Synthesis and Electroluminescent Properties of OLED Green Dopants Based on BODIPY Derivatives

Experimental

General Procedures. All reagents and solvents were obtained from commercial suppliers (Aldrich and TCI Chem. Co., Seoul, Korea) and were used without further purifi-cation. 1H and 13C NMR spectra were recorded on a JEON JNM-ECP FT-NMR spectrometer operating at 500 and 125 MHz, respectively. IR spectra were measured on a Shimadzu Prestige-21 FT-IR spectrophotometer. The samples were prepared as a KBr pellet and scanned against a blank KBr pellet background at a wave number ranging from 4000 to 400 cm−1. UV-vis absorption spectra were recorded on a Scinco S-3100 spectrophotometer while photoluminescence (PL) spectra were measured on a CARY Eclipse Varian fluorescence spectrophotometer. The HOMO levels were calculated from the oxidation potentials, while the LUMO levels were calculated based on the HOMO levels and the lowest-energy absorption edges of the UV-vis absorption spectra. Thermal gravimetric analysis (TGA) was conducted on a TG 209F1 (NET-ZSCH) thermal analysis system under a heating rate of 20 °C;min−1.

Synthesis. Compounds 2, 3 and 4 were known and syn-thesized by the following the previously reported methods in the literatures.16,17

Typical Procedure for Synthesis of Compounds (5). To a deoxygenated solution of aldehydes 4 (0.2 g, 0.539 mmol) and 3-ethyl-2,4-dimethyl-1H-pyrrole (0.15 mL, 1.304 mmol) in dichloromethane (10 mL) was added a catalytic amount of trifluoroacetic acid (3 drops). The reaction mixtures were stirred under nitrogen atmosphere at room temperature for 10 h. The resulting solution were treated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (0.12 g, 0.539 mmol) and stirred for 3 h. Then triethylamine (10 mL) and boron tri-fluoride diethyletherate (10 mL) were added. The reaction mixtures were stirred at room temperature for 10 h. After washing with deionized water, the organic phases were separated, dried over magnesium sulfate, filtered, and con-centrated. The residues were subjected to flash column chromatography to give the requisite products 5.

2,6-Diethyl-4,4-difluoro-1,3,5,7-tetramethyl-8-[4-(2-(4-ethynyl-phenyl)-9H-carbazole)phenyl]-4-bora-3a,4a-diaza-s-indacene (5a). Yield: 68%; red solid; mp 257-259 °C; FT-IR (KBr pellet): νmax 1420 cm−1 (boron); 1H NMR (500 MHz, CDCl3) δ 8.15 (d, J = 8.3 Hz, 2H), 7.79 (d, J = 8.3 Hz, 2H), 7.71 (d, J = 8.3 Hz, 2H), 7.60 (d, J = 8.3 Hz, 2H), 7.44 (m, 4H), 7.32 (m, 4H), 2.54 (s, 6H), 2.33 (q, J1 = 7.3 Hz, J2 = 14.7 Hz, 4H), 1.36 (s, 6H), 1.00 (t, J = 7.3 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 154.2, 140.6, 139.2, 138.3, 138.0, 136.2, 133.3, 133.1, 132.4, 130.6, 129.6, 128.7, 127.0, 126.2, 123.7, 121.9, 120.5, 120.4, 109.8, 90.0, 89.8, 17.2, 14.7, 12.7, 12.0.

2,6-Diethyl-4,4-difluoro-1,3,5,7-tetramethyl-8-[4-(2-(4-ethynyl-N,N-diphenylaniline)phenyl]-4-bora-3a,4a-diazas-indacene (5b). Yield: 66%; red solid; mp 261-262 °C; FT-IR (KBr pellet): νmax 1405 cm−1 (boron); 1H NMR (500 MHz, CDCl3) δ 8.18 (d, J = 8.3 Hz, 2H), 7.80 (d, J = 8.7 Hz, 2H), 7.70 (d, J = 8.3 Hz, 2H), 7.61 (d, J = 8.7 Hz, 2H), 7.45 (m, 5H), 7.29 (m, 5H), 2.60 (s, 6H), 2.31 (q, J1 = 7.4 Hz, J2 = 14.7 Hz, 4H), 1.39 (s, 6H), 1.16 (t, J = 7.4 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 154.2, 140.6, 138.3, 138.0, 136.2, 133.3, 132.6, 130.6, 128.7, 127.0, 126.2, 123.7, 120.5, 120.4, 109.8, 31.7, 22.8, 17.4, 17.2, 14.7, 14.2, 12.7, 12.0.

OLED Fabrication and Characterization. Glass sub-strate covered with indium tin oxide (ITO having a sheet resistance of 10 Ω/m2) was cleaned in ultrasonic baths con-taining acetone and 2-propanol, rinsed in deionized water. The substrate dried under a stream of nitrogen and subjected to a UV-ozone treatment. All organic and cathode metal layers were deposited by vacuum deposition technique under a pressure of ~1 × 10−7 Torr. The deposition rate of organic layers was about 0.5 Å/s. Then, LiF and Al were deposited in another vacuum deposition system without breaking vacuum. Deposition rates of LiF and Al were 0.1 Å/s, 5 Å/s, respectively. After deposition, the device was encapsulated in ambient nitrogen immediately. Current den-sity-voltage (J-V) and luminance-voltage (L-V) characteri-stics of device were measured by using a Keithley 2635A Source Meter Unit (SMU) and Konica Minolta CS-100A. Electroluminescence (EL) spectra and CIE color coordinate were obtained using a Konica Minolta CS-2000 spectro-radiometer.