A photoswitchable methylene-spaced fluorinated aryl azobenzene monolayer grafted on silicon

Article abstract of DOI:10.1039/c0cc00521e

Aryldiazonium-terminated methylene-spaced trifluoromethylazobenzene derivatives have been synthesized. Their self-assembled monolayers (SAMs) on a silicon surface allow for a stable and reversible molecular photoswitch.

Full text of DOI:10.1039/c0cc00521e

Chemical Communications
www.rsc.org/chemcomm
Volume 46 | Number 29 | 7 August 2010 | Pages 5177–5380
ISSN 1359-7345
FEATURE ARTICLES
Xue Duan et al.
COMMUNICATION
Layered double hydroxide films
Hyoyoung Lee, Byung-Chan Yu et al.
A photoswitchable methylene-
spaced fluorinated aryl azobenzene
monolayer grafted on silicon
Wenping Hu et al.
High performance organic
semiconductors
1359-7345(2010)46:29;1-D

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A photoswitchable methylene-spaced fluorinated aryl azobenzene
monolayer grafted on siliconw
Misook Min,ab Gyeong Sook Bang,b Hyoyoung Lee*b and Byung-Chan Yu*a
Received 22nd March 2010, Accepted 10th May 2010
First published as an Advance Article on the web 26th May 2010
DOI: 10.1039/c0cc00521e
Aryldiazonium-terminated methylene-spaced trifluoromethyl-
azobenzene derivatives have been synthesized. Their self-
assembled monolayers (SAMs) on a silicon surface allow for a
stable and reversible molecular photoswitch.
to prevent the steric constraints of photo-induced trans/cis
isomerization of the azobenzene SAMs. The methylene-spaced
alkyl chains of compound 1 were adopted for generating
high-ordered SAMs by using van der Waals interaction of
the neighboring alkyl chains, as well as suppressing the
excited-state surface quenching caused by the electronic
coupling between the molecule and substrate during irradiation
of the azobenzene SAM on the silicon substrate. Additionally,
the hydrophobic trifluoromethyl group was introduced to
increase the photo-switching ON/OFF ratio measured for
the hydrophilic water drop using the water contact angle
(WCA) method (see ESIw for control experiments with
compounds 2 and 3)8 and to protect the degradation of the
fluorinated SAMs from the photo irradiation since the bond
dissociation energy of C–F (B130 kcal molÀ1) is much higher
than that of the C–H bond.9
With the advent of molecular photoelectronic devices for
future applications in microelectronics and bioelectronics,1,2
development of functionalized and robust organic monolayers
that can be directly grafted onto silicon substrates via liquid
phase reactions is essential. The covalent bonding between the
silicon substrate and the carbon atom of the organic molecules
makes them very stable, even under harsh conditions (i.e.
sonication in organic solvents and boiling water temperature).3,5
This constitutes a key advantage when compared with self-
assembled monolayers (SAMs) of organic thiols on gold
surfaces whose stability seems less sound and subordinate to
several factors.4 For these reasons, the direct covalent grafting
of alkyl-tailed azobenzenes, with the help of light and catalysts,
onto a hydride-passivated silicon surface has been reported.5
In recent years, air-stable and easily synthesized aryldiazonium
salts have provided organic monolayers that encourage strong
bonds between the carbons of p-conjugated organic molecules
and surface atoms.6 In particular, the noble method provides
spontaneous and room-temperature routes to direct covalent
bonds.
For compounds 1, 2 and 3 shown in Fig. 1 (see ESIw,
Schemes S1 and S2), para-nitrophenol was dimerized by
addition of dibromodecane in the presence of Cs2CO3. The
resultant dinitrobenzene was then reduced to dianiline.
Addition of trifluoromethylnitrosobenzene to the dianiline in
the presence of acetic acid afforded the monoazobenzene,
which was then diazotized to give azobenzene diazonium salt
1 in 76% overall yield. Compound 2 was prepared according
to literature procedures in 70% yield.10
Upon UV and visible light irradiation, shown in Fig. 2,
changes in the UV-Vis absorption spectra of compound 1 in
CH3CN were observed (see ESIw). Compound 1 exhibited its
absorption maxima near 316 nm with a weak band at about
437 nm, corresponding to the p–p* and n–p* transition bands
of the trans azobenzene, respectively. Upon UV irradiation
over 10–20 min, the intensity of the p–p* transition band at
