Spectral Characteristics of Chemical Enhancement on SERS of

Nov 12, 2015 - Going beyond the vertical approximation with time-dependent density functional theory. Fabrizio Santoro , Denis Jacquemin...
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Spectral Characteristics of Chemical Enhancement on SERS of Benzene-like Derivatives: Franck−Condon and Herzberg−Teller Contributions Liqian Liu,† Danping Chen,† Huili Ma,‡ and WanZhen Liang*,† †

State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, People’s Republic of China ‡ Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: A systematically theoretical investigation has been performed to study the dynamic chemical enhancement of surfaceenhanced Raman spectroscopy (SERS) of pyridine, pyrimidine, 2mercaptopyridine, and 4-mercaptopyridine absorbed on a silver cluster of Ag20. The influences of different structural configurations (V and S), different intermolecular charge-transfer (CT) excited states, and the different approximations [Franck−Condon (FC) or FC + Herzberg− Teller (FCHT) approximations] to spectral cross sections have been examined. It is found that the photoexcitation can easily produce the intermolecular CT excited states, leading to the absorption maxima red-shift, and their intensities decrease compared with that of Ag20. Furthermore, we observe that the absolute Raman intensities are sensitive to the systems’ structural configurations, exchange-correlation functionals, CT excited states, and the FC/FCHT approximations as well. However, the relative Raman intensities and dominant vibrational structures of CT resonance RS (RRS) are mainly determined by the adsorbates. The modes which can have a larger enhancement in all CT RRS are those related to ring stretch and ring breathing. The ring-stretching mode at around 1600 cm−1 in four molecule−Ag20 systems is evidently enhanced compared to that of bare molecules which can be considered as a hint of the presence of the CT resonance enhancement. Additionally, we observe that HT effects dominate the resonance enhancement and it could explain the coupling between the plasmonic and chemical enhancement mechanisms, but the FCHT approximation does not significantly change the relative RRS intensities obtained through the FC approximation. The first one is the static chemical enhancement caused by the interaction between the nanostructures and adsorbates without resonance excitations of cluster−molecule systems. The second one is the resonance-like Raman enhancement where the excitation wavelength is resonant with an electronic excitation.10 For example, in the resonance-like CT enhancement mechanism, the excitation wavelength is resonant with a CT electronic transition between the nanostructures of the metallic surfaces and the adsorbates. The CT enhancement has been experimentally observed in the SERS spectra of aromatic molecules.11−13 In the experiments, the transient CT states are produced by tuning the laser photon frequency or the metal electrode potential. It was found that the vibrations in SERS features related to the resonance Raman CT process are closely related to Franck−Condon (FC) factors of a resonant photoinduced CT mechanism. Later, Otero et al. explained

1. INTRODUCTION Light can be guided and localized by the noble metal nanostructures that support surface plasmon. The surface plasmon resonance (SPR) is formed by the resonant interaction between photon and collective electron charge oscillations in metallic nanoparticles (NPs), which not only makes NPs possess the brilliant optical properties but also offers a strongly enhanced localized field to enhance spectroscopic signals of nearby molecules. Currently, the surface-enhanced Raman spectroscopy (SERS) has become one of the most sensitive spectroscopic techniques which can be used to detect surface species and to study adsorption on metal surfaces. The enormous enhancement has been ascribed to two kinds of mechanisms:1−4 one is electromagnetic field enhancement, and the other one is the chemical enhancement. The electromagnetic enhancement (EM) is caused by the strong SPR of rough metal surfaces which is coupled to the incident light. The EM model is very popular, and usually the EM dominates almost total enhancement of SERS.1−3,5−9 The chemical enhancement can be classified into two categories. © XXXX American Chemical Society

