Knowledge Bank

We have examined a Cr(III) octaethylporphin and a Cr(III) tetra-phenylporphin at temperatures ranging between $4\circ K$ and $300\circ K$. Both species exhibit abnormal absorption and emission spectra. The room temperature absorption spectra contain extra bands in the uv and near ir. The weak red shifted luminescence shows marked temperature dependence of quantum yields, band profiles, and lifetimes. We explain the electronic spectra by a model in which three unpaired d electrons on the chromium are coupled with the porphyrin $\pi$ levels. All singlet states become quartets and the triplet states split into doublets, ``tripquartets”, and sextets. The emission properties are believed to result from tripquartet and sextet states in thermal equilibrium. The nature of the porphyrin ring substituents appears to affect the tripquartet-sextet energy gap.