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. 2020 Sep 19;23(10):101587. doi: 10.1016/j.isci.2020.101587

Restriction of Conformation Transformation in Excited State: An Aggregation-Induced Emission Building Block Based on Stable Exocyclic C=N Group

Wentao Yu 1,5, Han Zhang 2,5, Ping-An Yin 2,5, Fan Zhou 2, Zhiming Wang 2,6,∗, Wanqing Wu 1,∗∗, Qian Peng 3,∗∗∗, Huanfeng Jiang 1,∗∗∗∗, Ben Zhong Tang 2,4,∗∗∗∗∗
PMCID: PMC7566090  PMID: 33089098

Summary

The development of aggregation-induced emission (AIE) building block and deciphering its luminescence mechanism are of great significance. Here a feasible strategy for the construction of AIE unit based on E-Z isomerization (EZI) of exocyclic C=N double bond is proposed. Taking [1,2,4]thiadiazole[4,3-a]pyridine (TZP) derivative as an example, its aryl-substituted derivative (TZPP) shows obvious AIE character. The analysis of spectral data and theoretical calculations indicates that fast structural relaxation of TZPP in the emissive state plays a key role in a low fluorescence quantum yield in dilute solution, which should be caused by the small energy gap between locally excited (LE) state and twisted intramolecular charge transfer state. When in solid state, the bright emission with LE state characteristic reappears due to the large shift barrier of geometry transformation. As a potential building block for AIEgens with special heterocyclic structure, these findings would open up opportunities for developing various functional materials.

Subject Areas: Theoretical Methods in Chemical Physics, Molecular Electronic Structure, Chemical Synthesis

Graphical Abstract

graphic file with name fx1.jpg

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Highlights

  • •

    A new aggregation-induced emission building block

  • •

    A novel AIE mechanism with spectral measurements and theoretical calculations

  • •

    Available starting materials resulting in convenient synthesis and modification

  • •

    A stable exocyclic C=N double bond in heterocycles

Theoretical Methods in Chemical Physics; Molecular Electronic Structure; Chemical Synthesis

Introduction

Aggregation-induced emission (AIE) (Luo et al., 2001; Mei et al., 2015), a novel photo-physics concept, has led to great interest in the past two decades. Generally, the luminogens with AIE characteristic (AIEgens) are weak fluorescent or non-fluorescent in solution state, but emit intense fluorescence when they aggregate in solid state, which is different from the traditional organic emitters with excellent photoluminescence (PL) performance in dispersive state rather than in aggregates. Based on these remarkable advantages, AIEgens are widely applied to various fields depending on aggregate luminescence, such as organic light-emitting diodes (OLEDs) and organic nano-dots for bio-imaging (Wei et al., 2018a; Hu et al., 2018; Feng and Liu, 2018; Kwok et al., 2015; Zhang et al., 2019; Qian and Tang, 2017).

Recently, many great efforts have been made to explore deeply AIE working processes as well as their mechanisms, so that a large number of material systems have been enriched following these rules by chemists (Chen et al., 2019; Kokado et al., 2019; Gon et al., 2018; Sasaki et al., 2016; Tu et al., 2019; Zhou et al., 2019; Wang et al., 2018; He et al., 2019). Tetraphenylethylene (TPE), as a star building block in AIE family is studied and applied widely (Feng et al., 2018; Lu et al., 2019; Wang et al., 2019a). It is currently believed that the free rotation restriction of the double bond at the excited state is the key factor for AIE activity besides restricted rotation of the phenyl group (Scheme 1A) (Kokado et al., 2018; Cai et al., 2018). In solution state, the central olefinic double bond of TPE unit will be opened and two diphenylmethylene (DPM) units are generated in the excited state. The rotations, twisting motions, and inter-frictions between each other or solvent media of DPM moieties would dissipate the excited state energy, which would compete with the radiation transition process (Shustova et al., 2012; Sturala et al., 2017; Zhang et al., 2017). Thus, the E-Z isomerization (EZI) in the central double bond has a critical contribution in the emission quenching (Xie and Li, 2019; Xiong et al., 2018; Tseng et al., 2012; Wei et al., 2018b; Zhang et al., 2014). However, when the EZI process is restricted, the bright emission reappears in solid state. In fact, when a large steric group is introduced or the rigid cycle between two phenyl groups in DPM is formed, the conversion between E- and Z-isomers would be largely blocked, and the AIE phenomenon becomes less obvious. Employing systematic photophysical studies and simulation calculation, this limited photo-switches process is confirmed. According to this theory, there should be enhanced effect on EZI process if the peripheral substituted groups become small enough, and a better AIE-active building block would be possibly obtained.

