Structural, photophysical, and electrophosphorescent properties of platinum(II) complexes supported by tetradentate N2O2 chelates

Article abstract of DOI:10.1002/chem.200390143

We present an examination of the structural and photophysical characteristics of [Pt(N₂O₂)] complexes bearing bis(phenoxy)diimine auxiliaries (diimine = 4,7-Ph₂phen (1) and 4,4′-tBu₂bpy (2)) that are tetradentat

Full text of DOI:10.1002/chem.200390143

C.-M. Che et al.
Chem. Eur. J. 2003, 9, No. 6
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1263

FULL PAPER
Structural, Photophysical, and Electrophosphorescent Properties of
Platinum(ii) Complexes Supported by Tetradentate N2O2 Chelates
Yong-Yue Lin,[a, b] Siu-Chung Chan,[a] Michael C. W. Chan,[a] Yuan-Jun Hou,[a]
Nianyong Zhu,[a] Chi-Ming Che,*[a] Yu Liu,[c] and Yue Wang[c]
Abstract: We present an examination of
the structural and photophysical charac-
teristics of [Pt(N2O2)] complexes bear-
ing bis(phenoxy)diimine auxiliaries
(diimine ˆ 4,7-Ph2phen (1) and 4,4'-
tBu2bpy (2)) that are tetradentate rela-
tives of the quinolinolato (q) ligand.
These neutral derivatives display high
thermal stability (>4008C in N2). While
the crystal lattice in 1 consists of (head-
to-tail)-interacting dimers, molecules of
2 are arranged into infinitely stacked
planar sheets with possible p p inter-
actions but no close Pt ¥¥¥ Pt contacts.
Complexes 1 and 2 exhibit moderately
intense low-energy UV/Vis absorptions
around l ˆ 400 500 nm that undergo
negative solvatochromic shifts. Both de-
rivatives are highly luminescent in sol-
ution at 298 K withemission lifetimes in
the ms range, and mixed 3[l ! p*(di-
imine)] (l ˆ lone pair/phenoxide) and
3[Pt(d) ! p*(diimine)] charge-transfer
states are tentatively assigned. The ex-
cited-state properties of 2 are also in-
vestigated by time-resolved absorption
spectroscopy and by quenching experi-
ments withpyridinium acceptors to
estimate the excited-state redox poten-
tial. These emitters have been employed
as electrophosphorescent dopants in
multilayer OLEDs. Differences be-
tween the brightness, color, and overall
performance of devices incorporating 1
and 2 are attributed to the influence of
the diimine substituents.
Keywords: charge transfer ¥ electro-
luminescence ¥ N,O ligands ¥ phos-
phorescence ¥ platinum
diodes (OLEDs) have resulted. More recently, intense efforts
have been devoted to electrophosphorescent emitters because
of the theoretical augmentation in device efficiency relative to
established fluorescent OLEDs.[1, 2] Phosphorescent emitters
containing heavy transition metals, which promote spin-orbit
coupling, have the potential to harness both singlet (25%) and
triplet (75%) excitons after charge recombination. In this
regard, several accounts of platinum(ii)[3, 4] and iridium(iii)[5]
complexes supported by organic ancillary ligands as dopants
in small molecule- and polymer-based OLEDs have ap-
peared, and exceptional device performances have been
reported.[6]
Researchinto ligand design for EL materials has been
dominated by the 8-hydroxyquinoline moiety, as in Alq3.[7] Its
use as an emissive and electron-transporting material has
been widespread because Alq3 combines many chemical and
physical attributes that are requisite for such a function,
including sufficient electron mobility, high photolumines-
cence (PL) quantum efficiency, and stability for vacuum
deposition coupled withresistance to subsequent recrystalli-
zation of the amorphous film. Surprisingly, optimization
studies involving systematic variation of the quinoline ligand
structure have not proven particularly fruitful and have often
afforded adverse device parameters.[8] Attempts to develop
alternative q-containing metal complexes for EL applications
Introduction
The pursuit of highly luminous and efficient electrolumines-
cent (EL) materials withsuitable chemical and operational
stability has proliferated over the last decade, and commerci-
alization of technologies based on organic light-emitting
[a] Prof. Dr. C.-M. Che, Dr. Y.-Y. Lin, S.-C. Chan, Dr. M. C. W. Chan,
Dr. Y.-J. Hou, Dr. N. Zhu
Department of Chemistry and HKU-CAS Joint Laboratory on New
Materials
The University of Hong Kong
Pokfulam Road
Hong Kong SAR (China)
Fax : (‡852)2857-1586
E-mail: cmche@hku.hk
[b] Dr. Y.-Y. Lin
Department of Chemistry
Yunnan Normal University
Kunming 650092 (China)
[c] Prof. Y. Wang, Y. Liu
Key Laboratory for Supramolecular Structure and Spectroscopy of
Ministry of Education
Jilin University
Changchun 130023 (China)
Supporting information for this article is available on the WWW under
http://www.wiley-vch.de/home/chemistry/ or from the author.
