A porphyrin-based molecular tweezer: Guest-induced switching of forward and backward photoinduced energy transfer

A bisindole-bridged-porphyrin tweezer (1), a pair of zinc porphyrins (P_Zn's) connected to bisindole bridge (BB) via the Cu ^I-mediated alkyne-azide click chemistry, exhibited unique switching in forward and backward photoinduced energy transfer by specific guest bindings. The addition of Cu²⁺ caused a change in electronic absorption and fluorescence quenching of 1. MALDI-TOF-MS and FT-IR analyses indicated the formation of stable coordination complex between 1 and Cu²⁺ (1-Cu(II)). Without Cu²⁺ coordination, the excitation energy flows from BB to P_Zn's with significantly high energy transfer efficiency. In contrast, the direction of energy flow in 1 was completely reversed by the coordination of Cu²⁺. The difference in fluorescence quantum yield between 1 and 1-Cu(II) indicates that more than 95% of excitation energy of P_Zn flows into Cu(II)-coordinated BB. The energy transfer efficiency was further controlled by bidentate ligand coordination onto 1-Cu(II). When pyrophosphate ion was added to 1-Cu(II), the recovery of fluorescence emission from P_Zn was observed. The quantum mechanical calculations indicated that the Cu(II)-coordinated BB has square planar geometry, which can be distorted to form octahedral geometry due to the coordination of bidentate ligands.