Self-aggregation of synthetic zinc perfluoroalkylated bacteriopheophorbide-d in fluorous organic solvents

Abstract

Bacteriochlorophylls-d self-aggregated in less polar organic solvents, aqueous organic solvents and aqueous micellar solutions as well as in chlorosomes, major light-harvesting antenna systems of green photosynthetic bacteria.. Here we report synthesis of zinc 31-demethyl-bacteriopheophorbides-d 1 possessing perfluoroalkyl groups at the 17-propionate and their self-aggregation in fluorous organic solvents. Pyropheophorbide-d was prepared by modification of chlorophyll-a and the resulting acid was esterified with perfluoroalkylated alcohols, followed by reduction of the 3-formylgroup and zinc-metallation to give desired compounds 1. Zinc complex 1 was monomeric in THF and the solution was mixed with perfluoro-2-butyltetrahydrofuran (FC-75). After stirring, the solution was separated into two phases: the upper THF phase was blue and the lower FC-75 phase was colorless. This observation indicates that monomeric 1 remained in THF. In 1%(v/v) THF¿hexane, 1 self-aggregates to form oligomers. When the green solution was shaken with FC-75, the oligomeric 1 completely moved to the fluorous FC-75 phase and could be solvated in FC-75. Such a selective extraction showed that self-aggregates 1 were consisted of a core part of chlorin p-conjugates (by a special bonding, 13-C=OÉH-O(31)É Zn, and p-p interaction) and outer sphere of perfluoroalkyl groups.