Switching a pH indicator by a reversible photoacid: A quantitative analysis of a new two-component photochromic system

Article abstract of DOI:10.1016/j.dyepig.2015.10.025

A UV/visible and pH-metric quantitative analysis of the spiropyran reversible photoacid 1-MEH allowed us to determine the pK_a's of the open and closed forms, the rate constants of relaxation, the quantum yield of decolouration and the UV/visible spectra of the protonated and deprotonated open and closed isomers. The coupling between 1-MEH and a selected pH-indicator acidochromic dye via proton exchange has also been analysed. The initial pH, stoichiometric ratio between the photoacid and the pH-indicator, light intensity and pK_a range of the pH-indicator have been established to maximize the proton transfer from the photoacid donor to the pH-indicator acidochromic dye acceptor. When the pK_a's are matched, the two-component system behaves like a single photochromic dye. The proposed model explains why 1-MEH photo-switches BromoCresol Green but not Methyl-Orange.

Full text of DOI:10.1016/j.dyepig.2015.10.025

Dyes and Pigments 125 (2016) 179e184
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Switching a pH indicator by a reversible photoacid: A quantitative
analysis of a new two-component photochromic system
*
J. Vallet, J.-C. Micheau, C. Coudret
Laboratoire des IMRCP, UMR 5623, Universit ꢀe Paul Sabatier (Toulouse III), 118, route de Narbonne, F-31062, Toulouse Cedex 4, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 September 2015
Received in revised form
A UV/visible and pH-metric quantitative analysis of the spiropyran reversible photoacid 1-MEH allowed
us to determine the pK 's of the open and closed forms, the rate constants of relaxation, the quantum
a
yield of decolouration and the UV/visible spectra of the protonated and deprotonated open and closed
isomers. The coupling between 1-MEH and a selected pH-indicator acidochromic dye via proton ex-
change has also been analysed. The initial pH, stoichiometric ratio between the photoacid and the pH-
20 October 2015
Accepted 22 October 2015
Available online 30 October 2015
indicator, light intensity and pK
proton transfer from the photoacid donor to the pH-indicator acidochromic dye acceptor. When the pK
a
range of the pH-indicator have been established to maximize the
's
a
Keywords:
are matched, the two-component system behaves like a single photochromic dye. The proposed model
explains why 1-MEH photo-switches BromoCresol Green but not Methyl-Orange.
© 2015 Elsevier Ltd. All rights reserved.
Photochromism
pH photocontrol
Photoacid
Chemical dynamics
Coupled reactions
Spiropyran
1. Introduction
have designed a simple water-soluble spiropyran merocyanine
derivative (Scheme 1).
Irreversible photoacids such as cationic onium salts [1], tri-
azines [2], arylsulfonyl esters [3] or caged sulfate [4] release a
strong acid upon photolysis. They find their main applications as
initiators and resists [5]. On the other side, reversible photoacids
Because of its easy accessibility, 1-MEH is now the centre of a
growing interest. For instance, it was recently used to switch a
water-soluble pH-responsive triethylene glycol monomethyl ether
of an hydrazone [17]. This result is particularly appealing since it
provides an experimental attempt to couple a reversible photoacid
with a pH-responsive equilibrium. Actually, 1-MEH has been sub-
mitted to some physicochemical studies, but only two significant
parameters have been obtained namely the quantum yield of
(
such as hydroxypolyaromatics derivatives [6]) exhibit in their
excited states, a lower pK than in their corresponding ground
a
state. Their reversibility is interesting since the proton accumula-
tion disappears when the light irradiation is switched-off. How-
ever, due to their very short lifetimes (from 10e7 to 10e9 s), the
released protons cannot accumulate in large amount under low
power continuous irradiation. In order to favour proton accumu-
lation under low power continuous irradiation, their release must
be coupled with a slow isomerisation process. Beside some
naphthol derivatives [7], long-lived reversible photo-acids have
been designed by modifying already existing photochromic mol-
photoreaction in DMSO [18] (Phi ¼ 0.37) and the pK
a
of 1-MEH
(pK
a
¼ 7.8) [14].
