Open Access
Add to BibSonomyAbstract
We report the design, synthesis and characterization of a series of electro-optic chromophores comprised of amino-benzene with a benzyloxy group as the donor units, thienyl-di-vinylene with ethylenedioxy as the π-conjugation bridge, and tricyanofuran derivatives as the acceptor units. The improvement of linear and nonlinear optical properties was found in electro-optic chromophores with ethylenedioxy compared with the chromophores without ethylenedioxy. The data obtained from hyper-Rayleigh scattering, absorption spectrum, and 1H-NMR measurements indicate the improved structural stability of π-conjugation unit, which may be induced by the steric effect and partial intra-molecular hydrogen bonding related to ethylenedioxy.
© 2016 Optical Society of America
1. Introduction
In recent decades, polymeric electro-optic (EO) materials have been the subject of in-depth study due to their potential and broad applications in high-speed optical modulators with very high bandwidth and low drive voltage, high-speed optical switches, high-speed digital signal processing, electric field sensing, and terahertz generation/detection [1–5]. To make highly effective polymeric electro-optic materials, a lot of effort has been put into developing EO chromophores with large molecular first hyperpolarizablity (β). In particular, the development of strong electron acceptors such as 2-(dicyanomethylene)-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF), 2-(dicyanomethylene)-3-cyano-4,5-dimethyl-5-trifluoromethyl-2,5-dihydrofuran (CF3-TCF), and 2-(dicyanomethylene)-3-cyano-4-methyl-5-phenyl-5-trifluoromethyl-2,5-dihydrofuran (CF3-Ph-TCF) represents one of the most important advances in the design of EO chromophores [6,7]. Furthermore, EO chromophores with TCF derivatives as the electron acceptor, thienyl-di-vinylene as the π-conjugated bridge, and amino-benzene derivatives as the electron donors showed excellent thermal and chemical stability along with large hyperpolarizablity (β) [7]. We previously made a report on a series of EO chromophores that have amino-benzene with methoxy or benzyloxy groups as the donor units, thienyl-di-vinylene as the π-conjugated bridge, and TCF derivatives as the electron acceptor units, which showed improved linear and nonlinear optical properties [8,9]. For improvement, the contribution of intra-molecular hydrogen bonding for the structural stabilization of the π-conjugated bridge near the electron donor unit was indicated. The methoxy and benzyloxy groups may lead to the structrural stability in the planar trans-configuration as well as reduce dynamical fluctuation of the conjugated system near the donor unit due to its intra-molecular hydrogen bonding. The EO guest/host and side-chain polymers with these EO chromophores exhibited excellent EO properties [10,11].
In this paper, we examine the design, synthesis, and characterization of a series of electro-optic chromophores comprised of amino-benzene with a benzyloxy group as the donor units, thienyl-di-vinylene with ethylenedioxy as the π-conjugated bridge, tricyanofuran derivatives as the acceptor units. EO chromophores with ethylenedioxy were newly designed and synthesized. Although there have been reports covering similar EO chromophores with thienyl-di-vinylene with alkyl groups or alkoxy groups, butadienyl-vinyl-thiophoene with alkyl groups, bi-ethylenedioxythiophene, bi-propylenedioxythiophene as the π-conjugated bridge [12–21], there have been no reports covering EO chromophores, which have amino-benzene with methoxy or benzyloxy groups as the donor units, thienyl-di-vinylene with ethylenedioxy as the π-conjugated bridge, and TCF derivatives as the electron acceptor. In particular, we discuss the role of ethylenedioxy bound to the thienyl-di-vinylene π-conjugated bridge, which affects the linear and nonlinear optical properties. Hyper-Rayleigh scattering (HRS), absorption spectrum, and 1H-NMR measurements were performed for a series of EO chromophores, including newly designed and synthesized EO chromophores. We found improved linear and nonlinear optical properties for EO chromophores with ethylenedioxy compared with chromophores without ethylenedioxy. The improved structural stability of the π-conjugated bridge near the TCF derivative acceptor unit was indicated, which may be induced by the steric effects and partial intra-molecular hydrogen bonding related to ethylenedioxy. Therefore, the addition of ethylenedioxy to thienyl-di-vinylene as the π-conjugated bridge is also an important advance in the design of EO chromophores as well as the previous design involving the addition of methoxy or benzyloxy groups to the amino-benzene donor unit [8,9].
2. Experimental
2.1. Materials
Figure 1 shows the chemical structures of the EO chromophores used in this study. The EO chromophores are composed of amino-benzene (with or without a benzyloxy group) as the donor units, a thenyl-di-vinylene bridge (with or without ethylenedioxy) as the π-conjugation bridge, and TCF, CF3-TCF, CF3-Ph-TCF as the acceptor units. In regards to the chromophores, those with ethylenedioxy were newly designed and synthesized in this paper.
Figure 2 schematically shows the synthesis procedure including the reagents and conditions to obtain the chromophores with ethylenedioxy. We referenced the literature [22] for the synthesis of diethyl[(2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl)methyl]phosphonate in the scheme (i) in Fig. 2(a) and the scheme (ii) in Fig. 2(b). The synthesis procedure for the chromophores without ethylenedioxy was shown elsewhere [8,9]. All the chromophore samples shown in Fig. 1 were obtained in powder form and thoroughly dissolved in chloroform.
Fig. 2 Schematic illustration of the synthesis procedure, including the reagents and conditions to obtain EO chromophores with ethylenedioxy.
2.1.1. N,N-dibuthyl-4-[2-(2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl)vinyl]aniline (S1)
Diethyl[(2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl)methyl]phosphonate (2.11 g, 7.22 mmol) and 4-(dibutylamino)benzaldehyde (1.68 g, 7.2 mmol) were dissolved in THF (25 ml) (solution a). Separately, potassium t-butoxide (0.83 g, 7.40 mmol) was dissolved in THF (35 ml) (solution b). The solution b was added to the solution a in a dry-ice/acetone bath. After stirring for 100 minutes, the solution temperature was slowly elevated up to 0°C. The solution was added into water (100 ml) and extracted with ethyl acetate. The extracted solution was washed with brine, dehydrated with sodium sulfate, and then evaporated. The residue was purified with the silica gel column chromatography (ethyl acetate/hexane = 1/5) to afford S1 (4.1 g, 78.5%).
