Influence of Heterocyclic Spacer and End Substitution on Hole

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Influence of Heterocyclic Spacer and End Substitution on Hole Transporting Properties Based on Triphenylamine Derivatives: A Theoretical Investigation Yang Ding, Yue Jiang, Wenhui Zhang, Linghai Zhang, Xubing Lu, Qianming Wang, Guofu Zhou, Jun-Ming Liu, Krzysztof Kempa, and Jinwei Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06335 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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The Journal of Physical Chemistry

Influence of Heterocyclic Spacer and End Substitution on Hole Transporting Properties Based on Triphenylamine Derivatives: A Theoretical Investigation Yang Ding1, Yue Jiang1∗, Wenhui Zhang1, Linghai Zhang2, Xubing Lu1, Qianming Wang3, Guofu Zhou4, Jun-ming Liu5, Krzysztof Kemp1,6 and Jinwei Gao1∗ 1

Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum

Engineering and Quantum Materials, Academy of Advanced Optoelectronics, South China Normal University, Guangzhou510006, P. R. China 2

School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong, P. R. China

3

School of Chemistry & Environment, South China Normal University, Guangzhou 510006, P. R. China

4Electronic Paper Displays Institute, South China Normal University, Guangzhou 510006, P. R. China

ACS Paragon Plus Environment

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Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China 6

Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA

ABSTRACT: Hybrid organic-inorganic halide-perovskite based solar cells have achieved outstanding progress, approaching one of the most competitive photovoltaic technologies. One of the hot topics is to develop inexpensive and efficient hole-transporting materials to improve the performance of devices for practical applications. In this paper, we theoretically design a series of hole transporting materials based on triphenylamine backbone through varying the spacer and the end substitution. The properties of frontier molecular orbital, ionization potential, reorganization energy, and charge mobility have been calculated and analyzed. The results show that the spacer and the end functional groups strongly influence the molecular geometry, stacking, electron density distribution, especially hole mobility. The best hole-transporting material with furan as spacer and hydroxyl or methoxyl as substitution is proposed due to its highest hole transporting mobility induced by the planar conformation and tight π-π stacking, which potentially could enable the highly efficient perovskite solar cells.


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1. Introduction Perovskite solar cells (PSCs) have attracted lots of attention due to their high power conversion efficiency (PCE) (more than 22%)

1,2

and low-cost processes. Perovskite, as a light

absorber was firstly used in DSSCs in 2009 with an efficiency of ~ 4%,3 which was limited by the instability caused by the liquid electrolyte. This issue was solved by the introduction of the solid-state hole-transporting materials (HTMs), which brings the efficiency to as high as 10%.4–6 Currently except for the chasing of high quality perovskite films,7 the investigation of simple and cheap HTMs is also considered as an important solution to push PSCs to commercial application.8 Spiro-OMeTAD is one of the widely used HTMs for PSCs with the efficiency around ~20%.9,10 However, it presents a potential hurdle to the further commercialization due to its expensive price induced by the complicated synthesis and purification process. Meanwhile, its intrinsic low electrical conductivity requires additive or salt for doping, which reduces the stability of PSC devices.11 The general strategies to design a dopant-free HTM are as follows, (1) proper HOMO and LUMO energy levels for efficient hole transportation and electron blocking, (2) high hole transporting properties to reduce the charge recombination, (3) the stability against moisture and oxygen. Lots of new HTMs with the typical hole transporting blocks including triphenylamine (TPA),12 naphthalene,13 carbazole,14 quinozino,15,16 triptycene,17 etc.18 have been studied. Among them, TPA blocks, with a good hole transporting property, proper ionized potential, as well as good solubility and stability, were grafted to various frameworks or as the central core which has been used as HTMs for solar cells.19 Among them, a new simple and cost-effective material named F101 (shown as A3 in Figure 1), is reported as the HTM in PSCs with a PCE of 13%.20 Same doped molecular system with fluorene–dithiophene (FDT) spacer is conducted and the efficiency of 20.2% has been achieved,21 suggesting the significant improvement of this system


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on the photovoltaic properties. Therefore, theoretically understanding the effect of the central spacers and the end functional groups on the performance of TPA backbone based HTMs is of great importance, which provides a clue for the rational design of HTMs for highly efficient solar cells.22,23 Herein, 18 new HTMs, based on triphenylamine backbone have been properly designed (Figure 1) by two strategies: 1) varying the central spacer “Ar” with furan (A), thiophene (B), selenophene

