Molecular organized isoindigo based small molecules with terminal thermally cleavable protecting group

Abstract

The potential of a new series of H-bonded isoindigo (ID) based small molecules as charge transporting materials was examined. The soluble precursor units contained two terminal isoindigo units bearing tert-butoxycarbonyl (Boc) solubilizing substituents. Thiophene, thienothiophene and/or diketo pyrrolopyrrole are used as π-conjugated spacers between the isoindigo units. The protected Boc-ID units are converted to the hydrogen-substituted counterparts by annealing the spin-coated precursor films up to 220 °C. The solid state molecular ordering after Boc thermal decarboxylation was investigated using thermogravimetric analysis, UV–vis spectroscopy and X-ray diffraction analysis. The thermogravimetric analysis confirmed the full removal of Boc groups. It was revealed that the formation of terminal intermolecular hydrogen bonds gave the ability to the deprotected materials to change their molecular ordering to edge-on or face-on orientation based on the spacer size and the π-conjugated length. The [-NH----OC] H-bonding dramatically improved the strength of the intermolecular interactions and induced a significant shift on the wavelength of annealed films. The hole mobilities of isoindigo based organic materials measured by the space charge-limited current (SCLC) method and showed a tenfold improvement after thermal decarboxylation.