The photochemistry of 'super' photoacid n-methyl-6-hydroxyquinolinium and other novel photoacids

The present research began with the study of excited-state proton transfer (ESPT) in chiral environments; experiments were conducted that examined the ESPT to/from chiral proton donors to chiral and achiral acceptors. The role of the exergonicity of the reaction and the transition-state position along the reaction coordinate for the existence of an enantiomeric effect was established and compared to previously studied "super" photoacids. A photoracemization was observed for the chiral photoacids (BINOLs), which was ultimately attributed to a "late" transition state similar to a planar achiral binaphtholate anion.;While this work proved fruitful and interesting, the research was limited by the excited-state acidity of the BINOLs, which have excited-state pK a*s greater than zero. Interest in studying photoacids capable of more facile deprotonation in the excited-state led to the design and synthesis of the "super" photoacid, N-methyl-6-hydroxyquinolinium (MHQ).;With the successful synthesis of various MHQ salts, we explored the ESPT dynamics of MHQ and discovered that it is the strongest photoacid reported in the literature to-date with a pKa* ≈ -7. Because little work has been conducted on photoacid salts, one goal of the research was to explore the effects of the counteranion on the ESPT. While we found that the proton-transfer step is independent of the counteranion, results indicate that the counteranion does affect some of the dynamics in these systems. As stated above, understanding the proton-transfer kinetics in these systems was a major goal of our research; therefore, the ESPT reaction of MHQ was studied using both fluorescence upconversion and time-correlated single photon counting techniques. The ultrafast kinetics were investigated in various alcohols and water and determined to be solvent-controlled. The ESPT temperature dependence of MHQ was also studied in various alcohols and compared to the ESPT temperature dependence observed for another "super" photoacid, 5,8-dicyano-2-naphthol (DCN2). A full set of kinetic and thermodynamic parameters describing the ESPT was obtained. The protolytic photodissociation rate constant for MHQ was higher than for DCN2, while the ESPT activation energies of MHQ were smaller. These findings were attributed to the approximately 3 orders of magnitude differences in excited-state acidities of MHQ and DCN2.;The research was then extended to the marriage of experiment and theory in order to further characterize the MHQ kinetics. Due to the complexity of the system upon photodissociation, the typical description of the reversible ESPT within the framework of the Spherically Symmetic Diffusion Problem (SSDP) was not possible. Further, the system is a three-body problem and the presence of counteranion causes additional complications that affect the proton mobility. Consequently, Brownian dynamics (BD) simulations were performed as a tool to reveal the interaction potentials and to elucidate the role of the counterion on the diffusion and reactive properties in the system. The results indicated that the anisotropy of the potential force can be taken into account after adapting this force for use in SSDP. The results of both the BD simulations and the SSDP calculations with the adapted potential were used to fit the experimental kinetics with excellent agreement.;Concurrently, we conducted several experiments exploring potential application of MHQ. Because MHQ exhibits such a dramatic pKa drop in the excited state, its use as a photoinitiator in cationic polymerization was of interest. The photoinitiated cationic polymerization of cyclohexene oxide (CHO) by MHQ through direct ESPT was explored. Although MHQ did, in fact, polymerize CHO upon irradiation in acetone solution, control experiments revealed that the polymerization occurs via irreversible ground-state acid formation. Further, control studies on several reversible photocatalysts for cationic polymerization revealed similar latent acid production. (Abstract shortened by UMI.)...