Tailoring the chemical structures and nonliear optical properties of julolidinyl-based chromophores by molecular engineering

Dyes and Pigments 173 (2020) 107876

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Tailoring the chemical structures and nonliear optical properties of julolidinyl-based chromophores by molecular engineering

T

Zhuo Chena, Airui Zhangb,∗∗, Hongyan Xiaoa, Fuyang Huoa,c, Zhen Zhena, Xinhou Liua, Shuhui Boa,∗ a

Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, PR China Division of Nanometrology and Materials Measurement, National Institute of Metrology, Beijing, 100029, PR China c University of Chinese Academy of Sciences, Beijing, 100043, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Nonlinear optical materials Chromophores Second-order Julolidine Electro-optic coefficient

To study the impact of component on performance of the nonlinear (NLO) optical properties, a series of NLO chromophores contained julolidinyl groups as the electron donor, two kinds of pi-conjugated bridge (divinylisophorone, divinylthiophene), two kinds of electron acceptor (TCF (2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran) and CF3-Ph-TCF (2-dicyanomethylene-3-cyano-4,5-dimethyl-5-trifluoromethyl-2,5dihydrofuran)) were synthesized and characterized. The microscopic nonlinear optical properties including solvent dependence of dipole moment (μ), hyperpolarizability (β), static polarizability (ɑ), and bond length alternation (BLA) of all chromophores were demonstrated by density functional theory (DFT) calculations. The macroscopic properties of EO effect were measured by simple reflection method, and the highest EO coefficient (r33) was 256 p.m./V. The solvatochromic behavior and thermal stability were also evaluated to study the structure-property relationships. This study explained the influence of microstructure on macroscopic performance in depth for the julolidinyl-based chromophores.

1. Introduction Second-order nonlinear optical (NLO) materials have been intensively studied for the past two decades due to their potential applications used in many areas, such as computing, tele-communications, microwave photonic system and the terahertz field [1–4]. Because of the large nonlinear optical coefficients, ultrafast response times, ease of synthetic design, simple preparation and low cost, organic electro-optic (EO) materials were studied and applied in many areas, especially the high-speed EO modulators, optical switches and frequency converters [5–9]. Furthermore, the dielectric constant and refractive index make the organic EO materials easy to realize the high frequency devices. For organic materials, the most distinctive thing is that the flexible molecular design and engineering can help to achieve better thermal stability and NLO properties. To meet the active requirements for the use of devices, one of the most crucial challenges is to develop NLO chromophores with large EO activity, excellent stability and good transparency [10,11]. Besides, the microscopic hyperpolarizability (β) must be translated to macroscopic EO activity effectively [12,13]. Typical organic EO material (chromophores molecules) has a ∗

π–electron conjugated structure coupled to electron donor and acceptor units which usually possesses a rod-like structure leading to strong intermolecular dipole–dipole interactions in the polymeric matrix, and the noncentrosymmetric alignment of the chromophores through poling method is very difficult [14,15]. In order to obtain the highly efficient NLO chromophores, optimization of the π-conjugated bridge, electrondonor and electron-acceptor is necessary [16,17]. The large β values of the chromophores can be achieved by careful modification of the strength of electron donor and acceptor units, as well as the ability to transfer electrons of π-conjugated. In addition, introducing suitable isolation groups was efficient to minimize intermolecular dipole–dipole interaction and enhance the poling efficiency, which can increase the EO activities effectively [18–20]. So optimizing the chromophore system and the surrounding environment may be the reasonable method to achieve excellent performance. However, most research is focused on the improving the performance of these units separately, and the overall molecular optimization of NLO chromophores is rarely studied. In recent years the stronger electron donor julolidinyl group which was firstly reported in our lab [15,21,22], electron acceptor attaching the halogen atom like 2-

Corresponding author. Corresponding author. E-mail addresses: [email protected] (A. Zhang), [email protected] (S. Bo).

∗∗

https://doi.org/10.1016/j.dyepig.2019.107876 Received 11 July 2019; Received in revised form 9 September 2019; Accepted 9 September 2019 Available online 10 September 2019 0143-7208/ © 2019 Published by Elsevier Ltd.

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Scheme 1. The Synthetic route of Chromophore B and Chromophore C.

reference). The MS spectra were obtained on MALDI-TOF (matrix assisted laser desorption/ionization of flight) on a BIFLEXIII (Broker Inc.) spectrometer. The UV–Vis spectra were recorded on a Cary 5000 photo spectrometer. The TGA was determined using TA5000-2950TGA (TA Co) at a heating rate of 10 °C/min under the protection of nitrogen.

dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) and 2-dicyanomethylene-3-cyano-4,5-dimethyl-5-trifluoromethyl-2,5-dihydrofuran (CF3-Ph-TCF) [23], or twisted π-conjugated systems all display just passable EO coefficient [24–26]. In some conditions, if the electron donor, acceptor and the π-conjugated bridge of chromophores are too strong, the high macroscopic NLO properties (r33 value) may not be available. We focused our attention at the effect on the microscopic and macroscopic NLO properties by changing the combination of the electron donor, π-conjugated bridge and electron acceptor. To achieve the high macroscopic NLO properties, tailoring the structures of the chromophore units is very important [27–29]. In this paper, using the julolidinyl groups as the electron donor, six types of chromophores were designed and synthesized with either divinylisophorone and divinylthiophene as the π-conjugated bridge, with TCF and CF3-Ph-TCF as the electron acceptor (As showed in Chart 1). By comparing the microscopic properties including dipole moment (μ), static polarizability (ɑ), hyperpolarizability (β) and bond length alternation (BLA) demonstrated by density functional theory (DFT) with the macroscopic property of EO effect (r33), the effect of molecular structure and engineering on the performance of organic EO materials was discussed. The best combination of the three parts was optimized and the biggest electro-optic coefficient of 256 p.m./V was obtained.

2.2. Synthesis and characterization 2.2.1. Preparation of compound 2 Compound 1 (8-hydroxy-1,1,7,7-tetramethyljulolidine-9-carboxaldehyde, 5.46 g, 0.02 mol), tert-butyl ((6-chlorohexy)oxy) dimethylsilane (7.50 g, 0.03 mol) and K2CO3 (4.16 g, 0.03 mol) were added into anhydrous DMF under N2 atmosphere. The mixed solution irradiated under microwaves for 40 min at 140 °C, then cooled externally to stop the reaction. When the mixture was cooled to the room temperature, filtered to remove the K2CO3, and then extracted with ethyl acetate. The combined organic solution was washed with saturated brine and then dried by MgSO4 overnight and the filtered. After the removal of the solvent, the residue was purified by column chromatography using hexane and acetone (V/V, 4/1) as eluent to give the product yellow solids (7.46 g, yield: 82%). 1H NMR (400 MHz, CDCl3): δ 9.92 (s, 1H), 7.57 (s, 1H), 3.96 (t, J = 6.8 Hz, 2H), 3.66 (t, J = 6.2 Hz, 2H), 3.39–3.31 (m, 2H), 3.31–3.22 (m, 2H), 1.96–1.84 (m, 2H), 1.76–1.66 (m, 4H), 1.56 (dd, J = 14.8, 8.1 Hz, 4H), 1.47 (d, J = 6.6 Hz, 2H), 1.42 (s, 6H), 1.24 (s, 6H), 0.89 (s, 9H), 0.04 (s, 6H). 13C NMR (101 MHz, CDCl3): δ 187.83, 162.08, 148.38, 126.09, 125.52, 120.73, 117.03, 78.60, 62.44, 47.46, 46.81, 39.34, 35.62, 32.63, 32.43, 32.02, 30.22, 30.00, 29.73, 25.76. MS (MADIL-TOF) calcd for C29H29NO3Si (M+): 487.35, found: 487.19. Anal. Calcd (%) for C29H29NO3Si: C, 71.41; H, 10.13; N, 2.87; found: C, 71.31; H, 10.18; N, 2.93.

2. Experimental section 2.1. Materials and instrument All chemicals are commercially available and are used without further purification unless otherwise stated. Compound (8-hydroxy1,1,7,7-tetramethyljulolidine-9-carboxaldehyde) was purchased in the J & K company. N,N-Dimethylformamide (DMF) and tetrahydrofuran (THF) were distilled overcalcium hydride and stored over molecular sieves. The synthesis of CF3-Ph-TCF acceptor and Chromophore A, D, E and F were synthesized according to the previous published work in our group [30]. TLC analyses were carried out on 0.25 mm thick precoated silica plates and spots were visualized under UV irradiation. Chromatography on silica gel was carried out on Kieselgel (200–300 mesh). 1 HNMR spectra were recorded using a Bruker Advance 400 (400 MHz) NMR spectrometer (tetramethylsilane as an internal

2.2.2. Preparation of compound 3 Compound 2 (4.87 g, 0.01 mol) and thiophene phosphate salt (5.26 g, 0.012 mol) were added into anhydrous ether (50 mL), after NaH (2.4 g, 0.1 mol) was added to the solution, the vessel was sealed and the mixed solution was stirred under room temperature by 24 h. The resulting solutions were filtered and then extracted with ethyl acetate three times. The combined organic solution was washed with saturated brine and then dried by MgSO4 overnight and the filtered. 2

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Fig. 1. The UV–Vis–NIR spectra of the chromophores.

