Spectroscopy and Photophysics of Monoazaphenanthrenes III. Luminescence of Phenanthridine and 7,8-benzoquinoline in Crystalline State

Fluorescence and phosphorescence spectra and the decay proflles of both these emissions have been investigated for the polycrystals of phenanthridine and 7,8-benzoquinoline, in the liquid helium (5 K) { room temperature range. These two monoazaderivatives of phenanthrene, which difier only by the position of N-heteroatom in the aromatic ring skeleton of phenanthrene, were found to exhibit very difierent ∞uorescence spectra, which also difier greatly in their temperature behavior. Supplementary investigations of the ∞uorescence of single crystals of 7,8-benzoquinoline have supported classiflcation of observed ∞uorescence as an excimer ∞uorescence (caused by the speciflc arrangement of molecules of 7,8-benzoquinoline in the crystal). In contrary ∞uorescence of phenanthridine crystals is of the monomeric type. Phosphorescence spectra observed for the crystals of both molecules are very similar, but their temperature dependence is also difierent. This may be considered as an indication of a difierent physical mechanism of nonradiative intersystem crossing processes, which are operating between the lowest excited singlet state and the lowest excited triplet state in the crystals of both molecules.