Recent advances in circularly polarized luminescence generated by inorganic materials

Intrinsically chiral luminophores (type I)

The chirality in inorganic materials can appear in various forms. For crystalline materials, point and space groups are basic tools to describe crystal structures. There are 230 group spaces, which are created via the combination of 32 point groups and 14 types of Bravais lattices. Point and space groups are also useful to judge chirality and chiroptical activity. Chiral crystals can exist in 65 Sohncke space groups, which are divided into two categories: I) 22 chiral groups with helical operations that thus contain 11 enantiomeric pairs (e.g., P3₁ and P3₂); II) the remaining 43 groups are achiral; however, if the unit (i.e., the smallest collection of atoms from which the whole crystal is restored by the application of all symmetry operations of the space group and translation) is chiral, the crystals can be chiral^[32]. Early in 1948, chiral crystals with CPL activity were demonstrated in sodium uranyl acetate Na[UO₂(CH₃COO)₃] crystals formed from achiral molecular subunits. The space group of Na[UO₂(CH₃COO)₃] crystals belongs to the chiral space group of P2₁3 and the chirality is caused by the helical canting of the three [UO₂(CH₃COO)₃] rings and the noncentrosymmetric spatial arrangement of four coordination clusters. Moreover, the crystallographic structure is cubic, which is advantageous to observe CPL with small interruptions caused by birefringence. With such unique structures, a high g_lum of 1.31 was observed in Na[UO₂(CH₃COO)₃] crystals by Riehl and Richardson^[33] and Moran et al.^[34].

Another method to endow crystals with optical activity is by creating crystalline defects (e.g., screws and distortions), which are caused by chiral additives regulating the nucleation-growth process^[35]. In addition to the lattice chirality described above, chiral nano-/micromorphologies (e.g., copying the helical patterns of chiral organic templates) on a higher length scale can also induce chiroptical activity in crystalline or amorphous materials^[31]. Duan et al.^[36] demonstrated chiroptically-active crystalline ZnO films with multiple chiralities on different scales. Using chiral amino acids of L-/D-methionine (Met) as both structure-directing and symmetry-breaking agents, chiral hydroxide zinc carbonate (HZC) nanostructures were formed on quartz substrates. After removing organic species by calcination at 550 °C in air, HZC was transformed into chiral nanostructured ZnO films (L-CNZF, obtained with L-Met) with three levels of chirality [Figure 3]. On the primary level, the crystal structure of the ZnO nanoplate shows a subtle helical distortion along the [10-11] axis in a left-handed coiled pattern together with some lattice defects, which is caused by the chiral center of the methionine molecules. On the secondary level, several dozens of ZnO nanoplates are bent into a ZnO particle with a right-handed helix form. On the tertiary level, the arrangement of several ZnO particles forms a left-handed circinate helical morphology on the microscale. For the ZnO films (D-CNZF) obtained using D-Met, the handedness one of each level is just opposite to the corresponding one of L-CNZF. These ZnO films showed various chiroptical activities in their CD, CPL and Raman optical activity spectra. Under a 325 nm excitation, D-CNZF and L-CNZF exhibited antipodal CPL signals centered at 510 nm, consistent with the fluorescence band maximum.

Figure 3. (A) SEM image of L-CNZF. (B) TEM image and (C) SAED patterns of a single ZnO nanoplate. (D) Schematic of hierarchical chirality in L-CNZF. (E) PL and CPL spectra of L- and R-CNZF. Reproduced with permission from Ref.^[36]. Copyright (2015) John Wiley and Sons. SAED: Selected area electron diffraction; CPL: circularly polarized luminescence; PL: photoluminescence; CNZF: chiral nanostructured ZnO films.

Chiral ligand-induced CPL (type II)

Small chiral organic molecules are widely found and are powerful for producing chiroptical signals on inorganic substances. As shown in Figure 4, for example, CD signals are observed on plasmonic metal (Au and Ag) and semiconductor (e.g., CdS, CdSe and CdTe) NPs capped with chiral ligands^[40-42]. The chirality transfer from ligands to inorganics can proceed through various mechanisms, including surface atom distortion with asymmetric clusters, the chiral arrangement of ligands on the surface, crystalline defects and the electronic interaction or coupling between chiral molecules and inorganic species^[43,44].

Figure 4. (A) Optimized geometries and induced CD spectra. (B) CPL (top) and total luminescence (bottom) spectra of L- and D-cysteine-capped CdSe quantum dots. Reproduced with permission from Ref.^[41]. Copyright (2013) American Chemical Society. (C) Chiral riboflavin molecule-induced CD optical activity in Au islands deposited on a glass substrate. Reproduced with permission from Ref.^[42]. Copyright (2013) American Chemical Society. CD: Circular dichroism; CPL: circularly polarized luminescence.

The combination between inorganic nanomaterials and chiral ligands can be fulfilled by two synthetic routes, namely, ligand-guided one-step synthesis or post-modification by ligands exchanges. Although these methods appear easy to carry out, the chirality induction is dependent upon many factors, such as the types of ligands, the sizes/shapes of the NPs and the synthetic conditions. Furthermore, although the chirality is mainly judged by the CD signals, CD activity does not guarantee CPL activity. This is understandable based on the differences between the CD and CPL mechanisms, i.e., CD is associated with absorption from the ground states, while CPL is associated with emissions in the excited states.

