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Bipolar Host Molecules for Efficient Blue Electrophosphorescence: A Quantum Chemical Design Xin Gu,† Houyu Zhang,*,† Teng Fei,† Bing Yang, Hai Xu,† Yuguang Ma,*,† and Xiaodong Liu‡ State Key Laboratory of Supramolecular Structure and Materials and College of Chemistry, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: May 18, 2009; ReVised Manuscript ReceiVed: NoVember 10, 2009
On the basis of density functional theory (DFT) calculations, a new series of bipolar host molecules for efficient blue electrophosphorescence devices are designed by linkage of hole-transporting moiety carbazole (CZ) and electron-transporting unit diphenylphosphoryl (ph2PdO) to the core molecules with high triplet energies. The electronic structures in the ground states, cationic and anionic states, and lowest triplet states of the designed molecules have been studied with emphasis on triplet energies, spin density distributions, ionization potentials, electron affinities, and the influence of molecular topology. Designed bipolar host molecules possess the following features: (1) relatively higher highest occupied molecular orbital (HOMO) for hole injection and, relatively lower lowest unoccupied molecular orbital (LUMO) for electron injection; (2) HOMO and LUMO separation and localization in the respective hole- and electron-transporting moieties; (3) dramatic bond length changes in ionic states occurring at different parts of the bipolar molecules with respect to their neutral states; (4) keeping higher triplet energy. The DFT results provide deep insight into the nature of bipolar molecules and show that the designed molecules are feasible to meet the requirements of the host materials for blue triplet emissions. Introduction The efficiencies of organic light-emitting devices (OLEDs) have been boosted rapidly in recent years since the first efficient device, which was based on phosphorescent metal complex (PhMC), was reported.1 Unlike traditional organic small molecules and polymers, PhMCs can harvest both electrogenerated singlet and triplet excitons for emission, resulting in a theoretically maximum internal efficiency of 100%.2,3 In typical, the PhMCs are used as the guest dopants dispersed into organic conductive host matrix to avoid concentration quenching.4 Therefore, appropriate host materials are of equal importance to PhMCs for highly efficient OLEDs. Such host-guest systems are well-established in green and red organic electrophosphorescent devices with efficiencies as high as 19 (green)2,5 and 18% (red).6 Unfortunately, the efficient host materials for blue PhMCs are still scarce and are in high demand. The design and development of suitable host materials for blue PhMCs are highly desirable and pursued challenging tasks. An ideal host material must fulfill four basic requisites. First, the triplet energy, T1, of the host must be higher than that of the guest to favor an exothermic energy transfer from the host to the guest and effectively confine triplet excitons within the emitting guest molecule. Second, the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of designed host materials should match the Fermi levels of commonly used electrode (or HOMO/LUMO levels of neighboring layers). Third, the host materials should have good and balanced hole and electron mobility. Lastly, the host materials should possess good thermal stability and be capable of forming good morphological films. All of these requirements * Corresponding authors. E-mail:
[email protected] (H.Z.);
[email protected] (Y.M.). † State Key Laboratory of Supramolecular Structure and Materials. ‡ College of Chemistry.
