Steric hindrance of photoswitching in self-assembled monolayers of azobenzene and alkane thiols

Article abstract of DOI:10.1021/la402144g

Surface-bound azobenzenes exhibit reversible photoswitching via trans-cis photoisomerization and have been proposed for a variety of applications such as photowritable optical media, liquid crystal displays, molecular electronics, and smart wetting surfac

Full text of DOI:10.1021/la402144g

Article
pubs.acs.org/Langmuir
Steric Hindrance of Photoswitching in Self-Assembled Monolayers of
Azobenzene and Alkane Thiols
David T. Valley, Matthew Onstott, Sergey Malyk, and Alexander V. Benderskii*
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States
*
S Supporting Information
ABSTRACT: Surface-bound azobenzenes exhibit reversible photo-
switching via trans−cis photoisomerization and have been proposed for
a variety of applications such as photowritable optical media, liquid
crystal displays, molecular electronics, and smart wetting surfaces. We
report a novel synthetic route using simple protection chemistry to
form azobenzene-functionalized SAMs on gold and present a
mechanistic study of the molecular order, orientation, and con-
formation in these self-assembled monolayers (SAMs). We use
vibrational sum-frequency generation (VSFG) to characterize their
vibrational modes, molecular orientation, and photoisomerization
kinetics. Trans−cis conformational change of azobenzene leads to the
change in the orientation of the nitrile marker group detected by VSFG.
Mixed SAMs of azobenzene and alkane thiols are used to investigate
the steric hindrance effects. While 100% azobenzene SAMs do not exhibit photoisomerization due to tight packing, we observe
reversible switching (>10 cycles) in mixed SAMs with only 34% and 50% of alkane thiol spacers.
1
9,20
INTRODUCTION
phase separation,
ogy,
effects of composition on morphol-
■
2
1,22
19
and relative device performance using a variety of
methods including surface wetting angle,
reflection IR, and XPS.
azobenzene-functionalized component have been built with
an additional spacer component for chemical diversity of the
A class of versitile photoswitchable molecular devices is based
on surface functionalization with azobenzenes. Incident light
induces reversible switching between the trans and the cis
conformations of the azobenzene. Light-induced changes on
azobenzene monolayers have been applied to a variety of
18,20
8,9,21,23
1
STM,
Binary SAMs with an
20
21,22,24,25
1
5,26,27
2
device,
group to that of the azobenzene,
hindered isomerization from the neat azobenzene films.
to pair the chemical funcionalization of a second
devices including optical information storage, controllable
2
8,29
3
,4
5
as well as to free up
wettable surfaces, liquid crystal alignment, and molecular
2
6,30,31
6
−9
electronic switches.
These applications all rely on the
In this study we are looking to get a molecular picture of the
azobenzene isomerization in these binary monolayers that
guide macroscopic device performance.
Vibrational sum-frequency generation (VSFG) is a surface-
selective spectroscopic technique that has previously been
shown to yield detailed information on molecular organization
electrical, mechanical, or optical property changes upon
isomerization between the cis and the trans isomers of the
azobenzene. The ultimate performance of the devices is
dependent on a number of factors that affect isomerization
such as stability, transition times and doses, and monolayer
1
0
formation. To improve these photoactive surfaces and
devices, it is first important to characterize the molecular
organization and understand critical factors affecting isomer-
ization such as packing, steric effects, and transition kinetics.
One widely utilized method of functionalizing a surface with
azobenzene is to form a self-assembled monolayer (SAM) on a
gold surface by chemisorption of a molecule with a thiol
3
2
33−35
35−37
such as chemical structure, packing,
orientation,
3
3,38−42
43,44
conformation,
and ultrafast dynamics
in molecular
monolayers on surfaces. These properties make it an
appropriate tool to characterize the surface coverage and
molecular orientation of azobenzene-containing SAMs. There
has been a limited number of papers which have utilized VSFG
to characterize the isomerization of azobenzene in monolayers
at the air−water interface
azobenzene-containing adamantane SAM on gold.
In this paper we present a novel synthesis using simple
protection chemistry to synthesize an azobenzene-function-
8
,11−14
headgroup and azobenzene-functionalized tail.
The
45−48
and only one report on
strong gold−thiolate bonds allow for high-density, aligned
49
1
1
monolayers to form well-controlled surfaces. Although not
the only method of functionalizing a surface with azoben-
4
,15
zene,
gold−thiol SAMs allow for a uniquely robust method
11,16 14,17
of coating a gold surface
or nanoparticle
with a variety
of molecules. Specifically, for binary SAMs (SAMS composed
Received: June 6, 2013
Revised: August 2, 2013
of two chemical constituents), there has been a great deal of
18
studies looking at the preferential coadsorbtive properties,
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/la402144g | Langmuir XXXX, XXX, XXX−XXX

Langmuir
Article
alized alkane thiol molecule which can be used to form SAMs
that either are 100% functionalized with azobenzene or contain
a mixture of azobenzene-functionalized thiols and simple alkane
thiol ‘spacers’. We then use VSFG to characterize the spectra of
the monolayers and quantitatively determine the relative
concentrations of azobenzene-functionalized thiol to alkane
thiol spacer. VSFG orientational analysis is applied to
investigate the mechanism of isomerization. We demonstrate
that isomerization is effectively arrested by steric hindrance in
then dissolved at a 1.2:1 molar ratio in 50 mL of acetic acid and stirred
under N for 3 days. The solid product (I) was filtered, washed with
2
water, dried, and recrystallized in ethyl acetate (EtOAc) resulting in in
3
6% yield product.
