Preparation and Characterization of Light-Switchable Polymer

Mar 5, 2013 - The polymer films were prepared by spin-coating the functional polymers from solution and by ultraviolet light (UV)-induced cross-linkin...
1 downloads 10 Views 3MB Size
Download PDF
Article pubs.acs.org/Langmuir

Preparation and Characterization of Light-Switchable Polymer Networks Attached to Solid Substrates Helge Schenderlein,† Agnieszka Voss,‡ Robert W. Stark,‡ and Markus Biesalski*,†,‡ †

Ernst-Berl-Institute of Technical and Macromolecular Chemistry, Chair for Macromolecular & Paper Chemistry, School of Chemistry, and ‡Center of Smart Interfaces and Department of Material Sciences, Physics of Surfaces, Technische Universität Darmstadt, Petersenstrasse 22, 64287 Darmstadt, Germany S Supporting Information *

ABSTRACT: Surface-attached polymer networks that carry light-responsive nitrospiropyran groups in a hydrophilic PDMAA matrix were prepared on planar silicon and glass surfaces and were characterized with respect to their switching behavior under the influence of an external light trigger. Functional polymers bearing light-responsive units as well as photo-cross-linkable benzophenone groups were first synthesized using free radical copolymerization. The number of spiropyran groups in the copolymer was controlled by adjusting the concentration of the respective monomer in the copolymerization feed. The polymer films were prepared by spin-coating the functional polymers from solution and by ultraviolet light (UV)-induced cross-linking utilizing benzophenone photochemistry. On substrates with immobilized benzophenone groups, the complete polymer network is linked to the surface. The dry thickness of the films can be controlled over a wide range from a few nanometers up to more than 1 μm. The integration of such light-switchable organic moieties into a surface-attached polymer network allows one to increase the overall number of light-responsive groups per surface area by adjusting the amount of surfaceattached polymer networks. The spiropyran’s function in dry (solvent-free) and swollen polymer films can be reversibly switched by UV and visible irradiation. In addition, the switching in water is faster than in the dry state. Therefore, implementing lightresponsive spiropyran functions in polymer films linked to solid surfaces could allow for switching of the chemical and optical surface properties in a fast and spatially controlled fashion.

■

hydrophobic (cell-attractive) state.19−21 This result shows the potential of such light-responsive polymer films. However, purely physisorbed layers always suffer from poor chemical and thermal stability. The potential desorption of molecules or the delamination of the whole film can be avoided by chemically attaching the polymer film to the substrate. To this end, Fries et al. prepared polymer brushes bearing spiropyran functional groups with a grafting-from strategy. Such spiropyran polymer brushes exhibit interesting switching behavior and are promising surface coatings that can act as ion sensors.22−25 Another strategy for attaching polymer films to solid surfaces that involves photochemistry has been developed by Rühe and co-workers.26 Surface-attached benzophenone linkers are used to immobilize polymer films to various solid substrates. Under UV irradiation, the benzophenone groups undergo an n−π* or π−π* transition to form a biradical that can react with nearby aliphatic CH groups to form a C−C bond via a H-abstraction/ recombination mechanism. With this technique, thin polymer monolayers with dry film thicknesses (i.e., the mass of surfaceimmobilized polymer) of a few nanometers can be conveniently

INTRODUCTION Functional polymers that respond to an external trigger by changing their chemical or structural properties are often referred to as smart materials. By confining such polymers in thin layers on solid substrates, the surface properties, such as wettability, adhesion, and adsorption,1−3 may become switchable by external triggers. Although the switching of stimuliresponsive polymer films by triggers such as temperature, ionic strength, or pH, has been the focus of a number of previous scientific studies,3−8 understanding the behavior of polymer films that can respond to light as an external trigger has received only limited attention. As an external trigger, light has the advantage of switching polymer films in a spatially controlled manner because it does not depend on diffusioncontrolled mechanisms. The incorporation of spiropyran functions into the surface-attached polymer film is a versatile strategy for preparing light-responsive films.9−12 The spiropyran group can be switched between a nonpolar, closed-ring spiropyran form and a polar, open-ring merocyanine form under the influence of UV and visible light, respectively.13−18 Using physisorbed spiropyran-functional copolymer films at a surface, Higuchi et al., Edahiro et al., and Kikuchi et al. were able to control cell adhesion on solid substrates by switching the films from a more hydrophilic (cell-repelled) to a more © 2013 American Chemical Society

