Visible-light-responsive reversible photoacid based on a metastable carbanion

Article abstract of DOI:10.1002/chem.201304226

A new photoacid that reversibly changes from a weak to a strong acid under visible light was designed and synthesized. Irradiation generated a metastable state with high C-H acidity due to high stability of a trifluoromethyl-phenyl- tricyano-furan (CF₃PhTCF) carbanion. This long-lived metastable state allows a large proton concentration to be reversibly produced with moderate light intensity. Reversible pH change of about one unit was demonstrated by using a 0.1 mM solution of the photoacid in 95 % ethanol. The quantum yield was calculated to be as high as 0.24. Kinetics of the reverse process can be fitted well to a second-order-rate equation with k=9.78×10² M ^-1 s^-1. Response to visible light, high quantum yield, good reversibility, large photoinduced proton concentration under moderate light intensity, and good compatibility with organic media make this photoacid a promising material for macroscopic control of proton-transfer processes in organic systems. Copyright

Full text of DOI:10.1002/chem.201304226

DOI: 10.1002/chem.201304226
Communication
&
Photochemistry
Visible-Light-Responsive Reversible Photoacid Based on
a Metastable Carbanion
Valentine K. Johns,[b] Ping Peng,[c] Joseph DeJesus,[c] Zhuozhi Wang,[a] and Yi Liao*[a]
ciation state is limited by the lifetime of the excited state,
Abstract: A new photoacid that reversibly changes from
which makes it difficult to accumulate a large photoinduced
a weak to a strong acid under visible light was designed
proton concentration. Another type of photoacid has a photo-
and synthesized. Irradiation generated a metastable state
generated metastable high-acidity state, which has a much
with high CꢀH acidity due to high stability of a trifluoro-
longer life time than excited states. Therefore, they can gener-
methyl-phenyl-tricyano-furan (CF3PhTCF) carbanion. This
ate a large proton concentration upon irradiation (as PAGs)
long-lived metastable state allows a large proton concen-
and possesses good reversibility (as excited-state photoacids).
tration to be reversibly produced with moderate light in-
The potential of metastable photoacids in controlling acid-cat-
tensity. Reversible pH change of about one unit was dem-
alyzed reactions, volume-change hydrogels, polymer conduc-
onstrated by using a 0.1 mm solution of the photoacid in
tivity, and bacterial killing have been demonstrated recent-
95% ethanol. The quantum yield was calculated to be as
ly.[12–14]
high as 0.24. Kinetics of the reverse process can be fitted
Previously, our group demonstrated that a metastable-state
well to
102 mꢀ1
a second-order-rate equation with k=9.78ꢀ
photoacid can be designed based on a photoinduced intramo-
lecular reaction (Scheme 1a).[12] The merocyanine photoacid
can reversibly alter pH by over two units in an aqueous solu-
tion. However, the photoacid cannot be used in low-polarity
organic media, not only due to its low solubility in organic
media, but also because the thermal equilibrium shifts to the
high-acidity spiropyran form in low-polarity media, which re-
sults in high dark acidity. This problem limits its application in
not only organic but also biomedical materials, because many
biological systems, such as membranes, have low polarity.
Herein, we report the development of a new photoacid based
on a metastable carbanion by using the same principle. This
photoacid showed both a large visible-light-induced proton
concentration and good compatibility with organic media.
