*2.3. Stoichiometry*

Spectrophotometric titration measurements were carried out for the reactions of PA with both ChA and DDQ, where acetonitrile was used as a blank solvent at wavelengths of 515 and 490 nm, respectively. A 0.25, 0.50, 0.75, 1.00, 1.50, 2.0, 2.50, 3.00, 3.50, and 4.00 mL aliquot of a standard solution (1 × 10−<sup>3</sup> M) of the appropriate acceptor in acetonitrile was added to 1.00 mL of 1 × 10−<sup>3</sup> M PA (in acetonitrile), and the total volume of the mixture was 5 mL. The concentration of PA (CD) in the reaction mixture was maintained at 1 × 10−<sup>3</sup> M, whereas the concentration of the acceptors (CA) changed over a wide range of concentrations (from 0.25 × 10−<sup>3</sup> M to 4 × 10−<sup>3</sup> M) to produce solutions with an acceptor molar ratio that varied from 1:4 to 4:1. The stoichiometry of the CT complexes was obtained from the determination of the conventional spectrophotometric titration method.
