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Ionic complexes of azobenzenes and dendritic structures are shown to exhibit efficient light-induced mass transport upon irradiation with a light interference pattern. Surface-relief gratings (SRGs) with modulation depths of up to 550 nm were successfully inscribed. We compare the SRG formation in three generations of supramolecular dendrons, dendrimers, and dendronized polymers and demonstrate that the grating formation process is destructed by the existence of self-assembled structures as well as by overly large size of the dendronic complexes.
© 2013 Optical Society of America
1. Introduction
Dendritic structures, having well-defined, monodisperse branched architectures, are of great interest for designing functional self-assemblies [1, 2]. Being able to densely accommodate many functional groups on their peripheral units, these unique molecular architectures offer versatile opportunities in designing light-responsive materials. Coupling of luminescent molecules into dendritic structures may lead to new materials for light harvesting and metal sensing applications [3]. Functional dendrimers have also been extensively studied for nonlinear optical applications [4, 5], due to their capability of providing site isolation for the photoactive units and tendency to exhibit large second-order nonlinear optical activities. Yet another interesting class of stimuli-responsive materials combines dendritic molecules with photoisomerizable azoben-zene derivatives that undergo conformational changes under light illumination [6]. The photoresponsive behavior of azobenzene-containing dendritic molecules depends on the position of 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 the periphery allows for the design of photocontrollable membranes and drug-delivery systems [8]. Lastly, ionic dendrimer–azobenzene complexes have been used in designing liquid-crystalline materials for efficient photoalignment [9, 10].
Dendritic molecules may also provide new insights into light-induced surface patterning, a phenomenon occurring predominantly in azobenzene-containing material systems [11, 12]. Applying a light-interference pattern onto a thin polymer film containing azobenzene moieties can generate regular sinusoidal surface-relief gratings (SRGs) due to light-induced mass transport, initiated by trans–cis–trans isomerization of the azobenzene molecules. The periodicity and the modulation depth of the formed gratings can be easily controlled by varying the inscription conditions and the materials design, thus providing a facile method for fabricating diffractive optical elements [11, 13] and nanostructures [14, 15]. At present, no common consent on the theoretical basis for the mechanism of the photoinduced mass transport exists, despite extensive modelling work by several research groups [16–22]. In recent years, polymer–azobenzene complexes, 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 efficient 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 the polymer backbone and the azobenzene units lead to distinct characteristics for the material system: hydrogen bonding results in dynamic, modularly tunable complexes [29–31], whereas the stronger 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 inscription compared to the less directional noncovalent interactions [28]. Supramolecular approach allows easy construction of molecular libraries without compromising the optical performance. This is important in terms of potential practical applications of photoinduced SRGs, but equally pertinent in complementing fundamental understanding on the structure–performance relationships of the photoinduced surface patterning process.
All the studies referred to in the previous paragraph used linear polymer backbones as supramolecular hosts for azobenzenes with complementary functional groups. But linear backbones may sometimes not be optimal for efficient light-induced mass transport, and the SRG formation can diminish with increasing molecular weight of the polymer host [34–36], possibly due to entanglement of long linear polymer chains. Hence, branched molecular architectures may pose certain advantages, and dendritic molecules, due to their well-defined monodisperse structure, allow for a systematic study of the effect of molecular architecture and bulkiness 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 generation of the dendrimer, the second generation bearing 8 azobenzenes on its periphery reaching a remarkable surface modulation depth of 1500 nm. The latter report, on the other hand, found no significant correlation between the polymer architecture and the SRG formation.
In this work, we compare the light-induced surface patterning in ionic complexes between Ethyl Orange (EO), a common azo dye, and three types of dendritic cationic molecules — dendrons [39], dendrimers [39], and dendronized polymers [40]. Each of the three types comprises three generations, hence we have a library of nine dendritic supramolecules that vary in their architecture and bulkiness. The type and generation of the corresponding systems allow for controlling and tailoring the assembly of the pendant units [40, 41], which in turn is anticipated to influence the photoinduced surface patterning efficiency. The structures were investigated using small-angle X-ray scattering and UV-Vis spectroscopy, and the inscribed gratings were inspected by in-situ diffraction measurements and ex-situ AFM observation.
