PAPER
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| Dalton Transactions
Synthesis, structure and electroluminescent properties of cyclometalated
iridium complexes possessing sterically hindered ligands
Marappan Velusamy,
aK. R. Justin Thomas,
aChih-Hsin Chen,
aJiann T. Lin,*
aYuh S. Wen,
aWan-Ting Hsieh,
bChin-Hung Lai
band Pi-Tai Chou*
bReceived 15th December 2006, Accepted 13th March 2007 First published as an Advance Article on the web 30th March 2007 DOI: 10.1039/b618311e
New CN donor ligands incorporating pyridine or benzoimidazole N donors and a sterically hindered cyclometalating aromatic core featuring a polyphenylenephenyl, fluoranthene, or triphenylene segment are prepared and successfully converted into heteroleptic iridium(III) cyclometalated complexes with
acetylacetone auxiliary ligands. The X-ray structure of the complex, derived from a ligand containing a fluoranthene fragment, has been solved to unveil the corresponding structure. The results clearly demonstrate that the nature of the r-coordinating ligand segment plays a key role in dictating the emission profile and peak position, such that the emission hue has been successfully tuned ranging from green to red. Supplementary support of this viewpoint is also rendered by computational (DFT) approaches. Electroluminescent devices fabricated using a complex as dopant in the PVK matrix were found to exhibit bright greenish yellow emission with promising device characteristics (maximum brightness 26450 cd m−2at 30 V and a maximum current efficiency of 40 cd A−1).
Introduction
There is considerable interest in the synthesis and characterization of cyclometalated iridium(III) complexes due to their use in light-emitting diodes (LED)1and as biological labelling agents.2
Light-emitting organometallic complexes such as Ru(II) polypyridine
complexes3 and Ir(
III) complexes1,4 are explored for LED
appli-cations because they offer the possibility of reaping both singlets and triplets formed during the LED operation.5In contrast, while
some polymers have been reported to facilitate triplet harvesting,6
small organic molecules usually lack this feature.
In view of iridium(III) complexes, iridium(III)
cyclometa-lated complexes containing N-donor motifs such as pyridine,7
benzothiazole,8benzoimidazole,9quinoline,10and pyrazole11were
prepared. Additional anionic auxiliary ligands such as acetylace-tone, pyridine-2-carboxalic acid,12triazoles13and pyrazoles14were
also used in the synthesis of heteroleptic complexes. Recently, dendritic15 and polymer16 based ligands have also been
devel-oped and used for the construction of Ir(III) complexes. These macromolecules inherit additional benefits such as chromophore isolation (preventing unwanted interchromophoric interactions that often lead to emission quenching and emission profile alteration), energy harvesting emission enhancement, and simple device fabrication procedures.
Polyphenylenephenyl dendrons, often termed as M ¨ullen-type dendrons, have been widely used in recent designs to probe inter-chromophoric interactions such as energy and electron transfer.17Organic molecules incorporating these dendrons were
found to emit intense blue18or red19colors in electroluminescent
aInstitute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan.
E-mail: [email protected]; Fax: +886 2 278313771; Tel: +886-2-27898522
bDepartment of Chemistry, National Taiwan University, Taipei, Taiwan.
E-mail: [email protected]; Fax: +886-2-23695208; Tel: +886-2-33663894
devices. M ¨ullen-type dendrons are rigid and voluminous, thus raising the Tgand thermal stability in addition to minimising the
non-radiative pathways. Attempts have also been made to replace the central phenyl core in the polyphenylenephenyl dendrons with other extended conjugated aromatic segments. Accordingly, fluoranthene cored bright blue emitters have been reported recently.20 Herein, we report a series of new CN donor ligands
and their Ir(III) cyclometalated complexes (Charts 1 and 2).
Chart 1
In view of the structure, these ligands can be classified into two categories: in the first, the central phenyl/aryl of the segment participates in cyclometalation (A in Scheme 1), and in the second, a peripheral phenyl farthest from the centroid is involved in coordination (B in Scheme 1). As for the case of B type ligands, Burn et al.15have initiated elegant work regarding the effect of
Chart 2
Scheme 1
made further investigations to probe both A and B type ligands, and found that, depending on the cyclometalating segment, the emission properties can be fine-tuned ranging from green to red. Consequently, the nature of the r-coordinating ligand segment plays a key role in dictating the emission properties.
Experimental
Methods and materialsAll reactions and manipulations were carried out under N2with
the use of standard inert atmosphere and Schlenk techniques. Solvents were dried by standard procedures. The entire column chromatographic studies were performed under N2with the use of
silica gel (230–400 mesh, Macherey–Nagel GmbH & Co.) as the stationary phase. The1H NMR spectra were obtained by using
Bruker AC300 or AMX400 spectrometers. Mass spectra were recorded on a JMS-700 double focusing mass spectrometer (JEOL, Tokyo, Japan). Elemental analyses were performed on a Perkin-Elmer 2400 CHN analyzer. Cyclic voltammetry experiments were performed with a BAS-100 electrochemical analyzer at room temperature with a conventional three-electrode configuration consisting of a glassy carbon working electrode, a platinum wire auxiliary electrode, and a non-aqueous Ag/AgNO3 reference
electrode. The solvent in all experiments was dichloromethane, and the supporting electrolyte was 0.1 M tetrabutylammonium hex-afluorophosphate. The E1/2 values were determined as 1/2(Epa+
Epc), where Epaand Epcare the anodic and cathodic peak
poten-tials, respectively. The potentials are quoted against the ferrocene internal standard. Electronic absorption spectra were obtained on a Perkin Elmer Lambda 900 UV-Vis-NIR spectrophotometer for dichloromethane solutions. Emission spectra were recorded
in deoxygenated toluene solution at 298 K with a Jobin Yvon SPEX Fluorolog-3 spectrofluorimeter. The emission spectra were collected on samples with O.D. (optical density) <0.1 at the excitation wavelength. The spectra were corrected for instrumental response. UV-Visible spectra were checked before and after irradi-ation to monitor possible sample degradirradi-ation. Emission maxima were reproducible to within 2 nm. Luminescence quantum yields (Uem) were calculated using quinine sulfate as the primary standard
(Uem= 0.577 in 0.1 M sulfuric acid) and Ir(ppy)3(Uem = 0.40 in
toluene) as the secondary reference.21The lifetime was measured
with the laser photolysis technique, in which the third harmonic of an Nd:YAG laser (8 ns, Continuum Surlite II) was used as the excitation source, coupled with a fast response photomultiplier (Hamamatsu model R5509-72) operated at−80◦C. Typically, an average of 512 shots was acquired for each measurement.
