Charge-transfer complexation of theobromine with two strong electron-accepting molecules (DDQ AND TCNQ): Spectral, optical, and physical parameter calculations

This study investigates the charge-transfer (CT) interactions of the purine alkaloid theobromine (TB) as an electron donor with two high-affinity organic acceptors, 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). DDQ and TCNQ are well-established and widely used as benchmark organic acceptors in materials science due to their superior electron-accepting properties. The primary objectives of this research were: i) to explore the liquid-liquid CT interactions between TB and DDQ/TCNQ by reacting their solutions in a 1:1 molar ratio; ii) to explore the solid-solid CT interactions between TB and DDQ/TCNQ by reacting their solids in a 1:1 molar ratio; iii) to confirm the complexation stoichiometry of TB with DDQ and TCNQ; iv) to calculate key spectroscopic and physical parameters, including the optical band gap, using the Benesi-Hildebrand equations; and v) to characterize the resulting TB-DDQ and TB-TCNQ complexes using elemental, UV-Visible, and FT-IR spectroscopic data. The experimental results demonstrate a strong interaction between TB and both DDQ and TCNQ, leading to the rapid formation of stable, brightly colored complexes through both liquid-liquid and solid-solid reactions. Results confirm that the complexation of TB with both acceptors occurs in a 1:1 stoichiometry, proceeding via an n→π* transition in an exothermic and spontaneous manner. KEY WORDS: Theobromine, Charge-transfer complex, DDQ acceptor, TCNQ acceptor, Physical parameters Bull. Chem. Soc. Ethiop. 2026, 40(4), 795-810. DOI: https://dx.doi.org/10.4314/bcse.v40i4.6