Liquid Crystal Institute, Chemical Physics Interdisciplinary Program and ‡Department of Chemistry and Biochemistry, Kent State University , Kent, Ohio 44242-0001 United States.
ACS Nano. 2016 Jan 26;10(1):1552-64. doi: 10.1021/acsnano.5b07164. Epub 2016 Jan 12.
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.
手性是一个基本的科学概念，最好通过缺乏镜像对称性和无法通过平移和旋转将物体叠加到其镜像上来描述。手性表现在几乎所有的分子水平上，从单个分子到超分子系统，并且在自然界中无处不在。在这里，为了探索手性如何从手性纳米级表面传播，我们研究了用轴向手性联萘分子功能化的金纳米粒子。特别是，我们合成了三对对映体手性配体封端的金纳米粒子，它们在尺寸、曲率和配体密度上有所不同，以调节手性从纳米级固体表面传递到各向异性液晶介质。最终，我们正在研究纳米粒子表面的手性能延伸到多厚的块状材料中。用联萘硫醇修饰的金纳米粒子的圆二色光谱证实，联萘部分在各向同性有机溶剂中形成顺式构象。在手性向列液晶相中，通过将金纳米粒子分散到各向异性非手性向列液晶溶剂中诱导形成，纳米粒子表面上的联萘部分形成反式构象，这是通过成像诱导胆甾相的螺旋扭曲方向来确定的。这表明纳米级金属表面上的配体密度提供了一个动态空间，可以改变和调整联萘衍生物的螺旋度，以响应周围介质的有序性。确定了诱导的手性向列相的螺旋节距值，并计算了手性金纳米粒子的螺旋扭曲功率（HTP），以阐明联萘配体封端的金纳米粒子的手性转移效率。值得注意的是，HTP随粒子直径的增加而增加，即与粒子尺寸减小相比，结合到纳米粒子表面的联萘单元的手性转移效率降低。然而，与游离配体相比，每个测试的金纳米粒子都会在围绕每个粒子的液晶主分子数量增加 10 到 50 倍的情况下诱导螺旋扭曲，这表明手性相关长度显著增加。我们提出，通过调整粒子尺寸和曲率以及手性配体的数量和密度，可以在手性纳米粒子表面控制轴向手性联萘衍生物的螺旋度和手性转移效率，最终测量和调节手性相关长度。
