Photoreaction of Matrix-Isolated Dihydroazulene-FunctionalizedMolecules on Au{111}Bala Krishna Pathem,†,‡ Yue Bing Zheng,†,‡,§ Seth Morton,∥ Michael Åxman Petersen,⊥ Yuxi Zhao,†,‡

Choong-Heui Chung,†,§ Yang Yang,*,†,§ Lasse Jensen,*,∥ Mogens Brøndsted Nielsen,*,⊥

and Paul S. Weiss*,†,‡,§

†California NanoSystems Institute, ‡Department of Chemistry and Biochemistry, and §Department of Materials Science andEngineering, University of California, Los Angeles, Los Angeles, California 90095, United States∥Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States⊥Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, Copenhagen, Denmark

*S Supporting Information

ABSTRACT: Dihydroazulenes are photochromic moleculesthat reversibly switch between two distinct geometric andconductivity states. Molecular design, surface attachment, andprecise control over the assembly of such molecular machinesare critical in order to understand molecular function andmotion at the nanoscale. Here, we use surface-enhancedRaman spectroscopy on special atomically flat, plasmonicallyenhanced substrates to measure the photoreaction kinetics ofisolated dihydroazulene-functionalized molecules assembledon Au{111}, which undergo a ring-opening reaction uponillumination with UV light and switch back to the initial isomervia thermal relaxation. Photokinetic analyses reveal the highefficiency of the dihydroazulene photoreaction on solidsubstrates compared to other photoswitches. An order ofmagnitude decrease in the photoreaction cross section ofsurface-bound dihydroazulenes was observed when comparedwith the cross sections of these molecules in solution.

KEYWORDS: Photoswitch, dihydroazulene, self-assembly, self-assembled monolayers, surface-enhanced Raman spectroscopy,plasmonics, molecular devices, density functional theory

Functionalization of solid substrates with molecules capableof performing specific functions when subjected to external

stimuli is of great interest in the fields of electronics (displaytechnologies),1−5 biology,6−9 medicine,10−13 energy,14,15 andnanoelectromechanics.16−22 The functional molecules shouldbe able to switch reversibly between two or more stable ormetastable states. Moreover, these molecules must retain theirfunctionality when they are assembled on solid substrates inorder to be efficiently utilized in electromechanical applications.The conductivity of the underlying substrate also plays a criticalrole in the photoisomerization of functional molecules. Forinstance, it is well established that the photoisomerization offunctional molecules is quenched when deposited onconductive substrates due not only to the underlying substratesbut also to the surrounding environment.22−25 It has beenshown that photoisomerization of azobenzene molecules iscompletely quenched when they are adsorbed directly on theAu{111} surface. However, when azobenzene was function-alized with bulky 3,3′,5,5′-tert-butyl legs, the azobenzenemoiety was lifted off the substrate, and did photoisomerize.23

Similarly, we have demonstrated that the functional moleculescan be decoupled with either nonconductive alkyl chains orconductive phenyl rings and their photoisomerizations weremeasured both by surface-enhanced Raman spectroscopy(SERS) and scanning tunneling microscopy (STM) atensemble and single-molecule scales, respectively.23,26,27 Anapproximately 4-fold decrease in photoisomerization crosssection was observed when the conductivity of the tether (thatis used to decouple the functional moiety from the substrate)was increased.27 Hence, it is desirable to decouple (electroni-cally) the functional group from the surface and to isolate themolecules from each other in order to elucidate the singlemolecular photokinetic behavior. Much research has focused onidentifying, understanding, and optimizing molecular switchcandidates capable of responding to a variety of external stimulisuch as light,23,24,26,28−36 electric field,19,37−40 electrochemical

