We describe syntheses of six triptycene-containing molecular rotors with several single-crystal X-ray diffraction analyses. These rod-shaped molecules carrying an axial rotator are designed to interleave on an aqueous surface into Langmuir-Blodgett (LB) monolayers containing a two-dimensional trigonal array of dipoles rotatable about an axis normal to the surface. Monolayer formation was verified with the simplest of the rotor structures. On an aqueous subphase containing divalent cations (Mg(2+), Ca(2+), Zn(2+), Sr(2+), or Cd(2+)), the LB isotherm yielded an area of 53 ± 3 Å(2)/molecule (monolayer of type A), compatible with the anticipated triangular packing of axes normal to the surface. On pure water, the area is 30 ± 3 Å(2)/molecule, and it is proposed that in this monolayer (type B), the molecular axes are tilted by 40-45° to a structure similar to those observed in single crystals of related triptycenes. After transfer to a gold surface, ellipsometry and PM IRRAS yield tilt angles of 29 ± 4° (monolayers of type A) and 38 ± 4° (type B). A full-scale examination of monolayers from all the rotors on a subphase and after transfer is underway and will be reported separately.
Treatment
of cleaned gold surfaces with dilute tetrahydrofuran
or chloroform solutions of tetraalkylstannanes (alkyl = methyl, ethyl, n-propyl, n-butyl) or di-n-butylmethylstannyl tosylate under ambient conditions causes a self-limited
growth of disordered monolayers consisting of alkyls and tin oxide.
Extensive use of deuterium labeling showed that the alkyls originate
from the stannane and not from ambient impurities, and that trialkylstannyl
groups are absent in the monolayers, contrary to previous proposals.
Methyl groups attached to the Sn atom are not transferred to the surface.
Ethyl groups are transferred slowly, and propyl and butyl rapidly.
In all cases, tin oxide is codeposited in submonolayer amounts. The
monolayers were characterized by ellipsometry, contact angle goniometry,
polarization modulated IR reflection absorption spectroscopy, X-ray
photoelectron spectroscopy, and electrochemical impedance spectroscopy
with ferrocyanide/ferricyanide, which revealed a very low charge-transfer
resistance. The thermal stability of the monolayers and their resistance
to solvents are comparable with those of an n-octadecanethiol
monolayer. A preliminary examination of the kinetics of monolayer
deposition from a THF solution of tetra-n-butylstannane
revealed an approximately half-order dependence on the bulk solution
concentration of the stannane, hinting that more than one alkyl can
be transferred from a single stannane molecule. A detailed structure
of the attachment of the alkyl groups is not known, and it is proposed
that it involves direct single or multiple bonding of one or more
C atoms to one or more Au atoms.
n-Alkyl self-assembled monolayers can be directly attached to gold through C−Au bonds by the deposition of organomercury salts on gold substrates, as shown here using nbutylmercury and n-octadecylmercury tosylate derivatives. The Hg atoms, which are codeposited during this process, are removed by thermal annealing at 95 °C, resulting in alkyl monolayers having a significantly enhanced thermal stability compared with alkanethiol monolayers, however, a lower degree of conformational order. The monolayer properties are elucidated by X-ray photoemission and IR spectroscopy, STM, ellipsometry, and contact-angle goniometry.
Treatment of a gold surface with a solution of C18H37HgOTs under ambient conditions results in the formation of a covalently adsorbed monolayer containing alkyl chains attached directly to gold, Hg(0) atoms, and no tosyl groups. It is stable against a variety of chemical agents. When the initial deposition is performed at a positive applied potential and is followed by oxidative electrochemical stripping, the mercury can be completely removed, leaving a gold surface covered only with alkyl chains. The details of the attachment structure are not known. The conclusions are based on infrared spectroscopy, X-ray and UV photoelectron spectroscopy, ellipsometry, contact angle goniometry, differential pulse polarography, and measurements of electrode blocking and electrochemical admittance.
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