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Figure 12
Residual electron density maps generated in the plane of the pyridinium C—N—C atoms and in the region of the O—H⋯N hydrogen bond formed between the hydrogen squarate and the un-protonated 4,4′-bipyridinium nitro­gen atom for (a), (b) the before-voltage (0 V) yellow form, (c), (d) the during-high-voltage (2400 V) red-shifted yellow form and (e), (f) the after-voltage-off (0 V) yellow form. Residuals are indicated as maxima (red regions) in the Fourier difference electron density maps (a), (c), (e) or as Q peaks (brown spheres) visualized in Olex2-1.3 (b), (d), (f). The O—H⋯N H atom is omitted from the model.

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