General Principles for the Design of Visible-Light-Responsive Photoswitches: Tetra-ortho-Chloro-Azobenzenes

Article abstract of DOI:10.1002/anie.202008700

Molecular photoswitches enable reversible external control of biological systems, nanomachines, and smart materials. Their development is driven by the need for low energy (green-red-NIR) light switching, to allow non-invasive operation with deep tissue p

Full text of DOI:10.1002/anie.202008700

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Accepted Article
Title: General Principles for the Design of Visible-Light-Responsive
Photoswitches: Tetra-ortho-Chloro-Azobenzenes
Authors: Lucien N. Lameijer, Simon Budzak, Nadja A. Simeth, Mickel
J. Hansen, Ben L. Feringa, Denis Jacquemin, and Wiktor
Szymanski
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10.1002/anie.202008700
Angewandte Chemie International Edition
General Principles for the Design of Visible-Light-Responsive
Photoswitches: Tetra-ortho-Chloro-Azobenzenes
Lucien N. Lameijer,1,2 Simon Budzak,3 Nadja A. Simeth,2 Mickel J. Hansen,2 Ben L.
Feringa,2 Denis Jacquemin,*,4 Wiktor Szymanski*,1,2
1 Medical Imaging Center, University Medical Center Groningen, University of Groningen,
Hanzeplein 1, 9713GZ Groningen, The Netherlands
2 Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747AF
Groningen, The Netherlands
3 Department of Chemistry, Faculty of Natural Sciences, Matej Bel University, Tajovkého 40,
97401 Banska Bystrica, Slovakia
4 CEISAM Lab, UMR 6230, Universitꢀ de Nantes, CNRS, F-44000 Nantes, France.
* Denis.Jacquemin@univ-nantes.fr; w.szymanski@umcg.nl
Abstract
Molecular photoswitches enable the reversible external control of biological systems,
nanomachines, and smart materials. Recently, their developments are driven by the need for
low energy (green-red-NIR) light switching, to allow for non-invasive operation with deep
tissue-penetration. The lack of clear design principles for the adaptation and optimization of
such systems strongly limits further applications. Here we provide such design rulebook for
tetra-ortho-chloroazobenzenes, an emerging class of visible-light-responsive photochromes,
by elucidating the role that substituents play in defining their key characteristics: absorption
spectra, band overlap, photoswitching efficiencies, and half-lives of the unstable cis isomer.
This is achieved through joint photochemical and theoretical analyses of a representative
library of molecules that feature substituents of varying electronic nature. A set of guidelines
is presented that enables tuning of properties to the desired application through informed
photochrome engineering.
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Introduction
Molecular photoswitches form the basis of light-responsive systems that are designed to
enable reversible control of function with high spatiotemporal resolution.[1] They have found
application in remotely manipulating biological systems,[2,3] “smart” materials,[4,5] and
molecular machines.[6,7] In particular, their potential in biomedical context, along the
principles of photopharmacology,[8–11] evoked considerable interest in recent years. The
available panel of molecular photoswitches features many established architectures that
mainly rely on double bond isomerisation (azobenzenes,[12] azoheteroarenes,[13] stilbenes,[14]
hemithioindigos[15]), electrocyclisation (diarylethenes[16]
(spiropyrans[17]). In addition, various novel designs[18] have appeared during the last decade,
including Donor-Acceptor Stenhouse Adducts (DASAs),[19,20] hydrazone[21]
and
)
or mixed mechanisms
-
acylhydrazone[22]-based switches, BF2-coordinated azo compounds,[23] diazocines,[24]
indigos[15,25] and iminothioindoxyls.[26] The development of new molecular photoswitches is
largely driven by the challenge of enabling the use of visible, and red or even near-IR (NIR)
light for operation in both directions.[27–29] This is relevant especially in biological
applications, where red/NIR light enables deep (1 cm) tissue penetration without the toxic
effects induced by higher energy light.[30]
The successful application of the new visible-light-responsive photoswitches depends on
establishing their design principles, based on the understanding of the interplay between the
nature of the substituents and the key photochemical properties. This understanding is enabled
through synthesis, spectroscopic studies, and theoretical investigations.[31–33] It ultimately
allows both the tuning of these properties, and the effective choice of substituents determining
the function of the photoresponsive unit in a biological system, material, or a molecular
machine.
