UCLA

Crystalline solids are a promising platform for the development of molecular machines due to their ability to take advantage of the preorganization and proximity of molecular elements and their potential for displaying emergent properties. Chapter 1 introduces a framework that can be used to design amphidynamic crystals and explain their dynamic behavior. A brief discussion of the history of molecular machinery is followed by the motivation for studying them in the solid state. Previous strategies for designing amphidynamic crystalline molecular rotors are discussed in light of an overarching comparison between packing coefficients and rotational rate constants of existing systems, and then a new framework is proposed. In this framework, extended porous solids with large cavities that can accommodate rotational motion and molecular crystals formed by close packing interactions are considered separately and their dynamics are explained by different factors. The rotational rates of extended porous solids are attributed to the intrinsic barriers of rotation around the bonds that form the axle of rotors, and these systems are considered to be activation-controlled. The rotational rates of close-packed molecular crystals are attributed to the ability of atoms surrounding rotators to undergo small-amplitude displacements and create transient cavities that accommodate rotational motion, an ability termed “crystal fluidity.” These systems that rely on crystal fluidity are considered to have inertial rotation and be diffusion-controlled. The chapter finishes with a discussion of the implications of these classifications.

While solid-state molecular machines have the potential to display unique properties, the observation of their motion can be challenging. Chapters 2 and 3 address methods to sense molecular motion using two different approaches based on fluorescence spectroscopy. In Chapter 2, the synthesis and characterization of a crystalline molecular rotor featuring an aggregation-induced emission (AIE) fluorophore is reported. Due to the reliance of both rotational motion and fluorescence intensity on crystal fluidity, we envisioned that the system would display an inverse relationship between rotational dynamics and fluorescence intensity, allowing for the measurement of rotational rate with fluorescence spectroscopy. The fluorescence lifetime of a powdered sample was on the order of 4–5 ns but showed no discernable trend between 77 K and 298 K, which was interpreted to indicate that the AIE fluorophore and its surroundings were essentially static over the fluorescence lifetime at all temperatures. This was supported by solid-state NMR studies that showed rotational dynamics between 198 K and 298 K in the kHz (microsecond) regime, which is three orders of magnitude too slow to affect fluorescence emission.

While Chapter 2 focuses on an indirect method for measuring a potential relationship between rotational dynamics and crystal fluidity using fluorescence spectroscopy, the work in Chapter 3 explores the use of a more direct method: fluorescence anisotropy decay. Six different acenes with axially linked trialkylsilylethynes of different sizes were synthesized and characterized in high-viscosity mineral oil solutions. The steady-state and time-resolved anisotropy of the systems were observed in the mineral oil solutions in order to calculate the rotational correlation times of each molecule.

Chapter 4 describes an experiment designed for the high school level to help promote interest in nanoscience and expose younger students to the fundamentals of absorption spectroscopy, color, enzymes, and sensing in a qualitative, low-cost manner. The experiment features a glucose sensing assay encapsulated in a hydrogel bead. The students swell the beads in the assay and then expose them to a variety of conditions in a well plate in order to explore the sensing pathway of the assay, elucidate the advantages of encapsulating it in a nanostructured environment, create a calibration curve for sensing sugar, and determine the amount of sugar in sweetened drinks.