Chiroptical Materials Research Articles

Chiral Expression and Morphology Control in Polymer Dispersion Systems.

The chiral self-assembly of polymers in dispersions plays an important role in chiral chemistry and self-assembly. In general, both polymerization-induced self-assembly (PISA) and post-polymerization self-assembly can be used to prepare polymer nanoassemblies. In chirality induction or transfer processes, the self-assembly manner of the polymers greatly affects the chiral expression of the nanostructure and cannot be ignored in the chiral fields. Moreover, unique chiral expression and morphological transition of polymer assemblies in dispersions enable the preparation of advanced functional chiroptical materials. Herein, this Review discusses recent advances in chiral expression and morphology control in polymer dispersion systems, particularly the comparison between traditional post-polymerization self-assembly and in situ PISA strategies, to predict and advance chirality control in polymers.

Folded Propeller Chiral Structures Exclusively Adaptive to Chloroform.

Intramolecular folding is a strategy to construct aryl chiral compounds with applications in chiroptical materials and asymmetrical catalysts. However, beyond polarity the role of solvent in controlling the folded chirality is ambiguous. Here, we report a simple folding protocol to build chiral benzimidazole skeleton with propeller chirality, which could be adaptive to chloroform (CHCl3) with high selectivity. Benzimidazole conjugated with diamino acid arms underwent folding driven by hydrogen bonds, exhibiting propeller chirality of which handedness could be tuned by the absolute chirality of amino acids. Reversible unfolding/folding behavior was realized by heating/cooling process, giving rise to the thermomediated chiroptical switch. Among up to 32 common solvents, chloroform exclusively inverted the propeller chirality. The geometry and hydrogen bonding sites of chloroform allow rearrangement of diamino acid arms into an opposite packing propensity. The chloroform behaves as an invasive linker between diamino acid arms to replace the pristine hydrogen bonds. This work reports the fabrication of chiral aryl compounds by simple folding, which shows the adaptiveness to the chloroform. It demonstrates that not only polarity but also the active participation of solvent could change the chirality and optical activities of small folded molecules.

Reversible Controlling the Supramolecular Chirality of Side Chain Azobenzene Polymers: Chiral Induction and Modulation.

Chirality represents a fundamental structure in nature and the induction and reversible modulation of supramolecular chirality with feasible techniques is of great value in the design of new chiroptical smart materials. Herein, two kinds of azobenzene side-chain polymers (without spacer: Azo-PMA0; with 6 spacers: Azo-PMA6) are synthesized, the length of spacer and azobenzene chromophores play a vital role in chirality transfer and modulation. The supramolecular chiral arrangement of Azo-PMA0 (amorphous phase) can be completely controlled and reversibly modulated over multiple cycles by 450nm circularly polarized light driven by the supramolecular interaction between azo groups of polymer chains, with an absorption dissymmetry factor (g) value of 0.0019. The chiroptical properties of Azo-PMA6 (liquid crystal state) can also be reversibly modulated by UV light and thermal annealing treatment during trans-cis isomerization of azo chromophore, with the g-value changes from 0-0.038. The successful construction of reversible chiral induction and modulation based on side chain azobenzene polymers may pave the way for designing photo-switchable functional materials.

Homeotropic Concentric Helix Orientations in Chiral Nematic Cellulose Nanocrystal Films by Local Magnetic Fields

AbstractMany fascinating device applications of chiral nematic materials are determined by their helical axis orientations. Cellulose nanocrystals (CNCs) self‐assemble producing left‐handed chiral nematic films with predominant planar textures. While varied techniques have been exploited to tune the helical axis orientations, it remains challenging to produce concentrically aligned chiral nematic state. Herein, the authors report novel findings that left‐handed chiral nematic CNC films formed with homeotropic concentric helix orientations in the planar textures can be realized by local radial magnetic field‐directed self‐assembly of CNCs. These chiral nematic CNC films display left‐handed circular polarization patterns and transform spontaneous luminescence to right‐handed circularly polarized luminescence on both film plane and lateral surfaces owing to the underlying photonic bandgaps. These plane‐lateral chiroptical properties can be tuned simultaneously from the visible to the near‐infrared spectral regime. Their work presents a step towards the development of self‐organized chiroptical materials encoded with complex polarization features for photonic applications.

