Tailoring the Properties of Surface-Immobilized Azobenzenes byMonolayer Dilution and Surface CurvatureThomas Moldt,† Daniel Brete,† Daniel Przyrembel,† Sanjib Das,‡ Joel R. Goldman,‡ Pintu K. Kundu,‡

Cornelius Gahl,*,† Rafal Klajn,*,‡ and Martin Weinelt*,†

†Fachbereich Physik, Freie Universitat Berlin, Arnimallee 14, 14195 Berlin, Germany‡Department of Organic Chemistry, Weizmann Institute of Science, 76100 Rehovot, Israel

*S Supporting Information

ABSTRACT: Photoswitching in densely packed azobenzeneself-assembled monolayers (SAMs) is strongly affected by stericconstraints and excitonic coupling between neighboringchromophores. Therefore, control of the chromophore densityis essential for enhancing and manipulating the photoisomeriza-tion yield. We systematically compare two methods to achievethis goal: First, we assemble monocomponent azobenzene−alkanethiolate SAMs on gold nanoparticles of varying size.Second, we form mixed SAMs of azobenzene−alkanethiolatesand “dummy” alkanethiolates on planar substrates. Bothmethods lead to a gradual decrease of the chromophore densityand enable efficient photoswitching with low-power lightsources. X-ray spectroscopy reveals that coadsorption from solution yields mixtures with tunable composition. The orientationof the chromophores with respect to the surface normal changes from a tilted to an upright position with increasing azobenzenedensity. For both systems, optical spectroscopy reveals a pronounced excitonic shift that increases with the chromophore density.In spite of exciting the optical transition of the monomer, the main spectral change in mixed SAMs occurs in the excitonic band.In addition, the photoisomerization yield decreases only slightly by increasing the azobenzene−alkanethiolate density, and weobserved photoswitching even with minor dilutions. Unlike in solution, azobenzene in the planar SAM can be switched backalmost completely by optical excitation from the cis to the original trans state within a short time scale. These observationsindicate cooperativity in the photoswitching process of mixed SAMs.

■ INTRODUCTION

Self-assembled monolayers (SAMs) are prime candidates forthe modification of surface properties such as polarity, chemicalreactivity, or charge transfer characteristics at interfaces.1−6

Integration of molecular switches into SAMs is an importantissue since it opens the possibility to reversibly change theseproperties by external stimuli, e.g., light.7−9

Azobenzene represents the most commonly used andinvestigated molecular switch.10−13 However, directly adsorbedon a metal surface, it exhibits strong substrate-inducedquenching of the photoisomerization yield.14,15 Therefore,effective decoupling of the switch from the substrate isrequired.16 A promising approach is the use of alkyl chains aslinkers between the chromophore and the surface,17−19 astrategy we also pursue in this work. Besides the verticaldecoupling of the photoswitch from the substrate, one has toaccount for lateral intermolecular interactions within the SAM.The trans−cis isomerization of azobenzene involves largegeometrical changes. In addition, excitonic coupling amongthe azobenzene molecules in the SAM modifies the opticalproperties of the ensemble.20 As a consequence, sterichindrance and excitonic band formation are expected tostrongly influence the photoisomerization yield.19,21,22 Both

effects can be analyzed and manipulated by tuning the densityof the chromophores. For this purpose we employed twostrategies: First, we studied SAMs of azobenzene−alkanethio-lates on the curved surface of gold nanoparticles (NPs). Placingchromophores on curved surfaces can decrease the chromo-phore density and thereby enhance the photoisomerizationefficiency.13,23,24 As illustrated in Figure 1a, the averagechromophore distance increases with decreasing NP size.Consequently, we examined 11-(4-(phenyldiazenyl)phenoxy)-undecane-1-thiol25−27 (Az11, structural formula shown inFigure 1b) bound to gold nanoparticles of different sizes.Irrespective of the NP size, we observed pronouncedphotoswitching, with the changes in optical spectra approx-imately as large as those for free molecules in solution. Second,on a planar gold substrate, simple (unfunctionalized)alkanethiolate ligands were incorporated into the azobenzeneSAM as lateral spacers to decrease the average density of thechromophores. The static dilution of chromophores in planarSAMs has been studied using an asymmetrical disulfide

Received: November 4, 2014Revised: December 18, 2014Published: December 29, 2014

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consisting of an azobenzene-terminated and an unfunctional-ized alkanethiolate counterpart; Tamada and co-workersshowed that upon adsorption on planar gold the S−S bondbreaks and leaves behind two thiolates, corresponding to a 50%dilution of azobenzene.28 A variable dilution of the azobenzenechromophores is possible by coadsorption from a solution oftwo thiols.13,29 The main problem of using this approach is thatone must ensure proper mixing of the two structurally differentconstituents on the surface. Sometimes the different thiolatessegregate and form islands,30,31 or one component is even fullydisplaced from the surface.32 An earlier publication showedindications of photoswitching in a mixed SAM where theazobenzene chromophores were strongly diluted.33 Recently,azobenzene SAMs of different dilutions have been prepared bycoadsorption of two thiols: Photoisomerization in the resultingmixed SAMs was observed using surface plasmon resonancespectroscopy,29 photoelectrochemical measurements,29,34 vibra-tional sum-frequency generation,35 scanning tunneling micros-copy,36 and surface-enhanced Raman spectroscopy.37

