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

Photon Upconversion Cooperates with Downshifting in Chiral Systems: Modulation, Amplification, and Applications of Circularly Polarized Luminescence

Circularly polarized luminescence (CPL)-active materials are increasingly recognized for their potential applications such as 3D imaging, data storage, and optoelectronic devices. Typically, CPL materials have required high-energy (HE) photons for excitation to emit low-energy (LE) circularly polarized light, a process known as downshifting CPL (DSCPL). However, the emergence of upconverted CPL (UCCPL), where the absorption of multi LE photons results in the emission of a single HE photon with circular polarization, has recently attracted considerable attention. This minireview highlights the intricate relationship between upconversion and CPL phenomena. During upconversion, the dissymmetry factor (glum) value can be improved in certain systems. Additionally, the integration of both LE and HE photons in upconversion-downshifting-synergistic systems offers avenues for dual-excitation or dual-emission CPL functionalities. More in detail, the emerging UCCPL based on various photon upconversion mechanisms and their synergy with DSCPL are introduced. Additionally, several examples that demonstrate the applications of UCCPL are presented to highlight the future opportunities.

Amplified Circularly Polarized Luminescence Behavior in Chiral Co-assembled Liquid Crystal Polymer Films via the Strategic Manipulation of Chiral Inducers

One of the most important factors for the future application of circularly polarized luminescence (CPL) materials is their high dissymmetry factors (gem), and more and more studies are working tirelessly to focus on increasing the gem value. Herein, we chose an achiral liquid crystal polymer (LC-P) and two chiral binaphthyl-based inducers (R/S-3 and R/S-6) with different substitution positions (3,3' positions for R/S-3 and 6,6' positions for R/S-6) to construct chiral co-assemblies and explored their induced amplification CPL behaviors. Interestingly, after the thermal annealing treatment, this kind of chiral co-assembly (R/S-3)0.05-(LC-P)0.95 can emit a superior CPL signal (|gem| = 0.31 and λem = 424 nm), which achieves about 13-fold signal amplification in the spin-coated film, compared to (R/S-6)0.1-(LC-P)0.9 (|gem| = 0.023 and λem = 424 nm). This is because (R/S-3)0.05-(LC-P)0.95 could further co-assemble to form a more ordered arrangement LC state and generate regular helix nanofibers than that of (R/S-6)0.1-(LC-P)0.9. This work provides an efficient method for synthesizing high-quality CPL-active materials through the strategic manipulation of the structure of chiral binaphthyl-based inducers in chiral co-assembled LCP systems.

One-Dimensionally Arranged Quantum-Dot Superstructures Guided by a Supramolecular Polymer Template

Colloidal quantum dots (QDs) exhibit important photophysical properties, such as long-range energy diffusion, miniband formation, and collective photoluminescence, when aggregated into well-defined superstructures, such as three-dimensional (3D) and two-dimensional (2D) superlattices. However, the construction of one-dimensional (1D) QD superstructures, which have a simpler arrangement, is challenging; therefore, the photophysical properties of 1D-arranged QDs have not been studied previously. Herein, we report a versatile strategy to obtain 1D-arranged QDs using a supramolecular polymer (SP) template. The SP is composed of self-assembling cholesterol derivatives containing two amide groups for hydrogen bonding and a carboxyl group as an adhesion moiety on the QDs. Upon mixing the SP and dispersed QDs in low-polarity solvents, the QDs self-adhered to the SP and self-arranged into 1D superstructures through van der Waals interactions between the surface organic ligands of the QDs, as confirmed by transmission electron microscopy. Furthermore, we revealed efficient photoinduced fluorescence resonance energy transfer between the 1D-arranged QDs by an in-depth analysis of the emission spectra and decay curves.

A supramolecular assembly-based strategy towards the generation and amplification of photon up-conversion and circularly polarized luminescence

For the molecular properties in which energy transfer/migration is determinantal, such as triplet-triplet annihilation-based photon up-conversion (TTAUC), the overall performance is largely affected by the intermolecular distance and relative molecular orientations. In such scenarios, tools that may steer the intermolecular interactions and provide control over molecular organisation in the bulk, become most valuable. Often these non-covalent interactions, found predominantly in supramolecular assemblies, enable pre-programming of the molecular network in the assembled structures. In other words, by employing supramolecular chemistry principles, an arrangement where molecular units are arranged in a desired fashion, very much like a Lego toy, could be achieved. This leads to enhanced energy transfer from one molecule to other. In recent past, chiral luminescent systems have attracted huge attention for producing circularly polarized luminescence (CPL). In such systems, chirality is a necessary requirement. Chirality induction/transfer through supramolecular interactions has been known for a long time. It was realized recently that it may help in the generation and amplification of CPL signals as well. In this review article we have discussed the applicability of self-/co-assembly processes for achieving maximum TTA-UC and CPL in various molecular systems.

