Self-aggregation of 32-substituted bacteriochlorophyll-d analogs in aqueous micelle

As naturally occurring bacteriochlorophyll-d models, zinc methyl (3(1)R)-bacteriopheophorbides-d possessing ethyl, vinyl, and ethynyl groups at the 3(2)-position were prepared by chemically modifying chlorophyll-a. The synthetic molecules self-aggregated in an aqueous Triton X-100 micelle solution similarly as in the major light-harvesting antenna of photosynthetic green bacteria to give supramolecularly ordered large oligomers with red-shifted and broadened visible absorption bands, compared with the monomeric bands. The sequential hydrogenation of the 3(2)-ethynyl to ethyl group via the vinyl group gradually suppressed the chlorosomal self-aggregation ability. The successive elongation of the 3(1)-alkyl chain from a methyl to propyl group via an ethyl group also diminished the J-aggregation ability. These decreases in the self-aggregation were ascribable to the steric enhancement of the 3(2)-substituents near the 3(1)-hydroxy group requisite for the J-aggregation.