Synthesis of π-Extended Heterocycles via Rh(III)-Catalyzed Oxidative Annulation of 5-Aryl Pyrazinones with Alkynes.

The Rh(III)-catalyzed C-H functionalization and subsequent oxidative annulation between 5-aryl pyrazinones and internal alkynes are reported. This protocol provides facile access to a wide range of pyrazinone-linked naphthalenes via the C(sp2)-H alkenylation and subsequent annulation. This transformation is characterized by mild conditions, simplicity, and excellent functional group compatibility. Notably, it is a first report of the utilization of pyrazinones as directing groups in C-H functionalization.

Ratiometric Turn-On Fluorophore Displacement Ensembles for Nitroaromatic Explosives Detection.

There is a recognized need in the area of explosives detection for fluorescence-based sensing systems that are capable of not only producing a turn-on response but also generating a distinctive spectral signature for a given analyte. Here, we report several supramolecular ensembles displaying efficient fluorophore displacement that give rise to an increase in fluorescence intensity upon exposure to various nitroaromatic compounds. The synthetic supramolecular constructs in question consist of a tetrathiafulvalene (TTF)-based pyrrolic macrocycle, benzo-TTF-calix[4]pyrrole (Bz-TTF-C4P), and fluorescent dyes, monomeric or dimeric naphthalenediimide (NDI) and perylenediimide (PDI) derivatives, as well as chloride or hexafluorophosphate (PF6-) salts of rhodamine 6G (Rh-6G). In chloroform solution, these assemblies exist in the form of discrete supramolecular complexes or oligomeric aggregates depending on the specific dye combinations in question. Each ensemble was tested as a potential explosive-responsive fluorescence indicator displacement assay (FIDA) by challenging it with a series of di- and trinitroaromatic compounds and examining the change in fluorescence spectral characteristics. Upon addition of nitroaromatic compounds (NACs), either a "turn-on" or a "turn-off" fluorescent response was observed depending on the nature of the constituent fluorophore and, where applicable, the counteranion. The FIDAs based on the PDI derivatives were found to display not only a ratiometric fluorescence enhancement but also analyte-dependent spectral changes when treated with NACs. The NAC-induced fluorescence spectral response of each ensemble was rationalized on the basis of various solution-phase spectroscopic studies, as well as single-crystal X-ray diffraction analyses.

Tetrathiafulvalene (TTF)-Annulated Calix[4]pyrroles: Chemically Switchable Systems with Encodable Allosteric Recognition and Logic Gate Functions.

Molecular and supramolecular systems capable of switching between two or more states as the result of an applied chemical stimulus are attracting ever-increasing attention. They have seen wide application in the development of functional materials including, but not limited to, molecular and supramolecular switches, chemosensors, electronics, optoelectronics, and logic gates. A wide range of chemical stimuli have been used to control the switching within bi- and multiple state systems made up from either singular molecular entities or supramolecular ensembles. In general, chemically triggered switching systems contain at least two major functional components that provide for molecular recognition and signal transduction, respectively. These components can be connected to one another via either covalent or noncovalent linkages. Of particular interest are switchable systems displaying cooperative or allosteric features. Such advanced control over function is ubiquitous in nature and, in the case of synthetic systems, may allow the capture and release of a targeted chemical entity or permit the transduction of binding information from one recognition site to another. Allosterically controlled complexation and decomplexation could also permit the amplification or deamplification of analyte-specific binding affinity, lead to nonlinear binding characteristics, or permit a magnification of output signals. Our own efforts to develop chemically driven supramolecular switches, advanced logic gates, and multifunction cascade systems have focused on the use of tetrathiafulvalene (TTF) annulated calix[4]pyrroles (C4Ps). These systems, TTF-C4Ps, combine several orthogonal binding motifs within what are conformationally switchable receptor frameworks. Their basic structure and host-guest recognition functions can be controlled via application of an appropriate chemical stimulus. Homotropic or heterotropic allosteric molecular recognition behavior is often seen. This has allowed us to (1) produce self-assembled structures, (2) control switching between bi- and multistate constructs, (3) generate chemical logic gates performing chemical-based Boolean logic operations, (4) create ionically controlled three-state logic systems that release different chemical messengers and activate disparate downstream reactions, and (5) encode a variety advanced functional operations into what are relatively simple molecular-scale devices. Looking to the future, we believe that exploiting allosteric control will expand opportunities for supramolecular chemists and allow some of the complexity seen in biology to be reproduced in simple constructs. Of particular appeal would be a capacity to release chemical messengers at will, perhaps after a prior capture and chemical modification step, that then encode for further downstream functions as seen in the case of the small molecules, such as neurotransmitters and pheromones, used by nature for the purpose of intraentity communication. Molecular scale logic devices with allosteric functions are thus the potential vanguard of a new area of study involving interactions between multiple discrete components with an emphasis on functional outcomes.

