Circularly polarized luminescence of coordination aggregates

The development and applications of materials with efficient circularly polarized luminescence (CPL) have become an interdisciplinary frontier research topic. We summarize herein the recent advance in the development and applications of CPL‐active aggregates based on metal‐ligand coordination materials (termed as “coordination aggregates”). The materials surveyed are classified as aggregates of small‐molecular metal complexes, which include monocomponent assemblies of Pt(II) complexes and other complexes and binary aggregates of metal complexes, and CPL‐active metal‐ligand coordination helicates, polymers, and frameworks. The efforts in improving the dissymmetry luminescence factors and quantum yields of these materials and the use of the aggregation strategy in enhancing the performance of isolated molecules are discussed. The recent applications of chiral metal complexes in circularly polarized organic light‐emitting diodes (OLEDs) based on solution‐ or evaporation‐processed procedures are surveyed. In addition, the uses of lanthanide complexes in CPL‐contrast imaging and as CPL probes are highlighted. The common discussion on the mechanism of aggregation‐enhanced CPLs and a perspective on future works of CPL‐active coordination aggregates are finally given.


INTRODUCTION
Circularly polarized luminescence (CPL) is generated by chiral materials that can emit differential left-and right-handed circularly polarized (CP) light, and it reflects the structural information of chiral emitters in excited states. [1][2][3][4] Since the first CPL measurement was recorded for the solution of transβ-hydrindanone by Emeis and Oosterhoff, [5] the exploitation of CPL-active molecular materials has attracted tremendous attention, in view of their applications in three-dimensional (3D) display, chiral optoelectronic devices, chiral recognition and catalysis, contrast imaging, security-enhanced information storage and transportation, etc. [1][2][3][4] Especially in the past decade, a large amount of CPL-active molecules and materials have been developed and investigated, including chiral organic small molecules and conjugated polymers, chiral lanthanide (Ln) and transition-metal complexes, supramolecular assemblies and liquid crystalline (LC) materials, as well as chiral metal clusters and inorganic nanomaterials.  CPL performance is generally evaluated by the luminescence dissymmetry factor (g lum ) and quantum yield (Φ). The former refers to the degree of "enantiorichness" of the emitted CP partially because of the size mismatch between the excitation wavelength (hundreds of nanometers) and that of common isolated molecular materials (typically a few of nanometers or less). [3,4,11] In this context, aggregation has become an efficient strategy for the chirality amplification and |g lum | enhancement of molecular materials, including supramolecular assembly and polymerization, [2,18,19] the construction of energy/electron transfer system, [38][39][40][41] and the formation of LC phase. [23,24] Remarkably, |g lum | values as high as 1. 79 have been recorded for CPL-active polymers combined with chiral nematic LC (N*-LC) materials, based on the excellent capability of N*-LC to selectively reflect/transmit handedness-specific CP light. [42] It is of note that aggregated or solid samples often suffer from the issue of aggregation-caused emission quenching, which however can be addressed by the combination of chromophores with aggregation-induced emission (AIEgens) [43][44][45][46][47] and chiral inducers. [20,21] As the homemade or commercial CPL measurement systems are gradually available in the past 10 years, the development of CPL-active materials has been largely boosted. This is evidenced by appearance of numerous reviews on related topics, including CPL-active organic small molecules, [4,12,13] metal complexes, [14][15][16][17] conjugated polymers, [18,19] supramolecular assemblies, [2,22,23] chiral metal clusters and inorganic nanomaterials, [25][26][27]48] and AIEgens and mesogens, [20,21,[32][33][34] as well as optoelectronic and bioimaging applications of these materials. [6][7][8][9][10] Considering the importance of metal complexes in the development of CPL-active materials and the aggregation strategy in enhancing the performance of isolated molecules, we summarize herein the recent advance in the development of CPLactive aggregates based on metal-ligand coordination materials (termed as "coordination aggregates"). Another reason for focusing on this particular topic is that materials are typically in aggregate state (including thin films) in practical applications. Metal complexes that only show CPL activity in dispersed solutions have been well discussed in previous reviews and is thus not repeated herein. [14][15][16][17] In addition to the brief introduction of the general principle and characterization method of CPL, we will mainly discuss the CPL performance of coordination aggregates based on discrete chiral metal complexes, chiral metal-ligand helicates, and coordination polymers and frameworks. This is followed by discussion of the representative applications of these materials in CP organic light-emitting diodes (CP-OLEDs) and CPLcontrast imaging and probes. The review is concluded with the perspective on the future development of this area.

