GAN Dong-Er ZHANG Zhng, LI Qio-Hong LI Wen-Mu

Oligo(aromatic ether sulfone)-F as a Nonlinear Polarized Polymeric Material: an Experiment and DFT Study①

GAN Dong-Era,cZHANG Zhangb, cLI Qiao-HongbLI Wen-Mua②

a(350002)b(350002)c(100049)

Polymeric membranes with diverse structures have attracted much attention as new materials for nonlinear optical devices.In this report, a novel oligomer of poly (aromatic ether sulfone) (OAES) has been synthesized and characterized.The electronic structure as well as linear and nonlinear optical properties has been studied by density function theory.The effect for general nonlinear optical polarizability of various condition has been further researched such as absence of side chains and introduction of phenyl substituent on side chains.The static and frequency-dependent hyperpolarizabilities of OAES and its derivativeshave been calculated.This work interprets an efficient adjustment for the frequency response and the intensity of nonlinear optical polarizability can be achieved by regulating the structure of system, which provides a new potential for the application of oligomeric materials on nonlinear optical field.

DFT method, NLO properties, oligo(aromatic ether sulfone)-F, sum-over-states method;

1 INTRODUCTION

The development of polymeric materials with the second and third nonlinear optical properties has been the focus of optical polymer materials research for last decades[1-5].In comparison to inorganic crystals, the organic macromole-cules have considerable advantages of easier synthesis, liable modification, higher optical-damage toleration and lower cost[6-8].

Majority of reports in this field focus on the-conjugated polymeric nonlinear optical materials, which are of a critical competence in larger hyperpolarizability and rapid response time owing to their multiple donor-acceptor systems[9-11].The works of Muhammad Ramzan Saeed Ashraf Janjua and Miroslav Medved have studied the amplification of hyperpolarizability and electron mobility with-spacers in polymer side chains or backbones by the analysis of density function theory.These polarized polymer materials perform well in nonlinear properties such as the second harmonic generation and all-optical matching[12, 13].

However, reports on the third-order nonlinearity of polarized non-conjugated polymers are relatively rare.The theoretical calculation of the relationship between structure and optical properties is very important for understanding the NLO properties of polymer materials[14, 15].Some works employing the DFT to thermal degradation research of the resinic polymers made an interesting explore in this subject.Enlightened by a study under the guidance of Debesh R.Roy on the NLO analysis of poly (methyl methacrylate) (PMMA), which utilizes the DFT calculations to predict the thermal and structural properties of PMMA,another traditional plastic polymer of poly (aromatic ether sulfone) (PAES) with fluoric side chain as a remarkable grafting site can also be researched by the DFT method[16].

PAES and its derivant have always been the excellent candidate materials in the fabrication of diverse polymer membranes.In the pyramid diagram of common thermosplas-tic materials adapted by Peiyuan Zuo, PAES belongs to the top marked as high performance polymers for its chemical, thermal, and mechanical stability[17].Good proton conduc-tivity can be obtained by conveniently introducinghydro-philic structure such as sulfonic groups[18].Owing to the great compatibility, PAES can easily form complexes with various organic and inorganic molecules, which achieved many noteworthy results in studying nanofiltration membranes and medical applications[19].

This novel PAES derived from the work of Zhao Li and Michael D.Guiver[20].After a series of modifications on monomer like introducing massive methyl, removing the electron withdrawing group and cancelingcomonomer as shown in Fig.1, the electronic cloud density and structural regularity of backbone have an obvious promotion.The consequential polymer of oligo(aromatic ether sulfone)-F provides an outstanding potential for further graft and substitution, which has many prospective applications in electrolyte membranes for fuel cells.

Fig.1.Synthesis of themonomer

The NLO properties of the polymer mainly depend on the microstructure of molecular and measurement conditions.This report has systematically studied the nonlinear optical properties of OAESF and calculated the first and second hyperpolarizability on its static and frequency by DFT method.

This study aims to determine the size of oligomers and the effects of side chain substitution and absence on nonlinear optical properties.The results may provide valuable guidance for the design and synthesis of polymeric NLO materials.