In this work we applied X-ray photoelectron spectroscopy(XPS), near-edge X-ray absorption fine structure (NEXAFS)spectroscopy, and UV/vis differential reflectance (DR) spec-troscopy in order to examine bicomponent SAMs of Az11 and1-dodecanethiol (C12) on planar gold substrates, prepared bycoadsorption from solution. XPS allowed us to determine therelative Az11 coverage in the bicomponent SAMs. Thecoverage could be tuned between 0 and 100% by adjustingthe mole fractions in solution, despite an observed preferentialadsorption of Az11. UV/vis spectroscopy of both curved andplanar SAMs revealed pronounced excitonic shifts of the opticalabsorption bands compared with the free molecule. This effectincreases with increasing chromophore density, which indicatesthe tunability of the coupling between the chromophores andthus their optical properties. On planar SAMs we found hints ofsmall Az11 aggregates at low azobenzene densities already.However, segregation of the mixed planar SAMs into large C12and Az11 domains did not occur. This was corroborated bydetermining the molecular orientation using NEXAFS spec-troscopy. Upon dilution of Az11 with C12 the azobenzene

moiety tilts toward the surface plane, which cannot occur indensely packed homogeneous domains. In curved SAMs onsmall NPs the formation of aggregates is likely inhibited byintercalation of solvent molecules.The mixed planar SAMs exhibited reversible photoisomeriza-

tion, in contrast with the single-component Az11 SAM. The cisand trans photostationary states could be reached withinminutes, using power densities of a few mW cm−2 that could bereadily delivered by LEDs. The cis form was stable for hours inthe dark under ambient conditions, and mixed SAMs could beswitched for several cycles without appreciable fatigue. Despitethe fact that we illuminated the diluted SAMs with a photonenergy corresponding to the main absorption band of Az11 insolution, we always observed the largest spectral change in itsblue-shifted aggregate band. On the basis of these results, weconclude that the photoisomerization of Az11 in mixed SAMstakes place in a stepwise cooperative process.

■ EXPERIMENTAL SECTIONAll experiments involving azobenzene compounds were carried outunder yellow light with a cutoff wavelength of 500 nm, well above theabsorption bands relevant for azobenzene photoisomerization. Samplepreparation, DR measurements, and illumination experiments wereconducted under ambient conditions. Az11 and 11-phenoxyundecane-1-thiol (P11, cf. Figure 1b) were synthesized as described in theSupporting Information. 1-Dodecanethiol (98%, C12) was used asobtained from Alfa Aesar.

NP Synthesis. Fairly monodisperse gold NPs of various diameters(≈ 2.5−12 nm) were synthesized using a previously describedtechnique.38 Briefly, 2.58 nm NPs were prepared by reducing a toluenesolution of HAuCl4 with tetrabutylammonium borohydride in thepresence of surfactants (see Supporting Information for detailedprocedures). These small NPs were functionalized with thiols or usedas seeds for the synthesis of larger particles, up to 12 nm in diameter.As-prepared NPs were stabilized with dodecylamine (DDA) anddidodecyldimethylammonium bromide (DDAB)weakly boundligands that could readily be displaced with thiols in a place-exchangereaction. The ligand exchange did not affect the sizes of the particles.

Preparation of Curved SAMs on NPs. The obtained NPs werefunctionalized with monocomponent monolayers of either Az11 orP11. Transmission electron microscopy (TEM) images of thefunctionalized NPs are shown in Figure 2. Following precipitationand a thorough washing to remove any excess of unbound thiols, wetested the solubility of Az11-functionalized NPs in a variety ofsolvents. We chose chloroform as optimal solvent because it stabilizesNPs coated with both trans and cis isomers of Az11; i.e., the UV-induced isomerization is not accompanied by aggregation of NPs.39−41

Unfortunately, even chloroform could not dissolve Az11-function-alized NPs larger than 8 nm, for which no suitable solvent wasidentified. This poor solubility can be attributed to the densely packedmonolayers of π−π stacked azobenzene groups that the solventmolecules could not solvate.a

Preparation of Planar SAMs. We prepared SAMs on 300 nmthick gold films on thin sheets of mica that had been annealed aftergold deposition (Georg Albert, PVD coating). The polycrystallinesurfaces exhibit large Au(111) terraces of a few hundred nanometers inwidth.42

For SAM preparation, immersion solutions of a 0.1 mM total thiolconcentration in methanol were prepared by mixing and diluting theappropriate amount of stock solution of each component. Afterimmersion for 20 h, the samples were copiously rinsed with methanoland blown dry with argon. XPS and DR measurements were carriedout on twin samples cut from the same piece of gold/mica substrateand immersed back-to-back into the same solution. The concentrationof the thiol solutions can be assumed to remain constant during SAMformation because a 150-fold excess of thiol was employed.b

In order to verify the quality of the SAMs, sulfur 2p XP spectra wererecorded for all samples. From samples examined at the synchrotron

Figure 1. (a) Schematic representation of azobenzene−alkanethiolateson a flat gold surface and on the curved surfaces of two differentlysized nanoparticles. The density of chromophores is largest for theplanar substrate and decreases with decreasing NP diameter. (b)Structural formulas of the compounds used in this work.