The starting materials, 3,4-bis(4-methoxy-phenyl)-2,5-diphenyl-cyclopenta-2,4-dienone,19
7,9-diphenyl-cyclopenta[a]acenaphthy-len-8-one,22 1,3-diphenyl-cyclopenta[l]-phenanthren-2-one,23and
2-ethynyl-pyridine,24 were prepared by adopting the literature
procedures.
Synthesis of the ligands 1–4
Ligands 2[{3,4-(4-methoxyphenyl)-2,5-diphenyl
}phenyl]pyri-dine (1), 2-(7,10-diphenyl-fluoranthen-8-yl)-pyri}phenyl]pyri-dine (2), and 2-(1,4-diphenyl-triphenylen-2-yl)-pyridine (3) were synthesized from 2-ethynylpyridine on reaction with 3,4-bis-(4-methoxy-phenyl)-2,5-diphenyl-cyclopenta-2,4-dienone, 7,9-diphenyl-cyclo-penta[a]acenaphthylen-8-one, and 1,3-diphenyl-cyclopenta[l]-phenanthren-2-one, respectively. Ligand 2-[(3,4-bis(4
-methoy-phenyl)-2,5
-diphenyl)-biphenyl-4-yl]-1-phenyl-1H-benzo(d)imida-zole 4 was synthesized from 3,4-bis-(4-methoxy-phenyl)-2,5-diphenyl-cyclopenta-2,4-dienone in reaction with 2-ethynyl-1-phenyl-1H-benzoimidazole. The detailed procedure for the preparation of ligand 1 is discussed here and the same procedure is followed for the other ligands.
Synthesis of ligand 1. A stirred mixture of 2-ethynylpyridine (0.61 g, 6 mmol) and 3,4-bis-(4-methoxy-phenyl)-2,5-diphenyl-cyclopenta-2,4-dienone (2.2 g, 5 mmol) in degassed diphenyl ether (15 mL) was refluxed for 15 h. At the completion of the reaction water was added to quench the reaction. The product was extracted with diethyl ether. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under vacuum. The solid was
adsorbed on silica gel and purified by column chromatography, using a CH2Cl2–hexane mixture as eluent to give 1 as a white solid
(2.0 g, 80%).1H NMR (d, CDCl
3): 3.61 (s, 3H, OCH3), 3.63 (s, 3H,
OCH3), 6.40–6.47 (m, 4H, OCH3-Ar), 6.63–6.71 (m, 4H, OCH3
-Ar), 6.85–6.87 (m, 3H, -Ar), 6.93–6.94 (m, 3H, -Ar), 7.04–7.13 (m, 6H, Ar), 7.33 (t, J= 7.3 Hz, 1H, pyridyl), 7.73 (s, 1H, central Ar), 8.55 (d, J= 4.56 Hz, 1H, pyridyl).13C NMR (d, CDCl
3): 55.0,
55.1, 112.4, 112.6, 121.3, 125.5, 125.9, 126.2, 127.2, 127.6, 130.1, 131.2, 131.5, 132.5, 132.6, 132.7, 135.3, 139.5, 140.1, 140.6, 141.5, 141.7, 142.0, 148.9, 157.3, 157.5, 158.5; FAB MS(m/z): 520.1 (M + H)+. FAB-HRMS calc. for C
37H30NO2[M + H]+: 520.2277 found:
520.2290.
Ligand 2. 1H NMR (d, CDCl
3): 6.71 (d, 1H, J = 7.2 Hz,
pyridyl), 7.0–7.05 (m, 2H, pyridyl), 7.26–7.30 (m, 2H, Ar), 7.34– 7.42 (m, 7H, Ar), 7.46–7.54 (m, 3H, fluoranthene and Ar), 7.59 (s,1H, central Ar), 7.68–7.74 (m, 4H, fluoranthene), 8.56 (d 1H,
J= 4.5 Hz, pyridyl).13C NMR (d, CDCl
3): 121.4, 123.4, 123.5,
125.5, 126.8, 127.0, 127.6, 127.7, 127.8, 127.9, 128.7, 128.8, 129.3, 129.4, 129.8, 130.4, 131.3, 133.3, 135.5, 135.9, 136.1, 136.6, 137.0, 138.2, 138.3, 139.2, 140.8, 148.9, 158.6. FAB MS(m/z): 431 (M+).
FAB-HRMS calc. for C33H22N [M + H]+ : 432.1752 found:
432.1760.
Ligand 3. 1H NMR (d, CDCl
3): 6.63 (d, 1H, J = 7.2 Hz,
pyridyl), 6.98 (m, 1H, pyridyl), 7.08–7.20 (m, 7H, Ar), 7.34–7.51 (m, 9 H, Ar), 7.75 (d, 1H, J = 8.43 Hz, pyridyl), 7.85 (s, 1H, central Ar), 8.42 (d, 2H, J= 8.2 Hz, Ar), 8.62 (d, 1H, J = 4.9 Hz, pyridyl).13C NMR (d, CDCl
3): 121.3, 123.3, 123.4, 125.4, 125.7,
126.6, 126.9, 127.0, 127.3, 128.8, 129.1, 129.8, 130.0, 130.3, 130.8, 131.2, 131.6, 131.7, 131.8, 132.0, 132.1, 135.1, 136.7, 138.7, 142.1, 144.5, 149.2, 159.6. FAB MS(m/z): 457.1 (M+). FAB-HRMS calc.
for C35H24N [M + H]+: 458.1909 found: 458.1909.
