Spectroscopic study of charge transfer complexation between doxepin and π–acceptors and its application in quantitative analysis

Abstract

Charge transfer complexes of the doxepin with π – acceptors such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetracyanoquinodimethane (TCNQ) and p-chloranilic acid (p-CA) have been studied spectrophotometrically. The stoichiometry of the reactions was studied by Job's method of continuous variations which show that the complexation ratio was 1:1 (acceptor: donor) for DDQ, TCNQ and p-chloranilic acid with doxepin base. The data were discussed in terms of association constant (K), molar absorptivity (ε), free energy change (ΔG), oscillator strength (f), transition dipole moment (μEN), resonance energy (RN) and ionization potential (ID). Three simple, fast and sensitive spectrophotometric methods were proposed for the quantification of doxepin hydrochloride in pure form and drug formulations. The methods (A, B and C) were based on the reaction of doxepin base as n-donor with DDQ, TCNQ and pCA resulting in coloured species which absorbed maximally at 570nm, 840nm and 530nm, respectively. Beer's law was obeyed in the concentration range of 5–45μgmL−1, 40–360μgmL−1, and 40–400μgmL−1 for the methods A, B, and C, respectively. A series of variables were studied to optimize the reaction conditions. The developed methods were successfully applied for the determination of doxepin in its formulations with high accuracy and precision and without interference from common excipients.