Highly branched green phosphorescent tris-cyclometalated
iridium(III) complexes for solution-processed organic light-emitting diodes
Wei-Sheng Huang
a, Chia-Wei Lin
a, Jiann T. Lin
b,*, Jen-Hsien Huang
c, Chih-Wei Chu
c,
Ying-Hsien Wu
d, Hong-Cheu Lin
a,*a
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC b
Institute of Chemistry, Academia Sinica, Taipei, Taiwan, ROC c
Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC d
Electro-optical Engineering and Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan, ROC
a r t i c l e
i n f o
Article history:
Received 14 October 2008
Received in revised form 28 November 2008 Accepted 24 February 2009
Available online 3 March 2009
PACS: 78.60-Fi Keywords: Iridium complexes Dendrimers Phosphorescence
Organic light-emitting diodes
a b s t r a c t
A series benzoimidazole-based dendritric complexes of iridium dendrimers containing Fré-chet-type dendrons with peripheral fluorenyl surface groups have been synthesized. These iridium dendrimers are green-emitting with high phosphorescence quantum yield, and can be spin-coated as films of good quality. From cyclic voltammograms (CV), high onset potentials at 1.42–1.58 V due to the peripheral fluorene group were observed. Device from a second generation dendrimer 17 with structure of ITO/PEDOT:PSS/CBP: 20 wt% 17/TPBI/ LiF/Al (PEDOT:PSS = poly(ethylene dioxythiophene): polystyrenesulfonate and CBP = bis(N-carbazolyl)biphenyl) has the best performance: maximum external quantum efficiency of 13.58% and maximum current efficiency of 45.7 cd/A. Space-charge-limited current (SCLC) flow technique was used to measure the mobility of charge carriers in the blend films of the compounds in CBP. Blend films of higher generation dendrimers have lower hole mobility, albeit with higher device efficiencies.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
Since Tang and coworkers reported electroluminescent devices based on tris(8-hydroxyquinoline) aluminum (Alq3) in 1987, organic light-emitting diodes (OLEDs) have
attracted great attention [1]. In recent years, there are increasing numbers of solution-processed OLED devices fabricated from fluorescent polymers or dendrimers[2]. However, the devices exhibit only low efficiencies in most cases. A lot of efforts have been directed to phosphorescent materials in order to improve device efficiencies[3]. Be-cause both singlet and triplet excitons can be harvested, theoretical 100% internal quantum efficiency is possible
to be achieved in electrophosphorescent devices[4]. How-ever, intermolecular interaction frequently leads to quenching of excited states and reduces the performance of OLEDs, fabricated via either vacuum deposition or solu-tion-processing.
An ideal approach to suppress intermolecular interaction and retain high emission quantum yields is to use bulky and/ or rigid peripheries to encapsulate the emitting core, i.e., dendritic approach [5,6]. Indeed, dendritic LEDs (DLEDs) using electrophosphorescent iridium dendrimers as emit-ters were reported to exhibit high luminous efficiency even without any host. For example, a maximum external quan-tum efficiency (EQE) of 13% and a maximum luminous effi-ciency of 34.7 cd A1were reported for green light-emitting
iridium dendrimers with benzoimidazole-based ligands containing carbazolyl dendrons[7]. Similarly, a high EQE va-lue of 13.6% (30 lm/W, 47 cd/A, 110 cd/m2) was also achieved on a host-free DLED based on a dendrimer with a 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2009.02.022
* Corresponding authors. Fax: +886 2 27831237 (J.T. Lin); +886 3 5724727 (H.-C. Lin).
E-mail addresses: [email protected](J.T. Lin), [email protected].
edu.tw(H.-C. Lin).
Contents lists available atScienceDirect
Organic Electronics
fac-tris(2-phenylpyridyl)iridium(III) core[8]. Possibly due to the presence of void space and the insulating linkages in-side a dendrimer, the carrier mobility generally decreases as the dendrimer generation increases[9]. Consequently, the dendrimers are commonly doped in host materials, such as bis(N-carbazolyl)biphenyl (CBP)[10], in order to improve the device efficiency. Red- [11], green-[7,12], and blue-emitting[13]DLEDs have been fabricated to demonstrate very promising efficiencies. It is worthy to note that besides encapsulation, dendrons surrounding the phosphorescent core also allow one to tether with suitable surface groups for enhancing the solubility of the dendrimer to facilitate spin-coating of the film[14], or tether with carrier-transport units for improving charge transporting[11,15].
Previously we synthesized a series of phosphorescent cyclometalated iridium complexes containing benzoimi-dazole-based ligands[16]. High performance DLEDs based on the complexes were fabricated via vacuum deposition. In a recent report we extended our study to Fréchet-type dendritic benzoimidazole ligands[17], and electrolumines-cent (EL) devices with good efficiencies can be achieved by solution-processing. In an attempt to further enlarge the size of Fréchet-type dendron, we tethered periphery with a fluorene moiety which was beneficial to raising the solu-bility and reducing the intermolecular interactions[12b]. Furthermore, fluorene moiety is also possible to assist in carrier hopping [18]. In this paper, we report the first-and second-generation cyclometalated iridium dendrimer, in which Fréchet-type benzyl ether-based dendrons were tethered with peripheral alkylated fluorenyl groups. DLEDs fabricated from these dendrimers by spin-coating tech-nique will also be discussed.
2. Experimental 2.1. Characterization
The1H NMR spectra were recorded on a Bruker AMX400
spectrometer. FAB-mass spectra were collected on a JMS-700 double focusing mass spectrometer (JEOL, Tokyo, Ja-pan) with a resolution of 3000 for low resolution and 8000 for high resolution (5% valley definition). For FAB-mass spectra, the source accelerating voltage was operated at 10 kV with a Xe gun, using 3-nitrobenzyl alcohol as the matrix. MALDI-mass spectra were collected on a Voyager DE-PRO (Applied Biosystem, Houston, USA) equipped with a nitrogen laser (337 nm) and operated in the delayed extraction reflector mode. Elemental analyses were per-formed on a Perkin–Elmer 2400 CHN analyzer. Cyclic vol-tammetry experiments were performed with a BHI-621B electrochemical analyzer. All measurements were carried out at room temperature with a conventional three-trode configuration consisting of a platinum working elec-trode, an auxiliary elecelec-trode, and a nonaqueous Ag/AgNO3
reference electrode. The E1/2 values were determined as
1=2ðEapþ E c pÞ, where E a pand E c
p are the anodic and cathodic
peak potentials, respectively. The solvent used was CH2Cl2
and the supporting electrolyte was 0.1 M tetrabutylammo-nium hexafluorophosphate. Electronic absorption spectra were obtained on a Cary 50 Probe UV–visible spectrometer.
Emission spectra were recorded in deoxygenated solutions at 298 K by a JASCO FP-6500 fluorescence spectrometer. The emission spectra were collected on samples with o.d. 0.1 at the excitation wavelength. UV–visible spectra were checked before and after irradiation to monitor any possi-ble sample degradation. Emission maxima were reproduc-ible within 2 nm. Luminescence quantum yields (Uem)
were calculated relative to Ir(ppy)3(Uem= 0.40 in toluene) [19]. Luminescence quantum yields were taken as the aver-age of three separate determinations and were reproduc-ible within 10%. Luminescence lifetimes were determined on an Edinburgh FL920 time-correlated pulsed single-pho-ton-counting instrument. Samples were degassed via freeze–thaw–pump cycle at least three times prior to mea-surements. Samples were excited at 337 nm from a nitro-gen pulsed flashlamp with 1 ns FWHM pulse duration transmitted through a Czerny-Turner design monochroma-tor. Emission was detected at 90o via a second
Czerny-Turner design monochromator onto a thermoelectrically cooled red-sensitive photomultipler tube. The resulting photon counts were stored on a microprocessor-based multichannel analyzer. The instrument response function was profiled using a scatter solution and subsequently deconvoluted from the emission data to yield an undis-turbed decay. Nonlinear least square fittings of the decay curves were performed with the Levenburg–Marquardt algorithm and implemented by the Edinburgh Instruments F900 software. The reported values represent the average of at least three readings.
