Electrostatic Control of Excitonic Energies and Dynamics in a CdS

Letter pubs.acs.org/JPCL

Electrostatic Control of Excitonic Energies and Dynamics in a CdS Quantum Dot through Reversible Protonation of Its Ligands Christopher M. Thompson, Mohamad Kodaimati, Dana Westmoreland, Raul Calzada, and Emily A. Weiss* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States S Supporting Information *

ABSTRACT: This paper describes the pH dependence of the excitonic energies and dynamics of CdS quantum dots (QDs) capped with phosphonopropionate (PPA) in water. QDs capped with PPA carry a negative charge on their surfaces upon deprotonation of PPA above pH ∼ 8.5; the resultant electric field induces large changes in the QD’s optical properties. Between pH 5.6 and 12.0, an increase in pH is accompanied by a 47-meV bathochromic shift in the bandgap of the QDs and a decrease in the Stokes shift by ∼4.3 meV/pH unit. An increase in the radiative recombination rate by a factor of 20.9 occurs on increasing the pH from 5.6 to 9.4. These observations are attributed to a shifting of the energy levels within the first exciton manifold, and are simulated using time-dependent density functional theory calculations on model Cd29S29 clusters surrounded by point charges.

C

spherical metal chalcogenide QDs both increase with QD radius. In this work, we demonstrate control over the optical band gap, Stokes shift, and radiative recombination rates of excitons in colloidal CdS QDs capped with phosphonopropionic acid (PPA) through the pH of the aqueous suspension. The density of negative charges on the surface of the QDdetermined by protonation equilibrium of the phosphonate anchoring groupcreates a radial electric field that modifies the confining potential of the QD exciton (Figure 1A). The modification of the potential manifests marked changes in “core properties” of the QD, properties that are typically only changed by changing the size or shape of the QD core or by adsorption of ligands that electronically couple to the QD core states enough to introduce new interfacial states that dictate the degree of quantum confinement.14,15,17−20 Specifically, we observe that an increasing pH from 5.6 to 12 results in a 47 meV bathochromic shift in the first exciton peak of the absorption spectrum (decreased band gap) as well as a change in the Stokes shift of −4.3 meV/pH unit. We also see an increase in the radiative exciton decay rate by a factor of 20.9 with increased surface charge density, which we explain by the relative shifting of bright and dark states within the first exciton manifold. We show that we can reversibly control these properties by modifying the pH of the solution. Our results demonstrate a rare use of surface chemistry to accomplish dynamic and reversible control of the QD’s core properties, and provides a strategy for monitoring the photoluminescence wavelength of QDs to sense changes in pH in, for example,

olloidal semiconductor nanocrystals (quantum dots, QDs) are valued as chromophores, fluorophores, and electronic components for the high degree of tunability of their electronic properties through small synthetic modifications. The strong influence of the size, shape, and material of the QD on its optical and electronic properties has been exhaustively studied over the past three decades, and it has been a goal of many researchers in the QD community to expand the extent of this tunability and exploit it in a range of applications.1−11 Some properties, like photoluminescence quantum yield (PL QY), blinking behavior, trapping rates, and solubility are readily modified by a ligand exchange or enrichment or selective termination of the surface with either the cationic or anionic component of the crystal.12,13 Other properties, hereafter referred to as “core properties”, depend mainly on the material, shape, and size of the inorganic QD core and are minimally affected by changes in the QD surface. These properties include band gap, extinction coefficient and exciton radiative lifetime; in spherical metal chalcogenide QDs, larger particles generally have a lower band gap, higher extinction coefficients, and shorter radiative lifetimes.2,7,9,11 Demonstrations of changes in core properties by choice of ligand14,15 are few, and reversible, dynamic sensitivity of core properties to the QD surface chemistry16 is even more rare, because the eigenstates of the exciton are generally only weakly dependent on the surface structure and ligand shell. Increasing the number of available strategies for ligand-based control over core properties is attractive, because it allows for both (i) design of QDs with dynamically tunable, environmentally responsive properties (using the ligands as couplers between the core and the environment) to be used as sensors, and (ii) the independent variation of properties that are otherwise correlated with core size; for example, the extinction coefficient and band gap of © 2016 American Chemical Society

Received: August 22, 2016 Accepted: September 20, 2016 Published: September 20, 2016 3954

