Synthesis and spectroscopic studies of charge transfer complex between dihydroxy-p-benzoquinone and 4-dimethylaminopyridine in different solvents

Charge transfer (CT) complex formation between 4-dimethylaminopyridine (4-DMAP) as the electron donor and 2,5-dihydroxy-p-benzoquinone (DHBQ) as the π-electron acceptor has been investigated spectrophotometrically in methanol (MeOH), ethanol (EtOH) and acetonitrile (AN). The stoichiometry of the complex has been identified by Job’s and photometric titration methods to be 1:1. The Benesi–Hildebrand equation has been applied to estimate the formation constant (K_CT) and molecular extinction coefficient (ε). It was found that the value of K_CT is larger in AN than in MeOH and EtOH. The thermodynamic parameters are in agreement with the K_CT values in that the enthalpy of formation (?ΔH) has a larger value both in EtOH and MeOH than in AN, suggesting higher stability of the complex in EtOH. The complex formed between 4-DMAP and DHBQ has been isolated as a solid and characterised using elemental analysis, FTIR and ¹H NMR measurements. Moreover, it has been found that the formed complex involves proton transfer in addition to CT.