Benzophenones as Generic Host Materials for ... - ACS Publications

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Benzophenones as Generic Host Materials for Phosphorescent Organic Light-Emitting Diodes Samik Jhulki,† Saona Seth,† Avijit Ghosh,‡ Tahsin J. Chow,*,‡ and Jarugu Narasimha Moorthy*,† †

Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115, Republic of China

‡

S Supporting Information *

ABSTRACT: Despite the fact that benzophenone has traditionally served as a prototype molecular system for establishing triplet state chemistry, materials based on molecular systems containing the benzophenone moiety as an integral part have not been exploited as generic host materials in phosphorescent organic light-emitting diodes (PhOLEDs). We have designed and synthesized three novel host materials, i.e., BP2−BP4, which contain benzophenone as the active triplet sensitizing molecular component. It is shown that their high band gap (3.91−3.93 eV) as well as triplet energies (2.95−2.97 eV) permit their applicability as universal host materials for blue, green, yellow, and red phosphors. While they serve reasonably well for all types of dopants, excellent performance characteristics observed for yellow and green devices are indeed the hallmark of benzophenone-based host materials. For example, maximum external quantum efficiencies of the order of 19.2% and 17.0% were obtained from the devices fabricated with yellow and green phosphors using BP2 as the host material. White light emission, albeit with rather poor efficiencies, has been demonstrated as a proof-of-concept by fabrication of co-doped and stacked devices with blue and yellow phosphors using BP2 as the host material. KEYWORDS: benzophenone, host materials, triplet energy, phosphorescence, PhOLED, band gap and phosphor

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INTRODUCTION Organic light-emitting diodes (OLEDs) have become an important commercial enterprise today due to the advantages they offer in terms of brightness, wide angle viewability, power consumption, and contrast ratio, etc.;1−10 flexibility is an added attribute to their technological repertoire, which is not rivaled by any of the other existing technologies today.11,12 Not surprisingly, the hunt for better and newer OLED materials with improved properties when applied in devices continues unabated. In recent times, there has been a progressive shift from fluorescence- to phosphorescence-based devices in pursuit of high efficiencies.13−17 In the latter, the phosphorescence emittersusually organometallic complexes of Ir, Os, and Pt, etc.allow realization of maximum internal quantum efficiency of 100% by sequestration of emission from both singlet and triplet states via facile spin−orbit coupling.16,17 In general, the high lifetimes of triplet emitters invariably lead to nonradiative deactivation by triplet−triplet annihilation and concentration quenching.16−18 To minimize the latter, the triplet emitters popularly known as dopantsare dispersed in suitable matricespopularly called as host materials.16 Some of the well-known and commercially available dopants with high internal quantum efficiencies are bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic), tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3), bis[2-(2-pyridinylN)phenyl-C](2,4-pentanedionato-O 2 ,O 4 )iridium(III) (Ir© 2015 American Chemical Society

(ppy) 2 acac), acetylacetonatobis(4-phenylthieno[3,2-c]pyridinato-N,C2′)iridium (PO-01), and bis(2-(2′-benzothienyl)-pyridinato-N,C3′)-iridium(acetylacetonate) (Ir(btp)2acac), etc. Insofar as host materials are concerned, there are certain criteria that must be fulfilled for any organic compound to be employed as a host material in OLED devices.16 First, the triplet energy of the host must be higher than that of the guest to prevent reverse energy transfer. Second, the HOMO and LUMO energies of the host should be commensurate with those of the adjacent hole- and electron-transporting materials, respectively, to aid facile charge injection into the host. Third, they should possess high thermal and morphological stabilities. Literature survey reveals that a variety of host materials have indeed been developed based on carbazole,19,20 di-/triarylamines,21,22 benzimidazole,23,24 oxadiazole,25,26 triazole,27 triazine,28 phosphine oxide,29,30 sulfone,31 silane,32 dibenzofuran,33 fluorene,34 and xanthene,34 etc., as triplet sensitizers. Benzophenone is a prototype compound for triplet-excited states in organic photochemistry.35−37 Surprisingly, one observes that the benzophenone moiety has been exploited only in limited instances for creation of OLED materials.38−47 Adachi and co-workers developed benzophenone-based bipolar Received: November 20, 2015 Accepted: December 21, 2015 Published: December 21, 2015 1527

DOI: 10.1021/acsami.5b11232 ACS Appl. Mater. Interfaces 2016, 8, 1527−1535

Research Article

ACS Applied Materials & Interfaces Chart 1. Structures of Electroluminescent Materials Containing the Benzophenone Moietya

a

Functional utilities are indicated in parentheses.

