ThiolâYne Click

Article pubs.acs.org/cm

Facile Image Patterning via Sequential Thiol−Michael/Thiol−Yne Click Reactions Haiyan Peng,†,‡,§ Chen Wang,∥ Weixian Xi,† Benjamin A Kowalski,⊥ Tao Gong,† Xiaolin Xie,§ Wentao Wang,‡ Devatha P. Nair,† Robert R. McLeod,⊥ and Christopher N. Bowman*,† †

Department of Chemical and Biological Engineering, University of Colorado, UCB 596, Boulder, Colorado 80309, United States Guangzhou Institute of Advanced Technology, Chinese Academy of Science, Guangzhou 511458, China § School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ∥ Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, Shaoxing University, Shaoxing 312000, China ⊥ Department of Electrical, Computer and Energy Engineering, University of Colorado, Boulder, Colorado 80309, United States ‡

S Supporting Information *

ABSTRACT: Freestanding substrates with high refractive index modulation, good oxygen resistance, and low volume shrinkage are critical in photolithography for the purpose of high density data storage, image patterning and anticounterfeiting. Herein, we demonstrate a novel paradigm of direct holographic image patterning via the radical-mediated thiol− yne click reaction subsequent to the base-catalyzed thiolMichael addition reaction. With the benefit of a newly synthesized alkyne monomer, 9-(2-((2-(prop-2-yn-1-yloxy)ethyl)thio)ethyl)-9H-carbazole (POETEC), holograms with as high as 96% diffraction efficiency, refractive index modulation of 0.0036, dynamic range of 5.6 per 200 μm and volume shrinkage of 1.1%, are successfully patterned in an aerobic environment. Uniquely and distinctly, an inhibitor is unnecessary to prevent the initiation of the sequential reaction in this framework.

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INTRODUCTION Holography, i.e., an interference process of multiple coherent beams,1 represents a powerful technique with vast capabilities useful for complex 3D fabrication over large areas, production of high density data storage and useful for high speed data processing. As such, research in this area from both the scientific and industrial realms has focused on applications such as 3D displays,2,3 3D image recording,4,5 graded rainbow patterns,6,7 anticounterfeiting,8 and data storage.9,10 During the holographic photopolymerization, which is used to construct permanent interference patterns, monomers diffuse from unexposed areas to irradiated areas, yielding material and corresponding refractive index differences between the constructive and destructive interference regions. The custom-designed features and performance necessitate exquisite control of the photopolymerization kinetics and network formation/gelation with both spatial and temporal control.5 Nevertheless, there are still vast challenges and beneficial opportunities to tailor the holographic process in thick freestanding substrates that possess mechanical toughness, low grating distortion, and large area production capability.11−14 In an attempt to conduct holographic recording on selfsupporting matrices, monomers for holography must be stable during the matrix formation step while also reacting rapidly during the subsequent patterning step. Toward this end, we © 2014 American Chemical Society

recently explored a novel two-stage orthogonal thiol-Michael/ thiol−ene click approach and formulated Bragg gratings in an aerobic environment with excellent oxygen tolerance and light sensitivity.6 Yet, straightforward designs that complement the fundamental understanding and practical applications of the two-stage orthogonal click framework have been barely examined. Click chemistry continues to attract considerable attention after its inception in 200115 because of its desirable characteristics such as high chemoselectivity, wide functional group tolerance, rapid reaction kinetics, and the ability to be performed at ambient conditions.16−24 Further, the various extensions to photomediated click reactions provide added beneficial routes to achieve spatiotemporal control and the design of new materials with uniform geometry and high glass transition temperatures.24−30 In light of the advantage of their orthogonal nature, the combination of two or more “click” reactions (either sequential or simultaneous) enables expansive applications including but not limited to the synthesis of dendrimers,31,32 hyperbranched polymers,33,34 main-chain cationic polymers,35 block copolymers,36,37 amphiphiles,38 and the implementation of biochemical patterning,39,40 microReceived: September 18, 2014 Revised: November 8, 2014 Published: November 20, 2014 6819

