CdSe Quantum Dots Functionalized with Chiral, Thiol-Free Carboxylic

Sep 28, 2017 - CdSe Quantum Dots Functionalized with Chiral, Thiol-Free Carboxylic Acids: Unraveling Structural Requirements for Ligand-Induced Chiral...
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CdSe Quantum Dots Functionalized with Chiral, Thiol-Free Carboxylic Acids: Unraveling Structural Requirements for Ligand-Induced Chirality Krisztina Varga,*,† Shambhavi Tannir,‡ Benjamin E. Haynie,† Brian M. Leonard,‡ Sergei V. Dzyuba,§ Jan Kubelka,*,‡ and Milan Balaz*,∥ †

Department of Molecular, Cellular, and Biomedical Sciences, University of New Hampshire, 46 College Road, Durham, New Hampshire 03824, United States ‡ Department of Chemistry, University of Wyoming, 1000 East University Avenue, Laramie, Wyoming 82071, United States § Department of Chemistry and Biochemistry, Texas Christian University, Fort Worth, Texas 76129, United States ∥ Underwood International College, Integrated Science and Engineering Division, Yonsei University, Seoul 03722, Republic of Korea S Supporting Information *

ABSTRACT: Functionalization of colloidal quantum dots (QDs) with chiral cysteine derivatives by phase-transfer ligand exchange proved to be a simple yet powerful method for the synthesis of chiral, optically active QDs regardless of their size and chemical composition. Here, we present induction of chirality in CdSe by thiol-free chiral carboxylic acid capping ligands (L- and D-malic and tartaric acids). Our circular dichroism (CD) and infrared experimental data showed how the presence of a chiral carboxylic acid capping ligand on the surface of CdSe QDs was necessary but not sufficient for the induction of optical activity in QDs. A chiral bis-carboxylic acid capping ligand needed to have three oxygen-donor groups during the phase-transfer ligand exchange to successfully induce chirality in CdSe. Intrinsic chirality of CdSe nanocrystals was not observed as evidenced by transmission electron microscopy and reverse phase-transfer ligand exchange with achiral 1-dodecanethiol. Density functional theory geometry optimizations and CD spectra simulations suggest an explanation for these observations. The tridentate binding via three oxygen-donor groups had an energetic preference for one of the two possible binding orientations on the QD (111) surface, leading to the CD signal. By contrast, bidentate binding was nearly equienergetic, leading to cancellation of approximately oppositely signed corresponding CD signals. The resulting induced CD of CdSe functionalized with chiral carboxylic acid capping ligands was the result of hybridization of the (achiral) QD and (chiral) ligand electronic states controlled by the ligand’s absolute configuration and the ligand’s geometrical arrangement on the QD surface. KEYWORDS: chiral quantum dots, ligand-induced optical activity, nanoparticles, quantum chemical simulations, density functional theory, malic acid, tartaric acid

T

spectra revealing the importance of not only the absolute configuration of the capping ligand but also of its chemical structure (amount and position of different functional groups).8 Our quantum chemical simulations suggested that the chirality of the QDs was the result of hybridization of electronic states of

he physical, chemical, electronic, and chiroptical properties of quantum dots (QDs) can be easily modified by small organic molecule capping ligands.1−7 We have shown that D- and L-cysteine-induced mirror-image circular dichroism (CD) and circularly polarized luminescence (CPL) in CdSe QDs via post-synthetic phase-transfer ligand exchange.1,2 Moreover, functionalization of CdSe and CdS QDs with cysteine and its derivatives (i.e., N-acetyl-cysteine, homocysteine, and penicillamine) resulted in different CD © 2017 American Chemical Society

Received: May 21, 2017 Accepted: September 28, 2017 Published: September 28, 2017 9846

