Light Triggers Molecular Shuttling in Rotaxanes

Abstract

In this work on rotaxanes comprising a strapped-porphyrin as a ring molecule and a dumbbell bearing one fullerene stopper, coordination forces are crucial for the excited state behavior. Distinct differences in the lifetime of the charge-separated state and the presence of more than a single lifetime were observed. Here, the coordinating properties and the polarity of the solvent have the strongest impact. For example, in apolar, non-coordinating toluene, a single lifetime was found for the charge-separated state, whereas in polar, coordinating PhCN, a short-lived and a long-lived charge-separated state evolved. In toluene and PhCl, these effects were also induced when a coordinating species, such as pyridine, was added. Depending on the solvent polarity, the shuttling is either directional or random. In apolar toluene and in the presence of pyridine, Coulombic interactions in the ZnP•+-C60 •-radical ion pair dominate and, in turn, force a close proximity between the oxidized porphyrin and the reduced fullerene. This sets up the means for a directional molecular shuttling, which results in a drastically reduced lifetime of the charge-separated state. In highly polar benzonitrile, shuttling is random and allows the fullerene anion to move further away from the porphyrin cation. This enables the system to overcome the barrier of Coulombic attractions. As such, larger porphyrin-to-fullerene distances go hand-in-hand with a longer lifetime of the charge-separated state in a high-polarity coordinating solvent; longer relative to that in either non-coordinating solvents or low-polarity solvents. Independent confirmation for our hypothesis came from the absence of a longer and/or a shorter-lived charge-separated state in the presence of a coordinating solvent in analogous experiments with a system, which features a strapped free-base porphyrin rather than a zinc porphyrin. Our results emphasize the importance of coordination interactions. To gain further insight, rotaxanes with longer, more rigid axles, which should enhance the effects on the distribution of lifetimes, are currently being designed and studied in our laboratories. Finally, a porphyrin with a metal center that binds pyridine more strongly than zinc, such as ruthenium, could exhibit interesting behavior.