Surface Plasmon-Polariton: A Novel Way To Initiate Azide–Alkyne

Article Cite This: Langmuir 2019, 35, 2023−2032

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Surface Plasmon-Polariton: A Novel Way To Initiate Azide−Alkyne Cycloaddition Olga Guselnikova,†,‡ Pavel Postnikov,*,†,‡ Mohamed M. Chehimi,§ Yevgeniya Kalachyovaa,‡ Vaclav Svorcik,‡ and Oleksiy Lyutakov*,†,‡ †

Department of Solid State Engineering, University of Chemistry and Technology, 16628 Prague, Czech Republic Research School of Chemistry and Applied Biomedical Sciences, Tomsk Polytechnic University, Tomsk 634050, Russian Federation § Université Paris Est, UMR 7182 CNRS, UPEC, 94320 Thiais, France

Langmuir 2019.35:2023-2032. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 04/05/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Plasmon catalysis has recently generated tremendous interest in the field of modern chemistry. Application of plasmon introduces the principally new stimulus for the activation of organic reactions, keeping the optical energy concentrated in the vicinity of plasmonic structure, creating an optical near-field enhancement as well as hot electron injection. In this work, for the first time, we presented a new way for the initiation of the azide−alkyne cycloaddition (AAC) using the surface plasmon-polariton wave, supported by the gold grating. With this concept in hand, the plasmon-active gold grating was functionalized with 4-ethynylbenzenediazonium compound. Then, surface-grafted 4-ethynylphenyl groups were plasmon activated and clicked with 4-azidobenzoic acid. Additional experiments allowed to exclude the potential effect of photon, heating, and metal impurities confirmed the key role of surface plasmon-polariton AAC activation. For the investigation of plasmon-induced AAC mechanism, 4-azidophenyl groups (instead of 4-ethynylphenyl groups) were also grafted to the grating surface. Further careful evaluation of reaction kinetics demonstrates that the AAC reaction rate is significantly higher in the case of acetylene activation than in the case of azide activation.

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INTRODUCTION The search of novel physical methods for the initiation of chemical transformations is considered to be one of the most important challenges in modern science for sustainable development and technology.1 As main examples, the establishment of temperature, light, microwave, or electrochemical reaction activation can be mentioned.2−7 However, there is a continuous demand in the new methods for the initiation of chemical reactions, which are consistent with the timely requirements of applied science. Recently, a novel and extremely perspective way for the activation of chemical processes was proposed: utilization of plasmon-based phenomenon as the driving force for chemical reactions.8−12 Plasmon is the collective oscillation of electrons, excited by the external electromagnetic wave on the metal−dielectric boundary.13,14 Since the plasmon is featured by the strongly localized evanescent wave, it can be considered as the extremely effective lens, which focuses light beyond the diffraction limit.15−18 Unique properties of plasmon have been extensively studied for Raman spectroscopy, plasmonic devices, and chemical transformation processes.19−23 Especially in the last case, the activation by plasmon energy has a great potential for overcoming many intrinsic limitations of © 2019 American Chemical Society

conventional catalysts that made plasmon-driven catalysis one of the hottest topics.9,24 There is a range of outstanding examples where plasmon was utilized for the initiation of chemical reactions, including the oxidation of p-alkyl thiophenols and alcohols,25 azo-coupling of amino groups,26,27 dissociation of hydrogen,28−32 and many synthetic methods.25−41 Nevertheless, for plasmon catalysis, the expansion of reaction scope will propel to the next level of applicability. In this work, we present the click chemistry reaction of azide− alkyne cycloaddition (AAC) as a unique example of plasmonics possibilities in the field of organic chemistry. Since the introduction of the click chemistry concept by Sharpless and co-workers,42,43 the click reactions have found their utilization in many applications.44,45 Through the diversity of possible click chemistry transformations, azide−alkyne cycloaddition (AAC) reaction has become a paragon of click chemistry.45−47 Becoming a prime example of click chemistry, AAC demonstrates great usability for modification of biomolecules Received: September 6, 2018 Revised: January 7, 2019 Published: January 18, 2019 2023

