Arylazopyrazoles: Azoheteroarene photoswitches offering quantitative isomerization and long thermal half-lives

Article abstract of DOI:10.1021/ja505444d

Arylazopyrazoles, a novel class of five-membered azo photoswitches, offer quantitative photoswitching and high thermal stability of the Z isomer (half-lives of 10 and ～1000 days). The conformation of the Z isomers of these compounds, and also the arylazop

Full text of DOI:10.1021/ja505444d

Communication
pubs.acs.org/JACS
Arylazopyrazoles: Azoheteroarene Photoswitches Offering
Quantitative Isomerization and Long Thermal Half-Lives
Claire E. Weston, Robert D. Richardson, Peter R. Haycock, Andrew J. P. White, and Matthew J. Fuchter*
Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom
S
* Supporting Information
heteroaromatic compounds have incomplete E−Z conversion
and fast thermal half-lives (usually under 1 min).19,20,22 Velasco
ABSTRACT: Arylazopyrazoles, a novel class of five-
membered azo photoswitches, offer quantitative photo-
switching and high thermal stability of the Z isomer (half-
lives of 10 and ∼1000 days). The conformation of the Z
isomers of these compounds, and also the arylazopyrroles,
is highly dependent on the substitution pattern on the
heteroarene, allowing a twisted or planar geometry, which
in turn has a significant impact on the electronic spectral
properties of the compounds.
and co-workers recently exploited this property to develop
extremely fast photoswitches (t1/2 values of microseconds to
nanoseconds) that tolerate thousands of switching cycles
without decomposition.25,26 Conversely, for applications
requiring long half-lives and high E−Z conversion, the
arylazoimidazoles are the only five-membered azoheteroarenes
to show promise,27−29 with Herges and co-workers demon-
strating the ability to photoswitch 1-methyl-5-phenylazoimida-
zole to a photostationary state (PSS) containing 98% Z isomer,
which possesses a thermal half-life of 528 h.27 However, the
best PSS for the backswitching only contained 55% E isomer.
We report a new class of highly promising photoswitchable
azoheteroarenes, the arylazopyrazoles (Figure 1),30 which we
have directly compared to the related arylazopyrroles. Our
arylazopyrazoles show high thermal stability in solution at room
temperature (t1/2 up to 1000 days), comparable to the highest
known value for an azobenzene. A large separation of the λmax
values of the E and Z isomers allows good to quantitative two-
way photoswitching. By virtue of their five-membered rather
than six-membered aromatic ring, we find that such compounds
can access a Z isomer conformation not accessible to
azobenzenes, a conformation that can be sterically tuned by
substitution on the heterocyclic ring. Such conformational
properties significantly affect the intensity of the n−π*
absorbance, thus azoheteroarenes offer photophysical and
photochemical properties not achievable with azobenzenes.
hotoswitchable compounds have diverse applications, from
Pphotopharmacology and optochemical genetics to data
storage.1−8 Azobenzenes are easily synthesized, highly versatile
photoswitches. They have high extinction coefficients and
quantum yields, allowing photoswitching with low-intensity
light, and are stable to repeated switching.9 Photochemical E−Z
isomerization induces a large change in the shape of these
molecules, including a significant difference in the end-to-end
distance.9 This has been exploited, for example, in the direct
conversion of light to mechanical energy.10−12 Generally, the E
isomers are more thermodynamically stable than the Z isomers,
the notable exception being Temps and co-workers’ tricyclic
ethylene-bridged azobenzene, which has reversed stability due
to ring strain in the E isomer.13
A large array of azobenzenes have been reported, with an
assortment of applications; however there are still a number of
drawbacks that limit their use for certain purposes. First,
incomplete photoswitching is often observed due to over-
lapping absorbances in the irradiation regions of interest.3
Second, the Z isomer often rapidly thermally converts back to
the E isomer. High thermal stability is particularly required in
ultrahigh-density optical data storage, an area currently
dominated by diarylethylenes.7,8 Since the properties of
azobenzenes can be varied substantially by altering the
substituents on the aromatic rings, a number of specific
azobenzenes have been recently reported that address such
limitations.14−17 For example, Hecht and co-workers recently
reported that 2,2′,6,6′-tetrafluoroazobenzene shows near-
quantitative photoswitching and the longest thermal half-life
reported for an azobenzene molecule (∼700 days at 25 °C in
DMSO).16
Figure 1. Azoheteroarenes with extended thermal half-lives and good
Five-membered azoheteroarenes (and their photochromism)
are far less studied. Much of the work on azoheteroarenes has
concentrated on developing donor−acceptor systems with
strong nonlinear optic and solvatochromic properties.18−24 Of
the photochromic compounds studied, generally, these
photoswitching.
