Electron-Rich and Electron-Deficient Heteroaromatics
Yu-Chang Chang,
†Yu-Da Chen,
‡Chih-Hsin Chen,
†Yuh-Sheng Wen,
†Jiann T. Lin,*
,†Hsing-Yin Chen,
§,†Ming-Yu Kuo,
|,†and Ito Chao*
,†Department of Chemistry and Biochemistry, National Chung Cheng UniVersity, Chia-Yi, 621, Taiwan, Institute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan
[email protected]; [email protected] ReceiVed March 10, 2008
New dipolar compounds containing alternating electron-rich thieno[3,2-b]thiophene units and electron deficient units have been synthesized. Compounds with 5-pyrimidinyl (compound 2) or benzothiazole (compound 5) as the electron-deficient unit were structurally characterized by the single-crystal X-ray diffraction method. The arrangement of the molecules is found to be one-dimensional slipped-π-stack for 2. That of 5 is of slipped-π-stack, albeit with a tilt angle between neighboring π-stacks. The π-π interfacial distances of the molecules in the crystal lattice are 3.47 and 3.59 Å for 2 and 5, respectively. On the basis of the crystal structure, compound 2, with negligibleπ-π slip along the short axis of the molecules, has a calculated electronic coupling value (0.153 eV) twice as large as that of the largest coupling of pentacene. Accordingly, the theoretically estimated hole mobility (µ+) for 2 (2.32 cm2s-1 V-1) compares favorably with that of pentacene (1.93-5.43 cm2 s-1 V-1), despite of the larger reorganization energy for hole transport in 2. The symmetric intrastack S · · · C contacts found between the thieno[3,2-b]thiophene and pyrimidinyl units explain the unique features of the crystal structure of 2 and the resulting large electronic coupling.
Introduction
There have been a growing number of efforts in the pursuit of a face-to-faceπ-π stacking array in the solid-state packing
of organic semiconductors, due to the potentially improved electrical characteristics largely promoted by the good inter-molecularπ-π interaction.1–4Herringbone packing arrangement
of the promising p-type semiconductors such as oligoacenes
constricts the realization of their potentially high carrier mobil-ity.5Recent studies demonstrate that through structural modi-fication of the commonly employed oligoacenes, for example, the herringbone packing could be forced into aπ-stacked manner
by destroying their C-H · · ·π interaction between acenes, and
may lead to better mobility.4Anthony and co-workers success-fully showed that functionalizing the bulky alkynyl groups at
†Academia Sinica.
‡National Chung Cheng University.
§Current address: Faculty of Medicinal and Applied Chemistry, Kaohsiung
Medical University, Kaohsiung 807, Taiwan,
|
Current address: Department of Applied Chemistry, National Chi Nan University, Nantou 545, Taiwan
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10.1021/jo800546j CCC: $40.752008 American Chemical Society
the 6,13 positions of pentacene leads to good face-to-faceπ-π
interaction.4a–d Kobayashi and co-workers exploited the
non-bonded chalcogen-chalcogen interaction6to force the
9,10-bis-(methylchalcogeno)anthracenes4e,f and 6,13-bis(alkylthio)pen-tacenes4gbeing crystallized into face-to-faceπ-π stacking in
the solid state. Meng, Bao, Wudl, and co-workers reported the hexathiapentacene molecules stacking face-to-face along the
a-axis facilitated by the intermolecular S · · · S chalcogen-chalcogen
interaction.4hForπ-stacked 5,11-dichlorotetracene, Bao et al.
reported a measured mobility that excelled that of tetracenes.4i
By employing a donor-acceptor strategy to promote antiparallel
π-π stacking, Swager and co-workers demonstrated that
fluorine- and alkyl/alkoxy-functionalized tetracenes confine their solid-state packing to a slipped cofacial motif.4j Similarly,
Nuckolls’ group showed that the donor-acceptor interactions between dipolar acenes afforded antiparallelπ-π stacking, and
the mobility and Ion/Ioff of its OFET device fabricated on an
OTS-treated silicon oxide were 10-2cm2s-1V-1and g10-6,
respectively.4k
Due to our interest in crystal engineering and OFET materials, we set out to synthesize compounds that have the tendency to form a face-to-face π-π stacking structure. The strategies to
obtaining face-to-faceπ-π stacked aromatic rings with tethers7
or hydrogen bonds8often suffer from unfavorable π-electron
repulsions and result in offset stacking of aromatic rings which decreases the effective overlap of theπ orbitals. In contrast, an
electron-rich aromatic ring and an electron-deficient perfluoro aromatic ring interact favorably and have a good tendency to form face-to-faceπ-π interaction.9
In this study, we strategi-cally construct bipolar molecules containing alternating electron-rich and electron-deficient moieties, aiming at favorable face-to-faceπ-π stacking for possible OFET applications. Structural
determinations on two of the compounds were conducted by the single-crystal X-ray diffraction method. Theoretical carrier mobility based on the determined structures of the two compounds also will be described.
