Singlet oxygen radiationless decay

Merkel PB, Kearns DR. Radiationless decay of singlet molecular oxygen in solution. An experimental and theoretical study of electronic-to-vibrational energy transfer. J Am Chem Soc 1972 94 7244-7253. 

The low fluorescence quantum yield and the fact that no phosphorescence could be observed in oligothio-phenes leads to the conclusion that most of the electronic excitation energy decays radiationless. There are different views of the participation of triplet states in this decay process in thin films. In solution, however, the relaxation via triplet states is well agreed. Tliis is due to the fact that singlet oxygen is produced and the absorption attributed to triplet-triplet excitation (i.e. at 1.80 eV [139] respectively 1.92 eV [38], theoretically predicted value around 2.7 eV [172]) is completely quenched by exposure to air. 

Oclkrug et al. could not find any evidence of singlet oxygen in 5T aggregates, in contrast to solution [ 152]. Therefore a radiationless deactivation via internal conversion or sudden polarization (see Section 4.2) is concluded. The latter intermolecular electron-hole pair creation as decay channel is also proposed by the group... 

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