Received: December 21, 2012 Revised: March 4, 2013 Published: March 5, 2013 4525

dx.doi.org/10.1021/la305073p | Langmuir 2013, 29, 4525−4534

Langmuir

Article

Ellipsometry. The dry thickness of the surface-attached films on silicon wafers was measured using a Nanofilm EP3 imaging ellipsometer. One zone angle of incidence (AOI) variation measurements were captured between AOIs of 40 and 80° with a 658 nm laser. The apparent film thickness was calculated from the measured angles Ψ and Δ using the EP4 analysis software supplied with the instrument. The fitting parameters for the silicon oxide layer thickness (tSiOx = 2.5 nm measured separately prior to polymer film immobilization) and the refractive index of the polymer layer (npolymer = 1.5) were held constant. Light Source for UV and Visible Light Irradiation. For the irradiation of the polymer films (switching of the light-sensitive spiropyran-functions), a Lumatec Superlite SUV-DC light source (200 W superpressure short mercury lamp) equipped with liquid fiber optics (Ø = 0.5 cm) was used. Two different filters, one for UV irradiation (λmax = 365 nm) and one for irradiation with light in the visible light range (λ = 400 to 700 nm), were employed. The maximum UV flux density of approximately 150 mW/cm2 at a distance of 3 cm from the tip of the fiber optics was measured with a photometer (International Light ILT 1400-A). According to the manufacturer’s data, the maximum flux density of the visible light is approximately 300 mW/cm2 at a distance of 3 cm from the tip of the fiber optics. The flux can be continuously varied between 0 and 100% of the maximum with a control unit supplied with the instrument. In general, values of 10, 25, 50, or 100% that were relative to the maximum value were chosen. UV−Vis Spectroscopy. A Varian Cary 50 SCAN UV−vis spectrometer with a Lumatec Superlite SUV-DC light source (described above) was used for UV−vis spectroscopy. The outlet of the fiber optics was placed 3 cm from the sample at an angle of approximately 45° to switch the spiropyran groups during the measurements. The samples were illuminated with UV (λmax = 365 nm) or visible light (λ = 400 to 700 nm). The UV−vis spectra were recorded in a range of 300 to 800 nm. For kinetic measurements, the change in the absorbance at a fixed observation wavelength (λmax of the merocyanine form of the nitrospiropyran group) was measured as a function of time (λmax= 565 nm for dry films and 540 nm for films in water). For the UV−vis measurements of covalently attached films, functionalized glass slides (0.8 × 2 cm2) were placed in a cuvette and measured against air or solvent (water). All measurements were performed at 22 °C. Synthesis of (4-(3-(Triethoxysilyl)propoxy)phenyl)(phenyl)methanone (Benzophenone Silane). Benzophenone silane was synthesized according to a protocol published by Prucker et al.26 In brief, 0.29 mol of triethoxysilane, 22 mols of 4-allyloxy-benzophenone, and 70 mg of platinum on activated charcoal were refluxed in a Schlenk flask for 5 h at 120 °C. The excess triethoxysilane was removed by recondensation. The crude product was dissolved in dry toluene, and the catalyst was separated by filtration through a frit. The toluene was removed by evaporation, and the product was dried under high vacuum to yield a slightly brownish oil (76% yield). All of the synthesis steps were performed in the absence of water and oxygen with nitrogen as the inert gas. The product was characterized by 1H NMR. Synthesis of 1′-(2-(Propylcarbamylmethacrylamide)ethyl)3′,3′-dimethyl-6-nitrospiro[2H-1]benzopyran-2,2′-indoline (3). The synthesis of nitrospiropyran monomer 1′-(2(propylcarbamylmethacrylamide)ethyl)-3′,3′-dimethyl-6-nitrospiro[2H-1]benzopyran-2,2′-indoline was similar to that described by Nayak et al.42 with a variation in the final steps to improve the yield and simplify purification. 1-(β-Carboxyethyl)-3′,3′-dimethyl-6nitrospiro(indoline-2′,2[2H-1]benzopyran (2) was synthesized according a previously published protocol.43 Next, 2 was reacted with N(3-aminopropyl)methacrylamide hydrochloride to yield monomer 3. In brief, 11.40 g (30.0 mmol) of 1-(β-carboxyethyl)-3′,3′-dimethyl-6nitrospiro(indoline-2′,2[2H-1]benzopyran (2) and 5.48 g (30.7 mmol) of N-(3-aminopropyl)methacrylamide hydrochloride were dissolved under a nitrogen atmosphere in 200 mL of absolute dimethylformamide (DMFabs) and cooled to 0 °C in an ice bath. Then, 6.89 g (45.0 mmol) of 1-hydroxybenzotriazole monohydrate

attached to solid substrates in an experimentally simple fashion. If polymer layers with larger dry thicknesses are desired, then the incorporation of the same photoreactive group (i.e., benzophenone) into the polymer allows for the attachment of polymer networks. The dry thickness of such polymer networks can be adjusted over a wide range from tens of nanometers up to several micrometers by controlling the amount of polymer that is transferred in the coating step (e.g., spin-coating).27−30 The principles of using benzophenonefunctionalized co- and terpolymers for the photoattachment of thin polymer films to solid substrates that also bear additional functionalities (e.g., redox-responsive groups, charged moieties, or active esters) have been investigated.30−37 Polymer networks containing spiropyran moieties are reported in the literature as bulk materials.38−41 However, thin films of surface-attached spiropyran-functionalized networks also deserve attention. Compared to the preparation of thin films of polymer brushes, which are typically polymerized directly at the solid surface, the preparation of spiropyran-functional polymers is beneficial because they can be attached to a solid substrate as a crosslinked polymer network. Therefore, film parameters, such as the cross-linking density, dry film thickness, and concentration of spiropyran functions inside the films, can be controlled by tuning the chemical composition during the bulk preparation of the copolymers (e.g., concentration of the polymers in dip-/ spin-coat formulation) and adjusting the processing parameters (e.g., UV irradiation). Herein we describe the preparation and characterization of spiropyran-functionalized, surface-attached polymer networks. We prepared different co- and terpolymers based on a hydrophilic poly(dimethyl acrylamide) matrix that has lightresponsive nitrospiropyran as well as photoreactive benzophenone groups. We investigated the formation of surface-attached spiropyran-functionalized polymer networks with respect to controlling the chemistry (i.e., composition) and overall adsorbed mass (i.e., dry thickness) of the polymer film. Finally, we studied the switching behavior of the spiropyran functions incorporated into the surface-attached networks under dry and wet conditions using UV−vis spectroscopy.

■

EXPERIMENTAL SECTION

Materials. All of the chemicals were purchased from Sigma-Aldrich (Munich, Germany) and used as received unless otherwise noted. Dimethylformamide (DMF) was dried over CaH2 for 24 h and distilled under reduced pressure. Toluene was dried over CaCl2 for 24 h and distilled under reduced pressure. Triethoxysilane was purchased from Alfa Aesar (Karlsruhe, Germany) and purified by distillation under reduced pressure. N,N-Dimethylacrylamide (DMAA) was destabilized by flushing through an alkaline alumina column followed by distillation in the presence of copper(I) chloride. N-(3Aminopropyl)methacrylamide hydrochloride was purchased from Polysciences Europe GmbH (Eppelheim, Germany). 1-Ethyl-3-(3′dimethylaminopropyl)-carbodiimide*HCl (EDC*HCl) was purchased from VWR (Darmstadt, Germany). 1-Hydroxybenzotriazole monohydrate (HOBt*H2O) was purchased from Iris Biotech GmbH (Marktredwitz, Germany). Benzophenonemethacrylate (MABP) was synthesized according to previously published protocols.28 Size Exclusion Chromatography (SEC). SEC was performed with a Hewlett-Packard Agilent 1200 series equipped with two columns (PSS GRAM VS 3a and PSS GRAM AS 3b) in series at a constant temperature of 30 °C. DMF containing 3 g/L LiCl was used as the mobile phase (flow rate 0.5 mL/min). The system was calibrated with a Ready Cal-Kit using narrowly dispersed poly(methylmethacrylate) standards purchased from PSS GmbH. 4526

