Photochemical supramolecular devices processes

Outline examples of how photochemical supramolecular devices can be used for energy- and electron-transfer processes, for information processing and as light-driven machines. 

When interaction between the metal-based components is weak, polynuclear transition metal complexes belong to the field of supramolecular chemistry. At the roots of supramolecular chemistry is the concept that supramolecular species have the potential to achieve much more elaborated tasks than simple molecular components while a molecular component can be involved in simple acts, supramolecular species can performIn other words, supramolecular species have the potentiality to behave as molecular devices. Particularly interesting molecular devices are those which use light to achieve their functions. Molecular devices which perform light-induced functions are called photochemical molecular devices (PMD). Luminescent and redox-active polynuclear complexes as those described in this chapter can play a role as PMDs operating by photoinduced energy and electron transfer processes. ... 

The photochemical and photophysical processes discussed above provide illustrations and incentives for further studies of photoeffects brought about by the formation of supramolecular species. Such investigations may lead to the development of photoactive molecular and supramolecular devices, based on photoinduced energy migration, electron transfer, substrate release, or chemical transformation. Coupling to recognition processes may allow the transduction of molecular infor-... 

Based on their easily tunable photophysical and redox properties, transition metal complexes are versatile components to be used in the construction of photochemical molecular devices. The studies presented in this article show that the combination of the Ru(bpy)22+ photosensitizer and cyanide bridging units allows the synthesis of a variety of polynuclear systems that exhibit interesting photochemical properties. Depending on the nature of the attached metal-containing units, supramolecular systems can be obtained that undergo efficient photoinduced intramolecular energy or electron transfer processes. 

Supramolecular complexes as described herein are composed of an assembly of individual inorganic and organic components that act together to provide an overall function to the molecule. Such systems have been also proposed as photochemical molecular devices by Balzani. The overall function of the supramolecular complex can be modulated by careful selection of the individual components. Because the individual components within the supramolecular complex are chemically bonded, bimolecular reaction for energy or electron transfer is eliminated, increasing the efficiency of the photochemical processes. The covalent coupling of subunits provides significant perturbation of the component s basic properties. 

In this report, we will describe some of our studies aimed at (i) obtaining new inorganic photosensitizers by second-sphere modification of known ones, and (ii) assembling photosensitizer units with other molecular components in discrete, covalently bound supramolecular structures. Studies of type (i), besides their intrinsic interest, have some relevance to the problem of how the properties of a photosensitizer are modified by inclusion in a supramolecular structure. Systems of type (ii) would be useful to study the basic processes of intramolecular electron and energy transfer involved in the performance of molecular photochemical devices. 

Photochromic processes are often observed both in solution and in the solid state, thus making for facile incorporation of photochromies in films, in membranes, and as dopants in host matrices—prerequisites for the construction of molecular optoelectronic devices. Section 2.3.1 focuses on the materials and supramolecular systems prepared from photochromic systems. For more comprehensive descriptions of the basic photochemical processes the reader is referred to any of the numerous reviews on the subject [47, 51, 89, 159-162]. 

In the framework of research on supramolecular chemistry the idea began to arise in a few laboratories that molecules are much more convenient building blocks than atoms for construction of nanoscale devices and machines. The main foundations of this idea were (a) molecules are stable species, whereas atoms are difficult to handle (b) Nature starts from molecules, not from atoms, to construct the great number and variety of nanodevices and nanomachines that sustain life (vide infra) (c) most laboratory chemical processes deal with molecules, not with atoms (d) molecules are objects that already have distinct shapes and carry device-related properties (e.g. properties that can be manipulated by photochemical and electrochemical inputs) and (e) molecules can self-assemble or can be connected to make larger structures. 

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