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Effects of Ortho-linkages on the Molecular Stability of Organic Light-Emitting Diode Materials Rui Wang, Yi-Lei Wang, Na Lin, Ruoyun Zhang, Lian Duan, and Juan Qiao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03142 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on December 3, 2018

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Chemistry of Materials

Effects of Ortho-linkages on the Molecular Stability of Organic LightEmitting Diode Materials Rui Wang,† Yi-Lei Wang,† Na Lin,†,‡ Ruoyun Zhang,† Lian Duan,† and Juan Qiao*,† †Key

Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China ‡Institute

of Materials, China Academy of Engineering Physics, Jiangyou 621907, China

ABSTRACT: Ortho-linkages are effective in improving the performance of organic light-emitting diode (OLED) materials but may weaken the local chemical bonds and even incur chemical degradation due to bond cleavage reactions in operating OLEDs, especially for blue-emitting materials. Here, we conducted a systematic theoretical study on a series of carbazole (Cz)-based molecules with typical donor (D)-π-acceptor (A) structures to investigate the influence of ortho-substituents on the bond dissociation energies (BDEf) of their fragile D-π bonds (i.e., C-N bonds), where ‘fragile’ means that the bonds have the minimum or a comparable-to-the-minimum BDE in the molecule. Through calculations, it was found that different substituents or even the same substituent on different sites of the molecule could bring variable effects on the polar D-π bond. Specifically, in the π-group, the effects of a substituent on BDEf mainly depend on the steric hindrance that it brings to the D-π bond. Thus, ortho-CH3 groups hardly affect BDEf, while ortho-phenyl and Cz groups both decrease the BDEf by more than 0.3 eV. Ortho-CH3 and phenyl groups scarcely affect the singlet excitation energies [E(S1)] of the molecule, while ortho-Cz groups decrease E(S1). Non-ortho-substituents on the π-group can freely tune E(S1) with little effect on BDEf. On the donor group (Cz), substituents at 1,8-positions usually decrease BDEf. On 3,6-positions, electron-donating groups can decrease both the BDEf and E(S1), while electron-withdrawing groups can increase both. Importantly, a positive correlation between the BDEf and the Hirshfeld charge of the N atom (qN) of the C-N bond is found, which provides a convenient way to predict the BDEf by the easily acquired qN. According to these results, several feasible strategies are proposed to manage BDEf and E(S1) for the design of robust OLED materials.

1. INTRODUCTION Organic light-emitting diodes (OLEDs) have achieved great advances since development and have become the display of choice for mobile phones, wearables, TVs and VR headsets in recent years. From conventional fluorescent materials in early ages to phosphorescent organic metal complexes and today's popular thermally activated delayed fluorescent (TADF) materials, the efficiency of OLEDs throughout the whole visible region has improved substantially.1-4 However, the issue of the lifetime of blue OLEDs remains one of the key limiting factors for the wide commercialization of OLEDs,5-8 owing to the large amount of energy (usually ≥2.7 eV) stored in the excited states. In general, it has been known that intrinsic device degradation is the result of the chemical deterioration of organic or metal−organic materials in operating OLEDs.5−11 For blue phosphorescent OLEDs, annihilations between excitons and polarons (especially triplet-polaron annihilations) have been proposed to be the dominant mechanism of chemical deterioration.6,11−13 They can generate energetically hot states (>6.0 eV), thus inducing bond dissociations.13 In blue TADF-OLEDs, in addition to

the annihilation processes above,10,14,15 synergistic degradation processes of photo- and electro-oxidations have been proposed, which can efficiently decompose high-energy triplets.16 Considerable investigations have also demonstrated in experiments and/or theoretical calculations direct unimolecular dissociations in excited or charged states for various OLED materials, including hosts, emitters, and charge-transporting materials.5−10,17−23 Each of these mechanisms could be the main degradation source of operating OLEDs under certain circumstances.11,15−22 Generally, when these mechanisms incur bond dissociations, the initial step would often be a bond-rupture process, followed by complex reactions with surrounding molecules.5−9,17−23 In the view of thermodynamics, the bond dissociation energy (BDE) is the key parameter in the bond rupture process. Of particular concern are weak bonds which have the minimum or a comparable-to-the-minimum BDE in a molecule. We define these weak bonds as potentially ‘fragile bonds’ and denote their BDEs as ‘BDEf’ for discussion. For instance, 4,4’,4”-tri(Ncarbazolyl)triphenylamine (TCTA) has two types of exocyclic C−N bonds, C(sp2)−N(sp3) bonds from the

