Light-emitting platinum(ii) complexes supported by tetradentate dianionic bis(N-heterocyclic carbene) ligands: Towards robust blue electrophosphors

Article abstract of DOI:10.1039/c3sc21822h

The synthesis, structures and photophysical properties of the charge-neutral Pt(ii) complexes (1-6) and their Pd(ii) (7) and Ni(ii) (8) congeners supported by tetradentate dianionic bis[phenolate-(N-heterocyclic carbene)] ligands are described. The X-ray

Full text of DOI:10.1039/c3sc21822h

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Light-emitting platinum(II) complexes supported by
tetradentate dianionic bis(N-heterocyclic carbene)
ligands: towards robust blue electrophosphors†
Cite this: DOI: 10.1039/c3sc21822h
Kai Li,a Gang Cheng,a Chensheng Ma,a Xiangguo Guan,a Wai-Ming Kwok,b
a
Yong Chen,a Wei Luac and Chi-Ming Che
*
The synthesis, structures and photophysical properties of the charge-neutral Pt(II) complexes (1–6) and their
Pd(II) (7) and Ni(II) (8) congeners supported by tetradentate dianionic bis[phenolate-(N-heterocyclic
carbene)] ligands are described. The X-ray crystal structures of two solvatomorphs of 2, which has p-F
substituents on the tetradentate ligand, have been determined. The photophysical properties of all the
complexes were examined. In THF solutions, 1–4 display deep blue phosphorescence (lmax ¼ ꢀ440–
460 nm, Fe ¼ 3–18% and s ¼ 0.5–3.5 ms). In solutions at room temperature, 5–8 show profoundly
different luminescence properties from being virtually non-emissive (Fe < 10ꢁ3) for 6–8 to highly
emissive (Fe ¼ 15%) with much red-shifted phosphorescence (lmax ¼ ꢀ530 nm) and a long emission
lifetime (s ¼ 47.2 ms) in the case of 5. Time-dependent density functional theory (TDDFT) calculations
reveal that the tetradentate bis(phenolate-NHC) ligands in 1–4 provide a rigid scaffold for preserving a
tightly bound Pt(II) in a square-planar coordination geometry in the T1 as in the S0 states and the blue
emission is derived from the T1 state having predominant ligand (pAr–O)-to-ligand (p*NHC) charge
transfer (LLCT) character. A switch of orbital parentage from LLCT to ligand-centred (LC) p–p* is
responsible for the long emission lifetime and vibronically structured emission displayed by 5 when
compared to that of 1–4 and 6. Both femtosecond time-resolved fluorescence (fs-TRF) and nanosecond
time-resolved emission (ns-TRE) measurements were conducted on 2 and 4 to directly probe the
excited-state dynamics after photoexcitation. Excellent thermal stability of the fluorine-free complex 4
and its higher emission quantum yield (relative to 1 and 3), and using 9-(4-tert-butylphenyl)-3,6-
bis(triphenylsilyl)-9H-carbazole (CzSi) as host material, led to the fabrication of highly efficient deep blue
OLEDs with peak current efficiency of 24 cd Aꢁ1 and white organic light-emitting devices (WOLEDs)
Received 25th October 2012
Accepted 18th March 2013
DOI: 10.1039/c3sc21822h
www.rsc.org/chemicalscience
with peak current efficiency of 88 cd Aꢁ1
.
organic light-emitting diodes (OLEDs)2–4 that hold potential for
the next generation of illumination sources and at-panel-
display portable electronic devices. Meanwhile, Pt(II) complexes
with long-lived emissive excited states have been applied as
efficient chemosensors, photocatalysts and photosensitizers.5,6
The phosphorescence of Pt(II) complexes is attributed to heavy-
atom induced spin–orbital coupling (SOC) which facilitates
efficient intersystem crossing (ISC) from the photogenerated
excited singlet state to lower-energy triplet state. Through high-
resolution optical spectroscopic measurements and/or indirect
tting approach based on temperature-dependent emission
data recorded with a number of Pt(II) and Ir(III) complexes,
Yersin and co-workers reported that: (1) SOC is a critical
parameter governing the rate of ISC and radiative decay of the
triplet excited state; and (2) zero-eld splitting (ZFS) is an
effective parameter for evaluating the suitability of luminescent
metal complexes as OLED phosphors.7 Thermal deactivation
through low-lying metal-centred (MC) ligand-eld (LF) d–d
Introduction
Phosphorescent organoplatinum(II) complexes have been
receiving a surge of interests for their intriguing photophysical
and photochemical properties and for their wide range of
potential practical applications.1 For instance, Pt(II) complexes
exhibiting high phosphorescence quantum efficiency and
microsecond lifetime can be used to fabricate high performance
aState Key Laboratory of Synthetic Chemistry, Institute of Molecular Functional
Materials, Department of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong, China. E-mail: cmche@hku.hk
bDepartment of Applied Biology and Chemical Technology, The Hong Kong Polytechnic
University, Hung Hom, Kowloon, Hong Kong SAR, China
cDepartment of Chemistry, South University of Science and Technology of China, 1088
Xueyuan Boulevard, Shenzhen, Guangdong 518055, China
† Electronic supplementary information (ESI) available: Additional experimental
details, gures and tables. CCDC reference numbers 907132 and 907133. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3sc21822h
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states constitutes an important non-radiative decay channel ligands are particularly suited to this purpose. To have the
affecting the emission and photophysical properties of transi- NHCs-based molecular orbital(s) involved in the low-energy
tion-metal complexes. Due to the population of electron in the excited states of transition-metal complexes may provide a
antibonding Pt(II) 5dx2ꢁy2 orbital, the d–d states of Pt(II) solution to this previously intractable issue. Indeed, the tetra-
complexes are severely distorted from the ground state and kis(NHC) Pt(II) complexes are known to exhibit phosphores-
hence are subjected to efficient non-radiative decay. Thus the cence in the UV region in the solid state.14 Very recently, charge-
energy of the low lying d–d states is an important parameter neutral green-to-blue light emitting Pt(II) complexes supported
affecting the efficiency and lifetime of the emissive excited by cyclometalating and NHC ligands have been reported.4 In
states.
addition, the [Pt(NHC)2(C^CAr)2] complexes were found to
To improve the photoluminescent properties of transition- emit in the deep blue region in uid solutions.12d Platinum(II)
metal compounds, intense efforts have been directed towards complexes that emit in the blue spectral region and are robust
searching for effective methods to regulate the electronic enough to be incorporated into high-performance OLEDs are
structures including the frontier molecular orbitals of the presently receiving a surge of interest.
ground state and excited states, to optimize the SOC and ZFS
We have previously communicated15 that the Pt(II) complexes
and to minimize the undesired effect of low lying d–d states. supported by tetradentate dianionic bis(phenolate-NHC)-based
Indeed, tuning luminescence in the context of emission energy ligands (ligands L1–L4; Pt(II) complexes 1–4, Chart 1) show
and emission quantum efficiency has been proved viable with phosphorescence in the blue region, and complex 3 can be
sagacious choice of ligands.1 One common strategy in this vacuum-deposited into multilayer OLEDs to emit blue light. We
regard is to prepare metal complexes using strong-eld multi- now present a full account to elucidate the structural features
dentate chelating ligand(s) so as to widen the energy gap responsible for achieving efficient blue emissions. Complexes 5
between the emitting state and the deactivating d–d excited and 6 containing the modied tetradentate bis(phenolate-NHC)
state(s). Another approach is to introduce an emissive ligand- ligands, L5 and L6, respectively, were prepared and their pho-
centred excited state at energy lower than the d–d excited state.1 toluminescence properties were examined. The Pd(II) (7) and
These approaches have led to a number of room-temperature Ni(II) (8) complexes of the tetradentate bis(phenolate-NHC)
phosphorescent Pt(II) complexes. For example, by using tri- ligand L3 have also been prepared for a comparison study. To
dentate cyclometalating ligands, our group8 and others9 have account for the emission properties, density functional theory
shown that Pt(II) complexes can display intense phosphores- (DFT) and time-dependent density functional theory (TDDFT)
cence in solutions at ambient temperature. Tetradentate dia- calculations on complexes 2–6 were performed. Through opti-
nionic ligands are a good choice of ligands for the development mization on the device conguration, we have successfully
of robust, highly phosphorescent Pt(II) complexes due to the fabricated highly efficient blue and white electro-
strong donor strength as well as improved rigidity of the tetra- phosphorescent devices using 4 as a phosphor.
