Research on charge-transport properties of TTF–TTP derivatives and organic interfaces

Abstract

The electronic and charge transport properties of four derivative groups containing nineteen different tetrathiafulvalene (TTF) and tetrathiapentalene (TTP) derivative compounds were theoretically investigated by density functional theory (DFT) based on the Marcus–Hush theory. In particular, we have considered TTF or TTP systems where some hydrogen atoms are substituted with methoxy or halogen moieties since it is well known that these electron-withdrawing groups are used to reduce the energies of frontier orbitals (HOMO and LUMO) and to increase the stability of the TTF derivative. A comparative analysis of the crystal structures reveals that the face-to-face layer structure and the herringbone structure with slip-stacks of the dimers exhibit higher charge transfer values compared with the disordered structures, and that the face-to-face π–π interaction and S–S interactions are favorable for molecular stacking and charge transport behavior. The computed values of the transfer integrals show that variation in calculated transfer integral values is likely along specific directions corresponding to the π–π stacking degree of the molecules. The interface charge conduction mechanism between two different molecule crystals is investigated, and we find there are high-conducting interfaces and low resistances of about 10–50 kΩ. Furthermore, the calculated data demonstrate that the TTF derivatives should be candidates for high-performance organic materials with high mobility values and good stability, and that the predicted highest hole and electron mobility values are 1.821 cm² V⁻¹ s⁻¹ and 1.709 cm² V⁻¹ s⁻¹, respectively, at 300 K. The high mobility combined with simple processing make TTF derivatives promising candidate materials for electronic devices where low cost and flexibility are required.

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Introduction

In recent years, organic field-effect transistors (OFETs) have attracted a lot of attention due to their improved electronic mobility and unique characteristics which are of potential application to electronics.^1–6 Over the past 20 years, a variety of organic semiconductors have been reported in use in OFETs, such as acene, pentacene, fullerenes, and thiophenes.^7–11 Although the performance of OFETs has been improved with the development of device fabrication techniques, there are still many problems such as low stability and processability. The surfaces of different organic molecular-based materials have rich electronic properties, which achieve great conductivity and low resistance. The development of new organic semiconductors with high stability, low-cost circuits and high charge carrier mobility is thus particularly important for the development of OFETs.

As Coffen and Hunnig point out in their papers, one of the most important organic molecules in charge-transfer (CT) semiconductors, tetrathiafulvalene (TTF), exhibited excellent field-effect transistor performances in single crystals in 1971.^12–16 Charge carrier mobility can be used as a measure of the quality of organic semiconductor molecules as it as an important parameter determining the charge transfer ability of OFETs. The highest mobility is to be found in single crystals which permit the best overlap of the π–π orbitals in molecular ordering. On the one hand, the mobility values of TTF derivatives have been reported to be as high as 1–11 cm² V⁻¹ s⁻¹ (hexamethylene-tetrathiafulvalene (HM-TTF): μ_max = 11 cm² V⁻¹ s⁻¹, dibenzo-tetrathiafulvalene (DB-TTF): μ_max = 1 cm² V⁻¹ s⁻¹, and dithiophene-tetrathiafulvalene (DT-TTF): μ_max = 1.4 cm² V⁻¹ s⁻¹), which should be a great advantage in producing potential high-performance materials.^17–19 As we all know, the higher the solubility of the organic molecules and the simpler the room-temperature techniques, the more advantageous it is for low-cost processing of electronic devices. On the other hand, most TTF crystals were synthesised in the vapor phase, which will make the TTF derivatives advantageous in applications to low-cost electronics. Used as building blocks for OFETs, molecular machines, electrochemical sensors and solar cells, TTF and its derivatives can be produced as components of organic conductors, superconductors, and electronic devices. It has been shown that TTF derivatives are promising materials in the field of OFETs owing to their outstanding device performance and good processability.

The driving force behind electronic transfer is the intermolecular interaction between the π–π stacking layers responsible for their transport properties in the molecular crystallization. The TTF molecule (Fig. 1) incorporates two sulfur atoms into the dithiole rings, which will greatly enhance the S–S interaction and the intermolecular interaction. Naraso and his experimental group combined different fused aromatic rings, benzene rings, and electron-deficient nitrogen heterocycles with the TTF skeleton to produce one promising organic conductor molecule family with high performance.^20,21 This study raised new questions about whether additional structural modifications in the design of the TTF skeleton could greatly improve the CT performance to meet the demands of practical application. It is thus important to gain a thorough and systematic theoretical understanding of the fundamental electronic properties of organic materials from first-principles calculation, rather than relying only on laboratory equipment to predict the mobility values.

