ORGANIC
`LETTERS
`
`2008
`Vol. 10, No. 6
`1199-1202
`
`Synthesis, Structures, and Properties of
`Unsymmetrical Heteroacenes Containing
`Both Pyrrole and Furan Rings
`
`Keiko Kawaguchi, Koji Nakano,* and Kyoko Nozaki*
`
`Department of Chemistry and Biotechnology, Graduate School of Engineering,
`The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
`knakano@chembio.t.u-tokyo.ac.jp; nozaki@chembio.t.u-tokyo.ac.jp
`
`Received January 11, 2008
`
`ABSTRACT
`
`Unsymmetrical heteroacenes, 11-phenylbenzofuro[3,2-b]carbazole (Ph- BFC) and its alkoxylated derivatives, were readily synthesized by palladium-
`catalyzed double N-arylation of arylamines. They characteristically form antiparallel cofacial (cid:240)-stacking arrangements, which may result from
`their unsymmetrical structures. Their physical properties show their potential for application as active layers in organic field-effect transistors.
`
`The exploration of new organic semiconducting molecules
`is a crucial topic in organic-based electronic devices such
`as organic field-effect transistors (OFETs).1 Recently, in-
`tensive research efforts in this field have led to the develop-
`ment of a number of organic semiconductors with high
`carrier mobilities comparable to those of amorphous silicon.2-12
`
`(1) For recent reviews on OFETs, see: (a) Katz, H. E.; Bao, Z.; Gilat,
`S. L. Acc. Chem. Res. 2001, 34, 359. (b) Dimitrakopoulos, C. D.; Malenfant,
`P. R. L. AdV. Mater. 2002, 14, 99. (c) Sun, Y. M.; Liu, Y. Q.; Zhu, D. B.
`J. Mater. Chem. 2005, 15, 53. (d) Organic Electronics, Materials,
`Manufacturing and Applications; Klauk, H., Ed.; Wiley-VCH Verlag GmbH
`& Co. KGaA: Weinheim, 2006. (e) Anthony, J. E. Chem. ReV. 2006, 106,
`5028. (f) Takimiya, K.; Kunugi, Y.; Otsubo, T. Chem. Lett. 2007, 36, 578.
`(2) (a) Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Radlik, W.;
`Weber, W. J. Appl. Phys. 2002, 92, 5259. (b) Kelley, T. W.; Boardman, L.
`D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. P. J. Phys.
`Chem. B 2003, 107, 5877.
`(3) (a) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Vitaly, M.; Etienne,
`W.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science
`2004, 303, 1644. (b) Podzorov, V.; Menard, E.; Borissov, A.; Kiryukhin,
`V.; Rogers, J. A.; Gershenson, M. E. Phys. ReV. Lett. 2004, 93, 086602.
`(4) Yamada, K.; Okamoto, T.; Kudoh, K.; Wakamiya, A.; Yamaguchi,
`S.; Takeya, J. Appl. Phys. Lett. 2007, 90, 072102.
`(5) (a) Takimiya, K.; Kunugi, Y.; Konda, Y.; Niihara, N.; Otsubo, T. J.
`Am. Chem. Soc. 2004, 126, 5084. (b) Takimiya, K.; Kunugi, Y.; Konda,
`Y.; Ebata, H.; Toyoshima, Y.; Otsubo, T. J. Am. Chem. Soc. 2006, 128,
`3044. (c) Takimiya, K.; Ebata, H.; Sakamoto, K.; Izawa, T.; Otsubo, T.;
`Kunugi, Y. J. Am. Chem. Soc. 2006, 128, 12604. (d) Yamamoto, T.;
`Takimiya, K. J. Am. Chem. Soc. 2007, 129, 2224.
`
`10.1021/ol703110g CCC: $40.75
`Published on Web 02/16/2008
`
`© 2008 American Chemical Society
`
`Among them, pentacene has achieved both an exclusively
`high hole mobility of (cid:24)3 cm2(cid:226)V-1(cid:226)s-1 and a high on/off ratio
`of 108.2 However, pentacene possesses the disadvantage of
`undergoing air degradation and/or photooxidation, which
`poses problems for its practical use in organic devices.
`Recently, heteroacenes with heteroatoms in fused frame-
`works, such as thiophene-based dinaphtho[2,3-b:2¢ ,3¢ -f]-
`
`(6) (a) Wu, Y.; Li, Y.; Gardner, S.; Ong, B. S. J. Am. Chem. Soc. 2005,
`127, 614. (b) Boudreault, P. L. T.; Wakim, S.; Blouin, N.; Simard, M.;
`Tessier, C.; Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2007, 129, 9125.
