Supplementary information for

Efficient up-conversion of triplet excitons into singlet state and the application for organic light emitting diodes

Ayataka Endo1, Keigo Sato1, Kazuaki Yoshimura2, Takahiro Kai3, Atsushi Kawada3, Hiroshi Miyazaki3 and Chihaya Adachi1

1Center for Organic Photonics and Electronics Research (OPERA),

and Center for Future Chemistry, Kyushu University

744 Motooka, Nishi, Fukuoka 819-0395, Japan

2Basic Research Laboratories, Nippon Steel Chemical Co., Ltd., 46-80, Nakabaru, Sakinohama, Tobata, Kitakyushu, Fukuoka 804-8503, Japan

3Functional Materials Laboratories, Nippon Steel Chemical Co., Ltd., 46-80, Nakabaru, Sakinohama, Tobata, Kitakyushu, Fukuoka 804-8503, Japan

Materials and Methods

Synthesis and Characterization

1H and 13C-nuclear magnetic resonance (NMR) spectra were recorded on a JEOL JNM-LA400. NMR spectra of S1, S2 and S3 were obtained in tetrahydrofuran (THF)-d8 at room temperature. NMR spectra of S4 were obtained in dimethyl sulfoxide (DMSO)-d6 at 70 °C. Mass spectra were measured on a JEOL JMS-AX505HA. Silica-gel column chromatography was performed using Merck Kieselgel 60 (70-230 mesh). All reagents used in this work are commercially available and were not purified before use.

The reaction scheme to synthesize 2-biphenyl-4,6-bis(1,2-phenylindolo[2,3-a] carbazol-11-yl)-1,3,5-triazine (PIC-TRZ) is shown in Scheme S1.

Scheme S1

Sulfuric acid 3.0 g (30.6 mmol) was added dropwise to an ethanol solution (1000ml) containing 1,2-cyclohexanedion 33.3 g (297.0 mmol) and phenylhydrazine hydrochloride 86.0 g (594.7 mmol) for 5min and the solution was stirred for 4 h at 65 °C. The solution was cooled at room temperature and the brown precipitate was filtered and washed with ethanol two times and dried in vacuum, resulted in the product (80 g, 109.4 mmol, 96.3 %). Then, the product (72.0 g, 261.5 mmol) was dissolved in the mixture of acetic acid (720 g) and trifluoro acetic acid (72.0 g), and stirred for 15 h at 100 °C. The solution was cooled at room temperature and the yellow precipitate was filtered and rinsed with acetic acid (200 ml) and hexane (200 ml) and dried in vacuum, resulted in the white powder S1 (28.0 g, 109.4 mmol, 41.8 %).

Under a nitrogen atmosphere, S1 (26.0 g, 101.4 mmol), iodobenzene (122.7 g, 601.4 mmol), copper iodide (54.7 g, 287.2 mmol), Potassium carbonate (66.7 g, 482.6 mmol) in quinoline (800ml) were stirred for 72 h at 190 °C. Upon cooling to room temperature, water (500ml), dichloromethane (500 ml) was added and stirred into the solution. After filtration the solution, the solution was partitioned between dichloromethane and water. The product was extracted from the organic layer and evaporated under reduced pressure. The product was purified by column chromatography on silica gel, resulted in the product S2 (12.7 g, 38.3 mmol, 37.8%)

Under nitrogen atmosphere, S2 (12.7 g, 38.3 mmol) in dehydrated N,N- dimethylformamide (70 ml) was added dropwise into a dehydrated N,N- dimethylformamide (70 ml) solution containing sodium hydride (55%, 2.16 g, 49.5 mmol) for 15 min and stirred for 1 h. Then, cyanuric chloride (3.54 g, 19.2 mmol) in dehydrated N,N-dimethylformamide (70 ml) was added dropwise for 15 min. After the stirring for 2 h, water (350 g) was added into the solution and the precipitate was filtered and dried in vacuum. The product was purified by column chromatography on silica gel, resulted in the product S3 (11.3g, 14.5 mmol, 75.5%).

