Reversible photochemical isomerization of N, N ′-Di(t -butoxycarbonyl)indigos

Article abstract of DOI:10.1021/jp512346z

We report on the photophysics of N,N′-di(t-butoxycarbonyl)indigos (tBOC indigos), finding that reversible photochemical trans-cis and cis-trans isomerization reactions proceed with high quantum yields (0.10-0.46). Absorption of wavelengths in the 500-600 nm region induces trans-cis isomerism, while blue light leads to the reverse cis-trans process. Like their parent indigos, trans-BOC indigos have low fluorescence yields (～1 × 10^-3), while the cis isomers have no measurable emission. These compounds are the first examples of photoisomerizable indigoid dyes in which photochemical isomerism effectively outcompetes radiative decay processes. Though indigo dyes typically have poor solubility in organic solvents, tBOC indigos can be dissolved at concentrations up to 8 w% in common organic solvents like acetone. Furthermore, unlike other photoisomerizable indigoids, tBOC indigos are not sensitive to quenching by proton and electron donors. These features, combined with high quantum yields of reversible photoisomerism induced by relatively low-energy photons (～2 eV), make tBOC indigo derivatives potentially interesting for photochromic applications, such as photomechanically actuated materials.

Full text of DOI:10.1021/jp512346z

Article
pubs.acs.org/JPCA
Reversible Photochemical Isomerization of
N,N′‑Di(t‑butoxycarbonyl)indigos
Dominik Farka, Markus Scharber, Eric Daniel Głowacki,* and Niyazi Serdar Sariciftci
Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Linz, Altenbergerstrasse 69, A-4040
Linz, Austria
ABSTRACT: We report on the photophysics of N,N′-di(t-butoxycarbonyl)indigos
(
tBOC indigos), finding that reversible photochemical trans−cis and cis−trans
isomerization reactions proceed with high quantum yields (0.10−0.46). Absorption of
wavelengths in the 500−600 nm region induces trans−cis isomerism, while blue light
leads to the reverse cis−trans process. Like their parent indigos, trans-BOC indigos
−3
have low fluorescence yields (∼1 × 10 ), while the cis isomers have no measurable
emission. These compounds are the first examples of photoisomerizable indigoid dyes
in which photochemical isomerism effectively outcompetes radiative decay processes.
Though indigo dyes typically have poor solubility in organic solvents, tBOC indigos can be dissolved at concentrations up to 8 w
%
in common organic solvents like acetone. Furthermore, unlike other photoisomerizable indigoids, tBOC indigos are not
sensitive to quenching by proton and electron donors. These features, combined with high quantum yields of reversible
photoisomerism induced by relatively low-energy photons (∼2 eV), make tBOC indigo derivatives potentially interesting for
photochromic applications, such as photomechanically actuated materials.
INTRODUCTION
around the central double bond, has fluorescence yields of
13,3
■
around 0.9.
Indigoids are molecules with a rich cultural and scientific
history. The use of indigo and derivatives, such as 6,6′-
dibromoindigo (Tyrian purple), has been a part of civilization
from ancient Egypt and the Mediterranean Greco-Roman
world, through southeast Asia, and the Americas for thousands
of years. In 19th century Europe, the commercial significance of
indigo became one of the driving forces behind the develop-
ment of modern organic synthetic chemistry and chemical
Trans−cis photoisomerizable compounds convert light
energy into mechanical work and thus have found application
in photoalignment coatings, especially for liquid-crystal
14
optoelectronics, optical data storage, photoactuating poly-
meric materials, photoswitchable optical elements, and
15
permeation membranes. In these applications, azobenzene
and its derivatives dominate, with spiropyrans and spiroox-
16
17
18
azines, fulgides, and various diarylethenes being other
notable photochromic material classes. Indigoids, especially
thioindigos, had been explored relatively early in some of these
applications and were thought attractive because of lower-
energy green or red light providing photoswitching, as opposed
to the UV light normally necessary for azobenzenes and other
photochromic moieties. Thioindigo, selenoindigo, and several
N,N′-substituted indigos with dimethyl or diacyl substituents all
undergo reversible photochemical trans−cis and cis−trans
photoisomerization. A major disadvantage of the photo-
isomerizable indigos, however, was low photochemical isomer-
ization yields, combined with the dominance of other undesired
photophysical processes, e.g. thioindigo was reported to have a
1
,2
industry. Indigos, in their hydrogen-bonded form (Figure 1a)
are remarkable for their photostability and low fluorescence
−
3
yield (ϕ ∼ 1 × 10 ). This property has been attributed to
f
rapid excited-state proton transfer, leading to a keto−enol
tautomerization process that efficiently dissipates the excited-
3
−5
state energy.
