Proton-Stabilized Photochemically Reversible E/ Z Isomerization of Spiropyrans

Article abstract of DOI:10.1021/acs.jpcb.8b03528

Spiropyrans undergo C_spiro-O bond breaking to their ring-open protonated E-merocyanine form upon protonation and irradiation via an intermediate protonated Z-merocyanine isomer. We show that the extent of acid-induced ring opening is controlled by matching both the concentration and strength of the acid used and with strong acids full ring opening to the Z-merocyanine isomer occurs spontaneously allowing its characterization by ¹H NMR spectroscopy as well as UV/vis spectroscopy, and reversible switching between Z/E-isomerization by irradiation with UV and visible light. Under sufficiently acidic conditions, both E- and Z-isomers are thermally stable. Judicious choice of acid such that its pK_a lies between that of the E- and Z-merocyanine forms enables thermally stable switching between spiropyran and E-merocyanine forms and hence pH gating between thermally irreversible and reversible photochromic switching.

Full text of DOI:10.1021/acs.jpcb.8b03528

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution
Proton Stabilized Photochemically
Reversible E/Z Isomerization of Spiropyrans
Luuk Kortekaas, Juan Chen, Denis Jacquemin, and Wesley Richard Browne
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b03528 • Publication Date (Web): 30 May 2018
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L. Kortekaas,a J. Chen,a D. Jacquemin,b,* and W. R. Brownea,*
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a Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Mathematics and Natural Sciences,
University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. E-mail: w.r.browne@rug.nl
b Chimie Et Interdisciplinarité, Synthèse, Analyse, Modélisation (CEISAM), UMR CNRS no. 6230, BP 92208, Univer-
sité de Nantes, 2, Rue de la Houssinière, 44322 Nantes Cedex 3, France. E-mail: denis.jacquemin@univ-nantes.fr
ABSTRACT: Spiropyrans undergo Cspiro-O bond breaking to their ring-open protonated E-merocyanine form upon proto-
nation and irradiation via an intermediate protonated Z-merocyanine isomer. We show that the extent of acid induced
ring opening is controlled by matching both the concentration and strength of the acid used and with strong acids full
1
ring opening to the Z-merocyanine isomer occurs spontaneously allowing its characterization by H NMR spectroscopy as
well as UV/vis spectroscopy, and reversible switching between Z/E isomerization by irradiation with UV and visible light.
Under sufficiently acidic conditions both E- and Z-isomers are thermally stable. Judicious choice of acid such that its pKa
lies between that of the E- and Z-merocyanine forms enables thermally stable switching between spiropyran and E-
merocyanine forms and hence pH gating between thermally irreversible and reversible photochromic switching.
Introduction
exist as any of several distinct isomers, denoted common-
ly by reference to the cis/trans orientation around each of
the three bonds starting at the Cspiro-C bond, indicated in
Scheme 1 by α, β and γ. The preferred conformations and
the complex photochromic pathways for several (neutral)
spiropyrans have been studied using various ab initio
models.24–29 The orientation of the β-bond affects the
thermal stability of the merocyanine with regard to rever-
sion to the spiropyran form. Although multiple colored
isomers have been observed at cryogenic temperatures,30
the low thermal stability of the cis-β configurations (re-
ferred to as the Z-isomer) means that only the more sta-
ble trans-β configurations (E-isomer) are observed at
ambient temperatures.31 Indeed, the TTC form, specifical-
ly, was shown by Ernsting and co-workers to be the ther-
mally most stable isomer.32
Smart systems built from molecular switches1 that re-
spond to external triggers such as light,2 electricity,3
heat,4,5 sound,6–8 and pH,9 through changes in molecular
properties (color, polarity, shape, conductivity, reactivity
etc.) are highly desirable due to the possibility to tune
these physical responses through structural (synthetic)
modification.10 Building such molecular switches into
materials can impart responsiveness at the macroscopic
level, often amplifying the changes in physical properties
ranging from sensing11 and surface properties12 to lumi-
nescence13,14 and electrochromism.15,16 Photochromes,
including dithienylethenes,17,18 azobenzenes19,20 and spiro-
pyrans,21 are amongst the most widely applied due to their
modularity and flexibility towards modification and the
possibility to combine them with other responsive units,
e.g., multiphotochromes.22 The combination of synthetic
versatility, and thermo- and acidochromism shown by
spiropyrans in addition to their well-known photochrom-
ism reported first in 1952,23 has made this class amongst
the most important in applications in smart materials and
systems to date.
