Efficient dipyrrin-centered phosphorescence at room temperature from bis-cyclometalated iridium(III) dipyrrinato complexes

Article abstract of DOI:10.1021/ic100633w

A series of seven dipyrrin-based bis-cyclometalated Ir(III) complexes have been synthesized and characterized. All complexes display a single, irreversible oxidation wave and at least one reversible reduction wave. The electrochemical properties were found to be dominated by dipyrrin centered processes. The complexes were found to display room temperature luminescence with emission maxima ranging from 658 to 685 nm. Through systematic variation of the cyclometalating ligand and the meso substituent of the dipyrrin moiety, it was found that the observed room temperature emission was due to phosphorescence from a dipyrrin-centered triplet state with quantum efficiencies up to 11.5%. Bis-cyclometalated Ir(III) dipyrrin based organic light emitting diodes (OLEDs) display emission at 682 nm with maximum external quantum efficiencies up to 1.0%.

Full text of DOI:10.1021/ic100633w

Inorg. Chem. 2010, 49, 6077–6084 6077
DOI: 10.1021/ic100633w
Efficient Dipyrrin-Centered Phosphorescence at Room Temperature from
Bis-Cyclometalated Iridium(III) Dipyrrinato Complexes
Kenneth Hanson, Arnold Tamayo, Vyacheslav V. Diev, Matthew T. Whited, Peter I. Djurovich, and
Mark E. Thompson*
Department of Chemistry, University of Southern California, Los Angeles, California 90089
Received April 2, 2010
A series of seven dipyrrin-based bis-cyclometalated Ir(III) complexes have been synthesized and characterized. All
complexes display a single, irreversible oxidation wave and at least one reversible reduction wave. The
electrochemical properties were found to be dominated by dipyrrin centered processes. The complexes were found
to display room temperature luminescence with emission maxima ranging from 658 to 685 nm. Through systematic
variation of the cyclometalating ligand and the meso substituent of the dipyrrin moiety, it was found that the observed
room temperature emission was due to phosphorescence from a dipyrrin-centered triplet state with quantum
efficiencies up to 11.5%. Bis-cyclometalated Ir(III) dipyrrin based organic light emitting diodes (OLEDs) display
emission at 682 nm with maximum external quantum efficiencies up to 1.0%.
Introduction
nato compounds where the ligand is coordinated to a metal
cation. Prior examples include complexes with Zn(II),9,10
Rh(III)7 and group 13 ions (Al(III),11 Ga(III), or In(III)12).
The majority of the metal dipyrrinato complexes are non-
emissive, and little is known about the triplet excited state of
these species. In fact, one of the attractive features noted for
efficient fluorescence from BODIPY dyes is the negligible
intersystem crossing into the triplet state.4 Phosphorescence
from the dipyrrinato dyes has been observed at 77 K through
formation of the triplet excited state either by charge separa-
tion and then recombination,13 triplet energy transfer,14 or
sensitization by a heavy atom solvent.15 To facilitate the
investigation of the triplet properties of the dipyrrinato
ligand it is important to efficiently produce a triplet excited
state so that room temperature phosphorescence can be
observed.
Since their initial synthesis by Hans Fischer in 1934,1
complexes with dipyrrinato (dipy) ligands have been widely
studied because of their synthetic utility as porphyrin pre-
cursors and their rich photophysical properties.2 Interest in
the photophysics of these complexes continues to grow
predominantly thanks to the discovery of the highly absorp-
tive, strongly emissive boron difluoride dipyrrinato complex
(BODIPY)3 and its more recent applications as a biological
label, tunable laser dye, and dopant in electroluminescent
devices.4 In addition to BODIPY, the monoanionic dipyrri-
nato ligand has been used as a bidentate chelate for numerous
metal cations including Cr(III), Mn(II þ III), Fe(II þ III),
Pd(II), Rh(II),2 and more recently Ru(II),5,6 Rh(III),7 and
Ir(III).8
While a vast number of emissive BODIPY dyes have been
prepared, there are very few reports of luminescent dipyrri-
Recently, room temperature phosphorescence from dipy
(Φ = 0.013) has been observed by coordinating the ligand
directly to a platinum(II) cyclometalate (C∧N) complex.16
Much like platinum(II) cyclometalates, tris- and bis-
cyclometalated iridium(III) complexes are known to have
*To whom correspondence should be addressed. E-mail: met@usc.edu.
(1) Fischer, H.; Orth, H. Die Chemie des Pyrrols; Akademische
Verlagsgesellschaft: Leipzig, Germany, 1937; Vol. 2.
(2) Wood, T. E.; Thompson, A. Chem. Rev. 2007, 107, 1831–1861.
(3) Treibs, A.; Kreuzer, F.-H. Liebigs Ann. Chem. 1968, 718, 208–223.
(4) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47,
1184–1201.
(10) Sazanovich, I. V.; Kirmaier, C.; Hindin, E.; Yu, L.; Bocian, D. F.;
Lindsey, J. S.; Holten, D. J. Am. Chem. Soc. 2004, 126, 2664–2665.
(11) Ikeda, C.; Ueda, S.; Nabeshima, T. Chem. Commun. 2009, 2544–2546.
(12) Thoi, V. S.; Stork, J. R.; Magde, D.; Cohen, S. M. Inorg. Chem. 2006,
45, 10688–10697.
(5) Smalley, S. J.; Waterland, M. R.; Telfer, S. G. Inorg. Chem. 2009, 48,
13–15.
(6) Yadav, M.; Singh, A. K.; Maiti, B.; Pandey, D. S. Inorg. Chem. 2009,
48, 7593–7603.
(7) Hall, J. D.; McLean, T. M.; Smalley, S. J.; Waterland, M. R.; Telfer,
S. G. Dalton Trans. 2010, 39, 437–445.
(13) Galletta, M.; Campagna, S.; Quesada, M.; Ulrich, G.; Ziessel, R.
Chem. Commun. 2005, 4222–4224.
(14) Nastasi, F.; Puntoriero, F.; Campagna, S.; Diring, S.; Ziessel, R.
Phys. Chem. Chem. Phys. 2008, 10, 3982–3986.
(8) Yadav, M.; Singh, A. K.; Pandey, D. S. Organometallics 2009, 28,
4713-4723.
(9) Sutton, J. M.; Rogerson, E.; Wilson, C. J.; Sparke, A. E.; Archibald,
S. J.; Boyle, R. W. Chem. Commun. 2004, 1328–1329.
(15) Harriman, A.; Rostron, J. P.; Cesario, M.; Ulrich, G.; Ziessel, R.
J. Phys. Chem. A 2006, 110, 7994–8002.
(16) Bronner, C.; Baudron, S. A.; Hosseini, M. W.; Strassert, C. A.;
Guenet, A.; De Cola, L. Dalton Trans. 2010, 39, 180-184.
r
2010 American Chemical Society
Published on Web 06/08/2010
pubs.acs.org/IC

6078 Inorganic Chemistry, Vol. 49, No. 13, 2010
Hanson et al.
mixture of 2-ethoxyethanol and deionized water as previously
reported by Nonoyama.21
Synthesis of (C∧N)2Ir(dipyrrinato) Complexes. To a solution
of dipyrromethane (0.5 mmol) in 20 mL of dry tetrahydrofuran
(THF) was added (0.5 mmol) 2,3-dicloro-5,6-dicyano-1,4-ben-
zoquinone (DDQ) and allowed to stir at room temperature for
1 h. A large excess of potassium carbonate (1 g) was then added,
and the mixture was stirred for 15 min followed by the addition
of [(C∧N)2Ir(μ-Cl)]2 (0.25 mmol). The solution was then re-
fluxed under N2 overnight. After cooling to room temperature,
solids were removed by vacuum filtration and washed with
dichloromethane (3 ꢀ 50 mL). The collected filtrate was then
evaporated to dryness under reduced pressure. The crude pro-
duct was then passed through a silica gel column using dichlor-
omethane/hexane (9:1) as eluent. Solvent from the first orange
fraction was then evaporated to dryness under reduced pressure.