DOI: 10.1021/acsnano.7b03555 ACS Nano 2017, 11, 9846−9853

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ACS Nano achiral QD and chiral ligand,1 where the sign of the resulting induced CD of the QDs was determined by both the geometrical arrangement of the ligand on the QD surface and the ligand’s absolute configuration.8 Combined with the quantum size effect, the post-synthetic functionalization of achiral QDs with chiral ligands represented a simple and straightforward method for the synthesis of colloidal QDs with tunable chemical, optical, and chiroptical properties. Chiral QDs are attractive building blocks for the bottom-up nanofabrication of chiral materials as well as promising candidates for a broad range of applications such as sensing, stereoselective synthesis, and bioimaging.9−19 To date, the syntheses of all reported optically active colloidal QDs utilized chiral cysteine derivatives (i.e., chiral thiols). Although QDs functionalized by thiol-free carboxylic acids ligands bound to the QDs surface via carboxylate functional group have been reported,20−22 their chiroptical properties were not explored in detail. To expand on our understanding of ligand-induced chirality in QDs, we studied a series of 13 chiral, thiol-free Cα- and Cβ-substituted mono- and bis-carboxylic acids as chiral capping ligands for CdSe QDs. Unlike the chiral cysteine derivatives which were limited structurally,8 the pool of chiral carboxylic acids is quite large and structurally diverse. Functionalizing CdSe QDs with carefully selected chiral carboxylic acids allowed us to explore in more detail the role of each capping ligand’s chemistry and structure on the chiroptical activity of QDs. Namely, we studied the effect of the type, number and position of functional groups (e.g., carboxyl, hydroxyl, amine, alkyl, and carbonyl) within the chiral capping ligand and the length of the carbon chain as well as the distance between functional groups and the anchoring group or the chiral center. Together, the experiments and simulations provided important insights into chiral, thiol-free ligand-induced chirality of QDs.

to yield predominantly monomalate (pKa = 3.4 and 5.2). Minimal phase transfer was observed under pH conditions that preferably generated (i) a fully deprotonated dianion of MA (i.e., pH > 5.5) or (ii) a neutral diacid (i.e, pH < 2.5). Partial deprotonation of MA ensured formation of amphiphilic species with a single negative charge that had increased solubility in an organic-aqueous biphasic system. Similarly, L- and D-Asp-CdSe, L- and D-MSA-CdSe, and L-BrSA-CdSe QDs were prepared at pH ∼ 4.4 (Asp; pKa = 2.1, 3.9, and 9.8), ∼4.5 (MSA; pKa = 4.1, 5.6), and ∼4.0 (BrSA; pKa = 2.6, 4.4), respectively. UV−vis absorption spectra revealed a high degree of aggregation of all carboxylic acid capped CdSe, as evidenced by the increased baseline at wavelengths above the band gap absorption (Figures 1c and S2). The addition of a fluoride anion as a hydrogen-

RESULTS AND DISCUSSION We started our studies with four chiral bis-carboxylic butanedioic acids (Chart 1) that shared the same carbon Chart 1. Structures of Chiral Carboxylic Acids with Different Functional Groups on Cα

Figure 1. CD spectra of L-MA-CdSe and D-MA-CdSe (a) ØCdSe = 3.0 nm and (d) ØCdSe = 4.4 nm. (b) CD spectra of L-Cys-CdSe and DCys-CdSe (ØCdSe = 3.0 nm). (c) UV−vis spectra of L-MA-CdSe and L-Cys-CdSe (ØCdSe = 3.0 nm).

bond acceptor23 failed to reduce the aggregation of carboxylic acid capped CdSe. Comparison of X-ray powder diffraction (XRD) data of OA-CdSe and carboxylic acids functionalized CdSe showed, in accordance with previous reports,8 that (i) phase-transfer ligand exchange did not induce any observable changes to the zinc blende CdSe crystal structure, and (ii) the primary peak at 2θ ≈11.4° was indexed to the (111) lattice plane (Figures S1 and S3). MA was the only acid that successfully induced chirality in CdSe QDs, while CdSe QDs (ØCdSe = 3.0 nm) functionalized with L-Asp, D-Asp, L-MSA, D-MSA, or L-BrSA did not give rise to any reproducible CD signal (Figure 1a).24 The CD spectrum of L-MA-CdSe (Figure 1a, red curve; ØCdSe = 3.0 nm) displayed an alternating (+/−/+/−/+; from long to short wavelengths) CD pattern with positive Cotton effects at 538, 490, and 438 nm and negative Cotton effects at 517 and 463 nm. As expected, D-MA and L-MA-induced mirror image CD spectra in CdSe QDs (Figure 1a, blue and red curves) confirming that

framework, but differed in the functional group attached to the C2 (i.e., Cα) stereogenic center, that is, L- and D-malic acid (2hydroxy group), L-and D-aspartic acid (2-amino group), L- and D-methyl-succinic acid (2-methyl group), and L-bromo-succinic acid (2-bromo group). All chiral ligand functionalized CdSe QDs were prepared by an organic-aqueous phase-transfer ligand exchange procedure from OA-CdSe QDs.1,2 The successful phase-transfer reactions were performed at acidic pH using tetramethylammonium hydroxide (TMAH) to adjust the pH (see SI for full experimental details). L- and D-MA-CdSe QDs were prepared at the acidic pH of ∼4.3, where MA was partially deprotonated 9847

