Solid-state white light-emitting electrochemical cells using iridium-based cationic transition metal complexes

Article abstract of DOI:10.1021/ja076051e

White electroluminescent (EL) emission from single-layered solid-state light-emitting electrochemical cells (LECs) based on host-guest cationic iridium complexes has been successfully demonstrated. The devices show white EL spectra (Commission Internationale de l'Eclairage coordinates ranging from (x, y) = (0,45, 0.40) to (0.35, 0.39) at 2,9-3.3 V with high color rendering indices up to 80. Peak external quantum efficiency and peak power efficiency of the white LEC reach 4% and 7.8 lm/W, respectively. These results suggest that white LECs based on host-guest cationic transition metal complexes may be a promising alternative for solid-state lighting technologies.

Full text of DOI:10.1021/ja076051e

Published on Web 02/27/2008
Solid-State White Light-Emitting Electrochemical Cells Using
Iridium-Based Cationic Transition Metal Complexes
Hai-Ching Su,† Hsiao-Fan Chen,‡ Fu-Chuan Fang,‡ Chih-Che Liu,†
Chung-Chih Wu,*,† Ken-Tsung Wong,*,‡ Yi-Hung Liu,‡ and Shie-Ming Peng‡
Department of Electrical Engineering, Graduate Institute of Electro-optical Engineering and
Graduate Institute of Electronics Engineering, National Taiwan UniVersity, Taipei 10617,
Taiwan, and Department of Chemistry, National Taiwan UniVersity, Taipei 10617, Taiwan
Received August 11, 2007; E-mail: chungwu@cc.ee.ntu.edu.tw; kenwong@ntu.edu.tw
Abstract: White electroluminescent (EL) emission from single-layered solid-state light-emitting electrochemi-
cal cells (LECs) based on host-guest cationic iridium complexes has been successfully demonstrated.
The devices show white EL spectra (Commission Internationale de l’Eclairage coordinates ranging from
(x, y) ) (0.45, 0.40) to (0.35, 0.39) at 2.9-3.3 V with high color rendering indices up to 80. Peak external
quantum efficiency and peak power efficiency of the white LEC reach 4% and 7.8 lm/W, respectively.
These results suggest that white LECs based on host-guest cationic transition metal complexes may be
a promising alternative for solid-state lighting technologies.
Introduction
The only previous report of solid-state white LECs was based
on phase-separated mixture of a polyfluorene derivative and
poly(ethylene oxide) (PEO)3c However, the fluorescent nature
of conjugated polymers would limit the eventual electrolumi-
nescence (EL) efficiency. Recently, cationic transition metal
complexes have also been used in solid-state LECs,4 which show
two major advantages over conventional polymer LECs: (i) no
ion-conducting material (e.g., PEO) is needed since these metal
complexes are intrinsically ionic; (ii) higher EL efficiencies
could be achieved due to the phosphorescent nature of the
transition metal complexes. Inspired by previous works regard-
White organic light-emitting diodes (OLEDs) based on
polymers and small-molecule materials have attracted much
attention due to their potential applications in flat-panel displays
and solid-state lighting.1 Compared with conventional white
OLEDs,2 solid-state white light-emitting electrochemical cells
(LECs)3 possess several promising advantages. LECs generally
require only a single emissive layer, which can be easily
processed from solutions, and can conveniently use air-stable
electrodes. The emissive layer of LECs contains mobile ions,
which can drift toward electrodes under an applied bias. The
spatially separated ions induce doping (oxidation and reduction)
of the emissive materials near the electrodes, that is, p-type
doping near the anode and n-type doping near the cathode.3
The doped regions induce ohmic contacts with the electrodes
and consequently facilitate the injection of both holes and
electrons, which recombine at the junction between p- and n-type
regions. As a result, a single-layered LEC device can be operated
at very low voltages (close to Eg/e, where Eg is the energy gap
of the emissive material and e is elementary charge) with
balanced carrier injection, giving high power efficiencies.
Furthermore, air-stable metals, for example, Au and Ag, can
be used since carrier injection in LECs is relatively insensitive
to work functions of electrodes.
(4) (a) Lee, J. K.; Yoo, D. S.; Handy, E. S.; Rubner, M. F. Appl. Phys. Lett.
1996, 69, 1686. (b) Lyons, C. H.; Abbas, E. D.; Lee, J. K.; Rubner, M. F.
J. Am. Chem. Soc. 1998, 120, 12100. (c) Rudmann, H.; Rubner, M. F. J.
Appl. Phys. 2001, 90, 4338. (d) Kalyuzhny, G.; Buda, M.; McNeill, J.;
Barbara, P.; Bard, A. J. J. Am. Chem. Soc. 2003, 125, 6272. (e) Slinker, J.
D.; Bernards, D.; Houston, P. L.; Abrun˜a, H. D.; Bernhard, S.; Malliaras,
G. G. Chem. Commun. 2003, 2392. (f) Rudmann, H.; Shimada, S.; Rubner,
M. F. J. Appl. Phys. 2003, 94, 115. (g) Slinker, J. D.; Gorodetsky, A. A.;
Lowry, M. S.; Wang, J.; Parker, S.; Rohl, R.; Bernhard, S.; Malliaras, G.
G. J. Am. Chem. Soc. 2004, 126, 2763. (h) Slinker, J. D.; Malliaras, G. G.;
Flores-Torres, S.; Abrun˜a, H. D.; Chunwachirasiri, W.; Winokur, M. J. J.
