Detection of trace amount of impurity in smectic liquid crystals

An effective method for detecting a trace amount of chemical impurity, e.g., a few ppm or less, that degrades the charge carrier transport properties in smectic liquid crystalline (LC) semiconductors was investigated using a model LC semiconductor and a chemical impurity system, i.e., a 2-phenylnaphthatene derivative of 2-(dodecyloxy)-6-(4-octylphenyl)naphthalene (8-PNP-O12) and a terthiophene derivative of 2,5-bis(5-hexylthiophene-2-yl)thiophene (6-TTP-6), respectively. A transient photocurrent measurement disclosed the contamination of chemical impurity of a few ppm or less that could not be detected by conventional analytical methods such as. high-performance liquid chromatography (HPLC) and gas chromatography (GC): the slow transit induced by the ionic conduction of ionized impurity molecules allowed us to detect it, which was clearly distinguished from a fast transit induced by the electronic conduction-of the host LC semiconductor. This systematic study provided a semiquantitative basis for evaluating the contamination of chemical impurity.