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Effects of Sulfur Oxidation on the Electronic and Charge Transport Properties of Fused Oligothiophene Derivatives Ping Li, Yahui Cui, Chongping Song, and Houyu Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03007 • Publication Date (Web): 23 Jun 2016 Downloaded from http://pubs.acs.org on June 26, 2016

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The Journal of Physical Chemistry

Effects of Sulfur Oxidation on the Electronic and Charge Transport Properties of Fused Oligothiophene Derivatives Ping Li, Yahui Cui, Chongping Song and Houyu Zhang∗ State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun 130012, P. R. China E-mail: [email protected]

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Abstract In present work, the comparative studies of a series of oligothiophenes and their oxidized compounds are carried out by means of quantum chemical calculations. Taking aim at the effects of chemical oxidation of thienyl sulfur on the modulation of electronic structures and charge transport properties of oligothiophenes, the geometrical structures, molecular reorganization energies upon gaining or losing electrons, molecular ionization potentials (IPs) and electron affinities (EAs), molecular aromaticities, frontier molecular orbitals, as well as charge mobilities are analyzed in detail to determine the structure-property relationships for the investigated oligothiophenes and their corresponding oxidized counterparts. The calculated results show that the oxidation of thienyl sulfur into the corresponding S,S-dioxide could possibly change the charge transport characteristics of fused oligothiophene from hole transporting materials to bipolar or electron transporting materials, shedding light on the exploration of n-type thiophene-based semiconductors.


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1. INTRODUCTION Nowadays organic semiconductors (OSCs) have been explored extensively in optoelectronic applications, such as organic light-emitting diodes (OLEDs), 1–3 organic field-effect transistors(OFETs), 4–10 and organic solar cells. 11–13 Among these organic materials, fused oligothiophenes and their derivatives have drawn considerable attention owing to their chemical versatility, and favorable electrical and optical properties. 14,15 As the heteroatom analogues of linear acenes, the fused oligothiophenes (thienoacenes) have extended π conjugation and rigid planarity, thus resulting in high charge transporting properties and chemical stability. 16–18 The sulfur atom in thiophene ring has the high polarizability and could facilitate electrondonating properties. Moreover, multiple short intermolecular S· · · S nonbonding interaction originated from sulfur atoms at the molecular periphery can make the molecules form highly aligned molecular arrays in the solid state, achieving a high charge mobility along π-stacking direction. With these advantages, more and more high performance oligothiophene-based derivatives have been produced through chemical approaches of synthesis and modification . The nonsubstituted oligothiophenes all behave as p-type (hole-transporting) materials; 19,20 However, n-type thiophene-based semiconductors are less explored and are in great demand for the fabrication of bipolar transistors, and may find an interest in the development of photovoltaic cells. The strategy for inducing electron transport in organic semiconductor is increasing the molecular electron affinity. In this regards, the incorporation of electronwithdrawing groups, such as perfluoroalkyl and cyano groups, to the molecular backbone of oligothiophenes could alter the charge transport properties. 21 More recently great efforts had been also made on the oxidation of oligothiophenes towards developing new organic semiconductors. The chemical oxidation of the thienyl sulfur into the corresponding S,S-dioxide could modulate the electronic structure of oligothiophene and significantly affect the fluorescence and redox properties. 22 It has been demonstrated that oligothiophene S,S-dioxides are considered as excellent candidates for application in light-emitting diodes. 23–25 Such oxidation could also change the nature of thienyl sulfur from electron donating to electron 3 ACS Paragon Plus Environment


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accepting, thus indicating that oligothiophene sulfones would be potential candidates for ntype semiconductors. 26,27 It is encouraging that oligothiophene S,S-dioxides have been also tested as a class of electron-acceptor materials for organic photovoltaics. 24 Though many experimental trials have been performed to investigate the charge transport properties of the oxidized oligothiophenes, the questions on how and to what extent the oxidation affect the electronic structure of oligothiophenes are still elusive. 28 To study the effect of the chemical oxidation on the oligothiophenes, the fundamental relationships between the molecular structures of the oxidized oligothiophenes, their supramolecular arrangements in solid state, and their charge transport properties need to be elucidated. To achieve this aim, quantum chemical calculations have the advantages on learning the electronic properties of OSCs and predicting their charge transport properties. 29–37 Tanaka et al. studied the band gap and stability of S,S-dioxide derivatives in comparison to oligothiophenes by semi-empirical theoretical methods. 38 Geng and coworkers have investigated the influence of oxidation on the photoluminescence and charge transport properties of diindenodithienothiophenes. 39 Such approach can be of great help in evaluating the potential candidates for OFETs and avoid the expensive and time consuming trial-and-error methods in material design. In the present work, a comparative study of the fused oligothiophenes and their oxidized compounds (shown in Figure 1) are performed with emphasis on the effect of the sulfur oxidation on the charge transport propeties. The fused oligothiophenes, dithienothiophene (1-1) and its phenyl substituted compound (2-1), pentathienoacene (3-1), dimer of thienothiophene (4-1) are selected as references for investigating their oxidized compounds. We are interested in how the oxidation change the geometrical and electronic structures of oligothiophenes. The molecular frontier orbitals, molecular IPs and EAs, and reorganization energies will be definitely influenced by the sulfur oxidation. The oligothiophenes have different π-conjugation and exhibit the aromaticity in the molecular backbone. While the oxidation of sulfur in the thiophene ring will partly break down the π-conjugation, and