Received: July 13, 2012Revised: December 31, 2012Published: January 3, 2013

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potential,16,19−21 or heat.41,42 Mapping microwave polarizabilityand local barrier height have been used to measure theconductances and locations of buried bonds.43,44 Of all theexternal stimuli mentioned above, light offers a straightforwardmeans to excite and to probe the functional moleculesassembled on solid substrates, because it is noninvasive,preserves the surface integrity, and is easily addressable. Variousfamilies of molecular switches have been synthesized to meetsuch requirements, including, azobenzenes,23,24,26,31,32 spiropyr-ans,45−47 stilbenes,48−50 diarylethenes,51−54 rotaxanes,16,19−21,55

oligo(phenylene ethynylene)s,37,38,56−59 dithienylethenes,60−63

and dihydroazulenes34,42,48,64−70 (DHAs). However, the questfor highly efficient surface-bound molecular switches continues.Photoreaction quantum yields of DHA molecules are higher

than those of equivalent azobenzenes and stilbenes, and yetDHA systems have not been extensively studied as molecularswitches on solid substrates, until recently in single-moleculejunctions.34,70 When irradiated with UV light, DHA undergoesa 10-electron retro-electrocyclization to vinylheptafulvene(VHF) that exists in equilibrium between s-cis and s-transconformations.71 It is also known that VHF undergoes a ring-closing reaction to DHA via thermal relaxation through the s-cisconformation. Unlike many other photoisomerization reactions,this reconfiguration has little impact on the external contours ofthe functional molecules indicating that steric hindrance due tointeractions with neighboring molecules and the concurrentefficiency reduction may be limited (cf. azobenzene).22,24,72 Ithas been shown that the absorption maximum of DHA can betuned by tailoring the substituents on the functional moiety.Although reversible transformation between DHA and VHFisomers has been established in solution,48,66,67,71,73−80 photo-chromism has not been extensively studied when thesemolecules are bound to solid substrates.34 To that end, wehave designed a S-(4-((4-(1,1-dicyano-1,8a-dihydroazulen-2-yl)phenyl)ethynyl)phenyl) ethanethioate molecule (henceforthDHA′) and studied its photokinetics by assembling these asisolated single molecules on Au{111} substrates patterned asnanohole arrays. By attaching an acetyl-protected thiolate via atolane linker to the five-membered ring of DHA, we were notonly able to separate the functional moiety spatially from theunderlying substrate but also to restrict the photoreaction tojust the s-cis conformation.23,24,26 We employed highly sensitiveplasmonically SERS to monitor the kinetics of photoreactiondue to its high sensitivity to molecular conformation,nondestructive nature, and high throughput.22,26,81 While theeffect of UV intensity on photoisomerization does play a role,

this Letter limits the measurements and analyses todemonstrate the photoswitching of novel functional moleculesassembled on solid substrates and to compare the quantitativephotoisomerization cross sections with other classes ofphotoswitches studied under similar irradiation conditions.We interpret and compare our experimental measurements andanalyses with supporting density functional theory calculations.We have recently demonstrated a sensitive means to follow

the photoisomerization kinetics of functional moleculesassembled on atomically flat surfaces using SERS.22,26,27,82 Byemploying Au nanohole array substrates, we have shown thatthe Raman spectra of the molecules are enhanced usingplasmonics, which are otherwise not detectable when substrateswithout nanoholes are used.26 We employ the same techniquehere to follow the photoreaction kinetics of DHA′ moleculesassembled on Au{111} nanohole array substrates. Focused ionbeam (FIB) lithography (Nova 600 NanoLab, FEI Company,Hillsboro, OR, U.S.A.) was used to fabricate cylindricalnanoholes (with diameters of 175 nm) in square arrays (withperiods of 300 nm) into Au thin films. Note that the observedRaman enhancement is from the areas between the holes withthe majority of the enhancement due to the flat areas next tothe holes.83

