Photoinduced energy transfer, supramolecular

Fig. 16. Representation of photoinduced energy transfer and electron transfer processes involved in supramolecular photochemistry. Generation of R S, RS, R+S", or R-S+ may be followed by a chemical reaction.

Class 3 fluorophores linked, via a spacer or not, to a receptor. The design of such sensors, which are based on molecule or ion recognition by a receptor, requires special care in order to fulfil the criteria of affinity and selectivity. These aspects are relevant to the field of supramolecular chemistry. The changes in photophysical properties of the fluorophore upon interaction with the bound analyte are due to the perturbation by the latter of photoinduced processes such as electron transfer, charge transfer, energy transfer, excimer or exciplex formation or disappearance, etc. These aspects are relevant to the field of photophysics. In the case of ion recognition, the receptor is called an ionophore, and the whole molecular sensor is... 

B. Valeur, J. Pouget, and J. Bourson, Photoinduced electronic energy transfer in a supramolecular complex between a coumarin bichromophoric molecule and lead(II), 7. Lumin. 52, 345-347 (1992). 

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-... 

Ward, M. D. Photoinduced electron and energy transfer in noncovalently bonded supramolecular assemblies ,... 

The most prevalent photoinduced processes in supramolecular and interfacial systems are electron transfer, energy transfer and nuclear motion, such as proton transfer and isomerization. Before discussing these processes, it is important to outline the fundamental properties of electronically excited states. 

Figure 2.13 Schematic energy level diagrams illustrating the photoinduced (a) electron and (b) energy transfers in the supramolecular dyad A-L-B straight lines indicate radiative transfers, while wavy lines represent non-radiative reactions...

Figure 2.14 Schematic representations of the mechanisms of photoinduced (a) electron transfer, (b) Dexter (electron-exchange) energy transfer, and (c) Fdrster (dipole-dipole) energy transfer mechanism processes in the supramolecular dyad A-L-B spheres represent electrons, while curved arrows indicate the directions of transfer...

For a photoinduced electron transfer and charge separation to be efficient in a supramolecular device, some structural and energetic prerequisites must be fulfilled. First, the electron transfer must be thermodynamically feasible, i.e. it must be exergonic. The free energy of a photoinduced electron transfer, AGpet, may be calculated according to the following equation ... 

Experimental methods for studying photoinduced electron transfer reactions are discussed in detail in Chapter 3. In an excited-state species, energy and electron transfer compete with other photophysical events, including emission. Consequently, energy and electron transfer are often detected by comparing the emission properties of the supramolecular species, using suitable model compounds as described below. 

The study of photoinduced ET in covalently linked donor-acceptor assemblies began with comparatively simple dyad systems which contain a transition metal center covalently linked to a single electron donor or acceptor unit [26]. However, work in this area has naturally progressed and in recent years complex supramolecular assemblies comprised of one or more metal complexes that are covalently linked to one or more organic electron donors or acceptors have been synthesized and studied [27-36]. Furthermore, several groups have utilized the useful photoredox properties of transition metal complexes to probe electron and energy transfer across spacers comprised of biological macromolecules such as peptides [37,38], proteins [39,40], and polynucleic acids [41]. 

Hunter C. A. and Hyde R. K. Photoinduced Energy and Electron Transfer in Supramolecular Porphyrin Assemblies. Angew. Chem., Int. Ed. Engl. 35 (1996) pp. 1936-1939. 

IV. FUNCTIONAL ASPECTS OF SUPRAMOLECULAR ASSEMBLIES A. Photoinduced Electron and Energy Transfer... 

As already mentioned, the supramolecular assemblies can lead to new properties and photoinduced processes which have been studied in great details in solution (see previous section). On the contrary, only few cases of intermolecular photoinduced processes, and in particular, energy transfer, involving transition metal complexes in pure crystalline phases have been reported and will be now discussed. Here, relative orientation and distance between interacting species can be more precisely determined by means of single-crystal X-ray difiractometric analysis. 

Finally, it is important to note (Section 5.3.6) that electrochemistry and UV-Vis absorption spectra of molecular dyads or triads based on metal polypyridines show that electronic interactions between the components of the systems discussed above are too small to influence ground-state behavior. Nevertheless, they are sufficient to allow for very fast intramolecular electron transfer when electronically excited. In fact electronic coupling of 0.002-0.005 eV would be quite enough, but hardly detectable electrochemically. Detailed studies of electrochemistry and spectroscopy of these supramolecular systems and their components are, nevertheless, essential for the understanding of the energetics of photoinduced intramolecular electron and energy transfer reactions. 

The various examples of photoresponsive supramolecular systems that have been described in this chapter illustrate how these systems can be characterized by steady-state and time-resolved spectroscopic techniques based on either absorption or emission of light. Pertinent use of steady-state methods can provide important information in a simple vay stoichiometry and stability constant(s) of host-guest complexes, evidence for the existence of photoinduced processes such as electron transfer, energy transfer, excimer formation, etc. Investigation of the dynamics of these processes and characterization of reaction intermediates requires in most cases time-resolved techniques. Time-resolved fluorometry and transient absorption spectroscopy are frequently complementary, as illustrated by the study of photoinduced electron transfer processes. Time-resolved fluorometry is restricted to phenomena whose duration is of the same order of magnitude as the lifetime of the excited state of the fluorophores, whereas transient absorption spectroscopy allows one to monitor longer processes such as diffusion-controlled binding. 

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