Article pubs.acs.org/Macromolecules
Structurally Controlled Dynamics in Azobenzene-Based Supramolecular Self-Assemblies in Solid State Mikko Poutanen,† Olli Ikkala,*,† and Arri Priimagi*,‡ †
Department of Applied Physics, Aalto University, P.O. Box 15100, FI-00076 Aalto, Espoo, Finland Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FI-33101, Tampere, Finland
‡
S Supporting Information *
ABSTRACT: Light-responsive supramolecular self-assemblies exhibit interplay between order and dynamics of the selfassembling motifs, through which the thermal isomerization rate of azobenzene chromophores can be tuned by orders of magnitude. By using supramolecular complexes of 4-(4-alkylphenylazo)phenols hydrogen-bonded to poly(4-vinylpyridine) as model systems, we demonstrate that the thermal isomerization rate of the hydroxyazobenzene derivatives increases 5700-fold when the material undergoes a transformation from a disordered, low-azobenzene-concentration state to a high-concentration state exhibiting lamellar, smectic-like self-assembly. Drastically smaller thermal isomerization rates are observed in disordered structures. This allows us to attribute the change to a combination of increased number density of the hydroxyazobenzenes inducing plasticization, and cooperativity created by the chromophore−chromophore interactions through self-assembled molecular order and alignment. Our results pinpoint the importance of molecular self-assembly and intermolecular interactions in modifying the dynamics in supramolecular complexes in a controlled manner. We foresee this to be important in lightcontrolled dynamic materials.
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INTRODUCTION Light is an attractive means to control order and disorder as well as functional properties of soft materials by incorporating suitable chromophores that allow light-induced shape changes.1−4 For controlling functionalities, the interplay of light and order is pertinent for amplifying the molecular-scale response through the cooperativity of the self-assembled motifs. In liquid crystals the mesogens are densely laterally packed and affect each other, which leads to cooperative effects. Therefore, the shape changes of only a small fraction of mesogens can lead to a macroscopic, light-induced order−disorder transition.5 The dynamics of the macroscopic responses are typically related to the dynamics of the molecular constituents. However, any macroscopic change simultaneously alters the local microenvironment of the molecular constituents, resulting in interlinked dynamics. Understanding and controlling the mechanisms underlying this interplay is essential for designing material systems with desired light responses. Azobenzene derivatives have been widely used as lightresponsive units in self-assembled structures,6 owing to their © XXXX American Chemical Society
reversible switching, i.e., a change in dipole moment and shape upon the trans−cis photoisomerization. The rod-like transisomers are feasible motifs for liquid crystalline or selfassembled structures, whereas the kinked cis-isomers typically suppress the order due to reduced packing tendency. This allows to control order with light.5 The cis-isomer is metastable, which is a drawback if bistable photoswitchable systems are pursued. On the other hand, several light-induced processes occur as a result of “dynamic photoisomerization”, i.e. continuous cycling between the trans- and cis-isomers, benefiting from short lifetime of the cis-isomer.7−9 The chemical substitution of the aromatic rings and the environment of the azobenzene molecules affect the thermal isomerization process.10 The isomerization has been studied in different solvents,11,12 polymers,13−15 and as liquid crystal dopants,16 but the cooperativity, i.e., effects due to the Received: March 19, 2016 Revised: May 2, 2016
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varied with the degree of complexation, and the absorbance maximum ranged from 0.3 to 1.5 depending on the sample. Fourier Transform Infrared Spectroscopy. FTIR was measured using Nicolet 380 infrared spectrometer in ATR mode by averaging 64 spectra with 2 cm−1 resolution. Small-Angle X-ray Scattering. SAXS measurements were performed with a rotating anode Bruker Microstar microfocus X-ray source (Cu Kα radiation, λ = 1.54 Å) with Montel Optics. The beam was further collimated with four sets of slits, resulting in a beam area of about 1 mm × 1 mm at the sample position. Scattering intensities were measured using a 2D area detector (Bruker AXS). Differential Scanning Calorimetry. DSC was measured with a Mettler Toledo Stare with Stare 8.10 software. The samples were placed in perforated aluminum crucibles and successively heated/ cooled with a rate of 10 °C/min. The second heating scan is used for the analysis. UV−vis Spectrometry. The initial absorbance spectra were recorded with a PerkinElmer Lambda 950 UV/vis/NIR absorption spectrophotometer and the isomerization experiments were carried out using a USB2000+ spectrometer (Ocean Optics, Inc.) with a deuterium−halogen light source. The samples were studied at 25 °C under nitrogen atmosphere. The intensity of the measuring beam was set such that it did not cause significant isomerization in the samples in the used time scales. In long measurements a shutter was used to prevent any isomerization effects of the measuring beam. For the trans−cis isomerization, the films were irradiated with a 365 nm UVLED (Thorlabs, Inc.) equipped with a 10 nm bandpass filter (Edmund Optics, Ltd.). The irradiation intensity varied in the range 5−50 mW/ cm2 between the experiments. The thermal cis−trans isomerization was followed from the time development of the absorbance at 340 nm.