Received: May 30, 2014
Published: August 6, 2014
© 2014 American Chemical Society
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Journal of the American Chemical Society
Communication
deficient azopyridines, having a blue-shifted π−π* λmax in
comparison to azobenzene.36 E-pyrazoles (8 and 11) showed a
weak n−π* absorbance, well separated from the π−π* band,
whereas E-2 and E-5 had a slight shoulder on their π−π* band,
due to a weak n−π* absorbance, poorly separated from the
π−π* absorbance. The separation of the n−π* and π−π* λmax
bands is qualitatively consistent with the TDDFT calculations
(Figure S21).
Scheme 1. Synthesis of Photoswitches
Exciting the π−π* transition of azopyrazoles E-8 and E-11 at
355 nm resulted in complete photoswitching (>98% Z isomer
in both cases). Near-quantitative switching was also achieved
with broad-band 330−400 nm light (Figures S8,9). To our
knowledge, the only azo compound for which ∼100% E−Z
photoswitching has been achieved is the ethylene-bridged
azobenzene reported by Temps and co-workers.13 Z-11 has a
stronger n−π* absorbance than E-11, while the n−π*
absorbance of Z-8 was similar to that of E-8 (Table 1).
Irradiating the tail of the n−π* absorbance of azopyrazole Z-11
at 532 nm switched it back to >98% E isomer. Thus, 11 can be
quantitatively switched in both directions, a result which
surpasses the state-of-the-art ethylene-bridged azobenzene13
and the 2,2′6,6′-tetrafluoroazobenzene reported by Hecht and
co-workers.16
Varying the substitution pattern or heterocycle resulted in
less complete photoswitching. Azopyrazole Z-8 only reached a
532 nm PSS containing 70 3% E isomer, due to overlap of
the Z and E isomer n−π* absorbances. Irradiation of
azopyrroles E-2 and E-5 at 415 nm led to PSSs containing 84
2% and 85 3% Z isomer, respectively (Figure 2). Similar to
the case for the azopyrazoles, azopyrrole Z-5 had a relatively
intense n−π* absorbance in comparison to Z-2 and E-5 (Table
1). Excitation of the n−π* transition of azopyrrole Z-5 at 532
nm led to quantitative (>98%) photoswitching to the E isomer.
Prolonged irradiation of the tail of the n−π* absorbance of Z-2
at 532 nm led to a PSS containing 82 3% E isomer. Quantum
yields were determined where possible (Table 1). All
compounds were photoswitched 20 times in each direction,
and no significant degradation was observed, demonstrating
high fatigue resistance (Figure S7).
Reagents and conditions. (a) concentrated HCl, aqueous NaNO2,
acetone/H2O, 0 °C, then add to Na2CO3, N-methylpyrrole, acetone/
H2O, 0 °C to room temperature, 52%; (b) concentrated HCl, aqueous
NaNO2, H2O, 0 °C, then 2,4-dimethylpyrrole, MeOH/pyridine, 0 °C,
71%; (c) NaH, MeI, THF, 0−60 °C, 58%; (d) K2CO3, MeI, MeCN,
room temperature; (e) Pd/C, H2, MeOH, room temperature, 58%
from 6; (f) nitrosobenzene, 40% aqueous NaOH/pyridine, 80 °C,
33%; (g) concentrated HCl, aqueous NaNO2, AcOH, 0 °C, then add
to NaOAc, acetylacetone, EtOH/H2O, 0 °C to room temperature,
quantitative; (h) NH2NHMe, EtOH, reflux, quantitative.
