Designing Janus Ligand Shells on PbS Quantum ... - ACS Publications

Mar 11, 2019 - cinnamic acids,18 which undergo simple 1:1 X-type ligand exchange with oleic ... large electron withdrawing character (negative dipole-...
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Designing Janus Ligand Shells on PbS Quantum Dots using Ligand−Ligand Cooperativity ACS Nano 2019.13:3839-3846. Downloaded from pubs.acs.org by UNIV OF MELBOURNE on 04/23/19. For personal use only.

Noah D. Bronstein,†,○ Marissa S. Martinez,†,‡,○ Daniel M. Kroupa,†,‡ Márton Vörös,§,∥ Haipeng Lu,† Nicholas P. Brawand,∥ Arthur J. Nozik,†,‡ Alan Sellinger,†,# Giulia Galli,§,∥,⊥ and Matthew C. Beard*,† †

Chemistry & Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States § Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States ∥ Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States ⊥ Department of Chemistry, University of Chicago, Chicago, Illinois 60637, United States # Department of Chemistry and Materials Science Program, Colorado School of Mines, Golden, Colorado 80401, United States ‡

S Supporting Information *

ABSTRACT: We present a combined experimental and theoretical study of ligand−ligand cooperativity during X-type carboxylate-to-carboxylate ligand exchange reactions on PbS quantum dot surfaces. We find that the ligand dipole moment (varied through changing the substituents on the benzene ring of cinnamic acid derivatives) impacts the ligand-exchange isotherms; in particular, ligands with large electron withdrawing character result in a sharper transition from an oleate-dominated ligand shell to a cinnamate-dominated ligand shell. We developed a two-dimensional lattice model to simulate the ligand-exchange isotherms that accounts for the difference in ligand binding energy as well as ligand−ligand cooperativity. Our model shows that ligands with larger ligand−ligand coupling energy exhibit sharper isotherms indicating an order−disorder phase transition. Finally, we developed an anisotropic Janus ligand shell by taking advantage of the ligand−ligand cooperative ligand exchanges. We monitored the Janus ligand shell using 19F nuclear magnetic resonance, showing that when the ligand−ligand coupling energy falls within the order region of the phase diagram, Janus ligand shells can be constructed. KEYWORDS: PbS quantum dots, ligand−QD interactions, ligand coupling, QD optical properties, QD surface science

Q

uantum dots1 and other colloidal nanocrystals have seen a tremendous amount of research over the last three decades.2 A large body of work has focused on synthesis with shape, size, and composition control of the inorganic core.3 Subsequent work4 has examined the surface chemistry and shown that the surface chemistry is itself responsible for much of the very same synthetic control. Often, important features of the nanocrystal shape can be traced to an impurity in the solvent, precursor, or preparation,5 which can result in various molecules binding preferentially to particular surface facets during NC synthesis. Beyond the preparation of nanocrystals, control of the ligand shell is a prerequisite for any application: the surface of the quantum dot is key to QD-QD electronic coupling to make conductive films,6−8 and ligandshell vacancies can be the source of electronic defects that degrade performance.9 Despite their importance, ligand © 2019 American Chemical Society

binding and exchange isotherms on nanocrystal surfaces remain under-explored. The most recent work on the QD surface chemistry has focused on understanding the binding headgroup chemistry through Green’s definition of X-, L-, and Z-type interaction,10,11 while the interaction of ligands with neighboring ligands has seldom been reported. Widmer-Cooper and coworkers12 unveiled some of the richness of the interactions of long aliphatic chains, showing coverage- and size-dependent phase transitions, differences in the binding site versus ligand tail symmetry, and coverage- and size-dependent interparticle Received: January 8, 2019 Accepted: March 11, 2019 Published: March 11, 2019 3839

DOI: 10.1021/acsnano.9b00191 ACS Nano 2019, 13, 3839−3846

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Scheme 1. Functionalized Cinnamic Acids Used in This Study, along with Their Dipoles Normal to the QD Surface Computed by DFTa

a

The molecules are either electron-donating (negative dipole) or electron-withdrawing (positive dipole).

