Hypothetical Efficiency of Electrical to Mechanical Energy Transfer during Individual Stochastic Molecular Switching Events

Abstract

There are now many examples of single molecule rotors, motors, and switches in the literature that, when driven by photons, electrons, or chemical reactions, exhibit well-defined motions. As a step toward using these single molecule devices to perform useful functions, one must understand how they interact with their environment and quantify their ability to perform work on it. Using a single molecule rotary switch, we examine the transfer of electrical energy, delivered via electron tunneling, to mechanical motion and measure the forces the switch experiences with a noncontact q-plus atomic force microscope. Action spectra reveal that the molecular switch has two stable states and can be excited resonantly between them at a bias of 100 mV via a oneelectron inelastic tunneling process which corresponds to an energy input of 16 zJ. While the electrically induced switching events are stochastic and no net work is done on the cantilever, by measuring the forces between the molecular switch and the AFM cantilever, we can derive the maximum hypothetical work the switch could perform during a single switching event, which is ∼55 meV, equal to 8.9 zJ, which translates to a hypothetical efficiency of ∼55% per individual inelastic tunneling electron-induced switching event. When considering the total electrical energy input, this drops to 1 × 10 −7 % due to elastic tunneling events that dominate the tunneling current. However, this approach constitutes a general method for quantifying and comparing the energy input and output of molecular-mechanical devices.

Autonomous use of electrical energy by an artificial molecular machine

“…Remarkably, such energy efficiency is significantly higher than those of other autonomous artificial systems reported to date. 3,4,5,6,49 We have operated our system also under opposite boundary conditions, i.e., Etip = -0.8 and Esub = 0 V, proving that multiple non-equilibrium steady-states are experimentally accessible using this system. 50 While the recorded current (-0.8 nA) is similar to the one measured under opposite boundary conditions, the distribution of species is completely different.…”

Autonomous Non-Equilibrium Self-Assembly and Molecular Movements Powered by Electrical Energy

“…Remarkably, such energy efficiency is significantly higher than those of other autonomous artificial systems reported to date. 15,18,24,25,48 We have operated our system also under opposite boundary conditions, i.e., Etip = -0.8 and Esub = 0 V, (see SI Section 6,7, Figures S8,S18,S19) proving that multiple non-equilibrium steady-states are experimentally accessible using this system. 49 While the recorded current (-0.8 nA) is similar to the one measured under opposite boundary conditions, the distribution of species is completely different.…”