Supramolecular Nanohelix Fabricated by Pillararene-Based Host–Guest System for Chirality Amplification, Transfer, and Circularly Polarized Luminescence in Water

Supramolecular Nanohelix Fabricated by Pillararene-Based Host–Guest System for Chirality Amplification, Transfer, and Circularly Polarized Luminescence in Water

Asymmetric Twisting of C-Centered Octahedral Gold(I) Clusters by Chiral N-Heterocyclic Carbene Ligation.

Asymmetric induction of metal clusters by ligation of chiral ligands is intriguing in terms of the mechanism of chirality transfer and the stability of the resulting chiral structure. Here we report the asymmetric induction of C-centered hexagold(I) CAuI6 clusters into an asymmetrically twisted structure through monodentate, chiral benzimidazolylidene-based N-heterocyclic carbene (NHC) ligands. X-ray diffraction analysis revealed that the NHC-ligated CAuI6 cluster was diastereoselectively twisted with directionally selective, bond length expansion, and contraction of the Au···Au contacts and that the original cluster with high symmetry was transformed into an optically pure, asymmetric CAuI6 cluster with C1 symmetry. Moreover, the circular dichroism spectroscopy and the time-dependent density functional theory calculation confirmed that the asymmetrically twisted CAuI6 structure was maintained even in solution. Such asymmetric induction of configurationally stable metal clusters would greatly expand the molecular design possibilities of asymmetric catalysts and chiroptical materials by utilizing library chiral NHC ligands.

Optical Mode Tuning of Monolayer Tungsten Diselenide (WSe2) by Integrating with One-Dimensional Photonic Crystal through Exciton-Photon Coupling.

Two-dimensional materials, such as transition metal dichalogenides (TMDs), are emerging materials for optoelectronic applications due to their exceptional light–matter interaction characteristics. At room temperature, the coupling of excitons in monolayer TMDs with light opens up promising possibilities for realistic electronics. Controlling light–matter interactions could open up new possibilities for a variety of applications, and it could become a primary focus for mainstream nanophotonics. In this paper, we show how coupling can be achieved between excitons in the tungsten diselenide (WSe2) monolayer with band-edge resonance of one-dimensional (1-D) photonic crystal at room temperature. We achieved a Rabi splitting of 25.0 meV for the coupled system, indicating that the excitons in WSe2 and photons in 1-D photonic crystal were coupled successfully. In addition to this, controlling circularly polarized (CP) states of light is also important for the development of various applications in displays, quantum communications, polarization-tunable photon source, etc. TMDs are excellent chiroptical materials for CP photon emitters because of their intrinsic circular polarized light emissions. In this paper, we also demonstrate that integration between the TMDs and photonic crystal could help to manipulate the circular dichroism and hence the CP light emissions by enhancing the light–mater interaction. The degree of polarization of WSe2 was significantly enhanced through the coupling between excitons in WSe2 and the PhC resonant cavity mode. This coupled system could be used as a platform for manipulating polarized light states, which might be useful in optical information technology, chip-scale biosensing and various opto-valleytronic devices based on 2-D materials.

Self-Assembled Helical Structures of Pyrene-Conjugated Amino Acids for Near-Infrared Chiroptical Materials and Chiral Photothermal Agents

Self-Assembled Helical Structures of Pyrene-Conjugated Amino Acids for Near-Infrared Chiroptical Materials and Chiral Photothermal Agents

Arene-perfluoroarene interaction induced chiroptical inversion and precise ee% detection of chiral acids in a benzimidazole-involved ternary coassembly.

