The remarkable effect of the manganese ion with dioxygen on the stability of π-conjugated radical cations

Article abstract of DOI:10.1002/chem.201201328

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 (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. 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

Full text of DOI:10.1002/chem.201201328

FULL PAPER
DOI: 10.1002/chem.201201328
The Remarkable Effect of the Manganese Ion with Dioxygen on the Stability
of p-Conjugated Radical Cations
Ali El-Agamey*[a, b] and Shunichi Fukuzumi*[a, c]
C
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-buta-
diene (14DPB), 1,6-diphenyl-1,3,5-hex-
atriene (16DPH), 4-methoxy-trans-stil-
bene (4MeOSt), and trans-stilbene
(trans-St)) in benzonitrile. Radical cati-
ons have been generated by means of
photosensitized electron-transfer (ET)
using 1,4-dicyanonaphthalene (14DCN)
CAR +, Ni
tardation of the decay of CAR +. It is
also interesting to note that Mn2+
(ClO4)2 causes a strong re-
remarkable effects are greatly dimin-
ished if either oxygen or metal ions are
removed from the investigated solu-
tions. Therefore, it can be concluded
that oxygen and metal ions interact co-
operatively to induce the observed sub-
C
,
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
stantial effects on the stabilities of
+
+
C
C
U
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 stabil-
ity of radical cations are discussed and
the mechanism that shows the role of
oxygen and metal ions in the enhance-
ment of radical-cation stability is de-
scribed.
ence of metal-ion and oxygen concen-
trations on the kinetics of CAR re-
+
C
veals their pronounced effect on the ki-
netic decay of CAR +. However, these
C
and biphenyl (BP). The kinetic decay
+
C
of CAR shows a strong dependence
Keywords: carotenoids
tion · dismutation · manganese ·
oxygen · radical ions
· conjuga-
on the identity of the examined metal
ion. For example, whereas NaClO4
has a weak effect on the kinetics of
Introduction
Winstein et al. attributed the initial sharp rise in the rate
constant, which he called a special salt effect, to the preven-
tion of the return of the solvent-separated ion pair (SSIP) to
contact ion pair (CIP) (Scheme 1).[1]
A special salt effect was first observed by Winstein et al.
during their study of the salt effect on the solvolysis reac-
tions of alkyl arylsulfonates. They observed a sharp rise in
the rate constant at low salt concentrations, which cannot be
explained by normal salt effects. However, at higher salt
concentrations, an increase in the rate constant follows the
normal salt effect, which depends on the ionic strength.[1]
Scheme 1. Various species formed following the heterolytic cleavage of
RX.
[a] Dr. A. El-Agamey, Prof. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering
Osaka University, ALCA, JST, Suita
Osaka 565-0871 (Japan)
For photoinduced electron transfer (PET) reactions, there
are many examples of the remarkable effects of various salts
on these reactions and the dependence of their efficiencies
on the nature of the metal ion used.[2–8] For example, Moore
E-mail: a_el_agamey@yahoo.co.uk
fukuzumi@chem.eng.osaka-u.ac.jp
et al. studied the influence of salts on the intramolecular
+
C
Cꢀ
charge-separated state (CAR -P-Q ) of a carotenopor-
phyrin-quinone triad (i.e., tetraarylporphyrin (P) covalently
linked to both a carotenoid (CAR) and a quinone (Q)) in-
duced by visible-light excitation of the porphyrin moiety in
the triad (Scheme 2). They found that saturation of a solu-
tion of the triad with tetra-n-butylammonium tetrafluorobo-
rate in chloroform significantly enhanced the yield and the
half-life of carotenoid radical cations.[9] Also, Mizuno et al.
investigated extensively the role of metal salts on photoiso-
[b] Dr. A. El-Agamey
Chemistry Department, Faculty of Science
Damietta University
New Damietta, Damietta (Egypt)
[c] Prof. S. Fukuzumi
Department of Bioinspired Science
Ewha Womans University
Seoul 120-750 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201328.
Chem. Eur. J. 2012, 00, 0 – 0
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ide.[2,10–13] Furthermore, a similar mechanism was also used
to explain the influence of salts in the presence of oxygen
on the photooxidation of tertiary methylamines when using
9,10-dicyanoanthracene as sensitizer.[2,3] Therefore, accord-
ing to these results, oxygen was proposed to have a signifi-
cant role through the formation of superoxide anion.
Scheme 2. Different pathways obtained following excitation of the por-
phyrin moiety (p) in the carotenoporphyrin–quinone triad (CAR-P-Q).
BET: back electron transfer.
In PET reactions in the presence of oxygen, superoxide
Cꢀ
anion (O2 ) can be formed if the ET reaction from the ini-
tially formed radical anion to oxygen is thermodynamically
merization, photoxygenation, and TiO2-catalyzed reactions,
and they found that metal salts have significant effects on
the quantum yields of various products.[4–7]
The significant effects produced by the salts in photoin-
duced electron-transfer reactions can be interpreted as de-
scribed in Scheme 3. Following the PET reaction, the initial-
Scheme 5. Proposed interactions of O2 and salt [M+,Xꢀ] with the CIP
+
C
Cꢀ
[D ,X
]
formed following photoinduced electron transfer between
donor (D) and acceptor (X).
feasible (Scheme 5).[14–16] Therefore, according to Scheme 4,
addition of salts to these reactions is expected to increase
the lifetime of the initially formed radical cation (D +) due
C
to the suppression of BET by means of metal-induced dis-
Cꢀ
Scheme 3. Influence of salt [M+,Xꢀ] on the contact ion pair (CIP)
mutation of O2 (Scheme 5). However, this assumption has
+
C
Cꢀ
never been directly tested before.
