Synthesis of a Strained Acetylenic Macrocycle Incorporating a para-Oligo[2]cruciform Bridge Bent over Nanoscopic Dimensions: Structural, Electronic, Spectroscopic, and Ion-Sensing Properties

An Eglinton–Galbraith diethyne cyclization preferentially yielded a structurally unusual macrocycle, comprising a strained conjugated oligo[2]cruciform wire, forced into a 2.2 nm bow‐shape by a terpyridine rein or tether, and stabilized towards light and heat by four insulating triisopropylsilylacetylene (TIPSA) substituents. Spectroscopic ion‐binding studies revealed the macrocycle to exhibit a particularly high UV/Vis selectivity for PdII in dilute solution, and one of its precursors to afford a variety of luminescence quenching and color responses to particular metals, suggestive of promising ion‐sensor applications. Under more concentrated conditions, the new macrocycle is able to bind specific metals (e.g., AuI) within its cavity despite the steric constraints. Intriguingly, variable‐temperature (VT) UV/Vis/1H NMR investigations showed the TIPSA substituents to undergo restricted intramolecular motions along with reversible changes in the spectroscopic bandgap of the compound with temperature. In line with the theoretical calculations, the VT UV/Vis observations are consistent with a thermal modulation of the electronic conjugation through the strained oligo[2]cruciform bridge, which is coupled with redistributions within a mixture of conformational isomers of the macrocycle with differing relative twisting between the TIPSA‐substituted phenyl rings. Overall, the generation of a para‐oligo[2]cruciform, bent and flexed over nanoscopic dimensions through conformational tethering within the macrocyclic ring is noteworthy, and suggests a general approach to nanosized, curved, and strained, yet heat‐ and light‐stable, para‐phenyleneethynylene oligomers with unique physicochemical properties and challenging theoretical possibilities. Reining in the strain: An Eglinton–Galbraith diethyne cyclization yielded an unusual bow‐shaped cycle (see scheme), incorporating a strained yet stable conjugated oligo[2]cruciform wire, bent over nanoscopic dimensions by a terpyridine tether. As well as spectroscopically sensing metal ions, it undergoes thermo‐controllable bandgap changes in solution, consistent with hindered relative twisting of the substituted phenyl rings, thus functioning as an environmentally adaptive conjugation switch.