Unraveling the mechanism of the photodeprotection reaction of 8-bromo- and 8-chloro-7-hydroxyquinoline caged acetates

Article abstract of DOI:10.1002/chem.201200366

Photoremovable protecting groups (PPGs) when conjugated to biological effectors forming "caged compounds" are a powerful means to regulate the action of physiologically active messengers in vivo through 1-photon excitation (1PE) and 2-photon excitation (2PE). Understanding the photodeprotection mechanism is important for their physiological use. We compared the quantum efficiencies and product outcomes in different solvent and pH conditions for the photolysis reactions of (8-chloro-7-hydroxyquinolin-2-yl) methyl acetate (CHQ-OAc) and (8-bromo-7-hydroxyquinolin-2-yl)methyl acetate (BHQ-OAc), representatives of the quinoline class of phototriggers for biological use, and conducted nanosecond time-resolved spectroscopic studies using transient emission (ns-EM), transient absorption (ns-TA), transient resonance Raman (ns-TR²), and time-resolved resonance Raman (ns-TR³) spectroscopies. The results indicate differences in the photochemical mechanisms and product outcomes, and reveal that the triplet excited state is most likely on the pathway to the product and that dehalogenation competes with release of acetate from BHQ-OAc, but not CHQ-OAc. A high fluorescence quantum yield and a more efficient excited-state proton transfer (ESPT) in CHQ-OAc compared to BHQ-OAc explain the lower quantum efficiency of CHQ-OAc relative to BHQ-OAc. Copyright

Full text of DOI:10.1002/chem.201200366

FULL PAPER
DOI: 10.1002/chem.201200366
Unraveling the Mechanism of the Photodeprotection Reaction of
and 8-Chloro-7-hydroxyquinoline Caged Acetates
ACHTUNGTRENN8UNG -Bromo-
Jiani Ma,[a] Adam C. Rea,[b] Huiying An,[a] Chensheng Ma,[a] Xiangguo Guan,[a]
Ming-De Li,[a] Tao Su,[a] Chi Shun Yeung,[a] Kyle T. Harris,[b] Yue Zhu,[b]
Jameil L. Nganga,[b] Olesya D. Fedoryak,[b] Timothy M. Dore,*[b] and
David Lee Phillips*[a]
Abstract: Photoremovable protecting
groups (PPGs) when conjugated to bio-
logical effectors forming “caged com-
pounds” are a powerful means to regu-
late the action of physiologically active
messengers in vivo through 1-photon
excitation (1PE) and 2-photon excita-
tion (2PE). Understanding the photo-
deprotection mechanism is important
for their physiological use. We com-
pared the quantum efficiencies and
product outcomes in different solvent
and pH conditions for the photolysis
reactions of (8-chloro-7-hydroxyquino-
lin-2-yl)methyl acetate (CHQ-OAc)
and (8-bromo-7-hydroxyquinolin-2-yl)-
methyl acetate (BHQ-OAc), represen-
tatives of the quinoline class of photo-
triggers for biological use, and conduct-
ed nanosecond time-resolved spectro-
scopic studies using transient emission
(ns-EM), transient absorption (ns-TA),
transient resonance Raman (ns-TR2),
and time-resolved resonance Raman
(ns-TR3) spectroscopies. The results in-
dicate differences in the photochemical
mechanisms and product outcomes,
and reveal that the triplet excited state
is most likely on the pathway to the
product and that dehalogenation com-
petes with release of acetate from
BHQ-OAc, but not CHQ-OAc. A high
fluorescence quantum yield and a more
efficient excited-state proton transfer
(ESPT) in CHQ-OAc compared to
BHQ-OAc explain the lower quantum
efficiency of CHQ-OAc relative to
BHQ-OAc.
Keywords: cage compounds · pho-
todeprotection · proton transfer ·
quinoline · time-resolved spectros-
copy
Introduction
near-UV light. Recently, there has been increasing interest
in developing PPGs that can be excited by two-photon exci-
tation (2PE) because of their special advantages (such as
minimizing damage to the biological system by using lower
energy photons in the near infrared region and improved
spatial resolution) for the release of biological effectors in
biological systems and one can refer to two very recent re-
views for details.[2] Only a few PPGs contain chromophores
with large enough two photon excitation cross sections that
can be used effectively in physiology experiments; therefore,
there are relatively few classes of PPGs that have been de-
veloped for 2PE compared with those developed for 1PE
excitation.
Photoremovable protecting groups (PPGs) have become
powerful tools for probing biological systems through their
conjugation to biological effectors, forming “caged com-
pounds”.[1] The photorelease reactions of PPGs are mostly
initiated by one-photon excitation of ultraviolet (UV) or
[a] J. Ma, Dr. H. An, Dr. C. Ma, Dr. X. Guan, Dr. M.-D. Li, T. Su,
C. S. Yeung, Dr. D. L. Phillips
Department of Chemistry
The University of Hong Kong
Pokfulam Road, Hong Kong S.A.R. (P.R. China)
Fax : (+852)2957-1586
E-mail: phillips@hku.hk
Dore and co-workers discovered that the quinoline-based
phototriggers have promise in mediating the release of phys-
iologically active messengers through both one-photon exci-
tation (1PE) and 2PE.[3] (8-Chloro-7-hydroxyquinolin-2-yl)-
methyl acetate (CHQ-OAc) and (8-bromo-7-hydroxyquino-
lin-2-yl)methyl acetate (BHQ-OAc) photolyze in response
to irradiation with ultraviolet (1PE) or near-visible light
(2PE), releasing acetate and the remnant of the protecting
[b] A. C. Rea, K. T. Harris, Dr. Y. Zhu, J. L. Nganga, Dr. O. D. Fedoryak,
Dr. T. M. Dore
Department of Chemistry
University of Georgia
Athens, Georgia 30602-2556 (USA)
Fax : (+1)706-542-9454
E-mail: tdore@uga.edu
group 8-chloro-2-(hydroxymethyl)
ACHTUNGERTNqNUNG uinolin-7-ol (CHQ-OH)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200366.
and 8-bromo-2-(hydroxymethyl)quinolin-7-ol (BHQ-OH),
AHCTUNGTRENNUNG
respectively (Scheme 1). BHQ in particular has been report-
ed to be a good phototrigger for mediating the release of
biologically relevant effectors in vivo through 2PE. It has
Re-use of this article is permitted in accordance with the Terms and
Conditions set out at http://onlinelibrary.wiley.com/journal/10.1002/AHCTUNG-
TRENNUNG(ISSN)1521-3765/homepage/2111_onlineopen.html.
