Diagnostic Absolute Configuration Determination of

Article pubs.acs.org/JPCC

Diagnostic Absolute Configuration Determination of Tetraphenylethene Core-Based Chiral Aggregation-Induced Emission Compounds: Particular Fingerprint Bands in Comprehensive Chiroptical Spectroscopy Dan Li,† Rongrong Hu,*,‡ Dong Guo,† Qiguang Zang,‡ Jianhui Li,† Yuekui Wang,§ Yan-Song Zheng,⊥ Ben Zhong Tang,*,‡,∥ and Hui Zhang*,† †

Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China ‡ State Key Laboratory of Luminescent Materials and Device, South China University of Technology, Guangzhou 510640, People’s Republic of China § Key Laboratory of Chemical Biology and Molecular Engineering of the Education Ministry, Institute of Molecular Science, Shanxi University, Taiyuan, Shanxi 030006, People’s Republic of China ⊥ Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China ∥ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China S Supporting Information *

ABSTRACT: Tetraphenylethylene (TPE) and its derivatives are the typical aggregation-induced emission (AIE) compounds with helical chirality, and they have enormous applications potential in chiral sensor, asymmetric catalyst, optoelectronic materials, and so on. The aggregation-induced helical chirality of TPE and its derivatives can only be observed in condensed phase such as crystalline or film state. The accurate determination of absolute configuration (AC) of TPE core compounds is complicated and difficult, especially when there is no heavy atoms exist in their structures. Herein, we demonstrate a powerful and convenient method to quickly determine the helical chirality of these stereochemically labile TPE core compounds through the combination of comprehensive solid-state chiroptical spectroscopy and theoretical calculation. Our study reveals that from lower to higher energy of the fingerprint region, the first electronic circular dichroism (ECD) band in the 300−450 nm or the first most intensive vibrational circular dichroism (VCD) peak in the 875−730 cm−1 can be used as the diagnostic bands for the AC determination of the TPE core with the fixed chirality. If these diagnostic bands cannot be observed in ECD or VCD spectra, the helical chirality of the atropisomers of TPE derivatives is not fixed. The explicit AC determination of TPE core contributes to the study of the helical chirality of supramolecular TPE core derivatives and allows for the further fabrication of novel chiral AIE functional materials.

1. INTRODUCTION π-Conjugated luminescent compounds with chirality are a series of promising candidates for applications in advanced chiral luminescent devices due to their tailored synthetic processability, low cost, and high feasibility. However, the notorious aggregation-caused quenching (ACQ) effect of many π-conjugated luminophors has greatly hampered the applications of the self-assembled luminescent materials.1 In 2001, we have demonstrated that a series of propeller-shaped molecules such as hexaphenylsilole is virtually nonluminescent when the molecules are dissolved in good solvents, but becomes highly emissive when it is fabricated into solid thin films or aggregated in poor solvents.2 This unusual photophysical phenomenon is coined as aggregation-induced emission (AIE). Since then, a © 2017 American Chemical Society

great number of organic luminophors were reported to possess AIE characteristics and were demonstrated to play valuable roles in various fields without extra effort to prevent ACQ or other consequent side effects.3−5 Tetraphenylethene (TPE), the most typical AIE molecule, is a versatile building block in the fabrication of numerous functional materials, owing to its high fluorescence quantum yield in the solid state, stable chemical structure, facile synthesis, and modification. 6 Furthermore, chiral crystals of TPE can be used as the chiral Received: July 7, 2017 Revised: August 22, 2017 Published: August 28, 2017 20947