The photochromism of spiropyrans arises from a light
driven interconversion between a ‘ring closed’ spiropyran
(SP) structure and the zwitterionic ‘ring open’ merocya-
nine (MC) state in which the C-O bond of the spiro motif
is cleaved, and is stabilized by an accompanying Z/E-
isomerization around the spiro- and pyran-bridging dou-
ble bond (Scheme 1). The merocyanine ‘open form’ can
Scheme 1. Structures and photochromism of the colorless
spiropyran (SP) and nitrospiropyran (NSP). Photo-
induced ring-opening can lead to any of 8 distinct isomers
differing in conformation, cis (C) and trans (T), around
the α, β and γ bonds. Protonation of the colored merocy-
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anine form inhibits thermally induced reformation of the
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served for the related photochromic spirooxazines.47 The
formation of the Z-MCH+ form has been proposed else-
where as well,48 and has been reported in gas-phase stud-
ies.49
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spiro form (ring closing).
Protonation of the phenol moiety of the merocyanine
form impacts the chemistry of spiropyrans and indeed
acidochromism has been reported for several spiropyran
structures whereby addition of acid results in thermally
induced conversion to the protonated merocyanine form
(MCH+).33,34,43–46,35–42 However, the mechanism and nature
of the intermediate species formed throughout these
reports are not consistent, and several conflicting mecha-
nistic scenarios have been proposed to date.
Here, we show through a combination of spectroscopy
and theory that the observed pH induced switching of
both spiropyrans (SP) and nitrospiropyrans (NSP) in
solution can be rapid and complete (Scheme 2) but is
highly dependent on acid strength in non-aqueous sol-
vents. The extent of reaction with acids follows the order
of pKa values reported, including the intermediate pKa of
HNO3 in the middle, for which an excess amount is re-
quired in order to generate the desired response.50–52 Fur-
thermore, we show unambiguously that the initial step is
cleavage of the C-O bond to form a relatively stable Z-
isomer of the merocyanine that undergoes thermal as well
as photochemically induced Z/E-isomerization to the
more stable E-form. Overall, we show that fully reversible
isomerization is retained upon acid/base switching pro-
vided that the acid used is stronger than the phenol moie-
ty in at least the E-isomer. The demonstration of pH in-
duced switching between spiro and Z-merocyanine forms
as well as photochemical E/Z switching opens new oppor-
tunities in the application of spiropyrans as molecular
switches. It is especially relevant in applications where
large local changes in pH can occur such as at electrodes,
e.g., during cyclic voltammetry,53,54 which can affect the
observed photochemistry profoundly.
