Reversing the thermal stability of a molecular switch on a gold surface: Ring-opening reaction of nitrospiropyran

Article abstract of DOI:10.1021/ja901238p

The ring-opening/closing reaction between spiropyran (SP) and merocyanine (MC) is a prototypical thermally and optically induced reversible reaction. However, MC molecules in solution are thermodynamically unstable at room temperature and thus return to t

Full text of DOI:10.1021/ja901238p

Published on Web 08/13/2009
Reversing the Thermal Stability of a Molecular Switch on a
Gold Surface: Ring-Opening Reaction of Nitrospiropyran
Marten Piantek, Gunnar Schulze, Matthias Koch, Katharina J. Franke,
Felix Leyssner, Alex Kru¨ger, Cristina Nav´ıo, Jorge Miguel, Matthias Bernien,
Martin Wolf, Wolfgang Kuch, Petra Tegeder, and José Ignacio Pascual*
Institut fu¨r Experimentalphysik, Freie UniVersita¨t Berlin, Arnimallee 14, 14195 Berlin, Germany
Received May 8, 2009; E-mail: pascual@physik.fu-berlin.de
Abstract: The ring-opening/closing reaction between spiropyran (SP) and merocyanine (MC) is a prototypical
thermally and optically induced reversible reaction. However, MC molecules in solution are thermodynami-
cally unstable at room temperature and thus return to the parent closed form on short time scales. Here
we report contrary behavior of a submonolayer of these molecules adsorbed on a Au(111) surface. At 300
K, a thermally induced ring-opening reaction takes place on the gold surface, converting the initial highly
ordered SP islands into MC dimer chains. We have found that the thermally induced ring-opening reaction
has an activation barrier similar to that in solution. However, on the metal surface, the MC structures turn
out to be the most stable phase. On the basis of the experimentally determined molecular structure of
each molecular phase, we ascribe the suppression of the back reaction to a stabilization of the planar MC
form on the metal surface as a consequence of its conjugated structure and large electric dipole moment.
The metal surface thus plays a crucial role in the ring-opening reaction and can be used to alter the stability
of the two isomers.
Introduction
been employed in order to balance the thermodynamic equi-
librium toward the open isomer. For example, chelation by metal
The chemical transformation between spiropyran (SP) and
merocyanine (MC) molecules has attracted great attention
because the molecular properties change enormously through a
very basic intramolecular reaction. The SP-to-MC transformation
follows a multistep pathway in which the fission of the spiro
C-O bond is the rate-determining step,1 triggering a complete
change in molecular structure and redistribution of charge. While
spiropyran molecules are three-dimensional, inert, and colorless,
the (open) merocyanine isomers are planar, chemically highly
active, conjugated, and colored. Furthermore, some merocyanine
compounds are usually found in a zwitterionic state and thus
have a large dipole moment. The large change in the molecular
properties has led to great interest in the use of these molecules
to form part of sensors and detectors,2 perform logic operations
in molecule-based devices3,4 and induce reversible changes of
chemical or optical properties of organic-inorganic interfaces.5
To date, most of the experimental studies on these molecules
have been performed in solution, where the open merocyanine
isomer is thermally unstable at room temperature: after photo-
coloration of the parent spiropyran compound, merocyanine
molecules undergo a thermal back-reaction, returning to the
more thermodynamically stable closed form on a time scale of
seconds or minutes at room temperature.6 Several strategies have
ions at the indole moiety inhibits thermal reversion of the MC
isomer to its closed form,7 allowing the back reaction to be
triggered by photodetachment of the metal cations.8 Tuning the
polar character of the solvent or the molecular substituents can
also retard the thermal back-reaction of merocyanine into
spiropyran. One of the most stable merocyanine isomers is
obtained by functionalization of the benzopyran moiety with a
nitro group. In polar solutions, this compound relaxes after
photocoloration into a thermodynamic equilibrium between the
two forms, which can be further decolored by irradiation with
visible light.6
To incorporate the molecular functionality into functional
devices, it is important to understand how such a thermodynamic
equilibrium is affected when the molecules are condensed into
structurally ordered films on top of a surface.9,10 It is known
that the interaction with a metal surface is capable of modifying
theequilibriumstructureofmolecularconformationalswitches.11-13
(6) Go¨rner, H. Phys. Chem. Chem. Phys. 2001, 3, 416.
(7) Wojtyk, J. T. C.; Kazmaierb, P. M.; Buncel, E. Chem. Commun. 1998,
1703.
