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Synthesis of chromophores with ultrahigh electro-optic activity: Rational combination of the bridge, donor and acceptor groups
Dyes and Pigments 136 (2017) 182e190
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Synthesis of chromophores with ultrahigh electro-optic activity: Rational combination of the bridge, donor and acceptor groups Fenggang Liu a, b, Hongyan Xiao a, Huajun Xu a, b, Shuhui Bo a, **, Chaolei Hu a, b, Yanling He a, b, Jialei Liu a, Zhen Zhen a, Xinhou Liu a, Ling Qiu a, * a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China University of Chinese Academy of Sciences, Beijing 100043, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 July 2016 Received in revised form 16 August 2016 Accepted 22 August 2016 Available online 24 August 2016
A series of high performance chromophores P1eP3, based on N-(4-dibutylaminophenyl) tetrahydroquinolinyl or julolidinyl donors, and phenyl-trifluoromethyl-tricyanofuran acceptors linked together via p-conjugation through thiophene or vinylene moieties as the bridges, have been designed and synthesized. Density functional theory (DFT) was used to calculate the HOMO-LUMO energy gaps and first-order hyperpolarizability (b) of these chromophores. Chromophores P1 and P3 had good thermal stabilities with glass transition temperature higher than 110 C. Most importantly, the high molecular hyperpolarizability of these chromophores can be effectively translated into large electro-optic (EO) coefficients (r33) in poled polymers with rational chromophore designs. In electro-optic activities, the poled films of P1/APC, P2/APC and P3/APC afforded r33 values of 223, 283 and 278 pmV1, respectively, at 1310 nm at the doping concentration of 25 wt%, which showed significant enhancement over the similar EO polymer systems previously reported. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Nonlinear optical materials Chromophore Second-order Electro-optic Bridge Acceptor
1. Introduction In the past three decades, organic and polymeric electro-optic (EO) materials which displaying nonlinear optical (NLO) activities are extensively studied because of their potential applications in the field of optical switches, high-speed and broadband information technology, and sensing technologies [1e5]. In earlier research, the majority of EO materials were inorganic second-order NLO crystals, including LiNbO3 and GaAs [6]. However, electro-optic activity (r33) of them remained below the 30 p.m./V value [7], their lower EO coefficient, higher dielectric constant and half-wave voltage, make them difficult to meet the requirements of electrooptic modulator with large area [8]. Compared with inorganic materials, organic materials have the potential advantages in faster response time, ease of processing, higher nonlinear optical activity [9]. Currently, the most commonly used materials for polymeric EO devices are based on poled
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Bo), [email protected] (L. Qiu). http://dx.doi.org/10.1016/j.dyepig.2016.08.048 0143-7208/© 2016 Elsevier Ltd. All rights reserved.
polymers with r33 values around 50e150 p.m. V1 at wavelengths of 1.31 or 1.55 mm [10,11]. In these materials, dipolar nonlinear optical (NLO) chromophores have been doped or incorporated at a level of ca. 20e25 wt % to reach their maximum r33 values [12,13]. EO coefficients greater than 300 p.m. V1 have been achieved through rational chromophore design and supra-molecular engineering like molecular self-assembly and binary chromophorecontaining dendrimer glasses and polymers [14,15]. However, compared to the simple guest-host EO polymers, these EO material systems are often more complex, the type of poled guest-host polymeric material in which an EO chromophore is dispersed in a polymer matrix is the most highly studied material in this field [16]. Generally, an organic NLO material is composed of push-pull organic chromophores, in which a p-conjugated bridge is endcapped by a donor and an acceptor (D-p-A) [17]. In order to achieve large EO activities, much effort has been made to design and synthesize novel NLO chromophores, seeking to achieve large r33 [18,19]. Rational design and synthesis of dipolar NLO chromophores with high hyperpolarizability (b) and robust thermal stability still represents one of the most critical challenges, optimization of the p-conjugated bridge, electron-donor and electron-acceptor characteristics of the substituents is needed [20]. In addition,
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introducing a huge steric hindrance group to isolate chromophores is the most popular and easy way to attenuate the dipole-dipole interactions of chromophores, significantly improved the poling efficiency and thus the resulting EO coefficient (r33)of materials [21]. Most of the high-performance organic EO materials developed so far generally include a primary class of high first-order hyperpolarizability (b) chromophores containing aryl-amine type donors, the tricyanovinyldihydrofuran (TCF) or CF3-TCF acceptors, and ring-locked tetraene (CLD) or 2,5-divinylthienyl (FTC) bridges [1]. There remains strong continued interest in improving the properties of these materials as well as the synthetic strategies for their preparation. Further enhancement of NLO activity of such structures can be achieved by fine-tuning the strength of bridge, donor and acceptor groups as well as considering the additional molecular characteristics of chromophores [22]. In this regard, we have designed and synthesized a series of high performance chromophores P1eP3 with strong CF3-TCF fixed as an acceptor, but with different electron donors, including N-(4dibutylaminophenyl) tetrahydroquinolinyl and julolidinyl groups and different bridge including thiophene and vinylene bridges (Chart 1). The ring-fused aminophenyl structures in tetrahydroquinolinyl and julolidinyl donors facilitate the overlap of the porbital of the amino atom with the phenyl ring thus providing a good mechanism to increase the electron-donating strength [23]. Meanwhile, the donors in P1eP3 contain two electron rich heteroatoms (nitrogen or oxygen) thus has strong electron-donating ability. We choose thiophene or vinylene moieties as the bridge rather than CLD bridge, since previously study showed that chromophores using julolidinyl as donor can show large r33 values with TCF acceptor. However, chromophore based on julolidinyl-based donor, CLD-type bridge and stronger CF3-TCF acceptor showed much lower b and r33 than the one with the same donor and CLD bridge but with TCF acceptor [24,25]. The N-(4-dibutylaminophenyl) substituent located on the nitrogen atom in chromophore P3 and the side thiophene ring group in chromophore P2 both shows dihedral angle of about 90 with average plane of the chromophore. This large dihedral angle caused large steric hindrance that suppressed aggregations among molecules. Meanwhile, the steric hindrance and tension of the rigid structure from the julolidinyl-based donor in chromophore P1 and P2 can also suppress aggregations [26,27]. All these modifications of chromophores can increase the b as well as reduce dipole-dipole interactions so as to translate their mb values into bulk EO performance more effectively. The UVeVis, solvatochromic behavior, DFT quantum mechanical calculations, thermal stabilities and EO activities of these chromophores were systematically studied and compared to understand their structure-property relationships. 2. Experimental 2.1. Materials and instrument All chemicals are commercially available and are used without further purification unless otherwise stated. N, N-dimethylformamide (DMF), Phosphorusoxychloride (POCl3), tetrahydrofuran (THF) and ether were distilled over calcium hydride and stored over molecular sieves (pore size 3 Å). Compound 2a was prepared according to the literature [21]. Compound 2d was prepared according to the literature [28]. Compound 3a, 4a and 4b were prepared according to the literature procedures, respectively [29,30]. Compound 4e was prepared according to the literature [31]. TLC analyses were carried out on 0.25 mm thick precoated silica plates and spots were visualized under UV light.