Photoacid 1 was designed and synthesized according to
Scheme 1d. The design is based on a negative photochromic
compound 3 recently discovered by our group (Scheme 1b).[15]
Compound 3 undergoes a cyclization reaction under visible
light to form a metastable zwitterionic structure 4, which was
fully characterized by NMR analysis. This work shows that the
tricyanofuran (TCF) structure can form a metastable anion with
a long lifetime. Compound CF3PhTCF is known as a stronger
electron acceptor than TCF (Scheme 1c).[16] Therefore, it was ra-
tionalized that reaction of 1 with CF3PhTCF can form an acid
with strong CH acidity after a photoreaction similar to that of
3 and the merocyanine-type photoacids (Scheme 1a),[12] be-
cause the corresponding carbanion in the proton-dissociation
state is resonance stabilized (Scheme 1c). Carbon-based excit-
ed-state photoacids have been previously reported.[10] For
compound 1, the high-acidity state 2 is not an excited state,
but a metastable state generated from an intramolecular pho-
toreaction. The key starting material, methyl CF3PhTCF, was
synthesized by following a literature procedure.[16] Reaction of
methyl CF3PhTCF at reflux with salicylaldehyde in ethanol fol-
lowed by concentrating the solution gave 1 as a precipitate.
s
ꢀ1. Response to visible light, high quantum yield,
good reversibility, large photoinduced proton concentra-
tion under moderate light intensity, and good compatibili-
ty with organic media make this photoacid a promising
material for macroscopic control of proton-transfer pro-
cesses in organic systems.
Photoacids are molecules that transform to strong acids upon
photoirradiation. They are promising materials for remote, spa-
tial, and temporal control of numerous proton-transfer pro-
cesses in nature. Irreversible photoacids are usually called pho-
toacid generators (PAGs). They photochemically change to
strong acids, for example, triflic acid, and generate a large
proton concentration. PAGs have been extensively studied as
photoinitiators for cationic polymerization and been applied in
photolithography.[1–5] Although some PAGs have reversible
pathways in their mechanisms,[6] the overall reactions are gen-
erally irreversible. In literature, photoacids often refer to mole-
cules that have high-acidity excited states, for example, deriva-
tives of naphthol. Excited-state photoacids have been studied
since 1970s.[7–11] They are reversible, because relaxation from
the excited state to the ground state changes these molecule
back to the low-acidity state. The lifetime of the proton-disso-
[a] Z. Wang, Dr. Y. Liao
Department of Chemistry, Florida Institute of Technology
Melbourne, FL 32901 (USA)
Fax: (+1)321-674-8951
E-mail: yliao@fit.edu
[b] Dr. V. K. Johns
Department of Chemistry, Eastern Kentucky University
Richmond, KY 40475 (USA)
[c] Dr. P. Peng, J. DeJesus
Department of Chemistry, University of Central Florida
Orlando, FL 32816 (USA)
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Communication
Scheme 1. a) Photoreaction of a merocyanine photoacid; b) known photore-
action that forms the TCF anion; c) photoreaction of 1; d) synthesis of 1.
Compound 1 is insoluble in water, but soluble in common or-
ganic solvents, such as ethanol, DMSO, and dichloromethane.
In ethanol, compound 1 showed a major absorption peak at
l=474 nm in the UV/Vis spectrum with a molar absorption co-
efficient of e=2.83ꢀ104 mꢀ1 cmꢀ1 (Figure 1). When the solution
was irradiated with a 470 nm LED light source (photon flux ca.
500 mmolm2 sꢀ1), the 474 nm peak sharply decreased, and
a peak at 314 nm appeared. The position of the appeared ab-
sorption peak is close to that of 4 (l=310 nm). In addition,
the behavior of the compound matches well with those de-
scribed in Scheme 1c, as will be described below. Therefore, it
was assigned to the anionic spiropyran 2. After the light was
switched off, the UV/Vis spectrum of 1 was recovered in about
5 min.
Figure 1. UV/Vis spectra of 1 in 100% EtOH before irradiation (b), 5 s after
irradiation (c; note that taking the spectrum took 5 s, during which time
some amount of 2 reversed to 1), and in 95% EtOH without irradiation
(a; top); and pH cycles of a 1.03ꢀ10ꢀ4 m solution of 1 (bottom).
To evaluate the photoinduced acidity, a 1.03ꢀ10ꢀ4 m solu-
tion of 1 in 95% ethanol was irradiated with a 470 nm LED
light (photon flux ca. 500 mmolm2 sꢀ1). Before irradiation, the
pH of the solution was 4.94. After irradiation for 5 min, the pH
dropped about one unit to 3.98 (Figure 1). Consistent with the
UV/Vis study, the photoinduced pH change was reversible.
