Theoretical Study on Charge Transport Properties of Intra- and Extra

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Theoretical Study on Charge Transport Properties of Intra- and Extra-Ring Substituted Pentacene Derivatives Jian-Xun Fan, Xian-Kai Chen, Shou-Feng Zhang, and Ai-Min Ren J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b12641 • Publication Date (Web): 30 Mar 2016 Downloaded from http://pubs.acs.org on April 4, 2016

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The Journal of Physical Chemistry

Theoretical Study on Charge Transport Properties of Intraand Extra-Ring Substituted Pentacene Derivatives Jian-Xun Fan,†,‡ Xian-Kai Chen, † Shou-feng Zhang, † and Ai-Min Ren,* † †

‡

Institute of Theoretical Chemistry, Jilin University, Changchun, 130023,China.

College of Chemistry and Life Sciences, Weinan Normal University, Weinan, 714000, China.

ABSTRACT: A series of pentacene derivatives, halogen-substituted and thiopheneand pyridine-substituted, have been studied on the electronic properties and charge transport properties using the density functional theory and classical Marcus charge transfer theory. The transport properties of holes and electrons have been focused to get insight into the effect of halogenation and heteroatom substitution on transport and injection of charge carrier. The calculation results revealed that fluorination and chlorination can effectively lower LUMO level, modulate the hole/electron reorganization energy (λh/λe), improve the stacking mode of crystal structure and enhance the ambipolar characteristic. Chlorination gives a better ambipolar characteristic. On the basis of halogen-substitution, the substitution of terminal benzene ring of triisopropyl-silylethynyl-pentacene (TIPS-PEN) by a thiophene or pyridine will largely lower the LUMO level, improve stacking mode, and become more suitable ambipolar materials. Hence, both intra- and extra-ring substitution are favorable to enhance the ambipolar transport property of TIPS-PEN. 1.

INTRODUCTION 1

ACS Paragon Plus Environment


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Organic field-effect transistors (OFETs) have attracted great interest because of their potential applications in cost-effective flexible organic electronics.1-3 As a driving component in many device applications, the field-effect mobility that can be extracted from the transistor is one of the major concerns in its development. Important progress has been made in this field due to the enhancement of charge carrier mobility through the development of new organic semiconductor materials and new device structure designs involving new dielectric materials and electrode selection and/or its modification.4 For organic semiconductors, many new material systems emerged besides the most common oligoacenes in recent years. Pentacene, an oligoacene, which is regarded as a benchmark due to its high charge carrier mobility, has been widely investigated by various experimental and theoretical methods.5-7 But the main inherent drawbacks of pentacene are the low stability, unoptimized π-overlap and poor solubility. Efforts to improve the stability of pentacene lead to the development of a variety of heteroacene-aromatic systems where carbocyclic rings of the pentacene are replaced by heteroatom-containing rings. In general, those derivatives do show improved stability, but still suffer from poor solubility. A more versatile substitution of pentacene is trialkylsilylethynyl group, e.g. t r i i s o p r o p y l - s il yl e t h yn y l ( T I P S ) , t r i m e t h yl - s i l yl et h y n y l ( T M S ) a n d triethyl-silylethynyl (TES). In this approach, the solubilizing substituent is held away from the active electronic component by a small, flexible spacer (the C-C triple bond), 2


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allowing the aromatic faces of adjacent molecules to achieve very close contacts. Furthermore, adjustments to the alkyl groups on the silane allow subtle tuning of the solubility and crystal packing of the material, permitting precise tuning for optimum performance in a variety of applications. Anthony and coworkers’ study indicated that the TIPS group is prior than other trialkylsilylethynyl groups for its length ratio of the substituents and two-dimensional brick stacking at about one half of acene cores. It has been demonstrated that this mode of substitution can avoid interrupting π-π interaction while enhancing solubility and crystallinity, which are critical for high-performance devices. 8,

9

In particular, density functional theory (DFT)

calculation and experimental measurements have also shown that the bis-silylethynylation makes the disubstituted acenes much more stable, especially with regard to oxidative degradation.10, 11 It has previously been shown by Fudickar et al.12 that the triple bond in the alkyne side chains acenes acts to stabilize the molecule by preventing the molecule from decomposing as a result of oxidation. Ambipolar semiconductors are important for complementary-like inverters that enable robust, low-power circuits with wide noise margins without using advanced patterning techniques to selectively deposit n- and p-channel materials.13 It is recognized that the type of charge carrier for a given molecule mainly depends on its molecular frontier orbital energy levels. Therefore, strategies for designing and

