Modelling of synthetic molecular motors and self-assembled monolayers
Open Access
- Author:
- Barbu, Corina Madalina
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 08, 2008
- Committee Members:
- Vincent Henry Crespi, Dissertation Advisor/Co-Advisor
Vincent Henry Crespi, Committee Chair/Co-Chair
Paul S Weiss, Committee Member
Jorge Osvaldo Sofo, Committee Member
Richard Wallace Robinett, Committee Member - Keywords:
- self-assembled monolayers
rotary motors
electric field
dipolar rotor
power law - Abstract:
- The main goals of this dissertation work are the modelling of the static and dynamic properties and mechanisms of energy dissipation of synthetic molecular motors, as well as modelling of the kinetics of exchange reaction processes in self-assembled monolayers. Chapter 1 presents a brief overview of the field of synthetic molecular motors and some of the theoretical methods used to evaluate their static and dynamic properties: density functional theory and universal force field classical potential. Artificial molecular motors have been created by scientists in order to develop better understanding of the biological ones, to mimic and to augment their functions. We discuss several different types of motors, classified according to the sources of fuel used as input energy, the environment in which they operate, and the type of mechanical-like motion they produce. In chapter 2, we study the static and dynamic properties of a synthetic caltrop-based rotary molecular motor chemically attached to a surface and driven by external rotational electric fields. Our theoretical calculations and simulations show that external rotating electric fields with magnitudes accessible experimentally induce unidirectional and repetitive rotation of the dipole-carrying rotator of the motor. The rotation occurs about the triple bond within the shaft of the motor. Resonances between the external drive and the soft modes associated with the deviation of the shaft of the motor with respect to the vertical axis give rise to a dramatic increase in friction within the motor. In chapter 3, we present a novel mechanism of dissipation in nanoscale and molecular-scale motors. We investigate a situation in which one degree of freedom is pulled out from the thermal bath and given an explicit equation of motion, interposed between the bath and the motor. We describe a regime in which the deceleration of an unpowered motor, coupled to a thermal bath via an explicit degree of freedom, follows a power law in time with universal exponent of $t$ equal to -1, rather than a standard exponential decay. We find that the span of the power law regime depends only on four dimensionless parameters and it can cover up to a few hundred elastic collisions between the motor and the damper. Surfaces self-limited to a single layer of molecules on a substrate, known as self-assembled monolayers, have important applications in nanotechnology. Experimental investigations show evidence that extit{n}-dodecanethiol molecules in solution displace mbox{1-adamantanethiolate} self-assembled monolayers on Au{111}, leading to complete molecular exchange. In chapter 4, we attempt to model the kinetics of the displacement process and we find that it can be described by the Johnson-Mehl-Avrami-Kolmogorov model of perimeter-dependent island growth for the whole range of mbox{ extit{n}-dodecanethiol} solution concentrations studied. Rescaling the growth rate at each concentration collapses all the data onto a single universal curve, suggesting that the displacement is a purely geometrical, scale-free process. Synthetic rotary motors, consisting of planar organic ligands with metal ions sandwiched in between, have gained a lot of attention recently. In chapter 5, we present some preliminary results for barriers to rotation in lanthanide double-decker complexes. Density functional theory calculations performed on isolated luthetium double-decker complexes, with no side-substituents added on the ligands, reveal substantial barriers to rotation. The modulation of the rotational barrier with size and position of ligand side-substituents, or metal ion, is proposed as a next step.