Tetrathiafulvalene- (TTF-) Derived Oligopyrrolic Macrocycles.

After the epochal discovery of the "organic metal", namely, tetrathiafulvalene (TTF)-7,7,8,8-tetracyano-p-quinodimethane (TCNQ) dyad in 1973, scientists have made efforts to derivatize TTF for constructing various supramolecular architectures to control the charge-transfer processes by adjusting the donor-acceptor strength of the dyads for numerous applications. The interesting inherent electronic donor properties of TTFs control the overall electrochemical properties of the supramolecular structures, leading to the construction of highly efficient optoelectronic materials, photovoltaic solar cells, organic field-effect transistors, and optical sensors. Modified TTF structures thus constitute promising candidates for the development of so-called "functional materials" that could see use in modern technological applications. The versatility of the TTF unit and the pioneering synthetic strategies that have been developed over the past few decades provide opportunities to tune the architecture and function for specific purposes. This review covers the "state of the art" associated with TTF-annulated oligopyrrolic macrocyclic compounds. Points of emphasis include synthesis, properties, and potential applications.

Disparate Downstream Reactions Mediated by an Ionically Controlled Supramolecular Tristate Switch.

The use of chemical messengers to control multiple and often disparate downstream events is a hallmark of biological signaling. Here, we report a synthetic supramolecular construct that gives rise to bifurcated downstream events mediated by different stimulus-induced chemical messengers. The system in question consists of a supramolecular redox-ensemble made up of a tetrathiafulvalene (TTF)-based macrocycle, benzo-TTF-calix[4]pyrrole, and an electron deficient partner, 7,7,8,8-tetracyanoquinodimethane (TCNQ). Different tetraalkylammonium halide salts are used to trigger the reversible switching between neutral (No-ET), charge transfer (CT), and electron transfer (ET) states. The result is an effective tristate switch that provides chemical access to three different forms of TCNQ, namely, a released neutral, radical anionic (TCNQ•-), or bound CT forms. The ionically induced switching chemistry is linked separately through the neutral and radical anion TCNQ forms to two distinct follow-on reactions. These reactions consist, respectively, of styrene polymerization, which is triggered only in the "1" (TCNQ radical anion ET) state, and a cycloaddition-retroelectrocyclization (CA-RE) reaction, which is mediated only by the neutral TCNQ "0" (No-ET) state. Neither downstream reaction is promoted by the CT form, wherein the TCNQ is receptor bound. The three states that characterize this system, their interconversion, and the downstream reactions promoted by TCNQ•- and free TCNQ, respectively, have been characterized by single-crystal X-ray diffraction analyses and various solution phase spectroscopies.

Light-Emitting Diodes with Hierarchical and Multifunctional Surface Structures for High Light Extraction and an Antifouling Effect.

Bioinspired hierarchical structures on the surface of vertical light-emitting diodes (VLEDs) are demonstrated by combining a self-assembled dip-coating process and nanopatterning transfer method using thermal release tape. This versatile surface structure can efficiently reduce the total internal reflection and add functions, such as superhydrophobicity and high oleophobicity, to achieve an antifouling effect for VLEDs.

Mid-IR spectroscopy of Fe:ZnSe quantum dots.

We report spectroscopic characterization of Fe:ZnSe quantum dots (for 2% of Zn/Fe molar ratio) fabricated by microemulsion hydrothermal synthesis. Mid-IR photoluminescence of the 5E↔5T2 transition of Fe2+ ions over 3.5-4.5 µm spectral range was observed in Fe:ZnSe quantum dot samples and kinetics of luminescence have been characterized at temperatures of 30-300 K under direct (2.788 µm) mid-IR excitation and indirect (0.355 µm) photoionization excitation. The radiative lifetime (τrad) was estimated from these measurements to be 48 µs while lifetime at room temperature was measured to be 440 ns. This agrees closely with the behavior of bulk material.

Redox- and pH-Responsive Orthogonal Supramolecular Self-Assembly: An Ensemble Displaying Molecular Switching Characteristics.