2.2.3. Preparation of compound 4 Compound 3 (2.69 g, E/Z, 4.8 mmol) was dissolved into THF (30 mL), then the solution was cooled to −78 °C, the n-BuLi in hexanes (4.5 mL, 1.6 M, 7.2 mmol) was added into the solution with drop under this temperature. When the solution was stirred by 1 h, DMF (0.45 g, 6.2 mmol) was added to the solution with drop in 5min, and then the mixed solution was stirred for 1 h. When the temperature of the solution was raised to the room temperature slowly, 10 mL deionized water was added to stop the reaction. The resulting solution was filtered and then extracted with ethyl acetate three times. The combined organic solution was washed with saturated brine and then dried by MgSO4 overnight and the filtered. The solvent was removed by vacuum distillation, and the product was purified by column chromatography

The solvent was removed by vacuum distillation, and the product was purified by column chromatography using hexane and acetone (V/V, 6/ 1) as eluent to give the oily orange solid (4.76 g, yield: 84.2%, E/Z mixture). 1H NMR (400 MHz, CDCl3): δ 7.10 (s, 0.6H), 7.03 (dd, J = 9.5, 5.4 Hz, 1.2H), 7.00–6.96 (m, 1H), 6.92 (dd, J = 9.6, 6.4 Hz, 1.4H), 6.87–6.79 (m, 0.6H), 6.57 (d, J = 11.9 Hz, 0.6H), 6.47 (d, J = 11.9 Hz, 0.6H), 3.86–3.78 (m, 2H), 3.56 (q, J = 6.9 Hz, 2H), 3.04 (ddd, J = 18.2, 11.6, 5.8 Hz, 4H), 1.75–1.61 (m, 6H), 1.50 (ddd, J = 21.0, 14.0, 7.1 Hz, 4H), 1.37 (d, J = 4.2 Hz, 6H), 1.35–1.27 (m, 2H), 1.25 (s, 3H), 1.12 (s, 3H), 0.85 (s, 9H), 0.03 (s, 3H), 0.00 (s, 3H). MS (MADIL-TOF) calcd for C34H53NO2SSi (M+): 567.36, found: 567.58. Anal. Calcd (%) for C34H53NO2SSi: C, 71.90; H, 9.41; N, 2.47; found: C, 71.74; H, 9.48; N, 2.57. 3

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When the reaction was completed, the solution was cooled down to the room temperature. The solvent was removed by vacuum distillation, and the product was purified by column chromatography using hexane and acetone (V/V, 8/1) as eluent to give the brown solid (1.82 g, yield: 75%), 1H NMR (400 MHz, CDCl3): δ 7.72 (d, J = 15.1 Hz, 1H), 7.53 (s, 5H), 7.53 (s, 5H), 7.38 (d, J = 15.8 Hz, 1H), 7.38 (d, J = 15.8 Hz, 1H), 7.28 (s, 3H), 6.98 (d, J = 15.3 Hz, 2H), 6.58 (d, J = 15.1 Hz, 1H), 3.83 (t, J = 6.5 Hz, 2H), 3.64 (t, J = 6.4 Hz, 2H), 3.28 (t, J = 5.9 Hz, 2H), 3.23–3.19 (m, 2H), 1.91 (s, 2H), 1.73 (s, 4H), 1.60 (d, J = 6.4 Hz, 6H), 1.53–1.44 (m, 3H), 1.42 (s, 6H), 1.29 (s, 6H). 13C NMR (101 MHz, CDCl3): δ 172.08, 155.12, 149.48, 144.71, 142.87, 136.52, 134.24, 132.94, 132.63, 128.82, 127.78, 127.35, 127.06, 126.27, 125.29, 123.13, 120.70, 119.94, 118.34, 114.65, 111.97, 88.16, 72.69, 63.04, 54.04, 48.98, 40.94, 38.61, 37.47, 30.77, 28.80, 28.64, 28.14, 26.58, 25.58, 20.61, −4.33. MS (MADIL-TOF) calcd for C51H59F3N4O3SSi (M+): 892.40, found: 892.03. Anal. Calcd (%) for C51H59F3N4O3SSi: C, 68.58; H, 6.66; N, 6.27; found: C, 68.48; H, 6.71; N, 6.33.

Table 1 The maximum UV–Vis–NIR absorption wavelength in different solvents. Chromophore Chromophore Chromophore Chromophore Chromophore Chromophore Chromophore

A B C D E F

λa)

λb)

λc)

λd)

λe)

Δλf)

647 713 730 655 765 755

673 778 775 685 805 803

727 855 868 744 975 966

661 809 802 686 982 978

667 812 803 688 975 973

80 142 138 89 225 224

a) b) c) d) e) The maximum absorption wavelength (nm) of chromophores measured in dioxane, toluene, chloroform, acetone, acetonitrile, respectively. f) Δλ = λ(max)–λ(min). λ(max) and λ(min) are the maximum and minimum absorption wavelength of five solvents, respectively.

Table 2 The thermal properties of the chromophores. Chromophore Chromophore Chromophore Chromophore Chromophore Chromophore Chromophore

