Competitive chiral induction in a 2D molecular assembly: Intrinsic chirality versus coadsorber-induced chirality

Revolutionizing the structural design and determination of covalent-organic frameworks: principles, methods, and techniques

Covalent organic frameworks (COFs) represent an important class of crystalline porous materials with designable structures and functions. The interconnected organic monomers, featuring pre-designed symmetries and connectivities, dictate the structures of COFs, endowing them with high thermal and chemical stability, large surface area, and tunable micropores. Furthermore, by utilizing pre-functionalization or post-synthetic functionalization strategies, COFs can acquire multifunctionalities, leading to their versatile applications in gas separation/storage, catalysis, and optoelectronic devices. Our review provides a comprehensive account of the latest advancements in the principles, methods, and techniques for structural design and determination of COFs. These cutting-edge approaches enable the rational design and precise elucidation of COF structures, addressing fundamental physicochemical challenges associated with host-guest interactions, topological transformations, network interpenetration, and defect-mediated catalysis.

Homochiral hierarchical molecular assemblies through dynamic combination of conformational states of a single chiral building block at the liquid/solid interface

We herein report the construction of homochiral, hierarchical self-assembled molecular networks (SAMNs) at the liquid/graphite interface using a single molecular building block, a chiral dehydrobenzo[12]annulene (cDBA) derivative with three chiral alkoxy and three hydroxy groups positioned in an alternating manner on the DBA core. The cDBA molecules form homochiral hierarchical SAMNs consisting of triangular clusters of several sizes, the size of which can be tuned by solvent polarity and solute concentration, reaching periodicities as large as 9.3 nm. We demonstrate the successful transmission of chirality information from the single molecular level to the hierarchical SAMN level, in a process that is mediated by dynamic self-sorting.

Excitation-Dependent Fluorescence with Excitation-Selective Circularly Polarized Luminescence from Hierarchically Organized Atomic Nanoclusters

Nanometer-scaled objects are known to have dimension-related properties, but sometimes the assembly of such objects can lead to the emergence of other properties. Here, we show the assembly of atomically precise gold nanoclusters into large fibrillar structures that are featuring excitation-dependent luminescence with an excitation-selective circularly polarized luminescence (CPL), even though all components are achiral. The origin of CPL in the assembly of atomic clusters has been attributed to the hierarchical organization of atomic clusters into fibrillar structures, mediated via a hydrogen bonding interaction with a surfactant. We follow the assembly process both experimentally and computationally showing the advance in the structural formation along with its chiroptical electronic properties, i.e., circular dichroism (CD) and CPL. Our study here can assist in the rational design of materials featuring chiroptical properties, thus leading to a controlled CPL activity.

Two-dimensional molecular networks at the solid/liquid interface and the role of alkyl chains in their building blocks

Nanoarchitectonics has attracted increasing attention owing to its potential applications in nanomachines, nanoelectronics, catalysis, and nanopatterning, which can contribute to overcoming global problems related to energy and environment, among others. However, the fabrication of ordered nanoarchitectures remains a challenge, even in two dimensions. Therefore, a deeper understanding of the self-assembly processes and substantial factors for building ordered structures is critical for tailoring flexible and desirable nanoarchitectures. Scanning tunneling microscopy is a powerful tool for revealing the molecular conformations, arrangements, and orientations of two-dimensional (2D) networks on surfaces. The fabrication of 2D assemblies involves non-covalent interactions that play a significant role in the molecular arrangement and orientation. Among the non-covalent interactions, dispersion interactions that derive from alkyl chain units are believed to be weak. However, alkyl chains play an important role in the adsorption onto substrates, as well as in the in-plane intermolecular interactions. In this review, we focus on the role of alkyl chains in the formation of ordered 2D assemblies at the solid/liquid interface. The alkyl chain effects on the 2D assemblies are introduced together with examples documented in the past decades.

