Electronic Properties of Mixed-Stack Organic Charge-Transfer Crystals

Article pubs.acs.org/JPCC

Electronic Properties of Mixed-Stack Organic Charge-Transfer Crystals Lingyun Zhu,†,‡ Yuanping Yi,†,§ Alexandr Fonari,† Nathan S. Corbin,† Veaceslav Coropceanu,*,† and Jean-Luc Brédas*,† †

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ‡ National Center for Nanoscience and Technology, Beijing, 100190, P. R. China § Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China S Supporting Information *

ABSTRACT: The electronic structures of a series of donor− acceptor mixed-stack crystals have been investigated by means of density functional theory calculations. The results highlight that a number of the donor−acceptor crystals under consideration are characterized by wide valence and conduction bands, large hole and electron electronic couplings, and as a result very low hole and electron effective masses. The fact that the effective masses and electronic couplings for holes and electrons are nearly equal along the stacking directions implies that the hole and electron mobilities in these systems are also similar. In addition, in several of these crystals, charge transport has a two-dimensional character. The impact on the charge transport properties of the electronic couplings between donor and acceptor frontier orbitals and of the related energy gaps is also discussed. Au or Ag as electrodes; mobilities as large as 1 cm2/(V s) have been found for both electrons and holes in DBTTFTCNQ.13−15 There is a strong relationship between the electrical properties and the packing motif of DA crystals. Semiconducting properties are usually displayed by mixed-stack systems in which the donor and acceptor molecules alternate along the stacking (...-D-A-D-A-...) directions. Our recent Density Functional Theory (DFT) study of the DBTTFTCNQ, DMQtT-F4TCNQ (DMQtT = dimethylquaterthiophene, F4TCNQ = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), and STB-F4TCNQ (STB = stilbene) crystals indicated that DA crystals can exhibit very small effective masses (large electronic couplings) for both holes and electrons, which suggests that these materials can display excellent ambipolar charge-transport properties.4 In that study, we pointed out that the electronic couplings along the stacking directions are superexchange in nature, i.e., the electronic coupling for holes results from the mixing of the frontier orbitals of two closest donor molecules with the orbitals of the “bridging” acceptor molecule and vice versa for electrons. These superexchange couplings, in general, depend on the

1. INTRODUCTION Organic semiconductors based on a single molecular building block such as pentacene and rubrene have been extensively used in recent years for optoelectronic applications.1 In the search for new materials with improved performance there is now an increasing interest in understanding the semiconducting properties of binary charge transfer organic crystals, where one component acts as an electron donor (D) and the other as an acceptor (A).2−10 Hasegawa and co-workers6 used (BEDTT T F ) ( F 2 T C N Q ) ( B E D T = bi s ( e t h y l e n e -d it h i o ) tetrathiafulvalene, F2TCNQ = 2,5-difluorotetracyanoquinodimethane) as an active layer for the fabrication of ambipolar single crystal field-effect transistors (FETs). Very recently, Zhang et al. developed a mixed-stack annulene-TCNQ crystal; OFET devices based on this compound exhibit ambipolar behavior with hole mobility of 0.04 cm2/(V s) and electron mobility of 0.03 cm2/(V s).5 Large hole mobilities of 0.3 cm2/ (V s) and 0.5 cm2/(V s) were also found in coronene-TCNQ11 and (coronene tetracarboxylate)-(methyl viologen)12 crystals, respectively. It was found that the performance of organic DA based devices can be enhanced when organic DA materials are also used as source-drain electrodes. For instance, organic FETs based on the DBTTF-TCNQ (DBTTF = dibenzotetrathiafulvalene, TCNQ = 7,7,8,8-tetra-cyanoquinodimethane) mixed-stack single crystals as channel and TTF-TCNQ films as source and drain electrodes overperform similar devices with © 2014 American Chemical Society

Received: March 10, 2014 Revised: June 10, 2014 Published: June 11, 2014 14150

dx.doi.org/10.1021/jp502411u | J. Phys. Chem. C 2014, 118, 14150−14156

The Journal of Physical Chemistry C

Article

1 1 ∂ 2E = 2 mij ℏ ∂kj∂ki

electronic interactions (tDA) between the donor and acceptor frontier orbitals and on the related energy gaps (CT energies, ΔEDA).4 In the present work, we extend our study to a series of mixed-stack DA crystals made of TCNQ or PMDA (PMDA= pyromellitic dianhydride) as acceptor and various donor molecules (see Figure 1). As we show below, the tDA values

