Communication
doi.org/10.1002/chem.202005486
Chemistry—A European Journal
Fluorinated Azobenzenes Switchable with Red Light
[
a]
[a]
[a]
[a]
[a]
[a]
[b]
[a, c]
The structures of established photoswitches, like
Abstract: Molecular photoswitches triggered with red or
NIR light are optimal for photomodulation of complex
biological systems, including efficient penetration of the
human body for therapeutic purposes (“therapeutic win-
dow”). Yet, they are rarely reported, and even more rarely
functional under aqueous conditions. In this work, fluori-
nated azobenzenes are shown to exhibit efficient E!Z
[9]
[1 ]
azobenzenes, spiropyrans, ° or diarylethenes, as well as
[11]
[12]
emerging scaffolds, like indigoids, or Stenhouse adducts,
are constantly modified in order to tune their photophysical
properties to particular applications. While the majority of
reported photochromes is triggered with UV light, photocontrol
of biological systems is more optimal with visible
[9,13]
wavelengths.
Photoswitches triggered with light within so-
photoisomerization with red light (PSS
>75% Z) upon
60nm
6
called “therapeutic window” (600-900 nm) are particularly
attractive for applications in photopharmacology, as these
conjugation with unsaturated substituents. Initially demon-
strated for aldehyde groups, this effect was also observed
in a more complex structure by incorporating the chromo-
phore into a cyclic dipeptide with propensity for self-
assembly. Under physiological conditions, the latter mole-
cule formed a supramolecular material that reversibly
changed its viscosity upon irradiation with red light. Our
observation can lead to design of new photopharmacology
agents or phototriggered materials for in vivo use.
wavelengths enable deep penetration of soft tissues in human
[14]
organism.
chloro-
This property has been reported for alkoxy-,
[14c,15]
[15a]
and – recently – mixed dichlorodifluoro
azoben-
[16]
[17]
zenes, as well as for diazocines, arylhydrazones, Stenhouse
adducts (DASA),
[18]
[19]
and hemithioindigos.
However, due to
synthetic difficulties, or incompatibility with physiological
[14a,2
]
conditions their use remains limited,
° and novel photo-
chromic scaffolds operational in this range of light are of high
demand.
Azobenzene derivatives modified with fluorine atoms
adjacent to the azo bond (“ortho-fluoroazobenzenes”) belong to
the most thermostable photochromic systems triggered with
visible light (E!Z 530 nm, Z!E 410 nm). They combine efficient
synthesis with pronounced geometry changes upon
Molecular photoswitches undergo reversible light-induced
transformations between two forms that differ in their
absorption spectra (photochromism) and molecular properties,
[1]
[21]
[6b]
such as geometry, polarity, or rigidity. They found a vast
isomerization, and biological stability.
This chromophore
[2]
[22]
number of applications, ranging from material sciences to
found numerous applications in photocontrol of materials
[3]
[23]
biological systems. A few selected examples are: photocontrol
and biological systems with visible light. However, efficient
[
4]
[5]
over molecular movement, assembly of nanoparticles and
(>50% Z-isomer) switching with light above 600 nm was not
yet described for purely fluorinated azobenzenes, with excep-
tion of a two-photon excitation process with high-power NIR-
[6]
[7]
soft materials, or activity of biopolymers and pharmacologi-
[8]
cally relevant substances.
[
24]
laser beam performed on a sensitizer-coupled derivative.
Here we report that functionalization of the tetra-ortho-
fluoroazobenzene (TFAB) chromophore with sp -hybridized
[
a] A.-L. Leistner, S. Kirchner, J. Karcher, T. Bantle, M. L. Schulte, P. Gödtel,
2
Dr. Z. L. Pianowski
Institut für Organische Chemie
Karlsruher Institut für Technologie
Fritz-Haber-Weg 6, 76131 Karlsruhe (Germany)
E-mail: pianowski@kit.edu
conjugated substituents leads to efficient E!Z isomerization
upon irradiation with red light (>630 nm), and that the red
light-induced photoisomerization can be used to elicit a macro-
scopic effect – namely a reversible viscosity change - in a
supramolecular material under physiological conditions.
[
b] C. Fengler
Institut für Technische Chemie und Polymerchemie
Karlsruher Institut für Technologie (KIT)
Engesserstraße 18, 76128 Karlsruhe (Germany)
Our group has previously investigated drug-releasing
supramolecular hydrogels consisted of cyclic dipeptide-based
[
c] Dr. Z. L. Pianowski
Institute of Biological and Chemical Systems – FMS
Karlsruhe Institute of Technology
Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen (Ger-
many)
[2g]
[6b,25]
gelators bearing non-fluorinated (1a) and fluorinated (1b-c)
azobenzenes. The cargo release was triggered, respectively, with
UV, or with green light. In search for new phototriggered
supramolecular materials, we wanted to exchange one of the
stereocenters in 1c for a double bond. For that, we planned to
couple a protected cyclo(Lys-Gly) with the aldehyde 4, previously
synthesized by oxidation of the TFAB-bearing alcohol 2 (Figure 1).
As expected, the alcohol 2 photoisomerized (E!Z) upon irradi-
©
2021 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided the
original work is properly cited.
ation with green light (λ =530 nm) (Table 1).
max
Chem. Eur. J. 2021, 27, 1–7
1
© 2021 The Authors. Published by Wiley-VCH GmbH
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