Photo-Induced or Plasmon-Induced Reaction: Investigation of the

Article pubs.acs.org/JPCC

Photo-Induced or Plasmon-Induced Reaction: Investigation of the Light-Induced Azo-Coupling of Amino Groups Zhenglong Zhang,† Daniel Kinzel,‡ and Volker Deckert*,†,‡ †

Leibniz Institute of Photonic Technology, Albert-Einstein-Strasse 9, 07745 Jena, Germany Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich-Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany

‡

ABSTRACT: The plasmon-mediated azo-coupling of p-aminothiophenol (PATP) and p-nitrothiophenol (pNTP) was recently achieved using gold and silver nanostructures with the help of hot electrons. To further investigate the underlying mechanisms the azo-coupling of dibenzo-1,2dithiine-3,8-diamine (D3ATP) was investigated under distinct environmental conditions. Depending on the presence or absence of oxygen, either a plasmon-mediated or a photochemical reaction can be inferred. O2 plays an active role in the case of photoinduced azo-coupling of D3ATP but is not required under plasmonic conditions. Consequently, two different reaction channels are proposed. In the photoreaction, NH2 needs to react with oxygen to finally form the coupling-molecule and H2O, while under the plasmonic conditions the −NH2 groups can directly couple to form the azo compound and H2. This study suggests that plasmon-induced hot electrons provide the necessary activation energy for the azo-coupling of D3ATP without the need for O2.

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(TERS).9 In addition to an electromagnetic field enhancement, surface plasmons can also induce hot electrons with comparatively high energy between the vacuum energy and Fermi level.10−12 These hot electrons scatter into the adsorbed molecules and trigger chemical reactions, thus promoting the reaction via plasmonic catalysis.13−15 This so-called plasmonic catalysis gains significant attention due to its high throughput and low energy requirements as reported in works on molecular coupling and dissociation reactions, e.g., dissociation of hydrogen,16,17 water,18,19 and hydrocarbon conversion,20,21 etc. Plasmonic nanostructures are employed not only to initiate plasmonic catalytic reactions but also to sensitively monitor the related processes by exploiting the electromagnetic field enhancement. As a new and exciting topic in heterogeneous surface catalysis, plasmonic catalysis has attracted increasing attention due to the possibility of replacing the typical UV light source with a visible light source.16,18 For instance, plasmon-catalyzed azo-coupling of p-aminothiophenol (PATP) or p-nitrothiophenol (pNTP) to 4,4-dimercaptoazobenzene (DMAB) was recently achieved using gold and silver nanoparticles with the SERS and TERS techniques.20−28 In 2002, Kim et al. reported the SERS spectra of PATP, and found it to be different from the standard Raman spectra.29 In 2010, Huang, Fang, and Capean reported the azo-coupling reaction of PATP to DMAB

INTRODUCTION Photochemical reactions are initiated by the absorption of energy in the form of light and are governed by an excited state pathway. The energy of the absorbed photon provides an alternative reaction pathway in the excited state of the reactants that would otherwise be obstructed by very high-energy barriers in the respective ground states. In general, the light source must provide the energy, a specific wavelength, to efficiently transfer the population into the excited state of interest to initiate the desired reaction. For small organic compounds, this excitation wavelength is typically in the ultraviolet (UV) range. However, the excitation energy can be decreased toward the visible range in the presence of a catalytically active species other than the reactant. In photocatalytic reactions, chemical reactions are not only accessible but are also accelerated in the presence of both light and a molecular catalyst.1,2 A prominent example is water splitting in the presence of TiO2 as a catalyst. This artificial photosynthesis process provides clean and renewable energy in the form of hydrogen fuel.3 Reducing these high excitation energies (i.e., initial photon energies in the UV) is important for accessible photoreactions. Surface plasmons are collective oscillations of free electrons that occur on the interface between a metal and a dielectric upon irradiation.4,5 In general, a localized surface plasmon resonance (LSPR) excited by an incident laser beam at a specific wavelength leads to a large enhancement in the localized electromagnetic field around the surface of the nanostructure.6,7 This enhancement in the electromagnetic field is utilized, for instance, in surface-enhanced Raman scattering (SERS)8 and tip-enhanced Raman scattering © XXXX American Chemical Society

Special Issue: Richard P. Van Duyne Festschrift Received: March 30, 2016 Revised: June 6, 2016

