Theoretical study on charge injection and transport properties of six emitters with push–pull structure

Chemical Physics 440 (2014) 47–52

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Theoretical study on charge injection and transport properties of six emitters with push–pull structure Tao Lin a, Xiaojun Liu a,b,⇑, Zhidong Lou a, Yanbing Hou a, Feng Teng a,b,⇑ a b

Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS, PR China

a r t i c l e

i n f o

Article history: Received 14 April 2014 In final form 4 June 2014 Available online 12 June 2014 Keywords: Charge transport Density functional theory Electronic structure Reorganization energy

a b s t r a c t The charge injection and transport properties of six organic light-emitting molecules with push–pull structures were studied by theoretical calculations. The ground-state geometries for the neutral, cationic and anionic states were optimized using density functional theory. Subsequently, the ionization potentials and electron affinities were calculated. We computed the reorganization energies and the transfer integrals based on the Marcus electron transfer theory. It was found that in addition to being emitters the six compounds are multifunctional materials being capable of transport for both holes and electrons. Moreover, the double-branched compound DCDPC2 was found to have higher charge injection ability and better balanced charge transport properties than single-branched compounds. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The development of organic light-emitting diodes (OLEDs) has been progressing tremendously due to their potential applications in many fields [1,2], particularly thin layer OLEDs which as a promising candidate for the next-generation full-color flat-panel displays. In recent years, the investigation of single layer devices with improved light-emission efficiency has become a hot topic. Compared with traditional multilayer devices, single layer devices have the advantage of simple preparing process. To obtain optimized single layer devices, it is necessary to design and synthesize efficient emitting layer materials which have the capability of transport for both holes and electrons with excellent performance [3,4]. For the last few years, several blue emitters with push–pull structure and being capable of transport for both holes and electrons were under research [5–7]. Meanwhile, a series of red emitters also with push–pull structure were synthesized, named 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM), 3-(dicyanomethylene)-5,5-dimethyl-1-(4-[9-carbazol]-styryl)cyclohexene (DCDCC), 3-(dicyanomethylene)-5,5-dimethyl-1(4-dimethylamino-styryl)cyclohexene (DCDDC), 3-(dicyanomethylene) -5,5-dimethyl-1(4-[(2-hydroxy-ethyl)-methyl-amino]-styryl) cyclohexene (DCDHC), 3-(dicyanomethylene)-5,5-dimethyl-1(4⇑ Corresponding authors at: Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China. Tel.: +86 010 51685072. E-mail addresses: [email protected] (X. Liu), [email protected] (F. Teng). http://dx.doi.org/10.1016/j.chemphys.2014.06.005 0301-0104/Ó 2014 Elsevier B.V. All rights reserved.

diphenylamino-styryl) cyclohexene (DCDPC) and the doublebranched compound DCDPC2 [8–11]. These polar compounds have high luminescence efficiency, good color purity and can be easily synthesized and purified [10]. For DCDDC, the PL spectrum shows a peak at 650 nm with an excitation wavelength of 400 nm [8]. For DCDPC and DCDPC2, there are two absorption bands: one near 350 nm and another in the range of 495–505 nm; and the materials showed appropriate red emission at 647–670 nm [9]. For DCDCC, there are also two absorption bands: one lying at 340–370 nm and the other locating at 420 nm; and the compound has strong fluorescence in solid state with peak wavelength at 575 nm [10]. The emitting properties of these materials were proved to be appropriate for potential practical applications [12]. However, their charge injection and transport properties are still uncertain. Theoretical calculation has been demonstrated success in studying charge properties for various organic light emitting materials [13–17], especially for multifunctional blue emitters [2,18– 20]. Since that, it would be useful in the investigation of red emitters. In this article, we presented a theoretical calculation method to investigate the charge injection and transport properties of the six red emitters. We calculated the ionization potential (IP) and the electronic affinity (EA) to evaluate energy barrier for the injection of holes and electrons. According to the Marcus electron transfer theory [21], we also computed the reorganization energy (k) and the transfer integral (V) to investigate the charge transport properties. In comparisons among the six molecules, further insights into the relationship of molecular structure and charge properties were given.

