Photoinduced Surface Catalytic Coupling Reactions of

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Photoinduced Surface Catalytic Coupling Reactions of Aminothiophenol Derivatives Investigated by SERS and DFT Rui Jiang, Meng Zhang, Shu-Li Qian, Feng Yan, Lin-Qi Pei, Shan Jin, Liu-Bin Zhao, De-Yin Wu, and Zhong-Qun Tian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04638 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 10, 2016

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The Journal of Physical Chemistry

Photoinduced Surface Catalytic Coupling Reactions of Aminothiophenol Derivatives Investigated by SERS and DFT Rui Jiang,† Meng Zhang,‡ Shu-Li Qian,† Feng Yan,† Lin-Qi Pei,† Shan Jin,*,† Liu-Bin Zhao,*,§ De-Yin Wu,*,‡ and Zhong-Qun Tian‡ †

Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of

Chemistry, Central China Normal University, Wuhan 430079, China ‡

State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of

Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China §

Department of Chemistry, School of Chemistry and Chemical Engineering, Southwest

University, Chongqing 400715, China *Corresponding

Authors,

Email:

[email protected];

[email protected]

1

ACS Paragon Plus Environment

[email protected];


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ABSTRACT: p-aminothiophenol (PATP) is an important probe molecule in surface-enhanced Raman spectroscopy. The unique and strong SERS signals of PATP distinguished from its normal Raman spectrum was considered as a signal of existing of charge transfer mechanism. Recent theoretical and experimental studies demonstrate that PATP undergoes surface catalytic coupling reaction to produce an aromatic azo species p,p′-dimercaptoazobenzene (DMAB), which should be responsible for the abnormal signals in the observed SERS spectra of PATP. In this work, three aminothiophenol derivatives with different substitute position and conjugation degree between amino group (-NH2) and mercapto group (-SH) were chosen to study the effects of substituent including adsorption orientation effect and conjugation effect on the reactivity of photoinduced surface catalytic coupling reactions. A combined SERS and DFT study indicated that no surface reactions were occurred for compound C1 and compound C2, while compound C3 was converted to the corresponding azo species during their SERS measurements. The differences in reactivity of the selected probe molecules were investigated on the basis of proposed photoexcitation and photoreaction mechanism.

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Introduction Surface-enhanced Raman spectroscopy (SERS) provide a reliable highly sensitive approach to characterize molecular structures of the surface species, which can be widely applied in chemistry, physics, environmental, material, medical sciences and biosciences.1-3 SERS is currently the only one technique that can simultaneously detect a single molecule and provide its chemical fingerprints.4,5 The ultra-high sensitivity of SERS technique arises from its giant enhancement of Raman signals of molecules on the surfaces. Normally, SERS enhancement effect are distributed into major two aspects, electromagnetic (EM) and chemical enhancement (CE) mechanism.6,7 EM is a result of surface plasmon resonance (SPR) on the surfaces of nanostructures and is typically considered as the main contribution to SERS signals in most SERS systems. CE is generally contributed from ground state charge transfer via surface bonding interaction and photon-driven charge transfer (CT) between adsorbate and substrate. Despite its relative weak enhancement compared with EM, chemical enhancement will significantly influence the pattern of SERS spectra in the frequency shift and the relative intensity of the spectral bands. However, there is a recent trend that abnormal SERS signals that cannot be fully understood by EM mechanism are attributed to the chemical enhancement or CT mechanism without carrying out delicate electrochemical SERS measurements or performing detailed quantum chemical calculations.8 P-aminothiophenol (PATP) is an important probe molecule in SERS and nanoscience fields.9-12 When it adsorbs on nanoscale roughed metal surfaces, ones can detect unique and strong surface-enhanced Raman signals.13-24 Osawa et al. observed for the first time 3