Received: June 20, 2015 Revised: November 12, 2015

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wavelengths, vibrational modes, and molecular derivatives. The enhancement factors and the selection rules for the SERS spectra of pyridine, pyrimidine, 2MPY, and 4MPY will be given. In the calculations of SERRS spectra, we adopted our recently developed time-dependent approach to molecular RRS spectra with inclusion of FC, HT, and mode-mixing effects.30,31 There are two possible photoinduced CT pathways in the interface between the metal and adsorbate. The first one is that the photoabsorption creates the excited states of the metal, and sequentially, the electron on the excited state injects into the molecule. The second one is that the photoexcitation directly induces the charge separation between the metal and molecule, named as CT excited state. In this study, we calculate the RRS spectra of hybrid systems by specifying the resonant states of RRS process to be the one-step photoinduced CT states and the ground state of hybrid systems. Therefore, the RRS approach for the molecules can be applied to describe the CT RRS spectra of hybrid systems. We will separately show the contributions of FC and HT effects on the SERRS spectra, so that the HT vibronic coupling effect on CT RRS spectra can be quantified, and the HT surface selection rules for the SERRS will be provided. A previous report32 advocated that FC should dominate the SERRS by comparing the calculation at FC approximation with the experimental SERRS measurement. It is interesting to provide a quantitative demonstration of both FC and HT contributions to the SERRS. The paper is organized as follows. In section 2, we outline the theoretical and computational models. Section 3 presents the corresponding SERS spectra for the model systems. The concluding remarks are given in section 4.

and determined the SERS features by the differences between the electronic properties of the neutral and anionic molecular species. On the basis of these results, they therefore proposed to use SERS as a tool to characterize the doublet states of adsorbates.14,15 The theories and computational methods for the molecular RRS spectra have been well developed and have been successfully applied to many systems. However, it is more difficult to calculate Raman scattering from adsorbates by CT states since the overall process is a time evolution with the Hamilton operator of the total interacting system (metal and molecule) with electronic and vibrational states. Two kinds of simplified treatments have been usually adopted. One is to simplify the problem by assuming molecules bound to very small metal clusters.16−19 The other is to use relatively simple but more general theories for the interaction of the electric field with the electrons in the metal and the adsorbate20−23 or the News−Anderson theory of electron transfer between metal and adsorbate with the parameters.24−26 With the first treatment, the Raman scattering from adsorbates by CT excitation can be calculated like the molecular RRS, and the previous studies27−29 have found that the electrode potential dependence of the SERS intensity and Herzberg−Teller (HT) effects in Raman scattering can be calculated and compared relatively well with the experiments. For example, Schatz’s group17 and Lombardi’s group18 adopted the molecule−cluster model to study the CT enhancement mechanism of SERS spectra for the pyridine absorbed on Ag cluster based on the time-dependent density functional theory (TDDFT). They thought that the laser photon can produce a complete electron transfer from the metal to the vacant orbitals of the adsorbed molecule, which can be considered to be equivalent to a resonance CT Raman mechanism, and thus suggested that the enhanced Raman scattering due to atomic clusters is comparable to findings on single nanoparticle. However, in most cases, the calculated Raman scattering spectra with respect to the small metal NPs are incomparable with the experimental measurements because the small NPs cannot produce the plasmon of the bulk metal. The largest challenge on theoretical calculation of SERS is to simultaneously account for the effects from the chemical enhancement and electromagnetic enhancement. In this work, we focus on CT enhancement of SERS for four hybrid systems composed of Ag20 and the organic adsorbate (pyridine, pyrimidine, 2-mercaptopyridine (2MPY), or 4-mercaptopyridine (4MPY)). Here, we will adopt FC and FCHT approximations to calculate RRS so that we can identify the roles of FC and HT and check whether the HT effect could explain the coupling between the plasmonic and chemical enhancement mechanisms. The molecule−Ag20 cluster model will be adopted. The CT RRS spectra of pyridine−Ag2017 and pyridine−Ag1018 as well as the NRS spectrum of pyrimidine− Ag 20 19 have been theoretically investigated. Here, we simultaneously study the pyridine−Ag20, pyrimidine−Ag20, 2MPY−Ag20, and 4MPY−Ag20 systems. Their electronic absorption spectra, the surface enhancement RRS (SERRS), and SENRS will be calculated. The comparison will be made with the results of bare metal cluster or molecules as well as the other experimental or theoretical results. The main purpose of this work is to make an investigation on how large the Raman intensities can be influenced by the chemical substitutes of adsorbates, binding sites, and excitation wavelengths. In the end, it is expected to depict the dependence of absolute and relative Raman intensities on the binding sites, excitation