must be taken into account for the design and synthesis of the host materials for PhMCs. Both experimental and theoretical efforts have been made to develop and design efficient host materials for blue electrophosphorescence. The host materials reported so far are mainly limited to carbazole derivatives7-10 and silicon11-14 containing compounds. The existing host molecules have the disadvantages of either low triplet energy or thermal and morphological instability. A commonly used host material, 4,4′-bis(N-carbazolyl)-2,2′-biphenyl (CBP),15 shows a pronounced backward triplet excitons transfer from a guest blue-emitting phosphorescent dye (iridium(III) bis[(4,6-difluorophenyl)-pyridinatoN,C2′] picolinate (FIrpic), T1 ) 2.65 eV) due to lower triplet energy of CBP (T1 ) 2.56 eV). The carbazole-based materials, 4,4-bis(9-carbazolyl)-2,2′-dimethyldiphenyl (CDBP),16 3,5-bis(9carbazolyl) benzene (mCP),17 and 9′-(2-ethyl-hexyl)-9′-H[9,3′;6′,9′′]-tercarbazole (TCz1)9 have large triplet energies of 2.9 to 3.0 eV and inhibit the triplet excitons transfer from the guest to the host, exhibiting dramatically improved performance in device as compared with CBP. Organosilicone compounds diphenyldi(o-tolyl)silane (UGH1) and p-bis(triphenylsilyly)benzene (UGH2) were reported to have large triplet energies (>3.5 eV).12 Although those carbazole- and organosilicone-based hosts can meet the requirement for high triplet energy, they suffer from thermal and morphological instability. The glass-transition temperature (Tg) for tetraphenylsilane and related UGHs is in the range of 26-53 °C,12 and that of mCP is 55 °C.18 Relatively low thermal and morphological stability may hinder their practical applications as a host material in high-performance blue phosphorescent OLEDs. 3,5-Bis(9-carbazoyl)tetraphenylsilane (SimCP),13 which is considered to be a combination of mCP and UGH1, shows significantly improved performance in phosphorescent blue OLEDs as compared with mCP and UGH1. A host with a balanced charge transport property plays an important role for its applicability in an electroluminescent (EL)
10.1021/jp904610s 2010 American Chemical Society Published on Web 12/28/2009
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device. It is well known that most organic EL materials are in favor of the injection and transport of holes rather than electrons;19 in general, extra layers are often introduced to confine the positive charge carriers in the emission zone.20 Many carbazole7-9,21 and triphenylamine22,23 derivatives are known to transport predominantly holes rather than electrons. Electrontransporting structural moieties such as 1,2,4-triazole,24 oaxadiazole,25 and triazine26 are introduced to design carbazole-based bipolar hosts with enhanced electron mobility to improve OLED performance further.23,27 Burrows et al. investigated that organic phosphine oxide can be used as hosts for blue electrophosphorescent materials, exhibiting high triplet energy, T1, low operating voltage, and enhanced electron injection and transport in devices.28 Furthermore, they connected organic phosphine oxide with carbazole unit to build bipolar host molecules.29 Such bipolar hosts are expected to be the electron-hole balance materials,23,30,31 which can transport the electron and hole to EL materials simultaneously, improve the single-layer device performance, and simplify the fabrication procedure.32 In contrast with the experimental studies, the theoretical contributions on deep understanding the nature of relationship between structures and properties of the host materials seem deficient, especially on the bipolar host materials. Beljonne group investigated the lowest-lying triplet excited states of commonly used conjugated host molecules in phosphorescent light-emitting diodes by means of correlated semiempirical and ab initio quantum-chemical methods.22 They also designed suitable host materials based on the carbazole and spirobifluorene for full color triplet emission.7 The calculated data were comparable to experimental data and showed that the density functional theory (DFT) method was reliable when evaluating the energy of lowest-lying triplet excited state. On the basis of the aforementioned literature survey and analysis, the hybrid host molecules would be expected to have different functional units and satisfy more requirements for ideal host materials. From the molecular design point of view, incorporating hole-transporting and electron-transporting moieties to the core molecules possessing high triplet energies may be a good strategy for designing bipolar host molecules for the use of blue OLEDs. Inspired by Burrows’s work, we designed a series of host molecules for blue triplet guest emitters, which were based on quantum-chemical investigation. The electronic structures of designed host molecules were studied to reveal the nature of relationship between the molecular topology and triplet energy (T1). The design route was based on some experimental findings, which, in return, gave hints to synthesize new host materials for blue triplet emissions. Computational Details DFT calculations were performed on the designed host molecules using the B3LYP33 hybrid functional with the 6-31g(d)34 basis set. For comparison, the B3PW9135 method was used to investigate the dependence of the functional. The lowest triplet excited states (T1) of compounds were investigated using spin-unrestricted B3LYP calculations, whereas the singlet ground states (S0) calculations were carried out at the restricted SCF level, followed by geometry optimization in vacuo. The triplet energy was evaluated as the total energy difference between the states T1 and S0, E(T1 - S0). A Mulliken population analysis was performed for the spin density (SD) to quantify the relative distribution of unpaired electrons in the triplet state. Although DFT calculations introduce systemic errors (∼0.1 eV) on evaluating triplet state energy compared with experimental measurements,22 this theoretical method can still be regarded
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Figure 1. Designed bipolar molecular model and chemical structures of the core molecules investigated in this work.
