Significant Enhancement of the Chiral Correlation Length in Nematic Liquid Crystals by Gold Nanoparticle Surfaces Featuring Axially Chiral Binaphthyl Ligands Taizo Mori,*,† Anshul Sharma,† and Torsten Hegmann*,†,‡ †

Liquid Crystal Institute, Chemical Physics Interdisciplinary Program and ‡Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242-0001 United States S Supporting Information *

ABSTRACT: Chirality is a fundamental scientific concept best described by the absence of mirror symmetry and the inability to superimpose an object onto its mirror image by translation and rotation. Chirality is expressed at almost all molecular levels, from single molecules to supramolecular systems, and present virtually everywhere in nature. Here, to explore how chirality propagates from a chiral nanoscale surface, we study gold nanoparticles functionalized with axially chiral binaphthyl molecules. In particular, we synthesized three enantiomeric pairs of chiral ligand-capped gold nanoparticles differing in size, curvature, and ligand density to tune the chirality transfer from nanoscale solid surfaces to a bulk anisotropic liquid crystal medium. Ultimately, we are examining how far the chirality from a nanoparticle surface reaches into a bulk material. Circular dichroism spectra of the gold nanoparticles decorated with binaphthyl thiols confirmed that the binaphthyl moieties form a cisoid conformation in isotropic organic solvents. In the chiral nematic liquid crystal phase, induced by dispersing the gold nanoparticles into an achiral anisotropic nematic liquid crystal solvent, the binaphthyl moieties on the nanoparticle surface form a transoid conformation as determined by imaging the helical twist direction of the induced cholesteric phase. This suggests that the ligand density on the nanoscale metal surfaces provides a dynamic space to alter and adjust the helicity of binaphthyl derivatives in response to the ordering of the surrounding medium. The helical pitch values of the induced chiral nematic phase were determined, and the helical twisting power (HTP) of the chiral gold nanoparticles calculated to elucidate the chirality transfer efficiency of the binaphthyl ligand capped gold nanoparticles. Remarkably, the HTP increases with increasing diameter of the particles, that is, the efficiency of the chirality transfer of the binaphthyl units bound to the nanoparticle surface is diminished as the size of the particle is reduced. However, in comparison to the free ligands, per chiral molecule all tested gold nanoparticles induce helical distortions in a 10- to 50-fold larger number of liquid crystal host molecules surrounding each particle, indicating a significantly enhanced chiral correlation length. We propose that both the helicity and the chirality transfer efficiency of axially chiral binaphthyl derivatives can be controlled at metal nanoparticle surfaces by adjusting the particle size and curvature as well as the number and density of the chiral ligands to ultimately measure and tune the chiral correlation length. KEYWORDS: gold nanoparticle, liquid crystal, chirality, binaphthyl, chiral correlation length reactions,13 or light14−17 as in the development of optical data storage or molecular machines. Liquid crystal (LC) phases are highly suitable materials to study, visualize, and measure chiral transfer mechanisms from chiral molecules and interfaces to the surrounding bulk. Chiral LC phases such as the chiral nematic (N*-LC) or the chiral smectic-C phase (SmC*), induced by adding small quantities of

U

nderstanding and controlling the chirality of molecules is a central topic in various fields of science, from fundamental research on elucidating the origin of homochirality on our planet1,2 to the application of chiral molecules and structures as enantio- as well as diastereoselective catalysts,3−5 in chiral molecular recognition,6−8 and in optical memory devices.9,10 Molecular chirality can be controlled by synthetically favoring one configuration over the other or by changing the conformation of molecules using external stimuli such as host−guest interactions,11,12 redox © XXXX American Chemical Society

Received: November 13, 2015 Accepted: January 6, 2016

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Figure 1. Helicity and dihedral angle control in binaphthyl derivatives: (a) Schematic structures of cisoid and transoid forms of binaphthyl with (R)-configuration. (b) Schematic view of the compression and expansion of a monolayer of amphiphilic binaphthyl derivatives at a 2D air−water interface. (c) Schematic illustrations of Au-NPs functionalized with axially chiral binaphthyl (Au-R1, Au-R2, and Au-R3 as well as Au-S1, Au-S2, and Au-S3) and the chiral nematic liquid crystal (N*-LC) phase induced by the addition of Au-NPs functionalized with chiral binaphthyl moieties into an achiral nematic LC (N-LC) solvent.

achiral N-LCs32−34 and that N-LCs can be employed to sense, visualize, and measure NP chirality.35 Thus, the initial achiral N-LC solvent should allow us to study and measure the efficiency of chirality transfer from chiral nanoscale surfaces to the LC bulk. By selecting well-studied and well-understood axially chiral binaphthyl ligands on the Au-NP surfaces, we should also be able to study the effects of NP size (and concurrently the number, curvature, and density of binaphthyl ligands), the binaphthyl dihedral angle, and ultimately the chiral correlation length once the well-dispersed binaphthyl chiral ligand-capped Au-NPs induce a bulk N*-LC phase. Axially chiral binaphthyl derivatives36 are often synthesized and studied to control axial chirality, also called helicity, because their helicity depends on a dihedral angle between the two naphthyl rings.37−40 For binaphthyl with (R)-configuration, the cisoid conformation with a dihedral angle between 0° and 90° gives left-handed helicity, and the transoid conformation with a dihedral angle between 90° and 180° leads to righthanded helicity, respectively (Figure 1a).41−44 The dihedral angle of binaphthyl can be statically confined by a short molecular bridge between the 2- and 2′-position of the two naphthyl rings (e.g., −OCH2CH2O−) or by introducing reasonably bulky substituents.39 Photoresponsive appendices like azobenzenes and diarylethenes have also been introduced to dynamically switch the helicity of binaphthyl molecules.38,40 Binaphthyl derivatives exhibit a cisoid conformation in isotropic solvents and a transoid conformation in anisotropic liquid crystalline solvents.45 Specific environments can affect the conformation of binaphthyl molecules; cisoid and transoid are energetically only 1 to 2 kcal mol−1 apart. For amphiphilic chiral binaphthyl derivatives, for example, Mori et al. reported that the dihedral angle can be controlled by simply applying a mechanical force to a Langmuir−Blodgett monolayer of binaphthyl molecules at the 2D air−water interface (Figure 1b).46 Similar to bridged binaphthyl derivatives,44 two hydrophilic oligooxyethylene substituents in the 2 and 2′ position