Fig. 1 Molecular structure of TTF.

Herein, we have carried out a systematic theoretical study of the dependency on the crystal structure of the charge transport properties of OFETs based on TTF-derivative single crystals. The electronic, geometrical and optical properties of the organic crystals of the systems here considered were studied in the framework of a quantum chemical approach based on the density functional theory. First, the solid-state structures of all TTFs derivatives were studied in order to assess the preferential directions for charge transport packing. At the same time, the charge transfer couplings between neighboring molecules in single crystals were calculated. Then we discuss and analyze frontier molecular orbital energies (HOMO and LUMO), reorganization energies (λ_hole and λ_ele) ionization potentials (IP) and electron affinities (EA) for groups of the compounds. Results of simulated absorption spectra in carbon tetrachloride were compared for all compounds. At the end of the article, the crystalline anisotropic mobility is calculated by Marcus theory and the Einstein equation. We also develop a system at the interface between two crystals of organic molecules to investigate the electronic transport properties at the interface. The resistance of two different typical molecules at the interface is predicted ab initio. The comparison of similar molecule crystals for the same group with different packing allows for an investigation of the influence of S–S intermolecular interactions and different substituents on the charge transport properties. Our theoretical study will provide much essential information to improve the charge transport efficiency of organic semiconductor materials and to find more promising materials for application.

Theory and computational details

The theoretical research into the physical properties of seventeen TTF derivatives and two tetrathiapentalene (TTP) semiconductor materials in this article is based on molecular orbital theory, first-principles and semiconductor CT theory.^22–24 A similar theoretical model approach was successfully applied to an investigation of the CT properties of organic semiconductors. As far as anyone is aware, the CT process occurs via hopping between adjacent molecules in materials. The CT rate constant (W) can be described by the Marcus–Hush equation:^25,26
where λ is the reorganization energy, k_B is the Boltzmann constant, V is the effective charge transfer coupling between neighboring molecules in the organic single crystal, T is the temperature, and ħ is the Planck constant. The formula shows that the values of λ and V are the two most important parameters in determining the CT rate W at room temperature (300 K). To achieve a higher value of W, the λ should be as small as possible, while the V needs to be as large as possible. After calculating the value of the W, the charge carrier mobility, μ, can be expressed with the Einstein–Smolechowski relation:²⁷
where D, the isotropic diffusion constant, is simulated by summing over possible hopping pathways and related to W by a random walk technique. Any one molecule can be regarded as a reference structure or initial CT center in the symmetric crystal structure. In the semiconductors, the conductive ratio is influenced by both the carrier concentration and by transferral. The drift directions of electrons and holes are both the same as the current density, and the current density J = J_e + j_q = (eC₁μ₁ + qC₂μ₂)|E|, where C₁ and C₂ are the electron and hole densities per unit square, and μ₁ and μ₂ are the electron and hole mobilities of the molecules. In a weak electric field, the conductivity σ = eC₁μ₁ + qC₂μ₂.^28–30 As we all know, the resistance and conductivity are reciprocal, so the resistance of interfaces between two different materials can be calculated based on the charge mobility values:
where e and q are equal to 1.6 × 10⁻¹⁹ C, σ is the material interface conductivity, μ are the electron and hole mobilities of the two different molecules surfaces and C are the densities at the interface of the two molecules. The electrons and holes are confined at the interface of the two layers, and the charge density follows a homogeneous distribution model. We can obtain the density per unit plane molecules based on the following equation , where Q is the charge transfer per molecule and S is the area per unit plane. The charge transfer at the interface of the molecular complexes crystal is analyzed by a molecular orbital approach in the ADF program.³¹