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the SERS spectra of the adsorbed PATP on a rough silver electrode.14 The significant frequency shift of intense Raman peaks were considered as a consequence of chemical adsorption, while the giant enhancement of “b2-modes” was interpreted to be the CT mechanism through the Herzberg-Teller vibronic coupling term in the previous study.14 From the potential dependent SERS experiments at different excitation wavelengths, a metal-to-molecule CT was proposed on the basis of the positive movement of potential maximum with the increasing excitation energies.14,25 This mechanism was accepted by subsequent SERS studies on PATP.10,22,23,26-29 However, theoretical calculations gave entirely different results. The fundamentals of so-called “b2-modes” in PATP SERS spectra cannot be found in the calculated Raman spectra.30 The CT direction explored from TD-DFT calculations was assigned to molecule-to-metal transition, which is opposite to the experimental deduction.31 What’s more important, the b2 modes of PATP were not enhanced in the simulated frequency-dependent Raman spectra by considering the Herzberg-Teller vibronic coupling effect.32 A novel mechanism was presented to understand the observed SERS for PATP adsorbed on silver surfaces.30 On the basis of theoretical calculations combined with experimental results in the literature,33-40 Wu et al. proposed that PATP molecules adsorbed on the nanoscale rough surfaces of noble metals undergoing a surface catalytic coupling

reaction

to

selectively

produce

a

new

surface

species

p,p′-dimercaptoazobenzene (DMAB), which should be responsible for the abnormal signals in the observed SERS spectra of PATP. Further experimental studies proved that the so-called abnormal SERS bands are in fact the fundamentals of DMAB molecules evidenced by mass spectroscopy, IR spectroscopy,41 electrochemistry, EC-SERS,20 4


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time-dependent SERS,42 and remote excitation polarization-dependent SERS.43 Recently, a photoinduced CT model was presented to explain the conversions of aromatic amines and aromatic nitro compounds to the corresponding azo species during SERS measurements.44-51 The photo-oxidation of aromatic amines on silver was illustrated as photoinduced CT from molecule to metal while the photo-reduction of aromatic nitro compounds was related to the photoinduced metal-to-molecule CT. It is found that the photocatalytic conversion of PATP to DMAB strongly depends on experimental conditions, such as air/N2 atomsphere,45 solution pH,52-55 irradiation wavelength,18,22 and irradiation power.20 The surface plasmon-mediated photocatalytic reactions have attracted great attentions in recent SERS studies.56,57 However, despite these facts, reports on systematic studies of the influence of different substitute position (ortho-, meso-, or para-substituted) and conjugation degree between amino group (-NH2) and mercapto group (-SH) on the reactivity of photoinduced surface catalytic coupling reactions are still scarce. In this work, three aminothiophenol derivatives with different substitute position and conjugation degree between amino group (-NH2) and mercapto group (-SH) as shown in Scheme 1 were chosen to systemic study the effects of substituent including adsorption orientation effect and conjugation effect on the reactivity of photoinduced surface catalytic coupling reactions. A combined SERS and DFT study indicated that no surface reactions were occurred for compound C1 and compound C2, while compound C3 was converted to the corresponding azo species during their SERS measurements. The difference in reactivity of the selected probe molecules was investigated on the basis of proposed photoexcitation and photoreaction mechanisms. 5



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Scheme 1 The molecular structure of compounds C1-C3

Experimental Water (18.2 MΩ) was purified by Milli-Q ultra-pure water purifier (Millipore Milli-Q System). Perchloric acid (HClO4, 70%) and Sodium perchlorate (NaClO4, 98%) were purchased from Sigma-Aldrich and used as received. Absolute ethanol, triethylamine,

NaOH

and

other

chemicals

were

analytical

reagent

grade.

