Preparation of Novel Fused Ring Spiro[benzotetraphene-fluorene] Derivatives and Application for Deep-Blue Host Materials

Abstract

A series of novel fused-ring spiro compounds, spiro[benzo[ij]tetraphene-7,9'-fluorene] (SBTF) derivatives containing an end-capping aryl substituent at both the C3 and C10-positions hasbeen designed and synthesized via multi-step Suzuki coupling reactions. 3-(1-Naphthyl)-10-phenylSBTF (1N-PSBTF), 3-(2-naphthyl)-10-phenylSBTF (2N-PSBTF) and 3-[4-(1-naphthyl)phenyl]-10-phenylSBTF (NP-PSBTF) showed improved glass transition temperatures ($T_g$) with good thermal stability. Their photophysical, electrochemical, and electroluminescent properties were investigated and were used to construct blue organic light emission diodes (OLEDs). The typical OLED devices showed excellent performance; the NP-PSBTF-based device exhibited highly efficient deep blue-light emission with a maximum efficiency of 5.27 cd/A (EQE, 4.63%) with CIE (x = 0.133, y = 0.144). According to these characteristics, these deep-blue light emitting materials have sufficient potential for fluorescent OLED applications.

Introduction

Spiro compounds with specific steric configurations have been attracting attention as an organic functional material in terms of specific physical properties of the material. The important class of spiro compounds as organic molecular materials with high glass transition temperatures that evolv-ed as a very promising approach for optoelectric materials has been investigated.1-5 Much of the recent research into light-emitting materials has centered on spiro-based deriva-tives, because of higher quantum electroluminescence effici-ency and better color purity in an organic light emission diodes (OLED) structure.6 Representative organic spiro materials as blue light-emitting materials include spirobi-fluorenes containing anthracene, pyrene, aromatic amines, spirofluorene, and their oligomers.7-19 Spiro-type host and dopant materials containing benzofluorene as OLED fluore-scent materials have recently received a great deal of atten-tion because they provide a variety of substituents on the spiro[benzo[c]fluorene-7,9'-fluorene] (SBFF) due to their asymmetric spiro core structure with naphthalene, phenyl rings of spiro molecules, and conjugation controlled OLED host materials, as shown in Scheme 1.20-24 More recently spiro[benzo[de]anthracene-7,9'-fluorene] (SBAF) derivatives were prepared and adopted to OLED host and dopant materials. The highly conjugated spiro molecules allow their optical and electrochemical properties to be tuned delicately over a wide range following appropriate chemical modifi-cation. 25-28 More fused and conjugated spiro-compounds are less studied, particularly spiro[benzo[ij]tetraphene-7,9'-fluor-ene] (SBTF) and its derivatives. No examples of applications of SBTF derivatives in organic electronics have been published.

Scheme 1.Various spiro molecules with fused aromatic rings.

In this study, a novel SBTF core structure was developed as a highly fluorescent deep blue host material with good thermal/morphological stability, and its physical properties were investigated. Three new blue host materials consisting of 3-(1-naphthyl)-10-phenylSBTF, 3-(2-naphthyl)-10-phenyl-SBTF, and 3-[4-(1-naphthyl)phenyl]-10-phenylSBTF were prepared and characterized using 1H nuclear magnetic re-sonance (NMR), 13C NMR, Fourier transform-infrared (FT-IR) and mass spectroscopy (MS), thermal analysis, UV–vis, and photoluminescence (PL) spectroscopy. The electrolumine-scent properties of the fabricated multilayered OLEDs were evaluated.