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Both Fissi et al.45 and Wojtyk et al.,33 for example, pro-
posed that
a rapid equilibrium between the non-
protonated and N-protonated spiropyran is established in
the presence of trifluoroacetic acid, lying in favor of the
protonated E-merocyanine form. The immediate conver-
sion the protonated E-merocyanine form upon protona-
tion of a spiropyran impregnated polydimethylsiloxane
polymer films was noted by Nam and co-workers also, but
with subsequent thermal reversion back to the spiropyran
form.46 Later Genovese et al. reported that reversion to
the ring-closed form could be induced by irradiation with
visible light also.34 Rémon et al., on the other hand, pro-
posed that, in aqueous media, N-protonation only occurs
at pH below 0.5, and that the only protonated species
present is the ring open merocyanine form which is in
thermal equilibrium with the non-protonated closed
form.36 Later Schmidt et al. noted the formation of the
protonated E-merocyanine in ethanol upon addition of
trifluoroacetic acid.37 Collectively, these reports indicate
that ring opening to the stable protonated E-merocyanine
occurs upon protonation. However, over two decades ago,
Zhou and co-workers reported a species "Y" while investi-
gating the pH dependence of the negative photochrom-
ism of a 6’,8’-dinitrospiropyran, for which the merocya-
nine form is most stable even when unprotonated.43 Spe-
cies Y was assigned to the protonated form of species “X”,
a transient spiropyran species which has a broken Cspiro-O
bond and a geometry intermediate to the perpendicular
spiro and the planar merocyanine form, and postulated
that a rapid equilibrium is established between species Y
and the spiro form under sufficiently acidic conditions. In
the same period, Roxburgh et al. reported the trifluoroa-
cetic acid induced thermal ring-opening of spiropyrans to
their protonated E isomer, which was proposed to be via
either the unprotonated or protonated Z form.44 The pro-
posed intermediacy of the protonated Z form was sup-
ported by Shiozaki subsequently, who proposed that pro-
tonation of spiropyran in ethanol with sulfuric acid, a
stronger acid than the trifluoroacetic acid, generated the
Z-merocyanine form, which could not only undergo sub-
sequent thermal but also photochemical Z/E-
isomerization (Scheme 2).38 Shiozaki’s interpretation of
the changes observed by UV-vis absorption spectroscopy
were supported by theoretical studies and is analogous to
the acid induced ring opening (C-O bond cleavage) ob-
Scheme 2. pH-gated photochromism of the colorless
spiropyran (SP). Protonation of SP results in spontaneous
ring opening to the Z-merocyanine (Z-MCH+). UV irradia-
tion to the thermally stable E-merocyanine (E-MCH+) is
reversed by visible irradiation.
It is notable that in most earlier studies of the pH de-
pendence of spiropyrans, triflouroacetic acid33–35,37,39,40 as
well as HCl,21,36,41,42 were the acids of choice employed.
Notably, however, in studying pH-gated spiropyran pho-
tochromism, Shiozaki employed sulfuric acid.38 Here, we
show that the choice of acid is crucial to fully understand-
ing the acido-photochromism of spiropyrans. We show
that addition of acid to SP and NSP induces a batho-
chromic shift, the extent of which is dependent on the
strength of the acid used. With relatively weak acids, such
as trifluoroacetic and HCl acid in acetonitrile, acido-
chromism in SP is weak with only a minor bathochromic
shift upon UV-irradiation. For NSP, the changes are even
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less apparent due to its already red-shifted absorption the merocyanine forms were used for reference. Irradia-
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(due to the electron withdrawing nitro group), which
rationalizes why the acidochromism of the spiro form has
thus far gone essentially unnoticed. We show here that
with stronger acids, protonation induced changes are
clear for both SP and NSP and that both show pH-gated
photochromism to the protonated merocyanine forms.
tion was carried out with LEDs (Thorlabs) at 365 nm (4.1
mW, M365F1), 455 nm (3200 mW, M455L3-C5), 565 nm
(2.0 mW, M565F1), and 660 nm (14.5 mW, M660F1). For
details of theoretical calculations see the SI.
Results and Discussion
Acidochromism of SP and NSP with weak acids.
Although the photochromism of spiropyran (SP) was
Spontaneous ring opening to the Z-MCH+/Z-NMCH+
states is observed upon addition of strong acid, with sub-
sequent reversible photo induced isomerization to the E-
MCH+/E-NMCH+ states. The higher acidity of the proto-
nated Z-merocyanine form is demonstrated by adding an
acid with a pKa intermediate of those of Z-MCH+ and E-
MCH+, enabling direct photochemical switching between
the SP and thermally stable E-MCH+ form at room tem-
perature. Understanding the acid/base switching of spi-
ropyrans, and the requirement for matching of the pKa of
the acid with that of the merocyanine forms, allows for
stable access of a total of four photochromic states and
opens up a wide range of possibilities for applications as
functional units. Finally, although only the spiropyran
form is thermally stable at room temperature in the ab-
sence of acid and addition of acid induces spontaneous
ring opening to the Z-MCH+ and E-MCH+ forms, by using
an acid with a pKa between that of the Z- and E- merocya-
nine isomers, reversible photochemical switching be-
tween thermally stable colorless SP and colored E-MCH+
forms can be achieved at room temperature.