chromophore−chromophore interactions induced by molecular order, has received less attention, especially in supramolecular self-assemblies. Supramolecular chemistry allows a modular approach for self-assembled materials, and light-responsive systems based on complexing chromophores to polymer chains through, e.g., ionic interactions,17,18 halogen bonding19 and hydrogen bonding20,21 have been developed. Therein, hydrogen bonding between phenols and poly(4-vinylpyridine) is particularly useful.21,22 Considering the photoresponsive properties, hydrogen-bond-assisted tautomerization10,23 of the phenol group renders hydroxyazobenzenes particularly sensitive to environmental factors. Therefore, they could allow probing the effect of cooperativity and intermolecular interactions on the dynamics of the chromophore. These effects are generally present in light-responsive supramolecular self-assemblies, and are, therefore, important for controlling their functionality. Herein, we use supramolecular hydrogen-bonded complexes (Figure 1) of 4-(4-alkylphenylazo)phenols with poly(4-vinyl-
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Figure 1. Molecular structures of the compounds used in this work.
RESULTS The hydrogen bonding between the pyridine nitrogen and the phenolic hydroxyl of nPAP can be verified by FTIR (Figure 2), where the absorbance corresponding to the free pyridine moieties at 993 cm−1 decreases and the absorbance of the
pyridine), P4VP, to study the effects of cooperativity and selfassembly on the kinetics of the thermal cis−trans isomerization in glassy polymeric matrices. Hydrogen bonding of 4-(4octylphenylazo)phenol (8PAP) to P4VP renders a smectic-like self-assembly involving lamellae of azobenzenes, promoted by repulsion between the nonpolar octyl chains and polar polymer backbone.20 By contrast, the complex of 4-(4-ethylphenylazo)phenol (2PAP) and P4VP renders a disordered material, due to insufficient repulsion between the short ethyl chain and P4VP. Yet also for 2PAP, hydrogen-bonded complexes are formed, which leads to bottle-brush structures with approximately radially aligned azobenzene moieties around the polymer backbone, with reduced lateral packing of the chromophores in comparison with the smectic packing of 8PAP. The two molecules, in combination with controlling their number density in P4VP, provide a powerful platform for exploring the effect of chromophore density and packing on the dynamics.
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EXPERIMENTAL SECTION
Sample Preparation. The poly(4-vinylpyridine), P4VP (Mw = 50 000 g/mol), and solvents were purchased from Polysciences, Inc. and Sigma-Aldrich, respectively, and were used without further purification. The 4-(4-alkylphenylazo)phenols (nPAP) were synthesized through diazotization reaction of 4-ethylaniline or 4-octylaniline and phenol according to previously reported procedures.24 The samples were prepared by dissolving P4VP and nPAP into chloroform as 15 mg/mL solutions, and mixing them at desired molar ratios. 4-(4octylphenylazo)phenol (n = 8) and 4-(4-ethylphenylazo)phenol (n = 2) were selected, as they should give rise to self-assembled smectic and disordered complexes upon hydrogen bonding to P4VP, respectively. The nominal molar ratios of nPAP vs pyridines of P4VP were x = 0.01, 0.05, 0.10, 0.25, 0.50, 0.75, and 1.00. Throughout this paper, the complexes are denoted as P4VP(nPAP)x. From the mixtures, thin films were spin-coated on quartz substrates. The absorbances of the films
Figure 2. Infrared spectra of (a) P4VP(8PAP)x and (b) P4VP(2PAP)x complexes at different nominal mole fractions x, confirming the complexation of 8PAP and 2PAP to the pyridines. B
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Figure 3. (a) SAXS data for P4VP(8PAP)x. Note that pure 8PAP reference is a crystalline material, and the data depicts the packing in the crystal. (b) Schematic structures of P4VP(8PAP)x at low and high degrees of complexation, and (c) the corresponding polarized optical micrographs of the complexes x = 0.1 (bottom) and x = 1.0 (top), imaged after thermal annealing.