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change the aromaticity in the location where the oxidation occur in the backbone, which will certainly affect the electronic properties in the molecules. Not only the electronic properties of the single molecule but also the intermolecular arrangements will be influenced by the oxidation. All these properties need to be investigated to further understand their structureproperty relationship upon oxidation. The question of whether the chemical oxidation could produce new ambipolar or n-type semiconductor will be answered by theoretical prediction. By means of density functional theory (DFT) calculations, we intend to illustrate the effects of chemical oxidation and establish structure-property relationships, and further probe a rational route to design ambipolar or n-type materials.

Figure 1: The chemical structures of the fused oligothiophene derivatives and their oxidized compounds investigated in this work. The letter denote symmetry-unique rings in these molecules.

2. THEORETICAL AND COMPUTATIONAL METHOD The molecular geometries of neutral and charged states are optimized at the DFT level using B3LYP hybrid functional 40,41 and 6-31G** basis set, as implemented in the Gaussian 09 package. 42 Vibrational frequencies are calculated at the same computational level on the basis of the resulting optimized geometries. The total density of state (DOS) and projected density of state (PDOS) for sulfone group are obtained with GaussSum 2.25 program. 43 To study the charge transport properties of fused oligothiophene S,S-dioxides at room temperature, the incoherent hopping mechanism is adopted to describe the sequential charge 5 ACS Paragon Plus Environment


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jumps between adjacent molecules. 5,44,45 The charge transfer process can be considered as a nonadiabatic hole/electron-transfer reaction involving a self-exchange charge from a charged molecule to an adjacent neutral one: M + M + −→ M + + M ; M + M − −→ M − + M

(1)

The charge transfer rate can be expressed by the Marcus-Hush equation 46,47 in terms of reorganization energy λ and electronic coupling Vab between neighboring molecules a and b: 48 k=

2πVab2 π 1/2 λ ( ) exp(− ) h λkB T 4kB T

(2)

where, T is room temperature (298K), and kB is the Boltzmann constant. The charge hopping rate benefits from larger Vab and smaller λ. The reorganization energy λ includes contributions from the inner reorganization energy (which is induced by intramolecular vibrations) and the external reorganization energy (which is caused by polarization of the surrounding medium). 49 For organic solids and weak polar media, the contribution to the reorganization energy from electronic polarization of surrounding molecules is quite small (typically lower than 0.01 eV), so the external reorganization energy can be neglected. 50–52 Herein, only the intramolecular reorganization energy is calculated, which can be directly derived from the relevant points on the adiabatic potential energy surfaces (PES) using the standard procedure detailed in the literature 53–56 or from the normal-mode analysis (NMA) method with the DUSHIN code. 57 Total relaxation energy is obtained by NMA approach as summation of the contributions from each vibrational mode: 53,58 λ= λi =

∑

∑

h ¯ ω i Si

(3)

λi ki ∆Q2i , Si = 2 h ¯ ωi

(4)

λi =

Here, ∆Qi is the displacement along normal mode i between the equilibrium positions of


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the neutral and charged states; Si is the Huang-Rhys factor; ki and ωi are the corresponding force constants and vibrational frequencies. The electronic coupling Vab describes the degree of molecular orbital overlapping between two adjacent molecules, and can be calculated through a direct approach using Fock operator: 59–61 0,b 0 Vab = ⟨Ψ0,a i |F |Ψi ⟩

(5)

where Ψi0,a and Ψ0,b represent the molecular frontier orbitals of isolated molecules a and b, i where i denote the highest occupied molecular orbitals (HOMO) for hole transfer and lowest unoccupied molecular orbitals (LUMO) for electron transfer. F 0 is the Fock operator for the dimer in a specific pathway, the superscript zero indicates that the molecular orbitals appearing in the operator are unperturbed. The Fock matrix can be evaluated by F = SCεC −1 , where S is the intermolecular overlap matrix, and C and ε are the molecular orbital coefficients and eigenvalues, respectively. Hybrid functionals such as B3LYP, 62–64 M062X, 65 and MPWB1K 35 and combined exchange-correlation functional PW91PW91 33,66,67 are used to calculate the electronic couplings in the literatures. So B3LYP is employed to calculate the ground-state electronic structures and electronic couplings in the dimer structures.