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Figure 1. Photochromism and applications of tetra-ortho-chloro-azobenzenes. A) The trans-isomer
can be switched to the cis isomer using green or red light. The metastable cis isomer can be switched
back using violet or blue light. B) The spectra of both isomers feature the high energy absorption band
in the UV region, associated with the symmetry-allowed -* transition, and a low energy band in the
visible region, associated with the symmetry-forbidden n-* transition. C) The operation of tetra-
ortho-chloro-azobenzenes with visible light is enabled due to the separation of n-* bands. D)
Examples of application of tetra-ortho-chloro-azobenzenes for visible-light-regulation of processes in
biology and supramolecular chemistry.
Here we present a systematic spectroscopic and theoretical investigation into the
photochemistry of tetra-ortho-chloro-azobenzenes, with the aim to provide a guide for their
design. In general, tetra-ortho-substituted azobenzenes emerged as privileged light-responsive
molecular photoswitches, with good band separation and half-lives of the metastable cis
isomer in the range that enables multiple applications.[27–29,34–36] Among them, azobenzenes
with all four ortho positions substituted with chlorine atoms (Figure 1A), have already
enabled using green - and even red - light to control peptide conformation,[27] antibiotic
potency,[37] ion channel activity,[38–40] and the function of nucleic acids[41] and ion
receptors[42,43] for controlling the transport through biological membranes[44] (Figure 1D).
However, while for normal azobenzenes several relationships between structure and
photochemical properties have been defined, only little systematic information is available for
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10.1002/anie.202008700
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the tetra-ortho-substituted systems, making their design largely a trial-and-error endeavour.
The main difference between those switches and classical azobenzenes comes from the fact
that, for their operation in both directions, visible-light absorption bands are used, which
correspond to weakly-allowed transitions of n-* character (Figure 1B,C). This presents a
challenge for informed design of photoresponsive units for applications necessitates the
systematic study on parameters that govern the key properties, such as band separation,
switching efficiency, photostationary state distributions and thermal stability of the metastable
isomer.
Results and discussion
A library of ten compounds with different groups in one of the para positions was designed,
spanning the range of Hammet para constants from the most electron-donating (-NMe2, para
= -0.83) to the most electron-withdrawing (-NO2, para = 0.78). We focused on the para-
substituents, since meta ones show less pronounced resonance effects, and all ortho positions
are occupied in the studied molecules. Furthermore, in all the applied molecules (Figure 1D),
para-substituents are used.
The synthesis of tetra-ortho-substituted azobenzenes is known to be challenging due to the
highly sterically congested nature of the central N=N double bond that is surrounded with
four large chlorine substituents. This limits the use of classical methods for azobenzene
synthesis, such as the Baeyer-Mills reaction,[45] diazonium coupling[27] or oxidative coupling
of anilines,[46] and has inspired the development of methods better suited for these targets:
late-stage C-H chlorination[38,47,48] and the reaction of diazonium salts with lithiated aromatic
compounds, reported recently by our group.[49] Here, we use the latter method to prepare a
versatile library of tetra-ortho-chloro-azobenzenes (Figure 2, Table 1), additionally
highlighting the robustness of this method. In addition, this substrate scope was acquired
without the use of transition metals, instead using a Smiles rearrangement to synthesize
compound 1. (See Supporting Information).
For the photochemical evaluation, we have chosen DMSO as the solvent, because it facilitates
the solubility needed throughout the analytical methods used (UV-vis spectrophotometry,
NMR spectroscopy) and, with its intermediate polarity, it approximates well both organic and
aqueous systems well. Even more importantly, in photopharmacology it is often used as a
solvent for stock solutions, which after irradiation are diluted into aqueous media for
4
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10.1002/anie.202008700
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biological evaluation.[37,50–52] Hence, it is often the photochemistry in DMSO that determines
the properties of molecules in final applications.
The photochemistry of tetra-ortho-substituted azobenzenes in the visible range of the
electromagnetic spectrum is related to the presence of S0-S1 absorption bands that are
traditionally associated with n-* transitions.[53] The installation of ortho substituents induces
a significant distortion of the geometry, which in turn, allows for the separation of those bands
and thus enables their selective excitation with light of specific wavelengths.[29] This selective
addressing is crucial, because the ratio of molecular attenuation coefficients  of the two
forms at the irradiation wavelength is one of the two key factors (the other being the ratio of
quantum yields  for the photoisomerisation in both directions) determining the
photostationary state distribution (PSD) of isomers under irradiation at that wavelength.
