Photoinduced Pedalo-Type Motion in an Azodicarboxamide-Based Molecular Switch

Article abstract of DOI:10.1002/anie.201709666

Well-defined structural changes of molecular units that can be triggered by light are crucial for the development of photoactive functional materials. Herein, we report on a novel switch that has azodicarboxamide as its photo-triggerable element. Time-resolved UV-pump/IR probe spectroscopy in combination with quantum-chemical calculations shows that the azodicarboxamide functionality, in contrast to other azo-based chromophores, does not undergo trans–cis photoisomerization. Instead, a photoinduced pedalo-type motion occurs, which because of its volume-conserving properties enables the design of functional molecular systems with controllable motion in a confined space.

Full text of DOI:10.1002/anie.201709666

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Accepted Article
Title: Photo-induced Pedalo-type Motion in an Azodicarboxamide-
based Molecular Switch
Authors: Saeed Amirjalayer, Alberto Martinez-Cuezva, Jose Berná,
Sander Woutersen, and Wybren Jan Buma
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709666
Angew. Chem. 10.1002/ange.201709666
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Angewandte Chemie International Edition
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COMMUNICATION
Photo-induced Pedalo-type Motion in an Azodicarboxamide-
based Molecular Switch
[a]
[b]
[b]
[c]
Saeed Amirjalayer,* Alberto Martinez-Cuezva, Jose Berna, Sander Woutersen,* and Wybren
[c]
Jan Buma,*
Abstract: Well-defined structural changes of molecular units that
can be triggered by light are crucial for the development of
photoactive functional materials. Here we report on a novel switch
that has azodicarboxamide as its photo triggerable element. Time-
resolved UV-pump/IR probe spectroscopy in combination with
quantum chemical calculations shows that the azodicarboxamide
functionality -in contrast to other azo-based chromophores- does not
environment or by immobilizing it at interfaces or in confined
[18-26]
environments such as crystals.
In all cases, however, it is
clear that coupling inevitably leads to spatial restrictions on the
switching process. One of the key challenges is thus the
development of externally controllable molecular units with
specific volume-conserving motions that are able to influence
the local environment. Tuning of the expansion behavior of
systems using a thermally induced pedalo-motion in a co-
undergo trans-cis photoisomerization. Instead,
a photo-induced
[
27-29]
pedalo-type motion occurs, which due to its volume-conserving
properties enables to design functional molecular systems with
controllable motion in a confined space.
crystal
may in this respect serve as an illustrative example
of the potential of such an approach. At the same time, the
reported photoinduced hula twist or bicycle pedal type of motion
further highlights the relevance of volume-conserving
[30,31]
dynamics.
Switching between two or more states on the molecular level is
fundamental for many biological phenomena including regulation
In the present study we aim to transfer this concept to new light-
responsive functional materials. To this purpose we focus on
[
1-4]
of muscle contraction, blood pressure and vision.
In the latter
[32,33]
azodicarboxamide-based molecular systems (Figure 1).
case a light-triggered conformational change of a molecular
chromophore results in a cascade of subsequent processes that
enable human sight. Embedded in a protein, the switching of the
photoactive molecular unit between two well-defined structural
states causes by specific interactions with the protein
environment a long-range effect that is the basis for the
These systems feature well-defined hydrogen bond interaction
sites that provide anchor points to the molecular environment
and an azo-moiety that might induce light-induced structural
changes. Normally, such an azo-moiety is used in conjunction
with aromatic substituents that lead to planar geometries around
[34-36]
the N=N bond.
The conjugation of the same function
[
5]
biological activity. Such a level of control of molecular motion
on the nanoscale is not only crucial for biological systems but
also for the development of new efficient functional materials for
a large range of applications (e.g. catalysis, drug delivery,
through carboxamide groups makes one expect different
conformational behaviour for azodicarboxamide-based systems,
potentially resulting in other types of molecular motions.
[
6-16]
molecular computers).
The functional performance of these
materials depends not only on the intrinsic switching properties
of the employed molecular units, but -as illustrated by the
[
5,17]
molecular-level description of human sight
switching is transmitted to the macroscopic world. This is done
either by coupling the switch to dynamic molecular
-also on how
a
[a]
Dr. S. Amirjalayer,
Physikalisches Institut
Westfälische Wilhelms-Universität Münster
Figure 1. Azodicarboxamide-based molecular thread: Lewis-structure of the
azodicarboxamide functional group (left, photoactive unit (blue) and potential
hydrogen bonding sites (red)); Ball and stick representation of
tetramethylazodicarboxamide (middle) and tetrabenzylazodicarboxamide
(right) optimized at the B3LYP/6-31G(d,p) level (color coding: C: black, O: red,
N: blue, H: white).
