Molecular Conformation in Organic Films from Quantum Chemistry ab

Article pubs.acs.org/JPCC

Molecular Conformation in Organic Films from Quantum Chemistry ab Initio Calculations and Second Harmonic Spectroscopy Roberto Macovez,*,†,‡ Nuria Lopez,§ Marina Mariano,† Marc Maymò,† and Jordi Martorell†,‡ †

ICFOInstitut de Ciencies Fotoniques, Avinguda Canal Olímpic, 08860 Castelldefels (Barcelona), Spain Departament de Fisica i Enginyeria Nuclear, Universitat Politecnica de Catalunya, 08034 Barcelona, Spain § Institute of Chemical Research of Catalonia, ICIQ, Avinguda Paisos Catalans 16, 43007 Tarragona, Spain ‡

ABSTRACT: Identifying molecular species and conformations at surfaces is of interest for both catalytic and biointerfacial research. Here, we present an extinction and second harmonic spectroscopy study of films of crystal violet together with quantum chemistry ab initio calculations on several conformation geometries of this dye. By comparing experimental and theoretical data, we identify the molecular species responsible for the spectral resonances, and we ascribe the nonlinear optical generation to pyramidal conformers with nonzero dipolar hyperpolarizability. Our results, which are consistent with recent findings on alcohol solutions of the same dye, help to settle a long-standing debate and demonstrate that nonlinear second harmonic spectroscopy is a valuable tool for identifying and probing molecular conformation in interfacial systems.

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INTRODUCTION Nonlinear optical techniques are valuable sensing and microscopy tools in chemistry and biology. Second harmonic microscopy, for instance, has been demonstrated to be extremely useful for high-resolution marker-free biomicroscopy.1 Second harmonic spectroscopy, on the other hand, has been rarely applied to the characterization of molecular specimens (see ref 2 for one of the few examples), despite its potential to probe electronic levels at buried interfaces.3−7 We show here that this technique can yield valuable information on molecular conformation at surfaces, presenting as a case study the characterization of a well-known chromophore, the cationic crystal violet dye, also known as methyl violet 10B. Commercially available as a chloride salt, crystal violet (CV) is used as a general biological stain, fluorescent marker, and acid−base indicator, as well as in the demonstration and primary classification of bacteria and even as an anti-infection or antifungal agent.8 Thanks to its optical properties and its relatively high hyperpolarizability,9 it is also one of the most commonly employed dyes in molecular optics.10−12 Despite its wide range of light-related applications the linear and nonlinear optical properties of crystal violet are not yet fully understood and have been the subject of controversy. Due to its symmetric shape, the isolated CV dye should possess a zero ground-state dipole moment. The most stable isomeric form of the cation has a propeller-like shape with the three phenyl rings tilted at the same angle with respect to the plane defined by the four centermost carbon atoms. The resulting D3 point-group symmetry leads to a mainly octupolar character of the quadratic hyperpolarizability with no dipolar character.13 The push−pull polar conjugated molecules which were for a long time the main paradigm for quadratic nonlinear optics have instead mainly dipolar hyperpolarizability and were © 2012 American Chemical Society

initially expected to display higher nonlinear activity than octupolar molecules. This view was challenged by the observation of efficient nonlinear optical conversion by CV and other octupolar species,14 and by a number of recent studies which have pointed out that the hyperpolarizability of octupolar systems can be comparable to that of dipolar ones.9,15 The case of CV is however not yet settled, as a recent hyperRayleigh scattering study in solution reports16 that the symmetry of the hyperpolarizability tensor β is lower than expected for a D3 structure. Moreover, gas-phase calculations predict a nonzero dipole moment of the CV−chloride charge transfer complex,17 which indicates that the basic properties of the dye can be modified by interaction with counterions. The linear extinction spectrum (UV−vis) of CV (and of triphenylmethane dyes in general) has similarly been the subject of a 60-year-long controversy.18 CV displays two main absorption bands in both solution and condensed phases. Since the photodissociation spectrum of CV in the gas phase exhibits only one peak at the position of the lower-wavelength absorption band,19 the presence of two absorption bands in condensed phases is due to local interactions. A study on liquid solutions of CV has found20 that the conformation and coordination of the dye depend strongly on solute−solute and solute−solvent interactions: solvents with low dielectric constants (ε < 10) favor the formation of ion pairs of CV cations with chlorine counterions; in alcohols with 10 < ε < 40 (ethanol, methanol, acetone, 1-propanol), solvent−solute interactions dominate; in water, for which ε = 80, aggregation Received: July 29, 2012 Revised: October 19, 2012 Published: November 30, 2012 26784