dentate ligand framework.2,3
As up to now, efficient red2c,3,10 and green11 phosphorescent
Pt(II) complexes have been developed. However, Pt(II) complexes
Results
showing decent blue phosphorescence, which is essential for
full-colour display and white-light illumination, are not
common. And achieving stable, highly efficient blue phosphors
remains a formidable challenge.4,12 In light of frontier orbital
Preparation of complexes 1–4 and the ligand precursors have
been described previously.15 Synthetic routes to complexes 5–8
are depicted in Scheme 1. The ligand precursors, aryl-
substituted imidazoles, were prepared in ca. 50% yields by
analysis, a large energy gap between the HOMO and LUMO of a Cu2O/KOH mediated coupling reaction between aryl bromides
Pt(II) complex is required for producing high energy blue and imidazole. The reaction between aryl-substituted imidaz-
emission. The emitting states of most of the literature phos- oles and dibromomethane under reuxing conditions
afforded bis(imidazolium) salts as white solid.16 Similarly,
phorescent Pt(II) complexes have mixed LC p–p* and metal-to-
ligand charge transfer (MLCT) characters. Accordingly,
employing chromophore ligands bearing a large LC p–p*
energy gap is a feasible approach in the development of blue
phosphorescent Pt(II) complexes. For instance, the coordination
of pyridyltriazolate12i,13 or pyridylpyrazolate12c ligand(s) to Pt(II)
affords phosphorescence in the sky blue to blue spectral region.
Nonetheless, when chromophore ligands having large energy
gap are used, thermal population of the relatively low lying d–d
state(s) tends to become increasingly involved. Hence, the crux
in the design of blue phosphorescent Pt(II) complexes is to seek
suitable ligand systems that have large HOMO–LUMO energy
gap to allow for emission to occur in the blue wavelength region
but at the same time have adequate donor strength to push up
the energy of d–d state(s). Having both weak p-accepting and
strong s-donating properties, N-heterocyclic carbene (NHC) Chart 1 Chemical structures of 1–8.
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Fig. 1 Crystal structures of 2-B: (a) perspective view with hydrogen atoms and
solvated molecules omitted for clarity; (b) and (c) packing diagrams.
Fig. 2) was obtained by dispersing a sublimed powder into a
mixed methanol–dichloromethane solution overnight; and (3)
2-VD (VD for vacuum-deposition) was a crystalline powder
obtained aer sublimation and without further contact with any
organic solvents. The form of 2-B crystallizes in a monoclinic
space group, contains a solvated water molecule, and features a
bent molecular framework. The coordination plane around Pt
ion in crystal 2-B adopts a square-planar geometry with negli-
gible deviation (ꢃ2ꢂ) from idealized angles of 90ꢂ. The short Pt–
Scheme 1 Synthetic routes to 5–8: Reagents and conditions: (i) imidazole, Cu2O,
KOH, DMSO, N2, 140 ꢂC, 24 h; or imidazole, CuI, L-proline, K2CO3, 100 ꢂC, 60 h; (ii)
CH2Br2, THF, 110 ꢂC, 48 h; (iii) HBr (48 wt% aq.)/HOAc, 120 ꢂC, 48 h; (iv)
Pt(DMSO)2Cl2, Et3N, EtOH (or DMF), 80 ꢂC, 6 h; (v) CH2BrCH2Br, 110 ꢂC, 48 h; (vi)
Pd(OAc)2, Et3N, EtOH, 80 ꢂC, 6 h; (vii) Ni(OAc)2$4H2O, Et3N, EtOH, 80 ꢂC, 6 h.
˚
C(NHC) distance of 1.91 A and slightly longer Pt–O (phenolate)
˚
distance of 2.03 A resemble the related values previously
reported for crystal 3.15 The bent molecules stack into innite
columns in a head-to-tail manner with intermolecular p–p
naphthalene-substituted imidazole used for the synthesis of 5
was prepared using Cu2O/KOH as catalyst. The use of CuI/L-
proline/K2CO3 improved the product yield (75 vs. 47%) even at
lower temperature (100 vs. 140 ꢂC).17 The metal complexes were
prepared with reference to literature methods.18 A mixture of
the bis(imidazolium) salt and Pt(DMSO)2Cl2 (or Pd(OAc)2 and
Ni(OAc)2$4H2O in the cases of 7 and 8, respectively) was allowed
to react in ethanol (or in DMF for 5) in the presence of trieth-
˚
distances of ꢀ3.5 A and alternating Pt/Pt distances of 3.563
˚
and 3.589 A (Fig. 1b). There are intermolecular Ar–F/H(NHCꢂ)
˚
interactions in the crystal structure (Fig. 1c) (2.385 A/165.22
ꢂ
˚
and 2.413 A/150.42 ) leading to an innite two-dimensional
network. In contrast, the form of 2-F crystallizes into a triclinic
space group, contains a solvated dichloromethane molecule,
and features a at molecular framework, as depicted in Fig. 2.
The differences in bond distances and bond angles of the
ꢂ
ylamine at 80 C. All complexes were characterized by spectro-
scopic means. The methylene protons of the NHC carbene
ligands in each of these complexes appear as a singlet at 6.3–6.4
ppm whereas those of the free ligand precursors, i.e. bis(imi-
dazolium) salts, are at 7.0 ppm, revealing successful metalation.
For 6, the proton signals of the ethylene linkage appear as a
singlet at 4.0 ppm. This is probably due to rapid motion of the
ethylene unit in solutions.
X-Ray crystal structures
The X-ray crystal structures of 2 have been determined (ESI†).
Crystal data and selected parameters are given in ESI† (Tables
S1 and S2, respectively). Depending on the conditions of
recrystallization, three crystal forms (pseudo-polymorphs) of 2
with different molecular shapes and packing arrangements
have been observed: (1) 2-B (B for bent, Fig. 1) was obtained
Fig. 2 Crystal structures of 2-F: (a) perspective view with hydrogen atoms and
from recrystallization in a DMF solution; (2) 2-F (F for at, solvated molecules omitted for clarity; (b) and (c) packing diagrams.
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planar PtC2O2 core (Table S2, ESI†) between crystal forms 2-F
and 2-B are negligible. Close scrutiny of the crystal structure
(Fig. 2b) reveals dual hydrogen bonds between every two adja-
ꢂ
ꢂ
˚
˚
cent molecules (2.482 A/174.07 and 2.606 A/168.86 ), with F
and Ar–H in the same molecule acting as acceptor site and
donor site, respectively. The hydrogen-bonding interactions
render the at molecules to align into innite one-dimensional
arrays. These one-dimensional arrays contain pairs of the Pt
˚
complex with Pt/Pt distance of 3.578 A and the pairs stack with
˚
p–p distances of 3.3 A (Fig. 2b and c). The structure of the third
crystal form, 2-VD, is different from that of 2-B and 2-F, as
revealed by a comparison of the power X-ray diffraction patterns
of these three forms (Fig. S1, ESI†). Attempts to obtain crystals
of form 2-VD suitable for X-ray crystal structure determination
have not been successful.
Fig. 3 UV-vis absorption spectra of 3, 5, 6, 7 and 8 in THF–DMF (19 : 1, v/v) or in
DMF (for 5 only) solutions (concentration ꢀ2 ꢄ 10ꢁ5 M).
UV-vis absorption and emission spectra
Photophysical data of 1–8 in THF–DMF (19 : 1, v/v) solutions are
listed in Table 1. The UV-vis absorption and emission data of 4
recorded in a panel of solvents are given in Table S3 (ESI†).
Apart from those measured in solutions, the emission proper-
ties of all of the complexes have also been examined in solid
state at both ambient and low temperature (77 K) as well as in
glassy solutions (MeOH–EtOH–DMF ¼ 4/1/1, v/v), except for 5
due to its low solubility. These emission data are also included
in Table 1.
As shown in Fig. 3, the Pt(II), Pd(II) and Ni(II) complexes
exhibit intense absorption bands at energy below 300 nm,
which can be attributed to ligand-centred p–p* transitions.
