Supramolecular green phosphorescent polymer iridium complexes for solution-processed nondoped organic light-emitting diodes

Journal of Organometallic Chemistry 804 (2016) 1e5

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Supramolecular green phosphorescent polymer iridium complexes for solution-processed nondoped organic light-emitting diodes Aihui Liang a, *, Gui Huang a, Zhiping Wang a, Wenjin Wu a, Yu Zhong a, Shan Zhao a, Renping Cao b, Shuiliang Chen a, Haoqing Hou a a b

College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, 330022, PR China College of Mathematics and Physics, Jinggangshan University, Ji'an, 343009, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 September 2015 Received in revised form 15 December 2015 Accepted 21 December 2015 Available online 24 December 2015

In order to develop a novel class of solution-processable electroluminescent (EL) emitters, a bis(dibenzylammonium)-tethered host was synthesized and two green emission supramolecualr phosphorescent polymers (SPPs) were formed by utilizing the efficient non-bonding self-assembly between bis(dibenzo-24-crown-8)-functionalized iridium complex and bis(dibenzylammonium)-tethered monomers. The photophysical, thermal and electroluminescent properties of the SPPs were discussed in detail. These SPPs exhibit good thermal properties with high glass-transition temperature (Tg). The photophysical and electroluminescent properties are strongly dependent on the host unit structure of supramolecular polymers. The present work may provide a useful way to afford a new class of highperformance electroluminescent emitters for green emission PLEDs. © 2015 Elsevier B.V. All rights reserved.

Keywords: Supramolecular polymer Iridium complex Phosphorescence Organic light-emitting diode

1. Introduction Solution-processable electrophosphorescent devices that contain transition-metal complexes as promising candidates in practical application such as full-color flat panel or flexible display devices have attracted considerable attention in recent years, as they can harvest both singlet and triplet excitons to realize highly efficient organic light-emitting devices (OLEDs), which can ensure nearly 100% internal quantum efficiency [1e7]. Iridium complexesbased phosphorescent materials are of paramount importance in the field because of their high photoluminscence quantum yields and appropriate excitons lifetimes [8e11]. As we know, the most widely adopted method to obtain solution processed phosphorescent organic light-emitting devices (PhOLEDs) is the dispersion of iridium complexes in a polymer host matrix which not only separates phosphors and avoids self-quenching, but also contributes to charge transport. The doping technique is extremely attractive due to their ease of fabricated by spin-coating or ink-jet printing technologies, and potential applications in large-area flat panel displays [12,13]. But the doped thin films prepared by this method are often subject to phase segregation and concentration quenching that

* Corresponding author. E-mail address: [email protected] (A. Liang). http://dx.doi.org/10.1016/j.jorganchem.2015.12.031 0022-328X/© 2015 Elsevier B.V. All rights reserved.

would deteriorate the device stability and lifetime. In order to overcome these problems, the iridium complexes were incorporated into the polymer backbone [14e16] or grafted to a polymer by a long alkyl side chain [17e19]. Although this strategy can effectively prohibit phase separation, many challenges still exist in developing high performance PhOLEDs. For example, trace of metal catalyst remaining in the polymers would inevitably impede the improvement of device performances [20e22]. In addition, it is hard to define the distribution and the ratio of iridium complex in the polymer that plays an important role in the properties of the resulting PhOLEDs [15,23,24]. Fortunately, the new emerged supramolecular polymers, which are formed from well-defined monomeric units by directional and reversible secondary interaction such as hydrogen bonding [25,26], pep stacking [27], or host-guest interaction [28e30], open up a new avenue towards high-efficiency solution-processable OLEDs. The supramolecular polymers exhibit many advantages such as it is easy to control the ratio of monomers in the alternating copolymer and there is no residual metal catalyst contamination, with respect to conventional polymers, prepared through metal-catalyzed polymerization [28]. In 2009, the self-assembled light-emitting polymers based on UPy-functionalized fluorescent monomers (UPy ¼ 2-ureido-4[1H]-pyrimidinone) were first utilized by Meijer, while the resulting luminous efficiency (LE) was less than 0.1 cd A1 [31]. In 2012, Huang et al. synthesized supramolecular fluorescent