Willhelm-Klemm-Strasse 10, 48149 Münster (Germany)
and
Center for Nanotechnology (CeNTech)
Heisenbergstrasse 11, 48149 Münster (Germany)
E-mail: s.amirjalayer@wwu.de
[
b]
c]
Dr. A. Martinez-Cuezva, Prof. Dr. J. Berna
Departamento de Química Orgánica, Facultad de Química
Universidad de Murcia
[
37]
Apart from one photolysis study, the photodynamic properties
of azocarboxamide systems have so far not been studied, and
practical applications are largely unexplored. We have therefore
combined picosecond time-resolved vibrational spectroscopy of
the electronically excited states with quantum-mechanical
calculations to elucidate in detail the photodynamics of
tetrabenzylazodicarboxamide (Figure 1), which turns out to be
indeed completely different from the usually observed trans-cis
isomerization in azobenzene-based molecular systems.
30100 Murcia (Spain)
[
Prof. Dr. S. Woutersen, Prof. Dr. W. J. Buma
Van ’t Hoff Institute for Molecular Sciences
University of Amsterdam
Science Park 904
1098 XH Amsterdam (The Netherlands)
E-mail: s.woutersen@uva.nl
E-mail: w.j.buma@uva.nl
Supporting information for this article is given via a link at the end of
the document.
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this case rotated by about 75° with respect to the N=N bond
(
MP2/6-31++G**) and point in opposite directions. Further
support for this conclusion is provided by our experimental
UV/Vis and IR absorption spectra which are in excellent
agreement with the corresponding results obtained by Density
Functional theory calculations (B3LYP/6-31G(d,p), (see Figures
S1 and S2 in the Supporting Information). The main path
between the two conformers primarily involves the torsion
around the C-N bond (see Figure 2, Figure S3 and S4 in the
Supporting Information). The resulting dynamics can be
described by a pedalo-type motion of the central nitrogen atoms.
In the context of molecular switches, two unique aspects are
associated with this effective motion. Firstly, it is volume-
conserving and therefore also possible in confined space.
Secondly, motion along this path changes the direction of the
carbonyl groups (Figure 2b) and thus modulates hydrogen
bonding with the molecular environment.
Figure 2. Symmetry breaking of the azodicarboxamide-based molecular
switch in the electronic ground-state: (a) Scanning of the conformational phase
space along the two torsion angles αNCNN and βNCNN (depicted in green and
orange in the Lewis structure formula); potential energy surface at the MP2/6-
3
1G level reveals two stable conformers; (b) Energy profile along the reaction
coordinate connecting these two conformers (CCSD(T)/6-31++G**//MP2/6-
1++G**) together with the ball and stick representation of the two
3
energetically equal conformers and the planar transition state structure along
the reactions coordinate.
To gain insight into the electronic and structural properties of
azodicarboxamide derivatives, we have performed quantum
mechanical calculations on tetramethylazodicarboxamide.
Detailed investigations of the conformational phase space in the
electronic ground-state reveals, in contrast to the parent
Figure 3. FTIR and time-resolved IR spectra of the N,N,N’,N’-
tetrabenzylazodicarboxamide molecular switch in chloroform (c=6.6 mM) at
room temperature: (a) FTIR spectra (black, solvent corrected) and calculated
stick spectra of the ground state conformer (gray) (top); UV-pump/IR-probe
transient IR spectra (λUV=350 nm) at indicated time delays after UV excitation
[
38,39]
compound 1,1’-azobisformamide,
a significantly non-planar
structure of the molecule (Figure 2a). The carbonyl groups are in
(middle); Species Associated Difference Spectra of species
1 (dark-red)
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assigned to the electronically excited-state species and species 2 (dark-blue)
assigned to the hot ground-state species together with the calculated stick-
spectra of the corresponding species (scaling factor: 0.965) (bottom); (b)
Kinetic data at 1647 cm (red) and 1711 cm (blue) together with the fit based
on the global analysis of entire dataset.
planar structure with a carbonyl stretch frequency that is in
perfect agreement with the frequency of the induced absorption
band observed in the SADS of Sp1 (Figure 4a). Further support
for planarization of the molecule in the excited state is found in
frequency calculations at the non-planar Franck-Condon
geometry. In this case only a small shift of the carbonyl band is
predicted (∆ν=38 cm ), which is not in line with the measured
data but would be consistent with the pedalo-type motion. It is
important to realize that one can only decisively conclude which
type of motion occurs by identifying the structure of the excited-
state species, and not by merely considering the initial and final
ground-state species (see S5 in the SI for further discussion).
-
1
-1
-1
To determine the light-triggered properties of the
azodicarboxamide functional group we have performed ps time-
resolved IR absorption spectroscopy which has been shown to
[
40-48]
provide a high temporal resolution and structural sensitivity.
