Possible singlet oxygen generation from the photolysis of indigo dyes in methanol, DMSO, water, and ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate

Article abstract of DOI:10.1016/j.tet.2006.08.095

We suggest that singlet molecular oxygen [¹O₂ (¹Δ_g)] is formed upon irradiation of indigo 1 [in air or O₂-saturated DMSO and DMSO (0.5% H₂SO₄)] and indigo carmine 2 [in air or O₂-saturated CH₃OH, D₂O, and 1-butyl-3-methylimidazolium tetrafluoroborate (BmIm-BF₄)]. The quantum yield for production of ¹O₂ is estimated to be 0.6 for 1 and 0.3-0.5 for 2. The rates of reaction of ¹O₂ with 1 and 2 were determined by monitoring the emission of ¹O₂ at 1270 nm over time. Low molar absorptivities (at 532 nm) and rapid physical quenching caused by 1 and 2 limit their utility as ¹O₂ photosensitizers in solution. Compounds 1 and 2 degrade slowly during the photolysis due to a self-sensitized (type I or II) photooxidation reaction. Oxidative cleavage of 1 by singlet oxygen and superoxide, and 2 by superoxide has been noted before (Kuramoto, N.; Kitao, T. J. Soc. Dyers Color. 1979, 95, 257-261; Kettle, A. J.; Clark, B. M.; Winterbourn, C. C. J. Biol. Chem. 2004, 279, 18521-18525).

Full text of DOI:10.1016/j.tet.2006.08.095

Tetrahedron 62 (2006) 10771–10776
Possible singlet oxygen generation from the photolysis of
indigo dyes in methanol, DMSO, water, and ionic liquid,
1-butyl-3-methylimidazolium tetrafluoroborate
Naveen Gandra,a Aaron T. Frank,b Onica Le Gendre,b Nahed Sawwan,b David Aebisher,b
a,
Joel F. Liebman,c K. N. Houk,d Alexander Greerb, and Ruomei Gao
*
*
aDepartment of Chemistry, Jackson State University, Jackson, MS 39217, USA
bDepartment of Chemistry, Graduate Center and The City University of New York (CUNY), Brooklyn College,
Brooklyn, NY 11210, USA
cDepartment of Chemistry and Biochemistry, University of Maryland, Baltimore County, Baltimore, MD 21250, USA
dDepartment of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095, USA
Received 7 July 2006; revised 25 July 2006; accepted 8 August 2006
Available online 25 September 2006
Abstract—We suggest that singlet molecular oxygen [1O2 (1Dg)] is formed upon irradiation of indigo 1 [in air or O2-saturated DMSO and
DMSO (0.5% H2SO4)] and indigo carmine 2 [in air or O2-saturated CH3OH, D2O, and 1-butyl-3-methylimidazolium tetrafluoroborate
(BmIm–BF4)]. The quantum yield for production of 1O2 is estimated to be 0.6 for 1 and 0.3–0.5 for 2. The rates of reaction of 1O2 with 1
and 2 were determined by monitoring the emission of 1O2 at 1270 nm over time. Low molar absorptivities (at 532 nm) and rapid physical
quenching caused by 1 and 2 limit their utility as 1O2 photosensitizers in solution. Compounds 1 and 2 degrade slowly during the photolysis
due to a self-sensitized (type I or II) photooxidation reaction. Oxidative cleavage of 1 by singlet oxygen and superoxide, and 2 by superoxide
has been noted before (Kuramoto, N.; Kitao, T. J. Soc. Dyers Color. 1979, 95, 257–261; Kettle, A. J.; Clark, B. M.; Winterbourn, C. C. J. Biol.
Chem. 2004, 279, 18521–18525).
