Quenching and photosensitization

Another class of carbonyl compounds that has received much attention is the cyclohexadienones. The primary photolysis product of 4,4-diphenylcyclohexadienone, for example, is 5. Quenching and photosensitization experiments have indicated that the... 

Quenching and photosensitization experiments on representative examples have indicated that the reaction proceeds through a triplet excited state. A scheme that delineates the bonding changes is outlined below ... 

Irradiation of olefins and dienes in the presence of oxygen and various sensitizers produces hydroperoxides and cyclic peroxides. Effective sensitizers include not only high-energy carbonyl triplets, but also low-energy organic dyes such as methylene blue, rose bengal, chlorophyl, and riboflavin. The same species that quench the phosphorescence of these complex molecules also quench their photosensitizing ability. 

Simons, J.P. and Tasker, P.W. (1973). Energy partitioning in photodissociation and photosensitization Part 2. Quenching of Hg(63Pi) by CO and NO and the near ultra-violet photodissociation of ICN, Mol. Phys. 26, 1267-1280. 

The non-exponential decay of the triplet excited states of the photosensitizers is observed for Chi-, Phe- and ZnTPP-containing vesicles [132-135], The reason for the non-exponentiality may be, first, a statistical distribution of the concentrations of porphyrin molecules in the membrane, and, second, a simultaneous decay of the triplet excited states via several parallel channels such as spontaneous deactivation, concentration quenching and triplet-triplet annihilation which are known to be characteristic of porphyrins in organic solvents [129]. For ZnTPP and Phe in vesicles, the process of triplet-triplet annihilation is indeed observed [56, 134], while according to [132] this process is surprisingly absent for Chi. 

Juan C, Stefflova K, Niedre K, et al. Protease-triggered photosensitizing beacon based on singlet oxygen quenching and activation. J Am Chem Soc 2004 126 11450-1. 

Zheng G, Chen J, Stefflova K, Jarvi M, Li H, Wilson BC. Photodynamic molecular beacon as an activatable photosensitizer based on protease-controlled singlet oxygen quenching and activation. Proc Natl Acad Sci USA 2007 104 8989-94. 

The photolysis of organic materials has also been accomplished by the oxidative quenching of photosensitizers. For example, oxidative quenching of Ru(bpy)3 by MV + in the presence of NADH leads to MV+ and NAD+ through the sequence of reactions outlined in Eqs. 15-17. The reduced photoproduct, MV, mediates H2 evolution in the presence of Pt colloid (Eq. 18), while NAD mediates the two-electron oxidation of ethanol (Eq. 19) or lactic acid (Eq. 20) biocatalyzed by the respective enzyme [225]. 

Ayman, A., Abdel Shah, A. A., Ward, M. D., Schmidt, R., Mechanism of Quenching by Oxygen of the Excited States of Ruthenium(II) Complexes in Aqueous Media. Solvent Isotope Effect and Photosensitized Generation of Singlet Oxygen, 02( Ag), by [Ru(diimine)(CN)4 2 Complex Ions, Dalton Trans. 2007, 2517 2527. 

Pena, J.M., Allen, N.S., Edge, M., Liauw, C.M., Roberts, 1., Valange, B. Triplet quenching and antioxidant effect of several carbon black grades in the photodegradation of LDPE doped with benzophenone as a photosensitizer. Polym. Degrad. Stab. 70, 437-454 (2000)... 

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