18718
J. Phys. Chem. B 2006, 110, 18718-18723
Ruthenium(II) Complexes with the Mixed Ligands 2,2′-Bipyridine and 4,4′-Dialkyl Ester-2,2′-bipyridine as Pure Red Dopants for a Single-Layer Electrophosphorescent Device Hong Xia, Yingying Zhu, Dan Lu, Mao Li, Chengbo Zhang, Bing Yang, and Yuguang Ma* Key Lab for Supramolecular Structure and Materials of Ministry of Education, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: March 12, 2006; In Final Form: July 20, 2006
The mixed-ligand polypyridine ruthenium(II) complexes, [Ru(bpy)2(dmeb)]2+(PF6-)2 (Ru(dmeb)2+) and [Ru(bpy)2(dbeb)]2+(PF6-)2 (Ru(dbeb)2+), where bpy is bipyridine, dmeb is 4,4′-dimethyl ester-2,2′-bipyridine, and dbeb is 4,4′-dibutyl ester-2,2′-bipyridine, are synthesized and characterized, and their spectroscopic, electrochemical, and electroluminescent properties are reported. Both Ru(II) complexes showed strong emission from the triplet metal-to-ligand charge-transfer excited state, red-shifted emission spectra (λmax ) 642 nm), and good solubility in organic solvents compared to the frequently used tris(bipyridine) Ru(II) complexes. The electrochemical measurements for these Ru complexes showed reversible and quasi-reversible redox processes, implying a potential improvement in the stability of the electroluminescent device. The electrophosphorescent devices were fabricated by doping them in a polymer host using a simple solution spin-coating technique. For a single-layer device with the 1.0 wt % Ru(dbeb)2+-doped polymer blends of poly(vinylcarbazole) (PVK) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD) as the emitting layer and with the metal Ba as the cathode, an external quantum efficiency of 3.0%, a luminous efficiency of 2.4 cd/A, and a maximum brightness of 935 cd/m2 are reached with an electroluminescence (EL) spectral peak at 640 nm and Commission Internationale de L’Eclairage chromaticity coordinates of x ) 0.64 and y ) 0.33, which were comparable with standard red color.
Introduction Organic and polymer light-emitting diodes (LEDs) have attracted great interest because of their potential applications in flat-panel displays.1,2 Utilizing triplet-based emitting centers in LEDs, and thereby capturing both singlet and triplet excited states, researchers have found that the internal quantum efficiency can, in principle, be 100%.3,4 Recently, considerable progress has been reported with electrophosphorescent OLEDs (based on small molecules) and PLEDs (based on polymers) that employ organometallic emitters as dopants.5-23 It has been demonstrated that efficient electrophosphorescence is obtained from a class of heavy-metal complexes that are characterized by a triplet excited state from the metal-to-ligand charge-transfer (MLCT) with a short lifetime of phosphorescence.6 Red-emitting LEDs based on triplet emitters such as trivalent Ir complexes,8,14,15 Pt porphyrin derivatives,5,19 tri-carbonyl (didmine) Re(I) complexes,20,21 tris(polypyridyl) Os(II) complexes,16 and Eu(III) chelates22,23 have been demonstrated. Compared with these phosphorescent materials, tris(polypyridyl) Ru(II) complexes are interesting because they exhibit strong emission and reversible redox states, rendering a potential improvement in device stability because redox processes frequently occur in electrochemical cells and LEDs.24-26 These Ru(II) complexes have been widely used to make light-emitting electrochemical cells (LECs) both in solution and in the solid state.27-35 High-brightness and high-efficiency emissions with a low-driving voltage of these Ru-complex-based solid-state electrochemical luminescence (ECL) cells have been achieved.31,33 However, the stability of these cells is far from that of the * Corresponding author. Telephone number: +86-431-5168480. Fax number: +86-431 5168480. E-mail:
[email protected].