Chart 2. Structures of Benzophenone Derivatives Investigated in This Study

investigation of benzophenone-based materials for application as universal host materials for blue, green, yellow, and red dopants is lacking, and attempts to elucidate their applicability for single host white emission are heretofore unknown to the best our knowledge. In continuation of our de novo approaches to the development of functional organic materials as applied to OLEDs,48−55 we were motivated by the surprisingly limited investigations with benzophenones to explore the latter as generic host materials. Herein, we demonstratebased on the results of a minimum fabrication of devicesthat durene, mesitylene, and bimesitylene functionalized 2-, 3-, and 4-fold with benzophenone, respectively (BP2−BP4, Chart 2) serve as excellent generic host materials for blue, green, yellow, and red phosphors; the sterics in these molecular systems preclude conjugation thereby conserving their high triplet energies. Indeed, characterization of their photophysical properties reveals that all of them possess a high band gap as well as triplet energies. In particular, they are found to be remarkable

materials 1−4 in Chart 1 for thermally activated delayed fluorescence (TADF) in OLEDs.38 Insofar as their utility as host materials is concerned, only a few reports are known.40−47 The seemingly popular benzophenone-based host is 5,40−44 which is constructed from two spirobifluorenes. The extended conjugation in the latter leads to a low triplet energy of 2.62 eV.40 Consequently, it cannot be employed as a host material for blue dopants but has been utilized for green42−44 and red43 dopants, and also as a co-host40,41 with another host material. Benzophenone-based compounds 6 and 7 are bipolar materials reported recently by Kido and co-workers.45−47 The triplet energy of 6, as that of 5, is only 2.61 eV46 and has therefore been applied as a co-host in solution processed devices. The benzophenone 7 functionalized by carbazoles through linkage at meta positions exhibits, however, a higher triplet energy of 2.91 eV.47 While it should in principle be applicable for blue dopants, the authors have investigated its ability to serve as a host material for only the green dopant, i.e., tris[2-(4tolyl)pyridine]iridium ([Ir(mppy)3]).45,47 Clearly, a systematic 1528


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°C, those of BP3 and BP4 were found to be 445 and 466 °C, respectively; cf. Table 1 and Supporting Information Figure S1. Increasing order of thermal stabilities from BP2 to BP4 is clearly a consequence of increasing molecular weight. DSC analysis (Supporting Information Figure S2) indicated no conclusive glass transition temperature (Tg) for BP2 and BP3 but was very conspicuous for BP4 at 122 °C. It is noteworthy that the Tg of BP4 is, in fact, almost double that of CBP (4,4′bis(N-carbazolyl)-1,1′-biphenyl) and mCP (1,3-bis(Ncarbazolyl)benzene), which are the two most popular commercially available hosts with Tgs of 62 and 60 °C, respectively. Both BP2 and BP3 were found to melt at 272 and 291 °C, respectively; BP4, however, indicated no melting phenomenon. These observations are consistent with the fact that BP2 and BP3 are microcrystalline, while BP4 is amorphous in nature; cf. Supporting Information Figure S3. Notably, none of the compounds exhibited any tendency of crystallization, as revealed by the DSC analysis. These molecules are characterized by orthogonal rigid planes, which exhibit difficulty for close packing.56 Electroluminescence Properties. Given the high band gap and triplet energies of benzophenones BP2−BP4, their ability to serve as host materials was investigated by fabrication of doped devices of different configurations. To begin with, the abilities of BP2 and BP3 to function as a host material for the green dopant, i.e., Ir(ppy)3, was probed by fabrication of the following two devices: (G) ITO/NPB (40 nm)/mCP (10 nm)/ BP2/BP3:Ir(ppy)3 (9−10%, 30 nm)/TmPyPB (45 nm)/LiF (2 nm)/Al (150 nm) and (G′) ITO/NPB (40 nm)/mCP (10 nm)/BP2/BP3:Ir(ppy)3 (9−10%, 30 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (2 nm)/Al (150 nm), where N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) and 1,3bis(N-carbazolyl)benzene (mCP) serve as hole-transporting materials, 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) functions as an electron-transporting material, and LiF/Al serves as the composite cathode. It was found that the maximum efficiencies are much better in configuration G, while the maximum luminances are almost comparable (cf. Table 2); the former is presumably a consequence of better matching of the LUMO levels between TmPyPB and that of BP2/BP3 (cf. Figure 2). From these preliminary experiments, it was inferred that the general device configuration, i.e., ITO/NPB (40 nm)/mCP (10 nm)/host:dopant (9−10%, 30 nm)/TmPyPB (45 nm)/LiF (2 nm)/Al (150 nm), could be adopted for other dopants and hosts as well; of course, the latter stems from the fact that the HOMO/LUMO energies of all hosts are similar, which should lead to suitable matching between the energy levels of the applied materials. Thus, devices of configurations B, G, Y, and R were fabricated by variation of dopants of different colors, viz., FIrpic (blue), Ir(ppy)3 (green), PO-01 (yellow), and Ir(btp)2acac (red). The I−V−L characteristics and electroluminescence spectra of the devices B, G, Y, and R are shown in Figures 3 and 4, respectively, and the electroluminescence data are collected in Table 2. As can be perused from Table 2, the turn-on voltages of all the devices are low/moderate, which suggests facile injection of holes and electrons from the respective electrodes and their transport across the layers to cause radiative emission. All benzophenones BP2−BP4 nicely function as host materials for blue, green, yellow, and red dopants, which attests to the fact that they are indeed universal host materials. The device performance results are highly impressive for yellow and green

