- The remarkable effect of the manganese ion with dioxygen on the stability of π-conjugated radical cations
-
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 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 (t1/2) of CAN.+ (t1/2>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. Life changes: Mn ions show a substantial effect on the lifetime of π-conjugated radical cations (D .+) in the presence of oxygen (see scheme). However, this remarkable effect is greatly diminished if either oxygen or manganese ions are removed from the solution. These observations can be attributed to the inhibition of the back electron transfer between O2. - and D.+. Copyright
- El-Agamey, Ali,Fukuzumi, Shunichi
-
-
Read Online
- Effect of oxygen on the formation and decay of stilbene radical cation during the resonant two-photon ionization
-
Formation and decay of radical cations of trans-stilbene and p-substituted trans-stilbenes (S.+) during the resonant two-photon ionization (TPI) of S in acetonitrile in the presence and absence of O2 have been studied with laser flash photolysis using a XeCl excimer laser (308 nm, fwhm 25 ns). The transient absorption spectra of S.+ were observed with a peak around 470-490 nm. The formation quantum yield of S.+ (0.06-0.29) increased with decreasing oxidation potential CEox) and increasing fluorescence lifetime (τf) of S, except for trans-4-methoxystilbene which has the lowest Eox and longer τf among S. The considerable low yield and fast decay in a few tens of nanoseconds time scale were observed for trans-4-methoxystilbene .+ in the presence of O2, but not for other S .+. It is suggested that formation of the ground-state complex between trans-4-methoxystilbene and O2 and the distonic character of trans-4-methoxystilbene.+ with separation and localization of the positive charge on the oxygen of the p-methoxyl group and an unpaired electron on the β-olefinic carbon are responsible for the fast reaction of trans-4-methoxystilbene.+ with O2 or superoxide anion, leading to the considerable low yield and fast decay of trans-4- methoxystilbene.+. The mechanism based on the transient absorption measurement of S.+ during the TPI is consistent with the relatively high oxidation efficiency of trans-4-methoxystilbene among S based on the product analysis during the photoinduced electron transfer in the presence of a photosensitizer such as 9,10-dicyanoanthracene and O2 in acetonitrile.
- Hara, Michihiro,Samori, Shingo,Xichen, Cai,Fujitsuka, Mamoru,Majima, Tetsuro
-
p. 4370 - 4374
(2007/10/03)
-
- Reactivities of Isomerization, Oxidation, and Dimerization of Radical Cations of Stilbene Derivatives
-
Reactions of radical cations of eight stilbene derivatives (S(.+)) have been studied using pulse radiolysis and γ-ray radiolysis in 1,2-dichloroethane or butyl chloride.Unimolecular isomerization from cis-S(.+) to trans-S(.+) and bimolecular reactions with O2 (oxidation) and a neutral stilbene (dimerization) occur depending on the substituents.The unimolecular c-t isomerization and the oxidation proceed preferably in S(.+) substituted with a p-methoxyl group (as an electron-donating substituent) with rate constants of ki = 4.5E6 to 1,4E7 s-1 and k02 = (1.2-4.5)E7 M-1 s-1, respectively.On the basis of transient absorption measurements, it is concluded that separation and localization of a positive charge and an unpaired electron play the most important role as the controlling factors in the reactivities of the unimolecular isomerization and the oxidation.The dimerization involves initial formation of a ?-complex with overlapping of two benzene rings and is inhibited by steric hindrance of substituents on the benzene rings and olefinic carbons.
- Majima, Tetsuro,Tojo, Sachiko,Ishida, Akito,Takamuku, Setsuo
-
p. 13615 - 13623
(2007/10/03)
-
- Photochemical generation, isomerization, and oxygenation of stilbene cation radicals
-
The cation radicals of cis- and trans-stilbene and several of their ring-substituted derivatives have been generated in solution directly by means of pulsed-laser-induced electron transfer to singlet cyanoanthracenes or indirectly via electron transfer from biphenyl to the singlet cyanoanthracene followed by secondary electron transfer from the stilbenes to the biphenyl cation radical. Transient absorption spectra of the cis- and trans-stilbene cation radicals generated by secondary electron transfer are similar to those previously obtained in 77 K matrices. Quantum yields for radical ion-pair cage escape have been measured for direct electron transfer from the stilbenes to three neutral and one charged singlet acceptor. These values increase as the ion-pair energy increases due to decreased rate constants for radical ion-pair return electron transfer, in accord with the predictions of Marcus theory for highly exergonic electron transfer. Cage-escape efficiencies are larger for trans- vs cis-stilbene cation radicals, possibly due to the greater extent of charge delocalization in the planar trans vs nonpolar cis cation radicals. Cage-escaped stilbene cation radicals can initiate a concentration-dependent one way cis- → trans-stilbene isomerization reaction. In the presence of oxygen, isomerization is inhibited and oxidative cleavage of either trans- or cis-stilbene to benzaldehyde occurs. Correlation of the measured quantum yields for oxygenation and isomerization with quantum yields for cage escape establishes that oxygenation is a nonchain process whose efficiency shows little variation with stilbene substituent or configuration, whereas isomerization is a chain process whose chain length is dependent upon substituent.
- Lewis, Frederick D.,Bedell, Ann M.,Dykstra, Ruth E.,Elbert, Jeffrey E.,Gould, Ian R.,Farid, Samir
-
p. 8055 - 8064
(2007/10/02)
-
- Substituent effects for the olefination of aldehydes by zirconium metalloazines
-
The reaction between Zr(IV) metalloazines and aldehydes affords unsymmetrical olefins and azines. Relative rates for formation of these species (and corresponding yields) were found to depend on the electron-donating ability of the metalloazine and on the electron-accepting ability of the aldehyde. Second-order rate constants were determined, and a reaction mechanism for olefin synthesis is proposed based on observed electronic effects.
- Arvanitis, Georgia M.,Schwartz, Jeffrey
-
p. 421 - 423
(2008/10/08)
-
