J. Phys. Chem. C 2009, 113, 105–108
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Outstanding Role of Silver Nanoparticles in the Surface-Enhanced Resonance Raman Scattering of p-Benzosemiquinone Francisco Avila, Juan Soto, Juan F. Arenas, Juan A. Rodrı´guez, Daniel Pela´ez, and Juan C. Otero* Department of Physical Chemistry, UniVersity of Ma´laga, E29071-Ma´laga, Spain ReceiVed: July 28, 2008
The surface-enhanced Raman scattering (SERS) of p-benzoquinone (Q) adsorbed on silver nanoclusters (Agn) reported by Tripathi (J. Am. Chem. Soc. 2003, 125, 1179) is analyzed by means of high-level ab initio calculations. CASPT2 energies of the Ag2-Q model confirm the charge transfer character of the adsorption (Agn + Q f Agn+-Q-•), yielding the semiquinone radical anion (Q-•). On the contrary, we demonstrate that the selective enhancement shown by mode 7a in SERS is due to a resonant process between the 2B2g ground state and the 2Au excited doublet of Q-•. This is an unexpected result given that the electronic spectrum is dominated by the strong dipole-allowed 2B2g f 2B3u transition. Therefore, that spectrum is, to our knowledge, the only reported surfaceenhanced resonance Raman scattering spectrum involving two doublet states of an adsorbate. Introduction This paper contains the evidence we have found supporting the presence of resonant processes in the Raman spectrum of p-benzoquinone (Q) recorded by Tripathi on silver nanoparticles (Figure 1a).1 It is demonstrated that such a spectrum corresponds with a surface-enhanced resonance Raman scattering (SERRS) involving two electronic states of the metal (M)-adsorbate (A) surface complex. Both of them are charge transfer (CT) levels (CT:M+A-) in which the electronic structure of the adsorbate is similar to that of the p-benzosemiquinone radical anion (Q-•) in its ground 2B2g and excited 2Au electronic doublet states. This conclusion is a new step to clarify the controversial origin of the SERS enhancement mechanism and shows the usefulness of this technique to study electronic processes involving states of nanocluster-adsorbate systems.2 Tripathi has reported the observation of very strong Raman bands at 1625 (Wilson mode 8a), 1425 (7a), and 1190 (9a) cm-1 in the surface-enhanced Raman scattering (SERS; 514.5 nm excitation) spectrum of either p-benzoquinone (Q) or quinhydrone (Q, QH2) on a colloidal silver aqueous solution (Figure 1a).1 These bands are not observed in the Raman spectra of either Q or QH2, while they correlate very straightforwardly with the bands at 1620, 1435, and 1161 cm-1 of the ground state of p-benzosemiquinone radical anion (Q-•; 2B2g) recorded in the resonance Raman (RR) spectrum with the excited 2B3u state (441 nm excitation, Figure 2a).3 Therefore, the ground electronic state of the surface complex is a CT state where an electron is transferred from the metal to the adsorbed molecule (CT:M+A-). Experimental facts demonstrate that the observed SERS was due to a reaction product obtained when Q is adsorbed on the nanoparticle (Agn + Q f Agn+-Q-•) rather than to the anion produced in the bulk solution and then adsorbed on the surface (Agn + Q-• f Agn-Q-•). Consequently, the adsorption is a CT process, and the metal stabilizes the radical whose concentration in aqueous solution is only significant under basic conditions but then becomes unstable. * To whom correspondence should be addressed. E-mail: jc_otero@ uma.es.
Figure 1. (a) Raman spectrum of p-benzosemiquinone on silver nanoclusters (514.5 nm excitation) Reprinted from ref 1. Copyright 2003 American Chemical Society. Preresonance Raman intensities calculated for the (b) 2B2g f 1Ag transition involving Q-• and Q, respectively, (c) 2B2g f 2Au transition of Q-•, and (d) 1B1 f 1A2 transition of the Ag2-Q system. The wavenumbers of the calculated spectra have been scaled to the experimental values.
Figure 2. (a) Time-resolved RR spectrum of p-benzosemiquinone (441 nm excitation). Reprinted from ref 3. Copyright 1987 American Chemical Society. (b) Calculated preresonance Raman spectrum for the 2B2g f 2B3u transition of Q-•. The wavenumbers of the calculated spectra have been scaled to the experimental values.