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Chromatography on silica gel was carried out on Kieselgel (200e300 mesh). 1 HNMR spectra were determined on an Advance Bruker 400 M (400 MHz) NMR spectrometer (tetramethylsilane as internalreference). The MS spectra were obtained on MALDI-TOF (Matrix Assisted Laser Desorption/Ionization of Flight) on BIFLEXIII (Broker Inc.,) spectrometer. The UVeVis spectra were performed on Cary 5000 photo spectrometer. The Tg or melt points was determined by TA DSC Q10 with a heating rate of 10 C min1 under the protection of nitrogen. 2.2. Syntheses 2.2.1. Synthesis of 1-(4-(dibutylamino)phenyl)-2,2,4,7-tetramethyl1,2,3,4-tetrahydroquinoline-6-carbaldehyde (compound 3d) DMF (1.71 g, 23.40 mmol) was added to freshly distilled POCl3 (3.59 g, 23.40 mmol) under an atmosphere of N2 nitrogen at 0 C, and the resultant solution was stirred until its complete conversion into a glassy solid. After the addition of 2d (4.60 g, 11.70 mmol) in DMF (20 mL) dropwise, the mixture was stirred at room temperature overnight, and then poured into a saturation aqueous solution of sodium bicarbonate (300 mL). After 2 hour stirring at room temperature, the mixture extracted with chloroform (5 30 mL), and the organic fractions were collected and dried over anhydrous MgSO4. The crude product was purified through a silica gel chromatography eluting with (Acetone: Hexane ¼ 1:5) to afford a yellow oil in 77.6% yield (3.82 g, 9.08mmol). MS (EI) (Mþ, C28H40N2O): calcd: 420.63; found: 420.43. 1H NMR (400 MHz, acetone-d6) d 9.80 (s, 1H), 7.50 (s, 1H), 6.82 (m, 2H), 6.66 (m, 2H), 5.72 (s, 1H), 3.27e3.23 (m, 4H), 2.96 (m, 1H), 2.18 (s, 3H), 1.65e1.44 (m, 5H), 1.37e1.22 (m, 8H), 1.19 (s, 3H), 0.97 (s, 3H), 0.87 (t, J ¼ 7.4 Hz, 6H). 13 C NMR (101 MHz, acetone-d6) d 190.49, 151.92, 148.72, 140.21, 133.68, 132.68, 132.26, 130.11, 124.48, 117.22, 113.48, 113.20, 56.36, 51.70, 47.05, 31.26, 27.93, 26.80, 21.27, 20.96, 20.26, 14.67. Anal. Calcd (%) for C28H40N2O: C, 79.95; H, 9.59; N, 6.66; found: C, 80.04; H, 9.52; N, 6.73. 2.2.2. Synthesis of (E)-N,N-dibutyl-4-(2,2,4,7-tetramethyl-6-(2(thiophen-2-yl)vinyl)-3,4-dihydroquinolin-1(2H)-yl)aniline (compound 4d) NaH (0.68 g, 23.80 mmol) was added to a stirred solution of compound 3d (1.00 g, 2.38 mmol) and 3e (2.09 g, 4.76 mmol) in THF (100 mL) under nitrogen. The solution was stirred for 24 h and then poured into water. The organic phase was extracted by AcOEt, washed with brine and dried over MgSO4. After removal of the solvent under reduced pressure, the crude product was purified by silica chromatography, eluting with (Acetone: Hexane ¼ 1:20) to give compound 4d as yellow oil in 86.1% yield (1.03 g, 2.05 mmol). MS (MALDI-TOF) (Mþ, C33H44N2S): calcd: 500.78; found: 500.36. 1 H NMR (400 MHz, CDCl3) d 7.43e7.32 (m, 1H), 7.14 (d, J ¼ 15.2 Hz, 1H), 7.01 (d, J ¼ 5.1 Hz, 1H), 6.98 (d, J ¼ 3.2 Hz, 1H), 6.93e6.86 (m, 3H), 6.85 (m, 1H), 6.58 (d, J ¼ 8.3 Hz, 2H), 5.84 (d, J ¼ 15.2 Hz, 1H), 3.25e3.19 (m, 4H), 3.03e2.96 (m, 1H), 2.08 (s, 3H), 1.64e1.46 (m, 5H), 1.40e1.27 (m, 8H), 1.20 (s, 3H), 0.99 (s, 3H), 0.92 (t, J ¼ 7.2 Hz, 6H). 13C NMR (101 MHz, CDCl3) d 147.00, 146.41, 144.60, 134.25, 133.15, 131.23, 128.63, 128.51, 127.39, 126.87, 126.00, 124.10, 123.39, 122.63, 117.82, 115.64, 112.19, 111.83, 54.32, 50.75, 46.71, 30.88, 29.43, 27.13, 25.63, 20.62, 20.35, 19.79, 13.97. Anal. Calcd (%) for C33H44N2S: C, 79.15; H, 8.86; N, 5.59; found: C, 79.08; H, 8.91; N, 5.62. 2.2.3. Synthesis of (E)-5-(2-(1-(4-(dibutylamino) phenyl)-2,2,4,7tetramethyl-1,2,3,4-tetrahydroquinolin-6-yl)vinyl)thiophene-2carbaldehyde (compound 5d) Under a N2 atmosphere, 4d (0.59 g, 2.00 mmol) was dissolved in
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150 mL freshly distilled THF and cooled to 78 C. Approximately 2 equivalents of n-BuLi in hexanes (16 mL, 4 mmol) was added drop wise over 20 min. Reaction continued at 78 C for 1 h at which time DMF (0.29 g, 4.00 mmol) was added over 1 min. The reaction was allowed to reach RT while the solution stirred for 1h. The organic phase was extracted by AcOEt, washed with brine and dried over MgSO4. After removal of the solvent under reduced pressure, the crude product was purified by silica chromatography, eluting with (Acetone: Hexane ¼ 1:5) to give compound 5d as a red oil with 76.9% yield (0.50 g, 1.54 mmol). MS (MALDI-TOF) (Mþ, C34H44N2OS): calcd: 528.79; found: 528.20. 1H NMR (400 MHz, CDCl3) d 9.80 (s, 1H), 7.62 (d, J ¼ 3.9 Hz, 1H), 7.46 (s, 1H), 7.33 (d, J ¼ 15.8 Hz, 1H), 7.04 (d, J ¼ 3.9 Hz, 1H), 6.98e6.86 (m, 3H), 6.65 (d, J ¼ 8.6 Hz, 2H), 5.87 (s, 1H), 3.33e3.25 (m, 4H), 3.16e2.98 (m, 1H), 2.17 (s, 3H), 1.71e1.55 (m, 5H), 1.49e1.41 (m, 4H), 1.41e1.35 (m, 4H), 1.28 (s, 3H), 1.07 (s, 3H), 0.99 (t, J ¼ 7.3 Hz, 6H). 13C NMR (101 MHz, CDCl3) d 182.05, 154.95, 147.44, 147.02, 139.65, 137.54, 135.35, 132.80, 131.43, 131.27, 129.66, 124.54, 124.07, 123.73, 121.62, 115.93, 115.65, 112.08, 111.73, 54.66, 50.74, 46.33, 30.63, 29.34, 26.99, 25.74, 20.47, 20.28, 19.63, 13.94. Anal. Calcd (%) for C34H44N2OS: C, 77.23; H, 8.39; N, 5.30; found: C, 77.25; H, 8.41; N, 5.26. 2.2.4. Synthesis of 2-(4-((E)-2-(5-((E)-2-(8-((6-chlorohexyl)oxy)1,1,7,7-tetramethyl-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9yl)vinyl)thiophen-2-yl)vinyl)-3-cyano-5-phenyl-5-(trifluoromethyl) furan-2(5H)-ylidene)malononitrile (chromophore P1) Compound 4a (0.20 g, 0.40 mmol) and compound 4e (0.14 g, 0.44 mmol) were mixed with anhydrous ethanol (5 mL). The mixture was allowed to stir at 65 C for 2h. The solvent was removed under vacuum and the residual mixture was purified by flash chromatography on silica gel using ethyl acetate and hexane (v/v, 1: 4) to give chromophore P1 as a deep green solid in 76.8% yield (0.24 g, 0.31 mmol). HRMS (ESI) (Mþ, C45H44ClF3N4O2S): calcd: 797.2904; found: 797.2891. 1H NMR (400 MHz, acetone-d6) d 7.84 (d, J ¼ 15.2 Hz, 1H), 7.79e7.73 (m, 2H), 7.67e7.61 (m, 4H), 7.51 (s, 1H), 7.43 (d, J ¼ 15.8 Hz, 1H), 7.30 (d, J ¼ 15.8 Hz, 1H), 7.21 (d, J ¼ 4.2 Hz, 1H), 6.70 (d, J ¼ 15.2 Hz, 1H), 3.87 (t, J ¼ 6.5 Hz, 2H), 3.60 (t, J ¼ 6.6 Hz, 2H), 3.35e3.29 (m, 2H), 3.28e3.22 (m, 2H), 1.93 (m, 2H), 1.83 (m, 2H), 1.77e1.69 (m, 4H), 1.67e1.53 (m, 4H), 1.42 (s, 6H), 1.28 (s, 6H). 13C NMR (101 MHz, acetone-d6) d 177.09, 169.85, 162.43, 159.65, 158.88, 146.43, 144.18, 142.48, 141.60, 138.87, 134.66, 133.75, 132.70, 131.05, 130.98, 130.59, 129.91, 129.21, 128.43, 128.20, 127.05, 124.57, 123.19, 117.57, 116.57, 115.05, 112.39, 96.30, 77.08, 63.24, 58.29, 48.28, 47.68, 45.97, 40.80, 39.94, 37.16, 33.57, 33.15, 31.43, 31.22, 30.92, 27.84, 26.68. Anal. Calcd (%) for C45H44ClF3N4O2S: C, 67.78; H, 5.56; N, 7.03; found: C, 67.81; H, 5.54; N, 7.05. 2.2.5. Synthesis of 2-(4-((1E,3Z)-4-(8-((5-chloropentyl)oxy)-1,1,7,7tetramethyl-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)-3(thiophen-2-yl)buta-1,3-dien-1-yl)-5-phenyl-5-(trifluoromethyl) furan-2(5H)-ylidene)malononitrile (chromophore P2) The procedure for Chromophore P1 was followed to prepare Chromophore P2 from 4b and 4e as a deep green solid in 71.3% yield (0.21 g, 0.28 mmol). HRMS (ESI) (Mþ, C45H44ClF3N4O2S): calcd: 797.2904; found: 797.2923. 1H NMR (400 MHz, acetone-d6) d 8.12 (d, J ¼ 13.8 Hz, 1H), 7.72 (d, J ¼ 5.1 Hz, 1H), 7.68e7.55 (m, 5H), 7.49 (s, 1H), 7.33e7.22 (m, 1H), 7.02 (d, J ¼ 3.2 Hz, 1H), 6.84 (s, 1H), 6.01 (d, J ¼ 13.8 Hz, 1H), 3.95 (t, J ¼ 6.6 Hz, 2H), 3.66 (t, J ¼ 6.6 Hz, 2H), 3.51 (m, 4H), 1.98e1.83 (m, 4H), 1.79e1.72 (m, 2H), 1.63 (m, 6H), 1.41 (s, 6H), 0.89 (s, 6H). 13C NMR (101 MHz, CDCl3) d 176.50, 161.66, 160.43, 149.17, 148.51, 136.93, 130.83, 130.72, 129.29, 128.82, 128.26, 128.05, 127.71, 127.58, 127.52, 127.24, 126.39, 125.64, 124.92, 123.36, 122.41, 121.08, 117.03, 112.47, 112.31, 111.74, 111.37, 94.96, 77.89, 54.55, 48.08, 47.41, 45.15, 38.77, 34.99, 32.43, 32.40, 31.64, 29.90,
