A Molecular Nanotube with Three‐Dimensional π‐Conjugation

A π‐conjugated twelve‐porphyrin tube is synthesized in 32 % yield by a template‐directed coupling reaction that joins together six porphyrin dimers, forming twelve new CC bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm 3 . Its UV/Vis/NIR absorption and fluorescence spectra resemble those of a previously reported six‐porphyrin ring, but are red‐shifted by approximately 300 cm −1 , reflecting increased conjugation. Ultrafast fluorescence spectroscopy demonstrates extensive excited‐state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis ( z  axis) to an excited state polarized in the xy plane occurs within 200 fs, resulting in a negative fluorescence anisotropy on excitation at 742 nm.