A systematic study on the influences of linkage length on phase behaviours and charge carrier mobilities of discotic dimers

Abstract

<p>Two series of discotic dimers <b>T3Dn</b> and <b>T5Dn</b> based on hexapropoxytriphenylene (<b>HAT3</b>) and hexapentyloxytriphenylene(HAT5), respectively, with polymethylene linkage O(CH₂)<i>_n</i>O (<i>n</i> = 3–12) have been synthesised. Their mesomorphism was investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The results showed that side chains induced a phase transition from col_hp phase to col_h phase, namely dimers <b>T3Dn</b> (<i>n</i> = 6–12) based on <b>HAT3</b> exhibiting a single col_hp phase, dimers <b>T4Dn</b> (<i>n</i> = 6, 7, 11, 12) based on <b>HAT4</b> showing a highly ordered col_hp phase in low-temperature region and a col_h phase in high-temperature region, dimers <b>T4Dn</b> (<i>n</i> = 8–10) based on <b>HAT4</b> displaying a single col_hp phase and dimers <b>T5Dn</b> (<i>n</i> = 6–11) based on <b>HAT5</b> indicating a single col_h phase. Dimers <b>T4Dn</b> showed a phase competition between col_h phase and col_hp phase induced by linkages. Surprisingly, a unique phenomenon was found by us, that is, those compounds in which linkage lengths were twice those of side chains showed the largest enthalpies, the smallest intracolumnar spacings and the highest charge-carrier mobilities among their homologues, respectively, which implied that they formed the most highly ordered phase among their homologues.</p