Chemiluminescence During Photooxidation of Melanins and Soil Humic Acids Arising From a Singlet Oxygen Mechanism*

Photooxidation of synthetic melanins and natural soil humic acids in a model system: polymer + OH‐+ O2+ solvent, initiated by irradiation with wavelengths longer than 300 nm, is accompanied by chemiluminescence with the maximum absolute intensity 104− 5 × 105 photons s‐1. Prolonged photoirradiation results in the decrease of both dark and photoinduced EPR signals and decrease of absorption in the spectral range 240–600 nm, indicating destruction of the aromatic core of polymers. Rate of photooxidation and chemiluminescence intensity increases in D2O, in comparison with H2O, by a factor of about 3. Inhibitors of free radicals and O21Δg‐quenchers diminish chemiluminescence by 20–97%. Chemiluminescence spectra measured with cut‐off filters and a calibrated photo‐multiplier operating in the single photon counting mode have emission bands at 480–500, 570 and 615–650 nm. The red band has been assigned to the emission from 2 O2 (1Δg 1Δg) dimols, and the blue‐green band to the sensitized chemiluminescence of fluorescing photooxidation products.