Tuning the emission colour of triphenylamine-capped cyclometallated platinum(II) complexes and their application in luminescent oxygen sensing and organic light-emitting diodes

FULL PAPER

DOI: 10.1002/ejic.201000327

Tuning the Emission Colour of Triphenylamine-Capped Cyclometallated

Platinum(II) Complexes and Their Application in Luminescent Oxygen Sensing

and Organic Light-Emitting Diodes

Wenting Wu,^[a]Chuanhui Cheng,^[b]Wanhua Wu,^[a]Huimin Guo,^[a]Shaomin Ji,^[a]

Peng Song,^[c]Keli Han,^[c]Jianzhang Zhao,*^[a]Xin Zhang,^[a]Yubo Wu,^[a]and

Guotong Du^[b,d]

Keywords: Phosphorescence / Luminescence / Sensors / Density functional calculations / OLEDs / Platinum

[(Aryl-ppy)Pt(acac)] (ppy = 2-phenylpyridine, acac = acetyl- ents on the ppy ligand or a decreased LUMO energy caused

acetonato) derivatives with triphenylamine (TPA) substitu-

ents on the ppy ligand have been prepared. The TPA frag-

ment is either directly cyclometallated (Pt-1) or attached to

the ppy ligand through a C–C single bond (Pt-2) or a novel

α-diketo group (Pt-3). All the complexes show room-tempera-

by the electron-trap effect of electron-withdrawing substitu-

ents; both result in a smaller HOMO–LUMO energy gap and

thus red-shifted emission. The complexes show extended lu-

minescence lifetimes (τ = 3.0–5.5 µs) relative to the parent

complex [ppyPt(acac)] (τ = 2.6 µs). The luminescent oxygen-

ture phosphorescence in fluid solution with emission bands sensing properties of the complexes were studied in solution

in the range of 530–590 nm, which are red-shifted relative to

the model complex [ppyPt(acac)] (λ_em= 486 nm). This emis-

sion colour tuning effect is attributed to either an elevated

HOMO energy caused by electron-donating TPA substitu-

and polymer films. White light emission was observed with

an OLED device fabricated with complex Pt-3 with CIE coor-

dinates of (0.32, 0.32).

Thus, these materials show promise for application as lumi-

nescent O₂-sensing materials, which require long-lived trip-

Introduction

Cyclometallated Pt^IIor Ir^IIIcomplexes are attractive trip- let emitters.^[20–39]However, to our surprise, no systematic

let emitters and have been extensively studied as phos- study has been carried out on the application of these com-

phorescent materials for electroluminescence (organic light- plexes as luminescent O₂-sensing materials. Furthermore,

emitting diodes, OLEDs), molecular sensors and photo- we have noticed that OLEDs and luminescent O₂sensing

responsive molecular devices.^[1–19]The emissive excited present the same challenge, that is, to tune the emission

3

states of these complexes are the mixed MLCT/³IL state, colour of the phosphorescent materials, for example, to

thus the emissions are characterized by structured emission achieve white light-emitting diodes or monochromic emis-

bands and large Stokes shifts.^[3]Usually, cyclometallated sion in the blue, green and red regions. Furthermore, it is

Pt^II/Ir^IIIcomplexes with C N ligands show high lumines- important to be able to vary the luminescent lifetimes of

∧

cent quantum yields and good photostability as well as long phosphorescent dyes for oxygen sensing and for use as

luminescent lifetimes in the range of microseconds (µs). OLEDs.^[25]

Triplet emitters (phosphorescent dyes) with long lumin-

escence lifetimes are used for luminescent O₂sensing, for

[a] State Key Laboratory of Fine Chemicals, School of Chemical

example, ruthenium polydiimine complexes such as

[Ru(bpy)₃]²⁺or platinum porphyrin complexes such as

PtOEP.^{[20–25,32,33]}Luminescent O₂sensing is based on the

quenching effect of O₂on the phosphorescence emission of

the complexes.^[21,34–38]To address the drawbacks of emis-

sion-intensity-based O₂sensing, lifetime-based sensing or

ratiometric sensing is desired.^[21–24]Lifetime-based O₂sens-

ing requires phosphorescent dyes with longer luminescent

lifetimes.^[25]In contrast, a short luminescent lifetime is pre-

ferred for OLED purposes to avoid saturation effects. How-

ever, it has been proven that the tuning of luminescent life-

times of transition-metal complexes is elusive. To the best

Engineering, Dalian University of Technology,

158 Zhongshan Road, Dalian 116012, P. R. China

Fax: +86-411-3960-8007

E-mail: zhaojzh@dlut.edu.cn

[b] School of Physics and Optoelectronic Technology,

Dalian University of Technology,

Dalian 116024, P. R. China

Fax: +86-411-8470-7865

[c] State Key Laboratory of Molecular Reaction Dynamics, Dalian

Institute of Chemical Physics, Chinese Academy of Sciences,

457 Zhongshan Road, Dalian 116023, P. R. China

[d] State Key Laboratory of Integrated Optoelectronics, College of

Electronic Science and Engineering, Jilin University,

2699 Qianjin Street, Changchun 130012, P. R. China

Supporting information for this article is available on the

WWW under http://dx.doi.org/10.1002/ejic.201000327.

View this journal online at

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FULL PAPER

of our knowledge, very little effort has been made to tune

the luminescent lifetimes of cyclometallated Pt complexes.

Herein we describe the synthesis of cyclometallated

[Pt^II(acac)] complexes with the triphenylamine (TPA)-inte-

grated ppy ligand (Pt-1) (ppy = 2-phenylpyridine, acac =

Results and Discussion

Design and Synthesis of the Ligands and Cyclometallated

Pt^IIComplexes

The design rationale of the Pt complexes lies in the no-

acetylacetonato)^[4]or with the TPA fragment attached to tion that attaching an electron-donating group to the

the phenyl group of the ppy moiety either through a C–C phenyl group of ppy will increase the energy of the HOMO,

single bond (Pt-2) or a novel α-diketo moiety (Pt-3). The but the energy of the LUMO will be hardly affected; thus,

photophysics of the complexes were studied by UV/Vis ab- the band gap (∆E_HOMO–LUMO) of the complex will be re-

sorption, luminescence emission spectra and theoretical cal- duced and red-shifted emission will be observed.^[1,4]Dif-

culations (DFT/TDDFT). The complexes show phos- ferent linkers, such as a C–C single bond and a α-diketo

phorescence emission bands in the range of 490–561 nm. moiety, were used to connect the triphenylamine (TPA) and

The red-shifted emission of the complexes relative to the the phenylpyridine (ppy) to investigate the effect of coupled

parent complex [ppyPt(acac)] (λ_em= 486 nm) is attributed and decoupled π-conjugated systems on the photophysical

to the increased HOMO energy or the decreased LUMO properties of the complexes. Pd-catalysed Suzuki and Sono-

energy, as suggested by DFT calculations. The complexes gashira coupling reactions were used to prepare the li-

were found to be sensitive luminescent oxygen-sensing ma- gands.^[25]Metallation of the ligands with [K₂PtCl₄] and

terials. A white light-emitting OLED was fabricated with then reaction with Hacac led to the cyclometallated Pt com-

Pt-3. To the best of our knowledge no cyclometallated Pt^IIplexes (Scheme 1). We found an unexpected α-diketo prod-

complexes have been systematically studied for luminescent uct in the metallation with the ethynyl ligand (Pt-3). This

O₂-sensing purposes. The structure–emission relationship is probably due to Pt-catalysed oxidation of the –CϵC–

revealed by our study will be useful in the design of phos- bonds.^[40,41]All the intermediates and the final Pt com-

phorescent emissive materials.

plexes were obtained in satisfactory yields.