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synthesizing air-stable ambipolar OFET materials with higher charge mobilities are still being sought.14 Bao et al.15 concluded a qualitative relationship between molecular orbital energy levels and the charge carrier type in OTFTs on the basis of a series of functionalized acene derivatives, that the highest occupied molecular orbital (HOMO) energy level of a semiconductor molecule should be -5.6eV or higher to allow p-channel field effect, and the lowest unoccupied molecular orbital (LUMO) energy level of a semiconductor molecule should be -3.15eV or lower to allow n-channel field effect. Charge can easily transfer and inject from the Au electrode, when the HOMO and LUMO energy levels of the molecules locate between -3.15eV and -5.6eV. To get n-type pentacene semiconductor, a common approach to this type of system is involved full halogenated pentacene backbone, especially with fluorine atoms. Bao group found that chlorine substitution could yield stable n-type semiconductors as well.16 Some of electronic devices based on partly halogenated pentacene materials show an ambipolar characteristic. But the effects of halogen substitutions on the transport properties of TIPS-PEN and its derivatives have not been thoroughly studied. In addition, substitution of heteroatom for carbon atom on the ring backbone of TIPS-PEN also significantly affects the transport and injection of charge carriers in organic semiconductor materials. Our research group specially investigated this issues.17, 18 In this study, we only concentrate on the tunable effect of 4


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heteroatom substitution intra- or extra-ring backbone based halogenated TIPS-PEN on the transport and injection of charge carrier. There have been some theoretical studies on the halogenated and/or heteroatom oligoacenes19-22 and TIPS-PEN derivatives.23 However, these studies focus on oligoacenes and the relative nitrogen-doped derivatives.21 To the best of our knowledge, the computational investigations on charge transport properties of halogenated TIPS-PEN derivatives and heteroatom have not been thoroughly done so far. Thus, we focus on the TIPS-PEN derivatives with different halogen substituents and with heteroatom nitrogen or sulfur in benzene ring, investigate their transport properties of holes and electrons to get insight into the effect of halogenation and heteroatom substitution on transport and injection of charge carrier in this contribution. All studied molecules are based on pentacene, tetraceno[2, 3-b]thiophene and anthra[2, 3-g]quinoline and thus form five linearly fused rings with the TIPS substitution at the central position. These molecules are in the same acene family, and allow for a useful comparison. As is well known, three factors determine carrier mobility, i.e. the electronic affinity/ionization potential (EA/IP), electron/hole reorganization energies (λe/λh), and electronic coupling originated from dense π-stacking. We calculated the transport properties of holes and electrons of all the molecules, thus assessed the impact of halogenation and heteroatom substitution on

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the three factors, finally concluded the rule of the effect of intra- and extra-ring substitution on charge transport properties of pentacene. 2.

COMPUTATIONAL DETAILS Our simulation model is based on first-principle calculations combined with

Marcus theory.24, 25 The electronic hopping rate, W, in standard Marcus equation, is closely related to reorganization energy λ, the transfer integral V, and the temperature T as: 0.5

 2π V 2  π  λ  W=   exp  −  h  λ k BT   4 k BT 

(1)

Here h and kB are the Plank constant and Boltzmann constants. Given a fixed temperature(here is 298K), the large transfer rate can be attributed to the larger transfer integral and the smaller reorganization energy. Assuming the hopping motion of electrons or holes is an isolated random walk and there is no correlation between charge hopping motions, their diffusion coefficient could be evaluated from the hopping rate as: D=

1 2 ∑ ri Wi P i 2n

(2)

Here, n is the spatial dimensionality, Wi denotes the hopping rate along the specific hopping path i , ri is the intermolecular center mass distance between two adjacent molecules, and Pi is the hopping probability for the specific path i which can be calculated as:

6


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Pi =

Wi ∑ Wi

(3)

i

Then, the drift mobility of hopping rate µ can be evaluated from Einstein relation:

µ =

eD k BT

(4)

Han et al.26 has used a mobility orientation function which could deduce the angular resolution anisotropic mobility:

µΦ =

e 2k BT

∑k r

2

i i

Pi cos 2 γ i cos 2 (θ i − Φ )

(5)

i

Here, there are two cosine expressions, γ i is the angle of hopping path relative to the plane of interest, θ i is the angle of the projected hopping path relative to the specific axis, and Φ is the orientation angle of the transistor channel relative to the specific crystallographic axis. Thus, θ i − Φ here denotes the angle between the hopping path and the conducting channel. The transfer integral V characterizes the strength of electronic coupling between the two adjacent molecules. It could be calculated from the site-energy correction method27 using the PW91/TZ2P of density functional theory (DFT) in the Amsterdam Density Functional (ADF2010) program,28 using the formula:

Veff =

J ij − 0.5(ei + e j ) Sij 1 − Sij2

(6)