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Table 1. CD Anisotropy g factors for MA-CdSe, TA-CdSe and Cys-CdSe QDs (ØCdSe = 3.0 nm)

thiol-free ligands successfully induced optical activity in CdSe QDs. To determine whether the induction of the CD (or lack of thereof) upon CdSe functionalization with MA, Asp, MSA, and BrSA was CdSe size dependent, we performed ligand exchange on CdSe QDs of different diameters. The L- and D-MA successfully induced mirror-image CD spectra in all studied diameters of CdSe QDs from D = 2.3 nm (λmax = 491 nm) to D = 4.4 nm (λmax = 596 nm) (Figures 1a and S4). The CD anisotropy g factor values of L-MA-CdSe of different diameters are listed in Table S2. The UV−vis absorption and emission spectra of MA-CdSe revealed that the band gap absorption and emission wavelengths have only marginally been affected by the phase-transfer ligand exchange procedure (Figure S5). However, the ligand exchange caused a strong decrease of the fluoresce intensity of MA-CdSe in comparison to OA-CdSe for all studied diameters. Small diameter CdSe displayed a larger proportion of the deep trap state emission similar to the staring material OA-CdSe (Figures S1 and S5). On the other hand, no detectable CD was induced by Asp, MSA, or BrSA regardless of the diameter of CdSe QDs. It should be noted that free MA, Asp, MSA, and BrSA chiral carboxylic acid capping ligands (i.e., not bound to the CdSe QDs) displayed CD signals only below 250 nm (Figure 2). Therefore, the observed CD signal of MA-CdSe > 400 nm originated from the interactions between the chiral carboxylic acids and CdSe QDs.

QDsa L-MA-CdSe L-Cys-CdSe L-TA-CdSe

g(CD+)b −5

g(CD−)c

+4.4 × 10 (539 nm) +12.4 × 10−5 (518 nm) +5.6 × 10−5 (515 nm)d d

−4.8 × 10−5 (517 nm)d −8.9 × 10−5 (536 nm) −4.3 × 10−5 (541 nm)d

a

ØCdSe = 3.0 nm. bPositive CD band. cNegative CD band. dUV−vis spectra have been adjusted to account for aggregation.

electron-deficient, 2- and 3-coordinated Cd atoms on the surface of CdSe nanocrystal. L-BrSA also had three donor groups at the pH of the exchange (two carboxyl/carboxylate groups and one bromine) but only two O-donor groups and failed to induce chirality. Asp and MSA that possessed two Odonor groups each (i.e., two carboxyl/carboxylate groups) at the pH of the exchange (the Cα amino group of Asp was fully protonated), and both failed to induce chirality in CdSe. It appears that two carboxyl groups and a Cα hydroxyl group must interact with the surface of the CdSe in the phase-transfer ligand exchange to successfully induce chirality in CdSe. To examine the possibility of chiral ligand-induced intrinsic chirality26,27 of CdSe nanocrystals, we have performed highresolution transmission electron microscopy (TEM) and reverse phase-transfer ligand exchange experiments. TEM images of chiral L-MA-CdSe and achiral BrSA-CdSe both show particles with an average particle size of 4.4 nm (Figure S6). All samples showed some degree of aggregation confirming the results observed via UV−vis spectroscopy. The CdSe particles showed predominantly 111 lattice planes with a spacing of 3.48 Å as determined by analysis of the highresolution TEM data (inset Figure S6). A few TEM images showed wavy or curved lattice lines that we hypothesize originated from the convergence of different crystal surfaces (edge sites) or curvature of the particle projected onto a 2D image (Figure S7). However, these curved lattice lines were rarely observed and surrounded by several particles with straight lattice lines. Further, after careful analysis of hundreds of QDs, nearly all particles showed straight lattice lines. Moreover, replacement of the L-MA in L-MA-CdSe with achiral 1-dodecanethiol (DDT) by reverse phase-transfer ligand exchange yielded achiral, toluene soluble DDT-CdSe QDs as evidenced by the absence of CD signal (Figure S8). TEM and reverse phase exchange thus did not provide supporting evidence for the L-MA-induced intrinsic chirality of CdSe nanocrystals (i.e., no ligand-induced chiral distortion of the CdSe surface). To further explore whether two carboxyl and/or O-donor groups were necessary for chirality induction in CdSe, we prepared QDs functionalized with eight derivatives of butanoic and propanoic carboxylic acids (Chart 2). These eight acids shared a common framework with either MA or Asp: They all had chiral centers at Cα with either a Cα hydroxy group (MA derivatives) or a Cα amino group (Asp derivatives) but differed in the type and number of functional groups (carboxyl/ carboxylate, amino/ammonium, methyl and hydroxyl) at Cβ and Cγ carbons (see SI for phase-transfer ligand exchange conditions, the UV−vis absorption spectra, and XRD profiles). Out of seven derivatives tested, only TA-induced chirality successfully in CdSe QDs, whereas remaining derivatives of MA (i.e., L-LA, L-HMBA, L-AHBA and L-Ise) and all derivatives of Asp (i.e., L-Ser, L-Hse and L-Thr) failed to induce optical activity in CdSe, and no CD signal was observed. L-TA functionalized

