Improved poling efficiency of polyurethanes containing spindle-like chromophores by a functional group tuning for nonlinear optic (NLO) materials

Abstract

Novel polyurethanes NLO materials (IPDI-1 to IPDI-3) containing different spindle-like chromophores were designed and synthesized. Spindle-like chromophores (OHSTC-1 to OHSTC-4) bearing 3,4,5-trifluorophenyl, phenyl, trifluoromethylphenyl and 4-methoxyphenyl on the conjugated bridge as lateral groups, respectively, have been successfully synthesized. Selected chromophores were used as monomers to prepare polyurethane NLO materials. The chemical structure of the chromophores and polyurethanes was characterized by ¹H-NMR, FT-IR and UV-Vis spectroscopy. Thermal and nonlinear optical activity characterizations of the chromophores and polyurethanes IPDI-1 and IPDI-2 were carried out. Both the chromophores and the polyurethanes NLO materials possessed relatively high thermal stability. The chromophores OHSTC-1 and OHSTC-2 displayed high thermal stability with 5% weight loss temperature (T_d) as 324 °C and 327 °C, respectively. Novel polyurethanes NLO materials containing different spindle-like chromophores could be oriented without any other doped polymers and exhibited a satisfactory temporal stability in the NLO response (the electro-optic coefficients of IPDI-2 maintained up to 90% of the initial value after being baked at 85 °C in air for 200 hours).

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Introduction

Organic nonlinear optic (NLO) materials possess many advantages over inorganic materials, such as large electro-optic coefficients (r₃₃), ultrafast response, and low cost, and they have been widely investigated^1–4 because of their potential applications in telecommunications, optical information processing and sensors.^5–7 Designing and preparing NLO materials that contain a high β chromophore with high poling efficiency and exhibit good temporal stability is of great significance for NLO materials.^8–14 Dipole–dipole interactions between chromophores have been widely recognized as a negative factor that not only diminishes the poling efficiency, but also increases optical loss due to guest/host phase separation and increased material inhomogeneity.¹⁵ Introduction of an isolation group or tuning the chromophore shape has proven to be efficient methods to reduce the dipole–dipole interactions by keeping the two dipoles away from each other.^16–19 Our group has successfully developed a series of novel two-dimensional spindle-like chromophores that can not only prevent the aggregation caused by dipole–dipole interactions but also exhibit good thermal stability and temporal stability.^20–22 Moreover, a series of polyurethanes utilizing these chromophores as monomers have been synthesized, in which conjugated bridge of the chromophore is constitutional unit of the polymer backbone. However, it was found that the reported polyurethanes (PU-1 to PU-4) containing a spindle-like chromophore should be doped with other polymers to obtain good poling efficiency. More factors need to be considered for this multi-component system such as compatibility of the components and poling temperature.^23–27 Poor poling efficiency of the reported polyurethanes could be attributed to insufficient rotational freedom of the chromophores for the NLO polymers poling.

Herein, we tried to develop spindle-like chromophore-based polyurethane NLO materials with improved poling efficiency. “Y-type” NLO polymers combined the advantages of side-chain and main-chain NLO materials, i.e., the “Y-type” NLO polymers were easier to process and polarize compared with main-chain NLO polymers and exhibited improved temporal stability compared with guest–host or side-chain NLO polymers.²⁶ The performance of spindle-like chromophores with different lateral groups covalently linked in polyurethane NLO materials has not yet been investigated. Thus, a proper design would be useful in designing new NLO materials with enhanced NLO responses and temporal stabilities.

The spindle-like chromophore suspended polyurethanes utilizing the novel spindle-like chromophores as monomers were designed to take the advantage of “Y-type” NLO polymers and spindle-like chromophores for NLO materials. First, a series of novel hydroxyl functionalized two-dimensional spindle-like chromophores OHSTC-1 to OHSTC-4 with different lateral groups were designed and synthesized. The lateral groups of chromophore OHSTC-1 to OHSTC-4 were 3,4,5-trifluorophenyl, phenyl, trifluoromethylphenyl and 4-methoxyphenyl, respectively. The structures of chromophores OHSTC-1 to OHSTC-4 are shown in Fig. 1. Differing from our previous study, two hydroxyl groups for polymerization were located at the donor side instead of the lateral side, the structure of the chromophore STC for the previous study is also shown in Fig. 1 for comparison. Second, novel polyurethanes were synthesized by polymerization between isophorone diisocyanate (IPDI) and novel spindle-like chromophores, affording novel polyurethanes IPDI-1 to IPDI-3, for nonlinear optical (NLO) materials. It was reported that the hydrogen bonding between the polyurethane linkages can form the physical crosslinking network. The physical crosslinking can not only improve the temporal stability to some degree but also improve the homogeneity and processability compared with the chemical crosslinking. Furthermore, the optical losses and decomposition of the chromophores caused by the chemical crosslinking could be eliminated at the same time.^28,29 Thermal and nonlinear-optic activity characterizations of the chromophores and the polyurethanes NLO materials IPDI-1 and IPDI-2 were carried out. Both the chromophores and the polyurethanes were proven to possess relatively high thermal stability. The 5% weight loss temperature (T_d) of the chromophores OHSTC-1 and OHSTC-2 were 324 °C and 327 °C, respectively. The spindle-like chromophores suspended polyurethanes could be oriented without any other doped polymers and exhibited a satisfactory performance for temporal stability (electro-optic coefficients (IPDI-2) maintained up to 90% of the initial value after being baked at 85 °C in air for 200 hours).