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triphenylamine subunit and C(sp2)−N(sp2) bonds between the carbazole groups (Cz) and the phenyl groups (Ph). In blue OLEDs employing TCTA as the host material, the degradation of TCTA was found to be mainly due to the cleavage of C(sp2)−N(sp3) bonds rather than C(sp2)−N(sp2) bonds,5−7,23 which can be rationalized by the lower BDEs of C(sp2)−N(sp3) bonds (~3.2 eV vs. ~3.5 eV of C(sp2)−N(sp2) bonds, calculated by Lee et al.6). In our previous work,9 we comparatively investigated the molecular stability of several donor (D)-π-acceptor (A) molecules with similar excitation energies for blue OLEDs, which contain typical A moieties like sulfonyl (SO), phosphine-oxide (PO), etc. In UV degradation experiments, molecules with lower BDEf values exhibited poorer stability, especially for those containing PO and SO subunits whose BDEf values are even lower than their excitation energies. These examples have clearly shown that the fragile bonds are the shortcoming in the molecular stability. Accordingly, the BDEf could be an important factor in evaluating the molecular stability of OLED materials. Currently, in the development of new D-π-A molecules—in particular, for blue OLED materials— potentially unstable acceptors such as PO and SO are gradually being replaced by stable acceptors such as cyano and triazine groups, which do not bring extra weak bonds.9,24−29 To further avoid the formation of weak bonds, linking tactics with different subunits should be considered, as well, especially for ortho-linkages, which could allow stable subunits to form rather weak bonds in some cases. Not coincidentally, ortho-substituents are widely used in OLED molecules to enhance the performance, particularly for TADF materials.24,26−28,30−35 They can efficiently suppress the overlap between HOMO and LUMO, which can decrease the energy gap between the S1 and T1 (ΔEST) of TADF molecules.4,30−34 Furthermore, ortho-substituents can cause rigid structures, which can restrain intramolecular motions and structural relaxations upon excitation, thereby improving the efficiency and color purity of the emitters.34,35 However, when orthosubstituents cause considerable distortion of the molecule, local chemical bonds might be stretched, and the conjugation might be suppressed; consequently, the BDEs of local bonds would be reduced. Thus, it is imperative to study the effects of ortho-linkages on the molecular stability of OLED materials. To date, precise determination of the BDE is still very formidable.36,37 Elucidation of the degradation products of OLED molecules through experiments is complicated and challenging.5−8,23,38 Under this circumstance, quantum chemical calculations have become a very efficacious tool for estimating BDEs and identifying the fragile parts of OLED molecules.5−10,17−19 In this work, we conducted quantum chemical calculations to investigate the influence of ortho-substituents on BDEf values of organic molecules with typical D-π-A structures. Considering that D-π bonds are usually weak exocyclic C−N single bonds5−10,17−23 and that the most widely used D and π subunits are Cz and Ph groups,4−6,24−35,39,40 our research was mainly based on the

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derivatives of 9-phenylcarbazole (9-PhCz). We studied the influences of various typical substituents on BDEf values according to their positions with respect to the fragile D(Cz)-π bond. The effects of non-ortho-substituents on the π-side were first studied through the molecules in Figure 1, group A, which contains several non-TADF molecules, including 4,4’-bis(9H-carbazol-9-yl)biphenyl (CBP); 1,3-bis(9H-carbazol-9-yl)benzene (mCP); 9,9’-(5(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(9Hcarbazole) (DCzTrz); and bis(4-(9H-carbazol-9yl)phenyl)methanone (Cz2BP). Then, several TADF molecules (mostly blue emitters) with ortho-connections on the π-side were studied, as shown in Figure 1, group B, which contains several representative TADF molecules including 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN); 2,3,4,5,6-penta-(9H-carbazol-9-yl)benzonitrile (5CzBN); 4,5-di(9H-carbazol-9-yl)-phthalonitrile (2CzPN); and 9,9’,9’’-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzene1,2,3-triyl)tris-(9H-carbazole) (TCzTrz). After finding that non-ortho substituents on the π-side have negligible effects on BDEf values, we designed a couple of model molecules with substituted Cz groups to study the effects of substituents on the D-side. The acceptors were omitted to simplify the calculations, as shown in Figure 1, groups C, D, E, and G. The excitation energies of the molecules in Figure 1, group A, B, and F were also calculated. Based on these results and findings, it is convenient to evaluate the specific effects of a substituent on BDEf values and excitation energies. Therefore, this work would provide feasible strategies to manage the BDEf and excitation energies of molecules for the rational design of new organic materials with desirable chemical stability.

2. COMPUTATIONAL METHODS Calculations and analysis were performed with the Gaussian 09 software suite41 and Multiwfn42,43. The density functional theory (DFT) with the widely used B3LYP44,45 functional and the 6-31G(d) basis set was chosen to perform geometry optimization and frequency analysis. Many works have pointed out that C−N bonds in Czbased molecules are unstable toward excitons and may undergo homolysis when the corresponding molecules are excited and/or exciton-exciton annihilations (EEA) occur.5−10,15,17−23,46 Thus, exciton-localized homolysis and EEA can be critical pathways for the degradation of these materials. In particular, for hosts and emitters that suffer continuous excitations and de-excitations, these excitonbased degradation pathways are almost impossible to avoid. Under this circumstance, this work focused on studying the effects of different substituents on the homolytic cleavage of the C−N bond. The BDE was calculated as the enthalpy change in the corresponding bond homolysis reaction of the molecule in the gas phase at 298.15 K and 1 atm. Through comparative calculations (the detailed data are shown in Figure S1, S2, and Table S1), it was found that the variation tendency of the B3LYP/631G(d) BDEs was in good agreement with the experimental data from iBonD47 and those obtained using the larger 2


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basis set 6-311+G(d,p) and the M06-2X functional. The latter functional is believed to be precise for thermal properties.48 Recently, Brédas et al. confirmed the reliability of the B3LYP/6-31G(d) level for calculating the relative BDEs of OLED molecules.49 To understand the effects of substituents on BDEs, the Hirshfeld charges of certain atoms and subunits were calculated with Multiwfn. In addition to the BDEs and Hirshfeld charges, the 0−0 transition energies of S1 states [E0−0(S1)] of the molecules were also calculated because S1 states are believed to be the initial states for most photochemical processes, according to Kasha’s rule.50 Calculations of E0−0(S1) referred to the method established by Adachi et al.,51,52 known as the qOHF method, which is based on time-dependent DFT (TDDFT) calculations. The Becke and Lee-Yang-Parr functionals were used as the exchange and correlation functional. The optimal percentage of Hartree−Fock exchange functional (OHF%) for the calculation of each molecule was determined based on the amount of charge (q) transferred from donors to acceptors. The OHF% values of the corresponding molecules are listed in Table S2, with most values at approximately 30−40%. For

comparison, the excitation energies of several molecules were also calculated by range-separated functionals ωB97XD and LC-ωPBE (see Table S2). It was found that the values obtained from q-OHF are closer to the experimental values, which accords well with the related literatures.51−53

3. RESULTS AND DISCUSSION In many D-π-A OLED molecules, acceptors are not adjacent to donors. In this way, acceptors are often nonortho-substituents on the π-side of D-π bonds. It is necessary to study whether non-ortho-substituents on the π-side have effects on the BDEf. Accordingly, we first evaluated the effects of non-ortho-substituents on the πside, then the effects of ortho-substituents on the π-side, and finally the effects of the substituents on the D-side, specifically including electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) on the 3,6-, 2,7, 4-, or 1,8-positions of Cz. Based on the results, we attempted to find the correlation between the BDEf values and the characteristics of the substituents. The effects of the substituents on the excitation energies in some typical OLED molecules were also investigated.