for sensitization of green and yellow dopants, i.e., Ir(ppy)3 and PO-01, respectively. As a proof-of-concept, their utility for white light emission is demonstrated from devices constructed with BP2 in which the blue and yellow phosphors are either codoped in one layer or doped in two independent layers that are juxtaposed one over the other.

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RESULTS AND DISCUSSION Synthesis. The synthesis of the benzophenone derivatives, i.e., BP2−BP4, was readily accomplished by a straightforward Friedel−Craft benzoylation reaction in carbon disulfide; cf. Scheme 1. The precursor arenes, namely, 1,4-diphenyldurene, Scheme 1. Synthesis of Benzophenone-Functionalized Host Materials

1,3,5-triphenylmesitylene, and 3,3′,5,5′-tetraphenylbimesitylene, were accessed by Suzuki coupling reactions; cf. the Supporting Information. Photophysical Properties. UV−vis and fluorescence spectra of the compounds BP2−BP4 recorded in dilute DCM (ca. 10−5 M) solutions are shown in Figure 1. Insofar as their absorption spectra are concerned, all of them exhibit similar spectral patterns with a λmax at 257 nm. All of the compounds possess very large band gap energies in the range of 3.91−3.93 eV (Table 1), which is important in view of their application as host materials. Fluorescence spectra of the compounds recorded for excitation at 330 nm have similar patterns as well. There exist a superimposable band at ca. 389 nm and two nonsuperimposable bands of different optical intensities at 410 and 430 nm. Triplet energies of benzophenones BP2−BP4 were determined to be in the range of 2.95−2.97 eV from their highest energy 0−0 vibrational energy transitions in the phosphorescence spectra, which were recorded in dilute 2-methyltetrahydrofuran solutions (ca. 10−5 M) at 77 K; cf. Figure 1c. The triplet energies are higher than that of the conventional blue dopant, i.e., FIrpic, which justifies their exploration as universal hosts; vide infra. Thermal Properties. TGA and DSC analyses were carried out to examine thermal properties of BP2−BP4. While the decomposition temperature (Td) for BP2 was found to be 337 1529


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Figure 1. Normalized absorption (a) and emission (b) spectra of BP2−BP4 in DCM for λex = 330 nm. (c) Phosphorescence spectra of BP2−BP4 recorded in 2-methyltetrahydrofuran at 77 K.

