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

A series of dendritic multiporphyrin arrays (P_ZnTz-nP_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_ZnTz-nP_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_ZnTz and P_ZnTz-nP_FB's, strong bathochromic shifts of P_Zn absorption were observed, indicating the HOMO-LUMO gap (ΔE_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_ZnTz-nP_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.