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
1O2 by photosensitization? The yield of 1O2 formed by irra-
diation of triplet O2 plus sensitizer varies depending on the
structure of the photosensitizer, the nature of the photosensi-
tizer’s excited state, and on the solvents used. Studies of
indigo dye spectroscopy,8–12 excited state chemistry,13–24
and photofading25,26 date back to the 1950s. The mechanism
of quenching of electronically excited states of thioindigo
dyes (S replacing NH) by triplet molecular oxygen has
been studied.27–35 Wyman and co-workers observed that
dissolved oxygen quenched the photoisomerization of 3
without diminishing the intensity of the fluorescence.36
This suggested that the trans–cis isomerization of 3 may
involve the excited triplet state. The triplet energy of 3 was
determined to be 27–32 kcal/mol,16,17 which is greater
than the 22.5 kcal/mol required to excite O2 from its ground
state to its first excited singlet state. The triplet state energies
of 1 and 2 have not yet been reported.
Indigo (1) possesses a blue color, and has been of scientific
interest given its historic provenance and esthetic hue. In-
digo is a naturally derived dye that comes from a variety
of plant species.1,2 Biosynthetic and laboratory preparations
of 1 have been reported.3–6 Indigo 1 and indigo carmine 2
represent stable dyes and colorants, and are composed of
a central carbon double bond with two imine nitrogens and
two carbonyl groups affixed (Chart 1).7
Both 1 and 2 are quite photostable8 and this led us to raise
the following question: How efficiently might they generate
O
O
H
N
H
N
NaO3S
SO3Na
N
N
H
H
O
O
O
S
1
2
Chart 1. Structures of indigo and indigo carmine in their keto forms.
S
O
* Corresponding authors. E-mail addresses: agreer@brooklyn.cuny.edu;
ruomei.gao@ccaix.jsums.edu
thioindigo, 3
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.08.095

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N. Gandra et al. / Tetrahedron 62 (2006) 10771–10776
The photostability of indigo 1 has been attributed by
Kobayashi and Rentzepis to the high internal conversion
from excited singlet state S1 to ground singlet state S0 rather
than trans to cis isomerization.14 The internal conversion
process may be enhanced by excited state intramolecular
proton transfer. However, Elsaesser et al. have concluded
that the internal conversion is related to the vibrations of
the N–H bonds, which are part of the intramolecular hydro-
gen bonds.13
Information regarding the quantum efficiency of 1O2 gener-
ation in ionic liquids is unavailable.
We report that absorption of visible light by 1 and 2 in
the presence of O2 leads to the production of O2. Singlet
1
oxygen generation was measured for 1 in DMSO, and 2
in CH3OH, D2O, and BmIm–BF4 using a time-resolved
method. Oxidizing intermediates produced during the reac-
tion led to the decomposition of 1 and 2.
Despite the importance of indigo dyes, little information is
available about whether they may serve as 1O2 sensitizers.36
Recent work by Seixas de Melo and co-workers showed that
the fluorescence efficiency FF is favored by the leuco, but
not keto forms, e.g., FF of indigo is 0.0023 for the keto
form, but 0.348 for the leuco form in DMF.37 The structures
of the keto and leuco forms of indigo are shown below. The
low fluorescence of indigo dyes in their keto form was ex-
plained by the small energy gap between S0 and S1. Accord-
ing to the golden rule for radiationless transitions, a small
value in the S1–S0 energy gap favors a nonradiative decay
pathway by increasing the vibrational overlap (Franck–
Condon integrals) of the wave functions for the nonradiative
modes of these two states.22,23
2. Experimental
2.1. General aspects
Indigo 1, indigo carmine 2, isatin 4, rose bengal, methylene
blue, and di-n-butyl sulfide were purchased commercially
and used without further purification. The solvents used
(DMSO, D2O, CH3OH, and BmIm–BF4) were of spectro-
scopic or equivalent grade and were used as received. Light
at 532 nm was obtained from a time-resolved Nd:YAG laser
set-up with pulse width 3–4 ns and maxima energy 50 mJ at
532 nm (Polaris II-20, New Wave Research Merchantek
Products). A liquid N2 cooled germanium photodetector
(Applied Detector Corporation) was used for the determina-
tion of quantum yields and luminescence quenching rates of
1O2. A Rayonet 250 W tungsten lamp equipped with a 1 cm
1.5 M NaNO3 solution cutoff filter was used for steady-state
H
N
O
H
H
O
N
N
1
N
generation of O2. UV–visible spectra were recorded on
O
H
O
a Varian 300 Bio (Cary) spectrophotometer. Mass spectro-
metry data was collected on an Agilent 1100 LC/MSD
instrument using either electrospray ionization or
atmospheric pressure photoionization methods. All experi-
ments were carried out at room temperature and with air
or O2-saturated solutions. Shaking, stirring, and/or sonica-
tion were conducted to dissolve 2 and methylene blue in
BmIm–BF4.