`(7) (a) Sheraw, C. D.; Jackson, T. N.; Eaton, D. L.; Anthony, J. E. AdV.
`Mater. 2003, 15, 2009. (b) Payne, M. M.; Parkin, S. R.; Anthony, J. E.;
`Kuo, C. C.; Jackson, T. N. J. Am. Chem. Soc. 2005, 127, 4986. (c) Li, Y.
`N.; Wu, Y. L.; Liu, P.; Prostran, Z.; Gardner, S.; Ong, B. S. Chem. Mater.
`2007, 19, 418.
`(8) van Breemen, A. J. J. M.; Herwig, P. T.; Chlon, C. H. T.; Sweelssen,
`J.; Schoo, H. F. M.; Setayesh, S.; Hardeman, W. M.; Martin, C. A.; de
`Leeuw, D. M.; Valeton, J. J. P.; Bastiaansen, C. W. M.; Broer, D. J.; Popa-
`Merticaru, A. R.; Meskers, S. C. J. J. Am. Chem. Soc. 2006, 128, 2336.
`(9) Meng, H.; Sun, F.; Goldfinger, M. B.; Jaycox, G. D.; Li, A.; Marshall,
`W. J.; Blackman, G. S. J. Am. Chem. Soc. 2005, 127, 2406.
`(10) Drolet, N.; Morin, J. F.; Leclerc, N.; Wakim, S.; Tao, Y.; Leclerc,
`M. AdV. Funct. Mater. 2005, 15, 1671.
`(11) Yoon, M. H.; DiBenedetto, S. A.; Facchetti, A.; Marks, T. J. J.
`Am. Chem. Soc. 2005, 127, 1348.
`(12) (a) Ando, S.; Nishida, J.; Tada, H.; Inoue, Y.; Tokito, S.; Yamashita,
`Y. J. Am. Chem. Soc. 2005, 127, 5336. (b) Ando, S.; Murakami, R.; Nishida,
`J.; Tada, H.; Inoue, Y.; Tokito, S.; Yamashita, Y. Y. J. Am. Chem. Soc.
`2005, 127, 14996.
`
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`
`

`
`Table 1. Synthesis of Ar-BFC 1 by Double N-Arylation of
`Aniline with 6
`
`entry
`
`ligand
`
`1
`2
`3
`4
`
`L1
`L2
`L2
`L2
`
`R
`
`H
`H
`H
`OMe
`
`yield of 1 (%)
`
`60
`67
`97 a
`72
`
`a Use of 15 mol % of Pd(dba)2 and 30 mol % of L2.
`
`First, the substrate for the double N-arylation was synthesized
`as shown in Scheme 1. Commercially available 3-amino-2-
`
`Scheme 1.
`
`Synthesis of Bistriflate 6
`
`thieno[3,2-b]thiophene (DNTT)5d and pyrrole-based indolo-
`[3,2-b]carbazoles (ICs)6 have been shown to be promising
`candidates as hole-transporting materials. OFETs based on
`these heteroacenes have shown high mobilities ((cid:237)FET (cid:25) 2.9
`cm2(cid:226)V-1(cid:226)s-1 for DNTT, (cid:237)FET (cid:25) 0.2 cm2(cid:226)V-1(cid:226)s-1 for ICs)
`and good environmental stabilities because of their lower
`HOMO energy levels and larger band gaps than those of
`pentacene.1f,6a Thus, the development of new heteroacene
`molecules should be important for the rapid improvement
`of heteroacene-based OFET materials.
