Direct photooxidation and xanthene-sensitized oxidation of naphthols: Quantum yields and mechanism

Article abstract of DOI:10.1021/jp108832x

The photoinduced oxidation of 1-naphthol to 1,4-naphthoquinone and of 5-hydroxy-1-naphthol to 5-hydroxy-1,4-naphthoquinone was studied by steady-state and time-resolved techniques. The direct photooxidation of naphthols in methanol or water takes place by reaction of the naphoxyl radical ( ^·ONaph) with the superoxide ion radical (O₂ ^·-), the latter of which results from the reaction of the solvated electron with oxygen after photoionization. The sensitized oxidation takes place by energy transfer from the xanthene triplet state to oxygen. From the two oxygen atoms, which are consumed, one is incorporated into the naphthol molecule giving naphthoquinone and the second gives rise to water. The effects of eosin, erythrosin, and rose bengal in aqueous solution, pH, and the oxygen and naphthol concentrations were studied. The quantum yield of the photosensitized transformation was determined, which increases with the naphthol concentration and is largest at pH > 10. The quantum yield of oxygen uptake is similar. The pathway involving singlet molecular oxygen is suggested to operate for the three sensitizers. The alternative pathway via electron transfer from the naphthol to the xanthene triplet state and subsequent reaction of ^·ONaph with O₂^·-, the latter of which is formed by scavenging of the xanthene radical anion by oxygen, does also contribute.

Full text of DOI:10.1021/jp108832x

2
80
J. Phys. Chem. A 2011, 115, 280–285
Direct Photooxidation and Xanthene-Sensitized Oxidation of Naphthols: Quantum Yields
and Mechanism
†
,‡
§
Michael Oelgem o¨ ller, Jochen Mattay,* and Helmut G o¨ rner
School of Pharmacy and Molecular Sciences, James Cook UniVersity, TownsVille, QLD 4811, Australia,
Organische Chemie I, Fakult a¨ t f u¨ r Chemie, UniVersit a¨ t Bielefeld, D-33501 Bielefeld, Germany, and
Max-Planck-Institut f u¨ r Bioanorganische Chemie, D-45413 M u¨ lheim an der Ruhr, Germany
ReceiVed: September 16, 2010; ReVised Manuscript ReceiVed: NoVember 26, 2010
The photoinduced oxidation of 1-naphthol to 1,4-naphthoquinone and of 5-hydroxy-1-naphthol to 5-hydroxy-
1
,4-naphthoquinone was studied by steady-state and time-resolved techniques. The direct photooxidation of
•
naphthols in methanol or water takes place by reaction of the naphoxyl radical ( ONaph) with the superoxide
ion radical (O
•-
2
), the latter of which results from the reaction of the solvated electron with oxygen after
photoionization. The sensitized oxidation takes place by energy transfer from the xanthene triplet state to
oxygen. From the two oxygen atoms, which are consumed, one is incorporated into the naphthol molecule
giving naphthoquinone and the second gives rise to water. The effects of eosin, erythrosin, and rose bengal
in aqueous solution, pH, and the oxygen and naphthol concentrations were studied. The quantum yield of the
photosensitized transformation was determined, which increases with the naphthol concentration and is largest
at pH > 10. The quantum yield of oxygen uptake is similar. The pathway involving singlet molecular oxygen
is suggested to operate for the three sensitizers. The alternative pathway via electron transfer from the naphthol
•
•-
to the xanthene triplet state and subsequent reaction of ONaph with O
2
, the latter of which is formed by
scavenging of the xanthene radical anion by oxygen, does also contribute.
S
Introduction
quantum yield (Φ ) of the photosensitized transformation of
1
-naphthol to parent NQ in air-saturated aqueous solution is known
Singlet molecular oxygen, O
the photosensitized oxidation of biomolecules and has been
2
(1∆
g
), is a key intermediate in
19,21
S
to increase strongly with increasing pH.
larger for the phenolate. These effects are mainly due to the much
larger rate constant k
Φ is likewise much
1
6
2-14
frequently used to initiate photooxidation of phenols.
In
19-21
3
for the phenolate/napththolate.