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solved resonance Raman (ns-TR3) spectroscopies. Density
functional theory (DFT) calculations were utilized to help
make assignments of the experimental Raman spectra to
probable intermediate species and to study the solvolysis re-
action that follows the deprotection reaction.
Scheme 1. Photolysis of CHQ-OAc and BHQ-OAc.
been used to pinpoint the timing and location of morpholino
activation in zebrafish with 2PE,[4] regulate the action of
a thrombin aptamer (HD1),[5] and initiate the expression of
GFP.[6] In response to these successful applications, it is im-
portant to characterize and understand the deprotection re-
action mechanism, so that biological experiments can be
conducted free of doubt about the performance of the pho-
totrigger, and improvements to the design of quinoline-
based protecting groups can be made.
Results and Discussion
Synthesis: BHQ-OAc was synthesized from bromoquinoline
1[3b] by the route shown in Scheme 2, which is a modification
of the route previously reported.[3a]
A methoxymethyl
According to initial investigations, the photodeprotection
reaction proceeds through a singlet transient species and the
release of the protected group likely takes place through
a solvent-assisted photoheterolysis (SN1) reaction mecha-
nism.[3b] This conclusion was based on indirect information
such as an oxygen quenching experiment conducted in ace-
tonitrile (MeCN), rather than in water-containing solutions.
Recent work has shown that BHQ-OAc and CHQ-OAc ex-
hibit totally different behavior in water-containing systems
compared with their behavior in inert organic solvents like
MeCN.[7] The 1PE quantum efficiency (Qu) and the 2PE un-
caging action cross-section (du) for the photolysis of CHQ-
OAc was found to be noticeably lower than that for BHQ-
OAc (Qu =0.29 for BHQ-OAc[3a] and 0.10 for CHQ-OAc;[3c]
du =0.59 GM for BHQ-OAc[3a] and 0.12 GM for CHQ-
OAc[3c]). If a singlet state transient species acts as the pre-
cursor for the deprotection reaction, the Qu of CHQ-OAc
could be expected to be significantly higher than that of
BHQ-OAc since the introduction of the heavy atom bro-
mine into its structure would better facilitate a competing
intersystem crossing (ISC) process in BHQ-OAc, and the
depletion of the singlet state would be more efficient in the
bromine containing system than the chlorine containing
compound. In addition, the excited state proton transfer
(ESPT) process, which has been extensively studied for the
parent compound 7-hydroxyquinoline (7-HQ),[8] was not ex-
amined in detail for BHQ-OAc, and the role of ESPT in the
water-containing solutions has not yet been examined. In
view of these issues, it is desirable to explicitly investigate
the reaction mechanism of BHQ-OAc in neutral aqueous
solutions not only to help gain a better understanding of the
reaction mechanism, but also to acquire information that
may assist in the design of improved two-photon phototrig-
ger compounds.
Scheme 2. Preparation of BHQ-OAc.
(MOM) ether was used as a protecting group instead of
a tert-butyldiphenylsilyl (TBDPS) group, and the bromine
was carried through the sequence instead of being added
near the end of the synthesis. This alternative synthesis of
BHQ-protected acetate represents an improvement over the
originally reported route.[3a] It is high yielding and eliminates
the use of an expensive tert-butyldiphenylsilyl protecting
group. This new route will be useful when protecting other
molecules with carboxylate and other functional groups.
CHQ-OAc was synthesized as previously described in the
literature.[3c] Authentic samples of the photolysis products
BHQ-OH and CHQ-OH were synthesized from the corre-
sponding acetates as previously described for BHQ-OH.[3a]
Solvent effects on quantum efficiency: The quantum effi-
ciencies (Qu) for the photolyses of BHQ-OAc and CHQ-
OAc at 254 nm in each of the three solvents were deter-
mined by methods previously described (Table 1).[3b,9] Pho-
tolysis reactions were conducted at 254 nm, rather than near
the lmax value of 369 nm to more closely match the time-re-
Table 1. Quantum efficiencies of BHQ-OAc and CHQ-OAc photolyses.
To help resolve these issues, we examined the photolysis
reactions of BHQ-OAc and CHQ-OAc to determine the
quantum efficiency (Qu) and initial product outcome under
different solvent and pH conditions, and we conducted com-
parative nanosecond time-resolved spectroscopic studies
using transient emission (ns-EM), transient absorption (ns-
TA), transient resonance Raman (ns-TR2), and time-re-
Chromophore
BHQ-OAc
Solvent
Qu at 254 nm
60:40 H2O/MeCN, pH 7
60:40 H2O/MeCN, pH 4
KMOPS buffer, pH 7.2
60:40 H2O/MeCN, pH 7
60:40 H2O/MeCN, pH 4
KMOPS buffer, pH 7.2
0.25
0.17
0.26
0.09
0.10
0.14
CHQ-OAc
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Photodeprotection of Caged Acetates
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solved studies, which used 266 nm for excitation (see
below). The value of Qu was lower for CHQ-OAc than that
for BHQ-OAc. At low pH, Qu for CHQ-OAc is relatively
unchanged compared to the value measured in the neutral
mixed H2O/MeCN solvent, but Qu for BHQ-OAc is lower
under acidic conditions compared with a neutral environ-
ment.
eluate from the column. BHQ-OAc required only 15 s of ir-
radiation, whereas CHQ-OAc needed 30 s. This is consistent
with the relative sensitivities to 1PE (eꢁQu) of the chromo-
phores: 754 for BHQ-OAc[3a] versus 280 for CHQ-OAc.[3c]
CHQ-OAc is less sensitive to photolysis than BHQ-OAc.
The product outcome of the photolysis of CHQ-OAc
under different solvent conditions was determined by irradi-
ating 100 mm samples of CHQ-OAc in 60:40 H2O/MeCN
pH 4, 60:40 H2O/MeCN pH 7, KMOPS buffer pH 7.2, and
dry MeCN and analyzing the reaction by HPLC. Photolysis
in a high pH environment (e.g., 60:40 H2O/MeCN pH 10)
was not possible because spontaneous hydrolysis of the ace-
tate at pH 10 was sufficiently rapid to compete with photoly-
sis. Irradiation of CHQ-OAc in dry MeCN did not lead to
observable photolysis products, even after several minutes.
