Platinum complexes of 4-hydoxy-1,5-naphthyridines as emitting dyes

Article abstract of DOI:10.1002/jccs.201100671

4-Hydoxy-1,5-naphthyridines (HNt) are derivatives of 8-hydroxyquinolines, yet possess a wider HOMO and LUMO band gap than the latter. The cyclometalated complex of platinum(II) with Nt exists in a square planner geometry, and has a high tendency to aggregate in condensedmedia. Such a phenomenon was verified by examining its photo-luminescence spectra in different concentrations. In a dilute solution, it exhibits a yellow phosphorescence centered at 530∑555 nm, yet red-shifted to ∑660 nm in a concentrated solution or in the solid state. This series of compounds can be used as emitting dyes in light-emitting diodes (LED). The LED devices with a configuration ITO/NPB/dye (6%) in CBP/BCP/AlQ₃ or TPBI/LiF/Al displayed a yellow to red color, where NPB, CBP, BCP, AlQ₃ and TPBI denote 4,4′-bis[N-(1-naphthyl), Nphenylamino] biphenyl, 4,4′-bis(carbazol-9-yl)biphenyl, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, tris(8-hydoxyquinolinato) aluminium, and 2,2′,2′-(1,3,5-benzenetriyl)tris(1-phenyl-1H- benzimidazole), respectively. The maximal light intensity exceeds 2.6 × 104 cd/m2 with an external quantum efficiency up to 5.8%.

Full text of DOI:10.1002/jccs.201100671

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Platinum Complexes of 4-Hydoxy-1,5-naphthyridines as Emitting Dyes
a
Ching-Ting Chien, Jin-Ruei Shiu, Chih-Ping Chang, Yung-Son Hon, Duo-Fong Huang,
b
a
b
a
c
Po-Ting Chou, Ching-Yang Liu and Tahsin J. Chow *
d
a,c,
a
Institute of Chemistry, Academia Sinica, Nankang, Taipei, Taiwan 115, R.O.C.
Department of Chemistry and Biochemistry, National Chung Cheng University, Chia-Yi, Taiwan 621, R.O.C.
b
c
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, R.O.C.
Department of Applied Chemistry, Chinese Culture University, Taipei 111, Taiwan, R.O.C.
d
(Received: Nov. 18, 2011; Accepted: Jan. 18, 2012; Published Online: Feb. 8, 2012; DOI: 10.1002/jccs.201100671)
4
-Hydoxy-1,5-naphthyridines (HNt) are derivatives of 8-hydroxyquinolines, yet possess a wider HOMO
and LUMO band gap than the latter. The cyclometalated complex of platinum(II) with Nt exists in a square
planner geometry, and has a high tendency to aggregate in condensed media. Such a phenomenon was ver-
ified by examining its photo-luminescence spectra in different concentrations. In a dilute solution, it ex-
hibits a yellow phosphorescence centered at 530~555 nm, yet red-shifted to ~660 nm in a concentrated so-
lution or in the solid state. This series of compounds can be used as emitting dyes in light-emitting diodes
(
LED). The LED devices with a configuration ITO/NPB/dye (6%) in CBP/BCP/AlQ
3
or TPBI/LiF/Al dis-
played a yellow to red color, where NPB, CBP, BCP, AlQ and TPBI denote 4,4¢-bis[N-(1-naphthyl),N-
3
phenylamino]biphenyl, 4,4¢-bis(carbazol-9-yl)biphenyl, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthro-
line, tris(8-hydoxyquinolinato)aluminium, and 2,2¢,2²-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimid-
4
2
azole), respectively. The maximal light intensity exceeds 2.6 × 10 cd/m with an external quantum effi-
ciency up to 5.8%.
Keywords: Platinum complexes; 4-Hydoxy-1,5-naphthyridines; Phosphorescence; OLED; Broad
band emission.
INTRODUCTION
The high triplet emission of platinum complexes has
been used effectively in the fabrication of light-emitting di-
shifted 124 nm with respect to that of Pt(PPy)Q. The emis-
sion colors of these materials, i.e., 1a~d, are variable de-
pending on the doping concentrations. Their characteristics
are investigated in this report.
1
,2
odes (LEDs). The platinum complexes form a square pla-
nar geometry around the metallic center, that renders them
easier to stack together in condense media. Upon photo-ex-
citation, the emissions of both the monomer and the aggre-
gate display together forming a broad band spectrum. For-
rest and Thompson et al. have taken advantage of the broad
band emission of platinum complexes for the fabrication of
white light LED devices. These type of devices may find
useful applications on illumination.