Ligand 4. 1H NMR (d, CDCl
3): 3.62 (s, 3H, OCH3), 3.65
(s, 3H, OCH3), 6.44–6.52 (m, 4H, OCH3-Ar), 6.68–6.77 (m, 4H,
OCH3-Ar), 6.81–6.82 (m, 2H, Ar), 6.93–6.94 (m, 3H, Ar), 7.08–
7.18 (m, 7H, Ar), 7.25–7.28 (m, 4H, Ar), 7.31–7.34 (m, 1H, Ar), 7.36–7.39 (m, 2H, Ar), 7.46–7.48 (m, 3H, Ar), 7.53 (s, 1H, central-Ar), 7.89 (d, 1H, J = 8.2 Hz, benzimidazole-Ar). 13C NMR
(d, CDCl3): 55.1, 110.5, 112.4, 112.7, 119.0, 119.8, 123.2, 123.5,
125.8, 126.3, 127.2, 127.5, 127.6, 127.8, 128.6, 128.9, 129.9, 130.0, 131.2, 131.6, 132.4, 132.5, 132.6, 132.8, 137.0, 137.2, 139.7, 139.8, 140.0, 141.2, 141.8, 142.0, 143.0, 143.3, 152.4, 157.3, 157.5. FAB MS(m/z): 710.1 (M+). FAB-HRMS calculated for C
51H39N2O2
[M + H]+: 711.3012 found: 711.3019.
Synthesis of the complexes 5–8
Complexes 5–8 were synthesized by similar procedures, as followed for complex 5.
Synthesis of complex 5. A mixture of 1 (0.62 g, 1.2 mmol), IrCl3·xH2O (0.15 g, 0.5 mmol), 2-methoxyethanol (6 mL), and
distilled water (3 mL) was heated at reflux for 12 h. After cooling, the yellow precipitate which formed was filtered off and washed with methanol (50 mL), diethyl ether (50 mL), and hexane (20 mL). The dried chlorobridged dimer was suspended in 2-methoxyethanol (5 mL) and treated with acetylacetone (0.10 g, 1 mmol) and anhydrous Na2CO3(0.16 g, 1.5 mmol). The reaction
mixture was stirred at 80 ◦C for 8 h. The insoluble products
were filtered off, washed with water (10 mL), methanol (10 mL), and diethyl ether (20 mL), and dried. These insoluble products were dissolved again in dichloromethane (20 mL), adsorbed on silica gel, and rapidly purified by column chromatography using a hexane–dichloromethane mixture (3 : 2). The yellow solid obtained was recrystallized from a dichloromethane–hexane mixture. Yield: 0.10 g (80%).1H NMR (d, CDCl
3): 1.56 (s, 6H, CH3-acac), 3.55
(s, 6H, OCH3), 3.59 (s, 6H, OCH3), 5.02 (s, 1H, CH-acac), 5.84
(d, 2H, J = 8.4 Hz, Ar), 6.21–6.22 (m, 2H, Ar), 6.31–6.39 (m, 16H, Ar), 6.50–6.57 (m, 4H, Ar), 6.62–6.73 (m, 8H, Ar), 7.04– 7.12 (m, 10H, Ar), 7.72–7.74 (m, 2H, Ar).13C NMR (d, CDCl 3): 14.2, 28.6, 31.0, 53.5, 54.9, 55.0, 99.5, 111.5, 111.8, 111.9, 119.2, 123.1, 123.9, 125.4, 125.5, 126.0, 128.0, 128.1, 130.1, 130.7, 131.0, 132.1, 132.5, 132.6, 132.7, 133.0, 134.0, 134.3, 134.6, 135.3, 139.8, 142.5, 143.8, 145.6, 149.1, 156.4, 156.5, 167.7, 184.0, 206.9; FAB MS(m/z): 1328.2 (M+). Anal. Calcd for C
79H63IrN2O6: C, 71.42;
H, 4.78; N, 2.11. Found: C, 71.21; H, 4.88; N, 2.05%. Complex 6. Yield: 55%.1H NMR (d, CDCl
3): 1.59 (s, 6H, CH3
-acac), 4.87 (s, 1H, CH--acac), 5.53 (d, 2H, J= 7.2 Hz, Ar), 6.08 (d, 2H, J= 7.2 Hz, Ar), 6.27 (d, 2H, J = 8.5 Hz, Ar), 6.59–6.63 (m, 4H, Ar), 6.76–6.89 (m, 2H, Ar), 6.9–7.02 (m, 10H, Ar), 7.14–7.16 (m, 2H, Ar), 7.47–7.55 (m, 6H, Ar), 7.61–7.62 (m, 4H, Ar), 7.72– 7.74 (m, 4H, Ar), 8.13–8.15 (m, 2H, Ar).13C NMR (d, CDCl 3): 14.2, 22.7, 28.5, 31.7, 99.6, 118.9, 121.5, 122.6, 123.4, 124.9, 125.5, 125.9, 126.9, 127.2, 127.4, 127.5, 129.0, 129.2, 129.5, 129.6, 129.7, 130.2, 130.6, 133.5, 134.9, 136.0, 137.2, 137.4, 138.0, 142.1, 143.0, 144.5, 146.5, 149.0, 149.5, 167.3, 184.5. FAB MS(m/z): 1152.3 (M+). Anal. Calcd for C
71H47IrN2O2: C, 74.00; H, 4.11; N, 2.43.
