Electronic Structure of Carbazole-Based Phosphine Oxides as

Jun 13, 2012 - *Email: [email protected], ... The hosts under investigation contain either 1, 2, or 3 carbazole subunits attached to the phen...
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Electronic Structure of Carbazole-Based Phosphine Oxides as Ambipolar Host Materials for Deep Blue Electrophosphorescence: A Density Functional Theory Study Dongwook Kim,*,†,‡ Lingyun Zhu,† 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 ‡ Department of Chemistry, Kyonggi University, San 94-6 Iui-Dong, Yeongtong-Gu, Suwon 443-760, Korea S Supporting Information *

ABSTRACT: We report the results of Density Functional Theory calculations on a series of carbazole-based phosphine oxides that experimental data have shown to be promising ambipolar host molecules for deep blue electrophosphorescence. The hosts under investigation contain either 1, 2, or 3 carbazole subunits attached to the phenyl rings of a triphenylphosphoryl group, with the carbazoles acting as hole transporters/acceptors and the triphenylphosphoryl groups as electron transporters/acceptors. The results underline that, in addition to the strong inductive effect of the phosphoryl groups, the LUMO of these hosts is further stabilized by the molecular orbital interactions among the phenyl rings of the triphenylphosphoryl group, which is modulated by the electron-withdrawing inductive effects of the carbazole subunits. The lowest triplet state of the hosts correspond to localized transitions within the carbazole units, which leads to a high triplet energy on the order of 3 eV. We describe the important buffer role of the phenyl rings in preventing the phosphoryl moiety from negatively affecting the hole-accepting characteristics and high triplet energies of the carbazole units. KEYWORDS: deep blue OLED, electroluminescence, ambipolar hosts, phosphine oxides, DFT calculation

I. INTRODUCTION Highly efficient electroluminescent materials are required for the application of organic light-emitting diodes (OLEDs) in full-color display and solid-state lighting.1 Small-molecule OLED emitters generally correspond to phosphorescent coordination complexes based on heavy metal elements, such as Ir and Pt, in order to harness both triplet excitons and singlet excitons and to reach internal quantum efficiencies approaching 100%. However, to prevent concentration quenching of the phosphorescence, the emitters need to be incorporated into an organic host matrix. Therefore, the performance of OLED devices substantially relies on the electronic properties of the host materials. Since luminescence in OLEDs takes place via the recombination of holes and electrons, the host materials are required to present good charge-carrier mobilities for both holes and electrons. Also, to maximize the electron−hole recombination at a given voltage, a balanced charge injection into the host from the hole- and electron-transport layers © 2012 American Chemical Society

should be realized. In addition, the hosts should possess a higher triplet energy than the emitters to ensure exothermic energy transfer from the host to the guest and prevent back energy transfer. The development of host materials for blue OLEDs, in particular deep-blue devices, remains challenging. On the one hand, the high exciton energy for blue-light emission requires a large optical band gap, which necessarily limits the πconjugation length of the system. On the other hand, materials with a reduced HOMO−LUMO gap (that is, a low ionization potential and high electron affinity) are needed for facile and balanced charge injection. As a result, these somewhat contradictory conditions can hardly be achieved with molecules based on units with a similar chemical nature, since in this Received: May 8, 2012 Revised: June 13, 2012 Published: June 13, 2012 2604

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Figure 1. Chemical structures of the molecules discussed in this study: (a) triphenyl-phosphine oxide-based host molecules with one carbazole (CBZ) unit, N-(4-diphenylphosphoryl phenyl) carbazole (host 1), with two CBZ units, bis-4-((N-carbazolyl)phenyl)phenylphosphine oxide (host 2), and with three CBZ units, 4,4′,4″-tri(N-carbazolyl)triphenylphosphine oxide (host 3); (b) N,N′-dicarbazolyl-3,5-benzene (mCP) as a reference host molecule for blue electrophosphorescence; (c) 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) as an electron-transport molecule; (d) N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPD) as a hole-transport molecule; and (e) iridium(III) bis((4,6difluorophenyl)pyridinato-N,C2′) picolinate (FIrpic) and (f) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6), as blue emitters.

