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.