Photoprocesses of p-Benzoquinones in Aqueous Solution

Article abstract of DOI:10.1021/jp030789a

The photochemistry of 1,4-benzoquinone (BQ) and several derivatives, for example, duroquinone, trimethyl-2,5- or 2,6-dimethyl-, and methyl-BQ in aqueous solution or mixtures with polar media, for example, acetonitrile or 2,2,2-trifluoroethanol, was studied by time-resolved UV-vis spectroscopy after pulses at 248 and 308 nm. The triplet state and the semiquinone radical (?QH/Q?^-) of BQs are spectroscopically and kinetically separated intermediates. The radical yield in the absence of H-atom donors is low and significantly increased in the presence of alcohols. Efficient photoinduced charge formation, because of Q?^- and H⁺ after H-atom transfer from 2-propanol to the triplet state, and small effects in the absence of a donor were observed by transient conductivity. The quantum yield of photodecomposition, λ_irr = 254 nm, is substantial for BQ, MeBQ, and Me₂BQs in aqueous solution, but small for Me ₄BQ. To account for the efficient photoconversion of BQs into hydrobenzoquinones and 2-hydroxy-1,4-benzoquinones, a novel water-mediated reaction not involving free radicals is proposed as major step. This mechanism is consistent with the prediction that the observed triplet state is monomeric and the yield of Q?^-, detected by both transient absorption and conductivity, is low for sub-millimolar BQ, MeBQ, and Me₂BQs at pH 5-6. In addition, H-atom abstraction from a polar organic solvent or by self-quenching plays a role in mixtures with water or at enhanced quinone concentration, respectively.

Full text of DOI:10.1021/jp030789a

J. Phys. Chem. A 2003, 107, 11587-11595
11587
Photoprocesses of p-Benzoquinones in Aqueous Solution
Helmut G o1 rner
Max-Planck-Institut f u¨ r Bioanorganische Chemie, D-45413 M u¨ lheim an der Ruhr, Germany
ReceiVed: June 25, 2003
The photochemistry of 1,4-benzoquinone (BQ) and several derivatives, for example, duroquinone, trimethyl-
,5- or 2,6-dimethyl-, and methyl-BQ in aqueous solution or mixtures with polar media, for example, acetonitrile
2
or 2,2,2-trifluoroethanol, was studied by time-resolved UV-vis spectroscopy after pulses at 248 and 308 nm.
-
The triplet state and the semiquinone radical (•QH/Q• ) of BQs are spectroscopically and kinetically separated
intermediates. The radical yield in the absence of H-atom donors is low and significantly increased in the
-
+
presence of alcohols. Efficient photoinduced charge formation, because of Q• and H after H-atom transfer
from 2-propanol to the triplet state, and small effects in the absence of a donor were observed by transient
conductivity. The quantum yield of photodecomposition, λirr ) 254 nm, is substantial for BQ, MeBQ, and
2 4
Me BQs in aqueous solution, but small for Me BQ. To account for the efficient photoconversion of BQs into
hydrobenzoquinones and 2-hydroxy-1,4-benzoquinones, a novel water-mediated reaction not involving free
radicals is proposed as major step. This mechanism is consistent with the prediction that the observed triplet
-
state is monomeric and the yield of Q• , detected by both transient absorption and conductivity, is low for
sub-millimolar BQ, MeBQ, and Me BQs at pH 5-6. In addition, H-atom abstraction from a polar organic
2
solvent or by self-quenching plays a role in mixtures with water or at enhanced quinone concentration,
respectively.
Introduction
SCHEME 1
Quinones function as electron carriers through the membrane
of various proteins and enzymes.1,2 The photochemistry of 1,4-
benzoquinone (BQ) and derivatives, such as methyl-1,4-ben-
zoquinone (MeBQ), 2,5- or 2,6-dimethyl-1,4-benzoquinones
3
-25
(
Me2BQs), or duroquinone (Me4BQ)
as well as 1,4-
2
6-30
naphthoquinone (NQ) and methyl-1,4-naphthoquinone,
has
been intensively investigated. Often studied water-soluble 9,10-
anthraquinone (AQ) derivatives are 1- or 2-sulfonated (1-SO3-
AQ , 2-SO3AQ ) and 2,6-disulfonated [2,6-(SO3)2AQ ]
which, however, reveal characteristic differences with respect
to parent BQ.
1
3-19,37-40
-
-
2- 30-42
has been considered by various groups.
This has been
questioned for sulfonated AQs, MeBQ,21 and 2,6-Me2BQ.
The mechanism is still under debate, but the free OH radical
hypothesis has been excluded for parent BQ.
Here, upon pulsed excitation at 248 and 308 nm and
continuous irradiation at 254 nm, the reactions of 1,4-benzo-
quinones were studied in aqueous solution or 1:1 (vol) mixtures
with polar organic solvents which do not favor H-atom transfer.
The organic solvents as additives were acetonitrile, tert-butyl
alcohol, 2,2,2-trifluoroethanol (TFE) or 1,1,1,3,3,3-hexafluoro-
3
2
20
2
2
Most quinones exhibit a high quantum yield of intersystem
3,4
crossing (Φisc) in solution at room temperature. The triplet
3
state ( Q*) is the major species after pulsed excitation (steps 1
and 2, Scheme 1). For BQ and AQ, the maximum quantum
yield of formation of hydroquinones (QH2) in the presence of
5
a H-atom donor, for example, 2-propanol, is close to unity.
The photoreduction of quinones by amines, such as triethylamine
2
-propanol (HFP). These fluoroalcohols are weakly nucleophilic
(
TEA) or DABCO, is the subject of various investi-
4
7
2
4,27-29,35,36
and strongly polar solvents. A series of quinones, such as
parent BQ, MeBQ, 2,5-, 2,6-Me2BQ, trimethyl-1,4-benzo-
quinone (Me3BQ), Me4BQ, 2,6-dimethoxy-BQ [(OMe)2BQ],
and phenyl-BQ (PhBQ), were chosen. The effects of quinoid
structure and medium on the photoreactions were studied by
time-resolved UV-vis spectroscopy and conductivity.
gations.
