photoinduced surface patterning of azobenzene-containing supramolecular dendrons, dendrimers and...

Photoinduced surface patterning ofazobenzene-containing supramoleculardendrons, dendrimers and dendronized

polymers

Jaana Vapaavuori,1 Arri Priimagi,1,∗ Antti J. Soininen,1

Nadia Canilho,2 Edis Kasemi,3 Janne Ruokolainen,1

Matti Kaivola,1 and Olli Ikkala1

1Department of Applied Physics, Aalto University, P.O. Box 13500, 00076 Aalto, Finland2Universite de Lorraine/CNRS, UMR7565, F-54506 Vandoeuvre-les-Nancy cedex, France

3Department of Materials, ETH Zurich, 8093 Zurich, Switzerland∗[email protected]

Abstract: Ionic complexes of azobenzenes and dendritic structures areshown to exhibit efficient light-induced mass transport upon irradiation witha light interference pattern. Surface-relief gratings (SRGs) with modulationdepths of up to 550 nm were successfully inscribed. We compare the SRGformation in three generations of supramolecular dendrons, dendrimers, anddendronized polymers and demonstrate that the grating formation process isdestructed by the existence of self-assembled structures as well as by overlylarge size of the dendronic complexes.

© 2013 Optical Society of America

OCIS codes: (160.5335) Photosensitive materials; (050.1950) Diffraction gratings.

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#186947 - $15.00 USD Received 15 Mar 2013; revised 16 Apr 2013; accepted 25 Apr 2013; published 3 May 2013(C) 2013 OSA 1 June 2013 | Vol. 3, No. 6 | DOI:10.1364/OME.3.000711 | OPTICAL MATERIALS EXPRESS 711

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1. Introduction

Dendritic structures, having well-defined, monodisperse branched architectures, are of great in-terest for designing functional self-assemblies [1,2]. Being able to densely accommodate manyfunctional groups on their peripheral units, these unique molecular architectures offer versa-tile opportunities in designing light-responsive materials. Coupling of luminescent moleculesinto dendritic structures may lead to new materials for light harvesting and metal sensing ap-plications [3]. Functional dendrimers have also been extensively studied for nonlinear opticalapplications [4,5], due to their capability of providing site isolation for the photoactive units andtendency to exhibit large second-order nonlinear optical activities. Yet another interesting classof stimuli-responsive materials combines dendritic molecules with photoisomerizable azoben-zene derivatives that undergo conformational changes under light illumination [6]. The pho-toresponsive behavior of azobenzene-containing dendritic molecules depends on the positionof the azobenzene units within the molecules: dendrimers with an azobenzene core have been


proposed as light-harvesters of low-energy photons [7], whereas having the azo moieties in theperiphery allows for the design of photocontrollable membranes and drug-delivery systems [8].Lastly, ionic dendrimer–azobenzene complexes have been used in designing liquid-crystallinematerials for efficient photoalignment [9, 10].

Dendritic molecules may also provide new insights into light-induced surface patterning, aphenomenon occurring predominantly in azobenzene-containing material systems [11,12]. Ap-plying a light-interference pattern onto a thin polymer film containing azobenzene moieties cangenerate regular sinusoidal surface-relief gratings (SRGs) due to light-induced mass transport,initiated by trans–cis–trans isomerization of the azobenzene molecules. The periodicity andthe modulation depth of the formed gratings can be easily controlled by varying the inscriptionconditions and the materials design, thus providing a facile method for fabricating diffractiveoptical elements [11, 13] and nanostructures [14, 15]. At present, no common consent on thetheoretical basis for the mechanism of the photoinduced mass transport exists, despite extensivemodelling work by several research groups [16–22]. In recent years, polymer–azobenzene com-plexes, in which photoactive units are attached to a polymer host via noncovalent interactions,have emerged as viable alternatives to covalently-functionalized side-chain azopolymers as effi-cient SRG-forming materials. The complexation can take place via hydrogen bonding [23–25],ionic bonding [26,27], or halogen bonding [28]. Different noncovalent interactions between thepolymer backbone and the azobenzene units lead to distinct characteristics for the material sys-tem: hydrogen bonding results in dynamic, modularly tunable complexes [29–31], whereas thestronger ionic interactions give rise to surface patterns with excellent thermal stability [26,27].Halogen bonding, on the other hand, is a highly directional noncovalent interaction [32, 33],which may enhance light-induced mass transport and result in more efficient SRG inscriptioncompared to the less directional noncovalent interactions [28]. Supramolecular approach allowseasy construction of molecular libraries without compromising the optical performance. This isimportant in terms of potential practical applications of photoinduced SRGs, but equally perti-nent in complementing fundamental understanding on the structure–performance relationshipsof the photoinduced surface patterning process.