“Host-guest”-induced CPL (type III)

Based on the concept of chirality induction in type II, it is found that various luminescent guests show chiroptical activity after being embedded in many chiral materials as chiral hosts. Moreover, several different guests can be simultaneously integrated into one host to acquire white CPL. In particular, with this method, emerging novel inorganic nanomaterials have found broad applications in CPL in recent years by encapsulating inorganic and/or organic luminescent guests. As a result, this strategy is more flexible for producing multicolor CPL by a rational combination between hosts and guests, thus widening the scope of inorganic CPL-active systems.

Inorganic luminophore-containing systems

Carbon dots

Since the discovery of carbon dots in 2006, their luminescence has gained increasing attention as they possess many important features, such as easy production, rich raw materials, multicolor emission, high stability and versatile surface chemistry. In some early reports, carbon dots with chiroptical activity in their CD spectra were easily prepared using chiral molecular precursors; however, the CPL activity was not easily demonstrated^[51]. The CPL of carbon dots was recently activated using the simple “host-guest” strategy^[52-55].

One special characteristic of carbon dots is that their emission wavelength can be tuned by changing their excitation wavelength. Building on this phenomenon, Li et al.^[54] showed an excitation-dependent CPL example. A chiral L-/D-glutamic acid gelator (L-/D-GAm) was assembled into a helical structure that was used as a chiral host to co-assemble with the nitrogen-doped carbon dot guest. The as-formed composites (L-and D-co-gels) appeared as spiral nanotubes [Figure 5A]. The maximum emission wavelength changed from 400 to 460 nm as the excitation wavelength increased from 300 to 400 nm [Figure 5B]. Similarly, the CPL signals were also red-shifted with increasing excitation wavelength from 350 to 370 nm [Figure 5C] and the |g_lum| was ~ 2×10⁻³-4×10⁻³ under a 360 nm irradiation.

Figure 5. Morphologies and optical properties of D-co-gel: (A) FE-SEM image; (B) PL; (C) CPL spectra with the fitting line (inset: plot of the maximum CPL emission wavelength versus the excitation wavelength) under different excitations. Reproduced with permission from Ref.^[54]. Copyright (2020) American Chemical Society. (D) Mirror image CPL spectra of three carbon dot-doped co-gels. (E) PL spectra of white-emitting co-gel (insets: a photograph of the co-gel under 365 nm (top right) and its CIE coordinate value (bottom left). (F) g_lum of white-emitting co-gel from D-GAm (black) and L-GAm. Reproduced with permission from ref.^[55]. Copyright (2021) John Wiley and Sons. PL: Photoluminescence; CPL: circularly polarized luminescence.

Again, using L-/D-GAm co-gels as the hosts to encapsulate three kinds of carbon dots with blue, yellow and red emission colors, Ru et al.^[55] demonstrated a white CPL system. These carbon dots displayed CPL signals at 450, 550 and 610 nm, with a |g_lum| of ~10^-2 [Figure 5D]. Furthermore, by simply blending these three kinds of carbon dots at a mass ratio of 4.63:15.75:1 (blue:yellow:red), white emission was fulfilled with a CIE chromaticity diagram coordinate value of (0.334, 0.329) [Figure 5E]. This as-obtained mixture showed circularly polarized white-light emission with a g_lum of ~ 2.5×10^-3 [Figure 5F], which is smaller than the g_lum of the individual carbon dots.

Metal clusters

Fluorescent metal clusters composed of a few to a hundred atoms represent a class of special luminophores that provide a link between nanoparticles and the atomic behavior of metals. These ultrasmall metal clusters exhibit molecule-like properties, including size-regulated fluorescence and discrete electronic states, which are promising for a broad range of biological applications. The ultrasmall size of the metal clusters is also possibly advantageous for the combination between metal clusters and ligands (or hosts), meaning that diverse CPL-active metal clusters can be synthesized by the three strategies discussed above (types I-III)^[56].

Wu et al.^[57] synthesized spirocyclic Ag clusters [2-(R) and 2-(S), Figure 6A] using two types of chiral diide ligands and 2-(pyridin-2-yl)-1H-benzo[d]imidazole (PBZ). The intrinsic chirality origin of 2-(R) and 2-(S) can be ascribed to the chiral diides, the spirocyclic arrangement of a Ag₉ cluster and the helical arrangement of PBZ molecules. In addition, 2-(R) and 2-(S) can crystalize into crystals with the chiral space group of P2₁. In a mixed solvent of water and methanol, with an increase in the water fraction, 2-(R) and 2-(S) assemble into μm-long helical nanofibers. Furthermore, the solid films of 2-(R) and 2-(S) show strong and mirror-imaged CPL signals centered at 558 nm, with a high g_lum of 0.16 [Figure 6B]. Such a high g_lum may benefit from the exciton couplings between the helically arranged PBZ molecules of individual spirocyclic clusters and the microscopic helical assembly. In other words, both the molecular-level and microscale chirality contribute to the high g_lum. The assembly- and aggregate-induced CPL of metal clusters is also found in gold clusters of Au₃[R/S-BINAP]₃Cl and copper clusters of [Cu₁₄(R/S-DPM)₈]⁶⁺ (DPM = R/S-2-diphenyl-2-hydroxylmethylpyrrolidine-1-propyne), which are CPL inactive when dispersed in good solvents (e.g., CH₂Cl₂) but become CPL active in the aggregate states after the addition of poor solvents (e.g., n-hexane)^[58,59].

