Self‐aggregation of zinc bacteriochlorophyll‐d analogs with an acylhydrazone moiety as the 13‐keto‐carbonyl alternative

Abstract

AbstractZinc methyl 3‐hydroxymethyl‐pyropheophorbides‐a possessing an acylhydrazinylidene group at the 131‐position were prepared by chemically modifying chlorophyll‐a, which were models of bacteriochlorophyll‐d as one of the light‐harvesting pigments in photosynthetic green bacteria. Similar to the self‐aggregation of natural bacteriochlorophyll‐d in the antenna systems called chlorosomes, some of the synthetic models self‐aggregated in an aqueous Triton X‐100 solution to give red‐shifted and broadened visible absorption bands. The newly appeared oligomeric bands were ascribable to the exciton coupling of the chlorin π‐systems along the molecular y‐axis, leading to intense circular dichroism bands in the red‐shifted Qy and Soret regions. The self‐aggregation in the aqueous micelle was dependent on the steric size of the terminal substituent at the 13‐acylhydrazone moiety. An increase in the length of the oligomethylene chain as the terminal moved the red‐shifted Qy maxima to shorter wavelengths, and branched alkyl and benzyl substitutes afforded no more self‐aggregates to leave monomeric species in the hydrophobic environment inside the micelle. These results indicated that the acyl groups on the 13‐hydrazone as the alternative of the natural 13‐ketone regulated the chlorosome‐like self‐aggregation.