Release of Cyanopyridine from a Ruthenium Complex Adsorbed on

Article pubs.acs.org/JPCC

Release of Cyanopyridine from a Ruthenium Complex Adsorbed on Gold: Surface-Enhanced Raman Scattering, Electrochemistry, and Density Functional Theory Analyses Dieric dos S. Abreu,† Tércio de F. Paulo,†,‡ Rômulo A. Ando,‡ Márcia L. A. Temperini,‡ Elisete A. Batista,∥ Elisane Longhinotti,§ and Izaura C. N. Diógenes*,† †

Departamento de Química Orgânica e Inorgânica, Universidade Federal do Ceará, Cx. Postal 6021, Fortaleza, Ceará, Brasil 60455-970 ‡ Instituto de Química, Universidade de São Paulo, Cx. Postal 26077, São Paulo-SP, Brasil 05508-000 § Departamento de Química Analítica e Físico-Química, Universidade Federal do Ceará, Cx. Postal 12200, Fortaleza, Ceará, Brasil 60455-960 ∥ Departamento de Físico-Química, Instituto de Química, Universidade Estadual Paulista, Cx. Postal 355, Araraquara, São Paulo, Brasil 14800-060 S Supporting Information *

ABSTRACT: The results presented in this work definitely show that the stability of the SAM formed with [Ru(NH3)4(CNpy)(pyS)]2+ on gold, where CNpy = 4-cyanopyridine and pyS = 4-mercaptopyridine, is dependent on the applied potential and on the chemical properties of the solution in the solid/liquid interface. By means of SERS spectroscopy, it was found that CNpy ligand is released from the coordination sphere if no reducing condition is imposed to the system, i.e., citrate solution or applied potential lower than the formal potential of the complex. Theoretical Raman spectra obtained from DFT presented reasonable correlation with the experimental spectra and gave support for the assignments. The relative intensities of the bands in the SERS spectra showed to be dependent on the applied potential as well as on the wavelength of the exciting radiation, indicating the contribution of a charge transfer process to the SERS intensification. In fact, the shift of the potential of maximum SERS intensity (Emax) to negative values as the radiation energy increases indicates a charge transfer process from the HOMO orbitals of the complex to the Fermi level.

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strengthening the bonding with the surface.9−15 Coordination compounds have been used as SAMs on gold to study electron transfer of metalloproteins, coordination reactions on surface, conduction as molecular wires, and so on.6−19 The SAM formed with [Ru(NH3)4(CNpy)(pyS)]2+ on gold, where CNpy = 4-cyanopyridine and pyS = 4-mercaptopyridine, was first reported by our group in 2006 and was used to assess the heterogeneous electron transfer of the metalloprotein cytochrome c (Cyt c).16 For this SAM, SERS (surface-enhanced Raman scattering) spectra, which were obtained in air and without applied potential, showed, as expected, that the molecule adsorbs on gold through the sulfur atom of the pyS moiety. By accounting that the ammonia ligands, which are located in the equatorial plane, do not possess orbitals of π symmetry (at least not at appropriate energy), all the back-

INTRODUCTION

It is widely accepted that self-assembling of sulfur-containing molecules on gold usually results in ordered arrays of monolayer dimensions (∼10−11 mol cm−2). Because of the typical thickness (1−3 nm) of these so-called self-assembled monolayers (SAMs), they are considered as the most elementary form in the area of thin film materials (nanoscale). In the field of molecular electronics, where the charge transfer is a crucial process and has to be controlled, the insertion of redox compounds in SAMs formed on metallic surfaces represents a great advantage as it can serve as model for studying the fundamentals of electron transfer since the distance to the underlying gold surface, for instance, is known.1−8 The insertion of redox moieties in SAMs can be achieved by the formation of coordination compounds on surface which, in comparison to the organic counterpart, are likely to form more robust molecular assemblies due to the metal-to-ligand back-donation. This interaction enhances the electron density on the adsorption fragment, thereby © 2014 American Chemical Society

