Acene Size-Dependent Transition of The Radical Centers From the Metal to The Acene Parts In Monocationic Dinuclear (Diethynylacene)diyl Complexes.

Controlling radical localization/delocalization is important for functional materials. The present paper describes synthesis and results of electrochemical, spectroscopic, and theoretical studies of diruthenium (p-diethynylacene)diyl complexes, Me3 Si-(C≡C)2 -Ru(dppe)2 -C≡C-Ar-C≡C-Ru(dppe)2 -(C≡C)2 -SiMe3 (1-6) (dppe: 1,2-bis(diphenylphosphino)ethane), and their monocationic radical species ([1]+ -[6]+ ). The HOMO-LUMO energy gaps can be finely tuned by the acene rings in the bridging ligands installed, as indicated by the absorption maxima of the electronic spectra of 1-6 ranging from the UV region even to the NIR region. The cationic species [1]+ -[6]+ show two characteristic NIR bands, which are ascribed to the charge resonance (CR) and π-π* transition bands, as revealed by spectroelectrochemistry. Expansion of the acene rings in [1]+ -[6]+ causes (1) blue shifts of the CR bands and red shifts of the π-π* transition bands and (2) charge localization on the acene parts as evidenced by the ESR, DFT and TD-DFT analyses. Notably, the monocationic complexes of the larger acene derivatives are characterized as the non-classical acene-localized radicals.

Substrate-Rhodium Cooperativity in Photoinduced ortho-Alkynylation of Arenes.

In the realm of metallaphotocatalytic C-H activation strategy, the direct excitation of the transition metal which plays the dual role of light energy harnessing alongside performing the bond breaking and forming is a rare phenomenon. In this context we have developed the first photo-induced Rh-catalyzed ortho-alkynylation under ambient conditions without the requirement of silver salt, photocatalyst (PC) or any engineered substrate or catalyst. The transformation functions by the specific cooperative effect of a six-membered rhodacycle which is the photo-responsive species. The catalytic system allows the conjugation of arenes with sp3 -rich pharmacophoric fragments. The control experiments as well as the computational studies resolve the mechanistic intricacies for this transformation. An outer sphere electron transfer process from Rh to alkynyl radical is operative for the present photo-induced transformation over the more common oxidative addition or 1,2-migratory insertion pathways.

Dimesitylborylethynylated Arenes: Unique Electronic and Photophysical Properties Caused by Ethynediyl (C≡C) Spacers.

Herein, we report the synthesis and electrochemical and photophysical properties of aromatic hydrocarbons having one or two dimesitylborylethynyl peripherals. The mono- (1) and diboryl compounds (2), readily prepared by nucleophilic substitution reaction, are fairly stable to air and moisture in the solid state. The inserted ethynediyl (C≡C) spacer cancels the steric hindrance between the bulky dimesitylboryl groups and aromatic rings, leading to effective π conjugation over the B-C≡C-Ar linkages, as revealed by cyclic voltammetry. Despite the small structural differences, the photophysical properties of the benzene, naphthalene, and anthracene derivatives are different. Virtually no emission was observed from the benzene derivatives, whereas the anthracene derivatives emitted with high quantum yields both in solution and in the solid state. Notably, the naphthalene derivatives showed aggregation-induced emission behavior. Unlike the common sterically congested triarylborane derivatives reported so far, the anthracene derivatives showed π-π*-type absorption and emission bands, which derive from efficient intramolecular orbital interactions between the boron centers and anthracene moieties, as supported by DFT calculations. As a result, the dimesitylborylethynyl substituents effectively lower the LUMO levels of the aromatic hydrocarbon parts, whereas the HOMO levels are almost unaffected, thereby leading to materials with controllable HOMO-LUMO gaps.

Organometallic Molecular Wires with Thioacetylene Backbones, trans-{RS-(C≡C)n }2 Ru(phosphine)4 : High Conductance through Non-Aromatic Bridging Linkers.