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Here ei ( j ) = 〈 Φ i ( j ) | H | Φ i ( j ) 〉 , S ij = 〈 Φ i | Φ j 〉 J ij = 〈Φ i | H | Φ j 〉 where Φ i ( j ) is the frontier molecular orbital of isolated molecule; i( j ) in the dimer representation which means the monomer HOMO (for hole transport) or LUMO (for electron transport). H is the Kohn-Sham Hamiltonian of the dimer system; ε i ( j ) is the site energy corresponding to monomer i( j ) ; Sij is the spatial overlap integral between the HOMOs (LUMOs) of two monomers; and J ij is the transfer integral considering no electrostatic polarization effect. Finally, the effective transfer integral Veff

is

obtained by considering the electrostatic polarization effect. The reorganization energy λ which measures the strength of the so-called local electron-phonon coupling,29 consists of intra- and intermolecular contributions determined by fast changes in molecular geometry (the inner contribution, λi ) and by slow variations in polarization of surrounding medium (the outer contribution, λ0 ) respectively, λ = λ0 + λi . The intermolecular contributions of reorganization energy are often small and can be neglected.30, 31 The intramolecular reorganization energy can be estimated from calculations in two ways: (1) from adiabatic potential energy surfaces of isolated monomers or (2) normal-mode analysis. Here we perform the former method throughout the paper. Calculations by Brédas et al. have shown that the two methods yield similar results.29, 32 λ can be expressed as follows

λh = [ E(+M ) − E(+M ) ] + [ E( M ) − E( M ) ]

(7)

λe = [ E(−M ) − E(−M ) ] + [ E( M ) − E( M ) ]

(8)

+

−

+

−

8


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Where λh / λe is the reorganization energy for hole/electron, E( M ) , E (+M + ) and E (−M − ) represent the total energies of the optimized neutral, cation and anion structures, respectively, while E ( M + ) / E ( M − ) represents the neutral energy of optimized cationic/anionic structure, and E (+M ) / E (−M ) represents the cationic/anionic energy of optimized neutral structure. Full geometry optimizations of the monomer molecule and the reorganization energy calculations are carried out using the B3LYP/6-31G(d,p). B3LYP was proved to have the good description of the relaxation processes in oligoacenes;33, 34 as a result, we have chosen the B3LYP functional for the calculations of the electronic properties at the molecular level. These calculations are performed with the Gaussian09 program.35 The crystal structure of compound 3 is constructed on the basis of homology modeling from the crystal structure of compound 8, and crystal optimization are performed using the pure functional (PBE36, 37) and a plane-wave basis set at cut-offs energy of 420 eV in Vienna ab initio simulation package (VASP) program.38

3.

RESULTS AND DISCUSSION The structures of the compounds studied in this paper are shown in Figure 1. The

crystal structures of compounds 1, 2, 4, 5, 6, 7, 8, and 9, were retrieved from the Cambridge Crystallographic Database.16, 39-43

3.1. Geometric Structures. 9



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The ground state geometries for studied compounds were determined at B3LYP/6-31G(d,p) level followed by vibrational frequency analysis and no imaginary frequency has been found. Single-point calculation was performed at the optimized geometries using B3LYP/6-31++g(d,p), with tight SCF convergence criteria (10-8 on root-mean-square deviation in electron density matrix elements).

Figure 1. Chemical structures of the selected compounds in this paper

10


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All the molecules share a similar acene core framework, the introduction of halogenated atoms at the terminal benzene ring didn’t change the planarity of the core part. But this modulated the bond lengths of the acene core. To clarify the effect of the halogenated atoms on the studied molecules, the geometric parameter of the neutral molecular structures is shown in Figure 2. Substitutions of fluorine atoms possessing strong electron-withdrawing ability lead to the reduction of the adjacent C-C bond lengths of about 0.001-0.005 Å. But substitutions of chlorine atoms possessing weak electron-donating ability lead the elongation of the adjacent C-C bond length of about 0.007-0.012 Å. This phenomenon is attributed to the conjugation effects from the vacant 3d orbitals of the Cl atom to acene core, which is mentioned in Bao’s report on other chloro-acenes16 and Zhang’s research.23 The substituent of pyridine gives rise to little change on the adjacent acene part, but the introduction of thiophene brings about the change by about 0.022 Å of the corresponding adjacent C-C bond length.

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Figure 2. Geometrical parameter (bond length/Angstroms) of their neutral structure optimized for the studied molecules.

