Ion Pairing of Cationic and Anionic Ir(III) Photosensitizers for Photocatalytic CO2 Reduction at Lipid-Membrane Surfaces.

We synthesized an ion pair comprising cationic and anionic Ir(III) photosensitizers ([Ir1+][Ir2-]) for photocatalytic CO2 reduction and showed that the cationic component imparts stability, while the cyclometalating ligands in the anionic component ensure effective visible-light absorption. The triplet excited state of [Ir1+] is the key photoredox species in this system and is mainly generated through the transfer of triplet excitation energy from the anionic moiety due to Coulombic interactions and appropriate triplet energy alignment between the two ionic components. The positive photosensitization effect of ion pairing was demonstrated by photocatalytic CO2 reduction in cooperation with a Re(I) molecular catalyst incorporated into a vesicle membrane.

Multistate Structural Switching of [3]Catenanes with Cyclic Porphyrin Dimers by Complexation with Amine Ligands.

Catenanes with multistate switchable properties are promising components for next-generation molecular machines and supramolecular materials. Herein, we report a ligand-controlled switching method, a novel method for the multistate switching of catenanes controlled by complexation with added amine ligands. To verify this method, a [3]catenane comprising cyclic porphyrin dimers with a rigid π-system has been synthesized. Owing to the rigidity, the relative positions among the cyclic components of the [3]catenane can be precisely controlled by complexation with various amine ligands. Moreover, ligand-controlled multistate switching affects the optical properties of the [3]catenanes: the emission intensity can be tuned by modulating the sizes and coordination numbers of integrated amine ligands. This work shows the utility of using organic ligands for the structural switching of catenanes, and will contribute to the further development of multistate switchable mechanically interlocked molecules.

Fabrication of Photoprocessable Materials via Photopolymerization Using an Acid-Induced Photocleavable Platinum-Acetylide Crosslinker.

Photopolymerization and photoprocessing are core technologies for molding and tuning polymer materials. However, they are incompatible with single materials owing to their contradictory photoreactivity. Herein, an acid-induced photocleavable crosslinker, a platinum-acetylide complex covered by permethylated cyclodextrins, enables the fabrication of photoprocessable materials via photopolymerization with N-(2-hydroxyethyl)acrylamide. The polymer networks are molded by 365 nm irradiation as well as softened and degraded by a cooperative reaction with HCl as an acidic additive under 365 nm UV light, or 470 nm visible light in the presence of a photosensitizer. Moreover, the crosslinker is applied to a photoadhesive triggered by 365 nm irradiation. The adhesion is detachable on-demand through acid-induced photodegradation with the same wavelength and intensity of irradiation. Thus, acid-induced photocleavage allows the integration of light-induced molding and processing under various lights of various wavelengths, opening up new strategies for polymer technologies.

Linked Rotaxane Structure Restricts Local Molecular Motions in Solution to Enhance Fluorescence Properties of Tetraphenylethylene.

The restriction of local molecular motions is critical for improving the fluorescence quantum yields (FQYs) and the photostability of fluorescent dyes. Herein, we report a supramolecular approach to enhance the performance of fluorescent dyes by incorporating a linked rotaxane structure with permethylated α-cyclodextrins. Tetraphenylethylene (TPE) derivatives generally exhibit low FQYs in solution due to the molecular motions in the excited state. We show that TPE with linked rotaxane structures on two sides displays up to 15-fold higher FQYs. Detailed investigations with variable temperature 1 H NMR, UV-Vis, and photoluminescence spectroscopy revealed that the linked rotaxane structure rigidifies the TPE moiety and thus suppresses the local molecular motions and non-radiative decay. Moreover, the linked rotaxane structure enhances the FQY of the dye in various solvents, including aqueous solutions, and improves the photostability through the inhibition of local molecular motions in the excited TPE.

Systematic Synthesis of Macrocycles Bearing up to Six 2,2'-Bipyridine Moieties through Self-Assembled Double Helix Structure.

