Synthesis and Self-Aggregation of Chlorophyll-a Derivatives with Ethynylene and Phenylene Groups Inserted Between the Hydroxymethyl Group and the Chlorin π-Skeleton

Chlorophyll(Chl)-a derivatives containing some rigid linkers in the C3-substituent, inserted between a hydroxymethyl group and a zinc 13(1)-oxo-chlorin moiety, were synthesized as models of bacteriochlorophyll-c/d/e molecules in the main light-harvesting antennae (chlorosomes) of photosynthetic green bacteria. These model compounds were synthesized from a C3-ethynylated Chl-a derivative via several coupling reactions, and the lengths of the linkers were controlled by ethynylene and p-phenylene groups. In less polar organic solvents or an aqueous micellar solution, some derivatives self-aggregated in a J-type fashion similar to that observed in natural chlorosomes, which was confirmed with UV/Vis absorption and CD spectroscopies. Their self-aggregation abilities were dependent on the length of the inserted linkers and the conformation of the propargylic/benzylic alcoholic hydroxy groups.