 

Experimental

Materials and Measurements. Tetrakis(triphenylphos-phine) palladium(0), (Aldrich Chem. Co., St. Louis, MO, USA), 4-(1-naphthyl)phenyl boronic acid, naphthalene-1-boronic acid and naphthalene-2-boronic acid (Frontier Sci-entific Co., West Logan, UT, USA) were used as received. 8-[2-(6-Phenyl)]naphthyl-1-bromonaphthalene were prepared by successive Suzuki coupling reaction using corresponding organic boronic acids and halides. 9,10-Di(2-naphthyl)-anthracene (β-ADN) and 1,6-bis[(p-trimethylsilylphenyl)-amino]pyrene (LBD; band gap, 2.73 eV; HOMO, 5.47 eV; λmax (Absorption) = 426 nm; λmax (Emission) = 460 nm) were used as host and dopant materials,29 respectively. n-Butyl-lithium (2.5 M solution in hexane), potassium carbonate, sodium hydroxide and HCl (Duksan Chem. Co., Seoul, South Korea) were used without further purification. Tetrahydro-furan (THF) was purified by distillation over sodium metal and calcium hydride. PL spectra were recorded on a fluore-scence spectrophotometer (Jasco FP-6500; Tokyo, Japan) and the UV-vis spectra were obtained by means of a UV-vis spectrophotometer (Shimadzu, UV-1601PC; Tokyo, Japan). Energy levels were measured with a low-energy photo-electron spectrometer (AC-2; Riken-Keiki, Union City, CA, USA). The FT-IR spectra were obtained with a Thermo Fisher Nicolet 850 spectrophotometer (Waltham, MA, USA), and the elemental analyses were performed using a CE Instrument EA1110 (Hindley Green, Wigan, UK). The differ-ential scanning calorimeter (DSC) measurements were performed on a Shimadzu DSC-60 DSC under nitrogen. The thermogravimetric analysis (TGA) measurements were performed on a Shimadzu TGA-50 thermo gravimetric analyzer. High resolution mass spectra were recorded using an HP 6890 and Agilent 5975C MSD in FAB mode.

Representative Preparation of 10-Phenylspiro[benzo-[ij]tetraphene-7,9'-fluorene] (PSBTF) and 3-Br-PSBTF. A solution of 8-[2-(6-phenyl)]naphthyl-1-bromonaphthalene (12.53 g, 30.6 mmol) in THF (100 mL) was added to a 250 mL two-necked flask. The reaction flask was cooled to −78 °C, and n-BuLi (2.5 M in n-hexane, 14.68 mL) was added slowly in a dropwise fashion. The solution was stirred at this temperature for 1 h, followed by adding a solution of 9-fluorenone (5.51 g, 30.6 mmol) in THF (30 mL) under an argon atmosphere. The resulting mixture was gradually warmed to ambient temperature and quenched by adding saturated aqueous NaHCO3 (90 mL). The mixture was extracted with CH2Cl2. The combined organic layers were dried over magnesium sulfate, filtered, and evaporated under reduced pressure. A yellow powdery product was obtained. The crude residue was placed in another two-necked flask (100 mL) and dissolved in acetic acid (50 mL). A catalytic amount of aqueous HCl (5 mol %, 12 N) was then added, and the whole solution was heated under reflux for 12 h. After cooling to ambient temperature, the compound was purified as a white powder by silica gel chromatography using dichloromethane/n-hexane (2/1).

10-Phenylspiro[benzo[ij]tetraphene-7,9'-fluorene] (PSBTF, 4.93 g, 10 mmol) was dissolved in carbon tetrachloride in a two-necked flask; bromine (2.36 g, 15 mmol) was then added slowly in a dropwise fashion over a period of 20 min. The mixture was stirred at room temperature for 3 days. The precipitated solid was filtered and dried in vacuo to give the crude product, which was purified by recrystallization from ethyl acetate/n-hexane (1/2) to give a white powder.

3-Br-PSBTF: Yield 87%. mp 339 °C. FT-IR (KBr, cm−1) 3060, 3020 (aromatic-C-H). 1H-NMR (500 MHz, CDCl3) δ 8.37 (d, 1H), 8.14 (d, 1H), 8.01-7.88 (m, 6H), 7.53 (d, 2H), 7.39 (m, 4H), 7.29-7.21 (m, 2H), 7.14-7.05 (m, 4H), 6.90 (d, 2H), 6.64 (d, 1H). 13C-NMR (CDCl3) δ 150.2, 148.2, 147.6, 143.0, 142.4, 142.2, 140.0, 138.5, 137.7, 137.4, 136.8, 136.6, 135.2, 134.3, 133.9, 133.7, 130.8, 129.8, 129.2, 128.3, 128.0, 127.8, 127.6, 127.1, 126.2, 125.8, 125.3, 125.0, 124.3, 124.0, 123.5, 123.0, 122.4, 120.4, 120.2, 66.5. MS (FAB) m/z 572.10 [(M+1)+]. Anal. Calcd. for C39H23Br (571.50): C, 81.96; H, 4.06; Br, 13.98. Found: C, 82.00; H, 4.02; Br, 13.94. UV-vis (THF): λmax (Absorption) = 357, 375 nm, λmax (Emission) = 420, 433 nm.

