Photophysical Properties of Chiral Tetraphenylethylene Derivatives

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Photophysical Properties of Chiral Tetraphenylethylene Derivatives with the Fixed Propeller-Like Conformation Chunyu Liu, Guochun Yang, Yanling Si, and Xiu-Mei Pan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11647 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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The Journal of Physical Chemistry

Photophysical

Properties

of

Chiral

Tetraphenylethylene

Derivatives with the Fixed Propeller-Like Conformation Chunyu Liu,† Guochun Yang,*,‡ Yanling Si,§ and Xiumei Pan*,† †

Institute of Functional Material Chemistry, National & Local United Engineering Lab for Power Battery, Faculty

of Chemistry, Northeast Normal University, Changchun, 130024 Jilin, China. ‡

Centre for Advanced Optoelectronic Functional Materials Research and Key Laboratory for UV Light-Emitting

Materials and Technology of Ministry of Education, Northeast Normal University, Changchun, 130024 Jilin, China. §

College of Resource and Environmental Science, Jilin Agricultural University, Changchun, 130118 Jilin, China.

ABSTRACT: The recent synthesized helical tetraphenylethylene (TPE) exhibits broad application prospects such as display, catalysis and medical imaging. Fully understanding of the intricate relation between structure and property is rather important to structural design and performance improvement. Here, we employed density functional theory (DFT) and time-dependent DFT to calculate their ground and excited state structures, electron transition properties, optical rotation (OR), and second-order nonlinear optical (NLO) properties. For compound 1, the simulated UV-Vis/CD spectra and calculated OR value are in reasonable agreement with the experimental ones, allowing us to reliably assign the electron transition and determine the absolute configuration. Intriguingly, TPE derivatives are the excellent candidates for the second-order NLO materials in view of the large first hyperpolarizability values and intrinsic asymmetric structures. The intramolecular charge transfer cooperativity for this kind of compounds was achieved through involvement of the donor and acceptor substituent groups or their combinations. The charge transfer within TPE plays a key role in determining the chiral origin and electron transition properties, whereas the contribution of peripheral phenyl rings is fairly small. Moreover, the designed compounds 5 and 7 are the potential materials for fluorescent probe.

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1. INTRODUCTION Continuous performance improvements of various devices strongly rely on the discovery of new materials with excellent and unique properties. As a consequence, much effort has been made to design and synthesize various functional materials with the desirable properties.1-3 Among them, chiral compounds4-6 have drawn great attention due to not only the importance of fundamental science but also the wide range of practical applications in such areas as catalysis, medicine, polymer, materials science, molecular devices and nonlinear optics.7-14 In particular, chiral compounds exhibit unique advantage (i.e. inherent asymmetric structures) for the second-order NLO materials, making that their second-order NLO response could be detected even in highly symmetric media, such as chiral isotropic liquids.15-18 Recently, some chiral compounds with high second-order NLO performance have been discovered.19-21 TPE is the parent compound of a family of polyaromatic compounds, exhibiting intriguing chemical and physical properties22-24 and broad applications such as electron transfer catalysts25 and aggregation-induced emission (AIE) materials.26-28 Moreover, TPE, having extended π-conjugation, can be used as the basic building block to construct organic photovoltaic materials29 or supramolecular framework.30-32 For

instance,

TPE

substituted

phenanthroimidazoles

show

reversible

mechanochromism between blue and green colors and can be used as luminogens for organic light-emitting diodes.33 Amine functionalized tetraphenylethylene (TPE-NH2), acting as a high fluorescent probe, successfully detect trace amounts of nitrite ions in water.34 On the other hand, TPE-NH2 possesses the large second-order NLO value of 22.29 × 10−30 esu, which becomes an excellent candidate for second-order NLO materials.35 To further expand the applicable range of TPE, efforts to immobilize the propeller-like conformation and simultaneously possess helical chirality have been made. Unfortunately, once the phenyl rotation of TPE is fixed, the intrinsic propeller-like conformation is broken, and the wanted helical chirality disappears.36, 37 Very recently, the helical TPE compound, named as compound 1 (Figure 1), with the 2


