Realization of highly photoresponsive azobenzene-functionalized monolayers

Article abstract of DOI:10.1039/c0jm03697h

We report the successful fabrication of azobenzene-functionalized self-assembled monolayers (SAMs) exhibiting high and reversible photoswitching between trans and cis states on a flat gold surface. Azobenzene thiols (MeSH and EtSH) containing meta and/or

Full text of DOI:10.1039/c0jm03697h

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PAPER
Realization of highly photoresponsive azobenzene-functionalized monolayers†
a
Mina Han, Takumu Honda,b Daisuke Ishikawa,b Eisuke Ito,c Masahiko Harabc and Yasuo Norikaned
*
Received 29th October 2010, Accepted 23rd December 2010
DOI: 10.1039/c0jm03697h
We report the successful fabrication of azobenzene-functionalized self-assembled monolayers (SAMs)
exhibiting high and reversible photoswitching between trans and cis states on a flat gold surface.
Azobenzene thiols (MeSH and EtSH) containing meta and/or ortho substituents were chosen based on
the occupied area per molecule as well as intermolecular interactions between the azobenzene aromatic
rings (formation of H-aggregates). Theoretical predictions of the geometrical structures were
performed to clarify the correlation between the molecular structure and photoisomerization
characteristics in monolayer systems. The striking difference in absorption spectra of a trans-EtSH
SAM and a cis-EtSH SAM by alternating UV and visible light irradiation was in good agreement with
that in their contact angles for water, strongly indicating that the structural changes were controlled by
light wavelength. By contrast, despite there being sufficient free space for each MeSH molecule, the
strong tendency of the planar azobenzene units to generate H-aggregates even during cis-MeSH SAM
formation lessened the trans-to-cis photoisomerization yield in a monolayer.
packing density of the molecules in the solid state, etc.).6,7
Introduction
Understanding the correlations between the molecular structure
Manipulating the surface properties of organic thin films in
and trans/cis photoisomerization characteristics of azobenzene
response to light wavelengths has attracted a great deal of
may open up applications in a wide range of fields, such as
interest for fundamental research as well as for practical appli-
molecular motors,8 photoswitching,9,10 optical data storage,11,12
cations.1 A self-assembled monolayer (SAM) system utilizing the
and optoelectronic devices.13
adsorption of sulfur head groups onto metal surfaces is a versa-
Efforts to extend reversible photoisomerization of azobenzene
tile method for fabricating a supported organic film, since the
to a flat gold surface have commonly encountered a problematic
chemical properties of the topmost surface layer can be adjusted
lack of photoresponsiveness due to the too-dense molecular
by simply modifying the functional tail group of organic thiols
packing caused by van der Waal interactions between the long
and disulfides.2,3
alkyl chains and p-stacking interactions between the azobenzene
Photochromic azobenzene molecules undergo trans-to-cis
aromatic rings.14 To promote the trans-to-cis photoconversion
isomerization upon irradiation with UV light, which can be
yield in monolayers, several investigations have been performed
reversed by either heating or irradiation with visible light.4,5 The
so far, mainly from the viewpoint of molecular free space.15 Even
rod-shaped planar trans form is thermodynamically more stable
though photoresponses of the azobenzene units could be moni-
by about 12 kcal molꢀ1 than the bent-shaped nonplanar cis form.
tored in real time by surface plasmon resonance and ellipsometry
measurements,15b,16 there still remain significant challenges
The lifetime of the cis form is mainly influenced by the molecular
structure as well as the surrounding environment (including such
associated with improving the trans-to-cis photoisomerization
factors as solvent properties, temperature, polymer matrix,
yield and controlling the lifetime of the two states without phase
separation.17
aDepartment of Chemistry and Department of Electronic Chemistry, Tokyo
In this investigation we have employed EtSH and MeSH
Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama,
azobenzene thiols (Scheme 1) to consider the following three
points. First, the respective single-component azobenzene thiols
are expected to generate a rather homogeneously distributed
monolayer and not a phase-separated system. Second, sufficient
free space (more than 0.45 nm2 of a critical free space18) required
for photoisomerization can be secured through the introduction
of sterically bulky alkyl substituents into the azobenzene unit.