All the studies referred to in the previous paragraph used linear polymer backbones assupramolecular hosts for azobenzenes with complementary functional groups. But linear back-bones may sometimes not be optimal for efficient light-induced mass transport, and the SRGformation can diminish with increasing molecular weight of the polymer host [34–36], possi-bly due to entanglement of long linear polymer chains. Hence, branched molecular architec-tures may pose certain advantages, and dendritic molecules, due to their well-defined monodis-perse structure, allow for a systematic study of the effect of molecular architecture and bulki-ness on the light-induced mass transport. The SRG formation has been studied in covalently-functionalized azobenzene-containing dendritic molecules [37], and in star-branched polymers[38]. The first report concluded that the SRG formation efficiency depends on the generationof the dendrimer, the second generation bearing 8 azobenzenes on its periphery reaching a re-markable surface modulation depth of 1500 nm. The latter report, on the other hand, found nosignificant correlation between the polymer architecture and the SRG formation.

In this work, we compare the light-induced surface patterning in ionic complexes betweenEthyl Orange (EO), a common azo dye, and three types of dendritic cationic molecules — den-drons [39], dendrimers [39], and dendronized polymers [40]. Each of the three types comprisesthree generations, hence we have a library of nine dendritic supramolecules that vary in theirarchitecture and bulkiness. The type and generation of the corresponding systems allow forcontrolling and tailoring the assembly of the pendant units [40,41], which in turn is anticipatedto influence the photoinduced surface patterning efficiency. The structures were investigatedusing small-angle X-ray scattering and UV-Vis spectroscopy, and the inscribed gratings were


Table 1. Molecular Weights of the Dendritic Hosts, Numbers of Their Peripheral UnitsUsed for Supramolecular Complex Formation with EO, and the Nominal EO Weight Frac-tion Within the Complexes

Code Type(Generation) M [g/mol] # of peripheral units EO fraction [wt %]DD1-EO Dendron(1) 282 2 70.2DD2-EO Dendron(2) 753 4 63.9DD3-EO Dendron(3) 1604 8 62.4DM1-EO Dendrimer(1) 1048 6 65.6DM2-EO Dendrimer(2) 2550 12 61.0DM3-EO Dendrimer(3) 5145 24 60.8DP1-EO Dend. polymer(1) 1551000 2 67.3DP2-EO Dend. polymer(2) 2347000 4 62.6DP3-EO Dend. polymer(3) 330000 8 61.8

inspected by in-situ diffraction measurements and ex-situ AFM observation.

