Light-driven molecular switches in azobenzene self-assembled monolayers: Effect of molecular structure on reversible photoisomerization and stable cis state

Article abstract of DOI:10.1039/b921801g

Both the reversible trans ? cis photoisomerization and slow thermal back cis-to-trans isomerization of azobenzene-functionalized self-assembled monolayers on gold surfaces have been achieved by rationally designed single-component azobenzene thiol.

Full text of DOI:10.1039/b921801g

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Light-driven molecular switches in azobenzene self-assembled
monolayers: effect of molecular structure on reversible
photoisomerization and stable cis statew
Mina Han,*a Daisuke Ishikawa,a Takumu Honda,a Eisuke Itob and Masahiko Hara*ab
Received 19th October 2009, Accepted 16th March 2010
First published as an Advance Article on the web 8th April 2010
DOI: 10.1039/b921801g
Both the reversible trans 2 cis photoisomerization and
slow thermal back cis-to-trans isomerization of azobenzene-
functionalized self-assembled monolayers on gold surfaces
have been achieved by rationally designed single-component
azobenzene thiol.
photoconversion, the effects of molecular structure on the
lifetime of cis-azobenzene in combination with reversible
photoisomerization on solid surfaces have not yet been
reported.
We have designed single-component azobenzene substituted
with sterically bulky ethyl groups at the ortho positions with
respect to the azo group (Et-SH, Fig. 1 and see ESIw), which
exhibits excellent reversible photoswitching properties as well
as slow thermal cis-to-trans isomerization over a one-day
period in SAMs. A comparison with Me-SH containing
methyl group at the meta position has also been made to
clarify the influence of free volume and molecular structure on
both the trans-to-cis photoisomerizability and the lifetime of
the cis form.
Design and fabrication of a smart surface capable of control-
lable and reversible switching between two states in response
to light have attracted much attention in the fields of photo-
chemistry, nanotechnology, biology, organic chemistry, and so
on.1 Among the various types of photochromic molecules,
considerable interest has been focused on azobenzene due to
its structural change between the trans and cis forms and its
ease of chemical modification.2 Controlling the trans 2 cis
photoisomerization and the lifetime of the two different states
is highly desirable for potential applications into optical
information storage and an optical signal generated by photo-
switching between the trans and cis states. In particular, the
lifetime of the cis form is greatly influenced by the surrounding
environment as well as the molecular structure.1a,3
Ortho-diethylated azobenzene (Et-SH) in dichloromethane
solution showed a strong p–p* absorption band at 354 nm
(e = 2.6 Â 104 L molÀ1 cmÀ1) in the as-prepared initial state
(Fig. S1 and Table S1, see ESIw). NMR and UV-vis absorp-
tion spectroscopic studies indicated that approximately 95%
of the cis form was produced at a photostationary state of UV
light irradiation at 365 nm. Subsequent irradiation with visible
light at 436 nm induced cis-to-trans isomerization, giving a
photostationary state containing both the major trans form
(B70%) and the minor cis form.1,9 Alternatively, thermal
back cis-to-trans isomerization of Et-SH proceeded much
Even though many azobenzene molecules in dilute organic
solutions undergo reversible photoisomerization and show a
lifetime of the cis form on the time scale of minutes to hours, it
has been reported that densely packed azobenzene thiol self-
assembled monolayers (SAMs) are photochemically unreactive
on flat gold surfaces.4,5 In this context, efforts to afford free
volume required for the conformational change between the
trans and cis forms during photoisomerization have been made
by fabricating (1) unsymmetrical azobenzene disulfide SAMs6
and (2) mixed azobenzenethiol and alkanethiol SAMs.7 How-
ever, another instability problem accompanied by cleavage of
the S–S disulfide and phase separation through favorable
intermolecular interactions between the same chemical com-
ponents emerges in these SAMs consisting of two components
on a gold surface.6b,8 Moreover, obviously different from
investigations of the effects of free volume on the trans-to-cis
a Department of Electronic Chemistry, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan.
E-mail: mrhan@echem.titech.ac.jp, masahara@echem.titech.ac.jp;
Fax: 81-45-924-5447; Tel: 81-45-924-5447
b Flucto-Order Functions Research Team, RIKEN-HYU Collaboration
Research Center, RIKEN Advanced Science Institute, 2-1 Hirosawa,
Wako, Saitama 351-0198, Japan
w Electronic supplementary information (ESI) available: Detailed
experimental procedures (instrumentation, monolayer preparation,
synthesis and characterization), UV-vis absorption spectra of dichloro-
methane solutions, XPS data, absorption spectra of Me-SH SAMs.
See DOI: 10.1039/b921801g
Fig. 1 Upper: Molecular structure. Lower: Schematic representation
of molecular switches of Et-SH SAMs on a gold surface.
