English

• Coumarin molecule is a natural product occurring in some plants^[1]. Many coumarin derivati-ves not only exhibit good activities in biology and medicine, but also have excellent fluorescence pro-perties^[2]. Therefore, they are widely used in biology, chemistry, physics, laser dyes, sensors and light-emitting devices^[3~5]. It is easy to introduce different substituents into the coumarin skeleton, changing their photophysical properties and spectral properties to achieve the desired effect, thereby making them more flexible in various applications^[6~8].

  Thiophene has smaller distortion with adjacent groups, moderate electronegativity and inter-molecular S…S non-covalent weak interactions^[9]. It has been used as π bridge for optical and electronic materials due to their good electron-transferring ability, structural rigidity^[10], tunable spectroscopic and electrochemical properties^[11]. Their derivatives have been widely used in organic photovoltaic materials, thermoelectric materials, and light-emitting materials in OLEDs^{[12, 13]}.

  In this paper, we report the synthese of three N-coumarin derivatives (Py-Th-NC, NC-Th-Ph-NC and NC-Th-Ph-C) (Scheme 1), in which the thiophene as π bridge improve the hole transporting ability, luminescence efficiency and thermal stability.

  Scheme 1

  

  图式 1.  Py-Th-NC、NC-Th-Ph-NC和NC-Th-Ph-C的合成路线

  Scheme 1.  The synthetic routes of Py-Th-NC, NC-Th-Ph-NC and NC-Th-Ph-C

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  1.   Experimental

  1.1   Reagents and apparatuses

  1-Bromopyren, Pd(pph₃)₄, 4-(N, N-diethylamino) salicylaldehyde, salicylaldehyde, bis(pinacolato) diboron, Pd(dppf)Cl₂ were bought from Energy Chemical, and 2-(thiophen-2-yl)acetonitrile was purchased from Ark Pharm. 4-Bromophenyl-acetonitrile was purchased from Adamas Reagent Co., Ltd. Tetrabutyl Ammonium Bromide (TBAB) was bought from Tianjin BASF Chemical Co., Ltd. Potassium acetate was bought from Tianjin kaixin chemical industry Co. Ltd. N-Bromosuccinimide (NBS) was purchased from Shanghai Crystal Pure Reagent Co., Ltd. Ethanol, toluene and 1, 4-dioxane were purified and freshly distilled prior to use. All other chemicals used were analytical grade.

  ¹H NMR spectra were recorded using Varian Mercury Plus 400MHz and Agilent Technologies DDZ 600MHz nuclear magnetic resonance spectro-meters. UV-Vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer. IR spectra (400~4000cm^-1) were measured on a Shimadzu IRPrestige-21 FT-IR spectrophotometer with KBr pellets. Mass spectra were obtained from a Thermo Scientific Orbitrap Elite mass spectrometer. Thermogravimetric analysis (TGA) was performed on a Perkin Elmer Pyris system. Melting points were measured by using an X-4 microscopic melting point apparatus, which was made in Beijing Taike Instrument Limited Company, and the thermometer was uncorrected. Fluorescent lifetime and quantum yield were recorded on a FLSP920 type steady-state/transient fluorescence spectrometer (Edinburgh Instruments Ltd). Cyclic voltammetry (CH Instru-ments 760 B) was performed with a 0.10 mol/L solution of Bu₄NPF₆ in dichloromethane, with the analyte present in a concentration of 10^-3 mol/L and employing a scan rate of 100 mV/s at room temperature. A Pt electrode was used as the working electrode, while a Pt wire and a Ag/Ag⁺ electrode were used as the counter electrode and reference electrode, respectively.

