Host–guest complexation between the dicationic viologen 1-tri(ethylene glycol)-1′-methyl-m-xylyl-4,4′-bipyridinium and cucurbit[7]uril (CB7) was studied at pH = 4.5 in water. The stability constants of the mono- and bis-CB7 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: k1 = (6.01 ± 0.03) × 106 M–1 s–1 and k–1 = 52.7 ± 0.4 s–1, respectively. Possible mechanisms of complexation are discussed in view of the kinetic results.

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.

The cleavage of decanethioacetate (C10SAc) has been studied by 1H nuclear magnetic resonance (NMR) spectroscopy and scanning tunneling microscopy (STM) imaging of in situ prepared decanethiolate self-assembled monolayers (SAMs) on Au(111). Solutions of C10SAc (46 mM) and previously reported cleavage reagents (typically 58 mM) in CD3OD were monitored at 20 °C by NMR spectroscopy. Cleavage by ammonium hydroxide, propylamine, or hydrochloric acid was not complete within 48 h; cleavage by potassium carbonate was complete within 24 h and that by potassium hydroxide or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) within 2 h. Similar cleavage rates were observed for phenylthioacetate. The degree of molecular ordering determined by STM imaging increased with increasing extent of in situ cleavage by these same reagents (2.5 mM C10SAc and 2.5 mM reagent in ethanol for 1 h, then 16 h immersion of Au/mica). Less effective cleavage reagents did not cleave the C10SAc sufficiently to decanethiol (C10SH) and gave mostly disordered SAMs. In contrast, KOH or DBU completely cleaved the C10SAc to C10SH and led to well-ordered SAMs composed of (√3 × √3)R30° domains that are indistinguishable from SAMs grown from C10SH. Monolayer formation from thioacetates in the absence of cleavage agents is likely due to thiol or disulfide impurity in the thioacetates. Eliminating disulfide by using Bu3P as a sacrificial reductant also helped to produce good molecular order in the SAM. The methods presented here allow routine growth of molecularly ordered alkanethiolate SAMs from thioacetates using reagents of ordinary purity under ambient, benchtop conditions.

Improvements in optical properties of organic xanthene fluorophores through molecular encapsulation in cucurbit[7]uril (CB7) should enable better molecular probes and devices to be designed. Although the interactions of several dyes with CB7 have been studied, the data have often been incomplete. Uniformly applied ensemble spectroscopic studies are presented herein, including 1H NMR, UV–Vis, and fluorescence titration experiments of aqueous CB7 complexes with the monocationic xanthene dyes rhodamine 6G (Rh6G, 1), rhodamine B (RhB, 2), rhodamine B benzyl ester (RhBBE, 3), pyronin B (PyB, 4) and pyronin Y (PyY, 5). All of these cationic xanthene dyes formed 1:1 complexes with cucurbit[7]uril as evidenced by NMR data and Job’s plots of fluorescence changes upon addition of CB7. Non-linear regression analysis of the fluorescence titration curves gave precise Ka’s for RhB, RhBBE and PyB between 1.1 × 105 M−1 and 9.1 × 106 M−1. The fluorescence emission intensity of Rh6G was lowered 0.8-fold in the presence of CB7 while the other dyes examined showed an increase between 1.3 and 4.7-fold. NMR titration experiments from 0 to 2.0 equivalents of CB7 per equivalent of xanthene gave in only some cases very clear evidence of inclusion complexation. Non-specific adsorption of these xanthene dyes onto borosilicate glass was very pronounced and could be inhibited by dye inclusion into CB7.