Syntheses, photoluminescence and electroluminescence of some
new blue-emitting phosphorescent iridium(III)-based materials
Inamur R. Laskar, Shih-Feng Hsu, Teng-Ming Chen
*Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30055, Taiwan Received 20 August 2004; accepted 29 October 2004
Available online 22 December 2004
Abstract
The cyclometallated ligand 2-(40,6-difluorophenyl)-4-methoxypyridine (F
2MeOppyH), whose complexes with iridium(III) emit
bright blue to green light, was synthesized in five separate steps. Ir(F2MeOppy)2(acac), (acacH = 2,4-pentanedione), Ir(F2
MeOp-py)2(pic) (picH = 2-picolinic acid), fac-Ir(F2MeOppy)3and mer-Ir(F2MeOppy)3complexes were synthesized from solution and fully
characterized. The structures of Ir(F2MeOppy)2(acac) and fac-Ir(F2MeOppy)3were authenticated by X-ray single crystal structure
analysis. fac-Ir(F2MeOppy)3showed a much higher solution photoluminescence (PL) quantum efficiency and blue-shifted emission
compared to its counter mer-isomer. All of the complexes showed reversible oxidations between 0.3 and 0.7 V versus the ferrocene/ ferrocenium ion. The relative thermodynamic stability of the mer versus fac isomer was investigated and correlated to their corre-sponding redox and PL properties. Two electroluminescent (EL) devices (D-1 and D-2) were fabricated using the same blue Ir(F2MeOppy)2(acac) complex as a a dopant but with two different hole blockers, BCP and BAlq, and consequently BCP proved
itself a good hole blocker for this type of system. The fabrication of another EL device (D-3) was carried out by using the same dopant, only replacing the host CBP by a wider band gap host, mCP which showed improved luminance, luminance yield and power efficiency (D-2: 133 cd m2, 0.66 cd A1, 0.22 lm W1; D-3: 326 cd m2; 1.63 cd A1; 0.26 lm W1).
2004 Elsevier Ltd. All rights reserved.
Keywords: Facial; Meridional; Iridium(III) complex; Blue phosphorescent; Photoluminescence; Electroluminescence
1. Introduction
Syntheses of materials based on heavy metal (Ir(III), Pt(II), Os(II), Re(I)) complexes, used as phosphorescent dopants, have attracted a great deal of attention due to their potential applications in organic light emitting de-vices (OLEDs)[1–4]. Of the above heavy metal-contain-ing phosphor emitters that have been reported in OLEDs, cyclometallated complexes of iridium(III) materials have shown the most promising applications due to their higher stability, higher photoluminescence (PL) efficiency and relatively shorter excited state
life-time. Purely green- and red-emitting phosphorescent complexes of iridium(III) [5]are common, whereas the purely blue-emitting complex dopants [6] are scarcely found. Hence, the current effort of scientists has been fo-cused on the syntheses of blue-emitting iridium(III)-based materials.
Amongst the complexes, the syntheses of Ir(F2ppy)2
-(acac) and Ir(F2ppy)2(pic) (F2ppy = 2-(20, 40
-difluor-ophenyl)pyridine; acacH = 2,4-pentanedione; picH = 2-picolinic acid)[7]are notable examples for blue-emit-ting materials. Recently, Coppo et al. [8]reported the blue-emitting phenylpyridine iridium(III) complexes using triazolyl pyridine types as ancillary ligands. In this work, we have introduced strong electron donating sub-stituents at the 4-position of the LUMO containing pyr-idyl moiety in 2-(20,40-difluorophenyl)pyridine, which is 0277-5387/$ - see front matter 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2004.10.016
* Corresponding author. Tel.: +8865712121x56526; fax:
+88635723764.
E-mail address:[email protected](T.-M. Chen).
supposed to further raise the LUMO energy state of the corresponding iridium(III) complex. Hence, the energy gap will be enhanced and consequently result in a more hypsochromic shift when compared to the parent com-plex. Our first choice was to incorporate a methoxy substituent at the 4-position of the pyridyl moiety in 2-(2,40-difluorophenyl)pyridine. We have also synthesized
both the fac-/mer-isomers of the same ligand, investi-gated and differentiated their photoluminescence and redox behaviors as well as their thermal stability. We re-port on the syntheses of four blue-emitting iridium(III) complex dopants using the same cyclometallated ligand, 2-(20,40-difluorophenyl)-4-methoxypyridine, and
charac-terization and detailed studies of their photophysical as well as redox properties, X-ray single-crystal structure analyses of Ir(F2MeOppy)2(acac) and fac-Ir(F2
MeOp-py)3 and the application of Ir(F2MeOppy)2(acac) as a
dopant in electroluminescent (EL) devices.
2. Experimental 2.1. Materials
IrCl3Æ3H2O was purchased from Alfa Aesar, USA,
2,4-difluoroboronic acid, picolinic acid and glycerol from Aldrich Chemicals, USA and the remaining com-pounds from Tokyo Kasei Kogyo Co. Ltd., Japan, and all were used as received.