  1.2   Syntheses and characterizations of Py-Th-NC, NC-Th-Ph-NC and NC-Th-Ph-C

  1.2.1   3-(4-(4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl) phenyl)-7-(N, N′-diethylamino)coumarin (b)

  The suspension of 3-(4-bromo-phenyl)-7-(N, N′-diethylamino)coumarin (a)^[14] (1.0 g, 2.69 mmol), bis(pinacolato) diboron (1.10 g, 4.33 mmol), potassium acetate (0.8 g, 8.15 mmol) and Pd(dppf)Cl₂ (0.1 g, 0.14 mmol) in absolute 1, 4-dioxane (50 mL) was placed in a three-necked flask. The mixture was heated up to 95℃ and stirred for 48 h under N₂. After solvent was evaporated under vacuum, the mixture was dissolved in dichloromethane and washed with water (3×50 mL). The organic layer was dried over MgSO₄ and concentrated in vacuum. The crude product was purified by column chromato-graphy eluted with EtOAc/CH₂Cl₂/petroleum ether (1:8:30) to afford a yellow solid (yield: 74%). m.p.: 212~214 ℃. Anal. Calcd. for C₂₄H₂₇BO₄ (%):C 71.30, H 6.73. Found (%): C 71.33, H 6.70. ¹H NMR (400 MHz, CDCl₃)δ: 7.85 (d, J=7.9 Hz, 2H, Ar-H), 7.73 (s, 2H, Ar-H), 7.71 (s, 1H, coumarin H), 7.32 (d, J=8.8 Hz, 1H, coumarin H), 6.59 (dd, J=8.8, 2.1 Hz, 1H, coumarin H), 6.53 (s, 1H, coumarin H), 3.43 (q, J=7.0 Hz, 4H, CH₂), 1.35 (s, 12H, CH₃), 1.22 (t, J=7.0 Hz, 6H, CH₃).

  1.2.2   4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl-pyrene (c)

  The solution of bis(pinacolato)diboron (2.71 g, 10.67 mmol), Pd(dppf)Cl₂ (0.26 g, 0.36 mmol) and potassium acetate (2.1 g, 21.34 mmol) in absolute 1, 4-dioxane (60 mL) was added into 1-bromopyrene (2 g, 7.11 mmol). The mixture was stirred under N₂ at 80 ℃ for 35 h. The residue was added into saturated sodium chloride solution and the mixture was extracted with CH₂Cl₂ (3×50 mL). The combined organic extracts were dried (Na₂SO₄) and concentrated in vacuo. The residue was chromatographed on silica, eluting with ether/CH₂Cl₂ (15:1~5:1 gradient) to give the compound c as yellow-white powder solid (yield 77%). m.p.: 117~118 ℃. ¹H NMR (600 MHz, CDCl₃) δ: 1.49 (s, 12H, CH₃), 7.99~9.09 (m, 9H, Ar-H). Anal. Calcd. for C₂₂H₂₁BO₂(%):C 80.51, H 6.45. Found(%): C 80.53, H 6.44.

  1.2.3   7-(N, N-diethylamino)-3-(thiophen-2-yl)-coumarin (d)

  The preparation of d was similar to that described for a. ¹H NMR (600 MHz, CDCl₃) δ: 7.88 (s, 1H), 7.66 (dd, J=3.6, 0.9 Hz, 1H), 7.36~7.27 (m, 2H), 7.08 (dd, J=5.0, 3.8 Hz, 1H), 6.61 (dd, J=8.8, 2.4 Hz, 1H), 6.53 (d, J=2.2 Hz, 1H), 3.43 (q, J=7.0 Hz, 4H), 1.22 (t, J=7.2 Hz, 6H)

  1.2.4   3-(5-bromothiophen-2-yl)-7-(N, N-diethy-lamino)coumarin (e)

  Compound e was obtained by the reaction of d and NBS, ¹H NMR (400 MHz, CDCl₃) δ: 8.24 (s, 1H), 7.32 (dd, J=6.5, 3.8 Hz, 2H), 7.05~6.98 (m, 1H), 6.62 (dt, J=8.9, 2.1 Hz, 1H), 6.52 (d, J=2.4 Hz, 1H), 3.43 (dq, J=7.1, 3.5 Hz, 4H), 1.23 (td, J=7.1, 1.4 Hz, 6H).