Results and Discussion
Synthesis and characterization. The new compounds syn-thesized are shown in Figure 1. They were obtained in moderate yields by the stepwise synthetic protocol illustrated in Scheme 1. Compounds 1-4 were synthesized via palladium(0)-catalyzed Stille coupling of 2,5-bis(tributylstannyl)thieno[3,2-b]thiophene with appropriate aryl bromides. The prototype synthesis of benzothiazole moiety was followed to prepare 5:10condensation
from thieno[3,2-b]thiophene-2-carboxylic acid and 2-ami-nothiophenol in the presence of POCl3. Compound 6 was
obtained from the condensation of o-phenylenediamine with the Na2S2O5 adduct of thieno[3,2-b]thiophene-2-carbaldehyde.11
Relevant spectral data are listed in Table 1. The vibronic feature observed in both the absorption and emission spectra implies the rigidity of the compounds. These compounds are highly crystalline, and the melting temperatures measured by DSC are 397, 329, 318, 340, 205, and 281 °C for 1, 2, 3, 4, 5, and 6, respectively.
Analysis of X-ray Crystal Structures and Theoretical Calculations. The structures of 2 and 5 (Figure 2) were also confirmed by the single-crystal X-ray diffraction method. There exists an inversion center passing through the midpoint of the C3-C3′bond (ORTEP plot in Figure 2a) of the
thieno[3,2-b]thiophene unit in 2. The molecule is nearly coplanar with a
small twist angle (7.4°) between the central
thieno[3,2-b]thiophene ring and the peripheral pyrimidine ring. Compound
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FIGURE1. The structure of compounds 1-6. SCHEME1
5 is also nearly coplanar, and the twist angle between the thieno[3,2-b]thiophene ring and the benzothiazole ring is only 2.7°. The packing of the molecules inside the crystal lattice of 2 and 5 viewing along the long and short axes of the molecule is illustrated in Figure 3. For 2, no herringbone structure is observed and the arrangement of the molecules can be basically described as a one-dimensional slipped-stack, with the interfacial distance of adjacent molecules being 3.47 Å (see d in Figure 3). In 5, the edge-to-face interactions occur only at the terminal rings (i.e., at the short edges of the molecule, in direct contrast to pentacene and perfluoropentacene where the edge-to-face interactions occur along the long molecular edges). Nevertheless, theπ-π interfacial distance of the adjacent molecules is 3.59
Å within the same stack (Figure 3). Both crystals 2 and 5 therefore have favorableπ-π interaction to facilitate
charge-carrier hopping among molecules in the direction perpendicular to the molecular plane. It is also noted that π-π interaction
exists between the electron-rich moiety of each molecule and the electron-deficient moiety of the next stacked molecule. The thieno[3,2-b]thiophene (electron rich) unit is right above half of the pyrimidine (electron deficient) unit for 2, with a short on-top intrastack S · · · C contact of 3.34 Å, and
thieno[3,2-b]thiophene is above part of the benzothiazole moiety for 5.
No S · · · S contact has been found in either crystal, but the N · · · S interstack contact (3.27 Å) has been found for 5.
