1. Introduction
Organic light-emitting diodes (OLEDs) have received significant attention from industry and academia since their development was first reported in 1987 by Tang and Vanslyke[1]. OLEDs have been successfully applied to televisions, mobile phones, and lighting devices because of their low driving voltage, self-luminance, fast response, wide viewing angles, and excellent color purity[2-4]. As the OLEDs market grows, research and development on OLEDs light-emitting materials is being continuously conducted. In general, fluorescence has an internal quantum efficiency (IQE) of 25%, but for phosphorescence an IQE of 100% is theoretically possible. In the case of red and green phosphorescent materials, commercialization has been successful. However, blue phosphorescent materials have not been commercialized, owing to the wide intrinsic band gap and cost of heavy metals, as well as the unfavorable roll-off phenomenon. Therefore, there is a need for further research on blue fluorescent materials, with high efficiency and long lifetimes[5-11]. As shown in Scheme 1, blue light-emitting materials with high yield and high purity were synthesized using a catalyst-free Diels-Alder reaction. The synthesized materials contain a spirobifluorene (SBF) moiety, which possesses excellent luminous efficiency as a core emitting group. 3',6'-diphenyl-1,1':2',1'' -terphenyl (DPTP) and 1,4-diphenyltriphenylene (DPTPL) were employed as bulky side groups. Side groups with varied conjugation lengths were introduced into the SBF, to study the optical and electrical properties of the synthesized materials.
2. Materials and methods
2.1. Synthesis
2.1.1. 2,7-bis(3',6'-diphenyl-[1,1':2',1''-terphenyl]-4'-yl)-9,9'-spirobi [fluorene] (BTPSF)
2,7-diethynyl-9,9'-spirobi[fluorene] (0.73 g, 2 mmol) and 2,3,4,5-tetraphenylcyclopenta- 2,4-dienone (1.54 g, 4 mmol) were added to 50 mL xylene. The reaction mixture was heated under reflux at 180 °C for 24 h under an argon atmosphere. The solution was cooled to room temperature, and subsequently filtered and dried with MgSO4. The crude product was purified by recrystallization from chloroform and ethanol to give 0.66 g (33%) of the compound BTPSF. 1H NMR (300 MHz, CDCl3, δ): 7.62 (d, 2H), 7.56 (d, 2H), 7.41 (s, 2H), 7.21 (d, 2H), 7.08 (m, 13H), 6.86 (s, 6H), 6.78 (s, 6H), 6.73 (d, 5H), 6.70 (s, 2H), 6.64 (t, 8H), 6.57 (d, 4H), 6.41 (d, 2H), and 6.32 (s, 2H).
2.1.2. 2,7-bis(1,4-diphenyltriphenylen-2-yl)-9,9'-spirobi[fluorene] (BDTSF)
2,7-diethynyl-9,9'-spirobi[fluorene] (0.9 g, 2.5 mmol) and 1,3-diphenyl- 2H-cyclopenta[l]phenanthren-2-one (1.93 g, 5 mmol) were added to 50 mL xylene. The reaction mixture was heated under reflux at 180 °C for 24 h under an argon atmosphere. The solution was cooled to room temperature, and subsequently filtered and dried with MgSO4. The crude product was purified by recrystallization from chloroform and ethanol to give 1.15 g (43%) of the compound BDTSF. 1H NMR (300 MHz, CDCl3, δ): 8.35 (d, 4H), 7.66 (d, 2H), 7.59 (t, 4H), 7.52 (s, 2H), 7.38 (m, 19H), 7.18 (d, 2H), 7.09 (t, 2H), 7.03 (t, 2H), 6.94 (m, 9H), 6.86 (d, 4H), 6.56 (d, 2H), and 6.23 (s, 2H).
2.2. Measurements
The 1H-NMR spectra were recorded on Bruker Advance 300 spectrometers. The optical UV-Vis absorption spectra were obtained using a Lambda 1050 UV/Vis/NIR spectrometer (Perkin Elmer). Perkin Elmer luminescence spectrometer LS55 (Xenon flash tube) was used for photo-luminescence (PL) spectroscopy. The photoluminescence quantum yield (PLQY) was obtained using Quantaurus-QY Absolute PL quantum yield spectrometer C11347 (Hamamatsu). For the EL devices, all organic layers were deposited under 10~6 torr, with a rate of deposition of 1 Å/s to give a deposition area of 4 mm2. The current- voltage-luminance (I-V-L) characteristics of the fabricated EL devices were obtained by Keithley 2400 electrometer, and light intensity was obtained by Minolta CS-1000A.
