A Molecular Nanotube with Three‐Dimensional π‐Conjugation

Abstract

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 C—C bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm3. 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.

Synthesis and Properties of Porphyrin Nanotubes

“…The 1 H-NMR spectra are strikingly simple owing to their D 6h symmetry on the NMR timescale. The assignment of the aromatic protons is supported by NOESY and COSY experiments (the assignment of t-P12·(T6) 2 was published previously; [60] see the Supporting Information for a detailed assignment of t-P12*·(T6) 2 and pt-P18*·T6-a).…”

“…We previously prepared t-P12·(T6) 2 via a direct templated stave-joining synthesis from l-P2b. [60] The final oligomerization was performed in the presence of T6, a radial six-legged template. The key intermediate in the synthesis of l-P2b is the tetra-meso-substituted zinc porphyrin monomer P1c, which was prepared from P1 using a Senge arylation to introduce the bis [72,73] Arylation was followed by metalation with zinc, bromination at the remaining free meso-position and Sonogashira cross-coupling with TMS-acetylene to give P1c.…”

“…During the reaction, the color of the solution changed from dark red to deep pink, in line with previous observations of nanotube formations. [60] Reaction progress was monitored by UV/VIS-NIR spectroscopy and when no further change was observed, the reaction mixture was passed through a short SiO 2 plug in toluene/1 % pyridine. Analysis by GPC and MALDI-ToF spectrometry indicated successful formation of t-P18·(T6) 3 .…”

“…This metal-ligand interaction has been widely exploited in the templatedirected synthesis of various porphyrin nanostructures by the groups of Sanders, [53,54] Lindsey, [55][56][57] Gossauer, [58,59] and Anderson. [60,61] The porphyrin subunits in most of the nanostructures studied in Oxford are linked by butadiyne (C 4 ) bridges, leading to strong electronic communication between the porphyrin centers. [62,63] Various fully π-conjugated porphyrin nanorings have been prepared and studied for their electronic and optical properties.…”

“…Recently, we reported the template-directed synthesis of a discrete molecular nanotube with threedimensional π-conjugation, constructed from 12 porphyrin units arranged in two conjoined 6-porphyrin nanorings; t-P12·(T6) 2 . [60] This 12-porphyrin nanotube was prepared via direct stave-joining (Figure 1,a) from a porphyrin dimer (l-P2b, Scheme 1); the structures of t-P12 and the hexapyridyl template T6 are shown in Figures 2 and 3, respectively. This discrete molecular nanotube has a length of 3.2 nm measured to the para hydrogen atoms of the aryl groups, which, to the best of our knowledge, is longer than all previously synthesized π-conjugated molecular belts, barrels, or nanotubes.…”