Td/°C A B C D E F

247 209 235 253 202 213

2.2.4. Preparation of Chromophore C The Chromophore B (1.1 g, 1.12 mmol) was dissolved into 10 mL acetone and 10 mL deionized water, HCl solution (5 mL, HCl:H2O = 1:10) was dropped into it. The solution was stirred at room temperature for 5 h. The solvent was removed by vacuum distillation, and the resulting solution was extracted with ethyl acetate three times. The combined organic solution was washed with saturated brine and then dried by MgSO4 overnight and the filtered. The solvent was removed by vacuum distillation, and the product was purified by column chromatography using hexane and acetone (V/V, 3/1) as eluent to give the black solid (0.795 g, yield: 90.0%). 1H NMR (400 MHz, CDCl3): δ 7.72 (d, J = 15.1 Hz, 1H), 7.53 (s, 5H), 7.38 (d, J = 15.8 Hz, 1H), 7.28 (s, 3H), 6.98 (d, J = 15.3 Hz, 2H), 6.58 (d, J = 15.1 Hz, 1H), 3.83 (t, J = 6.5 Hz, 2H), 3.64 (t, J = 6.4 Hz, 2H), 3.28 (t, J = 5.9 Hz, 2H), 3.24–3.17 (m, 2H), 1.91 (s, 2H), 1.73 (s, 4H), 1.60 (m, 4H), 1.53–1.44 (m, 2H), 1.42 (s, 6H), 1.29 (s, 6H). 13C NMR (101 MHz, CDCl3): δ 175.60, 161.58, 159.64, 158.08, 145.50, 141.53, 137.78, 133.86, 131.55, 129.80, 127.66, 127.24, 126.90, 123.28, 122.39, 116.38, 115.06, 111.42, 111.08, 76.38, 62.74, 47.27, 39.64, 35.99, 32.72, 32.24, 30.58, 30.23, 29.93, 26.32, 25.93. MS (MADIL-TOF) calcd for C45H45F3N4O3S (M+): 778.32, found: 778.90. Anal. Calcd (%) for C45H45F3N4O3S: C, 69.39; H, 5.82; N, 7.19. found: C, 69.23; H, 5.89; N,

using hexane and acetone (V/V, 4/1) as eluent to give the orange solid (2.21 g, yield: 74.2%, Z). 1H NMR (400 MHz, CDCl3): δ 9.82 (s, 1H), 7.64 (d, J = 3.9 Hz, 1H), 7.33 (d, J = 16.1 Hz, 1H), 7.29 (s, 1H), 7.06 (d, J = 3.9 Hz, 1H), 6.97 (d, J = 16.0 Hz, 1H), 3.85 (t, J = 8.6 Hz, 2H), 3.63 (t, J = 7.8 Hz, 2H), 3.26–3.19 (m, 2H), 3.19–3.10 (m, 2H), 1.94–1.86 (m, 2H), 1.80–1.74 (m, 4H), 1.61–1.55 (m, 4H), 1.46 (d, J = 12.4 Hz, 8H), 1.32 (s, 6H), 0.90 (s, 9H), 0.06 (s, 6H). MS (MADILTOF) calcd for C35H53NO3SSi (M+): 595.35, found: 595.36. Anal. Calcd (%) for C35H53NO3SSi: C, 70.54; H, 8.96; N, 2.35; found: C, 70.44; H, 8.99; N, 2.43. 2.2.4. Preparation of Chromophore B Compound 4 (2.98 g, 0.005 mol) and CF3-Ph-TCF (1.7 g, 0.0055 mol) was dissolved into ethanol (10 mL), and the mixture solution was put into the microwave reactor for 15 min under 120 °C.

Fig. 2. The front and side optimized structure of the chromophores. 4

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Fig. 3. The solvation effect of microcosmic electro-optic activity (a) μtot, (b) α, (c) βtot, (d) BLA for the six chromophores. Table 3 The values of microcosmic electro-optic activity in vacuum and solvents. Compd

Sol

μ (D)

α (a.u).

Β (10−30esu)

BLA (Å)

HOMO (eV)

LUMO (eV)

Eg (EV)

Chromophore A

VC DO CF AN VC DO CF AN VC DO CF AN VC DO CF AN VC DO CF AN VC DO CF AN

21.53 26.34 30.53 35.23 22.76 29.10 34.72 42.28 22.97 29.25 34.80 42.30 24.38 31.29 36.75 44.15 25.72 34.03 41.84 52.65 25.94 34.23 41.99 53.20

890.8 1094.5 1277.6 1524.6 988.0 1252.9 1494.8 1812.0 899.0 1152.6 1387.1 1688.7 831 1299.6 1547.0 1859.3 946 1465.5 1759.6 2022.1 905 1369.3 1655.2 1899.6

716.28 1542.07 2263.19 3922.83 971.79 2035.45 3320.18 4573.30 1000.62 2220.07 3364.66 4541.40 978.7 2240.2 3348.4 4429.6 1180.1 2349.6 2917.6 1327.8 1208.4 2478.9 3124.6 1314.5

−0.037 −0.029 −0.022 −0.013 −0.031 −0.019 −0.010 0.004 −0.031 −0.019 −0.009 0.003 −0.044 −0.035 −0.025 −0.012 −0.039 −0.024 −0.011 0.007 −0.039 −0.024 −0.011 0.007

−5.11 – – – −5.15 – – – −5.16 – – – −5.06 – – – −5.09 – – – −5.04 – – –

−3.20 – – – −3.30 – – – −3.30 – – – −3.23 – – – −3.31 – – – −3.25 – – –

1.91 – – – 1.85 – – – 1.86 – – – 1.83 – – – 1.77 – – – 1.79 – – –

Chromophore B

Chromophore C

Chromophore D

Chromophore E

Chromophore F

were filtered through a 0.22 μm filter. Thin films were prepared by spincoating onto indium–tin oxide glass substrates. Prior to the poling process, these thin films were dried in vacuum for 24 h to remove residual solvent, and the thickness of the films is about 1.5 μm. Poling process and EO coefficient: A thin layer of aluminum (50 nm)

7.29. 2.2.5. Preparation of thin films The chromophores (20 wt%) were dissolved into amorphous polycarbonate (APC) using dibromoethane as the solvent. The solutions 5

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Fig. 4. HOMO and LUMO energy level in vacuum of the chromophores.