interaction potentials. Other work by Jana and co-workers13 showed the importance of ligand coverage in creating chiral distortions of nanoplates, which can propagate through micron-long assemblies of those plates. The most-thorough characterization of ligand-exchange isotherms was done by Zeger Hens and co-workers using nuclear magnetic resonance (NMR), showing that ligand−ligand interactions are indeed important for the shape of the isotherm, as a Fowler isotherm reproduced the data much better than a Langmuir isotherm.14 In situ NMR is unfortunately limited to ligand exchange reactions in which both the initially bound and incoming ligand have distinct NMR peaks, and where the dynamics of the ligand exchange equilibrium are slow on the NMR time scale. In this work, we examine in situ ligand exchange isotherms via simple linear absorption spectroscopy. Giansante15 and Kroupa16−18 recently reported that some classes of ligands significantly enhance the absorption intensity of the QD/ ligand complex upon exchange. Similarly, Weiss and coworkers report a shift of the absorption onset upon ligand exchange.19−21 Kroupa introduced a library of functionalized cinnamic acids,18 which undergo simple 1:1 X-type ligand exchange with oleic acid16 and exhibit absorption enhancement that depends linearly on the cinnamate bandgap.17 Kroupa also demonstrated a simple analytical dependence of the extent of exchange with the absorption enhancement using in situ NMR spectroscopy.16 This indicates that the absorption enhancement acts as a simple feedback on the extent of exchange, allowing for the measurement of ligand exchange isotherms for ligands without easily distinguishable NMR peaks. Here, we take advantage of this feedback to study the exchange of the oleate surface ligands with eight different functionalized cinnamic acids (R-CAHs, Scheme 1) that are classified by their functional group on the aromatic ring.18 We developed a custom 2-dimensional lattice model that reproduces the salient features of the adsorption isotherms. The energy of exchange, ΔGexc , as well as a nearest-neighbor coupling energy between ligands, ΔJ , are outputs of the model. We find that the absolute magnitude of dipole moment of the cinnamic acid has a strong influence on the exchange energy, up to a few kBT, where kB is the Boltzmann constant and T is temperature, for the largest dipoles used in this study. We also observe an interesting correlation between ligand dipole moment and the nearest neighbor coupling, ΔJ : ligands with large electron withdrawing character (negative dipole-moment) show much higher cooperativity, ΔJ = −0.65 kBT, than

those with large electron donating character (positive dipolemoment), ΔJ = −0.15 kBT. We use these observations to design phase-segregated (e.g., patchy or Janus) ligand shells, which break the symmetry of an otherwise symmetrical nanocrystal. Patchy particles have received considerable attention in the literature, with the main focus on larger structures,22 or metallic nanoparticles,23,24 with one report showing the impact on the optical properties of CdS QDs when the exchanging ligands group together.25 There are numerous potential applications where controlling the electrostatics, morphology, and symmetry of the ligand shell is expected to have considerable influence over their emergent properties.

RESULTS We synthesized 3.2 nm diameter PbS QDs with first excition transition energy of 1.3 eV.26 They consist of a stoichiometric PbS core with Pb-oleate ligands. The number of OA− ligands initially bound to each QD is measured to be 100 ± 5 by quantitative 1H NMR, corresponding to 3.1 ligands per square nanometer, in agreement with literature values.4,18,19 This solution-phase ligand exchange procedure replaces the native oleate ligands with cinnamic acids to form cinnamate passivated PbS QDs through a 1:1 X-type ligand exchange.11 Incoming free cinnamic acid transfers a proton to a surface bound oleate to form free oleic acid and surface bound cinnamate. Adding excess cinnamic acid drives the exchange toward completion.16 For each of the functionalized cinnamic acids (Scheme 1), we performed quantitative spectrophotometric titrations (Figure S1a,b). We relate the absorption enhancement to the square root of the number of bound ligands (see the Supporting Information for details).16 Thus, to construct the adsorption isotherms (bound-ligands, R-CA−, versus added free ligands), we plot (Δα /α0)2 versus the number of free-ligand equivalents, where Δα is (α − α0), α0 is the integration of the as-synthesized OA−/QD absorbance spectrum, and α is the integration of the in situ QD spectrum (Figure 1a−h, filled circles for each of the cinnamate ligands); details and raw data are provided in the Supporting Information. Model of Ligand Exchanges. In our previous study, we found that a standard Langmuir isotherm for non-interacting ligands could not reproduce the adsorption isotherms,16,18 similar to the findings of Hens and co-workers.14 In fact, the θ , depends change in free energy upon ligand exchange, ΔGexc 3840