A
To verify the validity of this proposal (Scheme 5), we
have examined for the first time the influence of oxygen
concentration on the kinetic decay of various radical cations
in the presence of metal ion by using laser flash photolysis
(LFP) coupled with kinetic absorption spectroscopy. Also,
the influence of metal-ion concentration on the kinetic of
radical cations has been investigated. In addition, the effect
of the type of the metal ion used on the kinetic decay of
radical cations has been studied. Furthermore, the factors
that affect the stabilities of radical cations have been dis-
cussed. The structures of carotenoids and other compounds
used in this study are given in Figure 1.
+
C
Cꢀ
ly formed CIP [A ,B ] undergoes a fast, almost diffusion-
controlled double ion-exchange reaction with the salt
+
ꢀ
+
ꢀ
+
C
Cꢀ
ACHTUNGTRENNUNG[M ,X ] to form [A ,X ] and [M ,B ], which results in the
suppression of the back-electron-transfer (BET) reaction be-
tween the radical ions of the initially formed CIP
(Scheme 3).[2,4–7]
Of particular interest is the role of oxygen in the special
salt effect.[2–6,10–12] In a series of elegant studies by Astrucꢁs
group,[2,10–12] they investigated the influence of salts on the
reduction of O2 by iron(I) complexes (D), and they found
that in the presence of sodium hexafluorophosphate,
ꢀ
D+PF6 and Na2O2 were formed, whereas the formation of
cage products, which are formed in the absence of salt, was
totally inhibited (Scheme 4). These observations were attrib-
uted to the shift in the equilibrium of the ion-exchange reac-
Results
Influence of different metal ions on the ground-state spectra
of the carotenoids: Carotenoids are lipophilic pigments that
are responsible for the color of many vegetables and ani-
mals. They are characterized by long conjugated double
bonds, which result in a strong absorption in the visible
region.[17] The absorption maxima (lmax) and the molar ab-
sorption coefficients (e) for the carotenoids under investiga-
tion are given in Table 1.
+
ꢀ
+
C
Cꢀ
tion toward the formation of [D ,PF6 ] and [Na ,O2 ] due
to the metal-induced dismutation of superoxide to perox-
For astaxanthin (ASTA), lmax =495 nm, there is no signifi-
cant change in its spectrum in the presence of Li+, Na+, and
+
NH4 ions in benzonitrile. On the other hand, in the pres-
ence of certain metals (e.g., Mn2+, Zn2+, Ca2+, Ni2+, Mg2+
,
Lu3+, Sc3+, and Al3+), there is a red shift in its UV/Vis spec-
trum, and this shift is dependent on the type of metal ion
+
C
Cꢀ
Scheme 4. Influence of NaPF6 on the CIP [D ,O2 ] formed following
the reduction of O2 by iron (I) complexes (D).
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Stability of p-Conjugated Radical Cations
FULL PAPER
Scheme 6. Formation of the ASTA–Mn+ complex.
the formation constant (K) for the complex between Sc3+
ions and ASTA was determined in benzonitrile as (4.94ꢂ
0.50)ꢂ106 mꢀ2 using a modified Benesi–Hildebrand equation
[Equation (S4) in the Supporting Information; Figure 2 and
Figure S4B in the Supporting Information]. Also, the linear
plot observed in Figure S4B clearly indicates that the stoi-
chiometry of the ASTA–Sc3+ complex is 1:2.
On the other hand, CAN and b-CAR, unlike ASTA, do
not show any significant spectral change in their ground-
state spectra following the addition of various metal ions.
Figure 1. Structures of carotenoids and other compounds used in this
study.
+
C
Table 1. The absorption maxima (lmax) for CAR and CAR and the
Influence of metal ions on ASTA +: LFP (355 nm) of a solu-
molar absorption coefficients (e) for CAR in benzonitrile.
C
[a]
e [104 mꢀ1 cmꢀ1
]
lmax (CAR +) [nm]
tion of 5ꢂ10ꢀ3 m 1,4-dicyanonaphthalene (14DCN) and 0.3m
C
CAR
lmax (CAR) [nm]
8.41[b]
9.88
880
880
980
biphenyl (BP) in air-saturated benzonitrile results in the for-
ASTA
CAN
b-CAR
495
490
470
mation of biphenyl radical cation, BP +, through fast ET
C
11.64[b]
quenching of the singlet excited state of 14DCN (Scheme 7).
[a] e of CAR at lmax in benzonitrile. [b] Taken from ref. [18].
Scheme 7. Proposed pathways following photolysis of BP and 14DCN in
the presence of electron donor (D) and O2.
