Acid-catalysed liquid-to-solid transitioning of arylazoisoxazole photoswitches

Article abstract of DOI:10.1039/d1sc03308e

Molecular photoswitches play a vital role in the development of responsive materials. These molecular building blocks are particularly attractive when multiple stimuli can be combined to bring about physical changes, sometimes leading to unexpected properties and functions. The arylazoisoxazole molecular switch was recently shown to be capable of efficient photoreversible solid-to-liquid phase transitions with application in photoswitchable surface adhesion. Here, we show that the arylazoisoxazole forms thermally stable and photoisomerisable protonatedZ- andE-isomers in an apolar aprotic solvent when the pK_aof the applied acid is sufficiently low. The tuning of isomerisation kinetics from days to seconds by the pK_aof the acid not only opens up new reactivity in solution, but also the solid-state photoswitching of azoisoxazoles can be efficiently reversed with selected acid vapours, enabling acid-gated photoswitchable surface adhesion.

Full text of DOI:10.1039/d1sc03308e

Chemical
Science
View Article Online
View Journal
EDGE ARTICLE
Acid-catalysed liquid-to-solid transitioning of
arylazoisoxazole photoswitches†
Cite this: DOI: 10.1039/d1sc03308e
a
*
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Luuk Kortekaas,
Nikos L. Doltsinis
Julian Simke,a Niklas B. Arndt,a Marcus Bockmann,b
¨
b
a
*
‡
and Bart Jan Ravoo
Molecular photoswitches play a vital role in the development of responsive materials. These molecular
building blocks are particularly attractive when multiple stimuli can be combined to bring about physical
changes, sometimes leading to unexpected properties and functions. The arylazoisoxazole molecular
switch was recently shown to be capable of efficient photoreversible solid-to-liquid phase transitions
with application in photoswitchable surface adhesion. Here, we show that the arylazoisoxazole forms
thermally stable and photoisomerisable protonated Z- and E-isomers in an apolar aprotic solvent when
the pKa of the applied acid is sufficiently low. The tuning of isomerisation kinetics from days to seconds
by the pKa of the acid not only opens up new reactivity in solution, but also the solid-state
photoswitching of azoisoxazoles can be efficiently reversed with selected acid vapours, enabling acid-
gated photoswitchable surface adhesion.
Received 18th June 2021
Accepted 17th July 2021
DOI: 10.1039/d1sc03308e
rsc.li/chemical-science
(Scheme 1).14–20 The extent of catalytic reversion depends on the
exact acid strength and the amount added, as well as the solvent
type in conjunction with its limiting pKa.21 I.e., in 1938 Hartley
already reported that addition of several drops of acetic acid to
water accelerates the optical Z-to E-conversion to several
seconds,14 while Ciccone et al. found no effect upon addition of
acetic acid to Z-azobenzene in aqueous ethanol.15
Introduction
Over the past decades, molecular switches have become ubiqui-
tous in the development of smart materials, rendering reversible
changes in material function upon input stimuli.1,2 In addition to
designing new molecular switches and the synthetic optimisation
of existing structures, the deeper understanding of their funda-
mental responsivity has led to a vast body of research on small
molecular photoswitches, most notably on azobenzenes,3,4 spi-
ropyrans5,6 and diarylethenes.7 These exemplary molecular pho-
toswitches have proven to be excellent building blocks for
integration in materials owing in particular to their multi-
responsiveness, reacting not only to light,8,9 but also, e.g., heat,10
mechanical input,11 and pH-changes.12 It is particularly attractive
when there is synergy between distinct stimuli that leads to new
functionality, as is the case for the photo- and pH-response in
spiropyrans, where weak acids slow down the thermal reversion
and, as more recently shown, strong acids can open up to new
photophysics in the protonated isomers.6,13 Conversely, for the
azobenzene photoswitch, the interaction with acids has been long
shown to catalyse rather than retard the reversion of Z-to E-isomer
Acid-catalysed reversion rates in azobenzenes were reported
over the years to be independent of ionic strength of the
aCenter for So Nanoscience and Organisch-Chemisches Institut, Westfalische
¨
¨
¨
¨
Wilhelms-Universitat Munster, Busso-Peus-Straße 10, 48149, Munster, Germany.
E-mail: kortekaas@uni-muenster.de; b.j.ravoo@uni-muenster.de
bInstitute for Solid State Theory and Center for Multiscale Theory & Computation,
¨
¨
¨
Westfalische Wilhelms-Universitat Munster, Wilhelm-Klemm-Str. 10, 48149,
¨
Scheme 1 (A) Acid-catalysed Z-to E-reversion reported for azo-
benzenes. The isomerisation occurs spontaneously, a lingering red-
shifted azonium absorption is observed when strong acids are used. (B)
The (pH-gated) photochromism and acidochromism of azoisoxazole
reported here. We note that the depicted protonation sites are based
solely on an extensive theoretical analysis (see below).
Munster, Germany
† Electronic supplementary information (ESI) available: additional UV/vis and
NMR data, additional results of DFT calculations, methods. See DOI:
10.1039/d1sc03308e
‡ Lead contact.
© 2021 The Author(s). Published by the Royal Society of Chemistry
Chem. Sci.