S3 (10.0 g, 12.9 mmol), 4-biphenylboronic acid (3.25 g, 16.4 mmol), and Tetrakis(triphenylphosphine)palladium(0) (1.5 g, 1.3 mmol) were dissolved in the mixture of ethanol (50 ml) and toluene (100 ml). The, sodium carbonate (6.5 g, 47.0 mmol) in water (50 ml) was added into the solution and stirred for 5 h at 85C. Upon cooling to room temperature, water (100 ml), dichloromethane (100 ml) was added and stirred. After filtration of the solution, the solution was partitioned between dichloromethane and water. The product was extracted from the organic layer and evaporated under reduced pressure. The product was purified by column chromatography on silica gel, resulted in the yellow product S4 (6.9 g, 7.7 mmol, 59.7%)

S1. White Powder. 1H-NMR (THF-d8) d = 10.01 (s,2H), 8.08 (d, 2H, J = 7.8 Hz), 7.87 (s, 2H), 7.50 (d, 2H, J = 8.5 Hz), 7.31 (ddd, J = 8.5, 7.1, 1.22 Hz), 7.16 (ddd, 2H, 7.8, 7.1, 1.0 Hz).; 13C-NMR (THF-d8) d = 140.7, 127.0 125.7, 125.2, 122.2, 120.4, 119.8, 112.8, 111.7.;FD-MS m/z: 256 [M]+

S2. White Powder. 1H-NMR (THF-d8) d = 8.75 (s, 1H), 8.17 (ddd, 1H, J = 7.6, 1.2, 0.7 Hz), 8.09 (ddd, 1H, J = 7.8, 0.7, 0.6 Hz), 8.00 (s, 1H), 7.99 (s,1H), 7.74 – 7.62 (m, 4H), 7.62 – 7.58 (m, 1H), 7.36 – 7.23 (m, 5H), 7.15 (dd, J = 7.2, 1.2 Hz).; 13C-NMR (THF-d8) d = 142.1, 140.7, 139.6, 130.8, 128.8, 128.4, 127.4, 126.3, 125.9, 125.7, 125.5, 124.9, 123.6, 122.8, 121.0, 120.5, 120.2, 120.0, 113.5, 112.6, 120.1, 110.5.; FD-MS m/z: 332 [M]+

S3. White Powder. 1H-NMR (THF-d8) d = 8.54 - 8.49 (m, 2H), 8.27 – 8.23 (m, 2H), 7.81 (d, 1H, J = 7.5 Hz), 7.71 (d, 1H, J = 8.1 Hz), 7.61 – 7.55 (m, 2H), 7.29 (d, 1H, J = 8.1 Hz), 7.28 (t, 1H, J = 7.3 Hz), 7.17 (dt, 1H, J = 7.7, 1.2 Hz), 6.96 (brs, 1H), 6.80 (d, 1H, J = 8.3Hz), 6.73 (t, 1H, 7.5), 6.56 (brs, 1H), 5.93 (brs, 1H), 5.69 (brs, 1H) ; 13C-NMR (THF-d8) d = 170.3, 164.6, 141.5, 140.6, 140.3, 129.9, 129.5, 129.5, 128.1, 127.5, 127.4, 126.3, 126.2, 125.5, 125.2, 124.9, 124.7, 121.0, 120.6, 120.3, 117.7, 117.6, 112.4, 110.3. ; FD-MS m/z: 776 [M+1]+

S4. Yellow Powder. 1H-NMR (DMSO-d6) d = 8.69 (d, 2H, J = 8.1 Hz), 8.50 (d, 2H, J = 8.3 Hz), 8.38 (d, 2H, J = 7.6 Hz), 8.00 (d, 2H, J = 8.1 Hz), 7.84 (d, 4H, J = 7.6), 7.79 (d, 2H, J = 7.8 Hz), 7.67 (dd, 2H, J = 7.6, 6.8 Hz), 7.62 (dd, 4H, J = 7.8, 7.6 Hz), 7.57 (d, 2H, J = 7.6 Hz), 7.49 (d, 2H, J = 7.3 Hz), 7.33 (dd, 4H, J = 7.8, 6.8 Hz), 7.28 (d, 4H, J = 8.1 Hz), 7.22 (dd, 2H, J = 7.8, 7.3), 6.84 (brs, 1H), 6.72 (d, 2H, J = 7.8 Hz), 6.59 (dd, 2H, J = 7.3, 6.8) . ; FD-MS m/z: 894 [M+1]+

