Open Access
Kumar, Ajeet
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
April 29, 2010
Committee Members:
  • Paul S Weiss, Dissertation Advisor
  • Paul S Weiss, Committee Chair
  • Albert Welford Castleman Jr., Committee Member
  • John V Badding, Committee Member
  • Seong H Kim, Committee Member
  • single-molecule switch
  • single-molecule motor
  • molecular machines
  • molecular electronics
  • self-assembly
  • artificial molecular-muscle
  • scanning tunneling microscopy
We explore functional molecules and assemblies on substrates. We induce motion, structural changes, and electronic changes in molecules and assemblies, which we measured using novel instrumentation. We have controlled photo-induced isomerization of single azobenzene-functionalized molecules isolated in tailored n-alkanethiolate self-assembled monolayer matrices. We engineered the molecular design to suppress excited-state quenching from metal substrates and to form rigid assemblies of single tethered azobenezene molecules in the domains of the monolayer to limit steric constraints, and tip-induced and stochastic switching effects. We prepared one-dimensional chain assemblies of azobenzene-functionalized molecules in the domain boundaries of n-alkanethiol matrices. We switched the chains of molecules from trans to cis using UV light and observed that the molecules in the chains isomerize in concert. This concerted switching was attributed to electronic coupling between the molecules within the chains. We employed electron-induced isomerization of azobenzene in the chains and established that π orbital overlap of the molecules in the one-dimensional chain structures enables the electrons to delocalize along the chain. We created assemblies of redox-active bistable rotaxane molecules on microscale levers to produce forces that bend the levers reversibly and generate microscale motion. Assembled rotaxane molecules generate cooperative forces, much like artificial molecular muscles, and constitute a seminal step toward molecular-machine-based nanoelectromechanical systems. We also controlled the conformational changes in rotaxanes at the single-molecule level and observed that conformational changes correlate with the known redox states of rotaxanes. We observed that mechanical motions of these molecules are strongly influenced by their interactions with the surface and with neighboring molecules. Assemblies of double-decker molecules, having two parallel porphyrin or phthalocyanine rings connected by rare earth metal cations, were created to study both rotary motion dynamics of isolated double-deckers, and the collective interactions of ensembles whose rotors interact as intermeshed gears. We demonstrate the ability to control the placements in arrays by lateral manipulations using a scanning tunneling microscope probe tip. These studies demonstrate our abilities to create and to place isolated functional molecules and their assemblies. We are able to induce and to understand motion and structural and electronic changes in these nanoscale machineries assembled via bottom-up techniques.