Reduction of 4,4′-stilbenequinone and 4,4′-diphenoquinone upon reaction with photogenerated radicals

Article abstract of DOI:10.1111/j.1751-1097.2010.00798.x

The properties of 3,3′,5,5′-tetra-tert-butyl-4,4′- stilbenequinone (StQ) were studied by photochemical means. Acetone, acetophenone or benzophenone was photolyzed in the presence of both StQ and a donor, such as alcohols or triethylamine. This initiated reaction of a ketyl radical with StQ to form a semiquinone radical and eventually induce a permanent bleaching due to conversion of StQ to 4,4′-dihydroxystilbene (StQH₂). The quantum yield of conversion of StQ to StQH₂ increases with the donor concentration. Similar effects were found for the ketone-sensitized radical-induced conversions of the analogous diphenoquinone to the reduction product, diphenol.

Full text of DOI:10.1111/j.1751-1097.2010.00798.x

Photochemistry and Photobiology, 2010, 86: 1202–1207
Reduction of 4,4¢-Stilbenequinone and 4,4¢-Diphenoquinone upon
Reaction with Photogenerated Radicals
Helmut G o¨ rner*
Max-Planck-Institut f u¨ r Bioanorganische Chemie, M u¨ lheim an der Ruhr, Germany
Received 12 July 2010, accepted 8 August 2010, DOI: 10.1111/j.1751-1097.2010.00798.x
dimeric products of the 2,5-di-tert-butylphenoxyl radicals.
ABSTRACT
1,2-Benzoquinones and their reduced catechols play a specific
role as ligands in metal complexes. For 3,5-di-tert-butylcate-
The properties of 3,3¢,5,5¢-tetra-tert-butyl-4,4¢-stilbenequinone
StQ) were studied by photochemical means. Acetone, acetoph-
(
chol the catalytic catecholase activity of such metal complexes
(
enone or benzophenone was photolyzed in the presence of both
StQ and a donor, such as alcohols or triethylamine. This
initiated reaction of a ketyl radical with StQ to form a
semiquinone radical and eventually induce a permanent bleach-
Mn or Cu) has been demonstrated (23–28).
ing due to conversion of StQ to 4,4¢-dihydroxystilbene (StQH
2
).
The quantum yield of conversion of StQ to StQH increases with
2
the donor concentration. Similar effects were found for the
ketone-sensitized radical-induced conversions of the analogous
diphenoquinone to the reduction product, diphenol.
Chart 1.
INTRODUCTION
The photoprocesses of 1,4-benzoquinone (BQ) and numerous
derivatives involve a reactive triplet state (1,2). For several
BQs optimized conditions can be chosen, where the quantum
yield (F2H) of reduction is close to unity. The quantum yield of
intersystem crossing (Fisc) of a variety of quinones, as
established by several methods, is as high as 0.5–1 (1–3). The
In this work, the properties of StQ and DpQ and their doubly
reduced forms were studied by photochemical techniques. The
reduction products of StQ and DpQ are trans-3,3¢,5,5¢-tetra-
tert-butyl-4,4¢-dihydroxystilbene (StQH
) and 3,3¢,5,5¢-tetra-
). To induce photoreduction
2
tert-butyl-4,4¢-diphenol (DpQH
2
quantum yield (F
D
) of singlet molecular oxygen of quinones in
reactions for molecules with low Fisc, a ketone as sensitizer
together with a donor, e.g. an alcohol or triethylamine (TEA)
can be utilized (29–32). This method has recently been applied
for violanthrones (33,34). The quantum yield F2H of conversion
of the two tetra-tert-butylquinones to colorless and labile
organic solvents is also relatively large (4–7). These systems
with substantial F2H indicate the efficient formation of
semiquinone radicals (8–11). A few quinones, however, have
virtually no photochemical activity as Fisc in solution at room
temperature is too low.
StQH
UV irradiation.
2
2
⁄ DpQH increases with the donor concentration using
One class of quinones with rather low Fisc values contains
BQs with an isopropyl side chain: thymoquinone (2-Me-5-
isopropylBQ) and the 2,5-dibromo derivative (12). Another
class refers to quinones having a long side chain, e.g. vitamin
MATERIALS AND METHODS
StQ was a gift from Henry Gruen (35); a second sample was purchased
from TCI and no significant difference was found. DpQ was from
Alfa; the other compounds were the same as used elsewhere (33,34).