Received: September 11, 2014 Revised: November 11, 2014 Published: November 12, 2014 27925

dx.doi.org/10.1021/jp5092152 | J. Phys. Chem. C 2014, 118, 27925−27932

The Journal of Physical Chemistry C

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water. The activation of the SERS gold substrates was achieved by following the procedure of oxidation−reduction cycles (ORC) in 0.1 mol L−1 KCl in the potential range from −0.3 to +1.3 V vs Ag/AgCl at a scan rate of 0.1 V s−1 as can be found elsewhere.24 This procedure as well as the application of the potential for the acquisition of SERS with applied potential (EC-SERS) were obtained by using an Autolab PGSTAT 101 (Metrohm Autolab) potentiostat. The modifications of the gold surfaces for the acquisition of the SERS spectra were performed by following three protocols: (i) as in ref 16: immersion for 30 min of the nonpolarized electrode in an aqueous solution containing the complex (2.0 × 10−3 mol L−1) that was previously synthesized; (ii) immersion for 10 min of the electrode polarized at −0.4 V in 0.1 mol L−1 KCl solution containing the complex (1.0 × 10−3 mol L−1); (iii) immersion for 10 min of the electrode polarized at −0.4 V in 0.1 mol L−1 KCl solution containing the complex (1.0 × 10−3 mol L−1) and citrate. For protocol i, the modification step was followed by washing with water and drying under N2 flow, and the SERS spectra were acquired in air and without applied potential. SERS spectra were acquired on a Renishaw Raman InVia equipped with a CCD detector and coupled to an Olympus microscope (BTH2). The excitation radiations (λ0) were the 632.8 nm (He−Ne laser, Renishaw RL633) and 785 nm lines (diode laser). The laser beam was focused on the sample by a ×50 long distance objective (NA = 55) or by a ×63 water immersion objective (NA = 0.99). Chemicals. Aqueous solutions were prepared using Millipore water of at least 18 MΩ cm−1 resistance. KCl (0.1 mol L−1) was used as supporting electrolyte. 4-Mercaptopyridine (pyS), 4-cyanopyridine (CNpy), HCl, KOH, and KCl were purchased from Aldrich and used as received. The pH value was adjusted by the addition of HCl and KOH. Citrate-stabilized gold nanoparticles (AuNPs) of 45 ± 5 nm in diameter as determined by scanning electron microscopy (not shown) were synthesized according to Frens’s method25 with a few modifications proposed by Rodrigues et al.26 The isolated complex was added to the suspension of AuNPs in 1% w/w sodium citrate aqueous solution. The [Ru(NH3)4(CNpy)(pyS)](PF6)2 complex was synthesized according to the literature.16,27,28 Vibrational and electronic spectroscopic data as well as electrochemical were coincident with those reported,13,16 indicating the isolation of the desired compound. λ(MLCT, NaCH3COO, pH 3.0): 382 and 510 nm; E1/2 (V vs Ag/AgCl, KCl): 0.66 V; IR (KBr): ν(CN) 2201 cm−1, ν(CC + CN) 1594 and 1624 cm−1, ν(CS) 1110 cm−1. Computational Details. DFT calculations were performed using GAUSSIAN 03 (Gaussian Inc., Wallingford, CT) software.29−31 The ground state geometries were fully optimized by DFT with the B3LYP hybrid functional (Becke’s gradient corrected exchange correlation in conjunction with the Lee−Yang−Parr correlation functional with three parameters).32−34 The calculations of the molecules attached to gold were performed using the gen keyword, where the atoms (C, H, N, O, and S) were calculated with the 6-311++G(d, p) basis set and the ruthenium and gold atoms with the LANL2DZ basis set considering a pseudopotential. The vibrational frequency analyses were carried out under vacuum conditions where no imaginary frequencies were found, indicating that the optimized geometries were in a minimum of the potential energy surface. The simulated Raman spectra were plotted