Triplet quenching by anions, 3, and oxygen,
self-quenching, 4, and reaction 5 with H-atom donors are
-
competing steps. The semiquinone radicals (•QH/Q• ) play a
key role in the photoreduction of quinones and eventually yield
QH2 (reaction 6). The alcohol radical may yield another
semiquinone radical, 7, in competition to self-termination, 8
(
Scheme 1). In neutral aqueous solution, the semiquinone
Experimental Section
radicals are present as radical anions because of equilibrium
9
4
3-46
.
The compounds (Merck, EGA/Aldrich, Sigma, Fluka) were
purified by sublimation (Me4Q) or recrystallization (BQ, 2,6-
Me2Q) or were used as received; TFE and HFP (EGA/Aldrich)
and the other solvents (Merck, Uvasol quality) were checked
for impurities, and water was from a Milli Q system. The
absorption spectra were monitored on a UV-vis spectropho-
The photoreactions of parent BQ in aqueous solution lead to
,2,4-trihydroxybenzene (1,2,4-benzenetriol) as labile intermedi-
1
ate and eventually to 2-hydroxy-1,4-benzoquinone (Q′) and
hydrobenzoquinone as stable products.2
2,23
Formation of free
OH radicals in the photolysis of quinones in aqueous solution
1
0.1021/jp030789a CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/03/2003

11588 J. Phys. Chem. A, Vol. 107, No. 51, 2003
G o¨ rner
CHART 1
tometer (HP, 8453) and for photoconversion the 254-nm line
of a Hg lamp was used. Typically, absorbances of 1-3 were
used for λirr ) 254 nm. The conversion was carried out after
vigorous bubbling by argon prior and during irradiation. For
HPLC analyses, a 125 × 4.6 mm Inertsil ODS-3 5 µm column
was used and MeOH-water 1:2 or 2:1 as eluents. The quantum
yield of decomposition (Φd) was determined using the uridine/
water/air actinometer.48 The molar absorption coefficient of the
3
Q′ form of BQ at the two maxima is ꢀ260 ) 4 × 10 and ꢀ482 )
3
-1
-1 46
2
.2 × 10 M cm . Reaction of hydroquinones with oxygen
Figure 1. Absorption spectra of (a) BQ, (b) 2,6-Me
and (d) PhBQ in argon-saturated aqueous solution at pH 7 prior to
full) and after (dashed) irradiation at 254 nm; inset: absorption at
2 4
BQ, (c) Me BQ,
fully reverses the quinones back in the cases of AQs in polar
solvents,42 but not for BQs.
(
Two excimer lasers (Lambda Physik, pulse width of 20 ns
and energy <100 mJ) were used for excitation at 248 and 308
nm. The absorption signals were measured with two digitizers
maximum vs irradiation time for BQ (o), MeBQ (0), Me
PhBQ (4).
4
BQ (]), and
SCHEME 2
(Tektronix 7912AD and 390AD). Relative yields were obtained
from the appropriate absorption signals using optically matched
solutions. The experimental conditions were due to lower and
higher concentrations for λexc ) 248 and 308 nm, respectively.
Absorbances of 0.2-3 were used, corresponding to concentra-
tions of 0.02-0.1 mM for λexc ) 248 nm and 1-8 mM for λexc
)
308 nm, on the basis of molar absorption coefficients, for
4
2
-1
-1
a
example, ꢀ248 ) 1.9 × 10 and ꢀ308 ) 2.6 × 10 M cm for
BQ. For parent BQ, in contrast to several derivatives, the rate
constant for quenching by 2-propanol could not be determined
at a wavelength of triplet decay because of a too high absorption
of the semiquinone radical with respect to the triplet signal.
Instead, the grow-in kinetics were estimated. The molar absorp-
d
TABLE 1: Quantum Yields Φ of Decomposition of BQs
b
b
quinone
BQ
MeBQ
MeCN
M
e
C
N
-
H
2
O
2
-
p
r
o
p
a
n
o
l
-
H
2
O
H2O
0.10 (<0.01)c
0.06 (<0.01)
0.5
0.8
0.7
0.7 (0.4)
0.1
0.9
0.9
0.9
0.9
0.6
0.5 (0.4)
0.8 (0.8)
0.7 (0.5)
0.7 (0.5)
0.1
2
,5-Me2BQ 0.06 (<0.02)
2,6-Me2BQ 0.06 (<0.02)
Me BQ
-
0.01 (0.002)
0.005
tion coefficient of Q• and •QH for BQ in aqueous solution is
3
Me4BQ
0.1
3
3
-1
-1
ꢀ
430 ) 6.1 × 10 and ꢀ410 ) 4.3 × 10 M cm , respectively;
(
OMe)2BQ 0.002
0.03
0.2
43
the values for 2,6-Me2BQ are similar. For Me4BQ, the molar
PhBQ
0.05 (0.02)
0.2 (0.2)
3
-
absorption coefficients of Q*, Q• , and •QH are ꢀ500 ) 4.2 ×
a
In argon-saturated solution using λirr ) 254 nm. b In 1:1 (vol)
3
3
3
-1
-1
1
0 , ꢀ440 ) 7.6 × 10 , and ꢀ420 ) 4.7 × 10 M cm ,
mixtures. c Values in parentheses refer to oxygen saturation.
9
respectively. The fast (0.05-10 µs) and slow (5 µs to 1 s)
conductivity signals were measured by DC and AC bridges as
The quantum yields of decomposition were obtained from
plots of the absorption at the maximum versus irradiation time
which are initially linear (inset of Figure 1). The Φd values of
BQs (R: H, Me, Me2) in acetonitrile are much smaller than in
aqueous solution, where Φd is large but does not approach unity.
Conditions of efficient photoconversion are also acetonitrile/
water (1:1, vol) mixtures (Table 1); comparable effects were
found in mixtures of tert-butyl alcohol and TFE with water.
