Investigation on the effect of connected bridge on thermally activated delayed fluorescence property for DCBPy emitter

Accepted Manuscript Investigation on the effect of connected bridge on thermally activated delayed fluorescence property for DCBPy emitter Ying Gao, Yun Geng, Yong Wu, Min Zhang, Zhong-Min Su PII:

S0143-7208(17)30159-6

DOI:

10.1016/j.dyepig.2017.04.001

Reference:

DYPI 5895

To appear in:

Dyes and Pigments

Received Date: 24 January 2017 Revised Date:

30 March 2017

Accepted Date: 1 April 2017

Please cite this article as: Gao Y, Geng Y, Wu Y, Zhang M, Su Z-M, Investigation on the effect of connected bridge on thermally activated delayed fluorescence property for DCBPy emitter, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.04.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Previously, extensive efforts have been devoted to designing highly performance TADF material via varying the electron-donator (D) and electron-acceptor (A) units and tried the best to find a matching combination of D and A units with high external quantum efficiency. In present work, we have investigated the effect of modifying the connected bridge between D and A units on their

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electronic properties. Based on the reported thermally activated delayed fluorescence (TADF) molecule DCBPy (compound 1), four compounds 2-5 have been designed by modifying the connected bridge between D and A units. For predicting the accurate singlet-triplet energy gap (∆EST), the tuning range-separated functional has been utilized to calculate ∆EST. The calculated normal mode reorganization energy (λ) for the non-radiative decay process displays that the λ of compounds 2-5 in

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the high-frequency region is noticeably reduced compared with compound 1, suggesting that the high-frequency C=O stretching vibration is hindered through modifying the connected bridge between

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D and A units. Moreover, the radiative decay rate constant (kr) values of compound 2-5 are one order of magnitude higher than that of pristine compound 1. Besides, for our designed molecules, modifying the connected bridges noticeably increase their spin-orbital coupling matrix element () values, although the ∆EST values of compound 2-5 are greater than that of compound 1. As a consequence, for this kind of DCBPy compounds, modifying the connected bridge between D and A units maybe a valid approach to improve their TADF performances.

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Investigation on the Effect of Connected Bridge on Thermally Activated Delayed Fluorescence Property for DCBPy Emitter

[a]

Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University,

Changchun 130024, P. R. China E-mail: [email protected] [b]

School of Pharmaceutical Sciences, Changchun University of Chinese Medicine, 1035 Boshuo Road,

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Changchun, 130117, P. R. China E-mail: [email protected]

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College of Chemistry, Jilin University, Changchun 130012, P. R.

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[c]

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Ying Gao[a][c], Yun Geng*[a], Yong Wu*[b], Min Zhang[a], Zhong-Min Su[a][c]

1

ACCEPTED MANUSCRIPT Abstract One thermally activated delayed fluorescence (TADF) molecule DCBPy (compound 1) has been reported. Based on it, four compounds 2-5 have been designed by modifying the connected bridge

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between electron-donor (D) and electron-acceptor (A) units. The calculated normal mode reorganization energy (λ) for the non-radiative decay process shows that the λ of compounds 2-5 in the high-frequency region is noticeably reduced compared with compound 1, suggesting that the

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high-frequency C=O stretching vibration is hindered through modifying the connected bridge between

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D and A units. Moreover, the radiative decay rate constant (kr) values of compound 2-5 are nearly one order of magnitude higher than that of pristine compound 1. Besides, for TADF material, the reverse intersystem crossing (RISC) is dependent on a small singlet-triplet energy gap (∆EST) and large spin-orbital coupling matrix elements . For our designed compounds, modifying the

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connected bridges noticeably increase their values, although the ∆EST values of compound 2-5 are greater than that of compound 1. Especially, compound 2 has a comparable ∆EST with compound 1 and larger kr and than compound 1, which exhibits the best TADF

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efficiency among these compounds. As a consequence, for this kind of DCBPy emitter, modifying the

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connected bridge between D and A units is a valid approach to improve their TADF performances. Key words: reorganization energy, singlet-triplet energy gap, radiative decay rate constant, spin-orbital coupling matrix elements

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ACCEPTED MANUSCRIPT 1.

