Spontaneous Assembly and Three-Dimensional Stacking of Antiaromatic 5,15-Dioxaporphyrin on HOPG.

Antiaromatic compounds have recently received considerable attention because of their novel properties such as narrow HOMO-LUMO gaps and facile formation of mutual stacking. Here, the spontaneous assembly of antiaromatic meso-2-thienyl-substituted 5,15-dioxaporphyrin (DOP-1) is scrutinized at the liquid-solid interface by scanning tunneling microscopy (STM). Polymorphism in monolayers characterized by the orthogonal and parallel assemblies is found at the low concentration of 0.05 mM. The parallel assembly is more stable and dominantly formed at higher concentrations. Aggregation was observed at concentrations >0.2 mM, and the STM images of the aggregates implied the formation of stacked layers. The intrinsic electronic structures of the mutually stacked bilayer generated by applying an electric pulse to the monolayer were probed by scanning tunneling spectroscopy to reveal the narrowing of the HOMO-LUMO gap by about 20 % compared with the monolayer, thus suggesting significant molecular orbital interactions.

Template-Assisted Proximity for Oligomerization of Fullerenes.

Demonstrated herein is an unprecedented porous template-assisted reaction at the solid-liquid interface involving bond formation, which is typically collision-driven and occurs in the solution and gas phases. The template is a TMA (trimesic acid) monolayer with two-dimensional pores that host fullerenes, which otherwise exhibit an insignificant affinity to an undecorated graphite substrate. The confinement of C84 units in the TMA pores formulates a proximity that is ideal for bond formation. The oligomerization of C84 is triggered by an electric pulse via a scanning tunneling microscope tip. The spacing between C84 moieties becomes 1.4 nm, which is larger than the edge-to-edge diameter of 1.1-1.2 nm of C84 due to the formation of intermolecular single bonds. In addition, the characteristic mass-to-charge ratios of dimers and trimers are observed by mass spectrometry. The experimental findings shed light on the active role of spatially tailored templates in facilitating the chemical activity of guest molecules.

The Cyclic Hydrogen-Bonded 6-Azaindole Trimer and its Prominent Excited-State Triple-Proton-Transfer Reaction.

The compound 6-azaindole undergoes self-assembly by formation of N(1)-H⋅⋅⋅N(6) hydrogen bonds (H bonds), forming a cyclic, triply H-bonded trimer. The formation phenomenon is visualized by scanning tunneling microscopy. Remarkably, the H-bonded trimer undergoes excited-state triple proton transfer (ESTPT), resulting in a proton-transfer tautomer emission maximized at 435 nm (325 nm of the normal emission) in cyclohexane. Computational approaches affirm the thermodynamically favorable H-bonded trimer formation and the associated ESTPT reaction. Thus, nearly half a century after Michael Kasha discovered the double H-bonded dimer of 7-azaindole and its associated excited-state double-proton-transfer reaction, the triply H-bonded trimer formation of 6-azaindole and its ESTPT reaction are demonstrated.

Comprehensive characterization of polyproline tri-helix macrocyclic nanoscaffolds for predictive ligand positioning.

Multivalent ligands hold promise for enhancing avidity and selectivity to simultaneously target multimeric proteins, as well as potentially modulating receptor signaling in pharmaceutical applications. Essential for these manipulations are nanosized scaffolds that precisely control ligand display patterns, which can be achieved by using polyproline oligo-helix macrocyclic nanoscaffolds via selective binding to protein oligomers and cell surface receptors. This work focuses on synthesis and structural characterization of different-sized polyproline tri-helix macrocyclic (PP3M) scaffolds. Through combined analysis of circular dichroism (CD), small- and wide-angle X-ray scattering (SWAXS), electron spin resonance (ESR) spectroscopy, and molecular modeling, a non-coplanar tri-helix loop structure with partially crossover helix ends is elucidated. This structural model aligns well with scanning tunneling microscopy (STM) imaging. The present work enhances the precision of nanoscale organic synthesis, offering prospects for controlled ligand positioning on scaffolds. This advancement paves the way for further applications in nanomedicine through selective protein interaction, manipulation of cell surface receptor functions, and developments of more complex polyproline-based nanostructures.