as a reasonable tool to screen potentially interesting host materials from first principle investigation.7 Ionization potential (IP) and electron affinity (EA) were calculated to access the electron-donating and -accepting ability. All calculations were carried out using the Gaussian 03 package.36 Results and Discussion Designed Molecules. The host molecules we designed are to incorporate well-known hole-transporting carbazole (CZ) and electron-transporting diphenylphosphoryl (ph2PdO) groups on both end sides of core molecules so as to build bipolar molecules. The designed molecular model and core molecules are shown in Figure 1, and the designed bipolar host molecules are shown in Figure 2. The core molecules we choose are 2,2′dimethylbiphenyl (mbp), 2-phenylpyridine (ppy), 2,2′-bipyridine (bpy), 1-phenyl-pyrazole (ppz), 9,9-diphenyl-fluorene (dpf), and tetraphenylsilane (Siph4). The mbp is the central unit of CDBP molecule,16 whereas ppy, bpy, and ppz are usually used as wide-gap ligands for PhMCs.37,38 The common feature of these core molecules is that they possess single-bond-linked two aromatic rings, which can exhibit relatively conformationflexible behavior. The nonplanar molecular structures of dpf and Siph4 are in favor of preventing the close packing of the molecules in the solid state.31 Such core molecules are of benefit to the formation of smooth and pinhole-free thin films. Molecular Orbitals (MOs) and Triplet Energies (E(T1 S0)). The electronic structures of core molecules are analyzed in terms of energy levels and triplet excitation energies. The calculated HOMO/LUMO energies, HOMO-LUMO energy gaps, and triplet excitation energies E(T1 - S0) of the core units are shown in Table 1 and Figure 3. The calculated triplet energies of ppy and bpy are 3.05 and 2.97 eV, respectively, which are in agreement with experimental measurements of triplet energies of ∼3.0 eV for both of ppy and bpy.37 The Siph4 has a relatively higher triplet energy of 3.71 eV, which is also consistent with the experimental finding of ∼3.5 eV.12 All core units possess low HOMO energies (around -6.3 eV), high LUMO energies (up to 0 eV), wide HOMO-LUMO energy gaps (4.92 to 6.38 eV), and high triplet-singlet energies (2.97 to 3.71 eV). For ppy, bpy, and ppz, the larger the HOMOLUMO energy gaps, the higher (T1 - S0) energies they possess, as can be seen in Figure 3. The calculated triplet energies vary
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Figure 2. Chemical structures of designed bipolar host molecules investigated in this work.
TABLE 1: Calculated HOMO and LUMO Energies, HOMO-LUMO Energy Gaps E(H-L), Triplet Energies E(T1 - S0) and Dipole Moments (µ) for the Core Molecules molecules
HOMO (eV)
LUMO (eV)
E(H-L) (eV)
E(T1 - S0) (eV)
µ (debye)
mbp ppy bpy ppz dpf Siph4
-6.37 -6.15 -6.52 -6.08 -5.77 -6.58
0.01 -1.08 -1.17 -0.60 -0.85 -0.53
6.38 5.07 5.35 5.48 4.92 6.05
3.35 2.97 3.05 3.38 2.97 3.71
0.31 1.72 3.04 1.85 0.02 0.02
from 2.97 to 3.71 eV, which meet the requirements of being a host of blue phosphorescence, although the value will be tuned by substitution with the polar group of CZ and ph2PdO. The simultaneous interconnecting of hole-transporting moiety CZ and electron-transporting unit ph2PdO to the core units will destabilize the HOMO energy levels and stabilize the LUMO energy levels. The calculated HOMO and LUMO energies, HOMO-LUMO energy gaps, and triplet energies E(T1 - S0) of the designed bipolar host molecules are shown in Table 2 and Figure 4. Compared with core molecules, the HOMO