In the present paper, we want to show how should be matched
the properties of a target proton receiver to those of the 1-MEH
photoacid emitter in order to maximize the extent of photoinduced
proton transfer between both partners. In a first part, we have
modelled the behaviour of the 1-MEH photoacid in water. Our
approach is based on the proposition of a minimal but realistic
global chemical network. Reproduction of all the experimental
results in various situations via numerical fittings is then used to
confirm the plausibility of the above proposed network. The
different parameters obtained (rate constants, equilibrium con-
stants, spectral data) were then used to analyse the optimal con-
ditions to maximize the proton transfer yield from 1-MEH to an
arbitrarily chosen pH-sensitive dye. These theoretical results were
ecules such as 2-hydroxyazobenzenes compounds [8e10]. pK
a
photomodulation has also been observed in the spiro-compounds
family [11e14]. In polar acidic medium, spiropyrans behave like
long-lived reversible photoacids. Accordingly, Liao et al. [15,16]
*
Corresponding author.
E-mail address: coudret@chimie.ups-tlse.fr (C. Coudret).
http://dx.doi.org/10.1016/j.dyepig.2015.10.025
143-7208/© 2015 Elsevier Ltd. All rights reserved.
0

180
J. Vallet et al. / Dyes and Pigments 125 (2016) 179e184
(pss) which was reached using non filtered high power white light
irradiation. This pss spectrum is thus close to a mixture of the two
spiropyranes forms 1-SPH and 1-SP. During the thermal back-
reaction in the dark, an isosbestic point (quasi the same as on
Fig. 1A) insures that there is no accumulation of any significant
absorbing intermediate. This process is globally the reverse to that
witnessed under irradiation: there is a gradual decrease of the
proton concentration. Interestingly, here also the re-captured pro-
tons/recovered dye ratio is around 0.46. Such a value, less than
unity, confirms the previous assumption of the coupling between
two weak acid systems (1-MEH/1-ME vs 1-SPH/1-SP). Fig. 2B dis-
plays the simultaneous curve fitting of four selected wavelengths
together with the evolution of the proton concentration during the
thermal re-colouration in the dark (vide infra and Scheme 2 for the
details).
Scheme 1. Long-lived reversible 1-MEH photoacid. Protonated merocyanin isomer is
labelled “MEH”, while “SP” refers to the deprotonated spiropyran.
then applied to the experimental analysis of the photo-switching of
two pH-indicators. As predicted, 1-MEH was found to be able to
switch the BromoCresol Green pH-indicator but failed with Methyl-
Orange.
In the same way, the relaxation after KOH addition has been
carefully analysed. As shown on Fig. 3, there is an instantaneous
appearance of a spectrum likely to be due to the deprotonated
isomer 1-ME that decays with time. At the end of the relaxation,
residual absorbance at 540 nm indicates the presence of some
lasting 1-ME. The size and the shape of this final spectrum depend
on the amount of KOH equivalent added. Addition of an excess of
HCl restores the initial 1-MEH spectrum. We thus attributed the
observed features to the thermal ring-closure of the 1-ME form to
the colourless 1-SP one.
As it can be seen on Figs.1e3, typical monitored transformations
last for tens of minutes: it is therefore legitimate to consider that all
acido-basic equilibria (involving only proton exchanges with the
solution) are always reached during the photochemical trans-
formation. Furthermore, it is known that the isomerization
sequence (SP / ME) involves several short-lived intermediates
2
. Determination of the number of pertinent species and
establishment of a minimal kinetic network
2.1. Pertinent species
We have monitored the pH variations upon 365 nm irradiation of
a non-buffered aqueous solution of 1-MEH and observed that as
previously described, the bleaching of the solution (Fig. 1A) was
accompanied by a detectable acidification (Fig.1B). When the photo-
steady state (pss) was reached it was found as illustrated on Fig. 1A,
that a significant amount of the initial 1-MEH remained, revealing
the thermal reversibility of the photoisomerization. It is then ex-
pected that the photoconversion at the pss will depend on the 1-
MEH initial concentration, the light intensity and the wavelength of
irradiation. Fig. 1B shows the relationship between the proton
release and the 1-MEH photoconversion. From these data, it appears
that the released proton/converted dye ratio is only 0.49. This un-
expected result indicates that another species scavenges a part of the
released protons. As the decolouration is usually interpreted by the
photochemical conversion of the merocyanine into the spiropyrane
[19,20] that could be probed only by ultrafast techniques. Conse-
quently, we have considered that these transients were not kinet-
ically meaningful on the overall used time scale. Similar
characteristics can be foreseen for the MEH/SPH processes. An
analogous approach was used for the photochemical trans-
formation: the very fast elementary steps were considered to be
hidden into an apparent global photochemical process (see Supp.
info. part for a short discussion about the assumed fast photo-
chemical processes involving the 1-MEH species as reactant).
