Effectively Enhancing the Conductance of Asymmetric Molecular Wires by Aligning the Energy Level and Symmetrizing the Coupling.

An asymmetric structure is an important strategy for designing highly conductive molecular wires for a gap-fixed molecular circuit. As the conductance enhancement in the current strategy is still limited to about 2 times, we inserted a methylene group as a spacer in a conjugated structure to modulate the structural symmetry. We found that the conductance drastically enhanced in the asymmetric molecular wire to 1.5 orders of magnitude as high as that in the symmetric molecular wire. First-principles quantum transport studies attributed the effective enhancement to the synchronization of improved energy alignment and nearly symmetric coupling between the frontier orbitals and the electrodes.

Exploring a Linear Combination Feature for Predicting the Conductance of Parallel Molecular Circuits.

An accurate rule for predicting conductance is the cornerstone of developing molecular circuits and provides a promising solution for miniaturizing electric circuits. The successful prediction of series molecular circuits has proven the possibility of establishing a rule for molecular circuits under quantum mechanics. However, the quantitatively accurate prediction has not been validated by experiments for parallel molecular circuits. Here we used 1,3-dihydrobenzothiophene (DBT) to build the parallel molecular circuits. The theoretical simulation and single-molecule conductance measurements demonstrated that the conductance of the molecule containing one DBT is the unprecedented linear combination of the conductance of the two individual channels with respective contribution weights of 0.37 and 0.63. With these weights, the conductance of the molecule containing two DBTs is predicted as 1.81 nS, matching perfectly with the measured conductance (1.82 nS). This feature offers a potential rule for quantitatively predicting the conductance of parallel molecular circuits.

Biomedical Applications of Sulfonylcalix[4]arene-Based Metal-Organic Supercontainers.

Coordination cages sustained by metal-ligand interactions feature polyhedral architectures and well-defined hollow structures, which have attracted significant attention in recent years due to a variety of structure-guided promising applications. Sulfonylcalix[4]arenes-based coordination cages, termed metal-organic supercontainers (MOSCs), that possess unique multi-pore architectures containing an endo cavity and multiple exo cavities, are emerging as a new family of coordination cages. The well-defined built-in multiple binding domains of MOSCs allow the efficient encapsulation of guest molecules, especially for drug delivery. Here, we critically discuss the design strategy, and, most importantly, the recent advances in research surrounding cavity-specified host-guest chemistry and biomedical applications of MOSCs.

Hydroxyl Group as the 'Bridge' to Enhance the Single-Molecule Conductance by Hyperconjugation.

For designing single-molecule devices that have both conjugation systems and structural flexibility, a hyperconjugated molecule with a σ-π bond interaction is considered an ideal candidate. In the investigation of conductance at the single-molecule level, since few hyperconjugation systems have been involved, the strategy of building hyperconjugation systems and the mechanism of electron transport within this system remain unexplored. Based on the skipped-conjugated structure, we present a rational approach to construct a hyperconjugation molecule using a hydroxyl group, which serves as a bridge to interact with the conjugated fragments. The measurement of single-molecule conductance reveals a two-fold conductance enhancement of the hyperconjugation system having the 'bridging' hydroxyl group compared to hydroxyl-free derivatives. Theoretical studies demonstrate that the hydroxyl group in the hyperconjugation system connects the LUMO of the two conjugated fragments and opens a through-space channel for electron transport to enhance the conductance.

Attaining Exceptional Stable Copper(I) Metallacyclopentadiene Diradicaloids through Ligand Engineering.