DOI: 10.1021/acs.jpcc.7b06679 J. Phys. Chem. C 2017, 121, 20947−20954

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The Journal of Physical Chemistry C

tetrakis(4-ethynylphenyl)ethylene (TETPE) were selected for demonstration (Chart 1) since many AIE compounds were

initiators in the enantioselective asymmetric autocatalysis or synthesis.7 Plenty of novel luminescent micro/nanoarchitectures have been reported through the elaborative design of the peripheries based on a TPE scaffold, contributing to various efficient solidstate emitters with typical AIE effect.5,8−12 Owing to the remarkable spectral stability and enhanced luminescence efficiency in the aggregated states, TPE luminophore can be employed as powerful materials for the construction of efficient device with both electronic circular dichroism (ECD)/vibrational circular dichroism (VCD) and circularly polarized luminescence (CPL) properties.13−17 In fact, the aggregationinduced chirality effect of TPE core derivatives has been proved by the fact that TPE derivatives with chiral pendants are well assembled into left- or right-handed supra-helical ropes, helical nanoribbons, or organic polymers with large dissymmetry factors in the condensed phase.8,13,18,19 Therefore, it is crucial to determine the absolute configuration (AC) of the TPE core precisely in order to control the chirality and polarized fluorescence during the self-assembling process. However, to the best of our knowledge, there is no convenient and efficient strategy universally applies to the AC assignment up to now. The common method to determine the AC is single-crystal X-ray diffraction (XRD), which requires perfect crystal of sufficient size and quality for the analysis, while this is difficult to realize for some chiral compounds of interest, especially for the polymer systems. Furthermore, a heavy atom dopant (made up of atoms heavier than oxygen) is necessary in most cases to obtain the stereochemical information, otherwise the costly synchrotron radiation single crystal XRD characterization is needs to achieve accurate measurement.9 Hence, the AC determination of TPE core derivatives with only pure hydrocarbons is challenging. Comparing to the above-mentioned time-consuming and pricy methods, comprehensive circular dichroism spectroscopy combined with the theoretical simulation is a promising and convenient tool for AC determination in terms of its simple procedure, high-efficiency, and spectral stability.20−22 ECD represents a versatile and efficient tool to analyze chiral sample with suitable chromophores and has been considered as a common tool “to look at the stereochemistry of the molecule through the eyes of the chromophore”.23,24 VCD, on the other hand, as the extension of ECD into the infrared region, is characterized by differential absorption of left- and right-handed polarized IR light.25 Including many well-resolved IR bands, VCD spectroscopy has become an increasingly important method for AC assignment.26−28 In contrast to XRD technique, VCD and ECD do not require the presence of heavy atoms in the molecule and they avoid complicated sample pretreatment prior to analysis. Most of the current ECD and VCD spectra are recorded in solution, however, the solution measurement does not apply to TPE derivatives since the helical chirality is not locked up because of the quick and reversible configuration change.29 The P- or M-helical chirality of the TPE-containing atropisomer is originated from the mirror symmetry breaking (MSB) and the corresponding aggregation-induced CD signal can be characterized by solid-state chiroptical techniques.30−32 The solidstate CD spectroscopic analysis with KBr pellets is hence a promising tool to distinguish the P- or M-helicity of TPE core derivatives.33 In this study, three pure hydrocarbon AIE compounds TPE, 1,2-bis(4-ethynylphenyl)-1,2-diphenylethene (BETPE), and

Chart 1. Molecular Structures of TPE, BETPE, and TETPE

designed based on the TPE core and their spectra are clear without any chiral moieties’ interference. Herein, for the first time, we propose a chiroptical spectroscopy strategy to determine the AC of AIE compounds based on diagnostic ECD and VCD bands of TPE core.20−22,34 The method we provided is a sensitive and valuable tool for AC determination of these TPE core derivatives, which has been simultaneously validated by theoretical simulations. Owing to its simplicity, high-efficiency, and spectral stability, our method is expected to be widely applied in the AC determination of chiral AIE-based functional materials in the future.

