Red-light responsive metastable-state photoacid

Article abstract of DOI:10.1016/j.dyepig.2019.107719

Proton concentration is a important factor for various biological functions, and thus a method that allows spatial and temporal modulation of proton concentration with light has great potential in biomedical areas. Metastable-state photoacids (mPAHs) can

Full text of DOI:10.1016/j.dyepig.2019.107719

DyesandPigments171(2019)107719
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Red-light responsive metastable-state photoacid
Osamah Alghazwat, Adnan Elgattar, Thaaer Khalil, Zhuozhi Wang, Yi Liao∗
Biomedical and Chemical Engineering and Sciences Department, Florida Institute of Technology, Melbourne, FL, 32901, USA
A B S T R A C T
Proton concentration is a important factor for various biological functions, and thus a method that allows spatial and temporal modulation of proton concentration
with light has great potential in biomedical areas. Metastable-state photoacids (mPAHs) can reversibly produce a large proton concentration under visible light. For in
vivo applications, the activating wavelength needs to be in the tissue penetration window (650–1350 nm). The commonly used approach for increasing the absorption
wavelength based on donor-acceptor structure failed to yield an mPAH responding to a wavelength in this range. In this work, we developed a novel mPAH with a
donor-acceptor-donor structure, which increases the absorption wavelength without deactivating the photoreaction. The mPAH reversibly released a proton under
660 and 700 nm light, and effectively transferred the proton to a weak base.
1. Introduction
trans-cis isomerization, and nucleophilic cyclization [1,27]. Conjugated
donor-acceptor (DA) structures are commonly used to develop mole-
cules with long-wavelength absorption. In fact, the previously reported
mPAHs can absorb visible light due to their DA structures [1]. It is well-
known that strong donors and acceptors can increase absorption wa-
velength. However, we found that strong donors tend to deactivate the
mPAHs. For example, using phenothiazine as the donor resulted in a
photo-inactive molecule although it showed significant absorption
above 650 nm [28]. Regarding the acceptor part, the commonly used
indolinium moiety is already a strong acceptor. Increasing its electron
accepting strength with electron withdrawing substituents such as nitro
or carboxylic groups stabilizes the cyclic acidic form of mPAH, resulting
in significant amount of the acidic form in the dark equilibrium [29].
Recently, Read de Alaniz and coworkers reported a polyene photo-
switch that undergoes cyclization under 650 nm light. It possesses a
strongly electron donating amino group on one end and a strongly ac-
cepting Meldrum's acid on the other end [30]. Notably, there is a
moderately electron donating hydroxyl group between the strong donor
and acceptor. Since the hydroxyl group is on the acceptor side of the
active double bond, the molecule actually has a DDA structure. Apra-
hamian group recently reported an Azo-BF2 switch, which responses to
730 nm light and undergoes trans-cis isomerization [31]. Woolley
group have done a in vivo activation of an azo compound using red light
(635 nm) [32]. However, we found that it is difficult to design an mPAH
based on the structures of these photoswitches.
It is well-known that proton concentration is critical for the activity
of enzymes and proton gradient is the energetic form used by mi-
tochondrion for ATP synthesis. Therefore, a method that allows spatial
and temporal modulation of proton concentration with light has great
potential in biomedical areas. Metastable-state photoacids (mPAH) are
the molecules that can produce a large proton concentration with high
efficiency and good reversibility under photoirradiation [1]. They can
be conveniently incorporated into different systems to control various
proton transfer processes. Applications of mPAHs in chemical, material,
energy and biomedical areas have been demonstrated over the past
years [2–21]. For example, Gray, Dougherty and coworkers showed
that ion channels associated with vision and pain can be reversibly
activated with light using an mPAH [20]. Li group demonstrated that
chloroplast entrapped with an mPAH produced 2.9 times more ATP
than natural chloroplast [2]. Chumbimuni-Torres group developed a
Calcium biosensor by incorporating mPAHs in polymer thin films [5,6].