GENERAL PRINCIPLE AND CHARACTERIZATION METHOD OF CPL
Considering that the physical basis and related theory of CPL have been fully discussed in a number of reviews, [2,11,15,29,49] only a brief overview of basic principle and information is highlighted herein. CPL spectroscopy measures the intensity difference (ΔI) between left-and right-handed CPL (L-CPL and R-CPL) of a chiral emitter, as defined by Equation (1).
wherein I L and I R describe the intensity of L-CPL and R-CPL, respectively. The polarized degree of CPL is assessed by g lum (g EL is used for electroluminescence), as defined by Equation (2).
The extra factor of (1/2) in Equation (2) is purposely introduced to keep consistent with the absorption dissymmetry factor g CD (CD: circular dichroism; or g abs ) at definition, which is given by Equation (3).
where L and R refer to the molar absorption coefficients for the left-and right-handed CP light, respectively. Experimentally, the g CD value is determined according to Equation (4).
g CD = ellipticity 32,980 × absorbance (4) in which the values of "ellipticity" (unit: mdeg) and "absorbance" can be both directly obtained from the CD spectral measurement. The g lum factor could be theoretically expressed by Equation (5).
wherein μ and m represent the electric and magnetic transition dipole moment, respectively, and is the angle between those two vectors. The |μ| value of organic molecular materials is usually much larger than |m| and Equation (5) can be simplified to Equation (6).
However, Equation (6) is not applicable for materials, for example, chiral Ln(III) complexes, with a large |m|. Instead, another parameter r, which equals to |μ|/|m|, is introduced. In this case, Equation (5) is transformed to Equation (7).
It is clear from the above equation that when |μ| equals to |m| (r = 1) and is 0 o or 180 o , the theoretical maximal g lum of ±2 will be obtained.
In order to further assess the overall merit of CPL, the concept of CPL brightness (B CPL ), similar to the luminescence brightness (B) analog, is recently proposed by Di Bari and coworkers. [28] The B CPL value is defined as Equation (8).
wherein is molar extinction coefficient measured at the excitation wavelength (λ) and the fluorescence brightness B equals to × . Equation (8) is only applicable to solution-state samples. For solid-state samples, a related Equation (9) is introduced.
wherein the absorption efficiency abs is defined as the ratio between the absorbed and incident photon numbers, and measurable with an integrating sphere. Similar to B for solutionstate samples, B ′ describes the luminescence brightness of solid-state samples.

CPL-ACTIVE AGGREGATES OF SMALL-MOLECULAR METAL COMPLEXES
The exploration of CPL-active small-molecular metal complexes, mainly including chiral Ln(III) and transition-metal complexes, has attracted intense attention over the past few decades in view of their diverse coordination modes, rich excited-state properties, and high Φ or g lum values. [14][15][16][17]21] Among them, a certain number of materials display distinct aggregation-induced or enhanced CPL via crystallization or supramolecular assembly. Chiral square-planar Pt(II) complexes are one representative material in these studies. Other metal complexes, for example, Ln(III) and linear Au(I) complexes, have also been investigated in this context. In addition, apart from monocomponent assembly, bicomponent assembly involving chiral metal complexes provides another means for the development CPL-active aggregates. The details are discussed as follows.