Two heteroditopic monomers, namely a thiopropyl-functionalized tetrathiafulvalene-annulated calix[4]pyrrole (SPr-TTF-C[4]P 1) and phenyl C61 butyric acid (PCBA 2), have been used to assemble a chemically and electrochemically responsive supramolecular ensemble. Addition of an organic base initiates self-assembly of the monomers via a molecular switching event. This results in the formation of materials that may be disaggregated via the addition of an organic acid or electrolysis.

Ion-regulated allosteric binding of fullerenes (C60 and C70) by tetrathiafulvalene-calix[4]pyrroles.

The effect of ionic species on the binding of fullerenes (C60 and C70) by tetrathiafulvalene-calix[4]pyrrole (TTF-C4P) receptors and the nature of the resulting supramolecular complexes (TTF-C4P + fullerene + halide anion + tetraalkylammonium cation) was studied in the solid state through single crystal X-ray diffraction methods and in dichloromethane solution by means of continuous variation plots and UV-vis spectroscopic titrations. These analyses revealed a 1:1 stoichiometry between the anion-bound TTF-C4Ps and the complexed fullerenes. The latter guests are bound within the bowl-like cup of the C4P in a ball-and-socket binding mode. The interactions between the TTF-C4P receptors and the fullerene guests are highly influenced by both the nature of halide anions and their counter tetraalkylammonium cations. Three halides (F(-), Cl(-), and Br(-)) were studied. All three potentiate the binding of the two test fullerenes by inducing a conformational change from the 1,3-alternate to the cone conformer of the TTF-C4Ps, thus acting as positive heterotropic allosteric effectors. For a particular halide anion, the choice of tetraalkylammonium salts serves to modulate the strength of the TTF-C4P-fullerene host-guest binding interactions and, in conjunction with variations in the halide anion, can be exploited to alter the inherent selectivity of the host for a given fullerene. Differences in binding are reflected in the excited state optical properties. Overall, the present four-component system provides an illustration of how host-guest binding events involving appropriately designed artificial receptors can be fine-tuned via the addition of simple ionic species as allosteric modulators.

Laser fabrication of gold nanoparticle clustered tips for use in apertureless near-field scanning optical microscopy.

A laser fabrication method was developed to make gold nanoparticle clustered (GNC) tips for apertureless near-field scanning optical microscopes (ANSOMs) and tip-enhanced Raman spectroscopy (TERS). The near-field Rayleigh and Raman scattering of samples are highly enhanced when a gold nanoparticle cluster is synthesized on the end of the tip. This is due to the lightning rod effect in the sharp tips. The localized electromagnetic field enhancement and the spatial resolution (~30 nm) of the fabricated GNC tip were verified by TERS and ANSOM measurements of carbon nanotubes.

Chemoresponsive alternating supramolecular copolymers created from heterocomplementary calix[4]pyrroles.

The importance of noncovalent interactions in the realm of biological materials continues to inspire efforts to create artificial supramolecular polymeric architectures. These types of self-assembled materials hold great promise as environmentally stimuli-responsive materials because they are capable of adjusting their various structural parameters, such as chain length, architecture, conformation, and dynamics, to new surrounding environments upon exposure to appropriate external stimuli. Nevertheless, in spite of considerable advances in the area of responsive materials, it has proved challenging to create synthetic self-assembled materials that respond to highly disparate analytes and whose environmentally induced changes in structure can be followed directly through both various spectroscopic and X-ray diffraction analyses. Herein, we report a new set of artificial self-assembled materials obtained by simply mixing two appropriately chosen, heterocomplementary macrocyclic receptors, namely a tetrathiafulvalene-functionalized calix[4]pyrrole and a bis(dinitrophenyl)-meso-substituted calix[4]pyrrole. The resulting polymeric materials, stabilized by combination of donor-acceptor and hydrogen bonding interactions, undergo dynamic, reversible dual guest-dependent structural transformations upon exposure to two very different types of external chemical inputs, namely chloride anion and trinitrobenzene. The structure and dynamics of the copolymers and their analyte-dependent responsive behavior was established via single crystal X-ray crystallography, SEM, heterocomplementary isodesmic analysis, 1- and 2D NMR, and dynamic light scattering spectroscopies. Our results demonstrate the benefit of using designed heterocomplementary interactions of two functional macrocyclic receptors to create synthetic, self-assembled materials for the development of "smart" sensory materials that mimic the key biological attributes of multianalyte recognition and substrate-dependent multisignaling.

Dynamic mucus secretion in ventral surfaces of toe pads of the tree frog (Dryophytes japonica).