Figure 4. (a) SAXS data for P4VP(2PAP)x and (b) schemes of the corresponding structures formed at low and high degrees of complexation. (c) Polarized optical micrograph after thermal annealing.
hydrogen-bonded moieties at 1008 cm−1 strengthens with increasing nPAP content for both complexes. However, the peak at 993 cm−1 does not completely disappear even for x = 1.0, which indicates that a small fraction of the pyridine groups remains uncomplexed at the nominally equimolar ratio. All samples are macroscopically homogeneous, also the ones with x = 1.0. Note that the equimolar complexes have nPAP weight fractions even up to 80 wt %, and yet all the spin-coated films are of high optical quality, indicating the robustness and excellent film-forming properties of the supramolecular complexes. The structures of the complexes containing 8PAP and 2PAP were characterized by small-angle X-ray scattering (SAXS) and polarized optical microscopy. At low fractions of 8PAP (see Figure 3a), broad scattering maxima are observed, indicating correlation-hole behavior, typical for disordered material.25 At higher fractions (x ≥ 0.25), the reflections become narrower, and for P4VP(8PAP)1.0 a narrow reflection q* = 0.165 Å−1 and
second-order reflection 2q* are observed. This confirms a lamellar order with periodicity of 3.8 nm, indicative of end-toend packing of the 8PAP molecules. P4VP(8PAP)0.75 and P4VP(8PAP)1.00 also exhibit birefringent textures in polarized optical microscope (Figure 3c). Therefore, the lamellar order can be assigned to smectic-like self-assembly, where the azobenzene chromophores are laterally tightly packed, as shown schematically in Figure 3b. With low fractions of 8PAP, the complexes do not show birefringence in POM, confirming their disordered nature. These observations agree with previous structural findings.20 By contrast, hydrogen bonding of 2PAP to P4VP (see Figure 4) leads only to a broad correlation peak centered at 0.23 Å−1, even at nominal complexation degree of 1.00. The maximum of the broad peak corresponds to a dimension of 2.3 nm. The broadness of the peak indicates the lack of order, as also confirmed by POM showing no birefringence (Figure 4c) when viewed between crossed polarizers. As the FTIR data shows C
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Macromolecules 2PAP complexation to the polymer chain, the disordered nature seen in SAXS and POM data infers disordered bottlebrush-like structures, depicted schematically in Figure 4b. Bottle-brush structures have been previously reported for hydrogen bonded complexes of alkylphenols and P4VP.26 In the case of 2PAP, the repulsion of the ethyl chain is insufficient to drive for self-assemblies and cylindrical brushes are thus formed. Importantly, the 2PAP molecules are not ordered with respect to each other in this case, i.e., not laterally packed, whereas the 8PAP molecules are aligned in the smectic phase, i.e., tightly packed side by side. Hydrogen bonding of low-molecular-weight compounds into glassy polymer matrix can lead to plasticization and reduced glass transition temperature, Tg. The Tg’s for the complexes were analyzed by differential scanning calorimetry (DSC), as shown in Figure 5. Pure P4VP has a Tg of ca. 150 °C, and the
Figure 6. (a) Isomerization of azobenzene molecules. (b) Spectral changes upon thermal cis−trans isomerization for P4VP(8PAP)0.50. The inset shows the time development of the normalized absorbance and thermal relaxation for complexes P4VP(8PAP)x with x = 0.01, 0.50, 1.0.
