Olefin-Borane Interactions in Donor-pi-Acceptor Fluorophores that Undergo Frustrated-Lewis-Pair-Type Reactions

Olefin-borane pi-complexes have been postulated as intermediates for the addition of frustrated Lewis pairs (FLP) to olefins. In the present study, we have employed this weak interaction to modulate the electronic properties of boron-based pi-electron materials. A series of donor-pi-acceptor (D-pi-A) fluorophores that contain an alkenyl-bridged diarylboryl group is synthesized. A crystallographic analysis revealed that the olefin and boron moieties are held in close proximity. Upon addition of a Lewis base to a solution of these D-pi-A fluorophores, an FLP-type addition occurs with concurrent significant changes in the absorption and emission properties. The FLP-type reaction shifts the reaction site from the Lewis-acidic boron atom to a carbocationic center, and thereby even facilitates a reaction with bulky Lewis bases. For example, a tricyclohexylphosphine adduct thus generated exhibits temperature-dependent reversible dissociation/association behavior. These results provide a design strategy for stimuli-responsive emissive boron-based materials.