Materials Chemistry and Physics 124 (2010) 257–263 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Semiconductor behavior of 2,5-aromatic disubstituted pyrroles, viewed from an experimental and theoretical perspective Lioudmila Fomina, Gerardo Zaragoza Galán, Monserrat Bizarro, Jorge Godínez Sánchez, Irineo P. Zaragoza, Roberto Salcedo ∗ Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Coyoacán 04510, México DF, Mexico article info Article history: Received 21 May 2009 Received in revised form 8 June 2010 Accepted 13 June 2010 Keywords: Semiconductor polymers Thin films Computer modeling and simulation abstract Theoretical calculations were performed on 2,5-aromatic substituted pyrroles which have a nitro- benzene or a cyano-benzene link to the nitrogen atom of the pyrrole fragment. The molecules manifested interesting semiconductor behavior that was confirmed when thin films were prepared and their cor- responding electrical characterization was undertaken. The reason for this behavior is discussed, with reference to the electron-withdrawing feature of the substituents in the benzene chain. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Thin film transistors, made of inorganic semiconductor mate- rials have dominated the electronics industry, since the first transistor was invented by J. Bardeen, W. Shockley, and W. Brattain in 1947. Recently, organic materials have started to play an important role in electronic and photonic technolo- gies. Organic semiconductors are very useful because they have electrical properties similar to those of inorganic semiconduc- tors, but manifest the same properties as plastic (low cost, easily processed, flexibility and versatility of chemical synthe- sis). Interest in organic semiconductors began, following reports describing the electrical conductivity of the organic compound violanthrone in 1950, by Akamatu and Inokuchi [1]. Two decades later Shirakawa published information concerning the conduct- ing conjugated polymer, polyacetylene [2]. Since this discovery, many research groups have investigated -conjugated oligomers and polymers, mainly in relation to their nonlinear and semi- conducting properties. In 1987 Tang and Van Slyke at Kodak reported the first electroluminescent device, based on a - conjugated material, tris(8-hydroxyquinoline)aluminium [3] and in 1990, Friend’s group discovered electroluminescence in a con- jugated polymer, poly(paraphenylenevinylene) [4]. Until now, -conjugated oligomers and polymers have attracted considerable interest because of their potential applications in optoelectronic devices [5]. Many applications have been discovered, relating to ∗ Corresponding author. Tel.: +52 55 56224600; fax: +52 55 56161201. E-mail address: salcedo@servidor.unam.mx (R. Salcedo). the areas of light-emitting diodes [6], field effect transistors [7] and photovoltaic cells [8]. One of the most widely investigated -conjugated polymers is polypyrrole, due to its optical prop- erties and electrical conductivity [9]. Chemical functionalisation of -conjugated polymers by introducing substituents increases their potential for being processed and causes dramatic changes in structure and electronic properties [10]. Investigations con- cerning the substitution effects of polypyrrole oligomers, using quantum mechanical calculations have provided an understand- ing of the conducting and optical properties of these materials [11]. A narrow gap between HOMO and LUMO, indicating semi- conductor behavior [12] is the outstanding feature of molecules which represent apt targets regarding possible applications in the conducting area [12]. Most systems being researched comprise semiconductors with band gaps ranging from 2 to 4 eV [13]. Exten- sive research [5,12,14], using both experimental and theoretical approaches has been carried out in an attempt to endow a low HOMO/LUMO gap to organic materials. One strategy employed in order to decrease the band gap in organic molecular materials consists of substitution with electron-withdrawing groups [15]. Recently, it has been reported that molecules of N-substituted pyrroles with electron-withdrawing substituents decreased both HOMO and LUMO energies, when compared to their unsubstituted analogues [16]. Although previous works have analyzed the elec- tronic properties of N-substituted pyrroles, nobody had studied the case of nitro and cyano 2,5-disubstituted pyrroles (Fig. 1). In this work, these two different molecules were synthesized, electrically characterized and theoretically studied to explain the origin of their semiconducting behavior. The theoretical calculations were com- pared to the experimental results of the synthesized systems that 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.06.028