Nanoscale aryleneethynylene oligomers incorporating fluorenone units as electron-dopable molecular wires

Abstract

The first general synthetic procedures to obtain fluorenone-containing aryleneethynylene oligomers have been developed. The strategy involves stepwise Sonogashira cross-coupling methodology, with 2,7-diethynylfluorenone and 1,4-diiodo-2,6-di(hexyloxy)benzene as key building blocks, with terminal benzenethiol functionality protected as cyanoethyl derivatives. A second family of compounds contains a central 9-[(4-pyridyl)methylene]fluorene or 9-[di(4-pyridyl)methylene]fluorene unit in the backbone. UV-Vis absorption studies in solution establish that the increase of molecular lengths from ca. 4 nm (compounds 15 and 16) to ca. 7 nm (compounds 17, 27 and 28) results in only a very small red shift, with the effective conjugation length slightly longer than that of the 1,4-di(phenylethynyl)benzene (PEPEP) subunits. The pyridyl groups in 27 and 28 are only weakly conjugated to the π-electron backbone (UV-Vis data) which is consistent with the twisted conformation observed in the X-ray crystal structures of model compounds 20 and 24. Cyclic voltammetric studies reveal that the reduction waves of the fluorenone, 9-[(4-pyridyl)methylene]fluorene and 9-[di(4-pyridyl)methylene]fluorene units endow these oligomers with n-doping characteristics, with reversible reduction waves being observed for some compounds. The new aryleneethynylene oligomers have potential applications as molecular wires in practical devices.