We report quenching studies of the emissions from Cu(TPP), Cu(TMP), and Cu(OEP) in toluene where TPP, TMP, and OEP denote the doubly deprotonated forms of tetraphenylporphyrin, tetramesitylporphyrin, and octaethylporphyrin, respectively. The quenchers are a series of Lewis bases including acetonitrile, DMF, DMSO, three different pyridines, and THF. Results with a sterically encumbered pyridine along with the correlation between quenching ability and donor strength prove that formation of a five-coordinate adduct induces the quenching. Small positive enthalpies of activation for quenching suggest that adduct formation is reversible, at least with the weaker donors. We conclude that the quenching is not intrinsic to the intraligand excited state of the adduct but is due to the influence of another excited state. The available evidence suggests that a d-d state involving z(2) --> x(2) - y(2) excitation is probably responsible for the quenching.