Achiral Guest Research Articles

Temperature‐Dependent Left‐ and Right‐Twisted Conformational Changes in 1:1 Host‐Guest Systems: Theoretical Modeling and Chiroptical Simulations

An efficient strategy for high‐performance chiral materials is to design and synthesize host molecules with left‐ and right‐ (M‐ and P‐) twisted conformations and to control their twisted conformations. For this, a quantitative analysis is required to describe the chiroptical inversion, chiral transfer, and chiral recognition in the host‐guest systems, which is generally performed using circular dichroism (CD) and/or proton nuclear magnetic resonance (1H‐NMR) spectroscopies. However, the mass‐balance model that considers the M‐ and P‐twisted conformations has not yet been established. In this study, we derived the novel equations based on the mass‐balance model for the 1:1 host‐guest systems. Then, we further applied them to analyze the 1:1 host‐guest systems for the achiral calixarene‐based capsule molecule, achiral dimeric zinc porphyrin tweezer molecule, and chiral pillar[5]arene with the chiral and/or achiral guest molecules by using the data obtained from the CD titration, variable temperature CD (VT‐CD), and 1H‐NMR experiments. The thermodynamic parameters (ΔH and ΔS), equilibrium constants (K), and molar CD (Δε) in the 1:1 host‐guest systems could be successfully determined by the theoretical analyses using the derived equations.

Temperature-Dependent Left- and Right-Twisted Conformational Changes in 1:1 Host-Guest Systems: Theoretical Modeling and Chiroptical Simulations.

An efficient strategy for high-performance chiral materials is to design and synthesize host molecules with left- and right- (M- and P-) twisted conformations and to control their twisted conformations. For this, a quantitative analysis is required to describe the chiroptical inversion, chiral transfer, and chiral recognition in the host-guest systems, which is generally performed using circular dichroism (CD) and/or proton nuclear magnetic resonance (1H-NMR) spectroscopies. However, the mass-balance model that considers the M- and P-twisted conformations has not yet been established. In this study, we derived the novel equations based on the mass-balance model for the 1:1 host-guest systems. Then, we further applied them to analyze the 1:1 host-guest systems for the achiral calixarene-based capsule molecule, achiral dimeric zinc porphyrin tweezer molecule, and chiral pillar[5]arene with the chiral and/or achiral guest molecules by using the data obtained from the CD titration, variable temperature CD (VT-CD), and 1H-NMR experiments. The thermodynamic parameters (ΔH and ΔS), equilibrium constants (K), and molar CD (Δε) in the 1:1 host-guest systems could be successfully determined by the theoretical analyses using the derived equations.

Chirality Induction and Memory of Pillar[4]arene[1]quinone Derivatives in Visible-Light Range.

Pillar[4]arene[1]quinone derivatives (PQXs) were synthesized by the oxidation of pillar[5]arenes, which exhibited notable charge transfer (CT) transitions at approximately 485 nm. Successful chiral resolution of two pairs of enantiomeric conformers was achieved. Despite reduced binding affinity, PQXs demonstrated slower racemization kinetics. Visible-light chiroptical induction with a significant dissymmetry factor was attained by complexing PQXs with a chiral guest. The induced enantiomeric excess could be maintained through competitive binding with an achiral guest, offering a promising strategy for chiral sensing and memory.

Study of optical rotation based on the molecular structure in fused oligomers of macrocycles.

We designed a unique oligomer form in which several helically twisted macrocycles (M- or P-helicity) are arranged through fusion. We investigated the optical rotation of a series of fused oligomers of macrocycles with a difference in the number and arrangement of elements associated with point-chiral auxiliary. Some oligomers cooperatively attained a situation where an identical sense of twisting was preferred throughout the entire molecule. On the basis of these results, we estimated diastereomeric excess induced in each oligomer. We revealed that the molar optical rotation per element was modulated with a rotational angle between elements: an increase via 0° rotational arrangement, a decrease via 180° rotational arrangement, or a decrease via cyclic arrangement. Alternatively, for other oligomers in which several diastereomeric conformers coexist, we uniquely attempted to consider the optical rotation based on the molecular structure through the assessment of a change ratio of the absorption dissymmetry factor before and after complexation with an achiral guest. We found that molar optical rotation could be different based on the arrangement, although actual measured values were similar.

A Chiral Emissive Conjugated Corral for High-Affinity and Highly Enantioselective Recognition in Water.

