Synthesis and Self-Aggregation of π-Expanded Chlorophyll Derivatives to Construct Light-Harvesting Antenna Models

Shogo Matsubara, Hitoshi Tamiaki

Index: 10.1021/acs.joc.7b03212

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Abstract

Chlorosomes are one of the elegant light-harvesting antenna systems in anoxygenic photosynthetic bacteria, whose core is constructed from J-type self-aggregation of bacteriochlorophyll-c, bacteriochlorophyll-d, bacteriochlorophyll-e, and bacteriochlorophyll-f molecules without the influence of polypeptides. Chlorosomal supramolecular models were built up using synthetic porphyrin-type bacteriochlorophyll-d analogues with a methoxycarbonylethenyl, formyl, vinyl, or ethyl group at the 8-position. Their chlorosomal self-aggregates in an aqueous micelle solution showed relatively intense absorption bands around 500–600 nm where antennas of natural oxygenic phototrophs, as well as green sulfur bacteria possessing bacteriochlorophylls-c/d, absorb light less efficiently; this observation is called the “green gap”. Furthermore, the functional chlorosomal models were constructed by simple addition of a small amount of an energy acceptor model bearing a bacteriochlorin moiety to the pigment self-assemblies in an aqueous micelle. The resulting excited energy donor–acceptor supramolecules played the roles of chlorosomal light-harvesting and energy-transfer antenna systems and were efficient at light absorption in the “green gap” region.