Synthesis of Molecular Tripods Based on a Rigid 9,9′-Spirobifluorene Scaffold

The efficient synthesis of a new
tripodal platform based on a rigid
9,9′-spirobifluorene with three acetyl protected thiol groups
in the positions 2, 3′ and 6′ for deposition on Au(111)
surfaces is reported. The modular 9,9′-spirobifluorene platform
provides both a vertical arrangement of the molecular rod in position
7 and its electronic coupling to the gold substrate. To demonstrate
the validity of the molecular design, the model compound 24 exposing a para-cyanophenylethynyl rod is synthesized.
Our synthetic approach is based on a metal–halogen exchange
reaction of 2-iodobiphenyl derivative and his subsequent reaction
with 2,7-disubstituted fluoren-9-one to afford the carbinol 16. Further electrophilic cyclization and separation of regioisomers
provided the corresponding 2,7,3′,6′-tetrasubstituted
9,9′-spirobifluorene 17 as the key intermediate.
The molecular structure of 17 was determined by single-crystal
X-ray diffraction crystallography. The self-assembly features of the
target compound 24 were analyzed in preliminary UHV-STM
experiments. These results already demonstrated the promising potential
of the concept of the tripodal structure to stabilize the molecule
on a Au(111) surface in order to control the spatial arrangement of
the molecular rod.