Published: September 19, 2011 r2011 American Chemical Society 4367 dx.doi.org/10.1021/cm201819p | Chem. Mater. 2011, 23, 4367–4374 ARTICLE pubs.acs.org/cm Assembly and Characterization of Well-Defined High-Molecular-Weight Poly(p-phenylene) Polymer Brushes Jose Alonzo,* ,† Jihua Chen, ‡ Jamie Messman, ‡ Xiang Yu, § Kunlun Hong, ‡ Suxiang Deng, ^ Onome Swader, ^ Mark Dadmun, §,^ John F. Ankner, † Phillip Britt, § Jimmy W. Mays, ‡,§,^ Massimo Malagoli, #,|| Bobby G. Sumpter, ‡,|| Jean-Luc Br  edas, # and S. Michael Kilbey, II* ,‡,^ † Neutron Scattering Science Division, ‡ Center for Nanophase Materials Sciences, § Chemical Sciences Division, and || Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ^ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States # School of Chemistry and Biochemistry, Center for Computational Molecular Science and Technology, and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States b S Supporting Information ’ INTRODUCTION Conjugated polymers are the basis for many future develop- ments in areas such as energy conversion, energy storage, bio- sensors, and many devices that take advantage of their electrical, magnetic, and optical properties. 1À3 They are used as active layers in organic optoelectronic devices or organic integrated circuits. Although there has been considerable progress in these areas of research, many of the fundamental aspects that govern the performance of these devices are still largely unexplored or unresolved. Among these fundamental aspects, control of mor- phology at the nanoscale has been recognized as one of para- mount importance. 4À6 As many devices rely on tight control of interfaces, research focused on elucidating the links between nanoscale design, assembly, structure and properties is crucially important. Along these lines, conjugated polymers confined to interfaces in a “brushlike” configuration represent an important model system. Not only do these systems allow structureÀprop- erty relationships of well-defined systems to be studied, but the organized structure of a brush layer offers the possibility of improved or enhanced optoelectronic properties when π-con- jugated materials are used. For example, it has been shown by Niko et al. that the optical absorbance of oriented films of para-hexaphenyl, the six unit oligomer of poly(para-phenylene) (PPP), exhibits strong anisotropy effects. 7 PPP is regarded as one of the most promising polymers for optoelectronic applications 8 due to its high thermal and chemical stability 9 and high conductivity when doped. 10 Unfortunately, solubility issues inherent in high-molecular-weight unsubstituted PPPs (and other conjugated polymers) create processing and characterization problems that have hindered detailed studies of structureÀproperty relationships of these macromolecular ma- terials. To address this problem, low molecular weight PPPs or oligomeric paraphenylenes (OPPs) have been studied exten- sively, both in solution and as thin films, and the properties of high molecular weight PPPs have been extrapolated from these results. 11À13 Thus, not only is it challenging to create devicelike constructs based on high-molecular-weight PPPs, but the syn- thetic approaches typically used to create high molecular weight PPPs often preclude rigorous characterization of molecular parameters such as molecular weight, polydispersity, and chain Received: June 26, 2011 Revised: August 24, 2011 ABSTRACT: The assembly and characterization of well-defined, end-tethered poly- (p-phenylene) (PPP) brushes having high molecular weight, low polydispersity and high 1,4-stereoregularity are presented. The PPP brushes are formed using a precursor route that relies on either self-assembly or spin coating of high molecular weight (degrees of poly- merizations 54, 146, and 238) end-functionalized poly(1,3-cyclohexadiene) (PCHD) chains from benzene solutions onto silicon or quartz substrates, followed by aromatization of the end-attached PCHD chains on the surface. The approach allows the thickness (grafting density) of the brushes to be easily varied. The dry brushes before and after aromatization are characterized by ellipsometry, atomic force microscopy, grazing angle attenuated total reflectance Fourier transform infrared spectroscopy, and UV-Vis spectros- copy. The properties of the PPP brushes are compared with those of films made using oligo- paraphenylenes and with ab initio density functional theory simulations of optical proper- ties. Our results suggest conversion to fully aromatized, end-tethered PPP polymer brushes having effective conjugation lengths of 5 phenyl units. KEYWORDS: poly(para-phenylene), polymer brush, thin film, electronic structure