The pure product was precipitated with methanol (CH3OH),
collected by filtration, washed with CH3OH, and air-dried.
Figure 1. Structure of (C∧N)2Ir(5-R-dipy) complexes 1-7. See Scheme
1 for the isomers of 5.
long-lived excited states and highly efficient phosphorescent
emission. The high efficiency emission is attributed to inter-
system crossing from the singlet to the triplet excited state
facilitated by strong spin-orbit coupling of the iridium heavy
atom.17 Although a series of (η5-C5Me5)IrL(5-(4-R-Ph)dipy)
(L = Cl, phosphino, 2,20-bipyridyl; R = CN, NO2) com-
plexes have been recently prepared,8 these derivatives were
reported to be non-emissive. In this report we describe the
synthesis and characterization of a series of bis-cyclometa-
lated Ir(III) compounds containing dipyrrinato ligands
(Figure 1) that exhibit phosphorescence at room tempera-
ture. Through systematic variation of the cyclometalating
ligand and the meso substituent of the dipyrrin moiety, it was
found that the photochemical and electrochemical properties
were dictated by the dipyrrinato ligand.
20
Iridium(III) Bis(10-phenylpyrazolato-N,C )(dipyrrinato) (1).
1
Yield: 7% H NMR (400 MHz, CDCl3), δ 6.29 (dd, J = 4.0,
1.0 Hz, 2H), 6.39-6.41 (m, 4H), 6.78 (td, J = 7.5, 1.0 Hz, 2H),
6.86 (d, J = 1.0 Hz, 2H), 6.91 (dd, J = 7.5, 1.0 Hz, 2H), 6.94 (d,
J = 2.0 Hz, 2H), 6.97 (dd, J = 4.0 Hz, 2H), 7.17 (d, J = 8.0 Hz,
2H), 7.31 (s, 1H), 7.96 (d, J = 2.0 Hz, 2H). 13C NMR (100 MHz,
CDCl3) δ 106.9, 110.4, 116.8, 121.4, 125.1, 125.7, 130.5, 133.8,
134.2, 134.4, 137.7, 144.0, 151.8. Elemental analysis for
C27H21N6Ir: calcd: C 52.16, H 3.40, N 13.52; found: C 51.47,
H 3.40, N 12.72.
20
Iridium(III) Bis(10-phenylpyrazolato-N,C )(5-methyldipyrrinato)
(2). Yield: 13% 1H NMR (400 MHz, CDCl3), δ 2.76 (s, 3H), 6.30
(dd,J= 4.0, 1.0 Hz, 2H), 6.36 (dd, J=7.5, 1.0Hz, 2H), 6.39(t,J=
3.0 Hz, 2H), 6.77 (td, J = 7.5, 1.0 Hz, 2H), 6.89-6.93 (m, 6H), 7.18
(d, J = 8.0 Hz, 2H), 7.29 (dd, J = 4.0, 1.0 Hz, 2H), 7.94 (d, J = 3.0
Hz, 2H). 13C NMR (100 MHz, CDCl3) δ 19.4, 106.8, 110.4, 116.2,
121.2, 125.0, 125.7, 126.7, 134.1, 135.1, 137.6, 138.3, 143.8, 145.0,
151.1. Elemental analysis for C28H23N6Ir: calcd: C 52.90, H 3.65, N
13.22; found: C 52.75, H 3.54, N 12.93.
Experimental Section
Synthesis. Mesitylaldehyde, benzaldehyde, acetaldehyde,
pyrrole, pyrazole, trifluoroacetic acid, indium(III) chloride,
1-phenylpyrazole, 2-phenylpyridine, 2-phenylquinoline, (Aldrich),
paraformaldehyde (Fisher), 5-iodo-1,3-benzodioxole (Matrix
20
Iridium(III) Bis(10-phenylpyrazolato-N,C )(5-phenyldipyrrinato)
(3). Yield: 70% 1H NMR (400 MHz, CDCl3), δ 6.23 (dd, J = 2.5,
1.5 Hz, 2H), 6.39 (dd, J = 7.5, 1.0 Hz, 2H), 6.46 (t, J = 2.5 Hz, 2H),
6.49 (dd, J = 4.5, 1.5 Hz, 2H), 6.78 (td, J = 7.5, 1.0 Hz, 2H), 6.93
(td, J = 7.5, 1.5 Hz, 2H), 6.99 (t, J = 1.5 Hz, 2H), 7.00 (d, J = 1.5
Hz, 2H), 7.19 (dd, J = 8.0, 1.0 Hz, 2H), 7.37-7.47 (m, 5H), 7.99
(dd, J = 3.0, 1.0 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ 106.9,
110.5, 116.5, 121.3, 125.1, 125.8, 126.9, 127.9, 130.5, 130.9, 134.2,
135.1, 137.6, 137.9, 139.7, 143.9, 148.3, 152.1. Elemental analysis for
C33H25N6Ir: calcd: C 56.80, H 3.61, N 12.04; found: C 56.69, H
3.22, N 11.93.
Scientific), and IrCl3 nH2O (Next Chimica) were purchased from
3
the corresponding supplier (in parentheses) and used without
further purification. NMR spectra were recorded on a Bruker
AM 360 MHz or Varian 400 MHz NMR instrument as indicated,
and chemical shifts were referenced to residual protonated solvent.
Elemental analyses (CHN) were performed at the Microanalysis
Laboratory at the University of Illinois, Urbana-Champaign.
High-performance liquid chromatography (HPLC) analysis was
performed on a Shimadzu Prominance-LCMS 2020 equipped with
a column oven (T = 40 °C), a PDA photodetector, and a MS
spectrometer (LCMS 2020; m/z range: 0-2000; ionization modes:
ESI/APCI). HPLC was performed using an Inertsil ODS-3 C18 3 μ
3 ꢀ 150 mm column eluting with a 75:25 mixture of acetonitrile and
water.
20
Iridium(III) Bis(10-phenylpyrazolato-N,C )(5-mesityldipyrrinato)
(4). Yield: 73% 1H NMR (400 MHz, CDCl3), δ2.06 (s, 6H), 2.36 (s,
3H), 6.16 (dd, J = 4.0, 1.5 Hz, 2H), 6.38 (dd, J = 4.0, 1.5 Hz, 2H),
6.42-6.45 (m, 4H), 6.79 (td, J = 7.5, 1.0 Hz, 2H), 6.90-6.95 (m,
6H), 7.01 (d, J= 2.0 Hz, 2H), 7.19 (dd, J=8.0,1.0Hz,2H),7.99(d,
J = 2.0 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ 19.8, 21.1, 106.9,
110.5, 116.5, 121.3, 125.1, 125.8, 127.4, 129.3, 134.26, 134,28, 136.1,
136.3, 136.7, 137.5, 138.1, 144.0, 147.2, 151.6. Elemental analysis for
C36H31N6Ir: calcd: C 58.44, H 4.22, N 11.36; found: C 58.26, H 3.74,
N 11.02.