Fig. 1 Structures of novel spindle-like chromophores OHSTC-1 to OHSTC-4 and the previous spindle-like chromophore STC.

Experimental section

Materials and methods

Tetrahydrofuran (THF) was purified by fractional distillation over sodium. Potassium tert-butoxide (Acros Organics), isophorone diisocyanate and dibutyltin dilaurate were purchased form Aladdin. 1,3,3-Trimethyl-5-dicyanovinyl-1-cyclohexene (TDC) was synthesized according to a literature procedure. All the other solvents and chemical reagents were used as received without further purification. NMR spectra were obtained on a Bruker AVANCE NMR spectrometer at a 500 MHz resonance frequency for ¹H in CDCl₃, and tetramethylsilane (TMS) was used as the internal standard. An AVATAR 360 FTIR spectrometer was used to obtain the FT-IR spectra. Differential scanning calorimetry (DSC) was carried out on a NETZSCH 4 at a 10 K min⁻¹ scan rate under nitrogen. The decomposition temperatures were measured by a Perkin-Elmer TGA 7 thermogravimetric analyzer (TGA) in nitrogen from 50 °C to 750 °C at a 10 K min⁻¹ heating rate. Ultraviolet-visible (UV-Vis) absorption spectra were obtained on a SHIMADZU UV-3100 spectrophotometer.

General procedure for spindle-like chromophore molecules synthesis

Synthesis of compound 1a. 2,5-(3,4,5-Trifluorophenyl)-1,4-dicarbaldehyde (0.4 g, 1.02 mmol) was dissolved in 20 mL dichloromethane, (4-dihydroxyethylamino-benzyl)-triphenyl-phosphonium iodide (0.58 g, 1.02 mmol) and t-BuOK (0.172 g, 1.53 mmol) were added, then the mixture was stirred at room temperature for 2 h. A yellow solution was observed. Column chromatography eluting with petroleum–ethyl acetate (20 : 1) was used to purify the solid and the solvent was removed under reduced pressure. The target compound was a yellow solid (0.31 g, 53%). ¹H-NMR (500 M, CDCl₃, TMS): δ (ppm) 9.94 (s, 1H, CHO), 7.86 (s, 1H, Ar-Ar-Ar), 7.67 (s, 1H, Ar-Ar-Ar), 7.63–7.64 (d, 2H, N-Ar), 6.68–6.67 (d, 2H, N-Ar), 7.15–7.12 (d, 2H, CH=CH), 6.80–6.77 (d, 2H, CH=CH), 7.11–7.08 (t, 2H, Ar-F), 7.06–7.03 (t, 2H, Ar-F), 3.85–3.83 (t, 4H, N–CH₂–CH₂–OH), 3.61–3.59 (t, 4H, N–CH₂–CH₂–OH).

Synthesis of compound 2a. The synthesis procedure for 1a was repeated with 2,5-phenyl-1,4-dicarbaldehyde (0.29 g, 1.02 mmol) and the yellow solid 2a (0.246 g, 52%) was obtained. ¹H-NMR (500 MHz, CDCl₃, TMS): δ (ppm) trans: 9.99 (s, 1H, CHO), 8.00 (s, 1H, Ar-Ar-Ar), 7.79 (s, 1H, Ar-Ar-Ar), 7.55–7.39 (m, 10H, Ar-Ar-Ar), 7.28–2.26 (d, 2H, N-Ar), 6.66–6.64 (d, 2H, N-Ar), 7.16–7.13 (d, 2H, CH=CH), 6.99–6.96 (d, 2H, CH=CH), 3.13 (s, 6H, N–CH₃). cis: 9.96 (s, 1H, CHO), 7.99 (s, 1H, Ar-Ar-Ar), 7.88 (s, 1H, Ar-Ar-Ar), 7.57–7.59 (d, 2H, N-Ar), 7.02–7.04 (d, 2H, N-Ar), 6.65–6.67 (d, 2H, CH=CH, J = 16 Hz), 6.60–6.62 (d, 2H, CH=CH, J = 16 Hz), 3.87–3.85 (t, 4H, N–CH₂–CH₂–OH), 3.62–3.60 (t, 4H, N–CH₂–CH₂–OH), 5.32 (s, 2H, OH).