1264
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Chem. Eur. J. 2003, 9, No. 6

PtII Tetradentate N O Chelates Complexes
1263 1272
2
2
have focused on the closest relatives of Alq3, suchas trivalent
Ga, In, and Sc,[8, 9] as well as Znq2 (or (Znq2)4).[10]
Experimental Section
By integrating the two design features described above, we
have prepared phosphorescent platinum(ii) complexes ligated
by tetradentate q-related auxiliaries N2O2 (N2O2H2 ˆ bis(2'-
phenol)bipyridines and -phenanthrolines; Scheme 1) for em-
ployment as new dopants in EL devices. The N2O2 ligand
framework is devised withchemically inert functionalities and
the considerable chelate effect should ensure good thermal
stability. Phenyl and tert-butyl substituents are appended to
the Pt(N2O2) moiety, since coordinatively unsaturated square-
planar PtII compounds are known to engage in excited-state
and exciton self-quenching reactions in solution[11] and EL
devices,[12] respectively. The nature of these substituents may
therefore yield emitting layers with different quenching
behavior as well as other relevant physical properties. Both
complexes have been structurally characterized and an
assessment of their crystal packing arrangements is presented.
Investigations into the performances of these emitters in
OLEDs have revealed substituent-dependent electrophos-
phorescent energies and device characteristics.
General: All chemicals were obtained from commercial sources and used
as received. All reactions were performed under a nitrogen atmosphere
and solvents for syntheses (analytical grade) were used without further
purification. Solvents for photophysical measurements were purified
according to conventional methods.[13] 1H and 13C NMR spectra were
obtained on Bruker DRX 300, 500, and 600 FT-NMR spectrometers with
tetramethylsilane (TMS) as reference. Mass spectra (FAB and EI) were
obtained on a Finnigan Mat 95 mass spectrometer. Elemental analyses
were performed by the Institute of Chemistry at Chinese Academy of
Sciences, Beijing.
UV/Vis absorption spectra were obtained on a Perkin Elmer Lambda 19
UV/Visible spectrophotometer. Steady-state emission spectra were re-
corded on a SPEX 1681 Fluorolog-2 series F111 spectrophotometer
equipped witha Hamamatsu R928 PMT detector. Emission lifetimes were
obtained witha Quanta Ray DCR-3 pulsed Nd:YAG laser system (pulse
output 355 nm, 8 ns). Errors for l (Æ1 nm), t (Æ10%), and F (Æ10%)
values are estimated. Further details of instrumentation and emission
measurements have been given previously.[4a]
2,9-Bis(2'-methoxyphenyl)-4,7-diphenyl-1,10-phenanthroline:
A solution
of o-bromoanisole (15 g) in Et2O (30 mL) was added in a dropwise fashion
to a suspension of lithium metal pieces (2 g) in Et2O (20 mL) under a
nitrogen atmosphere. The mixture was refluxed for 2 h to yield a freshly
prepared solution of o-lithioanisole. This was slowly added to an ice-cooled
solution of 4,7-diphenyl-1,10-phenanthroline (4.4 g) in degassed toluene
(50 mL), and a wine-red solution was
obtained immediately. The resultant
mixture was refluxed for 24 hand
R1 R2
X
X
cooled in an ice bath, then deionized
water (25 mL) was added to hydrolyze
the products. The organic phase was
R1 R2
Li
MeO
2. MnO2
X
X
2
1.
separated and stirred withMnO 2 (50 g)
for 24 h, then filtered and dried with
MgSO4. A pale yellow solid was ob-
tained upon concentration of the sol-
ution, and this was collected by filtra-
tion, washed successively with MeOH
and Et2O, and dried under vacuum.
Yield: 3.7 g (51%). 1H NMR
(300 MHz, [D6]acetone): 8.41 (d, J ˆ
7.6 Hz, 2H; C6H4), 8.29 (s, 2H; phen),
7.89 (s, 2H; phen), 7.54 7.7 (m, 10H;
Ph), 7.49 (t, J ˆ 7.8 Hz, 2H; C6H4),
7.16 7.25 (m, 4H; C6H4), 3.97 ppm (s,
N
N
N
N
OMe MeO
R1,2 = HC=CH, X = Ph: 1
R1 = R2 = H, X = tBu: 2
pyHCl or HBr
R1 R2
X
X
R1 R2
X
X
K2PtCl4
N
O
N
‡
N
N
6H; OMe); EI-MS: m/z: 544 [M] .
glacial
acetic acid
Pt
2,9-Bis(2'-hydroxyphenyl)-4,7-diphen-
yl-1,10-phenanthroline (Ph2N2O2H2):
A mixture of the anisole precursor
(2 g, 3.7 mmol) and pyridinium hydro-
chloride (4.23 g, 37 mmol) was heated
under nitrogen at 2108C for 36 h. After
cooling, water (30 mL) was added and
O
OH
HO
Scheme 1. Synthesis of 1 and 2.
the aqueous solution was extracted with chloroform (3 Â 30 mL). The
combined organic extracts were washed with saturated sodium bicarbonate
solution (5 Â 30 mL) and water (3 Â 30 mL), dried over magnesium sulfate,
and evaporated to give a bright yellow solid. Chromatography on silica gel
with n-hexane/dichloromethane (1:2) as eluent afforded a yellow solid.