Dipyrromethane,18 5-methyl-dipyrromethane,18 5-phenyl-
dipyrromethane,19 and 5-mesityl-dipyrromethane19 were pre-
pared following procedures developed by Lindsey et al. 1-(4,5-
Methylenedioxopheny)pyrazole was prepared by Ullmann-type
coupling of 5-Iodo-1,3-benzodioxole with pyrazole.20 Cyclome-
talated Ir(III) dichloro-bridged dimers of general formula
Iridium(III) Bis[10-(4,5-methylenedioxophenyl)pyrazolato-N,
[(C∧N)2Ir(μ-Cl)]2 were synthesized by heating IrCl3 H2O to
20
C ](5-phenyldipyrrinato), Iridium(III) [10-(4,5-methylenedioxo-
3
20
110 °C with 2-2.5 equiv of cyclometalating ligand in a 3:1
phenyl)pyrazolato-N,C ][10-(4,5-methylenedioxophenyl)pyrazo-
60
lato-N,C ](5-phenyldipyrrinato), Iridium(III) Bis[10-(4,5-methy-
60
lenedioxophenyl)pyrazolato-N,C ](5-phenyldipyrrinato) (5(20,20), 5-
(17) Yersin, H. Highly efficient OLEDs with phosphorescent materials;
Wiley-VCH: Berlin, 2007.
(20,60), and 5(60,60), Respectively). Yield: 58% Product was further
purified by sublimation under reduced pressure. All characteriza-
(18) Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.;
Lindsey, J. S. Org. Process Res. Dev. 2003, 7, 779–812.
(19) Littler, B. J.; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea,
D. F.; Boyle, P. D.; Lindsey, J. S. J. Org. Chem. 1999, 64, 1391–1396.
(20) Klapars, A.; Antilla, J. C.; Huang, X.; Buchwald, S. L. J. Am. Chem.
Soc. 2001, 123, 7727–7729.
1
tion reported herein were performed on a sublimed sample. H
NMR (400 MHz, CDCl3), δ 7.90 (dd, J = 2.8, 0.8 Hz, 2H), δ 7.84
(21) Nonoyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 767–768.

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6079
(dd, J = 2.8, 0.8 Hz, 2H), δ 7.83 (dd, J = 2.8, 0.8 Hz, 2H), δ 7.76
(dd, J = 2.8, 0.8 Hz, 2H), 7.50-7.35 (m, 20H), 7.14 (t, J = 1.2 Hz,
2H), 7.12 (t, J = 1.2 Hz, 2H), 7.05 (t, J = 1.2 Hz, 2H), 7.03 (t, J =
1.2 Hz, 2H), 6.93 (dd, J = 2.8, 0.8 Hz, 2H), 6.91 (dd, J = 2.8, 0.8
Hz, 2H), 6.87 (dd, J = 2.8, 0.8 Hz, 2H), 6.85 (dd, J = 2.8, 0.8 Hz,
2H), 6.84 (s, 2H), 6.83 (s, 2H), 6.80(d, J= 8 Hz, 2H), 6.78 (d, J = 8
Hz, 2H), 6.52-6.48 (m, 8H), 6.46 (d, J = 8 Hz, 2H), 6.45 (d, J = 8
Hz, 2H), 6.41-6.38 (m, 4H), 6.31-6.28 (m, 4H), 6.28-6.23 (m,
8H), 5.88(d, J= 1.6 Hz, 2H), 5.87 (d, J= 1.6 Hz, 2H), 5.81 (d, J=
1.6 Hz, 2H), 5.80 (d, J= 1.6 Hz, 2H), 5.79 (s, 2H), 5.76 (s, 2H), 5.60
(d, J = 1.6 Hz, 2H), 5.58 (d, J = 1.6 Hz, 2H), 5.43 (d, J = 1.6 Hz,
2H), 5.40 (d, J = 1.6 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ
153.3, 152.6, 152.2, 152.1, 151.7, 148.3, 145.27, 145.0, 143.5, 143.3,
142.9, 142.8, 141.0, 140.4, 139.6, 139.5, 139.4, 138.3, 138.1, 137.2,
137.0, 136.4, 135.0, 134.9, 134.5, 134.4, 130.9, 130.8, 130.5, 130.4,
130.0, 128.6, 128.0, 127.9, 127.8, 126.9, 124.9, 124.6, 124.0, 123.7,
116.8, 116.7, 116.4, 116.3, 116.1, 114.7, 113.3, 113.1, 106.9, 106.7,
106.0, 105.8, 104.2, 103.9, 101.5, 101.4, 100.4, 100.3, 99.6, 99.5, 93.9,
93.7. Elemental analysis for C35H25N6O4Ir: calcd: C 53.49, H 3.21,
VWR) was used as the solvent under inert atmosphere with
0.1 M tetra(n-butyl)ammonium hexafluorophosphate (Aldrich)
as the supporting electrolyte or a solution of anhydrous 0.1 M
NBu4ClO4 in CH2Cl2 (VWR) as indicated. A glassy carbon rod,
a platinum wire, and a silver wire were used as the working
electrode, the counter electrode, and the pseudo reference
electrode, respectively. Electrochemical reversibility was estab-
lished using CV, while all redox potentials were determined
using DPV and reported relative to a ferrocenium/ferrocene
(Fcþ/Fc) redox couple used as an internal standard.24
The UV-visible spectra were recorded on a Hewlett-Packard
4853 diode array spectrophotometer. Steady state emission
experiments at room temperature and 77 K were performed
using a Photon Technology International QuantaMaster Model
C-60SE spectrofluorimeter. Phosphorescence lifetime measure-
ments were performed by a time-correlated single-photon
counting method using an IBH Fluorocube lifetime instrument
by equipped with a 405 nm LED excitation source. Quantum
efficiency measurements were carried out using a Hamamatsu
C9920 system equipped with a xenon lamp, calibrated integrat-
ing sphere and model C10027 photonic multichannel analyzer.
Computational Methods. All calculations were performed
using the Titan software package (Wavefunction, Inc.). The
gas phase geometry optimizations were calculated using B3LYP
functional with the LACVP** basis set as implemented in Titan.
The energy levels and orbital diagrams of the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) were obtained from the optimized geometry of
the singlet state. The spin-density of the triplet state was
calculated from the energy minimized triplet geometries.
N 10.69; found: C 53.16, H 2.84, N 10.42.
