Fluorescence-On Response via CB7 Binding to Viologen–Dye Pseudorotaxanes

Fluorescence-on sensors typically rely on disrupting photoinduced electron transfer quenching of the excited state through binding the electron donor. To provide a more general fluorescence-on signaling unit, a quencher–fluorophore dyad has been developed in which quenching by electron transfer to a tethered viologen acceptor can be disrupted through complexation of the viologen by cucurbit[7]uril (CB7). Dyads of benzyl viologen–rhodamine B or a BODIPY fluorophore gave upon CB7 complexation 14- and 30-fold fluorescence enhancement, respectively

Kinetics of Formation of the Host–Guest Complex of a Viologen with Cucurbit[7]uril

Host–guest complexation between the dicationic viologen 1-tri(ethylene glycol)-1′-methyl-<i>m</i>-xylyl-4,4′-bipyridinium and cucurbit[7]uril (<b>CB7</b>) was studied at pH = 4.5 in water. The stability constants of the mono- and bis-<b>CB7</b> adducts were determined at 25 °C by UV–vis spectroscopy. Stopped-flow kinetic experiments were performed to measure the formation and dissociation rate constants of the monoadduct: <i>k</i>₁ = (6.01 ± 0.03) × 10⁶ M^–1s^–1 and <i>k</i>_–1 = 52.7 ± 0.4 s^–1, respectively. Possible mechanisms of complexation are discussed in view of the kinetic results