Scheme 1. Synthesis of the ligands and the cyclometallated platinum complexes 1–3. Reagents and conditions: (a) i) nBuLi, THF, –78 °C,

1 h; ii) B(OCH₃)₃–78 °C, 2 h, HCl; (b) 2-(4Ј-bromophenyl)pyridine, Na₂CO₃/[Pd(PPh₃)₄] (3.6mol-%), toluene/H₂O (3:1, v/v), Ar, 90 °C,

20 h; (c) i) [K₂PtCl₄], 2-ethoxyethanol/water (3:1, v/v), Ar, 80 °C, 20 h; ii) Hacac/Na₂CO₃, 2-ethoxyethanol, 100 °C, 20 h; (d) ethynyltri-

methylsilane, [Pd(PPh₃)Cl₂] (2mol-%), CuI (4mol-%), PPh₃(4mol-%), NEt₃/THF (3:1 v/v), Ar, 90 °C, 12 h; (e) K₂CO₃, MeOH/diethyl

ether (2:1, v/v), room temp., 24 h; (f) 2-(4Ј-bromophenyl)pyridine, [Pd(PPh₃)₂Cl₂] (2mol-%), CuI (2mol-%), PPh₃(10mol-%), NEt₃/THF

(3:5, v/v), Ar, 90 °C, 8 h.

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Eur. J. Inorg. Chem. 2010, 4683–4696

Triphenylamine-Capped Cyclometallated Platinum(II) Complexes

X-ray Crystal Structures

The crystal structures of Pt-2 and Pt-3 were determined.

For Pt-2, half of the molecules in the crystal exist as head-

to-head dimers with the coordination plane of the mole-

cules parallel to each other. The Pt–Pt distance is 3.194 Å,

which is within the range of 2.7–3.5 Å and thus a weak

Pt^II···Pt^IIinteraction is possible in the crystal.^[4,42]

The platinum atom resides in a square-planar geometry

(Figure 1). The Pt–N distance is 1.995 Å, which is slightly

longer than that of common Pt–N bonds. The Pt–O and

Pt–C distances are in the range of 1.978–2.108 and 1.940–

1.967 Å, respectively. The C–Pt–N and O–Pt–O angles are

82.5 and 93.2°, respectively, which are similar to reported

values.^[2,4]

Figure 2. Perspective view of the crystal structure of Pt-3 (30%

probability ellipsoids, hydrogen atom labels have been omitted for

clarity).

a high ε value is beneficial for luminescent biosensing or

bioimaging applications.^[43]The absorption of Pt-1 is red-

shifted relative to L-1, which is due to the cyclometallation

of TPA and the MLCT absorption.

Figure 1. Perspective view of the crystal structure of Pt-2 (30%

probability ellipsoids, hydrogen atom labels have been omitted for

clarity).

The crystal structure of the α-diketo-containing Pt-3 was

also determined (Figure 2). In contrast to Pt-2, no dimer

packing was found for Pt-3. The Pt^II–Pt^IIdistance is

6.782 Å; thus, no Pt^II···Pt^IIinteraction exists in Pt-3. This

is probably due to the skewed geometry of the α-diketo

moiety [the torsion angle of O(3)–C(21)–C(20)–O(4) is

98.3°], which prevents tight packing of the molecules in the

crystal. We found that the two carbonyl groups adopt a

coplanar geometry with the connected aromatic moiety. For

example, the O(3)–C(21)–C(22)–C(23) and O(4)–C(20)–

C(17)–C(16) torsion angles are 176.8 and 13.7°, respectively.

The Pt–C and Pt–N bond lengths are 1.958 and 1.990 Å,

respectively. The C–Pt–N and O–Pt–O bond angles are

81.42 and 91.8°, respectively. These values are similar to

those of Pt-2.

Figure 3. UV/Vis absorption spectra of (a) the ligands and (b) the

Pt complexes; c = 1.0ϫ10^–5moldm^–3in dichloromethane. 25 °C.

UV/Vis Absorption Spectra of the Ligands and Complexes

The UV/Vis absorption behaviour of the ligands and

complexes were studied (Figure 3). The absorptions at 350–

400 nm are due to the π–π* transition in the TPA fragment.

On metallation, the absorptions show a redshift of around

Photoluminescence Spectra of the Ligands and Complexes

The emission behaviour of the ligands and complexes

50 nm (Figure 3, b). A shoulder appeared at the red end of were also studied (Figure 4). For the ligands, broad, struc-

the spectra (beyond 400 nm); this weak absorption is the tureless emission bands were observed. We found that L-

result of a spin-allowed ¹MLCT transition.^[1,3]We noted 4 shows the longest wavelength emission (489 nm). These

that the molar extinction coefficient (ε) of Pt-1 at around emissions are due to the S₁state of the ligands and are of

420 nm is in the range of 20000–30000 dmcm^–1mol^–1. Such π–π* character.

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[ppyPt(acac)]. For Pt-3, emission at 490 nm was observed.

The large Stokes shifts and the emissions at red-shifted

wavelengths suggest that the emission is due to the triplet

excited state, that is, the emission is phosphorescent in na-

ture. We propose the phosphorescence to be due to the

³MLCT/³IL mixed excited state.^[3,4,44,45]

The photophysical properties of the ligands and the Pt

complexes are compiled in Table 1. The fluorescence quan-

tum yields of the ligands are generally high and their fluo-

rescence lifetimes are in the range of ns, which is in agree-

ment with singlet emission. Pt-1, has a luminescent lifetime

of 3.7 µs at room temperature, which is longer than that of

the parent complex [ppyPt(acac)] (τ = 2.6 µs).^[3]The emis-

sion wavelength of Pt-1 is red-shifted relative to that of

[ppyPt(acac)], whereas the luminescent lifetimes are ex-

tended.

We found that the emission of the complexes is concen-

tration-independent. For example, the emission profile of

Pt-1 does not change with increasing concentration (Fig-

ure 5). Similar results were observed for Pt-2 (see the Sup-

porting Information).

Figure 4. Emission spectra of (a) the ligands and (b) the Pt com-

plexes; c = 1.0ϫ10^–5moldm^–3in dichloromethane. 25 °C.

For Pt-1, structured emission at 542 nm was observed;

the profile is typical of cyclometallated Pt complexes.^[1,3]

The emission is red-shifted by 60 nm relative to the parent

complex [ppyPt(acac)], which shows emission at 486 nm.^[3]

We propose that the red-shifted emission is due to the

strong electron-donating ability of the diphenylamine

group. The structured emission band, large Stokes shift and

long luminescent lifetime (3.7 µs) infer that the emission at

542 nm is phosphorescent in nature. This emission colour

tuning approach will be useful in the design of phosphor-

escent emitters.

Figure 5. Emission of Pt-1 in CH₂Cl₂solution at different concen-

trations; λ_ex= 425 nm, 27 °C.

Previously, Pt-1 was reported as a fluorescence/phospho-

rescence dual emissive material,^[4]but we observed only

phosphorescence (Figure 4). We tentatively propose the dis-

crepancy to be due to the highly sensitive nature of the

emission of this complex to oxygen. Trace amounts of oxy-

gen in the solution will diminish the emission and lead to a

DFT/TDDFT Calculations and the Low-Lying Excited

States of the Complexes

Recently, density functional theory (DFT) calculation

pseudo-dual emission (because the fluorescence emission methods were successfully used to study the photophysics

will be relatively high and the fluorescence/phosphorescence of fluorescent and phosphorescent chromophores.^[43,46–52]

emission intensity will become comparable).

The photophysics of lumophores can be better understood

For Pt-2, an emission band at around 561 nm was ob- by revealing the electronic structures of the excited

served, which is red-shifted by about 80 nm relative to states.^{[2–4,18,19,43]}In this work we carried out theoretical cal-

Table 1. Photophysical parameters of the platinum complexes Pt-1, Pt-2 and Pt-3 and the ligands.

λ_abs[nm]^[a]

λ_ex[nm]

λ_em[nm]

Φ_f^[b]

Φ_p

τ

[c]

[d]

L-1

L-2

L-3

L-4

Pt-1

Pt-2

Pt-3

307 (2.88), 346 (4.29)

294 (6.79), 350 (9.45)

294 (7.22), 367 (6.40)

297 (1.97), 383 (1.44)

308 (3.18), 338 (2.88), 420 (2.70)

294 (2.86), 371 (2.23)

293 (2.94), 336 (1.92), 377 (2.18)

364

376

388

425

372

376

433

456

466

0.8999

0.2562

0.6076

0.0008

–

3.34 ns

2.32 ns

2.07 ns

4.82 ns

3.70 µs

5.45 µs

3.07 µs

[d]

489

[d]

542, 600

561, 596

490, 532

–

0.36

0.21

0.03

[d]

–

[d]

–

[a] Extinction coefficients (10⁴^–1cm^–1) are shown in parentheses. [b] Fluorescence quantum yield in aerated CH₂Cl₂. [c] Phosphorescence

quantum yield in deaerated CH₂Cl₂. [d] Not applicable.

