Polymorph-Dependent Photophysics of Blue-Emitting Brominated Organic Crystals

Nonmetal-containing organic molecules typically do not emit blue phosphorescence efficiently due to weak intersystem crossing and the challenge of protecting high-energy triplet states from nonradiative decay. We show that use of triplet-promoting bromine atoms and careful control of molecular packing can lead to well-defined, efficient room-temperature phosphorescence (RTP) from organic crystals. We report on the distinct optical properties of a polymorphic bromine-containing organic molecule, 1,4-bis-(bromomethyl)-2,5-bis-(octyloxy)-benzene (Br8) in solid-state (micro)-crystal and thin-film forms. The two polymorphs, named Br8-H and Br8-J, exhibit different molecular packing arrangements in single crystals and thin films and, consequently, different photophysical behaviors. The Br8-J polymorph is characterized by a head-to-tail molecular arrangement in a triclinic crystal, while the Br8-H polymorph adopts a face-to-face arrangement (monoclinic crystal). Br8-J has a higher quantum yield (38% for the microcrystal form and 8% for the thin film), lower energy, and narrower emission, compared to Br8-H (quantum yield of just 2% for the microcrystal and 0.2% in the thin film). The higher quantum yields in the microcrystal forms indicate the key role of intermolecular interactions in reducing nonradiative recombination in these polymorphs. The presence of both prompt and delayed blue emission (in the range 420-470 nm) and large Stokes shifts from both polymorphs indicates the involvement of triplet excited states and the occurrence of RTP with lifetimes of ∼325 μs. With an emission wavelength of 470 nm and a quantum yield of 38%, the Br8-J polymorph demonstrates how specific head-to-tail packing can unlock highly efficient solid-state phosphorescence compared to its face-to-face counterpart. The optimal intermolecular halide bonding in the J-type packing arrangement combined with the heavy atom effect is key to unlocking efficient blue emission from Br8 crystals. Our findings uncover rich photophysics in halogenated organic molecules in the solid state and the importance of molecular packing in their emission characteristics.