Computational design of benzo [1,2-b:4,5-b′] dithiophene based thermally activated delayed fluorescent materials

Dyes and Pigments 127 (2016) 189e196

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Computational design of benzo [1,2-b:4,5-b0 ] dithiophene based thermally activated delayed fluorescent materials Jing Lu, Yiying Zheng, Jingping Zhang* Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 October 2015 Received in revised form 29 December 2015 Accepted 30 December 2015 Available online xxx

A series of benzo [1,2-b:4,5-b0 ]dithiophene based thermally activated delayed fluorescent molecules have been designed and investigated using density functional theory and time-dependent density functional theory. The theoretical calculations showed that the designed 4, 8-positions of benzo [1,2-b:4,5-b0 ] dithiophene substituted molecules exhibited a mixed states, which comprised a large proportion of charge transfer components and a small part of locally excited components. The calculated emission wavelengths of the investigated molecules may exhibit long wavelength emission characters. For the designed 4, 8-positions substituted molecule with two [1,2,5]thiadiazolo[3,4-c]pyridine-7-carbonitrile electron-accepting units, which exhibited zero singlet-triplet energy gap, the predicted hole and electron mobilities are 3.43 cm2 v1 s1 and 0.18 cm2 v1 s1, respectively. Our study may provide new ideas for the design of highly efficient thermally activated delayed fluorescent candidates by realizing the full potential of both singlet and triplet excitons. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Benzo [1,2-b:4,5-b0 ]dithiophene Thermally activated delayed fluorescence Density functional theory Time-dependent density functional theory Triplet-singlet conversion Charge transfer

1. Introduction In 2009, a novel promising mechanism to achieve highly electronic luminescent quantum efficiency, that is thermally activated delayed fluorescence (TADF), has been successfully exploited from Sn4þ-porphyrin complexes as emitters for organic light emitting diodes (OLEDs) [1e4]. A remarkable feature of TADF luminophores is up-conversion of excitons from the triplet to singlet excited state, which closely related to the small energy gap (DEST) between them [5e10]. Donoreacceptor molecular systems with conspicuous intramolecular charge transfer (ICT) character are quite suitable to realize small DEST value through the separation between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) [11e13]. Therefore, the smart choice of the connection and hybrid of donor and acceptor fragments are crucial for molecular design [14]. For the reported donoreacceptor type TADF molecules, the DEST can be further narrowed down by increasing steric hindrance between connecting donor and acceptor fragments, such as twist, bulky, or spirojunction [15e17]. The ideal donor fragments should

* Corresponding author. Tel.: þ86 431 85099372; fax: þ86 431 85099521. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.dyepig.2015.12.030 0143-7208/© 2016 Elsevier Ltd. All rights reserved.

have strong electron-donating ability as well as the stable and high triplet states. Currently, carbazole, diphenlamine, phenoxazine (PXZ), and their derivatives are commonly used as the D fragments [18,19]. However, In this study, benzo [1,2-b:4,5-b0 ]dithiophene (BDT) is employed as a new donor fragment to construct TADF molecules. BDT has lower HOMO energy level than the widely used PXZ fragment, and possesses multiple substitution sites for further molecular design [20]. In the case of TADF materials, a high molecular symmetry typically facilitates the reduction of number of active modes for vibronic couplings between the higher and lower states. Once the number of active modes and intensity of vibronic couplings are decreased, the nonradiative processes will be inhibited [21,22]. Consequently, the light emission is anticipated to occur from the higher excited states, which may give rise to smaller DEST or inverted singlet-triplet structures. Therefore, our main aim in this work is to design a series of symmetric molecules with branched twisted topology to design high efficient TADF molecules with nearly zero DEST value. As shown in Scheme 1, a series of BDT-mAn and BDT-mAn′ have been successfully designed, where the m and n refer to the number and series name of A fragments, respectively. For the BDT-mAn molecule, the An units are connected to the 4- or/ and 8-positions of the BDT core unit, while in BDT-mAn′ molecule, the An units are attached to the 2- or/and 6-positions.

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system [23]. As expected, the HOMO and LUMO energy levels are further reduced by introducing the cyano electron withdrawing group onto the A′ unit. [1,2,5]thiadiazolo[3,4-c]pyridine-4carbonitrile (A1) and [1,2,5]thiadiazolo[3,4-c]pyridine-7carbonitrile (A2) possess the lowest LUMO and HOMO energy levels among all the investigated acceptor fragments. In this study, we will choose A1 and A2 as the idea acceptor subunits to construct BDT based TADF molecules. 2.2. Calculation method

Scheme 1. Chemical structures of the donor (D), acceptors (An), and BDT-mAn and BDT-mAn′.

