Control of Exciton Confinement in Quantum Dot–Organic Complexes

Letter pubs.acs.org/NanoLett

Control of Exciton Confinement in Quantum Dot−Organic Complexes through Energetic Alignment of Interfacial Orbitals Matthew T. Frederick, Victor A. Amin, Nathaniel K. Swenson, Andrew Y. Ho, and Emily A. Weiss* Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, Illinois 60208-3113, United States S Supporting Information *

ABSTRACT: This paper describes a method to control the quantum confinement, and therefore the energy, of excitonic holes in CdSe QDs through adsorption of the hole-delocalizing ligand phenyldithiocarbamate, PTC, and para substitutions of the phenyl ring of this ligand with electron-donating or -withdrawing groups. These substitutions control hole delocalization in the QDs through the energetic alignment of the highest occupied orbitals of PTC with the highest density-of-states region of the CdSe valence band, to which PTC couples selectively.

KEYWORDS: Bathochromic shift, dithiocarbamate, carrier delocalization, confinement

T

his paper describes a method for controlling the size of an exciton within a semiconductor quantum dot (QD), not by changing the inorganic core of the QD but with small synthetic adjustments to the structure of the QD’s ligand shell. The set of ligands we use to achieve this control are parasubstituted derivatives of phenyldithiocarbamate, X-PTC (Chart 1), where X varies from a strongly electron-withdrawing

Coupling between the PTC HOMOs and the QD VB in the ground state of the complex creates hybrid orbitals at the inorganic/organic interface that present a lower potential energy barrier for the excitonic hole than do the interfacial states of the QD with its native ligands. The decrease in this confining barrier allows the hole to delocalize into the newly accessible states with mixed QD−surfactant character. Stabilization of the excitonic hole through this mechanism results in a bathochromic shift of the absorption spectrum of the QD− organic complex. Here, we achieve precise chemical control of the radius of the excitonic hole in CdSe QDsand further validate the orbital mixing mechanism for exciton delocalization in these materialsby demonstrating the sensitivity of the degree of hole delocalization to the energies of the HOMOs of X-PTC. Stabilization of the X-PTC HOMO by only 0.5 eV, on going from X = OCH3 to X = CF3, increases the effective delocalization radius of the CdSe QD exciton by more than a factor of 2 due to increased energetic resonance of interfacial orbitals, which allows for increased donation of electron density from PTC, a π-donor, to proximate Se2− ions. This deliberate control of the spatial distribution of a carrier wave function provides a means for coupling strongly confined carriers to their environments through postsynthetic modification of QDs. This coupling has the potential to vastly improve the yield of carrier extraction, in particular from short-lived excited states, like electronically “hot” (above-band-edge) and multiexciton states,5−7 and to facilitate inter-QD tunneling processes within

Chart 1. X-PTC Acid Where X = CF3, OCF3, F, Br, H, CH3, OCH3, or N(Me)2a

a

The molecule is added as an ammonium salt and binds to the surface of the QD as the anion.

group to a strongly electron-donating group (CF3, OCF3, F, Br, H, CH3, OCH3, N(Me)2).1 We have shown previously that PTC (where X = H) induces unprecedented bathochromic shifts in the optical bandgaps, Eg, of CdSe, CdS, and PbS QDs. We observed decreases in Eg of up to 970 meV for CdS and up to 220 meV for CdSe. These exciton stabilization energies are at least a factor of 6 larger than those induced by any other chemical treatments of these QDs; furthermore, they are larger than that induced by addition of a monolayer of the same semiconductor material onto the core of the QD.2 Exchange of the highly insulating native ligands of the QD (phosphonates for CdSe, oleate for CdS, and oleylamine/Cl− for PbS)3 for PTC ligands decreases Eg of the QD through mixing of the states in the valence band (VB) of the semiconductor with the highest occupied molecular orbitals (HOMOs) of PTC.4 © 2012 American Chemical Society

Received: November 6, 2012 Revised: December 11, 2012 Published: December 17, 2012 287

