Poly(quinoxaline-2,3-diyl) as a Multifunctional Chiral Scaffold for Circularly Polarized Luminescent Materials: Color Tuning, Energy Transfer, and Switching of the CPL Handedness

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.

Stimuli-responsive synthetic helical polymers

Synthetic dynamic helical polymers (supramolecular and covalent) and foldamers share the helix as a structural motif. Although the materials are different, these systems also share many structural properties, such as helix induction or conformational communication mechanisms. The introduction of stimuli responsive building blocks or monomer repeating units in these materials triggers conformational or structural changes, due to the presence/absence of the external stimulus, which are transmitted to the helix resulting in different effects, such as assymetry amplification, helix inversion or even changes in the helical scaffold (elongation, J/H helical aggregates). In this review, we show through selected examples how different stimuli (e.g., temperature, solvents, cations, anions, redox, chiral additives, pH or light) can alter the helical structures of dynamic helical polymers (covalent and supramolecular) and foldamers acting on the conformational composition or molecular structure of their components, which is also transmitted to the macromolecular helical structure.

Screw sense and screw sensibility: communicating information by conformational switching in helical oligomers

Biological systems have evolved a number of different strategies to communicate information on the molecular scale. Among these, the propagation of conformational change is among the most important, being the means by which G-protein coupled receptors (GPCRs) use extracellular signals to modulate intracellular processes, and the way that opsin proteins translate light signals into nerve impulses. The developing field of foldamer chemistry has allowed chemists to employ conformationally well-defined synthetic structures likewise to mediate information transfer, making use of mechanisms that are not found in biological contexts. In this review, we discuss the use of switchable screw-sense preference as a communication mechanism. We discuss the requirements for functional communication devices, and show how dynamic helical foldamers derived from the achiral monomers such as α-aminoisobutyric acid (Aib) and meso-cyclohexane-1,2-diamine fulfil them by communicating information in the form of switchable screw-sense preference. We describe the various stimuli that can be used to switch screw sense, and explore the way that propagation of the resulting conformational preference in a well-defined helical molecule allows screw sense to control chemical events remote from a source of information. We describe the operation of these conformational switches in the membrane phase, and outline the progress that has been made towards using conformational switching to communicate between the exterior and interior of a phospholipid vesicle.

Air-tolerant upconverted circularly polarized luminescence enabled by confined space of chiral micelle

Circularly polarized luminescence (CPL) has been widely demonstrated that the circular polarization in excited state can be significantly amplified through the triplet-triplet annihilation-based upconversion (TTA-UC) luminescence process in various chiral nano-assemblies. However, constructing such an upconverted circularly polarized luminescence (UC-CPL) system in the aqueous phase remains a challenge. In this work, a kind of amphiphilic chiral cationic gemini surfactant is utilized to construct chiral spherical micelle in the aqueous phase, whose internal chiral cavity can provide a hydrophobic and deoxygenated environment for air-sensitive TTA-UC system. In addition, due to the co-assembly process between the emitters and chiral micelles, achiral emitters of upconversion pairs exhibit induced chiroptical properties. More importantly, the luminescence dissymmetry factor (g_lum ) can be amplified by one order of magnitude through TTA-UC process. This work provides an effective and useful strategy for realizing UC-CPL in aqueous phase.

In Situ Regulation of Microphase Separation-Recognized Circularly Polarized Luminescence via Photoexcitation-Induced Molecular Aggregation

Circularly polarized luminescence (CPL) has attracted great interest owing to its extensive optical information and chiral structural dependence. However, rationally regulating solid-phase CPL signals remains difficult because of the close packing of molecules in solid-state materials and the lack of structural visualization. In this work, we proposed a microphase-separation-recognized CPL regulation strategy via coassembly of a hexathiobenzene-based luminophore and chiral block copolymer (cBCP) with in situ photocontrollability. As a consequence to the continuous increase in the luminophore-to-cBCP ratio, the CPL signal of the supramolecular system exhibited an increasing trend until a critical point. Then, further increasing the ratio stretched the helical pitch of cBCP, which led to CPL reduction. With the photoexcitation-induced molecular aggregation of the luminophore, which was implemented using in situ photoirradiation, the helical pitch was retracted along with the restoration of the CPL signal. These processes were fully recognized and monitored by the microphase-separated nanomorphological change of the coassembled system, which indicated that such a structural contrast could be an effective method for rationally regulating the supramolecular chiropticity of solid-state materials.

