Combining metalloporphyrins and cyclometalated complexes – A luminous pair?

Cyclometalation is investigated as a method to modulate the photophysical properties of phosphorescent (benzo)porphyrins. For cyclometalation with Pt(II), 2-pyridyl groups were incorporated into three Pd(II) porphyrin complexes. Integration of the freely rotating cyclometalated 2-phenylpyridyl based motif in meso-position does not lead to notable spectral changes and only affects the luminescence decay time (about 2-fold decrease) in solution at room temperature. The decay times, however, are not affected in frozen glass at 77 K or immobilized into a rigid polystyrene matrix. In contrast, cyclometalation involving a 2-pyridyl unit in meso-position and the β-H of the porphyrin results in a large bathochromic shift (∼ 1900 cm ⁻¹) of Q-bands in the absorption spectrum and the phosphorescence peak. A drastic decrease of the phosphorescence lifetime (about 20-fold) also is observed. Cyclometalation is accompanied by an increase of the singlet oxygen quantum yield to 85 % making the new dye a powerful red-light excitable ¹O ₂ photosensitizer.