a chiral molecule (i.e., as a chiral dopant) into the achiral LC host, produce unique optical and electro-optic effects based on the helical nature of the resulting molecular arrangement. Such effects include the selective reflection of light, unique defect textures, the origin of a spontaneous polarization, and optical bandgaps, among others, depending on the type of the induced chiral LC phase.18−20 Chirality transfer mechanisms of molecularly dispersed lowmolecular weight chiral dopants in numerous LC phases have been extensively studied and are generally well understood. In contrast, detecting and measuring bulk chiral interactions between chiral surfaces such as a flat metal substrate passivated with a chiral self-assembled monolayer (SAM) is extremely difficult. The primary reason for this is that chirality of 2D surfaces, such as a chiral SAM on gold, only induces short-range chiral surface−fluid interactions; that is, only the interfacial region between an achiral nematic LC (N-LC) and the chiral SAM would experience a helical director twist (short chiral correlation length, defined as the spatial distance over which a uniform helical pitch is maintained in the bulk). This effect is temperature dependent, and even as the temperature is lowered (well below the isotropic−nematic phase transition), a combination of short-range chiral surface−N-LC interactions and long-range anisotropic correlations combined lead to a smooth relaxation of the director twist as the nematic LC fluid approaches the achiral counter surface (e.g., air, glass) as shown by Zannoni and co-workers.21 Thus, chiral 2D surfaces only have a limited chiral correlation lengthwhich, in addition is difficult to measure. To address this problem, we here introduced chiral SAMs on curved nanoscale surfaces (i.e., nanoparticle surfaces with chiral ligand shells) into the bulk of nematic LC phases similar to molecular chiral additives. The chirality of metal clusters and nanoparticles (NPs) has become an intriguing field of research over the past decade.22−31 Our group verified that gold NPs (Au-NPs) capped with chiral ligands can transfer chirality to B

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Figure 2. Transmission electron microscopy (TEM) images and size histograms of the Au-NPs: (a) Au-R1, (b) Au-R2, (c) Au-R3, (d) Au-S1, (b) Au-S2, (f) Au-S3. Cartoons of the Au-NPs show potential shapes associated with the size and composition of the as-synthesized Au-NPs, some of which have compositions close to magic-sized gold clusters, for example, Au33, Au38, Au39, and Au55.

and compare it to the purely organic, molecularly dispersed analogues in a given N-LC medium. To realize this, we synthesized Au-NPs functionalized with three axially chiral binaphthyl derivatives differing in the length of the nontethered hydrocarbon chain attached at the 2′position of the binaphthyl unit. The quasicurvature of the polyhedral NP surfaces renders these systems 3D binaphthylLC interfaces in contrast to the 2D binaphthyl−air interface discussed earlier.46 N*-LC phases were induced by adding these Au-NPs as chiral dopants to a room temperature N-LC

assisted in forcing a cisoid conformation as the monolayer is compressed. On the basis of these experiments, we suggest that it should be possible to control and use the dihedral angle of binaphthyl molecules, tethered to a NP surface with only one flexible hydrocarbon chain, simply by its interaction with an anisometric achiral N-LC solvent. Such control over the molecular conformation together with the synthetic control over NP size that is coupled to curvature and ligand density should allow us to measure the elusive chiral correlation length C

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Table 1. Average Au-NP Size, Composition, Molecular Weight, and Molecular Concentration of the Au-NPs in Cyclohexane Used for UV−Vis and CD Spectra NP

D/nm

NAu

Nbinaphthyl

Au-R1 Au-S1 Au-R2 Au-S2 Au-R3 Au-S3

2.47 1.73 2.13 1.08 1.24 1.09

464 159 298 33 56 35

90 44 67 15 22 16

molecular concentration of NP/M

MW of NP 13.60 5.33 9.56 1.50 2.50 1.71

× × × × × ×

7.3 × 10−7 14 × 10−7 4.6 × 10−7 14 × 10−7 8.8 × 10−7 11 × 10−7

104 104 104 104 104 104

host (5CB; Cr 25 °C N 35 °C Iso) in order to visualize and measure the helicity change of the binaphthyl units at the AuNP−LC interface, measure the helical pitch, and then calculate the chiral correlation length (Figure 1c). The as-synthesized and purified Au-NPs were soluble in several organic solvents (halogenated, aliphatic), allowing us to draw conclusions about the dihedral angle of the binaphthyl helical conformation based on UV−vis spectroscopy and circular dichroism (CD) spectropolarimetry. The helical conformation of the binaphthyl moieties affixed to the Au-NP surface in the N-LC were then deduced from examining the twist direction of the induced N*-LC. Ultimately, the efficiency of chirality transfer from the suspended chiral binaphthylcapped Au-NPs to the N-LC was investigated by helical pitch measurements using fingerprint textures developing at N*-LCair interfaces (free surface) and via the Grandjean-Cano wedge cell method47 to calculate the helical twisting power (HTP).18 The HTP is a measure of the ability of a chiral dopant to twist a N-LC phase and is inverse proportional to the helical pitch p and the concentration c of the chiral additive (HTP = 1/(pc)). Hence, HTP values depend on the number and density of the binaphthyl moieties at the NP surface as discussed in the next section.

