Article pubs.acs.org/cm
Contorted Tetrabenzocoronene Derivatives for Single Crystal Field Effect Transistors: Correlation between Packing and Mobility Someshwar Pola,† Chi-Hsien Kuo,† Wei-Tao Peng,† Md. Minarul Islam,‡ Ito Chao,*,† and Yu-Tai Tao*,†,§ †
Institute of Chemistry, and ‡Taiwan International Graduate Program, Academia Sinica, Taipei 115, Taiwan § Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *
ABSTRACT: A series of contorted tetrabenzo[a,d,j,m]coronenes (TBCs) substituted with four fluoro-, chloro-, or methyl groups at 2,7,12,17-positions were synthesized and characterized. Except for the one with methyl substituents, which exhibits a shifted π−π stacking, the rest all show cofacial π−π stacking with small parallel displacements. One-dimensional growth along the stacking direction was observed in the single crystals for all derivatives. A systematic comparison of the crystal packing and the calculated electronic coupling/ mobility with the measured field-effect mobility for single-crystal field-effect transistors shows a good correlation. KEYWORDS: tetrabenzocoronenes, single crystal field-effect transistors
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INTRODUCTION Organic field-effect transistors have attracted much attention lately for their potential applications in cost-effective flexible organic electronics.1 As a driving component in many device applications, the field-effect mobility that can be extracted from the transistor is one of the major concerns in its development. Important progress has been made in this field because of the enhancement of charge carrier mobility through the development of new organic semiconductor materials and new device structure designs involving new dielectric materials and electrode selection and/or modification.2 For organic semiconductors, many new material systems emerged besides the most common oligoacenes in recent years. Yet a clear structure/property correlation is difficult to reveal, partly because the varied molecular packing associated with structural changes and partly because the complicated morphology normally involved in thin film forms of these materials. A mere substitution in a molecule can readily change the packing motif of organic molecules due to different steric requirements and/or electrostatic interactions. The film morphology depends not only on the molecular structure, but also the substrate nature, deposition temperature/rate and etc. A direct comparison of measured mobilities between devices prepared from different materials under different conditions and/or from different laboratories may not generate much insight to the property correlation. In contrast, organic single-crystal field effect transistors (SCFETs)3 offer the opportunity to explore the intrinsic charge transport properties in organic semiconductors, since the single crystals in principle are free from extrinsic effects of grain boundaries, molecular disorder and charge traps inevitably associated with polycrystalline films. Comparison of properties obtained with single-crystal devices would help to reveal the intrinsic structure−property © 2012 American Chemical Society
correlation and aid the material design, delineate the effect of dielectric surface on charge transport4 and so on. Among the SCFETs reported, rubrene yields the highest performance with a charge carrier mobility up to 20 cm2 V−1 s−1 and exhibits the expected anisotropy depending on the direction along which the measurement is made.5 The charge transport between organic semiconductors has been shown to relate to the electronic coupling between neighboring molecules, as well as the reorganization energy involved during neutral/charged or charged/neutral state transformation.6 While minimization of the latter is preferred, a maximization of the former is desirable. It is further demonstrated by theoretical calculation that if the π-planes of two pentacene molcules are held parallel at a constant distance, the electronic coupling is the highest if the two π-faces are stacked perfectly face-to-face,7 whereas decreasing or vanishing coupling results when the two molecules shift away from the perfect cofacial arrangement, due to a smaller or nodal overlap between the highest occupied molecular orbitals (HOMOs) or the lowest unoccupied molecular orbitals (LUMOs). Although perfect face-to-face packing is implied to give higher electronic coupling, it is in general not a favored packing for aromatic compounds due to quadrupole repulsion.8 Hence herringbone packings are commonly observed for most unsubstituted linear acenes and oligothiophenes. Substitution at the long edges of pentacene perturbs the packing from herringbone fashion to either shifted π-stacking or brick-wall type packing depending on the size of the substituent, which in turn very much affects the morphology of the crystal that is grown.9 Received: April 18, 2012 Revised: June 11, 2012 Published: June 11, 2012 2566