dx.doi.org/10.1021/la305073p | Langmuir 2013, 29, 4525−4534

Langmuir

■

(HOBt*H2O) and 12.5 mL (90 mmol) of triethylamine were added. After 15 min, 7.19 g (37.5 mmol) of N-(dimethylaminopropyl)-N′ethylcarbodiimid hydrochloride (EDC*HCl) was added. After another 15 min, the ice bath was removed, and the reaction flask was stirred at room temperature in the dark for 24 h. After the evaporation of DMF, the reaction mixture was dissolved in chloroform and washed several times with a saturated sodium bicarbonate solution, a saturated sodium chloride solution, and water, respectively. Next, the organic phase was dried over sodium sulfate and filtered. After removal of the chloroform by a rotary evaporator, the dark-red solid was dried under high vacuum to yield 1′-(2-(propylcarbamylmethacrylamide)ethyl)-3′,3′-dimethyl6-nitrospiro[2H-1]benzopyran-2,2′-indoline (2) in quantitative yield. 1 H NMR (300 MHz, CDCl3): δ 8.02−7.93 (m, 2H, 5-H and 7-H), 7.16 (td, J = 7.7, 1.3 Hz, 1H, 6′-H), 7.06 (dd, J = 7.3, 1.0 Hz, 1H, 4′H), 6.93−6.81 (m, 2H, 4-H and 5′-H), 6.77−6.69 (m, 1H, 8-H), 6.66 (d, J = 7.7 Hz, 1H, 7′-H), 6.60 (t, J = 6.3 Hz, 1H, NH), 6.50 (t, J = 6.1 Hz, 1H, NH), 5.85 (d, J = 10.4 Hz, 1H, 3-H), 5.76−5.69 (m, 1H, CH2 allyl), 5.36−5.29 (m, 1H, CH2 allyl), 3.67 (dt, J = 14.7, 7.3 Hz, 1H, CH2−N), 3.49 (dt, J = 14.8, 6.6 Hz, 1H, CH2−N), 3.31−3.08 (m, 4H, CH2), 2.55 (dt, J = 14.4, 7.1 Hz, 1H, CH2−CO), 2.49−2.34 (m, 1H, CH2−CO), 1.96 (dd, J = 1.4, 0.9 Hz, 3H, CH3), 1.66−1.49 (m, 2H, CH2), 1.25 (s, 3H, CH3), 1.13 (s, 3H, CH3). (For the number of dedicated protons, refer to Scheme S1 in the Supporting Information.) 13 C NMR (75 MHz, CDCl3): δ 18.7, 19.9, 25.9, 29.7, 35.8, 35.9, 36.1, 40.1, 53.0, 106.9, 107.0, 115.6, 118.8, 119.8, 120.0, 121.9, 122.1, 122.9, 126.0, 127.9, 128.4, 136.0, 139.8, 141.1, 146.5, 159.6, 169.0, 171.8. All NMR spectra are in agreement with those reported in the literature.42 Terpolymer Synthesis. For co- and terpolymerization of the nitrospiropyran monomer (NSp) with dimethylacrylamide (DMAA) and benzophenone-methacrylate (MABP) as the comonomers, a 50 mmol total monomer feed and 0.3 mol % AIBN (with respect to the monomer) were dissolved in 20 mL of chloroform. The reaction solution was degassed via three pump−freeze−thaw cycles and polymerized at a constant temperature of 60 °C under a nitrogen atmosphere. Co- and terpolymers with different compositions are obtained by a variation in the monomer ratio and holding all of the other polymerization parameters constant. The co- and terpolymers were characterized with respect to the molar mass and chemical identity (i.e., composition) by SEC and 1H NMR spectroscopy. Preparation and Characterization of Surface-Attached Polymer Networks. The photochemical surface attachment of the polymers by a benzophenone linkage was first described in detail by Prucker et al.26 In brief, silicon or glass substrates (2 × 2 cm2/0.8 × 2 cm2) were activated with a radio-frequency plasma treatment followed by deposition of a triethoxybenzophenone silane (BP-silane) layer using a spin-coating technique from a toluene solution (10 mg/mL) at 3000 rpm for 20 s. The covalent attachment of the silane groups to the hydroxyl groups of the surface was performed using a condensation reaction at 120 °C for 15 h. Nonbound BP silane was removed by Soxhlet extraction in toluene for 6 h. Next, P(DMAA-co-NSp-coMABP) was deposited onto the surface by spin-coating from a solution using 1-butanol as the solvent. Note that all of the surface-attached polymer networks reported herein had molar ratios of DMAA to NSp to MABP units of 92: 4: 4. A polymer solution (100 μL) with a fixed polymer concentration was deposited onto the surface of glass or silicon wafers and spin-coated at 3000 rpm for 20 s. Depending on the desired film thickness, the concentration of the polymer in the 1butanol solution was varied between 10 and 150 mg/mL. Surface attachment and cross-linking of the polymers was achieved by UV irradiation (254 nm, 400 mJ/cm2) in a Vilber Lourmat Bio-Link BLX UV exposure chamber followed by Soxhlet extraction of nonbound polymers in acetone for 6 h. All of the films were characterized to determine their chemical identity, homogeneity, and dry film thickness using FTIR spectroscopy, atomic force microscopy, and ellipsometry. All further experiments to investigate the light switching of the films were performed with surface-attached films that have a dry film thickness of approximately 450 nm.

Article

RESULTS AND DISCUSSION

Synthesis of Spiropyran-Functional Terpolymers. To prepare surface-attached polymer networks that respond to light as an external trigger, we first synthesized terpolymers using conventional free radical polymerization with AIBN as the initiator. The polymers were designed to consist of (i) lightresponsive spiropyran moieties, (ii) benzophenone groups for photoimmobilization on planar surfaces as well as cross-linking of the surface-confined polymer film, and (iii) dimethylacrylamide (DMAA) units that serve as a matrix to yield hydrophilic polymer films. DMAA was chosen to yield surface-confined, water-swellable polymer networks. After synthesis, the chemical composition and molar mass of the co- and terpolymers were analyzed by 1H NMR spectroscopy and SEC. By adjusting the concentration of the individual monomers in the polymerization feed, we were able to control the chemical composition of the terpolymers with respect to the amount of photolinker (benzophenone) as well as the lightswitchable spiropyran groups. The content of the lightresponsive spiropyran groups in the polymer was varied up to approximately 10 mol %, and the molar content of the benzophenone groups was maintained between 0 and 10 mol %. (For details of the chemical characterization and copolymer composition, see Table S1 and Figure S1 in the Supporting Information). After the characterization of the polymers, we prepared polymer films using P(DMAA92-co-NSp4-co-MABP4), where the molar content of the spiropyran and benzophenone groups was 4%. This concentration ensured a sufficiently large number of spiropyran functions within the surface-attached polymer network to monitor and investigate the switching processes spectroscopically. Polymer−Surface Attachment. For the preparation of surface-attached films (Scheme 1), silicon and glass slides were modified with a benzophenone silane linker, as described in the Experimental Section. The polymers were spin-coated from a 1butanol solution onto benzophenone-modified substrates. SpinScheme 1. Schematic Illustration of the Preparation of the Surface-Attached, Light-Switchable Polymer Networksa

a

The chemical structure of the light-sensitive polymers and the photochemical cross-linking of the macromolecules are shown as schematic insets.

