Synthesis, spectroscopic, thermal, structural characterization and DFT/TD-DFT computational studies for charge transfer complexes of 2,4-diamino pyrimidine with some benzoquinone acceptors

Abstract New charge transfer complexes between the donor 2,4-diamino pyrimidine (DAPY) and the π-acceptors, chloranilic acid (CHA) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) were synthesized and characterized. Investigations were conducted in liquid state using methanol as solvent at different temperatures and in the solid state. The stoichiometry of the complexes was identified as 1:1 using Job's and the photometric titration methods. The formation constant (KCT) of the complexes was determined, and it was revealed that the KCT of [DAPY-CHA] was slightly higher than that of [DAPY-DDQ]. The spectroscopic physical and thermodynamic parameters were also within the accepted values, confirming the stability of the formed CT complexes. A simple, rapid, and accurate spectrophotometric method for the quantitative analysis of DAPY in its pure form was applied and statistically validated. The solid complexes were prepared and characterized using various analytical tools, including FT-IR, 1H NMR, 13C NMR spectra, UV–Vis spectroscopy, TG analysis and powder X-ray diffraction (XRD). The structures of the [DAPY-CHA] and [DAPY-DDQ] complexes were determined using density functional theory (DFT) at wB97X-D and CAM-B3LYP level of theory employing 6-31++G(d,p) basis sets. The charge transfer process revealed that CHA and DDQ were the electron acceptor molecules, while the DAPY was the electron donor. In the [DAPY-CHA] and [DAPY-DDQ] complexes, the amounts of electrons (average value) transferred from the donor to the acceptor were 0.144 e and 0.063 e, respectively. The electronic properties and the UV–Vis spectra of the studied systems were also discussed, and we found that there was consistency between the experimental and theoretical results.