Long-lived photoacid based upon a photochromic reaction

Article abstract of DOI:10.1021/ja203851c

A visible-light activatable photoacid has been studied, which upon irradiation, changes from a weak acid, with a pK_a of 7.8, to a strong acid, which achieves nearly complete proton dissociation. This process is reversible and the half-life of t

Full text of DOI:10.1021/ja203851c

ARTICLE
pubs.acs.org/JACS
Long-Lived Photoacid Based upon a Photochromic Reaction
Zheng Shi,† Ping Peng,† Daniel Strohecker, and Yi Liao*
Department of Chemistry, University of Central Florida, Orlando, Florida 32816, United States
ABSTRACT: A visible-light activatable photoacid has been
studied, which upon irradiation, changes from a weak acid, with
a pKa of 7.8, to a strong acid, which achieves nearly complete
proton dissociation. This process is reversible and the half-life of
the proton-dissociation state is ∼70s. The long lifetime of the
proton-dissociation state is due to a sequential intramolecular
photochromic reaction. Using this photoacid, a pH change of
2.2 units has been achieved. In addition, we demonstrated that the photoinduced proton concentration can catalyze an esterification
reaction, and greatly alter the volume of a pH-sensitive polymer. This work shows that acid-catalyzed and pH-sensitive processes can
be photochemically controlled by using this type of photoacid.
induce proton transfer and consequent molecular events before,
but the lifetime of their proton-dissociation state was not re-
ported.8 These photochromic compounds include protonated
merocyanines and 2-hydroxyazobenzenes. Herein we report a
long-lived photoacid based on a photochromic reaction, and
demonstrate its applications by catalyzing an esterification reac-
tion, as well as altering the volume of a pH-sensitive polymer.
1. INTRODUCTION
Proton transfer is one of the most fundamental processes in
nature. It is involved in numerous chemical reactions, biological
functions, and material properties.1 Photoacids that undergo
proton dissociation upon irradiation promise spatial and tem-
poral control of these processes in a noncontact way, and could
provide a way to convert photoenergy into other types of energy.
Herein the term “photoacids” refers to the molecules that re-
versibly undergo proton photodissociation and thermal reasso-
ciation. Photoacid generators (PAGs) and other compounds that
are consumed in the photoprocesses are not considered. In fact,
photoacids have been studied since the 1970s, and reviews have
been published every decade since then.2À6 Photoacids have
been used to study molecular proton transfer, and have been
exploited to control molecular and supramolecular events.7,8
However, the photoinduced proton concentration of previously
reported photoacids has not been large enough to drive or
control many acid-catalyzed or pH-sensitive processes, and the
great potential of photoacid has yet to be explored. For example,
catalyzing reactions using photoacids was proposed in the 1970s
and is still described as a potential application in recent papers.9
The major challenge is that the lifetime of the proton-dissociation
state is limited by the lifetime of the conjugated base of the
photoacid in the ground state. The short lifetime of the proton
dissociation state is useful for studying some fast processes;
however, the liberated proton does not have enough time to diffuse
away from its counterion and thus cannot catalyze a chemical
process or significantly alter a macroscopic property even though
the theoretical excited-state pKa* can be very low. A recent work
reported a photoacid with a relaxation time of the proton-dissociation
state close to 1 s, which was estimated by pumpÀprobe absorp-
tion spectroscopy.9
2. EXPERIMENTAL PROCEDURE
General. The photoreactions were performed using a RPR-100
Rayonet photochemical reactor equipped with 16 lamps with emission
wavelength of 419 or 570 nm. Products of the esterification reaction were
analyzed by a PerkinElmer series 200 HPLC. Fisher Scientific Accument
AR15 benchtop meter and pH combination electrodes were used to
measure the pH change. The UV absorption spectral studies were
performed using a Varian Cary 50 Scan UVÀvis spectrophotometer.
Photoreaction Conditions. In the photocatalyzed esterification
reaction, a solution of the reactants and the photoacids was divided into
two solutions and placed in Pyrex test tubes. One of the test tubes was
wrapped with aluminum foil and used as the control. The two test tubes
were placed together in a RPR-100 Rayonet photochemical reactor and
were irradiated with a 419 and/or a 570 nm light. The photoreactor was
equipped with a ventilation fan, which kept the temperature close to
room temperature. The concentration of the solution, the irradiation
time, and the yields were given in the text. In the experiment that
demonstrated the photoinduced volume change, the hydrogel cuboid
was placed in an open beaker filled with an aqueous solution of the
photoacid. The mixture was irradiated in the photochemical reactor, and
the size of the cuboid was measured by placing the wet cuboid on a ruler.
