Helical fibrous nanostructures self-assembled from metal-free phthalocyanine with peripheral chiral menthol units

(D)- and (L)-enantiomers of a novel metal-free 2(3),9(10),16(17),23(24)-tetrakis(2-isopropyl-5-methylcyclohexoxyl)phthalocyanine (1) with four chiral menthol units attached at the peripheral positions of a phthalocyanine ligand have been synthesized, and characterized. Neither the (D)-1 nor the (L)-1 enantiomer display a circular dichroism (CD) signal in the Soret and Q absorption region of the phthalocyanine ligand, indicating the lack of effective chiral information transfer from the chiral menthol tails to the phthalocyanine chromophore at the molecular level. Their self-assembly properties were systematically studied by CD spectroscopy, transmission electron microscopy, scanning electron microscopy, and atom force microscopy technique. Although four constitutional stereoisomers of each enantiomer were synthesized, because the four chiral menthol substituents are randomly located at peripheral positions of the phthalocyanine ring, cooperation of intermolecular pi-pi interactions between the phthalocyanine rings with chiral discrimination of the chiral side chains of the (D)-1 and the (L)-1 enantiomer induces the formation of one-dimensional helices with left- and right-handed helical molecular arrangement, respectively, according to the CD spectroscopic results. This reveals the effective chiral information transfer from the chiral menthol tails to the phthalocyanine chromophore at the supermolecular level. The formed one-dimensional helices twist around each other to maximize the van der Waals interaction, leading to the formation of highly ordered fibrous nanostructures with both right- and left-handed helicity according to the staggering angles between the neighboring phthalocyanine molecules, indicating the hierarchical formation of these fibrous nanostructures. Careful inspection of these nanofibers indicates the majority of nanofibers with right- and left-handed helicity formed from (D)-1 and (L)-1 enantiomer, respectively, with the ratio of approximately 1.3-1.4:1 among all the fibrous nanostructures obtained. Electronic absorption spectroscopic and X-ray diffraction results reveal the H-aggregate nature of these nanofibers. The present results, showing part of our continuous effort towards preparation of self-assembled nanostructures with helical morphology through molecular design and synthesis, will be helpful on providing new insight into chiral information transfer and expression for synthetic conjugated systems at the supermolecular level.