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formation of drops that would lead to optically inhomogeneous films, and one side of the slide was carefully wiped with photographic paper to minimize second harmonic generation interferences between the two sides of the same substrate.33 UV−vis spectra were acquired with a commercial spectrophotometer. Second harmonic generation experiments were performed using an optical parametric oscillator (OPO) pumped by a Q-switched Nd:YAG laser providing pulses of 4.2 ns duration at 10 Hz repetition rate. The idler infrared beam from the OPO, whose wavelength was tuned between 750 and 1250 nm during the experiments, was filtered to get rid of residual UV and visible radiation and passed through a Fresnel rhomb to achieve a p-polarized pump beam in order to enhance the surface nonlinear signal. All nonlinear optical experiments were performed using p-polarized pump light. The generated second harmonic light was focused (after filtering out the fundamental beam) into a monochromator and detected at the exit slit with a photomultiplier tube (no polarization selection of the second harmonic light was performed). To acquire a second harmonic intensity spectrum, a molecular film was placed in the focal point of two lenses of 20 cm focal distance separated by 40 cm in a telescope configuration. The film normal lay at 45° with respect to the laser beam to maximize surface second harmonic generation. The second harmonic signal generated by a urea film on glass was used as reference for spectral calibration. For interferometric phase measurements, a lens with a 100 cm focal length was employed, yielding a weakly focused beam with a relatively long Rayleigh range. A molecular film was placed at the focal point, while a reference source (an 80 nm thick ITO film on glass) was mounted on a translation stage placed between the lens and the molecular film. The substrates’ normal lay at 45° with respect to the laser beam, and the two films faced one another. Second harmonic interferometry is a one-beam technique which probes nonlinear generation in the presence of the coherent second harmonic light generated by a reference source. Interferograms were acquired by scanning the position of the reference sample over a total distance of 300 mm along the beam direction in steps of 5 mm. Due to the phase mismatch between the fundamental and second harmonic light, either further phase second harmonic generation or backconversion into the fundamental can occur at the sample, leading to a characteristic sinusoidal dependence upon the distance between reference and sample, from which the phase of the second harmonic light generated by the crystal violet film could be extracted (see refs 11 and 34 for more details). Quantum chemical ab initio calculations were performed in the density functional theory scheme using the B3LYP functional.35 The basis set for all the atoms was 6-31g(d) quality, and the calculations were performed with the Gaussian package.36 The structure of the crystal violet cation was optimized allowing all degrees of freedom. The simulation of the absorption spectra was carried out by means of timedependent density functional theory with the same functional and basis set and including also solvent effects (ethanol).