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2. Theoretical and computational methodology

V e ¼ jELUMOþ1  ELUMO j=2

ð4Þ

The adequate injection and balanced transport for both electrons and holes are important in optimizing the performance of OLED devices. The ionization potential (IP) and electron affinity (EA) are well-defined properties that can be calculated to estimate the thermodynamic stability and the energy barrier for the injection of both holes and electrons into the compounds. According to the different calculation process, the ionization potential is divided into two categories: vertical ionization potential (IPv) and adiabatic ionization potential (IPa). IPv is the difference in total energies of cationic and neutral states in the optimized neutral geometries, while IPa is the difference in total energies of cationic state in the optimized cationic geometries and neutral state in the optimized neutral geometries. Similarly, electron affinity can be divided into vertical electron affinity (EAv) and adiabatic electron affinity (EAa). EAv is the difference in total energies of anionic and neutral states in the optimized neutral geometries, while EAa is the difference in total energies of anionic state in the optimized anionic geometries and neutral state in the optimized neutral geometries. The intermolecular charge transfer between organic molecules is an oxidation–reduction process, and its thermodynamic property is described by Gibbs free energy which has correlation with IP and EA. On the other hand, Marcus electron transfer theory is a semi-classical theory to investigate the dynamic property of the process. According to the Marcus electron transfer theory, the electron moves between neighboring molecules through a ‘‘hopping’’ process. In this process, the electron ‘‘hops’’ from a charged molecule to the adjacent one which is in the neutral state. The electron hopping rate k can be calculated using the following equation [22]:

V h ¼ jEHOMO  EHOMO1 j=2

ð5Þ

k¼



p

kkb T

1=2

  V2 k exp  h 4kb T

ð1Þ

where T is the temperature and kb is the Boltzmann constant. It can be seen that the k is determined by two factors: k and V. Here, k is the reorganization energy due to the geometric relaxation accompanying charge transfer, and V is the transfer integral. The reorganization energy k is composed by two energies: the inner reorganization energy of the molecule and the reorganization energy of surrounding medium. Considered of the weak polarity of the six molecules in this study, the reorganization energy of surrounding medium (due to the polarization of the medium) can be neglected, whereas the inner reorganization energy should be dominating. The inner reorganization energy reflects the change of molecular geometry associated with going from the neutral to the ionized state, or vice versa. Hence, it is the sum of the relaxation energies of the geometric relaxation following vertical ionization of a neutral molecule and vertical neutralization of a charged molecule. For electron transfer, it can be expressed as follows:

ke ¼ jE0  E0 j þ jE0  E j

ð2Þ

where E0 and E represent the energies of the neutral and anion species in their lowest energy geometries respectively. While E0 and E 0 represent the energies of the neutral and anion species with the geometries of the anion and neutral species, respectively. In this way, k for hole transfer can be expressed as follows:

kh ¼ jE0þ  E0 j þ jEþ0  Eþ j

ð3Þ

The transfer integral V can be obtained by using the direct coupling scheme HF–Koopman’s theorem (HF–KT). It describes V as half of the energy splitting, and the value of the energy splitting can be calculated as the energy difference between the LUMO (HOMO) and LUMO+1 (HOMO1) of the double molecular system for electron (hole) transfer:

The calculations described here were carried out with a hybrid density functional theory (DFT) method. The hybrid functional B3LYP, which corresponding to the combination of Becke’s three parameter exchange functional (B3) [23] with the Lee–Yang–Parr fit for the correlation functional (LYP) [24] has become a standard of DFT calculations for the ground state of organic molecules [25]. The 6-31G⁄ basis set [26] was used. All the calculations were carried out using the Gaussian09 program package [27]. 3. Results and discussion 3.1. Geometrical and electronic structure The Kekule structures of DCM, DCDDC, DCDHC, DCDCC, DCDPC, DCDPC2 and atomic labeling scheme are shown in Fig. 1. DCM and DCDDC share the same donor (i.e. dimethylamino) and acceptor (i.e. dicyanomethylene) units but different p-bridge group: 2methyl-6-styryl-4H-pyran in the case of DCM and 5,5-dimethyl1-styryl-cyclohexene for DCDDC. DCDDC, DCDHC, DCDPC and DCDCC have the same acceptor and p-bridge group but different donor: alkyl group in DCDDC and DCDHC; aromatic ring in DCDPC and DCDCC. DCDPC2 is the double-branched compound of DCDPC with an A–p–D–p–A structure. The bond lengths and the dihedral angles in the optimized geometries of DCDPC in the neutral, anionic and cationic states were summarized in Table 1 (the data of five other molecules can be found in the supplementary). For DCDPC, it can be seen that the structural parameters of the molecule in neutral state which were obtained from the theoretical calculation, consistent with the experimental observation. Since the absolute difference for the same bond lengths are no larger than 0.018 Å, which indicates that B3LYP/6-31G method is suitable for the ground state of these molecules. Table 1 also lists the variation of the parameters between the ionic states and the neutral states. It is observed that the difference is strongly affected by charge injection. For the cation of DCDPC, there is a hole in the molecule, the distances of N1–C13 and C16–C34 decrease more than 0.02 Å compared with the corresponding bond length of neutral geometry as a result of conjugation strengthening for the entire molecule. However, it is dissimilar in anion, where the bond length of C13–N1 increases and thus alleviates the conjugation between donor and acceptor. In fact, when an electron injected, charge distributions prefer the acceptor end due to the CN group is one of the strongest electron attract groups. As a result, the bond length along the p-bridge and acceptor averaged and thus increased the p-conjugation between them in the anion. Another interesting difference between the molecular structures of creating a hole and injecting an electron is the change of the dihedral angles for C23–N1– C13–C18 and C2–N1–C13–C14. In the cation, the variation of the dihedral angles for C23–N1–C13–C18 and C2–N1–C13–C14 are negligible (around 5°). In the anion, on the contrary, the variation of these dihedral angles are both up to around 27°. The large dihedral angles facilitate the weakening of the conjugation between donor and p-bridge in anion. It is clear that the modification of geometry structure in anion (both for the bond lengths and dihedral angles) is larger than in cation, which indicates that adding an electron has more effects on molecular structure than creating a hole. For the five other molecules, the geometric modification of ion compared with neural shares the same trend with DCDPC. Because the frontier molecular orbitals play an important role in charge transport, the highest occupied molecular orbital

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Fig. 1. The Kekule structure and the atomic labeling scheme of DCM, DCDCC, DCDHC, DCDDC, DCDPC, DCDPC2.

Table 1 Bond length (in Å) and dihedral angle (in °) for neutral, anionic and cationic states of DCDPC. DCDPC Neutral Bond lengths (Å) C23–N1 C2–N1 C13–N1 C13–C14 C14–C15 C15–C16 C16–C34 C34–C35 C35–C40 C40–C39 C39–C38 C38–C57 C57–C58 Dihedral angle (°) C23–N1–C13–C18 C2–N1–C13–C14 C17–C16–C34–C35 C34–C35–C40–C39 C39–C38–C57–C58

1.428 1.427 1.406 1.409 1.386 1.410 1.452 1.360 1.443 1.373 1.430 1.385 1.428 31.1 30.1 1.8 177.4 179.5

Anion 1.431a 1.418a 1.405a 1.392a 1.378a 1.395a 1.453a 1.342a 1.435a 1.358a 1.420a 1.375a 1.428a – – – – –

1.414 1.413 1.435 1.405 1.386 1.422 1.438 1.387 1.410 1.409 1.394 1.428 1.416 57.5 57.2 0.7 177.5 180.0

Cation 1.432 1.432 1.380 1.423 1.375 1.423 1.431 1.376 1.430 1.382 1.429 1.386 1.427

0.014 0.014 0.030 0.004 0.000 0.011 0.014 0.027 0.032 0.037 0.036 0.043 0.012 26.5 27.1 1.1 0.1 0.5

25.7 25.3 1.9 177.4 179.8

0.004 0.005 0.025 0.014 0.012 0.013 0.021 0.016 0.012 0.009 0.001 0.001 0.001 5.4 4.7 0.1 0.0 0.3

The value in the right column of anion and cation representing the difference compared with the theoretical calculated value of neutral. a The value of the experimental observation.

(HOMO) and lowest unoccupied molecular orbital (LUMO) were calculated for the six molecules. The HOMO and LUMO sketches of the six molecules are shown in Fig. 2. It is shown that the distribution of electrons of the six molecules shares the same trends. That is, the electrons populated on the whole molecule in HOMO while essentially centered on the p-bridge and acceptor unit in LUMO. The transfer of electron population from the donor group to the acceptor unit with electron promotion is attributed to the push–pull effect. 3.2. Ionization potential and electron affinity The calculated IP and EA of the six molecules were listed in Table 2. The corresponding vertical and adiabatic energies are quite similar, which indicates that the energy changes associated with structural relaxation are relatively small. For photoluminescent materials, the lower IP of the hole-transport layer (HTL) facilitates the entrance of holes from ITO to HTL; and the higher EA of the