3.2. Frontier Molecular Orbitals, Ionization Potential and Electron Affinity.

12


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Figure 3. The HOMOs and LUMOs of studied molecules calculated at B3LYP/6-31++g(d,p)//B3LYP/6-31g(d,p) level (isovalue surface 0.02). Hydrogen atoms omitted for clarity. As is well known, the HOMO and LUMO levels and EAs/IPs of material molecules directly connect to the injection barrier of electron/hole from electrodes, while the EA/IP of single molecule is closely related to the redox properties of formed material, thus IP/EA determine its stability, degradation and lifetime of molecular material. It is therefore necessary to theoretically investigate the levels of HOMO, LUMO and IP/EA. In this study, the neutral, cationic and anionic geometry structures of all the compounds have been optimized and the single point energy of various states was obtained, respectively. The molecular frontier orbitals of all the compounds are calculated at the level of B3LYP/6-31++g(d,p)//B3LYP/6-31g(d,p); parts of the 13



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results are shown in Figure 3. Both HOMOs and LUMOs are of π-characters and mostly locate at the conjugated cores. Comparing compound 2 with 1, the halogen substitution makes HOMO and LUMO energy levels significantly decrease. Compared with the fluorine-substituted 2, the chlorine-substituted 3 only slightly further reduces HOMO and LUMO energy levels, which is ascribed to enhance the overlap between the vacant 3d orbital in chlorine and π orbital in the acene core, _-extends π-conjugation. The substitutions of both thiophene (4, 5, 6) and pyridine (7,

8, 9) for one terminal benzene ring in the TIPS-PEN (1) and halogened TIPS-PEN (2, 3) result in more stable HOMO energy level. The introduction of terminal thiophene lifts the LUMO level and thus widens the HOMO-LUMO gap, but the introduction of the terminal pyridine decreases the LUMO energy level, resulting in the HOMO-LUMO gap almost unchanged relative to that of TIPS-PEN. Compared with compounds 1, 4, and 7, the HOMO and LUMO levels of their Cl substituted molecules (Compounds 3, 6, and 9) decrease significantly, but their HOMO-LUMO gaps change slightly. They have a low LUMO level, and almost the same HOMO level, compared with fluorinated counterpart.16 At the same time, the introduction of thiophene has a subtle lower HOMO level, and a higher LUMO level, but the introduction of pyridine has obviously dropped both the HOMO and LUMO level, but the gap remains unchanged. Overall, the introduction of thiophene (4, 5, 6) and pyridine (7, 8, 9) and halogenation can improve the air stability of acene material 14


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molecules, while the substitution of pyridine and halogenation for a terminal benzene ring in the acene can enhance antioxidability of themselves. The synergy effect of halogenation and heteroatom substitution further enhance above trend.

Figure 4. Contributions of the frontier orbitals for 1-9 used in Table 1; P2 refers to pentacene, thiophene- and pyridine-fused pentacene for 1-3, 4-6 and 7-9 accordingly. To understand the role of each substitution group in these frontier orbitals, molecular orbital contribution has been analyzed using the AOMix program.44, 45 The molecules are divided as figure 4. Molecular orbital compositions of 1‒9 are summarized in Table 1. In the first group (1-3), both F and Cl atoms have almost the same contribution of HOMO for compound 2 and 3, but they have more contribution of LUMO for 3 than that for 2. That would be the reason why compound 3 has a lower LUMO energy level than 2. The similar situation comes for compounds 5 and 6, 15



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8 and 9 relative to 4 and 7 respectively. It is found, after the introduction of halogen atoms, that there is appreciable charge transfer from the branch TIPS part to the center acene part according to the analysis of molecular moiety contribution for HOMO and LUMO. The P3 (TIPS part) component differentiation value between the HOMO and LUMO for 2 is about 1.6%, while it is 3% for 3. When the last ring of the acene is replaced by thiophene or pyridine, it gives about 1.36% and 1.83% for fluorine-substituted 5 and 8, and about 3.27% and 3.31% for chlorine-substituted 6 and 9, respectively. Results show from Table 1, although halogen atoms have small ingredient (about 1% for fluorine atoms, 2% for chlorine atoms), the introduction of halogen atoms especially the chlorine atoms, largely changes the molecular orbital compositions, and thus can tune the reorganization energy during accepting or losing an electron (detailed discussion see 3.3 section). In total, the halogenation greatly contributes to HOMO and LUMO of TIPS-PEN, the intra-hetero-substitution scarcely contribute to them. But the participation of thiophene- and pyridine-fused ring promotes and prevents the contributing of halogenation to the frontier molecular orbitals, respectively. Table 1 The HOMOs and LUMOs contributions of individual fragments (in%) to the FMOs of the investigated

molecules at the B3LYP/6-31G(d,p) level.