Curtis et al. have analyzed the spatial relationship of molecules in the crystal structures of substituted pentacenes, bithiazole- and thiophene-containing oligomers with “pitch” (P) and “roll” (R) angles.3 Briefly, the smaller the angles∠P or ∠R, the smaller the slip distances along the long (dp: the pitch distance) or short axis (dr: the roll distance) (Figure 3). One of
the key criteria for goodπ-π stacking is the molecular width
(w) to dr ratio (w/dr); for good π-π stacking, w/dr g 1 is
required, and the importance of small drhas been stressed.3Roll
distances larger than ca. 2.8 Å can seriously damage theπ-π
overlap between nearest-neighboring molecules because the widths of the organic materials Curtis et al. investigated are within the range of 2.8 Å (the approximate width of benzene). Table 2 shows the results of the same phenomenological analysis for pentacene, 2, and 5. The dpof 2 (2.78 Å) is similar
to that of the pentacene dimer (2.10 Å), while large slip along the long axis is observed for 5 (4.85 Å). Along the molecular short axis, an extremely small drof only 0.06 Å is observed for 2; for 5, a drof 0.55 Å also reveals a small short-axial slip. So
far, a drof 0.06 Å is the smallest slip distance found including
those data obtained in Curtis’ study.3Negligible slip along the
short axis and moderate slip along the long axis could be two of the factors that explains why 2 has a significant calculated electronic coupling value of 0.153 eV (t+in Table 3), which is twice as large as that of the largest coupling of pentacene. As the rate of charge transfer is proportional to the square of t in Marcus theory, the rate of hole transport of 2 could be four times that of pentacene, assuming factors other than t are kept the same.
It can be seen from Table 3 that t+values of 2 and 5 are appreciably larger than their respective t-values. This trend can be understood qualitatively according to the shapes of the frontier orbitals. Good orbital overlaps between the frontier orbitals of interacting molecules result in large t values. When an orbital (e.g., a p orbital) is above the nodal plane of the molecular orbital of another molecule (e.g., the π* orbital of
ethylene), the positive and negative overlaps between molecules cancel out and afford negligible electronic coupling. For LUMOs of 2 and 5 (related to t-), nodal planes are found parallel to both the long and short molecular axes, while nodal planes are only found along the short molecular axes for HOMOs (related to t+). (See Figure 4.) Therefore, when the slip distances are nonzero, it is of higher probability for 2 and 5 to have large t+ values than to have large t-values (due to fewer nodal planes in HOMO than in LUMO). This argument has been introduced by Troisi to explain results of cofacially stacked pentacene systems.14Nevertheless, the situation is reversed in the cases
of pentacene derivatives: nodal planes are found along both the short and long molecular axes for the HOMOs, but this is not the case for the LUMOs. Finally, it is noted that the donor and acceptor parts of 2 and 5 do not result in localized frontier orbitals; both the HOMOs and LUMOs spread out over the entire molecule, rather than localize at small regions. MOs localized in small regions are likely to afford ineffective electronic couplings. Therefore, the introduction of polar groups in 2 and 5 does not seem to have a negative effect on t.
The nearly on-top intrastack S · · · C contacts observed in 2 warrant further discussions in terms of crystal engineering and electronic coupling. For each dimer of 2, the relative spatial disposition is stapled by a pair of symmetrical S · · · C contacts (Figure 5a), so that dr is nearly zero and dpis moderate. The
interaction between the rich S atom and the electron-deficient C atom has to be quite favorable, as the pyrimidine plane becomes slightly out-of-plane with respect to the central thieno[3,2-b]thiophene unit. Considering the larger van der Waals radius of an S atom than that of a C atom, the S · · · C contact (3.34 Å) is significantly shorter than the nearby C · · · C distance (3.63 Å) (Figure 5a). In terms of crystal engineering, the above results imply the possibility to deem the nonbonded
(14) Troisi, A.; Orlando, G.; Anthony, J. E. Chem. Mater. 2005, 17, 5024. TABLE1. Absorption and Emission Spectral Data of
Thieno[3,2-b]thiophene Derivativesa compd λmax, nm λem, nm Φ, % 1 340, 360, 390 395, 420 21 2 347 387, 415 43 3 361, 380 414, 425 41 4 357 396, 427 44 5 352, 370 391, 415 42 6 325, 340, 360 371, 388, 413 49
aThe absorption and emission spectra were measured in CH
2Cl2. The
fluorescence quantum yield (Φ) was measured in CH2Cl2 relative to
coumarin-1 (73%) in EtOH.
relationship between the electron-deficient pyrimidine and the electron-rich thieno[3,2-b]thiophene a synthon for face-to-face
π-stacking. So far, no other crystal structure with the coexistence
of the pyrimidine and thieno[3,2-b]thiophene moieties can be found in the Cambridge Structure Database. Therefore, system-atic studies are needed to investigate whether the above two units can really be regarded as a synthon for face-to-face stacking. In Figure 5b, the HOMO-1 of dimers of 2 are shown (HOMO-1 of the dimer is the positive coupling between HOMOs of the monomer). It is clear that the nearly on-top S · · · C contacts facilitate good coupling between monomers.