levels of the designed molecules go upward (around -5.45 eV) and LUMO levels move downward (around -1.39 eV). As a result, the HOMO-LUMO energy gaps become narrow (3.85-4.05 eV). As can be seen from Figure 5 and Figure S1 (in the Supporting Information), the spatial distributions of the HOMOs are mainly contributed from the hole-transporting moiety CZ for the designed host molecules. The LUMOs locate at the core units (ppy, bpy, ppz, and dpf cored molecules) or ph2PdO and nearby aromatic rings moieties (PO-mbp-CZ and PO-Siph4-CZ), respectively. Compared with data in Tables 1
and 2, the HOMO energy levels increase ∼0.85 eV, and the electron-induced effects of ph2PdO stabilize the LUMO energy levels about 1.0 to 1.6 eV.28 Therefore, such HOMO-LUMO energy gaps narrowing can be ascribed to the high HOMO of CZ and the strong electron-induced effects of ph2PdO. It is interesting to note that all molecules show significant separation in the spatial distributions between HOMO and LUMO at their different moieties. This feature implies that the one-electron HOMO-LUMO transition becomes a typical charge transfer, which is preferable for efficient hole and electron transfer. The triplet energy correlates qualitatively well with the value of HOMO-LUMO energy gap except for mPO-ppy-mCZ, as illustrated in Figure 4a, which can be explained by stronger inductive effects of ph2PdO group. This is due to the coplanarity of PdO bond and core ppy unit (dihedral angle >154° at the triplet state). The triplet energies range from 2.65 to 3.39 eV, which are sufficiently high for the green and blue triplet emission host materials. From our calculated results, we find that the LUMOs of the designed molecules are much lower than that of N-(4-diphenylphosphoryl phenyl) carbazole (MPO12), except PO-mbp-CZ, which would be much facile for the electron injection.29 The HOMOs of designed molecules are suitable for hole injection for commonly used anode such as ITO (-4.7 eV), and the LUMOs are matchable with the cathode such as Al (-4.3 eV). Therefore, the HOMO and LUMO levels of designed host materials can match the Fermi levels of commonly used electrode. The point of emphasis is that such bipolar host molecules keep the relatively high triplet energies as their core molecules, whereas their HOMO and LUMO levels can be tunable to match charge injection from the electrodes. The HOMO-LUMO energy gaps change larger than the triplet
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Figure 3. (a) Calculated triplet energies E(T1 - S0) (red) and HOMO-LUMO energy gaps (black). (b) Calculated HOMO (black) and LUMO (red) energies of core molecules.
TABLE 2: Calculated HOMO and LUMO Energies, HOMO-LUMO Energy Gaps E(H-L), Triplet Energies E(T1 - S0), and Dipole Moments (µ) for the Designed Molecules
a
compound
HOMO (eV)
LUMO (eV)
E(H-L) (eV)
E(T1-S0) (eV)
µ (debye)
PO-mbp-CZ pPO-ppy-pCZ pPO-ppy-mCZ mPO-ppy-mCZ pPO-bpy-mCZ mPO-bpy-mCZ PO-ppz-CZ pPO-dpf-pCZ pPO-dpf-mCZ mPO-dpf-pCZ mPO-dpf-mCZ PO-Siph4-CZ MPO12
-5.36 -5.52 -5.65 -5.52 -5.63 -5.62 -5.44 -5.32 (-5.44)a -5.35 (-5.46) -5.30 (-5.42) -5.34 (-5.47) -5.37 -5.45
-0.92 -1.67 -1.60 -1.40 -1.63 -1.59 -1.36 -1.44 (-1.55) -1.44 (-1.54) -1.29 (-1.40) -1.28 (-1.39) -1.08 -1.03
4.44 3.85 4.05 4.11 3.94 4.03 4.07 3.88 (3.89) 3.91 (3.92) 4.01 (4.02) 4.06 (4.08) 4.29
3.11 2.65 3.18 2.97 3.03 3.10 3.38 2.71 (2.71) 2.83 (2.83) 3.17 (3.17) 3.17 (3.17) 3.39
3.56 3.57 3.68 3.26 4.18 4.28 4.35 3.56 (3.59) 4.94 (4.99) 3.67 (3.67) 4.23 (3.67) 3.86 3.88
Values in parentheses are from the B3PW91 method.
