UCLA

Artificial molecular machines have drawn much interest over the past twenty years. Several physical organic chemists have been actively involved in the study and development of different types of molecular machines. In nature many biological macromolecules have the ability to act as "molecular machines" and generate work. The most well known example of this is ATPsynthase, which functions by rotation of a central stock in order to convert ADP to ATP. These biological "molecular machines" are typically highly complex and consist of several subunits. However it is in this complexity that the function of these biological machines is manifested. In order to attain this level of complexity in artificial molecular machines, a gradual development of concepts and a deeper understanding of intermolecular interactions are required. Several molecular machine motifs have been identified and further functionality has been explored by many physical organic chemists, incorporating useful material properties. One way to create artificial molecular machines is by controlling non-brownian motion. A common motif is a system that has two distinct energy states that can interconvert between one another in response to an external stimulus. These types of molecular machine motifs can be used for generating materials that are capable of energy harnessing. Research in the Garcia-Garibay group has focused on the use of light as an external stimulus to facilitate a conformational change between two distinct states in a molecular rotor.

Chapter 2 of this thesis will discuss the functionalization of molecular rotors with salicylideneanilines rotators. In work done in collaboration with Dr. Braulio Rodriguez-Molina, three different photoresponsive molecular rotors were synthesized, characterized, and crystallized. The photochromism of these solids was tested, with all systems converting from the low energy cis-enol to the transient trans-keto. The best performing of these three different molecular rotors was identified by the generation of the most trans-keto upon photoexcitation. A deuterated rotator analog was synthesized and the activation energy for rotation in the solid state was determined. Prior to this work, the preferred isomerization pathway for salicylidenenanilines was through a bicycle pedal mechanism, which does not account for rotation. The ssNMR data for the deuterated analog indicated that isomerization could also occur through rotation.

Chapters 3 and 4 will discuss the functionalization of molecular rotors with salicylideneanilines stators to act as potential molecular brakes. In chapter 3, an amino substituted stator was synthesized and characterized. Crystallization proved challenging due to the preference of these compounds for the formation of spherulites, which are radial fibrous microcrystalline aggregate structures. Nonetheless, solid-state photochromism of the spherulites was determined for both systems by diffuse reflectance with the generation of a transient trans-keto form after photoexcitation. In chapter 4, an ester-substituted stator was synthesized and characterized. Single crystals could not be obtained and ssNMR was utilized to explore both the photochromism and thermochromism of the salicyladenanilines.

In chapter 5 of this thesis, I will discuss the synthesis and characterization of tetra-substituted salicylideneaniline adamantanes. Adamantanes are unique because they are the simplest diamonds and provide a rigid framework with minimal stress. Furthermore, tetra-substitution of adamantane generates a tetrahedral geometry around the adamantane core. Two adamantane systems were synthesized and single crystals were grown. However, only the connectivity was determined, due to poor quality crystals. These generated molecules could have up to 64 isomers, depending on the conformations of the four salicylideneanilines moieties. The photochromism of these solids was then tested with both systems generating the transient trans-keto species.