Solution Phase and Surface Photoisomerization of a Hydrazone Switch with a Long Thermal Half-Life

Article abstract of DOI:10.1021/jacs.9b07057

Photoswitches can be employed for various purposes, with the half-life being a crucial parameter to optimize for the desired application. The switching of a photochromic hydrazone functionalized with a C6 alkyl thiolate spacer (C6 HAT) was characterized on a number of metal surfaces. C6 HAT exhibits a half-life of 789 years in solution. Tip-enhanced Raman spectroscopy (TERS) was used to study the photoisomerization of the C6 HAT self-assembled monolayers (SAMs) on Au, Ag, and Cu surfaces. The unique spectroscopic signature of the E isomer at 1580 and 1730 cm^-1 in TER spectra allowed for its discrimination from the Z isomer. It was found that C6 HAT switches on Au and Cu surfaces when irradiated with 415 nm; however, it cannot isomerize on Ag surfaces, unless higher energy light is used. Based on this finding, and supported by density functional theory calculations, we propose a substrate-mediated photoisomerization mechanism to explain the behavior of C6 HAT on these different metal surfaces. This insight into the hydrazone's switching mechanism on metal surfaces will contribute to the further exploitation of this new family photochromic compounds on metal surfaces. Finally, although we found that the thermal isomerization rate of C6 HAT drastically increases on metal surfaces, the thermal half-life is still 6.9 days on gold, which is longer than that of the majority of azobenzene-based systems.

Full text of DOI:10.1021/jacs.9b07057

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Article
Solution phase and surface photoisomerization
of a hydrazone switch with long thermal half-life
Li-Qing Zheng, Sirun Yang, Jinggang Lan, Luzia Gyr, Guillaume
Goubert, Hai Qian, Ivan Aprahamian, and Renato Zenobi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07057 • Publication Date (Web): 12 Oct 2019
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Solution phase and surface photoisomerization of a hydrazone
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Li-Qing Zhenga, Sirun Yangb, Jinggang Lanc, Luzia Gyra, Guillaume Gouberta, Hai Qianb,d, Ivan
Aprahamianb* and Renato Zenobia*
a. Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 3, CH 8093, Switzerland
b. Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States
c. Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH 8057, Switzerland
d. Department of Chemistry, University of Illinois at Urbana-Champaign, 505 S Mathews Avenue, Urbana, 61801, United
States
ABSTRACT: Photoswitches can be employed for various purposes, with the half-life being a crucial parameter to optimize for the
desired application. The switching of a photochromic hydrazone functionalized with a C6 alkyl thiolate spacer (C6 HAT) was
characterized on a number of metal surfaces. C6 HAT exhibits a half-life of 789 years in solution. Tip-enhanced Raman
spectroscopy (TERS) was used to study the photoisomerization of the C6 HAT self-assembled monolayers (SAM) on Au, Ag and
Cu surfaces. The unique spectroscopic signature of the E isomer at 1580 and 1730 cm-1 in TER spectra allowed for its
discrimination from the Z isomer. It was found that C6 HAT switches on Au and Cu surfaces when irradiated with 415 nm,
however it cannot isomerize on Ag surfaces, unless higher energy light is used. Based on this finding, and supported by density
functional theory calculations, we propose a substrate-mediated photoisomerization mechanism to explain the behavior of C6 HAT
on these different metal surfaces. This insight into the hydrazone’s switching mechanism on metal surfaces will contribute to the
further exploitation of this new family photochromic compounds on metal surfaces. Finally, although we found that the thermal
isomerization rate of C6 HAT drastically increases on metal surfaces, the thermal half-life is still 6.9 days on gold, which is longer
than the majority of azobenzene-based systems.
INTRODUCTION
nanoscale can provide crucial information for future
implementations.
