Photocontrol of electrical conductance with a nonsymmetrical azobenzene dithiol

Article abstract of DOI:10.1055/s-0033-1340087

We report a method for preparing electrode-molecule-electrode junctions that incorporate nonsymmetrical azobenzene dithiols. Our approach is based on sequential deprotection of thiol moieties originally carrying two different protecting groups. The azoben

Full text of DOI:10.1055/s-0033-1340087

2370▌
LETTER
letter
Photocontrol of Electrical Conductance with a Nonsymmetrical Azobenzene
Dithiol
Photocontrol of Electrical Conductance
Tal Ely,a Sanjib Das,b,1 Wenjie Li,a Pintu K. Kundu,b Einat Tirosh,a David Cahen,a Ayelet Vilan,*a Rafal Klajn*b
a
Department of Materials and Interfaces, Weizmann Institute of Science, 76100 Rehovot, Israel
Fax +972(8)9344138; E-mail: ayelet.vilan@weizmann.ac.il
b
Department of Organic Chemistry, Weizmann Institute of Science, 76100 Rehovot, Israel
Fax +972(8)9344142; E-mail: rafal.klajn@weizmann.ac.il
Received: 30.07.2013; Accepted after revision: 14.10.2013
Abstract: We report a method for preparing electrode–molecule–
electrode junctions that incorporate nonsymmetrical azobenzene di-
thiols. Our approach is based on sequential deprotection of thiol
moieties originally carrying two different protecting groups. The
azobenzene derivatives retained their switching properties within
monolayers and permitted the photocontrol of electrical conduc-
tance.
Key words: azo compounds, thiols, protecting groups, self-assem-
bly, isomerization
The ability to modulate the function of electronic devices
Tal Ely received his B.Sc. in chemical engineering from Shenkar
College and then researched solar cells at the Weizmann Institute of
Science. He is now a process engineer at AMS Technologies, Israel.
Sanjib Das received his Ph.D. from the Indian Institute of Technolo-
by optical inputs is a key concept in the field of optoelec-
tronics.2 In this respect, photoswitchable molecules, as the
smallest functional components of such devices, have at-
tracted considerable interest. These molecules isomerize
reversibly between two (or more) different states upon ex-
posure to light of different wavelengths, and the underly-
ing assumption is that these distinct states, which often
have markedly different electronic structures, should have
different charge-transport properties.3–5 Accordingly,
many systems incorporating photoswitch-electrode junc-
tions have been prepared6 and the effects of light on their
conductances have been examined.
gy Kanpur, India, in 2007. After various postdoctoral positions, he
joined the Institute of Minerals & Materials Technology, India, in
2012.
Wenjie Li gained his Ph.D. from the Simon Fraser University. After
periods of postdoctoral research, he joined the Shenzhen Institute of
Advanced Technology in 2013.
Pintu Kumar Kundu did his Ph.D. work at the Bio-Organic Divi-
sion, Bhabha Atomic Research Centre, Mumbai, and is currently a
postdoctoral fellow at the Weizmann Institute of Science, Israel.
Einat Tirosh gained her Ph.D. from Tel Aviv University, and contin-
ued her postdoctoral studies at the Weizmann Institute of Science.
She is currently a research associate at Tel Aviv University.
David Cahen gained his Ph.D. from Northwestern University and did
postdoctoral research at the Hebrew University and the Weizmann In-
stitute of Science. He currently heads the Weizmann Institute’s Alter-
native Sustainable Energy Research Initiative.
Ayelet Vilan received her Ph.D. from the Weizmann Institute and did
postdoctoral research at Philips Research and Ben-Gurion University,
before returning to the Weizmann Institute as a Staff Scientist in
2005.
Rafal Klajn gained his Ph.D. from Northwestern University and,
since 2009, has been at the Weizmann Institute of Science. He was a
recipient of the 2010 IUPAC Prize for Young Chemists, the 2013
ACS Victor K. LaMer Award, and the 2013 ERC Starting Investiga-
tor Award.
Azobenzene represents perhaps the simplest of these pho-
toswitches, and its charge-transport properties have been
investigated in various device architectures. Scanning
tunneling microscopy7–9 and conducting-probe atomic-
force microscopy (CP-AFM)10 studies have shown that
the trans isomer has a higher conductance, whereas other
experiments11–14 have suggested that the opposite behav-
ior occurs, indicating an unexpected complexity in the
seemingly straightforward azobenzene system and sug-
gesting highly system-dependent behavior. Higher con-
ductivities through cis-azobenzene might be attributable
to the more compact structure of the isomer (the tunneling
current decays exponentially with distance), whereas low-
er conductances through this isomer might be
rationalized15,16 in terms of the nonplanar structure of the
cis-azobenzene moiety, which disturbs its conjugation. In
the vast majority of experimental systems reported to
date, however, only one side of the azobenzene group was
covalently coupled to an electrode. Earlier experiments
with other organic mono- and dithiols17–19 suggest that,
compared with covalent immobilization on one side of
azobenzene, immobilization of both sides by using appro-
priate dithiol building blocks should produce devices with
conductances that are 1 to 1.5 orders of magnitude greater
overall.
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DOI: 10.1055/s-0033-1340087; Art ID: ST-2013-B0719-L
© Georg Thieme Verlag Stuttgart · New York

