Effect of Substituents on the Electronic Structure and Degradation

Article pubs.acs.org/cm

Effect of Substituents on the Electronic Structure and Degradation Process in Carbazole Derivatives for Blue OLED Host Materials Minki Hong,†,‡ Mahesh Kumar Ravva,‡ Paul Winget,† and Jean-Luc Brédas*,‡,† †

School of Chemistry and Biochemistry Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ‡ Laboratory for Computational and Theoretical Chemistry of Advanced Materials, Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Kingdom of Saudi Arabia S Supporting Information *

ABSTRACT: We investigate the dissociation mechanism of the C−N bond between carbazole and dibenzothiophene in carbazole-dibenzothiophene (Cz-DBT) positional isomers, selected as representative systems for blue host materials in organic lightemitting diodes (OLEDs). The C−N bond dissociation energies, calculated at the density functional theory level, are found to depend strongly on the charge states of the parental molecules. In particular, the anionic C−N bond dissociations resulting in a carbazole anion can have low dissociation energies (∼1.6 eV) with respect to blue emission energy. These low values are attributed to the large electron affinity of the carbazole radical, a feature that importantly can be modulated via substitution. Substitution also impacts the energies of the first excited electronic states of the CzDBT molecules since these states have an intramolecular charge-transfer nature due to the spatially localized character of the frontier molecular orbitals within the carbazole moiety (for the HOMO) and the dibenzothiophene moiety (for the LUMO). The implications of these results must be considered when designing blue OLED hosts since these materials must combine chemical stability and high triplet energy. connected with an aromatic linking group or a π-conjugation inhibitor.6,7 While the quantum efficiency of OLED devices have reached a satisfactory level,8 their lifetime and long-term color purity are still problematic.9 The degradation of the organic materials is largely responsible for the deterioration of the operation of OLED devices over time,2,10 a problem especially acute for materials used for blue emission. Therefore, controlling the degradation of the materials in the blue emitting layers is critical to realize the full potential of OLED devices. Molecular mechanisms for the degradation of guest and host materials have been examined;11−15 in particular, the C−N bond of Nheterocyclic host materials has been reported as a potential failure point in OLED devices.16,17 The interaction between the lone pair of the nitrogen and surrounding radicals has been suspected as a source of material degradation.10 Kondakov et al. have reported that an exciton-induced dissociation of the C−N bond in a carbazole-biphenyl derivative could be the key step in the operational degradation of the corresponding OLEDs.16 Lin et al. have conducted similar studies on various hosts and found that the C−P or C−S bonds present in their materials can be vulnerable to dissociation, namely via singlet/triplet excitation and even in electron-only devices.18,19 These studies have

1. INTRODUCTION With phosphorescent organic light-emitting diodes (phOLEDs) now reaching ∼100% of charge-to-photon internal conversion efficiency via triplet harvesting,1 many of the early hurdles to commercialization have been overcome.2 OLED devices have entered various markets, including flat-panel/ flexible full-color displays for smart phones and televisions, and large-area solid-state lighting.3,4 In these OLED devices, the emission layer generally consists of a guest (dopant) emitting material and a host (matrix) material. Such a design helps minimize triplet−triplet annihilation (TTA) and concentration quenching of the emissive complexes, which would both lead to low quantum efficiencies.5,6 The electronic-structure characteristics of the host material are essential to ensure adequate performance of an OLED device. Appropriate host materials must have (i) high triplet energy (T1) that prevents energy back-transfer from the guest to the host material; (ii) appropriate energy-level alignment with the materials in the hole/electron transport layers to ensure good charge injection; and (iii) high charge-carrier mobilities for efficient carrier recombination. To fulfill these criteria, numerous ambipolar host materials have been synthesized, which consist of one or more electron-rich and electron-deficient functional groups. Common building blocks include diphenyl amine, carbazole, pyridine, triazole, benzimidazole, oxadiazole, and diphenyl phosphine; these blocks can be © 2016 American Chemical Society