Synthesis of compound 3a. The synthesis procedure for 1a was repeated with 2,5-trifluoromethylphenyl-1,4-dicarbaldehyde (0.43 g, 1.02 mmol) and a yellow solid 3a (0.35 g, 47%) was obtained. ¹H-NMR (500 MHz, CDCl₃, TMS): δ (ppm) trans : cis = 3 : 1; trans: 9.95 (s, 1H, CHO), 7.98 (s, 1H, Ar-Ar-Ar), 7.76 (s, 1H, Ar-Ar-Ar), 7.80–7.78 (d, 2H, Ar-CF₃), 7.75–7.73 (d, 2H, Ar-CF₃), 7.63–7.61 (d, 2H, Ar-CF₃), 7.58–7.56 (d, 2H, Ar-CF₃), 7.28–7.26 (d, 2H, N-Ar), 7.17–7.14 (d, 2H, CH=CH, J = 16 Hz), 6.88–6.85 (d, 2H, CH=CH, J = 16 Hz), 6.69–6.68 (d, 2H, N-Ar), 3.89–3.87 (t, 4H, N–CH₂–CH₂–OH), 3.64–3.62 (t, 4H, N–CH₂–CH₂–OH); cis: 9.96 (s, 1H, CHO), 8.03 (s, 1H, Ar-Ar-Ar), 7.50 (s, 1H, Ar-Ar-Ar), 7.69–7.80 (d, 2H, Ar-CF₃), 7.66–7.64 (d, 2H, Ar-CF₃), 7.58–7.56 (d, 2H, Ar-CF₃), 7.40–7.38 (d, 2H, Ar-CF₃), 7.14–7.12 (d, 2H, CH=CH, J = 9 Hz), 6.58–6.56 (d, 2H, CH=CH, J = 9 Hz), 6.62–6.61 (d, 2H, N-Ar), 3.89–3.87 (t, 4H, N–CH₂–CH₂–OH), 3.64–3.62 (t, 4H, N–CH₂–CH₂–OH).

Synthesis of compound 4a. The synthesis procedure for 1a was repeated with 2,5-methoxyphenyl-1,4-dicarbaldehyde (0.35 g, 1.02 mmol) and a yellow solid 4a (0.25 g, 47%) was obtained. ¹H-NMR (500 MHz, CDCl₃, TMS): δ (ppm) trans : cis = 1 : 3; cis: 9.95 (s, 1H, CHO), 7.91 (s, 1H, Ar-Ar-Ar), 7.70 (s, 1H, Ar-Ar-Ar), 7.37–7.36 (d, 2H, Ar-OCH₃), 7.34–7.24 (d, 2H, Ar-OCH₃), 7.24–7.23 (d, 2H, Ar-OCH₃), 7.03–7.01 (d, 2H, Ar-OCH₃), 6.97–6.95 (d, 2H, N-Ar), 6.60–6.58 (d, 2H, CH=CH, J = 9 Hz), 6.55–6.53 (d, 2H, CH=CH, J = 9 Hz), 6.60–6.58 (d, 2H, N-Ar), 3.88 (s, 3H, OCH₃), 3.85 (s, 3H, OCH₃), 3.81–3.80 (t, 4H, N–CH₂–CH₂–OH), 3.58–3.56 (t, 4H, N–CH₂–CH₂–OH); trans: 9.96 (s, 1H, CHO), 7.98 (s, 1H, Ar-Ar-Ar), 7.45 (s, 1H, Ar-Ar-Ar), 7.44–7.42 (d, 2H, Ar-OCH₃), 7.20–7.18 (d, 2H, Ar-OCH₃), 7.15–7.13 (d, 2H, Ar-OCH₃), 7.09–7.07 (d, 2H, Ar-OCH₃), 6.93–6.91 (d, 2H, N-Ar), 6.53–6.50 (d, 2H, CH=CH, J = 12 Hz), 6.24–6.21 (d, 2H, CH=CH, J = 12 Hz), 3.89–3.87 (t, 4H, N–CH₂–CH₂–OH), 3.64–3.62 (t, 4H, N–CH₂–CH₂–OH).

Synthesis of compound OHSTC-1. Compound 1a (0.27 g, 0.47 mmol) was dissolved in toluene (10 mL), TDC (0.115 g, 0.61 mmol) and piperidine (0.05 mL) were added, and the mixture was refluxed for 8 h with stirring. Column chromatography eluted with petroleum–ethyl acetate (16 : 1) was used for further purification and a red powder (0.22 g, 70%) was obtained. ¹H-NMR (500 MHz, CDCl₃, TMS): δ (ppm) 7.62 (s, 1H, Ar-Ar-Ar), 7.58 (s, 1H, Ar-Ar-Ar), 7.33–7.30 (d, 2H, N-Ar), 7.10–7.04 (m, 4H, Ar-F), 6.99–6.97 (1H, CH=CH–TDC), 6.84–6.82 (1H, CH=CH–TDC), 6.87–6.85 (d, 1H, N-Ar-CH=CH, 1H, CH), 6.57–6.55 (d, 1H, N-Ar-CH=CH), 6.96–6.94 (d, 2H, N-Ar), 3.90–3.88 (t, 4H, N–CH₂–CH₂–OH), 3.66–3.64 (t, 4H, N–CH₂–CH₂–OH), 2.58 (s, 2H, –CH₂–), 2.24 (s, 2H, –CH₂–), 1.04 (s, 6H, CH₃–C).