(1)

where subscripts i and j denote the Cartesian coordinates in reciprocal space, E is the band energy, ℏ is the Planck constant, and k is the electron wavevector. Subsequent diagonalization of m−1 ji provides the principal components and their orientations. The inverse effective mass tensor was calculated by means of the centered difference method with dk = 0.01/bohr. The effective transfer integrals for nearest-neighbor pairs of donor or acceptor molecules (denoted as tdirect) were evaluated at the optimized crystal geometry by using a fragment orbital approach in combination with a basis set orthogonalization procedure.18 The electronic couplings between A molecules [or between D molecules] along the stacking direction are obtained using an energy-splitting approach by considering the energy levels of an A−D−A [D−A−D] triad4 t eff =

E L + 1[H] − E L[H − 1] 2

(2)

where EL[H] and EL+1[H‑1] are the energies of the LUMO and LUMO+1 [HOMO and HOMO−1] levels taken from the neutral state of the A−D−A [D−A−D] triad. These calculations were performed with the B3LYP functional and 6-31G(d,p) basis set, using the Gaussian 03 package.19

3. RESULTS AND DISCUSSION The crystal structure data of all crystals investigated here (Figure 1) are taken from the Cambridge Structural Database (the unit-cell parameters are collected in the SI). All systems are characterized by a 1:1 stoichiometry and crystallized in mixed-stack arrays. The crystals of 4T-TCNQ (4T = quaterthiophene),20 4TA-TCNQ (4TA = thieno[2″,3″:4′,5′]thieno[2′,3′-d]thieno[3,2-b]thiophene, 21 naphthaleneTCNQ,22 anthracene-PMDA,23 tetracene-TCNQ,24 tetracenePMDA, 25 chrysene-TCNQ, 26 TTN-TCNQ (TTN = tetrathieno[2,3-a:3′,2′-c:2″,3″-f:3‴,2‴-h]naphthalene),27 phenazine-TCNQ,28 DBT-TCNQ (DBT = dibenzothiophene),29 and d14-p-terphenyl-TCNQ30 belong to the triclinic space group P1̅ , while the coronene-TCNQ,11 pyrene-TCNQ,31 and 4Me-porphine-TCNQ32 crystals belong to P21/c, P21/b, and P21/n, respectively. Anthracene-TCNQ33 and BZ-TCNQ (BZ = benzidine/4,4′-diaminobiphenyl)34 crystallize in the C2/m monoclinic space group. PTZ-TCNQ (PTZ = phenothiazine)35,36 and naphthalene-PMDA37,38 can display at room temperature two crystal polymorphs; they belong to the P1̅ and C2/c space groups for the former and C2/m and P21/c space groups for the latter. The electronic band structures of BZ-TCNQ, PTZ-TCNQ, and tetracene-TCNQ crystals are displayed in Figure 2; Figure S1 collects the results for all the other systems. The estimated valence band (VB) and conduction band (CB) widths of the crystals (Table 1) are in the range of 50−690 meV and 60−860 meV, respectively. Among the investigated crystals, BZ-TCNQ, PTZ-TCNQ, 4TA-TCNQ, and tetracene-PMDA possess large (about 400−860 meV) bandwidths for both valence and conduction bands. These bandwidths are comparable to those of the DBTTF-TCNQ, DMQtT-F4TCNQ, and STB-F4TCNQ crystals we investigated earlier.4 For the sake of comparison, we note that the VB and CB bandwidths in the pentacene crystal39,40 are 610 and 590 meV, respectively, at the same level of theory. Among all investigated DA systems, BZ-TCNQ

Figure 1. Chemical structures of the donor and acceptor molecules: naphthalene, anthracene, tetracene, benzidine (BZ), chrysene, pyrene, coronene, d 14 -p-terphenyl, dibenzothiophene (DBT), thieno[2″,3″:4′,5′]thieno[2′,3′-d]thieno[3,2-b]thiophene (4TA), quaterthiophene (4T), phenazine, phenothiazine (PTZ), tetrathieno[2,3-a:3′,2′c:2″,3″-f:3‴,2‴-h]naphthalene (TTN), 5,10,15,20-tetramethyl21H,23H-porphine (4Me-porphine), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and pyromellitic dianhydride (PMDA).

in these systems range from 2 meV to about 800 meV, while the variation among the ΔEDA values amounts to about 3 eV; this paves the way for a thorough investigation of the relationship between tDA and ΔEDA and the charge transport properties.