29.34, 29.32, 29.27, 29.23, 26.77, 25.42. Anal. Calcd (%) for C45H44ClF3N4O2S: C, 67.78; H, 5.56; N, 7.03; found: C, 67.76; H, 5.55; N, 7.04. 2.2.6. Synthesis of 2-(3-cyano-4-((E)-2-(5-((E)-2-(1-(4(dibutylamino)phenyl)-2,2,4,7-tetramethyl-1,2,3,4tetrahydroquinolin-6-yl)vinyl)thiophen-2-yl)vinyl)-5-phenyl-5(trifluoromethyl)furan-2(5H)-ylidene)malononitrile (chromophore P3) The procedure for Chromophore P1 was followed to prepare Chromophore P3 from 5d and 4e as a deep green solid in 77.1% yield (0.25 g, 0.30 mmol). HRMS (ESI) (Mþ, C50H50F3N5OS): calcd: 826.3766; found: 826.3793. 1H NMR (400 MHz, acetone-d6) d 7.88 (d, J ¼ 15.2 Hz, 1H), 7.78e7.72 (m, 2H), 7.66e7.60 (m, 3H), 7.57 (d, J ¼ 4.2 Hz, 1H), 7.47 (d, J ¼ 15.9 Hz, 2H), 7.29e7.20 (m, 2H), 6.95 (t, J ¼ 7.4 Hz, 2H), 6.78 (d, J ¼ 8.7 Hz, 2H), 6.67 (d, J ¼ 15.2 Hz, 1H), 5.85 (s, 1H), 3.40e3.33 (m, 4H), 2.11 (s, 3H), 1.94 (m, 1H), 1.75e1.58 (m, 5H), 1.50e1.34 (m, 8H), 1.30 (s, 3H), 1.08 (s, 3H), 0.98 (t, J ¼ 7.3 Hz, 6H). 13C NMR (101 MHz, CDCl3) d 175.43, 168.00, 161.56, 158.96, 149.01, 147.40, 141.54, 140.24, 137.81, 137.19, 134.69, 132.54, 131.49, 131.02, 130.62, 129.87, 129.75, 129.48, 128.64, 127.65, 126.87, 126.29, 125.67, 124.90, 124.56, 121.71, 116.09, 115.54, 112.22, 111.94, 111.80, 111.23, 110.93, 95.41, 57.38, 55.42, 50.89, 46.30, 30.90, 30.65, 29.47, 27.07, 26.20, 24.50, 20.41, 19.89, 14.41, 14.03. Anal. Calcd (%) for C50H50F3N5OS: C, 72.70; H, 6.10; N, 8.48; found: C, 72.73; H, 6.12; N, 8.46. 3. Results and discussion 3.1. Synthesis and characterization of chromophores Schemes 1 and 2 show the synthetic approach for the new chromophores P1, P2 and P3 containing the julolidine and tetrahydroquinoline donors. Starting from the amine donor intermediate compounds 1a or 1d, chromophores P1eP3 were synthesized in good overall yields through simple four or five step reactions: the free hydroxyl group on the julolidine donor was protected by the alkyl group to improve the solubility through Williamson ether synthesis. The free NeH group on the tetrahydroquinoline donor was protected by the aryl group to improve stability. Treatment of compound 2d with POCl3 and DMF gave the aldehyde 3d. After introduction of the bridge by Wittig condensation, compounds 3a or 4d was prepared with a high yield. Treatment of compound 3a or 4d with n-BuLi and DMF gave the aldehydes 4a or 5d. Treatment of compound 3a with POCl3 and DMF gave an aldehyde 4b. And the final condensations with the CF3-Ph-TCF acceptor give chromophores P1, P2 and P3 as green solids. All of the chromophores were fully characterized by 1H NMR, 13C NMR, and HRMS. These chromophores possess good solubility in common organic solvents, such as dichloromethane, chloromethane and acetone. 3.2. Thermal stability Thermal properties of chromophores P1eP3 were evaluated by differential scanning calorimetry (DSC) and the results are shown in Fig. 1 and Table 1. Chromophore P2 based on julolidinyl donor and vinylene bridge appeared as highly crystalline compound with a melting point at 212.1 C. While chromophores P1 and P3 were obtained as solids showing glass transition temperature (Tg) at 120.3 and 112.9 C, respectively. They are higher than many commonly used chromophores like AJC146 and YLD124 with the same CF3-TCF acceptor and thiophene or CLD bridges [32,33]. It is very attractive that they could act as a new class of chromophores.
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Scheme 1. Chemical structures and synthetic routes for chromophores P1 and P2.
Fig. 1. DSC curves of chromophores P1, P2 and P3 with a heating rate of 10 C min-1 in nitrogen atmosphere.
Scheme 2. Chemical structure and synthetic routes for chromophore P3.
3.3. Optical properties In order to reveal the effect of different electron donors and bridges on the charge-transfer (CT) absorption properties of the chromophores, UVeVis absorption spectra of the three chromophores were measured in a series solvent with different dielectric constants as shown in Figs. 2 and 3. The UV absorption spectra of chromophores P1eP3 in dilute chloroform solution (105 M) are shown in Fig. 2. Chromophore P2 shows a lmax of 760 nm which was the smallest because of shortest conjugate length. When comparing the lmax of chromophore P3 (841 nm) with chromophore P1 (849 nm) with the same thiophene bridge, P3 is slightly blue-shifted and the two compounds exhibit similar chargetransfer band shape.
Table 1 Thermal and Optical Properties data of the Chromophores. Cmpd
lmax (nm)a
lmax (nm)b
lmax (nm)c
Tg/Tm ( C)d
P1 P2 P3
849 791 841
770 735 798
730 723 749
120.3 212.1 112.9
a b c d
lmax was measured in chloroform. lmax was measured in toluene. lmax was measured in dioxane. Tg was the glass transition temperature. d Tm was the melting point.
The UVeVis absorption spectra were also measured in a series of aprotic solvents with different polarity so that the solvatochromic behavior of each chromophore could be investigated in a wide range of dielectric environments (Fig. 3). With the solvents varying from the slightly polar dioxane to the moderately polar dichloromethane, a continuous shift of the absorption maxima to longer wavelength was observed for all the three chromophores. More
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Fig. 2. UVeVis absorption spectra of chromophores P1, P2 and P3 in chloroform.
dramatic effects were observed while investigating the absorption spectra of P1eP3 in highly polar solvents, such as acetone and acetonitrile. In comparison with a rather broad absorption band recorded for chromophores P1 and P3, chromophore P2 showed a sharp and intense cyanine-type absorption band with a dramatically narrowed bandwidth. Furthermore, a saturation behavior was found for chromophores P2 in the more polar solvents, such as dichloromethane, acetone, and acetonitrile. The absorption peak of P2 in these solvents exhibited almost no shift with increase of solvent polarity. Chromophores P1 and P3 showed a bathochromic shift of its absorption maximum initially and then reversed to a hypsochromic shift for more polar solvents of acetone and acetonitrile, namely inverted solvatochromism. These interesting solvent-dependent spectral behaviors of chromophore P2 suggested that the more dipolar chromophore P2 can be polarized quite close to the cyanine limit, or even beyond that into the zwitter-ionic regime in the most polar solvents [34]. 3.4. Theoretical calculations To understand the ground-state polarization and microscopic NLO properties of the designed chromophores, the DFT calculations were carried out at the B3LYP level by employing the split valence 6-31G basis set using the Gaussian 09 program package [35,36]. The first hyperpolarizability (b) were carried out at the cam-B3LYP level by employing the split valence 6-31G basis set using the Gaussian 09 program package [36]. All molecules were assumed to be in trans-configurations [37,38]. The HOMO-LUMO energy gap, dipole moment (m), b and mb of the chromophores obtained from DFT calculations are summarized in Table 2. The HOMO-LUMO energy gap was used to understand the charge transfer interaction occurring in a chromophore molecule [39,40]. Fig. 4 depicts the electron density distribution of the HOMO and LUMO structures. It can be seen that the density of the ground and excited state electron is asymmetry along the dipolar axis of the chromophores. The HOMO-LUMO energy gaps DE (DFT) were calculated by DFT calculations as shown in Table 2. The energy gaps between the HOMO and LUMO energy for chromophores P1eP3 were 1.883, 2.271 and 1.914eV, respectively. As DE is reduced, resulting in a bathochromic shift of lmax within the series of compounds. These results correspond with the conclusion of UVeVis spectra analysis. To get more information from the frontier orbitals, the composition of the HOMOs and LUMOs has been calculated using the
Fig. 3. UVeVis absorption spectra of chromophores P1, P2 and P3 in six kinds of aprotic solvents with varying dielectric constants.
Table 2 Summary of DFT and EO coefficients of chromophores. Cmpd
DE(DFT) (eV)a
btot (1030 esu)b
m (D)c
mb (1048 esu)
r33 (pm/V)
P1 P2 P3
1.883 2.271 1.914
811.73 332.84 918.76
21.83 21.37 25.16
17719.98 6899.09 23116.00
223 283 278
a b c
DE(DFT) was calculated from DFT calculations. btot is the first-order hyperpolarizability calculated from DFT calculations. m is the total dipole moment.
