Red light-emitting Carborane-{BODIPY} dyes: Synthesis and properties of visible-light tuned fluorophores with enhanced boron content

A small library of 2,6- and 3,5-distyrenyl-substituted carborane-BODIPY dyes was efficiently synthesized by means of a Pd-catalyzed Heck coupling reaction. Styrenyl-carborane derivatives were exploited as molecular tools to insert two carborane clusters into the fluorophore core and to extend the π-conjugation of the final molecule in a single synthetic step. The synthetic approach allows to increase the molecular diversity of this class of fluorescent dyes by the synthesis of symmetric or asymmetric units with enhanced boron content. The structural characterization and the photoluminescence (PL) properties of synthesized dyes were evaluated, and the structure/properties relationships have been investigated by theoretical calculations. The developed compounds exhibit a significant bathochromic shift compared to their parent fluorophore scaffolds, and absorption and emission patterns were practically unaffected by the different substituents (Me or Ph) on the Ccluster atom (Cc) of the carborane cage or the cluster isomer (ortho- or meta-carborane). Remarkably, the presence of carborane units at 2,6-positions of the fluorophore produced a significant increase of the emission fluorescent quantum yields, which could be slightly tuned by changing the Cc-substituent and the carborane isomer, as well as introducing ethylene glycol groups at the meso-position of the BODIPY.