Found: C, 73.72; H, 4.24; N, 2.36%.
Complex 7. Yield: 70%.1H NMR (d, CDCl
3): 1.54 (s, 6H,
CH3-acac), 4.98 (s, 1H, CH-acac), 5.75–5.81 (m, 2H, Ar), 6.15–
6.28 (m, 2H, Ar), 6.40–6.41 (m, 1H, Ar), 6.43–6.49 (m, 3H, Ar), 6.50–6.83 (m, 10H, Ar), 6.98–7.01 (m, 5H, Ar), 7.22–7.41 (m, 12H, Ar), 7.53–7.54 (m, 2H, Ar), 7.71–7.73 (m, 1H, Ar), 8.05–8.07 (m, 1H, Ar), 8.28–8.34 (m, 4H, Ar), 8.54–8.56 (m, 1H, Ar).13C NMR
(d, CDCl3): 28.6, 28.8, 99.1, 99.6, 119.3, 120.8, 122.9, 123.4, 124.7,
125.0, 125.1, 125.5, 127.3, 127.6, 128.8, 129.2, 129.7, 130.3, 130.4, 130.5, 131.1, 131.2, 131.9, 132.2, 132.6, 134.3, 141.7, 143.5, 144.2, 145.0, 146.5, 147.8, 148.8, 150.1, 151.4, 167.2, 167.4, 183.8, 184.5. FAB MS(m/z): 1204.3 (M+). Anal. Calcd for C
75H51IrN2O2: C,
74.79; H, 4.27; N, 2.33. Found: C, 74.82; H.4.13; N, 2.24%. Complex 8. Yield: 60%.1H NMR (d, CDCl
3): 1.94 (s, 6H,
CH3-acac), 3.69 (s, 6H, OCH3), 3.73 (s, 6H, OCH3), 5.33 (s, 1H,
CH-acac), 6.22–6.25 (m, 2H, Ar), 6.40–6.42 (m, 2H, Ar), 6.37– 6.64 (m, 12H, Ar), 6.65–6.86 (m, 16H, Ar), 7.04–7.06 (m, 6H, Ar), 7.17–7.23 (m, 8H, Ar), 7.28–7.35 (m, 6H, Ar), 7.47–7.50 (m, 2H, Ar), 7.60–7.68 (m, 8H, Ar).13C NMR (d, CDCl 3): 28.5, 31.7, 55.0, 101.4, 110.2, 112.2, 112.3, 112.5, 116.7, 122.7, 123.8, 125.3, 125.9, 126.8, 127.5, 127.9, 128.5, 129.6, 130.1, 130.2, 131.4, 132.5, 132.7, 133.1, 133.6, 135.9, 136.6, 138.7, 139.4, 140.1, 140.5, 140.7, 141.2, 142.1, 148.5, 157.0, 157.3, 164.2, 184.9. FAB MS(m/z): 1711.5 (M+). Anal. Calcd for C
107H81IrN4O6: C, 75.11; H, 4.77; N, 3.27.
Found: C, 74.83; H, 4.83; N, 3.16%. LED Fabrication and measurements
Compound BCP (2.9-dimethyl-4,7-diphenyl-1,10-phenanthro-line) was purchased from Aldrich and used as received. The
electron-transporting materials, Alq3 (tris(8-hydroxyquinoline)
aluminium)25 and PPO
(2-(4-tert-butylphenyl)-5-(pyren-1-yl)-1,3,4-oxadiazole),26were synthesized according to literature
proce-dures and were sublimed prior to use. Prepatterned ITO substrates with an effective individual device area of 3.14 mm2were cleaned
as described in a previous report.27A PVK:Ir complex (device I)
or PVK:Ir Complex:PPO film (device II) was spin cast first. In device I, a 10-nm-thick BCP acting as a hole and exciton blocking layer (HBL) and 20-nm-thick Alq3layer as an electron transport
layer were then deposited sequentially. Finally, a thin layer of LiF (10 A˚ ) followed by aluminium (1000 A˚) was deposited as the cathode. The I–V curve was measured on a Keithley 2400 Source meter in the ambient environment. Light intensity was measured with a Newport 1835 optical meter.
Structural determination of complex 6
Red crystals of 6 were grown from a dichloromethane solution layered with hexane at room temperature. Relevant crystal data collection and refinement are summarized in Table 1. Selected bond lengths and bond angles are given in Table 2. The space group (P¯1) was determined from the systematic absence of specific reflections; successful refinement of the structure confirmed the space group assignment. Direct methods were used to locate the Ir atom, whereas subsequent cycles of least-squares refinements and difference Fourier map were used to locate the remaining non-hydrogen atoms. Hydrogen atoms were placed at calculated positions except for the O–H bonds of the water molecule which were fixed at 0.85 A˚ . All calculations were performed using the SHELX software package.
CCDC reference number 294665.