Iridium(III) Bis(2-phenylpyridinato-N,C2 )(5-mesityldipyrrinato)
0
1
(6). Yield: 66% H NMR (360 MHz, CDCl3), δ 7.89 (ddd, J =
5.9, 1.5, 0.7 Hz, 2H), 7.80 (dd, J = 8.05, 5.9 Hz, 2H), 7.61-7.54 (m,
4H), 6.90 (ddd, J = 7.8, 7.3, 1.5 Hz, 2H), 6.88 (s, 2H), 6.85 (ddd,
J = 7.6, 5.8, 1.5 Hz, 2H), 6.81 (ddd, J = 7.8, 7.3, 1.5 Hz, 2H), 6.71
(dd, J = 1.7, 1.2 Hz, 2H), 6.41 (dd, J = 7.6, 1.2 Hz, 2H), 6.37 (dd,
J = 4.2, 1.5 Hz, 2H), 6.14 (dd, J = 4.4, 1.2 Hz, 2H), 2.34 (s, 3H),
2.02 (s, 6H); 13C NMR (90 MHz, CDCl3) δ 168.8, 156.8, 151.8,
149.6 (2C), 147.3, 144.5, 136.7, 136.2, 135.9, 133.6, 132.3, 129.5
(2C), 127.4, 123.8, 121.6, 120.7, 118.6, 116.9. Elemental analysis for
C40H33N4Ir: calcd: C 63.05, H 4.37, N 7.35; found: C 62.90, H 4.11,
Device Fabrication. Prior to device fabrication, indium/tin
oxide (ITO) on glass was patterned as 2-mm-wide stripes with a
resistivity of 20 Ω 0-1. The substrates were cleaned by sonica-
tion in a soap solution, rinsed with deionized water, boiled in
trichloroethylene, acetone, and ethanol for 5-6 min in each
solvent, and dried with nitrogen. Finally, the substrates were
N 7.37.
0
Iridium(III) Bis(2-phenylquinalato-N,C2 )(5-mesityldipyrrinato)
(7). Yield: 71.5% 1H NMR (360 MHz, CDCl3), δ 8.06 (dd, J =
10.3, 8.8 Hz, 2H), 8.03 (dd, J = 10.3, 8.8 Hz, 2H), 7.81 (d, J = 3.4
Hz, 2H), 7.79 (d, J = 4.6 Hz, 2H), 7.66 (dd, J = 8.1, 1.5 Hz, 2H),
7.31 (dd, J = 8.1, 7.3 Hz, 2H), 7.06 (ddd, J = 9.3, 7.8, 1.5 Hz, 2H),
6.96 (dd, J = 8.3, 7.3 Hz, 2H), 6.72 (s, 2H), 6.70 (dd, J = 8.3, 7.3
Hz, 2H), 6.61 (d, J = 1.9 Hz, 2H), 6.47 (d, J = 7.6 Hz, 2H), 6.24
(dd, J = 4.4, 1.0 Hz, 2H), 6.11 (dd, J = 4.2, 1.0 Hz, 2H), 2.26 (s,
3H), 1.45 (s, 6H); 13C NMR (90 MHz, CDCl3), δ 171.21, 164.84,
157.93, 150.06, 149.18, 147.30, 146.930, 138.22, 136.50, 136.44,
136.09, 134.49, 133.94, 130.26, 129.80, 129.65, 127.94, 127.70,
127.47, 127.14, 125.86, 125.71, 120.86, 117.06, 21.02, 19.23. Ele-
˚
treated with UV ozone for 10 min. Layers of NPD (400 A), 10%
˚
3 or 5 doped into Alq3 (250 A) and BCP (400 A) were vapor-
˚
deposited onto the substrates in a high-vacuum chamber.
˚
˚
Lithium fluoride (10 A) and aluminum (1200 A) were then
vapor-deposited onto the substrates through a shadow mask,
that defined four devices per substrate with a 2-mm2 active area
each, in the same high-vacuum chamber. The devices were tested
within 3 h of fabrication. The electrical and optical intensity
characteristics of the devices were measured with a Keithly 2400
source/meter/2000 multimeter coupled to a Newport 1835-C
optical meter, equipped with a UV-818 Si photodetector. Only
light emitting from the front face of the device was collected and
used in subsequent efficiency calculations. The electrolumines-
cent (EL) spectra were measured on a PTI QuantaMaster model
C-60SE spectrofluorimeter, equipped with a 928 PMT detector
and corrected for detector response.25 The EL intensity was
found to be uniform throughout the area of each device.
mental analysis for C48H37N4Ir CH2Cl2: calcd: C 62.15, H 4.15, N
3
5.92; found: C 62.57, H 4.05, N 6.08.
X-ray Crystallography. Diffraction data for compounds 3, 4,
and 5(60,60) were collected on a Bruker SMART APEX CCD
diffractometer with graphite monochromated Mo KR radiation
˚
(λ = 0.71073 A). The cell parameters for the complexes were
obtained from a least-squares refinement of the spots (from 60
collected frames) using the SMART program. One hemisphere
of crystal data for each compound was collected up to a
˚
resolution of 0.80 A, and the intensity data were processed using
the Saint Plus program. All of the calculations for the structure
determination were carried out using the SHELXTL package
(Version 5.1).22 Absorption corrections were applied by using
SADABS.23 In most cases, hydrogen positions were input and
refined in a riding manner along with the attached carbons. A
summary of the refinement details and the resulting factors are
given in Supporting Information, Table 1.
Results and Discussion
Synthesis and Structure. The bis-cyclometalated Ir(III)
dipyrrinato complexes (Figure 1) were prepared by a
modified “one-pot” procedure introduced by Lindsey
for bis(dipyrrinato)metal complexes.26 In a stepwise manner,
dipyrromethane was first oxidized using DDQ followed by
sequential addition of K2CO3 and cyclometalated Ir(III)
Electrochemical and Photophysical Characterization. Cyclic
voltammetry (CV) and differential pulse voltammetry (DPV)
were performed using an EG&G Potentiostat/Galvanostat
model 283. N,N-dimethylformamide (DMF, purchased from
(24) Gagne, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19,
2854–2855.
(25) Forrest, S. R.; Bradley, D. D. C.; Thompson, M. E. Adv. Mater.
2003, 15, 1043–1048.
(26) Yu, L.; Muthukumaran, K.; Sazanovich, I. V.; Kirmaier, C.; Hindin,
E.; Diers, J. R.; Boyle, P. D.; Bocian, D. F.; Holten, D.; Lindsey, J. S. Inorg.
Chem. 2003, 42, 6629–6647.
(22) Sheldrick, G. M. SHELXTL, Version 5.1; Bruker Analytical X-ray
System, Inc: Madison, WI, 1997.
(23) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33–38.

6080 Inorganic Chemistry, Vol. 49, No. 13, 2010
Hanson et al.
Scheme 1
ligands in boron,31 zinc,26 and iron32 complexes. Instead,
the dipyrrinato ligand is bent in a manner similar that
seen in Co(III)33 and Pd(II)7,34 complexes. This distortion
of the pyrrole rings is defined here using a fold-angle
between the planes formed by the nitrogen, R and β
carbon atoms adjacent to the C5 carbon (Figure 1 and
Supporting Information, Figure S17). Fold-angles of
17.9(2)° and 17.7(5)° are found in complexes 3 and
5(60,60), respectively, while 4 contains two unique mole-
cules in the asymmetric unit cell, one with a bent dipyrri-
nato ligand (fold-angle = 11.1(9)°), the other near planar
(fold-angle = 1.0(8)°). The folding distortion is accom-
panied by a tilt of the dipyrrinato ligand away from a
square plane composed of the two coordinated carbons,
iridium center, and two Npyr atoms. This tilt-angle can be
quantified using two planes, one defined by the Ir and two
Npyr atoms versus the other that includes the two Npyr
atoms and the C5 carbon (see Supporting Information,
Figure S18). The tilt-angles were found to be 17.3(1)° in 3,
22.6(2)° in 5(60,60), 14.3(3)° in 4 (bent), and 0.3(3)° in 4
(planar). In addition to the buckling and tilting distor-
tions, a twist of the pendant aryl ring away from an
orthogonal conformation was observed. The aryl ring is
canted at an angle of 67.5(3)° in 3 and 72.3(5)° in 5(60,60),
74.0(7)° in 4 (bent) and 78.0(7)° in 4 (planar). These out-
of-plane distortions in the dipyrrinato ligand appear to
require low energy and are likely caused by crystal pack-
ing forces since both planar and nonplanar forms of the
ligand are present in the structure of 4.
dichloro-bridged dimers to give complexes 3-7 in 50-75%
yield. Using the same synthetic method, complexes 1 and 2
were obtained in 5-15% yield. The lower yields for com-
plexes 1 and 2 (comparable to those found for the boron
difluoride analogues27,28) are likely due to thermal instabi-
lity29 of the dipyrrin intermediate under the given reaction
conditions (refluxing THF).
