Photoinduced Formation of Bithiophene Radical Cation via a Hole

Article pubs.acs.org/JPCC

Photoinduced Formation of Bithiophene Radical Cation via a HoleTransfer Process from CdS Nanocrystals Alessandro Iagatti,† Rebecca Flamini, Morena Nocchetti, and Loredana Latterini* Dipartimento di Chimica and Centro Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy S Supporting Information *

ABSTRACT: The exciton dynamics in semiconductor nanocrystals can be strongly affected by coupling the nanocrystals to organic ligands. A deeper understanding of the interactions in semiconductor−organic hybrid systems is important for the design of functional devices. In the present work, the interactions between CdS quantum dots and bithiophene molecules have been investigated. In particular, the photophysical behavior of CdS nanocrystals has been investigated in n-heptane in the presence of increasing bithiophene concentration by use of steady-state and time-resolved measurements. Bithiophene is a well-known electron donor (or hole acceptor), and it has a good affinity with CdS surface for the presence of sulfur atoms. The nanocrystal luminescence was efficiently quenched upon addition of increasing concentration of the thiophene derivative, and modifications in the emission decay profiles of CdS were observed; the analysis of luminescence data suggests that quenching is mainly due to static interaction able to modify the dynamics of the exciton states of the hybrid nanomaterials. The transient absorption measurements enable to detect the bithiophene radical cation upon CdS excitation, thus revealing the occurrence of an efficient hole transfer process from the nanocrystals to the organic ligand, for which a quantum efficiency of 36% has been measured. The dependence of transient signal on bithiophene concentration and the formation of tetrathiophene intermediates indicate that CdS exciton states are able to photosensitize the polymerization of bithiophene after the hole transfer processes. The data indicate that in the investigated system the decay of charged species is not determined by back-reactions.

1. INTRODUCTION The design of hybrid nanomaterials based on colloidal semiconductor nanocrystals (or quantum dots, QDs) and organic moieties is an attractive research field in nanotechnology.1−4 This research area gives the possibility to obtain new nanocomposite materials with defined optoelectronic properties and photophysical behavior suitable for the advancement of nanodevices which can be used as emitting tools,5 sensing apparatus,6−8 or a photoelectron medium.2,9,10 In order to achieve hybrid nanomaterials with defined behavior and functions, the interactions between semiconductor nanocrystals and organic species have to be deeply understood. In many studies, the interactions between QDs and organic molecules have been monitored through the modifications of the nanocrystal luminescent properties. In some cases the QD emission behavior can be determined by the degree of crystallinity or the nanocrystal surface properties,11−13 which affect the monitoring signal and make difficult to reach a definitive assignment of the forces driving the interactions. Insights into the nature of the interactions can be achieved by a careful analysis of the time-resolved measurements,14−18 and the occurrence of energy and/or charge transfer process has been suggested. In a recent study, Bhattacharyya et al. observed a remarkable modification of the luminescent decay of CdTe© 2013 American Chemical Society

QDs in the presence of oligothiophene nanomaterials for the occurrence of hole transfer processes from the valence band of CdTe to the HOMO orbital of the thiophene system which compete with energy transfer processes.16 Xu et al. investigate the luminescence behavior of CdSe-ZnS−polymer in bulk and at single particle level and demonstrate that the hole transfer rate, in the CdSe-ZnS−polymer system, depends on the distance between donor and acceptor but also in the electronic coupling between the donor and acceptor components.17 The investigation of intermediate species, formed upon interaction of QDs exciting with organic or bio-organic molecules, and their assignment to chemical structures is extremely useful to establish the nature of the interactions.18−21 Huang et al. were able to reveal the nature CdSe− phenothiazine interactions by use of transient absorption measurements and observe that the dynamics of exciton bleaching was kinetically coupled to the formation of the radical cation of the organic moiety.21 Recently, Tseng et al. suggested the occurrence of a multistep electron transfer interactions initiated by a hole transfer process from CdS nanorod to a Ru− Received: June 19, 2013 Revised: October 19, 2013 Published: October 21, 2013 23996

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Figure 1. TEM image of CdS nanocrystals (a, scale bar is 20 nm). Inset: TEM higher resolution image of a single nanocrystal. Diameter distribution histogram determined by TEM (b) and (c) DLS size distribution of CdS colloids.