  1.2.5   7-(N, N-diethylamino)-3-(5-(4, 4, 5, 5-tetramethyl-1, 3, 2 -dioxaborolan-2-yl)thiophen-2-yl)coumarin (f)

  The preparation of f was similar to that described for b. The crude was purified by chromatography on silica gel using CH₂Cl₂/petroleum ether (1:20~1:5) as eluant to give an orange solid (yield 68%). m.p.: 81~83 ℃. ¹H NMR (400 MHz, CDCl₃) δ: 7.93 (s, 1H), 7.76 (d, J=3.7 Hz, 1H), 7.59 (d, J=3.7 Hz, 1H), 7.30 (t, J=6.8 Hz, 1H), 6.61 (dd, J=8.9, 2.5 Hz, 1H), 6.52 (d, J=2.4 Hz, 1H), 3.43 (q, J=7.1 Hz, 4H), 1.35 (s, 12H), 1.22 (t, J=7.1Hz, 6H).

  1.2.6   3-(5-(pyren-1-yl) thiophen-2-yl)-7-N, N-diethylamino coumarin (Py-Th-NC)

  Compound c (1 g, 3.05 mmol), e (1.49 g, 3.96 mmol), TBAB (0.05 g, 0.16 mmol) and Pd(PPh₃)₄ (0.17 g, 0.15 mmol) were dissolved in the mixture of 50mL toluene and 4.5 mL 2 mol/L Na₂CO₃ solution. The resulting mixture was refluxed with stirring for 24 h under N₂, and then the mixture was extracted with CH₂Cl₂ (3×50 mL). The organic extracts were combined, dried over anhydrous MgSO₄, and con-centrated in vacuo. The residue was chromato-graphed on silica, eluting with CH₂Cl₂/ether(1:10~2:3 gradient) to obtain an orange powder solid (yield 63%). m.p.: 247~249 ℃. IR (KBr pellet, cm^-1): 1710(ν_C=O), 1587 (ν_C=C), 1510 (ν_{Ar C=C}), 1137 (ν_C-O-C), 833 (ν_{Ar C-H}). ¹H NMR (400 MHz, CDCl₃) δ: 8.56 (d, J=9.3 Hz, 1H), 8.30 (s, 1H), 8.20 (t, J=7.7 Hz, 4H), 8.15~8.08 (m, 4H), 8.04 (dd, J=14.4, 6.8 Hz, 1H), 7.42~7.32 (m, 2H), 6.64 (dd, J=8.8, 2.4 Hz, 1H), 6.57 (d, J=2.2 Hz, 1H), 3.45 (q, J=7.1 Hz, 4H), 1.26~1.20 (m, 6H).¹³C NMR (151 MHz, CDCl₃)δ: 151.09 (s), 143.01 (s), 142.35 (s), 131.36 (d, J=10.5 Hz), 130.91 (d, J=4.1 Hz), 129.42 (s), 128.92 (s), 128.24 (s), 128.04 (d, J=7.4 Hz), 127.29 (s), 126.19 (s), 125.47 (s), 125.21 (s), 124.99 (s), 124.66 (d, J=7.3 Hz), 109.24 (d, J=5.7 Hz), 97.22 (s), 77.20 (s), 76.98 (s), 76.77 (s), 53.39 (s), 44.94 (s), 12.46 (s), 1.00 (s). MS (ESI) m/z: 500.16 [M+H]⁺.

  1.2.7   7-(N, N-diethylamino)-3-(4-(5-(7-(N, N-diethylamino)coumarin-3-yl) thiophen-2-yl)phenyl)-coumarin (NC-Th-Ph-NC)

