Long-Distance Rate Acceleration by Bulk Gold

Article abstract of DOI:10.1002/anie.201814342

We report on a very unusual case of surface catalysis involving azobenzenes in contact with a Au(111) surface. A rate acceleration of the cis–trans isomerization on gold up to a factor of 1300 compared to solution is observed. By using carefully designed molecular frameworks, the electronic coupling to the surface can be systematically tuned. The isomerization kinetics of molecules with very weak coupling to the metal is similar to that found in solution. For their counterparts with strong coupling, the relaxation rate is shown to depend on the spin-density distribution in the triplet states of the molecules. This suggests that an intersystem crossing is involved in the relaxation process. Aside from their impact on catalytic processes, these effects could be used to trigger reactions over long distances.

Full text of DOI:10.1002/anie.201814342

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Long-Distance Rate Acceleration by Bulk Gold
Authors: Rainer Herges, Alexander Schlimm, Roland Löw, Talina
Rusch, Fynn Röhricht, Thomas Strunskus, Tobias Tellkamp,
Frank Sönnichsen, Uwe Manthe, Olaf Magnussen, and Felix
tuczek
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201814342
Angew. Chem. 10.1002/ange.201814342
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10.1002/anie.201814342
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Long-Distance Rate Acceleration by Bulk Gold
Alexander Schlimma, Roland Löwb, Talina Ruschc, Fynn Röhrichtb, Thomas Strunskusd, Tobias
Tellkampb, Frank Sönnichsenb, Uwe Manthee, Olaf Magnussenc*, Felix Tuczeka*, Rainer Hergesb*
We report on a very unusual case of surface catalysis involving
azobenzenes in contact with a Au(111) surface. We observe a rate
acceleration (up to 1300-fold) of cis-trans isomerization on gold,
compared to solution. By using carefully designed molecular
frameworks we are able to systematically tune the electronic coupling
to the surface. The isomerization kinetics of molecules with very weak
coupling to the metal is similar to that found in solution. For their
counterparts with strong coupling the relaxation rate is shown to
depend on the spin density distribution in the triplet states of the
molecules. This suggests that an intersystem crossing is involved in
the relaxation process. Besides their impact on catalytic processes,
these effects could be used to trigger reactions over long distances.
isomerization on gold, compared to solution, even though the
reacting N=N double bonds are located 14 Å above and are
separated by eleven bonds from the Au(111) surface. To
elucidate this extraordinary, long-range acceleration of the
reaction rate we employ carefully designed molecular
frameworks.[3] The modularity of this unique system allows
precise adjustment of intermolecular distances and coupling.[4,5]
Furthermore, the electronic coupling to the surface is
systematically tuned by changing the π-conjugation of the
coupling unit while the distance between gold surface and
azobenzene remains almost the same. Surprisingly, the
isomerization kinetics of molecules with strong coupling to the
metal follows a different mechanism than observed for their
counterparts with no or very weak coupling. Specifically,
activation energies drop to very low values and frequency factors
become explicitly dependent on an electronic matrix element that
can be correlated with the spin density distribution in the triplet
states of the molecules. Besides their impact on catalytic
processes, these effects could be used to trigger chemical
reactions over a long distance. Similar remote effects have not
been considered in catalytic processes so far; however, they
could be operative in a number of reactions on metallic surfaces
or nanoparticles.