The samples were prepared by flame-annealing Au{111}nanohole array substrates and then subsequently immersingthem in 1:4 mixtures of ethanolic solutions of DHA′ anddodecanethiol (C12) in the dark such that the final solutionconcentrations were 1 mM. The samples were then stored inthe dark and under nitrogen atmosphere for 24 h.Subsequently, the samples were rinsed with ethanol and thenvapor annealed over C12 solutions at 80 °C for 2 h in order toincrease the crystallinity of the matrices by backfilling the C12molecules into existing defect sites. This procedure resulted inthe formation of tightly packed large domains of the hostmolecular monolayer matrix with dilute DHA′ moleculesisolated as single molecules within the domains of the C12matrix.24,26,84,85 Thus, random diffusion of isolated DHAmolecules on the substrates was restricted by the surroundingmatrix. The samples were stored in the dark until furtheranalysis. Figure 1 shows a schematic of the assembly of isolatedsingle molecules in a host C12 matrix. The isolation of DHA′was tested and confirmed by STM measurements (vide infra)and was consistent with prior results on the assembly andisolation of molecules in two-dimensional matrices.24,25 Themolecules initially exist in the thermodynamically favored,closed-ring DHA isomer and switch to the corresponding

Figure 1. Schematic of UV-light-induced photoreaction of functionalized dihydroazulene (DHA′, left) to a vinylheptafulvene (VHF′, right) isolatedas single molecules in a dodecanethiol (C12) matrix and the back reaction via thermal relaxation.

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vinylheptafulvene (henceforth VHF′) isomer upon illuminationwith 365 nm UV light. The thermal back reaction occurs at30 °C.A Renishaw inVia Raman system (Renishaw Inc., Hoffman

Estates, Illinois, U.S.A.) was employed for Raman analyses. A632.8 nm He−Ne laser was chosen as the Raman excitationsource taking into consideration the resonant plasmon energyselected for the Au substrates. Laser power and beam diameterwere ∼17 mW and ∼1 μm, respectively. All Ramanmeasurements were performed under ambient conditions.Each measurement was a convolution of 50 sweeps in thewavelength range of interest with a set integration time of150 s. Raman spectra, individual peak intensities, and the timeconstants from the exponential curves were calculated usingWire 3.2 (Renishaw Inc.) and OriginPro (OriginLab, North-ampton, MA, U.S.A.) software packages.Single-molecule STM measurements were performed on the

samples used for Raman analyses prepared under the sameconditions. Our results on these molecules and from ourprevious measurements on azobenzene molecules24 indicatethat majority of the molecules are isolated as single moleculeswithin the domains of the matrix molecules. Figure 2 showsSTM images collected at Vs = −1 V, and It = 5 pA in whicheach protrusion is attributed to a single dihydroazulenemolecule isolated with the alkanethiol matrix. The consistencyin the sizes and shapes of the features indicate that they arepredominantly due to single molecules. Since the features inSTM images are convolutions of topography and electronicstructure, protruding features were observed due to theenhanced conductance of the DHA′ molecules when comparedwith the surrounding nonconductive alkanethiol matrix. As theDHA′ molecules physically protrude out of the matrix, theyeffectively image STM tip when scanned, as in previouswork.56,86,87 The ordered, molecularly resolved alkanethiolmatrix surrounds the isolated DHA′ molecules (Figure 2B).The samples were analyzed in a dark room by exposing the

substrates to a ∼365 nm UV lamp source. Time coursemeasurements were performed over a period of 60 min. Thepower of the UV source was held constant (570 μW/cm2),similar to our previous measurements.26 After each successiveUV illumination time period, Raman spectra were collected atthe same location of the substrate. Since it is common toobserve a slight increase in temperature where the laser isfocused during the Raman measurements, the sample wasallowed to cool down naturally after the final UV measurementsbefore performing the thermal relaxation measurements. Thesample was then held at a constant temperature of 30 °C and

time course measurements were performed at regular timeintervals until the ratio of peak intensities saturated.The NWChem program package88 was used to calculate the

Raman spectra of DHA′ and VHF′ molecules attached to a Au3cluster. The ground-state geometry and normal modes werecalculated using the B3LYP functional89 and the LANL2DZeffective core potential for the Au atoms and the 6-311G* basisset for all other atoms. The vibrational frequencies have beenscaled by 0.98. The Raman spectra were calculated using finitedifferentiation of polarizabilities obtained using the LC-ωPBEhfunctional. This functional was used to avoid overpolarizationwhen calculating the Raman spectra of the molecules on theAu3 cluster.90 The differential Raman cross sections werecalculated assuming an incident laser wavelength of 633 nm.The differential Raman cross sections were broadened using aLorentzian function with a full width at half-maximum (fwhm)of 20 cm−1. The Raman intensities for mixtures of open andclosed isomers (Imixed) were calculated as:

= +I X I X Imixed open open closed closed

where Xopen and Xclosed are the mole fractions of the open andclosed isomers, respectively, and Iopen and Iclosed are the Ramanintensity of the open and closed isomers, respectively.The calculated DHA′ Raman spectrum (Figure 3) revealed

five distinct peaks in the range of 950−1800 cm−1, with peakpositions at 1068, 1140, 1532, and 1585 cm−1. The peak at1068 cm−1 is due to the ring breathing modes of the tether(parallel to the S−C bond) that was employed to attach thefunctional moiety to the underlying Au substrate. The motionsymmetrically localized around the CC stretch attached tothe functional moiety has an intense peak positioned at1140 cm−1. The C−C stretching modes and in-plane Hwagging motion of the five-membered ring of the functionalmoiety give rise to a distinct peak at 1532 cm−1 and theprominent C−C stretch in the five-membered ring of thefunctional moiety results in peak at 1585 cm−1 (SupportingInformation Figure S1). In order to follow the surfacephotokinetics, we simulated the Raman spectra of the VHF′isomer and the intermediate spectra of DHA′ and VHF′ atdifferent mole fractions. The corresponding vibrationalfrequencies of VHA are found at 1068, 1140, 1529, and 1593cm−1. As can be seen from Figure 3, the intensities of all thepeaks decrease dramatically as the photoreaction proceeds fromDHA′ to VHF′, except for the peak at 1068 cm−1, labeled P1.Since this peak arises from the vibrational modes of the tetherunit (Figure 4), the photoreaction has no effect on the peakintensity. The peak labeled P2, on the other hand, is directly

Figure 2. Scanning tunneling microscope images of (A) dihydroazulene-functionalized molecules isolated within the domains of dodecanetiol matrixin a 400 Å × 400 Å area and (B) high-resolution image showing a single molecule in 250 Å × 250 Å area. Imaging conditions: Vs = −1 V, It = 5 pA.

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affected by the cross-conjugation in the five-membered ring ofthe functional moiety and the intensity of C−H bendingvibrational modes decreases as the photoreaction proceedsfrom DHA′ to VHF′ (see Figure 4). Thus, experimentallymonitoring the intensities of peaks labeled P1 and P2 providesa direct measure of the fractions of reactant and product. Figure4 shows the vibrational modes of DHA′ and VHF′ isomers thatgive rise to peaks P1 and P2, respectively.Figure 5 shows a series of Raman spectra collected over time,

as described in the experimental section above. Four distinctpeaks in the 1000−1700 cm−1 range are observed. The spectraare measured as a function of illumination time and are offsetfor clarity. Although peaks P1 and P2 are well resolved, thepeaks at 1580 and 1610 cm−1 were not completely resolved andthus were not considered for quantification. Also, it can be seenthat the intensity of peak P1 remains constant, which is inagreement with simulations. We thus chose P1 as an internalstandard. Since the peak intensity of P1 is the least affected bythe DHA′→VHF′ photoreaction, comparison of the intensity

of P2 with that of P1 provides a direct measurement of theprogress of the photoreaction on the surface. Hence, wemeasured the ratio of the peaks P2 and P1 (P2/P1) andmonitored it as a function of UV illumination time.The series of Raman spectra in Figure 5 show that the

intensity of P2 decreases dramatically with UV illuminationtime, indicating the forward photoreaction of DHA′ to VHF′,while the intensity of P1 remains constant. The UVmeasurements were stopped when the ratio of peak intensitiessaturated. The samples were then cooled to room temperaturefor a few minutes and were maintained at a constanttemperature of 30 °C. Time-course Raman measurementswere performed at constant temperature. Relevant series ofRaman spectra can be seen in Figure 6; the intensities of peaksother than P1 increased as a function of time. The peakintensities in the Raman spectra after 60 min were observed tobe close to those of the initial (preillumination) measurements,however, the ratios of the peaks were still lower than the initialvalues, indicating that the thermal back reaction was not yetcomplete. By 120 min, the peak ratios saturated.

