Light-Switchable Buffers

Article abstract of DOI:10.1002/anie.202109250

A visible light-switchable buffer system based on a merocyanine photoacid is presented. Para-substitution of the indolium side with a methoxy group affords a compound suitable for making hydrolytically stable aqueous buffers whose pH can be tuned between 7 and 4 using 500 nm light.

Full text of DOI:10.1002/anie.202109250

Angewandte
Communications
Chemie
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International Edition: doi.org/10.1002/anie.202109250
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Molecular Switches
Hot Paper
Light-Switchable Buffers
Cesare Berton, Daniel Maria Busiello, Stefano Zamuner, Rosario Scopelliti, Farzaneh Fadaei-
Tirani, Kay Severin, and Cristian Pezzato*
Abstract: A visible light-switchable buffer system based on
a merocyanine photoacid is presented. Para-substitution of the
indolium side with a methoxy group affords a compound
suitable for making hydrolytically stable aqueous buffers
whose pH can be tuned between 7 and 4 using 500 nm light.
S
ince the introduction of the pH scale and the word “Puffer”
(in German) into the chemical lexicon by Søren P. L. Sør-
ensen[1] in 1909, buffer solutions constitute an essential tool in
the study of aqueous (bio)chemical systems.[2] By definition,
buffers can better resist pH changes when their actual pH
equals the pKa of the acid-base pair they are composed of—
i.e., their buffer capacity is maximized.[3] Herein, we intro-
duce the possibility of making buffer solutions whose pH can
be set at different constant values according to a light-
triggered acid-base pairꢀs pKa shift.
Figure 1. a) Four-state model describing the operation of MCHs in
water. Under dark conditions, dissociation (Ka) of MCH is followed by
Protonated merocyanines (MCHs) are spiropyran com-
pounds featuring negative photochromism.[4] They are com-
monly referred to as “metastable-state photoacids”[5]—i.e.,
chemical species whose proton photo-dissociation is persis-
tent enough to be coupled efficiently and reversibly with
proton transfer reactions in solution.[6] For this reason, MCHs
are attracting increasing attention across multiple research
fields spanning molecular machines,[7] supramolecular
chemistry,[8] nanotechnology[9] and materials science.[10]
Recently, we studied a series of MCHs bearing alkyl-1-
sulfonate bridges of variable length, providing insights on
their thermodynamics and kinetics in water (Figure 1a)
through cross-validation of 1H NMR, UV/Vis, and pH
measurements.[11] We showed that, under dark conditions,
MCHs are organic weak acids displaying a ground state
acidity constant close to neutrality (pKaGS = 6–7), whereas,
isomerization (Kc) of MC to the corresponding SP form—i.e., KaGS
=
Ka(1+Kc). Under steady light irradiation, photoproduct cis-MCH under-
goes ring-closing with concomitant release of a proton (KaMS). Liao’s
photoacid and all compounds discussed here bear a propyl-1-sulfonate
group (n=2). b) Effect of buffering on 500 nm light-triggered pH
jumps reported in this work.
under steady light irradiation, they became more acidic by ca.
4 pK units (pKaMS = 2–3).[11] The extent of proton release
(DpH) that a given MCH can achieve in solution results from
a
delicate interplay between its photoacidity (P =
GS
pKa ꢀpKaMS), quantum yield (F) and solubility, and usually
lays within 1.5 pH units. Here, we expand our experimental
and theoretical approaches to methoxy-substituted MCHs,
dissecting the possibility of achieving reversible pH changes
as high as P, by using buffered solutions (Figure 1b)—i.e.,
MCHsꢀ aqueous solutions whose dark equilibrium composi-
tion is not quantitatively shifted towards the undissociated
form, MCH. In particular, we will show that the pH in the
dark can be set at different constant values according to the
Henderson-Hasselbalch equation and that DpH can be
controlled—from 1 to 3 pH units—by carefully tuning the
intensity of the light source.