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fixed propeller-like conformation and intrinsic chirality, has been successfully synthesized by incorporating methylene, oxygen atom and benzene.38 It exhibits some unique photophysical properties, such as the almost quantitative fluorescence quantum yield, the immobilization of aggregation-induced emission effect, and the two enantiomers showing a large dissymmetric factor. In view of these merits, compound 1 is a promising material in fluorophore dyes, 3D display, catalysis and medical imaging.38 Nowadays, theoretical calculations have become powerful tools to explain the experimental phenomenon and investigate the photophysical behaviors of TPE derivatives. For example, the ultrafast deactivation mechanisms in isolated TPE was identified by a mixed quantum classical trajectory surface hopping method.39 Thiel et al. combined electronic structure calculations and nonadiabatic dynamics simulations revealed that the intrinsic excited-state decay paths of TPE compounds are closely related to their fluorescence properties and performance as AIE luminogens.40 The effect of aggregation on structure and fluorescent properties of TPE derivative, named as P4TA, has been confirmed by QM/MM methods.41 Mathematical expressions, describing the relationship between emission wavelength and molecular configuration of TPPE, were presented by TDDFT calculations, which provides a convenient way to predict the emission energy of other TPE derivatives.42 Kokado et al. reported that the twist of C=C bond is the major factor for quenching of AIE-active TPE derivatives in the solution state.43 Fox et al. raised the similar idea of phenyl ring torsion making primary contribution to singlet excited-state deactivation.44 By all appearances, it is necessary to study the structure-property relationship of this novel compound from standpoint of microscopic electronic structure level, so that expands its applications and elevates performance. On the other hand, based on above analysis, chiral TPE compound might provide some new chances for second-order NLO materials in view of inherent asymmetric electronic structure and the delocalized π-electron character. In this work, our goal is to shed light on electron transition property, chiral origin, the effect of different substituents on photophysical 3



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properties, and design novel second-order NLO materials, with the aid of DFT/TDDFT calculations.

2. COMPUTATIONAL DETAILS All calculations have been performed within the Gaussian 09 computational program package.45 The ground-state (S0) geometrical optimization of the studied compounds without any symmetry constraints was carried out by using the B3LYP functional, which is a combination of Becke’s three-parameter hybrid exchange functional46 and the Lee−Yang−Parr47 correlation functional. Basis sets of 6-31G(d) were used for H, C, N and O atoms. The absence of any imaginary frequencies confirms that the optimized structures are the local minima. To assign the absolute configuration (AC) of compound 1, the M enantiomer was used in our calculations. To reliably determine the electron excitation energies, oscillator strengths, and rotational strengths of the studied compounds, four popular DFT functionals have been chosen. The global hybrid B3LYP46 with 20% Hartree−Fock (HF) exchange fraction is the most widely used functional. Meta-GGA M06-2X48 functional with a high percentage of HF exchange developed by Zhao and Truhlar, which is usually combined with ultrafine integration grids. Also, the CAM-B3LYP49 and LC-BLYP50 including long range corrections were selected, which can reliably describe optoelectronic and excitonic properties of organic charge transfer systems.51,

52

Considering the accuracy and reasonable computational resource, the diffuse 6-31+G(d) basis set was used. Gaussian bandshapes53 with a bandwidth of 0.18 eV were used to simulate the UV-Vis/CD spectra. The first excited-state (S1) geometries of the studied compounds were optimized using the TDDFT method combined with 6-31G(d) basis set. Based on the first excited-state (S1), fluorescence emission energy was calculated by using the LC-BLYP functional. And basis set is same as electron transition calculation. Here, hyper-Rayleigh scattering (HRS) was used to measure the second-order NLO response. In the case of plane-polarized incident light and observation made perpendicular to the propagation plane without polarization analysis of the scattered 4


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beam, the second-order NLO response that can be extracted from HRS data can be described as:54, 55 β HRS (0;0,0) = 2 〈βZZZ 〉

and

2 〈βXZZ 〉

{〈 β

2 ZZZ

2 〉 + 〈 β XZZ 〉}

(1)

correspond to the orientational average of the β tensor. We only

concerned the static first hyperpolarizability. The reliability of different DFT functionals56 (B3LYP, CAM-B3LYP, and BHandHLYP) on first hyperpolarizability (βHRS) was also verified.