Theoretical Calculations of RR and SERRS Intensities The most striking feature of Figure 1a is the very strong intensity of the band at 1425 cm-1. One of the most important
10.1021/jp8088066 CCC: $40.75 2009 American Chemical Society Published on Web 12/09/2008
106 J. Phys. Chem. C, Vol. 113, No. 1, 2009
Avila et al.
Figure 3. Relative energies (∆E) and internuclear distances (Å) of the CASPT2(10,10)/ANO-optimized structures of the Ag2-Q conformers in the ground electronic state 1B1. Basis set: ANO-RCC for C,O[4s3p2d1f]/H[3s2p] and ANO-DK3 for Ag[5s3p2d].
applications of the RR spectra is the determination of structures in excited electronic states, given that the relative intensities are related to the differences between the equilibrium geometries of the resonant states |m〉 and |n〉 (A-term in RR).4 The preresonance Raman intensity of a band (Ii) can be estimated according to the expression5
Ii ) kγiωi2
(1)
where γi is the displacement parameter which depends on the differences between the geometries of the involved electronic states, ωi is the respective Raman wavenumber, and k can be arbitrarily adjusted to normalize relative intensities. γ is expressed as a function of B, a dimensionless parameter:6
γi ) 1/2Bi2
(2)
Bi ) 0.172ωi1/2[(Xm - Xn)µ1/2Ln]i
(3)
Figure 4. Relative energies of charge transfer (M+-A-, red) and the lowest non charge transfer (M-A, blue) states of the silver-adsorbate surface complex for pyridine, pyrazine, and p-benzoquinone, which are labeled according to the symmetry of the corresponding isolated molecule.
and
where µ is the 3N × 3N diagonal matrix of the atomic masses in atomic units, Ln is the eigenvector of the Hessian matrix associated with the ith normal mode, and (Xm - Xn) is the difference, Å, between the Cartesian coordinates of the resonant, |m〉 and |n〉, electronic states. Equations 1-3 allow for obtaining geometries of excited states from experimental intensities as well as prediction of RR intensities from ab initio calculations. We have previously used this capability to recognize unknown excited states involved in a resonant process, which has been very useful to clarify which enhancement mechanism operates in a particular SERS experiment. Computational Details All of the geometry optimizations of the relevant species involved in this work have been performed in Cartesian coordinates by using generally contracted basis sets of atomic natural orbitals (ANOs) (ANO-RCC for C, O, and H and ANODK3 for silver)7 and including the scalar relativistic effect with the Douglas-Kroll-Hess Hamiltonian.8 The contraction scheme of the basis sets will be specified in each case. Three conformers have been selected for the Ag2Q supermolecule where the adsorbate is located in the zy-plane and the silver cluster is
bonded to an oxygen atom. The Ag2 cluster is located along the x, y, or z axis for conformers I, II, and III, respectively. Conformers I and II are almost isoenergetic and are the most stable ones (Figure 3); therefore, the discussion has been focused on conformer I. To model a rather rigid metallic nanostructure, only the silver-silver internuclear distance has been fixed to the crystal X-ray experimental data9 during the optimization procedure. The geometry optimizations were performed at the complete active space self-consistent field (CAS-SCF)10 level of theory by computation of analytical energy gradients. Several optimizations have been done with the second-order multiconfigurational perturbation theory (CASPT2)11 by computation of numerical energy gradients. Both methods were applied as implemented in the MOLCAS 6 program.12 In the CASPT2 calculations, the 1s electrons of the carbon and oxygen atoms and the 1s, 2s, 2p, 3s, 3p, and 3d electrons of the silver atoms, determined in the SCF calculations, were kept frozen. The stationary points were characterized by their CASSCF analytic harmonic vibrational wavenumbers computed by diagonalizing the mass-weighted Cartesian force constant matrix. Finally, vertical excitation energies have been computed with the multistate extension of the multiconfigurational second-order perturbation theory (MSCASPT2).13
