Charge separation in the non-covalent adducts of aromatic amines and the rhenium tricarbonyl complex bearing a cyclodextrin unit on a ligand

Luminescence of the rhenium tricarbonyl complex bearing a cyclodextrin unit on a ligand (1) was quenched by the diphenylamine-4-sulfonic acid included in the cavity of the cyclodextrin unit. The origin of the quenching was attributed to the electron transfer from the amine to the triplet metal-to-ligand charge transfer (MLCT) state of the rhenium complex unit. The rate of the forward electron transfer was estimated at 6.2 × 10⁷ s^-1 on the basis of the luminescence lifetime for the inclusion complex. Charge separation after the photoinduced electron transfer was verified by the observation of the transient absorption for the charge separation products. Most of the charge separation products were generated in the electron transfer process within the inclusion complex, which was proven by a large decrease in the yield of the products on the addition of a competitive guest for the cyclodextrin cavity, 1-adamantanol. The rate of the back electron transfer was estimated by the decay of the transient absorption at 4.4 × 10⁴ s^-1. Both of the rates for forward and back electron transfer were by far slower than those for a covalently-linked aniline-rhenium complex molecule.