Autonomous artificial nanomotor powered by sunlight

Abstract

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 photoinduced 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 following 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 intramolecular 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 10 3 cycles. molecular machine ͉ nanoscience ͉ photochemistry ͉ rotaxane ͉ supramolecular chemistry T he miniaturization race is encouraging scientists to design and construct motors on the nanometer scale, that is, at the 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, and any attempt to construct systems of such a complexity, using the bottom-up molecular approach (10), would be challenging. What can be done, at present, is to construct simple prototypes of artificial molecular motors and machines (1-5, 11-19), consisting of a few components capable of moving in a controllable way, and to investigate the associated problems posed by interfacing them with the macroscopic world (20-25), particularly as far as energy supply is concerned.Natural motors are ''autonomous'': they keep operating, in a constant environment, as long as the energy source is available. By contrast, apart from a few recent examples (26-28), the fuel-powered artificial motors described so far are not autonomous because, after the mechanical movement induced by a chemical input, they need another, opposite chemical input to reset, which also implies generation of waste products. Addition of a fuel, however, is not the only means by which energy can be supplied to a chemical system. In fact, nature shows that, in green plants, the energy needed to sustain the machinery of life is ultimately provided by sunlight. Energy inputs in the form of photons can indeed cause mechanical movements by reversible chemical reactions without formation of waste products (13,14,16,17).In a previous work (29), we reported on the rotaxane 1 6ϩ (Scheme 1) that was carefully designed and synthesized to perform as a linear molecular...

Controlling the Pirouetting Motion in Rotaxanes by Counterion Exchange

“…Firstly, secondary amide thread E- [8] was prepared and subsequent nucleophilic attack by the in situ prepared anion on cyclic oxonium [9] yielded dianionic thread E- [10] with 'tethered' cobaltacarborane units. A hydrogen bond templated clipping rotaxane forming reaction followed by protonation of the pyridine moieties furnished rotaxane E- [11].…”

Nonlinear irreversible thermodynamics of single-molecule experiments

“…Many mesoscopic systems of great interest ranging from biology, such as proteins, RNA or DNA molecules [1][2][3][4][5][6][7][8][9][10], to nano-engeneering, like rotaxanes and catenanes [11][12][13], have similar dynamic behaviors due to their intrinsic properties and structure, as well as to their strong coupling with the surroundings. In particular, when they are subject to stressing conditions, like stretching forces or radiative interaction, the dynamics of these systems may present critical oscillations and stochastic resonance phenomena.…”

Artificial Photochemical Devices and Machines

“…Steady-state and time-resolved spectroscopic experiments together with electrochemical measurements in acetonitrile solution showed [64] that the absorption of a visible photon by 19 6þ can cause the occurrence of a forward and back ring movement, that is, a full mechanical cycle according to the mechanism illustrated in Figure 1.18 [65]. It was estimated that the fraction of the excited-state energy used for the motion of the ring amounts to $10% and the system can generate a mechanical power of about 3 · 10 À17 W per molecule.…”

“…The molecular shuttle 19 6þ can also be operated, with a higher quantum yield, by a sacrificial mechanism [63] based on the participation of external reducing (triethanolamine) and oxidizing (dioxygen) species and by an intermolecular mechanism [64] involving the kinetic assistance of an external electron relay (phenothiazine), which is not consumed. However, operation by the sacrificial mechanism does not afford an autonomous behavior and leads to consumption of chemical fuels and formation of waste products.…”