Carbazole-based coplanar molecule (CmInF) as a universal host for multi-color electrophosphorescent devices

Abstract

The synthesis, isomerism, photophysics and electrophosphorescent characterization of some functional cyclometallated iridium(III) complexes containing 2-[2-(N-phenylcarbazolyl)]pyridine and 2-[3-(N-phenylcarbazolyl)]pyridine molecular frameworks are described. A carbazole-based coplanar molecule (CmInF) obtained through the intramolecular ring closure of aryl substitutions at the C3 and C6 positions exhibits a high triplet energy (E_T = 2.77 eV), morphological stability (T_g = 195 °C) and hole mobility in the range of up to 5 × 10⁻³ cm² V⁻¹ s⁻¹. Highly efficient multi-color electrophosphorescent devices have been successfully achieved employing CmInF as the universal host material doped with phosphorescent dopants of various colors under the same device configuration of ITO/PEDOT:PSS (300 Å)/TCTA (250 Å)/CmInF: dopant (250 Å)/TAZ (500 Å)/LiF/Al (PEDOT:PSS = poly(ethylene dioxythiophene):polystyrene sulfonate; TCTA = 4,4′,4′′-tri(N-carbazolyl)triphenylamine; TAZ = 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole). Through the mixing of two phosphorescent dopants of complementary colors, we also fabricated a two-color white organic light-emitting device (WOLED) with the same device structure consisting of 12 wt% FIrpic and 0.3 wt% (Mpg)₂2Ir(acac) co-doped into CmInF as a single-emitting-layer, which exhibits peak WOLED efficiency of 13.4% (23.4 cd A⁻¹) and 11.2 lm W⁻¹ with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.33, 0.37). In addition, the use of such device structure in full-color OLEDs has the advantages of simplifying manufacturing process and reducing production cost that are the critical issues of commercialization.

---

Introduction

Organic light-emitting diodes (OLEDs) have drawn tremendous attention due to their potential applications in flat-panel displays and lighting sources.^1,2 For full-color OLED display applications, it is essential to deliver a set of primary red-green-blue (RGB) emitters with sufficiently high luminous efficiency. Notably, the use of phosphorescent materials, in which the strong spin–orbit coupling due to the heavy-atom effect leads to singlet–triplet state mixing, can boost the electroluminescence (EL) efficiency.³ In phosphorescent OLEDs (PHOLEDs), triplet emitters of heavy-metal complexes are normally used as the emitting guests in a host material to reduce the quenching effect associated with the relatively long excited-state lifetimes of triplet emitters as well as the triplet–triplet annihilation, etc. Consequently, the selection of a suitable host material dispersed with various phosphorescent guest dopants is of equal importance to achieve high device efficiency. The singlet and triplet levels of the host material should be higher than those of the phosphorescent guest dopants to prevent the reverse energy transfer from guest to host and also confine triplet excitons within the emitting layer.^4–7 For blue PHOLEDs, the design of such host material becomes a big challenge, particularly for a blue triplet emitter, in which the conjugation length of the host molecule must be extremely confined to achieve a triplet energy level higher than the photon energies of blue light (≥2.70 eV). In contrast, a host material with triplet energy higher than that of the blue phosphor may not be suitable for red, green, yellow emitters, etc., because of the unbalanced charge carrier transport in the emitting layer.

Recently, efficient green and red PhOLEDs with 100% internal quantum efficiency have been disclosed,^8–10 but highly efficient and stable blue PHOLEDs still remain a demanding issue. On the other hand, white light emission can be obtained by the mixing of several guest materials that give rise to RGB colors or two complementary colors (blue and red; blue and orange/yellow) through adoption of either the mixed fluorophore/phosphor or all-phosphor system. The design and synthesis of a host material that can work in various PHOLEDs are therefore important not only for blue, green and red phosphors but also for the realization of white OLEDs (WOLEDs) based on their suitable combinations. Up to now, reported literature examples of universal host materials for PHOLEDs are limited. For example, Cheng and Chou recently synthesized a bipolar host material bis-4-(N-carbazolyl)phenyl)phenylphosphine oxide (BCPO) containing a phenylphosphine oxide and two carbazole groups, which exhibits a high triplet energy (E_T) of 3.01 eV and a high glass transition temperature (T_g) of 137 °C.¹¹ By using BCPO as the universal host, high external quantum efficiencies (η_ext) of 23.5%, 21.6% and 17% were achieved for blue, green and red phosphorescent devices, respectively, using simple device architectures. Cheng et al. also reported three triptycene derivatives 2,6,14-tris-(diphenylamino)triptycene (TATP), 2,6,14-tris-carbazoyltriptycene (TCTP) and 2,6,14-tris-(diphenylphosphoryl)triptycene (TPOTP) with remarkably high E_T of 2.94–3.15 eV and T_g of 178–238 °C.¹²TATP can be employed as a general host for blue, green and red PHOLEDs, providing high EL efficiencies of 7.8%, 11.5% and 9.8%, respectively. Shu et al. reported the fluorene/triarylamine hybrid based host material tris[4-(9-phenylfluoren-9-yl)phenyl]amine (TFTPA) that exhibits a high E_T of 2.89 eV and a T_g of 186 °C.¹³ By employing TFTPA as the host, the devices display η_ext as high as 12.3% for blue, 11.8% for green and 9.2% for red. Yang et al. reported a silicon-bridged compound p-BISiTPA, combining p-type triphenylamine and n-type benzimidazole groups, showing a high E_T of 2.69 eV and a T_g of 102 °C accompanied by the bipolar charge transporting feature.¹⁴ Devices with p-BISiTPA as the host show excellent performance, with η_ext as high as 14.2% for blue, 18.9% for orange and 17.4% for white OLEDs at a practical brightness of 1000 cd m⁻². Most recently, the same research group has developed three new silicon-bridged compounds (m-BISiTPA, p-OXDSiTPA and m-OXDSiTPA) by integrating electron-donating arylamine and electron-transporting benzimidazole/oxadiazole functionalities into one molecule via a silicon-bridged linkage mode.¹⁵ By using these compounds as the universal hosts, they achieved multi-color PHOLEDs for blue, green, orange and all-phosphor white-light electrophosphorescence, exhibiting high η_ext.

