Preparation, spectroscopic and thermal characterization of charge-transfer molecular complexes formed in the reaction of 4-dimethylaminopyridine with π-electron acceptors

The interactions of the electron donor 4-dimethylaminopyridine (4DMAP) with the π-acceptors tetracyanoethylene (TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD) were studied spectrophotometrically in chloroform at room temperature. The electronic and infrared spectra of the formed molecular charge-transfer (CT) complexes were recorded. Photometric titration showed that the stoichiometries of the reactions were not fixed and depended on the nature of both the donor and the acceptor. The molecular structures of the CT-complexes were, however, affected by the amino group in 4-dimethylaminopyridine and the two methyl groups and were formulated as [(4DMAP)(TCNE)₂], [(4DMAP)(TCNQ)₂] and [(4DMAP)(TBCHD)]. The formation constant (K_CT), charge transfer energy (E_CT), molar extinction coefficients (ε_CT) and free energy change of the formed CT-complexes were obtained.