(C)24,

4H-cyclopenta[2,1-b:3,4-b']difuran

(D),

4H-cyclopenta[2,1-b:3,4-

b']dithiophene (E), and dithieno[3,2-b:2',3'-d]furan (F); 2) substituting the end functional group “R” with methoxyl groups (OMe, 3), hydroxyl (OH, 2) and cyano (CN, 1). The properties of frontier molecular orbital (FMO), ionization potential (IP), and reorganization energies (RE) of newly designed molecules were calculated by DFT methods. The charge mobility is derived from the calculations. In the end, we build a clear structure-properties relationship for the rational design of an efficient organic HTM.

Figure 1. Chemical structures of target compounds. “Ar” represents spacer (A, B, C, D, E, and F), and the “R” is the end functional group (1, 2, and 3). Combined with “Ar” and “R” in above


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target molecular structure, there are 18 new molecules, namely A1-A3, B1-B3, C1-C3, D1-D3, E1-E3, and F1-F3, in which F101 is A3. 2. Theoretical Basis The chemical structures are shown in Figure 1. “Ar” represents spacer (A, B, C, D, E, and F), and the “R” is the end functional group (1, 2, and 3). Combined with “Ar” and “R” in above target molecular structures, there are 18 new molecules, namely A1-A3, B1-B3, C1-C3, D1-D3, E1-E3, and F1-F3. The geometry and FMO of those monomers were optimized by B3LYP functional with 6-31g(d,p) basis set. Considering that B3LYP functional cannot describe the broken-symmetry effect of the bis-triarylamines in radical-cation state,25,26 the calculations for IP and RE is under ωB97xD/6-31g(d,p) method.27 All optimizing results show no imagery frequency, which means all optimized structures are in energy minima. The above calculations are based on Gaussian 09W software package.28 Then, the reorganization energy λ can be estimated by the adiabatic potential energy surface approach (Equation (1)).

λ = λ0 + λ± = ( E0* − E0 ) + ( E±* − E± )

(1)

where E0 and E± respectively represent the energies of neutral and charged species in their lowest energy geometries, while E0* and E±* are respectively the energies of the neutral molecule with a charged geometry and a charged molecule with the ground state geometry. The crystal structure prediction was performed based on Material Studio platform.29 All molecules were optimized by DMol3 module to obtain the electrostatic potential charges (ESP).30 The PBE functional and the Dreiding force field were employed to calculate the crystal structure of all molecules by performing the polymorph predictor module.31,32 Besides, in order to achieve a good efficiency and accuracy, space groups were restricted to five common space


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groups of P1, P-1, P21, P21/C and P212121.33 The M06-2X/6-31g(d,p) method was employed to calculate the energy and electronic structures of the dimers.34 Then, the Koopmans’s theorem (KT) approximation was further used to calculate the carrier transfer integral t.32–38 The transfer integral th of hole are given by the Equation (2),

th =

E HOMO − E HOMO −1 2

(2)

where th is the transfer integral of hole, EHOMO and EHOMO-1 are the energies of the HOMO and HOMO-1 in the closed-shell configuration of the neutral state, respectively. The charge transfer rate (k) was estimated by Marcus theory,39 and the charge hopping rate (k) is calculated by Equation (3).

k=

t2 h2

1 4πλk BT

exp(−

λ 4 k BT

)

(3)

Where th is the transfer integral, λ is the reorganization energy, h is the Dirac constant, T is the temperature in Kelvin, and kB is the Boltzmann constant. In the end, considering of a Brownian motion of charge carrier without electric field, the carrier mobility was described from the diffusion coefficient D with Einstein equation (Equation (4)).40,41

µ=

eD k BT

(4)

where e is the electron charge, and D is the diffusion coefficient which can be calculated by Equation (5).


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D=

(5)

r 2k 2d

where r represents the charge hopping centroid to the centroid distance, d is taken as 3 for the reason that the diffusion is regarded in three dimensions for compounds investigated.

