anwei Li

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.

Beyond Molecular Wires: Design Molecular Electronic Functions Based on Dipolar Effect

As the semiconductor companies officially abandoned the pursuit of Moores law, the limitation of silicone-based semiconductor electronic devices is approaching. Single molecular devices are considered as a potential solution to overcome the physical barriers caused by quantum interferences because the intermolecular interactions are mainly through weak van der Waals force between molecular building blocks. In this bottom-up approach, components are built from atoms up, allowing great control over the molecular properties. Moreover, single molecular devices are powerful tools to understand quantum physics, reaction mechanism, and electron and charge transfer processes in organic semiconductors and molecules. So far, a great deal of effort is focused on understanding charge transport through organic single-molecular wires. However, to control charge transport, molecular diodes, switches, transistors, and memories are crucial. Significant progress in these topics has been achieved in the past years. The introduction and advances of scanning tunneling microscope break-junction (STM-BJ) techniques have led to more detailed characterization of new molecular structures. The modern organic chemistry provides an efficient access to a variety of functional moieties in single molecular device. These moieties have the potential to be incorporated in miniature circuits or incorporated as parts in molecular machines, bioelectronics devices, and bottom-up molecular devices. In this Account, we discuss progress mainly made in our lab in designing and characterizing organic single-molecular electronic components beyond molecular wires and with varied functions. We have synthesized and demonstrated molecular diodes with p-n junction structures through various scanning probe microscopy techniques. The assembly of the molecular diodes was achieved by using Langmuir-Blodgett technique or thiol/gold self-assembly chemistry with orthogonal protecting groups. We have thoroughly investigated the rectification effect of different types of p-n junction diodes and its modification by structural and external effects. Through a combination of structural modifications, low temperature study, and quantum mechanical calculations, we showed that the origin of the rectification in these molecules can be attributed to the effect of dipolar field. Further studies on charge transport through transition metal complexes and anchoring group effect supported this conclusion. Most recently, a model system of molecular transistor was synthesized and demonstrated by STM-BJ technique. The gating effect in the molecular wire originated from the tuning of the energy levels via dipolar field and can be turned on/off by dipolar field and chemical stimulation. This is the first example of gated charge transport in molecular electronics.

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 carriers 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 Greens function/density functional theory was used to further investigate the transport process in these molecular wires.

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.

A Single-Molecular AND Gate Operated with Two Orthogonal Switching Mechanisms

Single-molecular electronics is a potential solution to nanoscale electronic devices. While simple functional single-molecule devices such as diodes, switches, and wires are well studied, complex single-molecular systems with multiple functional units are rarely investigated. Here, a single-molecule AND logic gate is constructed from a proton-switchable edge-on gated pyridinoparacyclophane unit with a light-switchable diarylethene unit. The AND gate can be controlled orthogonally by light and protonation and produce desired electrical output at room temperature. The AND gate shows high conductivity when treated with UV light and in the neutral state, and low conductivity when treated either with visible light or acid. A conductance difference of 7.3 is observed for the switching from the highest conducting state to second-highest conducting state and a conductance ratio of 94 is observed between the most and least conducting states. The orthogonality of the two stimuli is further demonstrated by UV-vis, NMR, and density function theory calculations. This is a demonstration of concept of constructing a complex single-molecule electronic device from two coupled functional units.