2. Materials and experiments

The molecular architectures studied — dendrons, dendrimers and dendronized polymers — areionically complexed with the well-known azobenzene dye Ethyl Orange (EO), which, togetherwith the structurally similar Methyl Orange, has previously been employed in various SRGstudies [42–44]. The chemical structures of the dendritic structures and EO are shown in Figs. 1and 2, respectively, and from now on the complexes are referred to as DDx-EO (dendrons),DMx-EO (dendrimers), and DPx-EO (dendronized polymers), where x denotes the generation(1-3). The synthesis of the dendritic molecules is described elsewhere [39, 40]. The molecularweights of the dendritic hosts, together with the nominal weight fraction and number of the pen-dant EO groups, are given in Table 1. The monodispersity of the dendrons and the dendrimershas been confirmed by 1H NMR, 13C NMR and MALDI mass spectra [Ref. 39, Supplemen-tary Information]. For polymers, weight-averaged molecular weight is given [Refs. 40 & 45,Supplementary Information]. The number of repeat units for DP1-EO, DP2-EO, and DP3-EOis 2250, 775, and 25, respectively.

The complexation between EO and the dendritic structures was carried out at a stoichiometricratio of the positive ammonium and negative sulfonate charges. 50-100 mg of lyophilized DDx,DMx, or DPx was dissolved in 50 ml of water under continuous stirring. An equivalent amountof EO needed to obtain stoichiometric complexation was separately dissolved in water. ThepH values of both solutions were adjusted with 2 M HCl to pH = 3-4 in order to maintainall amines positively charged. The solution containing the dendritic structures was then addeddropwise to the EO solution under continuous stirring. The ionic complexes precipitated andwere collected after removal of water from the centrifugated complex–water dispersion. Thecollected precipitates were dissolved in 1-butanol after which a large excess of acidic water(pH = 3-4) was added dropwise in order to avoid 1-butanol emulsification in water. Lastly, thewashed complexes were dried under vacuum at room temperature for 3 days to remove residualwater.

A Mettler TG50 unit with a Mettler TC11 TA Processor was used in thermogravimetrymeasurements. A bulk sample was placed in an open aluminium cup and heated from 40 ◦Cto 440 ◦C with a rate of 10 ◦C/min under a nitrogen purge. Differential scanning calorime-try (DSC) experiments were performed with Mettler Toledo Stare instrument. The temperatureramp profile consisted of three stages. First the samples were heated from 25 ◦C to 200 ◦C,after that they were cycled two times to 0 ◦C and back to 200◦C and finally cooled down to


HN NH

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n

Fig. 1. Chemical structures of the dendrons (DDx), dendrimers (DMx), dendronized poly-mers (DPx); EO denotes the Ethyl Orange pendant groups. Only the third-generationmolecules are entirely drawn. The corresponding first and second generations (excludingammonium charges) are highlighted in black and purple, respectively.


Fig. 2. Chemical structure of Ethyl Orange (EO).

25 ◦C. All the heating and cooling stages were done at a rate of 10 ◦C/min.For small-angle X-ray scattering (SAXS) measurements, the samples (about 1 mm thick in

the beam direction) were maintained between two Mylar films. The beam (copper Kα radiation,λ = 1.54 A) was generated by Bruker Microstar microfocus rotating anode source with Monteloptics and was collimated to ca. 1 mm in diameter at the sample position by four sets of JJ X-rayfour-blade slits. The scattering intensities were measured using Bruker HiStar 2D area detectorwith a sample-to-detector distance of approximately 60 cm. The frames recorded by the detectorwere spatially corrected and integrated in the radial direction to obtain the magnitude of thescattering vector (q) versus intensity curves. The magnitude can be written as q = 4π sin(θ)/λ ,where θ is half of the scattering angle.

Thin films for optical studies were drop-cast from dimethlyformamide solutions and werebaked at 70 ◦C for 24 h. The solution concentration was 10 mg/ml for SRG inscription studiesto yield samples of thickness on the order of 1 μm whereas the absorption spectra were takenfrom thinner films drop-cast from 2 mg/ml solutions. The absorption spectra were measuredwith a Perkin Elmer Lambda 950 spectrophotometer. The probe light was unpolarized and aclean microscope slide was used as a reference for reflection correction.

