Self-aggregation of a synthetic zinc chlorophyll derivative possessing a 131-dicyanomethylene group as a light-harvesting antenna model

Zinc methyl 3-hydroxymethyl-131-dicyanomethylene-pyropheophorbide-a, one of the bacteriochlorophyll-d analogs, was prepared by modifying chlorophyll-a. The semi-synthetic compound self-aggregated in an aqueous Triton X-100 solution to give large oligomers with red-shifted and broadened electronic absorption bands. The J-aggregates were similar to self-aggregates of bacteriochlorophyll-d molecules in natural light-harvesting antennas of green photosynthetic bacteria. The strongly electron-withdrawing dicyanomethylene group was an alternative functional moiety of the hydrogen-bond accepting 131-oxo group which had been necessary for such self-aggregation.