Computational Design of Host Materials Suitable for Green-(Deep

Article pubs.acs.org/JPCC

Computational Design of Host Materials Suitable for Green-(Deep) Blue Phosphors through Effectively Tuning the Triplet Energy While Maintaining the Ambipolar Property Jie Wu,†,‡ Yu-He Kan,*,†,‡ Yong Wu,† and Zhong-Min Su*,† †

Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, Jilin, People’s Republic of China ‡ Jiangsu Province Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huai’an 223300, Jiangsu, People’s Republic of China S Supporting Information *

ABSTRACT: We theoretically designed a series of ambipolar host materials (1−8) which incorporate phosphine oxide and carbazole groups to the two ends of diphenyl (DP)-like bridges by para- and meta-connections, respectively. Density functional theory calculations were performed to investigate the influence of altering the DP-like bridges of these molecules on electronic structures and properties, and further to predict their performances as host materials in organic light-emitting diodes. The investigated results show the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of 1−8, distributed at the phenylcarbazole and the DP-like bridge, are responsible for hole and electron injection properties, respectively. The difference in the energies of HOMOs or LUMOs for 1−8 may be derived from different degrees of conjugation effect and electrostatic induction with altering the DP-like bridges of 1−8. The singlet states (S1), arising from the HOMO → LUMO transition, have intramolecular charge transfer character, which determines the small and different values of S1 energies. On the other hand, altering the DP-like bridges brings a great effect on triplet exciton distributions, and consequently different triplet energies. The different singlet/triplet energies for 1−8 make hosts 1−8 suitable for four reference guests with green/deep-blue light when scientists consider the matching of host and guest in singlet/triplet energies for efficient energy transfer.

1. INTRODUCTION Phosphorescent organic light-emitting diodes (PHOLEDs) have been attracting a great deal of attention as a result of their harvesting of both singlet and triplet excitons to approach nearly 100% internal quantum efficiency.1,2 Phosphorescent emitters typically possess longer lifetimes for further diffusion, leading to undesired concentration quenching or triplet−triplet annihilation, and thereby declining performance. To overcome the drawback, the phosphor emitter is usually doped into an appropriate host, and then the host−guest system is realized. Efficient energy transfer from host to guest requires host material possessing higher triplet energies than dopant emitter to prevent reverse energy transfer from the guest back to the host. In addition, a good host material is also required to have favorable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels for charge injection from neighboring layers or electrodes, thus lowering the device driving voltage. Recent research trends have focused on the development of host materials possessing bipolar properties, because bipolar host materials are able to perform balanced injection/ transport/recombination of charge.3−35 Furthermore, to obtain © 2013 American Chemical Society

the host for blue phosphor, an efficient means for maintaining high triplet energy is to interrupt conjugation between the donor and acceptor groups to realize the confinement of triplet exciton to the donor or the acceptor. Phosphine oxide (PO) derivatives were confirmed as new host materials with evident improvement in electron injection and transport in addition to keeping up the high triplet bandgaps of the chromophore cores.36−39 This is because the PO group with tetrahedral structure acts as effective breaking points of π-conjugation between the core and the outer aryl groups, whereas the high electronegativity of oxygen makes the PO group highly polar and electron withdrawing. Recently, a series of PO/phenylcarbazole (PhCBZ) hybrids (PO-PhCBZs) were synthesized and examined as the hosts for blue PHOLEDs.7−13 We theoretically confirmed that the PO moiety directly linked to phenyl moiety of PhCBZ (e.g., PO-PhCBZ in Scheme 1) can effectively decrease the LUMO energy without influencing the HOMO and triplet energies due to the delocalization between Received: January 24, 2013 Revised: March 29, 2013 Published: April 1, 2013 8420