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factor that accounts for the fraction of ligands in solution with the identity of a new or incoming ligand ( fB when the incoming ligand has the identity B and fA when the incoming ligand has identity A). Thus, fA (θ , Nadd) =

θ*Nsites Nadd + Nsites

(1)

fB (θ , Nadd) =

Nadd − θ*Nsites Nadd + Nsites

(2)

The free energy of ligand exchange, ΔGexc is the difference in binding free energy between ligand B and A; we define NNB as the number of nearest ligand B neighbors and ΔGMF is a meanfield free energy difference between ligands of type B and type A when bound to the surface. The nearest neighbor ligand− ligand coupling energy is ΔJ , and the total number of nearest neighbors is 4 (i.e., we use a square lattice) thus NNA + NNB = 4 . Then the exchange energy ΔGAB associated with replacing a ligand of type A with a ligand of type B is ΔGAB = ΔGexc + θ ΔGMF + 4ΔJ(2 − NNB)

(3)

Similarly, the energy ΔG BA of replacing a ligand of type B with a ligand of type A is simply the negative of expression 3. The Boltzman factor is thus:

Figure 1. Adsorption isotherms (bound CA ligands vs free ligands) of the ligand exchange reactions constructed from the quantitative spectrophotometric titrations. The number of bound ligands is proportional to the (Δα /α)2 ; see the Supporting Information. Data are represented by filled circles, and the solid lines are the best-fit lines obtained from a nonlinear least-squares fitting of a 2d lattice simulation. The best-fit parameters are displayed in Figure 4. The dashed line is the number of binding sites available. The xaxis is the number of ligand equivalents added during the experiment.

| l exp( −ΔG /k T ) if ΔG > 0 o o AB B AB o o BfAB = m } o o o if ΔGAB ≤ 0 o o o n1 ~

(4)

and the probability of exchange is PAB = BfAB*fA (θ , Nadd), PBA is defined similarly; for more details, see section S2 of the Supporting Information. We use PAB and PBA defined above to carry out Monte Carlo simulations. At the beginning of the simulation, all binding sites are initially occupied by ligand A. The simulation progresses by randomly selecting a grid point (each grid point is associated with a ligand site) and computing the probability PAB of changing the grid point occupancy from ligand A to ligand B, or when the site is occupied by B and there is a probability of exchanging B for A, PBA . If PAB or PBA is larger than a random number between 0 and 1 the exchange occurs otherwise it does not. This random sampling is repeated a large number of times (between 106 and 109) to collect sufficient statistics on the extent of exchanges between ligands. The simulation is run for a wide range of different Nadd values to create isotherms of coverage versus the ligand addition. To model our measured adsorption isotherms we use nonlinear least-squares fitting (solid traces in Figure 1a−h) to find the computed isotherm with best-fit parameters ΔGexe , ΔGMF and ΔJ . We find that our model reproduces the trends in the data fairly well; however, at the low and high exchanges, the model deviates from the data. When setting the mean-field term ΔGMF to zero, the resulting best-fit values changed in a negligible way. In Figure 2, we show snapshots of a representative Monte Carlo simulation of the ligand exchange titrations when the ligand−ligand coupling energy is less than −0.44 kBT. At low additional equivalents (left box) the ligand shell is primarily oleate (ligand A, blue boxes). When the ligand concentration reaches 1 equiv, the ligand exchange reaction starts, and the cinnamates (ligand B, yellow boxes) tend to group together (they form patches) because of the ligand−ligand coopera-