improve the injection and charge-transfer properties of the host dramatically: Cho and Cheng developed bis-4-((N-carbazolyl)phenyl)phenylphosphine oxide (2) and incorporated it with FIrpic into devices that are highly efficient (ca. 23% maximum EQE).15 In addition, the lowest triplet energy of 2 is reported to be high enough (∼3.01 eV) to serve in deep-blue OLEDs. A third substitution with carbazole, however, leads to lower device performance: A device with 4,4′,4″-tri(N-carbazolyl)triphenylphosphine oxide (3) presents a maximum EQE of ca. 16% with FIrpic, although 3 exhibits a triplet energy on the order of 3.01 eV and good electron-transporting properties.16 A simple description of the HOMO/LUMO wave functions for 1 and 3 was presented previously;8,16 however, to the best of our knowledge, a systematic study of the effects of additional carbazoles on the charge injection and optical properties of hosts containing the triphenylphosphoryl moiety has not been reported. Here, we detail the results of Density Functional Theory (DFT) calculations on hosts 1−3. We address the effect of the various subunits (i.e., the carbazole, phenyl, and phosphoryl groups) on the charge-injection barriers with the holetransport/electron-transport layers. We also discuss the lowest triplet energies and the nature of the T1 states in these host molecules.

instance, the optical band gap and HOMO−LUMO gap are usually strongly correlated.2−9 This explains that considerable effort has been put forth to develop ambipolar host materials formed by the combination of hole-transport and electrontransport moieties with high triplet energies.6−8,10−20 For example, Kido and co-workers have reported the synthesis of ambipolar host molecules that combine carbazole and pyridinebased terphenyl-like subunits, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine and 3,5-bis(3-(carbazol-9-yl)phenyl)pyridine,10,11 while Hsu et al. have developed phosphine-oxide-based ambipolar hosts.13,14 In spite of excellent device performance, the lowest triplet energies of these molecules are on the order of 2.7−2.8 eV, which limits their applications for deep-blue OLEDs. In this context, much attention has been paid to triphenylphosphoryl-substituted carbazole molecules. For instance, the N-(4-diphenylphosphoryl phenyl) carbazole (1, see Figure 1), host synthesized by Sapochak et al., presents a lowest triplet energy as high as 3.10 eV, that is, high enough for deepblue OLED applications.6−8 However, the device performance remains rather poor (∼ 8% external quantum efficiency, EQE, with FIrpic), even though improvement in the balance of charge injection renders 1 an ambipolar host. Intriguingly, the introduction of an additional carbazole moiety appears to 2605

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Table 1. HOMO and LUMO Energies and Adiabatic Ionization Potential (IP) and Electron Affinity (EA) Values for Nphenylcarbazole (pCBZ), Host Molecules 1−3, mCP, NPD, TAZ, FIrpic, and FIr6.a LUMO EAb HOMO IPc

pCBZ

1

2

3

mCP

NPD

TAZ

FIrpic

FIr6

−0.86 0.50 −5.50 7.01

−1.23 −0.16 −5.61 7.10

−1.37 −0.43 −5.64 6.88

−1.45 −0.55 −5.67 6.86

−0.97 0.11 −5.63 6.94

−1.36d −0.46d −4.88d 6.14

−1.42 −0.46 −5.73 6.89

−1.88e −0.68e −5.82e 7.01

−1.87 −0.78 −5.89 7.56

a

All the values are in eV and were obtained from DFT calculations using the B3LYP functional and SV(P) basis set. bEA is defined as E(anionic state) − E(neutral state). cRadical-cationic structures were optimized at the HF level of theory. dData from ref 22. eData from ref 9.

Figure 2. Correlation diagram for the frontier molecular orbitals (FMOs) of N-phenylcarbazole and host 1. Orbitals of a similar nature are connected by dotted lines. localization of the positive charge on a single carbazole unit breaking the overall symmetry of the molecule), those structures were derived at the spin-unrestricted Hartree−Fock (HF) level of theory; HF calculations were carried out using the Gaussian 09 package,26 which turns out to be more robust in the evaluation of broken-symmetry structures. All the geometries were confirmed to be local minima via additional vibrational-frequency calculations. The adiabatic ionization potential (IP) and electron affinity (EA) values and the lowest triplet energies (ET) were determined via the ΔSCF method based on the optimized geometries for the respective electronic states. Zero-point vibrational energy (ZPVE) corrections were also taken into account for the triplet energies. To shed more light into the nature of the T1 states, natural orbital transition (NTO) analyses were performed via time-dependent (TD) DFT calculations based on the T1-state structures.