Protection, Esterization, and Deprotection. 15-Mercaptopen-
tadecanoic acid (Aldrich, 97%) and triphenylmethyl chloride (Alfa
Aesar, 98%) were dissolved in 8 mL of N,N-dimethylformamide
(
DMF) and stirred under N for 6 h. A 50 mL amount of of water was
2
added, the solution was extracted with ethyl acetate and washed with
water and brine, and the solvent was evaporated under vacuum,
resulting in product II with 67% yield. In 50 mL of 3:2
acetonitrile:DMF, 1 mol equiv of compound I, compound II, 1H-
benzotriazolium 1-(bis(dimethylamino)methylene)-5-chloro-, hexa-
fluorophosphate (1-),3-oxide (HCTU, Fluka, 98%), 4-dimethylami-
nopyridine (DMAP, Alfa Aesar, 99%) were combined. N,N-
Diisopropylethylamine (DIEA, Sigma-Aldrich, 99%) was added, and
1
00% azobenzene monolayers but proceeds readily when the
azobenze groups are separated by the spacers.
EXPERIMENTAL SECTION
Synthesis. Synthesis of the azobenzene-functionalized thiol is
accomplished in four steps (Scheme 1). First, we synthesize the
■
the solution was stirred under N for 24 h. The resultant solid (III)
2
was filtered, washed with acetonitrile, redissolved in DCM, and dried
under vacuum. Deprotection was performed by adding 1 mL of
trifluoroacetic acid to 0.2 g of compound III at 0 °C. A 100 μL amount
of triisopropylsilane was added dropwise. The solution was stirred for
Scheme 1. Synthetic Scheme for the Azobenzene-
Functionalized Alkane Thiol
a
1
0 min and dried under vacuum. The resultant product IV with 65%
yield was purified using EtOAc/hexanes column chromatography.
Formation of SAMs. Solutions were prepared of mixtures of
compound IV and 1-dodecanethiol of 1 mM total concentration in
absolute ethanol. Gold substrates were cleaned with hot, fresh piranha
solution (4:1 H SO :H O ) for 15 min and sonicated in ethanol for 2
2
4
2
2
min. The gold substrate was placed in solution for 24 h for SAM
formation. Afterward, the substrate was rinsed with ethanol and dried
with N2.
Spectroscopy. Our broad-band vibrational SFG (BB-SFG)
spectroscopy setup is based on a femtosecond Ti:Sapphire laser
system (Spectra Physics Spitfire) retrofitted with a Coherent Legend
regen cavity, which is pumped with a Nd:YLF laser (Evolution-30,
Spectra Physics) and seeded with a Ti:Sapphire oscillator (Kapteyn-
Murnane Laboratories) centered at ∼800 nm (full width at half-
maximum, fwhm ≈ 50 nm). Sixty percent of the uncompressed
fundamental output of the amplifier (4 mJ per pulse at 1 kHz
repetition rate) is sent through a compressor producing ∼60 fs pulses
(
1.8 mJ, ∼796 nm, fwhm ≈ 27 nm) and used to pump an optical
a
(
a) Oxidation of 4-aminobenzonitrile with 2 equiv of oxone (52%).
parametric amplifier (TOPAS-C, Light Conversion). The signal and
idler pulses (λ = 1.1−2.6 μm) produced from the TOPAS are mixed in
a difference frequency generator (NDFG, Light Conversion) to yield
(b) Addition of 1 equiv of 4-aminobenzoalcohol (36%). (c) Protection
of 15-mercaptopentadecanoic acid with trityl chloride (67%). (d)
Esterization between protected thiol and product I. (e) Deprotection
with trifluoroacetic acid (65% yield for d and e).
−1
tunable infrared (IR) pulses (500−4000 cm ). We are able to obtain
1
0 mW of IR centered at either 2900 (C−H stretch region) or 2200
−
1
−
1
cm (C−N stretch region) with fwhm ≈ 350 cm . The remaining
0% of the uncompressed fundamental pulse was directed into a
second compressor to produce 60 fs visible pulses, which were then
4
alcohol and nitrile-functionalized azobenzene (I) from precursors 4-
aminobenzonitrile and 4-aminobenzoalcohol.
group on the linker molecule 15-mercaptodecanoic acid is protected
with triphenylmethyl chloride (II). Third, the protected linker and
azobenzene are esterized (III). Finally a deprotection step yields the
product (IV).