1
3
10
various studies, ketones, quinones, tris(2,2′-bipyridine)ru-
27-32
The photophysical and photochemical properties of phenols
8
,9
11
thenium salts or a Cr(phen)
3
complex, methylene blue,
33-36
and 1- and 2-naphthol
solution have been intensively examined. The photooxidation
of phenol in water has been studied by electron paramagnetic
resonance and nuclear spin polarization. The direct photooxi-
dation of 1-naphthol and derivatives to NQs, that is, overall eq
in organic solvents and aqueous
porphyrins,11 riboflavin, eosin, and rose bengal5-7 served as
sensitizers. The photoinduced key step is energy transfer from
the triplet excited sensitizer to triplet oxygen. The related
photooxidation of monohydroxy- and dihydroxy-substituted
naphthalenes to naphthoquinones has been investigated and has
14
3,4
27
34,35
1
, has been studied by Richard and co-workers.
yield (Φ
-naphthol to parent NQ in deaerated and oxygen-saturated
The quantum
1
5-25
found widespread applications.
In particular, the photooxi-
Q
) of the direct photochemical transformation of
dation of 1-naphthol to 1,4-naphthoquinone (NQ) and of
-hydroxy-1-naphthol (1,5-dihydroxynaphthalene) to 5-hydroxy-
1
5
1
35
aqueous solution is Φ
Naphthalenes are also generators of O
naphthols (HONaph) is relatively low.
Q
) 0.003 and 0.03, respectively.
,4-naphthoquinone (5-HONQ: Juglone) has been intensively
1
2
( ∆ ), but Φisc of
g
17-25
studied.
Juglone serves as a valuable building block for
19,34-36
the synthesis of biologically active quinonoid compounds.
As sensitizers, eosin and rose bengal have frequently been
HONaph + O + hV f NQ + H O
(1)
Triplet quenching by oxygen can also yield the hydroperoxyl/
2
2
employed. In a typical experiment a sensitizer (S) is excited
1
and after intersystem crossing (isc) O
2
( ∆
g
) is formed from the
3
*
1
xanthene triplet state ( S), reaction 1 in Scheme 1. O
2
( ∆
g
)
reacts with the naphthol molecule (HONaph), step 3, or with
•
•-
superoxide ion radicals (HO
2
/O
2
), the properties of which are
1
the solvent, step 2. Generally, the quantum yield of O
2
( ∆
g
)
37,38
well documented, but its role in the photooxidation of
naphthols has been considered to be minor.
formation (Φ ) is only slightly smaller than that of intersystem
∆
15-21
26
3
crossing (Φisc). The rate constant k of reaction (3) of singlet
5
-Hydroxy-1-naphthol readily gives 4-20% of 5-HONQ
when directly irradiated in the presence of oxygen. Higher
molecular oxygen with (hydroxy)naphthols as quenchers de-
pends strongly on the applied conditions. For the 2-acetonaph-
thone/5-hydroxy-1-naphthol system in 1:1 dichloromethane/
24
yields have been obtained in acetone when UV-irradiated in a
22,24
quartz vessel.
No time-resolved measurements are available
7
-1 -1
19
acetonitrile k
3
) 0.7 × 10 M
s
has been reported. The
39
for acetone, in contrast to the benzophenone case. Electron
transfer rather than triplet energy transfer has been reported to
be operating, based on laser flash photolysis studies of the
*
To whom correspondence should be addressed.
James Cook University.
Universit a¨ t Bielefeld.
†
‡
§
interaction between triplet benzophenone and the phenolate or
39-41
Max-Planck-Institut f u¨ r Bioanorganische Chemie.
naphtholate anion.
The photoreaction of thioxanthone with
1
0.1021/jp108832x  2011 American Chemical Society
Published on Web 12/17/2010

Photooxidation and Oxidation of Naphthols
J. Phys. Chem. A, Vol. 115, No. 3, 2011 281
SCHEME 1
phenolic derivatives of biological relevance has been studied
using the magnetic field effect. To account for the role of
acetone as solvent and/or sensitizer, several mechanisms can
The oxygen uptake was measured using a Clark electrode cell.43
The oxygen concentration (under air atmosphere) was found to
remain essentially constant in water, 0.27 mM, when the light
was removed. The oxygen concentration decreases with irradia-
tion time and the slope of the initial linear dependence can be
taken as relative quantum yield of the transformation of the
naphthol to NQ. The sensitized oxidation was measured
4
1
be considered. One is energy transfer from the sensitizer to
1
oxygen, thereby generating O
2
( ∆
g
), Scheme 1. Another pathway
is electron transfer, reactions (4 + 5 or 5′ in Scheme 2), thereby
free radicals are expected to be involved. In principle, the
reactive oxygen could be generated by electron transfer.4
The photooxygenation generally refers to electron or H-atom
transfer to the sensitizer (type I) and energy transfer (type II),
2-44
Q
analogously. The absolute quantum yields for direct (Φ′ ) and
S
sensitized oxidation (Φ′ ) were obtained by calibration. The flash
photolysis was done using an excimer laser (Lambda Physik,
EMG 200, pulse width of 20 ns and energy <30 mJ) which
operated at λexc ) 308 nm. Alternatively, a Nd:YAG laser was
used for excitation at 530 nm. The absorption signals were
measured with two digitizers (Tektronix 7912AD and 390AD)
and an Archimedes 440 computer for data handling was used
as in previous work.4 The rate constant for quenching was
•
•-
1
1
yielding HO
2
/O
2
and O
2
g
( ∆ ), respectively. The reactive
oxygen species in the ketone-sensitized conversion to NQs is
open to question.