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derivatives were measured with a photoelectron spectrometer (AC-2, Riken Keiki) in ambient conditions with a UV source. Top-contact, top-gate SCFET was prepared by placing the single crystal on a glass substrate, which was modified with a monolayer of noctadecyltrichlorosilane(OTS). Painted colloidal graphite was used as the source and drain. A thin film of parylene (∼2−5 μm thick) was deposited on top of the crystal in a homemade reactor as insulating dielectric. Finally, colloidal graphite was painted on top of the parylene film as the gate electrode. The channel length and width of devices varied depending on the length and dimensions of crystals chosen. These parameters, as well as the parylene thickness, were determined for each individual device. The electrical characteristics of the devices were measured in ambient in a dark chamber using a computercontrolled Agilent 4156C Semiconductor Parameter Analyzer. The field-effect mobility of the OFET device was calculated from currents in the saturation region according to eq 115
Organic molecules pack based on steric interaction as well as electrostatic interaction. It is a challenging task to predict a priori the packing of organic molecules (molecular engineering). Contorted polyaromatics are conceived to have a high tendency to pack face-to-face because of the potential of selfcomplementary packing.10 A number of nonplanar polycyclic aromatic hydrocarbons have been shown to exhibit cofacial stacking in single crystals.11 The nonplanar tetraalkoxysubstituted cata-hexabenzocoronenes have been shown to exhibit columnar discotic liquid crystal phase, also suggesting cofacial π-stacking.11 Fiberlike structure has been fabricated into transistors. We demonstrated recently that the nonplanar pyreno[4,5-a]coronene molecules pack exactly face-on. Transistors based on its single crystal exhibit a high field-effect mobility of 0.89 cm2/(V s).12 The 1,2,3,4,7,8,9,10-tetrabenzocoronene (TBC) is another class of contorted molecules.13 The 2,7,12,17-tetraoctyl-substituted TBC showed strong long-range ordered π-stacking in solution state by forming nanofibers. A high hole mobility of 0.61 cm2/(V s) was measured by spacecharge-limited-current method. Nevertheless, no evidence of cofacial π−π stacking was found in this compound or the unsubstituted parent compound.13 In this paper, we report the synthesis and structure characterization of a series of 2,7,12,17-tetrasubstituted TBCs (1a−d, with R = H, Cl, F, CH3, Figure 1). Single-crystal
Ids,sat = (WC i /2L)μsat (Vgs − Vth)2
(1)
where W and L are the channel width and length respectively, and Ci is the capacitance per unit area of the parylene dielectric, Vgs is the gate voltage and Vth is the threshold voltage. Computational procedure for calculating charge carrier mobility has been elaborated previously.16 Briefly, the calculations of internal reorganization energy (λ) and relative energy of conformations of 1a− d were performed with Gaussian 09 program17 at the B3LYP/631G(d,p) level of theory.7,18 The transfer integral (t) between neighboring dimer pairs in the crystal structure was calculated with ADF19 at the PW91PW91/DZP level of theory.20 With λ and t, Marcus theory for self-exchange electron transfer was used to obtain the carrier transport rate at 300 K. With the rate and the intermolecular centroid-to-centroid distance obtained from the crystal structure, carrier diffusion coefficient was deduced and used in Einstein relation to derive carrier mobility. Although the Marcus theory has certain limitations such as the neglect of nuclear tunneling effect and the nonlocal electron−phonon couplings, it has been quite useful for the purposes of qualitative analysis and molecular design.21
Figure 1. Molecular structure of 2,7,12,17-tetra-substituted tetrabenzo[a,d,j,m]coronene(1).
Scheme 1. Synthesis of Contorted Tetrabenzocoronenes structures were grown and determined for all four derivatives. Cofacial stacking with small displacements was found for all derivatives, except for 1d with R=CH3, which shows a shifted πstacking and the molecular planes are not exactly parallel to each other. Needlelike or ribbon type crystals along the πstacking direction were obtained. Charge conduction along this direction is the major and probably the only direction in a single-crystal-based FET device. A systematic comparison of charge transport properties among various substituted derivatives may be elucidating. Theoretical hole mobility along the πstacking direction was calculated based on the single crystal structures. Single-crystal field-effect transistors were fabricated. The field-effect mobility was measured and its correlation with the theoretical results is discussed.