molecular concentration of binaphthyl on NP surface/M 2.1 2.6 1.2 1.3 1.1 1.1

× × × × × ×

10−5 10−5 10−5 10−5 10−5 10−5

on the size regime. Calculation and approximation methods are described in the Supporting Information (Table S1−S7). The number of gold atoms at the NP surface was calculated using the difference between the number of gold atoms of the particle and that of the “inner” particle with a radius that is 2.88 Å (the diameter of a Au atom) smaller than the particle. The number of binaphthyl thiol ligands at the NP surface was calculated by considering the molecular area and binding mode of thiols on a gold surface49 (Figure S6), and it closely matched with experimental data of ligand desorption and combustion recorded by thermogravimetric analysis (TGA, Figure S7). Combined, these calculations suggest that the quasi-spherical Au-R1 and -S1 as well as Au-R2 are on average similar to truncated icosahedra 489, 309, and 147, and that Au-S2 as well as Au-R3 and -S3 are on average similar to Ino’s decahedron 55 as well as Marks’ decahedron 29 and 39, respectively. The total molecular weight of all Au-NPs and their surface coverage with binaphthyl ligands are summarized in Table 1 (see also Table S1). Calculating the Au-NP composition helped estimate the molecular area of each binaphthyl ligand directly at the Au-NP surface (at the Au−S bonds) and further away at the 2,2′ position of the binaphthyl, separated from the Au-NP surface by an 11 carbon alkyloxy spacer emanating from the 2position (Table S8). The three enantiomeric pairs of binaphthyl ligands differ in the length of the alkyloxy chain attached to the 2′-position. Binaphthyl moieties with a longer, nontethered 2′ alkyl chain are characterized by a larger space between binaphthyl ligands at the 2,2′ position; those with shorter 2′ alkyl chains are more tightly packed on the Au-NP surface. Because all experimental parameters were set equally for each NP synthesis, it appears that the core size of the as-synthesized Au-NPs depends on length of the 2′ alkyl chain attached to the binaphthyl units. The space available for each binaphthyl core (or the space between them) is intricately linked to the number of thiols on the NP surface. Using simple geometric considerations of curvature, the molecular area available for each binaphthyl unit at the 2,2′-position becomes larger as the size of the Au-NP decreases. Assuming a cisoid conformation of the binaphthyl ligands in solution, the bulkiest ligands with the longest hydrocarbon chains at the 2′-position would then lead to the formation of smaller Au-NPs given that all other synthetic parameters are equal. Even the subtlest changes of the ligand chemical structure (e.g., changing the substitution pattern in methylbenzenethiols50,51 or the binding energy of ligands52) can have tremendous impact on the Au-NP size. Consequently, chiral ligand size and conformation are useful tools to tune the NP size and curvature as well as the space between chiral ligands to interact with host molecules. For example, Au-R3 and Au-S3 are the smallest NPs in the series featuring the bulkiest ligands R3 and S3.

RESULT AND DISCUSSION Synthesis and Characterization of the Au-NPs. Axially chiral binaphthyl derivatives [(R)- and (S)-1, 2, and 3)] with undecanoyl thiolate and an alkyloxy chain differing in length [ethyl, hexyl, and dodecyl (n = 2, 6, 12)] at the 2 and 2′ position of the binaphthyl, respectively, were synthesized as ligands for the Au-NPs (Scheme S1). The Au-NPs, Au-R and S1, 2, and 3 were prepared from these ligands as described in our earlier papers32,34 (see Supporting Information, section 3). As shown in Figure 2, the size and size distribution of the AuNPs were determined by transmission electron microscopy (TEM) and subsequent TEM image analysis. The larger NPs ranged in size from 1.7 to 2.5 nm (Au-R1 and -S1, and Au-R2). As expected for these sizes, the Au-NPs did not show any significant surface plasmon resonance (SPR) peak.48 The shapes of the larger NPs are quasispherical and icosahedral, and the size distributions were fairly narrow (less than ±0.20 nm), as shown in the NP size histograms. The smaller NPs (Au-S2 and Au-R3 and -S3) ranged in size from about 1.1 to 1.25 nm, and their size distribution is slightly larger (see detailed size histograms in Figure S1). On the basis of the calculated number of Au atoms in these Au-NPs (vide infra), Au-S2 as well as AuR3 and S3 have ellipsoidal and Ino’s or Marks’ decahedral structure (see SI, section 4). The number of Au atoms per NP was calculated assuming that the NPs are either quasispherical or ellipsoidal depending D

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Figure 3. UV−vis and circular dichroism (CD) spectra of the ligands and the Au-NPs in cyclohexane: (a) R1 and S1, (b) R2 and S2, (c) R3 and S3, (d) Au-R1 and Au-S1, (e) Au-R2 and Au-S2, and (f) Au-R3 and Au-S3. The spectra of the Au-NPs were normalized to the molecular concentration of the binaphthyl units on each Au-NP surface.

charge-transfer transitions as described earlier for other chiral ligand-capped Au-NPs.34,35 As a result, the shape of the CD spectra obtained from ligands and Au-NPs are similar, and CD signal intensities are nearly identical. Overall the solubility of the Au-NPs in several organic solvents is poorer in comparison to the ligands. Figure S10 shows the plots of the peak maxima in the UV−vis spectra against several parameters of the Au-NPs in order to compare the solubility of the Au-NPs. These plots suggest that the solubility of the Au-NPs depend on the radius and the molar ratio between binaphthyl moieties and gold atoms of the NPs, and the Au-NPs with a higher molar ratio between organic and inorganic constituents are better soluble in organic solvents. The correlation between the binaphthyl conformation and the CD spectrum based on the dihedral angle of the binaphthyl is interpreted in terms of the exciton coupling theory.53−55 The binaphthyl derivatives with (R)-configuration give a negative CD couplet for the dihedral angle in the range from 0° to 100°, and a positive CD couplet from 110° to 180°.39,46 Hence, we assume that the binaphthyl units form a cisoid conformation in cyclohexane as described earlier. The wavelengths of the peak maxima of the CD couplet shift to the red as the dihedral angle of the binaphthyl derivatives decreases in the range from 0° to 100° and does not shift in the range from 110° to 180°.46 For the Au-NPs in this series, the wavelength of the peak maxima shifts progressively to the red as the length of the 2′ hydrocarbon chain decreases, that is, from Au-S1 to Au-S2 to Au-S3. Thus, the dihedral angles of the binaphthyl unit in solution become smaller in this order. One would expect that the dihedral angle depends on the density of the binaphthyl units on the Au-NP surface, which coincides with the fact that the density of the binaphthyl ligands of the Au-NP surface is