2. EXPERIMENTAL SECTION 2.1. Solid-State UV−Vis and ECD Measurement. The solid state transmission electronic circular dichroism (ECD) and UV−vis spectra were performed on a JASCO J-810 spectrometer equipped with a pellet ECD attachment at room temperature in the range of 450−200 nm for BETPE and TETPE while 400−200 nm for TPE. All of the spectra were obtained in the 100 nm/min scanning speed, a bandwidth of 4 nm, a step size of 0.1 nm, and a response time of 1 s. Solid-state samples were mixed with dried KBr, adequately grounded in a Specac Mill and then pressed at 15 ton under vacuum for 3 min to make an almost transparent, spotless disk of 13 mm in diameter and 50 mg in weight. In order to make the sample evenly distributed in the KBr substance, stepwise dilutions were applied in the preparation of the solid-state pellet.29,35 Linear dichroism (LD), linear birefringence (LB), absorption flattening (AF), and scattering effect may impact the solid-state ECD spectra.24,36−38 To reduce these effects, the pellets were placed closely to the photomultiplier tube sensitive surface so as to diminish the scattering effect. Furthermore, rotation of the pellets around the incident-light axis (z axis) and flip (180° rotation) around the vertical y axis was necessary to ignore the LD and LB during the spectra measurements. Several pellets of the sample were prepared and measured for many times to guarantee identical spectra. Up to now solid-state ECD spectroscopy has been measured on large single crystals made up of oriented molecules, microcrystallines in a nujol mull or in a KBr matrix consisting of randomly oriented molecules.34 The diffuse reflectance (DR)-ECD demands a relatively large and perfectly flat plane of the crystal cluster, which immensely limits its wide applications.9 The nujol mull method, although informative, suffers from dispersion effect and interaction influences between the sample and nujol. 2.2. Solid-State IR and VCD Measurement. IR and VCD spectra were performed on a BioTools ChiralIR-2X dual PEM VCD spectrometer. Liquid nitrogen is used to cool the detector set as 4 cm−1 resolution. The ZnSe photoelastic modulator (PEM) was set to 1400 cm−1 in the fingerprint region. The 20948


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Figure 1. (a) Helical chirality of TPE core can be locked in the solid state to generate M- or P- helicity. (b) Conformations of TPE, BETPE, and TETPE optimized by DFT calculation.

Figure 2. Comparison of the experimental ECD and UV−vis spectra of (a) TPE, (b) BETPE, and (c) TETPE in KBr pellets with the corresponding simulated ECD and UV−vis spectra calculated at the B3LYP/6-311++G (2d, p) level of theory. A total of 100 excited states were included in the calculation.

solid-state samples were prepared as a pellet in the same way as ECD measurements mentioned. The pellets were mounted on an individual rotatable holder connected with Synchrocell rotated in a speed of 5 s/cycle. Measurement time was set as 15 h to improve the S/N ratio, thus, 15 VCD spectra for each sample were continuously collected, then averaged. To remove the baseline artifacts, we subtract the opposite enantiomer’s raw VCD spectra measured under the identical condition and divide the difference by two, that is, (R-S)/2 or (S-R)/2, and this is a commonly used method to remove the baseline artifacts. For the same sample, several pellets were made and the corresponding solid-state VCD and IR measurements were repeated several times to make sure the results were reliable and can be repeated. 2.3. Theoretical Calculation. Geometry optimizations, IR and VCD spectra calculations have been performed in the DFT

framework at B3LYP/6-311+G (2d, p) level of theory using Gaussian 09 D.01. All enantiomers in the gas phase have been used in the calculation. To account for the vibrational broadening, Lorentzian band shape with a half-width at halfheight of 4 cm−1 has been applied to the simulations of IR and VCD spectra. The time dependent density functional theory (TDDFT) method at B3LYP/6-311+G (2d, p) level of theory has been employed to the calculation of the excitation energies, oscillator strength and rotational strengths. The first 100 electronic states were taken into account for theoretical ECD spectral simulations. To best reproduce the experimental ECD spectra, the half bandwidths Γ of the Gaussians at the Δεmax/e were assumed as Γ = kλcalc1.5 with k = 2.887 × 10−3 (TPE), 3.849 × 10−3 (TETPE), and 4.811 × 10−3 (BETPE).39 20949


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Figure 3. Solid-state ECD spectra of (a) TPE, (b) P-BETPE, and (c) M-BETPE with different concentration gradient.

Figure 4. (a) Experimental ECD and UV−vis spectra of TPE, BETPE and TETPE in acetonitrile solution. (b) Experimental VCD and IR spectra of TPE, BETPE, and TETPE in deuterated chloroform solution.