A Flexible Circularly Polarized Luminescence Switching Device Based on Proton-Coupled Electron Transfer

Flexible circularly polarized luminescence (CPL) switching devices have been long-awaited due to their promising potential application in wearable optoelectronic devices. However, on account of the few materials and complicated design of manufacturing systems, how to fabricate a flexible electric-field-driven CPL-switching device is still a serious challenge. Herein, a flexible device with multiple optical switching properties (CPL, circular dichroism (CD), fluorescence, color) is designed and prepared efficiently based on proton-coupled electron transfer (PCET) mechanism by optimizing the chiral structure of switching molecule. More importantly, this device can maintain the switching performance even after 300 bending-unbending cycles. It has a remarkable comprehensive performance containing bistable property, low open voltage, and good cycling stability. Then, prototype devices with designed patterns have been fabricated, which opens a new application pattern of CPL-switching materials.

Helix Induction and Inversion of Polymeric Foldamer Regulated by the Single Enantiomers

In general, a single enantiomer can induce a foldamer into a preferred-handed helix, while another condition is required for the helical inversion. In this work, it is found that the helix induction and subsequent inversion of the poly(m-phenylene diethynylene)-based foldamer bearing aza-18-crown-6 pendants (Poly-1) can be realized by increasing the concentration of sterically hindered L-amino acid perchlorate salts. When the amount of chiral enantiomers is small, one enantiomer tends to complex with two non-adjacent aza-18-crown-6 rings via three N⁺ -H···O hydrogen bonds in a sandwich mode. Notably, the transition dipole moment is perpendicular to aza-18-crown-6 ring, so that the induced helical chirality in Poly-1 main chain is opposite to the chirality of enantiomers. When the amount of chiral enantiomers is large enough, each aza-18-crown-6 is occupied by one enantiomer, which causes the transition dipole moment in a parallel direction to aza-18-crown-6 ring. In this case, the increased steric hindrance can facilitate the inversion of screw sense of Poly-1 main chain, which is directed by chiral center of enantiomers. As a result, a helix inversion has been achieved successfully. This work not only provides a novel strategy for regulating the two-stage folded helical conformations by the single enantiomers, but opens a window to develop chiral recognition materials. This article is protected by copyright. All rights reserved.

Stoichiometric Ratio Controlled Dimension Transition and Supramolecular Chirality Enhancement in a Two-Component Assembly System

To control the dimension of the supramolecular system was of great significance. We construct a two component self-assembly system, in which the gelator LHC18 and achiral azobenzene carboxylic acid could co-assembly and form gels. By modulating the stoichiometric ratio of the two components, not only the morphology could be transformed from 1D nanaotube to 0D nanospheres but also the supramolecualr chirality could be tuned. This work could provide some insights to the control of dimension and the supramolecular chirality in the two-component systems by simply modulating the stoichiometric ratio.

Single-component color-tunable circularly polarized organic afterglow through chiral clusterization

Circularly polarized organic afterglow (CPOA) with both long-lived room-temperature phosphorescence (RTP) and circularly polarized luminescence (CPL) is currently attracting great interest, but the development of multicolor-tunable CPOA in a single-component material remains a formidable challenge. Here, we report an efficient strategy to achieve multicolor CPOA molecules through chiral clusterization by implanting chirality center into non-conjugated organic cluster. Owing to excitation-dependent emission of clusters, highly efficient and significantly tuned CPOA emissions from blue to yellowish-green with dissymmetry factor over 2.3 × 10-3 and lifetime up to 587 ms are observed under different excitation wavelengths. With the distinguished color-tunable CPOA, the multicolor CPL displays and visual RTP detection of ultraviolent light wavelength are successfully constructed. These results not only provide a new paradigm for realization of multicolor-tunable CPOA materials in single-component molecular systems, but also offer new opportunities for expanding the applicability of CPL and RTP materials for diversified applications.

Solvent-dependent helix inversion in optically active poly(diphenylacetylene)s and their chiral recognition abilities as chiral stationary phases for high-performance liquid chromatography

We report the first example of solvent-dependent helix inversion in poly(diphenylacetylene) (PDPA) derivatives. Asymmetrically substituted PDPAs bearing optically active substituents linked through amide bonds formed preferred-handed helical conformations because of the optically active substituents in the pendants, whose helix-senses were inverted upon thermal annealing in polar solvents such as N,N-dimethylformamide and dimethylsulfoxide and in nonpolar solvents such as tetrachloroethane. Unlike the solvent-dependent helix inversion reported for other dynamic helical polymers, the macromolecular helicity induced in the polymer backbone of these PDPAs upon thermal annealing was stably maintained at room temperature, independent of the solvent polarity. These diastereomeric PDPAs with opposite helix-senses generated almost mirror-imaged left- and right-handed circularly polarized light in the same solvent at room temperature. Taking advantage of this unique solvent-dependent helix inversion property, the diastereomeric PDPAs with opposite helix-senses were coated on macroporous silica gel and applied to chiral stationary phases for high-performance liquid chromatography. Despite having the same optically active substituents on the pendant phenyl rings, they showed completely different chiral recognition abilities toward many racemates depending on the helix-sense of the polymer backbone, and the elution order of the enantiomers was reversed for some racemates. The combination of the helix-sense of the polymer backbone and the chirality of the pendants, which afforded a higher chiral recognition ability, differed depending on the racemates.