dx.doi.org/10.1021/cm5034436 | Chem. Mater. 2014, 26, 6819−6826

Chemistry of Materials

Article

Chemetics. Triethylamine (TEA, purity: ≥98%) was purchased from Fluka. 2,4,6-Trimethylbenzoyldiphenylphosphine oxide (TPO, purity: 97%) was provided by BASF Corporation. All chemicals were used as received unless otherwise noted. Synthesis. The synthesis route for the high index yne-functionalized molecule, POETEC, is given as Scheme 2. To a 250 mL dry

arrays,41 Janus particles,42 3D lithography,43 and damage recovery.44 Among all orthogonal click chemistries, those approaches involving thiol-click reactions (e.g., photomediated thiol−ene reaction45) have drawn particular attention since the sulfur−hydrogen bond is amenable to a variety of different click reaction types.19,24 Though the orthogonal sequential thiol− Michael/thiol−yne click paradigm has been explored by Chan and others to form polyfunctional materials,46 cross-linked networks,47 and low shrinkage composites,33 its implementation in optics and engineering is still in its infancy. Herein, we report the synthesis of a novel alkyne monomer, 9-(2-((2-(prop-2-yn-1-yloxy)ethyl)thio)ethyl)-9H-carbazole (POETEC), with a high refractive index (∼1.68) and showcase a technique of holographic image patterning with high diffraction efficiency (96%) and low grating distortion via photomediated thiol−yne click chemistry on freestanding scaffolds, which are initially formulated through the thiol− Michael addition reaction (Scheme 1). We utilize the thiol−yne

Scheme 2. Synthesis of 9-(2-((2-(Prop-2-yn-1yloxy)ethyl)thio)ethyl)-9H-carbazole (POETEC)

Scheme 1. Overall Process for Holographic Patterning via Sequential Thiol−Michael/Thiol−Yne Click Reactions round-bottom flask, 20 g of N-vinylcarbazole, 7.2 mL of 2mercaptoethonal, 0.56 g of TPO, and 10 mL of anhydrous THF were added and allowed to stir at ambient temperature while being exposed to a 365 nm UV light (EXFO Acticure 4000 high-pressure mercury lamp equipped with a 365 nm short-pass filter) with an intensity of 20 mW/cm2 for 20 min. The full conversion of vinyl and thiol groups via the photomediated thiol−ene click reaction was confirmed by 1H NMR, and the intermediate product was used in subsequent reactions without further purification. The above-mentioned intermediate without additional purification was directly transported to another 1000 mL dry round-bottom flask and then located in an ice bath. An additional 90 mL anhydrous THF was added and then 6.7 g of sodium hydride was slowly added to the flask when stirring. The reaction mixture was allowed to warm to ambient temperature and kept stirring for another hour. Then, 16.8 mL of propargyl bromide was added dropwise, and the reaction was kept stirring overnight. Fifty milliliters of ethanol was added to quench the reaction. To purify the product, we added 200 mL of DI water to wash the mixture, filtered it with a filter paper, then extracted and washed the organic phase by using 1200 mL of ethyl acetate and 20 mL brine, respectively. The final pure product with a light yellow oil appearance (25 g, yield: 78%) was given after drying the crude by anhydrous Na2SO4 and separating by a silica gel column eluting with a mixture of hexane and ethyl acetate (20:1 in volume). 1H NMR (400 MHz, chloroform-d) δ 8.11 (d, J = 8.5 Hz, 2H), 7.52−7.41 (m, 4H), 7.33−7.21 (m, 2H), 4.60−4.48 (t, 2H), 4.15 (d, J = 2.4 Hz, 2H), 3.70 (t, J = 6.3 Hz, 2H), 3.06−2.97 (t, 2H), 2.76 (t, J = 6.3 Hz, 2H), 2.44 (t, J = 2.4 Hz, 1H). 13C NMR (101 MHz, chloroform-d) δ 140.08, 125.80, 123.01, 120.48, 119.22, 108.60, 79.43, 74.84, 69.88, 58.22, 43.41, 31.97, 30.89. HRMS calculated for [C19H20NOS]+ ([MH]+): m/z 310.1266, found: 310.1277. The NMR spectra are given as Figures S1−S3 in the Supporting Information. The refractive index of POETEC was determined to be ∼1.68.53 Sample Preparation. TMPTMP, DTPTA, POETEC, and 2 wt % of TPO were added to a 10 mL transparent vial, and then placed into a water bath at 45 °C. 0.3 wt % of TEA was added to trigger the basecatalyzed thiol-Michael addition reaction that allowed the formation of films in stage 1, a gel state before flood curing. The stage 2 films were prepared by irradiating the stage 1 films on each side for 10 min with an EXFO Acticure 4000 high-pressure mercury lamp. The light source was filtered through a 365 nm short pass filter, and the intensity was set as 20 mW/cm2. Kinetics. The reaction kinetics were monitored by a Nicolet 750 Magna FTIR spectrometer equipped with a KBr beam splitter and an MCT/A detector. We sandwiched samples between two NaCl plates

