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Cite this:Chem. Commun., 2014,

50, 10105

Photoisomerisation and ligand-controlledreversible aggregation of azobenzene-functionalised gold nanoparticles†

Anja Kohntopp, Alexandra Dabrowski, Michal Malicki‡ and Friedrich Temps*

The photochemical behaviour of functionalised gold nanoparticles

(AuNPs) carrying azobenzenethiolate–alkylthiolate monolayers was

investigated. Repeated trans–cis and cis–trans isomerisation cycles

could be performed in all cases with high efficiency. Reversible

photoinduced aggregation was observed when azothiolates with

long alkyl spacers (ZC7) were combined with short (C5) alkylthiolate

coligands. The choice of a coligand thus offers control over the

aggregation properties of the nanoparticles.

Functionalised nanoparticles have gained tremendous importancein fundamental and applied sciences because of their unique, size-dependent optical, electronic, magnetic and chemical propertieswith potential application in medical diagnostics, drug delivery,cancer therapy, nanoelectronics and information storage, imagingand super resolution microscopy, sensors, solar energy conversion,(photo)catalysis, surface coatings or the development of smartmaterials.1–3 Currently, particular interest exists in nanoparticleswith surface-bound molecular switches that can be controlled byultraviolet or visible light.4 Unlike in solution, however, the photo-switching properties of molecules on surfaces are subject to severalmechanisms affecting or hindering the outcome: first, densepacking of the surface-bound molecules may leave too little freespace for the transformation to proceed, especially when a photo-isomerisation is associated with a large-amplitude structuralchange.4–7 Second, energy transfer either between the attachedswitches or between the switch and the nanoparticle substrate maylead to a rapid electronic deactivation of the photoexcited mole-cules and even prevent the reaction altogether.7 A third major issuearises when a change in properties of NPs upon isomerisation of

the switches in the ligand shell leads to aggregation of the NPs.This effect may impede further switching after only a few cycles.5

In cases where aggregation is desired, it is of great importance tocontrol the process and the structure of the aggregates.8–10 Klajnand coworkers for example studied the behaviour of azobenzene(AB) functionalised AuNPs as a function of AB concentration andsolvent polarity and found different types of aggregates undervarying conditions.11,12

Here, we report on AB-functionalised AuNPs with high surfacecoverage in solution in toluene that can be repeatedly photoswitchedback and forth between their trans and cis forms and additionallyshow virtually fully reversible aggregation when they are in the cisstate. We evaluated the surface concentration of AB molecules onthe AuNPs independently by UV/Vis spectroscopy and by NMRspectroscopy and determined the cis/trans-AB compositions in thephotostationary states upon irradiation with UV and Vis light. Wefound that the aggregation potential of the functionalised AuNPscan be controlled through the choice of the alkylthiolate coligand.Evidently, the tendency for aggregation due to attractive dipole–dipole forces between the NP-bound cis-ABs depends strongly on thechain length of the coligand and besides the AB surface coverage.

For our investigation of the photoswitching properties of ABs onthe gold surface, we functionalised AuNPs with AB-derivatisedalkylthiols (azothiols) as ligands and normal alkylthiols as coligandsin mixed monolayers (see Fig. 1A and B) by ligand exchange fromamine-stabilised NPs in mixed solutions (molar fractions xAB = 0.3,0.5 or 0.7) of azothiol and alkylthiol as detailed in the ESI.† The alkyl(spacer) lengths of the thiols were systematically varied (Fig. 1B). Theobtained NPs were characterised using transmission electron micro-scopy (TEM), NMR and UV/Vis spectroscopy. A typical TEM image isdisplayed in Fig. 1C. It shows that the synthesis yielded nearlyspherical products with diameters typically of B4.0� 0.8 nm (singlestandard deviation). Considering the macroscopic density (rAu =19.3 g cm�3), the NPs should consist of B2000 Au atoms. 1H NMRspectra confirmed the purity of the functionalised NPs (see Fig. 1D).For organic molecules on AuNPs, the 1H NMR signals are stronglybroadened inhomogeneously and due to much faster spin–spinrelaxation13 so that surface-bound and free ligand molecules in

Institute of Physical Chemistry, Christian-Albrechts-University Kiel,

Olshausenstr. 40, D-24098 Kiel, Germany. E-mail: temps@phc.uni-kiel.de

† Electronic supplementary information (ESI) available: Syntheses details; detailson experimental methods; NMR spectra; evaluation of AB surface concentrationsby UV/Vis and NMR spectroscopy; aggregation at different AB surface coverages;and analysis of cis/trans-AB compositions in the photostationary states. See DOI:10.1039/c4cc02250e‡ Current address: European Space Research and Technology Centre, Keplerlaan 1,PO Box 299, NL-2200AG Noordwijk, The Netherlands.