In addition to size and ligand density considerations for the use of Au-NPs as chiral additives, the thermal stability of AuNPs is vital for the preparation of stable and reproducible LC mixtures, because most LC phases persist at temperatures above room temperature and sample preparation requires annealing and slow cooling from the isotropic liquid phase. In our experiments, both thermal and chemical stability of the AuNPs are more than sufficient because the Au-NPs were added to 5CB as achiral N-LC host forming a nematic phase between 25 and 35 °C. All mixtures were prepared at temperatures well below 50 °C, and TGA data showed that desorption of the binaphthyl ligands from the Au-NP surface sets in around 150 °C (Figure S7). Optical Properties of Au-NPs. Figure 3 shows the UV−vis and CD spectra of both the ligands and the Au-NPs in cyclohexane. The spectra of the Au-NPs were normalized according to molecular concentration of the binaphthyl moieties on the surface of the NPs. UV−vis spectra normalized to the molecular concentration of the Au-NPs are shown in Figure S8, and the CD spectra of all precursors used for the ligand synthesis are summarized in Figure S9. All spectra of ligands and Au-NPs show bands associated with the transitions of binaphthyl rings, the 1Bb transition at 200 to 250 nm, the 1La transition at 250 to 300 nm, and the 1Lb transition at 300 to 350 nm. The Au-NPs did not show any surface SPR peak due to their small size. In the CD spectra, the ligands and Au-NPs with (R)-configuration show a negative to positive CD couplet for the 1Bb transition and intense positive Cotton effects for the 1La and 1Lb transition, respectively. The absorbance (absorbance cross-section) of the binaphthyl moieties is too large to detect any peak absorption originating from metal-centered Au 5d10-6sp interband or ligand−metal E

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Figure 4. Polarizing optical microscopy (POM) images (crossed polarizers) of the N*-LCs induced by 5 wt % of (a) Au-R1, (b) Au-S1, (c) AuR2, (d) Au-S2, (e) Au-R3, and (f) Au-S3 at free surfaces (top interface is N*-LC/air) at 25 °C during cooling (insets show magnified section of each image).

reduced as we move from Au-R1 to Au-R2 to Au-R3 (similarly from Au-S1 to Au-S2 to Au-S3) because the binaphthyl ligands with longer 2′ hydrocarbon chains are bulkier and less densely packed on the smaller Au-NPs. Therefore, the order corresponds to the dihedral angles of the binaphthyl units on the Au-NPs. The molecular area of 1.13 nm2 for Au-R1 suggests that the dihedral angle is lower than 85° considering earlier molecular dynamics calculations.46 As a result, the binaphthyl units on the Au-NP surface adopt a cisoid conformation in isotropic solvents such as cyclohexane. Characterization of Au-NP Induced N*-LC Phases. The addition of 5 wt % of each of the Au-NPs as chiral dopants into 5CB as achiral N-LC host led in all cases to the induction of an N*-LC phase for the host 5CB. The induced N*-LC phases showed characteristic fingerprint textures at free surface as observed by polarized light optical microscopy (POM) at 25 °C on cooling as shown in Figure 4. The periodic distances between the bright (or dark) lines in such fingerprint textures corresponds to the helical half pitch of an N*-LC (Figure S11a). The temperature range of the N*-LC phase extended from about 15 to 33 °C (on average) on heating (Figure S12); wider than for the nondoped 5CB host with a more drastic reduction of the melting point. On cooling, the phase transition temperature from the isotropic to the N* phase occurred at 33 °C (like in the heating cycle), but the phase transition from the N* phase to crystalline solid could not be detected by differential scanning calorimetry (DSC) even at temperatures as low as −40 °C due to supercooling. POM observations between untreated glass slides and in cells with homeotropic boundary conditions (Figure S13 and S14) showed textures with characteristic differences to those observed at free surfaces (where the top of the film is an N*LC/air interface). The N*-LC induced by Au-R1 and Au-S1 showed Schlieren textures typical for an achiral N phase and cholesteric finger textures of an N* phase, respectively. The N*-LC induced by Au-R1 and Au-S2 showed textures typical for N and N* phases, respectively, in cells with homeotropic boundary conditions as well as untreated glass cells as shown in Figure S11b and d. The limited solubility of Au-R1, already observed in isotropic solvents (vide supra), appears to prevent

the formation of an N* phase with an observable helical pitch between untreated glass slides (planar alignment, cf. Figure S13a) and largely vertical alignment in homeotropic cells. The N*-LCs induced by doping Au-R2 into 5CB when observed between untreated glass slides showed a combination of homeotropic alignment domains and birefringent stripes resembling previously observed π-wall defects caused by the segregation of the Au-NPs to the interfacial regions of the cell, especially in the homeotropic domains, where they serve as homeotropic alignment layers32,33,56 (Figure S11c). The N*LCs induced by the addition of Au-S2 (the smaller of the S2 enantiomeric pair with a hexyloxy chain in the 2′-position; 1.08 nm for Au-S2 vs. 2.13 nm in diameter for Au-R2) showed characteristic cholesteric finger textures of an N* phase both between plain glass slides and in homeotropic cells with the helix axis parallel to the substrate (Figures S13d and S14d). Lastly, the N*-LCs induced by doping the smallest Au-NPs, Au-R3, and Au-S3 both with the longest alkyloxy chain in the 2′-position (n = 12), also displayed cholesteric finger textures between crossed polarizers in POM, similar to the almost equal-sized Au−S2. Thus, the smaller Au-NPs and those functionalized with the longer hydrocarbon chain in the 2′position promote or reinforce stronger homeotropic anchoring on plain glass or homeotropic alignment surfaces, respectively, facilitating the formation of cholesteric finger and fingerprint textures that in principle allow for an estimation of the helical pitch. Helical Twisting Power of Au-NPs. To obtain more precise helical pitch values of the induced N*-LC phases, we measured the distance between dark (or bright) lines of the fingerprint textures at free surfaces (Figure 4) as well as using the Cano wedge method47 (Figures S15 to S17). The Cano wedge method is based on the observation of discontinuity lines (so-called Grandjean−Cano steps) that appear when an N*-LC is inserted between two glass slides with a gradient thickness. The N*-LC induced by one of the smallest Au-NPs, Au-S2, was found to give the shortest helical pitch of 11.1 μm. The helical pitch of most other N*-LC phases induced by doping 5 wt % of the other Au-NPs were around 20 μm; values that were 1 order of magnitude larger than those obtained for F