reaction for holographic recording in accordance with three main aspects. First, the alkyne monomer, POETEC, is able to increase the refractive indices of the resulting cross-linked networks by improving the loading density of sulfur, which has a high atomic refraction,48 and accordingly, to offer a high grating diffraction efficiency. Second, the alkyne homopolymerization is negligible with a stoichiometric excess of thiols,49 and the total volume shrinkage of the thiol−yne systems is low. Finally, inhibitors are unnecessary to eliminate any thermally induced polymerization during the matrix formation step used to create the recording media prior to hologram writing.50 Because kinetic control plays a paramount role in the two-stage technique in which the first stage allows for explicit manipulation and processing while the second stage offers final and distinct structures,6,44,51,52 the results shown here are indicative of exciting opportunities afforded for designing and creating complex features and functional materials through carefully chosen monomer structures in a two-stage orthogonal click paradigm.

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EXPERIMENTAL SECTION

Materials. Butyl 3-mercaptopropionate (purity: 98%), di(trimethylolpropane)tetraacrylate (DTPTA) and N-vinylcarbazole (purity: 98%) were purchased from Sigma-Aldrich. Sodium hydride (purity: 60 wt % dispersion in mineral oil), propargyl bromide (purity: 80 wt % solution in toluene) and 2-mercaptoethanol (purity: 99%) were purchased from Acros Organics. Trimethylolpropanetris(3mercaptopropionate) (TMPTMP) was received as a gift from Evans 6820



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offer final, distinct and robust materials after the stage 2 cure, it is of vital importance to maximize the monomer diffusion so that monomers are capable of segregating in the irradiated regions before vitrification.6 Accordingly, the monomers must achieve a stringent set of criteria such as high refractive index, relatively low molecular weight, high solubility, low absorption, good stability, high reactivity, facile scale up and having the capability of forming cross-linked networks after reacting. Toward this end, a novel alkyne monomer designed on the basis of N-vinylcarbazole, POETEC, was successfully prepared through a radical-mediated thiol−ene click addition followed by a nucleophilic substitution with propargyl bromide (Scheme 2). We chose TMPTMP and DTPTA as the thiol and acrylate monomers, respectively, to form the initial matrix primarily because of their capability for readily controlling the matrix characteristics through compositional variation. Mechanisms. Thiols are readily dehydrogenated in the presence of a wide range of triggers such as bases, nucleophiles and radicals,19,24,54 and the orthogonal thiol−Michael/thiol− yne click strategy for holography is based on the fact that only the electron deficient Michael acceptors can react with the thiolate anion. We used a relatively weak base, TEA, to initiate the thiol−Michael addition reaction with a small reaction exotherm to prevent premature thiol−yne reactions in the absence of a radical inhibitor. As illustrated in Scheme 3a,