A new coordination polymer exhibiting unique 2D hydrogen-bonded (H2O)16 ring formation and water-dependent luminescence properties.

A new coordination polymer, [Zn(dpe)(bdc)]·4H(2)O (ZndB; dpe=1,2-bis(4-pyridyl)ethane, bdc(2-)=dianion of benzenedicarboxylic acid), which possesses a 3D metal-organic framework (MOF) has been synthesized and structurally characterized. This 3D MOF is constructed by the assembly of helical channels filled with guest water molecules in both inner and outer regions of the channel. The resulting network also creates a 2D water layer containing hydrogen-bonded (H(2)O)(16) rings as the basic building units. Thermogravimetric and powder X-ray diffraction measurements of ZndB revealed a two-step weight loss of water molecules with a reversible water adsorption/desorption process in the inner channel for the first stage, and irreversible water desorption in the outer channel for the second stage. This spongelike property is manifested by the excimer emission originating from interaction between dpe (π*) and the other dpe (π) of the proximal helical channel, which is highly sensitive to the environmental perturbation. Powder X-ray analyses reveal that the dehydration process induces the readjustment of dpe π-π stacking distance/orientation, which results in dramatic luminescence changes from dim pale blue (λ(em)≈470 nm) upon hydration to bright white-light generation (broad, λ(em)≈500-550 nm) upon water depletion, accompanied by a ≈100-fold increase in the emission intensity.

Reversible solid-state structural transformation of a 1D-2D coordination polymer by thermal De/rehydration processes.

A new coordination polymer, [Zn(HBTC)(BPE)0.5(H2O)]n·nH2O (1) with an extended 1D ladderlike metal-organic framework (MOF) has been synthesized and structural characterized by single-crystal X-ray diffraction method. Structural determination reveals that, in compound 1, the Zn(II) ion is four-coordinated in a distorted tetrahedral geometry, bonded to one nitrogen atom from one BPE ligand, and three oxygen atoms from two monodentate carboxylate groups of two HBTC(2-) ligands and one coordinated water molecule. The HBTC(2-) acts as a bridging ligand with a bis-monodentate coordination mode, connecting the Zn(II) ions to form a one-dimensional (1D) [Zn(HBTC)] chain. Two 1D chains are then interlinked via the connectivity between the Zn(II) ions and anti-BPE liagnds to complete the 1D ladderlike MOF. Adjacent 1D Ladders are further extended to a 2D hydrogen-bonded layered network through the intermolecular O-H · · · O hydrogen bond between the carboxylic group and carboxylate group of interladder HBTC(2-) ligand. Adjacent 2D layers are then packed orderly in an ABAB-type array via the intermolecular interactions of combined π-π interaction and O-H · · · O hydrogen bonds to form a 3D supramolecular architecture exhibiting 1D channels intercalated with guest water molecules. The reversible solid-state structural transformation between crystalline 1 with 1D ladderlike framework and its dehydrated powder 2, [Zn(HBTC)(BPE)0.5]n, with 2D framework via the displacement of coordinated water molecule to HBTC(2-) ligand, by thermal de/rehydrated processes has been verified by PXRD measurements. The emission of 1 and 2 is ascribed to a ligand-based transition.

Electric-field-induced supramolecular phase transitions at the liquid/solid interface: cat-assembly from solvent additives.

We demonstrate by using scanning tunneling microscopy that a series of trace organic solvent additives can efficiently promote the electrically triggered phase transition of trimesic acid (TMA), which would otherwise occur rather sporadically. DFT simulations taking into account the electric field effect elucidate such tailored phase transformations, based on the Gibbs activation and free energies of the deprotonation reactions of TMA.

Superstructure manipulation and electronic measurement of monolayers comprising discotic liquid crystals with intrinsic dipole moment using STM/STS.

In this work, we studied the discotic liquid crystals (DLCs) of dibenzo[a,c]phenazine at the liquid-solid interface using scanning tunnelling microscopy/spectroscopy, by which we show how to tailor the DLC assemblies and in turn their electron-transfer efficiency. This study presents an alternative method for phase control and electronic measurements for DLCs, especially at the microscopic level.

Electrochemical synthesis, characterization of Ir-Zn containing coordination polymer, and application in oxygen and glucose sensing.