Su group in collaboration with our group showed that the photoacidity
of an mPAH was enough to kill drug-resistant bacteria and assist the
antibacterial activity of Colistin [21]. Recently, our group showed that
pH pulses in a mPAH hydrogel can be repeated induced by light, even
when the hydrogel was placed in a pH buffer (PBS) [22]. For in vivo
application of mPAH, the activating wavelength must be in the tissue
penetration window (650–1350 nm). In fact, photosensitive molecules
responding to a wavelength in this window has been intensively studied
for drug delivery [23–26]. However, none of the previously reported
mPAHs can be activated by a wavelength in the tissue penetration
window.
Given the failures of the DA approach, we decided to try donor-
acceptor-donor (DAD) approach. Although DAD structures have been
widely used in the development of the polymers that absorb visible
light, it is not a common approach for designing small molecules with
long-wavelength absorption. The reason is that a DAD structure could
The photoreaction of an mPAH involves the absorption of light,
∗ Corresponding author.
E-mail address: yliao@fit.edu (Y. Liao).
https://doi.org/10.1016/j.dyepig.2019.107719
Received 25 June 2019; Received in revised form 11 July 2019; Accepted 15 July 2019
Availableonline16July2019
0143-7208/©2019ElsevierLtd.Allrightsreserved.

O. Alghazwat, et al.
DyesandPigments171(2019)107719
Scheme 1. The DAD structure of mPAH 1, (b) Photoreaction of mPAHs, and (c) synthesis of mPAH 1.
also be considered as a DA structure with a weaker acceptor, which
makes it difficult to predict whether the absorption wavelength can be
increase. What encouraged us to pursue the DAD approach is the work
by Martin and coworkers, which showed that photo trans-cis iso-
merization of some molecules with donor-donor (DD) structures are
better than that of similar molecules with DA structures [33]. This re-
sult together with the DDA photoswitch mentioned above made us
think that adding a donor to the acceptor part of an long-wavelength
absorbing mPAH could make it more photoactive.
take the proton released from the photoacid.
The synthetic route of mPAH 1 is also shown in Scheme 1. The 5-
anilino-2,3,3-trimethylindolenine was synthesized from N-phenyl-ben-
zene-1,4-diamine and 3-bromo-3-methyl-2-butanone following a lit-
erature procedure [34]. It was then reacted with propanesultone to
yield the zwitterionic indolinium. The indolium compound was heated
with 2-Hydroxy-9-methyl-9H-carbazole-3-carbaldehyde in methanol to
yield mPAH 1 as a purple precipitate. A small amount of ammonium
acetate was used as a catalyst. Given the photosensitivity, the reaction
was conducted in the dark. The procedures are given in the Experi-
mental section.
MPAH 1 is soluble in DMSO and alcohols. Dissolving mPAH 1 in
methanol or DMSO resulted in a dark purple solution (Fig. 1). The
UV–Vis spectrum showed a λmax at 557 nm, which is about 20 nm
longer than that of the mPAH without the phenylamino group. More
importantly, the absorption above the λmax gradually decreases and the
tail of the absorption band extends to near 800 nm. It has significant
absorption at 650 nm and small absorption at 700 nm. The molar ab-
sorptivities at 557 nm, 650 nm, and 700 nm are 4.1 × 104, 5.7 × 103,
and 6.9 × 102 L mol−1cm−1 respectively. Irradiation with a 660 nm
red-light LED diminished the absorption in visible range and increased
the absorption in UV range indicating the occurrence of the expected
photoreaction [1]. The most characteristic peak of the photoproduct is
2. Result and discussion
Therefore, mPAH 1 (Scheme 1) with a DAD structure was designed
and synthesized. Previously we found that using a carbazole (shown in
the structure of mPAH 1 in Scheme 1) as the electron donating nu-
cleophilic moiety resulted in a photoactive mPAH with a λmax as long as
538 nm [16]. However, the absorption above the λmax sharply de-
creased and there was little absorption above 600 nm. In this work, we
used the same carbazole structure as the electron donating nucleophilic
moiety. The electron accepting moiety is the commonly used indolium.
