Competitive decay pathways of the radical ions formed by photoinduced electron transfer between quinones and 4,4′-dimethoxydiphenylmethane in acetonitrile

Article abstract of DOI:10.1002/1521-3765(20010716)7:14<3005::AID-CHEM3005>3.0.CO;2-9

The reactivity of the cation radical of (4-MeOC₆H₄)₂CH₂ photosensitized by 1,4-benzoquinone (BQ), 2,5-dichloro-1,4-benzoquinone (Cl₂BQ), and tetrachloro-1,4-benzoquinone (chloranil, CA) was investigated in acetonitrile. The main photoreaction products obtained by steady-state irradiation were identified to be: (4-MeOC₆H₄)₂CHOC₆H₄OH, sensitized by BQ; (4MeOC₆H₄)₂CHCl, sensitized by Cl₂BQ; (4-MeOC₆H₄)₂CHOH, sensitized by CA. The mechanism of their formation was investigated by nanosecond laser flash photolysis that allowed transient species (radical ions, neutral radicals, and ions) to be detected and characterized in terms of absorption spectra, formation quantum yields, and decay rate constants. For all systems, the interaction between the triplet quinone (Q) and (4-MeOC₆H₄)₂CH₂ produced the corresponding radical ions (quantum yield φ ≥ 0.72) which mainly decay by back electron transfer processes. Less efficient reaction routes for the radical ions Q^.- and (4-MeOC₆H₄)₂CH₂^.+ were also: i) the proton-transfer process with the formation of the radical (4MeOC₆H₄)₂CH^. by use of Cl₂BQ; ii) the hydrogen-transfer process with the formation of the cation (4-MeOC₆H₄)₂CH⁺ in the case of CA. Instead, BQ sensitized a much higher yield of BQH^. and (4MeOC₆H₄)₂CH^., mainly by the direct interaction of triplet BQ with (4MeOC₆H₄)₂CH₂. It was also shown that the presence of salts decreases significantly the rate of the back electron transfer process and enhances the quantum yields of formation of the neutral radicals and ions when Cl₂BQ and CA are used, respectively. The behavior of BQ^.-, Cl₂BQ^.-, and CA^.- appears to be mainly determined by the Mulliken charges on the oxygen atom obtained from quantum mechanical calculations with the model B3LYP/6-311G(d,p). Spin densities seem to be much less important.

Full text of DOI:10.1002/1521-3765(20010716)7:14<3005::AID-CHEM3005>3.0.CO;2-9

FULL PAPER
Competitive Decay Pathways of the Radical Ions Formed by Photoinduced
Electron Transfer between Quinones and 4,4'-Dimethoxydiphenylmethane in
Acetonitrile
[a]
[b]
[c]
[a]
Tiziana Del Giacco, Enrico Baciocchi, Osvaldo Lanzalunga, and Fausto Elisei*
.
Abstract: The reactivity of the cation
rate constants. For all systems, the inter-
action between the triplet quinone (Q)
and (4-MeOC H ) CH produced the
a much higher yield of BQH and (4-
.
radical of (4-MeOC H ) CH photosen-
MeOC H ) CH , mainly by the direct
interaction of triplet BQ with (4-
MeOC H ) CH . It was also shown that
the presence of salts decreases signifi-
cantly the rate of the back electron
transfer process and enhances the quan-
tum yields of formation of the neutral
radicals and ions when Cl BQ and CA
are used, respectively. The behavior of
BQ , Cl BQ , and CA appears to be
6
4
2
2
6
4 2
sitized by 1,4-benzoquinone (BQ), 2,5-
dichloro-1,4-benzoquinone (Cl BQ),
and tetrachloro-1,4-benzoquinone
chloranil, CA) was investigated in ace-
6
4
2
2
corresponding radical ions (quantum
yield f ꢀ 0.72) which mainly decay by
back electron transfer processes. Less
efficient reaction routes for the radical
2
6
4
2
2
(
tonitrile. The main photoreaction prod-
ucts obtained by steady-state irradiation
were identified to be: (4-MeOC H ) -
.
�
.‡
ions Q and (4-MeOC H ) CH were
6
4
2
2
also: i) the proton-transfer process with
the formation of the radical (4-
6
4
2
2
CHOC H OH, sensitized by BQ; (4-
6
4
.
.�
.�
.�
MeOC H ) CHCl, sensitized by Cl BQ;
MeOC H ) CH by use of Cl BQ; ii) the
6
4
2
2
6
4
2
2
2
(
4-MeOC H ) CHOH, sensitized by
hydrogen-transfer process with the for-
mainly determined by the Mulliken
charges on the oxygen atom obtained
from quantum mechanical calculations
with the model B3LYP/6-311G(d,p).
Spin densities seem to be much less
important.
6
4 2
‡
CA. The mechanism of their formation
was investigated by nanosecond laser
flash photolysis that allowed transient
species (radical ions, neutral radicals,
and ions) to be detected and character-
ized in terms of absorption spectra,
formation quantum yields, and decay
mation of the cation (4-MeOC H ) CH
6
4 2
in the case of CA. Instead, BQ sensitized
Keywords: electron
photochemistry
radical ions ´ radicals
transfer
quinones
´
´
´
ground state,[2] and it can decay either by a H-atom transfer
process or, in the presence of aromatic substrates, by an
electron-transfer process with the formation of the corre-
sponding radical ions. The reactivity of the two radical ions is
of great interest because these intermediates are involved in
Introduction
Upon light absorption, fast intersystem crossing with almost
unitary efficiency[ results in quinones so that the lowest
excited triplet state (T ) is populated. T is a species with a
1]
1
1
significantly higher reduction potential with respect to the
photoinduced processes of biological (photosynthesis and
respiration) and industrial interest.[
3, 4]
In particular, the
substantial electron affinity of quinones and the low dissoci-
[
a] Prof. F. Elisei, Dr. T. Del Giacco
Dipartimento di Chimica, Universit aÁ di Perugia
Via Elce di Sotto 8, 06123 Perugia (Italy)
Fax : (‡39)075-5855598
ation energy of O�H bonds in their intermediates allow
quinones to function as hydrogen-atom transporters.
Generally, when the substrate is an alkylaromatic com-
pound, the corresponding radical cation is a strong carbon
E-mail: elisei@phch.chm.unipg.it
[
b] Prof. E. Baciocchi
acid and heterolytic cleavage of the C�H bond in the side
Dipartimento di Chimica, Universit aÁ La Sapienza
Piazzale Aldo Moro 5, 00185, Roma (Italy)
Fax : (‡39)06-490421
chain occurs (Scheme 1a).[
5±8]
The occurrence of homolytic
cleavage of the same C�H bond (Scheme 1b) was rarely
considered, and has not been experimentally investigated.
Recently, we reported that this kind of bond breaking can be
observed in the photoinduced electron-transfer reactions of
E-mail: baciocchi@axcasp.caspur.it
[
c] Dr. O. Lanzalunga
Dipartimento di Chimica
and Centro CNR di Studio sui Meccanismi di Reazione
Universit aÁ La Sapienza
Piazzale Aldo Moro 5, 00185, Roma (Italy)
Fax : (‡39)06-490421
(
4-MeOC H ) CH and aryl alkyl sulfides sensitized by
6
4
2
2
[
9, 10]
chloranil (CA).
