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, pseudorotaxane-type, mechanical molecular machines based on wires made up of an electron-transfer photosensitizer covalently linked to an electron acceptor. Compounds (2,2'-bipyridine)₂Ru(2,2'-bipyridine-5-(CH₂)-1-(4,4'- bipyridinium)-1'-CH₂-R)⁴⁺ (1⁴⁺), (4,4'-(Me)₂-2,2'-bipyridine)₂Ru(2,2'- bipyridine-5-(CH₂)₄-1-(4,4'-bipyridinium)1'-CH₂-Me)⁴⁺ (2⁴⁺), and (2,2':6',2''-terpyridine)Ru(2,2'.6',2''-terpyridine-4'-phenylene-2-(2,7- diazapyrenium)-7-CH₂-R)⁴⁺ (3⁴⁺), where R = -C₆H₄-(O-CH₂-CH₂)₂-O- Ph) have been prepared and their photochemical and photophysical processes have been investigated in butyronitrile fluid solution (room temperature) and rigid matrix (77 K). At room temperature the triplet metal-to-ligand charge- transfer (³MLCT) excited state of the Ru-based unit of 1⁴⁺ is quenched by a very fast (k(q) > 5 x 10⁹ s^-1) electron-transfer process. For 2⁴⁺, where the Ru-based and electron-acceptor units are separated by four methylene groups, the value of the quenching constant is 6.2 x 10⁸ s^-1. In 3⁴⁺, the potentially fluorescent S₁ excited state of the diazapyrenium unit is quenched by the Ru-based moiety with a rate constant ≥1 x 10¹¹ s^{- 1}. In rigid matrix at 77 K, the ³MLCT excited state of the Ru-based moiety is not quenched by the bipyridinium or diazapyrenium moiety, whereas both the fluorescence and phosphorescence of the diazapyrenium moiety of 3⁴⁺ are completely quenched by the MLCT levels of the Ru-based moiety through energy transfer. Excitation spectra of the Ru-based emission show that, in a rigid matrix at 77 K, the excitation of the bipyridinium moiety leads to population of the ³MLCT excited state of the Ru-based moiety. The above wires and a crown ether (1/5DN38C10) containing two 1,5-dioxynaphthalene electron-donor units self-assemble to give pseudorotaxane systems. Light-induced dethreading of a pseudorotaxane has been achieved and valuable information has been gathered concerning the design of more efficient systems. A spin-off of these studies has been the design of pseudorotaxanes in which the dethreading/rethreading process can be controlled by chemical stimuli.