that were separated by four 50-μm plastic slides. The sample was placed into a horizontal transmission apparatus and monitored at a rate of 1.8 s per scan under dry air. Functional group conversions were calculated from the absorption variations of thiol, acrylate, and alkyne functional groups at around 2570, 810, and 3286 cm−1, separately. Mechanics. Stage 1 and stage 2 films for dynamic mechanical analysis (DMA) were prepared between two glass slides in the shape of rectangular 10 mm × 5 mm × 1 mm. Analysis was conducted on a TA Q800 DMA with a ramp rate of 3 °C/min from −50 to 50 °C and 1 Hz frequency. The Tg and rubbery storage modulus were determined as previously reported.6 Holographic Recording. Stage 1 films of 100 μm thickness were formed and stored in the dark overnight prior to holographic recording. To record conventional gratings, light from a 405 nm Ondax wavelength-stabilized laser diode was spatially filtered and split into two beams with an equal intensity of 10 mW/cm2 for each, which were then interfered at an external half angle of ∼15° to produce an interference pattern with fringe spacing near 800 nm. The bisector of the two beams was normal to the recording plates. Subsequent to recording, the films were kept in the dark for 4 h and then uniformly flood-cured on each side for 10 min under a 365 nm light (an Oriel 66990 mercury lamp equipped with a 365 nm short-pass filter). Light intensities used to cure the samples were 20 mW/cm2. To understand the volume shrinkage, the recording plates were slanted to 20° of the angle between the sample normal and the bisector of the two writing beams, followed by holographic patterning for 5 s under 405 nm laser exposure and uniform irradiation using a 365 nm light. For the implementation of multiple weak overlapping angle-multiplexed gratings, each weak grating was exposed for 1 s, with the sample stage rotated from −21 to 20 degrees in 1 degree steps between successive exposures. Then, these holograms were left on the sample stage in situ for 30 min in the dark and cured with a uniform 365 nm light for 20 min. Since the material was insensitive to red light, a 633 nm He−Ne laser was used for nondestructive monitoring of the grating’s diffraction efficiency. Image Patterning. Computer generated images were directly patterned on the recording media by utilizing a Polygon400 Multiwavelength Dynamic Spatial Illuminator under 400 nm with an intensity of 8 mW/cm2 for 1 min. The image was postcured under a 365 nm light with an intensity of 20 mW/cm2 for 10 min and observed under 470 nm using a Nikon ECLIPSE microscope. All of the above experiments were repeated at least three times.

Scheme 3. Mechanisms for (a) Base-Catalyzed Thiol− Acrylate Michael Addition and (b) Radical-Mediated Thiol− Yne Reaction within the Two-Stage Methodology20,56

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RESULTS AND DISCUSSION It is a long-standing goal to develop enhanced, solid state photopolymers for data storage with large capacities, exceptional mechanical performance, and low distortion. We recently developed a two-stage orthogonal thiol−Michael/thiol−ene click paradigm that afforded as high as 2.4 cm/mJ of light sensitivity and excellent oxygen resistance.6 The thiol−yne reaction is an analogue to the thiol−ene reaction; however, each alkyne group is typically able to react twice with thiols to increase the cross-link density and the high refractive index sulfur content, leading to enhanced glass transition temperatures, rubbery moduli, and refractive indices, respectively.18,49 Thus, thiol−yne reactions provide distinct opportunities in hologram formation. Monomers. The monomers used here are shown in Chart 1. To conduct holographic recording in freestanding media and

during Michael addition the base abstracts a proton from the thiol, generating one thiolate anion that subsequently adds to the electron deficient double bond. Then, chain transfer occurs between the anionic adduct and another thiol group, regenerating a thiolate anion and the thio-ether product. Analogous to but distinct from the two-step anionic propagation process of the thiol−Michael reaction, the radical mediated thiol−yne addition proceeds in four steps because one yne group typically has the capability of sequentially reacting with two equivalents of thiol.55 As displayed in Scheme 3b, a thiyl radical directly adds to the alkyne bond, producing one thioether-vinyl radical that subsequently abstracts a hydrogen from another thiol to yield a new thiyl radical and one vinyl-thioether intermediate.20 Depending on the structure of the initial yne, the formed vinyl-thioether is highly reactive to the thiyl radical, often making it much more reactive with the