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Table 2. Characteristics of the Au-NPs, Helical Pitch Data of the Induced N*-LC Phases, and Helical Twisting Power, HTP, Data for All Au-NPs in 5CB NP

D/ nm

MW binaphthyl on NP × 104

number of binaphthyl on NP

mole % of binaphthyl on NP

Au-R1 Au-S1 Au-R2 Au-S2 Au-R3 Au-S3

2.47 1.73 2.13 1.08 1.24 1.09

4.47 2.19 3.70 0.857 1.41 1.03

90 44 67 15 22 16

32.9 41.2 38.7 57.0 56.3 60.3

p of N*LC/μm

HTP of NP/μm−1

HTP of binaphthyls on NP/μm−1

HTP of 1 binaphthyl on NP/μm−1

± ± ± ± ± ±

376 203 371 97.4 98.2 68.1

1146 495 959 171 175 112

12.8 11.3 14.5 11.1 7.98 7.03

27.7 20.0 19.6 11.8 19.3 19.2

3.6 2.5 3.2 1.6 2.6 2.7

Figure 5. POM images (crossed polarizers) of the miscibility test (contact preparations) between cholesteryl oleyl carbonate and the N*-LC induced by (a) Au-R1, (b) Au-S1, (c) Au-R2, (d) Au-S2, (e) Au-R3, and (f) Au-S3 in cell with homeotropic boundary conditions at 25 °C on cooling. The summary table (bottom right) and schematic illustrations (bottom left) show the conformation of the binaphthyl units with (R)configuration decorating the Au-NPs in cyclohexane and in the induced N*-LC. The neat ligands and the Au-NPs capped with the (S)configuration binaphthyl ligands are cisoid in cyclohexane and transoid in the N*-LC, but with a left-handed twist direction.

The HTP values for the Au-NPs regarded as a single molecule are summarized in Table 2 and range from 68 to 376 μm−1. Especially, Au-R1 and Au-R2 showed high HTP values of 376 and 371 μm−1, respectively. These values were much higher than those calculated for the free ligands and precursors, ranging from 4.0 to 17 μm−1 (summarized in Figure S18).

the N*-LC phases induced by doping 5 wt % of the free ligands (ranging from 3 to 4 μm). To compare the values of the helical pitch between ligands and Au-NPs, the molar concentration of the dopant in the N-LC host needs to be taken into consideration to ensure we compare HTP values based on the total number of chiral molecules in each mixture. G

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the miscibility test. As shown in Figure 5, the mixtures of COC and Au-R1, Au-R2, and Au-R3 showed a discontinuity with a contact zone giving a typical achiral nematic Schlieren texture. In contrast, the contact preparations of COC and Au-S1, AuS2, and Au-S3 showed no change in the optical texture. These results demonstrate that the twist directions of the N*-LC induced by Au-Rx and Au-Sx are opposite, namely they are right-handed for the (R)- and left-handed for the (S)enantiomers of binaphthyl ligands capping the Au-NP surface. The helical twist directions of the N*-LC induced by the free ligands were also determined by the miscibility test as shown in Figures S20 to S23. However, only the N*-LC induced by the ligand with the unmodified monohydroxyl group at the 2′position (R4) showed a left-handed twist, opposite to the N*LC induced by all other ligands and Au-NPs with (R)configuration. The ICD spectra and the POM images of thin N*-LC films after doping with the Au-NPs are shown in Figure S24. The thin films for ICD were prepared between two quartz substrates separated by a 10-μm fiberglass spacer. The ICD spectra were recorded only for the lower concentration of the Au-NPs (0.5 wt %) in the N-LC host to avoid CD reflection bands.32 The samples were rotated with respect to the light beam at 45° intervals from 0° to 315° in order to differentiate the intrinsic CD from linear dichroism and birefringence.32,35 The signs of the ICD signals of the N*-LC phase induced by Au-R1, -R2, and R3 as well as Au-S1, -S2, and -S3 are either positive or negative bands, at wavelengths above 320 nm, respectively, although the thin N*-LC films showed textures typical for a achiral N-LC phase in the POM images (i.e., the helical pitch values are too large to allow for the generation of chiral N*-LC textures at this low NP concentration). Nevertheless, the observed ICD signals originated from the twisted structure of 5CB in the N*-LC phase.59,60 As shown in Figure S25, we also recorded a CD spectrum of COC and an ICD spectrum of the N*-LC phase induced by doping 5CB with 5 wt % COC to elucidate the origin of the CD bands of the N*-LC doped with the Au-NPs. We observed a positive signal around 360 nm for neat COC and a negative signal above 320 nm for the COC-doped 5CB. A right-handed N*-LC phase reflects most of the right-handed circularly polarized light (CPL) corresponding to the helical pitch of the N*-LC and transmits left-handed CPL. The reflected wavelength depends on the helical pitch and the refractive index of the N*-LC. CD is defined as the difference between the absorbance of left-handed CPL and right-handed CPL, and thus, a right-handed N*-LC phase shows a negative peak at a wavelength corresponding to the helical pitch of the N*-LC. For neat COC, the intensity of this peak is quite large, and the signal at 360 nm is caused by selective reflection, because COC forms a left-handed helical structure and a short helical pitch commensurate to wavelengths of visible light. Hence, the CD band above 320 nm for COC-doped 5CB resulted from the left-handed, twisted structure of 5CB in the N*-LC phase. Overall, the ICD data support the assignment of the helical twist sense derived from miscibility tests (for some examples, see Figure S26), with the (S)-configuration of the binaphthyl ligands on the Au-NPs inducing a left-handed helix and the (R)-configuration a right-handed. All experimental data indicate that the binaphthyl moieties capping the Au-NPs form a transoid conformation in the N-LC matrix. Thin N*-LC films of 5CB doped with Au-R3 show positive CD signals at the 1Bb transition of binaphthyl