To the indolinium was added a phenyl amino group as the other elec-
tron donating moiety. It is worth mentioning that diphenylamine is a
very weak base with a pKa of 0.79. So the functional group does not
2

O. Alghazwat, et al.
DyesandPigments171(2019)107719
Fig. 1. UV–vis spectra of a methanol solution of mPAH 1 before, under, and 7 m after 660 nm irradiation [left, inserted figure: color change caused by irradiation],
and cycles of absorption change at 557 nm under irradiation and in the dark [right (for each cycle, the solution was irradiated with 660 nm light for 3 min and then
kept in the dark for 7 min)]. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
at 355 nm, which is close to that of the previously reported mPAHs
[1,27]. The color of the solution changed from purple to nearly color-
less. It is worth mentioning that the photoreaction occurred in DMSO
under 700 nm irradiation although the reaction was slow.
rate limiting step likely due to faster proton transfer in methanol than
that in DMSO [36].
The photo-induced proton release from mPAH 1 was demonstrated
by protonation of an acridine dye [16] (Fig. 3). This acridine dye
changes from orange to blue upon protonation and its λmax changes
from 440 nm to 604 nm. The pKa of its protonated form is about 5 [37].
As described above, proton transfer is fast in methanol. However, due to
the fast reverse reaction in methanol, we could not clearly record the
color change caused by photo-induced proton transfer between mPAH 1
and the acridine dye. Therefore, a mixture of methanol and DMSO
(volume ratio 2:1) was used as the solvent. A solution of mPAH 1
(3 × 10−5 M) and the acridine (1 × 10−4 M) was irradiated for 5 min.
The color of the solution changed from reddish brown to green (Fig. 3).
Before irradiation, UV–Vis spectrum showed the absorption of the ac-
ridine peaked at 442 nm, and the absorption of mPAH 1 peaked at
551 nm. The reddish brown color of the solution is a combination of the
purple color from mPAH 1 and the orange color from the acridine. After
irradiation, this absorption at 442 nm decreased from 1.53 to 1.04. The
reduction was about 1/3 of the absorption before irradiation. Given
The reversibility of an mPAH is important for many applications.
After the irradiation was removed, the absorption band of mPAH 1
gradually recovered to the original level (Fig. 1). The reversibility was
further tested by irradiating a solution of mPAH 1 for 3 min and then
keeping it in the dark for 7 min repeatedly for 10 cycles. As shown in
Fig. 1, there was no significant decrease of the absorption at 557 nm
after 10 cycles, which indicates good reversibility. Kinetic analysis
shows that the reverse reaction in DMSO was a 2nd order reaction with
a rate constant of 5.3 M−1 s−1 (Fig. 2). The 2nd order reaction implies
that recombination with the proton is the rate limiting step [35]. The
reverse reaction was also studied using a methanol solution of mPAH.
In this case, the data could not be fitted into a 2nd order equation. The
reaction followed
a first order kinetics with a rate constant of
7.8 × 10−3 s−1. The half life of the acidic state is calculated to be 89 s.
The kinetics indicates that recombination with proton is no longer the
Fig. 2. The 2nd order reverse reaction of mPAH 1 in DMSO (left) and 1st order reaction in methanol (right).
3

O. Alghazwat, et al.
DyesandPigments171(2019)107719
Fig. 3. UV–Vis spectra of a solution of mPAH 1 and an acridine dye before (purple) and after (green) irradiation (left), and the structures of the acridine dye and its
protonated form (right). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
that the concentration of mPAH 1 was also 1/3 of the concentration of
the acridine, the result shows that mPAH 1 efficiently protonated the
acridine upon irradiation. A new absorption band peaked at 602 nm
appeared due to the formation of the protonated acridine. The green
color of the solution after irradiation is due to a combination of the blue
color of the protonated acridine and the orange color of the non-pro-
tonated acridine.
spectrophotometer. NMR spectra were determined in deuterated sol-
vents on a Bruker av400 NMR spectrometer. Chemical shifts were re-
ported in delta (δ) units, parts per million (ppm) downfield from TMS.