Four heterocyclic compounds were chosen based on CAM-
B3LYP/6-311G(2df,2p) TDDFT predictions of their photo-
physics. All were N-methylated to prevent rapid Z−E thermal
isomerization.29,31 Arylazopyrroles 2 and 5 and arylazopyrazole
11 were prepared using standard diazo couplings. Arylazopyr-
role 8 was prepared from 4-amino-1-methylpyrazole and
nitrosobenzene via a modified Mills reaction (Scheme 1) in a
modest unoptimized yield, in line with highly substrate
dependent literature yields.32 The scope for this method has
previously included various aminopyridines and aminoquino-
lines;32,33 however, our study demonstrates it is also suitable for
the preparation of azo compounds containing five-membered
heteroarenes, of which few syntheses exist.25,27,34
For the arylazoimidazoles, Herges and co-workers reported
that both isomers have a weak n−π* absorbance and are
calculated to adopt a conformation in which the heteroarene
and azo function are coplanar.27 In our calculations, all E
isomers are predicted to be planar. For azopyrazole Z-8 and
azopyrrole Z-2 (without two “ortho” methyl groups on the
The UV/vis spectra of azoheteroarenes 2, 5, 8, and 11 are
shown in Figure 2. The pyrroles in 2 and 5 are better π donors
to the electron-withdrawing azo function (σp = +0.39),35
resulting in a red-shifted π−π* λmax. This is consistent with π-
Figure 2. UV/vis spectra of azoheteroarenes in acetonitrile. Inset: repeated photoswitching cycles of azopyrazole 11. The Z-2 and Z-5 spectra are
determined as described in the Supporting Information.
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Table 1. Spectral and Kinetic Data for Compounds
E isomer π−π*
E isomer n−π*
10−2ε/M−1
Z isomer π−π*
Z isomer n−π*
λmax
/
10−2ε/M−1
λmax
/
10−2ε/M−1
λmax/
10−2ε/M−1
h
π−π
n−π
nm
cm−1
λmax/nm
cm−1
nm
cm−1
nm
cm−1
half-life
Φ
*
Φ
*
Z−E
E−Z
a
d
e
e
d
e
2
385
394
328
335
221 18
282 23
176 14
227 18
∼413
a
a
333
346
275
296
157 13
423
479
403
441
17.9 1.4
2.95 0.05 h
0.50 0.07
0.41 0.05
0.41 0.05
0.60 0.06
0.56 0.04
a
b
b
c
5
∼430
70.4 5.7
80.6 6.4
57.7 4.6
60.8 4.9
21.2 0.5 s
f
g
g
8
417
6.44 0.51
9.63 0.76
6.66 0.53
∼1000 days
10 0.03 days 0.46 0.04
0.61 0.06
11
425
23.0 1.8
a
b
n−π* absorbance appears as a shoulder on the π−π* absorbance; hence, ε cannot be obtained. These values are for the estimated pure Z isomer
c
d
(see the Supporting Information). The rate was very sensitive to water present in the sample (see the Supporting Information). Excited at 415 nm.
e
f
g
h
Excited at 532 nm. Not determined due to competing thermal reaction. Excited at 355 nm. At 25 °C.
Figure 3. (A) Effect of dihedral angle on n−π* intensity for Z-11. (B) Crystal structures for Z-11 and Z-8. (C) Calculated conformations for Z
isomers (B3LYP/6-31G(d,p)), with dihedral angles annotated. All values, except ΦNNhet for Z-5, are given as 180 − Φ.
heterocycle) a conformation with the phenyl ring approx-
imately orthogonal to the planar heteroarene-azo function is
calculated, while the two methyl groups in azopyrazole Z-11
and azopyrrole Z-5 are predicted to force these species into a
twisted conformation. TDDFT predicts low-intensity n−π*
absorbances for all E isomers and for Z-2 and Z-8 (with the
heteroarene and azo group coplanaroscillator strengths of
0.0023 and 0.0026, respectively), as is seen in the experimental
spectra. The twisted azopyrazole Z-11 and azopyrrole Z-5 are
calculated and observed to have large n−π* absorbances.