3-aminothiophenol (C1, 99.5%, Sigma), 4-(aminomethyl)benzenethiol (C2, 99%, Aurora) were also used as received. S-4-((4-aminophenyl)ethynyl)thiophenol (C3)49 were prepared by the procedures described in literature methods (Scheme S1, Supporting Information). The SERS-active substrates were prepared according to the published protocol.20,49 The substrate was immersed in the 1 mM aminothiophenol derivatives ethanolic solution in a nitrogen atmosphere for 1 h at room temperature. The substrate was rinsed with ethanol and ultrapure water successively, and transferred into the spectroelectrochemical cell for SERS experiments. The environmental pH for the SERS measurements was adjusted by 0.1 M HClO4, 0.1 M NaClO4 and 0.1 M NaOH aqueous solutions, which correspond to the pH value of 1, 7 and 13, respectively. It is important to note that these solutions should be deaerated with Argon gas for 30 min before the SERS experiment. All the Raman spectra were recorded by an XploRA Raman Microscope from HORIBA Jobin Yvon and a 638 nm laser was used as the excitation line.

Computational Details Density functional calculations were carried out with Becke’s three-parameters 6


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hybrid exchange functional and Lee-Yang-Parr correlation functional (B3LYP)58,59. The basis sets for C, N, S, and H atoms of investigated molecules were 6-311+G(d, p), which included the polarization function in all the atoms and diffuse function in C, N, and S atoms60,61. For all metal atoms, the valence electrons and the inner shells were described by using the basis set, LanL2DZ, and the corresponding relativistic effective core potentials, respectively.62,63 Full geometry optimizations were carried out by using Gaussian 09 package.64 The scaled quantum-mechanical force field (SQMF) procedure65 was used to assign all the fundamental vibrational bands. We chose the scaling factors of 0.935 for N-H and C-H bonds and 0.963 for the other internal coordinators to the force constant matrix calculated at the B3LYP/6-311+G(d, p) level. These scaling factors were used to correct the incomplete property of theoretical approaches and basis sets, as well as the neglect of anharmonicity. Absolute Raman intensities are calculated on top of the differential Raman scattering cross section (DRSC), as published in our previous works30. In order to make direct comparison with the SERS experiments, the simulated Raman spectra were presented in terms of the Lorentzian expansion of the DRSC magnitudes from the Raman scattering factors (RSF) under the double-harmonic approximation. Electronic excitations were studied with time-dependent density functional theory, TD-B3LYP, which offers an orbital picture for a physical understanding of the excitation process.66

Results and Discussion SERS of Compounds C1-C3 Figure 1a shows the normal Raman spectrum of 3-aminothiophenol (C1) and its 7



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SERS spectra on roughened Au and Ag electrodes. The detailed assignments of main peaks are shown in Table S1. The bands at 681 and 990 cm-1 are assigned to the ring deformation mode α(ring). The bands at 878 and 1076 cm-1 is assigned to the C-S stretching mode, v(C-S) and the in-plane C-H bending mode, β(C-H), respectively. The relative intensities of these two modes are significantly enhanced in the SERS spectra of C1 on gold and silver. The band at 1266 cm-1 is assigned to the C-N stretching mode, v(C-N). No frequency shift of the v(C-N) mode was observed in the SERS spectra, indicating that the amino group does not directly interact with metal surfaces. The band at 1588 cm-1 is assigned to the C-C stretching mode, v(C-C). The frequency of this band was slightly shift to 1575 cm-1 in the SERS spectra. Such a frequency red-shift is also predicted by theoretical calculations. Figure 1b shows the SERS spectra of C1 on roughened Ag electrodes in different pH solutions. Unlike the pH-dependent SERS behavior of 4-aminothiophenol reported in literature,13,41,53,55,67 the SERS spectra of C1 were insensitive to the solution pH. No additional peaks, especially the characteristic Raman signals corresponding to the azo group, were apparent in acidic, neutral, or basic solutions. In addition, increasing laser power does not change the spectral feature of the SERS of C1. The above control experiments suggest that C1 molecules adsorbed on gold and silver surfaces does not occur photoinduced surface catalytic coupling reactions to produce azo-like species during SERS measurements.

