Negative Charge Management to Make Fragile Bonds Less Fragile toward Electrons for Robust Organic Optoelectronic Materials

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE10 Apr 2021Negative Charge Management to Make Fragile Bonds Less Fragile toward Electrons for Robust Organic Optoelectronic Materials Rui Wang, Qing-Yu Meng, Yi-Lei Wang and Juan Qiao Rui Wang Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 , Qing-Yu Meng Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 , Yi-Lei Wang Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 and Juan Qiao *Corresponding author: E-mail Address: [email protected] Key Lab of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 Center for Flexible Electronics Technology, Tsinghua University, Beijing 100084 https://doi.org/10.31635/ccschem.021.202100778 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail The operational stability of organic (opto)electronic devices largely depends on the intrinsic stability of organic materials on service. For organic light-emitting diode (OLED) materials, a key parameter of their intrinsic stability is the bond-dissociation energy of the most fragile bond (BDEf). Although rarely involved, many OLED molecules have the lowest BDEf in anionic states [BDEf(−) ∼1.6–2.5 eV], which could be a fatal short-slab for device stability. Herein, we separated BDEf(−) from other parameters and confirmed the clear relationship between BDEf(−), intrinsic material stability and device lifetime. Based on thermodynamic principles, we developed a general and effective strategy to greatly improve BDEf(−) by introducing a negative charge manager within the molecule. The manager must combine an electron-withdrawing group (EWG) with a delocalizing structure, so that it can firmly confine the negative charge and hinder the charge redistribution toward fragile bonds. Consequently, the use of this manager can substantially promote BDEf(−) by ∼1 eV for various fragile bonds and outperform the effect reported from solely employing EWGs or delocalizing structures. This effect was verified in typical phosphine-oxide and carbazole derivatives and backed up by newly designed molecules with multiple fragile bonds. This strategy provides a new way to transform vulnerable building blocks into robust organic (opto)electronic materials and devices. Download figure Download PowerPoint Introduction Operational stability is a crucial and common issue for organic (opto)electronic devices.1–4 In particular for organic light-emitting diodes (OLEDs), which have become the popular displays for mobile phones, wearables, televisions (TVs), and virtual reality (VR) headsets in recent years, operational stability is still one of the greatest impediments for large-scale commercialization in next-generation display and lighting technology. The intrinsic degradation of OLEDs is mainly ascribed to the accumulation of the chemical deterioration products of organic (or metal–organic) materials.4–9 Many (photo)physical processes could induce such chemical deterioration, like exciton–polaron and exciton–exciton annihilations (EPA and EEA), in which EPA has been confirmed as a dominant mechanism.10–16 In that process (Figure 1a), one exciton transfers energy to a polaron, generating an excited polaron whose energy can be high enough to break chemical bonds and incur chemical deterioration. In past decades, considerable effort was made to suppress these unwanted (photo)physical processes.10,17–23 However, completely avoiding them at the microscopic level was scarcely possible since even a small amount of deterioration product can result in a significant luminance loss.10,24 Therefore, finding a way to restrain the induced (photo)chemical deterioration is still a critical need. Figure 1 | (a) Schematic of the potential mechanism of EPA-induced bond-dissociation of the negative polaron. The asterisk refers to the excited state. BDEf(n) and BDEf(−) refer to the BDEs of the fragile C–X bonds in neutral and anionic states, respectively. (b) Chemical structures of typical OLED molecules 1–7. The fragile bonds are highlighted by the red markers. (c) Calculated BDEf values of the molecules of interest. All calculations are at the M06-2X/def2-SVP level. Download figure Download PowerPoint According to thermodynamics, the bond most likely to break is the fragile bond with the minimum (or comparable-to-the-minimum) bond-dissociation energy (BDE) in that molecule. Its BDE, denoted as BDEf, has been confirmed as a key parameter for the intrinsic stability of OLED materials by mounting evidence.25–33 In general, the chemical bonds of organic molecules are particularly vulnerable in anionic states. In Figures 1b and 1c, we list the BDE(n), BDE(+), and BDE(−) (n, +, and − refer to neutral, cationic, and anionic states) values of typical fragile exocyclic C–X single bonds (X = heteroatoms like N, P, S, etc.) in