keto form of indigo
leuco form of indigo
1
The formation of O2 is predominantly a triplet quenching
process. The 1O2 quantum yields (FD) were determined us-
ing Eq. 1.38 Eq. 1 represents a theoretical understanding of
quantum yields of 1O2 generation.
T;3O2
FD ¼ FTP
fT;D
ð1Þ
2.2. 1O2 Quantum yield measurements
Here FT is the quantum yield of triplet formation for the
photosensitizer, PT;3O the proportion of triplet quenched
by ground state oxygen and fT,D the fraction of triplet state
quenched by ground state oxygen that leads to the formation
of 1O2.
Absorbances of 1 and 2 ranged from 0.03 to 0.6 at the exci-
tation wavelength 532 nm. The absorbance of 1 and 2, and
rose bengal (known to possess a quantum yield of 1O2 gen-
eration 0.76 in DMSO, methanol, and heavy water)46,47 all
matched to within 80%. The quantum yield values may
2
1
change according to the reference used. The initial O2 in-
There is limited solubility of 1 in DMSO (w0.7 mM) and in
CHCl3, CH2Cl2, CH3CN, and CH3OH (<0.1 mM). How-
ever, because of its pair of sulfonate groups, indigo carmine
2 dissolves in water (w0.02 M) and MeOH (w0.8 M) to a
higher extent. A number of reports discussing photochemis-
try in ionic liquids have appeared in the past several years.39–43
The ability of ionic liquids to retain other ionic compounds
suggested the idea of using a photosensitizer bearing a
charge, thereby making it preferentially soluble in the ionic
liquid. Imidazole-tagged aryl ketones have been developed
that can efficiently sensitize photochemical reactions in
ionic liquids and are isolated simply by extraction of the
ionic liquid solution with an appropriate organic solvent after
tensity is extrapolated to t¼0. The data points of the initial
3–4 ns are not used due to electronic interference signals
from the detector. The intensity of the pulses at 532 nm
were controlled between 40 and 50 mJ. The quantum yield
for production of 1O2 is calculated according to Eq. 2. Unlike
Eq. 1, Eq. 2 is practical to use along with the time-resolved
method.
FDsample
Ssample
¼
ð2Þ
FDreference Sreference
Here FDsample and FDreference are the quantum yields for
samples and the reference. Rose bengal was used as a refer-
ence sensitizer. Ssample and Sreference represent the slopes
1
use.44 The [2+4] reaction between O2 and 1,4-dimethyl-
1
naphthalene has been investigated in several 1-methyl-3-
butylimidazolium cation (BmIm+) ionic liquids along with
obtained from the plot of initial intensity of O2 via the
absorbance at excitation wavelength 532 nm for the sample
and the reference, respectively.
ꢀ 45
the following counter anions: PF6ꢀ, SbF6ꢀ, and BF4
.

N. Gandra et al. / Tetrahedron 62 (2006) 10771–10776
10773
2.3. Quenching rate constants of 1O2
1
1
The phosphorescence of O2 at 1270 nm was measured as
previously described.48 The 1O2 quenching rates were mea-
sured by monitoring the deactivation of O2 by 1 and 2.
