Synthesis, characterization, and preliminary fluorescence study of a mixed-ligand bis(dicarbollyl)nickel complex bearing a tryptophan-BODIPY FRET couple

Abstract

In continuation of our work on nickelacarborane-based nanomolecular devices, the design and synthesis of a bis(dicarbollyl)nickel complex, in both formal Ni(III) and Ni(IV) oxidation states, bearing two fluorophore molecules capable of fluorescence resonance energy transfer (FRET) was performed. The FRET couple with a small Förster radius (R0 of 26 Å) consisted of tryptophan and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY). Each fluorophore was connected to the nickelacarborane core by a rigid linker containing two pairs of alternating ethynyl and para-phenylene groups, which created a specific distance (l) between the fluorophore molecules depending on the conformation of the nickelacarborane core. The presence (13 Å < l < 39 Å) or absence (l > 39 Å) of energy transfer in the designed system provides insight into the conformational changes of nickelacarboranes in solution. The target nickelacarboranes were prepared via a multistep organic/organometallic synthesis. The structures and compositions of the intermediates and final products were established by a combination of spectroscopic methods and X-ray structure analysis. A preliminary fluorescence study of the prepared nickelacarboranes was performed.