Synthesis of compound OHSTC-2. Compound 2a (0.27 g, 0.47 mmol) was dissolved in toluene (10 mL), and TDC (0.115 g, 0.61 mmol) and piperidine (0.05 mL) were added. A dark solution was obtained after refluxing for 6 h. Further purification were by column chromatography with petroleum ether–ethyl acetate (16 : 1) to give a red powder (0.22 g, 70%). ¹H-NMR (500 MHz, CDCl₃, TMS): δ (ppm) 7.75 (s, 1H, Ar-Ar-Ar), 7.71 (s, 1H, Ar-Ar-Ar), 7.54–7.43 (m, 10H, Ar-Ar-Ar), 7.31–7.29 (d, 2H, N-Ar), 7.15–7.12 (d, 1H, CH=CH–TDC), 7.10–7.07 (d, 1H, CH=CH–TDC), 7.02–7.00 (d, 1H, N-Ar-CH=CH), 6.99–6.97 (d, 1H, N-Ar-CH=CH), 6.81 (s, 1H, CH), 6.79 (s, 2H, N-Ar), 3.90–3.88 (t, 4H, N–CH₂–CH₂–OH), 3.66–3.64 (t, 4H, N–CH₂–CH₂–OH), 2.57 (s, 2H, –CH₂–), 2.23 (s, 2H, –CH₂–), 1.01 (s, 6H, CH₃–C).

Synthesis of compound OHSTC-3. Repeat the synthesis of OHSTC-1 with compound 3a (0.28 g, 0.47 mmol) and the product was a red powder (0.26 g, 73%). ¹H-NMR (500 MHz, CDCl₃, TMS): δ (ppm) 8.03 (s, 1H, Ar-Ar-Ar), 7.97 (s, 1H, Ar-Ar-Ar), 7.83–7.82 (d, 2H, Ar-CF₃), 7.79–7.78 (d, 2H, Ar-CF₃), 7.63–7.61 (d, 2H, Ar-CF₃), 7.58–7.56 (d, 2H, Ar-CF₃), 7.24–7.22 (d, 2H, N-Ar), 7.08–7.05 (d, 2H, CH=CH–TDC), 6.22–6.20 (d, 1H, N-Ar-CH=CH), 6.20–6.18 (d, 1H, N-Ar-CH=CH), 6.78–6.75 (d, 2H, CH=CH–TDC), 6.65–6.64 (d, 2H, N-Ar), 6.50 (s, 1H, CH), 3.85–3.83 (t, 4H, N–CH₂–CH₂–OH), 3.59–3.58 (t, 4H, N–CH₂–CH₂–OH), 2.56 (s, 2H, –CH₂–), 2.21 (s, 2H, –CH₂–), 1.01 (s, 6H, CH₃–C).

Synthesis of compound OHSTC-4. Compound 4a (0.3 g, 0.587 mmol) was taken as a reactant to repeat the OHSTC-1 synthesis. A red solid OHSTC-4 (0.27 g, 68%) was achieved. ¹H-NMR (500 MHz, CDCl₃, TMS): δ (ppm) 7.71 (s, 1H, Ar-Ar-Ar), 7.68 (s, 1H, Ar-Ar-Ar), 7.76–7.75 (d, 2H, Ar-OCH₃), 7.74–7.73 (d, 2H, Ar-OCH₃), 7.56–7.55 (d, 2H, Ar-OCH₃), 7.553–7.546 (d, 2H, Ar-OCH₃), 7.41–7.40 (d, 2H, N-Ar), 7.39–7.37 (d, 2H, CH=CH–TDC), 7.18–7.14 (d, 1H, N-Ar-CH=CH), 7.00–6.98 (d, 1H, N-Ar-CH=CH), 7.05–7.03 (d, 2H, CH=CH–TDC), 7.07–7.05 (d, 2H, N-Ar), 6.82 (s, 1H, CH), 4.12 (s, 6H, OCH₃), 4.10 (s, 3H, OCH₃), 3.93–3.92 (t, 4H, N–CH₂–CH₂–OH), 3.91–3.89 (t, 4H, N–CH₂–CH₂–OH), 2.57 (s, 2H, –CH₂–), 2.27 (s, 2H, –CH₂–), 1.03 (s, 6H, CH₃–C).

