Chiral Amplification in π-Conjugated Helical Polymers with Circularly Polarized Luminescence

Homochiral π-Rich Covalent Organic Frameworks Enabled Chirality Imprinting in Conjugated Polymers: Confined Polymerization and Chiral Memory from Scratch

Optically active π-conjugated polymers (OACPs) have garnered increasing research interest for their resemblance to biological helices and intriguing chirality-related functions. Traditional methods for synthesizing involve decorating achiral conjugated polymer architectures with enantiopure side substituents through complex organic synthesis. Here, we report a new approach: the templated synthesis of unsubstituted OACPs via supramolecularly confined polymerizations of achiral monomers within nanopores of 2D or 3D chiral covalent organic frameworks (CCOFs). We show that the chiral π-rich nanospaces facilitate the in situ enantiospecific polymerization and self-propagation, akin to nonenzymatic polymerase chain reaction (PCR) system, resulting in chiral imprinting. The stacked polymer chains are kinetically inert enough to memorize the chiral information after liberating from CCOFs, and even after treatment at temperature up to 200 °C. The isolated OACPs demonstrate robust enantiodiscrimination, achieving up to 85 % ee in separating racemic amino acids. This underscores the potential of utilizing CCOFs as templates for supramolecularly imprinting optical activity into CPs, paving the way for synthetic evolution and advanced functional exploration of OACPs.

Polymers with Circularly Polarized Luminescent Properties: Design, Synthesis, and Prospects

Chiral materials with circularly polarized luminescence (CPL) have garnered significant attention owing to their distinctive luminescent properties and wide array of applications. CPL enables the selective emission of left and right circularly polarized light. The fluorescence quantum yield and dissymmetry factor play pivotal roles in the generation of CPL. Helical polymers exhibit immense promise as CPL materials due to their inherent chirality, structural versatility, modifiability, and capacity to incorporate diverse chromophores. This Review provides a brief review of the synthesis of CPL materials based on helical polymers. The CPL can be realized by aggregation-induced CPL of non-emissive helical polymers, and helices bearing chromophores on the pendants and on the chain end. Furthermore, future challenges and potential applications of CPL materials are summarized and discussed.

Liquid Crystals Doped with Chiral Fluorescent Polymer: Multi-Color Circularly Polarized Fluorescence and Room-Temperature Phosphorescence with High Dissymmetry Factor and Anti-Counterfeiting Application

Chiral nematic liquid crystals (N*-LCs) can tremendously amplify circularly polarized luminescence (CPL) signals. Doped emissive N*-LCs have been substantially explored. However, their CPL performances still need to be improved, mainly due to the unsatisfying helical twisting power (HTP) of commonly used chiral fluorescent dopants. Chiral fluorescent helical polymers (CFHPs) have outstanding optical activity and CPL performance. The present contribution reports the first success in constructing emissive N*-LCs by doping CFHP into nematic liquid crystals (5CB, N-LCs). The helical assembly structures of N*-LCs effectively amplify the CPL signals of the CFHP. Owing to the high HTP of CFHP, the selective reflection band of N*-LC can be adjusted to fully cover its emission band. A nearly pure CPL with a dissymmetry factor (glum ) up to -1.87 is realized at 9 wt% doping concentration. Taking advantage of the selective reflection mechanism, multi-color CPL-active N*-LCs with high glum are fabricated via further adding achiral fluorophores. Also noticeably, circularly polarized room-temperature phosphorescence with glum up to -1.57 is achieved. Anti-counterfeiting application is demonstrated by exploiting multi-mode optical characteristics of the created N*-LCs. The established strategy for constructing emissive N*-LCs provides a platform for future exploring of CPL-active N*-LCs.

Switchable Circularly Polarized Signals with High Asymmetric Factor Triggered by Dual Photonic Bandgap Structure

Currently, the smart photonic materials that can switch circularly polarized signals in real-time have attracted extensive attention due to numerous potential applications in information storage and photonics displays. However, the dynamically reversible switching of circularly polarized signals requires precise structural reconfiguration, which is rarely achieved in traditional biomaterials. Herein, a dual photonic bandgap (PBG) structure is constructed based on the optical propagation principle of cellulose-based photonic crystals, enabling the flexible switching of the intensity, wavelength, and direction of circularly polarized luminescence (CPL). By adjusting the fluorescence intensity and the matching degree of chiral structure, the asymmetric factor value of dual PBG structure is up to -1.47, far exceeding other cellulose-based materials. Importantly, it is demonstrated that dual CPL emission can be efficiently induced by two different PBGs, opening a new approach for on-demand switching of single and dual CPL emission. In addition, the dual PBG structure exhibits dual circularly polarized reflected signals under the circular polarizer, which perfectly embodies the applicability of multiple encryptions in QR codes. This work provides new insights into the real-time manipulation of circularly polarized signals by chiral photonic materials.