In this study, we aim at a better understanding of the
mechanism of photooxidation of 1-naphthol and 1,5-dihydroxy-
3,44
naphthalene. The quantum yields in the absence Φ
Q
and
S
presence of a sensitizer Φ were measured under various
conditions. For the latter purpose, we employed the three most
common xanthene dyes: eosin, erythrosin, and rose bengal,
T
determined in argon-saturated solution from plots of 1/τ versus
the donor concentration. The samples were freshly dissolved
and measured at 24 °C.
1
,26,42-44
where Φisc and Φ
∆
are 0.5-0.8.
The reactive oxygen
species in the direct photoconversion of naphthols to NQs is
Results and Discussion
•
-
suggested to be O
2
. Electron transfer from HONaph to the
xanthene triplet state S in oxygen- or air-saturated aqueous
methanol could be proposed to initiate a subsequent reaction
3
*
Photooxidation of Naphthols. The absorption spectrum of
5
-hydroxy-1-naphthol in oxygen- or air-saturated methanol
•
-
exhibits maxima at 235 and 300 nm and no band above 340
nm. A new spectrum with a maximum at 250 nm and a shoulder
at 330 nm appears upon irradiation at 313 nm. Similar changes
were recorded in aqueous solution with 3% methanol (Figure
of the naphthoxyl radical with O
2
. This has to be excluded
for the xanthene-sensitized oxidation of phenols in solution.
3
*
Energy transfer from the S state to oxygen may successfully
compete and has to be considered as alternative pathway for
the present cases. The following questions were addressed: (i)
Which intermediates can be characterized by flash photolysis
of the two naphthols? (ii) What is the mechanism of sensitized
1
, inset). Juglone as the photoproduct of 5-hydroxy-1-naphthol
34-36
is well-established.
The isosbestic point at 277 nm excludes
a major contribution from secondary photolysis. The spectral
changes are similar in acetonitrile or dichloromethane and
initially also the same in aqueous solution at pH 6-9. The
spectra of the naphthols in aqueous solution at pH 10-12 are
different, that is, the maxima are strongly red-shifted. Note that
photooxidation? (iii) Which are the major factors influencing
S
the quantum yields Φ
Q
and Φ ? Furthermore, we would like to
understand (iv) the role of pH in water on the xanthene-
sensitized oxidation processes.
2
0
the pK
than for phenol, pK
a
of 1- and 2-naphthol is 9.2-9.5, that is, slightly lower
5
a
) 10.0. For the anion of 5-hydroxy-1-
Materials and Methods
naphthol, in contrast to that of 1-naphthol, a broad band centered
at 560 nm is formed thermally. Owing to this dark reaction,
which has already been reported,19 we did not study this
naphtholate.
The compounds (EGA, Sigma) and solvents (Merck, Fluka)
were checked for impurities and used as received except for
5-hydroxy-1-naphthol which was purified by repeated sublima-
tion. Water was from a Millipore Milli-Q system. The absorption
spectra were monitored on a UV/vis diode array spectropho-
tometer (HP, 8453). The molar absorption coefficient of
The absorption spectrum of 1-naphthol in acetonitrile or
methanol, with maxima at 298 and 330 nm and a minimum at
250 nm, is slightly hypsochromic shifted with respect to
5-hydroxy-1-naphthol. The spectral changes in oxygen-saturated
acetonitrile as a function of the irradiation time reveal a decrease
of A330 and an isosbestic point at 277 nm, again excluding
-1
-1 22-24
5
1
)
-hydroxy-1-naphthol is ε320 ) 3600 M cm .