2. Methodology 2.1. Selection of building fragments Prior to the designed molecules, the HOMO and LUMO energy levels of the individual BDT and An building fragments are investigated. The acceptor fragments with strong electron withdrawing strength are beneficial for building TADF molecules with smaller DEST values. As shown in Fig. 1, the HOMO and LUMO energy levels of benzo[c][1,2,5]thiadiazole (A) are 6.90 eV and 2.20 eV, respectively. Our previous work suggested that the “CH”/N substituted on the 5-position of the A unit gave much lower LUMO energy level, enhancing the electron-withdrawing ability [20]. The HOMO and LUMO energy levels of [1,2,5]thiadiazolo[3,4-c]pyridine A′ decreased due to “CH”/N substitution. On the other hand, the cyano group has a strong electron-withdrawing ability. Therefore, the cyano group may have a wide application in constructing sufficiently strong acceptor units for the donoreacceptor type TADF

Fig. 1. Energy levels of the HOMO and LUMO of BDT and the four acceptor fragments (H ¼ HOMO, L ¼ LUMO).

The calculated vertical excitation energies of the singlet state [EVA(S)] and triplet state [EVA (T)], and vertical singletetriplet gaps (DEST) using different DFT functionals including PBE0 [11,24], CAMB3LYP [25], LC-uPBE [26], M062X [27], HSEh1PBE [28], uB97XD [29], TPSSh [30], and B3LYP [31] are shown in Fig. 2 and Table S1. The calculated results show that all these functionals give the same trend on the prediction of DEST for symmetric and asymmetric structures, which is in the order of BDT-4An (BDT-2An) < BDT-An. For the symmetric molecules, DEST for molecule BDT-2A2 (BDT4A2) is smaller than that for BDT-2A1 (BDT-4A1). In addition, in our previous study, the computed DEST value (0.13 eV) at PBE0 theoretical level for 2PXZ-OXD is close to the experimental value (0.15 eV) [32]. Meanwhile, CASSCF and CASPT2 calculations also provide the same tendency with the TD-PBE0 calculations that the symmetric structure is more conducive than asymmetric one to construct TADF molecules with small DEST value. Considering the computing resources in our group, we choose PBE0 functional to predict the tendency of DEST values for these investigated BDT derivatives. The neutral geometric structures of the designed molecules were optimized using PBE0 functional and 6-31G (d) basis set. Then the vibrational frequency calculations were performed at the same theoretical level and the results showed that no imaginary frequencies were found. The singlet (S1) and triplet (T1) excited states were optimized with the TD-PBE0/6-31 G (d) method. On the basis of the optimized S1 states, the emission wavelengths were predicted using time-dependent density functional theory (TD-DFT). Finally, the ionic optimizations were treated as open shell systems using the unrestricted formalism (UPBE0). In the light of the optimized neutral, cationic and anionic geometries, reorganization energies were obtained from the singlet point energies calculations. All the calculations were carried out with Gaussian 09 program package [33]. The prediction for the charge transfer

Fig. 2. Calculated DEST using different DFT functionals ( B3LYP, LC-uPBE, M062X, HSEh1PBE, B3LYP).

PBE0,

uB97XD,

CAMTPSSh,

J. Lu et al. / Dyes and Pigments 127 (2016) 189e196

properties are performed using Materials Studio and our selfcompiled program. 2.3. Charge transfer theory Charge carrier transfer rate from the Marcus theory expression reads: [34].

Kif ¼

   2  Vij2 rffiffiffiffiffiffiffiffiffiffi p 4lkB T exp  DG0 þ l lkB T Z

Here Vij is the transfer integral, l is the internal reorganization energy, which is defined as the energy change associated with the geometry relaxation during the charge transfer, and DG0 is the relevant change of total Gibbs free energy. In the self-exchange reaction, DG0 equals zero. Transfer integral involves direct evaluation of the coupling element between frontier orbitals is calculated using the unperturbed density matrix of the dimer Fock operator.

