Effect of side chain length on film structure and electron mobility of core-unsubstituted pyromellitic diimides and enhanced mobility of the dibrominated core using the optimized side chain

Abstract

Pyromellitic diimides (PyDIs) are π-conjugated electron-transport materials based on an unusually small aromatic core (benzene), which provides low temperature processing and transparency in much of the visible range. We synthesized PyDI derivatives with a systematic series of fluoroalkyl side chains and investigated their film structures and electrical performances in thin-film transistors. The effect of the length of the fluorinated segment in fluoroalkylmethylene side chains was examined. Shorter side chains within this series induce higher electron mobilities, with a maximum of 0.026 cm² V⁻¹ s⁻¹ achieved with the perfluorobutylmethyl side chain. Atomic force microscopy images and X-ray diffraction peak widths were used as indications of crystallinity correlating with the mobility trend. The perfluorobutylmethyl side chain, when attached to 3,6-dibromo PyDI using a total of three synthetic steps, allowed nearly parallel PyDI cores and an exceptional mobility of 0.2 cm² V⁻¹ s⁻¹, accompanied by a correspondingly excellent morphology and effective intermolecular packing illustrated by a single crystal X-ray structure. This is the highest PyDI mobility yet reported, and is an unusually high mobility for a compound with such a small core, having such low visible range absorbance, and requiring so few synthetic steps.