Differing Isomerization Kinetics of Azobenzene-Functionalized Self

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Differing Isomerization Kinetics of Azobenzene-Functionalized Self-Assembled Monolayers in Ambient Air and in Vacuum Thomas Moldt, Daniel Przyrembel, Michael Schulze, Wibke Bronsch, Larissa Boie, Daniel Brete, Cornelius Gahl, Rafal Klajn, Petra Tegeder, and Martin Weinelt Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01690 • Publication Date (Web): 28 Sep 2016 Downloaded from http://pubs.acs.org on October 2, 2016

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Differing Isomerization Kinetics of Azobenzene-Functionalized Self-Assembled Monolayers in Ambient Air and in Vacuum Thomas Moldt,† Daniel Przyrembel,† Michael Schulze,‡ Wibke Bronsch,† Larissa Boie,† Daniel Brete,† Cornelius Gahl,∗,† Rafal Klajn,¶ Petra Tegeder,‡ and Martin Weinelt† Fachbereich Physik, Freie Universität Berlin, 14195 Berlin, Germany, Physikalisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany, and Department of Organic Chemistry, Weizmann Institute of Science, 76100 Rehovot, Israel E-mail: [email protected]

Abstract

tion of only a few minutes in vacuum and in a dry nitrogen atmosphere, but of more than one day in ambient air. Our results suggest that adventitious water adsorbed on the SAM’s surface stabilizes the polar cis configuration of azobenzene under ambient conditions. The back reaction rate constants differing by two orders of magnitude underline the huge influence of the environment and accordingly its importance when comparing various experiments.

Azobenzene-alkanethiols in self-assembled monolayers (SAMs) on Au(111) exhibit reversible trans–cis photoisomerization when diluted with alkanethiol spacers. Using these mixed SAMs, we show switching of the linear optical and second harmonic response. The effective switching of these surface optical properties relies on a reasonably large cross section and a high photoisomerization yield as well as a long lifetime of the metastable cis isomer. We quantified the switching process by X-ray absorption spectroscopy. The cross sections for the trans–cis and cis–trans photoisomerization with 365 and 455 nm light, respectively, are one order of magnitude smaller than in solution. In vacuum the 365 nm photostationary state comprises 50–74% of the molecules in the cis form, limited by their rapid thermal isomerization back to the trans state. In contrast, the 455 nm photostationary state contains nearly 100% trans-azobenzene. We determined time constants for the thermal cis–trans isomeriza-

Introduction Self-assembled monolayers (SAMs) are frequently proposed as robust molecular coatings to tune surface properties. 1–5 The incorporation of molecular switches into SAMs allows changing their properties by light. 6–10 Azo-biphenyl derivatives 11,12 or alkanethiols ωfunctionalized with azobenzene 13–17 are commonly used to achieve this goal. Both classes of compounds form densely-packed SAMs on gold. 11,17 For azo-biphenyl SAMs the photoisomerization has been demonstrated by Pace et al. 11,18 In azobenzene-alkanethiolate SAMs the chromophore density has to be reduced in order to enable photoisomerization. 13,16,19–22

∗

To whom correspondence should be addressed Freie Universität Berlin ‡ Universität Heidelberg ¶ Weizmann Institute of Science †

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Previously we have reported on the effective photoisomerization of mixed SAMs prepared by coadsorption of an azobenzene-alkanethiol (Az11) and 1-dodecanethiol (C12) (see Figure 1) from solution: 23 The density of photoswitches in these SAMs can be adjusted by varying the relative concentrations of Az11 and C12 in the adsorption solution. The C12 molecules act as lateral spacers, effectively diluting the photoswitches, which protrude from the alkanethiolate SAM. The modification of surface properties requires that a substantial fraction of azobenzene molecules in the SAM isomerize and that optical switching dominates over thermal isomerization. Therefore, two key characteristics of photochromic SAMs are the achievable photoisomerization yield and the thermal stability of the two isomers. Optical differential reflectance (DR) spectroscopy showed a clear signal of the photoisomerization of the chromophores. In this contribution we quantify the isomerization yields and rate constants in mixed Az11/C12 SAMs by near edge X-ray absorption fine structure (NEXAFS) in vacuum, DR in air, and second harmonic generation (SHG) measurements in air and dry nitrogen. Most strikingly, the rate constant of thermal cis– trans isomerization depends strongly on the environment: Whereas thermal isomerization takes more than a day in air, it proceeds within a few minutes in vacuum or in a dry nitrogen atmosphere. Such influences of the environment on the thermal relaxation have not yet been studied systematically for azobenzene-functionalized SAMs. The reported thermal relaxation times in various environments (e.g., ambient air, nitrogen atmosphere, and vacuum) vary by as much as three orders of magnitude, 11,16,23–25 but the differences in the experiments are too large to infer a correlation with the environment. However, for azobenzenes in solution it is well-known that different solvents alter the kinetic parameters of the isomerization and the effect is attributed to the polarity of the solvent. 26,27 The influence of the environment on the isomerization of azobenzene SAMs demands