ꢀc
This journal is The Royal Society of Chemistry 2010
3598 | Chem. Commun., 2010, 46, 3598–3600

View Article Online
more slowly at room temperature and the half-life time of
Et-SH (380 h) was approximately 30 times longer than that of
the comparable Me-SH (13 h), as shown in Fig. S3 and Table
S1 (ESIw). Such slow isomerization rate of Et-SH seems to
be due to bulky ethyl substituents that inhibit large-scale
distortion of the azo group during isomerization.1a,10
For X-ray photoelectron spectroscopy (XPS) and contact
angle measurements, polycrystalline gold films were prepared
by thermal evaporation of 100 nm of gold onto clean
glass substrates with a 5-nm Ti layer as an adhesion layer.
For UV-vis absorption spectroscopy measurements, gold films
with a thickness of 20 nm were prepared on clean quartz
substrates by vacuum sublimation. Azobenzene SAMs
were fabricated by immersion of gold substrates in 0.1 mM
azobenzene solution in dichloromethane for 24 h in the dark.
The chemisorption of azobenzene molecules to a gold sur-
face could be characterized by using XPS measurement. The
representative N 1s and S 2p XP spectra of azobenzene Et-SH
SAMs, together with the corresponding fits, are shown in
Fig. S4 (ESIw). The single peak at 399.8 eV in the N 1s spectrum
and 163.2 and 162.0 eV in the S 2p spectrum are assignable
to the nitrogen of the azo group, and the thiolate-type
sulfur bound to the metal surfaces,11 consistent with previous
data.12
Fig. 2 (a) UV-vis absorption spectral changes of Et-SH SAMs on a
gold surface after alternating UV and visible light irradiation. Inset:
Changes in absorbance at lmax obtained after each photoswitching
process. (b) Difference spectra obtained after alternating UV and
visible light irradiation in SAMs and in dichloromethane solution.
To examine reversible changes in the molecular confor-
mation and the resulting spectroscopic features of azobenzene
monolayers under UV and visible light irradiation, UV-vis
absorption spectroscopy measurements were employed. As
shown in Fig. 2, the spectrum of as-prepared Et-SH
monolayers reveals an intense p–p* absorption band at
350 nm, which is slightly blue-shifted by 4 nm with respect
to lmax = 354 nm in dichloromethane solution. Upon UV
light irradiation, a drastic reduction in the p–p* band is
observed as a result of the trans-to-cis photoisomerization.
Based on the assumption that the values of the molar extinc-
tion coefficients of the trans and cis forms are not altered
before and after the formation of SAMs, the estimated
trans-to-cis photoconversion was 88 Æ 5%. Subsequent visible
light irradiation gives rise to cis-to-trans back isomer-
ization, maximizing the p–p* absorption band at 350 nm
(Fig. 2a). Difference spectra in SAMs obtained after alter-
nating UV and visible light irradiation are in good agreement
with those in solution (Fig. 2b), obviously indicative of
excellent reversible nature of photoswitching between the trans
and cis states on a gold surface. More interestingly, no
prominent spectral change was observed in the UV-exposed
Et-SH SAMs over 180 min at ambient temperature in the
dark, and slow thermal cis-to-trans isomerization proceeded
over a one-day period (Fig. 3). These results clearly reflect
that cis-azobenzene of ortho-diethylated Et-SH exhibits high
stability even on a gold surface.
Fig. 3 Changes in (At À Aap) of Et-SH (triangles) and Me-SH
(circles) SAMs as a function of thermal cis-to-trans isomerization time
after UV light irradiation. Aap and At correspond to absorbance at
lmax of as-prepared SAMs and after dark incubation for time (t),
respectively.
Fig. 4 Reversible switching of contact angle for water of photo-
responsive Et-SH monolayer by UV and visible light irradiation.
To clarify factors related to controlling the trans-to-cis
photoisomerizability and stability of the cis form in azo-
benzene monolayers, we prepared Me-SH having methyl
group at the meta position (Fig. 1 and see ESIw). A p–p*
absorption band at 343 nm of as-prepared Me-SH monolayer
film was considerably blue-shifted by 25 nm with respect to
lmax = 368 nm in dichloromethane solution (see Fig. S2 and
S6, ESIw). In sharp contrast to the slight blue shift for Et-SH
containing two ethyl groups at the ortho positions, the large
spectral blue shift for Me-SH is attributed to H-aggregation
In addition to UV-vis absorption spectroscopy measure-
ments, the reversible conformation change of the azobenzene
unit was confirmed by means of contact angle (CA) measure-
ments (Fig. 4). The CA of as-deposited Et-SH SAMs for water
on a flat gold substrate was 94 Æ 11. Upon UV light irradiation,
the CA decreased to 90 Æ 11. Et-SH SAMs showed reversible
photoswitching of surface wettability under UV and visible
light irradiation.