2.2. Light-emitting devices fabrication
A layer of 70 nm thick
poly(ethylenedioxythio-phene):poly(styrene-sulfonic acid) (PEDOT:PSS) (Baytron PVP CH 8000) films was spin-coated on pre-cleaned ITO-coated glass substrates as the hole injection layer and then baked at 100 °C in air for 1 h. Next, the film of CBP containing iridium dendrimer or the neat iridium film (thickness at 45 nm for 16 and 18, 70 nm for 17 and 19, respectively) as the emitter was spin-coated using dichloroethane as the solvent (concentration: 10 mg mL1
for the host and wt% Ir dendrimer as the guest) at a spin rate of 2800 rpm (revolution per min.). Then, a
electron-transporting and hole blocking
1,3,5-tris(N-phen-ylbenzimidazol-2-yl)benzene (TPBI) film of 40 nm thick was vacuum deposited in a vacuum chamber less than 2.5 105torr. Finally, the device was completed by
thermal deposition of a LiF/Al (1 nm/120 nm) cathode. 2.3. Hole-only devices fabrication
The hole-only devices in this study consists of a 20 wt% of 16 (or 17–19) in CBP blend thin film sandwiched be-tween transparent indium tin oxide (ITO) anode and metal cathode. Before device fabrication, the ITO glasses (1.5 1.5 cm2) were ultrasonically cleaned in detergent,
de-ionized water, acetone and isopropyl alcohol before the deposition. After routine solvent cleaning, the sub-strates were treated with UV ozone for 15 min. Then a modified ITO surface was obtained by spin-coating a layer of poly(ethylene dioxythiophene): polystyrenesulfonate
(PEDOT:PSS) (30 nm). After baking at 130 °C for 1 h, the substrates were then transferred into a nitrogen-filled glove box. The active layer was spin coated (spin rate = 2800 rpm; spin time = 45 s) on top of PEDOT:PSS and then dried in covered glass Petri dishes. The film thick-ness of the active layer was measured to be 55, 50, 50 and 50 nm, for 16, 17, 18 and 19, respectively. Subsequently, a
20 and 100 nm thick of MoO3 and aluminum was
ther-mally evaporated under vacuum at a pressure below 6 106torr thorough a shadow mask. The active area of the device was 0.12 cm2.
3. Materials
Chemicals and solvents were reagent grades and pur-chased from Aldrich, Acros, TCI, and Lancaster Chemical Co. Solvents were dried by standard procedures. All reac-tions and manipulareac-tions were carried out under N2with
the use of standard inert atmosphere and Schlenk tech-niques. Solvents were dried by standard procedures. All column chromatography was performed by using silica gel (230–400 mesh, Macherey-Nagel GmbH & Co.) as the stationary phase in a column which is 25–35 cm in length and 2.5 cm in diameter.
3.1. 9,9-Dihexyl-9H-fluorene-2-carbaldehyde (1)
2-Bromo-9,9-dihexyl-9H-fluorene (20.2 g, 48.9 mmol) was dissolved in 100 mL of dry THF and the solution was cooled to 78 °C. n-Butyl lithium in hexane (1.6 M, 30.5 mL, 48.9 mmol) was added dropwise over a period of 30 min. The mixture was allowed to warm to 20 °C in the next 1 h, and 3.8 mL of dry DMF was added. The mix-ture was stirred at room temperamix-ture for 12 h. The reaction was quenched with water and the solution was extracted with diethyl ether. The combined organic extracts were washed with brine solution, dried over MgSO4, and
evapo-rated to dryness. The residue was purified by column chro-matography using a mixture of CH2Cl2and hexanes (1:1)
as the eluent to afford a white solid (13.4 g, 75%). 1H
NMR (CDCl3, 400 MHz, ppm): d 10.04 (s, 1H, CHO), 7.85
(s, 1H), 7.83–7.80 (m, 2H), 7.76–7.74 (m, 1H), 7.37–7.33 (m, 3H), 2.00–1.97 (m, 4H, CH2), 1.07–0.97 (m, 12H, CH2),
0.75 (t, J = 7.2 Hz, 6H, CH3), 0.56–0.53 (m, 4 H, CH2).
3.2. 4-(9,9-Dihexyl-9H-fluoren-2-yl)benzaldehyde (2) 4-Bromobenzaldehyde (9.25 g, 50 mmol), 9,9-dihexyl-9H-fluoren-2-yl-boronic acid (22.5 g, 1.2 equiv.), Na2CO3
(12.0 g, 2 equiv.), and Pd(OAc)2(112 mg, 0.01 equiv.) were
dissolved in a mixture of 30 mL of acetone and 35 mL of water. The mixture was stirred at room temperature for 16 h. The reaction was then quenched by pouring the solu-tion into water and the desired compound was extracted with diethyl ether. The collected organic extracts were col-lected, dried over anhydrous MgSO4. Filtration and
re-moval of the solvent provided a white solid. It was purified by column chromatography using a mixture of dichloromethane and hexanes (1:1) as the eluent to give a yellow oil in 73% yield (16.0 g). 1H NMR (CDCl3,
400 MHz, ppm): d 10.06 (s, 1H, CHO), 7.97 (d, J = 8.4 Hz, 2H, C6H4), 7.82 (d, J = 8.4 Hz, 2H, C6H4), 7.78 (d, J = 8.0 Hz, 1H, fluorene), 7.75–7.73 (m, 1H, fluorene), 7.62–7.60 (m, 2H, fluorene), 7.38–7.30 (m, 3H, fluorene), 2.04–1.99 (m, 4H, CH2), 1.12–1.04 (m, 12H, CH2), 0.75 (t, J = 7.2 Hz, 6H, CH3), 0.68–0.66 (m, 4H, CH2). 3.3. (9,9-Dihexyl-9H-fluoren-2-yl)methanol (3)
Compound 1 was dissolved in 40 mL of THF and 40 mL of methanol. Sodium borohydride (2 equiv.) was added slowly to the above solutions in portions, and the solution was allowed to stir for 24 h. The reaction was quenched by pouring the solution into water and the desired compound was extracted with diethyl ether. The organic extracts were collected and dried over anhydrous MgSO4. Filtration
and removal of the solvent provided a white solid. It was purified by column chromatography using a mixture of dichloromethane and hexanes (1:1) as the eluent to afford a white powder in 75% yield.1H NMR (CDCl3, 400 MHz,
ppm): d 7.67–7.64 (m, 2H), 7.32–7.25 (m, 5H), 4.75 (s, 2H, OCH2), 1.96–1.91 (m, 4H, CH2), 1.11–1.00 (m, 12H,
CH2), 0.75 (t, J = 7.2 Hz, 6H, CH3), 0.61–0.55 (m, 4H, CH2).
3.4. 4-(9,9-Dihexyl-9H-fluoren-2-yl)phenyl)methanol (4) Compound 4 was synthesized by the same procedure as illustrated for compound 3 except that compound 1 was used instead of compound 2. The compound was isolated as a white solid in 75% yield. 1H NMR (CDCl
3, 400 MHz, ppm): d 7.77–7.73 (m, 2H, fluorene), 7.69 (d, J = 8.4 Hz, 2H, C6H4), 7.59–7.57 (m, 2H), 7.48 (d, J = 8.4 Hz, 2H, C6H4), 7.39–7.31 (m, 3H, fluorene), 4.74 (d, J = 2.0 Hz, 2H, OCH2) 1.99–1.96 (m, 4H, CH2), 1.09–1.01 (m, 12H, CH2), 0.73 (t, J = 7.2 Hz, 6H, CH3), 0.66–0.64 (m, 4H, CH2). 3.5. 2-(Bromomethyl)-9,9-dihexyl-9H-fluorene (5)
A mixture of compound 3 (1.82 g, 5.0 mmol) and
tri-phenylphosphine (1.1 equiv.) was dissolved in THF
(15 mL) and cooled to 15 °C. N-bromosuccinimide
(1.1 equiv.) was added all at once. The reaction was stirred for additional 10 min and immediately quenched by cold water. The solids formed were extracted into dichloro-methane. The organic extracts were collected, washed with brine, and dried over anhydrous MgSO4. After filtration and
removal of the solvent, the crude product was further puri-fied by column chromatography on a silica gel column using a mixture of CH2Cl2 and hexanes (1:5 by volume)
as the eluent to afford the pure compound as a white solid in 85% yield.1H NMR (CDCl 3, 400 MHz, ppm): d 7.67–7.61 (m, 2H, fluorene), 7.36–7.30 (m, 5H, fluorene), 4.59 (s, 2H, CH2Br), 1.98–1.90 (m, 4H, CH2), 1.12–1.05 (m, 12H, CH2), 0.75 (t, J = 7.2 Hz, 6H, CH3), 0.59–0.50 (m, 4H, CH2). 3.6. 2-(4-(Bromomethyl)phenyl)-9,9-dihexyl-9H- fluorene (6)
Compound 6 was synthesized by the same procedure as illustrated for compound 5 except that compound 3 was used instead of compound 4. The product was isolated as
a white solid in 76% yield.1H NMR (CDCl
3, 400 MHz, ppm):
d7.73 (d, J = 8.0 Hz, 1H, fluorene), 7.70 (d, J = 7.2 Hz, 1H, fluorene), 7.62 (d, J = 8.4 Hz, 2H, C6H4), 7.54 (dd, J = 8.0 Hz
and 1.6 Hz, 1H, fluorene), 7.51 (d, J = 1.2 Hz, 1H, fluorene), 7.47 (d, J = 8.4 Hz, 2H, C6H4), 7.34–7.28 (m, 3H, fluorene), 4.55 (s, 2H, CH2Br), 1.99–1.96 (m, 4H, CH2), 1.09–1.01 (m, 12H, CH2), 0.73 (t, J = 7.2 Hz, 6H, CH3), 0.66–0.64 (m, 4H, CH2). 3.7. (3,5-Bis((9,9-dihexyl-9H-fluoren-2-yl)methoxy)phenyl)-methanol (7)
A mixture of 3,5-dihydroxybenzyl alcohol (2.80 g, 20 mmol), potassium carbonate (6.67 g, 40 mmol), com-pound 5 (2.1 equiv.), and 18-crown-6-ether (0.52 g, 0.2 mmol) in acetone (30 mL) was heated to reflux for 48 h. After being cooled, water was added and the solution was extracted with dicholormethane. The organic extracts were collected, washed with brine, and dried over anhy-drous MgSO4. After filtration and removal of the solvent,
the crude product was further purified by column chroma-tography using a mixture of CH2Cl2 and hexanes (1:1 by
volume) as the eluent. The product was isolated as a white solid in 70% yield.1H NMR (CDCl 3, 400 MHz, ppm): d 7.67 (d, J = 8.0 Hz, 4H, fluorene), 7.37–7.28 (m, 10H, fluorene), 6.64 (d, J = 2.0 Hz, 2H, C6H3), 6.62 (t, J = 2.0 Hz, 1H, C6H3), 5.10 (s, 4H, OCH2), 4.62 (s, 2H, OCH2), 1.95–1.91 (m, 8H, CH2), 1.12–0.97 (m, 24H, CH2), 0.74 (t, J = 7.2 Hz, 12H, CH3), 0.64–0.56 (m, 8H, CH2). 3.8. (3,5-Bis(4-(9,9-dihexyl-9H-fluoren-2-yl)benzyloxy)-phenyl)methanol (8)
Compound 8 was synthesized by the same procedure as illustrated for compound 7 except that 5 was used instead of 6. The product was isolated as a white solid in 70% yield.
