Ajeet's Research

  
Home About Me Photos Research Weiss Lab

  Curriculum Vitae

Controlled Motion of Single-Molecule Motors in Precise Nanoscale Assemblies
 

There has been significant interest and activities in understanding and mimicking nature’s complex molecular machines.  Artificial molecular machines, having a variety of movable parts, can be interlocked to realize machine-like directed motion, and thus, can be potentially used as components in the fabrication of intelligent nanodevices.  My research goal is to assemble various types of single-molecule motors in precise nanoscale assemblies on a substrate and to induce and to control motion via light, redox reactions, and ion binding.  We employ scanning tunneling microscopy (STM) to measure the motions generated at atomic scales, and microscopic methods to follow the actions of assemblies.

 We have driven reversible photo-induced switching of single azobenzene-functionalized molecules isolated in tailored alkanethiolate monolayer matrices on Au{111}.*  Reversible photo-isomerization of single molecules between trans and cis conformations was achieved by cycling exposure to visible and UV light, and were assigned to the ON and the OFF states of the molecule, respectively, when imaged with an ambient STM (Figure 1).  Control over stochastic switching was achieved by appropriately designing the tether and tuning the rigid assembly of the molecule to limit surface quenching and non-photo-induced switching effects.1  To test the effects of nanoscale environments on switching efficiencies, the azobenzene-functionalized molecules were also prepared as chains of molecules (1D) and islands of molecules (2D).  We observed that the efficiency of photo-switching decreases from isolated molecules to chains of molecules to islands of molecules. We are also attempting to measure the work function difference between two states due to the dipole moment change after isomerization using Kelvin probe microscopy.

 

 

 

 

Figure 1: Isolated azobenzene-functionalized molecules are embedded within 1-decanethiol monolayer domains photoisomerize from trans to cis and back, respectively, with UV and Vis light.
 

We drive microscale motions in a cantilever actuator using collective force generated by a monolayer of redox-controllable, palindromic bistable [3]rotaxane molecules (artificial molecular muscles) controlled electrochemically (Figure 2).  The deflections of the cantilever were monitored both as a function of (i) the scan rate (£25 mV s-1)  and (ii) the time for potential step experiments at oxidizing  (>+0.4 V) and reducing (<+0.2 V) potentials of rotaxanes.  The microcantilevers deflect in one direction following oxidation, and in the opposite direction upon reduction. The ~300 nm deflections were calculated to be commensurate with forces per molecule in excess of 100 pN.  The use of the cooperative forces generated by these self-assembled, nanometer-scale artificial molecular muscles that are electrically wired to an external power supply constitute a seminal step towards molecular-machine-based nanoelectromechanical systems (NEMS).  We have also been able to follow the motion of single rotaxane molecules under electrochemical control with a stabilized STM.

 

  
 

 

 

 

 

Figure 2: Schematic of the experimental setup used for in situ electrochemical activation of palindromic bistable [3]rotaxanes (R8+) molecules.
 

We have observed double-decker (DD) molecules, having two parallel pophyrin/phthalocyanine rings connected by rare earth metal cations, assembled on an HOPG substrate.  The spacing and orientations can be selectively controlled by tuning functionality on or off the surface.2  Using single-molecule STM manipulation, we can rearrange individual DD molecules within a chain and induce a concerted rotary motion.

 
 

References:

1. Kumar et al. Nano Lett. 2008, 8, 1644-1648.

2. Ye et al. J. Am. Chem. Soc. 2006, 128, 10984-10985.

*Editor’s choice for Science issue dated 05/30/2008.