dx.doi.org/10.1021/jp4008174 | J. Phys. Chem. C 2013, 117, 8420−8428

The Journal of Physical Chemistry C

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(FIr6)44 to represent the green/deep-blue guests in the host− guest system. To verify the ambipolar properties of our studied host materials, we also selected tris(8-hydroquinolinato)aluminum (Alq3) and N,N′-dicarbazolyl-3,5-benzene (mCP) as reference hosts with only electron-transport and only holetransport property, respectively. For all reference host and guest molecules, we calculated the HOMO/LUMO energies and singlet/triplet energies by employing different methods and further selected the most reliable methods by a comparison of calculated and experimental values. Then, using the reliable methods, we estimated the HOMO/LUMO energies and singlet/triplet energies for designed host materials further to predict their performances. From the investigated results, it is expected that the series of designed host materials have good electron/hole injection property and various triplet energies suitable for different color guests by means of altering the position and the number of N atom at the DP-like bridge. Our goal is to get a clear understanding of the following questions: (1) How does altering the position of N atom or increasing the number of N atom at the DP-like bridge influence the distributions and energies of the HOMO/LUMO and triplet state under the condition of a fixed molecular skeleton? (2) Why do hosts 1−8 have markedly different triplet localization and triplet energy under the condition of a fixed geometry skeleton?

Scheme 1. Chemical Structures and Triplet-State Distribution of the Hosts Studied in Previous Work

the PO and phenyl of PhCBZ in the LUMO but the localization of the HOMO and triplet excited state at the carbazole (CBZ) unit.40 Furthermore, from the investigated results by Ma and co-workers, when phenylpyridine (ppy) is introduced as a bridge between PO and CBZ groups (pPOppy-pCZ and pPO-ppy-mCZ in Scheme 1), the HOMOs and LUMOs are distributed at the pyridylcarbazole moiety and phenylpyridine bridge, respectively.41 Note that different linkage causes different localization of triplet excitons for pPO-ppy-pCZ and pPO-ppy-mCZ, as illustrated in Scheme 1. The triplet excitons are mostly localized at the phenylpyridine bridge for pPO-ppy-pCZ, whereas they are localized at the CBZ unit for pPO-ppy-mCZ, which leads to markedly different triplet energies between pPO-ppy-pCZ and pPO-ppy-mCZ (2.65 and 3.18 eV). However, Kim and others indicated that the T1 states of the hosts are confined into the meta-terphenyl (mTP)-like bridges with the lowest individual triplet energy but not the carbazole end group.42 This theoretical evidence aroused our attention. Therefore, in the present work, we designed a series of host materials which incorporate PO and CBZ groups to the two ends of diphenyl (DP)-like bridges by para- and meta-connections, respectively. All studied systems are displayed in Figure 1. In addition, we chose four reference guests, such as fac-tris(2-phenylpyridine)iridium (Ir(ppy)3),43 iridium(III) bis[4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic), 16 tris[(3,5-difluoro-4-cyanophenyl)pyridine]iridium (FCNIr), 7 and iridium bis[2-(4′,6′difluorophenyl)pyridinato-N,C2′]tetrakis(1-pyrazolyl)borate

2. THEORETICAL METHODS Density functional theory (DFT) has been remarkably successful at providing a means for computing a variety of ground state (S0) properties with an accuracy which rivals that of the post-Hartree−Fock methods.45−49 Calculated structures of organic molecules in the ground state (S0) with PBE0 hybrid functional often provide better agreement with crystal geometries versus other functionals.41,50,51 Thus, the geometric optimization of S0 for all the hosts and guests and reference molecules was performed using the PBE0 functional. Furthermore, we estimated HOMO and LUMO energies (EH and EL) for the eight reference molecules via directly calculating HOMO/LUMO eigenvalues from the optimized S0 results by various functionals including O3LYP, B3LYP, PBE0, and BH&HLYP (11.6%, 20%, 25%, and 50% HF exchange, respectively), compared with the experiment values. The tested and compared results indicate that, for the eight reference

Figure 1. Chemical structures of CBZ and its PO derivatives investigated in the work. 8421



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addition, to get a better insight into the nature of the S1 and T1 states, we performed TD-DFT calculations by employing PBE0/6-31+G**, on the basis of the optimized S0, S1, and T1 geometries. For the Franck−Condon S1 and T1 states involving more transitions with average contributions, the approach of natural transition orbitals (NTOs)61 based on TD-DFT calculation is performed to analyze the transition nature of the excited state. All calculations of geometry optimization were performed using 6-31G* basis set for the studied systems except Ir atoms of reference guests with LANL2DZ ECP basis set. All calculations on these systems under investigation were performed using Gaussian 09 program package.62

