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 b-carotene (b-CAR)) and various elec- tron 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 NaClO₄ has a weak effect on the kinetics of CAR^•+, Ni(ClO₄)₂ causes a strong retardation of the decay of CAR^•+. It is also interesting to note that Mn²⁺ , which is a biologically relevant metal ion, shows the strongest effect of all the investigated metal ions (e.g., in the presence of Mn²⁺ 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