3. Results and discussion 3.1 Geometrical optimization and the frontier molecular orbitals calculation Two types of chemical structures are shown in Figure 2a (a general molecular structure for A, B, and C) and Figure 2d (a general molecular structure for D, E, and F). Electron density distribution of HOMO and LUMO for two typical molecules A3 and D3 are shown in Figure 2b, c and Figure 2e, f, respectively. Here, “X”, “X1”, and “X2” represent the atoms of O, S, or Se in substitutions. Table 1 shows the optimized parameters of inter-ring distance: C1-C2 and dihedral angle: X-C1-C2-C3 for all molecules. Inter-ring distance has no change with variation of different end functional groups from cyano to methoxyl on the outer phenyl ring (for example, the distance for A1, A2 and A3 are all 1.453 Å), suggesting that the electron donating or withdrawing effect of end substitution has no influence on the C1-C2 distance. The dihedral angles also show trivial change with different end functional groups. With the same substitution, however, changing the heteroatom of O to S and Se, the C1-C2 bond length changes as the sequence of B≈C>A in the range of 1.45 to 1.47 Å. Similarly, dihedral angle showed the order of B (25°) > C (23°) >> A (2°). In all, the molecules with central furan spacer show the best planarity compared with those with thiophene and selenophene.


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Figure 2. General molecular structure (a) for A, B, and C, and (d) for D, E, and F. Electron density distribution of HOMO (b) and LUMO (c) of A3, and (e) HOMO and (f) LUMO for D3. “X, X2, and X3” donate heteroatom. “R” represents end substitution.

Table 1. Optimized inter-ring distance and dihedral angle (X-C1-C2-C3) at the B3LYP/631g(d,p) level. C1—C2/Å Dihedral angle/°

A3 1.453 2.1

A2 1.453 2.3

A1 1.453 2.1

B3 1.463 25

B2 1.463 25.1

B1 1.464 25.4

C3 1.461 20.3

C2 1.461 21.8

C1 1.463 23.0

C1—C2/Å Dihedral angle/°

D3 1.449 2.5

D2 1.45 2.1

D1 1.449 2.3

E3 1.461 24.4

E2 1.461 24.6

E1 1.462 23.1

F3 1.461 25.3

F2 1.461 25.4

F1 1.462 25.9

To investigate the influence of spacer size on the molecular conformation, geometrical optimization of D, E and F series are conducted and compared to its mono-ring counterpart A


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and B. Firstly, A and D, B and E, F series molecules show the almost same C1-C2 bond distance separately. Thus, it is clear that the C1-C2 bond distance is highly related to the character of nearby rings, not the spacer size. In addition, dihedral angles of D and A series are in the same region (~ 2°), while E series are slightly smaller than B series (23°~24° vs 25°) and F series are slightly larger than B series (25.5° vs 25.1°). Hence the DFT calculations show that the conformation of molecules is mainly controlled by the heteroatom in the X and X2 positions, while the size of the spacer has negligible influence.

0

-0.93 -0.97

-1

-2

Energy(eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3

-2.11

-2.28-2.31 MAPbBr3 MAPbI -3.36

A3 A2 -2.81 3.33 3.32

-4

-4.20

2.02 -5

-6

1.47

1.53

pp pm po 2.94 3.00 3.04

-4.24 -4.29

-5.22-5.31 -5.22 -5.43-5.38 -5.67 Spiro-OMeTAD

-1.17-1.21

-2.17 B3 B2 3.29 3.31

A1 3.40

3

FAPbI3 -3.90

-1.06-1.10

B1 3.45

-4.35-4.41

-1.11-1.15

-2.24 C3 C2 3.18 3.21 C1 3.39

-4.35 -4.43

-1.32-1.36

-2.11 D3 D2 2.97 2.98

-4.08 -4.12

-5.63

-5.63

-2.24

-2.26

F3 F2 E3 E2 3.04 3.05 D1 2.93 2.95 E1 3.00 F1 3.05 3.17

-4.26 -4.30

-5.12 -5.51

-1.32-1.35

-4.36 -4.41

-5.29

-5.43

Figure 3. Calculated FMO alignment of 18 molecules, as well as the experimental values of energy bands of FAPbI3, MAPbI3 and MAPbBr3 and spiro-OMeTAD (with three types of pp-, pm- and po-). Figure 3 shows the calculated LUMO and HOMO energy levels and Eg (LUMO-HOMO) for 18 molecules. The energy bands of spiro-OMeTAD, and FAPbI3, MAPbI3 and MAPbBr3 are also shown in Figure 3 42,43. Basically, for molecules with mono-ring spacers, HOMO is localized over the whole molecule, whereas LUMO is mainly localized on the central five-membered ring and the adjacent benzenes (Table S1 in Supporting Information). The electron distribution and HOMO of control molecule A3 are consistent with the previous result (-4.24 eV vs -4.23 eV).18