Flash photolysis experiments surprisingly
indicated the occurrence of hydrogen-atom transfer from the
Chem. Eur. J. 2001, 7, No. 14
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0714-3005 $ 17.50+.50/0
3005

F. Elisei et al.
FULL PAPER
Scheme 1. The heterolytic cleavage of the C�H bond in a side chain and
homolytic cleavage of the same bond.
.
�
aromatic radical cation to the radical anion CA . In this
context, it appeared interesting to investigate this process in
more detail by changing the molecular structure of the
photosensitizer.
This work deals with steady-state and laser photolysis
measurements carried out to investigate the influence of
chlorine substitution on the quinone ring on the reactivity of
the quinone/(4-MeOC H ) CH system in acetonitrile. In fact,
the photochemical behavior induced by CA was compared
with that of compounds sensitized by 1,4-benzoquinone (BQ)
6
4
2
2
�
3
Photochemical reactions: A solution of the quinone (5 Â 10 m) in
� 2
acetonitrile and (4-MeOC
6
H
4
)
2
CH
2
(1 Â 10 m), kept in a pyrex vessel
capped with a rubber septum and fluxed with argon or oxygen, was
irradiated without cooling at the instrument operating temperature in a
Rayonet photoreactor (lexc ˆ 310 ± 390 nm). After irradiation for 15 min
and 2,5-dichloro-1,4-benzoquinone (Cl BQ). The reactivity of
2
the radical ions was compared with the Mulliken charges and
spin densities of the radical anions obtained from quantum
mechanical calculations by use of the model B3LYP/6-
2
(BQ) or 10 min (Cl BQ or CA), an internal standard was added, and the
raw photolysate was analyzed by 1H NMR spectroscopy. All products
formed were identified by comparison with authentic specimens. The
[
11]
3
11G(d,p). The reaction pathways of the radical ions were
compounds 4,4'-dimethoxybenzhydrol [(4-MeOC
,4'-dimethoxybenzophenone (Aldrich), and tetrachlorohydroquinone
(CAH2, Sigma) were commercial products. The compound (4-
MeOC CHCl was prepared by reaction of (4-MeOC CHOH with
thionyl chloride; [(4-MeOC CH] O was prepared by reaction of (4-
MeOC CHOH and (4-MeOC CHCl in pyridine/MeCN. The
adduct (4-MeOC CHOC OH was isolated from the oxidation on a
6 4 2
H ) CHOH] (Aldrich),
also investigated in the presence of salts; particular attention
was paid to the effect of tetrabutylammonium perchlorate.
4
6
H
4
)
2
6 4 2
H )
6
H
4
)
2
2
6
H
4
)
2
6 4 2
H )
6
H
4
)
2
6 4
H
Experimental Section
preparative scale and characterized on the basis of the following spectral
data.
Starting materials: The compound 4,4'-dimethoxydiphenylmethane [(4-
1H NMR (300 MHz, CDCl
3
, 20 8C): d ˆ 7.3 ± 6.6 (m, 12H; ArH), 5.58 (s,
MeOC
6
H
4
)
2
CH
2
] was prepared from the corresponding ketone by the
13
1
1
H; OH), 6.01 (s, 1H; CH); C NMR (300 MHz, CDCl
52.3, 149.7, 133.8, 128.1, 117.4, 115.8, 113.8, 81.8, 55.2. A dimer of Cl
3
, 20 8C): d ˆ 158.9,
Huang ± Minlon modification of the Wolff ± Kishner reduction and purified
2
BQ
by recrystallization from hexane.[ The quinones 1,4-benzoquinone (BQ,
12]
with the loss of HCl (C12
preparative scale: H NMR (300 MHz, CDCl
H Cl O ) was isolated from the oxidation on a
3 3 4
Aldrich), 2,5-dichloro-1,4-benzoquinone (Cl
2
BQ, Aldrich), and tetra-
1
3
, 20 8C): d ˆ 7.19 (s, 1H), 7.05
chloro-1,4-benzoquinone (chloranil, CA, Fluka), the absorption spectra
of which are shown in Figure 1, were recrystallized before use. MeCN
‡
‡
(
s, 1H), 5.78 (s, 1H); MS (15 V, ESP ): 317 [M‡H] .
Quantitative analysis of reaction products was performed by 1H NMR
spectroscopy. A good material balance was obtained in all experiments
(
Aldrich, HPLC grade) was distilled before use. Hydroquinone (BQH
Aldrich), 2,5-dichlorohydroquinone (Cl BQH , Aldrich), and Bu NClO
Fluka) were used as received.
Methods: 1H NMR spectra were recorded with the use of CDCl
2
,
2
2
4
4
(
>80%). Further irradiation was avoided to limit photosensitized reactions
of the primary photoproducts. In the oxidation sensitized by BQ, the adduct
4-MeOC CHOC OH was formed (7%, yields referred to the
starting material). In the presence of oxygen, 4,4'-dimethoxybenzophenone
was formed (5%). In the oxidation sensitized by Cl BQ, the main products
observed were (4-MeOC CHCl (7%), [(4-MeOC CH] O (2%),
and the dimer of the sensitizer (5%). In the oxidation sensitized by CA, the
main products observed were (4-MeOC CHOH (16%), [(4-
MeOC CH] O (2%), and tetrachlorohydroquinone (14%).
(
3
on a
(
6
H
4
)
2
6 4
H
BruckerAC 300P spectrometer. MS analyses were performed on a Fisons
Instruments VG-Platform Benchtop LC-MS (positive ion electrospray
2
‡
mass spectra, ESP ) spectrometer.
6
H
4
)
2
6
H
4
)
2
2
6
4 2
H )
6
H
4
)
2
2
Laser flash photolysis: Excitation wavelengths of 355 nm from a Nd:YAG
laser (Continuum Surelite II, third harmonic, pulse width approximately
[
13]
7
ns and energy <3 mJ per pulse) and of 440 nm selected by a parametric
oscillator (OPO, Continuum, pulse width approximately 4 ns and energy
3 mJ per pulse) pumped by the Nd:YAG laser were used in nanosecond
<
flash photolysis experiments. The transient spectra were obtained by a
point-to-point technique, and the optical density changes (DA) were
monitored after the flash at intervals of 5 ± 10 nm over the spectral range
3
00 ± 850 nm; there were on average at least ten decays at each wavelength.
The lifetime values (the time at which the initial signal is reduced to 1/e)
and the t1/2 values (the time at which the initial signal is halved) were
reported for transients; these values showed first-order and second-order
kinetics, respectively. The quantum efficiencies of transient formation were
measured by calibrating the experimental setup with optically matched
solutions of references: benzophenone (B) and CA in MeCN at excitation
wavelengths of 355 and 440 nm, respectively. Transient quantum yields
Figure 1. Absorption spectra of BQ (dash), Cl
solid) in MeCN.