Accurately distinguishing between enantiomeric molecules is a fundamental challenge in the field of chemistry. However, there is still significant room for improvement in both the enantiomeric selectivity (KR(S)/KS(R)) and binding strength of most reported macrocyclic chiral receptors to meet the demands of practical application scenarios. Herein, we synthesized a water-soluble conjugated tubular host-namely, corral[4]BINOL-using a chiral 1,1'-bi-2-naphthol (BINOL) derivative as the repeating unit. The conjugated chiral backbone endows corral[4]BINOL with good fluorescent emission (QY = 34% ) and circularly polarized luminescence (|glum| up to 1.4×10-3) in water. Notably, corral[4]BINOL exhibits high recognition affinity up to 8.6×1010 M-1 towards achiral guests in water, and manifested excellent enantioselectivity up to 18.7 towards chiral substrates, both of which represent the highest values observed among chiral macrocycles in aqueous solution. The ultrastrong binding strength, outstanding enantioselectivity, facile accessibility, together with the superior fluorescent and chiroptical properties, endow corral[4]BINOL with great potentials for a wide range of applications.

A Chiral Emissive Conjugated Corral for High‐Affinity and Highly Enantioselective Recognition in Water

AbstractAccurately distinguishing between enantiomeric molecules is a fundamental challenge in the field of chemistry. However, there is still significant room for improvement in both the enantiomeric selectivity (KR(S)/KS(R)) and binding strength of most reported macrocyclic chiral receptors to meet the demands of practical application scenarios. Herein, we synthesized a water‐soluble conjugated tubular host—namely, corral[4]BINOL—using a chiral 1,1′‐bi‐2‐naphthol (BINOL) derivative as the repeating unit. The conjugated chiral backbone endows corral[4]BINOL with good fluorescent emission (QY=34 % ) and circularly polarized luminescence (|glum| up to 1.4×10−3) in water. Notably, corral[4]BINOL exhibits high recognition affinity up to 8.6×1010 M−1 towards achiral guests in water, and manifested excellent enantioselectivity up to 18.7 towards chiral substrates, both of which represent the highest values observed among chiral macrocycles in aqueous solution. The ultrastrong binding strength, outstanding enantioselectivity, and facile accessibility, together with the superior fluorescent and chiroptical properties, endow corral[4]BINOL with great potential for a wide range of applications.

Photocontrolled Reversibly Chiral-Ordered Assembly Based on Cucurbituril.

Chirality transfer and regulation, accompanied by morphology transformation, arouse widespread interest for application in materials and biological science. Here, a photocontrolled supramolecular chiral switch is fabricated from chiral diphenylalanine (l-Phe-l-Phe, FF) modified with naphthalene (2), achiral dithienylethene (DTE) photoswitch (1), and cucurbit[8]uril (CB[8]). Chirality transfer from the chiral FF moiety of 2 to a charge-transfer (CT) heterodimer consisting of achiral guest 1 and achiral naphthalene (NP) in 2 has been unprecedented achieved via the encapsulation of CB[8]. On the contrary, chirality transfer from chiral FF to NP cannot be conducted in only guest 2. Crucially, induced circular dichroism of the heterodimer can be further modulated by distinct light, attributing to reversible photoisomerization of the DTE. Meanwhile, topological nanostructures are changed from one-dimensional (1D) nanofibers to two-dimensional (2D) nanosheets in the orderly assembling process of the heterodimer, which further achieved reversible interconversion between 2D nanosheets and 1D nanorods with tunable-induced chirality stimulated by diverse light.

Alignment and Dynamic Inversion of Planar Chirality in Pillar[n]arenes.

Pillar[n]arenes are symmetrical macrocyclic compounds composed of benzene panels with para-methylene linkages. Each panel usually exhibits planar chirality and prefers chirality-aligned states. Because of this feature, pillar[n]arenes are attractive scaffolds for chiroptical materials that are easy to prepare and optically resolve and show intense circular dichroism (CD) signals. In addition, rotation of the panels endows the chirality of pillar[n]arenes with a dynamic nature. The chirality in tubular oligomers and supramolecular assemblies sometimes show time- and procedure-dependent alignment phenomena. Furthermore, the CD signals of some pillar[n]arenes respond to the addition of chiral guests when their dynamic chirality is coupled with host-guest properties. By using diastereomeric pillar[n]arenes with additional chiral structures, the response can also be caused by achiral guests and changes of the environment, providing molecular sensors.