upon the extent of exchange, θ Such an observation is indicative of associative ligand−ligand interactions that can promote ligand exchange.27 There are many empirical modifications to the non-interacting Langmuir isotherm model; however, none of them gives a good physical description of the surface dynamics. The coverage of surfaces by incoming gas molecules and atoms have been extensively studied, and various 2-dimensional (2D) statistical models have been developed to describe the dynamics of ligands, with the best-known being a lattice model isomorphic with the Ising spin−lattice model.28 We adopted a simple 2D lattice model27 and use that to perform a Monte Carlo simulation of the ligand exchange reactions at NC surfaces in solution. The simulations employ a square n × n binary lattice, where n*n = Nsites with periodic boundary conditions to simulate the spherical NC surfaces. Our simulations differ from traditional 2D lattice simulations in one key way: we consider a finite number of added ligands (Nadd), originally bound ligands (θ*Nsites), and binding sites (Nsites), where θ is the fraction of all binding sites occupied by the original ligands. For each binding site occupied by the orginal ligand (ligand A), we define a probability (PAB) of exchanging ligand A for ligand B, while for each binding site that becomes occupied with the new ligand (ligand B), we define the probability (PBA ) for the reverse reaction (i.e., the freed-up oleic acid can replace the newly bound cinnamates). The probabilities are a product of a Boltzman factor, Bf , and a 3841

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Figure 2. Isotherm for 4(CN)2-CAH exchanging with oleic acid. The red circles are the measured optical absorption values, scaled with the square-root dependence, and the solid red line is the simulated isotherm. The four square-lattice images show representative surface coverage patterns at equilibrium for four different ligand addition equivalents (“Equiv.”). The clustering of ligands with their own type clearly indicates strong nearest-neighbor interactions.

dipole, working against the acidity-driven binding enthalpy difference. As the absolute magnitude of the dipole moment increases, ΔGexc is reduced to ∼0.7−0.8 kBT. To account for this reduction, we consider three contributions: a dipole−dipole interaction energy for dipoles oriented perpendicular to the NC surface, Wevert , a dipole− dipole interaction energy for dipoles oriented parallel to the NC surface, Weplanar , and a term, ΔG0 , that represents the difference in binding free energy based on the different binding groups (cinnamate vs. oleate); thus, ΔGexc = Wevert + Weplanar + ΔG0 . The variations in pKa within the cinnamic acid derivatives (pKa ≈ 4)30 is much less than the difference in pKa between cinnamic acid and oleic acid (pKa ≈ 10),31 i.e., oleic acid is a much weaker acid than are the cinnamic acids. Thus, oleic acid binds lead cations much more strongly than cinnamic acid does, overwhelming any energetic difference based on proton transfer and causing oleate to bind ∼2.5 kBT more favorably than the cinnamic acid with near-zero dipole (35F-CAH). Furthermore, if the variations in acidity across our ligand library were to dominate the binding energy, we would expect ΔGexe to vary linearly with dipole moment (or Hammet parameter) rather than showing the observed dependence. For these reasons, we expect ΔG0 not to vary across the family of cinnamic ligands. Both electrostatic terms, Wevert and Weplanar are proportional to the magnitude of the dipole moment squared ( μ2 ). However, Wevert is purely repulsive and scales as sin(γ )/d3, where d is the distance between ligands, and γ is the angle formed by the ligand with the substrate (when γ = 0 the ligands are laying flat on the surface). Weplanar can either be repulsive or attractive depending on whether the ligands tilt together forming a head-to-tail alignment or tilt into each other forming a head-to-head alignment. We assume here that the ligands tilt together (Figure 3c), and thus, the interaction parallel to surface is attractive (Figure S5). When γ is less than 45°, |Weplanar| > |Wevert|, and the total interaction energy is attractive. We can estimate the angle the dipolar ligands form with the surface normal by recalling from our previous work that the