If irradiation was extended to 45 min or longer, the starting
material began to degrade and some new peaks were ob-
served by HPLC, but we could not identify the compounds
giving rise to these new peaks.
The values of Qu at 254 nm in the two neutral solvents are
similar to those reported for BHQ-OAc (0.29)[3a] and CHQ-
OAc (0.10)[3c] at 365 nm in KMOPS buffer. The lower Qu
for CHQ-OAc compared with BHQ-OAc and the previous
observation that CHQ-OAc is slightly more fluorescent
(lex =365 nm,) than BHQ-OAc[3c] is inconsistent with time-
resolved infrared spectroscopy (TRIR) and Stern–Volmer
quenching experiments, which indicated that BHQ-OAc
photolyzed through a singlet excited state.[3b] If the reaction
proceeded through a singlet excited state, the quantum effi-
ciency for CHQ-OAc should be higher than that for BHQ-
OAc, because the bromine would promote intersystem
crossing (ISC), which would deplete the singlet excited state
of BHQ-OAc. Chlorine does not significantly promote ISC.
The pKa of both BHQ-OAc and CHQ-OAc is 6.8, and the
UV/Vis absorbance spectra of BHQ-OAc and CHQ-OAc in
KMOPS are nearly identical.[3a,7c] In solution, 8-substituted-
7-hydroxyquinolines can exist as a single prototropic form
or a mixture of several (Figure 1). At neutral pH in 60:40
In each aqueous solvent, a single new peak was observed
at tR =4.1 min in the HPLC chromatogram (Figure 2a). ESI-
MS analysis of the eluate corresponding to that peak pro-
duced a signal at m/z 210, which corresponds to MH+ of
CHQ-OH. This assignment was confirmed by conducting
the same analysis on an authentic sample of CHQ-OH. In-
spection of the chromatograms in Figure 2a reveals that the
Figure 1. Prototropic states of XHQ-OAc.
H2O/MeCN, resonance Raman spectroscopy revealed that
BHQ-OAc and CHQ-OAc exist mainly as the neutral (N)
prototropic form with some contribution of the anionic (A)
form, but CHQ-OAc has a larger amount of the tautomeric
(T) form present.[7a,c] At low pH, less of the anionic (A)
form and more of the neutral (N) and cationic (C) forms
are expected to be present, which are the least absorptive
forms of the quinoline and would diminish Qu, as we ob-
served for BHQ-OAc. It is not clear why Qu values at neu-
tral and acidic pH are similar for CHQ-OAc.
Product outcome studies in different solvents: The initial
products formed in each photolysis reaction were deter-
mined by HPLC and MS analysis. Samples of BHQ-OAc
and CHQ-OAc in KMOPS buffer were irradiated at 254 nm
in a photoreactor to determine how much exposure time
was required to generate amounts of the initially formed
products of the reaction that could be observed by the UV/
Vis detector on the HPLC and ESI-MS analysis of the
Figure 2. HPLC chromatograms of the photolysis of a) CHQ-OAc (tR =
8.8 min) to CHQ-OH (tR =4.1 min) and b) BHQ-OAc (tR =11.0 min) to
BHQ-OH (tR =4.4 min) and HQ-OAc (tR =4.8 min).
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extent of disappearance of CHQ-OAc is greatest in aqueous
KMOPS buffer. The photolysis is less efficient in the mixed
H2O/MeCN solvents, and the reaction proceeds to the least
extent after 30 s in 60:40 H2O/MeCN pH 4 in which the less
absorbent neutral (N) and cationic (C) forms of CHQ-OAc
predominate. The relative photolysis efficiencies are consis-
tent with the distribution of prototropic ground states of
CHQ-OAc in neutral and basic water-rich solutions and
MeCN.[7c]
The photolysis of BHQ-OAc in the four solvents was
more complex than that of CHQ-OAc. Like CHQ-OAc,
BHQ-OAc did not photolyze in the dry MeCN solvent
system, and BHQ-OAc disappeared to the greatest extent in
KMOPS buffer and to the least extent in acidic media. In
the three water-rich solutions, HPLC analysis of the photo-
ACHTUNGTRENNUNGlysis reaction revealed that two different photolysis products
were formed with tR =4.4 and 4.8 min (Figure 2b), and ESI-
MS of the eluate corresponding to each peak showed signals
at m/z 254/256 and 218, respectively. The retention time of
4.4 min corresponds to that of an authentic sample of BHQ-
OH, and ESI-MS of BHQ-OH presents two peaks equal in
magnitude for MH+ at m/z 254 and 256, as a result of the
isotopic abundance of 79Br and 81Br.
Other work has suggested that photoinduced debromina-
tion of BHQ competes with the photolysis reaction,[7b] so we
synthesized a sample of the debrominated product (7-hy-
droxyquinolin-2-yl)methyl acetate (HQ-OAc) from TBDPS-
protected quinoline 6[3a] (Scheme 3). Analysis of this authen-
Scheme 3. Preparation of HQ-OAc.
Figure 3. a) Absorption spectra of BHQ-OAc with PS ([PS]ꢀ5ꢁ10À5 m,
[BHQ-OAc]ꢀ1ꢁ10À4 m) and without PS in H2O/MeCN ([BHQ-OAc]
ꢀ1ꢁ10À4 m, 3:2, v/v, pH 6–7) before irradiation, b) comparison of absorp-
tion spectra of BHQ-OAc without PS to those with 0.5 times [PS] upon
photolysis in H2O/MeCN (3:2, v/v, pH 6–7), and c) absorption spectra of
BHQ-OAc with 5 times [PS] ([PS]ꢀ5ꢁ10À4 m, [BHQ-OAc]ꢀ1ꢁ10À4 m)
in H2O/MeCN (3:2, v/v, pH 6–7).
tic sample of HQ-OAc revealed that the peak at tR =4.8 min
with m/z 218 for MH+ corresponds to HQ-OAc. Debromi-
nation appears to compete with photolysis of the acetate. At
first glance, this observation might preclude the use of BHQ
as a caging group for biological applications, but yields of
released products are consistently 60–70%.[3b] HQ-OAc is
also photoactive and releases acetate upon exposure to
light. These results indicate that the mechanism of light-
mediated carboxylate release is complex, but it does not di-
minish the potential for the use of quinoline-based protect-
ing groups in biological systems.
