Molecular Logic-based Computation

A Prasanna de Silva

• Publisher: Royal Society of Chemistry
• ISBN:
  9781782626237
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

We all learn - in schools, factories, bars and streets. We gather, store, process and transmit information in society. Molecular systems involved in our senses and within our brains allow all this to happen and molecular systems allow living things of all kinds to handle information for the purpose of survival and growth. Nevertheless, the vital link between molecules and computation was not generally appreciated until a few decades ago. Semiconductor-based information technology had penetrated society at many levels and the interest in maintaining momentum of this revolution led to the consideration of molecules, among others, as possible information handlers. Such an overlap between the recent engineering-oriented revolution with the ancient biology-oriented success story is very interesting and George Boole's times in Ireland 150 years ago produced the logic ideas that provide the foundations of computation to this day. Molecular logic and computation is a field which is 17 years young, has had a healthy growth and is a story which deserves to be told. It is a growing branch of chemical science which highlights the connection between information technology (engineering and biological) and chemistry. The author and co-workers of this publication launched molecular logic as an experimental field by publishing the first research in the primary literature in 1993 and are uniquely placed to recount how the field has grown. There is no other book at present on molecular logic and computation and is more comprehensive than that found in any review available so far. It shows how designed molecules can play the role of information processors in a wide variety of situations, once we are educated by those information processors already available in the semiconductor electronics business and in the natural world. Following a short history of the field, is a set of primers on logic, computing and photochemical principles which are an essential basis in this field. The book covers all of the Boolean logic gates driven by a single input and all of those with double inputs and the wide range of designs which lie beneath these gates is a particular highlight. The easily-available diversity of chemical systems is another highlight, especially when it leads to reconfigurable logic gates. Further on in the book, molecular arithmetic and other more complex logic operations, including those with a memory and those which stray beyond binary are covered. Then follows molecular computing approaches which lie outside the Boolean blueprint, including quantum phenomena and finally, the book catalogues the useful real-life applications of molecular logic and computation which are already available. This book is an authoritative, state of the art, reference and a 'one-stop-shop' concerning the current state of the field for scientists, academics and postgraduate students.

CHAPTER 1

A Little History

1.1 Introduction

The physicist and percussionist Feynman is widely credited with envisioning atom assembly as a way of building ultra-small devices.^1,2 While this vision can be realized by atom manipulation with scanning probe microscope tips,³ it is clear that chemists have a treasure chest of methods to arrange atoms in intricate arrays to produce molecules on a kilogram scale. Once synthesized, these molecules can be investigated one at a time – at the single molecule level – if necessary. More commonly, these molecules will be studied in large populations of many billions.

1.2 Early Proposals for Molecular Logic

Perhaps the best known early proposal for molecular logic and computation was announced in 1988 by Aviram^4,5 and is outlined in Figure 1.1. Given his affiliation with IBM, it was natural that the system 1 used electric voltages for both the inputs and the outputs. The vision was as follows. A π-system₁, an oligothiophene of ca. 25 nm length would be fixed via thiol terminals to two gold⁶ electrodes (electrode₁ and electrode₂). In this electrically neutral state, the oligothiophene would be non-conducting.^7–10 The middle of the oligothiophene contains a spiro linkage so that another π-system can be positioned orthogonally. The latter π-system₂ (different⁵ from or identical⁴ to the first oligothiophene) would be held in its radical cation form which is electrically conducting.^7–10 The spiro linkage would hinder electron transfer between the two π-systems so that the first oligothiophene remains electrically neutral. However if a strong electric field is applied to the spiro linkage via two orthogonal switching electrodes aimed at it, theoretical calculations^11,12 suggested that electron transfer would occur between the two π-systems. The first oligothiophene would then pass into its radical cation state which is an electron conductor, as mentioned earlier. Thus a voltage applied at one gold electrode₁ would appear at the other (output) gold electrode₂ in response to a voltage input applied across two switching electrodes aimed at the centre of 1, which are not shown in Figure 1.1. In other words, an electric field-induced insulator-to-conductor transition at the molecular scale would serve as the switching mechanism in this device. The above argument does not need electrode₃ and electrode₄ but they are required⁴ for building NOT logic gates, which are essential for general computing. Wiring of multiple copies of these switches was then expected to yield various logic functions.

