Fundamental Properties of Organic Nanocrystals and Photochemical Intermediates in their Crystal Lattices
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Fundamental Properties of Organic Nanocrystals and Photochemical Intermediates in their Crystal Lattices

  • Author(s): Hipwell, Vincent Michael
  • Advisor(s): Garcia-Garibay, Miguel A
  • et al.

This dissertation describes the use of organic nanocrystalline suspensions for the investigation of solid-state photochemical reaction mechanisms, the elucidation of fundamental properties of photochemical intermediates embedded within the crystal lattice, and a novel study of size-dependent photosalience in crystals of a compound subject to a solid state [2+2] photocycloaddition reaction. To perform these studies, the nanocrystalline suspensions formed from rapid precipitation of crystalline material via the addition of water-miscible solutions of the compound of interest to water or dilute surfactant solution uniquely enabled the use of transmission spectroscopy methods such as laser flash photolysis in order to gain detailed information about photochemical intermediates and the kinetics of solid-state reactions. In addition, the relatively new method of microcrystal electron diffraction was utilized for the first report of crystal structures solved before and after a single-crystal-to-single-crystal reaction in a single microcrystal specimen.Chapter One is a brief overview of the history of organic solid-state photochemistry, highlighting key advancements over many decades. A particular focus will be applied to the more recent advancements that have enabled various forms of solid-state photochemical mechanistic analysis, especially the use of nanocrystalline suspensions for transmission spectroscopy. Chapter Two describes a series of adamantylacetophenone compounds which serve as scaffolds for competitive -Hydrogen abstraction from multiple sites leading to the formation of divergent Norrish-Yang cyclization products. This work offered new insight into the nature of Norrish Type II reaction selectivity through a combination of laser flash photolysis, photoproduct determination, and other mechanistic analysis techniques in both solution and solid-state samples. It showed that the reaction selectivity is governed not only by the geometric parameters of the parent ketones, but also on varying C-H bond dissociation energies, and the steric parameters and lifetimes of biradical intermediates. Chapters Three and Four focus on the exceptional lifetimes of acyl-alkyl radical pair intermediates generated in crystals of diphenylmethyl- and triphenylmethyl-containing ketones, respectively. Diphenylmethyl ketones with various -adamantyl substituents were shown to be photostable with respect to decarbonylation, but still generated acyl-alkyl radical pairs which had triplet lifetimes an order of magnitude longer than those of any analogous radical pair or biradical previously reported. Upon this discovery, a series of triphenylmethyl ketones were synthesized in hopes of further extending this lifetime such that crystals of these compounds might serve as a promising platform for the generation of qubit pairs with correlation lifetimes on the millisecond scale. A subset of the triphenylmethyl ketones were photostable and displayed triplet radical pair lifetimes up to >4 ms by leveraging the stability of the iconic Gomberg radical. Chapter Five introduces preliminary results related to the effect of an external magnetic field on the lifetimes of radical pairs and biradicals in solution and the crystalline solid state. A macrocyclic ketone was used to generate an acyl-alkyl biradical tethered by a flexible aliphatic chain. In solution, this biradical has much greater conformational freedom than when it is embedded within a crystal lattice. This has a substantial effect on the singlet-triplet gap, as was shown by laser flash photolysis experiments under varying external magnetic field strengths. Chapter Six describes the discovery of a size-dependent threshold for the observation of photosalience in crystals that facilitate a [2+2] photocycloaddition reaction upon ultraviolet irradiation. Microcrystal electron diffraction was used to establish that the expected single-crystal-to-single-crystal reaction still occurs in small microcrystals, while transmission electron microscopy showed that they do not exhibit photosalience below a particular size. This novel discovery has potentially significant implications for the deployment of these types of crystals for use as nano- and micromechanical actuators controlled by external stimulus.

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