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The Journal of Physical Chemistry C on SERS-active Ag/Au substrates.21−23 In 2011, Dong reported the reaction of pNTP to DMAB, and Xie reported the reaction of pNTP to PATP.24,25 These examples nicely demonstrate how the field of plasmonic catalysis is evolving. According to previous reports, two amino (−NH2) or two nitro (−NO2) groups of adjacent PATP or pNTP, respectively, react to form an azo-bond (−NN−) on the surface of plasmonic nanostructures. However, azo-coupling was not observed in the absence of plasmonic nanostructures, primarily due to the required high excitation energies. Additionally, in the plasmon catalytic azo-coupling of PATP, O2 or surface bound oxide is an essential agent.30,31 Therefore, the reaction observed in the SERS and TERS experiments for the azo-coupling of PATP may be a combination of a O2-induced photoreaction and hot electron-induced plasmonic catalysis.32 To further determine which of these mechanisms plays a role in the azocoupling of amino compounds and study the role of O2 in the photoinduced reaction and plasmonic catalysis, we investigated the azo-coupling of dibenzo-1,2-dithiine-3,8-diamine (D3ATP, containing two amino groups) using normal Raman spectroscopy (NRS) and SERS in air (O2) and under inert gas (Ar) conditions.

a CCD detector (PIXIS400, Princeton Instruments) with an acquisition time of 1 s. Normal Raman measurements were performed by simply placing the polycrystalline D3ATP on a plain glass coverslip using the same setup but without the Ag nanoparticle film. All spectra were normalized to the Raman peak of the Si−Si bond at 520 cm−1. Raman simulations for the optimized structures of PATP and D3ATP were conducted using the Gaussian 09 program package.34 Density functional theory (DFT) with the hybrid exchange correlation functional B3LYP and the double-ζ-basis set 6-31G(d) was employed.35 We estimated the excitation energy based on the HOMO−LUMO gap of an individual molecule in the gas phase. In both cases, the HOMO−LUMO gap corresponds to a π → π* excitation. The corresponding orbital energy levels of PATP and D3ATP are shown in Figure 1b. The red and blue lines indicate the LUMO and HOMO energy levels, respectively, of PATP and D3ATP. The excitation energy from the HOMO to the LUMO was estimated to be ∼5 eV for PATP and ∼3.6 eV for D3ATP.

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RESULTS AND DISCUSSION

The azo-coupling of PATP to DMAB and the successful azocoupling reaction of D3ATP were characterized on the basis of the appearance of new Raman bands at 1387 cm−1 (νNN + νCC + νCN) and 1432 cm−1 (νNN + νCC + βCH), related to the appearance of the NN azo unit. The azo-coupling of D3ATP was monitored by time- and power-dependent NRS and SERS spectra using laser excitations at 457 nm (2.33 eV), 532 nm (2.33 eV) and 633 nm (1.96 eV), respectively. Initially, the normal Raman spectra of PATP and D3ATP were studied using 532 nm laser excitation in air. As shown in Figure 2a−e, the power-dependent NRS of PATP demonstrate that the Raman peaks corresponding to the azo-compound were not observed even at a very high laser powers (1.5 mW)

EXPERIMENT AND SIMULATION The SERS-active Ag nanoparticle film was prepared by evaporating 6 nm of silver onto precleaned glass substrates in high vacuum, followed by 30 s of annealing at 620 K under an argon atmosphere.9 An AFM image of the silver nanoparticle film is shown in Figure 1a. The diameter of the nanoparticles

Figure 1. (a) AFM image of the silver nanoparticle film used as the substrate. (b) Energy levels of the PATP and D3ATP frontier orbitals: red lines indicate the LUMO energy levels, and blue lines indicate the HOMO energy levels.

was approximately 50 nm. Full surface coverage of D3ATP (98%, Sigma-Aldrich, Germany) on the Ag nanoparticle substrate was achieved by immersing the substrates for 24 h in a 0.5 mM ethanolic D3ATP solution, followed by washing for 10 min with ethanol and deionized water to remove excess D3ATP and drying under vacuum. The fresh Ag nanoparticle films were kept in argon and rapidly immersed into solution, so that a Ag−S-molecule bond will be formed immediately, further protecting the silver surface from oxidation. The experimental setup has been previously described in detail33 and consists of an inverted Raman microscope with a ×60 oil objective (NA = 1.4, Olympus, Japan) using 457, 532, and 633 nm laser excitation. Both laser excitation and Raman (SERS) signal collection were performed along the same optical path, and the sample was always accessed from below. A notch filter was placed in front of the entrance of the spectrometer (SP2750i, Princeton Instruments), and Raman signals were detected using