Yield: 0.99 g (52%). 1H NMR (300 MHz, CDCl3): 14.69 (s, 2H; OH), 8.22
(s, 2H; phen), 8.00 (d, J ˆ 8.2 Hz, 2H; C6H4), 7.84 (s, 2H; phen), 7.58
(virtual s, 10H; Ph), 7.41 (t, J ˆ 7.7 Hz, 2H; C6H4), 7.30 (d, J ˆ 8.3 Hz, 2H;
C6H4), 6.98 ppm (t, J ˆ 7.5 Hz, 2H; C6H4); 13C NMR (151 MHz, CDCl3):
160.5, 157.7, 150.3, 142.8, 137.8, 132.2, 129.6, 128.9, 128.8, 127.1, 125.7, 123.7,
Abstract in Chinese:
‡
120.6, 119.4, 119.2, 118.9 ppm; FAB-MS: m/z: 517 [M ‡ H] .
6,6'-Bis(2''-methoxyphenyl)-4,4'-bis(tert-butyl)-2,2'-bipyridine: The proce-
dure for the 4,7-diphenyl-1,10-phenanthroline analogue was adopted by
using 4,4'-di-tert-butyl-2,2'-bipyridine. Upon workup, a yellow oily sub-
stance was obtained. This was purified by chromatography on a silica gel
column withpetroleum ether (60 80 8C)/ethyl acetate (8:1) as eluent. The
crude product was collected and recrystallized from hexane to give white
Chem. Eur. J. 2003, 9, No. 6
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C.-M. Che et al.
FULL PAPER
crystals. Yield: 3.3 g (35%). 1H NMR (300 MHz, CDCl3): 8.47 (d, J ˆ
1.8 Hz, 2H; bpy), 8.01 (d, J ˆ 7.6, 2H; C6H4), 7.91 (d, J ˆ 1.8 Hz, 2H;
bpy), 7.40 (t, J ˆ 7.8 Hz, 2H; C6H4), 7.15 (t, J ˆ 7.4 Hz, 2H; C6H4), 7.05 (d,
J ˆ 8.2 Hz, 2H; C6H4), 3.90 (s, 6H; OMe), 1.42 ppm (s, 18H; tBu); EI-MS:
(thickness 300 ä); electron-injection layer of lithium fluoride (LiF)
(thickness 5 ä), and aluminum layer as cathode (thickness 2500 ä).
The organic layers were laminated in sequence under 5 Â 10À6 mbar
without breaking vacuum between different vacuum deposition processes.
The cathode metal was deposited in another chamber under 5 Â 10À6 mbar
vacuum immediately after organic deposition. The layers were deposited at
rates of 2 5 äsÀ1. The emissive area of the devices as defined by the
overlapping area of the cathode and anode was 3 Â 3 mm2. EL spectra and
brightness-current density-voltage characteristics of the devices were
measured in air at room temperature witha Spectroscan PR650 spectro-
photometer and a computer-controlled direct-current power supply.
‡
m/z: 479 [M] .
6,6'-Bis(2''-hydroxyphenyl)-4,4'-bis(tert-butyl)-2,2'-bipyridine (tBu2N2O2H2):
A mixture of the anisole precursor (1 g) in hydrobromic acid (47%, 20 mL)
was refluxed for 12 h. This was cooled to room temperature and neutralized
witha saturated Na 2CO3 solution. The organic product was extracted with
chloroform, washed with deionized water (2 Â 50 mL), and dried over
Na2SO4. A solid residue was obtained by removal of volatiles, and this was
recrystallized from a methanol/dichloromethane solution to afford white
crystals. Yield: 0.56 g (60%). 1H NMR (300 MHz, CDCl3): 14.45 (s, 2H;
OH), 8.16 (d, J ˆ 1.4 Hz, 2H; bpy), 7.97 (d, J ˆ 1.3 Hz, 2H; bpy), 7.90 (d, J ˆ
8.0 Hz, 2H; C6H4), 7.34 (t, J ˆ 8.4 Hz, 2H; C6H4), 7.07 (d, J ˆ 8.2 Hz, 2H;
C6H4), 6.96 (t, J ˆ 8.1 Hz, 2H; C6H4), 1.47 ppm (s, 18H; tBu); 13C NMR
(151 MHz, CDCl3): 163.3 159.7, 157.5, 152.2, 131.5, 126.5, 119.2, 118.9, 118.4,
Results
Synthesis and characterization: The potentially tetradentate
bisphenols Ph2N2O2H2 and tBu2N2O2H2 are derived from 4,7-
Ph2phen and 4,4'-tBu2bpy, respectively, by modification of the
literature preparation for 2,9-bis(2'-hydroxylphenyl)-1,10-
phenanthroline.[14] By adapting the procedure reported by
Scandola et al.[15] for Ptq2, treatment of the ligands with
K2PtCl4 in refluxing glacial acetic acid yielded the neutral
platinum(ii) complexes [Pt(Ph2N2O2)] (1) and [Pt(tBu2N2O2)]
(2) (Scheme 1). Although 1 is readily soluble only in DMF,
CH2Cl2, and C6H6, th etert-butyl groups in 2 impart high
solubility in most common organic solvents, except aliphatic
hydrocarbons. These materials are indefinitely stable in the
solid state but undergo partial decomposition in solution
when exposed to light and atmospheric conditions. To
examine their suitability for vacuum deposition processes
during OLED fabrication, complexes 1 and 2 were studied by
thermal gravimetric analysis (Figure 1), and both materials
exhibited high thermal stability in nitrogen and air (heating
rate 158C minÀ1). Hence 1 and 2 are stable up to 440 and
5308C, respectively, in N2, while both undergo significant
weight loss from around 3808C in air.
‡
116.7, 116.4, 35.6, 30.6 ppm; EI-MS: m/z: 452 [M] .