Flexible regulation of chirality and handedness of chiral functional materials and quantitative sensing of natural chiral compounds remains a considerable challenge. Herein, an achiral fluorescent 1-pyrenecarboxylic acid-benzimidazole derivative (PBI) was synthesized and its chiroptical properties upon coassembly with chiral acids (TA and MA) and octafluoronaphthalene (OFN) through hydrogen bonds between benzimidazole and chiral acids as well as an arene-perfluoroarene (AP) interaction between a pyrene moiety and OFN were systemically studied. The binary assemblies of PBI/TA and PBI/MA displayed opposite chiroptical properties including circular dichroism (CD) and circularly polarized luminescence (CPL) signals. Interestingly, the handedness of CPL was further inverted in ternary assemblies due to the synergistic effect of the AP interaction and hydrogen bonds. Besides, the highly accurate chiral sensing of chiral acids via binary assemblies was successfully achieved with a high correlation coefficient. This work provides a simple method for regulating the handedness of chiroptical active materials and quantitative sensing of chiral acids through orthogonal multiple component coassemblies.

Topological-skeleton controlled chirality expression of supramolecular hyperbranched and linear polymers

The topology of polymers plays an essential role in their chemical, physical and biological properties. However, their effects on chirality-related functions remain unclear. Here, we reported the topology-controlled chirality expression in the chiral supramolecular system for the first time. Two topological supramolecular polymers, hyperbranched (HP) and linear (LP) supramolecular polymers produced by the host–guest interactions of branched and linear monomers, respectively, exhibited completely different chirality expressions despite the same molecular chirality of their monomers. Significantly, due to the branch points and strong steric hindrance existing in HP, cis-HP showed an enhanced and sign-inverted Cotton effect in the n–π* bands compared with cis-LP, as a result that the distinctive chirality induction and transfer were controlled by the topological skeletons. This topology-controlled chirality induction and transfer in the photoswitchable supramolecular polymers not only enables us to elucidate the in-depth effects of topology on the chiral expression in biopolymers but also inspires the design of chiroptical and bioinspired materials.

Phthalocyanine-Triggered Helical Dipeptide Nanotubes with Intense Circularly Polarized Luminescence.

Nanotubes with circularly polarized luminescence (CPL) are attracting much attention due to many potential applications, such as chiroptical materials, displays, and sensing. However, it remains a challenge to change the assemblies of ordinarily molecular building blocks into CPL supramolecular nanotubes. Herein, the regulation of quite common dipeptide (Fmoc-FF) assemblies into unprecedented helical nanotubes exhibiting intense CPL is reported by simply doping a few phthalocyanine (octakis(carboxyl)phthalocyaninato zinc complex (Pc)) molecules. Interestingly, altering the Fmoc-FF/Pc molar ratios over a wide range cannot change the nanotubes structures according to transmission electron microscopy (TEM) and atomic force microscope (AFM) measurements. Although molecular dynamics simulations suggest that the noncovalent interactions between Fmoc-FF and Pc are quite weak, few Pc molecules can still change the secondary structures of a large number of Fmoc-FF assemblies, which hierarchically form helical supramolecular nanotubes with long-range ordered molecular packing, leading to intense CPL signals with large luminescence dissymmetry factor (glum = 0.04). Consequently, the chiral reorganization of Fmoc-FF assemblies is dependent on the coassembly between Pc molecule and Fmoc-FF supramolecular architectures. These results open the possibility for the fine-tuning of helix and supramolecular nanotubes with CPL properties by using a small number of cofactors.

Stacked Reticular Frame Boosted Circularly Polarized Luminescence of Chiral Covalent Organic Frameworks

AbstractChiral covalent organic frameworks (COFs) with circularly polarized luminescence (CPL) are intriguing as advanced chiroptical materials but have not been reported to date. We constructed chiroptical COF materials with CPL activity through the convenient Knoevenagel condensation of formyl‐functionalized axially chiral linkers and C3‐symmetric 1,3,5‐benzenetriacetonitrile. Remarkably, the as‐prepared chiral COFs showed high absorption and luminescent dissymmetric factors up to 0.02 (gabs) and 0.04 (glum), respectively. In contrast, the branched chiral polymers from the same starting monomers were CPL silent. Structural and spectral characterization revealed that the reticular frame was indispensable for CPL generation via confined chirality transfer. Moreover, reticular stacking boosted the CPL performance significantly due to the interlayer restriction of frame. This work demonstrates the first example of a CPL‐active COF and provides insight into CPL generation through covalent reticular chemistry, which will play a constructive role in the future design of high‐performance CPL materials.