II. COMPUTATIONAL DETAILS The host molecules under investigation in this study are composed of the triphenylphosphoryl group with one (1), two (2), or three (3) additional carbazoles (Figure 1a). In order to gain better insight into the injection and charge-transfer processes, we also carried out calculations on the geometric and electronic structure of (i) N,N′dicarbazolyl-3,5-benzene (mCP), taken as a reference host molecule for blue electrophosphorescence, Figure 1b; (ii) 3-(4-biphenyl)-4phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), a representative electron-transport layer (ETL) material, Figure 1c; and (iii) bis(4′,6′difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6), a deeper blue phosphorescent emitter than FIrpic,21 Figure 1f. For the sake of completeness, computational data on N,N′-bis-(1-naphthyl)-N,N′diphenyl-1,1′-biphenyl-4,4′-diamine (NPD), as a representative holetransport layer (HTL) material, (Figure 1d) and on FIrpic, a widely used sky-blue phosphorescent emitter (Figure 1e), were reproduced from our previous work.9,22 All the calculations were conducted at the DFT level of theory using the B3LYP hybrid functional with the TurboMole package (version 6.0);23−25 we employed the default effective core potential (ECP) to deal with the core electrons of Ir and the split-valence SV(P) basis set for the valence electrons of Ir and all the electrons of the other atoms. The ground-state (S0) geometries were optimized via spin-restricted calculations while the optimal structures in the lowest triplet state (T1) and radical-ion states were evaluated via spin-unrestricted calculations. Since 2+ and 3+ are susceptible to broken-symmetry effects (that is,

III. RESULTS AND DISCUSSION 1. Frontier Molecular Orbital (FMO) Energies: Impact of the Subunits. Table 1 collects the IPs/EAs of the molecules studied in this work evaluated at both ΔSCF and Koopmans’ theorem (HOMO/LUMO) levels. In previous studies,22,27 we have shown that, in molecules composed of several distinct moieties, one subunit can affect the electronic structure of the others and of the whole molecule. When a phosphoryl (PO) group is attached directly to carbazole, 2606

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Figure 3. Correlation diagram for the frontier molecular orbitals (FMOs) of hosts 1−3. Orbitals of a similar nature are connected by dotted lines.

consistently decreasing by ca. 30 meV with each additional carbazole, see Figure 3 (and Table S1 in the Supporting Information). To rationalize this evolution, we note that the carbazole molecule presents a calculated dipole moment of ca. 1.77 D; thus, the electrostatic interactions among the carbazole dipoles can play a role. In order to confirm this point, we carried out single-point calculations of hosts 1−3, from which the central triphenylphosphoryl group is removed and the nitrogen of a carbazole unit is capped with a hydrogen. The results (see Figure S1, Supporting Information) show that the host molecules and the systems made simply of carbazoles present exactly the same trends with regard to the evolution of the orbital energies of the carbazole subunits. However, the impact of these electrostatic interactions remains limited due to the rather long distance between the carbazole subunits (ca. 10 Å between the centers of mass of the carbazole segments). On the other hand, the LUMO and LUMO+1 orbitals of the hosts are affected in a more complex way upon further addition of carbazole unit(s). In our previous works on ambipolar hosts,22,27 we have observed that a carbazole also exerts an electron-withdrawing inductive effect when it is linked via its N atom. As a result of such an inductive effect from the carbazole unit, the orbitals of carbazole-substituted phenyl rings get substantially stabilized. Hence, there occur significant energy offsets between the orbitals of a phenyl ring with a carbazole and those of a phenyl ring without a carbazole substituent, which severely limits any resonant interactions between these orbitals; on the other hand, significant resonant interactions are expected among the orbitals of either unsubstituted phenyl rings or substituted ones. In the case of host 1, see Figure 2, the LUMO is mainly localized within the carbazole-substituted phenyl ring, which is essentially due to the inductive effects of its substituents, while there is negligible interaction with the LUMOs of the other