Pt(Ph2N2O2) (1): A mixture of K2PtCl4 (0.10 g, 0.25 mmol) and Ph2N2O2H2
(0.13 g, 0.25 mmol) was refluxed in glacial acetic acid (10 mL) for 48 h.
After cooling, the resulting suspension was collected by filtration, washed
withacetic acid and water, and dried under vacuum to afford a brown solid.
The crude product was recrystallized by slow evaporation of a dichloro-
methane solution to afford red crystals. Yield: 0.11 g (62%). 1H NMR
(400 MHz, CD2Cl2): 8.67 (s, 2H; phen), 8.36 (d, J ˆ 8.8 Hz, 2H; C6H4), 8.02
(s, 2H; phen), 7.61 7.47 (m, 14H; Ph and C6H4), 6.88 ppm (m, 2H; C6H4);
‡
FAB-MS: m/z: 710 [M] ; elemental analysis calcd (%) for C36H22N2O2Pt ¥
0.5 CH2Cl2: C 58.29, H 3.08, N 3.72; found: C 58.50, H 3.03, N 3.81.
Pt(tBu2N2O2) (2): The same procedure was used as for 1. The product was
recrystallized by slow diffusion of diethyl ether into a dichloromethane
solution to afford yellow crystals. Yield: 25%. 1H NMR (300 MHz, CDCl3):
8.32 (d, J ˆ 1.3 Hz, 2H; bpy), 8.01 (d, J ˆ 8.5 Hz, 2H; C6H4), 7.86 (d, J ˆ
1.7 Hz, 2H; bpy), 7.48 (d, J ˆ 8.5 Hz, 2H; C6H4), 7.38 (t, J ˆ 7.6 Hz, 2H;
C6H4), 6.78 (t, J ˆ 7.5 Hz, 2H; C6H4), 1.54 ppm (s, 18H; tBu); 13C NMR
(151 MHz, CDCl3): 162.7, 159.1, 155.3, 149.9, 131.3, 128.0, 124.1, 120.5,
‡
120.4, 116.3, 36.0, 30.4 ppm; FAB-MS: m/z: 645 [M] ; elemental analysis
calcd (%) for C30H30N2O2Pt: C 55.81, H 4.68, N 4.34; found: C 55.39, H 4.70,
N 4.14.
X-ray crystallography: Crystal data for 1 ¥ CH2Cl2: C37H24N2O2Cl2Pt, Mr ˆ
≈
794.57, triclinic, P1, a ˆ 8.481(2), b ˆ 13.292(3), c ˆ 14.024(3) ä, a ˆ
71.48(3)8, b ˆ 87.14(3)8, g ˆ 72.20(3)8, Vˆ 1425.3(6) ä3, Z ˆ 2, 1calcd
ˆ
1.851 gcmÀ3
,
m(MoKa) ˆ 5.150 mmÀ1
F(000) ˆ 776, crystal size ˆ 0.40 Â
,
0.15  0.10 mm3, Tˆ 293(2) K, 2qmax ˆ 50.78, 4746 independent reflections
(Rint ˆ 0.032), 397 variable parameters, R1 ˆ 0.038 (I > 2s(I)), wR2 ˆ 0.099,
GOF(F 2) ˆ 1.09, max./min. residual electron density 0.928/ À 2.373 eäÀ3
.
For 2: C30H30N2O2Pt, Mr ˆ 645.65, monoclinic, P21/m, a ˆ 12.359(2), b ˆ
6.9720(14), c ˆ 14.438(3) ä, b ˆ 93.12(3)8, Vˆ 1242.2(4) ä3, Z ˆ 2, 1calcd
ˆ
1.726 gcmÀ3
,
m(MoKa) ˆ 5.678 mmÀ1, F(000) ˆ 636, crystal size ˆ 0.50 Â
0.20  0.03 mm3, Tˆ 301(2) K, 2qmax ˆ 50.98, 2074 independent reflections
(Rint ˆ 0.048), 205 variable parameters, R1 ˆ 0.044 (I > 2s(I)), wR2 ˆ 0.112,
GOF(F 2) ˆ 1.12, max./min. residual electron density 1.559/ À 1.789 eäÀ3
.
Data collection was performed on an MAR diffractometer witha 300 mm
image plate detector (l ˆ 0.71073 ä). Crysystallographic data (excluding
structure factors) for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as supple-
mentary publications no. CCDC-193494 (2) and CCDC-193493 (1). Copies
of the data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (‡44)1223-336-033 or
e-mail: deposit@ccdc.cam.ac.uk).
Device fabrication: ITO-coated glass slides (ITO ˆ indium tin oxide) witha
sheet resistance of 20 W/square were used as substrate and anode. Before
loading into a deposition chamber, the ITO-coated glass slides were
cleaned withorganic solvents (toluene, acetone, and metahnol) and
deionized water, dried using an infrared oven, and finally treated with
ultraviolet (uv)-ozone before use. Eachdevice was assembled in sequence
using the following materials: ITO electrode; hole-transport material N,N'-
di(a-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB) (thick-
ness 300 ä); emitting layer composed of host material beryllium bis(2,2'-
hydroxyphenyl)pyridine (Bepp2) and emitting dopant (complex 1 or 2)
Figure 1. TGA thermograms of complexes 1 and 2 under nitrogen and air.