Switching the light off resulted in reversal of the pH in about
5 min. The cycle can be repeated many times with no signifi-
cant fatigue. The pH value after irradiation was close to the
theoretical value for complete proton dissociation, indicating
that a strong acid was formed under irradiation. The pH before
irradiation (4.94) was lower than expected, because a pKa cal-
culated from the pH was 5.89, which is even much lower than
that of 4-nitro-phenol (7.16). Comparing the UV/Vis spectrum
of the solution in 95% ethanol with that in 100% ethanol
shows that only 1 can be observed in 100% ethanol, whereas
small amount of the strong acid 2 exists in the 95% ethanol
solution, which explains the relatively low pH value of the 95%
ethanol solution before irradiation (Figure 1).
In the dark, the high-acidity metastable state 2 changes
back to 1. Understanding the kinetics of this process is impor-
tant, because it allows to estimate the pH change with time
after irradiation, to obtain better understanding of the relaxa-
tion mechanism of 2, and to calculate the quantum yield of
the photoreaction as will be described below. If the rate-limit-
ing step of the reverse reaction (2+H+!1) involves protona-
tion of 2, and the majority of the protons in the solution come
from the proton dissociation of the metastable state (i.e.,
[H+]=[C2]), then the kinetics follows a second-order-rate equa-
tion:
2
d½C1ꢁ=dt ¼ k½C2ꢁ½Hþꢁ ¼ k½C2ꢁ
ð1Þ
If no other long-lived species is involved, [C2]=[C0]ꢀ[C1] (in
which [C0] is the total concentration), and Equation (1) can be
integrated to give Equation (2):
1=ð½C0ꢁꢀ½C1ꢁÞ ¼ kt þ 1=½C0ꢁ
ð2Þ
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Communication
in which [C2] is the concentration of the product 2, and k is the
rate constant of the reverse reaction, F is the quantum yield, I
is the photon flux, S is the irradiation area, and V is the volume
of the solution. The first term of the right side is the rate of
the forward photoreaction under total-absorption condition,
and the second term is the rate of the reverse reaction. As was
described above, k=9.78ꢀ10ꢀ2 mꢀ1
s
ꢀ1. Integrating Equation (3)
with the boundary conditions t=0, [C2]0 =0 gives Equation (4):
ð1=2 QÞln ½ðQꢀ½C2ꢁÞ=½ðQ þ ½C2ꢁÞꢁ ¼ ꢀkt
ð4Þ
p
in which Q= (FI/kl), and l=V/S.
Given [C2] and t, Equation (4) can be solved numerically to
give the quantum yield F. However, UV measurement took
about five seconds, during which time some product reversed
back. Given the large k, this factor cannot be ignored. There-
fore, [C2] immediately after irradiation was calculated from the
experimental [C2]exp by using the second-order-rate equation of
the reverse reaction, Equation (5):
1=½C2ꢁexp ¼ 1=½C2ꢁ þ kt0 ðin which t0 ¼ 5 sÞ
ð5Þ
Experimentally, ethanol solutions of 1 in quartz cells with
concentrations about 6ꢀ10ꢀ5 m (absorbance >1.5) were irradi-
ated with a 470 nm LED for 20, 40, and 60 s. The photon flux
was adjusted to be 50 mmolm2 sꢀ1. The concentrations of
1 before and after irradiation were calculated from the corre-
sponding UV/Vis absorption, and the change was used as
[C2]exp. The [C2] was calculated from [C2]exp by using Equa-
tion (5), and the quantum yield F was numerically solved by
using Equation (4). A multisample average of the quantum
yield was calculated to be F=0.238ꢂ0.036.
In summary, a metastable-state photoacid 1 based on
CF3PhTCF carbanion was designed and synthesized. The pho-
toacid reversibly changes from a weak acid to a strong acid
under visible light. Reversible pH change of about one unit
was demonstrated by using a 95% ethanol solution of 1.