Aggregates of other metal complexes
Apart from chiral Pt(II) complexes, CPL-active aggregates of Au(I) complexes, featuring linear molecular structures and potential Au-Au interactions, have received attention. For example, Tang and coworkers [70] presented two enantiomeric binuclear Au(I) complexes (R,R/S,S)-10 linked by the 1,2-diisocyano-1,2-diphenylethane ligand ( Figure 3A). Under the stimuli of mechanical force, the powders of 10 can be transformed from non-emissive isolated crystallites to highly emissive well-defined microcrystals, mainly as a result of subtle molecular motions driven by multiple aurophilic, C−H⋅⋅⋅F and π−π inter-/intramolecular interactions. The  [71] (C) (i) Molecular structures of (+)-12a-12d, (ii) scanning electron microscopy images of the helical assemblies of (+)-12a (left) and (+)-12c (right), and (iii) CPL and photoluminescence (PL) of (+)-12a-12d in chloroform (dashed lines) and CHCl 3 /hexane (1/24) mixture. Reproduced with permission: Copyright 2016, Royal Society of Chemistry [72] emission of 10 could be quenched by heating and further recovered by cooling and scratching. More interestingly, the emissive microcrystals displayed enhanced CPLs with |g lum | of 4 × 10 -3 at 500 nm and Φ of 15%. Chiral lanthanide complexes represent one important type of metal complexes for CPL studies. Considering their excellent performance in solutions, researchers are interested in knowing whether the CPL properties could be further improved by assembly or agglomeration. Okayasua and Yuasa [71] reported the chiral neutral Eu(III) complexes (R/S)-11a and 11b, in which 11a could form ensembles via coordination between the pendent NO 2 group and the Eu(III) center of adjacent molecules ( Figure 3B). The distinctly larger |g lum | observed for 11a in CH 3 CN (0.19 at 594 nm) with respect to that of 11b (0.11-0.13 at 594 nm) suggests the positive effect of ensembles in amplifying CPLs. These authors also prepared a series of chiral ionic Eu(III) complexes (+)-12a-12d with different metal cations (Na + , K + , Rb + , and Cs + ) and demonstrated a simple methodology to tune their CPLs by cation-dependent helical assemblies ( Figure 3C). [72] Compared to assemblies with the smaller cation K + (g lum = 0.43 at 595 nm, Φ = 0.31%), those with the larger cation Rb + or Cs + showed more favorable length and extension of twist as well as improved CPL g lum (1.21-1.45 at 595 nm) and Φ (0.68%-1.04%).

Binary aggregates of metal complexes
In comparison with the aforementioned self-assemblies, chiral co-assemblies possess several advantages, for example, the avoidance of tedious synthesis and the self-emission quenching of monocomponent assemblies, and the potential to tune CPLs by energy transfer. These advantages make coassembly an appealing strategy to obtain CPL-active aggregate materials. [2,73] Several CPL-active binary aggregates of metal complexes have been prepared in past few years either by direct supramolecular co-assembly with the assistance of various noncovalent interactions or by a doping method. For instance, You and coworkers [74] reported the CPL-active helical co-assemblies of the achiral bidentate Pt(II) complex 13 with a small fraction of the chiral Pt(II) analog (R/S)-14 ( Figure 4A). The co-assemblies displayed much improved |g lum | values (0.05-0.064 at 615 nm; Φ = 0.01) relative to those (<10 -3 ) of individual molecules of (R/S)-14. Additionally, Tang and coworkers [75] constructed helical structures via the hierarchical self-assembly of the chiral binuclear Au(I) complexes (R/S)-15, which could serve as the chiral template to co-assemble with other achiral dyes, for example, 9,10-bis(phenylethynyl)anthracene (BPEA), tetraphenyl ethylene (TPE), and tetra(4-methoxyphenyl)pyrazine (TPP-4M), to realize tunable CPLs with |g lum | of 1.0 × 10 -3 to 5.0 × 10 -3 at 410-560 nm ( Figure 4B and Table 1). Chen and coworkers [76] recently prepared the organometallic double salts of the chiral cationic Au

CPL-ACTIVE AGGREGATES OF COORDINATION HELICATES, POLYMERS, AND FRAMEWORKS
Aggregates of small-molecular metal complexes are assembled together by various noncovalent interactions. In comparison, those of metal-organic helicates, polymers (MOPs) and frameworks (MOFs) are linked together by coordination bonds with much improved stability. The constructions of CPL-active coordination helicates, MOPs, and MOFs have attracted much attention in the past decade and some of them are of practical use in enantioselective recognition and separation, asymmetric catalysis, nonlinear optics, etc. [77][78][79][80][81][82][83][84]

