CdSe Quantum Dots Functionalized with Chiral, Thiol-Free Carboxylic

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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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b03555 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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CdSe Quantum Dots Functionalized with Chiral, Thiol-Free Carboxylic Acids: Unraveling Structural Requirements for Ligand Induced Chirality Krisztina Varga,a* Shambhavi Tannir,b Benjamin E. Haynie,a Brian M. Leonard,b Sergei V. Dzyuba,c Jan Kubelka,b* and Milan Balazd* (a) Department of Molecular, Cellular, and Biomedical Sciences, University of New Hampshire, 46 College Road, Durham, New Hampshire 03824, USA. E-mail: [email protected] (b) Department of Chemistry, University of Wyoming, 1000 East University Avenue, Laramie, Wyoming 82071, USA. Email: [email protected] (c) Department of Chemistry and Biochemistry, Texas Christian University, Fort Worth, Texas 76129, USA. (d) Underwood International College, Integrated Science & Engineering Division, Yonsei University, Seoul, 03722, Republic of Korea. E-mail: [email protected] KEYWORDS chiral quantum dots, ligand-induced optical activity, nanoparticles, quantum chemical simulations, density functional theory, malic acid, tartaric acid.

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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 Dmalic 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 (TEM) and reverse phase transfer ligand exchange with achiral 1-dodecanethiol. Density Functional Theory (DFT) geometry optimizations and CD spectra simulations provide 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.


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The physical, chemical, electronic, and chiroptical properties of 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 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 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 thirteen 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


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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, 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. RESULTS AND DISCUSSION We started our studies with four chiral bis-carboxylic butanedioic acids (Chart 1) that shared the same carbon framework but differed in the functional group attached to the C2 (i.e., Cα) stereogenic center, i.e., L- and D-malic acid (2-hydroxy 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). CHART 1. Structures of chiral carboxylic acids with different functional groups on Cα. 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 ESI for full experimental details). L- and D-MA-CdSe QDs were prepared at the acidic pH of ~4.3 where MA was partially deprotonated to yield predominantly mono-malate (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 400 nm originated from the interactions between the chiral carboxylic acids and achiral CdSe QDs. Figure 2. CD spectra of L- and D-MA, L- and D-Asp, L- and D-MSA, L- and 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 off-set (+/-/+/-) 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 off-set, the L-MA-CdSe and L-Cys-CdSe showed mirror-image CD signs within the band gap absorption: the L-MA-CdSe displayed a positive CD


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band at 539 and a negative CD band at 517 nm (+/-), while L-Cys-CdSe 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. Table 1. CD anisotropy g factors for MA-CdSe and Cys-CdSe QDs (ØCdSe = 3.0 nm). QDsa

g (CD+)b

g (CD–)c

L-MA-CdSe

+4.4 × 10-5 (539 nm)d

-4.8 ×10-5 (517 nm)d

L-Cys-CdSe

12.4 × 10-5 (518 nm)

-8.9 ×10-5 (536 nm)

L-TA-CdSe

+5.6× 10-5 (515 nm)d

-4.3 ×10-5 (541 nm)d

a

ØCdSe = 3.0 nm. b Positive CD band. c Negative CD band. d UV-vis spectra have been adjusted to account for aggregation.

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 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 ligand25) that could interact with 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 O-donor groups each (i.e., two carboxyl/carboxylate groups) at the pH of the exchange (the Cα amino group of Asp was fully protonated) both failed to induce chirality in CdSe. It appears that two carboxyl groups and a Cα hydroxyl group must


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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 high resolution 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 3.48Å as determined by analysis of the high-resolution 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


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(carboxyl/carboxylate, amino/ammonium, methyl and hydroxyl) at Cβ and Cγ carbons (see ESI for phase transfer ligand exchange conditions, the UV-vis absorption spectra and XRD profiles). CHART 2. Structure of chiral capping ligands with different functional groups on Cβ and Cγ: five MA derivatives and three Asp derivatives. 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 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-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)). 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). 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