2. COMPUTATIONAL METHODS The time-dependent expression for the differential cross section of molecular RRS with inclusion of Duschinsky rotation (mode-mixing) as well as FC and HT vibronic coupling effects has been derived in our previous work30 in detail. The modemixing effect accounts for the differences between ground- and excited-state normal-mode coordinates and vibrational frequencies, and the HT vibronic coupling effect accounts for the dependency of transition dipole moments on the mode coordinates, μeg = μ0 + ∑l(∂μeg/∂Ql)Ql + .... Therefore, to predict the RRS, one is required to accurately compute the structure parameters, such as the equilibrium geometries and vibrational frequencies of resonant states, the excitation energies, and the transition dipole moments as well as their geometrical derivatives, etc. The ground-state properties can be easily obtained; however, it is difficult for us to calculate the excited-state properties. To avoid the difficulties, in this paper, we adopt the vertical gradient (VG) approximation which has been simply described in our previous paper.33 In VG approximation, physically, the excited-state potential surface is just a mere shift relative to the ground-state potential surface without any scrambling of the normal coordinates or a change in the harmonic frequencies, i.e., ωgj = ωej and Lg = Le. Here ω denotes the vibrational frequencies and L corresponds to the transform matrix, which is obtained by the diagonalization of the mass-weighted Hessian matrix H as LTHL = ω2. The displacements which are related to the structural change along the normal mode Qj upon excitation of the molecule read ⎛ ⎞ ⎜ ∂E ⎟ = (wjg )2 Δ̅ j ⎜ ∂Q ⎟ ⎝ j⎠ B

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Figure 1. Absorption spectra of pyrimidine−Ag20 complexes (red lines) and Ag20 cluster (black lines) calculated by TD-Cam-B3LYP (a, b) and TDB3LYP (c, d). The position and height of vertical stick below the absorption line shape represent the excitation energy and the oscillator strength of corresponding excited state, respectively. The sticks in pink represent the excited states with the obvious CT character, which will be considered as the resonant CT excited states in the RRS calculations.

Here ∂E/∂Qj is the first-order derivative of the vertical excitation energy with respect to the jth normal-mode coordinate Qj, and Δ̅ = (Le)Td. d = xg − xe denotes the corresponding shift, where xg and xe represent the massweighted coordinates of equilibrium ground and excited electronic states, respectively. Although the VG approximation could not give finer details like resolved vibrational spacings, it is quite adequate for the overall band line shape following multidimensional vibronic excitations, i.e., the envelop and width of the profile. All the related structure parameters are calculated by the density functional theory (DFT) or time-dependent DFT (TDDFT) within the Gaussian 09 software package.34 The hybrid DFT exchange-correlation (XC) functional B3LYP and the long-range-corrected XC functional Cam-B3LYP will be adopted. The basis set 6-31G** is used for carbon, nitrogen, and hydrogen atoms, while a pseudopotential basis set LANL2DZ is used for silver atom. All the calculated harmonic vibrational frequencies are scaled by a factor of 0.961. In HT approximation, one is required to calculate the geometric derivatives of transition dipole moments. Here we calculate them based on the finite-difference methods. The calculated normal RS (NRS) are all simulated at an incident wavelength of 514.5 nm, and the RRS apply a damping parameter of 100 cm−1 and Lorentz broadening of 10 cm−1.