A new synthetic strategy for macrocycles bearing multiple coordination moieties was developed. A self-assembled double helix structure, composed of two linear strands bearing 2,2'-bipyridine units and Cu(I) ions, provided access to macrocycles bearing a defined number of 2,2'-bipyridine moieties and a defined ring size, via an olefin-metathesis reaction between two linear strands in the helix. The double helix structure improved the selectivity of the macrocycle synthesis by bringing the reaction points in close proximity even in the case of large macrocycles.

Insulation of a coumarin derivative with [1]rotaxane to control solvation-induced effects in excited-state dynamics for enhanced luminescence.

A coumarin derivative bearing a [1]rotaxane structure with permethylated α-cyclodextrins suppressed unwanted solvation-induced effects and increased luminescent quantum yields in medium- and high-polarity solvents. The non-radiative decay was suppressed by the twist in the π-conjugated system and the radiative decay was enhanced by the suppression of the polarity-induced structural changes.

Maximizing Conversion of Surface Click Reactions for Versatile Molecular Modification on Metal Oxide Nanowires.

Click reactions (e.g., Huisgen cycloaddition) on metal oxide nanostructures offer a versatile and robust surface molecular modification for various applications because they form strong covalent bonds in a wide range of molecular substrates. This study reports a rational strategy to maximize the conversion rate of surface click reactions on single-crystalline ZnO nanowires by monitoring the reaction progress. p-Polarized multiple-angle incidence resolution spectrometry (pMAIRS) and Fourier-transformed infrared (FT-IR) spectroscopy were employed to monitor the reaction progress of an azide-terminated self-assembled monolayer (SAM) on single-crystalline ZnO nanowires. Although various reaction parameters including the concentration of Cu(I) catalysts, triazolyl ligands, solvents, and target alkynes were systematically examined for the surface click reactions, 10-30% of terminal azide on the nanowire surface remained unreacted. Temperature-dependent FT-IR measurements revealed that such unreacted residual azides deteriorate the thermal stability of the nanowire molecular layer. To overcome this observed conversion limitation of click reactions on nanostructure surfaces, we considered the steric hindrance around the closely packed SAM reaction points, then experimented with dispersing the azide moiety into a methyl-terminated SAM. The mixed-SAM method significantly improved the azide conversion rate to almost 100%. This reaction method enables the construction of spatially patterned molecular surface modifications on metal oxide nanowire arrays without detrimental unreacted azide groups.

Macroscopic Change in Luminescent Color by Thermally Driven Sliding Motion in [3]Rotaxanes.

Systematic investigation of rotaxane structures has revealed a rational design for thermally driven switching of their macroscopic properties. At low temperature, the luminophore is insulated by the macrocycles and displays monomer emission, whereas at high temperature, the luminophore is exposed owing to a change in the macrocyclic location distribution and interacts with external molecules, affording a thermally driven luminescent color change with high reversibility and responsiveness. This macroscopic switching through efficient thermal sliding was made possible by appropriate tuning of both the macrocycle-luminophore interactions within the rotaxane and the coupling between the excited luminophore and external molecules in an exciplex. The ability to switch properties by a simple and clean thermal stimuli should expand the utilization of rotaxanes as components of thermally driven molecular systems.

Synthesis of Insulated Heteroaromatic Platinum-Acetylide Complexes with Color-Tunable Phosphorescence in Solution and Solid States.

Phosphorescence colors of cyclodextrin-based insulated Pt-acetylide complexes were tuned by the molecular engineering of the chromophores. A series of Pt complexes bearing various acetylide ligands, including heteroaromatics, were prepared via self-inclusion of the linked macrocycles with the complexes. The decline in the inclusion efficiency derived from the heteroaromatics was overcome by the late-stage insulation via intramolecular slippage after the construction of the Pt-acetylide complexes. The cyclic protection of the thus-formed complexes prevented phosphorescence quenching via molecular interactions, even in the solid state. Accordingly, the tuned emission colors in a dilute system were replicated in the solid state.

Rational Method of Monitoring Molecular Transformations on Metal-Oxide Nanowire Surfaces.