The photoconversion of BQ in aqueous solution is well
characterized and the two photoproducts are QH2 and Q′; the
Φ values are equal and sum up to 0.42. Another literature
value is Φd ) 0.5 for 2,6-Me2BQ in aqueous solution at pH
5.5, λirr ) 265 nm. Saturation by oxygen reduces Φd only
moderately, in contrast to the absence of water. To further
confirm the quite large Φd in aqueous solution, the values were
also measured in 2-propanol/water (1:1) mixtures, where Φd )
25,42
reported elsewhere.
all measurements refer to 24 ( 2 °C and deoxygenated solution,
unless otherwise indicated.
The samples were freshly dissolved and
Results
Continuous Irradiation. Irradiation at 254 nm of BQ in
argon-saturated neat aqueous solution in relatively low concen-
tration of 0.1 mM reveals two characteristic wavelengths at 228
and 256 nm which on examination can be denoted as isosbestic
points as far as further conversion into secondary products is
avoided. The absorption decreases between these wavelengths
and increases below and above (Figure 1a). Comparable spectral
changes were recorded for MeBQ or 2,6-Me2BQ (Figure 1b),
whereas for Me4BQ (Figure 1c) and PhBQ (Figure 1d) the
isosbestic points are red-shifted. Photodecomposition of the
substrate was also measured by HPLC and hydroquinone, and
one further species, 2-hydroxy-1,4-benzoquinone (Q′, see
Scheme 2), was a major photoproduct absorbing above 250 nm.
Photoconversion into the corresponding QH2 in 2-propanol/
acetonitrile and into QH2 and Q′ in aqueous solution was
observed by HPLC for BQ, MeBQ, Me2BQs, or Me3BQ.
2
2
2
0
5
1.0 for BQ.
Anions are known to quench the triplet state of BQ, NQ,
1
2,23,30,33,34
and sulfonated AQs in aqueous solution.
Here, the
-
-
quenching of Φd by Cl and Br was probed. In fact, when
KCl was added to BQs, the slope of the absorption at the
maximum versus irradiation time (inset of Figure 2) strongly

Photoprocesses of p-Benzoquinones
J. Phys. Chem. A, Vol. 107, No. 51, 2003 11589
Figure 2. Plots of Φ
squares), 2,5-Me BQ (diamonds), and 2,6-Me
saturated aqueous solution, λirr ) 254 nm, pH 6-7; inset: absorption
vs irradiation time for 2,6-Me BQ and [KCl] ) 0 (O), 0.1 (b), 0.3
4), 1 (2), and 3 (0) mM.
d
as a function of [KCl] for BQ (circles), MeBQ
(
2
2
BQ (triangles) in argon-
Figure 4. Transient absorption spectra of 2,6-Me
saturated (a) aqueous solution at pH 7, (b) HFP-water (1:1) at 20 ns
2
BQ (1 mM) in argon-
2
(
O), 1 µs (4), 10 µs (0), and 1 ms (2) after the 308-nm pulse; insets:
(
triplet and radical decay at 430 nm.
Figure 5. Transient absorption spectra in argon-saturated acetonitrile-
Figure 3. Transient absorption spectra of BQ (1 mM) in argon-
saturated (a) aqueous solution at pH 7, (b) HFP-water (1:1) at 20 ns
2
water (1:1) of (a) (OMe) BQ and (b) PhBQ at 20 ns (O), 1 µs (4), 10
µs (0), 0.1 ms (b), and 1 ms (2) after the 308-nm pulse; insets: triplet
(
O), 1 µs (4), 10 µs (0), and 1 ms (2) after the 308-nm pulse; insets:
and radical decay at 350 nm.
triplet and radical decay at 420 nm.
state is the major species after the pulse (steps 1 and 2 in Scheme
1), as supported by faster decay on addition of oxygen, similar
to the case of various quinones in acetonitrile, where the rate
decreases; plots of Φd as a function of log [KCl] are shown in
-
Figure 2. The Cl concentration, where Φd is 50% of the
-
9
-1 -1 25
maximum value (half-concentration: [Cl ]1/2), can be compared
constant for triplet quenching is kox ) (0.5-3) × 10 M s .
T-T annihilation was reduced or avoided by a low excitation
intensity, whereby virtually no hydrated electron is formed, as
monitored for BQ and 2,6-Me2BQ in aqueous solution in the
600-700 nm range. In several cases, an effect of quinone
concentration on τT was observed. The plots of 1/τT versus
[quinone] are linear for 2,6-Me2BQ in aqueous solution and 1:1
(vol) mixtures with organic solvents (Figure 6) and the slope is
k4. This is also the case for BQ with exception of neat water,
where the plot is downward curved. Thus, quenching of the
triplet state by Q (self-quenching), reaction 4, has to be
with data from triplet quenching (see below).
Effects of Water on the Transient Properties upon
Excitation at 308 nm. Transient absorption spectra of BQs
(1-5 mM) in argon-saturated aqueous solution or 1:1 (vol)
mixtures with organic solvents are presented in Figures 3-5.
The spectra after the 308-nm pulse show the triplet state and
subsequently the semiquinone radical. The T-T absorption
spectrum of BQ in aqueous solution has two maxima at λTT )
2
75 and 400 nm (Figure 3a). For BQ in 1:1 (vol) mixtures with
HFP (Figure 3b) or TFE, the triplet state can be separated from
the semiquinone radical with maxima at λrad ) 320 and 410
nm. Transient absorption spectra of 2,6-Me2BQ show a red shift
of λTT to 450 nm in water (Figure 4a) and mixtures with
acetonitrile and HFP (Figure 4b). Further examples of the T-T
absorption spectra in water/acetonitrile mixtures are shown for
9
-1 -1
considered. The k4 values are up to 2 × 10 M s (Table 3).
The rate constant for triplet decay of BQs becomes larger on
addition of 2-propanol as donor (DH2) and the yield of the
semiquinone radical is enhanced. Formation of •QH via step 5
depends on the quinone structure and nature of solvent and
additives. For Me4BQ in 2-propanol, the triplet with λTT ) 470
(OMe)2BQ, λTT ) 480 nm (Figure 5a) and PhBQ, λTT ) 550
3
nm (Figure 5b). For MeBQ in acetonitrile and water, the
absorption of the semiquinone radical dominates with respect
to the triplet state.
nm as precursor and •QH as longer lived transient appear. The
long wavelength maximum of most BQs in aqueous solution is
λrad ) 410-420 nm and only slightly red-shifted for the Me4,
Ph, or (OMe)2 derivatives.