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990 1072 880

532

1069

1575

1252

879

1574

1261

682

(iii)

(ii) 1588

681 526

991

b

878

1076

(iii)

Raman intensity

a

Raman intensity

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991 1070 878

859

1266

996 930

1574 1251

1070

(ii) 1578

1200

(i)

(i) 600

900 1200 1500 -1 Raman shift / cm

1800

600

900 1200 1500 -1 Raman shift / cm

1800

Figure 1. (a) The normal Raman spectra (i) and the SERS spectra of 3-aminothiophenol (C1) on roughened Au (ii) and Ag (iii) electrodes. (b) The SERS spectra of 3-aminothiophenol on roughed Ag electrodes in pH = 1 (i), pH = 7 (ii), and pH = 13 (iii) solutions.

Figure 2a shows the normal Raman spectrum of C2 and its SERS spectra on roughened Au and Ag electrodes. The detailed assignments of main peaks are shown in Table S2. The strong bands at 1094 cm-1 and 1600 cm-1 are assigned to the C-S stretching mode, v(C-S) and the C-C stretching mode, v(C-C), respectively. The frequency red-shifts of these two peaks were observed in the SERS spectra of C2 on gold and silver. The bands at 1184 cm-1 and 1206 cm-1 are assigned to the C-H in-plane bending mode, β(C-H) and C-CH2 stretching mode, v(C-C). The weak bands at 1378 cm-1 and 1495 cm-1 are assigned to the CH2 wagging mode, ω(CH2) and the CH2 scissoring mode, δ(CH2), respectively. The band at 1641 cm-1 is assigned to the NH2 scissoring mode, δ(NH2). Figure 2b shows the SERS spectra of C2 on roughed Ag electrodes in different pH solutions. Similar to C1, the SERS spectra of C2 are also insensitive to the solution pH. Increasing laser power cannot result the appearance of new Raman signals corresponding 9



to the azo group, indicating that C2 molecules adsorbed on gold and silver surfaces do not convert to azo-like species during its SERS measurements. 1583

1083 1177 1214

1492

1185 1212

1486

(ii) 1600

1094 1184 1206

1000

1378 1495

1074 1185 1212

(iii) 1584

1074

1584

b

Raman Intensity

a

Raman intensity

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1641

1200 1400 1600 -1 Raman shift / cm

1411

1000

1800

(iii)

(ii)

1372

934

(i)

1409

(i) 1200 1400 1600 -1 Raman Shift / cm

1800

Figure 2. (a) The normal Raman spectra (i) and the SERS spectra of 4-(aminomethyl)benzenethiol (C2) on roughened Au (ii) and Ag (iii) electrodes; (b) The SERS spectrum of C2 on roughed Ag electrodes in pH = 1 (i), pH = 7 (ii), and pH = 13 (iii) solutions.

Figure 3a shows the normal Raman spectra of C3 and its SERS spectra on roughed Au and Ag electrodes. The mainly strong bands at 1090, 1140, 1583, and 2209 cm-1 are assigned to the C-S stretching, the phenyl-alkynyl C-C stretching, the C≡C stretching, and the ring C-C stretching, respectively. The weak bands at 1015, 1180, and 1605 cm-1 are assigned to the ring deformation mode α(ring), the C-H in-plane bending mode, β(C-H) and the NH2 scissoring mode, δ(NH2), respectively. The detailed assignments of main peaks are shown in Table S3. The SERS spectra of C3 on gold and silver are quite different from its normal Raman spectra. A sets of new bands appeared at 1135, 1399, 1457 cm-1. It is noted that the 1140 and 1583 cm-1 peaks in the normal Raman of C3 are broaden and split into two doublet peaks in the SERS spectra. The abnormal Raman 10


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signals appeared in SERS of C3 are very similar to that observed in SERS of PATP, which were considered as a spectroscopic indicator of formation of azo-species.44 The double peaks at 1135 and 1142 cm-1 are assigned to the C-N stretching mode from C3 and its N-N coupling product. The 1399 and 1457 cm-1 are assigned to the N-N stretching mode of the azo group. The double peaks at 1583 and 1598 are assigned to the C-C stretching mode from C3 and its N-N coupling product. Figure 3b shows the SERS spectra of C3 on roughed Ag electrodes in different pH solutions. It is noticed that the Raman signals from azo species can be detected in acidic, neutral, and basic solutions. In addition, the relative Raman intensities of 1135, 1399, and 1457 cm-1 bands increase with the increase of solution pH values. The appearance of characteristic Raman signals from azo species indicates that C3 molecules adsorbed on silver and gold surfaces undergo a photoinduced surface catalytic coupling reaction during its SERS measurements. a

b

1075 1131 1182

(iii)