Crystal structures and packing: The molecular structures of
complexes 1 and 2 were determined by X-ray crystallography
(Figure 2, top). Several similarities, as well as important
differences, are evident and are described below. The square-
1266
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Chem. Eur. J. 2003, 9, No. 6

PtII Tetradentate N O Chelates Complexes
1263 1272
2
2
Figure 2. Perspective views (50% thermal probability) and packing arrangements along the ab plane for 1 (left) and 2 (right). Selected bond lengths [ä] and
À
À
À
À
À
À
À
À
À
angles [8] for 1: Pt1 N1 1.978(6), Pt1 N2 1.941(6), Pt1 O1 1.983(6), Pt1 O2 1.972(5); Pt1 O1 C1 123.8(5), Pt1 O2 C20 124.1(5). For 2: Pt1 N1 1.97(1),
À
À
À
À
À
À
À
Pt1 N2 1.96(1), Pt1 O1 1.97(1), Pt1 O2 1.98(1); Pt1 O1 C1 123.4(9), Pt1 O2 C18 123(1).
À
À
planar geometry around the Pt cores and the Pt N and Pt O
distances (mean 1.960 and 1.978 ä in 1 and 1.963 and 1.975 ä
in 2) are generally unremarkable. The central Pt(N2O2)
frameworks in both structures are highly planar. However,
the two phenyl substituents of the phen ligand in 1 deviate
significantly (47.8 and 70.78) from the mean plane of the
molecule, hence the Ph groups may not participate in p
overlap with the phenanthroline moiety in the solid state.
Close examinations of the crystal lattices of 1 and 2 reveal
contrasting intermolecular contacts (Figure 2, bottom). Mol-
ecules of 2 form extended sheets that are stacked directly
along the c axis, and the interplanar distance is equivalent to
b/2, that is 3.486 ä. This allows the possibility of extended p-
stacking interactions, but a view of the ac plane shows a
noticeably displaced configuration and non-interacting metal
centers (Pt ¥¥¥¥ Pt 5.521 ä). In the crystal lattice of 1, molecules
are assembled into dimeric entities that are stacked in a head-
to-tail manner. The Pt(N2O2) planes of the dimers are
separated by 3.462 ä, which is comparable to 2. The distinctly
eclipsed and overlapping dimeric arrangement and the closest
Electronic absorption and emission spectra: excited-state
properties: The photophysical data of 1 and 2 are summarized
in Table 1 and their UV/Vis absorption spectra in CH2Cl2 are
depicted in Figure 3. Both derivatives exhibit intense high-
energy absorptions at lmax < 375 (e > 104 molÀ1 dm3 cmÀ1). In
addition, moderately intense bands are observed at lmax ˆ 420
and (488 sh) 504 nm (5300 and ꢀ7000 molÀ1 dm3 cmÀ1 respec-
tively) for 1 and lmax ˆ 397 and 479 nm (504 sh) (8400 and
ꢀ2700 molÀ1 dm3 cmÀ1, respectively) for 2. The effects of
solvent polarity on the UV/Vis absorptions of 1 and 2 were
investigated. While the energies of the absorption maxima for
2 at l ꢁ 315 nm remain virtually unchanged, the low-energy
absorption bands undergo significant blue shifts in solvents of
greater polarity (lmax): 408, 500 nm (533 sh) in C6H6/CH2Cl2
(10:1); 397, 479 nm (504 sh) in CH2Cl2; 390, 465 nm in
CH3CN; 385, 465 nm in CH3OH (Figure 4). Likewise, the low-
energy absorptions of 1 display similar shifts (lmax): 438, (504
sh) 535 nm in C6H6; 420 and (488 sh) 504 nm in CH2Cl2; 420
and (478 sh) 502 nm in DMF.
The photoluminescent data of 1 and 2 in various media are
listed in Table 1. Complexes 1 and 2 are highly luminescent in
solution at 298 K and display a structureless emission in
CH2Cl2 at lmax ˆ 586 and 595 nm, respectively (Figure 3), with
[16]
À
Pt Pt distance of 3.517 ä
in 1 may result in solid-state
photo- and electroluminescent properties that are different
from 2 (see below).
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C.-M. Che et al.
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Table 1. Photophysical data.
Complex
Medium
C6H6
T, K labs [nm] (e, Â104 molÀ1dm3cmÀ1
)
lem [nm] (t, ms; Fem)
Pt(Ph2N2O2) (1)
298
298
298
298 (2.50), 324 (2.68), 331(sh, 2.57), 363 (2.09),
384 (2.08), 438 (0.45), 504 (sh, 0.59), 535 (0.68)
291 (3.92), 315 (3.40), 325 (3.23), 352 (2.58), 375 (2.47),
420 (0.52), 488 (sh, 0.67), 504 (0.72)
599 (5.0; 0.58)
586 (5.3; 0.6)
589 (3.2; 0.45)
CH2Cl2
DMF
292 (3.45), 317 (3.64), 326 (sh, 3.54), 353 (2.84), 376 (2.69),
420 (0.64), 478 (sh, 0.73), 502 (0.78)
thin film
EtOH/MeOH (4:1)
solid
solid
298
77
298
77
-
-
-
-
651
555 (max, 13), 597
660 (2.0)
594 (max, 4.5), 644, 700
605 (1.7; 0.09)
Pt(tBu2N2O2) (2) C6H6/CH2Cl2 (10:1) 298
319 (1.69), 408 (0.773), 500 (0.240), 533 (sh, 0.203)
CH2Cl2
CH3CN
MeOH
thin film
EtOH/MeOH (4:1)
solid
298
298
298
298
77
253 (4.10), 313 (1.84), 397 (0.840), 479 (0.294), 504 (sh, 0.252) 595 (1.9; 0.12)
250 (4.45), 315 (1.85), 390 (0.847), 465 (0.309)
596 (1.3; 0.06)
247 (4.59), 310 (1.92), 385 (0.859), 465 (0.317)
592 (1.1; 0.05)
-
-
-
-
599
535 (max, 17), 575, 620
562, 593 (max, 1.5 and 0.3[a]), 656, 718
577 (max, 3.3 and 0.4[a]), 625, 682
298
77
solid
[a] Bi-exponential decay.
