Threading the Needle: Fluorescent Poly-<i>pseudo</i>-rotaxanes for Size-Exclusion Sensing

Abstract

Poly-pseudo-rotaxanes have been formed through the threading of cucurbit[n]urils (CB[n]) onto the cationic electron-poor poly(pyridyl vinylene), PPyV. The threading of CB[n] onto the PPyV backbone is confirmed by a broadening and upfield shift in the PPyV 1H NMR signals. Encapsulation of PPyV within the CB[n] macrocycles produces dramatic fluorescence enhancements with improved solubility. The threading ability of the CB[n] on the PPyV backbone is governed by the dimensions of the particular CB[n] portal, which grows with increasing number of methylene-bridged glycoluril repeat units. CB[5] is too small to thread onto the PPyV backbone. The portal of CB[6] requires extra time, suggesting high preorganization and/or macrocycle deformation are required to thread onto PPyV. Alternatively, the portal of CB[8] appears to be large enough such that it does not have sufficiently large dipole–dipole interactions with the PPyV chain to promote a strong threading equilibrium. However, we find that the portal of CB[7] is optimal for the threading of PPyV. The PPyV-CB[n] system was further exploited to demonstrate a dual-action sensor platform, combining the PL-responsive behavior demonstrated by PPyV toward electron-rich analytes with the size-exclusion properties imparted by volume of the respective CB[n] cavities. Thin films of PPyV-CB[7] were found to display reversible photoluminescence quenching when exposed to vapors of the biologically relevant molecule indole, which is recovered under ambient conditions, suggesting prospects for new size-exclusion based selective sensory schemes for volatile electron-rich analytes.