Luminescence of porphyrins and metalloporphyrins. Part 5.—Aluminium(III) and chromium(III) tetraphenylporphyrins

A1TPP exhibits intense fluorescence and low-temperature phosphorescence for which the observed properties are entirely consistent with (ππ*) excited states of a diamagnetic metalloporphyrin. In contrast, the paramagnetic CrTPP does not show normal (ππ*) fluorescence, but instead extremely weak triplet-state fluorescence from a tripquartet state is observed at room temperature. The lifetime of this tripquartet state is 295 ps, and its deactivation involves population of a lower-energy tripsextet state which shows inefficient low-temperature phosphorescence. Since the two states are in thermal equilibrium, delayed triplet fluorescence is observed at room temperature.