Fig. 2: ¹H NMR spectroscopic characterization of S-2⁶⁺·6Br^–.

A Structural formulae of S-2⁶⁺·6Br^–. The partial ¹H NMR spectra (600 MHz, 298 K) of B the precursor 1²⁺·2Br^– recorded in CD₃SOCD₃ and C S-2⁶⁺·6Br^– recorded in H₂O/CD₃SOCD₃ (v/v, 3:1). The sample used in C was obtained by heating a solution of 1²⁺·2Br^– (3 mM) and (SS)-CHDA (6 mM) in H₂O (1 mL) at 80 °C for 12 h, followed by adding CD₃SOCD₃ (0.33 mL) into the solution. D 2D DOSY spectrum (298 K, D₂O/CD₃SOCD₃ (v/v, 3:1)) of S-2⁶⁺·6Br^–, which was prepared in the same procedure as that used in C, using D₂O instead of H₂O. The protons “e” and “a” underwent deuteration and the corresponding resonances disappeared. E The partial 2D NOESY spectrum (298 K, D₂O/CD₃SOCD₃ (v/v, 1:1)) of S-2⁶⁺·6Br^–. NOE signals were clearly observed between the biphenyl protons (f and g) and those in either the pyridinium or the phthalaldehyde residues (including c, c’, b and h’), which are encircled with red rectangles. NOE signals were also observed between the protons c’ and h’, as well as h and d, indicating the two imine units orientate in exo and endo manners respectively with respect to the central proton d.