DOI: 10.1002/chem.200500371 A Photoactive Molecular Triad as a Nanoscale Power Supply for a Supramolecular Machine Sourav Saha, Erik Johansson, Amar H. Flood, Hsian-Rong Tseng, Jeffrey I. Zink,* and J. Fraser Stoddart* [a] Introduction Molecular recognition and self-assembly processes are now being harnessed [1] under the umbrella of template-directed synthesis to produce active supramolecular and molecular components in the rapidly evolving fields of nanoelectron- ics [2] and nanoelectromechanical systems (NEMS), [3] because of their interesting stereoelectronic and mechanical proper- ties [4] that are tunable even at a nanoscale level. Pseudoro- taxanes [5] and bistable rotaxanes [6] are amongst some of the best-qualified candidates for the active supramolecular or molecular components of artificial molecular machines [7,8] in fully integrated electronic circuitry. The structural unique- ness of interlocked molecules involving two or more nonco- valently bound components allow us to organize, power, and control their actuation at the molecular level in devices by employing external stimuli, such as those provided by chem- ical, electrochemical, and photochemical inputs. To maintain size and dimension, nanoscale machinery requires a nano- scale power supply. As a means of incorporating a source of electrical energy directly into molecular mechanical systems, we have studied a photoactive donor–chromophore–accept- or molecular triad [9,10] that mimics [11] the photosynthetic energy transduction process by harnessing light energy to generate electrical current. In this program of research, we have demonstrated that the photoactive molecular triad 1, whose structural formula is revealed in Figure 1a, can gener- ate electrical energy at the expense of light energy, the wavelength of which has been tuned specifically to the ab- Abstract: A tetrathiafulvalene–porphy- rin–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 compo- nents, 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-assem- bled 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 com- ponent (diameter ~ 1 nm). In a closed electronic circuit, a triad-SAM func- tionalized working-electrode generates a switchable photocurrent of ~ 1.5 mAcm 2 when irradiated with a 413 nm Kr-ion laser, a wavelength which is close to the porphyrin chro- mophore)s absorption maximum peak at 420 nm. The electrical energy gener- ated 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 p-electron-deficient tetracationic cyclobis(paraquat-p-phenylene) (CBPQT 4 + ) cyclophane and a p-elec- tron-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 fluores- cence intensity of the BHEEN unit. A gradual increase in the fluorescence in- tensity of the BHEEN unit concomi- tant 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 generat- ed by the triad-SAM. Keywords: actuation · electron transfer · fluorescence · photocur- rents · pseudorotaxanes [a] S. Saha, E. Johansson, Dr. A. H. Flood, Dr. H.-R. Tseng, Prof. J. I. Zink, Prof. J. F. Stoddart The California NanoSystems Institute and Department of Chemistry and Biochemistry University of California, Los Angeles 405 Hilgard Avenue, Los Angeles, CA 90095–1569 (USA) Fax:(+1)310-206-1843 E-mail: zink@chem.ucla.edu stoddart@chem.ucla.edu # 2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem.Eur.J. 2005, 11, 6846 – 6858 6846