Abstract

Abstract

Abstract

Abstract

Interlocked molecular and supramolecular actuators based on rotaxanes can act as gatekeepers at the entrances to nanopores in silica. Guest dye molecules can then be uploaded and released on demand. These molecular actuators, or nanovalves, reported on p. 685 by Fraser Stoddart, Jeffrey Zink, and co-workers, can be operated by using a wide range of stimuli, and can be regarded as prototypes for highly controllable drug-delivery systems.

This article features both molecular and supramolecular chemistry involving: i) stimuli-induced nanoscale movements within mechanically interlocked molecules; ii) the fabrication of mesoporous silica substrates; and iii) the integration of the mechanically interlocked molecular/supramolecular actuators to act as gatekeepers at the entrances to the silica nanopores into which guest dye molecules can be uploaded and released on demand from the mesoporous silica substrates. The supramolecular actuators are based on two [2]pseudorotaxanes—that is, 1:1 complexes that can be dissociated by external inputs, such as acid/base cycles, electrons, and light. The molecular actuators are based on bistable [2]rotaxanes and can be operated mechanically by using either redox chemistry or electrochemistry. After these pseudorotaxanes and bistable rotaxanes have been attached covalently to the orifices of the silica nanopores, stimuli-controlled mechanical movements within these mechanically interlocked molecules can be harnessed to close and open the nanopores. Therefore, these mechanically interlocked molecules have been employed as nanovalves for controlled sequestering and release of guest dye molecules into and out of the mesoporous silica substrates. These actuators can be regarded as the prototypes of highly controllable drug-delivery systems.

Mesostructured silica thin films and particles prepared by surfactant-templated sol–gel techniques are highly versatile substrates for the formation of functional materials. The ability to deliberately place molecules possessing desired activities in specific spatially separated regions of the nanostructure is an important feature of these materials. Such placement utilizes strategies that exploit the physical and chemical differences between the silica framework and the templated pores. As an example of placement of pairs of molecules, donor and acceptor molecules can be targeted to different regions of mesostructured thin films and energy transfer between them can be measured. The results not only demonstrate the spatial separation but also are used as a molecular ruler to measure the average distance between them. Mesostructured silica is also an excellent support for molecular machines. Molecules that undergo large amplitude motion, when attached to the silica, can function as impellers and nanovalves when activated by light, electrical (redox) and chemical (pH, competitive binding) energy. Derivatized azobenzene molecules, attached to pore walls by using one of the placement strategies, function as impellers that can move other molecules through the pores. Rotaxanes and pseudorotaxanes, placed at pore entrances, function as gatekeepers that can trap and release molecules from the pores when stimulated. Deliberately placed functional molecules on and in mesostructured silica offer many possibilities for both fundamental studies on the nanoscale and for applications in fields as diverse as fluidics, biological drug delivery and controlled release.

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⁺ [BHEEEN ≡ 1,5-bis[2-(2-(2-hydroxyethoxy)ethoxy)ethoxy]naphthalene and CBPQT⁴⁺ ≡ cyclobis(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(III), 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.

This article features both molecular and supramolecular chemistry involving: i) stimuli-induced nanoscale movements within mechanically interlocked molecules; ii) the fabrication of mesoporous silica substrates; and iii) the integration of the mechanically interlocked molecular/supramolecular actuators to act as gatekeepers at the entrances to the silica nanopores into which guest dye molecules can be uploaded and released on demand from the mesoporous silica substrates. The supramolecular actuators are based on two [2]pseudorotaxanes—that is, 1:1 complexes that can be dissociated by external inputs, such as acid/base cycles, electrons, and light. The molecular actuators are based on bistable [2]rotaxanes and can be operated mechanically by using either redox chemistry or electrochemistry. After these pseudorotaxanes and bistable rotaxanes have been attached covalently to the orifices of the silica nanopores, stimuli-controlled mechanical movements within these mechanically interlocked molecules can be harnessed to close and open the nanopores. Therefore, these mechanically interlocked molecules have been employed as nanovalves for controlled sequestering and release of guest dye molecules into and out of the mesoporous silica substrates. These actuators can be regarded as the prototypes of highly controllable drug-delivery systems.

