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

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 filled deep traps, hindering the electron transport and injection. Since TP3PO is more fragile toward electrons, the corresponding devices will generate many more defects on the same time scale, thus leading to bigger voltage increase and shorter device lifetime. Up to this point, we have comprehensively demonstrated the close relationship between BDEf(−), intrinsic material stability, and device lifetime, and revealed that active organic materials with lower BDEf(−) would result in poorer material stability and device lifetime. Notably, the comparison between TP3PO and PO-T2T demonstrates that with appropriate molecular design, an originally vulnerable group like PO can serve in robust materials, although that has rarely been recognized before. Therefore, it is important to show the relationship between BDEf(−) and molecular structure, and establish feasible strategies to improve BDEf(−). Key influence factors of BDEf(−) For the C–X (X = N, P, S, etc.) bonds in OLED molecules, X-sides usually have higher electronegativities, so the bond dissociations in anionic states usually result in anions containing X. According to Hess’s Law, an equation between BDEf(−) and BDEf(n) can be derived (Scheme 1), BDE f ( − ) = BDE f ( n ) + EA M − EA X (1)where EAX and EAM represent the electron affinity of the X radical and the intact molecule, respectively. For all C–X bonds in Figure 1b, it is their EAX values (2–2.6 eV) that are significantly higher than EAM values (0.3–1.5 eV) that lead to largely reduced BDEf(−) by over 1 eV. Most importantly, eq 1 suggests how to regulate BDEf(−). The first term BDEf(n) mainly depends on the type of C–X bond and is rarely influenced by other parts of the molecule except for ortho-substituents, as evidenced by the almost identical BDEf(n) of the same C–N or C–P bonds in molecules 4–7 (Figure 1c). Although ortho-substituents may cause large influences (up to 0.5 eV), the effects depend on case-specific spatial factors and the electronic character of the substituents.33 So, the ortho-effects are not involved here. EAM can certainly be increased by introducing EWGs; therefore, for the molecules with only one fragile bond, an ideal way to improve its BDEf(−) is to add EWG on the C-side of the C–X bond, which only increases EAM without changing EAX. However, many OLED molecules have multiple C–X bonds (like the other molecules in Figure 1b). In these cases, substitutions on the C-side of one C–X bond may be right on the X-sides of the others, which, in most cases, will simultaneously increase EAM and EAX. Consequently, such substitutions may not improve but even impair BDEf(−). For instance, the cyano on the 3-site of Cz in 2-Cz-DBT decreases BDEf(−) by 0.27 eV.34 As a result, the regulation of BDE(−) for a given molecule largely remains elusive. Scheme 1 | Derivation of the calculation formula of BDEf(−). Download figure Download PowerPoint Based on the foregoing discussion, we infer that the key to obtain high BDEf(−) is to control the variation of EAM and EAX simultaneously, which requires an appropriate EWG to manage the negative charge. Specifically, the manager should meet three primary criteria. First, it must be able to guarantee a high EAM; that is, it can successfully stabilize the negative charge in the intact molecule. Second, it must be able to control the increase of EAX; that is, it should have as little as possible effect on stabilizing the charge in the fragment. Third, it should be sufficiently stable without any fragile bonds in itself. Consequently, we next screened for suitable EWG to use as negative charge managers to improve BDEf(−) of various fragile bonds for robust OLED molecules. Improving BDEf(−) values of PO-derivatives We first took POPh3 (P1) as the parent molecule. Figure 3a shows that the negative charge in the POPh3 anion mainly distributes on its delocalized LUMO, while that of the POPh2 fragment anion mainly locates on the sp3-orbital of P. Thus, EAM and EAX are basically determined by the distinct orbitals. Two typical ways to increase EAM are introducing strong EWG and delocalizing structures. Meanwhile, these ways would have smaller impact on EAX due to the localization of the sp3-orbital. Accordingly, we designed P2–P4 with stable and strong EWGs, including trifluoromethyl (P2) or cyano (P3), or we replaced one of the phenyls with 1,3,5-triazine (P4); and P5–P6 with relatively weaker electron-withdrawing character but delocalizing structures, including pyridine (P5) and [1,1′:3′,1″-terphenyl]-5′-yl (PTP) (P6) (Figure 3b). Calculations showed that the EAM values of P2–P6 all increase by 0.6–1 eV, higher than the increase of EAX values (≤0.45 eV). Of note, EAX values of P5–P6 were ∼0.2 eV smaller than those of P2–P4. In total, BDEf(−) values of P2–P6 increased to 2.15–2.55 eV, which were considerably higher than that of P1 (1.87 eV), yet still lower or comparable with the BDEf(−) of the unstable TP3PO (2.49 eV). Thus, the substituents above did not satisfy the first criterion, and a greater enhancement of EAM is required. Incidentally, the same substituents at meta-positions of the P atom yield similar effects ( Supporting Information Table S4). Figure 3 | (a) Frontier orbital (isovalue = 0.03 au) and ESP maps of POPh3 and POPh2. (b) Chemical structures, EAM, EAX, and BDEf(−) values (with respect to C1–P bond) of molecules P1–P8. All calculations are at the M06-2X/def2-SVP level. Download figure Download PowerPoint Strong EWG and delocalizing structures can both increase EAM, while the latter has a smaller influence on EAX. Consequently, we combined the two methods, namely introducing delocalized strong EWG (D-EWG) as the negative charge manager to further improve BDEf(−). P7 and P8 were designed accordingly. Indeed, introduction of benzonitrile in P7 and 4,6-diphenyl-1,3,5-triazin-2-yl (Trz) in P8 remarkably increased EAM by over 1.2 eV. Moreover, they suppressed the increase of EAX. For instance, P3 and P7 both have a cyano; while P7 has an increased distance between the cyano and P atom, leading to a smaller EAX increase (0.32 vs 0.45 eV for P7 and P3). Meanwhile, neither benzonitrile nor Trz have fragile C–X bonds, so these D-EWGs met all the required conditions above. As anticipated, they significantly improved BDEf(−) to 2.73 and 3.04 eV for P7 and P8 (Figure 3b), much higher than that of the unstable TP3PO. Therefore, introducing D-EWG as the negative charge manager has an extraordinary effect on improving BDEf(−), outperforming that of solely introducing strong EWGs or delocalizing structures in P2–P6. For aryl amide and sulfone derivatives, these managers likewise increased their BDEf(−) values by ∼1 eV ( Supporting Information Table S5). Notably, the aforementioned TP3PO and PO-T2T involved P6 and P8 as the central substructures, respectively; so their stability difference can be likewise rationalized. Compared with TP3PO, PO-T2T with Trz as D-EWG has a significantly improved EAM (1.58 vs 0.77 eV), but a slightly decreased EAX (2.07 vs 2.22 eV), thus leading to ∼1 eV improvement for BDEf(−). This further supports our claim that D-EWGs meet the criteria of negative charge managers well. In addition, EAM of PO-T2T is 0.3 eV higher than that of P8 (1.58 vs 1.28 eV), demonstrating that the D-EWG Trz at the center of the molecule makes all the interconnected PO groups to further promote EAM, leading to the much higher BDEf(−) of PO-T2T (3.43 vs 3.04 eV). Moreover, the negative charge managers not only improve thermodynamic stability, as reflected by BDEf, but also suppress the kinetic process of bond cleavages. That is confirmed by the comparison between the rapid degradation of TP3PO-only EOD, and the robustness of PO-T2T-only EOD. During C–P bond cleavages toward electrons, the electron on LUMO redistributes to sp3-orbital of P. When there is a negative charge manager, the redistribution will become more difficult. To visualize this, potential energy curves (PECs) and electrostatic potential (ESP) maps are plotted during C1–P bond cleavages in TP3PO and PO-T2T anions. Figure 4a shows that the PECs of the two anions are almost identical when the C–P distance is ∼1.8 Å. However, when the distance is over 2.6 Å, PECs of PO-T2T clearly increase faster than those of TP3PO. In ESP maps (Figure 4b), when the C1–P distance is 1.8 Å, the negative ESP (in red) is mainly allocated around the center for both molecules (also see Supporting Information Figure S7). While at 2.6 Å, the red color in the central TP3PO becomes fainter; while that in PO-T2T scarcely changes. From 1.8 to 2.6 Å, the negative charge allocated around the central aryl (qAr) changes from −0.76 to −0.60 for TP3PO, while in PO-T2T, qA