Photochemistry
of Singlet Oxygen Sensor Green
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Abstract
To
detect singlet oxygen (<sup>1</sup>O<sub>2</sub>), the commercially
available fluorescent sensor named Singlet Oxygen Sensor Green (SOSG)
has been the most widely used from material studies to medical applications,
for example, photodynamic therapy. In light of the previous studies,
SOSG is a dyad composed of fluorescein and anthracene moieties. In
the present study, we carried out quantitative studies on photochemical
dynamics of SOSG for the first time, such as the occurrence of intramolecular
photoinduced electron transfer (PET), <sup>1</sup>O<sub>2</sub> generation,
and two-photon ionization. It was revealed that these relaxation pathways
strongly depend on the irradiation conditions. The visible-light excitation
(ex. 532 nm) of SOSG induced intramolecular PET as a major deactivation
process (<i>k</i><sub>PET</sub> = 9.7 × 10<sup>11</sup> s<sup>–1</sup>), resulting in fluorescence quenching. In
addition, intersystem crossing occurred as a minor deactivation process
that gave rise to <sup>1</sup>O<sub>2</sub> generation via the bimolecular
triplet–triplet energy transfer (<i>k</i><sub>q</sub> = 1.2 × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup>). Meanwhile, ultraviolet-light excitation (355 nm) of SOSG caused
the two-photon ionization to give a SOSG cation (Φ<sub>ion</sub> = 0.003 at 24 mJ cm<sup>–2</sup>), leading to SOSG decomposition
to the final products. Our results clearly demonstrate the problems
of SOSG, such as photodecomposition and <sup>1</sup>O<sub>2</sub> generation.
In fact, these are not special for SOSG but common drawbacks for most
of the fluorescein-based sensors