CPL-active coordination helicates
Chiral metal-ligand coordination materials with determined spatial geometrical structures, for example, molecular helicates and cages, are very promising in terms of creating novel CPL-active materials. Related works have been reported by several research groups separately or cooperatively and some materials show intense CPLs in solid states. Through the traditional chiral resolution by high performance liquid chromatography (HPLC), Ono et al. [78] prepared a series of stereoisomeric triple-stranded coordination helicates (P/M)-18a-18c comprised of Al(III) ions and achiral tetradentate ligands with methyl substituents at different positions ( Figure 5A). These enantioisomeric Al(III) helicates displayed mirror-imaged tunable CPLs with |g lum | in the range of 5 × 10 -4 to 4 × 10 -3 at 450-586 nm and Φ of 10%-50% both in solution and solid states ( Table 2). Kawai and coworkers [79] fabricated the D 2 -symmetrical homochiral circular Eu(III) helicates (R/S)-19 via ligand-to-ligand interactions between the chiral bis(4-phenyl-2-oxazolinyl)pyridine and achiral bis-β-diketonate ligands ( Figure 5B). The enantiomers (R/S)-19 showed distinct CPL with |g lum | of 0.22-0.24 at 591 nm and Φ of 5% in solid state with the identical spectral signatures as that in solution, indicating their stable helicate conformation in both states. In addition, Kawai and coworkers [80] constructed the D 4 -symmetrical octanuclear circular Ln(III) helicates (R/S)-20a and 20b composed of the achiral trianionic tris-β-diketonate (THP) and chiral bis(4-isopropyl-2-oxazolinyl)pyridine ( i Pr-pybox) ligands  [80] with Eu(III) or Tb(III) ion, respectively ( Figure 5C). Similar to the homochiral Eu(III) helicates 19 mentioned above, the homoconfigurational 20a and 20b are prone to be formed by the ligand-to-ligand interactions between THP and i Prpybox, and exhibit remarkable CPL with detected |g lum | values up to 1.25 at 592 nm for Eu(III) helicates (R/S)-20a (Φ = 14.5%) and 0.25 at 541 nm for Tb(III) analogs (R/S)-20b (Φ = 0.13%). The significant |g lum | values together with sufficient emission intensity of 20a in chloroform and PMMA thin films lead to the eye-recognition chiroptical dissymmetry through an optical filter setup.

Dye-encapsulated coordination frameworks
The high porosity and stability of MOFs is beneficial to act as a host system for the encapsulation of functional dyes to build host-guest materials. The host MOFs could be either selfemissive or non-emissive. After the incorporation of guest dyes, potential energy and chirality transfer processes will allow to greatly tune the photophysical properties of the obtained host-guest MOFs. [88][89][90][91][92] The above discussed MOF (−)-25 has been used to load different dyes to modulate the emission colors; however, no CPL properties were reported for the obtained host-guest MOFs. [87] Liu and coworkers [88] manufactured the chiral host-guest systems 26a-26g by loading various dyes into the emergent cubic void of the in situ formed MOFs from γCD and K + ions ( Figure 7A). These dye-encapsulated γCD-MOFs display tunable emissions and boosted CPLs. Notably, the sizes of the guest dyes significantly affect their located positions in the frameworks as well as the CPL signs and magnitudes (Table 2). With the inclusion of most of the dyes studied, the γCD-MOFs show negative CPLs. However, the CPL sign of the ruthenium trisbipyridine (Ru-BPY)-loaded MOF 26d is uncontrollable, possible due to the weak host-guest interaction in this case. [88] Another series of CPL-active MOFs 27a-27e were disclosed by Zang and coworkers [89] (Figure 7B). Two enantiomeric Zn-MOFs (D/L)-27a with inner helical channels were obtained from the reaction Zn 2+ ion with a chiral amino acid-derived ligand, which were used as chiral templates to incorporate a diversity of achiral dyes to realize red (27e), green (27d), blue (27c), and white (27b) CPLs with |g lum | of 1.0 × 10 -3 to 11.5 × 10 -3 at 435-618 nm and Φ of 19%-66% ( Table 2). The confinement effect of chiral nanopores in these frameworks plays a vital role in enhancing the tunable CPLs of encapsulated dyes.

