Circularly Polarized Luminescence Activity Research Articles

Chirality Transfer from Chiral Mesoporous Silica to Perovskite CsPbBr 3 Nanocrystals: The Role of Chiral Confinement

Chirality Transfer from Chiral Mesoporous Silica to Perovskite CsPbBr ₃ Nanocrystals: The Role of Chiral Confinement

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

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

Recent topics on circularly polarized luminescence generated by inorganic materials

Circularly polarized luminescence (CPL) is an interesting phenomenon representing the unequal emission of left-handed and right-handed polarized light from an emitter. CPL is promising in chirality characterizations and various optical applications. Traditionally, research on CPL is centered on organic substances. In recent years, CPL present on inorganic ones has also become a nascent topic, which is significant to explore novel chirality- and luminescence-related properties and applications in inorganic materials. This minireview summarizes the recent progress on the following two aspects: 1) how to endow common inorganic luminophores with CPL activity; 2) how to use emerging chiral inorganic nanomaterials to design CPL-active systems. The general synthesis strategies, optical properties, applications and outlook of CPL-active inorganic materials are demonstrated.

Endowing inorganic nanomaterials with circularly polarized luminescence

AbstractRecently, materials with circularly polarized luminescence (CPL) properties have attracted substantial attention because they offer new perspectives in fundamental research and wide applications in photonics, bio‐encoding, catalysis, and so forth. Such importance has recently promoted the development of CPL‐active materials from the traditional realm of organics to newly released areas in inorganics. Due to the advantages of inorganic nanomaterials in stability, high luminescent quantum efficiency, material diversity, and the diversity of shapes and sizes, extensive research about CPL active inorganic nanomaterials has been done in the past decades leading to great signs of progress on synthesis, characterizations, and potential applications. In this review, therefore, we will thoroughly describe the general design principles of inorganic nanomaterials with CPL activity, basically according to the origins of chirality in inorganic nanomaterials: intrinsic chirality in inorganic nanomaterials, ligand‐induced chirality, and structural chirality caused by the supramolecular assembly, respectively. The representative applications of the CPL‐active inorganic nanomaterials are presented with respect to challenges, prospective, and problems that unsolved to date.

Ultraviolet Light Detectable Circularly Polarized Room Temperature Phosphorescence in Chiral Naphthalimide Self-Assemblies.

The combination of circularly polarized luminescence (CPL) and pure-organic room temperature phosphorescence (RTP) potentially facilitates the construction of organic chiroptical optoelectronics and display materials, which however are challenging to use in realizing smart control of luminescent colors and switchable chiroptical properties. Here, we show a host-guest strategy to fabricate color-tunable RTP-based circularly polarized phosphorescence. Napthalimides were conjugated directly to chiral segments, of which supramolecular chirality and CPL activities in solid-states could be triggered by substituting bromine atoms on amines. Introducing tetracyanobenzene as an achiral host matrix via simple grinding would allow for the intersystem crossing to trigger red RTP and corresponding CPL by excitation lower than 320 nm, with a large Stokes shift more than 300 nm. The critical excitation wavelength of the RTP switch is determined by the absorbance of tetracyanobenzene. When the excitation wavelength was larger than 320 nm, blue fluorescence dominated with turned off RTP and CPL. The excitation wavelength-dependent RTP and CPL switch allows for detecting ultraviolet (UV) light, showing distinguishable red-blue luminescent color transition, accompanied by on/off RTP. Changing the host matrix from tetracyanobenzene to tricyanobenzene or dicyanobenzene could adjust the critical detecting wavelength limit from 320 to 300 nm. This work establishes a strategy to realize color-tunable, UV light detectable RTP and CPL under smart control.