The grating inscription was performed with a spatially filtered, p-polarized beam from a457 nm diode-pumped solid-state laser (Shanghai Dream Lasers Technology) using an irradia-tion intensity of 50 mW/cm2. The interference pattern was produced by splitting an expandedlaser beam with a mirror set at right angle with the sample, such that half of the beam reflectedfrom the mirror and interfered with the ”direct” half on the sample surface (Lloyd’s mirrorinterferometer). The incidence angle was set such that the grating period was ca. 1 μm. Theformation of phase gratings arising from the periodic surface modulation was monitored intransmission mode using a low-power, horizontally polarized, normally incident 680 nm beamfrom a diode laser. The wavelength of 680 nm lies well outside the absorption band of the EOcomplexes and hence does not affect the grating formation. Herein, we define the diffractionefficiency as the ratio of the power of the first-order diffracted beam to the power of the beamtransmitted through an unexposed spot on the sample. We also note that at the period–probewavelength combination used, only the first-order diffraction could be monitored. The modula-tion depths of the gratings were evaluated from atomic-force microscope (AFM) images takenwith Veeco Dimension 5000 SPM instrument.

3. Results and discussion

Based on the thermogravimetric analysis, all the complexes degrade at ca. 200 ◦C. Neitherglass-transition temperatures nor phase-transition peaks were observed in the differential scan-ning calorimetry curves for any of the samples in the range of 0 – 200 ◦C, suggesting that theglass-transition takes place only, if at all, very close to the degradation of the material. As a


DD1-EODD2-EODD3-EO

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Fig. 3. SAXS intensity curves of (a) dendron–EO, (b) dendrimer–EO and (c) dendronizedpolymer–EO complexes and (d) of pure EO as a reference. The curves have been verticallyshifted for the sake of clarity.

comparison, the glass-transition temperatures for the uncomplexed dendronized polymers are59 ◦C (DP1), 58 ◦C (DP2), and 82 ◦C (DP3) [40]. Hence, the lack of softening alkyl chainswithin the ionically complexed EO units renders the complexes very rigid.

The first generation dendron DD1 is known to be crystalline [39]. After complexation withEO, it forms a microphase-separated structure, as shown by the SAXS data in Fig. 3a. The scat-tering peak at 2.77 nm−1 corresponds to a scattering plane distance of 2.27 nm. Based on thesingle reflection peak, the structure cannot be determined unambiguously, but as the periodicity2.27 nm is close to the length of EO [27], we tentatively suggest the complex to have a lamellarsmectic-like structure. DD1-EO is the only nonpolymeric complex that forms a well-orderedstructure; DD2-EO and DD3-EO as well as all the dendrimer complexes (DMx-EO), organizerather poorly. For these samples the SAXS data (Figs. 3a and 3b) show a single broad, low-intensity peak centered at around 2.5 – 3.0 nm−1, corresponding to a characteristic length scaleof 2 – 3 nm. The structure size increases somewhat with generation, thus being directly linked tothe size of the dendritic host. The first- and second-generation dendronized polymer complexes,DP1-EO and DP2-EO, self-assemble into more organized structures (Fig. 3c). Both have twoscattering peaks: at 2.11 nm−1 and 2.67 nm−1 for DP1-EO and at 1.63 nm−1 and 2.53 nm−1 forDP2-EO. Without higher-order reflections the exact stuctures cannot be assigned, but as previ-ous reports on related dendritic complexes have revealed rectangular and tetragonal columnarstructures [40,41,45–50], we tentatively suggest tetragonal or oblique columnar structures alsoin the present case. DP3-EO, on the other hand, shows only a very broad negligible-intensity


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scattering peak, pointing again to a lack of well-ordered microphase-separated structure. Fig-ure 3d shows the SAXS curve of the pure EO as a reference. None of its scattering peaks appearin any of the complexes, which confirms that EO and the dendritic host molecules do not phaseseparate macroscopically.

