Zinc Porphyrin-Cored Dendrimers; Axial Coordination of Pyridine and Photoinduced Electron Transfer to Methyl Viologen

The porphyrin-incorporated arylether dendrimers ZnP-D1 and ZnP-D4 were investigated to discover the influence of dendritic environments for the axial ligation of pyridine and photoinduced electron transfer by methyl viologen. Absorption and fluorescence spectra of ZnP, ZnP-D1, and ZnP-D4 were measured in dichloromethane with the addition of pyridine or methyl viologen dichloride. Axial ligation of pyridine was confirmed by red-shifted absorption spectrum. The complex formation constants K_f (Table 1) for axial coordination of pyridine on ZnP, ZnP-D1, and ZnP-D4 were estimated to be 4.4 x 10³ M^-1, 3.3 x 10³ M^-1, and 1.7 x 10³ M^-1, respectively. The photoinduced electron transfer to methyl viologen dichloride was confirmed by fluorescence quenching. Stern-Volmer constants Ksv for ZnP, ZnP-D1, and ZnP-D4 were calculated to be 2.6 x 10³, 2.5 x 10³, and 2.1 x 10³, respectively. ZnP-D4 surrounded by 4 aryl ether dendrons shows the smallest K_f and Ksv values, with comparison to ZnP and ZnP-D1