O -fluoroazobenzenes as readily synthesized photoswitches offering nearly quantitative two-way isomerization with visible light

Article abstract of DOI:10.1021/ja310323y

Azobenzene functionalized with ortho-fluorine atoms has a lower energy of the n-orbital of the Z-isomer, resulting in a separation of the E and Z isomers' n→π* absorption bands. Introducing para-substituents allows for further tuning of the absorption spe

Full text of DOI:10.1021/ja310323y

Communication
pubs.acs.org/JACS
o‑Fluoroazobenzenes as Readily Synthesized Photoswitches Offering
Nearly Quantitative Two-Way Isomerization with Visible Light
David Bleger,*,† Jutta Schwarz,† Albert M. Brouwer,*,‡ and Stefan Hecht*,†
́
†Department of Chemistry, Humboldt-Universitat zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany
̈
‡Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, P.O. Box 94157, 1090 GD Amsterdam, The Netherlands
S
* Supporting Information
induced by irradiating in the visible region where the n→π*
ABSTRACT: Azobenzene functionalized with ortho-fluo-
rine atoms has a lower energy of the n-orbital of the Z-
isomer, resulting in a separation of the E and Z isomers’
n→π* absorption bands. Introducing para-substituents
allows for further tuning of the absorption spectra of o-
fluoroazobenzenes. In particular, electron-withdrawing
ester groups give rise to a 50 nm separation of the n→
π* transitions. Green and blue light can therefore be used
to induce E→Z and Z→E isomerizations, respectively. The
o-fluoroazobenzene scaffold is readily synthesized and can
be inserted into larger structures via its aryl termini. These
new azobenzene derivatives can be switched in both ways
with high photoconversions, and their Z-isomers display a
remarkably long thermal half-life.
bands of E and Z isomers mostly overlap, hence preventing E-
rich photostationary states (PSSs) after back-switching. These
limitations can be circumvented by modifying azobenzene such
that n→π* bands of E and Z isomers are separated.14 If the
separation is large enough, the two n→π* bands, and thus
visible light, can be used to isomerize both isomers selectively,
with high to complete E/Z photoconversions. Siewertsen et al.
recently identified such a system, i.e., an azobenzene covalently
bridged in ortho-positions by an ethylene linker, in which a 100
nm splitting of the n→π* transitions of E and Z isomers allows
for switching in both directions with visible light and almost
complete photoconversions (100% E→Z and 92% Z→E).15
Woolley’s group recently reported another interesting
azobenzene derivative,16 in which four methoxy groups
introduced in the ortho-positions led to a 36 nm splitting of
E and Z isomers’ n→π* bands and hence enabled photo-
switching in the visible range as well, although the reported
rganic photochromism makes use of molecules that
Otypically undergo pericyclic reactions or E/Z isomer- photoconversions (80% E→Z and 85% Z→E) were not as high
izations of double bonds.1 In the first case, the conjugation path
as for the cyclic azobenzene. In these two examples the
in photochromes such as spiropyrans,2 diarylethenes,3 or
substituents ortho to the NN bond cause a distortion of the
fulgides4 is dramatically affected, inducing marked changes in
typically planar E-isomers, which inevitably affects the energy of
the π and π* molecular orbitals. This distortion is most likely
the origin of the splitting of the n→π* bands of both isomers in
the case of the covalently bridged azobenzene,15 and it even
renders the Z-form the most stable, whereas usually the E-
isomer is thermodynamically favored. In the case of the ortho-
methoxy compound,16 the separation results from repulsive
interactions of the azobenzene n-orbitals with the relatively
high-energy lone pair orbitals of the oxygen atoms, which
increase the energy level of all orbitals.
