Guest-Induced Modulation of the Energy Transfer Process in Porphyrin-Based Artificial Light Harvesting Dendrimers

A series of dendritic multiporphyrin arrays ( P Zn Tz - n P FB; n = 2, 4, 8) comprising a triazole-bearing focal zinc porphyrin ( P Zn ) with a different number of freebase porphyrin ( P FB ) wings has been synthesized, and their photoinduced energy transfer process has been evaluated. UV/vis absorption, emission, and time-resolved fluorescence measurements indicated that efficient excitation energy transfer takes place from the focal P Zn to P FB wings in P Zn Tz - n P FB ’s. The triazole-bearing P Zn effectively formed host–guest complexes with anionic species by means of axial coordination with the aid of multiple C–H hydrogen bonds. By addition of various anionic guests to P Zn Tz and P Zn Tz - n P FB ’s, strong bathochromic shifts of P Zn absorption were observed, indicating the HOMO–LUMO gap (Δ E HOMO–LUMO ) of P Zn decreased by anion binding. Time-resolved fluorescence measurements revealed that the fluorescence emission predominantly takes place from P Zn in P Zn Tz - n P FB ’s after the addition of CN – . This change was reversible because a treatment with a silver strip to remove CN – fully recovered the original energy transfer process from the focal P Zn to P FB wings.