It is well known that the density functional theory (DFT) and GW approximation (GWA) are accurate in calculating the molecular electronic properties of organic semiconductors.^32–38 A recent study shows that the B3LYP density function of DFT combined with the 6-31g(d) basis set could be used to calculate accurately the energy values for studying TTF-derivative compounds. In order to provide accurate calculation results to measure the energy-related properties, the isolated molecules in the neutral, cationic and anionic states are optimized by the hybrid B3LYP function combined with the 6-31g(d) basis set. After the dimer pairs have been selected from the X-ray crystal system, the transfer integrals are calculated by the QZ4P basis set with PW91 gradient corrections in the ADF program.³⁹ In the article, all the quantum chemistry calculations were performed with the Gaussian 09 package.⁴⁰

We studied seventeen TTF derivatives and two TTP derivatives, which are shown in Fig. 2–5. This family of compounds can be classified into four different groups according to the different molecular structure and crystal packing mode and each group contains related TTF derivatives. The first group of symmetric TTF derivatives is based on hexamethylene-tetrathiafulvalene (1 HMTTF) single crystals.⁴¹ This classification contains three alkyl-substituted HMTTF derivatives (4–6)⁴² and two derivatives ((thiophene)(thiodimethylene)-TTF (2 TTDM-TTF) and (ethylenethio)(thiodimethylene)-TTF (3 ETTDM-TTF))¹⁹ with similar structures to HM-TTF. Six related TTF derivatives were chosen as the second group: the dinaphtho TTF derivative 7,²¹ the tetrahalogeno TTF derivatives 8 and 9,²⁰ the [2,2′]bi[naphtha[2,3-b]thiophenyl] (BNT) 10,⁴³ dibenzotetrathiafulvalene (DBTTF) bisimides 11,¹⁸ and tetramethoxy substituted DBTTF 12,⁴⁴ because of their symmetric six-membered ring-like structures on both sides of TTF. A new group of TTF derivatives which contain dimethoxycarbonyl and phthalimidyl groups (13–17)⁴⁵ belong to the third crystal structure group. The last group consists of two tetrathiapentalene (TTP) derivatives (18, 19).⁴⁶

Fig. 2 Molecular structures of TTF derivatives 1–6.

Fig. 3 Molecular structures of TTF derivatives 7–12.

Fig. 4 Molecular structures of TTF derivatives 13–17.

Fig. 5 Molecular structures of TTP derivatives 18–19.

Results and discussion

Single-crystal structure and transfer integral analysis

HMTTF (1 in Fig. 2) is a potential high-performance organic material, in which a mobility of 11 cm² V⁻¹ s⁻¹ was reported in the single-crystal transistor. Two similar molecules, TTDM-TTF and ETTDM-TTF, were chosen to compare the effect of structural symmetry on the charge carrier property. In previous cases, various alkyl substitutions were always incorporated with primitive molecules in order to improve the molecular packing, device stability and solubility.⁴⁷ Three different alkyl-substituted HMTTF derivatives are compared here (4–6).

The important geometric parameters of molecular type 1 are listed in Fig. 6 d(S–S) and d(S¹–S) are the intramolecular and intermolecular S–S shortest distances, and d(min) and d(cen) are the shortest intermolecular distance and the shortest distance between two centers of mass, respectively. The C–C and S–S chain distance values are around 1.35 Å and 3.24 Å.

Fig. 6 Molecular structures, face-to-face π-stacking, and important geometric parameters for the TTF derivatives of compound 1.