1H NMR (CDCl 3, 400 MHz, ppm): d 7.74–7.68 (m, 4H, fluo-rene), 7.67 (d, J = 8.4 Hz, 4H, C6H4), 7.56–7.49 (m, 4H, fluo-rene), 7.47 (d, J = 8.4 Hz, 4H, C6H4), 7.33–7.25 (m, 6H, fluorene), 6.67 (d, J = 2.0 Hz, 2H, C6H3), 6.60 (t, J = 2.0 Hz, 1H, C6H3), 5.07 (s, 4H, OCH2), 4.61 (s, 2H, OCH2), 1.95– 1.91 (m, 8H, CH2), 1.12–0.97 (m, 24H, CH2), 0.74 (t, J = 7.2 Hz, 12H, CH3), 0.64–0.56 (m, 8H, CH2). 3.9. Compound 9
Compound 9 was synthesized by the same procedure as illustrated for compound 5 except that compound 3 was used instead of compound 7. The product was isolated as a white solid in 70% yield. 1H NMR (CDCl
3, 400 MHz, ppm): d 7.68 (d, J = 8.4 Hz, 4H, fluorene), 7.41–7.25 (m, 10H, fluorene), 6.66 (d, J = 2.0 Hz, 2H, C6H3), 6.62 (t, J = 2.0 Hz, 1H, C6H3), 5.09 (s, 4H, OCH2), 4.39 (s, 2H, OCH2), 1.95–1.91 (m, 8H, CH2), 1.09–1.00 (m, 24H, CH2), 0.74 (t, J = 7.2 Hz, 12H, CH3), 0.56–0.53 (m, 8H, CH2). 3.10. Compound 10
Compound 10 was synthesized by the same procedure as illustrated for compound 5 except that compound 3 was used instead of compound 8. The product was isolated
as a white solid in 72% yield.1H NMR (CDCl
3, 400 MHz, ppm): d 7.74–7.69 (m, 4H, fluorene), 7.67 (d, J = 8.4 Hz, 4H, C6H4), 7.56–7.54 (m, 4H, fluorene), 7.51 (d, J = 8.4 Hz, 4H, C6H4), 7.34–7.26 (m, 6H, fluorene), 6.67 (d, J = 2.0 Hz, 2H, C6H3), 6.61 (t, J = 2.0 Hz, 1H, C6H3), 5.09 (s, 4H, OCH2), 4.43 (s, 2H, OCH2), 1.99–1.95 (m, 8H, CH2), 1.11– 1.03 (m, 24H, CH2), 0.73 (t, J = 7.2 Hz, 12H, CH3), 0.67– 0.63 (m, 8H, CH2). 3.11. 4-(1-Phenyl-1H-benzo[d]imidazol-2-yl)phenol (11) N-Phenyl-o-phenylenediamine (9.21 g, 50 mmol), and 4-hydroxybenzaldehyde (6.10 g, 50 mmol) were dissolved in 40 mL of 2-methoxyethanol. The mixture was heated to reflux for 48 h. After cooling, the deposited solids were filtered, washed with dichloromethane, and dried under vacuum to give the desired product (5.1 g, 35%).1H NMR (DMSO-d6, 400 MHz, ppm): d 7.71 (d, J = 7.6 Hz, 1H),
7.57–7.51 (m, 3H), 7.36 (d, 7.6 Hz, 2H), 7.32 (d, J = 8.4 Hz, 2H), 7.26 (t, J = 7.2 Hz, 1H), 7.20 (t, 7.2 Hz, 1H), 7.11 (d, J = 8.0 Hz, 1H), 6.68 (d, 8.4 Hz, 2H). FABMS: m/z 287.2 (M+H)+. Anal. Calcd for C
19H14N2O: C, 79.70; H, 4.93; N,
9.78. Found: C, 79.29; H, 5.04; N, 9.58.
Ligands 12, 13, 14, and 15 were synthesized by similar procedures, as described below for compound 12. Com-pound 11 (0.73 g, 2.5 mmol), K2CO3 (0.35 g, 2.5 mmol),
and compound 5 (1.06 g, 2.5 mmol) were dissolved in 30 mL of DMF. The mixture was heated at 100 °C for 24 h. After cooling, the reaction was quenched with water and the mixture was extracted with dicholormethane. The organic extracts were collected, washed with brine, and dried over anhydrous MgSO4. After filtration and removal
of the solvent, the crude product was further purified by column chromatography using a mixture of CH2Cl2 and
hexanes (3:1 by volume) as the eluent to provide 12 as a white solid in 73% yield.1H NMR (CDCl3, 400 MHz, ppm):
d 7.84 (d, J = 8.0 Hz, 1H), 7.67 (d, J = 8.4 Hz, 2H, C6H4),
7.55–7.43 (m, 5H), 7.35–7.27 (m, 8H), 7.20 (d, J = 8.0 Hz, 2H), 6.92 (d, J = 8.4 Hz, 2H, C6H4), 5.10 (s, 2H, OCH2),
1.94–1.90 (m, 4H, CH2), 1.10–1.00 (m, 12H, CH2), 0.74 (t,
J = 7.2 Hz, 6H, CH3), 0.60–0.58 (m, 4H, CH2). FABMS: m/z
633.3 (M+H)+. Anal. Calcd for C
45H48N2O: C, 85.40; H,
7.64; N, 4.43. Found: C, 85.24; H, 7.79; N, 4.35. 13: White solid. Yield = 75%.1H NMR (CDCl
3, 400 MHz, ppm): d 8.14 (d, J = 8.0 Hz, 1H), 7.68–7.60 (m, 8H), 7.54– 7.46 (m, 3H), 7.38–7.18 (m, 13H), 6.89 (d, J = 8.4 Hz, 2H), 6.67 (d, J = 2.0 Hz, 2H, C6H3), 6.62 (t, J = 2.0 Hz, 1H, C6H3), 5.09 (s, 4H, OCH2), 4.99 (s, 2H, OCH2), 1.94–1.86 (m, 8H, CH2), 1.09–0.99 (m, 24H, CH2), 0.73 (t, J = 7.2 Hz, 12H, CH3), 0.65–0.54 (m, 8H, CH2). FABMS: m/z 1101.9 (M+H)+.
Anal. Calcd for C78H88N2O3: C, 85.05; H, 8.05; N, 2.54.
Found: C, 85.15; H, 8.22; N, 2.60.