Carbazole-based host materials have been used extensively in PHOLEDs due to their large E_T and good hole-transporting properties.^16–18 Well-known carbazole derivatives are 4,4′-N,N′-dicarbazolebiphenyl (CBP; E_T = 2.56 eV)⁴ and N,N′-dicarbazolyl-3,5-benzene (mCP; E_T = 2.9 eV),^5,19 but both of them suffer from low thermal and morphological stability (T_g ≈ 65 °C), which are the essential properties for high performance OLED fabrication. To address these issues, many carbazole-based host materials with high E_T have been reported for blue PHOLEDs. Therefore, there is still a great demand for new universal host materials which can produce multi-color PHOLEDs including WOLEDs with proper doping concentrations of the emitters.^14,15 In this paper, we report an isomeric family of functional carbazole-derived organometallic phosphors and a carbazole-based coplanar host molecule (CmInF, see Scheme 1) obtained through the intramolecular ring closure of aryl substituents at the C3 and C6 positions and its applications in PHOLEDs based on various phosphorescent dopants in appropriate concentrations. The molecular design endows CmInF with several desirable properties: (i) the adoption of a coplanar chromophore, in which carbazole was fused to the neighboring phenylene rings through intramolecular annulation via sp³-hybridised carbon atoms bearing two p-tolyl groups as the peripheral substituents, makes the molecule rigid and bulky, and introduces molecular constraint; (ii) the carbazole unit can simultaneously possess sufficiently large E_T and carrier-transport properties, which is often used as the host material; and (iii) the introduction of aryl groups as peripheral substituents of the coplanar backbone also impedes the intermolecular interactions between the molecules leading to their exhibition of amorphous morphologies. We anticipate that the combination of carbazole and fluorene moieties in a coplanar π-conjugated backbone and the presence of two tolyl groups at sp³ C9 position in CmInF could prevent efficiently the intermolecular interactions between molecules, thereby increasing high morphological and thermal stability (T_g = 195 °C). Moreover, a moderate E_T value of 2.77 eV makes it a suitable host for PHOLEDs of various emission colors. The application of CmInF as the universal host doped with various phosphorescent emitters in the common device structure shows excellent device performance with η_ext of 9.2% for blue, 15.1% for green, 13.7% for red and 17.5% for yellow. A dual complementary emission based all-phosphor WOLED, hosted by CmInF, was achieved with a peak η_ext of 13.4%, a current efficiency (η_c) of 23.4 cd A⁻¹ and a power efficiency (η_p) of 11.2 lm W⁻¹.

Scheme 1 Chemical structure of CmInF and various color-tunable phosphorescent iridium(III) complexes used in this study.

Results and discussion

Synthesis and chemical characterization

Scheme 1 lists the structures of CmInF and various color-tunable phosphorescent iridium(III) complexes used in this study. The compound 4-bromo-2-nitrobiphenyl was obtained from the Suzuki–Miyaura coupling of phenylboronic acid with 2,5-dibromonitrobenzene in the presence of catalytic Pd(PPh₃)₄.²⁰ It was then heated in triethylphosphine under the Cadogan's reductive cyclization conditions to afford 2-bromocarbazole.²¹Ullmann condensation between 2-bromocarbazole and iodobenzene gave 2-bromo-N-phenylcarbazole in good yield,²² and the synthesis of the cyclometallating ligand was then completed by Stille coupling of 2-bromo-N-phenylcarbazole with 2-(tributylstannyl)pyridine (Scheme 2).²³

Scheme 2 Synthesis of 2-[2-(N-phenylcarbazolyl)]pyridine.

Following the ligand synthesis, 2-[2-(N-phenylcarbazolyl)]pyridine was easily converted to the new homoleptic iridium(III) complex Ir(2-Cz)₃3via a one-step cyclometallation with [Ir(acac)₃] (Hacac = acetylacetone) in refluxing glycerol at high temperature (>200 °C).²⁴ Here, the reaction was run with an excess of ligand, which can be easily recovered and reused. On the other hand, the iridium-μ-chloro-bridged dimer was synthesized by the reaction of IrCl₃·xH₂O with 2-[2-(N-phenylcarbazolyl)]pyridine according to a conventional procedure.²⁵ The diiridium(III) complex was then converted to the mononuclear iridium(III) complex (2-Cz)₂2Ir(acac) by replacing the two bridging chlorides with a bidentate monoanionic acac ligand. The synthetic pathways of the metal complexes are summarized in Scheme 3. Spectroscopic and elemental analyses of both complexes are consistent with the expected formulations of their structures. The facial geometry of the homoleptic complex is confirmed by the simplicity of ¹H NMR spectral pattern, which indicates that the number of coupled spins is equal to that of the protons on one ligand because the three ligands are magnetically equivalent due to the 3-fold symmetry of the molecule.²⁶ On the other hand, the preparation of Ir(3-Cz)₃3 (a geometrical linkage isomer of Ir(2-Cz)₃3) was reported previously.²⁷ The homoleptic series appears to show higher thermal stability than their heteroleptic analogue as revealed from the onset decomposition temperature (T_dec). Moreover, improvement in the thermostability was noted as we change the cyclometallating ligand from 2-[2-(N-phenylcarbazolyl)]pyridine to 2-[3-(N-phenylcarbazolyl)]pyridine in these complexes.

Scheme 3 Synthesis of new phosphorescent complexes Ir(2-Cz)₃3 and (2-Cz)₂2Ir(acac).

The synthesis of the host CmInF was carried out following our previously reported literature.²⁸ From the X-ray crystallographic analysis,²⁸ we observed the coplanar structure of CmInF. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to characterize the morphological property and thermal stability of CmInF. The results show a high onset decomposition temperature (T_d = 392 °C, corresponding to 5% weight loss) in the TGA curve and a distinctly high glass transition temperature (T_g) at 195 °C and melting point (T_m) at 346 °C from the DSC study. Therefore, this material can form homogeneous and stable amorphous films, which are the two critical factors for its successful use in thin film devices. The amorphous behavior and high T_g value of CmInF reveal that the rigidity of the conjugated backbone and the presence of the peripheral p-tolyl substituents effectively suppress intermolecular interaction.

Photophysical and charge transport properties

In this study, we have prepared two new cyclometallated iridium(III) complexes containing 2-[2-(N-phenylcarbazolyl)]pyridine and their photophysical properties were compared to their isomeric congeners derived from 2-[3-(N-phenylcarbazolyl)]pyridine (Table 1). The strongest absorption bands in the ultraviolet region are assigned to the spin-allowed intraligand ¹π–π* transitions. The next lower energy in the shoulder region of the ¹π–π* transitions and the weaker broad shoulders extending into the visible region with appreciable intensity are associated with an admixture of the typical spin-allowed metal-to-ligand charge transfer ¹MLCT, spin–orbit coupling enhanced ³π–π* and spin-forbidden ³MLCT transitions. These assignments are made by analogy with previously reported Ir^III complexes.²⁹ The weakest MLCT absorptions are red-shifted from the homoleptic to the corresponding heteroleptic complexes owing to the strong ligand field of acac. By comparing each of the isomeric complexes, different absorption profiles were also observed.

