Switching Devices. Signals and Information

If the operation of the devices considered above on their specific substrate (photon, electron, ion) is triggered by external optical, electrical or chemical stimuli, their features may be switched between two (or more) states presenting different characteristics. Such switching devices are therefore formed by two main components a trigger, the switching unit, activated by an external stimulus and a substrate, the switched species they should operate with efficiency, reversibility and resistance to fatigue. 

The substrate carries information defined by its molecular recognition features and constitutes a signal that may be modulated by the switching process. The stimulus to which the switch responds and the substrate on which it operates may both be either of photonic, electronic, ionic, magnetic, thermal or mechanical nature. Thus, a whole set of switches based on (substrate-stimulus) pairs, is defined by the combination of the various types opto-photo, opto-electro, electro-photo, etc. 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

Multistability is present in systems that are interconvertible between more than two states such as polynuclear-multivalence metal complexes or polytopic receptors where one or more substrates may occupy and transfer between several different binding sites. 

Molecular hysteresis results when the interconversion between the states does not follow the same pathway on sweeping the external stimulus or field in one or the opposite directions [8.240a, 8.241]. A hysteresis loop has, for instance, been described for the electrochemical interconversion of redox states in a dinuclear metal complex it involves the switching between two different binding sites [8.240b]. Spin 

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