Thus, the Au-NPs functionalized with chiral binaphthyl ligands, considered as a single supramolecular entity, are more efficient chiral dopants when compared to the free ligands. For instance, the helical pitch of the N*-LC phases induced by the R2 and Au-R2 pair compared at the same mole fraction of 1.4 × 10−2 mole % in 5CB shows values of 500 μm for R2 and 19.6 μm for Au-R2 (5 wt % Au-R2 = 1.4 × 10−2 mole % of Au-R2 in 5CB); a 25-times tighter helical pitch induced by the Au-NP. The high HTP values for the Au-NPs depend of course on the number of the binaphthyl units on the Au-NP surface. The HTP values of the single binaphthyl units at the Au-NP surface were calculated and varied from 7.0 to 14.5 μm−1, respectively, and were compared with the HTP values of the free ligands. The HTP of a single binaphthyl unit on the surface of Au-R1 for example (12.8 μm−1) is higher than that of the free ligand R1 (9.8 μm−1), that of Au-R3 (8.0 μm−1) is half the value of R3 (16.1 μm−1). Such comparison is of course flawed. The ligands on the NP surface are much less able to fully interact with the surrounding LC host molecules. The relationship between the HTP values and several parameters of the Au-NPs were compared to investigate the chirality transfer from the binaphthyl units capping the Au-NP surface to the N-LC host. As shown in Figure S19, the HTP values of a single binaphthyl unit depend on the molecular ratio and molecular area of the binaphthyl units at the Au-NP surface. HTP values of chiral dopants usually saturate with increasing concentration of the dopant in a given N-LC host. Thus, the efficiency of chirality transfer of the binaphthyl units on the Au-NP surface would decay for highly dense ligands on the Au-NP surface such as for Au-R3 and Au-S3. However, while ligands on the surface of Au-S2 and Au-S3 have similar surface areas (ca. 3.5 nm2), the chirality transfer of the Au-S2 appears to be more efficient in the case of Au-S3 (Table 2). It is feasible that the longer dodecyloxy chains (n = 12) of the binaphthyls capping Au-S3 (and Au-R3 as well) block the N-LC host molecules more effectively from approaching the chiral binaphthyl axis. We recently reported that Au-NPs functionalized with chiral cholesterol derivatives as mesogenic ligands are efficient chiral additives, and the measured helical pitch and calculated HTP values allowed us to draw conclusions about the overall chirality of the Au-NPs.35 The cholesterol-capped Au-NPs were found to be more efficient chiral dopants than their free chiral ligand counterparts because the chiral LC cholesterol derivatives were highly compatible with N-LC host, which allowed us to quantify the efficiency of chirality transfer. Future work will examine if Au-NPs functionalized with liquid crystalline (mesogenic) binaphthyl derivatives57 would be even more efficient chiral additives. Twist Direction of Induced N*-LC Phases. The twist directions of the induced N*-LCs were determined through the miscibility test58 and by induced CD (ICD) spectropolarimetry of the thin N*-LC films. The miscibility test is based on the observation of the mixing area (contact area) between the N*LC phase of interest and a known, standard N*-LC in contact preparations observed by POM. If the twist direction of the N*-LC is the same as that of the known N*-LC, the mixing area will show a continuous transition from one N*-LC to the other N*-LC of the same handedness. Otherwise, it will be discontinuous (i.e., opposite twist direction), showing a Schlieren texture typical for an achiral N-LC phase in the contact zone. Here, we used the left-handed N*-LC of cholesteryl oleyl carbonate (COC) as known standard LC for H

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Figure 6. Plots of: (a) Fs (ratio between the number of 5CB molecules near the Au-NP surface/number of binaphthyl molecules on the Au-NP surface) vs HTP and (b) Fa (ratio between the total number of 5CB molecules in a 3D voxel surrounding one Au-NP surface/number of binaphthyl molecules on the Au-NP surface, cf. panel c) vs HTP. (c) Schematic 2D representations highlighting the introduced parameters of 5CB molecules surrounding the Au-NP surface (top left) and in the 3D space (voxel, top right) defined by the interparticle distance (bottom) assuming reasonably well-dispersed Au-NPs in the 5CB matrix. For simplicity, the helical arrangement of the 5CB molecules in the induced N*-LC phase is not shown here. (d) Simplified cartoon showing the observed enhancement of the chiral correlation length when the binaphthyl ligands are on the Au-NP surface.

to S23). R4 forms a cisoid conformation in both isotropic and anisotropic solvents. The dihedral angle of R4 is restricted by the formation of dimers facilitated by hydrogen bonding between the monohydroxyl groups at the 2′-position in the NLC. The conformation of the ligands with (R)-configuration in isotropic and anisotropic solvents is summarized in Table S13. Chirality Transfer Efficiency. The efficiency of chirality transfer from a chiral dopant to an achiral N-LC is quantified by the helical twisting power (HTP) as defined earlier. The HTP depends on the twist elasticity of the N-LC and on the structural complementarity between dopant and nematic host to achieve chirality transfer. As another measure of the efficiency of chirality transfer, we calculated how many 5CB host molecules are affected by the presence of the chiral binaphthyl-capped Au-NPs, both in the direct vicinity of the ligand shell and in a 3D voxel defined by the average interparticle distance (assuming well-dispersed Au-NPs in the 5CB matrix (see Supporting Information, section 11). This directly relates to the chiral correlation length of the 3D curved chiral ligand-capped nanoscale surfaces. At 5 wt %, the larger Au-NPs (Au-R1, Au-S1, and Au-R2) with the highest per