4527

dx.doi.org/10.1021/la305073p | Langmuir 2013, 29, 4525−4534

Langmuir

Article

Figure 1. Top: Apparent dry film thickness of spin-coated films as measured by ellipsometry (closed symbols) and by AFM (open symbols) as a function of the variation in the concentration of P(DMAA92-co-NSp4-co-MABP4) in 1-butanol (constant spin-coating speed of 3000 rpm). Bottom: Photographic images of the films on silicon wafers and glass slides after irradiation with UV light.

coated films were more homogeneous and reproducible over a wide range of dry film thicknesses than dip-coated films (reference data not shown). Next, the surface-adsorbed polymer films were irradiated with UV light (254 nm, 400 mJ/cm2) to cross-link and covalently attach the polymer films to the substrate simultaneously. Subsequently, nonbound polymers were removed by Soxhlet extraction of the samples using a suitable solvent for the macromolecules. The radiant exposure of 400 mJ/cm2 used for linking the terpolymers to the substrate corresponds to approximately four half-life exposures of the benzophenone groups.27 Therefore, approximately 95% of the benzophenone groups were excited after this time. This ratio ensured that the degree of cross-linking within the film remained nearly constant during the subsequent photoswitching cycles. Next, the surface-attached networks were characterized to determine their chemical identity and dry layer thickness using FTIR spectroscopy and ellipsometry. An example of an IR spectrum that confirms the chemical identity of a surfaceattached P(DMAA92-co-NSp4-co-MABP4) network is shown in the Supporting Information. Figure 1 (top) shows the dry layer thickness of the surface-attached network as a function of the concentration of the polymer in the spin-coating solution. We observed that the dry thickness increases in a nearly linear fashion from tens of nanometers to more than 1 μm as the concentration of the polymer increases in the spin-coating solution. Therefore, by adjusting the concentration of the polymer in the solvent used in the spin-coating process, we were able to control the amount of surface-attached, lightresponsive polymers over a wide range. The films exhibited dry heights on the order of the wavelength of visible light and showed interference effects on reflective substrates such as silicon wafers (Figure 1, bottom).

On transparent glass substrates, we observed an enhanced darkblue to purple coloration of the films. The blue color can be explained by the fact that during surface attachment by UV light, spiropyran functions were also photochemically transferred to the merocyanine form, which has an enlarged πelectron system that absorbs light in the visible wavelength region (Scheme 2). Therefore, the coloration of the polymer Scheme 2. Schematic Illustration of the Light-Induced Switching of the Nitrospiropyran Group between a Nonpolar Spiropyran and a More Polar Merocyanine Form by Irradiation with Light of UV and Vis Wavelengths, Respectively

films provides qualitative proof of the presence of lightresponsive groups in the films. In addition, an enhancement of the intensity of the blue-to-purple color of the substrates qualitatively indicates an increase in the number of merocyanine groups (i.e., increased surface-attached polymer mass) on the substrates. The films prepared by spin-coating the spiropyran-functional polymer appeared to be very homogeneous on a macroscopic level, as observed in Figure 1. To assess the film homogeneity on a microscopic level, dry surface-attached polymer networks 4528

dx.doi.org/10.1021/la305073p | Langmuir 2013, 29, 4525−4534

Langmuir

Article

recorded after different exposure times, as denoted in the figure. After 1 s of exposure, an absorbance band at λmax = 565 nm appeared, which originated from the delocalized π-electron system of the merocyanine form. The intensity of the absorbance band increased as the irradiation time increased until a maximum in the integral intensity was reached after approximately 5 s. After subsequently treating the same film with a flat exposure to visible light, the absorbance at 565 nm decreased. Another equilibrium was reached 15 s later. Then, nearly no absorbance could be detected at this particular wavelength, indicating that the merocyanine groups were converted back to the closed spiropyran form. As long as the extinction coefficient of the analyzed species, the merocyanine form, is unknown, UV−vis experiments can provide only qualitative information. For small spiropyran molecules, the extinction coefficient can be calculated using a combination of NMR analysis and UV−vis spectroscopy, as described in the literature.44 Because of the small number of spiropyran moieties incorporated into our terpolymers, the direct quantification of the ratio between the two forms by NMR spectroscopy is not a trivial task. However, in the UV−vis measurements, we observed (almost) no merocyanine absorbance after visible light irradiation. Therefore, to a good approximation, all of the light-responsive functions were switched back to the spiropyran form. During exposure to UV light, the merocyanine absorbance increased, which indicates that the equilibrium between the two forms was shifted toward the merocyanine form. However, the transfer to the merocyanine form may have been complete or partial. To eliminate variations in the UV−vis measurements caused by minor differences in the film thickness, we normalized the absorbance data. In parts c and d of Figure 3, the normalized

on silicon and glass slides were analyzed with an atomic force microscope (AFM). An analysis of submicrometer surface areas exhibited an rms roughness of the films that was approximately 0.3 nm, which is typical for these substrates. AFM images of the films along with the details of the analysis are provided in the Supporting Information. Switching of Polymer Films in the Dry State. Next, we investigated the light-switching behavior of dry (i.e., solventfree) surface-attached polymer networks. To analyze this switching behavior spectroscopically, we attached polymer films to transparent glass slides. As previously noted, apparent color changes before and after irradiation with UV and visible light were observed with the naked eye (Figure 2). Irradiation with UV light through a lithographic mask (Figure 2, right) resulted in the spatially defined switching of defined areas.

Figure 2. Photographic images of a spiropyran-functional polymer network (450 nm dry thickness) attached to a glass substrate after irradiation with UV light (left), visible light (middle), and UV light through a (lithographic) mask (right).

