Diamine anchored molecular junctions of oligo(phenylene ethynylene) cruciform

Using diamine as anchoring group, the self-assembled monolayers (SAMs) based on oligo(phenylene ethynylene)s (OPEs) and cruciform OPEs with an extended tetrathiafulvalene (TTF) (OPE3 and OPE3-TTF) were successfully formed on the Au substrate and then utilized in molecular junctions by conductingprobe atomic force microscopy (CP-AFM).     

Upon the intrinsic optical properties of oligo(p-phenyleneethynylene)s (OPEs). Synthesis of OPE3 for experimental gas-phase absorption studies

A new oligo(p-phenyleneethynylene) (OPE3) was synthesized by step-wise Pd-catalyzed cross-coupling reactions. The molecule incorporates an amine functionality, which allows for transfer of the protonated molecule to the gas phase by electrospray ionization leaving the OPE entity neutral. This method has allowed for the first experimental gas-phase absorption spectrum of an OPE3 by action spectroscopy, employing an electrostatic ion storage ring in combination with a laser system. The studies reveal the effect of having a positive charge in proximity to the conjugated backbone of OPE3 in the absence of any interfering solvent molecules. In addition, ionization energies and electron affinities of OPE2-OPE17 were calculated at the density functional theory (DFT) level.     

Electronic and structural properties of oligophenylene ethynylenes on Au(111) surfaces

The interaction of oligophenylene ethynylene (OPE) on the (111) surface of a gold slab resembling a self-assembled monolayer (SAM) is investigated using ab initio density functional theory calculations. The authors performed a full optimization including all atoms in the OPE and in the slab to better understand OPE adsorption on the surface. It is found that OPE has two energetically favorable adsorption sites on the Au surface with relatively different molecular geometries: the nontop site adsorption greatly modifies the (111) surface structure; however, the extensive electron interactions enable a delocalized electron density distribution, implying an improved conductivity between OPE and Au, and the top site which is 0.9 eV higher in energy than the nontop and features weaker Au-S bonds. Interestingly the on top configuration shows a strong spin imbalance along the molecule and the nontop shows a small spin imbalance on the surface. This feature is of strong interest for the development of resonators for the detection of chemical and biological agents. They have also calculated the frequency spectrum of these SAMs, which yield deformations in the gold surface yielding peak frequency shifts specific to each absorption site.     

Interfacial electron-transfer kinetics of ferrocene through oligophenyleneethynylene bridges attached to gold electrodes as constituents of self-assembled monolayers: observation of a nonmonotonic distance dependence

The standard heterogeneous electron-transfer rate constants (k(n)0) between substrate gold electrodes and the ferrocene redox couple attached to the electrode surface by variable lengths of substituted or unsubstituted oligophenyleneethynylene (OPE) bridges as constituents of mixed self-assembled monolayers were measured as a function of temperature. The distance dependences of the unsubstituted OPE standard rate constants and of the preexponential factors (An) obtained from an Arrhenius analysis of the unsubstituted OPE k(n)0 versus temperature data are not monotonic. This surprising result, together with the distance dependence of the substituted OPE preexponential factors, may be assessed in terms of the likely conformational variability of the OPE bridges (as a result of the low intrinsic barrier to rotation of the phenylene rings in these bridges) and the associated sensitivity of the rate of electron transfer (and, hence, the single-molecule conductance which may be estimated using An) through these bridges to the conformation of the bridge. Additionally, the measured standard rate constants were independent of the identity of the diluent component of the mixed monolayer, and using an unsaturated OPE diluent has no effect on the rate of electron transfer through a long-chain alkanethiol bridge. These observations indicate that the diluent does not participate in the electron-transfer event.     