derivatives below 250 nm. This confirms that the positive CD signal of the Au-R3 doped sample originates from the transoid conformation of the binaphthyl moieties decorating the AuNPs in the N-LC host. Au-R3 in cyclohexane showed a negative CD couplet while showing a positive CD signal in the N-LC below 250 nm. Thus, the binaphthyl units of Au-R1, -R2, and -R3 have cisoid conformation in isotropic solvents such as cyclohexane but transform to a transoid conformation in an anisotropic solvent such as 5CB. In much the same way, the ligands used to synthesize the Au-NPs adopt a cisoid and transoid conformation in isotropic and anisotropic solvents, respectively. In previous work, the relative energy for this conformational change of binaphthyl derivatives from cisoid to transoid was estimated to be less than 2 kcal mol−1.45,46 It seems that the molecular interaction between the binaphthyl units on the surface of the Au-NPs and the surrounding selfassembled LC molecules stabilize the transoid conformation in the N-LC. Finally, all ligands with (R)-configuration (R1−R10) showed negative CD couplets in cyclohexane (except R4 showing lefthanded twist direction in the N*-LC, as shown in Figures S20 I

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Table 3. Relationship between the Number of 5CB Host Molecules in Direct Vicinity of the NP Surface, Fs, and in a 3D Voxel Determined by the Volume Fraction of the NPs in 5CB, Fa, and the HTP of One Binaphthyl on the Au-NP Surface at 5 wt % in 5CBa NP Au-R1 Au-S1 Au-R2 Au-S2 Au-R3 Au-S3 R1−R10 and S1−S10 a

D/ nm

Nbinaphthyl

N5CB on NP surface

N5CB around NP

Fs = N5CB on NP surface/ Nbinaphthyl

2.47 1.73 2.13 1.08 1.24 1.09

90 44 67 15 22 16

333 261 299 206 219 207

1.0 × 104 4.1 × 103 7.3 × 103 1.2 × 103 1.9 × 103 1.3 × 103 ∼ 200

3.7 6.0 4.5 13.3 9.9 12.8

Fa = N5CB around NP/ HTP of one binaphthyl on Nbinaphthyl NP/μm−1 116.4 92.4 109.3 74.5 86.0 80.8

12.83 11.31 14.45 11.13 7.98 7.03 4.0−17.1

For comparison, N5CB around the free ligands is also listed with their range of HTP.

binaphthyl units. CD spectra showed bands associated with the electronic transitions of the binaphthyl rings but did not show plasmonic CD signals due to the small size of the Au-NP. The solubility of the Au-NPs differed with NP size and the molecular ratio between organic and inorganic parts of the NPs. We clearly established that most, if not all, parameters of the Au-NPs (curvature, density of ligands, absorption maxima, etc.) are intricately linked to one another with respect to their solubility in organic solvent, miscibility in LC host, absorption maxima, etc. N*-LCs phases were then prepared by adding the Au-NPs into an achiral N-LC host to evaluate and rank their ability and effectiveness in inducing a chiral N*-LC phase, whose properties allow us to draw conclusions about changes in molecular conformation and effects of size and ligand density on this process. The chirality-transfer efficiency from the chiral binaphthyl ligand-capped Au-NPs to the achiral N-LC host generally depends on the size of the Au-NPs. More importantly, however, the chirality transfer efficiency depends on the local number and density of the chiral binaphthyl ligands. Although overall the largest Au-NPs showed a higher efficiency of chirality transfer characterized by higher HTP values, the solubility of these Au-NPs in the LC host is the lowest. Lessons learned from earlier work on cholesterol-capped Au-NPs35 will guide future experiments with binaphthyl-capped Au-NPs. For example, to increase the HTP of the binaphthyl ligand-capped Au-NPs even further, liquid crystal pendant groups could be introduced at the 2′-position of the binaphthyl unit. Particularly noteworthy is the fact that the same parameters affecting the solubility, also affect the calculated HTP values; i.e. the same Au-NPs (within the same size regime) occupy specific quadrants of the dependency plots (see Figures S10 and S19). We found that the conformation of the binaphthyl units on the Au-NP surface changes between isotropic and anisotropic solvent, which is remarkable considering the tight packing on the NP surface and the fact that one end of the molecule is tethered to the surface Au atoms. The binaphthyl units on the Au-NP form a cisoid conformation in isotropic organic solvent and a transoid conformation in the anisotropic N-LC solvent. The long-range elastic interactions within the N-LC host and interactions between host molecules and ligand shell lead to this conformational change, which in turn makes interactions with the N-LC host molecules possible and more effective. Consequently, the helicity of binaphthyl molecules forming a monolayer on the surface of the Au-NPs can be statically changed by controlling the molecular structure of the binaphthyl (vide supra) or by adjusting the size and surface

