Wholly Aromatic Ether-imides. Potential Materials for n-Type Semiconductors

We report on the synthesis and characterization of a series of low molar mass, high aspect ratio ether-imide compounds. All ether-imides were obtained by terminating the appropriate dianhydride, that is, pyromellitic dianhydride (PMDA), 1,4,5,8-naphthalenetetra-carboxylic dianhydride (NDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), and 3,3′,4,4′-oxydiphthalic dianhydride (ODPA), with three flexible aryl-ether tails of different chain lengths. Increasing the number of meta-substituted aryl-ether units reduces the melt transition temperatures and at the same time increases the solubility of the ether-imides. When the flexibility of the dianhydride moiety increases, the thermal behavior of the compounds becomes significantly more complex: The BPDA- and ODPA-based compounds form glasses and exhibit multiple crystal phases. Most compounds form isotropic melts upon heating; however, 2,7-bis(-4-phenoxy-phenyl)-benzo[lmn][3,8]phenanthroline-1,3,6,8-tetraone (NDA-n₀) displays a smectic A (S_A)-type texture when cooled from the isotropic phase, followed by what appears to be a smectic phase with a columnar arrangement of the mesogens inside the layers. Single-crystal X-ray diffraction analysis and cyclic voltammetry experiments indicate that the wholly aromatic ether-imides NDA and BPDA could be excellent candidates for n-type semiconductor applications.