## 3 Results and Discussion
### Development of MW-SPM
#### 3.1.2 UV-Visible Absorption Spectra and Gap Band Energy  
The UV-visible absorption spectra of methanolic solutions of DUV and SEL were recorded in the range of 200-800 nm (Figure 2). The spectrum of DUV showed dual discrete maxima (\(\lambda_{\text{max}}\)) at 269 and 341 nm. The spectrum of SEL showed three \(\lambda_{\text{max}}\) at 210, 231, and 291 nm. Both DUV and SEL did not show any UV-absorption at wavelengths \(>\)400 nm. The spectrum of DDQ did not show light-absorption at wavelengths \(>\)340 nm. Solutions of DUV and SEL were separately mixed with DDQ solution, and the reactions were proceeded at 25 \(\pm\) 2 \({}^{\circ}\)C (room temperature), and the light absorption spectra of the mixtures were recorded versus DDQ reagent as a blank solution. Reactions of both DUV and SEL gave red-colored products showing new absorption bands at 460 nm. The absorption intensity of these new bands increased with the increase of DUV and SEL concentrations in the reaction mixtures (Figure 3). This gradual ongoing increment in the new absorption bands was indicative of the yielding of new products. The shape and pattern of these new absorption bands were identical to that of the radical anion of DDQ reported in the literature [42; 43; 44]. Therefore, the reaction was suggested to proceed in polar solvent (methanol) via CT for each of DUV and SEL as an electron donor (D) to DDQ as a \(\pi\)-electron acceptor (A) to form the complex (D-A), which subsequently dissociated and eventually gave the red-colored radical anion of DDQ:Figure 3: Absorption spectra of reaction mixtures of varying concentrations of DUV (**A**) and SEL (**B**) and with a fixed concentration of DDQ (0.1%, \(w/v\)). The values given on the spectra are the concentrations of DUV and SEL in their reaction mixtures; concentrations were in \(\upmu\)g/mL). All solutions were in methanol.  
Figure 2: Absorption spectra of DUV (1), SEL (2), DDQ (3), reaction mixture of DUV with DDQ (4), and reaction mixture of SEL with DDQ (5). Concentrations of separate solutions of DUV, SEL, and DDQ were 20, 20, and 25 \(\upmu\)g/mL, respectively. Concentrations of DUV and SEL in their reaction mixtures with DDQ (0.5%, \(w/v\)) were 200 and 300 \(\upmu\)g/mL, respectively. All solutions were in methanol.  
In the additional confirmation for the CT property of the reaction, the red color of the reaction mixture disappeared upon its acidification with mineral acids. These results were approval of the formation of the CT complexes upon reactions of DUV and SEL with DDQ. Calculations were carried out to ascertain the band gap energy (Eg), which is the lowest amount of energy required to cause an excitation of an electron from its lower-energy valence band into its higher energy in order to participate in a conduction band formation [45]. Tauc plots (Figure 4) were generated for the Eg calculation by graphing energy values (h\(\upsilon\), in eV) against (\(\alpha\)h\(\upsilon\))\({}^{2}\). By projecting the linear portions of the graphs to (\(\alpha\)h\(\upsilon\))\({}^{2}\) = 0, the values of Eg were calculated [46]. For both DUV and SEL, the Eg was found to be 2.1 eV. This small value is indicative of the easiness of electron transfer from both DUV and SEL to DDQ and giving of the CTC absorption bands.