2 EXPERIMENTAL

2.1 Materials and measurements

4-Fluorobenzaldehyde(FBA), 4-fluorothiophenol (FBT) and 2,6-dimethylphenol were purchased from Shanghai Macklin Biochemical Technology Co, Ltd.Triflic acid, K2CO3and 4,4΄-difluorophenyl sulfone (DFPS) werecommercial from TCI Co, Ltd.DMAc wasavailable from Sinopharm Co, Ltd (Beijing, China) and redistilled before use.The1H NMR and19F spectra of monomer and oligomer were recorded on an AVANCE III at 400 MHz using deuterated dimethyl sulfoxide (DMSO-d6) and chloroform-d as solvents.The mechanical agitator of precipitating monomer was purchased from Shanghai Yike Instrument Co, Ltd.

2.2 Synthesis of monomer

The synthesis of 4-((4-fluorophenyl)thio)phenyl benzal-dehyde (FTPB)K2CO3(4.84 g, 35 mmol) was added to a solution of FBA (7 mL, 65 mmol) and FBT (7.45 mL, 70 mmol) in anhydrousDMAc (100 mL).The mixture with magnetic stirrer was then heated to 60°C under nitrogen overnight and precipitated in 500 mLwater.In the next step, the turbid mixture was extracted with 150 mL diethylether to get two homogeneous phases.The organic layer was concentrated with rotary evaporator to yield white oily solid.Yield: 93%.1H NMR(DMSO-6, ppm): 9.92(s, 1H), 7.81～7.83(d,= 8.3Hz, 2H), 7.62～7.66(dd,= 8.68Hz, 5.4Hz, 2H), 7.38(m, 2H), 7.27～7.28(d,= 8.3Hz, 2H).19F NMR (DMSO-6, ppm): –111.27(s, 1F).

The synthesis of 1,1-bis(2,6-dimethylphenol)-1-(4-(4-fluorophenyl)thio)phenyl ethane (BDFTPE) FTPB (7 g, 30 mmol) and 2,6-dimethylphenol (11 g, 90 mmol) were added to a 100mL round-bottomed flask under nitrogen atmosphere.Thenthe mixture was heated with magnetic stirrer to 50°C until the solids are dissolved.Triflic acid (0.7 mL, 7.5 mmol) as catalyst was added to the dissolved solution rapidly andthe temperature was increased to 60°C for 1.5h.Distilled water (100 mL) was poured into the resulting violet viscousliquid, obtaining orange solid after violent mechanicalstirring under 1200rpm.The product was washed and filtrated to give a yellow solid.Yield: 82%.1H NMR (DMSO-6, ppm): 8.06(s, 2H), 7.37～7.41(m, 2H), 7.18～7.25(m, 4H), 7.04～7.06(d,= 8.1Hz, 2H), 6.60(s, 4H), 5.20(s, 1H), 2.07(s, 12H).19F NMR (DMSO-6, ppm): –113.78(s, 1F).

2.3 Synthesis of oligo(aromatic ether sulfone)-F

As shown inFig.2, BDFTPE (0.912 g, 2 mmol), DFPS (0.508 g, 2 mmol) and K2CO3(0.69 g, 5 mmol) were added in 50 mL anhydrous DMAc and 10 mL toluene.A Dean-Stark trap was fitted on the 100 mLthree-necked round-bottomed flask in which the mixture was put.The temperature was raised to 140°C for 1h.Then the system was heated to 147°C to distill off water and toluene for about 10min.The mixture was precipitated in a mixture of methanol and distilled water (30/30) after concentration.The product was collected by centrifugal sedimentation and filtration.A white powder can be obtained by washing and drying at 80°C under vacuum.Yield: 71%.1H NMR (CDCl3, ppm): 7.72～7.75(m, Ar), 7.32～7.36(m, Ar), 7.10～7.12(m, Ar), 6.93～6.98(m, Ar), 6.74～6.76(m, Ar), 5.29(s, 1H), 1.94(s, 12H).19F NMR (CDCl3, ppm):–113.44(s, 1F).

