Substituted Methylenation at the 13²‐Position of a Chlorophyll‐a Derivative via Mixed Aldol Condensation, Optical Properties of the Synthetic Bacteriochlorophyll‐d Analogs, and Self‐aggregation of Their Zinc Complexes^†

Abstract

Chlorophyll-a derivatives possessing a substituted methylene group at the 13 2 -position were prepared by the mixed aldol condensation of methyl 3-hydroxymethyl-pyropheophorbide-a with aldehydes bearing a methyl, p-nitro/cyanophenyl, or pentafluorophenyl group. Their electronic absorption spectra were dependent on the substituents at the methylene terminal. The Soret bands were broadened by increasing the group electronegativity of the substituents, which was ascribable to the charge transfer from the core chlorin to the peripheral substituent in a molecule. Although their Qy absorption and fluorescence emission bands resembled each other, the emission intensities decreased with an increase in the electronegativity because of intramolecular electron transfer quenching. Some of their zinc complexes self-aggregated in a less polar organic solvent to give red-shifted and broadened absorption bands with intense circular dichroism couplets, which were similar to those of bacteriochlorophyll-c/d aggregates in natural chlorosomes as the main lightharvesting antennas of green photosynthetic bacteria and their models. The J-aggregation was suppressed with an enhancement in the size of the 13 2 -substituents.