electron-transport layer (ETL) facilitates the entrance of electrons from cathode to ETL [2]. It has been experimentally proved that BNPB is a good tri-functional molecule, which not only serves as light emitting material but also can be used in HTL and ETL [5,18]. As the IPs of the DCDPC and DCDPC2 are close to those of BNPB (6.1 eV), which manifest that the two emitters can be applied as the HTL materials [2]. DCDDC, DCDHC, DCM and DCDCC are a little more difficult to be injected with holes but still can be applied as the HTL materials [10]. By analyzing the values of EAs, the six compounds are easier to accept an electron than BNPB (0.77 eV) [18]. From this point of view, the six red emitters especially DCDPC and DCDPC2 are capable of transport for both holes and electrons. Compared with DCM, the ability of accepting electron and hole for DCDDC is better due to the presence of various p-bridges. The IPs and EAs of DCDDC and DCDHC are almost the same, which manifests that the effect of different alkyl donor on charge injection properties is negligible at this stage. It is clear that, DCDPC, with the smallest IPs, exhibits the most excellent properties as ETL

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material among the six compounds. Concerning characters as HTL, DCDPC2 outperforms the other molecules. Since DCDPC2 has the largest conjugation region among the six molecules, it is easier to be injected with both electron and hole. 3.3. Reorganization energy To obtain the reorganization energies k(h) for hole and k(e) for electron, the total energies of the six molecules for neutral, cationic and anionic states were calculated in their respective optimized geometries. In addition, the total energies of cation and anion in the optimized neutral geometries, the total energies of neutral in the optimized geometries of cation and anion were also calculated. The calculated k(h) and k(e) were collected in Table 3. As emitting layer material, it needs to achieve the balance between hole and electron transport [2]. The lower k value indicates the bigger charge-transport rate [2]. The data (Table 3) show that the k(h) for the six molecules are all smaller than their respective k(e). Compared with DCM, the k(e) for DCDDC is smaller but the k(h) is larger. The k(h) and k(e) of DCDDC and DCDHC are larger than those of DCDCC and DCDPC, which manifests that charge transfer properties of molecules with aromatic ring donor are better than those of molecules with alkyl group donor. The k(h) and k(e) of DCDPC2 are lower than those of the other molecules, indicating that double-branched molecule has a better charge-transport properties than the single-branched compounds. Moreover, the difference between the k(h) and k(e) for the six emitters is not large (from 0.12 eV for DCDPC2 to 0.37 eV for DCM) which demonstrates these materials are available for the charge transporting layer with relatively high efficiencies [8–11]. Especially, the difference between the k(h) and k(e) for DCDPC2 is the smallest, only 0.12 eV, implying that DCDPC2 has better equilibrium properties than the other emitters. In general, the larger conjugation region improves the charge transport as well as the balance between hole-transfer and electron-transfer, which is a great benefit to increase the produce of the exciton and enhance the luminescent efficiency. 3.4. Transfer integral

Fig. 2. The electron distribution of HOMO (left) and LUMO (right) level of DCM (a, b), DCDDC (c, d), DCDHC (e, f), DCDCC (g, h), DCDPC (i, j) and DCDPC2 (k, l) (from up to down). Isosurface value 0.02 au.

Table 2 The calculated IP and EA (in eV) of DCM, DCDDC, DCDHC, DCDCC, DCDPC and DCDPC2.

DCM DCDDC DCDHC DCDCC DCDPC DCDPC2

IPv/eV

IPa/eV

EAv/eV

EAa/eV

6.58 6.52 6.53 6.71 6.35 6.37

6.50 6.43 6.42 6.65 6.29 6.32

0.94 1.22 1.24 1.67 1.44 2.09

1.19 1.43 1.42 1.86 1.68 2.19

When the charges hop between neighboring molecules, there are a wide variety of possible intermolecular hopping pathways. Here, two main hopping pathways were studied: the vertical way and the parallel way (see Fig. 3). As the structures of the molecules would be a little changed when the crystalline phase was formed, we put two molecules together refer to their crystal unit cell parameters and optimized their structures then calculated the transfer integral. For DCDPC and DCDCC, we used the initial lattice parameters came from Ju et al. and the results were not far different from the experimentation [28]. We also calculated the four other molecules in the same way, but the results of DCDHC and DCDPC2 failed to be converged. It seemed due to the longer side chain which made the molecules failed to form crystalline. In Fig. 3, the molecules showed strong interaction when they were laminated arrangement (vertical way). They were bent due to the strong Van der Waals force. Otherwise, the molecules which were parallel (parallel way) kept initial structures. The energies of the HOMO, HOMO1, LOMO and LOMO+1 were calculated for the optimized structures, and the V(h) and V(e) of DCDPC, DCDCC, DCDDC and DCM were collected in Table 4. Table 3 Reorganization energies (in eV) of DCM, DCDDC, DCDHC, DCDCC, DCDPC and DCDPC2.