According to TDDFT calculations reported in a previous
communication,15 the absorption bands of 1–4, 6 and 7 at
around 350 nm (3max in the order of 104 Mꢁ1 cmꢁ1) are attrib-
uted to transitions involving phenolate-based HOMO and NHC-
based LUMO with small contribution from metal d-orbitals. The
transitions can be described to have mainly LLCT character.
The red-shied absorption of 8 at ꢀ400 nm with molar
absorptivity of 3500 Mꢁ1 cmꢁ1 is likely to involve transitions of
ligand-based orbitals, and the absorption tail beyond 450 nm
with molar absorptivity less than 1000 Mꢁ1 cmꢁ1 is attributed to
d–d transitions. The low-energy absorption of 5 at ꢀ395 nm is
red-shied by ꢀ50 nm from that of 1. In view of the larger extent
of aromatic conjugation in 5, the absorption spectrum of 5 is
dominated by naphthalenolate-based IL p–p* transitions, an
inference that has also been veried by TDDFT calculations (see
the later calculation section).
For these Pt(II) complexes, there is a pronounced depen-
dence of emission properties on the tetradentate ligand.
Complexes 1–4 exhibit phosphorescence with maximum at
ꢀ450 nm in both uid solution and in solid state when
dispersed into poly(methylmethacrylate) (PMMA). The ꢀ450 nm
emission is attributed to triplet monomer. However, the emis-
sion quantum yield of 2 (Fe ¼ ꢀ18% vs. 3–8% for 1, 3 and 4,
Table 1) in degassed THF solutions is high. This is likely a result
of minimal structural change of 2 in the excited state with
respect to the ground state, as supported by the DFT calcula-
tions described in the later section.
The absorption and emission data of 4 in a panel of solvents
were recorded (Table S3, ESI†) to examine the effect of solvent
polarity on the spectroscopic properties. As shown in Fig. 4, the
Table 1 Photophysical data of complexes 1–8
Emission lmax/nm; s/ms; F
Absorbance, lsmoalnx/nm (10ꢁ33/Mꢁ1 cmꢁ1 a
)
Solution at 298 Ka
Glass at 77 Kb
Solid state at 298 K
Solid state at 77 K
1
2
3
277 (sh, 8.4), 302 (7.3), 342 (7.1), 353 (7.1) 457; 0.5; 3%
286 (8.5), 310 (9.7), 353 (10.4), 365 (10.5) 443, 459; 3.5; 18%
425; 12.9
415, 440, 466 (sh); 19.9
436; 18.0
506; 0.3
434, 453, 524; 1.3
512; 2.3
464; 0.7
470; 1.5
480; 0.7
284 (sh, 10.0), 308 (9.4), 351 (9.7), 362
(9.7)
460; 1.8; 7%
4
282 (sh, 11.5), 308 (10.5), 352 (12.3), 363 461; 1.8; 8%
(12.7)
431; 15.6
421, 435, 502 (sh); 0.6 411, 436, 464; 3.6
5
6
7
8
327 (26.8), 379 (15.6), 395 (16.6)
310 (8.8), 340 (10.4), 372 (7.4)
327 (sh, 8.4), 345 (10.0), 364 (8.0)
294 (sh, 6.1), 322 (6163), 349 (8.7), 392
(3.5)
531, 572; 47.3; 15% 521, 562, 612 (sh); 513.4 630; 1.1
560, 606, 660 (sh); 207.4
—
—
—
403, 426, 454 (sh); 12.4
501; 134.8
—
480; 0.1
515; 0.8
—
453; 2.5
630; 68.3
—
a
Solutions in THF–DMF (19 : 1, v/v) with a concentration of ꢀ2 ꢄ 10ꢁ5 M for all complexes except 5 which was measured in DMF. b Glassy solution
in frozen MeOH–EtOH–DMF (4 : 1 : 1, v/v) (ꢀ2 ꢄ 10ꢁ5 M) for all complexes except for 5 which was measured in DMF.
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emission maximum of 4 is red-shied from 455 nm in DMF
solution to ca. 470 nm in less polar dichloromethane and
toluene, revealing a negative solvatochromism. This is in line
with the charge-transfer nature of the excited state which
features a smaller dipole moment when compared to that of the
ground state.
Fig. 5 shows comparison between the emission spectra of 3
recorded in THF–DMF (19 : 1, v/v) at room temperature and in
the glassy state (MeOH–EtOH–DMF, 4/1/1, v/v) at 77 K. The
glassy state emission shows a peak maximum at 435 nm, being
ꢀ25 nm blue-shied from that recorded in THF–DMF (19 : 1,
v/v) solution. Also displayed in Fig. 5 are the emission spectra of
3 in solid state at both ambient and low temperatures. The
solid-state emission at ambient temperature is composed of one
broad, structureless band centred at ꢀ512 nm with a shoulder
peak at ꢀ430 nm. In literature, such red-shied emission with
broad bandwidth is usually associated with intermolecular
interactions between Pt(II) complexes in the ground state, that is
emission arising from aggregated state(s) (dimer or oligo-
mer).8a,b This assignment appears to also apply in the present
case by the following ndings. First, the excitation spectra
recorded at the two emission peak maxima (430 and 512 nm)
are identical (Fig. S2, ESI†), indicating their same electronic
origin. In addition, the excitation spectra feature a low-energy
peak maximum at ꢀ400 nm, signicantly red-shied from
absorption of the monomeric form of 3 in the dilute solution,
favouring the origin of the solid-state emission from an aggre-
gated state. Upon cooling to low temperature (77 K), the solid-
state emission becomes more intense and blue-shis in energy,
which might be provisionally taken as monomeric emission
given its energy is close to that recorded in dilute solution. In
contrast, the solid-state emission spectra of 4 at room temper-
ature and at 77 K both resemble the corresponding monomeric
emission (Fig. S3, ESI†). This can be attributed to the existence
of bulky tBu substituents rendering the stacking interactions in
solid state less favourable.
Fig. 5 Emission spectra of 3 recorded in THF–DMF (19 : 1, v/v) solution, glassy
solution and solid state at ambient temperature and 77 K; lex ¼ 350 nm.
solid state emission spectra recorded with the three crystal
forms of 2 obtained under different conditions: 2-B, 2-F and 2-
VD. All of these three samples display aggregation-type emis-
sion. As depicted in Fig. 6, 2-B displays a broad emission con-
taining one band centred at ꢀ524 nm with a shoulder at
ꢀ430 nm. In contrast, both 2-F and 2-VD show an emission peak
maximum at 495 nm with a much narrower band-width. The
packings of molecules in crystals of 2-B and 2-F as revealed by
their X-ray crystal structures are different (Fig. 1 and 2).
Complex 5 shows emission maximum at 530 nm with a
quantum yield of ꢀ15% and an emission lifetime of ꢀ47 ms in
DMF (Fig. 7a). The structured emission band with vibrational
spacings of ꢀ1400 cmꢁ1 suggests the emitting excited state to
be p–p* in nature. The exceedingly long lifetime (s ¼ ꢀ47 ms)
imparts the potential of this complex as a luminescent sensor
for oxygen.5,6f As depicted in Fig. 7a, the emission intensity at
the peak maximum is decreased by 324-fold upon saturation
with air. The oxygen emission quenching rate constant kq is 7 ꢄ
109 sꢁ1 and the proportion of oxygen-quenched triplet states is
near unity [PT,O2 ¼ 1 ꢁ (saerated/sdegassed)]5c as estimated by
The emission prole of organic/organometallic materials in
solid state is closely associated with the packing of molecules in
the crystal structure leading to pseudo-polymorphic behav-
iour.19–21 The emission spectra of solid samples of 2 prepared by
different processing conditions are compared. Fig. 6 shows the
Fig. 6 Solid-state emission spectra of 2-B, 2-F, 2-VD (lex ¼ 350 nm) recorded at
Fig. 4 Emission spectra of 4 recorded in various solvents; lex ¼ 350 nm.