1H-NMR (600 MHz, CDCl3) δ ppm: 0.95 (6H, t, J = 7.6 Hz, Me), 1.32-1.38 (4H, m, CH2), 1.56-1.60 (4H, m, CH2), 3.27 (4H, t, J = 7.6 Hz, NCH2), 4.20-4.22 (2H, m, OCH2), 4.26-4.27 (2H, m, OCH2), 6.13 (1H, s, thiophene-H), 6.59 (2H, d, J = 9.0 Hz, Ar-H), 6.77 (1H, d, J = 16.5 Hz, CH = CH(E)), 6.90 (1H, d, J = 16.5 Hz, CH = CH(E)), 7.30 (2H, d, J = 9.0 Hz, Ar-H)
13C-NMR (150 MHz, CDCl3) δ: 14.02, 20.36, 29.48, 50.77, 64.75, 95.90, 111.58, 113.26, 118.29, 124.48, 126.73, 127.39, 137.84, 141.96, 147.56
2.1.2. (E)-7-[4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-carbaldehyde (S2)
N,N-dibuthyl-4-[2-(2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl)vinyl]aniline (2.05 g, 5.52 mmol) was dissolved in THF (40 ml) under Ar atmosphere. 4.5 ml (7.2 mmol) of n-butyl lithium (1.6 mol in hexane) was added into the solution in a dry-ice/acetone bath. After stirring for 45 minutes, N,N-dimethylformamide (0.53 ml) was added into the solution. After stirring for 1.5 hours, the solution temperature was slowly elevated up to 0°C, and then water (5 ml) was added into the solution. The solution was added into brine (120 ml) and extracted with ethyl acetate. The extracted solution was dehydrated with sodium sulfate (anhydrous), and then evaporated. The residue was purified with the silica gel column chromatography (ethyl acetate/hexane = 2/3) to afford S2 (1.72 g, 78.2%).
2.1.3. 2-[4-[(E)-2-[7-[(E)-4-(Dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-yl]vinyl]-3-cyano-5,5-dimethylfuran-2(5H)-ylidene]malononitrile (1b)
(E)-7-[4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-carbaldehyde (388 mg, 0.97 mmol), 2-(3-cyano-4,5-dimethyl-5-trifluoromethyl-2(5H)-furanylidene)propanedinitrile (213 mg, 1.07 mmol), and ammonium acetate (82.4 mg, 1.07 mmol) were added in a mixed solvent of ethanol (25 ml) and THF (5 ml). The solution was stirred for 18.5 hours at 50°C. The solution was evaporated, and the residue was purified with the silica gel column chromatography (chloroform/methanol = 100/1), and washed with ethanol to afford 1b (237 mg, 42%).
1H-NMR (600 MHz, CDCl3, 50.0°C) δ ppm: 0.97 (6H, t, J = 7.6 Hz, Me), 1.34-1.40 (4H, m, CH2), 1.57-1.62 (4H, m, CH2), 1.71 (6H, s, Me), 3.32 (4H, t, J = 7.6 Hz, NCH2), 4.33-4.34 (2H, m, OCH2), 4.40-4.41 (2H, m, OCH2), 6.46 (1H, d, J = 15.1 Hz,, CH = CH(E)), 6.62 (2H, d, J = 9.0 Hz, Ar-H), 6.94 (1H, d, J = 15.8 Hz, CH = CH(E)), 7.06 (1H, d, J = 15.8 Hz, CH = CH(E)), 7.35 (2H, d, J = 9.0 Hz, Ar-H), 7.74(1H, d, J = 15.8 Hz, CH = CH(E))
13C-NMR (150 MHz, CDCl3, 50.0°C) δ: 13.92, 20.39, 26.87, 29.66, 50.91, 54.66, 64.70, 65.93, 93.70, 96.32, 108.81, 111.52, 111.87, 111.93, 112.02, 112.77, 113.91, 123.43, 129.14, 132.02, 134.48, 135.09, 138.63, 147.80, 149.43, 172.83, 176.17
2.1.4. 2-[4-[(E)-2-[7-[(E)-4-(Dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-yl]vinyl]-3-cyano-5-methyl-5-(trifluoromethyl)furan-2(5H)-ylidene]malononitrile (2b)
(E)-7-[4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-carbaldehyde (300 mg, 0.75 mmol) and 2-(3-cyano-4,5-dimethyl-5-trifluoromethyl-2(5H)-furanylidene)propanedinitrile (209 mg, 0.83 mmol) were added in a mixed solvent of ethanol (15 ml) and THF (5 ml). The solution was stirred for 2.5 hours at 50°C. The solution was evaporated, and the residue was purified with the silica gel column chromatography (chloroform/methanol = 100/1), and washed with ethanol to afford 2b (438 mg, 91.9%).
1H-NMR (600 MHz, CDCl3, 50.0°C) δ ppm: 0.97 (6H, t, J = 7.6 Hz, Me), 1.35-1.41 (4H, m, CH2), 1.57-1.62 (4H, m, CH2), 1.86 (3H, s, Me), 3.33 (4H, t, J = 7.6 Hz, NCH2), 4.34-4.35 (2H, m, OCH2), 4.35-4.43 (2H, m, OCH2), 6.39 (1H, br, CH = CH), 6.63 (2H, d, J = 8.9 Hz, Ar-H), 6.96 (1H, d, J = 15.8 Hz, CH = CH(E)), 7.15 (1H, d, J = 15.8 Hz, CH = CH(E)), 8.38 (2H, d, J = 8.9 Hz, Ar-H), 8.14 (1H, d, J = 14.5 Hz, CH = CH)
13C-NMR (150 MHz, CDCl3, 50.0°C) δ: 13.91, 19.58, 20.37, 29.67, 50.94, 56.13, 64.70, 66.13, 92.82, 93.03, 93.25, 111.20, 111.75, 111.83, 112.07, 115.62, 116.68, 121.45, 123.29, 129.69, 135.86, 136.38, 136.77, 138.97, 149.37, 149.89, 161.44, 175.75
2.1.5. 2-[4-[(E)-2-[7-[(E)-4-(Dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-yl]vinyl]-3-cyano-5-phenyl-5-(trifluoromethyl)furan-2(5H)-ylidene]malononitrile (3b)
(E)-7-[4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-carbaldehyde (300 mg, 0.75 mmol) and 2-(3-cyano-4-methyl-5-phenyl-5-trifluoromethyl-2(5H)-furanylidene)propanedinitrile (260 mg, 0.82 mmol) were added in a mixed solvent of ethanol (15 ml) and THF (4 ml). The solution was stirred for 1.5 hours at 50°C. The solution was evaporated, and the residue was purified with the silica gel column chromatography (chloroform/methanol = 100/1), and washed with ethanol to afford 3b (470 mg, 89.9%).