Molecular switches play an important role in various fields,
including biology, chemistry, materials science and
electronics, because[1] their structures and functions can be
precisely controlled at the molecular level, using external
stimuli, such as light, pH, or electric current.[2] These
capabilities have made them very promising candidates for
molecular electronics and high-density data storage
applications.[3] Moreover, surfaces modified with molecular
switches have been patterned and functionalized to act as
regulators in organic field-effect transistors and organic light
emitting diodes.[4] Various types of photo switches have been
investigated over the years for such and other applications,
including azobenzenes[7], diarylethenes[8], spiropyrans[9], and
more recently hydrazones. [10] The thermal isomerization half-
life of these systems is a crucial characteristic that determines
the type of application they can be used for.[5] While these
switches have a wide range of thermal half-lives, ranging from
microseconds to thousands of years in solution,[11,12] these
values do not translate well when they are attached to metal
When exposed to an external stimulus, molecular switches
typically isomerize, for example by undergoing a ring-
closing/opening reaction, [14] or a cis/trans isomerization. [10,15]
A great deal of work has been devoted to understanding the
[17]
switching mechanism of molecules in solution,
and the
influence of solvent, temperature and substituents on the
outcome.[17a] However, when a molecular switch is attached to
a metal surface, the new molecular environment and surface
effects, further complicate the switching mechanism.[18] For
example, switching in the solid state is inhibited to a certain
degree as a result of steric hindrance. The metal surface can
alter the switching mechanism of the adsorbed species. One
reason for this is that excited adsorbates can relax via non-
radiative decay, namely via electron transfer to the conduction
band of the surface and energy transfer to the Fermi level of
19]
the surface,[18b,
leading to a decrease in the switching
efficiency. For example, the observation that azobenzene fails
to isomerize on Au (100) shows a strong surface effect.[20]
Since molecular adsorbates can couple with the high density
of states in the metal conduction band, the question arises
whether there are other effects that interfere with the switching
of molecules on metal surfaces upon light irradiation. For
instance, Van Duyne and coworkers recently employed pump-
probe surface-enhanced Raman spectroscopy (SERS) to
surfaces, and in general
a dramatic enhancement in
isomerization rates is observed .[12a] For example, the thermal
half-life of azobenzene derivatives decreases from 18.7h in
solution to 45.3s when self-assembled on gold surfaces.[13]
Consequently, photoswitches with very long thermal half-lives
(e.g., bistable) are needed to build future devices. Investigating
their molecular switching behavior on metal surfaces at the
demonstrate that photoinduced plasmons at
a
gold
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nanoparticle surface can drive the trans-to-cis isomerization of
C6 HAT-Z shows a maximum (max) at 368 nm (Figure 1b).
Irradiation the solution with 410 nm results in a shift of the
max to 333 nm. The photoisomerization efficiency of C6 HAT
trans-1,2-bis(4-pyridyl) ethylene (BPE).[21]
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
To reveal the isomerization mechanism of molecular
switches on a metal surface down to the single molecular layer
level, analytical tools with high surface sensitivity and a low
detection limit are needed. Tip-enhanced Raman spectroscopy
(TERS) combines plasmon-enhanced Raman spectroscopy
with scanning probe microscopy, and therefore provides
chemical fingerprint information with high sensitivity and
morphological information on the sample with nanometer
spatial resolution.[22] TERS has recently been used to probe the
isomerization of adsorbed azobenzene thiol (ABT), which was
triggered by the bias voltage of a scanning tunneling
microscope (STM) in ultra-high vacuum.[23] Using ambient
TERS, we reported that the photoisomerization yield of ABT
molecules at Au grain boundaries is higher than on Au
terraces, due to reduced cis-to-trans back-reaction rate at the
grain boundaries. The reduced back-reaction rate is a result of
the lower reaction enthalpy of trans-to-cis isomerization at Au
steps.[24] However, the lifetime of cis ABT on Au surfaces was
found to be only 5 h, which is comparable to TERS acquisition
times making the measurements difficult.
1
was studied using H NMR spectroscopy. Upon 410 nm light
irradiation a sample of C6 HAT-Z yields a photostationary
state (PSS) consisting of 98% C6 HAT-E isomer (Figure S6 in
the Supporting Information). In addition to NMR
spectroscopy, the ratio of E and Z isomers was calculated by
fitting two Gaussian peaks to the UV-Vis absorption spectrum.
PSS410 of 96.5 % C6 HAT-E were obtained by this fit, which
is in good agreement with the NMR results (Figure S7 in the
Supporting Information). The quantum yield () of the
process was determined to be 4.8 ± 0.1 % (Figure S8 in the
Supporting Information). The reverse switching can be
induced by irradiation of the E rich sample with 340 nm light.
The absorption maximum (max) red-shifts to 364 nm with a
PSS340 consisting of 74% (NMR spectroscopy) C6 HAT-Z
isomer, and a E → Z of 9.4 ± 0.4 % (Figure S9 in the
Supporting Information). Fitting the absorption spectrum
(dotted line in Figure 1B) yields a PSS340 containing 84 % of
the Z isomer (Figure S10 in the Supporting Information). The
switching process was repeated 10 times by alternation of the
irradiation wavelength between 410 and 340 nm. No signs of
sample degradation were observed (Figure S11 in the
Supporting Information). The half-life (1/2) of the E isomer
was measured to be 789 ± 52 years at 298 K using model
compound 2 (Table S1 in the supporting information).
