Synthesis and Properties of Functionalized Oligo(arylene) Molecular Wires with Thiolated Termini: Competing Thiol-Au and Nitro-Au Assembly

We report the synthesis by Suzuki cross-coupling methodology of oligo(arylene) molecular wires with protected thiolates at both termini and a central electron-acceptor unit (3,5-dinitrofluorenone, compounds 10−12) or an electron-donor unit [9-(1,3-dithiol-2-ylidene)fluorene, compounds 14−17] in the backbone. Core reagents are 2,7-dibromo-3,5-dinitrofluorenone 7 (obtained by nitration of 2,7-dibromofluorenone) and 2,7-dibromo-9-(4,5-dimethyl-1,3-dithiol-2-ylidene)fluorene 13. The solution electrochemical redox properties of these oligo(arylene) derivatives have been studied. The reduction CVs of the dinitrofluorenone-containing molecules display three reversible/quasireversible couples yielding, sequentially, radical anion, dianion, and radical trianion species, e.g., for 11 E^1red −1.02 V (vs. Ag/Ag⁺ in THF). The 1,3-dithiol-2-ylidene unit imparts good electron donor properties to molecules 14, 15, and 16 with radical cation formation observed at E^ox ca. 0.65 V (vs. Ag/Ag⁺ in DCM). We also report studies on the assembly of 11 and 15 on gold substrates. Current−voltage (I−V) characteristics and X-ray photoelectron spectra of the monolayers reveal that 11 assembles via competing S−Au and NO₂−Au interactions. This unusual phenomenon is ascribed to the very electron deficient dinitrofluorenone core of 11 weakening the S−Au interaction. An important conclusion is that thiolated molecules which possess strongly electron-withdrawing core units, especially those containing nitro groups, may not be suitable for controlled assembly in junctions. In contrast, 15 assembles via conventional S−Au interactions.