`One of the key factors responsible for high electronic per-
`formance is the solid-state packing structure.7,13-15 In general,
`strong electronic interactions between the (cid:240)-electron-rich
`frameworks of adjacent molecules leads to high charge-
`carrier mobility. One promising arrangement for strong elec-
`tronic interaction is a cofacial (cid:240)-stacking structure. Nuckolls
`et al. have investigated a series of substituted pentacenes.15
`Among them, a thienyl-substituted compound formed a
`cofacial (cid:240)-stacking structure and showed high OFET per-
`formance. In contrast, other derivatives with phenyl substit-
`uent(s) was packed with the dominant edge-to-face interac-
`tion between the pentacene core and the phenyl substituents,
`that is, without efficient interaction between the pentacene
`cores, resulting in much lower OFET performance. Anthony
`et al. also investigated anthradithiophene derivatives with a
`cofacial arrangement in the solid state, which displayed high
`OFET performance.7b
`In this paper, we report the synthesis of 11-arylbenzofuro-
`[3,2-b]carbazole (Ar-BFC), their solid-state structures, and
`their physical properties. We have recently reported the
`synthesis of 5,11-diphenylindolo[3,2-b]carbazoles (DPh-
`ICs) and dibenzo[d,d¢ ]benzo[1,2-b:4,5-b¢ ]difurans (DBBDFs)
`via palladium-catalyzed double N-arylation of aniline and
`intramolecular O-arylation, respectively.16,17 Photophysical
`and electrochemical studies of DPh-IC and DBBDF have
`indicated that DBBDF is more stable than DPh-IC toward
`photooxidation. Accordingly, we present here a new hetero-
`acene framework based on 11-phenylbenzofuro[3,2-b]car-
`bazole (Ph-BFC) as a molecule with enhanced environ-
`mental stability compared to DPh-IC. Unlike DPh-IC and
`DBBDF, the crystal structures of Ar-BFCs show antiparallel
`cofacial (cid:240)-stacking arrangements between heteroacene cores,
`which are expected to cause significant orbital interaction.
`
`The synthesis of BFCs was conducted with palladium-
`catalyzed double N-arylation as a key reaction (Table 1).17
`
`methoxydibenzofuran 2 was converted into iodide 3 via the
`Sandmeyer reaction (72%). The following Suzuki-Miyaura
`cross-coupling reaction with 2-methoxyphenylboronic acid
`produced a 92% yield of 4. Demethylation of 4 and
`
`(13) Bredas, J. L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem. ReV.
`2004, 104, 4971.
`(14) Moon, H.; Zeis, R.; Borkent, E. J.; Besnard, C.; Lovinger, A. J.;
`Siegrist, T.; Kloc, C.; Bao, Z. N. J. Am. Chem. Soc. 2004, 126, 15322.
`
`(15) Miao, Q.; Chi, X. L.; Xiao, S. X.; Zeis, R.; Lefenfeld, M.; Siegrist,
`T.; Steigerwald, M. L.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128, 1340.
`(16) Kawaguchi, K.; Nakano, K.; Nozaki, K. J. Org. Chem. 2007, 72,
`5119.
`
`1200
`
`Org. Lett., Vol. 10, No. 6, 2008
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`DUK SAN NEOLUX
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`

`
`subsequent esterification with triflic anhydride produced a
`68% yield of bistriflate 6 (2 steps). The double N-arylation
`of aniline with 6 was initially examined in the presence of
`
`Pd(dba)2 (10 mol %), ligand L118 or L2
`19 (20 mol %), and
`K3PO4 (2.8 equiv) in toluene at 100 (cid:176) C (Table 1). The use
`of ligand L2 resulted in a slightly higher yield of the desired
`1a (67%) than that using ligand L1 (60%) (entries 1 and 2).
`A small amount of 5 was observed as a byproduct in both
`reactions, which would be produced via the hydrolysis of
`bistriflate 6. Optimally, a 97% yield of 1a was obtained when
`using 15 mol % of the palladium catalyst (entry 3). The
`coupling of 6 with p-anisidine was also successful, resulting
`in a 72% yield of 1b (10 mol % Pd) (entry 4). Methoxy-
`substituted Ar-BFC 1b is a useful platform for further
`transformation. Thus, alkoxylated derivatives 1c and 1d were
`obtained via demethylation of 1b and subsequent alkylation
`with the corresponding iodoalkanes (Scheme 2). All
`
`Scheme 2.
`
`Synthesis of Ar-BFCs 1c and 1d
`
`Figure 1. Solid-state orderings of Ar-BFCs (a) 1a and (b) 1d.
`
`arrangement.20 The antiparallel pairs further stack in a
`cofacial manner, resulting in a one-dimensional stacking
`column. The interplanar (cid:240)-stacking distance between two
`molecules in the same stacked pair ((cid:240)a-(cid:240)a¢ , (cid:240)b-(cid:240)b¢ : 3.47
`Å) is slightly greater than that between different pairs
`((cid:240)a¢ -(cid:240)b: 3.43 Å). The pendant phenyl group is inclined at
`approximately 57.4(cid:176) with respect to the acene plane. One
`phenyl group also has an edge-to-face interaction with the
`acene core (C(cid:240)-Cph: 3.61, 3.80 Å) and the phenyl groups
`(Cph-Cph: 3.67 Å) in neighboring columns (Figure S1,
`Supporting Information). Single-crystal structures of 1c and
`1d were also obtained, and the molecular arrangement was
`found to be very similar between the two (Figure S2 for 1c;
`Figures 1b and S3 for 1d; see Supporting Information).