shown in the insets of Figure 1. For the S-complex, there is an on-top binding to one of its four faces, while for the latter structural configuration, the binding site is one of its vertexes, representing an adatom site. In both complexes, the molecule binds to the Ag20 cluster through the nitrogen atom in a perpendicular manner. Pyrimidine−Ag20. Electronic Excitations. The insets of Figure 1 show the geometries of S- and V-complexes of Pyrimidine-Ag20 system. The closest N−Ag distance in the Vcomplex is shorter than that in the S-complex, indicating the different static chemical interaction between Pyrimidine and Ag20. B3LYP produces a slightly longer N−Ag distance than Cam-B3LYP. At the optimized ground-state geometries of the complexes, we calculate the vertical excitation energies and corresponding oscillator strengths of 200 low-lying excited states. The simulated absorption spectra of pyrimidine−Ag20 are shown in Figures 1a−d. For comparison, the absorption spectrum of Ag20 cluster is also shown. TD-Cam-B3LYP overestimates the excitation energies and blue-shifts the absorption maxima about 0.2 eV compared to those from TD-B3LYP. TD-B3LYP produces an absorption maximum at 3.59 eV (345 nm) for the Ag20 cluster while it produces a absorption maximum at 3.55 eV (349 nm) and 3.56 eV (348 nm) for the S- and V-complexes, respectively. The location of absorption maximum of Ag20 calculated by TD-B3LYP is closer to the experimental plasmon peak of 3.5 eV of Ag nanoparticles. The strong chemical interaction between the molecule and the metallic cluster red-shifts the spectra and decreases the intensity of absorption maximum of Ag20, indicating that the photoninduced electronic transfer from the metal cluster to the molecule takes place. Those spectral characters exhibited on pyrimidine−Ag20 complexes can also be observed in other molecule−Ag20 systems.

3. RESULTS AND DISCUSSION Four organic molecules, pyrimidine, pyridine, 2MPY, and 4MPY, are bounded to an Ag20 cluster. The tetrahedral structure of Ag20 is used, which is known as one of the local minima of Ag20 clusters. Although the structure of Ag20 is unique, there are two different binding sites, denoted as the Sand V-type connections, to form the S- and V-complexes as C

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Figure 2. Simulated NRS of pyrimidine (a) and S- and V-complexes (b, c). The dark/red line denotes the result with respect to the structure parameters produced by B3LYP/Cam-B3LYP.

Figure 3. Chosen normal modes of pyrimidine.

vibrational structures are quite similar as a whole, but the peak positions shift a little and the relative Raman intensities change significantly due to the static chemical interaction between pyrimidine and Ag20. The enhancement ratio for each vibrational band is different, which is very much dependent on the vibration manner. The vibrations at around 1600 and 1050 cm−1 become dominant in NRS of the complexes. The main vibrational band at around 1050 cm−1 is magnified about 4−6-fold when pyrimidine binds to Ag20. We then calculate RRS corresponding to the different “resonant” CT excited states of pyrimidine−Ag20 systems. Totally, eight RRS spectra have calculated as shown in Figures 4a−h, which considered the influence from the different XC functionals, CT excited states, and the structural configurations of molecule−Ag20 systems. For example, for the S-complex with B3LYP, the first RRS corresponds to the CT excited state 62 and the second one corresponds to the nearly degenerated excited states 67 and 68. In the RRS calculations concerned with the degenerated excited states, the incident light energy is set to be the average excitation energy of those excited states. It is obvious that the RRS are sensitive to CT states, especially the Raman intensities. The spectral characters of both S- and V-complexes are dominant with the significantly increasing intensities of vibrational bands at around 1600, 1400, and 980 cm−1. For the S-complex calculated by TD-CamB3LYP, the maximum enhancement factor of 105 appears at the mode with the frequency of 1560 cm−1 while it is about 106 in the V-complex. Our calculations support the conclusion that the ring-stretching mode around the 1600 cm−1 region can be considered as a hint of the presence of the CT resonance process of SERS.27,32,37 The SERRS spectra of the pyrimidine molecule absorbed on silver clusters by changing the silver electrode voltage have been experimentally studied by Zhang et al.19 They found that with the increase of the voltage the intensities of the normal modes at around 1000 and 1600 cm−1 are gradually enhanced, and when the applied electrode voltage reaches 0.5 V, the two bands got the maximum enhancement