Triptycene-derived TADF enantiomers displaying circularly polarized luminescence and high-efficiency electroluminescence

A pair of novel circularly polarized thermally activated delayed fluorescence (CP-TADF) enantiomers (+)-( S , S )- CTRI-Cz and (−)-( R , R )- CTRI-Cz based on chiral triptycene scaffold were designed and synthesized. The obtained triptycene-derived enantiomers displayed obvious TADF activities with small singlet-triplet energy gap value (Δ E ST ) of 0.20 eV and characteristic microsecond delayed lifetime of 15.4 μs. Moreover, the TADF enantiomers showed mirror-image circular dichroism (CD) and circularly polarized luminescence (CPL) activities, and their luminescence dissymmetry factors ( g lum ) were about ±0.9 × 10 −3 . Finally, by using the TADF enantiomers as emitters, the optimized organic light-emitting diodes (OLEDs) achieved maximum external quantum efficiency (EQE max ), current efficiency (CE max ) and power efficiency (PE max ) of 15.0%, 48.8 cd/A and 46.9 lm/W, respectively. A pair of CP-TADF enantiomers based on chiral triptycene scaffold were synthesized, and they displayed CPL and high efficiency electroluminescence properties. • A pair of CP-TADF enantiomers based on chiral triptycene scaffold were conveniently synthesized. • The TADF enantiomers showed stable chiroptical properties with mirror-imaged CD and CPL activities. • The OLED devices based on the enantiomers achieved EQE max of 15.0% and 14.7%, respectively.

Chiral Self‐Assembled Film from Semiconductor Nanorods with Ultra‐Strong Circularly Polarized Luminescence

AbstractChiroptical nanomaterials have generated significant levels of interest for generating strong circularly polarized luminescence (CPL) signals. We used the Langmuir–Schaeffer technique to generate the continuous and compact assembly of CdSe/CdS chiral film. We assembled achiral CdSe/CdS nanorods by controlling the number of layers and angles between different layers. This allowed us to tailor chiroptical properties to achieve high CPL signals. The chiral film was symmetrical and had the highest circular dichroism (CD) response and CPL signals with ten layers (RH (right‐handed)‐/LH (left‐handed)‐5 + 5 layers) and a 45° inter‐angle. Specifically, RH‐5+5 of the chiral film exhibited 1431 mdeg of CD activity and strong CPL signals with a dissymmetry factor (glum) of 0.0997. The helical stacked crystal plates with linear birefringence resulted in strong circular birefringence, as determined by the Reusch model. Electromagnetic simulations indicated that such remarkable optical activity was attributed to the birefringence and dichroism of the well‐aligned CdSe/CdS nanorod layers in the chiral films. Under right/left circular polarized (RCP/LCP) light excitation, the well aligned semiconductor nanorods exhibited differences in the coupling efficiencies to RCP and LCP light. Our CdSe/CdS chiral films, which exhibit ultra‐strong CPL activity, will provide a novel strategy for the fabrication of chiroptical devices.

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

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

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

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

Preparation of twisted organic–inorganic hybrid silica bundles with circularly polarized luminescence by supramolecular templating polymerization

Circularly polarized luminescence (CPL) materials have attracted increasing attention because of their promising applications in optoelectronics and chiroptical sensing. However, silica-based organic–inorganic hybrid nanomaterials with CPL properties have been rarely reported. In this work, twisted hybrid silica bundles were prepared by supramolecular templating polymerization of tetraphenylethene-containing bis(triethoxysilane) and tetraethoxysilane using the self-assemblies of chiral low-molecular-weight gelators as templates. It was found that the chirality of the supramolecular templates was transferred to the hybrid silicas. The hybrid silicas emitted green CPL with the luminescence dissymmetry factor up to 1.2 × 10−3 and high fluorescence efficiency. This work presents a facile method to fabricate silica-based hybrid nanomaterials with CPL activity.

