Synthesis and properties of porphyrin nanotubes

Discrete π‐conjugated zinc porphyrin nanotubes are investigated as molecular analogues of carbon nanotubes. These porphyrin nanotubes have a diameter of 2.4 nm (Zn–Zn distance) and lengths of up to 3.6 nm, measured to the van der Waals surfaces of the outer β‐pyrrole hydrogen atoms, or 4.5 nm measured to the para hydrogen atoms of the aryl groups. We explore three different strategies for synthesizing these nanotubes. The first two strategies use a template to achieve direct or sequential stave‐joining, respectively, and proceed via linear oligomers that pre‐define the length of the nanotube. These strategies are applied to synthesize porphyrin nanotubes containing 12‐ or 18‐porphyrin subunits, with ethynylene (C2) or butadiynylene (C4) links between the 6‐porphyrin nanorings. The third strategy involves the covalent stacking of pre‐formed 6‐porphyrin nanorings to form a 12‐porphyrin nanotube, without using a template to guide this coupling reaction. The nanotubes show strongly red‐shifted absorption spectra and low fluorescence quantum yields, indicating structural rigidity and extensive π‐conjugation