A platinum(ii) complex bearing deprotonated 2-(diphenylphosphino)benzoic acid for superior phosphorescence of monomers

Article abstract of DOI:10.1039/c4dt00417e

The (ppy)-based Pt(ii) complex (Pt-1) with deprotonated 2-(diphenylphosphino)benzoic acid as an anionic ligand displays phosphorescence of monomers with a remarkably higher quantum yield than that of the corresponding iridium complex (Ir-1). A prototype OLED using Pt-1 exhibits high performance with an external quantum efficiency of 4.93%. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00417e

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A platinum(II) complex bearing deprotonated
2-(diphenylphosphino)benzoic acid for superior
phosphorescence of monomers†
Cite this: Dalton Trans., 2014, 43,
7704
Received 9th February 2014,
Accepted 14th March 2014
Bin Zhang,a Lina Zhang,a,b Chunmei Liu,a Yanyan Zhu,a Mingsheng Tang,a
Chenxia Du*a and Maoping Song*a
DOI: 10.1039/c4dt00417e
www.rsc.org/dalton
The (ppy)-based Pt(II) complex (Pt-1) with deprotonated 2-(diphe- electronic factors of the PPh2 group can efficiently enhance
nylphosphino)benzoic acid as an anionic ligand displays phosphor- the phosphorescent properties of cyclometalated complexes.
escence of monomers with a remarkably higher quantum yield However, developing new phosphorescent complexes contain-
than that of the corresponding iridium complex (Ir-1). A prototype ing PPh2 group ligands is not always feasible because of the
OLED using Pt-1 exhibits high performance with an external difficulty to synthesize them. Another effective strategy is the
quantum efficiency of 4.93%.
use of anionic ligands containing a diphenylphosphine struc-
ture to gain new complexes with high phosphorescent
efficiency. Until now, the utilization of anionic ligands modi-
fied with a diphenylphosphine moiety for phosphorescent
material is still rare.8 Moreover, to the best of our knowledge,
no effort has been made to investigate the influence of this
kind of ligands on the photophysical properties of both Pt(II)
and Ir(III) complexes. Herein, deprotonated 2-(diphenylpho-
sphino)benzoic acid (HL1) is chosen as a new anionic ligand
system in this paper to conjecture whether promising lumines-
cent Pt(II) and Ir(III) complexes can be obtained based on this
ligand. Two heteroleptic platinum and iridium complexes with
the formula (C^N)Pt(P^O) and (C^N)2Ir(P^O) are synthesized
respectively, where P^O is L1− and C^N is 2-phenylpyridine
(ppy). Comprehensive investigations including structural
characterizations, photophysical properties and quantum
chemical calculations of these complexes are presented. Inter-
estingly, unlike the rather low photoluminescent quantum
yield (Φ) of Ir-1 (Φ = 0.91% in CH2Cl2), Pt-1 exhibits efficient
green phosphorescence of monomers character both in solu-
tion (Φ = 18.32%) and in powder (Φ = 23.30%). The impressive
external quantum efficiency of 4.93% for a prototype phos-
phorescent OLED ranks Pt-1 as one of the most efficient
Pt(ppy)-based electroluminescent materials.
The Pt-1 and Ir-1 complexes are prepared by the reaction
of the corresponding chloride-bridged dimers [(C∧N)Pt(μ-Cl)]2
and [(C∧N)2Ir(μ-Cl)]2 with the HL1 in the presence of a base.
