## 3 Results and Discussion  
Host-guest interaction was studied between \(\beta\)-CD and 4-aminothiophen (4-ATP). The host-guest complex was assembled by hydrophobic interaction for each other. Structural analysis of the complex comprises of IR, \({}^{1}\)H NMR and single crystal X-ray diffraction studies. The guest 4-ATP is insoluble in aqueous solution, while the host \(\beta\)-CD is soluble in water (1.08 mg/mL). The complexation was induced by mixing an ethanolic solution of 4-ATP in an aqueous solution of \(\beta\)-CD. IR spectrum of the complex preserves most features of \(\beta\)-CD, while the bands at 1623 (w, sh), 1596 (w, sh), 1494 (w, sh) were unique. These were assigned due to the N-H bending modes and C-C stretching frequency of the aromatic ring as part of 4-ATP. The NMR spectrum of the complex bears a stronger indication of complex formation. \({}^{1}\)H NMR of the complex had two doublets at \(\delta\) 6.6 and 7.0. These are signals from the aromatic guest molecule. The \({}^{1}\)H NMR of \(\beta\)-CD and the complex were recorded in D\({}_{2}\)O. In D\({}_{2}\)O the guest molecule is not soluble and hence there was signal from the aromatic proton of the guest molecule in the inclusion complex. The NMR spectra recorded were very noisy in the \(\delta\)3.5-5 region because of the water molecules in the hydrophilic region of the complex shadowing the signals from the \(\beta\)-CD ring. It is a common practice to use thiols for the fabrication of nanomaterials [7, 8, 16] as linkers. In a separate study 4-ATP/\(\beta\)-CD complex was bonded to gold nanoparticles [5]. Thiols are known to oxidize to become disulfides. Evidently, previous reports ignored this common structural modification of the thiol molecules. Suitable crystals of the complex were isolated for single crystal X-ray diffraction studies. Scheme 1 illustrates the structural modification of 4-ATP to its disulfide form. The reason for oxidation was determined, though it was not measured if the oxidation of the guest molecule, 4-ATP, was accelerated in the \(\beta\)-CD environment. However, the suspected causes could be solely, or by a combination of, dissolved oxygen and atmospheric oxygen in the drying process. The solid-state structure of the complex demonstrated the host-guest interactions between the modified 4-ATP and \(\beta\)-CD (Figure 2). Each \(\beta\)-CD ring has one 4,4'-diaminodiphenyl disulfide guest molecule. Two of these complexes are in close proximity indicating noncovalent interactions. The \(\beta\)-CD rings maintain their preferred head-to-head interaction. Here, the hydrogen bonding is dominant between the -OH groups in the bigger rim of the cyclodextrin ring. It is also likely that there might be active \(\pi\)-\(\pi\) stacking interactions between the aromatic rings of two guest molecules from the neighboring complexes, as well. On average, a rough measurement of distance between such rings is 3.7(\(\pm\)4) A. Usually, parallel aromatic rings separated by a distance less than 4 A are considered to be involved in \(\pi\)-\(\pi\) stacking interactions. These rings are parallel but not completely superimposable to each other with a slight skid and twist away for carbon-to-carbon alignment of the two rings. Generally, the T-type \(\pi\)-\(\pi\) stacking is more stable than overlap-type \(\pi\)-\(\pi\). In this situation, the \(\pi\)-\(\pi\) stacking interaction is a later type. This was probably due to the strong H-bonding of the cyclodextrin rim controlling the \(\pi\)-\(\pi\) stacking interaction. However, the guest molecule when crystallized by itself also demonstrates overlap-type \(\pi\)-\(\pi\) stacking interaction. The amine group of the guest molecules is positioned outside the \(\beta\)-CD ring. This perhaps contributes to the slight enhancement of the solubility of the complex (1.18 mg/mL) compared to the host \(\beta\)-CD (1.08 mg/mL). The crystal lattice had disordered water molecules (not shown in Figure 2).