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Sketch of all observed reaction pathways. Values close to the arrows are calculated reaction thresholds assuming simple bond cleavages calculated at B3LYP/aug-cc-pvtz level of theory in Gaussian16.[46] The thickness of the arrows reflects qualitatively the relative intensities of the reaction channels.
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The reduction of 4-nitrothiophenol (NTP) to 4-4'-dimercaptoazobenzene (DMAB) on laser illuminated noble metal nanoparticles is one of the most widely studied plasmon mediated reactions. The reaction is most likely triggered by a transfer of low energy electrons from the nanoparticle to the adsorbed molecules. Besides the formation of DMAB, dissocia...
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... with high yields within two resonant signals close to 0 eV with a shoulder at 0.6 eV and at 3.6 eV. However, it is likely that the signal close to 0 eV is an artifact originating from the NTP -due to the low mass resolution of the used quadrupole mass filter. This is confirmed also by calculated threshold of the reaction, which is 0.15eV (see Fig. 3) So far, the formation of (NTP-H) -has not been observed by SERS measurements of NTP reactions on NPs. On the one hand the SERS spectrum of the dehydrogenated species is expected to be very similar to the respective NTP and NTP -signal [20] and on the other hand the dehydrogenation in the gas phase experiments most likely originates ...
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... anion at 138 u, formed by the loss of an oxygen atom from the nitro group and additional hydrogen, is observed from two resonances at 3.7 eV and 7.5 eV with low yield in a good agreement with calculated threshold of the reaction 2.02eV (see Fig. 3). The energy of these two resonances is exceeding the energy of plasmonically generated electrons (colored region in Fig. 2) and the formation of (NTP-O) -by the attachment of a single plasmonic electron is therefore unlikely. This is in a good agreement with studies of plasmon induced reactions showing that (NTP-O) -is formed ...
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... with three times higher intensity through the resonance at 3.5 eV than at 1.0 eV, whereas the 108/109 u signal appears with slightly higher intensity at 1.0 eV than at 3.5 eV, despite much lower electron affinity of the (NTP-NO 2 ) 109 u anion. The signals may be explained on the basis of calculated energetic thresholds of the reactions (see Fig. 3). While the threshold for direct bond cleavage with charge remaining on the ring (109 u) is 1.54 eV, formation of phenylthiol-radical anion (108 u) requires only 0.66 eV. Therefore, the 0.9 eV resonance enhancement is the contribution of the 108 u ...
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... it is a common reaction channel after electron attachment to nitro-substituted compounds. [40,51] The fragment anion with mass of 124 u is formed by the loss of NO from the nitro group and hydrogen from sulfur through a single resonant feature with maximum at 4.4 eV well above the energetic threshold of the reaction of only 0.3 eV (see Fig. 3). This difference may be explained by additional energy required for the rearrangement of the nitro-group forming a barrier for the observed process. The fragment ion at 92 u originates from the cleavage of nitric oxide and the thiol group and is formed from a single resonance at 3.7 eV with low yield. Energetic threshold for this ...
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Diaryldisulfides are known to undergo S-S bond cleavage upon one-electron reduction, which is called mesolysis of radical anions, to form the corresponding arylthiyl radical and anion. In this study, we prepared (4-cyanophenyl)(4'-methoxyphenyl)disulfide (MeOSSCN), and the mesolytic profiles were investigated by γ-ray and pulsed-electron radiolyses...
Citations
... 22 For 4-NTP, it is well known that even though the electron induced reaction channel leading to the cleavage of the nitro group exists, the formation of stable anionic intermediate states is far more relevant for the reactions on plasmonic NPs leading to intermolecular coupling. 23,51 Hence, it is possible that on the surface of AuNPs, the purely electron-based reaction pathways might be less relevant than on AgNPs and thermal contributions have a relevant role in the formation of amorphous networks. The heating of the AuNPs mainly depends on the absorption of light, which is the highest for blue and green light and low for red and near-infrared and which is generally independent on the functional groups of the ligand molecules. ...
Localized surface plasmon resonances on noble metal nanoparticles (NPs) can efficiently drive reactions of adsorbed ligand molecules and provide versatile opportunities in chemical synthesis. The driving forces of these reactions are typically elevated temperatures, hot charge carriers or enhanced electric fields. In the present work the dehalogenation of halogenated thiophenols on the surface of AuNPs has been studied by surface enhanced Raman scattering (SERS) as a function of the photon energy to track the kinetics and identify reaction products. Reaction rates are found to be surprisingly similar for the different halothiophenols studied here, although the bond dissociation energies of the C-X bonds differ significantly. Complementary information about the electronic properties at the AuNP surface, namely work-function and valence band states, have been determined by X-ray photoelectron spectroscopy (XPS) of isolated AuNPs in the gas-phase. In this way, it is revealed how the electronic properties are altered by the adsorption of the ligand molecules, and we conclude that the reaction rates are mainly determined by the plasmonic properties of the AuNPs. SERS spectra reveal differences in the reaction product formation for the different halogen species and on this basis the possible reaction mechanisms are discussed to approach an understanding of opportunities and limitations in the design of catalytical systems with plasmonic NPs.
