Influence of triplet-triplet absorption processes on the delayed fluorescence emission of polyriboadenylic acid

A biphotonic delayed fluorescence emission (DF) is observed from polyriboadenylate helices. The strong dependence of the DF quantum yield upon the excitation wavelength can be explained taking into account triplet-triplet absorption and photoionization processes which occur during the UV excitation. The DF emission is explained by an anion-cation recombination leading to excited adenine rings. ne union-cation pair formation is due to the trapping of photoejected electrons by a parent stacked adenine ring in the polyrA helix. Monovalent cations have a marked effect an the DF quantum yield. A model, involving the electric Field of monovalent cations bound to the negatively charged phosphates of the polyrA chain, is presented which accounts for their effect on the DF quantum yield. Solvated electrons are also produced during the photoionization process. no-it visible-light-stimulated recombination with photoionized adenine rings leads to the emission of phosphorescence and fluorescence of adenine. The variation of the photoionization cross section with UV excitation wavelength parallels that observed with the DF quantum yield except at wavelengths longer than 290 nm.