Appl. Phys. 2004, 95, 4381. (i) Hosseini, A. R.; Koh, C. Y.; Slinker, J. D.;
Flores-Torres, S.; Abrun˜a, H. D.; Malliaras, G. G. Chem. Mater. 2005, 17,
6114. (j) Slinker, J. D.; Koh, C. Y.; Malliaras, G. G.; Lowry, M. S.;
Bernhard, S. Appl. Phys. Lett. 2005, 86, 173506. (k) Parker, S. T.; Slinker,
J. D.; Lowry, M. S.; Cox, M. P.; Bernhard, S.; Malliaras, G. G. Chem.
Mater. 2005, 17, 3187. (l) Lowry, M. S.; Goldsmith, J. I.; Slinker, J. D.;
Rohl, R.; Pascal, R. A., Jr.; Malliaras, G. G.; Bernhard, S. Chem. Mater.
2005, 17, 5712. (m) Tamayo, A. B.; Garon, S.; Sajoto, T.; Djurovich, P.
I.; Tsyba, I. M.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 8723.
(n) Coppo, P.; Duati, M.; Kozhevnikov, V. N.; Hofstraat, J. W.; De Cola,
L. Angew. Chem., Int. Ed. 2005, 44, 1806. (o) Nazeeruddin, Md. K.; Wegh,
R. T.; Zhou, Z.; Klein, C.; Wang, Q.; Angelis, F. De; Fantacci, S.; Gra¨tzel,
M. Inorg. Chem. 2006, 45, 9245. (p) Su, H. C.; Fang, F. C.; Hwu, T. Y.;
Hsieh, H. H.; Chen, H. F.; Lee, G. H.; Peng, S. M.; Wong, K. T.; Wu, C.
C. AdV. Funct. Mater. 2007, 17, 1019. (q) Su, H. C.; Wu, C. C.; Fang, F.
C.; Wong, K. T. Appl. Phys. Lett. 2006, 89, 261118. (r) Soltzberg, L. J.;
Slinker, J. D.; Flores-Torres, S.; Bernards, D. A.; Malliaras, G. G.; Abrun˜a,
H. D.; Kim, J.-S.; Friend, R. H.; Kaplan, M. D.; Goldberg, V. J. Am. Chem.
Soc., 2006, 128, 7761. (s) Slinker, J. D.; Kim, J.-S.; Flores-Torres, S.;
Delcamp, J. H.; Abrun˜a, H. D.; Friend, R. H.; Malliaras, G. G. J. Mater.
Chem., 2007, 17, 76. (t) Slinker, J. D.; Rivnay, J.; Moskowitz, J. S.; Parker,
J. B.; Bernhard, S.; Abrun˜a, H. D.; Malliaras, G. G. J. Mater. Chem. 2007,
17, 2976.
† Department of Electrical Engineering.
‡ Department of Chemistry.
(1) (a) Kido, J.; Hongawa, K.; Okuyama, K.; Nagai, K. Appl. Phys. Lett. 1994,
64, 815. (b) Tokito, S.; Iijima, T.; Tsuzuki, T.; Sato, F. Appl. Phys. Lett.
2003, 83, 2459. (c) D’Andrade, B. W.; Forrest, S. R. AdV. Mater. 2004,
16, 1585. (d) Gong, X.; Wang, S.; Moses, D.; Bazan, G. C.; Heeger, A. J.
AdV. Mater. 2005, 17, 2053. (e) Huang, J.; Li, G.; Wu, E.; Xu, Q.; Yang,
Y. AdV. Mater. 2006, 18, 114.
(2) (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (b)
Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay,
K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539.
(3) (a) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269,
1086. (b) Pei, Q.; Yang, Y.; Yu, G.; Zhang, C.; Heeger, A. J. J. Am. Chem.
Soc. 1996, 118, 3922. (c) Yang, Y.; Pei, Q. J. Appl. Phys. 1997, 81, 3294.
9
10.1021/ja076051e CCC: $40.75 © 2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 3413-3419
3413

A R T I C L E S
Su et al.
General Procedures for Synthesis. The compound 1-(2′,4′-difluo-
rophenyl)pyrazole was prepared following a literature procedure.7 4,5-
Diaza-9,9′-spirobifluorene (dasb)8a and 9,9-bis(4-methoxyphenyl)-4,5-
diazafluorene (bmpdaf)8b were prepared according to literature procedures.
All experiments involving IrCl3‚H2O or any other Ir(III) species were
carried out in an inert atmosphere. Cyclometalated Ir(III) dichloro-
bridged dimers of the general formula (C∧N)2Ir(µ-Cl)2Ir(C∧N)2 (where
C∧N represents a cyclometalating ligand) were synthesized by a
literature procedure.9
ing energy transfer between ionic complexes reported by
Malliaras4i and De Cola,4n we had previously demonstrated
highly efficient solid-state LEC devices by adopting host-guest
cationic metal complexes.4q Yet to our knowledge, there is no
white LEC based on cationic transition metal complexes reported
to date, despite their high potential.
Efficient white light emission may be most easily achieved
by mixing two complementary colors, such as blue-green and
red emission. Thus the development of efficient blue-green-
Synthesis of 9,9-Diethyl-4,5-diazafluorene (dedaf). A mixture of
4,5-diazafluorene8c (600 mg, 3.57 mmol), ethylbromide (0.8 mL, 10.7
mmol), and potassium tert-butoxide (1.2 g, 10.7 mmol) was dissolved
in 15 mL of THF and stirred for 3 h at room temperature. The solvent
was evaporated, and the reaction mixture was extracted with dichlo-
romethane and dried with MgSO4 to afford pure product (733 mg, 92%
4l-o
and red-emitting4a-i,m cationic transition metal complexes
is highly desired. In this paper, we report the synthesis and
characterization of efficient blue-green- and red-emitting
cationic iridium complexes and their successful application in
white LECs with adopting the effective host-guest strategy.4i,q
The white LECs show EL with Commission Internationale de
l’Eclairage (CIE)5 coordinates ranging from (x, y) ) (0.45, 0.40)
to (0.35, 0.39) at 2.9-3.3 V, high color rendering indices (CRI)6
up to 80, external quantum efficiency (EQE) and power
efficiency of up to 4% and 7.8 lm/W, respectively. These results
demonstrate the high potential of white LECs based on host-
guest cationic transition metal complexes for solid-state lighting.