General procedure for the synthesis of the polyurethanes

Synthesis of polyurethane IPDI-1. Compound OHSTC-1 and isophorone diisocyanate (IPDI) were dissolved in dry THF (20 mL) at a 1 : 1.1 molar ratio and purged with nitrogen for 30 min to remove the atmospheric moisture. The whole reaction should be guaranteed absolutely no water. Then, dibutyltin dilaurate (DBTDL) was added. The reaction mixture was heated to 80 °C and maintained at this temperature for 2 d. The viscous solution was allowed to cool to the ambient temperature and then added drop wise to methanol. Purification by precipitation was carried out twice to remove the monomer and low molecular-weight oligomers. The deep red compound was filtered and dried in a vacuum desiccator to give IPDI-1. ¹H-NMR (500 MHz, CDCl₃, TMS): δ (ppm) 7.70 (s, 1H, Ar-Ar-Ar), 7.66 (s, 1H, Ar-Ar-Ar), 7.69–7.68 (d, 2H, N-Ar), 7.51–7.49 (m, 5H, Ar-F, CH=CH–TDC), 7.08–7.03 (m, 3H, N-Ar, CH=CH–TDC), 7.02–6.97 (m, 1H, N-Ar-CH=CH, 1H, CH), 6.89–6.87 (d, 1H, N-Ar-CH=CH), 3.68–3.57 (m, 8H, N–CH₂–CH₂–O), 2.61 (s, 2H, –CH₂–), 2.27 (s, 2H, –CH₂–), 1.73 (s, 2H, –CH₂–NHCOO), 1.06 (s, 6H, CH₃–C–CH₂–C=C, 6H, CH₃–C–CH₂–C–NHCOO), 0.95–0.90 (m, 9H, CH₃–C–CH₂–C–NHCOO).

Synthesis of polyurethane IPDI-2/IPDI-3. The OHSTC-2/OHSTC-3 reacted with IPDI, the synthesis and purification routes were the same as IPDI-1 to obtain the deep red compound IPDI-2/IPDI-3.

Polyurethane IPDI-2. ¹H-NMR (500 MHz, CDCl₃, TMS): δ (ppm) 7.74 (s, 1H, Ar-Ar-Ar), 7.69 (s, 1H, Ar-Ar-Ar), 7.52–7.45 (m, 10H, Ar-Ar-Ar), 7.38 (s, 2H, N-Ar), 7.14–7.11 (d, 1H, CH=CH–TDC), 7.07–7.04 (d, 1H, CH=CH–TDC), 7.04–7.02 (d, 1H, N-Ar-CH=CH), 6.99–6.97 (d, 1H, N-Ar-CH=CH), 6.81 (s, 1H, CH), 6.67 (s, 2H, N-Ar), 4.22–4.20 (t, 4H, N–CH₂–CH₂–O), 3.59–3.57 (t, 4H, N–CH₂–CH₂–O), 2.56 (s, 2H, –CH₂–), 2.23 (s, 2H, –CH₂–), 1.65 (s, 2H, –CH₂–NHCOO), 1.05–1.01 (d, 6H, CH₃–C–CH₂–C=C, 6H, CH₃–C–CH₂–C–NHCOO), 0.91–0.88 (m, 9H, CH₃–C–CH₂–C–NHCOO).

Polyurethane IPDI-3. ¹H-NMR (500 MHz, CDCl₃, TMS): δ (ppm) 7.71 (s, 1H, Ar-Ar-Ar), 7.67 (s, 1H, Ar-Ar-Ar), 7.69 (s, 2H, Ar-CF₃), 7.79–7.74 (m, 6H, Ar-CF₃), 7.60–7.58 (d, 2H, N-Ar), 7.57–7.55 (d, 1H, CH=CH–TDC), 7.51–7.49 (m, 2H, N-Ar-CH=CH, N-Ar-CH=CH), 7.01 (s, 1H, CH=CH–TDC), 6.86–6.84 (d, 2H, N-Ar), 6.71 (s, 1H, CH), 4.22–4.18 (t, 4H, N–CH₂–CH₂–O), 3.61–3.57 (t, 4H, N–CH₂–CH₂–O), 2.58 (s, 2H, –CH₂–), 2.23 (s, 2H, –CH₂–), 1.70 (s, 2H, –CH₂–NHCOO), 1.06–1.03 (d, 6H, CH₃–C–CH₂–C=C, 6H, CH₃–C–CH₂–C–NHCOO), 0.92–0.90 (m, 9H, CH₃–C–CH₂–C–NHCOO).

Preparation of polymer thin films. To prepare the polymer films, the polyurethanes IPDI-1, IPDI-2, IPDI-3 were dissolved in DMAC with a 0.15 g mL⁻¹ weight concentration. The solutions were syringed through a 0.22 μm pore size filter followed by quick spin-coating onto indium/tin oxide (ITO)-coated glass substrates at 800–1500 rpm and baked at 60 °C on a heating stage for 1 h to remove the majority of any volatiles. The residual solvent was removed by heating the films in a vacuum oven at 70 °C for 12 h.

