The Fluorescence Quenching of Zinc Porphyrins with the ω-[4-(4-Pyridyl)pyridinio]alkyl Group

The fluorescence of parent porphyrins in ZnPC2bpy+(n), (PC2bpy+(n), 5-[n-[2-[4-(4-pyridyl)pyridinio]ethoxyl]phenyl]-10,15,20-triphenylporphyrin bromide, n=2,3, and 4), and ZnPC8bpy+(3), (PC8bpy+(3), 5-[3-[8-[4-(4-pyridyl) pyridinio]octyloxy]phenyl]-10,15,20-triphenylporphyrin bromide), was effectively quenched in non-coordinating solvents, such as o-dichlorobenzene and dichloromethane, because the pyridyl group in bpy+ (ω-[4-(4-pyridyl)pyridinio]alkyl group) was coordinated to zinc in porphyrin. On the other hand, the fluorescence of parent porphyrins in ZnPC2bpy+(3), ZnPC2bpy+(4), and ZnPC8bpy+ (3) was partly quenched in such coordinating solvents as N,N-dimethylformamide and pyridine. However, only the fluorescence of the parent porphyrin in ZnPC2bpy+(2) was effectively quenched, even in coordinating solvents. The hydrophobic interaction between the porphyrin ring and bpy+ in ZnPC2bpy+(2) may be considerably large, for bpy+ in ZnPC2bpy+(2) is covalently linked to the ortho position of the phenyl group in ZnTPP, (TPP 5,10,15,20-tetraphenylporphyrin), by a short chain. The emission was effectively quenched by the photoinduced electron transfer from the porphyrin ring to bpy+, because the axial coordination or the hydrophobic interaction gave bpy+ access to the porphyrin ring