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Figure 1: 11-(4-(phenyldiazenyl)phenoxy)undecane-1-thiol (Az11) and 1-dodecanethiol (C12). careful consideration when comparing various methods. We attribute the much slower thermal relaxation in air compared with vacuum or a dry nitrogen atmosphere to water adsorbed from ambient air, which stabilizes the more polar cis state of azobenzene.

Experimental Section All samples were prepared and handled under yellow light (λ > 500 nm) to prevent photoisomerization. Mixed SAMs of 11(4-(phenyldiazenyl)phenoxy)undecane-1-thiol (Az11) and 1-dodecanethiol (C12) (see Figure 1) on Au(111) were prepared by coadsorption from a solution of both thiols, as described previously. 23 For DR, NEXAFS and X-ray photoelectron spectroscopy (XPS) the samples had a relative Az11 coverage of (20 ± 5)%; in SHG measurements samples with a coverage of (84 ± 5)% were investigated. As substrates we used 300 nm thick gold films deposited on thin sheets of mica (DR and SHG). The surface of these substrates exhibits large Au(111) terraces of a few hundred nanometers in width. 28 SAMs for NEXAFS and XPS measurements were prepared on single-crystalline gold because it has a better thermal coupling to the cryostat than gold evaporated on mica. The Au(111) single crystal surfaces were prepared following standard procedures (see Supporting Information). NEXAFS and XPS were performed at beamline UE56-2_PGM-2 of the BESSY II synchrotron facility (Helmholtz-Zentrum Berlin). The angle of incidence of the X-rays was fixed at 70◦ with respect to the surface normal. The NEXAFS spectra were taken by recording the KLL Auger electron yield as a measure of the X-ray absorption. We observed rapid thermal


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Table 1: Time constants τcth of thermal cis– trans isomerization

cis–trans isomerization in the SAM, therefore we continuously illuminated with UV light during the measurements in order to obtain spectra of mainly cis-Az11. Because X-ray beam damage to SAMs is reduced at cryogenic temperatures, 29 we performed most NEXAFS measurements at 110 K. In order to further minimize beam damage in transient NEXAFS measurements, data were acquired for 1 s in intervals of 5.5 s. During the remaining time the X-ray beam was blocked. DR spectra were recorded under ambient conditions using p-polarized light at an angle of incidence of 45◦ . More details regarding the NEXAFS and DR spectroscopy are given elsewhere. 23 For SHG measurements a Ti:sapphire laser system with a regenerative amplifier was used. The pulses at a central wavelength of 800 nm had a duration of 50 fs with a repetition rate of 300 kHz. By combining a Glan-Thompson polarizer with an achromatic half-wave plate, the pulse energy was set to 133 nJ. The secondharmonic signal was focused into a monochromator and detected by single-photon counting using a photomultiplier tube. Samples were measured under a protective-gas atmosphere (nitrogen, purity 5.0) or under ambient air. All SHG experiments were carried out at room temperature. Full experimental details are given in Ref. 30 The relative humidity in air during our DR and SHG experiments was typically in the range of 30 to 50%. For photoisomerization experiments with UV and blue light we illuminated the samples with unpolarized light at an angle of incidence of 45◦ (DR and NEXAFS) or in normal incidence (SHG). The following light sources with central wavelengths (full widths at half maximum in parentheses) and intensities on the sample were used: UV: Thorlabs LED source M365L2, 365 (9) nm, 3.4 mW cm−2 (DR), 4.1 mW cm−2 (NEXAFS), 118 mW cm−2 (SHG); Blue: Thorlabs LED source M455L3, 455 (22) nm, 16 mW cm−2 (DR and NEXAFS), Newport laser diode LQN445-50C, 445 nm, 33 mW cm−2 (SHG).