Synthesis of Single‐Handed Helical Spiro‐Conjugated Ladder Polymers through Quantitative and Chemoselective Cyclizations**

AbstractWe report the unprecedented synthesis of one‐handed helical spiro‐conjugated ladder polymers with well‐defined primary and secondary structures, in which the spiro‐linked dibenzo[a,h]anthracene fluorophores are arranged in a one‐handed twisting direction, through quantitative and chemoselective acid‐promoted intramolecular cyclizations of random‐coil precursor polymers composed of chiral 1,1′‐spirobiindane and achiral bis[2‐(4‐alkoxyphenyl)ethynyl]phenylene units. Intense circular dichroism (CD) and circularly polarized luminescence (CPL) were observed, whereas the precursor polymers exhibited negligible CD and CPL activities. The introduction of 2,6‐dimethyl substituents on the 4‐alkoxyphenylethynyl pendants is of key importance for this simple, quantitative, and chemoselective cyclization. This strategy is applicable to the defect‐free precise synthesis of other varieties of fully π‐conjugated molecules and coplanar ladder polymers that have not been achieved before.

Synthesis of Single-Handed Helical Spiro-Conjugated Ladder Polymers through Quantitative and Chemoselective Cyclizations*.

We report the unprecedented synthesis of one-handed helical spiro-conjugated ladder polymers with well-defined primary and secondary structures, in which the spiro-linked dibenzo[a,h]anthracene fluorophores are arranged in a one-handed twisting direction, through quantitative and chemoselective acid-promoted intramolecular cyclizations of random-coil precursor polymers composed of chiral 1,1'-spirobiindane and achiral bis[2-(4-alkoxyphenyl)ethynyl]phenylene units. Intense circular dichroism (CD) and circularly polarized luminescence (CPL) were observed, whereas the precursor polymers exhibited negligible CD and CPL activities. The introduction of 2,6-dimethyl substituents on the 4-alkoxyphenylethynyl pendants is of key importance for this simple, quantitative, and chemoselective cyclization. This strategy is applicable to the defect-free precise synthesis of other varieties of fully π-conjugated molecules and coplanar ladder polymers that have not been achieved before.

Circularly Polarized Luminescence Liquids Based on Siloxybinaphthyls: Best Binaphthyl Dihedral Angle in the Excited State.

A series of axially chiral 1,1'-binaphthyls with trialkylsiloxy (OSiR3 ) groups were synthesized. Among them, 1 a-c possessing OSiR3 groups at the 7,7'-positions and methyl groups at the 2,2'-positions were liquids at room temperature, and the neat liquids showed circularly polarized luminescence (CPL) (R=Bu; Φfl,liquid =0.21, |glum,liquid |=1.6×10-3 ). The |glum,liquid | value is the highest of pure liquids. These compounds remained liquid over a broad range of temperatures, down to -50 °C. Time-dependent DFT calculations indicated that in the excited state, the binaphthyls adopt a transoid conformation with a small angle between the electric and magnetic transition dipole moments (θμ,m =77°), which is a key factor in their CPL activity. The best binaphthyl dihedral angle in the excited state is approximately 110°.

Circularly Polarized Luminescence Liquids Based on Siloxybinaphthyls: Best Binaphthyl Dihedral Angle in the Excited State

AbstractA series of axially chiral 1,1′‐binaphthyls with trialkylsiloxy (OSiR3) groups were synthesized. Among them,1 a–cpossessing OSiR3groups at the 7,7′‐positions and methyl groups at the 2,2′‐positions were liquids at room temperature, and the neat liquids showed circularly polarized luminescence (CPL) (R=Bu;Φfl,liquid=0.21, |glum,liquid|=1.6×10−3). The |glum,liquid| value is the highest of pure liquids. These compounds remained liquid over a broad range of temperatures, down to −50 °C. Time‐dependent DFT calculations indicated that in the excited state, the binaphthyls adopt atransoidconformation with a small angle between the electric and magnetic transition dipole moments (θμ,m=77°), which is a key factor in their CPL activity. The best binaphthyl dihedral angle in the excited state is approximately 110°.

Regulating Circularly Polarized Luminescence of Axially Chiral Anthracene Derivatives through Solvatochromism and Supramolecular Self‐assembly

AbstractTwo axially chiral fluorophores derived from anthracene were designed and synthesized, and the chiroptical behaviour were thoroughly investigated including solvent‐dependent emission and circularly polarized luminescence (CPL). Interestingly, unexpected supramolecular self‐assembly of the chiral molecules driven by strong π−π interaction can enable significant hypsochromic shift and fluorescence intensity enhancement. Meanwhile, the luminescence dissymmetry factor of CPL activity exhibited significant enhancement through self‐assembly process. This work provides a novel approach to achieve the supramolecular CPL materials possessing both high luminescence quantum efficiency and high circular polarization.