1H-NMR (600 MHz, CDCl3, 50.0°C) δ ppm: 0.97 (6H, t, J = 7.6 Hz, Me), 1.34-1.40 (4H, m, CH2), 1.57-1.62 (4H, m, CH2), 3.32 (4H, t, J = 7.6 Hz, NCH2), 4.30-4.31 (2H, m, OCH2), 4.37-4.38 (2H, m, OCH2), 6.51 (1H, br, CH = CH), 6.61 (2H, d, J = 9.0 Hz, Ar-H), 6.93 (1H, d, J = 15.8 Hz, CH = CH(E)), 7.11 (1H, d, J = 15.8 Hz, CH = CH(E)), 7.36 (2H, d, J = 9.0 Hz, Ar-H), 7.47-7.524 (5H, m, Ph), 7.90 (1H, br, CH = CH)
13C-NMR (150 MHz, CDCl3, 50.0°C) δ: 13.90, 20.37, 29.66, 50.94, 56.25, 64.65, 66.06, 95.60, 111.25, 111.75, 111.81, 112.06, 115.75, 123.29, 126.86, 129.61, 129.69, 130.65, 131.19, 136.00, 136.38, 137.28, 138.99, 149.31, 149.90, 161.31, 175.92
2.1.6. 3-(Benzyloxy-N,N-dibuthyl-4-[2-(2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl)vinyl]aniline (S3)
Diethyl[(2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl)methyl]phosphonate (3.43 g, 11.735 mmol) and 4-dibutylamino-2-benzyloxybenzaldehyde (3.58 g, 10.546 mmol) were dissolved in THF (25 ml) (solution a). Separately, potassium t-butoxide (1.32 g, 11.764 mmol) dissolved in THF (35 ml) (solution b). The solution b was added to the solution a in a dry-ice/acetone bath. After stirring for 1 hour, the solution temperature was slowly elevated up to 0°C. The solution was added into water (100 ml) and extracted with ethyl acetate. The extracted solution was washed with brine, dehydrated with sodium sulfate, and then evaporated. The residue was purified with the silica gel column chromatography (ethyl acetate/hexane = 1/5) to afford S3 (4.15 g, 82.4%).
1H-NMR (600 MHz, CDCl3) δ ppm: 0.93 (6H, t, J = 7.6 Hz, Me), 1.27-1.33 (4H, m, CH2), 1.47-1.52 (4H, m, CH2), 3.21 (4H, t, J = 7.6 Hz, NCH2), 4.20-4.25 (4H, m, O(CH2)2O), 5.14 (2H, s, PhCH2O), 6.11 (1H, s, thiophene-H), 6.13 (1H, d, J = 2.1 Hz, Ar-H), 6.24 (1H, dd, J = 2.1Hz, 8.9 Hz, Ar-H), 7.02 (1H, d, J = 16.5 Hz, CH = CH(E)), 7.19 (1H, d, J = 16.5 Hz, CH = CH(E)), 7.29-7.48 (6H, m, Ar-H)
13C-NMR (150 MHz, CDCl3) δ: 14.01, 20.34, 29.50, 50.88, 64.78, 70.45, 85.88, 97.07, 113.96, 119.10, 121.93, 126.92, 127.38, 127.57, 128.49, 137.70, 148.63
2.1.7. 7-[2-(Benzyloxy)-4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-carbaldehyde (S4)
3-(benzyloxy)-N,N-dibuthyl-4-[2-(2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl)vinyl]aniline (3.53 g, 7.39 mmol) was dissolved in THF (65 ml) under Ar atmosphere. 6 ml (0.96 mmol) of n-butyl lithium (1.6 mol in hexane) was added into the solution in a dry-ice/acetone bath. After stirring for 25 minutes, N,N-dimethylformamide (0.7 ml) was added into the solution. After stirring for 1 hour, the solution temperature was slowly elevated up to 0°C, and then water (5 ml) was added into the solution. The solution was added into water and extracted with ethyl acetate. The extracted solution was washed with brine, dehydrated with sodium sulfate (anhydrous), and then evaporated. The residue was purified with the silica gel column chromatography (ethyl acetate/hexane = 2/3) to afford S4 (3.55 g, 95%).
2.1.8. (E)-7-[2-(Benzyloxy)-4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-carbaldehyde (S5)
7-[2-(Benzyloxy)-4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-carbaldehyde (Z/E) (3.55 g, 7.02 mmol) was dissolved in ether (260 ml) and then iodine (0.1 g) was added. After stirring for 30 minutes, the solution was washed with 5% sodium bisulfite aqueous solution (40 ml), washed with brine, dehydrated with sodium sulfate (anhydrous), and then evaporated. The residue was purified with the silica gel column chromatography (ethyl acetate/hexane = 2/3) to afford S5 (3.19 g, 89.9%).
1H-NMR (600 MHz, CDCl3) δ ppm: 0.93 (6H, t, J = 7.6 Hz, Me), 1.28-1.34 (4H, m, CH2), 1.48-1.53 (4H, m, CH2), 3.23 (4H, t, J = 7.6 Hz, NCH2), 4.29-4.30 (2H, m, OCH2), 4.35-4.37 (2H, m, OCH2), 5.15 (2H, s, PhCH2O), 6.11 (1H, d, J = 2.1 Hz, Ar-H), 6.24 (1H, dd, J = 2.1Hz, 8.9 Hz, Ar-H), 6.05 (1H, d, J = 15.8 Hz, CH = CH), 7.31-7.33 (1H, m, Ar-H), 7.37 (1H, d, J = 9.0 Hz, Ar-H), 7.38-7.41 (2H, m, Ar-H), 7.46 (2H, d, J = 7.6 Hz, Ar-H), 7.47 (1H, d, J = 15.9 Hz, CH = CH), 9.828 (1H, s, CHO)
13C-NMR (150 MHz, CDCl3) δ: 13.99, 20.31, 29.48, 50.88, 64.42, 65.42, 70.34, 96.34, 104.91, 112.63, 113.11, 113.83, 126.97, 127.79, 127.98, 128.61, 131.74, 137.05, 137.25, 149.71, 158.14, 178.90
2.1.9. 2-[4-[(E)-2-[7-[(E)-2-(benzyloxy)-4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl]vinyl]-3-cyano-5,5-dimethylfuran-2(5H)-ylidene]malononitrile (1d)
(E)-7-[2-(benzyloxy)-4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-carbaldehyde (502 mg, 0.993 mmol) and 2-(3-cyano-4,5,5-trimethyl-2(5H)-furanylidene)propanedinitrile (0.27 g, 1.104 mmol) were added in a mixed solvent of ethanol (15 ml) and THF (4 ml). The solution was stirred for 1 night at room temperature, stirred for 8 hours at 50°C, and stored at room temperature. The obtained crystals were filtered and washed with ethanol. The residue was purified with the silica gel column chromatography (chloroform/methanol = 200/1), and then washed with ethanol to afford 1d (432 mg, 63.1%).