As the separation of n→π* bands of both isomers is key to
switching performance in the visible, we envisioned controlling
the n→π* excitation energies by influencing the energy level of
the n-orbitals, without inducing a dramatic distortion of the azo
compounds. Indeed, for some applications it is desirable to
maintain the planarity of the photochromes, especially when an
effective conjugation along the π-system is targeted, or an
assembly by π,π-stacking is required. Here we describe a new
class of azobenzenes (Chart 1), in which the n→π* bands of E
and Z isomers are well separated by the introduction of fluorine
atoms ortho to the azo moiety, allowing for selective excitation
of the two isomers in the visible range of the spectrum.17 In
contrast to ortho-methoxy groups, which destabilize the n-
their color and frontier molecular orbitals. In the other case,
molecules with the ability to photoisomerize (at least) one
double bond undergo large geometrical changes that can be
collected and further amplified, for example in the process of
vision, resulting in mesoscopic/macroscopic phenomena.5,6
The simplest aromatic compounds presenting one isomerizable
double bond are stilbenes and azobenzenes, the latter being
readily synthesized and highly fatigue-resistant.7 These
advantages have rendered azobenzenes one of the most
attractive classes of molecular switches. For example, azo
compounds have been used to modify biological systems8,9
such as peptides, oligonucleotides, and transmembrane
proteins, allowing photocontrol over conformation, function,
and even neuronal activities in vivo.10 In the context of
materials science, azo compounds have found several
applications based on their structural changes,11 one of the
most appealing being the direct conversion of light into
mechanical energy.6,12,13
Despite the advantages mentioned above, azobenzenes
generally suffer from two drawbacks that limit their practical
use in biological and materials sciences. The first one is the
utilization of UV light, necessary to induce the E→Z
isomerization via π→π* excitation, but potentially interfering
with and destroying the surrounding environment. The second
is the typically incomplete reverse Z→E photoisomerization,
Received: October 24, 2012
© XXXX American Chemical Society
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in the E-isomer, and the net result is that the n→π* excitation
energies are very similar for both E and Z isomers.19 The n-
orbital can be considered as a linear combination of the lone
pair orbitals on the two neighboring N atoms. In the E-isomer,
the lone pair orbitals interact through bonds, which renders the
symmetric combination the highest in energy due to its mixing
with the N−N σ-bond orbital.20 The splitting of the bonding
and antibonding combinations is large enough to place the n-
MO (HOMO) well above the highest π-MO (HOMO-1). In
the Z-isomer, the lone pair orbitals interact much more strongly
through space, forming two combinations that resemble π and
π* MOs. The lower of these two cannot be identified in the
MO calculations because it is of low energy and mixed with
many other basis orbitals. The highest-energy combination is
the HOMO of the molecule (Figure 1). In view of the relatively
high n-MO levels in azobenzene caused by the repulsive
interaction of the nitrogen lone pairs, we reasoned that
reduction of the n-electron density by nearby σ-electron-
withdrawing groups should lower the n-orbital energy. This can
effectively be achieved by the introduction of fluorine
substituents in the ortho-positions (see Figure 1b). Note that
fluorine atoms have a superior inductive effect, while its
Chart 1. Synthesized and Investigated o-Fluoroazobenzene
Derivatives 1−3
orbital of Z-azobenzene by repulsive interactions between O
and N lone pair electrons, ortho-fluorine atoms reduce the
electron density in the nearby NN bond, hence lowering the
n-orbital energy.18 Moreover, because the fluorine atom has a
relatively small radius, they barely distort the planar geometry
of E-azobenzene. Electron-donating and -withdrawing groups
(EDGs and EWGs) have additionally been incorporated in
para-positions to tune the spectral features, i.e., maxima of
absorption and separation of the n→π* bands, and to introduce
functional substituents, useful to attach the photochromes to
other entities. Our design strategy, guided by molecular orbital
(MO) theory calculations, has led to a series of readily
synthesized azobenzene building blocks, which allow for nearly
quantitative photoswitching in both directions via n→π*
excitation in the visible range of the spectrum, and can readily
be modified to incorporate them into functional macro- and
supramolecular systems.
Within the framework of MO theory, the lowest excited
states of azobenzenes can be quite well described using singly
excited n→π* and π→π* configurations. Although the
separation of n and π orbitals by symmetry holds only for
the planar E-isomers, the relevant orbitals, i.e., π, n, and π*, are
easily recognized in MO calculations even for the helically
shaped Z-isomer. In azobenzene, the n→π* excitation energies
of the two isomers are similar due to coincidentally similar
shifts of π*- as well as n-MOs (Figure 1a). In the Z-isomer, the
π-electron delocalization is reduced due to the large dihedral
angles about the N−C single bonds. As a result, the π* orbital
level is much higher than in the planar E-isomer. However, the
n-orbital energy level in the Z-isomer is also much higher than
mesomeric effect is rather negligible (Hammett constant σpara
=
0.062).21
o-Tetrafluoroazobenzene 1 was synthesized in one step
starting from commercially available 2,6-difluoroaniline.22 The
UV−vis spectrum of 1 in acetonitrile (see Figure 2) resembles
that of the parent azobenzene, with a strong π→π* absorption
band at short wavelengths (λπ→π* ≈ 305 nm) and a weaker n→
π* band centered around 458 nm. Recording the absorbance of
1 in different solvents resulted in essentially identical spectra;
i.e., no solvatochromism was observed.22 The very slight blue
shift in the π→π* absorption when compared to the parent
compound (for azobenzene λπ→π* ≈ 316 nm in acetonitrile)
indicates that E-1 is not significantly distorted from planarity.23
Irradiation with UV light at 313 nm causes E→Z isomerization,
resulting in the decrease of the π→π* band typically observed
in azobenzenes, producing a PSS containing 84% of the
corresponding Z-isomer.24 In the visible part of the spectrum,
E→Z isomerization leads to the characteristic increase of the
n→π* absorption, whichin contrast to the parent azoben-
zeneis accompanied by a significant blue-shift of 42 nm; i.e.,
the n→π* bands of the two isomers were effectively separated.