14: White solid. Yield = 56%.1H NMR (CDCl
3, 400 MHz, ppm): d 7.84 (d, J = 8.0 Hz, 1H), 7.72 (d, J = 8.0 Hz, 1H), 7.69 (d, J = 7.2 Hz, 1H), 7.65 (d, J = 8.0 Hz, 2H), 7.55–7.45 (m, 9H), 7.31–7.28 (m, 6H), 7.22–7.18 (m, 2H), 6.91 (d, J = 8.8 Hz, 2H), 5.09 (s, 2H, OCH2), 1.99–1.96 (m, 4H, CH2), 1.09–1.01 (m, 12H, CH2), 0.73 (t, J = 6.8 Hz, 6H, CH3), 0.66–0.64 (m, 4H, CH2). FAB MS: m/z 709.5 (M+H)+. Anal.
Calcd for C51H52N2O: C, 86.40; H, 7.39; N, 3.95. Found: C,
15: White solid. Yield = 56%.1H NMR (CDCl 3, 400 MHz, ppm): d 8.02 (d, J = 8.0 Hz, 1H), 7.71–7.61 (m, 10H), 7.55– 7.47 (m, 12H), 7.34–7.27 (m, 9H), 7.20 (d, J = 8.4 Hz, 1H), 6.92 (d, J = 8.4 Hz, 2H), 6.67 (d, J = 2.0 Hz, 2H, C6H3), 6.62 (t, J = 2.0 Hz, 1H, C6H3), 5.11 (s, 4H, OCH2), 4.43 (s, 2H, OCH2), 1.99–1.96 (m, 8H, CH2), 1.09–1.01 (m, 24H, CH2), Br C6H13 C6H13 C6H13 C6H13 CHO 1 (a) C6H13C6H13 OH C6H13C6H13 Br (b) (c) 3 5 B(OH)2 C6H13 C6H13 Br OHC + C6H13 C6H13 CHO C6H13C6H13 Br (d) (c) 2 C6H13C6H13 OH (b) 4 6 5 + OH OH HO OH O O 7 (c) O O + D = , N N O D H2N HN + OHC OH N N OH O O C6H13 C6H13 C6H13 C6H13 C6H13 C6H13 C6H13 C6H13 c6h13 C6H13 C6H13 C6H13 12 13 , 14 (e) (f) 11 11 (g) 5 , 6, 9, or 10 C6H13 C6H13 C6H13 C6H13 Br O O C6H13 C6H13 C6H13 C6H13 9 6 + OH OH HO (e) O O Br , C6H13 C6H13 C6H13 C6H13 10 O O OH C6H13 C6H13 C6H13 C6H13 8 (c) 15 , ,
(a) (i) BuLi, THF, -78 °C, (ii) DMF (iii) H
+; (b) NaBH
4, THF: MeOH= 1: 1, rt, 16h; (c)
NBS, PPh
3, THF, rt, 20 min; (d) Pd(OAc)
2, acetone, 35 °C, 16 h; (e) K
2CO
3,
18-crown-6-ether, acetone, reflux, 24h; (f) 2-methoxyethanol, reflux, 16 h; (g) K
2CO
3,
DMF, 100 °C, 16 h.
0.73 (t, J = 7.2 Hz, 12H, CH3), 0.66–0.64 (m, 8H, CH2).
FAB-MS: m/z 1253.8 (M+H)+. Anal. Calcd for C
90H96N2O3: C,
86.22; H, 7.72; N, 2.23. Found: C, 85.88; H, 7.58; N, 2.00. Tris-Ir complexes 16, 17, 18, 19 were synthesized by similar procedures, as described below for 16. To a flask
containing IrCl3 3H2O (176 mg, 0.50 mmol) and 12
(1.10 g, 1.0 mmol) was added a 3:1 mixture of 2-ethoxy-ethanol and water (25 mL). The mixture was refluxed for 48 h. After cooling, the reaction was quenched by water, and the mixture was filtered and washed with diethyl
+ IrCl3. nH2O N N O D Ir N N O D Ir Cl Cl 2 2 4 HCl + n H2O S / H2O (3:1) S= 2-methoxyethanol 12 , 13, 14 or 15 K2CO3, glycerol N N O D N N O D Ir 3 16, 17, 18 ,and 19 N N O O O C6H13 C 6H13 C6H13 C6H13 N N O C6H13 C6H13 N N O N N O O O C6H13 C6H13 C6H13 C6H13 C6H13C 6H13 Ir 3 Ir 3 Ir Ir 16 18 17 19 3 3 , , ,
Scheme 2. Synthesis of dendritic iridium complexes.
Table 1
Physical data of the compounds.
cpd kabs(loge),a(nm) kem,(Up)in solution nm (%) kem,c(Up) film nm (%) s,d , ls sr,e, ls Eoxf(DEp). mV Egg, eV HOMOh , eV LUMOi , eV 12 275 (4.63), 307 (4.73) 354a 1580 3.64 6.11 2.47 13 273 (4.82), 306 (4.67) 355a 1520 3.64 6.05 2.41 14 308 (4.69) 356a 1420 3.64 5.95 2.31 15 282 (4.90), 314 (4.70) 357a 1420 3.64 5.95 2.31 (G0)3Irj 298 (4.6), 313 (4.6), 375 (4.1), 410 (3.8), 453 (3.5) 517 (45) 534 (15) 1.07 2.37 370 (76) 2.90 4.95 2.0 16 268 (4.66), 308 (4.69), 374 (3.96), 407 (3.74), 432 (3.57) 512b (55) 529 (16) 1.46 1.78 394 (63) 2.90 4.92 2.0 17 271 (4.87), 306 (4.64), 366 (3.88), 427 (3.34) 512b(63) 525 (30) 1.46 2.31 397 (76) 2.90 4.92 2.0 18 292 (4.60), 317 (4.69), 356 (3.99), 420 (3.34) 513b(65) 522 (17) 1.05 1.78 413 (93) 2.90 4.94 2.0 19 296 (4.83), 312 (4.77), 354 (3.77), 407 (3.07) 519b (74) 523 (33) 1.02 1.37 414 (124) 2.90 4.94 2.0 a
Measured in CH2Cl2at 298 K at a concentration of 105M.eis the absorption coefficient. b
Recorded in toluene solutions at 298 K. Excitation wavelength was 410 nm for all compounds. Quantum yield was measured in toluene relative to fac-Ir(ppy)3(Up= 0.40).
c Neat-film data measured at 298 K. PL quantum efficiencies in films were measured in an integrating sphere. d Measured in toluene solutions at 298 K.
e sr=s/Up. f
Oxidation potential reported is adjusted to the potential of ferrocene (E1/2= 270 mV vs. Ag/AgNO3) which was used as an internal reference. Conditions of cyclic voltammetric measurements: platinum working electrode; Ag/AgNO3reference electrode. Scan rate: 100 mV/s. Electrolyte: tetrabutylam-monium hexafluorophosphate.
g
Eg: bandgap. Egwas obtained from the absorption spectra. h
HOMO level were calculated from CV potentials using ferrocene as a standard [HOMO = 4.8 + (Eox EFc)]. i
LUMO derived via equation, Eg= HOMO LUMO. jRef.[7].
ether to give
l
-chloro-bridged Ir(III) dimer. One equiv. ofl
-chloro-bridged Ir(III) dimer was mixed with 2.5 equiv. of K2CO3, 2.0 equiv. of 12, and glycerol (5.0 mL) in a flask.The mixture was heated at 190 °C for 24 h. After cooling, the reaction was quenched with water and the mixture was extracted with dicholormethane. The combined ex-tracts were then washed with brine, dried over MgSO4,
fil-tered, and dried under vacuum. The crude product was isolated by column chromatography using a mixture of CH2Cl2and n-hexane (1:1 by volume) as the eluent.