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Solid-state emission of 7 can be observed at ambient
temperature and at low temperature. The solid-state emission
of 7 at 77 K (lmax ¼ 630 nm) is red-shied by over 100 nm from
that recorded at room temperature (lmax ¼ 515 nm). Similar
thermochromism has previously been observed for Pt(II)
3
complexes exhibiting MMLCT emission in solid state and the
red-shi in emission is attributed to lattice contraction upon
cooling.24 Taking into account the large shi of over 100 nm and
long emission lifetime of 68.3 ms, the red-emission of 7 at 77 K is
assigned to triplet excited states of intermolecular aggregates
driven by ligand–ligand interactions. The high-energy, better
vibronic resolved emission probably originates from mono-
meric form of 7 present in solid state at low temperature. No
emission from 8 has been observed in this study.
Time-resolved emission
To characterize the excited state dynamics leading to the blue
emission, fs-TRF and ns-TRE measurements were conducted on
2 and 4 in neat DMF at ambient temperature. Representative
data acquired are depicted in Fig. 8 for 4 and in Fig. S5 (ESI†) for
2. Fig. 8a and b, S5a and b (ESI†) show temporal evolution of fs-
TRF spectra and the correlated uorescence decay proles for 4
and 2, respectively. The transient uorescence, which arises
from the photo-populated S1 state, features lmax at ꢀ425/415 nm
for 4/2, and the TRF spectra for both complexes diminish rapidly
in <1 ps. Kinetic analysis performed by tting the experimental
decay curves with exponential function convolved with the
respective uorescence lifetime (sf) of 0.19 and 0.23 ps for 4 and
2 determined by instrumental response function.
The fs-TRF data reveal the following features. First, for both
complexes, the TRF is apparently associated with a fairly small
Stokes shi from the corresponding absorption spectra. This
reects a minor structural variation from S0 to S1. Given the
even smaller extent of Stokes shi in the case of 2 than 4, the
structural relaxation appears to be lesser in the former than in
the latter. Second, the ultra-short lifetime of S1 reects a minute
Fig. 7 Emission of (a) 5 in DMF solution (concentration ꢀ 2.0 ꢄ 10ꢁ5 M) under
degassed and aerated conditions (lex ¼ 400 nm); inset: photographs of the
solution under UV-light irradiation (365 nm). Emission of (b) 6 and (c) 7 in glassy
solution and in the solid state at 298 K and at 77 K (lex ¼ 350 nm).
monitoring the phosphorescence decay in degassed and in
aerated solutions. The emission property of 5 in solid state is
also signicantly affected by aggregation, like that observed for
3 (Fig. S4, ESI†). Complex 6 is non-emissive in degassed solu-
tions. Importantly, a deep blue emission is observed for 6 in
77 K glassy solution (Fig. 7b). In solid form, this complex shows
a weak, broad emission centred at 480 nm. Similar to most of
the reported Pd(II) complexes,22 7 is non-emissive in solutions at
room temperature. Low-lying d–d state(s) is usually responsible
for the absence of room-temperature emission from Pd(II)
complexes.23 However, this complex exhibits an intense green
luminescence (lmax ¼ 501 nm) in 77 K glassy solution with a
long lifetime of ꢀ135 ms revealing that the emission is phos-
phorescence in nature.
Fig. 8 fs-TRF and ns-TRE spectra (a and c) and decay profiles (b and d) recorded
at the various denoted delay times after 355 nm excitation of 4 in DMF.
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contribution of uorescence in the steady-state emission
spectra of 2 and 4. More importantly, the ultrafast decay of TRF
implies the presence of efficient non-radiative channel(s) in
depleting the S1 population. It is known that, aside from radi-
ative decay, the S1 state upon formation, decays by two possible
non-radiative pathways; one is ISC to the triplet manifold and
the other is internal conversion (IC) within the singlet manifold
to the S0 state. Given the large energy gap between the S0 and S1
states and the rigid molecular frameworks of 2 and 4, it is
unlikely that the IC of these complexes proceeds in the sub-ps
timescale. Thus, the prompt decay of S1 is caused by ISC for
both 2 and 4. With this assignment, the ISC can be derived to
take place at rate constants (kISC ¼ ꢀ1/sf) of ꢀ(4–5) ꢄ 1012 sꢁ1
and with an essential unity efficiency (FISC ¼ 1) leading to Fig. 9 Side views of the DFT optimized S0 and T1 geometries for (a) 2, (b) 5 and
(c) 6.
formation of the T1. It is worth noting that similar ultrafast ISC
has been reported for other Pt(II) complexes25 and for complexes
of other metals (Ru(II),26 Re(I)27 and Fe(II)28) and polypyridyl
2, 3 and 5 are similar, featuring a reection plane passing
ligands. Given the unity yield of ISC, the overall emission effi-
ciency (Fe) of the complex is affected by the radiative decay of
the T1 state (i.e., Fe ¼ FISCkrsT ¼ krsT, where kr and sT are the
radiative rate constant and lifetime of T1, respectively).
through the Pt(II) centre and methylene carbon (Pt1 and C10 for
2, C15 for 5, Fig. 9 and Table 2). In these complexes, the carbene
and phenolate pairs are slightly out of the nearly square Pt–
O/Pt–C coordination plane (Pt1–O1–C9–C7 of ꢀ15ꢂ and Pt1–O1–
C9–C4 of ꢀ17–21ꢂ). In contrast, the S0 structure of 6 has a
markedly distorted geometry due to the ethylene-containing
seven-membered ring and the two adjoining NHC moieties. To
accommodate the ethylene bridge, the Pt–C bonds of 6 are
Fig. 8c and d, S5c and d (ESI†) show the ns-TRE spectra and
kinetic decay curves obtained for the T1 states of 4 and 2,
respectively. The spectra were recorded with sample solutions
being deoxygenated in order to diminish dynamic quenching of
T1 by oxygen. Exponential tting of the emission decay traces
gave sT values of 4 and 2 of ꢀ2.2 and 4.1 ms, respectively. Taking
together the data of fs-TRF and ns-TRE, a simple picture of the
excited state decay can be drawn: sub-ps ISC from the S1 to
triplet excited states that are close-lying to S1 followed by decay
of the overall excited state population from the T1 on timescale
of several ms. The emission is assigned to come from the T1 as
revealed by DFT calculations.‡ According to the DFT calcula-
tions, the second lowest triplet state (T2) lies at energy ꢀ4900
and 4700 cmꢁ1 above the T1 of 2 and 4, respectively. This
nding, combined with the calculated radiative rate constants
of T1 and T2, predicts a negligibly small contribution of the T2
(ꢅꢀ0.01%) to the room-temperature emission.
˚
slightly longer (by 0.03–0.04 A) and the C–Pt–C angles are larger
(by ꢀ10ꢂ) than those of 2, 3 and 5. There is only little variation of
the Pt–O distances among all these Pt(II) complexes but the O–
Pt–O angle in 6 is smaller by ꢀ6ꢂ than that of 2, 3 and 5; the Pt–C
bonds also deviate subtly (O1–Pt1–O2–C10 of ꢀ4ꢂ) from the
plane dened by the O–Pt–O bonds. Unlike that in 2, 3 and 5,
the two pairs of phenolate and NHC moieties of 6 are not
symmetric to each other, each deviating largely from the Pt(II)
coordination plane (Pt1–O1–C10–C8 of ꢀ21ꢂ and Pt1–O1–C10–
C4 of ꢀ40ꢂ).
According to the results of TDDFT calculations, the lowest
energy absorption (S0 / S1) of 2, 3, 5 and 6 (Table S4†) are all
DFT and TDDFT calculations
To gain insight into the emission properties, DFT and TDDFT
calculations on the electronic excited states of complexes 2, 3, 5
and 6 were carried out. The calculations were performed using a
PBE0 method with a basis set of 6-31+G* for C, H, O and N
atoms and a pseudo-potential Stuttgart/Dresden (SDD) basis set
for Pt atom.15 Fig. 9 displays the optimized S0 and T1 geometries
calculated for 2, 5 and 6. The frontier orbitals of 2 and 5 that are
mainly involved in the transitions of absorption (S1 ) S0) and
phosphorescence emission (T1 / S0) are depicted in Fig. 10.
The optimized geometries of 3 and the corresponding orbitals
of 6 are given in Fig. S6 and S7 (ESI†).