+
C
The absorption spectrum of BP (Figure S5 in the Support-
ing Information), which exhibits two distinct absorption
bands in the visible region at 380 and 680 nm, is similar to
+
C
that reported previously by Lew et al. for BP in oxygen-sa-
Figure 2. The influence of scandium-ion concentration on the ground-
turated acetonitrile.[21] The 1,4-dicyanonaphthalene radical
state spectra of ASTA (1ꢂ10ꢀ5 m) in a 1 cm path-length cell.
Cꢀ
anion (14DCN ), which is characterized by a strong band at
390 nm and a weak band at approximately 500 nm, does not
interfere with the observed kinetics, since it is rapidly
quenched by oxygen (k=1.3ꢂ1010 mꢀ1 sꢀ1).[22–25] Furthermore,
it is important to note that laser photolysis (355 nm) of solu-
tions of 14DCN and BP in air-saturated benzonitrile in the
presence or absence of metal ions does not give rise to any
transient absorption features in the NIR region.
used (Figure 2 and Figures S1 and S2 in the Supporting In-
formation). The largest shift has been observed in the pres-
ence of Sc3+ ions (lmax =539 nm), which is employed as
ScACHTUNGTRENNUNG
the latter metal ions to form metal complexes. According to
the literature,[19,20] the complex formation is achieved
through the chelation of metal ion through the oxygen
atoms on the cyclohexene rings (Scheme 6). Furthermore,
Following LFP (355 nm) of a solution of 5ꢂ10ꢀ3 m 14DCN
and 0.3m BP in air-saturated benzonitrile that contained 1ꢂ
10ꢀ4 m ASTA, a strong NIR absorption around 880 nm at-
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A. El-Agamey and S. Fukuzumi
tributed to astaxanthin radical cation (ASTA +) was ob-
C
served (Figure 3 and Scheme 7). This transient spectrum
(Figure 3) is in good agreement with that observed following
Figure 3. Transient spectra obtained following LFP (355 nm) of a solution
of 14DCN (5ꢂ10ꢀ3 m) in air-saturated benzonitrile containing BP (0.3m)
and ASTA (1ꢂ10ꢀ4 m).
Cꢀ
ASTA oxidation with Br2 [Eq. (1)] in aqueous 2% Triton
X-100 (lmax =875 nm).[26] Also, the ET reaction from ASTA
+
C
to BP is supported by the fact that the oxidation potential
of BP (Eox =1.96 V versus SCE in acetonitrile) is significant-
ly higher than those reported for all carotenoids investigated
in this study (Eox <0.9 V versus SCE in various sol-
vents).[27,28] In addition, no transient absorption features
have been observed in the NIR region following laser irradi-
ation (355 nm) of air-saturated benzonitrile solutions that
contain any of the carotenoids investigated in this study.
ꢀ
Cꢀ
Cþ
Br2 þ ASTA 2 Br þ ASTA
ð1Þ
!
+
C
The influence of metal ions on ASTA in air-saturated ben-
Figure 4. Influence of 0.002m of different salts (A, salts with strong
zonitrile, generated as described earlier, has been investigat-
effect; B, salts with moderate to weak effect; and C, salts with very weak
+
+
C
C
effect) on the kinetic decay of ASTA at 900 nm generated following
ed. Figure 4 shows that the lifetime of ASTA is strongly
LFP (355 nm) of a solution of 14DCN (5ꢂ10ꢀ3 m) in air-saturated benzo-
dependent on the identity of the metal ion used. Since the
nitrile containing BP (0.3m) and ASTA (1ꢂ10ꢀ4 m). For comparison, the
+
C
kinetic decay of ASTA in the presence of metal ions is
+
C
kinetic absorption profile of ASTA in the absence of salts was added to
+
C
complicated, the half-life of ASTA (t1/2) has been taken as
each plot.
a qualitative measure to compare the influence of various
metal ions on the stability of ASTA +. Figure 4A and Fig-
C
ure S6 in the Supporting Information show clearly the re-
markable effect of Mn2+ on the stability of ASTA +. For ex-
C
ample, in the presence of Mn2+ ions (as Mn
ACHTUNTRGNEUNG(ClO4)2), which
induces the strongest effect of all the metal ions under in-
+
C
vestigation, the half-life (t1/2 ꢁ30 ms) of ASTA is three
orders of magnitude higher than that formed in the absence
of the metal ions (t1/2 ꢁ14 ms). To the best of our knowledge,
no similarly substantial effect has been reported for the in-
fluence of metal ions on the stabilities of radical cations.
According to half-life measurements (Figure 5), the influ-
+
C
ence of metal ion on the stability of ASTA can be divided
into three categories. The first category, which includes
Mn
Ca(ClO4)2 (or Ca
ity of ASTA (Figure 4A). The second category, which in-
cludes Sc(OTf)3, Zn(ClO4)2 (or Zn(OTf)2), Mg(ClO4)2,
G
ACHTNUGTNER(NUGN OTf)2), NiACHTUNRTGEG(NNNU ClO4)2, LuACHTNUGTREN(UNGN OTf)3, and
Figure 5. The influence of various metal ions on the half-life (t1/2) of
E
ACHTUNGTRENNUNG
+
C
ASTA at 900 nm in air-saturated benzonitrile. The inset shows the in-
+
+
C
C
fluence of various metal ions on the half-life (t1/2) of ASTA on shorter
timescales.