2.2. Syntheses
2-Chloropyridine-N-oxide (yield 95%): 2-Chloropyri-dine was allowed to react with excess m-chloroperoxy-benzoic acid in dichloromethane at 0 C, for 16 h. Then, the volume of the solvent was reduced to half and it was passed through a basic alumina bed using dichloromethane as an eluent, which resulted in pure 2-chloropyridine-N-oxide. 1H NMR (300 MHz, CDCl3): d 8.39 (t, 1H, J 3.9 Hz), 7.54 (t, 1H, J 5.1
Hz), 7.27 (m, 2H). EIMS: m/z 130, Calc. 130.5.
2-Chloro-4-nitropyridine-N-oxide (yield 70%) [9]: 2-chloropyridine-N-oxide (10 g) was added to conc. sulfu-ric acid (15 ml) at 0–5C. Then, a mixture of conc. sulfuric acid (15 ml) and fuming nitric acid (30 ml) (sp. gr. 1.5) was added dropwise with stirring at 1–2C over a 45-min per-iod. The mixture was heated slowly to 90C over 1 h and then maintained at 90C with stirring for an additional hour after which the mixture was cooled to 10C, poured into a stirred ice–water mixture, and neutralized with sodium carbonate. The yellow precipitate was collected, air-dried, and then recrystallized from an ethanol–chloro-form mixture. 1H NMR (300 MHz, CDCl3): d 8.45
(m, 2H), 8.04 (m, 1H). EIMS: m/z 176, Calc. 175.5. 2-Chloro-4-nitropyridine (yield 90%) [9]: 2 g of 2-chloro-4-nitropyridine-N-oxide was taken in 30 ml of
ethyl acetate and 3 ml of phosphorous tribromide was added slowly. Then, the mixture was heated with stirring at 70 C for 10 min. The mixture was cooled, poured into an ice–water mixture, made alkaline to lit-mus with 10% aqueous sodium hydroxide and extracted with chloroform. The solvent was evaporated and puri-fied by column chromatography. 1H NMR (300 MHz, CDCl3): d 8.67 (dd, 1H, J 0.5, 5.4 Hz), 8.20 (d, 1H, J
1.8 Hz), 7.99 (dd, 1H J 1.8, 5.4 Hz). EIMS: m/z 158, Calc. 158.5.
4-methoxypyridine (yield 55%): 2-Chloro-4-nitropyridine (1 eq.) was reacted with sodium methox-ide (1.3 eq.) in dioxane at 100C for 12 h. The reaction mixture was cooled to room temperature and poured into water. Then, the organic layer was separated out and the aqueous layer was washed by ethyl acetate. The combined organic extracts were dried and evapo-rated to dryness. The compound was purified by column chromatography. 1H NMR (300 MHz, CDCl3): d 8.43
(d, 1H, J 6 Hz), 7.26 (d, 1H, J 2.1 Hz), 7.06 (m, 1H). EIMS: m/z 143, Calc. 143.5.
2-(20,40-Difluorophenyl)-4-methoxypyridine (yield
80%) [10]. 5 g of 2-chloro-4-methoxy pyridine (1 eq.), 8.8 g of 2,4-difluorophenylboronic acid (1.6 eq.) and 1.15 g of triphenylphosphine (0.1 eq.) were dissolved in 1,2-dimethoxyethane (50 ml). 60 ml of 2 M K2CO3 (2.7 eq.) aqueous solution was added
and the mixture was purged with argon gas. 0.25 g of palladium acetate (0.025 eq.) was added and the mixture was refluxed for 18 h. The two phases were then separated and the aqueous phase was extracted with ethyl acetate. The combined organic phases were washed with water and brine, successively, and then dried over MgSO4. After evaporation of the solvent,
the pure product was obtained by column chromatog-raphy. 1H NMR (300 MHz, CDCl3): d 8.51 (d, 1H, J
5.7 Hz), 7.98 (m, 1H), 7.26 (t, 1H, J 2.1 Hz), 6.94 (m, 2H), 6.78 (dd, 1H, J 2.4, 5.7 Hz), 3.87 (s, 3H). EIMS: m/z 221, Calc. 221.
(F2MeOppy)2Ir(l-Cl)Ir(F2MeOppy)2 (yield 75%)
[5e–5f]. Calc. for C48H32N4O4F8Cl2Ir2: C, 43.1; H, 2.4;
N, 4.2. Found: C, 43.0; H 2.3; N 4.0%. A solution of IrCl3Æ3H2O (1 mmol) and 2-(20,40
-difluorophenyl)-4-methoxypyridine (3 mmol) in 2-ethoxyethanol (30 ml) was refluxed for 24 h. The pale green mixture was cooled to room temperature and 20 ml of 1 M HCl was added to precipitate the product. The mixture was filtered and washed with 100 ml of 1 M HCl followed by 50 ml of methanol, and then dried. The product was obtained as a light green powder.1H NMR (300 MHz, CDCl3):
d 8.91 (d, 4H, J 6.6 Hz), 7.80 (s, 4H), 6.43 (m, 4H), 6.31 (t, 4H, J 9.9 Hz), 5.37 (d, 4H, J 9.0 Hz), 4.01 (s, 12H). FABMS: m/z 1335, Calc. 1335.