All of the complexes were fully characterized by 1H and
13C NMR spectroscopy and elemental analysis. The
relatively simple NMR spectra of complexes 1-4, 6, and 7
are indicative of a C2-symmetric species having the same
trans-N disposition for the C∧N chelate as the dimeric Ir
starting material. However, the NMR spectra of (ooppz)2-
Ir(5-Ph-dipy) (5) was more complex. Analysis by HPLC-MS
indicated the presence of three species, in an approximate
1:2:1 ratio, which had identical mass and similar absorption
spectra (Supporting Information, Figure S16). Using a com-
bination of 1H-1H COSY (Supporting Information, Figure
S10) and H-1H decoupled NMR (Supporting Informa-
1
DFT Calculations. Density functional theory (DFT)
calculations of complex 1-7 were performed using the
B3LYP functional with the LACVP** basis set. Theore-
tical investigation of cyclometalated iridium complexes
using DFT calculations has repeatedly demonstrated
good correlation with experimental observations.35-38
Metrical parameters for the geometry-optimized struc-
tures of 3, 4, and 5(60,60) are well-correlated with equiva-
lent bond lengths and angles obtained by X-ray crystal-
lography. For example, the calculated bond lengths
1
tion, Figure S11), along with comparison to H NMR
spectra of complexes 1-4, three regioisomeric forms of
5 were established: bis(20-ooppz)Ir(5-Ph-dipy) 5(20,20), (20-
ooppz)(60-ooppz)Ir(5-Ph-dipy) 5(20,60), and bis(60-ooppz)Ir-
(5-Ph-dipy) 5(60,60) (Scheme 1). The statistical ratio of the
regioisomers indicates that there is no preference for either the
20 or 60 positions during cyclometalation of the ooppz ligand.
Crystals of complexes 3, 4, and 5(60,60) suitable for
X-ray diffraction analysis were obtained by zone subli-
mation. The structures show ligands arranged in a pseu-
do-octahedral geometry around the metal center with the
expected trans-configuration of pyrazolyl groups (Figure 2).
Bond lengths and angles for the C∧N ligands are comparable
to those found in literature for similar heteroleptic complexes
(Supporting Information, Table S2).30 Likewise, the bond
lengths and angles of the dipyrrinato moiety are similar to
those of other metal dipyrrinato-based complexes. The
iridium-pyrrolic nitrogen (M-Npyr) bond lengths (Ir-Npyr
˚
˚
(Ir-Npyr = 2.18 A, Ir-Npz = 2.05 A, and Ir-CPh
=
˚
2.05 A) and bond angles (Npyr-Ir-Npyr =86.1°, Npz-
Ir-Npz = 174.4°, and CPh-Ir-Npz = 79.5°) for complex
3 are within 0.06 A and 1° of the X-ray structure
˚
(Supporting Information, Table S2). One notable differ-
ence is that the dipyrrinato ligand is planar (fold-angle =
0°) in the calculated geometries, as opposed to bent in the
crystal structure.
For complexes 1-4, 6, and 7, the calculations show that
both valence orbitals are localized on the dipyrrinato
5
˚
≈2.11 A) are comparable to those found in the (η -C5Me5)Ir-
(Ph3P)(5-(4-NO2Ph)dipy) complex (Ir-N = 2.074(5) and
2.090(5)). Similar distances have also been observed in
(ppy)Pt(5-mesityldipy) complex where the Pt-Npyr bond
(31) Li, F.; Yang, S. I.; Ciringh, Y.; Seth, J.; Martin, C. H., III; Singh,
D. L.; Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am.
Chem. Soc. 1998, 120, 10001–10017.
(32) Cohen, S. M.; Halper, S. R. Inorg. Chim. Acta 2002, 341, 12–16.
(33) Telfer, S. G.; Wuest, J. D. Cryst. Growth Des. 2009, 9, 1923–1931.
(34) March, F. C.; Couch, D. A.; Emerson, K.; Fergusso., J. E.; Robinson,
W. T. J. Chem. Soc., A 1971, 440–448.
˚
˚
lengths are 2.091(6) A and 2.027(6) A.
The two pyrrole rings in complexes 3, 4, and 5(60,60) are
not coplanar, unlike what is observed for dipyrrinato
(35) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.;
Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125,
7377–7387.
(36) Sajoto, T.; Djurovich, P.; Tamayo, A. B.; Oxgaard, J.; Goddard,
W. A., III; Thompson, M. E. J. Am. Chem. Soc. 2009, 131, 9813–9822.
(37) Nie, D.; Liu, Z.; Bian, Z.; Huang, C. J. Mol. Struct., THEOCHEM
2008, 861, 97–102.
(27) Tram, K.; Yan, H.; Jenkins, H. A.; Vassiliev, S.; Bruce, D. Dyes
Pigm. 2009, 82, 392–395.
(28) Schmitt, A.; Hinkeldey, B.; Wild, M.; Jung, G. J. Fluoresc. 2009, 19,
755–758.
(29) Van Koeveringe, J. A.; Lugtenburg, J. Rec. Trav. Chim. Pays-Bas
1977, 96, 55–58.
(30) Tamayo, A. B.; Garon, S.; Sajoto, T.; Djurovich, P. I.; Tsyba, I. M.;
Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 8723–8732.
(38) Lowry, M. S.; Hudson, W. R.; Pascal, R. A., Jr.; Bernhard, S. J. Am.
Chem. Soc. 2004, 126, 14129–14135.

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6081
Figure 2. ORTEP drawings of compounds 3, 4, and 5(60,60). (carbon (black), nitrogen (blue), oxygen (red), iridium (purple)). A second unique structure
for 4 found in the unit cell is not shown.
Figure 3. (a) Qualitativeorbitalenergy diagram illustratingthe HOMO (transparent) and LUMO(mesh) orbitalsof3and 5(20,20), 5(20,60), and 5(60,60). All
values reported in eV. (b) Triplet spin-density surface of 3.