  Absolute toluene (60mL) was added into the mixture of e (1 g, 1.87 mmol), b (1.78 g, 4.24 mmol), Na₂CO₃ (1 g, 9.43 mmol), TBAB (0.05 g, 0.16 mmol) and Pd(PPh₃)₄(0.2 g, 0.17 mmol) under N₂. The suspension was heated up to 115 ℃ and stirred vigorously. After 10min, distilled water (2 mL) was added. The resulting mixture was stirred 115 ℃ for 30 h. After solvent was evaporated under vacuum, the mixture was dissolved in dichloromethane and washed with water (3×50 mL). The organic layer was dried over MgSO₄ and concentrated in vacuum. The residue was chromatographed on silica, eluting with EtOAc/CH₂Cl₂/petroleum ether (1:100:300~2:100:150) to give an orange solid. Yield: 70%. m.p.: 139~141℃. IR (KBr)/cm^-1: 1717(ν_C=O), 1584 (ν_C=C), 1517 (ν_{Ar C=C}), 1137 (ν_C-O-C), 827 (ν_{Ar C-H}). ¹H NMR (400 MHz, CDCl₃)δ: 8.23 (s, 1H), 7.75 (d, J=8.5 Hz, 3H), 7.66 ~7.60 (m, 2H), 7.32 (dd, J=23.3, 13.4 Hz, 4H), 6.61 (dd, J=10.9, 4.6 Hz, 2H), 6.53 (s, 2H), 3.42 (dd, J=10.5, 6.5 Hz, 8H), 1.23 (dd, J=9.3, 4.6 Hz, 12H).¹³C NMR (151 MHz, CDCl₃) δ: 161.47 (s), 160.89 (s), 156.17 (d, J=12.7 Hz), 151.05 (s), 150.61 (s), 143.81 (s), 142.10 (s), 140.32 (s), 135.59 (s), 132.40 (s), 130.66 (s), 129.41 (s), 129.04 (s), 128.61 (s), 126.46 (s), 125.36 (s), 119.79 (s), 112.69 (s), 109.69 (s), 109.27 (s), 109.04 (d, J=6.3 Hz), 108.31 (s), 97.09 (d, J=13.8 Hz), 77.23 (s), 77.02 (s), 76.80 (s), 44.89 (d, J=11.3 Hz), 12.46 (d, J=1.6 Hz). MS (ESI) m/z: 591.23 [M+H]⁺.

  1.2.8   3-(4-(5-(7-(N, N-diethylamino)coumarin-3-yl) thiophen-2-yl)phenyl)coumarin (NC-Th-Ph-C)

  This procedure was similar to that described for NC-Th-Ph-NC. The residue was chromatographed on silica, eluting with EtOAc/CH₂Cl₂/petroleum ether (1:100:300~2:100:150) to give an orange solid. Yield: 65%. m.p.: 128~131 ℃. IR (KBr)/cm^-1: 1717(ν_C=O), 1587 (ν_C=C), 1517 (ν_{Ar C=C}), 1133 (ν_C-O-C), 820 (ν_{Ar C-H}). ¹H NMR (400 MHz, CDCl₃) δ: 7.93~7.81 (m, 1H), 7.78~7.63 (m, 2H), 7.6~7.44 (m, 4H), 7.44~7.34 (m, 3H), 7.30 (t, J=7.5 Hz, 1H), 7.15 (t, J=4.3 Hz, 1H), 7.07 (d, J=9.1 Hz, 1H), 6.54~6.45 (m, 2H), 3.45~3.33 (m, 4H), 1.20 (t, J=7.1 Hz, 6H).¹³C NMR (151 MHz, CDCl₃) δ: 153.45 (s), 142.99 (s), 139.59 (d, J=13.8 Hz), 137.57 (s), 131.34 (s), 129.31 (s), 129.09 (s), 128.75 (d, J=4.0 Hz), 127.90 (d, J=11.3 Hz), 125.71 (s), 124.48 (s), 119.68 (s), 116.44 (d, J=9.1 Hz), 108.98 (s), 108.48 (s), 97.11 (s), 44.84 (s), 44.45 (s), 12.47 (d, J=11.0 Hz), -0.03 (s). MS (ESI) m/z: 520.15 [M+H]⁺.