tivity. Around 2 additional equivalents, the ligand shell is roughly 50:50, and the ligands are completely segregated. ΔGexc and ΔJ vs Ligand Dipole. The resulting model outputs (fitted parameters), ΔGexc and ΔJ , are plotted against the ligand dipole computed using DFT calculations in Figure 3a,b. The computed dipole moments are linearly related to the tabulated Hammet parameters29 (see Figure S4). It is important to note that ΔGexc and ΔJ do not represent the binding free energy and nearest neighbor interactions separately. Instead, ΔJ contains only the change in nearest neighbor interaction that does not contribute to the total free energy of exchange. Any component of the nearest neighbor coupling that does contribute to total binding free energy is included in ΔGexc . This can be understood as follows: the total energy for exchanging all of ligand A with all ligand B on a nanocrystal surface is a sum of all individual ligand exchanges, i.e., ΔGtot = ∑ ΔGAB. Because the lattice model used here is isomorphic with the Ising model,28 and the site-to-site coupling energy in the Ising model is symmetric, the coupling term is equal for A−A and B−B interactions. In the sum ∑ ΔGAB, the coupling terms cancel because the coupling energy is the same for A−A and B−B interactions (see the Supporting Information). However, for intermediate exchanges, this is not the case because it takes a coupling energy ΔJ to break or form A−A interactions. Thus, this term represents the differences in nearest neighbor coupling free energy between the oleate ligands and the cinnamate ligands that drive the cooperative ligand exchange. Larger negative values cause the ligand exchange isotherm to exhibit a sharper transition from all-oleate coverage at low ligand addition to allcinnamate coverage at high ligand addition. This can be clearly seen for the 4CN2 and 4CN species (Figure 1a,b) compared with the 4OCH3 and N(CH3)2 (Figure 1g,h) and in the clustering evident in the surface coverage graphs for the 4(CN)2 exchange depicted in Figure 2. With this in mind, we can now interpret the behavior of ΔGexc as a function of the ligand dipole (Figure 3a). For values of the dipole moment close to zero, the ΔGexc is maximum at ∼+2−3 kBT. However, as the cinnamic acid dipole increases, the polarization of the QD surface and ligand shell acts to stabilize the molecular 3842

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Figure 3. Parameters of the 2D lattice simulation for ligand exchange. (a) The change in free energy for the ligand/QD binding vs the cinnamate dipole moment (triangles) and dashed line is a calculation of the electrostatic dipole−dipole interaction energy (described in text). (b) The ligand/ligand nonbinding interaction energy vs the dipole moment. (bottom) A cartoon depiction of the dipoles and tilt angle (measured from the NC surface) considered in the interaction energy between dipoles at the surface of the NCs (see the text).

dipolar ligands cause the bandedges of the QD films to shift by up to 2 eV.18 The shift of the band-edges (Δϕ) is related to the magnitude of the ligand dipole, μ, and the ligand tilt angle, γ: Δϕ = μcos(90 − γ )/(εd 2).32 The dielectric constant of the ligand monolayer, ε , is calculated accounting for cooperative effects (see the Supporting Information).33 We find that the ligands should be tilted by ∼70°−40° from the direction normal to the surface (see the Supporting Information for estimation), so γ varies between 20° and 50°. Using these angles as starting values we can estimate both the attractive and repulsive terms34 (see the Supporting Information for details), and we find a general agreement between the calculation (Figure 3a, dashed line) and our data (Figure 3a, triangles) with slightly adjusted tilt angles (Figure S6). We also find a correlation between the nearest neighbor coupling (ΔJ ) and the ligand dipole moment (Figure 4b). There is a consistent trend in ΔJ versus the ligand dipole, with the most electron withdrawing species having the largest negative ΔJ . As the ligand dipole changes from negative to positive, the ΔJ reduces in magnitude, approaching zero. We have not determined a clear physical reason responsible for the observed trend of ΔJ with the dipole moment. While recent literature35 argues that π-stacking interactions are really dominated by dipole interactions, there is no obvious reason why the direction of the dipole should change the magnitude of the coupling between ligands. As the ligand is titled (Figure 3, bottom), next-nearest neighbor interactions between the substituent group and the benzene ring may produce the observed trend. Constructing a Janus Ligand Shell. The negative coupling term, ΔJ , is indicative of how likely patches form during the ligand exchanges. In the Ising model, an order− disorder phase transition occurs when the coupling energy is less than −0.44 kBT.28 For our modified model, the phase transition occurs at the same coupling energy (Figure S3). The implications are that for ligands whose coupling energy is less than −0.44 kBT it is very unlikely to find one cinnamate ligand surrounded by only oleate ligands. Rather, cinnmates will tend