HOMOs

LUMOs

compounds P1 a

P2 b

P3 c

P1 a

P2 b

P3 c 16


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a

1

0.09

85.36

14.55

0.09

85.41

14.5

2

1.51

83.12

15.37

1.14

85.13

13.73

3

1.85

82.41

15.75

2.12

85.1

12.78

4

0.09

85.02

14.89

0.1

84.61

15.29

5

1.64

82.47

15.89

1.25

84.62

14.13

6

2.01

81.48

16.52

2.35

84.8

12.85

7

0.1

84.03

15.87

0.08

86.38

13.54

8

1.76

81.09

17.15

1.09

85.92

12.99

9

2.17

79.98

17.85

2.02

85.76

12.22

P1 : the four atoms moieties; b P2 : acene part except four H, F or Cl atoms moieties; c P3 : TIPS branch moieties. as showed in

Figure 4.

For OFET or OLED materials, the IP and EA values of material molecules are important parameters to characterize the charge injection ability which is closely related to the organic device performance and its redox stability. In general, the n-type semiconductor material has high EAs while the p-type semiconductor material has low IPs. In the ambipolar semiconductor material, both high EA and low IP are needed. High EA ensures that electrons can overcome the energy barrier and be efficiently injected into the empty LUMO while low IP ensures the hole can overcome the energy barrier and be efficiently injected into the HOMO.46,

47

Therefore, adiabatic/vertical IP (IP(a)/IP(v)), and adiabatic/vertical EA (EA(a)/EA(v)) 17



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calculated using DFT at B3LYP/6-31++g(d,p)//B3LYP/6-31g(d,p) level are showed in Table 2. According to Table 2, both the ionization potentials and electron affinities increase obviously with the introduction of halogenated atoms. But the chlorinated derivatives have a lower IP and higher EA than their fluorinated counterparts. For compounds 1‒3, the IP(v) value changes from 5.961 eV of 1 up to 6.268 eV of 2, and then drops to 6.239 eV of 3 and the EA(v) value increases from 1.927 eV of 1 up to 2.230 eV then to 2.315 eV of 3 accordingly. With the IP value increasing, the molecules in ambient become more and more stable. And at the same time, increased EAs are getting close to the work function of widely using a metal electrode (Au), thus enhance the electron injection ability from the metal electrode and then improve the electron transport. IP and EA values of chlorinated compound are more close to the Au electrode than those for their fluorinated counterpart. That suggests that the chlorinated molecules are more suitable for the ambipolar charge transfer than their fluorinated counterpart. At the same time, the introduction of thiophene or pyridine tunes the IP or EA values, gives further choices for ambipolar materials.

3.3. Reorganization Energy. The reorganization energy is one of the key parameters that control the charge transfer rates. It describes the sum of internal site energy relaxation during charge transport. The low-reorganization energy is expected for the ease of the hopping 18


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charge transport. As seen in Table 2, for the studied molecules, the reorganization energies for electron transport are larger than from those for hole transport. The fluorinated molecule 2 has the larger hole and electron reorganization energy and the chlorinated molecule 3 has the smallest electron reorganization energy among compounds 1‒3. The same rule can be achieved in comparison of compounds 4‒6 or compounds 7‒9. For example, compounds 2, 5, and 8 have the larger λhole value (largest one is 0.168 eV) compared with the compounds 1, 4, and 7 (0.144 eV) and the compounds 3, 6 and 9 (0.156 eV), respectively. Meanwhile, compounds 2, 5, and 8 have the largest λelectron (greatest is 0.220 eV) in compounds 1‒9. Among the derivatives with chlorine, compound 9 gives the smallest λelectron and the largest λhole . The difference between the λelectron and the λhole of 9 is only 0.021eV, suggesting compound 9 with terminal pyridine will give a more balanced charge transfer than compounds 3, 6 on neglecting the effect of molecular packing. Table 2 Calculated ionization potentials(IPs), electron affinities (EAs) and the reorganization energies(λ) of nine molecules [all in eV] at B3LYP/6-31++G (d, p)//B3LYP/6-31G (d, p) level. λhole

λelectron

1.927

0.134

0.194

2.328

2.230

0.155

0.203

6.239

2.391

2.315

0.145

0.180

6.041

1.864

1.760

0.139

0.211

Compounds

IP(a)

IP(v)

EA(a)

1

5.889

5.961

2.019

2

6.192

6.268

3

6.174

4

5.966

EA(v)

