Molecular Rectification Tuned by Through-Space Gating Effect

Inspired by transistors and electron transfer in proteins, we designed a group of pyridinoparacyclophane based diodes to study the through-space electronic gating effect on molecular rectification. It was shown that an edge-on gate effectively tunes the rectification ratio of a diode via through-space interaction. Higher rectification ratio was obtained for more electron-rich gating groups. The transition voltage spectroscopy showed that the forward transition voltage is correlated to the Hammett parameter of the gating group. Combining theoretical calculation and experimental data, we proposed that the change in rectification was induced by a shift in HOMO level both spatially and energetically. This design principle based on through-space edge-on gate is demonstrated on molecular wires, switches, and now diodes, showing the potential of molecular design in increasing the complexity of single-molecule electronic devices

Edge-on Gating Effect in Molecular Wires

This work demonstrates edge-on chemical gating effect in molecular wires utilizing the pyridinoparacyclophane (PC) moiety as the gate. Different substituents with varied electronic demands are attached to the gate to simulate the effect of varying gating voltages similar to that in field-effect transistor (FET). It was observed that the orbital energy level and charge carrier’s tunneling barriers can be tuned by changing the gating group from strong electron acceptors to strong electron donors. The single molecule conductance and current–voltage characteristics of this molecular system are truly similar to those expected for an actual single molecular transistor

Exceptional Single-Molecule Transport Properties of Ladder-Type Heteroacene Molecular Wires

A series of ladder-type fused heteroacenes consisting of thiophenes and benzothiophenes were synthesized and functionalized with thiol groups for single-molecule electrical measurements via a scanning tunneling microscopy break-junction method. It was found that this molecular wire system possesses exceptional charge transport properties with weak length dependence. The tunneling decay constant β was estimated to be 0.088 and 0.047 Å^–1 under 0.1 and 0.5 bias, respectively, which is one of the lowest β values among other non-metal-containing molecular wires, indicating that a planar ladder structure favors charge transport. Transition voltage spectroscopy showed that the energy barrier decreases as the length of the molecule increases. The general trend of the energy offsets derived from the transition voltage via the Newns–Anderson model agrees well with that of the Fermi/HOMO energy level difference. Nonequilibrium Green’s function/density functional theory was used to further investigate the transport process in these molecular wires

Rational Design of Porous Conjugated Polymers and Roles of Residual Palladium for Photocatalytic Hydrogen Production

Developing highly efficient photocatalyts for water splitting is one of the grand challenges in solar energy conversion. Here, we report the rational design and synthesis of porous conjugated polymer (PCP) that photocatalytically generates hydrogen from water splitting. The design mimics natural photosynthetics systems with conjugated polymer component to harvest photons and the transition metal part to facilitate catalytic activities. A series of PCPs have been synthesized with different light harvesting chromophores and transition metal binding bipyridyl (bpy) sites. The photocatalytic activity of these bpy-containing PCPs can be greatly enhanced due to the improved light absorption, better wettability, local ordering structure, and the improved charge separation process. The PCP made of strong and fully conjugated donor chromophore DBD (M₄) shows the highest hydrogen production rate at ∼33 μmol/h. The results indicate that copolymerization between a strong electron donor and weak electron acceptor into the same polymer chain is a useful strategy for developing efficient photocatalysts. This study also reveals that the residual palladium in the PCP networks plays a key role for the catalytic performance. The hydrogen generation activity of PCP photocatalyst can be further enhanced to 164 μmol/h with an apparent quantum yield of 1.8% at 350 nm by loading 2 wt % of extra platinum cocatalyst

Photocatalysts Based on Cobalt-Chelating Conjugated Polymers for Hydrogen Evolution from Water

Developing photocatalytic systems for water splitting to generate oxygen and hydrogen is one of the biggest chemical challenges in solar energy utilization. In this work, we report the first example of heterogeneous photocatalysts for hydrogen evolution based on in-chain cobalt-chelating conjugated polymers. Two conjugated polymers chelated with earth-abundant cobalt ions were synthesized and found to evolve hydrogen photocatalytically from water. These polymers are designed to combine functions of the conjugated backbone as a light-harvesting antenna and electron-transfer conduit with the in-chain bipyridyl-chelated transition metal centers as catalytic active sites. In addition, these polymers are soluble in organic solvents, enabling effective interactions with the substrates as well as detailed characterization. We also found a polymer-dependent optimal cobalt chelating concentration at which the highest photocatalytic hydrogen production (PHP) activity can be achieved

Controlled Self-Assembly of Cyclophane Amphiphiles: From 1D Nanofibers to Ultrathin 2D Topological Structures

A novel series of amphiphilic <b>TC-PEG</b> molecules were designed and synthesized based on the orthogonal cyclophane unit. These molecules were able to self-assemble from 1D nanofibers and nanobelts to 2D ultrathin nanosheets (3 nm thick) in a controlled way by tuning the length of PEG side-chains. The special structure of the cyclophane moiety allowed control in construction of nanostructures through programmed noncovalent interactions (hydrophobic–hydrophilic interaction and π–π interaction). The self-assembled nanostructures were characterized by combining real space imaging (TEM, SEM, and AFM) and reciprocal space scattering (GIWAXS) techniques. This unique supramolecular system may provide a new strategy for the design of materials with tunable nanomorphology and functionality

Synthesis and Search for Design Principles of New Electron Accepting Polymers for All-Polymer Solar Cells

New electron withdrawing monomers, thieno­[2′,3′:5′,6′]­pyrido­[3,4-<i>g</i>]­thieno­[3,2-<i>c</i>]­isoquinoline-5,11­(4<i>H</i>,10<i>H</i>)-dione (TPTI) and fluorenedicyclopentathiophene dimalononitrile (CN), have been developed and used to form 12 alternating polymers having different monomer combinations: (a) weak donating monomer–strong accepting monomer, (b) weak accepting monomer–strong accepting monomer, (c) weak accepting monomer–weak accepting monomer, and (d) strong donating monomer–strong accepting monomer. It was found that lowest unoccupied molecular orbital (LUMO) energy levels of polymers are significantly determined by stronger electron accepting monomers and highest occupied molecular orbital (HOMO) energy levels by the weak electron accepting monomers. In addition, fluorescent quantum yields of the TPTI-based polymers in chloroform solution are significantly decreased as the LUMO energy levels of the TPTI series of polymers become deeper. The quantum yield was found to be closely related with the photovoltaic properties, which reflects the effect of internal polarization on the photovoltaic properties. Only the electron accepting polymers showing SCLC mobility higher than 10^–4 cm²/(V s) exhibited photovoltaic performance in blend films with a donor polymer, and the PTB7:PNPDI (1:1.8 w/w) device exhibited the highest power conversion efficiency of 1.03% (<i>V</i>_oc = 0.69 V, <i>J</i>_sc = −4.13 mA/cm², FF = 0.36) under AM 1.5G condition, 100 W/cm². We provide a large set of systematic structure–property relationships, which gives new perspectives for the design of electron accepting materials