Photochromic effects were reported in various
6
indigoids already in the 1930s and 1940s, with the generation
of a cis isomer proposed as the possible origin of this effect.
Wyman and Zenhausern showed in 1965 that the optical
7
spectra of N,N′-oxalylindigo, an indigoid that is covalently
locked in the cis configuration, closely match the photoinduced
spectral changes observed in the case of photochromic
indigoids. This led to the conclusion that indeed two
photoisomers exist. Subsequent experimental and theoretical
work on indigoids proved this to be the case. Substituting the
NH protons with methyl, acyl, aryl, chalcogenides like −S−
leads to indigoids (Figure 1b) where photochemical isomer-
ization occurs, competing with fluorescence and nonradiative
decay. Trans−cis quantum yields range from around 0.1 in the
1
1
19
ϕ of 0.47. N,N′-Dimethylindigo affords a ϕtrans−cis of ∼0.01
f
with a very rapid thermal reverse isomerization. N,N′-
Diacetylindigo has been reported to have a ϕtrans−cis of ∼0.1
as well, with higher yields of fluorescence. The low solubility
8
9,10
11
of indigos in organic solvents and polymers also hampered
applications. Photochromic thioindigo was explored as a
12
potential dye for optical information recording, and N,N′-
11
case of N,N′-diacetyl indigo and thioindigo to a maximum of
around 0.3 in the case of some thioindigo derivatives. By way
of comparison, the fused-ring cibalackrot indigo (Figure 1c),
which lacks NH protons and also does not allow rotation
12
Received: December 11, 2014
Revised: March 11, 2015
Published: March 26, 2015
©
2015 American Chemical Society
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acetate. Spectroscopic-grade anhydrous acetonitrile, acetone,
toluene, chlorobenzene, and bromobenzene were used for all
optical experiments. UV−vis spectra were obtained using a
PerkinElmer Lambda 1050. Photoluminescence spectra were
recorded using a PTI fluorometer equipped with a photo-
multiplier tube. Fluorescence quantum yield was calculated
using Rhodamine 6G in ethanol as a standard, according to the
25
IUPAC standardized method. Photochemical switching was
achieved using either a 532 nm diode laser or light from a
xenon lamp coupled through a grating monochromator, as
indicated. To correctly calculate both the trans−cis and cis−
trans photochemical isomerization yields, the extinction
coefficients for each species must be known. The extinction
coefficients of the pure trans and cis isomers were calculated
11
analytically using the method introduced by Blanc and Ross.
This method can be applied provided that at least one of the
two interconvertible isomers is luminescent at a wavelength
where the emission is not reabsorbed and at which there is no
luminescence from the other isomer. The method also assumes
that the Beer−Lambert law holds. Both conditions are satisfied
in the case of molecules 1−3 evaluated here, and as discussed in
the results and discussion section, only the trans isomers are
luminescent, emitting at wavelengths >600 nm, where no
reabsorption occurs. The method for calculating the extinction
coefficient for the cis isomer is described as follows (assuming
optical path length 1 cm): While the fluorescence intensity is
measured, with a constant excitation intensity, the concen-
tration [trans] changes to [trans] , and the optical density of
1
2
the sample changes from D to D . We define the ratio, R, of
1
2
the trans isomer concentrations as
Figure 1. (a) Photophysics of indigo. The dominant process upon
excitation is radiationless internal conversion, believed to occur via
excited-state proton transfer followed by a keto−enol−keto tautome-
rization process. Indigoids without NH protons (b), such as N,N′-
dimethylindigo and thioindigo, have competing luminescence and
trans−cis photoisomerization. Removing protons and blocking possible
isomerism results in the highly luminescent cibalackrot compound (c).