Third and most importantly, steric hindrance arising from the
bulky alkyl groups at the ortho position with respect to the azo
group19,20 leads to significant distortion from the typical planar
226-8502, Japan. E-mail: han.m.ab@m.titech.ac.jp; Fax: +81 45 924
5447; Tel: +81 45 924 5447
bDepartment of Electronic Chemistry, Tokyo Institute of Technology, 4259
Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan
cRIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama, 351-
0198, Japan
dPhotonics Research Institute, National Institute of Advanced Industrial
Science and Technology (AIST), Central 5, Higashi 1-1-1, Tsukuba,
Ibaraki, 305-8565, Japan
† Electronic supplementary information (ESI) available: thermal
cis-to-trans isomerization of EtSH and MeSH in dichloromethane and
SAMs. See DOI: 10.1039/c0jm03697h
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solubility in organic solvents such as dichloromethane, acetone
and DMF to give transparent solutions before and after UV light
irradiation. By comparison, H-C10 lacking the meta and ortho
substituents (Scheme 1) showed very poor solubility in common
organic solvents. Accordingly, the corresponding alkanethiol
could not be isolated pure. NMR data of EtSH, MeSH and
H-C10 taken before and after 365 nm light irradiation indicated
that more than 95% of the cis form was produced in each pho-
tostationary state, irrespective of the type and position of
substituent introduced. The half-life times of the cis form of
EtSH were found to be 380 h, which was about 30–50 times
longer than those of comparable MeSH (13 h) and H-C10 (7 h)
(see ESI†). Slow thermal cis-to-trans isomerization of ortho-
diethylated EtSH can be explained by steric hindrance origi-
nating from bulky ethyl substituents at the ortho position,19d
which restrict large-scale distortion of the azo group during
isomerization.
The following two important observations can be made on the
basis of the spectroscopic data obtained for MeSH and EtSH
(Fig. 1 and Table 1). First, in contrast to H-C10 and MeSH
possessing strong p–p* absorption bands with maxima at 373
(3 ¼ 41 000 L molꢀ1 cmꢀ1) and 368 nm, respectively, ortho-
alkylated azobenzene (EtSH) underwent a substantial blue-shift
in the p–p* absorption band to 354 nm, together with a notice-
able reduction of about 30–40% in its molar extinction coefficient
(3). Second, whereas the trans forms of H-C10 and MeSH dis-
played weak featureless n–p* bands at around 450 nm,4a,21 the
shape and intensity of the n–p* band in the trans state of EtSH
were quite comparable to that of the corresponding cis form
(inset of Fig. 1).
These observed spectroscopic features can be interpreted in
terms of the steric and electronic effects originating from the
introduction of sterically bulky ethyl groups at the ortho posi-
tions. Our density functional theory (DFT, B3LYP/6-31G(d,p))
calculation22 clearly indicates that meta-methylated azobenzene
(MeSH) adopted a planar structure in the azobenzene unit23 with
the dihedral angles C–C–N]N and C–N]N–C of 179.61ꢁ and
179.99ꢁ, respectively (Scheme 1A). However, ortho-diethylated
EtSH was distorted from the coplanarity of the two benzene
Scheme 1 (A) Molecular structures and optimized structures (long alkyl
chains are omitted for clarity) of EtSH, MeSH, and H-C10. (B)
Schematic representation of self-assembled systems prepared from
respective trans-azobenzene (trans SAM†) and cis-rich azobenzene thiol
(cis SAM‡) solutions and their reversible photoswitching. Unlike ortho-
alkylated EtSH SAMs exhibiting high and reversible photoisomerization,
and despite there being the increased free space between individual MeSH
molecules, a strong tendency to aggregate suppressed reversible photo-
switching between the trans-rich and cis-rich states.
trans-azobenzene structure, which would influence p-stacking
interactions. Our systematic investigation emphasizes that both
free space and suitable regulation of the intermolecular inter-
action between the planar trans azobenzene units are essential
prerequisites for creating an azobenzene-functionalized mono-
layer exhibiting highly efficient photoswitching between the two
states.
Results and discussion
Fig. 1 UV–vis absorption spectra of EtSH, MeSH and H-C10 in
dichloromethane. Inset: absorption spectral changes of EtSH upon UV
light irradiation.