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conductive paste. The laser beam (780 nm) was focused on the sample surface close to the working edge of the thermocouple, and temperature changes were monitored until a saturated value was reached (3 min of illumination was found to be enough for the achievement of stabilized temperature value). The experiments were performed in two arrangements: in the air or the water/methanol solution. In the latter case, the temperature of water/methanol solution was also measured independently using a second thermocouple (Thomas Traceable Waterproof Type K Thermometer probe) placed in the solution at a distance of 0.2 cm from the illuminated place. Direct Experiment at Increased Temperature. For experiments under increased temperature (50 °C, this temperature exceed the maximal surface heating under the laser illumination by 10 timessee the manuscript text), the SERS measurements were performed with the laser intensity of 18 μW/μm2 using a hot plate. Experiments with Silver Grating. Silver gratings modified by ADT−CCH were immersed in the solution of 1 mM 4azidobenzoic acid and illuminated with 785 nm under the same conditions as described above. Subsequent SERS control experiments (with 532 nm excitation wavelength) were performed using Advantage NIR Raman spectrometer (532 nm) on dry samples after their removal from the reaction mixture and rinsing with water/ methanol mixture to detect the possible trace concentrations of triazole moieties. Influence of Surface Groups on AAC. For this experiment, gold gratings modified by ADT−N3 were used. For the AAC between phenylacetylene and azide groups on the surface, 10 mL of 1 mM solution of phenylacetylene in methanol/water (1:1) was added to the gold grating. All other conditions are the same as described before. Control of Byproduct Formation. 4-(4-Phenyl-1H-1,2,3-triazole-1yl)benzoic acid (1) was prepared according to literature.63 Water/ methanol solution, 10−8 M, of 1 was deposited dropwise on pristine gold grating, and Raman scattering was measured on ProRaman-L spectrometer (785 nm excitation wavelengths, laser power 15 mW). For the gas chromatography−mass spectrometry (GC−MS) analysis of the reaction mixture after plasmon-induced AAC, a solution of 4azidobenzoic acid was collected after continuous illumination with 785 nm laser (18 μW/μm2) for 40 s and analyzed by GC−MS. The identification of dehydrogenation products was unequivocally established by a GC−MS ISQ TRACE 1300 (Thermo) with a TRACE TR-5 GC Column. For the GC−MS analyses, the column was coupled to a VG 70SE dual-sector high-resolution mass spectrometer. Measurement Technique. The surface chemical composition was determined by X-ray photoelectron spectroscopy (XPS) using an Omicron Nanotechnology ESCAProbeP spectrometer fitted with monochromated Al Kα X-ray source (hν = 1486.6 eV, spot size = 2 × 3 mm2). Binding energy positions were calibrated against Au 4d7/2 peak set at 84.0 eV. For characterization of the sample surface and nanomechanical mapping, the peak force atomic force microscopy (AFM) technique was applied using the Icon (Brucker) microscope. The UV−vis spectra were measured using Spectrometer Lambda 25 (Perkin-Elmer) in 300−1000 nm wavelength range. Concentration of metals in all solutions was measured by atomic adsorption spectroscopy on an AGILENT 280 FS AA spectrometer.

and natural products, polymer transformations, drug synthesis, and surface modification.45,48−50 Nowadays, several basic approaches for AAC activation can be distinguished:51−57 thermal activation, utilization of metals catalysts,42−45 strainpromoted AAC,46 and UV-induced catalysis.47,48 As a consequence of listed advantages and variety of possible activation ways, they have found comprehensive applications in material science.46,58,59 Taking into account the growing number of scientific articles regarding the use of click chemistry for materials science, there is a strong need to design novel alternative approaches catalyzing AAC on the surface with robustness to the existing repertoire of reactions.60 In this contribution, for the first time, we demonstrate a new approach to the plasmonic activation of AAC. In our experimental concept, the reaction was performed through the grafting of 4-ethynylphenyl groups to surface plasmonpolariton-supported gold grating and their activation by excited plasmonic waves. The proposed approach expands the reaction scope of plasmon-induced chemical transformation as well as introduces an alternative way for AAC activation.