2. METHODOLOGY Geometry optimizations of the crystal structures were performed using the B3LYP functional and the 6-31G basis set as implemented in the CRYSTAL06 package.16 During the optimization, the positions of the atoms in the unit cell were relaxed, while the cell parameters were kept fixed at the experimental values. However, due to the large number of atoms in the unit cell in the case of 4Me-porphine-TCNQ (4Me-porphine = 5,10,15,20-tetramethyl-21H,23H-porphine), the experimental geometry was considered for this system. In all other instances, the electronic band structures and density of states (DOS) were calculated using the optimized crystal structures. We found no major differences between the experimental and optimized crystal structures, see the Supporting Information (SI). The inverse effective mass tensor 17 in a three-dimensional crystal, m−1 ji , is defined as 14151



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Figure 2. B3LYP/6-31G electronic band structure and density of states (DOS) of BZ-TCNQ, PTZ-TCNQ, and tetracene-TCNQ crystals. The points of high symmetry in the first Brillouin zone are labeled as follows: Γ = (0,0,0), X = (0.5, 0, 0), Y = (0, 0.5, 0), Z = (0, 0, 0.5), U = (0, 0.5, 0.5), V = (0.5,0,0.5), T = (0.5,0.5,0), and R = (0.5, 0.5, 0.5), all in crystallographic coordinates. The zero of energy is taken as at the top of the valence band.

respectively. Our results thus indicate that all the DA systems discussed above have small effective masses for both holes and electrons, which underlines that these systems could exhibit ambipolar charge-transport properties. As seen from Table 2, BZ-TCNQ, PTZ-TCNQ, d14-p-terphenyl-TCNQ, and tetracene-PMDA have just one small effective mass component; therefore, they are expected to exhibit mainly one-dimensional (1D) charge-transport characteristics. In contrast, 4TA-TCNQ and PTZ-TCNQ have two small effective mass components for both holes and electrons; thus, these systems could display not only ambipolar characteristics but also two-dimensional (2D) charge-transport features. In the case of several other systems, a 2D transport (see Table 2) is expected either for holes or electrons. For instance, in 4T-TCNQ, anthracene-PMDA, phenazine-PMDA, and pyrene-PMDA, transport is predicted to be 2D for holes and 1D for electrons. In contrast, in the case of chrysene-TCNQ and TTN-TCNQ, transport is predicted to

displays the largest width for both VB and CB, i.e., 690 and 860 meV, respectively. In most of the systems considered here, the largest VB and CB dispersions occur along the stacking directions. The computed effective masses for both holes and electrons are collected in Table 2 (see also Table S2). The BZ-TCNQ, PTZ-TCNQ, 4TA-TCNQ, 4T-TCNQ, d14-p-terphenyl-TCNQ, and tetracene-PMDA crystals, as a consequence of their large bandwidths, have very small effective masses that are in the range of 0.24−0.86 m0 for holes and 0.22−0.82 m0 for electrons. Remarkably small effective masses for holes, 0.24 m0, and electrons, 0.22 m0, are found in BZ-TCNQ. These masses are comparable to those estimated recently for the DMQtTF4TCNQ crystal (0.20 m0 and 0.26 m0, for holes and electrons).4 For the sake of comparison, we recall that the effective masses estimated at the same level of theory for holes in pentacene41 and rubrene42 are about 1.6 m0 and 0.94 m0, 14152



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Table 1. Energy of the Charge-Transfer States (ΔEDA), Electronic Coupling between the HOMO of a Donor and the LUMO of the Adjacent Acceptor (tHD−LA), Effective Transfer Integral (tef f), and the Valence and Conduction Bandwidths of the Investigated Systems (All in eV) crystal phenazine-TCNQ naphthalene-TCNQ DBT-TCNQ d14-p-terphenylTCNQ chrysene-TCNQ pyrene-TCNQ 4TA-TCNQ anthracene-TCNQ coronene-TCNQ TTN-TCNQ tetracene-TCNQ PTZ-TCNQ (P-1) PTZ-TCNQ (C2/c) 4T-TCNQ BZ-TCNQ 4Me-porphineTCNQ phenazine-PMDA naphthalene-PMDA (C2/m) naphthalene-PMDA (P21/c) chrysene-PMDA pyrene-PMDA anthracene-PMDA tetracene-PMDA PTZ-PMDA