Thermally activated delayed fluorescence material-sensitized helicene enantiomer-based OLEDs: a new strategy for improving the efficiency of circularly polarized electroluminescence

A new strategy of thermally activated delayed fluorescence (TADF) material-sensitized circularly polarized luminescence (CPL) has been proposed for improving the efficiencies of fluorescent circularly polarized organic light-emitting diodes (OLEDs) (CP-OLEDs). Consequently, a pair of helicene enantiomers, (P)-HAI and (M)-HAI, were synthesized. The helicene enantiomers with the rigid helical π-skeleton had highly thermal and enantiomeric stabilities, and they also showed excellent photophysical properties, especially, intense mirror-image CPL activities with large luminescence dissymmetry factor (|glum|) values of about 6 × 10−3. Notably, the CP-OLEDs with the helicene enantiomers as emitters and a TADF molecule as sensitizer not only displayed better performance of lower turn-on voltage (VT) of 2.6 V, four-fold maxmium-external quantum efficiency (EQEmax) of 5.3%, and lower efficiencies roll-off of 1.9% at 1000 cd m−2, than those of the devices without TADF sensitizer, but also exhibited intense circularly polarized electroluminescence (CPEL) with the electroluminescence dissymmetry factor (gEL) values of −2.3 × 10−3 and +3.0 × 10−3. Meanwhile, this study also represents the first thermally activated sensitized fluorescent CP-OLEDs with markedly enhanced efficiencies and intense CPEL.

Ligand-Induced Chirality in Asymmetric CdSe/CdS Nanostructures: A Close Look at Chiral Tadpoles.

Ligand-induced chirality in asymmetric CdSe/CdS core-shell nanocrystals (NCs) has been extensively applied in chiral biosensors, regioselective syntheses and assemblies, circularly polarized luminescence (CPL), and chiroptic-based devices due to their excellent physiochemical properties, such as the tunable quantum confinement effects, surface functionality, and chemical stability. Herein, we present CdSe/CdS NCs with various morphologies such as nanoflowers, tadpoles, and dot/rods (DRs) with chirality induced by surface chiral ligands. The observed circular dichroism (CD) and CPL activities are closely associated with the geometrical characteristics of the nanostructures, such as the shell thickness and the aspect ratio of the CdSe/CdS NCs. Furthermore, in situ observations of the growth of tadpoles with a single tail indicate that the CD response is mainly attributed to the CdS shell, which has a maximum tail length of ∼45 nm (approximately λ/10 of the incident light wavelength). On the other hand, the CPL activity is only related to the CdSe core, and the activity benefits from a thin CdS shell with a relatively high photoluminescence quantum yield (QY). Further theoretical models demonstrated the aspect-ratio-dependent g-factor and QY variations in these asymmetric nanostructures. These findings provide insights into not only the asymmetric synthesis of CdSe/CdS NCs, but also the rational design of CdSe/CdS nanostructures with tunable CD and CPL activities.

New Perspectives to Trigger and Modulate Circularly Polarized Luminescence of Complex and Aggregated Systems: Energy Transfer, Photon Upconversion, Charge Transfer, and Organic Radical.