Scheme 1.

Scheme 1

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The Development of AIE Based on Double Dound

(A) AIE mechanism of TPE based on C=C double bond.

(B) Two construction strategies (ESIPT/exocyclic) for AIE formation based on the C=N double bond.

As we known, the lone pair electrons (LPEs) are considered as the smallest substitution group, so that it is even possible to achieve “turn-off” luminescence output when the C=C double bond is replaced by the C=N double bond in solution. While in solid, this switch process as described above would be blocked, and the strong emission would reappear. Standing point of view, this kind of C=N double bond is indeed an ideal model to form the ideal AIEgens' building block. However, the C=N double bond in non-cyclic structure is a relatively unstable unit, leading to difficulty in expanding its functional applications due to the sensitivity to acid, base, and water. In addition, this EZI process between LPEs and another substituent attached to the N atom is sometimes difficult to be suppressed in aggregates, leading to an AIE effect not as good as expected (Belowicha and Stoddart, 2012). A general strategy for inhibiting these solid motions is to insert an ortho-hydroxyl group at the adjacent position of the C=N group, and the generated intramolecular hydrogen bonding via the formation of five- or six-membered heterocyclic intermediates would stabilize the C=N group to some degree. So, a series of AIEgens based on excited state intramolecular proton transfer (ESIPT) process (Tong et al., 2017; Zhang et al., 2015; Suzuki et al., 2014; Tseng et al., 2016; Wang et al., 2019b; Berenbeim et al., 2019) have been developed and applied in organic materials and bioactive compounds, such as carrier transport materials in OLEDs and biological active materials (Scheme 1B) (Sedgwick et al., 2018; Zhou et al., 2018; Padalkar et al., 2016).

In this contribution, a feasible strategy for constructing a stable AIEgens' building block is attempted based on [1,2,4]thiadiazole[4,3-a]pyridine (TZP) unit, which is developed in our group as a new-born heterocycle unit with special exocyclic C=N double bond (Yu et al., 2018b; Tang et al., 2018). On the one hand, in its main molecular conjugated π-system, 11 π-electrons are shared by 9 atoms to form π911 electron-rich structure, which would increase the electron cloud density of the carbon atom in the C=N group and thus reduce its chance of being attacked by nucleophiles, and the stability of C=N group would be improved. On the other hand, the repulsive force on the hydrogen-hydrogen atom between the TZP ring and the substituent on C=N moieties could provide certain locking capabilities, and result in the Z-conformation at ground state. In addition, owing to the intermolecular interactions, the EZI process of C=N moiety may be further restricted in aggregates, which is conductive to the AIE behavior. So, this re-modification for TZP is meaningful to obtain AIE building block. After one-pot protocol, (Z)-N-phenyl-3H-[1,2,4]thiadiazolo[4,3-a]pyridine-3-imine (TZPP) and (Z)-N-ethyl-3H-[1,2,4]thiadiazolo[4,3-a]pyridin-3-imine (TZPE) are constructed efficiently to discuss the aryl or alkyl group substitution effect on their respective luminescence behavior. TZPP shows obvious AIE character, whereas TZPE gives bright emission in both solution and solid. Based on the spectral measurement and theoretical analysis, it is found that the stereochemistry of exocyclic C=N double bond in the first excited state (S1) leading to the transformation between local excited (LE) state and twisted intramolecular charge transfer (TICT) state is the crucial factor for AIE formation, which could be vividly described as restriction of conformation transformation in excited state (RCTES). Enjoying this process, a series of TZPP-based derivates are attempted, and all of them still maintain the AIE characteristics, demonstrating that TZPP based on E/Z transformation of the C=N bond is indeed a novel building block for AIEgens construction.