Table 1 Photophysical and thermal data of Ir(2-Cz)₃3, (2-Cz)₂2Ir(acac), Ir(3-Cz)₃3 and (3-Cz)₂2Ir(acac)

	Absorption (293 K)	Emission (293 K)			T d ^d/°C
	λ abs ^a/nm	λ em ^a/nm	Φ P ^b	τ P ^c/μs
a Measured in CH2Cl2. b Phosphorescent quantum yield measured in degassed CH2Cl2 relative to fac-[Ir(PPy)3] (ΦP = 0.40), λex = 400 nm. c Triplet emission lifetime measured in degassed CH2Cl2. d Onset decomposition temperature at 5% weight reduction. sh = shoulder peak.
Ir(2-Cz)33	266, 324, 359sh, 432	578	0.32	0.18	458
(2-Cz)22Ir(acac)	287, 341, 510sh	591	0.29	0.21	388
Ir(3-Cz)33 ^27a	291sh, 316, 398sh	515	0.43	0.46	477
(3-Cz)22Ir(acac) ^27b	301, 325, 358sh, 435sh	515	0.41	0.42	404

---

All of the iridium(III) complexes emit strong phosphorescence arising from the lowest energy ³MLCT transition at room temperature in CH₂Cl₂. The substitution position of the pyridyl ring on N-phenylcarbazole has a great influence on the photoluminescence (PL) spectra of the heavy-metal complexes. Clearly, the PL maximum of the 2-substituted homoleptic derivative is more red-shifted relative to the 3-substituted counterpart (from 515 nm for Ir(3-Cz)₃3 to 578 nm for Ir(2-Cz)₃3) due to the more conjugated structure of the former group over the latter one (Fig. 1). This is also the case for the heteroleptic species, (3-Cz)₂2Ir(acac)versus(2-Cz)₂2Ir(acac) (from 515 to 591 nm). Here, a prominent color-tuning of phosphorescence emission from green to orange/red is achieved by simply altering the ligation of metal with carbon atom at 2- or 3-position of carbazole unit in these linkage isomers. While the carbon atoms at 2-/7- and 3-/6-position of carbazole have different electronic density, where the 3-/6-position can be activated by the nitrogen atom and hence more electron-rich than the 2-/7-site, the energy level of carbazole-containing complexes can be changed by substitution at the 3-/6- or 2-/7-position. The more electron-donating 3-position on carbon of carbazole pushes up the energy of metal d orbital when it is ligated to the metal ion, giving rise to a higher energy level of the highest occupied molecular orbital (HOMO) than that of the less electron-donating 2-position and this is accompanied by significant bathochromic shifts in λ_em for these iridium(III) complexes.³⁰ This approach is different from those commonly used for other complexes in which the emission wavelength can be tuned by varying the electronic nature and/or position of the substituents on the cyclometallated ligands.³¹

Fig. 1 Absorption and PL spectra of the iridium(III) complexes with 2-[2-(N-phenylcarbazolyl)]pyridine and 2-[3-(N-phenylcarbazolyl)]pyridine groups.

The HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels of Ir(2-Cz)₃3 and (2-Cz)₂2Ir(acac) were determined electrochemically (HOMO/LUMO: −4.81/–2.31 eV for Ir(2-Cz)₃3 and −4.83/–2.43 eV for (2-Cz)₂2Ir(acac)). As compared to [Ir(PPy)₃] and (PPy)₂2Ir(acac) (PPy = 2-phenylpyridyl), the HOMOs of Ir(2-Cz)₃3 and (2-Cz)₂2Ir(acac) are elevated when the electron-rich and electron-donating carbazolyl units are attached to the pyridyl rings, signaling that both complexes have a lower ionization potential than their PPy congeners. This leads to a better hole injection/hole transport ability and hence these triplet emitters are potentially dual functional in nature.

Fig. 2 presents the UV-Vis absorption and PL spectra of CmInF in dilute solution (CH₂Cl₂) and neat film, as well as the corresponding phosphorescence (Phos) spectrum recorded from its EtOH solution at 77 K. The absorption spectra of CmInF in solution and neat film are very similar with peaks appearing at 328, 366 and 385 nm, which imply that no significant intermolecular interactions occur in the ground state. The absorption peaks at 366 and 385 nm are attributed to the π–π* transition of the extended conjugation of coplanar carbazole–fluorene chromophore of CmInF. Its emission spectra in solution and neat film are also similar with peaks appearing at 394 nm and 415 nm. The phosphorescence spectrum of CmInF in EtOH at 77 K indicates the high-energy phosphorescence emission at 448 nm, corresponding to the E_T value of 2.77 eV. The decrease in triplet energy agrees with the extended π-conjugation at C3 and C6 positions on the central carbazole core and reveals that CmInF has a lower E_T value as compared to CzSi (E_T = 3.02 eV),³² but a higher value than those in the earlier reported host materials DFC (E_T = 2.53 eV)³³ and CPhBzIm (E_T = 2.48 eV).³⁴ This E_T value is sufficiently high to prevent the reverse triplet energy transfer from the dopant back to CmInF. Thus, CmInF should be a suitable host for various phosphorescent devices from blue to red.

Fig. 2 Absorption and PL spectra of CmInF in solution and neat film, as well as the corresponding phosphorescence (Phos) spectrum recorded from its EtOH solution at 77 K.

We also studied the electrochemical properties of CmInF using cyclic voltammetry (CV) and its cyclic voltammogram is shown in Fig. 3. CmInF exhibits one reversible oxidation process at 1.09 eV (vs. Ag/AgCl). No reduction peak was observed. It is worth mentioning that no electropolymerization occurred during multiple cycles of CV scanning. It was previously reported that without blocking the active sites C3 and C6 of carbazole, the oxidation process is not reversible.³⁵ The observed reversible oxidation potential of CmInF indicates that carbazole is π-conjugated with fluorene at 3 and 6 positions. Thus, the rigid peripheral fluorene moieties conjugated to the carbazole core not only emerge with a significant impact on the oxidation potential of the whole molecule, but their addition also efficiently blocks the electrochemically active sites (C3 and C6) of carbazole and may improve its electrochemical stability in the EL devices. Moreover, the existence of stable radical cationic species suggests that it can be useful for potential hole-transporting material. The HOMO energy level (−5.4 eV) of CmInF was estimated by atmospheric photoelectron spectroscopy. From the equation LUMO = HOMO + ΔE_g, where ΔE_g (3.1 eV) is the optical band gap determined from the absorption threshold, we estimated the LUMO energy level to be −2.3 eV.

Fig. 3 Cyclic voltammogram of CmInF.

To evaluate the carrier mobility of CmInF, we carried out time-of-flight (TOF) measurements using the device structure of ITO/CmInF (3 μm)/Ag (100 nm). Fig. 4(a) shows a representative TOF transient for holes which reveals the nondispersive hole-transport characteristics and this implies that the carriers move with constant velocity across the organic film. To determine the carrier mobilities, the transit time, t_T, can be clearly evaluated from the intersection point of two asymptotes in the double logarithmic representation of the TOF transient as shown in the inset of Fig. 4(a). The hole mobilities of CmInF thus determined as a function of the electric field are shown in Fig. 4(b) and the hole mobilities are in the range from 2 × 10⁻³ to 5 × 10⁻³ cm² V⁻¹ s⁻¹ for fields varying from 10⁵ to 3.3 × 10⁵ V cm⁻¹. It is worth noting that a hole mobility of up to 5 × 10⁻³ cm² V⁻¹ s⁻¹ is very high among the carbazole-based oligomers reported in the literature.