Ir(F2MeOppy)2(acac) (yield 85%) [5e–5f]. Calc. for
C29H23N2O4F4Ir: C, 47.6; H, 3.1; N, 3.8. Found: C,
MeOp-py)2(1 mmol), acetylacetone (3 mmol) and sodium
car-bonate (10 mmol) were mixed in 10 ml of 2-ethoxyetha-nol. The mixture was refluxed under nitrogen for 12 h. The reaction mixture was then cooled and the pale green precipitate was filtered off. The product was washed by methanol several times, followed by hexane. Then it was recrystallised from the mixture of dichloromethane and methanol (1:1).1H NMR (300 MHz, CDCl3): d 8.19
(d, 2H, J 6.6 Hz), 7.75 (s, 2H), 6.76 (dd, 2H, J 2.5, 6.3 Hz), 6.30 (t, 2H, J 10.5 Hz), 5.70 (d, 2H, J 7.2 Hz), 5.22 (s, 1H), 4.20 (s, 6H), 1.85 (s, 6H). FABMS: m/z 731, Calc. 731.
Ir(F2MeOppy)2(pic) (yield 92%) [6d]. Calc. for
C30H20N3O4F4Ir2: C, 47.7; H, 2.7; N, 4.8. Found: C,
47.6; H 2.6; N 4.8%. 2.2 mmol of picolinic acid was added to a room temperature solution of 0.8 mmol of (F2MeOppy)2Ir(l-Cl)Ir(F2MeOppy)2in 60 ml of
dichlo-romethane. The mixture was heated to reflux under nitrogen in an oil bath for 16 h. The reaction mixture was cooled to room temperature and the pale yellow precipitate was filtered off. The pure product was ob-tained by flash chromatography. 1H NMR (300 MHz, CDCl3): d 8.48 (d, 1H, J 6.9 Hz), 8.32 (d, 1H, J 7.2 Hz), 7.92 (t, 1H, J 6.6 Hz), 7.76 (m, 3H), 7.40 (td, 1H, J 1.5, 5.7 Hz), 7.17 (d, 1H, J 6.6 Hz), 6.73 (dd, 1H, J 2.7, 6.9 Hz), 6.53 (dd, 1H, J 2.7, 6.6 Hz), 6.42 (m, 2H), 5.88 (dd, 1H, J 2.3, 8.7 Hz), 5.64 (dd, 1H, J 2.6, 8.7 Hz), 4.06 (s, 6H). FABMS: m/z 754, Calc. 754.
mer-Ir(F2MeOppy)3 (yield 60%) [6a]. Calc. for
C36H24N3O3F6Ir: C, 50.7; H, 2.8; N, 4.2. Found: C, 50.5;
H 2.7; N 4.0%. 1 equivalent of (F2MeOppy)2Ir(l-Cl)
Ir-(F2MeOppy)2, 2 equivalents of the cyclometallated
ligand (F2MeOppyH) and 10 equivalents of sodium
car-bonate were mixed and refluxed in 2-ethoxyethanol for 20 h. The reaction was then allowed to cool to room temperature and distilled water was poured down on it. Immediately, the impure product was precipitated out and it was purified by flash chromatography by using dichloromethane as an eluant. 1H NMR (300 MHz, CDCl3): d 7.77 (m, 3H), 7.64 (m, 2H), 7.31 (t,
1H, J 7.5 Hz), 6.64 (td, 1H, J 2.1, 7.9 Hz), 6.52 (td, 2H, J 3.0, 6.6 Hz), 6.39 (m, 4H), 6.06 (dd, 1H, J 2.4, 7.5 Hz), 5.88 (dd, 1H, J 2.4, 9.3 Hz), 4.14 (s, 9H). FABMS: m/z 852, Calc. 852.
fac-Ir(F2MeOppy)3 (yield 50%) [6a]. Calc. for
C36H24N3O3F6Ir: C, 50.7; H, 2.8; N, 4.2. Found: C,
50.4; H 2.7; N 4.1%. 1 equivalent of Ir(F2
MeOp-py)2(acac) and 1.5 equivalents of the cyclometallated
ligand (F2MeOppyH) were refluxed in glycerol for
24 h. Then the reaction mixture was cooled to room temperature and distilled water was added to it. The impure product was separated out and purified by flash chromatography. 1H NMR (300 MHz, CDCl3):
d 7.39 (s, 3H), 7.28 (d, 3H, J 6.6 Hz), 6.50 (t, 3H, J 4.2 Hz), 6.31 (m, 2H), 4.11 (s, 9H). FABMS: m/z 852, Calc. 852.
2.3. Crystallography
Single crystal diffraction data for Ir(F2MeOppy)2
(acac) and fac-Ir(F2MeOppy)3were collected on a
Bru-ker Smart CCD diffractometer equipped with a normal focus, 3kW sealed-tube X-ray source (k = 0.71073 A˚ ). The intensity data were collected in the x scan mode (width of 0.3/ frame) and corrected for Lp and absorp-tion effects by using theSAINTSAINT[11]program. Cell refine-ment and data reduction were carried out by using the program BrukerSHELXTLSHELXTL [12]and the crystal structure was solved by direct methods using the SHELXTLSHELXTL [12] version 5.1 software packages. The structure was further refined by full-matrix least-squares methods based on F2 usingSHELXTLSHELXTLversion 5.1[12]. Positions of non-hydro-gen atoms were refined anisotropically, whereas the hydrogen positions were not refined.