Table 1. Calculated HOMO/LUMO Values and Oxidation/Reduction Potentials
for Complexes 1-7
ligand. A representative example of the HOMO and
LUMO is illustrated for complex 3 in Figure 3a. Roughly
calculationa
electrochemistryb
similar HOMO (-4.96 to -5.01 eV) and LUMO (-1.71
to -1.79 eV) energies are also calculated for these deri-
vatives (Table 1). Likewise, the LUMOs for the three
regioisomers of 5 have similar energy and spatial distri-
bution as the other complexes. However, the HOMOs
differ depending on the site of metalation. Cyclometala-
tion at the 20 position of the ooppz ligand has minimal
affect on the location/energy of the HOMO in 5(20,20),
whereas cyclometalation at the 60 position in 5(60,60) and
5(20,60) shifts the HOMO from dipyrrinato to the 60-
ooppz ligand (Figure 3a). The change in location of the
HOMO also shifts the orbital energy to more positive
values (-4.73 eV). However, these differences in the
HOMO character are not reflected in the triplet state. A
representative example of the spin density distribution for
the HOMO and LUMO is illustrated in Figure 3b for
complex 3. All of the complexes have nearly identical spin
density contours that are localized on the dipyrrinato
ligand; little-to-no spin density is observed on either the
C∧N ligand or the iridium atom.
HOMO LUMO
(eV)
ΔE
(V)
complex
(eV)
Eox (V)
E1/2red (V)
1
2
3
4
-4.99
-4.97
-4.98
-4.98
-4.73
-4.72
-5.01
-4.99
-4.96
-1.72
-1.71
-1.76
-1.79
-1.77
0.52
0.52
0.53
0.55
-1.95
-1.98
-1.96
-1.89
2.47
2.50
2.49
2.44
5(60,60)
5(20,60)
5(20,20)
6
-1.75 0.39, 0.56c
-1.79
-1.92 c
2.31
-1.80
-1.79
0.51
0.56
-1.91
2.42
7
-2.70, -2.46, -1.96 2.52
a B3LYP/LACVP**. b Redox measurements were performed in an
anhydrous 0.1 M NBu4PF6 DMF solution and reported relative to
internal Fcþ/Fc. c Performed on a mixture of regioisomers of 5 in an
anhydrous 0.1 M NBu4ClO4 CH2Cl2 solution.
reduction process associated with the dipyrrinato ligand.
The assignment is consistent with the similarity in the
reduction potentials for 1-7 (Table 1) and supported by
the results from DFT calculations which show closely
related LUMO energies for all of the complexes. Two
additional reversible reduction peaks were observed with
complex 7 at higher potential (-2.46 V and -2.70 V).
These processes are assigned to the reduction of the two
quinolinyl moieties since they occur at potentials similar to
values reported for the (pq)2Ir(acetylacetonate) complex.40
Complexes 1-4, 6, and 7 display irreversible oxidation
waves from 0.51 to 0.56 V. Irreversible or quasi-reversible
Electrochemistry. The electrochemical properties of the
complexes 1-7 were investigated using cyclic voltamme-
try and differential pulsed voltammetry; results of these
measurements are listed in Table 1. The complexes all
display a reversible reduction wave between -1.89 V and
-1.96 V. These potentials are much less cathodic than
what is found for related Ir(C∧N)2(acetylacetonate) com-
red
plexes (E1/2 = -3.1 V to -2.45 V)39 and indicate a
(39) Fei, T.; Gu, X.; Zhang, M.; Wang, C.; Hanif, M.; Zhang, H.; Ma, Y.
(40) Wu, F.-I.; Su, H.-J.; Shu, C.-F.; Luo, L.; Diau, W.-G.; Cheng, C.-H.;
Synth. Met. 2009, 159, 113–118.
Duan, J.-P.; Lee, G.-H. J. Mater. Chem. 2005, 15, 1035–1042.

6082 Inorganic Chemistry, Vol. 49, No. 13, 2010
Hanson et al.
Table 2. Photophysical Properties of Complexes 1-7
emission at rtb
emission at 77 Kb
complex
absorbance λ (nm) (ε, ꢀ104 M-1 cm-1 a
228 (3.13), 245 (2.94), 311 (0.74), 474 (3.31) 672
229 (3.67), 243 (3.20), 310 (0.82), 480 (3.80) 658
229 (3.67), 244 (3.48), 304 (1.36), 481 (3.52) 683 (675) 5.3 (8.1)
228 (4.58), 245 (3.46), 310 (0.76), 483 (3.59) 676 (673) 12.7 (12.6) 0.115 (0.116) 0.91 (0.92)
229 (4.37), 260 (2.88), 300 (2.17), 481 (3.53) 685
246 (3.48), 342 (0.85), 404 (0.94), 483 (3.84) 677
269 (5.03), 342 (1.81), 388 (0.97), 485 (3.60) 675
)
λmax(nm)
τ (μs)
12.9
9.9
ΦPL
kr(104 s-1 c
)
knr(105 s-1 d
)
λ
max(nm)
τ (μs)
1
2
3
4
5e
6
7
0.075
0.094
0.58
0.95
0.72
0.92
1.8 (1.1)
0.70 (0.70)
2.2
661
644
668
664
670
665
659
23.1
22.3
16.0
22.4
16.0
12.9
18.7
0.060 (0.092) 1.1 (1.1)
4.3
6.3
9.6
0.060
0.099
0.092
1.4
1.6
0.96
1.4
0.95
d
a In CH2Cl2. b In 2-MeTHF deaerated with N2. Data in parentheses recorded in PMMA (2% w/w). c kr = Φ/τ. knr = (1 - Φ)/τ. e Mixture of
regioisomers.
oxidative processes have also been observed in Fe(5-
Ph-dipy)3,32 Zn(5-Ph-dipy)2,26 and (η5-C5Me5)IrCl(5-(4-
cyanoPh)dipy)8 at similar potentials. On the other hand,
bis- and tris-(C∧N) Ir(III) complexes typically undergo a
one-electron, reversible oxidation assigned to the metal-
aryl ligand portion of the molecules.35,39,41 The irrever-
sible response for 1-4, 6, and 7, and close similarity in
potential to other dipyrrinato containing materials, sug-
gests that oxidation is associated with the dipyrrinato
ligand. The DFT calculations support this interpretation
as they show that the HOMO of 1-4, 6, and 7 is localized
on the dipyrrinato ligand (Figure 3). In contrast, a
regioisomeric mixture of 5 displays a reversible oxidation
wave at a potential ∼130 mV more negative than that of
Figure 4. Absorption (in CH2Cl2, filled symbol) and emission (in
2-MeTHF, open symbol) spectra of 6 (square) and 7 (circle) at room
temperature.
the other complexes (Table 1). Further, a small, irrever-
sible oxidation wave can be resolved at 0.56 V in CH2Cl2
(Supporting Information, Figure S19). The data corre-
spond well with the HOMO energies obtained by DFT
calculations. Both 5(60,60) and 5(20,60) have HOMOs
localized on the C∧N, rather than the dipyrrinato ligand
(Figure 3) and HOMO energies (-4.73 eV) that are
destabilized compared to the other complexes (-4.96
eV to -5.01 eV). For these two isomers, the data indicate
that the initial oxidation process is localized on the C∧N
ligand. The second smaller peak at 0.56 V is tentatively
assigned to oxidation of the 5(20,20) regioisomer since the
calculations show the HOMO is located at a similar
location (dipyrrinato) and energy (-5.01 eV) as com-
plexes 1-4, 6, and 7. The lower intensity of this second
oxidation wave can be attributed to the lower abundance
of the 5(20,20) isomer (∼25%) relative to the other two
isomers of 5.