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“delocalization radius”.13,15 The delocalization radius equals the radius by which the CdSe core of the QD would have to grow in order to produce the decrease in Eg that we observe upon adsorption of OCH3-PTC. We note, however, that the core of the QD does not grow during this process;13,15 the shift is purely attributable to delocalization of the excitonic state out of its confining core and into the interfacial region and, possibly, the ligand shell. Figure 1 shows a plot of ΔR vs [OCH3-PTC], a type of adsorption isotherm for the OCH3-PTC/QD system. We created such an isotherm for each of the X-PTC/QD systems, X = CF3, OCF3, F, Br, H, CH3, OCH3, and N(Me)2 (see Supporting Information for spectra and isotherms). We attempted to synthesize NO2-PTC, which would be more electron withdrawing than CF3-PTC, but it was chemically unstable over the time period of ligand exchange.1 In analyzing the degree of delocalization afforded by X-PTC as a function of the substituent X, we used the value of ΔR from the saturated region of the isotherm because this region represents full occupancy of available binding sites for the X-PTC ligand on the QDs. Although different solubilities of the X-PTC molecules in methanol could lead to different concentrations of X-PTC at which saturation is achieved, the surface coverage at saturation should only be determined by the number of available adsorption sites on the QDs. We ensure that the number of adsorption sites does not change with X by using a single synthetic batch of QDs for all of the experiments. To ensure that the QD surface was not changing in time due to surface reorganization or oxidation, we repeated measurements of ΔR for each X-PTC several times over the several-week course of the study.16,17 Figure 2 is a plot of the saturated value of ΔR for each XPTC as a function of the energy of the HOMO of the acid of X-

thin films,8−12 without sacrificing the chemical passivation of an organic adlayer. Figure 1 (inset) shows ground-state absorption spectra for a set of thin films of CdSe QDs (first absorption peak at 542 nm,

Figure 1. A plot of the measured values of ΔR for a series of films of CdSe QDs immersed in different concentrations of OCH3-PTC in methanol for 24 h. We determine ΔR for a given concentration of XPTC by converting the measured decrease in optical bandgap of the QDs upon ligand exchange with OCH3-PTCdetermined from the shift in the ground-state absorption spectrum (inset)to an increase in exciton radius, using a previously measured calibration curve of exciton energy vs exciton radius. The black curve in the inset is the spectrum of the QDs after 24 h of exposure to neat methanol and is identical to the spectrum of the QDs dispersed in toluene in the absence of OCH3-PTC. The spectrum shifts to lower energy with increasing concentration of OCH3-PTC. It initially broadens, due to heterogeneity in surface coverage of PTC on the QDs in the film (blue curve), and then renarrows as the PTC surface coverage approaches saturation. The adsorption isotherm shows that the value of ΔR is saturated by [OCH3-PTC] = 3 mM. The Supporting Information contains the spectra and isotherms for all of the X-PTC molecules.

physical radius, R = 1.4 nm, ∼300 nm thickness) on glass, immersed in a methanolic solutions of OCH3-PTC (one of the eight substituted PTCs we used in this study) of various concentrations for 24 h. We chose to use QDs with R = 1.4 nm because we have shown previously that excitonic holes of CdSe QDs with R ≤ 1.8 nm are within the strong confinement regime (where confinement energy of the hole is much greater than the electron−hole Coulomb energy13), and strongly confined holes are most vulnerable to the extra delocalization volume provided by adsorption of PTC. The first excitonic peak of these QDs shifts to lower energy as [OCH3-PTC] increases. The shift in the spectrum usually saturates by 15 h, but we wait 24 h to ensure the system has equilibrated. In addition to shifting with increasing [OCH3-PTC], the peak first broadens as the OCH3-PTC diffuses to some QDs in the film and not others and then narrows again until its peak wavelength and line width remain constant, at which point [OCH3-PTC] is high enough such that surface coverage of PTC on the QDs is homogeneous throughout the film. For each spectrum within the series shown in Figure 1 (inset), we calculated the corresponding decrease in Eg of the QDs upon adsorption of OCH3-PTC (in energy units). From the decrease in Eg, we use a preconstructed calibration curve of Eg vs R for CdSe QDs14 to calculate the apparent increase in excitonic radius of the QDs, ΔR, which we refer to as the

Figure 2. Plot of the delocalization radius, ΔR, as a function of the energy of the HOMO of the isolated X-PTC acid, calculated using DFT (B3LYP, 6-311+G**). Each data point and error bar are the average and standard deviation, respectively, of absorption measurements on 9−12 separately prepared films of CdSe QDs. The point that is the “outlier” on this plot corresponds to F-PTC (HOMO = ∼−6.2 eV, ΔR = 0.39 nm).