For
-naphthol ε300 ) 5000 M- cm was taken and for NQ ε360
1
-1
5000 M- cm . For photoconversion λirr ) 313 nm was used
1
-1
either from a 1000 W Hg-Xe lamp and a monochromator or a
secondary photolysis. The photoproduct of 1-naphthol in
oxygen- or air-saturated solution is NQ.1
5-25
Examples of the
2
50 W Hg lamp and a suitable filter. The slope of the linear
part of Aobs as a function of irradiation time is a measure of the
quantum yield Φ . As reference for λirr ) 308/313 nm, Φ
.03 for 1-naphthol in oxygen-saturated aqueous solution was
UV-light-induced absorption spectra of 1-naphthol in aqueous
solution (including 3% methanol) at pH 6 and 10 are shown in
Figure 2 (inset). The increase of A252 (Figure 1) or the decrease
of A290/A330 (Figure 2) versus the irradiation time is generally
largest in oxygen-saturated solution and lowest under argon.
The quantum yield of conversion increases with the oxygen
concentration and reaches a maximum value. This is similar in
Q
Q
)
0
3
5
taken. For actinometry with λirr ) 500-560 nm, a system
containing rose bengal and 0.2 mM 5-hydroxy-1-naphthol in
air-saturated methanol was taken: Φ ) 0.003. In the case of
S
18
air saturation, a homogeneous solution was achieved by stirring.

2
82 J. Phys. Chem. A, Vol. 115, No. 3, 2011
Oelgem o¨ ller et al.
SCHEME 2
aqueous solution at pH 6-11. The maximum values are Φ
Q
)
0
.02-0.03 for the neutral form in organic solvents and in
aqueous solution. The values are much larger, Φ ) 0.09-0.12
Q
for the naphtholate (Table 1).
Figure 3. Plot of the oxygen concentration (initially 0.28 mM) as a
function of the time of irradiation in air-saturated methanol-water (1:
A longer-lived transient with maxima at 350 and 430 nm was
observed upon excitation at 308 nm of 1-naphthol in argon-
saturated methanol-water, 1:30, pH 6-8. It is assigned to the
3
0) of 1 mM 5-hydroxy-1-naphthol at pH 6.6, λirr ) 313 nm (1) and of
rose bengal (2) and eosin (3) plus 1 mM 1-naphthol at pH 10, λirr
)
•
naphthoxyl radical ONaph, in agreement with the literature,
5
00-560 nm.
1
-1 35
ε
430 ) 9000 M- cm . The corresponding radical is also
formed upon photolysis of 5-hydroxy-1-naphthol. The radicals
decay with a half-life of 0.1-0.2 ms and are not markedly
affected by oxygen. The spectra also contain the solvated
SCHEME 3
-
electron (e
s
) with a maximum at 700 nm as short-lived species.
-
The half-life of e
s
is 0.2-1 µs and the monophotonic quantum
yield of ionization (Φe-) of 5-hydroxy-1-naphthol in argon-
saturated aqueous solution is 0.01. For 1-naphthol, Φe- ) 0.016
35
-
s
has been reported. Note that e is not observable in acetonitrile
Figure 1. Absorption at 252 nm of 5-hydroxy-1-naphthol in oxygen-,
air-, and argon-saturated methanol-water (1:30, vol) at pH 6.6 (circles,
triangles, and squares, respectively) as a function of the time of
irradiation at 313 nm; inset: spectra under oxygen at 0, 20, 50, and
because it is scavenged by the solvent within 10 ns. The triplet
state of 5-hydroxy-1-naphthol at pH 6-8 is not detectable, in
agreement with the same finding for 1-naphthol in aqueous
35
solution, where Φisc ) 0.02.
2
00 s, 1-4, respectively.
The photooxidation of naphthols to NQs can also be analyzed
by observation of the oxygen concentration. An example of the
oxygen uptake upon irradiation at 313 nm of 1-naphthol in air-
saturated aqueous solution is shown in Figure 3. The quantum
yields Φ′
Q
are compiled in Table 1. The values using oxygen
values obtained from the spectra.
uptake are similar to the Φ
Q
The photoprocesses of naphthols in solution are fluorescence,
isc, and ionization. The quantum yield of fluorescence of
3
3
2
-naphthol has been determined as 0.1-0.3. The nonsensitized
photoreaction of 1-naphthol or 5-hydroxy-1-naphthol in aqueous
solution is mainly monophotonic photoionization, reaction (6)
-
in Scheme 3. Reaction (7) of e
s
with oxygen is efficient, the
Figure 2. Absorption of 1-naphthol in air-saturated methanol-water
1:30) at 290 nm, pH 6.6 (circles), and 330 nm, pH 10 (triangles),
9
-1 -1 37
rate constant in aqueous solution is k
7
) 19 × 10 M s .
