20-Substitution effect on self-aggregation of synthetic zinc bacteriochlorophyll-d analogs

Abstract Zinc methyl 20-substituted 3 1 -demethyl-bacteriopheophorbides- d were prepared through palladium-catalyzed cross-coupling of chlorophyll- a derivatives. The synthetic zinc 3 1 -hydroxy-13 1 -oxo-chlorins possessing a(n) (un)substituted ethynyl or butadiynyl group at the 20-position were good models for bacteriochlorophylls- c / d found solely in chlorosomes, the main light-harvesting antennas of green photosynthetic bacteria. Similarly as in natural chlorosomes, the synthetic models self-aggregated in an aqueous Triton X-100 solution to form large oligomers with red-shifted and broadened electronic absorption bands. While a phenyl group directly connected with the chlorin π-system at the 20-position disturbed the chlorosomal self-aggregation, the insertion of an ethynylene group between the two functional groups reduced the steric hindrance around the 20-position and was useful for the facile formation of the self-aggregates. A similar substitution effect was observed in the 20-ethynyl to butadiynyl moieties bearing a sterically bulky trimethylsilyl group at the terminal position.