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EXPERIMENTAL AND COMPUTATIONAL DETAILS
RESULTS AND DISCUSSION Synthesis Scheme 1 Shows the Synthetic Route to Compound 1. Synthesis of contorted coronene derivatives, such as hexabenzocoronenes (HBCs), has been demonstrated recently,22,23 starting from pentacene quinone derivatives. For the four TBC derivatives studied here, the parent compound 1a was first reported in 197224 with little characterization. Recently Wu’s group synthesized compound 1a in eight steps starting from benzophenone.13 Our procedure started from anthraqui-
The series of derivatives of 1 were synthesized in the laboratory (the synthetic details are provided in the Supporting Information). Compounds 1a, 1b, and 1c, which are sparingly soluble in common organic solvents, were characterized by mass, elemental analyses and X-ray crystallography. Compound 1d was additionally characterized by NMR. Single crystals were grown in a temperature-gradient copper tube by vapor-phase transfer method with Argon as the carrier gas.14 The X-ray diffraction was carried out on a Bruker X8APEX X-ray diffractometer with Mo Kα radiation (λ = 0.71073 Å) and the structure was solved by SHELX 97 program. The HOMO levels of the 2567
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none. After Corey-Fuchs25 reaction using triphenylphosphine and carbon tetrabromide to afford compound 2, the SuzukiMiyaura26 reaction between 2 and substituted boronic acid led to the key intermediate bisolefin 3. Oxidative dehydrogenation gave a mixture of half-cyclized products which, without isolation, was converted to fully cyclized products 1a, 1b, and 1c respectively by Scholl cyclization27 using FeCl3 as the catalyst. However, it is noted that with 1d, the last step yielded a mixture of chlorinated side products. It was found that with Cu(OTf)2 as the catalyst, the fully cyclized product was obtained in good yield. All products were purified by vacuumsublimation. Characterization. The compounds appear yellow (1a and 1d) to orange yellow (1b and 1c). Similar UV−vis absorption spectra in dichlorobenzene exhibiting multiple λmax were obtained in the 300−500 nm region (Figure 2). The p-band
Figure 3. Thermal gravimetric analyses of 1a−d.
Crystal structure analyses. All four compounds give needlelike crystals. X-ray analyses revealed a contorted geometry as expected. However, the contorted molecules have two stereoisomers. One has the two benzenoids in the right side pointing in the same direction and opposite to the two in the left side (Figure 4a). That is, the molecule possesses
Figure 2. UV−vis spectra for 1.
(π→π* transistion) in long wavelength region for 1a occurs at 426 nm. A small substituent effect on the absorption maximum can be seen, with chloro and methyl substitution giving a redshift (∼6−7 nm) and fluoro substitution giving a blue-shift (3 nm) relative to that of the parent compound. Very weak α-band (n→π*) was discernible for all four compounds between 457 to 465 nm. Among the derivatives, 1a and 1d exhibit ambient stability in that their solution UV−vis spectra did not change and no discoloration occurred after 2 days, whereas slow degradation was observed with 1b and 1c, whose UV absorption decayed to some uncharacterized species in 2 days (see the Supporting Information, Figure S1). The degradation process was nevertheless much slower compare to pentacene, which degraded completely within an hour under similar conditions (see the Suppotying Information, Figure S2). Thermal gravimetric analyses showed the series of compounds have excellent thermal stability with decomposition temperature (5−8% weight loss) at ∼460 °C and above in nitrogen atmosphere (see Figure 3). The electrochemical measurements of 1 were performed in a 1 × 10−3 M dichloroethane solution containing 0.1 M of tetran-butylammoniumhexafluorophosphate (TBAPF6) as electrolyte. While oxidation peaks were observed (see the Supporting Information, Figure S3), the onset oxidation potentials were less well-defined, probably because of the low solubility (incompletely dissolved). The HOMO energy levels for 1a−d were nevertheless measured by photoelectron spectrometer (AC2) on the solid samples to be 5.41, 5.85, 5.88, and 5.05 eV, respectively. Thus the electron-withdrawing groups lower the HOMO level and electron-donating raises the HOMO level.
Figure 4. (a) The C2 h structure observed for 1a−1c, (b) The D2 structure of 1d, (c) crystal packing structure of 1b viewing down the long molecular axis (b axis) and from the top of the molecules (a axis) respectively, (d) crystal packing of 1d viewing from the short axis (a axis) and from the top of the molecule(b axis) respectively.
