*3.1. Synthesis of Schi*ff *Bases 1–4*

The spectroscopic data from the 1H-NMR and FT-IR spectra, elemental analysis results, and physical properties (e.g., melting points and colors) of the synthesized Schiff bases **1–4** were identical to those of the previously reported bases [47]. Figure 1 represents the chemical structures of the synthesized Schiff bases **1–4**. Schiff base 1 contained a nitro group, while Schiff bases 2–4 contained a hydroxyl group with different arrangements (*ortho*, *meta*, and *para*).

**Figure 1.** Schiff bases **1–4** [47].