Stern–Volmer analyses were conducted with rose bengal as
the sensitizer in DMSO, methanol, and D2O.
0.8
0.6
0.4
0.2
0
1
3. Results and discussion
3.1. Spectral properties of 1 and 2
indigo
rose bengal
Normalized UV–visible spectra for 1 and 2 are shown in
Figure 1. The lmax and absorptivity at maxima of 1 and 2
and their 532 nm excitation wavelengths are collected in
Table 1. The absorption of 1 and 2 consists of a band in the
visible region between 500 and 700 nm with a small inflec-
tion or shoulder at shorter wavelengths (Fig. 1). The high
absorptivity indicates that 1 and 2 can be excited upon the
absorption of light in this range.
0
0.2
0.4
OD at 532 nm
0.6
0.8
Figure 2. 1O2 Emission intensity as a function of absorbance in DMSO with
the excitation wavelength of 532 nm.
through zero in Figure 2, and we attribute the deviation to
systematic errors. Thus, the quantum yields were determined
from the slopes of the plots. Triplet–triplet annihilation ap-
pears to be negligible at absorbances ranging from 0.03 to
1.6
1.2
0.8
1
0.60 from excitation at 532 nm, as indicated by the O2 in-
tensity showing a linear correlation with the absorption of
the complexes. We estimate that the FD for 1 is 0.6, and
for 2 is 0.3–0.6, where these FD’s were obtained in the com-
mon solvents noted above. No reference photosensitizer is
available for ionic liquids. Thus, the relative FD for 2 in
BmIm–BF4 was calculated assuming FD¼1.0 for methylene
blue. Our results show that FD for 2 is about half that of
methylene blue (Table 2).
3
0.4
0
4
1
1
The total O2 quenching rate constants (kT) of 2 with O2
were determined in D2O and CH3OH by Stern–Volmer anal-
yses, since a large quenching rate would severely limit their
potential as photosensitizers. The quenching rate constants
range from 5ꢁ107 to 3ꢁ108 Mꢀ1 sꢀ1 for 2 in CH3OH and
D2O. This is similar to other photosensitizers, e.g., rose
bengal (2ꢁ107 Mꢀ1 sꢀ1 in CH3OH49) and methylene blue
(2.3ꢁ108 Mꢀ1 sꢀ1 in CH3OH50 and 4ꢁ108 Mꢀ1 sꢀ1 in
2
1
400
500
600
Wavelength, nm
700
800
Figure 1. Normalized absorption spectra of 1 and 2. (1) DMSO solution of
1. (2) D2O solution of 2. (3) BmIm–BF4 solution of 2. (4) CH3OH solution
of 2.
1
D2O51). The quenching rates of O2 by 1 in DMSO and 2
in BmIm–BF4 cannot be observed because of their limited
solubility. The self-decay rate constants (kd, sꢀ1) measured
in the present work are comparable to published values
of 7.1ꢁ104 sꢀ1 in DMSO,52 8.9ꢁ104 sꢀ1 in CH3OH,53
1.8ꢁ104 sꢀ1 in D2O,54 and 6.3ꢁ104 sꢀ1 in BmIm–BF4.45
Table 1. Absorption maxima and extinction coefficients of solutions con-
taining 1, 2, and methylene blue
Sensitizer
Solvent
lmax, nm 3max
M
,
3532 nm
M
,
ꢀ1 cmꢀ1
ꢀ1 cmꢀ1
1
2
2
2
DMSO
CH3OH
D2O
620
598
607
2.26ꢁ103
4.76ꢁ103
7.92ꢁ103
—
3.26ꢁ102
1.59ꢁ103
2.02ꢁ103
—
Indigo 1 and indigo carmine 2 exhibit absorption bands in
the range of 500–700 nm, and can be activated to higher
energy levels by absorbing excitation light in this region.