Figure 1.1 Artist’s impression of a proposed molecular electronic switch 1. Another pair of switching electrodes which is aimed at the centre of 1 is not shown.
Adapted from L. F. Lindoy, Nature, 1993, 364, 17 with permission from Nature Publishing Group.

As we go to press 24 years later, Aviram’s proposal remains a proposal. Much effort has been expended by Tour, for example, to synthesize close relatives of 1 and many other derivatives in resourceful ways.^13,14 Tour and his collaborators Allara, Weiss and Reed also performed pioneering two-electrode experiments on molecules simpler than 1,^15–18 some of these results turning out to be controversial.^19–21 Nevertheless, he concluded that it was too difficult to focus more than two metal electrodes selectively onto a small molecule such as 1 in order to perform the crucial test.^22,23 Fabricating three-electrode devices based on single molecules remains a difficult art.^24,25 Logic gates based on graphene and (bundled or single) carbon nanotubes have also been constructed.^26,27

Though not proposed for molecular logic-based computation as such, Aviram and Ratner’s suggestion (made in 1974) for a molecular diode/rectifier²⁸ reached a happier conclusion. An electron donor π-system linked via a σ-bridge to an electron acceptor π-system such as 2 was Aviram’s and Ratner’s original suggestion. After a long odyssey and many controversies concerning the nature of the metal electrode–organic molecule interface in determining current output–voltage input profiles,²⁹ the rectification behaviour of monolayers of, for example, 3 was demonstrated.^30–32

Aviram’s baton has been picked up by several others to develop different approaches to molecular electronic logic and computation. These have focused on molecules different from 1, e.g. rotaxanes,³³ catenanes,³⁴ carbon nanotubes^26,27 and graphene.^26,27

This electronic approach to molecular logic and computation was and is very popular, perhaps because of its direct connection with the burgeoning semiconductor computer industry and also because it engaged the concepts and techniques of chemists, physicists and engineers in equal measure. Naturally, there have been many good discussions in the literature.^35–37 Well-resourced national programmes were launched in several countries. The USA operation occurred in two waves, with the first centred in the early 1980s^38,39 and the second starting in the late 1990s and continuing to this day.⁴⁰ The entire field of molecular logic-based computation has benefited as a result, but also (undeservedly) shared in the trauma of exposed hype at various times.

Hole-burning spectroscopy was another early approach which involved discussions on molecular computation. In the 1980s, Wild’s laboratory demonstrated computing functions such as elementary addition on molecular substrates.⁴¹ This was a photochemical method. A laser picked out a few molecules in a specific characteristic microenvironment in a rigid medium on the basis of their characteristic absorption lines, to the accuracy of 0.0001 cm⁻¹. These molecules were converted after photoselection to their tautomeric forms, which would persist. However, the ...

Table of contents

• Cover
• Title
• Copyright
• Foreword
• Preface
• Contents
• Chapter 1 A Little History
• Chapter 2 Chemistry and Computation
• Chapter 3 A Little Logic and Computation
• Chapter 4 A Little Photochemistry and Luminescence
• Chapter 5 Single Input–Single Output Systems
• Chapter 6 Reconfigurable Single Input–Single Output Systems
• Chapter 7 Double Input–Single Output Systems
• Chapter 8 Reconfigurable Double Input–Single Output Systems
• Chapter 9 Double Input–Double Output Systems
• Chapter 10 More Complex Systems
• Chapter 11 History-dependent Systems
• Chapter 12 Multi-level Logic
• Chapter 13 Quantum Aspects
• Chapter 14 Applications
• List of Abbreviations and Glossary
• Subject Index