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Figure 2: Visible-light band separation in compounds 1-10 for the trans isomers (blue spectra) and cis
isomers (orange spectra) (DMSO). The x-axis depicts the wavelength  (nm) and the y-axis depicts the
molar attenuation coefficient  (M-1 cm-1). On the right side of each spectrum, a panel is provided
6
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showing the band separation in more detail in the 550-650 nm range. The spectra of pure cis isomers
were calculated by irradiation of the sample in DMSO-d6 with λexc = 526 nm (FWHM = 90 nm) until
1
reaching PSS. HNMR spectra of aliquots (0.6 mL) were then taken to determine the cis/trans ratio,
followed by calculation of the cis spectra based upon the molar extinction coefficients of the trans-
species. (See the Supporting Information for full spectra). The pie charts show the content of cis
isomer at PSS that can be achieved under irradiation with exc = 426 nm (blue chart), exc = 526 nm
(green chart) or 625 nm (red chart) LEDs, as determined by NMR spectroscopy in DMSO-d6 (see the
Supporting Information).
The spectra of compounds 1-10 are presented in Figure 2 and their properties are summarized
in Table 1. In almost all cases, we have observed n-* absorption bands for the trans isomer
in the  = 450-465 nm region. Only for compound 1, which features a very strong electron-
donating –NMe2 group, we did not observe a well-resolved band in this region, probably due
to the overlap with a very strong -* band. The position of the bands was well reproduced
theoretically (Table 1).
To shed more light onto the experimental results, we have performed theoretical calculations
on all trans isomers (see the Supporting Information for details). It should be noted that all
compounds trans-1-10 strongly depart from the planarity of standard azobenzenes, with a
48.8° twisting of the aromatic rings with respect to the diazo bond in 4 (Figure 3A). In Table
S1 in the SI, we provide the transition energies determined with TD-DFT and with additional
CC2 corrections for all structures (see the SI for the details of technical details). For
compound 4, the best estimate for the S0-S1 excitation is 466 nm, in obvious agreement with
the experimental value 457 nm, (see Table 1). As can be seen in Figure 3B, this transition
has an n-* topology, mainly localized on the diazo bond, though it is slightly dipole-allowed
due to the above-mentioned non-planarity (f=0.05). According to theory, this transition is
separated by more than 1 eV from the following excitation (S0-S2, f=0.03), and even by 1.5 eV
from the intense S0-S4 absorption (f=0.41). Amongst all studied compounds, the most red-
shifted n-* transition should occur in the NMe2-bearing compound 1 (497 nm, f=0.11)
according to theory, but in that case it is likely buried under the very probable S0-S2 excitation
(f=0.86), that is much closer-lying than in compound 4. The second most red-shifted n-*
transition returned by theory is obtained for the NO2-substituted compound 10 (482 nm,
f=0.09), which fits the experimental ordering (see Table 1). As can be seen in Figure 3B, the
addition of strong donating or accepting groups does not fundamentally change the nature of
the transition, although one notice small red lobes (accepting character) on the nitro group of
7
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10.1002/anie.202008700
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compound 10. For 1, in contrast, it is mostly the planarization on one side of the compound
that accounts for the improved delocalization and the observed red-shift, rather than the
“direct” donating nature of the amino moiety.
Figure 3. A) Representation of the DFT optimal geometries for the two stable isomers as well as
inversion and rotation transition states, together with relative free energies in parenthesis (in kJ.mol-1)
and key dihedral angles for 4; B) electron density difference plot for the lowest transitions in three
selected trans compounds. The blue and red lobes indicate regions of decrease and increase of density
upon excitation, respectively. Representation threshold 15 x 10-4 au; C) same for the cis isomers; D)
spin density difference for the rotation transition state of 4 as given by BS-DFT. Representation
threshold 10 x 10-3 au.
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Table 1: Properties of compounds 1-10 (in DMSO) relevant for their photochromism: position of absorption maxima (max), molar attenuation coefficients
(), photostationary state distributions (PSD) determined under blue (exc = 426 nm), green (exc = 526 nm) and red (exc = 625 nm) light irradiation, quantum
yields for forward switching determined at  = 532 nm irradiation (tc), and for reverse switching (ct) determined at  = 445 nm irradiation,
photoswitching cross sections ( x ) at blue (exc = 426 nm), green (exc = 526 nm) and red (exc = 625 nm) light, experimentally determined half-life of the
metastable cis isomer at 25 C, and the calculated and measured activation barrier parameters for the thermal cis-to-trans isomerisation.