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To address this issue we decided to investigate the
isomerization mechanism of recently developed bistable
hydrazone photoswitches that exhibit thermal half-lives of up
to 5000 years in solution.[5,12c] Herein, we first describe the
molecular design of a hydrazone thiol (HAT) switch having a
6-carbon spacer (C6 HAT), allowing it to be conjugated to Au
via thiol chemistry. Subsequently, we elaborate on the
photoisomerization of HAT switches in solution, the solid-
state, and assembled as a monolayer on a metal surface, using
ultraviolet visible (UV-Vis) spectroscopy and TERS. We used
characteristic peaks in the Raman spectrum of C6 HAT to
demonstrate its photoisomerization, and quantitatively
measured the extent of its switching using UV-Vis
spectroscopy. Furthermore, to reveal the photoisomerization
mechanism of C6 HAT in a monolayer, we studied the
isomerization of HAT on different metals (i.e., Au, Ag and
Cu). These studies, in corroboration with density functional
theory (DFT) calculations, allowed us to propose a substrate-
mediated isomerization mechanism to explain the different
isomerization rates of C6 HAT on various metal surfaces.
RESULTS AND DISCUSSION
Molecular design, synthesis and solution behavior. We
chose a hydrazone skeleton[5] that has a half-life of 255 years
in toluene in our design of C6 HAT. To minimize any
electronic and steric effects on the hydrazone, the alkanethiol
group was attached to the ester moiety of the switch. A 6
carbon chain linker (C6 HAT, Figure 1A) was selected to
minimize the length of the molecule, but still allow it to form a
well-organized SAM on the surface.[25] C6 HAT was
synthesized using
a
Mitsunobu reaction between an
appropriate hydrazone derivative (Scheme S1) and 6-
bromohexan-1-ol, followed by reaction with potassium
thioacetate and deprotection with hydrazine in MeCN (56 %
yield). C6 HAT was fully characterized using NMR
spectroscopy and high-resolution mass spectrometry (Figures
S2-S5 in the Supporting Information).
Figure 1. A. Light induced Z/E isomerization of C6 HAT; B.
UV-vis spectra of C6 HAT in toluene (1x 10-5 M) before (solid
line) and after irradiation with 410 nm light (dashed line),
followed by irradiation with 340 nm light (dotted line).
UV/Vis spectroscopy was utilized to study the switching
behavior of C6 HAT in toluene. The absorption spectrum of
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Raman
spectroscopic
markers
and
solid-state
the Z isomer. The yellow bars indicate the Raman peak at
1562 and 1704 cm-1 which are characteristic for the E isomer.
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photoisomerization. DFT calculations (B3LYP/6-31G*) were
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performed to predict the differences in the Raman spectrum
between the Z and E isomers of C6 HAT (Figure 2a).
Only the peak shift at 1522 cm-1, predicted by DFT, is not
observed. Overall, the Raman spectrum of C6 HAT, after
irradiation at 415 nm, is in very good agreement with the
calculated spectrum of the E isomer. Based on the intensity
decrease in the bands at 1431, 1472 and 1677 cm-1, almost
100% conversion from Z to E is achieved in the solid-state
after 415 nm light irradiation. Upon further irradiation with
340 nm light, the intensities of the bands at 1431, 1472 and
1677 cm-1 partially recover. Meanwhile, the intensities of the
peaks at 1562 and 1704 cm-1 decrease. Based on the peak area
ratio between 1677 and 1704 cm-1 in the Raman spectrum, we
can calculate the E:Z and E ratio to be 66:34 (see Figure S12),
which is smaller than that observed in solution. Previously, we
reported on the photoisomerization of another hydrazone
switch in solution and in the solid state, monitored by UV-Vis
spectroscopy.[28] The switching efficiencies of C6 HAT
obtained by UV-Vis spectroscopy and Raman spectroscopy
are comparable to our earlier results.[28]
[26]
Theoretical frequencies were scaled by a factor of 0.98 to
correct for anharmonicity.[27] The intensity of the Raman
spectrum of the E isomer is half as strong as that of the Z one,
most likely as a result of the loss of extended conjugation in
the E isomer, which leads to a smaller Raman cross-section.[26]
The Raman bands at 1436 and 1475 cm-1 ((C=N)) of the Z
isomer shift to 1576 cm-1 upon isomerization. Moreover, the
band at 1668 cm-1 ( (C=O)) blue shifts to 1738 cm-1 because of
the disruption of the intramolecular H-bond (see Movie S1 and
S2 in the Supporting Information). Upon Z→E isomerization,
the C-N stretching mode slightly shifts from 1508 to 1522 cm-
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1
and a small peak appears at 1027 cm-1. The bands at 1600
and 989 cm-1 correspond to the C=C stretching mode and the
ring breathing mode of the benzene, respectively. Upon
isomerization, these two peaks do not shift. Therefore, these
two peaks were chosen as references.
