Photoinduced Electron Transfer Reactions as Photosynthetic Models: Reduction of Anthraquinones in SiO2 and ZrO2 Colloids

Abstract

AbstractThe photoinduced reduction of anthraquinone‐2‐sulfonate, AQS−, and anthraquinone‐2,6‐disulfonate AQS2‐2, in a negatively charged SiO2 colloid has been examined. Ru(bipy)2+3 was used as sensitizer and triethanolamine, TEOA, as an ultimate electron donor. The reduction of the quinones to the corresponding semiquinone radicals, AQS2‐ and AQS3‐2 proceeds only with propylviologen sulfonate, PVS0, (1), as a mediating electron acceptor. The primary electron acceptor, PVS0, functions as an efficient quencher of the excited species. The formation of AQS3‐2 is 4‐fold enhanced as compared to that of AQS2‐. This enhancement is attributed to the additional stabilization of AQS3‐2 from recombination with the oxidized species, Ru(bipy)3+3, due to electrostatic repulsions from the colloidal interface. Consequently, the subsequent oxidation of TEOA is facilitated. Similarly, a positively charged ZrO2 colloid has been used in controlling photosensitized electron transfer reactions. Anthraquinone‐2,6‐disulfonate, AQS2‐2, is photoreduced in the ZrO2 colloid to the respective AQHS2‐2 with the neutral sensitizer, Ru(bipy)2(CN)02, and cysteine as electron donor (Φ = 3 × 10−2). No photoreduction of the quinone is observed in a homogeneous aqueous phase. The success in driving the photoinduced reaction in the ZrO2 colloid is again attributed to the stabilization of the intermediate photoproducts from recombination by means of electrostatic interactions with the charged interface.