16: Yellow solid. Yield = 56%.1H NMR (CDCl
3, 400 MHz, ppm): d 7.57–7.45 (m, 21H), 7.28–7.24 (m, 12H), 7.18 (d, J = 8.0 Hz, 3H), 7.06–7.02 (m, 6H), 6.81 (t, J = 6.4 Hz, 3H), 6.72 (s, 3H), 6.60 (d, J = 8.4 Hz, 3H), 6.39 (d, J = 8.0 Hz. 3H), 6.21 (d, J = 8.4 Hz, 3H), 4.87 (d, J = 12.0 Hz, 3H, OCH2), 4.79 (d, J = 12.0 Hz, 3H, OCH2), 1.90–1.86 (m, 12H, CH2), 1.04–0.97 (m, 36H, CH2), 0.74 (t, J = 7.2 Hz, 18H, CH3), 0.60–0.56 (m, 12H, CH2). MADLI-TOF: m/z 2087.5
(M+H)+. Anal. Calcd for C
135H141N6O3Ir: C, 86.22; H, 7.72;
N, 2.23. Found: C, 85.97; H, 7.42; N, 2.20. 17: Yellow solid. Yield = 15%.1H NMR (CDCl
3, 400 MHz, ppm): d 7.63–7.56 (m, 18H), 7.52 (t, J = 7.5 Hz, 9H), 7.41 (d, J = 8.0 Hz, 3H), 7.30–7.26 (m, 30H), 7.00–6.94 (m, 6H), 6.79 (t, J = 7.2 Hz, 3H), 6.57–6.54 (m, 12H), 6.30 (s, 3H), 6.04 (s, 3H), 4.95 (s,12H, OCH2), 4.74 (d, J = 12.0 Hz, 3H, OCH2), 4.68 (d, J = 12.0 Hz, 3H, OCH2), 1.92–1.88 (m, 24H, CH2), 1.06–0.97 (m, 72H, CH2), 0.71–0.67 (m, 36H, CH3), 0.58– 0.53 (m, 24H, CH2). MADLI-TOF: m/z 3494.8 (M+H)+. Anal.
Calcd for C234H261N6O9Ir: C, 80.44; H, 7.53; N, 2.41. Found:
C, 80.21; H, 7.33; N, 2.25.
18: Yellow solid. Yield = 40%.1H NMR (CDCl
3, 400 MHz, ppm): d 7.67–7.63 (m, 6H), 7.52–7.46 (m, 12H), 7.41 (d, J = 7.6 Hz, 3H), 7.40–7.32 (m, 9H), 7.32–7.28 (m, 9H), 7.24–7.22 (m, 9H), 7.00 (t, J = 7.6 Hz, 3H), 6.92 (d, J = 7.6 Hz, 3H), 6.75 (t, J = 7.6 Hz, 3H), 6.66 (d, J = 2.4 Hz, 3H), 6.53 (d, J = 8.4 Hz, 3H), 6.28 (d, J = 8.4 Hz, 3H), 6.22 (dd, J = 8.4 and 2.4 Hz, 3H), 4.77 (d, J = 12.0 Hz, 3H, OCH2), 4.74 (d, J = 12.0 Hz, 3H, OCH2), 1.88–1.80 (m, 12H, CH2), 1.10–1.02 (m, 36H, CH2), 0.72 (t, J = 7.2 Hz, 18H, CH3), 0.66–0.63 (m, 12H, CH2). MADLI-TOF: m/z 2316.4
(M+H)+. Anal. Calcd for C
153H153N6O3Ir: C, 79.34; H, 6.66;
N, 3.63. Found: C, 78.97; H, 6.33; N, 3.35. 19: Yellow solid. Yield = 10%.1H NMR (CDCl
3, 400 MHz, ppm): d 7.74–7.69 (m, 12H), 7.65–7.60 (m, 12H), 7.57–7.50 (m, 21H), 7.45–7.42 (m, 15H), 7.36–7.28 (m, 21H), 6.98– 6.92 (m, 6H), 6.88–6.83 (m, 3H), 6.71–6.64 (m, 6H), 6.54– 6.45 (m, 15H), 4.95 (s,12H, OCH2), 4.74 (d, J = 12.0 Hz, 3H, OCH2), 4.68 (d, J = 12.0 Hz, 3H, OCH2), 1.92–1.88 (m, 24H, CH2), 1.06–0.97 (m, 72H, CH2), 0.71–0.67 (m, 36H, CH3), 0.58–0.53 (m, 24H, CH2). MADLI-TOF: m/z 3951.3 (M+H)+.
Anal. Calcd for C270H285N6O9Ir: C, 82.09; H, 7.27; N, 2.13.
Found: C, 81.71; H, 7.23; N, 2.25.
4. Results and discussion 4.1. Syntheses
Bouveault’s synthesis[20]and palladium-catalyzed Su-zuki reaction [21] were used to prepare two aldehydes, 9,9-dihexyl-9H-fluorene-2-carbaldehyde (1), and
4-(9,9-dihexyl-9H-fluoren-2-yl)benzaldehyde (2) (Scheme 1). Reduction of 1 and 2 with NaBH4gave the benzyl alcohol
3 and 4 in 80% yield, which was then treated with N-bro-mosuccinimide and triphenylphosphine to form the benzyl bromides, 5 and 6[4]. Reaction of compound 5 (or 6) with 3,5-dihydroxybenzyl alcohol provides compound 7 (or 8) containing two fluorene branches. Similar to the synthesis
250 300 350 400 450 500 0.0 0.2 0.4 0.6 0.8 1.0 Absorbance Wavelength (nm) 12 13 14 15 0.0 0.2 0.4 0.6 0.8 1.0 PL Intensity (a.u.)
Fig. 1. The absorption spectra and PL of 12–15 in CH2Cl2solution.
250 300 350 400 450 500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0 Absorbance Wavelengh (nm) 16 17 18 19 0.0 0.2 0.4 0.6 0.8 1.0 PL Intensity (a.u.)
Fig. 2. The absorption spectra of 16–19 measured in CH2Cl2solution and the normalized PL spectra of 16–19 measured in toluene.
1E-7 1E-6 1E-5 1E-4
35 40 45 50 55 60 65 70 75 80 Quantum Yield (%) Concentation (M) (G0)3Ir 16 17 18 19
Fig. 3. The variation of quantum yields for (G0)3Ir, and 16–19 in different concentration in toluene.
of 5 from 3, compounds 7 and 8 can be converted to 9 and 10, respectively. Compound 11, prepared from N-phenyl-o-phenylenediamine and 4-hydroxybenzaldehyde, was then allowed to react with 5, 6, 9, and 10, respectively, using
Williamson ether synthesis to form dendritic benzoimidaz-ole ligands 12–15 in 30–90% yields. The preparation of tris-cyclometalated iridium complexes 16–19 involved a two-step synthesis (seeScheme 2): (1) reaction of IrCl3 3H2O
with dendritic ligands 12–15 to from a chloro-bridged di-mer intermediate; (2) reaction of the didi-mer with additional 12–15 in glycerol at 190 °C. Only the facial (fac) isomer was isolated for 16–19, as evidenced from the NMR spectra. 4.2. Optical properties
The photophysical data of compounds 12–19 are sum-marized inTable 1. The absorption spectra and photolumi-nescence (PL) spectra of compounds 12–15 and 16–19 are shown inFigs. 1 and 2, respectively. Absorption bands at 310 nm (
e
104–105M1cm1) arep
–p
* transitioncharacteristic of the benzoimidazolyl moiety, and those at 275 nm (
e
104–105M1cm1) can be attributed top
–p
* the transition of the peripheral fluorene. Besidesthe
p
–p
* transition bands of the ligands, the dendriticirid-ium complexes 16–19 also exhibit weak absorption bands in the range of 350–450 nm due to metal-to-ligand charge transfer transitions,1MLCT and3MLCT.
Fig. 4. AFM images from spin-casting films: blend film of CBP: 20 wt% 16 (a); 17 (b); 18 (c); 19 (d) and neat film of 16 (e); 17 (f); 18 (g); 19 (h).
N N N N N N N N PEDOT: PSS ITO (70 nm) Glass x wt% Ir complexes in CBP or neat Ir complexes (45~70 nm) Al (120nm) TPBI (40 nm) LiF (1nm) CBP TPBI S O O n m PEDOT : PSS SO3 -+
Fig. 5. The configuration of EL devices and the molecular structures of the compounds used.
2.7 5.2 CBP TPBI 5.5 6.2 2.0
h
+e
-4.95energy levels of iridium complexes (16-19) Al 4.2
LiF ITO/
PEDOT:PSS
The dendritic ligands 12–15 emit in the violet-purple region (kem 355 nm) in CH2Cl2. Because of
non-conju-gated nature between the iridium center and the dendron, all dendritic iridium complexes emit green light both in toluene solution and neat film state (kem= 510–529 nm),
similar to their non-dendronized congener, (G0)3Ir [7].