1
yield) as a sticky dark fluid. H NMR (CDCl3, 400 MHz) δ 8.66 (dd,
J ) 4.8, 1.6 Hz, 2H), 7.72 (dd, J ) 8.0, 1.6 Hz, 2H), 7.31 (dd, J )
8.0, 4.8, 4H), 2.07 (q, J ) 7.2, 4H), 0.38 (t, J ) 7.2, 6H); 13C NMR
(CDCl3, 100 MHz) δ 149.3, 131.1, 123.2, 117.8, 115.2, 63.8, 28.4,
8.6; MS (m/z, ESI+) 62 (35), 195 (100), 224 (20); HRMS (m/z, FAB+)
calcd for C15H16N2 224.1313, found 224.1316.
Synthesis of [Ir(dfppz)2(dasb)]+(PF6-) (1). Bis-(µ)-chlorotetrakis-
(1-(4,6-difluorophenyl)-pyrazolato-C2,N)diiridium(III) (1.17 mg, 1.0
mmol) and 4,5-diaza-9,9′-spirobifluorene8a (668 mg, 2.1 mmol) were
dissolved in 1,2-ethanediol (70 mL) under Ar, and the solution was
kept at 150 °C for 16 h. The solution was cooled to room temperature,
and an aqueous solution of NH4PF6 (2.5 mg in 20 mL of deionized
water) was added to yield a yellow suspension. The solid was then
filtered and dried in an oven (80 °C) for 12 h. The crude product was
purified by column chromatography on silica gel (CH2Cl2/MeCN )
Experimental Section
General Experiments. 1H and 13C NMR spectra of compounds were
collected on a 400 MHz spectrometer at room temperature. Photo-
physical characteristics of complexes in solutions were collected at room
temperature by using 10-5 M dichloromethane (DCM) solutions of all
complexes, which were carefully purged with nitrogen prior to
measurements. The neat, 1-butyl-3-methylimidazolium hexafluoro-
phosphate [BMIM+(PF6-)] blended films (∼100 nm) for photolumi-
nescence (PL) studies were spin-coated onto quartz substrates from
acetonitrile (MeCN) solutions. UV-visible absorption spectra were
recorded on a spectrophotometer. PL spectra were measured with a
cooled charge coupled device (CCD) coupled to a monochromator using
the 325-nm line of the He-Cd laser as the excitation. Photolumines-
cence quantum yields (PLQYs) for solution and thin-film samples were
determined with a calibrated integrating sphere system. Excited-state
lifetimes of samples were measured using the time-correlated single-
photon counting technique.4p Oxidation and reduction potentials of all
complexes were determined by cyclic voltammetry (CV) at a scan rate
of 100 mV/s in MeCN or DCM solutions (1.0 mM). A glassy carbon
electrode and a platinum wire were used as the working electrode and
the counter electrode, respectively. All potentials were recorded versus
the Ag/AgCl (sat’d) reference electrode. Oxidation CV was performed
using 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6)
in MeCN (for 1 and 2), 0.1 M TBAPF6 in DCM (for 3), and 0.1 M
tetra-n-butylammonium perchlorate (TBAP) in MeCN (for 4) as the
supporting electrolyte. For reduction CV, 0.1 M TBAP in MeCN (for
all complexes) was used as the supporting electrolyte.
Fabrication and Characterization of LEC Devices. LEC devices
were fabricated by spin-coating the mixed solutions of complexes 3
and 4 and BMIM+(PF6-) (80.5, 0.4, and 19.1 wt.%, respectively) on
indium-tin-oxide coated glass substrates to form a ∼100-nm thick film,
followed by thermal evaporation of a 150-nm Ag top contact (cathode).
BMIM+(PF6-) was added to provide additional anions, which shortened
the device response time.4k The electrical and emission characteristics
of LECs were measured under constant bias voltages. The device was
driven with a source-measurement unit (SMU) and the emission
intensity was measured using a Si photodiode calibrated with Photo
Research PR650. EL spectra of devices were measured by a calibrated
spectrometer with a CCD array detector.
1
10/1) to give compound 1 (1.5 g, 87%) as a yellow solid. H NMR
(CDCl3, 400 MHz) δ 8.40 (d, J ) 2.4 Hz, 2H), 7.89-7.85 (m, 4H),
7.46 (t, J ) 7.8 Hz, 2H), 7.41 (d, J ) 2.4, 4H), 7.37 (d, J ) 2.4, 2H),
7.22 (d, J ) 8.0, 2H), 6.78-6.76 (m, 4H), 6.69 (td, J ) 10, 2.6 Hz,
2H), 5.85 (dd, J ) 8.0, 2.4 Hz, 2H); 13C NMR (CDCl3, 100 MHz) δ
162.1, 149.8, 148.6, 143.6, 141.7, 140.9, 139.0, 135.2, 131.7, 131.5,
129.6, 128.9, 128.1, 123.9, 120.8, 115.5, 115.3, 109.7, 99.9, 99.7, 67.1;
MS (m/z, ESI+) 381 (15), 869 (100); HRMS (m/z, FAB+) calcd for
C41H24F4IrN6 869.1628, found 869.1644.