Synthesis of CdS Nanocrystals. CdS nanocrystals were prepared in water compartment of water-in-oil microemulsions, following a procedure previously described,26 although some modifications were introduced. Two microemulsions were prepared by mixing 1.28 g of AOT as surfactants, 50 mL of nheptane as organic phase, and 125 μL of Milli-Q H2O. In one microemulsion, 50 μL of a Cd(NO3)2·4H2O (0.6 M) solution was added under vigorous stirring. The same preparation was carried out to obtain Na2S (0.6 M) in the second microemulsion. In these conditions, the microemulsions were characterized by a water content (W = [H2O]/[AOT]) value of 3.3. Subsequently, in a two-neck flask, 25 mL of Cd2+ microemulsion and 25 mL of S2− microemulsion were mixed under a nitrogen flow and stirred at room temperature for 10 h. After the mixing a droplet fusion process occurs, and the formation of nanocrystals is observed. In order to avoid droplet coalescence and nanocrystal clustering, a reflux treatment for 30 min under a nitrogen flow to remove the water content. Morphological Characterization. A Philips model 208 transmission electron microscope (operating at 80 kV of beam acceleration) was used to image the nanocrystals and analyze their size distribution. The size distributions were obtained by analyzing 100−150 nanocrystals in each sample; the data were used to build up histograms in terms of frequency of a dimension (Ni) compared to the total number of particles (NT), which were analyzed by a Gaussian function. The fitting parameters (peak position and width) are reported together with their statistical errors. The volume-mean diameter of CdS nanocrystal was determined by dynamic light scattering (DLS) with a Malvern Nano ZS instrument equipped with a 633 nm laser diode. Measurements were carried out at 25 °C in air-equilibrated suspensions placed in 1 cm quartz cuvettes. X-ray powder diffraction (XRPD) patterns were taken with a Philips X’PERT PRO MPD diffractometer operating at 40 kV and 40 mA, with a step size 0.0170 2θ degree, and step scan 20 s, using Cu Kα radiation and an X’Celerator detector. Photophysical Characterization. UV−vis absorption spectra were recorded with a PerkinElmer Lambda 800 spectrophotometer on air-equilibrated solutions. Photoluminescence spectra, corrected for the instrumental response, and quantum yields were measured with a Fluorolog (Spex F112AI) spectrophotofluorometer using quinine sulfate in H2SO4 (0.5 M), ΦF = 0.55,27 as standard. The luminescence decay, τL (mean deviation of three independent experiments, ca. 5%),

mononuclear complex following the exciton dynamics by ultrafast transient absorption.18 Using a nanosecond transient absorption setup, Sharma et al.19 were able to monitor the fate of charged species formed by hole transfer from CdSe to phenylenediamine after exciton excitation in the hybrid system, which is mainly controlled by back-reactions. Despite the effort to investigate the dynamics of charged species, the quantum efficiencies of hole transfer process in hybrid systems are rarely being quantified. The II−VI semiconductor structures are among the most studied materials for their band gap tunability; the wide interests in this family of semiconductors are because they can be prepared efficiently and in a cost-effectiveness way through chemical reactions in mild conditions. CdS nanocrystals have the wider band gap in the cadmium chalcogenide family (Eg = 2.2 eV22), and they have been tested for different applications ranging from photovoltaic to photocatalysis. To investigate the photophysical behaviors and the dynamics of charged species in hybrid materials based on CdS nanocrystals and organic molecules, CdS nanoparticles were coupled to a specific thiophene oligomer, such as bithiophene (α-2). Thiophene oligomers are well-known semiconductor organic materials, which attracted much attention for their remarkable electrical and optical features.23,24 The smallest oligomer in the family, bithiophene (α-2), has a relatively low ionization potential (1.1 V, whose absolute value is strongly dependent on torsional angle and on the substation/ conjugation)24 and a quite high band gap (4.4 eV24,25), thus making negligible any transition overlap with the CdS exciton and offering the possibility for a selective excitation of CdS exciton. Thus, it appears a suitable candidate for the study of charge transfer interactions and charge carrier mobility in a CdS−organic molecule system. In the present study, the behavior of CdS exciton in the presence of α-2 has been investigated through steady-state and time-resolved absorption and emission techniques to achieve direct evidence on the nature of the interactions in the hybrid nanomaterials, to quantify the efficiency of the process, and to obtain insight into the dynamics of charge transfer species.