PTC (vs vacuum); we calculated the optimized geometries and orbital energies of the X-PTC acid molecules with density functional theory (B3LYP, 6-311+G**). The data points and error bars in Figure 2 are averages and standard deviations, respectively, of 9−12 absorption measurements, each on a separately prepared QD film treated with X-PTC. We are confident that (i) the calculations yield sufficiently optimized geometries because the geometries they predict are very similar to the crystal structure that we acquired for the ammonium salt of CF3-PTC and (ii) the calculations yield a reasonable trend in 288

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HOMO energies because the predicted HOMO energies are correlated with the Hammett coefficients of the parasubstituents. The Supporting Information contains the crystal structure, the plot of ΔR vs Hammett coefficient, and details of the DFT calculations. We see from Figure 2 that the value of ΔR increases by approximately a factor of 2 upon stabilizing the X-PTC HOMO from an energy of −5.8 eV (where X is most electron-donating) to −6.5 eV (where X is most electron-withdrawing). The value of ΔR saturates at a maximum of ∼0.41 nm (a factor of 8 larger than an analogous thiol, benzylmercaptan), which corresponds to a decrease in Eg of the QDs of 140 meV.18 The value of ΔR saturates at a minimum of ∼0.18 nm, which corresponds to a decrease in Eg of 80 meV. For H-PTC, ΔR = 0.29 ± 0.04 nm, which is within experimental error of the value we obtained in our previous measurement15 (ΔR = 0.26 ± 0.03 nm) for a CdSe QD/H-PTC mixture dispersed in CH2Cl2. We have shown previously13 that batch-to-batch variations in the surface chemistry of the QDsdue to variability in, for example, the concentration of the impurities in the coordinating solvent technical grade trioctylphosphine oxideare a greater source of variability in the value of ΔR than are changes to the local dielectric environment. In order to explain the trend in ΔR with X shown in Figure 2, it is useful to connect the molecular orbital picture of the coupling between the orbitals of X-PTC and orbitals of the QD valence band (Figure 3, left) with the particle-in-a-sphere picture of carrier confinement (Figure 3, right). These pictures are shown schematically in Figure 3 for three representative ligands: octylphosphonate (OPA, the insulating native ligands

of the QD), OCH3-PTC (which, along with N(Me)2-PTC, results in the smallest value of ΔR for the X-PTCs we studied), and CF3-PTC (which results in the largest value of ΔR for the X-PTCs we studied). We first describe the MO diagram of QD−ligand mixing (Figure 3, left). In this diagram, we show the density of states of the VB of CdSe with a gray band and the HOMOs of the ligands in black (at −6.0, −6.4, and −7.6 eV). Despite its small size, the QD is large enough to approximate the bulk VB in its electronic structure;19 there are ∼1200 Se 4p orbitals (from ∼400 Se atoms), distributed over ∼4 eV, with an average of spacing of 3.3 meV.20 Importantly, the density of states for the CdSe VB is not evenly distributed over this energy range; its calculated density of states as a function of energy is detailed in ref 20 and illustrated schematically in Figure 3 by different degrees of shading along the energy axis (darker ≡ higher density of states). These calculations determine the density of states relative to the Fermi level, so we utilized reported ultraviolet photoelectron spectroscopy measurements to place the band edge relative to vacuum.21 At the QD−ligand interface, those orbitals of the semiconductor VB that have density at the surface of the QD mix with the orbitals of the isolated ligands.22,23 This mixing creates many new states (bonding and antibonding) with mixed QD/ligand character. Although the ligands couple directly only with VB states that have density on the surface of the QD, we can assume that VB orbitals with surface density are distributed evenly throughout the VB, since the Cd2+-enriched surfaces of these QDs ensure that Se2− ions near the surface are as coordinatively saturated as those in the core.16 A higher overall density of states in a particular energy range is therefore equivalent to a higher density of surface states in that energy range. Within second-order perturbation theory, the new hybridized orbitals of the system and the original isolated orbitals are split from the energies of isolated orbitals by the sum of the pairwise splittings between states [i] of the VB and the orbitals of the ligand [j] (eq 1) ΔEij =

