Quinine Charge Transfer Complexes with 2,3-Dichloro-5,6-Dicyano-Benzoquinone and 7,7,8,8-Tetracyanoquinodimethane: Spectroscopic Characterization and Theoretical Study

The molecular charge transfer reactions of quinine (Q) with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) as a π-acceptor to form charge transfer (CT) complexes have been studied. The CT complexes were characterized by infrared spectra, NMR, mass spectrometry, conductometry and spectrometry. The Q-DDQ and Q-TCNQ charge transfer complexes were monitored at 480 and 843 nm, respectively. The results confirm the formation of CT complexes. The molar ratio of Q:DDQ and Q: TCNQ assessed using Job’s method was 1:1, which agrees with the results obtained by the Benesi-Hildebrand equation. The stability of the formed CT complexes was assessed by measuring different spectroscopic parameters such as oscillator strength, transition dipole moment, ionization potential, the energy of CT complex, resonance energy, dissociation energy and standard free energy change. The DFT geometry optimization of quinine, DDQ and TCNQ, its charge transfer complex, and UV theoretical vs. experimental comparative study were carried out. The theoretical and experimental results agreed. DFT/B3LYP/6-311++G(d,p) level of theory was used for the investigation of charge transfer between quinine as electron donor and (DDQ and TNCQ) as electron acceptors. The geometric structures, orbital energies, HOMO, LUMO and energy gaps were determined. The transition energies of the charge transfer complexes were computed using the TD-DFT/B3LYP/6-311++G(d,p) level of theory. The computed parameters were comparable to the experimental parameters, and the computational results aided in the analysis of the data.