bonding interaction is expected to occur along the axial axis with the electron delocalization being affected by the π acid strength of the ligands located on the z-axis. For the [Ru(NH3)4(CNpy)(pyS)]2+ complex in solution, the results of electrochemistry and electronic spectroscopy suggest similar π acid strength for the CNpy and pyS ligands. More recently,13 we published a paper on the in situ formation of the SAM of [Ru(NH3)4(CNpy)(pyS)]2+ on gold. Based on electrochemical, SERS, and SPR (surface plasmon resonance) results, it was possible to follow the formation of this SAM since the selfassembling of the sulfate starting complex [Ru(SO 4)(NH3)4(pyS)]+ that, upon reduction of the metal center, undergoes the substitution reaction of (SO4)2− by H2O which, in turn, is replaced by CNpy. The published data indicated an increase in the π-back-bonding interaction upon adsorption due to the suggestion of the existence of gold adatoms. This result is in accordance with the now widely accepted idea that the interaction between gold- and sulfur-containing molecules involves a gold−thiolate covalent bond.20−22 In the time scale of the experiments conducted in such study, no instability of the monolayer was observed, i.e., no deterioration of the SERS bands typically assigned to the complex nor the decrease of the faradaic current attributed to the adsorbed compound. Surprisingly, however, the SAM formed with the [Ru(NH3)4(CNpy)(pyS)]2+ complex previously synthesized in homogeneous medium showed to be not stable. The aim of this paper is to try to understand why different approaches on the formation of the SAMs of the same compound imply different stabilities upon surface modification. To reach this purpose, we will correlate computational calculations with electrochemical and SERS data.

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EXPERIMENTAL SECTION Apparatus. Electronic spectra in the ultraviolet and visible (UV−vis) regions were acquired with a UV−vis Hewllet Packard, photodiode-array, model 8453 spectrophotometer. The infrared spectra of the compounds dispersed in KBr were obtained by using a ABB Bomen FTLA 2000-102 spectrometer. Reflection−absorption Fourier transform infrared spectroscopy (FTIRAS) was performed in a Thermo Scientific Nicolet 6700 FTIR spectrometer equipped with an MCT detector cooled with liquid nitrogen. For the spectroelectrochemical measurements, a thin layer cell compartment over CaF2 crystal with a policrystalline gold electrode (Ageom = 0.8 cm2) was used as described elsewhere.23 Gold ring and Ag/AgCl (KCl 3.0 mol L −1 ) were used as counter and reference electrodes, respectively. A reference spectrum was collected at −0.4 V. Following, potential steps of 0.2 V were applied, and a spectrum was collected at each potential (from −0.3 to +0.6 V). Each registered spectrum was an average of 100 interferograms, and the resolution was 4 cm−1. Electrochemical measurements were performed with a conventional glass cell with Au and Pt as working and auxiliary electrodes, respectively, using an EPSLON potentiostat (Bioanalytical Systems Inc., BAS, West Lafayette, IN) at 25 ± 0.2 °C. All electrochemical data, unless otherwise specified, are cited against the Ag/AgCl electrode (3.5 mol L−1 KCl, BAS). The supporting electrolyte was purged with nitrogen or argon for 20 min prior to experiments, and an inert atmosphere was maintained during all the experiments. The polycrystalline gold electrodes used for SERS acquisition were polished with 600 mesh and subsequently with 1200 and 2000 mesh sandpaper and thoroughly rinsed with deionized 27926