Diradicaloids are generally high-energy molecules with open-shell configuration and are quite reactive. In this work, we report a feasible synthetic approach to attaining exceptionally stable copper(I) metallacyclopentadiene diradicaloids through ligand engineering. Copper(I)-hybrid cyclopentadiene diradicaloids 1c-6c that absorb intensely in visible regions were successfully prepared in stoichiometrical yields under UV light irradiation. The diradicaloids originate from the C-C bonding coupling of two side-by-side-arranged ethynyl groups in complexes 1-6 upon photocyclization. By rational selection of substituents in triphosphine ligands, we systematically modulate the kinetic behavior of diradicaloids 1c-6c in the thermal decoloration process. With precise ligand design, we are able to obtain exceptionally stable copper(I)-hybrid cyclopentadiene diradicaloids with a half-life as long as ca. 40 h in CH2Cl2 solution at ambient temperature. As demonstrated by electron paramagnetic resonance (EPR) and variable-temperature magnetic studies, the diradicaloids manifest a singlet ground state, but they are readily populated to a triplet excited state through thermal activation in view of a small singlet-triplet energy gap of -0.39 kcal mol-1. The diradicaloids show two-step quasi-reversible reduction waves at about -0.5 and -1.0 V ascribed to successive one-electron-accepting processes, coinciding perfectly with the characteristics of diradicals.

Scissor-like Au4Cu2 Cluster with Phosphorescent Mechanochromism and Thermochromism.

Reaction of [Au(tht)2](ClO4) (tht = tetrahydrothiophene), [Cu(CH3CN)4](ClO4), 3,6-di-tert-butyl-1,8-diethynyl-9H-carbazole (H3decz), and bis(2-diphenylphosphinophenyl)ether (POP) in the presence of triethylamine (NEt3) gave the cluster complex Au4Cu2(decz)2(POP)2 as yellow crystals. As revealed by X-ray crystallography, the Au4Cu2 cluster exhibits scissor-like structure sustained by two decz and two POP ligands and stabilized by Au-Cu and Au-Au interactions. The Au4Cu2 cluster shows bright yellow to orange photoluminescence upon irradiation at >300 nm, arising from 3[π (decz)→5d (Au)] 3LMCT (ligand-to-metal charge transfer) and 3[π→π* (decz)] 3IL (intraligand) triplet states as revealed by theoretical and computational studies. When it is mechanically ground, reversible phosphorescence conversion from yellow to red is observed owing to more compact molecular packing and thus stronger intermetallic interaction. Variable-temperature luminescence studies reveal that it displays distinct red-shifts of the emission whether the temperature is elevated or lowered from ambient temperature, suggestive of exceptional thermochromic phosphorescence characteristics.

Transport Modulation Through Electronegativity Gating in Multiple Nitrogenous Circuits.

Investigating the correlations of electron transport between multiple channels shows vital promises for the design of molecule-scale circuits with logic operations. To control the electron transport through multiple channels, the modulation of electronegativity shows an effective frontier orbit control method with high universality to explore the interactions between transport channels. Here, two series of compounds with a single nitrogenous conductive channel (Sg) and dual-channels (Db) are designed to explore the influence of electronegativity on electron tunneling transport. Single-molecule conductance measured via the scanning tunneling microscope break junction technique (STM-BJ) reveals that the conductance of Db series is significantly suppressed as the electronegativity of nitrogen becomes negative, while the suppression on Sg is less obvious. Theoretical calculations confirm that the effect of electronegativity extends to a dispersive range of molecular frameworks owing to the delocalized orbital distribution from the dual-channel structure, resulting in a more significant conductance suppression effect than that on the single-channel. This study provides the experimental and theoretical potentials of electronegativity gating for molecular circuits.

Amino-Acid-Induced Circular Polarized Luminescence in One-Dimensional Manganese(II) Halide Hybrid.

Circular polarized luminescence (CPL)-active materials attract great attentions owing to their widely applications in 3D optical displays and encrypted transmission. Inspired by the strategies adopted in perovskite based CPL materials, herein, CPL-active hybrids (D)- and (L)-(tert-butyl prolinate)MnCl3 were successfully prepared by assembling chiral D/L tert-butyl prolinate with manganese (II) chloride. Single crystal structures show the as-formed hybrids possess one-dimensional (1D) structure containing linear chains of face-sharing MnCl6 octahedral surrounded by prolinate cations. The 1D Mn(II) hybrids display strong red emission peaked at 646 nm with PLQY of 67.1 % and 57.2 % for d-type and l-type, respectively, representing the highest PLQY for 1D MnII hybrids. Interestingly, the 1D Mn(II) hybrids exhibit prominent circular dichroism (CD) signals and remarkable CPL activity with the dissymmetry factor g of 6.1*10-3 and -6.3*10-3 from 550 to 800 nm for (D)- and (L)-(tert-butyl prolinate)MnCl3 , respectively, owing to the existence of chiral cations. It is worthy noted the obtained g represents the highest value for non-lead organic-inorganic hybrids.