2
BQ (dash dot), and CA
(
3006
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0714-3006 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 14

Competitive Decay Pathways of Radical Ions
3005±3013
(
f
Tr) were obtained by use of the relationship [Eq. (1)] between quantum
(16%), [(4-MeOC H ) CH] O (2%), and tetrachlorohydro-
6
4
2
2
yields (f), absorption coefficients (e), and changes of absorbance (DA),
measured at the corresponding absorption maxima of the transient (Tr) and
the reference (ref), namely triplet benzophenone (f
6
7
quinone (14%). It was shown that the ratio [(4-MeOC H ) -
6
4 2
CHOH]/[[(4-MeOC H ) CH] O] decreases significantly with
6
4
2
2
T
(B) ˆ 1 and e
(CA) ˆ 1 and e (CA) ˆ
700m cm at 520 nm). The experimental errors on t1/2 and fTr were
T
(B) ˆ
�
1
� 1
[14]
the increase of the irradiation time, which suggests that the
alcohol is formed first and then converted to the ether under
the reaction conditions.
500m cm at 520 nm) or triplet chloranil (f
T
T
�
1
� 1
[15]
estimated to be Æ10% and Æ15%, respectively.
e
T
ꢁref† DATr
f
Tr ˆ f
T
(ref)
(1)
e
Tr DAꢁref†
Laser flash photolysis
All solutions flowed through a quartz photolysis cell while
argon was bubbled through them. All measurements were
carried out at 22 Æ 28C.
Triplet properties and quenching: The only transient formed
by laser irradiation of quinones in MeCN in the absence of
other additives was in the lowest triplet state T and showed
1
Quantum mechanical calculations: The hybrid Hartree ±
Fock/DFT (density functional theory) method B3LYP was
used to study the charge and spin distributions of the radical
anions of quinones. In particular, the molecular structures
were optimized by the B3LYP/6-31G method, and the
Mulliken charges and spin densities of the radical anions
were determined by the B3LYP/6-311G(d,p) method of the
two broad absorption bands, one below 400 nm and the other
in the 400 ± 550 nm range; the triplet state decayed with a
lifetime on the microsecond timescale (Table 2).
Addition of (4-MeOC H ) CH significantly quenched the
6
4
2
2
state T with rate constants close to the diffusional limit, at
1
least for Cl BQ and CA, the quenching rate constants of
2
which were measured. As shown in Table 2, these values are in
agreement with the high reduction potentials of the triplet
[
11]
package Gaussian98.
Table 2. Triplet properties of quinones [absorption maxima, lmax, and lifetimes, t
T
], together with the quenching
Results and Discussion
rate constants (k
q
) for (4-MeOC
6
H
4
)
2
CH
2
in MeCN and the redox properties of quinones.
[1010
m
� 1
s
� 1
]
E
�ꢃ
red (Q/Q ) [V vs SCE]
.�
E
�ꢃ red (3Q*/Q.� ) [V vs SCE]
Q
l
max [nm]
t
T
[ms]
k
q
Steady-state irradiation: The
main photoproducts obtained
by steady-state irradiation of
the quinone/(4-MeOC H ) CH
systems are shown in Table 1,
together with the chemical
yield (%) obtained after fifteen
minutes (BQ) or ten minutes
[a]
BQ
Cl
CA
360, 410
390, 480
370, 510
1
1
3.5
� 0.50
� 0.18
0.02
1.80
2.52
2.72
2
BQ
1.9
2.8
6
4
2
2
.
.
6 4 2
[a] The strong overlap between the absorption spectra of triplet BQ, BQH , (4-MeOC H ) CH , and radical ions
together with the high absorption coefficient of (4-MeOC
quenching rate constant (see text).
.
6 4 2
H ) CH prevented the determination of a reliable
(
Cl BQ or CA) of irradiation.
2
In particular, the adduct (4-MeOC H ) CHOC H OH (7%,
quinones. In the case of triplet BQ, this determination was
6
4
2
6
4
yield refers to the starting material) was formed in the
oxidation of (4-MeOC H ) CH sensitized by BQ. In the
presence of oxygen, 4,4'-dimethoxybenzophenone (5%) was
prevented by the strong overlap between the absorption
.
spectrum of T
and those of the radicals (4-MeOC H ) CH
6 4 2
6
4
2
2
1
.
and BQH , which were also detected at low (4-
formed. In the oxidation sensitized by Cl BQ, the main
MeOC H ) CH concentrations because of their high absorp-
6 4 2 2
2
products formed were (4-MeOC H ) CHCl (7%) and a dimer
tion coefficients (see below).
6
4 2
of the sensitizer (5%), which was probably derived from its
decomposition. A smaller amount of [(4-MeOC H ) CH] O
BQ/(4-MeOC H ) CH : Upon irradiation of BQ in the
6
4
2
2
6
4
2
2
�
2
(2%) was also observed. In the oxidation sensitized by CA,
presence of (4-MeOC H ) CH (1.5 Â 10 m) in MeCN, sig-
6
4
2
2
the main products observed were (4-MeOC H ) CHOH
nals at 330, 350, 420, and 450 nm were observed within the
laser pulse (Figure 2). The signals at 330, 420, and 450 nm
decay significantly (second-order kinetics with t ˆ 0.71 ms
6
4 2
1/2
1
0
� 1 � 1
and 2k ꢂ 4 Â 10 m s , Table 3) between 0.16 to 1.3 ms after
2
Table 1. Main photoproducts obtained by irradiation of Q in MeCN in the
the laser pulse, whereas the absorption at 350 nm practically
remains unchanged for the first 2 ms (see inset of Figure 2) and
decays with second-order kinetics with t ˆ 23 ms and 2k ꢂ
presence of (4-MeOC
indicated).
6
H
4
)
2
CH
2
(Ar-saturated solutions unless otherwise
1/2
2
Q
photoproduct
yield [%]
9
� 1 � 1
6
 10 m
s
(Table 3).
BQ
(4-MeOC
4-MeOC
(4-MeOC
Cl
(4-MeOC
(4-MeOC
(4-MeOC
Cl (OH)
6
6
6
H
H
H
4
4
4
)
)
)
2
2
2
CHOC
CO[
CHCl
6
H
4
OH
7
5
7
5
2
16
2
14
The short-lived transients produced within the laser pulse
are assigned to the radical ions BQ
and (4-MeOC H ) CH
6 4 2
50 nm)[ by comparing their absorption spectra with those
a]
.�
(
(lmax ˆ 330 and
Cl
2
BQ
.‡
[
16]
4
4
20 nm)
(lmax ˆ 330 and
2
C
[
12
H
3
3
O
4
8]
6
H
4
)
2
CH]
CHOH
CH]
2
O
CA
6
H
4
)
2
already reported. The electron-transfer process between
triplet BQ and the ground state of (4-MeOC H ) CH occurs
[
C
6
H
4
)
2
2
O
6
4
2
2
6
4
2
with a high efficiency (f ˆ 0.72, Table 3). The long-lived
.