Oligo(p‐phenylene‐ethynylene)‐Derived Super‐π‐Gelators with Tunable Emission and Self‐Assembled Polymorphic Structures

Linear π‐conjugated oligomers are known to form organogels through noncovalent interactions. Herein, we report the effect of π‐repeat units on the gelation and morphological properties of three different oligo(p‐phenylene‐ethynylene)s: OPE3 , OPE5 , and OPE7 . All of these molecules form fluorescent gels in nonpolar solvents at low critical gel concentrations, thereby resulting in a blue gel for OPE3 , a green gel for OPE5 , and a greenish yellow gel for OPE7 . The molecule–molecule and molecule–substrate interactions in these OPEs are strongly influenced by the conjugation length of the molecules. Silicon wafer suppresses substrate–molecule interactions whereas a mica surface facilitates such interactions. At lower concentrations, OPE3 formed vesicular assemblies and OPE5 gave entangled fibers, whereas OPE7 resulted in spiral assemblies on a mica surface. At higher concentrations, OPE3 and OPE5 resulted in super‐bundles of fibers and flowerlike short‐fiber agglomerates when different conditions were applied. The number of polymorphic structures increases on increasing the conjugation length, as seen in the case of OPE7 with n=5, which resulted in a variety of exotic structures, the formation of which could be controlled by varying the substrate, concentration, and humidity.     

Characterization of self-assembled monolayers of oligo(phenyleneethynylene) derivatives of varying shapes on gold: effect of laterally extended pi-systems

Fully conjugated organic molecules, such as the oligo(phenyleneethynylene) (OPE) systems, are of growing interest within the field of molecular electronics, as is the self-assembly of well-defined molecular thin films with predefined functions. The structure and function of such films are intimately related and governed by the structures of their molecular constituents, through the intermolecular interactions and the interactions between the molecules and the substrate, onto which the film is assembled. Here we report on the synthesis of a series of three OPE derivatives, with the general structure phenylethynylene-aryl-ethynylenephenylene-headgroup, and the structural investigation of the self-assembled monolayers (SAMs) formed from them on Au(111) surfaces. The SAMs were characterized by infrared reflection-absorption spectroscopy, spectroscopic ellipsometry, high-resolution X-ray photoemission spectroscopy, and near-edge X-ray absorption fine structure spectroscopy. The effective thickness of the SAMs was observed to decrease as the pi-system of the aryl moiety of the OPE adsorbate was extended perpendicular to its molecular long axis. Changing the aryl moiety from benzene to naphthalene to anthracene resulted in lower molecular surface densities and larger molecular inclination. The average tilt angles for the benzene, naphthalene, and anthracene SAMs were found to be about 30 degrees , 40 degrees , and 42 degrees from the surface normal, respectively. For the largest adsorbate, the anthracene derivative, there is spectroscopic evidence suggesting the existence of nonequivalent binding sites. The differences observed between the SAMs are rationalized in terms of the shape of the adsorbates and the strength of the pi-pi interactions between them.     

Orientational ordering of the second layer of C60 molecules on Au(111)

We have studied the orientational ordering of the second layer of C(60) molecules on Au(111) using scanning tunnelling microscopy (STM) at 77 K. The orientation of individual molecules within the second layer follows a regular pattern, giving rise to a 2 × 2 superlattice. The long-range order of the 2 × 2 lattice depends on the structure of the first molecular layer with the best ordering found inside the R14° domain. The second layer formed on top of the contrast-disordered R30° domain consists of patches of bright and dim molecules. The contrast between bright and dim patches shows a clear dependence on the sample bias. This bias-dependent contrast is explained by considering the contributions to tunnel current from HOMO and LUMO mediated electron transfer processes. Scanning tunnelling spectroscopic measurement reveals the narrowing of the HOMO-LUMO gap for the layer of molecules in direct contact with the Au(111) substrate.     