Synthesis of [Ir(dfppz)2(bmpdaf)]+(PF6-) (2). Bis-(µ)-chlorotet-
rakis(1-(4,6-difluorophenyl)-pyrazolato-C2,N)diiridium(III) (350 mg, 0.3
mmol) and 9,9-bis(4-methoxyphenyl)-4,5-diazafluorene8b (239 mg, 0.63
mmol) were dissolved in 1,2-ethanediol (25 mL) under Ar, and the
solution was kept at 150 °C for 16 h. The solution was cooled to room
temperature, and an aqueous solution of NH4PF6 (750 mg in 7.5 mL
of deionized water) was added to yield a yellow suspension. The solid
was then filtered and dried in an oven (80 °C) for 12 h. The crude
product was purified by column chromatography on silica gel (CH2-
Cl2/MeCN ) 10/1) to give compound 2 (525 mg, 82%) as a yellow
1
solid. H NMR (CDCl3, 400 MHz) δ 8.34 (d, J ) 2.4 Hz, 2H), 8.05
(d, J ) 7.2 Hz, 2H), 7.85 (d, J ) 4.4 Hz, 2H), 7.58 (dd, J ) 8.0, 5.2
Hz, 2H), 7.16 (d, J ) 2.4 Hz, 2H), 7.09 (dd, J ) 6.8, 2.0 Hz, 4H),
6.85 (dd, J ) 6.8, 2.0 Hz, 4H), 6.70-6.64 (m, 4H), 5.80 (dd, J ) 8.0,
2.4 Hz, 2H), 3.77 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ 160.4, 159.5,
148.3, 145.7, 142.6, 138.8, 136.9, 131.9, 131.6, 131.4, 130.3, 128.2,
127.9, 117.1, 115.4, 115.2, 114.7, 109.5, 93.6, 86.6, 67.7, 55.5; MS
(m/z, ESI+) 261 (10), 381 (20), 931 (100); HRMS (m/z, FAB+) calcd
for C43H30F4IrN6O2 931.1996, found 931.2019.
(7) (a) Finar, I. L.; Rackham, D. M. J. Chem. Soc. B 1968, 211. (b) Finar, I.
L.; Godfrey, K. E. J. Chem. Soc. 1954, 2293.
(8) (a) Wong, K. T.; Chen, R. T.; Fang, F. C.; Wu, C. C.; Lin, Y. T. Org. Lett.
2005, 7, 1979. (b) Ono, K.; Yanase, T.; Ohkita, M.; Saito, K.; Matsushita,
Y.; Naka, S.; Okada, H.; Onnagawa, H. Chem. Lett. 2004, 33, 276. (c)
Thummel, R. P.; Lefoulon, F.; Mahadevan, R. J. Org. Chem. 1985, 50,
3824.
(5) Colorimety; Commission Internationale de l’Eclairage (CIE): Paris, 1986.
(6) Method of Measuring and Specifying Colour Rendering Properties of Light
Sources; Commission Internationale de l’Eclairage (CIE): Paris, 1974.
(9) (a) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem.
Soc. 1984, 106, 6647. (b) Lohse, O.; Thevenin, P.; Waldvogel, E. Synlett
1999, 1, 45. (c) Nonoyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 767.
9
3414 J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008

Solid-State White Light-Emitting Electrochemical Cells
A R T I C L E S
Scheme 1. Synthetic Pathways and Structures of Complexes 1, 2, 3, and 4
Synthesis of [Ir(dfppz)2(dedaf)]+(PF6-) (3). Bis-(µ)-chlorotetrakis-
(1-(4,6-difluorophenyl)-pyrazolato-C2,N)diiridium(III) (373 mg, 0.32
mmol) and 9,9-diethyl-4,5-diazafluorene (150 mg, 0.67 mmol) were
dissolved in 1,2-ethanediol (25 mL) under Ar, and the solution was
kept at 150 °C for 16 h. The solution was cooled to room temperature,
and an aqueous solution of NH4PF6 (750 mg in 7.5 mL of deionized
water) was added to yield a yellow suspension. The solid was then
filtered and dried in an oven (80 °C) for 12 h. The crude product was
purified by column chromatography on silica gel (CH2Cl2/MeCN )
10/1) to give compound 3 (480 mg, 82%) as a yellow solid. 1H NMR
(CDCl3, 400 MHz) δ 8.29 (d, J ) 8.0 Hz, 2H), 8.08 (dd, J ) 8.0, 0.8
Hz, 2H), 7.80 (t, J ) 8.0 Hz, 2H), 7.72 (dd, J ) 5.2, 0.8 Hz, 2H),
7.64-7.58 (m, 2H), 7.09 (t, J ) 6.4 Hz, 2H), 6.57 (td, J ) 10.6, 2.4,
2H), 5.77 (dd, J ) 8.0, 2.4 Hz, 2H), 2.21 (q, J ) 7.2 Hz, 4H), 0.53 (t,
J ) 7.2 Hz, 6H); 13C NMR (CDCl3, 100 MHz) δ 161.3, 147.7, 147.3,
144.5, 139.1, 138.4, 135.3, 131.6, 131.4, 127.9, 127.4, 123.6, 115.5,
109.3, 99.8, 99.6, 61.1, 29.9, 9.0; MS (m/z, ESI+) 551 (15), 775 (100);
HRMS (m/z, FAB+) calcd for C33H26F4IrN6 775.1784, found 775.1780.