CP-OLEDs
Compared to the traditional method of obtaining CP light by using a polarizer and quarter-wave plate to filter out equal amount light with an opposite CP direction, devices with self-emissive CPL, for example, CP-OLEDs, can avoid such complicated filtering process and corresponding energy loss. Consequently, great effort has been devoted to the fabrication TA B L E 2 Circularly polarized luminescence (CPL) parameters of metal-ligand coordination helicates, polymers, and frameworks Materials λ em (nm) Φ (%) g lum Ref.
On the basis of the chiral doping strategy, Di Bari and coworkers [97,98] manufactured the solution-processed CP-OLEDs by blending the CPL-active ionic Eu(III) complex (-)-31 into polyvinylcarbazole (PVK) and 1,3-bis[2-(4-tertbutylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7) as the active emissive layer (Figure 8). The corresponding optimized devices with a semitransparent thin layer (around 6 nm) of aluminum back cathode exhibited a high g EL value of -1.0 at 595 nm (comparable to the PL g lum , g PL , of -1.21), albeit with a low maximum external quantum efficiency (EQE max ) of 0.05% (Table 3). In contrast, when a thick layer (110 nm) of aluminum cathode was used, the g EL value dropped to -0.15 at the same emission wavelength. This decrease of circular polarization is caused by the detrimental effect of the back electrode (cathode) reflection, which can reverse the spiral direction of CP light. [98] The use of a semitransparent back electrode would be helpful to reduce this effect. As shown in Table 3, the |g EL | factors of most CP-OLEDs are smaller (comparable in a few of cases) with respect to their |g PL | values. [1,6,11] Apart from the back electrode reflection, other factors such as CP light scattering, interlayer reflections, and inefficient chirality transfer of multicomponent systems are believed to be responsible for this difference. [98] In addition to Eu(III) complexes, doped films with the enantiomeric platinahelicene (+/-)-32 were used in CP-OLEDs to show g EL of 0.22 and -0.38, respectively, at 615 nm. [99] Using the doped axially chiral platinabinaphthalenes (R/S)-33 as the emitters, solution-processed CP-OLEDs with |g EL | of ca. 1 × 10 -3 at about 640 nm and the maximum luminance (L max ) of 3500 cd m −2 and EQE max of 2.15% were demonstrated. [100] Based on a chiral poly(3vinylcarbazole) as the host and the axially chiral platinabinaphthalenes (R/S)-34 as the dopants, You and coworkers [101] manufactured the multilayer CP-PLEDs to exhibit |g EL | of ca. 1 × 10 -4 at 540 nm and EQE max of 1.2%.
In spite of the above advance, the g EL and EQE max of CP-OLEDs are unsatisfactory. Wang and coworkers [102] recently employed the chiral LC Pt(II) complexes (R/S)-35 as the dopants in PVK/OXD-7 blend to fabricate CP-OLEDs. Upon annealing treatment at 100 • C, the obtained devices presented large |g EL | value of 0.06 and high EQE max of 11.3%. In addition, the CP-OLEDs with the axially chiral dinuclear Pt(II) complexes (R/S)-36a and 36b were demonstrated by Xu and coworkers [103] to give |g EL | of 3.0 × 10 -3 and EQE max of 3.1%.
Though the solution processes are advantageous in simple device fabrication procedures, the EQEs of corresponding CP-OLEDs are not satisfactory. In comparison, the evaporation-processed CP-OLEDs usually show better device performances attributed to the effective balance of the hole and electron injection and transport. To meet the require-ment of evaporation, chiral emitters should feature relatively small molecular weight and weak intermolecular π-π interaction. Among them, small molecular organic compounds, including those with thermally assisted delayed fluorescence (TADF) properties, and chiral neutral metal complexes have been commonly used. [1,6,105] Some representative and very recent examples of evaporation-processed CP-OLEDs with chiral metal complexes are highlighted below. Other examples, including those with TADF molecules, have been discussed in a previous review. [11] Chiral octahedral Ir(III) complexes have been frequently used in evaporation-processed CP-OLEDs. For instance, Zheng and coworkers [106] firstly introduced a series of stereoisomeric Ir(III) complexes, including the classical Ir(III) trisphenylpyridine complexes 38, for evaporationprocessed multilayered CP-OLEDs. The obtained devices displayed |g EL | of 2.8 × 10 -4 to 6.8 × 10 -4 at 512-522 nm with high EQE max over 21%. The stereoisomeric Ir(III) complexes (Δ,R/S)-39 with the axially chiral disulfide ligand were found to give comparable performance (Table 3). [107] By using a semitransparent aluminum or silver cathode, Yang and coworkers [108] constructed the CP-OLEDs based on the stereoisomeric iridium(III) complexes (Δ/Λ,R)-40 with the chiral β-diketonate ligand to exhibit improved |g EL | of 7.7 × 10 -3 at 520 nm with EQE max of 18.8%. The obtained |g EL | values are about one order of magnitude larger than those measured from the conventional devices without the use of semitransparent cathodes (6.1 × 10 -4 ). The decrease of |g EL | with nontransparent cathodes was caused by the spiral direction reversion of CPL by the flection of cathode, as has been discussed above on the devices with the Eu(III) complex (−)-31. [98] Other than Ir(III) complexes, neutral Pt(II) and Zn(II) complexes have been tested in evaporation-processed CP-OLEDs. Enantiomeric platinahelicene deriatives with a similar structure as 32 have been examined as the chiral emitters in evaporation-processed CP-OLEDs to exhibit |g EL | in the 10 -3 order of magnitude and EQE max of over 15%. [109] You and coworkers [110] employed the evaporated chiral tridentate Pt(II) complexes (R/S)-41 as the neat emissive layer to construct CP-OLEDs, yielding |g EL | of 1.2 × 10 -4 at ca. 650 nm and EQE max of 9.7%. In addition, the CP-OLEDs with the enantiomeric tetradentate Pt(II) complexes (P/M)-42 were shown recently to give |g EL | of 6.8 × 10 -4 at 600 nm and EQE max of 12.6%. [111] One rare example using chiral Zn(II) complexes as the emitters in evaporation-processed CP-OLEDs were reported by Tang and coworkers, [112] who used the enantiomeric tetradentate Zn(II) salen complexes (R,R/S,S)-43 to achieve high |g EL | of up to 4.4 × 10 -2 at 490 nm albeit with low EQE max of 0.042%. Interestingly, the |g EL | factors of (R,R/S,S)-43 are around one order of magnitude higher with respect to their |g PL | values (3 × 10 -3 ). However, the reason for this difference has not been discussed by the authors of this work, which remains to be an intriguing topic to be investigated in the future.