The decay kinetics in most cases are first-order (rate
constant: 1/τT), when the intensity is low enough and a second
long-lived component due to radicals in the absence of oxygen
was subtracted. The triplet lifetimes of the quinones examined
in strongly polar media are compiled in Table 2. The triplet
The triplet decay is also accelerated on addition of amines,
-
for example, DABCO or TEA. The quinone radical anion (Q• )
and the triplet state are separated intermediates, whereas the
•
+
radical cation (DH2 ) of several amines has a too low molar

1
1590 J. Phys. Chem. A, Vol. 107, No. 51, 2003
TABLE 2: Triplet Lifetime τ
(in µs) of BQs in the Absence and Presence of Watera
quinone
BQ
G o¨ rner
T
λ
exc(nm)
MeCN
MeCN-H
2
Ob
TFE-H
2
Ob
HFP-H
2
Ob
H
2
O
2
D O
308
48
308
48
2.5
5
0.9
1.2
0.7
0.5
1.2
0.4
1.4
4
1
2
1-2
1.2
2
2
0.8
MeBQ
0.6
1
3
<0.7
3
0.6
0.6
0.5
0.7
1.2
5
2
<0.2
0.2
1.0
<0.5
1.2
1.8
c
2
248
308
0.3
1.0
2
2
,5-Me
,6-Me
2
BQ
BQ
2
2
248
1.4
Me
Me
3
BQ
BQ
248/308
248/308
248/308
248/308
8
4
15
10
3
15
6
2
8
3
1.5
6
4
2
(
OMe)
PhBQ
2
BQ
a
In argon-saturated solution at low intensity. b The columns refers to 1:1 (vol) mixtures with water, pH 7-8. c Not transparent enough.
TABLE 3: Rate Constants for Triplet Quenching by BQs, 2-propanol, and Cl , and Cl Half-Concentrationsa
-
-
(×10 M s-1
9
-1
)
k
(×10 M s-1
6
-1
)
k
(×10 M s-1
9
-1
)
1/(τ
× k
) (mΜ)
[Cl-]1/2 (mM)
quinone
BQ
solvent
k
4
5
3
T
3
H
2
O
2.4 [4]b
1.4
2.4 [4]
2.5 [4]
2.8 [4]
3.5
0.008
0.3
0.04
1
1.2
0.5
2.3
0.5
0.4
0.2
MeCN-H
2
Oc
<1
<2
2.0
2.3
1.6
<0.1
<0.1
<0.1
0.5
2
0.4
0.3
0.2
MeBQ
H
2
H
2
H
2
O
O
O
>30
20
20
2
,5-Me
,6-Me
2
BQ
BQ
2
2
MeCN-H
MeCN-H
MeCN-H
MeCN-H
2
O
2
O
2
O
2
O
40
Me
4
BQ
BQ
<1
<1
<1
(OMe)
2
>100
PhBQ
10
a
O, and <1% acetonitrile, at pH 6-8 using λexc ) 248 nm. b In brackets: KCl is replaced by KBr. c In 1:1
In argon-saturated solution, H
mixtures with acetonitrile.
2
TABLE 4: Relative Absorbances of Radical Versus Triplet
(
∆A
quinone
BQ
r
/∆A
T
) of BQs in (1:1) Aqueous Mixturesa
MeCN
TFE
HFP
neat H
2
O
neat D
2
Ob
0.5
0.5
0.1
0.2
0.6
0.4
0.15
0.2
0.2
0.5
0.1
0.2
0.15
0.5
0.1
0.2
0.5
0.2
MeBQ
,6-Me
2
2
BQ
4
Me BQ
a
In argon-saturated solution, λexc ) 308 nm. b Using λexc ) 248 nm.
(rather inert) organic solvents change as concentration effects
play a marked role. The much lower concentration applicable
for λexc ) 248 nm leads in most cases to longer τT values (Table
2
). The triplet lifetime in the presence of water (>20 M),
extrapolated to zero concentration, is 5-20 µs in most cases,
Figure 6. Plots of 1/τ
T 2
versus [quinone] for BQ (open) and 2,6-Me -
BQ (full) in argon-saturated (circles), aqueous solution at pH 7
but ca. 2 µs for BQ and Me2BQs or <1 µs for MeBQ. The rate4
for quenching of triplet BQ by water can be estimated as 10
(
(
triangles), acetonitrile-water (1:1) (squares), and TFE-water (1:1)
diamonds), λexc ) 248 nm (<1mM) and 308 nm.
-
1 -1
M s .
The spectra and maxima remain in principle the same, but
the contribution of the semiquinone radical with respect to the
triplet, just after the 248 nm pulse, is lower than for λexc ) 308
nm. Examples are shown in Figure 7 for BQ, MeBQ, and Me2-
BQs. In particular, under the low-intensity conditions, no
hydrated electron was observed for BQs in neat water. The
triplet lifetime is virtually unchanged, when water is replaced
by heavy water (Table 2). Variation of pH from 3 to 9 causes
only minor changes in the spectra and kinetics of the triplet
state and the semiquinone radical of BQ or 2,6-Me2BQ.
The triplet lifetime of BQs becomes shorter on addition of
anions. Linear dependences of 1/τT on [KCl] were found in
aqueous solution and 1:1 (vol) mixtures with acetonitrile (Figure
8). For AQs and selected quinones, a quenching reaction 3 of
absorption coefficient to be observed.25 After reaction 5, the
decay of •QH/Q• under argon is essentially second-order and
-
radical termination 6 yields Q and QH2. The first half-life (t1/2)
of BQs in acetonitrile is in the 0.1-10 ms range, depending on
3
,4,25
the system.
Reaction 7 is a second reduction step of the
alcohol or amine radical which competes with reaction 8 of
4
2
radical termination, not yielding QH2 (Scheme 1). Oxygen
also quenches the decay of the semiquinone radical.4
3-45
Both
subsequent intermediates are also spectroscopically separated
for (OMe)2BQ and PhBQ in the absence of H-atom donors
(Figure 5) but not sufficiently separated for BQ in aqueous
solution. The absorption signal at λrad (after triplet decay) with
respect to T-T absorption at λTT, expressed by ∆Ar/∆AT, is
taken as a measure of radical formation (Table 4). The
absorption signal of the radical is largest for BQ in 1:1 mixtures
of water with TFE.