N
A
R
ACHTUNGTRENNUNG
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Stability of p-Conjugated Radical Cations
FULL PAPER
AlACHTUNGTRENNUNG(ClO4)3, NH4ClO4, LiClO4, and NaClO4 has a moderate
+
to weak effect on the stability of ASTA (Figure 4B).[29] Fi-
nally, the last group, which includes tetra-n-butylammonium
perchlorate (TBAClO4) and tetra-n-butylammonium hexa-
fluorophosphate (TBAPF6), shows a very weak effect on the
C
stability of ASTA +. Therefore, it can be concluded that
C
ꢀ
ꢀ
TBA+, PF6 , and ClO4 have a very weak influence on the
+
C
stability of ASTA (Figure 4C). This is in agreement with
previous results, which showed that the perchlorate anion
has almost no effect on the kinetics of stilbene radical-
cation derivatives.[31] Furthermore, the influence of metal
Figure 7. The influence of oxygen concentration in the presence of
ions (e.g., MnACHTUNGTRENNUNG(ClO4)2 and ScACHTUNTGERN(NGUN OTf)3) on the transient spec-
+
C
MnACHTUGNTRENNNUG
+
C
trum of ASTA has been investigated. From Figures S7 and
S8 in the Supporting Information, it is clear that there is no
+
+
C
C
significant change in the transient spectrum of ASTA in
on the stability of ASTA (Figure 7 and Figure S13 in the
the presence of these salts within the timescale used in these
experiments.
Supporting Information). At oxygen concentrations of 5%
+
C
or higher, the transient absorption profiles of ASTA at
Furthermore, the influences of Mn
on the transient absorption profiles of ASTA have been
G
E
900 nm, show almost similar kinetic behavior. However, at
+
+
C
lower oxygen concentration, the decay of ASTA is greatly
investigated. It is clear that the kinetics of ASTA are not
significantly changed in the presence of either Mn(ClO4)2 or
Mn(OTf)2 (Figure S9 in the Supporting Information). In ad-
accelerated. The influence of oxygen on the stability of
+
C
ASTA in the presence of the other first-category metal
R
ions (Lu3+, Ni2+, Ca2+), which show a strong effect on the
stability of ASTA +, was also examined. Figure 8 clearly
C
dition, similar results were also obtained for the influence of
other metal perchlorate and triflate (e.g., Ca2+ and Zn2+
shows that oxygen plays an essential role in the stabilization
+
+
C
C
ions) on the decay of ASTA (see Figures S10 and S11 in
of ASTA in the presence of these metal ions (0.002m). To
the Supporting Information). Consequently, perchlorate or
triflate salts of a certain metal ion can both be used with
clarify whether or not metal ions have any role in the stabili-
zation of ASTA +, laser photolysis (355 nm) of air- and
C
equal success to investigate its role on the stability of
argon-saturated solutions of 5ꢂ10ꢀ3 m 14DCN, 0.3m BP, and
1ꢂ10ꢀ4 m ASTA in benzonitrile was carried out. Figure S14
in the Supporting Information clearly shows that in the ab-
+
C
CAR
.
In addition, the influence of metal-ion concentration on
+
the kinetics of ASTA in air-saturated benzonitrile was in-
vestigated. Mn(ClO4)2 was chosen to examine the effect of
sence metal ions, there is no stabilization for ASTA +, that
C
C
+
C
G
is, transient profiles of ASTA at 900 nm have almost simi-
the metal-ion concentration because of its remarkable effect
lar kinetic behavior. This clearly indicates that both oxygen
on the stability of ASTA +. At low metal-ion concentrations
and metal ions are essential to the creation of substantial
C
+
(ꢃ1ꢂ10ꢀ5 m), there is little change in the kinetics of ASTA
stabilization for ASTA +. Also, these results give the first
C
C
. However, at higher concentrations (>1ꢂ10ꢀ5 m), the life-
time of ASTA is significantly increased (Figure 6 and Fig-
direct observation of the role of oxygen in the special salt
effect, which was previously proposed by Astrucꢁs group
(Scheme 4).[2,10–12]
+
C
ure S12 in the Supporting Information).
Laser photolysis (355 nm) of solutions of 5ꢂ10ꢀ3
m
14DCN, 0.3m BP, 1ꢂ10ꢀ4 m ASTA, and 0.002m Mn
(ClO4)2
Influence of metal ions on CAN and b-CAR +: Laser irra-
+
C
C
in benzonitrile in the presence of various oxygen concentra-
tions shows a substantial effect of the oxygen concentration
diation (355 nm) of air-saturated solutions of 5ꢂ10ꢀ3
m
14DCN and 0.3m BP in benzonitrile in the presence of 7ꢂ
10ꢀ5 m of various carotenoids (CAN and b-CAR) resulted in
+
+
C
C
(lmax =880 nm) and b-CAR
the formation of CAN
(lmax =980 nm) (see Figure 9A and B, respectively). In addi-
+
C
tion, in the absence of oxygen, the kinetics of CAN and
b-CAR are not significantly changed (see, for example,
+
C
Figure S15 in the Supporting Information). The influences
+
+
C
C
of various metal ions on CAN and b-CAR stabilities
were studied. Figures S16 and S17 in the Supporting Infor-
mation show that metal ions behave in a similar way to that
+
C
observed with ASTA
.
