Photonics of dissolved oxygen molecules. Comparison of the rates of direct and photosensitized excitation of oxygen and reevaluation of the oxygen absorption coefficients

• Oxygenation rates of singlet oxygen traps were compared upon direct and photosensitized excitation of dissolved oxygen.
• Absorption coefficients in IR maxima of oxygen absorption bands were evaluated from relative oxygenation rates.
• Relative intensities of the oxygen bands at 765 and 1273 nm strongly increase with the increase of solvent polarity.
• Trap oxygenation rates are identical upon oxygen excitation by continuous and pulse laser radiation.
• The 765 nm light is more appropriate for direct oxygen excitation in aqueous and biomedical systems.

The rates of photooxygenation of singlet oxygen traps (1,3-diphenylisobensofuran, tetracene and rubrene) were compared in air-saturated carbon tetrachloride upon porphyrin-photosensitized and direct excitation of oxygen molecules. At equal power of incident radiation, the ratio of oxygen excitation rates normalized to porphyrin absorbance was found to be ∼104. Continuous and pulse (peak power ≤20 MW/cm2) laser radiation of equal average power caused similar rates of direct oxygen excitation. Improved procedure of data analysis was developed and accurate values of absorbance Amax, molar absorption coefficient εmax and the cross section of light absorption σmax were obtained for the oxygen absorption maxima at 1273, 765 nm and 1073 nm. The results were employed for reevaluation of the absorption coefficients of oxygen in other solvents, which were studied using carbon tetrachloride as the reference. It was found that the ratios A1273/A765; ε1273/ε765 and σ1273/σ765, which were equal to 7/1 in carbon tetrachloride, decreased in polar solvents and reached 1.5/1 in water. This effect was shown to be due to the decrease of ε1273 and σ1273 on going from non-polar solvents to water, whereas ε765 and σ765 are less sensitive to solvents and slightly increased with the increase of solvent polarity. The obtained data are important for dosimetry of laser radiation in biomedical experiments and suggest that the radiation at 765 nm is more appropriate for oxygen excitation in biological systems since its efficiency is similar to that of the radiation at 1273 nm, but dark red light penetrates deeper into tissues and causes weaker heating of water.

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