Received: May 23, 2016 Revised: July 25, 2016 Published: July 25, 2016 5791

DOI: 10.1021/acs.chemmater.6b02069 Chem. Mater. 2016, 28, 5791−5798

Article

Chemistry of Materials

2. COMPUTATIONAL DETAILS

mainly utilized LDI-TOF-MS (laser desorption ionization timeof-flight mass spectroscopy)13 or similar laser-based mass spectroscopies to identify the dissociation products and pathways. However, it remains often challenging to detect and identify chemically unstable dissociation products due to experimental limitations.17 As a result, a detailed bond dissociation mechanism for those host materials has not been fully described, and the role of the material electronic structures on the bond dissociation process is far from being completely understood. Here, we present a systematic computational study at the density functional theory level, of the dissociation of the C−N bond for a series of carbazole (Cz)−dibenzothiophene (DBT) positional isomers (x-Cz-DBTs, where x = 1, 2, 3, and 4; see Figure 1), taken as representative examples. Dibenzothiophene

Geometry optimizations and evaluations of vibrational frequencies are performed at the density functional theory (DFT) level using the B3LYP hybrid functional29,30 (the reliability of which for the task on hand is discussed below), in conjunction with the standard 6-31G(d) basis set, as implemented in the GAUSSIAN 09 package.31 Spinrestricted calculations are conducted for the ground states (S0), while spin-unrestricted calculations are used for the ionic states and radicals. A subset of reactions are calculated using a triple-ζ basis set using diffuse functions, 6-311+G(3df,2p) to determine the effect of the basis set size. Matrix effects are taken into account using the polarizable continuum model (PCM).32 Excited singlet and triplet energies are calculated by time-dependent (TD) DFT33 on the basis of the S0 geometries using the Tamm-Dancoff approximation.34 Spin densities are determined as the differences between the α and β electron densities. As host materials exist as neutral molecules as well as the corresponding radical cations and anions during typical OLED operation, we evaluated not only the energy required for homolytic C−N bond cleavage at 0 K, i.e., the enthalpy of reaction 1, ΔrxnH0, but also the energies required for the cleavage of the cation and anion, i.e., the enthalpies of reactions 2 and 3:

x ‐9‐Cz‐DBT → [9 − Cz]• + [x ‐DBT]•

(1)

[x ‐9‐Cz‐DBT]+ → [9‐Cz]+ + [x ‐DBT]• or [9‐Cz]• + [x ‐DBT]+

(2)

[x ‐9‐Cz‐DBT]− → [9‐Cz]− + [x ‐DBT]• or [9‐Cz]• + [x ‐DBT]−

(3)

These reactions are related through a thermodynamic cycle, which contains either the ionization energy (IE, the enthalpy required to remove an electron from a molecule at 0 K) or the electron affinity (EA, negative of the 0 K enthalpy change for the electron attachment reaction). On the adiabatic surface, the fragment with the smallest IE or largest EA determines the charged fragment. A recent study for homolytic C−N BDEs of aliphatic and aromatic amines has shown that the C−N BDEs calculated at the B3LYP/631G(d) level have a mean unsigned error of ∼0.17 eV compared to experimental data35 and of ∼0.1 eV compared to benchmark CBSQB3 values (Complete Basis Set-Quadratic Becke3; with the CBSQB3 values themselves having a mean unsigned error of 0.1 eV compared to experiment).36,37 We recall that the computational cost of highly accurate ab initio methods (e.g., CCSD(T), the “goldstandard”) or of composite protocols (e.g., Gaussian-3 with B3LYP/ 6-31G(d) geometries, G3//B3LYP,38 or the quadratic extrapolation to a complete basis set with B3LYP/6-31G(d) geometries, CBS-QB3), quickly becomes prohibitively expensive when screening large

Figure 1. Chemical structures of the carbazole−dibenzothiophene positional isomers: x-Cz-DBTs, x = 1, 2, 3, and 4.

and its derivatives have been extensively used in OLEDs,20−22 owing to their relatively high triplet energy (∼3.15 eV) and good carrier-transfer characteristics; carbazole has been used to improve hole transport, as either direct substituents23−27 or indirect ones connected by a benzene meta-linkage.28 In order to better understand how functionalization alters the electronic structure of the Cz-DBT compounds and their bond dissociation energies (BDEs), we also examined the effects of electron-donating and electron-withdrawing groups.