For crystallographic data in CIF or other electronic format see DOI: 10.1039/b618311e
Table 1 Crystal data and structure refinement details for 6
Empirical formula C71H47IrN2O2·3CH2Cl2·H2O
M 1425.10
T/K 100 (1)
k/A˚ 0.71073
Crystal system Triclinic
space group P¯1 a/A˚ 12.2327(4) b/A˚ 13.1476(4) c/A˚ 20.8703(6) a/◦ 83.244(2) b/◦ 73.8030(10) c /◦ 77.3430(10) V /A˚3 3139.30(17) Z 2 Calcd density/Mg m−3 1.508 l/mm−1 2.433 F (000) 1432 Crystal size/mm 0.38× 0.24 × 0.20
h range, data collection/◦ 1.02 to 25.03,−13 ≤ h ≤ 14,
−15 ≤ k ≤ 15, 0 ≤ l ≤ 24
No. of reflns collected 11103
No. of indep. reflns 11103
Refinement method Full-matrix least-squares on F2 No.of data/restraints/parameters 11103/2/687
Goodness-of-fit on F2 1.071
Final R indices (I> 2r(I)): R1, wR2 0.0278, 0.0755
R indices (all data): R1, wR2 0.0293, 0.0768
Table 2 Selected bond lengths (A˚ ) and angles (◦) for 6
Ir–C(11) 1.995(3) Ir–N(2) 2.017(3) Ir–C(61) 1.996(3) Ir–O(1) 2.128(2) Ir–N(1) 2.016(3) Ir–O(2) 2.140(2) C(11)–Ir–C(61) 96.77(13) N(1)–Ir–O(1) 92.83(10) C(11)–Ir–N(1) 80.69(13) N(2)–Ir–O(1) 81.84(10) C(61)–Ir–N(1) 103.68(13) C(11)–Ir–O(2) 87.12(12) C(11)–Ir–N(2) 104.46(13) C(61)–Ir–O(2) 173.31(12) C(61)–Ir–N(2) 80.26(13) N(1)–Ir–O(2) 82.28(10) N(1)–Ir–N(2) 173.25(11) N(2)–Ir–O(2) 93.52(11) C(11)–Ir–O(1) 173.17(12) O(1)–Ir–O(2) 89.87(9) C(61)–Ir–O(1) 86.83(12) Computational methodology
To obtain the excitation energies of complex 6, we applied time-dependent density function theory (TDDFT), in which the B3LYP functional was adopted. The geometry of complex 6 was based upon its X-ray structure. The basis set used is LANL2DZ for iridium, and 6-31G for the other elements. The calculation was performed with the Gaussian 03 program.28Additionally, though
being time consuming, the solvent effect on the absorption energies in CH2Cl2are also considered by using an implicit solvation model
IEFPCM with the same DFT functional and basis set.29To save
the computational resource, we only consider the solvent effect on the absorption energy of the lower lying states up to S2and T2.
Results and discussion
Synthesis and structureThe structures of ligands and complexes are depicted in Charts 1 and 2. The ligands were synthesized by employing a Diels– Alder [4 + 2] cycloaddition reaction22involving the appropriately
substituted cyclopentadienone and the corresponding terminal acetylene in diphenyl ether at 200◦C (Scheme 2).
Scheme 2
The ligands were obtained in moderate yields (60–80%) and were either colorless or yellow. The required complexes were obtained in two steps: (i) chloro-bridged dinuclear complexes by the treatment of the ligands with IrCl3·xH2O in 2-methoxyethanol.
(ii) These dinuclear complexes were cleaved by reaction with acetylacetone in the presence of sodium carbonate to obtain the mononuclear heteroleptic complexes, featuring the sterically demanding CN donor ligands and acetylacetonate auxiliary
ligand. The as prepared complexes were purified by column chromatography, and the yield ranged from 55 to 80%.
All the new compounds were thoroughly characterized by NMR (1H and 13C), mass spectra and elemental analyses. The
spectral characteristics were in accordance with the structural compositions expected for the compounds. Since no broadening of the peaks was noticed in the1H NMR spectra it is believed that
there exists only one isomer for these complexes. The structure of complex 6 was also confirmed by single crystal X-ray diffraction studies. In addition to the molecule 6, each asymmetric unit also contains three dichloromethane and one water molecules. As for the molecular structure displayed in Fig. 1, the iridium adopts an octahedral coordination environment with cis-C–C, cis-O–O and
trans-N–N donor dispositions. The average Ir–C, Ir–N and Ir–O
distances are 1.996 (3), 2.017 (3), and 2.134 (2) A˚ , respectively. These values are close to that observed for Ir(ppy)2(acac) (2.003
(9), 2.010 (9), 2.146 (6)).30 A slight deviation of the perfect
octahedron is evident from the C–Ir–C (96.77 (13)◦), O–Ir–O (89.87 (9)◦), and N–Ir–N (173.25 (11)◦) angles. The C–Ir–C angle is slightly wider when compared to that observed for Ir(ppy)2(acac)
(95.2 (5)◦). Consequently, the N–Ir–N angle is narrowed (3◦) as compared to that observed in the Ir(ppy)2(acac) complex. We
attribute the above observations to the steric effect of the rigid, bulky ligand. The steric dictation on the coordination sphere is more evident from the N–Ir–C bite angles (av. 80.48 (13)◦) and the N–Ir–C inter-ligand angles (av. 104.07 (13)◦). The former is slightly shorter (cf. 81.7 (4)◦), and the latter is wider (cf. 95.8 (4)◦) than in Ir(ppy)2(acac). All other distances and angles of the ligand
are within the normal range. It is important to note that the pyridyl ring of each cyclometalated ligand is somewhat parallel to one of the dangling phenyl rings from the other cyclometalated ligand, and the distance between the centroids of the two rings is∼3.7 A˚. Such a p–p interaction may result in low emission quantum yields for the complexes 5–7 in solution (vide infra). In comparison, the intermolecular p–p interactions seem negligible.
Fig. 1 Molecular structure of the complex 6 (the solvent molecules and
hydrogen atoms are removed for clarity). The thermal ellipsoids are drawn at the 30% probability level.
