DUAN's Group

reference

Angew. Chem. Int. Ed, 2022, 61, e202115600

(2) Gels

Self-assemble refers to the spontaneous formation of ordered structures from building blocks with the help of non-covalent bonds, such as hydrogen bond, van der waals forces, halogen bond, etc. Supramolecular self-assembly has become a universal method to construct CPL materials. In this way, the complex hierarchical chiral organization processes and chiral structures can be built rapidly to get a higher degree of chirality with minimal synthetic effort, and through co-assembly process, achiral molecules can be involved in fabricating a chiroptical system. Through the chirality transfer process, the resulting assembly could easily induce or amplify the chirality of chromophores. Today, supramolecular gels are widely used as chiral host matrixes to endow both organic and inorganic emitters with CPL activity.

picture 4: Schematic diagram of fabricating CPL-active gels encapsulating AIE dyes.

In 2017, we have developed a general method for constructing CPL-active materials by encapsulating achiral aggregation-induced emission (AIE) dyes into the chiral space of supramolecular gel nanotubes. During the formation of gel, the chirality transfer from host chiral space to achiral AIE guest induced the chirality of the achiral components, the calculated value of g_lum of the CPL signal is about 1.3 × 10⁻³. Because of the AIE effect, the luminescence intensity increased simultaneously in the gel.

picture 5: Schematic diagram of CPL gels encapsulating QDs.

Inorganic quantum dots (QDs) could also exhibit CPL properties through this method. We coassembled core-shell-type QDs with organic lipid gelator N, N’-bis(octadecyl)-L(R)-glutamic diamide. Attribute to the electrostatic and hydrogen-bonding interactions between the capping reagent of the QDs and the amine groups of the chiral gelator, QDs arranged along the helical gel nanotube and were endowed with chirality. But long spacer length of capping reagents between QDs and gels will prevent chiral induction process. Besides, by tuning the type or the blending ratio of QDs, we fabricated QDs with full-color and white CPL materials. The g_lum we got here had the same order of magnitude: 10⁻³.

With this supramolecular gelation method, perovskite nanocrystals (NCs) could also be endowed with CPL ability. Perovskite NCs were synthesized by a polar-solvent-free approach. The resulting CsPbX₃ (X = Cl, Br, I, Cl/Br, and Br/I) perovskite could emit different colors by varying the halide composition. Considering that the surface of the as-synthesized NCs is capped with oleic acid and oleylamine, we selected amine-containing lipid (LGAm/DGAm) for replacing some of the surface ligands and facilitating the formation of chiral co-assemblies. Mirror-image CPL signals could be gotten in various co-assemblies from 410 to 600 nm, with the maximum g_lum value up to 7.3 × 10⁻³. However, in a mixture of perovskite NCs and lipids, there was no CPL signal, which indicated that helical cogels played an important role in the achievement of CPL.

picture 6: Schematic diagram of CPL gels with UCNPs.

Upconversion nanoparticles (UCNPs) were another prominent inorganic luminescent nanomaterials, which could also be endowed with CPL properties in LGAm/DGAm nanotube. In our current work, two kinds of UCNPs were successfully helically encapsulated into chiral nanotubes. Similarly, the assembly exhibited strong CPL signal in all emission peaks of two kinds of UCNPs (average g_lum was 5.48 × 10^-3), and kept CPL silence in a non-assembly system.

reference

Adv. Mater, 2017, 295, 1606503

Angew. Chem. Int. Ed, 2017. 56, 12174 –12178

Adv. Mater, 2018. 30, 1705011

ACS Nano, 2019. 13, 2804−2811

(3) Liquid crystals

Liquid crystals can be induced into chiral nematic liquid crystals (N*LCs ) and form a stop band by adding chiral molecular. Generally, there are two mechanisms of CPL in N*LCs. When the stop band is far away from the emission peak, the obtained CPL is transmitted along the helical axis of the N*LC getting the polarized direction same as the handedness of the N*LC. When the stop band is matching with the emission peak, the circularly polarized light with the same handedness will be reflected while the one with opposite handedness will be transmitted, resulting in relatively high g_lum but low emission intensity. For the CPL research field, N*LCs have gained more attention because they have unique optical properties of generating CPL with an incomparably high g_lum value, they are remarkably general for almost all emitters and they perform excellent stimulus responsiveness.

picture 7: Schematic diagram of the enlarged g_lum at the center of the band gap of the chiral liquid crystal..

