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Abstract
4-phenylethylene derivatives (B1, B2 and B3) with π conjugated systems were designed and synthesized by Sonogashira coupling reaction and Schiff base reaction. Under the ns laser source, the nonlinear absorption coefficients (β) of B2 and B3 are 1.85 × 10−10 m/W and 1.9 × 10−10 m/W. B2 and B3 have good light limit performance. Furthermore, the third harmonic strength of B2 and B3 is 181 and 215 times that of SiO2, respectively. The optical limiting threshold and third harmonic signal show that 4-phenylethylene derivatives will have good application potential in laser protection and frequency conversion.
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1. Introduction
In recent years, organic nonlinear materials have attracted extensive attention due to their advantages of fast response speed, high damage threshold and easy processing [1,2], such as azo compounds, heterocyclic compounds and Schiff base compounds, which own π electronic conjugated structure and are easy to design and synthesize [3–6]. Schiff base compounds are composed of primary amines and carbonyl groups, where in the C = N bond forms a π electronic bridge, which makes the compounds have larger nonlinear optical response [5,7,8].
Meanwhile, people are committed to looking for materials with large π conjugate structures in order to further increase nonlinear optical properties [9–11]. Tetraphenylethylene compounds have large π conjugated planes, in which delocalized π electrons contribute to enhancing the optical nonlinear response of the materials. In addition, as the most extensive group in organic materials, alkyne bonds have been widely used in the effective connection of conjugated units. Alkynes are electron-rich groups, which not only effectively extend the π conjugate plane of molecules, but also enhance electron transport in molecular systems [12–18]. In this context, we have designed and synthesized 4-phenylethylene derivatives with D-π-A structure using tetraphenylethylene as charge transfer acceptor, imine and alkynyl as conjugate bridge, and diethylamine as charge transfer donor. The structure-activity relationship between the structure and nonlinear properties of the compounds was studied in detail, which provided guidance for the design and preparation of nonlinear compounds with excellent nonlinear optical properties in the future.
2. Experiment
2.1 Materials
1,1,2,2-tetrakis(4-bromophenyl)ethene was purchased from Henan Psai Chemical Products Co., Ltd. 4-ethynylaniline, 4-diethylaminobenzaldehyde, 4-(diethylamino)salicylaldehyde and other raw materials were purchased from Shanghai Macklin Biochemical Co., Ltd. All raw materials are commercially purchased and used without any further purification.
2.2 Synthesis of 4-phenylethylene derivatives
B1, B2 and B3 were synthesized according to the synthetic route of Fig. 1. The three compounds were characterized by NMR.
2.2.1 Synthesis of B1
1,1,2,2-tetrakis (4-bromophenyl)ethene (5.18 g, 8 mmol), 4-ethynylaniline (5.64 g, 48 mmol), tetra(triphenylphosphine)palladium (Pd(PPh3)4, 462 mg, 0.4 mmol) and CuI (76 mg, 0.4 mmol) were added into a 150 mL round bottom flask, and then the degassed dry solvents tetrahydrofuran (THF, 25 mL) and triethylamine (Et3N, 25 mL) were added. The whole reaction was stirred in N2 atmosphere at 80°C for 48 h, and then cooled to room temperature and filtered to obtain the crude product. The mixture of ethyl acetate and petroleum ether (1:1) was used as eluent. The orange yellow solid B1 (4.56 g, 72%) was obtained by silica gel column chromatography. The NMR spectrum of B1 is shown in Fig. S1.
2.2.2 Synthesis of B2
B1 (1.5 g, 1.89 mmol) and 4-diethylaminobenzaldehyde (1.6 g, 9.08 mmol) were dissolved in dried ethyl acetate (EA, 15 mL) respectively. After complete dissolution, they were mixed in a 100 mL round bottom flask. A drop of acetic acid (HOAc) was added as catalyst and stirred at 80°C for 72 h. After the reaction, the solution was cooled to room temperature, dried with anhydrous Na2SO4, and then the solvent was removed by rotary evaporator. The mixture of ethyl acetate and petroleum ether (1:1) was used as eluent and silica gel column chromatography was used to obtain yellowish brown solid B2 (1.9 g, 73%). The NMR spectrum of B2 is shown in Fig. S2.