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source (for the spectra see Supporting Information), we couldconclude that the main contribution to the spectra originated from thegold-bound thiols, as expected. The peak area commonly attributed toatomic sulfur was well below 10% of the total peak area, whereas therewas no indication for unbound thiol. For the samples examined withthe lab-based XPS setup, contributions of atomic sulfur and unboundthiol were below the S 2p detection limit of 10% of the total peak area.X-ray Spectroscopy. XPS and Auger yield NEXAFS spectroscopy

were carried out at the beamline UE56-2_PGM-2 of the synchrotronfacility BESSY II (Helmholtz-Zentrum Berlin), using an ultrahigh-vacuum (UHV) apparatus described in an earlier work.19 Additionally,lab-based XPS measurements were performed at room temperature ina UHV setup consisting of a mu-metal chamber equipped with amonochromatized X-ray source (VG Scienta MX650, Al KαI, hν =1486.7 eV) and a high-resolution electron analyzer (VG Scienta SES-200). The background pressure during all measurements was below 2× 10−10 mbar. The radiation was focused on a sample area of 3 × 9mm2. The samples were oriented toward the detector for a meantakeoff angle of 65° with respect to the surface normal. The totalenergy resolution was better than 400 meV. All XP spectra werereferenced to the substrate Au 4f7/2 peak at a binding energy of 83.96eV.44 To evaluate peak positions and peak areas, the signals were fittedwith Voigt line profiles and Shirley45 backgrounds. To compensate forX-ray intensity losses due to the aging of the laboratory X-ray source,the spectral intensities were corrected according to regularly measuredreference samples.UV/Vis Measurements. Absorption spectra of Az11 in methanol

were recorded at a concentration of approximately 0.03 mM in cellswith an absorption length of 10 mm. Differential reflectance (DR)spectra of SAMs were measured using a PerkinElmer Lambda 850spectrometer. For this purpose a reflection unit was designed, whichallows measurements using s- and p-polarized light to get insight intothe average molecular orientations. The SAM-induced change inreflectivity was determined from the difference between the reflectivityof the gold substrate before and after SAM preparation (seeSupporting Information). Illumination experiments on Az11 insolution and on SAMs with normal incidence were carried out usingtwo LED sources with the following central wavelengths and photonenergies (full width at half-maximum in parentheses) and intensities

on the sample: 365 (9) nm, 3.40 (0.08) eV, 7 mW cm−2 and 460 (26)nm, 2.70 (0.15) eV, 12 mW cm−2. Chloroformic solutions of NPs wereilluminated with a mercury discharge lamp (365 nm, 1 mW cm−2).

UV/vis spectra of nanoparticle solutions were recorded on aShimadzu UV-2700 spectrophotometer using standard quartz cellswith an absorption length of 10 mm. Spectra of P11-coatednanoparticles were subtracted from those of Az11-coated nanoparticles(see details below).

■ RESULTS AND DISCUSSION

Curved SAMs on Nanoparticles. Optical Properties. TheUV/vis absorbance spectra of Az11-decorated NPs (Az11-NPs)and free Az11 are shown in Figure 3a in the range of thedominating azobenzene S2 (π−π*) absorption band at 3.5 eV(354 nm). A comparable contribution to the absorbance of NPsoriginates from the localized surface plasmon resonance(LSPR) at a photon energy of 2.4 eV (520 nm),46,47 whichincreases with increasing NP diameter and masks the weak S1absorption band of Az11 at 2.7 eV (460 nm). To separate theoptical response of the surface-bound chromophores from thatof the gold NPs, we functionalized the same batches of NPswith a nonabsorbing ligand. For this purpose, the phenoxy-terminated alkanethiol (P11) was chosen because it endows theparticles with excellent solubility in chloroform and isstructurally similar to Az11 (cf. Figure 1b) while not absorbing

Figure 2. Transmission electron microscopy (TEM) images of Az11-functionalized gold nanoparticles of different diameters. All imageswere taken at the same magnification; the scale bars correspond to 10nm. The errors given are the standard deviations in the NP diameterdistribution.

