Formation of intersystem crossing transitions in Pd(II) and Pt(II) porphyrins: Nonplanar distortions of the macrocycle and charge transfer states

We found new bands in the absorption spectra of Pt(II) and Pd(II) complexes of octaethylporphyrin and tetraphenylporphyrin that differ in the nature, number, and position of their side substituents. The bands are observed at 295 K in the range 570–690 nm and are attributed to spin-forbidden transitions from the ground S ₀ state to the excited T ₁ and T ₂ triplet states (the internal heavy atom effect). We determined the frequency distribution, number, and nature of these transitions, as well as their extinction coefficients (? = 6.0–210.0 M^?1 cm^?1), using computer decomposition of complex contours into Gaussian components and additional data obtained from the phosphorescence and phosphorescence excitation spectra of these complexes (295–77 K). In comparison to Pd complexes of porphyrins with planar macrocycles, nonplanar distortions of the tetrapyrrole macrocycle in the ground S ₀ state of the sterically hindered PdOETPP molecule cause a bathochromic shift of the bands of the electronic spin-forbidden S ₀ → T ₁ and S ₀ → T ₂ transitions, as well as an increase in their extinction coefficients. For the PdOEP-Ph(o-NO₂) molecule, which contains the electron acceptor nitro group, an absorption band attributed to an electronic transition from the ground state S ₀ to a charge transfer state (λ_max = 905 nm, ? = 10.0 M^?1 cm^?1) is observed at 295 K.