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EXPERIMENTAL SECTION

Materials. Acetic acid (reagent grade, ≥99%), diethyl ether, deionized water, methanol (puriss, absolute, ≥99.8% (GC)), 4ethynylaniline (97%), p-toluenesulfonic acid monohydrate (ACS reagent, ≥98.5%), 4-azidoaniline hydrochloride (97%), and phenylacetylene (98%) were purchased from Sigma-Aldrich and used without further purification. Solution of Su-8 was purchased from Microchem, Germany. Samples Preparation. Grating Preparation. Su-8 films were spin-coated onto freshly cleaned glass substrates and patterned using the linearly polarized excimer laser irradiation according the reference.21 The gold and silver (purity of 99.99%, provided by Safina, Czech Republic) were deposited onto a patterned surface by vacuum sputtering, with thickness of approximately 25 nm for gold and 16 nm for silver. 4-Ethynylbenzenediazonium tosylate (ADT− CCH) and 4-azidobenzenediazonium tosylate (ADT−N3) were synthesized according to the slightly modified published procedure61 (for detailed information, see the Supporting Information (SI)). The metal surfaces were spontaneously modified by soaking in 2 mM freshly prepared aqueous solution of ADT−CCH or ADT−N3 for 20 min.62 After modification, the metal substrates were rinsed under sonication sequentially with water, methanol, and acetone for 10 min and dried in a desiccator. Plasmon-Driven Azide−Alkyne CycloadditionDirect Experiment. The plasmon-assisted AAC and the corresponding timedependent surface enhanced Raman spectroscopy (SERS) measurements were carried out on gold grating modified by ADT−CCH using portable ProRaman-L spectrometer (785 nm). Every time before starting the process, spectra of the modified gold surface were recorded. For the surface AAC between 4-azidobenzoic acid and ethynyl groups on the surface, 5 mL of 1 mM solution of 4azidobenzoic acid in methanol/water (v/v = 1:1) was added to the gold-grating surface. Next, the image of the selected modified gold grating was focused using a 40× microscope (immersed in solution) objective. The laser beam was switched on to start the coupling reaction. The SERS online measurements were performed in the solution using 785 nm excitation wavelength with the laser beam focused to a spot 25 μm in diameter. The laser power was set to be 6, 7, 8, 10, 14, and 18 μW/μm2. The conversion of surface groups was determined by characteristic Raman peak, located at 2098 cm−1 for the gold gratings and 2006 cm−1 for the silver gratings. Temperature Control Experiments. Schematic representation of control experiments for the estimation of plasmon-induced heating of samples is presented in Figure S1. The gold gratings grafted with −Ar−CCH were placed in the Petri dish and the ultrathin leaf-type thermocouple was placed on the sample surface and fixed using the

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RESULTS AND DISCUSSION For a detailed evaluation of plasmon-induced AAC, we utilized SPP-supporting gold gratings with parameters that are able to effectively support the excitation and propagation of plasmon waves.21,23 Schematic representation of a proposed experimental concept is given in Figure 1. We started with the creation of a periodical gold grating surface that provides efficient excitation of SPP waves and a huge concentration of electromagnetic field near the noble-metal surface. The gold grating surface was grafted with 4-ethynylbenzenediazonium tosylate (ADT−CCH)62 and then immersed in the solution 2024

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confirms the gold surface grafting. The XPS results were also used for the calculation of the density of the grafted organic functional groups on the noble-metal surface, which was found to be 6.22 molecule/nm2 (detailed description is given in the SI). The presence of carbon on the pristine grating surface can be attributed to a contribution from the underlying polymer film and slight samples contamination during the sputtering procedure or carbon sorption from the air. The gold grating parameters were evaluated using AFM technique (Figure 2A). It is evident that the created structure represents the wellordered sinusoidal pattern with amplitude of ca. 55 nm and periodicity of 280 nm. As was demonstrated in our previous works,23 such a periodical structure can efficiently support the SPP excitation and propagation. Utilization of SPP for plasmon-driven catalysis, in turn, prolongs the lifetime of hot electrons, which contributes to better process efficiency, through the compensation of mismatch between the hotelectron relaxation time and the time of chemical transformation elementary act. The ability of gold gratings to support the SPP was confirmed by the UV−vis spectroscopy (Figure S4). The wavelength position of SPP was found to be near 785 nm, and therefore, this wavelength was applied for the plasmon-induced AAC on the gold grating surface. In the next step, we applied the laser irradiation for the SPPbased initiation of AAC on the gold grating surface through the illumination of samples immersed in the freshly prepared solution of 4-azidobenzoic acid. It must be noted that the