The solution PL quantum yields (0.55–0.74 in toluene) of 16–19 compared favorably with that (0.45 in toluene) of the non-dendronized congener, (G0)3Ir[7], indicating that
the encapsulation indeed more effectively suppresses trip-let–triplet annihilation of the emitting core. The phospho-rescent lifetimes (1.02–1.46
l
s) of these complexes fall into the range of prototype non-dendronized tris-cyclo-metalated iridium complexes[19]. Apparently, the incor-poration of flexible alkyl group or ether linkage does not lead to facile non-radiative decay pathways. The encapsu-lation efficiency increases as the surface group becomes bulkier (i.e.,UPL(18) >UPL(16) andUPL(19) >UPL(17)),or the generation of the dendron increases (i.e., UPL
(17) >UPL (16) and UPL (19) >UPL (18)). Similar to the
quantum yields of the solutions, the trend retains in the film state. The studies of concentration quenching on 16–19 and non-encapsulated complex (G0)3Ir further
wit-ness the merit of encapsulation from the dendron, that is, the solution quantum yield of (G0)3Ir decreases more
rap-idly than 16–19 as the concentration increases from 106M to 5 104M (seeFig 3). Compared to the solution
state inTable 1, there is a significant drop of PL quantum yields in the solid film state (UPL= 0.15–0.33). This is
con-sistent with concentration quenching observed in the tol-uene solution (vide supra). Similar behavior was also reported in other phosphorescent dendrimers [5]. No
emission from the peripheral fluorene was noticed, indi-cating that energy transfer from the peripheral fluorene to the iridium center may occur, or there exists inner filter
effect. We found that excitation at the 1MLCT band
(410 nm) of 17 in the film state led to phosphorescent emission of higher intensity by 2.02.5 times compared to excitation at the peripheral fluorene (250–300 nm). Therefore, the energy transfer from the dendron to the iridium center is not likely to be the main cause of the in-creased quantum yields in the larger dendrimers. This is in
contrast to FIrpic (iridium(III)
bis[(4,6-difluoro-phenyl)pyridinato-N,C2’]-3-hydroxypicolinate) derivatives
with N,N’-dicarbazolyl-3,5-benzene-based dendrons [22], which were described to have efficient singletsinglet and triplettriplet energy transfer from the dendron to the iridium center.
4.3. Film morphology
Good thin film quality is prerequisite for good perfor-mance of OLEDs. Therefore, AFM was used to examine the morphology of the spin-casting films for these com-plexes. All compounds can be fabricated as good-quality films by spin-coating technique. Fig. 4 shows the AFM images of the spin-coating films (45–70 nm thick), ob-tained from iridium complexes 16–19, on plasma treated indium tin oxide (ITO) substrates. The root mean square (RMS) surface roughness of neat film in 16–19 was found to be 0.489, 1.055, 1.287, and 0.667 nm, respectively. The blend films of CBP with 20 wt% of 16–19 had slightly larger RMS surface roughness at 1.175, 0.741, 2.264, and 3.121 nm, respectively.
Table 2
EL data of DLEDs with different composition in the emitting layer. Emitting-layer VON, V max. L (at V), cd/m2 max. gext, % max. gc, cd/A max.gp, lm/W J = 20 mA/cm2 ; J = 100 mA/cm2 k em,max (fwhm), nm CIE, x,y L (at V) cd/m2 g ext,% gc, cd/A gp, lm/W Blend film 20 wt% 16 (4.65 mmol) 4.0 14005 (16.0) 9.8 33.8 21.2 3534 (11.6); 10,954 (14.4) 5.7; 3.2 19.5; 11.2 5.3; 2.4 516 (74) 0.29, 0.62 20 wt% 17 (2.77 mmol) 5.0 16229 (16.0) 13.6 45.8 20.6 6926 (8.7); 15,711 (12.5) 10.4; 4.7 35.1; 15.7 12.7; 3.9 514 (72) 0.27, 0.61 40 wt% 17 (5.54 mmol) 5.5 6321 (20.0) 9.4 33.0 9.4 3766 (16.2); – 5.2; – 18.4; – 3.5; – 520 (76) 0.30, 0.62 20 wt% 18 (4.20 mmol) 5.0 19217 (20.0) 8.28 27.9 13.48 3395 (12.5); 10,396 (18.1) 5.04; 3.10 17.0; 10.5 5.1; 2.4 520 (74) 0.31, 0.61 20 wt% 19 (2.45 mmol) 6.0 13,876 (18.0) 9.0 31.1 15.0 4025 (11.5); 10,830 (14.8) 5.8; 3.2 15.0; 20.1 5.5; 2.3 514 (72) 0.28, 0.62 40 wt% 19 (4.91 mmol) 8.0 5814 (20.0) 8.6 26.5 7.6 3073 (15.8); – 5.0; – 31.1; – 3.1; – 514 (72) 0.28, 0.61 Neat film 16 3.0 4895 (10.0) 3.6 11.4 10.3 1735 (6.0); 4375 (8.6) 2.7; 1.4 8.7; 4.4 4.5; 1.6 516 (76) 0.30, 0.58 17 3.0 993 (10.5) 7.1 23.1 14.5 990 (10.3); 502 (16.9) 1.5; 0.15 5.0; 0.5 1.5; 0.1 516 (74) 0.30, 0.60 18 3.0 5711 (11.5) 4.6 13.7 12.3 1964 (5.8); 4340 (8.2) 3.3;1.5 10.0; 4.4 5.5;1.7 516 (76) 0.30, 0.57 19 5.0 1235 (13.5) 5.0 16.6 9.5 1013 (10.2); 1081 (15.8) 1.5; 0.33 5.1; 1.1 1.6; 0.22 512 (70) 0.26, 0.60 Von, turn-on voltage, at a brightness of 1 cd/m2; L, luminance; V, voltage;gext, external quantum efficiency;gc, current efficiency;gp, power efficiency; fwhm, full width at half-maximum, max., maximum.
4.4. Electrochemical studies
The electrochemical properties of the ligands (12–15) and the complexes (16–19) were studied by cyclic voltam-metric (CV) method, and the electrochemical data are sum-marized in Table 1. All ligands show an irreversible oxidation wave with an onset potential at 1.42–1.58 V vs. Ag/AgNO3, which is characteristic of the peripheral
fluo-rene group. Besides the irreversible oxidation wave of the ligands, a quasi-reversible one-electron oxidation wave attributed to the oxidation of the iridium(III) was detected in the range of 0.39–0.41 mV vs. Ag/AgNO3. The
negligi-ble influence of the dendrons on the oxidation potential of the iridium center is likely due to non-conjugated nature of the spacer between the dendron and the iridium center. No reduction waves up to -2.0 V were detected in these iridium dendrimers. The HOMO (highest occupied molecu-lar orbital) energy levels of compounds 12–19 were calcu-lated from cyclic voltammogram in comparison with ferrocene (4.8 eV) [23]. The thus obtained HOMO levels, (6.0 eV for 12–15 and 4.95 eV for 16–19) in combina-tion with the optical bandgaps which were derived from the optical edges of absorption spectra, were used to calcu-late the LUMO (lowest unoccupied molecular orbital)
en-ergy levels [24]. Both HOMO and LUMO data are
collected inTable 1.
4.5. Electroluminescent properties
Electroluminescent (EL) devices of bilayered configura-tion, ITO/PEDOT:PSS/neat 16–19 or x wt% of dopant 16–19 in CBP (45–70 nm)/TPBI (40 nm)/LiF (1 nm)/Al (120 nm), were fabricated via spin-coating technology (see Fig. 5). Spin-coating technology was used for device fabrication except for the vacuum deposition of TPBI (the electron-transporting and hole-blocking layer). The LED devices without the use of TPBI will not be discussed because the efficiency dropped at least two orders in both cases. The energy band structures of the devices are shown inFig. 6, and the performance data are presented inTable 2. The EL spectra are shown inFig. 7. All devices emitted green light and the EL spectra were superimposed with the PL spectra. The absence of emission from CBP suggests that either energy transfer from CBP to the complexes is plete or the trapping of electrons and holes by the com-plexes is efficient.
Although neat films of good quality can be obtained by spin-coating technique (vide supra), the devices using the neat films of the complexes have efficiencies far from ideal: the maximum external quantum efficiencies are lower than 7.1%. The following four factors are probably responsible for the low device efficiencies: (1) T–T
annihi-400 500 600 700 800 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 EL intensity (a.u.) Wavelengh (nm) 16 20 wt% 17 20 wt% 18 20 wt% 19 20 wt% 350 400 450 500 550 600 650 700 750 800 0.0 0.2 0.4 0.6 0.8 1.0 EL intensity (a.u.) Wavelengh (nm) neat 16 neat 17 neat 18 neat 19
a
b
Fig. 7. EL spectra at a driving voltage of 12 V of DLEDs containing (a) 16– 19 doped with CBP, (b) 16–19 neat film.