The solvents (Aldrich, Merck) were of the purest spectroscopic quality
(Uvasol) and used as received. The absorption spectra were monitored
on a spectrophotometer (HP, 8453). The molar absorption coefficient
K (13,14). Intramolecular proton ⁄ H-atom transfer success-
fully competes with intermolecular photoprocesses. A third
class is constituted by BQs with butyl groups, examples are:
1
2
-tert-butylBQ, di-tert-butylBQs (15–18) and 3,3¢,5,5¢-tetra-
tert-butyl-4,4¢-stilbenequinone (StQ). The latter compound has
been studied over the years (18–21).
5
)1
)1
of StQ at 440 nm is e440 = 1.0 · 10
M
cm (21). For photocon-
version kirr = 313 nm from a 1000 W Hg-Xe lamp and a monochro-
mator was used. The quantum yield F2H of conversion was obtained
using aberchrome 540 as reference (36); the experimental error is
± 20ꢀ. For an efficient light-induced decoloration reaction the
solutions were thoroughly purged by argon. An excimer laser
The photoprocesses and the radiation chemistry of StQ
have also been examined (20,21). 3,3¢,5,5¢-Tetra-tert-butyl-
4,4¢-diphenoquinone (DpQ) (see Chart 1) is expected to
behave similarly (22). StQ and DpQ can be considered to be
(
<
Lambda Physik, EMG 200, pulse width of 20 ns and energy
30 mJ) was used for excitation at 308 nm. For relative yields
optically matched solutions, typically with absorbances at kexc of 0.3–3
per 1 cm), were used. The absorption signals were measured with
(
*
Corresponding author email: goerner@mpi-muelheim.mpg.de
Helmut Gorner)
two digitizers (Tektronix 7912AD and 390AD). The measurements
refer to 24ꢀC.
(
¨
2010 The Author. Journal Compilation. The American Society of Photobiology 0031-8655/10
ꢁ
1
202

Photochemistry and Photobiology, 2010, 86 1203
RESULTS
k
a
= 420 nm. Examples of the acetophenone-sensitized reduc-
tion of DpQ are shown in Fig. 3. Generally, F2H increases with
the donor concentration and is low when oxygen is not fully
excluded. Plots of F2H vs the concentrations of two donors are
Photoinduced reduction of tetra-tert-butylquinones
StQ in argon-saturated acetonitrile has an absorption band
with a maximum at k = 450 nm. Irradiation at 400–500 nm
a
a
does not show a marked effect on the absorbance at k (A450)
2
Table 2. Quantum yield of transformation of quinones to QH s in the
presence of TEA.*
and irradiation at 313 nm leads only to a relatively small
decrease of A450. A strong decrease of A450, however, was
observed upon UV-irradiation of a ketone (acetone, acetoph-
enone and benzophenone) in the presence of StQ. An even
stronger decrease of A450 was found upon ketone-excitation in
the presence of 2-propanol or other alcohols. The process is
Ketone
Solvent
[TEA] (M)
StQ: F2H
DpQ: F2H
Benzophenone Acetonitrile
Acetophenone
0.01
0.01
0.7
0.7
0.8
0.8
Acetone
Benzene
Acetonitrile
0.01
0.0001
0.001
0.5
0.03
0.5
0.5
0.02
0.6
attributed to ketone-sensitized conversion of StQ to StQH
see below). Alternatively, photoreduction was achieved in
2
(
0
.01
0.6 [0.7]†
0.7 [0.8]
argon-saturated benzene or acetonitrile in the presence of
TEA. Examples of the acetone- and benzophenone-sensitized
excitation are shown in Figs. 1 (inset) and 2a. The time
dependences of A450 in acetonitrile are shown in Figs. 1 and 2c.
The slope of the initial linear part is a measure of the quantum
*
3
Using argon-saturated solution kirr = 313 nm; †in brackets: kexc =
08 nm.
yield F2H of conversion of StQ to StQH . The F2H values are
2
compiled in Tables 1 and 2.
