Synthesis, physical and electroluminescence properties of 3,6-dipyrenylcarbazole end capped oligofluorenes

Abstract

A series of oligofluorenes bearing two 3,6-dipyrenylcarbazole units as the terminal substituents, namely BPCFn (n = 1–3), were successfully synthesized and characterized as non-doped hole-transporting blue emitters. These molecules showed strong blue emission with good solubility, and thermally stable amorphous and excellent film-forming properties. OLEDs using these materials as the emissive layers were fabricated by a simple solution spin-coating process. The blue OLED with excellent device performance (brightness of 6085 cd m⁻², luminance efficiency of 4.13 cd A⁻¹ and turn-on voltage of 3.4 V) was attained from BPCF3.

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Introduction

Over the last decade, organic light-emitting diodes (OLEDs) with high efficiencies have attracted considerable attention for their potential applications to full color ultra-thin flat panel displays and solid-state lighting.¹ These devices have many advantages such as fast response, low voltage drivers, and wide viewing angles compared with the traditional displays. According to many reported results, it is clear that if there is one strong need in the field of OLEDs it continues to be in the area of blue emitters and devices.² A blue emissive material with good color coordinates coupled with a long device lifetime and high luminance efficiency is the challenging goal of materials chemists in this field. Moreover, by using a guest–host system blue light from the appropriate blue emitter (host) can be converted into green or red light with the use of suitable dyes (guest) giving the possibility of generating all colors including white light.³ Many large band-gap small molecules,⁴ dendrimers⁵ and polymers⁶ have been developed for blue emission. To encapsulate, they are the distyrylarylene series,⁷ fluorenes and spirofluorenes,⁸ polycyclic aromatic hydrocarbons,⁹ pyrenes and perylenes,¹⁰ anthracenes,¹¹ aromatic amines,¹² organosilicons,¹³ heterocyclic compounds,¹⁴ and metal complexes.¹⁵ Though, some of these materials offer high-efficiency blue OLEDs such as di-spiro-9,9′-di-fluorene-9′′,9′′′-(9,10-dihydro-anthracene) (DSFA)¹⁶ and 2-methyl-9,10-di(2-naphthyl)anthracene (MADN),¹⁷ bis[4-(3,6-dimethoxycarbazole)phenyl]sulfone (DMOC-DPS),¹⁸ developing new blue emitters and OLEDs that match the performance of red and green is still an essential challenge.

To explore a new blue emitter and gain better insight of the structure–property relationships in the material, we present the designs, syntheses and properties of a series of new pyrene/carbazole/fluorene hybrid materials (Fig. 1), and the results of a study of their electroluminescence properties as solution processed non-doped hole-transporting blue light-emitting materials. We chose to use pyrene and fluorene units because of their strong blue fluorescence and their successful use as highly efficient blue fluorescent emitters,¹⁹ while carbazole has high triplet energy and hole-transport capability.²⁰ The end positions of oligofluorenes connected with carbazole moieties through the N-atom would result in a sterically bulky structure, which not only hinders their close packing and crystallization but also increases their molecular rigidity, leading to amorphous materials having distinct morphological stability. Attaching pyrene chromophores at 3,6-positions of each carbazole aims to reduce the aggregate fluorescence quenching of pyrene in solid-state. Finally, varying the number of fluorene units in the oligomer would allow a fine tuning the properties of the molecule. Herein, we report the detail synthesis, characterizations and properties of three new blue emitters.

Fig. 1 Molecular structures of BPCFn.

Results and discussion

Synthesis

The structures of new blue light-emitting 3,6-dipyrenylcarbazole end capped oligofluorenes BPCFn (n = 1–3) are shown in Fig. 1, and the synthetic routes of the compounds are depicted in Scheme 1. Mono and di coupling of 2,7-dibromofluorene 1 with carbazole under mild Ullmann-type coupling conditions catalyzed by CuI/K₃PO₄/±trans-1,2-diaminocyclohexane gave the relevant 2,7-bis(carbazol-N-yl)fluorene 2 and 2-bromo-7-carbazol-N-ylfluorene 4 in 87% and 75% yields, respectively. Double palladium catalyzed coupling of the bromide 4 with either hexabutylditin and 9,9-dihexylfluorene-2,7-diboronic acid in the presence of Pd(PPh₃)₄ as a catalyst provided the carbazole end-capped bisfluorene 5 and trisfluorene 7 in reasonable yields, respectively. NBS bromination of 2, 5 and 7 afforded the related tetrabromides 3, 6 and 8 in good yields. Finally, Suzuki cross-coupling of 3, 6 and 8 with an excess of 1-pyrene boronic acid catalyzed by Pd(PPh₃)₄/Na₂CO₃ gave the target BPCFn (n = 1–3) as light yellow solids in 61–70% yields. The structures and purities of all synthesized compounds were verified using ¹H NMR, ¹³C NMR, IR and MALDI-TOF MS. BPCFn show high solubility in chlorinated solvents and toluene allowing their thin films to be fabricated by cheap solution-based deposition methods, such as direct dip coating, spin-coating and paste coating.

Scheme 1 Synthesis of BPCFn.