19



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5

6.276

6.354

2.188

2.075

0.160

0.220

6

6.242

6.319

2.265

2.171

0.147

0.193

7

6.016

6.095

2.141

2.044

0.144

0.188

8

6.326

6.406

2.441

2.338

0.168

0.199

9

6.281

6.362

2.494

2.405

0.156

0.177

3.4. Transfer Integral and Mobility. The transport property of organic semiconductor materials can be characterized by carrier mobility, the magnitude of the carrier mobility for solid materials largely depends on two factors of reorganization energy and the transfer integral, an electronic coupling of neighbor molecules. The intermolecular electronic coupling and the mobility of holes and electrons are strongly dependent on the molecular structure and their packing motif. So in this study, the intermolecular electronic coupling and the mobility of holes and electrons are calculated based on the experimentally synthesized single crystal structures of 1, 2, 4, 5, 6, 7, 8, 9 and the optimized structures of 3, 6, 9. The optimized structure and crystal parameters are showed in Table S1 (Supporting Information). The lattice volumes of 9 of the optimized structure and experimental structure are 2104.83 Å3 and 2119.89 Å3 respectively. The relaxation gives rises to the change only about 0.7%, so, the optimized structure parameters don’t vary too much. The method and basis set used for crystal construction and optimization is suitable. Although all the single crystals 20


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show a similar triclinic space group P 1(compound 7 is in P1, it might not be the most perfect crystal), and adopt a two-dimensional π -stacked arrangement, the introducing of tetra-halogen atoms does have an important effect on the molecular packing. Compared to compound 1 with an average interplanar space of 3.43 Å, 40

compound 2 is in an average interplanar space of 3.36 Å. From the crystal data, crystals for chlorinated compounds seem to have a different packing, starting from the central molecule, there are five different paths, it differs from the parent compound and their fluorinated counterpart. As the intrinsic driving force of charge transport, the π-conjugated coupling interactions between the neighboring molecules are very strong for chlorinated compound crystals. Different carrier transport paths in each organic crystals are found and named as D1, D2, D3, (D4-D5). The electronic couplings (hole and electron transfer integrals) of these dimers are calculated and the mobilities along specific directions of these dimers. The angular resolution anisotropic mobilities and some experimental results of the mobilities are computed and the results are listed in Table 3. Table 3 The calculated hole and electron transfer integrals (V) and angular resolution anisotropic mobilities (µ)(some experimental results are given in parenthesis)

m

Given as the center-of-mass

distance (Å) in the ith hopping pathways. PBE refers to the structure construction and optimization from VASP V (meV)

µ(cm2 V-1 s-1)

21



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

Compounds

Type

rim

Hole

Electron

1

D1

7.57

-24.78

-128.83

D2

10.21

4.02

67.22

D3

16.22

1.48

-7.90

D1

7.55

31.00

-119.66

D2

10.54

-4.92

19.90

D3

16.61

-1.30

-1.72

D1

5.78

82.13

1.60

D2

7.49

59.28

74.26

D3

10.67

62.57

44.69

D4

14.51

4.48

-15.53

D5

17.37

-2.03

5.10

D1

7.66

-13.58

-113.41

D2

9.97

-18.29

57.06

D3

16.05

-1.66

-6.70

D1

7.61

-34.34

-115.31

D2

10.23

-39.74

46.36

D3

16.37

2.36

2.08

D1

5.00

-27.43

-110.89

D2

10.75

-54.45

-45.02

2

3(PBE)

4

5

6

Hole 0-0.83(1.8[a]; 0.72[b] ;0.95[c])

0-0.97(0.014[d]; 0.072 [e]

)

Page 22 of 36

Electron 0.60-8.88(0.05[c])

0-7.99( 0.105[e])

1.06-4.76(0.111[e])

0.20-3.51 (0.054[e])

0.02-0.55(1.25[f])

0.12-5.95

0.24-1.89(0.113[g])

0.14-5.56(0.216[g])

0.31-4.10(0.225[e])

0.75-2.39(0.561[e])

22


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7

8

9

D3

6.93

-54.36

-67.26

D4

16.94

-2.72

-5.11

D5

15.53

-2.70

-9.99

D1

7.84

-6.00

-108.61

D2

10.05

-12.15

68.79

D3

16.33

-1.10

-6.80

D1

7.83

8.30

-75.79

D2

10.38

-8.30

19.50

D3

16.72

0.67

1.27

D1

5.63

58.15

-20.91

D2

7.76

-52.74

-67.85

D3

10.59

-61.50

-43.10

D4

14.68

-2.31

9.83

D5

17.35

1.69

5.33

0.01-0.26 (0.22[h], 0.42[c])

0.01-0.08(0.08[h])

0.52-3.87(0.12[i], 0.23[j])

0.83-7.06(0.10[c])

0.02-3.63(0.09[h])

0.21-2.96(0.14[i], 0.21[j])