In 2020, we added CsPbBr₃ perovskite nanocrystals and upconversion nanoparticles (UCNPs) into commercial N*LCs, and regulate the stop band by chiral molecular S811. In this system, the energy was transferred from UCNPs to perovskite nanocrystals. The emission peak of UCNPs was located at the center of the stop band while the emission peak of perovskite nanocrystals was located at the edge. In this way, band edge enhanced CPL was gotten with the g_lum arriving 1.1. Moreover, the CPL signal can be controlled by the electric field and mechanical force, realizing a voltage-force-regulated switch of UC-CPL.

picture 8: Schematic diagram of chiral chromophore induced chiral liquid crystal.

In 2019 a kind of chiral binaphthyl derivatives substituted at 3,3’ positions or 2,2’ positions of binaphthyl rings, abbreviated as R/S-1 and R/S-2, were used as both chromophores and chiral dopants for N*-LCs. Interestingly, the chiral binaphthyl derivatives with the same (R)-configuration could induce different chiroptical properties of the N*-LCs, which was assigned to the different dihedral angles of binaphthyl derivatives. N*-LCs induced by R-1 exhibited negative CPL signals, while N*LC induced by R-2 exhibited positive CPL signals, the g_lum were around 0.25 and 0.27, respectively.

reference

Chem. Commun, 2019, 55, 5914--5917

Adv. Mater, 2020, 32, 2000820

(4) Perovskite nanocrystals

Perovskite nanocrystals are attracting great interest due to their excellent photonic properties. One of the main advantages of these materials is the fact that the optical band gap can be easily tuned by exchanging individual components. Hot-injection method is a universal approach to synthesize perovskite, but high temperature will result in racemization of chiral amine molecules. Fortunately, though chiral-ligand-assistant method by ultrasound, we successfully endowed perovskite nanocrystals with CPL property.

picture 9: Schematic diagram of CPL in perovskite nanocrystals systems.

In 2022, we applied a polar-solvent-free approach to synthesize chiral perovskite nanocrystals, and we gained UCNPs@PKNC system by adding UCNPs in precursor solutions. In this system, chirality transferred from capping ligands to perovskite nanocrystals and UCNPs, and g_lum was further amplified to 4.5×10^-3 through the energy transfer from UCNPs to perovskite nanocrystals.

reference

Nano Research, 2022, 15, 1047–1053

(5) Metal-organic frameworks

Porous crystalline nanomaterials (PCNMs) include metal-organic frameworks (MOFs), metal-organic cages (MOCs), covalent organic frameworks (COFs), etc. PCNMs can be assembled with the corresponding organic and inorganic components, providing a powerful solution for precisely controlling the arrangement of building blocks. Moreover, PCNMs have applications as host matrices owing to their porosity, variable pore size, and ultrahigh surface area. Such features enable PCNMs to enhance certain properties in the solid-state. For instance, ordered structures with high crystallinity can suppress the aggregation-caused quenching effect of luminophores. By endowing PCNMs with chirality, exceptional properties could be obtained, so PCNMs are widely investigated for their chiroptical applications.

picture 10: Schematic diagram of CPL in MOF.

Compared with other porous materials, MOFs have special applications as host matrices due to their unique features, including high crystallinity, regular and tunable structures, ultrahigh porosities and specific surface areas, and tunable pores. In our work, we utilized a facile and economically feasible strategy to fabricate the chiral zeolitic imidazolate framework (ZIFs) by incorporating L-/D-histidine (His) into the ZIF-8 framework using a mixed-ligand coassembly pathway. Achiral dyes, quantum dots (QDs), and upconversion nanoparticles (UCNPs) were easily loaded into the chiral ZIFs during the synthetic process, and achieved solid-state CPL-active materials with high luminescence efficiency. The emitting colors can be flexibly tuned by changing the categories of dyes and QDs. In addition, white light CPL emitting MOF materials are obtained by introducing various dyes or QDs simultaneously. Meanwhile, UC-CPL with a high dissymmetric factor(1.2×10^-2) is achieved through loading UCNPs into the chiral MOF matrixes. It was found that chiral ZIFs in this work could endow chirality to the organic dyes which dispersed in cages by chiral confined space or environment. But QDs were too large to be encapsulated into these chiral cages, which were distributed into the framework, and surrounded by the chiral ZIFs showing anticlockwise polarization as a consequence.

reference

Research, 2020, 12, 6452123

Although there are many methods to design the CPL materials, the g_lum is also relatively small for the application. In our research, we developed numerous methods to further amplify the g_lum, giving new approaches for future construction of CPL materials.

(1) Energy transfer enhanced circular polarization

According to the earlier research g_lum is determined by both the electric dipole transition moment (μ^gn) and the magnetic dipole transition moment (m^gn). In the organic system, compared with μ^gn, m^gn is too small to account, so the g_lum is mainly determined by μ^gn.

The radiationless energy transfer process will obviously decrease μ^gn. Based on this concept, a new method for amplifying the g_lum is built.

picture 11: Schematic diagram of FRET enhanced CPL.