−D2
⎛
⎞⎛ ⎞⎛
⎞
[
trans]1
IF1 D1 1 − 10−
≡ R⎜ ⎟⎜ ⎟⎜
⎟
⎠
D1
[
trans]2
⎝ I ⎠⎝ D ⎠⎝ 1 − 10
F2
2
(1)
λ
The optical density at a given wavelength, D , will constitute
the sum of the absorption contributions of the trans and cis
isomers, according to eq 2, where ε is the molar extinction
coefficient:
(
d) The three indigos used in this work (1−3) contain bulky tBOC
groups and undergo reversible photoisomerism.
λ
λ
λ
λ
D = (ε
− ε )[trans] + ε [total]
(2)
trans
cis
cis
disubstituted indigos were proposed for storage of solar energy
in the form of the photochemically generated higher-energy cis
The value of R, the knowledge of the total concentration, and
the absorption spectra in states 1 and 2 are sufficient to
construct the absorption spectrum of the cis isomer by
combining eqs 1 and 2 to give eq 3.
2
0
isomer. However, because of the low ϕtrans−cis and limited
solubility, these indigoids were overlooked in favor of some of
the material classes mentioned above. Recent work published
21
by de Melo and co-workers showed that indigo in its reduced
form, leuco-indigo, can have a ϕtrans−cis of 0.9, with the
photochemically generated cis isomer being fluorescent.
These results, combined with recent demonstrations of
λ
λ
R(D ) − (D )
[
λ
2
1
ε =
cis
total](R − 1)
(3)
22
indigoids as promising semiconducting materials, motivate
their further exploration as photoswitching chromophoric
systems. The tBOC indigos reported here, 1−3, combine
excellent solubility, relatively high trans−cis and cis−trans yields,
negligible fluorescence, and resilience to quenching and might
thereby revitalize exploration of the indigo family of molecules
for photoswitching applications.
To establish the quantum yield of photoisomerization, the
setup shown in Figure 2 was used. A monochromator is used to
tune the wavelength of light from a xenon lamp. The light was
focused onto the sample: a 0.1 mM solution in a 1 cm quartz
cuvette. This served as the pump to induce the desired
photoisomerization process. Changes in transmission of the
sample over time were measured using chopped light from a
tunable supercontinuum white light laser source (NKT SuperK
Extreme) detected using a Si photodiode (Hamamatsu), which
was synchronized to the chopper frequency using a lock-in
amplifier. Intensity of the probe is maintained much less than
that of the pump to not contribute to the photochemical
process. Another calibrated photodiode was placed in such a
way that it could be slid directly into the pump beamline in the
position where the sample was placed to measure the photon
MATERIALS AND METHODS
■
Indigo was obtained from BASF and purified by temperature
gradient sublimation. Both 5,5′- and 6,6′-dibromoindigo were
obtained from the corresponding 2-nitrobenzaldehydes accord-
ing to the Baeyer−Drewsen synthesis. All were converted to
their N,N′-diBOC forms 1−3 according to published
23
2
4
procedures and purified by recrystallization from ethyl
3
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We observed photochromism in 1−3. The absorption
spectrum of a 0.2 mM solution of 1 in acetonitrile is shown
in Figure 3 before and after irradiation with a 532 nm laser (2
Figure 2. Setup for measuring photoisomerization quantum yield.
Changes in transmission over time of the sample during excitation by
the pump are measured using chopped laser light and a Si-photodiode
with lock-in detection.
Figure 3. Reversible photoisomerization of a 0.2 mM solution of 1 in
acetonitrile. Black arrows show the thermal recovery of the original
absorption.
mW), followed by the thermal relaxation in the dark of the cis-
rich solution back to trans over 120 min. Isosbestic points are
observable at 398 and 498 nm. This process is reversible for at
least 5 cycles under these conditions. As mentioned above,
indigoids lacking intramolecular H-bonding are photochromic
compounds, undergoing reversible trans−cis isomerization.
Single-crystal X-ray diffraction revealed that N,N′-diBOC
indigos are highly strained with respect to the central ethylenic
carbons connecting the two indole rings, which helps to explain
why the barrier for isomerization is easily overcome by energy
from photons ∼2 eV. The absorption spectra for 1−3 are
shown as solid red traces in Figure 4. Irradiation with the 532
nm laser leads within 120−150 s to a photostationary state
shown in solid blue lines. Photoluminescence spectra were
recorded before and after photoirradiation. A weak emission
without visible vibrational structure was recorded in all cases.