(8) Byrne, R. J.; Stitzel, S. E.; Diamond, D. J. Mater. Chem. 2006, 16,
1332.
(9) Hagen, S.; Kate, P.; Peters, M. V.; Hecht, S.; Wolf, M.; Tegeder, P.
Appl. Phys. A: Mater. Sci. Process. 2008, 93, 253.
(10) Wolf, M.; Tegeder, P. Surf. Sci. 2000, 603, 1506.
(11) Henningsen, N.; Rurali, R.; Franke, K. J.; Ferna´ndez-Torrente, I.;
Pascual, J. I. Appl. Phys. A: Mater. Sci. Process. 2008, 93, 241.
(12) Henzl, J.; Bredow, T.; Morgenstern, K. Chem. Phys. Lett. 2007, 435,
278.
(1) Cottone, G.; Noto, R.; La Manna, G. Chem. Phys. Lett. 2004, 388,
218.
(2) Byrne, R.; Diamond, D. Nat. Mater. 2006, 5, 421.
(3) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. ReV. 2000, 100, 1742.
(4) Raymo, F. M.; Giordani, S. Proc. Natl. Acad. Sci. U.S.A. 2002, 99,
4941.
(13) Piantek, M.; Miguel, J.; Bernien, M.; Navío, C.; Krüger, A.; Priewisch,
B.; Ru¨ck-Braun, K.; Kuch, W. Appl. Phys. A: Mater. Sci. Process.
2008, 93, 261.
(5) Radu, A.; Scarmagnani, S.; Byrne, R.; Slater, C.; Tong Lau, K.;
Diamond, D. J. Phys. D: Appl. Phys. 2007, 40, 7238.
9
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Piantek et al.
Figure 1. Structure of 1,3,3-trimethylindolino-6-nitrobenzopyrylospiran
(SP) and one of its merocyanine (MC) zwitterionic isomers. While SP is
chiral and possesses two halves with orthogonal planes, the conjugated MC
form is planar and prochiral. At the metal surface, each of the four different
conformations of the MC isomer (given by combinations of the two possible
rotations of both the phenolate and indole groups) is expected to appear
indistinctly under its two chiral forms.
Figure 2. (a) STM image of an adsorbate-covered Au(111) surface annealed
to Tann ≈ 240 K. The height of the molecular islands oscillates around 0.3
( 0.1 nm depending on the applied sample bias. (b) High-resolution STM
image showing the alternating alignment of molecules along rows and their
intramolecular chiral structure. The unit cell indicated as a dashed rectangle
amounts to 1.2 nm × 1.1 nm (It ) 4.0 pA, Vs ) 1.0 V). The STM results
were analyzed using WSxM freeware.34
Even a chemically inert substrate like gold is active enough to
influence the molecular conformation by interacting nonco-
valently with conjugated molecular species.14 Spiropyran mol-
ecules are known to form ordered structures on a Au(111)
surface,15,16 but reports of similarly stable phases of the open
isomer remain absent.
otherwise specified, subsequently cooled to the lowest possible
temperature for its inspection.
STM experiments were performed at a temperature of 5 K using
a custom-made low-temperature UHV STM attached to the sample
preparation stage. XPS and NEXAFS spectra were acquired using
linearly p-polarized light from the beamline UE56/2-PGM2 of
BESSY in Berlin. XPS spectra were measured under normal
emission at 45° incidence. Absorption spectra were acquired in total-
electron-yield mode by recording the sample drain current as a
function of photon energy. HREEL spectra were recorded at 100
K in both specular (θi ) θr ) 60°) and off-specular (θi ) 50.8°, θr
) 60°) scattering geometries. The experimental results are supported
by force-field calculations of the structure of the molecular layer
and quantum-chemical simulations of the NEXAFS spectra. Further
experimental and theoretical details can be found in the Supporting
Information.
Here we show that trimethyl-6-nitrospiropyran (C19H18N2O3)
molecules (Figure 1) undergo a thermal ring-opening reaction
when adsorbed on a Au(111) surface. Through the combination
of several surface-science techniques, we have found that the
ring-opening SP f MC conversion proceeds to completion in
a narrow temperature window just above room temperature and
follows a dissociation pathway similar to that in solution,
namely, cleavage of the C-O bond followed by relaxation into
planar MC conformers. However, on the gold surface, the
reaction is complete and the conjugated open form turns out to
be more stable, reversing the situation usually found in solution.