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dibutylaminophenyl) substituent located on the nitrogen atom made a large dihedral angle of 85.51 with average plane of the chromophore. These large dihedral angles caused large steric hindrance that suppressed aggregations among molecules. As reported earlier, the b value has a close relationship with the strength of the donor and acceptor end groups and on the nature and length of the bridge, substituents, steric hindrance and intramolecular charge-transfer and so on [42,43]. The b value of chromophore P1 and P3 were larger than that of chromophore P2 due to narrower energy gap between HOMO and LUMO. The lmax of chromophore P1 and P3 were larger than that of chromophore P2, so the trend of increasing lmax is in good agreement with the trend of increasing b value. With additional electron-rich N-(4dibutylaminophenyl) substituent into the donor moiety, the chromophore P3 show larger b and mb than that of chromophore P1. 3.5. Electro-optic performance
Fig. 4. Frontier molecular orbitals HOMO and LUMO of chromophores P1, P2 and P3.
Multiwfn program [41]. As shown in Table 3, the whole chromophore molecule was segmented as donor, p-bridge, and acceptor. At the same time, the attribution of the thiophene (T) located in bridge moiety of chromophore P2 and the N-(4-dibutylaminophenyl) substituent (N) located in donor moiety of chromophore P3 was listed separately. For the three chromophores, the LUMO was largely stabilized by the contributions from acceptor (45.27%e 48.40%) and the p-bridge (25.62%e38.38%), while the HOMO was largely stabilized by the contributions from donors (52.43%e 59.35%). For the chromophore P2, when compare LUMO to HOMO level, the contribution of thiophene ring decreased to 0.23% from 2.38%. The comparison of HOMO and LUMO electron distribution in the thiophene ring indicated the easy delocalization of electrons in the thiophene ring. For the chromophore P3, when compare LUMO to HOMO level, the contribution of N-(4-dimethylaminophenyl) substituent decreased to 0.46% from 4.23%. The comparison of HOMO and LUMO electron distribution in the N-(4dibutylaminophenyl) substituent indicated the easy delocalization of electrons in the N-(4-dibutylaminophenyl) substituent. Consequently, it can be treated as another donor, which efficiently enhances the electron density of the conjugated system and increases the polarizability of chromophore P3 [26]. The geometrically optimized structures of the molecules were investigated by calculating the dihedral angles of the donor moieties as shown in Fig. 5. In chromophore P1, the alkyl chain substituent located on the oxygen atom of the donor made a large dihedral angle of 73.80 with average plane of the chromophore. As for chromophore P2, the lateral group thiophene and the alkyl chain made dihedral angle of 84.63 and 77.14 with average plane of the chromophore, respectively. As for chromophore P3, the N-(4-
Table 3 The molecular orbital composition (%) in the ground state for chromophores P1eP3. Cmpd
P1
P2
P3
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
donor p bridge acceptor T N
59.35% 24.32% 16.33%
16.58% 38.15% 45.27%
56.69% 10.11% 30.82% 2.38%
35.46% 15.91% 48.40% 0.23%
52.43% 25.62% 17.72%
14.82% 38.38% 46.34%
4.23%
0.46%
The molecular orbital composition was calculated using the Multiwfn program with RoseSchuit (SCPA) partition.
In order to investigate the translation of the microscopic hyperpolarizability into macroscopic EO response, the polymer films doped with 25 wt% chromophores into amorphous polycarbonate (APC) were prepared using dibromomethane as solvent. The resulting solutions were filtered through a 0.2-mm PTFE filter and spin-coated onto indium tin oxide (ITO) glass substrates. Films of doped polymers were baked at 80 C in a vacuum oven overnight. The corona poling process was carried out at a temperature of 10 C above the glass transition temperature (Tg) of the polymer. The r33 values of poled films were measured by Teng-Man simple reflection method at a wavelength of 1310 nm using carefully selected thin ITO electrode with low reflectivity and good transparency in order to minimize the contribution from multiple reflections [35,44]. The measured r33 values depend on the chromophore number density (N), b value, and poling efficiency, described by the cos3 (q) order parameter, as indicated by Ref. [45].
E. D r33 z2Nbf cos3 q n4 where the f term describes electric-field factors and n is the refractive index of the film, both of which remain relatively constant for related chromophores at similar loading densities. cos3 (q) is the acentric order parameter. q is the angle between the permanent dipole moment of chromophores and the applied electric field. At low concentration, the electro-optic activity increased with chromophore density, dipole moment and the strength of electric poling field. However, when the concentrations of chromophores increased to a certain extent, the N and cos3 (q) are no longer independent factor. Then,
D E h i cos3 ðqÞ ¼ ðmF=5kTÞ 1 L2 ðW=kTÞ where k is the Boltzmann constant and T is the Kelvin (poling) temperature. F ¼ [f (0) EP] where Ep is the electric poling field. L is the Langevin function, which is a function of W/kT, the ratio of intermolecular electrostatic energy (W) to the thermal energy (kT). L is related to electrostatic interactions between molecules. This relationship succeeds in qualitatively predicting the important trends involving electro-optic activity although not the quantitative values of electro-optic coefficients. When the intermolecular electrostatic interactions are neglected, the electro-optic coefficient (r33) should increase linearly with chromophore density, dipole moment, first hyperpolarizability and the strength of electric poling field. But chromophores with large dipole moment generate intermolecular static electric field dipole-dipole interaction, which
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Fig. 5. Optimized structures of chromophores P1, P2 and P3.
Chart 1. Chemical structures for chromophores P1, P2 and P3.
leads to the unfavorable antiparallel packing of chromophores. So the number of truly oriented chromophore (N) is small. In molecular optimization, introducing the huge steric hindrance group to isolate chromophores is the most popular and easy way to attenuate the dipole-dipole interactions of chromophores [46,47]. The poled films of P1/APC, P2/APC and P3/APC afforded r33 values of 223, 283 and 278 pmV1 at 1310 nm, respectively. It should be mentioned, the r33 values of P1/APC, P2/APC and P3/APC are within the highest ones ever reported for simple guestehost EO polymers when the polymer films doped with 25 wt% chromophores. The ring-fused aminophenyl structures in tetrahydroquinolinyl and julolidinyl donors facilitate the overlap of the porbital of the amino atom with the phenyl ring thus providing a good mechanism to increase the electron-donating strength. The donors strength of tetrahydroquinolinyl and julolidinyl group are much stronger than often used 4-(dialkylamino)phenyl groups. The increases donor strength of the chromophores significantly increases their macroscopic EO activities. The stronger the electrondonating power of the donor leads to higher lmax and r33 value [32]. The introduction of some isolation groups into the chromophore moieties to further control the shape of the chromophore could be an efficient approach to minimize interactions between the chromophores [8]. Except for the strong donor and strong acceptor, the isolated groups in the chromophores play an important role in the ultrahigh electro-optic activities. The N-(4-dibutylaminophenyl) substituent located on the nitrogen atom in chromophore P3 and the side thiophene ring group in chromophore P2 both shows dihedral angle of about 90 with average plane of the chromophore. This large dihedral angle caused large steric hindrance that suppressed aggregations among molecules. Meanwhile, the steric hindrance and tension of the rigid structure from the julolidinylbased donor in chromophore P1 and P2 can also suppress aggregations [48,49]. All these modifications of chromophores can increase the b as well as reduce dipoleedipole interactions so as to translate their mb values into bulk EO performance more effectively. The poled films of P2/APC and P3/APC afforded r33 values of 283 and 278 pmV1 at 1310 nm, respectively. Although chromophore P2 has much smaller b value than that of chromophore P3, the r33 value is even larger due to a better isolation effect. The dipoledipole interactions are reduced by isolating chromophores, through addition of steric bulk to approximate a spherical shape [50]. As for chromophore P2, it may be revealed by the optimized configurations (Fig. 5) that the side thiophene ring group was perpendicular to the direction of the dipole moment of the
F. Liu et al. / Dyes and Pigments 136 (2017) 182e190
chromophore which could act as the isolation group to suppress the possible aggregation. The larger r33values indicates that film-P2/ APC has weaker inter chromophore electrostatic interactions than film-P3/APC in this density.
[17]
[18]
4. Conclusion [19]
A series of high performance chromophores based on two different types of electron donors, including N-(4dibutylaminophenyl) tetrahydroquinolinyl and julolidinyl groups, with the same CF3-TCF acceptors and different bridge including thiophene and vinylene bridge, has been synthesized and systematically investigated. The high molecular hyperpolarizability of these chromophores can be effectively translated into electro-optic (EO) coefficients (r33) in poled polymers. In electro-optic activities, the poled films of P1/APC, P2/APC and P3/APC afforded r33 values of 223, 283 and 278 pmV1, respectively, at 1310 nm at the doping concentration of 25 wt%, which showed significant enhancement over the similar EO polymer systems previously reported. Ultrahigh r33 values, high thermal stability, together with good solubility, suggested that these chromophores could be useful for next generation EO materials and devices.
[20]
[21]
[22]
[23]
[24]
Acknowledgments [25]
We are grateful to the National Natural Science Foundation of China (No. 21473227, No. 61101054, No. 51503215 and No. 21003143) and China Scholarship Council for the financial support.
