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Aromatic Helicenes
Published in Klaus D. Sattler, st Century Nanoscience – A Handbook, 2020
Functionalized derivatives of helicenes were already applied to different branches of science that were enabled by the remarkable development in their synthesis discussed above. The unique structure, inherent chirality, extended 3D aromatic system, intriguing physico-chemical properties as well as plasticity of their design make them not only challenging synthetic targets but also attractive materials for further exploration.
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Published in Luis Liz-Marzán, Colloidal Synthesis of Plasmonic Nanometals, 2020
Andrés Guerrero-Martinez, José Lorenzo Alonso-Gómez, Baptiste Auguie, M. Magdalena Cid, Luis M. Liz-Marzán
Coinciding with early studies on chirality, but until recently following a completely different path, the study of colloidal metal particles traces back to the 19 th and early 20th centuries. The seminal works of Faraday [21] and Mie [22] provided a scientific ground to the phenomenal attraction of metal nanoparticles and their unique optical properties. Noble metals - silver and gold in particular - exhibit interesting optical properties at the nanoscale that differ markedly from those of the bulk material, and also from those of the isolated atomic constituents, due to the excitation of localized surface plasmon resonances (LSPRs) [23]. Following recent advances in the synthesis and surface modification of metallic nanocrystals [24], the ease of their integration into nanocomposites [25], and the control over their plasmonic properties [26], chiral metal nanostructures have emerged, and justify the present review. Whereas excellent reviews by Gautier and Bürgi [15] and Noguez and Garzon [27] have been specifically devoted to sub-nanometer metal clusters with inherent chirality and a strong associated optical activity, a report highlighting advances in the field of chiral plasmonic nanoparticles is still missing. This gap in the literature has occurred due to difficulties in the chemical synthesis of nanoparticles with intrinsic chiral morphology, or the reliable assembly of achiral particles into chiral superstructures [28]. Since the field of plasmonics is currently undergoing fast development, we intend to focus this review on the recent work with isotropic and anisotropic plasmonic nanoparticles, and only briefly mention chiral metal nanoclusters, placing each system within the context of the corresponding origin of the observed chirality. Future prospects in this field will also be discussed; we hope that through this perspective and the diversity of structures conveyed in this review article the reader will gain a greater appreciation of chirality in metal nanoparticles.
Gold Nanoclusters with Atomic Precision: Optical Properties
Published in Yan Zhu, Rongchao Jin, Atomically Precise Nanoclusters, 2021
Thiol ligand-protected gold clusters for use in various imaging technologies have become a research hotspot due to the high stability and biocompatibility of ligands, and they are promising candidates for imaging applications. However, the SHG of thiolate-protected gold clusters is rare. Knoppe et al. reported SHG for Au25(SCH2CH2Ph)18 and Au38(SCH2CH2Ph)24 [140]. Because SHG is directly related to the symmetry of the crystal structure, Au25(SCH2CH2Ph)18, which has an inversion center, does not produce a significant SHG signal. As a control, the chiral Au25(Capt)18 (Capt: captopril) has a SHG response. In contrast, the inherent chirality of Au38(SCH2CH2Ph)24 leads to its optical activity of the second harmonic. These experimental phenomena all indicate that SHG is sensitive to the structural symmetry of the material. In addition, two-photon fluorescence and third harmonics of Au25 and Au38 were also observed. Recently, Knoppe et al. investigated the first hyperpolarizability of a series of model clusters [Aum(SH)n]z (m = 18–38) through DFT [142]. Note that this work focuses on the contribution of the cluster itself to the hyperpolarizability, rather than the contribution of the ligand. The calculated results showed that there is no correlation between the first hyperpolarizability and the size, but it instead relies on the symmetry of clusters. The chiral Au38 cluster has a strong nonlinear optical response, whereas that of the centrosymmetric Au25 cluster is close to zero; this finding is consistent with previous experimental results [140]. Nevertheless, the central symmetry of the Au25 cluster can be effectively destroyed by ligand exchange, which causes the strong nonlinear optical response of Au25, even surpassing inherent chiral clusters such as Au38. These nonlinear optical responses of gold clusters render them interesting candidates for biological tissue imaging applications. It is worth mentioning that, compared to two-photon absorption, no photon absorption occurs during the second harmonic generation, and no energy level transition occurs.
Testing different supervised machine learning architectures for the classification of liquid crystals
Published in Liquid Crystals, 2023
Ingo Dierking, Jason Dominguez, James Harbon, Joshua Heaton
Liquid crystals (LCs) are fluids with partial order, thermodynamically located between that of the isotropic and the crystalline phases [1,2]. Whilst perfect crystals have full three‐dimensional order and isotropic fluids have none, LC phases have orientational order and sometimes one‐, two‐ or three‐dimensional positional order. Thermotropic liquid crystal phases can be categorised into several different classes. The phase with the lowest order and highest symmetry is the nematic, N, phase, which solely exhibits orientational order of the long molecular axis along a preferred direction called the director. This phase often shows characteristic topological defects in the form of two or four brushes, called the Schlieren texture [3], which makes it easy to be identified via machine learning. If the nematic phase is chiral, either by inherent chirality due to chiral elements within the molecular structure or through the addition of a chiral dopant, one speaks of the cholesteric, N*, phase. This exhibits a helical superstructure of the director and gives rise to characteristic oily-streak defects, which are also easily observed in polarised microscopy [3].
Supramolecular self-assembly of cytidine monophosphate-di-copper building blocks
Published in Journal of Coordination Chemistry, 2022
Teresa F. Mastropietro, Giovanni De Munno
Besides being the constituents of life able to exert important biological functions, biomolecules also represent an excellent source of building blocks which can replace synthetic organic molecules in the fabrication of functional bio-inspired materials with enhanced bio- and eco-compatibility [1–4]. Within this context, the coordination of natural biomolecules to transition metal ions offers a reliable approach for modular assembly of simple components into flexible architectures of increasing complexity, under mild conditions [5]. In these hybrid bio-inorganic coordination polymers, biomolecules are mainly responsible for the resultant supramolecular architecture and, thanks to their inherent chirality, also represent effective chirality delivery [6,7]. On the other hand, the presence of the metal centers provides introduction of additional functional properties, including luminescent, catalytic, and magnetic properties, which can be exploited for different applications, ranging from biosensing, catalysis, to cancer therapy [8,9].
A chiral Salen-based Zn(II)-Cd(II) heterometallic metal-organic framework: synthesis, crystal structure, and optical properties
Published in Journal of Coordination Chemistry, 2022
Bei Wang, Xue-Zhi Wang, Dong Luo, Xiao-Ping Zhou, Dan Li
UV–vis spectra of ZnL and 1 were recorded in the solid state. The results showed that similar absorption bands at 315 and 380 nm were obtained. The absorption bands around 250–450 nm in the spectra are ascribed to π→π* transitions of L [15]. Due to the d10 configuration of Zn(II), d-d transitions were not observed in the spectra (Figure 5a). For comparisons, the circular dichroism (CD) spectra of ZnL and 1 were measured in the solid state. As shown in Figure 5b, obvious chiroptical signals are observed around 420 nm, perhaps due to the π→π* transitions of ZnL [16]. Both ZnL and 1 have negative dichroic signals at 418 and 428 nm, respectively, suggesting that the inherent chirality of ZnL was transmitted to the resulting MOF 1.