A tetrathiafulvalene–porphyrin–fullerene (TTF–P–C₆₀) molecular triad, which generates electrical current by harnessing light energy when self-assembled onto gold electrodes, has been developed. The triad, composed of three unique electroactive components, namely, 1) an electron-donating TTF unit, 2) a chromophoric porphyrin unit, and 3) an electron-accepting C₆₀ unit, has been synthesized in a modular fashion. A disulfide-based anchoring group was tagged to the TTF end of the molecule in order to allow its self-assembly on gold surfaces. The surface coverage by the triad in a self-assembled monolayer (SAM) was estimated to be 1.4 nm² per molecule, a density which is consistent with hexagonal close-packing of the spherical C₆₀ component (diameter ∼1 nm). In a closed electronic circuit, a triad-SAM functionalized working-electrode generates a switchable photocurrent of ∼1.5 μA cm⁻² when irradiated with a 413 nm Kr-ion laser, a wavelength which is close to the porphyrin chromophore's absorption maximum peak at 420 nm. The electrical energy generated by the triad at the expense of the light energy is ultimately exploited to drive a supramolecular machine in the form of a [2]pseudorotaxane comprised of a π-electron-deficient tetracationic cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) cyclophane and a π-electron-rich 1,5-bis[(2-hydroxyethoxy) ethoxy]naphthalene (BHEEN) thread. The redox-induced dethreading of the CBPQT⁴⁺ cyclophane from the BHEEN thread can be monitored by measuring the increase in the fluorescence intensity of the BHEEN unit. A gradual increase in the fluorescence intensity of the BHEEN unit concomitant with the photocurrent generation, even at a potential (0 V) much lower than that required (−300 mV) for the direct reduction of the CBPQT⁴⁺ unit, confirms that the dethreading process is driven by the photocurrent generated by the triad-SAM.

Das Titelbild zeigt molekulare Motoren auf einem Silicatträger „bei der Arbeit“: Cyclobis(paraquat-p-phenylen)-Gürtel (blau) bewegen sich auf fadenartigen Polyetherketten (rosa) mit elektronenreichen Naphthalin-Einheiten (rot) in der Mitte hin und her. Diese Bewegung wird deutlich an der Lumineszenz der Naphthalin-Einheiten, die auftritt, wenn die Gürtel entfernt werden. Der zeitliche Ablauf der Bewegung ist von links nach rechts dargestellt; man erkennt, wie sich ein Gürtel auf einen Faden setzt und unter Lichteinwirkung wieder von ihm abgestoßen wird. Dies lässt sich durch Lumineszenzspektroskopie genau verfolgen, denn die Intensität nimmt zu, wenn der Gürtel angezogen wird, und ab, wenn er ausgezogen wird. Mehr über diesen Prozess erfahren Sie von Zink, Stoddart und Mitarbeitern auf S. 2513 ff.

Zwei oberflächengebundene Nanomotoren wurden konstruiert: Bei beiden dient das π-elektronenarme tetrakationische Cyclophan Cyclobis(paraquat-p-phenylen) als Zylinder (blau), in dem sich die Bewegung des Kolbens (π-elektronenreiche Dioxynaphthalin-Einheiten; rot und rosa) induzieren lässt, wenn Reduktionsmittel entweder durch Chemikalien zur Verfügung gestellt werden oder durch eine Lichtquelle, wobei dann 9-Anthracencarbonsäure (gelb) als Photosensibilisator dient und Ethylendiamintetraacetat (grün) als „Opferreagens“.

Abstract— Raman spectra of inorganic complexes in excited electronic states are discussed. A brief overview of the field of transient Raman spectroscopy and experimental considerations are presented. Two examples from the author's laboratory are used to illustrate the type of information that can be obtained. The first example, an excited-state Raman spectroscopic study of K₃[Mn(CN)₅NO], is chosen because it illustrates the connections between excited-state molecular structure and vibrational properties. The pump pulse causes a change from a linear sp-hybridized NO containing a triple bond to a bent sp²-hybridized NO containing a double bond. Both the NO stretch and normal modes involving other ligands are measured and interpreted. The second example is chosen to illustrate the vibrational consequences of photoinduced electron transfer. The Raman spectra of W(CO)₄(diimine) complexes (diimine = 2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine, and isopropyl-pyridine-2-carbaldehyde imine) in the lowest tungsten to diimine charge transfer excited state are discussed. The excited-state peaks are assigned to ligand ring deformation modes and to carbonyl stretching modes. The totally symmetric cis-carbonyl stretching mode in the charge transfer excited state is about 50 cm' higher in energy than that of the molecule in the ground electronic state. The increase is interpreted in terms of loss of metal-car-bonyl back-bonding in the charge transfer excited state. Finally, a summary of the field's strengths and difficulties and a brief discussion of the future perspectives are presented.