Controlling the Structural and Electrical Properties of Diacid Oligo(Phenylene Ethynylene) Langmuir–Blodgett Films

The preparation, characterization and electrical properties of Langmuir–Blodgett (LB) films composed of a symmetrically substituted oligomeric phenylene ethynylene derivative, namely, 4,4′‐[1,4‐phenylenebis(ethyne‐2,1‐diyl)]dibenzoic acid (OPE2A), are described. Analysis of the surface pressure versus area per molecule isotherms and Brewster angle microscopy reveal that good‐quality Langmuir (L) films can be formed both on pure water and a basic subphase. Monolayer L films were transferred onto solid substrates with a transfer ratio of unity to obtain LB films. Both L and LB films prepared on or from a pure water subphase show a red shift in the UV/Vis spectrum of about 14 nm, in contrast to L and LB films prepared from a basic subphase, which show a hypsochromic shift of 15 nm. This result, together with X‐ray photoelectron spectroscopic and quartz crystal microbalance experiments, conclusively demonstrate formation of one‐layer LB films in which OPE2A molecules are chemisorbed onto gold substrates and consequently ? COO? Au junctions are formed. In LB films prepared on a basic subphase the other terminal acid group is also deprotonated and associates with an Na⁺ counterion. In contrast, LB films prepared from a pure water subphase preserve the protonated acid group, and lateral H‐bonds with neighbouring molecules give rise to a supramolecular structure. STM‐based conductance studies revealed that films prepared from a basic subphase are more conductive than the analogous films prepared from pure water, and the electrical conductance of the deprotonated films also coincides more closely with single‐molecule conductance measurements. This result was interpreted not only in terms of better electron transmission in ? COO? Au molecular junctions, but also in terms of the presence of lateral H‐bonds in the films formed from pure water, which lead to reduced conductance of the molecular junctions.     

Charge transfer interactions of a Ru(II) dye complex and related ligand molecules adsorbed on Au(111)

The interaction of the dye molecule, N3 (cis-bis(isothiocyanato)bis(2,2(')-bipyridyl-4,4(')-dicarboxylato)-ruthenium(II)), and related ligand molecules with a Au(111) surface has been studied using synchrotron radiation-based electron spectroscopy. Resonant photoemission spectroscopy (RPES) and autoionization of the adsorbed molecules have been used to probe the coupling between the molecules and the substrate. Evidence of charge transfer from the states near the Fermi level of the gold substrate into the lowest unoccupied molecular orbital (LUMO) of the molecules is found in the monolayer RPES spectra of both isonicotinic acid and bi-isonicotinic acid (a ligand of N3), but not for the N3 molecule itself. Calibrated x-ray absorption spectroscopy and valence band spectra of the monolayers reveals that the LUMO crosses the Fermi level of the surface in all cases, showing that charge transfer is energetically possible both from and to the molecule. A core-hole clock analysis of the resonant photoemission reveals a charge transfer time of around 4 fs from the LUMO of the N3 dye molecule to the surface. The lack of charge transfer in the opposite direction is understood in terms of the lack of spatial overlap between the π?-orbitals in the aromatic rings of the bi-isonicotinic acid ligands of N3 and the gold surface.     

介孔SiO_2薄膜增敏SERS基底在消逝波激励下的特性表征

采用溶胶-凝胶分子模板法在50 nm厚金膜表面制备约40 nm厚介孔二氧化硅(MPS)薄膜,然后在MPS薄膜表面静电自组装金纳米粒子(GNP)单层膜,形成的多层膜结构用作表面增强拉曼散射(SERS)基底.利用扫描电镜观测到MPS薄膜具有表面开口多孔结构,有助于小分子向薄膜内快速扩散.基于时域有限差分(FDTD)方法对电场分布的仿真结果指出,在表面等离子体共振(SPR)条件下分布于金膜与GNP之间的消逝场显著增强.由于空间重叠,该增强场能够高效激发MPS内富集的小分子拉曼信号,产生的拉曼信号还可免受金属作用的干扰.利用Kretschmann结构和尼罗蓝(NB)拉曼活性分子测试了Au/MPS/GNP基底在785 nm激发波长下的SERS效果,并与Au/GNP基底进行了比较.结果表明,在SPR条件下,Au/MPS/GNP基底能够导致较强的定向和背向拉曼信号,而且在586 cm-1处的背向拉曼信号强度是Au/GNP基底的40倍,这归功于MPS薄膜.进一步测试表明背向拉曼信号强度与NB浓度成正相关.这意味着Au/MPS/GNP基底具有良好的半定量检测本领.     