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 3598–3600 | 3599

View Article Online
through strong intermolecular interaction between trans-
Me-SH adopting a planar conformation.13 Under UV light
irradiation the estimated trans-to-cis conversion was found to
be ca. 51 Æ 3%. Fast thermal cis-to-trans back isomerization
proceeded, especially, in the early stages of dark incubation for
30 min (Fig. 3). Considering that the occupied areas are
enlarged up to 0.55 nm2 for Et-SH and 0.50 nm2 for Me-SH
(estimated by the X-ray crystal analysis14 and a CPK model)
from 0.24 nm2 for the unsubstituted azobenzene,4,15 and that a
critical free volume required for trans-to-cis photoisomeri-
zation in densely packed monolayers is 0.45 nm2 per alkyl-
azobenzene unit,15b it seems likely that both the low trans-to-cis
conversion and fast thermal cis-to-trans isomerization of
Me-SH SAMs is not primarily due to a lack of free volume,
but rather due to molecular structure related to favorable
intermolecular interaction between the planar trans form of
Me-SH.
2 (a) R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni,
E. Kojima, A. Kitamura, M. Shimohigoshi and T. Watanabe,
Nature, 1997, 388, 431–432; (b) K. Ichimura, S. K. Oh and
M. Nakagawa, Science, 2000, 288, 1624–1626; (c) M. J. Cook,
A. M. Nygard, Z. X. Wang and D. A. Russell, Chem. Commun.,
2002, 1056–1057; (d) Y. L. Yu, M. Nakano and T. Ikeda, Nature,
2003, 425, 145–145; (e) S. Sortino, S. Petralia, S. Conoci and S. Di
Bella, J. Mater. Chem., 2004, 14, 811–813; (f) X. J. Feng, L. Feng,
M. H. Jin, J. Zhai, L. Jiang and D. B. Zhu, J. Am. Chem. Soc.,
2004, 126, 62–63; (g) M. Ito, T. X. Wei, P. L. Chen, H. Akiyama,
M. Matsumoto, K. Tamada and Y. Yamamoto, J. Mater. Chem.,
2005, 15, 478–483; (h) H. S. Lim, J. T. Han, D. Kwak, M. H. Jin
and K. Cho, J. Am. Chem. Soc., 2006, 128, 14458–14459;
(i) H. Nishioka, X. G. Liang, H. Kashida and H. Asanuma, Chem.
Commun., 2007, 4354–4356; (j) H. Asanuma, X. Liang,
H. Nishioka, D. Matsunaga, M. Liu and M. Komiyama, Nat.
Protoc., 2007, 2, 203–212; (k) G. Pace, V. Ferri, C. Grave,
M. Elbing, C. von Hanisch, M. Zharnikov, M. Mayor,
M. A. Rampi and P. Samori, Proc. Natl. Acad. Sci. U. S. A.,
2007, 104, 9937–9942; (l) V. Ferri, M. Elbing, G. Pace,
M. D. Dickey, M. Zharnikov, P. Samori, M. Mayor and
M. A. Rampi, Angew. Chem., Int. Ed., 2008, 47, 3407–3409;
(m) J. M. Mativetsky, G. Pace, M. Elbing, M. A. Rampi,
M. Mayor and P. Samori, J. Am. Chem. Soc., 2008, 130,
9192–9193.
3 (a) C. S. Paik and H. Morawetz, Macromolecules, 1972, 5, 171;
(b) E. V. Brown and G. R. Granneman, J. Am. Chem. Soc., 1975,
97, 621; (c) J. M. Nerbonne and R. G. Weiss, J. Am. Chem. Soc.,
1978, 100, 5953; (d) Y. Tada, T. Morita, J. Umemura, M. Iwamoto
and S. Kimura, Polym. J., 2005, 37, 599–607.
