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.

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.

Estimation of Electron Distribution over Dinuclear Organometallic Molecular Wires by "IR Tag" Analysis of Ancillary Acyl-Cp Ligands.

Understanding the details of electronic properties of mixed-valence (MV) states of organometallic molecular wires is essential to gain insights into electron transfer phenomena. Although the field of MV chemistry is matured, there remain issues to be solved, which cannot be accessed by the conventional analytical methods. Here, we describe the synthesis and properties of diruthenium bridging (diethynylbenzene)diyl wires, (µ-p and m-C≡C-C6H4-C≡C){RuCpR(dppe)}2 1, with the acyl-substituted cyclopentadienyl rings [CpR: Cpe; η5-C5H4COOMe (a-series: ester derivatives), Cpa; η5-C5H4COMe (b-series: acetyl derivatives)], which are installed as IR-tags to estimate electron densities at the metal centers in the MV species. The electrochemical and IR/near IR-spectroelectrochemical studies reveal that the two metal centers in the para-isomers p-1a,b + interact with each other more strongly than those in the meta-isomers m-1a,b + . Electron-spin resonance study also supports the radicals being delocalized over the Ru-(p-C≡C-C6H4-C≡C)-Ru moieties in p-1a,b + . The spectroelectrochemical IR study shows significant higher-energy shifts of the ν(C=O) vibrations brought about upon 1e-oxidation. Spectral simulation on the basis of the Bloch equations allows us to determine the electron transfer rate constants (k et) between the two metal centers being in the orders of 1012 s-1 ( p-1 + ) and 109 s-1 ( m-1 + ). The shifts of the ν(C=O) bands reveal that the charge densities on the para-isomers p-1a,b + are widely delocalized over the Ru-(p-C≡C-C6H4-C≡C)-Ru linkages in contrast to the meta-isomers m-1a,b + , where the electron densities are mainly localized on the metal fragments, as supported by the density functional theory and time-dependent density functional theory studies as well as comparison with the reference mononuclear acetylide complexes, C6H5-C≡C-RuCpR(dppe) 2. We have successfully demonstrated that the carbonyl groups (>C=O) in the ancillary Cp ligands also work as IR-tags to report detailed information on the electron densities at the metal centers and the electron distribution over MV complexes as well.

Control of dominant conduction orbitals by peripheral substituents in paddle-wheel diruthenium alkynyl molecular junctions.

Control of charge carriers that transport through the molecular junctions is essential for thermoelectric materials. In general, the charge carrier depends on the dominant conduction orbitals and is dominantly determined by the terminal anchor groups. The present study discloses the synthesis, physical properties in solution, and single-molecule conductance of paddle-wheel diruthenium complexes 1R having diarylformamidinato supporting ligands (DArF: p-R-C6H4-NCHN-C6H4-R-p) and two axial thioanisylethynyl conducting anchor groups, revealing unique substituent effects with respect to the conduction orbitals. The complexes 1R with a few different aryl substituents (R = OMe, H, Cl, and CF3) were fully characterized by spectroscopic and crystallographic analyses. The single-molecule conductance determined by the scanning tunneling microscope break junction (STM-BJ) technique was in the 10-5 to 10-4 G 0 region, and the order of conductance was 1OMe > 1CF3 ≫ 1H ∼ 1Cl, which was not consistent with the Hammett substituent constants σ of R. Cyclic voltammetry revealed the narrow HOMO-LUMO gaps of 1R originating from the diruthenium motif, as further supported by the DFT study. The DFT-NEGF analysis of this unique result revealed that the dominant conductance routes changed from HOMO conductance (for 1OMe) to LUMO conductance (for 1CF3). The drastic change in the conductance properties originates from the intrinsic narrow HOMO-LUMO gaps.

Single-molecule junctions of multinuclear organometallic wires: long-range carrier transport brought about by metal-metal interaction.

Here, we report multinuclear organometallic molecular wires having (2,5-diethynylthiophene)diyl-Ru(dppe)2 repeating units. Despite the molecular dimensions of 2-4 nm the multinuclear wires show high conductance (up to 10-2 to 10-3 G 0) at the single-molecule level with small attenuation factors (ß) as revealed by STM-break junction measurements. The high performance can be attributed to the efficient energy alignment between the Fermi level of the metal electrodes and the HOMO levels of the multinuclear molecular wires as revealed by DFT-NEGF calculations. Electrochemical and DFT studies reveal that the strong Ru-Ru interaction through the bridging ligands raises the HOMO levels to access the Fermi level, leading to high conductance and small ß values.

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.

Simple generation of various α-monofluoroalkyl radicals by organic photoredox catalysis: modular synthesis of ß-monofluoroketones.

A metal-free and operationally simple strategy for the generation of various α-monofluoroalkyl radicals has been developed. A combination of 1,4-bis(diarylamino)naphthalene photocatalyst and sulfoximine-based fluoroalkylating reagents is the key to success. The protocol can be applied to modular synthesis of ß-monofluoroketones through radical monofluoroalkylation of alkenyl acetates.

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.

Photoluminescent properties and molecular structures of dinuclear gold(I) complexes with bridged diphosphine ligands: near-unity phosphorescence from 3XMMCT/3MC.