DOI: 10.1021/acs.jpclett.6b01899 J. Phys. Chem. Lett. 2016, 7, 3954−3960

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The Journal of Physical Chemistry Letters

into a flask of cadmium oleate in octadecene at 230 °C using a procedure reported previously.22 The concentration of QDs was determined with absorption spectroscopy and a calibration curve reported by Yu and co-workers.9 The QDs were transferred from hexanes, in which they are soluble assynthesized, to water using a ligand exchange procedure recently developed in our group. This procedure, and detailed chemical characterization of PPA-capped QDs, are described elsewhere.23 Briefly, we diluted 1 mL of 10 μM QDs into 5 mL of hexanes and added 30 uL of 0.1 M phosphonopropionic acid (PPA) while stirring. Upon addition of PPA the QDs flocculated. We recovered the QDs by adding 1 mL of N,Ndimethylformamide (DMF), removing the hexanes layer, and adding 4 mL of 10 mM KOH in water (which had been bubbled with N2) to the QDs in DMF. This solution was then washed with CHCl3 to extract the DMF, and excess CHCl3 was evaporated by bubbling the solution with N2. Following phase transfer, the QDs were diluted in a buffer of PPA/KOH to achieve the desired pH. We have shown previously by NMR23 that, using our ligand exchange procedure with QDs of this size, oleate ligands are completely displaced by PPA, and that, after exchange, each QD has approximately 250 bound PPA ligands. The aqueous PPA-capped QDs are colloidally stable and resist etching for at least 5 days when stored in the dark (even in the presence of O2), or when stored in room light, if O2 is excluded and a sacrificial reductant is present to scavenge photogenerated holes.23 Experiments in the present study were performed within 12 h of phase transfer, on QDs that were stored in brown glass vials to protect them from room light. All samples sustained equal exposure to O2 and light during spectroscopic measurements, and all changes in optical properties were reversible and therefore not due to changes in the physical structure of the QD core (vida inf ra). Water-soluble QDs capped with mercaptopropionic acid (MPA), the thiolate analog of PPA, were prepared for comparison with PPA-capped QDs. We followed the same procedures described above, but diluted the QDs in DMF with 2 mL of 10 mM KOH and 2 mL of water in order to achieve similar postdilution pHs. Figure 1B shows the absorption spectra of CdS QDs capped by PPA for several values of pH between 5.6 and 12. Spectra of CdS QDs capped by the analogous thiol ligand, MPA, are shown in the Supporting Information, Figure S1. We observe a reproducible pH-dependent shift of the absorbance maximum of the first exciton peak of PPA-capped QDs from 2987 meV (415.1 nm) at pH 5.6 to 2940 meV (421.7 nm) at pH 12.0 (Figure 1C). The average pH-dependent exciton energy from the titrations of multiple samples with standard deviations are shown in the Supporting Information, Figure S2. The higherenergy absorption features also shift with pH in the same direction as the first exciton peak. The pH dependence of the first exciton energy is not linear; there is an inflection in its response between pH 8 and 9. By contrast, the energy of the first exciton of the same QDs, but capped with MPA, shifts less than 5 meV from pH 5.1 to pH 10.3, and shifts another 3 meV between pH 10.3 and 12.2. The magnitude of this shift is less than 20% of that observed for PPA-capped QDs (Figure 1C, open circles). The emission peak of PPA-capped QDs shifts with pH in the same direction, but with smaller magnitude, than the absorption peak (Figure 1C); thus, the Stokes shift of these QDs is also pH dependent. It decreases approximately linearly with increasing pH with a slope of −4.3 ± 0.6 meV/pH unit.