2.4. Optical measurements and compositions analysis The ultraviolet-visible (UV–Vis) spectra of the phosphorescent Ir(III) complexes were measured on an UV–Vis spectrophotometer (Agilent model 8453) and corrected for background due to solvent absorption. Photoluminescence (PL) spectra were carried out with a spectrophotometer (Jobin-Yvon Spex, model Fluoro-log-3). Emission quantum yields were measured by the method of Demas and Crosby [13] using fac-Ir(ppy)3
as a reference[14]. NMR spectra were recorded on Var-ian 300 MHz. MS spectra (EI and FAB) were taken by micromass TRIO-2000. Cyclic voltammetry (CV) analy-ses were performed by using CHI 2.05; dichloromethane was used as a solvent in an inert atmosphere and 0.1 M tetra(n-butyl)ammonium tetrafluoroborate was used as the supporting electrolyte. A glassy carbon rod was used as the working electrode, platinum was used as the counter electrode and a silver wire was used as a pseu-do-reference electrode. The TG-DTA analysis was car-ried out by using a thermal analyzer (SEIKO 1TG/ DTA model 200). Emission lifetimes were obtained by exponentially fitting the emission decay curves recorded on Continuum model NY61 spectrofluorometer. 2.5. OLED fabrication and testing
In the fabrication of OLEDs organic layers were ther-mally evaporated onto a glass substrate precoated with an indium–tin–oxide (ITO) layer with a sheet resistance of 20 X under high-vacuum. Prior to use, the ITO sur-face was ultrasonicated in a detergent solution followed by rinsing with deionized (DI) water, dipped into ace-tone, trichloroethylene and 2-propanol, and then de-greased with a vapor of 2-propanol. After degreasing, the substrate was oxidized and cleaned in a UV-ozone chamber before it was loaded into an evaporator. In a vacuum chamber at a pressure of 106 Torr, 500 A˚ of
NPB as the hole transporting layer; 200 A˚ of the com-plex doped (7%) CBP as the emitting layer; 100 A˚ of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as a hole and exciton blocking layer (HBL); 650 A˚ of Alq3as the electron transport layer; and a cathode
com-posed of 10 A˚ lithium fluoride and 2000 A˚ aluminum were sequentially deposited onto the substrate to give the device structure. The current–voltage (I–V) profiles and light intensity characteristics for the above-fabri-cated devices were measured in a vacuum chamber of 106 Torr at ambient temperature using a Keithley 2400 Source Meter/2000 Multimeter coupled to a PR 650 Optical Meter.
3. Results and discussion 3.1. Synthesis
The ligand, 2-(20,40
-difluorophenyl)-4-methoxypyri-dine (F2MeOppyH) has been synthesized in five separate
steps smoothly (Scheme 1; see experimental part). The yield of 4-methoxy-2-chloro-pyridine (4th step) is rela-tively poor (55%) as compared to the other intermedi-ates and the final compound (F2MeOppyH).
IrCl3Æ3H2O was allowed to react with an excess of
F2MeOppyH to give chloride-bridged dinuclear
com-plex [5e,5f]. The mononuclear complexes, Ir(F2
MeOp-py)2(acac) and Ir(F2MeOppy)2(pic) have been
synthesized [5e,5f] by replacing the bridging chlorides from the dinuclear species (Scheme 2), with bidentate, monoanionic acetylacetonate and picolinate, respec-tively. The proton NMR spectra of these complexes are consistent with the heterocyclic rings of the cyclo-metallated ligand being in a trans-disposition. The tris-isomeric complexes (facial/meridional ) have also been
synthesized by varying the temperature and using the cyclometallated ligand, F2MeOppyH. Thompson et al.
[6a] reported the syntheses of the mer-isomer at 140C and the fac-isomer of iridium(III) complexes at 200C along with little amount of mer-isomer as a by-product. In our case, the meridional form has been synthesized at a relatively lower temperature (120 C), whereas the synthesis at higher temperature (200 C) generates absolutely the facial form (Scheme 2). We have always isolated a mixture of fac and mer isomers at 140C. Fur-thermore, pure mer-isomer was taken in glycerol and re-fluxed for 24 h. It was observed that the mer-isomer converted to the fac-isomer almost completely, inferring that the mer-isomer was a kinetically controlled product, whereas the fac-isomer was favored thermodynamically. These isomers could also been differentiated clearly on the TLC plate due to their large polarity differences.
1
H NMR and19F NMR spectra (Fig. 1) clearly distin-guished the fac-isomer from the mer-form. In 19F spec-tra, we observed two peaks for the inherent-C3
symmetric facial isomer and the six peaks for the asym-metric mer-isomer. Furthermore, TGA-DTA studies showed that the complexes are stable up to 340– 360C and they can be sublimed easily under a pressure of 103mm Hg.