ΔEDA

tHD−LA

thef f

teeff

VBW

CBW

4.42 4.25 4.15 3.70

0.136 0.021 0.046 0.572

0.002 0.008 0.026 0.043

0.014 0.002 0.060 0.057

0.17 0.06 0.13 0.26

0.24 0.18 0.34 0.31

3.60 3.51 3.50 3.46 3.45 3.41 2.93 2.92 2.92 2.79 2.63 2.61

0.069 0.239 0.456 0.136 0.364 0.235 0.170 0.409 0.385 0.404 0.764 0.117

0.020 0.018 0.058 0.027 0.052 0.013 0.010 0.078 0.068 0.062 0.131 0.016

0.049 0.026 0.078 0.031 0.037 0.029 0.027 0.074 0.066 0.052 0.184 0.047

0.16 0.13 0.39 0.11 0.28 0.17 0.15 0.46 0.41 0.49 0.69 0.09

0.28 0.21 0.45 0.20 0.16 0.24 0.15 0.52 0.52 0.28 0.86 0.19

5.41 5.25

0.297 0.400

0.028 0.023

0.019 0.057

0.28 0.14

0.12 0.28

5.25

0.060

0.002

0.002

0.26

0.22

4.59 4.50 4.46 3.92 3.91

0.013 0.315 0.375 0.419 0.002

0.009 0.027 0.053 0.061 0.019

0.007 0.039 0.050 0.087 0.044

0.13 0.28 0.31 0.40 0.05

0.06 0.20 0.24 0.41 0.19

Table 2. Description of the Smallest Two Effective Mass Components for Hole (h) and Electron (e) in the Investigated Systemsa crystal

m1(h)

m2(h)

m1(e)

m2(e)

phenazine-TCNQ naphthalene-TCNQ DBT-TCNQ d14-p-terphenylTCNQ chrysene-TCNQ pyrene-TCNQ 4TA-TCNQ anthracene-TCNQ coronene-TCNQ TTN-TCNQ 4T-TCNQ tetracene-TCNQ 4Me-porphineTCNQ PTZ-TCNQ (P-1) PTZ-TCNQ (C2/c) BZ-TCNQ phenazine-PMDA naphthalene-PMDA (C2/m) naphthalene-PMDA (P21/c) chrysene-PMDA pyrene-PMDA anthracene-PMDA tetracene-PMDA PTZ-PMDA

1.48 5.62 2.84 0.86

13.91 9.84 3.35 2.21

1.07 1.21 1.07 0.82

3.26 9.54 5.11 2.56

1D(h;e) 1D(e) 1D(e) 2D(h;e)

1.39 3.39 0.77 2.91 1.19 1.22 0.36 1.18 4.12

3.97 3.89 1.15 48.47 3.81 3.67 0.95 8.28 4.52

1.11 2.38 0.73 1.75 1.79 1.37 0.55 2.02 1.68

1.73 2.44 1.21 5.29 2.43 2.21 6.51 5.19 6.83

1D(h);2D(e) 1D(e) 2D(h;e) 1D(e) 1D(h;e) 1D(h;e) 2D(h);1D(e) 1D(h;e) 1D(e)

0.40 0.50 0.24 1.67 3.00

1.58 2.38 6.53 2.00 27.4

0.45 0.51 0.22 2.93 1.10

0.91 1.09 8.94 6.38 12.54

2D(h;e) 2D(h;e) 1D(h;e) 2D(h) 1D(e)

1.46

9.36

2.04

5.56

1D(h;e)

2.53 1.64 1.15 0.77 3.49

12.07 2.34 1.71 3.07 4.77

7.14 1.44 1.12 0.65 2.00

7.75 11.39 8.91 8.15 12.98

a

transport

1D(h) 2D(h);1D(e) 2D(h);)1D(e 1D(h;e) 1D(e)

In units of m0, where m0 is the electron mass in vacuum.

basis of the ionization energy of an isolated donor and the electron affinity of an isolated acceptor (ΔEDA = E(D+1A−1) − E(D0A0)) are shown in Table 1. In the weak electronic coupling limit where tHD−LA ≪ ΔEDA, the effective transfer integrals are given by