2.2.3 Synthesis of B3
The synthesis of B3 and B2 is similar. B1 (1.5 g, 1.89 mmol) and 4-(diethylamino)salicylaldehyde (1.74 g, 9.08 mmol) were dissolved in dry EA (15 mL) respectively. Then the yellowish brown solid B3 (2.1 g, 75%) was obtained according to the synthesis method of B2. The NMR spectrum of B3 is shown in Fig. S3.
2.3 Characterization and methods
The 1H-NMR spectra were recorded on a Bruker Avance 300 MHz NMR spectrometer. UV-Vis absorption spectra were recorded on a Hitachi U-3900H spectrophotometer. The nonlinear absorption of the samples was investigated by an open-aperture Z-scan testing platform (WNLO-IDZ) with different laser pulse widths: 15 ps (FWHM) pulses of 532 nm were obtained from a Q-switched and mode-locked Nd: YAG laser (1064 nm, 15 ps, 10 Hz), and 4 ns (FWHM) pulses of 532 nm were extracted from a Q-switched Nd: YAG laser (1064 nm, 4 ns, 10 Hz). The laser source used for optical limiting measurements (NLO-IOL) was the same as that used for the open aperture Z-scan technique. The sample was dissolved in DMF solvent with the concentration of 1 mg/mL. The 2 mm thick quartz dish was used as the container for testing. The harmonic signal of the sample is measured by harmonic testing platform (WNXB) under laser source (1550 nm, 100 fs, 6 MHz), and the harmonic detection range is 300-900 nm.
3. Results and discussion
3.1 UV-Vis analysis
The UV-Vis absorption spectra of B1, B2 and B3 are shown in Fig. 2. B1, B2 and B3 were dissolved in DMF. It can be seen from Fig. 2 that the strong absorption bands of B1, B2 and B3 are located at 345 nm, 391 nm and 414 nm, respectively, which is mainly attributed to n-π* and π-π* electron transition. Compared with B1, the absorption peak of B2 has a red shift of 46 nm and that of B3 has a red shift of 69 nm. As the wavelength moves towards the long wave, the energy required for electron transition decreases. Therefore, the red shift of B2 and B3 maximum absorption peak is largely due to the decrease of HOMO-LUMO energy gap [19,20].
3.2 Quantum chemistry calculation
In order to further study the relationship between the molecular structure and nonlinear optical properties of B1, B2 and B3, the density functional theory (DFT) was used to calculate the quantum chemistry of B1, B2 and B3 at the B3LYP/6-31G (d) level on the Gauss 09 program [21,22], and the two components of frontier molecular orbitals (HOMO and LUMO) were simulated, as shown in Fig. 3. The occupancy of each component in the frontier molecular orbital is determined by Gauss sum, as shown in Table 1. In Fig. 3, B1, B2 and B3 can be approximately regarded as D-π-A planar molecule, and donor and receptor through π conjugated bridges (alkynes, imines and benzene rings) are connected. Obviously, the frontier orbital distribution of B1 is transferred from aniline to 4-phenylethylene through the conjugate bridge, and the frontier orbital distribution of B2 and B3 is transferred from diethylamino group to 4-phenylethylene through the conjugate bridge, which proves the intramolecular charge transfer (ICT) interaction. B1 has a higher intramolecular charge transfer (ICT), and the intramolecular charge transfer (ICT) of B2 is stronger than that of B3.
Table 1. The possession percentage that each component occupied in the frontier molecular orbitals of B1, B2 and B3.
Finally, it can be concluded that the HOMO-LUMO transitions of B1, B2 and B3 are controlled by ICT and π-π* transition. In addition, aniline and diethylamino groups act as electron donor and 4-phenylethylene acts as electron acceptor. As can be seen from Fig. 3, the HOMO-LUMO energy gaps of B1, B2 and B3 are 3.03 eV, 2.92 eV and 2.91 eV, respectively. B2 and B3 are large π conjugated structures, and 4-phenylethylene has obvious π-π* transitions accompanied by charge transfer. Compared with B2, B3 has a smaller HOMO-LUMO energy gap. This is mainly because the hydroxyl group on B3 receptor is connected with benzene ring, part of π-π* transition is transformed into n-π* transition, and the energy required for transition is reduced. Moreover, the benzene ring itself is π conjugate, and benzene ring and conjugated bridge increases the length of the π conjugate plane, so it greatly reduces the energy of the whole system. Theoretically, the nonlinear optical properties are affected by the energy gap between HOMO and LUMO. The smaller the energy required for the transition from the ground state to the excited state, the better the NLO performance. B3 has a smaller energy gap than B1 and B2, and B3 is expected to have better NLO properties, which is also verified in later experiments.