+
C
In addition, similar results to that of ASTA were ob-
served for the influence of metal-ion and oxygen concentra-
+
+
C
C
tions on the stabilities of CAN and b-CAR (Figur-
es S18–S23 in the Supporting Information). On the other
hand, whereas Mn2+ ions show substantial effects on the sta-
Figure 6. The influence of Mn2+ concentration, used as Mn
(ClO4)2, on
+
C
the kinetics of ASTA at 900 nm in air-saturated benzonitrile.
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A. El-Agamey and S. Fukuzumi
+
C
90 ms) and ASTA
(t1/2
ꢁ30 ms) (Figure 5, Figures S24
and S25 in the Supporting In-
formation). By comparing the
structures of these carotenoids,
it is clear that the only feature
that distinguishes ASTA and
CAN from b-CAR is the pres-
ence of heteroatoms, oxygen
atoms, in their structural for-
mulas (Figure 1). This suggests
that the presence of heteroa-
tom plays a role in the stabili-
zation of CAR +, possibly by
C
means of metal-ion–heteroa-
tom interactions.
Furthermore, as shown in
Figure 8 and Figures S21 and
S23 in the Supporting Informa-
tion, there is some stabilization
Figure 8. Transient profiles of ASTA +, at 900 nm, in air- or argon-saturated benzonitrile in the presence of
C
+
C
of CAR in argon-saturated
+
0.002m A) Mn2+, B) Ni2+, C) Lu3+, or D) Ca2+. For comparison, the kinetic absorption profile of ASTA in
C
solutions in the presence of
salts relative to those formed
in the absence of salts. This
the absence of metal ions, in air-saturated benzonitrile, was added to each plot. For a colored version of this
figure, see the Supporting Information.
stabilization can be attributed to the salt interactions with
CIP. In accordance with these observations, some radical
cations were reported to have some stabilization in the pres-
ence of Mg2+ ions in argon-saturated acetonitrile.[31,32]
Figure 9. Transient spectra obtained following LFP (355 nm) of 14DCN
(5ꢂ10ꢀ3 m) and BP (0.3m) in air-saturated benzonitrile containing
A) CAN (7ꢂ10ꢀ5 m) or B) b-CAR (7ꢂ10ꢀ5 m).
+
C
+
bilities of CAN and b-CAR +, as illustrated in Figure 10
Figure 10. Transient profiles of solutions of A) CAN (at 890 nm) or
C
C
+
C
B) b-CAR (at 980 nm) in air- or argon-saturated benzonitrile contain-
ing 0.002m MnACHTUGNTERN(UNNG ClO4)2. For comparison, the kinetic absorption profiles of
CAN or b-CAR in the absence of metal ions and in air-saturated ben-
and Figures S16A, S17A, S18, and S19 in the Supporting In-
+
+
+
C
formation, their stabilization effects on b-CAR (t1/2 ꢁ3 ms)
C
C
+
C
are still weaker than those observed with CAN (t1/2
>
zonitrile were added to plots A and B, respectively.
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Stability of p-Conjugated Radical Cations
FULL PAPER
Table 2. Oxidation potentials (Eox)[a], lmax of electron-donor radical cati-
+
Since BP is the precursor of CAR +, it was important to
C
C
ons (D +), and free-energy change (DG) for the electron-transfer reac-
C
examine the influence of metal ions on its kinetics. Figur-
es S26 and S27 in the Supporting Information show that the
lifetime of BP increases slightly in the presence of Mn-
+
C
tions between BP and various electron donors (D).
+
C
+
D
lmax of D [nm]
Eox [V]
DG [kcalmolꢀ1
]
C
1.21[41]
1.02[39]
1.18[35]
1.52[39]
ꢀ17.30
ꢀ21.68
ꢀ17.99
ꢀ10.15
+
14DPB
16DPH
4MeOSt
trans-St
560
600
500
480
C
ACHTUNGTRENNUNG
comparing the influence of Mn2+ ions on CAR and BP
(Figures 8A, 10 and Figures S6, S26 in the Supporting Infor-
mation), it can be concluded that the overall metal-ion inter-
actions with BP are not responsible for the remarkable ef-
+
+
C
C
[a] Oxidation potentials (Eox) were measured against SCE in acetonitrile.
+
C
+
corresponding radical cations (D +) (Figures S29–S31 in the
C
C
fects observed on the stabilities of CAR
.
Supporting Information).[40]
+
C
Influence of metal ions on other electron donors: In this
part, different electron donors (D) other than carotenoids
were investigated to examine the influence of Mn2+ ions
In a similar trend to that observed with CAR, 14DPB
16DPH , and 4MeOSt show significant stabilization in
the presence of oxygen and 0.002m Mn(ClO4)2 (Figure 12A–
,
+
+
C
C
AHCTUNGTRENNUNG
and oxygen on the stabilities of their radical cations (D +).