Fig. 4 (a) Representative TOF transient for CmInF (hole, 3 μm thickness, E = 2.3 × 10⁵ V cm⁻¹). Inset: double logarithmic plot. (b) Hole mobility vs. E^1/2.

Electrophosphorescent properties

To evaluate the performance of CmInF as a universal host material for multi-color emission, several PHOLED devices have been fabricated with the same device configuration of ITO/PEDOT:PSS (300 Å)/TCTA (250 Å)/CmInF doped with various phosphor dyes (250 Å)/TAZ (500 Å)/LiF (5 Å)/Al. The conducting polymer poly(ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) was used as the hole-injection layer,³⁶4,4′,4′′-tri(N-carbazolyl)triphenylamine (TCTA) as the hole-transport layer as well as an exciton blocker (E_T: 2.76 eV, HOMO/LUMO: −5.7/–2.4 eV) to confine triplet excitons within the emitting layer (EML).^37,38 The 1,2,4-triazole-based material, 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) (E_T: 2.7 eV, HOMO/LUMO: −6.3/–2.7 eV), was used as the electron-transport and hole-blocking layer to provide exciton and carrier confinement.³⁹ The EML consists of the large-gap CmInFhost doped with the blue iridium(III) bis[(4,6-di-fluorophenyl)-pyridinato-N,C²′]picolinate FIrpic,⁴ green bis(2-phenylpyridine)iridium(III) acetylacetonate (PPy)₂2Ir(acac),⁴⁰ red (Mpq)₂2Ir(acac),⁴¹ and a series of our tailor-made dopants (Ir(3-Cz)₃3, Ir(TPA)₃3,⁴²Ir(2-Cz)₃3 and (2-Cz)₂2Ir(acac), see Scheme 1) emitting from green to red. The green dopant (3-Cz)₂2Ir(acac) was not tested for PHOLED here since it has very similar emission features as Ir(3-Cz)₃3. The E_T estimated from the highest energy peaks of the phosphorescence spectra for light from blue to red colors are 2.65–2.00 eV, which are lower than CmInF (E_T = 2.77 eV) and can be confined in EML. There are no residual emissions from the host and/or adjacent layers, even at high drive currents, indicating that the EL emission originates from the dopant and there is complete energy and/or charge transfer from the host exciton to the phosphor upon electrical excitation (see Fig. 5).

Fig. 5 EL spectra of devices incorporating the host CmInF doped with various dopants.

Fig. 6 shows the current density–voltage–luminance (J–V–L) characteristics, and external quantum and power efficiencies as a function of brightness for each of the PHOLED devices. The pertinent EL properties analyzed from these graphs are summarized in Table 2. All of the devices in this study exhibited low turn-on voltages (3–3.5 V) and the difference in the J–V performance is presumably because of the different emitting dopants that influence the charge carrier transport of EML. We initially examined the performance of the FIrpic-based blue device with Commission Internationale de L'Eclairage (CIE) coordinates of (0.17, 0.37). The η_ext can reach 9.2% photons/electron with the η_c and η_P values of 20.2 cd A⁻¹ and 11.8 lm W⁻¹. These values are higher than those obtained using the same triplet emitter in CBP (6.1%, 7.7 lm W⁻¹) and mCP (7.5%, 8.9 lm W⁻¹) hosts.⁵ The values are moderate among FIrpic-based PHOLEDs, presumably because the E_T of CmInF is not high enough relative to FIrpic. However, this is a worthy sacrifice to acquire a compromise among the E_T of RGB phosphors.

Fig. 6 (a) J–V–L characteristics, (b) external quantum efficiency and (c) power efficiency versus brightness for PHOLED devices with different dopants.

Table 2 Device characteristics of PHOLEDs with different phosphorescent dopants

Dopant (%)	V on ^a/V	Voltage and ηext at 1000 nit^b/V, %	L max/cd m^−2	I max/mA cm^−2	η ext (%)	η c/cd A^−1	η p/lm W^−1	CIE (x, y)
a Turn-on voltage at which emission became detectable. b The values of driving voltage and ηext of device at 1000 cd m^−2.
(PPy)22Ir(acac), 9%	3	5.5, 14.5	122 000 (13 V)	700	15.1	55.6	42.2	0.34, 0.62
FIrpic, 12%	3	8.3, 6	19 800 (14.5 V)	704	9.2	20.2	11.8	0.17, 0.37
(Mpq)22Ir(acac), 9%	3	8.3, 10	21 300 (14.5 V)	820	13.3	14.1	9.7	0.65, 0.35
Ir(3-Cz)33, 9%	3.5	7, 11.8	130 000 (15 V)	910	12.4	43.6	27	0.30, 0.62
Ir(2-Cz)33, 9%	3	7.5, 16.2	80 200 (14.5 V)	880	17.5	44	29	0.56, 0.44
Ir(TPA)33, 8%	3.5	7.45, 11.1	95 000 (16 V)	860	12	45	24	0.38, 0.60
(2-Cz)22Ir(acac), 8%	3	6.5, 10.8	51 000 (15 V)	1240	13.7	25.6	19	0.60, 0.39
FIrpic, 12% + (Mpq)22Ir(acac) 0.3%	3	9, 7.7	15 900 (14.5 V)	570	13.4	23.4	11.2	0.33, 0.37

---

Next, we employed CmInF as a green host for (PPy)₂2Ir(acac), Ir(3-Cz)₃3 and Ir(TPA)₃3 dopants with the same device configuration. The (PPy)₂2Ir(acac)-based device exhibited a maximum brightness (L_max) of 122 000 cd m⁻² at 13 V and attractive EL efficiencies (15.1%, 55.6 cd A⁻¹, 42.2 lm W⁻¹) that are superior to those of the Ir(3-Cz)₃3-based device (12.4%, 43.6 cd A⁻¹, 27 lm W⁻¹) and Ir(TPA)₃3-based device (12%, 45 cd A⁻¹, 24 lm W⁻¹). It is worth noting that these green-emitting devices show a low roll-off in η_ext (Fig. 6(b)). When the brightness reaches 1000 cd m⁻², η_ext still remains as high as 14.5% with a low efficiency roll-off value of 4% for the (PPy)₂2Ir(acac)-based device. The Ir(3-Cz)₃3- and Ir(TPA)₃3-based devices also exhibited low efficiency roll-off values of only 4.8% and 7.5%, respectively, at a practical brightness of 1000 cd m⁻². To further evaluate the suitability of CmInF as the host material for low-energy triplet emitters, we fabricated PHOLED devices by using Ir(2-Cz)₃3 (orange), (2-Cz)₂2Ir(acac) (red) and (Mpq)₂2Ir(acac) (red) as phosphorescent dopants. The Ir(2-Cz)₃3-based device showed an impressive performance with a low turn-on voltage of 3 V, L_max of 80 200 cd m⁻² at 14.5 V, and peak EL efficiencies of 17.5%, 44 cd A⁻¹, and 29 lm W⁻¹ with the CIE coordinates at (0.56, 0.44). It is also worth mentioning that the Ir(2-Cz)₃3-based device gave better performance than other recently reported PHOLEDs in the orange chromaticity region.^43–45 For the red emitters, the (2-Cz)₂2Ir(acac)-based device exhibited a L_max of 51 000 cd m⁻² at 15 V and respectable EL efficiencies (13.7%, 25.6 cd A⁻¹, 19 lm W⁻¹) with the CIE coordinates of (0.60, 0.39) while (Mpq)₂Ir(acac)-based device exhibited a L_max of 21 300 cd m⁻² at 14.5 V and very good EL efficiencies (13.3%, 14.1 cd A⁻¹, 9.7 lm W⁻¹) with the CIE coordinates of (0.65, 0.35).

