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

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 ${\text{2 × 10}}^{\text{−1}}{\mathrm{\ge slant\; \phi }}_f{\text{≥slant 2 × 10}}^{\text{−4}}$ ; phosphorescence yields ${\text{7 × 10}}^{\text{−2}}{\mathrm{\ge slant\; \phi }}_p{\text{≥slant 3 × 10}}^{\text{−3}}$ ; and phosphorescence lifetimes $\text{100\hspace{0.17em}}\mathrm{msec}{\mathrm{\ge slant\; \tau }}_p\text{≥slant 1\hspace{0.17em}}\mathrm{msec}$ . The contrasting vibronic envelopes of phosphorescence for octaethylporphin and tetraphenylporphin derivatives is explained by attributing the former to transitions ${}^3{\Theta }_{\text{± 1}}{\to }^1{\Theta }_{\mathrm{GND}}$ and the latter to ${}^3{\Theta }_{\text{± 9}}{\to }^1{\Theta }_{\mathrm{GND}}$ , 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.