Abstract
We have prepared a series of MII4L6 tetrahedral cages containing one or the other of two distinct BODIPY moieties, as well as mixed cages that contain both BODIPY chromophores. The photophysical properties of these cages and their fullerene-encapsulated adducts were studied in depth. Upon cage formation, the charge-transfer character exhibited by the bis(aminophenyl)-BODIPY subcomponents disappeared. Strong excitonic interactions were instead observed between at least two BODIPY chromophores along the edges of the cages, arising from the electronic delocalization through the metal centers. This excited-state delocalization contrasts with previously reported cages. All cages exhibited the same progression from an initial bright singlet state (species I) to a delocalized dark state (species II), driven by interactions between the transition dipoles of the ligands, and subsequently into geometrically relaxed species III. In the case of cages loaded with C60 or C70 fullerenes, ultrafast host-to-guest electron transfer was observed to compete with the excitonic interactions, short-circuiting the I → II → III sequence.
Highlights
Supramolecular chemistry has enabled the synthesis of intricate and functional structures
A recent example is the family of metal−organic polyhedra[2−13] formed by subcomponent self-assembly,[14−18] which have porous surfaces and central cavities reminiscent of biological recognition sites. These cages form inclusion complexes with a wide variety of guest species.[19−27] The modular design of these materials enables incorporation of dyes and fluorophores into the architecture, resulting in a high local density of chromophores held in a fixed geometry through noncovalent interactions.[28−31] The interaction of these structures with light is a topic of growing interest.[32−42] An understanding of their photophysical behavior, and how it arises from the properties of the subcomponents, could unlock the potential of these functional materials in applications from tailored photochemical reactors and sensitive chemical detection to hybrid photovoltaic and light-emitting devices
We report the synthesis and photophysical characterization of a family of metal−organic cage frameworks based on emissive BODIPY chromophores (Figure 1a)
Summary
Supramolecular chemistry has enabled the synthesis of intricate and functional structures. We observe the same basic three-state progression in homo- and heteroleptic samples: the initial delocalized bright state (species I) rapidly relaxes into a nonemissive delocalized state (giving species II), prompting slight geometric relaxation (generating species III) in response to the redistribution of the electronic wave function These effects appear to depend only weakly on the metal (ZnII or FeII) at the vertices or electronic nature of the chromophore edges and are posited to arise from the architecture of the cage framework itself.
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