Results and discussion

Synthesis and characterizations

The donor and acceptor were attached to the terphenyl structure with different substituent groups through a Wittig reaction and a Knoevenagel condensation reaction, respectively. The desired chromophores OHSTC-1 to OHSTC-4 could be obtained. The Wittig reaction occurred between the aldehyde group and (4-dihydroxyethylamino-benzyl)-triphenyl-phosphonium iodide in the presence of t-BuOK as the catalyst. The following Knoevenagel condensation reaction used piperidine to catalyze the reaction between TDC and the aldehyde group. The full synthesis route can be explained, as shown in Fig. 2. The target chromophores were all dark red powders and the FT-IR and ¹H-NMR spectra were all consistent with the expected structures. As shown in Fig. 3, the chromophore monomers and IPDI polymerized under nitrogen protection for 48 h until the FT-IR spectra indicated that the absorption peak of the isocyanate group (NCO) in 2266 cm⁻¹ disappeared. Taking the polymerization of polyurethane IPDI-2, for example, the FT-IR spectra were obtained during the polymerization process every 24 h.

Fig. 2 Synthesis of chromophores OHSTC-1 to OHSTC-4.

Fig. 3 Polymerization route of the polyurethane IPDI-1 to IPDI-3.

As shown in Fig. 4, the absorption peak at 2266 cm⁻¹, which was attributed to isocyanate group, disappeared completely after reacting for 48 h. FT-IR spectra of polyurethanes are shown in Fig. 5. The characteristic absorptions of the cyano group appeared at approximately 2220 cm⁻¹ and the phenyl group displayed a strong absorption peak around 3030 cm⁻¹. There was no absorption at 2266 cm⁻¹, which was attributed to the isocyanate group. The band around 3380 cm⁻¹ was attributed to the N–H stretching and another strong absorption peak appeared at about 1720 cm⁻¹ that was attributed to C=O stretching. The FT-IR spectra of the polymers clearly indicated the formation of urethane linkages. The GPC measurements of the polyurethanes showed that the number-average-molecular weights were 1676 Da, 2660 Da, and 1214 Da for IPDI-1, IPDI-2, and IPDI-3, respectively. M_w/M_n of the polyurethanes was 1.49, 1.40, and 2.19 for IPDI-1, IPDI-2, and IPDI-3, respectively.

Fig. 4 FT-IR spectra during the polymerization (0 h, 24 h, and 48 h) of polyurethane IPDI-2.

Fig. 5 FT-IR spectra of polyurethanes IPDI-1 to IPDI-3.

Thermal properties

Thermal properties of the chromophores. Thermal properties of the chromophores were measured by thermogravimetric analysis (TGA). As shown in Fig. 6, the T_d values for OHSTC-1 and OHSTC-2 are 324 °C and 327 °C, respectively. This illustrates that the chromophores OHSTC-1 and OHSTC-2 displayed very high thermal stability. However, this temperature dropped to 186 °C for OHSTC-3 and 176 °C for OHSTC-4.

Fig. 6 TGA thermograms of chromophores OHSTC-1 to OHSTC-4 in a nitrogen atmosphere at a 10 °C min⁻¹ heating rate.

This dramatic difference in T_d could be attributed to the influence of different lateral groups attached to phenyl conjugation bridge. It was well known that the π–π interaction tends to occur among benzene rings and fluorinated benzene rings; the carbon atom of trifluoromethyl and methoxy group is sp³ hybridized, which displays the stereochemical structure and thus introduction of the two groups to a benzene ring resulted in the increase of the distance between benzene rings and the reduction of supramolecular interactions. Therefore, we suppose that the supramolecular interactions, which are caused by the benzene ring and fluorinated benzene ring between the chromophores, play an important role to improve thermal stability. The phenyl and 3,4,5-trifluorophenyl lateral groups contained chromophores (OHSTC-1 and OHSTC-2) exhibited higher decomposition temperatures due to the π–π or ArF–ArH supramolecular interactions. The other two lateral groups made the chromophores possess nonplanar structures, which blocked the π–π interaction, and thus displayed lower thermal stability. The DSC thermograms of the OHSTC-1 to OHSTC-4 are shown in Fig. 7; there was a melting peak at about 250 °C for OHSTC-1 and OHSTC-2. However, the OHSTC-3 or OHSTC-4 displayed a T_g instead of a melting peak. We supposed the melting peaks to be a reflection of the packing ability of OHSTC-1 or OHSTC-2.

Fig. 7 DSC thermograms of chromophores OHSTC-1 to OHSTC-4 in a nitrogen atmosphere at a 10 °C min⁻¹ heating rate.