Figure 2. CD spectra of L- and D-MA, L- and D-Asp, L- and D-MSA, Land D-TA. [Amino acid] = 1.0 mM in Na-cacodylate buffer (1 mM, pH = 7.0).

A comparison of CD spectra of L-MA-CdSe and L-Cys-CdSe1 (Figure 1a,b; ØCdSe = 3.0 nm) revealed similar but offset (+/−/+/−) CD patterns. The CD signal for the band gap absorption appeared to be net positive for L-MA-CdSe (negative for D enantiomer), and a CD couplet was observed for Cys-CdSe. As the result of the offset, the L-MA-CdSe and LCys-CdSe showed mirror-image CD signs within the band gap absorption: the L-MA-CdSe displayed a positive CD band at 539 and a negative CD band at 517 nm (+/−), while L-CysCdSe displayed a negative CD band at 536 nm and a positive CD band at 518 nm (−/+). CD anisotropy g-factors showed that MA induced a 2−3 times lower degree of dissymmetry in CdSe QDs than the Cys (Table 1). CD anisotropy g-factors for D-MA-CdSe of different diameters (ØCdSe = 2.2, 2.5, 3.5, and 4.4 nm) are listed in Table S2. The structure of the capping ligand (i.e., functional groups at Cα carbon) had a clear effect on induction of chirality in CdSe QDs by phase-transfer ligand exchange. MA had two carboxyl/ carboxylate groups and one hydroxyl group at the pH of ligand exchange (pH ∼ 4.3) and successfully induced chirality in CdSe QDs. In other words, MA had three oxygen-containing electron-donor groups (i.e., possessing at least one electron pair each; a tridentate ligand)25 that could interact with 9848

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Based on our current CD experimental data, our previous results8 and literature reports on MA and TA on metal surfaces,28−32 we have formulated the following working hypothesis for carboxylic acid-induced chirality in CdSe QDs: The interaction of two carboxyl groups and one hydroxyl group with the electron-deficient, 2- and 3-coordinated Cd atoms in the CdSe nanocrystal surface is imperative for induction of chirality. The hydroxyl group could additionally be involved in hydrogen bonding with neighboring, surface carboxylic acid ligands. The binding geometry of alkane-biscarboxylic acids (alkanedioic acids) on QD surfaces has not been explored previously, but experimental studies have shown that simple carboxylic acids (e.g., acetic acid) bound to the surface Cd atoms in a carboxylate form,33−36 while DFT theoretical simulations have identified bridge (two carboxylate oxygens linked to two Cd atoms) or chelate (two carboxylate oxygens bound to one Cd) binding modes as the most energetically favorable on the (111) CdSe surface.37 MA and TA have been reported to form multidentate binding on Cu surfaces via one or two carboxyls depending on the experimental conditions and to form well-defined patterns.28−32,38 Our previous studies on cysteine functionalized CdSe and CdS have shown that multidentate binding was energetically favorable and could led to the patterned organization of ligands on the QDs’ surface and chirality.8 Additional information about MA binding to CdSe surface came from attenuated total reflectance infrared (ATR-IR) data of solid L-MA-CdSe together with (i) neat L-MA (neutral diacid) and (ii) L-MA salt samples prepared at different pH using tetramethylammonium hydroxide (TMAH) as a base (Figure 4). IR spectrum of neat L-MA was dominated by the