 Vij ¼ ji jFjjj where ji and jj are the frontier orbitals of the two isolated molecules i and j in the dimer. F ¼ SCεC1 is the Fock operator, where S is the overlap matrix, and C and ε represents the KohneSham orbital coefficients and energies obtained from one-step diagonalization without interaction. 3. Results and discussion 3.1. Frontier molecular orbital The HOMOs and LUMOs isosurfaces of the optimized ground (S0) states for the investigated molecules are shown in Fig. 3, which shows an effective separation between HOMO and LUMO. The HOMOs are mainly distributed on the BDT fragments, while the

191

LUMOs are localized on the A1 or A2 fragments. There is a small overlap between BDT and A1 for both HOMO and LUMO. On the other hand, the overlap values between BDT and A2 are less than those between BDT and A1 for both HOMO and LUMO, indicating that the combination of BDT and A2 is more advantageous topology to realize TADF phenomenon (Table S2). We also analysed the frontier molecular orbital (FMO) characteristics of the excited states for all the designed molecules. As shown in Fig. 4, the S1 states consist of mixed states of the locally excited (LE) and CT, so called the hybridized local and charge transfer (HLCT). The dominance of CT can be explained upon careful examination of the dihedral angles (ɤ) between donor and acceptor fragments in the S1 states (Table S3). The ɤ value between BDT and A1 (A1′) for BDT-A1 (BDT-A1′) is 111.42 (33.12 ), indicating that the CT component of BDT-A1 (BDT-A1′) are larger than those of the LE component. BDT-2A1′ molecule exhibited the more LE on account of the planar structure than BDT-2A1. The ɤ value of BDT-2A1 is 56.88 due to the repulsion of neighbouring hydrogen atoms, promoting the growth of CT components. For the designed BDT-4A1, the ɤ value between BDT and A1, which connected at the 4, 8-position of BDT moiety, is 63.02 and 63.02 , respectively, while the ɤ value between BDT and A1′, which connected at the 2, 6-positions of BDT moiety, is 15.96 and 15.96 , respectively. Therefore, the 4, 8position substitutions are favourable for attaining large twisted angles and interrupting the electronic communication between donor and acceptor fragments. The calculated results on the electronic density distribution pattern of the molecules with BDT and A2 are also summarized in Fig. 4 and Fig. S1. The ɤ values for BDT-mA2 slightly reduced relative to the BDT-mA1 series, owing to the different positions of N atom (Table S3). For the 2- or/and 6- positions substituted derivates BDTA2′ and BDT-2A2′, the ɤ values are close to zero. Our calculations and discussions on the frontier molecular orbital indicated that the 4, 8-positions of BDT moiety might play more significant role in the small energy gap between singlet and triplet states.

Fig. 3. The HOMO (Green) and LUMO (Red) isosurfaces of the designed molecules in the ground states (S0). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 4. The energy levels and molecular orbital character of the excited states for the designed molecules.

3.2. The path of triplet-singlet conversion According to Kasha's rule, a light-emitting state is the lowest excited state, such as the S1 and T1 states. However, it is well known that there exist exceptions to Kasha's rule. In some HLCT compounds, it is possible that a reverse intersystem crossing (RISC) from T2 to S1 [21,22]. Conceivably, it is of great significance to construct a HLCT state to realize fully exciton-utilizing emitting materials. According to the state-mixing principle in quantum chemistry, hybridized state can be described as a linear combination of two states of j(CT) and j(LE), and the degree of mixing of the two states is related to the mixing coefficient (l). The energy gap between the two states at the initial configurations and the magnitude of the coupling matrix element between these two

states, which depends on the spatial wave function overlap of two states, symmetry properties, and the nature of H, are the two important factors.

JðS1 Þ ¼ JðLEÞ þ ljðCTÞ

(1)



j jHjjCT

l ¼

LE ECT  ELE

(2)

In order to deeply understand the reverse intersystem crossing (RISC) process from the triplet to singlet states for the designed TADF molecules, we calculated the first twenty singlet and triplet states for all the molecules, respectively. The calculated results are summarized in Fig. 4, including the energy gaps for singlet and

J. Lu et al. / Dyes and Pigments 127 (2016) 189e196 Table 1 The predicted emission wavelength (lem) of the designed molecules using TD-PBE0/ 6-31G (d) method.