Red circularly polarized luminescence from intramolecular excimers restricted by chiral aromatic foldamers

Aromatic oligoamide foldamers are highlighted as a verstile paltform for developing single-handed foldamers with two aromatic acetenyl groups at the same side. The foldamers with pyrene acetenyl units exhibit red excimer emissions, which were circularly polarized and show interesting circularly polarized luminescence properties with high CPL brightness B_CPL up to 109.8 M^-1.cm^-1. The red excimer CPL was attributed to the extended conjugations and the spatial restriction of pyrene units at the same side of foldamers.

A chemosensor-based chiral coassembly with switchable circularly polarized luminescence

Fluorescent chemosensors represent fast response to analytes with pronounced luminescent variations. They are promising as potential candidates in controlling luminescence and chiroptical activities of self-assembled chiral systems, which however have not been accomplished to date. We present a coassembled multiple component system that could respond to SO2 derivatives, giving rise to dynamic aggregation behaviors and switchable luminescence as well as circularly polarized luminescence (CPL). Cholesteryl-naphthalimide and coumarin derivatives coassemble into vesicles and nanohelices under the solvent strategy, behaving as energy transfer donor and accepter respectively. Energy transfer enables CPL transition from green to red depending on the molar fraction. After the addition of SO2 derivatives, hypochromic shifts occur to CPL due to the nucleophilic addition reaction to coumarin domain, hindering energy transfer and allow for the emergence of pristine luminescence. Here, we show a protocol to control over luminescence and chiroptical features of supramolecular chiral self-assemblies using fluorescent chemosensors.

Solvent-regulated chiral exciton coupling and CPL sign inversion of an amphiphilic glutamide-cyanostilbene

The chiral exciton couplings within a Y-shaped amphiphilic glutamide-cyanostilbene (GCS) could be significantly biased by solvent polarity and hydration effects, which led to sign inversion of both the circular dichroism and circularly polarized luminescence of the GCS assemblies.

Transfer, Amplification, Storage, and Complete Self-Recovery of Supramolecular Chirality in an Achiral Polymer System

Supramolecular chirality and its complete self-recovery ability are highly mystical in nature and biological systems, which remains a major challenge today. Herein, we demonstrate that partially cross-linked azobenzene (Azo) units can be employed as the potential chiral trigger to fully heal the destroyed helical superstructure in achiral nematic polymer system. Combining the self-assembly of Azo units and terminal hydroxyl groups in polymer side chains allows the vapor-induced chiral nematic phase and covalent fixation of the superstructure via acetal reaction. The induced helical structure of Azo units can be stored by inter-chain cross-linking, even after removal of the chiral source. Most interestingly, the stored chiral information can trigger perfect chiral self-recovery (CSR) behavior after being destroyed by UV light, heat, and solvents. The results pave a new way for producing novel chiroptical materials with reversible chirality from achiral sources.

Protected amino acids as a nonbonding source of chirality in induction of single-handed screw-sense to helical macromolecular catalysts

Chiral nonbonding interaction with N-protected amino acid methyl esters used as chiral additives in achiral solvents allows dynamic induction of single-handed helical conformation in poly(quinoxaline-2,3-diyl)s (PQX) bearing only achiral substituents. Ac-l-Pro-OMe, for instance, allows induction of energy preference of 0.16 kJ mol-1 per monomer unit for the M-helical structure over the P-helix in t-butyl methyl ether (MTBE). With this new mode of screw-sense induction, homochiral screw-sense has been induced in virtually achiral poly(quinoxaline-2,3-diyl)s 1000-mer containing phosphine pendants (PQXphos). Use of PQXphos as a helically dynamic ligand along with Ac-Pro-OMe (l or d) as a chiral additive in MTBE allowed a highly enantioselective Suzuki-Miyaura coupling reaction with up to 95% enantiomeric excess.