CPL-contrast imaging
CPL-contrast imaging is another appealing application for CPL-active metal complexes. [10] One prerequisite for CPLcontrast imaging is that the materials should possess emissions with large circular polarization. Thanks to the high |g lum | of Ln(III) complexes, several interesting proofof-concept studies have been demonstrated recently. For instance, based on the enantiomeric Eu(III) complexes (Δ/Λ)-44 with both intense emission (Φ = 47%) and high |g lum | (0.15 at 599 nm, 0.19 at 655 nm, and 0.32 at 708 nm), Pal and coworkers [113] demonstrated the pioneering exam-ple in 2016 ( Figure 9A). With the adaptation of an epifluorescence microscope for CPL analysis, time-resolved images and corresponding CPL spectra of (Λ/Δ)-44 showed a contrast ratio (CR Λ:Δ ) of 3.23:1 via the right CP light channel, meaning that the L-CPL features were 3.23 times brighter than the R-CPL features. On the other hand, a CR Λ:Δ ratio of 1:3.64 was detected through the left CP light channel. Very recently, Hasegawa and coworkers [114] fabricated a chiral transparent lumino-glass (+/-)-45 composed of the chiral [Eu(tfc) 3 ) complexes and the achiral glass promoter tris(2,6dimethoxyphenyl)phosphine oxide ligand ( Figure 9B). The high brightness (Φ = 13%) and CPL (|g lum | = 1.2 at 594 nm) of 45 favored its application as CPL-imaging security devices. Upon operation in a special CPL detection setup, in which biased L-CPL and R-CPL could selectively be obtained and imaged, a brightly visible encrypted sun or moon pattern was differentiated.

