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

Abstract

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.

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“…1a,b, and their design comprises a firmly immobilized platform, exposing a rigidly mounted freely suspended cantilever. They were deposited onto clean Au(111) surfaces by a spray deposition method described previously27, followed by mild annealing in ultra-high vacuum (UHV), which promotes the deprotection of the sulfur anchor groups and the proper binding to the gold substrate. Both molecules arrange in ordered islands with a well-defined adsorption geometry (Fig.…”

“…1c,d). While the synthesis of spiro A is already reported27, the synthesis of spiro B is provided in the Methods section. Details of the adsorption geometry are presented in Supplementary Fig.…”

“…1. The first modular platform based on a rigid 9,9′-spirobifluorene with three acetyl protected thiol groups in the positions 2, 3′ and 6′ exposing a para -cyanophenylethynyl rod in the position 7 ( spiro A ) was reported recently27. Our synthetic approach is based on a metal-halogen exchange reaction of the 2-iodobiphenyl derivative and its subsequent reaction with 2,7-disubstituted fluoren-9-one to afford the carbinol.…”

“…This can be achieved by three-dimensional molecular carrier platforms that allow for a well-defined arrangement of molecular subunits with respect to the substrate. Tremendous progress in the synthesis of such molecules has been made recently2526272829.…”

“…In this work, we present low-temperature STM measurements and density functional theory (DFT) calculations on tripodal spirobifluorene derivatives (spiro), which are designed to firmly bind to a gold surface via three sulfur atoms at the feet27. They carry, as functional subunit, a freely suspended nitrile group that is well separated from the metallic substrate, freely accessible by the STM tip and mounted on a rigid subunit.…”