The conditions of conversion of DpQ to DpQH
2
are
analogous to those of the StQ ⁄ StQH couple. In particular,
2
irradiation at 400–500 nm of DpQ in argon-saturated aceto-
nitrile does not show a marked effect on the band centered at
Figure 2. (c) Time-dependences of A450 for irradiation at 254 nm of
mM benzophenone in 2-propanol-water 9:1 at pH 11 in the presence
of StQ and under argon (open) and thermal back-reaction under air
3
(
(
full, two cycles); (a) absorption spectra and (b) fluorescence excitation
left, k = 430 nm) and emission (right, kexc = 320 nm) spectra, three
f
cycles with the same sample, 1–6, respectively.
Figure 1. Time-dependence of the absorption difference at 450 nm
upon irradiation at 313 nm of 0.3 M acetone in argon-saturated
acetonitrile in the presence of 20 lM StQ and 0.1 (h), 0.3 (D), 1 (s) and
8
(d) M 2-propanol; inset: absorption spectra of StQ ⁄ StQH
acetone ⁄ 2-propanol (0.3 ⁄ 8 M) at 0, 20 s and 4 h after admission of air,
–3, respectively.
2
with
1
2
Table 1. Quantum yield of transformation of quinones to QH s in the
presence of alcohols.*
Ketone
Donor
[Donor] (M)
StQ: F2H
<0.01
DpQ: F2H
Benzophenone None
<0.01
2-Propanol
10
10
0.8 (0.01)†
0.8
0.8 (0.01)
Acetophenone
Acetone
None
Methanol
<0.01
0.7
0.01
0.07
0.6
<0.01
0.7
0.02
0.08
0.7
10
0.1
1
2-Propanol
Figure 3. Time-dependence of A425 for irradiation at 313 nm of 0.03 M
acetophenone in argon-saturated acetonitrile in the presence of 20 lM
DpQ with 1 (h), 3 (D), 10 (s) and 90 (d) mM TEA; inset: absorption
spectra in the presence of 0.1 M TEA at 0 and 30 s irradiation, at 1 and
4 h after admission of air, 1–4, respectively.
1
0
*In argon-saturated acetonitrile,
air-saturated.
kirr = 313 nm; †in parentheses:

1
204 Helmut G o¨ rner
shown in Fig. 4 for acetone as sensitizer. The F2H values are
similar for StQ and DpQ, showing a large variation and a
characteristic shift between amine and alcohol. Finally, the
conversions are similar upon pulsed excitation or continuous
irradiation at 308 ⁄ 313 nm of a ketone plus donor and either
StQ or DpQ. Examples for acetone ⁄ TEA are included in
Table 2.
for the same sample is shown in Fig. 2c. Interestingly, the
absorption change due to the reduction of quinones in
acetonitrile can be achieved thermally with Zn powder and
perchloric acid. These conversions, which occur within a few
minutes on purging with argon but not in the presence of
air ⁄ oxygen and in the absence of acid, are shown for StQ and
DpQ in Fig. 5b,a, respectively.
Thermal processes
Fluorescence
The thermal back-reactions of StQH
2
to StQ and DpQH
2
to
The photoinduced conversion to dihydroproducts and the
thermal back-reaction can also be followed by fluorescence
DpQ upon admission of oxygen are generally slow, e.g. a few
days in the absence of base. A much faster back-reaction was
achieved in the alkaline range, e.g. at pH 10–12, see Figs. 2c
and 5. The photoinduced process and the oxygen-mediated
back-reaction constitute a cycle; an attempt with three cycles
spectroscopy. The trans-stilbenes, in particular StQH
fluorescence. The excitation maximum of StQH is centered at
340 nm, that of the emission is k = 380 nm. An example of
2
exhibit
2
f
2
the spectra of StQ ⁄ StQH prior and after irradiation as well as
after admission of oxygen is shown in Fig. 2b. StQ, in contrast
to DpQ, does not emit at all. Apparently, the absence of
fluorescence of StQ has to be attributed to rapid internal
2
conversion. For DpQ the cycle involving DpQH can also be
recorded by fluorescence spectroscopy (data not shown) but
the changes are much smaller than for StQ.