(
having a basic indoline nitrogen), we attributed this buffering effect
to the building up of the SP form. This thus introduces a new acid-
obasic couple: 1-SPH/1-SP (see Scheme 2 for the structures).
When the light was turned-off, a colour recovery of the 1-MEH
occurred. Fig. 2A displays the corresponding thermal evolution
after irradiation. In this particular case, the starting point of the
Therefore the only photochemical process considered is the
þ
following 1-MEH þ h
n
/ 1-SP þ H , keeping in mind that the
þ
equilibrium K
1
: 1-SP þ H ¼ 1-SPH is always reached. This
evolution was
a
high photoconversion photosteady state
1
.4
.2
1
a
A
B
8
6
10
10
1
hν
[
1-MEH]
0.8
0.6
0.4
0.2
+
[
H ]
4 10
2
10
pss
0
2
0
00 250 300 350 400 450 500 550 600
0
20
40
60
time (s)
ꢁ6
80
100
wavelength (nm)
ꢁ
5
ꢁ1
ꢁ1 ꢁ1
Fig. 1. A: Decolouration of 1-MEH (8.2 ꢀ 10 mol.L in aqueous solution) under 365 nm irradiation; I
0
(365 nm) ¼ 2.5 ꢀ 10 mol L .s ; a: before irradiation; pss: photo-steady
þ
state after 500 s of irradiation; (
D
t ¼ 30 s). B: Independent experiment showing the simultaneous [1-MEH] and [H ] monitoring under 436 nm light irradiation in aqueous solution.
(436) ¼ 4.1 ꢀ 10ꢁ mol L .s . pH(initial) ¼ 5.3; pH(pss) ¼ 4.39. Continuous lines are from the numerical fitting by the model (see below and supp. info. part).
6
ꢁ1 ꢁ1
I
0

J. Vallet et al. / Dyes and Pigments 125 (2016) 179e184
181
Scheme 2. Global kinetic scheme used in this work: A: Structural changes of the reversible photoacid 1-MEH with or without irradiation with visible light. B: independent pH-
sensitive equilibrium of an acidochromic dye. Networks A and B are merged for proton transfer experiments.
apparent photochemical transformation is also the one that has
been assayed directly on Fig. 1. Indeed this figure allows also a
rough graphic estimation of the quantum yield of 1-MEH photo-
values of the open and closed forms and the quantum yield of
decolouration as defined above (Table 1). This calculation also
helped us to confirm the molar extinction coefficients of the species
from which the whole spectra of the four species have been re-
calculated (Fig. 4).
decoloration. Thus from the slope of the decay,
t*I z 0.36 (i.e. close to 0.38 ± 0.03: the value found from the
curve fitting, vide infra).
F z D[1-MEH]/
D
0
Contrarily to the initial report, the 1-SPH form is not a strong
The simplest global kinetic network that takes into account all
these experimental observations is thus the following (Scheme 2A).
a
acid as a pK z 4.3 was determined. Upon irradiation, the
appearance of the closed form (1-SPH), more acidic by z 3.5 units
than the initial 1-MEH dye, and the change of composition of the
mixture explain the photoacid behaviour. Speciation diagrams in
the dark and at the pss are displayed on Fig. 4S in the
Supplementary information part. Results show that the revers-
ibility of the 1-MEH photoacid is not solely due to the ring-opening
2.2. Analysis of the kinetic network
Using homemade software, the consistency of the above pro-
posed network was assayed by fitting a set of independent exper-
imental kinetics. Results of such fits are displayed as solid lines in
Fig. 2B and in Fig. 3S in supp. info. part. A stable set of parameters
of the protonated forms (1-SPH / 1-MEH (k
change between the open and closed deprotonated isomers (1-SP
/1-ME (k ; k )). Any attempt to remove one parameter or to fix
6
)) but also to an ex-
)
4
5
i a
has been extracted, namely the relaxation rate constants k , pK
ꢁ
5
ꢁ1
Fig. 2. A: Spectral evolution (
D
t ¼ 20 s) of 1-MEH (8.5 ꢀ 10 mol.L in aqueous solution) during the thermal relaxation after white light irradiation; pss: photo-steady state; d:
þ
equilibrium state in the dark. B: Absorbances and [H ] monitoring (pH from 4.22 to 5.40) during the thermal relaxation. Continuous lines are from the numerical fitting of the model
shown on Scheme 2 (part A).