1
2,34
the triplet state by anions has been reported.
The rate
-
7
constant for triplet quenching of Me4BQ by Cl , k3 ) 1 × 10
-
1
-1
12
M
s , is in agreement with the literature. Larger values
9
were found with the other BQs (Table 3) and k3 ) 3 × 10
-
1
-1
34
Transient Properties for Low BQ Concentrations, λexc )
48 nm. The properties of the transients of BQs in argon-
M
s
for NQ. The half-concentration, where Φd is 50% of
2
the maximum value, agrees with data from triplet quenching,
-
saturated aqueous solution or 1:1 (vol) mixtures with polar
[Cl ]1/2 ) 1/(k3 × τT), indicating mainly physical quenching.

Photoprocesses of p-Benzoquinones
J. Phys. Chem. A, Vol. 107, No. 51, 2003 11591
Figure 9. Plots of ∆Arad vs [2-propranol] for BQ (circles), MeBQ
2
(squares), and 2,6-Me BQ (triangles) in argon-saturated aqueous
solution (open) and mixed with acetonitrile (50%, full) at pH 6-7,
λ
exc ) 248 nm; insets: signals for 2,6-Me
2
BQ in the presence of
-propanol 0.05 M (left) and 1 M (right) at 320 nm (upper) and 420
2
nm (lower).
0
max, of Radical
TABLE 5: Relative Absorbances, ∆A /∆A
r r
a
Formation for BQs in the Presence of TEA and 2-Propanol
Figure 7. Transient absorption spectra of (a) BQ, (b) MeBQ, (c) 2,5-
Me
2
BQ, and (d) 2,6-Me
2
BQ in argon-saturated aqueous solution at pH
quinone MeCN/TEAb MeCN/Pb MeCN-H O/Pc MeCN-H O/Pd
2
2
6
-7 at 20 ns (O), 1 µs (0), 10 µs (4), and 0.1 ms (b) after the 248-
BQ
MeBQ
0.2
0.2
0.17
0.17
0.5
1
0.1 (0.1)e
0.1
nm pulse; insets: triplet decay at 290 nm.
2
2
,5-Me2BQ
,6-Me2BQ
0.3
0.3
0.5
1
0.1
0.1
0.18
1
0.25
1
0.4
2
Me4BQ
OMe)2BQ
(
PhBQ
0.3
a
Argon-saturated. b In acetonitrile, λexc ) 308 nm, cf. ref 25. c For
acetonitrile-water, 1:1, λexc ) 308 nm. d For acetonitrile-water, 1:1,
λ
exc ) 248 nm. e Values in parentheses refer to relative conductivity
signal, λexc ) 248 nm.
Figure 8. Plots of the inverse lifetime on [KCl], λexc ) 248 nm, in
argon-saturated aqueous solution (open) and mixed with acetonitrile
2
(50%, full) for BQ (circles), MeBQ (squares), 2,6-Me BQ (triangles),
2 4
and 2,5-Me Q (diamonds) at pH 7; insets: Me BQ.
-
For example, [Cl ]1/2 ) 1 mM for BQ, larger for MeBQ and
smaller for Me2BQs; slightly smaller half-concentrations were
measured with KBr (Table 3).
The rate constant k5 for triplet quenching by 2-propanol of
Figure 10. Dependences of the conductivity signal at 0.1 ms on
8
-1 -1 22
[2-propanol] for BQ (circles), MeBQ (squares), and 2,6-Me BQ
BQ in aqueous solution is 1.0 × 10 M
s
and smaller for
2
(triangles) in argon-saturated aqueous solution (open) and mixed with
methylated BQs. In 1:1 (vol) mixtures with acetonitrile, k5 is
larger than in water (Table 3). Plots of ∆Ar after triplet decay
versus [2-propanol] are shown in Figure 9 for BQ, MeBQ, and
acetonitrile (1%, full) at (a) pH 5 and (b) pH 9, λexc ) 248 nm; insets:
conductivity signals for BQ at pH 5, 8, and 9.5 (from left to right).
2
,6-Me2BQ in aqueous solution and 1:1 (vol) mixtures with
for BQ and 2,6-Me2BQ. The signal at 0.01-0.1 ms increases,
0
0
acetonitrile. The curves increase linearly from the initial ∆Ar
when 2-propanol is added, from the initial ∆κ value to
value and approach saturation, ∆Armax. The ∆Ar /∆Ar
0
max
ratio,
maximum reactivity, ∆κ . In particular, for 2,6-Me2BQ the
max
which is taken as a measure of radical formation in neat water
with respect to the presence of H-atom donors (Table 5), is
generally smaller for BQs in water/2-propanol than in aceto-
nitrile in the presence of TEA or 2-propanol.
Transient Conductivity. The conductivity signal of BQs in
aqueous solution in the presence of 2-propanol (0.1-2 M) at
pH 4-6 is substantial at a few µs after the pulse under both
excitation conditions. To diminish the effects caused by self-
quenching, data presented here refer to λexc ) 248 nm. The
signals decay by second-order kinetics with half-lives in the 1
ms range; examples are shown in Figures 10 and 11 (insets)
peak signal increases by a factor of ca. 50. Similar dependences
were found for several BQs (Figure 10a). This shows that
-
+
photoreduction by alcohols leads to Q• and H (see Scheme
3) and indicates that protons are virtually not formed in the
absence of a donor. Neutralization at pH 8-10, thereby
removing protons from the bulk of solution, causes negative
signals (Figure 10b).