Representative Preparation of 3-(1-Naphthyl)-10-phen-ylspiro[ benzo[ij]tetraphene-7,9'-fluorene] (1N-PSBTF). A solution of 3-bromo-10-phenylspiro[benzo[ij]tetraphene-7,9'-fluorene] (5.71 g, 10 mmol), tetrakis(triphenylphosphine)-palladium(0) (0.59 g, 0.51 mmol), and naphthalene-1-boronic acid (1.72 g, 10 mmol) dissolved in THF (150 mL) was stirred in a double-necked flask for 30 min. Potassium carbonate (2 M, 150 mL) was added dropwise over 20 min. The resulting reaction mixture was refluxed overnight at 80 °C and then extracted with ethyl acetate and water. After the organic layer was evaporated with a rotary evaporator, the resulting powdery product was purified by column chromato-graphy from dichloromethane/n-hexane (1/1) to give the yellow 1N-PSBTF crystalline product. 2N-PSBTF and NP-PSBTF were prepared using similar procedures described above.

1N-PSBTF: Yield 78%. mp 336 °C. FT-IR (KBr, cm−1) 3060, 3018 (aromatic-C-H). 1H-NMR (500 MHz, CDCl3) δ 8.52 (d, 1H), 8.43 (d, 1H), 7.94 (m, 6H), 7.64 (d, 1H), 7.59 (m, 5H), 7.48-7.25 (m, 7H), 7.12 (m, 6H), 7.05 (d, 1H), 6.94 (t, 1H), 6.58 (d, 1H). MS (FAB) m/z 628.23 [(M+1)+]. Anal. Calcd. for C49H30 (618.76): C, 95.11; H, 4.89. Found: C, 95.08; H, 4.86. UV-vis (THF): λmax (Absorption) = 363 nm, λmax (Emission) = 436 nm.

2N-PSBTF: Yield 81%. mp 279 °C. FT-IR (KBr, cm−1) 3060, 3018 (aromatic-C-H). 1H-NMR (500 MHz, CDCl3) δ 8.50 (d, 1H), 8.41 (d, 1H), 7.94 (m, 6H), 7.62 (m, 5H), 7.55 (4H), 7.42 (m, 5H), 7.36 (d, 1H), 7.15-7.03 (m, 6H), 6.60 (d, 1H). MS (FAB) m/z 628.23 [(M+1)+]. Anal. Calcd. for C49H30 (618.76): C, 95.11; H, 4.89. Found: C, 95.08; H, 4.86. UV-vis (THF): λmax (Absorption) = 368 nm, λmax (Emission) = 439 nm.

NP-PSBTF: Yield 76%. mp 353 °C. FT-IR (KBr, cm−1) 3060, 3018 (aromatic-C-H). 1H-NMR (500 MHz, CDCl3) δ 8.50 (d, 1H), 8.41 (d, 1H), 8.06 (d, 1H), 7.97 (m, 5H), 7.83 (d, 1H), 7.65 (m, 5H), 7.53 (m, 5H), 7.39 (m, 4H), 7.26 (t, 1H), 7.13 (m, 5H), 7.04 (d, 2H), 6.64 (d, 1H). MS (FAB) m/z 795.86 [(M+1)+]. Anal. Calcd. for C53H32 (694.27): C, 95.07; H, 4.93. Found: C, 95.04; H, 4.89. UV-vis (THF): λmax (Ab-sorption) = 368 nm, λmax (Emission) = 436 nm.