Figure 1. Chemical structures of the molecules of interest. These molecules are divided into several groups according to the substituted positions with respect to D-π bonds. BDEf values of the fragile D-π bonds (those labeled in red) were calculated.

3



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Table 1. B3LYP/6-31G(d) Absolute (BDEf) and Relative (ΔBDEf) Bond Dissociation Energies of the Fragile D-π Bonds, Dihedral Angles Between Carbazoles and the Phenyl Groups ( ∠ D-π), and 0−0 Transition Energies [E 0−0 (S 1 )] of Carbazole-Based Molecules with Substituents on the π-Side E0-0(S1) (eV) molecule 9-PhCz

group A

BDEf (eV) 3.54

ΔBDEf (eV) a 0

∠D-π (°) 56

calcd 3.59

exptl b 3.58

Ref. 51

CBP

A

3.55

0.01

55

3.60

3.54

51

mCP

A

3.50

−0.04

54

3.26

3.57

Figure S3

DCzTrz

A

3.50

−0.04

54

3.17

3.22

25

Cz2BP

A

3.54

0

51

3.30

3.18

55

p-CzTrz

B

3.55

0.01

52

3.31

DMCzTrz

B

3.49

−0.05

90

3.44

BPCzTrz

B

3.22

−0.32

72

3.26

TCzTrz

B

3.22 (1) c 3.32 (2) c

−0.32 (1) −0.22 (2)

61 (1) 66 (2)

3.07

3.06

33

4CzIPN

B

3.13 (1) c 3.22 (2) c

−0.41 (1) −0.32 (2)

63(1) 64 (2)

2.61

2.63

51

2.83

2.97

24

2.96

2.94

51

5CzBN 2CzPN

B B

3.35 (3) c

−0.19 (3)

71 (3)

3.04 (1) c 3.09 (2) c

−0.50 (1) −0.45 (1)

63 (1) 65 (2)

3.17 (3)

−0.37 (1)

66 (3)

−0.18

60

3.36

c

a For

one molecule, ΔBDEf = BDEf − BDEf(9-PhCz). b the experimental E0−0(S1) of 9-PhCz is determined from the peak of its emission spectra in toluene. Others are determined from the crossing point of the absorption spectra and emission spectra in toluene solution (for mCP, CBP, Cz2BP, 2CzPN, and 4CzIPN) or in polystyrene film (for DCzTrz, TCzTrz, and 5CzBN). c Numbers in the bracket correspond to the specific bonds labeled in Figure 1, group B.

1. Substituents on the π-Side. 1.1. Non-Ortho-Substituents on the π-Side. The molecules in group A, Figure 1 were taken as examples to study the effects of non-ortho-substituents on the BDEf. 9PhCz is the parent molecule, and all others can be regarded as its derivatives with variable substituents on the meta- or para-position of Cz. The fragile bonds in all these molecules are the D-π bonds (i.e., the exocyclic C−N bonds between the Cz and Ph groups). The calculated results are shown in Table 1. It can be observed that non-orthosubstituents have little influence on the BDEs of D-π bonds. The differences between the BDEf values of 9-PhCz and its derivatives are less than 0.05 eV. The dihedral angles between the Cz and Ph groups show very little change (within 5°). In addition to the four molecules in group A, more Czbased molecules with non-ortho-substituents were investigated. Their chemical structures are shown in Figure S4, and the data are listed in Table S3. The same results were found, with changes of the BDEf values (in comparison with 9-PhCz) within 0.05 eV and changes of the dihedral angles within 6°. Likewise, the molecules with the commonly used 9,9-dimethyl-9,10-dihydroacridine (DMAC) and phenoxazine (Px) donors also follow this result. The investigated DMAC- and Px-based molecules and their BDEf values are shown in Figure S5 and Table S4.

This result provides a convenient way to calculate the BDEf values of D-π-A molecules by omitting non-ortho groups on the π-side. In contrast, the excitation energies of these molecules exhibit wide-range tuning with changes of the non-ortho groups on the π-side. Accordingly, it can be proposed that introducing non-ortho-substituents on the π-side would be a simple and practical way to tune the excitation energies of molecules without influencing (reducing) the BDEf values.

1.2 Ortho-Substituents on the π-Side. When a substituent is introduced into a molecule, its volume, electron character and other properties may affect the BDEf and E0−0(S1) of the molecule. Herein, three commonly used representative substituents with different characteristics were chosen: the methyl group (CH3), Ph, and Cz (a donor). CH3 has a small volume and a weak electron-donating character, and it cannot conjugate with the neighboring groups. In comparison, Ph has a larger volume, and it can conjugate with the neighboring groups. Cz has a much larger volume, a weak electron-withdrawing character in the ground state and a stronger electrondonating character in excited states,54 and it can also conjugate with neighboring groups. We studied the influence of these three substituents on the BDEf and E 0−0 (S 1 ) by introducing them into the parent molecule pCzTrz (see Figure 1, group B). 4


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As shown in Table 1, two ortho-CH 3 groups on the π-side bring three kinds of effects. (i) They slightly reduce the BDE f by approximately 0.05 eV. (ii) They considerably increase the dihedral angle between Cz and the central Ph from 52° to 90°, which would significantly decrease the HOMO−LUMO overlap and further decrease the ΔEST, thus enhancing the TADF character of the corresponding molecule.30 (iii) They slightly increase the E0−0 (S 1 ) (approximately 0.13 eV for DMCzTrz). In comparison, ortho-Ph groups significantly reduce the BDEf by over 0.3 eV but slightly decrease the E0−0(S1) within 0.05 eV. Cz groups give similar effects on the BDEf to those of Ph groups, while two ortho-Cz groups can remarkably decrease the E0−0(S1) by over 0.2 eV.