The 660 nm LED array was purchased from < www.elixa.com > . The
source of 700 nm light was a house-made array of twenty 700 nm LED,
which was purchased from < www.mouser.com > . Photon flux was
measured by an apogee quantum meter.
The quantum yield of the photoreaction was measured by irra-
diating a solution of mPAH 1 in DMSO. As described above, the reverse
reaction is slow in DMSO. Therefore, we ignored the effect of the re-
verse reaction, which means the quantum yield reported here is a little
lower than the real value. Since our quantum meter cannot measure the
photon flux of 660 nm light, we used a 525 nm LED. The photon flux
was adjusted to be 25 μmol m2 s−1. UV–Vis spectra were quickly taken
after 3-min irradiation. The amount of the reacted mPAH 1 was cal-
culated from the decrease of the UV–Vis absorbance. The quantum yield
was calculated to be 0.7% from the photon flux and the amount of the
reacted molecules. The relative low quantum yield could be due to two
reasons. The phenyl amino group decreases the electrophilicity of the
indolinium group and disfavors the nucleophilic cyclization. In addi-
tion, it is known that a strong push-pull electron configuration can
significantly increase the rate of cis-trans isomerization [38]. Therefore,
the strong carbazole electron donor may shorten the life time of the cis-
conformer, which is the intermediate before the cyclization reaction. A
more reactive electrophilic and/or a nucleophilic moiety is required to
increase the quantum yield, which will be investigated in the future.
In summary, a novel mPAH that responds to red light in the tissue
penetration window has been developed. The DAD structure increases
absorption wavelength without deactivating the photoreaction. The
mPAH reversibly released a proton under 660 nm light. The quantum
yield of the photoreaction was measured to be 0.7%. Photo-induced
protonation of an acidochromic dye with a pKa of ∼5 was demon-
strated. Further studies are necessary to understand the detailed me-
chanism and improve the quantum yield.
3.2. Synthesis of the mPAH 1 (Scheme 1)
3.2.1. Synthesis of 5-anilino-2,3,3-trimethyl-indolium-1-(3-sulfopropyl),
inner salt
The
starting
material
5-anilino-2,3,3-trimethyl-indolenine
(0.6 mmol, 0.150 g) and 1,3-propane sultone (0.9 mmol, 0.109 g) were
dissolved in minimum amount of toluene and heated at 90 °C for 12 h.
The white precipitate was collected by filtration and washed with THF
to yield the final product. (0.19 g, 86% yield). 1H NMR (400 MHz,
DMSO‑d6): δ = 8.72 (s, 1H), 7.83 (d, 1H, J = 8.8 Hz), 7.38 (s, 1H), 7,30
(t, 2H, J = 7.6 Hz),7.15(d, 1H, J = 8.8 Hz), 7.16 (d, 2H, J = Hz), 6.95
(t, 1H, J = 7.3 Hz), 4.57 (t, 2H, J = 7.3 Hz), 2.74 (s, 3H), 2.60 (t, 2H,
J = 6.4 Hz), 1.75 (m, 2H), 1.49 (s, 6H).
3.2.2. Synthesis of mPAH 1
The indolium sulfopropyl compound synthesized in the last step
(0.134 mmol, 50 mg) and 2-hydroxy-9-methyl-9H-carbazole-3- carbal-
dehyde [16] (0.134 mmol, 30 mg) were dissolved in 1 mL of methanol.