`Similar to 1a, two molecules of 1d form a cofacial and
`antiparallel pair, and the pairs further stack in a cofacial
`manner to form a (cid:240)-stacked one-dimensional column. Alkyl
`side-chains interdigitate, giving a lamellar structure (Figure
`S3, Supporting Information). The interplanar (cid:240)-stacking
`distances between two molecules in the same pair ((cid:240)c-(cid:240)c¢ ,
`(cid:240)d-(cid:240)d¢ ) and between different pairs ((cid:240)c¢ -(cid:240)d) are the same
`(3.41 Å). One phenyl group has an edge-to-face interaction
`with the acene core in neighboring columns (C(cid:240)-Cph: 3.48
`Å) (Figure S3, Supporting Information). Compared to 1a,
`both the cofacial and the edge-to-face distances are slightly
`shorter. The closer packing structure may be due to the self-
`assembly property of long alkyl chains.
`The photophysical and electrochemical data of Ar-BFCs
`are summarized in Figure 2 and Table 2. As shown in the
`UV/vis spectra, the absorption maximum of 1a is slightly
`blue-shifted compared to that of DPh-IC and red-shifted
`compared to that of DBBDF. The HOMO-LUMO energy
`band gap of 1a (Eg-1a), as evaluated from an absorption edge
`((cid:236) ) 393 nm), is 3.15 eV, which is larger than Eg-DPh-IC
`
`(20) Miao, Q.; Lefenfeld, M.; Nguyen, T. Q.; Siegrist, T.; Kloc, C.;
`Nuckolls, C. AdV. Mater. 2005, 17, 407.
`
`1201
`
`Ar-BFCs dissolved in common organic solvents such as
`CHCl3, THF, and toluene. Therefore, they were purified via
`silica-gel column chromatography and characterized by 1H
`and 13C NMR spectroscopy and HRMS.
`The solid-state ordering of 1a, as determined by single-
`crystal X-ray crystallography, is shown in Figure 1a. As de-
`scribed in our previous report,16 DBBDF showed a molecular
`ordering dominated by edge-to-face interactions between
`heteroacene cores, leading to a herringbone arrangement.
`DPh-IC also formed dominant edge-to-face interactions in
`the solid state. However, the two pendant phenyl groups on
`the nitrogen atoms of DPh-IC caused weaker interaction
`between the heteroacene cores. In sharp contrast, the solid-
`state ordering of 1a is completely different from that of
`DBBDF and DPh-IC. Two molecules form a pair with
`cofacial and antiparallel stacking (Figure 1a). Such a stacking
`structure is probably due to the steric effect of the unsym-
`metrically placed phenyl group. A small molecular dipole
`moment of 1a may also contribute to such an antiparallel
`
`(17) For double N-arylation in carbazole synthesis, see: (a) Nozaki, K.;
`Takahashi, K.; Nakano, K.; Hiyama, T.; Tang, H. Z.; Fujiki, M.; Yamaguchi,
`S.; Tamao, K. Angew. Chem., Int. Ed. 2003, 42, 2051. (b) Kuwahara, A.;
`Nakano, K.; Nozaki, K. J. Org. Chem. 2005, 70, 413. (c) Kitawaki, T.;
`Hayashi, Y.; Ueno, A.; Chida, N. Tetrahedron 2006, 62, 6792.
`(18) Tomori, H.; Fox, J. M.; Buchwald, S. L. J. Org. Chem. 2000, 65,
`5334.
`(19) Charles, M. D.; Schultz, P.; Buchwald, S. L. Org. Lett. 2005, 7,
`3965.
`
`Org. Lett., Vol. 10, No. 6, 2008
`
`DUK SAN NEOLUX
`EXHIBIT 1016
`PAGE 000003
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`

`
`Supporting Information). The alkoxy substituents on the
`phenyl group have little effect on the photophysical properties
`of the molecules (Table 2, Figure S5, Supporting Informa-
`tion),21 whereas they do affect the electrochemical properties;
`HOMO and LUMO levels of the alkoxy derivatives 1c and
`1d were found to be higher than those of parent 1a (Table
`2 and Figure S4).