Transient absorption properties
Pulsed 308 nm excitation of StQ in inert solvents of different
polarity, e.g. benzene or acetonitrile, reveals no triplet state or
radical absorption under our experimental conditions. Upon
ketone-sensitized excitation of StQ in argon-saturated meth-
anol or other alcohols, however, a longer-lived transient with a
maximum at kR = 600 nm and a half-life of t1 ⁄ 2 = 0.01–
0
.1 ms was observed. Similar results were found in acetonitrile
in the presence of TEA. Examples with alcohols are shown in
Fig. 6. The transient is ascribed to the semiquinone radical,
)
)
StQH
10 nm (21). In addition, a strong bleaching at k
with t = 0.01–0.1 ms and ketone-mediated kinetics in the
Æ
⁄ StQ
Æ
(see below). StQ
Æ
in cyclohexane has k
R
=
Figure 4. Double-logarithmic plot of F2H vs the TEA (circles) and
6
a
= 450 nm
2
0
-propanol (triangles) concentrations for irradiation at 313 nm of
.3 M acetone in argon-saturated acetonitrile in the presence of 20 lM
1 ⁄ 2
StQ (open) and DpQ (full).
submillisecond range were recorded. The absorption increases
and the bleaching disappears if oxygen is admitted.
In the case of benzophenone (and argon), the triplet state
with a maximum at 320 ⁄ 525 nm is efficiently quenched by the
donor and the signal after the pulse is already converted into
Figure 6. Transient absorption spectra in the presence of argon-
saturated 9 lM StQ and (a) 0.03 M benzophenone in methanol and
(b) 0.03 M acetophenone in acetonitrile ⁄ 2-propanol 1:9, at 20 ns (s),
1 ls (D), 10 ls (h), 0.1 ms (d) and 1 ms (m) after the 308 nm pulse;
inset: kinetics as indicated.
+
Figure 5. Plots of the Zn-H -induced reduction (open) of StQ (circles)
and DpQ (triangles) at pH 2.5 and after addition of oxygen and NaOH
at pH 12–13 (full); inset: absorption spectra of (a) DpQ at 0, 5, 50, 300
and 500 s and (b) StQ at 0, 50 and 300 s, 1–3, respectively.

Photochemistry and Photobiology, 2010, 86 1205
steps 1 and 2, but H-atom or electron transfer from the donor
)
yield the ketone-type, KH
Æ
⁄ K
Æ
, and donor-type, DH
and
are identical. The benzophenone radical has
Æ
⁄
+
DH2
DH
540 = 3.2 · 10 and e600 = 4.8 · 10
37,38). For benzophenone in methanol the rate constant k
Æ
, radicals (step 3). For acetone ⁄ 2-propanol KH
Æ
Æ
3
3
)1
)1
)
e
M
cm , for KH
Æ
⁄ KÆ
(
3
of
step 3 is small enough that the conversion of the triplet to the
ketyl radical is detectable at 500–530 nm (Figs. 6a and 7a).
The key intermediate is the semiquinone radical, QH
Æ
⁄ Q
),
Æ
formed via step 4 and converted via step 5 into the relatively
stable dihydro form. The structure and role of this radical is
illustrated in Scheme 2. The involvement of the semiquinone
radical with kR = 600 nm for StQH
DpQH is shown in Figs. 6 and 7, respectively. A few values
concerning related radicals are available in the literature
21,39–41). The radical anions of 3,3¢,5,5¢-tetrametyl-4,4¢-
Æ
and kR = 470 nm for
Æ
Figure 7. Transient absorption spectra in the presence of argon-
saturated DpQ of (a) benzophenone in methanol and (b) 0.03 M
acetophenone plus 0.03 M TEA at 20 ns (s), 1 ls (D), 10 ls (h),
(
stilbenequinone and the tetrametyl analog of DpQ have
maxima at 610 and 520 nm, respectively (41). Note that the
longest wavelength maxima of these radicals with similar e
0
.1 ms (d) and 1 ms (m) after the 308 nm pulse; inset: kinetics as
indicated.
4
)1
)1
of (2–3) · 10
M
cm are at 1000 and 850 nm, respectively.
the ketone radical, KH
is observed at 325 ⁄ 550 nm (34,35), but when StQ is present the
semiquinone radical, StQH
tion overlaps the 550 nm band (Fig. 6a). In the cases of
acetophenone (Fig. 6b) and acetone the TT-maximum is
shorter than 360 nm and the absorption of the ketyl radical
is weaker.
Æ
⁄ K
Æ
). The benzophenone ketyl radical
The semiquinone radical of 4,4¢-dihydroxydiphenol has
4 )1
)1cm
cm (39).
e620 = 0.9 · 10
M
and the radical anion e730 =
)
4
)1
)1
Æ
⁄ StQ
Æ
is formed and its absorp-
1.7 · 10
M
The transient difference spectra of DpQ, the bleaching at
k
a
= 420 nm and the kinetics (Fig. 7a,b) are similar to those
of StQ. The semiquinone radical of DpQ has a maximum at
= 470 nm. This maximum is significantly more blueshifted
k
R
a
for DpQ vs StQ than k .