of the dyes takes place, with the formation of CV dimers and trimers.21 Also in solid phases the arrangement of the dyes depends on local interactions: intermolecular, with counterions, and with solvent molecules trapped during the crystallization process, if present. The conformation reported in bulk solid phases is the propeller-like D3 structure,22−26 and the observation of two absorption bands has been attributed to a pairwise arrangement of cations in dimer structures that mimic those found in water solutions.21 In hydride CV crystals formed from water solutions, for example, the lower-wavelength absorption band is ascribed to the coupled electronic levels27 of CV dimers,18 an assignment corroborated also by theoretical calculations.21,25 Recent experiments on CV-intercalated polymer films, however, have found that the lower-wavelength absorption band is very intense in these systems.28 The dye’s local environment in a polymer matrix is clearly different from that of a hydride crystal, and it is not clear that cationic pairs should form. As mentioned earlier, moreover, two absorption bands are observed also in ideal solutions where neither ion-pair formation nor agglomeration occurs. Hence the assignment of the lower absorption band to dimer-like aggregates may not apply to all condensed phases. Absorption and transient-absorption studies19,29−31 have concluded that the two absorption bands observed in alcohol solutions originate from distinct CV conformers. Of these, one corresponds to the D3 structure, while the other is thought to have C3 symmetry which results from the bending of the D3 isomer into a pyramidal shape.25,30 Isomers of C1 or C2 symmetry with one of the phenyl rings tilted in the opposite direction from the other two have been also considered.32 As pointed out earlier, a recent hyper-Rayleigh scattering study on CV solutions demonstrated that their nonlinear activity cannot be due entirely to a D3 conformer, thus giving support to the view that a conformer of lower symmetry is present in solution, and further showing that other conformers are (at least partially) responsible for the quadratic nonlinear optical response.16 The interpretation of the optical properties of CV systems is thus complicated by the fact that several different mechanisms may contribute to the multiplicity of the spectral features. We report here on the extinction and second harmonic spectra of thin CV films obtained from alcohol solutions. While two main features were observed in extinction, as in other solid phases of CV, the second harmonic signal exhibited a single component at the spectral position of the lower-wavelength feature. In order to assign the observed resonances to precise molecular species, we performed ab initio calculations of the absorption levels of several CV conformers, based on which we propose a metastable pyramidal C3 conformer as mainly responsible for second harmonic generation. The contribution of a nonoctupolar conformer with nonzero dipolar hyperpolarizability helps rationalize the anomalously high nonlinear activity of the films as compared to those of other similar nonlinear chromophores, as well as the observation of a significant decay of the nonlinear response over time.

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RESULTS AND DISCUSSION Figure 1a shows the extinction (one-photon) spectra of crystal violet (CV) films deposited from 1-propanol solutions of differing molarity. Similar results were obtained on CV films grown from ethanol solutions. Two main absorption bands are clearly distinguishable in all spectra. The overall absorption intensity increases with increasing concentration, while the

METHODS Molecular films were deposited by immersion of microscope glass slides in crystal violet solutions in 1-propanol or ethanol. The films formed spontaneously upon immersion due to electrostatic interaction between substrate, cations, and counterions. Controlled retrieval was employed to avoid the 26785

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similar shapes, with the thin-film spectrum appearing slightly broader than the one measured in solution. Both spectra could be fitted as the sum of two Gaussian components centered at 562.6 and 595.2 nm, respectively, with the shorter-wavelength component significantly wider than the longer-wavelength one. Although the Gaussian functions used to fit the thin-film spectrum were slightly broader than those of the solution spectrum, the ratio between the intensity and width of the two components was the same in both spectra. This similarity of shape and relative intensity and width indicates that the species present in the lower-thickness films are the same as those present in solution, namely CV monomers of different conformation. The larger width of both components in the film spectrum reflects the wider variety of local environments available at the glass surface compared to the bulk solvent. The absorbance maximum of the CV films is approximately 39 times smaller than the maximum absorbance in solution, which was measured on a 10−6 M solution in a 1-cm-long quartz cuvette. Since according to Beer’s law the absorbance is directly proportional to the molecular density, we can estimate that the number density of molecules in the thinnest film is 0.16 nm−2. Assuming that in the full molecular layer on top of the glass substrate each molecules lies flat on the surface and occupies roughly an area of 1 nm2 (as on metal surfaces, where the experimental value is 0.96 nm2/molecule42), the molecular coverage for the films obtained from 1 × 10−4 or 5 × 10−4 M solutions are 0.16 and 0.6 monolayer, respectively. For thicker films this calculation is less reliable since the shape and width of the absorption spectrum of the film differ increasingly from the solution one, but we estimate the coverage to be of 1 monolayer or less for the film deposited from the 8 × 10−4 M solution.

