Synthesis of bacteriochlorophyll-c analogs possessing thio-functionalized substituents at the 20-position and their self-aggregation

Methyl pyropheophorbides-a possessing some substituents at the 3-position as chlorophyll-a derivatives were regioselectively trifluoromethylthiolated and thiocyanated at the 20-position by heating with N-trifluoromethylthio- and thiocyanato-dibenzenesulfonimide, respectively, in N,N-dimethylformamide. Based on the novel thio-functionalization at the 20-position, zinc methyl 3-hydroxymethyl-pyropheophorbides-a bearing the 20-SCF₃ or SCN group were prepared as the models of naturally occurring bacteriochlorophyll(BChl)-c with the 20-Me group. The resulting model compounds self-aggregated in an aqueous Triton X-100 micellar solution to give red-shifted and broadened electronic absorption bands, similar to the self-aggregation of BChl-c in chlorosomes, major light-harvesting antennae of photosynthetic green bacteria. The electronic and steric effects of the 20-SX groups on the absorption bands of their monomeric and aggregated states as well as their self-aggregation abilities are discussed.