Fabrication and Postmodification of Nanoporous Liquid Crystalline Networks via Dynamic Covalent Chemistry

Cluster-Triggered Self-Luminescence, Rapid Self-Healing, and Adaptive Reprogramming Liquid Crystal Elastomers Enabled by Dynamic Imine Bond

The integration of advanced functions and diverse practical applications calls for multifunctional liquid crystal elastomers (LCEs); however, the structure-intrinsic luminescence and excellent mechanical properties of LCEs have not yet been explored. In this study, clusteroluminescence (CL)-based LCEs (CL-LCEs) are successfully fabricated without depending on large conjugated structures, thereby avoiding redundant organic synthesis and aggregation-caused quenching. The experimental and theoretical results reveal that secondary amine (-NH-) and imine (-C = N-) groups play vital roles in determining the presence of fluorescence in CL-LCEs. Based on the above observation, the strategy universalization and a molecular library for constructing CL-LCEs are further demonstrated. Meanwhile, the dynamic bond of imine bonds endows the CL-LCE system with rapid self-healing under mild conditions (70 °C in 10 min), excellent stretchability, and adaptive programmable characteristics. Furthermore, the self-luminescent performance enables visual detection of the self-healing process. Finally, CL-based information storage and anticounterfeiting are successfully realized and their applications in fiber actuators and fluorescent textiles are demonstrated. The distinctive luminescence and dynamic chemistry presented in this work has significant implications in elucidating the mechanism of CL and providing new strategies for the rational design of novel multifunctional LCE materials.

Tuning the Fluorescence in Dynamic Covalent Bonded Liquid Crystals

A series of emissive liquid crystalline materials based on salicylidene derivatives is reported and investigated with respect to their thermoresponsive and mechanochromic properties. Single-crystal analysis and temperature-dependent powder X-ray diffraction measurements allowed us to correlate the intermolecular organization of the mesogens with thermoresponsive changes in the fluorescence behavior. As a proof-of-principle study, we employed the dynamics of the imine bond in transamination reactions for postsynthetic tuning of the fluorescence behavior as a further step toward the development of adaptive materials.

Manipulation of Liquid Crystalline Properties by Dynamic Covalent Chemistry─En Route to Adaptive Materials

Dynamic covalent bonds bear great potential for the development of adaptive and self-healing materials. Herein, we introduce a versatile concept not only for the design of low-molecular-weight liquid crystals but also for their in situ postsynthetic modification by using the dynamic covalent nature of imine bonds. The methodology allows systematic investigations of structure-property relationships as well as the manipulation of the materials' behavior (liquid crystallinity) and the introduction of additional properties (here, fluorescence) by a solvent-free method. For the first time, the transamination reaction is followed by variable-temperature ¹⁹F solid-state NMR in the mesophase, providing insights into the reaction dynamics in a liquid crystalline material. Finally, the application potential for the design of liquid crystalline materials with adaptive properties is demonstrated by a sequential combination of these reactions.

Polarized Fluorescence of N-Salicylideneaniline Derivatives Formed by In Situ Exchange from N-Benzylideneaniline Side Groups in Photoaligned Liquid Crystalline Copolymer Films

Polarized fluorescence of oriented N-salicylideneaniline (SA) derivatives is explored based on the thermally stimulated photoinduced molecular reorientation of liquid crystalline (LC) copolymethacrylate with N-benzylideneaniline derivative (NBA2) and benzoic acid (BA) side groups. The LC copolymer films show significant cooperative molecular reorientation of the NBA2 and BA side groups (D > 0.7). Subsequent thermal hydrolysis of the NBA2 side groups yields free phenylamine moieties. These moieties can form oriented SA derivatives via in situ condensation with 2-hydroxybenzaldehyde derivatives. The excited-state intermolecular proton transfer of the oriented SA molecules induces polarized fluorescence at 510-548 nm with a polarization ratio up to 6.2. Direct in situ exchange from the oriented NBA2 to SA derivatives achieves polarized fluorescence similar to that of the SA side groups.