Through the mixing of two dopants showing complementary colors,⁴⁶ we also fabricated a two-color white OLED (WOLED) under the same device structure consisting of 12 wt% FIrpic and 0.3 wt% (Mpg)₂2Ir(acac) co-doped into CmInF as a single-emitting-layer. The WOLED exhibited a L_max of 15 900 cd m⁻² at 14.5 V, with maximum efficiencies (η_ext, η_c, and η_p) of 13.4%, 23.4 cd A⁻¹, and 11.2 lm W⁻¹ (Fig. 7). Although a gradual decrease in efficiency with an increase of current density was observed, the EL efficiency of WOLED remained at 7.7% at the practical brightness of 1000 cd m⁻². The inset of Fig. 7 displayed the EL spectra of the WOLED device recorded at various brightness levels. The relative intensity of the red emission decreased slightly when the brightness increased, leading to a slight shift in the CIE coordinates from (0.36, 0.37) at a value of L = 1000 cd m⁻² to (0.33, 0.37) at 11 300 cd m⁻². This color shift originated from the shifting of the recombination zone upon increasing the applied voltage and from the easier formation of high-energy excitons at higher voltage.^47,48 Although the performance of these devices has yet to be optimized, it is our intention to highlight the potential merits of CmInF as a universal host towards color-tunable OLED applications.

Fig. 7 EL efficiencies for two-color WOLED. Inset: EL spectra of WOLED device at various brightness levels.

Conclusions

In summary, we have investigated the effect of the position of pyridyl ring substituent at C2 and C3 sites of the carbazole ring on the emission color, photophysics and electrophosphorescent properties of some iridium(III) cyclometallates bearing N-phenylcarbazole moieties. An interesting color-tuning strategy was developed by a change of the chelating carbon atom by a different binding site of the carbazolyl group. The origin of the wavelength shift arises from the striking difference of electron density on the carbon atom bound to the iridium centre. We also reported nicely a prominent multi-color spectrum employing CmInF as a universal host material doped with various phosphor dyes with the same device configuration to reach the ultimate target for highly efficient, color-switchable OLEDs. The present work furnished OLED colors spanning from sky-blue to red (475–620 nm) with high EL efficiencies (9.2–17.5%) which have great potential for application in multi-color displays and possibly in white light illumination sources through the mixing of two phosphorescent dopants of complementary colors.

Experimental

Photophysical and thermal measurements

All reactions were carried out under nitrogen atmosphere with the use of standard Schlenk techniques. Glassware was oven-dried at about 120 °C. Analytical grade solvents were purified by distillation over appropriate drying agents under an inert nitrogen atmosphere prior to use. All reagents and chemicals, unless otherwise stated, were purchased from commercial sources and used without further purification. The positive-ion fast atom bombardment (FAB) mass spectra were recorded in m-nitrobenzyl alcohol matrices on a Finngin-MAT SSQ710 mass spectrometer. NMR spectra were measured in deuterated solvents as the lock and reference on a JEOL JNM-EX270 FT NMR system or a Varian INOVA 400 MHz FT-NMR spectrometer, with ¹H and ¹³C NMR chemical shifts quoted relative to Me₄Si standard. Steady-state spectroscopic measurements were conducted both in solution and solid films prepared by vacuum (2 × 10⁻⁶ Torr) deposition on a quartz plate. Absorption spectra were recorded with a U2800A spectrophotometer (Hitachi). Fluorescence spectra at 300 K and phosphorescent spectra at 77 K were measured on a Hitachi F-4500 spectrophotometer upon excitation at the absorption maxima. The differential scanning calorimetry (DSC) analyses were performed on a Universal V2,6D TA Instrument at a heating rate of 10 °C min⁻¹ from 50 to 400 °C under a nitrogen atmosphere. Thermogravimetric analysis (TGA) was undertaken with a Universal V2,6D TA Instrument or a Perkin-Elmer TGA6 thermal analyzer. The thermal stability of the samples under a nitrogen atmosphere was determined by measuring their 5% weight loss while heating proceeded at a rate of 10 °C min⁻¹ up to 700 °C.

Cyclic voltammetry

The oxidation potential was determined by cyclic voltammetry (CV) in CH₂Cl₂ solution (1.0 mM) containing 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF₆) as a supporting electrolyte at a scan rate of 100 mV s⁻¹. A glassy carbon electrode and a platinum wire were used as the working and counter electrodes, respectively. The ferrocene/ferrocenium (Fc/Fc⁺) redox couple in THF/TBAP occurs at E_1/2 = +0.51 V. The potential value was recorded relative to the oxidation potential of ferrocene, which was added to the electrolyte as an internal standard. All potentials were recorded versusAg/AgCl (saturated) as a reference electrode.

Time-of-flight (TOF) mobility measurements

The samples for the TOF measurements were prepared through vacuum deposition in the configuration of ITO/CmInF (3 μm)/Ag (100 nm) and then placed inside a cryostat and kept under vacuum. The thickness of the organic film was monitored in situ using a quartz crystal sensor and calibrated using a thin film thickness measurement system (K-MAC ST2000). A nitrogen laser was used as the excitation source (λ = 337 nm) and was incident on the sample through the ITO. Under an applied dc bias, the transient photocurrent was swept across the bulk of the organic film towards the collection electrode (Al); it was then recorded using a digital storage oscilloscope. Depending on the polarity of the applied bias, the selected carriers (holes or electrons) were swept across the sample with a transit time of t_T. For an applied bias V and a sample thickness D, the applied electric field E is equal to V/D; the carrier mobility is then given by the equation μ = D/(t_TE) = D²/(Vt_T) from which the carrier transit times, t_T, can be extracted from the intersection points of the two asymptotes to the plateau and tail sections in the double logarithmic plot.

