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

Abstract

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 Pd(II) 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., Au(I)) within its cavity despite the steric constraints. Intriguingly, variable-temperature (VT) UV/Vis/(1)H 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.

Supramolecular Luminescent Sensors

“…Baxter and co-workers reported two systems in which the strained acetylenic hosts exhibited photophysical changes upon complexation with Hg 2+ . However, the hosts in these systems were either nonselective toward Hg 2+ or not compared against other, potentially interfering, heavy metal cations. , Ganapathi et al synthesized two S- and N-containing porphyrin-like macrocycles with BODIPY or BODIPY-like fluorophore appendages, 268 and 269 (Figure ) which bound selectively to Hg 2+ to produce increases in emission intensity . A final example of cation sensing reported the synthesis of three phenyl-urea based macrocycles that were shown to complex a range of cation and anion analytes with various changes in host fluorescence…”

π‐Conjugated Macrocycles Bearing Angle‐Strained Alkynes

“…Dehydrobenzopyrid[15]annulenes 123 a and 123 b bearing a proton‐accepting pyridine ring showed dynamic proton‐induced emission switching properties. Baxter and co‐workers reported terpyridine‐containing macrocycle 124 . The bond angles of a butadiyne moiety are strained (165.4 and 170.1°), but two of the four monoyne moieties are not significantly curved (∠ sp ≥174°; CCDC: 913133).…”

“…Baxter and co-workersr eported terpyridine-containing macrocycle 124. [203] The bond angles of ab utadiyne moiety are strained (165.4 and1 70.18), but two of the four monoyne moieties are not significantly curved( ff sp ! 1748; CCDC:913133).…”

Ion and Molecular Recognition Using Aryl–Ethynyl Scaffolding

“…This class of aryl–ethynyl cruciforms has seen extensive exploration in the literature from fundamental structure–property studies as substructures of larger dehydrobenzannulene derivatives, [49, 50] as well as by the group of Baxter in their work on strained- and oligo-cruciform aryl–alkynyl systems. [51, 52] …”