Chart 1. Monomers Used and Their Abbreviations

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thiyl radical than the original yne.49 Because the thiol−yne reaction behavior is highly dependent on the alkyne substitutes,55 we are able to design alkyne monomer structures to control the reaction kinetics during holography. Model Reaction. To understand the reactivity of the novel alkyne monomer POETEC with thiols, we conducted a model reaction with butyl 3-mecaptopropionate under a 365 nm light exposure with an intensity of 20 mW/cm2 (Scheme 4), and this

associated polymerization volume shrinkage,57 alkyne-limited formulations were used to record holograms. On the other hand, residual unreacted monomer embedded in the matrix plasticizes the network and maximizes monomer diffusion and polymerization during holographic recording.6 In this scenario, an optimized concentration of the residual monomer embedded in the stage 1 matrix is critical for controlling the final properties after the stage 2 photopolymerization. Here, we focus on a single formulation that is composed of an offstoichiometric mixture of TMPTMP/DTPTA/POETEC (with a functional group ratio of 2.1:1.0:0.3), because this composition yielded the best results after several experiments. Orthogonal Kinetics. To verify that POETEC is stable during the holographic matrix formation while affording fast reaction kinetics to record holograms, we performed an orthogonal reaction kinetics investigation. As illustrated in Figure 2, the thiol−acrylate Michael addition reaction proceeds

Scheme 4. Model Reaction of POETEC with Stoichiometrically Balanced Butyl 3-mercaptopropionate under a 365 nm Exposure with an Intensity of 20 mW/cm2

process was expected to be quantitative and efficient as a clicktype reaction. The FTIR kinetics visualized in Figure 1 (original

Figure 2. Kinetic profiles for orthogonal, sequential reactions of the mixture composed of TMPTMP/DTPTA/POETEC with a functional group ratio of 2.1:1.0:0.3, suggesting that no alkyne group was consumed during the initial base-catalyzed thiol-Michael addition reaction, whereas the yne reacted quickly with the thiol once the light was turned on. Figure 1. Kinetic profiles of the model reaction between POETEC and stoichiometrically balanced butyl 3-mercaptopropionate in the presence of 2 wt % TPO under a 365 nm exposure with an intensity of 20 mW/cm2, visualizing the quantitative addition within 1 min of exposure.

gradually in the presence of TEA, giving 100% acrylate conversion and 39% thiol conversion after 300 min. Because no residual CC double bonds are observed after the thiol− Michael addition reaction, 48% thiol conversion is predicted, indicating that the initial reaction in a short time during the sample preparation for infrared analysis is not detected (see Figure S7 in the Supporting Information). It is typical that some conversion cannot be recorded during characterization of the thiol-Michael addition because these reactions usually occur rapidly in a step-growth manner after addition of the catalyst but prior to putting the sample in the FTIR chamber,56 which is one reason for the extensive exploration of phototriggered catalysts for achieving spatial and temporal control of these reactions.25,58,59 The radical mediated thiol−yne reaction does not occur prior to light exposure but starts when the light is turned on, giving 56% alkyne monomer conversion in 7 min of exposure. The conversion of the thiol−yne reaction continues to increase under the light exposure, which benefits from monomer diffusion prior to vitrification that affords more significant refractive index gradients during holography. Overall, conversions of 77% for the alkyne group and more than 62%

data are given in Figure S4 in the Supporting Information ) clearly demonstrate that there is no reaction in the dark while the POETEC quantitatively reacts with 2 equiv of monothiol within 1 min of exposure without obvious alkyne homopolymerization, yielding only a single product as demonstrated by the NMR and mass spectra analysis (see Figures S5 and S6 in the Supporting Information). The same consumption rate of thiol and yne functional groups implies that there is no accumulation of the vinyl−thioether intermediate, which is indicative that the thiol adds to the vinyl−thioether much more rapidly than to the original propargyl ether.49 Formulation Optimization. Because either decreasing the initial concentration of unsaturated bonds or reducing the volume shrinkage factor by depressing homopolymerization (e.g., utilizing the thiol-excess formulations49) will reduce the 6822



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(including the conversion prior to characterization) for the thiol group are found after 400 min of monitoring, which are typical in cross-linked networks.49 Therefore, this offstoichiometric formulation is suitable for holographic reconstituting. Two-Stage Mechanics. The two-stage (dual cure) reaction paradigm has been used extensively to offer facile processing and distinct material properties,51,52,60,61 including Kloxin and co-workers who nicely formed photodirected wrinkles through this strategy.51 Recently, White and co-workers implemented large area restorations over damaged polymer materials by combing the two-stage and orthogonal click methodologies.44 Due to the relative independence of the material behavior in the two stages, this paradigm provides a robust route to design freestanding holograms. As shown in Figure 3, the mechanical