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Synthesis of chromophores with ultrahigh electro-optic activity: Rational combination of the bridge, donor and acceptor groups Fenggang Liu a, b, Hongyan Xiao a, Huajun Xu a, b, Shuhui Bo a, **, Chaolei Hu a, b, Yanling He a, b, Jialei Liu a, Zhen Zhen a, Xinhou Liu a, Ling Qiu a, * a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China University of Chinese Academy of Sciences, Beijing 100043, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 July 2016 Received in revised form 16 August 2016 Accepted 22 August 2016 Available online 24 August 2016
A series of high performance chromophores P1eP3, based on N-(4-dibutylaminophenyl) tetrahydroquinolinyl or julolidinyl donors, and phenyl-trifluoromethyl-tricyanofuran acceptors linked together via p-conjugation through thiophene or vinylene moieties as the bridges, have been designed and synthesized. Density functional theory (DFT) was used to calculate the HOMO-LUMO energy gaps and first-order hyperpolarizability (b) of these chromophores. Chromophores P1 and P3 had good thermal stabilities with glass transition temperature higher than 110 C. Most importantly, the high molecular hyperpolarizability of these chromophores can be effectively translated into large electro-optic (EO) coefficients (r33) in poled polymers with rational chromophore designs. In electro-optic activities, the poled films of P1/APC, P2/APC and P3/APC afforded r33 values of 223, 283 and 278 pmV1, respectively, at 1310 nm at the doping concentration of 25 wt%, which showed significant enhancement over the similar EO polymer systems previously reported. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Nonlinear optical materials Chromophore Second-order Electro-optic Bridge Acceptor
1. Introduction In the past three decades, organic and polymeric electro-optic (EO) materials which displaying nonlinear optical (NLO) activities are extensively studied because of their potential applications in the field of optical switches, high-speed and broadband information technology, and sensing technologies [1e5]. In earlier research, the majority of EO materials were inorganic second-order NLO crystals, including LiNbO3 and GaAs [6]. However, electro-optic activity (r33) of them remained below the 30 p.m./V value [7], their lower EO coefficient, higher dielectric constant and half-wave voltage, make them difficult to meet the requirements of electrooptic modulator with large area [8]. Compared with inorganic materials, organic materials have the potential advantages in faster response time, ease of processing, higher nonlinear optical activity [9]. Currently, the most commonly used materials for polymeric EO devices are based on poled
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Bo), [email protected] (L. Qiu). http://dx.doi.org/10.1016/j.dyepig.2016.08.048 0143-7208/© 2016 Elsevier Ltd. All rights reserved.
polymers with r33 values around 50e150 p.m. V1 at wavelengths of 1.31 or 1.55 mm [10,11]. In these materials, dipolar nonlinear optical (NLO) chromophores have been doped or incorporated at a level of ca. 20e25 wt % to reach their maximum r33 values [12,13]. EO coefficients greater than 300 p.m. V1 have been achieved through rational chromophore design and supra-molecular engineering like molecular self-assembly and binary chromophorecontaining dendrimer glasses and polymers [14,15]. However, compared to the simple guest-host EO polymers, these EO material systems are often more complex, the type of poled guest-host polymeric material in which an EO chromophore is dispersed in a polymer matrix is the most highly studied material in this field [16]. Generally, an organic NLO material is composed of push-pull organic chromophores, in which a p-conjugated bridge is endcapped by a donor and an acceptor (D-p-A) [17]. In order to achieve large EO activities, much effort has been made to design and synthesize novel NLO chromophores, seeking to achieve large r33 [18,19]. Rational design and synthesis of dipolar NLO chromophores with high hyperpolarizability (b) and robust thermal stability still represents one of the most critical challenges, optimization of the p-conjugated bridge, electron-donor and electron-acceptor characteristics of the substituents is needed [20]. In addition,
F. Liu et al. / Dyes and Pigments 136 (2017) 182e190
introducing a huge steric hindrance group to isolate chromophores is the most popular and easy way to attenuate the dipole-dipole interactions of chromophores, significantly improved the poling efficiency and thus the resulting EO coefficient (r33)of materials [21]. Most of the high-performance organic EO materials developed so far generally include a primary class of high first-order hyperpolarizability (b) chromophores containing aryl-amine type donors, the tricyanovinyldihydrofuran (TCF) or CF3-TCF acceptors, and ring-locked tetraene (CLD) or 2,5-divinylthienyl (FTC) bridges [1]. There remains strong continued interest in improving the properties of these materials as well as the synthetic strategies for their preparation. Further enhancement of NLO activity of such structures can be achieved by fine-tuning the strength of bridge, donor and acceptor groups as well as considering the additional molecular characteristics of chromophores [22]. In this regard, we have designed and synthesized a series of high performance chromophores P1eP3 with strong CF3-TCF fixed as an acceptor, but with different electron donors, including N-(4dibutylaminophenyl) tetrahydroquinolinyl and julolidinyl groups and different bridge including thiophene and vinylene bridges (Chart 1). The ring-fused aminophenyl structures in tetrahydroquinolinyl and julolidinyl donors facilitate the overlap of the porbital of the amino atom with the phenyl ring thus providing a good mechanism to increase the electron-donating strength [23]. Meanwhile, the donors in P1eP3 contain two electron rich heteroatoms (nitrogen or oxygen) thus has strong electron-donating ability. We choose thiophene or vinylene moieties as the bridge rather than CLD bridge, since previously study showed that chromophores using julolidinyl as donor can show large r33 values with TCF acceptor. However, chromophore based on julolidinyl-based donor, CLD-type bridge and stronger CF3-TCF acceptor showed much lower b and r33 than the one with the same donor and CLD bridge but with TCF acceptor [24,25]. The N-(4-dibutylaminophenyl) substituent located on the nitrogen atom in chromophore P3 and the side thiophene ring group in chromophore P2 both shows dihedral angle of about 90 with average plane of the chromophore. This large dihedral angle caused large steric hindrance that suppressed aggregations among molecules. Meanwhile, the steric hindrance and tension of the rigid structure from the julolidinyl-based donor in chromophore P1 and P2 can also suppress aggregations [26,27]. All these modifications of chromophores can increase the b as well as reduce dipole-dipole interactions so as to translate their mb values into bulk EO performance more effectively. The UVeVis, solvatochromic behavior, DFT quantum mechanical calculations, thermal stabilities and EO activities of these chromophores were systematically studied and compared to understand their structure-property relationships. 2. Experimental 2.1. Materials and instrument All chemicals are commercially available and are used without further purification unless otherwise stated. N, N-dimethylformamide (DMF), Phosphorusoxychloride (POCl3), tetrahydrofuran (THF) and ether were distilled over calcium hydride and stored over molecular sieves (pore size 3 Å). Compound 2a was prepared according to the literature [21]. Compound 2d was prepared according to the literature [28]. Compound 3a, 4a and 4b were prepared according to the literature procedures, respectively [29,30]. Compound 4e was prepared according to the literature [31]. TLC analyses were carried out on 0.25 mm thick precoated silica plates and spots were visualized under UV light.
