Uric acid and/or its monoanion has long been used as chemical-trapping agents to demonstrate the presence of singlet oxygen, O (a Δ ), in aqueous systems. "Oxidative bleaching" of uric acid, generally monitored through changes in the uric acid absorption spectrum, is often used in support of claims for the intermediacy of O (a Δ ). The bleaching of uric acid has also been used to quantify photosensitized O (a Δ ) yields in selected systems. Unfortunately, experiments performed to these ends often neglect processes and phenomena that can influence the results obtained. For the present study, we experimentally examined the behavior of uric acid under a variety of conditions relevant to the photoinitiated creation and subsequent removal of O (a Δ ). Although the oxidative destruction of uric acid can indeed be a useful tool in some cases, we conclude that caution must be exercised such as not to incorrectly interpret the data obtained.

Fluorescein, and derivatives of fluorescein, are often used as fluorescent probes and sensors. In systems where pH is a variable, protonation/deprotonation of the molecule can influence the pertinent photophysics. Fluorination of the xanthene moiety can alter the molecule’s pKa such as to render a probe whose photophysics remains invariant over a wide pH range. Di-fluorination is often sufficient to accomplish this goal, as has been demonstrated with compounds such as Oregon Green in which the xanthene moiety is symmetrically difluorinated. In this work, we synthesized a non-symmetrical difluorinated analog of Oregon Green which we call Athens Green. We ascertained that the photophysics and photochemistry of Athens Green, including the oxygen-dependent photophysics that results in the sensitized production of singlet oxygen, O2(a1Δg), can differ appreciably from the photophysics of Oregon Green. Our data indicate that Athens Green will be a more benign fluorescent probe in systems that involve the production and removal of O2(a1Δg). These results expand the available options in the toolbox of fluorescein-based fluorophores.

Spectra and absorption coefficients of the forbidden 765 nm X 3 Σ g − → b 1 Σ g + transition of molecular oxygen dissolved in organic solvents at atmospheric pressure were recorded over a 5 m path length using a liquid waveguide capillary cell. The results show that it is possible to investigate this weak near-infrared absorption transition in a common liquid hydrocarbon solvent without the need for a potentially dangerous high oxygen pressure. Proof-ofprinciple data from benzene, toluene, chlorobenzene, bromobenzene, and iodobenzene reveal a pronounced heavy atom effect on this spin-forbidden transition. For example, the absorption coefficient at the band maximum in iodobenzene, (28.9 ± 3.3) × 10 −3 M −1 cm −1 , is approximately 21 times larger than that in benzene, (1.4 ± 0.1) × 10 −3 M −1 cm −1 . These absorption measurements corroborate results obtained from O 2 (X 3 Σ g, which depended on data from a plethora of convoluted experiments. Spectroscopic studies of molecular oxygen in liquid solvents can help evaluate aspects of the seminal Strickler−Berg approach to treat the effect of solvent on Einstein's A and B coefficients for radiative transitions. In particular, our present results are a key step toward using the O 2 (X 3 Σ g − ) → O 2 (b 1 Σ g + ) transition to evaluate the speculated limiting condition of applying the Strickler−Berg treatment to a highly forbidden process. This latter issue is but one example of how an arguably simple homonuclear diatomic molecule continues to aid the scientific community by providing fundamental physical insight.