Synthesis, characterization and electronic applications of novel calamitic azulenes

Abstract

Due to its inherent dipole moment and unique electronic properties the azulene moiety is of interest in the development of novel organic semiconductors for optoelectronic applications. Molecular orientation of the applied organic semiconductors is crucial for the performance of organic electronics and can be controlled to some extent e.g., through self-assembly of liquid crystals. We developed a divergent synthetic route starting from 2,6-dibromoazulene to access novel, calamitic mesogens and investigated these azulenes on their mesomorphic behavior and -for selected examples- on their suitability as organic semiconductors. Azulenes with only one side chain tethered to the 7-membered ring exclusively formed soft-crystalline smectic E-phases (SmE), with 2-(6-(dodecyloxy)azulen-2-yl)benzo[b]thiophene(12O-Az-BT) exhibiting the broadest phase width between 51 °C and 270 °C during cooling. Additionally, derivatives with two side chains and a lateral substituent at the 5-membered ring were prepared. Based on 6-dodecyloxy-2-(4-dedecyloxyphenyl)azulene-1-carbonitrile (12O-AzCN-PhO12), parameters such as chain length, size, and polarity of the lateral substituent, and (hetero)aromatic moiety in 2-position were systematically varied to understand the complex structure-property-relationship governing mesomorphic properties of our novel azulenes. Mainly SmC phases were observed for derivatives with unhindered interaction of the aromatic azulene cores. Upon disturbance of the stacking due to bulky substituents, less polar side chains or the electronic influence of heteroatoms, less ordered SmA phase were the preferred mesophase. Some azulene derivatives with a SmA-SmC phase sequence were found to show de Vries-like properties, which are heavily investigated in the context of next generation liquid crystal displays. The best values for important de Vries parameters, namely a maximum layer shrinkage of only 0.16 % resulting in reduction factors of down to 0.14 compete with state of the art deVries materials. Additionally, 6-hexadecyloxy-2-(4-octyloxyphenyl)azulene-1-carbonitrile (16O-AzCN-PhO8) exhibited a SmA-SmC-SmA phase sequence. The rarely observed SmA re-entrant phase was explained by a reorientation of the azulene cores which was also identified as a key factor for de Vries-like behavior. A selected series of azulenes modified with thiophene units was investigated on their suitability as semiconducting layer in organic field effect transistor (OFET) devices. Spin-coating of these azulene derivatives at mesophase temperature resulted in uniform polycrystalline thin films. In the corresponding OFET devices, 12O-Az-BT achieved the best hole mobility with μ+ = (3.1 ± 0.5)∙10-3cm2V-1s-1.