Ionic Organic Small Molecules as Hosts for Light-Emitting

Jun 28, 2018 - (1) Ulmann coupling then gave the neutral benzimidazole material,(1) which ... by the presence of a small oxidative backwave at approxi...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Ionic Organic Small Molecules as Hosts for Light-Emitting Electrochemical Cells Matthew D. Moore,† Melanie H. Bowler,‡ Joseph E. Reynolds III,† Vincent M. Lynch,† Yulong Shen,‡ Jason D. Slinker,*,‡ and Jonathan L. Sessler† †

Department of Chemistry, The University of Texas at Austin, 105 East 24th Street, Austin, Texas 78712-1224, United States Department of Physics, The University of Texas at Dallas, 800 West Campbell Road, PHY 36, Richardson, Texas 75080-3021, United States

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‡

S Supporting Information *

ABSTRACT: Light-emitting electrochemical cells (LEECs) from ionic transition-metal complexes (iTMCs) offer the potential for high-efficiency electroluminescence in a simple, single-layer device. However, LEECs typically rely on the use of rare metal complexes. This has limited their cost effectiveness and put constraints on their applicability. With a view to leveraging the efficient emission of these complexes while mitigating costs, we describe here a host/guest LEEC strategy that relies on the use of carbazole (Cz)-based organic small-molecule hosts and iTMC guests. Three cationic host molecules were prepared via the coupling of 1-(4bromophenyl)-2-phenylbenzimidazole (PBI-Br) with Cz. This has allowed a comparison between the hosts bearing methoxy (PBI-CzOMe) and tert-butyl (PBI-CztBu) substituents, as well as an unsubstituted analogue (PBI-CzH). Cyclic voltammetry and UV−visible absorption revealed that all three host materials have wide band gaps characterized by reversible oxidation and irreversible reduction events. On the basis of electronic structure calculations, the host highest occupied molecular orbital (HOMO) resides primarily on the Cz moiety, whereas the lowest unoccupied molecular orbital (LUMO) is located primarily on the phenyl-benzimidazolium unit. Photoluminescence analysis of thin-film blends of PBI-CzH with iTMC guests confirmed that the emission was blue-shifted relative to pristine iTMC films, which is consistent with what was seen in dilute dichloromethane solution. LEEC devices were prepared based on thin films of the pristine hosts, pristine guests, and 90%/10% (w/w) host/guest blends. Among these host/guest blends, LEECs based on PBI-CzH displayed the best performance, particularly when an iridium complex was used as the guest. The system in question yielded a luminance maximum of 624 cd/m2 at an external quantum efficiency of 3.80%. This result stands in contrast to what is seen with typical organic light-emitting diode host studies, where tert-butyl substitution of the host generally leads to a better performance. To rationalize the present observations, the host materials were subject to single-crystal X-ray diffraction analysis. The resulting structures revealed clear head-to-tail interactions in the case of both PBI-CzH and PBI-CzOMe. No such interactions were evident in the case of PBI-CztBu. Furthermore, PBI-CzH showed a relatively smaller spacing between the successive HOMO and successive LUMO levels relative to PBI-CzOMe and PBI-CztBu, a finding consistent with more favorable charge transport and energy transfer. The results presented here can help inform the design and preparation of host materials suitable for use in single-layer iTMC LEECs. KEYWORDS: OLED, LEEC, electroluminescence, crystal structure, host/guest systems, ionic transition-metal complexes

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INTRODUCTION

materials. In LEECs, ionic rearrangement typically assists charge injection and facilitates efficient light emission.5−9 LEECs have been made from polymers, organic small molecules, and ionic transition-metal complexes (iTMC).3,10,11 In iTMC-based LEECs, the metal complexes facilitate ion transport (by virtue of a mobile counterion), electronic transport, and light emission.12−14 LEECs from

