Negative shift of chlorophyll a oxidation potential by aggregation in acetonitrile/ionic liquid mixed solvents

Spectroscopic and electrochemical properties of chlorophyll (Chl) a aggregates in mixed solvents of acetonitrile and an ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4), were examined. The red-shifted absorption peak of the Qy band due to Chl a aggregation as well as the log[Chl a aggregate] against log[Chl a monomer] plot suggested the formation of low-order aggregates such as a dimer. Square wave voltammetry revealed that the first oxidation potential of the Chl a aggregate in the AN/EMIBF4 mixed solvents is shifted negatively by 130 mV from that of Chl a monomer. In comparison with the redox potential of Chl a in vivo, a tuning mechanism of the redox potentials for photosynthetic species due to dimerization/aggregation is discussed.