1457

1135 1145 1073 1182

2213

1457 1399

1586 1599 2213

(iii) 1074 1135

1142

1583 1598 1399 2213

(ii) 1583

1140 1090

(i) 1015

1000

1376 1180

1605

1200 1400 1600 -1 Raman Shift / cm

2209

Raman Intensity

Raman Intensity

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1142

(ii)

1147

(i) 1000

2200

1200

1400

1600

Raman Shift / cm

2200

-1

Figure 3. (a) The normal Raman spectra (i) and the SERS spectra of C3 on roughened Au (ii) and Ag (iii) electrodes; (b) The SERS spectrum of C3 on roughed Ag electrodes in pH = 1 (i), pH = 7 (ii), and pH = 13 (iii) solutions.

11



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Simulated Raman spectra of C1-C3 Figure 4 shows the simulated normal Raman spectra of free C1-C3 compounds. The Raman spectrum of C1 is dominated by a very intense peak at 998 cm-1, which is assigned to the ring deformation. The calculated frequency matches well with experimental observation at 990 cm-1 as shown in Figure 1a. The medium strong peaks at 1080, 1272, and 1605 cm-1 agree with experimental results at 1076, 1266, and 1588 cm-1, which are assigned to the C-H bending mode, the C-N stretching mode, and the C-C stretching mode, respectively. The Raman spectra of C2 and C3 are characterized by two strong peaks at about 1100 and 1600 cm-1, and they are the C-S stretching mode and the C-C stretching mode. The frequency of the C-S stretching mode of C2 is lower than that of C3 because the C-S bond length in C2 is 0.004 Å longer than C3. Also note that a set of weak Raman peaks in the Raman spectrum of C2. A possible reason is that the aminomethyl group in C2 breaks the C2v molecular symmetry and some non-Raman active modes in C3 turns to be Raman active modes in C2. The calculated frequencies are also in good consistent with experimental results presented in Figures 2a and 3a. A detailed comparison of calculated and experimental frequencies and vibrational assignments can be found in Table S1-S3 (Supporting Information).

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Figure 4. Optimized structures and simulated normal Raman Spectra of (a) C1, (b) C2, and (c) C3 calculated at the B3LYP/6-311+G(d,p) level.

Figure 5 shows the simulated surface Raman spectra of C1-C3 adsorbed on a silver cluster. After adsorption on silver surface, the Raman intensities of C1 at 875, 1070, and 1578 cm-1 are strongly enhanced, which are in agreement with experimental observation as shown in Figure 1. The frequencies of the most intense Raman peaks in C2 at 1088 and 1608 cm-1 red-shift to 1070 and 1601 cm-1 as a result of chemical bonding interaction. In Figure 2, the C-S stretching at 1094 cm-1 moves to 1074 cm-1 and the C-C stretching at 1600 cm-1 moves to 1584 cm-1. Unlike such a good agreement between experimental and theoretical surface Raman spectra of C1 and C2, the lineshape of the simulated Raman spectrum of the C3-Ag13 complex is quite different from the observed SERS from C3 adsorbed on gold and silver surfaces, as shown in Figure 3. In the SERS of C3, two 13



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shoulder peaks appear near the original 1140 and 1583 cm-1 bands and two additional peaks are observed at 1399 and 1457 cm-1. However, these spectral changes cannot be observed in the simulated Raman spectrum of the C3-Ag13 complex.

Figure 5. Optimized structures and simulated surface Raman Spectra of (a) C1-Ag13, (b) C2-Ag13, and (c) C3-Ag13 by B3LYP/6-311+G(d,p)/Lanl2DZ.