OLED Fabrication. A basic device configuration of indium tin oxide (150 nm)/N,N′-bis-[4-(di-m-tolylamino)-phenyl]-N,N′-diphenylbiphenyl-4,4′-diamine (DNTPD, 60 nm)/bis[N-(1-naphthyl)-N-phenyl]benzidine (NPB, 30 nm)/ PSBTF hosts: LBD (30 nm, 5%)/aluminum tris(8-hydroxy-quinoline)( Alq3, 20 nm)/LiF (1 nm)/Al (200 nm) was used for device fabrication, as shown in Figure 1. The organic layers were deposited sequentially onto the substrate at a rate of 1.0 Å/s by thermal evaporation from heated alumina crucibles. The doping concentration of the dopant materials was varied at 5%. The devices were encapsulated with a glass lid and a CaO getter after cathode deposition. Current density–voltage luminance and EL characteristics of the blue fluorescent OLEDs were measured with a Keithley 2400 source measurement unit (Cleveland, OH, USA) and a CS 1000 spectroradiometer.

Figure 1.The device configuration and the chemical structure of the materials used in the devices.

 

Results and Discussion

Synthesis and Characterization. PSBTF was synthesiz-ed as shown in Scheme 2. In the first step, 6-bromo-2-naphthol was converted to 6-phenyl-2-bromophenol via a selective Suzuki coupling reaction after using triflic an-hydride, followed by a reaction with phenylboronic acid and palladium catalyst. In the second step, 6-phenyl-2-bromo-naphthalene was boronylated using triethylphosphite to generate 6-phenyl-naphthalene-2-boronic acid. 6-Phenyl-naphthalene-2-boronic acid was reacted with 1,8-dibromo-naphthalene to afford 8-[2-(6-phenyl)]naphthyl-1-bromo-naphthalene, which was converted to 8-[2-(6-phenyl)]naph-thyl-1-lithionaphthalene using BuLi. In the last step, spiro-formation reaction was employed for synthesis of PSBTF using 8-[2-(6-phenyl)]naphthyl-1-lithionaphthalene and 9-fluorenone, followed by an acid catalyzed cyclization reac-tion. 3-Bromo-PSBTF was prepared by selective bromina-tion of PSBTF in carbon tetrachloride solvent, as shown in Scheme 2. The molecular structures of aryl end-capped PSBTF derivatives and their synthetic routes are shown in Scheme 3. These compounds can be obtained with moderate yields using typical multi-step Suzuki coupling reactions. The chemical structures and composition of the resulting precursors and spiro-compounds were characterized by 1H NMR, 13C NMR, FT-IR, gas chromatography-MS, and ele-mental analysis.

Scheme 2.Synthetic schemes for the prepartion of spiro[benzo[ij]tetraphene-7,9'-fluorene].

Scheme 3.Preparation of 1NA-PSBTF, 2NA-PSBTF and NP-PSBTF host materials.

Thermal Properties. The thermal properties of the result-ing blue host materials were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis. Table 1 summarizes the DSC data for the two host materials. The 1N-PSBTF, 2N-PSBTF and NP-PSBTF showed melt-ing points (Tm) of 336 °C, 279 °C and 353 °C, respectively, but no melting points were observed on the second heating scan, even though they were given sufficient time to cool in air. Once the compounds became amorphous solids, they did not revert to the crystalline state. After the samples had cooled to room temperature, a second DSC scan showed glass transition temperatures (Tg) of 189 °C, 175 °C and 180 °C for 1N-PSBTF, 2N-PSBTF and NP-PSBTF, respec-tively, due to their rigid spiro-type backbone. This result suggests that their thermal stability improved significantly by introducing the rigid aromatic moiety into the spiro backbone. As a result, the amorphous glassy state of the transparent films of the three host materials showed that they are good candidates for use as electroluminescent (EL) materials.