Synthetic construction of the molecule is such that the nitrile group
on the azobenzene moiety acts as a convenient vibrational
chromophore, a spectroscopic marker which would change its
orientation with respect to the plane of the substrate upon
photoisomerization. Because of the high affinity of the thiol group
for the gold surface, SAMs of binary composition can be formed by
incubating a clean gold substrate in a mixed solution of the
azobenzene-functionalized thiol with dodecane thiol in ethanol at
15,50
Second, the thiol
−1
sent into a high-power air-spaced etalon (TecOptics; fwhm = 17 cm ,
51
−1
free spectral range ≈ 480 cm , and finesse ≈ 65) at 11° incidence
angle from the surface normal to produce picosecond narrow-
bandwidth pulses. IR and visible pulses were focused onto the sample
surface by a 25 cm focal length CaF lens and 45 cm BK7 lens,
respectively, to a spot size of 150 μm. Incidence angles of the visible
15
2
and IR beams are α = 63° and α = 66° from surface normal,
1
2
respectively. The laser power at the sample was 8−9 μJ per pulse for
IR and up to 20−22 μJ per pulse for the visible at a 1 kHz repetition
rate. The SFG signal was recollimated, spatially and frequency filtered,
focused onto the entrance slit of a 300 mm monochromator (Acton
Spectra-Pro 300i), and detected using a liquid nitrogen-cooled CCD
(Princeton Instruments, Spec-10:100B, 1340 × 100 pixels). We used
PPP (SFG-visible-IR), SSP, and SPS polarizations for the experiments.
Polarization of the visible beam is controlled using a zero-order quartz
half-wave plate (800 nm, CVI Melles Griot), while IR beam
1
1
various ratios while the total concentration of both thiols is 1 mM.
S y n t h e s i s o f 4 - ( 4 - ( H y d r o x y m e t h y l ) p h e n y l ) -
diazenylbenzonitrile. 4-Aminobenzonitrile(g) (Aldrich, 98%) was
dissolved in 10 mL of dichloromethane and stirred with 2 equiv of
oxone in 45 mL of water for 3 h at room temperature under N . The
polarization was controlled using a zero-order MgF half-wave plate
2
2
solution was extracted with dichloromethane (DCM) twice, and the
organic phase was washed with 1 M hydrochloric acid, saturated
bicarbonate solution, water, and brine. DCM was then evaporated
under vacuum, resulting in 76% yield of 4-nitrosobenzonitrile. 4-
Nitrosobenzonitrile and 4-aminobenzoalcohol (Aldrich, 98%) were
(150−6500 nm, Alphalas), and the SFG polarization was controlled
using a zero-order quartz half-wave plate (670 nm, CVI Melles Griot).
To eliminate polarization contamination, we used a wire-grid polarizer
(Specac) for the IR beam and a polarizing beam splitter cube
(Newport) for the SFG beam.
B
dx.doi.org/10.1021/la402144g | Langmuir XXXX, XXX, XXX−XXX

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Article
(
2)
(2) 2
Spectra of the narrow-band visible and broad-band IR pulses were
recorded using the same signal collection optics and the same
monochromator by replacing the sample surface with a gold substrate
for the total SFG signal intensity of ISFG(ω) ∝ |χ + χ | . The
NR
Res
shape of the nonresonant contribution was determined from
the SFG signal off a reference clean Au substrate. Because the
signal is proportional to surface coverage of the nitrile
chromophore, we can determine the surface fraction of
azobenzene of different samples by fitting the amplitude BCN
of the resonant term. Figure 2 shows the correlation between
(BioGold Microarray Slides, Thermo Scientific). Spectra of the
(2)
χ
Res
narrow-bandwidth visible pulse were recorded using the same grating
and CCD as for SFG detection. Spectra of the IR pulses were
measured using an IR grating blazed at 5 μm and a liquid nitrogen-
cooled mercury cadmium telluride detector (IR Associates). For all
experiments presented here, the time delay between the visible and the
IR pulses was set to maximize nonresonant SFG signal. Data was taken
also with a delay between the visible and the IR pulses for reduction of
nonresonant signal; however, this method reduced the overall signal-
to-noise ratio and is not presented.
RESULTS AND DISCUSSION
■
We prepared binary azobenzene/alkane thiol SAMs of various
fractions of azobenzene and observed their spectroscopic
properties and isomerization kinetics with VSFG. Figure 1
Figure 2. Amplitude BCN of the resonant CN stretch signal in PPP-
polarized VSFG spectra (normalized to linear relation) as a function of
the solution fraction of azobenzene-functionalized precursor molecules
used to form SAM. Different symbols correspond to different synthetic
batches and experimental preparations of the SAMs.
the amplitude BCN of the resonant signal χ(2) and the solution
Res
concentration of azobenzene-terminated thiol used to form the
SAM. If the azobenzene-terminated thiol and alkane thiol had
equal binding constants and noncooperative absorption then
(
2)
the amplitude of the resonant χ signal of the nitrile marker
Res
would be directly proportional the azobenzene mole fraction in
precursor solution. While there may be a deviation from linear
behavior at low azobenzene fractions (below 25%), possibly
indicating the cooperativity in alkane thiol adsorption to
exclude azobenzene-functionalized thiols, the correlation
between BCN and solution mole fraction is linear within the
signal-to-noise at azobenzene fraction of 50% and above used in
the measurements described below. We therefore assume that
the azobenzene mole fraction in solutions used for SAM
deposition represents the average surface coverage of the
azobenzene thiols in the formed SAMs on the surface.