As summarized in Tables 2 and S2 (ESI†) for 2 and in the
previous communication15 for 3, the optimized S0 structures of
2 and 3 are in good agreement with the crystal structures of 2-B
and 3. The calculated bond lengths and bond angles are within
ꢂ
˚
0.01 A and 1.5 from the measured values. The S0 geometries of Fig. 10 Plots of frontier orbitals of 2 and 5.
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ꢂ
ꢂ
˚
Table 2 Calculated bond distances (A), bond angles ( ) and dihedral angles ( ) for 2, 5 and 6 in ground- and excited-states
S0
T1
S0
T1
S0
T1
Pt1–C9
Pt1–C90
Pt1–O1
Pt1–O10
1.926
1.926
2.041
2.041
1.930
1.951
2.069
2.067
Pt1–C13
Pt1–C130
Pt1–O1
1.923
1.923
2.042
2.042
1.922
1.922
2.042
2.042
Pt1–C10
Pt1–C15
Pt1–O1
Pt1–O2
1.955
1.969
2.055
2.036
1.959
1.978
2.075
2.059
Pt1–O10
C9–Pt1–C90
C9–Pt1–O1
O1–Pt1–O10
C90–Pt1–O10
91.73
89.58
89.00
89.58
93.02
89.70
90.53
86.74
C13–Pt1–C30
C13–Pt1–O1
O1–Pt1–O10
C130–Pt1–O10
91.74
89.09
90.07
89.09
92.02
88.87
90.06
88.87
C10–Pt1–C15
C10–Pt1–O1
O1–Pt1–O2
C15–Pt–O2
101.28
85.63
83.91
89.32
104.80
87.52
81.42
86.57
Pt1–C9–N2–N1
177.85
177.85
179.70
142.40
Pt1–C13–N1–N2
O1–Pt1–O10–C10
178.95
154.78
175.39
163.42
Pt1–C15–N3–N4
Pt1–C10–N2–N1
171.18
176.69
177.74
152.61
Pt1–C90–N10–N20
derived primarily (by >95% contribution) from the HOMO / Unlike the LUMO at S0 geometry where the two NHC moieties
LUMO transition. The transition energy calculated for each of are electronically equivalent, the SOMO of T1 features electron
the complexes (ꢀ351–379 nm for 2, 3 and 6, ꢀ408 nm for 5) density concentrated on p* of one of the NHC motifs. According
agrees well with the corresponding experimental data. For 2 and to the frontier orbitals of the T1 and S0 (Fig. 10), the phospho-
6 (Fig. 10 and S7 (ESI†)) and in each of the cases, the electron rescence of 2 and 3 can be attributed to an LLCT (pO–Ar
density in HOMO localizes mostly on p orbital of the two p*NHC) transition.
/
phenolate moieties (O–Ar) and that in LUMO localizes on p*
To assess the origin of non-radiative decay of T1, it is of
orbital of the two NHCs aer electronic transition; in addition, interest to check the energy splitting of Pt d–d orbitals. Due to
the HOMO and LUMO both contain a small fraction of contri- the strong donor strength of the tetradentate ligand and the
bution (no greater than ꢀ4–8%) from Pt dp (dxz and dyz) planar geometry in T1 as a result of the strong Pt–O/Pt–C bonds,
orbitals. Thus, the S0 / S1 transition can be ascribed to have the lowest unoccupied d-orbital was calculated to be ꢀ60 kcal
mainly ligand-to-ligand charge-transfer (LLCT) pO–Ar / p*NHC molꢁ1 above the highest occupied d orbital in T1 states of 2 and
character. For 5, both the HOMO and LUMO (Fig. 10) feature 3. This large value of d–d splitting implies a limited role of the
electron density localized (for HOMO) or largely localized (for d–d state in deactivating the T1 state. Instead, ligand-mediated
LUMO) on the p and p* orbitals of the two naphthalenolate non-radiative decay process promoted by symmetry breaking
groups. As a result, the absorption transition of 5 is assigned to and ligand distortion could be the non-radiative decay pathways
have predominant naphthalenolate ligand-centred (LC) p / p* for the T1 excited states of these Pt(II) complexes.
character with an admixture (<ꢀ30% contribution) of the LLCT
The frontier orbital of 6 in T1 appears comparable to those of
2 and 3. The ligand skeleton of 6 in T1 features a large degree of
transition.
For 2 and 3, a distinct feature of the geometry of their T1 geometrical distortion relative to that of 2 and 3 (Fig. 9 and S6
excited states is the loss of structural symmetry due to asym- (ESI†)). Notably, the Pt–C bond distances of 6 in both S0 and T1
˚
metric distortion of the two NHC moieties from S0 to T1 (Fig. 9). are longer by 0.03–0.04 A than that of 2 and 3 (Table 2). In line
One of the NHC moieties twists out of the Pt–O/Pt–C plane with this, the d–d splitting of 6 is narrowed by several kcal
(dihedral angle Pt1–O1–C9–C7 increases by ꢀ20ꢂ from S0 to T1) molꢁ1 compared to that of 2 and 3.
˚
accompanied by a ꢀ0.08 A lengthening of the Ccarbene–N bonds;
In contrary to the asymmetry of T1 excited states of 2 and 3,
the other NHC moiety ips towards the Pt(II) plane (the dihedral the optimized T1 geometry (Fig. 9) and SOMO (Fig. 10) of 5
angle is reduced by ꢀ13ꢂ) with little structural changes of the remain symmetric and are close to its S0 geometry. The bond
NHC ring. On the other hand, the coordination geometry of distances, bond angles and dihedral angles of T1 are similar to
Pt(II) is largely preserved in T1 as in S0 except a small elongation that of S0 except that the two naphthalenolate moieties unfold
(by ꢀ0.03 A) of the Pt–C and Pt–O bonds. The structural towards a more planar geometry by ꢀ10ꢂ (dihedral angle O1–
˚
distortion from S0 to T1 is observed to occur to a lesser extent in Pt1–O10–C10 changes from 154.8ꢂ for S0 to 163.4ꢂ for T1), without
the case of 2 than 3. As depicted in Fig. 10, the break of struc- breaking the symmetric geometry. The emission of T1 is derived
tural symmetry in T1 appears to be a result of the electronic mainly from the naphthalenolate based LC(p / p*) transition
property of the singly occupied molecular orbital (SOMO) of T1. with ꢀ20% contribution of the LLCT character (Fig. 10).
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One of the key sources of error in DFT is the self-exchange, yellow phosphorescent Pt(II) complex (see Chart S1 in ESI† for
which is responsible for the poor performance of TDDFT for the chemical structure)40 was combined with 4 to realize white-
charge-transfer excited states.29 One practical way to ameliorate light emission. A notably high efficiency up to 88 cd Aꢁ1 and
the self-exchange problem is to increase the percentage of maximum luminance over 30000 cd mꢁ2 were achieved. By
Hartree exchange of the DFT functional. M06L (0% HF), PBE0 adjusting the doping ratios of these two dopants, optimized
(25% HF), M06 (27% HF) and M06-2X (54%) were used for device emanating white-light with CIEx,y ¼ (0.32, 0.42) was
prediction of the emission energy of 4. Among these func- obtained when complex 4 and the yellow Pt(II) phosphor were
tionals, the one with the highest HF component, M06-2X, was doped at 6% and 0.5% weight ratio. The performance data are
found to be the best performer for the emission energy, with an summarized in Table S6 (ESI†) and Fig. 12.
error of 0.4 eV. All other functionals gave errors over 0.76 eV.
Therefore, we used M06-2X (54% HF) to calculate the emission
properties of the Pt(II) complexes at optimized T1 geometries.