organic electroluminescence (EL) devices, and the turn-on time to reach maximum emission is relatively long (associated with the low mobility of the counterion in the solid state). In addition, the strong interaction between Ru complexes in the device may decrease the emission efficiency because of the self-quenching and triplet-triplet annihilation.26 In principle, a molecular dispersion of the Ru complex in the semiconductive matrix and using a carrier injection diode structure may overcome these drawbacks. Recently, LEDs using the Ru(II)-doped emitting layer have been fabricated and exhibited instantaneous light output and good device performance.36-39 Though chargeneutral Ru(II) emitters have been reported as being used for a vacuum deposited device, the most luminescent Ru(II) complexes were ion-like, which were not suitable for vacuum deposition in OLED fabrication because of decomposition during thermo-evaporation. Fortunately, the chemical modification can make some of the Ru(II) complexes very soluble in organic solvents; thus, they can be doped into semiconductive polymers for PLEDs fabrication. In doped LEDs, the Ru complexes may be excited through energy transfer from the host and charge-trapping induced direct recombination on the dopant site, differing in mechanism from the ECL cell where an electrochemical redox pathway is required.19 We have reported that [Ru(4,7-Ph2-phen)3]2+(ClO4-)2 is very soluble in common organic solvents and is suitable for fabrication of PLEDs. In these devices, a high luminous efficiency of 8.6 cd/A was reached, but its emission is not an ideal standard red emission with a peak at 612 nm and Commission Internationale de L’Eclairage (CIE) coordinates of (0.62, 0.37).37 It is well known that [Ru(2,2′-bipyridine)3]2+ (Ru(bpy)32+) is only soluble in polar solvents such as water and methanol and that its emission color is pink with a λmax ) 600 nm in
10.1021/jp0615149 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/25/2006
Ru Complexes with Mixed Ligands as Dopants water at room temperature.24 Significantly, replacing one bpy ligand in Ru(bpy)32+ with a 4,4′-dimethyl ester-2,2′-bipyridine (dmeb) or a 4,4′-dibutyl ester-2,2′-bipyridine (dbeb) ligand results in the new complexes, affording a great improvement in the solubility in organic solvents and a red-shifted emissive spectrum, which is close to the standard red color. In this paper, we report results about the synthesis, optical and electrochemical properties, and the electroluminescent device performance of these mixed-ligand ruthenium(II) complexes. Experimental Section Synthesis of 4,4′-Dicarboxy-2,2′-bipyridine. 4,4′-Dimethyl2,2′-bipyridine (1.02 g, 5.43 mmol) and potassium permanganate (3.20 g, 20.38 mmol) were heated at reflux in 35 mL of water for 12 h. Removal of the brown precipitate by filtration gave a yellowish solution, which was extracted with ether to remove unreacted starting material. Addition of concentrated hydrochloric acid precipitated the product, which was washed with water. Yield of white powder: 94% (1.25 g, 5.10 mmol). 1H NMR (500 MHz, NaOD/D2O, δ): 7.64 (d, 2H, CHbpy), 8.15 (d, 2H, CHbpy), 8.55 (s, 2H, CHbpy). Synthesis of 4,4′-Dimethyl Ester-2,2′-bipyridine (dmeb). 