Figure 1. Normalized PPP-polarized SFG spectra from a set of
different SAMs with percentage of azobenzene formulated SAMs.
Spectral peak at 2220 cm is from the nitrile group on the
functionalized azobenzene. Broad background is the nonresonant SFG
signal from gold.
−1
shows the trend of the VSFG spectra for the PPP polarization
The ratio of nitrile signals taken at different SFG polarization
combinations, PPP and SSP, can be used to determine the
ensemble orientation. Figure 3 shows both the experimentally
−1
in the nitrile stretch region (∼2220 cm ) from samples formed
with 100% alkane thiol SAM (zero solution volume percent
azobenzene functionalzed SAM) to neat (100%) azobenzene-
functionalized SAMs. The nitrile stretch at 2224 cm− appears
as a negative peak against the nonresonant background signal of
the gold substrate. The peak’s amplitude decreases as the
azobenzene percentage in the precursor solution is reduced. A
similar trend is observed in the SSP spectra presented in the
Supporting Information.
(
2)
(2)
determined χ /χ amplitude ratio for different azobenzene
PPP SSP
1
samples and the theoretical calculations described below. The
experimental values in the left side of the panel were collected
for PPP and SSP polarizations at six different dilutions of the
azobenzene on the surface. The error bars shown account for
(
2)
the error in both the fitting of the χ amplitudes and the
res
multiple measurements on the same azobenzene concentra-
tions. Below 40% azobenzene concentration the relative error
VSFG provides the ability to quantitatively determine the
concentration of a vibrational chromophore on a surface.
(
2)
in the ratio is large primarily due to the low level of the χ
SSP
52
(
2)
PPP
Spectra are fit using the standard VSFG formalism, where the
signal. There appears to be a significant difference in the χ
/
(2)
(2)
SSP
surface second-order nonlinear susceptibility χ is represented
χ
ratio measured for 100% and those of less than 100%,
as a sum of a nonresonant term χ( describing the background
2)
NR
although, within error, the mixed SAMs do not exhibit
(
2)
(2)
signal from the gold surface and a resonant Lorentzian term
significantly different response. The χ /χ ratio for 100%
PPP SSP
(
2)
χ
representing the nitrile vibrational resonance
is 21.5 ± 1.2, and for <100% it is 18 ± 1.0.
Res
From these ratios we can obtain quantatively the orientation
of the nitrile moiety relative to the surface normal using a
procedure developed by Zhuang et al. This can be
determined by comparing the ratios of the effective nonlinear
susceptibilities
(
2)
iϕ
χ
= ANR
e
(
1)
NR
3
6
(2)
BCN
χ
=
Res
(
ω − ωCN) + iΓ
(2)
C
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Article
The “L” terms are the Fresnel factors in eqs 8−10, which
depend on the light frequency. These are derived using a three-
layer model with the index of the two stacked layers, n and n ,
1
2
36
as well as the index of the monolayer represented by n ′.
2
n1(ω)cos γ
Lxx(ω) =
Lyy(ω) =
n (ω)cos γ + n (ω)cos α
(8)
(9)
1
2
2n1(ω)cos α
n (ω)cos α + n (ω)cos γ
1
2
2
⎛
⎜
⎞
⎟
2
n2(ω)cos α
n1(ω)
Lzz(ω) =
n (ω)cos γ + n (ω)cos α ⎝ n′(ω)⎠
1
2
(10)
The refractive index values used were n = 1 for all wavelength
air
values and nAu = 0.16 + 3.55i at 675 nm, nAu = 0.18 + 5.11i at
8
00 nm, and nAu = 2.66 + 31.55i at 4500 nm from the Palik
53
Handbook of Optical Constants of Solids. The effective index of
the interfacial layer n′ was taken to be 1.458, the index of bulk
dodecane thiol.
On the right-hand side of Figure 3, the calculated χ(2) /χ(2)
PPP SSP
Figure 3. (Left) Calculated nitrile χ (P 2P )P/χ(2) amplitude ratio for
different ensemble angles of the nitrile moiety off of the surface
SSP
ratio is plotted for tilt angle θ values between 0° and 90° from
surface normal. The ratio is calculated for three values of n′
normal. (Right) Experimental ratios for 100% and <100% azobenzene
(
1.358, 1.458, and 1.558) to show how differences in the index
percentages compared to the theoretical χ( /χ
2)
(2)
for samples of
PPP SSP
of the monolayer affect the estimated fit angle of the nitrile. As
a result of this procedure, we can bracket a range of
orientational angles of the nitrile. For 100% azobenzene θ =
different azobenzene percentages. Three values of n′ graphed are
bracketed around the index of bulk dodecane thiol of 1.458. Fit values
are 37−53° for 100% azobenzene and 45−60° for <100% azobenzene.
3
7−53° and for <100% from θ = 45° to 60°. Others have
(
2)
reported orientational analysis on different trans-azobenzene
SAMs using NEXAFS and quantitative XPS methods
χ
= χ L (ω)L (ω )L (ω )sin α
yyz yy
SSP
yy
1
zz
2
2
(3)
24,25
in
qualitative agreement with our study.