Figure 1. Schematic illustration of the plasmon-assisted azide−alkyne cycloaddition on the gold grating surface.

of 4-azidobenzoic acid for post-cycloaddition click reaction triggered through the SPP excitation. The Au surface grafting with −C 6 H 4−CCH was confirmed by XPS and Raman (results are presented in Table S1 and Figure S2, S3). Apparent increase of carbonrelated peak C (1s) at 284.7−293 eV and the growth of C (1s)/Au (4f) intensity ratio owing to the aryl layer indicate the successful attachment of organic functional groups. The appearance of specific Raman bands, characteristic for aromatic acetylenes (2098 cm−1 CC str) and aromatic azides (2006 cm−1 azide antisym str) (see Table S2, Figure S3), additionally

Figure 2. AFM topography measured on gold grating grafted by ADT−CCH before (A) and after (B) plasmon-assisted reaction with 4azidobenzoic acid at laser power of 8 μW/μm2 after 200 s. 2025

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Figure 3. Time-dependent SERS spectra of plasmon-assisted AAC under continuous exposure to a 785 nm laser intensities of 8 μW/μm2 (A, B), 14 μW/μm2 (C, D), and 18 μW/μm2 (E, F) on gold grating grafted by ADT−CCH.

plasmon evanescent wave decreases exponentially in the surrounding dielectric environment and, thus, the surfacegrafted 4-ethynylphenyl groups were dominantly activated by SPP. We carried out in situ SERS survey of plasmon-catalyzed AAC induced by different laser power densities (Figures 3 and S5). As is evident from Figure 3, the Raman spectra of 4ethynylphenyl and appropriate triazole functional groups are featured by different peaks, whose changes were further applied for the kinetics investigation of AAC. For all laser power densities, the Raman peak intensities attributed to the CC bond gradually decreased when the new peak, attributed to the triazole ring and −COOH groups (Table S2), appeared and gradually increased according to the conversion of the starting materials to the reaction product. In situ SERS monitoring

over different laser powers demonstrated the strong dependence of reaction rate on the laser power densitythe time required for the complete reaction was found to be strongly influenced by the laser intensities (Figure 3). In must be noted that for all applied laser power densities, we observed full conversion, and full conversion was achieved almost immediately (40 s) when the reaction is induced by the highest laser power density (18 μW/μm2). However, when the lower laser power density was applied, the time required for 100% conversion (according to absolute disappearance of the CC adsorption band) increases. For better “visibility”, we present the SERS peak intensity of 2098 cm−1 peak, attributed to the CC bond and the SERS peak at 2098 cm−1, characteristics of the triazole ring in Figure 2026

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Figure 4. (A) Time-dependent intensities of the disappearance (at 2203 cm−1, C−H stretching of −CCH) and appearance (2098 cm−1, imidazole ring stretching) of Raman bands for different laser powers (8, 10, 14, and 18 μW/μm2). (B) Kinetic curves of plasmon-induced AAC for different laser powers (8, 10, 14, and 18 μW/μm2) and reaction constants calculated according to the first-order kinetic equation.