Electronic Spectroscopy. Absorption and emission data
for complexes 1-7 recorded at room temperature and 77
K are summarized in Table 2. Representative spectra are
shown in Figure 4. The absorption spectra of the com-
plexes in dichloromethane show intense bands (ε > 104
M-1 cm-1) between 200-400 nm that are assigned to
both spin-allowed π-π* ligand-centered (LC) and metal-
to-ligand charge transfer (MLCT) transitions associated
with the C∧N ligand (Supporting Information, Figure
S20). A distinct feature is a very intense absorption band
(ε∼3.3-4.0 ꢀ 104 M-1 cm-1) extending from 400 to 540 nm.
This band has similar absorption intensity and wavelength to
boron- and metal-dipyrrinato dyes5,26,42-44 and is thus as-
signed to the 1π-π* ligand-centered transition of the dipyrri-
nato ligand. The assignment is further supported by com-
paring derivatives having either different C∧N ligands (4-7)
or substituents at the C5 position of the dipyrrinato chromo-
phore (1-4). Variation in the C∧N ligand has minimal affect
on the absorption wavelength and intensity of the peak at
∼480 nm in complexes 4-7 (Figure 4 and Supporting
Information, Figure S20b). However, a small red shift (∼7
nm) in the peak is observed when the parent dipyrrinato
ligand in 1(λmax = 474 nm) is substituted with either a methyl
or aryl group (2-4). Another feature associated with the
dipyrrinato ligand can be observed near 310 nm in complex 3,
where the absorptivity of this peak is nearly double that of 4
(Table 2 and Supporting Information, Figure S20a). A similar
enhanced absorbance found in analogous BF2-dipyrrinato
compounds has been attributed to an in-plane rotation of the
phenyl group, which increases probability of this transition.45
All complexes display vibronically structured lumines-
cence in fluid solution at room temperature and in glassy
media at 77 K. The emission peak maxima fall in a narrow
range at both room temperature (658-685 nm) and 77 K
(644-670 nm), with microsecond lifetimes (room tem-
perature, 5-13 μs; 77 K, 12-23 μs) at 77 K. Lower energy
vibrational modes extend out past 800 nm (Figure 4 and
Supporting Information, Figure S21). For complexes
1-7, large apparent Stokes shifts (5500-6200 cm-1
)
(41) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.;
Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg.
Chem. 2001, 40, 1704–1711.
(42) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891–4932.
(43) Wagner, R. W.; Lindsey, J. S. Pure Appl. Chem. 1996, 68, 1373–1380.
(44) Bruckner, C.; Zhang, Y.; Rettig, S. J.; Dolphin, D. Inorg. Chim. Acta
1997, 263, 279–286.
(45) Kee, H. L.; Kirmaier, C.; Yu, L.; Thamyongkit, P.; Youngblood,
W. J.; Calder, M. E.; Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt, W. R.;
Birge, R. R.; Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2005, 109, 20433–
20443.

Article
Inorganic Chemistry, Vol. 49, No. 13, 2010 6083
and long excited-state lifetimes are indicative of phos-
phorescence. In contrast, metal- and boron-containing
dipyrrinato complexes typically display fluorescent emis-
sion centered around 510 nm with only small Stokes shifts
(<1000 cm-1) from the lowest energy absorption
band.26,42,45
deaerated solution. A pronounced difference in efficiency
is seen between 5-phenyl substituted complexes 3 and 5
(Φ = 0.06) versus the 5-mesityl derivatives 4, 6, and 7
(Φ = 0.09-0.115). A related, albeit much larger, increase in
quantum yield has also been observed in fluorescent zinc
bis-dipyrrinato complexes when the phenyl group (Φ =
0.006) is replaced with a mesityl group (Φ = 0.36).10
Detailed studies by Li et al. on related boron compounds
concluded that the reduced efficiency is due to rotation of
the aryl ring, accompanied by a corresponding puckering
distortion of the dipyrrinato ligand. The distortion results
in an excited-state conformer that undergoes facile non-
radiative deactivation to the ground state.31,45 A similar
deactivation pathway, albeit less pronounced, is most
likely active in the aryl substituted complexes 3-7. This
conclusion is supported by the lower efficiency and short-
er lifetimes in 3 and 5 as compared to 4, 6, and 7. The
effects of aryl rotation can also be observed when com-
paring the differing response of emission behavior to
solvent rigidity. While the excited state lifetimes of 4, 6,
and 7 double upon cooling to 77 K, a 3-fold increase in
lifetime is found for 3 and 5, presumably because of the
rigid environment hindering rotation of the phenyl group.
Similar behavior is also observed at room temperature
when the photophysical properties of complexes 3 and 4
are examined in rigid media (polymethylmethacrylate,
PMMA, Supporting Information, Figure S22). The ra-
diative rates (kr) of both 3 and 4 remain constant upon
doping in PMMA (2%, w/w). However, while the non-
radiative rate (knr) of the mesityl derivative (4) is un-
changed in PMMA, the knr of the phenyl derivative (3) is
lowered from 1.8 to 1.1 ꢀ 105 s-1 (Table 2) resulting in an
increased efficiency (Φ = 0.092). Again, this behavior
coincides with the rigid media inhibiting the rotation of
the phenyl ring and thus, decreasing the rate of non-
radiative deactivation to the ground state.
Complexes 1-7 have relatively high non-radiative rates
(kr ∼1 ꢀ 105 s-1) and display only a minimal increase
(ꢀ2-3) in lifetime upon cooling from room temperature
to 77 K. Similar behavior has also been observed for the
red phosphor, bis[2-(20-benzothienyl)pyridinato-N,C30]-
iridium acetylacetonate, and is attributed to an intrinsic
temperature-independent non-radiative decay process.47,48
Temperature-independent non-radiative processes typically
involve geometric distortions caused by low energy molecu-
lar vibrations of the emissive ligand.
Dipyrrins can roughly be described as being half of a
porphyrin ring, and the triplet emission energies for com-
plexes 1-7 are comparable to those of Pt(porphyrins).49
Despite this similarity, phosphorescent emission from
Ir(5-Ph-dipy) 3 (Φ = 0.06) is less than one-third as efficient
than from platinum tetraphenylporphyrin (PtTPP, Φ =
0.19).49 A comparison of radiative and non-radiative rates
between 3 and PtTPP shows higher values for both rates in 3
(kr =1.1ꢀ 104 s-1;knr =1.8ꢀ 105 s-1) thaninPtTPP(kr =
3.5 ꢀ 103 s-1; knr = 1.5 ꢀ 104 s-1).49 The difference in
quantum yields is therefore caused by the 10-fold higher knr
values in 3. A likely origin of the high non-radiative rates in 3
is the out-of-plane distortion of the pyrrole rings observed in
The Ir complexes all emit at similar wavelength regard-
less of the cyclometalating ligand. This is best exemplified
by comparing the emission spectra of the (ppy)2Ir and
(pq)2Ir dipyrrinato complexes (6 and 7) to analogous
derivatives coordinated with diketonate ligands. Bis-cy-
clometalated iridium acetylacetonate complexes emit from
a triplet state with 3LC/MLCT character at energies that are
strongly dependent on the nature of the cyclometalat-
ing ligand. As a result, emission maxima of (ppy)2Ir(acac)
(λmax = 516 nm) and (pq)2Ir(acac) (λmax = 597 nm) differ
considerably.41 In contrast, the emission maxima of 6 and 7
are similar (677 and 675 nm) as can be seen in Figure 4.