The tree frog is a prominent amphibian among terrestrial vertebrates known for its ability to adhere to various surfaces through the capillary forces of water in the microchannels between micropillars on its disc-shaped toe pads, a phenomenon known as wet adhesion. However, the secretion pattern of mucus on the attachment surface of living tree frog toe pads and the distribution of active mucus pores (AMPs) have not yet been fully elucidated. In this study, we utilized synchrotron X-ray micro-computed tomography and interference reflection microscopy to obtain the spatial distribution of the entire population of ventral mucus glands on the toe pads of living tree frogs and the real-time mucus secretion patterns from the ventral mucus pores on the contact surface under different environmental conditions. We observed that the number and secretion frequency of AMPs on the toe pad are regulated according to environmental conditions. Such dynamic mucus secretion on the tree frog's toe pad could contribute to the understanding of capillary force regulation for wet adhesion and the development of adhesive surfaces by mimicking the mucus-secreting toe pad.

Three distinct equilibrium states via self-assembly: simple access to a supramolecular ion-controlled NAND logic gate.

During the past several decades, considerable effort has focused on self-assembled systems. However, most work has been directed toward understanding the equilibrium between two major chemical entities, namely the dissociated components and the corresponding associated complex. While there are quite a few examples of 'multiresponsive' materials, control over 'multistate' materials has proved difficult to achieve. Here, we report the formation and the interplay of a self-assembled calix[4]pyrrole array that exhibits three limiting forms, namely a 1:1 self-assembled oligomer, a 2:1 capsule, and the corresponding monomers. Interconversion between these states may be controlled by using the tetraethylammonium cation (TEA(+)) and/or iodide anion (I(-)) as chemical inputs. The combination of self-assembly and ion-based control may be used to create systems that display NAND logic behavior. The system outputs have been confirmed by a variety of analytic methods, including UV-vis and 2D (1)H DOSY, NOESY NMR spectroscopy, scanning electron microscopy, and single crystal X-ray diffraction analyses.

Modulation of electronics and thermal stabilities of photochromic phosphino-aminoazobenzene derivatives in weak-link approach coordination complexes.

A series of d(8) transition-metal (Pt(II) and Pd(II)) coordination complexes incorporating phosphine-functionalized aminoazobenzene derivatives as hemilabile phosphino-amine (P,N) ligands were synthesized and studied as model weak-link approach (WLA) photoresponsive constructs. The optical and photochemical properties of these complexes were found to be highly influenced by various tunable parameters in WLA systems, which include type of metal, coordination mode, type of ancillary ligand, solvent, and outer-sphere counteranions. In dichloromethane, reversible chelation and partial displacement of the P,N coordinating moieties allow for toggling between aminoazobenzene- or pseudostilbene- and azobenzene-type derivatives. The reversible switching between electronic states of azobenzene can be controlled through either addition or extraction of chloride counterions and is readily visualized in the separation between π-π* and n-π* bands in the complexes' electronic spectra. In acetonitrile solution, the WLA variables inherent to semiopen complexes have a significant impact on the half-lives of the corresponding cis isomers, allowing one to tune their half-lives from 20 to 21000 s, while maintaining photoisomerization behaviors with visible light. Therefore, one can significantly increase the thermal stability of a cis-aminoazobenzene derivative to the extent that single crystals for X-ray diffraction analysis can be grown for the first time, uncovering an unprecedented edge-to-face arrangement of the phenyl rings in the cis isomer. Overall, the azobenzene-functionalized model complexes shed light on the design parameters relevant for photocontrolled WLA molecular switches, as well as offer new ways of tuning the properties of azobenzene-based, photoresponsive materials.

Pd(II)-catalyzed direct C-H acylation of N-Boc hydrazones with aldehydes: one-pot synthesis of 1,2-diacylbenzenes.

A palladium(II)-catalyzed direct acylation of acetophenone N-Boc hydrazones with aldehydes via C-H bond activation is described. This protocol provides direct access to a range of 1,2-diacylbenzenes, which are useful precursors to construct biologically interesting and pharmaceutically important compounds.

Reusable Wrinkled Nanoporous Silver Film Fabricated by Plasma Treatment for Surface-Enhanced Raman Scattering Applications.

