Star‐Shaped, Dendrimeric and Polymeric Triarylamines as Photoconductors and Hole Transport Materials for Electro‐Optical Applications

Recent developments in the synthesis and application of hole conducting oligomeric and polymeric triarylamines are reviewed. The materials are classified as Star‐shaped molecules, Spiros and dendrimers, Side‐chain polymers, and Main‐chain polymers and copolymers. This paper concentrates on the research results of our group on the synthesis of a variety of such compounds, their structure‐property relationship and their application in devices like organic light emitting diodes, solar cells and photorefractive systems. The thermal properties and electronic properties of these compounds were varied by changing the chemical structure and nature of substituents. In the case of low molecular weight star‐shaped molecules the glass transition temperature could be increased to above 140°C by suitable structural design. Similarly, for polymeric triarylamines the variation of glass transition temperature was achieved over a wide range from 92 to 237°C. This is especially necessary for the wide spectrum of applications of these materials as hole conductors in low‐ T g photorefractive composites to high‐ T g materials in OLEDs. Moreover, the electronic energy levels and the band gap in these compounds can be manipulated to optimize the hole injection or electron transfer or emission properties or even photocurrent generation to make them suitable for various applications. Especially, the concept of copolymerization with other functional monomers results in multifunctional copolymers with good hole injection and transport properties. The polymer networks involving triarylamine structures are not included in this Review, because this constitutes the subject‐matter of insoluble hole transport materials and will be published elsewhere.