Results

The TZPP and TZPE are synthesized under mild conditions from commercial 2-amiopyridines (1) and the corresponding isothiocyanates (2) in 83% and 86% yields, which are not sensitive to air and water (Scheme 2A and Figures S1–S5). The two compounds are characterized using 1H NMR, 13C NMR, and mass spectra. Satisfactory data corresponding to their structures are obtained. The crystal structures from X-ray crystallography analysis are displayed in Scheme 2B-2E and Figure S6. The angles (ϕ) of the C(6)–N(3)–C(7) in TZPP and TZPE are 121.5° and 114.7°, respectively, indicating their preferential Z-conformation. The molecular packing of TZPP shows an antiparallel arrangement, in which TZP and phenyl units form a head-to-tail stacking. In addition, apart from H-H repulsive force, multiple intermolecular interactions, including the weak π-π stacking interactions among TZP units, C–H···N interactions, N···N interactions, and C–H···π interactions appearing in the four adjacent benzene rings in the crystal, also play an important role in locking the rotation of benzene ring and further retaining the absolute Z-configuration (Viglianti et al., 2017; Mei et al., 2014; Xu et al., 2015). By contrast, the TZP units in TZPE show a face-to-face stacking with a distance of 3.403 Å, and the distances of intermolecular interactions, including C–H···N, S···S, and C–H···π are 2.631 Å, 3.876 Å, and 3.180 Å/2.896 Å, respectively. The different packing mode and intermolecular interactions in TZPP and TZPE could bring different photophysical behavior in aggregates.

Scheme 2.

Scheme 2

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Synthesis and Characteristic of TZPP and TZPE

(A) Synthetic routes of TZPP and TZPE.

(B and C) Crystal structures of TZPP and TZPE with the thermal ellipsoids set at 50% probability, respectively.

(D and E) Packing pattern of TZPP in crystals.

As shown in Figures 1A and 1D, the UV absorption and PL spectra of TZPP and TZPE are investigated in dilute THF solution and solid states. They exhibit similar absorption bands around at 290 and 380 nm. Compared with TZPE, the former absorption peak of TZPP has greater absorption intensity due to the conjugation expanding effect from phenyl substituents, implying that these absorption bands might be originated from the π-π∗ transition of TZP. Interestingly, the PL emission of TZPP in THF could be detected until its concentration was up to 10−3 M, locating at around 487 nm with quite weak intensity, and its fluorescence quantum yield (ΦF) was only 0.6%. In contrast, TZPE gives bright sky-blue emission at 490 nm with ΦF of 10.0%. When they are fabricated into neat films, the emission intensity of TZPP is enhanced obviously with peaking at around 486 nm, and its ΦF reaches 15.4% (25.7 times increment), which is indicative of its AIE nature. The ΦF of TZPE increases to 16.8%, presenting aggregation-enhanced emission behavior to some extent (Reddy et al., 2019; Zhang et al., 2018). TZPP and TZPE could also luminesce strongly in crystal states, showing PL peaks at 519 and 508 nm with enhanced ΦF of 16.2% and 17.9%, respectively. Their constants of the radiative rate (kr) and non-radiative rate (knr) in different states are estimated according to the kinetic parameters from lifetime measurement (Table 1 and Figure S7). It can be seen that the knr of both the compounds show decrement from solutions to solid states, corresponding to the process of restriction of intermolecular motions (RIMs) (Feng et al., 2018; Hong et al., 2009), which is conductive to the higher ΦF in the solid state. However, different from the fast and steady kr of TZPE in various states, the kr of TZPP is quite slow in solution and exhibits a dramatic increase from solution to solid state, which probably gives rise to the “turn-on” luminescence. Therefore, besides the RIM mechanism, we supposed that a new mechanism could exist and play a significant role in the AIE behavior of TZPP.

