox activity. These findings led to the idea of utilizing the tetraazacyclophane unit as a component for a belt-shaped polymacrocyclic oligoarylamine. Polymacrocycle 1 (Scheme 1), which is classified as a pseudobeltane according to Vçgtle s nomenclature, 9] can be viewed as a kind of molecular belt containing six PD units connected by four 1,3,5-benzenetriyl ferromagnetic couplers, and thereby the higher oxidation states of 1 can lead to multispin systems.

N‐Substituted Dicyanomethylphenyl Radicals: Dynamic Covalent Properties and Formation of Stimuli‐Responsive Cyclophanes by Self‐Assembly

Dynamic covalent bonds and their chemistry have been of particular interest both from a fundamental and materials science aspect. Demonstrated herein is that triphenylamine (TPA) and carbazole (Cz), substituted with a dicyanomethyl radical, are useful motifs for dynamic covalent chemistry as they have the appropriate bond strength between monomer units as well as high stability and synthetic simplicity. TPA and Cz units substituted by two dicyanomethyl radicals formed macrocyclic oligomers classified as novel types of azacyclophanes, and in particular, the TPA-based diradical gave a cyclic dimer in almost quantitative yield. The cyclic oligomers exhibited thermo- and mechanochromic behavior resulting from the generation of radical species by intermonomer C-C bond cleavage.

Meta–Para-Linked Octaaza[18]cyclophanes and Their Polycationic States

Octaazacyclophanes, octaaza[1(8)]m,p,m,p,m,p,m,p-cyclophane (2) and octaaza[1(8)]m,p,p,p,m,p,p,p-cyclophane (3), as ring-size extended congeners of tetraaza[1(4)]m,p,m,p-cyclophane were synthesized, and the electronic states of their polycationic species were investigated by quantum chemical calculations, electrochemical measurements (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)), UV-vis-NIR spectroelectrochemical measurements, and pulsed electron spin resonance (ESR) spectroscopy. These octaazacyclophanes exhibited multiredox activities depending on different linkage patterns along the macrocyclic molecular skeletons, and both molecules were oxidizable up to their respective octacations. Spectroelectrochemical measurements demonstrated that p-phenylenediamine (PD) moieties in 2 could be converted from the semiquinoidal structure to the quinoidal sturcture with increasing oxidation number, whereas higher oxidation states of 3 did not show definite quinoidal deformation of PD moieties. A pulsed ESR spectrum gave evidence about formation of the almost pure spin-triplet state for 3(2+), whereas the high-spin states of 2(2+) and 2(4+) are virtually degenerate with the competing low-spin states even at low temperatures, probably due to the fragility of spin-coupling pathway caused by facile conformational changes.

Highly emissive excited-state intramolecular proton transfer (ESIPT) inspired 2-(2′-hydroxy)benzothiazole–fluorene motifs: spectroscopic and photophysical properties investigation

Tuning or switching of the solid state luminescence of organic materials is an attractive target for both basic research and practical applications. In the present study, solid state emissive compounds with very high quantum efficiencies (ΦF up to 68%) were achieved by chemical alteration of the excited state intramolecular proton transfer (ESIPT) 2-2′-hydroxy benzothiazole (HBT) unit. Five ESIPT inspired compounds based on fluorene were synthesized via Suzuki coupling reaction. Their photophysical properties were studied by means of steady state absorption, emission spectra and a time resolved emission method in solid as well as in solution of different polarities. The fluorophores showed absorption in the UV region and emission in the visible region with large Stokes shift (∼232 nm). Efficient yellow emissive compounds showed very high quantum yields (ΦF = 55–68%) in the solid state, which are the highest quantum yields in the solid state to the best of our knowledge, for fluorene based ESIPT molecules. The fluorescence lifetime in the solid state is between 3.48–5.21 ns, while it is 5–10 fold less in chloroform (0.52–0.75 ns) solution. The optical properties of these compounds are sensitive towards the polarity of the medium. The structural properties, such as X-ray single crystal analyses, DSC and TGA were studied, and the lack of stacking and/or hydrogen bonding interactions around HBT motifs reveals enough room for ESIPT in the series of molecules even in their solid state. The DFT computations were performed to support experimental results and the calculations are well in line with the experimental results. These suggest high quantum efficiency ascribed to the large orbital energy difference between HOMOs and LUMOs of enol and keto forms transformed via ESIPT, and hence, singlet energy localization onto the keto form. The intra-molecular charge transfer nature between fluorene and HBT units plays a key role for the localization of energy on HBT motifs in their excited states.

Redox modulation of para-phenylenediamine by substituted nitronyl nitroxide groups and their spin states.

Three kinds of para-phenylenediamine (PDA) derivatives bearing nitronyl nitroxide (NN) groups were prepared and characterized on the basis of the electrochemical, electron spin resonance (ESR) spectroscopic, absorption spectroscopic, and magnetic susceptibility measurements. It was clarified that the oxidation potential of the central PDA unit is strongly influenced by the numbers of substituted electron-withdrawing NN groups. In addition, the intervalence charge transfer in the central PDA unit was detected in the monocationic states of the PDAs with two NN groups, indicating the coexistence of the localized spins and the delocalized spin on theses molecules. Moreover, pulsed ESR measurements confirmed that the delocalized spin on the central PDA unit and the localized two spins on the NN groups were ferromagnetically coupled in the monocationic states.