Figure 2: Optimized structures in the neutral state of the oxidized molecules. Assuming no correlation between charge hopping events and charge motion is a homo7 ACS Paragon Plus Environment


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geneous random walk, 30 the drift mobility, µ, is related to the diffusion coefficient D and charge transfer rate k as: µ=

e 1 ∑ 2 D; D = d ki Pi kB T 2n i i

(6)

where e is the electronic charge. Considering the charge motion as a random walk in three dimensions (n = 3), D is summation over all possible hops. Pi is the probability (Pi = ∑ ki / ki ) for charge transfer to ith neighbor and di is the intermolecular center-to-center distance. The analyses of the local aromaticity are carried out by means of nucleus independent chemical shifts (NICS) and harmonic oscillator model of aromaticity (HOMA) index. The calculated NICS and HOMA values provide a relative comparison of aromaticity among all compounds. In the NICS procedure suggested by Schleyer et al., 68 the absolute magnetic shielding is computed at the ring center (NICS(0)) and 1 Å above the ring center (NICS(1) (for the heterocyclic ring in this work, we define the center as the ring bonding critical point). The HOMA index is calculated as 1∑ HOMA = 1 − α(Ropt − Ri )2 n i n

(7)

where n is the number of bonds of the ring considered, α is a normalization constant, Ropt is the optimal bond length for a fully delocalized π-electron system, and Ri stands for an actual bond length. In the present work, the parameters needed for the HOMA calculations CC CS were proposed by Krygowski: 69 αCC = 257.7, αCS = 94.09, Ropt = 1.388, and Ropt = 1.677.

3. RESULTS AND DISCUSSION 3.1 Molecular Geometries and Reorganization Energies The oligothiophenes and their oxidized derivatives are fully optimized without any structural constraints. All the optimized structures of the oxidized oligothiophenes, together with some 8 ACS Paragon Plus Environment

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Figure 3: Bond-length changes (in Å) upon oxidation and reduction for selected oligothiophenes and their S,S-dioxides. The bond indices are labelled on the molecular structures. selected dihedral angles, are shown in Figure 2. The fused thiophene units in oligothiophenes exhibit rigid planar structures (see Figure S1 in the supporting information). The chemical oxidation of the sulfur atom of the thiophene ring does not change the planar structure of fused thiophene. The sulfone groups are located in the plane perpendicular to the fused oligothiophene. The dihedral angle between the central trithiophene unit and phenyl group in 2-2 is about 27.7 o , which is almost not influenced by the oxidation and is nearly the same as that in 2-1. In the single-bond linked dimer of thienothiophenes (4-1), the dihedral angle between two dithiophenes is about 20.6o . While for the oxidized dimers, the twist angles are different and dependent on the position of oxidation on the sulfur atoms. The dihedral angle is smaller for the oxidation in the innner sulfur (15.8o ) than the outer one (21.8o ) of compounds. For the two internal sulfur atoms are oxidized in the compound 4-4, the twist angle is even smaller (13.2o ). Upon oxidation, the geometries of 4-1 are significantly changed.



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Table 1: The calculated internal reorganization energy (λh and λe ), ionization potential (IP) and electron affinity (EA) of adiabatic/vertical (A/V) with basis set of 6-31++G(d, p) and 6-31G(d, p) (in parentheses) in the unit of eV Mol.

λh

IP

λe

EA

PES

NMA

PES

NMA

AIP

VIP

AEA

VEA

1-1

0.34 (0.35)

0.34 (0.35)

0.31 (0.32)

0.31 (0.32)

7.38 (7.20)

7.55 (7.38)

0.04 (0.45)

0.19 (0.62)

1-2

0.39 (0.40)

0.39 (0.40)

0.41 (0.43)

0.44 (0.45)

7.95 (7.76)

8.14 (7.95)

−1.12 (−0.74)

−0.91 (−0.52)

2-1

0.31 (0.31)

0.38 (0.29)

0.41 (0.40)

0.48 (0.45)

6.55 (6.34)

6.70 (6.50)

−0.97 (−0.61)

−0.72 (−0.38)

2-2

0.37 (0.37)

0.42 (0.42)

0.43 (0.42)

0.50 (0.46)

6.94 (6.72)

7.14 (6.92)

−1.62 (−1.28)

−1.38 (−1.05)

3-1

0.30 (0.31)

0.30 (0.31)

0.26 (0.27)

0.26 (0.27)

6.78 (6.60)

6.93 (6.76)

−0.58 (−0.24)

−0.45 (−0.11)

3-2

0.33 (0.34)

0.33 (0.34)

0.35 (0.35)

0.35 (0.36)

7.20 (7.01)

7.36 (7.18)

−1.45 (−1.12)

−1.27 (−0.94)

4-1

0.37 (0.37)

0.38 (0.38)