EXPERIMENTAL
Instrumentation
Platinum complexes coordinated to either a phenyl-
Absorption spectra were taken on a Hewlett-Packard
8453 spectrophotometer and emission spectra on a Hitachi
F-4500 fluorescence spectrophotometer. Infrared spectra
were taken on a Perkin-Elmer L118-F000 FT-IR spectrom-
pyridine (PPy) ligand or a 8-hydroxyquinoline (HQ) ligand
3
,4
have been known for some time. The emission from com-
plex Pt(PPy)Q (679 nm) displayed a red color due to the
quinoline ligand. By replacing the C(4) atom of HQ with a
nitrogen, the HOMO-LUMO band gap of 4-hydoxy-1,5-
naphthyridine (HNt) is substantially increased. The emis-
sion maximum of Pt(PPy)Nt (1a) (555 nm, Fig. 1) is blue-
1
13
eter. H and C NMR spectra were recorded on a Bruker
AMX400 super-conducting FT NMR spectrometers. The
signals of tetramethylsilane at d 0.00 ppm was used as an
1 13
internal standard for both H and C NMR spectra. Mass
Dedicated to the memory of Professor Yung-Son Hon (1955–2011).
*
Corresponding author. E-mails: tjchow@chem.sinica.edu.tw and cheysh@ccu.edu.tw
J. Chin. Chem. Soc. 2012, 59, 357-364
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
357

Article
Chien et al.
–
ULVAC cryogenics at a chamber pressure of 10 Torr.
6
Typical devices were configured as ITO/NPB (40 nm)/dye
(
6%) doped in CBP (10 nm)/BCP(6 nm)/AlQ
3
or TPBI (40
nm)/LiF (1 nm)/Al (150 nm), where NPB, CBP, BCP, AlQ
3
and TPBI are 4,4¢-bis[N-(1-naphthyl)-N-phenylamino]bi-
phenyl, 4,4¢-bis(carbazol-9-yl)biphenyl, 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline, tris(8-hydoxyquinolinato)-
aluminium, and 2,2¢,2²-(1,3,5-benzenetriyl)tris(1-phenyl-
1
H-benzimidazole), respectively. After the deposition of
the electron-transporting layer, a thin film of LiF was fol-
lowed. The devices were then capped with a thick layer of
aluminum, which was served as cathode. Current voltage
and light intensity measurements were done on a Keithley
Fig. 1. Absorption (left), phosphorescence spectra in
2
400 Source meter and a Newport 1835C Optical meter
degassed dichloromethane solutions (center)
equipped with a Newport 818-ST silicon photodiode, re-
spectively. All measurements were completed under ambi-
ent conditions.
and as solid films (right) of complexes 1a
(
-o-), 1b (-D-), 1c (-s-), and 1d (-n -).
spectra were taken on a Jeol JMS-700 double focusing
spectrometer with a resolution of 8000 (5% valley defini-
tion). For fast atom bombardment (FAB) spectra, the source
accelerating voltage was operated at 10 kV with a Xe gun
and using 3-nitrobenzyl alcohol as the matrix. Microanaly-
ses were completed on a Perkin-Elmer 2400 elemental ana-
lyzer. Melting points of compounds were uncorrected.
X-Ray diffraction analyses were done on a Brucker AXS
X8APEX spectrometer. Differential scanning calorimetry
was done on a Perkin-Elmer DCS-7 instrument. Cyclic
voltammetry measurements were carried out on a BAS
(4-Hydoxy-1,5-naphthyridine)(2-phenylpyridine)plati-
num (1a)
To a two-necked round bottom flask (250 mL) con-
taining sodium carbonate (4.35 g) in 2-ethoxyethanol (100
mL) were added compound 6a (1.20 g, 8.22 mmol) and
2
[(2-phenylpyridyl)PtCl] (3.00 g, 3.90 mmol) under a ni-
trogen atmosphere. It was heated at 90 ºC for 16 h, while
the solution turned reddish brown. To the cooled solution
was added distilled water, which was then extracted with
dichloromethane. The combined organic solution was
washed with brine, dried over anhydrous magnesium sul-
fate and dried in vacuo. The product was purified by a silica
gel column chromatography eluted with dichloromethane/
1
00B electrochemical analyzer equipped with a conven-
tional three-electrodes system, i.e., glassy carbon, platinum
wire and Ag/AgCl as working, counter, and reference elec-
trodes respectively. Degassed dichloromethane solution
with 0.1 M tetra-n-butylammonium hexafluorophosphate
was used as electrolyte. Eox, Ered data were measured with
reference to ferrocene (Fc) internal standard which was
calibrated to +350 mV against to a Ag/AgCl reference elec-
trode. Measurements were taken at a scan rate of 100
methanol (50/1) to yield complex 1a (2.36 g) in 61% yield.
1
H NMR (CDCl , 400 MHz) d 9.27 (d, J = 5 Hz, 1H), 9.16
3
(d, J = 5 Hz, 1H), 8.48-8.45 (m, 2H), 7.89-7.87 (m, 1H),
7.69-7.64 (m, 2H), 7.54 (t, J = 8 Hz, 2H), 7.28-7.14 (m,
1
3
3
3H), 6.82 (d, J = 5 Hz, 1H). C NMR (CDCl , 100 MHz,
H-decoupled) d 173.6, 166.7, 154.1, 149.3, 146.9, 146.8,
146.7, 146.1, 139.8, 138.9, 137.3, 131.5, 129.3, 124.2,
123.8, 123.6, 121.8, 118.3, 112.2. IR (KBr) 3046, 1583,
1553, 1494, 1384, 1262, 1196, 1196, 916, 823, 753, 720
-
mV·s .