Quantum chemical calculations

To study the electronic structures and understand the nature of excited states of our new blue emitters, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed.²¹ In the optimized structures of BPCFn, the terminal 3,6-dipyrenylcarbazole moieties adopt a non-planar conformation to the backbone of oligofluorene core (Fig. S1†). Hence, the bulky 3,6-dipyrenylcarbazole substitutions on the oligofluorenes could play a vital role in reducing an unwanted intermolecular interaction in the solid state, and expediting the glass-forming ability and enhancing the thermal stability of the materials. Our TDDFT calculations also show that the S₀ → S₁ transitions mainly correspond to the HOMO → LUMO transitions with non-zero oscillator strengths (Table S1†). Therefore, we will place our special emphasis on the HOMO–LUMO analyses in the following discussion. The results of DFT calculations reveal that BPCFn have similar HOMOs and LUMOs in terms of their orbital characters (Fig. 2). The contour plots show that the HOMOs are mainly distributed along the 3,6-dipyrenylcarbazole backbone but partly on the central oligofluorene, whereas the LUMOs are located primarily on the pyrene peripheries. From Fig. 2, it is clear that the fluorene group do not significantly contribute to the HOMOs and LUMOs of BPCFn, thus, suggesting the little dependence of the HOMO and LUMO levels on the variation of number of fluorene. In contrast to their 3,6-dipyrenylcarbazole-free counterparts 2, 5 and 7, in the HOMOs and LUMOs the π-electrons are delocalized over the entire oligofluorene backbone and end-capped carbazole moieties through the lone electron pair of the nitrogen of the carbazole (Fig. 2). Hence, the HOMO stability and the emission energy gap of 2, 5 and 7 are greatly controlled by the number of fluorene. The effect of oligomer length on the HOMO and LUMO levels is also anticipated to be very significant. As shown in Table 1, the calculated HOMO and LUMO energy levels, energy gaps, excitation energies and maximum absorption wavelengths of BPCFn are nearly the same, while those of 2, 5 and 7 are gradually decreased as the number of the fluorene units per molecule increased.

Fig. 2 The HOMO and LUMO orbitals of 2, 5, 7 and BPCFn calculated by B3LYP/6-31G(d,p) method in CH₂Cl₂ solvent.

Table 1 The calculated HOMO, LUMO and HOMO–LUMO energy gap (Δ_H–L) of BPCFn by B3LYP/6-31G(d,p) in CH₂Cl₂ solvent

Compd	HOMO (eV)	LUMO (eV)	ΔH–L (eV)	Eex^a (eV, nm)
a Excitation energy calculated by TD-B3LYP/6-31G(d,p) method in CH2Cl2 solvent.
BPCF	−5.15	−1.68	3.47	3.13 (397)
BPCF2	−5.14	−1.68	3.46	3.11 (399)
BPCF3	−5.13	−1.68	3.45	3.10 (400)
2	−5.35	−1.28	4.07	3.52 (352)
5	−5.30	−1.54	3.76	3.30 (376)
7	−5.26	−1.63	3.64	3.19 (388)

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Optical properties

Fig. 3 presents the absorption and emission spectra of BPCFn, 2, 5 and 7 in diluted CH₂Cl₂ solutions and as spin-coated thin films. The pertinent data are listed in Table 2. BPCFn in solution exhibit absorption maxima around 352–362 nm, due to a π–π* transition contributed from the conjugated dipyrenylcarbazole–fluorene segments. There is an additional absorption at 277 nm contributed from the carbazole moieties. Consistent with the DFT results, the absorption maxima of BPCFn exhibit a slight blue-shift of 3–5 nm when increasing number of the fluorene units per molecule, while the absorption maxima of 2, 5 and 7 show a progressive red-shift of 10 nm per one fluorene unit increased in the molecule. This due to the π-electron system along the backbone of 2, 5 and 7 is extended. Upon photoexcitation at the absorption maximum, the BPCFn solutions exhibit blue fluorescence with emission peaks in the range of 406–436 nm. Similar to the absorption, a slight blue-shift in the PL spectra of BPCFn and a significant red-shift in the PL spectra of 2, 5 and 7 are observed. The PL spectra of these BPCFn in thin films are very similar to those in solution, except for a red-shift. In fact, the red-shifts of the emission maxima (Δλ_em) of BPCFn from solution to film reduce from 32 nm for BPCF to 17 nm for BPCF2 and BPCF3, as shown in Table 2. The fluorescence quantum yields (Φ_F) of BPCFn in CH₂Cl₂ solutions at ambient conditions are given in Table 1; outstanding quantum yields are found for these blue emitters (Φ_F = 0.89–0.94). Pyrene and fluorene derivatives generally have quantum yields in solution.²²

Fig. 3 Plots of UV-vis absorption (solid line) and PL (dotted line) spectra of (a) BPCFn in CH₂Cl₂, (b) 2, 5 and 7 in CH₂Cl₂ and (c) BPCFn as thin films coated on quartz substrates.

Table 2 Optical, physical and electrochemical data of BPCFn

Compd	λ^soluabs/λ^soluem^a (nm)	λ^filmem^b (nm)	Stokes shift (nm)	ΦF^c	Δλem^d (nm)	E1/2 (ox) vs. Ag/AgCl^e (V)	Tg/T5d^f (°C)	E^optg^g (eV)	HOMO/LUMO^h (eV)
a Measured in CH2Cl2.b Measured as thin films spin-coated on quartz substrates.c Measured in CH2Cl2 using quinine sulfate as a standard.d λ^filmem − λ^soluem.e Obtained from CV measured vs. Ag/Ag^+ in CH2Cl2 at a scan rate of 50 mV s^−1 and n-Bu4NPF6 as electrolyte.f Obtained from DSC and TGA measured at 10 °C min^−1 under N2.g Calculated from E^optg = 1240/λonset.h Estimated from HOMO = −(4.44 + E^oxonset); LUMO = E^optg − HOMO.
BPCF	277, 362/436	468	86	89	32	0.62 (Epc), 1.16 (Epa), 1.54	154/360	3.18	−5.52/−2.34
BPCF2	277, 357/430	447	73	93	17	0.58 (Epc), 1.18 (Epa), 1.49	212/410	3.14	−5.47/−2.33
BPCF3	276, 352/406sh, 424	442	61	94	18	0.58 (Epc), 1.18 (Epa), 1.45	242/425	3.12	−5.48/−2.36

