Spectral, electrochemical and theoretical studies on the charge transfer complexes of azacyclonol with novel substituted 1,4-benzoquinones possessing tunable electron acceptor properties

Abstract

The charge transfer (CT) complexes of a series of novel electron acceptors, 1,4-benzoquinones possessing a variable number of chloro and methoxy groups, with azacyclonol were investigated using various spectral techniques, such as UV-vis, ¹H NMR, FT-IR, LC-MS and fluorescence. The stoichiometry of the CT complexes was determined by Job's continuous variation method and was found to be 1 : 1 in all cases. The rate of the CT interaction was observed to decrease from CHL to MQ₄ and this variation is well supported by the formation constant and enthalpy of activation values. The half wave potentials, E_1/2, for the electro-reduction of these acceptors, evaluated using cyclic voltammetry, were 91, −69, −232, −383 and −556 mV, indicating progressive weakening of the electron accepting properties of the acceptors from CHL to MQ₄. Ab initio DFT calculations predicted that the variation in the bond lengths of the carbonyl moieties and the charge densities on the carbonyl oxygen atoms depend largely on the nature of the substituent present in the quinone ring. These variations have been in line with the observed FT-IR spectra of the acceptors. Also the HOMO_Donor–LUMO_Acceptor energy gaps linearly correlate with the association constants evaluated using fluorescence data. The equilibrium, kinetic, electrochemical and theoretical investigations of the CT interaction of these quinones indicated that progressive replacement of an electron withdrawing chlorine atom (−I effect) by an electron releasing methoxy group (+M effect) makes these acceptors progressively weaker.