SERRS of p-Benzosemiquinone
J. Phys. Chem. C, Vol. 113, No. 1, 2009 107
TABLE 1: Multistate CASPT2(10,10) Vertical Excitation Energies for the Electronic States of Conformer I of the Ag2-Q System (Basis Sets ANO-RCC for C,O[4s3p2d1f]/H[3s2p] and ANO-DK3 for Ag[5s3p2d]) state
configurationa
wb
qc
energyd
11B1 21B1
(5s+)1(π*CO)a1 (5s+)1(πCO)b1(π*CO)a2 (5s+)1(π*CO)b1 (5s+)1(5s-)1 (5s+)1(πCO)b1(π*CO)a1(5s-)1 (5s+)1(5s-)1 (5s+)1(πCO)b1(π*CO)a1(5s-)1 ref conf (5s+)0(5s-)2 (5s+)0(5s-)1(π*CO)a1 (5s+)0(5s-)2 (5s+)0(5s-)2 (5s+)0(5s-)1(π*CO)a1 (πCO)b0(π*CO)a2 (5s+)1(π*CO)a2(πCC)1 (5s+)1(π*CC)1 (5s+)1(π*CC)1 (5s+)1(π*CO)a2(πCC)1 (5s+)1(π*CO)a1(5s-)1(πCC)1 (5s+)1(πCO)b1(π*CO)a2(πCC)1(5s-)1 (π*CO)a1(πCC)1 (5s-)1(π*CC)1
84 51 16 32 36 43 22 58 19 47 15 37 21 19 55 23 58 23 70 47 54 49
-0.75 -0.21
3.06 (407.2)
0.324 × 10-1
-0.29
4.33 (286.4)
0.796 × 10-1
-0.25
4.64 (267.5)
0.951 × 10-2
+0.09
1.27 (973.2)
0.382 × 10-1
-0.88
1.80 (688.3)
0.374
-0.03
5.53 (224.3)
0.517 × 10-3
+0.01 -0.36
6.00 (207.5) 2.38 (521.5)
0.533 × 10-3 0.624 × 10-2
-0.80
3.10 (399.6)
0.722 × 10-1
-0.27 +0.06 +0.05 -0.74
4.81 (257.8) 7.20 (172.1) 4.36 (284.7) 4.68 (264.7)
0.378 × 10-3 0.101 × 10-3 0 0
31B1 41B1 11A1 21A1 31A1 41A1 11A2 21A2 31A2 41A2 11B2 21B2
f
e
a Reference configuration 11A1: (5s+)2(πCO)a2(πring)2(πCO)b2(π*CO)a0(5s-)0(π*CO)b0(π*ring)0(πCC)2(π*CC)0. b Perturbatively modified CAS-CI weight of the configuration (%). c Charge of the adsorbate at the CASSCF/ANO level. d Energy in electronvolts (nanometers) from the 11B1 ground state optimized at the CASPT2(10,9) level with the basis sets ANO-RCC for C,O[3s2p1d]/H[2s1p] and ANO-DK3 for Ag[5s3p2d]. e Oscillator strength.
Results and Discussion Previous to this work, our group has published several papers with the aim of demonstrating the relevance of CT processes in SERS (SERS-CT).14 In the SERS of molecules such as pyridine or pyrazine, we have detected the formation of the surface complex (M-A) as well as RR processes involving photoinduced transfer of an electron from the metal to the adsorbate in the CT excited state:
M-A + hν f M+-AIn these cases the SERS-CT mechanism gives rise to the formation of the radical anion of neutral adsorbates in the transient excited state (Figure 4). Moreover, we have proposed a methodology to analyze conveniently a particular SERS by comparing the experimental intensities with the theoretically calculated values for such an SERS-CT process.15 These intensities are estimated from the ab initio geometries of the adsorbate in the M-A and M+-Asurface states, i.e., in the ground electronic estates of the neutral species (A) and its usually not well-known anion (A-). The RR spectrum in Figure 2a can be used as a test to check the predictive capabilities of the procedure. That spectrum is characterized for the strong intensity of mode 8a and was obtained under preresonance conditions with the strongly adsorbing 430 nm (2B2g f 12B3u) transition of Q-•. It can be seen that the experimental RR features are very well reproduced with the calculated intensities by using eq 1 (Figure 2b). To apply such an equation, geometries of the involved states have been optimized at the CASSCF(9,8) level in conjunction with basis sets of ANO type with the C,O[4s3p2d1f]/H[3s2p] contraction scheme obtained from the C,O[14s9p4d3f2g]/ H[8s4p3d1f] primitive sets, the so-called ANO-RCC basis sets. A similar agreement is found for the respective spectra under resonance with the 2B2g f 2Au and 2B2g f 22B3u transitions of Q-• (see the Supporting Information).