3. RESULTS AND DISCUSSION 3.1. Geometrical Structures In this work, eight chiral TPE compounds were considered. Their ground-state (S0) geometrical structures of the studied compounds (Figure 1) were fully optimized at the B3LYP/6-31G(d) level. Among them, compound 1 has been synthesized and characterized, which possess the fascinating structure character.38 The four propeller-like phenyl rings in TPE are bridged between four phenyl rings by short tethers, exhibiting a stable TPE tetracycle propeller-like structure with helical chirality.38 Compounds 2-8 were designed to investigate the effect of different substituent groups and substitution positions on the photophysical properties, and find an effective way to enhance the NLO response. According to the structural character of compound 1, there are two different substitution positions. Specifically, R1 and R2 are directly connected to the central TPE, whereas R3 and R4 are separated from the TPE by O atom. In more detail, the introduction of Ph-NH2 (donor) and Ph-NO2 (acceptor) or their combination at R1 and R2 positions in compound 1 is named as compounds 2-4. With the same rule, compounds 5-7 can be obtained through substituting at R3 and R4. Substituting at R1, R2, R3 and R4 positions with Ph-NH2 and Ph-NO2 groups is compound 8. Based on the helicity of compound 1, absolute configuration M was used in the following calculations. Compound 1 exhibits rigid structure resulting from the interconnected benzene rings. Notably, besides the helical chirality, compounds 2-8 have the other stereogenic centers (denoted as *) due to the involvement of the substituents. For compounds 2-7, the three chiral centers lead to 5



four stereoisomers (MSS, MSR, MRR, and MRS) in theory. However, the structural symmetry makes MSR and MRS stereoisomers indistinguishable. As a consequence, the MSS, MSR, and MRR stereoisomers of compounds 2-7 were considered. This is the same for compound 8. Specifically, the MSSSS, MSSSR, MSSRR, MSRRR, and MRRRR stereoisomers were taking into account. All the stable possible configurations have been listed in Figure S1. The total energies of MSS and MSSSS stereoisomers are much lower than MSR, MRR, MSSSR, MSSRR, MSRRR, and MRRRR (Table S1). Thus, we mainly considered the electron transition properties of compounds with MSS or MSSSS stereoisomers in the main text. In the second-order NLO part, we considered all the possible stereoisomers. Here, compound 1 was taken as an example to test the reliability of our adopted method. The optimized structure and the atomic label of compound 1 were shown in Figure S2. It is found that the difference of main structural parameters between the experiment and calculation is rather small (Table S2). Thus, the adopted basis set and functional can reasonably describe ground-state molecular structures of the studied compounds. Moreover, the positive vibrational frequencies verify that our optimized structures are dynamically stable.

Figure 1. Chemical structures of the studied compounds. The stereogenic centers of compounds 2-8 are denoted as *. 6