Figure 2. Laser power-dependent normal Raman spectra of PATP and D3ATP powder in air. (a−d) Raman spectra of PATP from the same sample spot using 532 nm excitation at 5, 20, 200 μW, and 1.5 mW. (e) 1.5 mW for 3 min. (f−j) Raman spectra of D3ATP from the same sample spot using 532 nm excitation at 5, 20, 50, 200, and 5 μW. The azo-coupling Raman bands at 1387 and 1432 cm−1 are marked by red lines. B

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To confirm our hypothesis and further investigate the role of oxygen in the photoreaction of D3ATP, the laser powerdependent normal Raman spectra of D3ATP were investigated under inert atmosphere (dry argon) conditions. As shown in Figure 3f−j, Raman peaks corresponding to the azo-coupling product were not observed using 532 nm laser excitation in argon even at a very high laser power (1 mW) and long irradiation time (3 min). These results indicate that oxygen plays an important part in the photoinduced azo-coupling of D3ATP. Next, we examined the azo-coupling of D3ATP employing SERS substrates to investigate the plasmon-catalyzed azocoupling with and without oxygen. The SERS spectra of D3ATP adsorbed on the Ag nanoparticle film using 532 and 633 nm incident lasers are shown in Figure 4. At 2 μW using

and long irradiation times (3 min). Hence, the azo-coupling of PATP cannot be achieved via a photoreaction at these wavelengths. Figure 2f−j shows the laser power-dependent NRS of D3ATP. Initially, at low laser powers (5 μW), peaks of neat D3ATP were observed with no evidence of the azocompound, thus indicating that the molecules do not react at low laser powers (5 μW). When the laser power was increased to 20 μW, characteristic Raman bands indicating the azocoupling were observed at 1387 and 1432 cm−1. The photoreaction product was stable even when the laser power was increased to 200 μW (see Figure 2f−i). Figure 2j shows the Raman spectrum when the laser power was attenuated to 5 μW. The results in Figure 2j are different from those in Figure 2f and confirm the azo-coupling of D3ATP at increased laser power. In contrast, no azo-coupling reaction was observed upon 633 nm incident irradiation. As shown in Figure 3a−e, no azo-coupling of D3ATP was observed during the measurements even at comparatively high laser powers (1.5 mW).

Figure 4. Laser power-dependent SERS spectra of D3ATP adsorbed on the silver nanoparticle film in air. (a−c) SERS spectra of 532 nm excitation at 5, 20, and 50 μW. (d−f) SERS spectra of 633 nm excitation at 50, 150, and 500 μW.

532 nm excitation, characteristic bands at 1387 and 1432 cm−1 were observed. The laser power threshold of the reaction was reduced from 20 to 2 μW compared to the photoinduced reaction of D3ATP under normal conditions. As shown in Figure 4d−f, bands at 1387 and 1432 cm−1 were also observed under 633 nm laser excitation when the laser power was set to 150 μW, a higher power than that used for the 532 nm laser excitation. This behavior differs from that observed in the previous 633 nm experiment in the absence of a plasmonic substrate. Under normal Raman, the photoinduced reaction of D3ATP did not occur under 633 nm laser excitation (Figure 3a−e). Here, azo-coupling of D3ATP was observed, which indicates that the plasmonic conditions substantially lower the excitation energy into resonance. These results suggest that oxygen may not be required under plasmonic conditions. To reveal the role of oxygen in the azocoupling reaction under these conditions, we also studied the plasmon-catalyzed reaction of D3ATP using SERS in the absence of oxygen. The SERS spectra under argon using 532 nm laser excitation of D3ATP adsorbed on the Ag nanoparticles are shown in Figure 5. Similar to the spectra shown in Figure 4a−c, the characteristic bands at 1387 and 1432 cm−1 were observed even in the absence of oxygen, which indicates that plasmonic catalysis of the D3ATP azo-coupling can be initiated in the absence of oxygen even though oxygen is a

Figure 3. Laser power-dependent normal Raman spectra of D3ATP. (a−e) Raman spectra using 633 nm excitation at 0.05, 0.15, 0.5, 1, and 1.5 mW in air. (f−j) Raman spectra using 532 nm excitation at 5, 20, 50, 200, and 1000 μW in an argon atmosphere.