Using Rhodamine 6G in ethanol according to IUPAC
flux. Photon flux was calculated on the basis of the
manufacturer responsivity calibration data for the photodiode,
corrected for reflection losses in the setup, and kept around 1 ×
−5
−1
10
einsteins s . The well-known thioindigo was used as a
“standard” to cross-check our methodology.
With the knowledge of extinction coefficients for each
isomer, photoisomerization quantum yields can be calculated
26
according to the method of initial rates. The photochemical
isomerization is a first-order process, and the quantum yield
ϕi→f is thus given by eq 4, where Iabs is the moles of photons
absorbed by the sample per second and V is the total volume of
the solution.
⎛
⎞
∂
c
⎜
i ⎟
ϕi→f = −
× V
⎜ ∂t ⎟
25
−3
⎝
I
⎠
standards, a ϕ
f
of ∼1 × 10 was calculated for all three
abs
(4)
t=0
compounds. The excitation spectra, shown as dotted colored
lines in Figure 4, were found to follow the absorption spectrum
of the trans-rich solutions, suggesting that luminescence
originates from only the trans isomer. No detectable
luminescence attributable to the cis-isomer was found, including
when irradiating cis-rich solutions in the 420−450 nm region,
where cis absorption is at its maximum.
RESULTS AND DISCUSSION
■
In recent publications, the synthesis of N,N′-diBOC indigos
2
4,27
was described.
The point of that work was to use the tBOC
function as a protecting group that would render indigo
derivatives transiently soluble in organic solvents and thus
amenable to further chemical manipulation. Indigos are
normally remarkably planar molecules because of O···HN−
hydrogen-bonding. Introduction of tBOC groups eliminates
these H-bonding interactions and introduces substantial steric
bulk as well. For various applications of photoisomerizable
dyes, solubility is important. It is the low solubility of
photoswitching indigoids, in fact, that was the major factor in
A method for calculating the absorption spectra of two
isomers reversibly interchangeable by a photochemical isomer-
ization where only one isomer is luminescent was introduced
11
by Ross and Blanc. We followed this methodology to
calculate the extinction coefficients of the pure trans and cis
species (Figure 5). The cis absorption profile is similar to that
of oxalylindigo, an indigoid that is known to have cis
2
0,28
7
them being overshadowed by other molecular systems.
Compound 1 was found to produce stable solutions up to 75−
5 mg/mL in acetone, dichloromethane, and chloroform, with
configuration. The absorption maxima of both trans and cis
isomers follow the principles of H-chromophore absorption
8
̈
first outlined by Luttke and co-workers, i.e., that the
slightly lower solubility (∼65−70 mg/mL) in aromatic solvents
like toluene and chlorobenzene. Brominated 2 and 3 have lower
solubility, 25−35 mg/mL in the same solvents, with 3 being the
least soluble of the series. In contrast, thioindigo can be
dissolved with maximum concentration of ∼1 mg/mL in
organic solvents.
fundamental indigo chromophore is the cross-conjugated
system of electron-rich NH groups and electron-poor carbonyl
groups, and the highest occupied molecular orbital−lowest
unoccupied molecular orbital (HOMO−LUMO) transition
2
9,30
features a charge-transfer from NH to CO.
Thus, π-
electron donors at 5,5′ and 6,6′ positions yield bathochromic
3
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Figure 5. Molar extinction coefficients of the trans and cis isomers of
1
−3 calculated according to the analytical method reported in ref 11.
Figure 4. Original trans-rich solutions (solid red traces) for 1−3 in
acetonitrile solution. Photostationary states obtained with 532 nm
laser irradiation are shown in solid blue traces. Solid black lines show
the photoluminescence of the original trans-rich solutions, while the
dotted black traces show the photoluminescence of the photosta-
tionary cis-rich solutions. The dotted red and blue lines show the
excitation spectra for luminescence at 620 nm of the trans-rich and cis-
rich solutions, respectively.
measured using a Si photodiode with phase-sensitive detection.
The calculated photoisomerization yields, shown in Table 1, are
found to be significantly higher in the case of 1 than for
brominated 2 and 3.