Various surface spectroscopies have also provided us with clear
insight into the adsorption structure of the different molecular
phases. In particular, they have revealed the planar adsorption
geometry of the MC isomer on the metal surface, which appears
to be responsible for its stabilization. From our results, the use
of metal surfaces and clusters to mediate the reaction and
stabilization of metastable states of molecular switches is
envisaged.
Results and Discussion
We first studied the structure of the different molecular phases
prepared at a gradually increasing temperature using STM. A
molecular layer of SP deposited on a cold Au(111) surface
undergoes an ordering transition at Tann ≈ 220 K, leading to
the formation of extended self-assembled domains. STM images
of the adsorbate-covered surface annealed above this temperature
show that the islands are composed of molecular rows (Figure
2a) along which the molecules orientation alternates (Figure 2b).
In spite of such an anisotropic structure, the domains show an
overall rounded shape, suggesting that intermolecular interac-
tions within the layer are of similar strength both along and
across the molecular rows. The observation of the unperturbed
Au(111) herringbone reconstruction underneath the layers
suggests that the molecules populate a weakly bonded adsorption
state, as has been found for several low-temperature phases of
organic adsorbates on Au(111).17
We cannot a priori relate the observed structures to the closed
or open form, as equilibrium between the two species might
have been reached during thermal sublimation. High-resolution
STM images (Figure 2b) show that all of the molecules exhibit
the same shape and orientation on the surface, demonstrating
that only one type of molecular species is present. The chiral
intramolecular features visible in Figure 2b could equally be
explained by the presence of the chiral SP or prochiral MC
isomers. The observed row structure and the dimensions of its
Experimental Methods
All of the experiments were performed under ultrahigh vacuum
(UHV) conditions using three different setups, each holding one of
the complementary experimental techniques used in this experiment:
scanning tunneling microscopy (STM), X-ray photoelectron spectros-
copy (XPS) and near-edge X-ray absorption fine structure spectroscopy
(NEXAFS), and high-resolution electron energy loss spectroscopy
(HREELS). As required for a temperature-dependent study, each setup
included cryogenic facilities capable of maintaining the substrate at
temperatures below 150 K and the ability to perform controlled
annealing to temperatures well above 300 K.
Atomically clean (111)-oriented Au single crystals prepared using
standard sputtering-annealing methods were used as the substrate.
In all of the experiments described, a submonolayer of 1,3,3-
trimethylindolino-6-nitrobenzopyrylospiran molecules (TCI Europe)
was first deposited on the cold metal substrate (T < 150 K) under
UHV by warming a custom-made Knudsen cell containing the
spiropyran powder to ∼380 K. The adsorbate-covered surface was
then annealed up to the desired temperature Tann and, unless
(14) Henningsen, N.; Franke, K. J.; Torrente, I. F.; Schulze, G.; Priewisch,
B.; Ru¨ck-Braun, K.; Dokic, J.; Klamroth, T.; Saalfrank, P.; Pascual,
J. I. J. Phys. Chem. C 2007, 111, 14843.
(15) Huang, T.; Hu, Z.; Zhao, A.; Wang, H.; Wang, B.; Yang, J.; Huo,
J. G. J. Am. Chem. Soc. 2007, 129, 14843.
(17) Ferna´ndez-Torrente, I.; Franke, K. J.; Henningsen, N.; Schulze, G.;
Alemani, M.; Roth, Ch.; Rurali, R.; Lorente, N.; Pascual, J. I. J. Phys.
Chem. B 2006, 110, 20089.
(16) Huang, T.; Hu, Z.; Wang, B.; Chen, L.; Zhao, A.; Wang, H.; Huo,
J. G. J. Phys. Chem. B 2007, 111, 6973.
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Reversing the Thermal Stability of a Molecular Switch
A R T I C L E S
Figure 3. (a) STM image of an adsorbate-covered Au(111) surface after annealing to temperatures above Tann ≈ 300 K (It ) 20 pA, Vs ) 1.1 V). (b)
High-resolution STM image of the molecular chains showing that the molecular species exhibit an intramolecular pattern composed of a 0.1 nm high planar
oval structure with a 0.25 nm high internal lobe (It ) 50 pA, Vs ) 1.0 V). (c) STM image of the end of a molecular chain. The arrows indicate lateral
manipulation experiments with the STM tip. (d) Consecutive STM image showing the manipulated molecular dimers separated from the chain.
unit cell (1.2 nm × 1.1 nm) agree with previous observations
of this system,15 which were attributed to the closed SP isomer.
