Studies of the charge-transfer complexes of 1,4-naphthoquinone acceptors

Abstract

The naphthoquinone acceptors form stable charge-transfer complexes in solutions of aprotic solvents with aromatic hydrocarbons as donors. From the charge-transfer transition energies of the complexes as well as from the polarographic half-wave reduction potentials of the acceptors relative electron affinities of the acceptors are determined. In addition, the association constant, molar extinction coefficients, oscillator strengths, and enthalpies of formation of the complexes were obtained from charge-transfer spectral studies with hexamethylbenzene as donor. The average electron affinities of 2,3-dichloro-(0·90 eV), 2,3-dichloro-5-nitro-(1·18 eV), 2,3,5,6-tetrachloro-7-nitro-(1·30 eV), 2,3-dicyano-(1·53 eV), 2,3-dicyano-5-nitro-(1·68 eV), and 2,3-dicyano-5,6-dichloro-7-nitro-1,4-naphthoquinone (1·75 eV) obtained from the charge-transfer spectral studies clearly show the cumulative effects of electron-withdrawing substituents on the naphthoquinone π-system. The π-acid character of 1,4-naphtho-quinone aceptors are also correlated with the calculated molecular orbital energies and with the experimental polarographic half-wave reduction potential.