2. EXPERIMENTAL SECTION Chemicals. n-Heptane, dioctyl sulfosuccinate sodium salt (AOT), 2,2′-bithiophene (α-2), Cd(NO3)2·4H2O, and Na2S· 9H2O were purchased by Sigma-Aldrich. Milli-Q water was freshly prepared in-house. 23997

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was measured by single photon counting method using an Edinburgh Instrument 199S setup. A pulsed lamp filled with hydrogen was used as excitation source, and a Hamamatsu R7400U-03 detector acquired the signal. The transient behavior was investigated using a flash photolysis setup previously described28 based on an Nd:YAG Continuum laser (Surelite II, third harmonics, λexc = 355 nm, pulse width ca. 7 ns and energy ca. 1 mJ pulse). The transient spectra were obtained by monitoring the optical density changes every 5−10 nm over the 300−800 nm range and averaging at least 10 decays at each wavelength. The kinetic analysis of the signals at selected wavelengths allowed the transient decay time to be determined. The calibration of the experimental setup, for quantum yield determinations, was carried out with an optically matched solution of benzophenone in acetonitrile (triplet quantum yield, ΦT = 1 and εT = 6500 M−1 cm−1).29 The experimental errors on the transient decay time values are estimated to be about 10% while those on the quantum yields are about 15%. All the measurements were performed in air-equilibrated samples.

Figure 2. Absorption and luminescence spectra of CdS nanocrystals. Inset: absorption and luminescence spectra of α-2.

trapping processes has been confirmed by the quite low luminescence efficiency measured for the CdS suspension, since a value in the order of 5% has been measured for the photoluminescence quantum yield. Single photon counting measurements show that the luminescence decays have a nonexponential behavior, as previously observed for colloidal samples.11,16,30,31 The luminescence decays could be reproduced by biexponential functions which likely reflect the distribution of decay times related to the distribution of exciton populations, as previously reported.11,16,32 The decay parameters are collected in Table 1 together with the averaged decay time value (5.9 ns), which has been calculated to make easier the data analysis (see below). Interactions of CdS Nanocrystals with Bithiophene Molecules. With the aim to investigate the capabilities of the nanocrystals to transfer charge carrier species and the possible role of the surface defects, the interactions between bithiophene molecules and CdS nanoparticles have been studied. Thiophene oligomers are well-known electron donors, and bithiophene (α-2) presents electronic transitions in the UV region;25 thus, a selective excitation of CdS band gap was carried out. These experimental conditions enable to neglect the occurrence of radiative energy transfer processes between the excited CdS and the thiophene derivative because no spectral overlap occurs (see Figure 2). The luminescence behavior of CdS QDs upon addition of increasing concentration of α-2 was investigated, and an efficient emission quenching was observed. It has to be noted that at the excitation wavelength (λexc = 370 nm) the absorption of α-2 is negligible, as shown by the spectra reported in the inset of Figure 2. The emission quenching occurred without any changes in the CdS spectral features, even in the red edge of the spectrum (where the surface defects emit11), which suggests that the quenching occurs with similar efficiency of the different emitter’s populations or the emissions of the different populations are linked. In the inset of Figure 3 the plot of I0/I as a function of the quencher concentration (from 0 to 1 × 10−3 M) is reported; at high α-2 concentrations ([α-2] > 5.0 × 10−4 M), the data deviate from a linear trend, thus suggesting that in these conditions static interactions are taking place. The correlation of the data at lower quencher concentrations (0−4 × 10−4 M) with the Stern−Volmer model enables to determine a quenching rate constant value of 2.2 × 1011 M−1 s−1, using the average luminescence decay time (Table 1). The obtained quenching rate constant value is higher than the collision rate constants estimated through the