Figure 3. Series of theoretical simulations of SERS spectra as a mixtureof different mole fractions of functionalized dihydroazulene (DHA′)and corresponding vinylheptafulvene (VHF′), spectra are offset forclarity. Peaks labeled P1 and P2 were used to follow the photoreactionspectroscopically.

Figure 4. Vibrational modes P1 of (A) functionalized dihydroazulene (DHA′) and (B) vinylheptafulvene (VHF′) isomers. Vibrational modes P2 of(C) DHA′ and (D) VHF′ isomers.

Figure 5. Series of experimental Raman spectra as a function of365 nm UV light illumination time (the legend shows the duration oflight illumination). The ratio of peaks labeled P1 and P2 was used tofollow the kinetics of surface-bound molecular photoreaction. Thespectra are offset for clarity.

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Figure 7A shows an exponential fit to the decay curve plottedfor P2/P1 peak ratios with respect to UV illumination time.The error bars are from five measurements performed on fivedifferent nanohole array substrates. The P2/P1 ratio saturatedafter about 25 min of UV illumination. The minimal deviationin the data points after the 25 min mark from five differentmeasurements indicates the robustness of the SERS measure-ment technique. The data were fit with an exponential curvewith the formula Y = Y0 + A*exp(R0t), where R0 is the decayconstant of the best fit curve with units of time−1. The timeconstant was then calculated by taking the inverse of the decayconstant in units of minutes. The time constant extracted forthe DHA′ to VHF′ photoreaction was 9 ± 2 min. This is inagreement with our previous single-molecule measurementswhere the onset of photoreaction was observed to occur after10−20 min of UV illumination.34 The correlation of the single-molecule measurements with those of the ensemble measure-ments of surface-bound molecules reveals that the photo-reaction quantum yield of the DHA moieties is higher even

when assembled and isolated on surfaces, compared to otherphotochromic molecules measured, including azobenzenes andstilbenes.34 As a comparison figure of merit, isolatedazobenzenes with saturated tethers exhibit photoisomerizationtime constants of ∼38 min studied using the same measure-ment conditions and techniques26 and ∼54 min when studiedusing high-resolution STM.24

Figure 7B shows the exponential increase in P2/P1 as afunction of thermal relaxation time. The ratio of peakintensities increased as a function of time and eventuallysaturated after 120 min, indicating completion of the surface-bound VHF′→DHA′ thermal back reaction. The time constantcalculated was similar to that of the UV-induced forwardreaction measurement, 38 ± 7 min. The ratio (P2/P1) of peakintensities from theoretical calculations was also plotted as afunction of VHF′ mole fractions in order to compare with theexperimentally observed trend. A decreasing trend in the peakratio with increasing mole fraction of VHF′ was observed,consistent with our experiments. Since we codeposited toisolate the DHA′ molecules, we thereby restricted theadsorption of the DHA′ molecules largely to within thedomains of the host alkanethiolate molecules. As can be seen inFigure 2A, the functional molecules are distributed withindomains and are not isolated at domain boundaries and stepedges under these conditions (vs postmatrix depositioninsertion33,56,85,86). We predominantly observe moleculesisolated as single molecules on the surface. However, there isno doubt that some molecules can assemble as dimers, trimers,and/or, clusters, but these were rarely observed in STM imageshere compared to the number of isolated DHA moleculesfound (Figure 2A). We note that such clustering also leads tospectral shifts, which can be used to separate clustered fractionsif they appear in sufficient numbers.22 The result is that thesignals from various clustered molecules would be spread outand would not contribute significantly to the results reported(it is possible in some cases to direct assembly to a particularclustered state).25

In order to improve the efficiency of controlled molecularmotion and function of surfaces functionalized with switches,molecules with higher photoisomerization quantum yield have

Figure 6. Series of experimental Raman spectra as a function ofthermal relaxation time (the legend shows the thermal relaxation timeat 30 °C). The spectra are offset for clarity.