Figure 3. UV/vis spectra of gold nanoparticles (NPs) with differentdiameters, in comparison with free Az11 in chloroform (dashed): (a)Az11-functionalized NPs, normalized to the S2 band height; offsetswere added. The peak at ≈2.4 eV (520 nm) originates from a localizedsurface plasmon resonance (LSPR of gold NPs). (b) P11-function-alized NPs, normalized to the LSPR band heights in (a). (c)Photoswitching experiment on Az11-functionalized NPs, pristine(black) and the photostationary state (PSS) at 365 nm (gray). Thesignal originating from the surface plasmon resonance has beensubtracted, and the resulting difference spectra were normalized to theS2 peak height.

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light above ≈280 nm. In the spectra of these P11-functionalizedNPs (Figure 3b) the optical response originates from themetallic cores of the particles, which is altered by the SAMacting as a dielectric layer. We subtracted this “background”from the spectra of Az11-decorated NPs for varying curvatures(the solid lines in Figure 3c).To determine the number of adsorbed molecules, N = 4πR2/

F0, on a NP of radius R, we assumed a footprint of F0 = 0.217nm2 of alkanethiolates25 on gold(111).c Then we calculated thechromophore density, ρ = N/4π(R + d)2, in a sphere of radiusR + d, where d is the distance between the chromophore andthe NP surface. For d we chose the distance between the Auatom of the substrate and the “lower” N atom in the diazobridge (Figure 1b) of 2.25 nm, assuming that the moleculeshave a stretched upright geometry.In Figure 4 the S2 band position maxima for pristine NPs

(obtained from Figure 3c) are plotted versus the chromophoredensity, ρ. The S2 band shifts to higher energies with increasingchromophore density. This hypsochromic (blue) shift isattributed to the formation of H-aggregates, as discussed lateron in the comparison of excitonic shifts in planar and curvedSAMs.

Photoisomerization. We examined the trans−cis photo-isomerization of Az11 on NPs and compared it to thephotoisomerization of free Az11 in solution. The bottom partof Figure 3c shows the absorbance spectrum of trans-Az11 insolution and a spectrum after illumination with UV light (365nm). The UV light triggers the trans-to-cis isomerization. Thisleads to a weakening and a hypsochromic shift of the S2 band,whereas the S1 transition appears slightly stronger. Because theS2 bands of trans and cis isomers partially overlap, only aphotostationary state (PSS) with residual 10−15% of the transisomer is achieved.18

Also on the NPs we observed pronounced photoisomeriza-tion of Az11. The photoinduced relative change in the S2 bandintensity is very similar to that of free Az11, indicating acomparable photoswitching efficacy. For NPs of smallerdiameters, the relative change of the S2 band increases onlyslightly. Thus, it can be concluded that a lower chromophoredensity results in a little more effective photoswitching. Themaximum density studied on the NPs corresponds to around

43% dilution of a SAM on a flat gold substrate. Therefore, wesystematically tested mixing Az11 and C12 to create SAMs witha chromophore density similar to the density on NPs and toenable efficient photoswitching.

Structure and Optical Properties of Planar Bicompo-nent SAMs on Au(111). Composition of Mixed SAMs.Bicomponent SAMs on Au(111)/mica were made from mixedsolutions of Az11 and C12, with varying mole fractions χsol ofAz11. We analyzed the miscibility and component ratio of thetwo thiol species in SAMs by XPS of the N 1s core level. Asdepicted in Figure 5, the Az11 diazo bridge gives rise to a singlephotoemission peak. Its intensity decreases upon dilution ofAz11, whereas its energetic position shifts continuously towardhigher binding energies (for a plot of the peak positions, seeSupporting Information). The change in binding energy resultsfrom electrostatic interaction of the molecules with theirenvironment. The continuous shift indicates largely statisticalmixing of the two thiol species: In the case of two-dimensionalisland growth, we could not obtain a continuous shift of theXPS binding energy when we changed the Az11 coverage from0 to 100%, as substantiated in a previous study on mixed SAMsof azobenzene-alkanethiols with fluoromethyl and cyano endgroups.50

The N 1s XPS peak area represents the relative Az11coverage Θ of a mixed SAM, when normalized to the peak areaof a single-component Az11 SAM. Nitrogen atoms are only

Figure 4. S2 band position on NPs and in mixed SAMs versuschromophore density, ρ. The S2 band position of Az11 in solution isadded for comparison. All data points correspond to absorptionmaxima directly read from the spectra. The solid line represents a fit tothe SAM data points, the dashed line has the same slope, and an offsetwas added to compare with the NPs.

Figure 5. Series of N 1s XP spectra of mixed SAMs for different molefractions χsol(Az11) in the adsorption solution. The fits (solid lines)were obtained as described in the Experimental Section.

