## 3 Results and Discussion  
### Pyridones: UV-Vis and Electrochemical Measurement Perspective  
Since findings on the electrochemical behavior of pyridone derivatives are scarce in the literature, we thereby selected compound 9 due the peculiarity of its structure. The synthesis of **9** was reported earlier [34] and its crystal structure was revealed in our previous work [23]. Namely, **9** was proven to exist in a deprotonated zwitter-ionic form in the solid state (Figure 2) [23]. On the other hand, in solutions, this compound exists in an equilibrium of its zwitter-ionic and protonated forms (Figure 2). The relative ratio of the forms depends on the pH value of the solution, favoring the protonated form in an acidic and the zwitter-ionic one in an alkaline medium. It should be pointed out that the protonated form is positively charged, while the zwitter-ionic form is formally neutral. Namely, by UV-Vis spectrophotometric titration, the pKa value of compound **9** was calculated to be 3.5. Below this calculated value, the positively charged protonated form (Figure 2) is dominant, wherein above a pH value of 3.5, the ratio of the zwitter-ionic form increases. The stated can be seen from the UV-Vis spectra in Figure 2, wherein at pH 1.81, the peak at 315 nm is ascribed solely to the protonated form. On the other hand, the increment in the ratio of the zwitter-ionic form can be evidenced by the appearance of a shoulder (pH values 2.66 and 3.03) and later of a peak at 390 nm, when the pH value increases above pKa. The emergence of the new peak is followed by bathochromic shift of the dominant peak at lower wavelengths. Furthermore, to elucidate the electrochemical activity-structure relationship, bearing in mind the pH dependency of compound **9**, CVs were recorded in a BR electrolyte with pH varying from 1.81 to 11.98. Forward scans of the CVs are shown in Figure 3. Arrows in Figure 3 depict a trend following the pH value increase. In an acidic medium (pH = 1.81-6.4), peak current values obtained for the oxidation of **9** decrease with the increase of the pH value, the highest value being 107 \(\upmu\)A cm\({}^{-2}\) for pH 1.81. In an alkaline medium (pH = 8.79-11.98), the reaction currents increase with the rise in pH up to 130 \(\upmu\)A cm\({}^{-2}\). The increase in pH value shifts the peak potential (Ep) in a negative direction. Considering the above, as well as the fact that the structure of **9** is pH-dependent (Figure 2), the involvement of protons in the electrooxidation of **9** can be assumed. Previously reported data on the pH-dependent oxidation of organic compounds show that the deprotonation reaction occurs during the oxidation process [35]. The peak potential of compound **9** correlates linearly with pH value (inset in Figure 3), with a linear regression equation of Ep(mV) = \(-\)28.85 pH + 986.02 (R\({}^{2}\) = 0.992). The calculated slope of 28.8 mV/pH is very close to the Nernst value of a two-electron and one-proton process [35]. According to this, a putative electrochemical oxidation mechanism is presented in Figure S2 (Supplementary Materials) in correspondence to data found in the literature [36]. Figure 2: UV-Vis spectra of pyridine **9** at different pH values, obtained by titration using drops of NaOH and HCl solutions with designated structures of protonated and deprotonated forms. In order to reveal the structural fragment responsible for the oxidation of **9**, pyridones **1-8** were systematically chosen for further CV experiments (Figure S1, Supplementary Materials). Firstly, **1** and **2** were used as examples of unsubstituted pyridones and were inactive in the BR buffer over the investigated pH range, indicating that neither the amide fragment (NH-C=O) nor the separated -C=O and -N-H groups could be solely responsible for the electrochemical activity in **9**. Next, substituted pyridones (**3-8**) were chosen with different substituents in positions 1, 3 and 6.