The majority of chemical processes in nature and in industry
involve catalysis. There is no other way to form stable products
from stable reactants under mild conditions. Most technologically
relevant reactions employ heterogeneous catalysis, where the
reacting molecules interact with atoms or ions on surfaces,
forming intermediates.[1] This opens up reaction pathways with
lower barriers and faster reaction rates than in the uncatalyzed
processes. A number of reactions on surfaces have been studied
and their mechanisms have been elucidated in detail.[1,2] Here we
report on a very unusual case of surface catalysis involving
azobenzene units in contact with a Au(111) surface. We observe
a dramatic rate acceleration (up to 1300-fold) of cis-trans
Catalysts increase the rates of chemical reactions by creating a
new and energetically more favourable reaction path with lower
activation energy. In heterogeneous catalysis, this new path
involves adsorption of reactants at the surface, where chemical
[a]
[b]
Alexander Schlimm, Prof. Dr. Felix Tuczek
Christian Albrechts University Kiel
Institute of Inorganic Chemistry
Max-Eyth-Str. 2, 24118 Kiel
E-mail: ftuczek@ac.uni-kiel.de
Roland Löw, Fynn Röhricht, Dr. Tobias Tellkamp, Prof. Dr. Frank
Sönnichsen, Prof. Dr. Rainer Herges
Christian Albrechts University Kiel
Institute of Organic Chemistry
Otto-Hahn-Platz 4, 24118 Kiel
E-Mail: rherges@oc.uni-kiel.de
Talina Rusch, Prof. Dr. Olaf Magnussen
Christian Albrechts University Kiel
Institute of Experimental and Applied Physics
Leibnizstr. 19, 24118 Kiel
[c]
E-Mail: magnussen@physik.uni-kiel.de
Dr. Thomas Strunskus
Christian Albrechts University Kiel
Institute for Materials Science
Kaiserstr. 2, 24143 Kiel
Prof. Dr. Uwe Manthe
[d]
[e]
University of Bielefeld
Institute of Theoretical Chemistry
Universitätsstr. 25, 33501 Bielefeld
Figure 1. Modular design to tune the electronic coupling of azobenzene
units with the metal surface. TATA platforms define the intermolecular
distances on the surface (by the size of the side chain: a=octyl, b=propyl) and
ethinyl spacers provide a freestanding upright orientation of the reacting
azobenzene molecules. Coupling units (green) control the electronic coupling
of azobenzenes with the surface. Methoxy groups on top, report (IRRAS) on the
configurations of azobenzene (cis or trans).
Supporting information for this article is given via a link at the end of
the document.
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Figure 2. STM images, time-dependent IRRAS spectra, and cis→trans kinetics of compound 3a adsorbed on Au(111). a) STM image and model of a
monolayer of azo-TATA 3a on Au(111). The molecules exhibit an intermolecular distance of 12.6 Å b) PM-IRRAS signal of the C-O stretching vibration region,
before irradiation (grey) and the signal intensity after irradiation with light of 365 nm as a function of time (red to blue). The blue spectrum was obtained after
irradiation with light of 440 nm. The irradiation time was in both cases 10 min. c) PM-IRRAS intensity of C-O stretching as a function of time at two different
temperatures (271.5 K and 293.0 K). Half-lives (t1/2) are derived by linear regression. d) Arrhenius plot of reaction rates k at 5 different temperatures T. The activation
barrier Ea and the frequency factor A are derived by linear regression.
bonds within the molecule are broken and new bonds with the
surface atoms are created. Whereas the majority of chemical
reactions follow Eyring theory or RKKM-type transition state
theories, there are a few examples of chemical reactions that
substantially deviate from the theoretically predicted behavior.[6]
Their reaction rates mainly depend on quantum mechanical
upon addition of an increasing number of methyl groups (n=0-4)
in ortho position the phenyl groups are twisted out of coplanarity
which decreases their conductivity (Figure S124).[11] To shut off
π-conjugation completely, we inserted a trioxabicyclo[2,2,2]unit
which separates the π systems of azobenzene and the ethinyl
spacer with sp3 hybridized C and O atoms. The new compounds
(3a-5a, 1b, 2b, 5b) were exhaustively characterized in solution or
bulk material with NMR, UV/vis and vibrational spectroscopy (see
supporting information). Scanning Tunneling Microscopy (STM)
shows that, in agreement with previous results,[3,4,12] optimized
self-assembly conditions result in monolayers of 1a – 5a with
transition probabilities and not the height of
a barrier.