Figure 2. Bond dissociation energies using G3//B3LYP vs B3LYP/6-31G(d); (left) for the C−N bond of amino-dibenzothiophene (x-NH2-DBT) isomers and (right) for the C−H bond of dibenzothiophene. 5792


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Chemistry of Materials numbers of compounds. The so-called composite methods combine more modest levels of theory with larger basis sets and more robust theories with smaller basis sets to approximate the results obtained at a much more rigorous level. The success of composite methods relies on the assumption that important electronic effects are additive and that incrementally larger basis sets can be used to extrapolate to the complete basis set (CBS) limit. Given that our main goal in this work is to understand variations in BDEs over a series of chemically similar substrates, these additive effects are expected to be similar; as a result, in the present context, B3LYP/6-31G(d) represents a reasonable alternative that can provide accurate relative BDEs. To confirm this point, we performed a series of B3LYP calculations on dibenzothiophene and on amino derivatives of dibenzothiophene (which have C−N bonds similar to those in the Cz-DBT series), and compared the results to those obtained with the G3//B3LYP method. We also performed similar calculations for selected Cz-DBT derivatives, using different exchange correlation functionals (i.e., M06-2X,39 ω-B97XD,40 and B3LYP) and basis sets. All results are shown in Figure 2 and in the Supporting Information (Tables S1 and S2 and Figures S1−S3). Although the absolute errors from B3LYP can be significant compared to other methods, the critical aspect in the present context is that the relative values are well reproduced. Negligible differences in the relative C−N BDE values are observed in the case of B3LYP when considering a larger basis set. Most importantly, EAs of the radicals retain the same trends as those predicted via the small basis set upon inclusion of basis functions of higher angular momentum and of diffuse character. Thus, in light of these results, we have used B3LYP/6-31G(d) for all thermochemical calculations carried out in this study. Finally, we note the difference between the enthalpic ΔrxnH0, IE, and EA values and their energetic analogues is the change in zeropoint vibrational energy, ΔZPE. For instance, ΔZPE is on the order of ∼0.35 eV for homolytic dissociation of the C−H bond in DBT. When accurate values of the BDEs are required, inclusion of this term is needed; however, as the range of ΔZPE variations for the four unique hydrogen atoms is only ∼0.004 eV (which remains well below thermal energy at room temperature), the relative BDE values remain unchanged. Moreover, when comparing these quantities over a series of molecules with similar bonds that are broken and analogous radicals formed, the enthalphic values are linearly related to the energetic ones; as the composite method presented in this work, i.e., G3//B3LYP, uses scaled B3LYP/6-31G(d) frequencies, the enthalpy−energy relationship holds there as well. Therefore, we use energies hereafter for the sake of convenience.

Table 1. Ionization Energies (IEs), Electron Affinities (EAs), Absorption Energies (Eabs), and Phosphorescence Energies (Ephos) of 3,6-diCz-DBTa method Exp. (THF) Exp. (thin-film) B3LYP (THF) B3LYP (gas)

IE

EA

5.42

2.26

5.40 6.37

1.44 0.24

Eabs b

3.67 3.76b 3.05d 3.41

Ephos 2.87c 2.53d 2.97

a

All values are in eV. All experimental values are taken from ref 23. Calculations are performed with B3LYP/6-31G(d) using the statespecific PCM model with ε = 7.43, to match the experimental solvent, tetrahydrofuran, THF. bFrom the lowest-energy peak of the absorption spectra. cFrom the highest-energy peak of the phosphorescent emission spectra. dThe state-specific method was used for excited-state calculations.

2.26 eV. The similarity of the calculated values in THF, on the one hand, of the IE with that of 9H-Cz, 5.62 eV, and on the other hand, of the EA with that of DBT, 1.07 eV, are consistent with the description of two nearly independent redox centers. Accordingly, the lowest singlet excited state of 3,6-diCz-DBT (3.05 eV by TDDFT/B3LYP in THF) is seen to be dominated by a HOMO-to-LUMO transition with intramolecular charge transfer (ICT) character. The monosubstituted Cz-DBTs exhibit electrochemical quantities similar to those of the 3,6-diCz-DBT; see Table 2. There are only small variations in the frontier molecular orbital energies (HOMO energy variations of 0.11 eV and LUMO energy variations of 0.04 eV), and the vertical S1 energies are similar as well. In contrast, there is a large variation in the vertical T1 energies, as there can be significant involvement of a local excitation (LE) of the DBT moiety (H-3 → L). The contribution of this LE excitation declines as the dihedral angle between the two moieties decreases, as shown in Table 2. The T1 state of 4-Cz-DBT, where the two moieties are nearly perpendicular to one another with a dihedral angle of 88.1°, consisting mostly of a LE. The contributions from CT and LE are nearly equal in 1-Cz-DBT, which a dihedral angle of 72.5°, while in 2-Cz-DBT and 3-Cz-DBT, where dihedral angles are lower than 60°, the CT excitation dominates. As shown in Figure 3, the HOMO and LUMO are located on spatially distinct portions of the x-Cz-DBT compounds. The pronounced dihedral angle between the two moieties, as reported in Table 2, prevents effective π-conjugation across the whole molecule. The bond distance for the C−N bond linking the Cz and DBT fragments increases in conjunction with the increase in dihedral angle. There are only minor changes in the bond lengths and angles of the DBT moiety upon forming CzDBT (see the Supporting Information, Tables S3 and S4). 3.2. Electronic Structure of the 9-Cz and x-DBT Fragments. A consideration of both the geometric and electronic properties of the radicals, carbazol-9-yl (9-Cz) and xDBT, can lead to a better understanding of the dissociation energy of the C−N bond. As shown in Figure 4, the unpaired electron in x-DBT radicals is in fact highly localized in an sp2orbital orthogonal to the π-system, which is similar to the situation in the phenyl radical.42 As a result, the bond angle at the radical site (θ in Figure 4) is increased by more than 5°, and the adjacent C−C bonds are slightly shortened compared to the corresponding values in the Cz-DBT compounds. In contrast, see Figure 4, in 9-Cz one of the lone pair electrons on the nitrogen redistributes to the broken sp2-orbital, leaving the