The conductivity signals of BQs in mixtures of water with
2-propanol (0.2 M) is substantial at pH 5-6 but low at pH <4.5
(Figure 11). This is due to formation of protons via step 9,
forward. The pKa values for the semiquinone radicals are 4.1,

11592 J. Phys. Chem. A, Vol. 107, No. 51, 2003
G o¨ rner
Figure 11. pH dependence of the conductivity signal in the presence
of 0.2 M 2-propanol at 0.1 ms (open) and 0.1 s (full) after the 248-nm
Figure 13. pH dependence of the conductivity signal of BQ (circles),
MeBQ (squares), and 2,6-Me BQ (triangles) in argon-saturated aqueous
2
solution in the presence of 2-propanol (0.2 M) at 500 ns (open) and 10
µs (full) after the 248-nm pulse; insets: signals for BQ at pH 3.8, 5,
and 9 (a-c, respectively).
pulse for BQ (circles), MeBQ (squares), and 2,6-Me
argon-saturated aqueous solution; insets: signals for 2,6-Me
, 8, and 9.5 (from left to right).
2
BQ (triangles) in
2
BQ at pH
5
Figure 10. On the other hand, the conductivity signal becomes
smaller on addition of KCl, indicating competition of photoin-
duced proton formation with reaction 3.
Discussion
Reactions of the Quinone Triplet with H-Atom Donors.
The photoreduction of quinones leads to the semiquinone radical
and the eventual products are hydroquinones, QH2, and acetone,
3
-5
when 2-propanol is the H-atom donor.
With amines, pho-
-
•+
toinduced electron transfer yields Q• and DH2 as secondary
intermediates. The rate constant for triplet quenching (kD) of
NQ in acetonitrile by N,N-dimethylamine and TEA, because
of appropriate redox properties, is close to the diffusion-
2
8
controlled limit. The concentration for 50% change, [2-pro-
panol]1/2, increases from 0.1 M for BQ or MeBQ to values larger
than 10 M for Me4BQ. For amines the half-concentrations are
much smaller, for example, [TEA]1/2 ) 0.3 mM for BQ, because
Figure 12. Conductivity signal vs pH for BQ (open) and 2,6-Me
full) in argon-saturated aqueous solution in the absence of additives
at 0.1 µs (circles), 1 µs (triangles), 10 µs (squares), and 100 µs
diamonds) after the 248-nm pulse; insets: signals for BQ at pH 3.8,
, and 8 (a-c, respectively).
2
BQ
(
(
5
25
of a larger rate constant. Quenching of the triplet state by the
3,4,8
hydroquinone
was avoided for a low overall conversion.
Further pathways into •QH are quenching of the triplet state by
Q and T-T annihilation under pulsed excitation with k ) 2 ×
SCHEME 3
9
-1 -1
8
1
0 M
s
for Me4BQ.
Where the triplet state and the semiquinone radical are
spectroscopically well-enough separated, for example, for Me4-
2
5
BQ, PhBQ, and (OMe)2BQ (Figure 5), reaction 5 (Scheme 1)
is accessible for variable donor concentration. In some cases,
the absorption signal of the semiquinone radical is much larger
than that of T-T absorption, at least in the presence of TEA
and 2-propanol or related alcohols in appropriate concentra-
.45, and 4.6 for BQ, MeBQ, and 2,6-Me2BQ, respectively.3
,43-45
4
The neutralization kinetics at pH 8-10 (insets in Figure 11)
indicate that a contribution from the radical anion is small.
For all BQs examined, for example, in 100:1 (vol) mixtures
of aqueous solution at pH 5-7 with acetonitrile or in neat water,
a conductivity signal could be detected on the 0.1-10 ms time
scale which, however, is small, when compared to those in the
presence of 2-propanol. For BQ, MeBQ, or Me2BQs in neat
aqueous solution, the signal (after triplet decay) is largest at
25
-
tions. The spectra of •QH and Q• are only moderately shifted
3
,43-46
for BQ and derivatives.
The alcohol radical may yield
via reaction 7 another semiquinone radical in competition to
self-termination 8; the rate constant with 2-propanol is k7 )
9
-1 -1 35
1.6 × 10 M s . Such a two-step reduction, because of
reactions 5 and 7, is indicated by the absorption signal for 2,6-
Me2BQ (inset of Figure 9). This second increase was not always
observable in mixtures with 2-propanol. The reason is probably
that reaction 8 successfully competes under pulsed excitation,
0
.3 µs and pH 6 and becomes negative below pH 8. From the
positive to negative conversion of the signal within a few µs in
the slightly alkaline pH range a proton is identified as major
conducting species (Figure 12). This proton appears for a few
microseconds (insets of Figure 12), that is, proton formation is
probably not fully resolved due to subsequent fast decay. As
competing process, this fast proton intermediacy is also present,
but the time resolution is not improved when 2-propanol is
added in small concentration (Figure 13). Instead, the signals
are much larger, in accordance with the dependence shown in
25
in contrast to steady-state conditions. Studies of BQs and
4
2
AQs in acetonitrile in the presence of DABCO or TEA in
appropriate concentrations have shown that the kinetics of
+
-
conductivity increase, because of formation of H and Q• , and
the triplet decay is in accord.
Effects of Water on the Photoreactions. The photoprocesses
of BQs in aqueous solution deviate in several aspects from those

Photoprocesses of p-Benzoquinones
J. Phys. Chem. A, Vol. 107, No. 51, 2003 11593
SCHEME 4
in the organic phase. In particular, they have not been fully
understood for parent BQ or most derivatives in the absence of
H-atom donors. The quantum yield (Φ∆) of formation of singlet
1
molecular oxygen, O2( ∆g) was reported to be ca. 0.1 for BQ
and MeBQ in aqueous solution at pH 7.19 Nevertheless,
fluorescence plays virtually no role and Φisc of most BQs is
expected to be large even in water.3 For NQ, Φisc is not reduced
on addition of water to acetonitrile.28 This was also found for
,4
SCHEME 5
2
,5- or 2,6-Me2BQ (not shown). The large Φd values of BQs at
low concentration and in neutral aqueous solution (Table 1) are
best interpreted by Φisc values close to unity. The weak T-T
absorption for MeBQ and several BQ derivatives in neutral
aqueous solution may be related to a spectroscopically hidden
triplet state rather than to a low Φisc as concluded from the
strongly increasing dependence of the ∆Ar values on [2-pro-
panol] in mixtures of water with organic solvents (Figure 9 and
Table 5).
to reaction 10 plus 11 (Scheme 2). Moreover, Φd strongly
-
decreases as a function of [Cl ] (Figure 2). The similarity of
-
the half-concentrations [Cl ]1/2 and the calculated 1/(k3 × τT)
For Me4BQ in 1:1 mixtures of water with ethanol, it has been
reported that the kinetics of conductivity increase and triplet
values (Table 3) clearly show that the major route of BQs (<1
mM) in aqueous solution originates from the observed triplet
state.