Absorption spectra
Complexes 5–8 are either deep yellow or red in color, depending on the cyclometalating ligands. This is clearly manifested in the
absorption spectra of the complexes. The absorption profiles of the ligands and the complexes recorded in dichloromethane solution are displayed in Fig. 2 and 3. All the ligands except 2 possess p–p* transition bands below 350 nm (Fig. 2).31 The absorption
wavelengths of the p–p* transition bands are in the order of 1< 4 ≈ 3< 2, consistent with the degree of electronic delocalization in the ligands. The peak wavelength at 374 nm for ligand 2 may originate from the fluoranthene segment.22Compound 3 also possesses a
fused aromatic segment, triphenylene. However, its associated p– p* transition bands appear to be at shorter wavelengths because the fluoranthene segment has a smaller p–p* transition energy than the triphenylene analogue.32 In comparison to compound
4, the shorter absorption wavelength in compound 1 may be attributed to its less effective conjugation length since no more than three aromatic rings are linked in para form for favourable conjugation. Comparing Fig. 2 and 3, it is obvious that the ligand centered peaks for all the complexes, to certain degrees, are perturbed due to the presence of the metal center. In complexes 5 and 8, an additional three less intense transitions were noticed,
Fig. 2 Absorption spectra of the ligands 1–4 recorded in CH2Cl2
solutions.
Fig. 3 Absorption spectra of the complexes 5–8 recorded in CH2Cl2(inset
while in complexes 6 and 7, a prominent red-shifted absorption was identified above 450 nm. These bands were reasonably ascribed to the MLCT transitions.1,4 It is interesting to compare the
MLCT absorption peak wavelengths in the control complexes Ir(ppy)2(acac)33 and Ir(pbi)2(acac)9with those of the complexes
prepared in the current work. The salient spectral features derived from absorption studies enable us to gain insights into the role of the sterically hindered ligand on the electronic structure of the metal fragment. There is only slight red-shift (ca. 10 nm) in the MLCT transitions of 5 and 8 when compared to the corresponding transitions with the control complexes. Apparently, the peripheral phenyl rings do not lead to much conjugation because they are
skew with respect to the central phenyl rings, as evidenced from
the structure of 6 (vide supra). In contrast, better conjugation is expected in ligands 2 and 3 due to the presence of polyaromatic segments. Accordingly, the resulting HOMO–LUMO energy gap leads to more pronounced red-shifted MLCT absorption peaks for their corresponding complexes 6 and 7 compared to the control complex Ir(ppy)2(acac).34,15a
Emission spectra
The luminescence spectra of the ligands and complexes were mea-sured in dichloromethane and toluene, respectively. The spectra are displayed in Fig. 4 and 5, and the pertinent photophysical data are collected in Table 3. The complexes emit green or red in both solution and the solid state at room temperature. The emission peak wavelength of the bulky complexes is red-shifted when compared to that of the control complexes, and the magnitude of the shift is dependent on the conjugation of the cyclometalating aromatic segment.1b,13b,35Accordingly, complexes
5 and 8 are green-emitting species, while complexes 6 and 7 emit in the red due to the presence of the fluoranthene and triphenylene segments. As supported by the theoretical approach (vide infra), the emission is principally derived from the MLCT and p–p* states of the cyclometalated ligand. Moreover, the emission quantum yield of the complexes seems to be dependent on the interaction mode between metal and ligand sites. Type “A” complexes (5– 7) possess low quantum yield, while the type “B” complex (8) displays moderate emission efficiency. The low solution quantum
Fig. 4 Emission spectra of the ligands 1–4 recorded in CH2Cl2solution
at room temperature.
Fig. 5 Emission spectra of the complexes 5–8 recorded in toluene
solution, 3% 5 doped in PVK film and EL spectra of device II.
yields of complexes 5–7 stem from the close contact of the pyridyl ring and the dangling phenyl ring nearby (vide supra). Support of this viewpoint is given by the significantly large non-radiative decay rate constant deduced for 5–7 (see Table 3). Complexes 5 and 8 display phosphorescence lifetimes similar to those of the control complexes (Ir(ppy)2(acac) and Ir(pbi)2(acac). However, a
significant two-fold (7) and four-fold (6) enhancement in lifetime is observed for the complexes which possess extended polyaromatic conjugation in the r-coordinating ligand segment. This result clearly supports the involvement of the ligand in the excited state properties of the complexes.36These lifetime values appear to fall in
the range of bis-cyclometalated31and tris-cyclometalated21iridium
complexes. The distinct nonradiative decay constant for 5 among the four complexes accounts for its relatively small quantum yield. Electrochemical properties
Electrochemical characteristics of the complexes were studied by using cyclic voltammetric methods in dichloromethane and the redox potentials are reported in Table 3. All complexes exhibit a quasi-reversible one-electron oxidation wave ranging from 0.25 to 0.29 V vs. ferrocene, which is attributed to the oxidation of iridium(III). The larger peak-to-peak separation (DEp) observed in
the present study suggests a significant structural reorganization during the redox process.37 The pyridine-containing complexes,
5–7 have a higher oxidation potential (0.25–0.27 V) than the control complex, Ir(ppy)2(acac) (0.46 V),33while complex 8 has
an oxidation potential (0.29 V) similar to that of Ir(pbi)2(acac)
(0.30 V).9We attributed this observation to the influence of the
sterically hindered moiety on the HOMO. Apparently the sterically hindered moiety is closer to the cyclometalated phenyl ring in 5– 7. All complexes exhibit an irreversible reduction wave attributed to the reduction of the ligand. Complexes 6 and 7 have more positive potential, possibly due to the more delocalized nature of the fluoranthene and triphenylene segments. The differences in the oxidation and reduction potentials for the complexes are in the order of 6< 7 < 8 < 5. Though the irreversible nature of the reduction peak prevents us from more accurate estimation, this trend, however, is consistent with the absorption and emission spectra (vide supra).