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Triphenylamine-Capped Cyclometallated Platinum(II) Complexes

culations on the complexes and the photophysical proper-

ties, such as the excitation and emission properties, were

investigated with time-dependent DFT (TDDFT) calcula-

tions. The main objective of the theoretical calculations was

to investigate the electronic structures of the low-lying ex-

cited states and to correlate them with their emission prop-

erties. This information will be helpful in the future design

of luminescent cyclometallated Pt complexes with predeter-

mined photophysical properties such as red-shifted emis-

sion wavelengths or long luminescent lifetimes.^[25]

The optimized ground-state geometry of Pt-1 indicates a

planar square-coordinated Pt centre with Pt–N and Pt–C

bond lengths of 2.02 and 1.99 Å, respectively. These bond

lengths are very similar to the values determined for the

single-crystal structure of the cyclometallated Pt complex.^[4]

The HOMO of Pt-1 is localized on the TPA moiety and

the LUMO is mainly distributed on the pyridine. The Pt

Figure 6. Selected frontier molecular orbitals of complexes Pt-1

calculated by DFT at the B3LYP/6-31G(d)/LanL2DZ level of

theory using Gaussian 09.

atom contributes to both the HOMO and the LUMO (Fig- sible for the emission (Kasha’s rule),^[21,43,53]the electronic

ure 6). To assign the UV/Vis absorption and emission spec- structure is characterized by HOMOǞLUMO and

tra, the vertical excitation energies of the complex were cal- HOMO-2ǞLUMO transitions, both are assigned as

culated with TDDFT on the basis of the ground-state ge- TPAǞpyridine (³IL) and diphenylamineǞpyridine (³IL)

ometry (Table 2).

transitions. The Pt atom is also involved in these transitions.

The calculated UV/Vis absorption of the S₀ǞS₁transi- These pictures are in good agreement with the accepted

tion is centred at 437 nm, which is in good agreement with photophysics of [ppyPt(acac)] complexes. The calculated ex-

the UV/Vis absorption at 420 nm (Figure 3). The S₁state citation energy for the T₁excited state is 545 nm, which is

is characterized by HOMOǞLUMO and HOMO- consistent with the emission band observed at 542 nm.^[54]

2ǞLUMO transitions, both are assigned as TPAǞpyr-

Note that the vertical excitation energies were calculated

idine charge transitions. The Pt atom is involved in this on the basis of the ground-state geometry of the complex.

transition (Figure 6). Other excited states, such as the S₁₁The agreement between the virtual S₀ǞT₁excitation en-

state (309 nm, due to the TPA localized excitation), corre- ergy and the experimental T₁ǞS₀emission energy infers

late well with the experimental UV/Vis absorption at that the geometry relaxation in the T₁excited state is not

308 nm (Figure 4). This assignment of the UV/Vis absorp- significant, probably due to the rigid structure of Pt-1.

tion is in agreement with the traditional understanding of

the photophysics of these complexes, that is, MLCT/IL Pt-2 (Figure 7 and Table 3). First the geometries of the

mixed transitions.^[3]

complex and the ligands were optimized. It was found that

Similar theoretical calculations were also carried out for

The triplet states of Pt-1 were also investigated. For the the dihedral angle between the TPA and the ppy groups is

lowest-lying triplet excited state, that is, T₁, which is respon- 33.8°. This value is very close to the torsion angle of 33.79°

Table 2. Electronic excitation energies and the corresponding oscillator strengths (f), main configurations and CI coefficients of the low-

lying electronically excited states of complex Pt-1 calculated with TDDFT//B3LYP/6-31G(d)/LanL2DZ on the basis of the DFT//B3LYP/

6-31G(d)/LanL2DZ-optimized ground-state geometries.

TDDFT//B3LYP/6-31G(d)

Composition^[c]

Electronic

transition

Energy^[a]

f^[b]

CI^[d]

Character^[e]

Singlet

Triplet

S₀ǞS₁

S₀ǞS₂

S₀ǞS₇

2.84 eV, 437 nm

0.2998

HǞL

H–2ǞL

HǞL

0.6568

0.1003

0.6422

0.1974

0.2920

0.5994

0.6210

0.6762

0.1457

0.1137

0.1114

0.4873

0.3669

IL

MLCT

IL

LLCT

MLCT

LLCT

3.21 eV, 387 nm

3.69 eV, 336 nm

0.0506

0.0707

H–2ǞL

H–1ǞL+1

HǞL+3

HǞL

H–2ǞL

HǞL+1

H–1ǞL

H–1ǞL+1

H–2ǞL+1

S₀ǞS₁₁

S₀ǞT₁

4.01 eV, 309 nm

2.28 eV 545 nm

0.1700

0.0000^[f]

S₀ǞT₂

2.84 eV, 437 nm

0.0000^[f]

[a] Only the selected low-lying excited states are presented. [b] Oscillator strength. [c] H represents the HOMO and L represents the

LUMO. Only the main configurations are presented. [d] The CI coefficients are absolute values. [e] IL: intraligand; LLCT: ligand-to-

ligand charge transfer; MLCT: metal-to-ligand charge transfer. [f] No spin–orbital coupling effect was considered, thus the f value is

zero.

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Figure 7. Frontier molecular orbitals for Pt-2 calculated by DFT at the B3LYP/6-31G(d)/LanL2DZ level of theory using Gaussian 09.

Table 3. Electronic excitation energies and the corresponding oscillator strengths (f), main configurations and CI coefficients of the low-

lying electronically excited states of complex Pt-2 calculated with TDDFT//B3LYP/6-31G(d)/LanL2DZ on the basis of the DFT//B3LYP/

6-31G(d)/LanL2DZ-optimized ground-state geometries.

TDDFT//B3LYP/6-31G(d)

Composition^[c]

Electronic

transition

Energy^[a]

f^[b]

CI^[d]

Character^[e]

Singlet

S₀ǞS₁

S₀ǞS₂

2.80 eV, 444 nm

3.07 eV, 404 nm

0.4071

0.0174

HǞL

0.6767

0.1227

0.6428

0.1333

0.675

IL/LMCT

IL/LLCT

LLCT/MLCT

LLCT/LMCT

LLCT/MLCT

LLCT/IL

IL/LLCT

LLCT/MLCT

IL/LLCT

LLCT/MLCT

LLCT/LMCT

LLCT/MLCT

IL/LMCT

H–4ǞL

H–1ǞL

H–2ǞL

HǞL+1

H–2ǞL

HǞL+2

H–7ǞL

H–4ǞL

H–2ǞL+2

H–4ǞL

H–2ǞL

H–1ǞL

HǞL

S₀ǞS₃

S₀ǞS₆

S₀ǞS₁₇

3.35 eV, 370 nm

3.50 eV, 354 nm

4.21 eV, 295 nm

0.0710

0.0795

0.0705

0.2043

0.6468

0.2887

0.2389

0.5079

0.1444

0.2760

0.2792

0.5627

0.1359

0.1618

0.5815

0.2631

Triplet

S₀ǞT₁

2.34 eV, 530 nm

0.0000^[f]

HǞL+4

H–2ǞL

H–1ǞL

HǞL

S₀ǞT₂

2.76 eV, 449 nm

0.0000^[f]

[a] Only the selected low-lying excited states are presented. [b] Oscillator strength. [c] H represents the HOMO and L represents the

LUMO. Only the main configurations are presented. [d] The CI coefficients are absolute values. [e] IL: intraligand; LLCT: ligand-to-

ligand charge transfer; MLCT: metal-to-ligand charge transfer. [f] No spin–orbital coupling effect was considered, thus the f values are

zero.

determined in the single-crystal structure analysis (Fig- the triplet state will have a long lifetime due to the weak

ure 1). The optimized O–Pt–O and C–Pt–N bond angles are spin–orbital coupling effect of the Pt metal.^[1,3]This is in

