Chemistry - A European Journal p. 2413 - 2422 (1998)
Update date:2022-07-31
Topics:
Ashton, Peter R.
Ballardini, Roberto
Balzani, Vincenzo
Constable, Edwin C.
Credi, Alberto
Kocian, Oldrich
Langford, Steven J.
Preece, Jon A.
Prodi, Luca
Schofield, Emma R.
Spencer, Neil
Stoddart, J. Fraser
Wenger, Sabine
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.
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Doi:10.1016/S0277-5387(98)00181-8
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