Fig.2.Synthesis of OAESF

3 THEORY AND COMPUTATION

Thecalculation method based on density functional theory (DFT)[21]is an effective tool for predicting the electronic and optical properties of compounds.The ground state geometry of OAESFnis fully optimized based on B3LYP functional[22]and def2-sv (p) double-zeta basis sets[23], with D3 dispersion correction of Grimme[24].The calculation of nonlinear hyperpolarizability is carried out in def2-svp basic sets with PBE0 hybrid functional[25].With the double zeta basic sets, PBE0 functional may have better performance in valence excitation.All calculations were performed with the ORCA 4.2.1 program packages[26].Multiwfn software was used to analyze the electronic structure, excitation characteristics and hyperpolariza-bility[27].

The sum-over-states (SOS) method is a common method of nonlinear optical calculation[28].Static and frequency-dependent polarizabilities and hyperpolarizabilities can be calculated from the energy of each excited state and the transition dipole moment between each excited state.As shown in the following formula[28]:

The calculation of nonlinear optical susceptibility is to calculate the response of molecular wave function to external field.The Finite Field (FF) and Coupled Perturbed Hartree-Fock/Kohn-Sham (CPHF/KS) methods are based on the influence of external electric field on the static Schrodinger equation to calculate the higher-order derivative of energy to the external field.The Sum-Over-States (SOS) method is based on the time-dependent density functional theory to calculate the response of wave function to external field perturbation.The advantage of SOS method lies in its high efficiency in calculating frequency dependent nonlinear polarizability.When the number of States is enough, the SOS method is theoretically accurate.

The magnitude of hyperpolarizability is used to quantita- tively study the nonlinear polarizability of the system[32]:

4 RESULTS AND DISCUSSION

4.1 Study of oligomer constitutional unit

Aiming to study the nonlinear optical properties of OAESFn, the unit spherical representation was utilized for probing the anisotropy ofpolarizabilityof OAESF constitutional unit, as showed in Fig.3.It illustrates that the constitutional unit has a large polarizability in the direction of backbone.A significant electricity-induced dipole moment indicates the anisotropy of the polarizability of the oligomer molecule.The optical response of OAESF system along the backbone is more prominent[29].

Fig.3.Anisotropy of polarizabilityof OAESF constitutional unit

In order to illustrate the accuracy of the calculated linear and nonlinear optical coefficients, CPHF method under CAM-B3LYP/6-31+G (d,p) and SOS method under PBE0/def2-SVP and SOS-ZINDO are compared.It can be seen that the calculation accuracy of Sum-Over-States method in this system can be guaranteed.

Table 1.Nonlinear Optical Coefficients of OAESFCalculated by Different Methods

Then the second and third nonlinear optical polarizability density along the backbone was calculated.Contrast between the dipole moment after Taylor expansion and electron density gives hyperpolarizability of oligomer unit as follows[30].

Using the above formula, the finite difference is performed with 0.003a.u.as the step size, and the hyperpolarizability density of OAESF is obtained.The calculated hyperpolariza-bility densityshown in Fig.4 confirms that the polymer unit molecules provide a general contribution to the second-order nonlinear optical polarizability.It can be seen from the atomic space integral that both the mainandside chains contribute to.For, the contribution of the side chain is relatively high.Consequently, the increasing degree of polymerization tends to make a positive influence to the NLOpolarizabilities.

Fig.4.The second-order (left) and third-order (right) nonlinear polarizability density of OAESF constitutional unit (Positive isosurfaces are shown in green)

4.2 Effect of degree of oligomerization

Fig.5shows the relationship between the degree of oligomerization and its energy gap ΔEH-L.The rate of decreasing band gap is declining as the degree of oligomerization grows.The NLO properties can be approximated by TD-ZINDO-SOS method with results shown in Fig.6.The alteration of nonlinear optical coefficient tends to be gentle with the expanding size of oligomer.Subsequent study was based on the modification of OAESF3for its highly typicality.

Fig.5.Relationship between the OAESFnpolymerization degree and HOMO-LOMO energy gap

Fig.6.Relationship between the OAESFnpolymerization degree and the second order () and third order () nonlinear polarizability

4.3 Static and frequency-dependent NLO properties of OAESF3 with side chain substituted and deleted

Table 2 shows the calculation of ΔEH-Lfor different proportions of substituted phenyl or the absence of side chain.It may be noticed that the electronic structure of the polymer OAESF3changes with the structure of the side chain.The replacement of the side chain affects the HOMO and LUMO orbitals, which lead to the gradual reduction of ΔEH-L, while the absence of side chain amplifies ΔEH-L.