(
2)
−1
χ
= −χ L (ω)L (ω )L (ω )cos α cos α sin α
We also recorded the VSFG spectra in the 2900 cm region
corresponding to the C−H stretch vibrations of the saturated
alkane chains. The primary sources of signal in this region are
PPP
xxz xx
xx
1
zz
2
1
2
−
χ L (ω)L (ω )L (ω )cos α sin α cos α
xzx xx
zz
1
xx
2
1
2
2
the terminal CH stretch vibrational modes of the alkane thiol
3
+
χ L (ω)L (ω )L (ω )sin α cos α cos α
zxx zz
xx
1
xx
2
1
and the CH stretch vibrational modes of the alkane chain of
2
both the alkane- and the azobenzene-functionalized thiols. In
the PPP spectra shown in Figure 4, the primary peaks observed
for the neat alkane thiol SAMs are usually assigned to the CH3
+
χ L (ω)L (ω )L (ω )sin α sin α sin α
zzz zz
zz
1
zz
2
1
2
(4)
There is only one nonzero χ(2) tensor element contributing to
the SSP and four elements for PPP given in eqs 5−7. The
effective nonlinear susceptibilities are composed of three pieces.
The trigonometric terms depend on the angles of incidence of
the three beams: α for outgoing SFG, α for visible, and α for
IR. The χ terms are the nonzero elements of the χ tensor for
a vibrational chromophore of C point symmetry on an
azimuthally isotropic surface
1
2
(2)
∞v
37
χxxz = χyyz = 1 Nβ[cos θ(1 + r) − cos θ(1 − r)]
3
s
(
(
5)
6)
2
χxzx = χzxx = 1 Nβ[cos θ − cos θ(1 − r)]
3
s
2
3
χzzz = Nβ[r cos θ + cos θ(1 − r)]
s
(7)
Here r is the ratio of molecular hyperpolarizability tensor
elements; for chromophores of cylindrical symmetry around
the c axis, r = (β /β ) = (β /β ). The value r = 0.26 was
aaa ccc
aaa ccc
36
used for the nitrile moiety, in agreement with literature. The
tilt angle of the moiety from surface normal is represented by θ,
Figure 4. Normalized SFG spectra of PPP (top) and SSP (bottom)
polarization of samples between 0% and 100% azobenzene-terminated
C−H stretch modes.
assuming a δ-function distribution, and N β, a scaling constant,
s
which cancels out upon taking a ratio.
D
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Langmuir
symmetric stretch (2875 cm ), CH asymmetric stretch (2968
Article
−1
Figure 6 shows the amplitude of the resonant SFG signal for
the nitrile resonance for consecutive isomerization switching
3
−1
−1
cm ), and CH Fermi resonance (2930 cm ) vibrational
3
3
3,40,54
modes.
In the SSP spectra, only the CH symmetric
3
−1
−1
(
2875 cm ) and CH Fermi resonance (2933 cm ) peaks are
3
observed. This is consistent with previously reported SFG
spectra for PPP and SSP polarization of well-ordered, vertically
3
3,40,54
oriented alkane thiol SAMs.
Underlying these peaks
there are broader spectral features from the CH modes of the
2
3
3,35,41,51
alkyl chains
which are present from the alkyl chains
present in both the azobenzene-terminated thiol and the
dodecane thiol. For the high azobenzene percentage films only
CH groups are observable. The qualitative trend observed with
2
these peaks confirms the expected decrease of CH signal of the
3
alkane thiol as the solution percentage of azobenzene-
terminated thiol increases. Due to the congestion of the CH-
stretch spectral region, we did not use these spectra for
isomerization studies. No appreciable changes were detected
upon UV illumination.
Figure 6. Change in the amplitude of nitrile χ(2) for 100%, 66% ,and
Res
ANALYSIS OF ISOMERIZATION
VSFG can be also used to monitor orientational changes of the
nitrile marker upon azobenzene trans → cis or cis → trans
■
50% azobenzene SAMs taken at 10 s intervals upon isomerization with
340 nm (10 min exposure, purple triangles) light and thermal
relaxation in the dark (20 min periods, black circles). Inset shows 66%
azobenzene SAM undergoing 10 concurrent isomerization cycles.
(
2)
isomerization. The magnitude and sign of the change in χ
Res
upon isomerization of azobenzene are not trivial as they depend
on the distribution of all three orientational Euler angles of the
reactant trans-azobenzene, as detailed below. However, we were
able to track in situ the χ( resonant and nonresonant
magnitudes as well as their relative phase upon exposure to UV
light (340 nm, 4.5 mW/cm ) to both quantify the magnitude of
change upon isomerization and extract kinetic parameters for
the monolayers. Figure 5 shows VSFG spectra of the nitrile
cycles alternating between 10 min exposures to 340 nm light
and 20 min thermal relaxation in the dark for 100%, 66%, and
50% azobenzene samples. Up to 10 concurrent isomerization
cycles were observed. Although the magnitude of change for
the first cycle is somewhat larger than the concurrent cycles,
repeated reversible isomerization steps can be performed with
little observable decay in the magnitude of switching.
