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

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 λmax with increasing
molecular length, which approaches saturation at an effective conjugation length of ca. 15−20 π-units in
the molecules, where each phenyl ring or a triple bond is counted as one π-unit.