2. Materials and experiments
The molecular architectures studied — dendrons, dendrimers and dendronized polymers —are ionically complexed with the well-known azobenzene dye Ethyl Orange (EO), which, together with the structurally similar Methyl Orange, has previously been employed in various SRG studies [42–44]. The chemical structures of the dendritic structures and EO are shown in Figs. 1 and 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 molecular weights of the dendritic hosts, together with the nominal weight fraction and number of the pendant EO groups, are given in Table 1. The monodispersity of the dendrons and the dendrimers has been confirmed by 1H NMR, 13C NMR and MALDI mass spectra [Ref. 39, Supplementary 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-EO is 2250, 775, and 25, respectively.
Fig. 1 Chemical structures of the dendrons (DDx), dendrimers (DMx), dendronized polymers (DPx); EO denotes the Ethyl Orange pendant groups. Only the third-generation molecules are entirely drawn. The corresponding first and second generations (excluding ammonium charges) are highlighted in black and purple, respectively.
Table 1. Molecular Weights of the Dendritic Hosts, Numbers of Their Peripheral Units Used for Supramolecular Complex Formation with EO, and the Nominal EO Weight Fraction Within the Complexes
The complexation between EO and the dendritic structures was carried out at a stoichiometric ratio 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 amount of EO needed to obtain stoichiometric complexation was separately dissolved in water. The pH values of both solutions were adjusted with 2 M HCl to pH = 3–4 in order to maintain all amines positively charged. The solution containing the dendritic structures was then added dropwise to the EO solution under continuous stirring. The ionic complexes precipitated and were collected after removal of water from the centrifugated complex–water dispersion. The collected 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, the washed complexes were dried under vacuum at room temperature for 3 days to remove residual water.
A Mettler TG50 unit with a Mettler TC11 TA Processor was used in thermogravimetry measurements. A bulk sample was placed in an open aluminium cup and heated from 40 °C to 440 °C with a rate of 10 °C/min under a nitrogen purge. Differential scanning calorimetry (DSC) experiments were performed with Mettler Toledo Stare instrument. The temperature ramp 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 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 Å) was generated by Bruker Microstar microfocus rotating anode source with Montel optics and was collimated to ca. 1 mm in diameter at the sample position by four sets of JJ X-ray four-blade slits. The scattering intensities were measured using Bruker HiStar 2D area detector with a sample-to-detector distance of approximately 60 cm. The frames recorded by the detector were spatially corrected and integrated in the radial direction to obtain the magnitude of the scattering 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 were baked at 70 °C for 24 h. The solution concentration was 10 mg/ml for SRG inscription studies to yield samples of thickness on the order of 1 μm whereas the absorption spectra were taken from thinner films drop-cast from 2 mg/ml solutions. The absorption spectra were measured with a Perkin Elmer Lambda 950 spectrophotometer. The probe light was unpolarized and a clean microscope slide was used as a reference for reflection correction.
The grating inscription was performed with a spatially filtered, p-polarized beam from a 457 nm diode-pumped solid-state laser (Shanghai Dream Lasers Technology) using an irradiation intensity of 50 mW/cm2. The interference pattern was produced by splitting an expanded laser beam with a mirror set at right angle with the sample, such that half of the beam reflected from the mirror and interfered with the ”direct” half on the sample surface (Lloyd’s mirror interferometer). The incidence angle was set such that the grating period was ca. 1 μm. The formation of phase gratings arising from the periodic surface modulation was monitored in transmission mode using a low-power, horizontally polarized, normally incident 680 nm beam from a diode laser. The wavelength of 680 nm lies well outside the absorption band of the EO complexes and hence does not affect the grating formation. Herein, we define the diffraction efficiency as the ratio of the power of the first-order diffracted beam to the power of the beam transmitted through an unexposed spot on the sample. We also note that at the period–probe wavelength combination used, only the first-order diffraction could be monitored. The modulation depths of the gratings were evaluated from atomic-force microscope (AFM) images taken with Veeco Dimension 5000 SPM instrument.
3. Results and discussion
Based on the thermogravimetric analysis, all the complexes degrade at ca. 200 °C. Neither glass-transition temperatures nor phase-transition peaks were observed in the differential scanning calorimetry curves for any of the samples in the range of 0 – 200 °C, suggesting that the glass-transition takes place only, if at all, very close to the degradation of the material. As a comparison, the glass-transition temperatures for the uncomplexed dendronized polymers are 59 °C (DP1), 58 °C (DP2), and 82 °C (DP3) [40]. Hence, the lack of softening alkyl chains within the ionically complexed EO units renders the complexes very rigid.
