Reunderstanding the photoinduced charge transfer process of ammonium polyoxomolybdate

Abstract

Photoinduced charge transfer (PCT) reactions account for the rich photochromic and photocatalytic properties of polyoxometalates (POMs). Yamase proposed a hypothesis that photoinduced O→Mo(VI) charge transfer reactions of ammonium polyoxomolybdate correlate with a proton transfer process from the ammonium cation to the polyoxomolybdate anion. Many subsequent experimental studies adopted this hypothesis to explain the photochromic phenomena of ammonium polyoxomolybdate. This research attempts to verify the appropriateness of this hypothesis experimentally. Firstly, studies on two adducts of 4-phenylpyridine (4pp) and carboxylic acids revealed that only protonation of 4pp can enable charge transfer from the carboxyl group to 4pp to achieve photochromism. Then, a new ammonium polyoxomolybdate [H(4pp)]₄Mo₈O₂₆·2(4pp) was synthesized using the protonated 4pp as an indicator for proton transfer. IR analysis and the above control experiment revealed that proton transfer did not occur in the photochromic process of [H(4pp)]₄Mo₈O₂₆·2(4pp), which proves that the PCT of ammonium polyoxomolybdate does not necessarily need proton transfer from the ammonium cation to the polyoxomolybdate anion.