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 <b>10</b>−<b>12</b>) or an electron-donor unit [9-(1,3-dithiol-2-ylidene)fluorene, compounds <b>14</b>−<b>17</b>] in the backbone. Core reagents are 2,7-dibromo-3,5-dinitrofluorenone <b>7</b> (obtained by nitration of 2,7-dibromofluorenone) and 2,7-dibromo-9-(4,5-dimethyl-1,3-dithiol-2-ylidene)fluorene <b>13</b>. 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 <b>11</b> <i>E</i><sup>1red</sup> −1.02 V (vs. Ag/Ag<sup>+</sup> in THF). The 1,3-dithiol-2-ylidene unit imparts good electron donor properties to molecules <b>14</b>, <b>15</b>, and <b>16</b> with radical cation formation observed at <i>E</i><sup>ox</sup> ca. 0.65 V (vs. Ag/Ag<sup>+</sup> in DCM). We also report studies on the assembly of <b>11</b> and <b>15</b> on gold substrates. Current−voltage (<i>I</i>−<i>V</i>) characteristics and X-ray photoelectron spectra of the monolayers reveal that <b>11</b> assembles via competing S−Au and NO<sub>2</sub>−Au interactions. This unusual phenomenon is ascribed to the very electron deficient dinitrofluorenone core of <b>11</b> 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, <b>15</b> assembles via conventional S−Au interactions.