0.34 (0.34)

0.36 (0.35)

6.75 (6.56)

6.96 (6.77)

−0.72 (−0.36)

−0.52 (−0.17)

4-2

0.39 (0.38)

0.40 (0.39)

0.43 (0.43)

0.45 (0.44)

7.21 (7.01)

7.43 (7.22)

−1.60 (−1.25)

−1.37 (−1.03)

4-3

0.35 (0.37)

0.35 (0.38)

0.35 (0.37)

0.36 (0.37)

7.15 (6.95)

7.32 (7.14)

−1.60 (−1.24)

−1.42 (−1.05)

4-4

0.39 (0.41)

0.40 (0.41)

0.33 (0.34)

0.33 (0.35)

7.55 (7.35)

7.75 (7.55)

−2.20 (−1.86)

−2.03 (−1.68)

When a molecule gains or loses charges, it will relax its molecular geometry for a new charge distribution. The cationic and anionic state of the compounds are also optimized to determine the molecular reorganization energy, which is a key parameter pertaining to the intrinsic charge transport property. Compared to their neutral compounds, their structures of ionic states trend to be more flattened. Besides the dihedral angles changes, the bond lengths changes are not negligible. The changes of bond lengths upon oxidation (losing electron from the neutral to the cationic state) and reduction (gaining electron from the neutral to the anionic state) of 1-1, 4-1 and their oxidized species are presented in Figure 3. The changes of bond lengths for other compounds are presented in Figure S3 in the supporting information. Because of π-conjugated structures of the compounds, the changes of bond lengths are found to occur over the entire molecule. The bond length changes upon losing or obtaining an electron are comparable for the selected compounds. In comparison with oligothiophenes, the bond length changes are more pronouced in oxidized compounds, especially for obtaining an electron. The change of bond length for S=O is slight larger for reduction than that for oxidation. The change of the structural parameter in the compounds upon oxidation and reduction is also a reflection of the reorganization energy. The calculated reorganization energies by both adiabatic PES method and NMA method for the investigated 10 ACS Paragon Plus Environment

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molecules are collected in Table 1. Both methods show reasonable agreement, which allow to analyze the individual vibrational modes for the contribution to the reorganization energy. The calculated results show that reorganization energies of oligothiophenes will change upon the oxidation for both electron and hole. In comparison to their oligothiophene counterparts, the oxidation increase reorganization energies for the relatively rigid molecules 1-2, 2-2 and 3-2; while the increase or slightly decrease reorganization energies are found for the relatively flexible molecules 4-2, 4-3 and 4-4, which might be attributed to the reason that the different position and extent of oxidation result in the different structural changes upon getting or losing electrons. The reorganization energy for electron and hole are comparable, which is in favor of the bipolar charge transport. With the increase of the extent of oxidation on the oligothiophene, the reorganization energy for electron is smaller than that of hole in 4-4. To further analyze the effect of thiophene-based oxidation on the reorganization energy, the contributions from vibrational normal modes are analyzed for the compounds. The contributions to the reorganization energy for electron and hole from individual vibrational modes for 1-1 and 1-2 are taken examples and depicted in Figure 4. The analysis for other compounds are collected in Figure S4 in the supporting information. The vibrational modes which make the largest contribution to the reorganization energies for hole are 1555 cm−1 and 1477 cm−1 for 1-1 and 1-2, respectively. The modes for the largest contribution for electron are 1545 cm−1 and 1519 cm−1 for 1-1 and 1-2, respectively. These modes belong to the stretching motions of backbone in the conjugated systems. The most vibrational modes can be ascribed to vibrational motion of the backbone, which indicate that the sulfur oxidation in the thiophene could not destroy the conjugated carbon channels (C=C) of the backbone in the oligomer. At the low frequency domain, there exist the vibrational modes with moderate contributions to the reorganization energy for 1-2 which can be referred to as scissoring vibrations of two S=O bonds.



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Figure 4: Contribution of the vibrational modes to the relaxation energy for molecules 1-1 and 1-2, embeded with the normal modes contributes the most for λh and λe