Metal-oxide nanowires have demonstrated excellent capability in the electrical detection of various molecules based on their material robustness in liquid and air environments. Although the surface structure of the nanowires essentially determines their interaction with adsorbed molecules, understanding the correlation between an oxide nanowire surface and an adsorbed molecule is still a major challenge. Herein, we propose a rational methodology to obtain this information for low-density molecules adsorbed on metal oxide nanowire surfaces by employing infrared p-polarized multiple-angle incidence resolution spectroscopy and temperature-programmed desorption/gas chromatography-mass spectrometry. As a model system, we studied the surface chemical transformation of an aldehyde (nonanal, a cancer biomarker in breath) on single-crystalline ZnO nanowires. We found that a slight surface reconstruction, induced by the thermal pretreatment, determines the surface chemical reactivity of nonanal. The present results show that the observed surface reaction trend can be interpreted in terms of the density of Zn ions exposed on the nanowire surface and of their corresponding spatial arrangement on the surface, which promotes the reaction between neighboring adsorbed molecules. The proposed methodology will support a better understanding of complex molecular transformations on various nanostructured metal-oxide surfaces.

Synthesis and Acid-Responsiveness of an Insulated π-Conjugated Polymer Containing Spiropyrans in Its Backbone.

A π-conjugated polymer containing spiropyrans (SPs), which could be almost completely converted to protonated merocyanines (MCH⁺) and back to the SP form by adding an acid and a base, respectively, was developed. The insulation of the π-conjugated polymer, referred to as insulated spiropyran-containing poly(p-phenylene ethynylene) (ins-SP-PPE), using permethylated α-cyclodextrins (PM α-CD) suppressed the π-π interaction between the polymer chains containing MCH⁺, and the installation of PM α-CD improved the switching ability of SPs. The polymer exhibited repeatable acidochromism with almost complete conversion between the SP and MCH⁺ forms. Photoluminescence measurements were conducted and the acid-induced luminescence quenching of the polymer in the solution was observed, which stemmed from energy transfer from the PPE to MCH⁺ moieties. In the solid state, the quantum yield of ins-SP-PPE was more than twice that of the uninsulated polymer, which derived from the insulation effects. The acid-induced luminescence quenching was also observed in the solid state.

Programmed Synthesis of Molecular Wires with Fixed Insulation and Defined Length Based on Oligo(phenylene ethynylene) and Permethylated α-Cyclodextrins.

The development of new tuning methods for π-conjugated insulated molecular wires with strictly defined axle lengths as well as positions and degrees of macrocycle coverage would provide unprecedented insight into insulation effects in functionalized materials. Herein, iterative reactions of oligo(phenylene ethynylene) (OPE) linked with permethylated α-cyclodextrins were carried out to fabricate insulated molecular wires with a defined length and insulation in desired areas. Insulated OPEs were elongated in a stepwise manner by performing sequential coupling/deprotection reactions. The insulated areas on the OPE units in each expansion step were selectively controlled by means of programmed solvent conditions (high/low polarity). Moreover, a completely insulated OPE (up to a linked [11]rotaxane) with high structural regularity and high covering ratio was synthesized by appropriate tuning of the Pd catalyst and an extension unit bearing a traceless capping unit based on a tert-butyldimethylsilyl group. This strategy may guide the development of the selective synthesis of fully insulated, partially insulated, and uninsulated molecular wires with well-defined lengths and covered/uncovered areas.

Regio- and Stereoselective Synthesis of Triarylalkene-Capped Rotaxanes via Palladium-Catalyzed Tandem Sonogashira/Hydroaryl Reaction of Terminal Alkynes.

Triarylalkene-capped conjugated rotaxanes were synthesized via a palladium-catalyzed tandem Sonogashira/hydroaryl reaction between aryl halides and terminal alkynes bearing two permethylated α-cyclodextrins (PM α-CDs) with high regioselectivity because of the insulation effect of the PM α-CDs. Moreover, sequential Sonogashira coupling and hydroarylation reactions using different aryl substrates afforded a regio- and stereoselective trisubstituted alkene as a single product. This new class of rotaxane-forming reactions can be used to increase the diversity of rotaxane skeletons, and thereby the material functionalities of rotaxanes.

Boraformylation and Silaformylation of Allenes.