Introduction

Recently, the thermally activated delayed fluorescence (TADF) materials have attracted researchers’ significant attention.[1-4] The organic lighting-emitting diode (OLED) based on TADF shows many

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advantages. On one hand, by the thermally reverse intersystem crossing (RISC) from the lowest triplet (T1) to singlet (S1) excited states, the TADF material can use both singlet and triplet excitons.[5-6] Therefore, the maximum internal electroluminescence efficiency of TADF material is nearly 100%

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which is comparable with that of phosphorescent OLED.[7] On the other hand, the TADF material is

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more inexpensive and lower pollution in view of practical application than phosphorescent OLED. Because the most TADF materials are metal-free organic electroluminescence molecules and abundant on the earth.[1]

The effective TADF materials need a small singlet-triplet energy gap (∆EST) because a small ∆EST

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value can achieve an easier RISC at a given temperature.[8-9] And the small overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is able to minimize the ∆EST. Thus, the traditional TADF materials are charge-transfer emitters containing an

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electron-donor (D) unit and an electron-acceptor (A) unit. For example, Adachi et al. have reported

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numerous D-A model TADF molecules with high quantum efficiency.[1, 5, 10-14] To improve the quantum efficiency, the design of TADF molecules subsequently extends to the D-π-A, D-A-D, D-π-A-π-D models and so on.[2, 15-19] It is found that the D-A-D molecules show more efficient TADF and higher photoluminescence quantum yields than D-A molecules.[17] Moreover, for D-A-D molecule, increasing the distance between D and A by introducing phenyl bridge is simultaneously achieving small ∆EST and large fluorescence rate.[18] All the time, the designs of TADF molecules are mostly focusing on the strategy of varying the D 3

ACCEPTED MANUSCRIPT and A units. However, the influence of connected bridges between D and A units is scarcely concerned. Different from previous view focusing on the D and A units, in this work, we intend to investigate the influence of varying the connected bridge between D and A units on their electronic properties. In 2015,

24%,

low

turn-on

voltage

and

reduced

roll-off.

In

this

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one TADF molecule DCBPy was reported,[20] which shows a very high external quantum efficiency DCBPy

molecule,

a

new

phenyl(pyridine-4-yl)methanone acceptor was introduced and two carbazolyl donors were ortho and

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meta connected to the phenyl ring of the electron-acceptor unit. Based on the DCBPy molecule

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(compound 1), four compounds 2-5 (shown in Figure 1) have been designed by varying the number of phenyl ring and different linkages to the electron-acceptor unit. In the following section, a series of properties associated with the TADF performances in OLED have been calculated through using

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density functional theory method.

Figure 1 Molecular structures of compounds 1-5. 4

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2.

Computational method

All calculations were performed using Gaussian 09 program.[21] The optimized ground state

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geometries (S0) of compounds 1-5 were obtained at B3LYP/6-31G(d) level, and the lowest singlet excited state geometries (S1) of compounds 1-5 were optimized at TD-CAM-B3LYP/6-31G(d) method, which has provided a better description on the systems with obvious charge-transfer characteristics.

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Then, a dynamic analysis on frequency was performed to confirm the stability of the optimized

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structures. For large systems, the time-dependent density functional theory (TD-DFT)[22] is a feasible choice to calculate their excited properties in seeking a compromise between the computational cost and accuracy.[23] However, the TADF molecules mostly have a characteristic of large charge-transfer between D and A units. The traditional density functionals always underrate the excited energy when

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dealing with such large charge-transfer systems.[24] It has been demonstrated that the errors mainly derive from the introduction of inappropriate exchange-correlation (XC) approximations.[25-27] Therefore, the introduction of an appropriate and fixed amount of exact-exchange (eX) will improve

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the description of excited state properties.[28] Recently, the range-separated exchange (RS) density

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functional is used to settle this issue.[29-30] The RS functional can be expressed by the following formula (1).[31]

