Luminescence of porphyrins and metalloporphyrins. Part 1.—Zinc(II), nickel(II) and manganese(II) porphyrins

Abstract

Metal-free and ZnTPP complexes exhibit both fluorescence and low temperature phosphorescence, Mn^IITPP shows only low temperature phosphorescence while Ni^IITPP is non-luminescent. The absorption spectra show that there is little interaction between the metal and the porphyrin π-system so that the excited states can be regarded as (ππ^*) in nature. Where phosphorescence occurs, the Stokes shift is 3900 ± 100 cm^–1 and decreases with increased metal–porphyrin interaction. The rate constants for intersystem crossing and for non-radiative decay from the lowest (ππ^*) triplet state depend upon spin–orbital coupling properties. Zn^II and Ni^I exhibit fairly weak spin–orbital coupling but the effectiveness of Mn^II is enhanced by the paramagnetic character and by favourable geometry. Ni^IITPP possesses a low energy (dd) excited singlet state which facilitates rapid internal conversion within the singlet manifold.