*3.1. Synthesis of CT Complexes PA-ChA and PA-DDQ*

The interaction of PA (donor) with ChA/DDQ (as π acceptors) in methanol produced colored CT complexes with high molar absorptivity. The synthesis of CT complexes was straightforward as explained in Scheme 1. Electron donor PA in methanol was simply added to the acceptor molecules (ChA/DDQ) to form colored CT complexes (Scheme 1).

## *3.2. Electronic Absorption Spectra*

CT complexes were characterized using the spectrophotometric method. As shown in Figure 2, the absorption intensity of the CT complexes was related to the formation of the radical anion. PA showed peaks at 206 (n→σ\* for C-N), 264 (n→π\* for C=O), and 299 (n→π\* for C=O) nm at the concentration of 8 × 10−<sup>5</sup> M; ChA at 230 (n→π\* for C=O), 282 (n→π\* for C=O), and 431 (n→π\* for C=O) nm; and DDQ at 249 (n→π\* for C=O) and 380 (n→π\* for C≡N) nm at the concentration of 1 × 10−<sup>4</sup> M. Absorption maxima for PA-ChA were found at 246 (n→π\* for C=O), 316 (n→π\* for C=O), and 525 (n→π\* for the visible region) nm and those for PA-DDQ were found at 246 (n→π\* for C=O), 306 (n→π\* for C=O), 458 (n→π\* for the visible region), 543 (n→π\* for the visible region), and 590 (n→π\* for the visible region) nm at the concentration of 4 × 10−<sup>4</sup> M (Figure 2). Acetonitrile was used as

solvent blank for all measurements. The resulting color was stable for more than five hours, indicating the high stability of these complexes. The band gaps of the formed complex were calculated from the formula Eg (eV) = hυ = hc/λae (nm) (2) where "h" is Planck's constant, c is the velocity of light, and λ is the wavelength. The energy gaps were 2.41 and 2.53 eV for PA-ChA and PA-DDQ complexes, respectively.

**Scheme 1.** Synthesis of CT complexes PA-ChA and PA-DDQ.

**Figure 2.** Absorption curve of CT complex: (**A**) absorption maximum of PA, ChA, and DDQ; (**B**) absorption maximum of PA-ChA and PA-DDQ complexes; and (**C**) expanded spectra shown at 400–650 nm for PA-ChA and PA-DDQ complexes (all spectra were taken in acetonitrile).

#### *3.3. Molecular Composition of the PA Complexes*

The stoichiometry of the investigated complexes was monitored and determined spectrophotometrically using spectrophotometric titration (Figure 3). As shown in Figure 4, PA reacts with CHA/DDQ in a 1:1 ratio.

**Figure 3.** Spectrophotometric titration curves of A PA-ChA and B PA-DDQ CT complexes in acetonitrile at the detectable peaks (donor and acceptor, 1 × 10−<sup>3</sup> M).

**Figure 4.** The 1:1 Benesi–Hildebrand plot of procainamide CT complexes at detectable peaks: (**A**) for PA-ChA and (**B**) PA-DDQ.
