Broad light harvesting under restricted environment: Photophysical understanding leading to enhanced reactive oxygen species generation

Abstract Surfactant self-assemblies are of great importance due to their suitable attachment affinity with the hydrophobic organic sensitizers in aqueous media. In this work, the two organic photo-sensitizers protoporphyrin (PPIX) and squaraine (SQ2), absorbing in the visible and near-infrared region of the solar spectrum respectively, have been used simultaneously as dual sensitizers. These two sensitizers behave as donor-acceptor pair for F o ¨ rster resonance energy transfer (FRET) in close proximity of restricted micellar environment. This process of dual sensitization on the surface of cationic Cetyltrimethylammonium bromide (CTAB) micelle has been found to generate enhanced reactive oxygen species (ROS) under white light irradiation due to the resonance energy transfer from donor to an acceptor molecule. The choice of the cationic micellar solution has also been justified on the basis of proper sensitizer attachment position with the micelle due to its suitable electrostatic interaction affinity with the sensitizers. The attachment position of the sensitizers with cationic, anionic and nonionic micellar solutions has been confirmed by picosecond resolve fluorescence transients of the sensitizers in solvents of different polarities. Several control studies have been performed with the respective sensitizers alone in presence of white light irradiation and separately in their own absorption regions to confirm the enhanced ROS generation with dual sensitizers under white light irradiation. This positively charged host micelle and generation of ROS can be useful in potential photo remediation application towards selective anionic pollutant. Some preliminary results are also discussed regarding the possibility of anionic pollutant degradation.