Steady-State and Transient Absorption Spectroscopic Studies of Photochemical Mechanisms in Crystalline Solids Utilizing Nanocrystalline Suspensions

Because of the rigidity of the crystal lattice, reactions in organic crystals often proceed with selectivity and efficiency not seen in the analagous solution reactions. However, although there is great promise in solid state photochemistry, it remains a relatively unexplored field. The main reason for this is the lack of mechanistic understanding required to rationally design photoreactions. The use of nanocrystalline suspensions allows for detailed investigation into photochemical reactions in crystalline solids. Traditional approaches and the recently developed nanocrystalline approached will be discussed.;Chapters 2 and 3 discuss the photochemical phenomenon known as a quantum chain reaction. Diarylcyclopropenones undergo an adiabatic decarbonylation upon photolysis. In Chapter 2, the steady state photochemistry of the diaryl cyclopropenones in solution and crystalline environments will be described. The details of the proposed mechanism for the quantum chain reaction in nanocrystalline suspensions of diarylcyclopropenones will be discussed, along with a detailed characterization of the nanocrystalline suspension used in these experiments. In Chapter 3, linked-dimers of diaryl cyclopropenones are explored in solution phase photolysis. Evidence for an unusual Dexter mediated energy transfer in solution is given.;In Chapter 4, the mechanism of decarbonylation of a 2-indanone derivative is explored using transient absorption spectroscopy. The decarbonylation produces two distinct products upon photolysis in solution and the solid state. The details of the reactive intermediates were explored. Although the products are state dependant, we will show that they each arrive from common reactive intermediates that react on similar timescales.;Chapter 5 describes the steady state and transient absorption spectroscopy (nanosecond and femtosecond) for Norrish Type II photoreactions of alpha-adamantyl acetophenones. Using nanocrystalline suspensions, we determined that the diastereoselectivity of these reactions is controlled by two independatant rates, cyclization of the 1,4 biradical, and rotational rate of the alpha-adamantyl moiety.;Chapters 6 and 7 explore the photophysical transient properties of triplet excited states in nanocrystalline suspensions. It was noted early on that the triplet lifetime in nanocrystals was extremely short compared to solution or bulk solids. Surface quenching effects were probed. It will be shown that intermolecular self-quenching is the cause of this short lifetime in the nanocrystalline suspensions.;Chapter 8 details the kinetic trapping of one of the most ubiquitous, but never observed, reactive intermediates oxyallyl. This chapter will detail the experimental and computational efforts to characterize the reactive intermediate, including solid state photochemistry, fs transient absorption spectroscopy, and computational work. Exploration into modifying the crystal structure indicates that the crystal lattice is unique in the ability to trap oxyallyl.