Chart 2. Structure of Chiral Capping Ligands with Different Functional Groups on Cβ and Cγ: Five MA Derivatives and Three Asp Derivatives

CdSe QDs displayed an alternating (−/+/−/+/−) CD spectra, while D-TA-CdSe showed (+/−/+/−/+) CD spectra in all studied diameters of CdSe (Figures 3 and S9). Importantly, L-

Figure 3. CD spectra of (a) L-TA-CdSe and D-TA-CdSe (ØCdSe = 3.0 nm) and (b) L-TA-CdSe and L-MA-CdSe (ØCdSe = 4.4 nm).

TA-CdSe and L-MA-CdSe CD spectra displayed a mirror-image CD profiles regardless of the CdSe diameter (Figure 3b) and opposite values of CD anisotropy g-factors (Table 1). These data could be easily rationalized by the opposite R/S absolute configurations yet identical functional groups at the stereocenters of L-MA (2S) and L-TA (2R,3R). Free L-MA and L-TA acids also displayed mirror-image CD spectra (Figure 2) and opposite signs of specific rotations (L-(−)-MA vs L-(+)-TA)). Comparison of MA with its derivatives from Chart 2 revealed that a virtual substitution of the Cγ carboxyl group by (i) a hydrogen (MA → LA), (ii) an ammonium-methanediyl group (MA → L-AHBA), (iii) a methyl group (MA → L-HMBA), or (iv) an ammonium group (MA → L-Ise) caused the carboxylic acid capping ligand to fail to induce the chirality in CdSe, while (v) an addition of hydroxyl group on the cβ carbon (MA → TA) did not impair the chirality induction in CdSe. None of the Asp derivatives with β-ammonium group gave rise to CD signal in CdSe regardless of the functional group attached on the Cβ carbon (Chart 2). As expected, due to the reaction conditions (pH < 5), the effect of the unprotonated amino group (i.e., Ndonor group) on the chirality induction in the post-synthetic phase-transfer ligand exchange was not possible to assess.

Figure 4. ATR-IR spectra of (a) L-MA (neat, neutral diacid), (b) LMA (prepared from a solution at pH = 4.3), (c) L-MA (pH = 5.2), and (d) L-MA-CdSe QDs (pH = 5.2; ØCdSe = 4.4 nm). The peak at ∼1489 cm−1 corresponds to asymmetric methyl deformation mode, δasym(CH3) of TMAH.42

ν(CO) stretching mode (1696 cm−1; Figure 4a).39,40 The LMA sample prepared at pH = 4.3 (i.e., pH of the ligand exchange; Figure 4b) displayed the ν(CO) stretching modes of L-MA (∼1693 cm−1) and mono-L-malate anion (1644 cm−1) together with asymmetric νas(OCO) stretching mode of the bis-L-malate dianion (1581 cm−1; Figure 4b).39,40 The IR spectrum of solid L-MA from pH = 5.2 (i.e., pH of the aqueous solution of L-MA functionalized CdSe) has shown mostly the νas(OCO) and νs(OCO) stretching modes of the bis-malate dianion (1574 and 1378 cm−1; Figure 4c). Both samples (pH 9849