both the LUMO and HOMO energies are significantly stabilized, to the point that carbazole, intrinsically a holetransporting specie, is transformed into an electron-accepting/ hole-blocking material.6,8,9 However, when a phenyl ring is inserted in between the phosphoryl group and carbazole, as in hosts 1−3, the evolution of the FMO energy levels appears to be significantly different. Indeed, when going from Nphenylcarbazole (pCBZ) to host 1, while the LUMO energy becomes strongly stabilized, as expected from the strong electron-pulling nature of the phosphoryl group (from −0.86 eV for pCBZ to −1.23 eV for host 1, see Table 1), the HOMO energy is only marginally altered (from −5.50 eV to −5.61 eV). This result implies that the inductive effect of the phosphoryl group is much less effective.8 To better understand this disparity in the impact of the phosphoryl group, we further examined the FMO wave functions of pCBZ and hosts 1, see Figure 2. The LUMO of pCBZ is clearly localized within the carbazole unit while the LUMO+1 and LUMO+2 reside mostly on the phenyl ring. The addition of the phosphoryl group is seen to affect much more significantly the orbitals of the subunit to which it is attached than those of the more distant carbazole. Indeed, the stabilizations of phenyl-based LUMO+1 and LUMO+2 orbitals are in the range 0.53−0.57 eV, while the carbazole-based LUMO and HOMO orbitals get stabilized only by ca. 0.1 eV. As a result, importantly, the LUMO+1 and LUMO+2 of pCBZ become the LUMO and LUMO+1 of host 1, respectively. On the other hand, the HOMO and HOMO−1 of host 1 remain localized within the carbazole unit, which explains that the effect of the phosphoryl group on these energy levels is modest. Although the influence of the phosphoryl group is limited, when going from host 1 to 3, the energies of the frontier orbitals localized within the carbazole subunits (i.e., the HOMO−1, HOMO, and LUMO+2 of the hosts) keep 2607

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In terms of electron injection from the electron-transporting TAZ to the host, the barrier decreases with the number of carbazole subunits. The EA difference between host 1 and TAZ is evaluated to be around 0.3 eV (−0.16 vs −0.46 eV), indicating a sizable barrier for electron injection. However, for host 2, the EA difference becomes negligible (0.03 eV); host 3 has even a larger EA value (by 0.09 eV) than TAZ. As a result, the charge-injection properties of the hosts are expected to improve as additional carbazole units are appended to the triphenylphosphoryl core. In view of the results described, the experimental data appear to be somewhat puzzling. Although hosts 2 and 3 work much better in FIrpic-based devices than host 1, host 3 was outperformed by 2.16 Chaskar et al. suggested that the relatively poor performance of the device with host 3 might be related to the shape of the molecule;17 as electron transport occurs via the central triphenylphosphoryl segment, which in the case of host 3 is fully surrounded by three carbazole units, the electrontransport pathways for host 3 might be less effective than those for host 2;17 indeed, host 3 was found to transport more holes than electrons.16 To the best of our knowledge, however, a detailed comparison of the charge-transport properties of hosts 2 and 3 remains to be performed. Furthermore, Kido and coworkers have questioned the importance of the electron mobility of the organic materials in the performance of devices based on FIrpic.31 Given that electrons are easily trapped by FIrpic and electroluminescence appears to take place mainly near the interface between the emissive and electron-transport layers,32 a direct electron−hole recombination on the guest would be the key process of exciton generation;18 hence, the electron-transport properties of the host might play only a limited role, as long as electron injection is efficient.31 Since our results indicate that host 3 would be more favorable in terms of electron-injection barrier, the reason for its relatively poor performance in blue devices remains ambiguous. Again, the shielding of the triphenylphosphoryl core by the carbazole units can be suspected to play a role. When comparing the two emitters, FIr6 presents a larger IP than FIrpic (7.56 vs 7.01 eV) and a larger EA (−0.78 vs −0.68 eV). This implies that, when FIr6 is employed in the emissive layer, it would be harder for holes to transfer to the guest, while electrons are likely trapped more deeply by the guest. Therefore, direct recombination on the guest is expected to be suppressed compared to the case of FIrpic; this appears to be consistent with the higher efficiency of FIrpic-based devices.15 3. Triplet Energy: Buffer Role of the Phenyl Groups. As mentioned in the Introduction, the lowest triplet energy of the host molecule is required to be higher than that of the guest emitter to favor exciton formation/transfer on the latter. In this regard, ambipolar host molecules for blue electrophosphorescence usually involve the combination of moieties with high triplet energy. However, as shown earlier,9,27 the subunits can mutually impact their electronic structures and, thus, the triplet energy of the whole molecule. To develop promising host materials for blue luminescence, substituent effects that undermine a high triplet energy should be prevented. Thus, it is important to evaluate the triplet energies and characterize the nature of the lowest triplet state of the host molecules. The calculated triplet energies of hosts 1−3 and of the blue emitters are compared to experimental values in Table 2. The calculated values are in excellent agreement with experiment; the experimental data are found to be bracketed between the