The absorbance of BHQ-OAc at longer wavelengths
AHCTUNGTREG(NNUN >300 nm) will be focused on here because PS has strong
absorption below 300 nm. Before irradiation, the spectra of
the solutions without PS and with half of the concentration
of PS as BHQ-OAc in the same solution showed analogous
features in the region above 300 nm. Photolysis gave rise to
an obvious increase of the approximately 370 nm absorb-
ance that is assigned to the absorption of the converted pho-
toproducts in both solutions, and this process was much
slower in the solution with PS than in the one without PS.
Increasing the concentration of PS in the solution, the ab-
sorption of the mixed aqueous solution showed no discerni-
ble increase in the 370 nm region from 0 to 30 min. Inhibi-
Quenching experiments in a mixed aqueous solvent: To in-
vestigate whether a singlet or triplet species is correlated
with the deprotection reaction, quenching experiments were
conducted employing the well-known triplet quenching
agent potassium sorbate (PS). We compared the UV/Vis ab-
sorption spectra of BHQ-OAc before and after irradiation
under 266 nm wavelength laser pulses with different concen-
trations of PS in
(Figure 3).
a neutral mixed aqueous solution
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Photodeprotection of Caged Acetates
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tion of the photochemical reaction by the excess PS suggests
a triplet species (e.g., A(T1) or other triplet intermediates)
serves as the precursor of the photochemical reactions of
BHQ-OAc in neutral aqueous solutions.
of fluorescence intensities of the approximately 410 and
585 nm bands could indicate that there is a higher concen-
tration of the T(S1) to N(S1) species for CHQ-OAc than for
BHQ-OAc and hence the ESPT process is favored in CHQ-
OAc compared with BHQ-OAc. Also, the lifetimes of both
these species of CHQ-OAc are longer than those of BHQ-
OAc, suggesting that the singlet excited state faces less com-
petition from other routes, especially the ISC process for the
T(S1) and N(S1) species of CHQ-OAc. Furthermore, the
dual fluorescence quantum yield of CHQ-OAc was about 3
times higher than that of BHQ-OAc, consistent with a previ-
ous study that reported CHQ-OAc has a stronger emission
than BHQ-OAc,[3c] which suggests that a larger ratio of sin-
glet-state energy was lost through fluorescence rather than
promoting a photochemical reaction. Taken together, these
results imply that the ISC process is more efficient for
BHQ-OAc, and the detection of T(S1) demonstrates the oc-
currence of an ESPT process for BHQ-OAc in water-con-
taining solutions.
Nanosecond transient emission spectra (ns-EM): Based on
steady-state studies of BHQ-OAc,[7b] the peak in the ns-EM
spectra in neutral H2O/MeCN (Figure 4a) with an emission
wavelength at approximately 400 nm can be assigned to
Nanosecond transient absorption spectra (ns-TA): Upon
laser excitation of BHQ-OAc in MeCN, the emission band
at 360 nm and a broad band around 500 nm appeared and
then decayed to zero (Figure 5a). Likewise, for CHQ-OAc
the ns-TA spectrum showed two peaks at 410 and 485 nm
(Figure 5b). The similar lifetime constants for decay (t=256
and 322 ns) of BHQ-OAc and CHQ-OAc, respectively
(insets of Figure 5a and 5b) with first-order kinetics at each
wavelength suggests the two bands result from one species.
In light of the steady-state studies suggesting N(S0) of
BHQ-OAc is the predominant species[7a,b] and the ns-EM
spectra showing N(S1) is the only species observed in MeCN
solution, we propose that N(T1) is the species observed in
the ns-TA spectra obtained in MeCN. The N(T1) species
arises from conversion of N(S1) through ISC. The decay
time of N(T1) detected here is shorter than a previous time-
resolved infrared absorption (TRIR) measurement (t=
590 ns, k=1.7ꢁ106 sÀ1).[3b] A plausible explanation for this
difference stems from the different experimental conditions:
the ns-TA experiment here was performed in air-saturated
MeCN, whereas the TRIR study was conducted in argon-sa-
turated MeCN. The shorter decay time constant measured
under the air-saturated MeCN condition (256 ns) compared
with the argon-saturated MeCN condition (580 ns) is consis-
tent with the triplet assignment for the observed transient
species. To further verify, ns-TA experiments were also con-
ducted in oxygen-saturated MeCN (Figure 2S in the Sup-
porting Information) and a decay time of 93 ns was mea-
sured. The shortened lifetime of N(T1) in oxygen-rich condi-
tions corroborates the hypothesis that the species observed
in MeCN has a triplet character.
Figure 4. The ns-EM spectra of a) BHQ-OAc and b) CHQ-OAc acquired
in H2O/MeCN (3:2, v/v, pH 6–7).
N(S1) and the one at approximately 500 nm as T(S1). N(S1)
is the exclusive species observed in MeCN (Figure 1S in the
Supporting Information), whereas both N(S1) and T(S1) are
generated in appreciable amounts from excitation in neutral
aqueous solutions. Since the steady-state studies demonstrat-
ed that the tautomerization of BHQ-OAc is much less effi-
cient compared with 7-HQ, and the population of T(S0) of
BHQ-OAc is rather limited,[7a,b] any T(S1) that contributes
to the ns-EM spectra was likely almost exclusively produced
via an ESPT process.
The ns-EM spectra of CHQ-OAc in neutral H2O/MeCN
(Figure 4b) show two species were detected around 410 and
485 nm that were respectively assigned as N(S1) and T(S1)
of CHQ-OAc. In so far as the fluorescence quantum yield
ratios for the T(S1) to N(S1) species for the two compounds
BHQ-OAc and CHQ-OAc are similar, the different ratios
The ns-TA experiments on BHQ-OAc in neutral mixed
solvent provided different results (Figure 5c). The observed
DA of the two bands at 375 and 425 nm did not decay to the
baseline as time progressed, indicating that at least two spe-
cies contribute to the absorption spectra and some species
has (have) a long lifetime that might result from the final
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T. M. Dore, D. L. Phillips et al.