•-
2
(
The naphthoxyl radical reacts either with O
via (8) and (9)
upon time of irradiation at 313 nm; inset: absorption spectra at pH 6
and 10 and 0 and 200 s, 1-4, respectively.
leading to NQs or by radical termination.35 Equation 10 of the
naphthoxyl radical with oxygen does not contribute, as compared
TABLE 1: Quantum Yield Φ
Q
of Conversion of
6
-1 -1 35
with the similar reaction of phenol with k10 < 1 × 10 M s .
5
-R-1-Naphthols to NQsa
•-
In competition, termination (eq 11) of O
considered.
2
into H
2
O
2
may be
R ) Η
oxygen
R ) OH
oxygen
solvent
air
air
•
•
dichloromethane
acetonitrile
0.04
0.03
0.03
0.03
0.03
0.02
0.018
0.02
0.02
0.09
0.12
O + ONaph f O ONaph f products
(2)
(3)
2
2
methanol
0.034
0.03
0.02
0.02
c0.03
b
MeOH-H
2
2
O pH 6-7
O2•- + 2H f H O + O
+
2
d
2 2 2
Φ′
Q
b
c
MeOH-H
O pH 10
0.12
d
Φ′
Q
Photoionization also takes place for phenol in aqueous
solution where Φe- is likewise low. Data concerning the triplet
state and radicals of phenols in nonaqueous and aqueous solution
a
Using λirr ) 313 nm. 1:30, vol. c Thermal oxidation at pH
b
d
9
-11. Oxygen uptake: Φ′
Q
.

Photooxidation and Oxidation of Naphthols
J. Phys. Chem. A, Vol. 115, No. 3, 2011 283
S
TABLE 3: Quantum Yield Φ of Sensitized Transformation
a
of 5-R-1-Naphthols to NQs
sensitizer
eosin
pH
R ) Η
0.022 [0.030]b
0.18
R ) OH
6-7
c0.024 [0.036]
1
0
erythrosin
6-7
0.027 [0.034]
0.28
c0.024 [0.035]
1
0
a
2
In air-saturated MeOH-H O, 1:30, vol, using concentrations of
8
5
-12 µM for the sensitizer and 1 mM for the naphthols λirr )
00-560 nm. In brackets: oxygen-saturated. Thermal reaction.
b
c
Figure 4. Plot of A260 for 0.2 mM 5-hydroxy-1-naphthol in air-saturated
methanol-water (1:30) at pH 6.6 as a function of the time of irradiation
of eosin (10 µM) at 500-560 nm; inset: spectra at 0, 30, and 100 s,
1
-3, respectively.
Figure 6. Plots of A290 (cirles) and A350 (triangles) for 0.2 mM
-naphthol in neutral methanol-D O (1:30) (oxygen- and air-saturated:
1
2
open and full, respectively) as a function of the time of irradiation of
eosin (10 µM) at 500-560 nm; inset: spectra under oxygen at 0, 10,
and 200 s, 1-3, respectively.
Figure 5. Plots of A335 for 0.2 mM 1-naphthol in methanol-water
1:30) at pH 10 (oxygen- and air-saturated: circles and triangles,
respectively) as a function of the time of irradiation of erythrosin at
(
9
-1 -1
TABLE 4: Rate Constant k
4
(10 M s ) for Quenching of
a
the Xanthene Triplet State by Phenol/Naphthols
5
00-560 nm; inset: spectra at 0 and 90 s (under oxygen- and air), 1,
2
′, and 2, respectively.
sensitizer
1-naphthol
5-OH-1-naphthol
phenol
rose bengal
eosin
erythrosin
2.0 (2.2)b
2.1 (2.5)
2.3
1.8 (1.5)
1.9
2.1
<0.01
<0.01
<0.01
S
TABLE 2: Quantum Yield Φ of Rose Bengal Sensitized
a
Transformation of 5-R-1-Naphthols to NQs
[
naphthol]
(mM)
a
b
In argon-saturated methanol and using λexc ) 500-550 nm. In
solvent
methanol
R ) Η
0.004
R ) OH
0.003
parentheses: MeOH-H O 1:30 at pH 6-7.