Polymerizable Photocleavable Columnar Liquid Crystals for Nanoporous Water Treatment Membranes

Here, we describe a strategy to obtain nanoporous liquid-crystalline (LC) membranes by incorporating a photocleavable ortho-nitrobenzyl group in polymerizable columnar liquid crystals. Two derivatives were synthesized with propylene and nonylene spacers, respectively, between the ionic and the photocleavable moieties to introduce various size nanopores after photocleavage. The membranes were prepared by photopolymerization in the LC states, followed by photocleavage and washing with methanol. The resulting membranes show a virus rejection of 99.99%. Although the rejection value remained almost the same for the two membranes, water flux increased with increasing the length of the alkyl spacers. These membranes were found to be almost free from pinhole defects. The present study offers a new methodology for the development of nanoporous membranes with organized nanostructures for separation technologies.

Modular Design of Porous Soft Materials via Self-Organization of Metal-Organic Cages

Metal-organic frameworks (MOFs) and porous coordination polymers (PCPs) have been well-recognized as emerging porous materials that afford highly tailorable and well-defined nanoporous structures with three-dimensional lattices. Because of their microporous nature, MOFs can accommodate small molecules in their lattice structure, thus discriminating them on the basis of their size and physical properties and enabling their separation even in the gas phase. Such characteristics of MOFs have attracted significant attention in recent years for diverse applications and have ignited a worldwide race toward their development in both academic and industrial fields. Most recently, new challenges in porous materials science demand processable liquid, melt, and amorphous forms of MOFs. This trend will provide a new fundamental class of microporous materials for further widespread applications in many fields. In particular, the application of flexible membranes for gas separation is expected as an efficient solution to tackle current energy-intensive issues. To date, amorphous MOFs have been prepared in a top-down approach by the introduction of disorder into the parent frameworks. However, this new paradigm is still in its infancy with respect to the rational design principles that need to be developed for any approach that may include bottom-up synthesis of porous soft materials. Herein we describe recent progress in bottom-up "modular" approaches for the synthesis of porous, processable MOF-based materials, wherein metal-organic cages (MOCs), alternatively called metal-organic polyhedra (MOPs), are used as "modular cavities" to build porous soft materials. The outer periphery of a MOP is decorated with polymeric and dendritic side chains to obtain a polymer-grafted MOP, imparting both solution and thermal processability to the MOP cages, which have an inherent nanocavity along with high tailorability analogous to MOFs. Well-ordered MOP assemblies can be designed to obtain phases ranging from crystals to liquid crystals, allowing the fabrication of flexible free-standing sheets with preservation of the long-range ordering of MOPs. Furthermore, future prospects of the modular design for porous soft materials are provided with the anticipation that the bottom-up design will combine porous materials and soft matter sciences, leading to the discovery and development of many unexplored new materials and devices such as MOF-based self-healing membranes possessing well-defined nanochannels. The macroscopic alignment of channels can be controlled by external factors, including electric and magnetic fields, external forces, and modified surfaces (templating and patterning), which are conventionally used for engineering of soft materials.

Nanoporous Polymers Based on Liquid Crystals

In the present review, we discuss recent advances in the field of nanoporous networks based on polymerisable liquid crystals. The field has matured in the last decade, yielding polymers having 1D, 2D, and 3D channels with pore sizes on the nanometer scale. Next to the current progress, some of the future challenges are presented, with the integration of nanoporous membranes in functional devices considered as the biggest challenge.

Pore size dependent cation adsorption in a nanoporous polymer film derived from a plastic columnar phase

Nanoporous polymer films were fabricated from a plastic columnar phase that showed pore size and shape (whether open or collapsed) dependent cation adsorption properties.

Paraffinic metal–organic polyhedrons: solution-processable porous modules exhibiting three-dimensional molecular order

Metal–organic polyhedral cages with paraffinic side chains are designed as “porous modules” that self-organize into three-dimensional ordered structures and form into a self-supporting film, affording solution processable porous materials.