Organic light-emitting diodes (OLEDs) have been adopted across a range of display technologies because of their high color purity and wide viewing angles.1 However, the devices are often expensive to fabricate because of multilayer processing. The use of multiple layers is needed to control strictly both charge transport and emission. An alternative to OLEDs are light-emitting electrochemical cells (LEECs).2−4 These devices are advantageous over traditional OLEDs by virtue of a simpler and more robust design in that they require only a single layer and can be made from solution-processable © XXXX American Chemical Society

Received: May 17, 2018 Accepted: June 28, 2018 Published: June 28, 2018 A

DOI: 10.1021/acsami.8b08176 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces iTMCs have shown high efficiencies,15,16 long lifetimes,17,18 a wide range of colors,15,19 and versatility in device architectures and fabrication modes.20−23 Recently, thermally activated delayed fluorescent organic materials24 and copper(I) complexes25−27 have begun to attract attention in the context of efforts being devoted to the development and evaluation of materials for use in LEEC devices.28,29 Nevertheless, at present, the most common emitters used in LEECs are iTMCs based on ruthenium(II) and iridium(III) complexes because they generally provide the most stable and efficient devices.30,31 However, these complexes are relatively expensive and often not capable of supporting prolonged high-energy excited states, a limitation that can lead to emitter degradation and quenching of light emission by charge trap formation.32−35 Self-quenching can also be seen in iTMC LEECs because the excited state often has a significant overlap with the absorption profile.36−38 One way to increase the efficiency of single-layer phosphorescent light-emitting devices is to use a host/guest system. Host/guest systems have been previously used in conjunction with iTMCs, particularly those where one iTMC is doped into another,30,39 or where the iTMC is used as the host in conjunction with a small-molecule guest.40 However, these approaches rely on the use of costly transition metals as host components. In contrast, conventional OLEDs often employ an organic host.41 Organic OLED host materials are usually composed of an electroactive organic polymer or a small molecule that can facilitate electron and hole transport and transfer an exciton to the guest metal complexes. This dilution of the emitter allows for overall device improvement by reducing the self-quenching effects. Generally, this strategy lowers the costs by requiring less of the expensive emissive material. It also allows transfer of the higher-energy excited states to more stable states, which results in longer device lifetimes.42 However, attempts at using host materials developed for OLEDs with iTMCs have yielded mixed results. Low performance in many cases can be traced to the fact that the materials can phase-segregate as the metal complex is ionic and the hosts are neutral.43 Carbazole (Cz)-derived materials are ubiquitous in the electronic device literature.44 The Cz core is relatively easy to functionalize, and its low cost makes it a highly desirable starting material. However, the Cz group is plagued by a relative electrochemical instability and can self-couple through the position para to the nitrogen under oxidative conditions. The addition of alkyl substituents can be used to prevent selfcoupling and tune the energy levels and stability.44 The use of substituted Czs has been shown to improve the device performance.45 This improvement is attributed to an increase in the electrochemical stability, as well as improved thin-filmforming characteristics.46 Here, we report on the preparation and study of three Cz derivatives that were designed to serve as suitable hosts for iTMCs (Figure 1 and Scheme 1). The hosts were designed using a donor Cz moiety with an acceptor benzimidazolium unit so as to permit the transport of both holes and electrons through the device while maintaining its overall ionic character. The Cz subunit was used in its unsubstituted form, along with its dimethoxy and di-tert-butyl derivatives. As detailed below, we have explored the structural, electrochemical, photoluminescent, and electroluminescent properties of these materials and their interactions with iTMCs, particularly in host/guest systems.

Figure 1. Approach used in the present study to create an ionic host/ guest LEEC. A wider band gap blue-emitting ionic organic smallmolecule host is blended with a yellow-emitting iTMC guest. Photographs show the thin-film PL from the host, guest, and host/ guest blend.