Ev

lem

Ev

lem

BDT-A1

BDT-A10

BDT-2A1

BDT-2A10

BDT-4A1

1.518 816.70

1.795 690.74

1.719 721.17

1.875 661.29

1.743 711.45

BDT-A2

BDT-A2′

BDT-2A2

BDT-2A2′

BDT-4A2

1.455 852.09

1.905 650.89

1.617 766.61

1.901 652.18

1.713 723.74

triplet states and the excited-state characters. The energy gaps between S1 and T1 for both BDT-A1 and BDT-A2 (0.23 eV and 0.27 eV, respectively). However, the energy gaps between S1 and T1 for BDT-A1′ and BDT-A2′ (0.80 eV and 0.81 eV, respectively) are too large to realize the up-conversion from T1 to S1 state. Therefore, for the same topology, the 4- or 8-position substituted molecules BDTA1 and BDT-A2 may exhibit more effective RISC process than 2- or 6-position substituted ones. Then we analysed the designed molecules with two An (n ¼ 1, 2) units. For BDT-2A1 and BDT-2A2, the RISC process may take place through the higher excited states channel T2 / S1. The energy gaps between S1 and T2 are 0.13 eV and 0.00 eV, respectively, and the energy gaps between T2 and T1 are 0.13 eV and 0.28 eV, respectively. The T1 states can be assigned to LE with small amount of CT character. However, the S1 and T2 possess more CT than LE components in their HLCT state, showing similar distribution pattern. The energy gap of BDT-2A2 between S1 and T2 is nearly zero due to the increased triplet energy level. On the other hand, the optimized BDT-2A1′ and BDT-2A2′ showed a planar structure, whose excited states exhibited more LE component than BDT-2An. Finally, for the X-shaped molecules BDT-4A1 and BDT-4A2 with high molecular symmetry, the characters of S1 and T4 can assigned to HLCT with the same distribution pattern, while those of T1, T2, and T3 are mainly as ascribed to LE states. The energy gaps between S1 and T4 for BDT-4A1 and BDT-4A2 are 0.13 eV and 0.00 eV, respectively. The RISC process may take place through the higher excited states channel T4 / S1. 3.3. Emission properties To investigate the effect of attaching numbers of A fragments to the BDT core on emission wavelength, we calculated the emission properties of the designed molecules using the TD-PBE0/6-31G (d) method on the basis of the optimized S1 states. According to the calculated results listed in Table 1, the designed BDT-mAn showed long wavelength emission bands located in the range of 650e852 nm. The emission wavelength (lem) of BDT-A1 and BDT-

193

2A1 are 816.70 nm and 721.17 nm, respectively, and the lem of BDTA2 and BDT-2A2 are 852.09 nm and 766.61 nm, respectively. The lem of BDT-mAn exhibited a significantly hypochromatic shift as the numbers of electron-withdrawing units increased. The lem of BDT-4A1 (BDT-4A2) was blue shifted about 9.72 (42.87) nm compared with that of BDT-2A1 (BDT-2A2), due to increased A1 (A2) units on the 2, 6-positions of BDT unit. However, the lem of BDT-4A1 (BDT-4A2) is markedly red shifted about 50.16 (71.56) nm compared with that of BDT-2A1′ (BDT-2A2′) due to increased A1 (A2) units on the 4, 8-positions of BDT units. It can be rationalized upon careful examination of the change of structural parameters from BDT-4An to BDT-2An and BDT-2An′ in the optimized S1 states. (Fig. 5) The optimized bond length values of the An fragments for BDT-4An are indeclinable compared with those for BDT2An, indicating that the introducing of An′ fragments on the 2,6positions of BDT unit has negligible influence on the structural parameters of An fragments. On the other hand, optimized bond length values of the An′ fragments for BDT-4An change significantly compared with those for BDT-2An′, manifesting that the introducing of An fragments on the 4,8-positions of BDT unit have a significant effect on the bond lengths of An′ fragments. Therefore, the 4 and 8-positions of BDT core may have a significant impact on the emission wavelength for the investigated BDT-mAn. In comparison with BDT-mA1 and BDT-mA1′, the lem values of BDT-mA2 and BDT-mA2′ are red and blue shifted, respectively. 3.4. Reorganization energies According to the semi-classical Marcus theory, a low reorganization energy value is required for an efficient charge transfer and balance between the hole and electron. All the calculated reorganization energies are listed in Table 2. The calculated hole and electron reorganization energy (lh and le) of the designed molecules are in the range of 0.15e0.20 eV and 0.15e0.32 eV, respectively. The le value of BDT-mAn decreased as the numbers of An units increased. In the case of the same numbers of A units, the 4or/and 8-position substituted molecules possess larger le values than 2- or/and 6-position substituted ones. Then we discussed the balance between the hole and electron. The lh and le values both are 0.19 eV for BDT-2A1, suggesting the excellent balance between hole and electron. As for BDT-4A1, the transfer ability of electron is stronger than that of hole, while BDT-A1 presents much weaker transfer ability of electron than that of hole. On the other hand, the BDT-4A2 with the zero DEST value also exhibits well charge balance, whereas the transfer rate of hole is stronger than that of hole for the other molecules of A2 series. The fact that the designed molecules showed different lh and le values can be rationalized upon careful examination from

