Molecule Length Directed Self-Assembly Behavior of Tetratopic Oligomeric Phenylene-Ethynylenes End-Capped with Carboxylic Groups by Scanning Tunneling Microscopy Jian-Feng Zhao, † Yi-Bao Li, ‡ Zong-Qiong Lin, † Ling-Hai Xie,* ,† Nai-En Shi, † Xing-Kui Wu, ‡ Chen Wang,* and Wei Huang* ,† † Key Laboratory for Organic Electronics and Information Displays, Institute of AdVanced Materials, Nanjing UniVersity of Posts and Telecommunications, 9 Wenyuan Road, Nanjing 210046, P. R. China, and ‡ National Center for Nanoscience and Technology, Beijing 100190, P. R. China ReceiVed: March 11, 2010; ReVised Manuscript ReceiVed: April 21, 2010 A series of tetratopic C2-symmetric quasi-planar oligomeric phenylene-ethynylenes bearing carboxylic groups (OPE-COOHs), including the shortest 1,4-di((3,5-dicarboxyphenyl)ethynyl)benzene (TCB), midlength 4,4′- di((3,5-dicarboxyphenyl)ethynyl)biphenyl (TCBP), and the longest 4,4′′-di((3,5-dicarboxyphenyl)ethynyl)- p-terphenyl (TCTP), were synthesized via Pd(PPh 3 ) 4 -CuI-catalyzed Sonogashira coupling reaction. Their molecular structures were characterized by nuclear magnetic resonance spectroscopy (NMR), matrix-assisted laser desorption-ionization time-of-flight mass spectroscopy (MALDI-TOF-MS), and element analyses. The molecule length effect of OPE-COOHs on two-dimensional (2D) assemblies at the octanoic acid-highly ordered pyrolytic graphite (HOPG) interface was investigated by scanning tunneling microscopy (STM) technique at ambient temperature and simulated by molecular modeling. The patterns and domain sizes of OPE-COOHs strongly depend on their molecule length with Kagome ´ network only for TCB and parallel network for TCBP and TCTP. TCTP with the longest rigid-rod OPE backbone exhibits clearly larger ordered domain size than that of TCB and TCBP. Besides, TCBP exhibits tight boundaries transition among different orientation domains via the acute or obtuse V-shaped chevron arrangements. Those different two-dimensional (2D) assembly behaviors will be favorable to get a further understanding of the condensed architectures in conjugated organic semiconductors. 1. Introduction Organic semiconductors based on π-conjugated oligomeric and polymeric phenylene-ethynylenes (OPEs and PPEs) 1-3 with linear rigid-rod conformations have been widely explored in the application of sensors 4 and molecular wires 5-7 because of their excellent signal amplification and electrical conductance with small attenuation factor. 8 However, there are many problems in the field of organic solid devices, including organic light-emitting diodes (OLEDs), 9,10 organic field effect transistors (OFETs), 11-14 photovoltaic cells (PVCs), 15,16 and memories 17 owing to their complicated intramolecular rotation of single bond with low-energy barrier, 18 large hole-injection barrier of triple bonds, 9,19 and strong tendency of interchain π-π stacking interaction to form aggregates and/or excimers. 20 In the context, one of the key issues is to control condensed architectures of PPEs or OPEs in order to obtain the high-performance opto- electronic devices besides band gap engineering, which also could expand their functionality and smart behaviors. For example, conception of supramolecular semiconductors 21-23 has been powerful tools to achieve white OLEDs through the adjustment of energy transfer 24,25 among chromophores, electri- cal switches based on unique shuttle behaviors, and highly insulated light-emitting nanowires. 26 Supramolecular assemblies have been served as “bottom-up” approaches to construct the novelty organic devices. 27,28 The layer-by-layer (LBL) tech- nique 29 could make PPEs-based electroytes 30 to fabricate sandwiched alternative layers with PCBM facilitating charge separation and transportation. Therefore, it is significant to correlate molecular structure with hierarchical architectures via the network of supramolecular association. To achieve these targets, well-defined two-dimensional (2D) self-assembled monolayers (SAMs) 31,32 determined by scanning tunneling microscopy (STM) 33,34 have become a fundamental platform to understand and pre-evaluate the crystalline 35 or amorphous polymorphism 36,37 owing to their simplified interactions and relationships with respect to three-dimensional (3D) crystal engineering systems. Until now, OPEs and other conjugated oligomers have been explored to construct molecular arrangements with various low- dimensional polymorphs, including rhombious, 38 “V”-shaped chevron, 39 chicken wire and flower, 40 row and hexagon, 41 and other nanostructures. 42 Several impacting factors of the self- assembly behaviors and relationships have been investigated. Alternation of backbone with tiny structural parallel dislocation of dipole isophthalic groups may lead to a close-packed network. 43 We have reported that carboxylic groups number and solvent effects direct racemic 2D self-assembled structures of star-shaped oligofluorenes. 44,45 Effects of side chains 43,46-48 tend to induce various self-assembled networks and folding of conjugated PPEs side chains, 47 but they could be circumvented with respect to naked OPEs. Without the above-mentioned, molecule length may play a key role in low-dimensional self- assembled structures of OPEs. Wang et al. reported that * Corresponding authors: e-mail iamlhxie@njupt.edu.cn, Tel +86 25 8586 6396, Fax +86 25 8586 6396 (L.H.X.); e-mail wangch@nanoctr.cn, Tel +86 10 8254 5561, Fax +86 10 6265 6765 (C.W.); e-mail wei-huang@njupt.edu.cn, Tel +86 25 8586 6008, Fax +86 25 8349 2333 (W.H.). J. Phys. Chem. C 2010, 114, 9931–9937 9931 10.1021/jp1022482  2010 American Chemical Society Published on Web 05/12/2010