the peak at 355 nm and as an increase of the peak at 450 nm. Isosbestic points are observed for all samples, indicating that no other spectral changes occur during the relaxation. Upon the trans-cis isomerization, the lamellar assembly of the P4VP(8PAP)x observed at high degrees of complexation may partially break due to the geometrical shape change of the 8PAP molecules into the kinked cis-state. The wavelength 340 nm is used to follow the time dependence of the spectrum as shown by the inset of Figure 6b. The photostationary states vary somewhat depending on the azobenzene concentration due to the changes in thermal isomerization speed. However, the absorbance change at 340 nm is in the range 55−75% in all the complexes (for both P4VP(8PAP)x and P4VP(2PAP)x). After the UV-illumination is stopped, the cis-isomers relax thermally back to the trans-form, with relaxation rates drastically increasing as a function of the azobenzene concentration as illustrated in the inset of Figure 6b. The thermal cis−trans isomerization in glassy polymeric matrices deviates from monoexponential behavior observed in isotropic solutions.28 In soft medium the isomerization follows first-order kinetics, but below the Tg, the constraints caused by the frozen polymer matrix cause deviations due to free-volume distributions and possible constrained states of the isomers. The isomerization in glassy matrices has previously been fitted with two- or multiexponent functions,28 or a stretchedexponent function.11 The present data is captured by the two-parameter fit (k, β) of the stretched-exponential function, which is here defined as
Figure 5. Glass transition temperatures for P4VP(nPAP)x as a function of the molar fraction of nPAP molecules.
addition of nPAP gradually lowers the Tg to as low as 50 °C. Even though the Tg’s were lowered significantly, all complexes are still in a glassy state at room temperature where the optical experiments are conducted. The Tg’s of the two complexes behave approximately similarly as a function of the nPAP content, and at x = 1.00 the Tg’s are practically the same, thus, allowing the comparison of self-assembled and disordered complexes at similar plasticization states. In addition to the Tg, an order−disorder transition for the P4VP(8PAP)1.0 is observed passing 103 °C, where the lamellar order is destroyed, seen also as disappearance of the birefringent texture in POM. The photoisomerization behavior (Figure 6a) of P4VP(nPAP)x was measured from thin films at room temperature (25 °C) under nitrogen atmosphere, in order to reveal the effects of the degree of complexation and self-assembly on the isomerization kinetics. The initial spectra of complexes of 2PAP and 8PAP are dominated by the π−π* absorption peak of the trans-isomer around 360 nm, and the weaker n-π* absorption peak at ca. 450 nm. Upon increasing the azobenzene concentration, the absorption maximum exhibits an 8 nm blue-shift from 362 to 354 nm in both complexes (see Supporting Information, Figure S1), indicative of stronger chromophore−chromophore interactions.27 Figure 6b shows the isomerization measurement using P4VP(8PAP)0.50 as a characteristic example (see Supporting Information, Table S1 for details for all complexes). The film is illuminated with 365 nm UV-light for 60 s, during which a cis-isomer-rich photostationary state is reached, which is seen as lowering of
β
A(t ) = −(A∞ − A 0)e−(k·t ) + A∞
where A∞ is the absorbance after full thermal isomerization (alltrans state), A0 is the absorbance in the photostationary state, k is the rate constant, and β is the stretching exponent. The fits are of good quality for all the complexes (see Supporting Information, Figure S2). The azobenzenes isomerize efficiently in all compositions, but importantly, the thermal cis-trans isomerization speeds up substantially with increasing nPAP fraction (Figure 7). At low x, D
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complexes, we account the concentration dependence of the rate constant to two different mechanisms: (1) increased number density of the nPAP resulting in plasticization and (2) cooperativity induced by self-assembly. The two mechanisms are interconnected, yet our data unequivocally shows that the former alone is insufficient to explain the experimental observations. Plasticization of the polymeric phase caused by the dynamic nature of the hydrogen-bonded low-molecular-weight nPAP is an essential phenomenon in these complexes. In contrast, in covalent side-chain polymers, the polymer backbone largely determines the Tg, and in conventional doped systems, the achievable chromophore concentration without phase separation is limited to moderate values. Previous reports indicate a dependence of kinetics of the thermal cis−trans isomerization on Tg, and for example a Williams−Landel−Ferry-type dependence15 has been reported. However, also a lack of