4A, as a function of reaction time. As is evident, the 2098 cm−1 Raman peak decreased gradually up to zero value, whereas the 2203 cm−1 peak simultaneously increased up to a constant value. The full conversion of surface groups takes place after 40 s for 18 μW/μm2, 220 s for 14 μW/μm2, 390 s for 10 μW/μm2, and 470 s for 8 μW/μm2 laser power densities. As the highest power density of 18 μW/μm2 enables to perform plasmoninduced AAC in just 40 s, this laser power was used in subsequent experiments. We also calculated the kinetic rate constants of all laser power densities (Figure 4B) using the equation: ln(Ct/C0) = ln(I2098(t)/I2098(0)) = kt + b, where I2098 is the intensity of the Raman peak, indicating the disappearance of phenylacetylene groups ring according to literature.64 It is evident from Figure 4 that the formation of triazole ring starts immediately with the initiation of laser illumination. With the increase of laser power density, the rate constants dramatically increase; for instance, with the increase of laser power from 8 to 18 μW/μm2, the reaction constant grew by 1 order of magnitude (Figure 4). The formation of triazole on the surface was additionally proved by XPS (Figure S2C). The XPS survey spectrum after AAC demonstrates a decrease of Au (83.6 eV) peak and an increase of triazole-attributed carbon peak C (1s) (285.6−293 eV). Moreover, cycloaddition led to the formation of a new carbon-related peak (Figure S2C, inset) observed at 287.2 eV (C (1s)), indicating a nitrogen−carbon bond (C−N). Additional nitrogen signal can also be used as a marker for successful plasmon-induced AAC, since there was no elemental nitrogen present on the 4-ethynyl grafted surface. The N (1s) spectra (Figure S2C, inset) showed two peaks centered at 400.6 and 402.9 eV with a ratio of around 2:1,65 indicating the presence of nitrogen atoms, consistent with the formation of a triazole moiety. It should be also noted that absence of nitrogen peak at 404.1 eV confirms the absence of azide groups on the surface. So, the XPS results strongly proved the formation of triazole on the gold grating surface.

In the next step, we intended to exclude the possible side effects and establish the plasmonic nature of AAC activation. One of the most discussed question, in the field of plasmondriven catalysis, is whether the reaction is occurring under the plasmon or whether there are additional effects associated with impurities,19 heating,20 or photon (not plasmon) initiation.21 For an additional check of metal trace amounts, we analyzed the solutions of diazonium salts and 4-azidobenzoic acid using atomic adsorption spectroscopy (AAS), which revealed the absence of metal impurities. For the evaluation of potential plasmonic heating effects, we measured the surface temperature with thermocouple probes in place of laser illumination and reaction medium after 3 min of irradiation. The results presented in Figure S6 indicate that the temperature increase does not exceed 4 °C even under illumination with the highest available laser intensity (18 μW/μm2) when the experiments were performed in water/methanol solution. Additionally, the simultaneous measurement of the water/methanol solution temperature does not indicate any change in temperature during illumination. It must be noted that the observed increase in surface temperature is insufficient for the AAC reaction, since the azide−alkyne Huisgen cycloaddition requires keeping the reaction mixture under 100 °C for few hours.66 To experimentally differentiate heating, we investigated the influence of increased temperature on the plasmoninduced AAC. Taking into account the measured value of surface heating under illumination (4 °C under 18 μW/μm2), we compared the reaction rates at room temperature and 50 °C as time-dependent intensities of 2203 and 2098 cm−1 bands (Figure 5). It is evident that the temperature does not significantly affect the reaction ratethe reaction constants were found to be k50 °C = −0.2104 s−1 and kRT = −0.2034 s−1 for 50 °C and RT, respectively. As the difference between reactions constants is negligible, one may conclude that the temperature effects associated with plasmon heating are negligible. 2027

