The pH dependence of singlet oxygen quenching by histidine, N-acetyltyrosine ethyl ester (ATEE), ascorbic acid, Trolox C, and tryptophan has been observed using time-resolved infrared luminescence measurements in a D2O/acetonitrile (50:50 v/v) solvent. Deprotonation of ascorbic acid, the protonated imidazole ring of histidine and the phenolic group of ATEE leads to an increase in the quenching rate constants by between 2 and 3 orders of magnitude. Such changes appear to be the basis for wide variations in quoted literature values of singlet oxygen quenching constants for these and related compounds. It is estimated that these pH-dependent quenching rate constants predict a modest (approximately 2- to 3-fold) change in singlet oxygen lifetime between the extremes of cellular pH. Activation data for singlet oxygen quenching show that the enthalpies of activation are low in all cases (between 0 and 11 kJ mol-1) and that substantially negative entropies of activation (between −49 and −116 J K-1 mol-1) result in rate constants being much lower than the diffusion-controlled limit. In all cases the data are consistent with quenching via reversible formation of an exciplex, all reactions being at the preequilibrium limit over the available temperature range.

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We investigated the effect of oxygen on the photostability of the laser dyes Pyrromethene 567, Perylene Orange, and Rhodamine 590 by determining their longevity of laser operation when pumped by the second harmonic of aQ -switched Nd:YAG laser. In solution, dissolved oxygen accelerated the photodegradation of Pyrromethene 567 and Perylene Orange but not Rhodamine 590. The photostability of Pyrromethene 567 was also found to be dependent on the solvent and on the lifetime of singlet oxygen. Deoxygenated Pyrromethene 567 doped polycom glass and modified poly(methyl methacrylate) (MPMMA) samples were tested for longevity of laser operation. A factor of 6 improvement in photostability was found for Pyrromethene 567 in MPMMA upon deoxygenation, and the total absorbed energy per mole of dye molecules to one-half output pulse energy was 36 GJ mol-1 .

In this paper we report a time-resolved investigation of transients derived from curcumin, which may be intimately involved in the processes leading to its biological activity. Fluorescence and triplet quantum yields are respectively 0.06 and 0.11. The high percentage of internal conversion is proposed to proceed via H-transfer within the thermodynamically favored enol structure of what is formally a 1,3-diketone. The triplet energy (191 +/- 2 kJ mol-1), natural lifetime (1.5 microseconds) and self-quenching rate constant (5.0 x 10(8) L mol-1 s-1) have been determined. Oxygen quenching of the triplet leads to the production of singlet oxygen with unit efficiency. Curcumin quenches the latter species very inefficiently (2.5 x 10(5) L mol-1 s-1). The curcumin radical has been produced via three mechanistically distinct methods. This species is unreactive toward oxygen but is repaired by vitamins C and E and anthralin.