Figure 7. (A) The peak area ratio (P2/P1) as a function of UV light irradiation time fit to an exponential decay. The extracted time constant fordihydroazulene to vinylheptafulvene (DHA′→VHF′) photoreaction was 9 ± 2 min. (B) Data showing the increase in P2/P1 ratio as a function ofthermal relaxation time at a constant temperature of 30 °C, also fit to an exponential curve. The time constant for the VHF′→DHA′ thermal backreaction was 38 ± 7 min. The errors bars are from five sets of measurements.

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to be employed. Trans→cis photoreaction of azobenzene, forexample, has an average quantum yield of ∼0.2 and a yield of∼0.5 for the reverse reaction,91,92 whereas DHA→VHFphotoreaction in acetonitrile has a quantum yield of 0.55.74

We calculated the photoisomerization cross sections of thecurrent DHA′ system and compared them to those of ourprevious measurements of azobenzene-functionalized mole-cules performed under identical conditions to get a quantitativeestimate of the photoisomerization efficiency. By using theformula σ = hc/(λτI0), where h is Planck′s constant, c is thespeed of light, λ is the wavelength of irradiation (365 nm), τ isthe time constant, and I0 the power of the UV light source(570 μW/cm2), we get the photoisomerization cross section (inunits of cm2). In solution, we observe switching times of1 min,34 corresponding to σsoln = 2 × 10−17 cm2; similarly, wefound that the photoisomerization cross section of surface-bound DHA′ molecules with a rate constant of 9 min underthese conditions gives σbound = 1.5 × 10−18 cm2, which is inreasonable agreement with our previous single-moleculemeasurements.34 These cross sections for surface-boundmolecules are nearly an order of magnitude lower than thoseobserved in solution. However, when compared with measure-ments of azobenzene-functionalized molecules studied underthe identical conditions and with those measured using STM,we find that the surface-bound photoreaction cross section forDHA′ is higher, where σAzo varied between 4 × 10−19 and 8 ×10−20 cm2 depending upon the conductivity of the tether andthe degree of spatial separation of the functional moiety fromthe underlying substrate.23,26

In conclusion, we have designed novel photochromicdihydroazulene-functionalized molecules and isolated them assingle molecules on Au{111} substrates patterned as nanoholearrays. The cross sections derived from SERS measurements asa function of UV illumination time reveal the high photo-switching efficiency of the functional molecules, compared toprevious studies of various other surface-bound photoswitches.Furthermore, the reversibility of the dihydroazulene photo-reactions via thermal relaxation has been established.

■ ASSOCIATED CONTENT*S Supporting InformationVibrational modes of prominent peaks of DHA′, plot of P2/P1peak area ratio of calculated spectra at different mole fractions,synthesis of DHA′ and corresponding UV, H NMR and 13CNMR data. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: yangy@ucla.edu (Y.Y.); jensen@chem.psu.edu (L.J.);mbn@kiku.dk (M.B.N.); psw@cnsi.ucla.edu (P.S.W.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSB.K.P., Y.B.Z., and P.S.W. thank the Department of Energy(Grant DE-FG02-07ER15877), the Penn State Center forNanoscale Science (a NSF-supported Materials ResearchScience and Engineering Center), and the Kavli Foundationfor support of this work. L.J. acknowledges the CAREERprogram of the National Science Foundation (Grant CHE-0955689) for financial support. S.M. acknowledges an

Academic Computing Fellowship from the Pennsylvania StateUniversity Graduate School and the Penn State Center forNanoscale Science and the National Science Foundation(Grant OCI−0821527). M.A.P. and M.B.N. thank theEuropean Community’s Seventh Framework Programme(FP7/2007-2013) under the Grant agreement “SINGLE” no213609 and The Danish Council for Independent ResearchNatural Sciences. We thank N. Bodzin at UCLA for assistancewith FIB lithography.

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Nano Letters Letter

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Nano Letters Letter

dx.doi.org/10.1021/nl304102n | Nano Lett. 2013, 13, 337−343343