Synthesis of [Ir(ppy)2(biq)]+(PF6-) (4). Bis-(µ)-chlorotetrakis(2-
phenylpyridinato-C2,N)diiridium(III) (398 mg, 0.37 mmol) and 2,2′-
biquinoline (200 mg, 0.78 mmol) were dissolved in 1,2-ethanediol (30
mL) under Ar, and the solution was kept at 150 °C for 16 h. The
solution was cooled to room temperature, and an aqueous solution of
NH4PF6 (800 mg in 8 mL of deionized water) was added to yield a red
suspension. The solid was then filtered and dried in an oven (80 °C)
for 12 h. The crude product was purified by column chromatography
on silica gel (CH2Cl2/MeCN ) 10/1) to give compound 4 (658 mg,
are essential for LEC applications, are limited.4l-o Among them,
the complex [Ir(dfppz)2(dtb-bpy)]+(PF6)- (where dfppz is 1-(2,4-
difluorophenyl)pyrazole and dtb-bpy is [4,4′-di(tert-butyl)-2,2′-
bipyridine]) exhibits the shortest neat-film emission wavelength
of 487 nm.4m The large energy gap (3.08 eV) of [Ir(dfppz)2-
(dtb-bpy)]+(PF6)- can be reasonably attributed to the tailor-
made dfppz ligand, which stabilizes the highest occupied
molecular orbital (HOMO) and destabilizes the lowest unoc-
cupied molecular orbital (LUMO) in a good manner.4m A small
bathochromic shift (only 3 nm) between neat-film emission and
solution emission of this complex is possibly associated with
the reduced intermolecular interactions provided by sterically
bulky di-tert-butyl groups of the bipyridine ligand. To increase
rigidity of the molecular structure, which could potentially result
in higher photoluminescence quantum yields (PLQY), we
introduced 4,5-diazafluorene (daf) as an ancillary ligand onto
the dfppz-based cationic iridium complex. However, the syn-
thesis of cationic iridium complexes using C9-unsubstituted daf
was unsuccessful. Therefore, daf derivatives with aromatic or
alkyl substitutions on C9 of daf, such as 4,5-diaza-9,9′-
spirobifluorene (dasb), 9,9-bis(4-methoxyphenyl)-4,5-diazafluo-
rene (bmpdaf), and 9,9-diethyl-4,5-diazafluorene (dedaf), were
used instead. The synthesis of cationic iridium complexes
[Ir(dfppz)2(dasb)]+(PF6-) (1), [Ir(dfppz)2(bmpdaf)]+(PF6-) (2),
and [Ir(dfppz)2(dedaf)]+(PF6-) (3) is shown in Scheme 1. The
reactions of daf derivatives with cyclometalated Ir(III) dichloro-
1
98%) as a red solid. H NMR (CDCl3, 400 MHz) δ 8.95 (d, J ) 9.2
Hz, 2H), 8.88 (d, J ) 9.2 Hz, 2H), 8.13 (d, J ) 8.0 Hz, 2H), 8.08 (d,
J ) 8.0 Hz, 2H), 7.87-7.83 (m, 6H), 7.77 (dd, J ) 8.0, 1.2 Hz, 2H),
7.57 (t, J ) 8.0 Hz, 2H), 7.14 (t, J ) 8.0 Hz, 2H), 7.04 (q, J ) 8.0 Hz,
2H), 6.95 (t, J ) 8.0 Hz, 2H), 6.85 (t, J ) 8.0 Hz, 2H), 6.12 (d, J )
8.0 Hz, 2H); 13C NMR (CDCl3, 100 MHz) δ 159.5, 149.7, 148.0, 147.7,
142.4, 141.5, 137.8, 134.1, 131.0, 130.4, 129.7, 128.6, 127.8, 127.1,
124.6, 124.1, 123.0, 122.5, 122.3, 119.3; MS (m/z, ESI+) 501 (100),
757 (70); HRMS (m/z, ESI+) calcd for C40H28IrN4 757.1943, found
757.1946.
-
bridged dimers followed by subsequent ion exchanges to PF6
afforded the desired cationic complexes with excellent yields
(>80%).
For the red-emitting complex, [Ir(ppz)2(biq)]+(PF6-) (where
ppz is 1-phenylpyrazole and biq is 2,2′-biquinoline) was
previously reported to show neat-film emission around 622
nm.4m To achieve white emission, the red-emitting guest is to
be doped into the blue (or blue-green)-emitting host at a low
concentration (generally <1 wt %) to induce partial energy
transfer. However, at such low concentrations, the emission
usually shows significant blue-shift to orange or even yellow
Results and Discussion
To date, reports on blue (or blue-green)-emitting cationic
transition metal complexes with stable redox properties, which
9
J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3415

A R T I C L E S
Su et al.
Scheme 2. The Crystal Structures of Complexes 3 (top) and 4
Table 1. The Crystal Data of Complexes 3 and 4
(bottom)a
3
4
empirical formula
formula weight
crystal dimensions (mm3)
crystal system
space group
a (Å)
C36H32F10IrN6OP
977.85
C40H28F6IrN4P
901.83
0.20 × 0.15 × 0.10
monoclinic
Cc
13.9100(2)
13.0166(2)
19.9665(4)
90
105.8720(10)
90
0.15 × 0.10 × 0.05
triclinic
P1h
10.04400(10)
14.09140(10)
14.4291(2)
67.0550(10)
87.2080(10)
84.6510(10)
1872.27(3)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
cell volume (Å)3
Z
3440.70(10)
4
1.741
density (calcd) (g/ cm3)
F(000)
1.735
960
1768
temp (K)
295(2)
295(2)
wavelength (Å)
no. of reflns collected
0.71073
15981
0.71073
12983
no. of indep reflns (Rint
R(F), wR2 [I > 2σ(I)]
R(F), wR2 (all data)
)
8532 (0.0282)
0.0339, 0.0893
0.0388, 0.0955
7447 (0.0467)
0.0417, 0.0989
0.0531, 0.1162
[2.235(10)Å], Ir(1)-N(2) [2.219(7)Å], respectively, which are
significantly longer than those between the Ir center and 1-(2,4-
difluorophenyl)pyrazole [Ir(1)-N(3) ) 2.020(4)Å, Ir(1)-N(5)
) 2.020(4)Å] or 2-phenylpyridine [Ir(1)-N(3) ) 2.058(7)Å,
Ir(1)-N(4) ) 2.050(7)Å]. This can be possibly attributed to
the anionic nature of the cyclometalated 1-(2,4-difluorophenyl)-
pyrazole and 2-phenylpyridine ligands, which have a stronger
interaction with the cationic Ir(III) ion.