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1500 to 1700 cm−1, where the bending mode of water and C C and CN bonding can be observed,37 the spectra are quite similar except for the band assigned38,39 to the ν(CN) mode of CNpy at 2201 cm−1. This mode is observed only at applied potentials more positive than 0.3 V, indicating the disappearance of this band as the potential is scanned in the positive direction where, according to the cyclic voltammogram illustrated in Figure 1B, the ruthenium metal center is oxidized to Ru3+. As it is well-known,40 in the 3+ oxidation state, the πback-bonding interaction from ruthenium is no longer operative and, in consequence, the bond strength between the CNpy ligand and ruthenium is decreased, making possible the release of this ligand and thus explaining the disappearance of the ν(CN) mode. SAM of [Ru(NH3)4(CNpy)(pyS)]2+ on Gold. The first SERS study of the SAM formed with [Ru(NH3)4(CNpy)(pyS)]2+ on gold was performed in air and without applied potential.16 In such work, the gold surface was modified by immersion in an aqueous solution containing the complex which had been previously synthesized in homogeneous medium. As already commented and as expected for sulfurcontaining molecules,9,13−16,20,21,41−43 the complex adsorbs on gold through the sulfur atom of the pyS moiety. In that study, the absence of the ν(CN) mode of CNpy was assigned to the fact that this fragment was far from the surface; i.e., the SERS intensity depends, among other factors, on the distance to the surface due to the increase in the molecular polarizability near the surface.44−46 In addition, in the experimental conditions and in the time scale of the experiments conducted in such study, no instability of the monolayer was observed, i.e., no deterioration of the SERS bands typically assigned13,16 to the complex nor the decrease of the faradaic current attributed to the adsorbed compound. This stability was also observed13 for the SAM of the complex produced on surface through the coordination of CNpy to the SAM of [Ru(NH3)4(OH2)(pyS)]2+ if reducing condition is applied, i.e., in citrate containing solution or at applied potentials more negative than the formal potential of the complex (Eap < 0.6 V vs Ag/ AgCl). Moreover, in such reducing conditions, the ν(CN) mode of CNpy coordinated to ruthenium is clearly seen in the SERS spectra. Experimental data obtained more recently, however, showed that the stability of the SAM formed with the complex previously synthesized in homogeneous medium is dependent on the oxidation state of the metal center. We present here a reinvestigation by using Raman, SERS, and DFT in order to understand the apparently inconsistence of the data on the SAM of [Ru(NH3)4(CNpy)(pyS)]2+ on gold. Figure 2 presents theoretical and experimental Raman and SERS spectra for this complex. The spectra illustrated in Figure 2 should be analyzed by parts. Beginning from those obtained for the nonadsorbed complex (Figure 2a,c), one can see the similarity between the theoretical and experimental spectra. The bands associated37,47 with the CC and CN stretching modes, ν(CN) + ν(CC), and the CH bending (β(CH)) modes of the pyridine rings are observed respectively at 1598 and 1195 cm−1 while that assigned37,42,47 to the ring breathing mode is observed at ca. 1000 cm−1. A detailed assignment of the bands observed in the SERS spectra of [Ru(NH3)4(CNpy)(pyS)]2+ can be found elsewhere.13,16 The ring breathing mode with contribution of the CS stretching mode of pyS,42,43,48 called X-sensitive band, is observed as a very weak band at ca. 1100 cm−1 in the

considering the calculated Raman activities using a bandwidth of 5 cm−1 and a scaling factor of 0.9679 for the calculated harmonic vibrational wavenumbers.35 The vertical excitation energies were determined by the time-dependent density functional theory protocol (TDDFT) using the B3LYP functional and mixed basis sets mentioned above. Particularly, in the case of the vertical excitation energies, the polarizable continuum model (PCM) was used in order to take into account the solvent effect, where it was considered the dielectric constant of water.36

