Singlet Oxygen Kinetics in Polymeric Photosensitizers

Nanoparticles built up from polythiophenes act as photosensitizers without embedding any additional chromophores. The self-organized water-soluble nanoparticles in this work are made from polythiophene precursors with alkyl side chains which were combined with diphosphatidylcholine-polyethylene glycol in different mass ratios. The fluorescence and singlet oxygen quantum yields of such nanoparticles can be tuned by varying the mass ratio of the two components. Two unique properties of these polymeric photosensitizers result in special singlet oxygen kinetics. First, the backbone orientation of the polythiophene influences the probability of triplet excitons, which have a high mobility across all the nanoparticle and, second, oxygen can diffuse in and out of the polymeric photosensitizer. Therefore, most of the singlet oxygen is generated inside the nanoparticle, close to the surface, soon after oxygen diffuses in. After generation, the majority of the singlet oxygen diffuses out of the nanoparticle. Using highly sensitive time- and spectrally resolved singlet oxygen phosphorescence detection, the oxygen diffusion can be confirmed, and the observed kinetics and quantum yield variations can be explained based on the polymer–semiconductor model. Whenever singlet oxygen kinetics in polymeric nanostructures are investigated, such oxygen diffusion effects have to be taken into account.