Date of Award
Spring 2018
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Steinmetz, Mark G.
Second Advisor
Yi, Chae
Third Advisor
Timerghazin, Qadir K.
Abstract
The triplet excited state of thioxanthone produced by photolysis undergoes reversible triplet energy transfer with a trimethylene - linked benzothiophene-2-carboxanilide ring system. The ensuing electrocyclic ring closure of the anilide moiety produces a putative zwitterionic intermediate that is capable of expelling leaving groups (LG-) from the C-3 position of the benzothiophene ring. Stern-Volmer quenching studies with cyclohexadiene as quencher furnish the rate constants for the triplet excitation transfer in the forward and reverse directions, which can be expressed as an equilibrium constant K = 0.058. Overall, the rate of triplet excited state reaction becomes K x kr = 5.7 x 104 s-1 for LG- = Cl-, where kr is the triplet decay rate of the C-3 chloro – substituted benzothiophene-2-carboxanilide, found through Stern-Volmer quenching. The high quantum efficiencies found for the trimethylene - linked systems are due to K x kr being competitive with triplet excited state decay of the thioxanthone of kd = 7.7 x 104 s-1. From isc = 0.68, the overall expected quantum yield for direct photolysis should be 0.50 for LG- = Cl- as compared to 0.41 at 25 °C experimentally. decreases with increasing basicity of the leaving group (LG- = (EtO)2PO2-, PhCH2CO2-, PhS-, PhCH2S-, Cl¬-). Photoremovable protecting groups are of great importance in biological, biochemical, and physiological applications. The current work has discovered a highly versatile photoremovable protecting group that can release a wide variety of potentially biologically active leaving groups with high efficiencies at long wavelengths of 395 nm, which will minimize damage to cells and tissue. The released leaving groups have a wide range of basicities. Thus, both weak bases such as carboxylates and phosphates and relatively strong bases such as thiolates are photochemically released.