Characteristic features are apparently small frequency factors
and low activation barriers, corresponding to reaction rates that
are almost temperature independent. Such processes have
mostly been observed at low temperatures (< 50 K).[7] Herein we
show that bulk gold can mediate similar phenomena at
temperatures around 300 K. Moreover, by modifying the
electronic coupling between the reaction center and the Au
surface, we are able to shift the isomerization kinetics from a
classical, solution-like behaviour to a new regime involving a
drastically lowered barrier, and dramatically increased reaction
rates.
( 19× 19) R23.4° superstructures and low defect densities (see
√
√
Figure 2 and Figure S125). X-ray spectroscopy studies (XPS,
NEXAFS) prove that the adlayers are of high purity with the
molecules being oriented perpendicular to the surface, as shown
in Figure 2 (Figure S126-127). The vertical alignment of the
azobenzene with regard to the surface has been proven in
previous studies as well (tilt angle: 6±16°).[12]
We employ in our studies azobenzene functionalized TATA (“azo-
TATA”) molecules (Figure 1, top). The cis-trans-isomerization
kinetics of 1a and 2a have recently been investigated in solution
and for adsorbed monolayers, finding that the thermal cis-to-trans
relaxation of molecule adsorbed on bulk Au(111) is considerably
faster than in solution.[8–10] Assuming that this rate acceleration
involves electronic coupling to the metal,[8] we now synthesized
three new azo-TATA compounds with different coupling units,
inserted between the platform and the azobenzene unit (Figure 1
and supporting information). Biphenyl groups were chosen to tune
the electronic coupling of the azo group to the surface because
4,4’-substituted biphenyls retain the upright alignment of the
azobenzene with respect to the surface, and most importantly,
The well-defined molecular orientation together with the
surface selection rule allows monitoring the photochemical (365
nm) trans-to-cis isomerization as well as the ensuing thermal cis-
to-trans relaxation by IRRAS.[13] The latter leads to orientational
changes of the Cphenyl-Omethoxy bond relative to the surface and,
thus, temporal changes in the corresponding band’s intensity in
the IRRA spectrum. The rates of thermal cis-to-trans relaxation
were determined at five different temperatures (Figure 2, Figure
S129). From these data activation energies and frequency factors
were obtained for compounds 1a – 5a (Figure 3, Figure S128).
For adsorbate layers of compounds 1a, 2a and 3a on Au(111) the
rates show only small changes in the investigated temperature
range. Correspondingly, the activation barriers and frequency
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Figure 3. Kinetic data of the cistrans isomerisation of compounds 1a-5a and 6 in solution and adsorbed on Au(111). Half-lives (t1/2), observed rates (kobs),
activation barriers (Ea) and frequency factors (A) for azo-TATA compounds (sorted by decreasing coupling between platform and azobenzene) are determined in
solution (grey), and adsorbed on Au(111) (yellow). Spin densities (δs) of 1a-5a in the triplet state are calculated with DFT (M08HX/def2TZVP) and are integrated
over all atoms within the platform unit and the ethinyl spacer. For compound 6 the half-life, activation barrier and frequency factor are presented in solution. More
data points are shown and error margins are given in the supporting information (Figure S129).
factors are exceptionally small and typical for non-adiabatic
reactions, whose rates are controlled by quantum mechanical
transition probabilities, rather than by classical activation barriers.