Figure 1. (a) Extinction spectra of CV films deposited from 1propanol solutions of differing molarity (M) as marked. The spectra are displayed with different offsets for clarity. (b) Extinction spectra of the lowest-thickness film (open circles) and of a 10−6 M 1-propanol solution (dots); the latter are normalized to allow visual comparison. The fits of both spectra as the sum of two Gaussian functions are also shown (continuous lines).

spectral position of both absorption bands gradually shifts to the red as previously observed also for other dyes such as Rhodamine B,37 Rhodamine 6G,38 and Malachite Green.39 The systematic red shift with increasing dye concentration likely reflects the environmental polarity, since CV films deposited from alcohol solutions contain chlorine ions with a chemical composition close to the nominal 1:1 ratio of the CV salt,40,41 and polarity effects are likely to be important in such ionic systems. Figure 1b shows the comparison between the extinction spectrum of the film of lowest thickness and that acquired on a solution of low molarity (10−6 M). The two spectra exhibit very

Figure 2. Upper panels: extinction and second harmonic (SH) intensity spectra of (a) CV and (b) Rh6G chloride films, grown from solutions of the indicated molarities. Lower panels: (c) second harmonic intensity and (d) phase spectra of a CV film grown from a 5 × 10−4 M solution in 1propanol. 26786

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Figure 3. (a) Simulated absorption spectra of the propeller-like conformers: D3 (red), fully planar D3h (black), and pyramidal C3 (dashed and dash− dotted lines), the latter for different pyramidal angles, as marked. A typical structure of the C3 conformer is also depicted. (b) Simulated absorption spectrum and structure of C1 (green) conformers, compared to those of the D3 and D3h conformers. In both panels the time-dependent density functional theory roots have been expanded with the transmission factors and smeared with a Gaussian function.

nonlinear spectrum of the Rh6G film has a line shape that is similar to its extinction spectrum, with two components at approximately the same wavelength and the same relative intensity. This meets theoretical expectations, since a second harmonic resonance should appear whenever the second harmonic wavelength matches that of an absorption level.44 In contrast, the nonlinear spectrum of the CV film exhibits a single peak in correspondence with the lower wavelength absorption feature, for both lower and higher film thicknesses (Figure 2b,c). The presence of a single nonlinear resonance in CV films is confirmed also by second harmonic phase measurements. It is expected that the phase of the generated nonlinear light undergoes a change of π across a resonance.44 The second harmonic phase profile, shown in Figure 2d for the same film as Figure 2c, indicates that there is a single major phase change of roughly π across the low-wavelength resonance, which is not followed by another one at longer wavelengths. The observation of a single nonlinear resonance indicates that the hyperpolarizability of the moieties contributing to the shortwavelength absorption is much larger than that of the species contributing to the absorption at longer wavelengths. Since a nonlinear signal is detected also for the lowest coverage, the species responsible for it cannot be dimers. Moreover, the CV dimer has a centrosymmetric structure, a feature which is a hindering factor for second harmonic generation. Hence the observation of a stronger nonlinear signal at shorter wavelengths lends further support to the thesis that no dimers are present at low surface coverage. The second harmonic signal was observed to decay over a period of several hours and was almost absent after a few days depending on the initial concentration, indicating that the species responsible for nonlinear generation are relatively unstable. The absorption spectra of the films do not instead show important changes after long periods of time (days), which shows that a small fraction of the monomers contributes most of the observed second harmonic signal. We point out that under our experimental conditions the second harmonic signal of CV films is, initially, significantly higher than that of Rh6G or of other similar dyes, as is visible also in Figure 2 from the noisier character of the Rh6G spectrum. Hence the observed second harmonic generation is due to metastable, highly nonlinear conformers.