Figure 5. Transient absorption spectra of a) BHQ-OAc and b) CHQ-OAc in MeCN following 266 nm laser irradiation (insets are the time evolution with
an initial growth followed by a decay of the DA monitored at 520 and 480 nm, respectively) and transient absorption spectra of c) BHQ-OAc and
d) CHQ-OAc in H2O/MeCN (3:2, v/v, pH 6–7) following 266 nm laser irradiation (insets are the time evolution with an initial growth followed by
a decay of the DA monitored at 370 nm in open air and oxygen-saturated conditions for BHQ-OAc and the time evolution with an initial growth fol-
lowed by a decay of the DA for CHQ-OAc monitored at 430 nm).
solvolysis product. Because debromination competes with
the acetate deprotection for BHQ-OAc in aqueous solu-
tions, it is necessary to discuss which species is (are) contri-
buting to the long-lived absorption. Dehalogenation does
not take place for CHQ-OAc under the same reaction con-
ditions, so the long-lived absorption under irradiation in
CHQ-OAc aqueous solution was probably the product gen-
erated from the deprotection of the acetate (Figure 5d). The
ns-TA spectra for BHQ-OH, the final product from the de-
protection reaction of BHQ-OAc, in H2O/MeCN (3:2,
pH 7) (Figure 3S in the Supporting Information) shows
close similarity with the spectra seen at later times for the
ns-TA spectra of BHQ-OAc in the same solvent. Hence, the
species appearing at later times in the ns-TA spectra of
BHQ-OAc is probably the final product BHQ-OH. This
result further supports the idea that the deprotection re-
conditions provides evidence that a triplet species is needed
for the deprotection reaction.
Nanosecond transient resonance Raman (ns-TR2) and nano-
second time-resolved resonance Raman (ns-TR3) experi-
ments: The ns-TR2 and ns-TR3 spectra of BHQ-OAc in
MeCN (Figure 6) revealed that the transient species probed
in the ns-TR2 and ns-TR3 experiments are essentially the
same with Raman bands around 642, 1282, 1576, and
1633 cmÀ1. Given the same time resolution on the nanosec-
ond scale for the ns-TR2 and ns-TR3 spectra with that of the
ns-EM and ns-TA spectra, it is reasonable to propose N(T1)
is the transient species detected here. The major bands at
1576 and 1633 cmÀ1 show good agreement with the IR
bands observed at 1574 and 1632 cmÀ1 for N(T1) in
MeCN.[3b] The lifetime of the species that was observed in
the first 10 ns and subsequently decays on the hundreds of
nanosecond timescales measured by the ns-TR3 spectra also
agrees well with the ns-TA and TRIR results obtained in
MeCN.[3b] The good correlation of the vibrational frequen-
cies and the decay lifetime of this species strongly support
that N(T1) is the intermediate species observed in MeCN.
The extra bands at later times around 652, 852, 1214, and
ACHTUNGTRENNUNGaction takes place in neutral water-containing solutions. The
different band shapes in Figure 5c compared with those in
Figure 5a suggest that N(T1) was not detected in the H2O/
MeCN (3:2, pH 7) solution. The ns-TA spectra acquired in
an oxygen-saturated H2O/MeCN (3:2, pH 7) solution shows
the DA decreases almost to the baseline (Figure 5c inset).
The observed disappearance of BHQ-OH under oxygen-rich
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1633 cmÀ1 arise from species formed due to residual water
in MeCN.[3b]
Unlike the observation in MeCN that the earlier species
in the ns-TR3 spectra is the same as that observed in the ns-
TR2 spectrum, the ns-TR2 spectrum and ns-TR3 spectra
(Figure 7) at early time delays are significantly different for
BHQ-OAc and CHQ-OAc in neutral H2O/MeCN (3:2, v/v,
pH 6–7). This indicates that different photochemical reac-
tions occur after photolysis in neutral aqueous system as
compared with MeCN solvent. For BHQ-OAc, two sets of
Raman bands are observed within 10 ns in the ns-TR3 spec-
tra (indicated with black and red lines, respectively, in Fig-
ure 7a) and appear to decay on different timescales, indicat-
ing that they belong to different transient species created
after photolysis. However, the complicated TR3 spectra with
partially overlapping bands from different transient species
make the assignments challenging to unravel. Similar result
was observed in the spectra of CHQ-OAc (Figure 7b).
Because the T(S1) species of BHQ-OAc and CHQ-OAc
were detected in neutral aqueous solution by the ns-EM
spectra, it is necessary to take the ESPT reaction into con-
sideration. Previous studies on 3-, 6-, and 7-hydroxyquino-
line[10] reported that the ESPT process for these compounds
takes place by first forming the anionic intermediate form
A(S1) (Scheme 4).[8 g] Since A(S1) was speculated to be the
Figure 6. The ns-TR2 spectrum (labeled “transient”) and ns-TR3 spectra
of BHQ-OAc acquired at varying time delays with the 266 nm/320 nm
pump/probe wavelengths in MeCN.
precursor for the deprotection reaction in
a previous
study[3b] (although the multiplet of the precursor is now sug-
Figure 7. The ns-TR2 spectra (labeled “transient”) and ns-TR3 spectra of a) BHQ-OAc and b) CHQ-OAc in H2O/MeCN (3:2, v/v, pH 6–7) solution with
varying time delays indicated next to the spectra with the 266 nm pump and 320 nm or 355 nm probe laser pulses for BHQ-OAc and CHQ-OAc, respec-
tively.
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T. M. Dore, D. L. Phillips et al.
species probed is probably the analogous transient species
in the alkaline solution, A(T1). The residual peaks of (a) (la-
beled as a red line) that are missing in (b) can be found in
spectrum (c), suggesting that some amount of N(T1) is also
detected in the ns-TR2 spectrum acquired in neutral aque-
ous solutions. The ns-TR2 spectrum obtained in neutral
aqueous solution was compared with the predicted DFT
Raman spectrum of A(T1) species of BHQ-OAc (Figure 9)
Scheme 4. ESPT process of 7-HQ in water.
gested in the present work to be a triplet state), it would be
helpful to examine the situation for BHQ-OAc and CHQ-
OAc in alkaline solution.
The ns-TR3 experiments for photolysis of BHQ-OAc in
alkaline aqueous solution (pH 11–12), where the anion form
predominates is shown in Figure 4S (in the Supporting In-
formation). Unfortunately, the hydrolysis in the dark of
BHQ-OAc to BHQ-OH in alkaline aqueous solution com-
petes significantly with photolysis and would be observed
during the ns-TR3 experiment. Inspection of Figure 4S (in
the Supporting Information) also suggests that the case in
alkaline solution is fairly complex. Because the ns-TR2 spec-
trum of BHQ-OAc in alkaline solution was obtained at the
very beginning of the experiment with fresh sample solu-
tions, it is assumed that the main species at the moment of
detection is still A(S0) of BHQ-OAc. Upon irradiation,
A(S0) will undergo ISC process generating A(T1) or T(T1).
