Ultra-low loading porphyrin-incorporated conjugated polymer dots as photocatalysts for aerobic oxidation of sulfides in water†

Abstract

The development of metal-free, low-loading catalysts, together with the use of water as a benign solvent is desirable for advancing facile and efficient catalysis of organic transformations in an environmentally friendly manner. The direct oxidation of sulfides to sulfoxides is a key process in organic synthesis and pharmaceuticals. This study introduces metal-free porphyrin-integrated poly[9,9′-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-thiadiazole)] polymer dots (PFBT-TPP Pdots) as photocatalysts for photo-aerobic sulfide oxidation in water. PFBT-TPP Pdots were synthesized by the Suzuki–Miyaura cross-coupling process. DFT calculations confirmed the conjugation and delocalization characteristics of TPP incorporated into PFBT. The conjugated polymers were converted into Pdots using coprecipitation with poly(styrene-co-maleic anhydride). Pdots with a diameter of around 30 nm demonstrated exceptional dispersion in water. PFBT-TPP Pdots efficiently photocatalyzed 0.1 M of thioanisole or other sulfide derivatives, attaining remarkable conversion efficiency (80–100%) and selectivity (88–100%) at a low catalytic loading (50 μg of conjugated polymer) in water, under 1 atm O₂, at ambient temperature, and under 3 h-LED illumination (20 W, λ = 456 nm). The photocatalytic process was scaled to produce gram quantities of the isolated product with high selectivity and no signs of over-oxidation. This demonstrates a sustained and practical application. The computational analysis and experimental findings illustrated that PFBT-TPP Pdots effectively generated reactive oxygen species during photocatalysis, leading to enhanced catalytic efficiency compared to PFBT Pdots and their metal-containing counterparts. This study highlights the potential of Pdots for efficient, selective photocatalysis in aqueous environments, overcoming common solubility challenges for substrates and products.