Both EtSH and MeSH containing alkyl substituents at the meta
and/or ortho positions with respect to the azo group showed good
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Table 1 Calculated dipole moments, spectroscopic data, contact angles, and photoconversion yields of EtSH and MeSH
EtSH
MeSH
Compound
trans form
cis form
trans form
cis form
dipole moment (m)a
0.37
5.42
—
cis SAM
—
352 ꢂ 1
0.0041 ꢂ 0.0005
89 / 89
90 ꢂ 5
1.00
5.98
—
cis SAM
—
347 ꢂ 1
0.0052 ꢂ 0.0005
93 / 90
65 ꢂ 5
b
)
lmax/nm (3/L molꢀ1 cmꢀ1
354 (26 000)
trans SAM
0.55
368 (37 000)
trans SAM
0.50
area-per-moleculec/nm2
lmax/nm
350
343
DA ¼ Atrans rich ꢀ AUV (at lmax
)
0.0040 ꢂ 0.0005
94 / 90
88 ꢂ 5
0.0044 ꢂ 0.0005
93 / 90
51 ꢂ 3
contact angle/ꢁ (as prepared / UV)
photoconversion yield/%
a
Theoretical (B3LYP/6-31G(d,p) calculations were performed by the GAUSSIAN 03W program package. b In dichloromethane. c Estimated from the
X-ray crystal structure,19c CPK model, and absorption spectral data.9,29
rings (the C–C–N]N dihedral angle was 159.14ꢁ) as a result of
the presence of the bulky ethyl substituents at the ortho posi-
tions,19c which caused the blue-shift in the p–p* band and the
remarkable decrease in 3.19,24 In addition, the relatively charac-
teristic n–p* band of trans-EtSH can also be closely associated
with the distortion of the azobenzene unit. Similar spectroscopic
data were observed for macrocyclic compounds containing dis-
torted azobenzene units as well.25 That is, the forbidden n–p*
transition for the conventional planar trans form becomes
partially allowed for the distorted trans form.4a
occurring at a binding energy of 399.8 eV in the N1s spectra of
both cis-SAMs is ascribed to the azo group in the azobenzene
unit. Characteristic doublet structures at 163.2 and 162.0 eV
(with peak area ratio with 1 : 2) arising from the presence of the
S2p1/2 and S2p3/2 peaks, respectively, in the S2p spectrum are
attributable to thiolate-type sulfur bound to the metal surfaces.
These emission peaks are consistent with those acquired for
alkanethiol SAMs,27,28 responsible for the chemisorption of the
azobenzene thiols to the gold surface without chemical
contamination.
To explore the effect of the molecular structure on both the
spectroscopic features and reversible photoisomerization by
UV–vis absorption spectroscopy in monolayer systems, we
prepared the following two types of azobenzene monolayer
systems. Polycrystalline gold films with a thickness of 20 nm were
immersed into 0.1 mM trans-azobenzene (for preparing trans-
azobenzene SAMs) and cis-rich azobenzene (for preparing cis-
azobenzene SAMs) solutions, respectively, in the dark. In
particular, to keep the azobenzene moiety in the cis form, UV
light was irradiated for 5 min every two hours. UV light irradi-
ation and dark incubation of the azobenzene monolayers was
performed under a nitrogen atmosphere to prevent undesirable
photoreaction and contamination.
While an intense p–p* transition at 350 nm and a second
p–p* transition at 250 nm were observed for trans-EtSH
monolayers, as-prepared cis-EtSH monolayers showed a rela-
tively weak absorption band at 352 ꢂ 1 nm along with an
apparent band at 274 nm (Fig. 3a). Irradiation with UV light
brought about a prominent reduction in the p–p* band as well as
emergence of an absorption band at 274 nm, which is compa-
rable to the spectral changes in solution (inset of Fig. 1). A
comparison of the absorption spectrum of the as-prepared cis
SAMs with that of the cis-rich state obtained after UV light
irradiation demonstrates that a considerable amount of cis-
azobenzene was present in the as-prepared cis-EtSH SAMs.
Under our assumption that the molar extinction coefficients of
the trans and cis forms do not significantly change before and
after the preparation of SAMs,9b,9d,29 the estimated content of cis-
azobenzene was ca. 60 ꢂ 5% in the as-prepared cis SAMs. The
ratio of the cis to trans forms was then increased to roughly 90/10
in the photostationary state of UV light, quite similar to the
trans-to-cis photoconversion yield (88 ꢂ 5%) in trans-EtSH
SAMs (Table 1).
The N1s and S2p X-ray photoelectron spectra of MeSH and
EtSH SAMs on gold surfaces are presented in Fig. 2, together
with the corresponding fits. The respective spectra of cis SAMs
were almost the same as those of trans SAMs.9d,26 A single peak
By contrast, there was spectroscopic similarity between the as-
prepared trans-MeSH and cis-MeSH SAMs on the gold surface,
as clearly seen in Fig. 3c. Strong p–p* absorption bands with
maxima at 343 and 347 ꢂ 1 nm observed for trans-MeSH and cis-
MeSH SAMs, respectively, were considerably blue-shifted by
25–20 nm with respect to lmax ¼ 368 nm in solution. These
spectral changes can be explained in terms of the formation of H-
aggregates. Moreover, unlike EtSH, the cis SAMs prepared from
cis-rich MeSH solutions were comprised not of cis-azobenzene,
but rather trans-azobenzene as the major component. This
conclusion was based on the following observations: (1) the
similarity of the absorbance at ꢃ250 nm between the as-prepared
cis SAMs and the trans-rich state obtained after sufficient
Fig. 2 N1s and S2p X-ray photoelectron spectra of as-prepared cis-
EtSH and cis-MeSH SAMs, together with the corresponding fits (solid
line).