In this paper, we are interested in the dynamic chemical enhancement on SERS which is specified by the contribution from the “resonant” CT excited states where the photoexcitation induces an obvious electron transfer from the metal cluster to the adsorbate. In Figure 1 and Table S1, we thus list the low-lying CT excited states which possess relatively large oscillator strengths. The CT excited states are determined by the charge distributions of involved occupied and virtual molecular orbitals (MOs) (see Figures S1 and S2) by following the suggestion of Zhao et al.17 The large differences between Mulliken’s charges of Ag20 and the adsorbate in CT excited states also manifest the CT character of selected excited states. As it is shown, the locations of these low-lying intermolecular CT states produced by TD-B3LYP and TD-Cam-B3LYP are significantly different. The standard hybrid functional B3LYP underestimates the excitation energies and puts the intermolecular CT states far away from the absorption maxima of the complexes, while Cam-B3LYP puts them above or within the range of absorption maxima. The different locations of CT states arisen by the DFT XC functionals will definitely impact the RRS, as addressed by some literatures.35,36 Here we thus calculate the SERS spectra with respect to the structural parameters produced by TD-B3LYP and Cam-B3LYP, respectively, which allows us to demonstrate the functionaldependent effect on the SERS of molecule−Ag20 systems. NRS and RRS. To understand the SERS enhancement mechanism and check how large the enhancement factor can be for that small metal cluster, we calculate the NRS and RRS of pyrimidine−Ag20 complexes. Figures 2a−c display the calculated NRS of pyrimidine and the S- and V-complexes of pyrimidine−Ag20. In NRS of pyrimidine, the strongest peaks appear at around 1000 cm−1, the mode of the ring breathing as shown in Figure 3. Other relatively stronger peaks appear at around 1561, 1216, 1123, 792, 667, 605, and 392 cm−1. Overall, the calculated harmonic frequencies coincide with the experimental result.37 Comparing the NRS of S- and Vcomplexes with that of pyrimidine, it is found that the D

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Figure 4. Simulated CT RRS of pyrimidine−Ag20 complexes with the FC (black lines) and FCHT approximations (red lines). (a)−(d) are the RRS spectra based on the structure parameters produced by TD-Cam-B3LYP while (e)−(h) are those based on TD-B3LYP. The label “V/S-x” in the figures denotes the resonant CT state which corresponds to the xth excited state of the V/S-complex.

Figure 5. Absorption spectra of S- and V-complexes of pyridine−Ag20 (red lines) and Ag20 cluster (black lines) calculated by TD-Cam-B3LYP.

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Figure 6. Simulated NRS of pyridine (a) and S- and V-complexes (b, c) with Cam-B3LYP.

Figure 7. Simulated CT RRS of pyridine−Ag20 complexes with the FC approximation (black lines) and FCHT approximation (red lines) . The structure parameters are produced by TD-Cam-B3LYP.

by TD-Cam-B3LYP in the paper, and those by TD-B3LYP are shown in the Supporting Information. Pyridine−Ag20. Electronic Excitations. Pyridine absorbed on Ag cluster has been extensively studied by the experiment27 and theory.17,18 The shortest N−Ag distances are 2.53/2.58 and 2.370 Å in the equilibrium geometries of the S- and Vcomplexes by Cam-B3LYP/B3LYP, respectively. The calculated absorption spectra of pyridine−Ag20 systems and Ag20 cluster by TD-Cam-B3LYP are shown in Figures 5a,b while those by TD-B3LYP are shown in Figure S4 of the Supporting Information. The absorption maxima of the complexes redshift 0.06 eV, and their intensities decrease compared with that of the bare cluster, indicating that an obviously photoinduced electron transfer takes place from the metal cluster to pyridine in the Pyridine−Ag20 systems. Tables S2 and S3 in the Supporting Information list the lowlying CT excited states with relatively large oscillator strengths. With TD-Cam-B3LYP applied, the higher-lying excited states 66 in the S-complex and 63 in the V-complex possess the larger oscillator strengths. The excited states 63 and 64 in the Scomplex are almost degenerate and are well separated from the