two, unsubstituted phenyl rings. On the other hand, the LUMO +1 is delocalized over the unsubstituted phenyl rings due to the strong interaction between them. With the additional carbazole unit in host 2, the orbital energies of the carbazole-substituted phenyl rings come in resonance, and as a result, the LUMO+1 for host 1 evolves into LUMO for host 2, see Figure 3. In the case of host 3, the LUMO and LUMO+1 resemble the corresponding orbitals of host 2, as shown in Figure S2 (Supporting Information), which suggests that the orbitals of each phenyl ring interact in the same way for both hosts. Since all the phenyl orbitals in host 3 are closer in energy, their interactions are stronger and result in further stabilization of the LUMO and LUMO+1. It is worth stressing that this wave function delocalization among the phenyl rings occurs across the phosphoryl group. A phosphoryl group is usually considered as an effective breaking point of π-conjugation.5,6,8 As discussed, however, this break in conjugation is effective mainly when a significant orbital energy offset is present among the phosphine oxide substituents, which then leads to vanishing MO interactions. In summary, at this stage, we have found that the HOMO levels of the hosts are localized within the carbazole unit(s) and are only slightly affected by the inductive effect of the phosphoryl group and the electrostatic interactions among the carbazole subunits; these effects remain modest indeed due to the buffer role of the phenyl rings and the rather long distance between the carbazole dipoles. However, the LUMOs of the hosts reside on phenyl rings and their energies are considerably influenced by the inductive effects of the phosphoryl group (from N-phenylcarbazole to host 1) and the carbazole units (for N-phenylcarbazole and hosts 1−3). Furthermore, the molecular orbital interactions among the phenyl rings entail further significant LUMO stabilizations (from host 1 to 2 and 3). 2. IPs and EAs: Charge-Injection Barriers. Because a reduction in charge-injection barriers from the adjacent holetransport/electron-transport layers (HTL/ETL) is one of the major drivers for the development of new ambipolar host materials, estimating the extent of those barriers is of high importance. This can be qualitatively done by comparing the ionization potentials (IPs) and electron affinities (EAs) of the relevant molecules.28 In the case of host 1, the IP value for the isolated molecule is calculated to be 7.10 eV, which is much higher than that of the hole-transporter NPD (6.14 eV) and even than that of the emitter FIrpic (7.01 eV).29 This points to an inefficient injection of holes from the hole-transport layer into the host matrix and the guest emitter. This situation, however, is seen to improve somewhat with additional carbazoles; the IP values for hosts 2 and 3 are calculated to be ca. 6.88 and 6.86 eV, respectively, and thus fall in between those of NPD and FIrpic. We note that, since the IP values for hosts 2 and 3 were obtained on the basis of geometries derived at the HF level of theory (in order to take into account the broken-symmetry effects), a quantitative comparison may not be adequate. In this regard, mCP was also examined, because it is a widely studied host for blue electroluminescence30 and is also subject to broken-symmetry effects. The mCP IP is calculated to be 6.93 eV, which is slightly higher than those of hosts 2 and 3. Given that in all of these molecules the hole is transferred via units with the same chemical nature, that is, carbazole (Figure S3, Supporting Information), these results suggest that hosts 2 and 3 are compatible with mCP as hole acceptors. 2608

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position with respect to C−C bond between the carbazole phenyl rings, see Figure 1) reduces the triplet energy by ca. 0.2 eV; the substitutions in 3,6 positions (meta positions) hardly affected the triplet energy in spite of the similar nature of the triplet state. This is due to the difference in mesomeric effect of directly appended phosphoryl groups on the hole and particle wave functions of NTOs.9 On the other hand, in the cases of hosts 1−3, a phenyl group is inserted in between the phosphoryl group and carbazole and actually serves as a buffer,33 in particular, vs. the mesomeric effect of the phosphoryl group. Therefore, to keep the triplet energy intact, it is crucial to minimize the substituent effect on the NTO wave functions that determine the triplet state; our results show that this can be achieved via the insertion of a buffer moiety.