[*] C. Berton, Dr. R. Scopelliti, Dr. F. Fadaei-Tirani, Prof. K. Severin,
Dr. C. Pezzato
Institut des Sciences et Ingꢀnierie Chimiques, ꢁcole Polytechnique
Fꢀdꢀrale de Lausanne (EPFL)
1015 Lausanne (Switzerland)
E-mail: cristian.pezzato@epfl.ch
Dr. D. M. Busiello, Dr. S. Zamuner
Institut de Physique, ꢁcole Polytechnique Fꢀdꢀrale de Lausanne
(EPFL)
Working with MCHs in aqueous environments can be
problematic due to their tendency to hydrolyze irreversibly.[12]
MCHs hydrolyze with a mechanism similar to that of Schiff
bases (Figure 2a).[11] We reasoned that ortho- or para-
substitution of either the indolium or the chromene moiety
with electron-donating groups might increase MCHsꢀ hydro-
lytic stability by disfavoring water nucleophilic attack. There-
fore, we decided to quest for favorable mesomeric effects by
screening four different MCHs bearing a methoxy group
directly conjugated to the ene-iminium core. Compounds 1–4
were synthesized according to standard literature procedures
1015 Lausanne (Switzerland)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202109250.
ꢂ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution Non-Commercial
NoDerivs License, which permits use and distribution in any
medium, provided the original work is properly cited, the use is non-
commercial and no modifications or adaptations are made.
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comparable photochemical performance (see below). As for
compound 4, the MC Ð SP isomerization was so fast that we
were not able to determine any thermodynamic and kinetic
constant under the same conditions. An alternative way to
assess kꢀ2 is to follow thermal relaxation kinetics after
irradiation. The observed rate constant of relaxation in the
MS
dark matches kꢀ2 in the pH range between pKa and pKa—
i.e., at around pH 5 (see SI for more details).[11] Therefore, we
set up kinetic analyses at pH 5.2, monitoring the recovery of
the corresponding MCH form at 258C after light irradiation
(Figure S3). We found that 1 undergoes quantitative photo-
isomerization (> 99%) within a few milliseconds, whereas 4
isomerizes to a dramatically lesser extent (ca. 13%). The
observed rate constants of relaxation in the dark were found
to be 0.0043 sꢀ1 for 1—in good agreement with the kꢀ2 value
estimated at pH 9.5—and 4.5 sꢀ1 for 4. Regrettably, this
means that the half-life of proton photo-dissociation of
compound 4 is nearly three orders of magnitude shorter
than that of 1 (milliseconds vs. seconds). It follows that 4
would operate similarly to 1 only by using more intense light
sources, which however we do not have. For this reason, we
continued our studies focusing on compound 1.
Figure 2. a) Mechanism of hydrolysis of Liao’s photoacid below pH 9
(see ref. [11]). b) Profiles of apparent first-order rate constant of
hydrolysis of Liao’s photoacid (blue), compound 1 (grey), 2 (green), 3
(red), and 4 (black) as a function of the pH. Solid black lines represent
the best fits to Equation (S1). c) Extrapolated kinetic parameters for
nucleophilic addition of water (top) and decomposition of the tetra-
hedral intermediate (bottom). Experimental conditions:
Acid dissociation constants of 1 in the dark and under
steady light irradiation were determined by UV/Vis spectros-
copy, probing the dark equilibrium composition or the
photostationary state at increasing pH values. According to
the MC Ð SP isomerization studies described above, all
spectra in the dark were recorded after an equilibration time
of at least 15 minutes at 258C. In fact, only by doing so the
obtained signals refer to an equilibrium situation. Under dark
conditions, UV/Vis pH titrations of 1 (Figure 3a) gave an
apparent pKaGS value of 6.95 ꢁ 0.03. This result was confirmed
[1–4]=25ꢁ2 mM, [phosphate buffers]=20 mM, T=258C.
and characterized by common techniques including X-ray
crystallography[13] (see SI for more details). We studied their
hydrolytic stability as a function of the pH in water by UV/Vis
spectroscopy, monitoring the decay of the corresponding
MC(H) form at 258C (Figure S1). The resulting hydrolysis
profiles are reported in Figure 2b together with that (high-
lighted in blue) previously obtained for Liaoꢀs photoacid,
which we take as reference.