significant effects on Tg28 or even an indirect evidence suggesting increasing thermal lifetime upon decreased Tg in supramolecular complexes33 have been reported. In all these cases, the changes of the cis-lifetime have been far less significant than in the present study. Hydroxyazobenzene derivatives form a special group among azobenzenes as they exhibit hydrogen-bond-assisted tautomerization.10,23 We expect this to play an important role in the isomerization behavior of our complexes which are in the solid glassy state. In polar protic solvents, the tautomerization is triggered by the hydrogen bonding of the solvent molecules to the −NN− moiety. This causes intrinsic changes in the isomerization, and allows the rotational pathway for the thermal isomerization instead of the inversional one.10,23,34,35 Therefore, thermal cis−trans isomerization of hydroxyazobenzene derivatives in protic solvents can be significantly faster than in aprotic solvents, even up to 5 orders of magnitude. In our solid state, solvent-free complexes, the only hydrogen-bond donors are the phenol groups of the nPAP molecules, and the complexes can exhibit tautomerization only through chromophore−chromophore hydrogen bonding to the −NN− moiety. Upon increasing the nPAP concentration, the probability of intermolecular hydrogen bonds between the nPAP molecules increases, and thus more nPAP molecules are able to tautomerize. Combined with the dynamic and interchanging nature of the hydrogen bonding, also seen as plasticization, this speeds up the overall thermal isomerization speed. This effect is present in both 2PAP and 8PAP complexes, but it is the main mechanism for the change of cis-lifetime in the disordered 2PAP complexes. The self-assembly of 8PAP molecules induces aligned packing, which leads to cooperativity between the mesogenic azobenzene molecules, as the chromophores interact with each other. The alignment and proximity increase the tendency of hydrogen bonding to the −NN− moiety especially after the UV-irradiation and trans−cis isomerization, which we believe to partially destroy the microstructure. This increases the tautomerization, seen as increased thermal isomerization speed. The self-assembly-induced structural features promoting hydrogen bonding create the 36-fold difference between two complexes as the disordered phase of 2PAP complexes lack these mechanisms. The dependence on the self-assembly process highlights the effects that specific intermolecular interactions, alignment, and local environments of the molecules can have on the kinetics of the thermal isomerization process. Such a strong dependence on the molecular order
Figure 7. Thermal isomerization constant as a function of the nPAP content for P4VP(nPAP)x.
both 2PAP and 8PAP complexes are disordered, and their isomerization kinetics are approximately similar. However, at higher molar ratios (x ≥ 0.25), there are significant differences. In complexes of 8PAP the relaxation coefficient increases from 2.1 × 10−5 (x = 0.01) to 0.12 s−1 (x = 1.00), which by taking the stretching exponents into account correspond to the mean lifetimes of 25 h and 8 s, respectively. Such 5700-fold difference in the relaxation coefficient of the same azobenzene molecule has not been previously observed in the solid state. As a result, the relaxation kinetics of the cis-isomer can be modularly tuned between metastable (low concentration) and “dynamic” (high concentration) by controlling the chromophore concentration in the supramolecularly bound complexes. With complexes of 2PAP, the corresponding change is from 3.1 × 10−5 to 3.4 × 10−3 s−1, which is also a notable increase, yet significantly less than for the self-assembled 8PAP complexes. Also the stretching exponents (see Supporting Information, Figure S3) vary with concentration, starting from values of 0.5 at low nPAP concentrations, and increasing close to 1 at high concentrations. This indicates smaller deviation from the first-order kinetics at higher nPAP contents, which is consistent with the plasticization, alleviating the constraints brought about by the polymer matrix. The effect of the self-assembly on the isomerization dynamics is clearly reflected in the difference between 2PAP and 8PAP complexes at high molar fractions. Above the threshold of 25 mol %, the complexes of 8PAP show significantly faster thermal isomerization. As was seen from the SAXS data, the 8PAP complexes start showing evidence of self-assembled order around the same molar fraction. The structural features of the smectic-like self-assembly of P4VP(8PAP)1.0 complex lead to 36-times faster isomerization when compared to the disordered P4VP(2PAP)1.0 complex. Besides the difference that the 8PAPcomplex has self-assembled order with molecular alignment whereas the 2PAP-complex does not, the complexes are comparable as they show similar Tg’s and also identical absorption spectra.
