Page 241 - Photoreactive Organic Thin Films

220 EUGENIf KATZ,ANDREW N. SHIPWAY.AND ITAMARWILLNER

7.5.3. Reversible Bioaffinity Interactions at Photoisomerizable
Interfaces
7.5.4. Complex Photochemical Biomolecular Switches
7.6 INTERLOCKED COMPOUNDS AS MECHANICAL
COMPONENTS AT PHOTOISOMERIZABLE INTERFACES
7.7 CONCLUSIONS

7.1 INTRODUCTION

Advances in supramolecular chemistry has led to ingenious molecular archi-
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tectures exhibiting unique binding, transport, ' ' and catalytic ' properties.
Supramolecular architectures that reveal basic mechanical functions such as
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rotors ' ' ' ' and translational movement have been developed, and more
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complex assemblies also have been reported. ' ' However, to realize the
nanotechnologists' dreams of constructing complex devices, it is imperative
that the supramolecular architectures exhibit external addressability and
allow multidirectional communication with the macroscopic environment.
The ultimate goal of miniaturization is to use molecular assemblies as
memory, processing, and mechanical devices. To reach these goals, it is essen-
tial to transform molecular structures between two or more states in response
to external signals such as photonic, chemical, electrochemical, or magnetic
stimuli, and to tailor readable output(s) such as electronic or optical signals
that reflect the molecular state. The molecular units in a memory device, for
instance, also must be individually addressable. To achieve this goal, the
organization of molecular units on surfaces in two- and three-dimensional
structures is of particular importance. Researchers have assembled functional
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monolayers and multilayers on electrodes ' ' ' and other transducers. '
Another desirable function of signal-activated chemical assemblies is the
property of chemical self-amplification, that is, the output signal (or readout
signal) is amplified as compared to the input stimuli. Amplification may be
achieved if one state of an isomerizable molecule exhibits catalytic properties.
Some of the fundamental features of these nanodevices are schematically pre-
sented in Figure 7.1. Numerous molecular systems that can be switched
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between two states by photonic, 22523 electrical, " pH, and chemical '
stimuli have been studied in solution, and their functions as "switches,"
"memories," and "logic gates" have been discussed. Ingenious interlocked
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molecular structures ' where threaded molecular units are translated
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between distinct molecular sites by electrical, pH, and photonic signals, 33
have been reported. Recently, some of these molecular structures have been
incorporated in device-like assemblies and their potential application in
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molecular-scale computation was noted. ' Many of the approaches to
organize nanoscale devices follow models inspired by nature, and indeed,
nature has provided us with definitive proofs that such goals may be realized.
For example, the vision process represents an optoelectronic system where
optical signals are transduced as electrical stimuli that activate the neural
response. Photomorphogenesis represents many biological processes in plants