Quantum yield of the photoreaction was carefully calculated
to be as high as 0.24. The kinetics of the reverse process can
be fitted well to a second-order-rate equation with k=9.78ꢀ
Figure 2. UV/Vis absorption of a solution of photoacid 1 (4.47ꢀ10ꢀ5 m) 4 s to
58 s after irradiation (scans were taken every 6 s; top), and the data fitted to
a second-order-rate equation (bottom).
102 mꢀ1 ꢀ1. The photoacid is well soluble in common organic
s
solvents. Unlike the merocyanine-type metastable-state photo-
acid, the low-acidity form of this photoacid is predominant in
organic solvents that are less polar than water in the dark,
which makes it useful for controlling the acidity of organic sys-
tems.
Experimentally, the kinetics was studied by monitoring the
reformation of 1 after irradiation by using UV/Vis spectroscopy
(Figure 2). UV/Vis spectra were collected every six seconds
after irradiation, and the data were fit into Equation (2). As
shown in Figure 2, the kinetics fits well into the second-order-
rate equation, and the rate constant was calculated to be
9.78ꢀ102 mꢀ1 sꢀ1.
Experimental Section
Fisher Scientific Accumet AR15 bench top meter and pH combina-
tion electrodes were used to measure the pH changes of the pho-
toacid. The pH measurement in 95% ethanol solutions was cali-
brated with different concentrations of HCl in 95% ethanol. UV/Vis
spectra were obtained from a Varian Cary 50 Scan UV/Vis spectro-
photometer. The light source was a 470 nm LED array with 56 LEDs
(Elixa Ltd.). The intensity of the light was adjusted by changing the
distance between the light source and the sample and/or blocking
Quantum yield is an important parameter for evaluating
a photoreaction. Measuring the quantum yield of 1 was not
straightforward due to the relatively fast reverse reaction. The
rate equation of the photoreaction can be expressed in Equa-
tion (3):
2
ð3Þ
d½C2ꢁ=dt ¼ FIS=Vꢀk½C2ꢁ
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some of the bulbs on the array. Photon flux was measured by an
Apogee MQ-200 quantum sensor.
Photoacid 1 was synthesized by using the reaction shown in
Scheme 1a. Starting material CF3PhTCF was synthesized by follow-
ing a literature method.[16] Thus, to CF3PhTCF (80 mg, 0.25 mmol)
was added a couple drops of dichloromethane, then ethanol
(2 mL). To this solution was added salicylaldehyde (31 mg, 1 equiv),
and the solution was gently heated at reflux under nitrogen over-
night. After reaction was complete, the solution was concentrated
to about 1 mL and kept in the freezer for one day. The product
was collected as a crystalline precipitate by filtration and washed
by approximately 1 mL of ice-cold methanol (55 mg, 52% yield).
1H NMR (500 MHz, [D4]MeOH): d=8.03 (d, J=16.3 Hz, 1H), 7.66–
7.60 (m, 5H), 7.46 (m, 2H), 7.34 (m, 1H), 6.90–6.85 ppm (m, 2H);
13C NMR (500 MHz, [D4]MeOH): d=177.03, 166.35, 160.73, 148.83,
136.09, 132.75, 131.56, 130.97, 130.77, 130.69, 128.06, 128.05,
124.72, 122.70, 122.44, 121.40, 117.55, 115.99, 111.69, 111.31, 111.06,
102.56, 60.79, 58.35, 18.38 ppm; HRMS (DART, MeOH solution):
calcd for 420.0882 [M+H]+, found: 420.0929; calcd for [M+
MeOH+H]+ 452.1144, found: 452.1143.
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Acknowledgements
Support from the Air Force Office of Scientific Research and
National Science Foundation is gratefully acknowledged.
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Keywords: carbanions · photoacids · photochemistry · UV/Vis
spectroscopy
Received: October 29, 2013
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Published online on December 6, 2013
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692
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim