Gating the photochromism of an azobenzene by strong host-guest interactions in a divalent pseudo[2]rotaxane

Article abstract of DOI:10.1039/c5cc02811f

The ability of an E-configured azobenzene guest to undergo photoisomerisation is controlled by the presence of a complementary host. Addition of base/acid allowed for a weakening/strengthening of the interactions in the divalent pseudo[2]rotaxane complex and hence could switch on/off photochromic activity.

Full text of DOI:10.1039/c5cc02811f

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Gating the photochromism of an azobenzene by
strong host–guest interactions in a divalent
pseudo[2]rotaxane†
Cite this:DOI: 10.1039/c5cc02811f
Received 4th April 2015,
Accepted 23rd April 2015
a
b
b
b
Mirko Lohse, Karol Nowosinski, Nora L. Traulsen, Andreas J. Achazi,
b
b
b
a
Larissa K. S. von Krbek, Beate Paulus,* Christoph A. Schalley* and Stefan Hecht*
DOI: 10.1039/c5cc02811f
www.rsc.org/chemcomm
The ability of an E-configured azobenzene guest to undergo photo- been paid to cases where the non-covalent interactions influence
11
isomerisation is controlled by the presence of a complementary the azobenzene’s photoswitchability.
host. Addition of base/acid allowed for a weakening/strengthening Here, we describe the first pseudo[2]rotaxane assembly invol-
of the interactions in the divalent pseudo[2]rotaxane complex and ving a divalent crown ether host and a divalent photochromic
hence could switch on/off photochromic activity.
azobenzene guest. By making or breaking of the host–guest
complex we can either lock or unlock the E-isomer and thereby
Photochromism, the reversible switching of a molecule between gate its photoisomerisation ability.
two states by light, provides an excellent tool for the development
The key components of our host–guest system are photochromic
of smart materials by gaining remote control over their properties axles 1 and 2, carrying two terminal secondary ammonium-binding
1
via an external stimulus. Being able to turn on and off the ability sites, which are based on azobenzene and stilbene, respectively
of a photoswitch to isomerise, so-called ‘‘gated photochromism’’, (Fig. 1). The latter has been prepared in particular to access Z-2 as a
2
12
is desirable to implement responsiveness to multiple stimuli
and has primarily been achieved by acid–base reactions or known and tuneable interaction between a secondary ammonium
thermally stable, structural analogue of Z-1. Based on the well-
3
13
coordination of (transition) metal ions. In addition, specific ion and a dibenzo-24-crown-8, anthracene-spacered divalent
1
4
host–guest interactions involving one isomer of the photoswitch crown ether 3 was chosen as the complementary divalent
selectively could in principle also be employed to gate the switch- host. In addition, monovalent ammonium guest 5 and mono-
ing process and in turn provide photocontrol over the association valent crown ether 4 were prepared for comparison purposes.
process. The underlying binding event should be readily modu-
Synthesis of the azobenzene axle 1 involves a straightforward
lated by large structural changes in either the host or guest. linear sequence of reductive amination of 4-nitrobenzaldehyde
Therefore, it seems reasonable to utilise a photoswitch, which with benzyl amine and BOC-protection, followed by reduction to
undergoes a light-induced E–Z double bond isomerisation. In the aniline derivative and oxidative dimerization. BOC-deprotection
4 6
this context, azobenzene can be considered as the ‘‘drosophila’’ and protonation with NH PF finally yields hexafluorophosphate
in the field. Its significant structural changes upon E–Z photo- salt E-1. Axles E-2 and Z-2 were derived from the corresponding
4
15
16
isomerisation have been used to modulate microscopic and even (E)- and (Z)-stilbene dialdehydes via reductive amination and
5
macroscopic property changes, which have been exploited in protonation. The synthesis and characterisation of host 3 have
6
1,5,7
14
various areas from chemistry over material science
to bio- been described previously. More information about the synthesis
logy and potentially pharmacology. Whereas authors typically and compound characterisation is provided in the ESI.†
care about the influence of azobenzene photoisomerisation on To investigate the host–guest interactions between guest E-1 and
8
9
10
1
the resulting supramolecular structure, much less attention has host 3, H NMR spectroscopy experiments have been carried out. In
an equimolar mixture, a new set of signals appears, which is related
a
b
to complex E-1@3 (Fig. 2 top, see also ESI†, Fig. S1 and S2).
Whereas significant chemical upfield shifts were observed for the
aromatic protons H (Dd E 0.35 ppm) and H (Dd E 0.7 ppm) of
the azobenzene core of axle E-1 due to their position atop the
anthracene unit of host 3 indicating exclusive formation of doubly
Department of Chemistry, Humboldt-Universit a¨ t zu Berlin, Brook-Taylor-Str. 2,
2489 Berlin, Germany. E-mail: sh@chemie.hu-berlin.de;
1
Fax: +49 (0)30 2093-6940; Tel: +49 (0)30 2093-7308
d
e
Institut f u¨ r Chemie und Biochemie, Freie Universit a¨ t Berlin, Takustraße 3,
1
4195 Berlin, Germany. E-mail: b.paulus@fu-berlin.de, christoph@schalley-lab.de;
Fax: +49 (0)30 838-55366; Tel: +49 (0)30 838-52639
b c
bound dimers. The benzylic protons H and H shifted downfield
†
Electronic supplementary information (ESI) available: Experimental procedures
(
Dd E 0.5 ppm) upon complexation. These large shifts can be
and characterisation data, ITC and photochemistry experiments. See DOI:
0.1039/c5cc02811f
1
explained by the location of the azobenzene protons above the
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Fig. 1 Isomerization of divalent azobenzene and stilbene axles 1 and 2, divalent host 3, monovalent reference compounds 4 and 5, as well as divalent
pseudo[2]rotaxanes E-1@3 and E-2@3.
Table 1 Thermodynamic binding data of E-1@3, 5@4, E-1@4
2
and 5
3
: CH CN = 2.2 : 1, 298 K)
2
@3
as obtained from ITC experiments (CHCl
3
a
DG
[kJ mol
DH*
[kJ mol
TDS*
À1
À1
À1
À1
Complex
K
a
[M
]
]
]
[kJ mol
]
5
4
E-1@3
2 Â 10 Æ 2 Â 10 À30.3 Æ 0.2 À52.1
À21.9
À20.6
À7.7
5@4
1800 Æ 200
3100 Æ 300
870 Æ 90
À18.6 Æ 0.2 À39.3
À19.9 Æ 0.3 À27.6
À16.8 Æ 0.2 À35.0
À20.4 Æ 0.2 À26.3
À12.7 Æ 0.3 À32.9
E-1@4
2
K
K
K
K
1
2
1
2
À18.1
À5.9
5
2
@3
3700 Æ 400
170 Æ 20
À20.2
a
DH and DS values have larger errors than those of DG and should be
regarded as estimate rather than precise values.
In order to investigate the association behaviour of the different
axles with the host more thoroughly, a detailed thermodynamic
analysis of the assembly was carried out involving isothermal
titration calorimetry (ITC), which has proven very insightful for
1
Fig. 2 H NMR experiment of the complexation of azobenzene axle E-1
top) as well as E-2 (bottom) to host 3 (500 MHz, CDCl : CD CN 2 : 1,
.4 mM). The dashed lines indicate the shifting of selected protons upon
formation of the complex.
(
3
3
1
7
studying strongly bound pseudorotaxanes. In contrast to
similar pseudo[2]rotaxanes based on crown ether–ammonium
1
1
7
interactions, a very strong binding was observed for complex
E-1@3, giving rise to an unusually high association constant of
À1
anthracene core of host 3 on the one hand and the deshielding Ka = 200000 M (Table 1). This strong interaction demonstrates
of the benzylic protons due to C–HÁ Á ÁO hydrogen-bonding with the perfect fit of axle E-1 to host 3. Additional secondary binding
the crown ether oxygen atoms as well as due to the aromatic events, such as acceptor–donor type p,p-stacking of the electron-
18
ring current of the anthracene spacer, on the other. Note that deficient azobenzene and the electron-rich anthracene cores,
the splitting of several peaks for crown ether methylene groups together with the divalent complexation contribute to a large
in the complex E-1@3 is caused by the threading of the axle effective molarity (EM) value of 380 mM and a highly positive
1
9
through the host, resulting in new sets of diastereotopic proton chelate cooperativity factor of Kmono  EM = 340 c 1.
signals. EXSY NMR studies show that in the chosen solvent Additional ITC experiments to deduce the association constants
mixture (CDCl : CD CN = 2 : 1) there is no detectable exchange of the stilbene-based complexes E-2@3 and Z-2@3 were not
3
3
between the bound and free axle E-1, indicating that dethreading successful due to insufficient solubility of the axle components.
is slow. In addition, ESI mass spectrometry supports the formation However, in an insightful competition experiment monitored
1
of the complex E-1@3 (see ESI†, Fig. S25).
by H NMR spectroscopy it could be shown that upon addition
Trying to elucidate the structure of the host–guest complex of E-1 to an equimolar solution of Z-2 and 3, E-1@3 was formed
formed by the photoisomerised axle Z-1 is practically impos- rapidly (see Fig. S30 in ESI†). This indicates that the association
sible due to its thermal Z - E back isomerisation (note that the constant of Z-2@3 containing the Z-configured (stilbene) axle
inevitably forming E-1 binds to the host 3 much more strongly). has to be much lower than the one of the complex composed of
Therefore, the isosteric stilbene axles E-2 and Z-2 have been the E-configured (azobenzene) axle. The association constants of
1
studied. While H NMR titration reveals formation of a stable the monovalent model complexes 5@4, E-1@4
2
and 5
values for the first
2
@3 compared to 5@4 are
2
@3 are all
1
: 1 complex E-2@3 (Fig. 2, bottom), the Z-configured axle lower and in a similar range. Higher K
Z-2 does not yield a well-defined complex upon addition of 3 binding step in case of E-1@4 and 5
a
2
(see ESI†, Fig. S27).
expected due to statistics. However, both show negative allosteric
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interaction with reduced K
a
values for the second binding
) = 0.8 and
step and allosteric cooperativity factors a(E-1@4
a(5 @3) = 0.2, which has been found before for similar systems.
2
17
2
We attribute the quite strong negative allosteric cooperativity of
52@3 to the polarization of the p-system of the crown ether’s
anthracene spacer by the first ammonium ion. This increases
positive or diminishes negative partial charges at the second crown
ether binding site and thus reduces the binding strength of the
second monovalent axle.
To gain further insight into the binding situation, in particular
of the illusive complexes composed of the Z-configured axles, the
Gibbs energies DG for the 1 : 1 association were calculated by DFT
20
using an approach developed by Grimme. The calculations were
21
performed with TPSS-D3(BJ)/def2-TZVP including solvent effects
22
with the COSMO-RS solvent model. In these calculations it is
À
assumed that one PF
6
anion is close to the free guest and has to
be moved away from the guest during the threading. We have
recently described this approach in detail for similar systems and
the computed results agree well with the experiment. For E-1@3
the calculated Gibbs energy of association DG (Table S2 in ESI†) is
close to the experimental value, validating our method. According
to these calculations it turns out that both divalent complexes
23
3 3
Fig. 3 E - Z photoisomerisation of E-1 (40 mM in CHCl : CH CN = 2 : 1)
upon irradiation with 334 nm in the absence (top) and presence (bottom)
of an equimolar amount of host 3 (dashed line shows spectrum of pure 3).
Z-1@3 and Z-2@3 are thermodynamically unfavourable. Hence, This gives evidence for the successful inhibition of azobenzene
they should not form in agreement with our experiments showing photochromism (Fig. 3, bottom).
no evidence for the formation of discrete 1 : 1 complexes in the case
Binding to host 3 selectively inhibits the forward E - Z
of the Z-configured axles. Furthermore, the calculations predict a photoisomerisation of 1, but does not affect the reverse process
weaker yet favourable interaction energy for E-2@3, which is in line since irradiation of a PSS mixture (containing 90% Z-1 and
with a weaker p,p-stacking of the more electron-rich stilbene moiety 10% E-1) in the presence of 3 at 436 nm showed clean Z - E
but cannot be compared with experiment since no quantitative photoisomerisation (see Fig. S37a in ESI†). However, the presence
data are accessible. The strong stabilisation of the E-isomer upon of host 3 does not accelerate the thermal Z - E isomerisation by
binding to the host is furthermore evident from comparison of selective stabilisation of E-1@3, but in contrast the aggregate
the Gibbs energies for the E-1 - Z-1 isomerisation in the free formed with Z-1 decelerates the thermal back reaction (for com-
À1
À1
(
DG = +28.8 kJ mol ) and in the complexed (DG = +115.3 kJ mol
)
parison of thermal Z - E isomerisation in the free and complexed
form see Fig. S35, ESI†). However, a clean E - Z photoisomerisa-
form (detailed description see ESI†).
After analysis of the binding of the axles in their two tion can be observed when irradiating E-1 in the presence of
isomeric forms to the host, we were interested in the effect of excess monovalent host 4, i.e. forming E-1@4 (see Fig. S36b in
2
this complexation on the photoisomerisation behaviour. In the ESI†). This clearly shows that complexation of the ammonium
absence of host 3, axle E-1 shows the typical spectroscopic ions is not responsible for the observed inhibition effect. Further-
signature of a regular azobenzene derivative with an intense more, it is possible to induce E - Z photoisomerisation of the
p,p* band at 333 nm and a weak shoulder at 450 nm associated dethreaded, unprotonated axle in the presence of 3 (see Fig. S36a
with the symmetry forbidden n,p* transition (Fig. 3, top). Upon in ESI†), showing that the overlapping spectra of both species are
irradiation at 334 nm, the p,p* band decreases while the n–p* neither the reason for the inhibition. Fluorescence measurements
band increases and a photostationary state (PSS) with 90% Z-1 (see ESI†, Fig. S39) of the E-1@3 complex show no indication of
is reached. The quantum yield of the E - Z photoisomerisation energy transfer at various excitation wavelengths but in contrast,
for axle 1 under these conditions is in the expected range complexation leads to fluorescence quenching of the host, pre-
2
5
F
E-Z = (6.8 Æ 0.1)%. The activation barrier for the thermal sumably due to photoinduced electron transfer. In view of the
‡
À1
Z - E isomerisation of Z-1 amounts to DG = (110 Æ 7) kJ mol
,
strong binding in E-1@3 and considering the tuneability of the
giving rise to a thermal half-life of t1/2 = (127 Æ 1) h at room ammonium ion–crown ether interaction by solvent polarity,
temperature (see ESI†, Fig. S33 and S34). When mixing axle E-1 and another switching experiment was carried out in a more polar
host 3, their absorption spectra largely overlap (for the absorption solvent mixture, i.e. 14:1 instead of 1:2 CH
3 3
CN : CHCl . In this
spectra of individual compounds see Fig. S38 in ESI†). To prevent case, the attractive interactions between axle E-1 and host 3 are
24
potential photooxidation of the anthracene moiety, all photo- reduced due to competition of their hydrogen bond acceptors and
chemical experiments were carried out in degassed solvents. Upon donors with the polar solvent. As a consequence, inhibition
irradiation of an equilibrated 1 : 1 mixture of the axle and host becomes much less effective and the photochromic behaviour is
compounds at 334 nm, only minor spectral changes were observed, recovered (see ESI†, Fig. S37b). Note that the weaker binding
indicating only little photoisomerisation taking place in E-1@3. under these conditions could not be quantified by ITC due to
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Notes and references
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2
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Fig. 4 E - Z photoisomerisation of E-1 in presence of 3 (1@3, solid black
line), deprotonated E-1 in presence of 3 (1–2H + 3, dashed black line),
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2
, solid red line) monitoring azobenzene
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4.1 equiv.) reprotonates the axle and resets the system. Note that the system
does not recover fully because of the photostationary state of deprotonated
and photochemical side reactions of 3.
9
1
1
0 (a) X. Chen, L. Hong, X. You, Y. Wang, G. Zou, W. Su and Q. Zhang,
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above experiments clearly indicate that formation of the E-1@3
complex is actually responsible for inhibition and suggest that in
the tight, vice-like pseudo[2]rotaxane binding scenario E - Z
photoisomerisation is shut down.
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1
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scale of minutes to few hours. For a notable recent exception, see:
absorption maximum at 350 nm, one can see that the decrease
after 60 min for the complex E-1@3 is approximately 7 times
(
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+
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2
1
(Fig. 4, first 60 min). Subsequent in situ deprotonation of the
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3
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2
1
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In the present study, we found a unique way in which strong
binding of a divalent azobenzene guest to a complementary
host was found to effectively inhibit E - Z photoisomerisation.
This complexation-induced gating effect can be tuned by changing
the strength of the non-covalent interactions, either by varying
solvent polarity or more effectively by adding base or acid.
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2
22 F. Eckert and A. Klamt, COSMOtherm, Version C3.0, Release 13.01,
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The authors thank the Deutsche Forschungsgemeinschaft
for generous financial support (SFB 765). Support by the High-
Performance Computing facilities of the Freie Universit ¨a t Berlin
2
2
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(ZEDAT) is acknowledged. L.v.K. is grateful to the Studienstiftung
2
des Deutschen Volkes for a PhD fellowship.
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Chem. Commun.
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