3.2. X-ray crystallography
Single crystals of Ir(F2MeOppy)2(acac) and
fac-Ir-(F2MeOppy)3 were grown from
methanol/dichlorome-thane (1:1) and chloroform, respectively, and character-ized using X-ray crystallography. The ORTEP drawings of Ir(F2MeOppy)2(acac) and fac-Ir(F2MeOppy)3 are
represented inFigs. 2(a) and (b), respectively. The crys-tal data and the selected bond lengths of both complexes are displayed inTables 1 and 2, respectively. The
dispo-N Cl Cl CO3H N Cl O H HNO3/H2SO4 f. N Cl NO2 O N Cl OMe NaOMe 1,4-dioxane reflux H N Cl NO2 PBr3 EA N O F F F F B(OH)2 PPh3/Pd(OAc)2 aq.K2CO3/ DME DCM, 25degreeC reflux Scheme 1.
N F F O N COOH IrCl3.3H2O + reflux F2MeOppy
(F2MeOppy)2Ir( Cl)2Ir(F2MeOppy)2
Ir(F2MeOppy)2(pic)
DCM
reflux
Ir(F2MeOppy)2(acac)
acacH
Na2CO3
2-Ethoxyethanol mer-Ir(F2MeOppy)3
F2MeOppy K2CO3 reflux fac-Ir(F2MeOppy)3 2-Ethoxyethanol F2MeOppy Glycerol reflux Glycerol 2-Ethoxyethanol reflux reflux mu Scheme 2.
Fig. 1. 1H and19F NMR spectra of fac-Ir(F
sitions of the ligands in both the complexes exhibit pseu-do-octahedral geometry around the metal center. The C–C and C–N intraligand bond lengths and angles are
within normal ranges expected for cyclometallated irid-ium(III) complexes and similar to the values reported for (CN)2Ir(acac) [5e], fac-Ir(CN)3 [15,16] and Fig. 2. The ORTEP drawings of (a) Ir(F2MeOppy)2(acac) and (b) fac-Ir(F2MeOppy)3; the thermal ellipsoids for the image represent 50%
probability.
Table 1
Crystal data and structure refinement for fac-Ir(F2MeOppy)3and Ir(F2MeOppy)2(acac)
fac-Ir((F2MeOppy)3 Ir(F2MeOppy)2(acac)
Empirical formula C36H28F6IrN3O5 C29H23F4IrN2O4
Formula weight 888.81 731.69
Temperature (K) 295(2) 296(2)
Wavelength (A˚ ) 0.71073 0.71073
Crystal system triclinic monoclinic
Space group P 1 P21/c
Unit cell dimensions
a (A˚ ) 12.2277(5) 8.1323(7) b (A˚ ) 12.3887(5) 18.9904(15) c (A˚ ) 24.4856(10) 17.2636 (14) a() 93.103(1) b() 103.427(1) 90.777(2) c() 104.939(1) Volume (A˚3) 3460.1(2) 2665.9(4) Z 4 4 Dcalc(Mg/m3) 1.706 1.823 Absorption coefficient (mm1) 3.938 5.075 F(000) 1744 1424 Crystal size (mm) 0.20· 0.20 · 0.10 0.30· 0.20 · 0.20
hRange for data collection 0.86–28.33 1.59–28.30
Index ranges 16 6 h 6 16, 16 6 k 6 16, 32 6 l 6 32 10 6 h 6 9, 24 6 k 6 24, 20 6 l 6 23
Reflections collected 41 099 17 219
Independent reflections [Rint] 17 088 [0.0514] 6331 [0.0432]
Completeness to h (%) 99.0 (23.33) 95.5 (23.3)
Absorption correction empirical empirical
Maximum and minimum transmission 0.97346 and 0.59790 0.93848 and 0.71891
Refinement method full-matrix least-squares on F2 full-matrix least-squares on F2
Data/restraints/parameters 17 088/1/941 6331/0/365
Goodness-of-fit on F2 0.917 0.892
Final R indices [I > 2r(I)] R1= 0.0443, wR2= 0.1034 R1= 0.0378, wR2= 0.0894
R indices (all data) R1= 0.0971, wR2= 0.1178 R1= 0.0779, wR2= 0.958
(CN)2Ir(l-Cl)Ir(CN)2 [16] complexes. The
mer-Ir-(F2MeOppy)3complex should have the same disposition
of F2MeOppy ligand as found in Ir(F2MeOppy)2(acac).
The mutually trans-disposed Ir–N bonds in Ir(F2
MeOp-py)2(acac) have shorter bond lengths (Table 2) in
com-parison with those in fac-Ir(F2MeOppy)3 where the
bond lies in a trans position (Fig. 2(b)) to the strong trans influencing phenyl group. Another point to be noted is that the weak trans influence of the acetylacet-onate ligand leads to shorter Ir–C bonds for the Ir-(F2MeOppy)2(acac) complex than those observed in
fac-Ir(F2MeOppy)3, as indicated in Table 2.