A nanoporous silver film (npAgF), a promising structure for surface-enhanced Raman spectroscopy (SERS), can be fabricated by using successive O2 and Ar plasma treatments on a planar silver film. The common dealloying method for producing an npAgF involves annealing at high temperatures to produce an alloy film, as well as harsh etching using corrosive chemicals. By contrast, the plasma-based method can be applied directly to various functional substrates to produce more sophisticated npAgF structures. Herein, we report a facile fabrication method for a wrinkled npAgF (w-npAgF) for SERS applications using a thermally contractible polystyrene substrate. The w-npAgF had 3D wrinkles of the nanoporous structure and showed approximately 8 times higher SERS enhancement than did the flat npAgF. Moreover, the w-npAgF could be reused for multiple SERS measurements of different molecules by mild O2 and Ar plasma treatments after each use, in which the O2 plasma effectively removed the adsorbed organic molecules and the Ar plasma reduced silver oxide to pristine silver for subsequent SERS measurements. The wrinkled nanoporous structure was maintained after multiple mild plasma treatments for reuse. The simplicity of plasma-based fabrication and high sensitivity of w-npAgFs are promising features for the green production of low-cost and reusable 3D SERS substrates.

Calix[4]pyrrole-based heteroditopic ion-pair receptor that displays anion-modulated, cation-binding behavior.

A new ditopic ion-pair receptor 1 was designed, synthesized, and characterized. Detailed binding studies served to confirm that this receptor binds fluoride and chloride ions (studied as their tetraalkylammonium salts) and forms stable 1:1 complexes in CDCl(3). Treatment of the halide-ion complexes of 1 with Group I and II metal ions (Li(+), Na(+), K(+), Cs(+), Mg(2+), and Ca(2+); studied as their perchlorate salts in CD(3)CN) revealed unique interactions that were found to depend on both the choice of the added cation and the precomplexed anion. In the case of the fluoride complex [1⋅F](-) (preformed as the tetrabutylammonium (TBA(+)) complex), little evidence of interaction with the K(+) ion was seen. In contrast, when this same complex (i.e., [1⋅F](-) as the TBA(+) salt) was treated with the Li(+) or Na(+) ions, complete decomplexation of the receptor-bound fluoride ion was observed. In sharp contrast to what was seen with Li(+), Na(+), and K(+), treating complex [1⋅F](-) with the Cs(+) ion gave rise to a stable, receptor-bound ion-pair complex [Cs⋅1⋅F] that contains the Cs(+) ion complexed within the cup-like cavity of the calix[4]pyrrole, which in turn was stabilized in its cone conformation. Different complexation behavior was observed in the case of the chloride complex [1⋅Cl](-). In this case, no appreciable interaction was observed with Na(+) or K(+). In addition, treating [1⋅Cl](-) with Li(+) produces a tightly hydrated dimeric ion-pair complex [1⋅LiCl(H(2)O)](2) in which two Li(+) ions are bound to the crown moiety of the two receptors. In analogy to what was seen in the case of [1⋅F](-), exposure of [1⋅Cl](-) to the Cs(+) ion gives rise to an ion-pair complex [Cs⋅1⋅Cl] in which the cation is bound within the cup of the calix[4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.

KOtBu-promoted C3-homocoupling of quinoxalinones through single electron transfer from an sp2 carbanion intermediate.

The C3-selective homodimerization of quinoxalinones is described. A C3-sp2 carbanion species generated through deprotonation of quinoxalinone using potassium tert-butoxide (KOtBu) transfers an electron (single electron transfer mechanism) to a second quinoxalinone, affording a radical-anion intermediate. The radical scavenging and electron paramagnetic resonance (EPR) experiments support the plausible radical reaction pathway. A mild reaction temperature and a short reaction time were attained under cost-effective conditions, which reveal the amenability of this protocol to pharmaceutical and chemical industries.

Ion-controlled on-off switch of electron transfer from tetrathiafulvalene calix[4]pyrroles to Li+@C60.

Binding of chloride anion to a tetrathiafulvalene calix[4]pyrrole (TTF-C4P) donor results in ET to Li(+)@C(60) to produce the radical pair (TTF-C4P(•+)/Li(+)@C(60)(•-)), the structure of which was characterized by X-ray crystallographic analysis. The addition of tetraethylammonium cation, which binds more effectively than Li(+)@C(60)(•-) as a guest within the TTF-C4P cavity, leads to electron back-transfer, restoring the initial oxidation states of the donor and acceptor pair.

Correction: Changes in macrocyclic aromaticity and formation of a charge-separated state by complexation of expanded porphyrin and C60.

Correction for 'Changes in macrocyclic aromaticity and formation of a charge-separated state by complexation of expanded porphyrin and C60' by Won-Young Cha et al., Chem. Commun., 2019, 55, 8301-8304, DOI: .