[54]
tc
ct

ct x

tc x

tc x
R
PSD
PSD
PSD
half-life
at 25 C
(days)
thermal cis to trans isomerisation
Gcalc Gexp Hexp Sexp
kJ/mol kJ/mol kJ/mol J/mol
p
S0-S1 max ()
c,426
t,526
 t,625
426 nm
(% cis)
526 nm
(% cis)
625 nm
(% cis)
(%)
(%)
trans
cis
-NMe2
-0.83
-
483 (6301)
66
22
46
34
nda
nda
683
10
0.70
69.1
101.6
119.5
60.0
1
-OMe
-Me
-0.27
-0.17
0.0
463 (1100)
461 (814)
457 (847)
465 (1616)
456 (866)
444 (1324)
461 (618)
459 (652)
467 (873)
452 (1352)
443 (1572)
441 (1569)
452 (2595)
441 (1546)
442 (1583)
441 (1216)
444 (1290)
448 (1673)
15
15
15
18
12
12
12
10
5
58
56
62
52
54
55
52
46
18
88
90
90
87
89
85
87
83
45
63
30
51
21
43
57
41
20
18
51
72
48
37
ndb
73
ndb
47
30
506
963
660
663
ndb
264
87
8
14.3
12.5
38.1
7.4
72.4
72.0
71.4
70.0
72.2
72.5
72.4
73.1
67.9
108.7
108.4
111.1
107.1
110.7
108.8
110.4
105.8
94.5
107.6
96.0
-3.9
-41.7
-2.9
-11.6
9.2
2
<1
3
3
-H
113
155
104
212
87
110.3
103.6
113.5
99.5
4
-SMe
-Cl
0.0
4
5
0.23
0.31
0.35
0.54
0.78
6
32.4
14.6
28.6
4.4
6
-OAc
-OCF3
-CF3
-NO2
1010
ndb
5
-31.0
1.9
7
2
111.0
118.1
100.0
8
511
384
47
<1
6
41.5
18.5
9
89
0.046
10
b Quantum yield for the reverse switching was not determined for compound 1 due to the presence of the overlapping, bathochromically-shifted
-* band at the  = 445 nm part of the spectrum.
a Quantum yield values could not be determined for compounds 6 and 8 due to lack of clear convergence of obtained data to a convincing fit.
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Upon irradiation with green and red light, we consistently observe the emergence of a
hypsochromically shifted band, mostly in the  = 440-455 nm region, which corresponds to
the cis isomer. The calculations indicate that the unsubstituted cis-4 is 26.3 kJ.mol-1 less stable
than its trans counterpart, with a geometry rather typical for these structure (Figure 3A). The
lowest excited state conserves its n-* character (Figure 3C), and the CC2-corrected vertical
excitation energy of 434 nm (f=0.03), again close to the experimental value (441 nm, see
Table 1). The most red-shifted transition computed in the cis series is obtained for 1 (461 nm,
f=0.15), which is again consistent with experimental data. As can be seen in Figure 3C, the
geometry of cis-1 resembles closely the one of cis-4 but the lone pair of the amino group now
recovers its clear donating character (blue lobe) explaining the red-shift. Data for the other
compounds can be found in the Supporting Information.
Photostationary state distributions (PSDs) that can be achieved under irradiation with visible
light are of crucial importance for applications, especially in biological context where the two
isomers are expected to have different potency in, e.g., binding to the cellular target.[11] Only
in very rare cases[55,56] it is possible to design molecules in which the stable isomer is almost
inactive at a given concentration, while the irradiation leads to the metastable isomer which is
orders of magnitude more potent, thereby making the application virtually independent from
PSD. In the majority of cases,[9,57] the difference in potency is much less pronounced,
requiring high efficiency for switching in both directions.
In series 1-10, we observe (Figure 2, green pie charts) that under irradiation with green light
( = 526 nm) for switching in the forward (trans to cis) direction, a photostationary state with
the distribution containing 46-62% cis isomer can be attained. The only exceptions were
compound 1 (likely due to the band overlap) and compound 10 - possibly due to short half-
life of the cis isomer, whose back-isomerisation competed with the photochemical
transformation towards this isomer. However, as the use of red light is of much more
biological relevance, we also evaluated the PSDs under  = 625 nm irradiation (Figure 2, red
pie charts). To our delight, we observed distributions mostly exceeding 80% cis, which is also
consistent with negligible absorptivity of this isomer at wavelengths corresponding to red
light. Again, lower values observed for compounds 1 and 10, can be explained by the
substantial band overlap in this spectral region (Figure 2). Altogether, the limitation that
remains to be solved for tetra-ortho-chloro-azobenzenes, similarly to almost all available
molecular photoswitches, is the overall low red-light absorptivity of the trans form, which is
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one-two orders of magnitude lower than that of wavelengths corresponding to green light,
leading to prolonged irradiation times[37] and sometimes compromising the PSD in cases
where fast thermal back-isomerisation of the metastable state is a competing process (e.g., in
the case of compound 10).
