Synthesis of zinc 20-substituted bacteriochlorophyll-d analogs and their self-aggregation

Zinc complexes of methyl 3¹-demethyl-bacteriopheophorbides-d possessing a 2-(un)substituted ethynyl or ethenyl group at the 20-position were prepared through palladium-catalyzed cross-coupling of their 20-bromide with the corresponding alkyne or alkene. The synthetic zinc 20-substituted 3-hydroxymethyl-13¹-oxo-chlorins self-aggregated in an aqueous micellar solution to give similar large oligomers as self-aggregates of bacteriochlorophylls in the core part of the main light-harvesting antenna of photosynthetic green bacteria. The zinc 20-ethynyl-chlorin fully gave the self-aggregates, but its 20²-substitution disturbed the self-aggregation. The 20-(2-phenyl)ethenyl moieties induced the formation of relatively smaller oligomers as well as residual monomers due to their steric bulkiness.