The properties of the molecular layer change drastically when
Tann reaches ∼300 K. In this case, the two-dimensional molec-
ular domains disappear, and a new phase composed of molecular
chains is observed (Figure 3a). The chains show a preference
for following the underlying Au(111) herringbone reconstruction
and avoiding lateral packing. The former can be understood as
a consequence of an enhanced interaction with the metal surface.
The latter is characteristic of long-range repulsive interactions
among chains, similar to those found in ensembles of organic
molecules with large dipole moments or charge transfer on metal
surfaces18-20 (we found this avoidance of lateral packing even
for molecular coverages much higher than in Figure 3a, close
to the regime presented in later sections). Hence, a stronger
interaction of the high-temperature phase with the metal surface
can be concluded from the STM observations. It is important
to remark that since the experiments were performed after
posterior cooling to low temperatures, the transformation from
2D domains to chains is not reversible, further confirming that
the chain structure corresponds to the thermodynamically more
stable phase.
Figure 4. XPS spectra of ∼1 ML of spiropyran on Au(111), taken during
A close-up view of such a molecular chain is shown in Figure
a heating sequence from 230 to 330 K (the overall XPS energy resolution
3b. The chain structures can in general be explained as the result
of packing of molecular dimers. Each molecule appears as an
asymmetric oval feature with a higher lobe (0.25 nm high) on
one side. The molecular dimers could be detached from the
chains by lateral manipulation using the STM tip (Figure 3c,d).
In contrast, the dimers themselves are stable units, which could
not be dissociated by neither the STM tip nor the tunneling
current without irreversibly destroying the molecular features.
Qualitatively, this fact can be interpreted as an indication that
the dimer bonding is stronger than the molecule-metal and
dimer-dimer interactions.
was 670 meV). A linear background was subtracted from each spectrum.
The spectra have been shifted vertically for clarity. Smaller components at
400.8 and 402.8 eV in the spectrum at 230 K (dashed lines) may be due to
either a minute amount of the high-temperature phase already present at
that temperature (e.g., at step edges or kinks), shakeup processes during
the emission of the indole N 1s electron, or partial cleavage of some of the
molecules.25
assessment of the molecular isomers or orientations that appear
in each phase. In order to identify them, XPS, NEXAFS, and
HREELS studies were performed as a function of the annealing
temperature.
In view of the well-known isomerizability of spiropyran, we
may associate each of the two observed phases with a structure
formed solely by one of the two different isomers of the parent
spiropyran compound, with a complete transition from one to
the other triggered by temperature and the presence of the metal
substrate. However, the STM images do not provide a definitive
Thermally Activated Ring-Opening Reaction. A clear fin-
gerprint of the existence of two chemically different species
was found by analysis of the binding energy of the nitrogen
core electrons. Figure 4 shows XPS results for the N 1s core
level acquired during an annealing sequence from 230 to 330
K using a photon energy of 500 eV. The spectrum at 230 K
shows two prominent peaks at 399.0 and 405.3 eV whose
positions and line widths closely resemble the N 1s photoemis-
sion spectrum obtained from a powder sample of a benzoox-
azolinic spiropyran.22 The sharper peak at 399.0 eV is due to
emission from the indoline nitrogen atom, while the broader
(18) Baber, A. E.; Jensen, S. C.; Iski, E. V.; Sykes, E. C. H. J. Am. Chem.
Soc. 2007, 129, 6368.
(19) Yokoyama, T.; Takahashi, T.; Shinozaki, K.; Okamoto, M. Phys. ReV.
Lett. 2007, 98, 206102.
(20) Torrente, I. F.; Monturet, S.; Franke, K. J.; Fraxedas, J.; Lorente, N.;
Pascual, J. I. Phys. ReV. Lett. 2007, 99, 176103.
(21) Ågren, H.; Roos, B. O.; Bagus, P. S.; Gelius, U.; Malmquist, P.-Å.;
Svensson, S.; Maripuu, R.; Siegbahn, K. J. Chem. Phys. 1982, 77,
3893.
(22) Delamar, M.; Aubard, J.; Albert, J.-L.; Dubois, J.-E. J. Electron
Spectrosc. Relat. Phenom. 1983, 28, 289.
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A R T I C L E S
Piantek et al.
peak at higher binding energy (Eb ) 405.3 eV) can be assigned
to N core levels in the nitro moiety,23 consistent with the greater
ionic character of this group.21,22 This assignment thus suggests
that the molecules are adsorbed in their spiropyran form at low
temperatures.