T able 3 Ph ysical pr operties of the comple x es Compound kmax a/nm (emax /× 10 3M − 1cm − 1) kem a/nm kem b/nm UF b s b/ns kr /s − 1 knr /s − 1 E1/2 c/V (D Ep /mV) 5 500 (1.1), 455 (4.0), 424 (6.5), 369 (11.4), 308 (50.8) 527, 555 sh 532, 560 sh 0.077 1358 5.7 × 10 4 6.8 × 10 5 0.27 (107), − 1.95 (i) 6 514 (5.5), 484 (6.5), 372 (50.1), 354 (52.7), 311 (49.6), 273 (55.5) 631 622 0.114 5808 1.9 × 10 4 1.5 × 10 5 0.25 (139), − 1.79 (i) 7 477 (5.7), 414 (12.3), 349 (70.7), 282 (77.0) 625 627 0.127 2903 4.4 × 10 4 3.0 × 10 5 0.26 (142), − 1.82 (i) 8 463 (5.3), 431 (6.9), 393 (11.5),318 (71.1) 544, 575 sh 543, 578 sh 0.470 1589 2.9 × 10 5 3.2 × 10 5 0.29 (98), − 1.89 (i) (Ir(pp y )2 (acac) d 497 (3.0), 460 (3.3), 412 (3.4), 345 (3.8), 260 (4.5) 516 — 0 .34 1600 2.1 × 10 5 4.1 × 10 5 0.46 e Ir(pbi) 2 (acac) 453 (3.0), 414 (3.1), 388 (3.2), 346 (3.5), 315 (4.6), 300 (4.6) — 523 0.73 1820 4.0 × 10 5 1.4 × 10 5 0.30 (81) aR ecor ded in CH 2 Cl 2 solution. bR ecor ded in toluene solution. cR ecor ded in dichlor omethane solution using T B A PF 6 as electr o ly te and g lassy carbon w o rk ing electr ode . T he potentials ar e reported with re fer ence to the internal ferr ocene standar d . i= ir reve rs ibl e. dF o r 2 -meth y ltetr a h y dr ofur an solutions . eThe v alue fr om re f. 33 has been corr ected to the refer ence (ferr ocene) used her e. The ferr ocene has a n Eox at 0.23 V rela ti v e to Ag/Ag +. MO Calculations
To gain more insight into the photophysical properties of the complexes, TDDFT calculations (see Experimental section) on the molecular orbitals involved in the transitions was carried out for 6. The features of the two lowest unoccupied (LUMO and LUMO + 1) and the two highest occupied (HOMO and HOMO− 1) frontier orbitals mainly involved in the transition are depicted in Fig. 6, while the descriptions and energy gaps of each transition are listed in Table 4. Qualitatively, the stronger singlet absorptions for 6 are calculated at∼478, 412, and 390 nm, the results of which are in agreement with the observed absorption spectral profiles. Moreover, the lowest triplet state is calculated to be∼572 nm, which is also consistent with the 0–0 onset (∼580 nm) of the corresponding phosphorescence spectrum (see Fig. 5), supporting the validity of the TDDFT approach. Evidently, the electron densities of both the HOMO and HOMO− 1 are located largely on the metal and the fluoranthene segment of 6, whereas those of the LUMO and LUMO + 1 are distributed largely on the pyridyl moiety and partly on the fluoranthene segment and nonbonding d orbital of the central metal, indicating that the lowest transition is ascribed to metal-to-ligand charge transfer (MLCT) mixed with p (fluoranthene)→ p* (pyridine) character. The dominance of the HOMO→ LUMO transition (see Table 4), in which MLCT has a great contribution, qualitatively explains the broad, featureless emission band with fast radiative decay time. Accordingly, it is reasonable to expect that by introducing more extended polyaromatic conjugation into the r-coordinating ligand segment, such as in 6 and 7, there should be more pp* contribution to the T1 state in complex 7. Since the T1 → S0
transition with a pp* configuration does not directly involve the heavy metal (Ir(III)) dporbital as well as lacking the orbital flipping
that can be borrowed by the flip of spin, the transition dipole is small hence the resulting radiative lifetime is expected to be longer, consistent with that observed experimentally (see Table 3). The
Table 4 Calculated energy levels of the lower lying transitions of complex 6
State E/eV k/nm (f )b Assignments
S1 2.49 498.9 0.0008 HOMO→ LUMO (+90%)
2.45a 507.0a 0.0028a HOMO→ LUMO (+88%a
S2 2.59 478.3 0.0582 HOMO→ LUMO + 1 (+90%)
2.50 495.0 0.0971 HOMO→ LUMO + 1 (+90%)
S3 3.01 412.0 0.1534 HOMO− 1 → LUMO (+90%)
T1 2.17 571.9 0.0000 HOMO→ LUMO (+76%); HOMO → LUMO + 1 (+8%); HOMO − 1 → LUMO + 1 (+6%)
2.16 574.1 0.0000 HOMO→ LUMO (+69%); HOMO → LUMO + 1 (+16%); HOMO − 1 → LUMO + 1(6%)
T2 2.22 557.5 0.0000 HOMO→ LUMO + 1 (+71%); HOMO → LUMO (+9%); HOMO − 1 → LUMO (+8%)
2.20 562.9 0.0000 HOMO→ LUMO + 1 (+65%); HOMO → LUMO (16%); HOMO − 1 → LUMO (7%)
aThe values in Italic type are the results considering solvent effects.bOscillator strength.
aforementioned theoretical approaches were performed under gas conditions. In order to evaluate the solvation effect, the absorption energy gap with the addition of CH2Cl2was also calculated and
listed in Table 4 (see Experimental section for details). As shown in Table 4, upon considering the solvent effect, the absorption peaks in both the singlet and triplet states are only slightly red-shifted and are closer to those observed experimentally. Nevertheless, the involved frontier orbitals remain unchanged. Considering the time-consuming computation upon incorporating the solvation effect, theoretical approaches in the gas phase seem to be sufficient for the rationalization.