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ACS Nano 4.3 and 5.2) have also shown the δasym(CH3) of TMAH (∼1489 cm−1). The IR spectrum of L-MA-CdSe (D = 4.4 nm) displayed strong asymmetric νas(OCO) and symmetric νs(OCO) stretching modes (1554 and 1416 cm−1, respectively; Figure 4d), while the ν(CO) stretching modes of L-MA (∼1696 cm−1) or surface anchored unidentate monomalate anion (∼1654 cm−1 on Cu surface)41 were missing altogether, suggesting the presence of bis-malate dianion. The separation between the asymmetric and symmetric stretching mode, Δν = νas(OCO) − νs(OCO), has successfully been used as an empirical tool to identify bridge, chelate, or unidentate binding modes of carboxylates to various surfaces including CdSe nanocrystals (see Figure 5).43 For L-MA-CdSe

described earlier, Asp had two carboxylate groups at the pH of the ligand exchange (pH = 4.5; pKa values of L-Asp = 2.1, 3.9, and 9.8) since the Cα amino group was protonated (NH3+). To access the N-donor group (NH2 group was reported to bind to the CdSe surface),44−47 we have performed the phase-transfer ligand exchange reactions with L-Asp at pH = 10, 11, or 12. Unfortunately, no phase transfer was observed. Next, we have adjusted the pH of the L-Asp-CdSe QDs solution postsynthetically from pH = 4.5 to either 10, 11, or 12 using TMAH and sodium hydroxide (NaOH). However, the ammonium group deprotonation did not result in chirality induction in L -Asp-CdSe QDs, as no CD signal was detected. It seemed likely that the chiral reorganization of the ligand’s binding geometry and binding pattern on the surface of QDs did not take place. In the third approach, N-acetylation of L-Asp prevented the protonation of the nitrogen during the phase-transfer ligand exchange and at the same time introduced a less basic carbonyl as the third O-donor group. The ligand exchange performed with N-acetyl-L-aspartic acid (N-Ac-L-Asp) at pH = 4.1 on OACdSe successfully induced chirality in QDs (Figure 6). We

Figure 5. Bridge (left), chelate (center), and unidentate (right) geometries of acetate anion of the (111) surface of CdSe fragment.

the Δν = 138 cm−1 suggested the bridging binding mode, where four oxygens of two carboxylate groups interacted with four cadmium atoms. The absence of a well-defined δ(OH) vibration mode of MA was caused either by a strong interaction of the OH group with the surface or by strong intra- or intermolecular hydrogen bonds. Furthermore, missing an asymmetric methyl deformation mode, δasym(CH3), of TMAH in the L-MA-CdSe sample suggested that TMAH has been removed, and the carboxylate groups are bound to positively charged Cd atoms on the QD surface. Overall, the IR data have supported our hypothesis that L-MA (in the form of the bismalate dianion) interacted with the CdSe surface via both carboxylate groups and the hydroxyl group, where the latter could concurrently participate in intra- or intermolecular hydrogen bonds. A very similar IR spectrum dominated by asymmetric νas(COO) and symmetric νs(COO) stretching bands was also obtained for L-TA-CdSe advocating for similar binding mode as MA (Figures S10−S11). To test our hypothesis of multidentate surface binding further, we explored three strategies. In the first approach, we attempted to introduce a N-donor group by preparing AspCdSe at pH > 10. In the second approach, we deprotonated the Cα ammonium group of L-Asp-CdSe by increasing the pH (Scheme 1) to introduce a N-donor group post-synthetically. In the third approach, we introduced an acetyl group (CH3CO) on the amino group of Asp prior to the ligand exchange. As

Figure 6. CD spectra of (a) N-Ac-L-Asp-CdSe and (b) L-MA-CdSe. ØCdSe = 3.4 nm.

hypothesize that the carbonyl oxygen of the acetyl group binds to the CdSe surface together with the two carboxylate groups. As predicted, unprotected L-Asp with two carboxyl groups failed to induce chirality in CdSe QDs. To further explore our hypothesis, we performed density functional theory (DFT), PBEPBE/3-21G/sbkjc-VDZ* level geometry optimizations (for details see Methods and SI) of chiral carboxylic acids (L-MA, D-TA, L-Asp, N-Ac-L-Asp) functionalized Cd9Se7 QD cluster constructed from the zinc blende crystalline form cut along the (111) surface.8 The DFT calculations showed a clear preference for bridge-type binding (Figure 5) on both carboxylate groups along with binding by the OH group of L-MA (Figure 7). Comparing multiple geometry optimizations starting from different initial arrangements, which converged to local minima with one or both groups forming chelate of unidentate-type bonds (Figure 5), the bis-carboxylate bridge/bridge/OH binding was favored by at least 10 kcal/mol. The same type of binding was consistently found to be the most favorable for other ligands with O-donor groups: D-TA and N-Ac-L-Asp (Figure S13). Note that while DTA has two OH groups (four O-donor groups), the binding of the fourth O (i.e., tetradentate binding) is not geometrically possible. By contrast, the L-BrSA, in which the Br is a third donor group, failed to show any propensity for tridentate binding, with the Br consistently shifting away from the QD surface during optimizations (Figure S14b).