View Article Online
Chemical Science
Edge Article
acids,15,16 with the isomerisation step of the Z-to E-cation induced out-of-plane bending of the Z-isomer the intermolec-
determined as the rate determining step.17 The accelerated ular interactions are incapacitated, leading to a drastic decrease
isomerisation is commonly attributed to the mesomerism of the in melting point and consequent liquecation of the azoisox-
azonium ion weakening the azo double bond,14–17,19 an effect azole upon UV-induced isomerisation. Previously, we have uti-
that was observed to be stronger when electron-rich substitu- lised this remarkable retention of photophysics in the solid
ents are conjugated to the azo double bond.14,16 Interestingly, state to develop photoreversible low molecular weight (LMW)
despite the strong pH dependence of the Z-isomer, the adhesives, cohering glass surfaces at 30.8 N cmꢀ2 with rapid
absorption of the plain E-azobenzene was found to be inde- debonding upon UV-irradiation.44 Although the control over the
pendent of aqueous HCl between pH 1 and 13.16 Indeed, the bonding/debonding process is remarkably swi and reversible
formation of azonium species in some (para-) substituted azo- even upon physical breaking of the cohesion, the photochem-
benzenes was observed to require much harsher conditions, ical approach requires the use of at least one transparent
such as a 6 M HClO4 solution,17,20 or gas phase ionisation.22
surface. In this contribution, we seek to expand on the prom-
Alternatively, aminoazobenzenes, such as the methyl orange ising reusable photophysical features of these low molecular
pH-indicator, have been shown to yield red-shied azonium weight azoisoxazoles by exploring the possibility to kinetically
ions on account of resonance contributions from the amino- control their solidication with protons. Though we demon-
substituent,19 with 4,40-diaminoazobenzenes even protonating strate that the acid-induced isomerisation takes place with
up to pH 4.23 Such red-shiing of absorption through azonium select acids for azoisoxazoles also, the terms “weak acid” and
formation runs not only parallel to the strategy of appending “strong acid” turn out to be imperative while use of acids
auxiliary functional groups,24–27 it also gives rise to new iso- stronger than the conventional HCl and CF3COOH, i.e., tri-
merisation kinetics altogether. The group of Woolley, for uoromethanesulfonic acid (CF3SO3H, pKa ¼ 0.7 in acetoni-
example, discovered that the pKa of the azo-moiety is signi- trile45), yield steady state Z-azonium (Z-AIZH+) and E-azonium
1
cantly increased owing to hydrogen-bonding in tetra-ortho- (E-AIZH+) species as observed by UV/vis and H NMR spectros-
methoxy azobenzenes, such that the protonated species is copy. Furthermore, the isomerisation kinetics are not only pKa-
preferred at physiological conditions.28–31 Their red-shied tuneable in solution, but also in the pristine liqueed Z-form by
absorption opens up to E-to Z-isomerisation with NIR light exposure to acid vapours. Ultimately, it empowers the versatile
with lifetimes in the order of milliseconds to seconds,28–30 adhesive with a rapid, non-destructive trigger for reversibly
leading to application in various smart materials.32–35 Impor- conjoining surfaces with remarkable adhesive strengths irre-
tantly, the tetra-ortho substitution was found to be vital in spective of transparency, and showcases the further potential
raising their pKa to such extent, while a symmetric shi of two that the added pH-gated control opens up to for this already
methoxy units to the meta-position caused a pKa drop of 5 widely versatile class of photochromes.
units.29
Despite the rapidly grown interest in photochromically
Results and discussion
Azonium formation in azoheterocycles
superior azoheterocycles as an alternative to azobenzenes,36
only few contributions have been concerned with the genera-
tion and stabilisation of their azonium species.37–39 For
example, Fuchter et al. showed that for azopyrazole40–42 (AAP)
the acid-catalysis of azobenzenes is retained when trichloro-
acetic (CCl3COOH), triuoroacetic (CF3COOH) and hydro-
chloric acid (HCl) are applied, and that the transient Z-azonium
AAP could even be observed during catalysis.37 In further work,
ortho-substitution of the azopyrazole benzene ring analogous to
the work of Woolley et al. was found to have an effect on the
lifetime of Z-azopyrazoles also, albeit without mention of its
effects on acid-catalysis.38 More recently, Walther et al. suc-
ceeded in demonstrating the general acid-catalysis of water-
soluble azopyrazoles, opening up to a tunability of Z-isomer
half-lives over 5 orders of magnitude depending on the pH
value.39 This retention of the acid catalysis in the AAP azohe-
terocycle brings about important implications for azohetero-
cycles in general, as their superior photophysics, and their
vastly increased thermal half-lives in particular, give rise to
a signicantly larger control over half-life times without the
need for synthetic adjustments.
Contrary to the catalysed azo-reversion with conventional acids
such as CF3COOH and HCl, we observe that the addition of the
more acidic triuoromethanesulfonic acid (CF3SO3H) to azoi-
soxazole AIZ 1 causes a shi to a new steady-state absorption
(Fig. S1†). Comparably, the analogous unsubstituted AAP 4 (ref.
41) also shows a steady-state absorption upon addition of
CF3SO3H (Fig. S2†) and, furthermore, distinct azonium chem-
ical shis in 1H NMR spectroscopy (Fig. S3,† for AIZ spectra see
below, “Thermally stable Z-and E-protonation with strong acids”).
Although the addition of CF3SO3H to the unsubstituted AIZ 1
yields a new absorption at 315 nm, the alkoxy AIZ 2 and 3
azonium ions (AIZH+) exhibit strong red-shied absorptions
instead (Fig. 1), while undergoing catalytic Z-to E-reversion in
response to HCl and CF3COOH. Hence, it is possible to capi-
talise on varying responses and response times by judicious
choice and quantity of acid, opening up to a wider range of
control over previously established as well as new physical
properties.
Azoisoxazoles (AIZs), as a relatively new type of azohetero-
cycle, show great promise as components in smart materials
Accelerated Z-to E-reversion with weak acids
owing to the photoinduced reversible solid-to-liquid phase Owing to mesomerism,46 the uncatalysed half-life of the 4-
transition demonstrated recently.43 Due to the sterically alkoxy-substituted Z-AIZs 2 and 3 in solution was determined to
Chem. Sci.
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Edge Article
Chemical Science
Fig. 1 Mechanistic pathways of AIZ for (A) acid catalysed Z-to E-reversion with use of relatively weak acids (HCl, CF3COOH) and (B) pH-gated
photochromism with strong acids (i.e., CF3SO3H). ‡ ¼ not observed. (C) UV/vis absorption of E-AIZ 3 (70 mM in acetonitrile) at thermal equilibrium
(black) and after irradiation at 365 nm in the Z-isomer (green). Addition of 1500 and 1000 eq. of CF3SO3H under these conditions yields E-AIZH+
(blue) and Z-AIZH+ (red), respectively. DFT predicts protonation adjacent to the heterocycle (Fig. S4†), causing a break in symmetry in each of the
AIZ isomers and greatly increasing the probability of the lower energy n–p* transition similar to the changes observed upon E-to Z-isomer-
isation. For AIZ 2, see Fig. S5.† For a comparison to theoretical TD-DFT spectra, see Fig. S6 and S7.†
be 7 d (ref. 44) rather than the 45 d for Z-AIZ 1.43 Although we CF3COOH causes the generated Z-AIZH+ isomer to rapidly
observe no reactivity of Z-AIZ 2 and 3 to a large excess of acetic equilibrate with E-AIZH+. However, although the pKa of HCl is
acid (CH3COOH, 5000 equivalents in acetonitrile, Fig. S8†), sufficient to interact with the Z-form, any E-AIZH+ generated is
addition of CF3COOH and HCl cause the half-life to diminish to spontaneously deprotonated due to its signicantly increased
the (sub)second timescale at near-stoichiometric amounts acidity. The thermodynamically trapped unprotonated E-AIZ
already (Fig. S9 and S10†). DFT calculations of activation free (barrier of 1.86 eV to Z-AIZ) that results from this energetically
energies conrm that the Z-to E-barrier is substantially reduced favourable protonation–isomerisation–deprotonation sequence
in the azonium species from 1.29 eV (versus 1.11 eV experi- is thus continuously and, moreover, rapidly formed owing to Le
mentally44) to 0.86 eV (Fig. 2A). Importantly, out of three Chatelier's principle (Fig. 1A; further supported by calculations
possible N-protonation sites, the azo-N at the phenyl ring is taking into account conjugate bases, Fig. S13†).
lowest in energy for the E-form, and the N-azo at the heterocycle
Notably, catalytic amounts of HCl or CF3COOH cause the
for the Z-form (Fig. S11,† the protonated heterocycle being reversion to occur gradually, allowing for the thermal reversion
highest in both cases; we note that our calculations on AAP 4 are to be moderated as described for azobenzenes and azopyrazoles
in line with those by Fuchter et al.,37 Fig. S12†). The greatly before (Fig. 2B, S14†).14–20,37,39 Though the tuneable half-life
reduced thermal Z/E barrier accessible at excess HCl or ranges from several seconds at stoichiometric equivalents of
Fig. 2 (A) Relative Gibbs free energies of the unprotonated and protonated isomers of AIZ 2 and their transition states. For details on the DFT
calculations see the experimental section. (B) UV/vis absorption spectra of AIZ 3 (70 mM in acetonitrile) with 0.05 eq. of HCl (black solid line). The
thermal reversion is sped up through the acid-catalysed reversion on the right, hence the PSS365nm (radiant flux 1.2 W) is compromised through
spontaneous reversal via the azonium ions (black dashed lines, spectra every 2 min). Note also that the isosbestic point at 403 nm, where the
molar absorptivity of both azonium species is significantly higher (Fig. 1C), rules out a steady state generation of the Z-AIZH+ isomer.
© 2021 The Author(s). Published by the Royal Society of Chemistry
Chem. Sci.

View Article Online
Chemical Science
Edge Article
HCl to minutes at catalytic amounts, the higher pKa and solu- despite the mismatch in pKa the E-AIZH+ becomes energetically
bility of triuoroacetic acid allow for more control at super- more feasible in presence of thousands of possible (transient)
stoichiometric quantities (Fig. S15†), yielding varying well- proton sources, increasing its life-time and thus delaying the
tuneable half-lives (Table 1). The reduction in half-life eventu- catalytic process.
ally stagnates upon increasing the acid content, analogous to
the earlier reports on the deviation of rst-order kinetics in
azobenzenes.14,15,17,37,39 We note, however, that this is likely due
to the weak but increasing involvement of the E-azonium
Theoretical analysis of the Z / E thermal conversion
The large change in thermal reversion rates upon protonation
was investigated in further detail through locating the acces-
sible regime of the corresponding potential energy hypersur-
faces employing DFT. For azobenzenes, thermal reversion
pathways have been subject to many theoretical and experi-
isomer, not least because an increase to 5000 eq. CF3COOH
causes the half-life to signicantly rise again. In other words,
Table 1 Thermal half-life of Z-AIZ 3 (70 mM in acetonitrile) at varying mental studies.4,47–50 The molecular motion during azo-
concentrations of CF3COOH as determined by UV/vis spectroscopy
isomerisation in general is typically described as comprising
rotation and inversion, and certain combinations thereof
CF3COOH
Thermal half-life
Rate constant
(Scheme S1†).4 Classically, quantum chemical calculations rely
on locating individual stationary points (minima, transition
states) and lack information on how these points are dynami-
cally connected. Here, we adopt a point of view more suited to
a dynamical interpretation and scan the potential energy
surface. Fig. 3 presents relaxed energy proles starting out of
the minimum energy valley of AIZ 2 at varying CNNC dihedral
angles and C(5-ring)NN bond angles.
0 eq. (ref. 44)
2 eq.
10 eq.
7 d
—
0.00115 ꢁ 10ꢀ3 sꢀ1
2.05 ꢁ 10ꢀ3 sꢀ1
2.86 ꢁ 10ꢀ3 sꢀ1
5.84 ꢁ 10ꢀ3 sꢀ1
15.94 ꢁ 10ꢀ3 sꢀ1
28.93 ꢁ 10ꢀ3 sꢀ1
27.51 ꢁ 10ꢀ3 sꢀ1
11.57 ꢁ 10ꢀ3 sꢀ1
338.4 s
242.1 s
118.8 s
43.5 s
24 s
ꢂ46.3
ꢂ11.1
ꢂ18.9
ꢂ8.2
50 eq.
200 eq.
1000 eq.
2000 eq.
5000 eq.
ꢂ1.2
25.2 s
59.9 s
ꢂ0.5
ꢂ3.5
Fig. 3 Relative potential energies of (A and B) AIZ 2 and (C and D) AIZH+ 2 along the coordinates of the dihedral angle CNNC (fixed at the values
specified), and either (A and C) C(5-ring)NN bond angle or (B and D) C(6-ring)NN bond angle. The amount of data was reduced here for better
visualisation, additional data at 10ꢃ intervals of CNNC dihedral angle is reported in Fig. S16.†
Chem. Sci.
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Edge Article
Chemical Science
Accordingly, for the neutral as well as the protonated AIZ Whereas neither of the azonium isomers are observed by UV/vis
species, the minimum energy pathway is dominated by an spectroscopy even at 500 equivalents of HCl, addition of
increase of the dihedral angle CNNC (commonly viewed as increasing amounts of CF3SO3H to E-AIZ 3 causes a stepwise
‘rotation’) up to 80ꢃ whereupon an opening of the bond angle formation of the E-AIZH+ azonium with saturation at about
CNN becomes feasible. While the neutral AIZ allows bond angle 1500 equivalents (Fig. S18†). Accordingly, by considering the
opening to both sides, for the protonated AIZ opening at the equilibrium being practically exclusively between E-AIZ and E-
non-protonated side is feasible only. Hence, the mechanism AIZH+ and assuming full dissociation of CF3SO3H, the pKa of
involved is labelled ‘rotation-assisted inversion’ for the neutral the E-isomer is approximated to be ꢀ2.48 ꢂ 0.16 by UV/vis
and ‘(pure) rotation’ for the protonated AIZ species (Scheme spectroscopy.
S1†). The corresponding results for the (neutral isomers of) AAP
Interestingly, a sharp cut-off is observed for protonation in
4 and the parent compound azobenzene are effectively the same 1H NMR spectroscopy instead, transitioning from unprotonated
as for the unprotonated AIZ 2 (Fig. S17†), and thus constitute to protonated between 5 and 6 equivalents of CF3SO3H
a general feature of the azo moiety, in line with recent experi- (Fig. S19†). We note, however, that adding multiple equivalents
1
mental49 and theoretical50 reports. Protonation of Z-AIZ can of CF3SO3H to 3.3 mM AIZ in H NMR spectroscopy (versus 70
occur at either of the two azo-nitrogens, adjacency to the 5-ring mM in UV/vis) results in a much lower pH and thus increased
being energetically favoured by less than 0.1 eV (Fig. S11†). possibilities of interaction. This observation was made by the
Additionally, the likelihood of protonation via the solvent, group of Fuchter also while protonating an AAP with HCl,
investigated by minimum energy path calculations for proton requiring several orders of magnitude less equivalents at 1H
transfer from either azo-nitrogen to a single expliꢀcit molecule of NMR concentration.37 Presumably, the high interaction is also
acetonitrile, CH3COOꢀ, CF3COOꢀ, or CF3SO3 (Fig. S13†), the reason for the sharper cut-off in chemical speciation than in
further supports our experimental observations. The acidity of UV/vis absorbance spectroscopy.
CH3COOH is far too weak to protonate AIZ, and while for CF3-
Alternatively, addition of CF3SO3H at the PSS365nm (97% Z-
COOH protonation of AIZ is considerably more likely, the isomer) yields an additional red-shied absorption in UV/vis
proton still slightly favours the conjugate base of the acid. In the spectroscopy (Fig. S20†) as well as distinct Z-AIZH+ chemical
1
case of CF3SO3H, however, the equilibrium strongly shis to shis in H NMR spectroscopy (Fig. 4, S21, Table S1†). Impor-
AIZ. Concerning a possible shi of the proton from one azo- tantly, neither the Z-AIZH+ nor E-AIZH+ chemical shis are
nitrogen to the other, Fig. S13† reveals that a frequent observed upon addition of equal amounts of CF3SO3Li to AIZ 3
exchange should take place via CF3COOꢀ due to a very shallow (Fig. S22†). Furthermore, whereas near-equivalent amounts of
potential landscape for both isomers, whereas for CF3SO3ꢀ it is CF3SO3H destabilise the Z-AIZ already, its presence is unaltered
only feasible for the E-isomer, though less frequent than for with 15 eq. of CF3SO3Li, excluding a signicant role of ionic
CF3COOꢀ.
strength on chemical speciation. The Z-AIZH+ species generated
in presence of CF3SO3H, unlike the unprotonated Z-form, is
metastable at room temperature and reverts to a thermody-
namic equilibrium with 94% E-AIZH+ over time. The larger
thermodynamic ratio of the Z-to E-azonium in presence of
CF3SO3H in 1H NMR spectroscopy (6% Z-form as opposed to ca.
1% in absence of a proton source44) is in line with the change in
energetic barriers observed by DFT (Fig. 2A). Moreover, only 2
new sets of signals are observed here, their kinetics being
Thermally stable Z-and E-protonation with strong acids
Although substitution patterns play a vital role in increasing the
pKa of E-azonium towards physiological conditions,19,23,28–30 the
access to a larger range of molecular properties in less complex
existing azo-structures is highly advantageous. To achieve this,
the pKa of the acid needs to be near the low pKa of the azo-bond.
Fig. 4 1H NMR spectroscopy of AIZ 3 (3.3 mM in CD3CN) (A) after immediate addition of 15 eq. of CF3SO3H and (B) after addition of 15 eq. of
CF3SO3H to the PSS365nm (radiant flux 1.2 W). Although the required number of CF3SO3H equivalents to form the azonium species is much
smaller at 1H NMR than UV/vis concentration, such stronger interaction at higher concentration ranges has been noted before.37 Note that the
delay between addition of CF3SO3H and measuring causes the predominantly present Z-isomer at the PSS365nm to revert largely to the E-
azonium. For a full comparison with chemical shifts of the unprotonated forms see Table S1 and Fig. S21.†
© 2021 The Author(s). Published by the Royal Society of Chemistry
Chem. Sci.

View Article Online
Chemical Science
Edge Article
indicative of the E-and Z-azonium observed in UV/vis spectros- components of the abovementioned four photochemical
copy. Additionally, double protonation would result in blue- systems are chemically identical, the relative composition
shiing of absorptions (Fig. S23†), and no such features are provides tuneable control over various absorptions without the
observed upon addition of a large range of acid equivalents need for synthetic alterations.
(Fig. 5). Instead, an interplay between protonation/
deprotonation and reversible Z/E-photo- and thermal isomer-
isation opens up to four distinct regimes of kinetic control in
this acidity range. At near-stoichiometric equivalents primarily
E-AIZ and Z-AIZH+ are observed (Fig. 5A), differing from catal-
ysis with weak acids as it transiently yields a strongly absorbing
Z-AIZH+ isomer upon UV-irradiation (Fig. 1B). Upon increasing
the equivalent CF3SO3H to ca. 30 eq. the absorption of the red-
shied Z-AIZH+ reaches a fully photo- (Fig. 5B) and thermor-
eversible (Fig. S24†) steady state, while a further increase to ca.
500 eq. leads to a more signicant share of E-AIZH+ and
a limitation of absorption changes primarily to the UV-range
(Fig. 5C). At ca. 800 eq. of CF3SO3H, the absorption is domi-
nated by the azonium species, their large overlap in absorption
providing subtle photoreversible changes in the UV- and visible
range (Fig. 5D). Thus, we nd that although the molecular
Acid-catalysed liquid-to-solid phase transitioning
Although observed in few AAPs also,51,52 AIZs exhibit a much
higher tendency to undergo solid state switching with reversible
solid-to-liquid phase transitions, owing to a drastic change in
melting point upon the out-of-plane bending in the Z-form.43
Fascinatingly, the pKa dependence of the liqueed AIZ is
completely orthogonal to that in solution, showing for all three
AIZs a swi catalytic reversion in response to CF3COOH
vapours, accompanied by a delayed liquid-to-solid phase tran-
sition (Video S1†) that was also observed photochemically.44
Analogous to the solution behaviour, CH3COOH fumes yield no
response in phase as well as absorption, while CF3SO3H vapours
are found to be sufficiently acidic to protonate either isomer,
merely resulting in slow Z/E-equilibration (Video S2†). Slow
dispersion of the CF3SO3H in air occurs over time, however,
Fig. 5 UV/vis absorption spectra of AIZ 3 (70 mM in acetonitrile) giving rise to varying photocontrol over azo-speciation at 365 nm (radiant flux
1.2 W, teal arrows) and at 520 nm (radiant flux 87 lm, orange arrows) depending on the amount of CF3SO3H present. 20 steps shown for (B)–(D), ‡
¼ not clearly observed. Since only 2 species are predominant at 30 and 800 equivalents of CF3SO3H, this allows for approximation of the PSSs
under these circumstances by comparing the combined absorption at l ¼ 340 and 460 nm with those observed in Fig. 1C: (B) PSS365nm z 62% Z-
AIZH+ and 38% E-AIZ, PSS520nm z 20% Z-AIZH+ and 80% E-AIZ; (D) PSS365nm z 37% Z-AIZH+ and 63% E-AIZH+, PSS520nm z 12% Z-AIZH+, 88%
E-AIZH+.
Chem. Sci.
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Edge Article
Chemical Science
ultimately leading to resolidication over ca. 45 min as stoi- photochemically.44 The larger error margin is due to the
chiometric quantities of acid still induce catalysed reversion adventitious coverage of the acid-catalysis sometimes leaving
(Fig. 5A). Only triuoroacetic acid and hydrochloric acid (Video residual Z-isomer, most notably upon the more rapid acid
S2†) cause an immediate red-shi in absorption followed up by catalysis with HCl which yields an overall lower glass-to-glass
a blue-shi (Fig. S25†), indicative of transient formation of the adhesive strength of 15.9 ꢂ 3.0 N cmꢀ2. The photochemical
metastable red-shied Z-AIZH+ isomer.
debonding in the acid-cured adhesives is fully retained, never-
theless, with identically swi response times of photochemical
debonding, i.e., ca. 0.5 seconds, under near-maximum loads
(Fig. 6B, Video S4†). To further probe the inuence of acid on
adhesive strength, introduction of CF3COOH during complete
thermal liquecation and resolidication, i.e., adding vapours
aer heating past the AIZ melting point and immediately con-
joining the slides, was found to yield a thermal adhesive
strength of 13.9 ꢂ 1.1 N cmꢀ2. Although the thermal adhesion
of 19.6 N cmꢀ2 without acid vapours was previously also shown
to be lower than in the photochemical pathway,44 the adhesive
strength is affected by acid vapours more strongly in the
thermal pathway. Thus it seems that the efficiency of the cata-
lytic reversion plays a key role in attaining adhesive strengths
nearing those obtained photochemically.
Acid-catalysed adhesion
The retention of acid-catalysis in the liqueed Z-isomer fol-
lowed by the delayed recrystallisation of the E-isomer opens up
further opportunities in the application of AIZs as adhesives
(Fig. 6A). The delay in nucleation of several seconds aer the
near-instant acid-catalysis with CF3COOH vapours provides
time to press the surfaces together, while the resulting ux of
still liquid E-isomer favourably delays the recrystallisation
further and allows for quick readjustments still (Video S3†).
Accordingly, the acid-catalysed curing yields glass-to-glass
adhesive strengths of 25.8 ꢂ 4.8 N cmꢀ2 and PMMA-to-PMMA
adhesion at 21.9 ꢂ 2.5 N cmꢀ2 compared to 29.7 ꢂ 0.1 N
cmꢀ2 and 22.9 ꢂ 1.2 N cmꢀ2, respectively, that were achieved
Fig. 6 (A) Strategy for acid-catalysed adhesion independent of the transparency of substrates. Acid-catalysis by CF3COOH acid vapours causes
near-instant Z-to E-reisomerisation of photoliquefied AIZ 3. The several seconds that separate the catalytic recovery of E-AIZ and the advent of
nucleation sites for recrystallisation provide time to press slides together before the solidification takes place. (B) Suspension of glass slides
conjoined with 2.2 mg of compound 3 at a 1.0 by 1.4 cm overlapping surface area, resulting in an adhesive strength of 25.8 ꢂ 4.8 N cmꢀ2 (shown
here to hold 3.8 kg). The UV-induced debonding44 is unaffected by the acid-catalysis over 5 cycles of acid-catalysed bonding and photochemical
debonding (Video S4†), fully retaining response times of ca. 0.5 s at near-maximum loads. (C) Suspension of aluminium slides conjoined with
3.4 mg AIZ 3 at a 1.0 by 1.4 cm overlapping surface area, capable of holding a load of 35.8 ꢂ 3.5 N cmꢀ2 (over 15 samples, shown here to hold 5.7
kg). Although the non-transparency rules out photochemical annealing, acid-catalysed bonding can be repeatedly performed at 35.0 ꢂ 2.8 N
cmꢀ2 over 5 cycles of physical debonding.
© 2021 The Author(s). Published by the Royal Society of Chemistry
Chem. Sci.

View Article Online
Chemical Science
Edge Article
Interestingly, the lingering presence of trace acid vapours the adhesive and the fundamental understanding thereof opens
between adhered slides also invokes a self-healing mechanism up further opportunities in the use of the azoisoxazole class of
in the AIZ adhesive (Video S5†), persisting aer 10 days still photoswitches, not least the successful adhesion of non-
(Video S6†). The debonding is found to be unaffected by the self- transparent substrates. Furthermore, the adhesive shows
healing, as the latter occurs over a longer timescale than the excellent reusability aer physical breaking of cohesion
near-instant photoliquecation of the interfacial thin layer that between non-transparent substrates, retaining its “green”
dictates the adhesive strength. The mechanism of the acid- application. Ultimately, the expanded understanding of the
catalysed reversion thus additionally holds promise for acid-reactivity in azoisoxazoles and the demonstrated advance
increased resilience against exposure to ambient UV. Further- in application have broad implications for the pH-tuneable
more, while the photoreversible nature of the solid-to-liquid functionality of the myriad of existing azobenzenes and
phase transition requires the use of at least transparent azoheterocycles.
surface to harden the versatile adhesive at a range of metallic
and polymeric substrates,44 the acid-catalysed recrystallisation
Data availability
is wavelength independent and opens up to non-transparent
surfaces being conjoined also.
The datasets supporting this article have been uploaded as part
of the ESI.
In this way, the acid-catalysed pathway further broadens the
scope to completely non-transparent substrates with, e.g., a pair
of aluminium slides being able to bear 35.8 ꢂ 3.5 N cmꢀ2
(Fig. 6C, S26†). Though the adhesion of non-transparent
surfaces cannot be reversed with light, physical separation of
such surfaces (i.e., breaking of the cohesion by force, or heating
over the melting point of the adhesive44) still yields the chemi-
cally unaltered AIZ 3. Accordingly, a pair of aluminium slides
was repeatedly glued together to successfully yield an adhesive
strength of 35.0 ꢂ 2.8 N cmꢀ2 over 5 bonding/debonding cycles
without increasing the amount of adhesive agent.
Author contributions
LK conceptualised the work and rened it with the help of JS,
NBA and BJR. JS performed the synthesis and characterisation
of compounds 1–4. LK performed the primary investigation,
NBA supported on the solution studies. MB performed the
theoretical investigation under supervision of NLD. BJR
provided supervision and reviewed and edited the dra, LK
wrote the manuscript.
Conclusions
Conflicts of interest
Previously, the arylazoisoxazole class of azo switches was shown
to possess promising photophysical characteristics in solution
as well as the solid-state. The solid-state E-to Z-AIZ photo-
switching accompanied by a reversible solid-to-liquid phase
transition made the photoswitch extremely suitable for appli-
cation in fully photocontrolled reversible adhesion. Here, we
demonstrate that the acid-catalysed Z-to E-AIZ reversion in
solution is tuneable by the pKa of the acid employed. We show
that although such acid-catalysis was observed for azobenzenes
and azopyrazoles before, the key to this reactivity is that the acid
strength is intermediate to the Z-and E-isomer. Commonly
employed acids possess a pKa lower than only that of Z-AIZH+,
thermodynamically driving the isomerisation over the signi-
cantly lower Z-AIZH+ to E-AIZH+ energy barrier as conrmed by
DFT. Conversely, when an acid is employed that possesses a pKa
that is lower than both the Z-and E-isomer, such as CF3SO3H, it
leads to a gradual establishment of the equilibrium between the
now thermally stable E-AIZH+ and Z-AIZH+, giving rise to several
regimes of photoreversible control over a 4-state switching
cycle. Most interestingly, the acid catalysis observed in the
solution studies applies to the pristine liqueed Z-isomer also,
There are no conicts to declare.
Acknowledgements
We are grateful for funding by the Alexander von Humboldt
Foundation (postdoctoral fellowship to LK) and the Deutsche
Forschungsgemeinscha (project Z01 in CRC 858 to MB and
NLD and project A01 in CRC 1459 to BJR).
Notes and references
1 A. Goulet-Hanssens, F. Eisenreich and S. Hecht, Adv. Mater.,
2020, 32, 1905966.
2 C. Buten, L. Kortekaas and B. J. Ravoo, Adv. Mater., 2020, 32,
1904957.
3 M. Baroncini, G. Ragazzon, S. Silvi, M. Venturi and A. Credi,
Pure Appl. Chem., 2015, 87, 537–545.
4 H. M. D. Bandara and S. C. Burdette, Chem. Soc. Rev., 2012,
41, 1809–1825.
5 R. Klajn, Chem. Soc. Rev., 2014, 43, 148–184.
6 L. Kortekaas and W. R. Browne, Chem. Soc. Rev., 2019, 48,
3406–3424.
7 M. Irie, T. Fukaminato, K. Matsuda and S. Kobatake, Chem.
Rev., 2014, 114, 12174–12277.
8 Z. L. Pianowski, Chem. - Eur. J., 2019, 25, 5128–5144.
9 A. Fihey, A. Perrier, W. R. Browne and D. Jacquemin, Chem.
Soc. Rev., 2015, 44, 3719–3759.
providing
a chemical trigger for rapid and wavelength-
independent hardening of the adhesive with HCl or CF3COOH
vapours. Moreover, although the E-isomer has no strong
enough interaction with CF3COOH vapours, traces of the acid
remain aer the acid-catalysed adhesion and provide
a continual self-healing mechanism without distorting the
adhesive properties. The non-destructive chemical hardening of
Chem. Sci.
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Edge Article
Chemical Science
10 A. Abdollahi, H. Roghani-Mamaqani, B. Razavi and 33 Q. Bian, M. Jin, S. Chen, L. Xu, S. Wang and G. Wang, Polym.
M. Salami-Kalajahi, Polym. Chem., 2019, 10, 5686–5720. Chem., 2017, 8, 5525–5532.
11 M. Stratigaki and R. Gostl, ChemPlusChem, 2020, 85, 1095– 34 A. K. Saydjari, P. Weis and S. Wu, Adv. Energy Mater., 2017, 7,
1103. 1601622.
12 H. Nie, J. L. Self, A. S. Kuenstler, R. C. Hayward and J. Read 35 D. Wang, M. Wagner, H.-J. Butt and S. Wu, So Matter, 2015,
de Alaniz, Adv. Opt. Mater., 2019, 7, 1900224. 11, 7656–7662.
13 L. Kortekaas, J. Chen, D. Jacquemin and W. R. Browne, J. 36 S. Crespi, N. A. Simeth and B. Konig, Nat. Rev. Chem., 2019, 3,
¨
¨
Phys. Chem. B, 2018, 122, 6423–6430.
14 G. S. Hartley, J. Chem. Soc., 1938, 633–642.
15 S. Ciccone and J. Halpern, Can. J. Chem., 1959, 37, 1903–
1910.
16 R. Lovrien and J. C. B. Waddington, J. Am. Chem. Soc., 1964,
86, 2315–2322.
133–146.
37 R. S. L. Gibson, J. Calbo and M. J. Fuchter, ChemPhotoChem,
2019, 3, 372–377.
38 J. Calbo, A. R. Thawani, R. S. L. Gibson, A. J. P. White and
M. J. Fuchter, Beilstein J. Org. Chem., 2019, 15, 2753–2764.
39 S. Ludwanowski, M. Ari, K. Parison, S. Kalthoum, P. Straub,
N. Pompe, S. Weber, M. Walter and A. Walther, Chem. - Eur.
J., 2020, 26, 13203–13212.
17 G. Wettermark, M. E. Langmuir and D. G. Anderson, J. Am.
Chem. Soc., 1965, 87, 476–481.
18 N. J. Dunn, W. H. Humphries, A. R. Offenbacher, T. L. King 40 C. E. Weston, R. D. Richardson, P. R. Haycock, A. J. P. White
and J. A. Gray, J. Phys. Chem. A, 2009, 113, 13144–13151. and M. J. Fuchter, J. Am. Chem. Soc., 2014, 136, 11878–11881.
19 A. M. Sanchez, M. Barra and R. H. de Rossi, J. Org. Chem., 41 L. Stricker, E.-C. Fritz, M. Peterlechner, N. L. Doltsinis and
1999, 64, 1604–1609.
B. J. Ravoo, J. Am. Chem. Soc., 2016, 138, 4547–4554.
¨
20 E. Sawicki, J. Org. Chem., 1957, 22, 1084–1088.
21 L. P. Hammett, J. Am. Chem. Soc., 1928, 50, 2666–2673.
42 L. Stricker, M. Bockmann, T. M. Kirse, N. L. Doltsinis and
B. J. Ravoo, Chem. - Eur. J., 2018, 24, 8639–8647.
22 M. S. Scholz, J. N. Bull, N. J. A. Coughlan, E. Carrascosa, 43 P. Kumar, A. Srivastava, C. Sah, S. Devi and
B. D. Adamson and E. J. Bieske, J. Phys. Chem. A, 2017,
121, 6413–6419.
23 S. Stoyanov, L. Antonov, T. Stoyanova and V. Petrova, Dyes
Pigm., 1996, 32, 171–185.
24 P. Weis and S. Wu, Macromol. Rapid Commun., 2018, 39,
1700220.
25 A. A. Beharry, O. Sadovski and G. A. Woolley, J. Am. Chem.
Soc., 2011, 133, 19684–19687.
S. Venkataramani, Chem. - Eur. J., 2019, 25, 11924–11932.
44 L. Kortekaas, J. Simke, D. W. Kurka and B. J. Ravoo, ACS Appl.
Mater. Interfaces, 2020, 12, 32054–32060.
¨
¨
45 A. Kutt, T. Rodima, J. Saame, E. Raamat, V. Maemets,
I. Kaljurand, I. A. Koppel, R. Y. Garlyauskayte,
Y. L. Yagupolskii, L. M. Yagupolskii, E. Bernhardt,
H. Willner and I. Leito, J. Org. Chem., 2011, 76, 391–395.
`
46 R. J. W. Le Fevre and J. Northcott, J. Chem. Soc., 1953, 867–
´
26 D. Bleger, J. Schwarz, A. M. Brouwer and S. Hecht, J. Am.
870.
Chem. Soc., 2012, 134, 20597–20600.
27 M. J. Hansen, M. M. Lerch, W. Szymanski and B. L. Feringa,
Angew. Chem., Int. Ed., 2016, 55, 13514–13518.
47 J. Fregoni, G. Granucci, M. Persico and S. Corni, Chem, 2020,
6, 250–265.
´
48 J. Dokic, M. Gothe, J. Wirth, M. V. Peters, J. Schwarz, S. Hecht
28 S. Samanta, A. Babalhavaeji, M. Dong and G. A. Woolley,
Angew. Chem., Int. Ed., 2013, 52, 14127–14130.
and P. Saalfrank, J. Phys. Chem. A, 2009, 113, 6763–6773.
49 L. Zhou, L. Chen, G. Ren, Z. Zhu, H. Zhao, H. Wang,
W. Zhang and J. Han, Phys. Chem. Chem. Phys., 2018, 20,
27205–27213.
´
´
29 M. Dong, A. Babalhavaeji, M. J. Hansen, L. Kalman and
G. A. Woolley, Chem. Commun., 2015, 51, 12981–12984.
30 M. Dong, A. Babalhavaeji, C. V. Collins, K. Jarrah, 50 D. L. Isac, A. Airinei, M. Homocianu, N. Fifere, C. Cojocaru
O. Sadovski, Q. Dai and G. A. Woolley, J. Am. Chem. Soc.,
2017, 139, 13483–13486.
and C. Hulubei, J. Photochem. Photobiol., A, 2020, 390,
112300.
31 M. Dong, A. Babalhavaeji, S. Samanta, A. A. Beharry and 51 M. A. Gerkman, R. S. L. Gibson, J. Calbo, Y. Shi, M. J. Fuchter
G. A. Woolley, Acc. Chem. Res., 2015, 48, 2662–2670. and G. G. D. Han, J. Am. Chem. Soc., 2020, 142, 8688–8695.
32 G. Wang, D. Yuan, T. Yuan, J. Dong, N. Feng and G. Han, J. 52 Y. Nagai, K. Ishiba, R. Yamamoto, T. Yamada, M. Morikawa
Polym. Sci., Part A: Polym. Chem., 2015, 53, 2768–2775.
and N. Kimizuka, Angew. Chem., Int. Ed., 2021, 60, 6333–
6338.
© 2021 The Author(s). Published by the Royal Society of Chemistry
Chem. Sci.