Ultrafast photoinduced electron transfer between an incarcerated donor and a free acceptor in aqueous solution

Supramolecular photoinduced electron transfer dynamics between coumarin 153 (C153) and 4,4′-dimethyl viologen dichloride (MV²⁺) across the molecular barrier of a host molecule, octa acid (OA), has been investigated with femtosecond time resolution. The ultrafast electron transfer from C153 to MV²⁺ followed excitation with 150 fs laser pulses at a wavelength of 390 nm despite the fact that C153 was incarcerated within an OA₂ capsule. As a result, the photoexcited coumarin did not show any of the typical relaxation dynamics that is usually observed in free solution. Instead, the excited electron was transferred across the molecular wall of the capsuleplex within 20 ps. Likewise, the lifetime of the charge transfer state was short (724 ps), and electron back-transfer reestablished the ground state of the system within 1 ns, showing strong electronic coupling among the excited electron donor, host, and acceptor. When the donor was encapsulated into the host molecule, the electron transfer process showed significantly accelerated dynamics and essentially no solvent relaxation compared with that in free solution. The study was also extended to N-methylpyridinium iodide as the acceptor with similar results.