Electronic and optical properties of functionalized pentacene compounds in the solid state q J.S. Brooks a,c, * , D.L. Eaton b , J.E. Anthony b , S.R. Parkin b , J.W. Brill a , Y. Sushko a a Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506-0055, USA b Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055, USA c Department of Physics and NHMFL, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA Received 8 May 2001; accepted 18 June 2001 Abstract We describe physical measurements on a unique class of pentacene-based compounds [e.g., 6,13-bistriisopropylsilylethy- nyl)pentacene] where additional symmetric side groups have been added to enhance their solubility in common organic solvents. Single crystals of these materials formed from precipitation consist of various p stacking con®gurations columnar) of the pentacene molecules which are dierent from the herringbone motif of unsubstituted pentacene. Here we report the anisotoropic resistivity of single crystals of these materials, including the determination of activation gaps of order 1 eV) obtained from Arrhenius analysis of the temperature dependent resistivity. We ®nd that the resistivity at room temperature lies in the range 10 5 ±10 10 X cm, depending on crystallographic direction and side-group placement and composition. We further describe the response of the conductivity to optical excitation, and present preliminary studies of the dependence of the conductivity on Iodine doping and hydrostatic pres- sure. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 72.20.)i; 78.90.+t Keywords: Acenes; Organic semiconductor; Resistivity; Optical excitation 1. Introduction Recent progress in the realization of organic-elec- tronic devices based on single crystal pentacene thin ®lms [1] has shown that the presence of p-orbitals at the interface of an organic±insulator±metal gate structure leads to a highly mobile two-dimensional electron gas [2]. However these structures must be formed under highly controlled conditions where sublimation of the pentacene takes place in the presence of a hydrogen partial pressure [3]. The purpose of this work has been to consider pentacene derivatives where single crystal structures involving stacked pentacene chains or planes may be formed from simple precipitation from common solvents, or where thin ®lms may be spin-coated on to substrates. As the ®rst step in the characterization of these new structures, we have evaluated a series of pentacene-pendant compounds in terms of their elec- tronic, and optical properties. 2. Synthesis The strategy for funtionalization of pentacene was to introduce solubilizing side groups to allow self-assembly of the aromatic moieties into p-stacked arrays, thereby enhancing intermolecular orbital overlap. The com- pounds may be prepared in near quantitative yield in a one-pot reaction from N ; M -pentacenequinone where N ; M 6; 13 or 5; 14, respectively. Crystals plates of order 1 mm 1 mm 100 lm in size) were precipitated from acetone solution at )20°C and analyzed by X-ray diraction. The samples appear to be air stable, as noted from their electrical and optical properties, with no noticeable degradation after many weeks. Details of the synthesis and structural characterization will be re- ported elsewhere [4]. In the present study, four compounds were studied: 1. 6,13-bistriisopropylsilylethynyl)pentacene hereafter TIPS ± Fig. 1a)). The TIPS crystals are dark blue Current Applied Physics 1 2001) 301±306 www.elsevier.com/locate/cap q Original version presented at the International Workshop on Quantum Transport in Synthetic Metals and Quantum Functional Semiconductors, Seoul, Korea, 8±11 May 2001. * Corresponding author. Tel.: +1-850-644-2836. E-mail address: brooks@magnet.fsu.edu J.S. Brooks). 1567-1739/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S1567-173901)00028-1