Figure 1. (A) Illustration of the acid−base equilibrium of PPA proposed as the mechanism for the observed pH-dependence of the optical properties of PPA-capped QDs (aq). PPA binds to the QD via the oxygen atom within the phosphonate group with pKa ∼ 1, where its charge is balanced by the Cd2+-enriched QD surface. The second acidic proton of the phosphonate (red), with a solution-phase pKa of 8.5, participates in acid/base equilibrium with changing pH. When deprotonated, PPA generates an electric field that reduces the energy of the QD’s exciton. (B) Absorption spectra of PPA-capped CdS QDs (aq) as the pH was raised from 5.6 to 11.7 by addition of KOH. The intensities of all spectra are corrected for dilution. (C) Energies of the first excitonic absorption (left axis), the band-edge emission (left axis), and the Stokes shift (right axis) of aqueous PPA-capped CdS QDs as a function of pH, adjusted using additions of HCl and KOH. The data is the average of three titrations on separately prepared samples, and error bars denote the standard deviations of measured values. Also shown is the response of the first excitonic absorption of MPA-capped QDs to changes in pH over this range (open circles).

biological environments, without the conformational changes and buried acidic sites associated with green fluorescent protein-based sensors.21 Fine tuning of QD energy levels using surface chemistry is also potentially useful in QD-based photoredox catalysis, where energy levels and exciton lifetimes are critical factors in charge transfer rate and yield. CdS QDs with radius 1.9 nm and capped with oleate were prepared by injecting elemental sulfur, dissolved in octadecene, 3955

DOI: 10.1021/acs.jpclett.6b01899 J. Phys. Chem. Lett. 2016, 7, 3954−3960

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The Journal of Physical Chemistry Letters

Figure 2. Energies of excitonic absorption features of PPA-capped CdS QDs in water during pH cycling between pH 5.8 and pH 12.0 through addition of aqueous solutions of HCl or KOH. Left: Energies of the first, second, and third exciton features throughout the pH cycling are plotted in colored squares (right y-axis) and the corresponding pH is plotted in a solid black line (left y-axis). Right: Exciton energies are plotted against pH. Exciton energies were obtained by fitting absorption spectra to three Gaussian peaks and an exponential background (see the Supporting Information, Figure S3).

constant for radiative decay using eq 1, where QY is the absolute QY and τave is the intensity-averaged PL lifetime

Figure 2 demonstrates the excellent reversibility of the shift in the optical bandgap of aqueous PPA-capped QDs with pH. We can repeatedly cycle between lower-energy and higherenergy absorption spectra by alternate additions of HCl and KOH. There is some reproducible hysteresis within these titration cycles: the exciton energies during the addition of acid are lower by ∼5 meV than during the addition of base. We suspect this hysteresis may be an effect of small changes in the surface environment that are not perfectly reversible on the time scale of our pH scan, such as coadsorption of other ions in solution. We used time-correlated single photon counting (TCSPC) and measurements of absolute PL quantum yield (QY) to determine the pH-dependence of the radiative recombination rate (kr) for the PPA-capped CdS QDs. TCSPC gives the timedependent probability of photon emission by an ensemble of QDs, which is directly translatable to the rates of decay of distinct populations of QDs within the ensemble. Emission in the single photon regime was collected at 430 nm using 375 nm excitation. 430 nm was not the emission maximum for all samples, but we observed 98% HOMO-to-LUMO transition, and inspection of the HOMO and LUMO ground state densities confirm that these orbitals are delocalized over the cluster. Once we obtained the optimized geometry, we used TDDFT to calculate the absorption spectrum in the presence of Q = −1, −2, −4, and −8 charges, dispersed evenly over 1024 grid points on a sphere concentric with the geometric center of the cluster, and located 1.0 Å from the cluster surface. Spectra encompassing transitions to the first 10 excited states of the Cd29S29 cluster, artificially broadened by Lorentzian functions with peak width Γ = 5 meV, are in Figure 4A. The spectra of Q = −1, −2, −4, and −8 clusters have very similar shapes to that of the uncharged cluster but, as we see experimentally upon deprotonation of PPA, are shifted to lower energies.

Figure 3. Intensity-averaged excited state lifetimes (A), absolute PL QYs (B), and radiative recombination rate constants (C), collected from five separately prepared batches of PPA-capped CdS QDs (aq), as a function of pH. The data in these plots are all scaled by their values at pH 8.5, to minimize noise due to batch-to-batch variability of their absolute values. Points in gray (above pH 9.4) lie in the range of conditions where picosecond-time scale hole transfer to OH− occurs, but is not recorded by the TCSPC measurement (IRF ∼ 250 ps). The resulting error in τave leads to erroneous estimation of kr, so we do not consider these data in our analysis.