3.3 Nonlinear optical properties
3.3.1 Z-scan analysis
Z-scanning curve is obtained by Z-scanning instrument platform (WNLO-IDZ), the nonlinear absorption of B1, B2 and B3 is studied by 532 nm laser sources with different pulse widths (15 ps and 4 ns). The nonlinear absorption of B1, B2 and B3 was studied by using ps and ns Z-scan at 532 nm. The same Z-scan measurement was carried out for pure DMF solvent, and no peak valley signal was observed, which ruled out the influence of solvent on the nonlinear absorption of the sample. The sample solutions were placed in a 2 mm thick quartz dish with the concentration of 1 mg/mL. Figure 4 shows the Z-scan curves of B1, B2 and B3 solutions at 532 nm, and the points are the experimental data and the solid lines are the theoretical fitting results. It can be seen from the Fig. 4 that B1, B2 and B3 show enhanced absorption near the focus, which are normalized single valley curves, indicating that the samples have the reverse saturable absorption (RSA) [23]. With the increase of laser energy, RSA intensity increases gradually. At the same incident energy, RSA of B1 is the smallest, B2 and B3 have the same strength of RSA, and B3 has a wider broadband RSA, which is more suitable for optical limiting materials.
Fig. 4. The open aperture Z-scan curves of B1 (a), (b) for ps, ns, B2 (c), (d) for ps, ns and B3 (e), (f) for ps, ns. The dots are the experimental data while the solid curves are the theoretical fit.
The experimental results are fitted by Z-scan theory [24], and the nonlinear absorption coefficient (β) is obtained under different strength. It can be seen from Table 2 that β increases in proportion to the incident energy, which is the obvious two-photon absorption induced excited state absorption (TPA-ESA) [25,26]. Under the ps laser source with 0.8 µJ energy, the β values of B1, B2 and B3 are 0.22 × 10−11 m/W, 0.29 × 10−11 m/W and 0.55 × 10−11 m/W respectively. However, with the increase of energy, the nonlinear absorption intensity of B2 and B3 increases significantly, and β increases to 0.49 × 10−11 m/W and 0.69 × 10−11 m/W at 1.2 µJ energy, respectively. Under the ns laser source with 11 µJ energy, the nonlinear absorption intensities of B1, B2 and B3 are very close, the β values are 9.0 × 10−11 m/W, 12.0 × 10−11 m/W and 12.5 × 10−11 m/W respectively. With the increase of laser energy, the increasing trend of nonlinear absorption intensity is consistent with that under ps laser. And at the same energy, the fitting β of B1 is the smallest, the fitting β of B3 is slightly higher than that of B2. Structurally, because B3 has one more hydroxyl group than B2. In these three compounds, there is not only charge transfer, but also the obvious π-π* transition. When the hydroxyl group in B3 compound is connected with the benzene ring, part of the π-π* transition is transformed into n-π* transition, which reduces the energy required for the transition and makes the molecule have stronger RSA. So, it can be concluded that although charge transfer and π-π* transition result in broadband RSA, the π-π* transition in the molecule plays a decisive role in the final broadband RSA of the material. This is because the alkyne bond and carbon nitrogen double bond as conjugation bridges not only improve the conjugation length of the molecule, but also keep pyrene and other groups on the π conjugated surface in the same plane as much as possible, and greatly increase the intramolecular charge transfer, effectively improving the RSA. Finally, B2 and B3 have excellent nonlinear absorption and broadband RSA, so they can be used as potential optical limiting materials, and the hydroxyl active groups in B3 molecule greatly expand its application range.
Table 2. Third-order optical nonlinear absorptive coefficients (β) of B1, B2 and B3 extracted from open-aperture ps, ns Z-scan experiments.
3.3.2 Optical limiting research
The light limiting curve was measured by the light limiting instrument platform (NLO-IOL), and the laser source is the same as that used in Z-scan test. The sample was dissolved in DMF solvent at the concentration of 1 mg/mL. Quartz dishes (2 mm thick) were used as test containers.