3. RESULTS AND DISCUSSION In this section, we first examine the ground-state electronic structure and excitations of the Cz-DBT compounds, followed by the electronic structure of the fragments. Next, the C−N bond dissociation energies for various charge states of the parent compounds are examined. Lastly, we report the effect of substitution on the dissociation energies and excited-state energies. 3.1. Electronic Structure of the Cz-DBT Compounds. The 3,6-bis(carbazol-9-yl)dibenzothiophene (3,6-diCz-DBT) derivative belongs to a class of fluorene-like molecules with symmetric carbazole side groups linked via C−N bonds. Since 3,6-diCz-DBT has been previously well characterized experimentally23,41 and shares features with the monosubstituted CzDBTs, it is useful to consider this molecule in order to be able to compare the calculated results to the experimental data; see Table 1. Experimental CV measurements23 in tetrahydrofuran (THF, ε = 7.43) indicate a first ionization at 5.42 eV, attributed to the peripheral Cz units; this value is well reproduced by the B3LYP-calculated IE of 5.40 eV in THF. The calculated EA of 1.44 eV is underestimated compared to the experimental reduction potential that leads to an experimental EA estimate of 5793


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Table 2. x-Cz-DBT Compounds: B3LYP/6-31G(d) Dihedral Angles (DA), Frontier Molecular Orbital Energies (HOMO/ LUMO), Ionization Energies (IEs), Electron Affinities (EAs), and Lowest Singlet/Triplet Excited-State Energies (Vertical, S1/ T1) with Configuration Contributionsa

a

compound

DA(deg)

HOMO

IE

LUMO

EA

S1

S1 transition

1-Cz-DBT 2-Cz-DBT 3-Cz-DBT 4-Cz-DBT

72.5 55.7 58.2 88.1

−5.39 −5.35 −5.33 −5.44

6.73 6.61 6.59 6.75

−1.22 −1.24 −1.21 −1.20

−0.13 −0.07 −0.11 −0.13

3.62 3.63 3.58 3.56

H→L H→L H→L H→L

(98%) (97%) (97%) (98%)

T1

T1 transition

3.35 3.16 3.31 3.44

H-3→L H-3→L H-3→L H-3→L

(41%) H →L (41%) (21%) H →L (70%) (13%) H →L (75%) (80%)

All energy values are in eV.

eV, respectively). The changes in the geometry upon electron attachment are significantly different in the 9-Cz and 2-DBT radicals. In the latter, the adjacent bond distances (α in Figure 4) expand from 1.37 to 1.42 Å, and the angle markedly decreases; the interphenyl bond associated with the π-system (β in Figure 4) contracts by a small amount, 0.01 Å. In 9-Cz, the C−N−C bond angle (c in Figure 4) decreases only slightly by 1°, and the C−N bond (a in Figure 4) contracts by 0.01 Å; the bond connecting the two phenyl rings in the Cz moiety (b in Figure 4) contracts by 0.03 Å, which is the largest change in the 9-Cz case. Thus, the geometry changes upon electron attachment to the radical are stronger in 2-DBT than in 9-Cz, a feature which is confirmed by the magnitude of the reorganization energies (0.89 and 0.16 eV in 2-DBT and in 9-Cz, respectively; see Figure S4). 3.3. C−N Bond Dissociation Energies of the Cz-DBT Compounds. Table 3 gives the homolytic BDEs for x-Cz-

Figure 3. Orbital isosurfaces (0.04 au) for (a) HOMO and (b) LUMO of 2-Cz-DBT, as calculated at the B3LYP/6-31G(d) level.