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DISCUSSION The 5700-fold increase in the rate constant as a function of concentration is overwhelmingly large as compared to previous articles, which have reported 2−100−fold differences between different polymeric phases.28,29 These differences are generally attributed to the different free volumes,13 chemical environments,30,31 and site- and order-specific properties.32 In our E
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(3) Iamsaard, S.; Aßhoff, S. J.; Matt, B.; Kudernac, T.; Cornelissen, J. J. L. M.; Fletcher, S. P.; Katsonis, N. Conversion of Light into Macroscopic Helical Motion. Nat. Chem. 2014, 6 (3), 229−235. (4) Liu, D.; Broer, D. J. New Insights into Photoactivated Volume Generation Boost Surface Morphing in Liquid Crystal Coatings. Nat. Commun. 2015, 6, 8334. (5) Yu, H.; Ikeda, T. Photocontrollable Liquid-Crystalline Actuators. Adv. Mater. 2011, 23 (19), 2149−2180. (6) Smart Light-Responsive Materials; Zhao, Y., Ikeda, T., Eds.; John Wiley & Sons, Inc.: 2009. (7) Lee, K. M.; Smith, M. L.; Koerner, H.; Tabiryan, N.; Vaia, R. A.; Bunning, T. J.; White, T. J. Photodriven, Flexural-Torsional Oscillation of Glassy Azobenzene Liquid Crystal Polymer Networks. Adv. Funct. Mater. 2011, 21 (15), 2913−2918. (8) Priimagi, A.; Shevchenko, A. Azopolymer-Based Micro- and Nanopatterning for Photonic Applications. J. Polym. Sci., Part B: Polym. Phys. 2014, 52 (3), 163−182. (9) Hrozhyk, U. A.; Serak, S. V.; Tabiryan, N. V.; Hoke, L.; Steeves, D. M.; Kimball, B. R. Azobenzene Liquid Crystalline Materials for Efficient Optical Switching with Pulsed And/or Continuous Wave Laser Beams. Opt. Express 2010, 18 (8), 8697−8704. (10) Bandara, H. M. D.; Burdette, S. C. Photoisomerization in Different Classes of Azobenzene. Chem. Soc. Rev. 2012, 41 (5), 1809− 1825. (11) Serra, F.; Terentjev, E. M. Effects of Solvent Viscosity and Polarity on the Isomerization of Azobenzene. Macromolecules 2008, 41 (3), 981−986. (12) Asano, T.; Okada, T. Thermal Z-E Isomerization of Azobenzenes. The Pressure, Solvent, and Substituent Effects. J. Org. Chem. 1984, 49 (23), 4387−4391. (13) Mita, I.; Horie, K.; Hirao, K. Photochemistry in Polymer Solids. 9. Photoisomerization of Azobenzene in a Polycarbonate Film. Macromolecules 1989, 22 (2), 558−563. (14) Sasaki, T.; Ikeda, T.; Ichimura, K. Photoisomerization and Thermal Isomerization Behavior of Azobenzene Derivatives in LiquidCrystalline Polymer Matrixes. Macromolecules 1993, 26 (1), 151−154. (15) Eisenbach, C. D. Effect of Polymer Matrix on the Cis-Trans Isomerization of Azobenzene Residues in Bulk Polymers. Makromol. Chem. 1978, 179 (10), 2489−2506. (16) Garcia-Amorós, J.; Finkelmann, H.; Velasco, D. Role of the Local Order on the Thermal Cis-to-Trans Isomerisation Kinetics of Azo-Dyes in Nematic Liquid Crystals. Phys. Chem. Chem. Phys. 2011, 13 (23), 11233−11238. (17) Zakrevskyy, Y.; Stumpe, J.; Faul, C. F. J. A Supramolecular Approach to Optically Anisotropic Materials: Photosensitive Ionic Self-Assembly Complexes. Adv. Mater. 