Fig. 5. The bond length differences between the optimized BDT-4An and BDT-2An (BDT-2An′) in the first singlet excited states.

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Table 2 Frontier molecular orbital and hole/electron reorganization energies of the investigated molecules (in eV). Molecules

EHOMO

ELUMO

ELUMOþ1

D(ELUMOþ1  ELUMO)

lh

le

BDT-A1 BDT-2A1 BDT-4A1 BDT-A2 BDT-2A2 BDT-4A2

6.03 6.28 6.58 5.98 6.18 6.48

3.34 3.57 4.01 3.29 3.56 3.99

1.52 3.48 3.83 1.53 3.35 3.75

1.82 0.09 0.18 1.76 0.21 0.24

0.18 0.19 0.20 0.18 0.19 0.17

0.32 0.19 0.15 0.31 0.22 0.17

the optimized neutral structures to the optimized cationic/ anionic structures (Fig. 6). Most of the geometry changes between neutral and cationic states localized on the BDT fragment, while the differences between neutral and anionic states occur on

the An (n ¼ 1, 2) fragments. For the BDT-4An, the variation between An′ and BDT are more remarkable than that between An and BDT. The HOMOs and LUMO/LUMOþ1 energies are given in Table 2. The calculated energy difference between LUMO and LUMOþ1 of BDT-2An and BDT-4An are in the range of 0.09e0.24 eV. The LUMO and LUMOþ1 are energetically nearly degenerate and represent linear combinations of LUMOs of the two acceptor subunits. However, the calculated energy difference values between LUMO and LUMOþ1 of BDT-A1 and BDT-A2 are 1.82 and 1.76 eV, respectively. Meanwhile, the LUMO wave functions of BDT-A1 and BDT-A2 in the neutral reside on the acceptor subunits, the LUMOþ1 wave functions are delocalized the whole molecules (Fig. S2), thus BDT-A1 and BDT-A2 may exhibit the weak electron transfer properties.

Fig. 6. The bond length differences between the optimized neutral and ionic states for BDT-mAn.

J. Lu et al. / Dyes and Pigments 127 (2016) 189e196

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Therefore, the increased numbers of acceptor units are conducive to improve the electronic transfer ability. Taking both reorganization energy and mobility into consideration, BDT-2A2 and BDT-4A2 may be highly efficient TADF candidates. We envision that this detailed comparative analysis may provide useful information to design new TADF materials not only for OLED but also for other electronic devices. 4. Conclusions

Fig. 7. The predicted super cell structure of BDT-2A2 with P1 space group along (a) aeb plane, (b) aec plane, and (c) bec plane.

Table 3 The estimated transfer integrals (V) and mobility values for BDT-2A2 for the twenty pathways in the crystalline states. Pathways 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 mobility

Distance (Å) 3.807 3.807 11.745 12.870 14.911 11.116 12.300 14.421 17.004 16.572 17.004 11.388 11.346 12.521 17.004 16.573 17.004 11.388 11.346 12.521

Vh

Ve 1

2.454  10 2.455  101 1.02  102 1.6791  104 7.0099  108 5.6  103 1.3754  106 3.6335  108 1.2  103 1.7244  105 7.6195  105 1.9  103 2.4  103 1.048  104 7.6220  105 1.7522  105 1.2  103 1.9  103 2.4  103 1.05  104 3.43 cm2 v1 s1

6.70  102 6.70  102 5.70  103 1.2832  104 2.1865  107 4.1  103 6.4743  104 1.1064  106 1.02  102 3.2  103 5.2589  104 1.3  103 3.1  103 1.5236  104 5.2664  104 3.2  103 1.02  102 1.3  103 3.2  103 1.5341  104 0.18 cm2 v1 s1