reported
earlier,23
the
photochromism
of
6’-
9
nitrospiropyran (NSP) is much more pronounced55 due to
resonance stabilization of the phenolate in the ring-open
form (see Scheme 1). The merocyanine form of NSP is
sufficiently thermally stable for photoswitching to be
observed at room temperature (Φ = 0.03), however, the
thermal reversion of SP is sufficiently delayed only at -30
oC to observe photoswitching to its merocyanine form (Φ
= 0.07, Figure S1). The acidochromism of NSP and SP has
been noted on several occasions, with trifluoroacetic acid
in large excess,33–35,37,39,40 manifested in a slight batho-
chromic shift, that is less pronounced for NSP than SP,
and in both cases results in only minor amounts of the
protonated species (Figure 1). The limited acidochrom-
ism of SP and NSP observed with CF3CO2H and HCl ear-
lier, was observed also with excess CCl3CO2H and, in the
case of NSP, with H3PO4 (Figure S2) also. Irradiation at
365 nm results in a further slight bathochromic shift,
which is reversed by irradiation with visible light. The
greater effect on SP than for NSP, is consistent with theo-
retical calculations (Table S1 in the SI).
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Experimental Section
Materials. NSP and chemicals used for the synthesis of
SP were purchased from Aldrich or TCI and were used
without further purification. The synthesis of SP is de-
scribed in the supporting information, as well as charac-
terization by NMR spectroscopy (1H, 13C APT, HSQC,
HMBC, COSY and NOESY) of its protonated Z and E
forms generated by trifluoromethanesulfonic acid and
phosphoric acid, respectively. HPLC grade acetonitrile
was used for the spectroscopic studies.
Physical Methods. NMR spectra were obtained on a
Bruker 600 NMR spectrometer. Chemical shifts (δ) are
reported in parts per million and coupling constants in
Hertz. Integrations are reported, with multiplicities de-
noted as: s = singlet, d = doublet, t = triplet, br = broad
singlet, m = multiplet. Chemical shifts are reported with
respect to tetramethylsilane and referenced to residual
solvent (CD2HCN) signals. UV/vis absorption spectra
were recorded using an Analytik Jena Specord600 spec-
trometer. Quantum yields were determined as described
in the supporting information, with calculations concern-
ing the protonated photochromism were performed by
determining a PSS (λexc 365 nm) in the presence of strong
acid of 58.9 % E-MCH+ and 68.3 % E-NMCH+ by scaled
subtraction. The absorption spectra immediately after
protonation to the Z-forms and both those resulting from
partial deprotonation at the PSS and direct protonation of
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to E-MC and E-NMC formed by irradiation of SP and NSP
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at -30 °C (to limit thermal reversion, Figure S4 and Fig-
ure 4, respectively). Subsequent irradiation at 365 nm re-
establishes the PSS obtained with acid (vide supra) with
reversion to the Z-isomers upon irradiation at 455 nm.
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Figure 1. UV/vis absorption spectra of (top) SP and (bottom)
NSP (62 µM in acetonitrile, black lines) upon addition of
excess CF3CO2H (10 and 52 equivalents, respectively, red
lines) and upon subsequent irradiation at 365 nm (blue
lines). Irradiation with visible light recovers the absorption
spectrum prior to irradiation at 365 nm.
pH-gated photochromism of SP and NSP with strong ac-
ids.