Tunable Circularly Polarized Luminescence from Inorganic Chiral Photonic Crystals Doped with Quantum Dots

Chiral semiconductor nanostructures have received enormous attention due to their emerging circularly polarized luminescence (CPL) properties. However, compared with well-studied photoluminescence (PL), the reported CPL is much weaker and more challenging to be modulated. Herein, we describe a new approach for acquiring the intense and tunable CPL from inorganic chiral photonic crystals (CPCs) doped with semiconductor quantum dots (QDs). Unprecedentedly, the sign, position and intensity of CPL peaks can be precisely controlled by manipulating either the photonic band gap of CPCs or luminescence wavelength of QDs and a giant absolute dissymmetry factor |g_lum | up to 0.25 is obtained. More importantly, the origin of the CPL modulation is clearly elucidated by both experiment and theory. This work lays the foundation for the construction of next-generation high-performance CPL-based devices.

Chiral polymer hosts for circularly polarized electroluminescence devices

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications. The fabrication of a CP PLED requires a polymer host that provides the appropriate chiral environment around the emitting dopant. However, chemical strategies for the design of chiral polymer hosts remain underdeveloped. We have developed new polymer hosts for CP PLED applications. These polymers were prepared through a free-radical polymerization of 3-vinylcarbazole with a chiral N-alkyl unit. This chiral unit forces the carbazole repeat units to form mutually helical half-sandwich conformers with preferred (P)-helical sense along the polymer main chain. Electronic circular dichroism measurements demonstrate the occurrence of chirality transfer from chiral monomers to achiral monomers during chain growth. The (P)-helical-sense-enriched polymer interacts diastereoselectively with an enantiomeric pair of new phosphorescent (R)- and (S)-dopants. The magnitude of the Kuhn dissymmetry factor (g_abs) for the (P)-helically-enriched polymer film doped with the (R)-dopant was found to be one order of magnitude higher than that of the film doped with the (S)-dopant. Photoluminescence dissymmetry factors (g_PL) of the order of 10⁻³ were recorded for the doped films, but the magnitude of diastereomeric enhancement decreased to that of g_abs. The chiral polymer host permits faster energy transfer to the phosphorescent dopants than the achiral polymer host. Our photophysical and morphological investigations indicate that the acceleration in the chiral polymer host is due to its longer Förster radius and improved compatibility with the dopants. Finally, multilayer CP PLEDs were fabricated and evaluated. Devices based on the chiral polymer host with the (R)- and (S)-dopants exhibit electroluminescence dissymmetry factors (g_EL) of 1.09 × 10⁻⁴ and −1.02 × 10⁻⁴ at a wavelength of 540 nm, respectively. Although challenges remain in the development of polymer hosts for CP PLEDs, our research demonstrates that chiroptical performances can be amplified by using chiral polymer hosts.

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications.

Chiral Helical Polymer/Perovskite Hybrid Nanofibers with Intense Circularly Polarized Luminescence

Chiral perovskites with circularly polarized luminescence (CPL) performance have attracted tremendous attention. This contribution reports a convenient and universal strategy for constructing chiral helical polymer/perovskite hybrid nanofibers with outstanding CPL properties. The hybrid nanofibers are prepared through a one-step electrospinning method in which chiral helical polyacetylenes, perovskite nanocrystals, and polyacrylonitrile serve as a handed-selective fluorescence filter, fluorescent source, and electrospinning matrix, respectively. Specially, perovskite nanocrystals are in situ formed during the electrospinning process, which avoids the tedious process for preparing and purifying perovskites. The prepared hybrid nanofibers all exhibit good long-time stability in air, owing to the effective protection effect of polymer matrix. More importantly, intense CPL emissions with high dissymmetry factor up to 10^-2 level are obtained in the hybrid nanofibers. Furthermore, the emission color of CPL can be easily tuned by adjusting the precursors of perovskites. This work provides an efficient technique toward various kinds of CPL-active perovskite nanomaterials for both scientific research and future practical applications.

Induction of Chirality in Supramolecular Coassemblies Built from Achiral Precursors

The emergence, transference, amplification, and memory of chiroptical activity in supramolecular assemblies, including circularly polarized absorbance and circularly polarized luminescence, remain significant challenges. Herein, an achiral pyridine-substituted coumarin derivative and chiral additives can coassemble into helical nanostructures with fine chiroptical activity via subtle hydrogen-bonding interactions. The resulting supramolecular assemblies remain optically active even after the removal of chiral additives, demonstrating supramolecular chirality can be remembered in the assemblies. More importantly, the removed chiral elements can be reused to achieve continuous circulation and amplification of chirality. This work presents insight into the emergence, transference, amplification, and memory of chirality in a supramolecular assembly system and could be applied to the manufacturing of chiroptical materials.