Figure 6. (A) Spirocyclic Ag cluster structure of 2-(S). (B) CPL spectra (solid line) and g_lum (dotted line) for film samples of 2-(R) and 2-(S). Reproduced with permission from ref.^[57]. Copyright (2021) John Wiley and Sons. (C) SEM image of ZAS(180, 4.34)-80 showing helices with yellow blocks. (D) CPL spectra of AgNC/ZAS(180, 4.34)-T under different temperatures (T). Reproduced with permission from Ref.^[60]. Copyright (2020) John Wiley and Sons. CPL: Circularly polarized luminescence; ZAS: zeolite LTA superball.

Using a host-guest method, Tao et al.^[60] reported the CPL activity of luminescent Ag nanoclusters (AgNC) incorporated in colloidal zeolite LTA superball (ZAS) assemblies. The macroscopic porous films built from ZAS showed 3D long-range periodicity featuring a photonic band gap (PBG) [Figure 6C]. For one-dimensional PBG structures, the maximum PBG wavelength λ_max is calculated as follows: λ_max = nPsinθ, where n and P are the average refractive index and helical pitch of a PBG structure, respectively, θ is the angle of the incident light with respect to the PBG structures. A PBG structure with a helical periodic profile can selectively reflect CP light, especially at λ_max, with the same handedness of the PBG structure while allowing the transmittance of the opposite-handed CP light^[61-63]. For example, LCP is blocked while RCP is transmitted within a left-handed helical PBG structure, and therefore a large difference between LCP and RCP was detected with a high g_lum. By embedding luminescent AgNC, the AgNC/ZAS enabled the selective reflection of R-CP light and allowed L-CP light, resulting in a g_lum of 0.013 at a 520 nm emission. The g_lum can be tuned by the assembly temperature to change the helical pitches of PBG structures [Figure 6D].

Semiconductor nanocrystals

Semiconductor nanocrystals (NCs), mainly comprised of elements from the II-VI, III-V and IV-VI groups of the periodic table, show many promising optical properties, including exceptional brightness, high quantum yield, superior photostability and size-tunable emission. Colloidal semiconductor NCs are usually capped with organic ligands for the purposes of surface passivation and good dispersibility. In addition, surface ligands also influence the photoluminescent properties. By a ligand-exchange step, chiral ligands can be absorbed onto the surfaces of NCs to make semiconductor NCs CPL active^[27,41,64].

As shown in the previous work by Tohgha et al.^[41], CPL signals at ~ 605 nm (|g_lum| of 3-4×10^-3) are observed on CdSe NPs modified with D- and L-cysteine. Furthermore, Cheng et al.^[64] conducted a systematic study of the CPL properties of cysteine-modified CdSe/CdS core-shell NCs with different shapes [Figure 7A]^[27]. The CdS shell could improve the luminescent quantum yield (QY) of the CdSe core. Interestingly, even if the CdSe core is separated from chiral ligands by the CdS shell, CdSe still shows CPL bands from 550 to 650 nm with a |g_lum| of 1-9×10^-4. The CPL performance may be caused by orbital coupling and Coulombic dipole interactions between CdSe and cysteine. The PL and CPL could be manipulated by the thickness and shapes of the CdS shell (g_lum increases with decreasing CdS shell thickness) but is proportional to the QY [Figure 7B and C]. This work confirmed that chiral ligands can control the whole wavefunction even though there exists an energy barrier of the thin CdS shell.

Figure 7. (A) Schematic anisotropic shapes of CdSe/CdS (gold-colored core: CdSe; orange-color shell: CdS). (B) Dependence of g_lum of D-Cys-CdSe/CdS, CdS shell thickness and (C) QY on different shapes. Reproduced with permission from Ref.^[27]. Copyright (2020) American Chemical Society. QY: Quantum yield.

Chiral inorganic host-based systems

Chiral silica

Over the past two decades, a variety of chiral silica nanomaterials have been reported and their chirality can be manifested by helical outwards or molecular-scale asymmetry on the siloxane framework^[68]. Compared with chiral organic molecules, a prominent feature of silica is its thermal stability (close to 1,000 °C), which is desirable for high-temperature synthetic procedures. It has been confirmed that ions, molecules and nanoparticles incorporated within these chiral nano-silica materials can become CD active^[69,70]. With the above success, it is of interest to explore chiral silica-based CPL-active systems.

Our group first demonstrated several entirely inorganic silica-lanthanide oxide CPL systems^[71]. In our previous work, we found that linear polyethylenimine and L- (or D-) tartaric acids can co-assemble into catalytic chiral templates to direct the formation of chiral SiO₂ nanofibers [Figure 8A]. Vibrational circular dichroism spectroscopic analysis revealed the asymmetric feature of Si-O-Si frameworks on the molecular scale, which can be maintained even up to 900 °C. These SiO₂ nanofibers are used as chiral nanoreactors for the formation of sub-10 nm Eu₂O₃ (or Tb₂O₃) NPs, which are transformed from the Eu³⁺ (or Tb³⁺) trapped in SiO₂ after heating at 900 °C in air [Figure 8B and C]. Interestingly, the CPL features of NPs are observed around the emission wavelengths of 615 nm (Eu₂O₃) and 545 nm (Tb₂O₃) with a |g_lum| of the order of 10^-3. However, the excitation wavelength using homemade CPL spectroscopy is 375 nm, which deviates from the optimized excitation wavelength from PL spectra. The influence of the excitation wavelength on the CPL performance was not probed.