In contrast to the changes observed with acids such as
CF3CO2H, addition of near stoichiometric amounts of
strong acids (H2SO4, pKa 8.7; CF3SO3H, pKa 0.7; and
HClO4, pKa -0.7 )51 to SP and NSP in acetonitrile resulted
in the appearance of a well-defined absorption band as-
signed to Z-isomer of the protonated merocyanine form
(Figure 2 and Figure S3), which is unaffected by the
addition of further equivalents of acid. A bathochromic
shift and increase in absorptivity was induced by subse-
quent irradiation at 365 nm (Φ푅=퐻 = 0.92, Φ푅=푁푂 = 0.82)
2
consistent with the observations of and mechanism pro-
posed by Shiozaki and assigned as due to Z/E isomerisa-
tion.38 This is also consistent with the theoretical calcula-
tions (Table S1 in the SI) that predict a 32 nm batho-
chromic shift and a strong increase of the oscillator
strength when going from Z-MCH+ to E-MCH+. Irradia-
tion at 455 nm results in recovery of the Z-isomer for both
NSP (Figure 2) and SP, even under continuous irradia-
tion of the spectrometer (Figure 3), and is analogous to
the behavior of spirooxazines.47 The thermal nature of the
ring-opening was confirmed by monitoring absorbance at
375 nm only (i.e. monochromatic light source) where the
SP form does not absorb but the Z-MCH+ form does ab-
sorb, before and after addition of acid. The change in
absorbance at 375 nm was immediate and complete with
the mixing time.
Figure 2. UV/vis absorption spectra of (top) SP and (bottom)
NSP (62 µM in acetonitrile, black lines) upon addition of 1
equiv. CF3SO3H (red lines). Irradiation at 365 nm induces a
red shift to 420 nm (blue lines), which is reversed by irradia-
tion at 455 nm (for E-MCH+ to Z-MCH+ see Figure 3).
The red-shift that manifests Z to E-isomerization is as-
cribed to the increase in electronic delocalization that
accompanies increased planarity. Indeed the maximum
visible absorption of the TTC isomer of the (deprotonat-
ed) merocyanine shifts from 550 nm to 595 nm in the TTT
isomer (Scheme 1).56,57 The protonated E-merocyanines
are also obtained by addition of 1 equiv. of a (strong) acid
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tion spectrum of the ring closed SP and NSP forms, when
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base is added stepwise substoichiometrically the conver-
sion of the Z-isomer is observed prior to that of the E-
isomer (Figure S5). The order of recovery is consistent
with the pKa of the Z-isomer being less than that of the E-
isomer, which is expected considering the contribution of
ring closing to the acid/base equilibrium. Furthermore, at
-30 °C, a temperature at which the E-MC isomers are
thermally stable, the visible absorption of the MC form
does not appear until the sufficient base has been added
to deprotonate all of the Z-MCH+ present (Figure S6).
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Φ
Φ
Figure 3. UV/vis absorption of the E-MCH+ form (blue line,
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generated by irradiation at 365 nm, inset) over time, showing
near complete reversion to the initial Z-MCH+ form upon
continuous visible light irradiation (by the spectrometer,
spectra at 100 s intervals, while no change in absorbance was
observed over 200 s in the dark).
Φ
Φ
Scheme 3. pH-gated photochromism of spiropyrans and
nitrospiropyrans, in absence and presence of strong acids.
Calculated energies and barriers for ring opening of SP
and NSP
The energies of the observed isomers were calculated
using a DFT known to be suited for spiropyrans, as de-
tailed in the SI.41 The calculated energies of the unproto-
nated spiropyran forms relative to their merocyanine
isomers are fully consistent with experimental data; i.e.
that the ring-open form is thermally accessible from the
NSP form. Interestingly, a comparison between the com-
puted Raman spectra with those measured experimentally
for the non-protonated SP and NSP forms (Figure S7 and
Figure S10), as well as for the protonated Z-MCH+ and E-
MCH+ show a good qualitative match, which supports
that the selected level of theory is well suited for our pur-
poses.
Figure 4. Low temperature UV/vis absorption spectrum of E-
NSP and of E-NMC generated at -30 °C by irradiation at 365
nm (grey solid line, initial concentration of NSP 52 μM).