At room temperature, the hopping mechanism for the charge-transport process is believed to be important for thin-film OFET.5a,b,15Under this mechanism, the rate of charge-transport
is described by Marcus theory,16in which both t and reorga-nization energy,λ, are important factors. The calculated internal
reorganization energies (λ() of 2 and 5 are shown in Table 3. Results of 2 for the fully optimized and the partially constrained quasiplanar geometries (the two terminal inter-ring dihedral angles were constrained to the value found in the crystal
structure) are both shown in the Table 3. Since the quasiplanar geometry of 2 may better reflect the molecular structure in the packing of an OFET device, we use the latter data in the following discussion.
The significantly larger λ( values (>0.26 eV) of 2 and 5 compared to that of pentacene (<0.14 eV) show the price one has to pay for employing thieno[3,2-b]thiophene, benzothiazole, and pyrimidine as charge-transport units. The largeλ(values (Table 3) for 2 and 5 can be associated with the large bond length alterations (BLAltr) between the neutral and charged states. For pentacene, the BLAltr are in the range of 0.004-0.016 Å upon oxidation and reduction.17Whereas, the BLAltr of 2
for hole- and electron-transfer processes are in the range of 0.001-0.034 and 0.005-0.034 Å, respectively; and those of 5 are 0.001-0.034 and 0.003-0.046 Å, respectively. Notably, significant BLAltr are observed for the inter-ring distances in both 2 and 5, in accord with the orbital distributions across the inter-ring regions (Figure 4). In the anionic state, the inter-ring BLAltr (0.034 Å for 2, and 0.046 Å for 5) are the largest among all BLAltr.
It has been shown in the literature that reasonable qualitative estimates of charge mobilities (µ() can be calculated withλ, t,
and experimentally determined intermolecular distance (see computational details).12,18 Despite of the inferior λ(
values compared to those of pentacene, the impressive t+value of 2 still renders good theoretical hole mobility (µ+, 2.32 cm2s-1
V-1). Theµ+of 5 (0.60 cm2s-1 V-1) andµ-(0.18 cm2s-1
V-1) of 2 are an order of magnitude smaller, and theµ-of 5 is the smallest due to an even smaller t- and a large λ-. As pentacene is basically a p-channel OFET material in a normal device configuration, it is of interest to learn the effect the heteroaromatic units in 2 and 5 in terms of polarity of the semiconductors. According to the calculated electron affinities (EA), the smaller EA values of 2 and 5 compared to that of pentacene (Table 3) prevent them from being n-channel materi-(15) (a) Mas-Torrent, M.; Hadley, P.; Bromley, S. T.; Ribas, X.; Tarre´s, J.;
Mas, M.; Molins, E.; Veciana, J.; Rovira, C. J. Am. Chem. Soc. 2004, 126, 8546. (b) Bromley, S. T.; Mas-Torrent, M.; Hadley, P.; Rovira, C. J. Am. Chem. Soc.
2004, 126, 6544. (c) da Silva Filho, D.; Olivier, Y.; Coropceanu, V.; Bre´das,
J.-L.; Cornil, J. Theoretical Aspects of Charge Transport. In Organic Semicon-ductors: A Molecular PerspectiVe. In Organic Field-Effect Transistors; Bao, Z., Locklin, J., Eds.; CRC: Boca Raton, FL, 2007; Chapter 1.
(16) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Marcus, R. A.; Sutin, N. Biochem. Biophys. Acta 1985, 811, 265. (c) Newton, M. D.; Sutin, N. Annu. ReV. Phys. Chem. 1984, 35, 437. (d) Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996, 100, 13148.
(17) Chen, H. Y.; Chao, I. Chem. Phys. Lett. 2005, 401, 539.
(18) (a) Lemaur, V.; da Silva Filho, D. A.; Coropceanu, V.; Lehmann, M.; Geerts, Y.; Piris, J.; Debije, M. G.; van de Graats, A. M.; Senthilkumar, K.; Siebbeles, L. D. A.; Warman, J. M.; Bre´das, J.-L.; Cornil, J. J. Am. Chem. Soc.
2004, 126, 3271. (b) CornilJ, J.; Lemaur, V.; Calbert, J. P.; Bre´das, J.-L. AdV.