Table 1. Photophysical and Thermal Characterization Data of the Benzophenones compound

λmax(UV)a (nm)

band gapb (eV)

λmax(PL)a (nm)

ETc (eV)

HOMOd/LUMOe (eV)

BP2 BP3 BP4

257 257 257

3.91 3.93 3.91

389, 410 389, 410 389, 410

2.97 2.97 2.95

5.80/1.89 5.76/1.83 5.78/1.87

Tgf/ Tmf/ Tdg (°C) h

/272/337 /291/445 122/h/466

h

a Absorption and fluorescence spectra were recorded in dilute DCM solutions (ca. 10−5 M). bBand gap energies were calculated from red edge absorption onset values using the formula E = hc/λ. cTriplet energies were calculated from the 0−0 transitions of phosphorescence spectra recorded in 2-MeTHF at 77 K. dHOMO energies were measured from UPS spectra. eLUMO energies were calculated by subtracting the band gap energies from HOMO energies. fFrom DSC. gFrom TGA. hNot observed.

constructed with BP2 as the host material are higher, the efficiency roll-off is highest for BP2; cf. Supporting Information Figures S7 and S8. In general, the devices fabricated with BP3 and BP4 are more stable with reduced efficiency roll-off. Insofar as red devices are concerned, the performance characteristics are superior for the devices fabricated with BP4 as the host relative to those constructed with BP2 and BP3; cf. Table 2. Concerning the blue devices in which FIrpic serves as the dopant, the performances exhibited by all of the devices are rather poor; cf. Table 2. Clearly, energy transfer is not very efficient when the dopant employed is FIrpic. As mentioned earlier, application of benzophenone-type host materials has so far been limited to green dopants;42−45,47 although the carbazole−benzophenone hybrid, i.e., 7, has significantly higher triplet energy of 2.91 eV,47 its ability to sensitize blue dopant has not been investigated by the authors to preclude comparison of results with BPs. Insofar as the application for green dopants is concerned, devices based on 7 show relatively low efficiency roll-off than those observed by us for BPs; the low efficiency roll-off observed for 7 has been attributed by the

dopants, while they are only reasonable for blue and red dopants. Insofar as the green devices of configuration G are concerned, maximum external quantum efficiency, luminous efficiency, and power efficiency for BP2 as the host were 17.0%, 46.8 cd/A, and 29.1 lm/W, respectively; those for BP3 and BP4 were also very good with values of 15.6 and 12.1%, 45.6 and 37.5 cd/A, and 26.2 and 31.9 lm/W, respectively. Although the maximum efficiencies are better for BP2 and BP3, the maximum luminance was found to be highest for devices constructed using BP4 as the host material with a value of 5020 cd/m2; the maximum luminance for the devices constructed with BP3 was slightly lower with a value of 4940 cd/m2, while those with BP2 produced the lowest intensity light of 3080 cd/ m2. A similar trend was also observed for the yellow devices of configuration Y. The maximum luminous efficiencies for the devices constructed using BP2−BP4 as host materials are 50.6, 50.8, and 30.5 cd/A, respectively. The maximum luminance was again found to be highest for BP4 with an impressive value of 10700 cd/m2. Although the values of maximum efficiencies obtained from the devices of configurations G and Y 1530


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Table 2. Electroluminescence Data of PhOLED Devices Constructed Using Benzophenones as Host Materials

a

B, G, G′, Y, and R refer to device configurations: (B) ITO/NPB (40 nm)/mCP (10 nm)/host:FIrpic (9−10%, 30 nm)/TmPyPB (45 nm)/LiF (2 nm)/Al (150 nm), (G) ITO/NPB (40 nm)/mCP (10 nm)/host:Ir(ppy)3 (9−10%, 30 nm)/TmPyPB (45 nm)/LiF (2 nm)/Al (150 nm), (G′) ITO/NPB (40 nm)/mCP (10 nm)/host:Ir(ppy)3 (9−10%, 30 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (2 nm)/Al (150 nm), (Y) ITO/NPB (40 nm)/mCP (10 nm)/host:PO-01 (9−10%, 30 nm)/TmPyPB (45 nm)/LiF (2 nm)/Al (150 nm), and (R) ITO/NPB (40 nm)/mCP (10 nm)/ host:Ir(btp)2acac (9−10%, 30 nm)/TmPyPB (45 nm)/LiF (2 nm)/Al (150 nm). bTurn-on voltage (V). cMaximum external quantum efficiency (%). dExternal quantum efficiency (%) at a luminance of 100 cd/m2. eExternal quantum efficiency (%) at a luminance of 500 cd/m2. fMaximum power efficiency (lm/W). gMaximum luminous efficiency (cd/A). hMaximum luminance achieved (cd/m2). iλmaxEL (nm). j1931 chromaticity coordinates measured at 8 V.