approximately 0.1 eV lower than that of Cz 2 (which has one ortho-Cz group and one cyano group) and approximately 0.2 eV lower than that of Cz 3 (which has two ortho-cyano groups). The BDEs of the C−N bonds in 5CzBN also follow this trend. In comparison, the BDEf of 2CzPN is as high as 3.36 eV due to the reduced steric hindrance. In addition, the E0−0(S1) of 2CzPN (2.94 eV) is remarkably higher than that of 4CzIPN (2.63 eV), because it has two fewer Cz groups than does 4CzIPN.

The effects of ortho-Ph and Cz groups on BDEf values could be correlated with steric and conjugation effects. Taking TCzTrz as an example (see Figure 2a), the dihedral angle between the central Ph and the middle Cz is 61° and that of the lateral Cz is 66°. If the middle Cz leaves, the dihedral angles between the lateral Cz groups and the central Ph will remarkably decrease to 41°. Because of the relieved steric hindrance, the conjugation is much enhanced. However, if one of the lateral Cz is left, the dihedral angles between the central Ph and the other two Cz groups hardly change because the hindrance between the two Cz groups still remains. This result clearly suggests that the middle Cz has a larger leaving tendency; thus, its exocyclic C−N bond has a lower BDE.

Figure 3. (a) Geometries of DMCzTrz, BPCzTrz, and TCzTrz. All H atoms are hidden. (b) Potential energy curves against dihedral angles between Cz and Ph for 9-PhCz and its derivatives as simplified models of DMCzTrz, BPCzTrz, and TCzTrz. The results were calculated at the B3LYP/6-31G(d) level.

Figure 2. (a) Geometries of TCzTrz before and after the specific carbazole group is taken off. (b) Geometries of 4CzIPN, 5CzBN, and 2CzPN, as calculated at the B3LYP/6-31G(d) level.

It should be noted that introducing ortho-Cz groups into TADF molecules is a very common strategy to enhance the efficiency of the emitter.4,24,33,54 Herein, the well-known TADF molecules 4CzIPN, 5CzBN, and 2CzPN were taken as examples to further clarify the influences of ortho-Cz groups on BDEs. Their geometries in ground states are shown in Figure 2b. In 4CzIPN, the exocyclic C−N bond of Cz 1 (labeled by the red number 1, with two ortho-Cz groups) has the lowest BDE (3.13 eV), which is

By comparing the dihedral angle between D and π groups of p-CzTrz with that of DMCzTrz, BPCzTrz, and TCzTrz (see Figure 3a), it can be found that ortho-CH3 groups dramatically increase the dihedral angle from approximately 50° to 90°, while ortho-Ph and Cz groups cause smaller increases (approximately 10− 20°). This is because ortho-Ph and Cz tend to conjugate with the central Ph; thus, the donor and its ortho-substituents arrange in a propeller shape (see Figure 3a). To further study their effects on the conformation of the donor, the conformational energies against dihedral angles between Cz and Ph in 9-PhCz and its derivatives with different ortho-substituents were calculated (see Figure 3b). It can be clearly observed that compounds with larger orthosubstituents (like Cz) have narrower basins near the minimal potential energies, which means that the 5



conformations of the donor are trapped in a steep potential well. Such confinement could suppress nonradiative decay pathways arising from vibrations and rotations of the donor, thus helping enhance the efficiency of the emitter. Meanwhile, such an effect might be conducive to narrowing the FWHM of the emitter and eventually leading to a high color purity of the corresponding OLED.34,35 In summary, among ortho-substituents on the π-side, CH3 basically does not influence the BDEf and E0−0(S1), while it can significantly increase the dihedral angle between D and π, which could decrease the overlap between the HOMO and LUMO and lead to a smaller ΔEST. Ph and Cz groups can reduce the BDEf by over 0.3 eV. Ph has little influence on the E0−0(S1) (approximately 0.05 eV), while more Cz groups will decrease the E0−0(S1). As long as the BDEf is high enough relative to E0−0(S1), these large substituents could bring several benefits, such as enhancing the efficiency and color purity of the emitters. It is worth noting that from the discussion above, it seems that on the π-side, the effects of substituents on BDEf values mainly depend on the steric hindrance rather than the electronic characters. To confirm this point, we designed four more derivatives of 9-PhCz with orthogroups on the π-side, including molecules containing fluorine atoms (F), cyano groups (CN), trifluoromethyl groups (CF3), and 4,6-diphenyl-1,3,5-triazin-2-yl groups (Trz). These substituents have very different volumes and electron-withdrawing characters. Their chemical structures and BDEf values are shown in Figure S6 and Table S5. For the derivative with the small orthosubstituents of CN groups, its BDEf is close to that of DMCzTrz, while for the derivative with the large orthosubstituents of Trz groups, its BDEf is close to that of TCzTrz. These results confirm that on the π-side, the effects of a substituent on the BDEf mainly depend on the steric hindrance that it brings to the D-π bond, rather than on its electronic character.