The production of the latter species requires an extra excit-
ed protonation process, and the proton concentration in al-
kaline solution is limited; hence, it is reasonable to assume
that the population of T(T1) at very early time is much
lower, and the species detected in the ns-TR2 in alkaline
aqueous solution is probably A(T1). Further support comes
from the great similarity between the ns-TR2 spectrum with
the calculated Raman spectrum of A(T1) (Figure 5S in the
Supporting Information).
Figure 9. a) The ns-TR2 spectrum of BHQ-OAc obtained in mixed H2O/
MeCN (3:2, v/v, pH 6–7) solution. b) The DFT calculated normal Raman
spectrum of A(T1).
and the two exhibit good agreement with the vibrational fre-
quency differences being smaller than 7 cmÀ1 on average. A
comparison of the experimental and calculated vibrational
frequencies, preliminary vibrational assignments, and quali-
tative descriptions of the vibrational modes in the 600 to
1800 cmÀ1 region (Table 1S in the Supporting Information)
consistently indicate that the main species observed in ns-
TR2 in neutral aqueous solution can be assigned to A(T1) of
BHQ-OAc.
After the formation of A(S1), an ESPT reaction can pro-
ceed to produce some T(S1) in competition with the ISC
process that forms A(T1). Since T(S1) was the other easily
observed species in the ns-EM spectra of BHQ-OAc ac-
quired in neutral aqueous solution, it is suggested that T(T1)
may be also be created through an ISC process and ob-
served in the ns-TR3 spectra. Since most of the reported
ESPT reactions for the parent compound 7-HQ take place
in the singlet excited state rather than triplet state, any
T(T1) observed in the ns-TR3 spectra would probably be
produced from the ISC process from T(S1), instead of being
produced from A(T1) through an ESPT reaction. In addition
to the T(T1) transient species, the species with Raman bands
at 842 and 1626 cmÀ1 was observed. Combining with the ns-
TA results that show the occurrence of the deprotection re-
action in neutral aqueous solutions, the species detected
here may correlate with a zwitterion-like BHQ intermediate
generated from the deprotection reaction of BHQ-OAc in
the physiologically relevant system.[3b] A comparison of the
500 ns experimental spectrum to the calculated normal
Raman spectrum of the triplet zwitterion-like BHQ inter-
The ns-TR2 spectra acquired in alkaline and neutral aque-
ous solutions and MeCN (Figure 8) reveal that the spectrum
(a) resembles (b) more closely while it has extra Raman
bands. This suggests that spectrum (a) has contributions
from several different forms of BHQ-OAc coexisting in the
neutral aqueous solution and that the predominant transient
Figure 8. The ns-TR2 spectra of BHQ-OAc acquired in a) H2O/MeCN
(3:2, v/v, pH 6–7) solution, b) NaOH-H2O/MeCN (3:2, v/v, pH 11–12) so-
lution, and c) MeCN.
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Table 2. Comparison of the experimental and calculated Raman bands of
the nominal ring C=C stretch of A(T1), T(T1), and Z(T1).
Species
Experimental Raman shift
[cmÀ1
Calculated Raman shift
[cmÀ1
N
]
N
]
A(T1)
T(T1)
Z(T1)
1576
1600
1626
1578
1598
1620
The ns-TR2 and ns-TR3 spectra of CHQ-OAc are not
complicated by the dehalogenation reaction that occurs with
BHQ-OAc and ns-TR3 spectra for CHQ-OAc under alka-
line conditions as that for BHQ-OAc (Figure 7S in the Sup-
porting Information). However, CHQ-OAc is rapidly hydro-
lyzed under these conditions to CHQ-OH and acetate,
therefore, it is likely that the spectra obtained have signifi-
cant contributions from CHQ-OH. Nevertheless, the transi-
ent spectrum obtained in neutral solution compared with
that obtained in freshly prepared alkaline conditions
(Figure 10) shows great similarity, demonstrating that the
Scheme 5. Proposed reaction routes of XHQ-OAc in neutral aqueous so-
lution.
mediate Z(T1) (Scheme 5) predicted from DFT calculations
(Figure 6S in the Supporting Information) reveals good
agreement for the experimental vibrational frequencies,
with the calculated frequencies being within about 9 cmÀ1
on average for the thirteen experimental Raman bands. This
supports the assignment of the experimental spectrum at
500 ns being mainly due to the Z(T1) species. A comparison
of the experimental and calculated vibrational frequencies
with preliminary vibrational assignments and a qualitative
description of the vibrational modes in the 600 to 1800 cmÀ1
region are given in Table 2S (in the Supporting Informa-
tion). The TR3 spectra of BHQ-OAc in the neutral aqueous
solution show that the triplet zwitterion-like BHQ inter-
mediate Z(T1), which has a characteristic band at 1626 cm
À1, appears within 5 ns due to the deprotection reaction that
will form this zwitterion species. Thus, the release of acetate
is indicated to occur on approximately a 5 ns timescale. It is
now reasonable to assign A(T1) as the major precursor of
the photolysis reaction of BHQ-OAc in neutral water-con-
taining solutions with the ESPT process to T(T1) as a com-
peting process. The introduction of the heavy atom bromine
into the compound BHQ-OAc not only facilitates the re-
lease of acetate, but also provides an opportunity for the
ISC process following the ESPT, transferring T(S1) to T(T1).
The calculated Raman spectra of T(T1) and Z(T1) repro-
duce the large vibrational frequency up-shift of the nominal
ring C=C stretch Raman bands observed in the ns-TR3 spec-
tra compared with the ns-TR2 spectrum with A(T1) of
BHQ-OAc as the predominant species (Table 2).
Figure 10. Comparison of the transient spectrum of a) CHQ-OAc ob-
tained in alkaline H2O/MeCN (3:2, v/v, pH 11–12) and b) CHQ-OAc in
neutral H2O/MeCN (3:2, v/v, pH 6–7) solutions.
main transient species observed at neutral pH is A(T1) of
CHQ-OAc. Two sets of Raman bands were detected in the
TR3 spectra (Figure 7b) with their most intense Raman
band at 1606 and 1632 cmÀ1, respectively. Based on the anal-
ysis of BHQ-OAc, the two sets of bands might be due to the
species T(T1) generated from the ESPT process and the
zwitterion species Z(T1) that results from the deprotection
reaction (Figure 8S (inset) in the Supporting Information).