Capturing the Rotation of One Molecular Crank by Single-Molecule Conductance.

Unveiling the internal dynamics of rotation in molecular machine at single-molecule scale is still a challenge. In this work, three crank-shaped molecules are elaborately designed with the conformational flipping between syn and anti fulfilled by two naphthyl groups rotating freely along 1,3-butadiynyl axis. By investigating the single-molecule conductance using scanning tunnelling microscope break junction (STM-BJ) technique and theoretical simulation, the internal rotation of these crank-shaped molecules is well identified through low and high conductance corresponding to syn- and anti-conformations. As demonstrated by theoretically computational study, the orbital energy changes with the conformational flipping and influences the intraorbital quantum interference, thus eventually modulating the single-molecule conductance. This work demonstrates single-molecule conductance measurement to be a rational approach for characterizing the internal rotation of molecular machines.

Heteroctanuclear Au4Ag4 Cluster Complexes of 4,5-Diethynylacridin-9-One with Luminescent Mechanochromism.

Two heteroctanuclear Au4Ag4 cluster complexes of 4,5-diethynylacridin-9-one (H2L) were prepared through the self-assembly reactions of [Au(tht)2](CF3SO3), Ag(tht)(CF3SO3), H2L and PPh3 or PPh2Py (2-(diphenylphosphino)pyridine). The Au4Ag4 cluster consists of a [Au4L4]4- and four [Ag(PPh3)]+ or [Ag(PPh2Py)]+ units with Au4L4 framework exhibiting a twisted paper clip structure. In CH2Cl2 solutions at ambient temperature, both compounds show ligand fluorescence at ca. 463 nm as well as phosphorescence at 650 nm for 1 and 630 nm for 2 resulting from admixture of 3IL (intraligand) of L ligand, 3LMCT (from L ligand to Au4Ag4) and 3MC (metal-cluster) triplet states. Crystals or crystalline powders manifest bright yellow-green phosphorescence with vibronic-structured emission bands at 530 (568sh) nm for complex 1 and 536 (576sh) nm for complex 2. Upon mechanical grinding, yellow-green emission in the crystalline state is dramatically converted to red luminescence centered at ca. 610 nm with a drastic redshift of the emission after crystal packing is destroyed.

Highly Efficient Light-Emitting Diodes Based on an Organic Antimony(III) Halide Hybrid.

As low-dimensional lead-free hybrids with higher stability and lower toxicity than those of three-dimensional lead perovskites, organic antimony(III) halides show great application potential in opt-electronic field owing to diverse topologies along with exceptional optical properties. We report herein an antimony(III) hybrid (MePPh3 )2 SbCl5 with a zero-dimensional (0D) structure, which exhibits brilliant orange emission peaked at 593 nm with near-unity photoluminescent quantum yield (99.4 %). The characterization of photophysical properties demonstrates that the broadband emission with a microsecond lifetime (3.24 µs) arises from self-trapped emission (STE). Electrically driven organic light-emitting diodes (OLEDs) based on neat and doped films of (MePPh3 )2 SbCl5 were fabricated. The doped devices show significant improvement in comparison to non-doped OLEDs. Owing to the much improved surface morphology and balanced carrier transport in light-emitting layers of doped devices, the peak luminance, current efficiency (CE) and external quantum efficiency (EQE) are boosted from 82 cd m-2 to 3500 cd m-2 , 1.1 cd A-1 to 6.8 cd A-1 , and 0.7 % to 3.1 % relative to non-doped devices, respectively.