Table 3. B3LYP/6-31G(d) C−N Bond Dissociation Energies (eV) of Cz-DBTs, 3,6-diCz-DBT, and CzDBTO reactant

Figure 4. Top: Bond lengths and bond angles of the 2-DBT (left) and 9-Cz (right) moieties; the structural values are given (from left to right) for the full molecule (X = 2-DBT or 9-Cz)/the neutral radical (X = •̇)/and the anion (X = −). Bottom: Illustration of the singly occupied MO (isosurface: 0.04 atomic units) of each radical.

products ·

·

[x-DBT] + Cz [x-DBT]· + Cz+ [x-DBT]+ + Cz· [x-DBT]· + Cz− [x-DBT]− + Cz·

(initially lone-pair) π-atomic orbital formally singly occupied, with the unpaired electron delocalized over the whole carbazole π-system.36 Analogous radical stabilization via the redistribution of lone-pair electrons has also been reported by Lauvergnat et al. in the case of ammonia dissociation.43 For DBT, however, a similar reorganization does not occur because it would mean breaking the π-sextet of a benzene ring. Thus, the unpaired electron remains localized in the σ-system of the dissociated carbon atom. The 9-Cz radical has a C−N bond length of 1.38 Å (a in Figure 4) and a C−N−C bond angle (c in Figure 4) of 105.1°, which are decreased by 0.02 Å and 3° compared to those of the parent compound. The Mulliken spin density on the nitrogen is 0.506, confirming that the radical is delocalized compared to situation in the x-DBT radicals, where the Mulliken spin densities of the carbon radical centers are on the order of 0.997−1.003. Electron attachment for both radicals is favorable; however, the EA of the 9-Cz radical, 1.95 eV, is significantly larger than those of all the x-DBT radicals, where EA ≈ 1 eV; see Figure S3 (we note that addition of diffuse functions by using the 631+G(d) basis set increases the EAs by ∼0.4 eV, as shown in Table S2; also, the absolute value of the EA in the x-DBT radicals is slightly smaller than that of prototypical aromatic σradicals; for example, the calculated EA for the phenyl radical is 1.11 eV, and the EAs for 1- and 2-naphthyl are 1.37 and 1.30

a

1-CzDBT

2-CzDBT

3-CzDBT

4-CzDBT

3,6diCzDBT

3.67 4.23 4.81 1.59 2.46

3.70 4.38 5.01 1.68 2.79

3.71 4.41 4.98 1.65 2.88

3.60 4.13 4.64 1.51 2.45

3.85 4.64 4.89 2.13 3.16

CzDBTOa 3.52 4.13 2.66

Ref 19.

DBT compounds and the energies for both of the possible cleavage pathways when the parent molecules are charged. The homolytic BDEs of the neutral parents range from 3.60 to 3.71 eV for the Cz-DBT compounds (the value increases to 3.85 eV for the 3,6-diCz-DBT and decreased to 3.52 eV for 3,6-di(9Hcarbazol-9-yl)dibenzothiophene-S,S-dioxide,19 CzDBTO). These values are comparable to those of other C−N BDEs of aromatic amines reported in the literature.35 The BDE values follow the same trend as the dihedral angle and bond length between the two fragments, with 4-Cz-DBT < 1-Cz-DBT < 2Cz-DBT ≈ 3-Cz-DBT, which reflects the overall degree of conjugation. When there are positively charged parents, the BDEs follow the same trend, although the range of values is greater than that in the neutral case. For a cationic parent, both possible dissociation pathways show larger BDEs than when the parent is neutral, reflecting the ease of ionization of the parent in comparison to either radical product. As the stability of the charged fragment often determines the most probable dissociation path,44 the identity of the products is also 5794