Table 2. Adiabatic Lowest Triplet Energies of Nphenylcarbazole (pCBZ), Hosts 1−3 and the Blue Emitters, FIrpic and FIr6a pCBZ 1 2 3 FIrpic FIr6

ΔEtripb

ΔE0tripb

expt.

ref.

3.14 3.14 3.14 3.13 2.67c 2.77

2.96 2.95 2.95 2.95 2.56c 2.65

3.04 3.10 3.01 3.03 2.65/2.66 2.72

34 6 15 16 35/36 21

All energies are in eV. bΔEtrip. and ΔE0trip denote the lowest triplet energy without and with zero-point vibrational energy correction. c Data from ref 9. a

ZPVE (zero-point vibrational energy)-corrected and uncorrected DFT values. All the host molecules exhibit higher triplet energies than those of the FIrpic and FIr6 blue emitters, on the order of 3 eV. These values are higher than those of previously reported ambipolar host molecules,10,11,13,14 in the range of 2.7−2.8 eV. We also note that the high triplet energies of 1−3 are nearly identical to that of N-phenylcarbazole (Table 2). To better understand the nature of the lowest triplet state, NTO (natural transition orbital) analyses were conducted. As shown in Figure 4, the lowest triplet state is very similar in pCBZ and hosts 1−3. In all instances, the hole-particle pair of the NTOs corresponds to a localized transition within a carbazole unit, that is, the HOMO−1-to-LUMO transition within carbazole. In addition, we note that the HOMO−1 and LUMO of pCBZ are stabilized by nearly the same amount upon the introduction of either the phosphoryl group or additional carbazole units; therefore, the energy gap between the relevant hole-particle wave functions is hardly altered. In our previous study of phosphine-oxide-based host molecules, it was found that the substitution pattern of the phosphoryl group affected the triplet energy.9 The attachment of phosphoryl groups to carbazole in 2,7 positions (para

IV. SYNOPSIS We have investigated by means of DFT calculations the geometric and electronic structures of carbazole-based phosphine oxides, 1−3, that can serve as host molecules for deep blue electroluminescence. Hosts 1−3 are found to present an ambipolar character as the carbazole units are responsible for hole transfer and the triphenylphosphoryl group for electron transfer. In addition, the lowest triplet state corresponds to a localized transition within one of the carbazole subunits. The carbazole moiety exerts an electron-withdrawing inductive effect on the electronic structure of the triphenylphosphoryl group, which modulates the interactions among the frontier molecular orbitals of the phenyl rings and stabilizes the LUMO and anion state. As a result, as more carbazole units are appended, the electron-accepting property of the host improves. On the other hand, the phenyl group inserted in between the carbazole and phosphoryl groups serves as a buffer with respect to the inductive and mesomeric effects of the phosphoryl group. As a consequence, the IPs and lowest triplet energies of the hosts, which are determined by the carbazole subunits, remain unaffected.

Figure 4. Representation of the hole-particle wave functions corresponding to the natural transition orbitals (NTOs) for the lowest triplet excited state of the phosphine-oxide-based host molecules. Hole [particle] wave functions are depicted below [above] the arrows. 2609

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We believe that the findings in this work on carbazole-based phosphine oxides have a general applicability. They will be useful for the development of new host molecules, in particular, for deep blue electroluminescence.