Figure 5. DFT energy levels and Kohn−Sham orbitals of (a) TPE, (b) BETPE, and (c) TETPE.

state.9,30−32 The helicity of these compounds origins from twisted phenyl rings toward the same direction based on the central olefinic plane in the crystal lattice. M-helicity is defined as the anticlockwise torsion of the phenyl rings and P-helicity is corresponded to the clockwise torsion of the phenyl rings

3. RESULTS AND DISCUSSION TPE, BETPE, and TETPE were obtained from MSB according to the reported synthetic procedures.7,40,41 Their chirality cannot be maintained in solution due to the quick rotations of phenyl rings, but these rotations can be restricted in the solid 20950


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Figure 6. Comparison of the experimental IR spectra of TPE, BETPE, and TETPE in solid state as KBr pellets (1/40) with the corresponding calculated IR spectra at the B3LYP/6-311++G (2d, p) level of theory.

TETPE to prepare the KBr pellets in different concentrations. The block diagram of the optical-electronic layout of ECD and VCD spectrometer are presented in Figure S1.42 Powder X-ray diffraction is conducted to prove the single phase of compounds (Figure S2). We also tried many times to get the reliable single-crystal structure of TPE through single-crystal Xray diffraction using very intensive X-ray, ultimately we get a relatively reliable result with a relatively low flack parameter after many measurements. The previously reported singlecrystal X-ray diffraction measurements and our results are summarized in a list (Table S1), but they are still ambiguous to identify AC. The experimental and calculated ECD and UV−vis spectra of TPE, BETPE, and TETPE in the solid states are illustrated in Figure 2. The experimental ECD spectra were recorded as the KBr pellet with the appropriate mass concentrations of 1/1000, 1/800, and 1/400 for TPE, BETPE, and TETPE, respectively. To find the appropriate concentrations for reliable results, we repeated the ECD measurements under the same measurement condition for many times just varying the concentration (stepwise dilution; Figure 3). It can be found that around the proper concentration, the ECD results can be repeated, proving the reliability of the KBr pellet method.29 The experimental solid-state ECD signals of the opposite atropisomer are perfect mirror-images. However, the chiral helicity of these TPE core derivatives cannot be retained in solution (Figure 4). For simplicity, the atropisomer of P helicity is used in the TDDFT calculation at B3LYP/6-311++G (2d, p) level of theory in Gaussian 09 D.01.43 For ECD, detailed information about the excited states, such as excitation wavelengths λ (nm), oscillator strength f, rotational strength R (in Debye-Bohr-Magnetons), and electronic transition assignments, is tabulated in Tables S2−S4 (see SI).44 Comparing the simulated ECD spectra of TPE, BETPE, and TETPE in Figure 2, from longer wavelength to shorter wavelength, the first two ECD bands containing simple electronic transitions are similar among these three TPE core derivatives and less sensitive to the peripheral modification of the TPE core skeleton. By means of detailed information about the excited states in the Tables S2−S4, it is easy to assign these two ECD bands. For TPE, the first negative peak at 337 nm is correlated to the 11B1 excited state, corresponding to the electronic transition from 88th/22b2 (HOMO) to 89th/22b3 (LUMO) orbital and the second negative peak at 287 nm is associated with the 31B3 excited state which is mainly from 88th/22b2 (HOMO) to 91st/23b1 (LUMO+2) transition (Table S2). For BETPE, the first two negative Cotton effects (CE) at 365 and 316 nm are associated with the 11B excited state of 367.3 nm from the 100th/50b (HOMO) to 101st/51a (LUMO) transition and 21A excited state of 312.7 nm mainly

Figure 7. Comparison of the experimental VCD spectra (top) in solid state as KBr pellets (1:40) with the corresponding calculated VCD spectra (bottom) at the B3LYP/6-311++G (2d, p) level of theory of (a) TPE, (b) BETPE, and (c) TETPE.