Scheme 2.
DISCUSSION
Photoinduced processes of tetra-tert-butylquinones
Conditions of photoreduction of Qs and subsequent oxidations
of QH
2
s
Pulsed excitation at kexc = 265 nm of StQ in cyclohexane
revealed a transient with a maximum at k
T
= 380 nm which
The results of the ketone-sensitized irradiation can be analyzed
on the basis of Schemes 1 and 2. The curves in Fig. 4 are
similar for either StQ or DpQ and shifted to smaller
concentrations for TEA by three orders of magnitude with
has been attributed to the lowest triplet state (21). The triplet
lifetime in argon-saturated cyclohexane is sT = 0.07 ls;
oxygen quenches the triplet, the rate constant is
9
)1 )1
k
ox = 1.3 · 10
M
s
(21). For StQ and DpQ there are no
respect to 2-propanol. This is due to the difference in rate
6
other triplet data known. Pulsed excitation at kexc = 308 nm
of StQ in acetonitrile does not yield any transient spectra
under our conditions. This is not surprising, as the absorption
constants: k
9
3
= (1–2) · 10 for 2-propanol and k
3
= (1.2–
for TEA (42). Thus the half-concentration
)1 )1
1.6) · 10
M
s
for alcohols is much larger than for an amine. The type of
ketone plays no marked role, but only with benzophenone can
the entire photoreaction sequence be followed by flash
photolysis, whereas for acetone or acetophenone the T-T and
ketyl radical maxima are overlapped by the ground state
absorption.
at 308 nm is ca 1ꢀ of that at k = 450 nm. No triplet
a
absorption spectrum was recorded for DpQ.
The radical-mediated conversion of the two tetra-tert-
butylquinones (Q) examined, to the dihydroquinones (QH
is illustrated in Scheme 1. The triplet ketone is populated via
2
),
Scheme 1.

1
206 Helmut G o¨ rner
Moreover, the results follow competition kinetics. Reaction 4
The reduction to their doubly reduced forms, trans-3,3¢,5,5¢-
tetra-tert-butyl-4,4¢-dihydroxystilbene, StQH , and 3,3¢,5,5¢-
of Q with KH
Æ
could be replaced by reaction 4¢ of Q with DH
A reasonable rate constant close to the upper limit is
Æ
.
2
tetra-tert-butyl-4,4¢-diphenol, DpQH2, was achieved by
UV-irradiation of several ketones in the presence of alcohols
or TEA as donors. The results can be analyzed on the basis of
competition kinetics of three types of radicals: ketyl, donor and
semiquinone. The quantum yields F2H of conversion were
determined and the spectroscopy and kinetics of the radical
10
)1 )1
k4 = 1 · 10
half-life of t1 ⁄ 2 = 10 ls. The spectroscopic conditions of pulsed
08 nm excitation of StQ are limited to [Q] = 3–10 lM as
M
s
which for [Q] = 10 lM corresponds to a
3
otherwise bleaching is not observable or termination of the other
radicals successfully competes with reactions 4 or 4¢.
The oxidation of the dihydroquinones involving two equi-
libria is illustrated in Scheme 3. Oxygen quenches the ketone
triplet and the ketyl radicals (1,2). Nothing can therefore be
predicted for reaction 6 of the semiquinone radical with
oxygen. The cobalt(II)-catalyzed oxidation of StQ (43) and
involved characterized. Oxygen induces the recovery of QH
2
to Q. The StQ ⁄ StQH or DpQ ⁄ DpQH couples are proposed to
2
2
have a wide application potential.
Acknowledgements—The author thanks Professor Wolfgang Lubitz
for his support, Mr. Henry Gruen for stimulating discussions, and
Mr. Leslie J. Currell and Norbert Dickmann for technical assistance.
DpQ (44) in benzene leads to StQH
2 2
and DpQH . An
enzymatic reduction of complex redox dyes using NADH-
dependent reductase from Bacillus subtilis coupled with
cofactor regeneration has been reported (45).
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)1 )1
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M
s
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6
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CONCLUSIONS
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