Molecules for organic electronics studied one by one

The electronic and geometrical structure of single difluoro-bora-1,3,5,7-tetraphenyl-aza-dipyrromethene (aza-BODIPY) molecules adsorbed on the Au(111) surface is investigated by low temperature scanning tunneling microscopy and spectroscopy in conjunction with ab initio density functional theory simulations of the density of states and of the interaction with the substrate. Our DFT calculations indicate that the aza-bodipy molecule forms a chemical bond with the Au(111) substrate, with distortion of the molecular geometry and significant charge transfer between the molecule and the substrate. Nevertheless, most likely due to the low corrugation of the Au(111) surface, diffusion of the molecule is observed for applied bias in excess of 1 V.     

Probing the Catalytic Activity of Reduced Graphene Oxide Decorated with Au Nanoparticles Triggered by Visible Light

Hybrid materials in which reduced graphene oxide (rGO) is decorated with Au nanoparticles (rGO–Au NPs) were obtained by the in situ reduction of GO and AuCl₄^?_(aq) by ascorbic acid. On laser excitation, rGO could be oxidized as a result of the surface plasmon resonance (SPR) excitation in the Au NPs, which generates activated O₂ through the transfer of SPR‐excited hot electrons to O₂ molecules adsorbed from air. The SPR‐mediated catalytic oxidation of p‐aminothiophenol (PATP) to p,p′‐dimercaptoazobenzene (DMAB) was then employed as a model reaction to probe the effect of rGO as a support for Au NPs on their SPR‐mediated catalytic activities. The increased conversion of PATP to DMAB relative to individual Au NPs indicated that charge‐transfer processes from rGO to Au took place and contributed to improved SPR‐mediated activity. Since the transfer of electrons from Au to adsorbed O₂ molecules is the crucial step for PATP oxidation, in addition to the SPR‐excited hot electrons of Au NPs, the transfer of electrons from rGO to Au contributed to increasing the electron density of Au above the Fermi level and thus the Au‐to‐O₂ charge‐transfer process.     

介孔SiO2薄膜增敏SERS基底在消逝波激励下的特性表征

采用溶胶-凝胶分子模板法在50 nm 厚金膜表面制备约40 nm 厚介孔二氧化硅(MPS)薄膜, 然后在MPS薄膜表面静电自组装金纳米粒子(GNP)单层膜, 形成的多层膜结构用作表面增强拉曼散射(SERS)基底.利用扫描电镜观测到MPS薄膜具有表面开口多孔结构, 有助于小分子向薄膜内快速扩散. 基于时域有限差分(FDTD)方法对电场分布的仿真结果指出, 在表面等离子体共振(SPR)条件下分布于金膜与GNP之间的消逝场显著增强. 由于空间重叠, 该增强场能够高效激发MPS内富集的小分子拉曼信号, 产生的拉曼信号还可免受金属作用的干扰. 利用Kretschmann 结构和尼罗蓝(NB)拉曼活性分子测试了Au/MPS/GNP基底在785 nm激发波长下的SERS效果, 并与Au/GNP基底进行了比较. 结果表明, 在SPR条件下, Au/MPS/GNP基底能够导致较强的定向和背向拉曼信号, 而且在586 cm^-1处的背向拉曼信号强度是Au/GNP基底的40 倍, 这归功于MPS薄膜. 进一步测试表明背向拉曼信号强度与NB浓度成正相关. 这意味着Au/MPS/GNP基底具有良好的半定量检测本领.     