For a detailed investigation of the switching process, absorption spectra of the films were recorded using UV−vis spectroscopy. Figure 3 shows the UV−vis spectra of a surfaceattached network in air after UV irradiation (a) and the same sample after visible light irradiation (b) (i.e., back-switching the merocyanine into the spiropyran functions). The spectra were

Figure 3. UV−vis spectra of a P(DMAA92-co-NSp4-co-MABP4) film in air (a) after irradiation with UV light (150 mW/cm2) and (b) after irradiation with visible light (300 mW/cm2) with exposure times denoted respectively. (c and d) Respective variations of the normalized absorbance at 565 nm as a function of time. (The black dots correspond to the measured absorbances, and the red lines show the first-order kinetic fits.) 4529

dx.doi.org/10.1021/la305073p | Langmuir 2013, 29, 4525−4534

Langmuir

Article

Figure 4. Measured absorbance of a P(DMAA92-co-NSp4-co-MABP4) film in air at 565 nm. (a) Comparison of the original spectra (black line) and the smoothed spectra (red line) for the first switching cycle and (b) multiple switching cycles by alternating irradiation with UV and visible light (1 min UV at 15 mW/cm2, 2 min visible light at 30 mW/cm2) separated by 1 min of darkness (smoothed data are shown).

Figure 5. UV−vis spectra of a P(DMAA92-co-NSp4-co-MABP4) film swollen in water after (a) irradiation with UV light (150 mW/cm2) and (b) irradiation with visible light (300 mW/cm2). The apparent dry film thickness was approximately 450 nm.

the film was first illuminated for 1 min with UV light (15 mW/ cm2), followed by 1 min of dark relaxation and 2 min of irradiation with visible light (30 mW/cm2). When the light is turned off, we noticed a slight decrease in absorbance after UV irradiation. This effect may originate from a thermal relaxation process, which has been observed for spiropyran functional polymers,40 that is much slower than the observed visible light switching. Next, multiple consecutive UV−vis switches were performed and recorded in situ (Figure 4b). The switching process was reversible for multiple cycles. However, the number of possible cycles was limited by film fatigue as a result of the intensive UV irradiation. It is known from the literature that spiropyrans can undergo side reactions (e.g., oxidation reactions or reactions driven by radical formation).9,46 We investigated the possibility of suppressing such side reactions to suppress the fatigue of the spiropyran groups in the films in the reference studies. We found that switching the surface-attached spiropyran network films in an inert gas atmosphere resulted in a substantial reduction in the fatigue of the films. However, the film fatigue cannot be fully prevented (details in the Supporting Information). Switching of Swollen Surface-Attached Polymer Networks. In particular, we were interested in understanding whether the swelling of such polymer films in water also influences the switching kinetics of the incorporated spiropyran functions. The surface-attached polymer films swelled to 2.0- to 2.5-fold thickness in water (Milli-Q quality, no added salt, pH

absorbance at 565 nm is shown as a function of time. Both UV and visible light irradiation appear to follow first-order kinetics, which is consistent with observations of the switching dynamics of physisorbed spiropyran-polymer films.45 The fitted equations for the UV−vis absorption measurements were

A = e−kt

(1)

and A = 1 − e−kt

(2)

Here, A is the normalized absorbance, t is the irradiation time, and k is the rate constant. Fitting eqs 1 and 2 to the measurements shown in Figure 3 yields photon-flux-dependent rate constants of kUV = 0.74 s−1 and kvis = 0.27 s−1 for UV and visible light switching, respectively. Note, to obtain a more quantitative picture, normalizing the switching rates to the flux density will be discussed later. To analyze the reversibility of the switching process, we studied the switching by measuring the in situ time-dependent absorption of the dry film at a constant wavelength of λ = 565 nm. Figure 4a shows the normalized absorbance for a single UV−vis switch. Such in situ measurements required low flux irradiation as well as FFT smoothing of the data because of the irradiation inside the spectrometer used for the absorbance analysis, which resulted in significant noise in the photodetector. This reduction in flux density also leads to slower switching of the spiropyran functions inside the films, as will be discussed in more detail below. For a complete UV−vis cycle, 4530

dx.doi.org/10.1021/la305073p | Langmuir 2013, 29, 4525−4534

Langmuir

Article

Figure 6. Normalized absorbance by irradiation with different flux densities of UV and visible light as a function of (a, b) irradiation time and (c, d) exposure to light. The dots are the measured values, and the solid lines represent the first-order kinetic fits. (e, f) Calculated k and kn values are shown as a function of the flux density.

7). Films with a typical dry thickness of approximately 400 nm swelled to 800 to 1000 nm. These values were inferred from measurements using optical waveguide spectroscopy (described in the Supporting Information) with a boxlike model for the description of the segment density of the surface-bound layers. Spiropyran functional films were prepared on transparent glass substrates and placed in a cuvette that was fit into the UV−vis spectrometer. The cuvette was filled with deionized water. The surface-bound networks were allowed to swell for approximately 30 min, followed by irradiation with UV and visible light. UV−vis absorption spectra were recorded after defined irradiation times. In Figure 5, a set of UV−vis spectra is shown after irradiation. Exposure to UV light yields an increase in the absorbance at 540 nm resulting from a shift in the equilibrium to the open merocyanine form. Exposure to visible light results in a decrease in this particular absorbance resulting from a shift in the equilibrium to the closed-ring spiropyran form. This result shows that the spiropyran function implemented in the surface-attached polymer network can be switched to the merocyanine form (and back) under wet conditions. However, in contrast to dry conditions, the

absorbance maximum of merocyanine is slightly shifted to approximately 540 nm. This blue shift can be attributed to the solvatochromism of the merocyanine group. Therefore, the position of the absorbance maximum depends on the polarity of the solvent surrounding the organic functions.9 The flux density affects the switching rates of the ensemble. Therefore, we analyzed the switching rate of the films swollen in an aqueous environment at different flux densities. Figure 6a,b shows the normalized absorption as a function of the exposure time for UV and visible light irradiation at various defined fluxes, as denoted in the figure. The flux density was varied between 15 and 150 mW/cm2 (UV) and 30 and 300 mW/cm2 (visible). At the strongest possible flux densities of the light source (150 mW/cm2 (UV) and 300 mW/cm2 (visible)), the spiropyran groups within the surface-attached network switched within less than 2 s from the spiropyran to the merocyanine form and vice versa. Saturation was not observed. We observed a linear relationship between the flux density and switching rate. At 10% of the maximum, switching from the spiropyran to the merocyanine form required 20 s. The switching of the polymer networks in water also followed 4531