3.3. Photophysical properties
The solution UV–Vis absorption and PL spectra of all these complexes have been measured. The UV–Vis absorption spectra of these complexes show intense bands appearing in the ultraviolet part of the spectrum between 240 and 340 nm. The measured energies and the extinction coefficients are comparable to those of the free ligand, which helped us to assign the bands as spin allowed1(p–p*) transitions of the ligand. These li-gand-centered bands are accompanied by weaker
transi-tions with lower energy extending into the visible region from 350 to 400 nm (Fig. 3). With reference to the pre-vious photophysical studies of cyclometallated com-plexes of the iridium(III) system [17], these absorption features are assigned to intra-ligand (IL) p–p* (F2MeOppy) and also spin-allowed metal-to-ligand
charge transfer1MLCT [dp (Ir)! p F2MeOppy)]
tran-sitions. In addition, spectra of all the complexes exhibit weaker absorption tails toward the further lower energy region (400–470 nm) (Fig. 3), which may be recognized as the spin-forbidden 3MLCT [dp (Ir)! p* (F2MeOppy)] transitions. The high intensity of these
MLCT bands (shown by the extinction coefficients in the Table 3) has been attributed to an effective mixing of these charge transfer transitions with higher lying spin-allowed transitions on the cyclometallated ligand, which is facilitated by the strong spin–orbit coupling of the iridium(III) center. The energy of the MLCT tran-sitions for the fac-isomer is comparatively higher than those of the rest of the complexes. These MLCT bands in the fac-isomer are more prominent and easily distin-guishable as compared to its mer-counterpart, which is indicated in Fig. 3. The lowest energy transitions show blue-shifted absorption with increasing polarity of the solvents (i.e., toluene! chloroform ! acetone ! meth-anol). As shown inFig. 4, the solution PL emission spec-tra of all the complexes exhibit structured features. These facts support that the lowest excited states of the complexes have a mixed ligand centered (LC) as well as MLCT character. As shown inFig. 4, the PL intensity of the fac-isomer is much higher and exhibits 6 nm hypsochromic shift with respect to its mer-counterpart. Similarly, the PL intensity of the Ir(F2MeOppy)2(acac)
complex is found to be much higher and its emission spectrum much sharper relative to that of Ir(F2
MeOp-py)2(pic), whose spectrum is red-shifted, broad and Table 2
Selected bond distances [A˚ ] for fac-Ir((F2MeOppy)3 and
Ir(F2MeOppy)2(acac)
fac-Ir((F2MeOppy)3 Ir(F2MeOppy)2(acac)
Atom (1)–atom (2) Distance (A˚ ) Atom (1)–atom (2) Distance (A˚ ) Ir(1)–C(1) 2.017(7) Ir(1)–C(13) 1.976(6) Ir(1)–C(25) 2.020(6) Ir(1)–C(1) 1.982(6) Ir(1)–C(13) 2.021(7) Ir(1)–N(1) 2.028(5) Ir(1)–N(3) 2.113(6) Ir(1)–N(2) 2.043(5) Ir(1)–N(1) 2.117(6) Ir(1)–O(1) 2.135(4) Ir(1)–N(2) 2.131(6) Ir(1)–O(2) 2.136(4) 300 320 340 360 380 400 420 440 460 480 500 520 0.0 0.5 1.0 1.5 2.0 2.5 3.0 4 3 2 1 Absorbance (a.u.) Wavelength (nm) 10-4M in Dichloromethane 1 Ir(F2MeOppy)2(acac)
2 Ir(F2MeOppy)2pic
3 fac-Ir(F2MeOppy)3
4 mer-Ir(F2MeOppy)3
featureless. The solution spectrum of Ir(F2MeOppy)
2-(pic) also markedly differs from its thin-film emission spectrum (Fig. 5), which shows blue-shifted emission to a larger extent (30 nm). It has also been observed that the nature of the emission spectra of Ir(F2
MeOp-py)2-(pic) is red-shifted in order of increasing polarity
of the solvents, which suggest that the energy of the low-est emitting state of Ir(F2MeOppy)2(pic) is highly
sol-vent dependent. The thin-film spectra for the other complexes, as shown inFig. 5, exhibit blue-shifted emis-sion to a smaller extent (5–8 nm) as compared to their respective solution spectra.
The complexes, Ir(F2ppy)2(acac) and Ir(F2ppy)2(pic)
[F2ppy = 2-(2,4-difluorophenyl)pyridyine], have been
re-ported [6d,7] and well characterized. The strong elec-tron-donating methoxy substituent was incorporated onto the LUMO containing pyridyl ring of F2ppy,
and, therefore, it raised the LUMO energy and thereby increased the HOMO–LUMO energy gap. Hence, the complex Ir(F2MeOppy)2(acac) (kmax= 471 nm) shows
the expected blue-shifted emission as compared to the reported complex, Ir(F2ppy)2(acac) (kmax= 476 nm). It
is also known that Ir(F2ppy)2(pic) (kmax= 456 nm)
exhibits blue shifted emission compared to its acetylacet-onate analogue, Ir(F2ppy)2(acac) (kmax= 476 nm). But
in our case, Ir(F2MeOppy)2(pic) (kmax= 509 nm) shows
a wide red shifted emission compared to its acetylaceto-nate analogue (kmax= 471 nm). We have measured the
solution quantum efficiency of the complexes and the re-sults are shown inTable 3. The fac-isomer shows a much higher quantum efficiency as compared to its counter mer-isomer. Furthermore, as shown inTable 3, the mea-sured lifetime for thin film samples falls into the micro-second regime, which evidence the lowest excited states are the triplet-emitting states for these complexes. The short life-time also indicates that there is strong spin–or-bit coupling in the presence of heavy metal iridium(III). 3.4. Electrochemistry
The electrochemical properties of the bis- and tris-cyclometallated complexes were examined by cyclic vol-tammetry. Redox potentials, given inTable 3, were mea-sured relative to an internal ferrocene reference (Cp2Fe/
Cp2Fe+= 0.62 versus SCE in dichloromethane solvent).