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Chemistry of Materials important. In each case, the products with a positively charged 9-Cz species are favored over the formation of a DBT cation, which is consistent with the smaller IE of carbazole (the IEs of Cz and DBT are 7.11 and 7.49 eV, respectively). The average BDEs of the neutral and cationic species are greater than the triplet energies of the host material and potential dopants; thus, degradation via thermal pathways of an exciton or exciton− positively charged polaron energy transfer10,45 would be limited. In contrast, for negatively charged parents, the BDEs are lower than those for neutral parents, as the EAs are larger in the fragments than in the parent compounds. The more facile dissociations of C−N bonds via anionic parents are consistent with experimental reports for other host materials with similar C−N bonds.46,47 Importantly, for both dissociation pathways, the anionic BDEs are calculated to be lower than the triplet energies of the host materials. The formation of 9-Cz anions have BDEs on the order of ∼1.5−1.6 eV, which are 0.9 to 1.2 eV lower than if the products form a x-DBT anion. Such energies are similar to those of red photons, which implies that serious degradation can be induced during OLED operation. Since the extra electron in each of the parent Cz-DBT anions is initially localized on the DBT fragment, a crossing of electronic states must occur to lead to the lowest BDE products. A sketch of the corresponding potential energy surfaces is given in Figure 5 for the 2-Cz-DBT anion.

estimate for the effect of the condensed phase on the C−N BDEs, we have calculated all of the parameters using PCM for two dielectric constants, ϵ = 2.37 and 4.27 (the values for toluene and thiophenol, respectively), which are expected to bracket the actual thin-film dielectric constant. As shown in Table S5, while there are only small changes in the homolytic BDEs (ca. 0.05 eV), the BDEs with anionic parents significantly decrease, on average by 0.3 eV for ϵ = 2.37. Importantly, the anionic BDEs are uniformly reduced. 3.4. Substitution Effects. In ambipolar OLED host materials, both electron-withdrawing substituents and electron-donating substituents have been used to manipulate the electronic energy levels.48−50 However, control of BDEs via substitution using electron-withdrawing or -donating substituents is a considerably more challenging task.51−53 We have learned from the results described in the previous sections that the C−N bond dissociations of anionic Cz-DBT parents are potentially more damaging than those in other parents: Since a redistribution of electron density occurs upon C−N bond dissociation, it is reasonable to expect that the anionic BDEs could be modulated by the presence of a substituent on the DBT moiety where the extra electron is localized. To assess how these aspects are related, we have considered the impact of a series of substituents, i.e., cyano (−CN), fluoro (−F), and hydroxyl (−OH), which cover a range of electron-withdrawing and -donating characteristics. In particular, we have selected the 6- and 7-positions of DBT, which are meta- and para- to the opposing phenyl ring, respectively. The results are given in Table 4. As expected, the electron-withdrawing CN group is calculated to stabilize the frontier molecular orbital energies, especially the LUMO. Particularly, the LUMO of 2-Cz-7-CNDBT is ∼0.3 eV lower than that in 2-Cz-6-CN-DBT; this is reasonable considering that the LUMO coefficient at the 7carbon (para) is high, while the LUMO has a node at the 6carbon (meta).54 As a result, the energies of the lowest singlet and triplet excitations decrease since these transitions are dominated by a HOMO−LUMO excitation (see the 2-Cz-DBT case in Table 2). The LUMO energy stabilization via CN substitution comes mainly from the DBT moiety, where the LUMO is localized and is reflected in the larger EAs of the 2Cz-x-CN-DBT radicals compared to that of the unsubstituted one. This result implies that the redistribution of the extra electron of the anion parent (from the DBT to the Cz moiety, Figure 5) would be made more difficult with CN substitution. Consequently, the BDEs of the anionic parents are greatly

Figure 5. Energy diagram along the reaction pathway for the C−N bond dissociation of the 2-Cz-DBT anion with illustration of the spin densities (isosurfaces: 0.008 atomic units). A crude estimate for the energy of the 2-[Cz]−-DBT state is given by adding the difference in the LUMO and LUMO+1 orbital energies to the anionic ground state.

The energies of reactions resulting in charged fragments are highly dependent on the surrounding medium. To provide an

Table 4. B3LYP/6-31G(d) Frontier Molecular Orbital Energies, Lowest Excited-State Energies (Vertical, S1, and T1), Radical Electron Affinities (EAs), and C−N Bond Dissociation Energies (BDEs) of the 2-Cz-DBT Derivatives for Different Substituent (−CN, − F, and − OH) Positionsa C−N BDE

a

structures

HOMO

LUMO

H-L gap

S1

T1

EA of DBT radical

anion (Cz−)

anion (DBT−)

neutral (homolytic)