1
Device Fabrication
-
cm . HRMS calcd for C19
1
+
Aglass coated with indium-tin oxide (ITO) with sheet
13 3
H N OPt: 494.0785 (M+H) ,
resistance of <50 W/Πwas used as substrate. The substrate
found: 495.0782.
was pre-patterned by photolithography to give an effective
2
device size of 3.14 mm . Pre-treatment of ITO includes a
(4-Hydoxy-8-methyl-1,5-naphthyridine)(2-phenylpyri-
dine)platinum (1b)
routine chemical cleaning using detergent and alcohol in
sequence, followed by oxygen plasma cleaning. Thermal
evaporation of organic materials was carried out using
The procedure was similar to that for the preparation
of 1a. Complex 1b was obtained as yellow solids in 74%
yield. Calcd for C H N OPt: C, 47.25; H, 2.97; N, 8.26%.
2
0
15
3
358
www.jccs.wiley-vch.de
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2012, 59, 357-364

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Platinum Complexes of 4-Hydoxy-1,5-naphthyridines
1
Found: C, 47.04; H, 2.98; N, 8.06%. H NMR (CDCl
3
, 400
lution was added distilled water, which was then extracted
with dichloromethane. The combined organic solution was
washed with brine, dried over anhydrous magnesium sul-
fate and dried in vacuo. The product was purified by a silica
gel column chromatography eluted with dichloromethane/
methanol (50/1) to yield complex 1d as orange solids (3.69
g, 5.85 mmol) in 45% yield, mp 193 °C. Calcd for
MHz) d 9.26 (d, J = 5 Hz, 1H), 8.99 (d, J = 5 Hz, 1H), 8.44
d, J = 5 Hz, 1H), 7.87 (t, J = 8 Hz, 1H), 7.67 (d, J = 8 Hz,
H), 7.56-7.49 (m, 3H), 7.25-7.13 (m, 3H), 6.81 (d, J = 5
(
1
1
Hz, 1H), 2.75 (s, 3H). C NMR (CDCl
3
3
, 100 MHz, H-de-
coupled) d 173.8, 166.6, 152.8, 150.9, 149.2, 146.1, 145.7,
1
38.7, 137.7, 131.4, 129.2, 124.3, 123.7, 123.4, 121.7,
18.2, 112.0, 17.6 (two carbon nuclei could not observed or
1
25 23 3 4
C H N O Pt: C, 47.59; H, 3.30; N, 7.24%. Found: C,
1
resolved). IR (KBr) 3040, 1561, 1509, 1476, 1402, 1240,
3
47.74; H, 3.12; N, 7.11%. H NMR (400 MHz, CDCl ) d
-
34, 838, 823, 753, 728, 683, 628 cm . HRMS calcd for
1
9
9.27 (br, 1H), 8.99 (br, 1H), 8.88 (d, J = 5.4 Hz, 1H),
7.76-7.74 (m, 1H), 7.48 (d, J = 8.0 Hz, 1H), 7.39-7.32 (m,
3H), 7.16-7.07 (m, 3H), 4.53 (t, J = 5.0 Hz, 2H), 3.87 (t, J =
5 Hz, 2H), 3.65 (q, J = 7.0 Hz, 2H), 2.71 (s, 3H), 1.26 (t, J =
+
20 15 3
C H N OPt: 509.0943 (M+H) , found: 509.0932.
(
(
3-Carbethoxy-4-hydroxy-8-methyl-1,5-naphthyridine)
2-phenylpyridine)platinum (1c)
1
7.0 Hz, 3H). C NMR (100 MHz, CDCl ) d 175.08,
3
To a two-necked round bottom flask (250 mL) con-
3
taining sodium carbonate (2.37 g) in ethanol (120 mL)
166.25, 165.84, 165.33, 154.87, 150.68, 149.66, 146.69,
were added compound 4b (1.65 g, 7.11 mmol) and [(2-
146.65, 146.01, 138.57, 131.16, 129.18, 125.90, 123.64,
123.53, 121.87, 117.85, 113.16, 68.63, 66.68, 63.43, 17.95,
15.31, 14.40. IR (KBr) 3047, 2974, 1684, 1561, 1516, 1242,
2
phenylpyridyl)PtCl] (1.00 g, 1.30 mmol) under a nitrogen
atmosphere. It was heated to reflux for 16 h, while the solu-
tion turned to reddish brown. The solution was cooled to
ambient temperature, to it was then added distilled water
and was extracted a few times with dichloromethane. The
combined organic solution was washed with brine, dried
over anhydrous magnesium sulfate and dried in vacuo. The
product was purified by a silica gel column chromatogra-
phy eluted with dichloromethane/methanol (50/1) to yield
complex 1c as orange solids (1.82 g, 3.14 mmol) in 44%
-
1199, 1163 cm . MS (FAB) m/z 625.14 (M + H) .