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Thermal and morphological properties

The thermal properties of BPCFn were investigated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) (Fig. 4a and Table 2). All of the new blue emitters BPCFn exhibit high thermochemical stability, as evidenced by their TGA 5%-weight-loss temperatures (T_5d) under a nitrogen atmosphere being as high as 425 °C, which is beneficial to the long-term stability of OLED devices fabricated from these materials.²³ Under DSC examination, BPCFn reveal a distinct glass transition in the range of 154–242 °C, and no crystallization and melting transitions were found upon heating beyond T_g, indicative of their amorphous character. The T_gs of BPCFn are much higher than that of their 3,6-dipyrenylcarbazole-free counterparts; carbazole end-capped bisfluorene (T_g = 69 °C),²⁴ poly(9,9-dioctylfluorene) (POF) (T_g = 75 °C).²⁵ It is evident that the introduction of two rigid dipyrenylcarbazole moieties at the ends of the molecule renders BPCFn rather bulky and rigid and leads to a much higher T_g. The morphology of molecular materials and their thin films is one of the key factors in influencing the performance of OLED. The thin films of BPCFn prepared by spin-coating from CHCl₃/toluene (1 : 1) solutions were examined by atomic force microscopy (AFM). The AFM images of the spin-coated thin films of BPCF3 and BPCF2 show a uniform, smooth and pinhole-free surface and slightly better film quality than thin film of BPCF (Fig. 5). The spin-coated films of BPCFn (T_g = 154–242 °C) are subjected to heating at 120 °C and inspected by AFM periodically. The films remain unchanged after several hours. This is very important for the emissive materials to be thermally stable amorphous to avoid grain-boundary defects, which could reduce the efficiency of the OLEDs by hindering a charge migration.²⁶

Fig. 4 (a) DSC (2^nd heating scan) and TGA thermograms measured at a heating rate of 10 °C min⁻¹ under N₂. Plots of CV traces measured in CH₂Cl₂/n-Bu₄NPF₆ at scan rate of 50 mV s⁻¹ of (b) BPCFn, (c) BPCF3 and (d) BPCF.

Fig. 5 Tapping mode AFM images of the spin-coated thin films of BPCFn.

Electrochemical properties

Cyclic voltammetry (CV) was employed to investigate the redox behavior of BPCFn (Fig. 4b and Table 2). During the initial forward anodic scan, all the compounds show two irreversible oxidation waves. The first oxidation is associated with the removal of electrons from the carbazole group, resulting in carbazole radical cations (CBZ˙⁺). On the return cathodic scan, they exhibit a distinct extra peak (E_pc = 0.58–0.62 V) corresponding to an electrochemical coupling reaction of the radical cations formed. The repeated CV measurements reveal an increasing change in the CV traces, indicating a series of electrochemical reactions led to electro-polymerization of those radical cation species taking place on the glassy carbon electrode surface (Fig. 4c and d and S2†). Usually, this type of electrochemical coupling reaction can be observed in most unsubstituted and less hindered carbazole derivatives such as in the case of 2,7-bis[4-(carbazol-9-yl)-4-biphenyl-4-yl]-9,9-bis(4-diphenylaminophenyl)fluorene.²⁷ In fact, from the repeated CV traces and size of the E_pc peaks, the degree of oxidation coupling reaction of the radical cations of BPCFn are in the order of BPCF > BPCF2 ≫ BPCF3, suggesting that the radical cations of BPCF3 is relatively more stable or less reactive than that of BPCF and BPCF2. Fig. 6 illustrates a proposed oxidation and electrochemical coupling reaction of BPCFn. In the first step of the oxidation, the electrons are removed from N-atom of the carbazole to give the CBZ˙⁺, which then takes on electron delocalization to either the central oligofluorene or peripheral pyrene groups resulting in fluorene radical cation (FL˙⁺) or pyrene radical cation (PRY˙⁺), respectively. FL˙⁺ is relatively more sterically shielded, stable radical than PRY˙⁺. The highly reactive radical cation PRY˙⁺ then undertake dimerization coupling to form stable neutral molecule on the surface of the working electrode as indicated by the presence of the cathodic peak (E_pc). According to the CV results we believe that CBZ˙⁺ of BPCF3 having trisfluorene as a central core takes on electron delocalization to form FL˙⁺, while that of BPCF favors the formation PRY˙⁺. Though, this kind of radical–radical coupling reaction will come to be lethargic in the device. Moreover, under the CV measurement conditions, reduction wave is not observed for any of BPCFn in solution. The energy levels of the HOMO and LUMO of these compounds were determined and are listed in Table 2. The HOMOs were calculated from their oxidation potentials, while the LUMO level was estimated based on the HOMO energy level and the lowest-energy absorption edge of the UV-vis absorption spectrum. The HOMO and LUMO energy levels of these compounds are at ca. 5.47–5.52 eV and 2.33–2.36 eV, respectively.

Fig. 6 A proposed reaction mechanism for electrochemical oxidation and reaction of BPCFn.