Fig. 5. The donors, bridges and acceptors of six chromophores.

was sputtered onto the film as the top electrode to perform the electricfield poing. The film was contact-poled at poling temperature (Tp), which was approximately 5 °C higher than Tg (130–145 °C) of the sample. When the temperature reached the Tp, the poling voltage ranging from 75 to 125 V μm−1 was applied and followed by maintaining the poling voltage at Tp for 10 min under a nitrogen atmosphere. Both poling fields and currents were monitored in situ to optimize the entire process. The EO coefficients of the poled films were determined at 1310 nm using the simple reflection technique proposed by Teng and Man [38].

CF3 substituents (CF3-Ph-TCF) showed better accepting strength than TCF, so the CF3-Ph-TCF was used to obtain the new materials with a higher electro-optic coefficient [32]. Starting from 8-hydroxy-1,1,7,7-tetramethyljulolidine-9-carboxaldehyde, chromophores were synthesized through several step reactions. The synthesis of Chromophore A was synthesized according to the previous published work in our group [30]. The synthetic route of Chromophore B and C was shown in Scheme 1. Compound 2 reacted with witting salt following Witting reaction to obtain Compound 3. Compound 4 was obtained by reaction of compound 3 with n-BuLi followed by reaction with DMF. The final condensations with the CF3Ph-TCF acceptor gave chromophore B and C as dark solids. During the synthesis process, Wittig salt was abtained from thiophene-3-methanol after salt formation. All the chromophores were completely characterized by 1H NMR, 13C NMR, MS, UV–Vis–NIR spectroscopic analysis and the data obtained were in full agreement with the proposed formulations. The microwave (MW) heating method was proved effectively for the substitution and condensation reactions. In this paper, the MW

3. Results and discussion 3.1. Synthesis and characterization of chromophores In this work, the similar julolidine group was selected as the electron donor it has been reported with the better solubility, stronger electro-donating ability and large steric hindrance than the classical dimethyl anilino moiety [30,31]. As we know, the TCF acceptors with 6

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The solvatochromic behavior was also explored to investigate the polarity of the chromophores. The Chromophore A, B and C showed bathochromic shifts of 80 nm, 142 nm and 138 nm when the solvent was changed from dioxane to chloroform, respectively. Chromophore A exhibited much weaker bathochromic shifts in comparison to Chromophore B and C. It indicated that Chromophore B and C were more polarizable than Chromophore A. The Chromophore D, E and F showed the similar situation. When the acceptor was normal TCF, the λmax of the Chromophore A and D exhibited a red-shift initially and then followed by a blue-shift with the increase of the dielectric constant from dioxane to chloroform along with the nearly identical spectral shape. As the acceptor was CF3Ph-TCF, the broad absorption bands were also observed in more polar solvents. However, in more polarity solvents from chloroform or acetone to acetonitrile, they could be both polarized close to the cyanine limit (the neutral and charge-separated (zwitterionic) resonance forms contributed equally to the ground state) because of their stronger donor and acceptor pair, and the Chromophore E and F had the shoulder peak in the stronger polarity solvent [33]. At the same time, the Chromophore B and C showed no shoulder peak in all solvents, which indicated that the amount of charge-separated (zwitterionic) resonance form was small because of the weaker transfer electronic ability of divinylthiophene than the divinylisophorone. As shown in Fig. 1 and Table 1, the order of ease of polarizability is Chromophore E≥Chromophore F > Chromophore B≥Chromophore C > Chromophore D > Chromophore A, according to the Δλ value. Furthermore, the Chromophore E and F have almost the same polarizaibility, and the Chromophore B and C are also the case.

Table 4 The Mulliken charge of six chromophores in vacuum and three kinds of solvents. Compound Chromophore A

Chromophore B

Chromophore C

Chromophore D

Chromophore E

Chromophore F

D1 B2 A1 D1 B2 A2 D2 B2 A2 D1 B1 A1 D1 B1 A2 D2 B1 A2

VC

DO

CF

AN

0.2227 −0.0423 −0.2662 0.1835 −0.1322 −0.2455 0.1802 −0.2011 −0.2776 0.2149 0.1180 −0.3038 0.2350 0.2056 −0.3800 0.2332 0.2029 −0.3864

0.2451 −0.0349 −0.3131 0.3101 −0.0023 −0.4380 0.3099 −0.0038 −0.4362 0.2624 0.2111 −0.3878 0.2965 0.2376 −0.4901 0.2974 0.2364 −0.4898