Figure 1.

Figure 1

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Optical Characteristics of TZPP and TZPE

(A and D) Absorption spectra of TZPP/TZPE in 10−5 M THF solution; photoluminescence (PL) spectra of TZPP in 10−3 M THF solution and solid state/TZPE in 10−5 M THF solution and solid state.

(B and E) TZPP and TZPE in THF-water mixtures with different water fractions (fw).

(C and F) Plots of I/Io versus the fw; Io is the PL intensity in pure THF. Concentration = 10−3 M. Inset: Photographs of the corresponding luminogens in THF-water mixtures (fw = 0–95%), taken under the illumination of a UV lamp (365 nm).

Table 1.

Optical Properties of Luminogens Based on TZPP and TZPE

Sample λabsa (nm) λem (nm) [ΦFd (%)]
 aAIEe τf (ns) [kr (ns−1), knr (ns−1)]
THF Filmc Crystal THF Film Crystal
TZPP 389 487b (0.6) 486 (15.4) 519 (16.2) 25.6 2.82 (0.002, 0.352) 4.83 (0.032, 0.175) 6.14 (0.026, 0.136)
TZPE 382 490 (10.0) 493 (16.8) 508 (17.9) 1.68 5.32 (0.019, 0.169) 7.50 (0.022, 0.111) 8.17 (0.022, 0.100)
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kr = radiative decay rate (kr = ΦF/τ); knr= nonradiative decay rate [knr = (1-ΦF)/τ]

a

In THF solution (10-5 M).

b

In THF solution (10-3 M).

c

Drop-casted film on quartz plate.

d

Fluorescence quantum yield, determined by a calibrated integrating sphere.

e

Values of AIE effect, calculated by ФF (film)/ ФF (soln).

f

Fluorescence lifetime, measured at room temperature in air.

To further validate their different AIE activities, the emission behaviors of the two compounds in the THF/water mixture are investigated (Figures 1B, 1C, 1E and 1F). The PL intensity of TZPP remains low when the water fraction (fw, vol %) is less than 60%. By adding a large amount of water (fw ≥ 60%) to the THF solution, the nanoaggregates of TZPP are readily formed, and the PL intensity increased accordingly. Finally, stronger PL intensity is observed with higher water fraction used (fw = 95%) (Figure S8). For TZPE, the aggregation process shows no significant effect on the intensity of the PL peaks. The slight PL drop as the fw increased may be ascribed to the aggregate size effect (Yu et al., 2018a; Jiang et al., 2017).

To gain a deeper insight into the photophysical behavior differences between TZPP and TZPE, theoretical investigation on these two compounds is performed in both solution and solid state by using density functional theory (DFT) and time-dependent DFT. The solvation effect and aggregation effect are considered by using the polarizable model and ONIOM method with QM and MM layers in Gaussian 16 program, respectively. The universal force field (UFF) is used with the restrained electrostatic potential partial charges for the MM treatment. The computation models (Figure S9) are built by digging a cluster from the X-ray crystal structures.