Figure 2. Energy level diagrams for the devices of configurations G (a) and G′ (b).

Figure 3. Current density vs voltage (a) and luminance vs voltage (b) profiles for the PhOLED devices fabricated with BP2−BP4 with configurations B, G, Y, and R.

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Table 3. Electroluminescence Data of WOLEDs Constructed Using BP2 as a Host Material

a W1−W6 refer to device configurations: (W1) ITO/NPB (40 nm)/ mCP (10 nm)/BP2:FIrpic (10%) + PO-01 (2%) (20 nm)/TmPyPB (40 nm)/LiF (2 nm)/Al (150 nm), (W2) ITO/NPB (40 nm)/mCP (10 nm)/BP2:FIrpic (6%) + PO-01 (1%) (20 nm)/TmPyPB (40 nm)/LiF (2 nm)/Al (150 nm), (W3) ITO/NPB (40 nm)/mCP (10 nm)/BP2:FIrpic (8%) + PO-01 (0.33%) (20 nm)/TmPyPB (40 nm)/ LiF (2 nm)/Al (150 nm), (W4) ITO/NPB (40 nm)/mCP (10 nm)/ BP2:FIrpic (10%, 9 nm)/BP2:PO-01 (6%, 4 nm)/TmPyPB (40 nm)/ LiF (2 nm)/Al (150 nm), (W5) ITO/NPB (40 nm)/mCP (10 nm)/ BP2:FIrpic (10%, 4 nm)/BP2:PO-01 (6%, 6 nm)/TmPyPB (40 nm)/ LiF (2 nm)/Al (150 nm), and (W6) ITO/NPB (40 nm)/mCP (10 nm)/BP2:FIrpic (10%, 2 nm)/BP2:PO-01 (6%, 8 nm)/TmPyPB (40 nm)/LiF (2 nm)/Al (150 nm). bTurn-on voltage (V). cMaximum external quantum efficiency (%). dMaximum power efficiency (lm/W). e Maximum luminous efficiency (cd/A). fMaximum luminance achieved (cd/m2). g1931 chromaticity coordinates measured at 10 V.

Figure 4. EL spectra captured from the devices with configurations B− R fabricated from BP2−BP4. Note that the EL emission profiles in all of the devices constructed from different hosts are similar.

authors to a balanced charge transport as a result of the bipolar nature of the compound.47 Because different dopant and holeand electron-transporting materials than those used by us were employed by the authors, a direct comparison of the outcomes is difficult. Similarly, 5 is also not a suitable host for blue dopants, as mentioned earlier, as its triplet energy is as low as 2.62 eV.40 Although few studies have been reported that describe application of 5 as a standard host material for green dopant to examine (i) the hole-transport property of a newly synthesized amine,44 (ii) optimization of dopant concentrations,43 and (iii) its supportive role as a host for fabrication of white OLEDs,42,46 there are no reports, to the best of our knowledge, that utilize compounds containing benzophenone moiety as a single host for white light emission. Materials with multifarious functions in OLEDs can render the device fabrication easy. In general, white light emission is captured from devices with complex configurations.57 Several fabrication techniques such as “single emissive layer white OELDs”, “tandem white OLEDs”, and “multiple emissive layer white OLEDs”, etc., are known.57 Fabrication of tandem white OLEDs is highly complicated, while the other two techniques are rather common.57,58 In a single emissive layer white OLED device, dopants of different colors dispersed in the same host emit such that the overall emission is white. On the contrary, in a multiple emissive layer white OLED device, several independently doped layers are stacked one over the other; the host for each layer may be the same or different.57−59 Delighted by the ability of BP2−BP4 to function as host systems for all dopants, we were motivated to demonstrate the generic applicability BPs in Chart 2 for capturing white light emission. Toward this end, we sought to integrate yellow and blue dopants in one host matrix to generate white emission. Of course, the difference in efficiencies for blue and yellow emissions is a debilitating factor. We chose BP2 as a representative host material for fabrication of devices to maneuver white emission by two different approaches: (i) codoping method in which yellow emissive PO-01 and blue emissive FIrpic were simultaneously doped in BP2 to yield an overall white emission (devices W1−W3) and (ii) stacking method in which two independent layers of BP2 dispersed with blue and yellow dopants were juxtaposed carefully to elicit emission from both layers such that the overall emission is white (devices W4−W6). The results of the fabricated devices are collected in Table 3. Devices W1−W3 are considered first.