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RESULTS AND DISCUSSION Before starting the discussion on the stability of [Ru(NH3)4(CNpy)(pyS)]2+ adsorbed on gold, it is worth commenting on the stability of the complex in aqueous solution toward the substitution reaction of CNpy. The aquation reaction was studied in conditions of pseudo-firstorder reaction in the presence of an excess of dimethyl sulfoxide (DMSO) as auxiliary ligand. Even after 3 h of reaction, no spectral change in the electronic absorption spectra in the ultraviolet and visible (UV−vis) regions was observed, indicating that the compound is kinetically inert in the time scale of the experiments performed in this work. The influence of power density and exposure time of laser radiation (632.8 nm, He−Ne) on the substitution reaction was also evaluated. By using the same experimental condition of the UV−vis study, the solution was exposed for 1 h to the laser irradiation with output powers of 0.5 and 5.3 mW. As observed for UV−vis spectra, no spectral change was observed in the Raman spectra (Figure S1 of the Supporting Information), indicating the stability of the complex in such condition. On the other hand, meaningful changes were observed in the FTIRAS (reflection− absorption Fourier transform spectroscopy) spectra of the complex obtained with applied potential, as illustrated in Figure 1. The FTIRAS spectra in solution were acquired by using the spectrum obtained at −0.4 V as reference; i.e., the bands of the spectrum at −0.4 V were subtracted as background from all the spectra shown above. Not taking into account the region from

Figure 1. (A) IR in the solid state (a) and FTIRAS spectra in 0.1 mol L−1 KCl at different applied potentials (b−h) of [Ru(NH3)4(CNpy)(pyS)]2+. The reference spectrum was acquired at −0.4 V. (B) cyclic voltammogram of a glassy carbon electrode at 0.1 V s−1 in 0.1 mol L−1 KCl containing [Ru(NH3)4(CNpy)(pyS)]2+. 27927



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Figure 2. DFT Raman spectra for [Ru(NH3)4(CNpy)(pyS)](PF6)2 (a) nonbonded and (b) bonded to one gold atom, (c) normal Raman spectrum of [Ru(NH3)4(CNpy)(pyS)](PF6)2 in solid state, SERS spectra of the SAM formed with [Ru(NH3)4(CNpy)(pyS)]2+ on gold (d) without applied potential (protocol i), (e) at −0.4 V (protocol ii), and (f) in solution containing citrate and without applied potential. Red and black lines are for AuNPs and Au solid electrode, respectively. λ0 = 632.8 nm. The calculated Raman spectra are plotted in function of Raman activities.

normal Raman spectrum (Figure 2c). The band at 2201 cm−1 (at 2185 cm−1 in the theoretical spectrum) is assigned38,39 to the CN stretching mode, ν(CN), of CNpy. Comparing the SERS spectrum illustrated in Figure 2d with the normal Raman (Figure 2c) and the theoretical spectrum for the adsorbed complex (Figure 2b), two aspects should be addressed. First, the inversion in the relative intensities of the bands in relation to the X-sensitive band, which is observed as the most intense band at 1097 cm−1 in Figure 2d. This observation is frequently observed for the adsorption of complexes that contain pyS as the adsorption moiety and indicates that the complex is adsorbed through the sulfur atom of this ligand.13−16,41 Second, the band assigned to the ν(C N) mode of CNpy, which is seen at 2201 and 2181 cm−1 in Figures 2c and 2b, respectively, is not observed in the SERS spectrum obtained without applied potential (Figure 2d), suggesting that the CNpy fragment is no longer coordinated to the ruthenium metal center. However, when the SERS spectrum is acquired under polarization (−0.4 V, Eap < E1/2), the ν(CN) mode of CNpy is clearly seen at 2201 cm−1 (Figure 2e), meaning that this ligand is indeed part of the coordination sphere of the complex in such condition. In addition, this spectrum (Figure 2e), in comparison to Figure 2d, presents, again, an inversion in the relative intensities with the bands assigned to the pyridine ring being more intense than that assigned to the X-sensitive mode. This behavior has been assigned41,49 to the increase in the aromaticity of the pyridine ring in consequence of the electron delocalization from both the electrode and the reduced ruthenium atom through π-backbonding interaction. Finally, the SERS spectra illustrated in Figure 2f were obtained for modified gold nanoparticles (AuNPs) and solid electrode without applied potential but in KCl solution containing citrate, a reducing medium. In such