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device performances were discussed. 2. Results and discussion 2.1. Synthesis and characterization The synthesis of dibenzylammonium-tethered co-monomers H2 was presented in Scheme 2. G2 [46] and H1 [33] were reported recently. 2,7-Dibromo-9H-fluorene (1) was coupled with 9-(6bromohexyl)-9H-carbazole (2) by using sodium hydroxide in dimethyl sulfoxide (DMSO) to generate 9,9-bis-(6-carbazol-9ylhexyl)-9H-fluorene-2,7-dibromide (3), which were reacted with N,N-dimethylformamide (DMF) and n-BuLi in tetrahydrofuran (THF) to provide 9,9-bis-(6-carbazol-9-yl-hexyl)-9H-fluorene-2,7dicarbaldehyde (4). According to the synthetic methodology of H1, H2 was synthesized in good yields, starting from compound 4. The SPPs were self-assembled by the host-guest interaction between equimolar dibenzo-24-crown-8-based monomers and bisammonium-based co-monomers at high concentration (Scheme 1). The linear self-assembly was evidenced by viscosity measurements. The double logarithmic plot of specific viscosity versus the monomer concentration in chloroform/acetonitrile was shown in Fig. S1 (See supporting information). In the low concentration ranges, the curves for SPP1 and SPP2 have slopes of 0.16 and 0.09, which is characteristic for non-interacting assemblies of constant size [29]. As the concentration increases, the curves exhibit slopes of 1.30 for SPP1 and 1.06 for SPP2. There are clear changes in slopes at the concentrations of 21 and 29 mMol L1 for SPP1 and SPP2, respectively, which indicate a transition from cyclic species to linear supramolecular polymers. Compared to monomer H2, monomer H1 is more susceptible to form linear supramolecular structure. The viscosity measurement supported the formation of supramolecular polymers.

Scheme 1. Schematic representation of the construction of SPPs from monomers.

polymers by harnessing the efficient non-bonding interaction between functionalized dibenzo-24-crown-8 and dibenzylammonium salt [32]. Later on, orange-, sky-blue- and white-emission supramolecular phosphorescent polymers (SPPs) with iridium complexes were realized by self-assembly between dibenzo-24crown-8 and dibenzylammonium salt [33e35]. It is well recognized that carbazole based compounds are highmobility hole-transporting materials with a tunable and high triplet energy level [36e40]. Moreover, the thermal stability or glassy-state durability can be greatly improved upon incorporation of a carbazole moiety in the organic compounds [41e44]. Organic carbazole based compounds are widely used as the host materials for electrophosphors emitting different colors [10,45]. In order to obtain solution-processed green PhOLEDs, in this contribution, we describe a new bis-ammonium-functionalized host with carbazole pendant and two green emission supramolecular phosphorescent polymers (Scheme 1). The photophysical, thermal and electroluminescent properties of the resulting supramolecular polymers as well as the effects of different hosts on the

O

O

C8H17 C8H17 C8H17 C8H17 C8H17 C8H17

O

O

O

O O

O

PF6

PF6

N H2

N H2

Ir N

2

N

H1

G2

N

N

Br + 1

N

n-BuLi DMF

NaOH Br

N

THF

N C6H12Br

Br

Br

2

OHC

3 N

CHO 4

N

N

N

1)TFA,CH2Cl2

1)phenylmethanamine,toluene 2)NaBH4,methanol

2)NH4PF6

3)(Boc)2O,dichlormethane N Boc

N Boc 5

Scheme 2. Synthetic routes to monomers.

PF6 N H2

PF6 N H2 H2

A. Liang et al. / Journal of Organometallic Chemistry 804 (2016) 1e5

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Table 1 Photophysical and thermal properties of SPPs. 

SPPs

labs (nm)a

lPL (nm)a

FPL (%)a

Tg ( C)

SPP1 SPP2

292, 356 391

420, 527 531

3 14

110.8 154.5

a

Measured in the neat film at room temperature.

Table 2 Electrochemical data. Compounds

Eox(V)

Ered(V)

HOMO(eV)

LUMO(eV)

Eg (eV)

G2 H1 H2

0.80 1.40 1.18

1.89 1.75 1.71

5.20 5.80 5.58

2.51 2.65 2.69

2.69 3.15 2.89

Eox was taken from the onset potential of oxidation. Eg was estimated from the absorption onset of solution UVevis spectra. EHOMO, ELUMO and Ered are calculated from the empirical formula: EHOMO ¼ (4.4þEox), ELUMO ¼ (4.4þ Ered) and Eg ¼ ELUMOEHOMO. Fig. 1. Normalized UVevis absorption of G2 in CH2Cl2, and PL spectra of H1 and H2 in CH3CN.

terfluorenyl units. 2.3. Electrochemical properties

2.2. Photophysical properties The absorption spectra of G2 in CH2Cl2 and photoluminescence (PL) of H1 and H2 in CH3CN are shown in Fig. 1. The iridium complex G2 displays a broad metal to ligand charge transfer (MLCT) absorption band which centering around 385 nm and ranging from 350 nm to 450 nm. The H1 and H2 exhibit the PL spectra peaks at 400 nm and 373 nm, respectively. The PL emission spectrum of H1 and the absorption spectrum of the guest (G2) show little spectroscopic overlap. In contrast, the PL spectrum of H2 overlaps well with the absorption spectrum of G2. This overlap should enable €rster energy transfer from the singlet excited state of the efficient Fo host to the MLCT state of the iridium complex. Therefore, the effi€rster energy transfer from H2 to G2 can be expected [47,48]. cient Fo The absorption and PL spectra of the SPPs in neat films are shown in Fig. 2, and the relevant data are presented in Table 1. The PL spectra of the SPPs show a similar intense emission at about 530 nm, characteristic of the iridium monomer. Nevertheless, in SPP1, the presence of an additional emission peak at ca. 420 nm indicates an incomplete energy transfer from the terfluorenyl to iridium units. On the other hand, the PL quantum efficiencies (ФPL) in neat film are 3% and 14% for SPP1 and SPP2, respectively (Table 1). Compared to SPP2, SPP1 exhibits much lower ФPL, which probably owing to the triplet energy back transfer from iridium to