All of these complexes showed reversible oxidation, in Table 3
Photophysical and electrochemical data for the bis and tris-(F2MeOppy) cyclometallated iridium(III) complexes
Complex Absorbanceak(nm) (loge) Emission k
max(nm) Relative quantum
efficiencya,c
Redox Eox 1=2ðVÞ
a HOMO (eV) Lifetime (ls) Solutiona,b Filmb
Ir(F2MeOppy)2(acac) 383 (4.1); 419 (3.8) 471 469 0.60 0.508 5.3 0.59
Ir(F2MeOppy)2(pic) 375 (3.8); 411 (3.5) 509 485 0.82 0.661 5.5 0.84
fac-Ir(F2MeOppy)3 348 (4.1); 383 (3.8) 471 464 0.95 0.551 5.4 0.74
mer-Ir(F2MeOppy)3 384 (4.0); 420 (3.6) 477 468 0.41 0.345 5.6 0.68
a
Solvent used dichloromethane.
b
Excitations used 384, 374, 378 and 380 nm for solutions and 279, 270, 273 and 276 nm for thin films.
c
Values are reported relative to Cp2Fe/Cp2Fe+.
400 500 600 700 800 0 1000000 2000000 3000000 4000000 5000000 4 3 2 1 Intensity (a.u.) Wavelength (nm) 10-4M in Dichloromethane 1 Ir (F2MeOppy)2(acac) 2 Ir (F2MeOppy)2(pic) 3 fac-I r( F2MeOppy)3 4 mer-Ir(F2MeOppy)3
the range from 0.34 to 0.67 V. The fac-isomer has oxida-tion potentials ca.200 mV more positive than the cor-responding mer-form (i.e., facox: 0.55 V; merox: 0.34 V),
whereas the reduction potential for the fac-form is slightly more negative than that of the mer-isomer (i.e., facred: 2.67 V; merred: 2.62 V). It was known
that for phenyl-pyridyl based complexes, the oxidation processes involve the Ir–phenyl center (major HOMO contribution), while reduction processes occur primarily on the heterocyclic portion (major LUMO contribution) of the ligand[6]. The difference in electrochemical prop-erties between facial and meridional isomers can be rationalized by the presence of mutually trans phenyl rings of the ligands in the mer-isomer, whereas in the fa-cial form all the phenyls are cis (Fig. 2(b)) with respect to one another. Electron-rich sigma phenyl ligands nor-mally exhibit a very strong trans influence and trans ef-fect, which results in a lengthening of the transoid Ir–C bonds and hence destabilization of the HOMO to a sig-nificant extent, which is supported by the HOMO– LUMO energy calculations using redox data and absorption wavelength, summarized inTable 3. There-fore, it is concluded that the mer-isomer is easier to oxi-dize than the fac-isomer.
3.5. Description and performance of OLED devices Initially, we fabricated an electroluminescent device D-1 with structure ITO/CFx/NPB (300 A˚ )/CBP + 7%
dopant (200 A˚ )/BAlq (300 A˚)/Alq3(150 A˚ )/LiF (10 A˚)/
Al (2000 A˚ ) (the molecular structures of each compound used in the EL device are shown inScheme 3) using the complex Ir(F2MeOppy)2(acac) as a dopant in the
emit-ting layer.Fig. 6shows the EL emission spectra of the device where a broad and long tail extending up to 735 nm was observed. Clearly, the emission appearing
in the green and red regions is attributed to the leakage of holes into the BAlq and Alq3layers and the
conse-quent recombination in those layers infers that BAlq is not an effective hole blocker (HB) for deep-blue electro-luminescent material (Scheme 4a). Consequently, we re-placed BAlq with BCP, which has a comparatively higher HOMO level (6.5 eV) as compared to BAlq (6.0 eV). Accordingly, we fabricated another device by incor-porating the same dopant, Ir(F2MeOppy)2(acac) (D-2)
into the emitting layer, with BAlq replaced by BCP as a hole blocker. No observation of a broad emission was found in this device, but another extra emission was observed at ca. 425 nm, which was attributed to the NPB hole transport material as shown in Fig. 6. The external luminance efficiency and the power effi-ciency is found to be 0.66 cd A1 and 0.22 lm W1, respectively, at a current density of 20 mA cm2, as indi-cated in Fig. 7. The lowest triplet energy levels for the dopant, Ir(F2MeOppy)2(acac) and the host, CBP are
2.64 1and 2.56 eV[12], respectively, which suggests that the endothermic energy transfer[7]is being operated be-tween the host and the dopant. This type of energy transfer in an EL device is not completely favorable for blue-emitting dopants (Scheme 4b)[c]. The fabrica-tion of another EL device was carried out with the host mCP, having a wider band gap, i.e., 2.90 eV [18]. The layer sequences and the thicknesses of each layer were kept constant as before. The comparisons of device per-formance of D-2 and D-3 have been shown inTable 4. As indicated inFig. 7, in this case the external quantum efficiency and the power efficiency are improved and found to be 1.63 cd A1 and 0.47 lm W1 at 20 mA cm2 current density, respectively, as compared
400 500 600 700 800 -500000 0 500000 100000 0 150000 0 200000 0 250000 0 300000 0 350000 0 400000 0 450000 0 500000 0 2 4 1 3 Intensity (a.u.) Wavelength (nm) 1 Thinfilm-Ir(FFMeOppy)2(acac)) 2 ThinFilm-Ir(FFMeOppy)2(pic)) 3 Solution-Ir(FFMeOppy)2(acac)) 4 Solution-Ir(FFMeOppy)2(pic))
Fig. 5. Comparison of thin-film and solution PL spectra for Ir(F2MeOppy)2X (X = acac and pic).