Electroluminescence (EL) properties

To understand the electroluminescence (EL) efficiency of BPCFn, two types of OLED devices using these compounds as host emitters (EML) were fabricated: single-layer OLED (ITO/PEDOT:PSS/EML/LiF:Al) and double-layer OLED (ITO/PEDOT:PSS/EML/BCP/LiF:Al). First type device (device I) was initially fabricated using BPCF3 as the emitter. The performance of this device is listed in Table 3. A single-layer OLED (device I) fabricated with BPCF3 (spin-coating, 40 nm) emits blue light at 437 and 464 nm with a brightness (L_max) of 4050 cd m⁻² and a maximum luminescence efficiency (η_max) of 1.86 cd A⁻¹ at 47.0 mA cm⁻² and a turn-on voltage (V_on) of 5.9 V. The EL spectrum does not change with applied voltage with CIE coordinates of (0.17, 0.17). The EL spectrum of device I matches with to the PL spectrum of BPCF3 (Fig. 7b). Fig. 7a shows the relative HOMO and LUMO energy levels of each layer of the OLED. The moderate efficiency of this diode (device I) is probably because of the unbalance of holes and electrons during operation.²⁸ The relatively high LUMO level of BPCF3 (−2.36 eV) compared to the work function LiF/Al cathode (−4.20 eV) leads to difficult migration of electrons from the cathode to BPCF3. On the other hand, the migration of holes from the ITO/PEDOT:PSS anode to the HOMO of BPCF3 and to cathode is expected to proceed smoothly in view of the low HOMO and the great hole-transporting ability of the BPCF3 layer. As a result, hole leaking from BPCF3 to the Al layer is high and the probability of trapping both electron and hole in the layer of BPCF3 is low resulting in a moderate efficiency of device I. The insertion of a layer of dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)²⁹ between the LiF/Al and the layer of BPCF3 that can act as an electron-transporting and hole-blocking layer and can also enhance the electron injection and transporting efficiency improves the efficiency of the device. Thus, three devices II–IV, consisting of two organic layers, BPCFn (spin-coating, 30–40 nm)/BCP (30 nm), were fabricated. The performance of these devices is summarized in Table 3. Fig. 8 shows the EL characteristics of the devices. Device II fabricated with BPCF3/BCP displays device performance relative to that of device I increased substantially (Table 3). An L_max of 6085 cd m⁻², a η_max of 4.13 cd A⁻¹ at 22.8 mA cm⁻² and a low V_on of 3.4 V are achieved for this device. The device emits a sky blue emission at 424 and 446 nm with CIE coordinates of (0.17, 0.15). Devices III and IV using BPCF2 and BPCF, respectively, as the emitters show light blue emission at 470 and 490 nm with CIE coordinates of (0.16, 0.22) and (0.17, 0.35), respectively. Compared to device II, device III exhibits somewhat lower device performance with a η_max of 2.56 cd A⁻¹ at 49.5 mA cm⁻² and a V_on of 3.6 V, while device IV shows a lowliest device performance (η_max = 1.54 cd A⁻¹, V_on = 4.0 V). The EL spectra of devices II–IV do not change with applied voltage and exhibit the FWHM values of 80–91 nm (Fig. S3†). The resemblance of the EL spectra of all devices to the PL spectra of their corresponding EML indicates that the emission of all diodes is solely from the layer of the EML and that the hole and electron recombination in the device occurred in this layer only. Undoubtedly, the non-planar molecular structures of the emitting materials (BPCFn) is responsible for the observed pure EML emission.³⁰ Although, the efficiency of BPCF3-based blue OLED is not competitive with reported best blue device,^17,18,31 the simplicity of the device structure and fabrication process by solution spin-coating in this study is surely an advantage.

Table 3 EL properties of the OLEDs fabricated with BPCFn as EML

Device	EML	λEL/FWHM (nm)	Von/V100^c (V)	Lmax at voltage^d (cd m^−2/V)	Jmax^e (mA cm^−2)	ηmax at J/η at L100/η at L1000^f (cd A^−1/mA cm^−2)	CIE (x, y)
a ITO/PEDOT:PSS/EML (spin-coating)/LiF:Al.b ITO/PEDOT:PSS/EML (spin-coating)/BCP/LiF:Al.c Turn-on voltages at 1 and 100 cd m^−2.d Maximum brightness at applied voltage.e Current density at maximum brightness.f Luminance efficiencies at maximum, at brightness of 100 and 1000 cd m^−2.
I^a	BPCF3	437, 464 (104)	5.9/7.2	4050 (11.6)	496	1.86 (47.0)/1.15/1.83	0.17, 0.17
II^b	BPCF3	424, 446 (84)	3.4/4.8	6085 (11.2)	510	4.13 (22.8)/2.75/4.13	0.17, 0.15
III^b	BPCF2	470 (91)	3.6/5.3	6194 (9.4)	470	2.56 (49.5)/1.44/2.55	0.16, 0.22
IV^b	BPCF1	490 (80)	4.0/5.8	4276 (10.6)	601	1.54 (41.4)/0.77/1.36	0.17, 0.35

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Fig. 7 (a) A schematic energy diagram of each layer of the OLED. (b) EL spectra and the device emission colors of the OLEDs.

Fig. 8 Plots of (a) current density–brightness–voltage (J–V–L) and (b) efficiency–current density (η–J) characteristics of the OLEDs.