∑∑ i

j

Vij 2 Δεij

(1)

where Vij is a measure of the spatial overlap of, and Δϵij is the energy gap between, pairwise interacting orbitals. Even though the VB orbitals may couple with multiple MOs on the ligands, the interaction with the HOMO of the ligand is the dominant contribution to the most destabilized interfacial MOs; therefore, for simplicity, we restrict our discussion to mixing of VB orbitals with only the HOMO of each ligand (see Supporting Information for discussion of interactions of other MOs with the QD). When we adjust the energetic position of the X-PTC HOMO (by changing X), we potentially adjust both the set of Vij’s and the set of Δϵij’s; however, we assume that V does not change with substituent because the X-PTC ligands all have the same shapethey mix effectively with Se 4p orbitals because they also have π symmetry (see Supporting Information)and all couple to the Se2− ions on the surface of the QD through electrostatic interactions with an intervening Cd2+ ion. The Cd2+ ions themselves have full d-type occupied valence orbitals that are energetically well-separated from the HOMOs of XPTC20 and therefore do not mix with these orbitals. The major effect, then, of changing the substituent X from electrondonating to electron-withdrawing is to stabilize the HOMO of

Figure 3. (left) Schematic representation of the energetic alignment and splitting of orbitals of the CdSe VB (represented by the gray bar, where higher density of states is darker) and three ligands: the native ligand OPA, CF3-PTC, and OCH3-PTC. (right) Zoomed-in views of the hybridized orbitals created by mixing of VB states with the HOMO of each ligand and the corresponding diagram for the confining potential for the excitonic hole (wave function sketched in red) resulting from these hybridized orbitals. The potential profile is defined by the VB edge and the hybridized HOMO. The particle-in-abox diagrams are “upside-down” because the profiles are for holes, not electrons. 289

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Figure 4. (A) Top: structure of the CdSe4(NH3)4(CH3PO2OH)2 cluster used in DFT calculations (H = white; C = black; N = blue; O = red; P = dark orange; S = yellow; Se = light orange; Cd = tan). Bottom: we replaced each of the methylphosphonate ligands, one at a time, with X-PTC and allowed the geometry of the cluster to relax upon each ligand exchange. This figure shows the fully exchanged H-PTC cluster. (B) Plot of the first excited state energy, calculated from TDDFT, as a function of number of PTC substitutions. The first excited state energy progressively decreases as the native ligands are replaced with X-PTCs. (C) Natural Bond Order analysis determined that the electron population of Se 4p orbitals increases upon exchange of native ligands with X-PTC and that the charge density on Se increases more dramatically as X becomes more electronwithdrawing. (D) The increase in the negative charge density on Se upon exchange of both methylphosphonates with X-PTC is correlated with the experimentally observed value of ΔR. The red dotted line is a linear fit to the plot, with the value of Δq fixed to 0 for methylphosphonate (the native ligand).

translate the MO picture to the one-dimensional particle-in-aspherical-box picture. Note that the box that represents the confining surface potential, indicated by the black dashed line for each ligand, is “upside-down” in the energy scale because we are confining a hole rather than an electron. In the particle-in-abox model, the edge of the bulk CdSe VB (the VB orbital closest to vacuum level) defines the bottom of the box (Figure 3, right). The interfacial HOMOthe most destabilized orbital formed from VB−ligand mixingdefines the magnitude of the confinement potential, shown as a large barrier for OPA, a small barrier for OCH3-PTC, and a potential well for CF3-PTC. We draw the interfacial region as a well, rather than a barrier, for the CF3-PTC/QD system because, from our best estimates of the HOMO energies of the X-PTC molecules and from the fact that adsorption of a monolayer of PTC produces a larger decrease in the optical bandgap of the QD than does adsorption of an additional monolayer of CdSe,2 we believe that the hybridized orbitals of the QD-PTC interface are more destabilized than the VB edge for at least some if not all of the X-PTC molecules. The resulting interfacial potential well gets deeper as X becomes more electron-withdrawing. Our analysis of the trend in ΔR with X does not, however, rely on whether the interfacial region is a barrier or well. Added to the confinement potential is a parabolic Coulomb potential due to the presence of the excitonic electron (not shown in the diagram in Figure 3). The sum of these two potentials defines the total potential surface for the excitonic hole. We believe that the Coulomb potential prevents the wave