Figure 4. Diffraction efficiency (D.E.) of the stage 1 media as a function of recording time under a 405 laser exposure with total intensity of 20 mW/cm2, showing that there is no significant Bragg shift. The formulation consisted of TMPTMP/DTPTA/POETEC with a functional group ratio of 2.1:1.0:0.3.

Figure 3. (a) Storage modulus, E′, and (b) loss tangent, tan δ, versus temperature for the stage 1 and stage 2 materials formed from the formulation composed of TMPTMP/DTPTA/POETEC with a functional group ratio of 2.1:1.0:0.3, indicating that the stage 2 material exhibits higher rubbery storage modulus and Tg while having similar homogeneity as compared to the stage 1 material.

profiles of the two-stage films formed through the sequential thiol−Michael and thiol−yne reactions are analogous to those of thiol-Michael and thiol−ene reactions.6 The stage 1 film has a rubbery storage modulus (E′) of 2 MPa and a glass transition temperature (Tg) of −14 °C. These properties for stage 2 dramatically increase to 6 MPa and 15 °C after flood cure. Therefore, the cross-link density has tripled according to rubber elasticity theory.6 Moreover, no significant change is observed in the full width at half maximum (FWHM) of the loss tangent profiles from the stage 1 to the stage 2, which is indicative of similarly homogeneous networks formed in each stage. Holographic Recording on the Stage 1 Film. At the end of stage 1 the loosely cross-linked network with low glass transition temperature and rubbery storage modulus is essentially ready for holographic writing. As shown in Figure 4, after being in the dark for 30 s, the diffraction efficiency at the Bragg angle gradually ramps upward with increased dose and reaches a plateau after 70 s of illumination. Because large volume shrinkage readily leads to significant grating distortion and Bragg shifts that subsequently result in dramatic diffraction decays at the Bragg angle, the diffraction profile shown in Figure 4 manifests that the volume shrinkage for this recording media based on thiol−yne click reactions is relatively insignificant. Additionally, the relatively slower thiol−yne reaction rate as compared to that of the thiol−ene reaction accounts for the slower diffraction response. Angle Selectivity of the Stage 2 Gratings. After recording gratings on the stage 1 film for 90 s, we implemented a uniform flood-cure to stabilize the permanent hologram. As displayed in Figure 5, the stage 2 diffraction efficiency (D.E.) of a single grating at the Bragg angle approaches as high as 96%.

Figure 5. Stage 2 diffraction efficiency as a function of the incidence angle, displaying a sharp and highly symmetric peak ascribed to the low volume shrinkage. The formulation consisted of TMPTMP/ DTPTA/POETEC with a functional group ratio of 2.1:1.0:0.3.

This larger number than that acquired from Figure 4 is indicative of a Bragg shift of ∼0.1° during holographic writing. Since the surface height variation is usually ∼2 nm,6 it is not expected to significantly affect the diffraction properties of these thick holograms. Thus, the newly synthesized alkyne monomer, POETEC, with its high refractive index accounts for this large diffraction efficiency. The highly symmetric and sharp angle selectivity profile with a bandwidth of around 1 degree is indicative of uniform holograms without significant volume shrinkage.14 The grating pitch of 760 nm and refractive index modulation of 0.0036 are also given from the fitting of the angle selectivity peaks in accordance with Kogelnik’s coupled wave theory,62 which are close to our previous results from the thiol−ene reaction,6 and higher than those reported elsewhere.14 Volume Shrinkage. As mentioned before, the volume shrinkage during holography is insignificant for the radically polymerizable thiol−yne system. We also performed in situ 6823