The electrochemical behaviors of G2, H1 and H2 were studied by cyclic voltammetry in order to estimate the potential charge injection/transport properties of the SPPs (Table 2). The EHOMO of 5.20 eV and 5.80 eV, and the ELUMO of 2.51 eV and 2.65 eV for G2 and H1 are respectively reported by us [33,46]. The reversible oxidation wave of H2 was observed at 1.18 V. The Eg was estimated from the onset of solution absorption spectra. On the basis of the Eg data and the formulas of EHOMO ¼ (Eox þ 4.4) eV, ELUMO ¼ (Ered þ 4.4) eV and Eg ¼ ELUMOEHOMO, EHOMO of H2 is calculated as 5.58 eV and ELUMO is 2.69 eV, respectively [49]. Thus, the iridium complex shows a substantially higher HOMO level by 0.6 and 0.38 eV than H1 and H2, implying that the iridium unit in the SPP1 and SPP2 could be a strong hole trap in OLEDs. 2.4. Thermal properties The thermal properties of the supramolecular polymers were evaluated by the differential scanning calorimetry (DSC) measurements. It shows that these SPPs are inherently amorphous with a glass-transition temperature (Tg) of 110  C for SPP1 and 154  C for SPP2 (Fig. 3 and Table 1), which is comparable to that of the covalently bonded green phosphorescent polymers containing iridium complexes [50]. This amorphous morphology is desirable

Fig. 2. Normalized UVevis absorption (a) and PL spectra (b) of SPP1 and SPP2 in neat film.

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Fig. 3. DSC plots of SPP1 and SPP2.

a phosphorescent emitter. Poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT:PSS) was used as a holeinjection layer. A poly(N-vinylcarbazole) (PVK) layer was used as hole-transporting layer. A 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBI) layer was deposited on the emitting layer as electron-transporting layer. A vacuum-deposited cesium fluoride (CsF) layer was used as an electron-injection layer. The electroluminescent (EL) spectra of SPP2 with emission peak of 516 nm arise solely from the iridium complexes, which is identical to the corresponding solid-state PL emission (Fig. 4). However, the EL spectra of SPP1 are different from the PL spectra, in which the emission peak at 420 nm emitting from H1 is completely quenched. The substantial differences between PL and EL spectra of SPP1 imply different energy transfer mechanisms are involved [47,51]. Under photoexcitation, the singlet excited states are created on host of H1 and then transfer to €rster energy transfer. However, guest of iridium monomer by Fo charge trapping is considered as dominant under electrical excitation, since the iridium monomer is an efficient hole trap demonstrated by the electrochemical experiments. In charge trapping mechanism, an excited dopant molecule is formed by the sequential trapping of a separate hole and electron onto the dopant iridium complex, leading to pure green electrophosphorescent emission. The device performances with the resulting supramolecular polymers as emissive layer are shown in Fig. 5 (the relevant data are listed in Table 3). The device based on SPP1 acquires a turn-on voltage (Von) of 9.7 V, a maximum LE (LEmax) of 1.48 cd A1 and an external quantum efficiency (EQE) of 0.59% at a current density of 0.43 mA cm2. In contrast with SPP1, supramolecular polymer SPP2 shows better device efficiencies with a Von of 9.0 V, LEmax of 1.91 cd A1 and EQE of 0.76%, as the energy levels of G2 and H2 are more suitable. 3. Conclusions

Fig. 4. EL spectra of SPP1 and SPP2.

for electroluminescent purpose. 2.5. Electroluminescent properties As a first attempt, we introduce the SPPs into non-doped PLEDs with a configuration of ITO/PEDOT:PSS/PVK/SPP1‒2/TPBI/CsF/Al as

In summary, two novel supramolecular green phosphorescent polymers for solution processed organic light-emitting diodes were synthesized by self-assembly of dibenzo-24-crown-8 and dibenzylammonium-functionalized monomer units. The molecular structures of the host units affected greatly on the optical, thermal and electroluminescent properties of the resulting SPPs. Remarkably, these SPPs exhibit good thermal properties. This work may open a new avenue to afford highly efficient solution-processable green electroluminescent emitters.

Fig. 5. (a) The luminous efficiencyecurrent density (LEeJ) characteristics and (b) The current density (J)evoltage (V)ebrightness (L) characteristics (JeLeV) of SPP1 and SPP2.

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Table 3 PLED performances. Devices

Vona (V)

Vb (V)

Jb (mA cm2)

EQEb (%)

Lmax (cd m2)

LEmax (cd A1)

SPP1 SPP2

9.7 9.0

12.0 12.2

0.43 0.84

0.59 0.76

530 397

1.48 1.91

a b

Turn-on voltage at a brightness of 1 cd/m2. Device data at LEmax.

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