In this work, the design, synthesis, and single-molecule conductance of ethynyl- and butadiynyl-ruthenium molecular wires with thioether anchor groups [RS=n-C6 H13 S, p-tert-Bu-C6 H4 S), trans-{RS-(C≡C)n }2 Ru(dppe)2 (n=1 (1R ), 2 (2R ); dppe: 1,2-bis(diphenylphosphino)ethane) and trans-(n-C6 H13 S-C≡C)2 Ru{P(OMe)3 }4 3hex ] are reported. Scanning tunneling microscope break-junction study has revealed conductance of the organometallic molecular wires with the thioacetylene backbones higher than that of the related organometallic wires having arylethynylruthenium linkages with the sulfur anchor groups, trans-{p-MeS-C6 H4 -(C≡C)n }2 Ru(phosphine)4 4n (n=1, 2) and trans-(Th-C≡C)2 Ru(phosphine)4 5 (Th=3-thienyl). It should be noted that the molecular junctions constructed from the butadiynyl wire 2R , trans-{Au-RS-(C≡C)2 }2 Ru(dppe)2 (Au: gold metal electrode), show conductance comparable to that of the covalently linked polyynyl wire with the similar molecular length, trans-{Au-(C≡C)3 }2 Ru(dppe)2 63 . The DFT non-equilibrium Green's function (NEGF) study supports the highly conducting nature of the thioacetylene molecular wires through HOMO orbitals.

Anisotropic Contraction of a Polyaromatic Capsule and Its Cavity-Induced Compression Effect.

Anisotropic contraction of a spherical polyaromatic capsule was demonstrated through simple meta-to-ortho modification of the bent polyaromatic ligands. The resultant capsule, composed of two metal ions and four ortho-substituted ligands, possesses a spheroidal cavity (1.1 nm × 1.5 nm × 1.5 nm) fully encircled by a polyaromatic framework. One large planar or bowl-shaped molecule (e.g., porphine or sumanene) is quantitatively bound by the capsule, in which the cavity-induced compression effect causes the acceleration of the bowl-to-bowl inversion of sumanene. Temperature-dependent 1H NMR analysis revealed that the activation energy of the inversion decreases greatly (ΔG⧧ = -2.8 kcal mol-1 at 318 K) upon encapsulation, whereas the opposite effect was observed in the spherical cavity of the previous polyaromatic capsule.

Laser Flash Photolysis Studies on Radical Monofluoromethylation by (Diarylamino)naphthalene Photoredox Catalysis: Long Lifetime of the Excited State is Not Always a Requisite.

Organic photoredox catalysis has become a useful tool for the development of metal-free radical reactions. Recently, we have reported that 1,4-bis(diphenylamino)naphthalene N serves as an efficient photoredox catalyst for radical monofluoromethylation with N-tosyl-S-monofluoromethyl-S-phenylsulfoximine 2. In this paper, we report the preparation and photo- and electrochemical properties of (diarylamino)naphthalene derivatives, 1,4-bis(di(p-tert-butylphenyl)amino)naphthalene 1a, 1,5-bis(di(p-tert-butylphenyl)amino)naphthalene 1b, and 1-(di(p-tert-butylphenyl)amino)naphthalene) 1c, as supported by density functional theory (DFT) and time-dependent-DFT calculations. In addition, their performance of photocatalysis has been evaluated by means of methoxy-monofluoromethylation of aromatic alkenes. Laser flash photolysis shows that the fluorescence of 1a in the excited state is efficiently quenched by 2 (quenching rate constant kq = ca. 2 × 109 M-1 s-1). Transient absorption spectroscopic analyses reveal that the excited species of 1a in the presence of 2 starts decreasing in ca. 100 ps, suggesting the occurrence of fast electron-transfer processes. These results lead to the unconventional concept for the catalyst design, that is, long lifetime of the excited state is not always a requisite for efficient photoredox catalysts.

Single-Molecule Junction of a Cationic Rh(III) Polyyne Molecular Wire.