Conclusion

In summary, we have prepared a series of hole-transporting blue light-emitting 3,6-dipyrenylcarbazole end capped oligofluorenes using mild Ullmann-type coupling, bromination and Pd-catalyzed cross-coupling reactions. We have experimentally and theoretically demonstrated that both 3,6-dipyrenylcarbazole terminal substituents have a strong influence on the absorption, emission, electrochemical, thermal and morphological properties of these materials. These oligofluorenes show unique optical properties compared to their 3,6-dipyrenylcarbazole-free counterparts. All molecules exhibit strong blue fluorescence with high T_g amorphous and outstanding film-forming properties. With blue emitter BPCF3, we achieved a high luminescence efficiency solution-processed blue OLED (η_max = 4.13 cd A⁻¹ at J = 22.8 mA cm⁻²) together with a low turn-on voltage of 3.4 V and CIE coordinates x = 0.17 and y = 0.15. The admirable electroluminescent performance of these oligomers demonstrates their promising features for OLED displays.

Experimental section

Materials and methods

2,7-Dibromo-9,9-didodecylfluorene (1) was synthesized by using the reported procedure.^{32 1}H and ¹³C NMR spectra were recorded with a 300 MHz spectrometer in CDCl₃ as a solvent with TMS as the internal standard. Infrared (IR) spectra were measured as KBr disc. UV-vis and fluorescence spectra were recorded in diluted CH₂Cl₂ solutions and as thin films spin-coated on quartz substrates. Fluorescence quantum yield (Φ_F) was measured in CH₂Cl₂ using quinine sulfate in 1 M H₂SO₄ (Φ_F = 0.54) as a standard.³³ Thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) were recorded at a heating rate of 10 °C min⁻¹ under nitrogen stream. Cyclic voltammetry (CV) was performed with a three electrode system (platinum counter electrode, glassy carbon working electrode and Ag/Ag⁺ reference electrode) in CH₂Cl₂ under argon stream with n-Bu₄NPF₆ as a supporting electrolyte at scan rate of 50 mV s⁻¹. High-resolution MALDI-TOF mass spectra were recorded without a matrix.

Synthesis

2,7-Bis(carbazol-N-yl)-9,9-didodecylfluorene (2). A mixture of 1 (5.00 g, 9.09 mmol), carbazole (4.56 g, 27.27 mmol), CuI (1.73 g, 9.09 mmol), K₃PO₄ (9.65 g, 45.46 mmol) and (±)-trans-1,2-diaminocyclohexane (0.57 g, 5.04 mmol) in toluene (100 ml) was degassed with N₂ for 5 min and then stirred at reflux under N₂ atmosphere for 24 h. After cooling, water (50 ml) was added, and the mixture was extracted with CH₂Cl₂ (50 ml × 2). The combined organic phase was washed with water (100 ml × 2) and brine solution (100 ml), dried over anhydrous Na₂SO₄ and filtered, and the solvent was removed to dryness. Purification by silica gel column chromatography eluting with hexane gave a light yellow oil (6.24 g, 87%); ¹H NMR (300 MHz, CDCl₃) δ 0.89–0.91 (10H, m), 1.23 (36H, bs), 2.08 (4H, bs), 7.17–7.38 (4H, m), 7.46–7.50 (8H, m), 7.61 (4H, d, J = 7.5 Hz), 8.00 (2H, d, J = 8.1 Hz), 8.24 (4H, d, J = 7.6 Hz) ppm; ¹³C-NMR (75 MHz, CDCl₃) δ 14.17, 22.73, 24.23, 29.37, 29.46, 30.08, 31.96, 40.27, 55.78, 109.82, 120.01, 120.47, 121.08, 123.50, 126.03, 136.79, 139.62, 141.07, 152.96 ppm; FTIR (KBr) ν 2927, 1617, 1598, 1476, 1450, 1309, 1234, 814 cm⁻¹; MALDI-TOF (m/z) calcd for C₆₁H₇₂N₂: 832.5696; found 832.5677 (M⁺).

2-Bromo-7-(carbazol-N-yl)-9,9-didodecylfluorene (4). 4 was synthesized in a similar manner to 2 but using carbazole of 0.5 equiv., and obtained as a light yellow oil (5.70 g, 75%); ¹H NMR (300 MHz, CDCl₃) δ 0.90 (4H, bs), 1.01 (6H, t, J = 5.7 Hz), 1.24–1.35 (36H, m), 2.08–2.11 (4H, m), 7.40–7.44 (2H, m), 7.52–7.72 (9H, m), 7.94 (1H, d, J = 7.8 Hz), 8.27 (2H, d, J = 7.8 Hz) ppm; ¹³C-NMR (75 MHz, CDCl₃) δ 14.31, 22.88, 24.11, 29.50, 29.53, 29.77, 29.79, 29.82, 30.13, 30.22, 32.11, 40.34, 55.85, 109.87, 120.13, 120.56, 121.36, 121.68, 121.96, 123.64, 126.06, 126.46, 130.43, 137.02, 139.43, 139.49, 141.16, 152.39, 153.40 ppm; FTIR (KBr) ν 2920, 1617, 1465, 1432, 1281, 1230, 1020, 804 cm⁻¹; MALDI-TOF (m/z) calcd for C₄₉H₆₄BrN: 745.4222; found 745.4278 (M⁺).