Scheme 1. Introduction of Three Donor Groups on L-Asp by Deprotonation of the Ammonium Group (Left) and Acetylation of the Amino Group (Right)

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further supported by the good agreement between the experimental and simulated CD spectra for L-MA (Figures 1a and 7c) as well as for D-TA and N-Ac-L-Asp functionalized CdSe (SI). By contrast, the absence of the third O-donor group lifted the geometric constraints of tridentate binding and the energy difference between the two arrangements was much smaller, ≲2 kcal/mol for L-Asp and L-BrSA (SI). Although the absolute energy values were not accurate enough to draw quantitative conclusions, the differences in the relative binding preferences for one ligand orientation vs the other highlighted the importance of the binding geometry in determining the induced CD signals, which is consistent with our previous report.8

CONCLUSIONS We performed a systematic study on thiol-free carboxylic acidinduced chirality in colloidal CdSe QDs via post-synthetic ligand exchange from OA-CdSe. We described the example of chiral, thiol-free ligand-induced optical activity in water-soluble CdSe QDs. The structure of the carboxylic acid capping ligand influenced the yield of the ligand exchange reactions as well as the appearance and shape of the CD signal. Our data showed that the presence of a chiral capping ligand on the surface of CdSe QDs was necessary but not sufficient for induction of optical activity in QDs. Replacement of the L-MA in L-MACdSe with achiral 1-dodecanethiol (DDT) yielded achiral QDs, and no intrinsic chirality was observed. Based on our experimental CD and IR data, we proposed that interaction of two carboxylate groups and either one hydroxyl (MA or TA) or one carbonyl group (N-Ac-L-Asp) with the electrondeficient, 2- and 3-coordinated Cd atoms of the nanocrystal surface was required for chirality induction in CdSe QDs. The DFT-based geometry optimizations and CD spectra simulations fully support this conclusion and, consistent with our previous report, highlight the significance of the ligand binding geometry (i.e., binding pattern) for inducing the characteristic CD signals of the QD. The resulting CD signal of CdSe capped with chiral carboxylic acids was the result of hybridization of the (achiral) QD and (chiral) ligand electronic states controlled by the absolute configuration of the ligand and its geometrical arrangement on the QD surface. Presented data on thiol-free chiral carboxylic acid functionalized CdSe QDs further expanded our comprehension of ligand-induced chirality in colloidal QDs and will contribute to tailor-made chiral semiconducting nanomaterials.

Figure 7. Theoretical modeling of distinct QD-ligand binding modes and resulting CD spectra. (a) Schematic illustration of two multidentate binding geometries of L-MA dianion (i.e., bis-malate) on the (111) CdSe surface, leading to the oppositely signed CD spectra. The most energetically favorable binding configuration (left) and the higher energy alternative (right). (b) Corresponding molecular structures of L-MA bound to the model CdSe cluster optimized by DFT. (c) Simulated CD spectra for the model structures in (b): CD for the minimum energy L-MA-CdSe structure (red) and the alternative (green).

Moreover, through comparison of the energetics of the different binding modes and simulation of the corresponding CD (Figure 7c) using time-dependent DFT (TD-DFT, see Methods), the theoretical modeling suggested an explanation for the absence of the CD in QD complexes with chiral ligands with only two O-donor groups. We previously proposed that the sign pattern of the induced CD depends on the specific mode of ligand binding.6 Specifically, there were two distinct orientations for L-MA,6 which resulted in approximately oppositely signed CD spectra (Figure 7). If one of these binding arrangements is significantly energetically favorable, it is expected to lead to the predominant, periodically repeating patterns of chiral ligand arrangement on the QD surface and consequently to a strong observable CD signal. On the other hand, if both orientations are comparable in energy, there will be a statistical mixture of both, effectively canceling the CD, similarly to the spectra of the racemic mixture. For the carboxylic acid ligands studied here, the third anchoring Odonor group seems to be critical for stabilizing one of the arrangements (Figure 7a,b; left) over the other (Figure 7a,b; right). The much better fit of the oxygen atoms to the surface Cd is evident already from the schematic representations (Figure 7a), but borne out quantitatively in DFT geometry optimizations (Figure 7b), which yielded the difference of ≳6 kcal/mol for the L-MA. The reliability of the modeling was