The first generation dendron DD1 is known to be crystalline [39]. After complexation with EO, it forms a microphase-separated structure, as shown by the SAXS data in Fig. 3a. The scattering peak at 2.77 nm−1 corresponds to a scattering plane distance of 2.27 nm. Based on the single reflection peak, the structure cannot be determined unambiguously, but as the periodicity 2.27 nm is close to the length of EO [27], we tentatively suggest the complex to have a lamellar smectic-like structure. DD1-EO is the only nonpolymeric complex that forms a well-ordered structure; DD2-EO and DD3-EO as well as all the dendrimer complexes (DMx-EO), organize rather 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 scale of 2 – 3 nm. The structure size increases somewhat with generation, thus being directly linked to the 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 two scattering 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 for DP2-EO. Without higher-order reflections the exact stuctures cannot be assigned, but as previous reports on related dendritic complexes have revealed rectangular and tetragonal columnar structures [40, 41, 45–50], we tentatively suggest tetragonal or oblique columnar structures also in the present case. DP3-EO, on the other hand, shows only a very broad negligible-intensity scattering peak, pointing again to a lack of well-ordered microphase-separated structure. Figure 3d shows the SAXS curve of the pure EO as a reference. None of its scattering peaks appear in any of the complexes, which confirms that EO and the dendritic host molecules do not phase separate macroscopically.
Fig. 3 SAXS intensity curves of (a) dendron–EO, (b) dendrimer–EO and (c) dendronized polymer–EO complexes and (d) of pure EO as a reference. The curves have been vertically shifted for the sake of clarity.
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-shift of more than 15 nm in the absorption maximum with respect to the absorption maximum of disordered samples. The shift is not remarkably large but is clearly evident in all the ordered complexes, i.e. in DD1-EO (Fig. 4a), DP1-EO, and DP2-EO (Fig. 4c). This indicates that 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 lack well-ordered structures. Second, based on the dendrimer complexes (Fig. 4b) the generation has a rather small impact on the absorption spectra, given that no well-ordered structures are formed. Hence, we assume that the local chromophore environment does not directly depend on the 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 SRG formation efficiency.
Fig. 4 The normalized absorption spectra for (a) DDx-EO, (b) DMx-EO, and (c) DPx-EO. The absorption maxima for the complexes are given in the figure legends.
The best way to study the dynamics and efficiency of photoinduced SRG formation is to monitor in real time the first-order diffraction of a non-resonant probe beam, even if quantitative connection between the surface deformation and the diffraction efficiency can be rather complicated [51, 52]. These measurements are summarized in Fig. 5. Each of the three series — dendrons, dendrimers and dendronized polymers — behave somewhat differently and hint for different factors affecting the surface patterning efficiency of the supramolecular dendritic structures under investigation. We note that the chromophore content is comparable, 60 – 70 wt %, for all the nine complexes and hence does not account for the observed differences. We rather argue that the two most important factors affecting the surface patterning efficiency are (i) the existence/lack of well-ordered microphase separation, and (ii) the molecular weight of the complex under investigation.
Fig. 5 The first-order diffraction efficiencies for (a) DDx-EO, (b) DMx-EO, and (c) DPx-EO. To facilitate a comparison between the complexes, the same y-axis scale is used for all the graphs. All the inscriptions were performed using a p-polarized inscription beam (457 nm, 50 mW/cm2) and a period of ca. 1 μm.
For dendrons (Fig. 5a), the SRG inscription is rather efficient for the second and third generations, and remarkably slow for the first-generation complex DD1-EO. Note that there is no minimum size for the migrating units to yield efficient mass transport: several molecular glasses and low-molecular-weight complexes have been shown to exhibit extremely efficient SRG formation [53–55]. Hence, we attribute the hindered light-induced material motions to the formation of a well-ordered, microphase-separated structure (see Fig. 3). The differences between the disordered DD2-EO and DD3-EO complexes are rather negligible, implying that the generation as such does not play a major role in the SRG formation process. This conclusion is also supported by the dendrimer complexes, none of which forms a well-ordered microphase-separated structure, and which are overall the best for SRG formation. As can be seen from Fig. 5b, for the dendrimer complexes, the SRG formation is quite similar for the first and second generations. The evolution of the diffracted signal is way faster for DM1-EO and DM2-EO than for DM3-EO. For the latter, the first-order diffraction efficiency saturates to 15 % after 75 min of irradiation (intensity 50 mW/cm2). The modulation depths for the gratings formed within the DMx-EO series, together with a 3D AFM view for an SRG on DM2-EO, are illustrated in Fig. 6.