3. RESULTS AND DISCUSSION Synthesis and Characterization of CdS Nanocrystals. CdS nanocrystals were prepared in water compartment of water-in-oil microemulsions in order to control their growth process following a literature procedure,26 although some modifications were introduced to reduce the nanocrystal dimensions and to avoid water droplet coalescence. In particular, a rapid and efficient reflux treatment was applied under a nitrogen flow to the suspension for 30 min in order to remove the water content. Although modifications of CdS size distribution cannot be excluded, the formation of nanocrystal clusters, as previously reported was not observed.26 TEM images (Figure 1) show the formation of spherical nanocrystals whose size distribution is reproduced by a Gaussian function centered at 2.9 ± 0.1 nm and with 0.9 ± 0.1 nm as full width at half-maximum (fwhm), indicating a good dispersion parameter. Dynamic light scattering (DLS) measurements show that the sample exhibited a monomodal distribution (Figure 1c), centered to a diameter value of 3.2 ± 0.2 nm, thus confirming the absence of clustering phenomena. The X-ray powder diffraction (XRD) of CdS NPs before and after calcinations at 350 °C has been recorded (Figure S2). The spectrum of the as-prepared CdS colloids shows two very sharp and intense reflections ascribable to AOT surfactant; no reflections corresponding to CdS are detectable. In order to remove the surfactant, the CdS NPs dispersion has been calcinated at 350 °C, giving a brown-yellow powder. The XRD pattern exhibits broad peaks assignable to wurtzite structure CdS (JCPDS card, File No. 41-1049, labeled with a star in Figure S2b) and reflections due to S6 molecules very likely deriving from AOT decomposition. These data suggest that either the CdS colloids have a low degree of crystallinity or the small dimensions of the nanoparticles limit their detection by XRD. The absorption spectrum of the nanocrystal suspension is characterized by a single band gap signal centered at 350 nm. The luminescence spectrum presents a maximum at 440 nm (Figure 2). Despite the satisfactory size distribution, the emission spectrum is quite broad and presents an important tail in the red edge; it is important to notice that the spectral features of the QDs luminescence are strongly influenced by the presence of defect states. The occurrence of dissipative or 23998

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Table 1. Luminescence Decay Parameters for CdS in n-Heptane

a

sample

λem (nm)

ΦL

τL1 (ns)

τL2 (ns) [A2]

av τL (ns)

CdS CdS + α-2a

420 420

0.05 ± 0.01 0.03 ± 0.01

2.7 ± 0.3 2.3 ± 0.2

6.6 ± 0.3 [83%] 6.2 ± 0.3 [67%]

5.9 ± 0.3 4.9 ± 0.3

[α-2] = 6.5 × 10−4 M.

determined for the CdSe−phenylenediamine hybrid,19 but it still reports a high affinity of CdS surface to form complex with α-2. This hypothesis is further supported by luminescence decay measurements in the presence of α-2; similarly to what observed in the pristine CdS sample, the luminescence of the nanocrystals in the presence of α-2 molecules decayed with a nonexponential behavior (Table 2), leading to an averaged Table 2. Maxima of Absorption, Half-Lifes, Absorption Coefficients, and Formation Quantum Yields of Transient Species Formed upon Excitation of CdS in the Presence of 6.5 × 10−4 M α-2 species

λmax (nm)

t1/2 (μs)

εa (M−1 cm−1)

Φ

450 630

0.12 ± 0.01 0.20 ± 0.02

15 900 33 500

0.36 ± 0.05 0.16 ± 0.02

•+

α-2 α-4•+

Figure 3. Luminescence spectra of CdS nanocrystals in the presence of increasing concentrations of α-2. Inset: Stern−Volmer plot. a

Smoluchowski and Stokes−Einstein equations (in this case a kq value in the order of 104 M−1 s−1 was determined; see Supporting Information for details). These findings suggest that the interactions between CdS colloids and bithiophene molecules are static even at the low concentration regime. In particular, the thiophene moieties can be adsorbed on the surface of QDs thanks to the sulfur groups that interact with secondary bonds. By use of a simple complexation model, similarly to the one proposed by Sharma et al. for CdSe− phenylenediamine hybrid,19 the association of α-2 with CdS has been determined by considering the association equilibrium:

From ref 34.