k(h)/eV k(e)/eV

DCM

DCDDC

DCDHC

DCDCC

DCDPC

DCDPC2

0.18 0.55

0.20 0.46

0.23 0.56

0.12 0.39

0.13 0.46

0.09 0.21

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Fig. 3. The optimized bimolecular structure of the vertical (left) and the parallel (right) of DCDPC (a, b), DCDCC (c, d), DCDDC (e, f) and DCM (g, h) (from up to down). The dotted line and the number nearby are representing the distance between the same labeling atoms of two molecules.

According to the Marcus electron transfer theory, the higher V indicates the higher charge hopping rate k. The data (see Table 4) show that the V(h) and the V(e) for the vertical way are all higher than their respective parallel way. It suggests that the rate of charge hopping via the vertical way is obviously higher than the parallel way. For the vertical way, the co-planarity structure form the p–p stack and hence facilitates the electron transfer. The intimate p–p interaction also makes the molecules close in the vertical way. Some values of V(h) and V(e) are even large enough to compare with the reorganization energy. And this

demonstrate that the calculation method based on the HF–KT need amendment when deal with the systems which the molecules are close. For the parallel way, it was worthy to note that the V(h) and V(e) for DCM (which has the quite similar structure with DCDDC), are about half of the values for DCDDC, implying that p-bridges have a big effect on both hole and electron transfer. Moreover, the differences between the V(h) and V(e) for each molecule are not large, which demonstrate that these molecules can act as the charge transporting layer materials for both hole and electron.

Table 4 Transfer integral (in 104 eV) of DCDPC, DCDCC, DCDDC and DCM. Molecule pathway

V(h)/104 eV V(e)/104 eV

DCDPC

DCDCC

DCDDC

DCM

Vertical

Parallel

Vertical

Parallel

Vertical

Parallel

Vertical

Parallel

1215.00 870.77

119.73 85.72

87.08 1375.55

80.27 50.34

1566.03 2194.62

179.60 107.49

708.86 903.43

59.87 54.42

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4. Conclusion The calculations on charge transport properties of six emitters with push–pull structure were carried out. According to the calculated EA, IP, k and V, the capability of transport for both holes and electrons was being found for all of the six compounds. In addition to being emitters, these multifunctional materials can also be appropriate for HTL and ETL. The ability of accepting electron and hole as well as the charge transport rate is affected by the donor, p-bridge and acceptor branches. It is not surprising that DCDPC2, which is the double-branched molecule and has larger conjugation region than single-branched molecules, emerges as the best material in our study for the charge injection as well as charge transporting rate and balance. Acknowledgements Financial support for this work was provided by National Natural Science Foundation of China (Grant Nos. 61125505, 61274063 and 61377028), Beijing National Laboratory for Molecular Sciences, and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS.

Tao Lin received his B.S. and M.S. degrees from Beijing Jiaotong University in 2010 and 2012, respectively. He is currently a Ph.D. candidate under the guidance of Professor Feng Teng in the school of science at Beijing Jiaotong University. He is involved in several research projects including the organic polymer optoelectronic devices.