The crystal structures of compounds 3 and 6, which are two representative molecular structures from the group 1–6, are shown in Fig. 7. The important geometric parameters and the maximum calculated hole and electron transfer integrals for group 1–6 are listed in Table S1 (ESI†). The C–C and S–S chain distance values are around 1.35 Å and 3.23 Å. The interplanar shortest distance (d(min)) for the interdimer of compound 1 is abnormally large at 4.15 Å compared with the d(min) values for compounds 2–6 (3.72 Å ≤ d(min) ≤ 3.82 Å). In addition, the minimum distance of the center of the nearest molecular mass of compound 1 is much larger than for the other compounds (4.11 Å ≤ d(cen) ≤ 6.32 Å). All pathways may affect intermolecular CT in the single-crystal structure, as indicated by arrows in Fig. 6. The maximum transfer values, V, for each crystal can be positioned on the basis of the strongest electronic coupling from all possible neighbors. It can be seen that the closest face-to-face transport pathway shows the largest integral (V) values, where the hole V_hole values are 107.64, 64.46, 80.46, 75.52, 75.36 and 49.99 meV for group 1–6, while the electron V_ele values are 11.19, 24.81, 26.69, 92.99, 33.25, 33.25 meV, respectively. The transfer integral, V_hole, of compound 1 is significantly larger than that of compounds 2–6. The ETTDM-TTF structure shows a better charge carrier transfer than the symmetric molecule TTDM-TTF and the dissymmetric molecule ETTDM-TTF. These calculations indicate that the method of molecular packing is greatly influenced by the different alkyl substituents, largely because variation in the molecular structures is sensitive to the alkyl groups. The center-to-mass and minimum interplanar distances along the stacking direction are relatively small, which to a certain extent strengthens the stacking interaction. Besides, the alkyl substitution realizes uniform interactions in all directions. Consequently, there is a simple two-dimensional network, and the main-chain interaction makes the transverse intermolecular interactions more balanced. The close side-to-side and face-to-face molecular packings contribute to the performance of the single crystal. Intermolecular side-to-side packing corresponds to strong π–π interactions with a close S¹–S distance of about 4 Å along the long axis. Moreover, the supramolecular organization consists of head-to-tail dimers sustained by C–H⋯C and C–H⋯S intermolecular hydrogen bonds.

Fig. 7 Molecular structures, face-to-face π-stacking, and predicted charge hopping pathways projection along the long axis for the TTF derivatives of compounds 3 and 6.

Recently, a new series of high-performance organic semiconductors has been synthesized by introducing different fused aromatic rings into the TTF skeleton, which is a π-conjugated electron-donating structure. This molecule group can be achieved by introducing a six-membered ring-like structure into the TTF skeleton, such as fused benzene rings or nitrogen heterocycles. One advantage of the six-membered ring-like structure is that these compounds are expected to be highly stable in air, because electron-deficient heterocycles and electron-rich groups can be directly fused into the TTF skeleton, such as chlorine-family substitution (in compounds 8, 9) or methoxy groups (compounds 11, 12). Another factor in favor of fused aromatic rings is that they can lead to larger intermolecular π–π interactions and increasing transfer integrals between molecules, which are very important for achieving higher carrier mobility.

The crystal and molecular structures of TTF derivatives 7 and 9, which are two representative molecular structures in group 7–12, are summarized in Fig. 8. The important geometric parameters are shown in Table S2 (ESI†). The dinaphtho TTF derivative 7, BDT derivative 10 and tetramethoxy substituted DBTTF 12 have a herringbone packing structure. Tetrahalogeno TTF derivatives 8, 9 and DBTTF bisimides 11 have chair-like molecular configurations with face-to-face π-stacking, especially the completely planar structure and standard face-to-face π-stacking in molecules of 8 and 9. It can be seen that the introduction of nitrogen heterocycles (such as a pyrazine ring) into the molecular backbone might induce face-to-face π-stacking which can enhance intermolecular CT interactions for compounds 8, 9, and 11. The molecules are stacked with the electron-withdrawing substituents F and Cl in the electron-donating TTF in the two-dimensional stacking-layer crystal structure of compounds 8 and 9. The intermolecular shortest S¹–S contact between two closest sheet-like networks is observed to be 3.86 Å for compound 8. The interplanar minimum distance in compound 9 of 3.45 Å is shorter than the 3.74 Å in compound 8, and a larger intermolecular S¹–S distance of 7.41 Å between molecules is observed in compound 9. In group 7–12, the shortest intermolecular S¹–S distance is 3.75 Å for compound 10, the minimum interplanar distance d(min) is about 3.45 Å and the shortest intermolecular cen-to-cen distance d(cen) is 3.85 Å between neighboring molecules for compound 7, as shown in Table 2.† In contrast, the shortest intramolecular C=C bond and S–S distance d(S–S) are 1.33 Å and 3.19 Å, respectively. Molecule 10 has a planar geometry with centrosymmetry based on the central bithiophene rings, with a shortest intermolecular S¹–S contact of 3.75 Å between major and nearby conformers. These crystal structure analyses suggest that compound BNT might have a large carrier mobility because of its high dimensionality in CT transportation. The molecule structure of compound 11 includes two phenyl rings sandwiched between an imide ring and a 1,3-dithiole ring, with minimum cen-to-cen d(cen) distances of 4.62 Å, and shortest interplanar distance d(min) of 3.58 Å. Interestingly, there is a chair-like configuration from the two terminal butyl chains out of the backbone. The shorter side-by-side S¹–S contacts and face-to-face method of molecule stacking will benefit CT performance and increase the effective dimensionality of the electronic structure. The molecule for compound 12 has a distinctly planar structure with adjacent molecules forming a herringbone packing shape in an orthogonal manner in the crystal structure. There are short center-to-center contacts of 6.09 Å, and the shortest S¹–S intermolecular distance is 3.76 Å. On the one hand, the DBTTF bisimides might have many different electron properties because the electronically active TTF core is different from that of known bisimides. On the other hand, a diverse series of DBTTF bisimide derivatives with imide rings for different functionalities are similar to known bisimides. Moreover, a large dipole–dipole interaction, imparted by rigid donor–acceptor (D–A) ensembles, will enhance intermolecular π–π interactions and increase transfer integrals between neighboring molecules.