Mater. 2002, 14, 726. (c) Song, Y.; Di, C.; Yang, X.; Li, S.; Xu, W.; Liu, Y.; Yang, L.; Shuai, Z.; Zhang, D.; Zhu, D. J. Am. Chem. Soc. 2006, 128, 15940. (d) Kuo, M.-Y.; Chen, H.-Y.; Chao, I. Chem. Eur. J. 2007, 13, 4750.
FIGURE3. Schematic illustrations of “pitch” and “roll” angles/distances (∠P/dp;∠R/dr) for 2 and 5 as defined by Curtis et al.3The interfacial
distances (d) are also given. Distances are in Å.
TABLE2. Structural Characterization of Pentacene, 2, and 5 with Pitch (P) and Roll (R) Angles and Distances
compd P (deg) R (deg) d (Å) dp(Å) dr(Å) z (Å)a
pentaceneb 39.1 64.2 2.58 2.10 5.35 6.30
2 38.7 1.0 3.47 2.78 0.06 4.44
5 53.5 8.8 3.59 4.85 0.55 6.06
az ) (d
p2 + dr2 + d2)1/2. The deviations of z values from the
crystallographic stacking distances (i.e., the distance between two identical parts of two adjacent molecules) are 0.4%, 0.0%, and 0.4% for pentacene, 2, and 5, respectively. These small values ensure the quality of estimated pitch/roll angles and distances measured with the program Mercury.bData taken from ref 3.
als in a regular OFET configuration, even though the electron-deficient moieties are present. On the other hand, the presence of electron-deficient moieties and the lack of extended conjuga-tion have caused the ionizaconjuga-tion potentials (IP) to be quite large, which may impart higher oxidative stability for 2 and 5 compared to that of pentacene, but also result in higher hole injection barriers and higher sensitivity of the charge carriers to the traps caused by impurities.
Conclusion
Face-to-faceπ-stacked structure in the crystal lattice has been
achieved with bipolar molecules containing alternate electron-rich thieno[3,2-b]thiophene units and electron-deficient pyri-midine or benzothiazole units. Owing to the favorable interac-tions between the thieno[3,2-b]thiophene and pyrimidine units (which is characterized by the symmetrical on-top S · · · C contacts), the nearly zero slip distance along the short molecular axis and the moderate slip distance along the long molecular axis in 2 result in a significant electronic coupling twice as large as that of pentacene for hole hopping. Good hole mobilities (µ+) for 2 and 5 of 2.32 and 0.60 cm2s-1V-1, respectively,
are predicted from the acceptable λ+(reorganization energy) and high t+(intermolecular electronic coupling). Overall, the attempt at coupling electron-rich and electron-deficient het-eroaromatics is successful in terms of crystal engineering and electronic coupling. A new π-stacking synthon in crystal
engineering has been proposed and will be examined further in the future. The theoretical analyses on internal reorganization energy, EA, and IP reveal the disadvantage of using small heteroaromatic rings. More extended rings will be considered in the future so that the reorganization energy can be lowered and the EA and IP can be tuned.
Experimental Methods
2,5-Di(pyrimidin-2-yl)thieno[3,2-b]thiophene (1). A
two-necked round-bottomed flask was charged with 2-bromo-pyrimidine (1.11 g, 7.0 mmol), 2,5-bis(tributylstannyl)thieno[3,2-b]thiophene (2.37 g, 3.3 mmol), and Pd(PPh3)4(2 mmol %, 76 mg). Dry DMF
(10 mL) was added, and the mixture was heated at 80°C under nitrogen for 24 h. After cooling, the solution was filtered, and the pale yellow precipitates collected were washed with acetone and EtOH several times. Crude product was further sublimed to afford pure 1 as bright yellow powders in 25% yield.1H NMR (δ,
DMSO-d6): 7.42 (t, 2 H, J ) 4.7 Hz, pyrimidine), 8.36 (s, 2 H, thiophene),
8.86 (d, 4 H, J ) 4.7 Hz, pyrimidine). The compound is not soluble enough for recording13C NMR soectra. EI MS (m/z): 296.0 [M+].
HRMS (m/z): calcd for C14H8N4S2296.0190, found 296.0193 [M+].
Anal. Calcd for C14H8N4S2: C, 56.74; H, 2.72; N, 18.90. Found:
C, 56.79; H, 2.61; N, 18.86.