2
0.2
5
1
1
1
0.036
0.032
b
0.022 (0.045)c 0.023 (0.038)
MeOH-H
Q
2
2
O , pH 6-7
and 5-hydroxy-1-naphthol in oxygen-saturated methanol. In
aqueous solution, Φ increases with the pH, approaching 0.3 at
d
Φ′
MeOH-H
0.03
0.25
0.30
0e .03
S
O, pH 10
d
pH 10. The values are similar for eosin and erythrosin (Table
Φ′
Q
1
S
3
). Φ is rather low in organic and neutral aqueous solution.
a
In air-saturated solution, 8-12 µM rose bengal, λirr ) 500-560
The spectral changes are similar under oxygen and air but
different under argon. The increase of A250 versus the time of
nm. b 1:30, vol. In D
c
O. Oxygen uptake: Φ′
d
.
e
Thermal reaction.
2
Q
irradiation is largest in oxygen-saturated solution and smallest
under argon. So far the Φ values refer to air, but they are
are available.28 Photoionization of phenols/naphthols in the
absence of oxygen does not yield benzoquinones or NQs; instead
radical combination into diphenol takes place.3
S
markedly larger in oxygen-saturated solution (Figures 5 and 6
1
S
and Table 3). Moreover, Φ is significantly larger, when H
2
O
Xanthene-Sensitized Oxidation of Naphthols. The absorp-
tion spectra of the two naphthols in methanol with several peaks
at 230-340 nm are not significantly affected on addition of
one of the three xanthene dyes, the latter of which show a
maximum at 530-550 nm. The spectra in the UV range reveal
conversion to NQ upon irradiation at 500-560 nm under air-
saturated conditions. This is similar for the naphthols in mixtures
of methanol and water. Examples with 5-hydroxy-1-naphthol
and 1-naphthol are shown in Figures 4 and 5 (insets). Note that
the spectra at pH 10-12 are strongly red-shifted. The slope of
the linear part of Aobs as a function of the time of irradiation at
2
is replaced by D O (Table 2), an example is shown in Figure 6.
Examples of the oxygen uptake upon irradiation of rose bengal
S
at 500-560 nm are shown in Figure 3. The values of Φ′ ,
obtained from the relative quantum yield of the sensitized
transformation of 1-naphthols to parent NQs, are similar to those
S
S
S
of Φ (Table 2). The effect of pH on Φ′ and Φ reflects the
much higher reactivity of the naphtholate and thus the higher
product yield.
A comparison with the literature shows some dependence of
S
Φ on the position of the second hydroxy group. For 1,7- 1,6-,
and 1,5-dihydroxynaphthalene in air-saturated methanol and rose
S
5
00-560 nm (Figures 4 and 5) is a measure of the quantum
bengal Φ ) 0.002-0.004, but larger, 0.07, for 8-hydroxy-1-
S
18
yield. The Φ values of the sensitized transformation of
-naphthol to parent NQs are listed in Tables 2 and 3. As
naphthol. Alternatively, one can consider a value that was
S
1
calculated from reactions: Φ ) 0.05 and 0.5 for the system
S
reference, Φ ) 0.003 was taken for the system containing rose
bengal and 0.2 mM 5-hydroxy-1-naphthol in air-saturated
methanol.18 Generally, Φ increases with the naphthol concen-
tration (Table 2), approaching, for example, 0.04 for rose bengal
containing rose bengal and 1 mM 1-naphthol in air-saturated
aqueous solution at pH 7 and 11, respectively.20
S
Triplet Quenching by Naphthols. The rate constants for
electron transfer from phenol to the eosin excited singlet and

2
84 J. Phys. Chem. A, Vol. 115, No. 3, 2011
Oelgem o¨ ller et al.
SCHEME 4
SCHEME 5
9
) 1 × 10 M s-1, respectively.2
6
-1
triplet states are 10 and k
4
neutral oxygen-saturated solution, however, is in agreement with
Note that the rate constant for electron transfer from naphthols
to the triplet state of erythrosin and rose bengal is not known
from literature. Therefore, time-resolved measurements with
xanthenes were carried out to test for triplet quenching by
naphthols. The lifetime of a xanthene triplet state in argon-,
air-, and oxygen-saturated methanol is 5-10, 2-3, and 0.3-0.4
µs, respectively.43 The transient absorption spectra of rose
bengal, eosin, or erythrosin in solution show a strong bleaching
both pathways because k
1
× [O
2
] is five times larger than under
S
air. The larger Φ values under oxygen versus air (Figures 5
and 6) are therefore not compatible with the radical mechanism.
