Two π-Electrons Make the Difference : From BODIPY to BODIIM Switchable Fluorescent Dyes

Abstract

(aza-)BODIPY dyes (boron-dipyrromethene dyes) are well established fluorophores due to their excellent quantum yields, stability and diversity, which led to promising applications including imaging techniques, sensors, organic (opto)electronic materials or biomedical applications. We herein present a novel approach to modify the 12 π-electron core of the dipyrromethene scaffold. The replacement of two carbon atoms in the β-position of a BODIPY dye by two nitrogen atoms afforded a 14 π-electron system, which was termed BODIIM (boron-diimidazolylmethene) in systematic analogy to the BODIPY dyes. Remarkably the BODIIM dye was obtained with a BH2-rigidifying entity, which is currently elusive and sought after for the BODIPY dye class. DFT-calculations confirm the (12 + 2) π-electron relationship between BODIPY and BODIIM and reveal a strong shape correlation between LUMO in the BODIPY and the HOMO of the BODIIM. The modification of the π-system leads to a dramatic shift of the fluorescent emission. Nucleophilic reactivity was found at the meso-carbon atom in the formation of stable borane adducts with a significant shift of the fluorescent emission, and this behavior contrasts the reactivity of conventional BODIPY systems. In addition, the reverse decomplexation of the borane adducts was demonstrated in reactions with a representative N-heterocyclic carbene to retain the strongly fluorescent BODIIM compound, which suggests applications as fully reversible fluorescent switch.