Irradiation with visible light was therefore used to isomerize 1
in both directions, producing PSSs containing 91% of Z-1 with
green light (λ > 450 nm) and 86% of E-1 with blue light (410
nm). Note that in addition to effectively inducing the E→Z
isomerization, visible light produces higher Z/E ratios when
compared to the π→π* excitation by UV light (91% Z with
green light vs 84% Z with UV light). Multiple E/Z
isomerization cycles did not result in any noticeable
degradation,22 highlighting the robustness of the o-tetrafluor-
oazobenzene scaffold. Remarkably, the Z-form of 1 is thermally
exceptionally stable, with a half-life of ca. 700 days at 25 °C,
corresponding to an activation barrier of 117 kJ/mol (i.e., 28
kcal/mol).22 To the best of our knowledge, such a high thermal
stability has not been reported for an azobenzene derivative. In
particular, the Z-form of the o-methoxyazobenzene derivative
reported by Woolley et al. has a half-life of 14 days,16 whereas
the half-life of the covalently bridged azobenzene described
earlier15 is only 5 h.25 The calculations predict that the σ-
electron-withdrawing substitution leads to a stabilization of the
Z-isomer. For the parent azobenzene, the energy difference
Figure 1. Energetic diagram of the π, n, and π* orbitals of (a)
azobenzene and (b) 1, and representation of the n-orbitals (HOMOs)
calculated at the B3LYP/6-31G(d) level of theory (arrows highlight
n→π* transitions).
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the effect of the σ-electron-withdrawing fluorine atoms; i.e., the
n→π* separation will be smaller than in the parent ortho-
tetrafluoro compound (see Table S4). On the other hand,
EWGs lower all relevant orbital energies in the order π*(E) >
π*(Z) > n(Z) > π(E) > π(Z) = n(E). The net result is a small
shift to longer wavelength of the n→π* absorption of the E-
isomer relative to that of the Z-isomer. In combination with the
fluorine effect, this should give rise to an increased splitting of
the n→π* bands of the E and Z isomers.
These computational predictions were verified experimen-
tally by introducing either N-acyl (compound 2) or ester
(compound 3) groups para to the azo linkage. Compound 2
was synthesized starting from 1a via a Buchwald-type N-
arylation procedure, whereas compound 3 was prepared from
2,6-difluoroaniline via a bromination/cyanation/hydrolysis/
esterification sequence followed by a final oxidative coupling.
As typically observed in azobenzene with EDGs in para-
positions,7 the π→π* band of E-2 is significantly red-shifted
relative to that of the parent compound 1, whereas the n→π*
band remains essentially unaffected (see Figure 2). Excitation of
the n→π* transition with light of λ > 500 nm effectively
induces the E→Z isomerization (85% of Z-2); however, the
separation of the n→π* bands of the isomers is reduced to 22
nm (compared to 42 nm in 1). Indeed, and as anticipated on
the basis of the calculations, EDGs in para-positions counter-
balance the effects of the ortho-fluorine atoms by pushing
electron density into the NN bond. Eventually the two
isomers cannot be selectively excited in the visible range,
leading to only 69% Z→E photoconversion (when irradiated at
440 nm).
The situation is reversed in compound 3, in which the esters
(EWGs) work in concert with the ortho-fluorine atoms,
decreasing further the repulsive interactions between the N
lone pair electrons in the Z-isomer. Indeed, azobenzene 3
presents the best photochromic characteristics of the series,
with a 50 nm separation of the n→π* bands, allowing for a
selective excitation of either isomer with visible light, resulting
in high (90% E→Z with λ > 500 nm) to almost complete (97%
Z→E at 410 nm) photoconversions. Azobenzene 3 is moreover
very robust, undergoing repeated isomerization cycles without
apparent photobleaching,22 and is synthesized in 8% overall
yield, allowing for a rapid, large-scale preparation. This last
point is of importance, since in the two previous examples, in
which azobenzenes addressable with visible light are function-
alized for further attachment,16,27 overall yields of less than 1%
preclude access to multigram quantities necessary in the
context of potential materials science applications.