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present in Az11 and the attenuation of the signal due toscattering at the outer phenyl ring should be weak. Therefore,changes in the molecular orientation are expected to have anegligible influence on the recorded N 1s XPS intensity. Thecoverage Θ is not identical to the mole fraction of Az11 in theSAM because the footprint of alkanethiolates is about 10%smaller than the footprint of azobenzene-functionalizedthiolates.d Therefore, the total number of thiolates in theSAM increases slightly while diluting Az11 with C12. Therelation between the mole fraction χsol of Az11 in theadsorption solution and the Az11 coverage Θ in the SAM isplotted in Figure 6. For mole fractions χsol ≤ 20%, the mixture

on the surface resembled that in solution (dashed line); i.e., wefound a nearly linear relation. At higher χsol, however, the Az11coverage in the SAM exceeded the mole fraction in solution.This preferential adsorption of Az11 most likely results fromthe different interactions between the molecules in the SAM.The van der Waals interaction of C12 with neighboring C12 orAz11 should be very similar because the length of the aliphaticchains is nearly the same. In contrast, two neighboring Az11molecules should interact more strongly due to additional π−πinteractions of the chromophores. This leads to preferentialadsorption of Az11 with increasing mole fraction of Az11 insolution, however, without substantial segregation andformation of large homogeneous domains. These processesmay be suppressed because of the smaller footprint of C12which leads to a gain in the total adsorption energy due to anincrease of the total thiol coverage when C12 mixes with Az11.An immersion time of 20 h was sufficient to reach the

equilibrium composition in the SAM.e

Orientation of the Azobenzene Moieties. In order to studythe chromophore orientation for different Az11 coverages Θ,we applied near-edge X-ray absorption fine structure(NEXAFS) spectroscopy by recording the Auger yield. Morespecifically, we analyzed excitations from the C 1s and N 1score levels into the unoccupied molecular orbitals. We used amethod developed by Stohr et al.52 to determine theorientation of the transition dipole moment (TDM) withrespect to the surface normal from the X-ray polarizationcontrast (for details see Supporting Information). This

approach has already been successfully applied to SAMs ofbiphenyl-based thiols on gold(111).53 In aromatic systems, theTDMs of excitations into π* orbitals are perpendicular to thering plane.52 This allows us to determine the mean tilt angle αbetween the normal of the ring plane and the surface normal,defined in Figure 7a.Figure 8 shows N 1s NEXAFS spectra for SAMs of varying

Az11 coverage Θ. The π* (LUMO) resonance at a photonenergy of 398.4 eV was used to determine the molecularorientation. For the 100% Az11 SAM the Auger yield isstronger for s-polarized than for p-polarized light; however, thispolarization contrast is inverted for smaller coverages Θ, whichindicates a significant change in the orientation of thechromophores. Evaluating the polarization contrast of thepeak areas yields the tilt angles α compiled in Table 1. The

Figure 6. Az11 coverage Θ in mixed planar SAMs plotted versus themole fraction χsol(Az11) in solution. The coverage Θ was determinedfrom the peak area of the N 1s XP spectrum. The dashed linerepresents ideal mixing, and data points above the line indicatepreferential adsorption of Az11.

Figure 7. Transition dipole moments (TDMs) in Az11. (a) The TDMof the π* transition probed in NEXAFS is oriented perpendicular tothe aromatic plane. α denotes the angle between the surface normal nand the normal of the aromatic plane. ϑ is the angle between thesurface normal and the axis through the N−C bond at the “upper”phenyl ring. (b) The optical TDMs μ(S2) and μ(S3) lie in the aromaticplane.19

Figure 8. N 1s NEXAFS of SAMs with varying Az11 coverage Θ, for s-and p-polarized light, and light polarized along the magic angle.f Thepeak at 398.4 eV originates from the excitation into the π* (LUMO)orbital, whereas the peaks in the [400, 403] eV range are assigned toexcitations into higher unoccupied π* orbitals. The broad featurearound 407 eV is a σ* resonance. Assignments are in accordance withearlier work for a similar molecule.19

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same method was applied to the C 1s π* resonances (seeSupporting Information). In NEXAFS spectroscopy we averageover a macroscopic area of the sample. Thus, this methodprovides only the mean tilt angle α of the aromatic plane,without the information on how strongly every single moleculedeviates from the average orientation.In contrast to our earlier works on SAMs of azobenzene

functionalized with specific end groups at the 4′-position,19,50we cannot determine the angle ϑ of Az11 (see Figure 7a),which gives approximately the orientation of the long molecularaxis with respect to the surface normal. But since α for the100% Az11 SAM is in excellent agreement with theaforementioned results, we expect an analogous angle ϑ ofabout 30° for the azobenzene moiety.g We can conclude thatfor the 100% Az11 SAM the plane of the phenyl rings isoriented predominantly perpendicular to the surface, whereasupon dilution, the angle α decreases significantly. This mustcoincide with an increase in ϑ and thus with a “bending down”of the azobenzene units upon dilution. Such a change in theaverage orientation in the mixed SAM would not occur in thecase of the formation of extended densely packed domains,which corroborates the largely statistical mixing of Az11 andC12 molecules, as deduced from the continuous N 1s peak shiftin XPS. The NEXAFS contrast and the tilt angle, α, changesignificantly even for a small dilution to 80%.Our NEXAFS data are compatible with the range of tilt