decay time value of 4.9 ns. This value is clearly reduced compared to the value obtained from the pristine CdS sample. The fitting parameters (Table 2) indicate that the shortening of the average decay time is mainly due a decrease of the long component’s contribution (A2). However, the marked effect, upon interaction with α-2 on the long luminescence decay component, suggests that this component is related to processes occurring on the nanocrystal surface, where the interactions with α-2 molecules are taking place. Steady-state and time-resolved data enable to conclude that in the present system the quenching interactions mainly modify the dynamic behavior of exciton states, probably opening new deactivation paths. The presence of a shorter decay component (shorter than the instrumental profile) cannot be excluded, and further investigations are currently being carried out. Transient Absorption Measurements on CdS−Bithiophene Hydrids. Nanosecond laser transient absorption measurements have been carried out to investigate the nature of the interactions between CdS and α-2 molecules. Upon excitation of CdS nanocrystals at 355 nm, a transient signal was detected only in the presence of α-2; in particular in Figure 5, the transient spectra recorded in the presence of a α-2 concentration equal to 6.5 × 10−4 M are presented. In the whole spectral range the signals were produced within the laser pulse and present the main maxima at 450 and 630 nm; at both maxima the transient signals decayed with mixed order kinetics, although with different times since at 450 and 630 nm half-lives of 120 and 200 ns (Table 2), respectively, were determined. Furthermore, the intensity of the transient absorption at 450 nm as well as the kinetic behavior was strongly affected by the α-2 concentration (Figure 6). Upon increasing the α-2 concentration, the transient absorption increased until reaching a maximum value (at [α-2] = 6.5 × 10−4 M), and its decay became faster. The further increase of α-2 concentration resulted in a decrease of the optical density while the half-life reached a plateau, close to the instrumental resolution. These

CdS + α ‐2 ↔ [CdS...α ‐2]

Kapp is the apparent association constant, which can be expressed as a function of emission efficiencies of the complexed (Φ′) and uncomplexed (Φ0) CdS species and in terms of observed emission efficiency (Φobs).19 The data analysis (Figure 4) enable to determine a value of Kapp as 4 × 103 M−1. This value is 1 order of magnitude lower than that

Figure 4. Plot of Φ0/(Φ0 − Φobs) vs the reciprocal concentrations of α-2. 23999

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The transient signal assignment agrees with the dependency of the transient signal on α-2 concentration and with the lack of observing remarkable effects from oxygen removal.35,39 In the present experimental conditions, the bithiophene radical cation decayed with kinetics faster than observed in homogeneous solutions or in microemulsions, where only back-electrontransfer processes were operative. The direct detection of the oxidized form of bithiophene proved that the hole transfer process from CdS nanocrystals to the organic molecules is taking place; however, the concentration effects deserve a deeper understanding. For the assignment of signal centered at 630 nm to a chemical species, a different hypothesis could be put forward; it could be due to α-2 di-cation species,40 to cation radical dimers,41 or to electropolymerization products.23 The irradiation conditions used in the present work (pulsed excitation at low laser fluence) and the decay kinetics observed for dithiophene radical cation make negligible the possibility to form detectable amounts of α2 bipolaron species. However, is necessary to consider that the spectral contribution of cation radical dimers cannot be completely neglected. It is well established in the literature that radical cations generated from short chain thiophene oligomers, such as α-2, can rapidly polymerize to give longer oligothiophene species,23 thus making difficult in many experimental conditions to observe the oxidized species. The spectral features and the kinetic behavior of the signal suggest that it could be tentatively assigned to a tetrathiophene (α-4) radical cation (α-4•+). Upon calibration of the instrumental setup, using benzophenone in acetonitrile and taking advantage of the absorption coefficient values reported in the literature for α-2 and α-4 radical cations,34 the formation quantum yields of the two species were determined (Table 2). In particular, quantum efficiency values as 0.36 and 0.16 for α-2 and α-4 radical cations, respectively, were obtained. These high values suggest that the bithiophene radical cation formation is an efficient process, which is not in competition with radiative exciton recombination (which is 2 orders of magnitude lower efficiency) but with nonradiative deactivation processes likely controlled by the surface properties and reactivity, as suggested by time-resolved data; furthermore, the relevant α-4 •+ formation quantum yield indicates that back-charge-transfer reactions are not the processes which control the dymamics of charge carriers in the present hybrid system. All these findings can be rationalized considering that after adsorption of α-2 on the QDs surface by secondary bond and excitation of the hybrid systems the α-2 radical cation is formed through a hole transfer process, acting as catalysts for the photoinduced polymerization of thiophene oligomers through a hole transfer mechanism, with a mechanism similar to the one observed in electropolymerization.23 To substantiate the formation of α-4 molecules, the CdS suspension in the presence of α-2 was spectrophometrically and fluorimetrically analyzed before and after steady state or pulsed photolysis at 355 nm, where only the CdS nanocrystals absorb. The absorption spectrum of the suspension presents permanent spectral changes upon irradiation (Figure 7); furthermore, when the photolyzed suspension was excited at 410 nm (absorption maximum of tetrathiophene), a structured fluorescence spectrum was observed (Figure 7), which can be ascribed to α-4 emission, and it was not detected before the photolysis process.25

Figure 5. Transient absorption spectra of CdS nanocrystals in the presence of 6.5 × 10−4 M α-2 recorded 50 ns (circles) and 250 ns (squares) after the laser pulse (λexc = 355 nm).