(1)

Where the r12 denotes the interelectronic distance, and α quantifies the fraction of eX in the short-range limit, whereas α+β shows the fraction of eX in the long-range limit. The ω represents the inverse of the distance at which the exchange changes from DFT to HF. The RS density functional corresponding to tune ω to satisfy a fundamental property that the exact functional must obey the exact 5

ACCEPTED MANUSCRIPT Kohn-Shan (KS) or generalized KS (GKS) theory.[32] In this work, the ω tuning can be done according to the equation (2).[33]

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(2) The lowest singlet and triplet excitation energies and corresponding ∆EST of compounds 1-5 were calculated using the tuned range-separated functional (LC-ωPBE)[34] in the Tamm-Dancoff

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approximation.[35-36] Meanwhile, the polarized continuum model (PCM)[37] with the toluene media is chosen to consider the influence of solvent. In addition, TD-DFT/B3LYP calculation was performed

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to predict the fluorescent radiative decay rate within the Amsterdam Density Functional program (ADF2013) code.[38] The spin-orbital coupling (SOC) matrix elements were calculated with zero-order regular approximation (ZORA)[39] Hamiltonian in scalar approximation in the ADF code. All electron TZP basis set and conductor like screening model (COSMO)[40-41] of solvation with toluene

3. Results and Discussion

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3.1 Optimized geometries

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parameters were applied for SOC calculation.

Based on compound 1, four compounds 2-5 have been designed by modifying the connected bridge

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between D and A units. Compounds 2 and 3 are constructed by the way that one phenyl ring is para and ortho connected to the phenyl ring of the electron-acceptor unit. Similarly, the compounds whose two phenyl rings are para and ortho connected to the phenyl ring of the electron-acceptor unit are named compounds 4 and 5. The molecular structures of compounds 1-5 are plotted in Figure 1.

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Figure 2 A comparison between the ground state (red) and lowest singlet excited state (white)

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geometries for compounds 1-5.

Generally, to enhance the luminescence efficiency of TADF material, it is important to suppress the non-radiative decay via restraining the geometric variation between their ground (S0) and lowest singlet (S1) excited states geometries.[1] Figure 2 displays a geometric comparison between S0 and S1 of

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compounds 1-5. For compounds 1-5, the deviations between S0 and S1 geometries are mainly attributed to the twist between carbazole unit and the middle electron-acceptor unit. Moreover, the deviations

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between S0 and S1 geometries for compounds 4 and 5 are more remarkable than other compounds. Especially, the electron-acceptor unit of compounds 4 and 5 in S1 geometry deviates from that in the S0

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geometry in a great degree. On the other hand, the non-radiative decay partly including the energy dissipation through vibronic coupling can be evaluated by the normal mode reorganization energy. The total reorganization energy (λ) at the ground and excited states can be obtained through summing the product of the Huang-Rhys factor and vibrational energy of each mode. The Huang-Rhys factor of every mode represents the vibrational quanta during emission or absorption process, which can be calculated by the DUSHIN program.[42] Figure 3 depicts the λ versus the normal-mode frequency. As displayed in Figure 3, the total λ values are in the order of 5 (2457) > 1 (2410) > 3 (2325) > 2 (2090) > 7

ACCEPTED MANUSCRIPT 4 (2061 cm-1). The total λ of compound 1 is 2410 cm-1, whereas it reduces to 2090 cm-1 and 2061 cm-1 for compounds 2 and 4, respectively. Correspondingly, the relaxation energies of compounds 2-4 are reduced after modifying the connected bridges between D and A units, indicating that properly

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changing the connected bridges between D and A units tends to reduce the non-radiative dissipation of the excited-state energy. In other word, for the compounds 2-4, modifying the connected bridge between D and A units is a valid method to prevent the non-radiative decay channels in the excited-sate

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relaxation process to some extent. It is also noticeable that the total λ values of compounds 2 and 4 are

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much smaller than these of compounds 3 and 5. Therefore, it can be concluded that the para connected phenyl will effectively suppress the non-radiative decay and is a better choice to improve the quantum efficiency of TADF material.

