Electron Paramagnetic Resonance Studies of Photoselected Triplet Molecules. II. Coronene and Phenanthrene

Photoselective excitation of magnetoselected triplet molecules in glassy rigid matrix at 77°K was used to determine the orientation of the molecular transition moments. Two aromatic molecules were thus selected: coronene which is highly symmetric and phenanthrene which has two different transition moments in its near-ultraviolet absorption spectrum. Because of the high symmetry of coronene, it was only possible to show that the singlet—singlet transition moments involved in the population of the phosphorescent triplet state lie in the molecular plane. For phenanthrene, the long-wavelength singlet—singlet excitation is known to have a transition moment parallel to the short molecular axis, the short-wavelength one a transition moment parallel to the long axis of the molecule. Paramagnetic resonance of triplet molecules photoselectively excited in the two bands with convenient filtered light confirmed these orientations.