Dendron-mediated control over self-assembly of chlorophyll rosettes into columnar vs. discrete aggregates

Abstract

Photosynthetic bacteria have evolved highly efficient light-harvesting systems by organizing chlorophyll (Chl) pigments into circular and tubular supramolecular arrays. To construct these surapmoelcular Chl arrays from the same molecular design, we synthesized two hydrogen-bonding chlorins using natural Chl-a as the starting material: free-base chlorin functionalized with hydrogen-bonding barbituric acid and second- or third-generation alkyl dendrons (G2 and G3, respectively). The barbituric acid moiety promotes the formation of a hydrogen-bonded cyclic hexamer known as rosette. In chloroform, both the synthetic Chl-a derivatives formed rosettes; however, in methylcyclohexane as a low-polarity solvent, the G2-dendron chlorin formed columnar structures by stacking rosettes, while the G3-dendron chlorin formed disc-shaped particles. AFM revealed the formation of extended helical fibers for the former and homogeneous nanoparticles, possibly single rosettes, for the latter. These results suggest that the third-generation of the dendron can inhibit the stacking of rosettes, leading to the formation of two distinct types of chlorin aggregates: circular and tubular.