Optical Properties and Energy Levels. The UV and photoluminescent (PL) spectra of the synthesized compounds in THF solution are shown in Table 1 and Figure 2. The absorption spectra had two maxima with similar absorption between 350–400 nm, which originated from the combi-nation of π-π* transitions of the side tetraphene and the PSBTF cores. The PSBTF mother molecule had UVmax of 357 and 375 nm and a blue PLmax of about 420 and 433 nm, respectively. However, 2N-PSBTF and NP-PSBTF had UVmax values of 369 and 385 nm, and 368 and 385 nm, and PLmax values of 436 and 437 nm, indicating a red shift of about 5–10 nm relative to PSBTF. Introducing naphthyl and naphthylphenyl groups to the 3-positions in PSBTF caused a slight red-shift in the absorption and PL spectra because conjugation length increased. 1N-PSBTF showed UV-vis absorption maxima of 362 and 382 nm. The PL emission maximum of 1N-PSBTF was located at 412 and 429 nm. All three synthesized compounds showed PL wavelengths that were red shifted about 12-15 nm in solution compared to the film. Since there is a π–π* interaction between mole-cules in a film, this state typically shows red shifts relative to molecules in solution.

Table 1.aThe purity of the samples were finally determined by high performance liquid chromatography (HPLC) using the above prepared samples after train sublimation. bBandgap. cFull width at half maximum.

Figure 2.UV-vis and PL spectra of three PSBTF host materials.

A molecular simulation of three PSBTF derivatives was carried out to understand the physical properties at the molecular level. Figure 3 shows the geometric structure of 1N-PSBTF, 2N-PSBTF, and NP-PSBTF. Rotation of the naphthyl or naphthylphenyl groups in the three host materials was limited due to steric hindrance between PSBTF and the aromatic substituent. These lead to distortion of the naphthyl or naphthylphenyl group, whose conjugation can be destroy-ed by the PSBTF group in 1N-PSBTF, 2N-PSBTF, and NP-PSBTF.30 The structure obtained by molecular calcula-tion suggests that the 1-naphthyl units incorporated at the 3-position of PSBTF can have a torsion angle of 45. It seemed that 1N-PSBTF deviated significantly from the coplanar conformation. Particularly, the 2-naphthyl and naphthyl-phenyl groups that formed with the adjacent PSBTF core had a torsion angle of 35°, which allows the PSBTF core and the 2-naphthyl group to form reduced conjugation systems.

Figure 3.HOMO and LUMO electronic density distributions of three PSBTF derivatives, calculated at the DFT/B3LYP/6-31G* for optimization and Time Dependent DFT (TDDFT) using Gaussian 03.

The molecular simulation also showed the electron di-stribution of 1N-PSBTF, 2N-PSBTF, and NP-PSBTF. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution of 1N-PSBTF, 2N-PSBTF, and NP-PSBTF are shown in Figure 3. The HOMO and LUMO orbitals of the two host materials were concentrated in the PSBTF core, and the substituent did not affect the HOMO and LUMO distribution in the host materials.

Figure 4.Energy diagram of the deep blue fluorescence devices using PSBTF host materials.

Figure 5.Electroluminescence spectra of the devices using three PSBTF and β-ADN host materials.

The energy levels of the three host and dopant materials used to fabricate the OLEDs are shown in Figure 4. A low-energy photoelectron spectrometer was used to obtain infor-mation on the HOMO energies of the host and dopant materials and to examine the charge injection barriers. Energy gaps for 1N-PSBTF, 2N-PSBTF, NP-PSBTF, and LBD were calculated as 3.09, 3.03, 3.02, and 2.73 eV, respectively. The HOMO energy levels were −5.90, −5.80, −5.80, and −5.47 eV for 1N-PSBTF, 2N-PSBTF, NP-PSBTF and LBD, respectively. The HOMO value for dopant LBD was quite high compared with those of the other three hosts. The HOMO values of the three hosts were similar to those of other anthracene derivatives (−5.5 to −6.0 eV) such as 2-methyl-9,10-di(2-naphthylanthracene) (MADN) and β-ADN.31

EL Properties. The EL spectra of the blue fluorescent 1NA-PSBTF, 2NA-PSBTF, and NP-PSBTF devices doped with LBD dopant at concentrations of 5% are shown in Figure 5. A deep-blue emission was observed from the LBD-doped PSBTF devices, and the color coordinates of the blue device were about x = 0.132 and y = 0.144. The EL spectra of host 1N-PSBTF was quite a deep-blue emission with x = 0.133 and y = 0.134 as summarized in Table 2, which was comparable to other spiro host materials.16,29 The peak maxi-mum of the EL spectra was also 465 nm, and the spectra were consistent with substitution of naphthyl or naphthyl-phenyl groups to PSBTF, indicating that the EL emission was dominated by the emission peak of the dopant. This result suggests that the energy transfer from the host to LBD dopant was quite efficient at the optimum dopant concen-tration employed in this experiment. The full width at half maximum of 37 nm was quite small, which leads to deep blue, as illustrated in the PL spectrum (49 nm).

