Unexpectedly Long Lifetime of the Excited State of Benzothiadiazole Derivative and Its Adducts with Lewis Acids.

We report a study of photoluminescent properties of 4-bromo-7-(3-pyridylamino)-2,1,3-benzothiadiazole () and its novel Lewis adducts: ()(ZnCl) and [CuCl(-){}]·2CH ( = tetraphenyldiphosphine monoxide), whose crystal structure was determined by X-ray diffraction analysis. - exhibits a lifetime of 9 microseconds indicating its phosphorescent nature, which is rare for purely organic compounds. This phenomenon arises from the heavy atom effect: the presence of a bromine atom in promotes mixing of the singlet and triplet states to allow efficient singlet-to-triplet intersystem crossing. The Lewis adducts also feature a microsecond lifetime while emitting in a higher energy range than free , which opens up the possibility to color-tune luminescence of benzothiadiazole derivatives.