emission lifetimes and luminescent quantum yields of 5.3 ms
and 0.6 for 1 and 1.9 ms and 0.1 for 2. The time-resolved
absorption-difference (TA) spectra for the excited state of 2 in
various solvents at 298 K were obtained. Blue-shifted absorp-
tion peak maxima were observed from C6H6/CH2Cl2 (10:1)
(450 nm) to CH2Cl2 (440 nm), and CH3CN and CH3OH
(430 nm respectively; see Supporting Information for details
and TA spectra in CH3CN). For all of the solvents studied, the
decay lifetimes (tTA) and emission lifetimes are in good
agreement, indicating that the observed excited-state absorp-
tion can be ascribed to the excited state of 2.
Upon examination, we found that the emission energies of 1
and 2 are influenced by solvent polarity. The emission
maximum blue-shifts from 599 (in C6H6) to 586 (CH2Cl2),
and 589 nm (DMF) for 1 and from 605 (in C6H6/CH2Cl2
(10:1)) to 595 (CH2Cl2), 596 (CH3CN), and 592 nm (CH3OH)
for 2. Compared withfluid solutions, the emissions of 1 and 2
in alcoholic glasses at 77 K are blue-shifted and highly
structured. For example, the emission of 1 exhibits peak
maxima at 555 and 597 nm (vibronic progression ˆ 1270 cmÀ1)
and a lifetime of 13 ms. While the solid-state emissions of 1 and
2 are relatively diffused, the 77 K emissions are distinctly
vibronic (progression about 1300 cmÀ1). In this case, the
emission of the Ph2phen analogue 1 appears at partially lower
energies (Figure 5).
Figure 3. Absorption and emission (normalized, lex ˆ 400 nm) spectra of 1
and 2 in CH2Cl2 at 298 K.
To understand the excited-state reactivities and photoredox
properties of the [Pt(N2O2)] complexes, it was beneficial to
establish their excited-state redox potentials. This was per-
formed for 2 by using spectroscopic and electrochemical data
to estimate E0[Pt2‡*/‡] and E0[Pt3‡/2‡*] (Pt ˆ 2). The cyclic
voltammogram of 2 shows irreversible reduction and oxida-
tion waves at À1.94 and ‡0.56 V versus [Cp2Fe]‡/0, respec-
tively, in CH2Cl2. The irreversible nature of these electro-
chemical waves means that the excited-state reduction
potential cannot be determined precisely. Therefore, quench-
‡
ing studies using a series of pyridinium acceptors [A ] with
different reduction potentials have also been employed.[17]
A
table listing the quenching data for 2 and the nonlinear least-
Figure 4. UV/Vis absorption spectra of 2 in various solvents at 298 K.
1268
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PtII Tetradentate N O Chelates Complexes
1263 1272
2
2
deposited on ITO glass, and various EL devices at different
doping concentrations of the platinum materials were fabri-
cated. Bis(2-(2'-hydroxyphenyl)pyridine)beryllium (Bepp2)[18]
and N,N'-di(a-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-
diamine (NPB) were employed as the host and hole-trans-
porting layers respectively.
All devices exhibited low turn-on voltages of 5 7 V and
generated yellow to yellow-green EL at various voltages. In
device A, at 2% ratio of 1, only the Bepp2 emission at around
450 nm was observed when driven under forward bias. When
the dopant concentration level of 1 was increased to 10% in
device B, the resultant yellow EL was dominated by 1 at lmax
588 nm. The EL of this device and the involvement from
Bepp2 was found to be voltage-dependent (Figure 6). Namely,
the Bepp2 emission at 444 nm was insignificant below 8 V, but
slowly became observable at voltages above 8 V. Maximum
luminance and power efficiency of 850 cdmÀ2 (at
360 mAcmÀ2) and 0.26 lmWÀ1 (at 14 mAcmÀ2), respectively,
were achieved (see Supporting Information).
Figure 5. Normalized solid-state 77 K emission spectra of 1 and 2 (lex
355 nm).
ˆ
‡
square fitting of lnkq' versus E0(A /A) using the Marcus
equation (1) is provided in the Supporting Information.
(RT/F)lnkq' ˆ (RT/F)ln(Kkn) À [l(1 ‡ DG/l)2/4]
(1)
Here, K ˆ kd/kÀd (which is approximately 1 2 dm3 molÀ1)
and k, n, and l are the transmission coefficient, nuclear
frequency, and reorganization energy for electron transfer,
respectively; DG is the standard free-energy change for the
reaction [Eq. (2)]; the work terms wp and wr associated with
bringing the reactants/products to mean separation are small
and are neglected.