On the other hand, the largely decoupled compounds 4a and 5a
show a significantly stronger temperature dependence of their
reaction rate, close to the rates observed in solution. The
intermolecular steric repulsion is larger in the bent cis than in the
slender trans configuration.[14] Even though unlikely to account for
our large rate accelarations we performed further experiments to
probe this hypothesis. The large octyl side groups at the TATA
platform were replaced by smaller propyl (1b, 2b, 5b) groups
which results in a smaller intermolecular distance in self-
assembled monolayers on Au(111) (octyl: 12.6 Å vs propyl 10.4
Å).[3–5,10] However, the measured rate accelerations on gold are
similar (supporting information, Figure S129). Steric constraints,
thus, can be excluded. Moreover, steric confinement would not
explain the very low barriers and frequency factors. Traces of
surface absorbed water are neither responsible for the rate
acceleration.[15] On the contrary, adventitious water on surface-
adsorbed azobenzenes decreases the relaxation rate.[16] Added
water rather also slows down the reaction rates in solution (see
SI figure S133). It is known that charge transfer can accelerate
cis-trans isomerisation rates of azobenzenes. Azobenzenes
either oxidized to the corresponding radical cations or reduced to
the radical anions exhibit very fast thermal isomerization rates.[17]
The low cis-trans activation barriers of azobenzenes directly
absorbed on gold nanoparticles[18,19], or lying flat on Au(111)[20] or
Bi(111)[21] surfaces, have been explained by a transfer of electron
density from the azobenzene into the metal.[19] To investigate if
such a charge transfer would be operative in our system, we
synthesized azobenzene 6 with an Au+ ion directly connected to
the ethinyl group (Scheme S5). According to the NBO analysis of
6 the azoethinyl group carries approximately half an elementary
charge (-0.508 e-). Kinetic analysis, however, reveals that the
activation barrier (20.3 kcal mol-1) and the frequency factor
(log(A)=13.78 s-1, (Figure 3, right) are in the usual range expected
for undisturbed azobenzenes.[22] The strongest argument against
static charge transfer is the fact that the IR vibrational frequencies
combined surface-kinetic data thus show that at
a fixed
temperature the half-life of the adsorbed species can be changed
by up to four orders of magnitude through adjusting the electronic
coupling between platform and azo-unit. In contrast, the half-lives
of the five compounds in solution vary only slightly with the
coupling unit.
The fundamental change in the cis-to-trans reaction
observed across compounds 1a – 5a can be correlated with the
degree of electronic coupling to the surface. Particularly
remarkable is the change in kinetics between compounds 3a and
4a (Figure 3, right). As both are structurally closely related and
only differ in a methyl group, kinetic data in solution are very
similar. Absorbed on the metal surface, however, their half-lives
(t1/2), frequency factors (A) and activation barriers (EA) differ
drastically (Figure 3). Electronic coupling to the metal surface is
still operative in 3a, whereas the almost orthogonal arrangement
of the phenyl rings in 4a interrupts π-conjugation and electronic
coupling. The change from non-standard kinetics in 3a to classical
Arrhenius behaviour in 4a thus is clearly related to the breakdown
of molecule-substrate coupling. The different activation barriers of
the compounds 4a - 5a in solution and adsorbed on a surface can
be explained by the dissimilar environments.
Different hypotheses for the acceleration of cis-to-trans
relaxation processes in molecular adsorbates were discussed. It
is known that dense monolayers of azobenzenes on surfaces
exhibit increased cis to trans isomerisation rates, because
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information, section 1). Since the ISC as well as the CT processes
are enabled by the coupling to the bulk gold, the value of the
coupling element is determined by the respective wavefunction
amplitudes localized in the bulk gold. The coupling matrix element
for the ISC involves the S0 and T1 wavefunctions and is thus
proportional to δs. In contrast, the coupling matrix element for the
CT involves the S0 wavefunction and the wavefunction of the CT
state and is proportional to the square root of δs (see supporting
information section 1). Thus, only the ISC mechanism is
consistent with the observed quadratic δs-dependence of k’.
The bulk gold induced rate enhancement described
above represents a new type of catalysis. Unlike in conventional
heterogeneous catalysis, there is no direct chemical involvement
of the surface (no chemical bonds to the surface are made or
broken). Instead, the bulk metal opens an alternative pathway
with almost vanishing barriers by the mixing of molecular states
with the quantum states of gold. Particularly exceptional is the fact
that the state transitions occur at room temperature, without
photochemical activation, over a large distance (14 Å, 11 bonds).
We used a well-defined, controllable environment to investigate
these non-classical effects systematically and propose that a
change in spin state is responsible for the drastic rate acceleration.