As is visible in Figure 1a, upon increasing the CV concentration the extinction profile undergoes a gradual transformation from a line shape resembling that of alcohol solutions to one displaying the same characteristics of hydride crystals, which exhibit a prominent band at 543 nm and a shoulder near 620 nm (independent of the water content).18 The relative intensity of the lower-wavelength band increases with increasing dye content until it becomes the dominant component for the highest concentration studied. The enhancement of the lower-wavelength absorption band at very high coverage might be due to a pairwise arrangement of the cations similar to that of CV dimers in aqueous solutions or hydride crystals.18,20,21,43 Such a pairwise arrangement can occur, however, only at a later stage of film growth, while our data show that no aggregation takes place at low surface coverage, since the absorption spectrum of ultrathin films is virtually identical to that of diluted solutions where dimers are absent. Moreover, at least for the thinnest films, the molecular coverage is below a full molecular layer, and no dimers are observed in monolayer films on gold.42 The close similarity between the two spectra of Figure 1b indicates that the absorption features of ultrathin films have the same origin as those of alcohol solutions; namely, they arise from different conformation geometries of the dye.9,16,30 It is reasonable to assume that one of the conformers present in solution and in the thinner films is the D3 structure, which is the most stable one in the gas phase. Another likely candidate is the conformer of C3 symmetry observed in alcohols.30 The dye conformation at the glass surface may in general be different from that found in other phases, due to interfacial electric fields and substrate−adsorbate and adsorbate−adsorbate interactions. As discussed above, the lower-wavelength absorption band is broader than the higher-wavelength one (especially at low coverage), suggesting that distinct CV structures with similar absorption levels contribute to the absorption at shorter wavelengths. In order to investigate the occurrence of distinct CV conformers in the films, we have employed second harmonic spectroscopy. The upper panels in Figure 2 display the second harmonic (two-photon) and extinction (one-photon) spectra of a CV film, compared with those of a Rhodamine 6G (Rh6G) film spin-casted from a 10−3 M ethanol solution. Two absorption bands are observed also in the case of Rh6G. The 26787

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Figure 4. Theoretical spatial distribution of the HOMO and LUMO orbitals of the four conformers studied.

By comparing the theoretical and experimental spectra, it is readily seen that the shorter-wavelength band corresponds (at least at low dye coverage) to a propeller-like conformer. The large width of this band suggests that several conformers (D3 or C3 with small pyramidal angle) contribute to it. Both D3h and C1 structures give rise instead to absorption at longer wavelengths. D3h species might be stabilized at the glass surface through substrate−adsorbate interactions. In the case of a crystalline gold substrate, for example, the interaction with the metal surface leads to a first molecular layer of monomers lying flat on the surface plane in a slightly distorted hexagonal-closepacked arrangement.42 Similarly, on top of graphene and multilayer graphene the π−π interactions of the phenyl groups of the adsorbate with the substrate are believed to align the phenyl rings parallel to the surface.10 We thus assign the band at shorter wavelength to the presence of both D3 and C3 conformers, and the long-wavelength band mainly to D3h species. The C3 structure is likely stabilized through the interaction with the chlorine counterions, as suggested for alcohol solutions,30 while the D3h species might be stabilized by interaction with the substrate. With this assignment, the second harmonic signal is seen to stem from D3 and C3 conformers. Since the nonlinear signal decays over time, the metastable C3 conformers must play a role. The contribution to second harmonic generation from isomers with symmetry different from D3 is consistent with a recent hyper-Rayleigh scattering study which has shown that lower-symmetry conformers contribute to the dye’s nonlinear activity in alcohol solutions.16 The pyramidal geometry of the C3 structure entails a less symmetric electronic configuration of the molecule leading to a nonzero dipole moment in the ground state (absent in the D3 structure). Both these features constitute enhancing factors for the nonlinear susceptibility of organic molecules, and would help to explain the anomalously large second harmonic signal from CV films. Based on our results of Figure 3b and comparison with Figure 1, the pyramidal angle of such conformers lies probably between 90 and 100°. Given that several studies have confirmed the presence of a lower-symmetry C3 conformer in alcohol solutions,16,30,31 it is not surprising that the same conformer occurs also in thin films