several representative OLED molecules. The BDEf(n) and BDEf(+) of most molecules of interest are 3.1–4.8 eV, while most BDEf(−) are only 1.6–2.5 eV. Many OLED molecules have been reported with comparable BDEf values.6,29–32,34,35 As a result, once such negative polarons are generated and/or get involved in EPA, the fragile bonds are apt to dissociate and incur chemical deterioration. Therefore, BDEf(−) would be a fatal short-slab of the intrinsic stability for OLED materials and deserves special attention in the study of the related material and device degradation. The intrinsic degradation of OLED materials in anionic states was first reported by Aziz et al.36 in tris(8-hydroxyquinoline) aluminum (Alq3)-based devices. They found that excessive electrons can induce significant irreversible photoluminescence degradation of the Alq3 layer. For OLED molecules, we first looked at the BDE values of charged states in the study of phosphine-oxide (PO) materials,28 and found that the typical host CzPO2, whose BDEf(−) is only half of its BDEf(n), showed serious C–P bond cleavage in the aging of electron-only devices (EODs). Since then, the notion has been accepted that PO undermines device stability due to its fragile C–P bonds.6,29,35,37 Later, Lee et al.38 found the carbazole (Cz)-based host with higher BDE in charged states contributed to longer device lifetime. Recent further exploration has demonstrated prolonged device lifetime based on the consideration of BDE(−).39,40 Nevertheless, studies on the rational regulation of BDE(−) for organics are mainly about organic halides.41–43 For OLED molecules, Brédas et al.34 recently looked into the effects of typical substituents cyano, fluorine, and hydroxyl on BDE values of the C–N bonds in Cz–dibenzothiophene (DBT) positional isomers. They found that the BDE(−) can be increased by improving the relative electron affinity of the dissociation fragments, so the electron-withdrawing cyano on the DBT moiety increases BDE(−) by more than 0.4 eV. But it is noteworthy that the same cyano on the 3-site of Cz in 2-Cz-DBT does not increase but decrease BDEf(−) by 0.27 eV.34 For OLED molecules with various and multiple fragile bonds, the rational regulation of BDEf(−) is even more complicated and remains largely elusive. A general and effective molecular design strategy toward high BDEf(−) is still needed. Herein, we first conducted comprehensive experiments and theoretical calculations, and revealed how BDEf(−) affects intrinsic material stability and device lifetime. Considering that the PO group is known to have BDEf(−) issues, but are still one of the most popular electron acceptors in high-efficiency hosts,44–46 PO-derivatives are meaningful representatives for studying BDEf(−). In addition, Cz is nearly the most common electron donor in OLED molecules, and the corresponding C–N bonds have demonstrated BDEf(−) issues in some cases,28–32,34,35 so Cz derivatives also deserve thorough study. Thereby, we undertook a systematic theoretical study on typical PO and Cz-derivatives, and developed an effective and general strategy to rationally manage BDEf(−) by introducing a negative charge manager within the molecule. According to fundamental thermodynamics, the manager must combine a strong electron-withdrawing group (EWG) with a delocalizing structure, so that it can firmly confine the negative charge and hinder the charge redistribution toward the fragile bonds. As a consequence, it can substantially improve BDEf(−) irrespective of the types of fragile bonds (often by ∼1 eV) involved, and outperform the reported effect of solely employing strong EWGs or delocalizing structures. That was further validated by the comparisons in several groups of reported newly designed molecules with various and multiple fragile bonds. Importantly, this strategy can enable the original fragile bonds (like the C–P bonds in CzPO2) to turn into stable ones, which, to the best of our knowledge, is rarely seen in the development of OLED materials. Thus, it provides a new strategy for transforming the originally vulnerable building blocks, thus significantly enriching the alternative blocks for the development of robust OLED and other organic (opto)electronic materials. Experimental Methods Laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) measurement The samples of 1,3,5-tri[3-(diphenylphosphoryl)phenyl]benzene (TP3PO) and (1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T) were purchased from commercial resources and were sublimed to guarantee their purity. Measurements were performed with a Shimadzu AXIMA Performance matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) instrument (Shimadzu Corp., Kyoto, Japan) under the negative detection mode. The voltage applied between the target and the TOF aperture was 25 kV. The samples were excited by the pulsed nitrogen laser beam (337 nm) with a spot size