molecules, the HOMO energy is positively associated with the fraction of the HF exchange in the functional, and that the PBE0 gives a smaller deviation from the experiment values than the other functionals (Table S1, Supporting Information). Compared with the HOMO results, the errors in the LUMO eigenvalues by various functionals are significantly higher in energy than those determined experimentally because the virtual orbitals are generally more difficult to describe theoretically than the occupied orbitals. If so, the calculated HOMO−LUMO gap will be bigger than its experimental value, as presented in Table S2 (Supporting Information), and similar results were reported by Frost and Zhang.52,53 So for the estimate of the LUMO energy, we followed the report by Zhang53 that the LUMO eigenvalues are calculated by calculating the HOMO−LUMO gap and then subtracting the HOMO eigenvalue. Besides, many researchers confirmed that the HOMO−LUMO gap can give a reasonable approximation to the vertical S1 energy when considering the S1 state originates from the transition from HOMO to LUMO.46,53−56 Herein, we computed the E(S0→S1) by employing TD-DFT functionals with a varying fraction of HF exchange, including O3LYP, B3LYP, PBE0, and BH&HLYP (11.6%, 20%, 25%, and 50% HF exchange, respectively). The results from Table S2 (Supporting Information) indicate the calculated E(S0→S1) value by TD-O3LYP shows a larger correlation with the experimental EH‑L value compared with those from other tested functionals. Therefore, for our studied host and guest molecules in the present work, the HOMO energy and EH‑L were estimated by PBE0 and TD-O3LYP functionals, respectively. The LUMO energy was then obtained by their sum. Fortunately, the LUMO energies estimated by this method are in good agreement with the experimental results (Table S2, Supporting Information). Meanwhile, we tested the conventional DFT-based methods with different exchange−correlation functionals on the adiabatic S1 and T1 energies (ES/ET) by means of the ΔSCF method57−59 on the basis of the optimized structures for the S1, T1, and S0 states. TD-DFT optimization calculations can give a reasonable geometry for S1 states of medium-sized molecules.58,60 For T1 states, both spin-restricted TD-DFT and spinunrestricted DFT (UDFT) can be employed to optimize T1 geometry. We employed both TD-PBE0 and UDFT with a varying fraction of HF exchange to optimize the T1 geometries. The results show that TD-PBE0 and UDFT (with a varying fraction of HF exchange) give the close energies and similar transition natures of T1 for 1−3 (Figure S2, Supporting Information). So we employed UDFT to optimize T 1 geometries when considering that TD-DFT is very timeconsuming compared with UDFT for T1-geometry optimization. Moreover, the tested ES/ET values from Figure S3 (Supporting Information) show that different HF exchange functionals give the similar orders of the ES/ET values for system 1−3 and Ir(ppy)3: 1 > 2 > 3 > Ir(ppy)3 for the ES and 1 ≈ 2 > 3 > Ir(ppy)3 for the ET. On the other hand, the longitudinal comparison shows the fraction of the HF exchange in the functional is positively associated with the ES while having no effect on the ET. Moreover, when O3LYP and B3LYP functionals are employed, the ES is obviously lower than the ET, which suggests the functionals with low percentages of HF exchange seriously underestimate the ES due to the overestimation of the magnitude of π-delocalization. So in our work the TD-PBE0/UPBE0 functionals are employed for calculating the relative ES/ET values for systems 1−8. In

3. RESULTS AND DISCUSSION I. Molecular Orbital, Charge Injection, and Lowest Singlet State (S1). A good host material for blue phosphor is required to have high HOMO and low LUMO levels for reducing charge injection barriers from neighboring layers and electrodes, thus lowering the device driving voltage. The contour plots and energies of the frontier molecular orbitals (FMOs) of studied systems are displayed in Figure 2. We

Figure 2. Contour plots and energies of HOMOs and LUMOs for systems 1−8 and Alq3 and mCP in the ground states.