to group together (Figures 2 and S3) (that is, form segregated ligand patches). We can use this knowledge to build a Janus ligand shell, which we demonstrate using CF3-CAH, which has a coupling energy of −0.46 kBT and 3,5-F-CAH with a coupling energy of −0.6 kBT using NMR. Patchy and Janus ligand structures can be studied using NMR spectroscopy based on the composition dependence of the chemical shift; that is, the chemical shift of the substituent groups depend sensitively upon their chemical environment, an isolated (or solvated) cinnamate ligand will exhibit a different shift than will cinnamates that are packed together with nearest neighbors.24,36 The 1D NMR chemical shift of the ligands is expected to exhibit a linear, sigmoidal, or inverse dependence on the coverage, depending on whether the ligand morphology is random, striped, or Janus structured, respectively.36 Here, we measured the 19F chemical shift dependence on the coverage of CF3-CA− (Figure 4a) and compare it to 2,6-F-CA− (Figure 4b) on 3.4 nm PbS QDs (see Figures S9 and S10 for the raw data). A random distribution of new ligands and oleate ligands would exhibit a linear dependence of its chemical shift as the environment is a linear extrapolation of purely isolated to all cinnamate neighbors. However, the NMR shift, δ , for the CF3CA− is not linear in the extent of exchange, xA , and can be modeled by eq 5, which accounts for the partitioning of the ligands:36 δ = δB +

(δ I − δ B)t 2rxA

(5)

where δ B is the shift when the ligand is surrounded by like species, δ I is the shift when the ligand resides at the interface with unlike species, t is the thickness of the interfacial region (taken here to be 0.61 nm, the average distance between ligands), and r is the QD radius. The best-fit (red-trace) parameters are δ I = −63.66 ppm and δ B= −64.38 ppm. The nonlinear dependence occurs because at the early exchanges the chemical environment is changing as the ligand patch is small in a sea of dissimilar ligands, while at larger extents of exchange, larger patches form and the chemical environment is 3843

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Figure 4. (a) 19F chemical shift as a function of CF3-CA− exchange. The 1/xA dependence is indicative of patchy and cooperative ligand exchange. (b) 19F chemical shift as a function of 2,6-2F-CA− exchange. The linear dependence is indicative of the isotropic ligand shell during exchange. (c)2D NOESY spectrum of 3,5-F-CA−(69%)/OA− mixed coverage and (d) 2,6-F-CA−(53%)/OA− mixed ligand coverage. Resonances arise from the oleate methyl and ethyl groups (δ = 1−2.5 ppm), oleate vinyl (δ = 5.4 ppm), cinnamate aryl group (δ = 5.8−8 ppm), and sharp peaks are from the standards (ferrocene, δ = 4.15 ppm; hexafluoro isopropanol, δ = 4.5 ppm) and solvent (chloroform-d, δ = 7.2 ppm).