0.3016 −0.0189 −0.3829 0.3599 0.0121 −0.5026 0.3119 −0.0012 −0.4619 0.3011 0.2334 −0.4534 0.3547 0.2586 −0.5748 0.3540 0.2594 −0.5746

0.3450 −0.0075 −0.4431 0.4284 0.0280 −0.5933 0.3363 0.0078 −0.5081 0.3525 0.2575 −0.5333 0.4342 0.2822 −0.6843 0.3174 0.2699 −0.5729

VC: Vacuum, DO: Dixoane, CF: CHCl3, AN: Acetonitrile.

methodology was introduced to the Knoevenagel condensation of the final step in the chromophore synthesis, and the reaction was carried out at 120 °C for 15 min with the high yields (75%) [31]. All the chromophores exhibited good solubility in common organic solvents, such as acetone, DMSO, CH2Cl2, DMF, CHCl3, and THF.

3.2. UV–Vis–NIR spectra analysis 3.3. Thermal analysis In order to reveal the charge-transfer (CT) absorption properties of each chromophore and explore the effect of the chemical structure on the intramolecular charge-transfer (ICT) of dipolar molecules, the chromphores A-F were dissolved in six solvents (10−5 M) with different dielectric constants and the UV–Vis–NIR spectra were recorded (Fig. 1). As shown in Fig. 1 and Table 1, the Chromophore B and C exhibited the similar maximum absorption (λmax) because of the similar π→π* ICT system. Compared to the Chromophore A, B and C showed the maximum absorption (λmax) of 855 and 868 nm in chloroform, and red shifted nearly 130 nm because of the different acceptors (TCF and CF3Ph-TCF). The same situation pertained with the Chromophore D. However, the Chromophore E and F showed the λmax of 982 nm and 978 nm in acetone, respectively. The Chromophore D with the divinylisophorone was red-shifted in their absorption spectra, compared to the Chromophore A with the divinylthiophene bridge, probably due to the divinylisophorone. Furthermore, the Chromophore E and F exhibited larger red shift than the Chromophore D because of the strong accoptor CF3-Ph-TCF. The siloxane group in the Chrmophore B and E caused the absorption maximum to undergo a small red-shift ompared with the Chromophore C and F.

The thermal properties of the Chromophore A–F were evaluated by the thermal gravimetric analysis (TGA) under a nitrogen atmosphere with the results shown and tabulated in Table 2. Compared with the Chromophore A and D, The Td of Chromophore B, C, E and F were lower, because the stability of the CF3-Ph-TCF acceptor was worse than that of the parent TCF group. As the chromophores with TCF as the acceptor, the Chromophore D with divinylisophorone as the bridge showed the higher thermal stability than the Chromophore A with divinylisophorone as the bridge. However, as the chromophores with the CF3-Ph-TCF acceptor, the Chromophore B and C with thiophene as the bridge showed the higher Td than the Chromophore E and F with divinylisophorone as the bridge. It indicated that the Chromophore E and F with divinylisophorone bridges (strong electron transfer ability), CF3Ph-TCF (strong electron-withdrawing ability) may be more prone to instability because of the more zwitterionic structure. The Td of Chromophore B and E was respectively lower than Chromophore C and F, which indicated that the introduction of the siloxane group could reduce the molecular pitch and stacking and led to reduce thermal stability of the chromophore.

Fig. 6. The possible states of the dipole moleculars. 7

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Chart 1. The structures of the chromophores.

divinylthiophene as the electron bridge, the βtot value rose with increasing of the solvent polarity, and the βtot value of the chromophores with the strong electron acceptor of CF3-Ph-TCF was higher than the chromophores with the electron acceptor of TCF, which showed that the increasing of the electron acceptor strength improved the microscopic nonlinear optical properties. As the chromophores with the divinylisophorone bridge, the βtot value of the Chromophore D with the ordinary TCF rose with the increasing of the solvent polarity, while the βtot value of the Chromophore E and F with CF3-Ph-TCF showed the trend of increasing first and then decreased and reached the biggest value in chloroform. Bond-length alternation (BLA) was defined as the average difference for length between adjacent carbon–carbon double bonds and single bonds in the chromophore molecule. The ratio of two limiting resonance forms (neutral and zwitterionic form) in ground state could be regulated by the molecular structure design, thus optimize the NLO performance of the chromophores [34,35]. As shown in Fig. 3 (d), the BLA value of the six chromophores was negative in low polar solvents, indicating that the neutral form was dominant in ground state. In high polar solvent (AN), the BLA value of the chromophore with strong CF3Ph-TCF acceptor was positive, while the chromophore with TCF acceptor was negative. This indicated that the zwitterionic form was dominant at ground state of the chromophore with strong CF3-Ph-TCF acceptor in the high polar solvents, and the neutral form was dominant at ground state of the chromophore with strong TCF acceptor. For the push-pull polyene chromophore, the βtot was influenced by ground state polarization as well as BLA [36]. When the chromophores with the divinylisophorone as the electron bridge, the BLA value of the Chromophore E and F was closer to zero than Chromophore D, leading to the lower βtot and attributing to the cyanine limit electron structure inferred by UV–Vis–NIR data. When the chromophore with the divinylthiophene as the electron bridge, the BLA value of the Chromophore B and C was closer to zero than Chromophore A, but the βtot was higher. The phenomenon was different from the chromophore with the divinylisophorone as the electron bridge, indicating that the chromophore with the heterocycle did not conform to the β and BLA rule which is just for the polyene bridge. On the other hand, the Chromophore E and F had almost the same BLA value, and the similar happened in the Chromophore B and C, indicating that the side chain had almost no effect on the main chain electron transfer. In summary, the strong electron donor and acceptor did not always lead to the large βtot value and it would vary with the change of the electron bridge. So the reasonable molecular design to seek the best combination of donor and acceptor strength was a very important premise for the preparation of

