© 2018 Wiley-VHCA AG, Zurich, Switzerland 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.