presented the contour plots of FMOs for representative 3, as all the 1−8 have similar distributions for HOMOs or LUMOs. The HOMOs are mainly localized at phenylcarbazole-like moiety (PhCBZ), with little contribution from the b-ring. The LUMOs are distributed at diphenyl (DP)-like bridges, as shown in Figure S1 of Supporting Information. However, the energies for HOMOs or LUMOs have an obvious difference from 1 to 8 with altering of the position and the number of N atoms at the DP-like bridge. These differences may be caused by many factors, such as conjugation effect, resonance, and electrostatic induction. For systems 1 and 4−6, the electrostatic effects of different b-rings on the HOMOs at PhCBZ are responsible for the difference of HOMO levels because the HOMO is not distributed at b-ring itself in 1 and 4−6. For 2, 3, 7, and 8, the different HOMO levels depend on the electron-withdrawing ability of the a-ring attached to carbazole. However, the LUMO levels for 1−8, which vary from −2.44 to −3.15 eV, directly depend on the N atom number of a-ring or b-ring at the DPlike bridge, with a negligible impact of the exact site of the N atom in the bridge stemming from the observation that the LUMO levels of 2−5 are much the same, and the levels are much less than that of 1, with one N atom, but much more than that of 8, with three N atoms. Moreover, the LUMO levels of 8422



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1−8 fluctuate around that of Alq3 (−2.84 eV) with excellent electron transporting property. At the same time, the HOMO levels (from −5.64 to −6.20 eV) have no significant deviations from that of mCP (−5.90 eV) with only hole transporting property. These features show that 1−8 can be expected to present reduced energy barriers for both hole and electron injection properties, thus lowering the device driving voltage. On the basis of the TD-PBE0 calculations, we investigated the energies and transition natures of S1 states for all the hosts. The results collected in Table S3 (Supporting Information) show that the Franck−Condon S1 state of the studied hosts has unchanged transition nature based on both S 0 and S 1 geometries, i.e., the HOMO → LUMO transition. So the S1 nature is determined by the distribution of HOMO and LUMO and thus the S1 energy correlates with the HOMO−LUMO gap. The S1 state for systems 1−8, related to electron transition from PhCBZ group to DP-like bridge, is an intramolecular charge transfer (ICT) state, which determines that systems 1−8 have a small S1 energy for both S1 vertical and adiabatic transitions (Table S3 in the Supporting Information and Figure 3), and similar conclusions have been drawn in our recent work.40

Table 2. Simulated EH and EL, the EH‑L, the Adiabatic ES/ET, and the ΔEST for Systems 1−8 (Units in eV) compounds

EH

EL

EH‑L

ES

ET

ΔEST

1 2 3 4 5 6 7 8

−5.64 −5.79 −5.72 −5.64 −5.75 −5.82 −6.02 −6.20

−2.44 −2.68 −2.69 −2.75 −2.72 −2.92 −2.98 −3.15

3.20 3.11 3.03 2.89 3.03 2.90 3.04 3.05

3.36 3.25 3.08 3.12 3.15 3.03 2.99 3.00

3.20 3.19 2.77 2.85 2.91 2.90 2.89 2.96

0.16 0.06 0.31 0.27 0.24 0.03 0.10 0.04

Because the distribution of the triplet excitons dominates the triplet energy, we carried out a Mulliken population analysis to characterize the spin density distribution of unpaired electrons in the triplet state. As shown in Figure 4, for 1 and 2, the triplet

Figure 3. Change of electron density distribution upon the vertical S1 transition based on the S1 geometry. Light blue and deep blue colors correspond to a decrease and increase of electron density, respectively.

Figure 4. Spin density (SD) distribution and adiabatic triplet energies (the energy difference between T1 and S0 states) of the host and guest systems.

II. Triplet Energy (ET) and Transition Nature of T1. Higher triplet energy (ET) than that of the phosphorescent guest, as one of the most essential requirements of an ideal host material, can efficiently prevent back-transfer of energy from the guest to the host. The calculated and experimental T1 energies of all reference systems and designed hosts 1−8 are collected in Tables 1 and 2. From Table 1, it is found that the calculated T1 energies for the reference systems are in agreement with experimental values except for mCP and FIr6 with a deviation from experimental values (0.34 and 0.40 eV, respectively) due to the different experimental approaches and conditions for measuring T1 energies of different reference systems. Table 2 shows the T1 energy for hosts 1−8 ranges from 3.20 to 2.77 eV as the position of the N atom changed or the number of N atoms increased at the DP-like bridge in hosts 1−8.