The boraformylation of allenes with B2 (pin)2 and a formate ester as boron and formyl source, respectively, proceeds in the presence of a copper catalyst. The reaction selectively affords the corresponding ß-boryl ß,γ-unsaturated aldehydes in good to high yields. Furthermore, the silaformylation of allenes was achieved with a formate ester and PhMe2 Si-B(pin) as the silicon source.

Carboxyzincation Employing Carbon Dioxide and Zinc Powder: Cobalt-Catalyzed Multicomponent Coupling Reactions with Alkynes.

Cobalt-catalyzed carboxyzincation reactions employing carbon dioxide and zinc metal powder are developed. By using alkynes as substrates, regio- and stereodefined (Z)-ß-zincated acrylates are provided. The corresponding alkenylzinc moiety can be converted to various substituents, affording multisubstituted acrylic acids. Furthermore, by adding electron-deficient alkene to the reaction system, the four-component coupling reactions of alkyne, alkene, CO2, and the Zn atom proceed via carboxyzincation.

Rational Design for Rotaxane Synthesis through Intramolecular Slippage: Control of Activation Energy by Rigid Axle Length.

We describe a new concept for rotaxane synthesis through intramolecular slippage using π-conjugated molecules as rigid axles linked with organic soluble and flexible permethylated α-cyclodextrins (PM α-CDs) as macrocycles. Through hydrophilic-hydrophobic interactions and flipping of PM α-CDs, successful quantitative conversion into rotaxanes was achieved without covalent bond formation. The rotaxanes had high activation barrier for their de-threading, so that they were kinetically isolated and derivatized even under conditions unfavorable for maintaining the rotaxane structures. (1) H NMR spectroscopy experiments clearly revealed that the restricted motion of the linked macrocycle with the rigid axle made it possible to control the kinetic stability by adjusting the length of the rigid axle in the precursor structure rather than the steric bulkiness of the stopper unit.

A Typical Metal-Ion-Responsive Color-Tunable Emitting Insulated π-Conjugated Polymer Film.

We report the synthesis of an insulated π-conjugated polymer containing 2,2'-bipyridine moieties as metal coordination sites. Metal coordination to the polymer enabled easy and reversible tuning of the luminescent color without changes to the main chain skeleton. The permethylated α-cyclodextrin (PM α-CD)-based insulation structure allowed the metalated polymers to demonstrate efficient emission even in the solid state, with identical spectral shapes to the dilute solutions. In addition, the coordination ability of the metal-free polymer was maintained in the solid state, resulting in reversible changes in the luminescent color in response to the metal ions. The synthesized polymer is expected to be suitable for application in recyclable luminescent sensors to distinguish different metal ions.

Cobalt- and Nickel-Catalyzed Carboxylation of Alkenyl and Sterically Hindered Aryl Triflates Utilizing CO2.

A highly efficient cobalt-catalyzed reductive carboxylation reaction of alkenyl trifluoromethanesulfonates (triflates) has been developed. By employing Mn powder as a reducing reagent under 1 atm pressure of CO2 at room temperature, diverse alkenyl triflates can be converted to the corresponding α,ß-unsaturated carboxylic acids. Moreover, the carboxylation of sterically hindered aryl triflates proceeds smoothly in the presence of a nickel or cobalt catalyst.

Synthesis of Molecular Wires Strapped by π-Conjugated Side Chains: Integration of Dehydrobenzo[20]annulene Units.

In this study, π-conjugated molecular wires strapped by cyclic π-conjugated side chains were efficiently synthesized by the integration of dehydrobenzo[20]annulene units by intramolecular Glaser-type cyclization under high dilution conditions.

Copper-catalyzed regiodivergent silacarboxylation of allenes with carbon dioxide and a silylborane.

A regiodivergent silacarboxylation of allenes under a CO2 atmosphere with PhMe2Si-B(pin) as a silicon source in the presence of a copper catalyst at 70 °C has been developed. The regioselectivity of the reaction is successfully reversed by the proper choice of ligand; carboxylated vinylsilanes are obtained with rac-Me-DuPhos as the ligand, whereas the use of PCy3 affords carboxylated allylsilanes. Thus, two different carboxylated silanes can be selectively and regiodivergently synthesized from a single allene substrate.