1
+
Diethyl (3-pyridyl)aminomethylenemalonate (3a)
To a three-necked round bottom flask fitted with a
condenser and a magnetic stirrer was charged with 3-ami-
nopyridine (2a) (3.45 g, 37 mmol) and diethyl ethoxymeth-
ylenemalonate (8.87 g, 41 mmol). The mixture was stirred
and heated to 150 ºC, at which temperature ethanol was
evolved. It was then stirred for a few hours until gas evolu-
tion was ceased. The product was cooled and purified by
silica gel column chromatography eluted with ethyl ace-
tate/hexane (1:4) to afford 3a as white solids (9.57 g, 98 %
19 3 3
yield, mp 254 °C. Calcd for C23H N O Pt: C, 48.08; H,
1
.71; N, 6.73%. Found: C, 48.29; H, 3.50; N, 6.52%. H
3
3
NMR (400 MHz, CDCl ) d 9.29 (br, 1H), 9.01 (br, 1H),
1
yield); H NMR (400 MHz, CDCl ) d 11.0 (d, J = 13 Hz,
8
.87 (d. J = 5.4 Hz, 1H), 7.79-7.75 (m, 1H), 7.51 (d, J = 8.0
3
Hz, 1H), 7.40-7.35 (m, 3H), 7.17-7.07 (m, 3H), 4.45 (q, J =
1H), 8.50 (s, 1H), 8.48 (d, J = 13 Hz, 1H), 8.42-8.40 (m,
1H), 7.49-7.46 (m, 1H), 7.34-7.26 (m, 1H), 4.32 (q, J = 7
Hz, 2H), 4.26 (q, J = 7 Hz, 2H), 1.38 (t, J = 7 Hz, 3H), 1.33
1
.1 Hz, 2H), 2.73 (s, 3H), 1.50 (t, J = 7.1 Hz, 3H). C NMR
3
7
(
100 MHz, CDCl ) d 174.92, 166.51, 165.84, 155.09,
3
1
(t, J = 7 Hz, 3H); C NMR (100 MHz, CDCl , H-decoup-
3
1
1
1
1
5
50.89, 149.50, 147.00, 146.83, 146.66, 146.11, 138.66,
37.12, 131.25, 129.29, 125.82, 123.75, 123.65, 121.68,
18.00, 113.49, 60.24, 17.94, 14.42. IR (KBr) 3046, 2979,
3
led) d 169.0, 165.4, 151.5, 146.1, 139.9, 136.0, 124.2,
123.7, 95.6, 60.7, 60.4, 14.5, 14.3; IR (KBr) 3448, 3254,
3188, 2986, 2900, 1682, 1638, 1605, 1576, 1458, 1409,
-1
1343, 1257, 1100, 1019, 982, 923, 794, 698, 621, 573 cm ;
-
1
678, 1561, 1517, 1241, 1199, 1166 cm . MS (FAB) m/z
+
81.11 (M + H ) .
+
MS (EI) m/z 264 (M , 96%), 218 (100), 175 (12), 162 (83),
(
3-Ethoxyethoxycarbonyl-4-hydroxy-8-methyl-1,5-
naphthyridine)(2-phenylpyridine)platinum (1d)
To a two-necked round bottom flask (250 mL) con-
taining sodium carbonate (4.35 g) in 2-ethoxyethanol (100
mL) were added compound 4b (3.04 g, 13.1 mmol) and
145 (50) and 118 (20).
Diethyl (4-methyl-3-pyridyl)aminomethylenemalonate
(3b)
The procedure was similar to that for the preparation
of 3a, starting from 3-amino-4-methylopyridine (2b). Com-
pound 3b was obtained as white powder in 98% yield.
Calcd for C H N O : C, 60.64; H, 6.18; N, 10.10%.
[
2
(2-phenylpyridyl)PtCl] (1.84 g, 2.39 mmol) under a ni-
trogen atmosphere. It was heated at 80~100 ºC for 16 h,
while the solution turned reddish brown. To the cooled so-
1
4
18
2
4
J. Chin. Chem. Soc. 2012, 59, 357-364
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
359

Article
Chien et al.
1
Found: C, 60.32; H, 6.42; N, 9.81%. H NMR (400 MHz,
CDCl ) d 11.05 (d, J = 13 Hz, 1H), 8.53 (s, 1H), 8.52 (d, J =
3 Hz, 1H), 8.31 (d, J = 5 Hz, 1H), 7.16 (d, J = 5 Hz, 1H),
.33 (q, J = 7 Hz, 2H), 4.25 (q, J = 7 Hz, 2H), 2.38 (s, 3H),
of 5a, starting from 3-amino-4-methylopyridine (4b). Com-
pound 5b was obtained as brownish powder in 68% yield.
3
1
4
1
8 2 3
Calcd for C10H N O : C, 58.82; H, 3.95; N, 13.72%.
Found: C, 58.36; H, 4.00; N, 13.16%. IR (KBr) 3610, 3099,
3025, 1738, 1572, 1524, 1450, 1301, 1270, 1200, 1152,
1
3
.39 (t, J = 7 Hz, 3H), 1.31 (t, J = 7 Hz, 3H); C NMR (100
, H-decoupled) d 169.3, 165.3, 152.3, 146.2,
38.2, 136.4, 135.4, 125.6, 95.2, 60.7, 60.3, 17.0, 14.5,
4.4; IR (KBr) 3151, 3062, 2988, 1697, 1603, 1472, 1413,
-
1
+
MHz, CDCl
3
846, 801, 761, 709 cm ; MS (EI) m/z 204.0534 (M ,
10 8 2 3
C H N O requires 204.0535).