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3.2. UV-Vis and Emission Spectra TDDFT, due to its efficiency and accuracy, has become a well-accepted method to investigate the electronic transition properties.57-62 Generally, electronic excitation energies and transition properties are sensitive to the specific DFT functional. To the best of our knowledge, the electronic excitation energies of the studied compounds have not been explored thus far. Thus, four popular functionals: global hybrid B3LYP,46 higher HF exchange M06-2X,48 the long-range corrected functionals CAM-B3LYP49 and LC-BLYP50 were selected to find the best suitable functional to reliably describe electronic transition properties. For compound 1, the simulated UV-Vis spectra using these functionals along with the experimental ones were given in Figure S3. It is found that its UV-Vis spectra strongly depend on the adopted functional. The absorption wavelengths obtained by M06-2X, CAM-B3LYP and LC-BLYP functionals exhibit a significant blue-shifted compared to the experimental data, so that only two absorption bands appear in the considered wavelength range. This observation can be attributed to the larger energy gaps (Table S3). However, the UV-Vis spectra computed with the B3LYP functional (Figure 2) perfectly reproduces the experimental ones, not only with respect to the relative peak intensities but also the band positions, which allows us to accurately assign electronic transition character of compound 1. Studies have shown that 6-31+G(d) basis set is sufficient to determine the nature of electronic transitions for organic compounds.63-65 Thus, the B3LYP/6-31+G(d) method was used in the calculations of UV-Vis spectra of the studied compounds. The calculated excitation energies, absorption wavelengths, and oscillator strengths of compounds 1-8 were summarized in Table 1 and Table S4. In order to analyze the nature of electron absorption, a natural transition orbital (NTO) analysis was performed.66 Here we referred to the occupied and virtual NTO pairs as “hole” and “electron” transition orbitals, respectively. The NTOs of compounds 1-8 were given in Figure 3 and Figure S4. Compound 1 exhibits three main absorption bands (i.e. 281.48, 331.96 and 394.22 nm), which are in good agreement with the experimental ones. Taking into account all the NTOs, these electronic transitions are 7



mainly assigned as π→π* within TPE part (Figure 3). The most intense absorption bands at 283.36, 386.64, 281.71, 283.98, and 387.43 nm for compounds 2, 4, 6, 7, and 8 can also be characterized as π→π* transitions within TPE part. For compound 3, the hole NTO contributing to intense band (285.01 nm) is mainly localized on TPE moiety while the electron NTO delocalizes on Ph-NO2 substituent groups. The crucial band (281.88 nm) of compound 5 can be assigned as TPE→Ph-NH2 charge transfer. Based on above analysis, the intense absorption bands of compounds 2-8 are red-shifted compared with that of compound 1. Overall, the introduction of different substituent groups at the different positions has remarkable effect on not only electron transition properties but also the absorption wavelengths. In addition, the Kohn-Sham molecular orbital isosurfaces involved in the main electron transitions of compounds 1-8 were shown in Figure S5. Subsequently, we investigated the excited-state geometry structure and emission properties. The excited-state (S1) geometry of compound 1 was optimized by using five different DFT functionals (e.g. B3LYP, PBE1PBE, M06-2X, CAM-B3LYP, LC-BLYP) combined with 6-31G(d) basis set. Compared between the ground and excited state structures, the notable difference occurs on the central ethylene bridge of TPE (Table S5), which can be used to rationalize the observed large Stokes shifts. The same functional as optimization, combined with 6-31+G(d) basis set, was employed to calculate the emission wavelengths. The resultant emission wavelengths were listed in Table S6. Apparently, the emission wavelengths obtained by the B3LYP and PBE1PBE have a significant red-shifted compared with the experimental value, which is up to 84 nm. With respect to the wavelengths of B3LYP and PBE1PBE, the results computed with M06-2X and CAM-B3LYP are closer to experimental value. However, they are still not in the acceptable range. Notably, the emission wavelength of 513.93 nm, obtained by LC-BLYP functional, is in reasonably good agreement with the experimental value of 492 nm. The NTOs involved in this emission band with π→π* character of TPE (Figure 4). In view of high fluorescence quantum yield and large Stokes shift of compound 1, we investigated the potential of the studied compounds as fluorescent probe. In 8


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general, an ideal fluorescent probe possesses the large Stokes shift.67 Stokes shift, the difference between the emission and absorption maxima, is the inherent feature of each fluorophore.67 At present, most of the available fluorophores (e.g. rhodamine, fluorescein and cyanines) usually exhibit the small Stokes shifts within 25 nm, leading to the serious self-quenching and fluorescence detection errors.68 The emission energies and oscillator strengths of compounds 5 and 7 were also calculated at LC-BLYP/6-31+G(d) level of theory. Their Stokes shifts are close to that of compound 1, reaching to 230 nm (Table 2). Interestingly, the luminous intensity of compounds 5 and 7 are much larger than that of compound 1 in view of their larger oscillator strengths. These results indicate that compounds 5 and 7 are the good candidates of fluorescent probe materials.