contour colors, is shown by the well-defined structure in the NOE cross-peaks for the 2,6-F-CA/OA− sample, which is not found in the 3,5-F-CA−/OA− spectrum. While there are weak cross-peaks found in the 3,5-F-CA−/OA− NOESY, they are not present at the magnitude shown in the 2,6-F-CA− NOESY, and this is a direct indication that there are much fewer through space interactions on the latter particles. The differences in ligand length is most likely the explanation for the weak cross-peaks in the 3,5-F-CA−/OA− NOESY. Because the NOE phenomenon is observed for distances up to 5 Å, it is highly likely that the floppy oleate ligands at the Janus CF3CA−/OA− interface are close enough to the cinnamate to give a small off-diagonal NOESY cross-peak.

not changing much as new ligands are added. In contrast, Figure 4b shows that the NMR shift exhibits a linear dependence with the exchange 2,6-F-CA− whose coupling energy is too low to form segregated patches and the shift is linearly proportional to the extent of exchange. To further confirm, we performed 2D NMR experiments on QDs with mixed ligand coverage. Adjacent ligands on a nanoparticle surface will demonstrate a nuclear Overhauser effect (NOE), which arises from through-space interactions, rather than through bond interactions.36,37 For a nanoparticle surface with mixed ligand composition, ligand shells with random arrangement will exhibit strong cross-peaks arising from interactions with the opposite ligand, whereas the Janus ligand morphology should show few to no cross-peaks. We prepared mixed ligand composition samples using the same procedure described above and acquired 2D NOESY (Figure 4b,c) of 3,5-F-CA−(69%)/OA− (Figure 4c) and 2,6-FCA−(53%)/OA− (Figure 4d) mixed compositions, respectively (a NOESY spectrum of CF3-CA−(43%)/OA− in shown in Figure S11) with a 2 s delay and 0.3 s mixing time. The 3,5-FCA−(69%)/OA− spectrum shows a weak NOE coupling (Figure 4c, off-diagonal components for the 3,5-F-CA−, green-shaded region, and oleate peaks, gray-shaded region) between the cinnamate and oleate ligands, while the 2,6-F-CA/ OA− spectrum shows strong coupling (Figure 4d). A measure of the difference in magnitude, aside from the difference in

CONCLUSIONS Quantitative spectrophotochemical titrations are employed to monitor ligand-exchange isotherms optically. We model these isotherms with a 2D square lattice model, which allows us to extract differences in trends of binding free energies and nearest neighbor coupling. As expected, oleate binds more strongly than any of the functionalized cinnamates, but the binding preference is mitigated by the dipole interactions for both large positive and large negative dipoles. We explain this trend in binding free energy as a function of dipole moment via a collective electrostatic interaction with the lattice and favorable head-to-tail dipole-dipole interactions as the dipolar 3844

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remaining dark pellets are discarded. To each centrifuge tube, approximately 30 mL of methyl acetate is added to precipitate the QDs and then centrifuged at 7000 rpm for 10 min. The cycle of precipitation and redissolution using toluene and methyl acetate is repeated three times. The QD product is dried under vacuum and suspended in hexane for storage.

ligands tilt on the QD surface. For cinnamic acids with electron withdrawing molecular dipoles (negative dipoles), the isotherms show behavior associated with strong nearest neighbor association that causes the ligand exchange reaction to display a phase transition from all-oleate coverage to allcinnamate coverage, with a sharpness dictated by the ligand dipole moment: more negative dipole moments leads to sharper order−disorder phase transitions than those observed with positive dipole moments as a function of ligand addition. Using these observations, we prepared PbS QD with Janus shell ligands.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b00191. Additional details on the methods for constructing the absorption isotherms, the 2D lattice model for simulating ligand exchanges, simulations of the order− disorder phase transition, the Hammet coefficient with the computed dipole moments, the derivation between the dipole moment and the free-energy of exchange, experimental calculations, and NMR results of the Janus ligand shell (PDF)