1
1
1
2
4-Hydroxy-1,5-naphthyridine (6a)
-
384, 1262, 1109, 1005, 919, 827, 602 cm ; MS (EI) m/z
1
In a round bottom flask compound 5a (20.0 g, 105
mmol) was heated in mineral oil (500 mL) to 330 ºC. Car-
bon dioxide gas evolved vigorously during the heating.
Heating and stirring was continued for 30 min, then it was
cooled and the precipitates were filtered off. It was washed
with hexane to remove mineral oil and then recrystallized
+
18 2 4
78.1270 (M , C14H N O requires 278.1267, 78%), 232
(
100), 176 (12), 159 (43), 131 (23) and 119 (23).
3
-Carbethoxy-4-hydroxy-1,5-naphthyridine (4a)
To a round bottom flask fitted with a condenser were
added diphenyl ether (15 mL) and compound 3a (1.00 g,
.8 mmol). The solution was heated to reflux (258 ºC) for 1
1
from 95% ethanol. Compound 6a was collected as yellow
3
h, while the solution turned brownish and in which precipi-
tates formed. It was filtered and the solids were washed re-
peatedly with hexane. Compound 4a was collected as dark
brown solids (0.67 g, 82%); IR (KBr) 3594, 3520, 3070,
power (10.8 g, 70%); IR (KBr) 3651, 3062, 2915, 1623,
-
1
1
1576, 1502, 1192, 1085, 812, 787 cm ; H NMR (400
MHz, CDCl ) d 8.74 (d, J = 4 Hz, 1H), 8.07 (d, J = 8 Hz,
1H), 8.04 (d, J = 7 Hz, 1H), 7.70 (dd, J = 7 & 8 Hz, 1H),
3
1
3
6.52 (d, J = 7 Hz, 1H). C NMR (100 MHz, CDCl
2
1
988, 2738, 1686, 1620, 1579, 1517, 1458, 1362, 1306,
3
, H-de-
-
166, 1023, 908, 868, 812, 761, 750, 687, 588, 532 cm ;
1
coupled) d 179.8, 148.6, 141.9, 141.2, 138.4, 129.2, 128.3;
+
MS (EI) m/e 146.0483 (M , C H N O requires 146.0480,
8 6 2
+
MS (EI) m/z 218 (M , 52%), 173 (62) and 146 (100).
-Carbethoxy-4-hydroxy-8-methyl-1,5-naphthyridine
4b)
3
100%) and 118 (40).
(
4-Hydroxy-8-methyl-1,5-naphthyridine (6b)
The procedure was similar to that for the preparation
The procedure was similar to that for the preparation
of 1a, starting from 3-amino-4-methylpyridine (5b). Com-
pound 6b was obtained as brownish powder in 69% yield.
of 4a, starting from 3-amino-4-methylopyridine (3b). Com-
pound 4b was obtained as dark brown solids in 74% yield.
Calcd for C12
H
12
N
2
O
3
: C, 62.06; H, 5.21; N, 12.06%.
9 8 2
C H N O: C, 67.49; H, 5.03; N, 17.49%. Found: C, 67.19;
Found: C, 62.17; H, 5.16; N, 12.02%. IR (KBr) 3614, 3143,
H, 5.03; N, 17.20%. IR (KBr) 3129, 3084, 2811, 1922,
1572, 1424, 1402, 1306, 1247, 1207, 1166, 1085, 1026,
-1 1
2
1
981, 1690, 1623, 1576, 1531, 1476, 1435, 1292, 1229,
-
159, 1026, 956, 919, 846, 816, 757, 687, 624 cm ; m/z
1
964, 816, 672 cm ; H NMR (400 MHz, CDCl
3
) d 8.57 (d,
+
(
12 2 3
EI) 232.0844 (M , C12H N O requires 232.0848).
J = 4 Hz, 1H), 7.99 (d, J = 7 Hz, 1H), 7.56 (d, J = 4 Hz, 1H),
1
3
3
-Carboxy-4-hydroxy-1,5-naphthyridine (5a)
6.51 (d, J = 7 Hz, 1H), 2.61 (s, 3H); C NMR (100 MHz,
CDCl , H-decoupled) d 178.4, 146.4, 140.0, 139.4, 138.1,
136.4, 127.3, 111.0, 15.6; MS (EI) m/z 160.0634 (M ,
O requires 160.0637, 100%), 132 (38).