89.6 and 81.2°, respectively. The bond lengths of C–Pt and agreement with the experimental observations, that is, lumi-

Pt–N are 1.991 and 2.018 Å, respectively. These values are nescent lifetimes of 3.7 and 5.5 µs were observed for Pt-1

close to the values determined in the single-crystal structure and Pt-2, respectively (Table 1).

analysis. The calculated UV/Vis absorptions in the visible

Similar calculations were performed on Pt-3 (Figure 8

range are mainly due to the S₀ǞS₁(444 nm), S₀ǞS₂and Table 4). We found that the α-diketo moiety adopts a

(404 nm) and S₀ǞS₃transitions (370 nm; Table 3). These trans skewed geometry in the ground state with a torsion

absorption maxima are in good agreement with the experi- angle of 123.8°. The DFT optimization indicates that the

mental results (412 and 371 nm). Based on the orbitals in- two carbonyl groups adopt a coplanar geometry with the

volved in the transitions (Figure 7) we conclude that the respective aromatic moieties to which the carbonyl groups

transitions are mainly TPAǞppy ¹IL and PtǞppy are attached; the torsion angles are 0.36 and 1.16°, respec-

³MLCT excitations. The PtǞppy ¹MLCT transition is tively. The calculated excitations with significant oscillator

found for the S₂state (Table 3).

strengths are the excited states of S₆, S₉and S₂₀with exci-

We found that the T₁state of Pt-2 is comprised mainly of tation energies of 372, 337 and 298 nm. These values are in

the HOMOǞLUMO transition. The Pt atom contributes good agreement with the experimental observations of 377,

slightly to this transition. Therefore it was proposed that 336 and 293 nm, respectively (Figure 1).

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Eur. J. Inorg. Chem. 2010, 4683–4696

Triphenylamine-Capped Cyclometallated Platinum(II) Complexes

Figure 8. Frontier molecular orbitals for Pt-3 calculated by DFT at the B3LYP/6-31G(d)/LanL2DZ level of theory using Gaussian 09.

Table 4. Electronic excitation energies and the corresponding oscillator strengths (f), main configurations and CI coefficients of the low-

lying electronically excited states of complex Pt-3 calculated with TDDFT//B3LYP/6-31G(d)/LanL2DZ on the basis of the DFT//B3LYP/

6-31G(d)/LanL2DZ-optimized ground-state geometries.

TDDFT//B3LYP/6-31G(d)

Composition^[c]

Electronic

transition

Energy^[a]

f^[b]

CI^[d]

Character^[e]

Singlet

Triplet

S₀ǞS₁

S₀ǞS₆

S₀ǞS₉

S₀ǞS₂₀

S₀ǞT₁

2.65 eV, 467 nm

0.0374

0.4274

0.0843

0.1806

0.0000^[f]

HǞL

0.6767

0.3456

0.5925

0.2092

0.6698

0.1407

0.4690

0.3617

0.4661

0.2703

0.2517

0.2973

0.2090

0.3530

0.5857

IL/LMCT

IL/LLCT

IL

IL/MLCT/LLCT

IL/LLCT

LLCT/MLCT

IL

H–3ǞL

HǞL+1

H–3ǞL+1

HǞL+2

H–1ǞL+2

HǞL+6

HǞL+5

H–3ǞL

H–3ǞL+1

H–2ǞL

HǞL

3.33 eV, 372 nm

3.68 eV, 337 nm

4.16 eV, 298 nm

2.23 eV, 557 nm

IL

IL/LLCT/MLCT

LLCT/MLCT

IL/LMCT

IL

LLCT/MLCT

IL/LLCT

HǞL+1

H–2ǞL

H–1ǞL

S₀ǞT₂

2.33 eV, 531 nm

0.0000^[f]

[a] Only the selected low-lying excited states are presented. [b] Oscillator strength. [c] H represents the HOMO and L represents the

LUMO. Only the main configurations are presented. [d] The CI coefficients are given as absolute values. [e] IL: intraligand; LLCT:

ligand-to-ligand charge transfer; MLCT: metal-to-ligand charge transfer. [f] No spin–orbital coupling effect was considered, thus the f

values are zero.

The S₀ǞT₁transition of Pt-3 was found to have an exci- shifted emission of Pt-2 can be rationalized similarly. The

tation energy of 557 nm, which is very close to the phos- HOMO and LUMO energies of Pt-2 were calculated to be

phorescence emission (532 and 490 nm).^[54]We found that –4.79 and –1.66 eV, respectively.

the T₁state is characterized by TPAǞppy/diketo (³IL),

For Pt-3, however, an electron-withdrawing α-diketo

3

PtǞppy/diketo (³MLCT) and diketoǞlocalized IL tran- group is attached to the phenyl fragment of the ppy ligand.

sitions.

Based on the traditional theory of the structure–emission

For electroluminescence, the energies of the HOMO and properties of cyclometallated Pt complexes,^[1,3]blue-shifted

LUMO of the phosphorescent dopants are important for emission should be expected for Pt-3 relative to that of

device fabrication.^[4]The HOMO and LUMO energies cal- [ppyPt(acac)]. Interestingly, red-shifted emission was ob-

culated for the parent complex [ppyPt(acac)] are –5.41 and served for Pt-3. We found that the α-diketo moiety is signifi-

–1.58 eV, respectively. With electron-donating substitution cantly involved in the LUMO (Figure 8) and thus we expect

on the ppy ligand, the HOMO and LUMO energies of Pt- the energy of the LUMO to be reduced, that is, the α-diketo

1 were calculated to be –4.76 and –1.53 eV, respectively. moiety may act as an electron trap to perturb the electronic

Thus, the LUMO energy is hardly affected but the energy of structure of the excited state of Pt-3 (Figure 8). The HOMO

the HOMO is significantly increased with electron-donating energy of Pt-3 (–5.20 eV) is similar to that of [ppyPt(acac)]

substitution on the phenyl group of the ppy fragment. Thus, (–5.41 eV), but the LUMO energy of Pt-3 (–2.51 eV) is sig-

the energy gap is smaller and red-shifted emission is ob- nificantly lower than that of [ppyPt(acac)] (–1.58 eV). Thus,

served for Pt-1 relative to that of [ppyPt(acac)]. The red- the HOMO–LUMO energy gap of Pt-3 is much smaller

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J. Zhao et al.

FULL PAPER

than that of [ppyPt(acac)] and therefore red-shifted emis-

sion is expected for Pt-3. This theoretical prediction is fully

proven by the experimental results. Thus, the α-diketo moi-

ety acts as an efficient electron trap to perturb the excited

state of cyclometallated Pt complexes thereby leading to

red-shifted emission.^[2]Tuning the emission wavelength of

cyclometallated Pt complexes with an electron trap is rarely

reported.^[2]Our finding of α-diketo as a new and efficient

electron trap for this purpose may prove significant in the

future molecular design of cyclometallated Pt complexes.

Luminescent Oxygen Sensing

Recently, luminescent O₂sensing has attracted consider-

able attention.^{[20,23,24,55–57]}Usually Ru^II–polypyridine or

Pt^II–porphyrin complexes are used for oxygen sensing. To

the best of our knowledge no cyclometallated Pt(acac) com-

plexes have been studied for O₂sensing.^[24]Thus, we carried

out an investigation of the luminescent O₂-sensing proper-

ties of the complexes Pt-1, Pt-2 and Pt-3.

First we studied the luminescent O₂-sensing properties of

Pt-2 in solution (Figure 9). We found that the phosphores-

cence emission band at 560 nm can be significantly

quenched by O₂. For example, the emission is significantly

quenched with 0.04% O₂(mixture with N₂, v/v). The emis-

sion is almost completely quenched with 1.5% O₂. A similar

quenching effect was observed for Pt-1 (see the Supporting

Information).

Figure 10. Phosphorescent intensity response of sensing films of

the complexes in IMPEK-C to saturation O₂/N₂cycles and small-

step variations of O₂concentrations. (a) Saturation switching and

(c) dynamic emissive intensity response of sensing films of Pt-1 in

IMPEK-C to O₂/N₂saturation cycles. λ_ex= 400 nm, λ_em= 535 nm,

33 °C. (b) Saturation switching and (d) dynamic emissive intensity

response of Pt-3 in IMPEK-C to O₂/N₂saturation cycles. λ_ex

372 nm, λ_em= 540 nm, 30 °C.