Figure 2. Calculated UV−Vis (left) and CD (right) spectra of compound 1 at the B3LYP/6-31+G(d) level of theory along with experimental spectra.

Table 1. Calculated excitation energies (∆Ege, eV), absorption wavelengths (λabs, nm) along with experimental data (in parentheses), and oscillator strengths (f) for the compounds 1, 4, 7, and 8 at the B3LYP/6-31+G(d) level of theory. 9



Compound 1

4

7

8

∆Ege 3.15 3.73 4.40 3.21 3.72 4.30 3.14 3.72 4.37 3.20 3.73 4.16

λabs 394.22(365) 331.96(320) 281.48(270) 386.64 332.89 288.01 394.84 332.86 283.98 387.43 332.37 297.91

f 0.227 0.091 0.481 0.230 0.065 0.201 0.238 0.115 0.351 0.248 0.061 0.073

Figure 3. Natural transition orbitals in the absorption bands for compound 1 at the B3LYP/6-31+G(d) level of theory.

Figure 4. Natural transition orbitals in the emission band for compound 1. 10


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Table 2. Calculated emission wavelength (λem, nm), oscillator strength (f), and stokes shift (nm) for compounds 1, 5, and 7 at the LC-BLYP/6-31+G(d) level. Compound 1 5 7

λem 513.93 510.19 510.22

f 0.221 0.230 0.229

Stokes shift 232.45 228.31 226.24

3.3. Chiroptical Properties Reliable determination of AC of the chiral molecule is rather important because different enantiomers exhibit completely different properties in pharmacy. For most of chiral compounds, it is not easy to obtain the crystal structures. As a consequence, the combination of experimental measurement and theoretical calculation becomes a useful method to determine AC of the chiral compound.69-72 As shown in Figure 2, our simulated CD spectra using B3LYP functional are in reasonably good agreement with the experimental ones, which allows us to assign M absolute configuration of compound 1 with high confidence. Moreover, the rotational strengths calculated using the length and velocity gauge representations of the electric dipole operator are nearly equal (Table S7), again validating the suitability of the method used for the CD calculation and AC assignment. To further understand the chiral origin of compound 1, the molecular orbitals involved in the main transitions were shown in Figure S6. The observed CD bands mainly result from exciton-coupling of the TPE part. The calculated OR values are sensitive to basis sets. Here, three basis sets (cc-pVDZ, cc-pVTZ, and cc-pVQZ) were used to test the suitability on the studied compounds. The calculated OR values at 589 nm using the B3LYP functional along with experimental values are given in Table 3. With the size extension of basis sets (from cc-pVDZ to cc-pVQZ), the OR values become larger and larger. Specifically, the difference of OR values at 589 nm between the cc-pVDZ and cc-pVTZ basis sets reach to 192 deg dm−1 (g/mL)−1. Whereas the OR value of cc-pVTZ is almost the same as that of cc-pVQZ, which means that OR value already converges at cc-pVTZ level. It is noted that the sign of the OR value does not change as the size of the basis set increases. For the tested compound 1, although there is a slight deviation between 11



the calculated and experimental OR values, the sign of the experimental OR is correctly reproduced. Therefore, these results can be used to derive AC information of the studied compounds. Moreover, the OR values at the other wavelengths (578, 546, 436, and 365 nm) were also calculated at the cc-pVTZ level. The variation trend of OR values at different wavelengths was shown in Figure 5. It is found that the calculated OR value of compound 1 becomes more positive as the wavelength decreases. Interestingly, the OR value is the largest at 436 nm. Table 3. Calculated OR values (deg dm−1 (g/mL)−1) with three different basis sets (cc-pVDZ, cc-pVTZ, and cc-pVQZ) and experimental data with excitation wavelengths of 589 nm for compound 1. Basis set cc-pVDZ cc-pVTZ cc-pVQZ Exp