182
J. Vallet et al. / Dyes and Pigments 125 (2016) 179e184
1
1
,4
,2
1
Open forms absorptions spectra are dominated by a low energy,
protonation sensitive band that shifts from 540 nm up to 420 nm
for respectively 1-ME and 1-MEH. On the other hand, spiropyrane
closed forms exhibit an absorption band around 300 nm, the
deprotonated form 1-SP being more intense than the 1-SPH acid.
Remarkably, their maxima are poorly affected by protonation: this
can be interpreted by the lack of any substitution on the indoline
moiety. This also is in agreement with the observed isosbestic
points in Figs. 1 and 2. All the compounds absorb at 260 nm, the
deprotonated forms (1-SP and 1-ME) being the strongest.
a
b
c
0
0
0
0
,8
,6
,4
,2
0
3
. Modelling of the coupling between 1-MEH and a pH-
indicator acidochromic dye
With the parameters in hand, we used kinetic numerical
modelling to analyse “in silico” the coupling between the 1-MEH
photoacid and a pH-indicator 2-AH according to the complete
(A þ B) kinetic network from Scheme 2.
2
50 300 350 400 450 500 550 600
Lambda(nm)
ꢁ
5
ꢁ1
Fig. 3. A: Spectral evolution (
solution) after addition of 1 equivalent of strong base ([OH ]
Note the appearance of the transient spectrum at 534 and 376 nm a: before base
addition or after HCl acidification; b: just after base addition; c: basic equilibrium.
D
t ¼ 10 s) of 1-MEH (8.1 ꢀ 10 mol.L in aqueous
ꢁ ꢁ5 ꢁ1
0
¼ 8.1 ꢀ 10 mol.L ).
3
.1. Determination of the optimal pK
a
range
Several constraints were examined: the pK
a
values of the pH
indicator 2-AH (pKind), the initial pH, the stoichiometric ratio 1-
MEH/2-AH and the light intensity. Such a situation involving the
coupling of several thermal and photochemical equilibria under
non-equilibrium conditions (i.e. at the photosteady state) has been
managed thanks to the numerical integration of a nonlinear dif-
ferential equations system including 9 species and 12 pseudo-
elementary processes (see Supp. info. part). Fig. 5A compares the
pH drop responses upon irradiation of the free 1-MEH with mix-
tures of 1-MEH plus 40% mole/mole of a pH indicator acidochromic
dye exhibiting a pKind value at either, 3, 6 or 9. When the pKind of
the pH-indicator 2-AH is too high or too low the pH drop is unaf-
fected by the presence of the additive. However, with a pH indicator
exhibiting a pKind at 6, the pH drop is quenched: the released
protons are partly absorbed by the basic form of the pH-indicator.
This can be also analysed by examining the % switching of the
pH-indicator acidochromic dye (i.e. the yield of the proton transfer).
As previously discussed, the coupling readily vanishes for pKind
values 3 and 9 (Fig. 5B) and is maximal for pKind values close to 6 (in
Table 1
Optimized thermodynamic and kinetic parameters values obtained
after numerical fitting.
Parameters
Value
pK
pK
a
a
(1-MEH/1-ME)
(1-SPH/1-SP)
7.75 ± 0.3
4.3 ± 0.3
ꢁ
2 ꢁ1
k
k
k
k
4
5
6
7
(3 ± 0.7) ꢀ 10
3
ꢁ1
s
s
ꢁ
3 ꢁ1
s
4.6 ꢀ 10
ꢁ
8 ꢀ 10
0
Phi (h
n)
0.38 ± 0.03
it at a different value resulted in a systematic loss of the fitting
accuracy. While more detailed since several new parameters have
been determined, there are no fundamental discrepancies between
our modelling approach and the second-order treatment by Johns
et al. [18]. For instance, our first order relaxation rate constant 1-
ꢁ
3 ꢁ1
SP / 1-ME (k
5
¼ 4.6 ꢀ 10
s
) is of the same order of magnitude
a
between the pK 's of the closed (4.3) and open (7.75) photoacid
than the apparent pseudo first-order rate constant that can be
isomers): the irradiation of 1-MEH induces the switching, albeit
non quantitative, of the pH-indicator additive. Practically, the two-
component system behaves then like a single new photochromic
dye. An empirical approach leading to the same conclusion was
recently proposed by Chen et al. [24]. However, looking carefully at
Fig. 4B, it can be seen that the optimal initial pH does not occur
systematically at the expected pH ¼ pKind. For instance, for
pKind ¼ 3 the optimal pH is around 5.5, for pKind ¼ 6 at 6.5 and at 7.5
for pKind ¼ 9.