The gold(i) complexes [µ-LiPr(AuX)2] {X = Cl (1D), Br (2D), and I (3D); LiPr = 1,2-bis[bis(2-isopropylphenyl)phosphino]benzene} were synthesised to investigate the photoluminescence properties of dinuclear Au complexes comprising weak Au(i)-Au(i) bonds. Single crystals of the tetrahydrofuran (THF) adducts 1DOR, 2DOR, and 3DOR were obtained by recrystallisation of 1D, 2D, and 3D from a mixed solution of THF and n-hexane. These THF adducts afford orange emission, with peak wavelengths ranging from 597 to 630 nm, in the crystalline state at 293 K. Recrystallisation of 3D from a mixed solution of acetone and n-hexane afforded single crystals of the acetone adduct 3DGR, which exhibits blue-green emission at 293 K. No crystals of the acetone adduct were obtained from 1D and 2D. The emission spectra and lifetimes of 1DOR, 2DOR, 3DOR, and 3DGR measured in the temperature range 77-293 K indicate that emission from these complexes in the solid state is due to phosphorescence. Notably, although the molecular structure of 3D in the 3DOR crystal is near-similar to that of 3DGR, the phosphorescence spectrum of 3DOR differs markedly from that of 3DGR, with peak wavelengths at 597 and 506 nm, respectively. Theoretical studies revealed that (1) phosphorescence occurs via the electronic transition from the excited triplet state, which is mainly composed of halogen-to-metal-metal charge transfer and metal-centered transitions and (2) the T1-optimised structure of 3D in the 3DGR crystals differs markedly from that in 3DOR, and the differences in the phosphorescence colour observed between 3DGR and 3DOR are ascribed to the differences in their stabilised structures in the excited triplet state.

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.

An aromatic micelle with bent pentacene-based panels: encapsulation of perylene bisimide dyes and graphene nanosheets.

For exploitation of a new class of aromatic micelles, we synthesized a bent pentacene-based amphiphilic molecule through Diels-Alder reaction. The amphiphiles bearing two trimethylammonium tethers assemble into a spherical aromatic micelle, with an average core diameter of 1.5 nm, in water at room temperature. The new aromatic micelle efficiently encapsulates perylene bisimide (PBI) dyes and graphene nanosheets (GNS) in water. The encapsulated PBI dyes form a parallel stacked dimer, exhibiting characteristic absorption and emission bands. In addition, the encapsulated GNS are composed of few-layer graphene sheets with an average lateral size of ∼7 nm, as confirmed by Raman spectroscopy. The resultant, aqueous host-guest complexes are stable even after three weeks in water under ambient conditions.

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.

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.

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.

Fluoroalkanesulfinate Salts as Dual Fluoroalkyl and SO2 Sources: Atom-Economical Fluoroalkyl-Sulfonylation of Alkenes and Alkynes by Photoredox Catalysis.

We disclose that fluoroalkanesulfinate salts ((RFSO2)nM) such as the Langlois reagent, CF3SO2Na, serve as dual fluoroalkyl (RF) and sulfur dioxide (SO2) sources by the action of photoredox catalysis. An operationally simple strategy for the vicinal installation of RF and SO2 groups onto unsaturated carbon-carbon bonds, i.e., fluoroalkyl-sulfonylation, has been developed. In particular, the present photocatalytic trifluoromethyl-sulfonylation can be applied to aromatic alkynes in addition to aliphatic and aromatic alkenes bearing various functional groups.

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.

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.

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.

Luminescence color alteration induced by trapped solvent molecules in crystals of tetrahedral gold(i) complexes: near-unity luminescence mixed with thermally activated delayed fluorescence and phosphorescence.

The tetrahedral gold(i) complex [Au(LtBu)2]Cl 1Cl [LtBu = 1,2-bis[bis(4-tert-butylphenyl)phosphino]benzene], having eight tert-butyl (tBu) groups, was synthesized and characterized. Emission color alteration caused by solvent molecules captured in the crystal lattice of 1Cl was investigated. The recrystallization of 1Cl in mixed solvents of tetrahydrofuran, THF, and various alkanes afforded two single crystals: 1Cl·THF (1BG), presenting intense blue-green luminescence with an emission peak wavelength, λmax, = 507 nm and 1Cl·THF·0.5n-hexane·H2O (1OR), exhibiting weak orange luminescence with λmax = 625 nm. The emission quantum yields, ΦPL, of the two complexes were 0.95 and 0.06, respectively. X-ray structural analysis revealed that all the solvent molecules captured in the crystal were located inside the spaces surrounded by the tBu groups of the 1+ cation. Further, an n-hexane molecule in 1OR was found to be sandwiched between two 1+ cations via CH/π interactions. The structure of 1Cl in 1OR could be distorted by the n-hexane molecule incorporated in the crystal, leading to the red-shift of the emission peak wavelength and the low quantum efficiency of 1OR over that of 1BG. On the other hand, [Au(L)2]Cl [L = 1,2-bis(diphenylphosphino)benzene], having no peripheral tBu groups, was unable to incorporate the solvent molecules in the crystal by recrystallization from the mixed solvents, and thus, the crystal solely yielded intense blue light emission. The results revealed that the tBu groups of the peripheral phenyl units are essential for the luminescence color alteration caused by the incorporation of organic solvent molecules into the crystal lattice. Finally, studies on the emission spectra and quantum yields in the temperature range 77-293 K revealed that luminescence from both 1BG and 1OR was composed of phosphorescence and thermally activated delayed fluorescence.