To help make tentative assignments, the Raman spectra of
T(T1) (Figure 9S in the Supporting Information) and Z(T1)
(Figure 8Sb in the Supporting Information) were calculated
and show intense signals at 1600 and 1630 cmÀ1, respectively.
Therefore, the species that contributes at 1600 cmÀ1 was ten-
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T. M. Dore, D. L. Phillips et al.
tatively assigned to the T(T1) of CHQ-OAc species. The ex-
perimental Raman spectrum at 1000 ns delay time obtained
in neutral aqueous solution was compared with the calculat-
ed Raman spectrum of Z(T1) of CHQ-OAc (Figure 8S in
the Supporting Information). There is a good similarity be-
tween the experimental and calculated Raman spectra, and
this lends some support to the assignment of the transient
species with Raman band at 1632 cmÀ1 to Z(T1) of CHQ-
OAc. The ns-TR3 results reveal that A(T1) of CHQ-OAc
acts as the precursor of the deprotection process, though it
faces a side reaction from the ESPT process. The detection
of Z(T1) of BHQ-OAc and CHQ-OAc only in neutral
water-containing solution demonstrates that water and the
neutral pH (or close to neutral) are important for the depro-
tection reaction to occur.
ful and profound applications in a physiological environ-
ment. The poor suitability of CHQ-OAc as a PPG appears
mainly due to its favored fluorescence and ESPT pathways
and less efficient ISC capability to make the A(T1) precur-
sor species for the deprotection reaction when compared
with BHQ-OAc. This suggests that the introduction of
a heavy atom is necessary for the desired photodeprotection
reaction to take place with a high efficiency. A previous
steady-state study on the comparison of BHQ-OAc with 7-
HQ[7b] demonstrates that the involvement of the bromine
atom inhibits the ground-state tautomerization reaction and
lowers the population of T(S0), which also reduces unwanted
side reactions of the deprotection reaction. On the other
hand, the observed dehalogenation reaction for BHQ-OAc
should be taken into consideration in future studies. This
work sheds light on the design of promising two-photon
PPGs based on the 7-hydroxyquinoline scaffold.
DFT calculations: DFT calculations were utilized to exam-
ine the activation barrier for the deprotection reaction from
A(T1) of BHQ-OAc to the triplet zwitterion-like BHQ in-
termediate Z(T1) at the (U)B3LYP/6–311G** level of
theory.[11] The optimized geometries for the A(T1), transition
state species, and the triplet zwitterion-like BHQ intermedi-
ate were readily found from the calculations. These data
provide additional support for a two-step solvent-assisted
heterolysis (SN1) mechanism for the deprotection reaction
(see the Supporting Information for details). The calculation
results revealed that the deprotection proceeds through
a heterolytic cleavage from A(T1) of BHQ-OAc to form
a triplet zwitterion-like BHQ intermediate, which undergoes
ISC to its singlet and then proceeds via a singlet water–sol-
volysis reaction to produce the BHQ-OH side product.
Experimental Section
Materials: Dry acetonitrile was obtained by distillation from calcium hy-
dride. Mixed solvents were prepared by adjusting the pH of 40% MeCN/
60% H2O to pH 4 using 1m HCl. Solutions for photolyses were prepared
as follows: A 10 mm stock solution of BHQ-OAc or CHQ-OAc was pre-
pared in dry MeCN, and dilutions to 100 mm with a final volume of 3 mL
were made using each respective solvent. KMOPS buffer consisted of
100 mm KCl and 10 mm MOPS titrated to pH 7.2 with KOH.
Synthesis: BHQ-OAc was prepared by modification of the previously re-
ported route[3a] using a MOM protecting group on the phenol instead of
TBDPS. CHQ-OAc was synthesized and purified as described previous-
ly.[3c] HQ-OAc was prepared in one step by removal of the TBDPS group
from a known compound.[3a] See the Supporting Information for details.
Photochemistry: Solutions of BHQ-OAc or CHQ-OAc were irradiated
in a quartz test tube in a Rayonet photoreactor equipped with 254 nm
bulbs. To monitor the progress of the photoreactions, aliquots (20 mL)
were removed periodically and analyzed by HPLC using a 30:70 isocratic
mix of MeCN and H2O containing 0.1% TFA mobile phase, a 4.8 mm
OD C-18 reverse phase column, and a diode array UV detector (observ-
ing at 254 nm). Analysis of the eluate from the HPLC column by mass
spectrometry was performed one of two ways: 1) the HPLC system was
directly interfaced in line with an ESI mass spectrometer, or 2) fractions
corresponding to peaks on the chromatogram were collected and ana-
lyzed by ESI-MS offline. The quantum efficiency (Qu) under different
solvent conditions was determined as previously described.[3b,9] Briefly,
aliquots (20 mL) of the photolysis at 254 nm were removed periodically
Conclusion
Combining the results from the time-resolved spectroscopy
experiments (ns-EM, ns-TA, ns-TR2, and ns-TR3) with re-
sults from the photochemical quantum efficiency, product
outcome, and quenching experiments and DFT calculations,
a proposed reaction mechanism for the photophysical and
photochemical reactions of BHQ-OAc after UV photolysis
in aqueous environment has been deduced (Scheme 5). The
results presented here indicate that A(T1) acts as the precur-
sor for the desired deprotection reaction in neutral aqueous
solution and has competition from an ESPT process that
will form the T(T1) species. A favorable pathway with a low
free energy barrier of approximately 4.6 kcalmolÀ1 (see the
Supporting Information for details) was assigned to a sol-
vent-assisted heterolysis from A(T1) to produce the triplet
BHQ intermediate Z(T1). The completely different behavior
of BHQ-OAc in MeCN and MeCN/H2O solvents demon-
strates that water is vitally important for the desired photo-
deprotection reaction, which not only facilitates the excited
state deprotonation reaction but also stabilizes the zwitter-
ion transient species after the removal of the acetate leaving
group. The occurrence of the photodeprotection reaction in
the neutral aqueous solution suggests it can make meaning-
and analyzed by HPLC. The quantum efficiency was calculated using the
À1 [12]
equation Qu =(Ist90%
)
,
in which I is the irradiation intensity in Ein-
steins cmÀ2 of the lamp measured by potassium ferrioxalate actinometry
in the same setup, s is the decadic extinction coefficient (1000ꢁe) at
254 nm, and t90% is the time in seconds required for the conversion of
90% of the starting material to product as measured by HPLC.