3.4. DFT calculations To study the effect on the chromophore microscopic parameters with the changing of the acceptors and bridges, structure optimization of the choromphores was carried out by DFT calculation. The structure optimization of the choromphores in three different solvents and vacuum was calculated by B3LYP/6-31G level. Based on the structure optimization, the microscopic properties of the chromophores including dipole moment (μ), polarizability (α), bond length alternation (BLA) and the static total first-order hyperpolarizability (βtot) parameters were calculated by CAM-B3LYP/6-31 + G* level. The electronic transfer shifting property of the chromophores was calculated by CAMB3LYP/6-31 + G* level (time-dependent, TD). 3.4.1. The microscopic nonlinear optical property of the chromophores As Fig. 2 showed, the conjugate plane planarity of all the chromophores was good and beneficial to ICT. There were two independent methyl on the divinylisophorone bridge of the chromophore D, E and F, which led to the bigger steric than the chromophore with the divinylthiophene as the electronic bridge. Among the chromophores with different acceptors, the distance between the chromophores side-chain and the conjugate plane of the chromophores with the CF3-Ph-TCF as the acceptor was greater than the chromophores with the common TCF as the acceptor, and the benzene of the CF3-Ph-TCF acceptor provided the more steric apartment. The values of microscopic nonliear properties in solvents and vacuum were shown in Fig. 3, and the data was shown in Table 3. As was shown in Fig. 3 (a) and (b), the μtot and α value were increasing with the addition of the solvent polarity and the electron acceptor strength. Meanwhile, the μtot and α values of the chromophores with the divinylisophorone as the electron bridge (Chromophore D, E and F) were higher than the chromophores with the divinylthiophene as the electron bridge (Chromophore A, B, and C). At the same time, the μtot and α value of the chromophores with the CF3-Ph-TCF as the electron acceptors were higher than the chromophores with the ordinary TCF as the electron acceptors. This phenomenon may be caused by the higher ICT ability of chromophores which led to the higher μtot and α value. The results of μtot and α corresponded with the conlusion of UV–Vis–NIR spectrum analysis. As shown in Fig. 3(c), it was noteworthy that the change trend of βtot value was different between the chromophores with the divinylisophorone as the electron bridge (Chromophore D, E, and F) and the chromophores with the divinylthiophene as the electron bridge (Chromophore A, B and C). When the chromophores with the 8

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and the solution polarity. As the chromophores with the divinylisophorone electron bridges, some different phenomenon appeared. In addition, the β value of Chromophore D with ordinary TCF acceptor increased with the increasing of the solution polarity. The β value of Chromophore E and F with strong CF3-Ph-TCF acceptor showed the phenomenon of the first increasing after the decreasing. To study the relationship between the electro-optic coefficient of the EO-polymer (macroscopic property) and the first-order hyperpolarizability β value of chromophore molecular (microscopic property), the chromophores were doped into APC (amorphous polycarbonate) film as the content 20 wt% for the electro-optic coefficient test. The EO coefficient of Chromophore A-E is 74 p.m./V, 256 p.m./V, 230 p.m./V, 192 p.m./V and 61 pm/V, respectively. The large molecular weight, the narrow ΔE and without the protection of TBDMS groups resulted in poor compatibility of Chromophore F in APC. Thus, low content (10 wt %) of Chromophore F was doped in APC, and the EO coefficient of Chromophore F is only 24 p.m./V [31]. The electro-optic coefficient from the EO-polymer and the firstorder hyperpolarizability β value of the chromophores containing the divinylthiophene bridge and divinylisophorone bridge were compared and all showed the similar trend. For the Chromophore A, B and C with the divinylthiophene bridge, the EO effect of Chromophore A with TCF acceptor was weaker than Chromophore B and C with strong acceptor CF3-Ph-TCF. So the Chromophore B showed the highest electro-optic coefficient. This is because the Chromophore B has the strong donor and acceptor and the relatively weak bridge (divinylthiophene), and the state B in Fig. 6 of the chromophore B is dominant. The electro-optic coefficient (230 p.m./V) of the Chromophore C is lower than the chromophore B because of the siloxane group which is the good steric group for reducing the dipole-dipole interaction. However, for the Chromophore D, E and F with divinylisophorone bridge, the EO effect of Chromophore D with TCF acceptor was stronger than Chromophore E and F with acceptor CF3-Ph-TCF. This may be because that every part (julolidine donor, divinylisophorone bridge or CF3-TCF) in Chromophore E was all too strong to approach unstable zwitterionic (the state C in Fig. 7) which in ture reduced the EO effect [35,36].