Confocal Raman measurements on solid C6 HAT (powder
of pure material, Figure 2B) are in good agreement with the
calculated spectra. After 415 nm light irradiation the intensity
of the peak at 1431 cm-1 decreases, while the signals at 1472
and 1677 cm-1 disappear. Moreover, two new peaks appear at
1562 and 1704 cm-1, and a smaller one at 1027 cm-1. All these
spectral changes indicate that the Z isomer underwent Z→E
isomerization. The peaks at 994 and 1605 cm-1 remain the
same.
Photo isomerization of a C6 HAT monolayer on Au
surface. TER spectra of a C6 HAT SAM on a template
stripped (TS) Au substrate before and after 415 nm light
irradiation were collected, as shown in Figure 3A (top panel).
After 415 nm light irradiation, the intensities of the peaks at
1430, 1472 and 1508 cm-1 decrease, while the intensities of the
peaks at 1013 and 1050 cm-1 increase, and new peaks appear at
1580 and 1730 cm-1. All these spectral changes indicate that
Z→E isomerization occurred. To switch molecules back to the
Z state, we heated the sample to 50 oC for 16 h and collected a
TER spectrum afterwards. The bands at 1430 and 1472 cm-1
recover, while the peak at 1580 cm-1 still remains, indicating
that part of C6 HAT-E molecules isomerized back to the Z
form (see Figure S13). In the TER spectra, no Raman peak
related to carbonaceous contaminants are observed, which
indicates that neither UV irradiation nor laser irradiation
induce any photodamage in C6 HAT. The assumption that
there is no desorption of C6 HAT from the Au substrate upon
UV irradiation was tested and proven by temperature-
programmed desorption mass spectrometry (see Figure S14-
S16).
We further collected TERS maps of C6 HAT SAMs on Au
before and after 415 nm light irradiation (1  1 µm2 size with a
32  32 nm2 pixel size) to estimate the isomerization ratio. All
1064 TERS spectra within a map are displayed as a waterfall
plot (Figure 3B) to highlight the spectral differences. In the
TERS image of a C6 HAT SAM on Au prior to 415nm light
irradiation, the characteristic Raman peaks at 1430, 1472 and
1508 cm-1 of Z-C6 HAT are observed. However, because of
the limited enhancement of the used Ag tip, the Raman band at
1680 cm-1, which corresponds to the C=O stretching mode of
the Z isomer, was invisible in this map (top panel of Figure
3B). We, therefore, recorded another TERS map, in which the
peak at 1680 cm-1 is clearly observed, using another Ag tip
with higher enhancement (Figure S17 in Supporting
Information). After irradiation at 415 nm, Raman peaks at 999,
1013, 1050, 1600 and 1680 cm-1 were clearly observed in the
image, as shown in Figure 3B (bottom panel). Moreover, we
could clearly see that the peaks at 1430 and 1472 cm-1
disappear and new signals arise at 1580 and 1730 cm-1
simultaneously. As shown in the TERS image (Figure S18 in
Figure 2. A. Simulated Raman spectra of the Z and E isomers
of C6 HAT; B. Confocal Raman spectra of C6 HAT in the
solid-state before and after irradiation at 415 nm, followed by
irradiation at 340 nm. The light-blue bars indicate the Raman
peak at 1431, 1472, and 1677 cm-1, which are characteristic for
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Supporting Information), the peak at 1730 cm-1 appears in
79% of
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Figure 3. (A) Baseline corrected TER spectra (top panel) of a C6 HAT monolayer on Au surface before (black) and after (red) 415
nm light irradiation, and average TER spectra (bottom panel) from the TERS maps of a C6 HAT monolayer on Au before (black)
and after (red) 415 nm light irradiation. (B) Waterfall plot of all TER spectra from maps of a C6 HAT monolayer on Au before (top
panel) and after (bottom panel) 415 nm light irradiation. The size of all maps is 1  1 µm2 with a 31 nm pixel size. All TER spectra
were normalized using the Raman peak at 1602 cm-1.