Figure 8. (A) Schematic of chiral SiO₂ nanofiber synthesis using PEI/D-tart (or PEI/L-tart) as chiral catalytic templates and their usage to prepare chiral lanthanide oxide (M₂O₃) NPs. (B) TEM image of Tb₂O₃/D-SiO₂ nanofibers. (C) TEM image of an individual Tb₂O₃ nanoparticle on Tb₂O₃/D-SiO₂ nanofibers. Reproduced with permission from Ref.^[71]. Copyright (2018) John Wiley and Sons. (D) Various CPL-active systems using chiral silica as hosts to incorporate organic fluorescent dyes, AIEgens and perovskites NCs. Reproduced with permission from Ref.^[72]. Copyright (2020) John Wiley and Sons. CPL: Circularly polarized luminescence.

In addition to lanthanide oxides, other luminophores are also endowed with CPL after being embedded within these SiO₂ nanofibers [Figure 8D]^[72]. As a unique luminescent phenomenon, aggregation-induced emission (AIE) has gained ever-increasing attention. Tetraphenylethene (TPE) and its derivatives are representative AIE luminogens (AIEgens) with high quantum yields and ready functionalization. TPE molecules can be easily absorbed onto SiO₂ nanofibers modified with phenyl groups and have already demonstrated AIE-based CPL signals of ~ 476 nm in the aggregated state. When SiO₂ was modified by amine groups, fluorescent carboxylic molecules (5,10,15,20-tetrakis(P-carboxyphenyl)porphyrin, 1-pyrenecarboxyl acid and fluorescein) were attached to SiO₂ and then showed CPL signals at their characteristic emission bands with a |g_lum| of 2-5×10^-3. In addition, these amine-modified SiO₂ materials could work as chiral reactors for the in-situ formation of perovskite NCs with CPL activity. These results imply the significant potential of chiral SiO₂ in designing CPL-related materials.

Harada et al.^[70] found that cationic gemini surfactants of C₂H₄-1,2-[(CH₃)₂N⁺C₁₆H₃₃]₂ self-assemble into helical structures in the presence of L- or D-tartrate counter ions. Using these organic helices as templates (dented by 16-2-16 L-tartrate or 6-2-16 D-tartrate), elegant right- and left-handed SiO₂ nanohelices could be prepared and further used as chiral hosts to synthesize lanthanide ion- and perovskite-containing CPL systems [Figure 9A]. After the absorption of Tb³⁺ onto SiO₂ nanohelices and calcination at 900 °C for 4 h in air, the small amount of Tb³⁺ (~ 0.31%) can display CPL outputs at ~ 545 nm (irradiated at 230 nm) with a |g_lum| of ~ 10^-3 due to the helical-morphological chirality of SiO₂. The chiroptical activity may be due to the 4f orbitals of Tb³⁺ being chirally distorted by the asymmetrically arranged Si-O framework. In addition, after modifying the surfaces of SiO₂ helices with amine by 3-aminopropyl-triethoxysilane, perovskite CsPbBr₃ nanocrystals (PNCs) are grafted onto SiO₂^[73]. When suspended in toluene, PNC-SiO₂ do not exhibit CD and CPL signals. In contrast, after casting the above suspension on quartz and drying, the PNC-SiO₂ in a dried film state became both CD and CPL active. The maximum CPL signals locate at 517.5 nm (under a 360 nm excitation) and the |g_lum| reaches 5.7-6.9×10^-3. It was found that PNCs are closely packed and helically arranged around SiO₂ helices during the solvent evaporation process, which facilitated the dipolar interaction between the PNCs and then produced CPL.

Figure 9. (A) Molecular structure of 16-2-16 L-tartrate (left) and TEM images of L-SiO₂:Tb³⁺ calcined at 900 °C (middle) and nanohelices@PNCs in a dried film and the cross sections (yellow and red frames) obtained with 3D tomography reconstructions (right). Reproduced with permission from Ref.^[70]. Copyright (2021) Royal Society of Chemistry. Reproduced with permission from Ref.^[73]. Copyright (2020) American Chemical Society. (B) SEM image of TPES-P. Reproduced with permission from Ref. ^[74]. Copyright (2019) Royal Society of Chemistry. (C) SEM image of L-CASN. Reproduced with permission from Ref.^[75]. Copyright (2019) Royal Society of Chemistry. TPES-P: TPE-silica with plus helice. L-CASN: left-handed chiral lamellar mesostructured AIEgen-silica hybrid hollow nanotubes;

The synthesis of the CPL-active materials above occurs via a two-step process, which requires the synthesis of SiO₂ in advance. In fact, the luminescent components could also be one-step incorporated into a siloxane framework during the condensation of silica sources (e.g., tetraethyl orthosilicate and multisilylated organic molecules). For example, Cai et al.^[74] designed TPE-containing bis(triethoxysilane) (BTSTPE) as the silica precursor and then prepared TPE-bridged polybissilsesquioxane (TPE-silica) nanotubes with single-handed helicity using the self-assemblies of a pair of chiral cationic low-molecular-weight gelators (L-ValPyBr and D-ValPyBr) as templates [Figure 9B]. Strikingly, the TPE-silica products showed high thermal stability with only a 5% weight loss at ~ 500 °C. The helical TPE-silica powders emit green light at ~ 500 nm with a quantum yield of over 21%, which is higher than that of its precursor of BTSTPE. The CPL outputs are found at 500 nm with a |g_lum| of 0.6-1.6×10^-3. In another work, Zhang et al.^[75] reported AIEgen-silica helical nanotubes displaying CPL signals at 453 nm with a g_lum up to 0.02 [under a 366 nm excitation, Figure 9C]. The CPL-active products were obtained by the co-assembly from 2-[4-(1,2,2-triphenylethenyl)-phenoxy]-acetic acid (as the luminophore), N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (as a co-structure directing agent) and tetraethylorthosilicate (as a silica precursor).