Detailed reaction conditions along with the NMR spectra,
elemental analysis and crystallographic data of the complexes
are available in the ESI.†
As shown in Fig. 1a, the platinum of Pt-1 maintains a good
square-planar coordination geometry with the P atom opposite
to the nitrogen atom of the ppy ligand. Additionally, the Pt-1
units are stacked in a head-to-tail fashion with a separation of
Interest in cyclometalated complexes based on iridium and
platinum has soared over the past decade following the dem-
onstration of their potential in the manufacture of highly
efficient organic light-emitting devices (OLEDs).1 The spin–
orbit coupling exerted by the heavy metal effect allows the
harvest of both singlet and triplet electro-generated excitons in
devices, leading to a theoretically achievable 100% internal
quantum efficiency.2 However, the luminance efficiencies of
devices containing such compounds are still low due to the
concentration quenching.3 In this respect, compared with
Ir(III) complexes, the square-planar Pt(II) complexes particularly
have a great tendency for aggregation through platinum–plati-
num and π–π interactions, resulting in detrimental effects on
photoluminescence quantum yields and color purity.4 It is
well understood that the introduction of steric hindrance into
the structure can hinder the intermolecular interactions. Phos-
phorescent quantum efficiencies of the complexes have been
significantly improved by the incorporation of bulky substitu-
ents.5 For example, the functionalization of 2-phenylpyridine
chelate with a dimesitylboryl moiety has been proven to greatly
enhance the phosphorescent efficiency of Pt(II) compounds.6
Very recently, emission quantum yield up to 100% has been
achieved by Ir(III) phosphor complexes with the steric C^P
ligand, where C^P = benzyldiphenylphosphine.7 The prelimi-
nary results demonstrate that the steric structural features and
aThe College of Chemistry and Molecular Engineering, Zhengzhou University,
Zhengzhou, 450052, P.R. China. E-mail: dcx@zzu.edu.cn, mpsong@zzu.edu.cn
bCollege of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454003,
P. R. China
†Electronic supplementary information (ESI) available: Synthesis and character-
ization details. CCDC 969995 for Pt-1 and 969996 for Ir-1. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c4dt00417e
7704 | Dalton Trans., 2014, 43, 7704–7707
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Fig. 3 The emission spectra of Pt-1 (a) and Ir-1 (b) in solution (1 × 10−5
mol L−1) at room temperature and 77 K.
Fig. 1 Molecular structures of the Pt-1 (a) and Ir-1 (b) complexes. Hydro-
gen atoms are omitted for clarity.
predominant π–π* transition of the ligands and minor metal
to ligand charge transfer in the singlet manifold. Meanwhile,
the main absorption at ∼260 nm for Ir-1 in this region can be
mainly assigned to spin-allowed π–π* transition of the ligands.
The less intense absorption bands (ε < 1.0 × 103 M−1 cm−1) for
Pt-1 and Ir-1 in the region of 360–450 nm can be assigned to
an admixture of metal-perturbed MLCT and LLCT transitions.
Characteristic phosphorescences in degassed dichloro-
methane solution are observed for both Pt-1 and Ir-1 at room
temperature and 77 K (Fig. 3). Pt-1 shows well-resolved vibro-
nic-structured emissions at 482 and 516 nm due to the
CvC and CvN stretching frequencies of the ligand both at
room temperature and 77 K. The highly structured emission
spectra, independent of temperature, indicate that the strong
spin–orbit coupling effect imposed by Pt atom promotes the
phosphorescence with substantial ligand-centered (LC) charac-
ter. Ir-1 displays a broad emission band (∼505 nm) at room
temperature and shows vibronic-structured emissions at 77 K,
owing to the increased rigidity of the solvent decreasing the
electronic mixing of states.
It is noteworthy that the photoluminescence quantum yield
of the complex Pt-1 in a dichloromethane solution at room
temperature is 18.32%, which is comparable to that of Pt(ppy)
(acac) (about 15%),12 while the quantum yield of Ir-1 is only
0.91% (Table S5, ESI†). Further studies for Pt-1 found that the
emission profile of Pt-1 is independent of the applied concen-
tration in the range from 10−5 mol L−1 to 10−2 mol L−1
(Fig. S3, ESI†). Remarkably, Pt-1 maintains the phosphor-
escence of monomers in powder with attractive luminescence
quantum yields of 23.30%.
The TD-DFT calculations on the optimized structures in the
triplet excited states for Pt-1 and Ir-1 in CH2Cl2 demonstrate
that the lowest energy triplet states (T1) are dominated by
HOMO–LUMO transitions (Table S5†). As shown in Table S6,†
the orbital distributions of Pt-1 suggest that the HOMO is com-
posed of platinum-dπ (16.20%) and ppy-π orbitals (77.90%),
while the LUMO is dominated by the ppy ligand (92.10%).