1
The triplet emitting state of Ir(F2MeOppy)2(acac) has been
to the previous devices. This device shows a luminance value of 326 cd m2at 20 mA cm2current density, that is also higher than that of the former device (133 cd m2 at 20 mA cm2) (Figs. 8(a) and (b)). Hence the
exother-mic triplet energy transfer to the blue dopant Ir(F
2-MeOppy)2(acac) is more efficient when using the wider
band gap host m CP. The comparisons of I–V curves have also been shown inFig. 8(a) and (b).
N N NPB CBP N N N N BCP N O Al N O N O N O Al N O O Alq BAlq N Ir O O N N N F F O N N mCP Scheme 3. 300 400 500 600 700 800 -0.002 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 EL-Intensity (a.u.) Wavelength (nm) D - 1 D - 2 D - 3
4. Conclusion
We have synthesized four iridium(III)-based phos-phorescent dopants that emit in the blue range, using the same cyclometallated ligand, 2-(20,40
-difluorophe-nyl)-4-methoxypyridine. These complexes show different maximum emission wavelengths and quantum efficien-cies with respect to the ancillary ligands (actylacetonate and picolinate) and the types of isomerism. The strong
trans-influence in the meridional isomer leads to less sta-ble, broader, red-shifted and lower quantum efficiencies than the facial counterpart. Three EL devices were fab-ricated. It can be stated that BAlq is not a good hole blocker in the EL device for a blue-emitting phosphores-cent dopant. It has also been shown that the wider band gap host mCP shows good EL performances with re-spect to the host CBP having a relatively lower band gap, for the same blue-emitting iridium(III) dopant.
5. Supplementary data
Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Scheme 4. 0 20 40 60 80 100 0.4 0.6 1.0 1.2 1.4 1.6 1.8 D-1 D-2
Luminance Efficiency (cdA
-1 )
Current density (mAcm-2)
Fig. 7. Comparative plot of luminescent efficiency vs. current density of D-2 and D-3 devices.
Table 4
Comparisons of the EL performances of the devices with structure: ITO/CHF3/NPB (300 A˚ )/Host + 7% dopant (200 A˚)/BAlq (300 A˚)/
Alq3(150 A˚ )/LiF (10 A˚)/Al (2000 A˚), Host CBP for D-2; mCP for D-3
D-2 D-3
EL color blue blue
Peak wavelength (nm) 472 472
CIE-x at 20 mA cm2 0.19 0.17
CIE-y 0.31 0.30
Luminance (cd m2at 20 mA cm2) 133 327 External quantum efficiency (cd A1at 20 mA cm2) 0.66 1.63 Power efficiency (lm W1at 20 mA cm2) 0.22 0.88 All these parameters have been recorded at 20 mA cm2.
6 8 10 12 0 100 200 300 400 500 600 700 (b) 6 8 1 0 1 2 1 4 1 6 0 20 40 60 80 100 Voltage (V) Voltage (V)
Current Density (mAc
m -2 ) 0 20 40 60 80 100
Current Density (mAc
m -2 ) 0 200 400 600 800 1 000 1 200 1 400 1 600 1 800 (a) Lu m inance (c d m -2 ) Lu m inance (c d m -2 )
Fig. 8. Comparative plot of current density, voltage and luminance characteristics for the devices 2 (presented in b) and 3 (presented in a).
Data Centre, CCDC Nos. 228968 and 228969 for com-pounds Ir(F2MeOppy)2(acac) and fac-Ir(F2MeOppy)3,
respectively. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44 1223 336033; email: [email protected] or www:http:// www.ccdc.cam.ac.uk).
Acknowledgement
This research is supported by the Program for Pro-moting University Academic Excellence from the Minis-try of Education, Taiwan, Republic of China under the Contract PPAEU91-E-FA04-2-4-(B).
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