Trace amount of ammonium acetate was added to the solution as the
catalyst. The mixture was heated at 60 °C overnight. The purple pre-
cipitate was collected by filtration and washed with cold methanol to
yield the final product. (45 mg, 66%). 1H NMR (400 MHz, DMSO‑d6):
δ = 11.04 (s, 1H), 9.21 (s, 1H), 8.73 (s, 1H) 8.63 (d, 1H, J = 16 Hz),
8.26 (d, 1H, J = 7.6 Hz), 7.87 (d, 1H, J = 16 Hz), 7.55 (d, 1H,
J = 8 Hz), 7.42 (m, 2H), 7,32 (t, 2H, J = 8.4 Hz), 7,24 (t, 2H,
J = 7.2 Hz), 7.17 (d, 2H, J = 7.6 Hz), 6.95 (d, 1H, J = 7.2 Hz), 6.92 (s,
1H), 4.75 (t, 2H, J = 8.4 Hz), 3.79 (s, 3H), 2.71(t, 2H, J = 6.4 Hz), 2.22
(m, 2H),1.76(s,6H). 13C NMR (400 MHz, DMSO) 176.66, 158.27,
146.79, 146.30, 144.95, 144.68, 142.17, 141.75, 133.24, 129.41,
125.70, 122.85, 122.39, 121.10, 120.54, 120.30, 117.96, 117.69,
116.22, 115.65, 115.26, 109.53, 109.39, 107.68, 94.43, 69.00, 50.87,
47.30, 44.68, 40.15, 29.19, 27.01, 24.46. HRMS (ESI): M+H+
(580.2280, cal. 580.2192).
3. Experimental section
3.1. General method
Unless otherwise noted, reagents and solvents were commercially
available and used as received without any further purification. UV–vis
spectra were obtained from
a Varian Cary 60 Scan UV–Vis
4

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DyesandPigments171(2019)107719
3.3. Quantum yield measurement
Udayabhaskararao T, Manna D, Klajn R. Nat Chem 2015;7:646–52.
[10] Zhang T, Sheng L, Liu J, Ju L, Li J, Du Z, Zhang W, Li M, Zhang XA. Adv Funct Mater
2018:1705532.
[11] Wang Z, Liao Y. Nanoscale 2016;8:14070–3.
A solution of mPAH 1 with a concentration of about 4 × 10−5 M in
DMSO was irradiated by a 525 nm LED. The photon flux was adjusted to
be about 25 μmol m2 s−1 by moving the LED to a certain distance. The
photo flux was measured by putting the probe of the quantum meter at
the same position as the sample. A low photon flux is necessary to as-
sume that all the photons are absorbed by the sample. UV–Vis spectra
were taken before irradiation and after 1, 2 and 3 min irradiation. The
amount of the mPAH 1 reacted was calculated from the product of the
decrease of the absorption at 557 nm, the extinction coefficient, and the
volume of the solution. The quantum yield was then calculated by di-
viding the amount of the reacted mPAH1 by the product of photon flux,
the irradiated area on the cell, and the irradiation time.
[12] Guo J, Zhang HY, Zhou Y, Liu Y. Chem Commun 2017;53:6089–92.
[13] Shi Q, Meng Z, Xiang JF, Chen, C.-F., 2018; 54:3536–3539.
[14] Yang L-P, Jia F, Cui J-S, Lu S-B, Jiang W. Org Lett 2017;19:2945–8.
[15] Tatum LA, Foy JT, Aprahamian I. J Am Chem Soc 2014;136:17438–41.
[16] Chen H, Liao Y. J Photochem Photobiol, A 2015;300:22–6.
[17] Mahvidi S, Takeuchi S, Kusumoto S, Sato H, Nakagawa T, Yokoyama Y. Org Lett
2016;18:5042–5.
[18] Samanta D, Galaktionova D, Gemen J, Shimon L, Petr Kral, Diskin-Posner Y, Avram
L, Kral P, Klajn R. Nat Commun 2018;9:641.
[19] Wang Z, Johns VK, Liao Y. Chem Eur J 2014;20:14637–40.