excitons are mainly localized on the carbazole unit; therefore, the ET values (3.19−3.20 eV) are very close to that of carbazole (3.18 eV). In contrast, the triplet excitons for 3−8 are mainly distributed at their respective bridges, with a small contribution from the carbazole unit, which leads to smaller ET (2.77−2.96 eV) for 3−8 compared with those of 1 and 2. The triplet exciton confinement at the DP-like bridge for 3−8 can be explained by a conclusion drawn by Cooper that in an asymmetric complex the triplet exciton is confined to the lowest energy ligand.63 However, the conclusion seems to be not adapted for 1 and 2, although 1−8 have the same T1 backbone consisting of a DP-like bridge and carbazole unit. Importantly, how can we give a reasonable understanding on the difference in triplet exciton distribution between two groups of hosts? First, we investigated the T1 geometry structures of

Table 1. Simulated HOMO and LUMO Energies (EH and EL), Adiabatic S1 and T1 Energies (ES/ET), and Available Experimental Values for the Reference Molecules (Units in eV) compounds

EH

EL

EH‑L

PO-PhCBZ12 PPO17 mCP7 Alq37 FIr644 FCNIr7 FIrpic16 Ir(ppy)325

−5.71 −5.85 −5.90 −5.25 −6.02 −6.46 −5.72 −5.11

−2.17 −2.06 −2.20 −2.84 −3.06 −3.57 −3.01 −2.52

3.54 3.79 3.70 2.41 2.96 2.89 2.71 2.59

ES

3.19 2.94 2.94 2.84 8423

ET

EH(exp)

EL(exp)

EH‑L (exp)

ET(exp)

3.17 3.15 3.24

−5.70 −6.16 −6.10 −5.80 −6.10 −5.80 −5.70 −5.60

−2.12 −2.60 −2.40 −3.00 −3.10 −3.00 −2.70 −3.00

3.58 3.56 3.70 2.80 3.00 2.80 3.00 2.60

3.10 3.02 2.90

3.12 2.86 2.76 2.61

2.72 2.80 2.65 2.42



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Table 3. Coutour Plots of the Pairs of “Natural Transition Orbitals” Based on TD-DFT Calculationa

a

λ represents natural transition orbital eigenvalue.

from the delocalized transition from the DP-like bridge/ carbazole to the DP-like bridge; the FC-T1 state for 6−8 is mainly from the transition HOMO → LUMO, corresponding to the ICT transition from PhCBZ to DP-like bridge, which is inconsistent with spin density distributions by UDFT, confined to a DP-like bridge or carbazole. Similar results were reported for the study on the T1 nature of Pt(P(Bu)3)2(ethynylbenzene)2 by Batista and Martin.64 However, based on the optimized T1 geometry by UDFT, the FC-T1 state has a great change in transition nature for hosts 1−7 except for 3 and 4. Especially for 1 and 2, the FC-T1 state at the UDFT-T1 geometry is dominated by localized electron transition from the HOMO to the LUMO+1, which differs from the delocalized FC-T1 state based on the S0 geometry. In addition, the FC state of 6 and 7 involves the transition with two average contributions (HOMO, HOMO−2 → LUMO), related to the transition from the DP-like bridge/carbazole to the DP-like bridge. The related orbital distribution of the FC-T1 state based on the UDFT-T1 geometry is in good agreement with the simulated spin density distribution above (Figure 4). The results indicate that the geometric transformation of the DPlike bridge from one plane at the S0 to an angle at the T1 significantly affects the nature of the T1 transition, so it may be inadvisable for any host material to investigate the transition nature of the T1 state just by TD-DFT calculation at the S0 geometry, especially for T1 with exciton localization character because current TD-DFT methods usually fail to account for such localization effects, as indicated by Brédas et al.42 More importantly, it is concluded that the triplet exciton is not always confined to the lowest energy subunit and significantly associated with the linkage mode between different subunits, and similar results have been found in our recent work.65 In addition, for the reference green/blue guests, Ir(ppy)3, FIrpic, FCNIr, and Fir6, the T1 states mainly arise from the HOMO → LUMO transition.

