High singlet oxygen yields from a polymer-supported photosensitizer via superhydrophobicity and control of photosensitizer morphology

Photosensitizers (PSs) dissolved in solvents generate reactive oxygen species, such as singlet oxygen (¹O₂), in high yields, especially when the PS is fully solvated and unaggregated. For many applications, such as water treatment, homogenous phase reactions are not practical because the PS will contaminate the solution and be difficult to recover and reuse. Immobilizing PSs on solid polymer supports is an emerging strategy for ¹O₂ applications, as it prevents the PS from entering the solution and thus enables PS reuse. However, ¹O₂ yields from polymer-supported PS surfaces are much lower than in solvated systems. In this paper, we employ novel approaches to modify surface topography and surface chemistry of the polymer support to significantly increase ¹O₂ yields. To fabricate the surfaces we deposit a fluorinated, water-insoluble porphyrin, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TFPP) onto polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS) surfaces. We demonstrate that superhydrophobic surfaces exhibit a 2.9-fold higher yield of ¹O₂ compared to planar, wetted surfaces, even when the planar surfaces exhibit significant roughness from the addition of silica particles. Modifying the polymer surface chemistry to accelerate PS solution spreading decreases PS crystallite size thereby increasing PS surface area and further increasing ¹O₂ yields. Surface chemistry also affects PS aggregation; the PS forms J-aggregates on PET, but crystallizes in an unaggregated (non-overlapping) form on PDMS. Contrary to conventional assumptions, the PS aggregate state and higher loadings of the PS are not correlated with higher ¹O₂ yields, whereas reducing the size of PS crystallites significantly increases yields.