X-PTC, such that this HOMO becomes aligned with the highest density of states of the Se 4p orbitals that compose the VB. As a larger number of VB states become resonant with the HOMO of X-PTC, a more destabilized interfacial state is formed (see Figure 3, left). For example, upon displacing the native ligand OPA with OCH3-PTC, the HOMO energy of the isolated ligand increases (becomes less negative) by 1.6 eV; however, the HOMO of OCH3-PTC is aligned with a portion of the CdSe VB specifically, the band-edgethat has a low density of states. The alignment of the OCH3-PTC HOMO with the lowdensity-of-states band-edge results in only a few non-negligible terms in the sum in eq 1, so the maximum energetic splitting due to the OCH3-PTC/VB interaction is small (Figure 3, red box). In contrast, the HOMO of CF3-PTC is aligned with the portion of the VB with a high density-of-states, so the sum in eq 1 will have many more non-negligible terms, and the resulting interfacial HOMO will be more destabilized for CF3-PTC than for OCH3-PTC (Figure 3, green box). The native OPA ligand also has π-type HOMOs that can mix with the Se 4p orbitals of the VB. We speculate that the matrix elements V for OPA are smaller than that for X-PTC (for reasons described in the Supporting Information); however, even if VOPA were large, the energy of the isolated OPA HOMO is too low to form interfacial states that are as destabilized as those formed by XPTC (Figure 3, black box). The importance of the magnitude of mixing between the HOMO of the ligand and the VB orbitals is apparent when we 290

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PTC, stabilization of the HOMO by only 0.5 eV, by changing X from electron-donating to electron-withdrawing, increases the effective delocalization radius of the CdSe QD exciton from ∼0.2 nm (ΔEg = 80 meV) to ∼0.4 nm (ΔEg = 160 meV). The trend of ΔR with X has allowed us to clarify further the chemical and electronic parameters of the QD−ligand interaction that facilitate strong coupling of interfacial orbitals and resultant exciton delocalization. In order to achieve dramatic hole delocalization, the highest lying occupied orbitals of a π-donating ligand must be both (i) more energetically proximate to a high-density-of-states region of the semiconductor VB and (ii) more strongly electronically coupled, as dictated by spatial overlap and symmetry, with the orbitals of the VB than the native ligands of the QD. From these requirements, we can see why common surfactants fail to provide a noticeable change in optical transition energies of CdSe QDs upon ligand exchange. For instance, alkylcarboxylates and phosphonates do not have orbitals within a few electronvolts of the VB edge. Hypothetically, even with large values of electronic coupling, the interfacial orbitals they form present a large barrier to the excitonic hole. In systems where the barrier is large, changes in mixing and alignment upon ligand exchange between carboxylates and phosphonates do not alter the barrier enough to produce a significant change in the absorption spectrum of the QD. Exchange of these insulating ligands for thiolates does produce small bathochromic shifts in QD absorption spectra, probably through the same mechanism as is operative in the PTC case.25,26 Benzylmercaptan, for example, has a HOMO energy of ∼−6.5 eV and produces ΔR of ∼0.1 nm. Because thiols have HOMO energies near those of the most-delocalizing X-PTC molecules, we can infer that V for thiols is much lower than V for dithiocarbamates, probably because thiols bind in a conformation that prevents their sulfur atoms from assuming the natural lattice position of the anion (in the semiconductor) that is so well-approximated by the chelating PTC anion. The CdSe QD−PTC complexes we study here are analogous to the “quasi-type II” CdSe/ZnSe heterostructures discussed by Klimov and co-workers,27 where, in our case, CdSe is the core and PTC is the shell. Klimov modifies the energy offset at the interface by changing the thickness of the shell and the radius of the core (and therefore the degree of confinement of the carriers); we modulate this offset by changing X, the substituent on PTC. The quasi-type-II arrangement includes an electron localized to the core and a hole delocalized over core and shell and is differentiated from a true type-II (“indirect gap”) structure in our system by the absence of charge-transfer-like features in the absorption or photoluminescence spectra (for PL spectra of QD−PTC complexes, please see our previous work15). The effects of a strong coupling interaction between the core of a QD and its organic ligands on the QD exciton are both static (i.e., the change in the energies of absorption and photoluminescence of the QD) and potentially dynamic. For example, we are investigating whether treatment of a QD with PTC changes the radiative decay rate through tuning of electron−hole coupling or whether linking of a hole-accepting redox center to the QD through PTC enhances electronic coupling for interfacial charge transfer. In general, control of the wave function outside of the inorganic core provides another tunable parameter in designing QD-based functional materials with true artificial atom character.