The emission properties of complexes 1-4 in solutions
(DCM, 10-5 M) or in thin films are summarized in Table 2.
For reducing the response time of LECs, in this work the ionic
liquid BMIM+(PF6-) was introduced in the emissive layer to
provide additional mobile anions.4k,p,q Thus, emission properties
of 1-3 in films in the presence of BMIM+(PF6-) (19 wt %)
were also examined. In general, complexes 1-3 show PL peaks
at 491-499 nm and rather high PLQYs of 0.46-0.66 in
solutions. The high PLQYs of these complexes indicate that
the rigidity of the daf ligand is beneficial for reducing nonra-
diative (e.g., vibration) deactivation processes. Among the three
blue-green complexes 1-3, 3 exhibits shortest emission
wavelengths of 488-491 nm and highest PLQYs of 0.28-0.30
in thin films (neat or dispersed with BMIM+(PF6-)) and thus
was chosen as the host material for white LEC studies. The
absorption and PL spectra of blue-green complex 3 and red-
emitting complex 4 in solutions and in neat films (∼100 nm)
are explicitly shown in Figure 1. Both complexes show intense
a Counter anion PF6- is omitted for clarity. Thermal ellipsoids are drawn
at the 50% probability.
due to reduced intermolecular interactions. Therefore, a red
emitter with an even lower energy gap is desired to ensure red
emission even at low doping concentrations. To achieve this,
[Ir(ppz)2(biq)]+(PF6-) was modified by replacing ppz with
2-phenylpyridine (ppy), which exhibits lower triplet energy than
ppz,4m to form [Ir(ppy)2(biq)]+(PF6-) (4) (Scheme 1). Crystals
of complex 3 and 4 suitable for the X-ray diffraction analysis
were obtained by slow evaporation of the solvent from the
acetone/pentane cosolvent. The structures of complexes 3 and
4 are depicted in Scheme 2, and the crystal data are summarized
in Table 1. The molecular structures shown in Scheme 2,
complex 3 with two cyclometalated 1-(2,4-difluorophenyl)-
pyrazole (C∧N) ligands and one 9,9′-diethyl-4,5-diazafluorene
(N∧N) ligand and complex 4 with cyclometalated 2-phenylpy-
ridine (C∧N) ligands and one 2,2′-biquinoline (N∧N) ligand,
exhibit a distorted octahedral geometry around the Ir center as
indicated by the small bite angles of C(33)-Ir(1)-N(5) [80.39-
(15)o], C(24)-Ir(1)-N(3) [79.88(15)o], and N(1)-Ir(1)-N(2)
[80.66(12)o] and twisted bond angles of N(5)-Ir(1)-N(3)
[171.81(13)o], C(33)-Ir(1)-N(1) [171.91(13)o], and C(24)-
Ir(1)-N(2) [173.59(13)o] in complex 3 and small bite angles
of C(40)-Ir(1)-N(4) [80.0(3)o], C(29)-Ir(1)-N(3) [80.7(4)o],
and N(1)-Ir(1)-N(2) [75.6(3)o] and twisted bond angles of
N(4)-Ir(1)-N(3) [171.2(3)o], C(40)-Ir(1)-N(2) [177.6(3)o],
and C(29)-Ir(1)-N(1) [174.0(4)o] in complex 4. The Ir-N
bond distances between the Ir center and 9,9′-diethyl-4,5-
diazafluorene and 2,2′-biquinoline are estimated to be Ir(1)-
N(1) [2.177(3)Å], Ir(1)-N(2) [2.189(3)Å] and Ir(1)-N(1)
absorption bands (with the extinction ꢀ > 3 × 104 M-1 cm-1
)
in the ultraviolet region ranging between 200 and ∼300 nm.
These bands are associated with the ligand-centered (LC)
transitions.4l,m These LC bands are accompanied by weaker and
broad bands extending from ∼300 nm to the visible region,
which are related to the metal-to-ligand-charge-transfer (MLCT)
transitions (both allowed and spin-forbidden ones) mediated by
the strong spin-orbit coupling of the Ir(III) center.4l,m Interest-
ingly, complex 3 with the alkyl substitution on daf exhibits
nearly the same emission wavelengths in solutions and in solid
films. Complex 4 exhibits more saturated red emission (with
PL peaks of 656 and 672 nm in solutions and in neat films,
respectively) compared with that of [Ir(ppz)2(biq)]+(PF6-). The
energy gaps estimated by cyclic voltammetry for complex 4
9
3416 J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008

Solid-State White Light-Emitting Electrochemical Cells
A R T I C L E S
Table 2. Summary of Physical Properties of Complexes 1-4
λ
max,PL (nm), Φ, τ (µ
s)a
Eo1x2 (V)f
Er1ed2 (V)f
∆E1 2 (V)
/
-
c
complex
solutionb
neat film or host−guest film
film with BMIM+(PF6
)
/
/
1
2
3
4
499, 0.46, 0.71
497, 0.66, 0.85
491, 0.54, 0.73
656, 0.20, 0.75
513, 0.20, 0.36
507, 0.22, 0.51
491, 0.28, 0.55
672, 0.09, 0.33
504, 0.22, 0.42
498, 0.26, 0.56
488, 0.30, 0.74
+1.29g
+1.28g
+1.20h
+0.86i
-1.70i
-1.72i
-1.79i
-1.37i
2.99
3.00
2.99
2.23
4 (0.2 wt %)/3
4 (0.4 wt %)/3
(493, 606), 0.26, (0.49d, 1.54e)
(493, 614), 0.27, (0.38d, 1.52e)
(488, 603), 0.29, (0.54d, 1.68e)
(488, 612), 0.28, (0.47d, 1.68e)
a At room temperature. b Measured in CH2Cl2 (10-5 M). c Films containing 19 wt % BMIM+(PF6-). d Measured at 480 nm. e Measured at 650 nm. f Potential
vs ferrocene/ferrocenium redox couple. g TBAPF6 (0.1 M) in acetonitrile. h TBAPF6 (0.1 M) in CH2Cl2. i TBAP (0.1 M) in acetonitrile.