2-Cz-DBT 2-Cz-6-CN-DBT 2-Cz-7-CN-DBT 2-Cz-6-F-DBT 2-Cz-7-F-DBT 2-Cz-6-OH-DBT 2-Cz-7-OH-DBT 2-(3-CN-Cz)-DBT

−5.35 −5.56 −5.58 −5.41 −5.39 −5.32 −5.28 −5.79

−1.24 −1.67 −2.01 −1.41 −1.27 −1.24 −1.06 −1.49

4.11 3.89 3.57 4.00 4.12 4.08 4.22 4.30

3.63 3.42 3.17 3.53 3.63 3.60 3.73 3.80

3.16 3.04 2.81 3.10 3.15 3.13 3.15 3.25

0.85 1.23 1.28 0.96 0.94 0.88 0.81 0.85

1.68 2.16 2.49 1.87 1.73 1.75 1.56 1.41

2.79 2.89 3.16 2.86 2.75 2.82 2.70 3.20

3.70 3.69 3.70 3.70 3.70 3.70 3.71 3.77

All values are in eV. 5795


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compounds, CN substitution increases the anionic C−N BDEs resulting in the formation of a 9-Cz anion by more than 0.4 eV. For the −F and −OH substitutions, the variations in anionic BDEs are relatively small, in the range ∼0.1−0.2 eV. The same patterns of positional dependence as in the 2-Cz-DBT compounds are obtained in the other Cz-DBT compounds. The anionic C−N BDEs leading to a 9-Cz anion and the energies of the lowest excited singlet and triplet states for all Cz-DBT compounds are plotted in Figure 6 as a function of the

increased, while those of the neutral cases remain nearly the same. A similar evolution is found for the other electronwithdrawing substituent, −F, i.e., although its effect is substantially smaller than that of −CN. We note that the LUMO energies of the Cz-DBTs substituted with −CN and −F follow the trend expected from a consideration of the Hammett constants.55 The third substituent, the −OH group, has its electronwithdrawing or -donating character depending on the substitution position. It has weak (inductive) electron-withdrawing character at the 6 (meta) position; thus, there are only modest changes with OH substitution in this position. However, for substitution in the 7 (para) position, its mesomeric electron-donating character is explicit, and the LUMO energy increases; this results in a slightly increased singlet excitation energy and a decreased anionic dissociation energy. We have also examined substitution on the Cz moiety. Substituting a CN on the 3-position of Cz stabilizes the HOMO to a large extent, with a smaller stabilization of the LUMO (this is expected given the distinct HOMO and LUMO distributions on the Cz and DBT moieties, respectively). As a consequence, the EA of the 9-Cz radical is increased by the −CN group, while that of the DBT radical is not altered. It implies that the electron redistribution on the anion parent is enhanced. Hence, the BDEs of anionic parents are reduced, even more so than in the −OH case. Finally, given the significant variations in excited-state energies and anionic C−N BDEs observed as a function of substituent position, in order to achieve a more comprehensive picture, we have chosen to investigate all possible substitutional derivatives of the Cz-DBT compounds. The results are tabulated in Table 5 for the −CN group and in the Supporting Information for the −F and −OH groups (Table S6). An important result is that, regardless of the parental Cz-DBT

Figure 6. Lowest excited-state energies (vertical, S1, and T1) of the CzDBT derivatives and B3LYP/6-31G(d) C−N bond dissociations energies (BDEs) of the corresponding anions with different substituents, as a function of the HOMO−LUMO gap.

HOMO−LUMO gap. All three series of data are seen to correlate with the HOMO−LUMO gap, with the anionic C−N BDEs and the lowest excited-state energies being inversely proportional to each other. This correlation can be rationalized by two observations: (i) The excited singlet and triplet states effectively correspond to intramolecular charge-transfer states. Thus, larger HOMO−LUMO gaps lead to increased excitedstate energies, though to a lesser extent for triplet states due to their contributions from local excitations, as discussed above. (ii) The extra electron present on a Cz-DBT anion is localized on the DBT moiety but moves to the 9-Cz fragment upon bond dissociation. Hence, a larger HOMO−LUMO gap, which can be related to either a smaller EA of the x-DBT radical or a larger EA of the 9-Cz radical, is reflected in a promoted electron redistribution (see Figure 5) and subsequently, in lower anionic C−N BDEs into a 9-Cz anion. The critical consequence of these two observations is that strategies that can be followed to increase the triplet energy, e.g., by attaching an electron-donor group on the DBT moiety, can lead to a lower anionic C−N BDE. Thus, appropriate tradeoffs must be considered when designing blue OLED hosts, given that these materials are required to combine chemical stability with high excited-state energies.

