Self-assembled monolayers (SAMs) formed by perfluoroterphenyl-substituted alkanethiols (C(6)F(5)-C(6)F(4)-C(6)F(4)-(CH(2))(n)-SH, FTPn) with variable length of the aliphatic linker (n = 2 and 3) were prepared on (111) Au and Ag and characterized by a combination of several complementary spectroscopic and microscopic techniques. A specific feature of these systems is the helical conformation of the FTP moieties, which, along with the high electronegativity of fluorine, distinguishes them from the analogous non-fluorinated systems and makes them attractive for different applications. The SAMs were found to be well-defined, highly ordered, and densely packed, which suggests a perfect correlation between the orientations and, in particular, twists of the FTP helices in the adjacent molecules. Significantly, the SAM exhibited pronounced odd-even effects, i.e. a dependence of the molecular orientation and packing density on the length of the aliphatic linker in the target molecules, with parity of n being the decisive parameter and the direction of the effects on Au opposite to that on Ag. The presence of the odd-even effects in the FTPn system brings new aspects into the discussion about the origin and mechanism of these phenomena. Specifically, the helical conformation of the FTP moieties in the dense phase excludes a variation of the intramolecular torsion and molecular twist as the mechanism behind the odd-even effects.

Self-assembled monolayers (SAMs) of hybrid ω-(4′-methylbiphenyl-4-yl) alkaneselenolates CH 3 (C 6 H 4 ) 2 -(CH 2 ) n Se (BPnSe, n ) 2-6) on Au(111) substrates, prepared at room temperature, were studied using scanning tunneling microscopy. Molecularly resolved images reveal that all BPnSe species form well-ordered SAMs on Au(111), with rotational domains having typically a size of ca. 30-80 nm, which is about 5 times larger than their thiol analogues prepared at the same conditions. Two types of structures are alternatingly adopted depending on the parity of the number of the methylene units in the aliphatic linker of the BPnSe molecules. The unit cell of BPnSe SAMs with n ) odd can be described as close to a commensurate oblique (2 3 × 3)R30°structure with two molecules per unit cell. The molecular arrangement in BPnSe SAMs with n ) even can be described by anisotropic expansion of the above structure along its shorter unit cell vector. This expansion is, however, irregular so that the respective unit vector in this direction cannot be defined. As a result of this expansion, BPnSe SAMs with n ) even are characterized by about 22-28% lower packing density than those with n ) odd. For all systems, partial reorientation of the Au(111) step edges was observed upon SAM formation, indicating significant mobility of the topmost gold atoms induced by the adsorbates. The results for BPnSe/Au(111) systems are discussed in view of the previously reported spectroscopic data and compared with the analogous results for the BPnS/Au(111) films.

Two fully analogue homologue series of thiol and selenol based aromatic self-assembled monolayers (SAMs) on Au(111) in the form of CH(3)-(C(6)H(4))(2)-(CH(2))(n)-S-Au(111) (BPnS/Au(111), n = 2-6) and CH(3)-(C(6)H(4))(2)-(CH(2))(n)-Se-Au(111) (BPnSe/Au(111), n = 2-6), respectively, have been used to elucidate the relative stability of the S-Au(111) and Se-Au(111) bonding by monitoring their exchange by alkanethiol and alkaneselenol molecules from their respective solutions. The exchange process was monitored using infrared reflection absorption spectroscopy (IRRAS). Two main results obtained by these study are: (1) the selenium-based BPnSe/Au(111) series is significantly more stable than their sulfur analogues; (2) a clear odd-even effect exists for the stability of both BPnS/Au(111) and BPnSe/Au(111) SAMs towards exchange processes with the even-numbered systems being less stable. The results obtained are discussed in view of previously reported microscopic and spectroscopic data of the same SAMs addressing the issue of the relative stability of S-Au(111) and Se-Au(111) bonding, which is an important factor for the rational design of SAMs.