316 nm decreased while the n–p* transition band at 437 nm
slightly increased (trans to cis isomerization). Reversibly, upon
visible irradiation over 15 min, the intensity of the p–p*
transition band at 316 nm increased while the n–p* transition
band at 437 nm decreased (cis to trans isomerization).
Until now, however, important issues such as the steric
constraints of trans/cis isomerization of the azobenzene,
excited-state quenching, and stability of the covalently grafted
SAMs on the silicon surface have not been addressed for full
realization of photo-driven molecular switching devices.7
In this communication, a novel aryldiazonium-terminated
methylene-spaced trifluoromethylazobenzene 1 was synthesized
and its reversibly photoswitchable SAMs on a silicon surface
were characterized. To the best of the authors’ knowledge, this is
the first reported observation of direct photoswitching of an
azobenzene SAM covalently grafted with an aryl carbon on a
silicon surface without even use of a mixed SAM.7
The aryldiazonium salt was designed for direct attachment
to the silicon surface to simultaneously provide a phenyl space
a Department of Chemistry, Mokwon University, Daejon 302-729,
Republic of Korea. E-mail: chanyu@mokwon.ac.kr;
Fax: +82-42-822-7560; Tel: +82-42-829-7565
b NCRI, Center for Smart Molecular Memory,
Department of Chemistry, Sungkyunkwan University,
Suwon 440-746, Republic of Korea. E-mail: hyoyoung@skku.edu;
Fax: +82-31-290-5934; Tel: +82-31-299-4566
w Electronic supplementary information (ESI) available: Details of
synthesis of azobenzene diazonium salt, 1H-NMR, 13C-NMR,
UV-Vis, FT-IR, and XPS of the azobenzene diazonium salt. See
DOI: 10.1039/c0cc00521e
Fig. 1 Molecular structures of photoswitches investigated.
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Fig. 2 UV-Vis absorption spectra of the compound 1 solution
(CH3CN) as irradiation time (A) at 360 nm and (B) at 430 nm after
irradiation at 360 nm for 10–15 min.
Compound 2 exhibited a UV-Vis absorption spectrum
similar to compound 1 in CH3CN, showing an absorption
maximum at about 330 nm and a weak band near 474 nm.
This also corresponds to the p–p* and n–p* transition bands
of the trans azobenzene, respectively. Upon UV irradiation
over 30 min, the intensity of the p–p* transition band at
330 nm greatly decreased while that of the n–p* transition
band at 474 nm slightly increased (trans to cis isomerization).
Upon visible irradiation over 30 min, the intensity of the p–p*
transition band at 330 nm increased, showing the cis form
isomerizing to the trans form (see ESIw, Fig. S3). The spectra
of compounds 1 and 2 in solution exhibited the photo-
isomerization as previously reported for the azobenzene-type
molecule.11
Fig. 3 A representation of the photoswitching of the azobenzene
monolayer of compound 1.
UV light (360 nm) for 30 min, the photochemical change from
trans to cis caused a change in wettability reflected in a water
contact angle (y) change from 87.821 to 82.741, while it is
completely reversed to the original value by 30 min irradiation
with visible light (430 nm). This kind of cycle can be performed
repeatedly with nearly a 51 difference in wettability upon
UV/vis irradiation as shown in Table S1.w Fig. 3 shows the
scheme of the effects of trans–cis isomerization on the relative
protrusion of the azobenzene monolayer.
For the azobenzene monolayer, the hydride-terminated
silicon surface was first prepared according to literature
procedures.6a The hydride-terminated silicon surface was then
exposed to the solution of the compounds 1 and 2 in
anhydrous CH3CN (0.15 mM) during a period of 2 to 3 h,
in a glove box and under an inert atmosphere (see ESIw). The
measured ellipsometric thicknesses of the modified film were
approximately 3.19 Æ 0.06 and 1.81 Æ 0.07 nm for compound
1 and 2 SAMs, respectively. These thickness values were
similar to the estimated value of a monolayer, 3.09 and 1.31 nm
for compounds 1 and 2. The molecular length was estimated
using Cambridge Scientific Chem 3D.
For the SAMs of compounds 2 and 3, however, only less
than 2 cycles in the contact angle measurement for the
repeated UV and visible light irradiation were found (see ESIw
for control experiments with compounds 2 and 3), indicating
that isomerization on the surface is not as stable as observed in
the solution phase. It is known that evidence of photoinduced
