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-Ired-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 I I and 15 on gold Substrates. Current-voltage (I-V) characteristics and X-ray photoelectron spectra of the monolayers reveal that I I 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.