The Remarkable Effect of the Manganese Ion with Dioxygen on the Stability of π-Conjugated Radical Cations

Abstract

In this paper, nanosecond laser flash photolysis has been used to investigate the influence of metal ions on the kinetics of radical cations of a range of carotenoids (astaxanthin (ASTA), canthaxanthin (CAN), and β-carotene (β-CAR)) and various electron donors (1,4-diphenyl-1,3-butadiene (14DPB), 1,6-diphenyl-1,3,5-hexatriene (16DPH), 4-methoxy-trans-stilbene (4 MeOSt), and trans-stilbene (trans-St)) in benzonitrile. Radical cations have been generated by means of photosensitized electron-transfer (ET) using 1,4-dicyanonaphthalene (14DCN) and biphenyl (BP). The kinetic decay of CAR(· +) shows a strong dependence on the identity of the examined metal ion. For example, whereas NaClO(4) has a weak effect on the kinetics of CAR(· +), Ni(ClO(4))(2) causes a strong retardation of the decay of CAR(· +). It is also interesting to note that Mn(2+), which is a biologically relevant metal ion, shows the strongest effect of all the investigated metal ions (e.g., in the presence of Mn(2+) ions, the half-life (t(1/2)) of CAN(· +) (t(1/2) >90 ms) is more than three orders of magnitude higher than in the absence of the metal ions (t(1/2) ≈16 μs)). Furthermore, the influence of metal-ion and oxygen concentrations on the kinetics of CAR(· +) reveals their pronounced effect on the kinetic decay of CAR(· +). However, these remarkable effects are greatly diminished if either oxygen or metal ions are removed from the investigated solutions. Therefore, it can be concluded that oxygen and metal ions interact cooperatively to induce the observed substantial effects on the stabilities of CAR(· +). These results are the first direct observation of the major role of oxygen in the stabilization of radical cations, and they support the earlier mechanism proposed by Astruc et al. for the role of oxygen in the inhibition of cage reactions. On the basis of these results, the factors that affect the stability of radical cations are discussed and the mechanism that shows the role of oxygen and metal ions in the enhancement of radical-cation stability is described.