Design and characterization of cyanoaza heterocycles for use as n -type organic semiconductor materials.

Abstract

A family of azanitrile compounds was developed as potentially useful organic molecular conductors of electrons. In order to transport electrons, an organic molecular conductor (OMC) must be easily reduced and remain thermodynamically stable. Additionally, high electron mobility is favored if the reduced OMC doesn't significantly distort. Chemical change or molecular distortion lead to deep trapping of charge carriers that reduce the conductivity. The azanitrile compounds were characterized with respect to the ease and reversibility of reduction, stability and structure in the reduced state; finally thin films were prepared of the materials and charge transport measurements were carried out. The target azanitrile compounds were synthesized, purified, and characterized by chemical analysis and spectrometry. The reduction potentials were combined with spectroscopic determinations of the HOMO-LUMO gap energies to develop correlations between the molecular structures and electron injection energies of the target compounds. The chemical stability and molecular geometry of the reduced species were examined by EPR. Analysis of the EPR spectra was achieved in most cases by combining DFT calculations of the electron spin densities with spectral simulation techniques. Generalized rules for correlating nuclear hyperfine splittings with spin densities were developed and compared with the work of earlier investigators. The EPR analyses demonstrate both the chemical stability of the reduced azanitrile as well as the preservation of the neutral molecular geometry in the reduced state. The crystal structure of HAT(CN)₆-TBA revealed a slipped-stack packing arrangement that suggested the possibility of nearest-neighbor magnetic interactions between radical anions. The complex temperature dependent magnetic susceptibility was discovered to be a combination of Curie paramagnetism and a temperature independent paramagnetic susceptibility arising from a Pauli contribution due to the presence of free charge carriers in the material. Thin films were prepared and the electron transport properties examined by variable temperature electron time-of-flight mobility measurements. The electron mobilities were on the order of 10E<super>-4</super> cm<super>2</super>/Vs. The TOF studies revealed that transport in the azanitrile films was by variable range hopping mechanism, and that the films typically contained a mixture of deep and shallow traps, characteristic of disordered films.