Photosensitization processes have been reviewed in detail
by Wilkinson and Brummer.27 A brief summary is given
below (Eqs. 3–10). AType II mechanism involves the gener-
ation of 1O2 via energy transfer between T1 of a photosensi-
BmIm–BF4 609
Methylene blue BmIm–BF4 661
—
—
3.2. Possible 1O2 quantum yields and quenching rates
3
tizer and ground state O2 (Eq. 7). A Type I mechanism
The quantum yields for 1O2 generation were determined by
time-resolved laser measurements of its near-infrared lumi-
nescence (1270 nm) upon excitation at 532 nm with rose
bengal FD¼0.76 as the reference in DMSO,46 CH3OH,
and D2O (Fig. 2).47 Ideally, the straight lines should go
involves hydrogen-atom abstraction or electron-transfer
between the excited photosensitizer and a substrate, and
the subsequent generation of oxygen radicals. One example
of the Type I reaction is an electron-transfer from an excited
sensitizer to 3O2, to form superoxide (Eq. 8).

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N. Gandra et al. / Tetrahedron 62 (2006) 10771–10776
Table 2. Summary of 1O2 quantum yields (FD), total quenching rate constants (kT, Mꢀ1 sꢀ1) and self-decay rate constants in various solvents (kd, sꢀ1 a
)
Photosensitizer
Solvent
FDꢃS.D.
Reference
kT, Mꢀ1 sꢀ1
kd, sꢀ1
1
1
2
2
2
DMSO
0.61ꢃ0.09
0.62ꢃ0.05
0.53ꢃ0.07
0.31ꢃ0.03
0.53ꢃ0.11
Rose bengal 0.76
Rose bengal 0.76
Rose bengal 0.76
Rose bengal 0.76
Methylene bluec
—
—
(9.1ꢃ2.0)ꢁ104
(6.0ꢃ1.0)ꢁ104
(8.1ꢃ0.5)ꢁ104
(1.9ꢃ0.3)ꢁ104
(6.1ꢃ0.9)ꢁ104
DMSO/0.5% H2SO4
CH3OH
D2O
(5.6ꢃ0.1)ꢁ107
(3.2ꢃ0.7)ꢁ108
—
b
BmIm–BF4
a
All experiments were measured at room temperature in air-saturated solutions with an excitation wavelength of 532 nm.
There is no reference 1O2 photosensitizer available for BmIm–BF4.
b
c
The 1O2 quantum yield for indigo carmine 2 in BmIm–BF4 was calculated assuming FD¼1.0 for methylene blue.
values measured for 1 and 2 are mainly the result of physical
quenching since photooxidation products are only observed
after extended irradiation periods (several hours) under vis-
hn
/
SenðS0Þ
SenðS1Þ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
ð8Þ
ð9Þ
ð10Þ
1
ible light and oxygen. Physical quenching of O2 may be
Fluorescence
ƒƒƒƒƒ!
SenðS Þ
SenðS0Þ
1
related to trans–cis isomerization or the presence of the
imide nitrogens. The kT value for 2 is 5.6ꢁ107 Mꢀ1 sꢀ1 in
CH3OH, and 3.2ꢁ108 Mꢀ1 sꢀ1 in D2O.
IC
SenðS Þ SenðS Þ
!
1
0
Due to the lack of reference photosensitizers in ionic
liquids, the relative FD values were determined by assuming
FD(methylene blue)¼1.0 in BmIm–BF4, yielding FD(methylene
blue)/FD(indigo carmine 2)¼2:1. In contrast, the FD ratio of meth-
ylene blue to indigo carmine 2 is 1:1 in CH3OH. Further-
more, the FD is 0.5 for methylene blue in CH3OH has
been reported previously.47 The change in relative 1O2 quan-
tum yields may result from a competition reaction for the
triplet state. The enhancement of the lifetimes of triplet state
and radical ions is observed for xanthone in 1-butyl-3-methyl-
imidazolium hexafluorophosphate (BmIm–PF4).43 This
statement is speculative since influence of ionic liquids on
the deactivation of triplet states is not established. Thus, it
ISC
!