of 0.01 mm2. The maximum pulsed laser power was 180 μJ/pulse (at 50 Hz). The sample powder was dissolved by chromatogram class acetone or dichloromethane without assistant matrix and dip-coated on the sample holder. After solvent evaporation, we collected the MS spectra from one selected area to another, along with increasing the laser intensity gradually from 90 to 110 μJ/pulse. Device fabrication and test For all devices, the indium tin oxide (ITO)-coated glass substrates were precleared and treated by UV-ozone for 30 min. The evaporation processes were performed at a pressure under 1 × 10−4 Pa. The deposition rates for organic materials, LiF, and Al were 0.1, 0.01, and 0.3 nm/s, respectively. The electronic characteristics of the devices were measured by a Keithley 2400 SourceMeter (Global Sources, Guangdong, China). The electroluminescence (EL) characteristics of the devices were obtained on a PR650 spectrometer (Inc. of Chatsworth, Chatsworth, CA). Computational details In this work, calculations and analysis were performed with Gaussian 09 (D.01) and Multiwfn (3.7) software.47–49 BDE and electron affinity (EA) values were calculated as the enthalpy changes of the bond cleavage and electron attachment reactions at 298.15 K and 1 atm (gas phase), respectively. The corresponding geometry optimizations and frequency analysis were performed at the density functional theory (DFT) level using the M06-2X functional and def2-SVP basis set. The functional is known to be competent at calculating main-group thermochemistry.50,51 The rationality of using a diffusion function-free basis set to study the variations of BDE(−) and EA values of OLED molecules were referred to in the literature,34 and we also reconfirmed that the BDEf(−) and EA values calculated through def2-SVP basis set retain the same trends as those calculated via the double-ζ basis set with diffuse functions 6-31+G(d,p) ( Supporting Information Figure S1), while the former basis set is more cost-effective. In addition, we compared the BDE values derived from M06-2X/def2-SVP, the common B3LYP/6-31G(d) and the benchmark complete basis set-quadratic Becke3 (CBS-QB3) method, the detailed data are shown in Supporting Information Table S1. In additions to the BDEs, other molecular parameters were calculated to help understand the correlations between the BDEs and molecular structures. Spin density distribution (SDD) is defined as the difference between the α and the β electron densities of each point in space. In negatively charged species, the amount of negative charge allocated on a certain group R (qR) is calculated by the following equation: q R = ∑ i ∈ R ( q i neg − q i neu ) Here, i includes each atom in group R; qineu and qineg are the Hirshfeld charges52 of atom i in the neutral and anionic states of the corresponding structure, respectively. Results and Discussion Comparative studies on TP3PO and PO-T2T To separate the concerned BDEf(−) from other material-related parameters (e.g., exciton energy and thermal stability), we first comparatively studied the intrinsic stability of two representative PO-based electron-transporting materials (ETM), TP3PO and PO-T2T, which have very similar chemical structures (Figure 2a).53,54 The structural similarity leads to many similar molecular parameters ( Supporting Information Figure S2 and Table S2). As for molecular stability, BDEf(n) values in TP3PO and PO-T2T are all close to 4.00 eV for the C–P bonds (more details are in Supporting Information Table S3), while their BDEf(−) values showed large disparity, as low as 2.49 and 2.78 eV for C1–P and C2–P bonds of TP3PO, but as high as 3.43 and 3.60 eV for those of PO-T2T. The low BDEf(−) of TP3PO would cause undesired chemical degradations, while the high BDEf(−) of PO-T2T was enough to afford exciton energies in most OLEDs, which may disburden this material of undesired degradations. Figure 2 | (a) Chemical structures of TP3PO and PO-T2T, the fragile bonds are highlighted by the red markers. (b) Device structures of the EODs (x = 0, 5, 20, or 120). (c) Change of the voltages of EODs during 5 h under a constant current density of 10 mA cm−2. (d) Device structures of the OLEDs (x = 5, 10, 30, or 35). (e) The operation lifetime of the OLEDs measured at a brightness of 500 cd m−2 under a constant current. Download figure Download PowerPoint LDI-TOF-MS tests To validate these speculations, we conducted LDI-TOF-MS tests, which have proved to be powerful to study the chemical degradations of OLED materials.24,27–30 Here, samples were the pure powder of TP3PO and PO-T2T. Under the negative detection mode, the laser intensity was set to increase gradually from 90 to 110 μJ/pulse to track the degradation process (all the MS spectra are shown in Supporting Information Figure S3). Even at the lowest intensity of 90 μJ/pulse, the MS spectrum of TP3PO showed weak molecular and quasi-molecular