  2.   Results and Discussion

  2.1   Optical properties

  The UV-Vis absorption and photoluminescence spectra of three compounds in diluted dichloro-methane solutions at a concentration of 5.0×10^-5mol/L are shown in Fig. 1. Py-Th-NC, NC-Th-Ph-C and NC-Th-Ph-NC have absorbance maximum at 424, 414 and 431 nm, respectively. For NC-Th-Ph-NC, the enlarged π-conjugation of the molecule improves the HOMO and LUMO levels, resulting in the red-shift of absorption. The absorption bands at 242 and 278 nm of Py-Th-NC are attributed to the characteristic n-π^* electron transition, and the absorption bands of Py-Th-NC and NC-Th-Ph-C at about 340 nm are assigned to π-π^* electron transition. Py-Th-NC has a Stokes shift of 86nm and an emission maximum of 510 nm. NC-Th-Ph-C has a Stokes shift of 71 nm and an emission maximum of 485 nm, and NC-Th-Ph-NC has a Stokes shift of 81 nm and an emission maximum of 512nm. Compared with NC-Th-Ph-C and NC-Th-Ph-NC, Py-Th-NC has a larger Stokes shift, which indicates less energy loss and higher fluorescence efficiency during relaxation. NC-Th-Ph-NC with a large degree of conjugation showed the strongest fluorescence intensity. A summary of photophysical data are listed in Tab. 1.

  Figure 1

  

  图 1.  N-香豆素衍生物二氯甲烷溶液(c=1.0×10^-5mol/L)的UV-Vis吸收和光致发光光谱

  Figure 1.  UV-Vis absorption and photoluminescence spectra of N-coumarin derivatives in diluted dichloromethane (c=1.0×10^-5 mol/L)

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  Table 1

  

  表 1  N-香豆素衍生物的光学和电化学数据

  Table 1.  Optical and electrochemical data of N-coumarin derivatives

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  Comp.	UV-Vis, λ/nm	PL, λ/nm	Stokes shift/nm	ε/[×104 L·mol-1·cm-1]	τ/ns	HOMO/eVa	LUMO/eVc	Eg, optical/eVb
  Py-Th-NC	424	510	86	3.74	7.3	-5.34	-2.82	2.52
  NC-Th-Ph-C	414	485	71	1.96	8.0	-5.32	-2.76	2.56
  NC-Th-Ph-NC	431	512	81	6.28	7.7	-5.30	-2.8	2.50
  aHOMO of the three compounds determined by cyclic voltammogram tests, EHOMO=-[Eox-E(Fc/Fc+)+4.8]eV). bEg, optical was estimated by the onset of UV-Vis spectra in CH2Cl2 solutions, Eg, optical=1240/λonset. cLUMO was calculated by HOMO + Eg, optical.

  2.2   Quantum chemical calculations

  Frontier molecular orbitals play an important role in electric and optical properties for materials. Quantum chemical calculations performed by using the DFT/B3LYP/6-31G(d, p) method revealed the total electronic distributions of HOMO and LUMO orbitals for the N-coumarin derivatives (Fig. 2).

  Figure 2

  

  图 2.  DFT/B3LYP/6-31G(d，p)方法计算得到的N-香豆素衍生物的HOMO和LUMO轨道

  Figure 2.  The HOMO and LUMO orbitals of N-coumarin derivatives calculated by DFT/B3LYP/6-31G(d, p) method

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  In the LUMO of Py-Th-NC, the excited electrons are located on the thiophene-substituted N-coumarin backbone, while in HOMO, the π electrons are delocalized on almost the entire molecule. For NC-Th-Ph-C, almost all electron densities in the LUMO orbital are on the benzene-substituted coumarin unit, while in HOMO, the π-electron is mainly located on the thiophene-substituted N-coumarin.

  The above results indicated that there is light-induced intramolecular charge transfer (ICT) action in Py-Th-NC and NC-Th-Ph-C molecules. In the LUMO and HOMO of NC-Th-Ph-NC, electrons are distributed throughout the molecule.