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Fig. 3 UV–vis absorption spectral changes of (a) EtSH and (c) MeSH SAMs on gold surfaces. (i) As-prepared cis-EtSH and cis-MeSH SAMs. (ii) After
irradiation with UV light. (iii) After sufficient dark incubation after UV light irradiation. Difference spectra obtained after alternating UV and visible
light irradiation of (b) cis-EtSH and (d) cis-MeSH in SAMs and in dichloromethane.
thermal cis-to-trans isomerization for 4 h, and (2) the ꢃ10%
reduction in absorbance at 250 nm and slightly red-shifted lmax
in comparison with that of trans-MeSH SAMs. The slight vari-
ation in the absorption spectra is likely due to the somewhat
loose packing of the azobenzene units in the cis SAM,16a as
illustrated in Scheme 1B. It is envisaged that the reduction in
packing density provided increased free space required for the
structural change between the trans and cis forms. Therefore, the
trans-to-cis photoconversion yield could be somewhat improved
in the cis-MeSH SAM, when compared with the more densely
packed trans-MeSH SAM.
On the other hand, considering that 0.45 nm2 corresponds to
the critical two-dimensional free space required for sufficient
trans-to-cis photoconversion,18 one can expect that both trans-
MeSH and cis-MeSH SAMs securing ca. 0.50 nm2 and 0.55 nm2
Fig. 3d shows the difference spectra taken after alternating UV
and visible light irradiation of cis-MeSH SAMs. Such obvious
spectral changes are attributed to reversible trans/cis photo-
isomerization of the azobenzene unit on a gold surface. More-
over, dark incubation of the UV-exposed MeSH SAMs for 1–4 h
fully reversed the absorption spectrum as a result of cis-to-trans
isomerization (Fig. 4). Simultaneously the p–p* absorption
bands was slightly red-shifted to ꢃ351 nm. The difference in
absorbance at lmax (DA ¼ Atrans
ꢀ AUV) obtained after
rich
sufficient thermal cis-to-trans isomerization and UV light irra-
diation was found to be ca. 0.0052 ꢂ 0.0005 for cis-MeSH SAMs,
higher than ca. 0.0044 ꢂ 0.0005 for trans-MeSH SAMs (inset of
Fig. 4 and Table 1). The results support our earlier expectation
that the low-density cis-MeSH SAM consists of trans-azo-
benzene with increased free volume and shows higher trans-to-cis
photoconversion.
Fig. 4 UV–vis absorption spectral changes of cis-MeSH SAMs. Inset:
changes in DA (¼ At ꢀ AUV) of cis-MeSH and trans-MeSH SAMs as
a function of thermal cis-to-trans isomerization time (t) after UV light
irradiation.
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(estimated from the X-ray crystal structure,19c CPK model, and
absorption spectral data9,29) as the area-per-molecule value,
respectively, would show a high yield of trans-to-cis photo-
conversion. However, the estimated trans-to-cis photoconversion
yields for trans-MeSH and cis-MeSH SAMs remained at ca. 51%
and 65% levels, respectively. These results imply that in addition
to free space, another factor plays a critical role in affecting trans-
to-cis photoisomerization of azobenzene molecules in mono-
layers. That is, despite there being sufficient free volume, strong
intermolecular interactions between the planar trans-MeSH
azobenzene units facilitated the formation of H-aggregates
during the SAM preparation, as evidenced by a significant blue-
shift of the p–p* band in the absorption spectrum. Conse-
quently, favorable p-stacking of the aromatic units restrained
trans-to-cis photoisomerization.
successfully maintained without serious deterioration even after
more than 1 week of dark incubation.
We next assessed switching of surface wettability controlled by
light wavelengths at which trans/cis isomerization occurs. The
contact angles (CAs) of as-prepared cis-EtSH and trans-EtSH
SAMs for water were 89 ꢂ 0.6ꢁ and 94 ꢂ 1ꢁ respectively (Fig. 5).
The lower contact angle of cis SAMs is closely related to the
presence of a considerable amount of the cis form, as deduced
from the characteristic absorption spectral data (Fig. 3a).
Further UV light irradiation of the cis-EtSH SAMs under
a nitrogen atmosphere did not decrease the CA significantly.