SenðS Þ
SenðT Þ
1
1
3
Type II; energy transfer
ƒƒƒƒƒƒƒƒƒƒƒ!
1
SenðT Þ þ O2
SenðS Þ þ O
2
1
0
SenðT Þ þ O2
SenꢂþðS Þ þ Oꢂꢀ
3
Type I; electron transfer
ƒƒƒƒƒƒƒƒƒƒƒƒ!
1
0
2
1
physical deactivation
ƒƒƒƒƒƒƒƒƒƒ!
3
SenðS Þ þ O2
SenðS Þ þ O
2
0
0
phosphorescence
ƒƒƒƒƒƒƒ!
SenðT Þ
SenðS0Þ
1
1
is difficult to assess whether the quantum yields for O2
generation from rose bengal and indigo carmine 2 are signif-
icantly increased or decreased in different solvents.
The apparent production of 1O2 by the photolysis of 1 and 2
suggests that a triplet energy transfer process takes place.
The proposal of energy transfer from excited 1 or 2 to O2
3
is consistent with measured ET 27–32 kcal/mol for thio-
indigo compounds since the triplet ground and first excited
singlet state of O2 is 22.5 kcal/mol.16,17 Indigo dyes in their
excited triplet states may react with 3O2 and generate 1O2 via
Eq. 7. Alternatively, the redox potential of O2/Oꢂ2ꢀ and
O2/HOꢂ2 are ꢀ0.60 V and 0.12 V versus NHE in DMF,55
respectively, also suggests that electron-transfer is thermo-
dynamically favored, in particular by increasing the acidity
3.3. Possible 1O2 sensitization
Singlet oxygen may be formed under the conditions we have
used in our experiments. Figure 3 contains the data of a rep-
resentative 1270 nm signal decay produced by irradiation of
2 in CH3OH. We attribute the 1270 nm signal to 1O2, which
is quenched by addition of NaN3 (0.20 mM) to the solution.
The self-decay rate constants kd measured in various sol-
vents are consistent with the literature values,45,52–54 where
the kd ranges from 104 to 105 sꢀ1, which correspond to 1O2
lifetimes between 10 and 100 ms (vide supra, Section 3.2).
The 1270 nm signal also decreases with a decreasing con-
centration of oxygen in the solution. The generation of 1O2
can be very sensitive to reaction conditions. The amount of
1O2 produced by 1 and 2 is likely influenced by the concen-
trations of the sensitizer and oxygen, temperature, and
irradiation time. Although the quenching rate constant of
1O2 by 2 measured in this paper (5.6ꢁ107 Mꢀ1 sꢀ1 in meth-
anol) is similar to that of rose bengal (2.0ꢁ107 Mꢀ1 sꢀ1 in
methanol), higher concentrations of 2 will be required to
obtained the same absorbance at the excitation wavelength
because of a lower absorptivity [compare 3 (2 at 532 nm)¼
ꢂ
of the solution. The excited state redox potentials E (Sen+ /
)
*
for 1 and 2 are unknown. Our results do not appear to support
the Type I mechanism (Eq. 8) due to the fact that constant
and unvarying high quantum yields (FD) for 1O2 generation
are observed in both of DMSO and DMSO/0.5% H2SO4 for
1
1. The quantum yields for O2 generation suggest intersys-
tem crossing and the formation of triplet states of 1 and 2
under these conditions. However, the quantum efficiency
of singlet oxygen can potentially vary with the excitation
wavelength, light intensity, solvent, solute, etc. Our results
support the notion of low fluorescence efficiency and the for-
mation of excited triplet state of 1 and 2 in their keto
form.16,17,37,56
1
The FD for O2 generation from indigo carmine 2 in D2O
1600 Mꢀ1 cmꢀ1 with
3
(rose bengal at 532 nm)¼
(0.31) is lower compared to that in CH3OH (0.53). This
may be explained by a larger acidic dissociation of indigo
carmine in water than methanol. We conclude that the kT
30,000 Mꢀ1 cmꢀ1]. Therefore, low concentration, high
excitation intensity, and long irradiation times should be
employed when using 1 or 2 as 1O2 photosensitizers.