Sample SAM∗ SAM∗ SAM† SAM† SAM∗

in vacuum in vacuum in nitrogen in ambient air in ambient air

Az11 in methanol

Method

T (K)

τcth

NEXAFS NEXAFS SHG SHG DR

110 ± 5 RT‡ RT‡ RT‡ RT‡

(340 ± 50) s (170 ± 50) s ¶ (70 ± 20) s > 10 h (30 ± 5) h

UV/vis

RT‡

(70 ± 2) h

∗

20% relative chromophore coverage 84% relative chromophore coverage ‡ Room temperature ¶ Data shown in the Supporting Information †

Results Optical DR Spectroscopy To gain insight into the thermally activated cis–trans reaction in air and vacuum, optical differential reflectance (DR) spectroscopy was performed. DR spectra of a SAM under ambient conditions are shown in Figure 2a. The most prominent feature in the UV/vis spectra of azobenzenes is the S2 band. It is shifted compared with Az11 in solution due to excitonic coupling. 23 In the pristine SAM all chromophores are in the trans configuration. Upon illumination with UV light (365 nm), a pronounced signal decrease in the S2 band is observed due to trans–cis isomerization. 23 The illumination was continued until no further spectral change was observed, i.e., the photostationary state (PSS) was reached. Thereafter, we examined the thermal cis–trans isomerization by evaluating a series of spectra at the maximum of the S2 band in the pristine SAM. The resulting transient DR is shown in Figure 2b. An exponential fit yielded a time constant of more than one day (see Table 1). Subsequently, we illuminated the sample again with UV light until the PSS was reached and we brought it into a high vacuum for 90 minutes.a This led to almost complete isomerization back to the trans isomer. Successive illumination with UV light We maintained a pressure of less than 3×10−6 mbar for 50 minutes. a


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NEXAFS Spectroscopy Spectral Features In order to examine the photoisomerization of Az11 SAMs in vacuum, we performed NEXAFS spectroscopy. The experiments provide information about the molecular orientation and the electronic structure of trans- and cis-Az11 in the SAM. Figure 3a shows N 1s NEXAFS spectra of a pristine SAM and Figure 3b depicts NEXAFS spectra of the UV PSS. The prominent peak at 398 eV originates from the N 1s → π ∗ (LUMO) excitation. The intensity of this LUMO resonance depends only weakly on the X-ray polarization angleb β: The spectra recorded with the electric field vector E of the incident X-rays approximately parallel to the surface normal (corresponding to β = 10◦ ) show only a slightly higher intensity compared with the spectra with a perpendicular orientation of E with respect to the surface normal (β = 90◦ ). In mixed SAMs of Az11 and C12 the azobenzene entities protrude from the spacer layer and have a bigger free volume than in a single-component Az11 SAM. This leads to a tilt of the chromophore towards the surface of the spacer layer and a more flat-lying orientation. 23 For this geometry the N 1s NEXAFS spectra will show little dependence on the light polarization for both the trans and the cis configuration. Indeed, we observe only a small change of the polarization contrast before and after UV illumination (Figure 3a vs. Figure 3b).

Figure 2: Differential reflectance (DR) measurements of a SAM recorded under ambient conditions, with p-polarized light and at an angle of incidence of 45◦ . (a) Spectra of the S2 band, details see text. (b) Transient of the DR showing the thermal relaxation in air, recorded at 320 nm (gray bar in Figure 2a). We obtain a time constant of ca. 30 h.

However, high-resolution NEXAFS spectra of the LUMO resonance show a clear signal of photoisomerization and give meaningful insight into the electronic structure of the two azobenzene isomers. In Figure 4a we show spectra of the pristine SAM and the photostationary states under illumination with UV and blue light, respectively. The LUMO resonance in the UV PSS is shifted towards higher photon energies compared with the pristine SAM and the blue PSS. This shift indicates that the cis species absorbs at a higher photon en-

in air yielded the same PSS as in the first illumination cycle (Figure 2a), which proves that the SAM had not been degraded or altered. In addition, the thermal relaxation was slow as before. Therefore, we conclude that the thermal isomerization is strongly dependent on the surrounding.

b

The polarization angle β is the angle between the electric field vector E of the linearly polarized X-ray light and the plane of incidence.