2006, 18 (16), 2133−2136. (18) Koskela, J. E.; Liljeström, V.; Lim, J.; Simanek, E. E.; Ras, R. H. A.; Priimagi, A.; Kostiainen, M. A. Light-Fuelled Transport of Large Dendrimers and Proteins. J. Am. Chem. Soc. 2014, 136 (19), 6850− 6853. (19) Saccone, M.; Cavallo, G.; Metrangolo, P.; Resnati, G.; Priimagi, A. Halogen-Bonded Photoresponsive Materials. Top. Curr. Chem. 2014, 359, 147−166. (20) de Wit, J.; van Ekenstein, G. A.; Polushkin, E.; Kvashnina, K.; Bras, W.; Ikkala, O.; ten Brinke, G. Self-Assembled Poly(4-vinylpyridine)−Surfactant Systems Using Alkyl and Alkoxy Phenylazophenols. Macromolecules 2008, 41 (12), 4200−4204. (21) Zhao, Y.; Thorkelsson, K.; Mastroianni, A. J.; Schilling, T.; Luther, J. M.; Rancatore, B. J.; Matsunaga, K.; Jinnai, H.; Wu, Y.; Poulsen, D.; et al. Small-Molecule-Directed Nanoparticle Assembly towards Stimuli-Responsive Nanocomposites. Nat. Mater. 2009, 8 (12), 979−985. (22) Ruokolainen, J.; ten Brinke, G.; Ikkala, O.; Torkkeli, M.; Serimaa, R. Mesomorphic Structures in Flexible Polymer−Surfactant Systems Due to Hydrogen Bonding: Poly(4-vinylpyridine)−Pentadecylphenol. Macromolecules 1996, 29 (10), 3409−3415. (23) Garcia-Amorós, J.; Velasco, D. Understanding the Fast Thermal Isomerisation of Azophenols in Glassy and Liquid-Crystalline Polymers. Phys. Chem. Chem. Phys. 2014, 16 (7), 3108−3114.
indicates that the interactions between 8PAP molecules, i.e., cooperativity, are essential for the thermal isomerization speed in supramolecular self-assemblies. This has implications especially in systems where the intermolecular hydrogen bonding has an effect on the kinetics, like with hydroxyazobenzene derivatives, but also on other azobenzene derivatives, even though the effect there should be smaller. The altered intermolecular interactions upon self-assembly are a general effect, and may affect also properties other than thermal isomerization.
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CONCLUSIONS In conclusion, we have shown in solid glassy supramolecular complexes that the intermolecular hydrogen bonding and the dense lateral packing in smectic self-assemblies increases significantly the kinetics of the thermal cis−trans isomerization of hydroxyazobenzene derivatives. The speed may be increased even by a factor of 5700, which is considerably larger than changes that have been reported for covalently functionalized azopolymers. The speed-up of the isomerization is attributed to two phenomena: (i) increased number density of the supramolecularly bound azobenzene derivatives resulting in plasticization, and (ii) cooperativity due to self-assembly of the tightly laterally packed chromophores within a smectic phase. We also show a 36-fold difference between comparable selfassembling and disordered complexes, demonstrating thus the general importance of alignment-induced specific intermolecular interactions and changed local microenvironment.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00562. Additional supporting data and figures of the optical characterization (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(A.P.) E-mail: arri.priimagi@tut.fi. *(O.I.) E-mail: olli.ikkala@aalto.fi. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Academy of Finland through its Centres of Excellence Program (2014-2019). AP greatly acknowledges the Academy Research Fellowship program (Decision numbers 277091 and 284553) and the Emil Aaltonen Foundation for financial support.
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REFERENCES
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DOI: 10.1021/acs.macromol.6b00562 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.6b00562 Macromolecules XXXX, XXX, XXX−XXX