It is known that the vibrational normal modes with large λ are considered to be the significant

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channels to nonradiatively dissipate the excited state energy. For compounds 1-5, the vibration normal modes with large λ occur both in the low and high frequency regions. The corresponding vibrational modes are shown in Supporting Information Figure S1-5. We found that these modes with

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low-frequency vibrations are assigned as the twisting motions of the D and A units, and the

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high-frequency vibrations is attributed to the C=O stretching vibration in the electron-acceptor unit except for compound 4 as shown in Figure S1-5. Therefore, for compounds 1-3 and 5, the large contributions to the total λ stem from the twisting vibration of D and A units and C=O stretching vibration in the electron-acceptor unit. But for compound 4, the λ with large contribution mainly originates from the twisting vibration of the D and A units. For compound 1, the mode with largest λ which reaches 517 cm-1 and appears in the high-frequency region. For compounds 2-5 with modified connected bridge between D and A units, the λ values in the high-frequency region are noticeably 8

ACCEPTED MANUSCRIPT reduced by twice or more compared with compound 1, suggesting that the high-frequency vibrations are hindered for compounds 2-5. Especially for compound 4, the λ corresponding to the C=O stretching vibration is quite small. Therefore, modifying the connected bridge between D and A units is efficient

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for suppressing the C=O stretching vibration during the relaxation process from the excited state to the ground state. Meanwhile, the decrease in the total reorganization energy stems from the high-frequency vibrations for compounds 2-4. However, the λ values of compounds 2-5 in low-frequency regions are

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slightly increased in comparison with compound 1. It attributes to the enhanced twist between the D

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and A units.

Figure 3. Reorganization energy as a functional of normal-mode frequency.

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3.2 Frontier molecular orbitals Table 1 Calculated HOMO and LUMO energy levels, H-L gapa, ω value and vertical singlet-triplet

1

2

3

4

5

HOMO (eV)

-5.505

-5.535

-5.421

-5.492

-5.328

LUMO (eV)

-2.246

-2.284

-2.355

H-L gapa (eV)

3.259

3.251

3.066

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ω

0.1853

0.1702

∆EST (eV)

0.181

0.246

H-L gap = LUMO - HOMO

-2.474

-2.413

3.018

2.915

0.1761

0.1561

0.1610

0.519

0.676

0.774

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energy gap (∆EST).

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To investigate the effect of modifying the connected bridges between D and A units on their electronic properties, the calculated HOMO and LUMO energy levels of compounds 1-5 are listed in Table 1 and

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depicted in Figure 4. As shown in Figure 4 depicting the HOMO and LUMO distributions, it can be seen that the compounds 1 and 2 have similar HOMO distributions which are mainly localized on two

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electron-donor units and connected phenyl ring. Consequently, the HOMO-LUMO (H-L) gap of compound 1 (3.259 eV) is comparable with that (3.251 eV) of compound 2, indicating that the para connected phenyl ring has slight influence on the HOMO and LUMO energy levels. However, the H-L gap of compound 1 is larger than that of compound 3 (3.066 eV). From Figure 4, it is clear that the decrease originates from the reason that the ortho connected phenyl ring raises the HOMO energy level and lowers the LUMO energy level. In addition, in comparison with compound 1, the compound 4 has a great decrease in H-L gap. The analysis indicates that the decrease of H-L gap mainly arises from the 10

ACCEPTED MANUSCRIPT declined LUMO energy level in a great degree. The H-L gap of compound 5 is much lower than that of compound 1, which is the lowest one among compounds 1-5. On the whole, the H-L gap is decreased with increasing the number of phenyl ring. In term of their HOMO distributions, such result attributes

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to the enlarged π conjugation by introducing more number of phenyl rings. Moreover, the H-L gap with ortho linkage is reduced greater than that with para linkage irrespective of the number of phenyl ring. As shown in Figure 4, the ortho linkage of phenyl ring has a HOMO distribution over the whole

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electron-donor units and a greater π conjugation than para linkage. For example, the HOMOs of

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compounds 2 and 4 slightly distribute on the para phenyl ring, while, the HOMOs of compounds 3 and

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5 spread out the ortho connected phenyl ring.