... 20,21 DEA in halo-and thio-compounds often proceeds via vibrational Feschbach resonances that are very sensitive to the molecular temperature. [22][23][24] Of the two molecules presented here, only nitrothiophenol was previously explored with respect to DEA. 25 Isolated NTP in the gas phase interacts with low-energy electrons primarily via associative electron attachment at electron energies close to 0 eV. DEA products were also observed, however, with less than 1% of the parent anion intensity. ...
... The negative ion mass spectrum of isolated NTP (Fig. 2a) is dominated by the parent anion (NTP À ), as previously observed. 25 Peaks associated with lower mass fragments can still be identified but they are about one to two orders of magnitude less intense than the parent anion peak. Among the fragments, the most intense are at m/z = 46 which can be assigned to NO 2 À , m/z = 108 and 109 associated with (M-HNO 2 ) À and (M-NO 2 ) À , m/z = 124 assigned to (M-HNO) À , and m/z = 92 which could be a contribution of (M-NOSH) À or the phenoxide ion following cleavage and rearrangement. ...
We investigate the effect of microhydration on electron attachment to thiophenols with halogen (Br) and nitro (NO2) functional groups in the para position. We focus on the formation of anions upon the attachment of low-energy electrons with energies below 8 eV to heterogeneous clusters of the thiophenols with water. For nitro-thiophenol (NTP), the primary reaction channel observed is the associative electron attachment, irrespective of the microhydration. On the other hand, bromothiophenol (BTP) fragments significantly upon the electron attachment, producing Br- and (BTP-H)- anions. Microhydration suppresses fragmentation of both molecules, however in bromothiophenol, the Br- channel remains intense and Br(H2O)n- hydrated fragment clusters are observed. The results are supported by the reaction energetics obtained from ab initio calculations. Different dissociation dynamics of NTP and BTP can be related to different products of their plasmon induced reactions on Au nanoparticles. Computational modeling of the simplified BTP(H2O) system indicates that the electron attachment products reflect the structure of neutral precursor clusters - the anion dissociation dynamics is controlled by the hydration site.
... Such stability may be important for the transport and radiosensitizing properties of the molecule 8 but may also allow for multiple electron reduction. 98 The threshold DFT calculations helped us to interpret the fragmentation pattern of the molecule with main fragmentation reactions occurring on the CONH 2 group. The estimation of vertical attachment energies demonstrates several possible virtual states available for attachment via shape resonances that may be related to favipiravir's antiviral action. ...
Electron attachment and its equivalent in complex environments, single-electron reduction, are important in many biological processes. Here, we experimentally study the electron attachment to favipiravir, a well-known antiviral agent. Electron attachment spectroscopy is used to explore the energetics of associative (AEA) and dissociative (DEA) electron attachment to isolated favipiravir. AEA dominates the interaction and the yields of the fragment anions after DEA are an order of magnitude lower than that of the parent anion. DEA primary proceeds via decomposition of the CONH2 functional group, which is supported by reaction threshold calculations using ab initio methods. Mass spectrometry of small favipiravir-water clusters demonstrates that a lot of energy is transferred to the solvent upon electron attachment. The energy gained upon electron attachment, and the high stability of the parent anion were previously suggested as important properties for the action of several electron-affinic radiosensitizers. If any of these mechanisms cause synergism in chemo-radiation therapy, favipiravir could be repurposed as a radiosensitizer.
... As recently shown, low energy carriers (with energies on the order of ∼0.15 eV) can initiate this reaction. 65 Considering that the maximum (theoretical) energy that a hot-carrier can harvest is the same as that of the incident wavelength, 9,66 this low energy barrier reaction serves as a perfect probe to test a broadband absorber catalyst as this one. In this reaction, the hot electron can be directly injected into the adsorbed pNTP molecules to drive the −NO 2 group to react with the water absorbed on the silver surface to dimerize into a DMAB molecule. ...