Fano Resonance in Single-Molecule Junctions.

The Fano resonance in single-molecule junctions could be created by interaction with discrete and continuous molecular orbitals and enables effective electron transport modulation between constructive and destructive interference within a small energy range. However, direct observation of Fano resonance remains unexplored because of the disappearance of discrete orbitals by molecule-electrode coupling. We demonstrated the room-temperature observation of Fano resonance from electrochemical gated single-molecule conductance and current-voltage measurements of a para-carbazole anion junction. Theoretical calculations reveal that the negative charge on the nitrogen atom induces a localized HOMO on the molecular center, creating Fano resonance by interfering with the delocalized LUMO on the molecular backbone. Our findings demonstrate that the Fano resonance in electron transport through single-molecule junctions opens pathways for designs of interference-based electronic devices.

Selective Ethane/Ethylene Separation in a Robust Microporous Hydrogen-Bonded Organic Framework.

The separation of ethane (C2H6) from ethylene (C2H4) is of prime importance in the production of polymer-grade C2H4 for industrial manufacturing. It is very challenging and still remains unexploited to fully realize efficient C2H6/C2H4 separation in the emerging hydrogen-bonded organic frameworks (HOFs) due to the weak nature of hydrogen bonds. We herein report the benchmark example of a novel ultrarobust HOF adsorbent (termed as HOF-76a) with a Brunauer-Emmett-Teller surface area exceeding 1100 m2 g-1, exhibiting the preferential binding of C2H6 over C2H4 and thus highly selective separation of C2H6/C2H4. Theoretical calculations indicate the key role of the nonpolar surface and the suitable triangular channel-like pores in HOF-76a to sterically "match" better with the nonplanar C2H6 molecule than the planar C2H4, thus affording overall stronger multipoint van der Waals interactions with C2H6. The exceptional separation performance of HOF-76a for C2H6/C2H4 separation was clearly demonstrated by gas adsorption isotherms, ideal adsorbed solution theory calculations, and simulated and experimental breakthrough curves. Breakthrough experiments on HOF-76a reveal that polymer-grade ethylene gas can be straightforwardly produced from 50/50 (v/v) C2H6/C2H4 mixtures during the first adsorption cycle with a high productivity of 7.2 L/kg at 298 K and 1.01 bar and 18.8 L/kg at 298 K and 5.0 bar, respectively.

Vapor-triggered Green-to-Yellow Luminescence Conversion due to the Variation of Ligand Orientations in Tetranuclear Copper(I) Complex.

The reaction of 3,6-ditert-butyl-1,8-bis(diphenylphosphino)-9-methyl-9H-carbazole (L) with CuBr resulted in the isolation of tetranuclear copper(I) complex Cu4Br4L2 as two colorless crystal morphs, i.e., green-emitting 1G and yellow-emitting 1Y. As demonstrated by X-ray crystallography, the Cu4Br4 moiety in both 1G and 1Y adopts the same chair conformations. When L is bonded perpendicularly to the Cu4 plane, 1G with green emission is obtained, while it gives a yellow emission of 1Y once the L is parallelly bonded to Cu4 plane. Theoretical computational studies suggest that the variation in ligand orientation results in a different degree of structural distortion in triplet state and thus different luminescent energy. Particularly, 1Y undergoes dramatic structural distortion from the ground (S0) to triplet excied state (T1). Interestingly, 1G can be converted into 1Y upon exposed to saturated hexane vapor, which would return to 1G upon exposure to acetonitrile vapor. As demonstrated experimentally and theoretically, the reversible luminescence transformation between 1G and 1Y is ascribed to the variation of ligand L orientations.

Coordination-Bond-Driven Dissolution-Recrystallization Structural Transformation with the Expansion of Cuprous Halide Aggregate.