Processable circularly polarized luminescence material enables flexible stereoscopic 3D imaging

Endowing three-dimensional (3D) displays with flexibility drives innovation in the next-generation wearable and smart electronic technology. Printing circularly polarized luminescence (CPL) materials on stretchable panels gives the chance to build desired flexible stereoscopic displays: CPL provides unusual optical rotation characteristics to achieve the considerable contrast ratio and wide viewing angle. However, the lack of printable, intense circularly polarized optical materials suitable for flexible processing hinders the implementation of flexible 3D devices. Here, we report a controllable and macroscopic production of printable CPL-active photonic paints using a designed confining helical co-assembly strategy, achieving a maximum luminescence dissymmetry factor (glum) value of 1.6. We print customized graphics and meter-long luminous coatings with these paints on a range of substates such as polypropylene, cotton fabric, and polyester fabric. We then demonstrate a flexible textile 3D display panel with two printed sets of pixel arrays based on the orthogonal CPL emission, which lays an efficient framework for future intelligent displays and clothing.

Three-dimensional (3D) bioprinting of medium toughened dipeptide hydrogel scaffolds with Hofmeister effect

Short peptide self-assembled hydrogels as 3D bioprinting inks show excellent biocompatibility and diverse functional expansion, and have broad application prospects in cell culture and tissue engineering. However, the preparation of biological hydrogel inks with adjustable mechanical strength and controllable degradation for 3D bioprinting still faces big challenges. Herein, we develop dipeptide bio-inks that can be gelled in-situ based on Hofmeister sequence, and prepare hydrogel scaffold by using a layer-by-layer 3D printing strategy. Excitingly, after the introduction of Dulbecco's Modified Eagle's medium (DMEM), which is necessary for cell culture, the hydrogel scaffolds show an excellent toughening effect, which matches the needs of cell culture. It's notable that in the whole process of preparation and 3D printing of hydrogel scaffolds, no cross-linking agent, ultraviolet (UV), heating or other exogenous factors are involved, ensuring high biosafety and biocompatibility. After two weeks of 3D culture, millimeter-sized cell spheres are obtained. This work provides an opportunity for the development of short peptide hydrogel bioinks without exogenous factors in 3D printing, tissue engineering, tumor simulant reconstruction and other biomedical fields.

Stepwise Amplification of Circularly Polarized Luminescence in Chiral Metal Cluster Ensembles

Chiral metal-organic frameworks (MOFs) are usually endowed by chiral linkers and/or guests. The strategy using chiral secondary building units in MOFs for solving the trade-off of circularly polarized luminescence (CPL)-active materials, high photoluminescence quantum yields (PLQYs) and high dissymmetry factors (|g_lum |) has not been demonstrated. This work directionally assembles predesigned chiral silver clusters with ACQ linkers through reticular chemistry. The nanoscale chirality of the cluster transmits through MOF's framework, where the linkers are arranged in a quasi-parallel manner and are efficiently isolated and rigidified. Consequently, this backbone of chiral cluster-based MOFs demonstrates superb CPL, high PLQYs of 50.3%, and |g_lum | of 1.2 × 10^-2 . Crystallographic analyses and DFT calculations show the quasi-parallel arrangement manners of emitting linkers leading to a large angle between the electric and magnetic transition dipole moments, boosting CPL response. As compared, an ion-pair-direct assembly without interactions between linkers induces one-ninth |g_lum | and one-sixth PLQY values, further highlighting the merits of directional arrangement in reticular nets. In addition, a prototype CPL switching fabricated by a chiral framework is controlled through alternating ultraviolet and visible light. This work is expected to inspire the development of reticular chemistry for high-performance chiroptical materials.

