The structures of 1X were confirmed by NMR spectroscopy and mass spectrometry. The as-synthesized products were mixtures of three geometric isomers, namely ZZ, EZ (=ZE), and EE, as revealed by a 1 H NMR analysis (Figure ). Most of the peaks observed in each spectrum were assigned using two-dimensional (2D) NMR (COSY and NOESY) (Figures , and S14). The protons of the central quinoid rings showed characteristic chemical shifts depending on their proximity to the Z-or E-configured double bonds. The protons adjacent to the Z-configured bonds appeared upfield (e.g., 6.44 and 6.56 ppm for ZZ-1S and EZ-1S, respectively, in CDCl3) due to the shielding effect from the diamagnetic ring current of the neighboring aryl groups. In contrast, the protons near the E-configured bonds resonated downfield (e.g., 7.45 and 7.33 ppm for EE-1S and EZ-1S, respectively, in CDCl3). The ratios of the geometric isomers remained essentially unchanged at room temperature over the course of several hours (Figures and). This indicates a high activation barrier for isomerization. Specially, in the case of 1O, when recrystallized from ethanol, the EZ isomer EZ-1O was selectively obtained. The 1 H NMR spectrum of pure EZ-1O showed negligible changes even after three days (Figure ). Furthermore, in the NOESY spectra of all the compounds, no peaks originating from the chemical exchange between the isomers were observed within a mixing time of 0.5 s (Figures and).