Alignment and Dynamic Inversion of Planar Chirality in Pillar[n]arenes

AbstractPillar[n]arenes are symmetrical macrocyclic compounds composed of benzene panels with para‐methylene linkages. Each panel usually exhibits planar chirality and prefers chirality‐aligned states. Because of this feature, pillar[n]arenes are attractive scaffolds for chiroptical materials that are easy to prepare and optically resolve and show intense circular dichroism (CD) signals. In addition, rotation of the panels endows the chirality of pillar[n]arenes with a dynamic nature. The chirality in tubular oligomers and supramolecular assemblies sometimes show time‐ and procedure‐dependent alignment phenomena. Furthermore, the CD signals of some pillar[n]arenes respond to the addition of chiral guests when their dynamic chirality is coupled with host–guest properties. By using diastereomeric pillar[n]arenes with additional chiral structures, the response can also be caused by achiral guests and changes of the environment, providing molecular sensors.

Biased Symmetry Breaking in the Formation of Intercalated Layered Double Hydroxides: toward Control of Homochiral Supramolecular Assembly.

Homochiral supramolecular assembly (HSA) based on achiral molecules has provided important clues to understand the origin of biological homochirality from the aspect of symmetry breaking. However, planar achiral molecules still face the challenge of forming HSA due to the lack of driving force for twisted stacking, which is a prerequisite for homochirality. Here, with the benefit of the formation of 2D intercalated layered double hydroxide (LDH, host-guest nanomaterials) in vortex motion, planar achiral guest molecules can form the chiral units with spatially asymmetrical structure in the confinement space of LDH. Once the LDH is removed, these chiral units are in a thermodynamic non-equilibrium state, which can be amplified to HSA by self-replicating. Especially, the homochiral bias can be predicted in advance by controlling the vortex direction. Therefore, this study breaks the bottleneck of complicated molecular design and provides a new technology to achieve HSA made of planar achiral molecules with definite handedness.

Host‐Guest Chemistry of Chiral MOFs for Multicolor Circularly Polarized Luminescence Including Room Temperature Phosphorescence

AbstractExploiting the chirality transfer and amplification in the hierarchical chiral systems by the visible and accurate structures is still a challenge. Herein, a pair of homochiral metal‐organic frameworks (MOFs) DCF‐12 and LCF‐12 with high rigidity and high porosity are synthesized via reticular chemistry. Interestingly, these two enantiomers can act as nano‐containers, in which four chromophores, covering acridine, pyrene, 9,10‐Bis(phenylvinyl) anthracene (BPEA), and coronene can be introduced by in situ encapsulation. Importantly, the precise single crystal structures of all guest‐loaded MOFs by X‐ray diffraction technique can be obtained smoothly. It not only clearly reveals the chirality transfer from chiral host framework to achiral guest emitters through space chirality transfer, but also circularly polarized luminescence can be achieved and modulated through the synergistic effect. Extraordinarily, both pyrene@DCF‐12 and pyrene@LCF‐12 exhibit fascinating multi‐color tunable room temperature phosphorescence (RTP) and dynamic circularly polarized luminescence. Besides, the RTP quantum yields of pyrene@DCF‐12 and pyrene@LCD‐12 are high up to 75.39% and 73.43%, which exceeds most of that of RTP materials. These results demonstrate that chiral MOFs can serve as an accurate platform to investigate the mechanism of chirality transfer and amplification and to prompt the development of CPL‐active materials.

Steroid-Aromatics Clathrates as Chiroptical Materials with Circularly Polarized Luminescence and Phosphorescence.

Solid-state host-guest complexation shows great potential in the fabrication of chiroptical and phosphorescent materials. Developing chiral hosts toward achiral guests with a wide guest scope would expand the chiroptical application, which however remains a major challenge. Here, we report the steroid-aromatic compound complexation in the solid state that could realize effective chirality transfer, circularly polarized luminescence, and room temperature phosphorescence (RTP). Progesterone shows cocrystallization behavior toward a wide scope of guests through CH-π interaction, which also offers a rigid yet chiral microenvironment to entrap aromatic luminophores within the cavities or channels. Depending on the geometry of the guests, the handedness of the Cotton effects and circularly polarized luminescence could be tuned. Host-guest complexation not only gave rise to Cotton effects and circularly polarized luminescence but also stabilized the triplet state of bromo-compounds to achieve RTP and circularly polarized phosphorescence. This work first illustrates the application of steroid complexation in the chiroptical and phosphorescent materials, which shows potential in the display and information aspects.

Induced Circular Dichroism and Circularly Polarized Luminescence for Block Copolymers with Chiral Communications.