[
a] In oxygen-saturated solution.
transient at 350 nm is assigned to (4-MeOC H ) CH (l
ˆ
6
4
2
max
Chem. Eur. J. 2001, 7, No. 14 ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0714-3007 $ 17.50+.50/0
3007

F. Elisei et al.
FULL PAPER
Table 3. Spectral and kinetic properties of the transients of BQ, Cl
2
BQ, CA, and (4-MeOC
6
H
4
)
2
CH
2
(1.5 Â
We suggest that the forma-
�
2
10
m) detected in MeCN, together with the quantum yields of formation.
.
tion of BQH
and (4-
�
1
� 1
� 1 � 1
.
Q
transient
l [nm]
e [m cm
]
t
1/2 [ms]
2k
2
[m
s
]
f
MeOC H ) CH is mainly due
6
4 2
BQ[a]
.�
10
to the occurrence of reac-
tion (2), a direct H-atom trans-
fer from the substrate to the
triplet, rather than a proton-
transfer reaction between the
two radical ions [Eq. (3)]. This
suggestion is supported by the
oxygen effect on the kinetic
properties of the transients re-
corded in air-equilibrated solu-
tion. In fact, the decay kinetics
of the radical ions are practi-
cally unaffected by molecular
oxygen, while the lifetime of
BQ
(
330,420
330,450
350,410
350
505
330,430
450
355,425
350
505
320,425,450
450
505
6000
1000
6000
47000
120000
6000
0.71
0.71
3.6 Â 10
5.0 Â 10
5.3 Â 10
6.6 Â 10
4.5 Â 10
4.0 Â 10
4.0 Â 10
0.72
0.72
0.15
‡
‡
‡
10
9
4-MeOC
BQH
4-MeOC
4-MeOC
BQ
4-MeOC
BQH
4-MeOC
6
H
4
)
2
CH
2.
.
[b]
23
23
22
.
9
[b]
(
(
6
6
H
H
4
)
)
2
2
CH
CH
0.15
‡
[c]
4[d]
10
10
4
ꢄ 0.006
BQ[e]
.�
[f]
Cl
2
Cl
2
0.86
0.86
22
22
1.0
(
6
H
4
)
2
CH
2.
1.0
0.03
0.03
.
Cl
2
.
10
(
(
6
6
H
H
4
)
)
2
2
CH
CH
8.5 Â 10
‡
4-MeOC
4
ꢄ 0.007
CA[g]
CA
.
�
9700
0.80
6.0 Â 10
6.0 Â 10
4.4 Â 10
10
1.0
2.
‡
.
10
(
(
(
4-MeOC
4-MeOC
4-MeOC
6
6
6
H
H
H
4
)
4
)
4
)
2
2
2
CH
CH
CH
0.80
1.0
0.02
< 0.01
[c]
4[d]
23
�
2
[
a] [BQ] ˆ 2.8  10 m. [b] This was f of radicals produced within the laser pulse; a further amount corresponding
to f ꢂ 0.05 (see text) was produced at longer delay times. [c] Lifetime in ms. [d] First-order decay rate constant in
�
1
� 3
.
s
. [e] [Cl
2
BQ] ˆ 1.2  10 m. [f] Absorption coefficient of the analogous anion radical of 2,6-dichloro-1,4-
the radicals BQH and (4-
�
3
benzoquinone taken from ref. [16]. [g] [CA] ˆ 3.2  10 m.
.
MeOC H ) CH is shortened
6
4 2
to approximately 100 ns. If the
radicals were produced mainly
by reactions of the radical ions, the upper limit of their
lifetime would correspond to that of the precursors (t
ˆ
1
/2
�
5
0
.71 ms). Moreover, when 2.7 Â 10 m (4-MeOC H ) CH
6
4
2
2
was irradiated, a concentration at which the triplet lifetime
of BQ (t ꢂ 500 ns) is much longer than the laser pulse, we
could observe that the signal at 350 nm (l for both radicals)
max
and that of the radical ions (f.i. at 450 nm) grew with kinetics
that correspond to the decay of triplet BQ measured at
380 nm (Figure 4). These kinetics clearly show that the triplet
Figure 2. Time-resolved absorption spectra obtained by laser photolysis of
�
2
� 2
BQ (2.8 Â 10 m) in the presence of (4-MeOC
6
H
4
)
2
CH
2
(1.5 Â 10 m) in
MeCN (lexc ˆ 355 nm). Inset: decay kinetics recorded at 350(1), 330(2),
4
10(3), and 500(4) nm.
�
1
� 1 [17]
.
3
4
50 nm, emax ˆ 47000m cm ) and BQH (l ˆ 350 and
max
10 nm, e410 ˆ 6000m� cm ; Figure 3).
1 [18]
� 1
�
2
Figure 4. Decay kinetics of BQ (2.8 Â 10 m) in the presence of (4-
�
5
MeOC
4
6
H
4
)
2
CH
2
(2.7 Â 10 m) in MeCN recorded at 350(1), 380(2), and
50(3) nm (lexc ˆ 355 nm).
BQ is a precursor of both the neutral and charged radicals.
The quantum yield of radicals directly produced by the triplet
BQ is 0.15 (Table 3).
Figure 3. Normalized absorption spectra obtained by laser photolysis of
3
.
.
BQ* ‡ (4-MeOC
6
H
4
)
2
CH
2
! BQH ‡ (4-MeOC
6
H
4
)
2
CH
(2)
(3)
�
3
tert-butylperoxide (1.0 Â 10 m) in the presence of hydroquinone (triangles,
�
2
� 2
1
.7 Â 10 m) and of 2,5-dichlorohydroquinone (circles, 2.7 Â 10 m) re-
.
�
.‡
.
.
corded in MeCN, 0.5 ms after the laser pulse (lexc ˆ 355 nm).
BQ ‡ (4-MeOC
6
H
4
)
2
CH
2
! BQH ‡ (4-MeOC
6 4 2
H ) CH
3008
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0714-3008 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 14

Competitive Decay Pathways of Radical Ions
3005±3013
.
However, it is probable that a small amount of BQH and
.
(
4-MeOC H ) CH also comes from reaction (3). This is
6
4 2
indicated by the observation that the signal at 350 nm
.
(
absorption maximum for both the radicals BQH and (4-
.
MeOC H ) CH ) remains constant for about 2 ms and then
6
4 2
decreases with a decay rate slower than the other transients.
The latter kinetic behavior suggests that the decay of the
radicals recorded at 350 nm is initially compensated by the
.
increase of the absorption of BQH and especially of (4-
.
MeOC H ) CH formed by reaction (3). Accordingly, the best
6
4 2
fit of the kinetics recorded at 350 nm for the BQ/(4-
MeOC H ) CH system, carried out by use of the second-
6
4
2
2
order laws that apply to the radicals produced directly from
triplet BQ (growth within the laser pulse and decay with t1/2
3 ms) and those formed from the radical ions (growth with
ˆ
2
t1/2 ˆ 0.71 ms and decay with t ˆ 23 ms), results in a yield of
1/2
radicals produced by radical ions of about 0.05 and therefore a
quantum yield of about 0.07 (0.05/0.72) for the proton-
transfer process. This value is significantly smaller than the
quantum yield of radicals (0.15) formed through reaction (2);
therefore, the proton-transfer process between radical ions
seems to be only a minor route for radical formation. The
quantum yield calculated for this process has to be considered
as a rough estimate; nevertheless we feel that it can be used to
.