Thermal properties of the polyurethanes. For the thermal stability reasons IPDI-1, IPDI-2, and IPDI-3 were used for further study. The TGA curve of the polyurethanes shown in Fig. 8 demonstrates all the polyurethanes had high thermal stability, they had the same tendencies as the chromophore monomers and the 5% weight loss temperatures were 245 °C, 299 °C, and 212 °C for IPDI-1, IPDI-2, and IPDI-3, respectively. The other monomer was the same (IPDI), therefore we believed that this difference in thermal stability was caused by the difference of the lateral group, which can influence intermolecular interaction and then influence thermal stability. Fig. 9 shows that the glass transition temperature of IPDI-1, IPDI-2, and IPDI-3 were 106 °C, 160 °C, and 90 °C, respectively.

Fig. 8 TGA thermograms of polyurethanes IPDI-1 to IPDI-3 in a nitrogen atmosphere at a 10 °C min⁻¹ heating rate.

Fig. 9 DSC thermograms of polyurethanes IPDI-1 to IPDI-3 in a nitrogen atmosphere at a 10 °C min⁻¹ heating rate.

Optical properties

UV-Vis absorption spectra of chromophores and polyurethanes. The UV-Vis absorption spectra of chromophores in different solvents with different dielectric constants are demonstrated in Fig. 10. Most reported chromophores exhibited a wide absorption band raised from intermolecular charge transfer. It is worth noting that OHSTC-1 to OHSTC-4 showed two absorption peaks, main body absorption and shoulder absorption. As we reported in previous study,³⁰ spindle-like chromophores with phenyl, thiophene and other aromatic side groups also exhibited two absorption peaks. The maximum UV-Vis absorption wavelengths of the chromophores main-body are given in Table 1. The solvatochromism of the four novel chromophores were almost the same, the bathochromic shift of the main-body absorption from polar solvents acetone to DMF were 14 nm, 12 nm, 10 nm, and 10 nm for OHSTC-1 to OHSTC-4, respectively. However, the absorption strengths were dramatically influenced by different lateral groups as the chromophores, which contained fluorinated lateral group, had weaker main-body push–pull absorption and stronger shoulder absorption compared to phenyl and 4-methoxylphenyl lateral groups, which indicated that even weak push/pull groups linked onto terphenyl structure, could have an effect on the intermolecular charge transfer of the chromophore. Therefore, we speculated the terphenyl structure participated in the resonance of the main-body conjugated structure and formed shoulder peaks. The fluorinated lateral group had strong an electron withdrawing property leading to the charge dispersion into the terphenyl structure, and thus strengthened the shoulder absorption. The UV-Vis absorption spectra of polyurethanes in different solvents of varying dielectric constants were measured, the absorption spectra and the UV-Vis absorption spectra of chromophores and polyurethanes in DMF are displayed in Fig. 11. The polyurethane matrix itself had no absorption from 300 to 500 nm, the absorption of polyurethanes IPDI-1 to IPDI-3 was attributed to the bound chromophore absorption. However, the main-body push–pull absorption of polyurethanes IPDI-1 to IPDI-3 showed a hypsochromic shift compared to the corresponding chromophores OHSTC-1 to OHSTC-3 in DMF, acetone and toluene. The main-body push–pull structure that was attached to the polyurethane, when dissolved in the solvent and affected by the polarity of the environment wherein the main-body push–pull structure exists may be influenced by both the solvent and the polyurethane matrix. The main-body push–pull UV-Vis absorption wavelengths of the polyurethanes may have displayed different characteristics compared to the corresponding chromophores because of the changed environment polarity.

Fig. 10 UV-Vis absorption spectra of chromophores OHSTC-1 to OHSTC-4 in different solvents.

Table 1 Characterization data of OHSTC-1, OHSTC-2, OHSTC-3 and OHSTC-4^a

	λmax^a	λmax^b	λmax^c	λmax^d	λmax^e	Td^f (°C)
a The maximum UV-Vis absorption wavelengths of novel chromophores main-body in a toluene.b Dichloromethane.c Chloroform.d Acetone.e DMF; the unit is nm.f The decomposition temperature of chromophores (TGA measurement under nitrogen at a 10 °C min^−1 rate).
OHSTC-1	476.5	469.0	476.0	455.5	469.5	324
OHSTC-2	480.0	469.5	472.0	470.0	481.5	327
OHSTC-3	479.3	470.0	472.5	472.5	482.5	186
OHSTC-4	472.5	468.0	471.5	462.5	472.5	176

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Fig. 11 UV-Vis absorption spectra of polyurethanes IPDI-1 to IPDI-3 in different solvents and polyurethanes IPDI-1 to IPDI-3 in DMF.

UV-Vis absorption spectra of the IPDI-1 thin film. Polarity of the environment wherein the chromophores existed can influence UV-Vis absorption. In Fig. 12, the absorption of the IPDI-1 thin film at 510 nm increased and exhibited almost same height as that of absorption at 375 nm, which was quite different with that of absorption in solution (Fig. 10).