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis of the three host materials used in this study is outlined in Scheme 1. Initially, Cz was modified to give CztBu47 and CzOMe48 according to the literature procedures. Three different Cz units were chosen because of their high stability and wide, tunable band gap. Coupling of p-dibromobenzene with 2-phenylbenzimidazole (PBI) gives a brominated coupling partner (PBI-Br) suited for coupling with the Cz derivatives.1 Ulmann coupling then gave the neutral benzimidazole material,1 which was subsequently methylated using methyl iodide in acetonitrile. Ion exchange with potassium hexafluorophosphate ([PF6]) gives the three products shown in Scheme 1 in the form of highly crystalline colorless-to-pale yellow solids. Electronic Properties. The electronic characteristics were studied by cyclic voltammetry (CV) and UV−visible absorption spectroscopy in dichloromethane (DCM). The electrochemical properties were measured by CV as shown in Figure 2. All the three PBI-Cz systems are characterized by reversible oxidation and irreversible reduction features. Oxidation of the electron-rich Cz subunits occurs according to the following order: PBI-CzH (0.87 V vs Fc/Fc+), PBICztBu (0.79 V vs Fc/Fc+), and PBI-CzOMe (0.55 V vs Fc/ Fc+). Such an ordering is expected if the Cz subunits are electrochemically independent of the benzimidazolium unit.48 The reductive wave is attributed to the benzimidazolium unit and seen at approximately −1.84 V versus Fc/Fc+ in all the three systems. This gives an electrochemical band gap of 2.71, 2.63, and 2.40 eV for PBI-CzH, PBI-CztBu, and PBI-CzOMe, respectively. The gap between the oxidation and the reductive backwave (or reduction and oxidative backwave) is often attributed to the structural relaxation of the radical cation (or anion) intermediate produced during the corresponding redox process; however, solvent polarity, concentration, surface interface effects, and experimental conditions can affect this gap. In general, the smaller the backwave gap (also a measure B

DOI: 10.1021/acsami.8b08176 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthesis of the PBI-CzR Compounds Used in This Studya

a

(i) NaH, CuI, 1,10-phenanthroline, and dimethylformamide (73%). (ii) CuI, K2CO3, 18-crown-6, N,N-dimethylpropylurea (R = H (80%), tBu (77%), and OMe (84%)). (iii) MeI and acetonitrile (ACN); KPF6 (>95% for all).

Figure 2. CV curves of PBI-CzH (blue), PBI-CztBu (green), and PBICzOMe (orange). The CVs were recorded in 0.1 M tetrabutylammonium hexafluorophosphate in DCM using a silver0/silver+ reference electrode, a platinum button working electrode, and a platinum coil counter electrode. The voltage values are relative to the Fc/Fc+ redox couple.

Figure 3. Electronic absorption spectra of PBI-CzH (blue), PBICztBu (green), and PBI-CzOMe (orange). The spectra were recorded in DCM at a concentration of 10−4 M.

regions is seen that differs slightly among the three compounds. In addition, slight peak shifts are noted that can be correlated with the electron density of the Cz unit. The corresponding spectral data are given in Table 1. The

of reversibility), the less the structural rearrangement occurs. This is considered favorable in terms of host design because geometric relaxation can reduce the efficiency of charge transfer events. In the present systems, small backwave gaps are seen for the Cz oxidations. Cz is known to undergo a highly reversible oxidation that occurs directly at the nitrogen center. Furthermore, because of the restricted nature of the aryl rings surrounding the nitrogen atom, Cz does not undergo a significant conformational change. This results in little to no relaxation of the material and a small gap between the oxidative events and their reductive backwaves. However, structural rearrangement is not the only cause for such gaps.49 The reduction side of the CV curves for PBI-CzH, PBICztBu, and PBI-CzOMe is characterized by the presence of a small oxidative backwave at approximately 0 V. This backwave is associated with the reduction event observed at 1.84 V for all of the three materials. A significant gap between the reductive and oxidative backwave events can reflect the formation of a long-lived reduced species. The imidazolium derivatives are known to have a vacant p-orbital that can be filled readily with an electron to give a radical species. The driving force for the reoxidation of such species is relatively small, which results in a large gap. The fact that the PBI band gap and redox waves are essentially independent of the Cz substituent provides further support for the proposed spatial separation between the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) in the present host systems. The electronic absorption spectra of PBI-CzH, PBI-CztBu, and PBI-CzOMe are similar, as can be seen from an inspection of Figure 3. However, spectral tailing into the lower energy