  2.3   Cyclic voltammetry

  The electrochemical behaviors of the three compounds were investigated by cyclic voltammetry (Fig. 4), and the resulting data were summarized in Table 1. The estimated ground state oxidation potentials corresponding to HOMO levels calculated from the equation: E_HOMO=-[E_ox-E(F_c/F_c⁺) + 4.8]eV, are -5.34, -5.32 and -5.30 eV for Py-Th-NC, NC-Th-Ph-C and NC-Th-Ph-NC, respectively. The LUMO energy levels were calculated according to the equation: E_LUMO=HOMO + E_{g, optical}, where the E_{g, optical} was calculated from the onset of the UV/Vis absorption, according to the equation: E_{g, optical}=1240/λ_onset. The optical energy gaps/LUMO energy levels are 2.52/-2.82 eV for Py-Th-NC, 2.56/-2.76 eV for NC-Th-Ph-C and 2.50/-2.80 eV for NC-Th-Ph-NC. The energy levels have a strong influence on the device performance of OLED.

  Figure 3

  

  图 3.  N-香豆素衍生物在0.1mol·L^-1 TBAPF₆的CH₂Cl₂溶液中扫速100mV·s^-1下的循环伏安图

  Figure 3.  Cyclic voltammograms of N-coumarin derivatives in CH₂Cl₂ solution with 0.1 mol·L^-1 TBAPF₆ at scan rate 100 mV·s^-1

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  Figure 4

  

  图 4.  N-香豆素衍生物的TGA曲线

  Figure 4.  TGA curves of N-coumarin derivatives under nitrogen flow

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  2.4   Thermal properties

  TGA were performed in flowing drying nitrogen atmosphere at the heating rate of 10 ℃/min. The N-coumarin derivatives exhibit excellent thermal stabilities (Fig. 4). The 5 % weight loss decomposition temperatures are 263, 238 and 258 ℃ for Py-Th-NC, NC-Th-Ph-NC and NC-Th-Ph-C, respectively. There are three sharp weight losses in the TGA curves, indicating that the derivatives undergo three large stage decomposition processes.

  3.   Conclusions

  In this paper, three N-coumarin derivatives with thiophene as π bridge were designed and synthesized by a convergent approach using stepwise palladium-catalyzed Suzuki cross-coupling reactions. NC-Th-Ph-C exhibited blue-green emission, while Py-Th-NC and NC-Th-Ph-NC exhibited green emission. The results indicated that derivatives with high π-conjugated molecule structure show good fluorescence emission abilities and good thermal stabilities. These materials have significant potential in fluorescence OLED application.

  
  
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• 

  Scheme 1  The synthetic routes of Py-Th-NC, NC-Th-Ph-NC and NC-Th-Ph-C

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  Figure 1  UV-Vis absorption and photoluminescence spectra of N-coumarin derivatives in diluted dichloromethane (c=1.0×10^-5 mol/L)

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  Figure 2  The HOMO and LUMO orbitals of N-coumarin derivatives calculated by DFT/B3LYP/6-31G(d, p) method

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  Figure 3  Cyclic voltammograms of N-coumarin derivatives in CH₂Cl₂ solution with 0.1 mol·L^-1 TBAPF₆ at scan rate 100 mV·s^-1

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  Figure 4  TGA curves of N-coumarin derivatives under nitrogen flow

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  Table 1.  Optical and electrochemical data of N-coumarin derivatives

  Comp.	UV-Vis, λ/nm	PL, λ/nm	Stokes shift/nm	ε/[×104 L·mol-1·cm-1]	τ/ns	HOMO/eVa	LUMO/eVc	Eg, optical/eVb
  Py-Th-NC	424	510	86	3.74	7.3	-5.34	-2.82	2.52
  NC-Th-Ph-C	414	485	71	1.96	8.0	-5.32	-2.76	2.56
  NC-Th-Ph-NC	431	512	81	6.28	7.7	-5.30	-2.8	2.50
  aHOMO of the three compounds determined by cyclic voltammogram tests, EHOMO=-[Eox-E(Fc/Fc+)+4.8]eV). bEg, optical was estimated by the onset of UV-Vis spectra in CH2Cl2 solutions, Eg, optical=1240/λonset. cLUMO was calculated by HOMO + Eg, optical.

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