Photophysical properties and study of the singlet oxygen generation of tetraphenylporphyrinato palladium(II) complexes

Three differently substituted Pd-tetraphenyl-porphyrins complexes were synthesized and their X-ray crystal and photophysical properties were studied. All compounds display intense phosphorescence in de-aerated solutions at room temperature with decay time in the ms range. An efficient spin transfer to oxygen in aerated solutions was monitored using singlet oxygen (O-1(2)) luminescence at 1275 nm. For all compounds, the quantum efficiency for generating singlet oxygen was found to be close to unity. Interesting differences in the pathways from the excited singlet state to singlet oxygen via the intermediate triplet state were observed. The excitation spectra obtained while recording the phosphorescence at 1275 nm closely matched the absorption spectra with two bands: the Soret (or B) band near 410-420 nm and a split Q-band at 510-520 nm. Noticeable differences in the B/Q intensity ratio between the absorption and excitation spectra (for both phosphorescence and singlet oxygen luminescence) were determined, indicating different S-x -> T-1 pathways.