Figure 3a displays transient IR spectra of tetrabenzyla-
-
zodicarboxamide in chloroform (c=6.6 mM) in the 1600-1775 cm
1
range associated with the carbonyl stretch region after
1
We thus associate Sp1 with S which decays on a 1.5 ps
[
51]
excitation at 350 nm. In agreement with our DFT calculations
that show that the first electronic transition with non-zero
oscillator strength is expected at around 340 nm, no transient
signals could be observed upon excitation at longer wave-
timescale back to the ground state. The excited-state lifetime
of 1.5 ps indicates that internal conversion of the excited state
occurs in regions of the potential energy surface of the excited
state where the energy gap with the ground state is relatively
small or even absent, as it would be in the case of a conical
intersection between the two states. Our computational
methodology is not adequate to locate conical intersections -this
would require multi-reference methods such as CASSCF with
large active spaces (22 π-electrons)- but the fact that our
calculations show that there is a stable minimum in the excited
state strongly suggests that a barrier needs to be overcome to
access such a conical intersection with the ground state.
This excited-state relaxation process results in a vibrationally hot
ground-state species, in which the carbonyl-stretch mode is
slightly redshifted (causing the Sp2 SADS to look like the first
derivative of the FTIR spectrum) due to anharmonic coupling
with the hot low-frequency modes. Upon vibrational cooling, the
thermally equilibrated ground state is recovered. Remarkably,
the overall picture that thus emerges from experiments and
calculations (Figure 4b) is one in which a non-planar ground
state is excited to an electronically excited state with an
equilibrium geometry that corresponds to the transition state
between the two equivalent non-planar ground-state conformers
(Figure 2). We therefore conclude that during the photocycle the
azodicarboxamide unit effectively performs a pedalo-type motion.
lengths. This transition corresponds to the S
3 0
 S transition;
the S and S states have zero oscillator strength and are thus
1
2
not directly accessible by vertical one-photon excitation.
Before analyzing the vibrational spectra in full detail, we notice
that one expects the IR spectrum in the carbonyl stretching
range to be quite different for the planar and non-planar
conformations depicted in Figure 2 as is indeed confirmed by
our calculations (vide infra). This part of the spectrum thus is an
excellent probe for following a pedalo-type motion. The transient
IR spectrum at a delay of 0.5 ps shows one intense bleach
signal at the position of the carbonyl stretch vibration in the FTIR
-1
spectrum of the ground-state conformer (1708 cm ) and an
-1
additional induced broader peak at around 1640 cm (Figure
[
49]
3a).
For longer delays the induced absorption signal
decreases in intensity while its maximum shifts to higher
frequencies. The same is observed for the ground state bleach
signal but on a longer time scale and with a smaller shift of the
bleach signal. After about 30 ps the transient signal is
completely vanished. A full global analysis of the transient data
shows that they can be well described by a bi-exponential decay
(
Figure 3b). A sequential kinetic model then leads to two species
Sp1 and Sp2 with lifetimes of 1.5±0.05 and 8.6±1.1 ps,
respectively, and species associated difference spectra (SADS)
that are shown in Figure 3. Our data demonstrate that
photoexcitation does not produce any long-lived species. The
final species in the kinetic model (SpC) can thus be associated
with the system in its electronic ground state.
By combining the derived SADS with the results of DFT
calculations, we can elucidate the photodynamics of the system
3
in full detail. As mentioned above, the initially excited state is S .
With our time resolution (ca. 150 fs) we are only able to observe
one excited-state species. We thus conclude that, similar to the
[
50]
related fumaramide system,
ultrafast internal conversion processes that bring the molecule to
and it is this state for which we observe the excited-state
dynamics. This conclusion finds further support in our
calculations on tetramethylazodicarboxamide. Geometry
optimization of S at the TD-DFT level shows that the molecule
3
excitation of S is followed by
S
1
3
initially becomes more planar but never converges, most
probably due to the proximity of a conical intersection.
Figure 4. Photodynamics of N,N,N’,N’-tetrabenzylazodicarboxamide molecular
switch: Proposed excited-state relaxation pathway for the pedalo-type motion
(k and k referring to the corresponding time constants).
1 2
TD-DFT calculations on the lowest electronically excited state S
1
,
on the other hand, find that in this state the molecule adopts a
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Entry for the Table of Contents (Please choose one layout)
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Motion without motion: By
combining time-resolved infrared
spectroscopy with quantum-
mechanical calculations a volume-
conserving pedal-type motion was
studied in a molecular switch. These
azodicaboxamide-based systems
have the potential to open a new
realm to achieve motion in confined
space.
S. Amirjalayer,* A. Martinez-Cuezva, J.
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Photo-induced Pedalo-type Motion in
an Azodicarboxamide-based
Molecular Switch
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