T
Table 1. Photochemical properties of C6 HAT
the spectra, indicating that some of the Z-C6 HAT molecules
have isomerized to the E state. The isomerization ratio of the
C6 HAT SAM is smaller than that of C6 HAT bulk material
(see Table 1), which could result from electron or energy
transfer from the excited states of C6 HAT to the Au surface.
Monolayer on
Au Cu
Solid-
statea
Parameter
Toluene


max, Z-isomer
b
368
333
_
378
343
364
(nm)
max, E-isomer
b
_
330
(nm)
( [Z]:[E] )PSS415
98:2
74:26
99: 1
82:18
62:38
(%)
( [E]:[Z] )PSS340
b
b
66:34
_
_
(%)
789  52
years
6.90  0.03
b
b
c
_
_
1/2
days
aPowder sample. bNot determined. c Half life of the thermal E-
to-Z transformation.
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Figure 4. UV-Vis spectra of a C6 HAT SAM on a Au
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surface before (black line) and after 415 nm light irradiation
(red line). The blue line is the sample after heating under dark
for 16 h.
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Figure 5. (A) Baseline corrected average TER spectra from the TERS maps of a C6 HAT SAM on a Ag surface before and after
415 nm light irradiation. (B) UV-Vis spectra of a C6 HAT SAM on a Ag surface before (black line) and after 415 nm light
irradiation (red and blue lines). (C) Baseline corrected average TER spectra from the TERS maps of a C6 HAT SAM on a Cu
surface before and after 415 nm light irradiation. (D) UV-Vis spectra of a C6 HAT SAM on a Cu surface before (black line) and
after 415 nm light irradiation (red line). The sample was then kept in the dark and heated to 50 ºC for 16 h (blue line). The size of all
the maps is 1  1 µm2.
All TER spectra within a map were accumulated and averaged
to obtain statistical information (bottom panel of Figure 3A).
The average TER spectra of C6 HAT SAM before and after
irradiation at 415 nm show similar spectral features compared
to the TER spectra shown in Figure 3A (top panel). Gaussian
functions were fitted to the peaks at 1680 and 1730 cm-1
corresponding to the Z and E isomers, respectively. The area
ratio of these two peaks was used to determine that 82 % of
the molecules in the SAM are in the E form (Table 1, Figure
S19 in Supporting Information), which is consistent with the
calculation based on the probability of the presence of the
peak at 1580 cm-1 in the TERS map (Figure S18 in Supporting
Information). We also performed TERS line scans along Au
grain edges and terraces with a 10 nm pixel size to measure
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the isomerization ratio at different sites (Figure S20 in
(see Figure S24). Gaussian functions were again fitted to the
Supporting Information). The presence of the peak at 1580 cm-
peaks at 1680 cm-1 and 1730 cm-1 corresponding to Z and E
isomers, respectively. Based on the area ratio of these two
peaks, 62 % of C6 HAT molecules in the SAM have
isomerized (Table 1, see Figure S25 in Supporting
Information). The lower isomerization efficiency is the main
reason why the average spectral change of the TERS spectrum
of C6 HAT on Cu is smaller than that on the Au surface. (see
Table 1). Moreover, the molecular orientation of C6 HAT on
Cu is different from that of C6 HAT on Au. In figure 5C, the
new peaks that appear at 1576 cm-1 and 1732 cm-1 are less
obvious than the peak intensity decrease at 1427 and 1470 cm-
1, which is likely due to the surface selection rules of TERS.
The UV-Vis spectroscopic results also support this
observation. As shown in Figure 5D, the blue shift of the
absorption maximum from 364 to 330 nm was seen after 415
nm light irradiation, which indicates that C6 HAT molecules
on Cu surface underwent Z→E isomerization. The obtained
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both on terraces and at grain edges indicates that
photoisomerization can happen at both sites. Because of their
low Raman cross-section, the peaks at 1680 or 1730 cm-1 were
not observed in line-scan TER spectra.
