Electron Transfer Interaction of Dihydroxyquinones with Amine Quenchers: Dependence of the Quenching Kinetics on the Aliphatic and Aromatic Nature of the Amine Donors

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Abstract
Studies on the electron transfer (ET) interaction of 1, 4‐dihydroxy‐9, 10‐anthraquinone and 6, 11‐dihydroxy‐5, 12‐naphthacenequinone with aliphatic and aromatic amine (AlA and ArA, respectively) donors have been investigated in acetonitrile solutions. Steady‐state (SS) measurements show quenching of the quinone fluorescence by amines, without indicating any change in the shape of the fluorescence spectra. No significant change in the absorption spectra of the quinones is also observed in the presence of the amines. For all the quinonamine pairs, the bimolecular quenching constants (kq) estimated from SS and time‐resolved measurements are found to be similar. Variation in the kq values with the oxidation potentials of the amines indicates the involvement of the ET mechanism for the quenching process. A reasonably good correlation between the kq values and the free energy changes (ΔG0) for the ET reactions following Marcus’ outer‐sphere ET theory also supports this mechanism. It is seen that for both the quinone‐ArA and quinone‐AlA systems, the kq values initially increase and then get saturated at some diffusion‐controlled limiting values (kqDC) as ΔG0 values gradually become more negative. Interestingly, however, it is seen that the kqDC value for the quinone‐AlA systems is substantially lower than that for quinone‐ArA systems. Such a large difference in the kqDC values between quinone‐AlA and quinone‐ArA systems is quite unusual. Present results have been rationalized based on the assumption that an orientational restriction is imposed for the encounter complexes in quinone‐AlA systems to undergo ET reactions, which arises because of the localized (at amino nitrogen) shapes of the highest‐occupied molecular orbitals (HOMO) of AlA in comparison to the π‐like HOMO of the ArA.