2. Substituents on D-Side. 2.1. Substituents on the 3,6-Positions of Carbazole.

Considering that the most widely used donor in TADF molecules is Cz and that non-ortho-substituents on π-side have negligible effects on the BDEs of D-π bonds, 9-PhCz and its derivatives were chosen as model molecules to simplify the calculations. Similar to the study on the π-side, the effects of non-ortho-substituents on the donor should be investigated first. Hence, derivatives with 3,6substituted Cz were first studied. Of note, 3,6-positions of Cz are electrochemically active.9,56 This is a widely used strategy to introduce 3,6-substituents to Cz groups to improve the electrochemical stability of the corresponding materials. Thus, it is important to study the effects of 3,6substituents. Commonly used substituents with different electronic characters were chosen, including EDGs such as CH3 and the methoxyl group (OCH3) and EWGs such as CF3 and CN. Five compounds studied in this section are shown in

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Figure 1, group C, and the calculated results are shown in Table 2. Unlike the situation on the π-side, non-orthosubstituents on D side have substantial effects on BDEf values. Specifically, EWGs such as CF3 and CN on the 3,6positions of Cz can enhance the BDEf by over 0.1 eV. In contrast, EDGs such as OCH3 reduce the BDEf, even by 0.28 eV. To understand such large disparity of the effects between EWGs and EDGs on BDEf values, we first calculated electrostatic potential (ESP) maps of these molecules, which could visualize the effects of substituents on the charge distribution. In Figure 4, the area mapped in red has a negative electrostatic potential, while that mapped in blue has a positive potential. It can be found that in 9-phenyl-9H-carbazole-3,6-dicarbonitrile (PDN) and 9-phenyl-3,6-bis(trifluoromethyl)-9H-carbazole (PDF), the EWGs have strongly withdrawn electrons from the entire molecule, while in 3,6-dimethoxy-9-phenyl-9Hcarbazole (PDO) and 3,6-dimethyl-9-phenyl-9H-carbazole (PDM), the EDGs on the 3,6-positions of Cz have slightly increased the electron density near the D-π bonds and the Ph groups. Such different ESP distributions may correlate with their different effects on BDEf values. Based on this observation, the Hirshfeld charges of N atoms (qN) in the Cz groups and those of the entire Cz groups (qCz) in these molecules were calculated by Multiwfn (see Table 2) to further clarify the different effects of EWGs and EDGs on the BDEf. The Hirshfeld charge can clearly reflect the effects of the substituents on the atomic charges.57 Additionally, it is easy to calculate.58 In Table 2, it can be found that in 9-PhCz, qN is −0.021 and qCz is −0.085, which means that Cz is an electronegative group in the ground state of the molecule (even though it is a typical donor when the molecule is excited). This finding could be understood based on the fact that the electronegativity of the N atom, χN, is much higher than χC. The qN indicates that in 9-PhCz and its derivatives, the negative charge is always accumulated on the N atom of the D-π bond. In PDF and PDN, EWGs on the 3,6-positions of Cz would raise qN to −0.015 by CF3 and −0.012 by CN, which means that the EWGs withdraw electrons from the N atom. As a “response”, the N atom would withdraw more electrons from the Ph group through the D-π bond, which leads to a lower qCz (−0.11 of PDF and −0.13 of PDN) and an enhanced conjugation between Ph and Cz, thus increasing the BDEf to 3.65−3.68 eV (in comparison with that of 9PhCz at 3.54 eV). In contrast, EDGs on the 3,6-positions of Cz further decrease qN and raise qCz. For instance, in PDO, qN is decreased to −0.026, which indicates that the EDGs have increased the electron density on the N atom. To avoid oversaturation of the negative charge on this atom, Cz with EDGs would withdraw less electrons from Ph. Thus, the qCz of PDO is raised to −0.068 in comparison with that of 9-PhCz (−0.085), and consequently, the D-π bond is weakened to 3.26 eV. Most notably, the BDEf values of these molecules seem to have a positive correlation with qN values, which will be discussed later. Compared with two substituents on the 3,6-positions, a single substituent at 6


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the 3-position of Cz basically produces a similar effect on

BDEf, but the effect is weaker by half (see Table S8).

Figure 4. Electrostatic potential maps of 9-PhCz and its derivatives, isovalue=0.002.

Table 2. B3LYP/6-31G(d) Absolute (BDEf) and Relative (Δ BDEf) Bond Dissociation Energies of the Fragile D-π Bonds, Dihedral Angles Between Carbazoles and the Phenyl Groups (∠D-π), Hirshfeld Charges of the N Atom (qN) and of Carbazole (qCz) in the Derivatives of 9-PhCz molecule substituted positions substituent 9-PhCz H PDO 3,6OCH3 PDM 3,6CH3 PDF 3,6CF3 PDN 3,6CN MDO 2,7OCH3 MDM 2,7CH3 MDF 2,7CF3 MDN 2,7CN ODO 1,8OCH3 ODM 1,8CH3 ODF 1,8CF3 ODN 1,8CN a For a molecule, ΔBDE = BDE − BDE (9-PhCz). f f f

BDEf (eV) 3.54 3.26 3.46 3.65 3.68 3.54 3.53 3.64 3.68 3.24 3.15 3.33 3.56

2.2. Substituents on the 2,7- and 4-Positions of Carbazole. In addition to those of substituents on the 3,6-

positions of Cz, the effects of substituents on the 2,7- and 4-positions were also studied. The structures of 2,7substituted derivatives are shown in Figure 1, group D. The results are listed in Table 2. In 2,7-dimethyl-9-phenyl-9Hcarbazole (MDM), the CH3 groups cause a negligible decrease of the BDEf, by only approximately 0.01 eV. This is because CH3 groups on 2,7-positions cannot increase the electron density of the N atom (i.e., its meta-position). Similarly, in 2,7-dimethoxy-9-phenyl-9H-carbazole (MDO), OCH3 groups scarcely affect the electron density of the N atom. The qN (−0.022) of MDO is almost identical to that of 9-PhCz (−0.021); thus, their BDEf values are also very close. In comparison, in 9-phenyl-9H-carbazole-2,7dicarbonitrile (MDN) and 9-phenyl-2,7bis(trifluoromethyl)-9H-carbazole (MDF), the CN and CF3 groups both remarkably raise qN and increase the BDEf by approximately 0.1 eV due to the strong electronwithdrawing inductive effects. A single substituent on the

ΔBDEf (eV) a 0 −0.28 −0.08 0.11 0.14 0 −0.01 0.10 0.14 −0.30 −0.39 −0.21 0.02

∠D-π (°) 56 53 54 60 63 57 56 59 61 90 90 78 90

102×qN

102×qCz

−2.1 −2.6 −2.2 −1.5 −1.2 −2.2 −2.1 −1.7 −1.5 −2.0 −2.8 −2.3 −1.2

−8.5 −6.8 −7.8 −11.3 −12.7 −8.0 −8.1 −11.4 −12.7 −7.6 −13.2 -9.0 −15.2

2- or 4-position of Cz shows similar but weaker effects on BDEf values. The detailed data are shown in Table S8. 4,5Substituted Cz are rarely used in OLED materials due to the synthesis difficulty; therefore, the related content was not studied.

