VIETNAM JOURNAL OF CHEMISTRY VOL. 50(5) 613-618 OCTOBER 2012
PREPARATION OF PHOSPHORESCENT ORGANOMETALLIC COMPLEXES OF IRIDIUM (III) AND PLATINUM (II) USED FOR THE
FABRICATION OF POLYMER LIGHT EMITTING DIODE DEVICE
Nguyen Tien Thao'*, Vu Nhu NangS Shigcyuki Yagi^
'Department of Petrochemistry, VNU University of Science, Vietnam National University-Hanoi, 19 Le Thanh Tong Street, Hoan Kiem, Hanoi. Vietnam
^Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, l-I Gakuencho, Nakaku, Sakai, Osaka 599 - 8351, Japan
Received 27 September 2012 Abstract
The phosphorescent organometallic complexes of iridium (III) (Ir-I) and platinum (II) (Pt-I) were prepared using 2-phenyl pyridine and acetyl acetone as ligands and their photophysical properties were characterized The Ir-l complex gave the green photoluminescence (XPL= 519 nm) and Pt-1 complex exhibited a little blue shift in photoluminescence (\pL = 484 nm) due to the appearance of metal cemer transitions (d - d ). In both cases, their lifetime was rather long.
The two obtained complexes were further used for the fabrication of simple polymer light emitting diodes (PLEDs) using PVCz as a host polymer. These devices have emined green electroluminescence corresponding to the maximum photoluminescence of each complex and their maximum brightness was observed at high voltage
Keywords. Phosphorescence, photoluminescence, Pt, Ir, complex, PLED.
1. INTRODUCTION
Organometallic complexes with transition metal centers have received considerable attention in recem years because their high quantum efficiencies have offered significant advantages in various applications, such as organic light emittmg diodes (OLEDs) [1,2], oxygen sensor [3-5], and bio- labeling materials [6,7]. Cyclometalated complexes of heavy metal, particularly those containing platinum and iridium, involving homoleptic M(C'^N)„ and heteroleptic (C^N)n.iM(O'K)) complexes have been widely studied (C^N and 0'X>
are 2-arylpyridinate and 1,3-diketonate ligands, respectively; hereby, designation of ^ represents for bridged bonding between two atoms). Their phosphorescent properties result from a strong spin orbit coupling (a heavy metal effect) as well as an increased d - d ' energy state due to the strong ligand field of C^N [8]. They are good phosphorescent materials for organic light emitting diodes devices (OLEDs). In the present study, we have prepared two transition metal complexes and designed Ir-l as (C'^N)2li<0'K)) complex and Pt-1 as (C'^N)Pt(O'^) complex using 2-phenyl pyridine as cyclometalated C^N ligand and diketone O-X) ligand base on acetyl
acetone (see figure 2 below). An other way, PLED is one kind of organic light emitting diode (OLED) that has paid much attention since it has low cost, and potential roll, simple thin layer structure to expand manufacturing of large areas [4], In PLEDs, holes and electrons are injected into opposite surfaces of a planar multilayer organic thin film [9].
The holes and electrons can migrate through the thin film to a material interface, where they combine to form radiative excited states or excitons. These excitons could be harvested for light emission and therefore, in principal, 100% intemal quantum efficiency can be achieved in PLEDs [1,10-11].
Thus, by doping PLEDs with heavy metal phosphorescent material, the operation of PLEDs is more efficient such as eliminating the singlet and triplet limitation, increasing internal quantum efficiency, and giving efficient color emission.
Indeed, PLEDs have widely used poly(N- vinyIcarbazole)(PVCz) as a host polymer because its excellent film forming properties, high glass transition temperature, high triplet energy level and good hole mobility [12-17]. The present report is to deal with the synthesis and the photoluminescence properties of Ir-l, Pt-1 and the applications of such complexes in PLEDs.
VJC, VoL 50(5), 2012 2. EXPERIMENTAL
2.1. Synthesis of phosphorescent organometallic complexes
For preparation of lr-1 and Pt-1, the starting materials are 2-phenyl pyridine, acetyl acetone and metal salts (IrClj.nHjO and K^PtCI.,). Water and ethoxyelhanol are used as solvents. Cyclometalated metal complexes were synthesize by refluxing metal salt with 2 - 2.5 equivalent of cyclomctalating ligand in a 3:1 mixture of 2 - ethoxyethanol and water.
These products were further refluxed in an inert atmosphere in 2 - ethoxyethanol for 3-11 hours.
