*3.2. Ultraviolet (UV) Spectroscopy*

To further understand the binding of these 4-pyrrolidinopyridinium salts to Q[7], we also investigated the systems by UV–Vis spectroscopy. The UV spectra were obtained using aqueous solutions containing a fixed concentration of guests C0–C6 and variable concentrations of Q[7] (Figures 5 and A3, Figures A4 and A5). All systems of action showed similar phenomena, and here, only the interactions between Q[7] and gues<sup>t</sup> C2 are described as an example. On gradually increasing the Q[7] concentration in the C2 solution, the absorption band of the gues<sup>t</sup> exhibited a progressively higher absorbance due to the formation of the host–guest complex Q[7]@C2. The absorbance vs. ratio of n(Q[7])/n(C2) data can be fitted to a 1:1 binding model. The pyrrolidinopyridinium part of the gues<sup>t</sup> was encapsulated into the cavity of the Q[7] host, whilst the alkyl moiety remained outside. This generated a 1:1 host–guest inclusion complex. The encapsulation by Q[7] of this gues<sup>t</sup> is presumably due to the favorable ion-dipole interactions between the positively charged gues<sup>t</sup> and the portal oxygen atoms of Q[7] in addition to hydrophobic effects. Moreover, the association constant (K) is calculated from the UV-vis spectroscopy data according to the modified Benesi–Hildebrand (B–H) equation. For these systems, the binding constants have determined as Ka(Q[7]@C0) = 8.0 × 106, Ka(Q[7]@C2) = 5.7 × 109, Ka(Q[7]@C4) = 3.6 × 10<sup>7</sup> and Ka(Q[7]@C6) = 2.5 × 109.

**Figure 5.** (Color online) (**A**) Electronic absorption of C2 (2 × 10−<sup>5</sup> mol <sup>L</sup>−1) upon addition of increasing amounts (0, 0.2, 0.4······2.6, 2.8, 3.0 equiv.) of Q[7]; (**B**) the concentrations and absorbance vs. NQ[7]/NC2 plots; (**C**) the corresponding ΔA–NQ[7]/(NQ[7] + NC2) curves.