In order to identify the species contributing to the absorption and second harmonic spectra, we carried out an ab initio theoretical study of several CV structures, focusing on conformers with D3h, D3, C3, and C1 symmetries. Given the shape of the molecule and the rigidity of the phenyl rings, the conformers can differ only in the direction and relative rotation of these groups. The D3h conformer corresponds to the case where all three are coplanar. In the propeller D3 structure, the rings are all tilted by the same angle (about 32°). In both these conformers the bonds between the central carbon and the phenyl groups lie at 90° with respect to the 3-fold-symmetry axis. The C3 symmetry corresponds to a “pyramidal” propeller structure, in which the angle is increased from 90° to a higher value (in our calculations, up to 109°). Finally, the C1 structure is obtained from the D3 structure by rotating one of the phenyl planes in the direction inverse to the other two groups (the four conformers are represented in Figure 3). The D3 and D3h structures are highly symmetric and do not display a permanent dipole moment,21 which entails that the nonlinear susceptibility of these structures has only octupolar character. Some symmetry elements are instead lost in the C3 and C1 conformers, which can thus display nonzero dipole moment and dipolar hyperpolarizability. According to our calculations, the D3 structure is the most stable one also in solution (as it is in gas phase calculations), while all the other structures have higher energy due to the increased repulsion between the phenyl groups. The stability of each conformer in the CV films may be different, since it is affected by the substrate−adsorbate coupling and the interactions with the counterions.30,40,41 The simulated absorption spectra of the various conformers in ethanol solution are displayed in Figure 3. The theoretical peaks at 500 and 560 nm correspond to the D3 and D3h structures, respectively. The theoretical absorption spectrum of the C3 conformer is shown in Figure 3a for different pyramidal angles. For angles close to 90° the C3 absorption level lies next to the D3 spectrum, as it should. The decrease in symmetry through the rotation of one of the phenyl groups (C1) leads instead to the appearance of two absorption bands at 540 and 580 nm (Figure 3b). 26788

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Notes

grown from them. The occurrence of C3 conformers would also explain the observed discrepancy between the experimental valence band photoemission spectrum of CV films grown from solution and the theoretical calculation for the occupied electronic density of states of planar conformers.40,41 While for vapor-condensed pure CV films that do not contain chlorine the valence band photoemission spectrum matches the theoretical density of states of the isolated monomer, the spectrum of the chloride film is much broader and displays a line shape which cannot be reconciled with the molecular orbital calculations on conformers without pyramidal curvature.40,41 The broadness of the photoemission features of the chloride film indicates the presence of more than one conformer, and the discrepancy with the theoretical density of electronic states of planar conformers strongly hints at the presence of pyramidal isomers in solution-processed films, in agreement with our results. The absorption features correspond to transitions between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the CV conformers, and the resonant second harmonic signal stems from virtual transitions involving these orbitals. The spatial distributions of the HOMO and LUMO orbitals of the various conformers are shown in Figure 4. The HOMO electron density of the C3 conformer is strongly localized on only one of the phenyl groups, a fact that might have consequences for local interactions especially with the negatively charged chlorine counterions.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Spanish Ministerio de Ciencia e Innovación, under Grant MAT2008-00910/NAN and CONSOLIDER NANOLIGHT Grant CSD2007-00046. R.M. acknowledges support from a “Juan de la Cierva” fellowship by the same institution.

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ABBREVIATIONS CV, crystal violet; Rh6G, Rhodamine 6G; M, molarity; SH, second harmonic; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital

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CONCLUSIONS We have studied the extinction and second harmonic spectra of crystal violet films as a function of film thickness. While the linear extinction spectra of the films exhibit two main absorption features, the second harmonic spectrum displays a single dominant component near the spectral position of the lower-wavelength absorption band, also confirmed by phase measurements. The nonlinear signal is observed to decay significantly over time, which indicates that a nonequilibrium structure is responsible for the observed nonlinear susceptibility. The comparison of experiment with quantum chemistry ab initio calculations indicates that a pyramidal conformer with C3 symmetry is responsible for the shorter-wavelength absorption and for the large quadratic hyperpolarizability of the films. The C3 conformer, possibly stabilized through the coupling to counterions, has a nonzero ground state dipole moment, resulting in at least partial dipolar character of the films’ nonlinear susceptibility. These results show that second harmonic spectroscopy can yield information on molecular conformation at interfaces that is hard to assess without making an appeal to nonlinear optical techniques, and which can be valuable for catalytic and biointerfacial studies. Compared with other nonlinear methods such as sum frequency generation, second harmonic spectroscopy has the advantage of being of easier implementation.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 26789

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dx.doi.org/10.1021/jp3075018 | J. Phys. Chem. C 2012, 116, 26784−26790