Cucurbit[7]uril Complexation Drives Thermal trans–cis‐Azobenzene Isomerization and Enables Colorimetric Amine Detection

Complexation of yellow diaminoazobenzenes 1 and 3 inside cucurbit7]uril (CB7]) results in the formation of purple‐colored CB7] ? cis‐ 1? 2 H⁺ and CB7] ? cis‐ 3? 2 H⁺ complexes, respectively. The high binding affinity and selectivity displayed by CB7] 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 cucurbitn]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 μM range.