2.3. Substituents on the 1,8-Positions of Carbazole.

To note, substituents on 1,8-positions may bring significant steric hindrance, which can affect the conjugation between Cz and Ph and may even alter their electronic effects. The distinct effects of 1,8-substituents were studied using the molecules from Figure 1, group E. In 1,8-dimethyl-9-phenyl-9H-carbazole (ODM), the CH3 groups on the 1,8-positions significantly decrease the BDEf by approximately 0.4 eV. Note that ortho-CH3 groups on the π-side (like in DMCzTrz) only decrease the BDEf by approximately 0.1 eV. Such a big difference suggests that an ortho-substituent may have very different effects on BDEs when it is substituted on different sides of the polar bond. The causes and significance of this difference are worth in-depth study. Here, we attempted to give a 7



preliminary explanation based on the changes of qN values. In ODM, the CH3 groups on the 1,8-positions of Cz can decrease qN to −0.028 due to electron-donating effects, thus leading to a very low BDEf (3.15 eV). In contrast, on the π-side (DMCzTrz), CH3 groups hardly influence qN (−0.020 vs. −0.021 of 9-PhCz) because of the negligible conjugation between Cz and the π subunit (the dihedral angle is 90°). In 1,8-dimethoxy-9-phenyl-9H-carbazole (ODO), the BDEf decreases by approximately 0.3 eV in comparison with that of 9-PhCz. Since qN of ODO is almost unchanged, such a decrease should be due to steric hindrance, i.e., the lone pair electrons of 1,8-OCH3 repel the π electrons of Ph. In 9-phenyl-1,8-bis(trifluoromethyl)-9H-carbazole (ODF), the BDEf decreases by approximately 0.2 eV, which could be ascribed to the large steric hindrance between CF3 and Ph. As a result, the N atom in Cz has some character of sp3 hybridization, which could be supported by the natural bond orbital (NBO, see Figure S7 and Table S6) and adaptive natural density partitioning (AdNDP, see Figure S8 and Table S7) analysis. In the case of 9-phenyl-9Hcarbazole-1,8-dicarbonitrile (ODN), the CN groups on the 1,8-positions of Cz remarkably increase qN to −0.012, which should have increased the BDEf. However, its BDEf is almost the same as that of 9-PhCz due to mutual repulsion of the π electrons between the CN and Ph groups. Compared with the effects of substituents on the 3,6positions of Cz, it can be concluded that moving the same substituent from 3,6-positions to 1,8-positions on Cz usually brings adverse effects on BDEf values because of steric hindrance.

Figure 5. The correlation between the B3LYP/6-31G(d) Hirshfeld charges of the N atom (qN) in C−N bonds and the BDEf values in derivatives of 9-PhCz. The black curve is fitted without considering the gray dots. The BDEf of 9-PhCz is taken as the zero point of the right vertical axis, as marked by the blue line.

2.4. Correlation Between the BDEf and the Substituent on the Donor. 9-PhCz has the typical D-π structure that frequently appears in OLED molecules. In

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the previous sections, we studied the effects of various substituents on different positions of Cz on the BDEf values of its derivatives and tried to understand the possible mechanisms of these effects with the aid of ESP and qN. It seems that in the derivatives of 9-PhCz, the BDEf and qN do have some correlation in most cases. Plotting the BDEf versus qN gives this correlation clearly (Figure 5). Herein, the BDEf refers to the BDE of the D-π bond between Cz and Ph, and qN is the Hirshfeld charge of the corresponding N atom. This charge can well reflect the electronic effects of substituents, as shown by previous sections. To note, compared with calculating the BDEf, calculating qN is much easier. In Figure 5, the gray dots represent the derivatives of 9PhCz with substituents on the 1- or 1,8-positions of Cz, blue dots represent those with substituents on the 3- or 3,6positions and purple dots represent those with substituents on the 2-, 2,7-, or 4-positions. In addition to the molecules with the aforementioned four substituents (CH3, OCH3, CF3, CN), those with Ph, tert-butyl groups, and bromine atoms are supplemented, since these are also commonly used substituents. Moreover, the molecules in groups A and F in Figure 1, represented by red dots, are also used as real OLED materials. The structures and detailed data are shown in Figure S9 and Table S8. Interestingly, the fitted black curve in Figure 5 shows that the BDEf increases with the increase of q N , while the rate of increase tends to be reduced when q N is larger than about −0.020. Such a correlation could be very practical in the study of the molecular stability of OLED materials. For large molecules, the calculations of BDEs are very timeconsuming because both geometry optimization and frequency analysis have to be performed. Now, through this curve, the BDEf values of many Cz-based OLED molecules could be conveniently predicted by the easily acquired q N . However, it should be noted the curve was fitted without considering the gray dots (1- and 1,8substituted derivatives of 9-PhCz) since q N could not reflect the steric effects brought by the ortho-substituents. Thus, for molecules with ortho-linkages near D-π bonds, whether the ortho-substituents are on the π-side or on the D-side, prediction of the absolute value of the BDEf by this correlation may not be very reasonable. Notably, as shown in Figure 5, the BDEf values of a majority of the gray dots are lower than that of 9-PhCz. This finding can be comprehended since substituents on the 1- and 1,8-positions of Cz would bring considerable steric effects, which usually weaken D-π bonds and lead to lower BDEf values. As an extreme example, the BDEf of 1,8,9-trisphenyl-9H-carbazole (ODP, the lowest gray dot) is only 2.98 eV. In ODP, the three Ph groups on the 1-, 8-, and 9-positions of Cz repel each other, with the dihedral angle between Cz and any of the Ph groups being approximately 65°, and none of the groups could conjugate with Cz effectively unless the Ph group on the 9-position of Cz is taken off (Figure S11). Consequently, in ODP, the BDEf is significantly decreased by over 0.5 eV. Thus, substituents on the 1,8-positions of Cz are usually 8