Figure 6. Effects of α-2 concentration on transient signal (λobs = 450 nm) intensity (upper panel) and half-life (lower panel).

observations suggest that when the surface of QDs is completely passivated, the α-2 molecules present in solution quench the transient species present on the CdS surface. The transient absorption data can be rationalized formulating the hypothesis that at least two transient species have been photochemically produced. On the basis of literature data,33−35 the signal with maximum at 450 nm has been assigned to oxidized species of the thiophene derivative (radical cation, α-2•+); the presence of this intermediate specie can be rationalized with the occurrence of a hole transfer process from the excited CdS nanocrystals to α-2. Using the model developed by Brus to describe the electronic properties of small semiconductor crystallites as a function of sizes36,37 and the numerical calculations for CdS colloids with 1.45 nm radius reported by Lian et al.,38 for the system under investigation an excited-state oxidation potential of −1.27 V (SCE) and reduction potential of +1.61 V (SCE) can be determined. These values together with the ionization potentials of α-2 molecules24 enable to conclude that the hole transfer process from the excited CdS nanocrystals to α-2 is an energetically possible process. 24000

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Sesto Fiorentino, Florence, Italy, and INO - CNR, Istituto Nazionale di Ottica - Consiglio Nazionale delle Ricerche, Largo Fermi 6, 50125 Florence, Italy. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Danilo Pannacci (Dipartimento di Chimica, Università di Perugia) for the technical help with the analysis of photoproducts. The authors gratefully acknowledge the support of the University of Perugia. L.L. thanks the financial support of Ministero per l’Università e la Ricerca Scientifica e Tecnologica (Rome, Italy) under the project PRIN 2010-2011, 2010FM738P.

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Figure 7. Absorption and fluorescence (λexc = 410 nm) spectra of CdS suspension in the presence of α-2 (6.5 × 10−4 M) before and after photolysis. Inset: emission and excitation spectra of tetrathiophene in heptane.

4. CONCLUSIONS The interactions between CdS quantum dots and an organic electron donor have been investigated. In particular, steadystate and time-resolved absorption and emission techniques were used to study photophysical behavior of CdS nanocrystals in n-heptane in the presence of increasing of concentration 2,2′bithiophene (α-2). The steady-state luminescence of the nanocrystals was efficiently quenched upon addition of increasing concentration of thiophene derivative, and the quenching occurred without any spectral modifications. The static nature of interactions between the nanocrystals and the α-2 molecules has been established through the analysis of the luminescence data, based on the Stern−Volmer model; the evaluation of CdS luminescence decays suggested that the presence of α-2 molecules modifies the population of emitting states of the nanocrystals. The transient absorption signals, obtained using a nanosecond laser transient spectroscopy setup, of CdS QDs, in the presence of α-2 molecules, have shown the capability of these nanocrystals to photosensitize the formation of bithiophene radical cation, through a hole transfer process, with a remarkable quantum efficiency value (36%). The dependence of transient signals on bithiophene concentration and the formation of tetrathiophene radical cation indicate that the dynamics of CdS exciton states are not controlled by backelectron-transfer processes but can assist the dimerization of bithiophene. Indeed, the formation of tetrathiophene molecules was confirmed by the detection of its photoluminescence spectrum.

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ASSOCIATED CONTENT

S Supporting Information *

Estimation of quenching rate constant, TEM image, XRD pattern, luminescence decay curves and phase-shift data, and instrumental setup description. 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] (L.L.). Present Address †

A.I.: European Laboratory for Non Linear Spectroscopy (LENS), Università di Firenze, via Nello Carrara 1, 50019 24001

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dx.doi.org/10.1021/jp406072n | J. Phys. Chem. C 2013, 117, 23996−24002