Semiconductor Behavior of 2, 5-Aromatic Disubstituted Pyrolles, an Experimental and Theoretical Study
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SEMICONDUCTOR BEHAVIOR OF 2, 5-AROMATIC DISUBSTITUTED PYROLLES, AN EXPERIMENTAL AND THEORETICAL STUDY L. Fomina1, C. Y. León Valdivieso1, G. Zaragoza Galán1, M. Bizarro1, I. P. Zaragoza1, J. Godínez Sánchez1, R. Salcedo1 and A. Baeza2 1
Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior s/n. C.U. A. Postal 70-360. Delegación Coyoacán. C.P. 04510. México D.F. México. 2
Facultad de Química- Universidad Nacional Autónoma de México, Circuito Exterior s/n. C.U. A. Postal 70-360. Delegación Coyoacán. C.P. 04510. México D.F. México.
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 pyrrol fragment. The molecules manifested interesting semiconductor behavior that was confirmed when thin films were prepared and their corresponding electrical characterization undertaken. The reason for this behavior is discussed, with reference to the electron withdrawing feature of the substituents in the benzene chain.
INTRODUCTION Recently, organic materials have started to play an important role in electronic and photonic technologies. Organic semiconductors are very useful because they have electrical properties similar to those of inorganic semiconductors, but manifest the same properties as plastic (low cost, easily processed, flexibility and versatility of chemical synthesis). Until now, π-conjugated oligomers and polymers have attracted considerable interest because of their potential applications in optoelectronic devices [1]. One of the most widely investigated π-conjugated polymers is polypyrrole, due to its optical properties and electrical conductivity [2]. Investigations concerning the substitution effects of polypyrrole oligomers, using quantum mechanical calculations have provided an understanding of the conducting and optical properties of these materials [3]. Most systems being researched comprise semiconductors with band gaps ranging from 2 to 4 eV [4]. Extensive research [1, 5], 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 electronwithdrawing groups [6]. In this work, nitro and cyano 2,5-disubstituted pyrroles were synthesized, electrically characterized and theoretically studied to explain the origin of their semiconducting behavior. The theoretical calculations were compared to the experimental results of the synthesized systems that were obtained using a modification of the SchulteReisch reaction, as previously described [7, 8].
EXPERIMENTAL DETAILS Reagents were provided by Aldrich Chemical Company and used as purchased. FT-IR spectra were taken, using Nicolet 510p spectrophotometer. NMR 1H and 13C spectra were recorded using a Bruker Avance 400 MHz spectrometer. The chemical shifts are reported in ppm, scaled relative to TMS. Melting points
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