1H-NMR(600 MHz, CDCl3, 50°C) δ ppm: 0.94 (6H, t, J = 7.6 Hz, Me), 1.29-1.35 (4H, m, CH2), 1.50-1.55 (4H, m, CH2), 1.70 (6H, s, Me), 3.25 (4H, t, J = 7.6 Hz, NCH2), 4.27-4.29 (2H, m, OCH2), 4.38-4.39 (2H, m, OCH2), 5.18 (2H, s, PhCH2O), 6.13 (1H, d, J = 2.8 Hz, Ar-H), 6.27 (1H, dd, J = 2.1Hz, 8.9 Hz, Ar-H), 6.43 (1H, d, J = 15.8 Hz, CH = CH(E)), 7.11 (1H, d, J = 15.8 Hz, CH = CH(E)), 7.33 (1H, t, J = 7.6 Hz, Ar-H), 7.36-7.40 (3H, m, Ar-H), 7.45 (2H, d, J = 7.6 Hz, Ar-H), 7.48 (1H, d, J = 15.8 Hz, CH = CH(E)), 7.73 (1H, d, J = 15.8 Hz, CH = CH(E))
13C-NMR (150 MHz, CDCl3, 50°C) δ: 13.91, 20.37, 26.90, 29.71, 51.04, 54.14, 64.61, 65.94, 70.72, 92.94, 96.18, 96.76, 105.69, 108.31, 111.71, 112.12, 112.56, 112.96, 113.61, 113.93, 127.10, 128.02, 128.71, 129.41, 130.42, 133.98, 135.03, 137.27, 138.41147.99, 150.78, 159.02, 172.67, 176.30
2.1.10. 2-[4-[(E)-2-[7-[(E)-2-(benzyloxy)-4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl]vinyl]-3-cyano-5-methyl-5-trifluoromethylfuran-2(5H)-ylidene]malononitrile (2d)
(E)-7-[2-(benzyloxy)-4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-carbaldehyde (497 mg, 0.981 mmol) and 2-(3-cyano-4,5-dimethyl-5-trifluoromethyl-2(5H)-furanylidene)propanedinitrile (273 mg, 1.078 mmol) was added in a mixed solvent of ethanol (10 ml) and THF (2 ml). The solution was stirred for 1 hour at room temperature, stirred for 2 hours at 50°C, and stored at room temperature. The solution was evaporated, and the residue was purified with the silica gel column chromatography (chloroform/methanol = 100/1), and then washed with ethanol to afford 2d (346 mg, 47.5%).
1H-NMR (600 MHz, CDCl3, 50°C) δ ppm: 0.94 (6H, t, J = 7.6 Hz, Me), 1.30-1.36 (4H, m, CH2), 1.51-1.56 (4H, m, CH2), 1.84 (3H, s, Me), 3.27 (4H, t, J = 7.6 Hz, NCH2), 4.28-4.30 (2H, m, OCH2), 4.41-4.42 (2H, m, OCH2), 5.18 (2H, s, PhCH2O), 6.12 (1H, s, Ar-H), 6.29 (1H, dd, J = 2.1Hz, 9.0 Hz, Ar-H), 6.33 (1H, br, CH = CH(E)), 7.14 (1H, d, J = 15.8 Hz, CH = CH(E)), 7.33 (1H, t, J = 7.6 Hz, Ar-H), 7.37-7.40 (3H, m, Ar-H), 7.45 (2H, d, J = 7.6 Hz, Ar-H), 7.58 (1H, d, J = 15.8 Hz, CH = CH(E)), 8.12 (1H, br, CH = CH(E))
13C-NMR (150 MHz, CDCl3, 50°C) δ: 13.89, 19.63, 20.36, 29.73, 51.10, 55.29, 64.60, 66.13, 70.72, 92.81, 93.03, 96.50, 105.92, 111.48, 112.03, 112.09, 112.56, 113.62, 115.85, 122.46, 127.11, 128.09, 128.75, 130.07, 132.65, 137.06, 138.25, 138.78, 151.42, 159.56, 160.94, 175.93
2.1.11. 2-[4-[(E)-2-[7-[(E)-2-(benzyloxy)-4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl]vinyl]-3-cyano-5-phenyl-5-(trifluoromethyl)furan-2(5H)-ylidene]malononitrile (3d)
(E)-7-[2-(benzyloxy)-4-(dibutylamino)stylyl]-2,3-dihydrothieno[3,4-b] [1,4]dioxine-5-carbaldehyde (1.19 g, 2.35 mmol) and 2-(3-cyano-4-methyl-5-phenyl-5-trifluoromethyl-2(5H)-furanylidene)propanedinitrile (0.81 g, 2.57mmol) were added in a mixed solvent of ethanol (15 ml) and THF (4 ml). The solution was stirred for 1 hour at room temperature and then stirred for 1 hour at 50°C. The solution was evaporated, and the residue was purified with the silica gel column chromatography (chloroform/methanol = 100/1), and washed with hot ethanol to afford 3d (1.32 g, 69.9%).