Table 2.Eelectroluminescence properties of the devices obtained from four PSBTF hosts and β-ADN doped with LBD

Figure 6.Current density-voltage-luminescence characteristics of the devices using PSBTF host materials doped with 5% LBD.

Figure 7.Current efficiency-luminance characteristics of the device using PSBTF derivatives doped with 5% LBD.

OLED Device Properties. 1N-PSBTF, 2N-PSBTF, NP-PSBTF, and commercial β-ADN were evaluated as host materials for blue fluorescent OLEDs. LBD was doped as a deep-blue dopant in the four host materials, and the device performance of the blue OLEDs was studied. Figure 6 shows the luminance–voltage–current density characteristics of the OLEDs with three hosts doped with 5% LBD dopant. Because 2N-PSBTF, and NP-PSBTF had a low band gap, they showed a slightly higher current density and luminance than that of 1N-PSBTF at the same driving voltage. As can be seen in the luminance efficiency curves of the three devices in Figure 7, high efficiencies > 5.27 cd/A were obtained for NP-PSBTF. A low efficiency was observed for the device using β-ADN doped with 5% dopant. In the case of the 1N-PSBTF device doped with 5% LBD, the maxi-mum brightness of 1285 cd/m2 and 4.78 cd/A was obtained at 7 V, as summarized in Table 2. 2N-PSBTF doped with 5% LBD showed high efficiency and a small decrease in efficiency as the current density was increased from 0 to 37.6 mA/cm2, i.e., a weak-current-induced fluorescent quen-ching. 32,33 It reached a current efficiency of 4.94 cd/A and CIE coordinates of 0.132 and 0.144 at 7 V. These results suggest that an exciton was formed and light was emitted at specific thresholds.

Figure 8.Quantum efficiency–luminance curves of the PSBTF devices.

The maximum quantum efficiencies of the SBTF devices were similar with each another. The NP-PSBTF device doped with 5% LBD showed an external quantum efficiency (EQE) of 4.63% as shown in Figure 8. Notably, the effici-encies of these devices remained stable when the luminance was increased to 877 cd/m2. It can be attributed that the holes injected from a hole transfer layer, NPB, were trans-ferred to the lighting emitting layer and trapped at dopant sites. The dopant LBD had a large capacity to catch the holes, and minimized the loss of holes resulting in good EL efficiency. Thus, the HOMO level of LBD was suitable for hole trapping and hole transporting as a dopant, and the PSBTF derivatives were moderate for balancing holes and electrons in the emitting layer. Based on this result, fused-ring spiro[benzotetraphene-fluorene]-type host materials are efficient to improve EL efficiency and color purity in the structure of PSBTF: 5% LBD devices.

 

Conclusion

According to a rational strategy of synthesizing fused-ring spiro compounds, we prepared four novel conjugated spiro-[benzo[ij]tetraphene-7,9'-fluorene] chromophores such as PSBTF, 1N-PSBTF, 2N-PSBTF, and NP-PSBTF, and investi-gated their optical and photophysical properties, electro-chemical properties, and EL properties. Structure-function relationships were further discussed, suggesting a rational strategy to develop conjugated fused-ring spiro molecules. PSBTF derivatives showed improved glass transition temperatures (Tg) for good thermal stability, and were used to construct deep-blue OLEDs. The typical OLED devices showed excellent performance; the NP-PSBTF-based device exhibited highly efficient blue-light emission with a maxi-mum efficiency of 5.27 cd/A (EQE, 4.63%) with x = 0.133 and y = 0.144. According to these characteristics, these blue light emitting materials have sufficient potential for fluore-scent OLED applications.