�
compare the properties of BQ with those of the radical
anions of the other quinones (see below).
Figure 5. Time-resolved absorption spectra obtained by laser photolysis of
�
3
� 2
The adduct (4-MeOC H ) CHOC H OH formed in the
Cl
2
BQ (1.2 Â 10 m) in the presence of (4-MeOC
6
H
4
)
2
CH
2
(1.5 Â 10 m) in
6
4
2
6
4
MeCN, recorded in the absence (a) and in the presence of Bu
4
NClO
4
(3.0 Â
photolysis experiment was likely to have been produced by
�
2
.
.
10 m) (b) (lexc ˆ 355 nm).
coupling of the two radicals (4-MeOC H ) CH and BQH .
6
4 2
When the photolysis was carried out in the presence of O , a
2
.
.
fast reaction of (4-MeOC H ) CH with O2 led to 4,4'-
radical (4-MeOC H ) CH , depends on the concentration of
6 4 2
6
4
2
dimethoxybenzophenone.
the ground state of the sensitizer Cl BQ; this behavior
2
It is remarkable that the decay kinetics and the yield in
radicals were about the same in the absence and in the
presence of Bu NClO . Thus, it would seem that the salt does
suggests the occurrence of a reaction (not investigated in
detail) that is likely to result from the attack of the radical on
4
4
the unsubstituted position of the ring of Cl BQ. This makes
2
not significantly affect the reactivity of the two radical ions. It
should be noted however that the huge absorption of the
radical species produced by reaction (2) can mask the salt
effect on the decay kinetics of this system.
the Cl BQ/(4-MeOC H ) CH system much more complex
2
6
4
2
2
than the other two, for which the interaction reaction between
.
(4-MeOC H ) CH and the sensitizer was not observed. In
6
4 2
7
� 1 � 1
fact, a quenching rate constant of 3 Â 10 m
s
was deter-
mined from the linear trend of the observed decay rate
constant versus [Cl BQ]. Thus, the concentration of Cl BQ
Cl BQ/(4-MeOC H ) CH : The laser photolysis of (4-
2
6
4
2
2
2
2
�
2
MeOC H ) CH (1.5 Â 10 m) sensitized by Cl BQ leads to
used in this work was kept low enough so that there was a
6
4
2
2
2
.
the time-resolved spectra in Figure 5a. Signals at 330, 420, and
negligible interaction with (4-MeOC H ) CH , especially for
6
4 2
4
40 nm are observed and are attributable to the pair of radical
transient measurements, for which the quantum yields and the
rate constants were determined.
.
‡
ions (4-MeOC H ) CH
(lmax ˆ 330 and 450 nm) and
6
4
2
2
.
�
Cl BQ (lmax ˆ 330 and 420 nm) formed with unitary effi-
Somewhat similar behavior was observed in the presence of
Bu NClO (Figure 5b). Formation of the two radical ions and
2
[
20]
ciency.
These transients decay by second-order kinetics
4
4
1
0
� 1 � 1
.
.
with a half-life of 0.86 ms and 2k ˆ 1  10 m s . For longer
of the two neutral radicals Cl BQH and (4-MeOC H ) CH
2 6 4 2
2
delay times, absorption around 350 and 425 nm (Figure 3) is
was again observed. Under such conditions, the yield of
.
.
.
also observed, which is assigned to both (4-MeOC H ) CH
and Cl BQH , formed by reaction (4); these radicals decay
Cl BQH and (4-MeOC H ) CH was sufficiently high (f ˆ
6
4
2
2
6
4 2
.
[18]
2
0.11), and it was possible to see an increasing amount of these
radicals coupled with the decay of the radical ions, according
to reaction (4). The effect of the concentration of Bu NClO
by second-order kinetics with a half-life of 22 ms and 2k ˆ
2
10
� 1 � 1
8
.5 Â 10 m s .
4
4
on the yield of radicals is shown in Figure 6. These results
contrast with those found for the BQ system, in which the
yields of radicals are not affected by Bu NClO .
.
�
.‡
.
.
Cl
2
BQ ‡ (4-MeOC
6
H
4
)
2
CH
2
! Cl
2
BQH ‡ (4-MeOC
6
H
4
)
2
CH
(4)
4
4
.
.
The radicals Cl BQH and (4-MeOC H ) CH were ex-
2
6
4 2
Furthermore, it is to be noted that the decay time of the
signal recorded at 350 nm, mainly due to the absorption of the
pected to form the adduct (4-MeOC H ) CHOC H Cl OH as
6 4 2 6 2 2
in the previous case. However, no such adduct was observed in
Chem. Eur. J. 2001, 7, No. 14
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0714-3009 $ 17.50+.50/0
3009

F. Elisei et al.
FULL PAPER
4 4
Figure 6. Effect of the concentration of Bu NClO on the absorbance
�
3
changes recorded at 350 nm of Cl
MeOC H ) CH
6 4 2 2
2
BQ (1.2 Â 10 m) in the presence of (4-
�
2
(1.3  10 m) in MeCN (lexc ˆ 355 nm).
the reaction products; the only observed products are (4-
MeOC H ) CHCl, C H Cl O , and [(4-MeOC H ) CH] O.
6
4
2
12
3
3
4
6
4
2
2
We suggest that under the reaction conditions the adduct
decomposes mainly to form (4-MeOC H ) CHCl by reacting
6
4 2
with HCl produced by decomposition of the sensitizer
indicated by the presence of the sensitizer dimer
C H Cl O ). However, it cannot be excluded that at the
(
1
2
3
3
4
Figure 7. Time-resolved absorption spectra obtained by laser photolysis of
relatively high concentration of Cl BQ used in the photo-
� 3
� 2
2
CA (3.2 Â 10 m) in the presence of (4-MeOC
6
H
4
)
2
CH
2
(1.5 Â 10 m) in
chemical experiments the products might be derived from the
MeCN, recorded in the absence (a) and in the presence of Bu NClO (3.0 Â
4
4
.
� 2
attack of (4-MeOC H ) CH on Cl BQ.
10 m) (b) (lexc ˆ 355 nm).
6
4
2
2
CA/(4-MeOC H ) CH : In the reaction of (4-MeOC H ) CH
2
6
4
2
2
6
4
2
�
2
(
1.5 Â 10 m) photosensitized by CA we obtain time-resolved
spectra (Figure 7a), in which the formation of the two radical
.
� [21]
.‡
ions CA
observed just after the laser pulse (lmax ˆ 330, 420, and
50 nm); these transients decay by second-order kinetics with
and (4-MeOC H ) CH
(f ˆ 1.0) is clearly
6
4
2
2
4
1
0
� 1 � 1
t1/2 ˆ 0.80 ms and 2k ˆ 6  10 m s .