LETTER
Photocontrol of Electrical Conductance
2371
Scheme 1 Strategy for synthesis of the protected nonsymmetrical azobenzene dithiol 1 and the preparation of the corresponding junction
To ensure uniform arrangements of molecules within bled monolayers (SAMs) on gold were prepared by co-
monolayers (i.e. only one of the two sulfur atoms binds to adsorption of 1 and 11-sulfanylundecan-1-ol in a 1:2 mo-
a given electrode with no looping20), it is necessary to re- lar ratio.23–25 11-Sulfanylundecan-1-ol was used as a
sort to a three-step strategy in which a monoprotected α,ω- background thiol to provide sufficient conformational
dithiol is attached to the first electrode, deprotected, and freedom for the trans-azobenzene groups to isomerize
finally coupled to the second electrode through the newly even after adsorption onto the solid substrate.4,26 We used
generated thiol group. The experimental realization of this long (CH2)11 spacers to permit dense packing of the mono-
strategy, exemplified by the synthesis of a 3,4′-nonsymet- layers and to ensure that the underlying gold surface did
rically disubstituted azobenzene and the fabrication of a not interfere27,28 with the azobenzene switching reaction.
metal–monolayer–metal junction, is illustrated in Scheme
1. Briefly, the thiol groups in each of the starting materials
5 and 2 were protected by acetylation and by oxidation to
We monitored the isomerization of 1 in the mixed SAMs
(mSAMs) by means of UV–visible light spectroscopy.
Upon exposure to UV radiation (λ = 365 nm; intensity 25
the corresponding disulfide, respectively. The resulting
mW/cm2) for three hours, the intensity of the band cen-
protected derivatives 6 and 3 were then converted into the
tered at about 350 nm, characteristic of trans-azobenzene,
4-hydroxyazobenzene 721 and the alkyl bromide 4, respec-
decreased markedly [Figure 1 (a); see also Supporting In-
tively. Finally, these were coupled to give 1.22 Self-assem-
formation, Figure S7].29 The isomerization yield, estimat-
Figure 1 (a) UV–vis spectra of azo compound 1 adsorbed on gold before (blue) and after (black) exposure to UV radiation for 3 h. (b) UV–
vis spectra of a 0.03 mM solution of azo compound 1 in CH2Cl2 before (blue) and after (black) exposure to UV radiation (λ = 365 nm;
intensity 0.7 mW/cm2) for 3 minutes. (c) Monitoring of on-surface deprotection of azo compound 1 by PM-IRRAS spectroscopy. The arrows
on the left and the right indicate bands arising from asymmetric and symmetric C–H stretching, respectively, of the methyl groups of the acetyl
moieties. Inset: the disappearance of the band arising from C=O stretching of the carbonyl group of the acetyl moiety. The photoelastic modu-
lator frequency was 2900 cm–1 (main spectrum) and 1600 cm–1 (inset). The spectra are baseline-corrected.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2370–2374