Discussion
Transition-metal complexes as blue electrophosphors for
Following the methodology we employed for computational
study of phosphorescence efficiencies of cyclometalated Pt(II)
complexes;30 the radiative decay constants (kr) for 1, 2 and 4
OLEDs have been attracting increasing attention32 since the
report of the sky-blue emitter FIrpic (iridium(III)bis[(4,6-
2
0
diuorophenyl)pyridinato-N,C ]picolinate, CIEx,y
¼
(0.16,
were obtained from the following equations:
0.29))33 which bears a diuorinated bidentate cyclometalating
phenylpyridine ligand. To develop deep blue electrophosphors
(ideal CIEx,y ¼ (0.14, 0.08) for true blue color32a), tremendous
studies have been made to tune the electronic properties of
phosphorescent Ir(III) complexes; among the various strategies
employed32a,b,d is the introduction of electron-withdrawing
substituent(s), most frequently uorine. Introducing cyano
groups to the 4,6-diuorophenyl rings of FIrpic afforded
FCNIrpic, the latter has been employed to fabricate
(
)
ꢀ
ꢁ
2
ꢂ
ꢃ
1=2
Tma |Hsoc|Sn
h2
fn
X
3
kra ðTm/S0Þ ¼
EðTmÞ
1:5
EðSnÞ ꢁ EðTmÞ EðSnÞ
n
(
)
ꢀ
ꢁ
2
Tma |Hsoc|Sn
c1n=2ðcn ꢁ 1Þ
h2
X
¼
fn1=2
1:5
n
(1)
(2)
X
kr ¼
kra
a
The calculated radiative decay constants are 1.20 ꢄ 104,
1.23 ꢄ 104 and 1.56 ꢄ 104 sꢁ1 for 1, 2 and 4, respectively, all
comparable with the experimental values of (4–6) ꢄ 104 sꢁ1. The
dependence of emission efficiency and lifetime on substituents
on the tetradentate ligand is due to the difference in non-radi-
ative decay rate constants.
OLEDs fabrication and performances
Due to the excellent chromaticity of the deep blue phosphores-
cence, and the slightly higher emission quantum yield than that
of 3, complex 4 was used in the fabrication of blue and white
OLEDs. The chemical structure of the materials used and the
device architecture are depicted in Chart S1 (ESI†). Upon
screening over tens of host materials, 9-(4-tert-butylphenyl)-3,6-
bis(triphenylsilyl)-9H-carbazole (CzSi) was selected due to its
wide band-gap and excellent suitability for fabricating efficient
blue OLEDs.31 The device data with 4 at various dopant levels are
given in Table S5 (ESI†). The best performance was achieved at a
dopant level of 4 wt% with maximum luminance, peak current
efficiency (CE) and peak power efficiency (PE) of 9500 cd mꢁ2
,
23.8 cd Aꢁ1 and 16.6 lm Wꢁ1, respectively, and the blue emission
(lmax 460 nm) features a chromaticity with CIEx,y (the Commis-
sion Internationale de L'Eclairage coordinate) being (0.19, 0.21).
The maximum external quantum efficiency (EQE) was estimated
to be ꢀ15%. The EL spectra of the 4 wt% doping device and CE
vs. operating voltage are depicted in Fig. 11a and b. The EL
spectra are quite stable over a wide range of operating voltages.
The other device characteristics are given in Fig. S8–S11 (ESI†).
The potential of using 4 in fabricating (all-phosphor) white
Fig. 11 (a) Current density–luminance–voltage characteristics (inset: normalized
EL spectra of the device operated at 6–11 V) and (b) current efficiency–power
OLEDs was investigated. A complementary highly efficient efficiency–voltage characteristics of the blue OLED doped with 4% of 4.
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for III (CIEx,y ¼ (0.18, 0.35))37a and IV (CIEx,y ¼ (0.18, 0.27)),
respectively,37b were attained. For the uorine-free electro-
phosphors VI (which bears a bidentate pyridyltriazolate ligand)
and VII (which bears a bidentate aryl NHC ligand), the resultant
OLEDs (maximum efficiencies: EQE ¼ ꢀ12%, PE ¼ 38–61 lm
Wꢁ1 for VI;12i EQE ¼ 5–6%, luminance ¼ 4900–6750 cd mꢁ2 for
VII4a) feature turquoise–blue (CIEx,y ¼ (0.19, 0.45) ꢁ (0.26,
0.51))12i or green (CIEx,y ¼ (0.16, 0.31))4a emission.
A Pt(II) deep blue-emitting OLED with CIEx,y ¼ (0.16, 0.16),
though with lower maximum luminance of 1200 cd mꢁ2 and CE
of 0.5 cd Aꢁ1 (in this work, both values have improved to 9500 cd
mꢁ2 and 23.8 cd Aꢁ1, respectively) was obtained using the
uorine-free emitter 3 bearing tetradentate bis(phenolate-NHC)
ligand, as reported in our previous communication.15 There-
aer, OLEDs with electrophosphors V,38a VIII4b and IX38b were
reported. A notable feature of the diuorinated tridentate phe-
nylpyridine complex V and the dimesitylboryl-substituted
bidentate phenyl NHC complex VIII, and also the above-
mentioned II and VII, is their high photoluminescent quantum
yields in uid solutions (Fe ¼ 80% (II),39 60% (V),38a 41% (VIII)4b)
and in PMMA lms (Fe ¼ 85% (II),39 90% (VII),4a 86% (VIII)4b).
The OLED using V (efficiencies at ꢀ500 cd mꢁ2: EQE ¼ 3.9%,
CE ¼ 6.9 cd Aꢁ1) shows CIEx,y ¼ (0.20, 0.30),38a whereas that
Fig. 12 (a) Normalized EL spectra of white OLED doped with 6% of 4 and 0.5% using VIII (maximum efficiencies: EQE ¼ 9.8%, PE ¼ 22.5 lm
Wꢁ1) features CIEx,y ¼ (0.27, 0.50).4b For the OLED using uo-
of yellow phosphor Pt(II) and (b) current efficiency–power efficiency–voltage
characteristics of this device.
rine-free IX which bears a tridentate phenyl NHC ligand, a deep
blue emission with CIEx,y ¼ (0.16, 0.13) was achieved, with
maximum efficiencies of EQE ¼ 15.7% and PE ¼ 22 lm Wꢁ1
.
38b
high-efficiency deep blue OLEDs,34 one of which exhibits
The OLED fabricated in this work using the uorine-free
maximum efficiencies of EQE ¼ 26.4% and PE ¼ 29.5 lm Wꢁ1 emitter 4 (a congener of 3) upon modication of host materials
with CIEx,y ¼ (0.14, 0.19) upon optimizing host materials.34c The
majority of well-documented Ir(III) blue electrophosphors32 bear
uorinated ligands. Fluorine may render the molecule electro-
chemically reactive,35b and C–F cleavage of FIrpic36a and thermal
deuorination of a blue–green phosphorescent Ir(III) complex36b
were observed. Strategies of achieving uorine-free Ir(III) blue
electrophosphors have been reported;31c,32,35 one strategy is the
replacement of the diuorinated phenylpyridine ligand with
aryl NHC ligand,35 and a blue-emitting OLED using Ir(dbfmi)
(mer-tris(N-dibenzofuranyl-N0-methylimidazole)iridium(III))
shows maximum efficiencies of EQE ¼ 18.6% and PE ¼ 35.9 lm
Wꢁ1 with CIEx,y ¼ (0.15, 0.19).35c
Pt(II) blue electrophosphors for OLEDs are less studied when
compared to Ir(III) counterparts, despite numerous reports on
photophysical properties of phosphorescent Pt(II) complex-
es.1,32c A considerable number of photoluminescent Pt(II)
complexes exhibiting blue or bluish emission are documented
in literature,4,12,13,15,37–39 of which I–IX (Chart 2) and 3 have been
employed to fabricate blue-emitting OLEDs. Complex I12a bears
the same diuorinated bidentate phenylpyridine ligand as that
in FIrpic.33 The blue-emitting device using I as the electro-
phosphor was reported in 2006, featuring maximum EQE of
4.3% and CIEx,y of (0.11, 0.24).11b Subsequently, by using elec-
trophosphors II–IV which bear diuorinated tridentate phe-
nylpyridine ligands, maximum efficiencies of EQE ¼ 16% and
PE ¼ 20 lm Wꢁ1 (CIEx,y ¼ (0.15, 0.26)) for II12b and efficiencies (at
Chart 2 Pt(II) complexes previously claimed to be blue electrophosphors in OLED
ꢀ500 cd mꢁ2) of EQE ¼ 13.0%, 6.1% and PE ¼ 9.3, 2.8 lm Wꢁ1 applications.