1H-NMR (600 MHz, CDCl3, 50°C) δ ppm: 0.94 (6H, t, J = 7.6 Hz, Me), 1.29-1.35 (4H, m, CH2), 1.50-1.55 (4H, m, CH2), 3.27 (4H, t, J = 7.6 Hz, NCH2), 4.24-4.25 (2H, m, OCH2), 4.34-4.35 (2H, m, OCH2), 5.17 (2H, s, PhCH2O), 6.11 (1H, d, J = 2.1 Hz, Ar-H), 6.27 (1H, dd, J = 2.1 Hz, 8.9 Hz, Ar-H), 6.44 (1H, br, CH = CH(E)), 7.11 (1H, d, J = 15.8 Hz, CH = CH(E)), 7.31-7.52 (11H, m, Ar-H), 7.55 (1H, d, J = 15.8 Hz, CH = CH(E)), 7.91 (1H, br, CH = CH(E))
13C-NMR (150 MHz, CDCl3, 50.0°C) δ: 13.88, 20.357, 29.72, 51.09, 55.23, 64.53, 66.08, 70.72, 91.65, 95.36, 95.57, 96.50, 105.95, 108.43, 111.58, 112.07, 112.54, 113.64, 116.07, 122.43, 126.86, 127.09, 128.08, 128.73, 129.54, 130.09, 130.89, 131.07, 132.79, 137.03, 138.66, 138.82, 149.58, 151.46, 159.60, 160.68, 176.09
2.2. Instruments
The apparatus used in the experiment and procedure for HRS, 1H-NMR, absorption spectrum, and thermogravimetric measurements were described in detail elsewhere [8,9]. A brief outline of the apparatus and procedure is presented here. For HRS in measuring β values, the fundamental light with a wavelength of 1952 nm, whose typical laser linewidth is less than 5 cm−1, was obtained by the optical parametric oscillation of a beta barium borate crystal pumped by third-harmonic light of a nanosecond pulsed Nd3+:YAG laser (Hoya continuum, Panther OPO). A near-infrared sensitive photomultiplier tube (Hamamatsu, H10330A-25) was used to detect the HRS light of 976 nm after it passed through a color filter and two or three sets of narrow interference filters (Semrock, 976 nm laser line filters). The use of relatively long-wavelength fundamental light (1952 nm) enabled us to measure HRS at almost non-resonant conditions for all sample solutions, and the self-absorption of sample solutions at the HRS wavelength (976 nm) was negligible. The contribution of two-photon induced fluorescence to the detected signal was assessed by conducting with a frequency domain measurement by using sets of narrowband interference filters or by measuring spectroscopic characteristics [8,9]. The absorption spectra were recorded by using a spectrophotometer (Hitachi, U-4000). The NMR spectrometer (JEOL, JNM-ECA600II) was used to obtain the 1H-NMR and 13C-NMR spectra. The chloroform (CHCl3) solutions of the chromophores were used for the HRS and absorption spectrum measurements, while deuterated chloroform (CDCl3) solutions were used for the 1H-NMR and 13C-NMR measurements. Thermogravimetric and differential thermal analysis (TG-DTA) was performed by using thermogravimetry (Rigaku, TG8120) in nitrogen atmosphere at a heating ratio of 5 °C/min.
3. Results and discussion
Table 1 lists the β values obtained from the HRS measurements, the maximal absorption wavelength, and its molar absorption coefficient. The typical experimental error in HRS measurements is 10 to 15%, depending on the EO chromophores. Our benchmark chromophores 3a and 1a exhibit nearly the same structures as YLD156 and FTC, respectively, in Davis et al. [23] respectively. As previously shown, our experimental data was consistent with the data in Davis et al. [23], taking into account the difference with the external standard, the small difference with the fundamental wavelength, and the small difference with the chemical structures.
Table 1. First hyperpolarizability and maximum absorbance of EO chromophores
EO chromophores (1b, 2b, 3b) with ethylenedioxy and without a benzyloxy group showed improved linear and nonlinear optical properties when compared to their benchmark EO chromophores (1a, 2a, 3a), respectively. The trend of the peak shift in maximal absorption wavelength was consistent with the nonlinear optical properties. EO chromophores (1d, 2d, 3d) with ethylenedioxy and a benzyloxy group showed improved linear and nonlinear optical properties when compared to the EO chromophores (1c, 2c, 3c) without ethylenedioxy and with a benzyloxy group. The trend of the peak shift in maximal absorption wavelength was consistent with the nonlinear optical properties. The data in Table 1 indicates that both the benzyloxy group bound to the aminobenzene donor unit and ethylenedioxy bound to the thenyl-di-vinylene π-conjugated bridge have a positive influence on the linear and nonlinear optical properties, and the effects are additive. We previously proposed the intra-molecular hydrogen bonding as shown in Fig. 3 as the role of alkyloxy groups such as methoxy and benzyloxy groups, which may lead to improved structural stability in the planar trans-configuration of the π-conjugated bridge near the electron donor units, resulting in a positive influence on the linear and nonlinear optical properties [8,9].
Fig. 3 Schematic illustration of the intramolecular hydrogen bonding caused by methoxy or benzyloxy group.
The chemical shift of the corresponding proton in 1H-NMR and absorption spectrum (absorption band shape and its intensity) also supported this idea. In the optimized structure at B3LYP/6-31G(d) level of theory by the density functional theory (DFT) calculations, the distance (2.17 Å) between “O” and the corresponding proton “H” in Fig. 3 is close enough to consider the intramolecular hydrogen bonding. The corresponding proton “H” showed larger chemical shift under the presence of the methoxy or benzyloxy group both for experiments and calculations.
Here, we consider the role of ethylenedioxy bound to the thenyl-di-vinylene π-conjugated bridge. The absorption spectra for CHCl3 solutions of the chromophores (1a, 1b, 1c, 1d, 2a, 2b, 2c, 2d, 3a, 3b, 3c, and 3d) with a molar concentration of 1 × 10−5 M are shown in Figs. 4(a), 4(b), and 4(c), in which the optical density after passing through a 1-cm-length long cuvette against photon energy (eV) are plotted.
Fig. 4 Absorption spectra for CHCl3 solutions of EO chromophores with a molar concentration of 1 × 10−5 M; (a) chromophore 1a (blue line), 1b, (yellow line), 1c (green line), and 1d (red line), (b) chromophore 2a (blue line), 2b, (yellow line), 2c (green line), and 2d (red line), and (c) chromophore 3a (blue line), 3b, (yellow line), 3c (green line), and 3d (red line).
As previously shown, the maximum absorbance of chromophores 1c, 2c, and 3c with benzyloxy is shifted more to the low-energy side or longer wavelength than the benchmark chromophores 1a, 2a, and 3a, respectively. Absorption at the peak positions for chromophores 1c, 2c, and 3c is greater than that of corresponding benchmark chromophores 1a, 2a, and 3a, although absorption of the chromophore 1c is slightly greater than that of the benchmark chromophore 1a. Absorption bandwidths of chromophores 1c, 2c, and 3c are narrower than those of corresponding benchmark chromophores 1a, 2a, and 3a, although the bandwidth of chromophore 1c is slightly narrower than that of the benchmark chromophore 1a. The maximum absorbance of chromophores 1b, 2b, and 3b with ethylenedioxy is shifted more to the low-energy side or longer wavelength than benchmark chromophores 1a, 2a, and 3a, respectively. The absorption at the peak positions for chromophore 1b, 2b, and 3b is clearly larger than that of benchmark chromophores 1a, 2a, and 3a, respectively. Absorption bandwidths of chromophores 1b, 2b, and 3b are narrower than those of the benchmark chromophores 1a, 2a, and 3a, respectively. The maximum absorbance of chromophores 1d, 2d, and 3d both with benzyloxy and ethylenedioxy is shifted more to the low-energy side than chromophores 1b(1c), 2b(2c) and 3b(3c), respectively. The absorption at the peak positions for chromophores 1d, 2d, and 3d is greater than that for chromophores 1b(1c), 2b(2c), and 3b(3c), respectively. Absorption bandwidths of chromophores 1d, 2d, and 3d are narrower than those of chromophores 1b(1c), 2b(2c), and 3b(3c), accompanied by a sharper decrease on the low energy side. The benzyloxy group bound to the aminobenzene donor unit and ethylenedioxy bound to the thenyl-di-vinylene π-conjugation bridge exert a positive influence on absorption and the effects are additive. The trend towards the improved oscillator strength, energy shift, and narrowing of the absorption band was consistent with the trend observed in the nonlinear optical properties by HRS measurements. Thus, ethylenedioxy bound to the thenyl-di-vinylene π-conjugated bridge is expected to contribute to the structural stability of the planar trans-configuration of the π-conjugation bridge, as well as help reduce dynamical fluctuation. We think that the improved oscillator strength, energy shift, and narrowing of the absorption band originate from the steric effect and/or intramolecular hydrogen bonding in conjunction with the experimental data of 1H-NMR below.