In 2017 we designed a self-assembly system based on the chiral donor and achiral acceptor. Because of the chiral confined space and environment, achiral acceptor could be induced with CPL property. Moreover, the designed gelator (L-1 or D-1 for short), which contains a donor chromophore, cyano-substituted stilbene could transfer its energy to acceptor: 9,10-bis(phenylethynyl)anthracene (BPEA). According to the measurement, attribute to the energy transfer process, g_lum was amplified from 1.2 × 10^-3 to 3.0 × 10^-3. This result proved that Förster resonance energy transfer (FRET) could significantly amplify the dissymmetric effect.

picture 12: Schematic diagram of TTA enhanced CPL.

Triplet−triplet annihilation-based photon upconversion (TTA-UC) was a kind of energy transfer process, which attracted abundant attention because of its high efficiency, noncoherent excitation source tunable excitation and emission wavelengths. In 2017, we used CPL active binaphthyldiamine moiety with two 4-(10-phenylanthracen-9-yl)phenyl groups as acceptors and achiral Pt(II) octaethylporphine as sensitizers. In this way, one order of magnitude amplification of the g_lum in TTA-UC CPL was obtained in comparison with the normal promoted CPL, which provided a new strategy to design CPL materials with higher dissymmetry factor.

picture 13: Schematic diagram of RET enhanced CPL.

In 2021, we loaded formylphenyl-substituted diarylethene derivatives (DAEC) and upconversion nanoparticles (UCNPs) into chiral MOFs to construct a dual upconversion (UC) and downshifting (DS) CPL switch. Upon visible light irradiation, closed-ring DAECc transformed into open-ring DAECo, with the fluorescence and CPL signals being silent. Reversible photoisomerization was driven by either UV or high-power NIR light. The corresponding signals of both DS-CPL and UC-CPL were obtained under UV and NIR light excitation, respectively. Through an energy transfer process, the g_lum value of UC-CPL (7.8 × 10⁻²) showed significant amplification in comparison with that of DS-CPL (1.7 × 10⁻²).

reference

Nat. Commun, 2017, 8, 15727

J. Am. Chem. Soc, 2017, 137, 9783−9786

Adv. Mater, 2021, 33, 2101797

(2) Charge-transfer enhanced circular polarization

As g_lum is directly determined by μ^gn and m^gn, charge-transfer (CT) complex with large m^gn and small μ^gn would produce a large g_lum in the circularly polarized emission. In our work in 2019, an interesting example of a CPL-active CT complex composed of a chiral emissive donor (R/S-Py) and achiral electron acceptors (TCNB) was given. The intermolecular π-π interaction between chiral molecules resulted in dimer formation. Then, the dimer and TCNB sandwiched one another with the CT interaction to form mixed-stack packing along the c axis, which leads to the long-range ordered fibers. More surprisingly, g_lum value of CT complexes was calculated to be 0.017, almost 200 times larger than that of the chiral donor.

reference

Angew. Chem. Int. Ed, 2019, 58, 7013 –7019

(3) Arene–perfluororene interactions enhanced circular polarization

In our current research, we found that Arene-perfluoroarene (AP) interaction, a kind of the electrostatic interaction between a non-fluorinated and a perfluorinated aromatic ring, could significantly improve the g_lum. In this study, chiral emissive polycyclic aromatic hydrocarbons (PAHs) worked as arene and achiral octafluoronaphthalene (OFN) as perfluoroarene. Chiral emissive PAHs showed a relatively small g_lum in the amorphous state, but g_lum increased from 8.0 × 10^-4 to 1.1× 10^-2 when it coassembled with OFN.

picture 15: Schematic diagram of arene-perfluororene interactions enhanced CPL.

reference

Angew. Chem. Int. Ed, 2021, 60, 4575-4580

(4) Plasmon enhanced circular polarization

In earlier research, surface plasmon resonance (SPR) had been demonstrated to enhance the CPL, however they all applied it in single organic molecule systems. Molecular self-assemblies, which had strong orbital angular momentum (OAM), possessed better CPL performance. Meanwhile, as an inherent property of electromagnetic fields, spin-orbit interaction (SOI) had been widely demonstrated in experiments and theories as an efficient physical process for enhancing OAM, through a plasmon involved process. So that we used chiral emitter R-OPAn as gelator, and achiral silver nanowires (AgNWs) to provide SPR. Fixing the concentration of R-OPAn-NPs at 5 × 10⁻⁴ M, the g_lum value was significantly amplified with the addition of AgNWs. The maximum value could reach 1.7×10^-3 (mAgNWs/mR-OPAn-NPs = 2.5), which was almost 5.0 times in magnitude compared with the g_lum value of R-OPAn-NPs. However, when the mixing mass ratio was over 2.5, the g_lum had a decrease because of the reabsorption. Moreover, the g_lum value of R-OPAn-NPs/AgNW composites significantly enhanced with the increase of magnetic field, which indicated that amplification could be attributed to the plasmon enhanced SOI.