=

For heterogeneous O₂-sensing films, a modified Stern–

Volmer or a two-site model is required to study the quench-

ing effect.^[20,23,24]In the two-site model, the O₂-sensitive

dyes are considered as two different portions. The fraction

of the two portions are defined as f₁and f₂, respectively

(f₁+ f₂= 1); the two portions have different quenching

constants (K_SV1and K_SV2); see Eq. (1).

(1)

With the home-assembled flow cell coupled to a spectro-

fluorimeter, the responses of the sensing films to different

O₂partial pressure were studied (Figure 10). First the O₂-

sensing films of Pt-1 and Pt-3 were tested against the satu-

ration cycle of O₂and N₂(Figure 10, a and b). The films

in N₂exhibit intense emission, but the phosphorescence is

substantially quenched in O₂. Furthermore, fast response

(t_ȇ95) and recovery times (t_Ȇ95) of 2 and 6 s were ob-

served, respectively.^[20,25]The response (t_ȇ95) and recovery

times (t_Ȇ95) are generally accepted as the times for lumines-

cence intensity changes to reach 95% of the whole variation

when switching from 100% N₂to 100% O₂or vice versa.

Pt-2 is also sensitive towards O₂(see the Supporting Infor-

Figure 9. Emission spectra of complex Pt-2 under different partial

pressures of O₂(λ_ex= 372 nm, 12 °C).

We noted that the O₂sensitivity of the emission of Pt-3

towards O₂is lower than that of Pt-2. This is consistent

with the fact that Pt-2 has longer luminescent lifetimes

(Table 1).

Luminescent O₂Sensing of the Pt^IIComplexes in Polymer

Films

For practical applications, the study of sensing films, for mation). Fast response times are usually observed for

example, prepared by the distribution of complexes in sup- porous supporting materials such as MCM-41 molecular

porting polymers, is more meaningful than the study of sieves.^[2,60–64]However, our polymer film preparation ap-

oxygen-sensing in solution. Thus, the O₂-sensing properties proach is straightforward.

of the complexes were also studied in polymer films

Next, the dynamic response of the O₂-sensing films was

tested against small steps of O₂partial pressure variation

(IMPEK-C; Figure 10).^{[20,21,23,25,58,59]}

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Eur. J. Inorg. Chem. 2010, 4683–4696

Triphenylamine-Capped Cyclometallated Platinum(II) Complexes

(Figure 10, c and d). Such a detailed study will reveal the

dynamic O₂partial pressure range and the data can be used

to evaluate O₂sensitivity.^[20,25]We found that the sensing

film of Pt-1 is more sensitive to O₂than Pt-3. For example,

the emission intensity of Pt-1 was quenched by 68.6% un-

der 3.5% O_2.For Pt-3, however, the emission is quenched

by only around 9% in the presence of 5% O₂. To compare

the O₂-sensing properties of the complexes quantitatively,

the O₂-sensing data were fitted to the two-site model (Fig-

ure 11) and the results are summarized in Table 5. We can

clearly see the apparent quenching constants (the sensitivity

app

towards O₂) of the complexes Pt-1 (K_SV= 0.045 Torr^–1)

and Pt-2 (0.030 Torr^–1) are nearly 10-fold higher than that

of the complex of Pt-3 (0.003 Torr^–1). This result shows that

the luminescence lifetime of Pt-3 is shorter than those of

the complexes of Pt-1 and Pt-2, which is supported by the

experimental results (Table 1).

Figure 12. The configuration of the electrophosphorescent OLED

devices and the structures of the compounds used in the devices.

tration. The emission at around 413 nm derives from NPB

{4,4Ј-bis[(1-naphthyl)phenylamino]biphenyl} and increases

in intensity with decreasing doping concentration.

Figure 11. Two-site model plots for the sensing films of Pt-1, Pt-2

and Pt-3 in IMPEK-C. Intensity ratios I/I₀vs. O₂partial pressure.

Table 5. Parameters for the O₂-sensing film of complexes Pt-1, Pt-

2 and Pt-3 with IMPEK-C as the supporting matrix (fitting of the

results to the two site model).

r^2[c]

K_SV

pO₂

[a]

[b]

app[d]

[e]

f₁

f₂

K_SV1

K_SV2

Pt-1 0.7891 0.2109 0.0564 0.0001 1.000 0.04453 22.5

Pt-2 0.5001 0.4999 0.0601 0.0000 0.999 0.03006 33.3

Pt-3 0.2898 0.7102 0.0112 0.0001 1.000 0.00332 301.5

[a] Ratio of the two portions of dyes. [b] Quenching constants of

the two portions. [c] Determination coefficients. [d] Weighted

app

quenching constant, K_SV= f₁K_SV1+ f₂K_SV2. [e] The oxygen par-

tial pressure at which the initial emission intensity of the film is

quenched by 50% and calculated as 1/K_SV. In Torr

Electroluminescent Properties

As a preliminary investigation, the electroluminescence

properties of Pt-2 and Pt-3 were studied by using the com-

plexes as dopants in the emission layer of OLEDs (Fig-

ure 12).^[4]

The EL spectra of the devices based on complexes Pt-2

and Pt-3 at different doping concentration are shown in

Figure 13. For the devices based on Pt-2 (Figure 13, a), the

Figure 13. EL spectra of OLED devices using (a) Pt-2 and (b) Pt-

3 as dopants at different doping concentrations.

For the devices based on Pt-3 (Figure 13, b), the emis-

emission at 548 nm is due to monomer emission and the sion at around 559 nm is assigned to the monomer and the

band at around 590 nm is assigned to excimer emission. The band at around 618 nm is assigned to excimer emission. The

EL spectra of the complexes changed with doping concen- emission at around 430 nm derives from NPB and the in-

tration as a result of excimer emission. The excimer emis- tensity increases with decreasing doping concentration. The

sion increases in intensity with increasing doping concen- emission peak from NPB exhibits a small blueshift with in-

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J. Zhao et al.

FULL PAPER

erly dried or distilled before use in the syntheses and spectroscopic

studies. NMR spectra were recorded with a 400 MHz Varian Unity

Inova spectrometer. Mass spectra were recorded with a Q-TOF

Micro spectrometer. Uv/vis absorption spectra were recorded with

a HP8453 Uv/vis spectrophotometer. Fluorescence spectra were re-

corded with a JASCO FP-6500 or Sanco 970 CRT spectrofluorime-

ter. Luminescent quantum yields were measured with [Ru(bpy)₂-

(phen)] as reference (Φ = 6.0%, in CH₃CN, under deaerated condi-

tions). Luminescent lifetimes were measured with a Horiba Jobin

Yvon Fluoro Max-4 (TCSPC) instrument.

creasing doping concentration due to the absorption of

complex Pt-3. The device based on complex Pt-3 emits pure

white light with CIE coordinates (x, y) of (0.32, 0.32) at the

5 wt.-% doping ratio under 12 V.

The current density–luminance (J–L) and J current effi-

ciency characteristics of the devices based on the complexes

Pt-2 and Pt-3 were also studied (see the Supporting Infor-

mation). The devices based on Pt-2 and Pt-3 show a dif-

ferent dependence on doping concentration. For Pt-2, the

device with 10 wt.-% doping concentration exhibits a much

better performance than that with a 5 wt.-% doping concen-

tration. The 10 wt.-%-doped device has a turn-on voltage

of 6 V and a maximum luminance (L_max) of 1298 Cdm^–2at

a current density of 374 mAcm^–2. The maximum luminance

efficiency is 3.96 CdA^–1at J = 183 mAcm^–2. In contrast,

for Pt-3, the device with 5 wt.-% doping concentration

shows a better performance than that with 10 and 2 wt.-%

doping concentrations. The 5 wt.-%-doped device has a

turn-on voltage of 6 V and a maximum luminance of

1931 Cdm^–2at a current density of 180.4 mAcm^–2. The

maximum luminance efficiency is 1.86 CdA^–1at J =

35 mAcm^–2. This phenomenon suggests that the device

with Pt-3 as the phosphorescent dopant is more prone to

concentration saturation than that with Pt-2.