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T. Ely et al.
LETTER
ed by integrating the trans-azobenzene peak areas before broader peaks at about 2922 cm–1 and 2852 cm–1, imply-
and after irradiation, was approximately 60% (that is, the ing a lower quality of SAM, probably as a result of the ad-
photostationary state contained about 40% of the trans ditional bulk introduced by the thioacetyl groups on the
isomer and about 60% of the cis isomer), compared with azobenzene moieties. Hydrolysis of the thioacetyl groups
>90% for solution isomerization [Figure 1 (b)]. Addition- led to a further broadening of the peaks [8 in Figures 2 (a)
al confirmation of successful isomerization within the and S8], suggesting a larger conformational freedom of
mSAMs was provided by electrical conductance measure- the alkyl chains following the deprotection reaction, or
ments (see below).
partial degradation of the monolayer under basic hydroly-
sis conditions. Interestingly, the PM-IRRAS results were
in qualitative agreement with ellipsometry measurements,
which confirmed the presence of a high-quality mSAM 10
with a thickness of about 2.2 ± 0.2 nm, and revealed a sub-
monolayer coverage for 9 and 8 (about 1.9 ± 0.2 nm and
1.1 ± 0.2 nm, respectively).
On-surface deprotection of the S-acetyl group was accom-
plished by using a dilute solution of sodium hydroxide in
aqueous ethanol. We monitored the reaction by polariza-
tion-modulation infrared reflection–absorption spectros-
copy (PM-IRRAS), as shown in [Figure 1 (c)]. Three
hours of incubation were sufficient to generate free thiol
groups in near-quantitative fashion, as evidenced by the Importantly, PM-IRRAS can also be used to determine
disappearance of the bands at about 2965 cm–1 and 2878 the orientation of the ligand molecules with respect to the
cm–1, which correspond, respectively, to the asymmetric surface. Performed in a grazing-angle configuration, PM-
and symmetric C–H stretching of the methyl groups of the IRRAS is sensitive only to vibration modes perpendicular
acetyl moiety, and of the band at about 1770 cm–1 corre- to the underlying surface. The observed ratios of methyl-
sponding to the C=O stretching of the carbonyl group of to-methylene stretching intensities were much higher than
the acetyl moiety. PM-IRRAS was also used to assess the expected from the stoichiometries of the adsorbed mole-
quality of our mSAMs (Supporting Information; Figure cules, indicating that the axes of the methylene C–H
S8); the positions and widths of the methylene-stretching stretching are roughly parallel to the gold surface; in other
vibrations reflect30 the density and uniformity, respective- words, the ligand molecules have their long axes oriented
ly, of the monolayer packing. The PM-IRRAS spectrum roughly normal to the gold surface. Specifically, on the
of an mSAM prepared from nonsubstituted azobenzene basis of the analysis reported in Section 5 of the Support-
thiol31,32 [10 in Figure 2 (a)] showed fairly sharp peaks ing Information, we found that the tilt angle was in the
centered at about 2919 cm–1 and 2851 cm–1 (correspond- range 60–90°.
ing to the asymmetric and symmetric CH2 stretching, re-
spectively), indicative of a densely packed monolayer.
The electrical properties of monolayers 8–10 were charac-
terized by means of CP-AFM (see Supporting Informa-
(The asymmetric stretching frequency for densely packed
tion for experimental details). The conductance of the as-
monolayers prepared from unsubstituted alkanethiols is
prepared (trans-azobenzene) SAMs decreased in the order
8 (solid green) > 9 (solid red) > 10 (solid blue), as expect-
ed on the basis of the interaction with the gold-coated
about 2917 cm–1).33 In contrast, mSAMs prepared from
azo compound 1 [SAM 9 in Figures 2 (a) and S8] showed
Figure 2 (a) Schematic representations of the molecular junctions investigated. (b) Current–voltage characteristics of SAMs 8–10 before (solid
lines) and after (dashed lines) UV irradiation. All averages and standard deviations are of ln(current). Because 10 + UV was extremely insulat-
ing and generally below the sensitivity limit, averages and standard deviations were calculated from only three curves in this case. For all other
junctions, 30–50 repeats for each state were considered (10–15% of junction formations failed, giving either short or open circuits, and 10% of
the recorded curves were excluded as outliers). (c) “On–off” ratios of 8–10 as a function of the applied bias.
Synlett 2013, 24, 2370–2374
© Georg Thieme Verlag Stuttgart · New York