When the temperature of the sample is increased above 300
K, the indoline N1s peak undergoes a clear change in energy:
a new component at Eb ) 400.5 eV arises, while the peak at
399.0 eV vanishes almost completely at 330 K. At the same
time, the nitro N 1s peak at 405.3 eV widens and splits into
two peaks with Eb ) 403.5 and 405.8 eV. The resulting spectrum
at 330 K is typical for conjugated merocyanine compounds.22,24
The double-peak structure in the N 1s emission from the nitro
moiety is a characteristic feature in the spectra of the mero-
cyanine molecule. It has been attributed to the appearance of a
prominent shakeup satellite involving charge transfer between
the electron-donor and -acceptor parts of the molecule22,24 and
reflects the very high degree of conjugation between the two
cycles. The shift of the indoline N 1s core electron to higher
binding energy indicates a decrease in the electronegativity of
this atomic site after the ring opening. This is expected for the
merocyanine zwitterion, where an additional π bond is created
at the triply bonded nitrogen atom and balanced with a negative
charge at the carbonyl oxygen (see the structure in Figure 1).
In fact, an opposite shift is observed in the temperature series
of O 1s photoemission spectra acquired during the same heating
sequence (see the Supporting Information). This series of XPS
spectra thus suggests that the phase transition found by STM
corresponds to a temperature-induced SP f MC reaction taking
place between 300 and 330 K.
Figure 5. HREEL spectra of 0.7 ML of SP molecules on Au(111) annealed
to (a) Tann ) 240 K and (b) Tann ) 323 K. The spectra were recorded in
specular (black) and 9.2° off-specular (red) scattering geometries with a
primary electron energy of 3.7 eV. In the specular spectra, the signals contain
both dipole and impact-scattering components,35 while the off-specular
spectra detect only impact-scattering components and therefore serve to
separate dipole-active modes.
Further evidence of a ring-opening reaction is provided by
HREELS measurements in a similar temperature sequence.
Figure 5 shows angle-dependent HREEL spectra after deposition
of SP molecules on a cold Au(111) surface followed by
annealing to 240 and 323 K. The vibrational fingerprints of the
molecular layer after annealing at the two temperatures are very
different. Their detailed assignment (see Table 1 in the Sup-
porting Information for details) supports the occurrence of a
spiropyran-to-merocyanine transition upon annealing. In brief,
the observation of O-C-N stretch vibrations [νs(O-C-N) at
947 cm-1] and butterfly torsion modes involving the pyran and
phenyl rings (τbutt at 256 and 279 cm-1) at low Tann indicates
the presence of the spiropyran isomer. Furthermore, the strong
On the basis of both the XPS and HREELS measurements,
we may conclude that a thermally induced ring-opening reaction
of spiropyran takes place on the Au(111) surface near room
temperature, with the MC isomer being the more thermody-
namically stable species. Thus, the effect caused by the
temperature increase in this case is the opposite of that typically
encountered in solution, where the MC isomer exhibits a
ubiquitous tendency to undergo a thermal ring-closing reaction.
This result thus introduces the conceptually attractive possibility
of using inorganic substrates to change the relative stability of
open and closed isomers simply by tuning the strength of the
interaction with the metal substrate.
The origin of the greater stability of the MC isomer on the
gold surface may be due to either the softening of the C-O
bond, resulting in a reduction of the activation energy for its
dissociation, and/or the larger adsorption energy of the open
isomer. In order to obtain a quantitative insight into the
activation barrier governing the ring-opening reaction, we
performed an analysis of the reaction kinetics using changes in
the vibrational structure of the adsorbate during the SP f MC
transition. In particular, we used the change in the intensity of
the C-H deformation mode at 754 cm-1, which showed the
largest change upon ring opening, as a quantitative measure for
the reaction. Thereby, we determined the activation energy Ea
by determining the relative change in peak intensity (∆I)
between the SP-covered surface (at 240 K) and the substrate
annealed to a particular temperature and relating it to the
Arrhenius-like expression
intensity of the C-N (1273 cm-1) and N-CH3 (1311 cm-1
)
stretch vibrations as well as the absence of the symmetric and
asymmetric stretch modes of the NO2 group (normally located
at ∼1336 and ∼1515 cm-1, respectively) suggest that the
spiropyran molecule is adsorbed with the indoline unit perpen-
dicular and the benzopyran moiety parallel to the surface.