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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., N-donor group) on the chirality induction in the post-synthetic phase-transfer ligand exchange was not possible to assess. 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


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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 ν(C=O) stretching mode (1696 cm-1; Figure 4a).39, 40 The L-MA 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 = 5.3 (i.e., pH of the aqueous solution of L-MA functionalized CdSe) have 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 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 cm−1 and 1416 cm−1, respectively; Figure 4d), while the ν(C=O) stretching modes of L-MA (~1696 cm−1) or surface anchored unidentate mono-malate anion (~1654 cm-1 on Cu surface)41 were missing altogether, suggesting the presence of bis-malate dianion. Figure 4. ATR-IR spectra of (a) L-MA (neat, neutral diacid), (b) L-MA (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 The separation between the asymmetric and symmetric stretching mode, ∆ν = 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 the ∆ν = 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


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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 inter-molecular hydrogen bonds. Furthermore, missing asymmetric methyl deformation mode, δasym(CH3), of TMAH in 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 bis-malate dianion) interacted with the CdSe surface via both carboxylate groups and the hydroxyl group where the latter could concurrently participate in intra- or inter-molecular Hbonds. 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). Figure 5. Bridge (left), chelate (center) and unidentate (right) geometries of acetate anion of the (111) surface of CdSe fragment. To test our hypothesis of multidentate surface binding further, we explored three strategies. In the first approach, we attempted to introduce N-donor group by preparing Asp-CdSe 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 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 surface44-47), 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-AspCdSe QDs solution post-synthetically from pH = 4.5 to either 10, 11, or 12 using TMAH and


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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. Scheme 1. Introduction of three donor groups on L-Asp by deprotonation of the ammonium group (left) and acetylation of the amino group (right). 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-LAsp) at pH = 4.1 on OA-CdSe successfully induced chirality in QDs (Figure 6). We 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. Figure 6. CD spectra of (a) N-Ac-L-Asp-CdSe and (b) L-MA-CdSe. ØCdSe = 3.4 nm. To further explore our hypothesis, we performed Density Functional Theory (DFT), PBEPBE/3-21G/sbkjc-VDZ* level geometry optimizations (for details see Methods and ESI) of chiral carboxylic acids (L-MA, D-TA, L-Asp, N-Ac-L-Asp) functionalized Cd9Se7 quantum dot 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 (Figure 5) type bonds, the bis-carboxylate


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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 NAc-L-Asp (Figure S13). Note that while D-TA 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). Figure 7. Theoretical modeling of distinct QD-ligand binding modes and resulting CD spectra. (a) Schematic illustration of two multi-dentate binding geometries of L-MA dianion (i.e., bismalate) 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 LMA-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 pattern of chiral ligand arrangement on the QD surface and consequently to a strong observable CD signal. On the other hand, if both


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orientations are comparable in energy, there will be a statistical mixture of both, effectively canceling the CD, similarly of the spectra of the racemic mixture. For the carboxylic acid ligands studied here, the third anchoring O-donor 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 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 (ESI). 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 (ESI). 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 acid induced 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


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ligand on the surface of CdSe QDs was necessary but not sufficient for induction of optical activity in QDs. Replacement the L-MA in L-MA-CdSe 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 electron-deficient, 2- and 3-coordinated Cd atoms of the nanocrystal surface was required for chirality induction in CdSe QDs. The DFTbased geometry optimizations and CD spectra simulations fully support this conclusion and, consistently 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 tailormade chiral semiconducting nanomaterials. 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 heterogenous mixture was deoxygenated 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 re-dissolved in deionized H2O and characterized.


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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 321G for all lighter atoms was employed. Aqueous solvent was approximated by the conductorlike 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 constant width of 20 nm. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 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. AUTHOR INFORMATION Corresponding Author * Dr. Krisztina Varga: [email protected] * Dr. Jan Kubelka: [email protected] * Dr. Milan Balaz: [email protected] ACKNOWLEDGMENT


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This work was supported in part by the National Science Foundation (award CBET 1403947; KV, SVD, JK, MB), Yonsei University (MB) and the University of New Hampshire (KV).