Therefore, the emission of Pt-1 is mainly derived from the
[Pt(ppy)] moiety based on LC and some contribution of
the MLCT transition. For Ir-1, the HOMO and LUMO levels are
mainly localized on the ligands (90.92% and 99.02%, respect-
ively). The drastic decreased contribution from Ir(dπ) to the
HOMO (9.08%) might be rationalized by the significant
changes of geometry in the T1 state (Table S7 vs. Table S2,
Fig. 2 Comparisons between the calculated (in gas (GAS) and in CH2Cl2
with PCM mode (PCM), respectively) and experimental absorption
spectra for Pt-1 (a) and Ir-1 (b) in CH2Cl2 solution (1.0 × 10−5 mol L−1).
about 3.51 Å between the only partially overlapping 2-phenyl-
pyridine rings of the complex (Fig. S1a†), indicating only weak
π–π interactions between the Pt-1 units.9 Furthermore, the
long Pt–Pt distance (9.37 Å) rules out the presence of Pt–Pt
bimetallic interactions. The iridium of Ir-1 is coordinated by
two N atoms and two C atoms of 2-phenylpyridines (ppy)
together with P and O atoms from HL1, forming a slightly
deformed octahedral coordination geometry (Fig. 1b). The
PPh2 fragment exerts a notable trans-effect and results in rela-
tively longer Ir–C(1) bond distance of 2.053(6) Å versus that of
Ir–C(12) (2.002(7) Å).8c,10 The crystal packing of Ir-1 (Fig. S1b†)
also shows that no apparent stacking interactions can be
detected due to the steric repulsion of the HL1 ligand.
Calculated bond distances and angles in gas as well as in
CH2Cl2 solution are in very good agreement with that of crystal
structure data (Table S2†), which ensure the accuracy of DFT
studies at the B3LYP level for Pt-1 and Ir-1 complexes.11 The
frontier orbital analysis indicates that the HOMO levels of
both Pt-1 and Ir-1 have significant contributions from the d
orbitals of the Pt(II) and Ir(III) centers, whereas the LUMO levels
are dominated by the ligands (Fig. S2, Table S4†).
The UV-Vis absorption spectra of both complexes recorded
in CH2Cl2 at room temperature are displayed in Fig. 2 with per-
tinent data summarized in Table S3.† Simulated absorption
curves based on the calculated results at the TD-DFT level
agree well with the corresponding experimental data,
especially by the PCM model for CH2Cl2. In general, both com-
plexes exhibit intense absorptions in the region below 350 nm
with maximum ε values being above 1.5 × 104 M−1 cm−1. Two
absorption bands are observed for the compound Pt-1
in this region, which can be ascribed to a mixed character of
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of Ir-1 is dominated by ILCT and LLCT transitions, which may
rationalize the surprising difference in the photophysics of
Pt-1 and Ir-1. The device based on Pt-1 exhibits outstanding
performances with maximal external quantum efficiency (ηext
)
of 4.93% and power efficiency (ηP) of 14.64 lm W−1, which
would be the highest reported to date for Pt(II)-ppy based com-
plexes. The results highlight the pivotal role of HL1 in the
structure and luminescence behavior of the Pt(II) complex.
Screening of better phosphorescence-emitting compounds
based on this anionic ligand system is in progress.
Fig. 4 Thecurrent density (J)–voltage (V)–luminance (L) characteristics
of device (a). The external quantum efficiency (ηext) and power efficiency
(ηp) of the EL device versus luminance (b).
This work was supported by the Specialized Research
Fund for the Doctoral Program of Higher Education
(20114101130002) and the National Natural Science Foun-
dation of China (21371154).
ESI†) and the trans-effect imposed by the bulky L1− ligand.
Knowing that the overlap between π-orbital of the ligand and d
orbital of the metal atom is a key factor to give out a much
allowed transition,13 the relatively small contributions of
MLCT transition can hence be attributed to the poor solution
quantum yield of Ir-1.14 Deduced from the calculated results
of Ir-1, another remark worth mentioning is that the HOMO is
predominantly composed of L1− (60.43%) with a smaller con-
tribution of ppy (30.49%), while the populations are different
from the LUMO with 34.44% L1− and 64.58% ppy. As a result,
the emissive excited states of Ir-1 contain considerable LLCT
(ligand-to-ligand charge transfer), a partially forbidden tran-
sition, which could further reduce the emission quantum yield
of Ir-1.15
Notes and references
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