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volumetric shrinkage characterization after rotating the sample for 20 degrees from the bisector of the two writing beams. As explored in Figure 6, the largest diffractive angle of the stage 2

using the yne monomer POETEC is able to afford data storage with low distortion. From the multiplexing results, dynamic range number M/# of 5.6 per 200 μm and refractive index modulation of 0.0036 are determined. Unambiguously, the refractive index modulation is identical to the value calculated from a single grating in Figure 5, implying that the refractive index modulation is independent of reaction conditions and close to the largest number which this system can afford to date. Direct Photopatterning. We then implemented the direct photopatterning of a computer-generated image. Figure 8a

Figure 6. Stage 2 normalized diffraction efficiency as a function of the sample rotation angle, implying that there is only a small Bragg shift ascribed to the low volume shrinkage. The formulation consisted of TMPTMP/DTPTA/POETEC with a functional group ratio of 2.1:1.0:0.3.

Figure 8. (a) Original image used as a digital mask and (b) the directly patterned image showing index patterning within the volume of the stage 2 film composed of TMPTMP/DTPTA/POETEC with a functional group ratio of 2.1:1.0:0.3.

holograms shifts to 20.2° from the theoretical value of 20° as a consequence of volume shrinkage, and the volume shrinkage σ during holography can be calculated as σ = 1−tan ϕtheoretical/tan ϕexperiment = tan(20°)/tan(20.2°) = 1.1%,11,63 where ϕ represents the slanted angle of the gratings. The volume shrinkage is dependent on the initial functional group concentration and conversion.57 Multiplexing of Holograms. Because of the high diffraction efficiency and low volume shrinkage, this media bearing the novel monomer POETEC is suitable for data storage. Figure 7 presents the angle-multiplexing of 41 weak gratings in the stage 2 film. Interestingly, the D.E. response varies for each written hologram, which is similar to the behavior of dendronized macromonomers12 though the precise mechanism requires further exploration. Ultimately, as shown, this orthogonal sequential thiol−Michael/thiol−yne paradigm

displays the original image of a University of Colorado Buffalo while Figure 8b explores the patterned image, demonstrating that we successfully achieved the image data storage through the orthogonal thiol−Michael/thiol−yne click reactions. More complex images are also able to be reconstituted through this technique (see Figure S8 in the Supporting Information).

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CONCLUSION In summary, we successfully implemented direct image patterning through the radical-mediated thiol−yne click reaction on self-supporting substrates that were formulated through the base-catalyzed thiol−Michael addition. In this process, a newly synthesized alkyne monomer, POETEC, formed holographic patterns with high diffraction efficiency (96%), refractive index modulation (0.0036), dynamic range (M/# of 5.6 per 200 μm), and low volume shrinkage (1.1%) because of its as high as ∼1.68 refractive index and clickable reactivity with mercaptopropionate under light exposure. Another notable advantage of the thiol−yne reactions for the orthogonal design is that no radical inhibitor is required, which displays a great potential for the purpose of forming, modifying and controlling the material structures. We also found that the refractive index modulation of 0.0036 was found to be identical in both a single grating and in angle-multiplexed holograms, which was the largest refractive index modulation that was achieved in this system to date. However, it is of great opportunity for improvement by precisely tuning the molecular structures of the thiol, acrylate, and alkyne for enhancing monomer diffusion and reactivity.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 7. Multiplexing of 41 weak holograms in the stage 2 film consisted of TMPTMP/DTPTA/POETEC with a functional group ratio of 2.1:1.0:0.3.

Figures S1−S8. This material is available free of charge via the Internet at http://pubs.acs.org/. 6824



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Gayla J. Berg for the assistance conducting the direct light patterning experiment. We are also thankful for the financial support from the National Science Foundation Grant of United States (CHE 1214109, 1307918, 1240374, 0954202), Major International Joint Research Project (51210004) and Key Program (51433002) of National Natural Science Foundation, and Guangzhou Nansha Research Funds (2013C007) of China. H.Y.P. appreciates the China Scholarship Council award (201206160040).

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ABBREVIATIONS DTPTA, di(trimethylolpropane)tetraacrylate; TMPTMP, trimethylolpropanetris(3-mercaptopropionate); POETEC, 9(2-((2-(prop-2-yn-1-yloxy)ethyl)thio)ethyl)-9H-carbazole; D.E., diffraction efficiency

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