Radical character quenches luminescence in the all-hydrocarbon radical benzoBDPA

We have synthesized and characterized a benzannulated analog, bBDPA, of the well-known stable radical bis(diphenylene)phenylallyl (BDPA). Benzannulation gives rise to fluorescence in the closed-shell precursor molecule bBDPAH, but luminescence is quenched in the radical species. We detail the synthesis, characterization by absorption, emission, and EPR spectroscopy, and examine the molecular orbital landscape dictating the properties of the radical. We find that benzannulation alters the energy of the radical's frontier molecular orbitals differently than in luminescent radicals brought about by incorporation of heteroatoms into their frameworks.