3. Results and discussion
3.1. Molecule design concept
The novel blue-emitting materials, BTPSF and BDTSF, were successfully synthesized, as shown in Scheme 1 and Figure 1. Spirobifluorene (SBF) exhibits high photoluminescence (PL) efficiency and a highly twisted structure achieved by sp3 bonding, which can be used as the main emitting moiety. In this study, we used sterically bulky side groups to prevent aggregation caused quenching (ACQ), which can occur in the film state due to the proximity of the molecules. DPTP and DPTPL were attached to the core of the molecules as bulky side groups. The two side groups have different rigidities, with DPTP having an open structure to rotate itself, while the triphenylene group of DPTPL has a fused ring system and, therefore, a relatively high rigidity. We believe that the two different side groups result in different electro-optical properties.
3.2. Optical properties of the synthesized compounds
Figure 2 and Table 1 show the ultraviolet-visible (UV-Vis) absorption spectra and PL spectra of the synthesized compounds. In the solution state UV-Vis spectra, BTPSF and BDTSF exhibited absorption wavelengths of 308 and 327 nm; and 308 and 339 nm, respectively. In the film state, BTPSF and BDTSF exhibited absorption maximum wavelengths of 311 and 329 nm; and 311 and 328 nm, respectively. For PL measurements recorded in the solution state, BTPSF and BDTSF exhibited ultraviolet and deep-blue emission at 381 and 407 nm, respectively, with full-width-at-half-maximum (FWHM) values of 54 nm. BTPSF and BDTSF exhibited high solution state photoluminescence quantum yield (PLQY) values of 88% and 93%, respectively. In the film state, BTPSF and BDTSF exhibited deep-blue emission at 395 and 434 nm, respectively. Compared with the solution state, the maximum emission values for the film state were redshifted by 14 and 27 nm in BTPSF and BDTSF, respectively. FWHM values for BTPSF and BDTSF were 62 and 77 nm, respectively, 8 nm and 23 nm broader than solution state values. This means that the molecular packing of BDTSF in the film state is more efficient than that of BTPSF. Furthermore, for BDTSF, weak excimer emission of 549 nm was observed due to the proximity of the molecules. PLQY values for BTPSF and BDTSF in the film state were 65 and 71%, respectively. These values were approximately 23% and 22% lower than those observed in the solution state for BTPSF and BDTSF, respectively, owing to the ACQ effect[12]. The PLQY value for BDTSF was higher than that of BTPSF because the DPTPL side group is more rigid than the DPTP side group. It effectively prevents the intermolecular interaction compared to the DPTP side group.
3.3. Electroluminescence (EL) performance of the synthesized compounds
The synthesized compounds were used as emitting layers (EML) to fabricate non-doped OLEDs devices, and their EL performance was measured. The devices were structured as follows: ITO / 4,4',4''-Tris [2-naphthyl(phenyl)amino]triphenylamine (2-TNATA) (60 nm) / N,N'-Di (1-naphthyl)-N,N'- diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB) (15 nm) / BTPSF or BDTSF (30 nm) / Alq3 (30 nm) / LiF (1 nm) / Al (200 nm). 2-TNATA was used as a hole injection layer, and NPB was used as both a hole transporting layer (HTL), and an electron blocking layer (EBL). Alq3 was used as an electron transporting layer (ETL). The OLEDs properties of the non-doped devices using the synthesized EML materials are summarized in Figure 3 and Table 2. As shown in Figure 3, the BDTSF device exhibited typical OLEDs performance, such as current density (I)-voltage (V)-luminance (L) curve, current efficiency (CE) versus current density and external quantum efficiency (EQE) versus current density. However, for the BTPSF device, a normal EL spectrum was not observed because of the wide band gap of more than 3.4 eV. This was due to the wide band gap not allowing recombination for emission, as well as the worse electron and hole injections in the BTPSF device. The BDTSF device exhibited CE of 0.61 cd/A and EQE of 0.46%. In addition, no roll-off was observed for the BDTSF device, even at high current densities. Therefore, unlike the BTPSF device, the BDTSF device displays smooth charge injection. The BDTSF device exhibited an ELmax value of 437 nm, and the CIE color coordinates were (0.20, 0.17). The BDTSF device exhibited deep-blue region emission at the ELmax wavelength, but the CIE y-value was not as low. This is due to the weak excimer peak for BDTSF, which is derived from the molecular packing in the device. However, this CIE y-value is close to the commercialized mobile phone specification of 0.15.
4. Conclusions
Two novel blue-emitting fluorescent compounds, BTPSF and BDTSF, were synthesized, based on the spirobifluorene moiety. BTPSF and BDTSF showed ultraviolet and deep-blue PL emission at 395 and 434 nm, respectively, in the film state. The EL performance of BDTSF based on a non-doped device showed a CE of 0.61 cd/A and an EQE of 0.46%. BDTSF exhibited deep-blue emission at an ELmax wavelength of 437 nm, and exhibited a CIE color coordinates of (0.20, 0.17). The non-doped device displayed a relatively wide FWHM value owing to the molecular packing, but the CIE y-value of 0.17 for the BDTSF device was close to the commercialized mobile phone requirement (0.15) for blue emission.