`the synthesis of Ar-BFCss
`In summary, we report
`unsymmetrical heteroacenes containing both nitrogen and
`oxygen atomssvia palladium-catalyzed double N-arylation
`of arylamines with dibenzofuran precursor 6. Ar-BFCs are
`soluble in common organic solvents and exhibit an antipar-
`allel cofacial (cid:240)-stacking structure in the solid state, which
`would cause strong electronic coupling. Such a (cid:240)-stacking
`pattern presumably results from their unsymmetrical struc-
`tures. Thus, the present molecular design of the unsym-
`metrical introduction of a substituent on a (hetero)acene core
`would provide a guide to a cofacial arrangement of the
`(hetero)acene core. Photophysical and electrochemical ex-
`periments suggest that BFCs should be expected to possess
`high oxidative stability. Further studies on the synthesis of
`a series of BFCs as well as their application to OFETs are
`now in progress in our laboratory.
`
`Acknowledgment. We are grateful to Prof. Takashi Kato
`and Dr. Takuma Yasuda (The University of Tokyo) for CV
`analysis and to Prof. Yoshiaki Nishibayashi and Prof.
`Yoshihiro Miyake (The University of Tokyo) for HRMS
`analysis. This work was partially supported by a Grant-in-
`Aid for Science Research in a Priority Area “Super-
`Hierarchical Structures” (No. 446), for Young Scientists (B)
`(No. 18750112), and for the Global COE Program for
`Chemistry Innovation from the Ministry of Education,
`Culture, Sports, Science and Technology, Japan. K.K. thanks
`the Hayashi Memorial Foundation for Female Natural
`Scientists for financial support. K.N. gratefully acknowledges
`the financial support provided by the Konica Minolta
`Imaging Science Foundation.
`
`Supporting Information Available: Experimental pro-
`cedures, solid-state orderings, cyclic voltammograms of 1a,
`1c, and 1d, UV/vis spectra in solutions (1a-1d) and in thin
`films (1c and 1d), 1H and 13C NMR spectra for synthesized
`compounds, and crystallographic information files for 1a,
`1c, and 1d. This material is available free of charge via the
`Internet at http://pubs.acs.org.
`
`OL703110G
`
`(21) The UV/vis spectra of 1c and 1d in thin films can be found in the
`Supporting Information.
`
`Figure 2. UV/vis spectra of 1a, DPh-IC, and DBBDF in CHCl3
`(1 (cid:2) 10-5 M).
`
`and smaller than Eg-DBBDF. Cyclic voltammetry experiments
`of parent Ph-BFC 1a (Figure S4, Supporting Information)
`show one reversible oxidation wave in the scan range of
`0-1.5 V, in contrast to DBBDF, which produces one quasi-
`reversible wave, and DPh-IC, which produces two reversible
`waves.16 The HOMO energy level of 1a (EHOMO-1a), as
`onset), is 5.59 eV
`evaluated from the first oxidation onset (Eox
`below the vacuum level. EHOMO-1a is between EHOMO-DPh-IC
`and EHOMO-DBBDF, indicating that Ph-BFC 1a is less sensitive
`to oxidative degradation than DPh-IC. The LUMO energy
`level of 1a (ELUMO-1a) is -2.44 eV, which is lower than
`ELUMO-DPh-IC and ELUMO-DBBDF. These experimentally esti-
`mated energy levels of the frontier orbitals of 1a are
`consistent with those obtained from MO calculations (see
`
`Table 2. Photophysical and Electrochemical Data of
`Heteroacenes
`
`compound
`
`1a
`1c
`1d
`DPh-ICf
`
`(cid:236)edge
`(nm)a
`
`393
`395
`395
`422
`
`Eg
`(eV)b
`
`3.15
`3.14
`3.14
`2.95
`
`DBBDFf
`
`354
`
`3.50
`
`peak
`Eox
`(V)
`
`onset
`Eox
`(V)c
`
`1.17
`1.03
`1.10
`0.67
`1.24
`1.59
`
`1.01
`0.90
`0.95
`0.46
`
`1.16
`
`ELUMO
`EHOMO
`(eV)e
`(eV)d
`-5.59 -2.44
`-5.48 -2.34
`-5.53 -2.39
`-5.08 -2.13
`
`-5.78 -2.28
`
`a Absorption edge. b Determined from the absorption edge. c Onset
`potentials (vs Ag/Ag+) of first oxidation wave determined by cyclic
`voltammetry: 0.1 M Bu4NClO4 in CH2Cl2, Pt as working and counter
`electrodes, scan rate of 50 mV(cid:226)s-1. d Calculated according to EHOMO )
`onset + 4.58). e All values were estimated from the optical band gaps
`-e(Eox
`and EHOMO. f See ref 16.
`
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