Fig. 8 Molecular structures, face-to-face π-stacking, and predicted charge hopping pathways projection along the long axis for TTF derivatives 7 and 9.

A different family of TTF derivatives (13–17) has also been studied here. Carbomethoxy groups and different electron-withdrawing groups were introduced into the TTF core to improve solubility and to increase electron affinity. The conjugated system structures of compounds 14–17 are very similar, containing one C=C bond and four S atoms in the center, with one COOMe substituent on the side. The molecular and crystal structures for compounds 13 and 15, which are two representative molecular structures from group 13–17, are shown in Fig. 9. The benzo moiety is coplanar with the four S atoms in the TTF backbone. The methyl ester groups are twisted to produce essentially perpendicular and asymmetric molecule structures with the shortest intramolecular C=C distance of 1.327 Å and S–S distance of 3.171 Å for all compounds 12–17. In these compounds, two neighboring molecules form dimers and interact via intermolecular π–π interactions combined with S¹–S interactions. The closest interdimer π–π distances d(min) range from 3.609 to 6.355 Å and the closest S¹–S distance d(S¹–S) range from 3.784 to 7.934 Å for compounds 12–17 in which the TTF backbones are approximately parallel. All compounds form head-to-tail dimers with the nearest neighbor molecules in the adjacent stacks. At the same time, the molecules stack to form a herringbone pattern, interacting with other molecules through weak C–H–O hydrogen-bonding between two ester moieties in compounds 13–17. Hydrogen-bond interactions are found in compounds 14 and 17, especially between F–H, the imide CO groups and the terminal CH₃ groups. The ester moieties are considered to be hydrogen-bond acceptors, and the methyl is used as a donor of the carbomethoxy groups in the molecules.

Fig. 9 Molecular structures, face-to-face π-stacking, and predicted charge hopping pathways projection along the long axis for TTF derivatives 13 and 15.

In our previous research, TTP derivatives were found to have better performance than TTP compounds. Single-crystal structural analyses of TTP derivatives 18, 19 are shown in Fig. 10 to investigate the relationship between molecular structure and intermolecular interaction. The molecular crystal structures of compounds 18 and 19 are different, with herringbone for 18 and planar layer for 19. Molecules 18 and 19 have face-to-face π-stacking with interplanar shortest distances d(min) of 4.52 and 3.628 Å. Molecules of compound 18 are observed to be considerably tilted from the stacking axis on the molecule crystal side, and this is different from the parallel structure of compound 19. As we all know, the charge transfer integral, V, depends to some extent on the degree of intermolecular overlap. The largest V_hole value (43.4 meV) of compound 18 is found to be smaller than that of compound 19 (99.22 meV). So compound 18 might have a relatively poorer transistor performance than compound 19 because of its tilted stacking structure, smaller CT integral and anisotropic S–S contacts.

Fig. 10 Molecular structures, face-to-face π-stacking, and predicted charge hopping pathways projection along the long axis for the TTP derivatives 18, 19.

