Versatile supramolecular nanovalves reconfigured for light activation

Abstract

All autonomous machines share the same requirement-namely, they need some form of energy to perform their operations and nanovalves are no exception. Supramolecular nanovalves constructed from [2]pseudorotaxanes-behaving as dissociatable complexes attached to mesoporous silica which acts as a supporting platform and reservoir-rely on donor-acceptor and hydrogen bonding interactions between the ring component and the linear component to control the ON and OFF states. The method of operation of these supramolecular nanovalves involves primarily the weakening of these interactions. The [2]pseudorotaxane [BHEEEN⊂CBPQT]4+ [BHEEENS l,5-bis[2-(2-(2-hydroxyethoxy)ethoxy) ethoxy]naphthalene and CBPQT4+≡acyclobis(paraquat-p-phenylene)] , when this 1:1 complex is tethered on the surface of the mesoporous silica, constitutes the supramolecular nanovalves. The mesoporous silica is charged against a concentration gradient with luminescence probe molecules, e.g., tris(2,2′-phenylpyridyl)iridium(ni), Ir(ppy)3 (ppy = 2,2′-phenylpyridyl), followed by addition of CBPQT·4Cl to form the tethered [2]pseudorotaxanes. This situation corresponds to the OFF state of the supramolecular nanovalves. Their ON state can be initiated by reducing the CBPQT4+ ring with NaCNBH3, thus weakening the complexation and causing dissociation of the CBPQT4+ ring away from the BHEEEN stalks on the mesoporous silica particles MCM-41 to bring about ultimately the controlled release of the luminescence probe molecules from the mesoporous silica particles with an average diameter of 600 nm. This kind of functioning supramolecular system can be reconfigured further with built-in photosensitizers, such as tethered 9-anthracenecarboxylic acid and tethered [Ru(bpy)2(bpy(CH2OH)2)]2+ (bpy = 2,2′-bipyridine). Upon irradiation with laser light of an appropriate wavelength, the excited photosensitizers transfer electrons to the near-by CBPQT4+ rings, reducing them so that they dissociate away from the BHEEEN stalks on the surface of the mesoporous silica particles, leading subsequently to a controlled release of the luminescent probe molecules. This control can be expressed in both a regional and temporal manner by the use of light as the ON/OFF stimulus for the supramolecular nanovalves. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.