Single-molecule conductance studies on metal-containing inorganic and organometallic molecular wires are relatively less explored compared to those on organic molecular wires. Furthermore, conductance and transmission profiles of the metal-containing wires insensitive to the metal centers often hinder rational design for high performance wires. Here, synthesis and single-molecule conductance measurements of the bis(butadiynyl)rhodium wires with tetracarbene ligands 1H and 1Au are reported as rare examples for Rh(III) diacetylide molecular wires. The rhodium wires derived from the terminal acetylene and gold-functionalized precursors show comparable, high single-molecule conductance ((6-7) × 10-3 G0) as determined by the STM break-junction measurements, suggesting formation of virtually the same covalently linked metal electrode-molecule-metal electrode junctions. The values for the metallapolyynes are larger than those of the organic polyyne wires having the similar molecular lengths. The hybrid DFT-NEGF calculations of the model systems suggest that profiles of transmission spectra are highly sensitive to the presence and species of the metal fragments doped into the polyyne molecular wire because the conductance orbitals of the metallapolyynes molecular junctions carry significant metal fragment characters. Thus, the metallapolyyne junctions turn out to be suitable platforms for rationally designed molecular wires.

Recognition and Stabilization of Unsaturated Fatty Acids by a Polyaromatic Receptor.

Selective recognition of natural fatty acids is intrinsically difficult owing to the long, flexible, and poorly interactive hydrocarbon chains. Inspired by biological recognition systems, we herein demonstrate the exclusive binding of a monounsaturated fatty acid by an artificial polyaromatic receptor from a mixture of the unsaturated and corresponding saturated substrates (i.e., oleic and stearic acids) in water. The selectivity stems from multiple CH-π/π-π interactions between the host framework and the guest in its roughly coiled conformation. Moreover, competitive binding experiments elucidate higher binding affinities of the receptor for oligo- and polyunsaturated fatty acids (e.g., α-linolenic acid and EPA). Within the receptor, the biosubstrates are remarkably stabilized against air, light, and heat owing to the polyaromatic shielding effect.

N-Doping of Polyaromatic Capsules: Small Cavity Modification Leads to Large Change in Host-Guest Interactions.

To gain insight into the host functions of a nanocavity encircled by both polyaromatic panels and heteroatoms, nitrogen-doped polyaromatic capsules were successfully synthesized from metal ions and pyridine-embedded, bent anthracene-based ligands. The new capsules display unique host-guest interactions in the isolated cavities, which are distinct from those of the undoped analogues. Besides the inclusion of Ag+ ions, the large absorption change of fullerene C60 and altered emission of a BODIPY dimer are observed upon encapsulation by the present hosts. Moreover, the N-doped capsule exhibits specific binding ability toward progesterone and methyltestosterone, known as a natural female and synthetic male hormone, respectively, in water.

A Redox-Active Heterocyclic Capsule: Radical Generation, Oxygenation, and Guest Uptake/Release.

For the development of a redox-active supramolecular capsule with host function, we synthesized a bent heterocyclic amphiphile using phenothiazine panels capable of adopting three different states, i.e., neutral, radical, and oxygenated states. In water, the new amphiphiles spontaneously and quantitatively assemble into a heterocycle-based capsule with an average diameter of ∼2 nm, through the hydrophobic effect and π-stacking interactions. The product structure was confirmed by the combination of NMR, UV-visible, DLS, AFM, and molecular modeling studies. Electrochemical and chemical oxidation of the capsule generates relatively stable radical cation capsules at room temperature in a reversible fashion. The neutral capsule efficiently takes up large hydrophobic compounds (e.g., pigment blue 15 and fullerene C60) into the heterocyclic cavity through a grinding protocol and subsequent chemical oxidation of the products generates radical host-guest complexes. Moreover, chemical oxygenation of the host-guest complexes was shown to induce guest release in water via disassembly of the capsular structure through dioxygenation of the phenothiazine panels.

Open versus Closed Polyaromatic Nanocavity: Enhanced Host Abilities toward Large Dyes and Pigments.

Host functions of polyaromatic nanocavities were revealed by using an M2 L4 molecular cage and capsule. On the basis of the previously reported M2 L4 capsule with a closed polyaromatic cavity, a new M2 L4 cage (as a mixture of the isomers) was prepared by the quantitative assembly of two metal ions and four desymmetrized bispyridine ligands with a single polyaromatic panel. The obtained, open nanocavity of the cage exhibited enhanced binding abilities toward large dyes and pigments in water. For example, two molecules of coumarin dyes were bound in the nanocavity and showed strong whitish emission (up to ΦF =34 %). Furthermore, metallopigments, the sizes of which are larger than the inner cavities of the cage and capsule, were bound only in the open polyaromatic nanocavity of the cage to give water-soluble 1:1 host-guest complexes.