In a round bottom flask compound 4a (3.0 g, 13
3
+
mmol) was heated with 10% sodium hydroxide (5.0 mL) to
reflux for 16 h. Distilled water was added to the mixture,
then the solution was filtered. The filtrate was acidified to
pH 3, while solids were precipitated. The solids were fil-
tered off, washed with water, and dried at 110 ºC. Com-
pound 5a was collected (1.83 g, 70%) as brownish power;
IR (KBr) 3675, 3647, 3091, 3036, 2969, 1624, 1578, 1555,
9 8 2
C H N
RESULTS AND DISCUSSION
Syntheses and physical properties
The preparation of the HNt ligands were accom-
plished starting from 3-aminopyridine (2a,b) according to
5
a published procedure (Scheme I). Condensations of 2a,b
1
7
482, 1456, 1428, 1391, 1338, 1312, 1207, 895, 820, 764,
-
1
+
37, 648, 584 cm ; MS (EI) m/z 190 (M , 6%), 172 (7), 146
with diethyl 2-(ethoxymethylene)malonate were achieved
by a thermal reaction at 150 ºC. Heating the purified prod-
uct 3a,b at 250 ºC induced a ring cyclization to yield the
fused 1,5-naphthyridine skeleton of 4a and 4b. The car-
(
100) and 118 (45).
3
-Carboxy-4-hydroxy-8-methyl-1,5-naphthyridine (5b)
The procedure was similar to that for the preparation
360
www.jccs.wiley-vch.de
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2012, 59, 357-364

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Platinum Complexes of 4-Hydoxy-1,5-naphthyridines
Scheme I The synthesis of 4-hydoxy-1,5-naphthyri-
ids, which were stable under ambient condition and can be
sublimed in vacuum chamber to condensed into thin films
during OLED fabrications.
dine and its 8-methyl derivative
Absorption and Emission Spectra
The absorption spectra of 1a,b and the emission spec-
tra of 1a~d are shown in Figure 1. Amajor absorption max-
imum of 1a appeared at 247 nm, which was assigned to the
1
(
p-p*) transition of the ligands. The broad absorption re-
gion at longer wavelengths of 350~460 nm are derived from
-8
charge transfer transitions associated with the metal.
6
Photoexcitation of 1a~1d in deoxygenated dichlorometh-
ane at 426 nm yielded emission bands at lmax 530~555 nm.
Mild blue shifts were observed in the emissions of 1b and
boxylate substituents at C(3) were extruded by first con-
1
c, compared with those of 1a. Adding an electron-with-
verting 4a,b to the acids 5a,b, followed by thermal extru-
drawing carboxylate group to the phenolic ring has an ef-
fect of reducing the potential level of HOMO, so that the
band gap is broadened. The emission spectra of 1a,b in the
solid state were distinctively different. The maxima of
emission was red shifted about 110 nm, from 555 nm to 666
nm, along with a significantly band broadening. The full
width at half maximum (fwhm) expanded from ca. 70 nm to
100 nm. This low energy broad emission is believed to
have derived from aggregates in the solid state.
sions of CO
2
. The solubility of both structures 4a,b and
5
a,b was very low in organic solvents, therefore their struc-
tures were confirmed mainly by elemental analyses. Ther-
mal fragmentations of 5a,b were accomplished by heating
at 330 ºC, while 6a,b were obtained in ca. 70% yield.
The platinum complexes 1a and 1b were prepared in
6
1% and 74% yields from 6a and 6b, respectively, by heat-
ing with [(2-phenylpyridyl)PtCl] in 2-ethoxyethanol.
2
Complex 1d was collected through the same procedure,
however a nucleophilic substitution at the carboxylate moi-
ety happened simultaneously accompanying the reaction.
The ethoxyl group was replaced by a 2-ethoxyethoxyl
group via solvation, so that 1d was produced in 45% yield
as an unexpected side product. Nevertheless this com-
pound was considered also as a usable material, therefore
its chemistry was explored along with other compounds.
The ethoxycarbonyl derivative 1c can made by simply
changing the solvent to ethanol. After heated to reflux in
ethanol for 16 hours, compounds 1c can be isolated in 44%
yield. All products 1a~1d were purified to form orange sol-
Redox Potentials
Their oxidation potentials were measured by cyclic
voltammetry (CV) in degased dichloromethane. Com-
pounds 1a~1d showed reversible anodic waves at 0.46~
0.57 V separated by 320 mV in an oxidative sweep (Table
1). In a reductive sweep of 1a~1d, quasi-irreversible waves
appeared at -1.47~-1.61 V with peaks separated by ca. 110
mV. The band gaps, estimated by the difference between
E and E , were defined as the energy difference between
ox
red
HOMO and LUMO. The band gaps values can also be esti-
mated by the 0-0 absorption energies in their UV spectra,
which were measured at the position of interception be-
Table 1. Absorption energy, oxidation and reductive potentials of compounds 1a~1d
a
max
c
d
ox
d
e
f
HOMO LUMO
g
l
l
0-0
E
0-0
E
E
red
Band gap
(eV)
b
F
(
nm)
(nm)
(eV)
(V)
(V)
(eV)
(eV)
h
h
1
1
1
1
a
a
b
c
553, 666
540, 663
532
0.07
0.06
0.10
0.11
475
485
466
467
2.61
2.56
2.66
2.65
0.46
0.53
0.57
0.54
-1.47
-1.59
-1.61
-1.60
1.93
2.12
2.18
2.14
b
-4.86
-4.93
-4.97
-4.94
-2.25
-2.37
-2.31
-2.29
d
532
Emission wavelength in dichloromethane, unless otherwise notified. Quantum yield in dichloro-
c
d
+
methane. Taken at the crossing point of absorption and emission spectra. Fc/Fc at 0.35 V against
e
Ag/AgCl as internal reference. Gap between E and Ered. Calculated as –(E + 4.40). Calculated
h
as (HOMO + l0-0). Emission wavelength of solid films.
f
g
ox
ox
J. Chin. Chem. Soc. 2012, 59, 357-364
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
361

Article
Chien et al.