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3.2 Molecular Ionization Potential and Electron Affinity The molecular redox stability and effective charge injection from electrode are important for the practical operating electronic devices. The molecular ionization potential (IP) and electronic affinity (EA) are the key parameters to estimate the energy barrier for injection of hole and electron into molecules and provide useful information regarding ambient stability. 70 The calculated IPs and EAs of the compounds for both vertical (at the geometry of the neutral molecule) and adiabatic (optimized structure for both the neutral and charged molecules) values, are presented in Table 2. From the calculated values with different basis sets of 6-31++G(d,p) and 6-31G(d, p) (values in parentheses), we can see that these results are very dependent on the diffuse function of basis set, especially for the EAs. The energy difference between adiabatic and vertical value could reflect the extent of the structural relaxation upon charge injection. 71 The high IP could ensure the antioxidation ability for the p-type materials and increase the hole injection barrier from anode to molecules; the EA is expected to be more exothermic for n-type semiconductor to ensure efficient injection of the electrons and improve the stability of their anions. 72 It can be seen that both the absolute values of IP and EA are increased obviously with chemical oxidation of the oligothiophenes to corresponding S,S-dioxides. The calculated adiabatic and vertical IPs are larger than the adiabatic experimental IP for a stable p-type OFET material sexthiophene (5.80 eV), 73 indicating that all the compounds are more stable and exhibit the antioxidative ability in air. However, higher IP also means the increase of the energy barrier of hole injection with respect to the commonly used Au electrode (5.0 eV). A negative value of EA indicates exothermicity for the reduction of a molecule. The more negative EAs, the more stable of their anions. Upon going from oligothiophenes to corresponding S,S-dioxides, the adiabatic EAs are lowered by 0.65–1.48 eV. These EAs are more close to the work function of calcium (−2.90 eV) and magnesium (−3.68 eV). The chemical oxidation of the oligothiophenes makes the EAs of these materials more negative, which provide a strategy to lower the energy barrier for electron injection and improve the stability of their anions by preventing reaction with 13 ACS Paragon Plus Environment


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water and oxygen. Table 2: NICS(0), NICS(1) and HOMA values of individual heterocyclic rings in the compounds presented in Figure1. Compd 1-1 1-2 2-1

2-2

3-1

3-2

4-1 4-2

4-3

4-4

Ring a b a b a b c a b c a b c a b c d e a b a b c d a b c d a b

NICS(0) −11.970 −9.761 −12.500 3.717 −10.401 −9.844 −8.7770 −10.980 2.277 −8.936 −18.188 −9.844 −10.011 −12.436 2.687 −11.021 −10.338 −11.815 −11.816 −10.276 −11.765 −10.500 −11.077 0.717 −12.476 0.643 −10.396 −11.546 −12.346 0.633

NICS(1) −8.625 −7.365 −8.972 2.007 −7.361 −7.103 −10.322 −7.843 1.237 −10.365 −8.506 −7.051 −6.878 −8.859 1.629 −7.483 −7.557 −8.594 −8.731 −7.438 −8.771 −7.804 −8.050 0.021 −9.311 −0.056 −7.717 −8.750 −10.257 −0.201

HOMA 0.726 0.681 0.796 0.168 0.699 0.687 0.960 0.759 0.196 0.961 0.728 0.698 0.713 0.797 0.192 0.769 0.707 0.731 0.721 0.681 0.724 0.697 0.758 0.008 0.792 −0.016 0.694 0.737 0.813 0.029

3.3 Structure and Aromaticity Aromaticity is the property of a planar, cyclic, conjugated molecule in which cyclic electron delocalization results in enhanced stability, bond length equalization, and special magnetic, chemical and physical properties. 74,75 The fused oligothiophenes are aromatic because the 14 ACS Paragon Plus Environment

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Figure 5: HOMO and LUMO energy levels and their electronic density contours of the investigated molecules molecules are cyclic and planar structures, and follow Huckel’s rule, having 4n+2 electrons in the delocalized π-orbitals. The oxidation of 3-1 was verified with a certain regioselectivity because of the aromaticity of three environmently different thiophene rings. 27 The electronic structures of fused oligothiophenes will be definitely tuned by chemical oxidation on the sulfur in the thiophene ring, thus resulting in the changes of the extent of π-conjugation in the backbones of the molecules. Furthermore, the changed aromaticity is bound to affect the charge transport properties of oligothiophenes. Hence we evaluate the aromaticity of all compounds for understanding the molecular structure-property relationship. The calculated NICS(0), NICS(1) and HOMA values are collected in Table 2. Both NICS and HOMA show obvious aromaticity for the thiophene rings in the fused oligomers. The aromaticity of the rings exhibit remarkable difference between the central ring (b) and the periphery ring (a) in 1-1, which leads to the selectivity of chemical oxidation on the sulfur atoms of thiophene rings. Upon oxidation, the aromaticity of the oxidized thiophene rings are dramatically reduced. The rings with sulfone group become antiaromatic because of their positive NICS(0) values.



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These oxidized rings render different extent of antiaromaticity. The NICS(0) values of the oxidized rings in 1-2, 2-2, and 3-2 are larger than those values in 4-2, 4-3 and 4-4, indicating that these two types of the oxidized rings are not highly antiaromatic and even approximate to non-aromaticity (small NICS(0) value and close to zero), respectively. 76 The strong electron-withdrawing sulfone group could slightly increase the aromaticity of its adjacent rings. For example, the aromaticity of periphery ring (a) in 1-2 is larger than that in 1-1 because of the more negative value of NICS(0). Thus the chemical oxidation can change the extent of π-conjugation. In the fused oligothiophene-based molecules, the delocalized electron can transfer through two conjugated channels: (i) through carbons and (ii) through carbon and sulfur atoms. Once the sulfur is oxidized, the conjugation through second channel will be blocked and the aromaticity of oxidized thiophene ring will decrease and change to antiaromacity. In a previous report, Chen et al. have demostrated that the molecular conductance correlates negatively with the aromaticity in a single-molecule conductance measurement. 77 So the significantly decrease of aromaticity in the oxidized molecular segments might be in favour of charge hopping between the heterocyclic rings.