dx.doi.org/10.1021/la305073p | Langmuir 2013, 29, 4525−4534

Langmuir

Article

Table 1. Rate Constants k and kn and Half-Life Times (t1/2) for UV−Vis Switching in Air and Watera UV irradiation

visible light irradiation

surroundings

flux density/mW/cm2

k/s−1

R2

t1/2/s

kn/cm2/mJ

flux density/mW/cm2

k/s−1

R2

t1/2/s

kn/cm2/mJ

air water

150 150 75 38 15

0.74 2.83 1.33 0.58 0.24

0.986 0.997 0.998 0.998 0.998

0.94 0.24 0.52 1.20 2.89

0.005 0.019 0.018 0.016 0.016

300 300 150 75 30

0.27 5.90 3.01 1.44 0.53

0.982 0.995 0.997 0.995 0.999

2.57 0.12 0.23 0.48 1.31

0.001 0.020 0.020 0.020 0.018

R corresponds to the deviation in the measured kinetic data and the fitted first-order kinetics. All measurements were performed at 22 °C.

a 2

Figure 7. Measured absorbance at 540 nm of a swollen P(DMAA92-co-NSp4-co-MABP4) film in water (apparent dry film thickness approximately 450 nm). (a) Comparison of the original spectra (black line) and the smoothed spectra (red line). (b) Alternating UV (15 mW/cm2) and visible light irradiation (30 mW/cm2). Irradiation cycles of 20 s were separated by 40 s of darkness (smoothed data are shown).

only the switching kinetics using films attached to transparent glass slides, and the kn values do show good reproducibility on these particular substrates (Supporting Information). The reversibility of the switching in the swollen films may be affected by the aqueous environment. The absorbance at λmax = 540 nm of a surface-attached polymer network in water was measured as a function of time during alternating UV and visible light irradiation cycles (i.e., 20 s of UV, 40 s of dark, 20 s of vis, 40 s of dark) (Figure 7). Again, we smoothed the raw data. Figure 7 shows that even after more than 10 switching cycles the swollen film does not exhibit fatigue. This reduced fatigue may be due to shorter irradiation times required for the switching of the networks under wet conditions as well as slower UV fatigue in water compared to irradiation in air (Supporting Information). The latter finding is consistent with the observation of the switching process being independent of the flux density, as described above.

first-order kinetics (Figure 6). Therefore, the data were fitted using eqs 1 and 2. The energy deposited in the film is given by the exposure (3)

E = Pt

The exposure E is given per surface area in mJ/cm , the flux density P is given in mW/cm2, and the exposure time t is given in seconds. Below the saturation flux, the measured switching rate can be normalized with the flux density P to yield the normalized rate constant kn in cm2/mJ, 2

kn =

k P

(4)

The absorbance as a function of the exposure (as calculated from eq 3) is shown in Figure 6c,d. The rate constants (k) and the normalized rate constants (kn) are shown in Figure 6e,f as a function of the light flux density during the individual switching cycles. Because switching is a linear function of the exposure, simple control of the transfer of spiropyran to the merocyanine form and back is possible by adjusting the exposure of UV− and vis−light, respectively. In Table 1, the calculated k and kn values for the dry films and the water-swollen films are summarized. In addition, the half-life time t1/2 (referring to the switching process) and R2 values (referring to the fits using eqs 1 and 2) are provided. By comparing the rate constants for dry film switching with those for the swollen film switching under similar conditions, we observed that UV and visible light switching values in water are approximately 5 and 20 times faster, respectively. A general increase in the switching rate constants of water-swollen films may be attributed to an increase in the free space of the spiropyran groups resulting from the considerable swelling of the networks in the polar environment. The environment may also act as a stabilizer for intermediate polar products. Thus far, we have investigated

■

CONCLUSIONS Surface-attached polymer networks bearing light-responsive spiropyran functions were prepared and characterized with respect to film characteristics (i.e., film thicknesses, chemical composition) and switching kinetics in the dry and solvent(water-) swollen states. Co- and terpolymers bearing nitrospiropyran and photo-cross-linkable benzophenone groups within a hydrophilic PDMAA matrix were prepared and transferred as thin films onto planar solid substrates using spin-coating. Because the polymers have photoreactive benzophenone groups, surface-attachment and film crosslinking can be simultaneously achieved. This cross-linking reaction does not interfere with the light-switchable spiropyran functions embedded in the polymer network, and the lightswitchable functions can be switched between the closed-ring 4532