be 2D for electrons and 1D for holes. As was found previously for DBTTF-TCNQ, DMQtT-F4TCNQ, and STB-F4TCNQ, the smallest effective mass component of most of the systems studied here is oriented along the staking (...-D-A-D-A-...) directions. However, a few systems such as tetracene-TCNQ and 4TA-TCNQ deviate from this trend. In order to gain a deeper understanding of the band-structure results, we now turn to a discussion of the electronic couplings along possible transport pathways. In our previous study,4 we found that the values of the effective (superexchange, tef f) transfer integrals along the stacking directions in DBTTFTCNQ, DMQtT-F4TCNQ, and STB-F4TCNQ are primarily defined by the electronic couplings between the HOMO of a donor and the LUMO of the adjacent acceptor (tHD−LA). Similar results are found for BZ-TCNQ, PTZ-TCNQ, 4TA-TCNQ, 4T-TCNQ, d14-p-terphenyl-TCNQ, and acene-PMDA, for which the tHD−LA value is in the range of 400−800 meV. We note that when tHD−LA is the only channel that contributes to the superexchange mechanism, the effective transfer integrals for holes and electrons are equal. In addition, in this case, mirror symmetry between the valence band (VB) and the conduction band (CB) is observed. Similar hole−electron symmetry is displayed in the band structures of BZ-TCNQ and PTZTCNQ (see Figure 2). The value of the superexchange electronic coupling also depends on the energy of the charge-transfer (CT) states, ΔEDA. The corresponding values of ΔEDA derived solely on the

theff = teeff =

t H2 D− LA ΔEDA

(3)

tefh f

tefe f ]

Beyond the weak electronic coupling limit, [or can be estimated using an energy-splitting approach (see eq 2); however, it is then necessary to consider the energy levels of an D−A−D [A−D−A] triad instead of those of a dimer as in the case of single-component crystals such as pentacene.4,41 As seen from Table 1, while a correlation between tHD−LA and tefh f [tefe f ] is evident, the effect of ΔEDA on the effective transfer integrals is less obvious. Also, it should be noted that in addition to tHD−LA, other donor and acceptor molecular orbitals can contribute to the superexchange mechanism. For instance, in the case of PTZ-PMDA, tefh f = 19 meV and tefe f = 44 meV despite the fact that tHD−LA is only 2 meV. Our calculations show that for this system the effective coupling for holes is, in fact, dominated by the interaction between the HOMO of the donor and the LUMO+1 of the acceptor (tHD−L+1A = 220 meV). In the case of electrons, there are at least three channels contributing to the superexchange mechanism; these involve, on the one hand, the LUMO of PMDA and, on the other hand, the HOMO-1, HOMO-2, and HOMO-3 of PTZ (tH−1D−LA = 220 meV, tH−2D−LA 14153



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Figure 3. Illustration of the main charge-transport pathways for holes and electrons in the tetracene-TCNQ and 4T-TCNQ crystals. The numbers in blue circles label the molecules used for the electronic coupling calculations. The red solid lines indicate the directions of the two smallest effective mass components m1 and m2.

= 100 meV, tH−3D−LA = 220 meV). A similar finding that more pathways should be considered to rationalize the effective couplings along the stacking (...-D-A-D-A-...) directions was also obtained for other systems such as DBT-TCNQ, chryseneTCNQ, and chrysene-PMDA. Large electronic couplings can occur along directions other than the stacking directions. For instance, in the case of tetracene-TCNQ (see Figure 3), the calculations show that the largest electronic couplings for electrons are along the stacking direction; however, the largest transfer integrals for holes are between molecules located on different stacks and the interaction is through space. As seen from Figure 3, in 4TTCNQ, the effective electronic coupling for holes along the stacking direction (62 meV) is comparable with that through space (48 meV); in the case of electrons, the coupling is large only along the stacking direction. This explains why there are two components of the effective mass with small values for holes and only one for electrons. Based on eq 3, one might expect that the charge-transport properties of DA systems with TCNQ as acceptor would be superior in comparison to their PMDA counterparts due to values of ΔEDA nearly 1 eV smaller. However, our calculations do not support the validity of this hypothesis. The reason is that the effect of the DA electronic couplings on the effective transfer integrals exceeds that of ΔEDA. For instance, the effective electronic couplings displayed by oligoacene-PMDA systems are much larger than in the corresponding oligoaceneTCNQ crystals. Interestingly, tHD−LA in naphthalene-PMDA, anthracene-PMDA, and tetracene-PMDA has nearly the same value. As seen from Figure 4, this is a consequence of the same overlap pattern between the LUMO of PMDA and the HOMO of naphthalene, anthracene, or tetracene. In contrast, the related overlap pattern in oligoacene-TCNQ systems is dependent on the nature of the oligoacene molecule.

Figure 4. Representation of the donor HOMO and acceptor LUMO in D−A dyads along the stacking direction (in each instance, the HOMO is on top).