Figure 4. Frontier molecular orbital energy level and electron density contours of HOMO and LUMO. It is well-known that the HOMO and LUMO distributions are important for designing efficient TADF material, which dominates the ∆EST and subsequently the RISC. As depicted in Figure 4, the main contribution of HOMO originates from the two carbazolyl groups and slight contribution comes 11

ACCEPTED MANUSCRIPT from the phenyl ring for compounds 1-5. The LUMO distributions for compounds 1-5 are almost the same, and they mainly localize on the BPys core and partly extend to the connected phenyl ring. It is noteworthy that a small overlap between HOMO and LUMO is existent for compounds 1-5. To

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quantify the HOMO and LUMO distributions, the fragment contribution including D, A units and bridge for compounds 1-5 is shown in Figure 5, which gives the same conclusion as HOMO and LUMO distributions. It is descripted in Figure 5 that for all compounds, the HOMO mainly distributes

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on two electron-donor units and extends partly to the bridge. The LUMO is localized on the

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electron-acceptor unit and bridge, and a quite small proportion resides on the electron-donor unit. The overlap between HOMO and LUMO is mainly centered on the connected bridge. Such small overlap between HOMO and LUMO indicates that compounds 1-5 have small ∆EST and are potential TADF

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materials. In the following section, the ∆EST of compounds 1- 5 will be discussed in detail.

Figure 5. Fragment contribution of bridge, donor and acceptor units in the HOMO and LUMO.

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ACCEPTED MANUSCRIPT 3.3 Singlet-triplet energy gap (∆EST) For efficient TADF material, a small ∆EST is essential for the RISC from T1 to S1 and the enhancement of luminescence efficiency. The calculated ∆EST values of compounds 1-5 are summarized in Table 1.

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Sun Hai-Tao et al. have proposed that the tuned range-separated functional is a better choice to predict the singlet-triplet energy gap in organic emitter for TADF emitting.[32] As a consequence, we have calculated the ∆EST values of compounds 1-5 with tuned LC-ωPBE* functional. It is well-known that

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the ω values are strongly dependent on systems, and the corresponding ω value for each compound is

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listed in Table 1. In this manuscript, the ‘Golden proportion’ method is utilized to calculate the final ω value for compounds 1-5. Because the ‘Golden proportion’ method can obtain a more accurate ω and need a less number of single point calculation. Taking compound 1 as example, the relationship between ω and J2 is depicted in Figure 6. Other ω functionals for compounds 2-5 are plotted in Figure

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S6 of Supporting Information. As seen in Table 1, the ∆EST of compound 1 is 0.181 eV, which is smaller than these of compounds 2-5. Therefore, the way of modifying the connected bridge between D and A units enhances their ∆EST values. Next, the effect of changing the number and different linkage

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of phenyl ring on their ∆EST is discussed in detail. It can be seen that the ∆EST values of compounds 4-5

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are greater than these of compounds 2-3, indicating that increasing the number of phenyl ring causes to the increase in ∆EST. A comparison between compounds 2 and 3, it can be found that the para linkage obtains a smaller ∆EST than ortho linkage. Meanwhile, the result from the ∆EST of compounds 4-5 is consistent with the above conclusion. As stated above, the ∆EST is related with the HOMO and LUMO distributions. Broadly speaking, a small ∆EST is obtained if the HOMO and LUMO distributions are in a good separation. To explain the increase in ∆EST for compounds 2-5, we connect it with the HOMO and LUMO distributions shown in Figure 4. As stated above, the overlap between HOMO and LUMO 13

ACCEPTED MANUSCRIPT is mainly centered on the bridge for compounds 1-5. For compounds 1 and 2, the overlap between HOMO and LUMO is nearly equal, and subsequently they have comparable ∆EST values. For compounds 3-5, the overlap between HOMO and LUMO is larger than one of compounds 1-2 due to

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the increased number of phenyl ring. As a consequence, the ∆EST values of compounds 3-5 are greater than those of compounds 1-2. Besides, excellent TADF requires not only a small ∆EST, but also a large radiative decay rate. In the following part, we continue to investigate the influence of modifying the

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connected bridge between D and A units on their radiative decay rate.

