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

Abstract

Chlorophyll(Chl)‐a derivatives containing some rigid linkers in the C3‐substituent, inserted between a hydroxymethyl group and a zinc 131‐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.

Synthesis and Self‐Aggregation of Chlorophyll‐a Derivatives Possessing a Hydroxymethyl Group in the C20‐Substituent with Ethynylene and/or Phenylene Linkers

“…This observation indicated the chlorosomal J ‐aggregation of 2‐Zn in the aqueous micellar solution ‡ . The superior self‐aggregation ability of 2‐Zn over 1‐Zn was found to be similar to our recent studies in the related 3‐substitutes (37). The benzyl alcoholic moiety, which is linked to the chlorin π‐skeleton via an ethynylene group, worked well for the self‐aggregation of 6‐Zn (see Fig.…”

“…Based on the red‐shift values of Qy maxima by self‐aggregation, the exciton splitting energies Δ E , in which the excitonic interaction between the chlorin π‐skeletons was reflected, are available for the structural and optical studies of the self‐assemblies derived from Chl‐ a derivatives. In our previous report, the Δ E values of self‐aggregates derived from 6‐Zn and 7‐Zn were calculated as shown in Table 1; Δ E = 1.9 × 10 −20 ( 6‐Zn ) and 3.8 × 10 −20 J ( 7‐Zn ) (37). § These differences arose due to the changes in the chlorin‐to‐chlorin distances in the assemblies, specifically the inserted linker between the hydroxymethyl group and chlorin π‐skeleton, which is one of the most important factors used to control exciton splitting energies (48).…”

“…1C) inserting a rigid linker between a hydroxymethyl group and the chlorin π‐skeleton were synthesized by the cross‐coupling of the 20‐bromide. These linkers, including ethynylene and p ‐phenylene moieties, were selected by referring to our previous report, in which the effects of the linker on the J ‐aggregation abilities were investigated in the C3‐substituents (37). Their self‐aggregation abilities were studied by electronic absorption, circular dichroism (CD) and infrared (IR) absorption spectra in an aqueous micellar solution.…”