where the electronic excitations lie in the region of CT excitation, leading to a resonance-like Raman scattering and a significant enhancement in the SERS intensity. Therefore, the characters of our calculated CT enhanced RRS spectra agree well with those produced by the experiment.19 The HT effect on RRS spectra is significant, which not only significantly enhances the Raman intensities but also makes the inactive modes in FC approximation become visible. The dominant HT effect comes from the intensity borrowing from the strongly allowed states. For example, for the RRS spectra corresponding to CT excited states 57 and 58 for the Vcomplex with TD-Cam-B3LYP, the peak intensity for the vibration at 1600 cm−1 at FCHT approximation is magnified by 104 compared to that at FC approximation. Figure 1b shows that the excitation energies of CT states 57 and 58 are very close to one of excited states of Ag20, and the intensity borrowing from the strongly allowed state can easily take place. With the same procedure, we calculate the SERS spectra of other model systems. We would not elaborate the details, but show the succinct results instead. Since then we show all the calculated results based on the structural parameters produced F

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Figure 8. Absorption spectra of 2MPY−Ag20 complexes (red lines) and Ag20 cluster (black lines) calculated by TD-Cam-B3LYP.

Figure 9. Simulated NRS of 2MPY and the complexes with Cam-B3LYP.

enhancement ratio is relatively small, while it reaches to about 106 in Figure 7b and about 105 in Figure 7d . The spectral characters of RRS in pyridine−Ag20 systems are dominant with the significantly increasing intensities of vibrational bands at around 1600, 1200, 1000, and 600 cm−1. Compared the spectra in Figure 7 with those in Figure 4, the significant differences appear in the vibrations at around 1400, 1200, and 600 cm−1. For the pyrimidine−Ag20 complexes, there is apparent peak at around 1400 cm−1, a mode of the ring stretch. However, the band is weak in the pyridine−Ag20 complexes; instead, the bands at 1200 and 600 cm−1 become dominant ones. The HT contribution on the RRS of pyridine−Ag 20 complexes is significant, which not only magnifies the intensities of observed Raman peaks at FC approximation but also change the relative intensities of RRS. With the HT effect getting involved, our calculated RRS spectrum shown in Figure S8 agrees well with the result produced by Zhao et al.17 with the short time approximation real-time TDDFT simulation.40 2-Mercaptopyridine−Ag20. Electronic Excitations. 2MPY binds to Ag20 to form S- and V-complexes of 2MPY−Ag20 as shown in the insets of Figures 8a,b. The calculated shortest N− Ag distances are 2.73/2.78 and 2.40/2.44 Å in the optimized geometries of S- and V-complexes by Cam-B3LYP/B3LYP, respectively. The simulated absorption spectra of 2MPY−Ag20 by TD-Cam-B3LYP and TD-B3LYP are shown in Figure 8 and Figure S10, respectively. The absorption maxima of the S- and V-complexes red-shift in comparison to that of bare cluster.