4 R. Wang, T. Iyoda, L. Jiang, D. A. Tryk, K. Hashimoto and
A. Fujishima, J. Electroanal. Chem., 1997, 438, 213–219.
5 (a) S. D. Evans, S. R. Johnson, H. Ringsdorf, L. M. Williams and
H. Wolf, Langmuir, 1998, 14, 6436–6440; (b) K. Tamada,
J. Nagasawa, F. Nakanishi, K. Abe, T. Ishida, M. Hara and
W. Knoll, Langmuir, 1998, 14, 3264–3271.
A significant variation in lmax from 368 nm for Me-SH to
354 nm for Et-SH in solution may be closely related to the
steric effect arising from the substitution of two ethyl groups at
the ortho positions with respect to the azo group. It is known
that unsubstituted trans-azobenzene adopts a planar confor-
mation, and that methyl substituents at the meta positions
have little influence on the planar structure of azobenzene.1,13
In sharp contrast, our X-ray crystal structure analysis revealed
that the substitution of two ethyl groups at the ortho positions
leads to a large distortion of the phenyl ring from coplanarity
(C–C–NQN dihedral angle is 156114). Consequently, strong
intermolecular interaction between azobenzene aromatic rings
is suppressed and free volume is efficiently procured in Et-SH
monolayers. Furthermore, two ethyl groups at the ortho
positions would restrict large-scale distortion (through either
the inversion mechanism or the rotation mechanism) of the
azo group for thermal cis-to-trans isomerization,1a,2i,10 thus
resulting in stability of the cis state.
6 (a) K. Tamada, H. Akiyama and T. X. Wei, Langmuir, 2002, 18,
5239–5246; (b) K. Tamada, H. Akiyama, T. X. Wei and S. A. Kim,
Langmuir, 2003, 19, 2306–2312; (c) L. F. N. Ah Qune,
H. Akiyama, T. Nagahiro, K. Tamada and A. T. S. Wee, Appl.
Phys. Lett., 2008, 93, 083109.
7 (a) S. Yasuda, T. Nakamura, M. Matsumoto and H. Shigekawa,
J. Am. Chem. Soc., 2003, 125, 16430–16433; (b) A. S. Kumar,
T. Ye, T. Takami, B. C. Yu, A. K. Flatt, J. M. Tour and
P. S. Weiss, Nano Lett., 2008, 8, 1644–1648.
In conclusion, we have reported that the ortho-diethylated
azobenzene molecule exhibits excellent reversible photoswitching
of its molecular conformation as well as the long-lived cis state
on gold surfaces. Not only is free volume required for photo-
isomerization but also a suitable design for the azobenzene
molecules based on (1) intermolecular interactions and (2) the
lifetime of the cis form is an important factor in controlling the
trans-to-cis photoisomerizability and the stabililty of the cis
state on a solid surface. Our current work provides a strategy
for developing new types of photoreactive surfaces potentially
applicable to optical data storage and photoswitching devices.
8 T. Ishida, S. Yamamoto, W. Mizutani, M. Motomatsu,
H. Tokumoto, H. Hokari, H. Azehara and M. Fujihira, Langmuir,
1997, 13, 3261–3265.
9 M. Han and K. Ichimura, Macromolecules, 2001, 34, 82–89.
10 (a) J. P. Ortruba III and R. G. Weiss, J. Org. Chem., 1983, 48,
3448–3453; (b) N. J. Bunce, G. Ferguson, C. L. Forber and
G. J. Stachnyk, J. Org. Chem., 1987, 52, 394–398.
11 (a) D. G. Castner, K. Hinds and D. W. Grainger, Langmuir, 1996,
12, 5083–5086; (b) K. Heister, H.-T. Rong, M. Buck,
M. Zharnikov, M. GrunzeL and S. O. Johansson, J. Phys. Chem. B,
2001, 105, 6888–6894.
12 M. Onoue, M. Han, E. Ito and M. Hara, Surf. Sci., 2006, 600,
3999–4003.
13 (a) E. J. Gabe, Y. Wang, L. R. C. Barclay and J. M. Dust, Acta
Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1981, 37,
978–979; (b) C. Ruslim and K. Ichimura, J. Mater. Chem., 1999, 9,
673–681.
Notes and references
1 (a) H. Rau, Photochromism: Molecules and Systems, ed. H. Durr
and
14 M. Han, D. Hashizume and M. Hara, Acta Crystallogr., Sect. E:
Struct. Rep. Online, 2006, 62, o3001–O3003.
¨
1990;
H.
Bouas-Laurent,
Elsevier,
Amsterdam,
(b) Photoreactive Organic Thin Films, ed. Z. Sekkat and W. Knoll,
Academic Press, Elsevier Science, USA, 2002; (c) K. Ichimura,
Chem. Rev., 2000, 100, 1847–1873; (d) A. Natansohn and
P. Rochon, Chem. Rev., 2002, 102, 4139–4175.
15 (a) H. Wolf, H. Ringsdorf, E. Delamarche, T. Takami, H. Kang,
B. Michel, C. Gerber, M. Jaschke, H. J. Butt and E. Bamberg,
J. Phys. Chem., 1995, 99, 7102–7107; (b) M. Nakagawa, R. Watase
and K. Ichimura, Chem. Lett., 1999, 1209–1210.
ꢀc
This journal is The Royal Society of Chemistry 2010
3600 | Chem. Commun., 2010, 46, 3598–3600