2
At longer delay times, a small absorption at 505 nm due to
‡
(
4-MeOC H ) CH is present. The carbocation, formed with
6
4 2
an efficiency of 0.016, decays with first-order kinetics (t ˆ
[
22]
2
3 ms).
In the presence of Bu NClO , again the formation of the
4
4
two radical ions is observed (Figures 7b and 8). Furthermore,
their decay leads to the formation of an intense signal
‡
centered at 505 nm due to (4-MeOC H ) CH obtained in
Figure 8. Effect of the concentration of Bu NClO on the absorbance
6
4
2
4
�
4
3
reaction (5); in fact, the decay of the ion radicals, recorded at
changes recorded at 505 nm of CA (3.2 Â 10 m) in the presence of (4-
�
2
1
0
� 1 � 1
MeOC
6
H
4
)
2
CH
2
(1.5  10 m) in MeCN (lexc ˆ 355 nm).
4
50 nm (t1/2 ˆ 2.7 ms and 2k ˆ 1.8  10 m s , Figure 9), is
2
coupled with an increase in the amount of the cation. Clearly,
the reaction between the radical cation and the radical anion
is a hydrogen atom transfer reaction [Eq. (5)], the efficiency
of which appears to be significantly increased by the presence
Reaction mechanism: The main reaction pathways of the
quinone/(4-MeOC H ) CH systems, identified by both the
�
2
6
4
2
2
of the added salt (f ˆ 0.12 at 3  10 m Bu NClO , Ta-
4
4
steady-state and transient techniques in oxygen-free solution
are summarized in Scheme 2. The decay rate constants and
the formation quantum yields of the transients detected by
laser flash photolysis allowed the second-order decay rate
constants for the decay of the radical ions to be determined
[
22]
ble 4). The lifetime of the cation remains at approximately
3 ms, even in the presence of the salt. The absorption
2
�
[9, 10]
spectrum of the anion CAH (lmax ˆ 320 and 450 nm)
was also obtained at long delay times, at which the radical
ions were completely decayed.
(
Table 5).
The decay processes of radical ions corresponding to back
electron transfer, proton transfer, and hydrogen transfer
.
�
.‡
�
‡
CA ‡ (4-MeOC
6
H
4
)
2
CH
2
! CAH ‡ (4-MeOC
6
H
4
)
2
CH
(5)
3010
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0714-3010 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 14

Competitive Decay Pathways of Radical Ions
3005±3013
1
0
� 1 � 1
.�
Table 5. Second-order decay rate constants [10
m
s
] of Q and (4-
.
‡
MeOC
6
H
4
)
2
CH
2
in MeCN in the absence and in the presence of
�
2
[a]
Bu
4 4
NClO
(3 Â 10 m) (see Scheme 2).
�
2
Q
Without Bu NClO
(2) 2k (4) 2k
4
4
With 3 Â 10 m Bu
(2) 2k (4)
4
NClO
2k (6)
4
2
k
2
2
2
(6)
2k
2
2
2
BQ
Cl BQ
CA
(4.0)
3.9
5.9
(0.30)
0.12
ꢄ 0.06
ꢄ 0.04
ꢄ 0.03
0.12
2
0.98
1.6
0.12
ꢄ 0.06
ꢄ 0.02
0.18
[
a] Values in parentheses were calculated by using the formation quantum
yield of radicals from radical ions (f ˆ 0.05) estimated by fitting procedures
see text).
(
In this equation, 2k (RI) stands for the overall second-order
�
3
2
Figure 9. Decay kinetics obtained by laser photolysis of CA (3.2 Â 10 m)
�
2
rate constant of the radical ions, and f(i) and f(RI) represent
the quantum yields of the i-fold step and of the formation of
radical ions, respectively (Tables 3 and 4). The efficiency of
step 2 was calculated by use of relationship (7); this is
supported by the lack of transient species that provide
evidence for further decay pathways of radical ions.
in the presence of (4-MeOC
50 nm in the absence (1) and in the presence of Bu
2) (lexc ˆ 355 nm).
6
H
4
)
2
CH
2
(1.5 Â 10 m) in MeCN, recorded at
� 2
4
4
NClO
4
(3.0 Â 10 m)
(
2
Table 4. Spectral and kinetic properties of the transients of Cl BQ, CA, and (4-
�
2
MeOC
6
H
4
)
2
CH
2
(1.5 Â 10 m) detected in MeCN in the presence of Bu
4
NClO
4
2
(
3 Â 10� m), together with the quantum yields of formation.
f(2) ˆ 1 � f(4) � f(6)
(7)
�
1 � 1
Q
transient
l [nm]
t
1/2 [ms] 2k
2
[m
s
]
f
.�
10
10
10
Cl
2
BQ[a] Cl
2
BQ
325, 430
450
350
2.2
2.2
6.0
1.1 Â 10
1.1 Â 10
7.1 Â 10
1.0
1.0
0.11
< 0.007
1.0
1.0
0.10
< 0.03
The interaction between triplet quinones and (4-
MeOC H ) CH efficiently produces radical ions (step 1) with
f ꢀ 0.72; only in the case of BQ does a parallel hydrogen-
transfer process (step 3) occur with f ˆ 0.15. In the absence of
salts, the main decay process of the radical ions is the diffusion
‡
‡
(
(
(
4-MeOC
4-MeOC
4-MeOC
6
6
6
H
H
H
4
4
4
)
)
)
2
2
2
CH
CH
CH
2.
.
6
4
2
2
‡
CA[b]
CA
.�
320, 425, 450 2.7
450
505
1.8 Â 10
1.8 Â 10
4.4 Â 10
10
10
(
(
(
4-MeOC
4-MeOC
4-MeOC
6
6
6
H
H
H
4
)
4
)
4
)
2
2
2
CH
CH
CH
2.
‡
.
2.7
[c]
4[d]
23
controlled back electron transfer process (2k ˆ 3.9 ± 5.9 Â
2
10
� 1 � 1
1
0 m s ), with slightly higher rate constants in the case of
�
3
� 3
[
a] [Cl
2
BQ] ˆ 1.2  10 m. [b] [CA] ˆ 3.2  10 m. [c] Lifetime in ms. [d] First-
CA. The other decay processes (steps 4 and 6) have rate
constants that are much smaller than that for step 2 by at least
�
1
order decay rate constant in s
.
1
0
� 1 � 1
one order of magnitude: 2k (4) ꢄ 0.3 Â 10 m
s
and
2
10
� 1 � 1
2
k (6) ꢄ 0.12 Â 10 m s . In fact, the proton-transfer process
2
(
step 4) is operative for BQ and Cl BQ, while the hydrogen-
2
transfer process (step 6) is only for CA.
The second-order decay kinetics of the radicals produced
by steps 3 and 4 suggest that their main decay process is the
recombination (step 5) with formation of the adduct. This was
also shown from steady-state experiments of BQ, in which
irradiation of free-oxygen solutions allowed us to isolate the
photoproduct (4-MeOC H ) CHOC H OH. In the case of
6
4
2
6
4
Cl BQ, the products isolated are probably derived from the
2
decomposition of the first adduct formed (4-MeOC H ) -
6
4 2
CHOC H Cl OH, which, however, was not observed among
6
2
2
the photoproducts (see above).