Figures 4a-4c present the normalized absorption spectra for thin films of all the complexes.Two observations can be made. First, microphase separation gives rise to a clear blue-shiftof more than 15 nm in the absorption maximum with respect to the absorption maximumof disordered samples. The shift is not remarkably large but is clearly evident in all the or-dered complexes, i.e. in DD1-EO (Fig. 4a), DP1-EO, and DP2-EO (Fig. 4c). This indicatesthat the intermolecular interactions between the EO units (presumably side-by-side packing)are more pronounced in the microphase-separated complexes than in the complexes that lackwell-ordered structures. Second, based on the dendrimer complexes (Fig. 4b) the generationhas a rather small impact on the absorption spectra, given that no well-ordered structures areformed. Hence, we assume that the local chromophore environment does not directly depend onthe generation, and the disordered complexes (DD2-EO, DD3-EO, DM1-EO, DM2-EO, DM3-EO) allow for assessing the role of the bulkiness/molecular weight of the complex on the SRGformation efficiency.

The best way to study the dynamics and efficiency of photoinduced SRG formation is tomonitor in real time the first-order diffraction of a non-resonant probe beam, even if quantita-


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tive connection between the surface deformation and the diffraction efficiency can be rathercomplicated [51, 52]. These measurements are summarized in Fig. 5. Each of the three se-ries — dendrons, dendrimers and dendronized polymers — behave somewhat differently andhint for different factors affecting the surface patterning efficiency of the supramolecular den-dritic structures under investigation. We note that the chromophore content is comparable60 – 70 wt %, for all the nine complexes and hence does not account for the observed dif-ferences. We rather argue that the two most important factors affecting the surface patterningefficiency are (i) the existence/lack of well-ordered microphase separation, and (ii) the molec-ular weight of the complex under investigation.

For dendrons (Fig. 5a), the SRG inscription is rather efficient for the second and third gen-erations, and remarkably slow for the first-generation complex DD1-EO. Note that there isno minimum size for the migrating units to yield efficient mass transport: several molecularglasses and low-molecular-weight complexes have been shown to exhibit extremely efficientSRG formation [53–55]. Hence, we attribute the hindered light-induced material motions tothe formation of a well-ordered, microphase-separated structure (see Fig. 3). The differencesbetween the disordered DD2-EO and DD3-EO complexes are rather negligible, implying thatthe generation as such does not play a major role in the SRG formation process. This con-clusion is also supported by the dendrimer complexes, none of which forms a well-orderedmicrophase-separated structure, and which are overall the best for SRG formation. As can beseen from Fig. 5b, for the dendrimer complexes, the SRG formation is quite similar for the


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first and second generations. The evolution of the diffracted signal is way faster for DM1-EOand DM2-EO than for DM3-EO. For the latter, the first-order diffraction efficiency saturates to15 % after 75 min of irradiation (intensity 50 mW/cm2). The modulation depths for the gratingsformed within the DMx-EO series, together with a 3D AFM view for an SRG on DM2-EO, areillustrated in Fig. 6.

When comparing the five disordered complexes (DD2-EO, DD3-EO, DM1-EO, DM2-EO,DM3-EO), their diffraction dynamics is comparable, though somewhat more efficient for den-drimers, for other samples apart from DM3-EO. Since (i) all five complexes are disordered and(ii) their absorption spectra are quite similar (suggesting that no major differences in the pack-ing of the EO units occur), the main difference between DM3-EO and the other complexes liesin bulkiness: the molecular weight of DM3-EO is 13 113 g/mol, more than twice that of DM2-EO, which is the best sample in terms of SRG formation efficiency. Note that the molecularweights given in Table 1 account for the dendritic hosts only, whereas the overall molecularweight comprises the hosts as well as the EO pendant units. Hence, total molecular weight of< 10 000 g/mol seems to be favorable for efficient SRG formation for the complexes under in-vestigation. This statement is supported by the fact that dendronized polymers are drasticallyinefficient compared to the lower-molecular-weight complexes (Fig. 5c), which is most likelyalso explained by the overly high molecular weight of the migrating units and slow dynamics. Itis also of interest to compare DD3-EO and DM1-EO, the nominal molecular weights of whichare quite similar, 4 260 g/mol and 3 040 g/mol, respectively, accounting for both the dendritichost and the EO pendant units. Their saturated diffraction efficiencies upon 30 min irradiationare 23 – 24 % and also the surface-modulation depths estimated from atomic-force micrographsare similar, ca. 360 nm and 330 nm, respectively.