However, upon visible light irradiation to induce cis-to-trans
isomerization the CA was increased to 92 ꢂ 0.7ꢁ. The CA change
on cis-EtSH SAMs was in agreement with that on trans-EtSH
SAMs. Explicit photoswitching of the surface wettability for
both SAMs is mainly due to the change in the dipole moment
between cis-EtSH (mcis ¼ 5.42) and trans-EtSH (mtrans ¼ 0.37),
accompanied by the reversible structural change controlled by
light wavelength. Nevertheless, the CA change on azobenzene
SAMs was not so large, only about 3ꢁ difference by alternating
UV and visible light irradiation. The small changes in the CA
may be ascribed to a homogeneously flat surface layer consisting
of single-component azobenzene molecules and a relatively
smooth surface.30
In sharp contrast to MeSH SAMs, for cis-EtSH SAMs, DA (¼
0.0041 ꢂ 0.0005) was almost the same as that for trans-EtSH
SAMs (DA ¼ 0.0040 ꢂ 0.0005), as shown in Table 1 and Fig. 5.
The trans-to-cis photoconversion yield in cis SAMs was esti-
mated to be ca. 90%, almost identical to that in trans SAMs.
Even though a considerable amount of the cis form existed in the
as-prepared cis-EtSH SAMs, the average packing density of
molecules was not significantly altered in comparison with that
of trans-EtSH SAMs. Steric hindrance originating from the ethyl
groups at the ortho positions lessens inordinate intermolecular
interactions between azobenzene molecules, and consequently
allows for high photoresponse in monolayers. Furthermore, the
reversible structural change of the azobenzene unit was
Conclusions
Both the obvious spectroscopic data and contact angle for water
provide convincing evidence for the experimental realization of
high and reversible trans/cis photoisomerization in single-
component azobenzene EtSH SAMs on a flat gold surface. In
addition to the high yield of trans-to-cis photoconversion, the
lifetime of the cis state was extended on a time scale of hours at
ambient temperature. Comparisons of the molecular structure
and photoisomerization characteristics in both a trans SAM and
a cis SAM indicated that not only insufficient free space between
the individual molecules but also the strong tendency to aggre-
gate during the SAM preparation can be a main disadvantage of
photoswitching systems.
Experimental
Preparation of the self-assembled monolayers
For X-ray photoelectron spectroscopy and contact angle
measurements, polycrystalline gold films were prepared by
thermal evaporation of 100 nm of gold onto clean glass
substrates with a 5 m Ti layer as an adhesion layer. For UV–vis
absorption spectroscopic measurements, gold films with a thick-
ness of 20 nm were prepared on quartz substrates by vacuum
sublimation. The ‘‘trans-azobenzene SAMs’’ were prepared by
immersion of gold substrates in 0.1 mM trans-azobenzene solu-
tion in dichloromethane for 6–24 h in the dark. The ‘‘cis-azo-
benzene SAMs’’16a were formed by dipping gold substrates into
UV-exposed (cis-rich) azobenzene solution for 6 h in the dark.
After immersion, the samples were sufficiently rinsed with
dichloromethane, and blown dry with nitrogen gas.
Fig. 5 Reversible switching of (upper) absorbance at lmax and (lower)
contact angle for water of photoresponsive trans-EtSH SAMs (open
circle) and cis-EtSH SAMs (filled circle) by alternating UV and visible
light irradiation.
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Instrumentation
The chemical composition of the azobenzene SAMs was char-
acterized using XPS measurements (Thermo Fisher Scientific,
Theta Probe) by a monochromatized AlKa X-ray source (1486.6
eV). The peak position and area in the spectrum were determined
by curve-fitting analysis (Thermo Avantage ver. 3.25). The sessile
contact angle method was used to measure the contact angles of
€
a water drop on the gold substrates in air using the Kruss drop
shape analysis system DSA10-Mk2. The UV–vis absorption
spectrum (Shimadzu UV3150 UV-VIS-NIR spectrophotometer)
of each sample was recorded in absorbance mode against air in
the reference path. For each spectrum of the azobenzene SAM,
the spectrum of the pure gold surface (measured prior to the
preparation of the azobenzene SAM) was subtracted. Azo-
benzene SAMs were exposed to UV light (365 nm, Mineralight
lamp, model UVGL-25, UVP, Upland, CA 91786) under
a nitrogen atmosphere to induce trans-to-cis isomerization or to
visible light (436 nm, high-pressure UV lamp, Ushio Inc.,
combination of Toshiba color filters, Y-43 + V-44) to induce cis-
to-trans isomerization.
Acknowledgements
This work was supported by the Global COE program from the
Ministry of Education, Culture, Sports, Science, and Technology
of Japanese Government.
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