DFT/TDDFT Calculations: The structures of the complexes were

optimized using density functional theory (DFT) with the B3LYP

functional and 6-31G(d)/LanL2DZ basis set. The vertical exci-

tation energies were calculated with time-dependent DFT

(TDDFT) on the basis of the optimized ground-state geometries.

The 6-31G (d) basis set was employed for C, H, N, O and the

LanL2DZ basis set was used for Pt^II. There are no imaginary fre-

quencies for any of the optimized structures. All these calculations

were performed with the Gaussian 09 software package.^[65]

X-ray Structural Analysis: Single-crystal X-ray diffraction data

were obtained with a Bruker SMART APEX CCD diffractometer

using graphite-monochromated Mo-K_αradiation (λ = 0.71073 Å)

with the SMART and SAINT programs. The data were collected

using the x/2 h scan mode and corrected for Lorentzian and polar-

ization effects during the data reduction using SHELXTL 97 soft-

ware the absorption effect was corrected for as well.^[66]

CCDC-762460 (for Pt-2) and -762459 (for Pt-3) contain the supple-

mentary crystallographic data for this paper. These data can be

obtained free of charge from The Cambridge Crystallographic

Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Conclusions

Triphenylamine (TPA)-substituted [ppyPt(acac)] (ppy =

2-phenylpyridine, acac = acetylacetonato) complexes with

different linkers between the TPA and the ppy moiety have

been prepared. The TPA fragment is either directly cyclo-

metallated (Pt-1) or attached to the ppy ligand through a

C–C single bond (Pt-2) or a α-diketo group (Pt-3). All the

Synthesis of 2-(4Ј-Bromophenyl)pyridine:^[7]At 0 °C, NaNO₂(4.14 g,

0.06 mmol, 8.0  solution in water) was slowly added to a suspen-

sion of p-bromoaniline (5.00 g, 0.03 mmol, 1 equiv.) in concen-

trated HCl (10 mL). The mixture was stirred for 1 h at 0 °C and

complexes show red-shifted room-temperature phosphores- then added dropwise into pyridine (125 mL). The brown solution

was stirred at 40 °C for 4 h and then Na₂CO₃(50.0 g) was added.

The slurry was stirred at the same temperature for a further 18 h.

The mixture was extracted with CH₂Cl₂(3ϫ50 mL) and the com-

bined organic layers were dried with Na₂SO₄and the solvents evap-

orated to dryness. The crude product was purified with column

chromatography (silica gel, toluene/MeOH = 95:5, v/v). An orange

solid was obtained; yield 1.64 g, 23.0%. ¹H NMR (400 MHz,

CDCl₃): δ = 8.68 (d, J = 4.0 Hz, 1 H), 7.86 (d, J = 8.4 Hz, 2 H),

7.77 (t, J = 6.4 Hz, 1 H), 7.73 (d, J = 8.4 Hz, 1 H), 7.58 (d, J =

8.4 Hz, 2 H), 7.25 (t, J = 6.0 Hz, 1 H) ppm. HRMS (EI): calcd. for

cence emission (centred at ca. 530–590 nm) relative to the

parent complex [ppyPt(acac)] (emission at 486 nm).

Through DFT calculations, the red-shifted emission of the

complexes Pt-1 and Pt-2 can be rationalized by their higher

HOMO energies, whereas a lower LUMO energy was found

for Pt-3 (due to the electron trap effect of the α-diketo frag-

ment); in each case there is a reduced HOMO–LUMO en-

ergy gap and red-shifted emission. The phosphorescence of

the complexes show extended luminescence lifetimes (3.0–

5.5 µs) compared with the parent complex [ppyPt(acac)] C₁₁H₈NBr [M]⁺232.9840; found 232.9849.

(τ_phos= 2.6 µs). The complexes were successfully used for

luminescent O₂sensing and the O₂sensitivity can be varied

by 13-fold (Stern–Volmer quenching constants). White light

emission was observed with the OLED fabricated with Pt-

3 with CIE coordinates of (0.32, 0.32). Our study of these

phosphorescent cyclometallated Pt complexes and their lu-

minescent O₂sensing and electroluminescent properties will

be useful in the design of new phosphorescent complexes

for application as luminescent oxygen-sensing or OLED

materials.

Synthesis of 4-Bromotriphenylamine:^[67]Triphenylamine (5.00 g,

20.4 mmol) in DMF (25 mL) was stirred at room temperature and

then N-bromosuccinimide (NBS, 3.60 g, 20.4 mmol) in DMF

(25 mL) was added in small portions. After stirring overnight at

room temp., the reaction mixture was poured into water and ex-

tracted with diethyl ether. The organic layer was dried with anhy-

drous Na₂SO₄. After removing the solvent by evaporation, the

crude white solid was purified by recrystallization from methanol.

A white solid was obtained; yield 3.86 g, 59.5%. ¹H NMR

(400 MHz, CDCl₃): δ = 7.31 (d, J = 8.8 Hz, 2 H), 7.24–7.28 (m, 4

H), 7.02–7.10 (m, 6 H), 6.93 (d, J = 9.2 Hz, 2 H) ppm. HRMS

(EI): calcd. for C₁₈H₁₄NBr [M]⁺323.0310; found 323.0320.

Experimental Section

The chemicals used in the syntheses were analytically pure and were

used as received without further purification. Solvents were prop-

Synthesis of 4-(Diphenylamino)-1-phenylboroic Acid:^[68]A solution

of 4-bromotriphenylamine (6.00 g, 18.5 mmol) in THF (40 mL)

was cooled to –78 °C under Ar. nBuLi (2.5  solution in hexane,

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Eur. J. Inorg. Chem. 2010, 4683–4696

Triphenylamine-Capped Cyclometallated Platinum(II) Complexes

9.0 mL, 22.5 mmol) was added slowly through a syringe. The mix-

ture was stirred for 1 h at –78 °C. Then B(OMe)₃(2.34 g, 2.5 mL,

22.5 mmol) was added quickly through a syringe. The reaction mix-

ture was stirred for a further 2 h before being allowed to warm to

room temperature. Aqueous HCl (5.0%) was added to bring the

pH to 5–6. Then the mixture was extracted with CH₂Cl₂

(3ϫ50 mL). The combined organic phase was washed with water

(100 mL) and brine (100 mL). Then the organic phase was dried

with anhydrous Na₂SO₄and the solvents were removed. A white

J = 7.2 Hz, 2 H) ppm. HRMS (EI): calcd. for C₂₉H₂₂N₂[M]⁺

398.1783; found 398.1789.

Pt-2: Under Ar, L-2 (0.11 g, 0.276 mmol) and [K₂PtCl₄](57.3 mg,

0.138 mmol) were mixed in 2-ethoxyethanol (6 mL) and water

(2 mL) and the suspension was heated at 80 °C for 20 h. After cool-

ing to room temperature, water (20 mL) was added and the yellow

precipitate was washed with water (2ϫ20 mL) and dried in vacuo

in an oven. The precipitate was heated in the presence of Hacac

(41.4 mg, 0.414 mmol) and Na₂CO₃(131.7 mg, 1.24 mmol) in 2-

ethoxyethanol (8 mL) at 100 °C for 20 h. Water (10 mL) was added

and the yellow precipitate was collected and washed with water

(2ϫ10 mL), After drying in vacuo in an oven, the precipitate was

purified by column chromatography (silica gel, CH₂Cl₂/petroleum

ether = 1:1, v/v). A yellow solid was obtained; yield 15.0 mg, 7.9%.

