Lachlan Everett Hall

Formalism, analytical model, and a priori Green’s-function-based calculations of the current–voltage characteristics of molecular wires

Various Green’s-function-based formalisms which express the current I as a function of applied voltage V for an electrode–molecule–electrode assembly are compared and contrasted. The analytical solution for conduction through a Huckel (tight binding) chain molecule is examined and only one of these formalisms is shown to predict the known conductivity of a one-dimensional metallic wire. Also, from this solution we extract the counter-intuitive result that the imaginary component of the self-energy produces a shift in the voltage at which molecular resonances occur, and complete analytical descriptions are provided of the conductivity through one-atom and two-atom bridges. A method is presented by which a priori calculations could be performed, and this is examined using extended-Huckel calculations for two gold electrodes spanned by the dithioquinone dianion. A key feature of this is the use of known bulk-electrode properties to model the electrode surface rather than the variety of more approximate schemes which are in current use. These other schemes are shown to be qualitatively realistic but not sufficiently reliable for use in quantitative calculations. We show that in such calculations it is very important to obtain accurate estimates of both the molecule–electrode coupling strength and the location of the electrode’s Fermi energies with respect to the molecular state energies.Various Green’s-function-based formalisms which express the current I as a function of applied voltage V for an electrode–molecule–electrode assembly are compared and contrasted. The analytical solution for conduction through a Huckel (tight binding) chain molecule is examined and only one of these formalisms is shown to predict the known conductivity of a one-dimensional metallic wire. Also, from this solution we extract the counter-intuitive result that the imaginary component of the self-energy produces a shift in the voltage at which molecular resonances occur, and complete analytical descriptions are provided of the conductivity through one-atom and two-atom bridges. A method is presented by which a priori calculations could be performed, and this is examined using extended-Huckel calculations for two gold electrodes spanned by the dithioquinone dianion. A key feature of this is the use of known bulk-electrode properties to model the electrode surface rather than the variety of more approximate schem...

Optimization and Chemical Control of Porphyrin‐Based Molecular Wires and Switches

ABSTRACT: The design rationale is described as are some experimental and theoretical developments concerning rigidly fused porphyrin molecular wires. These materials consist of porphyrin units fused to acene‐type bridges and have been synthesized in a range of topologies including linear porphyrin octamers of length ca. 120 Å. Next we demonstrate, for some linear oligoporphyrins, how the electronic coupling between the end porphyrin units can be modulated by simple (possibly in situ) chemical modulation of the bridging units. Specifically, the chemical systems considered involve either pH‐controlled protonation of bridge azines or conversion of bridge quinone or quinone dioxime rings to or from benzenoid or hydroquinone rings. In the most general terms, the electronic coupling through oligoporphyrin molecular wires is discussed in terms of a simple model in which complete end‐to‐end π electronic delocalization is required in order to provide strong long‐range interactions. Computationally, we monitor interorbital coupling using an appropriate mixture of density functional (B3LYP) and ab initio SCF computational schemes. Finally, we examine bridge modulation of the intermetallic coupling in three homovalent bis‐metallic oligoporphyrin systems, two in which Ru(CO)2 is complexed within terminal porphyrin rings, the other in which [Cu(I)Cl2]− is tethered using phenanthroline end groups. Results are obtained both using an effective two‐level model, appropriate for spectroscopic properties, and using a more general scheme, appropriate for molecular conduction.

Chemical Control of Tautomerization-Based Molecular Electronic and Color Switches

ABSTRACT: Many schemes which have been suggested for the chemical control of molecular electronic devices are impractical because of the large energy changes associated with bond breaking and reforming: these would result in unacceptably large amounts of energy being required for device operation. We have chosen to look in detail at a class of chemical reactions prominent in biological systems that, on energetic grounds, could produce a feasible device: tautomerization reactions of heterocyclic bases. In particular, we concentrate on a model reaction, the 4‐pyridinol/4(1H)‐pyridinone tautomerization, through which an aromatic pyridinol unit is converted into a π‐localized pyridinone unit. In these molecules the functionalities are arranged para, allowing the possibility that the molecules align in a solid phase in linear hydrogen‐bonded chains. The change in the π electronic structure associated with reactions of this type is quite large, as illustrated, for example, in the conversion of quinacridone, a brightly colored pigment, to quinacridol, an uncolored species with strong UV absorption. Our primary purpose is to calculate energy changes and reaction barrier heights to see whether such tautomerization reactions could provide the basis for an operational chemically controlled molecular electronic device. We also consider, briefly, other required properties in a practicable device.