[a] Park, S. K., et al.48 [b] Kim, D. H., et al. 49 [c] Liu, K., et al. 23 [d]Swartz, C.R. et al. 40 [e]Tang, M. L. et al. 16 [f]Tang, M. L.,et al.50 [g]Tang, M. L., et al. 41 [h]Liu, Y. Y., et al. 42 [i]Song, C. L.,et al.23 [j]Zeng, W. J., et al.51

All parent and fluorinated compounds have the larger transfer integral value for electron than that for hole (figure 5). Chlorinated compounds get more balanced charge transfer mobility than fluorinated counterparts. The substitution of the thiophene or pyridine for a terminal benzene of the pentacene (compounds 4 and 7) also lessens the electron transport. The halogenation with hetero-ring substitution at 23



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Page 24 of 36

both ends of the pentacene (compounds 5, 6, and 9) further decrease the electron transfer integral and increase the hole transfer integral, thus resulting in more balanced carrier transport rate.

Figure 5. Molecular stacking motifs, charge transfer pathways in single crystals (left) : a) 1. b) 2. c) 3. d) 6. Viewed along the short axis of the molecule. The frontier orbitals (right) of the main channels at the B3LYP/6-31g(d,p) level (isovalue surface 0.01). Hydrogen atoms omitted for clarity. It can be seen from Table 3 and Figure 5, the largest transfer integral values for hole and electron do not always appear in a same hopping path. Such as both electron 24


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and hole have the same transport channel D2 for compounds 2 and 3 (or for 5 and 6), while the electron transport has additional main path of D1. However, that the electron transport is suppressed dramatically compared with un-substituted pentacene is ascribed to the significant decrease of the transfer integral of leading electronic transport channel D1. It is interested that the chlorine substitution for terminal benzene of the pentacene adds extra pathways (D4, D5) of carrier transport, while fluorine and thiophene or pyridine remain the same transport channel with the pentacene. And the pyridine substitution, combining with chlorine substitution in a pentacene make electron and hole transport on each path largely balanced. Table 3 displays the anisotropic mobilities with the angular resolution change based on the crystal structures. According to our calculated results, the mobility ranges, and the extrema of anisotropic mobilities are listed. Some of the experimental mobilities may be inconsistent with the theoretical results, which should be caused by that the experimental mobilities are very sensitive to measurement conditions. Temperature, purity, electric field and so on could change the mobilities greatly. At the same time the interface and the bulk of organic semiconductors may play an effect on the transport.23, 52 Obviously, theoretical simulation in this study did not consider the influence of temperature and impurity. However, theoretical simulations ignoring the experimental condition details can offer a reasonable analysis of the intrinsic

25



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charge transport properties of semiconductor materials, providing useful information for the preferred material design of the organic electronic devices.

Figure 6. Calculated anisotropic mobilities of holes and electrons for with change of angular resolution based on experimental crystal structures for compounds 1-9 (except 3)(The angle of a conduction channel is relative to the reference crystallographic axis b) .

Figure 6 shows the anisotropic mobilities with the angular resolution change based on the crystal structures of compounds 1-9 (except for 3). Because of the large TIPS part, the charge transport between layers is less efficient. All compounds are in an anisotropic mobility of single channel, no matter for the hole or the electron transfer. All the parent and fluorinated compounds have larger inherent transfer 26


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mobility for electron than that for hole. The fluorination doesn’t greatly alter the packing and the maximum of the mobility, but the chlorination exhibits a different result. Because of the medium granularity and electronegativity of chlorine atoms, the shifts between adjacent layers in molecular packing gives more overlaps of the HOMOs, as a result, the hole transfer integrals of main channels and the hole carrier mobilities increase (figure 5c, 5d). So the compounds with chlorine have higher hole mobilities and more balanced carrier transport than the fluorinated counterparts in this study no matter from the maximum anisotropic mobility (Table 3) or from random walk simulations (see Table S2 in the Supporting Information). Table S2 lists all the calculated mobilities based on crystal structures (3 is built on homology modeling) using a random walk method. For the sake of comparison, the molecular group (1, 2, 3), which analysis of the charge mobilities can provide only halogenation effect on the value of mobility and ambipolar characteristic. And the comparison of group (1, 4, 7) can probe the relationship between the introduction of hetero atoms, the value of mobility and ambipolar characteristic. Similarly comparison of groups (1, 5, 6) and (1, 8, 9) can understand synergy effect of halogenation and hetero-atom in the acene skeleton. It can be seen from the Table S2,

3 shows balanced electron (μe =0.72 cm2 V-1 s-1) and hole (μh = 1.39 cm2 V-1 s-1) mobilities with lowerμe /μh ratio (0.52), while 2 shows electron-dominating (μe =1.78 cm2 V-1 s-1 , μh = 0.23 cm2 V-1 s-1) mobilities with higher (μe /μh = 7.74) 27



