Effect of the naphthylene linker on the J-aggregation abilities of chlorophyll-a derivatives

Chlorophyll(Chl)-a derivatives inserting an ethynylene-naphthylene linker between the chlorin π-skeleton and hydroxymethyl group were prepared as models of chlorosomal Chls. Their syntheses were achieved via Sonogashira coupling reaction. Their J-aggregation behaviors were investigated by electronic absorption and circular dichroism spectroscopic measurements. These studies revealed that the 2,6-naphthylene inserted Chl-a derivatives gave the single J-aggregation species in an aqueous Triton X-100 micellar solution with a larger red-shift value (1270 cm^-1) of the Qy band in spite of its longer linker compared with p-phenylene inserted Chl-a derivative (970 cm^-1). These unique optical properties were also discussed based on the computational studies, which indicated the different positional relation of chlorin rings in the assemblies by the linker structure.