Self-assembly of double-decker cages induced by coordination of perylene bisimide with a trimeric Zn porphyrin: study of the electron transfer dynamics between the two photoactive components

Abstract

We describe the thermodynamic characterization of the assembly process of a covalently connected trimeric Zn porphyrin 1 induced by coordination to a bispyridyl functionalized perylene bisimide 2. The perylene bisimide ligands act as pillars via two axial coordination bonds with the porphyrinic Zn(II) ions fixing the planes of the porphyrin units in a nearly co-facial orientation and inducing the formation of trigonal prism-like structures. The fully assembled 1₂·2₃ aggregate and the partially assembled one, 1₂·2₂, in which only two zinc porphyrin sites of trimeric 1 are axially coordinated to 2, are present in solution in equilibrium with freely diffusing species 1 and 2. The strong quenching observed in the mixture for the luminescence of the components 2 and 1 is ascribed to an efficient photoinduced electron transfer from the Zn porphyrin units of 1 to coordinated 2 occurring upon excitation of both components within the assemblies. In the formed assemblies, the Zn porphyrin units of the trimer 1 behave independently. Thus, the porphyrin units that are not coordinated with 2 in the partially assembled complex, 1₂·2₂, display the same photophysical behaviour registered for freely diffusing 1. The rate of charge separation within the cage is nearly independent on the polarity of the solvent (ca. 10¹⁰ s⁻¹) whereas the charge recombination process, leading to the ground state, has a lifetime of 110 ps in dichloromethane and ca. 6 ns in toluene, in agreement with a Marcus inverted behaviour.