Fluorescent Organic pi-Radicals Stabilized with Boron: Featuring a SOMO-LUMO Electronic Transition

We report on the fluorescence properties of a new class of emissive and stable pi-radicals that contain a boron atom at a position distant from the radical center. A fully planarized derivative exhibited an intense red fluorescence with high fluorescence quantum yields (Phi(F) > 0.67) even in polar solvents. To elucidate the origin of this phenomenon, we synthesized another boron-stabilized radical that contains a bulky aryl group on the boron atom. A comparison of these derivatives, as well as with conventional donor-rr-acceptor (D-pi-A)-type emissive pi-radicals, unveiled several characteristic features in their photophysical properties. A theoretical analysis revealed that the SOMO-LUMO electronic transition generates an emissive D-1 state. Unlike conventional D-pi-A-type pi-radicals, this state does not undergo significant structural relaxation. The boron-stabilized pi-radicals demonstrated promising potential for organic light-emitting diodes as an emitting material.