Date of Award
Summer 2018
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Steinmetz, Mark G.
Second Advisor
Yi, Chae S.
Third Advisor
Donaldson, William A
Abstract
Two bichromophoric systems were synthesized which have thioxanthone as the chromophore that absorbs the incident light and a naphthothiophene-2-carboxanilide, which serves as a triplet excited state energy acceptor. Upon excitation the triplet excited state of the thioxanthone is populated via intersystem crossing, which is efficient with isc = 0.68. Based on the relative energies of the triplet excited states of thioxanthone (ET = 65 kcal mol-1) and naphthothiophene (ET = 51 kcal mol-1), the subsequent triplet excited state energy transfer to generate the triplet excited state of the naphthothiophene-2-carboxamide should be a very rapid process that is 14 kcal mol1 exothermic. Computational studies have shown that the triplet excited state of the naphthothiophene-2-carboxanilide undergoes an electrocyclic ring closure to ultimately generate an intermediate with zwitterionic character. This intermediate is thought to be responsible for the elimination of a leaving group that is attached to the C-3 position of the thiophene ring. When the thioxanthone chromophore is linked to naphthothiophene-2-carboxanilide via a trimethylene linkage, the release of a leaving group originally attached to the C-3 position of the thiophene ring occurs with = 0.13 – 0.15. These efficiencies can be compared to the thioxanthone sensitized photolysis, in which the triplet energy transfer is intermolecular. The triplet sensitized quantum yield is = 0.13. The close similarity in efficiencies for the linked and unlinked systems indicates that the triplet energy transfer is as efficient when it occurs by a through – bond mechanism involving the trimethylene linker as when it is intermolecular and occurs collisionally between donor and acceptor, which instead requires overlap of the donor and acceptor orbitals. In both cases the energy transfer efficiency is et = 1, given that = iscetr. On the other hand, when the thioxanthone directly replaces the phenyl group of the naphthothiophene-2-carboxanilide the quantum yield is two-fold less than the trimethylene – linked system. Either et < 1 or r < 0.19 due to an unfavorable substituent effect that discourages electrocyclization with the thioxanthone ring.