Photodynamics in stable complexes composed of a zinc porphyrin tripod and pyridyl porphyrins assembled by multiple coordination bonds

Abstract

A tripod zinc porphyrin (TPZn₃) forms a stable 1 ∶ 1 complex with gold(III) tetra(4-pyridyl)porphyrin (AuTPyP⁺) and free-base tris(4-pyridyl)porphyrin (2H-Py₃P) in nonpolar solvents. The strong binding of TPZn₃ with AuTPyP⁺ or 2H-Py₃P results from the encapsulation of AuTPyP⁺ or 2H-Py₃P inside the cavity of TPZn₃ through multiple coordination bonds, as indicated by UV-vis-NIR, ESI-MS, ¹H NMR, electrochemistry and computational studies. The binding constants of monomer zinc porphyrin (MPZn) with AuTPyP⁺ and 2H-Py₃P drastically decrease as compared with TPZn₃. Detailed photophysical studies have been carried out on these composites using laser flash photolysis as well as emission spectroscopy. The efficient quenching of the singlet excited state of TPZn₃ occurs via a photoinduced electron-transfer pathway in the TPZn₃–AuTPyP⁺ complex. In contrast, energy transfer occurs in the TPZn₃–2H-Py₃P complex due to the smaller driving force of the photoinduced electron-transfer pathway. Neither electron transfer nor energy transfer occurs from MPZn to AuTPyP⁺ under the same experimental conditions due to the small association constant of the monomer zinc porphyrin.