Photoisomerization in indigos has been proposed to proceed
via a singlet intermediate excited state. The finding that both
brominated derivatives 2 and 3 have lower photoisomerism
yields is consistent with the singlet model: Bromine atoms are
expected to increase the population of triplet excited states
because of spin−orbit coupling, and bromination in this case is
found experimentally to decrease and not increase the
photoisomerism yield. Increasing triplet population decreases
the singlet excited state that can lead to isomerization. It is
important to note here that the comparison of 5,5′ and 6,6′
dibrominated derivatives helps to exclude other electronic
effects on the photoisomerization process, as substitution at
5,5′ versus 6,6′ is known to have opposite influences on the H-
3
1,32
and hypsochromic shifts, respectively.
For example, the Br
atoms at the 5,5′ positions of 2 inductively push electron
density onto the NH groups, giving a bathochromic shift.
Compounds 1−3 follow this H-chromophore principle in both
trans and cis isomeric forms. The inductive effects on the
photoisomerism yields, however, remain unclear and would
have to be resolved with the help of time-resolved optical
measurements on the intermediate transition states.
The knowledge of the extinction coefficients of both isomers
is necessary both to calculate the photoisomerism yield and to
choose photoswitching wavelengths that selectively excite one
or the other isomer as desired. The rate of trans−cis
photoisomerization is linear with respect to excitation intensity,
indicating a first-order photochemical reaction. Thus, based on
3
1,32
chromophore of indigos.
solvent” method introduced by Kasha and co-workers,
According to the “heavy-atom
33
employing solvents with heavier atoms are expected to increase
triplet population. We measured yields of compounds 1−3 in
the solvent series toluene, chlorobenzene, and bromobenzene.
The values of ϕtrans−cis decreased in the presence of heavier-
atom solvents (Table 2), supporting the hypothesis that
photoisomerism proceeds via a singlet excited state. Because
the combined quantum yields of both luminescence and
photoisomerism do not exceed 0.1−0.46, a natural question is
how the rest of the energy is dissipated. Based on the
differences in photoisomerization yields of 1 and brominated 2
26
the method of initial rates outlined by Lippert and Luder, the
quantum yields of trans−cis and cis−trans photochemical
isomerization could be calculated. For trans−cis reaction of 1,
for example, we used monochromatic light from a xenon lamp
in series with a grating monochromator at a frequency of 562
nm; for cis−trans, 433 nm was used. Molar absorptivity based
on calculated extinction coefficients (Figure 5) was assumed.
The setup (Figure 2) for these measurements featured a cw
pump illumination and a chopped probe, with transmission
3
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Table 1. Extinction Coefficients and Photophysical Data for 1−3 in Acetonitrile Solution, with Measurement of Thioindigo
Shown for Comparison
trans λmax (nm), ε (mol− cm−1 L)
1
cis λmax (nm), ε (mol−1 cm−1 L)
fluorescence λmax (nm)
ϕf
ϕtrans−cis
ϕcis−trans
compound
1
546, 7 626
557, 8 604
543, 6 840
542, 17 900
438, 5 885
450, 7 577
426, 4 246
483, 13 800
606
621
600
592
0.001
0.001
0.002
0.55
0.13
0.01
0.06
0.10
0.46
0.02
0.20
0.38
2
3
thioindigo
Table 2. Φ
for 1−3 in Solvents with Potential
photobleaching experiment was found to slow bleaching in the
case of 1 but had no effect on 2 or 3. The survival of these
samples under relatively intense illumination for such periods of
time is encouraging for many applications. Under normal room
lighting, samples were found to be stable for at least 6 months.
For photoswitching applications, understanding of the
isomerization process in solid films is likewise of interest.
The photochemical behavior of compounds 1−3 was measured
in spin-cast 100 nm-thick polymer films of poly(methyl
methacrylate) and poly(styrene). When these dyes are
dissolved in the polymer matrix at a ratio of 1:5 dye:polymer
by weight, their absorption and emission spectra resembled
closely those obtained from dyes in solution. Photochromism
was not observed for the brominated derivatives. In the case of
trans−cis
Increasing Heavy Atom Effect
ϕtrans−cis
solvent
toluene
1
2
3
0.13
0.09
0.08
0.09
0.04
0.04
0.05
0.04
0.04
chlorobenzene
bromobenzene
and 3, the contribution from population of the triplet states is
not a significant channel. The remaining option is efficient
vibrational relaxation. Yang et al. have recently reported on
the basis of theoretical calculations, that because of the
torsional degrees of freedom offered by the tBOC groups on
3
4
1
, however, photoswitching between the trans−cis and cis−trans
1, configurational relaxation processes are expected to be a
was observed, albeit with a much lower quantum yield ranging
around 1 × 10 . Conversely, as can be expected from the
suppression of isomerism, photoluminescence yield increases to
major pathway of nonradiative energy dissipation.