Small variations in λmax are found with differing sub-
stituents on the dipyrrinato ligand. Both the parent
complex 1 and mesitylene derivatives 4, 6, and 7 emit at
λ
em = 672-677 nm, whereas a small blue-shift (λem = 658
nm) and red-shift (λem = 683-685 nm) were observed for
the methyl (2) and the phenyl substituted (3 and 5)
complexes, respectively. The absence of any effect of the
cyclometalating ligand, and the influence of the R sub-
stituent on λmax, indicate that emission originates from
the dipyrrinato ligand. Phosphorescence from the dipyr-
rinato ligand is further supported by the fact that cyclo-
metalation at the 20- or 60-positions in 5 has minimal affect
on the photophysical properties of the complexes. The
emission spectrum for a regioisomeric mixture of 5 has
a near identical line shape to that of 3. The measured
lifetime for the regioisomeric mixture of 5 can also be
accurately fit to a single exponential decay, which implies
identical decay constants for the excited state of each
isomer. The assignment is consistent with DFT calcula-
tions that show the lowest triplet state localized on the
dipyrrinato ligand (Figure 3b).
Phosphorescence from compounds containing a dipyr-
rinato ligand has rarely been directly observed. The
phosphorescent spectra from BF2(dipy) compounds has
been reported at 77 K with the addition of iodoethane as a
heavy atom solvent.15 Triplet emission at 77 K has also
been observed in a multicomponent BF2(dipy)-M(II)
terpyridine complexes because of charge-separation fol-
lowed by recombination (M = Ru),13 or by triplet energy
3
transfer (M = Pt),14 resulting in a π-π* excited state
localized on the dipyrrinato moiety. Similarly, a boron
dipyrrinato chromophore appended to a (ppy)2Ir(bpy)
complex has been shown to display phosphorescence at
77 K through triplet energy transfer from the excited state
of the Ir fragment.46 For the series of complexes reported
here, direct coordination of the dipyrrinato ligand to
iridium, as in the previously reported (C∧N)Pt(dipy)
complexes,16 facilitates efficient intersystem crossing to
the triplet state and thus, phosphorescence from the
dipyrrinato ligand is observed at room temperature.
The luminescent quantum efficiencies (Φ) of complexes
1-7 range from 0.06 to 0.115 at room temperature in
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6084 Inorganic Chemistry, Vol. 49, No. 13, 2010
Hanson et al.
Figure 5. Luminance (red, cd/m2) and current density (black, mA/cm2)
as a function of voltage (V) for OLEDs using compound 3 (filled squares)
and 5 (open triangles).
Figure 6. EL spectra of OLEDs using dopants 3 and 5.
Conclusion. In summary, a series of dipyrrinato-based
bis-cyclometalated Ir(III) complexes has been synthesized
and characterized. Theelectrochemical, spectroscopic, and
electroluminescent properties were examined. The oxida-
tion and reduction potentials of this series of molecules,
with one exception, were found to be centered on the
dipyrrinato ligand. Cyclometalation at the 60-position of
ooppz ligand results in a shift in the location of the HOMO
from dipyrrinato to the cyclometalating ligand, and thus
the oxidation potential is shifted to more negative poten-
tials relative to the other molecules. All of the complexes
have high molar absorptivity (104 M-1 cm-1) at similar
wavelengths (∼480 nm) and exhibit phosphorescent emis-
sion at room temperature in the deep red part of the visible
spectrum (658-685 nm) with quantum efficiencies ranging
from 0.06 to 0.115. OLEDs made using two of the com-
plexes as dopants had external quantum efficiencies of 0.6
and 1.0%.
Despite the modest quantum efficiency, particularly
with respect to other cyclometalated iridium complexes,
these derivatives provide efficient phosphorescence from
a dipyrrinato ligand at room temperature. The (C∧N)2-
Ir(dipy) complexes have several advantages not present in
the fluorescent dipyrrinato analogues. (1) Selective mon-
itoring of emission with no absorption spectra overlap is
possible because of the large Stokes shift between absorp-
tion and emission maxima. (2) The red-shifted phosphor-
escent emission (λmax ≈ 680 nm) is in the biological tissue
window (650-900 nm) making these complexes possible
candidates for biological labeling applications. (3) Effi-
cient triplet excited state formation upon photoexcita-
tion is an important step in singlet oxygen formation
for both oxygen sensing and photodynamic therapy. (4)
The high energy absorption wavelength and oxida-
tion potential of the complexes can easily be modified
by changing the cyclometalating ligand with minimal
affect on the photophysical properties of the dipyrrinato
ligand.
the crystal structures of 3, 4, and 5(60,60) (Figure 2). The
presence of both a planar and nonplanar form of the
dipyrrinato ligand in the unit cell of 4 suggests a low energy
barrier for this type of distortion. For Pt(TPP), on the other
hand, the ability to traverse such a large amplitude distortion
is limited by the structural rigidity of the porphyrin ring
system.
OLED. Organic light emitting diodes (OLEDs) using
complexes 3 and 5 as emissive dopants were fabricated by
high-vacuum thermal evaporation. The device architec-
ture, shown in Supporting Information, Figure S23, is
identical to the one employed to make efficient OLEDs
doped with Pt(octaethylporphyrin).50 The voltage-cur-
rent density and voltage-luminance characteristics of the
fabricated OLEDs are shown in Figure 5. The turn-on
voltages were 5.9 and 6 V for devices with an aluminum
tris(8-hydroxyquinoline) (Alq3) host doped with 10% 3
and 5, respectively, and both reached a luminance of
100 cd/m2 at 12 V. Both devices exhibited narrow, deep-
red emission (λmax = 682 nm) when positive bias was
applied to the ITO electrode (Figure 6). The EL spectra
are coincident with the photoluminescent (PL) spectra of
the corresponding complexes at room temperature. How-
ever, for the device employing compound 3, it was
observed that, with increasing driving voltage, the EL
color slowly evolved from deep saturated red, to orange
to yellow. Examination of the EL spectra from this device
shows that this perceived color change at higher voltage is
due to mixed emission from both the phosphorescent
dopant and Alq3 host (Figure 6). In contrast, emission
from Alq3 is reduced significantly when 5 is used as the
dopant. The decreased contribution from Alq3 could be
due to better hole trapping by the dopant since both the
5(20,60) and 5(60,60) isomers are easier to oxidize than
dopant 3. Improved charge trapping could also be re-
sponsible for the lower conductivity of devices doped with
5 compared to those doped with 3 (Figure 5). The maxi-
mum external quantum efficiency was 0.6% and 1.0% for
the device using compound 3 and 5, respectively
(Supporting Information, Figure S24). The relatively poor
device performance, especially when compared relative to
other cyclometalated Ir-based devices, is likely due to the low
solution photoluminescent efficiencies of the dopants.
Acknowledgment. The authors would like to thank
Universal Display Corporation and the Department of
Energy for their financial support of this work.
Supporting Information Available: The 1H and 13C NMR of
1-7, crystallographic data for compounds 3, 4, and 5(60,60), the
CV traces of 3 and 5, the absorption/emission spectra of 1-7,
and the room temperature emission spectra of 3 and 4 in
PMMA. This material is available free of charge via the Internet
at http://pubs.acs.org.
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