After cooling to room temperature, a colored precipitate was filtered off and washed with distilled water, hexane, and clhci The crude products of Ir-I and Pt-1 were chromatographed on a silica column (for Pt-1) and aluminum column (for Ir-I) by dichloromethane mobile phase lo yield pure products, after solvent evaporation and drying.
2.2. Characterization
For characterization of the products, ' H NMR spectra were recorded on a Jeol LA-400 spectrometer (400 MHz), using chloroform D as an internal standard (0.00 ppm). Matrixes assisted desorption/ionization time of light (MALDl ToF- Mass spectra) mass Spectra were measured on a Kratos PC Axima CFRplus V2.4.0 using shinapinic acid as a matrix.
2.3. Spectroscopic measurement
UV-Vis and photoluminescence (PL) spectra were measured on Shimadzu UV-3100 and Jasco FP-6600 spectrophotometer, respectively, using a
PVCz
Nguyen Tien Thao, et ai quartz cell. PL lifetime was obtained Horiba Jobin Yvon equipped with DAS6 v6.5 monochrometer. PL quantum yield were measured on Hamamatsu Photonics C9920PL quantum yield measurement system. The solid samples were measured by dissolving In chloroform solvent and then bubbled by Argon following by complete sealing.
2.4. P L E D s fabrication and testing
The PLEDs were prepared according to a standard procedure [12, 13]. The indium tin oxide glass substrate (ITO) was cleaned by ultrasonic treatment in chloroform solvent, distilled water, acetone, hexane, and isopropanol, respectively. The ITO glass substrate was air-dried after UV-O3 treatment. A layer of poly(3,4- clhylcnedioxythiophene) polystyrenesulfonate (PEDOT-PSS) was first spin coated on the top of ITO glass and then dried at 1 2 0 ^ for 1 hour.
Mixture of metal complexes with mixture of PVCz, 2-(4-tert-bytylphenyl)-5-(4-diphenyI)-1,3,4- oxadiazole (PBD) and Mcompiex was dried onto the PEDOT: PSS layer under argon gas and compatible rotating speed in the glovebox (PBD is). The layers of CsF and Al as cathode were deposited onto the surface of organic layer by vacuum deposition method at pressure of 10"* Pa. Finally the PLED was covered by glass cap and encapsulated by UV epoxy resin in air to prevent the oxidation of cathode and organic layer. The structure of PLED was ITO (150 nm) /PEDOT-PSS (40 nm), PVCz:PVD:M««,pfc. (81- 88 nm), CsF (1 nm), Al (250 nm) (Fig. 1). The PLED properties were tested by Hamamatsu photonics C9920 to evaluate current density, electroluminescence, emission and power efficiency and external quantum efficiency.
C
Glass substritePVCi:PBD:M-compleK (81-88 nm)
PEDOT-PSS (40 nm)
Figure I: Chemical structures of PVCz, PBD and general structure for a PLED device
3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization
The synthesis of lr-1 and Pt-I complexes occurs
in two steps (Fig. 2). In the first step, metal salts react with an excess amount of C'^N ligand to give the cyclometalated complexes (C'^N)2(^- CI)2lr(C^N)2 and (C'^N)2ptCI. In the next step, the chloride bridge was broken and replaced by 614
VJC, Vol. 50(5), 2012
monoanionic diketonate ligand C^O to form monomeric complex (C'^N)2lr(C^0) for the case of iridium while the formation of (CN)Pt(0''0) is simpler due to the replacing of one C^N ligand and chloride atom by another ligand of diketonate ligand O ^ . The yield of the latter step was about 35-56%.
'H NMR spectra confirm the existence of the heterocyclic rings and diketone group in each complex sbiicture. The sfructure of Pt-1 and Ir-l was fiirther confirmed by the analysis of MALDI-TOF mass spectra.
IT - 1: 'H NMR (400 MHz, CDCI3): 5 1.73 (s.
Preparation of phosphorescent organometallic...