Many sophisticated chiral materials are found in living organisms, giving specific functions and required complexity. Owing to the remarkable optical properties of chiral materials, they have drawn significant attention for the development of synthetic materials to give optical activities for appealing applications. In contrast to a top-down approach, the bottom-up approach from self-assembled systems with chiral host-achiral guest and achiral guest-chiral host for induced circular dichroism and induced circularly polarized luminescence has greatly emerged because of its cost-effective advantage with easy fabrication for mesoscale assembly. Self-assembled hierarchical textures with chiral sense indeed give significant amplification of the dissymmetry factors of absorption and luminescence (gabs and glum ), resulting from the formation of well-ordered superstructures and phases with the building of chromophores and luminophores. By taking advantage of the microphase separation of block copolymers via self-assembly, a variety of well-defined chiral nanostructures can be formed as tertiary superstructures that can be further extended to quaternary phases in bulk or thin film. In this article, a conceptual perspective is presented to utilize the self-assembly of chiral block copolymers with chiral communications, giving quaternary phases with well-ordered textures at the nanoscale for significant enhancement of dissymmetry factors.

Chiral pillar[n]arenes: Conformation inversion, material preparation and applications

Chiral pillar[n]arenes have shown great research value and application prospect in construction of chiral materials and chiral applications, due to their inherent planar chiral configurations, chiral recognition ability, easy modification and highly symmetric hydrophobic cavity. This review systematically summarized the conformation inversion factors of planar chiral pillar[5]arenes (pR/pS), such as solvents, temperature, substituent size, alkyl chains, chiral and achiral guest molecules. We firstly introduced the applications of chiral pillar[n]arenes for constructing chiral materials and pointed out that planar conformation inversion showed a great potential role in constructing chiral materials. Then, we mainly concluded the chiral applications of chiral and planar chiral pillar[n]arenes like chiral enantiomer analysis by circular dichroism, electrochemistry or chiral fluorescence sensing. From this review, we found that the inherent planar chiral conformation of chiral pillar[n]arenes have played a very important role in chiral field in the future.

Static Retention of Dynamic Chiral Arrangements for Achiral Shear Thinning Metal-Organic Colloids.

Chiral compounds are known to be important not only because they are the fundamental components of living organisms, but also for their unique chiroptical properties. In recent years, scientists have fabricated several chiral organic supramolecular aggregates by using chiral physical fields, such as vortex flow. Herein, the relationship between dynamic chiroptical properties and rheological nature is discussed, suggesting the shear thinning properties of non-Newtonian fluids might help colloidal particles adopt a chiral arrangement in vortices. Furthermore, the storage modulus of colloids could be increased by adding a linking agent, which successfully kept the dynamic chiroptical properties in the static state. Moreover, the salt effect on the host-guest interaction involved in the colloids was studied, the results suggested a significant enhancement of the transferred dynamic circular dichroism for the achiral guest molecule.

Chiroptical Cross‐Linked Polymers Grown via Radical Polymerization around Chiral Nanosilica

AbstractChiral polymers are often prepared by two ways depending upon chiral monomers, chiral catalysts, and chiral auxiliaries which always come from organics: one is the enantio‐selective polymerization of prochiral or racemic monomers or polymerization of optically active monomers; and the other one is the molecular imprinting technique, which creates chiral recognition sites inside a densely cross‐linked polymer gel. Herein, a unique method for the synthesis of chiroptical cross‐linked polymers via radical polymerization of achiral divinyl monomers in the mediation of the inorganic chiral nanosilica is reported. The monomers that are cross‐linkable such as divinylbenzene (DVB) and N,N′ ‐methylenebisacrylamide (MBA) are physically adsorbed onto chiral silica bundles and then polymerized by dispersion of those monomers/silica bundles in the presence of radical initiators. The products are characterized by SEM, FT‐IR, solid‐state 13C NMR, TGA, and CD. Very interestingly, the cross‐linked polymers themselves obtained via final removal of the chiral silica show remarkable chiroptical activity with mirror relationship and can make achiral guest of porphyrin adsorbed on the polymers become optically active.

Steering Nanohelix and Upconverted Circularly Polarized Luminescence by Using Completely Achiral Components.

Enormous attention has been paid to upconverted circularly polarized luminescence (UC-CPL). However, so far, chiral species are still needed in UC-CPL materials, either through the covalent or noncovalent bond. Here, we report a general supramolecular coassembly approach for the fabrication of UC-CPL systems from completely achiral components. We have found that an achiral C3-symmetric molecule could form a chiral nanohelix through symmetry breaking, which could serve as a general helical platform to endow achiral guests with induced chirality and CPL activity. Two different photon upconversion systems, namely, triplet-triplet annihilation photon upconversion (TTA-UC) donor/acceptor pairs and inorganic lanthanide upconversion nanoparticles (UCNPs), are selected. When these two systems coassembled with the chiral nanohelix made from an achiral C3-symmetric molecule, hybrid nanohelix structures formed and UC-CPL activity was induced. Through such an approach, we demonstrated that the fabrication of the UC-CPL materials does not require any chiral molecules. Moreover, we have shown that the polarization of UC-CPL can be tuned by the helicity of the nanohelix, which could be controlled through the seeded vortex. Our work provides a general approach for designing tunable UC-CPL materials from completely achiral motifs, which largely expands the research scope of the CPL materials.

