Autonomous artificial nanomotor powered by sunlight Vincenzo Balzani†‡, Miguel Clemente-Leo´ n†§, Alberto Credi†‡, Bele´ n Ferrer†¶, Margherita Venturi†, Amar H. Floodʈ, and J. Fraser Stoddart‡ʈ †Dipartimento di Chimica ‘‘G. Ciamician,’’ Universita`di Bologna, via Selmi 2, 40126 Bologna, Italy; and ʈCalifornia NanoSystems Institute and Department of Chemistry and Biochemistry, 405 Hilgard Avenue, University of California, Los Angeles, CA 90095 Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved December 6, 2005 (received for review October 14, 2005) Light excitation powers the reversible shuttling movement of the ring component of a rotaxane between two stations located at a 1.3-nm distance on its dumbbell-shaped component. The photoin- duced shuttling movement, which occurs in solution, is based on a ‘‘four-stroke’’ synchronized sequence of electronic and nuclear processes. At room temperature the deactivation time of the high-energy charge-transfer state obtained by light excitation is Ϸ10 ␮s, and the time period required for the ring-displacement process is on the order of 100 ␮s. The rotaxane behaves as an autonomous linear motor and operates with a quantum efficiency up to Ϸ12%. The investigated system is a unique example of an artificial linear nanomotor because it gathers together the follow- ing features: (i) it is powered by visible light (e.g., sunlight); (ii)it exhibits autonomous behavior, like motor proteins; (iii) it does not generate waste products; (iv) its operation can rely only on in- tramolecular processes, allowing in principle operation at the single-molecule level; (v) it can be driven at a frequency of 1 kHz; (vi) it works in mild environmental conditions (i.e., fluid solution at ambient temperature); and (vii) it is stable for at least 103 cycles. molecular machine ͉ nanoscience ͉ photochemistry ͉ rotaxane ͉ supramolecular chemistry he miniaturization race is encouraging scientists to design Scheme 1. Structural formulas of rotaxane 16ϩ, its dumbbell-shaped com- Tand construct motors on the nanometer scale, that is, at the ponent 26ϩ, and model compound 32ϩ. molecular level (1–5). Such a daring goal finds its scientific origin in the existence of natural molecular motors (6–9). Natural molecular motors, however, are extremely complex, namely triethanolamine and oxygen, with concomitant genera- 6ϩ and any attempt to construct systems of such a complexity, using tion of waste products. In this work, we show that 1 does the bottom-up molecular approach (10), would be challenging. behave as an autonomous ‘‘four stroke’’ motor powered only by What can be done, at present, is to construct simple prototypes sunlight, without generation of waste products. We have mea- of artificial molecular motors and machines (1–5, 11–19), con- sured the rate constants and the efficiencies of the electronic and sisting of a few components capable of moving in a controllable nuclear processes on which the shuttling movement is based, and way, and to investigate the associated problems posed by inter- we also discuss the related energetic aspects. facing them with the macroscopic world (20–25), particularly as Just like their macroscopic counterparts, molecular-level ma- far as energy supply is concerned. chines have to be organized structurally and work as functionally 6ϩ Natural motors are ‘‘autonomous’’: they keep operating, in a integrated multicomponent systems (1). The rotaxane 1 con- constant environment, as long as the energy source is available. sists of as many as six molecular components suitably chosen and By contrast, apart from a few recent examples (26–28), the assembled to achieve the devised function. It comprises a fuel-powered artificial motors described so far are not autono- bis-p-phenylene-34-crown-10 electron donor macrocycle R mous because, after the mechanical movement induced by a (hereafter called the ring) and a dumbbell-shaped component chemical input, they need another, opposite chemical input to that contains two electron acceptor recognition sites for the ring, Ј 2ϩ Ј Ј namely a 4,4 -bipyridinium (A1 ) and a 3,3 -dimethyl-4,4 - reset, which also implies generation of waste products. Addition 2ϩ of a fuel, however, is not the only means by which energy can be bipyridinium (A2 ) units, that can play the role of ‘‘stations’’ for supplied to a chemical system. In fact, nature shows that, in green the ring R. Molecular modeling shows that the overall length of 6ϩ Ϸ plants, the energy needed to sustain the machinery of life is 1 is 5 nm and the distance between the centers of the two ultimately provided by sunlight. Energy inputs in the form of photons can indeed cause mechanical movements by reversible Conflict of interest statement: No conflicts declared. chemical reactions without formation of waste products (13, 14, This paper was submitted directly (Track II) to the PNAS office. 