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Chromatography on silica gel was carried out on Kieselgel (200e300 mesh). 1 HNMR spectra were determined on an Advance Bruker 400 M (400 MHz) NMR spectrometer (tetramethylsilane as internalreference). The MS spectra were obtained on MALDI-TOF (Matrix Assisted Laser Desorption/Ionization of Flight) on BIFLEXIII (Broker Inc.,) spectrometer. The UVeVis spectra were performed on Cary 5000 photo spectrometer. The Tg or melt points was determined by TA DSC Q10 with a heating rate of 10 C min1 under the protection of nitrogen. 2.2. Syntheses 2.2.1. Synthesis of 1-(4-(dibutylamino)phenyl)-2,2,4,7-tetramethyl1,2,3,4-tetrahydroquinoline-6-carbaldehyde (compound 3d) DMF (1.71 g, 23.40 mmol) was added to freshly distilled POCl3 (3.59 g, 23.40 mmol) under an atmosphere of N2 nitrogen at 0 C, and the resultant solution was stirred until its complete conversion into a glassy solid. After the addition of 2d (4.60 g, 11.70 mmol) in DMF (20 mL) dropwise, the mixture was stirred at room temperature overnight, and then poured into a saturation aqueous solution of sodium bicarbonate (300 mL). After 2 hour stirring at room temperature, the mixture extracted with chloroform (5 30 mL), and the organic fractions were collected and dried over anhydrous MgSO4. The crude product was purified through a silica gel chromatography eluting with (Acetone: Hexane ¼ 1:5) to afford a yellow oil in 77.6% yield (3.82 g, 9.08mmol). MS (EI) (Mþ, C28H40N2O): calcd: 420.63; found: 420.43. 1H NMR (400 MHz, acetone-d6) d 9.80 (s, 1H), 7.50 (s, 1H), 6.82 (m, 2H), 6.66 (m, 2H), 5.72 (s, 1H), 3.27e3.23 (m, 4H), 2.96 (m, 1H), 2.18 (s, 3H), 1.65e1.44 (m, 5H), 1.37e1.22 (m, 8H), 1.19 (s, 3H), 0.97 (s, 3H), 0.87 (t, J ¼ 7.4 Hz, 6H). 13 C NMR (101 MHz, acetone-d6) d 190.49, 151.92, 148.72, 140.21, 133.68, 132.68, 132.26, 130.11, 124.48, 117.22, 113.48, 113.20, 56.36, 51.70, 47.05, 31.26, 27.93, 26.80, 21.27, 20.96, 20.26, 14.67. Anal. Calcd (%) for C28H40N2O: C, 79.95; H, 9.59; N, 6.66; found: C, 80.04; H, 9.52; N, 6.73. 2.2.2. Synthesis of (E)-N,N-dibutyl-4-(2,2,4,7-tetramethyl-6-(2(thiophen-2-yl)vinyl)-3,4-dihydroquinolin-1(2H)-yl)aniline (compound 4d) NaH (0.68 g, 23.80 mmol) was added to a stirred solution of compound 3d (1.00 g, 2.38 mmol) and 3e (2.09 g, 4.76 mmol) in THF (100 mL) under nitrogen. The solution was stirred for 24 h and then poured into water. The organic phase was extracted by AcOEt, washed with brine and dried over MgSO4. After removal of the solvent under reduced pressure, the crude product was purified by silica chromatography, eluting with (Acetone: Hexane ¼ 1:20) to give compound 4d as yellow oil in 86.1% yield (1.03 g, 2.05 mmol). MS (MALDI-TOF) (Mþ, C33H44N2S): calcd: 500.78; found: 500.36. 1 H NMR (400 MHz, CDCl3) d 7.43e7.32 (m, 1H), 7.14 (d, J ¼ 15.2 Hz, 1H), 7.01 (d, J ¼ 5.1 Hz, 1H), 6.98 (d, J ¼ 3.2 Hz, 1H), 6.93e6.86 (m, 3H), 6.85 (m, 1H), 6.58 (d, J ¼ 8.3 Hz, 2H), 5.84 (d, J ¼ 15.2 Hz, 1H), 3.25e3.19 (m, 4H), 3.03e2.96 (m, 1H), 2.08 (s, 3H), 1.64e1.46 (m, 5H), 1.40e1.27 (m, 8H), 1.20 (s, 3H), 0.99 (s, 3H), 0.92 (t, J ¼ 7.2 Hz, 6H). 13C NMR (101 MHz, CDCl3) d 147.00, 146.41, 144.60, 134.25, 133.15, 131.23, 128.63, 128.51, 127.39, 126.87, 126.00, 124.10, 123.39, 122.63, 117.82, 115.64, 112.19, 111.83, 54.32, 50.75, 46.71, 30.88, 29.43, 27.13, 25.63, 20.62, 20.35, 19.79, 13.97. Anal. Calcd (%) for C33H44N2S: C, 79.15; H, 8.86; N, 5.59; found: C, 79.08; H, 8.91; N, 5.62. 2.2.3. Synthesis of (E)-5-(2-(1-(4-(dibutylamino) phenyl)-2,2,4,7tetramethyl-1,2,3,4-tetrahydroquinolin-6-yl)vinyl)thiophene-2carbaldehyde (compound 5d) Under a N2 atmosphere, 4d (0.59 g, 2.00 mmol) was dissolved in
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150 mL freshly distilled THF and cooled to 78 C. Approximately 2 equivalents of n-BuLi in hexanes (16 mL, 4 mmol) was added drop wise over 20 min. Reaction continued at 78 C for 1 h at which time DMF (0.29 g, 4.00 mmol) was added over 1 min. The reaction was allowed to reach RT while the solution stirred for 1h. The organic phase was extracted by AcOEt, washed with brine and dried over MgSO4. After removal of the solvent under reduced pressure, the crude product was purified by silica chromatography, eluting with (Acetone: Hexane ¼ 1:5) to give compound 5d as a red oil with 76.9% yield (0.50 g, 1.54 mmol). MS (MALDI-TOF) (Mþ, C34H44N2OS): calcd: 528.79; found: 528.20. 1H NMR (400 MHz, CDCl3) d 9.80 (s, 1H), 7.62 (d, J ¼ 3.9 Hz, 1H), 7.46 (s, 1H), 7.33 (d, J ¼ 15.8 Hz, 1H), 7.04 (d, J ¼ 3.9 Hz, 1H), 6.98e6.86 (m, 3H), 6.65 (d, J ¼ 8.6 Hz, 2H), 5.87 (s, 1H), 3.33e3.25 (m, 4H), 3.16e2.98 (m, 1H), 2.17 (s, 3H), 1.71e1.55 (m, 5H), 1.49e1.41 (m, 4H), 1.41e1.35 (m, 4H), 1.28 (s, 3H), 1.07 (s, 3H), 0.99 (t, J ¼ 7.3 Hz, 6H). 13C NMR (101 MHz, CDCl3) d 182.05, 154.95, 147.44, 147.02, 139.65, 137.54, 135.35, 132.80, 131.43, 131.27, 129.66, 124.54, 124.07, 123.73, 121.62, 115.93, 115.65, 112.08, 111.73, 54.66, 50.74, 46.33, 30.63, 29.34, 26.99, 25.74, 20.47, 20.28, 19.63, 13.94. Anal. Calcd (%) for C34H44N2OS: C, 77.23; H, 8.39; N, 5.30; found: C, 77.25; H, 8.41; N, 5.26. 2.2.4. Synthesis of 2-(4-((E)-2-(5-((E)-2-(8-((6-chlorohexyl)oxy)1,1,7,7-tetramethyl-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9yl)vinyl)thiophen-2-yl)vinyl)-3-cyano-5-phenyl-5-(trifluoromethyl) furan-2(5H)-ylidene)malononitrile (chromophore P1) Compound 4a (0.20 g, 0.40 mmol) and compound 4e (0.14 g, 0.44 mmol) were mixed with anhydrous ethanol (5 mL). The mixture was allowed to stir at 65 C for 2h. The solvent was removed under vacuum and the residual mixture was purified by flash chromatography on silica gel using ethyl acetate and hexane (v/v, 1: 4) to give chromophore P1 as a deep green solid in 76.8% yield (0.24 g, 0.31 mmol). HRMS (ESI) (Mþ, C45H44ClF3N4O2S): calcd: 797.2904; found: 797.2891. 1H NMR (400 MHz, acetone-d6) d 7.84 (d, J ¼ 15.2 Hz, 1H), 7.79e7.73 (m, 2H), 7.67e7.61 (m, 4H), 7.51 (s, 1H), 7.43 (d, J ¼ 15.8 Hz, 1H), 7.30 (d, J ¼ 15.8 Hz, 1H), 7.21 (d, J ¼ 4.2 Hz, 1H), 6.70 (d, J ¼ 15.2 Hz, 1H), 3.87 (t, J ¼ 6.5 Hz, 2H), 3.60 (t, J ¼ 6.6 Hz, 2H), 3.35e3.29 (m, 2H), 3.28e3.22 (m, 2H), 1.93 (m, 2H), 1.83 (m, 2H), 1.77e1.69 (m, 4H), 1.67e1.53 (m, 4H), 1.42 (s, 6H), 1.28 (s, 6H). 13C NMR (101 MHz, acetone-d6) d 177.09, 169.85, 162.43, 159.65, 158.88, 146.43, 144.18, 142.48, 141.60, 138.87, 134.66, 133.75, 132.70, 131.05, 130.98, 130.59, 129.91, 129.21, 128.43, 128.20, 127.05, 124.57, 123.19, 117.57, 116.57, 115.05, 112.39, 96.30, 77.08, 63.24, 58.29, 48.28, 47.68, 45.97, 40.80, 39.94, 37.16, 33.57, 33.15, 31.43, 31.22, 30.92, 27.84, 26.68. Anal. Calcd (%) for C45H44ClF3N4O2S: C, 67.78; H, 5.56; N, 7.03; found: C, 67.81; H, 5.54; N, 7.05. 2.2.5. Synthesis of 2-(4-((1E,3Z)-4-(8-((5-chloropentyl)oxy)-1,1,7,7tetramethyl-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)-3(thiophen-2-yl)buta-1,3-dien-1-yl)-5-phenyl-5-(trifluoromethyl) furan-2(5H)-ylidene)malononitrile (chromophore P2) The procedure for Chromophore P1 was followed to prepare Chromophore P2 from 4b and 4e as a deep green solid in 71.3% yield (0.21 g, 0.28 mmol). HRMS (ESI) (Mþ, C45H44ClF3N4O2S): calcd: 797.2904; found: 797.2923. 1H NMR (400 MHz, acetone-d6) d 8.12 (d, J ¼ 13.8 Hz, 1H), 7.72 (d, J ¼ 5.1 Hz, 1H), 7.68e7.55 (m, 5H), 7.49 (s, 1H), 7.33e7.22 (m, 1H), 7.02 (d, J ¼ 3.2 Hz, 1H), 6.84 (s, 1H), 6.01 (d, J ¼ 13.8 Hz, 1H), 3.95 (t, J ¼ 6.6 Hz, 2H), 3.66 (t, J ¼ 6.6 Hz, 2H), 3.51 (m, 4H), 1.98e1.83 (m, 4H), 1.79e1.72 (m, 2H), 1.63 (m, 6H), 1.41 (s, 6H), 0.89 (s, 6H). 13C NMR (101 MHz, CDCl3) d 176.50, 161.66, 160.43, 149.17, 148.51, 136.93, 130.83, 130.72, 129.29, 128.82, 128.26, 128.05, 127.71, 127.58, 127.52, 127.24, 126.39, 125.64, 124.92, 123.36, 122.41, 121.08, 117.03, 112.47, 112.31, 111.74, 111.37, 94.96, 77.89, 54.55, 48.08, 47.41, 45.15, 38.77, 34.99, 32.43, 32.40, 31.64, 29.90,