OLED device fabrication

All chemicals were purified through vacuum sublimation prior to use. The OLEDs were fabricated through vacuum deposition of the materials at 10⁻⁶ Torr onto ITO-coated glass substrates having a sheet resistance of 15 Ω sq⁻¹. The ITO surface was cleaned ultrasonically—sequentially with acetone, methanol, and deionized water—and then it was treated with UV-ozone. The deposition rate of each organic material was ca. 1–2 Å s⁻¹. Subsequently, LiF was deposited at 0.1 Å s⁻¹ and then capped with Al (ca. 5 Å s⁻¹) through shadow masking without breaking the vacuum. The J–V–L characteristics of the devices were measured simultaneously in a glove-box using a Keithley 6430 source meter and a Keithley 6487 picoammeter equipped with a calibration Si-photodiode. EL spectra were measured using a photodiode array (OTO SD1200).

Synthesis of 4-bromo-2-nitrobiphenyl

Phenylboronic acid (1.50 g, 12.3 mmol), 2,5-dibromonitrobenzene (3.45 g, 12.3 mmol) and Pd(PPh₃)₄ (1.42 g, 1.42 mmol) were dissolved in a mixture of toluene (250 mL) and 2 M aq. Na₂CO₃ (12 mL). The degassed mixture was heated at 90 °C for 5.5 h, cooled to 20 °C and then diluted with distilled H₂O. The organic phase was separated and the organic products were extracted into diethyl ether. The combined extracts were dried with MgSO₄ and concentrated under reduced pressure to give a dark brown liquid. The main product was separated by chromatography eluting with a mixture of hexane and CH₂Cl₂ and characterized by fast-atom bombardment (FAB) mass spectrometry. The mixture was used in the next step without further purification (yield: 70%). MS (FAB): m/z 278 (M⁺). ¹H NMR (CDCl₃): δ [ppm] 8.00 (d, J = 2.0 Hz, 1H, Ar), 7.75 (dd, J = 8.2 and 2.0 Hz, 1H, Ar), 7.45–7.41 (m, 3H, Ar), 7.33 (d, J = 8.2 Hz, 1H, Ar), 7.31–7.27 (m, 2H, Ar). ¹³C NMR (CDCl₃): δ [ppm] 136.5, 135.6, 135.5, 133.5, 131.8, 129.5, 129.1, 128.8, 128.0, 127.3, 124.2, 121.6 (Ar).

2-Bromocarbazole

4-Bromo-2-nitrobiphenyl (1.50 g, 5.42 mmol) and P(OEt)₃ (4.80 g, 37.9 mmol) were heated under reflux for 10 h. The excess P(OEt)₃ was distilled off under vacuum. The remaining residue was diluted with a 1 : 1 mixture of MeOH and distilled H₂O to give a precipitate which was filtered off and washed several times with a 1 : 1 mixture of MeOH/H₂O, followed by hexane. Column chromatography using hexane as eluent gave the title compound as a white solid (500 mg, 38%). MS (FAB): m/z 246 (M⁺). ¹H NMR (CDCl₃): δ [ppm] 11.40 (s, 1H, NH), 8.15–8.11 (dd, J = 8.0 Hz, 2H, Ar), 7.68 (d, J = 1.7 Hz, 1H, Ar), 7.54–7.19 (m, 4H, Ar). ¹³C NMR (CDCl₃): δ [ppm] 140.6, 139.9, 126.1, 121.9, 121.8, 121.6, 121.3, 120.3, 119.1, 118.2, 113.5, 111.2 (Ar).

2-Bromo-N-phenylcarbazole

The N-arylation of 2-bromocarbazole (2.50 g, 10.2 mmol) was carried out by the Ullmann condensation of 2-bromocarbazole with iodobenzene (3.12 g, 15.3 mmol) in the presence of CuI (1.35 g, 7.14 mmol), KOH (4.00 g, 71.4 mmol) and 1,10-phenanthroline (1.42 g, 7.14 mmol) in p-xylene (50 mL) and the mixture was stirred at 140 °C for two days. The cooled mixture was extracted with CH₂Cl₂. The organic layer obtained was washed with water and dried over anhydrous MgSO₄ and evaporated. The product was purified by flash chromatography on silica gel using hexane as eluent to give a pure white solid (90%, 2.95 g). MS (FAB): m/z 322 (M⁺). ¹H NMR (CDCl₃): δ [ppm] 8.10–8.08 (d, 1H, J = 7.6 Hz, Ar), 7.98–7.96 (d, 1H, J = 8.4 Hz, Ar), 7.63–7.59 (m, 2H, Ar), 7.53–7.48 (m, 4H, Ar), 7.42−7.35 (m, 3H, Ar), 7.30–7.27 (m, 1H, Ar). ¹³C NMR (CDCl₃): δ [ppm] 141.68, 141.08, 137.03, 130.05, 127.91, 127.11, 126.34, 123.05, 122.71, 122.25, 121.44, 120.36, 120.26, 119.50, 112.78, 109.95 (Ar).

Synthesis of 2-[2-(N-phenylcarbazolyl)]pyridine

2-Bromo-N-phenylcarbazole (1.43 g, 4.45 mmol) and 2-(tributylstannyl)pyridine (2.15 g, 6.68 mmol) were mixed in dry toluene (50 mL) and Pd(PPh₃)₄ (0.51 g, 0.45 mmol) was then added to the solution. The resulting mixture was stirred at 110 °C for 2 days. After cooling to room temperature, the reaction mixture was poured into a separating funnel and CH₂Cl₂ was added followed by washing with water. The organic phase was dried over MgSO₄. Solvent was then removed and the residue was purified by column chromatography eluting with CH₂Cl₂/hexane (1 : 1, v/v). The title product was obtained as a white solid (1.36 g, 95%). MS (FAB): m/z 320 (M⁺). ¹H NMR (CDCl₃): δ [ppm] 8.66–8.65 (d, J = 4.4 Hz, 1H, Ar), 8.22–8.14 (m, 2H, Ar), 8.04 (s, 1H, Ar), 7.87 (m, 1H, Ar), 7.76 (m, 2H, Ar), 7.72 (m, 4H, Ar), 7.47 (m, 3H, Ar), 7.30 (m, 1H, Ar), 7.19 (m, 1H, Ar). ¹³C NMR (CDCl₃): δ [ppm] 158.09, 149.60, 141.74, 141.42, 137.50, 136.68, 129.97, 127.59, 127.34, 126.24, 123.96, 122.97, 121.83, 120.99, 120.52, 120.48, 120.06, 119.09, 109.87, 108.34 (Ar).

Synthetic procedures of new iridium(III) compounds

Ir(2-Cz)₃3 . This homoleptic iridium(III) complex was synthesized by heating a solution of [Ir(acac)₃] (262 mg, 0.535 mmol) and 3.5 molar equivalents of 2-[2-(N-phenylcarbazolyl)]pyridine (600 mg, 1.87 mmol) in glycerol (15 mL) and the mixture was heated to 200 °C under a N₂ atmosphere. After 18 h, the reaction mixture was then cooled down to room temperature and water (20 mL) was added. The mixture was then filtered followed by repetitive washing with CH₂Cl₂. The reddish orange product was obtained in 43% yield. Its UV and PL spectra have confirmed the absence of any excess ligand after purification. ¹H NMR (THF-d₈): δ [ppm] 8.00–7.98 (d, J = 8.28 Hz, 3H, Ar), 7.92 (m, 3H, Ar), 7.71–7.62 (m, 24H, Ar), 7.76 (m, 3H, Ar), 7.55–7.53 (m, 6H, Ar), 6.87–6.83 (m, 6H, Ar). MS (FAB): m/z 1150 (M⁺). Anal. Calcd for C₆₉H₄₅N₆Ir: C, 72.04; H, 3.94; N, 7.31; found: C, 72.12; H, 4.18; N, 7.45%.