ꢁ
5
calculated in their 7.03
ꢀ
10
M
aqueous solution:
z 4.6 ꢀ 10
ꢁ
1
ꢁ1
ꢁ5
ꢁ3 ꢁ1
ꢁ3 ꢁ1
73s
M
ꢀ 7.03 ꢀ 10 M ¼ 5.13 ꢀ 10
s
s
1
1
1
,6 10
,4 10
,2 10
1
-MEH
1
-SP
3.2. Influence of the light intensity on the yield of proton transfer
1
8
6
4
2
10
The efficiency of the proton transfer from the photoacid to a pH-
000
000
000
000
0
1-ME
indicator acidochromic dye has been also investigated by varying
simultaneously the stoichiometric ratio and the light intensity. The
efficiency has been regarded either as a chemical yield (% of
switched pH-indicator) or as a protonation quantum yield (rate of
pH-indicator protonation vs absorbed light intensity by the pho-
toacid). Fig. 6A shows that the % of switched pH-indicator is higher
at low pH-indicator loading ([pH-indicator]/][1-MEH] stoichio-
metric ratio). The levels-off at high intensity is related to the limited
1
-SPH
250
300
350
400
450
500
550
600
wavelength (nm)
þ
amount of protons which can be released by the photoacid ([H ]
0
released maxi < [1-MEH] ). When the light intensity is decreased,
Fig. 4. Re-calculated UV/visible spectra of the four species involved in the 1-MEH
reversible photoacid system (see Table 1S in the supp. info. part).
the thermal proton re-capture becomes significant and limits the %

J. Vallet et al. / Dyes and Pigments 125 (2016) 179e184
183
6
0
free 1-MEH
1
1
1
.6
.4
.2
1
pK = 3
a
pK = 6
A
B
50
a
pK = 9
a
4
3
0
0
0
0
0
0
.8
.6
.4
.2
0
pK = 6
a
2
1
0
0
pK = 3
pK = 9
a
a
0
3
4
5
6
7
8
9
3
4
5
6
7
8
9
10
11
pH initial
pH initial
6
ꢁ1 ꢁ1
ꢁ5
Fig. 5. A: Numerical simulation of the pH drop upon irradiation until pss (I
0
¼ 1.46 ꢀ 10ꢁ mol L .s ). Dashed curve, free 1-MEH: 3.5 ꢀ 10 M in water; curve pK
a
¼ 3: 1-MEH
ꢁ
5
ꢁ5
3.5 ꢀ 10 M with 1.5 ꢀ 10 M of a pH-indicator AH exhibiting a pK
a
at 3; curve pK
a
¼ 6: same with AH (pK
a
¼ 6). Curve pK
a
¼ 9: with AH (pK ¼ 9). B: % of switched pH-indicator
a
(
([AH]pssꢁ[AH]initial)/[AH]
0
) upon irradiation. Note the buffering of the pH drop (A) and the significant switching effect (B) when the pK
a
of the added pH indicator is around 6.
70
0.34
low
A
B
0
.32
high
6
0
0
.3
50
40
30
20
medium
0
0
0
0
.28
.26
.24
.22
medium
low
high
1
0
0.2
0
0.18
0
2 10
4 10
6 10
8 10
1 10
1.2 10
0
2 10
4 10
6 10
8 10
1 10
1.2 10
-1
-1
-1 -1
I (mol.L .s )
I (mol.L .s )
0
0
¼ 6) upon irradiation of 1-MEH (3.5 ꢀ 10ꢁ M in water solution) at various light intensities (pH initial ¼ 6.71). Note the
5
Fig. 6. A: Variation of the % of switched pH-indicator (pK
a
better switching at low pH-indicator loading. B: Quantum yield of proton capture by the pH-indicator dye. Note the drop of the quantum at low intensity. pH-indicator
loading ¼ [pH-indicator]/[1-MEH]; low: 0.43; medium: 0.83; high: 1.67.
