Conformational and regioselective aspects on the photodegradation of organic pollutants by TiO2 composites of derivatives of meso-tetrathien-2-ylporphyrin

• Role of conformation on photosensitizing properties of thienylporphyrins.
• Role of position of substituents on photosensitizing properties of thienylporphyrins.
• Action of scavengers on photocatalytic activity of porphyrin-TiO2 composites.

The role of conformation and the effect of position of the substituent on the activity of copper(II) derivative of meso-5,10,15-tris(4- or 5- bromothien-2-yl)-20-(4-((thiophen-2-ylimino)methyl)benzoic acid)porphyrin (CuXB5TP, X = 4 or 5) as photosensitisers have been investigated. The higher photosensitizing activity of Cu4B5TP over its similar phenyl group containing porphyrins (meso-5,10,15-tris(4-bromothien-2-yl)-20-(4-((4-phenylimino)methyl)benzoic acid)porphyrinato copper(II) and meso-5,10,15-tris(4-bromothien-2-yl)-20-(4-carboxyphenyl)porphyrinato copper(II)) indicated the ease of electron flow through a conformationally favourable meso-group of the porphyrin and its role in making its TiO2 composite a better photocatalyst. The higher activity of TiO2 composites of Cu5B5TP over Cu4B5TP gives a clear evidence of the regioselective influences of the substituents on the porphyrins on their photosensitizing action. The difference in the photosensitizing activity of the various porphyrins in the present study is attributed to the significance of the orientation of the meso-linker (thienyl vs phenyl) group with respect to the porphyrin central macrocyclic ring in extending electronic conjugation. The photocatalytic result is supported by differential pulse voltammetry. The action of different scavengers on the photocatalytic activity indicated that the species which is responsible for the degradation of pollutants is not superoxide but are the holes generated in the valence band of porphyrin and the formed OH radicals.

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