Figure 4. (a) Calculated triplet energies E(T1 - S0) (red) and HOMO-LUMO energy gaps (black). (b) HOMO and LUMO energies of designed molecules.
energies, as can be seen from the comparison between data in Tables 1 and 2. To investigate the effect of different functional used for the DFT calculations on the charge-transfer-like molecules, we also used the B3PW91 method for the calculations of the dpf cored molecules, which are list in Table 2 and Figure S2-4 of the Supporting Information. The HOMO/LUMO energies calculated by the B3PW91 functional are in agreement with the results of the B3LYP method with the B3PW91 results being ∼0.11 eV lower in energy. For the energy difference, E(H-L) and E(T1 - S0), two different functionals give the same
results. In other words, changing the HF exchange in the functional does not affect the properties of the frontier orbitals. Comparison with the Experimental Data. To confirm the correctness of the DFT calculations, we calculate the real synthesized compounds in Burrow’s work. The calculated triplet energy for the above-mentioned bipolar MPO12 molecule is 3.15 eV, which is very close to its experimental value, 3.0 eV.29 Whereas for the organic phosphine oxide ph2PdO-substituted host 2,8-bis(diphenylphosphine oxide) dibenzofuran28c and 4,4′bis(diphenylphosphine oxide) biphenyl,28a the calculated triplet
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Figure 5. Spatial distributions of the HOMO and LUMO for the selected host molecules.
TABLE 3: Calculated Ionization Potentials (IPs) and Electron Affinities (EAs) of dpf-Based Molecules a
compound
VIP (eV)
dpf pPO-dpf-pCZ pPO-dpf-mCZ mPO-dpf-pCZ mPO-dpf-mCZ
7.22 6.56 (6.67)c 6.63 (6.74) 6.54 (6.65) 6.63 (6.75)
AIP(eV)
b
VEA (eV)
AEA (eV)
7.10 -0.59 -0.47 6.46 (6.57) 0.28 (0.39) 0.48 (0.58) 6.55 (6.66) 0.26 (0.37) 0.46 (0.56) 6.44 (6.55) 0.14 (0.25) 0.30 (0.41) 6.54 (6.65) 0.09 (0.21) 0.27 (0.38)
a Indicates vertical values (at the geometry of the neutral molecule). b Indicates adiabatic values (at the optimized structure for both the neutral and charged molecule). c Values in parentheses are from B3PW91 method.
energies are 3.16 and 2.87 eV, respectively, both are comparable to their corresponding experimental data 3.10 and 2.72 eV. Although our calculated data have ∼0.10 eV errors on evaluating triplet state energy in comparison to experimental measurements, this theoretical method can be regarded as a reasonable tool for the design of host materials. Ionization Potential and Electron Affinity. Both IP and EA, which reflect the ability of the host to donate and accept electrons, are very important parameters to assess electron- and hole-injection occurring in the OLEDs.39 According to Koopmanns’ theorem, the IP and EA are related to the HOMO and LUMO energies. The calculated IPs and EAs of dpf-based molecules are listed in Table 3. The calculated results are consistent with HOMO and LUMO energies with ∼0.1 eV deviations. The less the IP is, the easier the host is to accept a hole. After the CZ substitution of the dpf unit, both the vertical IPs and adiabatic IPs are reduced, which means that the energy barriers decrease, and the hole can be more easily injected from the electrode to the host. The greater the EA is, the easier the
host is to accept an electron. The introduction of ph2PdO moiety increases both vertical EAs and adiabatic EAs from negative to positive, and thus the ability of accepting an electron of the host molecule is enhanced. From the calculated IP and EA, we are convinced that our route for designing of such bipolar host molecules is feasible for charge injection. The noticeable bond length changes are observed between the ionic and neutral molecule mPO-dpf-mCZ, as can be seen in Figure 6. For the cationic molecule, the bond length changes occur on the CZ unit and nearby benzene ring of dpf unit with ∼0.01 Å lengthening or shortening; for the anionic molecule, the bond length changes take place on dpf unit with up to ∼0.04 Å alternation. The bond length changes indicate that the redox process of the molecule occurs at different parts of the bipolar host molecule. Spin Density Distribution in Triplet States. To explore the importance of localization of triplet wave function on the triplet energy, a detailed Mulliken population analysis is carried out to characterize the SD distributions of unpaired electrons in the triplet state. The SD plots of the selected representative compounds are depicted in Figure 7. The host molecules can be classified into two categories according to the localization of the triplet wave function: localization on the CZ units (i.e., pPO-ppy-mCZ, mPO-dpf-pCZ, mPO-dpf-mCZ, and PO-Siph4CZ) and the central units (i.e., PO-mbp-CZ, pPO-ppy-pCZ, pPOdpf-pCZ, pPO-dpf-mCZ, and bpy cored compounds). For the former, the associated E(T1 - S0) energies range from 3.17 to 3.39 eV and are mainly determined by the CZ moiety, which are very close to the triplet energy of CZ (∼3.26 eV). For the latter, the spin densities are delocalized over roughly two (POppz-CZ and bpy cored compounds) or three rings (pPO-ppypCZ, pPO-dpf-pCZ, and pPO-dpf-mCZ) in the central units.
