The synthesis of functionalised diaryltetraynes and their transport properties in single-molecule junctions

The synthesis and characterisation is described of six diaryltetrayne derivatives [Ar-(C≡C)4-Ar] with Ar=4-NO2-C6H4- (NO₂4), 4-NH(Me)C6H4- (NHMe4), 4-NMe2C6H4- (NMe₂4), 4-NH2-(2,6-dimethyl)C6H4- (DMeNH₂4), 5-indolyl (IN4) and 5-benzothienyl (BTh4). X-ray molecular structures are reported for NO₂4, NHMe4, DMeNH₂4, IN4 and BTh4. The stability of the tetraynes has been assessed under ambient laboratory conditions (20 °C, daylight and in air): NO₂4 and BTh4 are stable for at least six months without observable decomposition, whereas NHMe4, NMe₂4, DMeNH₂4 and IN4 decompose within a few hours or days. The derivative DMeNH₂4, with ortho-methyl groups partially shielding the tetrayne backbone, is considerably more stable than the parent compound with Ar=4-NH2C6H4 (NH₂4). The ability of the stable tetraynes to anchor in Au|molecule|Au junctions is reported. Scanning-tunnelling-microscopy break junction (STM-BJ) and mechanically controllable break junction (MCBJ) techniques are employed to investigate single-molecule conductance characteristics.