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45. Nose, K.; Fujita, H.; Omata, T.; Otsuka-Yao-Matsuo, S.; Nakamura, H.; Maeda, H. Chemical Role of Amines in the Colloidal Synthesis of CdSe Quantum Dots and Their Luminescence Properties. J. Lumin. 2007, 126, 21-26. 46. Bullen, C.; Mulvaney, P. The Effects of Chemisorption on the Luminescence of CdSe Quantum Dots. Langmuir 2006, 22, 3007-3013. 47. Donakowski, M. D.; Godbe, J. M.; Sknepnek, R.; Knowles, K. E.; Olvera de la Cruz, M.; Weiss, E. A. A Quantitative Description of the Binding Equilibria of Para-Substituted Aniline Ligands and CdSe Quantum Dots. J. Phys. Chem. C 2010, 114, 22526-22534. 48. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M. L., X.; ; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K., et al. Gaussian 09, Gaussian, Inc.: Wallingford CT, 2009. 49. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 50. Azpiroz, J. M.; Matxain, J. M.; Infante, I.; Lopez, X.; Ugalde, J. M. A DFT/TDDFT Study on the Optoelectronic Properties of the Amine-Capped Magic (CdSe)13 Nanocluster. Phys. Chem. Chem. Phys. 2013, 15, 10996-11005. 51. Kulakov, M. P.; Balyakina, I. V.; Kolesnikov, N. N. Phase-Diagram and Crystallization in the System CdSe-ZnSe Inorg. Mater. (Engl. Transl.) 1989, 25, 1386-1389.


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CHART 1. Structures of chiral carboxylic acids with different functional groups on Cα. 85x34mm (300 x 300 DPI)


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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 D-Cys-CdSe (ØCdSe = 3.0 nm). (c) UV-vis spectra of L-MA-CdSe and L-CysCdSe (ØCdSe = 3.0 nm). 84x91mm (300 x 300 DPI)


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Figure 2. CD spectra of L- and D-MA, L- and D-Asp, L- and D-MSA, L- and D-TA. [amino acid] = 1.0 mM in Na-cacodylate buffer (1 mM, pH = 7.0). 84x30mm (300 x 300 DPI)


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CHART 2. Structure of chiral capping ligands with different functional groups on Cβ and Cγ: five MA derivatives and three Asp derivatives. 85x65mm (300 x 300 DPI)


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Figure 3. CD spectra of (a) L-TA-CdSe and D-TA-CdSe (ØCdSe = 3.0 nm), and (b) L-TA-CdSe and L-MACdSe (ØCdSe = 4.4 nm). 84x49mm (300 x 300 DPI)


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Figure 4. ATR-IR spectra of (a) L-MA (neat, neutral diacid), (b) L-MA (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.41 84x57mm (300 x 300 DPI)


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Figure 5. Bridge (left), chelate (center) and unidentate (right) geometries of acetate anion of the (111) surface of CdSe fragment. 85x28mm (300 x 300 DPI)


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Scheme 1. Introduction of three donor groups on L-Asp by deprotonation of the ammonium group (left) and acetylation of the amino group (right). 85x30mm (300 x 300 DPI)


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Figure 6. CD spectra of (a) N-Ac-L-Asp-CdSe and (b) L-MA-CdSe. ØCdSe = 3.4 nm. 84x47mm (300 x 300 DPI)


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Figure 7. Theoretical modeling of distinct QD-ligand binding modes and resulting CD spectra. (a) Schematic illustration of two multi-dentate 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). 85x106mm (300 x 300 DPI)


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TOC Graphics 82x44mm (300 x 300 DPI)


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CdSe Quantum Dots Functionalized with Chiral, Thiol-Free Carboxylic

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