Using a visible light source under normal Raman conditions, no azo-coupling reaction of PATP to DMAB is observed because the required excitation energy is approximately 5 eV (248 nm), much higher than the provided photon energy at 633, 532, or even 457 nm. However, in the presence of oxygen, a plasmon-induced azo-coupling of PATP occurs, and then produces H2O (not detected). Consequently, oxygen must be involved in the reaction of PATP to DMAB. The excitation energy of D3ATP under noncatalytic conditions was approximately 3.6 eV, much lower than that of PATP but still not in resonance with the incident lasers used in this study. Nevertheless, at 532 nm, azo-coupling occurred. Therefore, oxygen plays an active role in initiating the photochemical azocoupling of D3ATP, and we propose that oxygen is involved in the light-induced azo-coupling of D3ATP, similar to the reaction of PATP. The required excitation energy in the presence of oxygen must be higher than 1.96 eV because no azo-coupling occurred at 633 nm. C

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Figure 5. Laser power-dependent SERS spectra of D3ATP under argon. (a−c) SERS spectra using 532 nm excitation at 2, 20, and 50 μW.

necessary ingredient for photoinduced azo-coupling under normal Raman conditions. For PATP, the oxide on the Ag/Au surface could also act as a reagent in the plasmon-catalyzed reaction.30 As shown in our experimental setup, the oxidization of Ag was at least minimized by avoiding oxygen exposure during all preparation steps and a fast and full surface coverage of molecules bonded via Ag−S bonds. While the presence of such oxides is nevertheless difficult to exclude, a comparison of Figures 4 and 5 shows no obvious difference between the SERS spectra excited by 532 nm in air and in Ar conditions. However, a difference between 532 and 633 nm excited spectra in air is obvious. This also strongly indicates a wavelength-dependent reactivity rather than dependence on oxygen. In this case, plasmon-induced hot electrons could play a key role in the plasmon-catalyzed reaction of D3ATP. Irradiation with an even higher energy at 457 nm shows the same results as for 532 nm exposure. Figure 6a summarizes the results of the above investigation regarding the azo-coupling of D3ATP by NRS and SERS. The hypothetical pathways are presented in Figure 6b−c. For the photoinduced reaction in air, the amino group reacts with oxygen first before proceeding toward the coupled molecule (and H2O). On the basis of our experiments the presence of oxygen is required for the photoinduced (absence of a plasmonic structures) azo-coupling of D3ATP, but is not required when plasmonic structures are present. In the plasmonic catalysis, the amino groups seem to combine directly. According to DFT calculations, the activation barrier of D3ATP is around 3.6 eV, which is higher than the photon energy of the employed excitation laser. In the presence of oxygen, 457 nm as well as 532 nm excitation can initiate the photoinduced reaction, but excitation at 633 nm cannot; this indicates that oxygen decreases the overall energy barrier of D3ATP by 1.3−1.6 eV. In the presence of plasmonic structures plasmon-induced hot electrons can provide the required energy

Figure 6. Summary and sketch of the photoinduced reaction and plasmonic catalysis of D3ATP. (a) The azo-coupling can always be initiated in the presence of plasmonic substrates, while 532 and 457 nm excitation can initiate a photoinduced reaction in the presence of oxygen without plasmonic nanoparticles. (b) Reaction channels of the photoinduced reaction and plasmonic catalysis of D3ATP. (c) Sketch of the estimated activation barrier for the azo-formation between D3ATP molecules.

(greater than 3.6 eV) to initiate the azo-coupling of D3ATP without the need for oxygen.

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CONCLUSION In this study, we report the photo- and plasmon- induced azocoupling of D3ATP. Raman experiments of D3ATP were performed under ambient and inert atmospheric conditions to investigate the influence of atmospheric oxygen. Photoinduced (no metal present) azo-coupling of D3ATP was observed using 532 and 457 nm laser excitation in the presence of oxygen only; 633 nm excitation did not provide sufficient energy for azocoupling, even in the presence of oxygen. The plasmonic catalytic azo-coupling of D3ATP was successfully confirmed by SERS experiments using 457, 532, and 633 nm laser excitation both in air and argon. The results provide an estimate of the amount of energy required for the respective mechanisms and how the activation energy is lowered in the presence of oxygen and plasmonic structures. Future investigations utilizing different laser excitations will allow the actual energies to be determined even more precisely to further elucidate the role of oxygen in both photoinduced and plasmonic catalysis of the azo-coupling of D3ATP. D

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +493641/948347. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the Deutsche Forschungsgemeinschaft and the Thüringer Aufbaubank (No. 2011SE9048). Z.Z. acknowledges financial support from the Alexander von Humboldt Foundation and National Science Foundation of China (No. 11504224). D.K. is grateful for financial support from the Thuringian State Government within its ProExcellence initiative (APC2020).

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