Control of photoinduced energy transfer between metal-polypyridyl luminophores across rigid covalent, flexible covalent, or hydrogen-bonded bridges

This review describes four recent examples of how photoinduced energy-transfer in dinuclear complexes can be manipulated or controlled according to the nature of the bridging pathway between the metal-polypyridyl luminophores. In the first examples, the interacting fragments [polypyridyl complexes of Ru(II), Os(II) or Re(I)] are covalently linked by the bridging ligand 2,2 ' :3 ' ,2 " :6 " ,2 ' " -quaterpyridine which has two inequivalent bipyridyl binding sites in close proximity, In heterodinuclear Ru-Os and Ru-Re complexes, efficient inter-component photoinduced energy-transfer occurs, with the emission characteristics being sensitive to the electronic difference between the two bipyridyl sites. This, in the Ru-Re diads either Ru --> Re or Re --> Ru energy transfer can occur depending on which metal fragment is in which binding site. The second example is of a supramolecular assembly in which the interacting Ru(II) and Os(II) mononuclear components are associated in CH2Cl2, solution via a reversible hydrogen-bonding interaction between peripheral nucleobase groups. Watson-Crick base-pair formation results in a Ru-Os diad showing efficient Ru -,Os energy-transfer across the hydrogen-bonded interface. The third example describes a Ru-Re diad in which the flexible bridging ligand incorporates a diazacrown macrocyclic unit. Binding of Ba2+ into this macrocycle at 77 K results in a decrease in the rate of Re --> Ru photoinduced energy-transfer by a factor of 30, probably because of a conformational change which causes the Ru and Re components to move further apart. The final example is of a Ru-Os diad in which the [Ru(bipy)(3)](2+) and [Os(bipy)(3)](2+) components are separated by a flexible poly(oxoethylene) 18-atom chain whose conformation is solvent dependent. Changing the solvent polarity results in a conformational change in the chain, and consequently a change in the Ru . . . Os separation and hence the Ru --> Os energy-transfer rate. Thus, the long-range energy-transfer interaction can be controlled by the polarity of the solvent. (C) 2001 Elsevier Science B.V. All rights reserved.