2,2′-Bis(carbazol-N-yl)-7,7′-bi(9,9-didodecylfluorene) (5). A mixture of 4 (2.75 g, 3.68 mmol), Bu₃Sn–SnBu₃ (0.85 g, 1.47 mmol), Pd(PPh₃)₄ (0.034 g, 0.03 mmol) in toluene (50 ml) was degassed with N₂ for 5 min and then stirred at reflux under N₂ atmosphere for 24 h. After cooling, CH₂Cl₂ (100 ml) was added. The organic phase was separated, washed with water (50 ml × 2) and brine solution (50 ml), dried over anhydrous Na₂SO₄ and filtered. The solvent was removed to dryness and the residue was purified by silica gel column chromatography eluting with a mixture of CH₂Cl₂ and hexane to yield light yellow solids (1.71 g, 87%); m.p. 85–87 °C. ¹H NMR (300 MHz, CDCl₃) δ 0.85–0.90 (20H, m), 1.17–1.28 (72H, m), 2.10–2.20 (8H, m), 7.32–7.36 (4H, m), 7.44–7.51 (8H, m), 7.58–7.60 (8H, m), 7.72–7.78 (4H, m), 7.89 (2H, d, J = 7.8 Hz), 7.96 (2H, d, J = 8.1 Hz), 8.20 (4H, d, J = 7.8 Hz) ppm; ¹³C-NMR (75 MHz, CDCl₃) δ 14.11, 22.68, 24.08, 29.34, 29.37, 29.62, 30.05, 31.92, 40.33, 55.58, 109.83, 119.88, 120.18, 120.39, 120.87, 121.51, 123.40, 125.86, 125.93, 126.40, 136.39, 139.65, 140.10, 140.71, 141.10, 151.88, 152.89 ppm; FTIR (KBr) ν 2920, 1617, 1465, 1432, 1281, 1230, 1020, 804 cm⁻¹; MALDI-TOF (m/z) calcd for C₉₈H₁₂₈N₂: 1333.0078; found 1333.0054 (M⁺).

2,7-Bis{2′-(carbazol-N′′-yl)9′,9′-didodecylfluoren-7′-yl}-9,9-dihexylfluorene (7). A mixture of 4 (1.05 g, 1.14 mmol), 9,9-dihexylfluorene-2,7-diboronic acid (0.28 g, 0.56 mmol), Pd(PPh₃)₄ (0.013 g, 0.01 mmol), 2 M Na₂CO₃ aqueous solution (5 ml) in THF (25 ml) was degassed with N₂ for 5 min and then stirred at reflux under N₂ atmosphere for 24 h. After cooling, CH₂Cl₂ (50 ml) was added. The organic phase was separated, washed with water (50 ml × 2) and brine solution (50 ml), dried over anhydrous Na₂SO₄ and filtered. The solvent was removed to dryness and the residue was purified by silica gel column chromatography eluting with a mixture of CH₂Cl₂ and hexane to yield light yellow solids (0.85 g, 76%); m.p. 90–92 °C. ¹H NMR (300 MHz, CDCl₃) δ 0.79–0.90 (30H, m), 1.16–1.30 (84H, m), 2.09–2.19 (12H, m), 7.34 (4H, dt, J = 6.3 Hz, J = 1.5 Hz), 7.34–7.51 (8H, m), 7.58–7.59 (4H, m), 7.71 (4H, s), 7.74 (4H, d, J = 7.2 Hz), 7.88 (4H, t, J = 7.5 Hz), 7.96 (2H, d, J = 8.4 Hz), 8.20 (4H, d, J = 7.8) ppm; ¹³C-NMR (75 MHz, CDCl₃) δ 14.03, 14.10, 22.56, 22.67, 23.86, 24.08, 29.32, 29.36, 29.62, 29.67, 29.70, 30.05, 31.47, 31.90, 40.33, 55.39, 55.55, 109.83, 119.86, 120.04, 120.15, 120.38, 120.84, 121.52, 121.90, 123.39, 125.83, 125.92, 126.23, 126.34, 136.33, 129.52, 140.13, 140.37, 140.90, 141.10, 151.85, 152.88 ppm; FTIR (KBr) ν 2920, 1617, 1465, 1432, 1281, 1230, 1020, 804 cm⁻¹; MALDI-TOF (m/z) calcd for C₁₂₃H₁₆₀N₂: 1665.2582; found 1665.2588 (M⁺).