That a larger oxygen concentration can increase at all indicates
a competing contribution of the radical mechanism, that is,
1
•-
O
2
( ∆
g
) increases at the expense of S .
S
Likewise, the strikingly larger Φ values in heavy versus
normal water (Figure 6) exclude the radical mechanism. It is
3
,4,44
1
with a maximum at 520-580 nm.
presence of a naphthol is the xanthene triplet state. The rate
constant for triplet quenching by 1-naphthol is k ) (2.0-2.5)
is only slightly
The first transient in the
noteworthy that the lifetime of O
2
( ∆
g
) in heavy versus normal
26
water is 10-15 times longer and therefore the competing step
2 is less efficient. On the other hand, quenching of the S state
of phenol in air-saturated methanol or in a mixture with water
3
*
4
9
-1 -1
×
10 M
s
and for 5-hydroxy-1-naphthol k
4
smaller (Table 4). The secondary minor transient under argon
is a xanthene radical, formed via steps (4) and (5).
4
cannot efficiently compete with reaction 2 because k is two or
3 orders of magnitude lower (Table 4).
For parent phenol we found a small k
4
and a much lower
The proposed singlet molecular oxygen mechanism of a
naphthol in neutral and alkaline solution is shown in Schemes
1 and 5, respectively. The rate constant of reaction k of singlet
3
molecular oxygen with phenols or naphthols as quenchers
depends strongly on the applied conditions, that is, substrate
triplet yield for the cases of eosin and erythrosin due to strong
fluorescence quenching. Concerning the possibility of quenching
the triplet state of xanthene dyes, the effect of pH on the
reactions with aromatic amino acids may also be instructive.
The values for quenching of the triplet state of eosin by tyrosine
concentration and pH. Selected rate constants k
3
and quantum
) 4 ×
for 1-naphthol in acetonitrile, but 50 times larger
7
-1 -1
and N-acetyltyrosine are k
and 5-8 times larger at pH 11.
Mechanism. The commonly assumed mechanism of the
4
) (4-7) × 10 M
s
at pH 8
yields Φ are available in the literature, for example, k
∆
3
3
6
-1 -1
10 M
for the naphtholate.
(Tables 2 and 3) may be a consequence of the much larger k
for the naphtholate. For phenol, an increasing reactivity of
s
19,20
S
The much larger Φ at pH 10-11
1
1-3
xanthene-sensitized oxidation of phenols is O
2
( ∆
g
) mediated.
3
Here, the reaction sequence of involvement of radicals is
considered for 1-naphthols in aqueous solution. This putative
photooxidation mechanism is illustrated in Scheme 4. The
naphthoxyl radical, formed via reactions (4) and (5), does not
react with oxygen, and therefore, step (10) is excluded. The
1
O
2
( ∆
g
) with the phenolate is well-known, for example, k
3
) 2
6
8
-1 -1
5
× 10 at pH 7-8 and 2 × 10 M
s
at pH 11. A strong
increase of k
3
(10-400-fold) with pH is known for tyrosine
3
3
and a derivative. Similar pH dependences of k have been
reported for 1-naphthols.
•
-
19-21
S
radical, however, reacts with oxygen, step (12), regenerating
•
-
the sensitizer and generating O
2
, which is then converted to
In principle, electron transfer (4) to the sensitizer triplet state
and energy transfer (1) to oxygen compete. The former reaction
2 2
H O
via step (11). Whether electron transfer is possible or not
in oxygen- or air-saturated solution has not been reported yet.
For rose bengal it has been shown that the transfer from the
donor to the acceptor triplet state needs some reorganization
energy to accommodate the additional electron in the dianion.42
The same can be assumed for eosin and erythrosin.