Optical and Electrical Characterization

Optical characteristics of vacuum deposited films were estimated using absorption spectra, photoluminescence (PL) spectra, and absolute PL efficiency (ФPL). The films were formed on quartz substrates by thermal vacuum deposition with a thickness of 50 nm. Absorption spectra and PL spectra were recorded using a UV spectrometer (UV-2550, Shimadzu) and a spectrophotometer (FP-6500-A-51, Jasco), respectively. For the PL measurement of solid films, we used an excitation wavelength of 337 nm. The ФPL values were measured using an absolute PL quantum yield measurement system (C9920-02, Hamamatsu Photonics Co.). The accuracy of absolute PL values is well recognized by our previous report1). Transient lifetimes were measured using a streak camera (C4334, Hamamatsu Photonics Co.) with a N2 gas laser (MNL200, Laser Technik Berlin, l=337 nm, pulse width »500 ps, repetition rate=20 Hz) as the excitation source and sample temperature was systematically changed between 5 K and 350 K.

Figure S1 shows PL characteristics of PIC-TRZ in a toluene solution with a PIC-TRZ concentration of 10-4 molL-1. In addition, transient PL intensities are visually demonstrated. The black line shows the fluorescence spectrum and the red line shows the TADF spectrum. Although the nitrogen degassed solution showed FPL=35±3% with an intense delayed emission component as indicated by the presence of strong green component (a), the solution that was not degassed gave only FPL=10±3% and the delayed component completely disappeared (b). The sensitivity of the emission to degassing with nitrogen provides clear evidence that the delayed component is caused by TADF which occurs via triplet excited states.

Organic light emitting diodes (OLEDs) were fabricated by conventional vacuum deposition of the organic layers and the cathode on a pre-cleaned 110 nm-thick indium-tin-oxide (ITO)-coated glass substrate. The active area of the devices was 1 mm in diameter. The current density (J), voltage (V), and luminance (L) characteristics of the OLEDs were measured using a semiconductor parameter analyzer (E5273A, Agilent) and an optical power meter (1930C, Newport). The EL spectra of the OLEDs were obtained using a multi-channel spectrometer (UBS2000, Ocean Optics).

Figure S2 shows photoluminescence characteristics of a 6wt%-PIC-TRZ:mCP co-deposited film at 5 K and 300 K, respectively. At 5 K, the black line shows the prompt component based on the PIC-TRZ fluorescence and the red line shows the delayed component based on the PIC-TRZ phosphorescence. Here, the streak image clearly indicates the phosphorescence has long transient decay time. At 300 K, on the other hand, the emission spectrum of the delayed component (red line) is nearly identical to the emission spectrum of the prompt component (black line), indicating that the delayed component is based on TADF. In addition, the transient decay of the delayed component is sub-micro second that is significantly short compared with that of the phosphorescence.

Figure S3 shows the OLED characteristics of ITO/N,N’-diphenyl-N,N’-bis (1-naphthyl)-1,10-biphenyl-4,4’-diamine (α-NPD)/m-CP/ 6wt%-PIC-TRZ:m-CP/ BP4mPy/LiF/Al. (a) External EL quantum efficiency, ΦEL(ext), versus current density characteristics of the device ITO/α-NPD/m-CP/6 wt% PIC-TRZ:m-CP/ BP4mPy/LiF/Al. The highest ΦEL of 5.1% was obtained at a low current density. (b) Streak image of transient EL decay showing the prompt component (black) and delayed component (red).

Reference

1)  Y. Kawamura, H. Sasabe, and C. Adachi, Jpn. J. Appl. Phys. 43, 7729-30 (2004).

2