dx.doi.org/10.1021/la305073p | Langmuir 2013, 29, 4525−4534

Langmuir

Article

(5) Kooij, E. S.; Sui, X.; Hempenius, M. A.; Zandvliet, H. J. W.; Vancso, G. J. Probing the Thermal Collapse of Poly(N-isopropylacrylamide) Grafts by Quantitative in Situ Ellipsometry. J. Phys. Chem. B 2012, 116, 9261−9268. (6) Bü nsow, J.; Enzenberg, A.; Pohl, K.; Schuhmann, W.; Johannsmann, D. Electrochemically Induced Formation of SurfaceAttached Temperature-Responsive Hydrogels. Amperometric Glucose Sensors with Tunable Sensor Characteristics. Electroanalysis 2010, 22, 978−984. (7) Beines, P. W.; Klosterkamp, I.; Menges, B.; Jonas, U.; Knoll, W. Responsive Thin Hydrogel Layers from Photo-Cross-Linkable Poly(Nisopropylacrylamide) Terpolymers. Langmuir 2007, 23, 2231−2238. (8) Biesalski, M.; Rühe, J. Tailoring the Charge Density of SurfaceAttached Polyelectrolyte Brushes. Macromolecules 2004, 37, 2196− 2202. (9) Such, G.; Evans, R.; Yee, L.; Davis, T. Factors Influencing Photochromism of Spiro-Compounds within Polymeric Matrices. J. Macromol. Sci., Polym. Rev. 2003, 43, 547−579. (10) Ercole, F.; Davis, T. P.; Evans, R. A. Photo-Responsive Systems and Biomaterials: Photochromic Polymers, Light-Triggered SelfAssembly, Surface Modification, Fluorescence Modulation and Beyond. Polym. Chem. 2010, 1, 37−54. (11) Davis, D. A.; Hamilton, A.; Yang, J.; Cremar, L. D.; Van Gough, D.; Potisek, S. L.; Ong, M. T.; Braun, P. V.; Martinez, T. J.; White, S. R.; Moore, J. S.; Sottos, N. R. Force-Induced Activation of Covalent Bonds in Mechanoresponsive Polymeric Materials. Nature 2009, 459, 68−72. (12) Berkovic, G.; Krongauz, V.; Weiss, V. Spiropyrans and Spirooxazines for Memories and Switches. Chem. Rev. 2000, 100, 1741−1754. (13) Lee, H.; Wu, W.; Oh, J. K.; Mueller, L.; Sherwood, G.; Peteanu, L.; Kowalewski, T.; Matyjaszewski, K. Light-Induced Reversible Formation of Polymeric Micelles. Angew. Chem., Int. Ed. 2007, 46, 2453−2457. (14) Vandewij, Ph.; Smets, G. Photochromic Polymers. J. Polym. Sci., Part C: Polym. Symp. 1968, 231−245. (15) Irie, M.; Menju, A.; Hayashi, K. Photoresponsive Polymers Reversible Solution Viscosity Change of Poly(methyl methacrylate) Having Spirobenzopyran Side Groups. Macromolecules 1979, 12, 1176−1180. (16) Ivanov, A. E.; Eremeev, N. L.; Wahlund, P. O.; Galaev, I. Y.; Mattiasson, B. Photosensitive Copolymer of N-Isopropylacrylamide and Methacryloyl Derivative of Spyrobenzopyran. Polymer 2002, 43, 3819−3823. (17) Sumaru, K.; Kameda, M.; Kanamori, T.; Shinbo, T. Reversible and Efficient Proton Dissociation of Spirobenzopyran-Functionalized Poly(N-isopropylacrylamide) in Aqueous Solution Triggered by Light Irradiation and Temporary Temperature Rise. Macromolecules 2004, 37, 7854−7856. (18) Darwish, T. A.; Tong, Y.; James, M.; Hanley, T. L.; Peng, Q.; Ye, S. Characterizing the Photoinduced Switching Process of a Nitrospiropyran Self-Assembled Monolayer Using in Situ Sum Frequency Generation Spectroscopy. Langmuir 2012, 28, 13852−13860. (19) Higuchi, A.; Hamamura, A.; Shindo, Y.; Kitamura, H.; Yoon, B. O.; Mori, T.; Uyama, T.; Umezawa, A. Photon-Modulated Changes of Cell Attachments on Poly(spiropyran-co-methyl methacrylate) Membranes. Biomacromolecules 2004, 5, 1770−1774. (20) Edahiro, J.; Sumaru, K.; Tada, Y.; Ohi, K.; Takagi, T.; Kameda, M.; Shinbo, T.; Kanamori, T.; Yoshimi, Y. In Situ Control of Cell Adhesion Using Photoresponsive Culture Surface. Biomacromolecules 2005, 6, 970−974. (21) Kikuchi, K.; Sumaru, K.; Edahiro, J. I.; Ooshima, Y.; Sugiura, S.; Takagi, T.; Kanamori, T. Stepwise Assembly of Micropatterned Cocultures Using Photoresponsive Culture Surfaces and Its Application to Hepatic Tissue Arrays. Biotechnol. Bioeng. 2009, 103, 552−561. (22) Fries, K.; Samanta, S.; Orski, S.; Locklin, J. Reversible Colorimetric Ion Sensors Based on Surface Initiated Polymerization of Photochromic Polymers. Chem. Commun. 2008, 6288−6290.

spiropyran form and the open merocyanine form by UV and visible light irradiation, respectively. Because very thick polymer networks can be attached to planar substrates, the switching can be easily monitored through a color change in the films via UV−vis spectroscopy or even by the naked eye. Because of the covalent attachment of the films to the surface, it is possible to switch polymer network films surrounded by solvents (i.e., swollen in water) without the surface detachment of the films. Switching of the swollen films occurs much faster compared to the switching of a dry film. Altering the environment of the films (e.g., by changing the surrounding media) can be used to adjust the switching kinetics for potential future applications. The change in the kinetics can be explained by an increase in the mobility of the spiropyran functions in the swollen films as well as the change in the polarity of the surroundings.

■

ASSOCIATED CONTENT

S Supporting Information *

Synthesis details. Copolymerization diagram of the copolymerization of the DMAA monomer and the spiropyran monomer. FTIR and AFM measurments. Optical waveguide mode spectroscopy. UV fatigue of the films during the photoattachment of the films and under air, water, and inert gas conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: +49 6151 16-2177. Tel: +49 6151 16-2479. E-mail: [email protected]. Author Contributions

The manuscript was written with contributions from all of the authors. All of the authors have approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge financial support from the LandesOffensive zur Entwicklung Wissenschaftlich-ö konomischer Excellenz (LOEWE) of the State of Hesse through research initiative Soft Control. We also thank Heike Herbert for technical support and the groups of Prof. Buntkowsky, Prof. Müller-Plathe, and Prof. Thiele of the LOEWE Soft Control Research Cluster at the Department of Chemistry, TU Darmstadt, for valuable discussions and ongoing collaborations. A.V. thanks DFG Exc. 259 for seed funds.