¹H NMR (400 MHz, CDCl₃): δ = 8.99 (d, J = 5.6 Hz, 1 H), 7.78–

7.83 (m, 2 H), 7.60–7.62 (m, 3 H), 7.47 (d, J = 8.0 Hz, 1 H), 7.26–

7.34 (m, 5 H), 7.14 (d, J = 7.6 Hz, 6 H), 7.10 (t, J = 6.0 Hz, 1 H),

7.01 (t, J = 7.6 Hz, 2 H), 5.48 (s, 1 H), 2.18 (s, 3 H), 2.02 (s, 3

H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 185.96, 184.44, 168.34,

148.02, 147.55, 147.40, 143.61, 141.42, 139.27, 138.24, 135.93,

129.45, 128.58, 128.28, 128.18, 124.665, 124.06, 123.55, 123.06,

122.40, 121.08, 118.51, 102.69, 29.92, 28.46 ppm. HRMS

(MALDI-TOF): calcd. for C₃₄H₂₉N₂O₂Pt [M]⁺691.1799; found

691.1816. C₃₄H₂₈N₂O₂Pt·0.25CH₂Cl₂(712.91): calcd. C 57.70, H

4.03, N 3.93; found C 57.76, H 3.94, N 3.77.

1

powder was obtained; yield 4.47 g, 83.0%. H NMR analysis was

performed on the 2,2-dimethylpropane-1,3-diol-protected 4-(di-

1

phenylamino)phenylboronic acid: H NMR (400 MHz, CDCl₃): δ

= 7.64 (d, J = 8.4 Hz, 2 H), 7.22–7.26 (m, 4 H), 7.09 (d, J = 7.6 Hz,

4 H), 7.02–7.04 (m, 4 H), 3.75 (s, 4 H), 1.02 (t, J = 7.6 Hz, 6

H) ppm. HRMS (EI): calcd. for C₂₃H₂₄BNO₂[M]⁺357.1900;

found 357.1909.

N,N-Diphenyl-4-(2-pyridinyl)benzeneamine (L-1):^[69]2-Bromopyr-

idine (0.240 g, 1.53 mmol), triphenyl boronic acid (0.49 g,

1.68 mmol), [Pd(PPh₃)₄] (88.5 mg, 0.086 mmol), K₂CO₃(2.76 g,

2.0  aqueous solution, 10 mL) and toluene (15 mL) were mixed.

The mixture was degassed and heated at refluxed at 90 °C for 20 h

under Ar. After being cooled, the solvent was evaporated under

reduced pressure, the organic product was extracted with CH₂Cl₂,

dried with Na₂SO₄and then evaporated to dryness. The crude

product was purified by column chromatography (silica gel,

CH₂Cl₂/petroleum ether = 2:1, v/v). A light-yellow solid was ob-

tained; yield 0.22 g, 44.0%. ¹H NMR (400 MHz, CDCl₃): δ = 8.63

(d, J = 4.8 Hz, 1 H), 7.85 (d, J = 8.8 Hz, 2 H), 7.70–7.63 (m, 2 H),

7.26 (t, J = 8.4 Hz, 2 H), 7.12–7.15 (m, 6 H), 7.03 (t, J = 7.2 Hz,

2 H) ppm. HRMS (EI): calcd. for C₂₃H₁₈N₂[M]⁺322.1470; found

322.1472.

4-Ethynyl-N,N-diphenylbenzeneamine (Ethynyl-TPA):^[70](Trimeth-

ylsilyl)acetylene (0.66 mL, 4.64 mmol) and CuI (16.0 mg,

0.08 mmol) were added to a degassed solution of 4-bromotri-

phenylamine (1.0 g, 3.09 mmol), [PdCl₂(PPh₃)₂] (125.6 mg,

0.179 mmol) and PPh₃(23.9 mg, 0.091 mmol) in triethylamine

(40 mL) and the reaction mixture was heated at reflux for 12 h.

Then the mixture was poured into water (50 mL) and extracted

with CH₂Cl₂(3ϫ50 mL). The organic layers were dried with

Na₂SO₄and the solvents evaporated to dryness. The crude product

was purified by column chromatography (silica gel, CH₂Cl₂/petro-

leum ether = 1:3, v/v). A yellow oil was obtained (the trimethylsilyl

protected acetylide); yield 621.8 mg, 58.9%. K₂CO₃(1.00 g,

Pt-1:^[4]Under Ar, L-1 (0.17 g, 0.527 mmol) and [K₂PtCl₄]

(109.0 mg, 0.26 mmol) were added to a mixture of 2-ethoxyethanol

(6 mL) and water (2 mL). The suspension was heated at 80 °C for

20 h. After cooling to room temperature, water (20 mL) was added

and the precipitate was filtered and washed with water (2ϫ20 mL)

and dried in vacuo oven at 50 °C for 5 h. The precipitate was

treated with Hacac (79.1 mg, 0.7905 mmol) and Na₂CO₃7.24 mmol) was added to a methanol solution (60 mL) of the yel-

(256.0 mg, 2.37 mmol) in 2-ethoxyethanol (6 mL) at 100 °C for

20 h. The product was purified by column chromatography (silica

gel, CH₂Cl₂/petroleum ether = 1:1, v/v). A yellow solid was ob-

tained; yield 33.0 mg, 10.0%. ¹H NMR (400 MHz, CDCl₃): δ =

8.87 (d, J = 5.6 Hz, 1 H), 7.71 (t, J = 7.2 Hz, 1 H), 7.45 (d, J =

8.0 Hz, 1 H), 7.25–7.29 (m, 6 H), 7. 18–7.22 (m, 4 H), 6.97–7.05

low oil obtained above (621.8 mg, 1.82 mmol) and the mixture was

stirred at room temp. for 24 h. The solvent was evaporated and

water (10 mL) was added and then the mixture was extracted with

CH₂Cl₂. The organic layer was dried and then the crude product

was purified by column chromatography (silica gel, CH₂Cl₂/petro-

leum ether = 1:3, v/v). A yellow solid was obtained; yield 373.5 mg,

1

(m, 3 H), 6.75 (d, J = 8.4 Hz, 1 H), 5.38 (s, 1 H), 1.96 (s, 3 H), 1.01 mmol, 76.2%. H NMR (400 MHz, CDCl₃): δ = 7.32 (d, J =

1.72 (s,

3

H) ppm. HRMS (MALDI-TOF): calcd. for

8.8 Hz, 2 H), 7.27 (t, J = 8.4 Hz, 4 H), 7.04–7.11 (m, 6 H), 6.95 (d,

J = 8.4 Hz, 2 H), 3.01 (s, 1 H) ppm. MS (EI): calcd. for C₂₀H₁₅N

[M]⁺269.1204; found 269.1210.

C₂₈H₂₅N₂O₂Pt [M]⁺615.1486; found 615.1505. C₂₈H₂₄N₂O₂Pt

(616.59): calcd. C 54.63, H 3.93, N 4.55; found C 54.80, H 3.88, N

4.47.

N-{4-[4-(2-Pyridyl)phenylethynyl]phenyl}-N,N-diphenylamine (L-3):

A solution of ethynyl-TPA (269.0 mg, 1.0 mmol) in THF (10 mL)

was added to a degassed solution of 2-(4-bromophenyl)pyridine

4-Diphenylamino-4Ј-(2-pyridyl)biphenyl (L-2): 2-(4Ј-Bromophenyl)-

pyridine (324.0 mg, 1.38 mmol), triphenyl boronic acid (439.0 mg,

1.52 mmol), [Pd(PPh₃)₄] (80.1 mg, 0.078 mmol), aqueous K₂CO₃(234.0 mg, 1.0 mmol), [PdCl₂(PPh₃)₂] (20.6 mg, 0.02 mmol) and