by 1H-NMR analyses, which revealed that 1 features a pKa =
GS
7.4 ꢁ 0.1 along with a Kc = 1.4 ꢁ 0.1, resulting in a pKa
=
In all cases, Equation (S1) fits the obtained bell-shaped
profiles with good confidence, confirming that the rate-
determining step of MCHsꢀ hydrolysis changes with pH from
decomposition of a tetrahedral intermediate (II) to nucleo-
philic addition of water (I) (see SI for more details).
Inspection of the obtained kinetic parameters (Figure 2c)
revealed that water nucleophilic addition proceeds most
slowly in the case of 1 (grey) and that the intermediate reacts
favorably—either backward or forward—only in the case of 4
(black). These preliminary findings indicate that para-sub-
stituted MCHs are better able to resist hydrolysis over the
entire pH window tested. Thus, we decided to continue our
investigations with compounds 1[14] and 4[12] for practical
convenience.
7.0 ꢁ 0.1 (Figure S5). On the other hand, photochemical
studies under steady light irradiation showed that 1 displays
slightly different photostationary states depending on the
wavelength of irradiation, featuring an apparent pKaMS value
decreasing from 3.43 ꢁ 0.04 at 425 nm (Figure S6) to 3.31 ꢁ
0.04 at 500 nm (Figure 3b). These studies show that the
photoacidity of compound 1 (P500 = 3.6 ꢁ 0.1) is similar to
that of Liaoꢀs photoacid (P425 = 3.7 ꢁ 0.1[11]) but more shifted
towards the neutral region by ca. 1 pK unit, and that 1 can be
easily activated by exposure to visible light of longer wave-
lengths.
We then examined the reversible proton release of
compound 1 in water. pH readings at equilibrium in the
GS
dark were fully consistent with pKa and the total concen-
We moved on studying the MC Ð SP isomerization,
monitoring the decay of the corresponding MC form at 258C.
Kinetic studies were carried out at pH 9.5, where competing
hydrolytic processes are limited, and the dark equilibrium is
reduced to the MC and the SP forms only. In the case of 1, we
determined an isomerization constant Kc = 1.46 ꢁ 0.07 (Fig-
ure S2). This value indicates that 1 has less tendency to
equilibrate towards SP in the dark as compared to Liaoꢀs
photoacid, whose Kc lays around 9 ꢁ 1.[11] However, the rate
constant of back-isomerization (kꢀ2) was found to be similar
(0.0044 sꢀ1 vs. 0.0045 sꢀ1), suggesting that 1 may possess
tration of 1 in solution. Simultaneous data fitting of our four-
state model to three independent variable-power pH-jump
experiments carried out using either 425 or 500 nm light
sources (Figures S8) revealed that the quantum yield does not
change significantly by shifting the wavelength of irradiation,
yielding an average value of F425 ꢂ F500 = 0.40 ꢁ 0.03. This
value is close to the one obtained for Liaoꢀs photoacid by
Coudret and co-workers[15] (F436 = 0.38 ꢁ 0.03) and confirms
the hypothesis that 1 does display comparable photochemical
performance.
2
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Figure 4. a) Apparent solubility of compound 1 in the dark as a func-
Figure 3. UV/Vis spectra of 1 in the dark (a) and under steady light
irradiation (b) as a function of the pH. The obtained absorbance (A)
profiles of MCH (437 nm), MC (534 nm), cis-MCH (375 nm), and SP
(225 nm) are reported on the right. Solid black lines represent the best
fit to Boltzmann’s sigmoidal equation. Experimental conditions:
[1]=24ꢁ2 mM, [phosphate buffers]=20 mM, T=258C. In the case of
(b), spectra were acquired using a 500 nm LED-light source (100 mW);
its emission spectrum is highlighted using the corresponding RGB
color code.
tion of the equivalents of NaOH (a) added to the system. b) The pH
of the resulting buffer solutions at equilibrium in the dark as a function
of a; solid black lines represent the best fit to Equation (1) and (2),
respectively. c) Reversible jumps of a solution of 1 at pH=pKaGS
(a=1). d) pH tuning by modulation of the light source power.
