Selbstorganisation molekular dimensionierter „Kästen”

A photoactive molecular triad as a nanoscale power supply for a supramolecular machine

A tetrathiafulvalene-porphyrin-fullerene (TTF-P-C(60)) 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 C(60) 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 nm(2) per molecule, a density which is consistent with hexagonal close-packing of the spherical C(60) component (diameter approximately 1 nm). In a closed electronic circuit, a triad-SAM functionalized working-electrode generates a switchable photocurrent of approximately 1.5 microA 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 pi-electron-deficient tetracationic cyclobis(paraquat-p-phenylene) (CBPQT(4+)) cyclophane and a pi-electron-rich 1,5-bis[(2-hydroxyethoxy) ethoxy]naphthalene (BHEEN) thread. The redox-induced dethreading of the CBPQT(4+) 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 CBPQT(4+) unit, confirms that the dethreading process is driven by the photocurrent generated by the triad-SAM.

Versatile Self-Complexing Compounds Based on Covalently Linked Donor-Acceptor Cyclophanes

A range of covalently linked donor-acceptor compounds which contain 1) a hydroquinone (HQ) unit, 2) a 1,5-dioxynaphthalene (DNP) ring system, or 3) a tetrathiafulvalene (TTF) unit as the pi-donor, and 4) cyclobis(paraquat-p-phenylene) (CBPQT(4+)) as the pi-accepting tetracationic cyclophane were prepared and shown to operate as simple molecular machines. The pi-donating arms can be included inside the cyclophane in an intramolecular fashion by virtue of stabilizing noncovalent bonding interactions. What amounts to self-complexing/decomplexing equilibria were shown to be highly temperature dependent when the pi-donating arm contains either an HQ or DNP moiety. The thermodynamic parameters associated with the equilibria have been unraveled by using variable-temperature (1)H NMR spectroscopy. The negative DeltaH degrees and DeltaS degrees values account for the fact that the "uncomplexed" conformation becomes the dominant species, since the entropy gain associated with the decomplexation process overcomes the enthalpy loss resulting from the breaking of the donor-acceptor interactions. The arm's in-and-out movements with respect to the linked cyclophanes can be arrested by installing a bulky substituent at the end of the arm. In the case of compounds carrying a DNP ring system in their side arm, two diastereoisomeric, self-complexing conformations are observed below 272 K in hexadeuterioacetone. By contrast, control over the TTF-containing arm's movement is more or less ineffective through the thermally sensitive equilibrium although it can be realized by chemical and electrochemical ways as a result of TTF's excellent redox properties. Such self-complexing compounds could find applications as thermo- and electroswitches. In addition, the thermochromism associated with the arm's movement could lead to some of the compounds finding uses as imaging and sensing materials.