Addition of 1.5 equiv. of CF3SO3H leads to formation of E-
NMCH+ (blue solid line) with reversion to a PSS upon irradi-
ation by the light source of the UV/Vis spectrometer (red
solid line).
The lowest energy MC form, the TTT, lies 3.2 kcal/mol
higher than the ring-closed SP form and has a 17.5
kcal/mol barrier from the E to Z form before undergoing
low-barrier ring-closing. The lowest energy NMC form,
on the other hand, is the TTC conformer, which is 0.8
kcal/mol lower in energy than the ring-closed form,
though closely followed by the TTT (1.8 kcal/mol). Fur-
thermore, the TTC form, which is the main species ob-
served experimentally also for nitrospiropyrans32,59–61 (with
in acetonitrile one order of magnitude greater abundance
than the TTT form)62, has a significantly higher barrier to
conversion to the Z-NMC form (26.9 kcal/mol for direct
conversion, 21.8 kcal/mol for conversion through the TTT
form). These data are consistent with its observed ther-
mal stability at room temperature. Notably, theory shows
that even if the indolinic nitrogen is protonated, as pro-
The acid/base dependence of the photochemistry of
both spiropyrans is summarized in Scheme 3. The data
presented here contradict earlier proposals that irradia-
tion of the protonated E-merocyanine leads to the for-
mation of the spiropyran form,33,34,36,58,46 based on the loss
of visible absorption (i.e. decoloration). Instead it is the
protonated Z-merocyanine that was obtained. Indeed that
this is the case is apparent in the blue shifted shoulder
due to the Z-(N)MCH+ form in spectra reported earlier.37
Although deprotonation of MCH+ at the photostation-
ary state (PSS365nm) results in full recovery of the absorp-
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posed earlier,33,36 proton transfer coupled with Cspiro-O
bond cleavage is barrierless and yields the thermally more
stable Z-merocyanine form (Scheme 5).
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Scheme 5. Computed free energy profiles for the opti-
mized lowest energy conformers of the protonated forms
of (a) SP and (b) NSP. See caption of Scheme 3.
The barriers are significantly higher than those of the
unprotonated forms, ranging from 29.6 (Scheme 5a) to
39.4 (Scheme 5b) kcal/mol, consistent with their experi-
mentally observed thermal stability. The photochemical
interconversion between the Z- and E-merocyanine forms
thus enables controlled access over two distinct protonat-
ed states as shown, through the energetic entrapment of
the respective isomers.
Scheme 4. Computed free energy profiles for the opti-
mized lowest energy conformers of (a) SP and (b) NSP in
the ring-closed spiropyran and ring-open merocyanine
forms, and the barriers to their interconversion. All ener-
gy differences given in kcal/mol.
Re-enabling room temperature switching of SP
The difference in the pKas of the Z and E-merocyanine
isomers additionally opens the possibility to gate the
photochromism of SP with pH. In large excess, H3PO4
induces formation of Z-MCH+ (Figure S7), whereas near
stoichiometric amounts have essentially no effect on the
absorption spectrum of SP. Remarkably, in presence of
equimolar phosphoric acid at room temperature solely
the E-MCH+ isomer is generated both upon irradiation at
300 nm (Figure 5) and thermally over time (Figure S8),
with full recovery to the unprotonated SP–form upon
subsequent irradiation at 455 nm. This effect is observed
because the pKa of H3PO4 lies between that of the E- and
-
Z-MCH+ isomers. Any H2PO4 present will deprotonate Z-
MCH+ spontaneously inducing ring closing, while any
photogenerated E-MC will undergo protonation by H3PO4
preventing thermal reversion and thus enables essentially
direct photoconversion between SP and E-MCH+ forms
through the Le Chatelier principle (Scheme 6) despite
that SP/E-MC photochromism is thermally inhibited at
room temperature. Furthermore, the thermal stability of
the E-MCH+ formed in the presence of near-
stoichiometric amounts of H3PO4 allows for its character-
ization by NMR spectroscopy (1H, 13C APT, HSQC, HMBC,
COSY and NOESY), as well as that of the Z-MCH+ form
generated by addition of CF3SO3H (see the SI).
