The field of organic solid state photochemistry has great potential for new and exciting discoveries as a result of its youth in being able to determine the identities of the intermediates produced upon irradiation of a photochemically active molecule. Studying solid state photochemical reaction mechanisms has proven quite challenging due to the inherent optical properties of crystalline solids, such as birefringence, dichroism, and scattering, that must be overcome in order to observe the true photochemistry of the system under investigation. The rigid environment within a solid, in particular crystalline solids, is advantageous because it essentially programs a certain amount of chemical information into the system that will dictate if and how the photochemical reaction proceeds. The work described in this dissertation demonstrates the utility of using nanocrystalline suspensions as a simple and robust method to significantly reduce the difficulties associated with studying reactions in crystals. Specifically the solid state kinetics and photochemical mechanism of diarylcarbenes and the spiropyranmerocyanine photochromic system have been investigated using nanocrystalline suspensions in order to expand our knowledge of photochemical reactivity in the solid state.
Chapter One introduces the field of organic solid state photochemistry and uses historical examples to demonstrate several of its fundamental principles, such as the topochemical postulate. The advantages to using solid state photochemistry is also discussed along with the challenges associated with studying photochemical reactions in solids. Specifically, the use of nanocrystalline suspensions is examined as a method capable of addressing the optical problems that are connected with studying solid state photochemical reactions spectroscopically. Additionally, the photochemistry of photochromic molecules and carbenes is highlighted in order to briefly illustrate the utility of solid state photochemistry.
In Chapter Two, the solid state photochromism of spiropyrans is investigated under steady-state irradiation conditions. A homologous set of nitro-substituted spiropyrans with different N-alkyl substituents was selected in order to study the effects of crystal packing on the thermal decay kinetics. It was found that the merocyanine has a biexponential lifetime 10–100 times longer in nanocrystalline suspensions than the single exponential lifetime found in solution, which indicated that the crystal lattice might impede multiple merocyanine structures from interconverting rapidly enough to produce a single kinetic signature.
In Chapter Three, nanosecond transient absorption spectroscopy was used to study the photochemical ring-opening reaction for a 6-nitroindolinospiropyran (SP1) in solution and in nanocrystalline (NC) suspension. The kinetics in argon-purged and air-saturated acetonitrile
were measured and evidence of a triplet excited state species was found along with evidence for two ground state species. Laser flash photolysis studies performed in NC suspensions initially showed a very broad, featureless absorption spectrum that decayed uniformly for ca. 70 ns before revealing a more defined spectrum that is consistent with a mixture of the ground state Z- and E-merocyanine structures. DFT calculations suggested that the broad, featureless transient absorption spectrum results from the contribution of the transition structure and other high-energy species during the Z- to E-merocyanine isomerization.
In Chapter Four, the nanosecond laser flash photolysis of a 1,1’-biphenyl-2-phenyldiazomethane was investigated both in solution and in the solid state. Our results showed the presence of a single transient species both in n-hexane and in nanocrystalline (NC) suspension that we hypothesize to be the triplet excited state of the 9-phenylfluorenyl product. UV-vis analysis of our solutions and NC suspensions pre and post laser flash photolysis strongly support the formation of the cyclized 9-phenylfluorenyl product, which also indicates that the photochemical mechanism could be the same in both media. The only difference we observed between the solution and solid state data was the decay lifetime for the transient species in NC suspension was much longer (47 μs) than the solution lifetime (1.3 μs).