state 66. The CT excited states in the S-complex mainly include the electronic transitions from the high-lying occupied MOs of the metal to the LUMO+1 of pyridine while those in Vcomplex mainly include the electronic transitions from the high-lying occupied MOs of the metal to the LUMO of pyridine. Then we calculate the RRS spectra corresponding to those CT excitations. NRS and RRS. The NRS spectra of pyridine and S- and Vcomplexes are displayed in parts a, b, and c of Figure 6, respectively. Like pyrimidine, the strongest peak in NRS spectrum of pyridine also appears at around 1000 cm−1, the mode of the ring breathing with N moving toward silver as shown in Figure S6. The second strongest peaks appear at around 1600, 1200, and 700 cm−1. Overall the harmonic frequencies coincide with the experimental results.38,39 The interaction between pyridine and Ag20 further enhances the vibrations at 1600, 1200, and 1000 cm−1 and makes these three modes become the dominant ones. A maximum enhancement appears at around 1000 cm−1. The existence of metal cluster slightly blue-shifts this peak and makes its peak intensity magnified by 3-fold. For two pyridine−Ag20 complexes, four RRS spectra have been calculated. Figures 7a,b correspond to the CT excited states 64 and 66 of the S-complex, and Figures 7c,d correspond to the CT excited states 56 and 63 of the V-complex. Because of the weak transitions of the state 63 in S-complex and the state 62 in V-complex (see Table S2), their contributions to RRS have been omitted. For the RRS shown in Figure 7a, the G

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Figure 10. Simulated CT RRS of 2MPY−Ag20 complexes with the FC approximation (black lines) and FCHT approximation (red lines). The structure parameters are produced by TD-Cam-B3LYP.

Figure 11. Absorption spectra of 4MPY−Ag20 complexes (red lines) and Ag20 cluster (black lines) calculated by TD-Cam-B3LYP.

Figure 12. Simulated NRS of 4MPY (a) and 4MPY−Ag20 complexes (b, c) with Cam-B3LYP.

peak near 1600 cm−1 for pyridine− and pyrimidine−Ag20 systems while there are two peaks for 2MPY−Ag20 systems. When 2MPY binds to Ag20 cluster, the relative intensities of the high-frequency mode near 1600 cm−1 and low-frequency modes are compressed. Overall, the calculated harmonic frequencies coincide with the experimental results.41 Figures 10a−c show the calculated RRS corresponding to different CT excited states of S- and V-complexes. Not like NRS of the complexes, all the peaks at around 1600 cm−1 are significantly enhanced, becoming one of the strongest bands in CT RRS. The maximum enhancement factor in the RRS

Figure 8 and Table S4 list the low-lying CT excited states with relatively large oscillator strengths calculated by TD-CamB3LYP. For the S-complex, the CT excited state 69 possesses the larger oscillator strength. In the V-complex, the excited state 58 possesses the larger oscillator strength and those of the states 62 and 63 are relatively smaller. NRS and RRS. The NRS spectra of the 2MPY molecule and S- and V-complexes of 2MPY−Ag20 are displayed in Figures 9a−c, respectively. Because of the lower molecular symmetry of 2MPY than pyridine and pyrimidine, more dominant active modes are observed in NRS of 2MPY molecule. There is one H

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Figure 13. Simulated CT RRS of 4MPY−Ag20 complexes with the FC approximation (black lines) and FCHT approximation (red lines). The structure parameters are produced by TD-Cam-B3LYP.

spectra of the S-complex reaches to 103 as shown in Figure 10a, while it is about 107 and 105 for the V-complex as shown in Figures 10b,c, respectively. 4-Mercaptopyridine−Ag20. Finally, we study 4-mercaptopyridine (4MPY), an isomer of 2MPY. The SERS of 4MPY has been extensively investigated experimentally.41−44 The different binding sites, silver substrates and environment pH values on the influence of SERS have been investigated. Here we not only calculate the SENRS but also the CT RRS. Electronic Excitation. The calculated absorption spectra of Ag20 and the S- and V-complexes of 4MPY−Ag20 by TD-CamB3LYP and TD-B3LYP are shown in Figures 11a,b and Figure S16, respectively. The existence of 4MPY red-shifts the absorption maxima about 0.06 eV and decreases the absorption maximum of Ag20. Tables S6 and S7 display the low-lying CT excited states with relatively large oscillator strengths. NRS and RRS of 4MPY−Ag20. The NRS spectra of 4MPY and 4MPY−Ag20 complexes are displayed in Figures 12a−c. The existence of metal cluster significantly changes the relative Raman intensities of vibrational bands in both complexes. The spectral characteristics of the complexes is dominant with the increasing intensities of the peaks at around 1600 and 1100 cm−1, which is consistent with the experimental results.41−44 The relative intensities of low-frequency modes are compressed. Because of higher molecular symmetry of 4MPY than 2MPY, one instead of two main peaks near 1600 cm−1 is observed in the NRS of 4MPY−Ag20 complexes. Then three RRS spectra of the complexes with the different incident energies have been calculated, and the results are shown in Figures 13a−c. For the S-complex, the excitation energies of the excited states 63, 64, and 65 are nearly same. For the V-complex, the excitation energies of the states 56 and 63 are different. Obviously, the relative intensities of vibrational structures in CT RRS spectra are sensitive to the CT excited states. In three RRS spectra, the vibration at around 1600 cm−1 is strongest. The maximum enhancement factor in RRS of the V-complex reaches to 106. The HT effect on RRS is significant, which not only obviously changes the Raman intensities but also makes some inactive modes become active.