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exhibited maximum efficiencies (EQE ꢀ 15%, PE ¼ 16.6 lm frontier orbitals of both the singlet and triplet states. First,
Wꢁ1) and chromaticity (CIEx,y ¼ (0.19, 0.21)) comparable to because of the large energy gap of the pO–Ar / p*NHC transition
those reported for the OLEDs using IX38b and the above- and the predominant LLCT character of the T1, T1 decays back
mentioned Ir(dbfmi).35c This lends credence to the possible to the ground state with emission at high energy. Second, the
application of a Pt(II) complex with tetradentate ligand, or “one involvement (though to a small extent) of metal orbital(s)
metal ion + one ligand” approach, in developing practical deep induces spin–orbit coupling (SOC) and hence promotes ISC (S1
blue electrophosphors for OLEDs.
/ Tn) as well as allowing adequately fast radiative decay rate (kr)
Tetradentate ligands can form robust Pt(II) complexes which for the T1 state. This corroborates with the ultrafast ISC
are applicable as electrophosphors for OLEDs.1b Previously we observed with complexes 2 and 4. The ISC of the other Pt(II)
have reported a number of OLEDs using Pt(II) electrophosphors complexes (1, 3, 5 and 6) having electronic structures similar to
bearing tetradentate Schiff-base ligands, but these devices do that of 2 and 4 are most likely to proceed in the ultrafast
not exhibit blue emission.1b In literature, a Pt(II) complex timescale and with unity efficiency though it is noted that both
bearing tetradentate bis(phenylpyridine) ligand was employed orbital spacings and the consequent state mixings may also
to fabricate OLEDs, which showed maximum EQE of up to affect the decay path. Taking into account the experimental fe
14.7% and featured a bright green emission (CIEx,y ¼ (0.32, values (Table 1), it follows that the kr values are ꢀ(4–6) ꢄ 104 sꢁ1
0.62)).3 The well-documented Ir(III) blue electrophosphors for for the T1 of complexes 1–4 in THF and ꢀ3 ꢄ 103 sꢁ1 in the case
OLEDs bear bidentate ligands.32
of 5 in DMF solutions. The one order of magnitude lower kr
The development of the tetradentate dianionic ligands in value for 5 is in line with a negligible MLCT character in the
115,16b and 2–415 was inspired by the literature reports on the emissive excited state of this complex.1,30 It should, however, be
photoluminescence in near-UV region (lmax 386–387 nm) pointed out that besides the SOC matrix elements due to the
exhibited by Pt(II) tetra(NHC) complexes consisting of two Pt(II) character, the oscillator strengths and energy ratios
bidentate bis(NHC) ligands,14a with the emission being shied between the coupling singlet and triplet excited states could
to the blue region (lmax 432–465 nm) upon replacing a CH group also affect the radiative decay rate constants.30 Last and fore-
of the NHC ve-membered ring with N atom14b or replacing one most, these Pt(II) complexes containing the tetradentate
of the two bis(NHC) ligands with bis(monodentate arylacety- bis(NHC) ligands feature modest degree of structural distortion
lide).12d The blue emission of 1–4 demonstrates another on going from S0 to T1. Of particular importance is the preser-
approach to blue-emitting Pt(II)-bis(NHC) complexes, i.e., by vation of the strongly bound planar coordination geometry of
functionalizing the bidentate bis(NHC) group with bis(pheno- Pt(II) in T1 as that in S0. The small structural distortion limits
late) to form
a tetradentate bis(phenolate-NHC) ligand. structural rearrangement induced energy relaxation in the
Through synthetic, structural, photophysical, and DFT calcu- excited state and hence facilitates emission at high energy. This
lation studies in this work, insights can be obtained about the agrees with the modest absorption–emission Stokes shis
origin of the blue phosphorescence and the effects of ligand (ꢀ3300–4600 cmꢁ1) observed for complexes 1–4. The strong Pt–
rigidity, p-conjugation, and metal ion, as discussed below.
O/Pt–C bonding in the T1 reects strong ligand eld strength
that increases the energy of non-radiative d–d excited state
hence disfavouring the involvement of the latter in quenching
the emission from T1. Of note, despite the large number of
Origin of the blue phosphorescence
In principle, the emission property of a molecular system is phosphorescent Pt(II) complexes reported in literature, non-
determined primarily by (i) electronic properties and structure porphyrin Pt(II) complexes supported by tetradentate ligands
of the emitting state(s) and (ii) dynamical interplay of the and with emission of primarily LLCT character are not common
emitting state(s) with nearby electronic state(s) that induces in the literature.
change in population of the emitting state(s). These two aspects
The Pt(II) complexes of this work display unique emission
are inter-related. Point (i) governs mainly the photophysical properties including lifetime, quantum yield and energy. The
parameters including the radiative rate constant (kr), emission dependence of the photophysical and luminescent properties
energy and energy distribution as revealed in the emission on ligand substituent(s) can be used for tuning the excited state
spectrum of the emitting state(s). Point (ii) dictates the dynamics of the Pt(II) complexes bearing the tetradentate
dynamics and lifetime of the emitting state(s). This point, bis(NHC) ligands. Complexes 2, 5 and 6 are particularly illus-
together with kr, is primarily responsible for the emission trative. Being the best blue phosphor among the complexes
quantum yield. The results we presented here for 1–4 together examined in this work, 2 with uorine substituent displays
with comparison studies on the related complexes 5–8 alto- emission in the deep blue region with emission lifetime and
gether reveal that, upon coordination to Pt(II), the tetradentate emission quantum yield being several-fold greater than that of
bis(phenolate-NHC) type ligands (such as those in 1–4) provide the parental complex 1. The inclusion of electron-withdrawing
a superior scaffold for achieving efficient deep blue light uorine substituent at the para-position of the bis(phenolate)
emission at ambient temperature.
groups of the tetradentate ligand stabilizes the pO–Ar orbital
A salient feature is that for the Pt(II) complexes (1–4 and 6) (donor ligand) leading to a larger pO–Ar / p*NHC energy gap
that show deep blue emission (at ambient and/or low temper- and therefore higher emission energy in the case of 2. Another
ature), their emissions are primarily attributed to LLCT (pO–Ar important effect by the uorine atom substituent is that there is
/ p*NHC) transitions with a slight admixture of Pt dp in the lesser degree of geometry distortion on going from S0 to T1. This
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may lead to the reduction of non-radiative decay rate, contrib- from the naphthalenolate based p / p* transitions with
uting to the improved emission yield of 2. Besides, the less modest LLCT contribution. In our case, the switch from the
overall structural distortion in the T1 of 2 appears to account for mainly LLCT (for 1–4 and 6) to a mainly LC p / p* character
the narrower emission band of 2 compared to those of 1, 3 and (for 5) is also responsible for the increased absorption coeffi-
4. Compared with the emissions of 1, 3 and 4 (Fig. S12 in ESI†), cient and the more structured prole of the emission displayed
the emission of 2 is narrowed by ꢀ20 nm in FWHM (full width by 5. In addition, since states derived from p / p* excitation
at half maximum), suggesting a better emission colour purity. usually feature substantial singlet-to-triplet energy separation
Indeed, recent studies by Yersin et al.39 on some Pt(II) complexes due to the sizeable electron exchange integral,41 the predomi-
containing bidentate or tridentate ligands suggested that nantly LC p / p* character in 5 may also contribute to its
molecular rigidity and structural distortion in T1 altogether play extensive Stokes shi (ꢀ6500 cmꢁ1) from the (S1 ) S0)
an important role in controlling the colour purity of the lumi- absorption to (T1 / S0) emission when compared to those
nescence of transition-metal complexes.
(ꢀ3300–4600 cmꢁ1) of complexes 1–4.