Synthesis of Photoacid 1. Scheme 1 shows the details. Starting
material 2,3,3-trimethyl-1-(3-sulfonatepropyl)-3H-indolium was synthe-
sized following a literature method.11 Thus, 2,3,3-trimethylindolenine
(1.65 g, 0.01 mmol, Aldrich) was added into propane sultone (1.26 g,
0.01 mmol, TCI). The mixture was stirred at 90 °C for 4 h under N2.
Using an intramolecular photoreaction to stabilize the proton
dissociation state can be an effective way to increase the lifetime
of the proton dissociation state. The most extensively studied
intramolecular photoreactions are photochromic reactions.10 As
a matter of fact, photochromic compounds have been used to
Received: April 26, 2011
Published: August 08, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis (Top) and Photochemical Reaction (Bottom) of Photoacid 1
Figure 1. UVÀvis absorption of a solution of photoacid 1 before irradiation, ∼10 s (time between the end of the irradiation and the end of UVÀvis
measurement) after irradiation at 419 nm for 3 min, and kept in the dark for 10 min after irradiation (left), and UVÀvis absorption of 2, an analogue of
the SP form of 1 (right).
The purple solid was collected by filtration, washed with cold diethyl
ether, and dried in vacuo (2.61 g, 89% yield). H NMR (500 MHz,
3. RESULTS AND DISCUSSION
1
Photoacid 1 is a protonated merocyanine with a propyl
sulfonate group on the nitrogen of the indoline moiety. It was
synthesized following the procedure in Scheme 1. In an aqueous
solution, the protonated merocyanine (MEH) form of 1 is
predominant at room temperature in the dark, which is proven
by UVÀvis (Figure 1) and NMR spectroscopy. In the UVÀvis
spectrum, the absorption of the deprotonated merocyanine
(ME) can also be observed, which indicates that 1 is weakly
acidic. A 5.88 Â 10À4 M aqueous solution of photoacid 1 has a
pH value of 5.5, from which the pKa of 1 was calculated to be
7.8. When the solution was irradiated with 419 nm light, which is
close to the λmax of MEH (424 nm), pH drops more than 2 units
to 3.3. This value is close to the theoretical value for complete
proton dissociation (3.2) indicating that 1 is a strong acid under
irradiation. When the light was turned off, pH value increased
quickly to ∼4.5 in ∼1 min, gradually returning to its original level
in ∼5 min. This cycle can be repeated many times (Figure 2).
Therefore, proton concentration can be simply controlled by
switching a light on and off. This process does not require control
CDCl3): δ = 8.04 (d, 1H, J = 7.0), 7.81 (d, 1H, J = 6.5), 7.61 (m, 2H),
4.65 (t, 2H, J = 8), 2.82 (s, 3H), 2.62 (t, 2H, J = 6.5), 2.14 (m, 2H), 1.52
(s, 6H).
For the synthesis of 1, 2,3,3-trimethyl-1-(3-sulfonatepropyl)-3H-
indolium (100 mg, 0.36 mmol) and 2-hydroxybenzaldehyde (48 mg,
0.39 mmol) were added into anhydrous ethanol (2 mL). The mixture
was allowed to reflux overnight. The orange solid was obtained by
1
filtration (110 mg, 80% yield). H NMR (500 MHz, d6-DMSO): δ =
11.05 (s, 1H), 8.60 (d, 1H, J = 16.5), 8.28 (d, 1H, J = 7.0), 8.02 (d, 1H,
J = 7.0), 7.87 (m, 2H), 7.62 (m, 2H), 7.45 (t, 1H, J = 8.4), 7.04 (d, 1H,
J = 8.5), 6.96 (t, 1H, J = 7.0), 4.80 (t, 2H, J = 7.5), 2.65 (t, 2H, J = 6.0),
2.15 (m, 2H), 1.77 (s, 6H). 13C NMR (500 MHz, d6-DMSO): δ = 25.1,
26.9, 45.9, 47.8, 52.3, 111.9, 115.5, 117.1, 120.5, 121.8, 123.4, 129.5,
129.6, 130.2, 136.2, 141.4, 143.9, 149.1, 159.5, 182.2. HRMS (ESI):
[M À H]À = 384.1275, [M À 2H + Na]À = 406.1094. UVÀvis absorption
in water: λmax= 424 nm. The solubility of MEH in water at room
temperature is ∼2 Â 10À4 M. Assisted by gentle heating, ∼6 Â 10À4
M solution can be obtained. However, the solution became slightly cloudy
after being kept overnight.