Figure 2. Absorption spectra of 1−3 as their E and Z isomers as well
as their PSS mixtures in acetonitrile at 25 °C. Insets show the n→π*
bands and indicate the compositions (as determined by liquid
chromatography) of the PSS mixtures upon irradiation with visible
light (λ > 450 or 410 nm for 1, λ > 500 or 440 nm for 2, and λ > 500
or 410 nm for 3). Pure Z-isomer spectra were calculated from spectra
of pure E-isomer and PSS mixtures with known PSS compositions.
between the two isomers is calculated to be 15.2 kcal/mol,26
whereas for the fluorine-substituted 1 it is reduced to 8.1 kcal/
mol. The reduced driving force is reflected in a higher
activation barrier.
Guided by simple MO theory considerations, we have
rationally synthesized a series of o-tetrafluoroazobenzenes in
which E and Z isomers display distinct n→π* transitions. This
separation allows one to selectively address both isomers in the
visible region of the spectrum, causing E/Z isomerizations with
nearly quantitative photoconversions. Substituents para to the
NN bond were introduced to further optimize the spectral
properties and provide functional groups suitable for
subsequent chemical modification. Importantly, all building
blocks are readily synthesized starting from commercially
available 2,6-difluoroaniline. Photoswitching via the n→π*
transitions offers new opportunities in bulk applications such as
photoswitchable MOFs28 and optomechanical systems,6 in
which the very strong absorption of azobenzenes in the UV
range29 drastically limits the light penetration into the materials
In order to prepare building blocks suitable for further
chemical modification, for example by catalyzed cross-coupling
reactions, halogen atoms have been attached at both ends of 1
(see 1a,b, Chart 1). The halogenated derivatives were
synthesized in two steps starting from 2,6-difluoroaniline.
Building blocks 1a,b present photochromic properties similar to
those of the parent compound 1, with a 40 nm splitting of the
n→π* bands and hence the possibility to photoswitch in both
directions in the visible range.22
With the intention to maximize the separation of the n→π*
bands, which would allow for exciting the two isomers more
selectively and hence reaching higher photoconversions, we
envisioned to introduce electroactive substituents para to the
azo moiety. Calculations indicate that EDGs in para-positions
raise the energy of the highest occupied orbitals and counteract
C
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(19) Note that in the parent azobenzene photoswitching upon n→π*
excitation is possible due to the fact that the Z-isomer exhibits a higher
molar absorption coefficient than the E-isomer.
and hence the amplitude of the photoinduced motion/
actuation.
(20) Hoffmann, R. Acc. Chem. Res. 1971, 4, 1.
(21) Hammett, L. P. J. Am. Chem. Soc. 1937, 59, 96.
(22) See Supporting Information.
ASSOCIATED CONTENT
* Supporting Information
Experimental details including synthesis, photochemistry, and
computational methods. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
(23) The calculations indicate that the energy surface for twisting
about the CC−NN bonds is shallow up to ∼30°; see Figure S7.
(24) All PSS compositions were determined by ultraperformance
liquid chromatography (UPLC) analyses using integration of the UV
signal at the wavelengths of the isosbestic points.
(25) Note that in the covalently bridged azobenzene the Z form is
thermodynamically favored over the E-isomer as a result of the ring
strain.
(26) Note that calculations performed at the G3MP2 level of theory
give an energy difference between the E and Z isomers in better
agreement with the experimental value of 11.4 kcal/mol previously
determined: Cammenga, H. K.; Emel’yanenko, V. N.; Verevkin, S. P.
Ind. Eng. Chem. Res. 2009, 48, 10120.
(27) Samanta, S.; Qin, C.; Lough, A. J.; Woolley, A; G. Angew. Chem.,
Int. Ed. 2012, 51, 6452.
(28) (a) Park, J.; Yuan, D. Q.; Pham, K. T.; Li, J. R.; Yakovenko, A.;
Zhou, H. C. J. Am. Chem. Soc. 2012, 134, 99. (b) Yanai, N.; Uemura,
T.; Inoue, M.; Matsuda, R.; Fukushima, T.; Tsujimoto, M.; Isoda, S.;
Kitagawa, S. J. Am. Chem. Soc. 2012, 134, 4501.
AUTHOR INFORMATION
Corresponding Author
david.bleger@chemie.hu-berlin.de; a.m.brouwer@uva.nl; sh@
chemie.hu-berlin.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Generous support by the German Research Foundation (DFG
via SFB 658, project B8) is gratefully acknowledged. BASF AG,
Bayer Industry Services, and Sasol Germany are thanked for
generous donations of chemicals.
(29) The molar absorption coefficient of azobenzenes 1 and 3 is
∼20 000 M−1 cm−1 for the π→π* transition vs ∼1000 M−1 cm−1 for
the n→π* transition.
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D
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