Table 2.ΔEH-L (eV) for OAESF3with Different Proportions of Substituted Phenyl or the Absence of Side Chain

Table 3 lists the vertical excitation features calculated by TDDFT under the substitution and absence of side chains.It states that the electronic excitation properties of OAESF3depend not only on the HOMO and LOMO orbitals, but also on the contributions of other excited states.OAESF3may have some potential in the application of frequency doubling due to its absorption in deep ultraviolet region.

Table 3.Crucial Vertical Excitation Properties of OAESF3 and Derivatives

*maxis the maximum absorption wavelength while f refers to the maximum of oscillation strength.H and L denote HOMO and LUMO, respectively

At the PBE0/def2-svp level, 100 excited states are calculated using TDDFT for the SOS method.The accuracy of SOS method largely depends on the convergence of the calculated results.Herein the convergence of the method was tested firstly.As shown inFig.7, 100 excited states can meet the requirements of computational precision and accuracy.

Fig.7.Convergence of applying SOS method to OAESF3

Then the static polarizabilities and the first and second hyperpolarizabilities of substituted and side chain deleted OAESF3were calculated, as shown in Table 2.What can be known is that the absence of side chain results in the reduction of ΔEH-L, which accordingly reduces the nonlinear polarizability along the backbone.The substitution on side chains is increasing in general, but the excessive substitution can lower the hyperpolarizability, which may be caused by the enhancement ofelectronic delocalization in the direction of side chains owing to the overmuch substituent.It illustrates that the absence/substituted proportion plays a key role in regulating the nonlinear optical properties of OAESF materials.We can be see from the fitting of ΔEH-Land static nonlinear optical coefficients that an approximate relative linearity exists respectively between static polarizability, hyperpolarizability and ΔEH-L, as depicted in Fig.8[31].

Fig.8.Relationship between the electronic polarizability and hyperpolarizability with energy gap ΔEH-Lfor OAESF3and derivatives

Subsequent calculation utilized SOS method to the nonlinear polarizability of the second harmonic generation, Kerr effect, electrically-induced second harmonic and third harmonic generation, and degenerated four wave mixing (DFWM) at the wavelength of 350～750 nm, as shown inFig.9.When the frequency of the incident light is equal to or close to the natural frequency of the harmonic oscillator in the medium, the abnormal dispersion will occur due to resonance.At this time, the value of nonlinear optical polarizability with frequency is very large, which has only qualitative significance.It can be noted that increasing the phenyl substituents of the side chain can significantly improve the static nonlinear polarizability and the frequency nonlinear polarizability.For OAESF system, the increase of side chain substitution can make the third-order nonlinear optical absorption peak red shift.However, the loss of side chain will cause the third-order nonlinear optical absorption peak to slightly blue shift.It is possible to change the nonlinear optical properties of the whole oligomer through side chain chemical regulation.These properties can be applied to optically modulate the OAESF polymer materials.The OAESFs-based polarized membrane materials may have promising applications in fields like adjustable electro-optic switching and fiber optic sensing.

Fig.9.Frequency-dependent NLO coefficients of OAESF3and derivatives

5 CONCLUSION

In this work, a novel oligomer of PAESF was synthesized and characterized.The electronic structure of OAESF was studied by DFT method.The effects of side chain absence and substitution on the linear optical coefficients and the second- and third-order nonlinear optical coefficients of OAESF were studied on the basis of OAESF3.The loss of side chain will reduce ΔEH-L, and then the nonlinear optical polarizability of the polymer.A reasonable range of substitution can improve ΔEH-Land the overall nonlinear polarizability of oligomers.At the same time, the loss and substitution of side chains can lead to the red/blue shift of nonlinear polarizability.The nonlinear optical properties of the oligomer can be adjusted by regulating the molecular structure and incident light frequency.This study provides a reference for the design and synthesis of nonlinear polarized membrane optical devices.

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15 May 2020;

23 December 2020

①The authors gratefully acknowledge the financial support of the 100-Talent Program of Chinese Academy of Sciences

.E-mail: liwm@fjirsm.ac.cn

10.14102/j.cnki.0254–5861.2011–2878