−8
In reports on thioindigo and similar indigoids, both proton
3
5,36
37
donors
and electron donors were found to quench trans−
∼
0.15. Photoluminescence yields of these samples were
estimated based on the method reported by Langhals et al.,
cis isomerism. Proton quenching is seen as evidence that in
indigo compounds photoinduced proton transfer is a prevalent
process that results in nonradiative deactivation of the excited
state mediated by keto−enol tautomerism. This mechanism has
been postulated for the negligible photoluminescence com-
using N,N′-bis(1-hexylheptyl)-3,4,9,10-perylene bis-
42
(
dicarboximide) as a standard. These results were the same
in both polymer matrices. On the basis of the very low
photoisomerization yields and the moderate photolumines-
cence efficiency, ∼85% of the excitation energy must be
dissipated by vibrational relaxation processes. However, these
experiments do not exclude the possibility that in the polymeric
matrix the photochemical forward and back conversion are too
rapid to observe photochromism.
38
bined with photostability of indigo itself. Indeed, 4,4′-
dihydroxythioindigo was recently found to be a mimetic of
indigo in that efficient photoinduced intramolecular proton
transfer from hydroxyl groups on the phenyl rings quenches
luminescence and isomerization. Theoretical studies also
support the model of excited-state proton transfer reactions
5
4
,39
outcompeting other photophysical processes.
In the case of
CONCLUSIONS
1−3, however, quenching was observed only with quencher
■
It has been shown that tBOC indigos have promising
photoisomerization properties, where relatively high photo-
isomerism quantum yields outcompete radiative processes. The
tBOC indigos are resilient to quenching by proton and electron
donors, which normally quench photoisomerizable indigos.
Combined with their high solubility in organic solvents and
ability to dissolve and even isomerize in glassy polymer
matrices, they are potentially advantageous for photoswitching
and photomechanical applications.
concentrations greater than ∼5000× that of 1−3. Phenol was
used as the proton donor as in the case of earlier reports. Thus,
the tBOC indigos are remarkably unaffected by proton-
quenching. It is likely that the steric bulk of the tBOC groups
hinders the phenol−indigo interaction that would cause
quenching. Stronger acids (e.g., mineral acids) were not used
because these would lead to hydrolysis of the tBOC functional
groups. Photoreduction has been reported when thioindigo and
other indigiods are irradiated in the presence of electron
3
7,40,41
donors.
For compounds 1−3, we found that these
materials show the same insensitivity to electron donors as they
do to proton donors. A ∼10 × excess of triethylamine was
AUTHOR INFORMATION
■
4
Corresponding Author
E-mail: eric_daniel.glowacki@jku.at.
needed to observe any quenching. Overall, the idea that N,N′-
substitution of bulky groups can help to preserve photo-
isomerism in environments with potential quenching species
may be useful for applications. Considering the stability of
tBOC indigos with respect to quenching, we evaluated also the
stability to photobleaching. It was found that solutions of
compounds 1 and 2, when placed on a solar simulator with an
*
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for support by the Austrian Science
Foundation, FWF, within the Wittgenstein Prize of N. S.
Sariciftci Solare Energie Umwandlung Z222-N19 and the
Translational Research Project TRP 294-N19 “Indigo: From
ancient dye to modern high-performance organic electronics
2
intensity of 100 mW/cm , began to show irreversible
photobleaching after ≈17−18 h. Compound 3 was less stable,
beginning to degrade already after ≈4 h of illumination.
Saturating the acetonitrile solutions with Ar or N prior to the
2
3
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circuits”. We thank Elisa Tordin and Gundula Voss for
synthesis support.
(23) Baeyer, A.; Drewsen, V. Darstellung von Indigoblau Aus
Orthonitrobenzaldehyd. Chem. Ber. 1882, 15, 2856−2864.
(24) Głowacki, E. D.; Voss, G.; Demirak, K.; Havlicek, M.; Sunger,
N.; Okur, A. C.; Monkowius, U.; Gąsiorowski, J.; Leonat, L.; Sariciftci,
N. S. A Facile Protection−Deprotection Route for Obtaining Indigo
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DOI: 10.1021/jp512346z
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