Instead, the cation (4-MeOC H ) CH formed by CA
‡
6
4 2
through step 6 decays with first-order kinetics; this is likely
to be through a reaction, with small traces of water still
present in the solvent (step 7), to form the alcohol (4-
MeOC H ) CHOH. In an analogous way to the case of
6
4 2
benzylic alcohols,[ (4-MeOC H ) CHOH is likely to react
23]
6
4 2
‡
Scheme 2. A summary of the main reaction pathways of the quinone/(4-
MeOC CH system.
with the cation (4-MeOC H ) CH to produce the ether [(4-
6
4 2
6
H
4
)
2
2
MeOC H ) CH] O.
6
4
2
2
�
2
For BQ in the presence of Bu NClO (3 Â 10 m), the decay
4
4
processes were essentially unchanged with respect to those
recorded in the absence of the salt, but small effects on the
rate constants of steps 2 and 4 could be hidden by the high
amount of radicals produced through triplet BQ.
(steps 2, 4, and 6 of Scheme 2, respectively) were kinetically
characterized for the three quinones by Equation (6).
2
k
2
(i) ˆ 2k
2
(RI) Â f(i)/f(RI)
(6)
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
Chem. Eur. J. 2001, 7, No. 14
0947-6539/01/0714-3011 $ 17.50+.50/0
3011

F. Elisei et al.
FULL PAPER
On the other hand, for Cl BQ and CA in the presence of
Mulliken charges are signifi-
cantly affected by the presence
and number of chlorine sub-
stituents, while the spin densi-
ties are not.
On the basis of these calcu-
lations, the different reactivity
of the three radical anions of
quinones appears therefore to
2
�
2
Bu NClO (3 Â 10 m), the quantum yields of the electron-
4
4
transfer process were unchanged (unitary), while the half-life
of the radical ions increased from approximately 0.8 ms to
approximately 2.5 ms (Tables 3 and 4). This behavior is similar
to that already found in the photoinduced electron transfer
between 1,4-dicyanonaphthalene and trans-stilbene or 4,4'-
dimethoxybiphenyl by addition of magnesium perchlorate.[
The longer decay times recorded in the presence of salts
suggest that the formation of complexes between the charged
species (radical ions and salt ions) takes place. In our case,
such complexes slow down the rate of the back electron
transfer process by a factor of 4 (Table 5). This explains the
significant increase in the quantum yields observed in the
presence of Bu NClO for the production of radicals from
24]
be mainly due to the decrease of the negative charge on the
oxygen atoms. In particular, the low negative charge on the
.
�
oxygen of CA is probably responsible for the fact that this
radical anion does not act as a base, but reacts with (4-
.
‡
MeOC H ) CH
2
by means of a hydrogen-transfer process
6
4
2
.
�
.�
(step 6 of Scheme 2); instead, for BQ and Cl BQ the
4
4
2
.
�
.‡
Cl BQ and (4-MeOC H ) CH [Eq. (4)] and of ions from
CA and (4-MeOC H ) CH [Eq. (5)]. If we also took the
charge is sufficiently high for a fast proton-transfer process
(step 4 of Scheme 2). These properties were more evident in
the presence of salts, for which the back electron transfer
process was less efficient.
2
6
4
2
2
.
�
.‡
6
4
2
2
experimental errors into account, a negligible salt effect was
observed on the rate constants of the proton- and hydrogen-
transfer processes (Table 5).
Quantum mechanical calculations: In order to try to under-
stand the effect of chlorine atoms on the reactivity of the
anion radicals of benzoquinone, ab initio quantum mechanical
calculations were carried out to determine Mulliken charges
and spin densities of the anion radicals by the model B3LYP/
Conclusion
This study has shown that the compound (4-MeOC H ) CH
2
6
4
2
is a suitable substrate to investigate the reactivity of radical
.
‡
ions because its transients (4-MeOC H ) CH
2
,
(4-
6
4
2
.
‡
6
3
-311G(d,p), after geometrical optimization with B3LYP/6-
1G. The results of such calculations are compiled in Table 6
MeOC H ) CH , and (4-MeOC H ) CH are easily detect-
6
4
2
6
4 2
able as a result of their well-separated absorption spectra and
high absorption coefficients. In fact, the electron-transfer
mechanism is operative with all sensitizers, as already found
.
�
together with those already reported for BQ . In practice,
except for the charge on C1 and C4 (0.16 instead of 0.30) the
present charge distribution and spin densities are very close to
those already obtained by PWP86/6-311G(2d,p) calcula-
tions. As shown in Table 6, the main effect of the increasing
number of chlorine atoms in the quinone structure is to
increase the positive charge on C1 and C4 (0.16 and 0.39 for
.
�
for CA , even if the direct hydrogen-transfer reaction from
the substrate to triplet BQ is a competitive process.
[
24]
.�
The results suggest that in CA , the negative charge is so
efficiently delocalized that hydrogen-atom transfer from the
radical cation prevails over the most common proton-transfer
.
�
.�
.
.
�
�
BQ and CA , respectively) and to reduce the negative
process. Instead, for BQ and Cl BQ the negative charge on
2
.
�
charges on the oxygen atoms (� 0.48 and � 0.38 for BQ and
the oxygen atoms means that the basicity of the radical anion
is sufficient and so the proton-transfer process is the dominant
one (Table 5). This suggestion is supported by the Mulliken
charges and spin densities obtained for the radical anions of
quinones by ab initio calculations with the model B3LYP/6-
311G(d,p).
The photoproducts sensitized by BQ and CA in steady-state
experiments are in agreement with the nature of the transients
(radicals and ions, respectively) detected by laser flash
.
�
CA , respectively). Instead, the spin density shown in the last
column of Table 6 is quite high and constant on the oxygen
atoms (0.3) and relatively small on the other atoms. Thus, the
Table 6. Mulliken charges and spin densities of anion radicals of quinones
calculated by B3LYP/6-311G(d,p) after geometrical optimization with
B3LYP/6-31G.[
a]
radical anion element n. atom
charge
spin
.
�
photolysis. In the case of Cl BQ, the photoreactivity appears
BQ
C
1, 4
0.16(0.30)
0.05(0.07)
2
C
2, 3, 5, 6
7, 8
9, 10, 11, 12
1, 4
� 0.14(� 0.13)
� 0.48(� 0.49)
0.05
0.08(0.10)
0.30(0.25)
� 0.01(<0.01)
0.04
much more complex probably because the stable products
originate from the unstable primary photoadduct and sub-
stantial decomposition of the sensitizer took place.
O
H
C
.
�
Cl
2
BQ
0.26
C
C
O
Cl
H
C
2, 5
3, 6
7, 8
9, 11
10, 12
1, 4
� 0.26
� 0.03
� 0.42
� 0.16
0.10
0.08
0.09
0.30
0.00
Acknowledgements
� 0.01
Thanks are due to the Italian Consiglio Nazionale delle Ricerche (CNR)
and Ministero della Universit aÁ e della Ricerca Scientifica e Tecnologica
(MURST) for financial support. F.E. also thanks the MURST and the
University of Perugia for the National Project.