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Figure 3: (a) N 1s NEXAFS spectra of a pristine SAM recorded for different X-ray polarization angles β. The peak at 398 eV is the N 1s → π ∗ excitation (LUMO resonance), the peaks in the 400–403 eV range are assigned to excitations into higher π ∗ orbitals. The broad feature around 407 eV is a σ ∗ resonance. Assignments are identical to those in Ref. 23. (b) Spectra of the photostationary state (PSS) under UV light exposure. The photoisomerization results mainly in a shift of the LUMO resonance to higher photon energy, which is shown in Figure 4.

Figure 4: High-resolution N 1s NEXAFS spectra of the LUMO resonance recorded at a sample temperature of ca. 110 K. (a) The pristine state (i.e., pure trans) and the photostationary states (PSSs) under UV and blue light exposures. The dashed line at 398.4 eV denotes the photon energy at which the isomerization kinetics were monitored, cf. Figure 5. (b) Spectrum of the UV PSS decomposed into its trans and cis components. (c) Constructed spectrum of the pure cis isomer. The range of uncertainty corresponds to the upper and lower limit of the amount of cis molecules given in (b). The spectrum of the trans isomer is shown for comparison.

ergy than the trans species. The spectra were recorded with an X-ray polarization angle of β = 52◦ . For this magic angle the X-ray absorption is independent of the molecular orientation.c Therefore, the intensity of a specific NEXAFS peak is proportional to the surface concentration of that species. Hence, we can decompose the spectrum of the UV PSS into a linear combination of the spectra of the trans and cis isomers. In this case the PSS contains at most 50% of the trans spectrum (otherwise, c

the remaining spectrum of the cis isomer would have negative signal contributions). A lower limit for the trans contribution results from the kinetics discussed in the next paragraph. Under the experimental conditions there remain at least 26% of the Az11 molecules in trans configuration due to thermal cis–trans isomerization. Accordingly, we obtain 50–74% cis molecules in the UV PSS. Figure 4b displays the two limiting spectral decompositions of the NEX-

See Supporting Information of Ref. 23


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AFS spectrum. By scaling the intensity of the cis component to 100%, we obtained the NEXAFS spectrum of the cis isomer within a certain range of uncertainty (see Figure 4c). In the cis configuration the LUMO resonance is broadened and shifted to higher photon energies by 0.2 eV. This peak shift is compatible with the observed binding-energy shift of the N 1s core level upon photoisomerization (see Supporting Information). When illuminating with blue light, the spectrum of the PSS is almost identical to that of the pristine SAM. Therefore, we can conclude that nearly all molecules in the SAM isomerized back to trans, whereas in solution in the blue PSS a sizable fraction of 27% of the molecules are in the cis form (see Supporting Information).

Figure 5: Transient NEXAFS measurements on a SAM at ca. 110 K, recorded at an X-ray polarization angle of β = 90◦ . (a) Data as measured. The general decrease in the signal by ca. 15% over 1 h is caused by X-ray beam damage and was described by an exponential fit (red line); details are given in the Supporting Information. (b) Corrected data. Each segment of the kinetics was fitted with a single-exponential function (pink, green, and blue lines).

Isomerization Kinetics The energetic difference between the LUMO resonance of the trans and the cis isomer can be exploited to examine the isomerization kinetics in vacuum. At a photon energy of 398.4 eV (cf. dashed line in Figure 4) the cis isomer has a higher absorption than the trans isomer. Figure 5 shows the transient of the X-ray absorption at this photon energy while illuminating with UV and blue light and upon thermal relaxation. In each segment the X-ray absorption evolves nearly exponentially towards a new stationary state (see Figure 5a). Additionally, we observed a general decrease of the signal over time due to X-ray beam damage of the SAM. This was corrected by subtracting a suitable background (for details see Supporting Information). The result is shown in Figure 5b. We assume that the cis–trans equilibria (photostationary states) are established by isomerization reactions that all obey first-order kinetics. The general differential rate equation depends on the respective numbers of cis and trans molecules Nc and Nt : dNc = − kcUV/blue + kcth Nc (t) dt UV/blue + kt Nt (t)

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UV/blue

is the trans–cis photoisomerizaHere kt tion rate constant for illumination with UV or UV/blue blue light, kc is the analogous cis–trans photoisomerization rate constant, and kcth is the thermal cis–trans isomerization rate constant. Assuming that the isomerization does not alter the total number of chromophores on the surface (Nt + Nc = const.), we get the following exponential function (for details see Supporting Information): UV/blue