4. CONCLUSIONS We have investigated the electronic structures of a series of donor−acceptor mixed-stacked crystals by means of DFT calculations. The band-structure calculations indicate large valence-band and conduction-band widths for BZ-TCNQ, PTZ-TCNQ, 4TA-TCNQ, and tetracene-PMDA. The calculations also predict very small effective masses for these systems; they are in the range of 0.2−0.8 m0 for holes and 0.2− 0.7 m0 for electrons; these values are smaller than the effective mass for holes in pentacene and rubrene and comparable with our recently published results in DMQtT-F4TCNQ, DBTTF14154



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Structure of Tetracene−Tcnq and Perylene−Tcnq Compounds. J. Phys. Chem. A 2008, 112, 2497−2502. (8) Sagade, A. A.; Rao, K. V.; Mogera, U.; George, S. J.; Datta, A.; Kulkarni, G. U. High-Mobility Field Effect Transistors Based on Supramolecular Charge Transfer Nanofibers. Adv. Mater. 2013, 25, 559−564. (9) Tsutsumi, J. Y.; Matsui, H.; Yamada, T.; Kumai, R.; Hasegawa, T. Generation and Diffusion of Photocarriers in Molecular DonorAcceptor Systems: Dependence on Charge-Transfer Gap Energy. J. Phys. Chem. C 2012, 116, 23957−23964. (10) Park, S. K.; Varghese, S.; Kim, J. H.; Yoon, S.-J.; Kwon, O. K.; An, B.-K.; Gierschner, J.; Park, S. Y. Tailor-Made Highly Luminescent and Ambipolar Transporting Organic Mixed Stacked Charge-Transfer Crystals: An Isometric Donor-Acceptor Approach. J. Am. Chem. Soc. 2013, 135, 4757−4764. (11) Chi, X.; Besnard, C.; Thorsmolle, V. K.; Butko, V. Y.; Taylor, A. J.; Siegrist, T.; Ramirez, A. P. Structure and Transport Properties of the Charge-Transfer Salt Coronene-Tcnq. Chem. Mater. 2004, 16, 5751−5755. (12) Sagade, A. A.; Rao, K. V.; George, S. J.; Datta, A.; Kulkarni, G. U. A Charge Transfer Single Crystal Field Effect Transistor Operating at Low Voltages. Chem. Commun. 2013, 49, 5847−5849. (13) Sakai, M.; Sakuma, H.; Ito, Y.; Saito, A.; Nakamura, M.; Kudo, K. Ambipolar Field-Effect Transistor Characteristics of (Bedt-Ttf)(Tcnq) Crystals and Metal-Like Conduction Induced by a Gate Electric Field. Phys. Rev. B 2007, 76, 045111−045115. (14) Takahashi, Y.; Hasegawa, T.; Abe, Y.; Tokura, Y.; Nishimura, K.; Saito, G. Tuning of Electron Injections for N-Type Organic Transistor Based on Charge-Transfer Compounds. Appl. Phys. Lett. 2005, 86, 063504−063506. (15) Takahashi, Y.; Hasegawa, T.; Abe, Y.; Tokura, Y.; Saito, G. Organic Metal Electrodes for Controlled P- and N-Type Carrier Injections in Organic Field-Effect Transistors. Appl. Phys. Lett. 2006, 88, 073504−073506. (16) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Harrison, N. M.; Doll, K.; Civalleri, B.; Bush, I. J.; D’Arco, P. et al. CRYSTAL06 User’s Manual; University of Torino, Torino, 2006. (17) Seeger, K. Semiconductor Physics: An Introduction, 9th ed.; Springer-Verlag: Berlin, 2004. (18) Valeev, E. F.; Coropceanu, V.; da Silva Filho, D. A.; Salman, S.; Brédas, J.-L. Effect of Electronic Polarization on Charge-Transport Parameters in Molecular Organic Semiconductors. J. Am. Chem. Soc. 2006, 128, 9882−9886. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C. et al. Gaussian, Inc.: Pittsburgh, PA, 2003. (20) Qian, M.; Fu, H.; Cao, Y. The Crystal Structure of Tetrathiophene Tcnq Complex. Chin. J. Struct. Chem. 1986, 5, 163− 165. (21) Mazaki, Y.; Kobayashi, K. Crystal and Molecular-Structure of Thieno[2″,3″/4′,5′]Thieno[2′,3′-D]Thieno[3,2-B]Thiophene as a Hydrogen-Poor Heterocycle. J. Chem. Soc., Perkin Trans. 2 1992, 761−764. (22) Shaanan, B.; Shmueli, U.; Rabinovich, D. Structure and Packing Arrangement of Molecular Compounds 0.7. 7,7,8,8-Tetracyanoquinodimethane-Naphthalene (1:1). Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1976, 32, 2574−2580. (23) Boeyens, J. C. A.; Herbstein, F. H. Molecular Compounds and Complexes. Iii. The Crystal Structures of the Equimolar Π-Molecular Compounds of Anthracene and Perylene with Pyromellitic Dianhydride1. J. Phys. Chem. 1965, 69, 2160−2176. (24) Buurma, A. J. C.; Jurchescu, O. D.; Shokaryev, I.; Baas, J.; Meetsma, A.; de Wijs, G. A.; de Groot, R. A.; Palstra, T. T. M. Crystal Growth, Structure, and Electronic Band Structure of Tetracene-Tcnq. J. Phys. Chem. C 2007, 111, 3486−3489. (25) Bulgarovskaya, I. V.; Zavodnik, V. E.; Vozzhennikov, V. M. Structure of the 1:1 Pi-Molecular Complex of Tetracene with 1,2−4,5-