1−8 and easily obtained a preliminary law that for 3−8 the DPlike bridge and the carbazole N atom are retained in one plane (in one straight line from side view) in T1, which is favorable for the delocalization of triplet exciton from the plane DP-like bridge into the adjacent carbazole. In contrast, in 1 and 2, there exists an angle between a- and b-rings on the DP-like bridge, which hinders the delocalization of triplet exciton and therefore confines the triplet exciton to the carbazole unit. It is concluded that the obvious geometric deformation from S0 to T1 at the DP-like bridge significantly affects the nature of T1 transition. Spin density distribution provided by UDFT calculation can shed light onto the localization of the triplet exciton. However, the TD-DFT calculation can provide a detailed multiconfigurational description of the excited states. For the excited states with main transition contributions, the transition nature and character are easily described and visualized. However, for the excited state involving more transitions with average contributions,64 the approach of natural transition orbitals (NTOs)61 based on TD-DFT is an efficient means of analyzing the transition nature of the excited state. Note that the NTO analysis is practicable only when each pair of NTOs (hole and electron) accounts for more than 90% of a transition (λ > 90%). Herein, to investigate the influence of altering the DPlike bridge on the T1 transition nature, we performed TD-PBE0 calculations for all studied systems at optimized S0 and T1 geometries, respectively. Table S4 (Supporting Information) collects the transition nature and main composition of the T1 of all host and guest systems. For the T1 involving two average contributions, we employed the NTOs approach based on TDDFT to realize the visualization of the dominant pair (hole/ electron) of NTOs of the T1 state, as presented in Table 3. It is found from Table S4 (Supporting Information) and Table 3 that, when based on the S0 geometry, the Franck−Condon (FC) T1 state for systems 1−5 is composed of two average contributions (HOMO, HOMO−2 → LUMO) mainly arising 8424



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Figure 5. (a) Orbital energy levels of host and guest molecules. (b) Singlet and triplet energies of host and guest molecules.

III. Match of Singlet/Triplet Energies between Host Material and Phosphorescent Guest. As mentioned above, a good host material itself is required to have feasible HOMO and LUMO energies and a higher T1 energy than that of the guest. Importantly, because of the inherent FMO energy levels and excited-state energy for the host and guest, the selection of the host and guest plays a significant role in determining the emission mechanism in phosphorescent devices.66−68 Generally, many mechanisms may lead to guest emission: Förster energy transfer (FET),69 Dexter energy transfer (DET),70 direct charge trapping (DCT),71 or their mixed mechanism.72−74 In experiments, the occurrence of singlet−singlet (S−S) FET can be judged by the overlapping of the host fluorescence emission and guest absorption spectra. However, the experiments fail to judge the possibility of triplet−triplet (T−T) DET by the host and guest spectra, because the triplet state of the host is a long life (microseconds and more), nonemissive “dark” state. For efficient T−T DET, a small distance between the host and guest molecule diameter is required to ensure overlapping of molecular orbitals to realize effective electronic coupling between the host and guest in adjacent host and guest molecules. Additionally, the small distance between the host and guest molecular diameters can be adjusted by changing the doping concentration of the guest in experiments. DCT requires that the host must have a lower HOMO energy and a higher LUMO energy than the guest for blocking the hole and electron from the anode and cathode on the guest, respectively. Regardless of any mechanism, the triplet energy of the host should be higher than that of the guest to prevent reverse triplet energy transfer from the guest back to the host. In theories, for efficient S−S FET and T−T DET, the host is required to have higher S1 and T1 energies than the

guest, respectively, and such theoretical characterizations gave a reasonable explanation for experimental phenomena.65 Herein, we qualitatively predicted possible emission mechanisms of the four groups of host−guest systems by investigating the FMO energies and the stable S1/T1 energies. That the bandgap results of Figure 5a indicate the HOMOs of all hosts except 3 are shallower than those of corresponding guests, which is unfavorable for blocking the hole from the anode on the guest, means the impossibility of direct hole trapping on the phosphorescent guest, although their shallower LUMOs than those of the guests allow for direct electron trapping on the guest. As shown in Figure 5b, considering the match of host and guest in T1 energy, hosts 1 and 2 with T1 energies of 3.19− 3.20 eV may be more suitable for deep-blue FIr6 (ET = 3.12 eV), while 5−8 (ET = 2.89−2.96 eV) may match better with blue FCNIr (ET = 2.86 eV). Likewise, 3 and 4 (ET = 2.77−2.85 eV) may be just suitable for green Ir(ppy)3 and sky-blue FIrpic (ET = 2.61−2.76 eV), respectively. It should be pointed out that, even if 1 and 2 can be selected as hosts for sky-blue FIrpic, a much higher ET for 1 and 2 than that for FIrpic may lead to the great loss of T1 energy of the host in the process of energy transfer from the host to the guest. Then we turn to the S1 energies of every combination of host and guest and try to predict the possibility of S−S FET. The higher S1 energy for the host than that of the guest for every combination in Figure 5b suggests the possibility of FET as a feasible emission mechanism. So in such host−guest systems, formation of singlet excitons on the guest is mainly from singlet excitons on the host molecule undergoing S−S FET; the formation of triplet excitons on the guest may be both from triplet excitons on the host molecule undergoing T−T DET and from singlet excitons on the guest via intersystem crossing. Consequently, 8425