Here, we will discuss the 1H-NMR spectra to further investigate the role of ethylenedioxy. Figures 5(a)-5(f) show the 1H-NMR spectra at 20°C for chromophores 1a, 2a, 3a, 1b, 2b, and 3b, respectively. The peaks in the NMR spectra and protons in the chemical structures that deserve special attention are emphasized by dotted circles and dashed circles. We can identify these protons and their chemical shifts by analyzing the 1H-NMR and 13C-NMR spectra of each chemical compound in the synthesis procedure of Fig. 1. In addition, the proton with the higher chemical shift and the proton with lower chemical shift are indicated by the dotted circles and dashed circles, the identification of which was also supported by DFT calculations at the B3LYP/6-31G(d) level of theory. As an example, the optimized structure and the calculated chemical shift for chromophore 3b in a vacuum are shown in Figs. 6(a) and 6(b), respectively. In the calculation, the butyl groups in the electron donor unit were replaced by ethyl groups for the sake of simplicity. The calculated chemical shift was obtained from the calculated NMR spectrum of tetramethylsilane in a vacuum. Although there is a somewhat difference on the absolute values between experiments and calculations, the identification of the protons near the electron acceptor unit, to which we paid special attention, seems to be valid. In Fig. 6(a), we found that “Os” (red ones) in ethylendioxy exist in the plane that includes π-conjugation, while CF3 and phenyl groups in the CF3-pheny-TCF acceptor unit exist outside the plane. The distance between “O” in ethylendioxy and the nearest “H” is 2.288 Å, which is slightly longer than the distance (2.17 Å) between “O” in the benzyloxy and the nearest “H” in Fig. 3. By comparing 1a, 2a, and 3a in Figs. 5(a), 5(c), and 5(e) with 1b, 2b, and 3b in Figs. 5(b), 5(d), and 5(f), we surprisingly notice the broadening of the peaks of corresponding protons. In particular, the peaks of 2b and 3b in Figs. 5(d) and 5(f) are very broad and barely discernible. In regards to the central values of the peaks with the higher chemical shift, when we compared 1a and 2a in Figs. 5(a) and 5(c) with 1b and 2b in Figs. 5(b) and 5(d), we found there were nearly the same. On the other hand, 3b in Fig. 5(f) has a slightly high chemical shift value, compared with 3a in Fig. 5(e). The broadening of the 1H-NMR spectra is related to the structural rigidity of the corresponding part in the molecule, that is, a solid-like state of the corresponding part, because inter or intra-molecular chemical exchange processes are not expected in our molecular systems. The data indicates that ethylenedioxy bound to the thenyl-di-vinylene π-conjugated bridge significantly contributes to the structural stability in the π-conjugated bridge near the TCF derivative acceptors. The steric effect is the most probable reason for the structural stability, while the weak intramolecular hydrogen bonding between the oxygen atom in the ethylenedioxy and the proton with a higher chemical shift in the π-conjugated bridge near TCF derivative acceptors may also be involved [25]. Thus, the broadening of the 1H-NMR spectra is attributed to the steric effect and/or intramolecular hydrogen bonding, which lead to the restricted rotation of the corresponding bond.
Fig. 5 1H-NMR spectra of EO chromophores in CDCl3 at 20°C; (a) chromophore 1a, (b) chromophore 1b, (c) chromophore 2a, (d) chromophore 2b, (e) chromophore 3a, and (f) chromophore 3b.
Fig. 6 (a) Optimized geometry of chromophore 3b calculated at the B3LYP/6-31G(d) level of theory, (b) Chemical structure with the calculated chemical shift (δ) [ppm] of protons for chromophore 3b.
Figures 7(a)-7(f) show the 1H-NMR spectra at 50°C for chromophores 1a, 2a, 3a, 1b, 2b, 3c, respectively. As clearly seen, the broad peaks in Figs. 5(b), 5(d) and 5(f) at 20°C are observed as sharper but slightly broad peaks in Figs. 7(b), 7(d) and 7(f). In particular, the peaks of chromophores 2b and 3b in Figs. 7(d) and 7(f) are still broad. Thus the structural rigidity of the corresponding part in the molecule was alleviated at 50°C to a certain extent. The data in Fig. 7 support the aforementioned statement on the role of ethylenedioxy bound to the thenyl-di-vinylene π-conjugated bridge. The 1H-NMR spectra for chromophores 1d, 2d, and 3d that have ethylenedioxy and benzyloxy groups basically exhibited the same tendencies as those of chromophores 1b, 2b, and 3b. As an example, Figs. 8(a) and 8(b) show 1H-NMR spectra for the chromophore 3d at 20°C and 50°C, respectively. In Fig. 8(a), the peak of the proton with a higher chemical shift is very broad and barely discernible, while the peak of the proton with a lower chemical shift is difficult to discern due to overlapping with the other peaks but does contributes to the small elevation of the baseline of other peaks. The broad peaks at 20°C in Fig. 8(a) are observed as sharper but still broad peaks at 50°C, as shown in Fig. 8(b). The data in Fig. 8 also support the aforementioned statement.
Fig. 7 1H-NMR spectra of EO chromophores in CDCl3 at 50°C; (a) chromophore 1a, (b) chromophore 1b, (c) chromophore 2a, (d) chromophore 2b, (e) chromophore 3a, and (f) chromophore 3b.
Fig. 8 1H-NMR spectra for chromophore 3d in CDCl3 at (a) 20°C and (b) 50°C. The inset in (a) show the magnified images for the proton with a higher chemical shift.