The photoisomerization of C6 HAT SAMs on Au was
further investigated by UV-Vis spectroscopy. As shown in
Figure 4, the maximum absorption peak of Z-C6 HAT SAM
appears at 378 nm. Upon 415 nm light irradiation it shifts to
343 nm, which indicates that Z→E isomerization has taken
place. The sample was then heated to 50 ºC for 16 h, which
resulted in a shift of the max back to 365 nm and a decrease of
absorbance. This result indicates that most of the E-C6 HAT
has isomerized back to the Z state. The absorbance decrease
could be due to the desorption of C6 HAT molecules from the
surface upon heating. The discrete steps at 370 and 420 nm
are due to the change of grating. A control experiment was
conducted to check whether irradiation by 340 nm can induce
the E-to-Z isomerization of C6 HAT on Au surface. A C6
HAT SAM on Au was first irradiated with 415 nm light and
then followed by irradiation at 340 nm. The absorption peak of
C6 HAT shifts back to 346 nm, which means that irradiation
at 340 nm barely induce E→Z isomerization on the Au surface
(see Figure S21).
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º
sample was then heated to 50 C for 16 h. The absorption
maximum was found to shift to 339 nm, which indicates only
a partially isomerization of E-C6 HAT to the Z form. The
discrete step at 330 nm (blue spectrum) is due to the change
from
a tungsten lamp to a deuterium lamp during
measurement. A control experiment was conducted to check
whether irradiation by 340 nm can induce the E-to-Z
isomerization of C6 HAT on a Cu surface. It is shown that
irradiation by 340 nm light barely induce E→Z isomerization
on the Cu surface, which is similar to the case of C6 HAT on
Au (see Figure S26 in Supporting Information).
The thermal lifetime of the E isomers on the Au surface is an
important characteristic of the surface bound switch. The
absorbance change at 340 nm of a C6 HAT SAM on a Au
surface after 415 nm light irradiation as a function of time is
shown in Figure S22. Analysis of the data shows that the
thermal back isomerization on the surface is dominated by
first-order kinetics with a rate constant of 1.16  10-6 s-1. The
half-life of the E isomer on the Au surface is thus around 6.90
 0.03 days, which is the longest value obtained for such
systems, and higher than a recently reported azobenzene
Substrate-mediated photoisomerization mechanism. The
first possible isomerization mechanism that we evaluated
(Figure 6A) is the direct photo-excitation of C6 HAT on the
metal surfaces. Because of the quenching effects of metal
surfaces, the isomerization of C6 HAT through direct
molecular photoexcitation is suppressed, although to different
degrees, by the metal surfaces. The failure of the isomerization
of C6 HAT on Ag surface and the decrease in the
isomerization efficiency of C6 HAT on the Au surface
compared to the solid-state are indications of this
phenomenon. A drop in isomerization efficiency could also
result from steric hindrance. To exclude this effect, we
investigated the isomerization of C6 HAT on Ag diluted in a
10-fold excess of hexanethiol using UV-Vis spectroscopy. As
shown in Figure S27, the maximum absorbance of C6 HAT
did not shift upon 415 nm light irradiation, indicating that C6
HAT molecules with a lower packing density on Ag cannot
derivative on Au surface.[13] As
a thermodynamically
unfavorable state, the long lifetime of the E form is attributed
to the high energy barrier for the E→Z isomerization.[5] With
such a relatively long thermal lifetime on the metal surface, it
is a promising system towards the development of optical data
storage devices.
Photo isomerization of a C6 HAT monolayer on Ag or
Cu surface. To study the effect of the nature of the metal on
the photoisomerization of a C6 HAT SAM, we also studied its
isomerization on Ag or Cu surface. A waterfall plot of the
spectra extracted from TERS images of C6 HAT SAM on Ag
before and after 415 nm light irradiation is shown in Figure
S23. The average TER spectra are shown in Figure 5A, in
which the Raman bands at 1430, 1473, 1506 and 1680 cm-1,
characteristic peaks for Z-C6 HAT, are still observed, and no
peaks appear at 1580 or 1730 cm-1. This suggests that no Z→E
isomerization of C6 HAT occurs on the Ag surface.