A similar trend is observed for the n−π* absorbance in
planar E-azobenzene and nonplanar Z-azobenzene, the former
has a lower intensity absorbance than the latter.9 If both arenes
are coplanar with, or orthogonal to, the azo group, a mirror
plane exists through the plane of the azo function and the n−π*
transition becomes symmetry-forbidden at the equilibrium
geometry. Vibrational and rotational motion results in the
molecule existing in conformations with weakly allowed n−π*
transitions, resulting in a small absorbance. The spectroscopic
findings suggest azopyrazole Z-8 and azopyrrole Z-2 exist in
planar (or near-planar) conformations, and the more sterically
congested bis(o-methylated) azopyrazole Z-11 and azopyrrole
Z-5 adopt twisted conformations. With six-membered rings, Z-
azobenzenes are always twisted,9 so these five-membered
heteroarenes allow steric tuning of the spectral properties not
available to azobenzenes. TDDFT calculations were performed
(on constrained-optimized structures of azopyrazole Z-11 for
illustration) to predict the effect of the dihedral angle between
the heteroarene and azo groups on the n−π* intensity. The
intensity is minimum at planar (Φ = 0, 180°) and orthogonal
geometries (Φ = 90°) (Figure 3A).
27.1° (Figure 3). Although the Z-8 crystal structure was found
to have a ΦNNhet dihedral angle of 10.8° (cf. the calculated value
of 0.0°), this was still considerably smaller than the value of
25.7° for Z-11, as predicted. The difference between the
calculated and X-ray structure dihedral angles is consistent with
the work by Jacquemin and co-workers on E-azobenzene,
which is believed to adopt a planar structure in solution and a
twisted structure in the solid state.37
Thermal isomerization kinetics were obtained for all
compounds, using UV/vis (5) or 1H NMR (2, 8, 11)
spectroscopy (Table 1), and all showed first-order isomer-
ization. The presence of two “ortho” methyls on the
heteroarene accelerates the isomerization. There is some
debate as to the mechanism by which thermal isomerization
of azoarenes occurs.38−40 However, both the inversion and the
rotation transition states would relieve the steric strain in the
bis(o-methylated) molecules, lowering the barrier for thermal
isomerization compared to the molecules lacking the bis(o-
methyls) (Figure S18). This has been studied for a range of
methylated azobenzenes.41
The azopyrazoles had much slower thermal rates than the
azopyrroles. An Eyring plot of the kinetics of thermal
isomerization of azopyrazole Z-8 in DMSO between 80 and
120 °C gave activation parameters for this process (ΔH⧧ =
+115 1 kJ mol−1, ΔS⧧ = −13 3 J K−1 mol−1) that are very
similar to those obtained by Hecht and co-workers for 2,2′,6,6′-
tetrafluoroazobenzene (ΔH⧧ = +109 3 kJ mol−1, ΔS⧧ = −29
9 J K−1 mol−1), the azobenzene having the slowest
isomerization to date.16 Using these, we estimate a half-life of
∼1000 days at 25 °C. This compares highly favorably to those
observed for diarylethenes and suggests that azoheteroarenes
may hold potential for optical data storage.7 Substituted
azopyrazole 11 was found to have a half-life of 10 days at 25
°C in acetonitrile. Azopyrrole 5 had a very fast thermal rate in
Crystal structures were obtained for pyrazoles Z-11 and Z-8.
The ΦNNhet dihedral angle of 25.7° for Z-11 observed in the
solid-state structure matched well with the calculated value of
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Journal of the American Chemical Society
Communication
acetonitrile, ∼103 faster than that for 4-aminoazobenzene in a
similar polar aprotic solvent.42 By substitution of the phenyl
ring of 5 with a para-electron-withdrawing group, push−pull
azopyrrole systems can be generated with rapid thermal
isomerization rates.25
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11 can be quantitatively photoswitched in both directions; to
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ASSOCIATED CONTENT
* Supporting Information
Text, tables, figures, and CIF files giving synthetic methods,
NMR and crystallographic data, photochemical methods
including additional spectra and kinetics, and computational
details. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail for M.J.F.: m.fuchter@imperial.ac.uk.
Notes
■
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the Engineering and Physical Sciences Research
Council for support. R.D.R. thanks the Leverhulme Trust
(Grant No. RPG-2012-441) for funding. We also thank Dr.
Marina Kuimova for equipment access.
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