Radical and non-radical carbazole derivatives formolecular electronics. Molecular Glasses and Liquid Crystals.

Abstract

Molecular electronics is an emergent area of the new technologies related to the use of organic and biological materials in optoelectronic and electronic devices. Among the materials used in electronic applications, low molecular weight materials, or molecular materials, became of great interest in the last years, due to the advantages they present in front of the traditional polymeric materials, such as, easier synthesis, purification and characterization, and a better processability.<br/><br/>The objectives of this thesis were the preparation and characterization of new molecular materials bearing the carbazole heterocycle (part 1), the evaluation of their charge transport properties (part 2) and their application in electronic devices, for the ones that presented the required properties (part 3).<br/><br/>1. Materials. New compounds belonging to two groups were prepared: non-radical carbazole derivatives, mainly based in the N-phenylcarbazole aromatic core, and radical carbazole derivatives, based on structures that combine the unit tris(2,4,6-trichlorophenyl)methyl radical and the carbazole fragment. Depending on the morphology of their solid state, the sought materials were classified in liquid crystals and molecular glasses, that is, molecular materials showing glassy states. The light absorption, photoluminescence and electrochemical properties of the different derivatives and their solid state phases were determined by different techniques. The studies proved that the mesogens deriving from N-phenylcarbazole with 7 alkoxyl chains presented ordered columnar phases. These columnar mesophases were stabilized by doping with 2,4,7-trinitrofluorenone and destabilized by the presence of terminal carboxílic acids. However, the derivatives with the same aromatic core but only 4 alkoxyl chains formed lamelar phases (layers). The analogous N-phenylcarbazole derivatives with 7 and 4 methoxy groups, though, presented glassy states. Derivatives of the 1,3,5-tris(N-carbazolyl)benzene bearing tricyanovinyl moieties, which showed reversible reductions -an opposite behavior to the 1,3,5-tris(N-carbazolyl)benzene hole-transporting precursor- and high thermal stability, were prepared. The first discotic radical bearing the unpaired electron in the center of the aromatic core and showing columnar liquid-crystalline phases was prepared. A change in the magnetic behavior due to changes in the molecular disposition was detected in this radical compound at temperatures under rt. and over rt.<br/>2. Charge transport properties. The most representative glassy molecules and columnar liquid crystals were evaluated as semiconductors. The used techniques were the xerographic time-of-flight (XTOF), for molecular glasses, and the time of flight (TOF), for liquid crystals. The obtained results indicated that some of the radical molecular glasses presented charge transport with high mobility values for both holes and electrons (bipolar materials), whereas some of the non-radical molecular glasses showed moderate hole mobility values by XTOF measurements. The charge mobility of liquid crystals could not be evaluated due to the highly dispersive regime of their charge transport. A methodology to attain homeotropic alignment of the radical mesogen columns within the measure cells was found.<br/>3. Electronic devices. Organic thin film transistors (OTFTs) of two radical compounds, as well as of an N-phenylcarbazole derivative were built. A p-type semiconductor behavior was observed in some cases. Schottky type and bulk-heterojunction organic photovoltaic devices (OPVs) with N-carbazolylTTM radical were prepared. In these devices the radical adduct worked as a donor material. Acceptable open circuit voltage values and symbathic behaviors were observed in the measured cells.

<I>De entre els materials que es fan servir per aplicacions electròniques, els materials de baix pes molecular, o materials moleculars, han estat el centre d'un gran interés en els últims anys al camp de l'electrònica molecular degut als avantatges que presenten enfront dels materials polimèrics tradicionals.<br/><br/>El treball realitzat durant aquesta tesi tingué com a objectiu la preparació i caracterització de nous materials moleculars que continguessin l'heterocicle de carbazole (part 1), l'avaluació de les seves propietats de transport de càrrega (part 2) i la seva aplicació en dispositius electrònics (part 3). <br/><br/>1. Materials. Es van preparar i caracteritzar nous compostos pertanyents a dos grups: derivats del carbazole no radicalaris, principalment basats en el nucli aromàtic d'N-fenilcarbazole i derivats del carbazole radicalaris, basats en estructures que combinen la unitat radicalària de tris(2,4,6-triclorofenil)metil i el fragment de carbazole. Es van determinar les propietats fotoluminiscents i electroquímiques dels diferents derivats i es van caracteritzar les seves fases cristall-líquid o vítries mijtançant diferents tècniques. Entre els materials preparats es van detectar des de cristalls líquids columnars que presentaven processos d'oxidació reversible fins a vidres moleculars amb capacitat de reduir-se reversiblement. A més, es va obtenir el primer radical discòtic que presenta l'electró desaparellat en el centre del nucli aromàtic. El comportament magnètic d'aquest radical variava en funció de la temperatura degut als canvis en la seva ordenació molecular.<br/><br/>2. Propietats de transport de càrrega. Es van avaluar els vidres moleculars per la tècnica de temps de vol xerogràfic (XTOF) i els cristalls líquids columnars per la tècnica de temps de vol (TOF). Els resultats obtinguts més rellevants indicaven que alguns dels compostos radicalaris actuaven com a vidres moleculars bipolars (transportardors de forats i electrons). <br/><br/>3. Dispositius electrònics. Es va observar un comportament de semiconductor tipus-p en alguns transistors de capa fina orgànics (OTFTs) basats en compostos radicalaris i en un derivat de l'N-fenilcarbazole. Es van preparar cel·les solars (OPVs) amb el radical N-carbazolilTTM que presentaren voltatges de circuit obert amb valors acceptables i comportaments simbàtics.</I>