2,5-Di(pyrimidin-5-yl)thieno[3,2-b]thiophene (2). Essentially
the same procedures as for the synthesis of 1 were used except that 5-bromopyrimidine was used instead of 2-bromopyrimidine. A bright yellow solid was obtained in 20% yield. 1H NMR (δ,
DMSO-d6): 8.23 (s, 2 H, thiophene), 9.16 (s, 2 H, pyrimidine),
9.29 (s, 4 H, pyrimidine).13C NMR (δ, DMSO-d
6): 163.1, 157.4,
153.2, 138.6, 128.2, 119.1. FAB MS (m/z): 297.1 [M++ H]. HRMS (m/z): calcd for C14H9N4S2297.0269, found 297.0271 [M++ H].
Anal. Calcd for C14H8N4S2: C, 56.74; H, 2.72; N, 18.90. Found:
C, 56.98; H, 2.74; N, 18.83.
2,5-Di(pyridin-5-yl)thieno[3,2-b]thiophene (3). Essentially the
same procedures as for the synthesis of 1 were used except that 2-bromopyridine was used instead of 2-bromopyrimidine. After cooling, the reaction mixture was extracted with a mixture of saturated KF aqueous solution and CH2Cl2. The organic extracts
were collected and dried over MgSO4. The solution was then
purified by column chromatography to provide 3 as a pale yellow powder in 25% yield.1H NMR (δ, CDCl 3): 7.16 (dd, 2 H, J ) 8.7, 4.8 Hz, 2 H, pyridine), 7.69 (m, 4 H, pyrimidine), 7.77 (s, 2 H, thiophene), 8.57 (d, 2 H, J ) 4.8 Hz, pyrimidine).13C NMR (δ, CDCl3): 152.1, 149.5, 141.6, 136.9, 122.3, 119.0, 117.7. FAB MS (m/z): 295.2 [M+ + H]. HRMS (m/z): calcd for C16H11N2S2
295.0364, found 295.0366 [M++ H]. Anal. Calcd for C16H10N2S2:
C, 65.28; H, 3.42; N, 9.52. Found: C, 65.01; H, 3.32; N, 9.34.
2,5-Bis(4-(trifluoromethyl)phenyl)thieno[3,2-b]thiophene (4).
Essentially the same procedures as for the synthesis of 3 were used except that 1-bromo-4-(trifluoromethyl)benzene was used instead of 2-bromopyridine. A pale yellow solid was obtained in 20% yield.
1H NMR (δ, CDCl
3): 7.57 (s, 2 H, thiophene), 7.65 (d, 4 H, J )
8.3 Hz, C6H4), 7.73 (d, 4 H, J ) 8.3 Hz, C6H4). 13C NMR (δ,
CDCl3): 126.1, 125.9, 125.8, 116.8, 115.0. FAB MS (m/z): 428.0
[M+]. HRMS (m/z): calcd for C20H10F6S2428.0128, found 428.0129
[M+]. Anal. Calcd for C20H10F6S2: C, 56.07; H, 2.35. Found: C,
56.09; H, 2.11.
2-Thieno[3,2-b]thiophene-2-yl-benzothiazole (5). A two-necked
round-bottomed flask was charged with thieno[3,2-b]thiophene-2-carboxylic acid (3.0 g, 16 mmol), 2-aminothiophenol (2.25 g,, 1.92 mL, 18 mmol), and POCl3(20 mL). The mixture was refluxed for
12 h. After cooling, iced water was added, and the solution was neutralized by adding aqueous NaOH. The solution was extracted with CH2Cl2. The organic extracts were collected and dried over
MgSO4. The solution was further purified by column
chromatog-raphy to provide 5 as a light brown powder in 11% (0.50 g) yield.
TABLE3. Calculated Adiabatic Ionization Potentials (IP) and Electron Affinities (EA), Reorganization Energies of the Charge Transport (λ(), Electronic Couplings (t(), and Mobilities (µ()
compd dc-cd(Å) IP (eV) λ+(eV) t+(eV) µ+(cm2s-1V-1) EA (eV) λ-(eV) t-(eV) µ-(cm2s-1V-1)
pentacenea 6.27 5.899 0.094 0.034 1.93 1.136 0.133 0.046 2.03 5.22 0.044 2.25 0.076 3.84 4.76 0.075 5.43 0.078 3.35 2b 4.44 7.287 0.262 0.153 2.32 1.138 0.274 0.046 0.18 2c 7.310 0.309 1.121 0.419 5 6.03 7.007 0.300 0.071 0.60 0.620 0.335 0.013 0.01
aMost data were taken from ref 12. The IP value of pentacene in ref 12 was incorrectly typed as 5.888 eV. Three distinct dimeric pairs were observed in the crystal structure of pentacene, and three sets of values were estimated for t(andµ(. The structure of pentacene was taken from ref 13. bTwo inter-ring dihedral angels were frozen at the angles observed in the crystal structure.cCalculated based on the gas-phase optimized geometry. dMolecular center-to-center distance used in the estimation of charge mobility.