Fig. 6 (a): Surface profiles of the gratings recorded on the dendrimer complexes. The curves are offset in the y-direction for the sake of clarity. The modulation depths for samples written until the diffraction efficiency has saturated are ca. 330 nm (DM1-EO), 550 nm (DM2-EO), and 420 nm (DM3-EO). (b): 3D AFM view of an SRG on the DM2-EO complex.
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 dendrimers, 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 packing of the EO units occur), the main difference between DM3-EO and the other complexes lies in 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 molecular weights given in Table 1 account for the dendritic hosts only, whereas the overall molecular weight 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 investigation. This statement is supported by the fact that dendronized polymers are drastically inefficient compared to the lower-molecular-weight complexes (Fig. 5c), which is most likely also explained by the overly high molecular weight of the migrating units and slow dynamics. It is also of interest to compare DD3-EO and DM1-EO, the nominal molecular weights of which are quite similar, 4 260 g/mol and 3 040 g/mol, respectively, accounting for both the dendritic host and the EO pendant units. Their saturated diffraction efficiencies upon 30 min irradiation are 23 – 24 % and also the surface-modulation depths estimated from atomic-force micrographs are similar, ca. 360 nm and 330 nm, respectively.
The suggested molecular-weight depedence is consistent with previous works [34, 36], but also counterexamples exist: most notably, high-modulation-depth SRGs have been successfully inscribed in ionic, lamellar-packed complexes of EO and high-molecular-weight cationic polyelectrolytes [27], and large modulation depths in azocellulose polymers with ultrahigh molecular weight have been obtained [56]. The distinct results obtained by different research groups using different materials emphasize the delicate nature of the light-induced surface patterning process. Details in experimental conditions (e.g. polarization and intensity of the inscription beam) as well as in sample preparation details (affecting the molecular packing and microphase-separation) certainly matter and complicate the comparison of the results obtained by different research groups. However, the experimental results presented here, supported by our own previous studies [31, 36], clearly hint that relatively small and disordered complexes are optimal for efficient SRG formation.
The efficient SRG inscription in the supramolecular dendritic complexes is enabled by the rigidity 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 material motions. As another benefit, rigid ionically complexed pendant groups increase the thermal stability of the resultant surface-relief structures. An interesting question is why the existence of a well-ordered self-assembled structure seems to have such a drastic effect on the light-induced macroscopic motions of our dendritic complexes. A comprehensive answer to this question may teach 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-relief gratings (SRGs) in complexes of Ethyl Orange (EO) and three types of dendritic cationic molecules — dendrons, dendrimers, and dendronized polymers — each of which comprises three generations. Such series allowed us to investigate the role of (i) the generation, (ii) the existence of well-ordered microphase-separated structures, and (iii) the bulkiness of the migrating units on the surface-relief formation efficiency. Overall, dendrimers were found to be most efficient for SRG formation, followed shortly by dendrons. Dendronized polymers, on the other hand, were drastically inefficient. The best complex in terms of SRG formation was the second-generation dendrimer–EO complex for which the surface-modulation depth reached a respectable 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 the SRG formation. On the other hand, an increased degree of ordering seemed to destruct SRG formation (based on the dendron-EO complexes). The disordered dendrimer-EO series in turn shows that the size of the migrating units (i.e. the dendritic complex) should not be overly large in order to achieve efficient light-induced mass migration; on the other hand smaller is not necessarily better, as seen by comparing the first- and second-generation dendrimer complexes. In general, dendritic light-responsive supramolecules are shown to provide a powerful and versatile class of materials for gaining fundamental understanding on the photomechanical response of azobenzene-containing materials.
Acknowledgments
Prof. Raffaele Mezzenga is greatly acknowledged for fruitful discussions in the course of this project, and for comments on the manuscript. Prof. A. Dieter Schlüter is thanked for his important contribution on synthesis of the dendritic molecules used. This project is partially funded by the Academy of Finland (project number 135106). J. Vapaavuori acknowledges the financial support of the National Graduate School in Material Physics.
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