high EO performance chromophore. 3.4.2. The energy gap analysis of the six chromophores The energy gaps of the chromophores in the vacuum are showed in Table 3. The ΔE value of the chromophores with the divinylthiophene as the electron bridge (Chromophore A, B and C) were higher than that of the chromophores with the divinylisophorone as the electron bridge, which agreed with the of the absorption maximum results of UV absorption spectrum analysis. As the chromophore with the same electron bridge, the ΔE value of the chromophores with the CF3-Ph-TCF as the acceptor were lower than the chromophores with the TCF as the acceptor, and this also agreed with the more redshift of the chromophores with the CF3-Ph-TCF as the acceptor analyzed by the UV–Vis–NIR absorption spectrum. VC: Vacuum, DO: Dixoane, CF: Chloroform, AN: Acetonitrile. The ground state electronic structure of dipolar molecules could be indicated directly by the Frontier molecular orbitals. Fig. 4 showed the HOMO and LUMO energy level in vacuum of the chromophores. It could be concluded that all the chromophores showed the good planarity, which did benefit to the intramolecular electron delocalization. The electron cloud of HOMO mainly localized on the julolidinyl-based donor and conjugated bridge, while the electron cloud of LUMO mainly localized on the TCF or CF3-Ph-TCF acceptor group and the conjugated bridge, especially in the acceptor. For the LUMO, the electron cloud was more concentrated in the acceptor of the chromophores with the strong acceptor CF3-Ph-TCF than the chromophores with the acceptor TCF. This was due to the strongger electron withdrawing ability of the CF3Ph-TCF acceptor. The ICT of the chromophores mainly took place in the transition from HOMO to LUMO, which was proved by the simulation calculation and the distribution diagram of the electron cloud. This explained the energy gap of the Chromophore D was bigger than the Chromophore E and F, and the energy gap of the Chromophore A was bigger than Chromophore B and C. This was consistent with the results of the chromophore's energy gap. 3.4.3. The analysis of the calculated Mulliken charges The Mulliken charges analysis was a useful method for studying the ground state electronic structure and measuring the molecular polarization (Fig. 5) [37]. The Mulliken charges of the chromophores in the vacuum and three kinds of solvent were shown in Table 4. For studying the solvation effect of the Mulliken charges, the Mulliken charges of six kinds of donor (D), bridge (B) and acceptor (A) in three solvents were calculated. With the increasing of the solvent polarity, the charge of the chromophores' donors became more positive, while the charge of the chromophores’ acceptors became more negative, indicating that the increasing of the solvent polarity could promote the increasing of ICT. On the other hand, the different phenomenon appeared in the different bridge. When the divinylthiophene was used as the charge bridge, the charge distribution of the bridge was influenced by acceptor. If the acceptor was TCF (Chromophore A), the charge of the bridge was negative and become less negative with the increasing of the solvent polarity. If the acceptor was CF3-Ph-TCF, it showed negative in the low polarity solvent and positive in high polarity solvent. When the divinylisophorone was used as the charge bridge, the part of bridge always showed positive charge and the positive charge increased with the solvent polarity. It all explained that the degree of the intramolecular charge transfer from donor to acceptor become much more with the increasing of the solvent polarity.

4. Conclusions Six kinds of chromophores with julonidine donor based on divinylisophorone/divinylthiophene π-bridges and TCF/CF3-Ph-TCF accpetor were prepared and investigated successfully. The trend of these properties with the changing of the stucture of the chromophore were compared, studied and discussed in deepth. The macroscopic and microscopic properties of the chromophores including thermal stability, photophysics and EO performance were explored by experimental and calculated data. The different component of the chromophores led to the difference of the space structure, physical property, the calculated resultsby DFT and so on. It can be learned from the results that the chromophores with divinylthiophene electronic bridge showed the better thermal stability, while the chromophores with divinylthiophene electronic bridge and CF3-Ph-TCF as the acceptor showed the highest EO coefficient with the highest value 256 p.m./V. This might offer a general principle for molecular design of the chromophore with specific requirement, a theoretical and practical value for the development of the new EO materials for high performance devices. Conflicts of interest We declare that we have no financial and personal relationships with other people or organizations that represents a conflict of interest in connection with our work submitted.

3.5. The macroscopic electro-optic coefficient measurements According to the above results and discussion, different electron bridges and acceptors made the chromophore different in the space structure, photophysical properties and DFT calculation results. As the chromophores with the thiophene electron bridges, the first-order hyperpolarizability β increased with the increasing of acceptor intensity

Acknowledgements We are grateful to the National Natural Science Foundation of China 9

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(No. 21504099 and U1733106), the National Key Research and Development Program of China (grant no. 2016YFB0402004), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2017033) and Key Laboratory of Optical Fiber Sensing and Communications (Education Ministry of China) for the financial support.

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