It is quite likely that similar effects are operative in a number of
catalytic processes where molecules react at metal surfaces.
Likewise, controlled coupling of molecules to the conducting band
of materials could open new ways to trigger reactions with
spatiotemporal control. Such effects may also be exploited to
efficiently catalyze chemical reactions or to remote control
molecular machines on surfaces.[23]
Figure 4. Log(kobs) as a function of log(δs) for compounds 1a – 5a. δs is the
spin density of the triplet state of 1a – 5a integrated over all atoms of the
platform and the ethinyl spacer, calculated at the M08HX/-def2TZVP level of
DFT. Compound 5 serves as a reference molecule without electronic coupling
to the surface and a very small value for δs (dashed lines). Continuous lines are
fitted according to eqn(1).
of the cis and the trans azo-TATA molecules in solution and
absorbed on the surface are virtually identical (Figure S123).
The appearance of two different regimes in Figure 3
suggests the existence of two distinct mechanisms. It will be
shown that the delocalization of the electronic wavefunction of the
azobenzene unit over the entire molecule into the gold plays a key
role in this context. A convenient measure of the delocalization is
the spin density δs at the TATA platform computed for triplet state
(Figure S124): the wavefunction amplitude localized in the bulk
gold is approximately proportional to the square root of δs for the
triplet (T1) as well as for the singlet (S0) electronic state (see
supporting information section 1). The plot of log(kobs) vs. log(δs)
shown in Figure 4 reveals that the observed rate constant kobs can
be written as a sum of a rate constant k which is associated with
a thermally activated process exhibiting standard Arrhenius-type
behaviour, and a rate constant k´ which in addition depends upon
the square of the spin density at the TATA platform:
Acknowledgements
We thank DFG (SFB 677) for funding this research. Furthermore we
would like thank HZB for the supply of beamtime at the HE-SGM
beamline (XPS & NEXAFS Measurements). The authors thank Martin
Kirk for fruitful discussions.
Keywords: Catalysis, Surface adsorbed azobenzenes, surface
spectroscopy, vibrational spectroscopy, Self-Assembled
Monolayers
−ꢉꢊ
−ꢉꢊ′
ꢀꢁꢂꢃ = ꢀ + ꢀ´ = ꢄ ꢅꢆꢇ ꢈ
ꢍ + ꢎꢃꢏ ꢄꢐ ꢅꢆꢇ ꢑ
ꢒ
(1)
ꢋꢌ
ꢋꢌ
The parameters A (105.6 s-1) and EA (14.7 kcal/mol) for the regime
associated with k are taken from compound 5a serving as a
reference system for a surface-deposited azo-TATA molecule
with an electronically decoupled azobenzene unit (Figure 3). For
k´, a significantly reduced apparent barrier height EA´ of 2.70.1
kcal/mol and a prefactor A´ of 894110 s-1 are found by fitting our
data to eqn(1) (Figure S130-131).
The described findings strongly suggest that an
electronic transition mediated by the bulk gold is involved in the
reaction mechanism associated with the rate constant k´. Two
different mechanisms can be imagined:
a gold-mediated
intersystem crossing (ISC) between the singlet (S0) and triplet (T1)
states of the azobenzene or a transient charge transfer (CT)
between the gold and azo-TATA. Considering the coupling
between the different electronic states as a weak perturbation, k'
can be assumed to be proportional to the square of the coupling
element enabling the electronic transition (see supporting
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Remote control of reactions:
Author(s), Corresponding Author(s)*
Azobenzenes standing upright on a
bulk gold surface isomerize within 45s
from cis to trans compared to 19h in
solution. A coupling unit allows tuning
between those extremes. Abnormally
low activation barriers, vanishing
frequency factors and the theoretical
treatment show that the 1300 fold rate
acceleration is due to a singlet-triplet-
singlet spin change, forbidden in
Page No. – Page No.
Title
solution but mediated by bulk gold
over 11 bonds and a distance of 14Å.
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