angles ϑ of 45°−60° determined in a complementary work35

for mixed SAMs of C12 and trans-p-cyanoazobenzenethiols.h

Optical Properties of Planar SAMs. Because the goldsubstrate is nontransparent, we used differential reflectance(DR) spectroscopy in order to examine the optical propertiesof planar SAMs. More specifically, we measured the change inreflectivity caused by the SAM on the gold surface (for detailsof the method see Supporting Information). The DR spectrawere recorded at an angle of incidence of 45° with respect tothe surface normal, using linearly polarized light. Figure 9shows DR spectra of mixed and single-component SAMsmeasured with p- and s-polarized light. The electric field vectorof s-polarized light is parallel to the surface, whereas p-polarizedlight at 45° has components parallel and perpendicular to thesurface (see schematic drawings in the figure). The generalshape of the DR spectra is given by the change in reflectivitydue to the SAM acting as a dielectric layer.19 This effect is bestdiscussed for the C12 SAM, which contains no chromophores(Θ = 0%), and the spectrum lacks any absorption bands. Weobserved a drop in the DR signal at 2.6 eV, a step at 3.6 eV, andan increase for photon energies higher than ≈4.5 eV. In themiddle of the figure, an absorbance spectrum of trans-Az11 insolution is plotted. The observed absorption bands aretypical18,54,55 for azobenzenes. Like in the SAMs on NPs thedominating transition is the S2(π−π*) excitation; the S1(n−π*)

excitation is only weakly observed.i The S3 absorption band isdue to higher π−π* excitations. As expected, the p-polarizedDR signal of the S2 band (top) increases with increasing Az11coverage Θ. Additionally, the absorption maximum shiftstoward higher photon energies. This hypsochromic shiftamounts to 0.64 ± 0.01 eV for a 100% Az11 SAM comparedwith the band position in solution. The S3 absorption band alsoincreases in intensity with increasing Θ. In contrast to the S2band, the center of the S3 band shifts toward lower energies(bathochromic shift) and a weak fine structure of three peaksemerges.The observed shifts in the S2 and S3 bands can be understood

using the theory of excitonic coupling.20 The S2 transitiondipole moment (TDM) of an isolated trans-Az11 molecule liesin the plane of the chromophore, almost parallel to the axis thatconnects the C4 and the C4′ atoms (see Figure 7b). We haveshown that in a 100% Az11 SAM the chromophores, and thusthe S2 TDMs, stand predominantly upright. In a 2Darrangement, they form an H-aggregate, leading to ahypsochromic shift of the respective band.20 The formation

Table 1. Average Orientations α (cf. Figure 7a) in SAMs ofVarying Az11 Coverage Θ, Determined from N 1s and C 1sNEXAFS Spectraa

α/deg

Θ(Az11)/% N 1s C 1s

100 73 ± 5 71 ± 5≈80 59 ± 5 55 ± 5≈15 45 ± 5 42 ± 5

aThe error of 5 includes systematic contributions and sample-to-sample variations.

Figure 9. Series of p- and s-polarized differential reflectance (DR)spectra of SAMs for different Az11 coverages Θ (angle of incidence45°, offsets were added). Schematic drawings indicate the orientationof the electric field vector of the incident light. An absorbancespectrum of trans-Az11 in methanol (middle, dashed) is plotted forcomparison. The inset shows the upscaled S1 absorption band insolution.

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of H-aggregates has been observed for many azobenzene-functionalized SAMs.19,56,57 The excitation into the S2 band isstrongest when the electric field vector of the incident light isoriented parallel to the TDM; thus, the S2 transition is muchmore pronounced in p- than in s-polarized spectra.The S3 band in Az11 SAMs exhibits a bathochromic shift

with respect to that in solution, in contrast to the S2 band. Thisshift can be explained by the formation of a J-aggregate, whichis composed of head-to-tail aligned TDMs.58 The S3 TDMs areoriented roughly parallel to the NN bond (see Figure 7b).The S3 band is only slightly more intense in p- than in s-polarized DR spectra; therefore, the NN bond has anintermediate orientation between perpendicular and parallel tothe surface. In J-aggregates vibrational excitations are sup-pressed,59 revealing the absorption fine structure visible for the100% SAM.In addition to the blue-shifted part of the S2 band, the p-

polarized DR spectra show a signal in the energy range of the S2band of Az11 in solution (≈3.6 eV). We attribute this mainly tothe absorption by molecules located at “defects”, e.g., in thevicinity of C12 or on gold step edges. Another contributionstems from the dielectric background.Comparison of Excitonic Shifts in Planar and Curved

