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.

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.

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.

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.

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.

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.

Efficient Red-Emissive Circularly Polarized Electroluminescence Enabled by Quasi-2D Perovskite with Chiral Spacer Cation.

Perovskites are promising environmentally sustainable materials for circularly polarized electroluminescence (CPEL). While another chiral nonemissive layer is required for the developed perovskite-based CPEL, we report herein a highly efficient circularly polarized electroluminescence based on a single layer of quasi-2D perovskite with achiral phenethylammonium iodide (PEAI) and chiral S/R-1-(1-naphthyl)ethylammonium iodide (S/R-NEAI) as dual spacer cations. The quasi-2D perovskite was further passivated by carbazole-functionalized phosphonium. The as-fabricated film exhibits not only a circular dichroism (CD) signal but also prominent circularly polarized luminescence (CPL) activity with a maximum photoluminescence dissymmetry factor (glum) of ∼2.1 × 10-3. More importantly, a highly efficient, spin-polarized light-emitting diode (LED) was fabricated based on the in situ passivated quasi-2D perovskite with a peak external quantum efficiency of 3.7% and a maximum electroluminescence dissymmetry factor (gEL) of ∼4.0 × 10-3.

DMSO/tBuONa/O2-mediated efficient syntheses of diverse quinoxalines through α-imino radicals.

Herein, we describe an efficient method involving the synthesis of diverse quinoxalines using the DMSO/tBuONa/O2 system as a single-electron oxidant to form α-imino radicals and nitrogen radicals for the direct construction of C-N bonds. This methodology provides a novel approach to form α-imino radicals with good reactivity.

Advances in electrically driven light-emitting diodes based on lead-free metal halides.

The emerging lead halide perovskites show great potential for their use as emitters in electrically driven light-emitting diodes (LEDs) with external quantum efficiency (EQE) over 25%. While the toxicity of lead and inferior device stability are the main obstacles for their commercialization, replacing Pb2+ with low- or non-toxic metal ions to form low- or zero-dimensional structures provides an alternative approach to effectively tackle these issues. Recently, luminescent lead-free metal halides have been increasingly developed toward eco-friendly and highly efficient electroluminescence. In this feature article, we give a brief overview of recent advances in luminescent lead-free metal halides and their applications in electrically driven LEDs. The challenges and prospects in this field are outlined at the end.

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.

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.

Highly Phosphorescent Dimers of PtAu2 Complexes and the Use in Solution-Processed OLEDs.

Two asymmetric PtAu2 complexes having HC≡CC6H4C≡CH (1,4-diethynylbenzene) or HC≡CCarbC≡CH (2,7-diethynyl-9-(2,3,5,6-tetrafluorophenyl)-9H-carbazole) and the corresponding bis(acetylide)-linked Pt2Au4 complexes are prepared and characterized. The structures of PtAu2 complexes 1 and 3 together with Pt2Au4 complex 2 are determined by X-ray crystallography. Relative to PtAu2 complexes, bis(acetylide)-linked Pt2Au4 complexes not only display a distinct red shift of the emission but also provide a much higher phosphorescent efficiency. Utilizing highly emissive Pt2Au4 complexes as phosphorescent dopants, high-efficiency solution-processed OLEDs are obtained with peak current efficiency of 75.9 cd A-1 and external quantum efficiency of 19.0% at luminance of 336 cd m-2 and voltage of 5.2 V. When two PtAu2 moieties are linked by a bis(acetylide) ligand, the corresponding Pt2Au4 complexes show a much improved electroluminescent performance compared with that of asymmetric PtAu2 complexes.

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.

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.

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.

Reversible Luminescent Vapochromism of a Zero-Dimensional Sb3+-Doped Organic-Inorganic Hybrid.