1H NMR (δ, acetone-d 6): 7.45 (td, 1 H, J ) 7.2, 1.2 Hz, C6H4), 7.51 (d, 1 H, J ) 5.2 Hz, thiophene), 7.55 (td, 1 H, J ) 7.2, 1.2 Hz, C6H4), 7.80 (d, 1 H, J ) 5.2 Hz, thiophene), 7.99 (d, 1 H, J ) 8.0 Hz, C6H4), 8.07 (d, 1 H, J ) 8.0 Hz, C6H4), 8.14 (s, 1 H, thiophene).13C NMR (δ, acetone-d 6): 163.7, 154.7, 145.0, 139.9, 136.4, 133.3, 132.0, 127.6, 127.0, 126.5, 123.7, 122.9, 122.6, 121.0. FAB MS (m/z): 274.1 [M++ H]. HRMS (m/z): calcd for C13H8NS3
273.9819, found 273.9818 [M++ H]. Anal. Calcd for C13H7NS3:
C, 57.11; H, 2.58; N, 5.12. Found: C, 56.37; H, 2.13; N, 5.30.
2-Thieno[3,2-b]thiophene-2-yl-1H-benzoimidazole (6). To a
flask containing thieno[3,2-b]thiophene (1.4 g, 10 mmol) in 40 mL of DMF immersed in a cold bath at 0°C was added n-BuLi (7.0 mL, 1.6 M in hexane) dropwise. After addition was complete, the solution was stirred for 1 h. Another 10 mL of DMF was added and the solution was stirred for an additional 20 min at 0°C. After the solution was warmed to room temperature, the solution was extracted with Et2O. The ether extracts were collected and dried
over MgSO4. The solvent was removed in vacuo to provide yellow
oily thieno[3,2-b]thiophene-2-carbaldehyde. To a flask containing thieno[3,2-b]thiophene-2-carbaldehyde (1.51 g, 9.0 mmol) in 30 mL of EtOH immersed in a cold bath at 0°C was added dropwise 30 mL of aqueous solution containing Na2S2O5(10.8 mmol). The
solution was stirred at 0 °C for 6 h and filtered. The yellow hydroxylthieno[3,2-b]thiophene-2-ylmethanesulfonate collected was washed with H2O and EtOH and pumped dry. A flask was charged
with hydroxylthieno[3,2-b]thiophene-2-ylmethanesulfonate and 17 mL of DMF. After being stirred at 130°C, the solution was cooled to room temperature. The solid formed upon addition of H2O was
collected by filtration. It was then extracted with a mixture of CH2Cl2and saturated brine. The organic extracts were collected
and dried over MgSO4. After removal of the solvent, the residue
was recrystallized from CH2Cl2/hexane to afford off-white 6 in 15%
yield.1H NMR (δ, acetone-d 6): 7.19-7.23 (m, 2 H, C6H4), 7.48 (d, 1 H, J ) 5.2 Hz, thiophene), 7.48-7.51 (m, 1 H, C6H4), 7.63-7.67 (m, 1 H, C6H4), 7.71 (d, 1 H, J ) 5.2 Hz, thiophene), 8.08 (s, 1 H, thiophene).13C NMR (δ, acetone-d 6): 148.3, 145.3, 141.8, 140.6, 136.9, 136.0, 130.5, 124.0, 123.0, 120.9, 120.0, 119.4, 111.9. FAB MS (m/z): 257.1 [M++ H]. HRMS (m/z): calcd for C13H9N2S2257.0207, found 257.0202 [M++ H]. Anal. Calcd for
C13H8N2S2: C, 60.91; H, 3.15; N, 10.93. Found: C, 61.26; H, 3.16;
N, 10.96.