SAMs. Both planar and curved SAMs show density-dependentexcitonic shifts of the S2 band. Figure 4 shows a plot of theobserved S2 band positions versus the chromophore density, ρ.In planar SAMs ρ is proportional to the coverage Θ.j For planarSAMs with vanishing ρ, one might expect an S2 band positionsimilar to that of free Az11 in solution. However, a linear fityields an offset of (0.21 ± 0.02) eV. This value is one order ofmagnitude larger than typical solvatochromic effects.18 We canconclude that the chromophores in the planar SAM tend toform small aggregates even for very low chromophoredensities.k This view is corroborated by the fact that modelsof azobenzene dimers and oligomers21 predict an excitonic shiftof this order of magnitude.A similar trend of the excitonic shift increasing with

chromophore density, ρ, can be observed for curved SAMs.In contrast to the planar SAMs, the extrapolation to infinitelysmall ρ values is in agreement with the S2 band position ofAz11 in solution. Thus, for small NPs the formation ofaggregates might be inhibited by solvation of the azobenzeneunits, in contrast with the planar SAMs, where no solvent ispresent. This is in agreement with the observation that NPswith diameters larger than ≈8 nm are insoluble in all solvents.Photoisomerization in Planar SAMs. Figure 10 shows p-

polarized DR spectra of pristine SAMs and spectra afterillumination with UV (365 nm) and blue (460 nm) light incomparison with the analogous photoswitching experiment ofAz11 in solution. The illumination with UV light triggers thetrans-to-cis isomerization. In solution this leads to a weakeningand a hypsochromic shift of the S2 band, whereas the S1transition appears slightly stronger because the respectivetransition is not symmetry-forbidden in the cis isomer. The S2bands of the trans and cis isomers partially overlap; therefore,only a photostationary state (PSS) with residual 10−15% ofchromophores in the trans form is reached.18 Uponillumination with blue light, we reach another PSS that isdistinct from the initial state (i.e., the PSS contains residual cisisomer). Experimentally, the samples were illuminated until nofurther change in the spectrum was observed. The followingphoton doses were sufficient to reach a PSS in solution: 3 ×1017 photons cm−2 for UV light and 9 × 1017 photons cm−2 for

blue light. These photon doses are equivalent to a few secondsof illumination with the LED lamps.For the 100% Az11 SAM, very small changes upon UV

illumination were observed with a total photon dose of 1019

photons cm−2 (several minutes). This is in line with previouswork on densely packed azobenzene SAMs.19,22 In contrastwith the 100% Az11 SAM, lower-coverage SAMs exhibit apronounced photoswitching. This can be seen best from thechange in the DR signal at the S2 band. For decreasing coverageΘ, i.e., for a higher dilution of Az11, a larger percentage ofchromophores can be optically switched. In each case 6 × 1018

photons cm−2 were sufficient to reach the UV PSS.Upon illumination with blue light, we reached a PSS after a

photon dose of 8 × 1018 photons cm−2, equivalent to a fewminutes of illumination time. We performed several switchingcycles on mixed SAMs; no appreciable fatigue could beobserved (see Supporting Information).The center wavelength of the UV light source used for this

work is 365 nm (3.4 eV)close to the S2 absorption bandmaximum of the Az11 monomers in solution. However, in allswitching SAMs we observed that illumination at thiswavelength has a significant effect on the S2 aggregate band(peak at ≈295 nm, 4.2 eV). This difference is very largecompared with the full width at half-maximum of the LEDsource of 80 meV. The response of the aggregate band whenexciting the monomer transition suggests a stepwise coopera-tive switching process of azobenzene stacks within the mixed

Figure 10. Photoswitching experiment on Az11 SAMs and Az11 insolution; pristine and photostationary states (PSS) after illuminationwith UV (365 nm) and blue (460 nm) light are shown. Top: p-polarized DR spectra of SAMs for different coverages Θ; verticaloffsets were added for clarification. Bottom: absorbance spectra ofAz11 in methanol. The illumination wavelengths are indicated byvertical lines.

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SAMs: isomerization of a chromophore at the edge of anaggregate reduces the coupling to its neighbor. Consequently, italters the latter’s absorption and opens space for switching.This describes a scenario where chromophores in the SAMswitch one after another in a domino-like manner.Furthermore, switching the SAM back with blue light leads

almost to the pristine spectrum, which is in contrast to the bluePSS in solution, in which a substantial amount of the cis isomercan be observed. We neglected thermal relaxation from the cisinto the trans ground state during the experiment becausespectra of mixed SAMs in the UV PSS were stable for severalhours after turning off the light source. The trans/cis ratio of theblue PSS depends on the absorption cross section σ and on thephotoisomerization quantum yield, Φ, of both isomers. Theabsorption cross section, σ, is a measure for the probability of achromophore to absorb a photon; Φ yields the probability ofisomerization upon absorption. The preferential adsorption ofAz11 and the zero-density offset of the S2 band position withrespect to that in solution indicate that the trans state isenergetically favored in the SAM due to the interaction of achromophore with its neighbor. This gives rise to a differencein the photoisomerization quantum yield between SAM andsolution. Upon illumination with blue light, the ongoing cis-to-trans isomerization in the SAM leads to an increasingpreference of the trans species. We can conclude that lateralinteractions foster the complete back switching of the SAM.This corroborates the proposed cooperativity in the photo-isomerization of our SAMs, in agreement with STM studies onthe photoisomerization of azobenzene SAMs.60−62