The remarkable enhancement of mode 7a in the SERS of Figure 1a points to a change in the resonance conditions with respect to the RR spectrum of Figure 2a. That enhancement has been tentatively explained by assuming that the electron goes back to the metal in the excited state:1
M+-A- + hν f M-A From the point of view of the adsorbate, the foregoing mechanism involves the electronic ground states of the anion and the neutral species, just the reverse situation described in the SERS of pyridine and analogues.14 By comparing Figures 1b and 2b, it can be seen that the intensities of the 2B2g f 1Ag (Q-• f Q) transition calculated on the basis of the respective CASSCF geometries are quite similar to those calculated for the 2B2g f 2B3u transition of Q-• and do not account for the enhancement of mode 7a in SERS. Therefore, this mechanism involving the neutral species has to be discarded. To clarify the origin of the SERS intensities, we have also carried out ab initio calculations of the surface M-A complex modeled as a Ag2-Q cluster similar to that used in ref 15 to study the SERS of pyrazine. CASPT2 calculations including the scalar relativistic effect with the Douglas-Kroll-Hess (DHK) Hamiltonian for predicting a CT ground state for the cluster (CT0:11B1:Ag2+-Q-•; Table 1) confirm the conclusion derived by Tripathi concerning the adsorption process. It must be noted that the CASSCF wave function of the ground state of this system including the scalar relativistic effect with the DHK Hamiltonian does not show CT character. Therefore, it is necessary to perform CASPT2 calculations in conjunction with relativistic corrections to get the proper ground state. Table 1 shows the MS-CASPT2 vertical transition energies of the lowest 13 excited singlet states of the Ag2-Q cluster; only S2 (21A1), S5 (21A2), and S9 (21B2) states with vertical energies of 1.80, 3.10, and 4.68 eV, respectively, are CT:M+Astates (CT:Ag2+-Q-•) and therefore have been called CT1, CT2, and CT3, respectively. The transferred charge from the metal
108 J. Phys. Chem. C, Vol. 113, No. 1, 2009 to the adsorbate in each CT0-3 state is 0.75, 0.88, 0.80, and 0.74 electron, respectively. This CT0 [S0, 1B1] ground state is 1.27 eV below the S1 (1A1) state, the lowest one which does not show CT character. On the contrary, the CIS-calculated energy between the minima of the S0 (1A1) and CT0 (1B1) states of the Ag2-pyrazine complex amounts to 3.14 eV,16 which indicates a much lower stability of the radical anion of pyrazine with respect to that of p-benzoquinone. This explains why the SERS wavenumbers correlate with those of the neutral species in the case of pyridine or pyrazine (ground state: S0:M-A) and with those of the radical anion in the case of p-benzoquinone (ground state: CT0: M+-A-), which is very stable as is well-known (Figure 4). All of the calculated RR intensities for transitions from the CT0 level to these non-CT states of the cluster are quite similar to that calculated for the 2B2g f 1Ag transition of the isolated molecule (Figure 1b). On the contrary, the spectrum calculated for the CT0:1B1 f CT2:21A2 transition (Figure 1d) is dominated by the strong intensity of mode 7a as occurs in SERS. The electronic structure of p-benzoquinone in both mentioned CT states is closely related to that of its radical anion in the 2B2g and 2Au levels, becoming B1 and A2 under C2V, respectively. Once again, the calculated intensities for the 2B2g f 2Au transition of Q-• predict that mode 7a should be the strongest one (Figure 1c). Moreover, the MS-CASPT2 energy for the CT0 f CT2 excitation compares nicely with the laser excitation as well as with the calculated one for the S0 f CT0 transition of the