Homolytic X-H Bond Cleavage at a Gold(III) Hydroxide: Insights into One-Electron Events at Gold

C(sp³)-H and O−H bond breaking steps in the oxidation of 1,4-cyclohexadiene and phenol by a Au(III)-OH complex were studied computationally. The analysis reveals that for both types of bonds the initial X−H cleavage step proceeds via concerted proton coupled electron transfer (cPCET), reflecting electron transfer from the substrate directly to the Au(III) centre and proton transfer to the Au-bound oxygen. This mechanistic picture is distinct from the analogous formal Cu(III)-OH complexes studied by the Tolman group (J. Am. Chem. Soc. 2019 , 141, 17236–17244), which proceed via hydrogen atom transfer (HAT) for C−H bonds and cPCET for O−H bonds. Hence, care should be taken when transferring concepts between Cu−OH and Au−OH species. Furthermore, the ability of Au−OH complexes to perform cPCET suggests further possibilities for one-electron chemistry at the Au centre, for which only limited examples exist.     

Electrochemical gating of single osmium molecules tethered to Au surfaces

The electrochemical study of electron transport between Au electrodes and the redox molecule Os[(bpy)₂(PyCH₂ NH₂CO-]ClO₄ tethered to molecular linkers of different length (1.3 to 2.9 nm) to Au surfaces has shown an exponential decay of the rate constant k _ET ⁰ with a slope β = 0.53 consistent with through bond tunneling to the redox center. Electrochemical gating of single osmium molecules in an asymmetric tunneling nano-gap between a Au(111) substrate electrode modified with the redox molecules and a Pt-Ir tip of a scanning tunneling microscope was achieved by independent control of the reference electrode potential in the electrolyte, E _ref ? E _s, and the tip-substrate bias potential, E _bias. Enhanced tunneling current at the osmium complex redox potential was observed as compared to the off resonance set point tunneling current with a linear dependence of the overpotential at maximum current vs. the E _bias. This corresponds to a sequential two-step electron transfer with partial vibration relaxation from the substrate Au(111) to the redox molecule in the nano-gap and from this redox state to the Pt-Ir tip according to the model of Kuznetsov and Ulstrup (J Phys Chem A 104: 11531, 2000). Comparison of short and long linkers of the osmium complex has shown the same two-step ET (electron transfer) behavior due to the long time scale in the complete reduction-oxidation cycle in the electrochemical tunneling spectroscopy (EC-STS) experiment as compared to the time constants for electron transfer for all linker distances, k _ET ⁰.     

Synthesis, spectroscopic and nonlinear optical properties of multiple [60]fullerene-oligo(p-phenylene ethynylene) hybrids

A series of multiple [60]fullerene terminated oligo(p-phenylene ethynylene) (OPE) hybrid compounds has been synthesized through a newly developed in situ ethynylation method. Structural and magnetic shielding properties of the highly unsaturated carbon-rich C(60) and OPE scaffolds were characterized by 1D and 2D NMR spectroscopic analyses. Electronic interactions between the [60]fullerenes and the OPE backbones were investigated by UV/Vis spectroscopic and cyclic voltammetry (CV) experiments. Our studies clearly show that although the multiple [60]fullerene groups are connected via pi-conjugated OPE frameworks, they present diminutive electronic interactions in the ground state, and the electronic behavior of the [60]fullerene cages are only affected by the OPE backbones through modest inductive effects. Interestingly, sizable third-order nonlinear optical (NLO) responses (gamma) and enhanced two-photon absorption (TPA) cross-sections (sigma((2))) were determined for the multifullerene-OPE hybrid 31 relative to its OPE precursor from differential optical Kerr effect (DOKE) experiments. Such enhanced NLO performance is presumably due to the occurrence of periconjugation and/or charge transfer effects in the excited state. In addition, comparatively strong excited-state absorption was observed and characterized for OPE pentamer 12. Thus, the use of such fullerene-derivatized conjugated oligomers aids the quest for molecules with large third-order NLO and TPA properties.     