To shed light on the stereochemistry of C=N double bond and its function during the photophysical process, the optimized Z- and E-geometries of TZPP are investigated at the same time. The results of geometry optimization with frontier molecular orbitals and energy gaps performed for S0 and S1 in solution state are summarized in Figure 2 and Table S1. At S0, the geometry of isomers changes mainly around the C=N double bond as the C6–N3 bond length decreases slightly with central dihedral angle S1–C6–N3–C7 (θ) twisted from 3.6° (S0-1) to 168.8° (S0-2). Meanwhile, the distribution characters of electronic orbitals (HOMO→ LUMO) are almost the same, which are consistent with the calculated absorption energy results of isomers, but the data differ widely at S1. These two optimized geometries at S1 can be easily distinguished as LE state and TICT state according to their specific electronic distribution (Naito et al., 2017; Li et al., 2018). The former one (S1-1) is mainly dominated by electronic local excitation on the whole molecular backbone, whereas the latter one (S1-2) is totally a charge transfer from the TZP unit to phenyl part. The optimization geometries of these two minimums are further compared at S1, which showed that the C6–N3 bond length changed from 1.271 Ǻ at S0 to 1.306 Ǻ at LE state/1.381 Ǻ at TICT state, together with the central dihedral θ twisted from 3.6° to 31.1°/112.1°, respectively. All these differences in geometries finally result in the practically different distribution character of the electrons (main bond length and angles are presented in Table S2). The LE state with a visible emission at 450 nm possesses a much larger oscillator strength (0.2038) than that of the TICT state (0.0011), which agrees well with the extremely weak experimental emission at 487 nm but fails to explain the low quantum yield in solution.

Figure 2.

Figure 2

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Optimized Geometries of TZPP at S0 and S1, Frontier Molecular Orbitals of TZPP at S0 and S1, and PESs along the Central Dihedral Angle S1-C6-N3-C7 (θ) of TZPP by Scanning Relaxed Geometries in THF

To solve the mystery of the almost no fluorescent behavior of TZPP in solution, the potential energy surfaces (PESs), as indicated by the optimized geometries of TZPP at S1 state, were obtained along the central dihedral angle θ via scanning relaxed geometries at S0 and S1 states, respectively. As suggested in Figure 2, “double wells” are exhibited in both PESs at S0 and S1 states of TZPP in solution. For S0 they correspond to the typical Z- and E-geometries, and the Z one is more stable, in good agreement with the single-crystal X-ray structure. Different from the large energy barrier (0.77 eV) for isomerization at S0 state, the two local minima of S1 have a very low barrier of 0.05 eV, which means they can undergo fast conformation conversion before deactivation process back to the ground state. Meanwhile, TICT state has an invisible emission at 869 nm (Figure S10), suggesting that it is a dark state inclined back to the ground state through a faster nonradiative transition process. It is confirmed that the weak emission in solution comes from the trace LE state (Figure S11), but most of the excited state energy “dropped” to the dark state (TICT). A conclusion could be drawn that the low energy gap between LE and TICT state in TZPP makes it easy for undergoing fast structural relaxation at S1 and finally results in a low fluorescence quantum yield in solution.

According to our calculation results in solid state of TZPP, it is impossible to proceed with the geometry transformation between LE and TICT at S1 state as it only has an LE emission at 417 nm due to the large shift barrier (Table S3, Figure S12), which further reinforces the correctness of our hypothesis that the C=N double bond structure is essential for the AIE phenomenon of TZPP. All calculations performed in solid state do not include the intermolecular vibrionic coupling, which would result in hypochromic shift of emission.

However, this advantage of structure becomes functionless when combined with ethyl group because no similar electronic distribution trend of TZPE could be found at the S1 state with the central dihedral angle θ gradually twisted in solution, and the distribution of electrons almost has no change at the same time, indicating that the de-excitation process of S1 always concentrates on the TZP moiety (Figure S13). Not surprisingly, the calculated results at S1 in solid state of TZPE show the same LE characteristics with the same emission at 422 nm (Table S4), which agrees well with the experimental fluorescence data. Comparing the geometries and transition orbitals of TZPP and TZPE, we find that the electronic effects of the substituent on C=N double bond could make big differences in the electronic distribution for photophysical behavior. It should be the stereochemistry of the C=N double bond and the electronic properties of the substituents attached to the N atom that co-determine the nature of the AIE phenomenon of TZPP, suppressing the conformation conversion between LE and TICT states from solution to solid state.