Biasing the Screw-Sense of Supramolecular Coassemblies Featuring Multiple Helical States

By enchaining a small fraction of chiral monomer units, the helical sense of a dynamic polymer constructed from achiral monomer units can be disproportionately biased. This phenomenon, known as the sergeants-and-soldiers (S&S) effect, has been found to be widely applicable to dynamic covalent and supramolecular polymers. However, it has not been exemplified with a supramolecular polymer that features multiple helical states. Herein, we demonstrate the S&S effect in the context of the temperature-controlled supramolecular copolymerization of chiral and achiral biphenyl tetracarboxamides in alkanes. The one-dimensional helical structures presented in this study are unique because they exhibit three distinct helical states, two of which are triggered by coassembling with monomeric water that is codissolved in the solvent. The self-assembly pathways are rationalized using a combination of mathematical fitting and simulations with a thermodynamic mass-balance model. We observe an unprecedented case of an "abnormal" S&S effect by changing the side chains of the achiral soldier. Although the molecular structure of these aggregates remains elusive, the coassembly of water is found to have a profound impact on the helical excess.

Amplifying the excited state chirality through self-assembly and subsequent enhancement via plasmonic silver nanowires

The development of circularly polarized luminescent materials with a large luminescence dissymmetry factor (g_lum) is continuing to be a big challenge. Here, we present a general approach for amplifying circular polarization of circularly polarized luminescence (CPL) through intergrating molecular self-assembly and surface plasmon resonance (SPR). Molecular self-assembly could amplify the CPL performance. Subsequently, the composites built of nanoassemblies and achiral silver nanowires (AgNWs) show intense CPL activity with an amplified g_lum value. By applying an external magnetic field, the CPL activity of the nanoassemblies/AgNWs composites has been significantly enhanced, confirming a plasmon-enhanced circular polarization. Our design strategy based on SPR-enhanced circular polarization of the chiral emissive systems suggests that combining plasmonic nanomaterials with chiral organic materials could aid in the development of novel CPL active nanomaterials.

Crystallization-Driven Asymmetric Helical Assembly of Conjugated Block Copolymers and the Aggregation Induced White-light Emission and Circularly Polarized Luminescence

Controlling the self-assembly morphology of π-conjugated block copolymer is of great interesting. Herein, amphiphilic poly(3-hexylthiophene)-block-poly(phenyl isocyanide)s (P3HT-b-PPI) copolymers composed of π-conjugated P3HT and optically active helical PPI segments were readily prepared. Taking advantage of the crystallizable nature of P3HT and the chirality of the helical PPI segment, crystallization-driven asymmetric self-assembly (CDASA) of the block copolymers lead to the formation of single-handed helical nanofibers with controlled length, narrow dispersity, and well-defined helicity. During the self-assembly process, the chirality of helical PPI was transferred to the supramolecular assemblies, giving the helical assemblies large optical activity. The single-handed helical assemblies of the block copolymers exhibited interesting white-light emission and circularly polarized luminescence (CPL). The handedness and dissymmetric factor of the induced CPL can be finely tuned through the variation on the helicity and length of the helical nanofibers.

Multifarious Chiral Nanoarchitectures Serving as Handed-Selective Fluorescence Filters for Generating Full-Color Circularly Polarized Luminescence

A series of full-color circularly polarized luminescence (CPL)-active materials are fabricated by judiciously combining multifarious chiral nanoarchitectures with achiral fluorescence dyes. The investigated nanoarchitectures include organic polymer nanofibers, organic-inorganic hybrid nanoflowers, and inorganic nanoflowers. The as-prepared chiral nanoarchitectures all can act as handed-selective fluorescence filters to powerfully transform unpolarized fluorescent light into circularly polarized luminescence. Also notable, no interaction is required between chiral and fluorescent components for achieving CPL emission. The present study provides a convenient and universal approach for preparing full-color CPL materials. Following the strategy, numerous chiroptical materials with CPL performance can be expected due to the abundant chiral matters and achiral fluorophores.