condition, the ν(CN) mode of CNpy is seen as well as the decrease of the intensity of the X-sensitive mode. This result indicates that the oxidation state of the ruthenium metal atom affects the electron density on the CN fragment of CNpy and, as consequence, the intensity of this mode. This conclusion is reinforced by the spectrum obtained for the modified AuNPs that shows that the reducing medium more than the distance from the surface affects the intensity of the ν(CN) mode of CNpy. The spectra illustrated in Figure 3 corroborate with this suggestion. All the spectra obtained at open circuit potential (ca. 0.08 V vs Ag/AgCl) present intensity decrease in comparison to those obtained at −0.4 V, regardless of citrate addition. This observation means that, for this complex, the oxidation state of the metal center plays a key role in the stability of the compound upon adsorption. In addition, it is observed a decrease in the intensity of the band assigned to the ν(CN) mode of CNpy in the set of spectra obtained without citrate in solution (Figure 3A). On the other hand, in solution containing citrate (Figure 3B), the intensity of this band is enhanced while that of the X-sensitive band at 1097 cm−1 decreases even for the spectrum at −0.4 V. It is well-known41,49 that the intensity of this mode is related to the bonding localization in the pyridine ring of the pyS moiety. The more localized the bonds in the pyridine ring, the less intense are the bands at ca. 1200 and 1590 cm−1. Accordingly, the results presented in the spectra shown in Figure 3 indicate that the gold atoms of the surface are inducing the oxidation of the ruthenium metal atom of the adsorbed complex. The gold atoms, which are likely to be oxidized, being attached to the sulfur atom of the pyS ligand enhance the withdrawing capability of this species, thus delocalizing the electron density toward the surface. As a consequence, it is observed the decrease of the aromaticity of 27928



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Figure 4. SERS spectra of the SAM formed with [Ru(NH3)4(CNpy)(pyS)]2+ on gold in 0.1 mol L−1 KCl at different applied potentials. Curves d, g, i, and k were obtained at −0.4 V just after the acquisition of the spectra at 0.0, 0.4, 0.6, and 0.8 V as indicated in the plot of potential vs acquisition sequence. The other spectra were obtained as indicated in the figure. λ0 = 632.8 nm. Figure 3. SERS spectra of the gold electrode modified with [Ru(NH3)4(CNpy)(pyS)]2+ at open circuit (black lines) and at −0.4 V (blue and red lines) in solutions without (A) and (C) and with (B) citrate. λ0 = 632.8 nm.

0.0 V to negative potentials while the intensity of that at 1192 cm−1 decreases at potentials more positive than 0.0 V at the expense of the gradual intensification of the bands at 1097 and 1612 cm−1, indicating the bonding localization. At λ0 = 785 nm, this behavior is also observed but at potentials more positive than 0.3 V, suggesting that the oxidation of the ruthenium metal atom is facilitated at the radiation line of higher energy. In addition, the observed dependence of the spectral profile on the wavelength of the exciting radiation indicates the existence of charge transfer between the adsorbed complex and the metal conduction band.44,46,50−56 Aiming to further investigate this process, potential profiles for different exciting radiations were plotted since it is well-known that the relation of the potential of maximum SERS intensity (Emax) with the energy of the exciting radiation allows the evaluation of the direction of the charge transfer mechanisms.44,46,50−56 Accordingly, a positive slope of Emax vs energy of the exciting radiation indicates a charge transfer transition from the Fermi level to the LUMO orbitals of the adsorbed molecule while a negative slope indicates a process from the HOMO orbitals to the Fermi level.52,56 The potential profiles of the studied complex (Figure S2 of the Supporting Information) shows an increase in the SERS intensity as the potential becomes negative. Although speculative since we could not apply potentials more positive than +0.8 V nor more negative than −0.4 V due to the stability of the SAM, the potential profiles of the adsorbed complex showed values of Emax at −0.58 and −0.49 V for 632.8 and 785 nm exciting radiations, respectively. Therefore, the shift of the Emax to negative values as the radiation energy increases indicates a charge transfer process from the HOMO orbitals of the complex to the Fermi level of the electrode. DFT Comments. Figure 5 shows the contour plots of the frontier orbitals as obtained from DFT calculations for the complex nonbonded and bonded to gold (as to mimic the adsorption situation).20,22,57,58 For the latter situation, the calculations were performed by considering just one gold atom in the +1 oxidation state by following recent works in which simulations of the adsorption of sulfur-containing molecules on gold were performed.20,22,58