(Figure 1a). Their structures are summarized in the Figure 1b. Normally, there are three possible results in the unit cell: all conformers are P, all conformers are M, or half conformers are P and half conformers are M, thus resulting to three different chiral crystals: P, M or racemic in the same batch. We pick up a pair of enantiomeric P and M crystals of TPE, BETPE, and 20951


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Figure 8. (a) Experimental solid-state ECD spectra of P-6 and M-6 in KBr pellets. (b) Experimental solid-state VCD and IR spectra of P-6 and M-6 in KBr pellets.

the diagnostic bands for the AC determination of TPE core compounds with P-helicity and vice versa. Including many well-resolved IR bands, VCD spectroscopy has become an increasingly important method for AC assignment. For the TPE core compounds, only solid-state VCD spectra is applicative considering the helical chirality cannot be locked up in the solution (Figure 4b), therefore, solid state VCD spectroscopy is utilized to study the fixed helical chirality of TPE, BETPE, and TETPE.45−47 Generally, there is a good agreement between the experimental and calculated IR (Figure 6) and VCD (Figure 7) spectra. Unlike ECD, VCD signals are quite weak, about 10−4−10−6 times lower than the corresponding IR intensity. In the VCD spectra of these TPE derivatives, the peaks in the higher wavenumber region (1650−1000 cm−1) involving plenty of mixed vibrations are very weak and mainly attributed to the stretching vibration of phenyl, ethylene, or acetylene groups. In contrast, the VCD bands in the lower fingerprint region (875− 730 cm−1) are intensively strong and associated with the bending vibration of the whole structure. Considering the poor transmission of ZnSe photoelastic modular element and of BaF2 polarizers in the lower wavenumber region, this fingerprint region may be silent to other chiral compounds, but for the TPE-core derivatives systems, experimental VCD signals in this lower wavenumber region is especially intensive and can be well simulated by the corresponding theoretical VCD spectra. The negative peaks at 752 (TPE), 810 (BETPE), and 845 cm−1 (TETPE) are the diagnostic bands for the Phelical chirality determination of TPE, BETPE, and TETPE, respectively. It is advisable to employ the first most intensive negative VCD peak in the range of 875−730 cm−1 as the diagnostic bands for the AC determination of the P-helical TPE core derivatives, and vice versa. Solid state vibrational circular dichroism spectroscopy is a new and valuable tool for the AC determination of these TPE core derivatives. One of the authors, Y.-S.Z., recently reported a pair of resolved TPE core compounds (denoted as P-6 and M-6), whose propeller-like conformations immobilized by an organic framework (Figure S3), result in a definite AC.15 Their solid state ECD and VCD spectra (Figure 8) agree with the predictions based on our proposed ECD and VCD diagnostic peaks, that is, the first negative ECD band in the 300−450 nm

from 100th/50b (HOMO) to 102nd/51b (LUMO+1) transition, respectively (Table S3). For TETPE, the negative peaks at 388 and 329 nm are related to the 11B1 excited state of 387.7 nm from 112nd/28b 2 (HOMO) to 113rd/28b 3 (LUMO) orbital and 21B3 excited state of 328 nm mainly from 112nd/28b2 (HOMO) to 114th/29b1 (LUMO+1) transition, respectively (Table S4). All these electron transitions around the HOMO and LUMO can be interpreted as π−π* transition in terms of the orbital features displayed in the Kohn−Sham orbitals (Figure 5a−c). The energy gap of TPE (4.17 eV) is larger than BETPE (3.82 eV) and TETPE (3.62 eV), since the electrons of TPE are relatively less delocalized among the whole orbitals due to the lack of conjugated acetylene substituents. It can be seen that there is a good agreement between the experimental and calculated ECD spectra except some trivial difference. For TPE (Figure 2a), five separate peaks at 340, 288, 262, 231, and 209 nm in the experimental ECD spectra are correlated to the peaks of 337, 287, 265, 244, and 218 nm in the calculate ECD spectra. For BETPE (Figure 2b), a very broad experimental band at 341 nm is assigned to the two peaks at 365 and 316 nm in the simulated ECD and other experimental ECD peaks at 270, 241, and 210 nm can be interpreted based on the simulate ECD peaks at 260, 241, and 221 nm, respectively. For TETPE (Figure 2c), distinctive experimental ECD peaks at 374, 328, 276, 238, and 211 nm are corresponding to the 388, 329, 246, 224, and 196 nm signals in the simulated ECD spectra. In the experimental ECD spectra of TPE, BETPE and TETPE, two separate negative peaks (Figure 2a), a broad negative band (Figure 2b) and a main peak combined with a small shoulder peak (Figure 2c) are corresponding to the first two simulated ECD bands. These differences can be interpreted in terms of the substitution of the TPE core. TPE and TETPE are of D2 symmetry whereas BETPE is of C2 symmetry. Asymmetric substituted acetylene in BETPE promotes the broadening of the peaks, result in the merged peak. Combined the simulated and experimental ECD spectra, bands in the shorter wavelength (200−275 nm) containing too many mixed transitions. In contrast, the first negative band in the 300−450 nm are simple and insensitive to the peripheral modification of the TPE core derivatives, can be identified as 20952