In this context, the quantum yield of the forward isomerisation becomes important, potentially
determining the usefulness of red-light operation of a photoswitch in a biological context.
While in the studied series of molecules no general trends can be observed (tc = 38 ± 16%)
(Table 1), we note that quantum yields observed for compounds with strong electron-donating
c
substituents (Compound 2, t = 34 – 63%) are somewhat higher than for those with
c
electron-withdrawing groups (9,10, t = 18 – 20%), although additional studies are still
needed to confirm this trend. In a broader context of photochemical process efficiency, we
also analysed the photoswitching cross section (Table 1) under green light irradiation (i.e.,
product of the quantum yield  and molar attenuation coefficient  at exc = 526 nm, the
maximum emission of the green LED used here). In general, values in the useful order of
magnitude (102-103) were found, again with the strong EDG-substituted compounds 1 and 2
showing the highest efficiency. The same trends are observed for irradiation with red light
(exc = 625 nm, Table 1), albeit with cross sections in the 100-101 order of magnitude.
The reverse (cis to trans) switching was studied by irradiation with blue light (exc = 426 nm,
Table 1). We were delighted to see that for most of the compounds it was possible to recover
>80% of the trans isomer (Figure 2, blue pie charts). This highlights the good dynamic range
that can be achieved with tetra-ortho-chloro-azobenzenes 2-9, which can be switched between
containing 82-90% trans isomer under blue light irradiation and 83-90% cis isomer under red
light irradiation. Compound 10 features the best PSD under blue light (95% trans), but its
forward switching is less pronounced (vide supra). Strikingly, due to the overlap with a strong
-* band in the blue region of the spectrum, the behaviour of compound 1 is essentially
inverted, as it can be most efficiently switched in the forward direction with blue light (66%
cis isomer) and in the reverse direction with green light (78% trans isomer). Thanks to
quantum yields exceeding 30% and strong absorptivities of all the studied compounds at  =
426 nm, the reverse switching is an efficient process, with cross sections in the 103-104 order
of magnitude.
The main motivation behind the introduction of tetra-ortho-substituted azobenzenes has been
the possibility to achieve visible-light-switching without compromising the half-life or the
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metastable isomer, which was the typical drawback of the more established azobenzene
architectures substituted with both an electron-withdrawing and electron-donating substituent
in the para positions (push-pull systems).[29] Indeed, our data (Table 1) for the tetra-ortho-
chloro-azobenzenes confirm that for most of the studied para-substituents (compounds 2-9),
the half-life of the cis isomer is between 4 to 38 d, which for all practical purposes translates
to bistable systems in biological applications, meaning that the thermal cis-trans isomerisation
can often be neglected for p between -0.27 - 0.54. However, compounds with strongly
electron-donating and –withdrawing groups (e.g. compounds 1 and 10) feature much lower
stability of the cis isomer – an effect especially pronounced for compound 10, where a half-
life of ~1 h was measured.
The thermal back isomerization in an azobenzene can typically take place through an
inversion or a rotation mechanism, and both have been found here through theoretical
investigation (Figure 3A and Table S1 in the Supporting Information). For all investigated
compound, the latter mechanism yields a more stabilised transition state and rotation is
therefore the most favoured pathway. We note that this mechanism comes with a rupture of
the  bond, and we therefore used broken-symmetry DFT to investigate it, which lead to the
expected spin distribution (Figure 3D). The theoretical back-isomerisation barriers are listed
in Table 1 and it can be seen that they are significantly smaller than their experimental
counterpart, but that the trends are nicely reproduced. Indeed, excluding the compound
substituted with a CF3 group, we obtain a determination coefficient, R2, between experiment
[Gexp] and theory [Gcalc] of 0.82. Compound 9 proved to be most difficult for theoretical
assessment. At this stage, it might be interesting to take a specific look at azobenzenes 1 and
10, as they are substituted with the prototypical strong donor (NMe2) and acceptor (NO2)
groups. As one might appreciate, both groups experimentally show quite small and similar
t1/2. However, the underlying reasons are different. Indeed, in compound 1, the cis form is
essentially non-stabilized, with a relative free energy of 31.4 kJ.mol-1 as compared to the trans
form, which is much higher than in the non-substituted case (26.3 kJ.mol-1). The cis-to-trans
barrier is small due to this lack of stabilisation (Table S1 in the Supporting Information). In
contrast, for compound 10, the relative free energy of cis as compared to the trans structure,
26.3 kJ.mol-1, is essentially unchanged from the unsubstituted case, but the rotational
transition state is itself much more stabilized (Table S1 in the Supporting Information). For
this reason, one needs to find a balance between the relative stabilities of the two isomers and
the stabilization of the rotational TS itself.
