UC Riverside

Solid-state nuclear magnetic resonance (NMR) has been frequently used in solving protein structures and material sciences. The benefits from solid-state NMR, such as Magic-Angle-Spinning, contribute a lot to improving the resolution of NMR spectrum, which is important in protein structure assignments. This thesis will bring another view of benefits from solid-state NMR. The photochemical reaction of materials can be clearly drawn by solid-state NMR during the photodimerization. Single-crystal NMR is another powerful tool for investigating the photochemical reaction, which has orientation dependence. The spin-lattice relaxation curve is also useful for determining the domain interactions and domain sizes.

Characteristics of anthracene-9-carboxylic acid tert-butyl ester (9TBAE) during the photodimerization were investigated in chapter two. The objective is to prove the photoreaction will form a metastable intermediate crystal and this intermediate structure will have the ester sidegroups pointing inward prior to rotating to the lower energy outward-facing position. A newly designed single crystal goniometer probe was used for 13C CP NMR experiments of 9TBAE. The design details, benefits and performance of this home-built probe are presented in this chapter. This probe has the two axes adjustment system and Hall Effect sensor for sample tube orientation. The ester sidegroups rotation was determined by the change of quartet peaks based on 13C dipolar coupling.

The photodimerization and T1 measurement of 9-methylanthracene (9-MA) by 13C solid-state NMR were investigated in chapter three. 1H T1 relaxation curves were obtained for both the monomer and the dimer. A series of Solid-state NMR spectra provide the process of photodimerization based on various UV exposure times. The spin diffusion rate between the monomer and the dimer is quite small from a global fitting of spin diffusion model. It will contribute to determine that the domain size is over 1000 nm, which indicates the formation of large dimer domains in our sample.

The photochemical dynamics of crystals composed of 4-chlorocinnamic acid (4Cl-CA) was investigated in chapter four, whose photochemistry is dominated by an irreversible {2+2} photodimerization reaction. The 13C CPMAS solid-state NMR spectra showed the whole progress of photodimerization. While 4Cl-CA is probably not of practical interest as a photomechanical material, our results provide evidence that the photoinduced twisting of this class of photoreactive crystals is a general phenomenon. We discuss possible mechanisms for this size dependence, and propose a mechanism by which the mechanical response of photoreactive molecular crystals may be enhanced by reducing the crystal dimensions.