C). Again, these results confirm that oxygen and certain
metals interact cooperatively to induce remarkable effects
C
Laser photolysis (355 nm) of air-saturated solutions of
14DCN and BP in benzonitrile in the presence of various
electron donors (14DPB, 16DPH, 4MeOSt, and trans-St)
on the stabilities of the radical cations. On the other hand,
+
C
trans-St shows little stabilization in the presence of 0.002m
+
C
gave rise to their corresponding radical cations (14DPB
,
MnACTHUNGTERN(NUG ClO4)2 (Figure 12D and Figure S32 in the Supporting In-
formation). Furthermore, in the presence of the metal ion,
the observed kinetics are hardly affected by the absence of
16DPH , 4MeOSt , and trans-St +) (Figure 11). The tran-
+
+
C
C
C
C
sient spectra of 14DPB , 16DPH , 4MeOSt +, and trans-
+
+
C
C
+
C
St are in good agreement with the previously reported
oxygen, which suggests that oxygen has almost no influence
+
spectra of these radical cations (Figure 11).[33–39] The calcu-
on the stabilization of trans-St (Figure 12D). Comparing
C
+
C
lated free-energy change (DG) for the ET reactions between
the influence of Mn2+ ions on trans-St
with that of
+
+
BP and various D, lmax values of D +, and the oxidation
4MeOSt again raises the possibility of the involvement of
C
C
C
potentials (Eox) of D are noted in Table 2. Moreover, the
metal-ion–heteroatom interactions in the stabilization of
+
+
C
C
transient absorption profiles of D show similar kinetic be-
4MeOSt
.
+
C
havior in air- and argon-saturated benzonitrile (Figure S28
in the Supporting Information). Furthermore, it is important
to note that laser irradiation (355 nm) of solutions of D
alone in air-saturated benzonitrile does not give any transi-
ent (or very weak transient) absorption at the lmax of their
Furthermore, the transient spectrum of BP overlaps
with the transient spectra of D +, therefore it is important to
C
+
C
compare the transient absorption profiles of D with that
of BP under the same experimental conditions. Figur-
+
C
es S33–S36 in the Supporting Information clearly reveal that
+
C
BP absorption interferes very
little with the observed kinetics
+
C
of D at their lmax value. In
addition, the intensity of the
transition absorption profile of
+
C
BP observed in the absence
of D will be significantly re-
duced in the presence of vari-
ous electron donors on account
of its quenching by these elec-
tron donors.
Discussion
In light of these results and the
work of Astrucꢁs group, the
principal steps that show the
role of oxygen in the presence
of salts [Mn+,Xꢀ] in photoin-
duced ET reactions are descri-
bed in Scheme 8. Photolysis of
14DCN in the presence of BP
and various electron donors
Figure 11. Transient spectra obtained following LFP (355 nm) of solutions of 14DCN (5ꢂ10ꢀ3 m) and BP
(0.3m) in air-saturated benzonitrile containing A) 14DPB (1ꢂ10ꢀ4 m) (laser energy=13 mJ), B) 16DPH (7ꢂ
10ꢀ5 m), C) 4MeOSt (1ꢂ10ꢀ3 m), or D) trans-St (1ꢂ10ꢀ3 m).
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A. El-Agamey and S. Fukuzumi
It is important to note that
the complex formation be-
Cꢀ
tween metal ion and O2
(Scheme 8,
pathway iii)
is
based on the results of many
reports about the ability of
Cꢀ
O2
to react with various
metal ions (e.g., Li+, Mg2+
Ca2+, Sr2+, Zn2+, Tl+, Ba2+
Cd2+, Mn2+, Fe2+, Co2+, Y3+
,
,
,
Sc3+, Lu3+, and Cr3+) to form
metal-ion–superoxo complexes
[(Mn+
)C
Also,
Cꢀ
(O2 )].[45–53,56,59]
the formation of metal-ion–su-
peroxo complexes was report-
ed to promote thermodynami-
cally unfavorable electron-
transfer reactions between
oxygen and various electron
donors.[60] In addition, the
binding energies of many
+
+
+
C
C
C
Figure 12. Transient profiles of solutions of A) 14DPB (at 560 nm), B) 16DPH (at 600 nm), C) 4MeOSt
+
Cꢀ
C
(at 500 nm), or D) trans-St (at 480 nm) in air- or argon-saturated benzonitrile containing 0.002m MnACTHNUTRGNE(UNG ClO4)2.
metal ions with O2 have been
+
+
+
+
C
C
C
C
For comparison, the kinetic absorption profiles of 14DPB , 16DBH , 4MeOSt , or trans-St in the absence
of metal ions, and in air-saturated benzonitrile were added to plots A, B, C, and D, respectively. For a colored
version of this figure, see the Supporting Information.
evaluated.[56,61,62]
It is also noteworthy that, in
addition to BET (Scheme 8,
pathways vi and vii), other re-
actions such as isomerization,
deprotonation,
and addition
dimerization,
reactions
(Scheme 8, pathway viii) can
+
C
compete for the decay of D
[63]
.
The contribution of these
+
C
reactions to the decay of D is
dependent on their relative
rate constants. Therefore the
weak effects of metal ions on
+
Scheme 8. The influence of O2 and salt [Mn+,Xꢀ] on the different transient intermediates formed following
photolysis of 1,4-dicyanonaphthalene (14DCN) and biphenyl (BP) in the presence of electron donor (D).