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Figure 6. Bond length changes of mPO-dpf-mCZ in cationic and anionic states with respect to neutral state.
Figure 7. Spin density (SD) distributions for the selected host molecules. The radius of the circles corresponds to the value of the SD on each atom.
The triplet energies decrease according to the degree of delocalization, varying from 3.38 eV for PO-ppz-CZ to 2.65 eV for pPO-ppy-pCZ. Substitution Effects. The molecular dipole moments are estimated by DFT calculations, which are listed in Tables 1 and 2. Compared with the corresponding core molecules, the substituted bipolar molecules possess stronger molecular dipole moments (3.20 to 4.92 D), which are determined by molecular topology according to different substitution positions of electrondonating and electron-withdrawing groups. This is consistent with the electron push-pull properties of the functional groups
in the molecules. From the structural point of view, the frontier MO energies and E(T1 - S0) energies can be tuned accordingly by proper handling of the molecular topology. To investigate the substitution effects of ph2PdO and CZ units systematically, we calculated all possible dpf cored molecular structures with substitution at ortho, meta, and para positions. Besides the designed molecules shown in Figure 2, other dpf cored molecule structures and calculated results are listed in Figure S5 and Table S1 in the Supporting Information. For ph2PdO subsitutions at para and ortho positions, it should be noted that the molecules of CZ substitutions at meta or ortho positons have higher triplet
Efficient Blue Electrophosphorescence energy than that of CZ substitution at para position by ∼0.12 eV. For example, the triplet energies of pPO-dpf-mCZ and pPOdpf-oCZ are higher than that of pPO-dpf-pCZ, and triplet energy of oPO-dpf-mCZ is higher than that of oPO-dpf-pCZ. This behavior is associated with the decrease in π-conjugation caused by CZ substitution at meta and ortho configurations, which is in agreement with previous report.7 This is because the nitrogen atom of CZ extends the π conjugations of the core molecules through its lone pair electrons. For the geometry in the triplet excited state, the dihedral angle of CZ and the dpf plane is about 45.7° in pPO-dpf-pCZ and is less than those angles in pPO-dpf-mCZ and pPO-dpf-oCZ, as can be seen from Figure S6 in the Supporting Information. The results clearly indicate that disrupting the molecular conjugation is an effective way to increase the triplet energy gaps. Whereas for ph2PdO subsitutions at meta positons, the triplet energies of molecules are almost the same as the triplet energy of CZ. In the triplet state, the PdO bond in ph2PdO on meta position is almost perpendicular to the conjugated dpf plane (dihedral angle >74°), and there is no obvious inductive electron-withdrawing effect of the ph2PdO group. Therefore, the SD distributions of triplet wave functions mainly locate at CZ unit, and the triplet energies of mPO-dpf-pCZ, mPO-dpf-mCZ, and mPO-dpf-oCZ are ∼3.17 eV. For fixed CZ substitution at para, meta, or ortho positions, three group molecules have different LUMO energy levels, although the LUMO is contributed by the core unit dpf. The LUMO energy levels in molecules with ph2PdO substitution on para are much lower than those for molecules with ph2PdO subsitution on meta and ortho positions by 0.15 and 0.12 eV, respectively. The PdO bond in ph2PdO on para position is nearly coplanar with the conjugated dpf plane (dihedral angle