of switched pH-indicator. On the other hand, Fig. 6B displays the
quantum yield of proton capture by the pH-indicator. As expected,
the situation is reversed, results show that for the highest loading
and the highest light intensity, the quantum yield of proton capture
reaches 0.33 i.e. a value close to the quantum yield of photoacid
isomerization (0.37) since in these conditions proton re-capture
within the photoacid manifold is negligible. On the contrary, at
the lowest light intensities, thermal relaxation of the photoacid
becomes significant and as a consequence proton release is limited:
the quantum yield of proton capture vanishes. This decrease is
more apparent at low loading.
pK
a
s
is the following: pKind(MO)
¼
3.39
<
a
pK (1-
SPH) ¼ 4.3 < pKind(BCG) ¼ 4.83 < pK
a
(1-MEH) ¼ 7.75. As expected
from Fig. 5B, preliminary experiments have confirmed that the % of
switched MO was limited (less than 0.3%) while, on the contrary, %
of switched BCG reached more than 40%. Therefore, most of the
experimental work has been carried-out with BCG. This pH-
indicator exhibits an absorption band centred at 614 nm associ-
ated with the deprotonated form (BCG) and a band centred at
444 nm associated with the protonated form (BCGH) (see supp.
info. part).
Fig. 7A shows the switching of BCG during irradiation of 1-MEH.
The decrease at 616 nm is mainly related to the protonation of BCG
while the decrease at 420 nm mostly corresponds to the photo-
induced ring closure of the 1-MEH photoacid. Without changing
These simulations show that the interplay between all processes
impedes a complete conversion of the pH indicator.
a
the extracted values of model parameters but putting the pK of the
4
. Experimental photoswitching of a pH-indicator
pH-indicator at its BCG value (pKind ¼ 4.8), allowed us to reproduce
the spectral evolution (Fig. 7B) using the molar extinction co-
efficients of the various species. This result is particularly inter-
esting since it confirms the predictability of the proposed model
when the 1-MEH long-lived reversible photoacid is put in a work-
ing condition i.e. to switch an acidochromic dye. The success of the
fitting of this independent experiment is a strong indication that
acidochromic dye by 1-MEH
To illustrate the previous study, we have checked experimen-
tally the pH switching capabilities of the 1-MEH spiropyrane pho-
toacid in presence of pH indicators with variable pK 's. For this
a
purpose, two pH indicators, namely methyl orange (MO) [21] and
bromocresol green (BCG) have been selected. The ordering of the

184
J. Vallet et al. / Dyes and Pigments 125 (2016) 179e184
¼ 1.31 ꢀ 10ꢁ5; [1-MEH]
¼ 2.9 ꢀ 10
ꢁ5
ꢁ1
mol L
(BCG loading ratio ¼ 0.45) under 405 nm irradiation
Fig. 7. A: Spectral evolution of a BCG þ 1-MEH mixture ([BCG]
0
0
ꢁ7
ꢁ1 ꢁ1
(
I
0
¼ 3.6 ꢀ 10 mol L .s ); arrows indicate the direction of evolution ( t ¼ 10 s). pH initial was around 5.2; B: Evolution of the absorbances at 6 selected wavelengths. Continuous
D
a
lines: numerical simulation using the proposed model with all the extracted parameters (pK 's, rate constants and molar extinction coefficients).
the main features of the photoacid mechanism of proton release
Appendix A. Supplementary data
have been interpreted. The model confirms that the pH variation
has crossed the BCG pK
a
value and has switched the [BCGH]/[BCG]
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.10.025.
ratio. Moreover the model was able to simulate accurately the
evolution of the concentrations of the various species either under
irradiation or during the thermal relaxation in the dark (see supp.
info part).
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[
dye is expected to be a useful component to photo-trigger pH
controlled phenomena in a similar way to related examples [22,23],
we have also defined the optimal operating conditions to maximize
1
2
[
þ
the observed effect, using a pH-indicator dye as H receptor. Not
2
a
only the receptor's pK must fall in a limited window conditioned
[
[
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photoacid can effectively be used to photoswitch efficiently a pH-
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[
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a
indicator dye whose pK lies in the 4e6 range. Our results pro-
́
̌
vide some theoretical guidelines to the question of the optimal use
of long-lived photoacids for light controlled pH-drop and actua-
tions [24]. For instance, artificial molecule-releasing devices
25e27] could be designed from long-lived reversible photoacids in
order that the effect of a drug will depends on the conditions where
this drug is released and re-captured.
[
[
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Acknowledgements
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CNRS and University Toulouse III - Paul Sabatier are acknowl-
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edged for financial support.
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