To explore the universality of this new AIEgen (TZPP), we try to decorate it at the C-3/C-4 position of the pyridine ring or C-10 position of the phenyl ring with commonly used electron-donating groups (triphenylamine and carbazole). The halo-substituted products can readily undergo Suzuki-Miyaura coupling reaction to afford the corresponding arylated products (Figure 3 and S14–S15). After a simple separation operation using silica gel column chromatography, the purified products are obtained in good yields. The structures of Cz-3-TZPP, TPA-4-TZPP, and TPA-10-TZPP are also clarified by single-crystal X-ray analysis. Excitingly, all TZPP-based derivatives have the stable Z-configuration in solid state. Then, a series of photophysical properties are also studied (Table 2, Figure 4, and S16–18). The TZPP-based derivatives show semblable AIE properties, in which weak fluorescence intensity (ΦF < 1%) is detected in THF solutions, whereas intensive emission is observed by forming aggregates. From solutions to neat films, all derivatives show an order of magnitude increase in the constants of kr, corresponding to the process of RCTES, which gives rise to the “turn-on” luminescence. In addition, these derivates are not sensitive to external stimuli, such as temperature, organic solvent vapors, and mechanical force (Figure S19). It should be noted that the enlarged conjugate system allows the molecule to emit more efficiently (ΦF = 28.1%) in modifying the pyridine ring at the C-3 position, which provides a new guiding direction for further molecular modification. Meanwhile, the packing mode of the substitutes in TZPP-based derivatives could also have an impact on the variation of knr, leading to their different ΦF. These PL spectra accord with the theoretical calculation that they exhibit LE and TICT transformation (Figure S20 and Table S5). Thus, the TZPP compound should not only be an AIEgen but also be a typical AIE building block.

Figure 3.

Figure 3

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Structures of TZPP-Based Derivates

Table 2.

Optical Properties of Luminogens Based on TZPP-Based Derivates

Sample λabsa (nm) λem (nm)
 ΦFd (%)
 aAIEe
 τf (ns) [kr (ns−1), knr (ns−1)]
THFb Filmc THFb Filmc THF Film
TPA-3-TZPP 375 517 532 0.7 10.0 14.3 1.08 (0.006, 0.919) 3.84 (0.026. 0.234)
Cz-3-TZPP 340 524 539 0.6 28.1 46.8 2.95 (0.002, 0.337) 4.74 (0.050, 0.152)
TPA-4-TZPP 342 504 517 0.5 3.4 6.8 4.07 (0.001, 0.244) 1.40 (0.024, 0.690)
Cz-4-TZPP 327 497 514 0.8 4.4 5.5 3.64 (0.002, 0.272) 1.69 (0.026, 0.565)
TPA-10-TZPP 398 509 521 0.2 2.4 12 2.49 (0.001, 0.400) 0.52 (0.046, 1.877)
Cz-10-TZPP 391 497 513 0.2 2.6 13 0.57 (0.004, 1.751) 0.54 (0.048, 1.804)
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kr = radiative decay rate (kr = ΦF/τ); knr= nonradiative decay rate [knr = (1 - ΦF)/τ]

a

In THF solution (10-5 M).

b

In THF solution (10-3 M).

c

Drop-casted film on quartz plate.

d

Fluorescence quantum yield, determined by a calibrated integrating sphere.

e

Values of AIE effect, calculated by ФF (film)/ ФF (soln).

f

Fluorescence lifetime, measured at room temperature in air.

Figure 4.

Figure 4

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Optical Characteristics of TZPP-Based Derivates

(A) PL spectra of Cz-3-TZPP in THF/water mixtures with different water fractions (fw).

(B) The plots of emission maximum and the relative emission intensity (I/I0) versus the composition of the aqueous mixture of TZPP-based derivates; concentration = 10−5 M. I0 = PL intensity in pure THF.

(C) Photographs of the corresponding luminogens (Cz-3-TZPP) in THF-water mixtures (fw = 0–95%), taken under the illumination of a UV lamp (365 nm).