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Figure 2. UVÀvis absorption of a solution of photoacid 1 25À340 s after irradiation (left) (the inset plot shows the data in the first 100 s fitted to a first
order reaction equation), and cycles of pH change under irradiation and in the dark (right). For each cycle, a solution of 1 (1.5 Â 10À4 M) was irradiated
with 419 nm light for 3 min and kept in the dark for 10 min.
of two light sources with different wavelengths, nor does it
involve irradiation with UV light.
The photochemical reaction was studied by UVÀvis absorp-
tion spectroscopy (Figures 1 and 2). Upon irradiation, the
absorption of MEH decreased, and an absorption peak at
300 nm appeared. This absorption peak is assigned to the SP
form of 1 based on the UVÀvis spectrum of an analogue, 2, in
Figure 1. The absorption of ME disappeared quickly upon
Figure 3. Derivatives of 1 synthesized and studied in this work.
irradiation, leaving only the MEH and SP absorption to be
observed. This indicates that ME is not likely as an intermediate
in the MEH-to-SP process. The disappearance of ME should
be due to the equilibrium between the MEH and ME, and the
increased acidity caused by the MEH-to-SP transformation,
which changes ME to MEH. In the dark, the absorption intensity
of MEH recovered to the level before irradiation in ∼5 min. The
absorption of ME increased with that of MEH with no isosbestic
point (Figure 2), which indicates that MEH is not generated
from ME in the SP-to-MEH transformation. On the basis of
these analyses and previous studies of photochromism,12 the
mechanism of the photoreaction is proposed as in Scheme 1.
Photoirradiation with 419 nm light induces a transÀcis isomer-
ization of MEH. Proton dissociation of the cis-MEH produces
cis-ME, which undergoes a nucleophilic ring closing reaction to
yield SP. The process is reversible, and SP reverts back to MEH in
the dark. In addition, irradiation at 570 nm, which is only
absorbed by ME, also induced MEH-to-SP transformation. This
supports the idea that photoirradiation induces transÀcis iso-
merization rather than proton dissociation. Absorption peaks of
cis-ME and cis-MEH were not observed, which may be due to
their low stability in addition to absorption overlaps with those of
MEH, ME, and SP.
The lifetime of the proton-dissociation state was calculated
from the rate of MEH formation after irradiation, which was
monitored by UVÀvis spectroscopy. UVÀvis spectra were
collected every 10 s after irradiation (Figure 2). The proton
concentration equals the overall concentration subtracted by the
MEH concentration. The experimental data in the first 100 s
were fit to a first order kinetic equation, and a rate constant k =
0.009 14 sÀ1 was obtained (Figure 2). The half-life of the proton
dissociation state (t1/2 = 0.693/k) was calculated to be 76 s. This
lifetime is only an estimation, since the process is a reversible
multistep process, and thus may not strictly follow first-order
kinetics. Nonetheless, the calculated half-life is consistent with
the pH measurements, which shows that first order decay is a
reasonable approximation for the early stage of the proton con-
centration decay.
To further improve the photoacidity and understand the
factors that affect it, analogues of 1 (3À5 in Figure 3) were
synthesized. Compound 3 has an electron-withdrawing NO2
group on the phenol moiety, and 4 has an electron-donating OH
group. Both 3 and 4 are less effective than 1 in lowering the pH
on photolysis. The electron-withdrawing group of 3 increased
its acidity in the dark. The pKa of 3 in the dark is 6.36, which is
1.5 units lower than that of 1. The pH of a 3.7 Â 10À4 M solution
of 3 changed from 4.9 to 3.9 upon irradiation, while a solution of
1 of the same concentration can change its pH from 5.5 to 3.5.