Holding of planar chirality of pillar[5]arene by kinetic trapping using host-guest interactions with achiral guest solvents.

We report a solvent-dependent switching and holding of planar chirality of pillar[5]arene with stereogenic carbons at both rims by host-guest complexation with achiral guest solvents. The planar chirality could be held for a given length of time at 25 °C in long linear guest solvents by kinetic trapping through host-guest complexation. The kinetic trapping worked at 25 °C, but not at 60 °C, thus a planar-chiral inversion using kinetic trapping based on host-guest complexation in the long linear solvents was demonstrated.

Transfer of Chiral Information from Silica Hosts to Achiral Luminescent Guests: a Simple Approach to Accessing Circularly Polarized Luminescent Systems

Systems that show circularly polarized luminescence (CPL) are usually constructed in one of two possible ways: either covalently binding the chiral moieties (usually organic compounds) to luminophores (inorganic or organic compounds) or associating the luminophores as guests with chiral hosts (usually organic compounds). Herein, we propose inorganic-based CPL-active systems constructed by the "chiral host-luminescent guest" strategy, in which silica acts as a chiral host to endow various luminescent guests with CPL. The chiral silica was modified by silane coupling with amino or phenyl groups to allow interaction with luminescent guests, and then used in combination with acidic achiral dyes, lead-halide type perovskites, and aggregation-induced emission luminogens (AIEgens). Interestingly, when these achiral guests were noncovalently confined in surface-modified chiral silica, the guests showed chiroptical behavior in the circular dichroism (CD) spectra, and thus became CPL active, even though they are not inherently chiral. The surface functional groups on the silica play very important roles in transferring the chiral information from the silica to the guests. This work provides a new concept for constructing CPL-active systems using inorganic materials as a chiral source.

Complexation of Chiral Zinc(II) Porphyrin Tweezer with Achiral Aliphatic Diamines Revisited: Molecular Dynamics, Electronic CD, and 1H NMR Analysis.

We have reported here the complexation and chiroptical behavior of the host-guest complexes using a new chiral Zn(II) bisporphyrin tweezer host and a series of achiral aliphatic diamine guests varying the chain length. (1R,2R)-Cyclohexanediamine covalently links two Zn(II) porphyrin moieties, which thereby produces a strong chiral field around the bisporphyrin tweezer framework. The chiral tweezer upon complexation with achiral guest exhibited large changes in the UV-vis spectra and CD exciton couplets due to a sudden change in the porphyrin disposition, which is controlled by the host-guest stoichiometry as well as the chain lengths of the diamine guest. Addition of smaller diamines (n: 2-5) to the host resulted in the formation of 1:1 sandwich and 1:2 open complexes, respectively, at the low and high guest concentration, which eventually display two-step inversions of the CD couplet. With longer diamines (n: 6-8), however, only 1:1 sandwich complexes are formed with retention of the CD sign. Similar observations were also reported by us recently using another chiral bisporphyrin tweezer having (1R,2R)-diphenylethylenediamine as the spacer. In an effort to obtain deeper insights into the sudden changes of interporphyrin disposition just by changing the length of the achiral diamines, we have extended a series of computational studies and correlated closely with the results obtained from the experiment. While the previously published study has relied on commonly applied Monte Carlo (MC) sampling of the potential energy surface in addition to being guided by porphyrin effective transition moment approximation, the present study uses a considerably more robust molecular modeling protocol, namely Molecular Dynamics (MD) simulations followed by full ab initio geometry optimization and TD-DFT CD prediction. The experimental data corroborate with the results obtained from the theoretical conformational analysis. The latter are also supported by experimental 1H NMR data empowered by the porphyrin ring-current effect. The NMR spectral patterns of pyrrolic protons of the free host and the 1:1 sandwich complexes appear very diagnostic and reflect the changes in the mutual porphyrin disposition on moving from the free host to the complexed ones with short and long diamines. Overall, the experimental NMR data underscore the sensitivity of pyrrolic protons chemical shifts to subtle alterations of the geometrical features, and as such, they come in agreement with the theoretically derived models.