Our previous studies demonstrated that PATP can be transformed to DMAB during SERS experiments.20,30,31 This suggests that the C3 molecule with similar structure of PATP may also be converted to an azo-like species upon laser irradiation. To verify this hypothesis, the Raman spectrum of the oxidative coupling product of C3 (C3-azo) is simulated as shown in Figure 6. Three very strong Raman peaks are observed at 1115, 14


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1386, and 1585 cm-1, which are assigned to the C-N stretching, the N-N stretching, and the C-C stretching, respectively. The frequencies of these three peaks match well the new peaks at 1135, 1399, and 1583 cm-1 observed in the SERS of C3 shown in Figure 3. It is also noted that the frequencies of C-N stretching (1115 cm-1) and C-C stretching (1585 cm-1) in C3-azo are quite close to those in C3. In case that C3 undergoes a photoreaction during the SERS experiment, the SERS of C3 should be a mixed spectrum from unreacted C3 and its coupling product C3-azo. This can explain the appearance of two broad doublet peaks (1135/1142 and 1583/1598) in the SERS of C3 shown in Figure 3a. By comparing the simulated surface Raman spectra with the experimental SERS spectra, we conclude no surface reactions are occurred for compounds C1 and C2, while the compound C3 is converted to the corresponding azo species during the SERS measurements.

Figure 6. Optimized structure and simulated Raman spectrum of oxidative coupling product of C3.

Photoreaction mechanism 15



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In our previous study, a photoinduced charge transfer model was used to describe the photoinduced reaction of aromatic amines adsorbed on silver surfaces.44 The proposed photoexcitation and photoinduced reaction mechanism for a given molecule absorbed on metal surfaces are schematically depicted in Figure 7. When the energy difference between the ground state R and the photon-induced CT excited state R* matches the energy of light irradiation, a resonant CT will take place (red arrow). The molecule in the CT excited states may undergo two different types of deexcitation. One involves a reverse CT back to the ground state, followed by a radiative process (black arrow). Raman scattering or fluorescence emission may be involved in this purely physical deexcitation channel. The other possibility is to transform to a new surface species P via a chemical reaction channel (blue arrow). For aromatic amines adsorbed on silver surfaces, an electron is excited from the occupied orbital of adsorbate to the unoccupied state of metal substrates by visible light irradiation. Ar-NH2 losses one electron to produce an Ar-NH2+• radical cation. The cation radical Ar-NH2+• in its excited state losses one proton to form an Ar-NH• neutral radical. This process is favorable in basic solutions. A head-to-head coupling of Ar-NH• neutral radicals gives hydrazo species Ar-NHNH-Ar, which can be further oxidized through a photoinduced molecule-to-metal CT transition to produce azobenzene species.

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Figure 7. Proposed photoexcitation and photoinduced reaction mechanisms for C3 molecules adsorbed on metal surfaces.

Comparison of SERS experiments and DFT calculations of model compounds C1-C3 indicates that no photoinduced reactions are occurred for C1 and C2 molecules adsorbed on silver or gold surfaces under laser irradiation, by contrast the C3 molecule can undergo a photoinduced surface catalytic coupling reaction to convert to azo-like species during SERS measurements. The difference in reactivity for the selected probe molecules C1-C3 must be strongly related to the difference in molecular electronic structures. The electron density diagrams of frontier orbitals of C1-C3 molecules are shown in Figure 8. The levels HOMO and LUMO levels of C1 are -5.81 and -0.51 eV, respectively. They are very close to the HOMO/LUMO levels of PATP, -6.00/-0.62 eV.30 Note that the HOMO of C1 is a mixed orbital from amino lone pair p orbital and benzene ring π orbital related to the p-π interaction. The HOMO/LUMO levels of C2 (-6.02/-0.64 eV) are slightly lower than that of C1. Because the methylene group blocks the p-π interaction between the amino group and the benzene ring. Meanwhile, the electron 17