ion peaks ([M ± H]−, etc.), and fragment peaks [M − POPh2]− corresponding to C2–P bond cleavage with the loss of a POPh2 subunit; but very strong fragment peaks [M − Ph]− and [M[O] − Ph]− corresponded to C1–P bond cleavage with the loss of the phenyl. Herein, [O] refers to an oxygen atom attached to [M − Ph]−, which might come from the tiny amount of residual gas in the LDI-TOF-MS chamber. As the laser intensity increased, the [M ± H]− and [M − POPh2]− almost disappeared, while the [M − Ph]− and [M[O] − Ph]− became very conspicuous. In comparison, at 90 μJ/pulse, the MS spectrum of PO-T2T showed strong molecular ion peaks [M ± H]−, but very weak fragment peaks [M − Ph]− and [M[O] − Ph]− corresponding to C1–P bond cleavage. It was not until the laser intensity exceeded 100 μJ/pulse that these fragment peaks became stronger than the molecular ion peaks. Of note, the fragment peak [M − POPh2]− corresponding to C2–P bond cleavage remained very weak in all the spectra of PO-T2T. The LDI-TOF-MS results accord well with BDE predictions. Specifically, (1) for both TP3PO and PO-T2T, C1–P bonds with lower BDE are more fragile than C2–P bonds. (2) TP3PO with lower BDEf(−) showed poorer material stability than PO-T2T. These results strongly support that BDEf is a key molecular parameter for the intrinsic material stability. Since the calculations and experiments both suggest that C1–P (C1 means the unsubstituted phenyl) are more fragile than C2–P bonds, the following studies on PO-derivatives mainly focus on C1–P bonds. Device degradation experiments We first fabricated TP3PO- and PO-T2T-based EODs with the structure ITO|PO-T2T (120 − x nm)|TP3PO (x nm)|TPBi (20 nm)|LiF (1 nm)|Al (120 nm) (Figure 2b). The current density–voltage curves of the devices were shown in Supporting Information Figure S4. Under electrical stress (10 mA cm−2), we found that the aging of the EODs became more rapid as the thickness of TP3PO increased. For x = 0, 5, 20, and 120, the voltage increases (ΔV) of the EODs after 5 h stress were 0.04, 1.57, 4.41, and 20.52 V, respectively (Figure 2c). This result clearly demonstrates that the voltage rise is strongly related to the bulk of the TP3PO layer, where the TP3PO with much lower BDEf(−) is more prone to degrade and thereby generate defects. Of note, since the EOD does not generate excitons or photons, what energy arouses the degradation remains unclear and needs further study. Next, we fabricated OLEDs with the structure ITO|1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile (HATCN) (10 nm)|N,N-diphenyl-N,N-bis1-naphthyl-1,1-biphenyl-4,4-diamine (NPB) (30 nm)|4,4′,4″-tris(carbazol-9-yl) triphenylamine (TCTA) (15 nm)|3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) (15 nm)|bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO):30 wt % 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9′,9″-diphenyl-9H,9′H,9″H-3,3′:6′,3″-tercarbazole (TCzTrz) (30 nm)|PO-T2T (40 − x nm)|TP3PO (x nm)|4,4′,4″-tris(N-carbazolyl)triphenylamine,1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) (5 nm)|LiF (1 nm)|Al (120 nm) (x = 5, 10, 30, or 35; Figure 2d). Of note, to prevent changing interfaces and energy levels, devices with only TP3PO (or PO-T2T) as the ETM were not fabricated. Chemical structures of the involved organic materials are shown in Supporting Information Figure S5. TCzTrz is a sky-blue emitter developed by Zhang et al.55 All these OLEDs demonstrated close maximum efficiencies (∼11%, Supporting Information Figure S5), which were comparable with reported values. Figure 2e shows their half-lifetime (LT50) at an initial brightness of 500 cd m−2. For the devices x = 5, 10, 30, and 35, LT50 values are 12.0, 11.0, 7.7, and 4.2 h, respectively (Figure 2e). Since the only difference between these devices is the thicknesses of TP3PO and PO-T2T, the results further support that the TP3PO with much lower BDEf(−) has a significant influence on the device lifetime. Even though the bulk of the TP3PO layer is isolated from the emission layer, it is possible that the degradation of TP3PO still involves its excited anion, which may be produced through photoexcitation from light emitted by the device.26,56 To rationally link the macroscopic device degradation with the microscopic bond cleavage, we further calculated energy levels of PO-containing anions resulting from the bond cleavage of TP3PO and PO-T2T. Supporting Information Figure S6 shows that the highest occupied molecular orbitals (HOMOs) of PO-containing anions (−1.72 to −2.20 eV) are much lower than lowest unoccupied molecular orbitals (LUMOs) (−0.79 to −1.27 eV) of the intact molecules. Hence, once generated, those anions would act as the electron and Since TP3PO is more fragile toward the corresponding devices generate many more on the same thus to voltage increase and device lifetime. to this we have demonstrated the close relationship between BDEf(−), intrinsic