Circularly Polarized Luminescence from Solvent-Free Chiral Organic π-Liquids

The solvent-free organic π-liquids have been attracting increasing attentions owing to the inherent optoelectronic properties accompanied by the advantages of non-volatility and high processability. Herein, we reported a series of naphthalene derivatives substituted with chiral branched alkyl chains, which are present as liquids (Nap1-3) or solid (Nap4) at room temperature, depending on the substitution positions. Circular dichroism (CD) and circularly polarized luminescence (CPL) were only observed for enantiomeric Nap2 (2,3-substituted) liquid. It is suggested that the chiral aggregation in the π-liquid leads to the CD signal and the chiral excimer resulting in the CPL performance. When achiral anthracene or pyrene was dissolved in Nap2, the π-liquid could serve as chirality and energy transfer media in which both CD and CPL emerged from the achiral anthracene. A CPL dissymmetry factor (|g_lum |) of anthracene reached to 5.2×10^-2 when dissolved in chiral Nap2 liquid, which is nearly two orders of magnitude higher than that of the pure Nap2 π-liquid.

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.

Template Assisted Generation of Chiral Luminescence in Organic Fluorophores

Development of efficient ways of fabricating chiral light emitting materials is an active area of research due to the vast potential offered by these materials in the field of optoelectronic devices, data storage, and asymmetric synthesis. Among the various methods employed, template assisted generation of chiral luminescence is gaining enormous attention due to its simplicity, applicability over a wide range of fluorescent molecules/dyes, and the display of high anisotropic values.

Rich-colour mechanochromism of a cyanostilbene derivative with chiral self-assembly

The tricolored fluorescence switching was realized in a novel chiral fluorophore. The fabrication of a helical assembly was proposed as a candidate strategy for attaining an additional metastable state, which contributed to enriched PL colors via pairwise excimer emission.

Helical polymer self-assembly and chiral nanostructure formation

This review surveys recent progress towards robust chiral nanostructure fabrication techniques using synthetic helical polymers, the unique inferred properties that these materials possess, and their intricate connection to natural, biological chirality.

pH-influenced handedness inversion of circularly polarized luminescence

Supramolecular co-assemblies between tolane-derived Phe–Phe dipeptides and 1,2-diaminoethane were fabricated, and CPL handedness inversion was achieved by regulating the pH value.

The Progress and Perspective of Organic Molecules With Switchable Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) has been under intense research for future applications in high-resolution 3D displays, smart sensors, and information technologies. Different types of CPL materials have been developed, but neither the handedness nor the asymmetrical luminescence degree can be inferred from the material composition or the components. CPL materials with switchable handedness or emission wavelength play an important role, reducing the need for repetitive bottom-up synthesis. Here, we have presented switchable CPL behaviors toward multiple reported stimuli, including light irradiation, host-guest interaction, metal ions, pH, solvent, temperature, etc. This summary and discussion of the effective stimuli is aimed to promote rational future material exploration and boost related multidisciplinary applications.

Supramolecular chiroptical switches

Recent progress in chiroptical switches including on/off, amplification, and inversion of the chiral signals such as ECD and CPL in supramolecular assemblies is shown.

Living Cyclocopolymerization through Alternating Insertion of Isocyanide and Allene via Controlling the Reactivity of the Propagation Species: Detailed Mechanistic Investigation

Living cyclocopolymerization through the alternating insertion of an isocyanide and allene into palladium-carbon bond was developed based on the controlling the reactivity of the propagation species using bidentate ligands. We revealed that the rate of the presented cyclocopolymerization was depended on the ligands of Pd-initiator. When the palladium-methyl complexes having appropriate cis-chelating ligand, such as 1,3-bis(diphenylphosphino)propane (dppp), were used as initiator, the cyclocopolymerization of bifunctional aryl isocyanides (1) that contain both isocyano and allenyl moieties polymerized to afford poly(quinolylene-2,3-methylene)s with controlled molecular weight and narrow molecular weight distributions. The resulting polymer was characterized by ¹H and ¹³C NMR analyses, which clearly showed that the terminal moiety of the polymer formed well-defined organopalladium complex as the resting state for the polymerization, which could undergo further polymerization; not only cyclocopolymerization with 1 but also homopolymerization of simple aryl isocyanide. In the analysis of the cyclocopolymerization mechanism, we conclusively demonstrated that the insertion reaction of isocyanide is the rate-determination step in the cyclocopolymerization, which proceeds via a five-coordinate intermediate with a geometrical change. The cis-chelating ligand controls the site interchange reaction, which dominates the reactivity of propagation species.