Figure 1. Absorption (left axis) and PL spectra (right axis) of complexes
3 and 4 in dichloromethane solutions (10-5 M) and in neat films.
Figure 2. PL spectra of host-guest films containing different guest
-
concentrations (0.4 and 0.2 wt %) without and with BMIM+(PF6
(19 wt %).
)
(2.23 eV, Table 2) and [Ir(ppz)2(biq)]+(PF6-) (2.45 eV)4m are
consistent with the photophysical observation. For [Ir(ppz)2(biq)]+-
(PF6-), the HOMO and the LUMO are predominantly localized
on the ppz and biq ligand, respectively.4m The smaller energy
gap of complex 4, which also contains biq, is thus associated
with destabilization of HOMO in replacing ppz with ppy.
Cyclic voltammetry (CV) was used to probe the electro-
chemical properties of these complexes. The results are sum-
marized in Table 2. For complexes 1, 2, and 3, one reversible
oxidation potential was detected, which can be attributed to the
oxidation occurring on the Ir center. Interestingly, the electronic
nature of ancillary daf ligands plays a subtle role on the
oxidation potential. For example, complex 3 containing a more
electro-rich dedaf ligand shows a lower oxidation potential (1.20
V) compared with those of complexes 1 (1.29 V) and 2 (1.28
V), both containing aryl substitutions on C9 of the diazafluorene
ligand. On the other hand, the reduction capability of the
diazafluorene ligand is presumably responsible for observed
reversible reduction of complexes 1, 2, and 3. Thus, the Ir
complex coordinated to an electron-rich dedaf ligand (complex
3) shows a higher reduction potential (-1.79 V) compared with
those complexed to the aryl-substituted daf ligand [complexes
1 (-1.70 V) and 2 (-1.72 V)]. These results thus suggest that
the energies of frontier orbitals in these complexes can be subtly
manipulated by tailoring the electronic properties of the auxiliary
daf ligands. For the red complex 4, one reversible oxidation
(0.86 V) and one reversible reduction (-1.37 V) were detected.
The electron-rich character of the ppy ligand and the more
π-delocalized biq ligand contribute to the higher oxidation and
lower reduction, respectively, thus leading to a small energy
gap of complex 4.
Table 3. Summary of LEC Device Characteristics
ηext,max
,
ηL,max
,
b
c
d
e
bias
(V)
tmax
Lmax
ηp,max
t1
/2
CIE (x, y)a
CRIa
[min]
[cd/m2]
[%, cd/A, lm/W]
[h]
2.9
3.1
3.3
(0.45, 0.40)
(0.37, 0.39)
(0.35, 0.39)
81
80
80
240
60
30
2.5
18
43
4.0, 7.2, 7.8
3.4, 6.1, 6.2
3.3, 5.8, 5.5
8.9
1.3
0.4
a Evaluated from the EL spectra. b Time required to reach the maximal
brightness. c Maximal brightness achieved at a constant bias voltage.
d Maximal external quantum efficiency, cd/A, and power efficiency achieved
at a constant bias voltage. e The time for the brightness of the device to
decay from the maximum to half of the maximum under a constant bias
voltage.
the PLQYs are slightly raised (Table 2), indicating the role of
BMIM+(PF6-) in suppressing intermolecular interactions.4q As
shown in Figure 2, white emission of different blue-green and
red compositions can be achieved by adjusting the guest
concentration. Raising the guest concentration effectively
enhances the host-to-guest energy transfer, accompanied by
shorter lifetimes of the host emission (Table 2). The host-guest
film containing 0.4 wt % 4 (guest) shows white emission having
CIE coordinates of (x, y) ) (0.34, 0.37), which is rather close
to the ideal equal-energy white (x, y) ) (0.33, 0.33), and
therefore was subjected to further EL studies.
The LECs have the structure ITO/emissive layer (100 nm)/
Ag (150 nm), where the emissive layer contains host 3 (80.5
wt %), guest 4 (0.4 wt %), and BMIM+(PF6-) (19.1 wt %).
Their EL properties are summarized in Table 3. The EL spectra
of the white LECs under various biases, along with the PL
spectra for comparison, are shown in Figure 3a. It is noted that
the peak wavelength of the blue component in the EL spectra
is 488 nm, which is one of the shortest EL wavelengths reported
to date for LECs based on cationic transition metal complexes.
Compared with PL, the relative intensity of the red emission
with respect to the blue emission is larger in EL and increases
as the bias decreases. Furthermore, the maximum current density
of the host-guest device (with 0.4 wt % guest) is lower than
Emission properties of the 3/4 host-guest films are also
summarized in Table 2. Figure 2 depicts the PL spectra of the
host-guest films with different guest 4 concentrations (0.4 and
0.2 wt %) with or without the presence of BMIM+(PF6-) (19
wt %). In general, with the presence of BMIM+(PF6-), both
the host and guest exhibit longer excited-state lifetimes, and
9
J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3417

A R T I C L E S
Su et al.