.
�
CA
0.39
0.03
C
O
Cl
2, 3, 5, 6
7, 8
9, 10, 11, 12
� 0.17
� 0.38
� 0.08
0.08
0.30
0.00
[
a] Values of charge and spin distributions in parentheses are taken from
[1] Handbook of Photochemistry (Eds.: S. L. Murov, I. Charmicael, G. L.
Hug), Marcel Dekker, New York, 1993.
ref. [24].
3012
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0714-3012 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 14

Competitive Decay Pathways of Radical Ions
3005±3013
.
.
[
[
2] G. J. Kavarnos, N. J. Turro, Chem. Rev. 1986, 86, 401.
3] a) Light-Induced Charge Separation in Biology and Chemistry (Eds.:
H. Gerischer, J. Katz), Verlag Chemie, Weinheim, 1978; b) A. J.
Swallow, Function of Quinones in Energy Conserving Systems (Eds.:
B. L. Trumpower), Academic Press, New York, 1982, Chapter 3, p. 66;
c) B. L. Trumpower, J. Biol. Chem. 1999, 265, 11409; d) M. Y.
Okamura, G. Feher, Annu. Rev. Biochem. 1992, 61, 861; e) A. R.
Crofts, C. A. Wraight, Biochim. Biophys. Acta 1983, 726, 149; f) P. R.
Rich, Biochim. Biophys. Acta 1984, 768, 53.
[18] Genuine absorption spectra of BQH and Cl
photolysis of the tert-butylperoxide/hydroquinone (QH
MeCN, which produced the absorption spectra shown in Figure 3
2
BQH were obtained by
2
) system in
(triangles and circles for BQ and Cl
2
BQ, respectively), assigned to the
doublet ± doublet transitions of the radical QH . As already report-
ed, irradiation of tert-butylperoxide with UV light promotes the
formation of QH by the following processes [Eqs. (8a ± c)].
.
[
19]
.
(tBuO)
2
2
‡ hn ! (tBuO)
2
*
(8a)
(8b)
(8c)
[
4] A. C. Buchanan, R. Livingston, A. S. Dworkin, G. P. Smith, J. Phys.
Chem. 1980, 84, 423.
* ! 2tBuO.
(
tBuO)
.
! tBuOH ‡ QH.
[
[
5] E. Baciocchi, Acta Chem. Scand. 1990, 44, 645, and references therein.
6] X.-M. Zhang, F. G. Bordwell, J. Am. Chem. Soc. 1994, 116, 904, and
references therein.
tBuO ‡ QH
2
.
The absorption spectrum of BQH shows two broad bands centered at
approximately 350 and 410 nm with practically the same absorption
coefficients. A value of the absorption coefficient of BQH at lmax
corresponding to 6000m cm was then obtained from the absorption
spectrum of the radicals BQH and (4-MeOC
after the laser irradiation of the BQ/(4-MeOC
(Figure 2, diamonds). The following are also taken into account:
i) the two radicals have the same concentration; ii) the absorption
[
[
[
7] A. Anne, P. Hapiot, J. Moiroux, P. Neta, J.-M. Sav e ant, J. Am. Chem.
Soc. 1992, 114, 4694, and references therein.
8] E. Baciocchi, T. Del Giacco, F. Elisei, J. Am. Chem. Soc. 1993, 115,
.
�
1
� 1
.
.
12290.
6
H
4
)
2
CH recorded 8 ms
CH system
9] E. Baciocchi, T. Del Giacco, F. Elisei, O. Lanzalunga, J. Am. Chem.
Soc. 1998, 120, 11800.
6
H
4
)
2
2
[
10] T. Del Giacco, F. Elisei, O. Lanzalunga, Phys. Chem. Chem. Phys.
000, 2, 1701.
.
� 1
� 1
2
coefficient of (4-MeOC
6
H
4
)
2
CH at 350 nm is 47000m cm ; iii) the
CH at 410 nm is negligible; iv) the
absorption of BQH at 350 nm is equal to that recorded at 410 nm
.
[
11] Gaussian98 (RevisionA.6), M. J. Frisch, G. W. Trucks, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A.
Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M.
Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi,
V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo,
S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K.
Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B.
Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A.
Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C.
Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen,
M. W. Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A.
Pople, Gaussian, Inc., Pittsburgh, PA, 1998.
absorption of (4-MeOC
6
H
4
)
2
.
.
[Eq. (9)] (DA350((4-MeOC
6
H
4
)
2
CH ) ˆ DA350 � DA410
)
.
.
e
410(BQH ) ˆ e350((4-MeOC
6
H
4
)
2
CH )
.
.
 DA410(BQH )/DA350((4-MeOC
6 4 2
H ) CH ) (9)
[19] P. K. Das, M. V. Encinas, S. Steenken, J. C. Scaiano, J. Am. Chem. Soc.
1981, 103, 4162.
.
�
[20] The absorption spectrum of Cl BQ obtained in this work is very
2
close to that of 2,6-dichloro-1,4-benzoquinone already reported.[16]
The quantum yield was calculated by using the same absorption
coefficients of the analogous anion radical of 2,6-dichloro-1,4-
.
� [16]
[
12] A. I. Vogel, Textbook of Practical Organic Chemistry, 5th ed.,
Longman, Essex, 1989, p. 831.
2
benzoquinone for Cl BQ .
[21] J. J. Andree, G. Weill, Mol. Phys. 1968, 15, 97.
[
13] a) A. Romani, F. Elisei, F. Masetti, G. Favaro, J. Chem. Soc. Faraday
Trans. 1992, 88, 2147; b) H. Görner, F. Elisei, G. G. Aloisi, J. Chem.
Soc. Faraday Trans. 1992, 88, 29.
[22] In our previous work on the CA/(4-MeOC H ) CH system in MeCN,
6
4
2
2
a significant yield of cation was also observed in the absence of added
salt.[9] We tentatively attribute this different behavior with respect to
[
[
[
14] I. Carmichael, G. L. Hug, J. Phys. Chem. Ref. Data 1986, 15, 1.
15] H. Kobashi, T. Nagumo, T. Morita, Chem. Phys. Lett. 1978, 57, 369.
16] S. Fukuzumi, I. Nakanismi, J. Maruta, T. Yorisue, T. Suenobu, S. Itoh,
R. Arakana, K. M. Kadish, J. Am. Chem. Soc. 1998, 120, 6673.
17] J. Bartl, S. Steenken, H. Mayr, R. A. McClelland, J. Am. Chem. Soc.
the present results to the use of a MeCN batch that contained
extremely small amounts of ammonium ions as an impurity.
[23] E. F. Pratt, P. W. Erickson, J. Am. Chem. Soc. 1956, 78, 76.
[24] B. Goodson, G. B. Schuster, Tetrahedron Lett. 1986, 27, 3123.
[
1990, 112, 6918.
Received: December 29, 2000 [F2976]
Chem. Eur. J. 2001, 7, No. 14
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0714-3013 $ 17.50+.50/0
3013