Abstract— The photooxygenation of allylthiourea (ATU) sensitized by thionine does not occur according to the singlet oxygen mechanism but rather proceeds via the formation of radicals. In oxygen‐free solution the primary process is a redox reaction between the thionine triplet and ATU where a semithionine‐ and an ATU‐radical are formed. In further reaction steps the leuco form of the dye is finally produced (reductive photobleaching; D
R mechanism after Koizumi). The primary process in an oxygen‐containing aqueous solution is the same, since at high concentrations of ATU (0·2M) the amount of semithionine formed by a photolytic flash, as well as the time course of disappearance of semithionine, does not depend on the oxygen content of the solution.
The reformation of thionine following flash photolysis has been investigated with regard to oxygen concentration and pH dependence. Two different excitation intensities were used. A quadratic dependence of thionine reformation on excitation intensity at high oxygen concentration was observed, indicating a reaction between two photoproducts.
The dependence of the reaction rate of semithionine on the ionic strength has been investigated. These experiments show that the reaction partner of semithionine carries a charge of + 1 in oxygen‐free as well as in oxygen‐saturated solution.
Abstract— As we have shown in previous papers, thionine‐sensitized photooxygenation reactions follow a Type I (radical) mechanism. We now demonstrate that, by an appropriate choice of the acceptor and its concentration (solvent: pyridine) or by working in a rigid matrix (ethyl cellulose), the reaction can be switched to a Type II (singlet oxygen) mechanism. The system studied in the present investigation, thionine and 9,10‐dimethylanthracene, represents to a certain extent an intermediate type. Photooxygenation at low DMA‐concentration occurs according to a Type II mechanism as verified by the method of competitive photooxygenation, while in oxygen‐free solutions, the sensitizer is partially photoreduced by the acceptor, which is typical for Type I systems. Whereas the photooxygenation of allylthiourea (ATU) with thionine as sensitizer takes place via radicals at high ATU concentrations, a change to the singlet oxygen mechanism could be observed at low ATU concentrations in pyridine solution.
Abstract— In order to gain further insight into the sensitized photooxygenation of the system thionine, allylthiourea, and oxygen, the influence of the leucothionine, which is formed during the photoreaction, was studied by flash photolysis. In the presence of leucothionine, additional thionine (Λobs= 598 nm) is reformed; i.e., leucothionine is oxidized to thionine by way of a semithionine intermediate (Λbs= 770 nm). This additional semithionine formation due to leucothionine is complete by 30 μsec after the flash. By varying the leucothionine concentration, the flash intensity and the pH it can be shown that the agent which oxidizes leucothionine to semithionine is identical to the agent which transforms semithionine to thionine.
Die durch Thionin photosensibilisierte Sauerstoffübertragung auf Allylthioharnstoff als. Akzeptor läuft nicht nach dem Kautsky‐Mechanismus (d.h. über Singulettsauerstoff) ab, sondern ist ein Redoxprozeß. Die Rückbildung von Thionin nach einem Photolyseblitz wurde in Abhängigkeit von der Sauerstoffkonzentration bei zwei verschiedenen Anregungsintensitäten und pH‐Werten in wäßriger Lösung untersucht. Bei hohen Sauerstoffkonzentrationen wurde eine quadratische Abhängigkeit der Thioninrückbildung von der Anregungsintensität beobachtet, was eine Reaktion zwischen zwei Photoprodukten anzeigt.Bei hohen Konzentrationen (0,2 m) an Allylthioharnstoff hängt weder die gebildete Menge von Semithionin noch der zeitliche Verlauf des Verschwindens von Semithionin vom Sauerstoffgehalt der Lösung ab. Die Abhängigkeit der Reaktionsgeschwindigkeit von Semithionin von der Ionenstärke wurde untersucht. Aus diesen Experimenten ergibt sich, daß der Reaktionspartner von Semithionin sowohl in sauerstofffreier als auch in sauerstoffgesättigter Lösung die Ladung + 1 trägt.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.