29.34, 29.32, 29.27, 29.23, 26.77, 25.42. Anal. Calcd (%) for C45H44ClF3N4O2S: C, 67.78; H, 5.56; N, 7.03; found: C, 67.76; H, 5.55; N, 7.04. 2.2.6. Synthesis of 2-(3-cyano-4-((E)-2-(5-((E)-2-(1-(4(dibutylamino)phenyl)-2,2,4,7-tetramethyl-1,2,3,4tetrahydroquinolin-6-yl)vinyl)thiophen-2-yl)vinyl)-5-phenyl-5(trifluoromethyl)furan-2(5H)-ylidene)malononitrile (chromophore P3) The procedure for Chromophore P1 was followed to prepare Chromophore P3 from 5d and 4e as a deep green solid in 77.1% yield (0.25 g, 0.30 mmol). HRMS (ESI) (Mþ, C50H50F3N5OS): calcd: 826.3766; found: 826.3793. 1H NMR (400 MHz, acetone-d6) d 7.88 (d, J ¼ 15.2 Hz, 1H), 7.78e7.72 (m, 2H), 7.66e7.60 (m, 3H), 7.57 (d, J ¼ 4.2 Hz, 1H), 7.47 (d, J ¼ 15.9 Hz, 2H), 7.29e7.20 (m, 2H), 6.95 (t, J ¼ 7.4 Hz, 2H), 6.78 (d, J ¼ 8.7 Hz, 2H), 6.67 (d, J ¼ 15.2 Hz, 1H), 5.85 (s, 1H), 3.40e3.33 (m, 4H), 2.11 (s, 3H), 1.94 (m, 1H), 1.75e1.58 (m, 5H), 1.50e1.34 (m, 8H), 1.30 (s, 3H), 1.08 (s, 3H), 0.98 (t, J ¼ 7.3 Hz, 6H). 13C NMR (101 MHz, CDCl3) d 175.43, 168.00, 161.56, 158.96, 149.01, 147.40, 141.54, 140.24, 137.81, 137.19, 134.69, 132.54, 131.49, 131.02, 130.62, 129.87, 129.75, 129.48, 128.64, 127.65, 126.87, 126.29, 125.67, 124.90, 124.56, 121.71, 116.09, 115.54, 112.22, 111.94, 111.80, 111.23, 110.93, 95.41, 57.38, 55.42, 50.89, 46.30, 30.90, 30.65, 29.47, 27.07, 26.20, 24.50, 20.41, 19.89, 14.41, 14.03. Anal. Calcd (%) for C50H50F3N5OS: C, 72.70; H, 6.10; N, 8.48; found: C, 72.73; H, 6.12; N, 8.46. 3. Results and discussion 3.1. Synthesis and characterization of chromophores Schemes 1 and 2 show the synthetic approach for the new chromophores P1, P2 and P3 containing the julolidine and tetrahydroquinoline donors. Starting from the amine donor intermediate compounds 1a or 1d, chromophores P1eP3 were synthesized in good overall yields through simple four or five step reactions: the free hydroxyl group on the julolidine donor was protected by the alkyl group to improve the solubility through Williamson ether synthesis. The free NeH group on the tetrahydroquinoline donor was protected by the aryl group to improve stability. Treatment of compound 2d with POCl3 and DMF gave the aldehyde 3d. After introduction of the bridge by Wittig condensation, compounds 3a or 4d was prepared with a high yield. Treatment of compound 3a or 4d with n-BuLi and DMF gave the aldehydes 4a or 5d. Treatment of compound 3a with POCl3 and DMF gave an aldehyde 4b. And the final condensations with the CF3-Ph-TCF acceptor give chromophores P1, P2 and P3 as green solids. All of the chromophores were fully characterized by 1H NMR, 13C NMR, and HRMS. These chromophores possess good solubility in common organic solvents, such as dichloromethane, chloromethane and acetone. 3.2. Thermal stability Thermal properties of chromophores P1eP3 were evaluated by differential scanning calorimetry (DSC) and the results are shown in Fig. 1 and Table 1. Chromophore P2 based on julolidinyl donor and vinylene bridge appeared as highly crystalline compound with a melting point at 212.1 C. While chromophores P1 and P3 were obtained as solids showing glass transition temperature (Tg) at 120.3 and 112.9 C, respectively. They are higher than many commonly used chromophores like AJC146 and YLD124 with the same CF3-TCF acceptor and thiophene or CLD bridges [32,33]. It is very attractive that they could act as a new class of chromophores.
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Scheme 1. Chemical structures and synthetic routes for chromophores P1 and P2.
Fig. 1. DSC curves of chromophores P1, P2 and P3 with a heating rate of 10 C min-1 in nitrogen atmosphere.
Scheme 2. Chemical structure and synthetic routes for chromophore P3.
3.3. Optical properties In order to reveal the effect of different electron donors and bridges on the charge-transfer (CT) absorption properties of the chromophores, UVeVis absorption spectra of the three chromophores were measured in a series solvent with different dielectric constants as shown in Figs. 2 and 3. The UV absorption spectra of chromophores P1eP3 in dilute chloroform solution (105 M) are shown in Fig. 2. Chromophore P2 shows a lmax of 760 nm which was the smallest because of shortest conjugate length. When comparing the lmax of chromophore P3 (841 nm) with chromophore P1 (849 nm) with the same thiophene bridge, P3 is slightly blue-shifted and the two compounds exhibit similar chargetransfer band shape.
Table 1 Thermal and Optical Properties data of the Chromophores. Cmpd
lmax (nm)a
lmax (nm)b
lmax (nm)c
Tg/Tm ( C)d
P1 P2 P3
849 791 841
770 735 798
730 723 749
120.3 212.1 112.9
a b c d
lmax was measured in chloroform. lmax was measured in toluene. lmax was measured in dioxane. Tg was the glass transition temperature. d Tm was the melting point.
The UVeVis absorption spectra were also measured in a series of aprotic solvents with different polarity so that the solvatochromic behavior of each chromophore could be investigated in a wide range of dielectric environments (Fig. 3). With the solvents varying from the slightly polar dioxane to the moderately polar dichloromethane, a continuous shift of the absorption maxima to longer wavelength was observed for all the three chromophores. More
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Fig. 2. UVeVis absorption spectra of chromophores P1, P2 and P3 in chloroform.
dramatic effects were observed while investigating the absorption spectra of P1eP3 in highly polar solvents, such as acetone and acetonitrile. In comparison with a rather broad absorption band recorded for chromophores P1 and P3, chromophore P2 showed a sharp and intense cyanine-type absorption band with a dramatically narrowed bandwidth. Furthermore, a saturation behavior was found for chromophores P2 in the more polar solvents, such as dichloromethane, acetone, and acetonitrile. The absorption peak of P2 in these solvents exhibited almost no shift with increase of solvent polarity. Chromophores P1 and P3 showed a bathochromic shift of its absorption maximum initially and then reversed to a hypsochromic shift for more polar solvents of acetone and acetonitrile, namely inverted solvatochromism. These interesting solvent-dependent spectral behaviors of chromophore P2 suggested that the more dipolar chromophore P2 can be polarized quite close to the cyanine limit, or even beyond that into the zwitter-ionic regime in the most polar solvents [34]. 3.4. Theoretical calculations To understand the ground-state polarization and microscopic NLO properties of the designed chromophores, the DFT calculations were carried out at the B3LYP level by employing the split valence 6-31G basis set using the Gaussian 09 program package [35,36]. The first hyperpolarizability (b) were carried out at the cam-B3LYP level by employing the split valence 6-31G basis set using the Gaussian 09 program package [36]. All molecules were assumed to be in trans-configurations [37,38]. The HOMO-LUMO energy gap, dipole moment (m), b and mb of the chromophores obtained from DFT calculations are summarized in Table 2. The HOMO-LUMO energy gap was used to understand the charge transfer interaction occurring in a chromophore molecule [39,40]. Fig. 4 depicts the electron density distribution of the HOMO and LUMO structures. It can be seen that the density of the ground and excited state electron is asymmetry along the dipolar axis of the chromophores. The HOMO-LUMO energy gaps DE (DFT) were calculated by DFT calculations as shown in Table 2. The energy gaps between the HOMO and LUMO energy for chromophores P1eP3 were 1.883, 2.271 and 1.914eV, respectively. As DE is reduced, resulting in a bathochromic shift of lmax within the series of compounds. These results correspond with the conclusion of UVeVis spectra analysis. To get more information from the frontier orbitals, the composition of the HOMOs and LUMOs has been calculated using the
Fig. 3. UVeVis absorption spectra of chromophores P1, P2 and P3 in six kinds of aprotic solvents with varying dielectric constants.
Table 2 Summary of DFT and EO coefficients of chromophores. Cmpd
DE(DFT) (eV)a
btot (1030 esu)b
m (D)c
mb (1048 esu)
r33 (pm/V)
P1 P2 P3
1.883 2.271 1.914
811.73 332.84 918.76
21.83 21.37 25.16
17719.98 6899.09 23116.00
223 283 278
a b c
DE(DFT) was calculated from DFT calculations. btot is the first-order hyperpolarizability calculated from DFT calculations. m is the total dipole moment.