11
decay correspond. Pulsed excitation of the semiquinone radical
-
for 2-SO3AQ in 2-propanol/water at 308 nm ejected an
Mechanisms which Can Be Excluded. The most cited
electron.41 This ionization was avoided here by applying low
enough pulse intensities and conversions, as monitored under
conditions, for example, in the absence of acetonitrile, where
the solvated electron is observable. The literature values for the
triplet lifetime in aqueous solution are rather short, for example,
possibility for photoreduction in aqueous solution involves
2
,3
-
OH radicals.
The photochemistry of 2-SO3AQ or
2
-
2
,6-(SO3)2AQ
was interpreted by involvement of OH
3
7-40
radicals.
Formation of OH radicals has also been postulated
in the photolysis of BQ and other quinones.
1
,13-18
However, a
6-8
6,7
0
.3 µs for MeBQ,
0.8-1.2 µs for 2,3-Me2BQ, and 0.8 µs
for 2,5-Me2BQ. Larger values at low concentrations of BQs
Figure 6 and Table 2) are due to better suppression of reaction
(Scheme 1). The low radical yield, as shown by the strongly
increasing dependence of the ∆Ar and ∆κ values on [2-propanol]
Figures 9 and 10 and Table 5), indicates a different mechanism
free OH radical mechanism has been questioned, on the basis
8
2
-
- 32
of results with 2,6-(SO3)2AQ and 2-SO3AQ . In a recent
modification of the water oxidation mechanism, the results with
MeBQ were interpreted by assuming a hydroxylating intermedi-
(
4
2
1
ate, different from the free OH radical.
(
In this work, no indication was found for such an intermedi-
ate; the triplet state as initial species and the radicals are
spectroscopically and kinetically separated. The ∆Ar/∆AT values
of BQ (Figure 3), MeBQ (Table 4), or 2,6-Me2BQ (Figure 4)
in aqueous solution are significantly suppressed with respect
to the presence of H-atom donors, indicating less efficient
formation of the semiquinone radical. This is supported by the
low conductivity signal of BQs in aqueous solution and the
increase on addition of 2-propanol (Figure 10). On the other
hand, the Φd values of BQ, MeBQ, and Me2BQs in aqueous
solution at pH 7 are much larger than in neat acetonitrile (Table
for BQ and several derivatives in water, not involving the free
semiquinone radical.
Suggested Mechanism for Low BQ Concentrations. To
account for the photoreactions of BQs at low concentration in
the absence of a H-atom donor, a novel charge separation in
the lowest triplet state followed by the water addition 10,
formation of 1,2,4-trihydroxybenzene, and reaction 11 with Q
Scheme 2) are now suggested.22 The water-mediated triplet
(
reaction does not involve radicals at all. This mechanism is
-
consistent with the results that the yield of Q• is low at pH
5
-9 (Figure 9) and the prediction that the observed triplet state
1
3
1), indicating efficient nucleophilic water addition with Φ )
is monomeric. Reaction 10 has been proposed for BQ and
d
20
0.5-0.8.
2,6-Me2BQ, but without further details, for example, specifica-
tion of the excited state.
One possibility for the water addition is step 10′ (Scheme
4
); it does not involve radicals at all and originates from either
On the basis of the fast conductivity signals of BQ, MeBQ,
or 2,6-Me2BQ in aqueous solution at pH 4-8 (Figure 12), step
0 (Scheme 3) is accompanied by a fast proton intermediacy
the excited singlet or a hidden upper excited triplet state. This
reaction, however, is unlikely since fluorescence is not measur-
1
3,4
able at all and the yield of the observed triplet state is virtually
not affected by the amount of water (not shown). Even for
MeBQ, where the triplet lifetime in aqueous solution is shorter
than for other BQs, population of an upper excited triplet state
should be excluded.
within a few microseconds. This holds also for those BQs which
were examined in mixtures of water with acetonitrile. Moreover,
the charge increase is essentially due to a proton, which adds
within a few microseconds to the 1,2,4-trihydroxybenzene anion,
4
9
the pKa ) 9. The pKa of 2-hydroxy-1,4-benzoquinone, Q′,
4
6
which, however, is not formed in this time range, is 4.2. As
competing process, longer lived protons as major conducting
species are produced via sequences 5 and 9 (Scheme 3) when
Involvement of OH radicals in the photolysis of parent BQ
in aqueous solution has recently been excluded by von Sonntag
and his group; instead, the suggested photoreactions lead via
2
-propanol is added (Figures 11 and 13). The dianion of
the semiquinone radical to Q and QH and via the 2-hydroxy-
2
trihydroxybenzene has been reported as a suggested intermediate
of the hydroxide ion induced oxygenation of BQ to rhodizon-
1,4-semibenzoquinone radical (•Q′H) to the 2-hydroxy-1,4-
benzohydroquinone and eventually into Q′ and QH2.22 The
5
0
ate.
mechanism for BQ could involve a triplet exciplex and two
-
-
The triplet state of BQs is quenched by anions, such as Cl
radicals, Q• /•QH and •Q′H. Decay of the latter occurs via
-
(Figure 8). At Cl concentrations of <10 mM the effect is due
reactions 15 and 16 (Scheme 5). However, a crucial point is
the effect of concentration on the triplet decay. Two triplet
to physical quenching 3 not yielding products in competition

11594 J. Phys. Chem. A, Vol. 107, No. 51, 2003
G o¨ rner
SCHEME 6
than 0.1 in acetonitrile (Table 1). Therefore, the observed triplet
state is an intermediate in the major part of the photoreaction
of BQs in aqueous solution. Formation of separated radical
cations and anions is inconsistent with the low yield of Q• as
observable species for BQ, MeBQ, and Me2BQs in water and
-
1:1 mixtures with polar solvents (Figures 3 and 4). Nevertheless,
the overall photoreduction is efficient and the Φd values are
much larger than in neat acetonitrile (Table 1).