‡
DG ˆ E0[Pt3‡/2‡*] À E0(A /A) ‡ wp À wr
(2)
Thus an excited-state reduction potential of À1.40 V versus
SSCE (or À1.71 V versus [Cp2Fe]‡/0) for E0[Pt3‡/2‡*] was
calculated. This value closely corresponds to that (ꢁ À 1.7 V
versus [Cp2Fe]‡/0) obtained from Equation (3), in which E0 0 is
approximated from the spectroscopic data (2.3 eV), and
E0[Pt3‡/2‡] is estimated from the Epa of the irreversible
oxidation wave in cyclic voltammetry measurements (0.56 V
versus [Cp2Fe]‡/0). The large negative values obtained for
E0[Pt3‡/2‡*] indicate that the excited state of 2 is strongly
reducing.
Figure 6. Voltage-dependent EL for [ITO/NPB/1-doped (10%) Bepp2/
LiF/Al].
The EL spectra of devices C E are depicted in Figure 7.
The EL spectrum of device C (0.3% of 2) displayed two peaks
at lmax ˆ 453 and 540 nm, due to emission from Bepp2 and 2
respectively, that culminated in strong yellow-green EL (CIE
0.33, 0.47). Saliently, maximum luminance (Figure 8) and
power efficiency of 9330 cdmÀ2 (at 330 mAcmÀ2) and
1.44 lmWÀ1 (at 40 mAcmÀ2), respectively, were detected.
Upon enhancing the concentration of 2 to 1% in device D,
the 550 nm emission from 2 became noticeably more intense
than the Bepp2 emission at 450 nm. However, this EL color
adjustment (CIE 0.39, 0.54) was accompanied by detrimental
device performance. For example, the maximum luminance
dropped to 6200 cdmÀ2 at 390mAcmÀ2 while the optimum
efficiency fell to 0.80 lmWÀ1 at 84 mAcmÀ2.
E0[Pt3‡/2‡*] ˆ E0[Pt3‡/2‡] À E0
(3)
0
Electroluminescent properties: The photophysical properties
and apparent stability of 1 and 2 signify that they are good
candidates as phosphorescent emitters in organic electro-
luminescent (EL) devices. The following EL device structures
were prepared: (ITO/NPB (300 ä)/1 (or 2) doped (x wt%)
Bepp2 (300 ä)/LiF (5 ä)/Al (2500 ä)) (devices A and B
correspond to Bepp2 layers doped with2 and 10 wt% of
complex 1 respectively; devices C E correspond to Bepp2
layers doped with0.3, 1, and 2 wt% of complex 2, respec-
tively). Hence, the device structure is a multilayer stack
In device E, a 2% dopant content of 2 generated yellow EL
(CIE 0.42, 0.56) withminimal contribution from Bepp
emission, but the observed optimal luminance (4480 cdmÀ22
at 280 mAcmÀ2) and power efficiency (0.51 lmWÀ1 at
280 mAcmÀ2) were comparatively poor. The EL band shape
Chem. Eur. J. 2003, 9, No. 6
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1269

C.-M. Che et al.
FULL PAPER
and more recently by Yersin et al.[19] In DMF solution, Ptq2
exhibits
a
low-energy
band
at
478 nm
(e ꢂ
6900 molÀ1 dm3 cmÀ1). Because corresponding transitions ap-
pear at higher energies for Alq3 and Rhq3 (lmax ˆ 388 and
425 nm respectively), a metal-perturbed, ligand-centered
1(l ! p*) (l ˆ lone pair/phenoxide) charge-transfer assign-
ment was proposed[15] and later confirmed by Shpol×skii
spectroscopic measurements.[19]
Because the absorption maxima, bandshape, and intensity
of the low-energy bands for Ptq2 and especially 2 bear great
resemblance, we tentatively assign the low-energy absorptions
at 400 500 nm in 1 and 2 to a 1[l ! p*(diimine)] CT transition
1
mixed withPt(d) ! p*(diimine) MLCT. Furthermore, like
Ptq2, 1 and 2 also display negative solvatochromic shifts (the
absorption of 2 at 24500 cmÀ1 (408 nm) in C6H6/CH2Cl2 (10:1)
blue-shifts to 26000 cmÀ1 (385 nm) in MeOH), and this is
consistent with the polar character of the electron-rich
oxygen/phenoxide fragment in the ground state. We suggest
Figure 7. EL spectra for devices C E: [ITO/NPB/2-doped (x%) Bepp2/
LiF/Al].
1
that mixing with the MLCT state is likely, and indeed, the
energies of the absorptions for 1 and 2 at 420 and 397 nm,
1
respectively, are typical of MLCT transitions in PtII diimine
complexes.[11, 20]
Finally, it is noteworthy that the above assignment is in
essence equivalent to the S(p)/Pt(d) ! p*(diimine) (i.e.
'charge-transfer-to-diimine') assignment designated by Eisen-
berg for the lowest energy absorption band in a related family
of PtII diimine derivatives containing aromatic dithiolate
ligands.[21] For example, the CT absorption for [Pt(4,4'-
tBu2bpy)(toluene-3,4-dithiolate)] appears at lmax ˆ 563 nm
(e ꢂ 7200 molÀ1 dm3 cmÀ1) in CH2Cl2 and similarly undergoes
negative solvatochromic shifts.[22]
The long emission lifetimes for 1 and 2 (in the ms range)
strongly indicate phosphorescence. In general, the structure-
less emissions of 1 and 2 undergo blue shifts on cooling from
298 to 77 K and become highly structured. Vibronic pro-
gressions of about 1300 cmÀ1 are evident in the 77 K solid-
Figure 8. I-V-L characteristics for device C: [ITO/NPB/2-doped (0.3%)
Bepp2/LiF/Al].