3.4 Molecular Orbitals and Density of States The chemical oxidation of thiophene rings will affect the frontier orbitals, especially for HOMOs and LUMOs, which are closely related to gain and loss electrons during the charge transport. The energy gap between HOMO and LUMO and the relative orderings of HOMO and LUMO levels will be modulated as well. The HOMO and LUMO plots and their corresponding orbital levels of the investigated molecules are shown in Figure 5. The HOMO and LUMO are distributed all over the molecules because the molecular planarity and extended π conjugation. Both HOMO and LUMO of the oligothiophene S,S-dioxides move downwards in comprison to those of the nonoxidized fused oligothiophenes; this can be ascribed to the introduction of strong electron-withdrawing group sulfones to the conjugated system. The decline of LUMO is larger than that of HOMO, resulting in a smaller HOMO-LUMO 16 ACS Paragon Plus Environment

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gap in oxidized molecules by narrowing down 0.21–0.67 eV. In comparison to mono-oxidized molecules (4-2 and 4-3), di-oxidized molecule (4-4) possesses much more lower LUMO (−3.11 eV) and smaller HOMO-LUMO gap (3.00 eV), indicating that the extent of the oxidation significantly affect the electronic structure of oligothiophene. The decrease of LUMO in the oxidized molecules is of importance for the electron injection, since the relative lower LUMO levels are more matchable to calcium and magnesium electrode in the devices. Moreover, the lowered LUMO can improve the stability of oxidized molecules in the practical device, 78 which is necessary for an n-type organic semiconductor material. It is worth noting that there are non-ignorable contributions from sulfone (S=O) groups to the frontier orbitals, especially for the LUMOs in the oxidized molecules. To further investigate the composition of orbitals near the HOMO-LUMO gap, we calculate the total DOS and PDOS for sulfone group for the molecules 4 series , as shown in Figure 6. We find that sulfone groups take part in the formation of LUMOs, rather than involve the formation of HOMOs. Sulfone groups appear in the participation of deeper occupied orbitals, as an example in HOMO-6. From the electronic structure point of view, the chemical oxidation will change the LUMO distribution in the molecules, leading to the sulfone groups directly involved in the transporting electrons. Such electronic structure changes affect the orbital interactions, resulting in the different intermolecular electronic couplings for hole and electron.

3.5 Electronic Coupling and Mobility The chemical oxidation of thiophene rings will not only affect the intrinsic charge transport parameter (e. g. molecular organization energy), but also the relative orentation and moleular stacking characters between molecules. 79 The structure of molecular single crystal could provide a reliable model to inspect how the oxidation affect the molecular arrangement in the solid state. Therefore, the charge transport properties of oligothiophene S,S-dioxides are analyzed on the basis of single crystal structures to reach a deeper understanding of the 17 ACS Paragon Plus Environment


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Figure 6: Total DOS and PDOS for the sulfone groups in the energy window of -12 and 2.5 eV for the series of 4.


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Figure 7: Charge hopping pathways for compounds of oligothiophene S,S-dioxides (a) 1-2, (b) 2-2, (c) 4-2, (d) 3-2, (e) 4-3 and (f) 4-4.



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Table 3: Electronic Couplings Vab of hole and electron for the different hopping pathways of all the oxidized compounds. compd. 1-2

2-2

3-2

4-2

4-3

4-4

pathway 1, 2 5, 6 7, 8 1 2 4 7 1 2 4 5, 6 1 3, 4 5, 6 1 2 3 4 5,6 7, 8 1, 2 3, 4, 5, 6 7, 8

d (Å) 3.87 9.74 8.92 3.91 9.51 8.47 5.23 3.74 3.68 8.68 13.33 4.97 6.06 5.13 4.00 7.94 5.94 8.38 10.04 8.63 7.89 8.77 6.10

Vhab (meV) Veab (meV) 175.1 346.5 13.8 5.5 0.07 17.7 81.8 40.5 58.3 10.0 5.5 16.1 5.5 60.2 171.1 272.2 192.1 274.5 0.9 24.1 10.6 4.7 92.7 80.5 6.5 13.6 12.6 102.7 158.3 57.5 146.7 7.6 4.0 86.3 1.4 16.7 22.4 12.1 27.8 7.9 14.3 46.6 61.7 19.6 59.8 34.5

