  Many quantum emitters have been measured close or near the grain boundaries
of the two-dimensional hexagonal boron nitride where various Stone-Wales
defects appear. We show by means of first principles density functional theory
calculations that the pentagon-heptagon Stone-Wales defect is an ultraviolet
emitter and its optical properties closely follow the characteristics of a
4.08-eV quantum emitter often observed in polycrystalline hexagonal boron
nitride. We also show that the square-octagon Stone-Wales line defects are
optically active in the ultraviolet region with varying gaps depending on their
density in hexagonal boron nitride. Our results may introduce a paradigm shift
in the identification of fluorescent centres in this material.