Effect of ligand rigidity
Effect of metal ion
In contrast to 1–4, 6 is non-emissive (i.e., fe < 10ꢁ3) in uid Apart from ligand substitution, the metal ion also has an effect
solutions at room temperature. This complex features absorp- on the spectral properties of d8 metal complexes supported by
tion and emission at energies comparable to those of 1–4, thus tetradentate bis(NHC) ligands. Although the bis(phenolate-
the electronic excited states of all of these complexes are NHC) type ligand system can accommodate the square-planar
conceived to be similar, a conclusion that has also been reached coordination geometry of Pt(II) both in the S0 and T1 states, this
by DFT calculations. The non-emissive behaviour of 6 can be may not be the case when the metal ion is Pd(II) or Ni(II). The
ascribed to a much increased efficiency of non-radiative decay presence of low-lying d–d excited states and the less strong Pd/
in comparison to those of 1–4. As revealed by the calculations, Ni–C and Pd/Ni–O bonding compared with the Pt–C and Pt–O
the ligand system of 6 suffers a severe structural distortion in T1 ones may contribute to the non-emissive behaviour of
excited-state vs. S0 ground-state; the Pt–O/Pt–C bonds show complexes 7 and 8 in solutions at room temperature. Within
elongation in these two states and there is a tendency of 6 in the this context, the larger Stokes shi (ꢀ6600 cmꢁ1) in energy
excited state to deviate from a planar geometry when comparing between the emission (in the glass solution at 77 K) and
to those of 1–4. The ligand skeleton of 6 in the T1 excited state absorption spectra of 7 when compared to that (ꢀ4600 cmꢁ1) of
shows deformation thereby promoting vibration-assisted radi- the Pt(II) counterpart (3) is attributed to the more extensive
ationless decay while the weakening of Pt–ligand bonds could excited state distortion in the former than in the latter
decrease the activation energy for accessing the d–d state thus complexes. As for the Ni(II) congener 8, the low energy absorp-
favouring thermal non-radiative decay in solution at ambient tion tail beyond 450 nm is attributed to d–d transition(s). It is
temperature. Putting these two factors together, the T1 of 6 known that the ligand-eld d–d splitting increases down the
upon formation is subjected to facile non-radiative process, group of transition metals, being much smaller for rst-row
accounting for its low emission quantum yield.
transition metals. The non-emissive behaviour of 8 is attributed
to the d–d excited states which are present in the low-energy
visible spectral region.
Effect of ligand p-conjugation
Complex 5 has a long emission lifetime and displays large red-
shis both in absorption and emission energies from that of the
Polymorphism and thermal processing
other platinum(II) complexes. The long emission lifetime is an Regarding the two different geometries of 2 (see Fig. 1 and 2),
indication of the slow rate of non-radiative decay in T1. This is the optimized S0 geometry by DFT calculations shows structural
in fact supported by the calculations revealing that the T1 of 5 parameters and bent framework close to that observed for 2-B,
has a structure closely resembling that of its S0. The T1 has not indicating that the at geometry of 2-F (Fig. 2) could be a high-
only the tightly bound square-planar Pt(II)–O/C coordination energy conformation. In the course of sublimation, however,
but also a symmetric conformation of the overall ligand system thermal activation to a higher-energy geometry becomes
with no asymmetric distortion of the NHC moieties as found in possible and the at conformation is kinetically trapped by
complexes 1–4. This effectively prohibits the structural distor- multiple hydrogen-bonding interactions in solid state (Fig. 2).
tion arising from the d–d excited state and ligand vibration Recrystallization from a DMF solution results in breaking of
thereby minimizing the non-radiative processes and allowing intermolecular hydrogen bonds; hence the thermodynamically
the radiative decay. The modest emission quantum yield of 5 stable and bent conformation is restored.
can be ascribed to a low value of kr of T1, the latter is lowered by
The profound inuence of intermolecular hydrogen-bonding
more than ten-fold in magnitude than those of complexes 1–4. on the molecular structure of luminescent transition-metal
The decrease in kr appears to have the same origin as that for complexes is of fundamental interest. In addition to the rich
the red-shi in absorption and emission energies, and this can solid-state polymorphism and luminescence-tuning of Pt(II)
be attributed to the extended p-conjugation of the tetradentate complexes that are related to intermolecular Pt/Pt and p–p
ligand in 5. The replacement of phenolate with naphthalenolate interactions,21 this observation is reminiscent of gaining insight
group causes the absorption and emission to derive mainly into the geometry and packing of molecules in organic
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electronic devices, such as organic thin-lm transistors (OTFTs) excited states in the blue spectral region are high energy and
and organic photovoltaic cells (OPVs), where vacuum-deposi- hence could be powerful photocatalysts.
tion technique is commonly employed. Charge-transporting
mobility, being an important factor in determining device
performance, is predominantly determined by packing of
Acknowledgements
molecules in the solid state. Single-crystal X-ray diffraction
studies on samples obtained from solutions, in conjunction
with thin-lm X-ray diffraction studies, are frequently used for
This work was supported by National Key Basic Research
Program of China (973 Program 2013CB834802 and
2013CB834803) and the Hong Kong Research Grants Council
the estimation and simulation of molecular packings in devices.
(HKU 7008/09 and AoE/P-03/08). This work was also supported
Nevertheless, our ndings reveal that the geometry and packing
by Guangdong Special Project of the Introduction of Innovative
R&D Teams, China. The computational part of this research is
conducted using the HKU Computer Centre research
computing facilities that are supported in part by the Hong
of molecules are not necessarily the same before and aer
thermal sublimation. Morphology control by thermal annealing
is an effective strategy for improving device performances. In
addition to change of molecular packing upon annealing
Kong UGC Special Equipment Grant (SEG HKU09). We thank
treatment, high temperature may also lead to a high-energy but
Miss Man Ying Ko, Drs. Steven C. F. Kui and Gary Chi-Chung
kinetically stable molecular geometry enforced by extensive
Kwok for providing the sample of the yellow Pt(II) electro-
phosphor used in the white OLEDs.
intermolecular interactions.
Conclusions
Notes and references
A series of Pt(II) complexes bearing tetradentate bis(NHC)
ligands were prepared and their photophysical properties were
examined. By ligand modication, the luminescence properties
can be altered dramatically. Studies by single-crystal diffraction,
‡ Calculation using M06-2X functionals revealed that the energy difference DE(T2
ꢁ T1) between the T2 and T1 is 4928 and 4717 cmꢁ1 for 2 and 4, respectively.
Whether or not the T2 emission is observable can be predicted using the
approximate relationship F(T2)/F(T1) ¼ k(T2)/k(T1)exp[ꢁDE(T1–T2)/kBT], where
ultrafast time-resolved spectroscopy and DFT calculations
altogether reveal that the s-donor strength, structural rigidity
and geometry distortion of the tetradentate ligands, and the
metal ion are essential in tuning the frontier molecular orbitals
and hence luminescent properties. The emission energy and
dynamics are governed by subtle interplays of several factors
including (1) the ligand conjugation-specic orbital parentage
and (2) dynamic quenching due to both the d–d excited state
and ligand centred excited state structural distortion.
We demonstrated that highly efficient blue- and white-light
emitting OLEDs can be fabricated by using our deep-blue phos-
phorescent complexes as phosphors. Meanwhile, the long-lived
triplet excited state of 5 can be developed into efficient lumi-
nescence sensor for oxygen. The serendipitous observation of a
at geometry of 2 shows that both the geometry and packing of
molecules could be modulated via installing intermolecular
non-covalent interactions.
The present work reveals that the “one metal ion + one
ligand” approach, i.e., using a tetradentate ligand to form
robust Pt(II) complex, is an appealing strategy to develop highly
efficient deep blue-emitting OLEDs with organoplatinum(II)
electrophosphors including those without uorine atoms.
Future development along this approach, based on insight
provided by the experimental and theoretical studies in this
work, could be directed to, for example, enhancing the rigidity
of the stable Pt(II) complex supported by tetradentate ligand
and/or increasing MLCT contributions, yet maintaining the
desired large energy gaps between HOMO and LUMO orbitals
and between the highest occupied and the lowest unoccupied
d-orbitals, to achieve deep blue emission with excellent
quantum yields. Apart from the use as electrophosphors for
OLEDs, robust blue-emitting organoplatinum(II) complexes can
also be used for many other purposes, because long-lived triplet
F(T2) and F(T1) is emission yield of the T2 and T1, respectively; k(T2) and k(T1) is
the radiative rate constant of the T2 and T1, respectively; kB is the Boltzmann
constant, and T is the absolute temperature under the examined experimental
condition. Based on the calculated value of k(T2)/k(T1) < 102, and the large energy
difference DE(T2 ꢁ T1), F(T2)/F(T1) is predicted to be <10ꢁ8. Generally, if F(T2)/
F(T1) is <10ꢁ2, it is difficult to observe the emission from upper state. Therefore, in
our case, we are unlikely to observe any emission from the upper T2 state.
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This journal is ª The Royal Society of Chemistry 2013
Chem. Sci.