A simple and sensitive biosensor array based on phosphorescence detection that is able to detect oxygen and glucose in human serum, respectively, has been developed. We demonstrate an electrochemical method as a fast, effective, tunable, and versatile means of growing phosphorescence sensing material. This sensing material, crystalline iridium(III)-Zn(II) coordination polymers, namely Ir-Zn(e), was grown on a stainless steel mesh and then doped in a sol-gel matrix. The emission of Ir-Zn(e) was ascribed to a metal-to-ligand charge transfer transition (MLCT). The noteworthy oxygen-sensing properties of Ir-Zn(e) were also evaluated. The optimal oxygen-sensing conditions of Ir-Zn(e) with a deduced K(SV) value of 3.55 were 5 V and 30 °C for 1 hour. Moreover, the short response time (23 s) and the recovery time (21 s) toward oxygen have been measured. The reversibility experiment was carried out for eleven cycles. The resulting >70% recovery of intensity for Ir-Zn(e) on each cycle demonstrated a high degree of reproducibility during the sensing process. The detection limit could be 0.050% for gaseous oxygen. The sensing substrate was subsequently built up under glucose oxidase encapsulated in hydrogel and then immobilized on an egg membrane by the layer-by-layer method. Once the glucose solution was injected into this array, oxygen content depleted simultaneously with a concomitant increase in the phosphorescence of coordination polymers. The linear dynamic range for the determination of glucose was 0.1-6.0 mM, the correlation coefficient (R(2)) was 0.9940 (y = 0.75 [glucose] + 0.539), and the response time was less than 120 s. The minimum detectable concentration for glucose was calculated to be 0.05 mM from three times signal to noise. The photophysical properties of the sensing material and the effects of buffer concentration, pH, interference, matrix effect, temperature, and the stability of the biosensor array have also been studied in detail. The biosensor array was successfully applied to the determination of glucose in human serum.

Synthesis, structure and oxygen-sensing properties of iridium(III)-containing coordination polymers with different cations.

Four iridium(III)-containing coordination polymers 1-4 using Ir(ppy)(2)(H(2)dcbpy)PF(6) (L-H(2), ppy = 2-phenylpyridine, H(2)dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) as the bridging ligand, [ZnL(2)]·3DMF·5H(2)O (1), [CdL(2)(H(2)O)(2)]·3DMF·6H(2)O (2), [CoL(2)(H(2)O)(2)]·2DMF·8H(2)O (3) and [NiL(2)(H(2)O)(2)]·3DMF·6H(2)O (4), have been synthesized and structurally characterized. The emissions from 1-4 are ascribed to a metal-to-ligand charge transfer transition (MLCT). The absolute emission quantum yields for 1-4 in single crystals were measured in air to be 0.274, 0.193, 0.001 and 0.002, respectively. The noteworthy oxygen-sensing properties of 1-4 as well as L-H(2) in a single crystal were also evaluated. The Stern-Volmer quenching constant, K(SV) values, of 1-4 and L-H(2) can be deduced to be 0.834, 2.820, 1.328, 1.111 and 2.476, respectively. The results show promising K(SV) values (e.g.2) that are competitive or even larger than those of many known Ir-complexes. Moreover, the short response time (e.g. compound 2) and recovery times toward oxygen of 1-4 have been measured in their single crystal forms. The reversibility experiments for 1-4 were carried out for seven repeated cycles. As a result, >75% recovery of intensity for 1 and 2 on each cycle demonstrates a high degree of reproducibility during the sensing process. It should be noted that iridium(III)-containing coordination polymers with high emission intensity and notable oxygen sensing properties are obscure, especially in the single crystal form. This, in combination with its fine reversibility, leads to success in single crystal oxygen recognition based on photoluminescence imaging. The detection limit could be 0.50% for gaseous oxygen. Moreover, the temperature effect of compound 2 in a single crystal upon application as an oxygen sensor was expected.

A new recognition concept using dye sensitized solar cell configuration.

A bifunctional compound 1 was designed and synthesized, in which the donor-π-conjugated-acceptor-cyanoacrylic acid configuration is suited for DSSC, while the tetraoxa-azacrown (donor) part also acts as the recognition unit for e.g. Ca(2+) and the output of DSSC is utilized as signal transduction, demonstrating a new recognition concept using DSSC.