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the transition natures of S1 and T1 of hosts 1−8, based on different geometries, by various functionals, contour plots, and energy diagrams. This information is available free of charge via the Internet at http://pubs.acs.org.

all triplet excitons generated on the guest may wait for phosphorescence radiation to occur, because the higher triplet energies of the hosts than those of the guest emitters are able to efficiently prevent reverse energy transfer from the guest back to the host, as well as confine triplet exciton in the emitter layer. In the viewpoint of match of host and guest in S1 energy, systems 2 and 6−8 have slightly higher S1 energies than their reference guests by 0.05−0.09 eV, which just allows for S−S FET occurring between the host and guest. By contrast, systems 1 and 3−5 in the ground state need a great energy to be excited into the high-energy S1 state. What is more, the great S1 energy difference of 0.16−0.21 eV may bring about a larger energy loss in the process of FET from 1 and 3−5 to Ir(ppy)3. Systems 2 and 6−8 may exhibit good performance as a host material for deep-blue guests as a result of the balanced charge injection property and the matching S1/T1 energies for FET/ DET between the host and guest. In summary, matching of the S1 and T1 energies between the host and guest should be taken into account toward efficiently realizing energy transfer (FET and DET) from the host to guest for pursuing a suitable and high-efficiency host for the guest emitter, although other factors such as chemical stability, doping concentration of the guest, and degradation of the used materials are associated with both efficiency and durability of OLEDs for practical application.

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*E-mail: Y.H.K., [email protected]; Z.M.S., [email protected]. Phone: +86-431-85099108. Fax: +86-431-85684009. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project No. 20903020, 21131001, and 21203019), National Basic Research Program of China (973 Program-2009CB623605), the Science and Technology Development Planning of Jilin Province (201201071), the Natural Science Foundation of Jiangsu Province (BK2011408), the Opening Project of Key Laboratory for Chemistry of Low-Dimensional Materials of Jiangsu Province (JSKC12109), and the Cultivation Fund of the Key Scientific Innovation Project of Huaiyin Normal University (11HSGJBZ11).

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4. CONCLUSIONS We have performed a detailed theoretical study on electronic structures and properties as host molecules for a series of ambipolar host materials (1−8) which incorporate phosphine oxide and carbazole groups to the two ends of DP-like bridges by para- and meta-connections, respectively. The results show the HOMOs and LUMOs of 1−8 are distributed at the PhCBZ and DP-like bridge. The LUMO levels of 1−8 fluctuate around that of Alq3 with excellent electron transporting properties while the HOMO levels oscillate around that of mCP as widely used hole transport material, which suggests that 1−8 have good hole and electron injection abilities. The S1 states for hosts 1−8 originate from the HOMO → LUMO transition and thus have ICT character, which determines the small S1 energies. The T1 states have a different transition nature, and the triplet excitons are distributed at the carbazole unit for 1 and 2 while at the DP-like bridge for 3−8. The different excited-state nature in 1−8 is visualized by spin density and NTOs analysis based on the optimized T1 geometries. From the viewpoint of match of S1/T1 energy between host and guest for efficient S−S FET/T−T DET, hosts 1−8 with different S1/T1 energies make themselves suitable for four reference guests with green/deep-blue light. In summary, the present work including characterization of electronic structure and prediction of performances as host materials, suggests that match of the S1 and T1 energies between host and guest should be taken into account toward efficiently realizing energy transfer (FET and DET) from host to guest for pursuing suitable and high-efficiency host for the guest emitter, although other factors such as chemical stability, doping concentration of guest, and degradation of the materials are associated with both efficiency and durability of OLED for practical application.

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REFERENCES

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S Supporting Information *

The distributions and energies of HOMO/LUMO, the HOMO−LUMO gap, the vertical/adiabatic S1 and T1 energies, 8426



Article

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Computational Design of Host Materials Suitable for Green-(Deep

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