4
can take place, when k is large and the naphthols are present
in large concentration. In the case of pure type II mechanism,
S
an increase of Φ by about 2.5 times should occur in going
from air- to oxygen-saturated solution and by about 10 times
in going from normal to heavy water. However, the increase of
S
We suggest that a naphthol molecule in a high concentration
Φ is only 1.2-1.5 and 1.7-2, respectively. In the case of pure
3
*
S
can quench the xanthene S state in air-saturated methanol or
in mixtures with water. This is possible because the term k
HONaph] × τ is rather high. A high substrate concentration
should favor NQ formation, for example, when [HONaph] ) 1
type I mechanism, a reduction of Φ by about 40% should occur
4
×
in going from air to oxygen, while it should remain the same
in going from H O to D O. Thus, only the assumption that both
2 2
[
T
type I and type II mechanisms contribute adequately to the
observed photosensitized production of NQs can account for
the experimental observations in total.
The triplet state of a sensitizer is quenched by 1-naphthol,
sequence (4), (5), (8), and (12). Such a mechanism could explain
why NQs are formed in acetonitrile where reaction (7) cannot
take place, because the solvated electron is successfully
6
-1
mM, k
4
× [HONaph] ) 2 × 10 s in air-saturated aqueous
solution at pH 7. This estimation is markedly larger than the
5
-1
rate k
1
× [O
2
] ) 5 × 10 s , taking k
1
) 1.3, 1.6, and 2.1 ×
9
-1 -1
43
1
0 M
s
for rose bengal, eosin, and erythrosin. This could
be in favor of the radical mechanism for oxidation of the two
naphthols under examination. The corresponding estimation in

Photooxidation and Oxidation of Naphthols
J. Phys. Chem. A, Vol. 115, No. 3, 2011 285
scavenged by the solvent. The observed higher yield of Juglone
upon UV irradiation of 5-hydroxy-1-naphthol in acetone24 is
ascribed to the electron transfer mechanism, analogous to that
of Scheme 4. Moreover, for this sequence, only a trace amount
of oxygen is required.
(14) D ´ı az, M.; Luiz, M.; Alegretti, P.; Furlong, J.; Amat-Guerri, F.;
Massad, W.; Criado, S.; Garc ´ı a, N. A. J. Photochem. Photobiol., A 2009,
02, 221.
2
(
15) (a) Tournaire, C.; Croux, S.; Maurette, M.-T.; Baun, A. M.;
Oliveros, E. New J. Chem. 1991, 15, 909. (b) Luiz, M.; Guti e´ rrez, M. I.;
Bocco, G.; Garc ´ı a, N. A. Can. J. Chem. 1993, 71, 1247. (c) Amat-Guerri,
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Conclusions
Based on the photolysis of 1-naphthol and 5-hydroxy-1-
naphthol in methanol and water we conclude the following: (i)
The naphthoxyl radical is the major intermediate in photooxi-
(
f) Miskoski, S.; Soltermann, A. T.; Molina, P. G.; G u¨ nther, G.; Zanocco,
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(
Commun. 1976, 676. (b) Duchstein, H.-J.; Wurm, G. Arch. Pharm. 1984,
Q
dation. (ii) A major factor influencing Φ is the pH, whereas
3
1
17, 809. (c) Baumann, J.; Wurm, G. Prostaglandins, Leukotrienes Med.
984, 14, 139.
the second hydroxy group has no significant effect. Concerning
point (iii) of the xanthene-sensitized oxidation of naphthols, the
pH plays a dominant role and concentrations of oxygen and
naphthol play an important role. In principle, the two pathways
via singlet molecular oxygen (reaction 1 and Scheme 5) or the
superoxide ion radical (Scheme 4) are kinetically possible. This
is in contrast to phenol, where electron transfer reaction (4) is
too inefficient. The proposed mechanism for naphthols is in
favor of both the singlet molecular oxygen route. From the two
oxygen atom of a consumed oxygen molecule, one is incorpo-
rated into the naphthol molecule, giving naphthoquinone, while
the second oxygen atom should give rise to water. The
involvement of the hydroperoxyl/superoxide ion radical is not
ruled out based on competition kinetics under the applied
conditions. Finally, (iv) the dominant role protonation state on
the sensitized oxidation processes of 1-naphthol in aqueous
solution is attributed to the much faster reactivity of the reactive
oxygen species in the alkaline pH range.
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(
A. M.; Oliveros, E. New J. Chem. 1990, 14, 161.
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J. Chem. 1996, 74, 49.
(
20) Soltermann, A. T.; Luiz, M.; Biasutti, M. A.; Carrascoso, M.; Amat-
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Acknowledgment. We thank Professor W. Lubitz for his
support, a referee for the helpful suggestions, and Mr. Leslie J.
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