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Page 28 of 36

ratio. It indicates that the halogenation can significantly change the unipolar character of materials and it is an important step toward ambipolar feature but it cannot make theμe/μh value reduce into 1. At the same time, chlorination prevails fluorination in the transition of charge transport from electron dominating to balanced ambipolar charge transport just considering from the intrinsic properties of the material. The impact of heteroatom substitution on the ambipolar characteristics is complicated with respect to halogenation. Observing the mobilities of group (1, 4, 7), it can be found that there is a relation with the mobility of electron and hole,μe (1) > μe (7) > μe (4); μh (4) ≈μh (1) > μh (7); μe/μh (7) = 37.17 > μe/μh (1) = 15.80 >μe/μh (4) = 8.33. The results indicate that ambipolar characteristic cannot be enhanced by only thiophene or pyridine-substitution. Pyridine-substitution will further help increase unipolar n-type feature of materials because of the increase of the ratio of μ e/μh.

Thiophene-substitution lessens unipolar n-type characteristic. As a result, there

is a different result of synergy effect between halogenation and heteroatom substitution on charge transport. In group (1, 5, 6) there is the relation as follows: μe /μh (1) = 15.80 > μe /μh (5) = 7.33 > μe/μh (6) = 0.98. It is obvious that the synergy effect of halogenation and hetero-ring thiophene-substitution may further improve ambipolar transport property, in particularly, the synergy effect of chlorination and thiophene substitution achieves optimum for improving ambipolar property of designed material. However, the synergy effect between halogenation and 28


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hetero-ring pyridine substitution does not necessarily benefit to the improvement of ambipolar characteristic of material. In group (1，8，9), µe/µh (8) = 24.50 > µe/µh (1) = 15.80 >> µe/µh (9) = 0.59. Overall, the halogenation of TIPS-pentacene is one of the most efficient methods toward ambipolar design of material, the synergy effect between chlorination and thiophene-substitution for terminal ring of TIPS-pentacene will make the studied system to achieve excellent ambipolar characteristic. At the same time, the terminal ring substituted is in an important effect on the transfer channel and results in different absolute value of mobility. For example, the introduction of thiophene gives a lower HOMO level, but for hole and electron the reorganization energy and transfer integral keep a balance, which makes the mobility change slightly. There is the different situation for pyridine-substituted one, the pyridine ring seems to do change the molecular stacking motif and then decrease the transfer integrals, so the charge mobility decreases. Compound 6 has the better ambipolar mobility, at about 2.30 cm2 V-1 s-1, but the maximum value of mobility appears at about 60 degrees for hole and electron transport (Figure 6). Compound 9 also has the balanced ambipolar charge transfer by about 0.20 cm2 V-1 s-1, but the maximum value of mobility appears at about 30 degrees for hole and electron transport. Zhang et al. give the electron/hole mobility ratio of about 1:1 in experimental.23, 42 Our study demonstrates that compound 9 may be an ambipolar but more for n-type material just from intrinsic characteristics. It suggests that the charge 29



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Page 30 of 36

injection in OFET of compound 9 may play an important role. Although the introduction of pyridine can lower the LUMO level making it easy to be injected, it does reduce the maximum of transfer integrals due to lessened electronic interaction between the neighboring molecules in crystal and thus pull down the mobility.

4.

CONCLUSION We have theoretically investigated the charge-carrier transport properties of

halogenated TIPS-PEN and series of derivatives with thiophene and pyridine separately substituted for one terminal benzene in acene backbone and halogenated TIPS-PEN. This study elucidates how to modify the TIPS-PEN to get preferred ambipolar transport materials for organic electronic devices. Based upon DFT calculation results, the origin of significant difference in hole and electron mobilities is revealed in the halogenated TIPS-PEN. The tetra-chlorine substituent of TIPS-PEN lowers LUMO and gets smaller reorganization energy and more balanced ambipolar mobility than their fluorinated counterparts, which originate from the overlap between delocalized π-orbitals of acene and the unoccupied 3d orbitals of the chlorine atom. At the same time, the introduction of thiophene is advantageous for improving packing effect, and the introduction of pyridine is beneficial for decreasing the energy barrier of electron injection, they both make the acene more air-stable to protect from attacking of oxygen and moisture.

30


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Supporting information description: Table S1 Optimized Cell parameters of molecules 6 and 9 Table S2 Summary of the electron and hole simulated mobilities , and mobility ratios (μe/μh) based on single crystals and the optimized structure.

Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS This work is supported by the Natural Science Foundation of China (21473071, 21173099, 20973078 and 20673045) and the Major State Basis Research Development Program (Grant 2013CB 834801).

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Theoretical Study on Charge Transport Properties of Intra- and Extra

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