2)
2
For the 50% and 66% samples, there is clearly observed
(
2)
isomerization with the total change of χ of the nitrile marker
Res
(
2)
of ∼10% saturating at about 0.9 of the initial χ amplitude of
Res
the all-trans state. Interestingly, for the 100% sample there is
virtually no change of the resonant nitrile VSFG signal upon
UV illumination. Only a very slight positive vertical offset, less
than 1%, is observed which can be ascribed to some leakage of
the UV lamp illumination detected by the CCD. Thus, we
observe a nearly complete suppression of photoisomerization
for a 100% azobenzene-functionalized SAM. The most likely
explanation is steric hindrance by tightly packed azobenzene
moieties which are all held at approximately the same height
above the gold surface by the alkanethiol linker. Previous STM
studies have shown tightly packed order in neat films of
1
2
azobenzene-functionalized SAMs. On the other hand, the
alkenathiol spacers, which in our case are shorter than the
azobenzene linkers (12 vs 15 carbons per chain, Figure 5),
relieve the mutual congestion of the azobenzene head groups
and allow isomerization to occur. Overall, the observation that
pure azobenzene SAMs have hindered isomerization is
consistent with the literature that postulates that at high
packing fraction isomerization is hindered. This behavior has
Figure 5. (Left) PPP SFG spectra for the nitrile stretch band of a 66%
azobenzene film. Displayed are the experimental data for the film
under 340 nm illumination (blue) and without (red). Spectra are fit
with the nonresonant portion shown in yellow and the resonant
portion shown on the bottom. (Right) Cartoon of mixed SAM on gold
surface, formed from a mixture of azobenzene-functionalized thiol and
dodecane thiol.
3
0,55
been assigned to both the intermolecular sterics
delocalized excitonic coupling at high densities to block trans
and the
24
stretch band of a 66% azobenzene film prior to UV light
exposure and during exposure to 340 nm light. Also shown is
the decomposition of the spectrum into the Gaussian-shaped
→ cis isomerization.
(
2) 2
ISOMERIZATION KINETICS
Figure 7 shows the kinetics of the χ change due to trans →
cis photoisomerization upon UV illumination (top panel) and
thermal cis → trans recovery in the dark (bottom panel).
Kinetic traces were obtained by averaging over several UV light
exposure periods for the photoisomerization and dark periods
nonresonant signal |χ | (yellow line) and the extracted
■
NR
(
2) 2
(2)
Res
resonant contribution intensity |χ | (amplified by a factor of
Res
1
0, red and blue lines on the bottom). There was little observed
(2) 2
change of the nonresonant background |χ | correlated with
UV illumination. However, the resonant contribution, χ ,
NR
(
2)
Res
shows a reproducible decrease of ∼10% upon UV exposure.
E
dx.doi.org/10.1021/la402144g | Langmuir XXXX, XXX, XXX−XXX

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Article
ISOMERIZATION MECHANISM
■
(
2)
The magnitude of change in χ signal upon isomerization can
Res
be modeled to derive more orientational information about the
surface-bound azobenzene moieties. Some orientation models
of the of azobenzene SAMs before and after isomerization exist
in the literature. These are based on either geometrical
26
arguments extrapolating from solution-phase fitting or explicit
56
ab initio simulation of azobenzene thiol cis configurations.
However, one of the difficulties in modeling isomerization is
that the geometry of the cis-azobenzene isomer orientation
inherently has more degrees of freedom than the trans isomer.
Even a small distribution of orientational angles for the trans-
azobenzene isomer therefore results in a bi- or trimodal
distribution of orientational angles for the cis configuration
after isomerization. Here we address two questions. First, we
seek to quantify the isomerized fraction at saturation. Second,
we want to determine if there is a preferential orientation of the
resulting cis isomer relative to the surface.
The same theory used to determine the tilt angle of the
nitrile moiety on the trans-azobenzene precursor can be used to
construct a model to determine the expected change of VSFG
signal level upon isomerization from trans to various possible
cis orientations. The model assumed an initial δ-function
distribution in θ of the nitrile chromophore of the trans isomer.
This, however, leaves uncertain the torsional orientation of the
trans-azobenzene around the long axis of the molecule, i.e., the
orientation of the NN bond relative to the surface
normal (Figure 8). We approximate this uncertainty by
assuming that upon isomerization the cis-azobenene molecules
assume two possible conformations: “Up” and “Down”. In the
Up conformation, the nitrile group has rotated by 118° away
from the plane of the surface, and in the Down conformation,
Figure 7. (A) Change in amplitude of the nitrile χ (R 2e )s under 340 nm
illumination for 100%, 66%, and 50% azobenzene samples versus time.
(
2)
(
B) Change in amplitude from the nitrile χ for thermal relaxation in
Res
the dark following 10 min exposure to 340 nm light for 100%, 66%,
and 50% azobenzene samples versus time. Data sets were averaged
over multiple isomerization cycles. Solid lines show exponential fits
described in the text.