Effects of Solvent and Substituent. Several attempts with
organic solvents as additives to BQ in water, for example,
dioxane, dimethyl sulfoxide, dimethylformamide, butyronitrile,
or formamide, failed since H-atom abstraction is enhanced and
absorption of the triplet is completely overlapped by that of
the semiquinone radical. This is also gradually the case with
acetonitrile, which for MeBQ has already been reported to be
SCHEME 7
21
somewhat susceptible to abstraction. For BQ and MeBQ even
in neat acetonitrile, because of Φd values of 0.05-0.1 (Table
1
), photoreduction has to be considered. On the other hand, tert-
exciplexes with carbonyl and benzenoid ring adducts of electron-
donating water molecules have been considered for 2,6-(SO3)2-
AQ .
butyl alcohol does not behave as other alcohols, where H-atom
abstraction is commonly efficient. Nevertheless, tert-butyl
alcohol is not an inert solvent, as shown by semiquinone radical
formation. The results with the two fluoroalcohols (Table 4)
and tert-butyl alcohol are rather similar. The latter weakly
nucleophilic polar solvents favor H bonds to oxygen and
2
- 40
Another possibility is a monomer-dimer equilibrium 12;
electron-transfer step 13 within the dimer triplet could account
-
for formation of radical Q• . This pathway via free radical ions
is illustrated in Scheme 4 for a ground-state dimer. The products
eventually derived from the radical cation are QH2 and Q′. No
indication for a dimer could be found in the absorption spectrum,
when the BQ or 2,6-Me2BQ concentrations were varied between
4
7
stabilize excited π,π states and ions.
The reactivity of the triplet state of BQs with 2-propanol is
quite large and the rate constant k5 is strongly reduced by four
2
5
methyl groups in acetonitrile and also in water-acetonitrile.
As indicators in the presence of water, both the triplet lifetime
and the relative radical yield (Tables 4 and 5) as well as Φd
0.01 and 10 mM. At least for BQ derivatives at low concentra-
tions (<0.3 mM), ground state and triplet dimers cannot play a
role.
(Table 1) can be used. The polarity may affect the triplet lifetime
Possible Mechanism for Higher BQ Concentrations. A
for BQ and 2,6-Me2BQ, but the variation is only gradual for
Me4BQ. Me3BQ and PhBQ are placed between and the smallest
effects were obtained for (OMe)2BQ. Generally, the effects of
water on the triplet and radical properties are small for these
BQs, in contrast to parent BQ, MeBQ, and the two Me2BQs.
The shortest triplet lifetime was found in water and a significant
shortening on addition of water to “inert” organic solvents.
3
further possibility is an electron transfer 4 from Q to Q*. An
analogous pathway into radical ions is T-T annihilation under
3
3
pulsed excitation, Q* + Q*, and this interaction has previously
8
been considered for Me4BQ. The linear plots of 1/τT versus
[
(
quinone] for 2,6-Me2BQ and the curved dependence for BQ
Figure 6) accounts for the radical cation derived products at
quinone concentrations larger than 1 mM. The possible pathway
of BQs into free radical ions are steps 4 and 17 (Scheme 6).
The radical cation/anion mechanism, however, conflicts with
Conclusions
-
the findings that the yield of Q• is low. The eventual products
The suggested water-mediated photoprocesses of BQ and
are of QH2 and Q′ type22 and reaction 18 (Scheme 6) can be
postulated. Reaction 18 competes successfully with electron
back transfer, when the Φd values are as large as 0.5-0.8. In
addition, the possibility of H-atom abstraction from the solvent,
such as acetonitrile, 5 (Scheme 7), establishes another pathway,
which however, should change the product pattern into higher
amounts of QH2 with respect to Q′.
derivatives, such as MeBQ and Me BQs, are either step 10
(Scheme 7) of reaction of the lowest triplet state with water at
2
low concentrations or one at high BQ concentrations which is
induced by self-quenching of the triplet state followed by
electron transfer 18. In both pathways, virtually no free radicals
are involved for BQs. A third free radical pathway, 5 and 6′
(Scheme 7), as minor step contributes in mixtures with “nearly
inert” organic solvents even in the absence of a H-atom donor.
A hypothesis via reaction 4 (Schemes 6 and 7) has been
rejected for 2,6-(SO3)2AQ2
-37-40
since no increasing dependence
of 1/τT versus [quinone] was found, that is, the rate constant k4
is too low in aqueous solution. However, this is not generally
the case, for example, the rate constant in aqueous solution is
Acknowledgment. The author thanks Professor Wolfgang
Lubitz for his support, Professor Clemens von Sonntag for
providing parent BQ and ref 22, and Mr. Leslie J. Currell, Lars
Kalender, and Horst Selbach for technical assistance.
9
-1 -1
22
9
-1 -1
larger than 5 × 10 M
s
for BQ and 4 × 10 M
s
for
NQ.28 This is in agreement with k4 values (Table 3), obtained
from the slopes of Figure 6. The model can account for products
of BQs at moderate or higher concentrations, taking k4 ) 1 ×
References and Notes
(1) Lubitz, W.; Lendzian, F.; Bittl, R. Acc. Chem. Res. 2002, 35, 313.
9
-1 -1
1
0 M
s
or larger and a limiting τT of 10 µs. Then, a
(2) Burie, J.-R.; Boussac, A.; Boullais, C.; Berger, G.; Mattioli, T.;
concentration of 0.1 mM is sufficient for more than 50% reaction
of the observed triplet state under continuous irradiation. One
could argue that for BQ or MeBQ, Φisc is low in water and an
upper excited triplet is involved. However, the yield of observed
triplet state in either water or acetonitrile is very similar in these
and most other cases, Φd is up to unity in water and smaller
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(
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(
Compounds; Patei, S., Rappaport, Z., Eds.; Wiley: New York, 1988; p
737 and refs cited therein.

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