(2.76 g, 2.0 , 10 mL) and toluene (15 mL) were mixed. The mix-

ture was heated at reflux at 85 °C for 24 h under Ar. After being

cooled, the solvent was evaporated and the organic product was

extracted with CH₂Cl₂and dried with Na₂SO₄. The solvent was

removed under reduced pressure and the crude product was puri-

fied by column chromatography (silica gel, ethyl acetate/hexane =

1:6, v/v). A light-yellow solid was obtained; yield 0.12 g, 19.8%. ¹H

NMR (400 MHz, CDCl₃): δ = 8.71 (d, J = 4.4 Hz, 1 H), 8.05 (d,

J = 8.0 Hz, 2 H), 7.77 (s, 2 H), 7.68 (d, J = 8.0 Hz, 2 H), 7.53 (d,

J = 8.4 Hz, 2 H), 7.22–7.30 (m, 5 H), 7.14–7.16 (m, 6 H), 7.04 (t,

PPh₃(27.0 mg, 0.1 mmol) in triethylamine (6 mL). Then CuI

(4.0 mg, 0.02 mmol) was added and the reaction mixture was

heated at reflux for 8 h. The mixture was poured into water (20 mL)

and extracted with CH₂Cl₂(3ϫ30 mL) and then the combined or-

ganic fractions were dried with Na₂SO₄and the solvents evapo-

rated to dryness. The crude product was purified by column

chromatography (silica gel, ethyl acetate/hexane = 1:6, v/v). A light-

yellow solid was obtained; yield 92.0 mg, 21.8%. ¹H NMR

(400 MHz, CDCl₃): δ = 8.70 (d, J = 4.8 Hz, 1 H), 7.99 (d, J =

8.4 Hz, 2 H), 7.78–7.75 (m, 4 H), 7.61 (d, J = 8.4 Hz, 2 H), 7.38

Eur. J. Inorg. Chem. 2010, 4683–4696

www.eurjic.org

4693

J. Zhao et al.

FULL PAPER

(d, J = 8.8 Hz, 2 H), 7.30–7.24 (m, 3 H), 7.11 (d, J = 7.6 Hz, 4 H),

7.07 (t, J = 7.2 Hz, 2 H), 7.01 (d, J = 8.8 Hz, 2 H) ppm. ¹³C NMR

(100 MHz, CDCl₃): δ = 156.52, 149.61, 148.01, 147.15, 138.42,

136.97, 132.57, 131.85, 129.40, 126.76, 125.01,124.33, 123.57,

which 4,4Ј-dicarbazolybiphenyl (CBP) served as the host for metal

complexes, 4,7-diphenyl-1,10-phenanthroline (Bphen) as the hole

blocker and electron-transporting material, 4,4Ј,4ЈЈ-tris(3-methyl-

phenylphenylamino)triphenylamine (m-MTDATA) and 4,4Ј-bis[(1-

122.32, 122.22, 120.59, 115.89, 91.11, 88.55 ppm. HRMS (EI): naphthyl)phenylamino]biphenyl (NPB) as the hole-injecting and

calcd. for C₂₁H₂₂N₂[M]⁺422.1783; found 422.1790.

-transporting materials, respectively. Organic layers were deposited

by vacuum (Ͻ1ϫ10^–3Pa) thermal evaporation on to a clean glass

substrate precoated with an indium/tin oxide (ITO) layer with a

sheet resistance of ca. 20 Ω/. The 30 nm of m-MTDATA, 20 nm of

NPB, 30 nm of CBP doped with Pt complex and 50 nm of Bphen

were sequentially deposited onto the substrate. Finally, the Al cath-

ode was deposited through a shadow mask by thermal evaporation

at 3ϫ10^–3Pa in another vacuum chamber. The active area of the

device was 9 mm². The thicknesses of the organic and metal films

were monitored by a quartz crystal microbalance. The current–volt-

age–brightness (I–V–B) characteristics and spectra of the EL de-

vices were measured with a Keithey 2400 Source meter and a PR-

650 Spectra Colorimeter under ambient conditions.

Pt-3: A mixture of L-3 (92.0 mg, 0.218 mmol) and [K₂PtCl₄]

(45.2 mg, 0.109 mmol) in 2-ethoxyethanol (6 mL) and water (2 mL)

was heated at 80 °C for 20 h. After cooling to room temperature,

the mixture was added to water (20 mL) and the precipitate was

washed with water (4ϫ20 mL) and dried in vacuo in an oven at

50 °C for 5 h. The precipitate was treated with Hacac (0.03 g,

0.30 mmol) in the presence of Na₂CO₃(103.0 mg, 1.00 mmol) in 2-

ethoxyethanol (8 mL) at 100 °C for 20 h to give a residue that was

purified by column chromatography (silica gel, CH₂Cl₂). A yellow

solid was obtained; yield 12.0 mg, 7.7%. ¹H NMR (400 MHz,

CDCl₃): δ = 9.05 (d, J = 5.6 Hz, 1 H), 8.17 (s, 1 H), 7.88 (t, J =

8.8 Hz, 1 H), 7.80 (d, J = 7.2 Hz, 2 H), 7.69 (d, J = 7.6 Hz, 2 H),

7.50 (d, J = 8.0 Hz, 1 H), 7.35–7.31 (m, 5 H), 7.21 (t, J = 6.8 Hz,

1 H), 7.17–7.15 (m, 7 H), 6.97 (d, J = 8.8 Hz, 2 H), 5.47 (s, 1 H),

2.01 (s, 3 H), 1.97 (s, 3 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ

= 196.40, 193.70, 186.09, 184.72, 167.40, 153.59, 150.83, 147.98,

146.25, 138.56, 132.99, 131.92, 131.69, 129.91, 126.57, 125.75,

125.53, 125.39, 122.90, 119.83, 119.34, 102.72, 29.89, 28.36 ppm.

HRMS (MALDI-TOF): calcd. for C₃₆H₂₈N₂O₄Pt [M]⁺747.1697;

Supporting Information (see also the footnote on the first page of

this article): Characterization and calculations for compounds.

Acknowledgments

found 747.1741. C₃₆H₂₈N₂O₄Pt·0.25CHCl₃(777.54): calcd.

56.00, H 3.66, N 3.60; found C 56.16, H 3.60, N 3.53.

C

We thank the National Natural Science Foundation of China

(NSFC) (Grants 20642003, 20634040 and 20972024), the Ministry

of Education, Scientific Research Foundation for the Returned

Overseas Chinese Scholars, the Specialized Research Fund for the

Doctoral Program of Higher Education (Grant 200801410004) and

the New Century Excellent Talents in University (Grant 08-0077),

Changjiang Scholars and Innovative Research Team in University

(PCSIRT) (Grant IRT0711), the State Key Laboratory of Fine

Chemicals (KF0710 and KF0802), the State Key Laboratory of

Chemo/Biosensing and Chemometrics (2008009), the Education

Department of Liaoning Province (2009T015) and the Dalian Uni-

versity of Technology (SFDUT07005 and 1000-893394) for finan-

cial support. We are grateful to the Royal Society (UK) and NSFC

for the ChinaϪUK Cost-Share Science Networks.

1-(4-Diphenylaminophenyl)-4-(2-pyridylphenyl)ethane-1,2-dione (L-

4): L-3 (200.4 mg, 0.47 mmol) in 2-ethoxyethanol (6 mL) was

heated to 80 °C and [K₂PtCl₄] (83.0 mg, 0.2 mmol) in water (2 mL)

was added slowly. Then the reaction mixture was heated at 80 °C

for 20 h. After cooling to room temperature the mixture was added

to water (20 mL) and the precipitate was washed with water

(20 mLϫ4) and dried in vacuo at 50 °C for 5 h. The precipitate

was treated with Hacac (71.0 mg, 0.71 mmol) in the presence of

Na₂CO₃(250.0 mg, 2.36 mmol) in 2-ethoxyethanol (6 mL) at

100 °C for 20 h to give a residue that was purified by column

chromatography (silica gel, ethyl acetate/hexane = 1:5, v/v). A yel-

low solid was obtained; yield 30.0 mg, 20.9%. ¹H NMR (400 MHz,

CDCl₃): δ = 8.74 (d, J = 4.8 Hz, 1 H), 8.15 (d, J = 8.4 Hz, 2 H),

8.10 (d, J = 8.8 Hz, 2 H), 7.78–7.81 (m, 4 H), 7.36–7.29 (m, 5

H), 7.20–7.17 (m, 6 H), 6.98 (d, J = 8.8 Hz, 2 H) ppm. ¹³C NMR

(100 MHz, CDCl₃): δ = 194.94, 192.65, 155.99, 153.89, 150.19,

146.01, 145.107, 137.13, 133.66, 131.89, 130.56, 129.96, 127.49,

126.68, 125.62, 125.07, 123.33, 121.36, 119.03 ppm. MS (ESI):

calcd. for C₃₁H₂₂N₂O₂[M + H]⁺455.1760; found 454.1743.

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Article Doi

Tuning the emission colour of triphenylamine-capped cyclometallated platinum(II) complexes and their application in luminescent oxygen sensing and organic light-emitting diodes

DOI: 10.1002/ejic.201000327

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