Abstract
Six singlet oxygen (1O2) photosensitizers (PS) - 5,10,15,20-tetrakis(1-methyl-4-pyridinio) porphine (TMPyP), meso-tetrakis(4-sulfonathophenyl)porphine (TPPS4), Al(III) phthalocyanine chloride tetrasulfonic acid (AlPcS4), eosin Y, rose bengal, and methylene blue - were investigated in terms of their ability to produce delayed fluorescence (DF) in solutions at room temperature. All the PS dissolved in air-saturated phosphate buffered saline ( pH 7.4) exhibit detectable DF, which can be quenched by 10 mM NaN3, a specific 1O2 quencher.
The DF kinetics has a biexponential rise-decay character in a microsecond time domain. Therefore, singlet oxygen-sensitized delayed fluorescence (SOSDF), where the triplet state of PS reacts with 1O2 giving rise to an excited singlet state of PS, is the prevailing mechanism.
It was confirmed by a monoexponential decay of triplet-triplet transient absorption kinetics, dependence of SOSDF kinetics on oxygen concentration, absence of SOSDF in nitrogen-saturated samples, or the effect of isotopic exchange H2O-D2O. Eosin Y and AlPcS4 show the largest SOSDF quantum yield among the selected PSs, whereas rose bengal possesses the highest ratio of SOSDF to prompt fluorescence intensity.
The rate constant for the reaction of triplet state with 1O2 giving rise to the excited singlet state of PS was estimated greater than or similar to 1×109 M-1 s-1. SOSDF kinetics contains information about both triplet and 1O2 lifetimes and concentrations, which makes it a very useful tool for monitoring photosensitizing and 1O2 quenching processes, allowing its detection in the visible spectral region, utilizing the PS itself as a 1O2 probe.
Under our experimental conditions, SOSDF was up to three orders of magnitude more intense than the infrared 1O2 phosphorescence and by far the most important pathway of DF. SOSDF was also detected in a suspension of 3T3 mouse fibroblast cells, which underlines the importance of SOSDF and its relevance for biological systems.