Porphyrins. XXVII. Spin-orbit coupling and luminescence of Group IV complexes

Abstract

Luminescence studies are reported on compounds M(IV)X2P: M is Si, Ge, Sn, Pb; X is F, Cl, Br, I, OH, benzoate; P is etioporphyrin or octaethylporphin. (One tetraphenylporphin is reported for comparison.) We find fluorescence yields 2 × 10−1 ≥slant φf ≥slant 2 × 10−4; phosphorescence yields 7 × 10−2 ≥slant φp ≥slant 3 × 10−3; and phosphorescence lifetimes 100 msec ≥slant τp ≥slant 1 msec. The contrasting vibronic envelopes of phosphorescence for octaethylporphin and tetraphenylporphin derivatives is explained by attributing the former to transitions 3Θ± 1 → 1ΘGND and the latter to 3Θ± 9→ 1ΘGND, where ± 1 and ± 9 are pseudoangular momentum quantum numbers. The spin-orbit interaction is calculated by the extended Hückel model, and it is found that the ligands have far more effect than the metal, in agreement with the data. However a simple relation between decay rates and spin-orbit coupling fails quantitatively, and the extended Hückel model appears to exaggerate the contribution of the ligand to the spin-orbit coupling.