the pyridine ring due to the decrease in the π-back-bonding capability of ruthenium due to its partial oxidation. On the other hand, the reducing effect of citrate favors the reduced state of ruthenium (RuII), making operative the π-back-bonding interaction. In addition, as illustrated in Figure 3C, the behavior of the complex on surface under the effect of the applied potential is not reversible if there is no citrate in solution. For this figure, the spectra were acquired in the following sequence: (i) −0.4 V (blue line); (ii) open circuit (black line), and (iii) −0.4 V (red line). The third spectrum (red line) is clearly different from the first one (black line) obtained at the same applied potential particularly concerning the bands at 2195 and 1192 cm−1 that are assigned respectively to the ν(CN) mode of CNpy and β(CH) mode of the cyanopyridine ring. This observation indicates that, after the open circuit step in which the ruthenium atom is likely to be oxidized, the CNpy moiety is released from the complex. This conclusion is in agreement with that suggested by the FTIRAS spectra obtained at different applied potentials (see Figure 1 and discussion). Another possibility for the oxidation of the complex on surface is the energy of the exciting radiation during the acquisition of the SERS spectra. Aiming to investigate this hypothesis, SERS spectra were acquired at different applied potentials and exciting radiations. Figure 4 shows the results obtained for the exciting radiation at 632.8 nm. The colored spectra were obtained at −0.4 V as indicated in the plot of potential vs acquisition sequence to check the reversibility of the spectral profile upon the application of positive potentials. Because of the sensitivity of the bands at 2195 and 1192 cm−1 to the oxidation state of ruthenium, these modes were used as marker bands for the oxidation state of the metal atom since, upon oxidation, they disappear or have their intensities strongly diminished. At λ0 = 632.8 nm, the line of higher energy used in this work, the band at 2195 cm−1 is only observed from 27929



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Assuming that the MLCT transition is indeed operative for the complex on surface as theoretically predicted (blue curve in Figure 6C), we believe that it is reasonable to consider the involvement of the Fermi level in the charge-transfer mechanism of SERS intensification, as indicated in the previous discussion on the potential profiles. This assumption, in turn, supports the partial oxidation of the ruthenium atom upon adsorption since the transition from the HOMO level (mainly ruthenium) to the Fermi level occurs at lower energies in comparison to the HOMO−LUMO transition. Accordingly, after adsorption, this process facilitates the release of the CNpy ligand due to the weakening of the Ru-CNpy bonding, thus explaining the instability of the adsorbed compound in relation to the complex nonbonded to gold in solution. In fact, the electron density delocalization toward the gold atom, illustrated in Figure 5, is reinforced by the percentage contributions shown in Figure 6 where the contribution of the ruthenium atom to the HOMO level decreases from 68% to 45% after adsorption. A similar trend is observed for the contribution of pyS moiety to the HOMO level. For the pyS fragment, the increase in the percentage contribution to the HOMO level upon adsorption explains the inversion in the relative intensities observed when the SERS spectra are compared to the normal spectrum, i.e., the relative intensification of the X-sensitive mode in relation to the pyridine ring modes, particularly without applied potential (see Figure 2d).