The EO chromophores comprised of amino-benzene derivative as the donor units, thienyl-di-vinylene with or without two alkylchains as the π-conjugation bridge, TCF as the acceptor unit, both which were excellent standard EO chromophores denoted by FTC [20], were reported. The introduction of two butyl-groups to thienyl-di-vinylene did not affect the wavelength of maximum absorbance and hyperpolarizability. The absorption maximal wavelength of FTC with two butyl-groups bound to thienyl-di-vinylene was 650 nm in CHCl3, which is close to that of EO chromophore 1a. The detailed 1H-NMR studies were not reported. On the other hand, in our study, the introduction of ethylenedioxy to thienyl-di-vinylene affected the wavelength of maximum absorbance, its shape, and the NMR spectrum of protons near the acceptor, resulting in improved nonlinear optical properties. It seems that it is difficult to explain our data by simple steric effect of ethylenedioxy, because two butyl groups are bulkier than ethylenedioxy. Recently, there was a report on EO chromophores comprised of double amino-benzene as the donor unit, thienyl-di-vinylene with an alkoxy group as the π-conjugation bridge, TCF as the acceptor unit [15]. The report discussed the influence of the different location of the alkoxy group on thiophene ring on the linear and nonlinear optical properties. Better data was obtained when the alkoxy group was closer to the donor unit. The wavelengths of maximum absorbance of the chromophores with the alkoxy group close to the donor unit or the acceptor unit were 793 nm and 724 nm, respectively. There were no reports on 1H-NMR and HRS studies. However, it is difficult to directly compare our data with the data in the literature [15] because the chromophores in the literature have the specific double amino-benzene as the donor unit. In our study, ethylenedioxy bound to thienyl-di-vinylene strongly affected the wavelength of absorbance maximum and its shape, resulting in improved nonlinear optical properties. In addition, it seems that ethylenedioxy contributed significantly to the structural stability of the π-conjugation bridge near the TCF derivative acceptors based on the data of the 1H-NMR spectra. Although EO chromophores with bi-propylenedioxythiophene as the π-conjugated bridge and alkylthiophene-based EO chromophores with a longer conjugation length were reported [16,19], it is difficult to compare our chromophores with the chromophores mentioned in previous reports because they have different π-conjugated structures and different conjugation lengths.
Table 2 shows the decomposition temperature (Td) for the chromophores (1a, 1b, 1c, 1d, 2a, 2b, 2c, 2d, 3a, 3b, 3c, and 3d) obtained by TG-DTA measurements. The chromophores (1b, 1d, 2b, 2d, 3b, and 3d) with ethylenedioxy had similar or slightly improved high-temperature resistance as compared with the chromophores (1a, 1c, 2a, 2c, 3a, and 3c) without ethylenedioxy, respectively.
Table 2. Thermal properties of EO chromophores
The benchmark EO chromophores such as 1a, 2a, and 3a are excellent EO chromophores for practical applications. The introduction of ethylenedioxy to thienyl-di-vinylene helps to improve the linear and nonlinear optical properties, as found in the EO chromophores (1b, 1d, 2b, 2d, 3b, and 3d). Notably, the narrowing of the absorption band accompanied by the sharp decrease in the low-energy side as well as the increase in the oscillator strength of the absorption band provide us with preferable characteristics for EO chromophores, because there is a trade-off between the absorption loss and the magnitude of hyperpolarizability. In addition, we found that the benzyloxy group bound to the aminobenzene donor unit and ethylenedioxy bound to the thenyl-di-vinylene π-conjugation bridge both exert a positive influence to the linear and nonlinear optical properties, these effects were additive. Thus, the benzyloxy and ethylenedioxy groups can be utilized to tune the linear and nonlinear optical properties. Furthermore, the introduction of substituents such as benzyloxy and ethylenedioxy would help prevent chromophore aggregations as well as the changing of the chromophore shape, which may lead to improvement in the degree of orientations in practical applications [7,26].
4. Conclusions
In this study, new EO chromophores with ethylenedioxy bound to thienyl-di-vinylene and/or benzyloxy bound to aminobenzen were designed, synthesized, and systematically investigated by using HRS, absorption spectra, and 1H-NMR spectra. It was found that the addition of ethylenedioxy to thienyl-di-vinylene helped to improve the linear and nonlinear optical properties. Most notably the narrowing of absorption band accompanied by the sharp decrease in the low energy side and increased oscillator strength produced advantageous properties for EO chromophores. The ethylenedioxy bound to the thenyl-di-vinylene π-conjugation bridge and the benzyloxy bound to the aminobenzene donor unit both exerted a positive influence on the linear and nonlinear optical properties, and the effects were additive. In this account, we provide a comprehensive picture about the role of ethylenedioxy and benzyloxy which is the structural stability of π-conjugation bridge near the acceptor and donor units through systematic studies by using the HRS, 1H-NMR, and absorption spectra. In addition, the chromophores with ethylenedioxy exhibited similar or slightly improved thermal stability, compared with those without ethylenedioxy. Thus, the data obtained in this study are useful in designing EO chromophores and will provide us with a new guide for further researches.
Acknowledgments
This work was supported by JSPS KAKENHI Grant Number JP16K04941.
