Photoinduced Processes in Some Mechanically Interlocked Supramolecules as Studied by Time-Resolved Electron Paramagnetic Resonance

Abstract

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used for studying photoinduced intramolecular electron transfer (ET) and energy transfer (EnT) processes in three mechanically interlocked molecules in which zinc(II)porphyrin (ZnP) and C60 fullerene moieties are arrayed around a central Cu(I) bisphenanthroline core used to assemble these donor–acceptor (D–A) systems. The specific molecules studied include a “long” (ZnP)2–Cu(I)(phen)2–C60 rotaxane as well as ZnP–Cu(I)(phen)2 and ZnP–Cu(I)(phen)2–C60 catenanes, embedded in different phases of nematic liquid crystal and frozen isotropic solvents. It was demonstrated that the routes and rates of the transfer processes in these supramolecules strongly depend on the characteristics of their microenvironment and the molecular entity which was selectively photoexcited. This is reflected by formation of distinct long-lived charge-separated species such as the radical ion pair (ZnP)2•+–Cu(I)(phen)2•– and the metal-to-ligand charge-separated state Cu(II)–(phen)2•– under the various experimental conditions. The results are discussed in terms of the correlation between the chemical structure, conformational mobility, and relaxation pathways of the photoexcited states in these mechanically interlocked systems. Results are compared with previously reported TREPR data on related interlocked D–A porphyrin/fullerene systems.