In summary, we have designed a series of BDT-mAn TADF molecules with high efficiency CT characteristic emission covering the long wavelength region. The DFT analysis indicated that the HOMOs and LUMOs of BDT-mAn exhibited a good separation in S0 states, where HOMOs were mainly distributed on the BDT fragments, while LUMOs are localized on the An fragments. Compared with 2, 6-positions of BDT substituted molecules, the 4, 8-positions of those are favourable to achieve small overlap and interrupt electronic communication between D and A. The TD-DFT calculations showed that the excited states of the designed BDT-mAn are HLCT states with a great proportion of CT components. The upconversion paths from triplet to singlet states for BDT-2An may take place through the higher excited states channel T2 / S1. For the BDT-4An with high molecular symmetry, the characters of S1 and T4 can be assigned to HLCT with the same distribution pattern. Combined with the energy gap rules, the RISC process may take place through the higher excited states channel T4 / S1. The BDT2A2 and BDT-4A2 exhibited nearly zero energy gap values between singlet and triplet excited states. BDT-A1 and BDT-A2, whose lem are 816.70 nm and 852.09 nm, may have a broad application prospect in near infrared TADF materials. The lem of BDT-2An and BDT-4An exhibited a significantly hypochromatic shift compared with that of BDT-An due to the increased numbers of A fragments. BDT-2An and BDT-4An presented red visible emission TADF characters. In addition, the calculated lh and le of the designed molecules are in the range of 0.15e0.20 eV and 0.15e0.32 eV, respectively. The predicted carrier mobilities are mh ¼ 3.43 cm2 v1 s1 and me ¼ 0.18 cm2 v1 s1 for BDT-2A2, respectively, and mh ¼ 0.67 cm2 v1 s1 and me ¼ 1.38 cm2 v1 s1 for BDT-4A2, respectively. Our results may provide an ideal strategy to design efficient TADF molecules. Acknowledgements

3.5. Mobility The calculations in the crystalline state were implemented to obtain quantitative charge transfer properties. BDT-2A2 and BDT4A2, which possess small DEST values, were selected to study their charge transport properties in the solid state. The predicted lattice parameters are a ¼ 3.81 Å, b ¼ 15.57 Å, c ¼ 31.35 Å, a ¼ 90.0 , b ¼ 103.9 , g ¼ 90.0 for BDT-2A2 with the P21/c space group (Fig. 7), and a ¼ 4.01 Å, b ¼ 26.39 Å, c ¼ 17.93 Å, a ¼ 98.3 , b ¼ 124.8 , g ¼ 86.2 for BDT-4A2 with P1 space group (Fig. S3). Then the corresponding coupling integrals for the nearest neighbouring pathways (Fig. S4) were calculated with our self-compiled program. In Table 3 and Table S4, pathway 1 and 2 have stronger coupling integrals for both hole and electron. These mobility calculations for the BDT-2A2 crystalline structure show that the transfer strength of hole is higher than that of electron. The calculated mobility are mh ¼ 3.43 cm2 v1 s1 and me ¼ 0.18 cm2 v1 s1 for BDT-2A2, respectively. For BDT-4A2, the predicted mobility values are mh ¼ 0.67 cm2 v1 s1 and me ¼ 1.38 cm2 v1 s1, respectively.

The financial support from NSFC (21173037 and 21203021) and Specialized Research Fund for the Doctoral Program of Higher Education for New Teachers (20120043120008) are gratefully acknowledged. We thank Professor Fang Weihai and Miss Chang Xueping for providing CASSCF and CASPT2 calculations. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.12.030. References [1] Endo A, Ogasawara M, Takahashi A, Yokoyama D, Kato Y, Adachi C. Adv Mater 2009;21:4802e6. [2] Tao Y, Yuan K, Chen T, Xu P, Li H, Chen R, et al. Adv Mater 2014;26:7931e58. [3] Xiao L, Chen Z, Qu B, Luo J, Kong S, Gong Q, et al. Adv Mater 2011;23:926e52. [4] Serevicius T, Nakagawa T, Kuo MC, Cheng SH, Wong KT, Chang CH, et al. Phys Chem Chem Phys 2013;15:15850e5. [5] Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C. Nature 2012;492:234e8. [6] Zhang Q, Li B, Huang S, Nomura H, Tanaka H, Adachi C. Nat Photonics 2014;8: 326e32.

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