In W1, even at a ratio of 10:2 for FIrpic:PO-01, the outcome is primarily dominated by the yellow dopant, i.e., PO-01; cf. Table 3. As the relative concentration of the blue dopant was further increased and the dopant ratio was changed to 6:1, the emission characteristics were still found to be controlled by PO-01. Presumably, energy transfer from the blue to yellow dopants takes place, thereby diminishing emission efficiency from the blue phosphor, i.e., FIrpic. However, a slight shift of the CIE coordinates toward the blue region was clearly evident (cf. Table 3), which indicated that the amount of blue dopant needed to be further increased. Thus, when the dopant ratio was changed to 8:0.33 in W3, white emission was indeed captured with CIE coordinates of 0.26, 0.37. In the second method, two independent emissive layers (BP2:FIrpic and BP2:PO-01) were juxtaposed with thicknesses carefully maneuvered such that emission occurs from both layers to yield white emission. In this setup, device W6 yielded emission whose CIE coordinates are closest to the pure white emission (0.33, 0.33), although efficiencies of the devices are unremarkable. The I−V−L characteristics and EL spectra of devices W3 and W6 are shown in Figure 5. Notwithstanding rather poor efficiencies of white light emission, it is compellingly evident that benzopheone-based hosts of wide band gap energies permit generic applicability for dopants of variable band gap energies to allow maneuvering of the device fabrications for desired light emission. It is needless to mention that better device performance results necessitate more experimentation with device engineering aspects. Be this as it may, the results obtained with electron-deficient benzophenones are particularly encouraging in light of the fact that universal host materials reported so far are mostly bipolar in nature.60−68 There is little to doubt that the results disclosed herein will pave the way for creation of newer host materials with benzophenone as the active molecular component. 1532


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Figure 5. (a) EL spectra captured from the devices of configurations W3 and W6. (b) I−V−L profiles of the corresponding devices.

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CONCLUSIONS Three novel host materials, i.e., BP2−BP4, functionalized 2-, 3and 4-fold, respectively, with benzophenone as the active triplet sensitizing moiety, were designed and synthesized. Rigid scaffolds, namely, durene, mesitylene, and bimesitylene that constitute the core of BP2−BP4, respectively, were found to impart high thermal stabilities. Twisting as a consequence of sterics built by the methyl groups restricts the conjugation leading to high band gap (3.91−3.93 eV) as well as triplet energies (2.95−2.97 eV), the two salient features that are essential for applicability of any compound to function as host material in PhOLED devices. Indeed, all of the benzophenonefunctionalized compounds, i.e., BP2−BP4, are demonstrated to serve as universal host materials with respectable performances for blue (FIrpic), green (Ir(ppy)3), yellow (PO-01), and red (Ir(btp)2acac) phosphors in simple PhOLED devices. The performance characteristics are shown to be excellent for yellow and green dopants; maximum external quantum efficiencies of the order of 19.2% and 17.0% were obtained from the yellow and green devices fabricated with BP2 as a host material. The potential of this new family of host materials is demonstrated by creating white light through fabrication of co-doped and stacked devices with blue and yellow phosphors. Although observed efficiency of the white light emission is limited, there is little to doubt that more device engineering will pave the way for better performance results.

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ACKNOWLEDGMENTS J.N.M. is thankful to SERB (Sanction Order No. SR/S1/OC84/2010), New Delhi, for generous financial support. S.J. and S.S. thank CSIR, New Delhi, for a senior research fellowship and SPM fellowship, respectively. We acknowledge the optoelectronic device fabrication and testing by the scientific instrument facility at the Institute of Chemistry, Academia Sinica, Taipei.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11232. CV, TGA, DSC, and PXRD profiles, EL and efficiency plots for the devices constructed, and 1H and 13C NMR spectral reproductions of the compounds reported (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(T.J.C.) E-mail: [email protected]. *(J.N.M.) E-mail: [email protected]. Notes

The authors declare no competing financial interest. 1533


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