Light-Responsive Supramolecular Nanotubes-Based Chiral Plasmonic Assemblies

We describe the fabrication of dual-responsive (thermo/light) chiral plasmonic films. The idea is based on using photoswitchable achiral liquid crystal (LCs) forming chiral nanotubes for templating helical assemblies of Au NPs. Circular dichroism spectroscopy (CD) confirms chiroptical properties coming from the arrangement of organic and inorganic components, with up to 0.2 dissymmetry factor (g-factor). Upon exposure to UV light, organic molecules isomerize, resulting in controlled melting of organic nanotubes and/or inorganic nanohelices. The process can be reversed using visible light and further modified by varying the temperature, offering a control of chiroptical response of the composite material. These properties can play a key role in the future development of chiral plasmonics, metamaterials, and optoelectronic devices.

Circularly Polarized Laser Emission from Homochiral Superstructures based on Achiral Molecules with Conformal Flexibility

Without external chiral intervention, it is a challenge to form homochirality from achiral molecules with conformational flexibility. We here report on a rational strategy that uses multivalent noncovalent interactions to clamp the molecular conformations of achiral D-A molecules. These interactions overcome the otherwise dominant dipole-dipole interactions and thus disfavor their symmetric antiparallel stacking. It in turn facilitates parallel packing, leading to spontaneous symmetry breaking during crystallization and thus the formation of homochiral conglomerates. When this emergent homochirality is coupled with optical gain characteristics of the molecules, the homochiral crystals are explored as excellent circularly polarized micro-lasers with low lasing threshold (16.4 μJ cm^-2 ) and high dissymmetry factor g_lum (0.9). This study therefore provides a facile design strategy for supramolecular chiral materials and active laser ones without the necessity of intrinsic chiral element.

Plasmonic Chiral Metasurface-Induced Upconverted Circularly Polarized Luminescence from Achiral Upconversion Nanoparticles

Chirality induction, transfer, and manipulation have aroused great interest in achiral nanomaterials. Here, we demonstrate strong upconverted circularly polarized luminescence from achiral core-shell upconversion nanoparticles (UCNPs) via a plasmonic chiral metasurface-induced optical chirality transfer. The Yb³⁺-sensitized core-shell UCNPs with good dispersity exhibit intense upconversion luminescence of Tm³⁺ and Nd³⁺ through the energy transfer process. By spin-coating the core-shell UCNPs on this chiral metasurface, strong enhancement and circular polarization modulation of upconversion luminescence can be achieved due to resonant coupling between surface plasmons and upconversion nanoparticles. In the UCNPs-on-metasurface composite, a significant upconversion luminescence enhancement can be achieved with a maximum enhancement factor of 32.63 at 878 nm and an overall enhancement factor of 11.61. The luminescence dissymmetry factor of the induced upconverted circularly polarized luminescence can reach 0.95 at the emission wavelength of 895 nm. The UCNPs-on-metasurface composite yields efficient modulation for the emission intensity and polarization of UCNPs, paving new pathways to many potential applications in imaging, sensing, and anticounterfeiting fields.

Tunable Circularly Polarized Luminescence via Chirality Induction and Energy Transfer from Organic Films to Semiconductor Nanocrystals

Circularly polarized luminescent (CPL) films with high dissymmetry factors hold great potential for optoelectronic applications. Herein, we propose a strategy for achieving strongly dissymetric CPL in nanocomposite films based on chirality induction and energy transfer to semiconductor nanocrystals. First, focusing on a purely organic system, aggregation-induced emission (AIE) and CPL activity of organic liquid crystals (LCs) forming helical nanofilaments was detected, featuring green emission with high dissymmetry factors g_lum ∼ 10^-2. The handedness of helical filaments, and thus the sign of CPL, was controlled via minute amounts of a small chiral organic dopant. Second, nanocomposite films were fabricated by incorporating InP/ZnS semiconductor quantum dots (QDs) into the LC matrix, which induced the chiral assembly of QDs and endowed them with chiroptical properties. Due to the spectral matching of the components, energy transfer (ET) from LC to QDs was possible enabling a convenient way of tuning CPL wavelengths by varying the LC/QD ratio. As obtained, composite films exhibited absolute g_lum values up to ∼10^-2 and thermally on/off switchable luminescence. Overall, we demonstrate the induction of chiroptical properties by the assembly of nonchiral building QDs on the chiral organic template and energy transfer from organic films to QDs, representing a simple and versatile approach to tune the CPL activity of organic materials.