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The Journal of Physical Chemistry C (ECD fingerprint region) or the first most intensive negative VCD peak in the 875−730 cm−1 (VCD fingerprint region) can be used as the diagnostic bands for the AC determination of the P-helical TPE core derivatives and vice versa. It can be found that for the P-6 enantiomer, the first ECD band in the 300−450 nm range is negative and the first most intensive VCD peak around 800 cm−1 is negative. The above results definitely confirms the effectiveness and universality of our proposed comprehensive diagnostic ECD and VCD bands in the chiroptical spectroscopy for the AC determination of the TPE core functional materials.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.Z., J.H.L., Y.K.W. and Y.S.Z. thank the National Natural Science Foundation of China (Grant Nos. 21273175, 21773195, 21273139, and 21072067). B.Z.T. and R.R.H. thank the National Basic Research Program of China (973 Program; 2013CB834701) and the Innovation and Technology Commission of Hong Kong (ITC-CNERC14SC01). The computations were made on the IBM servers of State Key Laboratory for Physical Chemistry of Solid Surfaces of Xiamen University. We thank Zier Yan, Juanzhu Yan, Lei Ding, Hongkun Li, Haoke Zhang, and Prof. Anjun Qin for the experimental support.

4. CONCLUSIONS In conclusion, the helicity of the TPE, BETPE, and TETPE were investigated by the comprehensive solid-state ECD, VCD, IR, and UV−vis spectroscopies, combined with theoretical calculations at the B3LYP/6-311G level in Gaussian 09. A convenient and efficient method is proposed based on the diagnostic peaks in the solid-state ECD or VCD spectra to quickly determine the AC of the TPE derivatives without expensive and time-consuming procedures. Specifically, the first negative ECD band in the 300−450 nm or the first most intensive negative VCD peak in the range of 875−730 cm−1 can be used as the diagnostic bands for the AC determination of the P-helical TPE core derivatives and vice versa. Furthermore, this method has been proved through the corresponding simulations and experiments. This comprehensive solid-state chiroptical technique is quite promising, providing great opportunity to identify the absolute configuration of chiral molecules, especially for the TPE-core AIE functional materials. It is anticipated that this method can open a wide range of applicability to the chiral TPE core derivatives and promote further fabrication of devices based on new chiral AIE compounds.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06679. Block diagram of CD spectrometer; powder X-ray diffraction patterns of TPE, BETPE, and TETPE; excitation parameters and involved transitions for the main excited states of TPE, BETPE, and TETPE (PDF). X-ray crystallographic data for TETPE (CIF). Crystallographic and structural refinement parameters of TETPE (PDF). X-ray crystallographic data for TPE (CIF). Crystallographic and structural refinement parameters of TPE (PDF).

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Rongrong Hu: 0000-0002-7939-6962 Yuekui Wang: 0000-0001-8435-1980 Yan-Song Zheng: 0000-0002-1807-4580 Ben Zhong Tang: 0000-0002-0293-964X Hui Zhang: 0000-0001-8192-1116 20953


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Diagnostic Absolute Configuration Determination of

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