Metal-organic frameworks (MOFs) with cuprous-halide-aggregates have shown superiority as organic LED (OLED) and semiconductor materials, while engineering MOF flexibility by involving the expansion of cuprous aggregates remains a great challenge. In this particular work, a dissolution-recrystallization structural transformation (DRST) with the dramatic growth of CuI-I aggregates, from 2D NJNU-100 to 3D NJNU-101 has been successfully realized. The unsaturated coordination nodes (2-positional nitrogen atoms) in NJNU-100 have been demonstrated to be the driven force for DRST to NJNU-101 via the formation of coordination bonds. The structural transformation process was irreversible and observed with optical microscopy and powder XRD. The expansion of CuI-I aggregates was also computational simulated accompanying with the rotation of the neutral tripodal TTTMB ligand (1,3,5-tris(1,2,4-triazol-1-ylmethyl)-2,4,6-trimethylbenzene) and the reduction of CuII to CuI. Moreover, the intermediate product NJNU-102 was captured by adding the planar molecular anthrancene to shut down the reaction, where only partial 2-positional nitrogen atoms coordinated to the aggregates and the anthrancene was oxidized to anthraquinone. NJNU-102 has further confirmed that DRST involved the breakage and recombination of coordination bonds and the electron transfer. NJNU-100 and NJNU-101 could be applied as semiconductor and OLED materials. This work has provided insights for crystal engineering, especially for the construction of the CuIxXy aggregates, and illustrated that DRST could be controlled with a rational design (as the unsaturated coordination modes).

Cooperative Binding and Stepwise Encapsulation of Drug Molecules by Sulfonylcalixarene-Based Metal-Organic Supercontainers.

The cooperative binding behavior of a face-directed octahedral metal-organic supercontainer featuring one endo cavity and six exo cavities was thoroughly examined in chloroform solution through ultraviolet-visible (UV-Vis) titration technique using two representative drug molecules as the guests. The titration curves and their nonlinear fit to Hill equation strongly suggest the efficient encapsulation of the guest molecules by the synthetic host, which exhibit interesting cooperative and stepwise binding behavior. Based on the control experiments using tetranuclear complex as a reference, it is clear that two equivalents of the guest molecules are initially encapsulated inside the endo cavity, followed by the trapping of six additional equivalents of the drug molecules through six exo cavities (1 eq. per exo cavity), and the remaining guests are entrapped by the external pockets. The results provide an in-depth understanding of the cooperative binding behavior of metal-organic supercontainers, which opens up new opportunities for designing synthetic receptors for truly biomimetic functional applications.

Sensitive and Specific Guest Recognition through Pyridinium-Modification in Spindle-Like Coordination Containers.

An elaborately designed pyridinium-functionalized octanuclear zinc(II) coordination container 1-Zn was prepared through the self-assembly of Zn2+ , p-tert-butylsulfonylcalix[4]arene, and pyridinium-functionalized angular flexible dicarboxylate linker (H2 BrL1). The structure was determined by a single-crystal X-ray diffractometer. 1-Zn displays highly sensitive and specific recognition to 2-picolylamine as revealed by drastic blueshifts of the absorption and emission spectra, ascribed to the decrease of intramolecular charge transfer (ICT) character of the container and the occurrence of intermolecular charge transfer between the host and guest molecules. The intramolecular charge transfer plays a key role in the modulation of the electronic properties and is tunable through endo-encapsulation of specific guest molecules.

Luminescent Vapochromism Due to a Change of the Ligand Field in a One-Dimensional Manganese(II) Coordination Polymer.

