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Energetics and lattice dynamics of the 2,5-distyrylpyrazine solid state reaction
Abstract
With the proliferation of possible applications for condensed-phase organic materials, solid-state reactions have attracted considerable interest. Unfortunately, much of what is known about these systems is limited to geometrical considerations of the crystalline environment. In this research, the energetics and lattice dynamics of the 2,5-distyrylpyrazine solid-state reaction are investigated in order to extend the physical understanding of solid-phase reactions. The first, crystal-to-crystal, oligomerization stage is the focus of most of the study. Low-temperature, specular reflection measurements revealed that the oligomerization of DSP is biexcitonic. Local $\pi\sp\*\gets$ n exciton-phonon coupling creates a lattice distortion which traps the delocalized $\pi\sp\*\gets\pi$ exciton to initiate the reaction. The second, polymerization, stage is induced by a single $\pi\sp\*\gets$ n excitation, connecting the unreacted styrylpyrazine ends. The pyrazine ring is crucially important in determining the reactivity of this crystal. The phonon-assistance and chemical pressure models were investigated to determine their applicability to the DSP photoreaction. Single-crystal, polarized, Raman spectroscopy of the lattice phonon modes at varying stages of the reaction demonstrated that the changing lattice potential energy matches the expectations of the chemical pressure hypothesis. Although several modes were found to soften early in the reaction, phonon assistance involving phonon-phonon interactions does not play an important role. This became obvious from the study of modal anharmonicity using piezomodulated Raman spectroscopy. Structural studies were done correlating energetic and lattice dynamical changes to the structural transformations. The monomer and the partially oligomerized crystals were studied using atomic force microscopy. This allowed investigation of the local effects of the reaction, revealing that the crystal is finely segregated into domains of reacted and unreacted molecules. This agrees with the X-ray crystallography of the partially oligomerized crystal. X-ray measurements of the completely oligomerized crystal showed that the lattice changes to reflect the geometry of the oligomers. The unreacted styrylpyrazine ends are severely distorted to fit the cavity allowed by the oligomer crystal. These findings further demonstrate the effects of the evolving chemical pressure upon the crystal lattice.
Subject Area
Chemistry
Recommended Citation
Peachey, Nathaniel Mark, "Energetics and lattice dynamics of the 2,5-distyrylpyrazine solid state reaction" (1994). ETD collection for University of Nebraska-Lincoln. AAI9416919.
https://digitalcommons.unl.edu/dissertations/AAI9416919