Bromination. A solution of 2, 5 or 7 (0.40 mmol) in THF (20 ml) and acetic acid (20 ml) was added with NBS (1.69 mmol) in small portions under N₂ atmosphere. The mixture was stirred at room temperature for 1.5 h and then water (20 ml) was added. The mixture was extracted with CH₂Cl₂ (50 ml × 2). The combined organic phase was washed with water (70 ml × 2) and brine solution (70 ml), dried over anhydrous Na₂SO₄, filtered, and the solvent was removed to dryness. The residue was purified by silica gel column chromatography eluting with a mixture of CH₂Cl₂ and hexane.
2,7-Bis(3′,6′-dibromocarbazol-N′-yl)-9,9-didodecylfluorene (3). As white solids (98%); ¹H NMR (300 MHz, CDCl₃) δ 0.84–0.88 (10H, m), 1.15–1.21 (36H, m), 2.06 (4H, bs), 7.32 (4H, d, J = 8.7 Hz), 7.53 (8H, d, J = 9.3 Hz), 8.01 (2H, d, J = 8.4 Hz), 8.23 (4H, s) ppm; ¹³C-NMR (75 MHz, CDCl₃) δ 14.17, 22.71, 24.20, 29.37, 29.43, 29.61, 29.68, 29.98, 31.94, 40.15, 55.89, 111.46, 113.19, 121.43, 123.35, 124.04, 125.95, 129.50, 136.02, 139.92, 139.97, 153.17 ppm; FTIR (KBr) ν 2920, 1617, 1465, 1432, 1281, 1230, 1020, 804 cm⁻¹; MALDI-TOF (m/z) calcd for C₆₁H₆₈Br₄N₂: 1144.2116; found 1144.2203 (M⁺).
2,2′-Bis(3′′,6′′-dibromocarbazol-N′′-yl)-7,7′-bi(9,9-didodecylfluorene) (6). As white solids (88%); m.p. 82–84 °C. ¹H NMR (300 MHz, CDCl₃) δ 0.87 (20H, t, J = 6.3 Hz), 1.15–1.26 (72H, m), 2.02–2.15 (8H, m), 7.30 (4H, d, J = 8.7 Hz), 7.49 (4H, d, J = 6.6 Hz), 7.53 (4H, dd, J = 11.7H, J = 1.8 Hz), 7.72 (2H, s), 7.75 (2H, d, J = 7.8 Hz), 7.88 (2H, d, J = 7.8 Hz), 7.95 (2H, d, J = 8.7 Hz), 8.23 (4H, d, J = 1.5 Hz) ppm; ¹³C-NMR (75 MHz, CDCl₃) δ 14.14, 22.70, 24.07, 29.36, 29.61, 29.67, 30.01, 31.93, 40.28, 55.66, 111.53, 113.06, 120.35, 121.10, 121.55, 121.70, 123.30, 123.97, 125.74, 126.53, 129.44, 135.41, 139.41, 140.10, 140.75, 151.87, 153.16 ppm; FTIR (KBr) ν 2920, 1617, 1465, 1432, 1281, 1230, 1020, 804 cm⁻¹; MALDI-TOF (m/z) calcd for C₉₈H₁₂₄Br₄N₂: 1644.6498; found 1644.6478 (M⁺).
2,7-Bis{2′-(3′′,6′′-dibromocarbazol-N′′-yl)-9′,9′-didodecylfluoren-7′-yl}-9,9-dihexylfluorene (8). As white solids (90%); m.p. 90–91 °C. ¹H NMR (300 MHz, CDCl₃) δ 0.76–0.88 (30H, m), 1.13–1.26 (84H, m), 2.06–2.18 (12H, m), 7.29 (4H, d, J = 8.7 Hz), 7.48 (4H, J = 6.6 Hz), 7.52 (4H, dd, J = 8.7 Hz, J = 1.8 Hz), 7.68 (4H, s), 7.72 (4H, d, J = 8.4 Hz), 7.86 (4H, t, J = 7.5 Hz), 7.94 (2H, J = 8.4 Hz), 8.24 (4H, d, J = 1.5 Hz) ppm; ¹³C-NMR (75 MHz, CDCl₃) δ 14.02, 14.10, 22.54, 22.66, 23.84, 24.05, 29.32, 29.62, 29.99, 31.46, 31.90, 40.26, 55.39, 55.61, 111.54, 113.02, 120.07, 120.28, 121.04, 121.52, 121.68, 123.28, 123.95, 125.71, 126.24, 124.44, 129.42, 135.30, 139.18, 140.12, 140.29, 140.82, 141.20, 151.80, 153.15 ppm; FTIR (KBr) ν 2920, 1617, 1465, 1432, 1281, 1230, 1020, 804 cm⁻¹; MALDI-TOF (m/z) calcd for C₁₂₃H₁₅₆Br₄N₂: 1976.9002; found 1976.9063 (M⁺).