Computational Methods
All the structures were optimized at the DFT-B3LYP/6-31G(d,p) level of theory and frequency calculations were performed to confirm whether the optimized geometry was an energy minimum on the potential energy surface with Gaussian 03.19(〈S2〉for the
open-shell species is in the range of 0.7615-0.7702.) At high temperature (including room temperature), the charge carriers in thin-film OFET are believed to be localized at individual molecules, and the charge-transport mechanism is a hopping mechanism described by Marcus theory.16The rate expression of self-exchange
process is: ket) 4π 2 h 1
√
4πλkBT t2exp(- λ 4kBT) (1)in which the internal reorganization energy (λ) and intermolecular
electronic coupling (t) decide the value of the electron transfer rate (ket).
Because the reorganization energies (λ) calculated at the
DFT-B3LYP/6-31G(d,p) level agree well with experimental values,15b,17,20
λ is estimated at the same theory level. The total internal
reorganization energy (λ( ) is the sum of λ1( and λ2( (see
eqs 2-4): λ() λ1 ( + λ2 ( (2) λ1) E((QN) - E((Q() (3) λ2) EN(Q() - EN(QN) (4)
where E((QN) is the total energy of the charged state in the neutral
geometry, E((Q() is the total energy of the charged state in the
charged state geometry, EN(Q() is the total energy of the neutral
state in the charged state geometry, and EN(QN) is the total energy
of the neutral state in the neutral geometry.
The t(values were obtained via the direct coupling method (eq 5) with the use of dimeric structures extracted from the X-ray structures. The fragmental orbital approach implemented in the Amsterdam density functional (ADF) program21was used to derive the components
in eq 5. The details are described in two recent papers.22The DZP
basis set in ADF was used and the local density functional VWN was employed in conjunction with the PW91 gradient corrections.
t()HRP- SRP(HRR+ HPP)/2
1 - SRP2 (5)
where HRPis the charge transfer integrals, SRPis the spatial overlap,
and HRRand HPPare site energies.
Once λ( and t( are obtained, one can calculate the electron transfer rate ketaccording to eq 1. The diffusion coefficient D of
charge carriers can then be given by eq 6, where L is the effective
(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.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O. ; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D.02; Gaussian, Inc.: Wallingford, CT, 2004.
(20) (a) Coropceanu, V.; Malagoli, M.; da Silva Filho, D. A.; Gruhn, N. E.; Bill, T. G.; Bre´das, J. L. Phys. ReV. Lett. 2002, 89, 275503. (b) Kwon, O.; Coropceanu, V.; Gruhn, N. E.; Durivage, J. C.; Laquindanum, J. G.; Katz, H. E.; Cornil, J.; Bre´das, J. L. J. Chem. Phys. 2004, 120, 8186. (c) Chen, H. Y.; Chao, I. ChemPhysChem 2006, 7, 2003.
(21) (a) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Guerra, C. F.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931. (b) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor. Chem. Acc. 1998, 99, 391. (c) ADF 2005 01, SCM, Theoretical Chemistry, Vrije Universiteit: Amsterdam, The Netherlands, http://www.scm.com.
(22) (a) Prins, P.; Senthilkumar, K.; Grozema, F. C.; Jonkheijm, P.; Schenning, A. P. H. J.; Meijer, E. W.; Siebbeles, L. D. A. J. Phys. Chem. B
2005, 109, 18267. (b) Senthilkumar, K.; Grozema, F. C.; Buckelhaupt, F. M.;
Siebbeles, L. D. A. J. Chem. Phys. 2003, 119, 9809.
FIGURE5. (a) A pair of on-top S · · · C contacts (Å) can be found in the dimer of 2 (crystal structure). The nearby C · · · C distances are also labeled.
length of electron transfer (L was approximated by the molecular center-to-center distance of a dimer). This allows us to evaluate the drift mobility of charge from the Einstein relation,23as shown
in eq 7, where e is the electronic charge, kBis Boltzmann constant,
D ) L2ket/2 (6)
µ ) eD/kBT (7)
and T is temperature in Kelvin (300 K in this study). This method has been employed for the calculation of the charge carrier mobility of organic seimconductors.12,18
Acknowledgement. We thank the National Science Council and Academia Sinica for the financial support. The computing time granted by the National Center for High-Performance Computing and the Computing Center of Academia Sinica is also acknowledged.
Supporting Information Available: Complete crystallo-graphic data and calculated coordinates of compounds 2 and 5 and copies of1H and13C spectra for the new compounds. This
material is available free of charge via the Internet at http://pubs.acs.org.
JO800546J
(23) Atkins, P. W. Physical Chemistry, 4th ed.; Oxford University Press: Oxford, UK, 1990.