a C2h symmetry and is achiral. Compounds 1a−c adopt this geometry. Compound 1d, on the other hand, has the two benzenoids in the right side pointing in the opposite direction so that the molecule has a D2 symmetry and is chiral (Figure 4b). All four compounds show monoclinic packing motif in crystal packing. Compounds 1a, 1b and 1c pack nearly face-toface with π−π stacking distances of 3.61−3.66 Å and centroid2568
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to-centroid distances of 3.72−3.77 Å (Figure 4c and Table 1). The displacements along the short molecular axis are rather Table 1. π−π Stacking Distances (dπ−π), Centroid-toCentroid Distances (dc−c), Short Molecular Axis Shift (ds), Long Molecular Axis Shift (dl) in the Crystal Structures and the Calculated Reorganization Energies (λ+), Electronic Couplings (t+), Mobilities (μ+) for Hole Transport compd 1a 1b 1c 1d
dπ−π (Å)
dc‑c (Å)
3.66 3.63 3.61
3.77 3.75 3.72 4.39
ds(Å)
dl (Å)
0.87 0.76 0.84
0.01 0.55 0.34
λ+ (meV)
t+ (meV)
μ+ (cm2/(V s))
132 154 169 136
41 51 41 20
0.58 0.68 0.36 0.18
Figure 5. Diagrams of π−π stacking for contorted TBC derivatives with (a) C2h and (b) D2 molecular symmetry. These diagrams are seeing molecules along the long molecular axis. The middle line means the molecular mean plane. The tilted solid and dotted lines represent the benzo groups in the front and in the back, respectively.
similar (0.76−0.87 Å). For the displacements along the long molecular axis (dl) of unsubstituted 1a and halogenated 1b/1c, the values are 0.01 and 0.55/0.34 Å, respectively. The methylated 1d, on the other hand, packs somewhat unparalleled and shifts more prominently along the long molecular axis (Figure 4d). The centroid-to-centroid distance increases to 4.39 Å. It is further noticed that for 1a and 1d, the unit cell contains four molecules at the four corners of a square (Figure 4d), whereas for 1b and 1c, the unit cell contains five molecules, with a molecule at the center of the square (Figure 4c). Thus the introduction of chlorine or fluorine atoms had little effect on the packing motif between layers, except making small shifts along the long molecular axis. Yet the presence of chlorine or fluorine atoms does change the packing arrangement within a layer. In a five-molecule-per-cell arrangement, the halogen−halogen interactions or the C−X/C−X dipole repulsions present in a four-molecule-per-cell packing can be alleviated. The cell parameters are listed in Table 2. The directions of the needlelike crystals are indexed to be along the π−π stacking direction.
alleviates all the steric interactions. When substituted with larger atoms such as fluorine or chlorine atoms, shifting slightly along the long molecular axis is also needed (dl = 0.34 and 0.55 Å, respectively). Thus with a slightly less favored conformation, these molecules can achieve near cofacial packing. However, methyl group is probably too big for 1d to pack effectively in a cofacial manner, so it adopts different packing with the more stable conformation. It is noted that a D2 structure cannot shift along the short axis as depicted in Figure 5a. In D2 structure, the benzo groups on each side are pointing to different directions, so that moving along a direction alleviates the congestion of a pair of benzo groups, and yet aggravates the steric interaction of another pair (Figure 5b). SCFET Device Property and Theoretical Calculation. Crystallites of 1a−d were carefully examined under microscope for their integrity and chosen for device fabrication. The single crystal field-effect transistors were prepared in otherwise the same fashion for all four compounds, which exhibited typical ptype behavior. Three to twelve samples were prepared and measured. The measured device characteristics are shown in Table 3. The averaged carrier mobilities are of similar
Table 2. Cell Parameters of Single Crystals of 1 a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) cell density (g/cm3) point group
1a
1b
1c
1d
16.25 3.77 18.72 90 106.55 90 1.35 P21/n
3.75 14.39 23.72 90 91.06 90 1.42 P21/c
3.72 11.31 27.72 90 92.67 90 1.32 P21/c
24.65 7.33 29.87 90 101.70 90 1.40 C2/c
Table 3. Device Characteristics of Single-Crystal Field-Effect Transistors Based on 1
Theoretical calculation shows that the C 2h and D 2 conformations differ by ∼1 kcal in energy, with D 2 conformation as the more stable one for all four derivatives. The interchange of the two involves a barrier of 9−10 kcal, which suggests the interchange may proceed even at room temperature. The reason that 1a−1c and 1d adopt different molecular symmetry may be understood from the packing. With C2h structure (1a−1c), a slightly shifted cofacial packing can be obtained. As shown in Figure 5a, the upward-pointing benzo groups of the bottom C2h molecule are having close contacts with the downward-pointing benzo groups of the top molecule. This congestion problem can be eased if the bottom molecule shifts to the right (i.e., shifts along the short molecular axis). For the unsubstituted 1a, shifting along the short axis