Suzuki-cross coupling reaction. A mixture of bromides 3, 6 or 8 (0.09 mmol), 1-pyrene boronic acid (0.48 mmol), Pd(PPh₃)₄ (0.01 mmol), 2 M Na₂CO₃ aqueous solution (5 ml) in THF (25 ml) was degassed with N₂ for 5 min and then stirred at reflux under N₂ atmosphere for 24 h. After cooling, CH₂Cl₂ (50 ml) was added. The organic phase was separated, washed with water (50 ml × 2) and brine solution (50 ml), dried over anhydrous Na₂SO₄ and filtered. The solvent was removed to dryness and the residue was purified by silica gel column chromatography eluting with a mixture of CH₂Cl₂ and hexane followed by recrystallization with a mixture of CH₂Cl₂ and methanol.
2,7-Bis(3′,6′-dipyrenylcarbazol-N′-yl)-9,9-didodecylfluorene (BPCF). As light yellow solids (69%); m.p. >250 °C. ¹H NMR (300 MHz, CDCl₃) δ 0.71 (6H, t, J = 6.9 Hz), 1.04–1.23 (40H, m), 2.23 (4H, bs), 7.76–7.87 (12H, m), 7.99–8.08 (8H, m), 8.11–8.22 (22H, m), 8.26 (4H, d, J = 7.8 Hz), 8.35 (4H, d, J = 9.3 Hz), 8.50 (4H, s) ppm; ¹³C-NMR (75 MHz, CDCl₃) δ 13.99, 22.54, 24.35, 29.22, 29.52, 29.56, 29.64, 29.67, 30.13, 31.76, 40.40, 55.99, 109.85, 121.36, 121.92, 122.55, 123.76, 124.68, 124.72, 125.02, 125.10, 125.59, 125.98, 127.28, 127.45, 127.49, 128.16, 128.89, 129.14, 130.42, 131.07, 131.57, 133.46, 136.85, 138.37, 139.82, 140.84, 153.20 ppm; FTIR (KBr) ν 2920, 1617, 1465, 1432, 1281, 1230, 1020, 804 cm⁻¹; MALDI-TOF (m/z) calcd for C₁₂₅H₁₀₄N₂: 1632.8200; found 1632.8214 (M⁺).
2,2′-Bis(3′′,6′′-dipyrenylcarbazol-N′′-yl)-7,7′-bi(9,9-didodecylfluorene) (BPCF2). As light yellow solids (61%); m.p. >250 °C. ¹H NMR (300 MHz, CDCl₃) δ 0.78 (12H, t, J = 6.9 Hz), 0.97 (8H, bs), 1.12–1.17 (72H, m), 1.59 (8H, bs), 7.71–7.81 (16H, m), 7.94–8.21 (32H, m), 8.26 (4H, d, J = 8.1 Hz), 8.35 (4H, d, J = 9.0 Hz), 8.50 (4H, s) ppm; ¹³C-NMR (75 MHz, CDCl₃) δ 14.05, 22.61, 24.18, 29.29, 29.44, 29.61, 29.66, 30.11, 31.84, 40.43, 55.72, 109.89, 120.31, 121.10, 121.59, 121.88, 122.51, 123.70, 124.67, 124.71, 125.02, 125.11, 125.62, 125.96, 126.51, 127.26, 128.17, 128.89, 129.09, 130.41, 131.08, 131.58, 133.35, 136.39, 138.42, 139.67, 140.37, 140.83, 140.91, 151.95, 153.14 ppm; FTIR (KBr) ν 2920, 1617, 1465, 1432, 1281, 1230, 1020, 804 cm⁻¹; MALDI-TOF (m/z) calcd for C₁₆₂H₁₆₀N₂: 2133.2582; found 2133.2593 (M⁺).
2,7-Bis{2′-(3′′,6′′-dipyrenylcarbazol-N′′-yl)-9′,9′-didodecylfluoren-7′-yl}-9,9-dihexylfluorene (BPCF3). As light yellow solids (70%); m.p. 138–140 °C. ¹H NMR (300 MHz, CDCl₃) δ 0.77–0.99 (30H, m), 1.14–1.18 (84H, m), 2.18 (12H, bs), 7.74–7.82 (20H, m), 7.87 (2H, d, J = 7.8 Hz), 7.93 (2H, d, J = 7.8 Hz), 7.99–8.22 (30H, m), 8.27 (4H, d, J = 8.1 Hz), 8.36 (4H, d, J = 9.0 Hz), 8.50 (4H, s) ppm; ¹³C-NMR (75 MHz, CDCl₃) δ 14.07, 22.62, 23.91, 24.19, 29.29, 29.45, 29.62, 29.66, 30.13, 31.51, 31.85, 40.43, 55.44, 55.70, 109.91, 120.10, 120.28, 121.08, 121.58, 121.88, 122.52, 123.71, 124.68, 124.72, 125.04, 125.12, 125.63, 125.85, 125.97, 126.30, 126.45, 127.26, 127.44, 127.50, 128.19, 128.91, 129.10, 130.42, 131.09, 131.59, 133.35, 136.34, 138.44, 139.52, 140.18, 140.42, 140.93, 141.06, 151.91, 153.14 ppm; FTIR (KBr) ν 2920, 1617, 1465, 1432, 1281, 1230, 1020, 804 cm⁻¹; MALDI-TOF (m/z) calcd for C₁₈₇H₁₉₂N₂: 2465.5086; found 2465.5081 (M⁺).

Quantum chemical calculation

The ground state geometries were fully optimized using B3LYP functional at 6-31G(d,p) basis set, while the excited state properties were done by TD-B3LYP/6-31G(d,p) on the ground state geometries. The solvent effect from CH₂Cl₂ solution was considered using the conductor-like polarizable continuum model (C-PCM). All calculations were performed by Gaussian09 program package.²¹

Fabrication and characterization of OLEDs

OLEDs were fabricated, with configurations of indium tin oxide (ITO)/PEDOT:PSS (50 nm)/EML (30–40 nm)/LiF (0.5 nm)/Al (150 nm), and ITO/PEDOT:PSS (50 nm)/EML (30–40 nm)/BCP (30 nm)/LiF (0.5 nm)/Al (150 nm), in which EML represents one of BPCFn. The EML layer was fabricated through solution spin-coating of the emissive materials (1.5–2% w/v) in CHCl₃/toluene solution (1 : 1) at a spin speed of 3000 rpm for 30 seconds onto 50 nm thick PEDOT:PSS coated ITO glass substrates. Then, BCP of 30 nm-thick, and LiF of 0.5 nm-thick were evaporation-deposited sequentially at a rate of 0.5–1.0 nm s⁻¹ under a base pressure of ∼10⁻⁵ mbar onto the EML layer. Finally, a 150 nm-thick Al was deposited as the cathode. Layer thickness was in situ measured using a quartz thickness monitor during deposition. The device area was 0.04 cm² determined by the overlap area of the anode and cathode. The EL spectra were measured using an Ocean Optics USB4000 multichannel spectrometer. The current–voltage–brightness relationships of the devices were measured using a Keithley 2400 source meter and a Newport 1835C optical meter equipped with an 818-UV/CM silicon photodiode.

Acknowledgements

This work was supported by the Thailand Research Fund (TRF) under the TRF Senior Research Scholar (Grant no. RTA5680008) and Rayong Institute of Science and Technology (RAIST) Foundation. We thank Department of Physic, Ubon Ratchathani University for providing AFM facility. We also gratefully acknowledge the scholarship support for T Sangchart from the Center of Excellence for Innovation in Chemistry (PERCH-CIC).

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Footnote

† Electronic supplementary information (ESI) available: Additional DFT calculation data, CV plot, EL spectra and NMR spectra. See DOI: 10.1039/c5ra02382c

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