The optical limiting properties of B1, B2 and B3 were studied by ns and ps laser sources. The optical limiting test curve is shown in Fig. 5. The optical limiting threshold (Fth) is defined as the incident energy flow when the nonlinear transmittance of the material is reduced to half of the original value. It can be seen from Fig. 5 that when the incident energy flow increases gradually, the linear transmittance of B1, B2 and B3 decreases in varying degrees. Under ns laser source, the transmittance of B1, B2 and B3 begins to decrease, and the corresponding incident energy is 0.41 J/cm2, 0.33 J/cm2 and 0.17 J/cm2. The attenuation trend of transmittance of B1, B2 and B3 under ps laser source is similar to that under ns laser source. In short, B1, B2 and B3 have certain optical limiting ability, and B2 and B3 begin to limit the passage of their laser while contacting smaller incident energy, which has a better protective effect in optical protection. Under the action of ns laser source, the optical limiting thresholds of B1, B2 and B3 are 3.3 J/cm2, 2.6 J/cm2 and 2.6 J/cm2, respectively, and the optical limiting thresholds of B2 and B3 are the same. Under the action of ps laser source, the decreasing trend of incident energy flow of B1, B2 and B3 under ps laser source is consistent with that under ns laser source. But, under the same incident fluence, the output fluence of B3 is less than B2, indicating that B3 has better optical limiting ability. This is mainly due to the stronger of the anti-saturation absorption capacity of B3 than B2. Besides, in the quantum chemistry calculation, due to structural reasons, B3 has smaller energy gap, which is consistent with the results of Z-scan. Therefore, B3 has good optical limiting performance under both ps laser source and ns laser source, which has a certain potential in the field of optical limiting.
3.3.3 Harmonic analysis
The third harmonic signal was measured by harmonic testing platform (WNXB). Laser light source (1550 nm, 100 fs, 6 MHz) was selected as the test light source to collect 300-900 nm signals. A small amount of solid powder compound was taken as test samples, and silica (SiO2) was used as performance reference to study the harmonic signals of B1, B2 and B3 [2, 27, 28]. The harmonic test collected the harmonic signal at each point of the sample plane. Finally, the distribution diagram of harmonic signal intensity of the sample was obtained, as shown in Fig. 6. (f, g, h). Figure 6 (a, b) show the highest signal intensity in the sample test process and schematic diagram of third harmonic generation (THG) process. Figure 6(c, d, e) are the physical images of B1, B2 and B3 under the microscope. It can be seen from Fig. 6 that B1, B2 and B3 have a strong sharp peak at 517 nm, which corresponds to one third of the incident light at 1550 nm, showing THG characteristics. THG is one kind of nonlinear optical phenomena. The generation of harmonic signal may be due to the anisotropy of the material, which leads to high second-order nonlinear polarizability (χ(2)) and higher third-order nonlinear polarizability (χ(3)) [29,30]. The third harmonic generation process can be described as the annihilation of three fundamental incident photons in the nonlinear material and the generation of a new frequency doubling photon, as shown in Fig. 6(b). And the third harmonic generation process does not have energy transition, which is very friendly to the test sample. At the same laser intensity, the THG intensity of B1, B2 and B3 is 89, 181 and 215 times that of SiO2. On the whole, B1, B2 and B3 are all nonlinear materials with good THG properties. The THG properties of B1, B2 and B3 can be used as frequency conversion and modulation materials in the field of optical communication and signal processing.
Fig. 6. Nonlinear spectra of SiO2, B1, B2 and B3 under excitation wavelength of 1550 nm (a). Schematic diagram of THG generation process (b). The physical map of B1 (c), B2 (d) and B3 (e) and the THG intensity mapping of B1 (f), B2 (g) and B3 (h).
4. Conclusion
In summary, three tetraphenylvinyl compounds (B1, B2 and B3) were designed and synthesized. B2 and B3 have a longer D-π-A structure, which not only improves the intermolecular charge transfer ability, but also forms a larger π conjugate planes in the structure, so they exhibit good nonlinear optical properties. Under ps laser source, B2 and B3 have the large the nonlinear absorption coefficient and good optical limiting performance, but B3 has better optical limiting ability than B2. Moreover, B2 and B3 have pure third harmonic signals. The nonlinear optical properties of these compounds show that they can be used in the field of laser protection and optical signal conversion.
Funding
National Natural Science Foundation of China (21571159, 21671178, U1704256); Zhongyuan Science and Technology Innovation Leading Talents (214200510017); Basic and Frontier Technology Research Program of Henan Province (162300410033).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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