Ultrafast Singlet–Singlet Energy Transfer between an Acceptor Electrostatically Attached to the Walls of an Organic Capsule and the Enclosed Donor

Ultrafast Forster resonance energy transfer (FRET) between a donor (coumarin dye) and a cationic acceptor (rhodamine 6G) situated within and on the exterior walls, respectively, of a bimolecular capsule made up of anionic octa-acid (OA) is studied by femtosecond up-conversion. 1D and 2D 1H NMR spectral analyses reveal that donors coumarin-1, coumarin-153, and coumarin-480 form a 1:2 complex (guest to host) and are enclosed within the OA capsule. NMR and emission spectra suggest that the cationic acceptor rhodamine 6G is attached to the anionic exterior walls of the OA capsule. The acceptor emission displays a rise component of 1–3 ps which corresponds to a donor–acceptor distance (RDA) of ∼13 A and thus indicates the occurrence of ultrafast FRET between the donor and acceptor at close contact.