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

摘要

A series of dendritic multiporphyrin arrays (P_(Zn)Tz-nP_(FB); n = 2, 4, 8) comprising a triazole-bearing focal zinc porphyrin (P_(Zn)) with a different number of freebase por-phyrin (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-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_(Zn)Tz and P_(Zn)Tz-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_(Zn)Tz-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.