OR 1105.25 1297.23 1345.52 479

Figure 5. The variation trend of OR values at five different excitation wavelengths (589, 578, 546, 436, and 365 nm) for compound 1. 3.4. Second-Order NLO Property Excellent second-order NLO materials should have nonsymmetrical electron structures and obvious intramolecular charge transfer character. Based on above 12


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analysis, our studied compounds meet these requirements, which inspires us to study their second-order NLO response. Appropriate choice of functional is the premise of accurate description of NLO properties. Here, three widely used DFT functionals56 (B3LYP, CAM-B3LYP, and BHandHLYP) in the NLO field were selected to test the reliability of the adopted method on prediction of first hyperpolarizability (βHRS). The calculated βHRS values with 6-31+G(d) basis set of compounds 1-8 (2-8 with the MSS and MSSSS stereoisomers) are given in Table 4. Among the calculated βHRS values of the adopted DFT functionals (Table 4), the B3LYP produces the largest βHRS value, whereas the CAM-B3LYP and BHandHLYP give the nearly identical βHRS values. These differences can be understood from the composition of the specific functional. For B3LYP, the incorrect electric field dependence of the “response part” of the exchange-correlation potential lacks a linear term counteracting the applied electric field,73 leading to overestimate the βHRS values. CAM-B3LYP with the long-range corrections and BHandHLYP containing high HF exchange fractions can provide semiquantitative accuracy with a reasonable computational cost.56,

74, 75

Currently,

CAM-B3LYP becomes a promising tool for predicting NLO properties. 76-78 Therefore, βHRS values obtained from the CAM-B3LYP were used in the following discussion. The βHRS values of the studied compounds range from 357 to 9498 × 10−33 esu, indicating that subtle structural modifications can substantially enhance second-order NLO response. Specifically, for compounds 2-4 (substituent positions at R1 and R2), the βHRS value of compound 2 is about 5 times that of compound 1, which means that introduction of the Ph-NH2 group has certain impact on the NLO response. The βHRS value of compound 3 is larger than that of compound 2, showing that the introduction of electron acceptor NO2 is much conducive to enhancing the NLO response. Compound 4 has the largest βHRS value. Thus, asymmetric substitution is the most effective way to increase the βHRS value. On the other hand, their βHRS values are also sensitive to the positions of the substitution groups. In other words, the βHRS values substituted at R3 and R4 positions are different from that of R1 and R2 positions. For example, compound 5 has the larger βHRS value than compound 6, which is reverse to the βHRS values of compounds 2 and 3. These results imply that the introduction of 13



different substituents and substituted at different positions have a significant effect on the NLO response. At last, simultaneously substituted at R1, R2, R3 and R4 positions, the βHRS value of compound 8 is the largest, which is about 55 times that of the average second-order polarizability of urea.79 Therefore, our studied chiral compounds are expected to become good potential second-order NLO materials. To better understand their NLO origins, the corresponding electron density difference maps of compounds 1, 7 and 8 were shown in Figure 6. It can be seen that NLO origin of compound 1 mainly derives from π→π* character of the TPE molecule. Whereas, there is the obvious charge transfer coming from the phenyl rings at 283.98 nm of compound 7, besides the π→π* for TPE at 332.86 nm and 394.84 nm. Notably, the charge transfer, from benzene to Ph-NO2 and Ph-NH2 to TPE, is mainly responsible for the NLO response of compound 8. The NLO properties of the studied compounds with MSR, MRR, MSSSR, MSSRR, MSRRR, and MRRRR stereoisomers were also calculated, as shown in Table S8. The overall variable trend of βHRS values for these stereoisomers is similar to those of MSS and MSSSS stereoisomers. For compounds 2'-a, 3'-a and 2'-b, 3'-b (MSR and MRR stereoisomers), the βHRS values are in the following order: 2'-a

Photophysical Properties of Chiral Tetraphenylethylene Derivatives

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