Three-state molecular shuttles operated using acid/base stimuli with distinct outputs

Abstract

This paper describes the acid/base-mediated three-state translational isomerization of two [2]rotaxanes, each containing N-alkylaniline and N,N-dialkylamine centers as binding sites for threaded dibenzo[24]crown-8 units. Under neutral conditions, the dialkylamine unit predominantly recognized the crown ether component through cooperative binding of a proton; when both amino units were protonated under acidic conditions, both translational isomers were generated; the addition of a strong base caused aniline-crown ether interactions to dominate. The three states of the [2]rotaxane featuring the 3,5-diphenylaniline terminus in its dumbbell-shaped component were accompanied by distinct absorptive outputs that were detectable using UV spectroscopy.

Four‐State Molecular Shuttling of [2]Rotaxanes in Response to Acid/Base and Alkali‐Metal Cation Stimuli

“…After sequential additions of Bu 4 NOH (1.5 equiv) andN aClO 4 (10 equiv), we added [15]crown-5 (14 equiv) to the mixture to remove the Na + cation from 1H 0 NaX 1 .T he resulting NMR spectrum( Figure 3h) was nearly identical to that of 1H 0 X 0 , thereby confirming the reversibility of the Na + -cation recognition. We also checked the details of the reverse process;r elocation of DB24C8 required more than 12 equivalents of [15]crown-5 after the addition of NaClO 4 (10 equiv;s ee the Supporting Information).…”

“…Notably, however, the chemical shift of the proton H r ( δ =4.44 ppm) was not at a significantly low field (for a previously reported, structurally similar rotaxane, the chemical shift was ca. 4.9 ppm) . Presumably, the macrocyclic component mainly encircled the arylamine station because of a somewhat strong hydrogen bond between the DB24C8 component and the NH group of the arylamine unit, but it could also reside around the tetra(ethylene glycol) and alkylamine units (see the discussion below).…”

“…4.9 ppm). [15] Presumably,t he macrocyclic component mainly encircled the arylamine station because of as omewhat strong hydrogen bond between the DB24C8 component and the NH group of the arylamine unit, but it could also reside aroundt he tetra(ethylene glycol) anda lkylamine units (see the discussion below).…”

“…Havingc onfirmed the three individual shuttling processes of the [2]rotaxane 1H 1 PF 6 in response to acid, base in the absence of metal ions, and base in the presence of Na + cations, we combined theses timuli to realize reversible four-state shuttling. First, we added Bu 4 NOH (1.5 equiv) to deprotonatet he dialkylammonium unit of 1H 1 PF 6 (from state 2t os tate 3); second, we added NaClO 4 (15 equiv) to form 1H 0 NaX 1 (from state 3t os tate 4);t hird, we added [15]crown-5 (20 equiv) to remove the Na + ion (from state 4t os tate 3);f ourth,w ea dded AcOH (1.7 equiv) to re-protonate the alkylamino group of 1H 0 X 0 (from state 3t os tate 2);f ifth, we protonated the arylamino group of 1H 1 X 1 with TfOH (20 equiv) (from state 2t o state 1);a nd, finally,w ea dded triethylamine (25 equiv) to deprotonate the arylamino group of 1H 2 X 2 (from state 1t o state 2). It was possible to monitor the cycling among these four shuttling states by using NMR spectroscopy (see the Supporting Information);t he NMR spectra of each state revealed its expected translational isomer(s).…”

“…Such multistate molecular shuttles can sense the degree or amplitude of the external stimulus, and they can be used to detect the nature of various external stimuli (e.g., chemical additives) that influence the shuttling behavior . In previous studies, we developed acid/base‐responsive multistate [2]‐, [3]‐, and [4]rotaxanes and an acid/base‐ and alkali‐metal cation‐dependent molecular shuttle based on crown ether/ammonium‐ion recognition . We suspected that if both systems could be combined, we could realize multistate molecular shuttles that respond to multiple stimuli.…”