*3.2. FESEM of Schi*ff *Bases 1–4*

The morphologies of Schiff bases **1–4** were investigated through FESEM. Figures 2–5 show that Schiff bases **1–4** had a relatively uniform and amorphous surface with micro-size particles. The pore dimensions of the Schiff base samples varied and were found to be 20–392 nm. It was clear that the particle size of Schiff base **1** (Figure 2) was smaller than those of Schiff bases **2–4**. Schiff bases **2** and **4** (Figures 3–5) have the largest pore dimensions. Schiff base **1** had a different morphology compared to the other Schiff bases because it contains a nitro group, which causes more noticeable irregularity in particle size and shape and, thus, a highly porous structure. The presence of the functional group that had a high content of nitrogen (nitro group) could improve not only the porosity but also the surface area and efficiency for CO2 uptake [33].

**Figure 2.** Field emission scanning electron microscopy (FESEM) image of Schiff base **1**.

**Figure 3.** FESEM image of Schiff base **2**.

**Figure 4.** FESEM image of Schiff base **3**.

**Figure 5.** FESEM image of Schiff base **4**.

The pore dimensions of Schiff bases **1–4** were smaller than those reported for some POPs and larger than those for telmisartan tin complexes [54–56]. For example, POPs containing polyphosphates derived from 1,4-diaminobenzene showed irregular and porous structures with pore dimensions of 49–981 nm [54]. In addition, polyphosphates derived from benzidine showed porous structures with pore dimensions of 28–806 nm [55]. In contrast, the pore dimensions of telmisartan tin complexes ranged from 20 to 51 nm.
