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.

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

“…The central ring is clearly strongly delocalized for the benzenoid (even grey shading), while the quinonoid contains isolated double and single bonds (black and light shading, respectively). The phenomenon is actually not restricted to oligoporphyrins, but is a general feature of electron transfer 23,29 through benzenoid/ quinonoid bridges.…”

“…However, the single-step interconversion of benzenoid and quinonoid forms thus envisaged is somewhat impractical, but the corresponding hydroquinones have benzenoid structures and may be readily converted using wet-chemical or electrochemical techniques to or from either the quinone or the corresponding dioxime, both of which have the quinonoid structure. 13 Indeed, interconversion of benzenoid and quinonoid forms may be feasible using very-low-energy hydrogen tautomerization reactions between pyridinones and pyridinols; our other article in this volume 29 is concerned with the details of such reactions and their possible application to fast color switching devices.…”

An Atomistic Approach to Conduction Between Nanoelectrodes Through a Single Molecule

“…It is for systems such as this that Hückel (tight binding) Hamiltonians are most appropriate, and there is much interest in electrode-molecule-electrode conduction through such systems. 19,46 It is highly desirable that any computational method, whether classical or quantum, for the electronic charge distribution in nanoelectronic structures be able to produce realistic results in this limit. FIGURE 5 shows Mulliken charges and electrostatic-potential (ESP) charges obtained by a number of quantum electronic structure methods for linear H 5 + and Au 5 + , as well as classical results obtained using the 2D uniform charge density per atom model.…”

“…The Hückel method is very important in the study of electrode-moleculeelectrode conduction, as it not only provides important analytically solvable paradigms 10,19 but also effective computational methodologies for large problems such as nanotube conduction. 18 One significant limitation of this approach is that it implicitly treats molecules as being metallic, though this can be alleviated in a man-,…”

“…We have expanded on the earlier work of Mujica et al, 10 developing both convenient and accurate (much more so than through use of the standard McConnell approximation) analytical solutions for nonresonant conduction of linear-chain molecules. 19 Hückel theory is, unfortunately, only qualitatively descriptive of basic molecular properties. This is well known, it being demonstrated, for example, by the widely different interatomic interactions required in order to fit data of different types (e.g., ionization energies and electronic transition energies) for series of closely analogous molecules.…”