6H); 5.20 (s, IH); 6.25 (d, / = 6.4 Hz, 2H); 6.67 (dt, J =1.4 and 7.8 Hz, 2H); 6.80 (dt, J= 0.9 and 7.8 Hz 2H); 7.12 ( d t , / = 1.8 and 7.3 Hz, 2H); 7.53 (dt,7 = 0.9 and 7.8 Hz, 2H); 7.72 (dt, y = 1.8 and 8.2 Hz, 2H); 7.84 (d, / = 8.2 Hz, 2H); 8.15 (d, J = 5.0 Hz, 2H) MALDI TOP MS m/z 600.6
Pt-1: 'H NMR (400 MHz, CDCI3): 5 2.00 (s, 6H); 5.46(s, IH); 7.07-7.12 (m, 2H); 7.18-7.22 (m, IH); 7.43 (d, J = 7.2 Hz, IH); 7.60-7.62 (m, 2H);
7.79 (dt, J = 7.8 Hz and 1.2 Hz, IH); 8.98-9.00 (m, IH) MALDI TOP MS m/z 448.5.
C N + Klm^
/ \ J NairajpEIOCHiCHjOH \ / \ .
"' ^ Nj, reflux, M°C,3h ^ Figure 2: Synthetic scheme of Ir-l complex and Pt-1 complex
3.2. ElectroDic absorptioD and photoluminesceDce properties of Pt-1 and Ir-l
The absorption and photoluminescence spectra of Ir-l and Pt-1 are show in the Fig 3. The data for hr-l and Pt-1 are summarized in the Table 1. For Ir- 1, the electronics transitions in the range between 250-319 nm can be assigned for metal to ligand charge transfer from metal to C N ligand (d - :i*).
The absorption observed in the higher energy region
between 320-550 nm is assigned to TI-TI' ligand centered transitions (Fig. 3a). Besides, Pt-1 shows the electronic transitions for metal ligand charge transfer and ligand center transitions in the range between 250-288 nm and 289-333 nm, respectively (Fig. 3b). Furthermore, Pt-1 absorbs the light and exhibits electronic transitions in the lower energy band of 335-399 run, which corresponds to the metal center transitions (d-d).
a 0 35 1 03 j 025 1 i 02 JD.IS
<
01 0,052S0 300 350 400 450 Wiiwlenglh(nin)
500 lr-1
- • • P M
550 600
b 'S
16 14
Intensity
° " 6
4
2
4 10 4S0 SOO 550 600 Wnelenglh(nin)
— ( i r - l ) (Pl-t)x3
6S0 700 750
Figure 3: (a) UV-vis absorption and (b) photoluminescence of Ir-l and Pt-1
V J C , Vol. 50(5), 2 0 1 2
More details, table 1 tabulated that for I r - I , Ir - complex emits green p h o t o l u m i n e s c e n c e at X,„„ •=
519 nm while Pt-I has blue shifted photoluminescence at Xmax - 484 n m . T h e photoluminescence spectra w e r e well known to be related to the combination between mixed energy level of metal ligand charge transfer and ligand center in each complex. T h e appearance of metal center transitions level in Pt-1 gave rise to a little blue shift in PL spectra.
Table 1: Photophysical characteristic of Ir-I and Pt-I c o m p l e x e s
Ir-I Pt-I
Absorption
>™.,(nm)
2 5 5 , 330 267, 299, 347
PL (nm)
519
484,514 Life- time (lis) 0.63 1.32
Q u a n t u m yield
0.410 0.242
3 . 3 . P h o t o l u m i n e s c e n c e lifetime a n d q u a n t u m yield o f l r - I a n d P t - 1
Nguyen Tien Thao, etal P h o t o l u m i n e s c e n c e lifetime and q u a n t u m yiel4, o f the t w o c o m p l e x e s m e a s u r e d in chloFoform solvent are s h o w n in T a b l e 1 with t h e data of Pt-1.
T h e p h o t o l u m i n e s c e n c e lifetime o f Ir-l is 0.63 ^s while that of Pt-1 ( 1 . 3 2 p s ) is a l m o s t 2 times longer, Ir-I exhibited highly q u a n t u m yield with value of 0.410 a l m o s t as t w i c e as that o f Pt-1 complex (0.242). T h e low q u a n t u m yield o f Pt-1 is explained by the blue shift in p h o t o l u m i n e s c e n c e .
3.4. P L E D s f a b r i c a t e d w i t h c o m p l e x e s
a n d Pt-I
T h e fabricated P L E D s contain polymeric layer P V C z : P B D : Mcompic with e q u i v a l e n t ratio 1 : 0.3 : 0 . 1 , I T O / P E D O T - P S S a n o d e and CsF/AI cathode. In this w a y , the P L E D h a s a layered structure with I T O / P E D O T - P S S / P V C z : P B D : M^^^C%¥IM (Fig.