■

REFERENCES

(1) Wischerhoff, E.; Badi, N.; Laschewsky, A.; Lutz, J.-F. Smart Polymer Surfaces: Concepts and Applications in Biosciences. In Bioactive Surfaces; Börner, H. G., Lutz, J.-F., Eds.; Springer: Berlin, 2010; Vol. 240, pp 1−33. (2) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Reversible Switching between Superhydrophilicity and Superhydrophobicity. Angew. Chem., Int. Ed. 2004, 43, 357−360. (3) Cole, M. A.; Voelcker, N. H.; Thissen, H.; Griesser, H. J. StimuliResponsive Interfaces and Systems for the Control of Protein-Surface and Cell-Surface Interactions. Biomaterials 2009, 30, 1827−1850. (4) Ayres, N.; Boyes, S. G.; Brittain, W. J. Stimuli-Responsive Polyelectrolyte Polymer Brushes Prepared via Atom-Transfer Radical Polymerization. Langmuir 2007, 23, 182−189. 4533

dx.doi.org/10.1021/la305073p | Langmuir 2013, 29, 4525−4534

Langmuir

Article

(23) Fries, K. H.; Driskell, J. D.; Samanta, S.; Locklin, J. Spectroscopic Analysis of Metal Ion Binding in Spiropyran Containing Copolymer Thin Films. Anal. Chem. 2010, 82, 3306−3314. (24) Samanta, S.; Locklin, J. Formation of Photochromic Spiropyran Polymer Brushes via Surface-Initiated, Ring-Opening Metathesis Polymerization: Reversible Photocontrol of Wetting Behavior and Solvent Dependent Morphology Changes. Langmuir 2008, 24, 9558− 9565. (25) Fries, K. H.; Driskell, J. D.; Sheppard, G. R.; Locklin, J. Fabrication of Spiropyran-Containing Thin Film Sensors Used for the Simultaneous Identification of Multiple Metal Ions. Langmuir 2011, 27, 12253−12260. (26) Prucker, O.; Naumann, C. A.; Rühe, J.; Knoll, W.; Frank, C. W. Photochemical Attachment of Polymer Films to Solid Surfaces via Monolayers of Benzophenone Derivatives. J. Am. Chem. Soc. 1999, 121, 8766−8770. (27) Berchtold, B. Oberflächengebundene Polymernetzwerke zur ReEndothelialisierung von procinen Herzklappenbioprothesen. Thesis, Albert-Ludwigs-Universität, Freiburg, Germany, 2005. (28) Schlemmer, C.; Betz, W.; Berchtold, B.; Rühe, J.; Santer, S. The Design of Thin Polymer Membranes Filled with Magnetic Particles on a Microstructured Silicon Surface. Nanotechnology 2009, 20, 255301. (29) Toomey, R.; Freidank, D.; Rühe, J. Swelling Behavior of Thin, Surface-Attached Polymer Networks. Macromolecules 2004, 37, 882− 887. (30) Komp, A.; Rühe, J.; Finkelmann, H. A Versatile Preparation Route for Thin Free-Standing Liquid Single Crystal Elastomers. Macromol. Rapid Commun. 2005, 26, 813−818. (31) Bunte, C.; Prucker, O.; König, T.; Rühe, J. Enzyme Containing Redox Polymer Networks for Biosensors or Biofuel Cells: A Photochemical Approach. Langmuir 2010, 26, 6019−6027. (32) Bunte, C.; Rühe, J. Photochemical Generation of FerroceneBased Redox-Polymer Networks. Macromol. Rapid Commun. 2009, 30, 1817−1822. (33) Junk, M. J. N.; Berger, R.; Jonas, U. Atomic Force Spectroscopy of Thermoresponsive Photo-Cross-Linked Hydrogel Films. Langmuir 2010, 26, 7262−7269. (34) Gianneli, M.; Roskamp, R. F.; Jonas, U.; Loppinet, B.; Fytas, G.; Knoll, W. Dynamics of Swollen Gel Layers Anchored to Solid Surfaces. Soft Matter 2008, 4, 1443−1447. (35) Melzak, K. A.; Mateescu, A.; Toca-Herrera, J. L.; Jonas, U. Simultaneous Measurement of Mechanical and Surface Properties in Thermoresponsive, Anchored Hydrogel Films. Langmuir 2012, 28, 12871−12878. (36) Brunsen, A.; Ritz, U.; Mateescu, A.; Höfer, I.; Frank, P.; Menges, B.; Hofmann, A.; Rommens, P. M.; Knoll, W.; Jonas, U. Photocrosslinkable Dextran Hydrogel Films As Substrates for Osteoblast and Endothelial Cell Growth. J. Mater. Chem. 2012, 22, 19590−19604. (37) Huang, J.; Cusick, B.; Pietrasik, J.; Wang, L.; Kowalewski, T.; Lin, Q.; Matyjaszewski, K. Synthesis and In Situ Atomic Force Microscopy Characterization of Temperature-Responsive Hydrogels Based on Poly(2-(dimethylamino)ethyl methacrylate) Prepared by Atom Transfer Radical Polymerization. Langmuir 2007, 23, 241−249. (38) Byrne, R.; Ventura, C.; Benito Lopez, F.; Walther, A.; Heise, A.; Diamond, D. Characterisation and Analytical Potential of a PhotoResponsive Polymeric Material Based on Spiropyran. Biosens. Bioelectron. 2010, 26, 1392−1398. (39) Garcia, A.; Marquez, M.; Cai, T.; Rosario, R.; Hu, Z.; Gust, D.; Hayes, M.; Vail, S. A.; Park, C.-D. Photo-, Thermally, and pHResponsive Microgels. Langmuir 2007, 23, 224−229. (40) Lyubimov, A. V.; Zaichenko, N. L.; Marevtsev, V. S. Photochromic Network Polymers. J. Photochem. Photobiol., A 1999, 120, 55−62. (41) Sumaru, K.; Ohi, K.; Takagi, T.; Kanamori, T.; Shinbo, T. Photoresponsive Properties of Poly(N-isopropylacrylamide) Hydrogel Partly Modified with Spirobenzopyran. Langmuir 2006, 22, 4353− 4356.

(42) Nayak, A.; Liu, H. W.; Belfort, G. An Optically Reversible Switching Membrane Surface. Angew. Chem., Int. Ed. 2006, 45, 4094− 4098. (43) Fissi, A.; Pieroni, O.; Ruggeri, G.; Ciardelli, F. Photoresponsive Polymers. Photomodulation of the Macromolecular Structure in Poly(L-lysine) Containing Spiropyran Units. Macromolecules 1995, 28, 302−309. (44) Raymo, F. M.; Giordani, S.; White, A. J. P.; Williams, D. J. Digital Processing with a Three-State Molecular Switch. J. Org. Chem. 2003, 68, 4158−4169. (45) Stitzel, S.; Byrne, R.; Diamond, D. LED Switching of Spiropyran-Doped Polymer Films. J. Mater. Sci. 2006, 41, 5841−5844. (46) Malatesta, V.; Hobley, J.; Salemi-Delvaux, C. The Chemistry of Photomerocyanines. Mol. Cryst. Liq. Cryst. 2000, 344, 69−76.

4534

dx.doi.org/10.1021/la305073p | Langmuir 2013, 29, 4525−4534