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dibutylaminophenyl) substituent located on the nitrogen atom made a large dihedral angle of 85.51 with average plane of the chromophore. These large dihedral angles caused large steric hindrance that suppressed aggregations among molecules. As reported earlier, the b value has a close relationship with the strength of the donor and acceptor end groups and on the nature and length of the bridge, substituents, steric hindrance and intramolecular charge-transfer and so on [42,43]. The b value of chromophore P1 and P3 were larger than that of chromophore P2 due to narrower energy gap between HOMO and LUMO. The lmax of chromophore P1 and P3 were larger than that of chromophore P2, so the trend of increasing lmax is in good agreement with the trend of increasing b value. With additional electron-rich N-(4dibutylaminophenyl) substituent into the donor moiety, the chromophore P3 show larger b and mb than that of chromophore P1. 3.5. Electro-optic performance
Fig. 4. Frontier molecular orbitals HOMO and LUMO of chromophores P1, P2 and P3.
Multiwfn program [41]. As shown in Table 3, the whole chromophore molecule was segmented as donor, p-bridge, and acceptor. At the same time, the attribution of the thiophene (T) located in bridge moiety of chromophore P2 and the N-(4-dibutylaminophenyl) substituent (N) located in donor moiety of chromophore P3 was listed separately. For the three chromophores, the LUMO was largely stabilized by the contributions from acceptor (45.27%e 48.40%) and the p-bridge (25.62%e38.38%), while the HOMO was largely stabilized by the contributions from donors (52.43%e 59.35%). For the chromophore P2, when compare LUMO to HOMO level, the contribution of thiophene ring decreased to 0.23% from 2.38%. The comparison of HOMO and LUMO electron distribution in the thiophene ring indicated the easy delocalization of electrons in the thiophene ring. For the chromophore P3, when compare LUMO to HOMO level, the contribution of N-(4-dimethylaminophenyl) substituent decreased to 0.46% from 4.23%. The comparison of HOMO and LUMO electron distribution in the N-(4dibutylaminophenyl) substituent indicated the easy delocalization of electrons in the N-(4-dibutylaminophenyl) substituent. Consequently, it can be treated as another donor, which efficiently enhances the electron density of the conjugated system and increases the polarizability of chromophore P3 [26]. The geometrically optimized structures of the molecules were investigated by calculating the dihedral angles of the donor moieties as shown in Fig. 5. In chromophore P1, the alkyl chain substituent located on the oxygen atom of the donor made a large dihedral angle of 73.80 with average plane of the chromophore. As for chromophore P2, the lateral group thiophene and the alkyl chain made dihedral angle of 84.63 and 77.14 with average plane of the chromophore, respectively. As for chromophore P3, the N-(4-
Table 3 The molecular orbital composition (%) in the ground state for chromophores P1eP3. Cmpd
P1
P2
P3
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
donor p bridge acceptor T N
59.35% 24.32% 16.33%
16.58% 38.15% 45.27%
56.69% 10.11% 30.82% 2.38%
35.46% 15.91% 48.40% 0.23%
52.43% 25.62% 17.72%
14.82% 38.38% 46.34%
4.23%
0.46%
The molecular orbital composition was calculated using the Multiwfn program with RoseSchuit (SCPA) partition.
In order to investigate the translation of the microscopic hyperpolarizability into macroscopic EO response, the polymer films doped with 25 wt% chromophores into amorphous polycarbonate (APC) were prepared using dibromomethane as solvent. The resulting solutions were filtered through a 0.2-mm PTFE filter and spin-coated onto indium tin oxide (ITO) glass substrates. Films of doped polymers were baked at 80 C in a vacuum oven overnight. The corona poling process was carried out at a temperature of 10 C above the glass transition temperature (Tg) of the polymer. The r33 values of poled films were measured by Teng-Man simple reflection method at a wavelength of 1310 nm using carefully selected thin ITO electrode with low reflectivity and good transparency in order to minimize the contribution from multiple reflections [35,44]. The measured r33 values depend on the chromophore number density (N), b value, and poling efficiency, described by the cos3 (q) order parameter, as indicated by Ref. [45].
E. D r33 z2Nbf cos3 q n4 where the f term describes electric-field factors and n is the refractive index of the film, both of which remain relatively constant for related chromophores at similar loading densities. cos3 (q) is the acentric order parameter. q is the angle between the permanent dipole moment of chromophores and the applied electric field. At low concentration, the electro-optic activity increased with chromophore density, dipole moment and the strength of electric poling field. However, when the concentrations of chromophores increased to a certain extent, the N and cos3 (q) are no longer independent factor. Then,
D E h i cos3 ðqÞ ¼ ðmF=5kTÞ 1 L2 ðW=kTÞ where k is the Boltzmann constant and T is the Kelvin (poling) temperature. F ¼ [f (0) EP] where Ep is the electric poling field. L is the Langevin function, which is a function of W/kT, the ratio of intermolecular electrostatic energy (W) to the thermal energy (kT). L is related to electrostatic interactions between molecules. This relationship succeeds in qualitatively predicting the important trends involving electro-optic activity although not the quantitative values of electro-optic coefficients. When the intermolecular electrostatic interactions are neglected, the electro-optic coefficient (r33) should increase linearly with chromophore density, dipole moment, first hyperpolarizability and the strength of electric poling field. But chromophores with large dipole moment generate intermolecular static electric field dipole-dipole interaction, which
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Fig. 5. Optimized structures of chromophores P1, P2 and P3.
Chart 1. Chemical structures for chromophores P1, P2 and P3.
leads to the unfavorable antiparallel packing of chromophores. So the number of truly oriented chromophore (N) is small. In molecular optimization, introducing the huge steric hindrance group to isolate chromophores is the most popular and easy way to attenuate the dipole-dipole interactions of chromophores [46,47]. The poled films of P1/APC, P2/APC and P3/APC afforded r33 values of 223, 283 and 278 pmV1 at 1310 nm, respectively. It should be mentioned, the r33 values of P1/APC, P2/APC and P3/APC are within the highest ones ever reported for simple guestehost EO polymers when the polymer films doped with 25 wt% chromophores. The ring-fused aminophenyl structures in tetrahydroquinolinyl and julolidinyl donors facilitate the overlap of the porbital of the amino atom with the phenyl ring thus providing a good mechanism to increase the electron-donating strength. The donors strength of tetrahydroquinolinyl and julolidinyl group are much stronger than often used 4-(dialkylamino)phenyl groups. The increases donor strength of the chromophores significantly increases their macroscopic EO activities. The stronger the electrondonating power of the donor leads to higher lmax and r33 value [32]. The introduction of some isolation groups into the chromophore moieties to further control the shape of the chromophore could be an efficient approach to minimize interactions between the chromophores [8]. Except for the strong donor and strong acceptor, the isolated groups in the chromophores play an important role in the ultrahigh electro-optic activities. The N-(4-dibutylaminophenyl) substituent located on the nitrogen atom in chromophore P3 and the side thiophene ring group in chromophore P2 both shows dihedral angle of about 90 with average plane of the chromophore. This large dihedral angle caused large steric hindrance that suppressed aggregations among molecules. Meanwhile, the steric hindrance and tension of the rigid structure from the julolidinylbased donor in chromophore P1 and P2 can also suppress aggregations [48,49]. All these modifications of chromophores can increase the b as well as reduce dipoleedipole interactions so as to translate their mb values into bulk EO performance more effectively. The poled films of P2/APC and P3/APC afforded r33 values of 283 and 278 pmV1 at 1310 nm, respectively. Although chromophore P2 has much smaller b value than that of chromophore P3, the r33 value is even larger due to a better isolation effect. The dipoledipole interactions are reduced by isolating chromophores, through addition of steric bulk to approximate a spherical shape [50]. As for chromophore P2, it may be revealed by the optimized configurations (Fig. 5) that the side thiophene ring group was perpendicular to the direction of the dipole moment of the
F. Liu et al. / Dyes and Pigments 136 (2017) 182e190
chromophore which could act as the isolation group to suppress the possible aggregation. The larger r33values indicates that film-P2/ APC has weaker inter chromophore electrostatic interactions than film-P3/APC in this density.
[17]
[18]
4. Conclusion [19]
A series of high performance chromophores based on two different types of electron donors, including N-(4dibutylaminophenyl) tetrahydroquinolinyl and julolidinyl groups, with the same CF3-TCF acceptors and different bridge including thiophene and vinylene bridge, has been synthesized and systematically investigated. The high molecular hyperpolarizability of these chromophores can be effectively translated into electro-optic (EO) coefficients (r33) in poled polymers. In electro-optic activities, the poled films of P1/APC, P2/APC and P3/APC afforded r33 values of 223, 283 and 278 pmV1, respectively, at 1310 nm at the doping concentration of 25 wt%, which showed significant enhancement over the similar EO polymer systems previously reported. Ultrahigh r33 values, high thermal stability, together with good solubility, suggested that these chromophores could be useful for next generation EO materials and devices.
[20]
[21]
[22]
[23]
[24]
Acknowledgments [25]
We are grateful to the National Natural Science Foundation of China (No. 21473227, No. 61101054, No. 51503215 and No. 21003143) and China Scholarship Council for the financial support.
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