UCLA

This thesis is presented as three independent sections, all dealing with the interaction of light with organic materials in the solid state. Section 1, consisting of chapters 1 and 2, deals with molecular gyroscopes: a class of amphidynamic materials. Molecular gyroscopes mimic the architecture of their macroscopic namesakes, having a freely rotating component (the rotator) surrounded and shielded by bulky static groups (the stators). Chapter 1 explores the development of a molecular gyroscope which contains the phenyleneethynylene chromophore along its axis of rotation. Chapter 2 discusses the phenyleneethynylene containing molecular gyroscopes with dipolar rotators. Chapter 3 presents a detailed characterization of the photodegradation of bird feathers. Chapter 4 examines the solid-state photodecarbonylation of tetraphenylacetone by laser flash photolysis. A more in-depth look at the contents of each chapter follows:

Chapter 1. Reports the synthesis, crystal structure, solid-state dynamics, and photophysical properties of 6,13-bis((4-(3-(3-methoxyphenyl)-3,3-diphenylprop-1-yn-1-yl)phenyl)ethynyl)-5,7,12,14-tetrahydro-5,14:7,12-bis([1,2]benzeno)pentacene (1), a molecular dirotor with a phenyleneethynylene chromophore. The incorporation of a pentiptycene into the molecular dirotor provides a central stator and a fixed phenylene ring relative to which the two flanking ethynylphenylene rotators can explore various torsion angles; this allows the phenyleneethynylene fluorophore dynamics to persist in the solid state. X-Ray diffraction studies show that molecular dirotor 1 packs so that all the phenyleneethynylene fluorophores adopt a parallel alignment, this is ideal for the development of functional materials. Variable temperature, quadrupolar echo 2H-NMR studies show that phenylene rotator flipping has an activation energy of 9.0 kcal/mol and a room temperature flipping frequency of ~2.6 MHz. Lastly, with measurements in solution, glasses and crystals, evidence was obtained that the fluorescence excitation and emission spectra of the phenyleneethynylene chromophores is dependent on the extent of conjugation between the phenylene rings, as determined by their relative dihedral angles.

Chapter 2. The molecular gyroscope architecture developed in chapter 1 is further elaborated to include a dipolar rotator. This is the first example of molecular gyroscopes that incorporate both the phenyleneethynylene chromophore and dipolar rotators. While the initial dipolar rotator design lead to molecules which failed to crystalize, a redesigned rotator gave crystals which were isomorphic with those discussed in chapter 1 and therefore retained the desired parallel alignment. Intriguingly, the optical properties of the dipolar molecular gyroscopes were less variable than those of their nonpolar parent compound.

Chapter 3. A study of the photo-induced degradation of bird feathers is performed. Unpigmented turkey feathers and carotenoid pigmented scarlet ibis feathers are aged in ultraviolet light, filtered sunlight, and UV-free museum lighting. Aged feathers are examined and photographed under both visible light and UV light; photo-induced changes that are readily apparent under UV illumination are undetectable under ordinary illumination. Aged feathers are characterized by XPS, FTIR, and fluorescence spectroscopy. Spectral changes caused by light aging are qualitatively similar to previously reported changes in light aged wool spectra and are indicative of increasing concentrations of cystine oxidation products in UV aged feathers. GCMS analysis confirms that the cysteic acid to cystine ratio increases with UV exposure. Pigment concentration decreases with exposure to both visible and UV light and loss of pigment is detectable by analysis of feather extract long before visible changes are observed in feathers.

Chapter 4. The first transient absorbance spectra and kinetic data for the photodecarbonylation of a dibenzyl ketone derivative in a microcrystalline suspension are reported. While this class of reaction has been extensively studied under other conditions (solutions, micelles, and in zeolites) the synthetic utility of the solid-state version made it worthy of additional scrutiny. A protocol for the preparation of minimally scattering microcrystalline suspensions of TPA is reported; the crystal packing of these suspensions is similar the bulk powder. Product analysis has shown that while these reactions go cleanly to TPE in the solid state, benzophenone is the major product in solution. The transient absorbance signal for TPA in microcrystalline suspension is slightly (10 nm) red shifted verses the solution spectrum, and the transient lifetimes are significantly longer.