TCNQ, and STB-F4TCNQ crystals. Overall, among the series investigated in this work, the BZ-TCNQ and PTZ-TCNQ crystals appear as the most promising materials with regard to charge-transport properties. They present large valence and conduction bandwidths and small hole and electron effective masses. Two small components of the effective masses are found for several crystals, suggesting that charge transport for both holes and electrons has a two-dimensional character. The similarity between the electronic couplings and effective masses for holes and electrons in BZ-TCNQ, PTZ-TCNQ, 4TATCNQ, tetracene-PMDA, 4T-TCNQ, and anthracene-PMDA indicate that the mobilities of holes and electrons could also be similar; therefore, these systems could be good candidates for ambipolar field-effect transistors.

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ASSOCIATED CONTENT

* Supporting Information S

Crystallographic parameters for the unit cells of the investigated crystals; hole and electron effective masses of the investigated crystals; RMSD values between the optimized geometries and the experimental crystal structures; B3LYP/6-31G-calculated electronic band structures and densities of states; illustration of the main charge-transport pathways for holes and electrons. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been funded by the National Science Foundation under Award No. DMR-1105147 and by the U.S. Army Research Laboratory and the U.S. Army Research Office under contract/grant number W911NF-13-1-0387. The computational resources have been made partly available via the CRIF Program of the NSF under Award No. CHE-0946869.

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REFERENCES

(1) Anthony, J. E. The Larger Acenes: Versatile Organic Semiconductors. Angew. Chem., Int. Ed. 2008, 47, 452−483. (2) Horiuchi, S.; Hasegawa, T.; Tokura, Y. Molecular DonorAcceptor Compounds as Prospective Organic Electronics Materials. J. Phys. Soc. Jpn. 2006, 75, 051016−051029. (3) Hasegawa, T.; Takeya, J. Organic Field-Effect Transistors Using Single Crystals. Sci. Technol. Adv. Mater. 2009, 10, 024314−024329. (4) Zhu, L. Y.; Yi, Y. P.; Li, Y.; Kim, E. G.; Coropceanu, V.; Bredas, J. L. Prediction of Remarkable Ambipolar Charge-Transport Characteristics in Organic Mixed-Stack Charge-Transfer Crystals. J. Am. Chem. Soc. 2012, 134, 2340−2347. (5) Zhang, J.; Geng, H.; Virk, T. S.; Zhao, Y.; Tan, J.; Di, C.-A.; Xu, W.; Singh, K.; Hu, W.; Shuai, Z.; et al. Sulfur-Bridged Annulene-Tcnq Co-Crystal: A Self-Assembled ‘‘Molecular Level Heterojunction’’ with Air Stable Ambipolar Charge Transport Behavior. Adv. Mater. 2012, 24, 2603−2607. (6) Hasegawa, T.; Mattenberger, K.; Takeya, J.; Batlogg, B. Ambipolar Field-Effect Carrier Injections in Organic Mott Insulators. Phys. Rev. B 2004, 69, 245115−245120. (7) Shokaryev, I.; Buurma, A. J. C.; Jurchescu, O. D.; Uijttewaal, M. A.; de Wijs, G. A.; Palstra, T. T. M.; de Groo, R. A. Electronic Band 14155



