Phenylene and biphenyl compounds with dibenzophosphole sulfide (DBPS) as an anchoring group for single molecule junctions were synthesized. The conductance measurements revealed that the phosphine sulfide indeed acts as an anchoring group for Au electrodes. Theoretical calculations including metal electrodes demonstrated that the LUMO level of the DBPS-terminated biphenyl is close to the Au Fermi level, leading to the electron conduction of the AumoleculeAu junction based on the resonance-tunneling mechanism.Electron-transport properties through single molecules have attracted growing attention as the basis for fabricating ultrasmall electronic devices. 1,2 Recent extensive studies on a variety of single molecule junctions revealed that the electron conduction through a molecule bonded between two metal electrodes highly depends on the nature of anchoring groups, which affects not only the HOMOLUMO energy gap of the whole molecular system, but also the alignment of the energy levels of these frontier MOs against the electrode Fermi level.2,3 The development of a new anchoring group as well as the deep understanding of the conduction mechanism are therefore urgent subjects for the evolution of single molecule electronics. Besides the combination of thiol (SH) and Au electrodes, a variety of anchoring groups has been investigated, such as pyridine, 16,17 for various electrodes. In this study, we explored the possibility of using a phosphine sulfide (P=S) bond as a new anchoring group to form single molecule junctions with Au electrodes. The P=S bond has several characteristics. First, a sulfur atom in the P=S bond has high affinities to various metals, such as Au, Ag, and Cu, leading to strong adsorption.18 Second, phosphine sulfides have high chemical and thermal stability among the various types of organophosphorus compounds, which guarantees their versatile use for the molecular junction. Moreover, the P=S group acts as a strong electron-withdrawing group. We envisioned that the incorporation of a P=S group as an anchor to a certain ³-conjugated skeleton would decrease the LUMO level of the molecular system, 19,20 and thus affect the electron-transport mechanism. To minimize the steric hindrance around the P=S moiety, we decided to employ a dibenzophosphole sulfide (DBPS) skeleton. As a model system, we synthesized the DBPSterminated phenylene 1 and biphenyl-4,4¤-diyl 2 (Figure 1b). We now disclose the electron conductance of these molecular systems and some insights into their conduction mechanism.Compounds 1 and 2 were synthesized in different ways, as shown in Schemes 1 and 2, respectively. The biphenyl derivative 2 was readily prepared by the reaction of P-chlorodibenzophosphole (DBPCl) (10) with 4,4¤-dilithiobiphenyl, followed by the oxidation with elemental sulfur (Scheme 2). However, a similar route based on the reaction between DBPCl and 1,4-dilithiobenzene did not afford 1 at all. Alternatively, by employing P-(4-bromophenyl)-DBP as a precursor, we in situ