■ SUMMARY AND CONCLUSIONSSteric hindrance and excitonic coupling in azobenzene−alkanethiolate (Az11) SAMs can be reduced by decreasingthe chromophore density. This has been demonstrated usingtwo approaches: first, we bound Az11 to microscopically curvedsurfaces, i.e., gold nanoparticles; second, we prepared mixedSAMs on planar substrates by coadsorption from a solution oftwo thiol species. We demonstrated that Az11 and C12 formmixed layers and that the average orientation of thechromophores changes from an almost upright-standingposition to a more flat-lying one upon dilution of Az11 withC12 spacer molecules.Increasing the chromophore density in curved single-

component Az11 SAMs by changing the NP diameter leadsto an increasing hypsochromic shift of the azobenzene S2absorption band. A similar density dependence is found forthe excitonic coupling among the chromophores in mixedplanar SAMs. An offset between planar and curved SAMsindicates the formation of small aggregates in mixed SAMshaving low Az11 coverage, but there is no sign of furthersegregation and the formation of large single-componentislands at higher coverages.The photoswitching of Az11 bound to nanoparticles is as

effective as for Az11 in solution. In contrast, the 100% Az11SAM on a planar substrate exhibits negligible photoisomeriza-tion yield. The dilution of Az11 with C12 is a prerequisite forefficient and reversible photoswitching of the planar SAM, withphotostationary states reached after illumination with (6−8) ×1018 photons cm−2. Coverage dependence, spectral response,and full reversibility of the trans-to-cis photoisomerization allindicate cooperativity in a step-by-step switching process.Isomerization from trans to cis of a given chromophore leads todecoupling of the neighboring Az11 molecules and reduces

steric hindrance, which promotes further switching. The bluePSS is almost identical to the pristine state. The cis state isstable for at least several hours under ambient conditions, andno appreciable fatigue was observed after several switchingcycles. Our results demonstrate that tuning the density andcomposition of azobenzene SAMs enables effective photo-switching and thus paves the way toward tailoring surface andinterface properties in a reversible fashion.

■ ASSOCIATED CONTENT*S Supporting InformationSynthesis of Az11 and P11; preparation of planar and curvedSAMs; additional UV/vis, XP, and NEXAFS spectra; methodused to determine the molecule orientation from NEXAFSspectra; setup used for UV/vis DR spectroscopy. This materialis available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail c.gahl@fu-berlin.de (C.G.).*E-mail rafal.klajn@weizmann.ac.il (R.K.).*E-mail weinelt@physik.fu-berlin.de (M.W.).

Present AddressS.D.: Dept. of Colloids and Materials Chemistry, CSIR-Instituteof Minerals and Materials Technology, Odisha, India.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSSupport by the Deutsche Forschungsgemeinschaft through Sfb658Elementary Processes in Molecular Switches at Surfaces,the Helmholtz Virtual InstituteDynamic Pathways in Multi-dimensional Landscapes, the Israel Science Foundation, and theMinerva Foundation are gratefully acknowledged.

■ ADDITIONAL NOTESaThis view is corroborated by the observation that NPs coatedwith mixed monolayers of Az11 and small amounts of P11 werehighly soluble in a wide range of solvents even at large particlesizes (e.g., 12 nm).bWe used 1 mL of 0.1 mM thiol solution per cm2 of sample,and assumed a surface area of 0.242 nm2 per Az11 molecule.43cOn NPs with about 2 nm diameter, footprints of about 0.15−0.17 nm2 have been reported.48,49 For larger NPs the footprintshould converge with that of planar gold. Using the value forplanar gold leads to a small error for small NPs, withoutaffecting the general trend.dSimple alkanethiolates on Au(111) form a (√3 × √3)R30°structure,25 which leads to a footprint of 0.217 nm2, whereas forSAMs of Az11 and also the equivalent compound with a shorteralkyl chain (6-4-[phenyldiazenyl]phenoxyhexane-1-thiol) avalue of about 0.242 nm2 has been determined.43,51eSamples that had been immersed for just 30 min or up to 8days showed no changes in Az11 coverage compared withsamples immersed for 20 h.fThe magic angle is the polarization angle for which the X-rayabsorption is independent of the molecular orientation.gThe relation of ϑ to α depends on the twist angle γ of thechromophore.19hHere, a longer linker of mercaptopentadecanoic ester was usedfor the azobenzene compound.

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iThe n−π* excitation is symmetry-forbidden in trans-azobenzene, but it is weakly visible due to molecularvibrations.54 In Az11 it is more intense because of theasymmetric substitution.jThe chromophore density was calculated using a footprint of0.242 nm2 for Az11.43kThe formation of larger aggregates would contradict thepreviously shown XPS and NEXAFS results.

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