Date of Award

Spring 2016

Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Steinmetz, Mark G.

Second Advisor

Donaldson, William A.

Third Advisor

Timerghazin, Qadir


Photochemical cleavage reactions have found a widespread used in biological applications that require the photorelease of biologically active molecules such as proteins, peptides, neurotransmitters, and nucleotide phosphates. This research focuses on the design of photoremovable protecting groups which can be utilized to release these biomolecules by photolysis. These biomolecules are attached to the photoremovable protecting group at the sites of functional groups that are present within these substrates. Such functional groups are carboxylates, phosphates, thiolates, or phenolates, which upon exposure to light, are released as anions of varying basicities. The photochemical reaction involved is an electrocyclic ring closure between aromatic groups that are bridged by a carboxamide linkage. The key intermediate produced by electrocyclization is thought to have zwitterionic character. This zwitterionic intermediate is believed to expel the leaving group as the anion. One of the aromatic groups attached to the amide carbonyl carbon has been a benzothiophene ring system with leaving groups such as LG- = Cl-, PhS-, HS-, PhCH2S- that are present at the C-3 position. The photochemical expulsion of these LG-s is experimentally known to occur in triplet excited state. Furthermore, the initial step in the mechanism has been shown to involve the transfer of triplet excitation energy from the chromophore to the benzothiophene ring. When this energy transfer is unfavorable, energetically, the quantum yield is expected to be low. This is the case when thioxanthone is the chromophore and benzothiophene is the energy acceptor. The projects described herein involve efforts to lower the triplet excited state energy of the energy acceptor to make the triplet excitation energy transfer step more efficient. With benzothiophene as energy acceptor this step is endothermic. This project replaces the benzothiophene ring system with a naphtho[1, 2-b]thiophene so that energy transfer will be somewhat exothermic. An additional replacement attempted to use phenyl-2-thienyl ketone as energy acceptor. With the naphtho[1,2-b]thiophene energy acceptor the expulsion of the C-3 LG- = Cl- is more efficient (Φ = 0.084) than with the benzothiophene ring system under comparable aqueous buffered conditions using 385 nm light to generate the initial triplet excited state of thioxanthone chromophore. With benzothiophene as energy acceptor Φ = 0.035. Quenching experiments show that the photocyclization occurs via a short-lived triplet excited state localized primarily on the naphthothiophene ring. The initial cyclization must therefore be a very fast reaction. The limiting step likely is the triplet energy transfer step, which is only 46% efficient. Another source of inefficiency is the intersystem crossing step of the thioxanthone, which is 67% efficient. Subsequent to energy transfer, the remaining reaction steps are 27% efficient. These efficiencies are obtainable through a series of sensitized photolyses using xanthone and thioxanthone and the anilide obtained by replacing the thioxanthone chromophoric group and are thought to be representative of those for the actual thioxanthone linked by carboxamide to naphthothiophene.