Charge transfer interaction between tetrahalo p-benzoquinones and polynuclear aromatic hydrocarbons

Abstract

The molecular charge-transfer absorption bands have been detected in mixture of tetrahalo p-bezoquinones and polynuclear aromatic hydrocarbons in chloroform, carbon tetrachloride and methyl acetate solutions. The plot of charge-transfer interaction energies against ionization energies of hydrocarbons was found to be linear. For the isolated complexes, the charge transfer interaction was found to be in the order hγ (iodanil) > (bromanil) > hγ (chloranil) for all hydrocarbons, although in solution the order was anomalous. For the complexes of the same donor with different acceptors the oscillator strength of the charge-transfer absorption band was found to increase with an increase in equilibrium constant. No such correlation was noticed with complexes of different donors with the same acceptor. The equilibrium constant increased with the lowering of the ionization energy of the hydrocarbon. There was nearly a hundredfold increase in dipole moment of the complexes on electronic excitation. The evaluation of Mulliken's a and b co-efficients showed that the charge transfer-complexes of tetrahalo p-benzoquinones with polynuclear aromatic hydrocarbons are weak complexes.