Table 5. B3LYP/6-31G(d) Lowest Excited-State Energies (Vertical, S1, and T1) of the Cz-DBTs with −CN and C−N Bond Dissociation Energies (BDEs) of the Corresponding Anions vs the HOMO-LUMO (H-L) Gapa

a

structures

H-L gap

S1

T1

anion C−N BDE (9-Cz−)

1-Cz-DBT 2-Cz-DBT 3-Cz-DBT 4-Cz-DBT 1-Cz-8-CN-DBT 2-Cz-8-CN-DBT 3-Cz-8-CN-DBT 4-Cz-8-CN-DBT 1-Cz-7-CN-DBT 2-Cz-7-CN-DBT 3-Cz-7-CN-DBT 4-Cz-7-CN-DBT 1-Cz-6-CN-DBT 2-Cz-6-CN-DBT 3-Cz-6-CN-DBT 4-Cz-6-CN-DBT 1-Cz-5-CN-DBT 2-Cz-5-CN-DBT 3-Cz-5-CN-DBT

4.18 4.11 4.11 4.24 3.67 3.66 3.63 3.85 3.59 3.57 3.51 3.67 3.94 3.89 3.84 3.93 3.56 3.51 3.36

3.62 3.63 3.58 3.56 3.20 3.26 3.17 3.23 3.10 3.17 3.05 3.06 3.39 3.42 3.32 3.26 3.08 3.11 2.88

3.35 3.16 3.31 3.44 3.10 2.93 3.01 3.22 2.99 2.81 2.87 3.05 3.23 3.04 3.12 3.26 3.01 2.80 2.73

1.59 1.68 1.65 1.51 2.19 2.30 2.28 2.12 2.41 2.49 2.48 2.33 2.08 2.16 2.13 1.97 2.33 2.41 2.36

4. CONCLUSIONS We have examined the C−N bond dissociation process in carbazole (Cz)-dibenzothiophene (DBT) positional isomers (x-Cz-DBTs, x = 1, 2, 3, and 4) with substitutions occurring on both the DBT and Cz moieties. Irrespective of substitution and Cz position, the C−N bond dissociation energies (BDEs) of

All values are in eV. 5796



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the anionic species are lower than that for neutral and cationic species. This characteristic can be attributed to the electron affinity of the fragments exceeding that of the neutral molecule. A series of substituents, −CN, −F, and −OH, have been attached to the DBT moiety to gain an understanding of the substituent effects. While the homolytic C−N BDEs are nearly independent of substitution, the anionic C−N BDEs strongly depend on the substituents. Importantly, we have found that the low values of the anionic C−N BDEs in the unsubstituted compounds, can be increased by manipulating the relative electron affinities of the dissociation fragments. For instance, the electron affinity of a CN-substituted x-DBT is relatively larger than that of unsubstituted x-DBT, compared to that of 9Cz. Hence, upon C−N bond dissociation, the extra electron localized in the CN-substituted DBT moiety has less tendency to redistribute toward the Cz moiety; consequently, the C−N bond dissociation resulting in a 9-Cz anion becomes less probable than that in the unsubstituted case, i.e., the anionic C−N BDE increases. The substituents also affect the electronic structures of the Cz-DBT compounds, in which the S1 and T1 excited states are characterized by an intramolecular charge transfer (ICT) transition. Interestingly, the substituent effects on the BDEs and on the excited-state energies show a trade-off. When the C−N BDEs increase due to substitution, which is favorable in terms of chemical stability, the excited-state energies decrease, which can be detrimental to blue emission. Although the simple functionalization of molecules considered in the present work might not be optimal to achieve both high excited-state energy and good chemical stability, our results point to the importance of systematic investigations of the relationships between excited states and BDEs in the design of blue OLED host materials.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02069. Vertical excitation energies, C−N homolytic bond dissociation energies using ω-B97XD, M06-2X, and G3//B3LYP with different basis sets, structural data for Cz-DBT compounds, and solvent effects on the anionic BDE (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by King Abdullah University of Science and Technology and in part at the Georgia Institute of Technology by the Global Research Outreach (GRO) Program of the Samsung Advanced Institute of Technology (SAIT). We acknowledge the IT Research Computing Team and Supercomputing Laboratory at KAUST for providing computational and storage resources. We thank Dr. Gjergji Sini for insightful discussions. 5797


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Effect of Substituents on the Electronic Structure and Degradation

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