58
the isomerization occurs toward the surface. We consider
three different scenarios: (1) assumes equal ratios of Up and
Down conformations after isomerization (labeled Both in
Figure 8); (2) assumes Up isomerization for 50% of the
molecules while the remaining 50% are frozen in the trans
form, e.g., as if Down isomerization is sterically hindered; (3)
assumes only Down isomerization for 50% of the molecules
with the remaining 50% frozen. While admittedly simplistic,
this model should capture the essential bias in orientational
changes upon isomerization. An alternative scenario of free
torsional rotation is presented in the Supporting Information
for thermal recovery (Figure 6). The trans-to-cis photo-
isomerization kinetics can be fit by an exponential function of
eff p
−σ n
photon dose (n ), ΔB (n ) = A(1 − e
)). The fit of the
p
CN
p
kinetics under 340 nm illumination yields the effective
−18
2
photoisomerization cross-section σeff = 2.3 × 10
cm for
−18
2
the SAM containing 50% azobenzene and 2.2 × 10 cm for
6
6% azobenzene SAM, which is comparable to literature values
27,49,56
for similar azobenzene systems.
It is interesting to compare the trans → cis photo-
isomerization quantum yield of azobenzene on the surface
under 340 nm illumination to that in solution. For this
comparison we assume that the absorption cross section of the
surface-conjugated azobenzene is the same as that measured in
the bulk. Taking the absorption cross section from solution at
and yields similar results for the calculated change in χ( as
2)
Res
scenario (1).
(
2)
Equations 11−13 derive the expected relative change in χ
signal for PPP polarization (i.e., the ratio of χ amplitude after
Res
(
2)
Res
isomerization to that of the all-trans monolayer before
isomerization) as a function of the initial trans tilt angle θ
and the fraction of isomerized molecules at saturation η. The
−
17
2
57
3
40 nm of 4.81 × 10 cm
and dividing by our measured
−18
2
photoisomerization cross section of 2.3 × 10 cm we get a
quantum yield of isomerization for the dilute SAMs (50% or
(
2)
(2)
first equation for χ /χ
describes scenario 1 that starts
Both Trans
6
6%) of ∼5% (on average per photon per molecule), i.e., about
with an orientation θ and after isomerization goes to equal
contributions of both θ + 118° and θ − 118°. The second
one-half of the trans → cis photoisomerization quantum yield
following S to S excitation in solution, measured to be 11%.
57
(2) (2)
equation for χUp/χTrans only allows Up isomerization for one-
0
2
The cis → trans thermal isomerization kinetics can also be fit
with a single exponential with a half-life of 49 s for 50% and 95
s for 66% azobenzene SAM. Although it is the subject of further
ongoing investigations in our group, we note that the thermal
recovery kinetics observed in SAMs on gold surfaces in this
work are significantly faster than the cis → trans thermal
isomerization of azobenzene in solution which proceed on the
half of the molecules while locking the orientation for the
downward isomerizing azobenzenes. The last equation for
(
2)
(2)
χ
Down/χTrans does the reverse, allowing only Down isomer-
(
2)
ization for one-half of the molecules. The χPPP(θ) dependence
is evaluated using eqs 4−7. For these calculations the effective
index of the substrate, n′, value used was 1.458, although nearly
all of the n′ dependence cancels when solving for a change in
signal.
57
time scale of many hours.
F
dx.doi.org/10.1021/la402144g | Langmuir XXXX, XXX, XXX−XXX

Langmuir
Article
orientation and fraction of isomerized molecules η for different
scenarios: (1) Both (Orange), (2) Up (Blue), and (3) Down
(
Red). Although in these calculations the error introduced by
the assumed n′ value is small, the uncertainty in the θ value
shown in Figure 3 remains. Our analysis allows for several
possible mechanisms of isomerization. If there is little
preference between the Up and the Down cis states, then the
fraction of isomerized molecules at saturation must be very low
(
<10%). This would be possible, for example, if phase
segregation occurs between the azobenzene and alkane thiol
spacers and isomerization occurs only at the edges of
azobenzene domains. Another possibility suggested by Figure
8
is that a large fraction of molecules undergoes isomerization
but the Up conformation is strongly preferred over the Down
conformation, e.g., due to steric constraint for the Down
configuration. This would result in a strong orientational
preference of the cis-Up conformation over the cis-Down
conformation after UV exposure. Since the model assumes 50%
of the molecules undergoing Up mechanism isomerization, this
is consistent with the measured quantum yield of 5% (per
molecule per photon) in the monolayer on the surface being
approximately one-half of that in solution (11%). Other studies
have also seen a high-isomerization fraction for similar binary
8
,9
azobenzene SAMs.
CONCLUSIONS
Figure 8. Three different scenarios for isomerization used: (1) where
isomerization yields equal amounts of Up and Down states, (2) where
only the Up states are free to isomerize, (3) where only the Down
molecules can isomerize. The three cones (Both, orange; Up, blue;
■
We synthesized and characterized a novel azobenzene-
functionalized SAM. Use of the same surface attachment
chemistry as in regular thiol SAMs on gold surfaces allows us to
prepare mixed monolayers where the azobenzene moieties are
interspersed with shorter alkanethiol spacers. This approach
enables investigation of mechanistic features of isomerization
such as steric hindrance effects due to packing. Using VSFG
spectroscopy of the nitrile marker attached to the azobenzene
moiety, we were able to observe the spectral modes of SAMs of
a binary composition of the azobenzene-functionalized and
alkylated thiols and quantitatively determine the relative surface
coverage in mixed SAMs. Using the polarization-dependent
VSFG measurements of the nitrile signal we were able to
quantitatively determine molecular orientation relative to the
surface normal for different dilutions of the binary SAM.