For Tann > 300 K, the butterfly torsion and the O-C-N stretch
modes disappear, indicating the occurrence of the ring-opening
reaction. In this phase, the strong dipole activity of the C-H
deformation and phenyl-ring torsion modes [out-of-plane γ(C-H)
at 754, 844, and 927 cm-1; τ(C-C) at 492 and 844 cm-1] clearly
indicate a parallel orientation of the merocyanine isomer with
respect to the surface (i.e., it lies flat on the surface).33
(23) The larger width of the N 1s peak from the nitro group can be assigned
to a shakeup band, as has been observed for a nitroaromatic powder.22
(24) Chehimi, M. M.; Delamar, M. J. Electron Spectrosc. Relat. Phenom.
1988, 46, 427.
Ea
∆I ) I0 exp -
(1)
(25) Roodenko, K.; Gensch, M.; Rappich, J.; Hinrichs, K.; Esser, N.;
Hunger, R. J. Phys. Chem. B 2007, 111, 7541.
(
)
kBTann
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Reversing the Thermal Stability of a Molecular Switch
A R T I C L E S
Figure 6. HREEL spectra of 0.7 ML of SP molecules deposited on Au(111)
as a function of annealing temperature. The spectra were recorded in the
specular scattering geometry with a primary electron energy of 3.7 eV. The
SP f MC reaction causes a gradual increase in the intensity of the out-
of-plane C-H bending mode at 754 cm-1 (shadowed). Inset: Intensity
change of this mode as a function of the substrate temperature. The activation
energy (Ea) for the ring-opening reaction of spiropyran on Au(111) was
obtained by using the Arrhenius expression to fit the data.
where I0 is an intensity pre-exponential factor and kB is the
Boltzmann constant. It should be noted that the measurements
were done on freshly prepared films using the same annealing
time (10 min) at each particular temperature. From these
measurements, we derived a value of 0.84 ( 0.05 eV for the
activation energy (see the inset of Figure 6). This value is
surprisingly close to the activation energy for the reaction
in solution (∼1 eV, depending on the solvent),6 which can
be attributed to the weakly adsorbed state of spiropyran on
Au(111).
Figure 7. (a) Experimental N K-edge absorption spectra of 1 ML of
spiropyran on Au(111), taken during a heating sequence from 230 to 330
K (simultaneously with the data in Figure 4). Continuous black (dashed
red) lines correspond to spectra acquired at 90° normal (20° grazing) X-ray
incidence. The photon energy resolution was 130 meV. The inset explains
the experimental geometry, including the light incidence angle θ, the sample
normal n, and the polarization vector Ehor. The spectra have been shifted
vertically for clarity. (b) Calculated NEXAFS spectra for the free SP and
MC molecules. For the calculated spectra, the angle of light incidence [90°
(continuous black) and 20° (dashed red)] was defined with respect to the
molecular plane of the nitrobenzene moiety.28
If the presence of a metal surface does not significantly reduce
the barrier for C-O bond cleavage, it may still balance the
energetics toward the merocyanine isomer if an additional mech-
anism contributes to the stabilization of this form on the surface.
The larger interaction of MC molecules with the Au(111) surface
observed by STM could well play such a stabilizing role. A full
understanding of its origin required the resolution of details about
the molecular orientation on the surface.
(DFT) simulations of N K-edge NEXAFS spectra for the
isolated MC and SP isomers (Figure 7b) obtained using the
StoBe code27 (details of the calculations are provided in the
Supporting Information).
For Tann g 300 K a second π* resonance develops at 399
eV, exclusively in the grazing-incidence spectra. These absorp-
tion peaks can only be caused by the formation of an additional
π bond to one of the nitrogen atoms. As already suggested by
the XPS data, the indoline nitrogen site is expected to develop
such a new bond in the merocyanine zwitterion (see Figure 1).
This π bond is linked to the conjugated system of the
nitrobenzene moiety, allowing electronic transitions from the
indole N 1s state into the extended π* molecular orbitals. Thus,
the two different π* resonances seen in the experimental
NEXAFS grazing-incidence spectrum at 330 K come from
transitions from the two unequal N 1s excitation centers into
the LUMO of the MC molecule. The lack of resonances in both
the experimental and simulated spectra at normal incidence
proves that the π orbital system of merocyanine lies perpen-
dicular to the surface and consequently that the whole molecule
lies flat and parallel to the metal surface, in agreement with the
angular dependence of the HREELS data.