16, 17). Abbreviation: SCE, saturated calomel electrode. In a previous work (29), we reported on the rotaxane 16ϩ ‡To whom correspondence may be addressed. E-mail: [email protected], (Scheme 1) that was carefully designed and synthesized to [email protected], or [email protected]. perform as a linear molecular motor powered exclusively by §Present address: Instituto de Ciencia Molecular, Universidad de Valencia, Dr. Moliner 20, visible light, with autonomous operation relying on intramolec- 46100 Burjassot, Spain. ular processes. In such a work, however, we only succeeded in ¶Present address: Departamento de Quı´mica,Universidad Polite´cnica de Valencia, Camino demonstrating the operation of the rotaxane as a light-driven de Vera s͞n, 46022 Valencia, Spain. mechanical switch that needed alternate addition of two ‘‘fuels,’’ © 2006 by The National Academy of Sciences of the USA 1178–1183 ͉ PNAS ͉ January 31, 2006 ͉ vol. 103 ͉ no. 5 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0509011103 Downloaded by guest on October 2, 2021 2ϩ acceptor power of the A1 station and therefore the ring R moves back again by Brownian motion to surround this station. We have carried out in acetonitrile solution spectroelectrochemi- cal experiments by exhaustive one-electron reduction of 16ϩ and 26ϩ at a potential of Ϫ0.50 V vs. SCE. The spectra so obtained (Fig. 1) display the characteristic absorption bands, at Ϸ390 and 600 nm, of Ј ϩ the one-electron reduced form of 4,4 -bipyridinium (A1 ) moiety. There is no evidence for the formation of the one-electron reduced Ј Ј ϩ formof3,3-dimethyl-4,4 -bipyridinium (A2 ), which has a substan- tially different absorption spectrum. 2ϩ 2ϩ Photochemical Switching. The [Ru(bpy)3] -type unit P (Scheme 1) is capable of playing the role of a photosensitizer for ring shuttling because visible light excitation leads to the formation of a long-lived strongly reducing excited state. Except for luminescence quantum Fig. 1. UV-visible difference absorption spectra obtained upon electrochem- yield and lifetime (see below), the photophysical and electrochem- ical reduction of 16ϩ (solid line) and 26ϩ (dashed line) at Ϫ0.50 V vs. SCE in a ical properties of the photosensitizer P2ϩ unit contained in 16ϩ and Ϫ Ϫ spectroelectrochemical thin-layer cell (5.0 ϫ 10 4 mol⅐liter 1 acetonitrile so- 26ϩ are the same as those exhibited by the model compound 32ϩ: lution at room temperature). The spectra were normalized to the intense ␭ ϭ ␧ ϭ ⅐ Ϫ1⅐ Ϫ1 6ϩ absorption, max 458 nm, max 14,800 liters mol cm ; band peaking at 392 nm for 1 .(Inset) Detail of the spectra in the 370–405 ␭ ϭ ␶ ϭ ⌽ ϭ nm region. emission, max 618 nm, 880 ns; em 0.065; excited-state Ϫ19 2ϩ ؉ energy Ϸ2.1eV(1eVϭ 1.602 ϫ 10 J); E1/2(P ͞P ) ϭϩ1.15 V vs. SCE. It is worthwhile recalling that the P2ϩ unit can harvest sunlight quite efficiently because it displays a broad and intense stations, measured along the dumbbell, is Ϸ1.3 nm. Further- absorption band in the visible region. more, the dumbbell-shaped component incorporates a 2ϩ The overall mechanism devised to perform the light-driven [Ru(bpy)3] -type (bpy, 2,2Ј-bipyridine) electron transfer pho- 6ϩ 2ϩ shuttling process in the rotaxane 1 is based on the following tosensitizer P that is able to operate with visible light and also four phases (Scheme 2a). plays the role of a stopper, a p-terphenyl-type rigid spacer S that (a) Destabilization of the stable conformation. Light excitation of has the task of keeping the photosensitizer far from the electron the photoactive unit P2ϩ (step 1) is followed by the transfer of an acceptor units, and finally a tetraarylmethane group T as the 2ϩ 2ϩ electron from the *P excited state to the A1 station, which is second stopper, crucial for the template-directed slippage syn- encircled by the ring R (step 2), with the consequent ‘‘deactivation’’ 6ϩ thesis of the rotaxane 1 from the ring R and its dumbbell- of this station; such a photoinduced electron-transfer process has to 6ϩ shaped precursor 2 . compete with the intrinsic decay of *P2ϩ (step 3). (b) Ring displacement. After reduction (‘‘deactivation’’) of the Results and Discussion 2ϩ ϩ 2ϩ CHEMISTRY A1 station to A1 , the ring moves by Brownian motion to A2 Electrochemical Switching. Electrochemical and NMR spectro- (step 4), a step that has to compete with back electron-transfer ϩ 16 ϩ 3ϩ scopic data showed (29) that the stable conformation of is by from A1 to the oxidized photoactive unit P (step 5). This far the one in which the R component is located around the better requirement is the most difficult one to meet because step 4 2ϩ electron acceptor station, A1 , as represented in Scheme 1. When involves only slightly exergonic nuclear motions, whereas step 5 2ϩ the interaction between R and A1 is switched off by selective is an exergonic outer-sphere electron transfer process.