METHODS General Procedure for Phase-Transfer Ligand Exchange Synthesis and Characterization of Colloidal CdSe QDs Functionalized with Chiral, Thiol-Free Carboxylic Acids. An aqueous solution of chiral carboxylic acid was added to a toluene solution of oleic acid-capped CdSe, and the resulting heterogeneous mixture was deoxygenated and then vigorously stirred at RT under N2 in the absence of light for 2 h. The aqueous layer was removed, and the CdSe QDs functionalized with chiral carboxylic acids were precipitated with ethanol and separated by centrifugation. Isolated CdSe QDs were redissolved in deionized H2O and characterized. Theoretical Modeling. All quantum chemical calculations were done at density functional theory (DFT) and time-dependent DFT (TD-DFT) levels using Gaussian 09 software.48 All geometries were optimized with PBEPBE functional,49 while TD-DFT calculations were carried out using CAM-B3LYP. The same basis set, sbkjcVDZ*,1,50 (see SI) for Cd, Se, and Br and 3-21G for all lighter atoms was employed. Aqueous solvent was approximated by the 9851

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ACS Nano conductor-like polarized continuum model (CPCM) with default parameters for water.51 The UV and CD spectral contours were simulated from the computed wavelengths and intensities of the individual transitions by assuming Gaussian line shapes with a constant width of 20 nm.

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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/acsnano.7b03555. Synthetic procedures; UV−vis absorption and emission spectra, size-dependent CD spectra, HRTEM images, ATR-IR spectra, and XRD data of CdSe QDs functionalized with chiral carboxylic acids; relative energies of QD-ligand optimized structures and simulations of CD spectra for QD-ligand geometries (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Krisztina Varga: 0000-0003-2810-0997 Brian M. Leonard: 0000-0002-9185-2473 Milan Balaz: 0000-0002-6108-3896 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported in part by the National Science Foundation (award CBET 1403947; K.V., S.V.D., J.K., M.B.), Yonsei University (M.B.), and the University of New Hampshire (K.V.). REFERENCES (1) Tohgha, U.; Deol, K. K.; Porter, A. G.; Bartko, S. G.; Choi, J. K.; Leonard, B. M.; Varga, K.; Kubelka, J.; Muller, G.; Balaz, M. Ligand Induced Circular Dichroism and Circularly Polarized Luminescence in CdSe Quantum Dots. ACS Nano 2013, 7, 11094−11102. (2) Tohgha, U.; Varga, K.; Balaz, M. Achiral CdSe Quantum Dots Exhibit Optical Activity in the Visible Region Upon Post-Synthetic Ligand Exchange with D- or L-Cysteine. Chem. Commun. 2013, 49, 1844−1846. (3) Baker, D. R.; Kamat, P. V. Tuning the Emission of CdSe Quantum Dots by Controlled Trap Enhancement. Langmuir 2010, 26, 11272−11276. (4) 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. (5) Evans, C. M.; Cass, L. C.; Knowles, K. E.; Tice, D. B.; Chang, R. P. H.; Weiss, E. A. Review of the Synthesis and Properties of Colloidal Quantum Dots: The Evolving Role of Coordinating Surface Ligands. J. Coord. Chem. 2012, 65, 2391−2414. (6) Zhou, Y.; Yang, M.; Sun, K.; Tang, Z.; Kotov, N. A. Similar Topological Origin of Chiral Centers in Organic and Nanoscale Inorganic Structures: Effect of Stabilizer Chirality on Optical Isomerism and Growth of CdTe Nanocrystals. J. Am. Chem. Soc. 2010, 132, 6006−6013. (7) Ben-Moshe, A.; Maoz, B. M.; Govorov, A. O.; Markovich, G. Chirality and Chiroptical Effects in Inorganic Nanocrystal Systems with Plasmon and Exciton Resonances. Chem. Soc. Rev. 2013, 42, 7028−7041. 9852

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DOI: 10.1021/acsnano.7b03555 ACS Nano 2017, 11, 9846−9853