Received 26th March 2014,Accepted 11th July 2014

DOI: 10.1039/c4cc02250e

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10106 | Chem. Commun., 2014, 50, 10105--10107 This journal is©The Royal Society of Chemistry 2014

the sample solution can be easily distinguished. The absence of sharpsignals of free AB thiols in the measured NMR spectrum thusdemonstrates that practically all AB molecules in the sample arebound to the AuNPs. An observed high-field shift (see Fig. S1, ESI†) ofthe aromatic proton signals with increasing AB surface concentrationsupports an irregular, statistical distribution of the azothiolate andalkylthiolate ligands in the mixed monolayer.

The UV/Vis spectra of the AB-functionalised NPs displayed inFig. 2 show two pronounced absorptions, the broad local surfaceplasmon resonance (LSPR) band at around l = 515 nm and the pp*band of the AB ligands in their trans form at l = 350 nm. The weakernp* AB band at around l = 455 nm is only barely visible. Sizeablesurface enhancement of the AB pp* absorption could be ruled out inour case by a combination of 1H NMR and UV/Vis spectroscopy asdetailed in the ESI.† Typical AB surface concentrations by bothmethods were found to be around B100 molecules per NP. Forphotoswitching, the solutions were irradiated at either l = 365 nmor at l = 455 nm. The results obtained with the six differentazothiolate–alkylthiolate combinations are given in Fig. 2. As canbe seen, UV-induced trans–cis isomerisation leads to a drasticdecrease of the intensity of the pp* band, which recovers uponback-isomerisation with visible light. The photostationary statesPSS365 and PSS455 reached at the two wavelengths (for temporalevolutions see Section S3, ESI†) represent the switching of the ABs onthe AuNPs to mostly the cis and mostly the trans forms, respectively.In contrast, with two exceptions (Fig. 2C and E, discussed below) littlechange was observed in the region of the LSPR bands. In all cases,between three and five switching cycles were performed without anynoticeable signs of degradation or reduction in switching efficienciesof the samples, as evident by the near-perfect agreement between themeasured spectra belonging to the respective photostationary stateswhich are plotted on top of each other.

While this general behaviour is the same for all investigatedsamples, significant differences are revealed depending on the

Fig. 1 (A) Schematic representation of the functionalised AuNPs with azothiolate ligands and alkylthiolate coligands (NPs and ligands not drawn to scale).(B) Structural formulas and abbreviations of the different thiols used in this work. (C) Typical TEM image of the AB-functionalised AuNPs (scale bar: 10 nm).(D) Comparison of the 1H NMR spectra of free AB-OC7SH in solution (blue) and AB-OC7S-/C5S-NPs (red). The lack of sharp 1H signals of the AB-function inthe NP spectrum confirms that the ligands are bound to the NPs (residual sharp lines in the NP spectrum belong to the solvent and a trace of H2O).

Fig. 2 Measured UV/Vis spectra of the AB-functionalised AuNPs beforeirradiation (all-trans, black) and in PSS365 (blue) and PSS455 (red). All sampleswere prepared using equimolar azothiol–alkylthiol solutions and switchedback and forth between the two photostationary states reproducibly at least3 to 5 times, the individual spectra belonging to those switching cycles areplotted on top of each other. In panels A, B, D and F, only photoisomerisationof the AB moiety is observed. Panels C and E additionally show the reversibleaggregation of the NPs in PSS365 demonstrated by the broadened LSPRbands; in this case the spectra show the overall extinction (sum of absorptionplus possible light scattering).

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alkyl chain lengths of the ligands: in PSS365, the NPs with theshortest azoligand (AB-OC3S) still show significant residual350 nm (pp*) absorption by trans-AB, indicating that not allAB units were isomerised (see Fig. 2A and B). Assuming that thespectra of the ABs on the NPs are similar in shape to those insolution, we found switching efficiencies from the pure transform at the start to 70% cis/30% trans in PSS365 and 10% cis/90%trans in PSS455 (for details see Section S3.4, ESI†). With longerazoligands, the trans-to-cis switching efficiency increased, while thecis-to-trans efficiency decreased. For the ligand–coligand combi-nation with the longest alkyl spacer (AB-OC11S/C10S, Fig. 2F), wereached switching to 95% cis/5% trans in PSS365 and 25% cis/75%trans in PSS455.

Much more striking are the differences after irradiation in theshape of the LSPR bands when the longer azoligands are combinedwith short alkylthiolates (Fig. 2C and E), whereas little change isobserved in all systems with decanethiolate coligands (Fig. 2B, Dand F), a strong broadening and decrease in intensity of the LSPRband together with a sizeable red-shift are found after isomerisa-tion of samples carrying ABs with long alkyl spacers (C7, C11) incombination with pentanethiolate coligands (Fig. 2C and E). Thisphenomenon indicates aggregation of the NPs after switching ofthe attached ABs to their cis form. As shown, it is completelyreversible. Fig. S5 (ESI†) demonstrates that aggregation occurswhether the sample solutions are stirred or not.