Nc (t) = A exp(−keff

t)

(2)

with an amplitude A and the effective rate conUV/blue stant keff : UV/blue

keff

UV/blue

= kt

+ kcUV/blue + kcth

(3)

Each of the kinetic curves in Figure 5b was fitted with a single-exponential decay according to (2). We determined the thermal cis–trans isomerization rate constant from the "dark" seg-

(1)


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(a)

σ=

k Eph =k , J Iph

Azobenzene SAM (in air)

1.00

SHG Intensity (normalized)

ment of the transient NEXAFS intensity,d at ca. 110 K as well as at room temperature (ca. 293 K, data see Supporting Information). The resulting time constants τcth = 1/kcth are (340 ± 50) s and (170 ± 50) s, respectively. Note that these time constants are more than two orders of magnitude smaller than in air (see Table 1). Additionally, the thermal relaxation at room temperature is faster than at 110 K, as one would expect. Applying the Arrhenius equation yields a potential barrier of 10 meV and an attempt frequency of 10−2 s−1 for the thermal cis–trans isomerization in the SAM. Both values seem implausibly small: For another azobenzene derivative adsorbed on Au(111) a barrier of 250 meV and an attempt frequency of 106±1 s−1 has been reported. 31 This discrepancy indicates that the thermal isomerization of Az11 in the SAM cannot be described by a simple Arrhenius model. From the single-exponential fits of the "UV" and "blue" segments in Figure 5b we determined the effective isomerization rate constants UV/blue keff under illumination with either UV or blue light. The photoisomerization cross sections (see Table 2) were calculated according to

0.95

blue PSS

1.5 h

0.90

Time

0.85 0.80

UV PSS

0.75

0

2

Illumination steps

4

6

(b) SHG Intensity (normalized)

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1.00 0.95

Azobenzene SAM (N2 atmosphere)

0.90 0.85

0.1 h Time

tUV PSS = 70 ± 20 s

0.80 0

2

Illumination steps

Figure 6: (a) Reversible photoswitching of the non-linear optical (NLO) response of an azobenzene SAM under ambient conditions (in air). Changes in the SHG signal amplitude upon illumination with UV or blue light (365 nm and 445 nm, respectively) demonstrate the light-induced reversible changes in the NLO interfacial response due to the trans–cis isomerization. When the UV light source is switched off after obtaining the UV PSS, the signal amplitude remains constant for at least 1.5 h. (b) In a nitrogen atmosphere thermally induced cis–trans isomerization occurs with a time constant of ca. 70 s at room temperature.

(4)

with the rate constant k, the photon flux J, the photon energy Eph , and the light intensity Iph . σtUV and σtblue are the cross sections of the trans–cis photoisomerization processes under UV and blue light, respectively; whereas σcUV and σcblue stand for the cis–trans photoisomerization cross sections under UV and blue light, respectively. These cross sections were UV/blue calculated using the rate constants kt and UV/blue kc , which were determined from the effecUV/blue tive rate constants keff , the isomerization yields, and the thermal isomerization rate constant kcth (see Supporting Information). In conUV blue trast, the effective cross sections σeff and σeff characterize how fast the system approaches the

photostationary state and depend on the experimental conditions, i.e., photon flux and samUV blue ple temperature. σeff and σeff are calculated directly from the respective effective rate conUV/blue stants keff .

Second-Harmonic Response In addition, we examined the photoisomerization of a SAM with a larger Az11 coverage of 84% by SHG. We used the SHG response to compare the isomerization rate constants in air and in a dry nitrogen atmosphere. Figure 6a shows the SHG intensity as a function of the illumination steps measured under ambient con-

d

In Figure 5b the asymptote observed in the "dark" segment is different than the asymptotes in the following "blue" segments. This might indicate deviations from first order kinetics.