Figure 6. J2 value as a functional of ω value.

3.4 Radiative decay rate (kr) It has been demonstrated that the method to design a high-efficiency TADF molecules generally combines a small ∆EST and a reasonable radiative decay rate. The singlet excited state radiative decay can be simply estimated by Einstein spontaneous emission.[43] Here, the radiative decay rate constant (kr) was calculated and listed in Table 2. It can be seen that the kr of compound 1 is 3.25×106 s-1, whereas the kr of compound 2-5 is nearly one order of magnitude higher than that of pristine compound 14

ACCEPTED MANUSCRIPT 1. Therefore, increasing the number of phenyl ring and modifying different linkages are valid methods to enhance the kr. For investigating the effect of different linkages on their kr, we have compared the kr of compounds 2 and 3 and compounds 4 and 5, respectively. As shown in Table 2, it can be found that

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the kr of compound 3 is almost twice than that of compound 2. Compared with compound 5, the kr of compound 4 is increased by twice. Thus, the para linkage enhances the kr much greater than the ortho linkage. Further, we investigate the effect of the number of phenyl ring on the kr by comparing

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compounds 2 and 4, and compounds 3 and 5. In comparison with compound 2, the kr of compound 4 is

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increased to twice. Given in Table 2, the kr of compounds 3 (2.96×107 s-1) is twice as much as that of compound 5 (1.16×107). For ortho and para linkages, increasing the number of phenyl ring produces inverse influences on their kr. For ortho linkage, increasing the number of phenyl ring from one to two leads to the decrease in kr. However, for para linkage, the kr is enhanced nearly one time with

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increasing the number of phenyl ring. Although compounds 2-5 have larger ∆EST values than that of compound 1, the kr of compounds 2-5 enhances much greater than that of compound 1. On the other hand, if the RISC rate constant is larger enough, more singlet excitons will be produced and thus the

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quantum efficiency enhance will be enhanced.

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Table 2. Calculated spin-orbital coupling matrix elements (n = 1-3) and radiative decay rate (kr).

2

3

4



0.1920

0.1819

0.3496

0.3528

0.5348



0.3978

0.5455

0.6880

0.3043

0.5823



0.1126

0.3678

0.9721

0.7371

0.2517



0.7794

1.1697

0.9259

1.3106

1.2689



0.1601

0.3235

0.1345

0.3385

0.3196



0.2437

0.9526

0.2483

0.1221

0.2705



0.1284

0.2098

0.2413

0.5449

0.3429



0.1117

0.1450

0.0241

0.1584

0.4965



0.0107

0.0982

0.1081

0.6607

0.0732

3.25×106

9.52×106

2.96×107

2.49×107

1.16×107

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kr (s-1)

5

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1

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The kRISC is inversely proportional to the ∆EST, and proportion to the spin-orbital coupling (SOC) matrix elements. Here, the spin-orbital coupling matrix elements () (n = 1-3) obtained in ADF software are listed in Table 2. The singlet (Sn) and triplet (Tn) excitation energy levels are depicted in Figure 7. From Figure 7, it can be seen that the T2 energy levels are very close to the S1 energy levels for compounds 1-2 and 4-5. For compound 3, its T3 energy level is approaching the S1 energy level. Traditionally, for TADF material, the RISC is deemed to be a process from the T1 to S1. Given in Table 2, the values of compounds 3-5 are greater than that of compounds 1 and 16

ACCEPTED MANUSCRIPT 2. As states above, although the ∆EST values of compounds 3-5 are large, the enhanced values suggest that the RISC from the T1 to S1 is probably realized for compounds 3-5. The intersystem crossing takes place simultaneously with internal conversion and the coupling between higher singlet

