Ru-II polypyridine complexes covalently linked to electron acceptors as wires for light-driven pseudorotaxane-type molecular machines

An investigation has been performed on the design of light-driven, pseudoro taxane-type, mechanical molecular machines based on wires made up of an ele ctron-transfer photosensitizer covalently linked to an electron acceptor. C ompounds (2,2'-bipyridine)(2)Ru(2,2'-bipyridine-5-(CH2)-1-(4,4'-bipyridiniu m)-1'-CH2-R)(4+) (1(4+)), (4,4'-(Me)(2)-2,2'-bipyridine)(2)Ru(2,2'-bipyridi ne-5-(CH2)(4)-1-(4,4'-bipyridinium)-1'-CH2-Me)(4+) (2(4+)), and (2,2':6',2 "-terpyridine)Ru(2,2':6',2"-terpyridine-4'-phenylene-2-(2,7-diazapyrenium)- 7-CH2-R)(4+) (3(4+)), where R= -C6H4-(O-CH2-CH2)(2)-O-Ph) have been prepare d and their photochemical and photophysical processes have been investigate d in butyronitrile fluid solution (room temperature) and rigid matrix (77 K ), At room temperature the triplet metal-to-ligand charge-transfer ((MLCT)- M-3) excited state of the Ru-based unit of 1(4+) is quenched by a very fast (k(q) > 5 x 10(9) s(-1)) electron-transfer process. For 2(4+), where the R u-based and electron-acceptor units are separated by four methylene groups, the value of the quenching constant is 6.2 x 10(8) s(-1). In 3(4+), the po tentially fluorescent S-1 excited state of the diazapyrenium unit is quench ed by the Ru-based moiety with a rate constant greater than or equal to 1x1 0(11) s(-1). In rigid matrix at 77 K, the (MLCT)-M-3 excited state of the R u-based moiety is not quenched by the bipyridinium or diazapyrenium moiety, whereas both the fluorescence and phosphorescence of the diazapyrenium moi ety of 3(4+) are completely quenched by the MLCT levels of the Ru-based moi ety through energy transfer. Excitation spectra of the Ru-based emission sh ow that, in a rigid matrix at 77 K, the excitation of the bipyridinium moie ty leads to population of the (MLCT)-M-3 excited state of the Ru-based moie ty, The above wires and a crown ether (1/5DN38C10) containing two 1,5-dioxy naphthalene electron-donor units self-assemble to give pseudorotaxane syste ms. Light-induced dethreading of a pseudorotaxane has been achieved and val uable information has been gathered concerning the design of more efficient systems. A spin-off of these studies has been the design of pseudorotaxane s in which the dethreading/rethreading process can be controlled by chemica l stimuli.