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Table 2: Photoisomerization Cross Sections Cross sections‡ σcUV σtblue σcblue −18 2 −18 2 (10 cm ) (10 cm )

Sample

Method

σtUV

SAM in vacuum∗ Az11 in methanol†

NEXAFS UV/vis

0.9 ± 0.2 27 ± 2

0.2 ± 0.2 0.82 ± 0.06

0.05 ± 0.01 1.8 ± 0.1

1.0 ± 0.3 5.2 ± 0.3

Effective cross sections¶ UV blue σeff σeff −18 2 (10 cm ) 1.4 ± 0.2 27 ± 2

1.2 ± 0.3 7.0 ± 0.3

∗

SAM with 20% relative chromophore coverage, measured at (110 ± 5) K in order to reduce X-ray beam damage measured at room temperature, data shown in the Supporting Information. UV/blue UV/blue ‡ Cross sections of the single processes: σt and σc are the trans–cis and cis–trans photoisomerization cross sections under exposure of UV and blue light, respectively. ¶ The effective photoisomerization cross sections are significantly larger than the cross sections of the single processes because they are calculated from effective rate constants, which consist of the thermal isomerization rate constant and the photoisomerization rate constants for both directions, cf. Equation 3. †

ditions. A large second-order non-linear optical (NLO) contrast upon switching between the trans and the cis isomer was observed. The signal amplitude changed reversibly by ca. 16% between the two PSSs. When generating the cis isomer in the PSS via UV light exposure and leaving the sample in the dark for 1.5 h, the SHG-signal amplitude remained constant, which indicates that the UV PSS was thermally stable on this timescale. Thus, we can assume a relaxation time constant of more than 10 h. This result is in agreement with the time constant obtained from DR spectroscopy in air (see Table 1). Repeating the SHG experiment under dry nitrogen atmosphere strikingly illustrated that the UV PSS is thermally unstable if the illumination is stopped (see Figure 6b): We observed a thermal relaxation within a few minutes, comparable to the result obtained from the NEXAFS measurement in vacuum.

time constants on the order of only minutes. Both vacuum and nitrogen are water-free environments, unlike ambient air. Thus, we attribute the large difference in the thermal relaxation times between air and vacuum or the dry nitrogen atmosphere to the presence of adventitious water on the sample in air: In the cis configuration Az11 has a larger permanent dipole moment than in the trans configuration (1.0 D vs 2.6 D). 25 Therefore, in the cis state the SAM is more hydrophilic and under an ambient atmosphere water should be present on the surface, stabilizing the cis state and thus hindering the thermal isomerization. Upon applying vacuum, the water desorbs from the SAM, which enables the fast thermal isomerization to the trans form. The full reversibility of the stabilization and the absence of any changes in photochromism, e.g., when changing the sample environment from ambient air to vacuum and vice versa (cf. Figure 2) rule out that the stabilizing effect is caused by (irreversible) chemisorption of airborne contaminants or oxidation reactions in the SAM. The interpretation of adventitious water being responsible for the stabilization of the cis state is supported by an earlier experiment on an almost identical SAMe where also a thermal relaxation constant on the order of minutes was observed in vacuum. 24 In contrast, for another mixed SAM consisting of a p-CN-

Discussion Using DR spectroscopy and SHG measurements, we examined the thermal cis–trans isomerization of Az11 in SAMs under ambient conditions. We determined a time constant of many hours. However, after exposing the switched SAM to high vacuum for approximately one hour, an almost complete isomerization back to the trans isomer was observed. From NEXAFS measurements in vacuum and from SHG measurements in a dry nitrogen atmosphere we obtained thermal isomerization

e

A mixed SAM of Az12 diluted with C12 as spacer. Az12 is identical to Az11 but has a dodecamethylene linker chain.


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ported for exposure to UV light. 13,16,25 These results are in line with our findings.f The effective cross sections determined for azobenzene attached to bulky molecular "platforms" on gold are on the order of 10−17 cm2 , similar to the value in solution. 32 In vacuum we determined a fraction of 50– 74% cis isomers in the PSS under UV light exposure. Note that this photoisomerization yield is limited by the rapid thermal relaxation. A higher fraction of 67–100% cis molecules would be obtained for conditions where the thermal isomerization could be neglected over the photoisomerization (see Supporting Information). This would be the case in air, or for a higher UV photon flux on the sample. This indicates that the vast majority of the azobenzene units in the mixed SAMs maintain their ability to isomerize.