Electroluminescent properties
The HOMO energy values derived from the oxidation potential (Table 3) fall in the range 5.05–5.09 eV. This is sufficiently high to facilitate hole injection in the molecular layer. Due to the favourable redox and emission characteristics, the complexes are perceived as suitable for electroluminescent applications. Subsequently, the complexes were applied in organic light-emitting diodes as dopants. Two types of devices were fabricated using 5 and 8 as dopant in a polyvinyl carbazole (PVK) layer (Fig. 7). They are: (I) ITO/3% 5 (or 8) in PVK (70 nm)/BCP (10 nm)/Alq3
(20 nm)/LiF (1 nm)/Al (100 nm), and (II) ITO/3% 5 (or 8) and 40% PPO in PVK (100 nm)/LiF (1 nm)/Al (100 nm). The first device adopts a multi-layer architecture consisting of an Ir doped hole-transport layer, a hole-blocking layer, and an electron-transport layer. In the second device, a single-layer configuration containing a blend of hole-transporting PVK and electron-transporting PPO was used to dope the Ir complex. It was found that the latter device did not require a hole-blocking layer to restrict the recombination zone inside the PVK layer. The EL spectra of the two devices closely resemble the solid state PL realized for the PVK : Ir complex blend (Fig. 5). A slight red-tailing noticed in the EL spectra of the devices may be attributed to the medium perturbation. The pertinent parameters are collected in Table 5, and the I–V –L characteristics observed for devices 5 are plotted in Fig. 8. Device II for 5 exhibited maximum brightness, 26450 cd m−2at 30 V, and a maximum current efficiency of 40 cd A−1. The better performance of device II as compared to that of
I is ascribed to the improved balance in the charge transport in the HTL : ETL blend layer.38The maximum external quantum
efficiency (I, 4.4%; II, 6.69%) for both devices of 5 are comparable to that (∼10%) of the vacuum-deposited multi-layer device of Ir(ppy)2(acac).33However, the maximum power efficiency for both
Fig. 7 Device configurations and the structure of the constituent
materials.
devices (I, 7.64 lm W−1; II, 6.69 lm W−1) are inferior to that (∼18 lm
W−1) of Ir(ppy)
2(acac). This may be due to the low
phospho-rescence quantum yield (7.7%) of 5 compared to Ir(ppy)2(acac)
(34%). In comparison, the device performance appears to be better than the single-layer device of the complex composed of a fac-tris(2-phenylpyridyl)iridium(III) core and similar sterically crowded dendron (2.9%) reported by Burn’s group.15aWe speculate
that the emission of the iridium core is well encapsulated by the three somewhat insulating ligands to allow efficient carrier injection. For both devices of 8, the maximum external quantum efficiency (I, 8.64%; II, 3.85%) and maximum power efficiency (I, 9.51 lm W−1; II, 5.79 lm W−1) seem to be inferior to those
(∼16.7%; 20 lm W−1) of the vacuum-deposited multi-layer device
of Ir(pbi)2(acac).9The cause of the low efficiency for the devices
of 8 may be due to both encapsulation of the emitting core with somewhat insulating ligands as well as the imbalance in the carrier mobility. However, more studies are needed to understand the
Table 5 Electroluminescent data for 5 and 8a Device I (5) II (5) I (8) II (8) Von/V 8.0 8.0 6.5 5.5 Lmax/cd m−2(V at Lmax, (V)) 15264 (28.5) 26498 (30) 7125 (22.5) 5407 (14.5) kem/nm 566 550 550 548 CIE (x,y) 0.44, 0.54 0.43, 0.56 0.40, 0.54 0.41, 0.56 fwhm/nm 92 84 72 70 gext,max(%) 11.53 11.17 8.64 3.85 gp,max/lm W−1 7.64 6.69 9.51 5.79 gc,max/cd A−1 40.12 40.23 30.33 13.99 Lb/cd m−2 15210 23058 6961 5119 gextb(%) 4.40 6.41 1.99 1.42 gpb/lm W−1 1.71 2.61 0.99 1.18 gcb/cd A−1 15.32 23.11 6.99 5.15 aLegends: L
max, maximum luminance; L, luminance; Von, turn-on voltage; V , voltage; gext,max, maximum external quantum efficiency; gp,max, maximum power efficiency; gc,max, maximum current efficiency; gext, external quantum efficiency; gp, power efficiency; gc, current efficiency; fwhm, full width at half maximum.bAt a current density of 100 mA cm−2. V
onwas obtained from the x-intercept of log(luminance) vs. applied voltage plot.
Fig. 8 I–V –L characteristics of the devices I and II containing the
complex 5 as a dopant.
factors affecting the device performance. Fabrication of the devices for complex 6 was also attempted; unfortunately, the performance was poor. Besides poor carrier mobility and poor balance of carrier mobility, the low device efficiency, in part, may be attributed to the intramolecular p–p interaction that leads to a rather low quantum yield of the complexes (vide supra). In yet another approach, we also carried out a concentration dependent experiment and found that the emission quantum yield of complex 5 in toluene dropped ∼25% as the concentration increased from 4.47 × 10−5 M to
1.35× 10−4M. The results, in combination with the negligible
improvement in device performance upon increasing the doping concentration, indicate that weak intermolecular interactions cannot be completely ruled out for explaining the low device efficiency.
Conclusion
In summary, we have synthesized and characterized a new series of cyclometalated iridium complexes bearing sterically demanding polyaromatic r-coordination sites. Comprehensive
structural characterization and photophysical behaviour of the corresponding complexes have been investigated. The results, in combination with theoretical approaches, address the correlation between the nature of the r-coordinating ligand segment and the absorption/emission characteristics such as spectral profile, peak wavelength and relaxation dynamics. For complexes 5–8, a wide spectral tunability ranging from green to red has been achieved. Electroluminescent devices using a selected complex (5) as dopant in a polymer or polymer : ETL blend matrix are demonstrated with promising device parameters. Work focusing on promoting device engineering and its associated physical and photophysical properties is underway.
Acknowledgements
Financial support from Academia Sinica and National Taiwan University is gratefully acknowledged.
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