Recent progress in photochemical radical di- and mono-fluoromethylation.

Recently, photoinduced radical difluoromethylation has emerged as a step-economical synthetic method of CHF2-containing compounds. In this article, difluoromethylation of alkenes, isonitriles and aryl bromides promoted by photoredox catalysis is described together with a non-catalytic photoinduced system. Representative reactions will be discussed for each highlighted difluoromethylating reagent. In addition, related monofluoromethylation with their corresponding monofluoromethylating reagents is also discussed.

A Polyaromatic Gemini Amphiphile That Assembles into a Well-Defined Aromatic Micelle with Higher Stability and Host Functions.

A gemini-type amphiphilic molecule, constituted of two V-shaped polyaromatic amphiphiles linked by a linear acetylene spacer, was synthesized. The gemini amphiphile assembles into a well-defined aromatic micelle (ca. 2 nm in core diameter), providing higher stability in water even at low concentration (0.09 mm) and high temperature (>130 °C). Unlike common gemini amphiphiles with aliphatic chains, the present amphiphile and its micellar assembly emit green and orange fluorescence (ΦF =33 and 9 %), respectively. Despite strong and multiple π-stacks of the polyaromatic panels of the amphiphiles, the water-soluble gemini aromatic micelle incorporates medium-size to large hydrophobic compounds into the frameworks. Interestingly, the guest binding capability toward large planar molecules was enhanced by more than two times through the pre-encapsulation of spherical molecules in the cavity.

A Pentacene-based Nanotube Displaying Enriched Electrochemical and Photochemical Activities.

Unlike previously well-studied, acyclic pentacene oligomers, the first synthesis of a cyclic pentacene trimer with a fixed tubular conformation is reported. A short-step synthesis starting from common pentacenequinone yielded the target molecule with a 1.5 nanometer length and a subnanometer pore. Steady-state spectroscopic analyses revealed that the close proximity of the non-conjugated, three pentacene chromophores allows the nanotube to display stepwise electrochemical/chemical oxidation characteristics. Furthermore, time-resolved transient absorption measurements elucidated the generation of an excited triplet state of the nanotube, with high quantum yield reaching about 180 % through intramolecular singlet fission and a very long triplet lifetime.

A Peanut-Shaped Polyaromatic Capsule: Solvent-Dependent Transformation and Electronic Properties of a Non-Contacted Fullerene Dimer.

Synthesis of molecular containers capable of incorporating multiple fullerenes remains challenging. Reported here is that room-temperature mixing of metal ions with W-shaped bispyridine ligands featuring polyaromatic panels results in the quantitative formation of a peanut-shaped M2 L4 capsule. The capsule reversibly converts into two molecules of an ML2 double tube in response to changes in the solvent. Notably, the capsule allows the incorporation of two fullerene molecules into the connected two spherical cavities at room temperature. The close proximity yet non-contact of the encapsulated C60 molecules, with a separation of 6.4 Å, was revealed by X-ray crystallographic analysis. The resultant, unusual fullerene dimer undergoes sequential reduction within the capsule to generate (C60 .- )2 , C60 .- ⋅C60 2- , and (C60 2- )2 species. Furthermore, temperature-controlled stepwise incorporation of two C60 molecules into the capsule is demonstrated.

"Doping" of Polyyne with an Organometallic Fragment Leads to Highly Conductive Metallapolyyne Molecular Wire.

Exploration of highly conductive molecules is essential to achieve single-molecule electronic devices. The present paper describes the results on single-molecule conductance study of polyyne wires doped with the organometallic Ru(dppe)2 fragment, X-(C≡C) n-Ru(dppe)2-(C≡C) n-X. The metallapolyyne wires end-capped with the gold fragments (X = AuL) are subjected to single-molecule conductance measurements with the STM break junction technique, which reveal the high conductance (10-3-10-2 G0; n = 2-4) with the low attenuation factor (0.25 Å-1) and the low contact resistance (33 kΩ). A unique "'doping'" effect of Ru(dppe)2 fragment was found to lead to the high performance as suggested by the hybrid density functional theory-nonequilibrium green function calculation.