Table 2. Working parameters of OLED devices. Typical configuration is ITO/NPB (40 nm)/dye (6%) doped in CBP (10
nm)/BCP (6 nm)/ETL (40 nm)/LiF (1 nm)/Al (150 nm), unless otherwise indicated. The parameters are quoted
2
at 20 mA/cm current density wherever applicable
Phosphorescent emitter
Device type
1a
1b
1c
1d
a
B
A
B
A
B
B
B
Wavelength maximum (nm)
Turn on voltage (V)
652
6.2
646
6.0
548
6.5
550
5.8
668
6.5
532
5.5
532
5.5
2
Maximum intensity (Cd/m )
7450
620
10.5
2.5
15410
660
8.7
14620
3300
13.7
5.3
26790
3680
11.5
5.8
12060
650
9.7
7799
1810
11.3
4.0
11118
3220
11.1
5.2
2
Intensity (Cd/m ) at 20 mA/cm
2
2
Voltage (V) at 20 mA/cm
External quantum efficiency (%)
Luminescence efficiency (Cd/A)
Power efficiency (lm/W)
2.4
5.2
3.08
1.0
3.35
1.2
16.55
3.8
18.40
5.0
3.30
1.1
9.1
16.1
4.6
2.5
b
FWHM (nm)
c
CIE
182
174
76
76
142
160
60
0.57, 0.43 0.56, 0.43 0.45, 0.54 0.45, 0.54 0.63, 0.37 0.45, 0.53 0.38, 0.58
a
b
c
NPB layer deposited in 30 nm. 20% doped in CBP. Full width at half maximum. Commission Internationale de
I’Eclairage 1931.
tween the absorption and emission spectra. The HOMO
levels can be calculated according to the equation EHOMO
therefore reduced the degree of aggregation.
=
Plots of intensity versus voltage for the devices made
with 1a~1d are shown in Fig. 3. The turn-on voltages of the
devices were at about 5.5~6.5 V, whilst at high voltages the
9
-
(Eox + 4.40) eV.
Electrophosphorescence Devices
Two standard types of devices were made with the
following configurations: (i) device A: ITO/NPB (40 nm)/
dye (6%) doped in CBP (10 nm)/BCP (6 nm)/AlQ
3
(40
nm)/LiF (1 nm)/Al (150 nm), and (ii) device B: ITO/NPB
(
(
40 nm)/dye (6%) doped in CBP (10 nm)/BCP (6 nm)/TPBI
40 nm)/LiF (1 nm)/Al (150 nm). In these devices NPB
worked as a hole transporter (HT), while BCP coupled with
either AlQ or TPBI as electron transporters (ET). All dyes
3
were doped 6% in CBP host matrix as the emitting medi-
ums, except in one case compound 1b was doped in 20%.
The two kinds of devices differed from each other on the
ET materials used, i.e. AlQ
3
and TPBI, where The HOMO
levels of AlQ (5.7 eV) is higher than that of TPBI (6.2
3
eV). In device B, a higher energy barrier retards the mi-
gration of holes across the film boundary between the emit-
Fig. 2. Electrophosphorescence spectra of devices, us-
1
0
ing complexes 1a (-¡- and -l-), 1b (-D-,
ting layer and ET. The parameters of all devices are listed
in Table 2.
-
s- and -t-), 1c (-I-), and 1d (-o-) as
emitting materials. The devices were config-
ured differently in type A (-¡- and -D-) and
type B (-l-, -s- and -t-). The short wave-
length bands (~550 nm) were derived from the
missions of the monomers, while the long
wavelength bands (~670 nm) were derived
from aggregates. Excimer emission became
predominated while the doping concentration
of 1b was increased to 20% (-t-), whereas a
highly intensive red color device was obtained.
There is no clear distinction between the performance
of devices type A and type B. In both type of devices, a
layer of BCP served as an effective electron transporter as
3
well as hole blocker, so that the influence of either AlQ or
TPBI was minimized. A noticeable feature was that the de-
vices made of 1a displayed a much broader spectrum than
those made with 1b (Fig. 2). The methyl substituent of 1b
increased the steric hindrance between adjacent molecules,
362
www.jccs.wiley-vch.de
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2012, 59, 357-364

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Platinum Complexes of 4-Hydoxy-1,5-naphthyridines
2
maximal intensity can exceed 26000 Cd/m . The external
changing the doping concentrations. For example, the
emission maximum of 1b was 550 nm at 6% doping, yet
red-shifted to 660 nm at 20% doping (Fig. 2). The visual
colors of the devices were distinctively different, i.e., the
former was yellow and the latter red.
quantum efficiencies of devices 1b~1d were in a range of
2
~6% at 20 mA/cm (Fig. 4 and Table 2). These values may
4
be further improved, since the fabrication processes have
not yet been extensively optimized. The EL spectra of these
compounds exhibited broad band emissions due to molecu-
lar aggregation. The emission maxima can be adjusted by
CONCLUSIONS
The coordination compounds of 4-hydoxy-1,5-naph-
thyridine (HNt) with platinum ions have proven to be effec-
tive phosphorescent materials for the fabrication of LED
devices. These complexes exhibited broad band emissions
covering the region from 550 to 750 nm. The maximal ex-
ternal quantum yield can reach to a level of ~6% with the
2
highest intensity exceeding 26000 Cd/m at 18V, while
doped 6% in CBP. The color of these devices can be tuned
from yellow to red by adjusting the doping concentrations.