Molecular Orientation and Structure of the Self-Assembled
Domains. Detailed insight into the orientations of the two isomers
on the Au(111) surface was obtained by inspecting the angular
dependence of NEXAFS spectra at the N K-edge, which
represents transitions from the N 1s core levels to unoccupied
molecular states. Figure 7a shows angle-resolved NEXAFS
spectra of 1 monolayer (ML) of spiropyran on Au(111)
measured using the same temperature sequence as for the XPS
data shown in the previous section. All of the spectra measured
at grazing incidence show a pronounced π* resonance at ∼403
eV that remains unaffected by the ring-opening reaction above
300 K. This resonance can be assigned to the transition from
the N 1s (NO2) core level into the lowest π* orbital.26 Since
NO2
this resonance is not present in any of the normal-incidence
spectra, we conclude that the nitrobenzopyran moiety lies flat
on the surface at all of the temperatures employed here and
hence for both isomeric forms. This assignment of the spectra
was successfully tested by means of density functional theory
(27) Hermann, K.; Pettersson, L. G. M. StoBe (Stockholm-Berlin) Program
Package; Fritz-Haber-Institut: Berlin, 2005.
(28) The peaks in the range from 405 to 407 eV result from the absence of
the surface in the calculations.29
(29) Kolczewski, C.; Williams, F. J.; Cropley, R. L.; Vaughan, O. P. H.;
Urquhart, A. J.; Tikhov, M. S.; Lambert, R. M.; Hermann, K. J. Chem.
Phys. 2006, 125, 034701.
(26) Turci, C. C.; Urquhart, S. G.; Hitchcock, A. P. Can. J. Chem. 1996,
74, 851.
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in the stabilization of these structures through the formation of
two hydrogen bonds with molecules in the neighboring chain.
Merocyanine molecules possess great conformational flex-
ibility, although on a metal surface the motion is restricted to
the four planar isomers, which maintain the central C-CdC-C
bridge in a trans configuration and thus maximize the interaction
with the metal surface. These are the most stable isomers also
for the free molecule.1,32 The conformational flexibility is
reflected in the observation of several dimer bonding structures
in the merocyanine chains, as they follow the underlying
herringbone reconstruction. Among all the possible combinations
of MC planar isomers, our calculations of a free two-dimensional
layer show a ubiquitous tendency for the molecular dimers to
be stabilized by the nitro end groups. As shown in Figure 8d,
the most stable dimers are formed when these groups are
involved in the formation of H bonds between them. According
to our model, the higher lobe observed in the intramolecular
structure marks the site of the indole nitrogen atom. Hence, its
higher appearance agrees with the partial positive charge
localized at this site in the MC zwitterion. This local charge
could also play an important role in the apparent electrostatic
repulsion between chains observed in the STM images.20
From these complete structural models, we are now able to
draw conclusions about the origin of the large stability of the
MC isomer in the Au(111) surface. The planar conjugated
structure of the MC form is prone to interact with electronic
states at the metal surface, thus hindering the back-reaction.
Furthermore, the XPS and NEXAFS results suggest that the
adsorbed merocyanine molecules exist in their zwitterionic form.
The large electric dipole of this form is expected to interact
with the metal through the creation of image charges, increasing
the strength of the metal-molecule bond.
Figure 8. Three-dimensional minimum-energy configuration of (a) nitro-
spiropyran and (b) nitromerocyanine molecular dimers. An underlying
surface parallel to the z ) 0 plane, in which the motion of some atoms is
confined, is drawn only for illustration purposes. (c, d) Top view of the
corresponding molecular domains, partially superimposed with the STM
images. Expected hydrogen bonds joining one molecule to its neighbors
are shown as dashed lines. In (c), molecular rows labeled 1 and 2 have the
j
same structure, whereas row 3 is rotated by 180°, causing a misalignment
between rows (vertical dashed line).
On the basis of the identification of both the molecules
forming each phase and their orientations at the surface, we
were able to construct complete structural models that further-
more explain the nature of the intermolecular forces that stabilize
the different molecular phases and account for the STM images.
For this purpose, we carried out molecular dynamics simulations
using the mm2 force field, as implemented in the TINKER
package.30 The orientation of the molecules at the surface
deduced from our NEXAFS and HREELS measurements was
fixed in the simulations by confining the motion of various atoms
to a plane (more details are given in the Supporting Information).
We used a two-step method based on an initial stabilization of
the preferred packing structure of two molecules followed by a
subsequent minimization of larger molecular ensembles con-
structed by fitting the relaxed dimers to the high-resolution STM
data. This approach allowed us to account for weak noncovalent
forces among a large ensemble of molecules, and its validity
was tested by checking that the resulting minimum energy
configurations agreed with the experimental data.