The suggested molecular-weight depedence is consistent with previous works [34, 36], butalso counterexamples exist: most notably, high-modulation-depth SRGs have been success-fully inscribed in ionic, lamellar-packed complexes of EO and high-molecular-weight cationicpolyelectrolytes [27], and large modulation depths in azocellulose polymers with ultrahighmolecular weight have been obtained [56]. The distinct results obtained by different researchgroups using different materials emphasize the delicate nature of the light-induced surface pat-terning process. Details in experimental conditions (e.g. polarization and intensity of the in-scription beam) as well as in sample preparation details (affecting the molecular packing andmicrophase-separation) certainly matter and complicate the comparison of the results obtainedby different research groups. However, the experimental results presented here, supported byour own previous studies [31, 36], clearly hint that relatively small and disordered complexesare optimal for efficient SRG formation.

The efficient SRG inscription in the supramolecular dendritic complexes is enabled by therigidity of the pendant EO units: the lack of flexible chains prevents the formation of well-ordered microphase-separated structures, which in turn enhances the light-induced materialmotions. As another benefit, rigid ionically complexed pendant groups increase the thermalstability of the resultant surface-relief structures. An interesting question is why the existence ofa well-ordered self-assembled structure seems to have such a drastic effect on the light-inducedmacroscopic motions of our dendritic complexes. A comprehensive answer to this question mayteach us a great deal about the light-induced mass transport process.

4. Conclusion

We have studied the light-induced macroscopic motions and the formation of surface-reliefgratings (SRGs) in complexes of Ethyl Orange (EO) and three types of dendritic cationicmolecules — dendrons, dendrimers, and dendronized polymers — each of which comprisesthree generations. Such series allowed us to investigate the role of (i) the generation, (ii) theexistence of well-ordered microphase-separated structures, and (iii) the bulkiness of the mi-grating units on the surface-relief formation efficiency. Overall, dendrimers were found to bemost efficient for SRG formation, followed shortly by dendrons. Dendronized polymers, on theother hand, were drastically inefficient. The best complex in terms of SRG formation was thesecond-generation dendrimer–EO complex for which the surface-modulation depth reached arespectable value of 550 nm upon 25 min inscription with moderate intensity of 50 mW/cm2.We observed no clear connection between the generation of the dendritic structures and theSRG formation. On the other hand, an increased degree of ordering seemed to destruct SRGformation (based on the dendron-EO complexes). The disordered dendrimer-EO series in turnshows that the size of the migrating units (i.e. the dendritic complex) should not be overly largein order to achieve efficient light-induced mass migration; on the other hand smaller is not nec-essarily better, as seen by comparing the first- and second-generation dendrimer complexes. Ingeneral, dendritic light-responsive supramolecules are shown to provide a powerful and versa-tile class of materials for gaining fundamental understanding on the photomechanical responseof azobenzene-containing materials.

Acknowledgments

Prof. Raffaele Mezzenga is greatly acknowledged for fruitful discussions in the course of thisproject, and for comments on the manuscript. Prof. A. Dieter Schluter is thanked for his impor-tant contribution on synthesis of the dendritic molecules used. This project is partially fundedby the Academy of Finland (project number 135106). J. Vapaavuori acknowledges the financialsupport of the National Graduate School in Material Physics.


photoinduced surface patterning of azobenzene-containing supramolecular dendrons, dendrimers and...

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