C
the stabilities of BP
and
+
C
trans-St (Figure 12D and Fig-
ures S26, S27 in the Supporting
Information), in the presence
+
Cꢀ
C
(D) results in the formation of 14DCN
and
D
of oxygen, can be partly attributed to the presence of other
major pathways (Scheme 8, pathway viii) available for the
decay of these radical cations, which is to say, BET is not
the major pathway for their decay.
Furthermore, the dependence of the kinetic decay of
CAR on metal concentration (Figure 6 and Figures S18,
Cꢀ
Cꢀ
(Scheme 8, pathway i). In the presence of oxygen, 14DCN
reacts rapidly (k=1.3ꢂ1010 mꢀ1 sꢀ1)[25] with O2 to form O2
Cꢀ
(pathway ii). This ET reaction from 14DCN (Eox =ꢀ1.31 V
versus SCE in acetonitrile)[42,43] to oxygen (Ered =ꢀ0.86 V
versus SCE in acetonitrile)[44] is thermodynamically feasible
+
C
Cꢀ
(DG=ꢀ10.4 kcalmolꢀ1). In the presence of [Mn+,X ], O2
S19 in the Supporting Information) in the presence of
oxygen can be attributed to the shift in the equilibrium of
the ion-exchange reaction (see Scheme 8, pathway iii)
ꢀ
reacts with the metal ion to form the corresponding metal-
Cꢀ
ion–superoxo complexes [(Mn+
)AHCUTNGERN(NUG O2 )] (Scheme 8, path-
toward the formation of [D +,Xꢀ] and [(Mn+
)
N
C
Cꢀ
(O2 )], which
way iii). Once formed, these metal-ion–superoxo complexes,
which were reported to be transiently stable, dismute to
oxygen and peroxide as described in Scheme 8 (pathways iv
leads to the retardation of BET.[2]
It should be noted that the enhancement of the transient
+
+
and v).[45–54] Overall, the inhibition of BET from O2 to the
absorption intensity of CAR and D observed in the pres-
ence of salts can be accounted for by the ionic environment
induced by the salts, which stabilizes the radical ion forma-
tion (see Figures 6, 8, 12, and Figures S18 and S19 in the
Supporting Information). There are many examples in the
Cꢀ
C
C
radical cation (Scheme 8, pathway vii) is mainly due to
metal-induced dismutation of O2 , that is, the degree of
BET inhibition depends mainly on the superoxide dismutase
Cꢀ
(SOD) activity of the metal ions.[55]
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Stability of p-Conjugated Radical Cations
FULL PAPER
literature[9,19,32,33,64] that showed an increase in the yield of
radical ions following the addition of salt.
Conclusion
In addition, the significant increase in the stability of het-
The conclusions that arise from the work reported in this
manuscript are as follows: 1) the direct observation of the
role of oxygen in the special salt effect is reported for the
first time; 2) many parameters are important in determining
the stability of radical cation: the nature of the metal ion,
the nature of the radical cation, metal-ion and oxygen con-
centrations, and relative contributions of the other competi-
tive pathways to the decay of the radical cation; 3) the
+
+
C
C
eroatom-containing radical cations (HA ) (see CAN
+
+
versus b-CAR and 4MeOSt versus trans-St +) has been
attributed to metal-ion–heteroatom interactions. The occur-
rence of such interactions might facilitate the presence of
C
C
C
metal ions near HA +, which results in the efficient dispro-
C
Cꢀ
portionation of O2 before undergoing BET to HA
C
.
+
[65]
Of special interest is the influence of Mn2+ ions on the
stability of p-conjugated radical cations. Manganese is an es-
sential element for many physiological processes because of
its role in the activity of many enzymes, for example, man-
aganese superoxide dismutase (MnSOD).[66–68] The well-
ꢀ
anions used (i.e., ClO4 and TfOꢀ) in this study have little
effect on the stabilization of the radical cation; 4) for heter-
oatom-containing radical cations, the heteroatom does seem
to have a significant effect on the stabilization of the radical
cation; 5) according to our results, the significant stabiliza-
tion of radical cation observed in air-saturated solutions that
contained metal ions can be attributed to the cooperative in-
Cꢀ
known reactivity of MnSOD toward O2 has drawn much
attention toward the investigation of the reactivity of Mn2+
Cꢀ [51–53,69,70]
ions toward O2
.