(2-Cz)₂2Ir(acac) . This heteroleptic iridium(III) complex was synthesized by reacting IrCl₃·3H₂O (151 mg, 0.51 mmol) with 3 equivalents of 2-[2-(N-phenylcarbazolyl)]pyridine (486 mg, 1.52 mmol) in a mixture of 2-ethoxyethanol and water (6 : 2, v/v). The mixture was refluxed for 24 h and then cooled to room temperature. A precipitate gradually formed during the reaction. The precipitate was collected by filtration and washed with ethanol followed by hexane. The solid was then pumped dry completely to give the crude iridium(III) dimer for subsequent reaction. Without further purification, this crude dimer was mixed with 10 equivalents of Na₂CO₃ (196 mg, 0.185 mmol) in 2-ethoxyethanol (8 mL) and 3 equivalents of acetylacetone (0.056 mL, 0.554 mmol) were added. The reaction mixture was then refluxed for 16 h. After the mixture was cooled to room temperature, water was added and extracted with CH₂Cl₂. The organic layer was collected and the solvent was subsequently removed. The residue was then purified using preparative TLC plates eluting with a mixture of CH₂Cl₂/hexane to give a reddish orange solid in 41% yield. MS (FAB): m/z 930 (M⁺). ¹H NMR (CDCl₃): δ [ppm] 8.67 (d, J = 4.0 Hz, 2H, Ar), 7.93–7.91 (m, 2H, Ar), 7.79–7.54 (m, 15H, Ar), 7.25 (m, 8H, Ar), 7.02–6.93 (m, 3H, Ar), 5.26 (s, 1H, acac), 1.83 (s, 6H, CH₃). ¹³C NMR (CDCl₃): δ [ppm] 184.55 (CO), 168.96, 148.36, 142.67, 141.33, 138.21, 137.68, 136.50, 134.94, 129.70, 127.05, 126.78, 125.63, 123.05, 122.57, 121.41, 120.45, 118.92, 118.62, 109.73, 109.02, 105.88 (Ar), 100.49 (CH), 28.85 (CH₃). Anal. Calcd for C₅₁H₃₇N₄O₂Ir: C, 65.86; H, 4.01; N, 6.02; found: C, 66.02; H, 3.92; N, 6.20%.

Acknowledgements

W.-Y. Wong acknowledges the financial support from Hong Kong Baptist University (FRG2/09-10/091), Hong Kong Research Grants Council (HKBU202709 and HKUST2/CRF/10) and Areas of Excellence Scheme, University Grants Committee of HKSAR, P.R. China (Project No. [AoE/P-03/08]). W.-Y. Hung and K.-T. Wong are grateful for the financial support from the National Science Council (NSC 98-2119-M-002-007-MY3, 100-2112-M-019 -002 -MY3) of Taiwan.