At 20% doping in CBP, a highly intensive red color was
produced.
ACKNOWLEDGEMENT
Financial supports from Academia Sinica and the Na-
tional Science Council of the Republic of China.
Fig. 3. Intensity vs. voltage plots for the devices made
with complexes 1a (-¡- and -l-), 1b (-D-,
-
s-, and -t-), 1c (-I-), and 1d (-o-) as
emitting materials. The devices were config-
ured differently in type A (-¡- and -D- in 6%
doping), type B (-l- and -s- in 6% doping)
and B* (-t- in 20% doping). Turn-on voltages
can be seen better in the inset.
REFERENCES
1
. Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F. Inorg.
Chem. 1986, 25, 3858-3865.
2
. (a) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson,
M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4-6. (b)
Kwong, R. C.; Lamansky, S.; Thompson, M. E. Adv. Mater.
2
000, 12, 1134-1138. (c) Lamansky, S.; Djurovich, P.;
Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.; Bortz,
M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg. Chem. 2001,
4
0, 1704-1711.
3
4
. (a) Meyers, A.; Weck, M. Chem. Mater. 2004, 16, 1183-
1188. (b) Chen, C. H.; Shi, J. Coord. Chem. Rev. 1998, 171,
1
61-174.
. (a) Halls, M. D.; Schlegel, H. B. Chem. Mater. 2001, 13,
632-2640. (b) Burrows, P. E.; Shen, Z.; Bulovic, V.;
2
McCarty, D. M.; Forrest, S. R.; Cronin, J. A.; Thompson, M.
E. J. Appl. Phys. 1996, 79, 7991-8006. (c) Burrows, P. E.;
Sapochak, L. S.; McCarty, D. M.; Forrest, S. R.; Cronin, J.
A.; Thompson, M. E. Appl. Phys. Lett. 1994, 64, 2718.
. (a) Adams, J. T.; Bradsher, C. K.; Breslow, D. S.; Amore, S.
T.; Hauser, C. R. J. Am. Chem. Soc. 1946, 68, 1232. (b) Eck,
T. D.; Wehry, Jr., E. L.; Hercules, D. M. J. Inorg. Nucl.
Chem. 1966, 28, 2439-2441.
Fig. 4. External quantum efficiency of the devices
5
made of complexes 1a (-¡- and -l-), 1b
(
(
-D-, -s-, and -t-), 1c (-o-), and 1d
-I-) as emitting materials. The devices were
configured differently in type A (-¡- and -D-
in 6% doping), type B (-l- and -s- in 6%
doping) and B* (-t- in 20% doping).
6. (a) Tsuboi, T.; Tanigawa, M. Thin Solid Films 2003, 438-
439, 301-307; (b) Holzer, W.; Penzkofer, A.; Tsuboi, T.
J. Chin. Chem. Soc. 2012, 59, 357-364
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
363

Article
Chien et al.
Chem. Phys. 2005, 308, 93-102.
. Maestri, M.; Sandrini, D.; Balzani, V.; von Zelewsky, A.;
Deuschel-Cornioley, C.; Jolliet, P. Helv. Chim. Acta 1988,
1, 1053-1058.
M. Y.; Parker, A. W.; Sun, X.-Z.; Towrie, M.; Weinstein, J.
A. Dalton Trans. 2005, 2092.
7
8
9. Janietz, S.; Bradley, D. D. C.; Grell, M.; Giebeler, C.;
Indasekaran, M.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 2453.
10. (a) Koene, B. E.; Loy, D. E.; Thompson, M. E. Chem. Mater.
1998, 10, 2235. (b) Tao, Y.-T.; Balasubramaniam, E.; Danel,
A.; Tomasik, P. App. Phy. Lett. 2000, 77, 933. (c)
Balasubramaniam, E.; Tao, Y.-T.; Danel, A.; Tomasik, P.
Chem. Mater. 2000, 12, 2788.
7
. (a) Shavaleev, N. M.; Adams, H.; Best, J.; Edge, R.;
Navaratnam, S.; Weinstein, J. A. Inorg. Chem. 2006, 45,
9
401-9415. (b) Mdleleni, M. M.; Bridgewater, J. S.; Watts,
R. J.; Ford, P. C. Inorg. Chem. 1995, 34, 2334. (c) Kovelenov,
Y. A.; Blake, A. J.; George, M. W.; Matousek, P.; Mel’nikov,
364
www.jccs.wiley-vch.de
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2012, 59, 357-364