switching of single molecules on metal electrodes is often
complicated by surface quenching7 and steric hindrance. The
previous failure of the surface isomerization led to the design
of new azobenzene diazonium salts bearing a molecular spacer
to suppress excited-state quenching from the metal substrate7
and form dense assemblies of single tethered azobenzene
molecules, such as compound 1. In contrast to the SAM 2,
azobenzene SAM 1 showed stable changes in surface properties
after repeated UV and visible light irradiation. This kind of
cycle can be performed repeatedly with nearly a 51 difference
in wettability. The reversible switching was again repeated
many times, and good reversibility of the surface wettability
was observed as depicted in Fig. 4.
The elemental composition of the azobenzene-modified
Si(111) surface was examined by XPS. Elements C, N, O, F,
and Si were monitored in the spectra (see ESIw, Fig. S4). In the
XPS spectra of the compound 1 film, the C 1s spectrum was
observed at 285.0 and 292.8 eV, and assigned to –CH2 and
–CF3, respectively. An N 1s peak (400.3 eV) and an F 1s
(688.6 eV) peak were also detected, confirming the presence of
–NQN– and –CF3 groups.
Photoisomerization via irradiation with a high-intensity UV
and visible light can be achieved to give the trans or cis form,
which can be utilized for a photoswitching. The contact angle
measurements were employed to observe the changes of
surface wettability of a self-assembled monolayer after UV
and visible light irradiation since the hydrophobicity of the
trans state exposed to a hydrophobic –CF3 group is a little
higher than that of the cis state.5,8 Thus, it is expected that the
contact angle of the trans monolayer increases, while that of
the cis monolayer deceases because of its higher dipole
moment and weaker hydrophobicity. Water contact angles
were measured after 30 min of exposure. With irradiation with
To verify the stability of SAM 1 to UV and visible light
irradiation, the N 1s XPS spectra were investigated. After
several UV light irradiations, each for 30 min, the intensities of
the N 1s peak from –NQN– remained unchanged, indicating
that the methylene-spaced azo monolayer was not damaged
during UV light irradiation even after 60 days (see ESIw, Fig.
S4(B)). However, the intensities of the N 1s peak of SAM 2
decreased after several UV light irradiations (see ESIw,
Fig. S4(C)). Thus the methylene spacer plays an important
role in the stability of the SAM under UV light irradiation.
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environment. The novel use of the methylene-spaced
fluorinated stable SAMs on sillicon surfaces is applicable in
developing miniaturized optical switches.
The authors thank the Creative Research Initiatives (Smart
Molecular Memory) of MEST/NRF for financial support.
Notes and references
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Fig. 4 Evolution of the water contact angle of the azobenzene
monolayer 1 with the methylene-spaced group. The columns labeled
cis and trans respectively refer to the sample after UV (360 nm) and
visible (430 nm) light irradiation.
5 X. Zhang, et al., J. Phys. Chem. C, 2008, 112, 8288.
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This stability was also tested through WCA.12 The wettability
showed no obvious changes in laboratory ambient conditions
for over 60 days even though the intensities of the ON/OFF
ratio were slightly reduced, demonstrating not only the stability
of the grafted monolayer, but also the reproducibility of the
isomerization performance of the azobenzene molecule.5
In conclusion, a novel aryldiazonium-terminated trifluoro-
methylazobenzene bearing
a carbon chain between the
azobenzene moiety and silicon surface (111) was successfully
prepared and provided a stable monolayer that prevented
possible surface quenching during irradiation of the azobenzene
SAM on the silicon substrate. A self-assembled monolayer of a
novel methylene-spaced fluorinated molecule was quite stable
with photo irradiation over a couple of months. The
wettability study demonstrated the trans/cis conformational
transformation that overcame the steric constraints, even
after repeated cycles lasting several months in a laboratory
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5234 | Chem. Commun., 2010, 46, 5232–5234