Frontier molecular orbital energies (HOMO and LUMO), reorganization energies (λ_hole and λ_ele) ionization potentials (IP) and electron affinities (EA)

The calculated energy parameter values of frontier molecular orbital energies, reorganization energies, ionization potentials and electron affinities for the TTF derivatives based on B3LYP/6-31 G(d) are shown in Table S5 (ESI†). The appropriate frontier orbital is crucial for the efficiency of intermolecular charge transfer, in which the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) correspond to hole transport and electron transport. The frontier orbital characteristics and reorganization energies are of great importance to an understanding of CT efficiency, so we have chosen the reorganization energy, IP, EA, and the energy of frontier molecular orbitals (HOMOs and LUMOs) as well as the energy gaps to discuss the CT processes of all the compounds.

In the charge carrier transport process, an increase in HOMO and a decrease in LUMO energy values can make the charge transfer more stable, and increases in EA levels can reduce the sensitivity of mobile electrons. The injection ability of holes and electrons is greatly improved with increased HOMO energies and decreased LUMO energies. The high HOMO energies suggest that the molecules are probably effective in hole injection for hole-transport. Similarly, the low LUMO energies indicate that the molecules might be effective n-type semiconductors for electron transport due to a large energy barrier for electron injection from the electrode to the organic layer. We can regulate the H–L energy levels to enhance stability. To our knowledge, the energy values of IP and EA are important parameters in determining CT performance because these energy values are closely related to the capacity to inject empty orbitals and whether the energies exceed the energy barrier for charge transition. The suitable HOMO range and LUMO range are −3 to −4 eV and −5.5 to −4.9 eV for good n-type and p-type organic semiconductors. As shown in Fig. 11, the HOMOs are localized in a similar position to that of the molecular skeleton TTF core in all of the investigated derivatives 1–17. The LUMOs are mainly localized in different positions of the electron-withdrawing groups, such as the bisimide rings, the halogenated groups, the COOMe and the phthalimide groups. In the first group, it is observed that compounds 2 and 3 have relatively lower-lying HOMO levels with −4.81 and −4.68 eV and LUMO levels being increased to −1.24 and −1.13 eV with two S atoms in different molecule skeletons. However, the introduction of bulky alkyl groups of different lengths for compounds 4–6 has slightly changed the similar molecular orbital energies (4.25 eV for HOMO and 0.68 eV for LUMO). There are unsymmetrical structures for the introduction of benzene and pyrazine rings on either side of the TTF core for the second group. As is known to all, the inclusion of electron-withdrawing groups can decrease the HOMO and LUMO levels and increase the stability of organic devices in which the molecules are connected to the TTF core. The incorporation of halogen (F and Cl) groups obviously lowers the HOMO and LUMO levels for compounds 7 and 8. It is worth noting that the HOMO levels for compounds 8–11 are lower than the −5 eV which is usual for p-type organic semiconductors. For the third group of compounds, the energies of the HOMO levels are in the range of −5.01 to −5.47 eV, and the energies of the LUMOs vary from −1.93 to −2.65 eV. These results show that the unilateral imide decreases the values of both HOMO and LUMO for compounds 14–17 compared with −5.01 eV and −1.93 eV for HOMO and LUMO in compound 13. At the same time, the substitution of fluorinated groups reduces the frontier orbital energies by a few points. The HOMO (−4.62 and −4.74 eV) and LUMO (−0.50 and −0.45 eV) levels are found to be very close to those of the two TTP derivatives 18 and 19. H–L gaps are estimated to be 4.12 and 4.29 eV, indicating that compounds 18 and 19 should have more stable performance as transistors than preceding molecules 1–17.

Fig. 11 HOMO and LUMO orbitals of compounds 1–19.

The reorganization energy is a key value to control the CT rate values in the rate expression given in the formula for calculating W. The larger the reorganization energy, the smaller the CT rate constant normally is. The semiclassical Marcus–Hush theory for CT processes is suitable for predicting charge mobility values when the intermolecular coupling V is far smaller than the calculated reorganization energy. Since the smallest calculated λ_hole (0.23 eV) and λ_ele (0.14 eV) for compound 2 are found to be much larger than the corresponding largest coupling values (64.46 meV and 24.81 meV) in various hopping pathways, this method is appropriate for TTF derivatives. Fig. S1 (ESI†) shows that most of the compounds exhibit a relatively large λ_ele value compared with λ_hole, especially for compounds 10 (λ_hole = 0.18 eV and λ_ele = 0.63 eV) and 17 (λ_hole = 0.31 eV and λ_ele = 0.79 eV). Most of the TTF derivatives should be p-type organic rather than n-type materials based on their calculated λ values.