LETTER
Photocontrol of Electrical Conductance
2373
AFM tip [Figure 2 (b)]. The highest conductance of 8 can most insulating component of the junction), V0 generally
be attributed to the formation of a covalent linkage be- expresses the characteristics of the alkane linker, which is
tween the CP-AFM tip and the SH group [Figure 2 (a)] not affected by light. The range of V0 values in our SAMs
that is liberated during the on-surface hydrolysis reaction. (~1.0 V < V0 < ~1.4 V) is in good agreement with those
No such covalent interaction can exist between 10 and the previously reported for related systems.39–41
AFM tip. The intermediate conductance shown by 9
In summary, we have developed a straightforward method
might originate from weak interactions between the thio-
for preparing electrode–molecule–electrode junctions in-
acetyl groups and the gold surface34–36 and/or the gold-in-
corporating nonsymmetrical azobenzene dithiols. Azo-
duced hydrolysis of thioacetyl groups, as reported
benzene retained its switching properties within the
previously.37 It is also possible that this effect might be as-
monolayers, although the isomerization yields were
smaller than those observed in solution. The trans→cis
isomerization induced a decrease in conductance, sug-
gesting that distortion of the planar structure of trans-azo-
cribed to the quality of the SAMs (determined by ellip-
sometry and PM-IRRAS; see above), which decreases in
the same order as their conductance values increase; in
other words, the increasing conductivities might be due to
benzene was a major contributor to the observed effects.
the presence of pinholes or other defects in monolayers 8
Interestingly, the substantial drop in conductance of about
30 times was brought about by the isomerization of only
about 60% of the more conductive isomer, suggesting that
and 9. However, the photoisomerization results (see be-
low) argue strongly against this explanation.
After exposure to UV radiation for three hours, all the the intrinsic charge transport through trans-azobenzene is
mSAMs showed weaker electron-transport properties significantly higher than through the cis isomer, as previ-
[Figure 2 (b)]. The average on–off ratios, calculated as the ously predicted theoretically42 and observed in experi-
ratios of the averaged currents in the trans configuration mental probes on individual molecules.8
to those in the cis state, are shown in Figure 2 (c). If the
differences in dark conductance of 8, 9, and 10 resulted
from differences in the SAM quality, the presumed cur-
Acknowledgment
R.K. acknowledges support from the Mel and Joyce Eisenberg-
Keefer Fund and the Israel Science Foundation, Jerusalem (grant
no. 1463/11). A.V. and D.C. thank the Israel Science Foundation
(through a Centre of Excellence grant), the Nanoscience E+ pro-
gram of the ERA Net (Transnational project MaECENAS), the
Grand Centre for Sensors and Security, the Kimmel Centre for Na-
noscale Science, and the Schmidt-Minerva Centre for Supramole-
cular Architecture for support. D.C. holds the Schaefer Chair in
Energy Research.
rent flow through defects and pinholes should have led to
the smallest photoswitching effect for the lowest-quality
films, whereas the observed differences were well within
the margins of experimental error (see also Figures S10
and S11). Importantly, the change in the transport charac-
teristics was reversible; when exposed to visible light (450
nm light-emitting diode; intensity = 5 mW/cm2; 1 h), all
the monolayers regained their original properties, as illus-
trated for monolayer 8 in Figure S10.
We analyzed the behavior of the samples in terms of their
equilibrium conductance Geq and their bias-scaling factor
V0,38 both of which can be determined by parabolic fitting
of the conductance, G ≈ Geq[1 + 3(V/V0)2], where V is the
applied bias, G is the conductance, and Geq is the equilib-
rium conductance for V→0. Geq decreases by a factor of
20–30 after exposure of the samples to UV radiation (see
Figure S11). This consistent behavior suggests that the
higher pre-irradiation conductances are due to better delo-
calization of frontier orbitals in the planar structure of
trans-azobenzene. Interestingly, a similar factor of ~33
was extracted for junctions fabricated by using hexadec-
ane-1,16-dithiol and hexadecane-1-thiol (Figure S11a),
suggesting that the effect of the trans→cis isomerization
on the conductance is comparable to that of removing one
of two covalent linkages to the electrodes. In contrast to
the large discrepancies in the Geq values, V0 showed little
or no dependence on irradiation or on the type of mono-
layer (Figure S11b). This can be understood if we consid-
er the net (electron tunneling) transmission across the
molecule as being equal to the product of transmission
across the alkane linkers (Talkane) and that across the azo-
benzene units (TAB), i.e., Tmol ≈ Talkane·TAB. Because the ap-
plied bias drops predominantly over the alkane linker (the
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
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LETTER
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hydrochloride 5 (136 mg; 0.49 mmol), K2CO3 (179 mg; 1.29
mmol), and DMF (3 mL) under N2, and the mixture was
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