Table 1. Electronic Absorption Spectral Data [PBI-CzH] [PF6] [PBI-CztBu] [PF6] [PBI-CzOMe] [PF6] a

λAbs (nm)a

molar absorptivity

232, 285, 321, 333 235, 284, 293, 329, 341 226, 294, 307, 347, 364

39 093 55 926 62 519

Spectra were recorded as dilute DCM solutions.

approximate lowest energy absorption features at 410, 390, and 385 nm are seen for PBI-CzOMe, PBI-CztBu, and PBICzH, respectively. On this basis, the band gaps could be calculated, and are found to be slightly larger than those found electrochemically. No significant peak shifts were observed with increasing concentrations, leading to the conclusion that little appreciable aggregation occurs, even at millimolar concentrations. Theoretical Calculations. Theoretical calculations were performed to determine the spatial distribution of the molecular orbitals and to obtain insights into the lowest energy conformation of the hosts (Figure 4). Using the X-ray crystal structures discussed below and shown in Figure S1 as the starting geometry, the organic materials were optimized at the B3LYP/6-31G* level of theory using the Firefly QC package,50 which is partially based on the GAMESS (US) source code.51 For all of the three molecules, the difference between the optimized structures and the X-ray crystal structures is minimal. The HOMO resides on the Cz moiety C

DOI: 10.1021/acsami.8b08176 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. Spatial distribution of the HOMO (bottom) and the LUMO (top) for each host. The individual images are shown at the same contour and grid values.

and the LUMO is located primarily on the phenyl benzimidazolium unit. Importantly, little significant overlap between the HOMO and LUMO is seen, a finding considered beneficial in the context of charge transfer. The HOMO shows a trend in energy that matches the electrochemical and optical band gaps (PBI-CzH = −7.27 eV, PBI-CztBu = −7.05 eV, and PBI-CzOMe = −6.83 eV), whereas the energetics of the LUMO are mostly independent of the Cz substituents (PBICzH = −4.79 eV, PBI-CztBu = −4.79 eV, and PBI-CzOMe = −4.68 eV), as would be expected from the electrochemical studies discussed above. Luminescent Properties. To investigate the luminescent properties of PBI-CzH, PBI-COMe, and PBI-CztBu and test their viability as hosts, the photoluminescence (PL) spectra were recorded in the solution and in the solid state, as well as for thin-films spin-coated from acetonitrile, both neat and doped with a phosphorescent emitter. The solid-state and thinfilm spectra are shown in Figure 5; the additional spectra and a summary of PL values can be found in Figures S3, S4, and Table S1 of the Supporting Information. No emission was observed in acetonitrile for the three compounds upon excitation at several wavelengths between 280 and 400 nm. However, PBI-CzH and PBI-CztBu gave rise to yellow and orange emissions in DCM, respectively (Figure S4), whereas PBI-CzOMe remained nonemissive in this latter solvent. The solution emission profiles are structureless, broad, and significantly red-shifted with respect to their excitation wavelength. These features are attributed to charge transfer from the Cz to benzimidazolium subunits. The host materials are largely insoluble in nonpolar solvents (ethyl acetate, toluene, diethyl ether, and tetrahydrofuran), soluble in polar organic solvents (acetonitrile, methanol, and acetone), and moderately soluble in chlorinated solvents (DCM and chloroform). Polar solvents tend to increase the lifetime of a charge-transfer event. This extension in lifetime can result in a simultaneous red shift in the emission energy and an increase in accessible nonradiative decay pathways. All three hosts were strongly emissive in the solid state when studied as microcrystalline powders. The excitation profiles were found to be shaped similarly, reflective of the electronic oxidation characteristics. However, the PL emission profiles (shown in Figure 5b) of PBI-CzH (λex = 365 nm) and PBICztBu (λex = 375 nm) were found to overlap significantly at around 455 nm, whereas that of PBI-CzOMe (λex = 375 nm) is characterized by a maximum emission at around 535 nm. The HOMO of the methoxycarbazole unit is significantly higher, which is thought to account for the observed red shift in the