Other chiral inorganic hosts

As discussed above, there are 22 chiral space groups in crystal structures, providing the possibility of generating intrinsically chiral crystalline materials. For example, the space groups of TbPO₄·H₂O crystals belong to the chiral group of P3₁21 or its enantiomorph P3₂21. Hananel et al.^[26] employed tartaric acid enantiomers as chirality inducers to produce chiral TbPO₄·H₂O doped with Eu³⁺. Under a 365 nm excitation, Eu³⁺ in the TbPO₄·H₂O host showed strong CPL signals between 590 and 720 nm corresponding to the ⁰D₅→⁷F_J transitions of Eu³⁺. The g_lum could be 0.06 at 612 nm and is comparable with many chiral Eu(III) complexes. In this work, CPL spectroscopy is also used as a tool to study the symmetry breaking behavior of chiral TbPO₄·H₂O crystals controlled by the enantiomeric excess (e.e.) of tartaric acid.

Unlike the case above where the symmetry breaking is regulated by chiral molecules, Zhang et al.^[76] found that oleyamine-coated GdOOH nanowires with a diameter of less than 1 nm could randomly transform into macroscopic left- and right-handed helical assemblies (MHAs) at the centimeter level by an evaporation-induced self-assembly process without the assistance of a chiral dopant [Figure 10A,B]^[76]. The spontaneous formation of helices is a result of an energy-minimizing process and the handedness is uncontrollable. Fortunately, the left- and right-handed helices can be observed by the naked eye and easily separated. Achiral fluorescent molecule guests of DACT and TMD embedded into MHAs showed CPL signals at their emission bands at 500 and 625 nm, respectively.

Figure 10. (A) Cryo-electron microscopy image and its large-magnification version (upper inset) of the sub-1 nm GdOOH nanowires (bottom: a photograph of the dispersion of nanowires). (B) CPL spectra of DACT-MHA. Reproduced with permission from Ref.^[76]. Copyright (2020) American Chemical Society. CPL: Circularly polarized luminescence; MHA: macroscopically spiral assemblies.

Up-conversion CPL

Up-conversion luminescence is capable of converting long-wavelength NIR light to shorter-wavelength narrow and sharp emission ranging from UV, to visible and even to shorter NIR light. Up-conversion luminescence is utilized widely in a broad range of areas, especially in biological and biomedical ones, due to low background fluorescence, weak photodamage, good penetration in tissues and high sensitivity.

Lanthanide ions (e.g., Yb³⁺, Er³⁺ and Tm³⁺) doped in fluoride-based matrixes (e.g., NaYF₄ and NaGdF₄) possess high up-conversion efficiencies because of sufficient metastable energy and excited states with a long lifetime. Yang et al.^[77] offered a dexterous solution for an up-conversion CPL based on a radiative energy transfer process from NaYF₄:Yb/Tm Upconverting nanoparticles (UCNPs) to CsPbBr₃ perovskite nanocrystals (PKNCs) in the chiral nematic liquid crystal (N*LC) featured with PBG [Figure 11A]. A high |g_lum| is possible in a PBG structure due to strong handedness-selective reflection of CP light. However, the light loss may occur and the emission of emitters is boosted when the emission wavelength is located at the edge of the photonic bandgap, while the emission is suppressed at the center of the bandgap. Therefore, it is a challenge to achieve both a large g_lum and high emission efficiency. To address this issue, an achiral nematic liquid crystal SLC1717 and chiral dopant R-(or S-)811 were used to form left- or right-handed helical N*LC, where two different emitters of NaYF₄:Yb/Tm and CsPbBr₃ were embedded. Under a 980 nm excitation, NaYF₄:Yb/Tm UCNPs showed emission bands at 450 and 475 nm near the edge of the bandgap of N*LC and these emissions were thus enhanced and the g_lum at 475 nm was 0.3. The emission of PKNCs was centered at 495 nm, which overlaps with the center of the bandgap. Fortunately, the emission of PKNCs could be enhanced via a radiative energy transfer with quenching the emission of NaYF₄:Yb/Tm at 475 nm. Meanwhile, a very high g_lum of 1.1 was still achieved for PKNCs at 495 nm.

Figure 11. (A) Schematic representation of RET process from UCNPs to CsPbBr₃ PKNCs in N*LC (the emissions of CsPbBr₃ PKNCs and UCNPs were located at the center and the edge of the photonic bandgap, respectively, left) and UC-CPL spectra of CsPbBr₃ PKNCs/UCNPs in N*LC with different incident power of a 980 nm laser (right). Reproduced with permission from Ref.^[77]. Copyright (2020) John Wiley and Sons. (B) NIR-CPL spectra (top) of both the enantiomers of Yb(TTA)₃ⁱPrPybox (left) and Yb(TTA)₃PhPybox (right) and total NIR emission (bottom). Reproduced with permission from Ref.^[78]. Copyright (2019) Royal Society of Chemistry. RET: Radiative energy transfer; PKNCs: perovskite nanocrystals; UCNP: upconverting nanoparticles; TTA: 2-thenoyltrifluoroacetonate.