Pulse radiolysis studies established
Cꢀ
Cꢀ
that Mn2+ ions react rapidly with O2 to form a short-lived
teraction between O2 and metal ions to prevent the BET
manganous-superoxide transient species [(Mn2+
)
G
from O2 to the radical cation; 6) the manganese ion, which
Cꢀ
(O2 )],[71]
Cꢀ
which decays along various pathways depending on the
is a biologically relevant metal ion, shows a substantial abili-
ty to stabilize radical cations. This ability can be directly at-
tributed to its well-known SOD activity observed either as a
part of SOD enzymes or as free metal ions.[45,46,51–53,67,68]; and
7) generation of carotenoid radical cations by laser photoly-
sis of 14DCN and BP in the presence of CAR can be used
as a basis for a new method of screening the relative SOD
activities of various metal ions.
nature of the anion used.[51–53,70] For example, in the pres-
Cꢀ
ence of phosphate as an anion, [(Mn2+
)AHCUTNGERNU(NG O2 )] dispropor-
tionates rapidly to form manganous phosphate, oxygen, and
hydrogen peroxide (Scheme 9, pathway i). On the other
We are currently extending these results to study the in-
fluence of manganous phosphate, which was reported to
Cꢀ
remove rapidly and catalytically O2 from aqueous solu-
tions,[53] and other salts on the stability of various radical
cations in biologically relevant environments, for example,
+
C
aqueous Triton X-100. In addition, many CAR are able to
Scheme 9. Various pathways for the decay of manganese–superoxo com-
plex.
oxidize various biological molecules such as tyrosine and
cysteine and this reaction can cause deleterious effects, for
example, enzyme inactivation, if it occurs in vivo.[28,72]
Therefore, it is important to re-examine the reactivity of
hand, if Mn2+ ions are bound to pyrophosphate, the
Cꢀ
C
G
N
CAR +, in the presence of metal ions, toward these mole-
pyrophosphate and hydrogen peroxide (Scheme 9, path-
way ii).[53] Therefore, in light of these results, it is reasonable
to suggest that the substantial effects induced by Mn2+ ions
on the stability of most of the investigated p-conjugated rad-
ical cations can be attributed to the efficient scavenging of
cules to shed light about the influence of metal ions on
these reactions. Finally, the role induced by the presence of
heteroatom in the structure of radical cations will be further
studied by investigating the influence of metal ions on vari-
ous radical cations and their corresponding heteroatom-con-
taining radical cations.
Cꢀ
O2 by Mn2+ ions, which results in the inhibition of the
BET reaction. To the best of our knowledge, the influence
of Mn2+ ions on photoinduced ET reactions has not previ-
ously been examined. However, the influence of other salts
Experimental Section
such as MgACHTUNGTRENNUNG(ClO4)2, NaClO4, LiClO4, KClO4, LiBF4, and
others on various reactions have been reporte-
d.[3,6a,7,9,10,31–33,36] Since Mn2+ ions show a remarkable effect
on the kinetics of radical cations, it will be of interest to re-
examine some of these reactions in the presence of Mn2+
ions.
Materials: 1,4-Diphenyl-1,3-butadiene, 1,6-diphenyl-1,3,5-hexatriene, cal-
cium perchlorate tetrahydrate, calcium triflate, scandium triflate, zinc
perchlorate hexahydrate, zinc triflate, manganese perchlorate hydrate,
manganese triflate, aluminium perchlorate nonahydrate, lutetium triflate,
biphenyl (99.5%), and benzonitrile (99.9%) were purchased from Al-
drich. Sodium perchlorate (anhydrous) and magnesium perchlorate were
purchased from Wako. trans-Stilbene, 1,4-dicyanonaphthalene (98%),
and tetra-n-butylammonium perchlorate were purchased from Tokyo
Chemical Industry (TCI). Nickel perchlorate hexahydrate (STREM
CHEMICALS), 4-methoxy-trans-stilbene (fluorochem), lithium perchlo-
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A. El-Agamey and S. Fukuzumi
rate (anhydrous, Nacalai Tesque), ammonium perchlorate (Nacalai
Tesque), astaxanthin (Alexis Biochemicals, ꢄ97%), and b-carotene
(Fluka, ꢄ 97%) were used as received. Canthaxanthin was kindly sup-
plied by Dr. David J. McGarvey (School of Physical and Geographical
Sciences, Keele University, Keele, Staffordshire ST5 5BG, UK) and was
used as received. Argon (99.99%) was supplied by Yamazaki Sangyo,
Japan. A gas blender (GB-3C) from KOFLOC, Japan, was used to obtain
various oxygen concentrations.
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A.E. is grateful to JSPS (Japan Society for the Promotion of Science) for
financial support and to Dr. David J. McGarvey (School of Physical and
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Stability of p-Conjugated Radical Cations
FULL PAPER
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[55] The relative influence of metal ions on the CAR stability is not
+
C
correlated with the Lewis acidity (see ref. [56]) or the charge density
of the metal ions (see refs. [31,57,58]).
1324–1328.
+
Cꢀ
(O2 ) can also be written as MnO2
C
[71] The (Mn2+
)
G
.
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Received: April 19, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
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A. El-Agamey and S. Fukuzumi
Life changes: Mn ions show a substan-
tial effect on the lifetime of p-conju-
Radical Cations
gated radical cations (D +) in the pres-
C
A. El-Agamey,*
ence of oxygen (see scheme). How-
ever, this remarkable effect is greatly
diminished if either oxygen or manga-
nese ions are removed from the solu-
tion. These observations can be attrib-
S. Fukuzumi* ................. &&&& — &&&&
The Remarkable Effect of the Manga-
nese Ion with Dioxygen on the
Stability of p-Conjugated Radical
Cations
uted to the inhibition of the back elec-
+
Cꢀ
C
tron transfer between O2 and D
.
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These are not the final page numbers!