Organic Crystals Light Up

Phosphorescent organic crystals can be chemically tuned to glow in different colors without the need for metals, researchers from the University of Michigan report (Nat. Chem., DOI: 10.1038/nchem.984).

“Traditionally, purely organic materials have been widely considered to be nonphosphorescent,” says Wai-Yeung Wong, a chemistry professor and member of the Centre for Advanced Luminescence Materials at Hong Kong Baptist University. The new organic crystals offer a simple, low-cost approach to activating solid-state phosphorescence at room temperature, Wong adds.

Phosphorescence is similar to fluorescence in that a substance absorbs and then reemits electromagnetic radiation. In phosphorescence, however, the excited electron flips its spin, making quantum mechanically forbidden transitions. Such transitions are easier in inorganic or organometallic compounds. Phosphorescent materials in applications such as light-emitting diodes or sensors are typically made from organometallic compounds incorporating precious metals such as iridium.

Michigan materials science professor Jinsang Kim and colleagues instead designed a molecule, 2,5-dihex­yloxy-4-bromobenzaldehyde, that incorporates both an aromatic carbonyl group and a bromine atom. In solution, the molecules are weakly fluorescent. When they crystallize, however, halogen bonds—noncovalent interactions akin to hydrogen bonds—form between the bromine of one molecule and the aldehyde oxygen of another. The arrangement promotes the spin-flipping transitions and results in crystals that glow green.

The phosphorescence can be enhanced by diluting the bromobenzaldehyde crystals with a dibromobenzene analog, which helps shut down self-quenching pathways but still makes plenty of bromine atoms available for halogen bonds to aldehyde groups.

Kim and coworkers were also able to tune the phosphorescence color by altering the structure of the molecule. Exchanging the alkoxy arms for alkyl or thioether groups produced blue or yellow-green phosphorescence, respectively. Replacing the central benzene ring with a naphthyl core yielded dark-orange phosphorescence.