Charge transfer dynamics of 3,4,9,10-perylene-tetracarboxylic-dianhydride molecules on Au(111) probed by resonant photoemission spectroscopy

Charge transfer dynamics across the lying-down 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) organic semiconductor molecules on Au(111) interface has been investigated using the core-hole clock implementation of resonant photoemission spectroscopy. It is found that the charge transfer time scale at the PTCDA∕Au(111) interface is much larger than the C 1s core-hole lifetime of 6 fs, indicating weak electronic coupling between PTCDA and the gold substrate due to the absence of chemical reaction and∕or bonding.     

A core-shell strategy for constructing a single-molecule junction

Understanding the effects of intermolecular interactions on the charge-transport properties of metal/molecule/metal junctions is an important step towards using individual molecules as building blocks for electronic devices. This work reports a systematic electron-transport investigation on a series of "core-shell"-structured oligo(phenylene ethynylene) (Gn-OPE) molecular wires. By using dendrimers of different generations as insulating "shells", the intermolecular π-π interactions between the OPE "cores" can be precisely controlled in single-component monolayers. Three techniques are used to evaluate the electron-transport properties of the Au/Gn-OPE/Au molecular junctions, including crossed-wire junction, scanning tunneling spectroscopy (STS), and scanning tunneling microscope (STM) break-junction techniques. The STM break-junction measurement reveals that the electron-transport pathways are strongly affected by the size of the side groups. When the side groups are small, electron transport could occur through three pathways, including through single-molecule junctions, double-molecule junctions, and molecular bridges between adjacent molecules formed by aromatic π-π coupling. The dendrimer shells effectively prohibit the π-π coupling effect, but at the same time, very large dendrimer side groups may hinder the formation of Au-S bonds. A first-generation dendrimer acts as an optimal shell that only allows electron transport through the single-molecule junction pathway, and forbids the other undesired pathways. It is demonstrated that the dendrimer-based core-shell strategy allows the single-molecule conductance to be probed in a homogenous monolayer without the influence of intermolecular π-π interactions.     

不同受限状态下链硫醇的非线性光学响应

以和频(SFG)振动光谱技术探测了正十二硫醇(DDT)在不同受限状态下的分子振动信号,包括金属基底上的自组装单层(SAM)分子,放置在二氧化硅基底上的表面DDT化的金纳米粒子以及金纳米粒子的甲苯溶液.在三种状态下都探测到了来自于DDT分子的振动光谱,振动光谱的区别提供了在不同受限态下DDT分子的结构信息.在金属基底上DDT分子排列规整,放置在二氧化硅基底上的金纳米粒子表面的DDT分子具有一定的柔性,在空气-甲苯溶液界面金纳米粒子表面的DDT分子高度无序.此外,光谱实验显示,金纳米粒子表面的分子振动信号产生了局域场增强的效应,相对于金基底上的自组装单层分子而言,增强系数为102-103,取决于光谱的偏振组合.     

不同受限状态下链硫醇的非线性光学响应

以和频(SFG)振动光谱技术探测了正十二硫醇(DDT)在不同受限状态下的分子振动信号, 包括金属基底上的自组装单层(SAM)分子, 放置在二氧化硅基底上的表面DDT化的金纳米粒子以及金纳米粒子的甲苯溶液. 在三种状态下都探测到了来自于DDT分子的振动光谱, 振动光谱的区别提供了在不同受限态下DDT分子的结构信息. 在金属基底上DDT分子排列规整, 放置在二氧化硅基底上的金纳米粒子表面的DDT分子具有一定的柔性, 在空气-甲苯溶液界面金纳米粒子表面的DDT分子高度无序. 此外, 光谱实验显示, 金纳米粒子表面的分子振动信号产生了局域场增强的效应, 相对于金基底上的自组装单层分子而言, 增强系数为10²-10³, 取决于光谱的偏振组合.