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For same spacer series containing CN, OH and OMe, the order of HOMO is OMe > OH > CN due to the decrease of the electron cloud density by increasing electron withdrawing effect. Carrying the same substitution, HOMO with furan spacer is the highest, followed by the thiophene and selenophene. This result is consistent with the above-mentioned conclusion that A series molecules have the best planarity thus the longest conjugation system, which contributes to the higher HOMO and the stacking properties as well. By sharp contrast, in these molecules (D, E, and F) with fused cycles, the electron density distribution of HOMO and LUMO are both mainly localized on the central conjugating rings while the four outer phenyl rings contribute less as compared to their counterparts A and B, shown in Table S1. Also, the HOMOs of molecules with fused cycles are relatively higher than those of molecules with mono cycle, for example, HOMO of D3 (-4.08 eV) is higher than that of -4.24 eV for A3 and E1 (-5.29 eV) is higher than B1 (-5.63 eV). Besides, HOMOs of E series are higher than those of F series due to its better planarity between the linker and adjacent phenyl rings (with dihedral angles of 23.1° vs 25.9°). Overall, except the CN substituted molecules, the energy difference between the HOMOs of the other molecules and the valence bands of perovskite materials is larger than the difference when compared to spiro-OMeTAD, which contributes to efficient hole extraction. Simultaneously, the calculated LUMOs are all higher than the conduction bands of perovskite, indicating the capability of blocking electron.

3.2 Structures, Ionization Potential and Reorganization Energy


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Figure 4. The general structure of (a) for A, B and C series and (b) for D, E and F series for calculations of IP and RE. The Bα, Bβ and Bγ mean the benzene rings at the different positions.

Figure 4 shows the general structure of (a) A, B and C series and (b) D, E and F series for calculations of IP and RE. The Bα, Bβ and Bγ indicate the benzene rings at the different position. The dihedral angles (θ) of neutral molecules, radical-cations at the ωB97xD/6-31g(d,p) level, are shown in Table 2, the detailed parameters are summarized in Table S2 in Supporting Information. Considering neutral molecules as references, θAr-Bα of B1, B2 and B3 decreases from 31.77°, 19.85° and 19.30° to 4.45°, 4.79° and 5.06° in the radical-cation state. θBα-Bβ and θBα-Bγ of B series show decrease by half. Other series of molecules present the same trend in dihedral angles between neutral and radical-cation states. Thus, as expected, the planarity of the molecules is much better in the radical-cation state than that in neutral state. Whereas with different substitutions, θBα-Bβ and θBα-Bγ vary with the order of 1>2≈3 in both neutral and radical-cation states, for instance θBα-Bβ of radical-cation A1(20.49°)>A2(16.76°) ≈A3(16.78°), suggesting that the electron donating effect of OH and OMe groups is beneficial for the molecular planarity improvement, especially in the radical-cation state. Regarding the variation of Ar core, the TPA


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block presents a flatter conformation with θBα-Bβ and θBα-Bγ in the sequence of C>B>A, while less difference was found in the D, E, F series. In all, the molecules combined with the electron donating groups and the furan spacer exhibit a good planarity in radical-cation state, potentially contributing to a good hole mobility.

Table 2. Dihedral angles (θ) of neutral molecules, radical-cations at the ωB97xD /6-31g(d,p) level.

Comp.

