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Chem. Mater. 2009, 21, 1400–1405
A Meaningful Analogue of Pentacene: Charge Transport, Polymorphs, and Electronic Structures of Dihydrodiazapentacene Qin Tang,† Dieqing Zhang,† Shenglong Wang,‡ Ning Ke,§ Jianbin Xu,§ Jimmy C. Yu,† and Qian Miao*,† Department of Chemistry, Department of Electronic Engineering, The Chinese UniVersity of Hong Kong, Shatin, Hong Kong, China, and Department of Chemistry, New York UniVersity, New York 10003 ReceiVed January 21, 2009. ReVised Manuscript ReceiVed February 18, 2009
6,13-Dihydro-6,13-diazapentacene (DHDAP) has two nitrogen atoms replacing two carbon atoms of pentacene, the leading organic semiconductor for organic thin film transistors (OTFTs). This report details a comprehensive investigation of DHDAP highlighting the relationship between charge transport, polymorphs, and electronic structures. Three crystalline polymorphs are found from the thin films of DHDAP according to their (001) spacing (layer periodicity). Our surprising finding is that the field effect mobility of DHDAP is extremely sensitive to the polymorphs with the “12.9 Å phase” yielding a mobility of 0.45 cm2 V-1 s-1, which is over 5000 times higher than those of the other two phases. This unusually large effect of the crystalline polymorph on charge transport can be understood in terms of molecular packing using the models developed by de Wijs and Bre´das. The comparable field effect mobilities and highly relevant structures of DHDAP and pentacene imply that the common structural features shared by the two molecules may be key factors that benefit the electrical performance while the particular structural features of pentacene should be unimportant to the electrical properties. In this point of view, DHDAP is a meaningful analogue of pentacene allowing better understanding on the structure-property relationship of pentacene. The electronic structure of DHDAP is studied in comparison with that of pentacene using both computational and experimental methods. It is found that DHDAP has a delocalized HOMO with the energy level essentially the same as that of pentacene, although the HOMO-LUMO gap of DHDAP is significantly larger. The environmental stability of DHDAP suggests that a relatively high HOMO energy level does not necessarily lead to environmentally unstable organic semiconductors. These results may lead to better understanding on the structure-property relationship of organic semiconductors.
Introduction Organic thin film transistors (OTFTs) offer promising applications in large-area, flexible, and low-cost electronics.1 Pentacene (shown in Figure 1) has led organic semiconductors with one of the highest charge carrier mobilities in OTFTs2 and is regarded as the benchmark for all newly developed small-molecule organic semiconductors.3 However, it remains unclear how the outstanding electrical performance of pentacene OTFTs exactly depend on the molecular structure, crystal structure, and thin film morphology.4 Better understanding these questions is a key to rational design of organic semiconductors better than pentacene. Atmospheric instability and very low solubility are two * Corresponding author. E-mail:
[email protected]. † Department of Chemistry, The Chinese University of Hong Kong. ‡ New York University. § Department of Electronic Engineering, The Chinese University of Hong Kong.
(1) For reviews of organic TFTs, see: (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99–117. (b) Katz, H. E. Chem. Mater. 2004, 16, 4748–4756. (c) Dodabalapur, A. Mater. Today 2006, 9, 24–30. (d) Muccini, M. Nat. Mater. 2006, 5, 605–613. (2) The highest field effect mobility of pentacene measured in thin film transistors is 5 cm2 V-1 s-1. See: Kelley, T. W.; Muyres, D. V.; Baude, P. F.; Smith, T. P.; Jones, T. D. Mater. Res. Soc. Symp. Proc. 2003, 771, 169. (3) Murphy, A. R.; Fre´chet, J. M. J. Chem. ReV. 2007, 107, 1066–1096. (4) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Bre´das, J.-L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436–4451.
factors that limit practical applications of pentacene in organic electronics, and great efforts have been devoted to molecular engineering of pentacene to improve stability and solubility.5 Nitrogen-containing heteroacenes are an interesting but much less studied class of π-functional materials.6 Only a few members of nitrogen-containing heteroacenes have been explored for the applications in OTFTs.7,8 6,13-Dihydro-6,13(5) (a) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J. J. Am. Chem. Soc. 1998, 120, 664–672. (b) Afzali, A.; Dimitrakopoulos, C. D.; Breen, T. L. J. Am. Chem. Soc. 2002, 124, 8812–8813. (c) Payne, M. M.; Parkin, S. R.; Anthony, J. E.; Kuo, C.-C.; Jackson, T. N. J. Am. Chem. Soc. 2005, 127, 4986–4987. (d) Okamoto, T.; Senatore, M. L.; Ling, M.-M.; Mallik, A. B.; Tang, M. L.; Bao, Z. AdV. Mater. 2007, 19, 3381–3384. (6) (a) Manassen, J.; Khalif, Sh. J. Am. Chem. Soc. 1966, 88, 1943–1947. (b) Jenekhe, S. A. Macromolecules 1991, 24, 1–10. (c) Riley, A. E.; Mitchell, G. W.; Koutentis, P. A.; Bendikov, M.; Kaszynki, P.; Wudl, F.; Tolbert, S. H. AdV. Funct. Mater. 2003, 13, 531–540. (d) Tadokoro, M.; Yasuzuka, S.; Nakamura, M.; Shinoda, T.; Tatenuma, T.; Mitsumi, M.; Ozawa, Y.; Toriumi, K.; Yoshino, H.; Shiomi, D.; Sato, K.; Takui, T.; Mori, T.; Murata, K. Angew. Chem., Int. Ed. 2006, 45, 5144– 5147. (e) Winkler, M.; Houk, K. N. J. Am. Chem. Soc. 2007, 129, 1805–1815. (7) (a) Ma, Y.; Sun, Y.; Liu, Y.; Gao, J.; Chen, S.; Sun, X.; Qiu, W.; Yu, G.; Cui, G.; Hu, W.; Zhu, D. J. Mater. Chem. 2005, 15, 4894–4898. (b) Nishida, J.; Naraso Murai, S.; Fujiwara, E.; Tada, H.; Tomura, M.; Yamashita, Y. Org. Lett. 2004, 6, 2007–2010. (8) Miao, Q.; Nguyen, T.-Q.; Someya, T.; Blanchet, G. B.; Nuckolls, C. J. Am. Chem. Soc. 2003, 125, 10284–10287.
10.1021/cm9001916 CCC: $40.75 2009 American Chemical Society Published on Web 03/19/2009
Meaningful Pentacene Analogue: Dihydrodiazapentacene
Chem. Mater., Vol. 21, No. 7, 2009 1401 Table 1. Field Effect Mobility for OTFTs of DHDAP and Pentacene Depending on Dielectric Surface and Substrate Temperature (Ts)a bare SiO2
Figure 1. Molecular structures of pentacene and 6,13-dihydro-6,13diazapentacene (DHDAP).
material
Ts ) 25 °C
Ts)100 or 80 °Cb
DHDAP pentacenec
(5-8) × 10-5 ∼0.11-0.14
