Polynuclear complexes energy transfer

This section summarizes work carried out on polynuclear complexes containing M(bpy)2 units, an area in which there is much interest, in particular with respect to energy transfer. Dendritic systems are excluded from this review, but are covered elsewhere in CCC The complexes-as-ligands strategy is commonly exploited for the controlled construction of multinuclear complexes and examples are seen in this section. 

As we have seen above (Section IV.C), in the polynuclear complexes dealt with in this review it is possible to identify components which can undergo photoexcitation independently from one another. The excited component can then give rise to intercomponent energy transfer processes, in competition with intracomponent decay. For most of the components which constitute the examined systems, the lifetime of the lowest excited state is long enough to allow the occurrence of energy transfer to nearby components when suitable energetic and electronic conditions are satisfied. This is not usually the case for upper excited states, which usually decay very rapidly (picosecond time scale) to the lowest excited state within each component. 

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 study of photoinduced processes in Ir(III)-based arrays has been exploited since facile synthetic methods for the preparation of Ir(III)-polyimine complexes became available. Polynuclear complexes containing, in addition to Ir(III) centers, either Os(II)-, Ru(II)-, Re(II) or Cu(I)-polyimine units linked by different bridging ligands, displayed a rich variety of photoinduced energy transfer processes. In Fig. 22 are reported some representative cases of early studies. 

Fig. 22 Schematic structures of polynuclear complexes, see text arrows denote energy transfer...

Photoinduced Electron and Energy Transfer in Polynuclear Complexes... 

The charge transfers in polynuclear complexes with metal centres ready to undergo redox reactions are called intervalence charge transfer (IT). These compounds are often characterized by low-energy M-M transitions and thereby have intense colour (eg Prussian blue and its analogues). 

In most cases, the metal atoms are in fact end groups necessary to define a starting site and an arrival site, while the spacer is usually purely organic. The concept of a molecular wire requires something to be transferred between these two sites more efficiently than if the two sites were separated by empty space. The present chapter will deal essentially with electron (or hole) transfer, but the related topic of energy transfer (treated extensively in Part 1, Chapter 3 of this volume) will be also evoked. Representative examples of electron and energy transfer in polynuclear transition metal complexes can also be found in recent reviews by Juris, Balzani et al. [3a] and by De Cola and Belser [3b. ... 

Before continuing, some words of clarification are due. Although there exist a limited number of polynuclear lanthanide complexes whose component metal ions are shown to participate in intramolecular energy transfer (Thompson et al., 2001), for most such complexes, there is no apparent metal-metal bonding, nor are there any significant interactions mediated by the commonly observed bridging ligands, a consequence of... 

In a polynuclear or polymetallic lanthanide complex, additional processes become feasible. Figure 3 is a simplified scheme that shows the energy transfer pathways available in a bimetallic lanthanide complex singlet-mediated energy transfer and competitive non-radiative quenching pathways are not shown in the scheme to facilitate interpretation. Not only can both lanthanide ions be sensitized by the excited states of the chromophore, but energy transfer between lanthanide ions is also possible where appropriate spectral overlap exists between the two lanthanide ions. 

These polynuclear gold(I) phosphine complexes have been found to possess rich photochemistry. The phosphorescence of 49 is found to be quenched by a series of energy acceptors such as trans- and cis-stilbene, styrene, hept-l-ene, and cyclohexene [129], The transient absorption difference spectrum of a degassed acetonitrile solution of 49 and irans-stilbene displays absorption bands at about 365 and 390 nm, typical of the triplet excited state absorption spectrum of trans-stilbene, indicating the energy transfer nature of the quenching mechanism, Be-... 

In this chapter three topics related to photosensitization and photocatalysis of polypyridyl complexes are discussed types of excited states encountered and "tuning" of their properties, light-induced electron and energy transfer reactions and their applications. Concepts are illustrated with select examples, with references made only to mononuclear complexes. A comprehensive review of the photophysics and photochemistry of various polypyridine complexes has been published recently [13]. In the accompanying chapter, Scandola et al. discuss extension of the photophysics and photoredox chemistry to polynuclear systems. 

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