Moreover, there is no absorption peak shift observed in the
UV-Vis spectrum of C6 HAT SAM on Ag after 415 nm light
irradiation (Figure5B), which is in agreement with the TERS
results. Next we studied the isomerization of a C6 HAT SAM
on a Cu surface. As shown in Figure 5C, after 415 nm light
irradiation, we can clearly see that the intensities of the peaks
at 1430, 1473 and 1680 cm-1 decrease, with small peaks
appearing at 1580 and 1730 cm-1 in the average TER spectrum
of C6 HAT on Cu. The spectral change is seen more clearly in
the waterfall plot of the TERS spectra extracted from images
isomerize to the
E form. Therefore, failure of the
photoisomerization of C6 HAT on Ag must stem from
electronic rather than steric effects. The fact that C6 HAT
SAM can isomerize on Au and Cu with 415 nm light
irradiation also supports this interpretation. The second
possible
photoisomerization mechanism
mechanism
is
a
substrate-mediated
[29]
in which the electronic
structure of the molecules, and the density of states (DOS) of
the substrate should be taken into account. Tegeder and co-
workers
proposed
such
a
mechanism
for
the
photoisomerization of TBA physically absorbed on a Au
surface.[29] UV light first generates hot holes in the Au d-band,
which subsequently relax to the top of the d-band through
Auger decay. These hot holes then undergo a charge transfer
to the HOMO of TBA, leading to the formation of transient
positive ions, which may subsequently result in the
isomerization of TBA. Based on this theory, we calculated the
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orbital energy levels of C6 HAT bound to the different metal
surfaces, and the DOS of the different metals using density
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functional theory (DFT). (see Figure S28 and Figure S29) As
shown in Figure 6B, the top of the d band of Au is at 2.2 eV,
which overlaps with the second HOMO (HOMO-2) of C6
HAT. We speculate that 415 nm light (E = 3 eV) irradiation
generates hot holes in the Au d-band, which first relax to the
top of the d-band, and can then transfer to C6 HAT to form
transient positive ions, leading to the isomerization of C6
HAT. In the case of Ag, because of
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Figure 7. (A) Baseline corrected average TER spectra from
the TERS maps of a C6 HAT SAM on a Au surface before
and after 340 nm light irradiation. (B) Baseline corrected
average TER spectra from the TERS maps of a C6 HAT SAM
on a Ag surface before and after 280 nm light irradiation. The
size of all maps is 1  1 µm2 with a 31 nm pixel size.
Figure 6. (A) Possible excitation and quenching mechanisms
in the photoisomerization of C6 HAT SAM at different metal
surfaces. (B-D) Proposed excitation mechanism for the
photoisomerization of C6 HAT SAM at different metal
surfaces.
To further verify the substrate-mediated photoisomerization
mechanism, we irradiated the C6 HAT SAM on Au with 340
nm light (E = 3.65 eV), and the C6 HAT SAM on Ag with
280 nm light (E = 4.4 eV), respectively, to induce the Z→E
photoisomerization. As shown in Figure 7A, after 340 nm
light irradiation, we can clearly see the intensity drop of the
peaks at 1430, 1472, 1506 and 1680 cm-1, and new peaks
appearing at 1580 and 1730 cm-1 in the average TER spectrum
of C6 HAT on Au. These spectral changes indicate that Z→E
isomerization took place, which is in agreement with the
substrate-mediated mechanism. In contrast, 340 nm light
irradiation induces E→Z isomerization in solution, showing
that there is a large difference in the solution phase and
monolayer switching behavior. According to the substrate-
mediated mechanism, light irradiation with energy above 3.9
eV should induce Z→E isomerization of C6 HAT on Ag
surface. The average TER spectra of C6 HAT SAM on Ag
before and after 280 nm light irradiation are shown in Figure
7B. The intensity of peaks at 1430, 1473 and 1506 cm-1 drops,
and new peaks come out at 1577 and 1730 cm-1, which is quite
different from the TER spectrum when irradiating with 415
nm light (Figure 5A). This suggests that C6 HAT molecules
isomerized to the E form with 280 nm light irradiation. In
contrast, solid-phase C6 HAT cannot isomerize from the Z to
the E form upon irradiation by either 340 or 280 nm light
(Figure S30). Moreover, irradiating a C6 HAT solution with
280 nm light cannot induce Z→E isomerization (Figure S31).
Only a very tiny and negligible proportion of Z-C6 HAT
isomerizes to the E form in solution upon irradiation with 340
the lower lying d-band (d-band edge of Ag is 3.9 eV), light-
induced switching cannot be achieved with 415 nm light
irradiation (Figure 6C). In contrast, the d-band edge of Cu is at
2.1 eV, so 415 nm light irradiation can induce the
isomerization of C6 HAT SAM on Cu. (Figure 6D) Regarding
the hot carrier transfer process from substrate to the C=N
switchable group of C6 HAT, our hypothesis is that it occurs
via direct electron tunneling. Our system is a typical donor-
bridge-acceptor system[30], in which the alkyl chain serves as
the bridge between the metal substrate and the switchable
group of C6 HAT. It is well known that in such a system,
coherent electron tunneling from donor to acceptor can take
place via virtual states in the bridge, referred to as
superexchange mechanism.[30,31] Based on Marcus theory, the
electron transfer rate decays exponentially with donor-
[30a, 30c, 32]
acceptor distance.