Hydrophilic Oligo(lactic acid)s Captured by a Hydrophobic Polyaromatic Cavity in Water.

Biologically relevant hydrophilic molecules rarely interact with hydrophobic compounds and surfaces in water owing to effective hydration. Nevertheless, herein we report that the hydrophobic cavity of a polyaromatic capsule, formed through coordination-driven self-assembly, can encapsulate hydrophilic oligo(lactic acid)s in water with relatively high binding constants (up to Ka =3×105 m-1 ). X-ray crystallographic and ITC analyses revealed that the unusual host-guest behavior is caused by enthalpic stabilization through multiple CH-π and hydrogen-bonding interactions. The polyaromatic cavity stabilizes hydrolyzable cyclic di(lactic acid) and captures tetra(lactic acid) preferentially from a mixture of oligo(lactic acid)s even in water.

Fine Design of Photoredox Systems for Catalytic Fluoromethylation of Carbon-Carbon Multiple Bonds.

Trifluoromethyl (CF3) and difluoromethyl (CF2H) groups are versatile structural motifs, especially in the fields of pharmaceuticals and agrochemicals. Thus, the development of new protocols for tri- and difluoromethylation of various skeletons has become a vital subject to be studied in the field of synthetic organic chemistry. For the past decades, a variety of fluoromethylating reagents have been developed. In particular, bench-stable and easy-to-use electrophilic fluoromethylating reagents such as the Umemoto, Yagupolskii-Umemoto, Togni, and Hu reagents serve as excellent fluoromethyl sources for ionic and carbenoid reactions. Importantly, the action of catalysis has become a promising strategy for developing new fluoromethylations. For the past several years, photoredox catalysis has emerged as a useful tool for radical reactions through visible-light-induced single-electron-transfer (SET) processes. Commonly used photocatalysts such as [Ru(bpy)3](2+) and fac-[Ir(ppy)3] (bpy = 2,2'-bipyridine; ppy = 2-pyridylphenyl) have potential as one-electron reductants strong enough to reduce those fluoromethylating reagents, resulting in facile generation of the corresponding fluoromethyl radicals. Therefore, if we can design proper reaction systems, efficient and selective radical fluoromethylation would proceed without any sacrificial redox agents, i.e., via a redox-neutral process under mild reaction conditions: irradiation with visible light, including sunlight, below room temperature. It should be noted that examples of catalytic fluoromethylation of compounds with carbon-carbon multiple bonds have been limited until recent years. In this Account, we will focus on our recent research on photoredox-catalyzed fluoromethylation of carbon-carbon multiple bonds. First, choices of the photocatalyst and the fluoromethylating reagent and the basic concept involving a redox-neutral oxidative quenching cycle are explained. Then photocatalytic trifluoromethylation of olefins is discussed mainly. Trifluoromethylative difunctionalization reactions, i.e., simultaneous introduction of the CF3 group and a different functional group across carbon-carbon double bonds, are in the middle of the discussion. Oxy-, amino-, and ketotrifluoromethylation allow us to synthesize various organofluorine compounds bearing C(sp(3))-CF3 bonds. In addition, the synthesis of valuable trifluoromethylated alkenes is also viable when the olefins have an appropriate leaving group or undergo deprotonation. The present reaction system features high functional group compatibility and high regioselectivity. Furthermore, future prospects, especially trifluoromethylative difunctionalization of alkynes and difluoromethylation of alkenes, are also discussed.

Inorganic and Organometallic Molecular Wires for Single-Molecule Devices.

Recent developments of single-molecule conductance measurements allow us to understand fundamental conducting properties of molecular wires. While a wide variety of organic molecular wires have been studied so far, inorganic and organometallic molecular wires have received much less attention. However, molecular wires with transition-metal atoms show interesting features and functions distinct from those of organic wires. These properties originate mainly from metal-ligand dπ-pπ interactions and metal-metal d-d interactions. Thanks to the rich combination of metal atoms and supporting ligands, frontier orbital energies of the molecular wires can be finely tuned to lead to highly conducting molecular wires. Moreover, the unique electronic structures of metal complexes are susceptible to subtle environmental changes, leading to potential functional molecular devices. This article reviews recent advances in the single-molecule conductance study of inorganic and organometallic molecular wires.