Figure 5. (a) Solution-phase PL spectra of PBI-CzH (blue) and PBICztBu (green). (b) Solid-state PL spectra of powdered crystals of PBI-CzH (blue), PBI-CztBu (green), and PBI-CzOMe (orange). (c) PL spectra of spin-coated films of PBI-CzH-doped 10 w/w % with [Ir(ppy)2(dtb-bpy)][PF6] (yellow) or [Ru(bpy)3][PF6]2 (red).

emission relative to PBI-CzH and PBI-CztBu. Surprisingly, the PL quantum yields (PLQYs) were significantly higher in the solid state than solution for all hosts. The highest PLQY (over 80%) was seen for PBI-CztBu in the form of powder samples. This high PLQY is ascribed to steric shielding provided by the −tBu groups; these substituents are likely to preclude the contact needed for intermolecular charge transfer, which can lead to unfavorable excited-state deactivation. Thin films of the complexes were spin-coated out of acetonitrile solutions onto quartz slides. Each host was deposited as a neat film, as a doped film with 10% (w/w) [Ir(ppy)2(dtb-bpy)][PF6] (where ppy is 2-phenylpyridine and dtb-bpy is 4,4′-di-tert-butyl-2,2′-bipyridine), or as a doped film with 10% (w/w) [Ru(bpy)3][PF6]2 (where bpy is 2,2′bipyridine). The PL emission spectra of neat and doped thin films of PBI-CzH are shown in Figure 5c. The emission profiles of PBI-CzH and PBI-CztBu were again found to be similar, while under these conditions, PBI-CzOMe proved nearly nonemissive. Using only the excitation wavelengths suitable for D