Article

Pyromellitic Dianhydride. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1987, 43, 764−766. (26) Munnoch, P. J.; Wright, J. D. Crystal-Structure of 1:1 Molecular Complex of Chrysene and 7,7,8,8-Tetracyanoquinodimethane. J. Chem. Soc., Perkin Trans. 2 1974, 13, 1397−1400. (27) Fischer, E.; Larsen, J.; Christensen, J. B.; Fourmigue, M.; Madsen, H. G.; Harrit, N. Synthesis of New Sulfur Heteroaromatics Isoelectronic with Dibenzo[G,P]Chrysene by Photocyclization of Thienyl- and Phenyl-Substituted Ethenes. J. Org. Chem. 1996, 61, 6997−7005. (28) Goldberg, I.; Shmueli, U. Structure and Packing Arrangement of Molecular Compounds. 3. (1:1) 7,7,8,8-TetracyanoquinodimethanePhenazine. Acta Crystallogr. Sect. B: Struct. Commun. 1973, B 29, 440− 449. (29) Wright, J. D.; Ahmad, Z. A. The Structure of the 1:1 MolecularComplex of Dibenzothiophene and 7,7,8,8-Tetracyanoquinodimethane. Acta Crystallogr., Sect.B: Struct. Crystallogr. Cryst. Chem. 1981, 37, 1848−1852. (30) Lisensky, G. C.; Johnson, C. K.; Levy, H. A. Neutron-Diffraction Study of 1:1 Molecular-Complex of 7,7,8,8-Tetracyanoquinodimethane with P-Terphenyl. Acta Crystallogr., Sect. B: Struct. Sci. 1976, 32, 2188−2197. (31) Prout, C. K.; Tickle, I. J.; Wright, J. D. Molecular Complexes. 16. Crystal-Structure of 1:1 Molecular Complex of Pyrene and 7,7,8,Tetracyanoquinodimethane. J. Chem. Soc., Perkin Trans. 2 1973, 48, 528−530. (32) Jose-Larong, J.; Takahashi, Y.; Inabe, T. Charge-Transfer Complexes of Meso-Substituted Porphines. Struct. Chem. 2013, 24, 113−122. (33) Williams, R. M.; Wallwork, S. C. Molecular Complexes Exhibiting Polarization Bonding. 11. Crystal and Molecular Structure of 7,7,8,8-Tetracyanoquinodimethane-Anthracene Complex. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1968, 24, 168−174. (34) Yakushi, K.; Ikemoto, I.; Kuroda, H. Crystal-Structures of Molecular Complexes between Benzidine and 7,7,8,8-Tetracyano-PQuinodimethane. 2. Benzidine-Tcnq Solvent-Free Complex. Acta Crystallogr., Sect. B: Struct. Chem. 1974, 30, 835−837. (35) Kobayashi, H. Sinusoidal Structure of 1−1 Complex of Phenothiazine and 7,7,8,8-Tetracyanoquinodimethane, Ptz-Tcnq. Acta Crystallogr.,Sect. B: Struct. Crystallogr. Cryst. Chem. 1974, 30, 1010−1017. (36) Toupet, L.; Karl, N. Nonsinusoidal Structure of the 1:1Complex of Phenothiazine and 7,7,8,8-Tetracyanoquinodimethane. Acta Crystallogr., Sect.C: Cryst. Struct. Commun. 1995, 51, 249−251. (37) Le Bars-Combe, M.; Chion, B.; Lajzerowicz-Bonneteau, J. Structure of 2 Forms of Naphthalene-Pyromellitic Dianhydride - Study of Disordered Orientation in the Orange Form. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1979, 35, 913−920. (38) Le Bars-Combe, M.; Lajzerowicz-Bonneteau, J. NaphtaleneDianhydride Complex of Pyromeltitic Acid. 1. Order-Disorder Transition and Low-Temperature Structure of the Orange Form. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1981, 37, 1707−1712. (39) de Wijs, G. A.; Mattheus, C. C.; de Groot, R. A.; Palstra, T. T. M. Anisotropy of the Mobility of Pentacene from Frustration. Synth. Met. 2003, 139, 109−114. (40) Hummer, K.; Ambrosch-Draxl, C. Electronic Properties of Oligoacenes from First Principles. Phys. Rev. B 2005, 72, 205205− 205214. (41) Coropceanu, V.; Li, Y.; Yi, Y. P.; Zhu, L. Y.; Bredas, J. L. Intrinsic Charge Transport in Single Crystals of Organic Molecular Semiconductors: A Theoretical Perspective. Mater. Res. Bull. 2013, 38, 57−64. (42) Li, Z. Q.; Podzorov, V.; Sai, N.; Martin, M. C.; Gershenson, M. E.; Di Ventra, M.; Basov, D. N. Light Quasiparticles Dominate Electronic Transport in Molecular Crystal Field-Effect Transistors. Phys. Rev. Lett. 2007, 99, 016403−016406.

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Electronic Properties of Mixed-Stack Organic Charge-Transfer Crystals

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