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states and triplet states is also possible. Thus, we calculated other spin-orbital couplings, too. It can be found that the values are noticeably increased after modifying the connected bridge. Therefore, taking both the ∆EST and values into account simultaneously, the designed

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compounds probably realize the RISC process from the triplet to singlet excited states. Especially, as

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shown in Table 2, the values of compounds 1-5 are quite large, which are nearly more than 1 cm-1. In comparison with compound 1, changing the connected bridges noticeably increase their values. For example, the value of compound 1 is 0.7794 and the compounds 2-5 have values which are in a range from 0.9259 to 1.3106 cm-1. In addition,

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it is found that the value of compound 2 is greater than that of compound 3, at the same time, the value of compound 4 is higher than that of compound 5. One consistent conclusion is obtained that the para linkage is a better method to enhance the parameter

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for this kind of TADF material.

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4.

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Figure 7. Singlet (Sn) and triplet (Tn) excitation energy level of compounds 1-5. Conclusions

In summary, based on the synthesized thermally activated delayed fluorescence (TADF) DCBPy molecule (compound 1) bearing a new benzoylpyridine core as an electron-acceptor (A) unit and two

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carbazolyl groups as electron-donator (D) units, compounds 2-5 have been designed to investigate the influence of modifying the connected bridge between D and A units on their properties. The results indicate that for compounds 1-5, the vibration normal modes with large reorganization energy (λ) occur

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in the low and high frequency regions, which are assigned as the twisting motions of the D and A units

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and the C=O stretching vibration in the electron-acceptor unit, respectively. Moreover, the λ of compounds 2-5 in the high-frequency region are noticeably reduced compared with compound 1, suggesting that the high-frequency corresponding to C=O stretching vibration is hindered after modifying the connected bridge between D and A units. Further, from the HOMO-LUMO (H-L) gap values of compounds 1-5, we found that the ortho linkage has greater influence than para linkage on the HOMO and LUMO energy levels. The same conclusion is obtained, for compounds 1-5, the main contribution of HOMO originates from the two carbazolyl groups and slight contribution comes from 18

ACCEPTED MANUSCRIPT the phenyl ring. The LUMO distributions are almost the same, and mainly localized on the BPys core and partly extended to the connected bridge. The calculated ∆EST values with tuned range-separated exchange density functional indicate that the compound 2 is comparable with compound 1, whereas

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these of compounds 3-5 are a little greater than that of compound 1. The calculated kr shows that the kr of compound 2-5 is nearly one order of magnitude higher than that of pristine compound 1. The para linkage enhances the kr much greater than the ortho linkage. But the increase is related with the number

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of phenyl ting. For ortho linkage, increasing the number of phenyl ring from one to two leads to the

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decrease in kr. However, for para linkage, the kr is enhanced nearly one time with increasing number of phenyl ring. In addition, in comparison with compound 1, modifying the connected bridges noticeably increase their values, which is advantageous for reverse intersystem crossing from triplet to singlet excited states. To conclude, for DCBPy, modifying the connected bridge between D and A

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units is a valid approach to improve their TADF performances. Acknowledge

The authors gratefully acknowledge financial support from National Natural Science Foundation of

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China (21131001, 21363025, 21203019, 21273030 and 21603018), National Basic Research Program

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of China (973 Program—2013CB834801), Specialized Research Fund for the Doctoral Program of Higher Education and Research Grants Council Earmarked Research Grants Joint Research Program (20120043140001), National Nature Science Foundation of Jilin Prov. (No.20150101006JC) and Thirteen Five-Year Sci-tech Research Guideline of the Education Department of Jilin Prov. China. Institute of Functional Material Chemistry and National & Local United Engineering Laboratory for Power Batteries.PR China References 19

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ACCEPTED MANUSCRIPT The effected of modifying connected bridge on TADF performances was investigated. The kr values of compound 2-5 are higher than that of compound 1. The values of compound 2-5 are noticeably increased compared with compound 1.

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The compound 2 has a comparable ∆EST with compound 1 and larger kr and .