Table 4: The predicted hole and electron diffusion mobilities for the S,S-dioxides. Compd. 1-2 2-2 3-2 4-2 4-3 4-4

µh (cm2 V−1 s−1 ) 0.16 0.08 0.32 0.08 0.40 0.08

µe (cm2 V−1 s−1 ) 0.46 0.02 0.63 0.06 0.09 0.06


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Figure 8: The stacking interaction in the dimer of 1-2. (a) the dimer structure viewed from long axis (left) and short (right) axis of molecule. (b) Pictorial HOMO-HOMO interaction. (c) Pictorial LUMO-LUMO interaction. chemical oxidation on the nature of the charge transport. The crystal structures of oxidized molecules are displayed in Figure 7, in which the charge hopping pathways in the same molecular layer are shown. One molecule is selected as the charge donor, then all the surrounding nearest neighbor molecules can be regarded as charge acceptors. The electronic couplings are evaluated based on the dimer configuration as in the crystal structure. The calculated electronic couplings (Vab ) are collected in Table 3. We notice that the S=O groups appear alternately in the two flanks of the molecule 1-2 in the crystal, which make molecule stacked on top of each other along c axis, as displayed in Figure 7(a). The same feature is also found in the crystal structure of 3-2 (Figure 7(d)). In comparison with herringbone arrangement of 3-1 in the crystal (see Figure S5 in the supporting information), the sulfone group adjust the molecular alignment and enhance the π − π stacking interaction in the oxidized molecules. The intermolecular distances along π-stacked direction are about 3.87 Å in 1-2 and 3.68 Å in 3-2. The calculated electronic couplings for hole are respective 175.1 and 192.1 meV for 1-2 and 3-2; while those values for



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electron are 346.5 and 274.5 meV, respectively. For the better understanding the electronic couplings in the π − π stacking interactions, the pictorial orbital interactions in the stacked dimer of 1-2 are shown in Figure 8. The S=O groups do not involve in the π orbtials of the HOMO, so they contribute little to the HOMO-HOMO overlaping; while they partly participate in the LUMO, hence enhance the LUMO-LUMO interaction. For the nonoxidized compound 4-1, it has been reported that it shows a herringbone packing arrangement in the crystal structure; 17 while the corresponding S,S-dioxides derivatives exhibit a marked difference in their solid-state packing. The compound 4-2 in which the oxidized sulfur is in the external region of the oligomer shows sandwich-herringbone packing consisting of a anti-parallel π-stacked dimers slipped along the long molecular axis (pathway 1 in Figure 7(c)). The electronic couplings for hole and electron of respective 92.7 and 80.5 meV. There also exist edge-to-face stacked dimers (pathway 5) which contribute a electronic coupling of 102.7 meV for electron. The compound 4-3 with the oxidation in the internal region of the molecule renders as face-to-face π-stacked dimers (pathway 1 in Figure 7(e)), having a slightly slipped configuration along the short molecular axis. The compound 4-4 with the oxidation of two thienyl sulfurs arranges in a slipped π-stacking along the short molecular axis (pathway 7 and 8 in Figure 7(f)). The S=O groups are not beneficial to the close π-interactions because of steric hindrance. Thus the oxidation of fused oligothiophenes makes significant effects on their solid state packing arrangement, further tuning the electronic couplings between molecules. Beside the effects on the π-stacking interactions, the introduction of S=O groups also affect the molecular packing by short O· · · S and O· · · H contacts. From the crystal structure, we notice that the short O· · · S and O· · · H contacts could not only make the molecules closely packed in the crystal structure, but also facilitate electron transport because of the high polarizability of oxygen and sulfur atoms. The selected short O· · · S and O· · · H contacts in compounds 1-2, 3-2, and 4-2 and corresponding LUMO interactions are shown in Figure 9. The intensive O· · · S interactions are responsible for their large electronic couplings of


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respective 346.5, 272.7 and 102.7 meV for electron. The predicted average mobilities of the oxidized oligothiophenes are listed in Table 4. The values of hole and electron mobilities of the compounds are comparable and are in the same order of magnitude, indicating the bipolar charge transporting abilities. Because of the existence of large electronic couplings for electron, the compound 1-2 and 3-2 show high electron mobilities of 0.46 and 0.63 cm2 V−1 s−1 , respectively. The phenyl substitution at the terminal of compound 2-2 do not favor the densely packing and intermolecular interactions, resulting in a relatively small charge transport abilities in comparison to 1-2. The position and extent of the oxidization also affect the predicted mobilities: 4-3 prefer transporting holes to electrons; while 4-2 and 4-4 exhibit similar hole and electron mobilities. The two oxidized units in molecule 4-4 decrease the reorganization energy, the electron injection barrier and aromaticity, which could be beneficial to increase the electron mobility. From the calculated data, chemical oxidation indeed achieve the conversion from p-type materials (oligothiophenes) to ambipolar or n-type materials (oligothiophene S,S-dioxides). The calculated results show that all the studied S,S-dioxides can be potential candidates as ambipolar or n-type materials in OFETs, which needs to be further verified experimentally. Considering the fact that the experimental mobilities are strongly influenced by the the microstructural characteristics of the dielectric layers in OFETs, such as film deposition temperature, film growth mode, and semiconductor phase composition, etc, 80 our calculated mobility based on the crystal structures can be regarded as a reference. It has been proved that the magnitude of the field effect mobility in a particular transistor channel depends on the specific surface of the organic crystal. When the charge transport is dominant within a two-dimensional molecular layer and less efficient between molecular layers, the angular resolution anisotropic mobility within a molecular layer can be predicted by the following formula: 73

