Convergent Synthesis of 10 nm Aryleneethynylene Molecular Wires by an Iterative Regioselective Deprotection/Sonogashira Coupling Protocol

Abstract

[structure: see text] The synthesis of a new series of rigid-rod aryleneethynylene derivatives of up to ca. 10 nm molecular length (compounds 16 and 17) is reported using iterative Pd-mediated Sonogashira coupling methodology combined with regioselective removal of the different protecting groups (namely, trimethylsilyl and 2-hydroxyprop-2-yl groups) from the terminal alkyne units. Additionally, the TMS-acetylene unit has been cleanly deprotected to afford a terminal alkyne in the presence of a cyanoethylsulfanyl group. Some of these molecular wires are functionalized with terminal protected thiophenol units for attachment to metal surfaces (compounds 16 and 17). Internal electron-acceptor units have been incorporated into their structures, namely, 9-[di(4-pyridyl)methylene]fluorene (compound 17) or fluorenone (compounds 19-22). Optical absorption and photoluminescence spectra reveal a red shift in the value of lambda(max) with increasing molecular length, which approaches saturation at an effective conjugation length of ca. 15-20 pi-units in the molecules, where each phenyl ring or a triple bond is counted as one pi-unit.