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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, GermanyDepartment 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 azobenzenealkanethiolate SAMs on gold nanoparticles of varying size.Second, we form mixed SAMs of azobenzenealkanethiolatesand 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 azobenzenealkanethiolate 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.
INTRODUCTIONSelf-assembled monolayers (SAMs) are prime candidates forthe modification of surface properties such as polarity, chemicalreactivity, or charge transfer characteristics at interfaces.16
Integration of molecular switches into SAMs is an importantissue since it opens the possibility to reversibly change theseproperties by external stimuli, e.g., light.79
Azobenzene represents the most commonly used andinvestigated molecular switch.1013 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,1719 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 transcis 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 azobenzenealkanethio-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-thiol2527 (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
2014 American Chemical Society 1048 DOI: 10.1021/la504291nLangmuir 2015, 31, 10481057
consisting of an azobenzene-terminated and an unfunctional-ized alkanethiolate counterpart; Tamada and co-workersshowed that upon adsorption on planar gold the SS 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 cm2 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( 2.512 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.3941
Unfortunately, even chloroform could not dissolve Az11-function-alized NPs larger than 8