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detrimental to BDEf values, an effect which is recommended to be taken into consideration during the design of materials for blue OLEDs. In addition to predicting the BDEf, novel stable donors could be screened according to this correlation. For the derivatives of 9-PhCz, introducing EWGs on the 3,6- or 2,7positions of Cz is one feasible way to raise q N and further enhance the BDE f . For example, the BDE f is raised from 3.54 eV for 9-PhCz to 3.68 eV for PDN due to the introduction of cyano groups. In addition, one could employ other donors with naturally higher q N values. They may form even stronger bonds with Ph according to this correlation. For instance, in 9-phenyl-α-carboline, the αcarboline has an sp2-hybridized N atom in its aromatic ring, which could raise q N of the N atom in the D-π bond to −0.019. As a result, the BDE f between α-carboline and Ph is high as 3.78 eV, which is 0.24 eV higher than that of 9PhCz. The detailed data of other carboline derivatives are shown in Figure S10 and Table S9. This result suggests that carbolines may have great potential to be stable donors for blue materials. Recently, carbolines have been used in host and TADF materials for blue OLEDs, owing to their high triplet energies and electron-transporting characters.59−61 Compared with their Cz-based counterparts, it can be anticipated that these carboline-based molecules would have enhanced molecular stability according to our result. In addition to the derivatives of 9-PhCz, DMAC- and Pxbased molecules were also studied (see Figure 1, Group G). It was found that their q N and BDEf values also have positive correlations (see Figure S12 and Table S10). This result highlights that the prediction of the BDEf by q N has some certain commonalities for the corresponding OLED materials. Further study to gain deeper insight into the correlation between q N and BDEf is underway.

2.5. Effects of Substituents on Excitation Energies. In

addition to BDEf values, excitation energies also correlate with the molecular stability of OLED molecules. Hence, to thoroughly understand the influence of substituents on Cz groups on molecular stability, excitation energies of several molecules with substituted Cz groups are calculated. Different from the BDEf, the excitation energy is determined by the entire molecule; thus, model molecules (like those in group C−E, Figure 1) could not be used, and p-CzTrz was chosen as the parent molecule in this section. Here, only the effects of substituents on the 3,6-positions of Cz were studied. The corresponding molecular structures are shown in Figure 1, group F. The results are shown in Table 3. The E0−0(S1) of p-CzTrz is 3.31 eV. By introducing CH3 and OCH3 on the 3,6-positions of Cz, E0−0(S1) is decreased to 3.21 eV in 9-(4-(4,6-diphenyl1,3,5-triazin-2-yl)phenyl)-3,6-dimethyl-9H-carbazole (PDMTrz) and to 3.00 eV in 9-(4-(4,6-diphenyl-1,3,5triazin-2-yl)phenyl)-3,6-dimethoxy-9H-carbazole (PDOTrz). In most Cz-based D-π-A molecules, the HOMO is mainly contributed by Cz. Hence, EDGs on Cz can raise the HOMO level, thus decreasing the band gap between the HOMO and LUMO. For instance, the HOMO level of p-CzTrz is −5.42 eV. It is raised to −4.90 eV by OCH3 in

PDOTrz. As a result, the energy gap of PDOTrz is as low as 3.02 eV (vs. 3.47 eV of p-CzTrz). In comparison, EWGs on the 3,6-positions of Cz can decrease the HOMO levels, widen the energy gaps and eventually raise the E0−0(S1) of the corresponding materials. For instance, by introducing CF3 and CN, the E0−0(S1) is raised to 3.64 eV in 9-(4-(4,6diphenyl-1,3,5-triazin-2-yl)phenyl)-3,6bis(trifluoromethyl)-9H-carbazole (PDFTrz) and to 3.71 eV in -(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9Hcarbazole-3,6-dicarbonitrile (PDNTrz), respectively. In addition, the BDEf values of these molecules were also calculated and are almost identical to those in Figure 1, group C. This result, once again, proves that non-orthosubstituents basically do not affect the BDEf. Table 3. Substituents (R), B3LYP/6-31G(d) Absolute (BDEf) and Relative (ΔBDEf) Bond Dissociation Energies of Fragile Bonds, 0−0 Transition Energies [E 0−0 (S 1 )] of the Molecules in Figure 1, group F Compound

Group

BDEf

ΔBDEf a

E0−0(S1)

R

(eV)

(eV)

(eV)

PDOTrz

OCH3

3.29

−0.26

3.00

PDMTrz

CH3

3.48

−0.07

3.21

p-CzTrz

H

3.55

0

3.31

PDFTrz

CF3

3.62

0.07

3.64

PDNTrz

CN

3.67

0.12

3.71

a For

a molecule, ΔBDEf = BDEf − BDEf(p-CzTrz).