Moreover, on the basis of the shape of the molecules on the
surface, it is also possible to argue for the larger stability of the
MC isomer. The opening of the ring simply requires the cleavage
of a C-O bond, and this can occur even in molecules embedded
in an ordered domain. In contrast, the closing reaction would
require the previous removal of the indole group from the
surface and the cleavage of the intermolecular hydrogen bonds
stabilizing a molecular dimer. The addition of these noncovalent
forces to the C-O bond formation signifies a substantial increase
in activation barrier for closing of the adsorbed MC isomer.
Summary
To simulate self-assembled islands of SP isomers, we imposed
a molecular orientation in which the benzopyran part of the
molecule lies parallel to the confining plane, with the dimethyl
moiety of the indoline group oriented toward the metal surface,
as shown in Figure 8a. Figure 8c shows a comparison of the
energy-minimized structure with the STM images. The resulting
most stable configuration comprises rows with SP molecules
of alternating chirality, in agreement with the chiral intramo-
lecular structure observed in the STM images in Figure 2b.15
Interestingly, the indoline groups lie perpendicular to the surface,
exposing their π orbitals to form π-H bonds among them.
Chiral recognition between the rows also exists, where the most
stable configuration is the one that bonds two rows with the
same chiral sequence (rows 1 and 2 in Figure 8c). Occasionally,
Molecular switches deposited on a metallic surface exhibit
different properties than in solution, essentially as a result of
their condensation into ordered molecular phases and the
interaction with the metal surface. Here we have shown that
the thermal stability of nitrospiropyran molecules is strongly
modified when they are adsorbed on a Au(111) surface. The
molecules undergo a complete ring-opening reaction near room
temperature, resulting in molecular phases of zwitterionic
(31) With some functionalized STM tips, such changes in the chiral
sequence of a row led to a change in the intramolecular contrast, as
was observed in ref 15.
(32) Takahashi, H.; Murakawa, H.; Sakaino, Y.; Ohzeki, T.; Abe, J.;
Yamada, O. J. Photochem. Photobiol., A 1988, 45, 233.
(33) Modes observed in the specular geometry and absent in the off-specular
geometry are dipole-active and correspond to atomic motion perpen-
dicular to the metal surface.
j
domain boundaries are observed (rows 2 and 3 in Figure 8c),
which can be explained by a change in the chiral sequence of
the rows.31 The nitro moiety appears to have a dominant role
(34) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Baro,
A. M. ReV. Sci. Instrum. 2007, 78, 013705.
(35) Ibach, H.; Mills, D. Electron Energy Loss Spectroscopy and Surface
Vibrations; Academic Press: New York, 1982.
(30) Ponder, J. TINKER, version 4.2, 2004.
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12734 J. AM. CHEM. SOC. VOL. 131, NO. 35, 2009

Reversing the Thermal Stability of a Molecular Switch
A R T I C L E S
Acknowledgment. The authors acknowledge fruitful discussions
with M. Karcher, Ch. Ru¨dt, and P. Fumagalli. We thank K.
Hermann for help with the calculations of NEXAFS spectra and
B. Zada and W. Mahler for technical support at the beamline
UE56/2 at BESSY. This research was supported by the DFG
through collaborative research center Sfb 658. G.S. acknowledges
his research contract with the DFG through the SPP 1243 Project.
merocyanine with completely different properties. In contrast
to the behavior found in solution, the open isomer turns out to
be more stable. The origin of its larger stability is associated
with both the larger interaction of the adsorbed merocyanine
with the metal and the formation of strongly hydrogen-bonded
molecular dimers. A temperature increase thus drives the
reaction in the direction opposite to that for the same compounds
in solution. On the basis of these results, we foresee a strategy
for designing the stability and the photoswitching properties of
spiropyran-based thin-film devices by choosing an optimum
combination of organic functionalization and inorganic substrate.
The selection of substrate materials with different bonding
strengths and electronic configurations should allow tuning of
not only the potential energy landscape guiding the isomerization
process but also the reversible photoisomerization, as the surface
provides new excitation/decay pathways not present in solution.
Supporting Information Available: Additional details about
the experiments and the theoretical simulations, results from
the O 1s XPS measurements supporting the identification of the
two isomers, DFT simulations of the NEXAFS data, and the
complete vibrational assignment of the HREEL spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JA901238P
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