Photooxidation of isobenzofurans. Dual mechanism process

Abstract

The photooxidation of several substituted isobenzofurans has been studied. Kinetic behavior at low substrate concentrations shows that a second mechanism other than that involving singlet oxygen is operative, and the absence of azulene quenching demonstrates that the second mechanism involves direct addition of oxygen to excited singlet isobenzofuran. Rate constants for this singlet substrate-ground state oxygen reaction are in the range of 10 M_1 sec-1 and the mechanism is calculated to contribute about 10% of the photooxidation at high isobenzofuran concentration, ranging upward to virtually 100% at lower substrate concentrations. The mechanism is suggested to be via charge-transfer rather than concerted addition and the possibility of such an oxidation mechanism is generalized to other aromatic systems.A great deal of evidence has been amassed in the past few years to indicate that photoinduced addition of molecular oxygen to several types of substrates proceeds primarily if not exclusively via excited oxygen molecules in the (^g) electronic state.2

Mechanism of ethidium bromide fluorescence enhancement on binding to nucleic acids

“…These results can be interpreted according to the following reaction scheme (Olmsted and Akashah, 1973;Kearns, 1971) ('So, 'S, and 3S refer to ground state, first excited singlet, and triplet state excited sensitizer molecules, respectively; D = DPI, the singlet oxygen acceptor).…”

“…Solutions were excited at 546 nm by a 200-W Hg lamp filtered by Corning CS 3-69 and 4-96 filters. The substrate was 10-6 M 1,3-diphenylisobenzofuran, and its disappearance by sensitizer-triplet-generated singlet oxygen photooxidation (Olmsted and Akashah, 1973) was determined by fluorescence intensity measurements using 404-nm excitation and viewing emission through a Corning 3-72 filter. Control solutions lacking sensitizer displayed no loss of fluorescence when irradiated under these conditions.…”

Photosensitized oxidation by singlet oxygen. An adaptable photochemical system

“…(1) even further. The mechanism of the reaction that transforms A into P is probably similar to other reactions of singlet oxygen with dienes to yield, initially, endoperoxides; quantum yield measurements on the appearance of P or careful isolation techniques to obtain and characterize intermediates, e.g., AO2, would be useful for further elucidation of the reaction mechanism for advanced students.5 Perhaps other 5 The pure endoperoxide intermediate, formally the monozonide of a bicyclobutadiene derivative, thermally reacts to give P in solution, but is explosive and must be handled only with great care, if isolated; see Ref. (£).…”

The role of geminate reaction in the self-sensitized photooxidation of arylazonaphtholate dyes

“…This behavior is similar to that of the acceptor 1,3-diphenylisobenzofuran (DPBF) reported by Wilson4 and by Olmsted and Akashah. 5 Wilson pointed out the possible involvement of a "cage" reaction. 4 The experiments of Olmstead and Akashah suggested that this residual oxidation involved direct addition of oxygen to the excited singlet state.…”