Heterodimers of zinc and free-base chlorophyll derivatives co-assembled in biomimetic chlorosomal J-aggregates.

Abstract

Light-harvesting antennas are one of the most important types of apparatus that use solar energy for natural and artificial photosynthesis. Bacteriochlorophyll (BChl) and chlorophyll (Chl) pigments play key roles in absorbing photons from the sun, migrating/transferring the singlet excitation energy, and transferring electrons. Chlorosomes, the main light-harvesting antennas of photosynthetic green bacteria, contain self-aggregates composed of a large number of BChl-c, d, e, and f molecules without any assistance from peptides, and such J-aggregates transfer the harvested energy to BChl-a pigments of baseplate proteins. In building such green bacterial light-harvesting antennas in artificial systems, self-aggregative model compounds of chlorosomal BChl-c-f pigments and their energy transferring systems have been reported to use synthetic (B)Chl derivatives in organic and aqueous solutions. In this study, we report the co-assembly and energy transfer of chlorin and porphyrin model compounds with covalently linked synthetic zinc and free-base (B)Chl heterodimers having different π-skeletons (bacteriochlorin, chlorin, and porphyrin). The co-assemblies of the self-aggregating BChl-d model having a porphyrin π-skeleton with heterodimers were first studied and showed singlet excitation energy transfer from the photoexcited chlorosomal J-aggregates to a free-base bacteriochlorin or chlorin moiety of the heterodimers. The present co-assembly is a good light-harvesting model system that mimics chlorosomes of photosynthetic green bacteria.