Liquid crystal-templated chiral nanomaterials: from chiral plasmonics to circularly polarized luminescence

Chiral nanomaterials with intrinsic chirality or spatial asymmetry at the nanoscale are currently in the limelight of both fundamental research and diverse important technological applications due to their unprecedented physicochemical characteristics such as intense light-matter interactions, enhanced circular dichroism, and strong circularly polarized luminescence. Herein, we provide a comprehensive overview of the state-of-the-art advances in liquid crystal-templated chiral nanomaterials. The chiroptical properties of chiral nanomaterials are touched, and their fundamental design principles and bottom-up synthesis strategies are discussed. Different chiral functional nanomaterials based on liquid-crystalline soft templates, including chiral plasmonic nanomaterials and chiral luminescent nanomaterials, are systematically introduced, and their underlying mechanisms, properties, and potential applications are emphasized. This review concludes with a perspective on the emerging applications, challenges, and future opportunities of such fascinating chiral nanomaterials. This review can not only deepen our understanding of the fundamentals of soft-matter chirality, but also shine light on the development of advanced chiral functional nanomaterials toward their versatile applications in optics, biology, catalysis, electronics, and beyond.

Hierarchical self-assembly into chiral nanostructures

One basic principle regulating self-assembly is associated with the asymmetry of constituent building blocks or packing models. Using asymmetry to manipulate molecular-level devices and hierarchical functional materials is a promising topic in materials sciences and supramolecular chemistry. Here, exemplified by recent major achievements in chiral hierarchical self-assembly, we show how chirality may be utilized in the design, construction and evolution of highly ordered and complex chiral nanostructures. We focus on how unique functions can be developed by the exploitation of chiral nanostructures instead of single basic units. Our perspective on the future prospects of chiral nanostructures via the hierarchical self-assembly strategy is also discussed.

Achiral copper clusters helically confined in self-assembled chiral nanotubes emitting circularly polarized phosphorescence

Achiral Cu5− cluster coassembled with a chiral amphiphile to afford helical nanotubes, where the Cu5− cluster was confined within the nanotubes, forming helical arrangement with emerged chiroptical activities (CD and CPP), with a large gabs up to 0.018.

Shining light on chiral inorganic nanomaterials for biological issues

The rapid development of chiral inorganic nanostructures has greatly expanded from intrinsically chiral nanoparticles to more sophisticated assemblies made by organics, metals, semiconductors, and their hybrids. Among them, lots of studies concerning on hybrid complex of chiral molecules with achiral nanoparticles (NPs) and superstructures with chiral configurations were accordingly conducted due to the great advances such as highly enhanced biocompatibility with low cytotoxicity and enhanced penetration and retention capability, programmable surface functionality with engineerable building blocks, and more importantly tunable chirality in a controlled manner, leading to revolutionary designs of new biomaterials for synergistic cancer therapy, control of enantiomeric enzymatic reactions, integration of metabolism and pathology via bio-to nano or structural chirality. Herein, in this review our objective is to emphasize current research state and clinical applications of chiral nanomaterials in biological systems with special attentions to chiral metal- or semiconductor-based nanostructures in terms of the basic synthesis, related circular dichroism effects at optical frequencies, mechanisms of induced optical chirality and their performances in biomedical applications such as phototherapy, bio-imaging, neurodegenerative diseases, gene editing, cellular activity and sensing of biomarkers so as to provide insights into this fascinating field for peer researchers.

Deep Blue Circularly Polarized Luminescence Response Behavior of an Achiral Pyrene-Based Emitter Regulated by Chiral Co-assembly Helical Nanofibers

Supramolecular co-assembly provides a brand-new powerful strategy for regulating simple organic molecules into various hierarchical nano- and microstructures as smart functional materials. In particular, chiral supramolecular assemblies with strong fluorescent emission have received extensive attention for their application as circularly polarized luminescence (CPL) emitters. Herein, we synthesized three achiral pyrene derivatives, but only the chiral co-assembly (R/S-NMe2-Py-2) can exhibit the regular and orderly helical nanofiber via π-π stacking interaction between chiral N,N'-dimethyl-binaphthyldiamine enantiomers (R/S-NMe2) and the achiral pyrene derivative (Py-2). Interestingly, this kind of 2:1 molar ratio (R/S-NMe2)2-Py-2 co-assembly with a helical nanofiber structure can emit a strong deep blue CPL signal from the achiral pyrene-based emitter, and the dissymmetry factor g_em value can reach 0.027 (λ_em = 423 nm) in the film from spin-coating.