The reactions of MnBr2 and ethane-1,2-diylbis(diphenylphosphine oxide) (dppeO2) in dichloromethane-methanol solutions gave colorless crystals with the general chemical formulas [MnBr2(dppeO2)] n (1), [MnBr2(dppeO2)(DMF)] n (1a), [Mn(dppeO2)3][MnBr4] (2), and Mn2Br4(dppeO2)2 (3) depending on the crystallization conditions. Compounds 1 and 1a display one-dimensional chain structures composed of Mn(II) ions linked by bridging dppeO2 to exhibit tetrahedral (1) or trigonal-bipyramidal (1a) coordination geometry, whereas 3 exhibits a cyclic dinuclear structure with two Mn(II) centers bridged by double dppeO2 to adopt tetrahedral geometry. Compound 2 consists of octahedrally coordinated cation [Mn(dppeO2)3]2+ and tetrahedrally arranged anion [MnBr4]2-. While 1 and 3 in crystalline and powder states are highly luminescent with green emission bands centered at ca. 510 nm, 2 shows intense orange luminescence peaking at 594 nm. Upon exposure of 1 to N, N-dimethylformamide vapor, the green emission centered at 510 nm is converted to red luminescence peaking at 630 nm, ascribed to the formation of DMF-coordinated compound 1a with a trigonal-bipyramidal ligand field, as demonstrated by X-ray crystallography. Red-emitting 1a could be reverted to the original green-emitting 1 with a tetrahedral ligand field upon heat at 160 °C, and such a reversible conversion could be perfectly repeated for several cycles. A new mechanism of luminescent vapochromism is thus proposed because of the reversible conversion of ligand fields in manganese(II) complexes.

Luminescent Mechanochromic Dinuclear Cu(I) Complexes with Macrocyclic Diamine-Tetracarbene Ligands.

Dinuclear Cu(I) complexes bearing hexadentate, macrocyclic N-heterocyclic carbene (NHC) ligands, [Cu2(L1)(CH3CN)][PF6]2 (1) and [Cu2(L2)(CH3CN)]2[Cu2(L2)(CH3CN)2][PF6]6 (2), have been synthesized by the reactions of [H4L][PF6]4 (L = L1, L2) with excess Cu2O in acetonitrile. Crystallizations of the heat-treated samples of 1 and 2 from acetone/methanol/ether or CH3NO2/ether result in [Cu2(L1)][PF6]2 (3) and [Cu2(L2)][PF6]2 (4). Complexes 1-4 are emissive with luminescent maxima at 464, 472, 540, and 488 nm in the solid state, respectively. The origin of the red shift of the emission maximum of 3 relative to the other three complexes has been studied by theoretical calculations, showing the cuprophilic interactions in the excited state of 3. The mechanochromic luminescent properties of 1-4 have been studied. After grinding in a mortar, a significant emission color change is found with a red shift of 98 nm for 1, 82 nm for 2, 20 nm for 3, and 64 nm for 4, respectively. These mechanochromic transformations are found to be a crystalline-to-amorphous conversion, which can be reverted by adding drops of the organic solvent or recrystallization. The possible correlations between the luminescent properties and structural modifications such as Cu···Cu distances are discussed.

Multiphotochromism in an Asymmetric Ruthenium Complex with Two Different Dithienylethenes.

An asymmetric bis(dithienylethene-acetylide) ruthenium(II) complex trans-Ru(dppe)2(L1o)(L2o) (1oo) incorporating two different dithienylethene-acetylides (L1o and L2o) was designed to modulate multistate photochromism in view of the well separated ring-closing absorption bands between L1o and L2o. Upon irradiation with appropriate wavelengths of light, complex 1 undergoes stepwise photocyclization and selective photocycloreversion to afford four states (1oo, 1co, 1oc, and 1cc). As a contrast, symmetric complexes trans-Ru(dppe)2(L1o)2 (2oo) and trans-Ru(dppe)2(L2o)2 (3oo) with two identical dithienylethene-acetylides were synthesized, and the corresponding photochromic behavior was investigated. The photochromic properties of the oxidized species (1oo+/1co+/1oc+/1cc+, 2oo+/2co+/2cc+, and 3oo+/3co+/3cc+) were also investigated. The ring-closing absorption bands of one-electron oxidized species 1oo+, 2oo+, and 3oo+ show obvious blue shifts relative to those of 1oo, 2oo, and 3oo, respectively. The ring-closing absorption bands in both neutral and oxidized species grow progressively following oo → oc/co → cc and oo+ → oc+/co+ → cc+. As revealed by spectroscopic, electrochemical, and computational studies, complex 1 displays eight switchable states through stepwise photocyclization, selective cycloreversion, and a reversible redox process.