In our case, the distance between
donor and accepter is only ca. 10 Å. For comparison, Murray
et al. reported that the conductivity of Au clusters protected by
an alkanethiol monolayer with a 6-carbon chain was ca. 1.8 x
10-4 Ω-1cm-1 and 6.6 x 10-4 Ω-1cm-1 depending on the size of the
Au clusters.[30a] Therefore, direct electron tunneling from the
Au surface to the C=N switch group of C6 HAT is clearly
feasible.
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nm light because of the small absorption of Z-C6 HAT at this
wavelength (see Figure S32) These results support our
proposed substrate-mediated photo-isomerization mechanism.
Corresponding Author
1
2
* Email: ivan.aprahamian@dartmouth.edu
* Email: renato.zenobi@org.chem.ethz.ch
3
4
5
6
7
8
9
CONCLUSIONS
Author Contributions
A new bistable (1/2 = 789 years) hydrazone photoswitch
having a C6 alkyl thiolate linker (C6 HAT) was synthesized,
and its switching behavior studied in solution, the solid-state
and as a monolayer on various metal surfaces using confocal
Raman TERS and UV-Vis spectroscopies. Interestingly, C6
HAT can isomerize on Au and Cu surfaces, but not on Ag
upon irradiation by 415 nm light (E = 3 eV). Higher energy
light is required to activate the switch on Ag. Based on these
findings, we propose a substrate-mediated charge transfer
mechanism for the isomerization of C6 HAT on metal
surfaces. This mechanism is supported by DFT calculations
and TERS results. In this process, the length of the carbon
linker plays an important role. When increasing the carbon
The manuscript was written through contributions of all authors.
Notes
The authors declare no competing financial interest.
The original data used in this publication are made available in a
curated data archive at ETH Zurich (https://www.research-
collection.ethz.ch) under the DOI: 10.3929/ethz-b-000353526.
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ACKNOWLEDGMENT
We thank Hao Yin (ETH Zurich, Xiamen University) for
insightful discussions, Tian Liu (ETH Zurich), Dr. Alessandro
Lauria (ETH Zurich), Madeleine Fellner (ETH Zurich) and Prof.
Markus Niederberger (ETH Zurich) for UV-Vis spectroscopy
measurements, Jacek Szczerbińsky (ETH Zurich) for TPD-MS
measurements. L.-Q. Z. is indebted to the Chinese Scholarship
Council for a Ph.D. student fellowship. We thank the High
Performance Computing Team at ETH Zurich for help with DFT
chain
length,
the
proposed
substrate-mediated
photoisomerization mechanism would stop working at a
certain distance, since the electron transfer rate decays
exponentially with donor-acceptor distance. However,
photoswitches are still able to isomerize at larger distances via
direct photoexcitation, because at larger distances there is less
quenching by the surface. Thus, the length of the carbon is a
critical parameter to maximize the isomerization efficiency on
metal surfaces. As for the E→Z isomerization, it can be
thermally activated, with a half-life of 6.9 days (at room
temperature) on the Au surface. This is the longest thermal
half-life measured so for photoswitches on metal surfaces. The
lifetime of the E molecules may depend on the nature of the
metal, and on the linker length,[13] which suggests a thermal
relaxation mechanism. The details of the relaxation
mechanism are a topic that will require future studies to be
clarified. This property, in addition to the ease of synthesis and
assembly of the switches on metal surfaces, and very good
separation of the Raman bands of the Z and E isomers, make
this system highly valuable in the development of photo-
controllable surfaces. One immediate application might be
surface patterning using the photoswitches and using TERS
for the in-situ imaging of the UV light initiated surface
isomerization.
calculations and the ERC program (grant
# 741431 -
2DNanoSpec) for financial support. I. A. is thankful to the
National Science Foundation (CHE-1807428) for the generous
support.
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ASSOCIATED CONTENT
Supporting Information
Experimental details, chemical synthesis, photoisomerization
studies in toluene solution, desorption analysis by MS, TERS
study of the isomerization of a C6 HAT SAM, thin-film UV-Vis
spectroscopic study, theoretical calculations, confocal Raman
spectrum analysis and the vibrational modes of the Z and E
isomers.
The Supporting Information is available free of charge on the
ACS Publications website.
All of the supported figures (PDF)
Vibrational modes of the Z and E isomers (movies)
AUTHOR INFORMATION
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