binaphthyl HTP values in the series induce a bulk helical distortion of the nematic director of approximately 4000 to 10 000 5CB host molecules in each 3D voxel surrounding them. The smaller Au-NPs (Au-S2, Au-S3, and Au-R3) induce a bulk helical distortion in only 1200 to 1900 N-LC host molecules (Figure 6, Table 3). The same number of free ligand molecules of R1−R10 and S1−S10, with HTP values of the same magnitude as the Au-NPs, however, affects less than 200 5CB host molecules in a 3D space defined by their mole fraction and intermolecular space (the molecular weight of the ligands is, on average, less than twice that of 5CB). Even more astounding, despite the smaller size and slightly lower HTP values, the lower number of chiral binaphthyl ligands on the surface of the smaller Au-NPs (Au-S2, Au-R3, and Au-S3) affect (twist) a 2- to 3-fold higher number of 5CB host molecules than the larger three Au-NPs (Au-S1, Au-R1, and Au-R2) in the direct vicinity of the Au-NP surface (i.e., their ratio between the number of 5CB molecules directly surrounding the Au-NP and the number of binaphthyl molecules on the Au-NP surface, Fs, is about 2- to 3-fold higher, see Figure 6a and b). In essence, in this specific case the data suggest that curved nanoscale chiral surfaces have larger chiral correlation lengths, defined by the number of surrounding molecules experiencing a nonvanishing helical director twist, than their related chiral molecular counterparts (i.e., the chiral ligands). This may well be facilitated by the intrinsic chirality of the Au-NP core, an additional chirality transfer from the small gold clusters (which are often intrinsically chiral, although racemic in the absence of chiral ligands) to the ligands as shown by Bürgi et al.61 or by a chiral pattern of the ligands on the surface of the Au-NP as shown Jin et al. for Au133 clusters.62 A perhaps related effect of chirality transfer and amplification has been described by Fujiki et al. for achiral, helical polysilanes spin-coated atop a monolayer of chiral polysilanes chemically grafted onto flat quartz surfaces. The anisotropy factors (or Kuhn dissymmetry factors, g = Δε/ε, where Δε and ε are the molar circular dichroism and molar extinction coefficient, respectively) are highest at film thicknesses around 12 nm and decrease significantly at distances around 40 to 50 nm.63,64

CONCLUSION We have synthesized three sets of Au-NPs functionalized with enantiomeric pairs of binaphthyl ligands differing in the length of the nontethered hydrocarbon chain and induced N*-LC phases by adding these Au-NPs as chiral dopants into an achiral N-LC host. All Au-NPs have narrow size distributions, and the core size (ranging from ∼1 to 2.5 nm) depends on the length of nontethered hydrocarbon chain at the 2′-position of the J

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ACS Nano coverage on the surface of NPs, in addition to dynamical switching via photoresponsive moieties.38,40 Overall, we showed that more effective chirality transfer to a bulk achiral LC phase is possible when a 2D-chiral surface is broken up into small nanoscale particle “pieces” whose volume fraction in a given 3D host matrix is high enough to overcome the smooth relaxation of the director twist that would lead to an achiral bulk configuration. Interestingly, the presence of chiral ligand-capped Au-NPs results in a helical distortion of a much larger (10- to 50-fold larger) number of LC host molecules surrounding each Au-NP in comparison to any of the free ligands. This leads to two important messages. First, the chiral correlation length for the same chiral structure is larger when it is confined at a certain density to a curved nanoscale interface, and second, well-dispersible NPs provide access to highly elusive and very difficult to measure chiral correlation lengths in LC phases. With careful design, we foresee that Au-NPs functionalized with such a highly sensitive and conformational responsive chiral ligand shell will allow for more experimental evidence and data of the highly elusive chiral correlation length in condensed matter systems and will likely be applied in several types of applications. For example, N*-LC patterns with controlled alternating helical sense, pitch, and alignment could be created on various substrates by inkjet printing of chiral AuNPs.65 Likewise, the enantioselectivity of LC physical gels containing such chiral ligand-capped Au-NPs could be altered by passing isotropic or anisotropic solvents.66

Additional information for materials, methods, and synthesis of ligands and nanoparticles; calculations for nanoparticle size and composition; additional UV−vis, CD, ICD, and polarized optical microscopy data; Cano wedge cell and helical twisting power data; calculations related to volume fraction and interparticle distances; NMR spectra. (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] Author Contributions

T.H. directed the research, T.M. and T.H. designed the experiments. T.M. and T.H. wrote the manuscript with input from the other author. T.M. was responsible for NP synthesis and characterization as well as data collection and analysis. A.S. and T.M. performed cell preparation and helical pitch measurements in Cano wedge cells. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF, DMR-1506018), the Ohio Third Frontier (OTF) program for Ohio Research Scholars “Research Cluster on Surfaces in Advanced Materials” (T.H.), which also supports the cryo-TEM facility at the Liquid Crystal Institute (Kent State University), where current TEM data were acquired. Finally, T.M. acknowledges financial support (postdoctoral scholarship) from the Japan Society for the Promotion of Science (JSPS). We also thank L.C. Chien at the Liquid Crystal Institute for access to cell gap measurements.

EXPERIMENTAL SECTION Materials. For general information, materials, synthesis and characterization of ligands and Au-NPs see Supporting Information. The Au-NPs were characterized by 1H NMR spectroscopy (see Supporting Information, section 12), HRTEM imaging/image analysis, TGA analysis, UV−vis spectroscopy, and CD spectropolarimetry (see Supporting Information, section 5). The average size of the Au-NPs was determined by HR-TEM. The weight ratios between gold and organic part were calculated and compared to TGA data. The complete characterization of the Au-NPs is summarized in the Supporting Information. Preparation of Induced N*-LC Phases. The N*-LC phases were prepared by mixing solutions of exactly weighed amounts of LC and Au-NPs. Mixture between 5CB and 5 wt % of the Au-NPs were dissolved in toluene followed by stirring and mild sonication. Thereafter, the solvent was completely evaporated under a stream of dry nitrogen at a temperature above the N/Iso phase transition temperature of the LC material, that is, at 50 °C for 5CB for 3 days. The obtained LC mixtures were finally subjected to pulsed sonication for a few seconds using a sonotrode in the isotropic phase of the LC. ICD of N*-LC Films. For the ICD measurements, thin films of the induced N*-LC phase were prepared between two precleaned quartz substrates (bottom square, 19 × 19 × 0.5 mm; top disc, 1 in. × 1/16 in.) separated by 10-μm fiberglass spacers. The cell gaps of the cells were measured following an established protocol.47

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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.5b07164. K

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DOI: 10.1021/acsnano.5b07164 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.5b07164 ACS Nano XXXX, XXX, XXX−XXX

Significant Enhancement of the Chiral Correlation Length in Nematic Liquid Crystals by Gold Nanoparticle Surfaces Featuring Axially Chiral Binaphthyl Ligands.

Chirality is a fundamental scientific concept best described by the absence of mirror symmetry and the inability to superimpose an object onto its mir...
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