The role of halogen bonding in metal free phosphors.

The enhanced spin-orbit coupling necessary for phosphorescence is thought to be due to the halogen bonding that is present in the all-organic crystalline systems. To elucidate the underlying mechanism, the electronic and optical properties of purely organic phosphor candidates are investigated using density functional theory calculations. The unit cell structure of a known organic phosphor containing bromine is used to validate the accuracy of the computational methodology. Compared to experiments, the calculated lattice constants deviate by less than 1 percent for each lattice constant. The same computational approach is then used to predict the lattice constants for molecular analogs containing fluorine, chlorine, and iodine. Electronic structure and photonic properties of the predicted crystals are computed. Finally, the presence of halogen bonding is corroborated, with fluorine forming the weakest and iodine the strongest halogen bonding interactions. Our findings demonstrate how computational methods can be effectively used for the predictive design of organic materials in lighting devices.