NIR emission CPL

The emission at the NIR range from luminophores is highly beneficial to optical applications in transparent biological windows because of the suppressed absorption or scattering from skin and blood. However, there is a lack of suitable, reliable detection instruments for weak NIR emission and reports on NIR-CPL are scarce. Yb³⁺ is a typical NIR emitter in the range of 900–1050 nm. The complex of Yb(TTA)₃ (TTA = 2-thenoyltrifluoroacetonate) shows relatively good quantum yields due to the antenna effect of TTA ligands. On the basis of Yb(TTA)₃, Zinna et al.^[78] introduced additional chiral ligands (ⁱPrPyBox or PhPyBox) to synthesize two groups of chiral Yb(TTA)₃ⁱPrPyBox and Yb(TTA)₃PhPyBox complexes [Figure 11B]. To measure the NIR-CPL of these complexes, the authors conducted modifications on spectrofluoropolarimeter apparatus using an Ag-O-Cs photomultiplier tube as the detector and a photoelastic modulator coupled with an uncoated Glan–Thompson polarizer for polarization discrimination. For Yb(TTA)₃ⁱPrPyBox, relatively sharp peaks at 972 nm (²F_5/2 → ²F_7/2 transition of Yb³⁺) appeared on their CPL spectra appeared with a g_lum of -0.025 and +0.029 for (R,R) and (S,S) enantiomers, respectively. Similar results were also found for Yb(TTA)₃PhPyBox enantiomers.

Thermally activated delayed fluorescence CPL

In principle, TADF enables an internal quantum efficiency of 100% by harvesting triplet (T1) excitons, which is fulfilled through the reverse intersystem crossing from the triplet (T1) to singlet (S1) state in a thermally activated process. Therefore, TADF has received widespread attention, especially in organic light-emitting diode displays. Recently, TADF-based CPL has also gained increasing interest in organic molecules^[79]. In contrast, TADF-CPL in inorganic emitters is rare. Han et al.^[80] synthesized octahedral Ag(I) clusters using chiral ligands named as L/D and PL/PD enantiomers [structures shown in Figure 12A]. In the case of PL/PD, Ag₆ cores in the Ag clusters of Ag₆PL₆ and Ag₆PD₆ are distorted and deviate from the regular octahedral Ag6 skeleton, and these clusters crystallize in the chiral monoclinic space group P2₁. Based on chiroptical spectra and DFT simulations, the chiroptical activity of the clusters is mainly attributed to the asymmetric Ag₆ core and the chiral interface between Ag and the coordinated C and N atoms. The PLQY of Ag₆PL₆/Ag₆PD₆ surpasses 95% at room temperature, which is a result of TADF confirmed by the temperature-dependent luminescence spectra and decays. Finally, these clusters display TADF-based CPL signals at 575 nm (excited at 370 nm) with a g_lum of ~ 4×10^-3.

Figure 12. (A) Structures of Ag₆L₆/D₆ and Ag₆PL₆/PD₆ enantiomers, L/D, PL/PD and octahedral Ag₆ framework in Ag₆L₆/D₆. Reproduced with permission from Ref.^[80]. (B) Images of various lanthanide-containing phosphor materials before and after modification with L/D-Cys via ball milling (top) and schematic of CaAl₂O₄:Eu,Nd phosphor attached by chiral Cys molecules on the surface and the induction of the phosphorescence-based CPL (bottom). Reproduced with permission from Ref.^[81]. Copyright (2021) John Wiley and Sons. (C) Schematic of hybrid chiral photonic showing dual CPL and CPRTP (left), an experimental setup for the measurement of CPRTP (middle) and phosphorescence emission spectra of hybrid chiral photonic films recorded with an L-CPF or R-CPF (right). Reproduced with permission from Ref.^[52]. Copyright (2020) American Chemical Society. CPL: Circularly polarized luminescence; CPRTP: circular polarized room-temperature phosphorescence; CPF: circularly polarized filter.

Phosphorescence CPL

As for the origin of photoluminescence, phosphorescence is an important complementary mechanism to fluorescence. Phosphorescence is the emission from triplet-excited states and its lifetime is longer than fluorescence. Therefore, phosphorescence is appealing in optical sensors, information security, optoelectronic devices and biomedical agents.

Phosphorescence can be observed in lanthanide and Cr ions at room temperature. As demonstrated in Figure 12B Hao et al.^[81] reported a simple method to obtain colorful CPL-active inorganic persistent lanthanide-based phosphor materials with a long afterglow, including a yellow-green SrAl₂O₄:Eu,Dy, purple CaAl₂O₄:Eu,Nd, blue-green Sr₄Al₄O₂₅:Eu,Dy, sky blue Sr₂MgSi₂O₇ and red Y₂O₂S:Eu,Mg,Ti. These commercial phosphor powders are mixed with D- (or L-) cysteine (Cys) in ethanol and subjected to ball-milling for several hours and then the D-/L-Cys-modified phosphors are collected. Using this simple process, these phosphors became CPL-active with a |g_lum| of 10⁻³-10⁻², of which a maximum value was observed in the CaAl₂O₄:Eu,Nd sample (2.10×10⁻²). The CD and CPL activity of these inorganic phosphors was induced by chiral Cys molecules, which were absorbed via metal-sulfur bonds.

