Highly Strained Nonclassical Nanotube End-caps. A Single-Step Solution Synthesis from Strain-Free, Non-Macrocyclic Precursors

Nonclassical nanotube end-caps have
been constructed from strain-free
heterocyclic precursors using a one-step synthetic procedure, involving
multiple nickel-mediated Ullmann couplings. These systems consist
of tubular macrocyclic sections that are tightly capped on one side
with a bridging benzene ring, forming deep, chemically accessible
cavities. The end-caps are characterized by exceptionally high internal
strain energies reaching 144 kcal/mol. The optical absorption and
emission properties of these molecules show a marked dependence on
conjugation length and geometrical factors. The mechanism of end-cap
formation, investigated using DFT calculations, relies on precise
timing of transmetalation and reductive elimination events.