METHODS Materials. All chemicals were used as received without further purification unless noted. Anhydrous octane (≥99%), anhydrous diethylene glycol dimethyl ether (diglyme, 99.5%), N,N′-diphenylthiourea (98%), anhydrous toluene (99.5%), anhydrous tetrachloroethylene (TCE, ≥ 99.9%), anhydrous methyl acetate (MeOAc, 99%), anhydrous hexane (≥99%), anhydrous dichloromethane (DCM, ≥ 99.8%), anhydrous acetonitrile (ACN, 99.8%), anhydrous isopropanol (IPA, 99.5%), anhydrous chloroform-d (CDCl3, ≥ 99.8%), acetone (≥99.9%, degassed), anhydrous tetrahydrofuran (THF, ≥ 99.9%), 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP, ≥ 99%, degassed), transcinnamic acid (4-H-CAH, ≥ 99%), trans-2,6-difluorocinnamic acid (2,6-F-CAH, 99%), trans-3,5-difluorocinnamic acid (3,5-F-CAH, 99%), trans-4-(trifluoromethyl)cinnamic acid (4-CF3-CAH, 99%), 4methoxycinnamic acid, predominantly trans (4-OCH3-CAH, 99%), 4(dimethylamino)cinnamic acid, predominantly trans (4-N(CH3)2CAH, 99%), ferrocene (Cp2Fe, 98%), triethylamine (TEA, ≥ 99%), benzenethiol (4-H-SH, ≥ 98%), 4-aminobenzenethiol (4-NH2-SH, 97%), and 4-methylbenzenethiol (4-CH3-SH, 98%) were obtained from Sigma-Aldrich. 4-(Trifluoromethyl)benzenethiol (4-CF3-SH, 97%) was obtained from Alfa Aesar. 4-(2,2-Dicyanovinyl)cinnamic acid (4-(CN)2-CAH). Solvent compositions are given in Table 1.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Noah D. Bronstein: 0000-0003-3657-2652 Márton Vörös: 0000-0003-1321-9207 Haipeng Lu: 0000-0003-0252-3086 Nicholas P. Brawand: 0000-0002-0624-3666 Arthur J. Nozik: 0000-0001-7176-7645 Alan Sellinger: 0000-0001-6705-1548 Matthew C. Beard: 0000-0002-2711-1355 Author Contributions

Table 1. Solvent compositions for the various ligand exchanges

○

N.D.B. and M.S.M. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge support through the photochemistry program within the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, Office of Science within the US Department of Energy through contract no. DE-AC36-08G028308 for the development of the Janus shell nanocrystals and the 2D lattice model. M.V., H.L., A.J.N., and G.G. aknowledge support from the Center for Advanced Solar Photophysics (CASP), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. N.D.B. acknowledges support from a Director’s fellowship at NREL. M.V. was supported by Laboratory Directed Research and Development (LDRD) funding from Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under contract no. DE-AC0206CH11357. A.S. was supported through Colorado School of Mines start-up funds.

**

Heating and sonication were sometimes necessary to solubilize the ligand. The addition of neat ligand solvent had no significant effect on QD absorbance spectrum in 6:1 ratios of DCM/ligand solvent.

Oleate-Capped PbS QD Synthesis. Oleate-capped PbS QDs were synthesized following the substituted thiourea protocol developed by Hendricks et al.29 In a nitrogen glovebox, 8.81 g of Pb(oleate)2 and 150 mL of anhydrous octane are added to a 2-neck 250 mL Schlenk flask and sealed. A total of 1.74 g of N,N′diphenylthiourea and 5 mL of diglyme are mixed in a 20 mL scintillation vial and sealed. Both vessels were brought to 95 °C in an oil bath under nitrogen and allowed to stir for ∼30 min (both solutions were clear). The N,N′-diphenylthiourea diglyme solution is injected into the Pb(oleate)2 octane solution under vigorous stirring and allowed to react for 60 s, removed, and allowed to cool to room temperature. The flask is transferred to a nitrogen-filled glovebox and dispersed in ∼40 mL of toluene and centrifuged at 7000 rpm for 10 min. The brown nanocrystal solution was decanted, and the

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DOI: 10.1021/acsnano.9b00191 ACS Nano 2019, 13, 3839−3846