VSFG is sensitive to the orientational changes of the
azobenzene moiety upon isomerization. Over 10 cycles of
reversible isomerization were observed for dilute azobenzene-
functionalized films. VSFG also allows measurements of the
photon dose required for photoisomerization from trans to cis
conformation and thermal relaxation rates back to trans. We
observe a pronounced steric hindrance of photoisomerization:
no conformational change occurs upon UV illumination in neat
Down, red) map out the expected χ( change upon UV exposure for
2)
Res
the three models as a function of isomerization fraction at saturation.
Cones represent the range of tilt angles θ = 45−60° of the trans
isomer derived above (3). Black horizontal bar represents the
(
2)
experimentally observed value of Δχ upon isomerization, 0.91.
Res
(
2)
χ
χ
Both
(θ, η)
(2)
Trans
1 − η)χ (θ) + η
(
2)
χ
(2)
(θ + 118°) + η
χ
(2)
(θ − 118°)
PPP
(
PPP
2
PPP
2
=
(
2)
|
χ
(θ)|
PPP
(11)
(
2)
χ
Up
(
θ, η)
(2)
χ
Trans
(
2)
η
2
(2)
η
(2)
(
1 − η)χ (θ) + χ (θ + 118°) + χ (θ)
PPP
PPP
2
PPP
=
(
2)
|
χ
(θ)|
PPP
(12)
(
100%) azobenzene films, presumably due to tight packing of
(
2)
the headgroups limiting the conformational mobility or through
excitonic coupling at high chromophore densities. However,
photoisomerization and thermal recovery readily proceed and
are reversible over multiple cycles when the azobenzene
fraction is lowered to 66% or 50% (i.e., for as little as 34% of
the shorter alkanethiol spacer). Perhaps this can be rationalized
by the large amount of excess energy deposited into the
azobenzene moiety upon 340 nm UV excitation into the S2
state (likely, over an eV above the curve crossing that leads to
χ
Down
(
θ, η)
(
2)
χ
Trans
(
2)
η
(2)
η
(2)
(
1 − η)χ (θ) + χ (θ) + χ (θ − 118°)
PPP
2
PPP
2)
2
PPP
=
(
|
χ
(θ)|
PPP
(13)
Figure 8 shows the solutions for eqs 11−13 for θ angles
within the 45−60° range derived above (Figure 3). The
5
9,60
either the cis or the trans ground state conformation
).
observed experimental value is 0.91 χ( modulation upon
2)
Lastly, orientational modeling of the VSFG signals allows us to
propose several possible isomerization mechanisms. The
change of the VSFG signal level upon UV exposure indicates
Res
isomerization, indicated by the horizontal black bar. Each cone
represents the calculated values with regard to starting
G
dx.doi.org/10.1021/la402144g | Langmuir XXXX, XXX, XXX−XXX

Langmuir
Article
that a significant fraction of the azobenzene molecules
isomerize with a cis conformation pointing away from the
surface strongly preferred. We hope that this work will help
increase the understanding of the isomerization mechanism by
highlighting the importance of steric effects, packing, and
crystallinity on the switching in dense films of actively isomeric
molecules with either azobenzene functionality or other
molecular switches that involve conformational change.
(18) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir
1
997, 13, 4502−4504.
19) Lee, L. Y. S.; Sutherland, T. C.; Rucareanu, S.; Lennox, R. B.
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893.
21) Kakiuchi, T.; Iida, M.; Gon, N.; Hobara, D.; Imabayashi, S.;
Niki, K. Langmuir 2001, 17, 1599−1603.
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(
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(
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Golledge, S. L.; Sasaki, T.; Castner, D. G.; Campbell, C. T.; Stayton, P.
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ASSOCIATED CONTENT
Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
*
S
(23) Jung, U.; Filinova, O.; Kuhn, S.; Zargarani, D.; Bornholdt, C.;
Herges, R.; Magnussen, O. Langmuir 2010, 26, 13913−13923.
(24) Gahl, C.; Schmidt, R.; Brete, D.; McNellis, E. R.; Freyer, W.;
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1
(
AUTHOR INFORMATION
Corresponding Author
E-mail: alex.benderskii@usc.edu.
■
*
3
94015.
Notes
(26) Tamada, K.; Akiyama, H.; Wei, T. Langmuir 2002, 18, 5239−
5
246.
The authors declare no competing financial interest.
(
27) Tamada, K.; Akiyama, H.; Wei, T.-X.; Kim, S.-A. Langmuir
003, 19, 2306−2312.
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2
(
ACKNOWLEDGMENTS
This research was supported by AFOSR Grant No. FA9550-09-
-0547. We thank Barry C. Thompson, Travis J. Williams, Dana
■
1
(
Mustafa, and Janet Olson for discussions and help with organic
synthesis. M.O. was supported as a summer undergraduate
researcher by NSF REU grant CHE-0552397.
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dx.doi.org/10.1021/la402144g | Langmuir XXXX, XXX, XXX−XXX