Discussion

In summary, a novel design of AIE building block based on EZI has been developed. Through stable exocyclic C=N double bond, the pristine TZPP is reported as an AIE core, which could be synthesized under mild conditions with commercial 2-amiopyridine and isothiocyanate. Theoretical calculations show that the crucial factor of AIE behaviors in TZPP is the stereochemistry change of intramolecular C=N double bond and the corresponding transformation between LE and TICT states from solution to solid state, which is consistent with a series of spectral experiments. In addition, these TZPP-based derivatives exhibit similar AIE behaviors, which affords a clear and specific strategy for constructing and decorating AIEgens in further work. It is believed that the compound TZPP and TZPP-based derivatives will not only be explored as the virgin territory of AIEgens but also be beneficial to provide design guidelines for the development of new AIE systems and practical applications.

Limitations of the Study

In this work, the fluorescence quantum yields of AIEgens based on TZPP core are relatively low in solid state. Subsequent work should be carried out on improving the fluorescence efficiency.

Resource Availability

Lead Contact

Further information and requests for resources should be directed to the Lead Contact, Zhiming Wang (wangzhiming@scut.edu.cn).

Materials Availability

This study did not generate new unique reagents.

Data and Code Availability

The crystallography data have been deposited at the Cambridge Crystallographic Data Center (CCDC) under accession numbers CCDC: 1921846 (TZPP), CCDC: 1588922 (TZPE), CCDC: 1921837 (TPA-4-TZPP), and CCDC: 1921840 (C2-3-TZPP) and can be obtained free of charge from www.ccdc.cam.ac.uk/getstructures.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.

Acknowledgments

This work is financially supported by the National Key Research and Development Program of China (2016YFA0602900), National Natural Science Foundation of China (21472051, 21788102, 51673118, 21975077), the Innovation and Technology Commission of Hong Kong (ITC-CNERC14SC01), Science & Technology Program of Guangzhou (201804020027, 201804010218), and the Fundamental Research Funds for the Central Universities (2019ZD04).

Author Contributions

W.Y. discovered the reaction, synthesized the reagents and substrates, and performed the experimental studies; H.Z. assisted the experimental studies, analyzed the data, and contributed to the redaction of the manuscript; P.-A.Y. performed the theoretical calculation and contributed to the writing of the theoretical calculation; F.Z. assisted the experimental studies; Z.W. devised and supervised the experiments and contributed to the redaction of the manuscript; Q.P. gave the guidance of theoretical calculation; W.W. gave the guidance of synthesis and contributed to the redaction of the manuscript; H.J. and B.Z.T managed the overall project, designed the reactions, planned the experiments, and participated in the redaction of the manuscript.

Declaration of Interests

The authors declare no competing interests.

Published: October 23, 2020

Footnotes

Supplemental Information can be found online at https://doi.org/10.1016/j.isci.2020.101587.

Contributor Information

Zhiming Wang, Email: wangzhiming@scut.edu.cn.

Wanqing Wu, Email: cewuwq@scut.edu.cn.

Qian Peng, Email: qpeng@iccas.ac.cn.

Huanfeng Jiang, Email: jianghf@scut.edu.cn.

Ben Zhong Tang, Email: tangbenz@ust.hk.

Supplemental Information

Document S1. Transparent Methods, Figures S1–S37 and Tables S1–S5
mmc1.pdf (2.5MB, pdf)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Document S1. Transparent Methods, Figures S1–S37 and Tables S1–S5
mmc1.pdf (2.5MB, pdf)

Data Availability Statement

The crystallography data have been deposited at the Cambridge Crystallographic Data Center (CCDC) under accession numbers CCDC: 1921846 (TZPP), CCDC: 1588922 (TZPE), CCDC: 1921837 (TPA-4-TZPP), and CCDC: 1921840 (C2-3-TZPP) and can be obtained free of charge from www.ccdc.cam.ac.uk/getstructures.

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