Synthesis of Triptycene-Based Molecular Rotors for Langmuir–Blodgett Monolayers

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 (Mg2+, Ca2+, Zn2+, Sr2+, or Cd2+), the LB isotherm yielded an area of
53 ± 3 Å2/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 Å2/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.