4,4′-Dicarboxy-2,2′-bipyridine (0.30 g, 1.23 mmol), 10 mL of methanol, and 1 mL of H2SO4 were added to a 50 mL flask, and the mixture was refluxed for 4 h. After the solution was cooled, the mixture was added to cold water, and a white precipitate formed immediately. The resulting precipitate was collected by filtration, washed with methanol, and dried under vacuum. Yield: 92% (0.31 g, 1.13 mmol). 1H NMR (500 MHz, CDCl3, δ): 4.01 (s, 6H, CH3), 7.95 (d, 2H, CHbpy), 8.90 (d, 2H, CHbpy), 9.01 (s, 2H, CHbpy). Synthesis of 4,4′-Dibutyl Ester-2,2′-bipyridine (dbeb). 4,4′Dibutyl ester-2,2′-bipyridine was prepared as above, using 4,4′dicarboxy-2,2′-bipyridine (0.22 g, 0.90 mmol), 7 mL of 1-butanol, and 0.70 mL of H2SO4. Yield: 85% (0.27 g, 0.77 mmol). 1H NMR (500 MHz, CDCl3, δ): 1.00 (t, 6H, CH3), 1.51 (m, 4H, CH2CH2CH2CH3), 1.81 (m, 4H, CH2CH2CH2CH3), 4.41 (t, 4H, CH2CH2CH2CH3), 7.91 (d, 2H, CHbpy), 8.88 (d, 2H, CHbpy), 9.00 (s, 2H, CHbpy). Synthesis of Ru(bpy)2 (dmeb)(PF6-)2 (Ru(dmeb)2+).40 A mixture of the Ru(bpy)2Cl2 precursor (121.7 mg, 0.25 mmol) and dmeb (105.5 mg, 0.39 mmol) in ethanol was refluxed for 48 h. The resulting mixture was evaporated until dry under reduced pressure. The crude product was purified by recrystallization from a KPF6 saturated aqueous solution to give a dark red powder. Yield: 82% (205.0 mg, 0.21 mmol). 1H NMR (500 MHz, DMSO-d6, δ): 3.97 (s, 6H, CH3), 7.49 (t, 2H, CHbpy), 7.56 (t, 2H, CHbpy), 7.71 (d, 2H, CHbpy), 7.74 (d, 2H, CHbpy), 7.90 (d, 2H, CHbpycooR), 7.98 (d, 2H, CHbpycooR), 8.20 (m, 4H, CHbpy), 8.86 (t, 4H, CHbpy), 9.35 (s, 2H, CHbpycooR). Anal. Calcd for C34H28F12N6O4P2Ru: C, 41.86; N, 8.61; H, 2.89. Found: C, 42.76; N, 8.95; H, 2.87. Synthesis of Ru(bpy)2 (dbeb)(PF6-)2 (Ru(dbeb)2+). The synthesis of Ru(dbeb)2+ was the same as that for Ru(dmeb)2+. A mixture of the Ru(bpy)2Cl2 precursor (38.0 mg, 0.08 mmol) and dbeb (52.8 mg, 0.15 mmol) in ethanol was refluxed for 48 h. The resulting mixture was evaporated until dry under reduced pressure. The crude product was purified by recrystallization from a KPF6 saturated aqueous solution to give an orange powder. Yield: 79% (66.8 mg, 0.063 mmol). 1H NMR (500 MHz, DMSO-d6, δ): 0.94 (t, 6H, CH2CH2CH2CH3), 1.44 (m, 4H, CH2CH2CH2CH3), 1.73 (m, 4H, CH2CH2CH2CH3), 4.38 (t, 4H, CH2CH2CH2CH3), 7.49 (t, 2H, CHbpy), 7.57 (t, 2H, CHbpy), 7.71 (d, 2H, CHbpy), 7.76 (d, 2H, CHbpy), 7.91 (d, 2H,
J. Phys. Chem. B, Vol. 110, No. 37, 2006 18719 CHbpycooR), 7.98 (d, 2H, CHbpycooR), 8.20 (m, 4H, CHbpy), 8.86 (t, 4H, CHbpy), 9.25 (s, 2H, CHbpycooR). Anal. Calcd for C40H40F12N6O4P2Ru: C, 45.33; N, 7.93; H, 3.80. Found: C, 46.51; N, 8.22; H, 3.86. UV-Vis and Photoluminescence (PL) Spectra. UV-vis absorption spectra were recorded on a UV-3100 spectrophotometer. Fluorescence measurements were carried out with a RF-5301PC fluorometer. The films for the PL experiments were formed on a precleaned quartz plate exposed to air atmosphere. Electrochemistry. Electrochemical measurements were performed with a BAS 100W Bioanalytical Systems device using a glass carbon disk (Φ ) 3 mm) as the working electrode, a platinum wire as the auxiliary electrode, and a porous glass wick (Ag/Ag+) as the reference electrode. Cyclic voltammetry (CV) studies of Ru(dmeb)2+, Ru(dbeb)2+, Ru(bpy)32+, dmeb, dbeb, and bpy were carried out at a scan rate of 100 mV/s in DMF solutions containing 0.1 M NBu4BF4 as the supporting electrolyte. Device Fabrication. Indium-tin oxide (ITO)-coated glass with a sheet resistance of