function of the excitonic hole in the QD-PTC system from localizing in the potential wells formed by X-PTC and preserves the distinctly quantum dot-centered absorption of the system; we observe no change in shape of the spectrum, just a shift, upon adsorption of PTC. The final component in the particle-in-a-box diagram in Figure 3 is the wave function for the excitonic hole (red line; sketched, not calculated, but calculated explicitly for several barrier heights in a previous publication13). As the confinement potential presented by the interfacial orbitals decreases, and eventually becomes a well rather than a barrier, the probability for tunneling of this wave function into the interfacial region comprising organic and inorganic orbitals increases, and the hole delocalizes. The greater this delocalization, the greater the stabilization of the excited state of the X-PTC−QD complex and the greater the value of ΔR for X-PTC. The MO/particle-in-a-box diagrams in Figure 3 provide a qualitative model for the trend of ΔR with X. In order to more quantitatively describe the mechanism by which changing the substituent X, and the energy of the X-PTC HOMO, changes the degree of mixing at the PTC−QD interface, we approximated the surface of the QD (the orbitals of which the ligands couple to directly) with a nonstoichiometric CdSe cluster, Cd5Se4 (Figure 4A). Like the QDs used in this study, the cluster is cadmium enriched and therefore requires passivation with anionic ligands.16,17,24 We first optimized the ground state geometry of this cluster passivated with two negatively charged methylphosphonate ligands as models for native OPA, and four ammonia molecules, in order to model dative passivation by hexadecylamine, using DFT with a B3LYP functional and the 6-31G*/LANL2DZ basis set with an effective core potential for S, Cd, and Se atoms. We then progressively displaced each methylphosphonate with an XPTC ligand (in its anionic state) and allowed the geometry of the cluster to relax after each substitution. Figure 4B shows that the lowest-energy electronic transition of the cluster (calculated by TDDFT using the same geometry, functional, and basis set as DFT) decreases with exchange of each methylphosphonate to X-PTC and that the magnitude of this decrease becomes larger as X becomes more electron-withdrawing, in general. Figure 4C shows explicitly that the ground-state interaction between X-PTC and the cluster that results in the lowering of the transition energy corresponds to an increase in the amount of electron density in Se 4p orbitals. The X-PTC ligand is a πdonor, and somewhat counterintuitively, the ability of X-PTC to donate electron density to Se 4p increases as X becomes more electron-withdrawing. This trend is reasonable, however, considering that the increased electron density in the Se 4p orbitals is due to mixing of these orbitals with those of X-PTC, and the magnitude of this mixing increases as the HOMO of XPTC becomes more resonant with the highest density of Se 4p orbitals. Figure 4D shows that the calculated charge redistribution in the clusters (upon substitution with PTC) is positively correlated with the experimentally measured value of ΔR for the QDs. We therefore conclude that X-PTC decreases the bandgap of the QD by increasing the electron density on surface Se2− ions, thereby lowering the potential barrier for delocalization of the excitonic hole through interfacial states that extend into the ligand shell. In summary, we have demonstrated control of the excitedstate energy of a CdSe QD by adjusting energetic alignment of ligand orbitals with orbitals at the surface of the QD. In the case of a series of substituted phenyldithiocarbamate ligands, X291

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(24) Owen, J. S.; Park, J.; Trudeau, P.-E.; Alivisatos, A. P. J. Am. Chem. Soc. 2008, 130, 12279. (25) Liang, Y.; Thorne, J. E.; Parkinson, B. A. Langmuir 2012, 28, 11072. (26) Munro, A. M.; Zacher, B.; Graham, A.; Armstrong, N. R. ACS Appl. Mater. Interfaces 2010, 2, 863. (27) Ivanov, S. A.; Piryatinski, A.; Nanda, J.; Tretiak, S.; Zavadil, K. R.; Wallace, W. O.; Werder, D.; Klimov, V. I. J. Am. Chem. Soc. 2007, 129, 11708.

ASSOCIATED CONTENT

S Supporting Information *

Details of experimental and computational procedures, saturation isotherms, additional absorption spectra, the crystal structure of CF3PTC, and Figures S1−S8. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the DOE through the Office of Science Early Career Research Award (DE-SC0003998) to E.A.W., by the NSF through the Northwestern University Materials Research Science and Engineering Center (MRSEC, DMR-1121262) (N.K.S.), and through an NSF Graduate Research Fellowship to V.A.A.

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REFERENCES

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