substituted azobenzene and C12 as a spacer, no stabilization of the cis state was observed under ambient conditions. 16 In this case the cis state might not be stabilized by adventitious water because, in contrast to the H-Azo compound, the CN-Azo compound has a smaller dipole moment in the cis state than in the trans state. 24 In solution the influence of the solvent on the isomerization of azobenzenes is well-known: For 4-methoxyazobenzene, a chromophore similar to Az11, the thermal isomerization is twice as fast in the nonpolar solvent benzene as in the polar solvent methanol. 26 However, the thermal isomerization rate constants for Az11 SAMs in vacuum exceed those in air by more than two orders of magnitude. This effect cannot be understood on the single molecular level, i.e., by comparing non-interacting azobenzene molecules in vacuum 27 and in solvents with different polarity. 26 In the latter cases relatively small changes of the thermal cis–trans isomerization rate constant were observed. In the SAM there are significant lateral interactions among the azobenzene chromophores, as evidenced by the excitonic coupling and the suppressed photoisomerization in pure Az11 SAMs. 23 In addition, upon dilution we observed a tilting of the chromophores towards the SAM surface, likewise indicating a stabilization by intermolecular interactions. 23 For the SAM in vacuum or in a dry nitrogen atmosphere we conclude that these lateral interactions promote the fast thermal isomerization and thus cause a destabilization of the cis form. However, under ambient conditions the thermal back reaction from the cis to the trans state is slow. Here the formation of a water adlayer at the SAM surface is a plausible scenario which would explain the stabilization of the cis form via dipole– dipole interactions modifying the potential energy landscape. Further systematic investigations on the role of adsorbed water, both experimental and theoretical, are necessary to reveal the microscopic process in more detail. The photoisomerization cross sections of Az11 in the SAM are about one order of magnitude smaller than those in solution. For mixed azobenzene/spacer SAMs on gold, effective cross sections of (2–4) × 10−18 cm2 were re-

Summary We have shown that the thermal isomerization in azobenzene SAMs is strongly dependent on the environmental conditions. We observed thermal relaxation times on the order of minutes in vacuum and in a dry nitrogen atmosphere. The thermal relaxation in air is hindered, occurring on a timescale of more than one day. We attribute this observation to the presence of adventitious water on the SAM under ambient conditions, which stabilizes the more polar cis configuration of the azobenzene chromophores in the SAM. The photoisomerization of the mixed SAM is very efficient. We obtained a yield of 50–74% for the photoisomerization to the cis state under UV light in vacuum. This yield is limited by the rapid thermal relaxation. For the reverse process induced by blue light we determined a yield of approximately 95% trans-Az11. The photoisomerization cross sections of azobenzene in the SAMs are about one order of magnitude f

Note that in Ref. 16 the thermal isomerization rate constant is on the same order of magnitude as the effective trans–cis photoisomerization rate constant observed under UV light exposure; therefore, the cross section σtUV would be significantly smaller than the effective cross section given in that paper.


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smaller than those of free Az11 molecules in solution. The hindered thermal relaxation in air allows us to effectively tune and reversibly switch the surface properties by light, as exemplified here by the linear and non-linear optical response of the SAMs.

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monolayers: methods and sensor applications. Chem. Soc. Rev. 2011, 40, 2567– 2592. (6) Ferri, V.; Elbing, M.; Pace, G.; Dickey, M. D.; Zharnikov, M.; Samorì, P.; Mayor, M.; Rampi, M. A. Light-powered electrical switch based on cargo-lifting azobenzene monolayers. Angew. Chem., Int. Ed. 2008, 47, 3407–3409.

Supporting Information Available: Gold crystal preparation, additional NEXAFS and XPS spectra, derivation of isomerization kinetics, isomerization kinetics of Az11 in methanol. This material is available free of charge via the Internet at http://pubs.acs.org.

(7) Crivillers, N.; Orgiu, E.; Reinders, F.; Mayor, M.; Samorì, P. Optical modulation of the charge injection in an organic field-effect transistor based on photochromic self-assembled-monolayerfunctionalized electrodes. Adv. Mater. 2011, 23, 1447–1452.

Acknowledgement Support by the Deutsche Forschungsgemeinschaft through the Sfb 658—Elementary Processes in Molecular Switches at Surfaces and the Helmholtz Virtual Institute—Dynamic Pathways in Multidimensional Landscapes is gratefully acknowledged.

(8) Smaali, K.; Lenfant, S.; Karpe, S.; Oçafrain, M.; Blanchard, P.; Deresmes, D.; Godey, S.; Rochefort, A.; Roncali, J.; Vuillaume, D. High onoff conductance switching ratio in optically-driven self-assembled conjugated molecular systems. ACS Nano 2010, 4, 2411–2421.

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Differing Isomerization Kinetics of Azobenzene-Functionalized Self

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