Nanosecond transient emission (ns-EM) and nanosecond transient ab-
sorption (ns-TA) experiments: The ns-EM and ns-TA measurements
were performed on an LP-920 Laser flash photolysis setup (Edinburgh
Instruments). The 266 nm pump laser pulse was obtained from the fourth
harmonic output of an Nd:YAG Q-switched laser, and the probe light
was provided by a 450 W xenon lamp. The sample was excited by the
pump laser, and the probe light from the xenon lamp was passed through
the sample at right angles to the path of the exciting pulse. The two
beams were focused onto a 1 cm quartz cell. After passing through the
sample the analyzing light was directed to a monochromator/spectro-
graph. The transmitted probe light was then measured either by a single
detector (for kinetic analysis at a single wavelength) or by an array detec-
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Photodeprotection of Caged Acetates
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tor (for spectral analysis at a given time). The transmission properties of
the sample before, during, and after the exciting pulse were converted by
the detector into electrical signals, which were measured by an oscillo-
scope (in the case of the single detector) or acquired by a CCD camera
(in the case of an array detector). The changes in the transmission prop-
erties were converted into changes of optical density. The signals ana-
lyzed by a symmetrical Czerny-Turner monochromator were detected by
a Hamamatsu R928 photomultiplier, and the signal processed via an in-
terfaced PC and analytical software. Unless otherwise indicated, the ns-
EM and ns-TA experiments were conducted in air saturated solutions
and the sample solutions were prepared to yield the same absorbance at
the specified lex. In that way, the same number of photons is absorbed
for the same irradiating conditions in each case.[13] For the oxygen
quenching ns-TA experiment, the sample solution was placed in a 1 cm
quartz cell where oxygen was bubbled constantly for 30 min through the
sample in that cell, and then the cell was sealed to prepare it for the ns-
TA experiment.
Nanosecond transient resonance Raman (ns-TR2) and time-resolved res-
onance Raman (ns-TR3) experiments: Resonance Raman experiments,
ns-TR2 and ns-TR3, were performed using our previously described ex-
perimental apparatus and method.[14] Briefly, the fourth harmonic of
a Nd:YAG nanosecond-pulsed laser supplied the 266 nm pump beam and
the third anti-Stokes hydrogen Raman-shifted laser line from the second
harmonic of a second Nd:YAG laser supplied the 319.9 nm probe beam
used in the ns-TR3 experiments. The ns-TR2 spectrum was obtained from
the difference of the spectra under 266 nm high power and low power of
the pump laser by taking an appropriately scaled difference spectrum to
subtract the precursor and solvent Raman bands. The 266 nm pump pulse
excited the sample to initiate the photochemical reactions, and the
319.9 nm probe pulse interrogated the sample and the intermediate spe-
cies produced by the pump pulse. The laser beams were lightly focused
and aligned so that they were overlapped onto a flowing liquid stream of
sample. The diameter of the pump beam was adjusted to be slightly
larger than that of the probe beam at the overlapping volume in the
liquid jet to minimize the ground-state normal Raman signal. A pulse
delay generator was employed to electronically control the time delay be-
tween the pump and probe laser beams from the two different Nd:YAG
lasers operated at a repetition rate of 10 Hz. The Raman scattered light
was acquired using a backscattering geometry, then detected by a liquid-
nitrogen-cooled charge-coupled device (CCD) detector. The TR3 signal
was acquired for 10 s by the CCD before being read out to an interfaced
PC computer. Ten scans of the signal were accumulated to produce a res-
onance Raman spectrum. The ns-TR3 spectra were obtained from sub-
traction of an appropriately scaled probe-before-pump spectrum from
the corresponding pump-probe resonance Raman spectrum to remove
non-transient bands. The Raman bands of acetonitrile were employed to
calibrate the Raman shifts of the ns-TR3 spectra with an estimated accu-
racy of 5 cmÀ1. A Lorentzian function was used to integrate the Raman
bands of the species of interest in the ns-TR3 spectra to determine their
areas and elucidate the growth and decay time constants of the species
observed in the experiments.
Acknowledgements
This work was supported by a grant from the Research Grants Council
of Hong Kong (HKU 7039/07P) and the University Grants Committee
Special Equipment Grant (SEG-HKU-07) to D.L.P. and from the Nation-
al Science Foundation (CHE-0349059 and CHE-1012412) grant to
T.M.D. Support from the University Grants Committee Areas of Excel-
lence Scheme (AoE/P-03/08) is also gratefully acknowledged. We thank
Dennis R. Phillips and the UGA Proteomic and Mass Spectrometry Core
Facility and Siming Wang and the Georgia State University Mass Spec-
trometry Facility for mass spectrometry support.
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Density functional theory calculations: DFT calculations employing
(U)B3LYP method with a 6–311G** basis set were done to determine
the optimized geometries and vibrational wavenumbers for all of the
likely species that may be potential intermediates associated with the
photochemical reaction of BHQ-OAc and CHQ-OAc. No imaginary fre-
quencies were observed at any of the optimized structures studied. A
Lorentzian function with a 15 cmÀ1 bandwidth was used for the Raman
vibrational frequencies and the relative intensities to obtain the computa-
tional Raman spectra that were compared to the experimental TR3 spec-
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Received: February 3, 2012
Published online: && &&, 0000
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Photodeprotection of Caged Acetates
FULL PAPER
The water-mediated triplet excited
state deprotection reaction of 8-
bromo- and 8-chloro-7-hydroxyquino-
line caged acetate (BHQ-OAc) in
water-rich neutral solutions was inves-
tigated through photochemical product
outcome studies and nanosecond time-
resolved spectroscopic measurements.
Combined with density functional
theory calculations, a detailed photo-
deprotection mechanism in neutral
aqueous solution was deduced (see
scheme).
Photochemistry
J. Ma, A. C. Rea, H. An, C. Ma,
X. Guan, M.-D. Li, T. Su, C. S. Yeung,
K. T. Harris, Y. Zhu, J. L. Nganga,
O. D. Fedoryak, T. M. Dore,*
D. L. Phillips* ................ &&&& — &&&&
Unraveling the Mechanism of the
Photodeprotection Reaction of ACHTUNGTRENNUNG8-
Bromo- and 8-Chloro-7-hydroxyquino-
line Caged Acetates
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
13
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These are not the final page numbers!