1) [9]. T h e characteristics o f P L E D s d o p i n g with Ir- I and Pt-1 w e r e s u m m a r i z e d in table 2. Figure 4 s h o w s the plots of l u m i n a n c e , current density versus voltage, and e l e c t r o l u m i n e s c e n c e spectra for PLED prepared from studied c o m p l e x e s . T h e applied voltage is the range b e t w e e n 0 - 2 0 V .
jj 10000 9000 8000 7000
Luminance /cd
2000 1000
0
• Lumitiance
— Curr density /
.7 ,•/
5 10 iS 20 Applied Voltage/V
300
250
2 0 0 ^
150 1
100 1 5 so
25
^ 2 5 0 0
2000
"•£
•SI 5 0 0
• ; ;
i
| | 0 0 0
5 0 0
Q
Luminance
— Curr density «
'
/ / _" /
S 10 IS Aplhed Voltage/V
180 160 / 140 1 I M ^
100^
80 1
60 1
4 0 20
20
Figure 4- Luminance and current density as function o f applied v o l t a g e of P L E D with (a) Ir-l and (b) Pt-1
Table 2: Characteristic of P L E D s d o p i n g with Ir-l and Pt-1
lr-1 Pt-1
Lm«, (cd/
m^) 9303 2287
Current efficiency
(cd/A) 6.4 5.5
P o w e r efficiency
( I m / W ) 1.4 1.9
riexi 1%
1.7 2.1
P L E D prepared with I r - l g a v e the quantum efficiency o f 1.7% a n d p o w e r efficiency of 1.4 Im/W while t h e s e v a l u e s are 2 . 1 % a n d 1.9 Im/W respectively for P L E D prepared with Pt-1 (Fig. 4).
T h e m a x i m u m b r i g h t n e s s o f t h e s e devices are typically a c h i e v e d at high v o l t a g e 9 3 0 9 cd/m^ for Ir- 1 a n d 2 2 8 7 cd/m^ for Pt-1 ( F i g . 4 ) . High voltage is contributed to t h e d e c r e a s i n g in luminance, power efficiency, q u a n t u m efficiency o f t h e s e devices, in a g r e e m e n t w i t h t h e results reported in Ref. [18].
616
VJCVol 50(5), 2012
External quantum efficiencies for Ir-l and Pt-1 devices as fimction of current density are shown in Fig 5b. The external quantum efficiency of these devices reaches a maximal value before going down
Preparation of phosphorescent organometallic...
with increasing in current density. These devices showed the electroluminescence X^ax corresponding to the maximum dopant PL at 525 nm for Ir-l and 486, 516 nm for Pt-1 (Fig. 5a).
a ' 0 0 ; 90 •
80 '•<
ntensity
s i 40
30 20 10 -
350
J
450
1 1 \
A
1 \ 1 \
550 Wavclcnglh(nn
(Ir-l) (Pt-1)\3
' ^ N . V ^
6 5 0 750
) 1
b 2 2 -
18 1 -- 16 l -
t 12
| o 8
04
tj V * * .
25 50
.
7S 100 125 liO I7S 200 Cu,r.mJ.n.t,y ^U^'
-'- lr-1 - ^ - P M
\
23 250 275
Figure 5: Electroluminescence of PLED with Ir-l complex and Pt-1 complex (a) and quantum efficiency as function of current density of PLEDs with Ir-l and Pt-1 (b) 4. CONCLUSION
Cyclometalated heavy metal complexes Ir-l and Pt-I bearing 2-phenyl pyridine and acetyl acetone ligand have been successfully synthesized and their photophysical properties such as photoluminescence, lifedme, and quantum yield for each complex were carefully investigated. These complexes were used to fabricate PLEDs devices The simple PLEDs with thin layer structure using PVCz as host polymer have been tested withm voltage of 0-20 V. The characteristic quantities of PLEDs were measured.
The results indicated that Ir-l device gave the green photoluminescence while Pt-1 devices exhibited a little blue shift photoluminescence.
Acknowledgements. The authors express great thank to JICA project for supporting student exchange program between Vietnam National University (VNU, Vietnam) and Osaka Prefecture University (Japan). The authors also acknowledge VNU-Hanoi for financial support under the Project Number of QG. 12.08.
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Corresponding author: Nguyen Tien Thao
Department of Petrochemistry, VNU University of Science, Vietnam National University, Hanoi.
19 Le Thanh Tong, Hoan Kiem, Hanoi, Vietnam Email: [email protected] / nguyentienthao(ggmail.com Tel. (04) 3933 1605. Fax: (04)3824 1140.