θAr-Bαα (°)

θBαα-Bββ (°)

θBαα-Bγγ (°)

Neutral

Radical-Cation

Neutral

Radical-Cation

Neutral

Radical-Cation

A1

6.36

1.17

48.47

20.49

48.75

20.50

A2

1.61

1.39

33.24

16.76

35.13

14.86

A3

1.63

1.38

28.32

16.78

34.95

14.97

B1

31.77

4.45

47.55

21.75

49.66

22.56

B2

19.85

4.79

38.76

18.07

42.05

18.79

B3

19.30

5.06

38.61

17.70

42.04

18.79

C1

36.86

2.01

51.03

22.08

51.08

21.91

C2

30.87

3.95

32.71

19.49

31.04

18.67

C3

31.37

4.18

33.34

19.58

32.41

18.54

D1

2.14

0.51

53.51

25.58

51.90

26.19

D2

0.28

1.87

35.49

15.99

33.42

14.20

D3

5.29

2.05

34.57

16.11

35.24

14.54

E1

30.82

12.56

48.59

24.63

48.79

24.97

E2

31.47

8.35

29.54

18.01

33.38

18.50

E3

32.18

8.19

28.67

17.43

33.50

18.52

F1

33.36

11.74

47.99

22.40

45.67

22.70

F2

31.60

8.94

33.21

16.58

29.87

17.35

F3

34.77

8.56

25.28

16.34

32.13

17.46


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The calculated adiabatic IP and the hole RE (λh) are listed in Table 3. The IP, the energy required to remove the loosely bound electron is an important criterion for the estimation of the hole mobility. Similar results are observed in each series of molecules. The substitution of cyano clearly increases the IP than hydroxyl. For example, order of IPs of B series is B3 (5.66 eV) F>A>B>C, which will be discussed in section 3.3. In this study, we mainly focus on the hole transportation property of TPA based materials, so electron transportation has not been discussed.

Table 3. Adiabatic IP and hole RE of series A, B, C, D, E and F calculated at the ωB97xD/631g(d,p) level.

Comp.

IP (eV)

λh (eV)

Comp.

IP (eV)

λh (eV)

A1

6.64

0.88

D1

6.23

0.77

A2

5.61

0.63

D2

5.36

0.67

A3

5.55

0.60

D3

5.31

0.70

B1

6.78

0.96

E1

6.42

0.80

B2

5.73

0.70

E2

5.58

0.72

B3

5.66

0.69

E3

5.52

0.73

C1

6.74

1.07

F1

6.59

0.84

C2

5.74

0.71

F2

5.73

0.70


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C3

5.67

0.72

F3

5.67

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0.70

Another important factor to assess the charge transportation is the RE (including external RE and internal RE). It is known that the external RE is much lower than the internal RE based on a polarized force field calculation.44 Therefore, the external part is neglected in this study. The calculated hole REs (λh) are listed in Table 3. From Marcus theory,39 it is rational to conclude that the lower RE favors the higher charge transporting rate. From Table 3, the order of λh is ArCN >Ar-OH ≈ Ar-OMe. All of cyano substitutions show larger λh than hydroxyl and methoxyl substitutions, which means the electron-withdrawing group restricts the hole transporting properties, while electron donating groups, e.g. OH and OMe favor hole transportation. Meanwhile, λh of molecules with various Ar behaves in the same trend due to the similar planarity structures, which is consistent with their geometrical variation in the radical-cation state that the molecules with OH and OMe substituting groups improve molecular planarity, favoring efficient hole hopping process.

3.3 Charge Mobility According to the Marcus theory, charge mobility is affected by transfer integral (t),39 which is the electron coupling and highly influenced by the dimer structure. The initial crystal structures of dimers were predicted in Materials Studio platform. In the calculations, the space groups were restricted to five common space groups of P1, P-1, P21, P21/C and P212121,33 in which structures with P-1 space group are the most stable among the predicted crystals (ΔE is the difference between ESG (energy of different space groups) and EP-1(energy of space group P-1), see Figure S1 in Supporting Information). Thus all of the hole mobility calculations are based on


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P-1 space group (shown in Table 4). The detailed parameters for predicting crystal structures are listed in Table S3 in Supporting Information. The transfer integrals of all hopping pathways were calculated by employing the M06-2X functional based on the direct coupling approach.45 Finally, the Koopmans’s theorem (KT) approximation has been used to calculate the carrier transfer integral t based on FMOs of neutral dimers.38

Table 4. The centroid to centroid distances (r, Å), the hole transfer integrals (th, meV), and hole mobilities (µh, cm2 V-1 s-1) based on the predicted crystalline structures

Comp.

r

th

µh

Comp.