terephthalic acid mixtures upon raising the pH of the mixtures to 9.5−10 (see the Supporting Information, Figure S5). We therefore explain the apparent drop in kr above pH 9.5 by quenching of the QD’s PL by picosecond-time scale hole transfer from the QD to OH−, and disregard data in this pH range, plotted in gray in Figure 3, in our analysis. The dependence of the absorption energy, emission energy, Stokes shift, and radiative rate of exciton decay of the CdS QD on pH leads us to attribute these responses to an acid−base equilibrium of the phosphonate headgroup of the PPA ligand (Figure 1A). Two pieces of evidence specifically implicate the phosphonate headgroup in modulating the optical properties of the QD: (i) the dramatically smaller optical response of the MPA-capped QDs to pH (Figure 1C, open circles), which eliminates the possibility that the carboxylate tailgroup is primarily responsible, and (ii) the increase in the rate of change of absorption energy, emission energy, and kr with pH above 8.5 (Figures 1−3), which is the pKa of the second acidic proton of the phosphonate group. The pKa of the first acidic proton is ∼1, so the oxygen that binds to Cd2+ on the surface of the QD is always deprotonated, and the second OH group is free to undergo deprotonation under basic conditions.26 We note that the pKa of PPA molecules bound to the surface is almost certainly modified by the local density of negative charge and the presence of counterions, which is why we observe a continuous shift in exciton energy even above 8.5. A reversible 3957

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sensitive to the size of the QD (i.e., the confining potential) and should also be sensitive to the imposition of a radial electric field from charged ligands. We propose that the electric field imposed by charged PPA ligands shifts both the ±1U state and the ±1L and/or ±2 state(s) closer to the ground state, and thereby decreases both the absorption energy and the emission energy, and that the charge distribution influences the ±1U state more than the ±1L and/or ±2 state(s) such that the Stokes shift also decreases. Within the established framework, emission from the lowest energy excited state, J = ±2, is optically forbidden and, in CdSe QDs, has a lifetime of microseconds, while emission from the “bright” ±1L state is optically allowed and has a lifetime of tens of nanoseconds. Above 10 K, the thermal equilibrium between the dark (long-lived) state and the nearest bright (short-lived) state, split by ΔEeq, causes a temperature dependence in kr.37 Our absorption and emission spectra clearly demonstrate that the energies of states within the first exciton manifold are shifted by tens of millielectron volts relative to one another by the electric field, so we also expect pH-induced changes in the distribution of population between the lowest bright and dark states within the exciton. Specifically, if we assume that the lowest dark and bright states, J = ±2 and J = ±1L, are the only states involved in emission, then kr is given by the Boltzmann-weighted average of rate constants in eq 2,

Figure 4. (A) TDDFT (B3LYP, LANL-2dz)-calculated absorption spectra for a Cd29S29 cluster with point charges (with total charge magnitude of Q = 0, −1, −2, −4, and −8), distributed isotropically at 1.0 Å from the QD surface. The charges were placed on 1024 grid points, such that the charge at each grid point is Q/1024. The Supporting Information contains analogous calculated spectra where the charges are distributed over 256 grid points, and placed at 2.0 Å from the QD surface. (B) Depiction of the ground state and first exciton manifold within PPA-capped CdS QDs. Energy differences within the first exciton manifold are exaggerated. The main observations in this work can be explained by a lowering of the ±1U state (ΔEabs), a lowering of the ±2 state (ΔEem), a decrease in ΔEstokes, and a less positive/more negative ΔEeq, with increasing pH.

k r = k brightfbright + kdarkfdark

(2)