... Dissociative electron attachment was also observed in gas phase experiments. 40 Determining the yield of a reaction from Raman spectra is generally difficult, as the precise Raman scattering cross-section of a single molecule is usually not known. The situation is even more complicated in SERS measurements due to very inhomogeneous amplification of the Raman signal in plasmonic hot spots. ...
div>The plasmon-driven dimerization of 4-nitrothiophenol (4NTP) to 4-4’-dimercaptoazobenzene (DMAB) has become a testbed for understanding bimolecular photoreactions enhanced by nanoscale metals, in particular, regarding the relevance of electron transfer and heat transfer from the metal to the molecule. By adding a methylene group between the thiol bond and the nitrophenyl, we add structural flexibility to the reactant molecule. Time-resolved surface-enhanced Raman-spectroscopy proves that this (4-nitrobenzyl)mercaptan (4NBM) molecule has a larger dimerization rate and dimerization yield than 4NTP and higher selectivity towards dimerization. X-ray photoelectron spectroscopy and density functional theory calculations show that the electron transfer would prefer activation of 4NTP over 4NBM. We conclude that the rate limiting step of this plasmonic reaction is the dimerization step, which is dramatically enhanced by the additional flexibility of the reactant. This study may serve as an example for using nanoscale metals to simultaneously provide charge carriers for bond activation and localized heat for driving bimolecular reaction steps. The molecular structure of reactants can be tuned to control the reaction kinetics.
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Surface-enhanced Raman scattering (SERS) is an effective and widely used technique to study chemical reactions induced or catalyzed by plasmonic substrates, since the experimental setup allows us to trigger and track the reaction simultaneously and identify the products. However, on substrates with plasmonic hotspots, the total signal mainly originates from these nanoscopic volumes with high reactivity and the information about the overall consumption remains obscure in SERS measurements. This has important implications; for example, the apparent reaction order in SERS measurements does not correlate with the real reaction order, whereas the apparent reaction rates are proportional to the real reaction rates as demonstrated by finite-difference time-domain (FDTD) simulations. We determined the electric field enhancement distribution of a gold nanoparticle (AuNP) monolayer and calculated the SERS intensities in light-driven reactions in an adsorbed self-assembled molecular monolayer on the AuNP surface. Accordingly, even if a high conversion is observed in SERS due to the high reactivity in the hotspots, most of the adsorbed molecules on the AuNP surface remain unreacted. The theoretical findings are compared with the hot-electron-induced dehalogenation of 4-bromothiophenol, indicating a time dependency of the hot-carrier concentration in plasmon-mediated reactions. To fit the kinetics of plasmon-mediated reactions in plasmonic hotspots, fractal-like kinetics are well suited to account for the inhomogeneity of reactive sites on the substrates, whereas also modified standard kinetics model allows equally well fits. The outcomes of this study are on the one hand essential to derive a mechanistic understanding of reactions on plasmonic substrates by SERS measurements and on the other hand to drive plasmonic reactions with high local precision and facilitate the engineering of chemistry on a nanoscale.
Light induced electron transfer reactions of molecules on the surface of noble metal nanoparticles (NPs) depend significantly on the electronic properties of the metal-organic interface. Hybridized metal-molecule states and dipoles at the interface alter the work function and facilitate or hinder electron transfer between the NPs and ligand. X-ray photoelectron spectroscopy (XPS) measurements of isolated AuNPs coated with thiolated ligands in a vacuum have been performed as a function of photon energy, and the depth dependent information of the metal-organic interface has been obtained. The role of surface dipoles in the XPS measurements of isolated ligand coated NPs is discussed and the binding energy of the Au 4f states is shifted by around 0.8 eV in the outer atomic layers of 4-nitrothiophenol coated AuNPs, facilitating electron transport towards the molecules. Moreover, the influence of the interface dipole depends significantly on the adsorbed ligand molecules. The present study paves the way towards the engineering of the electronic properties of the nanoparticle surface, which is of utmost importance for the application of plasmonic nanoparticles in the fields of heterogeneous catalysis and solar energy conversion.
The plasmon-driven dimerization of 4-nitrothiophenol (4NTP) to 4-4′-dimercaptoazobenzene (DMAB) is a testbed for understanding bimolecular photoreactions enhanced by nanoscale metals, in particular, regarding the relevance of electron transfer and heat transfer from the metal to the molecule. By adding a methylene group between the thiol bond and the nitrophenyl, structural flexibility is added to the reactant molecule. Time-resolved surface-enhanced Raman-spectroscopy proves that this (4-nitrobenzyl)mercaptan (4NBM) molecule has a larger dimerization rate and dimerization yield than 4NTP and higher selectivity toward dimerization. X-ray photoelectron spectroscopy and density functional theory calculations show that the electron transfer prefers activation of 4NTP over 4NBM. It is concluded that the rate limiting step of this plasmonic reaction is the dimerization step, which is dramatically enhanced by the additional flexibility of the reactant. This study may serve as an example for using nanoscale metals to simultaneously provide charge carriers for bond activation and localized heat for driving bimolecular reaction steps. The molecular structure of reactants can be tuned to control the reaction kinetics.