Chiral functional materials with circularly polarized luminescence (CPL) have risen rapidly in recent years because of their fascinating characteristics and potential applications in various research fields. CPL refers to the differential spontaneous emission of left (L)- and right (R)-handed circularly polarized light upon photon or electron excitation. Generally, an outstanding CPL-active material needs to possess a high luminescence dissymmetry factor (glum) (defined as 2(IL - IR)/(IL + IR) where I is the emission intensity), which is between -2 and +2. Although the exciting development in CPL-active materials was achieved, the modulation of CPL signs is still a challenge. For small organic systems, a relatively small glum value, one of the key parameters of CPL, limits their practical applications. Searching for efficient approaches for amplifying glum is important. Therefore, over the past decades, besides optimizing the structure of small molecules, many other strategies to obtain efficient CPL-active materials have been developed. For instance, self-assembly has been well demonstrated as an effective approach to amplify the supramolecular chirality as well as the glum values. On the other hand, chiral liquid crystals (CLCs), which are capable of selective reflection of left- and right-handed circularly polarized light, also to serve as a host matrix for endowing guest emitters with CPL activity and high glum values. However, self-assembly focuses on modulating the conformation and spatial arrangement of chiral emitters. And the CPL of a luminophore-doped CLC matrix depends on the helix pitch and band gap positions. Lately, novel photophysical approaches to modulate CPL signs have gradually emerged.In this Account, we discuss the recent progress of excited-state-regulation involved CPL-active materials. The emergence, amplification, and inversion of CPL can be adjusted through regulation of the excited state of chiral emitters. For example, Förster resonance energy transfer (FRET) can amplify the glum values of chiral energy acceptors in chiral supramolecular assemblies. By combining the concepts of photon upconversion and CPL, high-energy upconverted circularly polarized emission was achieved under excitation of low-energy light, accompanied by an amplified glum. In addition, the organic systems with unpaired electrons, i.e., charge transfer (CT) system and open-shell π-radical, show favorable CPL properties, which can be flexibly tuned with an applied magnetic field. It should be noted that these photophysical process are associated with the excited state of chiral emitters. So far, while the main focus is on the regulation of the molecular and supramolecular nanostructures, direct regulation of the excited state of the chiral system will serve as a new platform to understand and regulate the CPL activity and will be helpful to develop smart chiroptical materials.

Axially Chiral Biphenyl Compound-Based Thermally Activated Delayed Fluorescent Materials for High-Performance Circularly Polarized Organic Light-Emitting Diodes.

To boost intrinsic circularly polarized luminescence (CPL) properties of chiral emitters, an axially chiral biphenyl unit is inlaid in thermally activated delayed fluorescent (TADF) skeleton, urging the participation of chiral source in frontier molecular orbital distributions. A pair of enantiomers, (R)‐BPPOACZ and (S)‐BPPOACZ, containing the cyano as electron‐withdrawing moieties and carbazole and phenoxazine as electron‐donating units are synthesized and separated. The circularly polarized TADF enantiomers exhibit both high photoluminescence quantum yield of 86.10% and excellent CPL activities with maximum dissymmetry factor |g PL| values of almost 10−2 in solution and 1.8 × 10−2 in doped film, which are among the best values of previously reported small chiral organic materials. Moreover, the circularly polarized organic light‐emitting diodes based on the TADF enantiomers achieve the maximum external quantum efficiency of 16.6% with extremely low efficiency roll‐off. Obvious circularly polarized electroluminescence signals with |g EL| values of 4 × 10−3 are also recorded.

Chiral Reticular Self-Assembly of Achiral AIEgen into Optically Pure Metal-Organic Frameworks (MOFs) with Dual Mechano-Switchable Circularly Polarized Luminescence.

Circularly polarized luminescence (CPL) is attractive in understanding the excited-state chirality and developing advanced materials. Herein, we propose a chiral reticular self-assembly strategy to unite achiral AIEgens, chirality donors, and metal ions to fabricate optically pure AIEgen metal-organic frameworks (MOFs) as efficient CPL materials. We have found that CPL activity of the single-crystal AIEgen MOF was generated by the framework-enabled strong emission from AIEgens and through-space chirality transfer from chirality donors to achiral AIEgens via metal-ion bridges. For the first time, a dual mechano-switched blue and red-shifted CPL activity was achieved via ultrasonication and grinding, which enabled the rotation or stacking change of AIEgen rotors with the intact homochiral framework. This work provided not only an insightful view of the aggregation induced emission (AIE) mechanism, but also an efficient and versatile strategy for the preparation of stimuli-responsive CPL materials.