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530 nm as opposed to the gold gratings at 785 nm. Thus, illumination with 785 nm cannot excite any SPP wave in the case of silver grating. In the next step, the silver grating surface was modified by ADT−CCH (Figure S2B), immersed in the solution of 4-azidobenzoic acids, and illuminated with 785 nm (to avoid any possible mistakes, the control SERS measurements on the silver grating were performed with 532 nm, after the samples were illuminated with 785 nm, washed, and dried). As in previous cases, the reaction was controlled by increasing the peak intensity at 2203 cm−1. It is evident (Figure S3) that on silver grating, under illumination with 785 nm, which does not support the SPP excitation, the peak at 2203 cm−1 was not detected, indicating the absence of the AAC reaction. Moreover, significant increase of illumination time also does not lead to the appearance of 2203 cm−1 Raman band, confirming the key role of SPP in the AAC initiation. The control experiments also included the initiation of AAC using the illumination of grafted gold grating with 450 nm (this wavelength is far from the plasmon excitation wavelengths range). Further SERS measurements (performed after the samples were washed and dried) also indicate that no chemical transformation takes place, additionally confirming the plasmonic nature of AAC initiation (Figure S7). The potential formation of the reaction byproduct was proved using reversal procedures, confirming that there were no additional byproducts on the surface and in the reaction solution. For this purpose, we prepared the target triazole and deposited it dropwise on unmodified gold grating. The comparison of triazole SERS with our plasmon-induced results is given in Figure S8, and the characteristic Raman bands (2200, 1570, 1390, 1330, 1107, 1077, 1024, 856, 720, 631, and 513 cm−1) measured in both cases are located at the same position. So, we did not observe any additional peaks in the SERS spectra after plasmon-induced AAC, indicating that no byproducts are formed on the gold surface. Similar situation was observed in the case of MS analysis of the reaction mixture

Figure 5. Time-dependent intensity of the disappearance (at 2203 cm−1, C−H stretching of −CCH) and appearance (2098 cm−1, imidazole ring stretching) under the plasmon-triggering Raman bands measured at room temperature and 50 °C.

We also evaluate the effect of light illumination (i.e., traditional photochemistry effect). Toward this end, two types of grating were used: silver and gold gratings. Both gratings support the SPP excitation and propagation but under illumination with different wavelengths. Results of UV−vis spectroscopy (Figure 6) show that the SPP-attributed absorption peak in the case of silver grating is located near

Figure 6. (A) UV−vis spectra of gold and silver grating. (B) Time-dependent intensity of triazole-related peak appeared under the plasmoninduced AAC (illumination with 785 nm wavelength, laser power 18 μW/μm2) measured on the gold and silver gratings surfaces. 2028

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Figure 7. (A) Raman spectra of gold grating grafted by ADT−N3 after 40 and 380 s of plasmon-assisted reaction with phenylacetylene, laser power 18 μW/μm2. (B) Kinetic curves (according to the first-order kinetic equation) of plasmon-induced AAC for proper and improper reaction activation.

the SI). Plasmon-induced chemical transformation mechanism(s) occurs under the injection of hot electrons and/or plasmon-induced polarization of chemical bond. Both events suppose the close proximity of molecule, which should be activated, to the plasmonic surface. Thus, even grafted 4azidophenyl will be dominantly activated by SPP under illumination. However, the common mechanism of AAC presumes activation of the acetylene group and not azide. Thus, the plasmon-induced reaction of grafted azidophenyl groups with dissolved phenylacetylene is expected to be limited (we further use the term “improper” initiation in this case). Indeed, these limitations can be partially transcended due to plasmon activation of diffused phenylacetylene molecules from the solution to the gold surface. The successful grafting of 4-azidophenyl groups was confirmed by XPS and SERS (Figure S2, Tables S1 and S2). The narrow XPS spectrum of N 1s is fitted with three components assigned to NH/NC (399.8 eV), N−/NC (∼401.2 eV), and N+(∼404.1 eV) according to literature.67 The modified gratings were irradiated by a laser beam in the solution of phenylacetylene, and Figure 3A shows the SERS spectra measured after illumination with 18 μW/μm2 laser power for 40 s, i.e., the time required for full conversion in the case of “proper” initiation (for 4-ethynylphenyl-grafted surface). It is evident that illumination for 40 s almost did not affect the original SERS spectrum. An increase in the illumination time by several times led to a partial appearance of characteristic triazole-related peaks and a corresponding decrease of azidophenyl-related Raman peaks. Observed changes in SERS spectra, however, are significantly less pronounced than in the case of proper initiation. Moreover, the reaction is saturated before the full conversion of phenyl organic moieties (the conversion of functional groups in the case of improper initiation does not exceed 52%), a phenomenon that can be attributed to the steric hindering

