Synthesis, characterisation and analysis of triarylamines as potential charge transport materials
The application of triarylamines as charge transport materials is an important consideration in the modern electrophotographics industry. The ability of these materials to form stable amorphous glasses and the electrochemical properties they exhibit has led to the use of compounds such as 4,4'-bis-[N-(3-methylphenyl-N-phenylamino)]biphenyl (TPD) as charge materials in electrophotography and electroluminescence. In order to develop this science an understanding of the structural properties that facilitate the use of triarylamines in such devices is required so that new charge transport materials can be developed with improved performance and stability. With this intention Ullmann condensation reactions were employed to prepare a series of substituted triphenylamines and triarylamine starburst compounds containing differing structural units for which the synthesis and characterisation are detailed. Alkene linked triarylamines were prepared employing Wadsworth-Emmons reactions, Wittig reactions, McMurry coupling reactions and Grignard reactions. Further, palladium catalysed cross coupling reactions were undertaken to prepare acetylene linked triarylamines. Thermal analysis was undertaken using the differential scanning calorimetry (DSC) technique which identified those compounds which display an amorphous nature, indicated by a glass transition temperature. The importance of this property is linked to the requirement of charge transport materials to either formulate in an inert polymeric binder as part of a charge transport layer or form an amorphous thin film monolayer. Electrochemical analysis was undertaken by cyclic voltammetry (CV) and time of flight techniques (TOF). This analysis of triarylamines explores the viability of the concepts of polyfunctionality and intramolecular charge mobility as well investigating the effects of structure on the stability, electrochemical reversibility and oxidation potentials of the radical cations of triarylamines prepared.