References

1. C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett., 1987, 51, 913.
2. M. A. Baldo, M. E. Thompson and S. R. Forrest, Nature, 2000, 403, 750.
3. M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
4. C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett., 2001, 79, 2082.
5. R. J. Holmes, S. R. Forrest, Y.-J. Tung, R. C. Kwong, J. J. Brown, S. Garon and M. E. Thompson, Appl. Phys. Lett., 2003, 82, 2422.
6. S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki and F. Sato, Appl. Phys. Lett., 2003, 83, 569.
7. K. Goushi, R. Kwong, J. J. Brown, H. Sasabe and C. Adachi, J. Appl. Phys., 2004, 95, 7798.
8. S.-J. Su, C. Cai and J. Kido, Chem. Mater., 2011, 23, 274.
9. Y. Tao, Q. Wang, C. Yang, C. Zhong, K. Zhang, J. Qin and D. Ma, Adv. Funct. Mater., 2010, 20, 304.
10. Y. Tao, Q. Wang, L. Ao, C. Zhong, J. Qin, C. Yang and D. Ma, J. Mater. Chem., 2010, 20, 1759.
11. H.-H. Chou and C.-H. Cheng, Adv. Mater., 2010, 22, 2468.
12. H.-H. Chou, H.-H. Shih and C.-H. Cheng, J. Mater. Chem., 2010, 20, 798.
13. P.-I. Shih, C.-H. Chien, F.-I. Wu and C.-F. Shu, Adv. Funct. Mater., 2007, 17, 3514.
14. S. Gong, Y. Chen, C. Yang, C. Zhong, J. Qin and D. Ma, Adv. Mater., 2010, 22, 5370.
15. S. Gong, Y. Chen, J. Luo, C. Yang, C. Zhong, J. Qin and D. Ma, Adv. Funct. Mater., 2011, 21, 1168.
16. S. J. Lee, J. S. Park, K. J. Yoon, Y. I. Kim, S. H. Jin, S. K. Kang, Y. S. Gal, S. Kang, J. Y. Lee, J. W. Kang, S. H. Lee, H. D. Park and J. J. Kim, Adv. Funct. Mater., 2008, 18, 3922.
17. F. M. Hsu, C. H. Chen, P. I. Shih and C. F. Shu, Chem. Mater., 2009, 21, 1017.
18. T. J. Park, W. S. Jeon, J. J. Park, S. Y. Kim, Y. K. Lee, J. Jang and J. H. Kwon, Thin Solid Films, 2008, 517, 896.
19. V. Adamovich, J. Brooks, A. Tamayo, A. M. Alexander, P. I. Djurovich, B. W. D'Andrade, C. Adachi, S. R. Forrest and M. E. Thompson, New J. Chem., 2002, 26, 1171.
20. M. Tavasli, S. Bettington, M. R. Bryce, A. S. Batsanov and A. P. Monkman, Synthesis, 2005, 10, 1619.
21. (a) J. I. G. Cadogan and M. Cameron-Wood, J. Chem. Soc., 1962, 361; (b) J. I. G. Cadogan, M. Cameron-Wood, R. K. Mackie and R. J. G. Searle, J. Chem. Soc., 1965, 4831; (c) J.-F. Morin and M. Leclerc, Macromolecules, 2001, 34, 4680; (d) F. Dierschke, A. C. Grimsdale and K. Müllen, Synthesis, 2003, 2470.
22. (a) M. Sonntag and P. Strohriegl, Chem. Mater., 2004, 16, 4736; (b) F. Ullmann and M. Zlokasoff, Chem. Ber., 1905, 38, 2211.
23. J. K. Stille, Angew. Chem., Int. Ed. Engl., 1986, 25, 508.
24. K. Dedeian, P. I. Djurovich, F. O. Garces, G. Carlson and R. J. Watts, Inorg. Chem., 1991, 30, 1685.
25. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. F. Lee, C. Adachi, P. E. Burrows, S. R. Forrest and M. E. Thompson, J. Am. Chem. Soc., 2001, 123, 4304.
26. M. G. Colombo, T. C. Brunold, T. Riedener, H. U. Güdel, M. Förtsch and H. Bürgi, Inorg. Chem., 1994, 33, 545.
27. (a) W.-Y. Wong, C.-L. Ho, Z.-Q. Gao, B.-X. Mi, C.-H. Chen, K.-W. Cheah and Z. Lin, Angew. Chem., Int. Ed., 2006, 45, 7800; (b) C.-L. Ho, Q. Wang, C.-S. Lam, W.-Y. Wong, D. Ma, L. Wang, Z.-Q. Gao, C.-H. Chen, K.-W. Cheah and Z. Lin, Chem.–Asian J., 2009, 4, 89.
28. K.-T. Wong, T.-C. Chao, L.-C. Chi, Y.-Y. Chu, A. Balaiah, S.-F. Chiu, Y.-H. Liu and Y. Wang, Org. Lett., 2006, 8, 5033.
29. (a) H. Z. Xie, M. W. Liu, O. Y. Wang, X. H. Zhang, C. S. Lee, L. S. Hung, S. T. Lee, P. F. Teng, H. L. Kwong, H. Zhen and C. M. Che, Adv. Mater., 2001, 13, 1245; (b) Y. H. Song, S.-J. Yeh, C.-T. Chen, Y. Chi, C.-S. Liu, J.-K. Yu, Y.-H. Hu, P.-T. Chou, S.-M. Peng and G.-H. Lee, Adv. Funct. Mater., 2004, 14, 1221; (c) W.-Y. Wong and C.-L. Ho, J. Mater. Chem., 2009, 19, 4457; (d) H. Yersin, Top. Curr. Chem., 2004, 241, 1; (e) P.-T. Chou and Y. Chi, Chem.–Eur. J., 2007, 13, 380; (f) B. M. W. Langeveld and U. S. Schubert, Adv. Mater., 2005, 17, 1109; (g) M. A. Baldo, M. E. Thompson and S. R. Forrest, Pure Appl. Chem., 1999, 71, 2095; (h) R. C. Evans, P. Douglas and C. J. Winscom, Coord. Chem. Rev., 2006, 250, 2093; (i) P. L. Burn, S.-C. Lo and I. D. W. Samuel, Adv. Mater., 2007, 19, 1675; (j) Y. Chi and P.-T. Chou, Chem. Soc. Rev., 2007, 36, 1421; (k) J. A. G. Williams, S. Develay, D. L. Rochester and L. Murphy, Coord. Chem. Rev., 2008, 252, 2596; (l) H. Wu, L. Ying, W. Yang and Y. Cao, Chem. Soc. Rev., 2009, 38, 3391; (m) Y. Chi and P.-T. Chou, Chem. Soc. Rev., 2010, 39, 638.
30. (a) G. Zhou, W.-Y. Wong and X. Yang, Chem.–Asian J., 2011, 6, 1706; (b) C. Yang, X. Zhang, H. You, L. Zhu, L. Chen, L. Zhu, Y. Tao, D. Ma, Z. Shuai and J. Qin, Adv. Funct. Mater., 2007, 17, 651; (c) K. Zhang, Z. Chen, C. Yang, X. Zhang, Y. Tao, L. Duan, L. Chen, L. Zhu, J. Qin and Y. Cao, J. Mater. Chem., 2007, 17, 3451; (d) S. Bettington, M. Tavasli, M. R. Bryce, A. Beeby, H. Al-Attar and A. P. Monkman, Chem.–Eur. J., 2006, 13, 1423.
31. W.-Y. Wong and C.-L. Ho, Coord. Chem. Rev., 2009, 253, 1709.
32. M.-H. Tsai, H.-W. Lin, H.-C. Su, T.-H. Ke, C. Wu, F.-C. Fang, Y.-L. Liao, K.-T. Wong and C.-I. Wu, Adv. Mater., 2006, 18, 1216.
33. K.-T. Wong, Y.-M. Chen, Y.-T. Lin, H.-C. Su and C.-C. Wu, Org. Lett., 2005, 7, 5361.
34. W.-Y. Hung, L.-C. Chi, W.-J. Chen, Y.-M. Chen, S.-H. Chou and K.-T. Wong, J. Mater. Chem., 2010, 20, 10113.
35. J. He, H. Liu, Y. Dai, X. Ou, J. Wang, S. Tao, X. Zhang, P. Wang and D. Ma, J. Phys. Chem. C, 2009, 113, 6761.
36. A. Elscher, F. Bruder, H.-W. Heuer, F. Jonas, A. Karbach, S. Kirchmeyer, S. Thurm and R. Wehrmann, Synth. Met., 2000, 111, 139.
37. S.-J. Su, E. Gonmori, H. Sasabe and J. Kido, Adv. Mater., 2008, 20, 4189.
38. Y.-Y. Lyu, J. Kwak, W. S. Jeon, Y. Byun, H. S. Lee, D. Kim, C. Lee and K. Char, Adv. Funct. Mater., 2009, 19, 420.
39. J. Kido, M. Kimura and K. Nagai, Science, 1995, 267, 1332.
40. C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl. Phys., 2001, 90, 5048.
41. W.-Y. Hung, T.-C. Tsai, S.-Y. Su, K.-T. Wong and L.-C. Chi, Phys. Chem. Chem. Phys., 2008, 10, 5822.
42. G.-J. Zhou, Q. Wang, C.-L. Ho, W.-Y. Wong, D. Ma, L. Wang and Z. Lin, Chem.–Asian J., 2008, 3, 1830.
43. C.-H. Lin, Y. Chi, M.-W. Chung, Y.-J. Chen, K.-W. Wang, G.-H. Lee, P.-T. Chou, W.-Y. Hung and H.-C. Chiu, Dalton Trans., 2011, 40, 1132.
44. C.-L. Ho, W.-Y. Wong, Q. Wang, D. Ma, L. Wang and Z. Lin, Adv. Funct. Mater., 2008, 18, 928.
45. W.-Y. Wong, G.-J. Zhou, X.-M. Yu, H.-S. Kwok and B. Tang, Adv. Funct. Mater., 2006, 16, 838.
46. G.-J. Zhou, W.-Y. Wong and S. Suo, J. Photochem. Photobiol., C, 2010, 11, 133.
47. Y. Shao and Y. Yang, Appl. Phys. Lett., 2005, 86, 073510.
48. W.-Y. Hung, Z.-W. Chen, H.-W. You, F.-C. Fan, H.-F. Chen and K.-T. Wong, Org. Electron., 2011, 12, 575.

---