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Altogether, the data presented herein enables the formulation of certain general rules for the
design of tetra-ortho-chloro-azobenzene photoswitches for specific applications (Figure 4). In
situations where long half-lives of the cis isomer are required, e.g., when one studies the
effect of both isomers of a photopharmacological agent on a cell line for longer time, the use
of substituents from the middle of the Hammet p scale is recommended (e.g. those in
compounds 4 and 6), as it provides the metastable state that persists for multiple weeks,
similarly to those observed for hemiindigo photoswitches that also respond to red-light
irradiation.[58,59] Conversely, when life-times on the scale of hours are desired, as in the case
of photoswitchable antibiotics that are activated prior to administration and then should
spontaneously lose their activity,[60] the strongly electron-withdrawing groups (e.g.,
compound 10) are favoured.
In photopharmacology, a few azobenzene-based bioactive molecules have been described in
which the cis isomer shows potency several orders of magnitude higher than trans.[55,56] In
such cases, the photostationary state distribution that one can achieve is of less importance, as
even low concertation of the metastable state will result in localized activation. Here, the use
of tetra-ortho-chloro-azobenzenes with strong electron-donating groups (e.g., compound 1) is
recommended, as it offers the most efficient activation with visible light. However, such
systems are so far scarce, and most often, the difference in potency of the photoisomers is
limited, which requires that high photostationary states are achieved. Especially in these
cases, the intermediate substituents (e.g., compounds 3-8) should be considered.
Figure 4. Design guidelines for tetra-ortho-chloro-azobenzenes. Influence of the properties of
substituents in the para position (black arrows) of the azobenzene (A) on the position of the bands of
both isomers in the visible range of the spectrum (B) and on the kinetics of photoisomerisation (kp,
which is a function of molar attenuation coefficient  and quantum yield ) and thermal
reisomerisation kt (C).
13
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10.1002/anie.202008700
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In summary, we present here a systematic analysis of the photochromism of tetra-ortho-
chloro-azobenzenes, an emerging class of visible-light operated photoswitches with great
potential for use in biological and material sciences. Their versatility, underlined by the
possibility to tune their photochemical properties towards the desired application, renders
them a highly useful tool in a still limited repertoire of molecular photoswitches that respond
to low energy green and red light. Detailed understanding of the influence that substituents
play on key photochemical properties and thermal isomerisation barriers, as presented here,
will enable successful design of functional, photoresponsive systems. In a long-term
perspective, these insights provide a major step towards using light for the efficient regulation
of biological processes with outstanding spatiotemporal precision.
Acknowledgment. This research was financially supported by the Netherlands Organization
for Scientific Research (NWO‐CW), ECHO grant 711.017.012 to W.S. D.J. is indebted to the
CCIPL computational center in Nantes, for generous allocation of computational time. D.J.
and S.B. thank the Region des Pays de la Loire for supporting their collaboration in the
framework of the Opt-Basis project. S.B. acknowledges the support of the Scientific Grant
Agency VEGA 1/0562/20. Support from the Ministry of Science and Education (FMS
gravitation program) to B.L.F. is gratefully acknowledged. N.A.S. thanks the Humboldt
Foundation for a Feodor-Lynen Fellowship. Gabriele Straaß is greatly acknowledged for
helping with the assembly of the irradiation setup.
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Table of contents entry:
Molecular photoswitches that respond to visible light irradiation enable external, non-invasive
control over biological systems, materials, and molecular machines. Here we present a
systematic spectroscopic and theoretical investigation into the photochemistry of tetra-ortho-
chloro-azobenzenes and outline the design principles that allow the optimisation of those
emerging photochromes on a molecular level.
Table of contents graphics:
18
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