The acidity of 3 after irradiation is lower than that of 1, which
should be due to a less efficient nucleophilic cis-ME-to-SP
reaction. For compound 4, photoirradiation does not effectively
induce MEH-to-SP transformation, and thus produces little pH
change. This may be because the cis-MEH cannot undergo
proton dissociation efficiently to generate cis-ME. Detailed
mechanistic study is required to quantitatively define the suitable
photoreactivities and energy levels of MEH, SP, and ME, which is
beyond the scope of this article. Solubility of the photoacid is
important for inducing a large pH change in solution. The satura-
tion concentration of 1 in water is ∼6 Â 10À4 M, which limits the
photoinduced pH change to ∼2 units. The solubility of 5 is too
low in both organic solvents and water for a significant change of
proton concentration to be realized. Comparing 1 with 5, we can
see that the propyl sulfonate group increases the solubility of 1. In
addition, it has been reported that alkyl sulfonate groups on the
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These are only two examples of the potential of photoacids.
Given that proton transfer is one of the most fundamental
processes in nature, this type of photoacid may provide a way
to utilize photoenergy to control or drive numerous processes
that involve proton concentration.
’ AUTHOR INFORMATION
Corresponding Author
yi.liao@ucf.edu
Figure 4. pH sensitive hydrogel before (left) and after (right) being
irradiated in a solution of 1. (The cuboid was taken out of the solution
and measured on a ruler.)
Author Contributions
†These authors have made equal contributions.
nitrogen of the indoline moiety can stabilize the open ring forms
(MEH and ME).13 The positive charge of N+ on the indoline
moiety is shielded by SO3À, which disfavors the nucleophilic
ring-closing reaction of the phenoxy anion, and thus stabilizes
open ring structures.
’ ACKNOWLEDGMENT
Support from the Air Force Office of Scientific Research under
AFOSR-(FA9550-09-1-0628) is gratefully acknowledged.
Two experiments were conducted to demonstrate the applica-
tions of this type of photoacid. In the first experiment, 1 was used
to catalyze a Fisher esterification reaction, which often requires a
strong acid catalyst, such as sulfuric acid. Thus, a mixture of acetic
acid (12.5 mM) and 1 (3.1 mM) in ethanol was irradiated at 419
and 570 nm. Ethyl acetate was produced and analyzed by HPLC.
The yields were 33%, 50%, and 66% for 1, 2, and 3 h reaction,
respectively. No reaction was detected when the mixture was
kept in the dark. In a control experiment, 1 was substituted by 2.
No esterification reaction was detected after the mixture was
irradiated for 3 h. This experiment shows that acid-catalyzed
reactions could be photocatalyzed. In the second experiment, a
solution of 1 was used to alter the volume of a pH sensitive
polymer. The idea was proposed before, but no experimental
demonstration has been made to date.14 Thus, a hydrogel of
cross-linked polyacrylamide was prepared following a literature
procedure.15 It was partially hydrolyzed to a copolymer of
poly(acrylic acid) and polyacrylamide by 1 M NaOH solution.
The hydrogel was cut into cuboids, which were then soaked in a
mixture of 1 in water (1 mg/mL). After irradiation, the volume of
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1
the hydrogel changed to ∼ /8 of its initial volume (Figure 4).
The hydrogel did not change back to its original volume when left
in the dark although the pH of the solution did revert back to its
original value. This is due to the property of the hydrogel, not the
photoacid, since the hydrogel showed the same behavior when
placed in different buffer solutions with different pH values.
Given that these experiments are for demonstration only, no
effort was put forth to optimize the conditions. For real applica-
tions, the photoacid should be covalently linked to polymers,
which could lead to recyclable heterogeneous catalysts that
eliminate the requirements of strong acids, as well as photo-
responsive actuators and artificial muscles that can be controlled
using fiber optics or noncontact irradiation.
4. CONCLUSION
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In conclusion, we have discovered a visible-light activatable
photoacid that has a proton-dissociation state lifetime of ∼70 s
and can alter the pH value of its solution unprecedentedly. The
long lifetime of its proton-dissociation state is due to a sequential
intramolecular reaction. The magnitude of photoinduced proton
concentration reported in this work is so large that it can catalyze
an esterification reaction and alter the volume of a pH sensitive
polymer. None of these have been achieved with other photoacids.
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