As can be seen in Fig. S2 (ESI†), the trend of the calculated IP and EA values is consistent with the calculated HOMO and LUMO energies in TTF-derivative organic materials. The introduction of electron-attracting substituents increases the IP and EA markedly compared with those of the parent TTF, and the displacement clearly leads the higher EA and the lower LUMO energy levels of the imide-group substitutes, with fluorination and chlorination chains in different positions on the molecules. All the TTF derivatives show high IP values and high thermal stability.

Fig. 12 shows the simulated absorption spectra in carbon tetrachloride for all TTF-derivative compounds according to a TD DFT method. The maximal predicted absorption spectrum wavelength for TTF-derivative group 1 is around 350 nm. In the second group, the introduction of –F and –Cl can cause red-shifts in the absorption wavelength (12(375 nm) < 11(400 nm) < 7(470 nm) < 8(515 nm) < 9(545 nm)). For group 3, compounds 13–17 show a similar maximum absorption wavelength in the range of 320 to 350 nm, and there is another band appearing at 410 nm for compounds 14–17 due to the electron-withdrawing property of the fluorine-substituents. In the absorption spectrum of the last group, the two TTP derivatives show similar trends for the wave crests at around 335 nm.

Fig. 12 Simulated absorption spectra in carbon tetrachloride for all compounds using a TD (time dependent) DFT method.

Simulation of mobility values

Based on the semiclassical Marcus–Hush theory, the largest predicted charge mobility for the TTF-derivative single crystals reaches as high as 1.82 cm² V⁻¹ s⁻¹ for hole transfer in compound 1 and 1.71 cm² V⁻¹ s⁻¹ for electron transfer in compound 7, as shown in Fig. 13. As expected, the calculated hole mobility for compounds 1–3, 6, 9–10, 12–15 and 18–19 is more than 10 times larger than that of electron mobility, which suggests that these compounds could function as higher efficiency p-type semiconductors rather than as n-type semiconductors. In contrast, the largest electron mobility in compounds 7–8, 5 and 16 is more than 10 times larger than hole mobility, indicating that these compounds should be more inclined to function as n-type organic semiconductors. In addition, three compounds (4, 11, 17) with similar values for electron and hole mobility, may be more suitable as bipolar materials.

Fig. 13 Predicted anisotropic curves and 3D models of hole mobility for compound 1 (left) and electron mobility for compound 7 (right) in all compounds (the mobility in cm² V⁻¹ s⁻¹).

There are rather high electron mobilities and hole mobilities exceeding 1 cm² V⁻¹ s⁻¹ in compounds 7 dinaphtho TTF and 1 HMTTF, respectively. The large π–π intermolecular interactions in compound 1 HMTTF can contribute to its excellent performance as an organic semiconductor as well as to efficient carrier injections. Although the molecular packing pathways are considerably influenced by the introduction of different alkyl groups in compounds 4–6, the mobility values increase slightly. Two different TTF derivatives 8 and 9 with halogenosubstituted quinoxaline rings show excellent p- and n-type performances with high carrier mobilities (1.09 cm² V⁻¹ s⁻¹ for the electron mobility of compound 8 and 0.414 cm² V⁻¹ s⁻¹ for the hole mobility of compound 9). DBTTF combined with different electron-withdrawing groups such as bisimides (13–17) of π-conjugated molecules will provide new opportunities in organic electronics.

We selected two molecules 1 and 19 for this research to study the electronic transport in the interface between two different crystals, because they are similar semiconductors with close mobility values and well-defined structures (Fig. 14). Compounds 1 and 19 are semiconductor crystals that are similar to many other organic materials used for the fabrication of flexible field-effect transistors (FETs) with electronic properties. The energy of the charge-transfer state has to be smaller than the energy of the system when the molecules remain neutral (where 1 stands for donor and 19 for acceptor). The energetic balance is determined by the difference in the energy of the 1 HOMO and the 19 LUMO (which are similar at the interface and in bulk). As we all know, the electrons in the HOMO of 1 are transferred to the LOMO of 19, and the H–L gap of two corresponding molecules is equal to the bandgap for the conducting materials, which is larger than 2 eV, so there is a stable charge-transfer state.^48,49 1 and 19 chains behave as decoupled, and the material should be highly conducting in one-dimensional electronic systems at room temperature. As can be seen in Fig. 15, the electron occupancy in the molecular n orbitals becomes significantly lower with the an increase in the interval between the two molecules, and the molecule reactivity and charge transfer will be reduced (Table S5 (ESI†)). Charge transfer between molecules 1 and 19 can indeed take place at one interface a–b plane, which has the highest mobility values.