Figure 4. (a) Brightness (solid symbols) and current density (open symbols)
and (b) external quantum efficiency (solid symbols) and power efficiency
(open symbols) as a function of time under a constant bias voltage of 2.9-
3.3 V for the white LEC.
Figure 3. (a) The bias-dependent EL spectra of the white LECs compared
with the PL spectrum and (b) maximum current density vs voltage
characteristics for the host-guest (0.4 wt % doping concentration) and the
host-only LECs. The inset shows the energy level diagram of the host and
guest molecules.
in brightness and efficiencies under a constant bias was
irreversible and thus may be rationally associated with the
degradation of the emissive material during the LEC operation.4d
Furthermore, although the device clearly gave white emission,
the brightness currently achieved is still limited. The limited
brightness is to some degree associated with the relatively lower
emission quantum yields (e.g., hosts and guests). By further
improving the materials to get higher emission efficiencies, it
is believed that the brightness can be much further enhanced.
Peak brightness and turn-on time (the time required to reach
the maximal brightness) as a function of bias voltage for white
LECs are shown in Figure 5a. An electrochemical junction
between p- and n-type doped layers of LECs is formed during
device operation. As revealed in previous studies,4f as bias
voltage increases, the junction width decreases due to extension
of doped layers, consequently leading to higher current density
(Figure 4a) and higher brightness (Figure 5a). The higher electric
field in the device also accelerates redistribution of mobile ions,
which facilitates formation of ohmic contacts with the electrodes.
Thus, operation of LECs under higher bias voltages fastens the
device response (Figure 5a). However, higher brightness and
faster response are obtained at the expense of device stability.
As shown in Figure 5b, peak EQE and device lifetime (the time
for the brightness of the device decaying from the maximum
to half of the maximum under a constant bias voltage) of the
white LECs deteriorate under higher bias voltages. It may be
associated with the higher electric field or current density
accelerating degradation (e.g., multiple oxidation and subsequent
decomposition)4k of the emissive cationic transition metal
complexes. Similarly, previous studies showed that modulation
of the electronic properties of the emissive layer of LECs by
different additives has significant influence on the device
lifetimes.4b,c For instance, blending an inert polymer4c into the
emissive layer of LECs improves device lifetimes, while adding
electrolytes4b shortens device lifetimes. Blending with an inert
polymer reduces the electronic conductivity of the emissive layer
that of the host-only device under same bias conditions (Figure
3b). These results could be understood by energy level align-
ments of the host and guest (inset of Figure 3b). At lower biases,
such energy level alignments favor carrier injection and trapping
on the smaller-gap guest, resulting in direct carrier recombina-
tion/exciton formation on the guest (rather than host-guest
energy transfer). Therefore, larger fractions of guest emission
are observed at lower biases. Nevertheless, white emission with
CIE coordinates of (0.35, 0.39) could be achieved at higher
biases and brightnesses. Also, the present white LEC exhibits
a rather high CRI (up to 80), which is an important characteristic
for solid-state lighting.
Figure 4a shows the time-dependent brightness and current
density under constant biases of 2.9-3.3 V for the white LEC.
After the bias was applied, the current first rose and then stayed
rather constant. On the other hand, the brightness first increased
with the current and reached the maxima of 2.5, 18, and 43
cd/m2 under biases of 2.9, 3.1, and 3.3 V, respectively. The
brightness then dropped with time with a rate depending on
the bias voltage (or current).4 Corresponding time-dependent
EQEs and power efficiencies of the same device are shown in
Figure 4b. When a forward bias only was applied, the EQE
was rather low due to poor carrier injection. During the
formation of the doped regions near electrodes, the capability
of carrier injection was improved and the EQE thus rose rapidly.
The peak EQEs, cd/A, and power efficiencies at 2.9, 3.1, and
3.3 V are (4.0%, 7.2 cd/A, 7.8 lm/W), (3.4%, 6.1 cd/A, 6.2
lm/W), and (3.3%, 5.8 cd/A, 5.5 lm/W), respectively. The drop
of efficiencies and brightness after reaching the peak value, as
commonly seen in solid-state LECs,3,4 may be associated with
a few factors. Before the current reaches a steady value, the
carrier recombination zone may keep moving closer to one
electrode due to discrepancy in electron and hole mobilities,
which would induce exciton quenching. Further, the decrease
9
3418 J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008

Solid-State White Light-Emitting Electrochemical Cells
A R T I C L E S
recent reports4r,s have also revealed that electrical driving of
LECs based on cationic transition metal complexes induces oxo-
bridged dimers, which effectively quench electroluminescence.
Despite these sporadic studies, detailed degradation mechanisms
of the LECs based on cationic transition metal complexes remain
unclear, and further studies are still needed to achieve practical
device lifetimes.
In summary, we have reported the synthesis and characteriza-
tion of efficient cationic blue-green-emitting complexes,
[Ir(dfppz)2(dasb)]+(PF6-) (1), [Ir(dfppz)2(bmpdaf)]+(PF6-) (2),
and [Ir(dfppz)2(dedaf)]+(PF6-) (3), and a red-emitting complex,
[Ir(ppy)2(biq)]+(PF6-) (4). By rationally combining these emit-
ters in the host-guest systems, for the first time, we successfully
generated white EL from solid-state LECs based on host-guest
cationic iridium complexes, with decent color performances and
efficiencies. These results suggest that white LECs based on
host-guest cationic transition metal complexes may be a
promising alternative for solid-sate lighting technologies.
Acknowledgment. We are grateful for financial support from
National Science Council, Ministry of Education, and Ministry
of Economic Affairs of Taiwan.
Figure 5. (a) Peak brightness (solid symbols) and turn-on time (open
symbols) and (b) peak external quantum efficiency (solid symbols) and
lifetime (open symbols) as a function of bias voltage for the white LECs.
Supporting Information Available: CIF files of complexes
3 and 4. This material is available free of charge via the Internet
at http://pubs.acs.org.
and thus lowers the current density. In contrast, additional
mobile ions provided by the electrolyte increase the doping level
and conductivity, resulting in higher current density. A few
JA076051E
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J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3419