Dr. Xiaojun Liu is an Associate Professor from School of Science at Beijing Jiaotong University. She received her Doctor of Engineering in Material Science from Shandong University and Institute of Chemistry, Chinese Academy of Science. She was a postdoctoral research fellow and a Visiting Professor at Technische Universität München. Her research interests are nonadiabatic potential-energy surfaces and photochemical reaction paths of molecules and clusters.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemphys.2014. 06.005. References [1] C. Tang, S. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] L.Y. Zou, A.M. Ren, J.K. Feng, Y.L. Liu, X.Q. Ran, C.C. Sun, J. Phys. Chem. A 112 (2008) 12172. [3] A.P. Kulkarni, C.J. Tonzola, A. Babel, S.A. Jenekhe, Chem. Mater. 16 (2004) 4556. [4] J. Slinker, D. Bernards, P.L. Houston, H.D. Abruña, S. Bernhard, G.G. Malliaras, Chem. Commun. (2003) 2392. [5] W.L. Jia, X.D. Feng, D.R. Bai, Z.H. Lu, S. Wang, G. Vamvounis, Chem. Mater. 17 (2005) 164. [6] T. Noda, Y. Shirota, J. Am. Chem. Soc. 120 (1998) 9714. [7] W.L. Jia, M.J. Moran, Y.-Y. Yuan, Z.H. Lu, S. Wang, J. Mater. Chem. 15 (2005) 3326. [8] X. Tao, S. Miyata, H. Sasabe, G. Zhang, T. Wada, M. Jiang, Appl. Phys. Lett. 78 (2001) 279. [9] H.-D. Ju, X.-T. Tao, Y. Wan, J.-H. Shi, J.-X. Yang, Q. Xin, D.-C. Zou, M.-H. Jiang, Chem. Phys. Lett. 432 (2006) 321. [10] H. Ju, Y. Wan, W. Yu, A. Liu, Y. Liu, Y. Ren, X. Tao, D. Zou, Thin Solid Films 515 (2006) 2403. [11] C. Tang, S. VanSlyke, C. Chen, J. Appl. Phys. 65 (1989) 3610. [12] C.-T. Chen, Chem. Mater. 16 (2004) 4389. [13] X. Ren, D. Bo, A. Ren, J. Feng, L. Du, C. Sun, J. Mol. Struc.-Theochem. 904 (2009) 91. [14] L.Y. Zou, A.M. Ren, J.K. Feng, X.Q. Ran, Y.L. Liu, C.C. Sun, Int. J. Quantum Chem. 109 (2009) 1419. [15] D. Liang, S. Tang, J. Liu, J. Liu, L. Kang, J. Mol. Struct.-Theochem. 908 (2009) 102. [16] S. Mi, D. Chen, N. Lu, Int. J. Quantum Chem. 111 (2011) 2039. [17] S. Tang, J. Zhang, Int. J. Quantum Chem. 111 (2011) 2089. [18] X. Li, X. Wang, J. Gao, X. Yu, H. Wang, Chem. Phys. 326 (2006) 390. [19] T. Yamada, T. Sato, K. Tanaka, H. Kaji, Org. Electron. 11 (2010) 255. [20] X.-H. Fang, Y.-Y. Hao, P.-D. Han, B.-S. Xu, J. Mol. Struc.-Theochem. 896 (2009) 44. [21] R.A. Marcus, J. Chem. Phys. 24 (2004) 966. [22] R.A. Marcus, Rev. Mod. Phys. 65 (1993) 599. [23] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [24] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [25] D. Jacquemin, E.A. Perpete, I. Ciofini, C. Adamo, Acc. Chem. Res. 42 (2008) 326. [26] G. Petersson, A. Bennett, T.G. Tensfeldt, M.A. Al-Laham, W.A. Shirley, J. Mantzaris, J. Chem. Phys. 89 (1988) 2193. [27] M. Frisch, G. Trucks, H.B. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, Petersson Inc., Wallingford, CT 200, 2009. [28] H. Ju, Design, Synthesis and Properties of Isophorone-based light-emitting Materials, Shandong University, 2007.

Zhidong Lou, is a professor at Institute of Optoelectronic Technology, Beijing Jiaotong University. She has a PhD degree in physics from Changchun Institute of Physics, Chinese Academy of Sciences. From 2000–2004 she worked as a postdoctoral research fellow at Tohoku University (Japan) and the University of Guelph (Canada) on magnetic thin films and inorganic luminescent materials, respectively. She has been working in the field of organic optoelectronic devices since 2004.

Yanbing Hou was born in China. 2003.8- , Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing, China. 2003.8-present, he is a Professor in Institute of Optoelectronic Technology, Beijing Jiaotong University. Research interest of his group is organic/ inorganic thin-film light emitting diode (LED), thin-film electroluminescence, up-conversion luminescence of rare earth doped compound, polymer light emitting devices, electrooptical device, polymer photoamplifier and behavioral study of polymer excited state under electric fields.

Feng Teng received his Ph.D. degree from Changchun Institute of Physics, Chinese Academy of Sciences in 1998. From 1998 to 2000, he worked on the organic electro-luminescent materials at Institute of Chemistry, Chinese Academy of Sciences. Prof. Teng has received the support from the National Science Fund for Distinguished Young Scholars in 2012. His current research interests include organic electroluminescent materials and device physics, up-conversion luminescent materials and optical bistable devices.