Fig. 14 The charge transfer system in the molecules 1 and 19, (a) the molecules of 1 and 19, (b) a similar charge transfer device between 1 (green) and 19 (blue) crystals, (c) the 1–19 interface with the crystal a–b plane.

Fig. 15 The HOMO and LUMO of the 1–19 dimer in 0.2 nm and 1.2 nm.

The interface charge density decreases as the separation distance increases for the 1–19 dimer, and when the distance increases beyond 0.8 nm the density falls to zero as shown in Fig. 16a. The highest charge density is 2.5 × 10¹⁴ e cm⁻². At the same time, the interface electronic couplings for the dimers were calculated, and the results indicate that the charge coupling values are really small. So the electronic couplings do not make a contribution to the conduction at the interface between molecules 1 and 19. We also predict the resistance curve at the interface with a change in the distance between 1 and 19. The resistances are about 10 kΩ in 0.2 nm and 50 kΩ in 0.8 nm, and it can be concluded that the resistance will increase with decreasing separation distance at 300 K. We also predict that the resistance will decrease with increasing charge density at the interface of the compound 1 and 19 crystals. The theoretical explanation of the conduction mechanism is that the charge density (which comes from the charge transport and confinement between two crystals layers) rather than the charge coupling is responsible for the conduction process.

Fig. 16 The interface charge density and resistance of the separation distance for the 1–19 dimer.

Conclusion

A series of semiconductors based on a TTF core have been theoretically investigated at the first-principle DFT level based on Marcus–Hush theory. The high mobilities for different TTF derivatives were calculated by combining large transfer couplings and small reorganization energy values. The computation results (reorganization energies, charge transfer integrals, and frontier orbital characteristics) exhibit excellent performance, with hole mobility up to 1.821 cm² V⁻¹ s⁻¹ for compound 1 and electron mobility up to 1.709 cm² V⁻¹ s⁻¹ for compound 7. Compounds 5, 7 and 8 were found to be promising candidates for n-type organic materials. However, compounds 1, 6, 8 and 19 are more likely to function as high-performance hole-transport organic semiconductors. In addition, we systematically investigated the influence of the single-crystal CT structure on the performance of intermolecular charge transfer. The face-to-face layer structure of compound 1 and the herringbone structure of compound 7 exhibit high n-type and p-type organic performance. The packing structure combining face-to-face π–π interactions with S–S intermolecular interactions demonstrates good CT behavior. The introduction of aromatic rings and π-stacking structures into the TTF skeletons in the second group of compounds can effectively enhance the intermolecular interactions and strengthen the electron transfer property and stability of the molecules. At the same time, the incorporation of electron-withdrawing groups (such as bisimides or halogen groups) has the advantage of reducing the HOMO and LUMO energies to improve the stability of the molecules. Compounds 18 and 19 have the same TTP backbone and similar herringbone molecular packing and hole mobility, suggesting that the introduction of the outer groups has little effect on the crystal structure or CT property. The conduction mechanism and electron properties of the interface between two different organic molecules was investigated, and the values of interface density and resistance indicate that charge transfer could occur in the a–b plane between crystals of molecules 1 and 19, with a resistance of about 10–50 kΩ per square in a separation distance 0.2–0.8 nm.

This theoretical study of four groups of TTF derivatives demonstrates that most of the TTF derivatives are good p-type materials with high hole mobility and large values of molecular stability. The charge conducting interfaces show that the TTF and TTP derivatives can be applied to a new class of charge conduction devices. The systematic theoretical study of these TTF derivatives is beneficial to an understanding of the charge transport properties of organic semiconductors and design of high-performance molecular devices.

Conflict of interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with this work submitted.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07360c

‡ Project supported by Program for Innovative Research Team (in Science and Technology) in University of Henan Province (13IRTSTHN016), the National Natural Science Foundation of China (Grant no. 11274096) and the Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant no. 124200510013).

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