compd
mobility μ (cm2 V−1 s−1)
averaged mobility μavg (cm2 V−1 s−1)
1a
0.271−0.501
0.401 ± 0.118
1b
0.385−0.702
0.564 ± 0.162
1c
0.045−0.165
0.100 ± 0.0399
1d
0.123−0.188
0.155 ± 0.032
on/off ratio 1 × 102 to 1 × 105 1 × 103 to 1 × 105 1 × 103 to 1 × 105 1 × 102 to 1 × 104
thresh. voltage Vth (V) −27 to −36 −15 to −19 −30 to −41 −1 to −29
magnitude, but 1a and 1b (0.401−0.564 cm2 V−1 s−1) are noticeably better than that of 1c and 1d (0.100−0.155 cm2 V−1 s−1). The typical output and transfer characteristics for compound 1a are shown in Figure 6. Theoretical calculations have brought insights to experimental SCFET results and been used to screen potential organic compounds.21a,28 Similar to Sokolov’s recent effort, in this study we estimate carrier mobilities on the basis of Marcus theory of charge transfer rate. The calculated mobilities are all 2569
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prohibited by the size of substituent. Compounds 1a−c exhibit face-to-face π−π stacking and grow into needle-like singlecrystals. A shifted π-stacking in 1d also leads to ribbonlike single crystal. The small structural perturbation resulted from the substitution lead to variation in the transfer integral as well as reorganization energy and thus the mobility, which seems to be in agreement with the experimental observation. Among these, the SCFET with chlorinated 1b as channel material and parylene as gate insulator gave hole mobility as high as 0.7 cm2 V−1 s−1.
Figure 6. (a) Output and (b) transfer characteristics of 1b SCFETs.
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in the same order of magnitude (Table 1). As in the experimental observation, 1a and 1b have mobilities larger than 1c and 1d. For compounds with the same packing motif (1a−1c), the mobility order of 1b > 1a > 1c is found both experimentally and theoretically. Comparing the calculated reorganization energies, transfer integrals and mobilities to the device performances, one can rationalize the observed trend of mobilities for these compounds and achieve some insight of the structure−property relationship that may be beneficial for future design of high performance organic semiconductors. First, the smaller mobility of 1d can be readily attributed to the large shift stack in the crystal structure and hence the smallest transfer integral of these compounds in the charge transport direction. Second, compounds 1a−1c possess the similar value of transfer integrals, but 1c has the largest reorganization energy which suppresses the efficient charge transfer. Chao et al. had demonstrated that fluorination and chlorination would have an impact on the reorganization energy, which originated from the antibonding nature of the C-X bond in the frontier orbital.29 They also demonstrated that chlorination has a smaller effect on the reorganization energy than fluorination.27b This statement holds in this study. Third, although the reorganization energy of 1b is 22 meV larger than 1a, the slightly higher mobility of 1b can be rationalized by the larger transfer integral due to the smaller short axis shift (0.76 vs 0.87 Å). By examining the shape of HOMO, one will realize that the shift along the short axis will have a more significant impact on the transfer integral than the long axis shift for TBC derivatives (Figure 7), as the wave function has more nodal planes along
ASSOCIATED CONTENT
S Supporting Information *
Details of synthesis procedure, UV−vis spectra, and CV data are provided.This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Author Contributions
S.P. did materials syntheses and characterizations, C.H.K. and M.M.I. grew single crystals and fabricated devices, W.T.P. did the theoretical calculation, Y.T.T. and I.C. cowrote the paper. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the National Science Council, Taiwan, Republic of China (Grant NSC-98-2119-M-001-007-MY3 and NSC-100-2113-M-001-005-MY3) and Academia Sinica for the financial support. They also thank the National Center for High-Performance Computing and the Computing Center of Academia Sinica for providing computational resources.
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REFERENCES
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Figure 7. HOMO of TBC.
the short axis than along the long axis. In fact, when holding a dimer of 1a in the π−π distance found in the crystal structure and keep the short axis displacements as 0.4 and 0.0 Å, the calculated transfer integral surges to 148 and 201 meV, respectively. These values provide a surge of 9 to 16 folds of the current mobility if all other parameters are kept the same. Therefore, further minimization of the displacement is desirable. In conclusion, a series of nonplanar fused aromatic compounds, namely tetrabenzo[a,d,j,m]coronenes were prepared. The molecules tend to pack in a cofacial fashion unless 2570
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dx.doi.org/10.1021/cm301190c | Chem. Mater. 2012, 24, 2566−2571
