CT complexes of DDQ with amino molecules in polar solvents

The charge-transfer (CT) complexes formed from the reaction of electron acceptors with donors containing heteroatoms, such as nitrogen, sulphur or oxygen, have seen a growing importance in recent years. In particular, they can play an important role in analytical chemistry because they can be used in the quantitative estimations of drugs [1-6]. The CT complexes formed between the drug and the acceptor absorb radiations in the visible range and the absorbance values at the maxima absorption wavelengths are used for the quantification of the drugs. The linear range of the response and the sensitivity of the detection system are related to the stoichiometry and to the stability of complexes formed. The acceptors more used with this aim are the 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetracyanoethylene (TCNE), 2,3,5,6-tetrabromo-1,4-benzoquinone (bromanil), 2,3,5,6-tetrachloro-1,4- benzoquinone (chloranil). In this work the interaction of the acceptor DDQ with molecules containing nitrogen atoms in acetonitrile and ethanol was studied. We evaluated that the DDQ preferentially interacts with not aromatic amines, therefore the attention was focused on the study of the interaction of DDQ with two molecules containing these functional groups: a -adrenergic blocking drug, the atenolol, and a synthetic local anaesthetic drug, the procaine. In order to obtain stable and coherent results we evaluated preliminarily the stability of DDQ in the two polar solvents and the time necessary to obtain a stable and high response with the drugs. The stability of DDQ is not excellent and the deteriorated solution absorb in the same range of CT complexes. In order to allow the complex formation but to avoid an eccessive degradation of free DDQ, the solutions were prepared in inert atmosphere and the signals of the complexes were recorded after 15 minutes from the solutions preparation. The experiments were conducted at 25 ± 0.1°C. For both the systems, the stoichiometries of the complexes were defined with the Job’s plot method and the stability constants were calculated from spectrophotometric titration data using the software Hyspec [7]. The sui- table concentration working range for the quantification of each drug was determinated and the quantification of procaine and atenolol in a wastewater derived from a pharmaceutical laboratory were performed. In accordance with the previous works, the CT complexes formed show spectra with 6-7 bands in the range 350-600 nm. The stoichiometry of the complexes are in all cases 1:1 and logK ranging between 3 and 5. A good linearity was obtained for the two systems in the range between 50 and 200 mg/l of drugs. The linearity range is linked to the stoichiometry of the complexes and the signals increase linearly with the concentration of the drugs until the excess of DDQ has been reached.