r

th

µh

A3

4.97

10.08

1.01E-02

D3

5.01

7.85

2.24E-03

A2

4.94

10.91

9.05E-03

D2

5.97

8.26

4.76E-03

A1

4.96

2.69

3.74E-05

D1

4.99

3.01

1.63E-04

B3

4.38

6.56

1.30E-03

E3

4.81

6.04

8.99E-04

B2

4.46

6.65

1.28E-03

E2

4.86

6.05

1.04E-03

B1

4.48

0.99

1.92E-06

E1

4.36

3.88

1.44E-04

C3

3.98

4.63

3.98E-04

F3

4.95

6.57

1.55E-03

C2

3.45

5.09

3.91E-04

F2

5.00

6.05

1.27E-03

C1

4.26

0.08

3.67E-09

F1

4.99

2.11

3.56E-05

Table 4 lists the centroid distance between dimers (r), hole transfer integrals (th), and hole mobilities (µ µh). The detailed lattice parameters of all crystals on P-1 space groups are shown in Table S3 in supporting information. The order of centroid distance with the same substituting group shows as r (A)> r (B) > r (C), and r (D)> r (E) < r (F) whereas th (A)>th (B) >th (C), suggesting that although the bigger heteroatom on the five-atom rings drags molecules closer, it simultaneously induces to a larger stagger distance and reducing π-π stacking. The same conclusion can be drawn for other molecules listed Table S4 in Supporting Information.


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Meanwhile, with the same linker OMe substitution drags molecules closer than CN (r (Ar-OMe) < r (Ar-CN)). The slightly higher t of OH substitution than OMe is probably due to the weaker interaction of O…H between adjacent molecules, shown in Figure 5. The measured O…H distances between two A2 molecules are 2.531 Å, 2.670 and 2.673 Å at three positions, smaller than their Van der Waals Radius 2.72 Å. Apparently, the molecules with fused rings substituted with OH or OMe groups show increased stacking distance and decreased transfer integral.

Figure 5. Dimer stacking structure of A2 with measured distance of O…H. Hence the hole mobility follows the trend of transfer integral and the RE, with the order of A>B>C, and Ar-OMe ≈ Ar-OH> Ar-CN in the case of A, B, C series. Specially, TPA molecules with furan spacer and OMe or OH substitution show larger hole mobility. When fused rings are used as spacers, the molecules present the similar hole transportation behavior for thiophene derivatives at the magnitude of -3 to -4. However, the selenophene linker gives the worst


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performance, compared with other linkers, which is highly related to its poor π-π stacking due to torsional conformation in the dimer structures. These trends are generally consistent with the conclusion obtained from the IP data.

4. Conclusion A series of molecules have been designed based on F101 with the variation of furan linker to thiophene, selenophene and fused rings, and substitution of methyloxyl to hydroxyl and cyano functional groups. The newly designed molecules were theoretically studied in electronic structure, molecular orbital, and hole transportation by using DFT B3LYP and ωB97xD methods. These results suggest that the molecule with a furan as a linker has better conformational planarity and π-π stacking than those with thiophene and selenophene linkers. Regarding the substitution groups, electron withdrawing groups are obviously not suited for the hole transporting materials in view of both energy level and charge transportation property, whereas OH and OMe, two electron donating groups enhance the hole mobility. Overall, this study builds a clear relationship between molecular structure and hole transporting property, potentially offers a fundamental clue to design efficient TPA based HTMs.

AUTHOR INFORMATION Corresponding Author *E-mail for J. Y.: [email protected] *E-mail for J. G.: [email protected]

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT We thank the financial support from National Natural Science Foundation of China (51571094 and 21272080), National Key Research Program of China (2016YFA0201002), Guangdong Province Foundation (2016KCXTD009, 2014B090915005, and 2016A010101023), China Postdoctoral Science Foundation (2016M590795), and Program for Chang Jiang Scholars and Innovative Research Teams in Universities Project (No. IRT13064).

ASSOCIATED CONTENT Supporting Information. Electron cloud distribution of all molecules, Geometric parameters of (a) neutral molecules, (b) radical-cations at the ωB97xD /6-31g(d,p) level, Lattice parameter of all crystal on P-1 space groups, 3D graph of all dimers and The ∆E of each space group to P-1 group of every crystal (ESG - EP-1). The Supporting Information is available free of charge on the ACS Publications website at DOI:

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Table of Contents Graphic


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Influence of Heterocyclic Spacer and End Substitution on Hole

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