Achiral isomers controlled circularly polarized luminescence in supramolecular hydrogels

Although chiral functional materials showing circularly polarized luminescence (CPL) are being widely reported, it remains a challenge to provide convenient and universal strategies for constructing controllable CPL-active materials with a high luminescence dissymmetry factor (glum). Herein, the supramolecular chirality as well as the CPL handedness of phenylalanine-based hydrogels can be effectively regulated by commercial achiral naphthylamine isomers through non-covalent interactions. Typically, the co-assembled hydrogels exhibit considerably high |glum| values in the range of 5.62 × 10-3-8.74 × 10-3. The CPL inversion is mainly facilitated by intermolecular hydrogen bonds and π-π stacking between the achiral and chiral molecules, which might be induced by the reorientations of the assembled molecules. This work may provide an alternative method to construct tunable CPL-active materials.

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.

Optically Active Upconverting Nanoparticles with Induced Circularly Polarized Luminescence and Enantioselectively Triggered Photopolymerization

In this work, lanthanide-doped upconversion nanoparticles (UCNPs) showing upconverted circularly polarized luminescence were demonstrated in an organic-inorganic co-assembled system. Achiral UCNPs (NaYF₄:Yb/Er or NaYF₄:Yb/Tm) can be encapsulated into chiral helical nanotubes through the procedure of co-gelation. These co-gel systems display intense upconverted circularly polarized luminescence (UC-CPL) ranging from ultraviolet (UV, 300 nm) to near-infrared (NIR, 850 nm) wavelength. In addition, the UV part of UC-CPL can be used to initiate the enantioselective polymerization of diacetylene.

(R)-Binaphthyl derivatives as chiral dopants: substituent position controlled circularly polarized luminescence in liquid crystals

Chiral liquid crystals (N*-LCs) induced by binaphthyl derivatives which have the same (R)-configuration exhibited opposite circularly polarized luminescence.

Preparation of Carbodiimides with One-Handed Axial Chirality

The axial chirality of carbodiimide was proposed in 1932, but the synthesis of carbodiimide with one-handed axial chirality has not been achieved because of the low barrier of racemization. This work presents a strategy to use a conformationally restrained cyclic structure for creating carbodiimides whose biases of the axial chirality (labeled as S_NCN/ R_NCN) are higher than 100:1, as determined by vibrational circular dichroism spectroscopy and density functional theory calculations.

Electronic chirality inversion of lanthanide complex induced by achiral molecules

A novel mechanism for chiroptical activity inversion based on the electronic structure of metal complexes without Λ- or Δ-type structure change was demonstrated spectroscopically and theoretically. To demonstrate the mechanism, a europium (Eu(III)) complex with chiral (+)-3-(trifluoroacetyl)camphor (+tfc) and achiral triphenylphosphine oxide (tppo) was prepared. The steric and electronic structures of the Eu(III) complex were adjusted by additional achiral tppo and coordinating acetone molecules, and were characterised by 1H NMR, photoluminescence, and emission lifetime measurements. The optical activity of the Eu(III) complex in solution was evaluated by circularly polarized luminescence (CPL) measurements. CPL sign inversion, which was independent of Λ- or Δ-type structure changes from the spectroscopic viewpoint, and a drastic CPL intensity enhancement were observed depending on the external achiral molecules around Eu(III) ion. These phenomena provide the first clarification of optical activity change associated with electronic structure rather than chiral coordination structure-type (Λ or Δ) under external environments.

Cyclopolymerizations: Synthetic Tools for the Precision Synthesis of Macromolecular Architectures

Monomers possessing two functionalities suitable for polymerization are often designed and utilized in syntheses directed to the formation of cross-linked macromolecules. In this review, we give an account of recent developments related to the use of such monomers in cyclopolymerization processes, in order to form linear, soluble macromolecules. These processes can be activated by means of radical, ionic, or transition-metal mediated chain-growth polymerization mechanisms, to achieve cyclic moieties of variable ring size which are embedded within the polymer backbone, driving and tuning peculiar physical properties of the resulting macromolecules. The two functionalities are covalently linked by a "tether", which can be appropriately designed in order to "imprint" elements of chemical information into the polymer backbone during the synthesis and, in some cases, be removed by postpolymerization reactions. The two functionalities can possess identical or even very different reactivities toward the polymerization mechanism involved; in the latter case, consequences and outcomes related to the sequence-controlled, precision synthesis of macromolecules have been demonstrated. Recent advances in new initiating systems and polymerization catalysts enabled the precision syntheses of polymers with regulated cyclic structures by highly regio- and/or stereoselective cyclopolymerization. Cyclopolymerizations involving double cyclization, ring-opening, or isomerization have been also developed, generating unique repeating structures, which can hardly be obtained by conventional polymerization methods.