Ag2-pyrazine system in which the CT contribution to SERS has been shown.16 This is an unexpected result given that the resonance Raman spectra with the 2B2g f 2B3u and 2B2g f 2Au transitions of Q-• are recorded by excitation at ca. 440 and 350 nm, respectively, whereas the SERRS is recorded by using the far less energetic 514.5 nm laser line. Conclusions In summary, silver nanostructures play an unusual role in the SERS of p-benzosemiquinone and show the following outstanding features: they behave as a pool which provides electrons for CT processes, they form surface complexes stabilizing the radical anion, which is very unstable in aqueous solution, they modify the energies and intensities of particular electronic transitions of the adsorbate, and moreover, as usual in SERS, the Raman signal is enormously enhanced, which allows for the routine detection of spectra coming from minority species adsorbed on the metal. Finally, this work is a very illustrative example of how the electronic states of the surface complex determine both the SERS wavenumbers and intensities and shows again the usefulness of ab initio calculations in
Avila et al. understanding unexpected physical and/or chemical phenomena occurring in nanocluster-adsorbate systems.17 Acknowledgment. We thank the Spanish MEC for financial support of this research (Grants NAN2004-09312-C03-01 and CTQ2006-02330). Supporting Information Available: Geometries, vibrational frequencies, normal modes, and energies of Q, Q-•, and three conformers of the Ag2-Q system in the ground and excited electronic states as well as experimental and calculated resonance Raman intensities for selected electronic transitions. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Tripathi, G. N. R. J. Am. Chem. Soc. 2003, 125, 1179, and reference therein. (2) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. J. Phys. Chem. B 2003, 107, 6668. (3) Tripathi, G. N. R.; Schuler, R. H. J. Phys. Chem. 1987, 91, 5881. (4) Clark, R. J. H.; Dines, T. J. Angew. Chem., Int. Ed. Engl. 1986, 25, 131. (5) (a) Siebrand, W.; Zgierski, M. Z. J. Chem. Phys. 1979, 71, 551. (b) Negri, F.; Orlandi, G.; Langkilde, F. W.; Wildbrandt, R. J. Chem. Phys. 1990, 92, 4907. (6) Orlandi, G.; Zerbetto, F.; Zgierski, M. Z. Chem. ReV. 1991, 91, 867. (7) (a) Widmark, P.-O.; Malmqvist, P.-Å; Roos, B. O. Theor. Chim. Acta 1990, 77, 291. (b) Tsuchiya, T.; Abe, M.; Nakajima, T.; Hirao, K. J. Chem. Phys. 2001, 115, 4463. (8) Ross, O.; Lindth, R.; Malmqvist, P.-A.; Veryazov, V.; Widmark, P.-O. J. Phys. Chem. A 2004, 108, 2851. (9) Handbook of Chemistry and Physics, 83rd ed.; Lide D. R., Ed.; CRC Press: Boca Raton, FL, 2002. (10) Roos, B. O. In Ab Initio Methods in Quantum Chemistry II; Lawley, K. P., Ed.; Advances in Chemical Physics; John Wiley & Sons: Chichester, England, 1987; Chapter 69, p 399. (11) (a) Anderson, K.; Malmqvist, P.-Å; Roos, B. O.; Sadlej, A. J.; Wolinski, K. J. Phys. Chem. 1990, 94, 5483. (b) Anderson, K.; Malmqvist, P.-Å.; Roos, B. O. J. Chem. Phys. 1992, 96, 1218. (12) Anderson, K.; et al. MOLCAS 6; Department of Theoretical Chemistry, University of Lund: Lund, Sweden. (13) Finley, J.; Malmqvist, P.-Å.; Roos, B. O.; Serrano-Andre´s, L. Chem. Phys. Lett. 1998, 288, 299. (14) Arenas, J. F.; Soto, J.; Lopez-Tocon, I.; Otero, J. C.; Marcos, J. I. J. Chem. Phys. 2000, 112, 7669. (15) Centeno, S.; Lopez-Tocon, I.; Arenas, J. F.; Soto, J.; Otero, J. C. J. Phys. Chem. B 2006, 110, 14916. (16) Arenas, J. F.; Soto, J.; Lopez-Tocon, I.; Fernandez, D. J.; Otero, J. C.; Marcos, J. I. J. Chem. Phys. 2002, 116, 7207. (17) Jensen, L.; Aikens, C. M.; Schatz, G. C. Chem. Soc. ReV. 2008, 37, 1061.
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