Stacked Reticular Frame Boosted Circularly Polarized Luminescence of Chiral Covalent Organic Frameworks.

Chiral covalent organic frameworks (COFs) with circularly polarized luminescence (CPL) are intriguing as advanced chiroptical materials but have not been reported to date. We constructed chiroptical COF materials with CPL activity through the convenient Knoevenagel condensation of formyl-functionalized axially chiral linkers and C3-symmetric 1,3,5-benzenetriacetonitrile. Remarkably, the as-prepared chiral COFs showed high absorption and luminescent dissymmetric factors up to 0.02 (gabs ) and 0.04 (glum ), respectively. In contrast, the branched chiral polymers from the same starting monomers were CPL silent. Structural and spectral characterization revealed that the reticular frame was indispensable for CPL generation via confined chirality transfer. Moreover, reticular stacking boosted the CPL performance significantly due to the interlayer restriction of frame. This work demonstrates the first example of a CPL-active COF and provides insight into CPL generation through covalent reticular chemistry, which will play a constructive role in the future design of high-performance CPL materials.

B,N-Embedded Double Hetero[7]helicenes with Strong Chiroptical Responses in the Visible Light Region.

The development of helicene molecules with significant chiroptical responses covering a broad range of the visible spectrum is highly desirable for chiral optoelectronic applications; however, their absorption dissymmetry factors (gabs) have been mostly lower than 0.01. In this work, we report unprecedented B,N-embedded double hetero[7]helicenes with nonbonded B and N atoms, which exhibit excellent chiroptical properties, such as strong chiroptical activities from 300 to 700 nm, record high gabs up to 0.033 in the visible spectral range, and tunable circularly polarized luminescence (CPL) from red to near-infrared regions (600-800 nm) with high photoluminescence quantum yields (PLQYs) up to 100%. As revealed by theoretical analyses, the high gabs values are related to the separate molecular orbital distributions owing to the incorporation of nonbonded B and N atoms. The new type of B,N-embedded double heterohelicenes opens up an appealing avenue to the future exploitation of high-performance chiroptical materials.

Halogen Bonded Chiral Emitters: Generation of Chiral Fractal Architecture with Amplified Circularly Polarized Luminescence

AbstractSelf‐assembled chiroptical materials have attracted considerable attention due to their great applications in wide fields. During the chiral self‐assembly, it remains unknown how achiral molecules can affect the assembly process and their final chiroptical performance. Herein, we report an achiral molecule directed chiral self‐assembly via halogen bonds, exhibiting not only an unprecedented chiral fractal architecture but also significantly amplified circularly polarized luminescence (CPL). Two axially chiral emitters with halogen bond sites co‐assemble with an achiral 1,4‐diiodotetrafluorobenzene (F4DIB) and well‐ordered chiral fractal structures with asymmetry amplification are obtained. The enhancement of the dissymmetry factors of the assemblies was up to 0.051 and 0.011, which was approximately 100 folds than those of the corresponding molecules. It was found that both the design of the chiral emitter and the highly directional halogen bond played an important role in hierarchically chirality transfer from chiral emitters to the micrometer scale chiral fractal morphology and amplified dissymmetry factors. We hope that this strategy can give a further insight into the fabrication of structurally unique featured highly efficient chiroptical materials.

Halogen Bonded Chiral Emitters: Generation of Chiral Fractal Architecture with Amplified Circularly Polarized Luminescence.

Self-assembled chiroptical materials have attracted considerable attention due to their great applications in wide fields. During the chiral self-assembly, it remains unknown how achiral molecules can affect the assembly process and their final chiroptical performance. Herein, we report an achiral molecule directed chiral self-assembly via halogen bonds, exhibiting not only an unprecedented chiral fractal architecture but also significantly amplified circularly polarized luminescence (CPL). Two axially chiral emitters with halogen bond sites co-assemble with an achiral 1,4-diiodotetrafluorobenzene (F4 DIB) and well-ordered chiral fractal structures with asymmetry amplification are obtained. The enhancement of the dissymmetry factors of the assemblies was up to 0.051 and 0.011, which was approximately 100 folds than those of the corresponding molecules. It was found that both the design of the chiral emitter and the highly directional halogen bond played an important role in hierarchically chirality transfer from chiral emitters to the micrometer scale chiral fractal morphology and amplified dissymmetry factors. We hope that this strategy can give a further insight into the fabrication of structurally unique featured highly efficient chiroptical materials.