DOI: 10.1021/acsami.8b08176 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces the host materials, the emission profiles were taken of the doped films and compared with neat films of each iTMC (see Figure S5). Upon doping with the Ir(III) complex, there is an apparent blue shift in the emission spectra compared to the neat iTMC film. Notably, the emission profile of the film doped with the Ir(III) complex was found to resemble that seen in solution, which is consistent with effective dilution of the Ir(III) emitter within the host. In accordance with our design expectations, the PLQY of each Ir(III) complex-doped thin film was found to be higher than that of the neat host. Most impressively, when doped with iTMCs, both PBI-CzH and PBI-CztBu exhibit quantum yields (up to 79.6%) higher than those of either neat films of the metal complexes or pure host films. A similar behavior was seen for the ruthenium complexes. For instance, while the film containing [Ru(bpy)3][PF6] alone is only weakly emissive (with a PLQY of 2.9%), the corresponding doped films were highly emissive. The PBI-CzH host doped with 10% [Ru(bpy)3][PF6] displayed the highest PLQY at 32.8%; this corresponds to a >10× increase. Even the PBI-CzOMe-based films showed a higher PLQY when doped with this metal complex. In all cases, the increases in emissive features were considered to reflect efficient energy transfer between the host and the metal complex,52 which competes successfully with potential deactivation pathways. Device Performances. To evaluate the effectiveness of these host materials for LEECs, devices were prepared by blending the hosts into an emissive layer doped at a 10% (w/ w) level with either [Ir(ppy)2(dtb-bpy)][PF6] or [Ru(bpy)3][PF6]2. LEECs of pristine host materials were also studied to evaluate the extent of emission arising from the host alone. A summary of the resulting device performance driven under constant current (500 A/m2) is presented in Figure 6, and a full table of extracted parameters is shown in Table S3 of the Supporting Information. In the case of the devices containing [Ir(ppy)2(dtb-bpy)][PF6] (Figure 6a), the performance of the hosts is clearly distinguishable. Among the three hosts, the [PBI-CzH] host/[Ir(ppy)2(dtb-bpy)] guest device achieved a luminance maximum of 624 cd/m2 at an external quantum efficiency of 3.80%. By way of comparison, Wang et al. recently described a blend of a blue conducting polymer host with a ruthenium iTMC guest.53 The resulting device showed voltage-controlled color profiles ranging from blue to red, a maximum luminance of 364 cd/m2, and a maximum external quantum efficiency of 0.1%.53 Pertegás et al. detailed a Cz host modified with an imidazolium cation.54 The optimized blended devices showed a luminance maximum of 205 cd/m2 with a 0.8% external quantum efficiency maximum. These [PBICzH]/[Ir(ppy)2(dtb-bpy)] host/guest systems are brighter and substantially more efficient than previously reported organic host/iTMC guest devices.53,54 The [PBI-CzOMe]/[Ir(ppy)2(dtb-bpy)] host/guest and [PBI-CztBu]/[Ir(ppy)2(dtb-bpy)] host/guest LEEC luminance values peaked at 12 and 5 cd/m2, respectively. The [PBI-CzH] device was also characterized by a much faster response time relative to the devices constructed using the other two hosts; it reached a luminance maximum in under 2 min, whereas the other host devices required at least 20× longer to reach a corresponding level of peak performance. On the basis of these findings, we conclude that among the three test systems of this study, [PBI-CzH][PF6] constitutes the best host.

Figure 6. (a) Luminance versus time from the onset of emission (>1 cd/m2) for LEECs prepared with the three test hosts of the present study and an [Ir(ppy)2(dtb-bpy)][PF6] guest. The devices were run at 1.5 mA constant current (500 A/m2) with a 10 V compliance. (b) Luminance versus time from the onset of emission (>1 cd/m2) for LEECs prepared with a PBI-CzH host and either no guest (blue), a [Ir(ppy)2(dtb-bpy)][PF6] guest (yellow), or a [Ru(bpy)3][PF6]2 guest (red). The devices were run at 1.5 mA constant current with a 10 V compliance. (c) Electroluminescence spectra for LEECs prepared with a PBI-CzH host and either no guest (blue), an [Ir(ppy)2(dtb-bpy)][PF6] guest (yellow), or a [Ru(bpy)3][PF6]2 guest (red). The spectra were recorded under a 10 V constant bias.

In light of the above, [PBI-CzH] was further investigated as a host using another metal complexes (Figure 6b). It was found that the [PBI-CzH]/[Ru(bpy)3] host/guest device produced a maximum luminance of 359 cd/m2 at a maximum external quantum efficiency of 1.10%. The pristine [PBI-CzH] host was also tested in a device, yielding a maximum luminance of 26 cd/m2, underscoring the benefits of the iTMC guest in these systems. It was found that the emission color of the devices was similar to that of the thin films, as can be seen from the electroluminescence spectra shown in Figure 6c. A blue-green emission was observed for pristine PBI-CzH, a green emission E

DOI: 10.1021/acsami.8b08176 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

arrangements consistent with the π−π orbital overlap expected to support donor−acceptor-type interactions.55 In the solid state, PBI-CzH packs in the form of infinite chains, whereas PBI-CzOMe crystallizes as discreetly packed dimers. In contrast, in the case of PBI-CztBu, no obvious evidence of π−π interactions is seen. Rather, close contacts between the solvent and the Cz unit are observed along with evidence of close contacts (