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density of HOMO is mainly distributed on the benzene ring while amino lone pair electrons are localized in HOMO-1 (-6.86 eV). In contrast to C1 and C2, the HOMO-LUMO gap of C3 is obviously smaller than those of C1 and C2 due to a better conjugation system, in which the electron densities of HOMO/LUMO are delocalized in the whole molecule. The HOMO level of C3 increases to -5.36 eV and the LUMO level of C3 decreases to -1.39 eV. It is noted that the p-π interaction plays an important role in the electronic structure of C1-C3 molecules. The strength of p-π interaction can be characterized by the bond length of the C-N bond. For the investigated three molecules, the C-N bond length increases as an order of C3 (1.392 Å) < C1 (1.395 Å) < C2 (1.468 Å, no p-π interaction). The shorter the C-N bond length is, the higher the HOMO level is (HOMO-1 for C2).

Figure 8. Frontier molecular orbitals of C1-C3.

In order to investigate the influence of electronic structure on the photoexcitation and photoinduced chemical reaction of C1-C3, TD-DFT calculations were performed to study the photoinduced CT processes. Molecules adsorbed on silver surfaces are modeled 18


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as molecule-metallic cluster complexes. Ag13 cluster was used to mimic the active sites on surfaces, C1 to C3 molecules are adsorbed on the hollow site of the Ag13 cluster via thiol group. Table 1 shows the calculated vertical excitation energies and oscillator strengths of the low-lying CT transitions of the three molecules absorbed on silver. As shown in Table 1, all the low-lying CT transitions for C1-C3 molecules arise from molecule-to-metal excitations because the HOMO levels of the probe molecules are close to the Fermi level of silver. The lowest CT transition energies of C1 and C3 are 2.55 eV (486 nm), and 2.21 eV (562 nm), which are comparable to the CT transition energy of PATP (2.28 eV).32, 44 The transition energies match the laser photonic energy used in SERS measurements, thus the photoinduced CT transitions could occurred for C1 and C3 molecules adsorbed on silver. Although the calculated oscillator strengths of CT transitions are relative small, the photoinduced molecule-to-metal CT process can be strongly amplified as a result of significantly enhanced local optical electric field via surface plasmon resonance.68 It is noted that the HOMO level of C3 (-5.36 eV) is higher than that of PATP (-6.00 eV)30 and the lowest CT transition energy of C3-Ag13 complex (2.21 eV) is lower than that of PATP-Ag13 complex (2.28 eV).32 So C3 should be more likely than PATP to occur a photocatalytic oxidation reaction. Since the coupling of aromatic amines to its corresponding azo species is an oxidative dehydrogenation reaction, basic solution is facilitated for the hydrogen abstraction.69 The solution pH plays a very important role in the photoinduced conversion of PATP to DMAB. In the previous pH-dependent SERS study of PATP, it was found that the SERS signals from DMAB could be observed only in basic solution.52-55 However, the SERS signals from azo species are detected in both 19



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Page 20 of 32

acidic and basic solution in the SERS of C3 as shown in Figure 3b. The difference in reactivity between PATP and C3 is due to their different conjugation structures. The better conjugation structure of C3 gives rise to a lower HOMO level. So the C3 molecule is more easily oxidized than PATP even in the acidic solution.

Table 1. Calculated Low-Lying CT Transitions of Model Surface Complexes by TD-B3LYP. Excitation

Oscillator

energy

strength

Transitions

2.55 eV C1-Ag13

0.0010 (486 nm)

2.65 eV 0.0101 (468 nm) C2-Ag13 3.75 eV 0.1166 (331 nm)

2.21 eV C3-Ag13

0.0006 (562 nm)

The photoinduced CT transitions of the C2 molecule are quite different from C1 and 20


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C3. Two possible molecule-to-metal CT transitions were observed by TD-DFT calculations. One is assigned to the CT from the HOMO with an electron population on the benzene ring π orbital and the other is assigned to the CT from the HOMO-1 with the electron distribution mainly on the nitrogen lone pair orbital. The calculated transition energy for HOMO to silver CT is 2.65 eV (468 nm). However, such a CT transition will not lead to an oxidation of the amino group. The calculated transition energy for HOMO-1 to silver CT is 3.75 eV (331 nm), which is far away from the laser photonic energy used in the SERS measurements. As a result, C2 molecule is not able to convert to the corresponding azo species via a photoinduced CT transition. Previous theoretical studies illustrated that B3LYP functional trends to underestimate