References and links
1. D. Chen, H. R. Fetterman, A. Chen, W. H. Steier, L. R. Dalton, W. Wang, and Y. Shi, “Demonstration of 110 GHz electro-optic polymer mudulators,” Appl. Phys. Lett. 70(25), 3335–3337 (1997). [CrossRef]
2. C. T. Derose, R. Himmelhuber, D. Mathine, R. A. Norwood, J. Luo, A. K.-Y. Jen, and N. Peyghambarian, “High Deltan strip-loaded electro-optic polymer waveguide modulator with low insertion loss,” Opt. Express 17(5), 3316–3321 (2009). [CrossRef] [PubMed]
3. C.-Y. Lin, A. X. Wang, B. S. Lee, X. Zhang, and R. T. Chen, “High dynamic range electric field sensor for electromagnetic pulse detection,” Opt. Express 19(18), 17372–17377 (2011). [CrossRef] [PubMed]
4. X. Zheng, A. Sinyukov, and L. M. Hyden, “Broadband and gap-free response of a terahertz system based on a poled polymer emitter-sensor pair,” Appl. Phys. Lett. 87(8), 081115 (2005). [CrossRef]
5. C. V. McLaughlin, L. M. Hayden, B. Polishak, S. Huang, J. Luo, T.-D. Kim, and A. K.-Y. Jen, “Wideband 15 THz response using organic electro-optic polymer emitter-sensor pairs at telecommunication wavelengths,” Appl. Phys. Lett. 92(15), 151107 (2008). [CrossRef]
6. L. R. Dalton, “Polymeric Electo-Optic Materials: Optimization of Electro-Optic Activity, Minimization of Optical Loss, and Fine-Tuning of Device Performance,” Opt. Eng. 39(3), 589–595 (2000). [CrossRef]
7. L. R. Dalton, P. A. Sullivan, and D. H. Bale, “Electric Field Poled Organic Electro-Optic Materials: State of the Art and Future Prospects,” Chem. Rev. 110(1), 25–55 (2010). [CrossRef] [PubMed]
8. T. Yamada, I. Aoki, H. Miki, C. Yamada, and A. Otomo, “Effect of methoxy or benzyloxy groups bound to an amino benzene donor unit for various nonlinear optical chromophores as studied by hyper-Rayleigh scattering,” Mater. Chem. Phys. 139(2–3), 699–705 (2013). [CrossRef]
9. T. Yamada, H. Miki, I. Aoki, and A. Otomo, “Effect of two methoxy groups bound to an amino benzene donor unit for thienyl-di-vinylene bridged EO chromophores,” Opt. Mater. 35(12), 2194–2200 (2013). [CrossRef]
10. T. Yamada and A. Otomo, “Usefulness of transmission ellipsometric method for evaluation of electro-optic materials,” IEICE Trans. Electron. E98-C(2), 143–146 (2015). [CrossRef]
11. X. Piao, Z. Xianmin, Y. Mori, M. Konishi, A. Nakaya, S. Inoue, I. Aoki, A. Otomo, and S. Yokoyama, “Nonlinear optical side-chain polymers post-functionalized with high-β chromophores exhibiting large electro-optic property,” J. Polym. Sci. A Polym. Chem. 49(1), 47–54 (2011). [CrossRef]
12. L. Dalton, A. Harper, A. Ren, F. Wang, G. Todorova, J. Chen, C. Zhang, and M. Lee, “Polymeric electro-optic modulators: From chromophore design to integration with semiconductor very large scale integration electronics and silica fiber optics,” Ind. Eng. Chem. Res. 38(1), 8–33 (1999). [CrossRef]
13. M. He, T. M. Leslie, and J. A. Sinicropi, “Synthesis of chromophores with extremely high electro-optic activity. 1. Thiophene-bridge-based chromophores,” Chem. Mater. 14(2), 4662–4668 (2002). [CrossRef]
14. G. Deng, S. Bo, T. Zhou, H. Huang, J. Wu, J. Liu, X. Liu, Z. Zen, and L. Qiu, “Fasile synthesis and electro-optic activities of now polycarbonates containing tricyanofuran-based nonlinear optical chromophores,” J. Polym. Sci. A Polym. Chem. 51(13), 2841–2849 (2013). [CrossRef]
15. Y. Yang, J. Liu, M. Zhang, F. Liu, H. Wang, S. Bo, Z. Zhen, L. Qiu, and X. Liu, “The important role of the location of the alkoxy group on the thiophene ring in designing efficienet organic nonlinear optical materials based on double-donor chromophores,” J. Mater. Chem. C 3(16), 3913–3921 (2015). [CrossRef]
16. S. Liu, M. A. Haller, H. Ma, L. R. Dalton, S.-H. Jang, and A. K.-Y. Jen, “Focused microwave-assisted synthesis of 2,5-dihydrofuran derivatives as electron acceptors for highly efficient nonlinear optical chromophores,” Adv. Mater. 15(7–8), 603–607 (2003). [CrossRef]
17. J.-M. Raimundo, P. Blanchard, P. Frére, N. Mercier, I. Ledoux-Rak, R. Hierle, and J. Roncali, “Pushu-pull chromophores based on 2,2′-bi(3,4-ethylenedioxythiophene) (BEDOT) π-conjugating spacer,” Tetrahedron Lett. 42(8), 1507–1510 (2001). [CrossRef]
18. C. M. Isborn, A. Leclercq, F. D. Vila, L. R. Dalton, J. L. Brédas, B. E. Eichinger, and B. H. Robinson, “Comparison of static first hyperpolarizabilities calculated with various quantum mechanical methods,” J. Phys. Chem. A 111(7), 1319–1327 (2007). [CrossRef] [PubMed]
19. S. R. Hammond, O. Clot, K. A. Firestone, D. H. Bale, D. Lao, M. Haller, G. D. Phelan, B. Carlson, A. K.-Y. Jen, P. J. Reid, and L. R. Dalton, “Site-isolated electro-optic chromophores based on substituted 2,2′-Bis(3,4-propylenedioxythiophene) π-conjugated bridges,” Chem. Mater. 20(10), 3425–3434 (2008). [CrossRef]
20. H. Chen, B. Chen, D. Huang, D. Jin, J. D. Luo, A. K.-Y. Jen, and R. Dinu, “Broadband electro-optic polymer modulators with high electro-optic activity and low poling induced optical loss,” Appl. Phys. Lett. 93(4), 043507 (2008). [CrossRef]
21. S. Takahashi, B. Bhola, A. Yick, W. H. Steier, J. Luo, A. K.-Y. Jen, D. Jin, and R. Dinu, “Photo-stability measurement of electro-optic polymer waveguides with high intensity at 1550-nm wavelength,” J. Lightwave Technol. 27(8), 1045–1050 (2009). [CrossRef]
22. G. G. Rajeshwaran, M. Nandakumar, R. Sureshbabu, and A. K. Mohanakrishnan, “Lewis acid-mediated Michaelis-Arbuzov reaction at room temperature: a facile preparation of arylmethyl/heteroarylmethyl phosphonates,” Org. Lett. 13(6), 1270–1273 (2011). [CrossRef] [PubMed]
23. J. A. Davies, A. Elangovan, P. A. Sullivan, B. C. Olbricht, D. H. Bale, T. R. Ewy, C. M. Isborn, B. E. Eichinger, B. H. Robinson, P. J. Reid, X. Li, and L. R. Dalton, “Rational enhancement of second-order nonlinearity: bis-(4-methoxyphenyl)hetero-aryl-amino donor-based chromophores: Design, synthesis, and electrooptic activity,” J. Am. Chem. Soc. 130(32), 10565–10575 (2008). [CrossRef] [PubMed]
24. Ch. Bosshard, K. Sutter, Ph. Prêtre, H. Hulliger, M. Flörsheimer, P. Kaatz, and P. Günter, Organic Nonlinear Optical Materials (Gordon and Breach Scienece Phublishers SA, 1995).
25. V. Lazić, M. Jurković, T. Jednačak, T. Hrenar, J. P. Vuković, and P. Novak, “Intra- and intermolecular hydrogen bonding in acetylacetone and benzoylacetone derived enamione derivatives,” J. Mol. Struct. 1079, 243–249 (2015). [CrossRef]
26. H. L. Romml and B. H. Robinson, “Orientation of elctro-optic chromophores under poling conditions: A spheroidal model,” J. Phys. Chem. C 111(50), 18765–18777 (2007). [CrossRef]