CPL probes and sensors
Polarized light is a subtle and delicate investigator of molecular condition. CPL spectrum contains rich information of excited states including the sign and intensity for each transition and fine spectral resolution of overlapping multiple emissions. It also provides information on the local chiral environment of molecular and aggregated materials. On account of these reasons, CPL spectrum has emerged as a promising analytical method to probe the substrate-analyte interactions, in particular those between Ln(III) complexes and bio-related molecules. [9,[115][116][117][118][119][120][121][122] Ln(III) complexes typically possess large g lum factors and dynamic coordination structures, making them advantageous as CPL probes. In this context, CPL could be induced either from achiral metal complexes after association with chiral analytes or from chiral metal complexes responsive to analytes. For instance, Parker and coworkers [117] prepared the racemic Eu(III) complex 46 with a dangling dipyridylamino unit, which showed different affinities and CPL response upon the successive interaction with Zn(II) ions and nucleotides ( Figure 10A). The adducts generated by complexation of 46 with Zn(II) displayed induced CPL with opposite chiral signs upon the addition of adenosine monophosphate (AMP) or adenosine diphosphate (ADP) versus adenosine triphosphate (ATP). This phenomenon was explained by for the preferential formation of the chiral (Λ)-[46⋅Zn⋅AMP], (Λ)-[46⋅Zn⋅ADP], and (Δ)-[46⋅Zn⋅ATP] aggregate, respectively. Another interesting example is the racemic Eu(III) complex 47, which showed oppisite CPL signs for upon aggregation with human or bovine serum protein α 1 -AGP ( Figure 10B). [118] In addition to achiral complexes, chiral Ln(III) complexes have been used as CPL probes. The Parker group have reported the use of chiral Eu(III) and Tb(III) complexes to sense HCO 3 and CO 3 2by CPL signal changes. [119][120][121] One interesting example is the chiral Eu(III) complex 48 with a pendant pyrazine derivative which could selectively and reversibly bind to the "drug site II" of serum albumin ( Figure 10C). [122] The (S,S,S)-Δ form of 48 was shown to form aggregates with human or bovine serum albumin (BSA) to induce the inversion of CPL direction. In contrast, no CPL inversion occurred with the (R,R,R)-Λ form of 48, making it a unique chiroptical probe of albumin binding.  [117] (B) CPL spectra of racemic 47 upon forming aggregates with bovine or human α 1 -AGP at pH of 9.3. Reproduced with permission: Copyright 2018, The Royal Society of Chemistry. [118] (C) CPL spectra of (S,S,S)-Δ-48 with gradual addition of bovine serum albumin (BSA). Reproduced with permission: Copyright 2008, The Royal Society of Chemistry [122] 6

MECHANISM OF CPL ENHANCEMENT BY AGGREGATION
As was surveyed above, considerable progress has been made to date to enhance the |g lum | and Φ of coordination materials by aggregation. Depending on the specific material or system, the underlying mechanism of the aggregationenhanced CPL could be very different. Generally speaking, the CPLs of aggregated materials could be either originating from the amplification of intrinsic chirality or structural chirality. [3,21,23,39] For instance, the CPLs of supramolecular helical assemblies and energy-transfer systems are typical examples caused by the amplification of intrinsic chirality by exciton coupling. The materials discussed in Sections 3-6 are mostly associated with this type of mechanism. On the other hand, the CPL amplification by the circularly selective reflection with N*-LC is of origin of structural chirality. [42,123] Chiral supramolecular helical assembly has been proved to be a promising approach to amplify the chirality and |g lum | of chiral complexes. [3] The helical assembly increases the size and long-range order of aggregated materials with respect to dispersed molecules, which are beneficial for the biased lightmatter interactions to induce CPL. In addition, the exciton and magnetic coupling between neighboring chromophores are important for CPL performance. The CPL amplification is in particular distinct for the helical assemblies obtained from the metal complexes with aggregation-induced or enhanced emissions. For instance, the helical assemblies of the chiral Pt(II) complexes (R/S)-3a and 3b showed AIE CPLs with |g lum | of up to ca. 0.01 compared to their CPL-silent nonhelical assemblies. [54,55] The chiral Pt(II) complexes (R/S)-4 are CPL-silent in solution, while corresponding helical assemblies showed highly amplified CPLs with |g lum | of over 0.02. [56] In cases when monocomponent assembly of chiral chromophores fails for the CPL amplification, binary assembly provides an alternative to obtain CPL-active aggregates. In this regard, numerous examples have been discussed in this review. For instance, the physical doping of the CPLsilent assemblies of the chiral di-Au(I) complexes (R/S)-15 with various achiral emitters afforded tunable CPLs. [75] By incorporating selected achiral emitters into the inner helical channels of the crystalline MOFs (D/L)-27, amplified and tunable CPLs were achieved. [89] By incorporating various achiral Ln complexes into the pores of the chiral Zn-MOF 28, CPL-active MOF films with |g lum | of around 10 -2 were obtained, thanks to the effective host-guest energy transfer. [90] Furthermore, by integrating UC process into energy-transfer systems, chiral coordination aggregates 29b demonstrated enhanced CPLs. [91] All of these processes are associated with the chirality transfer in excited states, possibly as a result of the coupling of μ and m among individual components. [39] However, more in-depth mechanism of chirality transfer remains to be investigated in the future. [3] The structural chirality of CPL-active materials generally refers to the situation in N*-LCs, in which chiraloptical phenomena occurs when the helical pitch of chiral supramolecular assemblies matches the length scales of emissions. [3] The CPLs originated from structural chirality are mainly caused by circularly selective reflection/transmission of light (Bragg reflection) in N*-LCs. The |g lum | values in such circumstances are relatively high and strongly affected by the thickness of the chiral LC medium. [3,23,123] An extremely high |g lum | value of 1.79 has been reported by Akagi and coworkers [42] by the selective transmission of CPL emitted from chiral disubstituted polyacetylenes across the thermotropic N*-LCs. However, this method has not yet been used for the CPL amplification of chiral metal complexes. The known CPLactive LC materials of metal complexes, for example, platinum complexes 8 and 9 (Figure 2), are mainly a result of the amplification of intrinsic chirality in the highly ordered LC phases. [68,69]