Compared with lanthanides ions, the observation of room-temperature phosphorescence on other luminescent materials is not straightforward. Xu et al.^[52] demonstrated a three-component system with both fluorescence- and phosphorescence-based CPL of carbon dots (CDs) at room temperature [Figure 12C]. Cellulose nanocrystals are used to form chiral photonic films, in which poly(vinyl alcohol) (PVA) and luminescent CDs are integrated by a co-assembly process. Because of the PBG effect, CPL was observed when the fluorescence wavelength (460 nm, excited by 365 nm) of the CDs located within the bandgap. The employment of PVA seemed to restrict molecular motions and inhibit oxygen quenching to maintain triplet excitons, thus leading to phosphorescence afterglow (480-500 nm) lasting for 2 s, even after turning off the UV (260 nm) irradiation. However, there is an overlap between the fluorescence spectrum (with a peak located at 450-470 nm) and the room-temperature phosphorescence (RTP) spectrum (with a peak at 480-500 nm). In other words, the fluorescence bands are close to the RTP bands. Therefore, it is difficult to differentiate FL-based CPL from RTP-induced CPL on the as-used JASCO CPL-200 spectrophotometer. These authors designed an analytical device [shown in Figure 12C, middle] to measure circular polarized room-temperature phosphorescence (CPRTP), which is fulfilled by combining a fluorospectrophotometer and an L- (or R-) circularly polarized filter (CPF). The CPF was covered on the detector at the phosphorescence measurement mode to record the CPRTP spectra. In principle, L-CPRTP would be blocked by the RCPF, while R-CPRTP would be blocked by the LCPF. On this basis, it was found that the CPRTP intensity recorded with LCPF was weaker than RCPF. By borrowing the definition of g_lum, a so-called g_RTP is defined as g_RTP = [2×(I_L-I_R)]/(I_L+I_R), where I_L and I_R are the intensity of RTP recorded under LCPF and RCPF, respectively. The highest absolute value of g_RTP could reach 0.461.

Sensors

Fluorescence sensing is an important analysis technique with many merits, such as rapidness, sensitivity, nondestructiveness and the ability to obtain diverse information regarding molecular interactions and microenvironments. Since fluorescence is a prerequisite for CPL, the change in fluorescent properties leads to the variation of CPL signals. Therefore, the principles of fluorescence sensing may be borrowed to design CPL sensors. For example, Suo et al.^[82] reported a AuAg bimetallic cluster-based CPL sensing method by virtue of the fluorescence quenching effect [Figure 13A]. In the presence of metal ions (Sr²⁺ and K⁺), guanosine 5′-monophosphate can self-assemble into left- or right-handed helical nanofibers (g-fibers) with a width of 200 nm and a length of several micrometers. These g-fibers are used as chiral hosts to embed AuAg clusters stabilized with adenosine 5′-monophosphate. Consequently, helical nanostructured g-fiber-AuAg composites were obtained and showed CPL at ~ 475 nm under a 356 nm excitation, with a |g_lum| of 1-3×10^-2. Because of the fluorescence quenching of AuAg NCs by L-Cys, g-fiber-AuAg NCs could be used as CPL sensors for the selective detection of L-Cys (in the range of 0.1 to 8 μmol L⁻¹) with a limit of detection down to 95.7 nM, which is comparable to other common fluorometric and electrochemical methods.

Figure 13. Applications of inorganic-associated CPL-active systems. (A) CPL sensors of Cys. Reproduced with permission from Ref.^[82]. Copyright (2020) American Chemical Society. (B) A security paint based on CPL-active Eu(III) lumino-glass. Reproduced with permission from Ref.^[83]. (C) Electroluminescent device architecture containing chiral Eu(III) complexes and their circularly polarized electroluminescence (CPEL) spectra. Reproduced with permission from Ref.^[84]. Copyright (2015) John Wiley and Sons. (D) Conceptual description of illuminating PL/CPL dual-mode rewritable optical storage and real images of PL/CPL dual-mode patterns. Reproduced with permission from Ref. ^[87]. Copyright (2020) Royal Society of Chemistry. (E) Schematic of enantioselective photopolymerization setup of HA. Reproduced with permission from Ref.^[88]. Copyright (2019) American Chemical Society. PL: Photoluminescence; CPL: circularly polarized luminescenc.

Anticounterfeiting

Luminescent nanomaterials are widely employed in anticounterfeiting applications. To improve the security level, it is highly desirable that a luminescent material can possess more features. CPL-active materials encode hidden chiral fingerprint information into luminescence, which is not easily decoded by common optical read-out instruments. As a result, CPL offers a novel advanced anticounterfeiting technique with high expertise in chirality.

Kitagawa et al.^[83] firstly synthesized europium complexes with chiral (+/-)-3-(trifluoroacetyl)camphor (+/-tfc) ligands and then cast them on a glass substrate together with the achiral glass promoter tris(2,6-dimethoxyphenyl)phosphine oxide (tmpo) ligand [Figure 13B]. After solvent evaporation, two enantiomeric Eu(+tfc)-tmpo(2) and Eu (−tfc)-tmpo(2) transparent films with an extra-large |g_lum| of 1.2 at a 594 nm emission (the magnetic dipole transition ⁵D₀ → ⁷F₁ of Eu³⁺) were produced. Furthermore, the authors demonstrated a CP light-based security device model to visualize the CPL image by a camera equipped with a linear polarizer, a rotatable λ/4 plate and a bandpass filter (594 nm). Eu(+tfc)-tmpo(2) was cast into a sun pattern and Eu(−tfc)-tmpo(2) into a moon. Without the λ/4 plate, both the sun and moon are observed. By rotating the λ/4 plate 90° clockwise or anticlockwise using an angle controller, the photographs of the emission using L-CP light (“sun”) and R-CP light (“moon”), respectively, are exhibited.