Toward Large Dissymmetry Factor of Circularly Polarized Luminescence in Donor-Acceptor Hybrid Systems.

Chiroptical materials with circularly polarized luminescence (CPL) activity have aroused a lot of interest. One essential factor for evaluating the features of CPL-active materials is the dissymmetry factor (glum), which represents the circular polarization of emitted light. Essentially, for the practical application of CPL, the most important issue is to greatly improve the glum value. Recently, benefiting from the flexible and efficient design in hybrid donor-acceptor systems, more and more examples involving glum value amplification have been reported. In this Perspective, we highlight the proposed mechanism for the generation and amplification of CPL in these hybrid systems. We also present the corresponding design principles and potential pitfalls in experimental processes. We hope that this Perspective can shed light on the development of highly efficient CPL-active materials.

Halogen Bonding Mediated Hierarchical Supramolecular Chirality

As a highly directional force, halogen bonds based on σ-holes have potential to manipulate supramolecular chirality and build functional chiral systems, which however are largely unexplored. In this work, we report the manipulation of supramolecular chirality in hierarchically self-assembled systems via intracomponent halogen bonds. Cholesteryl cyanostilbene conjugates and 1,3,5-trifluoro-2,4,6-triiodobenzene formed a C3 symmetrical supramolecular complex with ultrahigh binding affinity and binding constants at 1011 order of magnitude. The halogen bonded propeller geometries exhibited inversed chirality as well as chiroptical activity compared to the pristine helically orientated aggregates. Halogen bonds enabled the engineering of nanoarchitectures, affording supercoiled helical structures and highly aligned nanotubes. This work unveils the role of halogen bonds in controlling supramolecular chirality, establishing a protocol to build functional chiroptical materials from species containing σ-holes and halogenated domains.

Manipulations of Chiroptical Properties in Belt‐Persistent Cycloarylenes via Desymmetrization with Heteroatom Doping

AbstractA desymmetrization strategy has been devised in the design of molecular cylinders to maximize the dissymmetry factor relevant to circularly polarized light. Although the highest dissymmetry factor of organic molecules was previously achieved with a chiral belt‐persistent cycloarylene having magnetic and electric transition dipole moments in parallel, we noticed that an unbalanced magnitude of two moments was detrimental for higher dissymmetry factors. In this study, a molecular cylinder was desymmetrized by arraying doped and undoped panels via stereoselective cross‐coupling macrocyclization. The desymmetrization succeeded in balancing two moments by reducing the electric transition moment at the global minimum but failed to maximize the dissymmetry factor. Structural studies revealed that the dissymmetry factor is sensitive to subtle structural fluctuations, while the rotatory strength is not affected. This study is important for the development of chiroptical materials.

Manipulations of Chiroptical Properties in Belt-Persistent Cycloarylenes via Desymmetrization with Heteroatom Doping.

A desymmetrization strategy has been devised in the design of molecular cylinders to maximize the dissymmetry factor relevant to circularly polarized light. Although the highest dissymmetry factor of organic molecules was previously achieved with a chiral belt-persistent cycloarylene having magnetic and electric transition dipole moments in parallel, we noticed that an unbalanced magnitude of two moments was detrimental for higher dissymmetry factors. In this study, a molecular cylinder was desymmetrized by arraying doped and undoped panels via stereoselective cross-coupling macrocyclization. The desymmetrization succeeded in balancing two moments by reducing the electric transition moment at the global minimum but failed to maximize the dissymmetry factor. Structural studies revealed that the dissymmetry factor is sensitive to subtle structural fluctuations, while the rotatory strength is not affected. This study is important for the development of chiroptical materials.