[20] Shafaat OS, Winkler JR, Gray HB, Dougherty DA. Chembiochem 2016;17:1323–7.
[21] Luo Y, Wang C, Peng P, Hossain M, Jiang T, Fu W, Liao Y, Su M. J Mater Chem B
2013;1:997–1001.
[22] Elgatta A, Abeyrathna N, Liao Y. J Phys Chem B 2019;123:648–54.
[23] Peterson JA, Wijesooriya C, Gehrmann EJ, Mahoney KM, Goswami PP, Albright TR,
Syed A, Dutton AS, Smith EA, Winter AH. J Am Chem Soc 2018;140:7343–6.
[24] Gorka AP, Nani RR, Schnermann MJ. Acc Chem Res 2018;51:3226–35.
[25] Al-Afyouni MH, Rohrabaugh TNJ, Al-Afyouni KF, Turro C. Chem Sci
2018;9:6711–20.
Acknowledgement
Support from the National Science Foundation (1565613) is grate-
fully acknowledged.
[26] Sun W, Li S, Haeupler B, Liu J, Jin S, Steffen W, Schubert US, Butt H-J, Liang X-J,
Wu S. Adv Mater 2017;29:1603702.
[27] Shi Z, Peng P, Strohecker D, Liao Y. J Am Chem Soc 2011;133:14699–703.
[28] Wang Z. PhD dissertation. Florida Institute of Technology; 2017.
[29] Liu J, Tang W, Sheng L, Du Z, Zhang T, Su X, Zhang SX-A. Chem Asian J
2019;14:438–45.
[30] Hemmer JR, Poelma SO, Treat N, Page ZA, Dolinski N, Diaz YJ, Tomlinson W, Clark
KD, Hooper JP, Hawker CJ, Read de Alaniz J. J Am Chem Soc 2016;138:13960–6.
[31] Yang Y, Hughes RP, Aprahamian I. J Am Chem Soc 2014;136:13190–3.
[32] Samanta S, Beharry AA, Sadovski O, McCormick TM, Babalhavaeji A, Tropepe V,
Woolley GA. J Am Chem Soc 2013;135:9777–84.
References
[1] Liao Y. Acc Chem Res 2017;50:1956–64.
[2] Xu Y, Fei J, Li G, Yuan T, Li Y, Wang C, Li X, Li J. Angew Chem Int Ed Engl
2017;56:12903–7.
[3] Go D, Rommel D, Liao Y, Haraszti T, Sprakel J, Kuehne AJC. Soft Matter
2018;14:910–5.
[4] Bao H, Li F, Lei L, Yang B, Li Z. RSC Adv 2014;4:27277–80.
[5] Johns VK, Patel PK, Hassett S, Calvo-Marzal P, Qin Y, Chumbimuni-Torres KY. Anal
Chem 2014;86:6184–7.
[6] Patel PK, Chumbimuni-Torres KY. Analyst 2016;141:85–9.
[7] Fu C, Xu J, Boyer C. Chem Commun 2016;52:7126–9.
[8] Maity C, Hendriksen WE, van Esch JH, Eelkema R. Angew Chem Int Ed
2015;54:998–1001.
[33] Martin RE, Bartek J, Diederich F, Tykwinski RR, Meister EC, Hilger A, Luthi HP. J
Chem Soc 1998;2:233–42.
[34] Sasaki N, Chichiishi K, Fujita S, Iwasaki Y. US 2004;6716993.
[35] Johns VK, Wang Z, Li X, Liao Y. J Phys Chem A 2013;117:13101–4.
[36] Pines E, Fleming GR. J Phys Chem 1991;95:10448–57.
[37] Lindauer H, Czerney P, Grummt UW. J Prakt Chem 1994;336:521–4.
[38] García-Amorós J, Velasco D. Beilstein J Org Chem 2012;8:1003–17.
[9] Kundu PK, Samanta D, Leizrowice R, Margulis B, Zhao H, Borner M,
5