excited states, and the different approximations to RRS spectral cross section (FC or FCHT approximations) have been examined. The comparison of the calculated results is made with those of bare clusters or isolated molecules as well as other theoretical and experimental results. The following conclusions are reached by this study: (1) The photoexcitation can easily induce the electron transfer from Ag20 cluster to the adsorbates, which is verified not only by the obvious charge displacement from Ag20 to adsorbates but also by the decreased intensities of absorption maxima of Ag20 and the large CT resonance enhancement factors in RRS. (2) The absolute RRS intensities are very much dependent on the excitation energies and oscillator strengths of CT excited states. TD-B3LYP underestimates the excitation energies of intermolecular CT states and puts CT states of V-complexes always far away from the absorption maxima of Ag20, leading to smaller absolute RRS intensities in V-complexes than those in S-complexes. TD-Cam-B3LYP slightly overestimates the excitation energies and puts the CT states in right positions, and therefore both V-complexes and S-complexes can possess large RRS intensities. (3) Although the absolute RRS intensities are sensitive to “resonant” CT excited states, the main vibrational structures and relative intensities of RRS are not so sensitive to the CT excited states. The vibrational structures of RRS are dominantly determined by the adsorbates. The lower the molecular symmetry is, the more active modes in SERRS are observed. (4) The HT effects on the CT RRS are significant, which can not only magnify the intensities of observed peaks but also make the inactive modes under the FC approximation become active. With the HT effect getting involved, the simulated RRS spectra agree better with other theoretical and experimental results. (5) The modes with a larger enhancement in CT RRS spectra are those related to ring stretch and ring breathing. For all the four molecule−Ag20 systems, the modes at around 1600 cm−1 are enhanced more significantly than the other modes. In SENRS spectra, the ring-breathing mode around 980 cm−1 can be easily enhanced; the enhancement is ascribed to the vibrational motion of the N atom along the N−Ag bond when the molecule is adsorbed on the Ag20.

4. CONCLUDING REMARKS In this work, we have performed a systematic theoretical study on SERS spectra of pyridine, pyrimidine, 2-mercaptopyridine, and 4-mercaptopyridine absorbed on a Ag20 cluster. The influences of different structural configurations formed between the adsorbates and Ag20 cluster, different intermolecular CT I

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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.jpcc.5b05910. Excitation energies, oscillator strengths, and the difference between Mulliken’s charges of Ag20 and absorbers of low-lying excited states with the obvious CT character for the S- and V-complexes of four molecule−Ag20 systems; the charge distribution of the related MOs; the selected normal modes of organic molecules as well as the simulated normal Raman and CT resonance-like Raman scattering spectra based on the structure parameters produced by (TD-)B3LYP (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone +86-592-2184300 (W.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation of China (Grants 21290193, 21373163, and 21573177) is acknowledged.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b05910 J. Phys. Chem. C XXXX, XXX, XXX−XXX