0 2 4 6 8 10 12 14 16 18 1 10 100 1000 10000 100000 Br ig h tn e ss ( c d /m 2) Voltage (V) 0 100 200 300 400 500 600 700 800 neat 16 neat 17 neat 18 neat 19 Cur rent densi ty ( m A /cm 2) 0.1 1 10 100 0.1 1 10 100 Ext er n al Q u ant u m Ef fi ci ency ( % )
Current density (mA/cm2)
0.01 0.1 1 10 100 neat 16 neat 17 neat 18 neat 19
Current efficiency (cd/A)
a
b
Fig. 8. The electro-optical characteristic of DLEDs containing neat 16–19 (a) brightness and current density as a function of voltage, and (b) current efficiency and EQE as a function of current density.
lation due to the high concentration of the guest; (2) the large energy gap between PEDOT:PSS and the HOMO level of peripheral fluorenyl group which hampers the hole injection into the emitting layer; (3) inefficient carrier mobility of the dendrons; (4) quenching of phosphores-cence via energy transfer from the iridium center to the dendron due to small gap (1000 cm1) between the
trip-let energies of the two. Foe example, 15 ET(77 K,
tolu-ene) = 2.55 eV; 10, ET (77 K, toluene) = 2.53 eV; 16, ET
(77 K, toluene) = 2.34 eV. The current–voltage–brightness (I–V–L) characteristic and the EQEs and current efficiency vs. current density for the devices based on neat films of 16–19 are shown inFig. 8. DLEDs based on 17 and 19 ex-hibit much lower current density and brightness. Our observations are consistent with previous report illustrat-ing that the carrier mobility decreases as the dendrimer generation increases[9]. The I–V–L characteristic and the EQEs and current efficiency vs. current density of DLEDs containing CBP doped with complexes 16–19 are shown inFig. 9. Among them, the best efficiencies were found to be
g
ext,max= 9.8%, 13.6%, 8.3%, and 9.0%, andg
c,max= 33.8,45.8, 27.9, and 31.1 cd/A for the devices with 4.65, 2.77, 4.20, and 2.45 mol% of 16, 17, 18, and 19, respectively. The significant drop of device efficiencies at dopant con-centrations above 40% for all the complexes is likely due to T–T annihilation. The best device performance was
achieved at a lower dopant concentrations (mol%) for the devices based on the second-generation dendrimers (17 and 19) compared to the devices based on the first-gener-ation dendrimers (16 and 18), which may be attributed to the inefficient carrier conductivity of the fluorene-contain-ing dendrons. Inefficient carrier conductivity of the den-dron is evident from the I–V plots (Fig. 10) of 16 (first generation dendrimer) and 19 (second generation dendri-mer) at different doping concentrations: the current de-creases as the doping concentration inde-creases at the same driving voltage. Though the current density of 18-based device was higher than that of 19-based device, 16- and 17-based devices were found to have comparable current density, where the conductivity might be affected by the film morphology. Nevertheless, the efficiency of the device increases as the generation of the dendrimer increases, i.e., 17 > 16 and 19 > 18. Arylamine-based dendrimers with stilbene dendrons were also reported to have better device efficiency and inferior carrier mobility as the generation increases[9]. It was suggested that the increased spacing of hopping sites in the bulkier dendrimers inhibited trans-port of majority charge carriers and resulted in a reduction of the mobility. The influence of the dendron on the device efficiency may also be accounted by the same token in our
0 5 10 15 20 1 10 100 1000 10000 100000 B rightnes s ( c d/m 2) Voltage (V) 0 50 100 150 200 250 300 350 400 450 500 16 20 wt% 17 20 wt% 18 20 wt% 19 20 wt% Cu rren t d e n s it y (mA/ cm 2) 0.1 1 10 100 1 10 100 Ex te rn a l Qu an tum Effic iency (%)
Current density (mA/cm2)
0.1 1 10 100 Cu rren t ef fi ci en cy ( c d /A) 16 20 wt% 17 20 wt% 18 20 wt% 19 20 wt%
a
b
Fig. 9. The electro-optical characteristic of DLEDs containing x wt% 16–19 in CBP: (a) brightness and current density as a function of voltage; (b) current efficiency and EQE as a function of current density.
0 100 200 300 400 500
CUrrent density (mA/cm
2) Voltage (V) 16 5 wt% 16 10 wt% 16 20 wt% 16 40 wt% 16 50 wt% 16 100 wt% 0 5 10 15 20 0 5 10 15 20 0 100 200 300 400 500
Current density (mA/cm
2) Voltage (V) 19 5 wt% 19 10 wt% 19 20 wt% 19 40 wt% 19 50 wt% 19 100 wt%
a
b
Fig. 10. The electro-optical characteristic of current–voltage (I–V) curves in DLEDs containing CBP with various dopant ratios of (a) compound 16 and (b) compound 19.
case. It is interesting to note that neat film devices also be-have similarly (seeFig. 11).
4.6. Hole mobility properties
Space-charge-limited current (SCLC) flow technique was used to measure the mobility of charge carriers in the films[25]. In spite of the ambipolar carrier-transport-ing characteristic of CBP[26], no discernible currents were detected for the blend film of CBP with the compounds synthesized in electron-only devices, indicating that the electron mobility was negligible in the film. This is consis-tent with the poor performance of DLEDs without the pres-ence of TPBI (vide supra). In comparison, from the hole-only devices the hole mobilities can be determined pre-cisely by fitting the dark current vs. voltage (J–V) curves for single carrier devices to SCLC model[27–28]. The dark current is given by J ¼ 9
e
0e
rl
V2=8L3[29], wheree
0e
ris thepermittivity of the dendrimer,
l
is the carrier mobility, and L is the device thickness. From the capacitance–voltage measurements we have obtained a relative dielectric con-stante
r of 0.65, 1.81, 1.30, 1.98 for the blend film of 20wt% of 16, 17, 18, and 19 in CBP, respectively. Therefore, the hole mobilities were calculated to be 1.85 107,
9.80 108, 4.76 108, and 2.43 108cm2
/Vs for the blend films of 16, 17, 18 and 19, respectively. These obser-vations are consistent with the lower current density mea-sured for DLEDs based on larger dendrimers, i.e., device of 17 < device 16 and device 19 < device 18 (vide supra). The lower hole mobility of the larger dendrimers (17 and 19) is likely due to increased spacing of hopping sites [9]. The better efficiencies of the devices for the larger dendri-mers may be stemmed from the increased waiting time of charge carriers which will increase the probability of recombination, similar to that reported for triarylamine-cored distyrylbenzene-based dendrimers [9]. The device based on the neat film has a lower efficiency than that based on the CBP blend film, which may also be attributed to the non-conductivity of the former, and the very narrow exciton recombination region near the interface between
the neat film and TPBI. Compared with the iridium com-plexes encapsulated with benzyl ether dendrons we re-ported earlier[17], the hole mobilities of the dendrimers in this study appear to be one order lower. The hole mobilities of the previous dendrimers were measured to be 2.7 106, and 9.2 107cm2/V s for the 20 wt% CBP
blend films of (G1)3Ir (first generation dendrimer) and
(G2)3Ir (second generation dendrimer), respectively.
Possi-bly the larger fluorene moiety increases spacing of hopping sites and results in a reduction of the mobility (vide supra). The somewhat lower device efficiencies in this study may be due to the less balanced hole and electron mobilities (the electron mobility of TPBI is 105cm2/(V s)[30]).
5. Conclusions
In conclusion, we have synthesized a series of benzoim-idazole-based dendritic complexes of iridium dendrimers containing Fréchet-type dendrons with peripheral fluoenyl surface groups. These iridium dendrimers emit green light with high PL quantum yields, and can be spin-cast as films of good quality. The electroluminescent devices fabricated by the spin-coating technique have high-performance of electro-optical properties. One of the devices with the structure of ITO/PEDOT:PSS/CBP: 17 (20 wt%)/TPBI/LiF/Al has a maximum EQE of 13.58% and a maximum current efficiency of 45.7 cd/A. The high HOMO level of the periph-eral fluoenyl surface group leads to a larger turn on volt-age. EL devices based on the dendrimers of higher generation have a lower current density because of the slower carrier mobility of the higher generation dendri-mer. However, EL devices from dendrimers of higher gen-eration exhibit higher EQEs and current efficiencies. Acknowledgments
We thank the Academia Sinica, National Chiao Tung University and the National Science Council for supporting this work. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0.01 0.1 1 0 2 4 6 8 0.0 2.0x10-5 4.0x10-5 6.0x10-5 8.0x10-5 1.0x10-4 1.2x10-4 1.4x10-4 Voltage (V2) J (A /m 2) J (A/m 2 ) Voltage (V) 16 20 wt% in CBP (1.85* 10-7 cm2 /Vs) 17 20 wt% in CBP (9.80* 10-8 cm2/Vs) 18 20 wt% in CBP (4.76* 10-8 cm2 /Vs) 19 20 wt% in CBP (2.43* 10-8 cm2/Vs)
Fig. 11. The current–voltage (J–V) plots of hole-only devices with 20 wt% of 16–19 in CBP. Insert is the log J–V2 curves.
