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

Abstract

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.