Design and Construction of Artificial Molecular Pumps

One of the distinguishing features of living organisms is their proclivity to employ molecular machinery on a grand scale to power their metabolic processes. During billions of years of evolution, carrier proteins have developed finely tuned secondary and tertiary structures, capable of harnessing external fuel to exert precise control over the noncovalent forces---the potential energy landscape of energy barriers and wells---experienced by their cargos in order to drive them energetically uphill, temporarily away from thermodynamic equilibrium.;A challenge in contemporary chemistry is the realization of artificial molecular machines that can perform work, in solution and at interfaces, on their environments. This Thesis outlines my efforts to understand the design and construction of artificial molecular machines that are capable of performing work using external energy inputs. In Chapter 1, I chronicle the development of wholly synthetic molecular machines and highlight some of the recent milestones reported in the literature. The point I try to make clear is that research into artificial molecular machines is moving towards building molecules that can perform work. Chapter 2 describes my initial attempts at building a pseudo[2]rotaxane-based unsymmetrical molecular transport system. The results show that harvesting light can continuously drive a system away from its thermodynamic equilibrium. My initial donor-acceptor system, however, lacked the ability to perform external work on account of its intrinsic properties. In Chapter 3, I investigate systematically a radical-based system, which provides pent up structure-function information that can be used to drive artificial molecular machines. Radical-radical interactions are introduced into an energetically demanding molecular transport system in Chapter 4. The key feature of the radical-radical interactions is the fact that their attractive radical-pairing interactions can be flipped to Coulombic repulsions by controlling the redox state of three bipyridinium units. In Chapter 5, I combined all the information I learned from the chemistry reported in Chapters 2--4 to design an artificial molecular pump that attracts a tetracationic cyclophane containing two bipyridinium units onto a cationic dumbbell incorporating one bipyridinium unit under reductive conditions and parks it against its will on a collecting polymethylene chain of a rotaxane. Two cycles have been achieved experimentally with high efficiencies, exhibiting almost identical kinetic profiles for the two successive pumping cycles, indicating that the machine can pump rings repetitively and progressively away from equilibrium.;In Chapter 6, I conclude by summarizing the fundamental progress I made in the design and synthesis of non-equilibrium systems, offering some insight and providing an outlook for the future. I believe the journey towards the goal of creating wholly synthetic molecular machines will doubtlessly result in remarkable tools for harnessing the potential of molecule-upon-molecule assembly, as well as leading to an ever-increasing understanding of how biological molecular machines carry out their remarkable functions.