Coumarins C-153, C-480, and C-1 formed 1:2 (guest:host) complexes ith a watersoluble cavitand having eight carboxylic acid groups (OA) in aqueous borate buffer solution. The complexes were photoexcited in the presence of electron acceptors (methyl viologen, MV 2+, or TiO 2) to probe the possibility of electron transfer between a donor and an acceptor physically separated by a molecular wall. In solution at basic pH, the dication MV 2+ was associated to the exterior of the complex C-153@OA 2, as suggested by diffusion constants (?1.2 × 10- 6 cm2/s) determined by DOSY NMR. The fluorescence of C-153@OA 2 was quenched in the presence of increasing amounts of MV 2+ and Stern-Volmer plots of Io/I and ?o/? vs [MV 2+] indicated that the quenching was static. As per FT-IR-ATR spectra, the capsule C-153@OA 2 was bound to TiO 2 nanoparticle films. Selective excitation (?exc = 420) of the above bound complex resulted in fluorescence quenching. When adsorbed on insulating ZrO 2 nanoparticle films, excitation of the complex resulted in a broad fluorescence spectrum centered at 500 nm and consistent with C-153 being within the lipophilic capsule interior. Consistent with the above results, colloidal TiO 2 quenched the emission while colloidal ZrO 2 did not. © 2012 American Chemical Society.