3
state emission spectra and this is emblematic of pp* excited
is similar to the corresponding thin-film photoluminescence at
lmax ˆ 599 nm but the EL energy (lmax ˆ 548 nm) is blue-
shifted.
states for monomeric PtII diimine compounds.[23] However,
the room-temperature fluid emissions of 1 and 2 are
excessively red-shifted for this assignment. In addition, the
negative solvatochromism observed for the PL of 1 and 2 is
also inconsistent witha ™normal∫ 3pp* state. By comparison
Discussion
withthe emission spectra of Ptq in absolute EtOH at 77 K
2
(lmax ˆ 625, 680 nm; progression ꢂ1300 cmÀ1), the equivalent
spectra of 1 and 2 in glassy EtOH/MeOH are blue-shifted but
the bandshape for 1 is remarkably similar and all three
complexes display matching vibronic progressions. We there-
fore tentatively assign the emissions of 1 and 2 to mixed
Singlet and triplet states: The intense absorptions in the high-
energy region (l < 375 nm) of the UV/Vis spectra of com-
plexes 1 and 2 are assigned to ligand-centered 1(pp*)
transitions by comparison with the corresponding bisphenol
ligands in CH2Cl2. However, the origins of the moderately
intense absorptions at lmax ˆ 420 and 504 nm (488 sh) for 1 and
lmax ˆ 397 and 479 nm (504 sh) for 2 are more problematic to
elucidate. These absorption bands are clearly too low in
energy for pp* transitions (the free ligands absorb very
weakly at l > 370 nm). We have plotted the absorbance of 2 at
510 nm against complex concentration and shown that this
band obeys Beer×s law, so the possibility of involvement by
oligomeric species may be eliminated.
3
3MLCT and [l ! p*(diimine)] (l ˆ lone pair/phenoxide) ex-
cited states.
Electrophosphorescence and substituent effects: The toler-
ance of complexes 1 and 2 to heat and vacuum deposition
conditions is thought to arise from the strong chelate effect and
the chemically inert nature of the N2O2 ligand. The novel
beryllium(ii) chelate Bepp2 (Hpp ˆ 2(2'-phenol)pyridine) was
recently employed by Wang and co-workers as an emitting
layer in high-performance blue OLEDs[18a] and as a host
material for the construction of orange-red light EL devices.[18b]
At this juncture, it is appropriate to discuss the photo-
physical and electronic nature of the closely related bischelate
Ptq2, which was first investigated in detail by Scandola et al.[15]
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PtII Tetradentate N O Chelates Complexes
1263 1272
2
2
[1] M. A. Baldo, D. F. O×Brien, Y. You, A. Shoustikov, S. Sibley, M. E.
Thompson, S. R. Forrest, Nature 1998, 395, 151 154.
In the devices containing complex 1 as dopant, only EL
from Bepp2 was observed at 2% (device A), suggesting
ineffective host dopant Fˆrster energy transfer. However,
this would be surprising because there is excellent overlap
between the host-emission and dopant-absorption spectra, so
the shorter-range Dexter process is presumably dominant in
the Bepp2:1 system. At 10% dopant level of 1 (device B),
yellow EL from 1 only was visible at low voltages but
contribution from the Bepp2 host became noticeable around
10 V (Figure 6). This is consistent with the long electro-
phosphorescent lifetime exhibited by 1, which leads to
saturated triplet exciton population at high voltages so that
energy transfer from Bepp2 excitons becomes blocked. While
the abundance of p electrons in 1 may be beneficial for charge
mobility, the favorable formation of dimeric assemblies (see
crystal lattice of 1) and the presence of pendant phenyl
substituents can facilitate intermolecular quenching (includ-
ing self-quenching) processes. This may explain the inferior
brightness and efficiency of devices derived from dopant 1.
The employment of ancillary ligands containing bulky
substituents has been demonstrated to be advantageous for
electrophosphorescent OLEDs with regards to the suppres-
sion of self-quenching activities[24] and the maintenance of
amorphous films.[25] In this work, modification of the diimine
fragment from Ph2phen in 1 to tBu2bpy in 2 results in
substantially improved OLED performance. At 0.3% dopant
level of 2, (device C), long-range Fˆrster energy transfer from
host excitons appears considerably more efficient compared
to 1 and EL from 2 at lmax ˆ 540 nm is observed, although the
Bepp2 emission at 450 nm remains prominent. Excellent
maximum luminance (9330 cdcmÀ2) and power efficiency
(1.44 lmWÀ1) have been realized, and this can be ascribed to
the molecular structure of 2, which may mediate effective
charge transport to yield relatively balanced charge recombi-
nation within the 2-doped Bepp2 layer without promoting
quenching mechanisms.
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We are grateful for financial support from the University of Hong Kong,
the Croucher Foundation (Hong Kong), Research Grants Council of Hong
Kong SAR, China [HKU 7298/99P], and the Innovation and Technology
Commission of the Hong Kong SAR Government (ITS/053/01). We also
thank Dr. S. C. Yu for technical assistance.
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