CT

excitation

energies.70

TD-DFT

calculation

with

other

correlation-exchange functionals such as CAM-B3LYP,71 wB97XD72 and M062X73 are also performed to investigate the CT properties of C1-C3 silver complexes (Table S4, Supporting Information). The CT excitation energies predicated by CAM-B3LYP and wB97XD functionals, which consider long range correlation, are higher than those computed by B3LYP functional. However, M062X functional gives very similar CT excitation energy to B3LYP. TD-DFT calculations show that C1 and C3 can be excited from the ground state (R) to the CT excited state (R*) via photoinduced amine-to-silver CT transitions. However, only C3 is observed to have a chemical transform to the corresponding azo species and no characteristic Raman signals from the azo group are observed in SERS of C1. The orientation effect of substituent group plays an important role to stabilize the CT excited state. The possibly existed resonance structures of 4-ATP, 3-ATP (C1), and 2-ATP radical 21



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cations are given in Figure 9. Among all the resonance structures of ATP, structure (iv) is extremely stable because the positive charge could be stabilized by the electron-donating thiol group. A similar stable resonance structure (structure v) can be also found in 2-ATP radical cation. The characteristic Raman bands of azo group have been observed in a series of ortho-substituted anilines such as 2-aminothiophenol,74 2-aminophenol,75,76 and 1,2-phenylenediamine.77 However, such a stable structure does not exists in the resonance structures of 3-ATP radical cation. So the CT excited state (radical cation) of C1 may be not stable enough to convert to the product through a non-radiative chemical reaction channel before it goes back to the ground state through a radiative physical channel. The instability of C1 excited state can explain why no Raman signals of azo-like species were observed in SERS of C1.

Figure 9. Resonance structures of the radical cations of 4-ATP, 3-ATP, and 2-ATP. 22


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Conclusion Three aminothiophenol derivatives C1-C3 with different electronic structures were designed to study the effects of substituent groups including the orientation effect and the conjugation effect on the reactivity of photoinduced surface catalytic coupling reactions. The surface catalytic coupling reactions of aromatic amines are illustrated as an initial photoinduced molecule-to-metal excitation and a subsequent chemical transformation process. A combined SERS and DFT study was performed to explore their differences in reactivity of the designed probe molecules. It is found that C1 and C2 cannot convert to the corresponding azo species during present SERS experiments. The orientation effect of the substituent group plays an important role to prevent the photoinduced reaction of C1, because the CT excited state (a radical cation) of C1 is not a stable structure on silver surfaces. The p-π conjugation effect between the amino group and the benzene ring is greatly weaken by the methylene group in C2. As a result, the incident laser energy used in SERS experiments is not sufficient to drive the molecule-to-metal excitation. In contrast to previous study that the formation of DMAB form PATP could be observed mainly in neutral and basic solutions, the SERS signals from azo species are detected in acidic, neutral, and basic solutions in the SERS of C3 with more extended π conjugation structures, which can be selectively transformed to the corresponding azo species during SERS measurements.

Acknowledgements. The authors acknowledge financial support from National Natural Science Foundation of China (Nos. 21173094, 21321062, 21573086, and 21533006) and 23



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Fundamental Research Funds for the Central Universities (SWU114076 and XDJK2015C100) and 2015CB932303 for financial support. Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Reference 1.

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Surface-Enhanced Raman Scattering of the Enzyme Reaction Product. Anal. Chem. 1997, 69, 1492-1495.

31



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Table of Contents Graphic NH2

×

e−

e− S

C2

e−

C3

e−

e−

C1

Ag/Au

NH2

S

Ag/Au

32


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Photoinduced Surface Catalytic Coupling Reactions of

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