Considering that substituents on the 1,8-positions had been demonstrated to be harmful to the BDEf, their effects on excitation energies were not studied. For substituents on the 4- and 2,7-positions of Cz, it is found that their effects on the excited states of the corresponding molecules are complicated. For instance, the S1 of p-CzTrz is a charge transfer state. By introducing CN on the 2,7positions of Cz, the S1 of the corresponding molecule (i.e., 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9Hcarbazole -2,7-dicarbonitrile, MDNTrz) becomes a locally excited state. Natural transition orbitals analysis (see Figure S13) clearly shows such character. Therefore, the effects of substituents on the 2,7-positions on excited states need further investigation. In summary, EDGs on the 3,6-positions of Cz will simultaneously decease the E0−0(S1) and BDEf of the corresponding molecules, while EWGs have opposite effects. In this work, we mainly focused on the BDEf. For practical applications, substituent effects on the energy levels of the HOMO/LUMO, the emission color and other properties should also be considered. Relevant studies are still in progress.

9



4. SYNOPSIS In this work, we first put forward the concept of ‘fragile bonds’. It refers to bonds which have the minimum or a comparable-to-the-minimum BDE in a molecule. The BDEs of these bonds were denoted as ‘BDEf’, which could be one important parameter for the molecular stability of OLED materials, as evidenced by the summarization of previous studies. We then studied the effects of different substituents on the BDEf values of a series of Cz-based OLED molecules with typical D-π substructures. Our research has two main sections according to the substitution position with respect to the fragile D(Cz)-π bond. The main conclusions and the corresponding implications for the development of new materials include the following. On the π-side, the effects of a substituent on the BDEf mainly depend on the steric hindrance that it brings to the D-π bond, rather than on its electronic character. Thus, non-ortho (with respect to the donor, Cz) substituents have little effect on the BDEf, normally changing it by less than 0.05 eV. Such a result allows one to fine tune the E0−0(S1) of a molecule without affecting the fragile bond by adding, reducing or replacing non-ortho-donor/acceptors. In addition, one could simplify the calculation of the BDEf by omitting non-ortho-substituents on the π-side of the corresponding molecules. CH3 has a small volume; thus, ortho-CH3 groups on the π-side can slightly decrease the BDEf by only 0.05 eV and increase the E0−0(S1) of the molecule by approximately 0.1 eV. Furthermore, they can remarkably increase the dihedral angle between D and π, which may efficiently restrain the overlap between the HOMO and LUMO. Therefore, for a TADF molecule with a relatively large ΔEST, introducing ortho-CH3 groups on its π-side would be a practical way to decrease its ΔEST and improve its TADF character with little adverse effect on the molecular stability. Ortho-Ph groups hardly affect the E0−0(S1), while ortho-Cz groups can decrease the E0−0(S1) depending on the number of donors in the corresponding molecule. On the other hand, both substituents can largely decrease the BDEf by over 0.30 eV, but as long as the BDEf is high enough, they can substantially improve the performance of the corresponding material. Specifically, these large orthosubstituents can cause a rigid structure and efficiently confine intramolecular motions, thus helping improve the efficiency and color purity of the emitter. On the donor-side, molecules with a 1,8-substituented Cz usually have low BDEf values due to their steric hindrance. Interestingly, it was found that the same orthosubstituent on different sides of the polar D-π bond could bring very different effects on this bond, which provides more spaces for synergistic modulation of the BDEf, E0−0(S1), and other photophysical properties of OLED materials. For molecules with a 3,6-, 2,7-, or 4-substituted Cz, the BDEf values are mainly influenced by the electronic characters of the substituents on Cz. Specifically, introducing EWGs on these positions of Cz would increase

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the BDEf of the corresponding material, while EDGs would decrease the BDEf. Importantly, a positive correlation between the BDEf and the Hirshfeld charge (qN) of the N atom in Cz was found for these molecules. Such a correlation provides a convenient and reasonable way to predict BDEf values by the easily acquired qN. It also allows one to design or screen stable donors with higher qN values, such as carbolines, and develop molecules with stronger Dπ bonds for blue OLEDs. Notably, such a relationship also exists in DMAC- and Px-based molecules, which implies a certain commonality of qN for predicting the BDEf values and molecular stability of OLED materials. In addition, EWGs such as CF3 on the 3,6-positions of Cz can increase the E0−0(S1), while EDGs such as OCH3 on the 3,6-positions of Cz can decrease the E0−0(S1), which provides an approach to tune the emission color of the corresponding material. We hope that these results can provide feasible strategies to modulate the BDEf and E0−0(S1) for the design of stable materials, in particular, for those used in blue OLEDs. Further experiments exploring the specific correlation between the BDEf values of OLED materials and the stability of corresponding devices are in progress. It should be noted that chemistry is mainly based on bond reorganization processes governed intrinsically by relevant BDEs;47,62 thus, we anticipate that this study on the modulation and prediction of BDEs could be beneficial for many studies that involve bond-rupture processes in chemistry and material sciences.

ASSOCIATED CONTENT

Supporting Information. Details of comparative calculations of BDEf values; calculated values and experimental values of EVA(S1) and OHF% of some molecules in Figure 1, group A, B, and F; absorption and emission spectra of mCP in toluene solution; calculated BDEf values of twelve Cz-based molecules with non-ortho-substituents on the πside; calculated BDEf values of six DMAC-based and six Pxbased molecules with non-ortho-substituents on the π-side; structures, calculated BDEf values and discussions of four Czbased molecules with ortho-EWGs on the π-side; NBO and AdNDP analysis and discussions of molecules in group E of Figure 1 and 9-PhCz; structures, calculated BDEf values, and qN of points in Figure 5; structures, calculated BDEf values, and qN of carboline-based model molecules; optimized geometry of ODP and its fragment; structures, calculated BDEf values, and qN of molecules in Figure 1, group G; NTOs of MDNTrz. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID

Juan Qiao: 0000-0002-9919-3927

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS 10


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This work was supported by the National Key R&D Program of China (No. 2016YFB0401003), the National Key Basic Research and Development Program of China founded by MOST (No. 2015CB655002), and Tsinghua Xuetang Talents Program.

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