(results are presented in Figure S9). In this case, no difference between the initial 4-aziodibenzoic acid solution and the residual solution after performing plasmon-induced AAC reaction was found. So, we can conclude that no byproducts are formed during the reaction, since no trace amounts of additional compounds were detected. The plausible mechanism of the plasmon-induced reaction is presented in Figure S10. According to the recent theories of plasmon-based chemical transformation initiation, the interaction of organic molecules with plasmon will lead to the injection of hot electrons from plasmonic surface to organic species and/or electron transition from lowest unoccupied molecular orbital to highest occupied molecular orbital molecular orbitals and/or to electric-field-induced stretching of chemical bonds. All of the above-mentioned mechanisms can potentially participate in the observed AAC initiation, including the injection of electron density, intramolecular electron transition or strong plasmon-related electric field activation of grafted chemical moieties. We also compared the present approach for plasmon-based initiation of AAC with another commonly used or recently proposed methods. The comparative analysis of the AAC initiation pathway is presented in Table S3, with special focus on the advantages and limitation of each method. As is evident, the plasmonbased initiation of AAC possess a range of additional advantages such as low increase of temperature during the reaction, possibility to avoid the utilization of toxic reagents, and short reaction time. In the next step, the reactivity of 4-azidophenyl groups under plasmon activation was investigated. The gold grating surface was grafted by 4-azidobenzenediazonium tosylate using the same procedure as in the case of 4-ethynylphenyl groups (the density of grafted OFGs, calculated from the attenuation of gold-related XPS peak was found to be 14.5 molecules/nm2, indicating the formation of multimolecular layer structure; see 2029

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Funds, OP RDE, funded project “CHEMFELLS4UCTP” (No. CZ.02.2.69/0.0/0.0/17_050/0008485).

due to the formation of the multilayer structure of phenyl azide (see Figure S11). Comparative reaction kinetics is presented in Figure 7. As is evident, the proper reaction initiation leads to a reaction with a much higher rate. Calculation of the reaction kinetic, performed according to literature,52 allows estimation of the reaction constants (K) for both cases of initiation: proper initiation K = −0.2034 s−1; improper initiation K = −0.023 s−1. A comparison of these constants shows that the reaction rate decreased by a factor of 10, indicating the great influence of exactly 4-ethynylphenyl functional group plasmonic activation.

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CONCLUSIONS We developed a new approach for the AAC reaction activation based on the surface plasmon-polariton triggering. In particular, the proposed initiation approach allows transformation of the grafted ethynyl moieties to appropriate triazoles with high efficiency in the absence of metal catalysts. We found that the reaction rate strongly depends on the laser power density. It was also demonstrated that the initiation of 4-ethynylphenyl groups dominantly promoted the reaction, whereas the application of plasmon activation of previously grafted 4-azidophenyl groups led to a significantly decreased reaction rate. Additional series of control experiments, including the potential metal impurities, heating effect, or diversity of plasmon absorption band and excitation wavelength, was performed and convincingly demonstrated that the reaction is exactly activated by surface plasmon polariton wave. We believe that the suggested approach enables to widen the modern tool box for plasmonic implementation in the field of chemical transformation.

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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.langmuir.8b03041.

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REFERENCES

General experimental remarks; synthesis of 4-ethynylbenzenediazonium tosylate; synthesis of 4-azidobenzenediazonium tosylate; calculation of the thicknesses of the grafted OFGs; calculation of the density of grafted OFGs; reproducibility of ADTs grafting on plasmonactive gold gratings; Raman peak affiliation and actual state of the art of the AAC reaction initiation (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.P.). *E-mail: [email protected] (O.L.). ORCID

Pavel Postnikov: 0000-0001-9713-1290 Mohamed M. Chehimi: 0000-0002-6098-983X Oleksiy Lyutakov: 0000-0001-8781-9796 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the GACR under the project P108/12/G108, Tomsk Polytechnic University (VIURSCABS-196/2018), and European Structural and Investment 2030

DOI: 10.1021/acs.langmuir.8b03041 Langmuir 2019, 35, 2023−2032

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DOI: 10.1021/acs.langmuir.8b03041 Langmuir 2019, 35, 2023−2032