Electroluminescent devices

Electroluminescent devices, where an emissive layer is sandwiched between an anode and a cathode, are promising for artificial light sources. In these devices, the charge carriers (electrons and holes) are generated at the electrodes under an external voltage, transported to and then recombined at the emissive layer to produce electroluminescence. Various luminescent materials are used as the emissive layers to construct electroluminescent devices, such as organic, CD-based and perovskite-based light emitting diodes. Furthermore, these devices, which are capable of directly emitting CP light, are of significant interest for their promising potential in displays with high-resolution, 3D imaging ability, improved brightness and lower energy consumption.

Zinna et al.^[84] used chiral europium complexes of CsEu(hfbc)₄ (hfbc = 3-heptafluoro-butylyrylcamphorate) with the highest available g_lum (g_lum = 1.38 at 595 nm) as circularly polarized emitters in electroluminescent devices [Figure 13C]. To reduce the emission from the matrix, the Eu(III) complexes were blended with a polyvinylcarbazole host and a soluble additive of 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene to form the emitting layer. To minimize the internal reflection of the emitted light on the back electrode, a Ba/Al cathode (50% transmittance at 612 nm) was employed. With these strategies, a good polarization degree of emission (as measured by the dissymmetry factor of g_EL in electroluminescence) is achieved, with g_EL values of ±0.75 and −0.09/+0.15 for the Eu ⁵D₀→⁷F₁ (at 595 nm) and Eu ⁵D₀→⁷F₂ transitions (at 612 nm), respectively. Furthermore, by optimizing the emissive layer formulations and fine-tuning the architectures of the devices, the absolute value of g_EL could even be increased to 1.0 (for Eu ⁵D₀→⁷F₁ at 595 nm)^[85].

Han et al.^[86] reported two enantiomeric gold clusters (Au₄L₄)_n/(Au₄D₄)_n consisting of a Au₄ core and a chiral ligand, (S)-/(R)-4-isopropylthiazolidine-2-thione [abbreviated as L and D, structures shown in Figure 12A]. Through an aggregation- and crystallization-induced emission process, these aggregated gold nanoclusters showed a strong green emission centered at 507 nm with a photoluminescence quantum yield of 41.4% and intense CPL signals with |g_lum| of 7.0×10⁻³. These nanoclusters were employed as the chiral emissive layer to fabricate electroluminescent devices. The as-obtained device exhibited obvious green CPEL signals (driven at a voltage of 10 V) with a maximum external quantum efficiency of 1.5% and a dissymmetry factor (|g_EL|) of 4-7×10⁻³.

Optical storage

The usage of CP light for optical storage is attractive due to the orthogonal characteristics of the polarization and uncomplicated optical system structures. Juan et al.^[87] proposed an optical storage model based on 980 nm NIR-light triggered reversible CPL switching. A cholesteric liquid crystal with a self-organized helical superstructure was used as the host to load lanthanide (Yb³⁺, Er³⁺ or Tm³⁺)-doped NaYF₄-based UCNPs and a chiral fluorescent photoswitch of 6 [Figure 13D]. When the UCNPs are irradiated by 980 nm at a low power (980-l), the 520 nm green emission (Yb³⁺ → Er³⁺ energy transfer process) dominated, while the 365 and 450 nm emissions (Yb³⁺ → Tm³⁺ energy transfer process) appeared at a high laser power density (980 h). Switch 6 showed a trans to cis photoisomerization under 520 nm light and the reverse isomerization upon irradiation at 365 or 450 nm. A strong emission peak at ~ 580 nm appeared for the trans state of switch 6 and greatly decreased for the cis state. Correspondingly, both the CPL signal and its absolute g_lum of ~ 580 nm decreased with low-power 980 nm NIR light, while partly recovering when exposed to high-power 980 nm light. Based on this switchable process controlled by laser power, a PL/CPL-encryption dual-mode rewritable optical storage device can be designed. The fluorescent dot array can be written on the cell using a photomask with a high-power laser and the information is erased under a low-power mode.

Asymmetric synthesis

CP light-induced asymmetric photochemical synthesis is an important type of absolute synthesis and even accounts for the homochirality origin on earth. Because of the unbalance between L- and R-CP emissions, CPL-active emitters are potential CP light sources to initiate enantioselective synthesis. Jin et al.^[88] showed an up-conversion CPL to trigger the enantioselective polymerization of diacetylene [Figure 13E]. Achiral lanthanide-doped UCNPs (NaYF₄:Yb/Er or NaYF₄:Yb/Tm) were co-assembled with chiral lipid gelator L-/D-GAm to create helical nanotubes. After excitation with a 980 nm laser, the emission peaks of UCNPs-Tm are mainly located at 360 nm (¹D₂ → ³H₆ transition), 476 nm (¹G₄ → ³H₆) and 802 nm (³H₄ → ³H₆). Interestingly, the UV part of CPL from UCNPs-Tm could induce the polymerization of 2,4-heneicosadiynoic acid into polydiacetylene (PDA). PDA formed using L-GAm/UCNPs-Tm showed positive CD signals, while negative CD signals were found with D-GAm/UCNPs-Tm. Therefore, the molecular chirality of the gelator controlled the handedness of the UC-CPL from lanthanide ions and then subsequently regulated the enantioselective polymerization of diacetylene.