picture 16: Schematic diagram of plasmon enhanced CPL.

reference

Nanoscale, 2020, 12, 19760-19767

(1) Information encryption

Circularly polarized light has special polarization properties, which provide us the idea of designing the information encryption device. We explored their application using CPL liquid crystals. According to the different CPL light intensity transmitted by different circular polarizers, two detection modes could be established to input different information, and different signals could be output through the photoelectric signal modulator. Based on this, we designed a simple encryption functional signal generator system. For right-handed liquid crystals, when the fast axis of the quarter-wave plate and the light transmission axis of the linear polarizer were at 45°, a high light intensity signal was input, and a sine wave signal was displayed after modulation. On the contrary, when the fast axis and the light transmission axis are at 135°, the output was weaker and the square wave signal was displayed to achieve the effect of data encryption. The left-handed liquid crystal showed the opposite law, and the achiral liquid crystal had no encryption effect.

reference

Mater. Adv, 2021, 2, 3851–3855

(2) Enantioselective photopolymerization

In 2020, we introduced TTA active pairs into a chiral nematic liquid crystal, resulting in a circularly polarized ultraviolet luminescence with a large g_lum value. With the help of high-energy UV light, the polymerization of diacetylene can be well initiated. Moreover, due to the higher g_lum after sequential amplification, the system can stably control chiral polymerization. Here we repeated this polymerization ten times. All of the resulting product showed the single chirality, which was the same as annihilator, indicating that CPL could stably regulate this reaction.

reference

Nat Commun, 2020, 11, 5659

(3) Logic circuit

Based on the fluorescence and CPL signals of chiral MOFs loaded diarylethene derivatives and upconversion nanoparticles in PMMA film in responsive to UV, visible, and NIR light, a chiral logic circuit with a 2D information output was designed. When irradiated with 365nm and 980nm light, these materials would exhibit smaller and larger g_lum, respectively. But if irradiated by 465nm light, DAEC would experience a close-ring process and expressed no luminescence. With three excitation lights as input signals, low-g_lum and high-g_lum circularly polarized light as output signals, a logic circuit with two-dimensional information output capability could be constructed as shown in the picture below.

reference

Adv. Mater, 2021, 33, 2101797

(4) Enantioselective recognition

In 2019 we used the ligand-exchange strategy to fabricate the first example of circularly polarized luminescent ZIFs. We chose 4-(10-phenylanthrancen-9-yl)phenyl and functional imidazole groups (R-/S-1) as chiral emitters. After blending with one kind of Zn-based ZIF (ZIF-8) nanoparticles, ligand-exchange happened and the g_lum was amplified from 7.0 × 10^-4 to 5.5 × 10^-3. When the ZIF-8 mixed with the a-hydroxycarboxylic acid (e.g. tartaric acid (TA)), fluorescence would be enhanced. More interestingly, the fluorescence of R-ZIF with L-TA showed a higher enhancement than D-TA, while the opposite trend in enantioselectivity was observed for the mixture of S-ZIF with TA. Two different diastereomeric complexes generated by the chiral sensors with the two enantiomers could be the reason for the different fluorescence enhancement. This phenomenon enabled enantioselective recognition.

reference

Angew. Chem. Int. Ed, 2019, 58, 4978–4982

(5)Long-persistent circularly polarized phosphorescence

Phosphorescence, transmitting from triplet to singlet, typically has longer lifetime than fluorescence. However, Long-persistent circularly polarized phosphorescence (LPCPP) is not that common to see. In this work, we designed and incubated a pair of chiral organic ionic crystals, terephtalic (R/S)-phenylethyamine(TPA-(R/S)-PEA), which generated LPCPP. This ionic crystals showed two phosphorescence peaks at 425nm and 500nm, which generated from two different triplet states and had lifetimes of 2.11ms and 862ms. The large g_lum was measured at 500nm ( 1.5×10^-2 and 2.0×10^-2 in TPA-(R)-PEA and TPA-(S)-PEA, respectively). The achiral TPA molecules possessed chiral structures with a mirror relationship in co-crystals with PEA. The induced chirality of TPA structures may be responsible for the generation of chiral phosphorescence signals. In conclusion, this work had demonstrated the first example of long-persistent circularly polarized phosphorescence in organic ionic crystals.

reference

Chem. Eur. J, 2018, 24, 17444-17448