Surface chiroselective assembly of enantiopure crystalline porous films containing bichiral building blocks

The development of chiral crystalline porous materials (CPMs) containing multiple chiral building blocks plays an important role in chiral chemistry and applications but is a challenging task. Herein, we report the first example of bichiral building block based enantiopure CPM films containing metal-organic cages (MOCs) and metal complexes. The functionalized substrate was immersed subsequently into homochiral metal complex (R)- or (S)-Mn(DCH)3 (DCH = 1,2-diaminocyclohexane) and racemic Ti4L6 cage (L = embonate) solutions by a layer-by-layer growth method. During the assembly process, the substrate surface coordinated with (R)- or (S)-Mn(DCH)3 can, respectively, layer-by-layer chiroselectively connect Δ- or Λ-Ti4L6 cages to form homochiral (R, Δ)- or (S, Λ)-CPM films with a preferred [111] growth orientation, tunable thickness and homogeneous surface. The resulting enantiopure CPM films show strong chirality, photoluminescence, and circularly polarized luminescence (CPL) properties as well as good enantioselective adsorption toward enantiomers of 2-butanol and methyl-lactate. The present in situ surface chiroselective strategy opens a new route to assemble homochiral CPM films containing multiple chiral building blocks for chiral applications.

Asymmetric metal–organic frameworks with double helices for enantioselective recognition

A pair of homochiral metal–organic frameworks are elaborated by employing flexible enantiopure ligands.

2D Co-crystallization of molecular homologues promoted by size complementarity of the alkyl chains at the liquid/solid interface

Co-crystallization of organic molecules is an important strategy for the fabrication of molecular materials. In this contribution, we investigated the mixing behavior of 5-(benzyloxy)-isophthalic acid homologues (BIC-Cn, n = 6, 8, 10, 12, and 14) at the liquid/solid interface using a scanning tunneling microscope. Deposition of the single component of BIC-Cn always results in typical honeycomb networks, whereas co-deposition of two BIC-Cn homologues leads to hybrid double-walled honeycomb networks or phase separation depending on the difference in the length of their alkyl chains. 2D co-crystallization can only be realized for BIC-C6/BIC-C10 or BIC-C8/BIC-C12 which have a four-methyl unit difference in their alkyl chains. The size complementarity of the alkyl chains in the two components suggests that it is responsible for the 2D co-crystallization, though hydrogen bonding contributes a lot both to the pristine honeycomb network and to the hybrid co-crystal. This result is of importance for understanding the role of van der Waals interaction and its interplay with hydrogen bonding in 2D co-crystallization.

Two-dimensional supramolecular crystal engineering: chirality manipulation

Two dimensional (2D) supramolecular crystal engineering, one of the most important strategies towards nanotechnology, is both a science and an industry. In the present review, the recent advances in 2D supramolecular crystal engineering through chirality manipulation on solid surfaces are summarized, with the aid of the scanning tunneling microscopy technique. On-surface chirality manipulation includes surface confined structural chirality formation, chirality transformation, chirality separation as well as chirality elimination, by using component exchange and different external stimuli. Under this principle, host-guest supramolecular interactions, solvent induction, temperature regulation and STM-tip driven orientation control and reorientation effects under equilibrium or out-of-equilibrium conditions, towards the generation of the best-adapted chiral or achiral 2D nanostructures, are mainly described and highlighted. Future challenges and opportunities in this exciting area are also then discussed.

Intermediate States Directed Chiral Transfer on a Silver Surface

Chiral synthesis on surfaces has acquired tremendous interest. We herein report a novel approach of two-dimensional chiral transfer directed by metal-organic intermediate states on a silver surface. With initial deposition at low temperature, the achiral 4,4'-dihydroxybiphenyl molecules self-assemble into large scale two-dimensional networks with 4-fold symmetry via intermolecular hydrogen bonding. Fine controlled annealing, however, leads to the formation of tetramer-like chiral metal-organic hybrids, which self-organize into enantiomeric islands on the Ag(100) surface. Subsequent ortho C-C couplings of the reactants lead to dimer products. Of great importance, the chirality expressions of the dimer products are observed to be transferred directly from that of the tetramer intermediate states. The detailed reaction pathways are rationalized by DFT calculations and synchrotron-based XPS experiments, demonstrating the mechanisms of the chiral transfer.