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

Abstract

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 1.4nm2 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 μAcm-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) (CBPQT 4+) 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. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.