CONCLUSION AND PERSPECTIVE
The research on the development and applications of CPLactive molecular materials has been largely boosted in recent years. Metal-ligand coordination aggregates are characterized by high |g lum | and Φ which are important from the viewpoint of practical optoelectronic and biological applications. This review highlights the recent progresses in CPL-active coordination aggregates, including those assembled from discrete small-molecular metal complexes and coordination helicates, polymers, and frameworks. Square-planar Pt(II) and linear Au(I) complexes are the mostly employed molecular complexes to make CPL-active aggregates, benefiting from their excellent assembly capability and tunable polymorphic emissions. Relative to solids with disordered molecular arrangement, the formation of supramolecular helical assemblies/coassemblies and LC phases can amplify the g lum values of chiral emitters by orders of magnitude. CPL-active coordination helicates, polymers, and frameworks have received increasing attention due to their extended structural features and wide potential applications. Among them, Ln(III), Zn(II), Cd(II), and Al(III) metal ions are often used as the coordination nodes, taking into account of their tunable coordinationenhanced emissions. In particular, luminescent MOFs have recently emerged as promising materials for the exploration of CPL activity by host-guest chemistry. The fast development of these materials are believed to further boost the applications of CPL-active aggregates in CP-OLEDs, CPLcontrast imaging, CPL probes, etc. The research on CPL is believed to attract continuing and growing interest in the coming decade. The focus will be placed on the development of high-performence materials and their explorations in new applications. In terms of coordination aggregates, a number of CPL-active assemblies of Pt(II), Au(I), and Eu(III) complexes have been disclosed in recent years; however, the relationship between the assembly structure and CPL property and the underlying mechanism of chirality transfer and amplification remain to be explored. In order to further improve the CPL parameters, new assembly building blocks, for example, those made of Cu(I), Ag(I), Au(III), and Pd(II) complexes with excellent luminescent properties, [124][125][126] are in demand to conquer the trade-off between g lum and Φ. In addition, the assembly method or strategy will play a crucial role in determining the performance. CPL-active LCs and conjugated polymers are promising materials for practical optoelectronic applications in view of their high g lum and Φ. [18,23,24] Recent works on metal complex-involved LCs, polymers, and MOFs have clearly demonstrated their advantages in the amplification of g lum , which will further stimulate the design and synthesis of these aggregated materials. [71,81,79,102] In addition to the direct self-assembly into mesophase and coordination assembly, chiral doping in LCs or helical polymers is another effective method to explore efficient CPL materials, and more attention needs to be paid in this aspect.
One appealing application of CPL-active coordination materials is CP-OLEDs; however, it still remains a challenge to realize good device performance with high g EL factors. This needs to be tackled from both the material and device structure optimization. As more and more chiral crystalline MOFs with excellent and tunable CPLs have been prepared, their potential applications in chiral recognition and separation, asymmetric catalysis, and nonlinear optics are expected to be studied. In addition, nano-and microcrystals are another type of important functional materials in photonic and electronic fields. [127,128] The constructions of CPL-active nano-/microcrystals from chiral metal complexes will be attractive for applications such as circular polarization organic lightemitting transistors, CP light detectors, and so on. [129][130][131] We believe that coordination aggregates will be promising for the development of high-performance CPL-active materials and related applications.

A C K N O W L E D G M E N T S
The authors are grateful to the funding supports from the National Natural Science Foundation of China (grants 21925112, 22090021, 21872154, and 21601194) and Beijing Natural Science Foundation (grant 2191003).

C O N F L I C T O F I N T E R E S T
The authors declare that is no conflict of interest.