Cucurbit[7]uril complexation drives thermal trans-cis-azobenzene isomerization and enables colorimetric amine detection.

Complexation of yellow diaminoazobenzenes 1 and 3 inside cucurbit[7]uril (CB[7]) results in the formation of purple-colored CB[7].cis-1.2H+ and CB[7].cis-3.2H+ complexes, respectively. The high binding affinity and selectivity displayed by CB[7] toward 1 and 3 pays the >10 kcal mol(-1) thermodynamic cost for this isomerization. We investigated the behavior of these complexes as a function of pH and observed large pK(a) shifts and high pH responsiveness, which are characteristic of cucurbit[n]uril molecular containers. The remarkable yellow to purple color change was utilized in the construction of an indicator displacement assay for biologically active amines 4-10. This indicator displacement assay is capable of quantifying the pseudoephedrine (5) content in Sudafed tablets over the 5-350 microM range.