Substituted Phosphorous Triazatetrabenzocorroles: Correlation Between Structure and Excited State Properties

The photophysical properties of five novel phthalocyanine analogues, dihydroxy phosphorus(v) triazatetrabenzocorrole (PTBC) substituted with –NO2, –SO3H, OiPr, and –NH2, respectively, were studied by a combination of absorption, steady-state emission, time-resolved fluorescence, and laser flash photolysis. All substituents, even for the strong electron-donating –NH2, cause only a slight red shift of their absorption and emission maxima. These complexes are generally monomeric in organic solution, whereas the sulfonated derivative, PTBC(SO3H)4, slightly aggregates in aqueous buffer. Distinct from phthalocyanines, PTBCs substituted with –NO2 or –NH2 still show high photo activities. The electron-withdrawing –NO2 and –SO3H decrease the fluorescence quantum yield but increase the triplet formation yield to 0.76 and 0.82, respectively. All PTBCs have long triplet lifetimes and hence generate singlet oxygen efficiently with a quantum yield from 0.43 to 0.75. Together with the ground-state absorption properties, the results suggest that these PTBCs may be used as excellent photosensitizers for photodynamic therapy.