The aggregation is attributed to the attractive dipole–dipoleforces between the cis-ABs in the ligand shells around the NPs,when they come into close proximity by Brownian motion. Thedipole moment of cis-AB (3.2 Debye) causes weaker solventstabilisation of the NPs in nonpolar toluene such that theyaggregate. Related mechanisms have been discussed for variousNP systems in the literature.12,14 In our case, longer alkylthiolatecoligands appear to better shield the cis-ABs and to lead to bettersolvation, thereby preventing association. Fig. S6 (ESI†) showsthat the aggregation tendency in a row of three AB-OC7S-/C5S-AuNP samples increases with higher AB surface concentration.This effect is rationalised by the higher dipole density on theNPs. In addition, the coligand chain length seems to play acrucial role, as aggregation occurs only when it is shorter thanthe alkyl spacer length in the azoligand. Two arguments come tomind: first, the shorter chain length of the coligand allows forcloser contact between the cis-switched NPs and thereby strongerinteraction of the dipoles. Second, the cis-switched NPs withshort alkylthiols are expected to be less well stabilised by thesolvent so that their tendency for aggregation is raised comparedto decanethiol-cofunctionalised NPs. The first hypothesis issupported by two facts: the AB-OC3S-functionalised NPs with apentanethiolate coligand show practically no aggregation evenafter prolonged exposure to UV light, demonstrating that pentane-thiol alone does not enable aggregation by lowering the solubility ofthe NPs. Further, the strength of aggregation seems to dependmainly on the surface concentration of AB on the NPs rather than

on the absolute length difference between the ligands andcoligands. This suggests that – at least in the present case –the solubility of the NPs is not the main driving force forthe aggregation, whereas the dipole–dipole forces are. Whenthe cis-ABs are isomerised back to trans, the aggregates are nolonger favoured and disaggregate.

In conclusion, we studied AuNPs with mixed azothiolate–alkylthiolate monolayers. The samples could be photoswitchedback and forth reproducibly between two photostationary states(mostly cis and mostly trans) without visible photodegradationafter several cycles. Observed differences in switching efficiencyindicate steric or electronic hindrance of the isomerisationwhen the alkyl spacer in the azoligand becomes too short.Furthermore, we observed a fully reversible aggregation of NPsswitched to the cis state when a long azoligand was combinedwith a coligand possessing a shorter alkyl chain. Together with thefinding that the tendency for aggregation depends on the cis-ABsurface concentration, our results lead to the conclusion that dipole–dipole attraction is the main driving force for the observed aggrega-tion. Another critical factor appears to be solubility, as we found thatazothiolate–AuNPs without alkylthiolates were practically insoluble.Our approach using mixed monolayers offers control of the photo-aggregation potential of functionalised AuNPs by optimising thechain length of the coligand and the surface concentration ofthe azoligand. The photoswitching dynamics of our functiona-lised AuNPs are now under investigation using femtosecondtime-resolved spectroscopy.

We gratefully acknowledge the support of this work by theCollaborative Research Centre 677 ‘‘Function by Switching’’.

Notes and references1 K. G. Thomas and P. V. Kamat, Acc. Chem. Res., 2003, 36, 888.2 S. Link and M. A. El-Sayed, Annu. Rev. Phys. Chem., 2003, 54, 331.3 C. Louis and O. Plouchery, Gold Nanoparticles for Physics, Chemistry

and Biology, Imperial College Press, London, 2012.4 R. Klajn, J. F. Stoddart and B. A. Grzybowski, Chem. Soc. Rev., 2010,

39, 2203.5 S. D. Evans, S. R. Johnson, H. Ringsdorf, L. M. Williams and

H. Wolf, Langmuir, 1998, 14, 6436.6 R. Wang, T. Iyoda, L. Jiang, D. A. Tryk, K. Hashimoto and

A. Fujishima, J. Electroanal. Chem., 1997, 438, 213.7 J. Zhang, J. K. Whitesell and M. A. Fox, Chem. Mater., 2001, 13, 2323.8 S. Das, P. Ranjan, P. S. Maiti, G. Singh, G. Leitus and R. Klajn,

Adv. Mater., 2013, 25, 422.9 Y. Wei, S. Han, J. Kim, S. Soh and B. A. Grzybowski, J. Am. Chem.

Soc., 2010, 132, 11018.10 J.-Q. Lin, H.-W. Zhang, Z. Chen, Y.-G. Zheng, Z.-Q. Zhang and

H.-F. Ye, J. Phys. Chem. C, 2011, 115, 18991.11 R. Klajn, P. J. Wesson, K. J. M. Bishop and B. A. Grzybowski, Angew.

Chem., Int. Ed., 2009, 48, 7035.12 R. Klajn, K. J. M. Bishop and B. A. Grzybowski, Proc. Natl. Acad. Sci.

U. S. A., 2007, 104, 10305.13 M. J. Hostetler, J. E. Wingate, C.-J. Zhong, J. E. Harris, R. W. Vachet,

M. R. Clark, J. D. Londono, S. J. Green, J. J. Stokes, G. D. Wignall,G. L. Glish, M. D. Porter, N. D. Evans and R. W. Murray, Langmuir,1998, 14, 17.

14 K. J. M. Bishop, C. E. Wilmer, S. Soh and B. A. Grzybowski, Small,2009, 5, 1600.

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