where f bright and fdark are the Boltzmann populations of the bright and dark states, and we approximate kbright and kdark using values similar to those reported for CdSe QDs (coated with either ZnS shell or a double shell of ZnSe and ZnS): kbright = 0.1 ns−1 and kdark = 1 μs−1.2,38,39 Using this two state model, ΔEeq must become at least 108 meV more negative on going from pH 5.6 to pH 9.4 in order to obtain the observed increase in radiative rate over this pH range (ΔEeq(5.6) = 111.1 meV, ΔEeq(9.4) = 3.1 meV). Based on the magnitude of shifts we observe in ΔEabs (47 meV) and ΔEem (24 meV), this change is unrealistically large, but our simple calculation is very sensitive to the ratio of kbright/kdark, which could be quite different from that for CdSe/ZnS and CdSe/ZnSe/ZnS QDs. An additional factor is that, while this simple model is based on just two emissive states, our Stokes shift measurements indicate that the ±1U state moves from being 100 meV (∼4 kBT) above the ±2 state at pH 6 to less than 85 meV (∼3.3 kBT) above the ±2 state at pH 9.4. With an energy spacing of 85 meV, roughly 4% of excited QDs should populate this strongly radiating ±1U state. The radiative contribution from this population could increase the measured radiative rate. We invoke energy level shifting rather than changes in the oscillator strengths to explain the observed changes in radiative rates, because not only are the changes in radiative recombination rates we observe much larger than those attributed to oscillator strength changes in previous work,14 but increasing oscillator strengths are typically accompanied by greater absorption intensities in colloidal QDs,11,40 whereas we observe a slightly lower extinction coefficient with increasing pH. In general, control of surface charge on QDs in water offers another method for controlling QD “core properties” independent of QD size, material and shape. Effects similar to the ones we describe here are likely to be observable in all

Distributions of charge with lower symmetries, produced by distributing the total charge over 1, 2, 4, 8, and 16 grid points, rather than 1024 grid points, resulted in major qualitative changes to the absorption spectrum (see the Supporting Information, Figure S9). The changes we observe experimentally best resemble those obtained in the systems with 1024 grid points; thus we believe that the deprotonated PPA ligands decrease the energy of excitonic states by creating a quasispherical negative charge distribution. In order to explain the dependence of the Stokes shift and the radiative lifetime of the QD on a spherically symmetric charge density, we must consider the fine structure of the first exciton of the QD. Within the effective mass approximation, the (1Se1S3/2) first exciton state is split into five energy levels by a combination of crystal field splitting (for hexagonal crystallites), the electron−hole exchange, and anisotropy of the QD shape, as shown explicitly by Efros.35,36 Figure 4B shows the relative energies of these five excitonic states, labeled by the projection of their angular momentum and a U or L (upper or lower), per Efros’ convention. Absorption in QDs occurs mainly to ±1U state within the manifold, because this state has the greatest oscillator strength. The energy between the ground state and this state is therefore labeled ΔEabs. Emission occurs from the lowest two states, ± 1L and ±2, after relaxation has occurred, with energy ΔEem. The difference between ΔEabs and ΔEem is the Stokes shift, labeled ΔEstokes. All of these values are 3958

