Self-aggregation of 13²,13²-disubstituted bacteriochlorophyll-d analog

Zinc methyl 13²,13²-disubstituted 3-hydroxymethyl-pyropheophorbides-a were prepared as models of bacteriochlorophyll-d, which self-aggregated in the main light-harvesting antenna (chlorosome) of photosynthetic green bacteria. The synthetic zinc 3¹-hydroxy-13¹-oxo-chlorins possessing methyl and methoxycarbonyl groups at the 13²-position could not self-aggregate in an aqueous Triton X-100 solution. However, another model compound bearing an ethane-1,2-diyl group at the 13²-position did self-aggregate under the same conditions to give red-shifted and broadened Qy and Soret absorption bands. The spiro-cyclopropane condensation slightly suppressed the chlorosome-like self-aggregation due to an increase in the steric hindrance around the 13-carbonyl group. The red-shifted and broadened values of these bands by the self-aggregation were dependent on the 13²-substituents. The 13²-substitution substantially controlled the aqueous J-aggregation.