Photoactive metal ions doping is an efficient way to modulate the photophysical properties of perovskite. Herein, we report a zero-dimensional (0D) InCl6(C4H10SN)4·Cl:Sb3+ by doping Sb3+ into InCl6(C4H10SN)4·Cl, which undergoes a significant enhancement of the emission peak at 550 nm with photoluminescence quantum yield boosting from 20% to 90%. Interestingly, a red-shifted emission is observed on InCl6(C4H10SN)4·Cl:Sb3+ upon exposure to ethanol and DMF vapor with the emission peak red-shifted from 550 to 580 and 600 nm, respectively. Furthermore, the transformation is reversed after drying the vapor-exposed InCl6(C4H10SN)4·Cl:Sb3+ at ambient conditions. Detailed characterizations reveal that the crystal packing and structure distortion account for the reversible luminescent vapochromism. Thanks to the superior stability and feasible transformation of InCl6(C4H10SN)4·Cl:Sb3+ at ambient conditions, a DMF sensor was fabricated by coating the mixture of InCl6(C4H10SN)4·Cl:Sb3+ and PMMA into patterned substrate, which exhibits an obvious luminescent change upon release and uptake of DMF and excellent stability and producibility in several cycles.

Elaborate Design of d8-d10 Heteronuclear Phosphors for Ultrahigh-Efficiency Solution-Processed Organic Light-Emitting Diodes.

Highly soluble d8-d10 heteronuclear phosphors afford an alternative approach to achieve high-efficiency organic light-emitting diodes (OLEDs) through a solution process. In this work, four highly phosphorescent d8-d10 heteronuclear complexes with significant Pt-Au interactions were prepared. By judicious selection of sterically hindered and π-conjugated substituents in triphosphine ligands, the phosphorescence is dramatically promoted through effectively prohibiting nonradiative thermal relaxation with an efficiency of 0.94-0.99 in doping films. Exploiting highly emissive Pt-Au complexes as phosphorescent dopants, ultrahigh-efficiency solution-processed OLEDs were attained. The peak current efficiency, power efficiency, and external quantum efficiency are 96.2 cd A-1, 65.0 lm W-1, and 26.4% for the green-emitting PtAu2 phosphor and 68.6 cd A-1, 42.5 lm W-1, and 25.1% for the orange-emitting Pt2Au phosphor, which represent the state-of-art for solution-processed OLEDs based on non-iridium phosphors.

Elaborate Design of Ag8Au10 Cluster [2]Catenane Phosphors for High-Efficiency Light-Emitting Devices.

In this work, rational design of highly soluble and phosphorescent Ag-Au cluster complexes with exceptional [2]catenane structures is conducted using 1,8-diethynyl-9H-carbazole (H3decz) as a rigid U-shaped ligand with a distinguished hole-transport character. The self-assembly reaction of H3decz, Au+, and Ag+ generated phosphorescent Ag4Au6 cluster 1 (Φem = 0.22 in CH2Cl2) with H2decz- having a free ethynyl (-C≡CH) group. When the four free C≡CH groups in the Ag4Au6 complex 1 are further bound to four (PPh3)Au+ and four (PPh3)Ag+ moieties through M-acetylide linkages, the formation of Ag8Au10 cluster 2 not only eliminates nonradiative ethynyl C-H vibrational deactivation process but also improves dramatically the molecular rigidity so that the phosphorescent efficiency of the Ag8Au10 cluster 2 (Φem = 0.63) is nearly 3 times that of the Ag4Au6 cluster 1. The Ag8Au10 cluster structure is further rigidified using diphsophine Ph2P(CH2)4PPh2 (dppb) in place of PPh3 so that the phosphorescence of the Ag8Au10 cluster 3 (Φem = 0.77) is more efficient than that of 2. Making use of the Ag8Au10 clusters as phosphorescent dopants, high-efficiency solution-processed organic light-emitting diodes (OLEDs) were achieved with current efficiency (CE) and external quantum efficiency (EQE) of 47.2 cd A-1 and 15.7% for complex 2 and 50.5 cd A-1 and 14.9% for complex 3.

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.