Figure 5. Contour plots of the frontier molecular orbitals (HOMOs and LUMOs) as obtained from DFT calculations considering the solvent effects for the [Ru(NH3)4(CNpy)(pyS)]2+ complex nonbonded (A) and bonded to a gold atom (B).

According to the DFT calculations considering the solvent effects (water molecules), the LUMO orbital of the complex, chiefly located on CNpy moiety (Figures 5 and 6, (A) and (B)), is not meaningfully affected by the interaction with gold. On the other hand, the HOMO orbital of the complex, which is mainly located on ruthenium and pyS moieties (Figures 5 and 6, (A) and (B)), is destabilized going from −6.20 eV for the nonadsorbed complex to −6.04 eV upon adsorption. Accordingly, the energy HOMO−LUMO gaps of the nonbonded and bonded complex to gold respectively, are 2.38 and 2.26 eV. The value calculated for the nonbonded complex (2.38 eV, 519 nm), therefore, is very near the metal-to-ligand chargetransfer (MLCT) transition from ruthenium to CNpy (Ru(dπ) → CNpy(pπ*), λmax = 510 nm ≅ 2.4 eV), as can be seen in Figure 6C.

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CONCLUSIONS The results presented in this work definitely show that the stability of the SAM formed with [Ru(NH3)4(CNpy)(pyS)]2+ on gold is dependent on the applied potential and on the chemical properties of the solution in the solid/liquid interface. On the contrary of what is observed for the complex in solution, where it is quite stable, the CNpy ligand is released from the coordination sphere upon adsorption if no reducing condition is applied to the system. The gold surface atoms in

Figure 6. Percentage contributions of Ru, CNpy, pyS, NH3, and Au fragments to HOMO and LUMO frontier molecular orbitals of [Ru(NH3)4(CNpy)(pyS)]2+ nonbonded (A) and bonded (B) to gold atom. (C) Overlay of experimental (black curve) and theoretical TD-DFT UV−vis spectra for [Ru(NH3)4(CNpy)(pyS)]2+ nonbonded (red curve) and bonded to gold (blue curve) in water. An water polarizable continuum model (PCM) was employed using the B3LYP functional and 6-311++G(d, p) (C,H,N,S) and LANL2DZ (Ru and Au) basis sets. Computed oscillator strengths have been rescaled so that the intensity of the absorption band of the experimental and calculated spectra match. 27930



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1+ oxidation state, being attached to the sulfur atom of the pyS, enhance the withdrawing capability of this species thus delocalizing the electron density toward the surface. This delocalization, in turn, induces the oxidation of the ruthenium metal atom of the adsorbed complex weakening the Ru-CNpy bonding and making possible the release of the CNpy ligand from the coordination sphere. On the other hand, at negative applied potentials (more negative than the E1/2 of the complex) or in citrate containing solution, the reduced state of ruthenium (RuII) is favored making operative the π-back-bonding interaction and keeping all the ligands in the coordination sphere of the metal. The intensity of the bands in the SERS spectra showed to be dependent on the applied potential as well as on the wavelength of the exciting radiation indicating the contribution of a charge transfer process to the SERS intensification. It was observed that the oxidation of the complex is facilitated at the higher energy line (632.8 nm vs 785 nm) indicating that the Fermi level is involved in the transition. Indeed, the shift of the Emax to negative values as the radiation energy increases indicates a charge transfer process from the HOMO orbitals of the complex to the Fermi Level.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +55(85)33669166 (I.C.N.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.F.P. is thankful for the grant from FAPESP. M.L.A.T., R.A.A., and I.C.N.D. are thankful to CNPq for the grants and FAPESP, FUNCAP (PRONEM PRN-0040- 00065.01.00/10 SPU No. 10582696-0), and CAPES for the financial support.

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REFERENCES

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Release of Cyanopyridine from a Ruthenium Complex Adsorbed on

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