material stability, and device and revealed that organic materials with lower BDEf(−) would result in poorer material stability and device lifetime. the between TP3PO and PO-T2T demonstrates that with molecular an originally vulnerable group like PO can in robust materials, that has rarely been Therefore, it is to the relationship between BDEf(−) and molecular structure, and to improve BDEf(−). Key influence of BDEf(−) For the C–X (X = N, P, S, etc.) bonds in OLED molecules, have higher so the bond in anionic states result in anions According to an between BDEf(−) and BDEf(n) can be derived BDE ( − ) = BDE ( ) EA − EA and the electron affinity of the and the intact respectively. For all C–X bonds in Figure it is their values eV) that are significantly higher than values eV) that to largely BDEf(−) by 1 eV. 1 how to BDEf(−). The first BDEf(n) mainly depends on the of C–X bond and is rarely by other of the for as by the almost BDEf(n) of the same C–N or C–P bonds in molecules (Figure Although may cause large to the effects on and the electronic of the the are not involved can be increased by introducing for the molecules with only one fragile an way to improve its BDEf(−) is to on the of the C–X which only increases without changing However, many OLED molecules have multiple C–X bonds (like the other molecules in Figure In these on the of one C–X bond may be on the of the which, in most increase and Consequently, such may not improve but even BDEf(−). For the cyano on the 3-site of Cz in 2-Cz-DBT BDEf(−) by 0.27 eV.34 As a result, the regulation of BDE(−) for a largely remains elusive. 1 | of the of BDEf(−). Download figure Download PowerPoint Based on the we that the key to high BDEf(−) is to the of and which an to manage the negative Specifically, the manager it must be to guarantee a high that it can the negative charge in the intact molecule. it must be to the increase of that it have as as possible effect on the charge in the it be stable without fragile bonds in Consequently, we for to use as negative charge to improve BDEf(−) of various fragile bonds for robust OLED molecules. BDEf(−) values of PO-derivatives We first as the molecule. Figure shows that the negative charge in the mainly on its while that of the POPh2 fragment mainly on the of Thus, and are by the typical to increase are introducing strong and delocalizing structures. these would have on due to the of the we designed with stable and strong or cyano or we one of the with and with electron-withdrawing but delocalizing and (Figure showed that the values of all increase by eV, higher than the increase of values Of note, values of were eV than those of In BDEf(−) values of increased to eV, which were higher than that of still lower or comparable with the BDEf(−) of the TP3PO Thus, the substituents not the first and a of is the same substituents at of the atom similar effects ( Supporting Information Table Figure | (a) = and of and (b) Chemical and BDEf(−) values to C1–P of molecules All calculations are at the M06-2X/def2-SVP level. Download figure Download PowerPoint and delocalizing structures can both increase while the has a influence on Consequently, we the two introducing strong as the negative charge manager to further improve BDEf(−). and were designed of in and in increased by eV. the increase of For and both have a while has an increased between the cyano and to a increase eV for and have fragile C–X bonds, so these all the As significantly BDEf(−) to and eV for and (Figure much higher than that of the Therefore, introducing as the negative charge manager has an effect on improving BDEf(−), that of solely introducing strong EWGs or delocalizing structures in For and these increased their BDEf(−) values by ∼1 eV ( Supporting Information Table the TP3PO and PO-T2T involved and as the so their stability difference can be with TP3PO, PO-T2T with as has a significantly but a thus to ∼1 eV for BDEf(−). This further our that the of negative charge In addition, of PO-T2T is 0.3 eV higher than that of that the at the of the all the PO groups to further promote to the much higher BDEf(−) of PO-T2T the negative charge not only improve thermodynamic stability, as by BDEf, but also suppress the process of bond That is confirmed by the between the rapid degradation of and the of C–P bond toward the electron on to of is a negative charge the redistribution become more To potential energy curves and potential are during C1–P bond in TP3PO and PO-T2T Figure shows that the of the two anions are almost the C–P is However, the is of PO-T2T clearly increase than those of In (Figure the C1–P is the negative is mainly allocated the for both molecules Supporting Information Figure at the red in the TP3PO while that in PO-T2T scarcely to the negative charge allocated the changes from to for TP3PO, while in PO-T2T,

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