References
[1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] (a) P.W. Wang, Y.J. Liu, C. Devadoss, P. Bharathi, J.S. Moore, Adv.
Mater. 8 (1996) 237;
(b) M. Halim, J.N.G. Pillow, I.D.N. Samuel, P.L. Burn, Adv. Mater. 11 (1999) 371;
(c) A.W. Freeman, S.C. Koene, P.R.L. Malenfant, M.E. Thomson, J.M.J. Fréchet, J. Am. Chem. Soc. 122 (2000) 12385;
(d) A. Adronov, J.M.J. Fréchet, Chem. Commun. (2000) 1701; (e) J.M. Lupton, I.D.W. Samuel, R. Beavington, M.J. Frampton, P.L. Burn, H. Bässler, Phys. Rev. B 63 (2001) 5206.
[3] (a) H.Z. Xie, M.W. Liu, O.Y. Wang, X.H. Zhang, C.S. Lee, L.S. Hung, S.T. Lee, P.F. Teng, H.L. Kwong, H. Zheng, C. Che, Adv. Mater. 13 (2001) 1245;
(b) J. Ostrowski, M.R. Robinson, A.J. Heeger, G.C. Bazan, Chem. Commun. (2002) 784;
(c) J.-P. Duan, P.-P. Sun, C.-H. Cheng, Adv. Mater. 15 (2003) 224; (d) Y.-J. Su, H.-L. Huang, C.-L. Li, C.-H. Chien, Y.-T. Tao, P.-T. Chou, S. Datta, R.-S. Liu, Adv. Mater. 15 (2003) 884;
(e) A.B. Tamayo, B.D. Alleyne, P.I. Djurovich, S. Lamansky, I. Tsyba, N.N. Ho, R. Bau, M.E. Thompson, J. Am. Chem. Soc. 125 (2003) 7377; (f) W. Lu, B.-X. Mi, M.C.W. Chan, Z. Hui, N. Zhu, S.-T. Lee, C.-M. Che, Chem. Commun. (2002) 206;
(g) B.W. D’Andrade, J. Brooks, V. Adamovich, M.E. Thompson, S.R. Forrest, Adv. Mater. 14 (2002) 1032;
(h) Y. Ma, H. Zhang, J. Shen, C.-M. Che, Synth. Met. 94 (1998) 245; (i) Y. Ma, C.-M. Che, H.-Y. Chao, X. Zhou, W.H. Chan, J. Shen, Adv. Mater. 11 (1999) 852.
[4] (a) M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151;
(b) M.A. Baldo, D.F. O’Brien, M.E. Thompson, S.R. Forrest, Phys. Rev. B 60 (1999) 14422;
(c) C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys.. 90 (2001) 5048.
[5] T.D. Anthopoulos, M.J. Frampton, E.B. Namdas, P.L. Burn, I.D.W. Samuel, Adv. Mater. 16 (2004) 557.
[6] S.-C. Lo, N.A.H. Male, J.P.J. Markham, S.W. Magennis, P.L. Burn, I.D.W. Samuel, Adv. Mater. 14 (2002) 975.
[7] J. Ding, J. Gao, Y. Cheng, Z. Xie, L. Wang, D. Ma, X. Jing, F. Wang, Adv. Funct. Mater. 16 (2006) 575.
[8] S.-C. Lo, T.D. Anthopoulos, E.B. Namdas, P.L. Burn, I.D.W. Samuel, Adv. Mater. 17 (2005) 1945.
[9] (a) J.M. Lupton, I.D.W. Samuel, R. Beavington, P.L. Burn, H. Bässler, Adv. Mater. 13 (2001) 258;
(b) J.M. Lupton, I.D.W. Samuel, R. Beavington, M.J. Frampton, P.L. Burn, H. Bässler, Phys. Rev. 63 (2001) 155206.
[10] P.L. Burn, S.-C. Lo, I.D.W. Samuel, Adv. Mater. 19 (2007) 1675. [11] (a) G. Zhou, W.-Y. Wong, B. Yao, Z. Xie, L. Wang, Angew. Chem., Int.
Ed. 46 (2007) 1149;
(b) B. Liang, L. Wang, Y. Xu, H. Shi, Y. Cao, Adv. Funct. Mater. 17 (2007) 3580.
[12] (a) J.P.J. Markham, S.-C. Lo, S.W. Magennis, P.L. Burn, I.D.W. Samuel, Appl. Phys. Lett. 80 (2002) 2645;
(b) R.N. Bera, N. Cumpstey, P.L. Burn, I.D.W. Samuel, Adv. Funct. Mater. 17 (2007) 1149.
[13] S.-C. Lo, G.J. Richard, J.P.J. Markham, E.B. Namdas, S. Sharma, P.L. Burn, I.D.W. Samuel, Adv. Funct. Mater. 15 (2005) 1451.
[14] (a) T.D. Anthopoulos, J.P.J. Markham, E.B. Namdas, J.R. Lawrence, I.D.W. Samuel, S.-C. Lo, P.L. Burn, Org. Electron. 4 (2003) 71; (b) N. Cumpstey, R.N. Bera, P.L. Burn, I.D.W. Samuel, Macromolecules 38 (2005) 9564.
[15] (a) T. Tsuzuki, N. Shirasawa, T. Suzuki, S. Tokito, Jpn. J. Appl. Phys. 44 (2005) 4151;
(b) S.-C. Lo, E.B. Namdas, C.P. Shipley, J.P.J. Markham, T.D. Anthopolous, P.L. Burn, I.D.W. Samuel, Org. Electron. 7 (2006) 85. [16] W.-S. Huang, J.T. Lin, C.-H. Chien, Y.-T. Tao, S.-S. Sun, Y.-S. Wen,
Chem. Mater. 16 (2004) 2480.
[17] W.-S. Huang, J.T. Lin, H.-C. Lin, Org. Electron. 9 (2008) 557. [18] (a) C.-C. Wu, T.-L. Liu, W.-Y. Hung, Y.-T. Lin, K.-T. Wong, R.-T. Chen,
Y.M. Chen, Y.-Y. Chieh, J. Am. Chem. Soc. 125 (2003) 3710; (b) C.-C. Wu, Y.-T. Lin, K.-T. Wong, R.-T. Chen, Y.-Y. Chieh, Adv. Mater. 16 (2004) 61;
(c) C.-C. Wu, T.-L. Liu, Y.-T. Lin, W.-Y. Hung, T.-H. Ke, Appl. Phys. Lett. 85 (2004) 1172;
(d) C.-C. Wu, W.-G. Liu, W.-Y. Hung, T.-L. Liu, K.-T. Wong, Y.-Y. Chieh, R.-T. Chen, T.-H. Hung, T.-C. Chao, Y.-M. Chen, Appl. Phys. Lett. 87 (2005) 052103.
[19] A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K. Ueno, J. Am. Chem. Soc. 125 (2003) 12971.
[20] (a) L. Bouveault, Bull. Soc. Chim. Fr. 31 (1994) 1306; (b) J.D. Einhorn, J. Org. Chem. 49 (1984) 1078; S.M. Denton, A. Wood, Synlett (1999) 55.
[21] L. Liu, Y. Zhang, B. Xin, J. Org. Chem. 71 (2006) 3994.
[22] T.-H. Kwon, M.K. Kim, J. Kwon, D.-Y. Shin, S.J. Park, C.-L. Lee, J.-J. Kim, J.-I. Hong, Chem. Mater. 19 (2007) 3673.
[23] J. Pommerehne, H. Vestweber, W. Guss, R.F. Mahrt, H. Bässler, M. Porsch, J. Daub, Adv. Mater. 7 (1995) 551.
[24] (a) B.E. Koene, D.E. Loy, M.E. Thompson, Chem. Mater. 10 (1998) 2235;
(b) M. Thelakkat, H.-W. Schmidt, Adv. Mater. 10 (1998) 219. [25] D. Hertel, H. Bässler, ChemPhysChem 9 (2008) 666. [26] H. Kanai, S. Ichinosawa, Y. Sato, Syn. Met. 91 (1997) 195. [27] P.W.M. Blom, M.J.M. De Jong, M.G. Van Munster, Phys. Rev. B 55
(1997) R656.
[28] D.H. Dunlap, P.E. Parris, V.M. Kenkre, Phys. Rev. Lett. 77 (1996) 542. [29] W.D. Gill, J. Appl. Phys. 43 (1972) 5033.
[30] Y. Li, M.K. Fung, Z. Xie, S.-T. Lee, L.-S. Hung, J. Shi, Adv. Mater. 14 (2002) 1317.