N. Gandra et al. / Tetrahedron 62 (2006) 10771–10776
10775
irradiation. These preliminary results may point to the inter-
mediacy of O2, which then oxidizes dibutyl sulfide. How-
1
ever, we have not explored this reaction in sufficient detail
to confirm whether O2 is trapped and responsible for the
oxidation of dibutyl sulfide.
1
4. Conclusion
Quantum yields for the possible generation of 1O2 by indigo
1 and indigo carmine 2 have been measured, which suggest
that ISC competes with fluorescence and internal conversion
for S1. We believe that 1 and 2 may serve as 1O2 photosen-
sitizers in conventional polar solvents and an ionic liquid.
However, low molar absorptivities and physical quenching
1
of 1 and 2 limits their utility as O2 photosensitizers. High
intensity of excitation light, longer irradiation times, and
low concentrations of 1 or 2 would be needed when attempt-
ing to use them as 1O2 photosensitizers. A better understand-
ing of the photooxidation mechanism of 1 and 2 under the
conditions will require further study.
Figure 3. Decay kinetics of 1O2 for 2 (OD¼0.4 at 532 nm; 40 mJ) in
CH3OH. The phosphorescence emission of 1O2 was monitored at
1270 nm. The dots are experimental data, and the solid line is calculated
from a 1st order kinetic fitting.
Acknowledgements
3.4. Indigo and indigo carmine sensitized
photooxidations
This work was supported by research grants to R.G. from
the NSF-PREM program (DMR-0611539), A.T.F. from
the NIH-MARC program (2T34 GM008078), and to A.G.
from the National Institutes of Health (S06 GM076168-01)
and PSC-CUNY. We thank Mark Kobrak (Brooklyn Col-
lege) for useful comments on the manuscript, Cliff Soll
(Hunter College Mass Spectrometry Facility) for conducting
several mass measurements, and Bin Ye and Matthias Selke
(California State University, Los Angeles) for carrying out
several laser experiments. We thank a reviewer for com-
ments on the manuscript.
In 1979, Kuramoto and Kitao attributed the photofading of
indigo 1 in solution to a self-sensitized photooxidation in-
volving singlet oxygen.25 We observed a cleavage of 1 into
isatin (4) after 8 h of visible light irradiation in an O2-satu-
rated solution of DMSO-d6, or in an O2-saturated solution
of CDCl3 in the presence of methylene blue. The LC–MS
data is indicative of isatin 4 since we find a molecular ion
peak m/z 148 (M+H)+, and under various ionization condi-
tions find m/z 146 (MꢀH)ꢀ, m/z 165 (M+NH4)+, m/z 180
(M+CH3OH2)+, m/z 183 (M+Cl)ꢀ representing the addition
of solvent molecules or ions. Isatin 4 was also identified by
1H NMR spectroscopy and the comparison to a commercial
sample of isatin 4. Singlet oxygen may contribute to the
oxidative degradation of 1 by a reaction with the central
C]C double bond of 1. Alternatively, oxidation of 1 may
take place via a Type I photooxidation pathway, similar to
the reaction of 2 with superoxide reported recently.57 The
mechanism by which active oxygen species initiates a pro-
cess leading to the oxidative cleavage of the double bond
of 1 and 2 will require further investigation. Greater yields
of 4 are observed when a co-sensitizer, such as methylene
blue is present.25
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