Tetra-2,3-pyrazinoporphyrazines with Externally Appended Pyridine Rings. 6. Chemical and Redox Properties and Highly Effective Photosensitizing Activity for Singlet Oxygen Production of Penta- and Monopalladated Complexes in Dimethylformamide Solution

Tetrakis-2,3-[5,6-di-(2-pyridyl)pyrazino]porphyrazinatopalladium(II) [Py8TPyzPzPd] (1) and the corresponding pentapalladated species [(PdCl2)4Py8TPyzPzPd] (2), dissolved (c ≈ 10−5−10−6 M) in preacidified dimethylformamide ([HCl] ≈ 10−4 M), behave as potent photosensitizing agents for the production of singlet oxygen, 1O2, with ΦΔ values of 0.89 ± 0.04 and 0.78 ± 0.05, respectively. The related octacation [(2-Mepy)8TPyzPzPd]8+ (3), examined under similar experimental conditions, exhibits lower ΦΔ values, that is, 0.29 ± 0.02 (as an iodide salt) and 0.32 ± 0.02 (as a chloride salt). In view of the very high values of ΦΔ, the photophysics of complexes 1 and 2 has been studied by means of pump and probe experiments using ns laser pulses at 532 nm as excitation source. Both complexes behave like reverse saturable absorbers at 440 nm because of triplet excited-state absorption. The lifetimes of the triplet excited states are 65 and 96 ns for the penta- and mononuclear species, respectively. Fluorescence quantum yields (Φf) are 0.1% for both 1 and 2. Such low Φf values for the two complexes are consistent with the high efficiency of triplet excited-state formation and the measured high yields of 1O2. Time-dependent density-functional theory (TDDFT) calculations of the lowest singlet and triplet excited states of the mono- and pentapalladated species help to rationalize the photophysical behavior and the relevant activity of the complexes as photosensitizers for the 1O2 (1Δg) generation.