Experimental conditions: T=258C, 500 nm LED-light source: (c)
195 mW and (d) 40.0, 8.2 and 7.0 mW.
GS
ð2Þ
pH ¼ pKa þ logðaÞ
We moved on assessing the possibility of controlling
reversible pH changes in buffered solutions. First, we wanted
to see whether and how the solubility of 1 changes as
a function of the equivalents of base added to the system—
indicated hereafter with the Greek symbol a. Solubility
experiments were carried out stirring an excess amount of
crystals into aqueous KCl containing HCl (1 mM) or increas-
ing sub-stoichiometric amounts of NaOH. These experiments
were performed at a constant temperature of 258C until
stabilization of the pH (Figure S9). Compared to Liaoꢀs
photoacid, we observed that the solubility of the undissoci-
ated form 1-MCH (SMCH) is higher, but still remains in the
sub-millimolar regime (0.37 mM vs. 0.19 mM). Interestingly,
however, we found that the total amount of 1 (Stot) that can be
dissolved in aqueous KCl increases linearly with a (Figure 4a)
according to the following equation:
All in all, these observations show that the pH in the dark
(and so the hydrolytic stability, see Figure 2b) can be
controlled and raised at will without diminishing the concen-
tration of the photoactive form (1-MCH). Independent pH-
jump experiments showed that DpH increases significantly
with a, passing from 1.5 to 3.0 pH units from a = 0 to a = 1.
Importantly, the obtained profiles of proton release/uptake
were fully in line with model predictions (Figure S11). In the
GS
case of the buffer solution at pH = pKa we were able to
trigger fully reversible jumps of 3 pH units for 2 consecutive
hours without significant loss (Figure 4c). In a separate
experiment, we also demonstrated that the amplitude of the
pH jump can be controlled at will by modulating the intensity
of the light source (Figure 4d).
The results presented here show that our four-state model
can be extended to describe MCHsꢀ proton release/uptake in
buffered systems. We believe our findings hold promise for
implementing MCHs in the remote control of any aqueous
acid-sensitive (bio)chemical system with visible light.
Stot ¼ SMCHð1 þ aÞ
ð1Þ
We ascribed the occurrence of such a linear regime to
a situation where the dissociated forms 1-MC and 1-SP are
effectively more soluble than 1-MCH, resulting in a Le
Chꢁtelier shift that maintains SMCH independent from a.
Acknowledgements
The pH values of the resulting buffer solutions at
equilibrium were found to be in good accordance (Figure 4b)
with those predicted by the Henderson-Hasselbalch approx-
imation,[16] which, after deriving the expression for the
concentration of both the dissociated and undissociated
forms from Equation (1), can be rewritten as:
This work was supported by the Swiss National Science
Foundation (SNSF “Ambizione” PZ00P2_180008). We thank
Prof. Paolo De Los Rios, for his support and invaluable
discussions.
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Conflict of Interest
The authors declare no conflict of interest.
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graphic data for this paper. These data are provided free of
charge by the joint Cambridge Crystallographic Data Centre and
Fachinformationszentrum Karlsruhe Access Structures service
www.ccdc.cam.ac.uk/structures.
Keywords: buffers · molecular switches · photoacidity ·
protonated merocyanines · water
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Manuscript received: July 12, 2021
Accepted manuscript online: August 12, 2021
Version of record online: && &&, &&&&
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Communications
Molecular Switches
We show that protonated merocyanines
can act as reversible photoacids even
when buffered with NaOH, attaining
tunable and fully reversible jumps of up
to 3 pH units between pH 7 and 4.
C. Berton, D. M. Busiello, S. Zamuner,
R. Scopelliti, F. Fadaei-Tirani, K. Severin,
C. Pezzato*
&&&& — &&&&
Light-Switchable Buffers
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