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(12) Sasabe, H.; Kido, J. Chem. Mater. 2011, 23, 621. (13) Hsu, F.-M.; Chien, C.-H.; Shih, P.-I.; Shu, C.-F. Chem. Mater. 2009, 21, 1017. (14) Hsu, F.-M.; Chien, C.-H.; Shu, C.-F.; Lai, C.-H.; Hsieh, C.-C.; Wang, K.-W.; Chou, P.-T. Adv. Funct. Mater. 2009, 19, 2834. (15) Chou, H.-H.; Cheng, C.-H. Adv. Mater. 2010, 22, 2468. (16) Ding, J.; Wang, Q.; Zhao, L.; Ma, D.; Wang, L.; Jing, X.; Wang, F. J. Mater. Chem. 2010, 20, 8126. (17) Chaskar, A.; Chen, H.-F.; Wong, K.-T. Adv. Mater. 2011, 23, 3876. (18) Sasabe, H.; Seino, Y.; Kimura, M.; Kido, J. Chem. Mater. 2012, 24, 1404. (19) Jeon, S. O.; Jang, S. E.; Son, H. S.; Lee, J. Y. Adv. Mater. 2011, 23, 1436. (20) Yook, K. S.; Lee, J. Y. Org. Electron. 2011, 12, 1711. (21) Holmes, R. J.; D’Andrade, B. W.; Forrest, S. R.; Ren, X.; Li, J.; Thompson, M. E. Appl. Phys. Lett. 2003, 83, 3818. (22) Kim, D.; Coropceanu, V.; Brédas, J.-L. J. Am. Chem. Soc. 2011, 133, 17895. (23) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454. (24) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1997, 264, 573. (25) Furche, F.; Ahlrichs, R. J. Chem. Phys. 2002, 117, 7433. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; J. A. Montgomery, J.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (27) Salman, S.; Kim, D.; Coropceanu, V.; Brédas, J.-L. Chem. Mater. 2011, 23, 5223. (28) We note that electrochemical measurements also provide the oxidation/reduction potentials of hosts 1−3; these are collected in Table S2 (Supporting Information). However, these experiments were not carried out in a consistent manner; in the case of the reduction potentials, in particular, different solvents were used for all three hosts and can be expected to exert different impacts on the stability of the charged molecules. As a result, the HOMO/LUMO values derived from these electrochemical data are consistent neither with our calculated values nor with device performance. (29) This is in fair agreement with the results based on the geometries derived at DFT level; IPs for host 1, NPD, FIrpic, and FIr6 are calculated to be 6.94, 5.74, 6.68, and 6.99 eV, respectively. (30) Williams, E. L.; Haavisto, K.; Li, J.; Jabbour, G. E. Adv. Mater. 2007, 19, 197. (31) Xiao, L.; Su, S.-J.; Lan, H.; Kido, J. Adv. Mater. 2009, 21, 1271. (32) Tsai, M.-H.; Lin, H.-W.; Su, H.-C.; Ke, T.-H.; Wu, C.-c.; Fang, F.-C.; Liao, Y.-L.; Wong, K.-T.; Wu, C.-I. Adv. Mater. 2006, 18, 1216. (33) The effects of the substituents include the inductive effects of the phosphoryl group and the carbazole subunits, the mesomeric effect of the phosphoryl group, and dipole−dipole interactions among carbazole moieties. (34) Bonesi, S. M.; Erra-Balsells, R. J. Lumin. 2001, 93, 51. (35) Holmes, R. J.; Forrest, S. R.; Tung, Y.-J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E. Appl. Phys. Lett. 2003, 82, 2422. (36) American Dye Source, Inc., Quebec, Canada.

ASSOCIATED CONTENT

S Supporting Information *

Calculated frontier molecular orbital energies of pCBZ and hosts 1−3 at the DFT B3LYP/SV(P) level of theory; comparison of experimental and computed HOMO/LUMO values; evolution of the frontier molecular orbital energies in the hosts and the corresponding carbazole-only compounds; illustration of the frontier molecular orbitals of pCBZ and hosts 1-3; pictorial comparison between HOMOs and cation-state wave functions of hosts 2 and 3. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected], jean-luc.bredas@ chemistry.gatech.edu. Present Address §

Also affiliated with Department of Chemistry, King Abdulaziz University, Jeddah 21589, Saudi Arabia. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge stimulating discussions with Dr. Chad Risko, Dr. John Sears, and Dr. Veaceslav Coropceanu. This work has been mainly supported by Solvay and by the National Science Foundation (NSF) under the STC Program (Award DMR0120967); the computer resources at Georgia Tech have been supported in part by the CRIF program of NSF (Award CHE0946869). The portion of this research carried out at Kyonggi University has been supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0025653).

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REFERENCES

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dx.doi.org/10.1021/cm301416n | Chem. Mater. 2012, 24, 2604−2610