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isomers through significantly higher thermal barriers
1
than in the deprotonated states. Additionally, in contrast
to conclusions drawn earlier regarding the protonation of
spiropyrans,33,34,36,37,45,46,48 theory suggests that the proto-
nation of the indolinic nitrogen leads to barrierless pro-
ton transfer to the phenolate. Hence, even if an acid is
strong enough to protonate the indolinic nitrogen, spon-
taneous ring-opening to the Z-(N)MCH+ form would
make such a protonation transient at most. Finally, by
selecting an acid such that its pKa lies between that of the
Z- and E-MCH+ isomers (e.g., H3PO4 in the case of SP),
enables gated photochromism and a unidirectional multi-
state interconversion between spiropyran and merocya-
nine species at room temperature. The nitro group of NSP
is electron withdrawing in comparison with SP and, as
such, stabilizes the merocyanine form both in the depro-
tonated form and as a leaving group in the ring opening
process. As a counter poise to these effects, however, the
nitro group decreases the pKa and hence the strength of
acid required to achieve thermal ring opening needs
therefore to be greater. Overall, however, the nitro-group
of NSP does not appear to affect the mechanisms involved
in comparison with SP.
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Figure 5. UV/vis absorption spectra of SP (62 µM in acetoni-
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trile, black line) with 1 equiv. H3PO4 (red line) followed by
Irradiation at 300 nm (to form E-MCH+, blue line), while
irradiation at 455 nm recovers the SP form.
The pKa dependent control of the state of the photo-
chrome opens new opportunities in the application of
spiropyrans in photocontrol of pH and in understanding
the influence local pH changes (e.g., at surfaces of elec-
trodes) have on spiropyran based electrochemical devices.
Details of synthesis and characterization of SP, with NMR
spectroscopic characterization (1H, 13C APT, HSQC, HMBC,
COSY and NOESY) of both Z-MCH+ and E-MCH+, actinome-
try, computational studies, experimental and calculated
Raman spectra of the SP/NSP forms and Z-MCH+/Z-NMCH+
and E-MCH+/E-NMCH+ forms, and additional UV/vis ab-
sorption spectral data can be found in the Supporting Infor-
mation. This material is available free of charge via the Inter-
net at http://pubs.acs.org.
Scheme 6. Unidirectional cyclic interconversion between
SP and E-MCH+ upon pH-gated photochromism in near
stoichiometric presence of H3PO4.
Conclusions
In conclusion, we show through a combined experi-
mental and theoretical study that the acidochromism of
spiropyrans is highly dependent on the strength of the
acid used in aprotic solvents, with acids, such as CF3CO2H
and HCl, used typically, being too weak to protonate the
Z-(N)MC isomer. Fully pH-gated photochromism of sim-
ple spiropyrans is achieved with stronger acids (e.g.,
CF3SO3H), manifested in the two-state photo-
isomerization of Z-(N)MCH+/E-(N)MCH+ and, together
with (N)SP/E-(N)MC photoisomerization, leads to an
overall four-state molecular switching cycle. In retrospect,
the reactivity reported in the present study implies that in
earlier studies the Z-(N)MCH+ form may have been ob-
served erroneously assigned as the expected ring-closed
spiropyran form. The theoretically computed energies
agree with the experimentally observed forms and, more-
over, underline the increased stability of the Z- and E-
AUTHOR INFORMATION
* Denis.Jacquemin@univ-nantes.fr, w.r.browne@rug.nl
All authors have contributed and given approval to the final
version of the manuscript.
Financial support was provided by The Netherlands Ministry
of Education, Culture and Science (Gravity Program
024.001.035 to L.K. and W.R.B.) and Chinese Scholarship
Council (J.C.). This work used computational resources of
the CCIPL in Nantes and the GENCI-IDRIS.
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