Highly Selective Na⁺‐Templated Formation of [2]Pseudorotaxanes Exhibiting Significant Optical Outputs

Abstract

The diverse range of binding geometries exhibited by metal ions and their relatively stable interactions with ligands can be exploited for the efficient template-directed synthesis of macrocycles,[1] supramolecular architectures, [2] and interlocked molecules.[3] Although the formation of rotaxanes or pseudorotaxanes has been demonstrated using transitionmetal ions coordinated to four (tetrahedral and square planar), [4] five (trigonal bipyramidal and square pyramidal), [5] and six (octahedral) [6] donor atoms, alkali-metal ions, which are useful templates for the synthesis of crown ethers, [7] are rarely used to direct the construction of such complexes. In one example, Li + ions have been used to assist the formation of a unique donor-acceptor pseudorotaxane, [8] but it appears that Na + ions play a similar role in that system. [9] We became interested in forming pseudorotaxanes in solution with high efficiency and selectivity toward a single species of alkalimetal ion-especially if we could differentiate between the physiologically important, [10] but chemically similar, Na + and K + ions.[11] Herein, we report a new molecular recognition system based on the molecular cage 1 (Scheme 1) and two different threadlike molecules (anthraquinone and squaraine), each of which forms a pseudorotaxane complex with 1 in the presence of templating Na + ions and exhibits high selectively over the other alkali-metal ions tested. This ionspecific templating effect was easy to monitor because the complexation and decomplexation of the pseudorotaxane complexes in solution occurred with color changes that were detectable to the naked eye. It appears from the solid-state structures that the dramatic Na Previously, we demonstrated that molecular cage 2 and anthraquinone (3, Scheme 2) can form pseudorotaxane-like complexes in solution when K + ions are present. [12] We suspected that greater ion-selective templating behavior would arise if we replaced the two openings resembling dibenzo [24]crown-8 (DB24C8) in 2 with two relatively smaller and more rigid units resembling dibenzo [18]crown-6

Rotaxane and Catenane Host Structures for Sensing Charged Guest Species

“…10 The system exhibits high selectivity for Na + over the other alkali metal cations: addition of K + results in no pseudorotaxane formation. The authors extended this principle to prepare a Na + responsive molecular shuttle, in which the same macrocycle translocates and encapsulates the integrated squaraine dye upon Na + addition in CD 3 CN, resulting in fluorescence enhancement ( Figure 4).…”

Squaraine‐Based [2]Rotaxanes that Function as Visibly Active Molecular Switches

“…[13] Thus, we anticipated that the , each featuring one squaraine and one bipyridinium station, but differing in the length of the alkyl spacer (C 16 and C 8 units, respectively) between them, through the reactions of the squaraine-containing pyridine derivatives 6-long·Br and 6-short·Br, respectively, with the benzyl bromide 7 in the presence of the molecular cage 1 and NaClO 4 (42:42:63:105 mm) in CH 3 CN. Our reasons for choosing to install bipyridinium moieties as competing recognition units in these [2]rotaxanes were twofold: 1) their potentially strong binding to the crown ether like (C À H···O hydrogen bonds) and catechol (p stacking) motifs of the molecular cage moiety would allow the macrocycle to reside at these stations after removing the Na + template ions and 2) their fluorescence quenching ability would allow the tuning of the fluorescence intensity of the squaraine units by controlling the distance between the two guest stations.…”