µϕ =

e ∑ 2 d ki Pi cos2 (θi − ϕ) 2kB T i i 23

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(8)


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where Pi cos2 (θi − ϕ) is the relative hopping probability of various transport pathways to the specific transistor channel. As exampled in Figure 10(a) for 3-2, ϕ is the orientation angle of the conducting channel relative to the crystallographic axis a and θi are the angles of different pathways relative to the reference axis. θi − ϕ is the angles between the different pathways and the conducting channel. The anisotropic mobilities for electron in 3-2 are shown in Figure 10(b) We can see clearly the electron mobilities are varied with the ϕ. The electron mobility has maximum values of 1.87 cm2 V−1 s−1 for 3.2 with ϕ of 0o or 180o , which is the hopping pathways possessing the maximal transfer integrals. Thus one could control the orientation of the molecules with correlation to the transistor channel to obtain high electron mobility performance.

Figure 9: Pictorial LUMO interactions for short O· · · S and O· · · H contacts for compound 1-2, 3-2 and 4-2.

4. Conclusions In summary, the electronic and charge transport properties of oligothiophenes and their corresponding S,S-dioxides have been investigated through comparative analysis by means of DFT calculations. The effects of the sulfur oxidation are itemized as follows: • The chemical oxidation of sulfur atom in the thiophene ring could keep the planar structure of fused oligothiophene, but slightly change the reorganization energy. The 24 ACS Paragon Plus Environment

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Figure 10: (a) The angle-dependent hopping paths projecting to a transistor channel in the ab plane and (b) the calculated angle-resolved anisotropic electron mobilities (b) of 3-2. increase the extent of oxidation will decrease the reorganization energy for electron. • The introduction of sulfone group in the conjugated backbone could enhance the redox stability of oligothiophene by increasing both IP and EA. The improved electron affinity are of great benefit to lowering the energy barrier for electron injection and transporting electron. • The sulfur oxidation change the aromaticity of thiophene ring to antiaromacity and partly break down the conjugation by blocking the channel through sulfur at the location of oxidized thiophene ring. It should be noted that the overall conjugation is increased as HOMO-LUMO gap is decreased. The decrease of aromaticity in the oxidized molecular segments might be in favour of charge hopping between the adjacent molecules. • The oxidation make prominent influence on the solid state packing arrangement, which is dependent on the position and extent of oxidation. The sulfone group directly involve in the formation of LUMO and enhance the electronic coupling for the electron. • The predicted mobilities of oxidized fused oligothiophenes show that the thienyl sulfur oxidation could achieve the conversion from p-type materials to ambipolar or n-type



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materials. The calculated results would hint us that chemical oxidation of the sulfur atoms to S,Sdioxides is an effective strategy to modulate the electronic and charge transport properties of fused oligothiophene. It is also demonstrated that such oxidation of thiophene could be a potential approach to transform p-type thiophene-based semiconductor to ambipolar or n-type OFET materials.

Acknowledgement We are grateful for the financial support from the National Nature Science Foundation of China (grant nos. 21173101 and 21073077).

Supporting Information Available The following files are available free of charge. • Figure S1: Optimized structures in the neutral state of the oligothiophene derivatives. • Figure S2: Optimized structures in the neutral state of the molecules 4-3 and 4-4 together with their respective isomers. • Figure S3: Bond-length changes upon oxidation and reduction for the oligothiophenes and their corresponding S, S-dioxides. • Figure S4: Contribution of the vibrational modes to the reorganization energy for all molecules except for 1-2 and 3-2. • Figure S5: Charge hopping pathways for compounds of oligothiophenes 3-1 and 4-1. • Table S1: The calculated energies and molecular internal reorganization energies for the trans- and syn-isomers of molecules 4-3 and 4-4. 26 ACS Paragon Plus Environment

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• Table S2: Electronic couplings Vab of hole and electron for the different hopping pathways of the non-oxidized compounds 3-1 and 4-1. This material is available free of charge via the Internet at http://pubs.acs.org/.

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