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(11) Gong, K.; Martin, J. E.; Shea-Rohwer, L. E.; Lu, P.; Kelley, D. F. Radiative Lifetimes of Zincblende CdSe/CdS Quantum Dots. J. Phys. Chem. C 2015, 119, 2231−2238. (12) Peterson, M. D.; Cass, L. C.; Harris, R. D.; Edme, K.; Sung, K.; Weiss, E. a. The Role of Ligands in Determining the Exciton Relaxation Dynamics in Semiconductor Quantum Dots. Annu. Rev. Phys. Chem. 2014, 65, 317−339. (13) Galland, C.; Ghosh, Y.; Steinbrück, A.; Hollingsworth, J. A.; Htoon, H.; Klimov, V. I. Lifetime Blinking in Nonblinking Nanocrystal Quantum Dots. Nat. Commun. 2012, 3, 908. (14) Jin, S.; Harris, R. D.; Lau, B.; Aruda, K. O.; Amin, V. a; Weiss, E. a. Enhanced Rate of Radiative Decay in CdSe Quantum Dots upon Adsorption of an Exciton-Delocalizing Ligand. Nano Lett. 2014, 14, 5323−5328. (15) Jethi, L.; Mack, T. G.; Krause, M. M.; Drake, S.; Kambhampati, P. The Effect of Exciton-Delocalizing Thiols on Intrinsic Dual Emitting Semiconductor Nanocrystals. ChemPhysChem 2016, 17, 665−669. (16) Liang, Y.; Thorne, J. E.; Parkinson, B. A. Controlling the Electronic Coupling between CdSe Quantum Dots and Thiol Capping Ligands via pH and Ligand Selection. Langmuir 2012, 28, 11072− 11077. (17) Amin, V. A.; Aruda, K. O.; Lau, B.; Rasmussen, A. M.; Edme, K.; Weiss, E. A. Dependence of the Band Gap of CdSe Quantum Dots on the Surface Coverate and Binding Mode of an Exciton-Delocalizing Ligand, Methylthiophenolate. J. Phys. Chem. C 2015, 119, 19423− 19429. (18) Frederick, M. T.; Amin, V. a; Swenson, N. K.; Ho, A. Y.; Weiss, E. a. Control of Exciton Confinement in Quantum Dot-Organic Complexes through Energetic Alignment of Interfacial Orbitals. Nano Lett. 2013, 13, 287−292. (19) Frederick, M. T.; Amin, V. a.; Cass, L. C.; Weiss, E. A. A Molecule to Detect and Perturb the Confinement of Charge Carriers in Quantum Dots. Nano Lett. 2011, 11, 5455−5460. (20) Frederick, M. T.; Weiss, E. A. Relaxation of Exciton Confinement in CdSe Quantum Dots by Modification with a Conjugated Dithiocarbamate Ligand. ACS Nano 2010, 4, 3195−3200. (21) Kneen, M.; Farinas, J.; Li, Y.; Verkman, A. S. Green Fluorescent Protein as a Noninvasive Intracellular pH Indicator. Biophys. J. 1998, 74, 1591−1599. (22) Yu, W. W.; Peng, X. Formation of High-Quality CdS and Other II-VI Semiconductor Nanocrystals in Noncoordinating Solvents: Tunable Reactivity of Monomers. Angew. Chem., Int. Ed. 2002, 41, 2368−2371. (23) Calzada, R.; Thompson, C. M.; Edme, K.; Westmoreland, D. E.; Weiss, E. A. Organic-to-Aqueous Phase Transfer of Cadmium Chalcogenide Quantum Dots Using a Sulfur-Free Ligand for Enhanced Photoluminescence and Oxidative Stability. Chem. Mater. 2016, DOI: 10.1021/acs.chemmater.6b03106. (24) Melhuish, W. H. Quantum Efficiencies of Fluorescence of Organic Substances: Effect of Solvent and Concentration of the Fluorescent Solute. J. Phys. Chem. 1961, 65, 229−235. (25) Simon, T.; Bouchonville, N.; Berr, M. J.; Vaneski, A.; Adrović, A.; Volbers, D.; Wyrwich, R.; Döblinger, M.; Susha, A. S.; Rogach, A. L.; et al. Redox Shuttle Mechanism Enhances Photocatalytic H2 Generation on Ni-Decorated CdS Nanorods. Nat. Mater. 2014, 13, 1013−1018. (26) Freedman, L. D.; Doak, G. O. The Preparation and Properties of Phosphonic Acids. Chem. Rev. 1957, 57, 479−523. (27) Fredoueil, F.; Evain, M.; Massiot, D.; Bujoli-Doeuff, M.; Janvier, P.; Clearfield, A.; Bujoli, B. Synthesis and Characterization of Two New Cadmium Phosphonocarboxylates Cd2(OH)(O3PC2H4CO2) and Cd3(O3PC2H4CO2)2 × 2H2O. J. Chem. Soc. Trans. 2002, 2, 1508− 1512. (28) Heller, W.; Bockelmann, U.; Abstreiter, G. Electric-Field Effects on Excitons in Quantum Dots. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 6270−6273. (29) Yaacobi-Gross, N.; Soreni-Harari, M.; Zimin, M.; Kababya, S.; Schmidt, A.; Tessler, N. Molecular Control of Quantum-Dot Internal

instances of QDs dispersed in water in the presence of solvated ions, where unbalanced (or partially balanced) charges may adsorb to the QD surface. Full optimization of these systems will require a more complete and quantitative theoretical picture than that presented here.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01899.

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Experimental details, Figures S1−10, and Table S1 (PDF)

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the David and Lucile Packard Foundation through a Packard Foundation Fellowship for Science and Engineering, by the Army Research Office via the Presidential Early Career Award for Scientists and Engineers (PECASE), and by the National Science Foundation through the Northwestern Materials Research Science and Engineering Center (Grant DMR-1121262).

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REFERENCES

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DOI: 10.1021/acs.jpclett.6b01899 J. Phys. Chem. Lett. 2016, 7, 3954−3960

Letter

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DOI: 10.1021/acs.jpclett.6b01899 J. Phys. Chem. Lett. 2016, 7, 3954−3960