Fig. 12 UV-Vis absorption spectra of the IPDI-1 thin film before and after poling.

This illustrated the polyurethane condition was a benefit for forming main-body push–pull resonant structures with the potential to benefit the electro-optic activity. According to the change of the UV-Vis absorption before and after poling, we can calculate the order parameter Φ, Φ = 1 − A₁/A₀, where A₁ and A₀ are the maximum UV-Vis absorption after and before poling, respectively. The order parameter was 0.15 for IPDI-1 and 0.16 for IPDI-2.

Nonlinear optical properties

Electro-optic activity. To exhibit an electro-optic effect, the chromophore units in the polyurethanes must be non-centrosymmetric. High electric field poling was utilized to orient the chromophore units along the applied electric field and thus produced a non-centrosymmetric arrangement. Poling conditions were as follows: 110 °C and applying a high dc voltage 3.0–5.0 kV at the tungsten wire across the films for about 1 h at a 1.0 cm gap distance. Finally, the temperature was decreased to room temperature with the electric field still applied. The studied poling results of IPDI-1 and IPDI-2 were due to the excellent stability of IPDI-1 and IPDI-2. The electro-optic coefficients r₃₃ of the thin films were measured with the Teng-Man setup at 1310 nm. The r₃₃ of IPDI-1 and IPDI-2 were 27 pm V⁻¹ and 30 pm V⁻¹, respectively. It is reasonable that IPDI-1 and IPDI-2, which possessed similar content of chromophore and similar main resonant push–pull structure of chromophore, showed similar r₃₃ values. It is worth noting that a slight difference in the lateral group of the chromophores could have an effect on the electro-optic coefficients. However, they can be different than the main resonant structures in different matrix polarities. In the case of our system, the electro-optic activities were close to each other. The related characterization data of the polyurethanes are shown in Table 2.

Table 2 Characterization data of IPDI-1, IPDI-2 and IPDI-3^a

	λmax^a (nm)	Tg^b (°C)	Td^c (°C)	Φ	r33 (pm V^−1)
a The maximum UV-Vis absorption wavelength of polyurethanes film materials.b The glass transition temperature of polyurethanes materials (DSC measurement at nitrogen condition at a rate of 10 °C min^−1).c The decomposition temperature of polyurethanes materials (TGA measurement at nitrogen condition at a rate of 10 °C min^−1).
IPDI-1	495	113	245	0.15	27
IPDI-2	485	155	299	0.16	30
IPDI-3	470	90	212

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Stability of the electro-optic activity. Temporal stability of the polyurethanes were researched by baking the oriented films at 85 °C in air for 200 hours and monitoring the change of r₃₃ every once in a while to obtain the r₃₃(t). The r₃₃(t)/r₃₃(t₀) as a function of time at 85 °C is given in Fig. 13. The r₃₃ of IPDI-2 presented a small amount of attenuation in the first 20 h and remained stable after heating for 50 h. The r₃₃ of IPDI-2 still retained more than 90% of the original behavior and the r₃₃ of IPDI-1 maintained more than 75% after the heating process. The temporal stabilities of both them were satisfactory. IPDI-1 displayed a slightly poorer temporal stability compared to IPDI-2 due to the lower glass transition temperature.

Fig. 13 Temporal stability of the IPDI-1 and IPDI-2 (baking the oriented films at 85 °C in air for 200 hours).

Conclusions

A series of novel hydroxyl functionalized two-dimensional spindle-like chromophores with different lateral groups OHSTC-1 to OHSTC-4 were designed and synthesized in a satisfactory yield. The thermodynamic stabilities of the chromophores with planar substituted benzene groups were greatly improved (about 140 °C higher than similar chromophores with nonplanar substituted benzene groups) due to the π–π or ArF–ArH supramolecular interactions, especially the T_d (5 wt% loss) of chromophores OHSTC-1 and OHSTC-2, which were 324 °C and 327 °C, respectively. The novel chromophores further polymerized with IPDI and the spindle-like chromophores suspended polyurethanes IPDI-1 to IPDI-3 were successfully prepared as nonlinear optical (NLO) materials for the first time. The spindle-like chromophores suspended polyurethanes displayed improved poling efficiency compared with the previous ones (the r₃₃ of polyurethanes NLO materials IPDI-1 and IPDI-2 were 27 pm V⁻¹ and 30 pm V⁻¹, respectively) and exhibited a more satisfactory temporal stability performance (electro-optic coefficients (IPDI-2) were maintained up to 90% of the initial value after being baked at 85 °C in air for 200 hours).

Acknowledgements

This article was supported by the National Natural Science Foundation of China (51173063) and (21204028). The authors would like to acknowledge the assistance and expertise of Miss Xiaoyu Shi, Miss Danfeng Cao and Mr Xuesong Wang.

Notes and references

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Footnote

† These authors contributed equally to this work and should be considered co-first authors.

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