Date of Award
Doctor of Philosophy
William S. Jenks
Dibenzothiophene oxide (DBTO) produces dibenzothiophene (DBT) as the only detectable sulfur containing photoproduct under certain conditions, but indirect evidence also indicates the formation of O(3P). The quantum yield of deoxygenation increases in the presence of oxygen trapping solvents. From these and other results, a mechanism for the deoxygenation was proposed which invokes a unimolecular S--O bond scission via a bond stretch coupled to intersystem crossing.;Heavy atom substituted DBTOs show higher efficiency of phosphorescence and a moderate increase in the deoxygenation quantum yield. The trend order of iodo > bromo > chloro ~ H allows assignment of this to a heavy atom effect.;Internal trapping groups were appended to the 4-position of DBTO to attempt to increase the efficiency of the deoxygenation. Photolysis of these substituted DBTOs showed an increase in the quantum yields and a detectable amount of intramolecular trapped products. The quantum yield of deoxygenation was less solvent dependent for this series of DBTOs. However, in cyclohexene no intramolecular trapped product was detected, presumably because, cyclohexene as a solvent is a better trapping group than the appended alkenyl and sulfenyl groups.;Nonetheless, detection of intramolecular trapped products suggests that solvent effects observed in previous studies of DBTO derive at least mainly from the reactivity between the oxidizing species that is released---presumably O(3P)---and the solvent's functional groups, rather than from other macroscopic solvent parameters, such as polarity and viscosity.;The S--O bond breaking energy in thiophene-S-oxide (TO) is estimated to be about 15 kcal/mol less than that of DBTO, whereas its first excited singlet state has almost the same energy as DBTO. Stable substituted TO derivatives were prepared with the hope that they would produce O(3P) with higher efficiency. The quantum yield of deoxygenation for 2,5-bis(trimethylsilyl) TO in benzene was about 14 times higher than for DBTO. However, only about half of the disappearance was accounted for by the appearance of the corresponding thiophene. Other thiophene oxides produced furan or unknown products. There could be an analogy between the deoxygenation mechanism of DBTO and TO but it is too early to speculate.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Nag, Mrinmoy, "Sulfoxides of thiophene and dibenzothiophene: a mechanistic study of photochemical deoxygenation " (2005). Retrospective Theses and Dissertations. 1843.