Helical Assemblies of One-Dimensional Supramolecular Polymers Composed of Helical Macromolecules: Generation of Circularly Polarized Light Using an Infinitesimal Chiral Source

We report the synthesis of one-dimensional supramolecular polymers composed of one-handed helical macromolecules bearing fluorescent pendant groups and the generation of circularly polarized light on the basis of hierarchical chiral amplification starting from a tiny amount of chiral substituent. Copolymerization of benzo[1,2-b:4,5-b']dithiophene-appended achiral/chiral isocyanides (99:1, mol/mol) with a solid-state photoluminescence feature afforded submicrometer supramolecular fibers, in which almost perfect single-handed helical polyisocyanides were noncovalently connected end to end. The resulting helical supramolecular polymers were further helically assembled to form a cholesteric liquid crystal film with an intense circularly polarized luminescence (CPL) signal. Surprisingly, the supramolecular system containing only 0.01 mol % of the chiral monomer unit also emitted the observable circularly polarized light owing to multiple chiral amplification from an infinitesimal point chirality to helical chirality and then to supramolecular chirality. Furthermore, chiral information was efficiently transferred from the helically assembled supramolecular system containing 1 mol % of the chiral unit to achiral dye molecules blended in the film, allowing full-color tunable induced CPL with luminescence dissymmetry factors greater than 1.0 × 10^-2. This unprecedentedly strong chiral amplification enables the creation of helical supramolecular polymers and chirally assembled systems with various chiral functions based solely on an infinitesimal chiral source.

Chiral helical polymer materials derived from achiral monomers and their chiral applications

Helix-sense-selective polymerization (HSSP) of achiral monomers and chiral post-induction of racemic helical polymers provide two alternative approaches for constructing chiral helical polymer materials.

Silylium cation initiated sergeants-and-soldiers type chiral amplification of helical aryl isocyanide copolymers

Silylium cations act as new highly efficient metal-free single-component cationic initiators for the cationic polymerization and copolymerization of chiral or achiral aryl isocyanides, preparing optically active polymers and copolymers obeying “sergeants-and-soldiers” rule.

Synthesis of helical π-conjugated polymers bearing pyridine N-oxide pendants and asymmetric allylation of aldehydes in the helical cavity

Catalytically active chiral π-conjugated polymers (poly-1(NO)r) bearing pyridine N-oxide pendants were synthesized by ternary copolymerization of a d-glucose-bound diethynyl compound with two types of thieno[3,4-b]thiophene comonomer, one of which contained a pyridine N-oxide group. When the pyridine N-oxide content in the copolymer was 10 mol% (poly-1(NO)0.10), the polymer backbone formed a one-handed helical structure in acetonitrile. Pyridine N-oxide pendants arranged inside the helical cavity of poly-1(NO)0.10 exhibited catalytic activity for the asymmetric allylation of benzaldehydes, producing the corresponding allyl alcohols with up to 43% ee.

Telechelic Helical Poly(quinoxaline-2,3-diyl)s Containing a Structurally Defined, Circularly Polarized Luminescent Terquinoxaline Core: Synthesis by Core-Initiated Bidirectional Living Polymerization

We have designed and synthesized divalent initiators that contain a fluorescent terquinoxaline unit with two palladium groups for the living polymerization of 1,2-diisocyanobenzenes. Using these divalent initiators, the bidirectional living polymerization of a monomer bearing (S)-butoxymethyl side chains afforded telechelic helical poly(quinoxaline-2,3-diyl)s (PQXs), which consist of a terquinoxaline unit at the center of the polymer chain and chiral oligomeric blocks on both sides. The location of the core unit was confirmed by NMR spectroscopy and photoluminescence measurements. Upon changing the solvent from CHCl₃ to 1,1,2-trichloroethane, these PQXs exhibit both left- and right-handed circularly polarized luminescence with dissymmetry factors of approximately 1.0 × 10^-3.

Synthesis of a one-handed helical polythiophene: a new approach using an axially chiral bithiophene with a fixed syn-conformation

An optically active polythiophene, which can fold into a one-handed helical conformation under good solvent conditions, has been developed.

We report an optically active polythiophene capable of forming a one-handed helically folded conformation without needing aggregate formation, poor solvent conditions, hydrogen-bonded ion-pair formation or guest addition. The target polythiophene (poly-T_R) with a static axial chirality in the main chain was synthesized via Stille coupling copolymerization of a glucose-linked chiral 5,5′-dibromobithiophene with 2,5-bis(stannyl)thiophene. Poly-T_R showed a characteristic circular dichroism and circularly polarized luminescence, which were completely different to those observed for an analogous polymer (poly-Ph_R) and the corresponding unimer/dimer model compounds. This chiroptical study, combined with the results of all-atom molecular dynamics simulations, revealed that poly-T_R can fold into a left-handed helical conformation under good solvent conditions. Partial conformational regulation derived from the fixed syn-conformation of the chiral bithiophene unit was considered a key factor in producing the one-handed helical polythiophene.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure-Property Relationships and Applications

Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4-11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure-property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.

Helicity control of π-conjugated foldamers containing d-glucose-based single enantiomeric units as a chiral source

We have succeeded in the helicity control of polymer backbones and their circularly polarized luminescence without the need for chirality of an unnatural antipode, l-glucose.